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.
3943 @item --batch-size=@var{nmerge}
3944 @opindex --batch-size
3945 @cindex number of inputs to merge, nmerge
3946 Merge at most @var{nmerge} inputs at once.
3948 When @command{sort} has to merge more than @var{nmerge} inputs,
3949 it merges them in groups of @var{nmerge}, saving the result in
3950 a temporary file, which is then used as an input in a subsequent merge.
3952 A large value of @var{nmerge} may improve merge performance and decrease
3953 temporary storage utilization at the expense of increased memory usage
3954 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3955 requirements and I/0 at the expense of temporary storage consumption and
3958 The value of @var{nmerge} must be at least 2. The default value is
3959 currently 16, but this is implementation-dependent and may change in
3962 The value of @var{nmerge} may be bounded by a resource limit for open
3963 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3964 OPEN_MAX} may display limits for your systems; these limits may be
3965 modified further if your program already has some files open, or if
3966 the operating system has other limits on the number of open files. If
3967 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3968 silently uses a smaller value.
3970 @item -o @var{output-file}
3971 @itemx --output=@var{output-file}
3974 @cindex overwriting of input, allowed
3975 Write output to @var{output-file} instead of standard output.
3976 Normally, @command{sort} reads all input before opening
3977 @var{output-file}, so you can safely sort a file in place by using
3978 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3979 However, @command{sort} with @option{--merge} (@option{-m}) can open
3980 the output file before reading all input, so a command like @code{cat
3981 F | sort -m -o F - G} is not safe as @command{sort} might start
3982 writing @file{F} before @command{cat} is done reading it.
3984 @vindex POSIXLY_CORRECT
3985 On newer systems, @option{-o} cannot appear after an input file if
3986 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3987 scripts should specify @option{-o @var{output-file}} before any input
3990 @item --random-source=@var{file}
3991 @opindex --random-source
3992 @cindex random source for sorting
3993 Use @var{file} as a source of random data used to determine which
3994 random hash function to use with the @option{-R} option. @xref{Random
4001 @cindex sort stability
4002 @cindex sort's last-resort comparison
4004 Make @command{sort} stable by disabling its last-resort comparison.
4005 This option has no effect if no fields or global ordering options
4006 other than @option{--reverse} (@option{-r}) are specified.
4009 @itemx --buffer-size=@var{size}
4011 @opindex --buffer-size
4012 @cindex size for main memory sorting
4013 Use a main-memory sort buffer of the given @var{size}. By default,
4014 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4015 @var{size} to be interpreted as a percentage of physical memory.
4016 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4017 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4018 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4019 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4022 This option can improve the performance of @command{sort} by causing it
4023 to start with a larger or smaller sort buffer than the default.
4024 However, this option affects only the initial buffer size. The buffer
4025 grows beyond @var{size} if @command{sort} encounters input lines larger
4028 @item -t @var{separator}
4029 @itemx --field-separator=@var{separator}
4031 @opindex --field-separator
4032 @cindex field separator character
4033 Use character @var{separator} as the field separator when finding the
4034 sort keys in each line. By default, fields are separated by the empty
4035 string between a non-blank character and a blank character.
4036 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4039 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4040 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4041 not considered to be part of either the field preceding or the field
4042 following, so with @samp{sort @w{-t " "}} the same input line has
4043 three fields: an empty field, @samp{foo}, and @samp{bar}.
4044 However, fields that extend to the end of the line,
4045 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4046 retain the field separators present between the endpoints of the range.
4048 To specify @acronym{ASCII} @sc{nul} as the field separator,
4049 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4051 @item -T @var{tempdir}
4052 @itemx --temporary-directory=@var{tempdir}
4054 @opindex --temporary-directory
4055 @cindex temporary directory
4057 Use directory @var{tempdir} to store temporary files, overriding the
4058 @env{TMPDIR} environment variable. If this option is given more than
4059 once, temporary files are stored in all the directories given. If you
4060 have a large sort or merge that is I/O-bound, you can often improve
4061 performance by using this option to specify directories on different
4062 disks and controllers.
4068 @cindex uniquifying output
4070 Normally, output only the first of a sequence of lines that compare
4071 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4072 check that no pair of consecutive lines compares equal.
4074 This option also disables the default last-resort comparison.
4076 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4077 this equivalence does not extend to arbitrary @command{sort} options.
4078 For example, @code{sort -n -u} inspects only the value of the initial
4079 numeric string when checking for uniqueness, whereas @code{sort -n |
4080 uniq} inspects the entire line. @xref{uniq invocation}.
4082 @macro zeroTerminatedOption
4084 @itemx --zero-terminated
4086 @opindex --zero-terminated
4087 @cindex process zero-terminated items
4088 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4089 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4090 and terminate output items with @acronym{ASCII} @sc{nul}.
4091 This option can be useful in conjunction with @samp{perl -0} or
4092 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4093 reliably handle arbitrary file names (even those containing blanks
4094 or other special characters).
4096 @zeroTerminatedOption
4100 Historical (BSD and System V) implementations of @command{sort} have
4101 differed in their interpretation of some options, particularly
4102 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4103 behavior, which is usually (but not always!) like the System V behavior.
4104 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4105 consistency, @option{-M} has been changed in the same way. This may
4106 affect the meaning of character positions in field specifications in
4107 obscure cases. The only fix is to add an explicit @option{-b}.
4109 A position in a sort field specified with @option{-k} may have any
4110 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4111 global ordering options are inherited by that particular field. The
4112 @option{-b} option may be independently attached to either or both of
4113 the start and end positions of a field specification, and if it is
4114 inherited from the global options it will be attached to both.
4115 If input lines can contain leading or adjacent blanks and @option{-t}
4116 is not used, then @option{-k} is typically combined with @option{-b} or
4117 an option that implicitly ignores leading blanks (@samp{MghnV}) as otherwise
4118 the varying numbers of leading blanks in fields can cause confusing results.
4120 If the start position in a sort field specifier falls after the end of
4121 the line or after the end field, the field is empty. If the @option{-b}
4122 option was specified, the @samp{.@var{c}} part of a field specification
4123 is counted from the first nonblank character of the field.
4125 @vindex _POSIX2_VERSION
4126 @vindex POSIXLY_CORRECT
4127 On older systems, @command{sort} supports an obsolete origin-zero
4128 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4129 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4130 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4131 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4132 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4134 This obsolete behavior can be enabled or disabled with the
4135 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4136 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4137 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4139 Scripts intended for use on standard hosts should avoid obsolete
4140 syntax and should use @option{-k} instead. For example, avoid
4141 @samp{sort +2}, since it might be interpreted as either @samp{sort
4142 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4143 support only the obsolete syntax, it can use a test like @samp{if sort
4144 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4147 Here are some examples to illustrate various combinations of options.
4152 Sort in descending (reverse) numeric order.
4159 Sort alphabetically, omitting the first and second fields
4160 and the blanks at the start of the third field.
4161 This uses a single key composed of the characters beginning
4162 at the start of the first nonblank character in field three
4163 and extending to the end of each line.
4170 Sort numerically on the second field and resolve ties by sorting
4171 alphabetically on the third and fourth characters of field five.
4172 Use @samp{:} as the field delimiter.
4175 sort -t : -k 2,2n -k 5.3,5.4
4178 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4179 @command{sort} would have used all characters beginning in the second field
4180 and extending to the end of the line as the primary @emph{numeric}
4181 key. For the large majority of applications, treating keys spanning
4182 more than one field as numeric will not do what you expect.
4184 Also note that the @samp{n} modifier was applied to the field-end
4185 specifier for the first key. It would have been equivalent to
4186 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4187 @samp{b} apply to the associated @emph{field}, regardless of whether
4188 the modifier character is attached to the field-start and/or the
4189 field-end part of the key specifier.
4192 Sort the password file on the fifth field and ignore any
4193 leading blanks. Sort lines with equal values in field five
4194 on the numeric user ID in field three. Fields are separated
4198 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4199 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4200 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4203 These three commands have equivalent effect. The first specifies that
4204 the first key's start position ignores leading blanks and the second
4205 key is sorted numerically. The other two commands rely on global
4206 options being inherited by sort keys that lack modifiers. The inheritance
4207 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4208 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4209 character position is not affected by whether initial blanks are
4213 Sort a set of log files, primarily by IPv4 address and secondarily by
4214 time stamp. If two lines' primary and secondary keys are identical,
4215 output the lines in the same order that they were input. The log
4216 files contain lines that look like this:
4219 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4220 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4223 Fields are separated by exactly one space. Sort IPv4 addresses
4224 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4225 because 61 is less than 129.
4228 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4229 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4232 This example cannot be done with a single @command{sort} invocation,
4233 since IPv4 address components are separated by @samp{.} while dates
4234 come just after a space. So it is broken down into two invocations of
4235 @command{sort}: the first sorts by time stamp and the second by IPv4
4236 address. The time stamp is sorted by year, then month, then day, and
4237 finally by hour-minute-second field, using @option{-k} to isolate each
4238 field. Except for hour-minute-second there's no need to specify the
4239 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4240 based on leading prefixes that cannot cross field boundaries. The
4241 IPv4 addresses are sorted lexicographically. The second sort uses
4242 @samp{-s} so that ties in the primary key are broken by the secondary
4243 key; the first sort uses @samp{-s} so that the combination of the two
4247 Generate a tags file in case-insensitive sorted order.
4250 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4253 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4254 that file names that contain blanks or other special characters are
4256 by the sort operation.
4258 @c This example is a bit contrived and needs more explanation.
4260 @c Sort records separated by an arbitrary string by using a pipe to convert
4261 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4262 @c and converting each @samp{\0} back to the original record delimiter.
4265 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4269 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4270 sort lines according to their length.
4273 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4276 In general this technique can be used to sort data that the @command{sort}
4277 command does not support, or is inefficient at, sorting directly.
4280 Shuffle a list of directories, but preserve the order of files within
4281 each directory. For instance, one could use this to generate a music
4282 playlist in which albums are shuffled but the songs of each album are
4286 ls */* | sort -t / -k 1,1R -k 2,2
4292 @node shuf invocation
4293 @section @command{shuf}: Shuffling text
4296 @cindex shuffling files
4298 @command{shuf} shuffles its input by outputting a random permutation
4299 of its input lines. Each output permutation is equally likely.
4303 shuf [@var{option}]@dots{} [@var{file}]
4304 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4305 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4308 @command{shuf} has three modes of operation that affect where it
4309 obtains its input lines. By default, it reads lines from standard
4310 input. The following options change the operation mode:
4318 @cindex command-line operands to shuffle
4319 Treat each command-line operand as an input line.
4321 @item -i @var{lo}-@var{hi}
4322 @itemx --input-range=@var{lo}-@var{hi}
4324 @opindex --input-range
4325 @cindex input range to shuffle
4326 Act as if input came from a file containing the range of unsigned
4327 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4331 @command{shuf}'s other options can affect its behavior in all
4336 @item -n @var{lines}
4337 @itemx --head-count=@var{count}
4339 @opindex --head-count
4340 @cindex head of output
4341 Output at most @var{count} lines. By default, all input lines are
4344 @item -o @var{output-file}
4345 @itemx --output=@var{output-file}
4348 @cindex overwriting of input, allowed
4349 Write output to @var{output-file} instead of standard output.
4350 @command{shuf} reads all input before opening
4351 @var{output-file}, so you can safely shuffle a file in place by using
4352 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4354 @item --random-source=@var{file}
4355 @opindex --random-source
4356 @cindex random source for shuffling
4357 Use @var{file} as a source of random data used to determine which
4358 permutation to generate. @xref{Random sources}.
4360 @zeroTerminatedOption
4376 might produce the output
4386 Similarly, the command:
4389 shuf -e clubs hearts diamonds spades
4403 and the command @samp{shuf -i 1-4} might output:
4413 These examples all have four input lines, so @command{shuf} might
4414 produce any of the twenty-four possible permutations of the input. In
4415 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4416 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4417 output permutations.
4422 @node uniq invocation
4423 @section @command{uniq}: Uniquify files
4426 @cindex uniquify files
4428 @command{uniq} writes the unique lines in the given @file{input}, or
4429 standard input if nothing is given or for an @var{input} name of
4433 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4436 By default, @command{uniq} prints its input lines, except that
4437 it discards all but the first of adjacent repeated lines, so that
4438 no output lines are repeated. Optionally, it can instead discard
4439 lines that are not repeated, or all repeated lines.
4441 The input need not be sorted, but repeated input lines are detected
4442 only if they are adjacent. If you want to discard non-adjacent
4443 duplicate lines, perhaps you want to use @code{sort -u}.
4444 @xref{sort invocation}.
4447 Comparisons honor the rules specified by the @env{LC_COLLATE}
4450 If no @var{output} file is specified, @command{uniq} writes to standard
4453 The program accepts the following options. Also see @ref{Common options}.
4458 @itemx --skip-fields=@var{n}
4460 @opindex --skip-fields
4461 Skip @var{n} fields on each line before checking for uniqueness. Use
4462 a null string for comparison if a line has fewer than @var{n} fields. Fields
4463 are sequences of non-space non-tab characters that are separated from
4464 each other by at least one space or tab.
4466 For compatibility @command{uniq} supports an obsolete option syntax
4467 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4470 @itemx --skip-chars=@var{n}
4472 @opindex --skip-chars
4473 Skip @var{n} characters before checking for uniqueness. Use a null string
4474 for comparison if a line has fewer than @var{n} characters. If you use both
4475 the field and character skipping options, fields are skipped over first.
4477 @vindex _POSIX2_VERSION
4478 On older systems, @command{uniq} supports an obsolete option syntax
4480 This obsolete behavior can be enabled or disabled with the
4481 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4482 conformance}), but portable scripts should avoid commands whose
4483 behavior depends on this variable.
4484 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4485 the ambiguous @samp{uniq +10}.
4491 Print the number of times each line occurred along with the line.
4494 @itemx --ignore-case
4496 @opindex --ignore-case
4497 Ignore differences in case when comparing lines.
4503 @cindex repeated lines, outputting
4504 Discard lines that are not repeated. When used by itself, this option
4505 causes @command{uniq} to print the first copy of each repeated line,
4509 @itemx --all-repeated[=@var{delimit-method}]
4511 @opindex --all-repeated
4512 @cindex all repeated lines, outputting
4513 Do not discard the second and subsequent repeated input lines,
4514 but discard lines that are not repeated.
4515 This option is useful mainly in conjunction with other options e.g.,
4516 to ignore case or to compare only selected fields.
4517 The optional @var{delimit-method} tells how to delimit
4518 groups of repeated lines, and must be one of the following:
4523 Do not delimit groups of repeated lines.
4524 This is equivalent to @option{--all-repeated} (@option{-D}).
4527 Output a newline before each group of repeated lines.
4528 With @option{--zero-terminated} (@option{-z}), use a zero
4529 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4532 Separate groups of repeated lines with a single newline.
4533 With @option{--zero-terminated} (@option{-z}), use a zero
4534 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4535 This is the same as using @samp{prepend}, except that
4536 no delimiter is inserted before the first group, and hence
4537 may be better suited for output direct to users.
4540 Note that when groups are delimited and the input stream contains
4541 two or more consecutive blank lines, then the output is ambiguous.
4542 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4543 each sequence of consecutive newlines with a single newline.
4545 This is a @sc{gnu} extension.
4546 @c FIXME: give an example showing *how* it's useful
4552 @cindex unique lines, outputting
4553 Discard the first repeated line. When used by itself, this option
4554 causes @command{uniq} to print unique lines, and nothing else.
4557 @itemx --check-chars=@var{n}
4559 @opindex --check-chars
4560 Compare at most @var{n} characters on each line (after skipping any specified
4561 fields and characters). By default the entire rest of the lines are
4564 @zeroTerminatedOption
4571 @node comm invocation
4572 @section @command{comm}: Compare two sorted files line by line
4575 @cindex line-by-line comparison
4576 @cindex comparing sorted files
4578 @command{comm} writes to standard output lines that are common, and lines
4579 that are unique, to two input files; a file name of @samp{-} means
4580 standard input. Synopsis:
4583 comm [@var{option}]@dots{} @var{file1} @var{file2}
4587 Before @command{comm} can be used, the input files must be sorted using the
4588 collating sequence specified by the @env{LC_COLLATE} locale.
4589 If an input file ends in a non-newline
4590 character, a newline is silently appended. The @command{sort} command with
4591 no options always outputs a file that is suitable input to @command{comm}.
4593 @cindex differing lines
4594 @cindex common lines
4595 With no options, @command{comm} produces three-column output. Column one
4596 contains lines unique to @var{file1}, column two contains lines unique
4597 to @var{file2}, and column three contains lines common to both files.
4598 Columns are separated by a single TAB character.
4599 @c FIXME: when there's an option to supply an alternative separator
4600 @c string, append `by default' to the above sentence.
4605 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4606 the corresponding columns (and separators). Also see @ref{Common options}.
4608 Unlike some other comparison utilities, @command{comm} has an exit
4609 status that does not depend on the result of the comparison.
4610 Upon normal completion @command{comm} produces an exit code of zero.
4611 If there is an error it exits with nonzero status.
4613 @macro checkOrderOption{cmd}
4614 If the @option{--check-order} option is given, unsorted inputs will
4615 cause a fatal error message. If the option @option{--nocheck-order}
4616 is given, unsorted inputs will never cause an error message. If
4617 neither of these options is given, wrongly sorted inputs are diagnosed
4618 only if an input file is found to contain unpairable lines. If an
4619 input file is diagnosed as being unsorted, the @command{\cmd\} command
4620 will exit with a nonzero status (and the output should not be used).
4622 Forcing @command{\cmd\} to process wrongly sorted input files
4623 containing unpairable lines by specifying @option{--nocheck-order} is
4624 not guaranteed to produce any particular output. The output will
4625 probably not correspond with whatever you hoped it would be.
4627 @checkOrderOption{comm}
4632 Fail with an error message if either input file is wrongly ordered.
4634 @item --nocheck-order
4635 Do not check that both input files are in sorted order.
4639 @item --output-delimiter=@var{str}
4640 Print @var{str} between adjacent output columns,
4641 rather than the default of a single TAB character.
4643 The delimiter @var{str} may not be empty.
4647 @node ptx invocation
4648 @section @command{ptx}: Produce permuted indexes
4652 @command{ptx} reads a text file and essentially produces a permuted index, with
4653 each keyword in its context. The calling sketch is either one of:
4656 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4657 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4660 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4661 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4662 limitations and changing several of the program's default option values.
4663 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4664 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4665 document. For the full list, see @xref{Compatibility in ptx}.
4667 Individual options are explained in the following sections.
4669 When @sc{gnu} extensions are enabled, there may be zero, one or several
4670 @var{file}s after the options. If there is no @var{file}, the program
4671 reads the standard input. If there is one or several @var{file}s, they
4672 give the name of input files which are all read in turn, as if all the
4673 input files were concatenated. However, there is a full contextual
4674 break between each file and, when automatic referencing is requested,
4675 file names and line numbers refer to individual text input files. In
4676 all cases, the program outputs the permuted index to the standard
4679 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4680 operates in traditional mode, there may be zero, one or two parameters
4681 besides the options. If there are no parameters, the program reads the
4682 standard input and outputs the permuted index to the standard output.
4683 If there is only one parameter, it names the text @var{input} to be read
4684 instead of the standard input. If two parameters are given, they give
4685 respectively the name of the @var{input} file to read and the name of
4686 the @var{output} file to produce. @emph{Be very careful} to note that,
4687 in this case, the contents of file given by the second parameter is
4688 destroyed. This behavior is dictated by System V @command{ptx}
4689 compatibility; @sc{gnu} Standards normally discourage output parameters not
4690 introduced by an option.
4692 Note that for @emph{any} file named as the value of an option or as an
4693 input text file, a single dash @kbd{-} may be used, in which case
4694 standard input is assumed. However, it would not make sense to use this
4695 convention more than once per program invocation.
4698 * General options in ptx:: Options which affect general program behavior.
4699 * Charset selection in ptx:: Underlying character set considerations.
4700 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4701 * Output formatting in ptx:: Types of output format, and sizing the fields.
4702 * Compatibility in ptx::
4706 @node General options in ptx
4707 @subsection General options
4712 @itemx --traditional
4713 As already explained, this option disables all @sc{gnu} extensions to
4714 @command{ptx} and switches to traditional mode.
4717 Print a short help on standard output, then exit without further
4721 Print the program version on standard output, then exit without further
4729 @node Charset selection in ptx
4730 @subsection Charset selection
4732 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4733 As it is set up now, the program assumes that the input file is coded
4734 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4735 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4736 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4737 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4738 of characters which are letters is different; this alters the behavior
4739 of regular expression matching. Thus, the default regular expression
4740 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4741 however, is still crude; it obeys the underlying character set ordering
4747 @itemx --ignore-case
4748 Fold lower case letters to upper case for sorting.
4753 @node Input processing in ptx
4754 @subsection Word selection and input processing
4759 @itemx --break-file=@var{file}
4761 This option provides an alternative (to @option{-W}) method of describing
4762 which characters make up words. It introduces the name of a
4763 file which contains a list of characters which can@emph{not} be part of
4764 one word; this file is called the @dfn{Break file}. Any character which
4765 is not part of the Break file is a word constituent. If both options
4766 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4767 @option{-b} is ignored.
4769 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4770 break character is to write all the break characters in the file with no
4771 newline at all, not even at the end of the file. When @sc{gnu} extensions
4772 are disabled, spaces, tabs and newlines are always considered as break
4773 characters even if not included in the Break file.
4776 @itemx --ignore-file=@var{file}
4778 The file associated with this option contains a list of words which will
4779 never be taken as keywords in concordance output. It is called the
4780 @dfn{Ignore file}. The file contains exactly one word in each line; the
4781 end of line separation of words is not subject to the value of the
4785 @itemx --only-file=@var{file}
4787 The file associated with this option contains a list of words which will
4788 be retained in concordance output; any word not mentioned in this file
4789 is ignored. The file is called the @dfn{Only file}. The file contains
4790 exactly one word in each line; the end of line separation of words is
4791 not subject to the value of the @option{-S} option.
4793 There is no default for the Only file. When both an Only file and an
4794 Ignore file are specified, a word is considered a keyword only
4795 if it is listed in the Only file and not in the Ignore file.
4800 On each input line, the leading sequence of non-white space characters will be
4801 taken to be a reference that has the purpose of identifying this input
4802 line in the resulting permuted index. For more information about reference
4803 production, see @xref{Output formatting in ptx}.
4804 Using this option changes the default value for option @option{-S}.
4806 Using this option, the program does not try very hard to remove
4807 references from contexts in output, but it succeeds in doing so
4808 @emph{when} the context ends exactly at the newline. If option
4809 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4810 are disabled, this condition is always met and references are completely
4811 excluded from the output contexts.
4813 @item -S @var{regexp}
4814 @itemx --sentence-regexp=@var{regexp}
4816 This option selects which regular expression will describe the end of a
4817 line or the end of a sentence. In fact, this regular expression is not
4818 the only distinction between end of lines or end of sentences, and input
4819 line boundaries have no special significance outside this option. By
4820 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4821 used, end of sentences are used. In this case, this @var{regex} is
4822 imported from @sc{gnu} Emacs:
4825 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4828 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4829 of lines are used; in this case, the default @var{regexp} is just:
4835 Using an empty @var{regexp} is equivalent to completely disabling end of
4836 line or end of sentence recognition. In this case, the whole file is
4837 considered to be a single big line or sentence. The user might want to
4838 disallow all truncation flag generation as well, through option @option{-F
4839 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4842 When the keywords happen to be near the beginning of the input line or
4843 sentence, this often creates an unused area at the beginning of the
4844 output context line; when the keywords happen to be near the end of the
4845 input line or sentence, this often creates an unused area at the end of
4846 the output context line. The program tries to fill those unused areas
4847 by wrapping around context in them; the tail of the input line or
4848 sentence is used to fill the unused area on the left of the output line;
4849 the head of the input line or sentence is used to fill the unused area
4850 on the right of the output line.
4852 As a matter of convenience to the user, many usual backslashed escape
4853 sequences from the C language are recognized and converted to the
4854 corresponding characters by @command{ptx} itself.
4856 @item -W @var{regexp}
4857 @itemx --word-regexp=@var{regexp}
4859 This option selects which regular expression will describe each keyword.
4860 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4861 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4862 disabled, a word is by default anything which ends with a space, a tab
4863 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4865 An empty @var{regexp} is equivalent to not using this option.
4866 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4869 As a matter of convenience to the user, many usual backslashed escape
4870 sequences, as found in the C language, are recognized and converted to
4871 the corresponding characters by @command{ptx} itself.
4876 @node Output formatting in ptx
4877 @subsection Output formatting
4879 Output format is mainly controlled by the @option{-O} and @option{-T} options
4880 described in the table below. When neither @option{-O} nor @option{-T} are
4881 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4882 output format suitable for a dumb terminal. Each keyword occurrence is
4883 output to the center of one line, surrounded by its left and right
4884 contexts. Each field is properly justified, so the concordance output
4885 can be readily observed. As a special feature, if automatic
4886 references are selected by option @option{-A} and are output before the
4887 left context, that is, if option @option{-R} is @emph{not} selected, then
4888 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4889 Emacs @code{next-error} processing. In this default output format, each
4890 white space character, like newline and tab, is merely changed to
4891 exactly one space, with no special attempt to compress consecutive
4892 spaces. This might change in the future. Except for those white space
4893 characters, every other character of the underlying set of 256
4894 characters is transmitted verbatim.
4896 Output format is further controlled by the following options.
4900 @item -g @var{number}
4901 @itemx --gap-size=@var{number}
4903 Select the size of the minimum white space gap between the fields on the
4906 @item -w @var{number}
4907 @itemx --width=@var{number}
4909 Select the maximum output width of each final line. If references are
4910 used, they are included or excluded from the maximum output width
4911 depending on the value of option @option{-R}. If this option is not
4912 selected, that is, when references are output before the left context,
4913 the maximum output width takes into account the maximum length of all
4914 references. If this option is selected, that is, when references are
4915 output after the right context, the maximum output width does not take
4916 into account the space taken by references, nor the gap that precedes
4920 @itemx --auto-reference
4922 Select automatic references. Each input line will have an automatic
4923 reference made up of the file name and the line ordinal, with a single
4924 colon between them. However, the file name will be empty when standard
4925 input is being read. If both @option{-A} and @option{-r} are selected, then
4926 the input reference is still read and skipped, but the automatic
4927 reference is used at output time, overriding the input reference.
4930 @itemx --right-side-refs
4932 In the default output format, when option @option{-R} is not used, any
4933 references produced by the effect of options @option{-r} or @option{-A} are
4934 placed to the far right of output lines, after the right context. With
4935 default output format, when the @option{-R} option is specified, references
4936 are rather placed at the beginning of each output line, before the left
4937 context. For any other output format, option @option{-R} is
4938 ignored, with one exception: with @option{-R} the width of references
4939 is @emph{not} taken into account in total output width given by @option{-w}.
4941 This option is automatically selected whenever @sc{gnu} extensions are
4944 @item -F @var{string}
4945 @itemx --flac-truncation=@var{string}
4947 This option will request that any truncation in the output be reported
4948 using the string @var{string}. Most output fields theoretically extend
4949 towards the beginning or the end of the current line, or current
4950 sentence, as selected with option @option{-S}. But there is a maximum
4951 allowed output line width, changeable through option @option{-w}, which is
4952 further divided into space for various output fields. When a field has
4953 to be truncated because it cannot extend beyond the beginning or the end of
4954 the current line to fit in, then a truncation occurs. By default,
4955 the string used is a single slash, as in @option{-F /}.
4957 @var{string} may have more than one character, as in @option{-F ...}.
4958 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4959 truncation flagging is disabled, and no truncation marks are appended in
4962 As a matter of convenience to the user, many usual backslashed escape
4963 sequences, as found in the C language, are recognized and converted to
4964 the corresponding characters by @command{ptx} itself.
4966 @item -M @var{string}
4967 @itemx --macro-name=@var{string}
4969 Select another @var{string} to be used instead of @samp{xx}, while
4970 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
4973 @itemx --format=roff
4975 Choose an output format suitable for @command{nroff} or @command{troff}
4976 processing. Each output line will look like:
4979 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
4982 so it will be possible to write a @samp{.xx} roff macro to take care of
4983 the output typesetting. This is the default output format when @sc{gnu}
4984 extensions are disabled. Option @option{-M} can be used to change
4985 @samp{xx} to another macro name.
4987 In this output format, each non-graphical character, like newline and
4988 tab, is merely changed to exactly one space, with no special attempt to
4989 compress consecutive spaces. Each quote character: @kbd{"} is doubled
4990 so it will be correctly processed by @command{nroff} or @command{troff}.
4995 Choose an output format suitable for @TeX{} processing. Each output
4996 line will look like:
4999 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5003 so it will be possible to write a @code{\xx} definition to take care of
5004 the output typesetting. Note that when references are not being
5005 produced, that is, neither option @option{-A} nor option @option{-r} is
5006 selected, the last parameter of each @code{\xx} call is inhibited.
5007 Option @option{-M} can be used to change @samp{xx} to another macro
5010 In this output format, some special characters, like @kbd{$}, @kbd{%},
5011 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5012 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5013 backslash and a pair of dollar signs (to force mathematical mode). The
5014 backslash itself produces the sequence @code{\backslash@{@}}.
5015 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5016 @code{~\@{ @}} respectively. Other diacriticized characters of the
5017 underlying character set produce an appropriate @TeX{} sequence as far
5018 as possible. The other non-graphical characters, like newline and tab,
5019 and all other characters which are not part of @acronym{ASCII}, are merely
5020 changed to exactly one space, with no special attempt to compress
5021 consecutive spaces. Let me know how to improve this special character
5022 processing for @TeX{}.
5027 @node Compatibility in ptx
5028 @subsection The @sc{gnu} extensions to @command{ptx}
5030 This version of @command{ptx} contains a few features which do not exist in
5031 System V @command{ptx}. These extra features are suppressed by using the
5032 @option{-G} command line option, unless overridden by other command line
5033 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5034 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5035 Here are the differences between this program and System V @command{ptx}.
5040 This program can read many input files at once, it always writes the
5041 resulting concordance on standard output. On the other hand, System V
5042 @command{ptx} reads only one file and sends the result to standard output
5043 or, if a second @var{file} parameter is given on the command, to that
5046 Having output parameters not introduced by options is a dangerous
5047 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5048 portably between @sc{gnu} and System V, you should always use it with a
5049 single input file, and always expect the result on standard output. You
5050 might also want to automatically configure in a @option{-G} option to
5051 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5052 that the installed @command{ptx} accepts @option{-G}.
5055 The only options available in System V @command{ptx} are options @option{-b},
5056 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5057 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5058 this enumeration. Moreover, some options have a slightly different
5059 meaning when @sc{gnu} extensions are enabled, as explained below.
5062 By default, concordance output is not formatted for @command{troff} or
5063 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5064 or @command{nroff} output may still be selected through option @option{-O}.
5067 Unless @option{-R} option is used, the maximum reference width is
5068 subtracted from the total output line width. With @sc{gnu} extensions
5069 disabled, width of references is not taken into account in the output
5070 line width computations.
5073 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5074 processed from input file with no adverse effect, even if @sc{gnu} extensions
5075 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5076 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5079 Input line length is only limited by available memory, even if @sc{gnu}
5080 extensions are disabled. However, System V @command{ptx} processes only
5081 the first 200 characters in each line.
5084 The break (non-word) characters default to be every character except all
5085 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5086 extensions are disabled, the break characters default to space, tab and
5090 The program makes better use of output line width. If @sc{gnu} extensions
5091 are disabled, the program rather tries to imitate System V @command{ptx},
5092 but still, there are some slight disposition glitches this program does
5093 not completely reproduce.
5096 The user can specify both an Ignore file and an Only file. This is not
5097 allowed with System V @command{ptx}.
5102 @node tsort invocation
5103 @section @command{tsort}: Topological sort
5106 @cindex topological sort
5108 @command{tsort} performs a topological sort on the given @var{file}, or
5109 standard input if no input file is given or for a @var{file} of
5110 @samp{-}. For more details and some history, see @ref{tsort background}.
5114 tsort [@var{option}] [@var{file}]
5117 @command{tsort} reads its input as pairs of strings, separated by blanks,
5118 indicating a partial ordering. The output is a total ordering that
5119 corresponds to the given partial ordering.
5133 will produce the output
5144 Consider a more realistic example.
5145 You have a large set of functions all in one file, and they may all be
5146 declared static except one. Currently that one (say @code{main}) is the
5147 first function defined in the file, and the ones it calls directly follow
5148 it, followed by those they call, etc. Let's say that you are determined
5149 to take advantage of prototypes, so you have to choose between declaring
5150 all of those functions (which means duplicating a lot of information from
5151 the definitions) and rearranging the functions so that as many as possible
5152 are defined before they are used. One way to automate the latter process
5153 is to get a list for each function of the functions it calls directly.
5154 Many programs can generate such lists. They describe a call graph.
5155 Consider the following list, in which a given line indicates that the
5156 function on the left calls the one on the right directly.
5162 tail_file pretty_name
5163 tail_file write_header
5165 tail_forever recheck
5166 tail_forever pretty_name
5167 tail_forever write_header
5168 tail_forever dump_remainder
5171 tail_lines start_lines
5172 tail_lines dump_remainder
5173 tail_lines file_lines
5174 tail_lines pipe_lines
5176 tail_bytes start_bytes
5177 tail_bytes dump_remainder
5178 tail_bytes pipe_bytes
5179 file_lines dump_remainder
5183 then you can use @command{tsort} to produce an ordering of those
5184 functions that satisfies your requirement.
5187 example$ tsort call-graph | tac
5207 @command{tsort} detects any cycles in the input and writes the first cycle
5208 encountered to standard error.
5210 Note that for a given partial ordering, generally there is no unique
5211 total ordering. In the context of the call graph above, the function
5212 @code{parse_options} may be placed anywhere in the list as long as it
5213 precedes @code{main}.
5215 The only options are @option{--help} and @option{--version}. @xref{Common
5221 * tsort background:: Where tsort came from.
5224 @node tsort background
5225 @subsection @command{tsort}: Background
5227 @command{tsort} exists because very early versions of the Unix linker processed
5228 an archive file exactly once, and in order. As @command{ld} read each object
5229 in the archive, it decided whether it was needed in the program based on
5230 whether it defined any symbols which were undefined at that point in
5233 This meant that dependencies within the archive had to be handled
5234 specially. For example, @code{scanf} probably calls @code{read}. That means
5235 that in a single pass through an archive, it was important for @code{scanf.o}
5236 to appear before read.o, because otherwise a program which calls
5237 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5238 reference to @code{read}.
5240 The way to address this problem was to first generate a set of
5241 dependencies of one object file on another. This was done by a shell
5242 script called @command{lorder}. The GNU tools don't provide a version of
5243 lorder, as far as I know, but you can still find it in BSD
5246 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5247 resulting sort to define the order in which you added objects to the archive.
5249 This whole procedure has been obsolete since about 1980, because
5250 Unix archives now contain a symbol table (traditionally built by
5251 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5252 linker uses the symbol table to effectively make multiple passes over
5255 Anyhow, that's where tsort came from. To solve an old problem with
5256 the way the linker handled archive files, which has since been solved
5260 @node Operating on fields
5261 @chapter Operating on fields
5264 * cut invocation:: Print selected parts of lines.
5265 * paste invocation:: Merge lines of files.
5266 * join invocation:: Join lines on a common field.
5270 @node cut invocation
5271 @section @command{cut}: Print selected parts of lines
5274 @command{cut} writes to standard output selected parts of each line of each
5275 input file, or standard input if no files are given or for a file name of
5279 cut @var{option}@dots{} [@var{file}]@dots{}
5282 In the table which follows, the @var{byte-list}, @var{character-list},
5283 and @var{field-list} are one or more numbers or ranges (two numbers
5284 separated by a dash) separated by commas. Bytes, characters, and
5285 fields are numbered starting at 1. Incomplete ranges may be
5286 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5287 @samp{@var{n}} through end of line or last field. The list elements
5288 can be repeated, can overlap, and can be specified in any order; but
5289 the selected input is written in the same order that it is read, and
5290 is written exactly once.
5292 The program accepts the following options. Also see @ref{Common
5297 @item -b @var{byte-list}
5298 @itemx --bytes=@var{byte-list}
5301 Select for printing only the bytes in positions listed in
5302 @var{byte-list}. Tabs and backspaces are treated like any other
5303 character; they take up 1 byte. If an output delimiter is specified,
5304 (see the description of @option{--output-delimiter}), then output that
5305 string between ranges of selected bytes.
5307 @item -c @var{character-list}
5308 @itemx --characters=@var{character-list}
5310 @opindex --characters
5311 Select for printing only the characters in positions listed in
5312 @var{character-list}. The same as @option{-b} for now, but
5313 internationalization will change that. Tabs and backspaces are
5314 treated like any other character; they take up 1 character. If an
5315 output delimiter is specified, (see the description of
5316 @option{--output-delimiter}), then output that string between ranges
5319 @item -f @var{field-list}
5320 @itemx --fields=@var{field-list}
5323 Select for printing only the fields listed in @var{field-list}.
5324 Fields are separated by a TAB character by default. Also print any
5325 line that contains no delimiter character, unless the
5326 @option{--only-delimited} (@option{-s}) option is specified.
5327 Note @command{cut} does not support specifying runs of whitespace as a
5328 delimiter, so to achieve that common functionality one can pre-process
5329 with @command{tr} like:
5331 tr -s '[:blank:]' '\t' | cut -f@dots{}
5334 @item -d @var{input_delim_byte}
5335 @itemx --delimiter=@var{input_delim_byte}
5337 @opindex --delimiter
5338 With @option{-f}, use the first byte of @var{input_delim_byte} as
5339 the input fields separator (default is TAB).
5343 Do not split multi-byte characters (no-op for now).
5346 @itemx --only-delimited
5348 @opindex --only-delimited
5349 For @option{-f}, do not print lines that do not contain the field separator
5350 character. Normally, any line without a field separator is printed verbatim.
5352 @item --output-delimiter=@var{output_delim_string}
5353 @opindex --output-delimiter
5354 With @option{-f}, output fields are separated by @var{output_delim_string}.
5355 The default with @option{-f} is to use the input delimiter.
5356 When using @option{-b} or @option{-c} to select ranges of byte or
5357 character offsets (as opposed to ranges of fields),
5358 output @var{output_delim_string} between non-overlapping
5359 ranges of selected bytes.
5362 @opindex --complement
5363 This option is a @acronym{GNU} extension.
5364 Select for printing the complement of the bytes, characters or fields
5365 selected with the @option{-b}, @option{-c} or @option{-f} options.
5366 In other words, do @emph{not} print the bytes, characters or fields
5367 specified via those options. This option is useful when you have
5368 many fields and want to print all but a few of them.
5375 @node paste invocation
5376 @section @command{paste}: Merge lines of files
5379 @cindex merging files
5381 @command{paste} writes to standard output lines consisting of sequentially
5382 corresponding lines of each given file, separated by a TAB character.
5383 Standard input is used for a file name of @samp{-} or if no input files
5405 paste [@var{option}]@dots{} [@var{file}]@dots{}
5408 The program accepts the following options. Also see @ref{Common options}.
5416 Paste the lines of one file at a time rather than one line from each
5417 file. Using the above example data:
5420 $ paste -s num2 let3
5425 @item -d @var{delim-list}
5426 @itemx --delimiters=@var{delim-list}
5428 @opindex --delimiters
5429 Consecutively use the characters in @var{delim-list} instead of
5430 TAB to separate merged lines. When @var{delim-list} is
5431 exhausted, start again at its beginning. Using the above example data:
5434 $ paste -d '%_' num2 let3 num2
5445 @node join invocation
5446 @section @command{join}: Join lines on a common field
5449 @cindex common field, joining on
5451 @command{join} writes to standard output a line for each pair of input
5452 lines that have identical join fields. Synopsis:
5455 join [@var{option}]@dots{} @var{file1} @var{file2}
5458 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5459 meaning standard input. @var{file1} and @var{file2} should be
5460 sorted on the join fields.
5463 Normally, the sort order is that of the
5464 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5465 the @option{-t} option is given, the sort comparison ignores blanks at
5466 the start of the join field, as in @code{sort -b}. If the
5467 @option{--ignore-case} option is given, the sort comparison ignores
5468 the case of characters in the join field, as in @code{sort -f}.
5470 The @command{sort} and @command{join} commands should use consistent
5471 locales and options if the output of @command{sort} is fed to
5472 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5473 sort a file on its default join field, but if you select a non-default
5474 locale, join field, separator, or comparison options, then you should
5475 do so consistently between @command{join} and @command{sort}.
5476 If @samp{join -t ''} is specified then the whole line is considered which
5477 matches the default operation of sort.
5479 If the input has no unpairable lines, a @acronym{GNU} extension is
5480 available; the sort order can be any order that considers two fields
5481 to be equal if and only if the sort comparison described above
5482 considers them to be equal. For example:
5499 @checkOrderOption{join}
5503 @item the join field is the first field in each line;
5504 @item fields in the input are separated by one or more blanks, with leading
5505 blanks on the line ignored;
5506 @item fields in the output are separated by a space;
5507 @item each output line consists of the join field, the remaining
5508 fields from @var{file1}, then the remaining fields from @var{file2}.
5511 The program accepts the following options. Also see @ref{Common options}.
5515 @item -a @var{file-number}
5517 Print a line for each unpairable line in file @var{file-number} (either
5518 @samp{1} or @samp{2}), in addition to the normal output.
5521 Fail with an error message if either input file is wrongly ordered.
5523 @item --nocheck-order
5524 Do not check that both input files are in sorted order. This is the default.
5526 @item -e @var{string}
5528 Replace those output fields that are missing in the input with
5533 Treat the first line of each input file as a header line. The header lines will
5534 be joined and printed as the first output line. If @option{-o} is used to
5535 specify output format, the header line will be printed according to the
5536 specified format. The header lines will not be checked for ordering even if
5537 @option{--check-order} is specified. Also if the header lines from each file
5538 do not match, the heading fields from the first file will be used.
5541 @itemx --ignore-case
5543 @opindex --ignore-case
5544 Ignore differences in case when comparing keys.
5545 With this option, the lines of the input files must be ordered in the same way.
5546 Use @samp{sort -f} to produce this ordering.
5548 @item -1 @var{field}
5550 Join on field @var{field} (a positive integer) of file 1.
5552 @item -2 @var{field}
5554 Join on field @var{field} (a positive integer) of file 2.
5556 @item -j @var{field}
5557 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5559 @item -o @var{field-list}
5560 Construct each output line according to the format in @var{field-list}.
5561 Each element in @var{field-list} is either the single character @samp{0} or
5562 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5563 @samp{2} and @var{n} is a positive field number.
5565 A field specification of @samp{0} denotes the join field.
5566 In most cases, the functionality of the @samp{0} field spec
5567 may be reproduced using the explicit @var{m.n} that corresponds
5568 to the join field. However, when printing unpairable lines
5569 (using either of the @option{-a} or @option{-v} options), there is no way
5570 to specify the join field using @var{m.n} in @var{field-list}
5571 if there are unpairable lines in both files.
5572 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5573 field specification notation.
5575 The elements in @var{field-list}
5576 are separated by commas or blanks.
5577 Blank separators typically need to be quoted for the shell. For
5578 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5579 2.2'} are equivalent.
5581 All output lines---including those printed because of any -a or -v
5582 option---are subject to the specified @var{field-list}.
5585 Use character @var{char} as the input and output field separator.
5586 Treat as significant each occurrence of @var{char} in the input file.
5587 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5588 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5589 the whole line is considered, matching the default operation of sort.
5590 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5591 character is used to delimit the fields.
5593 @item -v @var{file-number}
5594 Print a line for each unpairable line in file @var{file-number}
5595 (either @samp{1} or @samp{2}), instead of the normal output.
5602 @node Operating on characters
5603 @chapter Operating on characters
5605 @cindex operating on characters
5607 This commands operate on individual characters.
5610 * tr invocation:: Translate, squeeze, and/or delete characters.
5611 * expand invocation:: Convert tabs to spaces.
5612 * unexpand invocation:: Convert spaces to tabs.
5617 @section @command{tr}: Translate, squeeze, and/or delete characters
5624 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5627 @command{tr} copies standard input to standard output, performing
5628 one of the following operations:
5632 translate, and optionally squeeze repeated characters in the result,
5634 squeeze repeated characters,
5638 delete characters, then squeeze repeated characters from the result.
5641 The @var{set1} and (if given) @var{set2} arguments define ordered
5642 sets of characters, referred to below as @var{set1} and @var{set2}. These
5643 sets are the characters of the input that @command{tr} operates on.
5644 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5646 complement (all of the characters that are not in @var{set1}).
5648 Currently @command{tr} fully supports only single-byte characters.
5649 Eventually it will support multibyte characters; when it does, the
5650 @option{-C} option will cause it to complement the set of characters,
5651 whereas @option{-c} will cause it to complement the set of values.
5652 This distinction will matter only when some values are not characters,
5653 and this is possible only in locales using multibyte encodings when
5654 the input contains encoding errors.
5656 The program accepts the @option{--help} and @option{--version}
5657 options. @xref{Common options}. Options must precede operands.
5662 * Character sets:: Specifying sets of characters.
5663 * Translating:: Changing one set of characters to another.
5664 * Squeezing:: Squeezing repeats and deleting.
5668 @node Character sets
5669 @subsection Specifying sets of characters
5671 @cindex specifying sets of characters
5673 The format of the @var{set1} and @var{set2} arguments resembles
5674 the format of regular expressions; however, they are not regular
5675 expressions, only lists of characters. Most characters simply
5676 represent themselves in these strings, but the strings can contain
5677 the shorthands listed below, for convenience. Some of them can be
5678 used only in @var{set1} or @var{set2}, as noted below.
5682 @item Backslash escapes
5683 @cindex backslash escapes
5685 The following backslash escape sequences are recognized:
5703 The character with the value given by @var{ooo}, which is 1 to 3
5709 While a backslash followed by a character not listed above is
5710 interpreted as that character, the backslash also effectively
5711 removes any special significance, so it is useful to escape
5712 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5717 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5718 from @var{m} through @var{n}, in ascending order. @var{m} should
5719 collate before @var{n}; if it doesn't, an error results. As an example,
5720 @samp{0-9} is the same as @samp{0123456789}.
5722 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5723 brackets to enclose ranges. Translations specified in that format
5724 sometimes work as expected, since the brackets are often transliterated
5725 to themselves. However, they should be avoided because they sometimes
5726 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5729 Many historically common and even accepted uses of ranges are not
5730 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5731 range will not do what most would expect because @samp{A} through @samp{Z}
5732 are not contiguous as they are in @acronym{ASCII}.
5733 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5734 the best way to work around this is to use character classes (see below).
5735 Otherwise, it is most portable (and most ugly) to enumerate the members
5738 @item Repeated characters
5739 @cindex repeated characters
5741 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5742 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5743 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5744 to as many copies of @var{c} as are needed to make @var{set2} as long as
5745 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5746 octal, otherwise in decimal.
5748 @item Character classes
5749 @cindex character classes
5751 The notation @samp{[:@var{class}:]} expands to all of the characters in
5752 the (predefined) class @var{class}. The characters expand in no
5753 particular order, except for the @code{upper} and @code{lower} classes,
5754 which expand in ascending order. When the @option{--delete} (@option{-d})
5755 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5756 character class can be used in @var{set2}. Otherwise, only the
5757 character classes @code{lower} and @code{upper} are accepted in
5758 @var{set2}, and then only if the corresponding character class
5759 (@code{upper} and @code{lower}, respectively) is specified in the same
5760 relative position in @var{set1}. Doing this specifies case conversion.
5761 The class names are given below; an error results when an invalid class
5773 Horizontal whitespace.
5782 Printable characters, not including space.
5788 Printable characters, including space.
5791 Punctuation characters.
5794 Horizontal or vertical whitespace.
5803 @item Equivalence classes
5804 @cindex equivalence classes
5806 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5807 equivalent to @var{c}, in no particular order. Equivalence classes are
5808 a relatively recent invention intended to support non-English alphabets.
5809 But there seems to be no standard way to define them or determine their
5810 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5811 each character's equivalence class consists only of that character,
5812 which is of no particular use.
5818 @subsection Translating
5820 @cindex translating characters
5822 @command{tr} performs translation when @var{set1} and @var{set2} are
5823 both given and the @option{--delete} (@option{-d}) option is not given.
5824 @command{tr} translates each character of its input that is in @var{set1}
5825 to the corresponding character in @var{set2}. Characters not in
5826 @var{set1} are passed through unchanged. When a character appears more
5827 than once in @var{set1} and the corresponding characters in @var{set2}
5828 are not all the same, only the final one is used. For example, these
5829 two commands are equivalent:
5836 A common use of @command{tr} is to convert lowercase characters to
5837 uppercase. This can be done in many ways. Here are three of them:
5840 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5842 tr '[:lower:]' '[:upper:]'
5846 But note that using ranges like @code{a-z} above is not portable.
5848 When @command{tr} is performing translation, @var{set1} and @var{set2}
5849 typically have the same length. If @var{set1} is shorter than
5850 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5852 On the other hand, making @var{set1} longer than @var{set2} is not
5853 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5854 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5855 the last character of @var{set2} as many times as necessary. System V
5856 @command{tr} truncates @var{set1} to the length of @var{set2}.
5858 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5859 When the @option{--truncate-set1} (@option{-t}) option is given,
5860 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5861 instead. This option is ignored for operations other than translation.
5863 Acting like System V @command{tr} in this case breaks the relatively common
5867 tr -cs A-Za-z0-9 '\012'
5871 because it converts only zero bytes (the first element in the
5872 complement of @var{set1}), rather than all non-alphanumerics, to
5876 By the way, the above idiom is not portable because it uses ranges, and
5877 it assumes that the octal code for newline is 012.
5878 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5881 tr -cs '[:alnum:]' '[\n*]'
5886 @subsection Squeezing repeats and deleting
5888 @cindex squeezing repeat characters
5889 @cindex deleting characters
5891 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5892 removes any input characters that are in @var{set1}.
5894 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5895 @command{tr} replaces each input sequence of a repeated character that
5896 is in @var{set1} with a single occurrence of that character.
5898 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5899 first performs any deletions using @var{set1}, then squeezes repeats
5900 from any remaining characters using @var{set2}.
5902 The @option{--squeeze-repeats} option may also be used when translating,
5903 in which case @command{tr} first performs translation, then squeezes
5904 repeats from any remaining characters using @var{set2}.
5906 Here are some examples to illustrate various combinations of options:
5911 Remove all zero bytes:
5918 Put all words on lines by themselves. This converts all
5919 non-alphanumeric characters to newlines, then squeezes each string
5920 of repeated newlines into a single newline:
5923 tr -cs '[:alnum:]' '[\n*]'
5927 Convert each sequence of repeated newlines to a single newline:
5934 Find doubled occurrences of words in a document.
5935 @c Separate the following two "the"s, so typo checkers don't complain.
5936 For example, people often write ``the @w{}the'' with the repeated words
5937 separated by a newline. The Bourne shell script below works first
5938 by converting each sequence of punctuation and blank characters to a
5939 single newline. That puts each ``word'' on a line by itself.
5940 Next it maps all uppercase characters to lower case, and finally it
5941 runs @command{uniq} with the @option{-d} option to print out only the words
5947 | tr -s '[:punct:][:blank:]' '[\n*]' \
5948 | tr '[:upper:]' '[:lower:]' \
5953 Deleting a small set of characters is usually straightforward. For example,
5954 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5960 However, when @samp{-} is one of those characters, it can be tricky because
5961 @samp{-} has special meanings. Performing the same task as above but also
5962 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5963 that would fail because @command{tr} would try to interpret @option{-a} as
5964 a command-line option. Alternatively, we could try putting the hyphen
5965 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5966 it would make @command{tr} interpret @code{a-x} as the range of characters
5967 @samp{a}@dots{}@samp{x} rather than the three.
5968 One way to solve the problem is to put the hyphen at the end of the list
5975 Or you can use @samp{--} to terminate option processing:
5981 More generally, use the character class notation @code{[=c=]}
5982 with @samp{-} (or any other character) in place of the @samp{c}:
5988 Note how single quotes are used in the above example to protect the
5989 square brackets from interpretation by a shell.
5994 @node expand invocation
5995 @section @command{expand}: Convert tabs to spaces
5998 @cindex tabs to spaces, converting
5999 @cindex converting tabs to spaces
6001 @command{expand} writes the contents of each given @var{file}, or standard
6002 input if none are given or for a @var{file} of @samp{-}, to standard
6003 output, with tab characters converted to the appropriate number of
6007 expand [@var{option}]@dots{} [@var{file}]@dots{}
6010 By default, @command{expand} converts all tabs to spaces. It preserves
6011 backspace characters in the output; they decrement the column count for
6012 tab calculations. The default action is equivalent to @option{-t 8} (set
6013 tabs every 8 columns).
6015 The program accepts the following options. Also see @ref{Common options}.
6019 @item -t @var{tab1}[,@var{tab2}]@dots{}
6020 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6023 @cindex tab stops, setting
6024 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6025 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6026 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6027 last tab stop given with single spaces. Tab stops can be separated by
6028 blanks as well as by commas.
6030 For compatibility, GNU @command{expand} also accepts the obsolete
6031 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6032 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6038 @cindex initial tabs, converting
6039 Only convert initial tabs (those that precede all non-space or non-tab
6040 characters) on each line to spaces.
6047 @node unexpand invocation
6048 @section @command{unexpand}: Convert spaces to tabs
6052 @command{unexpand} writes the contents of each given @var{file}, or
6053 standard input if none are given or for a @var{file} of @samp{-}, to
6054 standard output, converting blanks at the beginning of each line into
6055 as many tab characters as needed. In the default @acronym{POSIX}
6056 locale, a @dfn{blank} is a space or a tab; other locales may specify
6057 additional blank characters. Synopsis:
6060 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6063 By default, @command{unexpand} converts only initial blanks (those
6064 that precede all non-blank characters) on each line. It
6065 preserves backspace characters in the output; they decrement the column
6066 count for tab calculations. By default, tabs are set at every 8th
6069 The program accepts the following options. Also see @ref{Common options}.
6073 @item -t @var{tab1}[,@var{tab2}]@dots{}
6074 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6077 If only one tab stop is given, set the tabs @var{tab1} columns apart
6078 instead of the default 8. Otherwise, set the tabs at columns
6079 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6080 beyond the tab stops given unchanged. Tab stops can be separated by
6081 blanks as well as by commas. This option implies the @option{-a} option.
6083 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6084 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6085 separated by commas. (Unlike @option{-t}, this obsolete option does
6086 not imply @option{-a}.) New scripts should use @option{--first-only -t
6087 @var{tab1}[,@var{tab2}]@dots{}} instead.
6093 Also convert all sequences of two or more blanks just before a tab stop,
6094 even if they occur after non-blank characters in a line.
6101 @node Directory listing
6102 @chapter Directory listing
6104 This chapter describes the @command{ls} command and its variants @command{dir}
6105 and @command{vdir}, which list information about files.
6108 * ls invocation:: List directory contents.
6109 * dir invocation:: Briefly ls.
6110 * vdir invocation:: Verbosely ls.
6111 * dircolors invocation:: Color setup for ls, etc.
6116 @section @command{ls}: List directory contents
6119 @cindex directory listing
6121 The @command{ls} program lists information about files (of any type,
6122 including directories). Options and file arguments can be intermixed
6123 arbitrarily, as usual.
6125 For non-option command-line arguments that are directories, by default
6126 @command{ls} lists the contents of directories, not recursively, and
6127 omitting files with names beginning with @samp{.}. For other non-option
6128 arguments, by default @command{ls} lists just the file name. If no
6129 non-option argument is specified, @command{ls} operates on the current
6130 directory, acting as if it had been invoked with a single argument of @samp{.}.
6133 By default, the output is sorted alphabetically, according to the locale
6134 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6135 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6136 produce output that is sorted differently than you're accustomed to.
6137 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6138 If standard output is
6139 a terminal, the output is in columns (sorted vertically) and control
6140 characters are output as question marks; otherwise, the output is listed
6141 one per line and control characters are output as-is.
6143 Because @command{ls} is such a fundamental program, it has accumulated many
6144 options over the years. They are described in the subsections below;
6145 within each section, options are listed alphabetically (ignoring case).
6146 The division of options into the subsections is not absolute, since some
6147 options affect more than one aspect of @command{ls}'s operation.
6149 @cindex exit status of @command{ls}
6154 1 minor problems (e.g., failure to access a file or directory not
6155 specified as a command line argument. This happens when listing a
6156 directory in which entries are actively being removed or renamed.)
6157 2 serious trouble (e.g., memory exhausted, invalid option, failure
6158 to access a file or directory specified as a command line argument
6159 or a directory loop)
6162 Also see @ref{Common options}.
6165 * Which files are listed::
6166 * What information is listed::
6167 * Sorting the output::
6168 * Details about version sort::
6169 * General output formatting::
6170 * Formatting file timestamps::
6171 * Formatting the file names::
6175 @node Which files are listed
6176 @subsection Which files are listed
6178 These options determine which files @command{ls} lists information for.
6179 By default, @command{ls} lists files and the contents of any
6180 directories on the command line, except that in directories it ignores
6181 files whose names start with @samp{.}.
6189 In directories, do not ignore file names that start with @samp{.}.
6194 @opindex --almost-all
6195 In directories, do not ignore all file names that start with @samp{.};
6196 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6197 option overrides this option.
6200 @itemx --ignore-backups
6202 @opindex --ignore-backups
6203 @cindex backup files, ignoring
6204 In directories, ignore files that end with @samp{~}. This option is
6205 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6210 @opindex --directory
6211 List just the names of directories, as with other types of files, rather
6212 than listing their contents.
6213 @c The following sentence is the same as the one for -F.
6214 Do not follow symbolic links listed on the
6215 command line unless the @option{--dereference-command-line} (@option{-H}),
6216 @option{--dereference} (@option{-L}), or
6217 @option{--dereference-command-line-symlink-to-dir} options are specified.
6220 @itemx --dereference-command-line
6222 @opindex --dereference-command-line
6223 @cindex symbolic links, dereferencing
6224 If a command line argument specifies a symbolic link, show information
6225 for the file the link references rather than for the link itself.
6227 @itemx --dereference-command-line-symlink-to-dir
6228 @opindex --dereference-command-line-symlink-to-dir
6229 @cindex symbolic links, dereferencing
6230 Do not dereference symbolic links, with one exception:
6231 if a command line argument specifies a symbolic link that refers to
6232 a directory, show information for that directory rather than for the
6234 This is the default behavior when no other dereferencing-related
6235 option has been specified (@option{--classify} (@option{-F}),
6236 @option{--directory} (@option{-d}),
6238 @option{--dereference} (@option{-L}), or
6239 @option{--dereference-command-line} (@option{-H})).
6241 @item --group-directories-first
6242 @opindex --group-directories-first
6243 Group all the directories before the files and then sort the
6244 directories and the files separately using the selected sort key
6245 (see --sort option).
6246 That is, this option specifies a primary sort key,
6247 and the --sort option specifies a secondary key.
6248 However, any use of @option{--sort=none}
6249 (@option{-U}) disables this option altogether.
6251 @item --hide=PATTERN
6252 @opindex --hide=@var{pattern}
6253 In directories, ignore files whose names match the shell pattern
6254 @var{pattern}, unless the @option{--all} (@option{-a}) or
6255 @option{--almost-all} (@option{-A}) is also given. This
6256 option acts like @option{--ignore=@var{pattern}} except that it has no
6257 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6258 (@option{-A}) is also given.
6260 This option can be useful in shell aliases. For example, if
6261 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6262 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6263 lists the file @file{README~} even though @samp{ly -A} would not.
6265 @item -I @var{pattern}
6266 @itemx --ignore=@var{pattern}
6268 @opindex --ignore=@var{pattern}
6269 In directories, ignore files whose names match the shell pattern
6270 (not regular expression) @var{pattern}. As
6271 in the shell, an initial @samp{.} in a file name does not match a
6272 wildcard at the start of @var{pattern}. Sometimes it is useful
6273 to give this option several times. For example,
6276 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6279 The first option ignores names of length 3 or more that start with @samp{.},
6280 the second ignores all two-character names that start with @samp{.}
6281 except @samp{..}, and the third ignores names that start with @samp{#}.
6284 @itemx --dereference
6286 @opindex --dereference
6287 @cindex symbolic links, dereferencing
6288 When showing file information for a symbolic link, show information
6289 for the file the link references rather than the link itself.
6290 However, even with this option, @command{ls} still prints the name
6291 of the link itself, not the name of the file that the link points to.
6296 @opindex --recursive
6297 @cindex recursive directory listing
6298 @cindex directory listing, recursive
6299 List the contents of all directories recursively.
6304 @node What information is listed
6305 @subsection What information is listed
6307 These options affect the information that @command{ls} displays. By
6308 default, only file names are shown.
6314 @cindex hurd, author, printing
6315 List each file's author when producing long format directory listings.
6316 In GNU/Hurd, file authors can differ from their owners, but in other
6317 operating systems the two are the same.
6323 @cindex dired Emacs mode support
6324 With the long listing (@option{-l}) format, print an additional line after
6328 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6332 The @var{begn} and @var{endn} are unsigned integers that record the
6333 byte position of the beginning and end of each file name in the output.
6334 This makes it easy for Emacs to find the names, even when they contain
6335 unusual characters such as space or newline, without fancy searching.
6337 If directories are being listed recursively (@option{-R}), output a similar
6338 line with offsets for each subdirectory name:
6341 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6344 Finally, output a line of the form:
6347 //DIRED-OPTIONS// --quoting-style=@var{word}
6351 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6353 Here is an actual example:
6356 $ mkdir -p a/sub/deeper a/sub2
6358 $ touch a/sub/deeper/file
6359 $ ls -gloRF --dired a
6362 -rw-r--r-- 1 0 Jun 10 12:27 f1
6363 -rw-r--r-- 1 0 Jun 10 12:27 f2
6364 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6365 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6369 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6373 -rw-r--r-- 1 0 Jun 10 12:27 file
6377 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6378 //SUBDIRED// 2 3 167 172 228 240 290 296
6379 //DIRED-OPTIONS// --quoting-style=literal
6382 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6383 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6385 The offsets on the @samp{//SUBDIRED//} line delimit the following
6386 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6388 Here is an example of how to extract the fifth entry name, @samp{deeper},
6389 corresponding to the pair of offsets, 222 and 228:
6392 $ ls -gloRF --dired a > out
6393 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6397 Note that although the listing above includes a trailing slash
6398 for the @samp{deeper} entry, the offsets select the name without
6399 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6400 along with an option like @option{--escape} (aka @option{-b}) and operate
6401 on a file whose name contains special characters, notice that the backslash
6406 $ ls -blog --dired 'a b'
6407 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6409 //DIRED-OPTIONS// --quoting-style=escape
6412 If you use a quoting style that adds quote marks
6413 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6414 So beware that the user may select the quoting style via the environment
6415 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6416 should either specify an explicit @option{--quoting-style=literal} option
6417 (aka @option{-N} or @option{--literal}) on the command line, or else be
6418 prepared to parse the escaped names.
6421 @opindex --full-time
6422 Produce long format directory listings, and list times in full. It is
6423 equivalent to using @option{--format=long} with
6424 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6428 Produce long format directory listings, but don't display owner information.
6434 Inhibit display of group information in a long format directory listing.
6435 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6436 provide this option for compatibility.)
6444 @cindex inode number, printing
6445 Print the inode number (also called the file serial number and index
6446 number) of each file to the left of the file name. (This number
6447 uniquely identifies each file within a particular file system.)
6450 @itemx --format=long
6451 @itemx --format=verbose
6454 @opindex long ls @r{format}
6455 @opindex verbose ls @r{format}
6456 In addition to the name of each file, print the file type, file mode bits,
6457 number of hard links, owner name, group name, size, and
6458 timestamp (@pxref{Formatting file timestamps}), normally
6459 the modification time. Print question marks for information that
6460 cannot be determined.
6462 Normally the size is printed as a byte count without punctuation, but
6463 this can be overridden (@pxref{Block size}). For example, @option{-h}
6464 prints an abbreviated, human-readable count, and
6465 @samp{--block-size="'1"} prints a byte count with the thousands
6466 separator of the current locale.
6468 For each directory that is listed, preface the files with a line
6469 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6470 for all files in that directory. The block size currently defaults to 1024
6471 bytes, but this can be overridden (@pxref{Block size}).
6472 The @var{blocks} computed counts each hard link separately;
6473 this is arguably a deficiency.
6475 The file type is one of the following characters:
6477 @c The commented-out entries are ones we're not sure about.
6485 character special file
6487 high performance (``contiguous data'') file
6491 door (Solaris 2.5 and up)
6493 @c semaphore, if this is a distinct file type
6497 @c multiplexed file (7th edition Unix; obsolete)
6499 off-line (``migrated'') file (Cray DMF)
6501 network special file (HP-UX)
6505 port (Solaris 10 and up)
6507 @c message queue, if this is a distinct file type
6511 @c shared memory object, if this is a distinct file type
6513 @c typed memory object, if this is a distinct file type
6515 @c whiteout (4.4BSD; not implemented)
6517 some other file type
6520 @cindex permissions, output by @command{ls}
6521 The file mode bits listed are similar to symbolic mode specifications
6522 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6523 third character of each set of permissions as follows:
6527 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6531 If the set-user-ID or set-group-ID bit is set but the corresponding
6532 executable bit is not set.
6535 If the restricted deletion flag or sticky bit, and the
6536 other-executable bit, are both set. The restricted deletion flag is
6537 another name for the sticky bit. @xref{Mode Structure}.
6540 If the restricted deletion flag or sticky bit is set but the
6541 other-executable bit is not set.
6544 If the executable bit is set and none of the above apply.
6550 Following the file mode bits is a single character that specifies
6551 whether an alternate access method such as an access control list
6552 applies to the file. When the character following the file mode bits is a
6553 space, there is no alternate access method. When it is a printing
6554 character, then there is such a method.
6556 GNU @command{ls} uses a @samp{.} character to indicate a file
6557 with an SELinux security context, but no other alternate access method.
6559 A file with any other combination of alternate access methods
6560 is marked with a @samp{+} character.
6563 @itemx --numeric-uid-gid
6565 @opindex --numeric-uid-gid
6566 @cindex numeric uid and gid
6567 @cindex numeric user and group IDs
6568 Produce long format directory listings, but
6569 display numeric user and group IDs instead of the owner and group names.
6573 Produce long format directory listings, but don't display group information.
6574 It is equivalent to using @option{--format=long} with @option{--no-group} .
6580 @cindex disk allocation
6581 @cindex size of files, reporting
6582 Print the disk allocation of each file to the left of the file name.
6583 This is the amount of disk space used by the file, which is usually a
6584 bit more than the file's size, but it can be less if the file has holes.
6586 Normally the disk allocation is printed in units of
6587 1024 bytes, but this can be overridden (@pxref{Block size}).
6589 @cindex NFS mounts from BSD to HP-UX
6590 For files that are NFS-mounted from an HP-UX system to a BSD system,
6591 this option reports sizes that are half the correct values. On HP-UX
6592 systems, it reports sizes that are twice the correct values for files
6593 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6594 it also affects the HP-UX @command{ls} program.
6603 @cindex security context
6604 Display the SELinux security context or @samp{?} if none is found.
6605 When used with the @option{-l} option, print the security context
6606 to the left of the size column.
6611 @node Sorting the output
6612 @subsection Sorting the output
6614 @cindex sorting @command{ls} output
6615 These options change the order in which @command{ls} sorts the information
6616 it outputs. By default, sorting is done by character code
6617 (e.g., @acronym{ASCII} order).
6623 @itemx --time=status
6626 @opindex ctime@r{, printing or sorting by}
6627 @opindex status time@r{, printing or sorting by}
6628 @opindex use time@r{, printing or sorting files by}
6629 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6630 print the status change time (the @samp{ctime} in the inode) instead of
6631 the modification time.
6632 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6633 or when not using a long listing format,
6634 sort according to the status change time.
6638 @cindex unsorted directory listing
6639 @cindex directory order, listing by
6640 Primarily, like @option{-U}---do not sort; list the files in whatever
6641 order they are stored in the directory. But also enable @option{-a} (list
6642 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6643 were specified before the @option{-f}).
6649 @cindex reverse sorting
6650 Reverse whatever the sorting method is---e.g., list files in reverse
6651 alphabetical order, youngest first, smallest first, or whatever.
6657 @opindex size of files@r{, sorting files by}
6658 Sort by file size, largest first.
6664 @opindex modification time@r{, sorting files by}
6665 Sort by modification time (the @samp{mtime} in the inode), newest first.
6669 @itemx --time=access
6673 @opindex use time@r{, printing or sorting files by}
6674 @opindex atime@r{, printing or sorting files by}
6675 @opindex access time@r{, printing or sorting files by}
6676 If the long listing format (e.g., @option{--format=long}) is being used,
6677 print the last access time (the @samp{atime} in the inode).
6678 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6679 or when not using a long listing format, sort according to the access time.
6685 @opindex none@r{, sorting option for @command{ls}}
6686 Do not sort; list the files in whatever order they are
6687 stored in the directory. (Do not do any of the other unrelated things
6688 that @option{-f} does.) This is especially useful when listing very large
6689 directories, since not doing any sorting can be noticeably faster.
6692 @itemx --sort=version
6695 @opindex version@r{, sorting option for @command{ls}}
6696 Sort by version name and number, lowest first. It behaves like a default
6697 sort, except that each sequence of decimal digits is treated numerically
6698 as an index/version number. (@xref{Details about version sort}.)
6701 @itemx --sort=extension
6704 @opindex extension@r{, sorting files by}
6705 Sort directory contents alphabetically by file extension (characters
6706 after the last @samp{.}); files with no extension are sorted first.
6711 @node Details about version sort
6712 @subsection Details about version sort
6714 Version sorting handles the fact that file names frequently include indices or
6715 version numbers. Standard sorting usually does not produce the order that one
6716 expects because comparisons are made on a character-by-character basis.
6717 Version sorting is especially useful when browsing directories that contain
6718 many files with indices/version numbers in their names:
6722 abc.zml-1.gz abc.zml-1.gz
6723 abc.zml-12.gz abc.zml-2.gz
6724 abc.zml-2.gz abc.zml-12.gz
6727 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6728 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6729 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6730 @var{ver1} < @var{ver2} implies that the name composed of
6731 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6732 ``@var{prefix} @var{ver2} @var{suffix}''.
6734 Note also that leading zeros of numeric parts are ignored:
6738 abc-1.007.tgz abc-1.01a.tgz
6739 abc-1.012b.tgz abc-1.007.tgz
6740 abc-1.01a.tgz abc-1.012b.tgz
6743 This functionality is implemented using gnulib's @code{filevercmp} function,
6744 which has some caveats worth noting.
6747 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6748 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6749 was set to @samp{C}.
6750 @item Some suffixes will not be matched by the regular
6751 expression mentioned above. Consequently these examples may
6752 not sort as you expect:
6760 abc-1.2.3.4.x86_64.rpm
6761 abc-1.2.3.x86_64.rpm
6765 @node General output formatting
6766 @subsection General output formatting
6768 These options affect the appearance of the overall output.
6773 @itemx --format=single-column
6776 @opindex single-column @r{output of files}
6777 List one file per line. This is the default for @command{ls} when standard
6778 output is not a terminal.
6781 @itemx --format=vertical
6784 @opindex vertical @r{sorted files in columns}
6785 List files in columns, sorted vertically. This is the default for
6786 @command{ls} if standard output is a terminal. It is always the default
6787 for the @command{dir} program.
6788 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6789 possible in the fewest lines.
6791 @item --color [=@var{when}]
6793 @cindex color, distinguishing file types with
6794 Specify whether to use color for distinguishing file types. @var{when}
6795 may be omitted, or one of:
6798 @vindex none @r{color option}
6799 - Do not use color at all. This is the default.
6801 @vindex auto @r{color option}
6802 @cindex terminal, using color iff
6803 - Only use color if standard output is a terminal.
6805 @vindex always @r{color option}
6808 Specifying @option{--color} and no @var{when} is equivalent to
6809 @option{--color=always}.
6810 Piping a colorized listing through a pager like @command{more} or
6811 @command{less} usually produces unreadable results. However, using
6812 @code{more -f} does seem to work.
6816 @itemx --indicator-style=classify
6819 @opindex --indicator-style
6820 @cindex file type and executables, marking
6821 @cindex executables and file type, marking
6822 Append a character to each file name indicating the file type. Also,
6823 for regular files that are executable, append @samp{*}. The file type
6824 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6825 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6826 and nothing for regular files.
6827 @c The following sentence is the same as the one for -d.
6828 Do not follow symbolic links listed on the
6829 command line unless the @option{--dereference-command-line} (@option{-H}),
6830 @option{--dereference} (@option{-L}), or
6831 @option{--dereference-command-line-symlink-to-dir} options are specified.
6834 @itemx --indicator-style=file-type
6835 @opindex --file-type
6836 @opindex --indicator-style
6837 @cindex file type, marking
6838 Append a character to each file name indicating the file type. This is
6839 like @option{-F}, except that executables are not marked.
6841 @item --indicator-style=@var{word}
6842 @opindex --indicator-style
6843 Append a character indicator with style @var{word} to entry names,
6848 Do not append any character indicator; this is the default.
6850 Append @samp{/} for directories. This is the same as the @option{-p}
6853 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6854 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6855 the same as the @option{--file-type} option.
6857 Append @samp{*} for executable regular files, otherwise behave as for
6858 @samp{file-type}. This is the same as the @option{-F} or
6859 @option{--classify} option.
6864 Print file sizes in 1024-byte blocks, overriding the default block
6865 size (@pxref{Block size}).
6866 This option is equivalent to @option{--block-size=1K}.
6869 @itemx --format=commas
6872 @opindex commas@r{, outputting between files}
6873 List files horizontally, with as many as will fit on each line,
6874 separated by @samp{, } (a comma and a space).
6877 @itemx --indicator-style=slash
6879 @opindex --indicator-style
6880 @cindex file type, marking
6881 Append a @samp{/} to directory names.
6884 @itemx --format=across
6885 @itemx --format=horizontal
6888 @opindex across@r{, listing files}
6889 @opindex horizontal@r{, listing files}
6890 List the files in columns, sorted horizontally.
6893 @itemx --tabsize=@var{cols}
6896 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6897 @command{ls} uses tabs where possible in the output, for efficiency. If
6898 @var{cols} is zero, do not use tabs at all.
6900 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6901 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6902 do not properly align columns to the right of a TAB following a
6903 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6904 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6905 @command{ls} to align using spaces, not tabs.
6908 @itemx --width=@var{cols}
6912 Assume the screen is @var{cols} columns wide. The default is taken
6913 from the terminal settings if possible; otherwise the environment
6914 variable @env{COLUMNS} is used if it is set; otherwise the default
6920 @node Formatting file timestamps
6921 @subsection Formatting file timestamps
6923 By default, file timestamps are listed in abbreviated form. Most
6924 locales use a timestamp like @samp{2002-03-30 23:45}. However, the
6925 default @acronym{POSIX} locale uses a date like @samp{Mar 30@ @ 2002}
6926 for non-recent timestamps, and a date-without-year and time like
6927 @samp{Mar 30 23:45} for recent timestamps.
6929 A timestamp is considered to be @dfn{recent} if it is less than six
6930 months old, and is not dated in the future. If a timestamp dated
6931 today is not listed in recent form, the timestamp is in the future,
6932 which means you probably have clock skew problems which may break
6933 programs like @command{make} that rely on file timestamps.
6936 Time stamps are listed according to the time zone rules specified by
6937 the @env{TZ} environment variable, or by the system default rules if
6938 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6939 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6941 The following option changes how file timestamps are printed.
6944 @item --time-style=@var{style}
6945 @opindex --time-style
6947 List timestamps in style @var{style}. The @var{style} should
6948 be one of the following:
6953 List timestamps using @var{format}, where @var{format} is interpreted
6954 like the format argument of @command{date} (@pxref{date invocation}).
6955 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6956 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6957 with @command{date}, @var{format}'s interpretation is affected by the
6958 @env{LC_TIME} locale category.
6960 If @var{format} contains two format strings separated by a newline,
6961 the former is used for non-recent files and the latter for recent
6962 files; if you want output columns to line up, you may need to insert
6963 spaces in one of the two formats.
6966 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6967 format with nanosecond precision, e.g., @samp{2002-03-30
6968 23:45:56.477817180 -0700}. This style is equivalent to
6969 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6971 This is useful because the time output includes all the information that
6972 is available from the operating system. For example, this can help
6973 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
6974 uses the full timestamp to determine whether a file is out of date.
6977 List @acronym{ISO} 8601 date and time in minutes, e.g.,
6978 @samp{2002-03-30 23:45}. These timestamps are shorter than
6979 @samp{full-iso} timestamps, and are usually good enough for everyday
6980 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
6983 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
6984 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
6985 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
6986 timestamps are uglier than @samp{long-iso} timestamps, but they carry
6987 nearly the same information in a smaller space and their brevity helps
6988 @command{ls} output fit within traditional 80-column output lines.
6989 The following two @command{ls} invocations are equivalent:
6994 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
6995 ls -l --time-style="iso"
7000 List timestamps in a locale-dependent form. For example, a Finnish
7001 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7002 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7003 timestamps typically consume more space than @samp{iso} timestamps and
7004 are harder for programs to parse because locale conventions vary so
7005 widely, but they are easier for many people to read.
7007 The @env{LC_TIME} locale category specifies the timestamp format. The
7008 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7009 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7010 @command{ls} invocations are equivalent:
7015 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7016 ls -l --time-style="locale"
7019 Other locales behave differently. For example, in a German locale,
7020 @option{--time-style="locale"} might be equivalent to
7021 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7022 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7023 @samp{30. M@"ar 23:45}.
7025 @item posix-@var{style}
7027 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7028 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7029 example, the @samp{posix-long-iso} style lists
7030 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7031 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7036 You can specify the default value of the @option{--time-style} option
7037 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7038 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7039 later use the @option{--dired} option and therefore can parse any date
7040 format, but if you are using Emacs 21.1 or 21.2 and specify a
7041 non-@acronym{POSIX} locale you may need to set
7042 @samp{TIME_STYLE="posix-long-iso"}.
7044 To avoid certain denial-of-service attacks, timestamps that would be
7045 longer than 1000 bytes may be treated as errors.
7048 @node Formatting the file names
7049 @subsection Formatting the file names
7051 These options change how file names themselves are printed.
7057 @itemx --quoting-style=escape
7060 @opindex --quoting-style
7061 @cindex backslash sequences for file names
7062 Quote nongraphic characters in file names using alphabetic and octal
7063 backslash sequences like those used in C.
7067 @itemx --quoting-style=literal
7070 @opindex --quoting-style
7071 Do not quote file names. However, with @command{ls} nongraphic
7072 characters are still printed as question marks if the output is a
7073 terminal and you do not specify the @option{--show-control-chars}
7077 @itemx --hide-control-chars
7079 @opindex --hide-control-chars
7080 Print question marks instead of nongraphic characters in file names.
7081 This is the default if the output is a terminal and the program is
7086 @itemx --quoting-style=c
7088 @opindex --quote-name
7089 @opindex --quoting-style
7090 Enclose file names in double quotes and quote nongraphic characters as
7093 @item --quoting-style=@var{word}
7094 @opindex --quoting-style
7095 @cindex quoting style
7096 Use style @var{word} to quote file names and other strings that may
7097 contain arbitrary characters. The @var{word} should
7098 be one of the following:
7102 Output strings as-is; this is the same as the @option{-N} or
7103 @option{--literal} option.
7105 Quote strings for the shell if they contain shell metacharacters or would
7106 cause ambiguous output.
7107 The quoting is suitable for @acronym{POSIX}-compatible shells like
7108 @command{bash}, but it does not always work for incompatible shells
7111 Quote strings for the shell, even if they would normally not require quoting.
7113 Quote strings as for C character string literals, including the
7114 surrounding double-quote characters; this is the same as the
7115 @option{-Q} or @option{--quote-name} option.
7117 Quote strings as for C character string literals, except omit the
7118 surrounding double-quote
7119 characters; this is the same as the @option{-b} or @option{--escape} option.
7121 Quote strings as for C character string literals, except use
7122 surrounding quotation marks appropriate for the
7125 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7126 Quote strings as for C character string literals, except use
7127 surrounding quotation marks appropriate for the locale, and quote
7128 @t{`like this'} instead of @t{"like
7129 this"} in the default C locale. This looks nicer on many displays.
7132 You can specify the default value of the @option{--quoting-style} option
7133 with the environment variable @env{QUOTING_STYLE}. If that environment
7134 variable is not set, the default value is @samp{literal}, but this
7135 default may change to @samp{shell} in a future version of this package.
7137 @item --show-control-chars
7138 @opindex --show-control-chars
7139 Print nongraphic characters as-is in file names.
7140 This is the default unless the output is a terminal and the program is
7146 @node dir invocation
7147 @section @command{dir}: Briefly list directory contents
7150 @cindex directory listing, brief
7152 @command{dir} is equivalent to @code{ls -C
7153 -b}; that is, by default files are listed in columns, sorted vertically,
7154 and special characters are represented by backslash escape sequences.
7156 @xref{ls invocation, @command{ls}}.
7159 @node vdir invocation
7160 @section @command{vdir}: Verbosely list directory contents
7163 @cindex directory listing, verbose
7165 @command{vdir} is equivalent to @code{ls -l
7166 -b}; that is, by default files are listed in long format and special
7167 characters are represented by backslash escape sequences.
7169 @node dircolors invocation
7170 @section @command{dircolors}: Color setup for @command{ls}
7174 @cindex setup for color
7176 @command{dircolors} outputs a sequence of shell commands to set up the
7177 terminal for color output from @command{ls} (and @command{dir}, etc.).
7181 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7184 If @var{file} is specified, @command{dircolors} reads it to determine which
7185 colors to use for which file types and extensions. Otherwise, a
7186 precompiled database is used. For details on the format of these files,
7187 run @samp{dircolors --print-database}.
7189 To make @command{dircolors} read a @file{~/.dircolors} file if it
7190 exists, you can put the following lines in your @file{~/.bashrc} (or
7191 adapt them to your favorite shell):
7195 test -r $d && eval "$(dircolors $d)"
7199 @vindex SHELL @r{environment variable, and color}
7200 The output is a shell command to set the @env{LS_COLORS} environment
7201 variable. You can specify the shell syntax to use on the command line,
7202 or @command{dircolors} will guess it from the value of the @env{SHELL}
7203 environment variable.
7205 The program accepts the following options. Also see @ref{Common options}.
7210 @itemx --bourne-shell
7213 @opindex --bourne-shell
7214 @cindex Bourne shell syntax for color setup
7215 @cindex @command{sh} syntax for color setup
7216 Output Bourne shell commands. This is the default if the @env{SHELL}
7217 environment variable is set and does not end with @samp{csh} or
7226 @cindex C shell syntax for color setup
7227 @cindex @command{csh} syntax for color setup
7228 Output C shell commands. This is the default if @code{SHELL} ends with
7229 @command{csh} or @command{tcsh}.
7232 @itemx --print-database
7234 @opindex --print-database
7235 @cindex color database, printing
7236 @cindex database for color setup, printing
7237 @cindex printing color database
7238 Print the (compiled-in) default color configuration database. This
7239 output is itself a valid configuration file, and is fairly descriptive
7240 of the possibilities.
7247 @node Basic operations
7248 @chapter Basic operations
7250 @cindex manipulating files
7252 This chapter describes the commands for basic file manipulation:
7253 copying, moving (renaming), and deleting (removing).
7256 * cp invocation:: Copy files.
7257 * dd invocation:: Convert and copy a file.
7258 * install invocation:: Copy files and set attributes.
7259 * mv invocation:: Move (rename) files.
7260 * rm invocation:: Remove files or directories.
7261 * shred invocation:: Remove files more securely.
7266 @section @command{cp}: Copy files and directories
7269 @cindex copying files and directories
7270 @cindex files, copying
7271 @cindex directories, copying
7273 @command{cp} copies files (or, optionally, directories). The copy is
7274 completely independent of the original. You can either copy one file to
7275 another, or copy arbitrarily many files to a destination directory.
7279 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7280 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7281 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7286 If two file names are given, @command{cp} copies the first file to the
7290 If the @option{--target-directory} (@option{-t}) option is given, or
7291 failing that if the last file is a directory and the
7292 @option{--no-target-directory} (@option{-T}) option is not given,
7293 @command{cp} copies each @var{source} file to the specified directory,
7294 using the @var{source}s' names.
7297 Generally, files are written just as they are read. For exceptions,
7298 see the @option{--sparse} option below.
7300 By default, @command{cp} does not copy directories. However, the
7301 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7302 copy recursively by descending into source directories and copying files
7303 to corresponding destination directories.
7305 When copying from a symbolic link, @command{cp} normally follows the
7306 link only when not copying
7307 recursively. This default can be overridden with the
7308 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7309 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7310 @option{-H} options. If more than one of these options is specified,
7311 the last one silently overrides the others.
7313 When copying to a symbolic link, @command{cp} follows the
7314 link only when it refers to an existing regular file.
7315 However, when copying to a dangling symbolic link, @command{cp}
7316 refuses by default, and fails with a diagnostic, since the operation
7317 is inherently dangerous. This behavior is contrary to historical
7318 practice and to @acronym{POSIX}.
7319 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7320 the target of a dangling destination symlink, in spite of the possible risk.
7321 Also, when an option like
7322 @option{--backup} or @option{--link} acts to rename or remove the
7323 destination before copying, @command{cp} renames or removes the
7324 symbolic link rather than the file it points to.
7326 By default, @command{cp} copies the contents of special files only
7327 when not copying recursively. This default can be overridden with the
7328 @option{--copy-contents} option.
7330 @cindex self-backups
7331 @cindex backups, making only
7332 @command{cp} generally refuses to copy a file onto itself, with the
7333 following exception: if @option{--force --backup} is specified with
7334 @var{source} and @var{dest} identical, and referring to a regular file,
7335 @command{cp} will make a backup file, either regular or numbered, as
7336 specified in the usual ways (@pxref{Backup options}). This is useful when
7337 you simply want to make a backup of an existing file before changing it.
7339 The program accepts the following options. Also see @ref{Common options}.
7346 Preserve as much as possible of the structure and attributes of the
7347 original files in the copy (but do not attempt to preserve internal
7348 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7349 directory in a different order).
7350 Try to preserve SELinux security context and extended attributes (xattr),
7351 but ignore any failure to do that and print no corresponding diagnostic.
7352 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7355 @itemx @w{@kbd{--backup}[=@var{method}]}
7358 @vindex VERSION_CONTROL
7359 @cindex backups, making
7360 @xref{Backup options}.
7361 Make a backup of each file that would otherwise be overwritten or removed.
7362 As a special case, @command{cp} makes a backup of @var{source} when the force
7363 and backup options are given and @var{source} and @var{dest} are the same
7364 name for an existing, regular file. One useful application of this
7365 combination of options is this tiny Bourne shell script:
7369 # Usage: backup FILE...
7370 # Create a @sc{gnu}-style backup of each listed FILE.
7372 cp --backup --force -- "$i" "$i"
7376 @item --copy-contents
7377 @cindex directories, copying recursively
7378 @cindex copying directories recursively
7379 @cindex recursively copying directories
7380 @cindex non-directories, copying as special files
7381 If copying recursively, copy the contents of any special files (e.g.,
7382 FIFOs and device files) as if they were regular files. This means
7383 trying to read the data in each source file and writing it to the
7384 destination. It is usually a mistake to use this option, as it
7385 normally has undesirable effects on special files like FIFOs and the
7386 ones typically found in the @file{/dev} directory. In most cases,
7387 @code{cp -R --copy-contents} will hang indefinitely trying to read
7388 from FIFOs and special files like @file{/dev/console}, and it will
7389 fill up your destination disk if you use it to copy @file{/dev/zero}.
7390 This option has no effect unless copying recursively, and it does not
7391 affect the copying of symbolic links.
7395 @cindex symbolic links, copying
7396 @cindex hard links, preserving
7397 Copy symbolic links as symbolic links rather than copying the files that
7398 they point to, and preserve hard links between source files in the copies.
7399 Equivalent to @option{--no-dereference --preserve=links}.
7405 When copying without this option and an existing destination file cannot
7406 be opened for writing, the copy fails. However, with @option{--force}),
7407 when a destination file cannot be opened, @command{cp} then removes it and
7408 tries to open it again. Contrast this behavior with that enabled by
7409 @option{--link} and @option{--symbolic-link}, whereby the destination file
7410 is never opened but rather is removed unconditionally. Also see the
7411 description of @option{--remove-destination}.
7413 This option is independent of the @option{--interactive} or
7414 @option{-i} option: neither cancels the effect of the other.
7416 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7421 If a command line argument specifies a symbolic link, then copy the
7422 file it points to rather than the symbolic link itself. However,
7423 copy (preserving its nature) any symbolic link that is encountered
7424 via recursive traversal.
7427 @itemx --interactive
7429 @opindex --interactive
7430 When copying a file other than a directory, prompt whether to
7431 overwrite an existing destination file. The @option{-i} option overrides
7432 a previous @option{-n} option.
7438 Make hard links instead of copies of non-directories.
7441 @itemx --dereference
7443 @opindex --dereference
7444 Follow symbolic links when copying from them.
7445 With this option, @command{cp} cannot create a symbolic link.
7446 For example, a symlink (to regular file) in the source tree will be copied to
7447 a regular file in the destination tree.
7452 @opindex --no-clobber
7453 Do not overwrite an existing file. The @option{-n} option overrides a previous
7454 @option{-i} option. This option is mutually exclusive with @option{-b} or
7455 @option{--backup} option.
7458 @itemx --no-dereference
7460 @opindex --no-dereference
7461 @cindex symbolic links, copying
7462 Copy symbolic links as symbolic links rather than copying the files that
7463 they point to. This option affects only symbolic links in the source;
7464 symbolic links in the destination are always followed if possible.
7467 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7470 @cindex file information, preserving, extended attributes, xattr
7471 Preserve the specified attributes of the original files.
7472 If specified, the @var{attribute_list} must be a comma-separated list
7473 of one or more of the following strings:
7477 Preserve the file mode bits and access control lists.
7479 Preserve the owner and group. On most modern systems,
7480 only users with appropriate privileges may change the owner of a file,
7482 may preserve the group ownership of a file only if they happen to be
7483 a member of the desired group.
7485 Preserve the times of last access and last modification, when possible.
7486 On older systems, it is not possible to preserve these attributes
7487 when the affected file is a symbolic link.
7488 However, many systems now provide the @code{utimensat} function,
7489 which makes it possible even for symbolic links.
7491 Preserve in the destination files
7492 any links between corresponding source files.
7493 Note that with @option{-L} or @option{-H}, this option can convert
7494 symbolic links to hard links. For example,
7496 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7501 Note the inputs: @file{b} is a symlink to regular file @file{a},
7502 yet the files in destination directory, @file{c/}, are hard-linked.
7503 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7504 tells @command{cp} to dereference command line arguments, it sees two files
7505 with the same inode number, and preserves the perceived hard link.
7507 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7509 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7515 Preserve SELinux security context of the file, or fail with full diagnostics.
7517 Preserve extended attributes of the file, or fail with full diagnostics.
7518 If @command{cp} is built without xattr support, ignore this option.
7519 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7520 they are preserved by this option as well.
7522 Preserve all file attributes.
7523 Equivalent to specifying all of the above, but with the difference
7524 that failure to preserve SELinux security context or extended attributes
7525 does not change @command{cp}'s exit status. In contrast to @option{-a},
7526 all but @samp{Operation not supported} warnings are output.
7529 Using @option{--preserve} with no @var{attribute_list} is equivalent
7530 to @option{--preserve=mode,ownership,timestamps}.
7532 In the absence of this option, each destination file is created with the
7533 mode bits of the corresponding source file, minus the bits set in the
7534 umask and minus the set-user-ID and set-group-ID bits.
7535 @xref{File permissions}.
7537 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7538 @cindex file information, preserving
7539 Do not preserve the specified attributes. The @var{attribute_list}
7540 has the same form as for @option{--preserve}.
7544 @cindex parent directories and @command{cp}
7545 Form the name of each destination file by appending to the target
7546 directory a slash and the specified name of the source file. The last
7547 argument given to @command{cp} must be the name of an existing directory.
7548 For example, the command:
7551 cp --parents a/b/c existing_dir
7555 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7556 any missing intermediate directories.
7563 @opindex --recursive
7564 @cindex directories, copying recursively
7565 @cindex copying directories recursively
7566 @cindex recursively copying directories
7567 @cindex non-directories, copying as special files
7568 Copy directories recursively. By default, do not follow symbolic
7569 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7570 @option{--dereference} (@option{-L}), @option{--no-dereference}
7571 (@option{-P}), and @option{-H} options. Special files are copied by
7572 creating a destination file of the same type as the source; see the
7573 @option{--copy-contents} option. It is not portable to use
7574 @option{-r} to copy symbolic links or special files. On some
7575 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7576 @option{-L} and @option{--copy-contents} for historical reasons.
7577 Also, it is not portable to use @option{-R} to copy symbolic links
7578 unless you also specify @option{-P}, as @acronym{POSIX} allows
7579 implementations that dereference symbolic links by default.
7581 @item --reflink[=@var{when}]
7582 @opindex --reflink[=@var{when}]
7585 @cindex copy on write
7586 Perform a lightweight, copy-on-write (COW) copy.
7587 Copying with this option can succeed only on some file systems.
7588 Once it has succeeded, beware that the source and destination files
7589 share the same disk data blocks as long as they remain unmodified.
7590 Thus, if a disk I/O error affects data blocks of one of the files,
7591 the other suffers the exact same fate.
7593 The @var{when} value can be one of the following:
7597 The default behavior: if the copy-on-write operation is not supported
7598 then report the failure for each file and exit with a failure status.
7601 If the copy-on-write operation is not supported then fall back
7602 to the standard copy behaviour.
7606 @item --remove-destination
7607 @opindex --remove-destination
7608 Remove each existing destination file before attempting to open it
7609 (contrast with @option{-f} above).
7611 @item --sparse=@var{when}
7612 @opindex --sparse=@var{when}
7613 @cindex sparse files, copying
7614 @cindex holes, copying files with
7615 @findex read @r{system call, and holes}
7616 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7617 does not occupy any physical disk blocks; the @samp{read} system call
7618 reads these as zeros. This can both save considerable disk space and
7619 increase speed, since many binary files contain lots of consecutive zero
7620 bytes. By default, @command{cp} detects holes in input source files via a crude
7621 heuristic and makes the corresponding output file sparse as well.
7622 Only regular files may be sparse.
7624 The @var{when} value can be one of the following:
7628 The default behavior: if the input file is sparse, attempt to make
7629 the output file sparse, too. However, if an output file exists but
7630 refers to a non-regular file, then do not attempt to make it sparse.
7633 For each sufficiently long sequence of zero bytes in the input file,
7634 attempt to create a corresponding hole in the output file, even if the
7635 input file does not appear to be sparse.
7636 This is useful when the input file resides on a file system
7637 that does not support sparse files
7638 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7639 but the output file is on a type of file system that does support them.
7640 Holes may be created only in regular files, so if the destination file
7641 is of some other type, @command{cp} does not even try to make it sparse.
7644 Never make the output file sparse.
7645 This is useful in creating a file for use with the @command{mkswap} command,
7646 since such a file must not have any holes.
7649 @optStripTrailingSlashes
7652 @itemx --symbolic-link
7654 @opindex --symbolic-link
7655 @cindex symbolic links, copying with
7656 Make symbolic links instead of copies of non-directories. All source
7657 file names must be absolute (starting with @samp{/}) unless the
7658 destination files are in the current directory. This option merely
7659 results in an error message on systems that do not support symbolic links.
7665 @optNoTargetDirectory
7671 @cindex newer files, copying only
7672 Do not copy a non-directory that has an existing destination with the
7673 same or newer modification time. If time stamps are being preserved,
7674 the comparison is to the source time stamp truncated to the
7675 resolutions of the destination file system and of the system calls
7676 used to update time stamps; this avoids duplicate work if several
7677 @samp{cp -pu} commands are executed with the same source and
7684 Print the name of each file before copying it.
7687 @itemx --one-file-system
7689 @opindex --one-file-system
7690 @cindex file systems, omitting copying to different
7691 Skip subdirectories that are on different file systems from the one that
7692 the copy started on.
7693 However, mount point directories @emph{are} copied.
7701 @section @command{dd}: Convert and copy a file
7704 @cindex converting while copying a file
7706 @command{dd} copies a file (from standard input to standard output, by
7707 default) with a changeable I/O block size, while optionally performing
7708 conversions on it. Synopses:
7711 dd [@var{operand}]@dots{}
7715 The only options are @option{--help} and @option{--version}.
7716 @xref{Common options}. @command{dd} accepts the following operands.
7722 Read from @var{file} instead of standard input.
7726 Write to @var{file} instead of standard output. Unless
7727 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7728 bytes (or the size specified with @samp{seek=}).
7730 @item ibs=@var{bytes}
7732 @cindex block size of input
7733 @cindex input block size
7734 Set the input block size to @var{bytes}.
7735 This makes @command{dd} read @var{bytes} per block.
7736 The default is 512 bytes.
7738 @item obs=@var{bytes}
7740 @cindex block size of output
7741 @cindex output block size
7742 Set the output block size to @var{bytes}.
7743 This makes @command{dd} write @var{bytes} per block.
7744 The default is 512 bytes.
7746 @item bs=@var{bytes}
7749 Set both input and output block sizes to @var{bytes}.
7750 This makes @command{dd} read and write @var{bytes} per block,
7751 overriding any @samp{ibs} and @samp{obs} settings.
7752 In addition, if no data-transforming @option{conv} option is specified,
7753 each input block is copied to the output as a single block,
7754 without aggregating short reads.
7756 @item cbs=@var{bytes}
7758 @cindex block size of conversion
7759 @cindex conversion block size
7760 @cindex fixed-length records, converting to variable-length
7761 @cindex variable-length records, converting to fixed-length
7762 Set the conversion block size to @var{bytes}.
7763 When converting variable-length records to fixed-length ones
7764 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7765 use @var{bytes} as the fixed record length.
7767 @item skip=@var{blocks}
7769 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7771 @item seek=@var{blocks}
7773 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7775 @item count=@var{blocks}
7777 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7778 of everything until the end of the file.
7782 Do not print the overall transfer rate and volume statistics
7783 that normally make up the third status line when @command{dd} exits.
7785 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7787 Convert the file as specified by the @var{conversion} argument(s).
7788 (No spaces around any comma(s).)
7795 @opindex ascii@r{, converting to}
7796 Convert @acronym{EBCDIC} to @acronym{ASCII},
7797 using the conversion table specified by @acronym{POSIX}.
7798 This provides a 1:1 translation for all 256 bytes.
7801 @opindex ebcdic@r{, converting to}
7802 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7803 This is the inverse of the @samp{ascii} conversion.
7806 @opindex alternate ebcdic@r{, converting to}
7807 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7808 using the alternate conversion table specified by @acronym{POSIX}.
7809 This is not a 1:1 translation, but reflects common historical practice
7810 for @samp{~}, @samp{[}, and @samp{]}.
7812 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7816 @opindex block @r{(space-padding)}
7817 For each line in the input, output @samp{cbs} bytes, replacing the
7818 input newline with a space and padding with spaces as necessary.
7822 Remove any trailing spaces in each @samp{cbs}-sized input block,
7823 and append a newline.
7825 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7828 @opindex lcase@r{, converting to}
7829 Change uppercase letters to lowercase.
7832 @opindex ucase@r{, converting to}
7833 Change lowercase letters to uppercase.
7835 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7838 @opindex swab @r{(byte-swapping)}
7839 @cindex byte-swapping
7840 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7841 when an odd number of bytes are read---the last byte is simply copied
7842 (since there is nothing to swap it with).
7846 @cindex read errors, ignoring
7847 Continue after read errors.
7851 @cindex creating output file, avoiding
7852 Do not create the output file; the output file must already exist.
7856 @cindex creating output file, requiring
7857 Fail if the output file already exists; @command{dd} must create the
7860 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7864 @cindex truncating output file, avoiding
7865 Do not truncate the output file.
7868 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7869 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7870 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7875 @cindex synchronized data writes, before finishing
7876 Synchronize output data just before finishing. This forces a physical
7877 write of output data.
7881 @cindex synchronized data and metadata writes, before finishing
7882 Synchronize output data and metadata just before finishing. This
7883 forces a physical write of output data and metadata.
7887 @item iflag=@var{flag}[,@var{flag}]@dots{}
7889 Access the input file using the flags specified by the @var{flag}
7890 argument(s). (No spaces around any comma(s).)
7892 @item oflag=@var{flag}[,@var{flag}]@dots{}
7894 Access the output file using the flags specified by the @var{flag}
7895 argument(s). (No spaces around any comma(s).)
7897 Here are the flags. Not every flag is supported on every operating
7904 @cindex appending to the output file
7905 Write in append mode, so that even if some other process is writing to
7906 this file, every @command{dd} write will append to the current
7907 contents of the file. This flag makes sense only for output.
7908 If you combine this flag with the @samp{of=@var{file}} operand,
7909 you should also specify @samp{conv=notrunc} unless you want the
7910 output file to be truncated before being appended to.
7914 @cindex concurrent I/O
7915 Use concurrent I/O mode for data. This mode performs direct I/O
7916 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7917 A file cannot be opened in CIO mode and with a standard open at the
7923 Use direct I/O for data, avoiding the buffer cache.
7924 Note that the kernel may impose restrictions on read or write buffer sizes.
7925 For example, with an ext4 destination file system and a linux-based kernel,
7926 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7927 output buffer size is not a multiple of 512.
7931 @cindex directory I/O
7933 Fail unless the file is a directory. Most operating systems do not
7934 allow I/O to a directory, so this flag has limited utility.
7938 @cindex synchronized data reads
7939 Use synchronized I/O for data. For the output file, this forces a
7940 physical write of output data on each write. For the input file,
7941 this flag can matter when reading from a remote file that has been
7942 written to synchronously by some other process. Metadata (e.g.,
7943 last-access and last-modified time) is not necessarily synchronized.
7947 @cindex synchronized data and metadata I/O
7948 Use synchronized I/O for both data and metadata.
7952 @cindex nonblocking I/O
7953 Use non-blocking I/O.
7958 Do not update the file's access time.
7959 Some older file systems silently ignore this flag, so it is a good
7960 idea to test it on your files before relying on it.
7964 @cindex controlling terminal
7965 Do not assign the file to be a controlling terminal for @command{dd}.
7966 This has no effect when the file is not a terminal.
7967 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
7972 @cindex symbolic links, following
7973 Do not follow symbolic links.
7978 Fail if the file has multiple hard links.
7983 Use binary I/O. This option has an effect only on nonstandard
7984 platforms that distinguish binary from text I/O.
7989 Use text I/O. Like @samp{binary}, this option has no effect on
7994 Accumulate full blocks from input. The @code{read} system call
7995 may return early if a full block is not available.
7996 When that happens, continue calling @code{read} to fill the remainder
7998 This flag can be used only with @code{iflag}.
8002 These flags are not supported on all systems, and @samp{dd} rejects
8003 attempts to use them when they are not supported. When reading from
8004 standard input or writing to standard output, the @samp{nofollow} and
8005 @samp{noctty} flags should not be specified, and the other flags
8006 (e.g., @samp{nonblock}) can affect how other processes behave with the
8007 affected file descriptors, even after @command{dd} exits.
8011 @cindex multipliers after numbers
8012 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8013 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8014 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8015 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8017 Use different @command{dd} invocations to use different block sizes for
8018 skipping and I/O@. For example, the following shell commands copy data
8019 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8020 4 KiB label at the start of the disk:
8023 disk=/dev/rdsk/c0t1d0s2
8026 # Copy all but the label from disk to tape.
8027 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8029 # Copy from tape back to disk, but leave the disk label alone.
8030 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8033 Sending an @samp{INFO} signal to a running @command{dd}
8034 process makes it print I/O statistics to standard error
8035 and then resume copying. In the example below,
8036 @command{dd} is run in the background to copy 10 million blocks.
8037 The @command{kill} command makes it output intermediate I/O statistics,
8038 and when @command{dd} completes normally or is killed by the
8039 @code{SIGINT} signal, it outputs the final statistics.
8042 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8043 $ kill -s INFO $pid; wait $pid
8044 3385223+0 records in
8045 3385223+0 records out
8046 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8047 10000000+0 records in
8048 10000000+0 records out
8049 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8052 @vindex POSIXLY_CORRECT
8053 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8054 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8055 environment variable is set.
8060 @node install invocation
8061 @section @command{install}: Copy files and set attributes
8064 @cindex copying files and setting attributes
8066 @command{install} copies files while setting their file mode bits and, if
8067 possible, their owner and group. Synopses:
8070 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8071 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8072 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8073 install [@var{option}]@dots{} -d @var{directory}@dots{}
8078 If two file names are given, @command{install} copies the first file to the
8082 If the @option{--target-directory} (@option{-t}) option is given, or
8083 failing that if the last file is a directory and the
8084 @option{--no-target-directory} (@option{-T}) option is not given,
8085 @command{install} copies each @var{source} file to the specified
8086 directory, using the @var{source}s' names.
8089 If the @option{--directory} (@option{-d}) option is given,
8090 @command{install} creates each @var{directory} and any missing parent
8091 directories. Parent directories are created with mode
8092 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8093 current umask. @xref{Directory Setuid and Setgid}, for how the
8094 set-user-ID and set-group-ID bits of parent directories are inherited.
8097 @cindex Makefiles, installing programs in
8098 @command{install} is similar to @command{cp}, but allows you to control the
8099 attributes of destination files. It is typically used in Makefiles to
8100 copy programs into their destination directories. It refuses to copy
8101 files onto themselves.
8103 @cindex extended attributes, xattr
8104 @command{install} never preserves extended attributes (xattr).
8106 The program accepts the following options. Also see @ref{Common options}.
8116 Compare each pair of source and destination files, and if the destination has
8117 identical content and any specified owner, group, permissions, and possibly
8118 SELinux context, then do not modify the destination at all.
8122 Ignored; for compatibility with old Unix versions of @command{install}.
8126 Create any missing parent directories of @var{dest},
8127 then copy @var{source} to @var{dest}.
8128 This option is ignored if a destination directory is specified
8129 via @option{--target-directory=DIR}.
8134 @opindex --directory
8135 @cindex directories, creating with given attributes
8136 @cindex parent directories, creating missing
8137 @cindex leading directories, creating missing
8138 Create any missing parent directories, giving them the default
8139 attributes. Then create each given directory, setting their owner,
8140 group and mode as given on the command line or to the defaults.
8142 @item -g @var{group}
8143 @itemx --group=@var{group}
8146 @cindex group ownership of installed files, setting
8147 Set the group ownership of installed files or directories to
8148 @var{group}. The default is the process's current group. @var{group}
8149 may be either a group name or a numeric group ID.
8152 @itemx --mode=@var{mode}
8155 @cindex permissions of installed files, setting
8156 Set the file mode bits for the installed file or directory to @var{mode},
8157 which can be either an octal number, or a symbolic mode as in
8158 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8159 point of departure (@pxref{File permissions}).
8160 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8161 execute for the owner, read and execute for group and other, and with
8162 set-user-ID and set-group-ID disabled.
8163 This default is not quite the same as @samp{755}, since it disables
8164 instead of preserving set-user-ID and set-group-ID on directories.
8165 @xref{Directory Setuid and Setgid}.
8167 @item -o @var{owner}
8168 @itemx --owner=@var{owner}
8171 @cindex ownership of installed files, setting
8172 @cindex appropriate privileges
8173 @vindex root @r{as default owner}
8174 If @command{install} has appropriate privileges (is run as root), set the
8175 ownership of installed files or directories to @var{owner}. The default
8176 is @code{root}. @var{owner} may be either a user name or a numeric user
8179 @item --preserve-context
8180 @opindex --preserve-context
8182 @cindex security context
8183 Preserve the SELinux security context of files and directories.
8184 Failure to preserve the context in all of the files or directories
8185 will result in an exit status of 1. If SELinux is disabled then
8186 print a warning and ignore the option.
8189 @itemx --preserve-timestamps
8191 @opindex --preserve-timestamps
8192 @cindex timestamps of installed files, preserving
8193 Set the time of last access and the time of last modification of each
8194 installed file to match those of each corresponding original file.
8195 When a file is installed without this option, its last access and
8196 last modification times are both set to the time of installation.
8197 This option is useful if you want to use the last modification times
8198 of installed files to keep track of when they were last built as opposed
8199 to when they were last installed.
8205 @cindex symbol table information, stripping
8206 @cindex stripping symbol table information
8207 Strip the symbol tables from installed binary executables.
8209 @itemx --strip-program=@var{program}
8210 @opindex --strip-program
8211 @cindex symbol table information, stripping, program
8212 Program used to strip binaries.
8218 @optNoTargetDirectory
8224 Print the name of each file before copying it.
8226 @item -Z @var{context}
8227 @itemx --context=@var{context}
8231 @cindex security context
8232 Set the default SELinux security context to be used for any
8233 created files and directories. If SELinux is disabled then
8234 print a warning and ignore the option.
8242 @section @command{mv}: Move (rename) files
8246 @command{mv} moves or renames files (or directories). Synopses:
8249 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8250 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8251 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8256 If two file names are given, @command{mv} moves the first file to the
8260 If the @option{--target-directory} (@option{-t}) option is given, or
8261 failing that if the last file is a directory and the
8262 @option{--no-target-directory} (@option{-T}) option is not given,
8263 @command{mv} moves each @var{source} file to the specified
8264 directory, using the @var{source}s' names.
8267 @command{mv} can move any type of file from one file system to another.
8268 Prior to version @code{4.0} of the fileutils,
8269 @command{mv} could move only regular files between file systems.
8270 For example, now @command{mv} can move an entire directory hierarchy
8271 including special device files from one partition to another. It first
8272 uses some of the same code that's used by @code{cp -a} to copy the
8273 requested directories and files, then (assuming the copy succeeded)
8274 it removes the originals. If the copy fails, then the part that was
8275 copied to the destination partition is removed. If you were to copy
8276 three directories from one partition to another and the copy of the first
8277 directory succeeded, but the second didn't, the first would be left on
8278 the destination partition and the second and third would be left on the
8281 @cindex extended attributes, xattr
8282 @command{mv} always tries to copy extended attributes (xattr), which may
8283 include SELinux context, ACLs or Capabilities.
8284 Upon failure all but @samp{Operation not supported} warnings are output.
8286 @cindex prompting, and @command{mv}
8287 If a destination file exists but is normally unwritable, standard input
8288 is a terminal, and the @option{-f} or @option{--force} option is not given,
8289 @command{mv} prompts the user for whether to replace the file. (You might
8290 own the file, or have write permission on its directory.) If the
8291 response is not affirmative, the file is skipped.
8293 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8294 when it might be a symlink to a directory.
8295 Otherwise, @command{mv} may do something very surprising, since
8296 its behavior depends on the underlying rename system call.
8297 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8298 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8299 renames not the symlink but rather the directory referenced by the symlink.
8300 @xref{Trailing slashes}.
8302 The program accepts the following options. Also see @ref{Common options}.
8312 @cindex prompts, omitting
8313 Do not prompt the user before removing a destination file.
8315 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8316 options, only the final one takes effect.
8321 @itemx --interactive
8323 @opindex --interactive
8324 @cindex prompts, forcing
8325 Prompt whether to overwrite each existing destination file, regardless
8327 If the response is not affirmative, the file is skipped.
8333 @opindex --no-clobber
8334 @cindex prompts, omitting
8335 Do not overwrite an existing file.
8337 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8343 @cindex newer files, moving only
8344 Do not move a non-directory that has an existing destination with the
8345 same or newer modification time.
8346 If the move is across file system boundaries, the comparison is to the
8347 source time stamp truncated to the resolutions of the destination file
8348 system and of the system calls used to update time stamps; this avoids
8349 duplicate work if several @samp{mv -u} commands are executed with the
8350 same source and destination.
8356 Print the name of each file before moving it.
8358 @optStripTrailingSlashes
8364 @optNoTargetDirectory
8372 @section @command{rm}: Remove files or directories
8375 @cindex removing files or directories
8377 @command{rm} removes each given @var{file}. By default, it does not remove
8378 directories. Synopsis:
8381 rm [@var{option}]@dots{} [@var{file}]@dots{}
8384 @cindex prompting, and @command{rm}
8385 If the @option{-I} or @option{--interactive=once} option is given,
8386 and there are more than three files or the @option{-r}, @option{-R},
8387 or @option{--recursive} are given, then @command{rm} prompts the user
8388 for whether to proceed with the entire operation. If the response is
8389 not affirmative, the entire command is aborted.
8391 Otherwise, if a file is unwritable, standard input is a terminal, and
8392 the @option{-f} or @option{--force} option is not given, or the
8393 @option{-i} or @option{--interactive=always} option @emph{is} given,
8394 @command{rm} prompts the user for whether to remove the file.
8395 If the response is not affirmative, the file is skipped.
8397 Any attempt to remove a file whose last file name component is
8398 @file{.} or @file{..} is rejected without any prompting.
8400 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8401 possible to recover the contents of that file. If you want more assurance
8402 that the contents are truly unrecoverable, consider using @command{shred}.
8404 The program accepts the following options. Also see @ref{Common options}.
8412 Ignore nonexistent files and never prompt the user.
8413 Ignore any previous @option{--interactive} (@option{-i}) option.
8417 Prompt whether to remove each file.
8418 If the response is not affirmative, the file is skipped.
8419 Ignore any previous @option{--force} (@option{-f}) option.
8420 Equivalent to @option{--interactive=always}.
8424 Prompt once whether to proceed with the command, if more than three
8425 files are named or if a recursive removal is requested. Ignore any
8426 previous @option{--force} (@option{-f}) option. Equivalent to
8427 @option{--interactive=once}.
8429 @itemx --interactive [=@var{when}]
8430 @opindex --interactive
8431 Specify when to issue an interactive prompt. @var{when} may be
8435 @vindex never @r{interactive option}
8436 - Do not prompt at all.
8438 @vindex once @r{interactive option}
8439 - Prompt once if more than three files are named or if a recursive
8440 removal is requested. Equivalent to @option{-I}.
8442 @vindex always @r{interactive option}
8443 - Prompt for every file being removed. Equivalent to @option{-i}.
8445 @option{--interactive} with no @var{when} is equivalent to
8446 @option{--interactive=always}.
8448 @itemx --one-file-system
8449 @opindex --one-file-system
8450 @cindex one file system, restricting @command{rm} to
8451 When removing a hierarchy recursively, skip any directory that is on a
8452 file system different from that of the corresponding command line argument.
8454 This option is useful when removing a build ``chroot'' hierarchy,
8455 which normally contains no valuable data. However, it is not uncommon
8456 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8457 use one's start-up file. The catch is that it's easy to forget to
8458 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8459 your normally throw-away chroot, that command will remove everything
8460 under @file{/home}, too.
8461 Use the @option{--one-file-system} option, and it will
8462 warn about and skip directories on other file systems.
8463 Of course, this will not save your @file{/home} if it and your
8464 chroot happen to be on the same file system.
8466 @itemx --preserve-root
8467 @opindex --preserve-root
8468 @cindex root directory, disallow recursive destruction
8469 Fail upon any attempt to remove the root directory, @file{/},
8470 when used with the @option{--recursive} option.
8471 This is the default behavior.
8472 @xref{Treating / specially}.
8474 @itemx --no-preserve-root
8475 @opindex --no-preserve-root
8476 @cindex root directory, allow recursive destruction
8477 Do not treat @file{/} specially when removing recursively.
8478 This option is not recommended unless you really want to
8479 remove all the files on your computer.
8480 @xref{Treating / specially}.
8487 @opindex --recursive
8488 @cindex directories, removing (recursively)
8489 Remove the listed directories and their contents recursively.
8495 Print the name of each file before removing it.
8499 @cindex files beginning with @samp{-}, removing
8500 @cindex @samp{-}, removing files beginning with
8501 One common question is how to remove files whose names begin with a
8502 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8503 function to parse its arguments, lets you use the @samp{--} option to
8504 indicate that all following arguments are non-options. To remove a file
8505 called @file{-f} in the current directory, you could type either:
8518 @opindex - @r{and Unix @command{rm}}
8519 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8520 predates the development of the getopt standard syntax.
8525 @node shred invocation
8526 @section @command{shred}: Remove files more securely
8529 @cindex data, erasing
8530 @cindex erasing data
8532 @command{shred} overwrites devices or files, to help prevent even
8533 very expensive hardware from recovering the data.
8535 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8536 not actually destroyed. Only the index listing where the file is
8537 stored is destroyed, and the storage is made available for reuse.
8538 There are undelete utilities that will attempt to reconstruct the index
8539 and can bring the file back if the parts were not reused.
8541 On a busy system with a nearly-full drive, space can get reused in a few
8542 seconds. But there is no way to know for sure. If you have sensitive
8543 data, you may want to be sure that recovery is not possible by actually
8544 overwriting the file with non-sensitive data.
8546 However, even after doing that, it is possible to take the disk back
8547 to a laboratory and use a lot of sensitive (and expensive) equipment
8548 to look for the faint ``echoes'' of the original data underneath the
8549 overwritten data. If the data has only been overwritten once, it's not
8552 The best way to remove something irretrievably is to destroy the media
8553 it's on with acid, melt it down, or the like. For cheap removable media
8554 like floppy disks, this is the preferred method. However, hard drives
8555 are expensive and hard to melt, so the @command{shred} utility tries
8556 to achieve a similar effect non-destructively.
8558 This uses many overwrite passes, with the data patterns chosen to
8559 maximize the damage they do to the old data. While this will work on
8560 floppies, the patterns are designed for best effect on hard drives.
8561 For more details, see the source code and Peter Gutmann's paper
8562 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8563 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8564 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8565 California, July 22--25, 1996).
8567 @strong{Please note} that @command{shred} relies on a very important assumption:
8568 that the file system overwrites data in place. This is the traditional
8569 way to do things, but many modern file system designs do not satisfy this
8570 assumption. Exceptions include:
8575 Log-structured or journaled file systems, such as those supplied with
8576 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8577 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8580 File systems that write redundant data and carry on even if some writes
8581 fail, such as RAID-based file systems.
8584 File systems that make snapshots, such as Network Appliance's NFS server.
8587 File systems that cache in temporary locations, such as NFS version 3
8591 Compressed file systems.
8594 In the particular case of ext3 file systems, the above disclaimer applies (and
8595 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8596 mode, which journals file data in addition to just metadata. In both
8597 the @code{data=ordered} (default) and @code{data=writeback} modes,
8598 @command{shred} works as usual. Ext3 journaling modes can be changed
8599 by adding the @code{data=something} option to the mount options for a
8600 particular file system in the @file{/etc/fstab} file, as documented in
8601 the mount man page (man mount).
8603 If you are not sure how your file system operates, then you should assume
8604 that it does not overwrite data in place, which means that shred cannot
8605 reliably operate on regular files in your file system.
8607 Generally speaking, it is more reliable to shred a device than a file,
8608 since this bypasses the problem of file system design mentioned above.
8609 However, even shredding devices is not always completely reliable. For
8610 example, most disks map out bad sectors invisibly to the application; if
8611 the bad sectors contain sensitive data, @command{shred} won't be able to
8614 @command{shred} makes no attempt to detect or report this problem, just as
8615 it makes no attempt to do anything about backups. However, since it is
8616 more reliable to shred devices than files, @command{shred} by default does
8617 not truncate or remove the output file. This default is more suitable
8618 for devices, which typically cannot be truncated and should not be
8621 Finally, consider the risk of backups and mirrors.
8622 File system backups and remote mirrors may contain copies of the
8623 file that cannot be removed, and that will allow a shredded file
8624 to be recovered later. So if you keep any data you may later want
8625 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8628 shred [@var{option}]@dots{} @var{file}[@dots{}]
8631 The program accepts the following options. Also see @ref{Common options}.
8639 @cindex force deletion
8640 Override file permissions if necessary to allow overwriting.
8643 @itemx -n @var{number}
8644 @itemx --iterations=@var{number}
8645 @opindex -n @var{number}
8646 @opindex --iterations=@var{number}
8647 @cindex iterations, selecting the number of
8648 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8649 overwrite. You can reduce this to save time, or increase it if you think it's
8650 appropriate. After 25 passes all of the internal overwrite patterns will have
8651 been used at least once.
8653 @item --random-source=@var{file}
8654 @opindex --random-source
8655 @cindex random source for shredding
8656 Use @var{file} as a source of random data used to overwrite and to
8657 choose pass ordering. @xref{Random sources}.
8659 @item -s @var{bytes}
8660 @itemx --size=@var{bytes}
8661 @opindex -s @var{bytes}
8662 @opindex --size=@var{bytes}
8663 @cindex size of file to shred
8664 Shred the first @var{bytes} bytes of the file. The default is to shred
8665 the whole file. @var{bytes} can be followed by a size specification like
8666 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8672 @cindex removing files after shredding
8673 After shredding a file, truncate it (if possible) and then remove it.
8674 If a file has multiple links, only the named links will be removed.
8680 Display to standard error all status updates as sterilization proceeds.
8686 By default, @command{shred} rounds the size of a regular file up to the next
8687 multiple of the file system block size to fully erase the last block of the file.
8688 Use @option{--exact} to suppress that behavior.
8689 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8690 blocks, the resulting file will be 512 bytes long. With this option,
8691 shred does not increase the apparent size of the file.
8697 Normally, the last pass that @command{shred} writes is made up of
8698 random data. If this would be conspicuous on your hard drive (for
8699 example, because it looks like encrypted data), or you just think
8700 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8701 all zero bits. This is in addition to the number of passes specified
8702 by the @option{--iterations} option.
8706 You might use the following command to erase all trace of the
8707 file system you'd created on the floppy disk in your first drive.
8708 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8712 shred --verbose /dev/fd0
8715 Similarly, to erase all data on a selected partition of
8716 your hard disk, you could give a command like this:
8719 shred --verbose /dev/sda5
8722 A @var{file} of @samp{-} denotes standard output.
8723 The intended use of this is to shred a removed temporary file.
8730 echo "Hello, world" >&3
8735 However, the command @samp{shred - >file} does not shred the contents
8736 of @var{file}, since the shell truncates @var{file} before invoking
8737 @command{shred}. Use the command @samp{shred file} or (if using a
8738 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8743 @node Special file types
8744 @chapter Special file types
8746 @cindex special file types
8747 @cindex file types, special
8749 This chapter describes commands which create special types of files (and
8750 @command{rmdir}, which removes directories, one special file type).
8752 @cindex special file types
8754 Although Unix-like operating systems have markedly fewer special file
8755 types than others, not @emph{everything} can be treated only as the
8756 undifferentiated byte stream of @dfn{normal files}. For example, when a
8757 file is created or removed, the system must record this information,
8758 which it does in a @dfn{directory}---a special type of file. Although
8759 you can read directories as normal files, if you're curious, in order
8760 for the system to do its job it must impose a structure, a certain
8761 order, on the bytes of the file. Thus it is a ``special'' type of file.
8763 Besides directories, other special file types include named pipes
8764 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8767 * link invocation:: Make a hard link via the link syscall
8768 * ln invocation:: Make links between files.
8769 * mkdir invocation:: Make directories.
8770 * mkfifo invocation:: Make FIFOs (named pipes).
8771 * mknod invocation:: Make block or character special files.
8772 * readlink invocation:: Print value of a symlink or canonical file name.
8773 * rmdir invocation:: Remove empty directories.
8774 * unlink invocation:: Remove files via the unlink syscall
8778 @node link invocation
8779 @section @command{link}: Make a hard link via the link syscall
8782 @cindex links, creating
8783 @cindex hard links, creating
8784 @cindex creating links (hard only)
8786 @command{link} creates a single hard link at a time.
8787 It is a minimalist interface to the system-provided
8788 @code{link} function. @xref{Hard Links, , , libc,
8789 The GNU C Library Reference Manual}.
8790 It avoids the bells and whistles of the more commonly-used
8791 @command{ln} command (@pxref{ln invocation}).
8795 link @var{filename} @var{linkname}
8798 @var{filename} must specify an existing file, and @var{linkname}
8799 must specify a nonexistent entry in an existing directory.
8800 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8803 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8804 --no-target-directory @var{filename} @var{linkname}}. However, the
8805 @option{--directory} and @option{--no-target-directory} options are
8806 not specified by @acronym{POSIX}, and the @command{link} command is
8807 more portable in practice.
8809 If @var{filename} is a symbolic link, it is unspecified whether
8810 @var{linkname} will be a hard link to the symbolic link or to the
8811 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8812 to specify which behavior is desired.
8818 @section @command{ln}: Make links between files
8821 @cindex links, creating
8822 @cindex hard links, creating
8823 @cindex symbolic (soft) links, creating
8824 @cindex creating links (hard or soft)
8826 @cindex file systems and hard links
8827 @command{ln} makes links between files. By default, it makes hard links;
8828 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8832 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8833 ln [@var{option}]@dots{} @var{target}
8834 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8835 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8841 If two file names are given, @command{ln} creates a link to the first
8842 file from the second.
8845 If one @var{target} is given, @command{ln} creates a link to that file
8846 in the current directory.
8849 If the @option{--target-directory} (@option{-t}) option is given, or
8850 failing that if the last file is a directory and the
8851 @option{--no-target-directory} (@option{-T}) option is not given,
8852 @command{ln} creates a link to each @var{target} file in the specified
8853 directory, using the @var{target}s' names.
8857 Normally @command{ln} does not remove existing files. Use the
8858 @option{--force} (@option{-f}) option to remove them unconditionally,
8859 the @option{--interactive} (@option{-i}) option to remove them
8860 conditionally, and the @option{--backup} (@option{-b}) option to
8863 @cindex hard link, defined
8864 @cindex inode, and hard links
8865 A @dfn{hard link} is another name for an existing file; the link and the
8866 original are indistinguishable. Technically speaking, they share the
8867 same inode, and the inode contains all the information about a
8868 file---indeed, it is not incorrect to say that the inode @emph{is} the
8869 file. Most systems prohibit making a hard link to
8870 a directory; on those where it is allowed, only the super-user can do
8871 so (and with caution, since creating a cycle will cause problems to many
8872 other utilities). Hard links cannot cross file system boundaries. (These
8873 restrictions are not mandated by @acronym{POSIX}, however.)
8875 @cindex dereferencing symbolic links
8876 @cindex symbolic link, defined
8877 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8878 a special file type (which not all kernels support: System V release 3
8879 (and older) systems lack symlinks) in which the link file actually
8880 refers to a different file, by name. When most operations (opening,
8881 reading, writing, and so on) are passed the symbolic link file, the
8882 kernel automatically @dfn{dereferences} the link and operates on the
8883 target of the link. But some operations (e.g., removing) work on the
8884 link file itself, rather than on its target. The owner and group of a
8885 symlink are not significant to file access performed through
8886 the link, but do have implications on deleting a symbolic link from a
8887 directory with the restricted deletion bit set. On the GNU system,
8888 the mode of a symlink has no significance and cannot be changed, but
8889 on some BSD systems, the mode can be changed and will affect whether
8890 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8891 libc, The GNU C Library Reference Manual}.
8893 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8894 occurs when the string in the symlink does not resolve to a file.
8895 There are no restrictions against creating dangling symbolic links.
8896 There are trade-offs to using absolute or relative symlinks. An
8897 absolute symlink always points to the same file, even if the directory
8898 containing the link is moved. However, if the symlink is visible from
8899 more than one machine (such as on a networked file system), the file
8900 pointed to might not always be the same. A relative symbolic link is
8901 resolved in relation to the directory that contains the link, and is
8902 often useful in referring to files on the same device without regards
8903 to what name that device is mounted on when accessed via networked
8906 When creating a relative symlink in a different location than the
8907 current directory, the resolution of the symlink will be different
8908 than the resolution of the same string from the current directory.
8909 Therefore, many users prefer to first change directories to the
8910 location where the relative symlink will be created, so that
8911 tab-completion or other file resolution will find the same target as
8912 what will be placed in the symlink.
8914 The program accepts the following options. Also see @ref{Common options}.
8925 @opindex --directory
8926 @cindex hard links to directories
8927 Allow users with appropriate privileges to attempt to make hard links
8929 However, note that this will probably fail due to
8930 system restrictions, even for the super-user.
8936 Remove existing destination files.
8939 @itemx --interactive
8941 @opindex --interactive
8942 @cindex prompting, and @command{ln}
8943 Prompt whether to remove existing destination files.
8949 If @option{-s} is not in effect, and the source file is a symbolic
8950 link, create the hard link to the file referred to by the symbolic
8951 link, rather than the symbolic link itself.
8954 @itemx --no-dereference
8956 @opindex --no-dereference
8957 Do not treat the last operand specially when it is a symbolic link to
8958 a directory. Instead, treat it as if it were a normal file.
8960 When the destination is an actual directory (not a symlink to one),
8961 there is no ambiguity. The link is created in that directory.
8962 But when the specified destination is a symlink to a directory,
8963 there are two ways to treat the user's request. @command{ln} can
8964 treat the destination just as it would a normal directory and create
8965 the link in it. On the other hand, the destination can be viewed as a
8966 non-directory---as the symlink itself. In that case, @command{ln}
8967 must delete or backup that symlink before creating the new link.
8968 The default is to treat a destination that is a symlink to a directory
8969 just like a directory.
8971 This option is weaker than the @option{--no-target-directory}
8972 (@option{-T}) option, so it has no effect if both options are given.
8978 If @option{-s} is not in effect, and the source file is a symbolic
8979 link, create the hard link to the symbolic link itself. On platforms
8980 where this is not supported by the kernel, this option creates a
8981 symbolic link with identical contents; since symbolic link contents
8982 cannot be edited, any file name resolution performed through either
8983 link will be the same as if a hard link had been created.
8989 Make symbolic links instead of hard links. This option merely produces
8990 an error message on systems that do not support symbolic links.
8996 @optNoTargetDirectory
9002 Print the name of each file after linking it successfully.
9006 @cindex hard links to symbolic links
9007 @cindex symbolic links and @command{ln}
9008 If @option{-L} and @option{-P} are both given, the last one takes
9009 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9010 are silently ignored. If neither option is given, then this
9011 implementation defaults to @option{-P} if the system @code{link} supports
9012 hard links to symbolic links (such as the GNU system), and @option{-L}
9013 if @code{link} follows symbolic links (such as on BSD).
9022 # Create link ../a pointing to a in that directory.
9023 # Not really useful because it points to itself.
9028 # Change to the target before creating symlinks to avoid being confused.
9034 # Hard coded file names don't move well.
9035 ln -s $(pwd)/a /some/dir/
9039 # Relative file names survive directory moves and also
9040 # work across networked file systems.
9041 ln -s afile anotherfile
9042 ln -s ../adir/afile yetanotherfile
9046 @node mkdir invocation
9047 @section @command{mkdir}: Make directories
9050 @cindex directories, creating
9051 @cindex creating directories
9053 @command{mkdir} creates directories with the specified names. Synopsis:
9056 mkdir [@var{option}]@dots{} @var{name}@dots{}
9059 @command{mkdir} creates each directory @var{name} in the order given.
9060 It reports an error if @var{name} already exists, unless the
9061 @option{-p} option is given and @var{name} is a directory.
9063 The program accepts the following options. Also see @ref{Common options}.
9068 @itemx --mode=@var{mode}
9071 @cindex modes of created directories, setting
9072 Set the file permission bits of created directories to @var{mode},
9073 which uses the same syntax as
9074 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9075 everyone) for the point of the departure. @xref{File permissions}.
9077 Normally the directory has the desired file mode bits at the moment it
9078 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9079 special mode bits, but in this case there may be a temporary window
9080 during which the directory exists but its special mode bits are
9081 incorrect. @xref{Directory Setuid and Setgid}, for how the
9082 set-user-ID and set-group-ID bits of directories are inherited unless
9083 overridden in this way.
9089 @cindex parent directories, creating
9090 Make any missing parent directories for each argument, setting their
9091 file permission bits to the umask modified by @samp{u+wx}. Ignore
9092 existing parent directories, and do not change their file permission
9095 To set the file permission bits of any newly-created parent
9096 directories to a value that includes @samp{u+wx}, you can set the
9097 umask before invoking @command{mkdir}. For example, if the shell
9098 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9099 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9100 To set a parent's special mode bits as well, you can invoke
9101 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9102 Setgid}, for how the set-user-ID and set-group-ID bits of
9103 newly-created parent directories are inherited.
9109 Print a message for each created directory. This is most useful with
9112 @item -Z @var{context}
9113 @itemx --context=@var{context}
9117 @cindex security context
9118 Set the default SELinux security context to be used for created directories.
9125 @node mkfifo invocation
9126 @section @command{mkfifo}: Make FIFOs (named pipes)
9129 @cindex FIFOs, creating
9130 @cindex named pipes, creating
9131 @cindex creating FIFOs (named pipes)
9133 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9134 specified names. Synopsis:
9137 mkfifo [@var{option}] @var{name}@dots{}
9140 A @dfn{FIFO} is a special file type that permits independent processes
9141 to communicate. One process opens the FIFO file for writing, and
9142 another for reading, after which data can flow as with the usual
9143 anonymous pipe in shells or elsewhere.
9145 The program accepts the following option. Also see @ref{Common options}.
9150 @itemx --mode=@var{mode}
9153 @cindex modes of created FIFOs, setting
9154 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9155 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9156 for the point of departure. @var{mode} should specify only file
9157 permission bits. @xref{File permissions}.
9159 @item -Z @var{context}
9160 @itemx --context=@var{context}
9164 @cindex security context
9165 Set the default SELinux security context to be used for created FIFOs.
9172 @node mknod invocation
9173 @section @command{mknod}: Make block or character special files
9176 @cindex block special files, creating
9177 @cindex character special files, creating
9179 @command{mknod} creates a FIFO, character special file, or block special
9180 file with the specified name. Synopsis:
9183 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9186 @cindex special files
9187 @cindex block special files
9188 @cindex character special files
9189 Unlike the phrase ``special file type'' above, the term @dfn{special
9190 file} has a technical meaning on Unix: something that can generate or
9191 receive data. Usually this corresponds to a physical piece of hardware,
9192 e.g., a printer or a disk. (These files are typically created at
9193 system-configuration time.) The @command{mknod} command is what creates
9194 files of this type. Such devices can be read either a character at a
9195 time or a ``block'' (many characters) at a time, hence we say there are
9196 @dfn{block special} files and @dfn{character special} files.
9198 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9199 @mayConflictWithShellBuiltIn{mknod}
9201 The arguments after @var{name} specify the type of file to make:
9206 @opindex p @r{for FIFO file}
9210 @opindex b @r{for block special file}
9211 for a block special file
9214 @c Don't document the `u' option -- it's just a synonym for `c'.
9215 @c Do *any* versions of mknod still use it?
9217 @opindex c @r{for character special file}
9218 @c @opindex u @r{for character special file}
9219 for a character special file
9223 When making a block or character special file, the major and minor
9224 device numbers must be given after the file type.
9225 If a major or minor device number begins with @samp{0x} or @samp{0X},
9226 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9227 as octal; otherwise, as decimal.
9229 The program accepts the following option. Also see @ref{Common options}.
9234 @itemx --mode=@var{mode}
9237 Set the mode of created files to @var{mode}, which is symbolic as in
9238 @command{chmod} and uses @samp{a=rw} as the point of departure.
9239 @var{mode} should specify only file permission bits.
9240 @xref{File permissions}.
9242 @item -Z @var{context}
9243 @itemx --context=@var{context}
9247 @cindex security context
9248 Set the default SELinux security context to be used for created files.
9255 @node readlink invocation
9256 @section @command{readlink}: Print value of a symlink or canonical file name
9259 @cindex displaying value of a symbolic link
9260 @cindex canonical file name
9261 @cindex canonicalize a file name
9265 @command{readlink} may work in one of two supported modes:
9271 @command{readlink} outputs the value of the given symbolic link.
9272 If @command{readlink} is invoked with an argument other than the name
9273 of a symbolic link, it produces no output and exits with a nonzero exit code.
9275 @item Canonicalize mode
9277 @command{readlink} outputs the absolute name of the given file which contains
9278 no @file{.}, @file{..} components nor any repeated separators
9279 (@file{/}) or symbolic links.
9284 readlink [@var{option}] @var{file}
9287 By default, @command{readlink} operates in readlink mode.
9289 The program accepts the following options. Also see @ref{Common options}.
9294 @itemx --canonicalize
9296 @opindex --canonicalize
9297 Activate canonicalize mode.
9298 If any component of the file name except the last one is missing or unavailable,
9299 @command{readlink} produces no output and exits with a nonzero exit
9300 code. A trailing slash is ignored.
9303 @itemx --canonicalize-existing
9305 @opindex --canonicalize-existing
9306 Activate canonicalize mode.
9307 If any component is missing or unavailable, @command{readlink} produces
9308 no output and exits with a nonzero exit code. A trailing slash
9309 requires that the name resolve to a directory.
9312 @itemx --canonicalize-missing
9314 @opindex --canonicalize-missing
9315 Activate canonicalize mode.
9316 If any component is missing or unavailable, @command{readlink} treats it
9322 @opindex --no-newline
9323 Do not output the trailing newline.
9333 Suppress most error messages.
9339 Report error messages.
9343 The @command{readlink} utility first appeared in OpenBSD 2.1.
9345 There is a @command{realpath} command on some systems
9346 which operates like @command{readlink} in canonicalize mode.
9351 @node rmdir invocation
9352 @section @command{rmdir}: Remove empty directories
9355 @cindex removing empty directories
9356 @cindex directories, removing empty
9358 @command{rmdir} removes empty directories. Synopsis:
9361 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9364 If any @var{directory} argument does not refer to an existing empty
9365 directory, it is an error.
9367 The program accepts the following options. Also see @ref{Common options}.
9371 @item --ignore-fail-on-non-empty
9372 @opindex --ignore-fail-on-non-empty
9373 @cindex directory deletion, ignoring failures
9374 Ignore each failure to remove a directory that is solely because
9375 the directory is non-empty.
9381 @cindex parent directories, removing
9382 Remove @var{directory}, then try to remove each component of @var{directory}.
9383 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9384 As such, it fails if any of those directories turns out not to be empty.
9385 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9386 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9387 exit unsuccessfully.
9393 @cindex directory deletion, reporting
9394 Give a diagnostic for each successful removal.
9395 @var{directory} is removed.
9399 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9404 @node unlink invocation
9405 @section @command{unlink}: Remove files via the unlink syscall
9408 @cindex removing files or directories (via the unlink syscall)
9410 @command{unlink} deletes a single specified file name.
9411 It is a minimalist interface to the system-provided
9412 @code{unlink} function. @xref{Deleting Files, , , libc,
9413 The GNU C Library Reference Manual}. Synopsis:
9414 It avoids the bells and whistles of the more commonly-used
9415 @command{rm} command (@pxref{rm invocation}).
9418 unlink @var{filename}
9421 On some systems @code{unlink} can be used to delete the name of a
9422 directory. On others, it can be used that way only by a privileged user.
9423 In the GNU system @code{unlink} can never delete the name of a directory.
9425 The @command{unlink} command honors the @option{--help} and
9426 @option{--version} options. To remove a file whose name begins with
9427 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9432 @node Changing file attributes
9433 @chapter Changing file attributes
9435 @cindex changing file attributes
9436 @cindex file attributes, changing
9437 @cindex attributes, file
9439 A file is not merely its contents, a name, and a file type
9440 (@pxref{Special file types}). A file also has an owner (a user ID), a
9441 group (a group ID), permissions (what the owner can do with the file,
9442 what people in the group can do, and what everyone else can do), various
9443 timestamps, and other information. Collectively, we call these a file's
9446 These commands change file attributes.
9449 * chgrp invocation:: Change file groups.
9450 * chmod invocation:: Change access permissions.
9451 * chown invocation:: Change file owners and groups.
9452 * touch invocation:: Change file timestamps.
9456 @node chown invocation
9457 @section @command{chown}: Change file owner and group
9460 @cindex file ownership, changing
9461 @cindex group ownership, changing
9462 @cindex changing file ownership
9463 @cindex changing group ownership
9465 @command{chown} changes the user and/or group ownership of each given @var{file}
9466 to @var{new-owner} or to the user and group of an existing reference file.
9470 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9473 If used, @var{new-owner} specifies the new owner and/or group as follows
9474 (with no embedded white space):
9477 [@var{owner}] [ : [@var{group}] ]
9484 If only an @var{owner} (a user name or numeric user ID) is given, that
9485 user is made the owner of each given file, and the files' group is not
9488 @item owner@samp{:}group
9489 If the @var{owner} is followed by a colon and a @var{group} (a
9490 group name or numeric group ID), with no spaces between them, the group
9491 ownership of the files is changed as well (to @var{group}).
9494 If a colon but no group name follows @var{owner}, that user is
9495 made the owner of the files and the group of the files is changed to
9496 @var{owner}'s login group.
9499 If the colon and following @var{group} are given, but the owner
9500 is omitted, only the group of the files is changed; in this case,
9501 @command{chown} performs the same function as @command{chgrp}.
9504 If only a colon is given, or if @var{new-owner} is empty, neither the
9505 owner nor the group is changed.
9509 If @var{owner} or @var{group} is intended to represent a numeric user
9510 or group ID, then you may specify it with a leading @samp{+}.
9511 @xref{Disambiguating names and IDs}.
9513 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9514 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9515 require support for that, but for backward compatibility @acronym{GNU}
9516 @command{chown} supports @samp{.} so long as no ambiguity results.
9517 New scripts should avoid the use of @samp{.} because it is not
9518 portable, and because it has undesirable results if the entire
9519 @var{owner@samp{.}group} happens to identify a user whose name
9522 The @command{chown} command sometimes clears the set-user-ID or
9523 set-group-ID permission bits. This behavior depends on the policy and
9524 functionality of the underlying @code{chown} system call, which may
9525 make system-dependent file mode modifications outside the control of
9526 the @command{chown} command. For example, the @command{chown} command
9527 might not affect those bits when invoked by a user with appropriate
9528 privileges, or when the
9529 bits signify some function other than executable permission (e.g.,
9531 When in doubt, check the underlying system behavior.
9533 The program accepts the following options. Also see @ref{Common options}.
9541 @cindex changed owners, verbosely describing
9542 Verbosely describe the action for each @var{file} whose ownership
9551 @cindex error messages, omitting
9552 Do not print error messages about files whose ownership cannot be
9555 @itemx @w{@kbd{--from}=@var{old-owner}}
9557 @cindex symbolic links, changing owner
9558 Change a @var{file}'s ownership only if it has current attributes specified
9559 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9561 This option is useful primarily from a security standpoint in that
9562 it narrows considerably the window of potential abuse.
9563 For example, to reflect a user ID numbering change for one user's files
9564 without an option like this, @code{root} might run
9567 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9570 But that is dangerous because the interval between when the @command{find}
9571 tests the existing file's owner and when the @command{chown} is actually run
9573 One way to narrow the gap would be to invoke chown for each file
9577 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9580 But that is very slow if there are many affected files.
9581 With this option, it is safer (the gap is narrower still)
9582 though still not perfect:
9585 chown -h -R --from=OLDUSER NEWUSER /
9589 @opindex --dereference
9590 @cindex symbolic links, changing owner
9592 Do not act on symbolic links themselves but rather on what they point to.
9593 This is the default.
9596 @itemx --no-dereference
9598 @opindex --no-dereference
9599 @cindex symbolic links, changing owner
9601 Act on symbolic links themselves instead of what they point to.
9602 This mode relies on the @code{lchown} system call.
9603 On systems that do not provide the @code{lchown} system call,
9604 @command{chown} fails when a file specified on the command line
9606 By default, no diagnostic is issued for symbolic links encountered
9607 during a recursive traversal, but see @option{--verbose}.
9609 @itemx --preserve-root
9610 @opindex --preserve-root
9611 @cindex root directory, disallow recursive modification
9612 Fail upon any attempt to recursively change the root directory, @file{/}.
9613 Without @option{--recursive}, this option has no effect.
9614 @xref{Treating / specially}.
9616 @itemx --no-preserve-root
9617 @opindex --no-preserve-root
9618 @cindex root directory, allow recursive modification
9619 Cancel the effect of any preceding @option{--preserve-root} option.
9620 @xref{Treating / specially}.
9622 @item --reference=@var{ref_file}
9623 @opindex --reference
9624 Change the user and group of each @var{file} to be the same as those of
9625 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9626 user and group of the symbolic link, but rather those of the file it
9633 Output a diagnostic for every file processed.
9634 If a symbolic link is encountered during a recursive traversal
9635 on a system without the @code{lchown} system call, and @option{--no-dereference}
9636 is in effect, then issue a diagnostic saying neither the symbolic link nor
9637 its referent is being changed.
9642 @opindex --recursive
9643 @cindex recursively changing file ownership
9644 Recursively change ownership of directories and their contents.
9647 @xref{Traversing symlinks}.
9650 @xref{Traversing symlinks}.
9653 @xref{Traversing symlinks}.
9662 # Change the owner of /u to "root".
9665 # Likewise, but also change its group to "staff".
9668 # Change the owner of /u and subfiles to "root".
9673 @node chgrp invocation
9674 @section @command{chgrp}: Change group ownership
9677 @cindex group ownership, changing
9678 @cindex changing group ownership
9680 @command{chgrp} changes the group ownership of each given @var{file}
9681 to @var{group} (which can be either a group name or a numeric group ID)
9682 or to the group of an existing reference file. Synopsis:
9685 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9688 If @var{group} is intended to represent a
9689 numeric group ID, then you may specify it with a leading @samp{+}.
9690 @xref{Disambiguating names and IDs}.
9692 The program accepts the following options. Also see @ref{Common options}.
9700 @cindex changed files, verbosely describing
9701 Verbosely describe the action for each @var{file} whose group actually
9710 @cindex error messages, omitting
9711 Do not print error messages about files whose group cannot be
9715 @opindex --dereference
9716 @cindex symbolic links, changing owner
9718 Do not act on symbolic links themselves but rather on what they point to.
9719 This is the default.
9722 @itemx --no-dereference
9724 @opindex --no-dereference
9725 @cindex symbolic links, changing group
9727 Act on symbolic links themselves instead of what they point to.
9728 This mode relies on the @code{lchown} system call.
9729 On systems that do not provide the @code{lchown} system call,
9730 @command{chgrp} fails when a file specified on the command line
9732 By default, no diagnostic is issued for symbolic links encountered
9733 during a recursive traversal, but see @option{--verbose}.
9735 @itemx --preserve-root
9736 @opindex --preserve-root
9737 @cindex root directory, disallow recursive modification
9738 Fail upon any attempt to recursively change the root directory, @file{/}.
9739 Without @option{--recursive}, this option has no effect.
9740 @xref{Treating / specially}.
9742 @itemx --no-preserve-root
9743 @opindex --no-preserve-root
9744 @cindex root directory, allow recursive modification
9745 Cancel the effect of any preceding @option{--preserve-root} option.
9746 @xref{Treating / specially}.
9748 @item --reference=@var{ref_file}
9749 @opindex --reference
9750 Change the group of each @var{file} to be the same as that of
9751 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9752 group of the symbolic link, but rather that of the file it refers to.
9758 Output a diagnostic for every file processed.
9759 If a symbolic link is encountered during a recursive traversal
9760 on a system without the @code{lchown} system call, and @option{--no-dereference}
9761 is in effect, then issue a diagnostic saying neither the symbolic link nor
9762 its referent is being changed.
9767 @opindex --recursive
9768 @cindex recursively changing group ownership
9769 Recursively change the group ownership of directories and their contents.
9772 @xref{Traversing symlinks}.
9775 @xref{Traversing symlinks}.
9778 @xref{Traversing symlinks}.
9787 # Change the group of /u to "staff".
9790 # Change the group of /u and subfiles to "staff".
9795 @node chmod invocation
9796 @section @command{chmod}: Change access permissions
9799 @cindex changing access permissions
9800 @cindex access permissions, changing
9801 @cindex permissions, changing access
9803 @command{chmod} changes the access permissions of the named files. Synopsis:
9806 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9809 @cindex symbolic links, permissions of
9810 @command{chmod} never changes the permissions of symbolic links, since
9811 the @command{chmod} system call cannot change their permissions.
9812 This is not a problem since the permissions of symbolic links are
9813 never used. However, for each symbolic link listed on the command
9814 line, @command{chmod} changes the permissions of the pointed-to file.
9815 In contrast, @command{chmod} ignores symbolic links encountered during
9816 recursive directory traversals.
9818 A successful use of @command{chmod} clears the set-group-ID bit of a
9819 regular file if the file's group ID does not match the user's
9820 effective group ID or one of the user's supplementary group IDs,
9821 unless the user has appropriate privileges. Additional restrictions
9822 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9823 @var{ref_file} to be ignored. This behavior depends on the policy and
9824 functionality of the underlying @code{chmod} system call. When in
9825 doubt, check the underlying system behavior.
9827 If used, @var{mode} specifies the new file mode bits.
9828 For details, see the section on @ref{File permissions}.
9829 If you really want @var{mode} to have a leading @samp{-}, you should
9830 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9831 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9832 file} (without the @option{--}) complains if it behaves differently
9833 from what @samp{chmod a-w file} would do.
9835 The program accepts the following options. Also see @ref{Common options}.
9843 Verbosely describe the action for each @var{file} whose permissions
9852 @cindex error messages, omitting
9853 Do not print error messages about files whose permissions cannot be
9856 @itemx --preserve-root
9857 @opindex --preserve-root
9858 @cindex root directory, disallow recursive modification
9859 Fail upon any attempt to recursively change the root directory, @file{/}.
9860 Without @option{--recursive}, this option has no effect.
9861 @xref{Treating / specially}.
9863 @itemx --no-preserve-root
9864 @opindex --no-preserve-root
9865 @cindex root directory, allow recursive modification
9866 Cancel the effect of any preceding @option{--preserve-root} option.
9867 @xref{Treating / specially}.
9873 Verbosely describe the action or non-action taken for every @var{file}.
9875 @item --reference=@var{ref_file}
9876 @opindex --reference
9877 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9878 @xref{File permissions}.
9879 If @var{ref_file} is a symbolic link, do not use the mode
9880 of the symbolic link, but rather that of the file it refers to.
9885 @opindex --recursive
9886 @cindex recursively changing access permissions
9887 Recursively change permissions of directories and their contents.
9894 @node touch invocation
9895 @section @command{touch}: Change file timestamps
9898 @cindex changing file timestamps
9899 @cindex file timestamps, changing
9900 @cindex timestamps, changing file
9902 @command{touch} changes the access and/or modification times of the
9903 specified files. Synopsis:
9906 touch [@var{option}]@dots{} @var{file}@dots{}
9909 @cindex empty files, creating
9910 Any @var{file} argument that does not exist is created empty, unless
9911 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9912 (@option{-h}) was in effect.
9914 A @var{file} argument string of @samp{-} is handled specially and
9915 causes @command{touch} to change the times of the file associated with
9918 @cindex permissions, for changing file timestamps
9919 If changing both the access and modification times to the current
9920 time, @command{touch} can change the timestamps for files that the user
9921 running it does not own but has write permission for. Otherwise, the
9922 user must own the files.
9924 Although @command{touch} provides options for changing two of the times---the
9925 times of last access and modification---of a file, there is actually
9926 a standard third one as well: the inode change time. This is often
9927 referred to as a file's @code{ctime}.
9928 The inode change time represents the time when the file's meta-information
9929 last changed. One common example of this is when the permissions of a
9930 file change. Changing the permissions doesn't access the file, so
9931 the atime doesn't change, nor does it modify the file, so the mtime
9932 doesn't change. Yet, something about the file itself has changed,
9933 and this must be noted somewhere. This is the job of the ctime field.
9934 This is necessary, so that, for example, a backup program can make a
9935 fresh copy of the file, including the new permissions value.
9936 Another operation that modifies a file's ctime without affecting
9937 the others is renaming. In any case, it is not possible, in normal
9938 operations, for a user to change the ctime field to a user-specified value.
9939 Some operating systems and file systems support a fourth time: the
9940 birth time, when the file was first created; by definition, this
9941 timestamp never changes.
9944 Time stamps assume the time zone rules specified by the @env{TZ}
9945 environment variable, or by the system default rules if @env{TZ} is
9946 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9947 libc, The GNU C Library Reference Manual}.
9948 You can avoid ambiguities during
9949 daylight saving transitions by using @sc{utc} time stamps.
9951 The program accepts the following options. Also see @ref{Common options}.
9957 @itemx --time=access
9961 @opindex atime@r{, changing}
9962 @opindex access @r{time, changing}
9963 @opindex use @r{time, changing}
9964 Change the access time only.
9969 @opindex --no-create
9970 Do not warn about or create files that do not exist.
9973 @itemx --date=@var{time}
9977 Use @var{time} instead of the current time. It can contain month names,
9978 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
9979 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
9980 specifies the instant of time that is 489,392,193 nanoseconds after
9981 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
9982 minutes east of @acronym{UTC}. @xref{Date input formats}.
9983 File systems that do not support high-resolution time stamps
9984 silently ignore any excess precision here.
9988 @cindex BSD @command{touch} compatibility
9989 Ignored; for compatibility with BSD versions of @command{touch}.
9992 @itemx --no-dereference
9994 @opindex --no-dereference
9995 @cindex symbolic links, changing time
9997 Attempt to change the timestamps of a symbolic link, rather than what
9998 the link refers to. When using this option, empty files are not
9999 created, but option @option{-c} must also be used to avoid warning
10000 about files that do not exist. Not all systems support changing the
10001 timestamps of symlinks, since underlying system support for this
10002 action was not required until @acronym{POSIX} 2008. Also, on some
10003 systems, the mere act of examining a symbolic link changes the access
10004 time, such that only changes to the modification time will persist
10005 long enough to be observable. When coupled with option @option{-r}, a
10006 reference timestamp is taken from a symbolic link rather than the file
10010 @itemx --time=mtime
10011 @itemx --time=modify
10014 @opindex mtime@r{, changing}
10015 @opindex modify @r{time, changing}
10016 Change the modification time only.
10018 @item -r @var{file}
10019 @itemx --reference=@var{file}
10021 @opindex --reference
10022 Use the times of the reference @var{file} instead of the current time.
10023 If this option is combined with the @option{--date=@var{time}}
10024 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10025 the origin for any relative @var{time}s given, but is otherwise ignored.
10026 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10027 equal to five seconds before the corresponding time stamp for @file{foo}.
10028 If @var{file} is a symbolic link, the reference timestamp is taken
10029 from the target of the symlink, unless @option{-h} was also in effect.
10031 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10032 Use the argument (optional four-digit or two-digit years, months,
10033 days, hours, minutes, optional seconds) instead of the current time.
10034 If the year is specified with only two digits, then @var{cc}
10035 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10036 69 @dots{} 99. If no digits of the year are specified,
10037 the argument is interpreted as a date in the current year.
10038 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10042 @vindex _POSIX2_VERSION
10043 On older systems, @command{touch} supports an obsolete syntax, as follows.
10044 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10045 @option{-t} options, and if there are two or more @var{file}s and the
10046 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10047 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10048 any, were moved to the front), and if the represented year
10049 is in the range 1969--1999, that argument is interpreted as the time
10050 for the other files instead of as a file name.
10051 This obsolete behavior can be enabled or disabled with the
10052 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10053 conformance}), but portable scripts should avoid commands whose
10054 behavior depends on this variable.
10055 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10056 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10062 @chapter Disk usage
10066 No disk can hold an infinite amount of data. These commands report
10067 how much disk storage is in use or available, report other file and
10068 file status information, and write buffers to disk.
10071 * df invocation:: Report file system disk space usage.
10072 * du invocation:: Estimate file space usage.
10073 * stat invocation:: Report file or file system status.
10074 * sync invocation:: Synchronize memory and disk.
10075 * truncate invocation:: Shrink or extend the size of a file.
10079 @node df invocation
10080 @section @command{df}: Report file system disk space usage
10083 @cindex file system disk usage
10084 @cindex disk usage by file system
10086 @command{df} reports the amount of disk space used and available on
10087 file systems. Synopsis:
10090 df [@var{option}]@dots{} [@var{file}]@dots{}
10093 With no arguments, @command{df} reports the space used and available on all
10094 currently mounted file systems (of all types). Otherwise, @command{df}
10095 reports on the file system containing each argument @var{file}.
10097 Normally the disk space is printed in units of
10098 1024 bytes, but this can be overridden (@pxref{Block size}).
10099 Non-integer quantities are rounded up to the next higher unit.
10101 @cindex disk device file
10102 @cindex device file, disk
10103 If an argument @var{file} is a disk device file containing a mounted
10104 file system, @command{df} shows the space available on that file system
10105 rather than on the file system containing the device node (i.e., the root
10106 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10107 on unmounted file systems, because on most kinds of systems doing so
10108 requires extremely nonportable intimate knowledge of file system
10111 The program accepts the following options. Also see @ref{Common options}.
10119 @cindex automounter file systems
10120 @cindex ignore file systems
10121 Include in the listing dummy file systems, which
10122 are omitted by default. Such file systems are typically special-purpose
10123 pseudo-file-systems, such as automounter entries.
10125 @item -B @var{size}
10126 @itemx --block-size=@var{size}
10128 @opindex --block-size
10129 @cindex file system sizes
10130 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10131 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10135 @cindex grand total of disk size, usage and available space
10136 Print a grand total of all arguments after all arguments have
10137 been processed. This can be used to find out the total disk size, usage
10138 and available space of all listed devices.
10144 Equivalent to @option{--si}.
10150 @cindex inode usage
10151 List inode usage information instead of block usage. An inode (short
10152 for index node) contains information about a file such as its owner,
10153 permissions, timestamps, and location on the disk.
10157 @cindex kibibytes for file system sizes
10158 Print sizes in 1024-byte blocks, overriding the default block size
10159 (@pxref{Block size}).
10160 This option is equivalent to @option{--block-size=1K}.
10166 @cindex file system types, limiting output to certain
10167 Limit the listing to local file systems. By default, remote file systems
10172 @cindex file system space, retrieving old data more quickly
10173 Do not invoke the @code{sync} system call before getting any usage data.
10174 This may make @command{df} run significantly faster on systems with many
10175 disks, but on some systems (notably SunOS) the results may be slightly
10176 out of date. This is the default.
10179 @itemx --portability
10181 @opindex --portability
10182 @cindex one-line output format
10183 @cindex @acronym{POSIX} output format
10184 @cindex portable output format
10185 @cindex output format, portable
10186 Use the @acronym{POSIX} output format. This is like the default format except
10191 The information about each file system is always printed on exactly
10192 one line; a mount device is never put on a line by itself. This means
10193 that if the mount device name is more than 20 characters long (e.g., for
10194 some network mounts), the columns are misaligned.
10197 The labels in the header output line are changed to conform to @acronym{POSIX}.
10200 The default block size and output format are unaffected by the
10201 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10202 variables. However, the default block size is still affected by
10203 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10204 otherwise. @xref{Block size}.
10211 @cindex file system space, retrieving current data more slowly
10212 Invoke the @code{sync} system call before getting any usage data. On
10213 some systems (notably SunOS), doing this yields more up to date results,
10214 but in general this option makes @command{df} much slower, especially when
10215 there are many or very busy file systems.
10217 @item -t @var{fstype}
10218 @itemx --type=@var{fstype}
10221 @cindex file system types, limiting output to certain
10222 Limit the listing to file systems of type @var{fstype}. Multiple
10223 file system types can be specified by giving multiple @option{-t} options.
10224 By default, nothing is omitted.
10227 @itemx --print-type
10229 @opindex --print-type
10230 @cindex file system types, printing
10231 Print each file system's type. The types printed here are the same ones
10232 you can include or exclude with @option{-t} and @option{-x}. The particular
10233 types printed are whatever is supported by the system. Here are some of
10234 the common names (this list is certainly not exhaustive):
10239 @cindex @acronym{NFS} file system type
10240 An @acronym{NFS} file system, i.e., one mounted over a network from another
10241 machine. This is the one type name which seems to be used uniformly by
10244 @item 4.2@r{, }ufs@r{, }efs@dots{}
10245 @cindex Linux file system types
10246 @cindex local file system types
10247 @opindex 4.2 @r{file system type}
10248 @opindex ufs @r{file system type}
10249 @opindex efs @r{file system type}
10250 A file system on a locally-mounted hard disk. (The system might even
10251 support more than one type here; Linux does.)
10253 @item hsfs@r{, }cdfs
10254 @cindex CD-ROM file system type
10255 @cindex High Sierra file system
10256 @opindex hsfs @r{file system type}
10257 @opindex cdfs @r{file system type}
10258 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10259 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10262 @cindex PC file system
10263 @cindex DOS file system
10264 @cindex MS-DOS file system
10265 @cindex diskette file system
10267 An MS-DOS file system, usually on a diskette.
10271 @item -x @var{fstype}
10272 @itemx --exclude-type=@var{fstype}
10274 @opindex --exclude-type
10275 Limit the listing to file systems not of type @var{fstype}.
10276 Multiple file system types can be eliminated by giving multiple
10277 @option{-x} options. By default, no file system types are omitted.
10280 Ignored; for compatibility with System V versions of @command{df}.
10285 Failure includes the case where no output is generated, so you can
10286 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10287 @var{dir}} to test whether @var{dir} is on a file system of type
10288 @samp{ext3} or @samp{reiserfs}.
10291 @node du invocation
10292 @section @command{du}: Estimate file space usage
10295 @cindex file space usage
10296 @cindex disk usage for files
10298 @command{du} reports the amount of disk space used by the specified files
10299 and for each subdirectory (of directory arguments). Synopsis:
10302 du [@var{option}]@dots{} [@var{file}]@dots{}
10305 With no arguments, @command{du} reports the disk space for the current
10306 directory. Normally the disk space is printed in units of
10307 1024 bytes, but this can be overridden (@pxref{Block size}).
10308 Non-integer quantities are rounded up to the next higher unit.
10310 If two or more hard links point to the same file, only one of the hard
10311 links is counted. The @var{file} argument order affects which links
10312 are counted, and changing the argument order may change the numbers
10313 that @command{du} outputs.
10315 The program accepts the following options. Also see @ref{Common options}.
10323 Show counts for all files, not just directories.
10325 @itemx --apparent-size
10326 @opindex --apparent-size
10327 Print apparent sizes, rather than disk usage. The apparent size of a
10328 file is the number of bytes reported by @code{wc -c} on regular files,
10329 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10330 For example, a file containing the word @samp{zoo} with no newline would,
10331 of course, have an apparent size of 3. Such a small file may require
10332 anywhere from 0 to 16 KiB or more of disk space, depending on
10333 the type and configuration of the file system on which the file resides.
10334 However, a sparse file created with this command:
10337 dd bs=1 seek=2GiB if=/dev/null of=big
10341 has an apparent size of 2 GiB, yet on most modern
10342 systems, it actually uses almost no disk space.
10348 Equivalent to @code{--apparent-size --block-size=1}.
10350 @item -B @var{size}
10351 @itemx --block-size=@var{size}
10353 @opindex --block-size
10355 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10356 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10362 @cindex grand total of disk space
10363 Print a grand total of all arguments after all arguments have
10364 been processed. This can be used to find out the total disk usage of
10365 a given set of files or directories.
10368 @itemx --dereference-args
10370 @opindex --dereference-args
10371 Dereference symbolic links that are command line arguments.
10372 Does not affect other symbolic links. This is helpful for finding
10373 out the disk usage of directories, such as @file{/usr/tmp}, which
10374 are often symbolic links.
10376 @c --files0-from=FILE
10377 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10383 Equivalent to @option{--dereference-args} (@option{-D}).
10387 @cindex kibibytes for file sizes
10388 Print sizes in 1024-byte blocks, overriding the default block size
10389 (@pxref{Block size}).
10390 This option is equivalent to @option{--block-size=1K}.
10393 @itemx --count-links
10395 @opindex --count-links
10396 @cindex hard links, counting in @command{du}
10397 Count the size of all files, even if they have appeared already (as a
10401 @itemx --dereference
10403 @opindex --dereference
10404 @cindex symbolic links, dereferencing in @command{du}
10405 Dereference symbolic links (show the disk space used by the file
10406 or directory that the link points to instead of the space used by
10411 @cindex mebibytes for file sizes
10412 Print sizes in 1,048,576-byte blocks, overriding the default block size
10413 (@pxref{Block size}).
10414 This option is equivalent to @option{--block-size=1M}.
10417 @itemx --no-dereference
10419 @opindex --no-dereference
10420 @cindex symbolic links, dereferencing in @command{du}
10421 For each symbolic links encountered by @command{du},
10422 consider the disk space used by the symbolic link.
10424 @item --max-depth=@var{depth}
10425 @opindex --max-depth=@var{depth}
10426 @cindex limiting output of @command{du}
10427 Show the total for each directory (and file if --all) that is at
10428 most MAX_DEPTH levels down from the root of the hierarchy. The root
10429 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10438 @opindex --summarize
10439 Display only a total for each argument.
10442 @itemx --separate-dirs
10444 @opindex --separate-dirs
10445 Normally, in the output of @command{du} (when not using @option{--summarize}),
10446 the size listed next to a directory name, @var{d}, represents the sum
10447 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10448 With @option{--separate-dirs}, the size reported for a directory name,
10449 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10454 @cindex last modified dates, displaying in @command{du}
10455 Show time of the most recent modification of any file in the directory,
10456 or any of its subdirectories.
10458 @itemx --time=ctime
10459 @itemx --time=status
10462 @opindex ctime@r{, show the most recent}
10463 @opindex status time@r{, show the most recent}
10464 @opindex use time@r{, show the most recent}
10465 Show the most recent status change time (the @samp{ctime} in the inode) of
10466 any file in the directory, instead of the modification time.
10468 @itemx --time=atime
10469 @itemx --time=access
10471 @opindex atime@r{, show the most recent}
10472 @opindex access time@r{, show the most recent}
10473 Show the most recent access time (the @samp{atime} in the inode) of
10474 any file in the directory, instead of the modification time.
10476 @item --time-style=@var{style}
10477 @opindex --time-style
10479 List timestamps in style @var{style}. This option has an effect only if
10480 the @option{--time} option is also specified. The @var{style} should
10481 be one of the following:
10484 @item +@var{format}
10486 List timestamps using @var{format}, where @var{format} is interpreted
10487 like the format argument of @command{date} (@pxref{date invocation}).
10488 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10489 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10490 with @command{date}, @var{format}'s interpretation is affected by the
10491 @env{LC_TIME} locale category.
10494 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10495 format with nanosecond precision, e.g., @samp{2002-03-30
10496 23:45:56.477817180 -0700}. This style is equivalent to
10497 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10500 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10501 @samp{2002-03-30 23:45}. These timestamps are shorter than
10502 @samp{full-iso} timestamps, and are usually good enough for everyday
10503 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10506 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10507 This style is equivalent to @samp{+%Y-%m-%d}.
10511 You can specify the default value of the @option{--time-style} option
10512 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10513 the default style is @samp{long-iso}. For compatibility with @command{ls},
10514 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10515 the newline and any later characters are ignored; if @env{TIME_STYLE}
10516 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10517 @env{TIME_STYLE} is @samp{locale} it is ignored.
10520 @itemx --one-file-system
10522 @opindex --one-file-system
10523 @cindex one file system, restricting @command{du} to
10524 Skip directories that are on different file systems from the one that
10525 the argument being processed is on.
10527 @item --exclude=@var{pattern}
10528 @opindex --exclude=@var{pattern}
10529 @cindex excluding files from @command{du}
10530 When recursing, skip subdirectories or files matching @var{pattern}.
10531 For example, @code{du --exclude='*.o'} excludes files whose names
10534 @item -X @var{file}
10535 @itemx --exclude-from=@var{file}
10536 @opindex -X @var{file}
10537 @opindex --exclude-from=@var{file}
10538 @cindex excluding files from @command{du}
10539 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10540 one per line. If @var{file} is @samp{-}, take the patterns from standard
10545 @cindex NFS mounts from BSD to HP-UX
10546 On BSD systems, @command{du} reports sizes that are half the correct
10547 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10548 systems, it reports sizes that are twice the correct values for
10549 files that are NFS-mounted from BSD systems. This is due to a flaw
10550 in HP-UX; it also affects the HP-UX @command{du} program.
10555 @node stat invocation
10556 @section @command{stat}: Report file or file system status
10559 @cindex file status
10560 @cindex file system status
10562 @command{stat} displays information about the specified file(s). Synopsis:
10565 stat [@var{option}]@dots{} [@var{file}]@dots{}
10568 With no option, @command{stat} reports all information about the given files.
10569 But it also can be used to report the information of the file systems the
10570 given files are located on. If the files are links, @command{stat} can
10571 also give information about the files the links point to.
10573 @mayConflictWithShellBuiltIn{stat}
10578 @itemx --dereference
10580 @opindex --dereference
10581 @cindex symbolic links, dereferencing in @command{stat}
10582 Change how @command{stat} treats symbolic links.
10583 With this option, @command{stat} acts on the file referenced
10584 by each symbolic link argument.
10585 Without it, @command{stat} acts on any symbolic link argument directly.
10588 @itemx --file-system
10590 @opindex --file-system
10591 @cindex file systems
10592 Report information about the file systems where the given files are located
10593 instead of information about the files themselves.
10596 @itemx --format=@var{format}
10598 @opindex --format=@var{format}
10599 @cindex output format
10600 Use @var{format} rather than the default format.
10601 @var{format} is automatically newline-terminated, so
10602 running a command like the following with two or more @var{file}
10603 operands produces a line of output for each operand:
10605 $ stat --format=%d:%i / /usr
10610 @itemx --printf=@var{format}
10611 @opindex --printf=@var{format}
10612 @cindex output format
10613 Use @var{format} rather than the default format.
10614 Like @option{--format}, but interpret backslash escapes,
10615 and do not output a mandatory trailing newline.
10616 If you want a newline, include @samp{\n} in the @var{format}.
10617 Here's how you would use @option{--printf} to print the device
10618 and inode numbers of @file{/} and @file{/usr}:
10620 $ stat --printf='%d:%i\n' / /usr
10629 @cindex terse output
10630 Print the information in terse form, suitable for parsing by other programs.
10634 The valid @var{format} directives for files with @option{--format} and
10635 @option{--printf} are:
10638 @item %a - Access rights in octal
10639 @item %A - Access rights in human readable form
10640 @item %b - Number of blocks allocated (see @samp{%B})
10641 @item %B - The size in bytes of each block reported by @samp{%b}
10642 @item %d - Device number in decimal
10643 @item %D - Device number in hex
10644 @item %f - Raw mode in hex
10645 @item %F - File type
10646 @item %g - Group ID of owner
10647 @item %G - Group name of owner
10648 @item %h - Number of hard links
10649 @item %i - Inode number
10650 @item %n - File name
10651 @item %N - Quoted file name with dereference if symbolic link
10652 @item %o - I/O block size
10653 @item %s - Total size, in bytes
10654 @item %t - Major device type in hex
10655 @item %T - Minor device type in hex
10656 @item %u - User ID of owner
10657 @item %U - User name of owner
10658 @item %x - Time of last access
10659 @item %X - Time of last access as seconds since Epoch
10660 @item %y - Time of last modification
10661 @item %Y - Time of last modification as seconds since Epoch
10662 @item %z - Time of last change
10663 @item %Z - Time of last change as seconds since Epoch
10666 When listing file system information (@option{--file-system} (@option{-f})),
10667 you must use a different set of @var{format} directives:
10670 @item %a - Free blocks available to non-super-user
10671 @item %b - Total data blocks in file system
10672 @item %c - Total file nodes in file system
10673 @item %d - Free file nodes in file system
10674 @item %f - Free blocks in file system
10675 @item %i - File System ID in hex
10676 @item %l - Maximum length of file names
10677 @item %n - File name
10678 @item %s - Block size (for faster transfers)
10679 @item %S - Fundamental block size (for block counts)
10680 @item %t - Type in hex
10681 @item %T - Type in human readable form
10685 Time stamps are listed according to the time zone rules specified by
10686 the @env{TZ} environment variable, or by the system default rules if
10687 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10688 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10693 @node sync invocation
10694 @section @command{sync}: Synchronize data on disk with memory
10697 @cindex synchronize disk and memory
10699 @cindex superblock, writing
10700 @cindex inodes, written buffered
10701 @command{sync} writes any data buffered in memory out to disk. This can
10702 include (but is not limited to) modified superblocks, modified inodes,
10703 and delayed reads and writes. This must be implemented by the kernel;
10704 The @command{sync} program does nothing but exercise the @code{sync} system
10707 @cindex crashes and corruption
10708 The kernel keeps data in memory to avoid doing (relatively slow) disk
10709 reads and writes. This improves performance, but if the computer
10710 crashes, data may be lost or the file system corrupted as a
10711 result. The @command{sync} command ensures everything in memory
10712 is written to disk.
10714 Any arguments are ignored, except for a lone @option{--help} or
10715 @option{--version} (@pxref{Common options}).
10720 @node truncate invocation
10721 @section @command{truncate}: Shrink or extend the size of a file
10724 @cindex truncating, file sizes
10726 @command{truncate} shrinks or extends the size of each @var{file} to the
10727 specified size. Synopsis:
10730 truncate @var{option}@dots{} @var{file}@dots{}
10733 @cindex files, creating
10734 Any @var{file} that does not exist is created.
10736 @cindex sparse files, creating
10737 @cindex holes, creating files with
10738 If a @var{file} is larger than the specified size, the extra data is lost.
10739 If a @var{file} is shorter, it is extended and the extended part (or hole)
10740 reads as zero bytes.
10742 The program accepts the following options. Also see @ref{Common options}.
10749 @opindex --no-create
10750 Do not create files that do not exist.
10755 @opindex --io-blocks
10756 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10758 @item -r @var{rfile}
10759 @itemx --reference=@var{rfile}
10761 @opindex --reference
10762 Set the size of each @var{file} to the same size as @var{rfile}.
10764 @item -s @var{size}
10765 @itemx --size=@var{size}
10768 Set the size of each @var{file} to this @var{size}.
10769 @multiplierSuffixesNoBlocks{size}
10771 @var{size} may also be prefixed by one of the following to adjust
10772 the size of each @var{file} based on their current size:
10774 @samp{+} => extend by
10775 @samp{-} => reduce by
10776 @samp{<} => at most
10777 @samp{>} => at least
10778 @samp{/} => round down to multiple of
10779 @samp{%} => round up to multiple of
10787 @node Printing text
10788 @chapter Printing text
10790 @cindex printing text, commands for
10791 @cindex commands for printing text
10793 This section describes commands that display text strings.
10796 * echo invocation:: Print a line of text.
10797 * printf invocation:: Format and print data.
10798 * yes invocation:: Print a string until interrupted.
10802 @node echo invocation
10803 @section @command{echo}: Print a line of text
10806 @cindex displaying text
10807 @cindex printing text
10808 @cindex text, displaying
10809 @cindex arbitrary text, displaying
10811 @command{echo} writes each given @var{string} to standard output, with a
10812 space between each and a newline after the last one. Synopsis:
10815 echo [@var{option}]@dots{} [@var{string}]@dots{}
10818 @mayConflictWithShellBuiltIn{echo}
10820 The program accepts the following options. Also see @ref{Common options}.
10821 Options must precede operands, and the normally-special argument
10822 @samp{--} has no special meaning and is treated like any other
10828 Do not output the trailing newline.
10832 @cindex backslash escapes
10833 Enable interpretation of the following backslash-escaped characters in
10842 produce no further output
10858 the eight-bit value that is the octal number @var{nnn}
10859 (zero to three octal digits)
10861 the eight-bit value that is the octal number @var{nnn}
10862 (one to three octal digits)
10864 the eight-bit value that is the hexadecimal number @var{hh}
10865 (one or two hexadecimal digits)
10870 @cindex backslash escapes
10871 Disable interpretation of backslash escapes in each @var{string}.
10872 This is the default. If @option{-e} and @option{-E} are both
10873 specified, the last one given takes effect.
10877 @vindex POSIXLY_CORRECT
10878 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10879 @command{echo}'s first argument is not @option{-n} it outputs
10880 option-like arguments instead of treating them as options. For
10881 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10882 plain @samp{hello}.
10884 @acronym{POSIX} does not require support for any options, and says
10885 that the behavior of @command{echo} is implementation-defined if any
10886 @var{string} contains a backslash or if the first argument is
10887 @option{-n}. Portable programs can use the @command{printf} command
10888 if they need to omit trailing newlines or output control characters or
10889 backslashes. @xref{printf invocation}.
10894 @node printf invocation
10895 @section @command{printf}: Format and print data
10898 @command{printf} does formatted printing of text. Synopsis:
10901 printf @var{format} [@var{argument}]@dots{}
10904 @command{printf} prints the @var{format} string, interpreting @samp{%}
10905 directives and @samp{\} escapes to format numeric and string arguments
10906 in a way that is mostly similar to the C @samp{printf} function.
10907 @xref{Output Conversion Syntax,, @command{printf} format directives,
10908 libc, The GNU C Library Reference Manual}, for details.
10909 The differences are listed below.
10911 @mayConflictWithShellBuiltIn{printf}
10916 The @var{format} argument is reused as necessary to convert all the
10917 given @var{argument}s. For example, the command @samp{printf %s a b}
10921 Missing @var{argument}s are treated as null strings or as zeros,
10922 depending on whether the context expects a string or a number. For
10923 example, the command @samp{printf %sx%d} prints @samp{x0}.
10927 An additional escape, @samp{\c}, causes @command{printf} to produce no
10928 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
10929 E} prints @samp{ABC}.
10932 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
10933 digits, as opposed to C where it can have an unlimited number of
10934 digits. For example, the command @samp{printf '\x07e'} prints two
10935 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
10940 @command{printf} has an additional directive, @samp{%b}, which prints its
10941 argument string with @samp{\} escapes interpreted in the same way as in
10942 the @var{format} string, except that octal escapes are of the form
10943 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
10944 If a precision is also given, it limits the number of bytes printed
10945 from the converted string.
10948 Numeric arguments must be single C constants, possibly with leading
10949 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
10953 @vindex POSIXLY_CORRECT
10954 If the leading character of a numeric argument is @samp{"} or @samp{'}
10955 then its value is the numeric value of the immediately following
10956 character. Any remaining characters are silently ignored if the
10957 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
10958 warning is printed. For example, @samp{printf "%d" "'a"} outputs
10959 @samp{97} on hosts that use the @acronym{ASCII} character set, since
10960 @samp{a} has the numeric value 97 in @acronym{ASCII}.
10965 A floating-point argument must use a period before any fractional
10966 digits, but is printed according to the @env{LC_NUMERIC} category of the
10967 current locale. For example, in a locale whose radix character is a
10968 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
10969 the command @samp{printf %g 3,14} is an error.
10973 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
10974 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
10975 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
10976 digits) specifying a character to print.
10981 @cindex ISO/IEC 10646
10983 @command{printf} interprets two character syntaxes introduced in
10984 @acronym{ISO} C 99:
10985 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
10986 characters, specified as
10987 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
10988 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
10989 @command{printf} outputs the Unicode characters
10990 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
10991 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
10992 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
10994 The processing of @samp{\u} and @samp{\U} requires a full-featured
10995 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
10996 or when @code{libiconv} is installed prior to this package. Otherwise
10997 @samp{\u} and @samp{\U} will print as-is.
10999 The only options are a lone @option{--help} or
11000 @option{--version}. @xref{Common options}.
11001 Options must precede operands.
11003 The Unicode character syntaxes are useful for writing strings in a locale
11004 independent way. For example, a string containing the Euro currency symbol
11007 $ env printf '\u20AC 14.95'
11011 will be output correctly in all locales supporting the Euro symbol
11012 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11015 $ env printf '\u4e2d\u6587'
11019 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11021 Note that in these examples, the @command{printf} command has been
11022 invoked via @command{env} to ensure that we run the program found via
11023 your shell's search path, and not a shell alias or a built-in function.
11025 For larger strings, you don't need to look up the hexadecimal code
11026 values of each character one by one. @acronym{ASCII} characters mixed with \u
11027 escape sequences is also known as the JAVA source file encoding. You can
11028 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11029 is how to convert a piece of text into a shell script which will output
11030 this text in a locale-independent way:
11033 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11034 '\u4e2d\u6587\n' > sample.txt
11035 $ recode BIG5..JAVA < sample.txt \
11036 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11043 @node yes invocation
11044 @section @command{yes}: Print a string until interrupted
11047 @cindex repeated output of a string
11049 @command{yes} prints the command line arguments, separated by spaces and
11050 followed by a newline, forever until it is killed. If no arguments are
11051 given, it prints @samp{y} followed by a newline forever until killed.
11053 Upon a write error, @command{yes} exits with status @samp{1}.
11055 The only options are a lone @option{--help} or @option{--version}.
11056 To output an argument that begins with
11057 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11058 @xref{Common options}.
11062 @chapter Conditions
11065 @cindex commands for exit status
11066 @cindex exit status commands
11068 This section describes commands that are primarily useful for their exit
11069 status, rather than their output. Thus, they are often used as the
11070 condition of shell @code{if} statements, or as the last command in a
11074 * false invocation:: Do nothing, unsuccessfully.
11075 * true invocation:: Do nothing, successfully.
11076 * test invocation:: Check file types and compare values.
11077 * expr invocation:: Evaluate expressions.
11081 @node false invocation
11082 @section @command{false}: Do nothing, unsuccessfully
11085 @cindex do nothing, unsuccessfully
11086 @cindex failure exit status
11087 @cindex exit status of @command{false}
11089 @command{false} does nothing except return an exit status of 1, meaning
11090 @dfn{failure}. It can be used as a place holder in shell scripts
11091 where an unsuccessful command is needed.
11092 In most modern shells, @command{false} is a built-in command, so when
11093 you use @samp{false} in a script, you're probably using the built-in
11094 command, not the one documented here.
11096 @command{false} honors the @option{--help} and @option{--version} options.
11098 This version of @command{false} is implemented as a C program, and is thus
11099 more secure and faster than a shell script implementation, and may safely
11100 be used as a dummy shell for the purpose of disabling accounts.
11102 Note that @command{false} (unlike all other programs documented herein)
11103 exits unsuccessfully, even when invoked with
11104 @option{--help} or @option{--version}.
11106 Portable programs should not assume that the exit status of
11107 @command{false} is 1, as it is greater than 1 on some
11108 non-@acronym{GNU} hosts.
11111 @node true invocation
11112 @section @command{true}: Do nothing, successfully
11115 @cindex do nothing, successfully
11117 @cindex successful exit
11118 @cindex exit status of @command{true}
11120 @command{true} does nothing except return an exit status of 0, meaning
11121 @dfn{success}. It can be used as a place holder in shell scripts
11122 where a successful command is needed, although the shell built-in
11123 command @code{:} (colon) may do the same thing faster.
11124 In most modern shells, @command{true} is a built-in command, so when
11125 you use @samp{true} in a script, you're probably using the built-in
11126 command, not the one documented here.
11128 @command{true} honors the @option{--help} and @option{--version} options.
11130 Note, however, that it is possible to cause @command{true}
11131 to exit with nonzero status: with the @option{--help} or @option{--version}
11132 option, and with standard
11133 output already closed or redirected to a file that evokes an I/O error.
11134 For example, using a Bourne-compatible shell:
11137 $ ./true --version >&-
11138 ./true: write error: Bad file number
11139 $ ./true --version > /dev/full
11140 ./true: write error: No space left on device
11143 This version of @command{true} is implemented as a C program, and is thus
11144 more secure and faster than a shell script implementation, and may safely
11145 be used as a dummy shell for the purpose of disabling accounts.
11147 @node test invocation
11148 @section @command{test}: Check file types and compare values
11151 @cindex check file types
11152 @cindex compare values
11153 @cindex expression evaluation
11155 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11156 evaluation of the conditional expression @var{expr}. Each part of the
11157 expression must be a separate argument.
11159 @command{test} has file status checks, string operators, and numeric
11160 comparison operators.
11162 @command{test} has an alternate form that uses opening and closing
11163 square brackets instead a leading @samp{test}. For example, instead
11164 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11165 brackets must be separate arguments; for example, @samp{[-d /]} does
11166 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11167 @var{expr} ]} have the same meaning, only the former form is discussed
11173 test @var{expression}
11175 [ @var{expression} ]
11180 @mayConflictWithShellBuiltIn{test}
11182 If @var{expression} is omitted, @command{test} returns false.
11183 If @var{expression} is a single argument,
11184 @command{test} returns false if the argument is null and true otherwise. The argument
11185 can be any string, including strings like @samp{-d}, @samp{-1},
11186 @samp{--}, @samp{--help}, and @samp{--version} that most other
11187 programs would treat as options. To get help and version information,
11188 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11189 the usual closing brackets. @xref{Common options}.
11191 @cindex exit status of @command{test}
11195 0 if the expression is true,
11196 1 if the expression is false,
11197 2 if an error occurred.
11201 * File type tests:: -[bcdfhLpSt]
11202 * Access permission tests:: -[gkruwxOG]
11203 * File characteristic tests:: -e -s -nt -ot -ef
11204 * String tests:: -z -n = !=
11205 * Numeric tests:: -eq -ne -lt -le -gt -ge
11206 * Connectives for test:: ! -a -o
11210 @node File type tests
11211 @subsection File type tests
11213 @cindex file type tests
11215 These options test for particular types of files. (Everything's a file,
11216 but not all files are the same!)
11220 @item -b @var{file}
11222 @cindex block special check
11223 True if @var{file} exists and is a block special device.
11225 @item -c @var{file}
11227 @cindex character special check
11228 True if @var{file} exists and is a character special device.
11230 @item -d @var{file}
11232 @cindex directory check
11233 True if @var{file} exists and is a directory.
11235 @item -f @var{file}
11237 @cindex regular file check
11238 True if @var{file} exists and is a regular file.
11240 @item -h @var{file}
11241 @itemx -L @var{file}
11244 @cindex symbolic link check
11245 True if @var{file} exists and is a symbolic link.
11246 Unlike all other file-related tests, this test does not dereference
11247 @var{file} if it is a symbolic link.
11249 @item -p @var{file}
11251 @cindex named pipe check
11252 True if @var{file} exists and is a named pipe.
11254 @item -S @var{file}
11256 @cindex socket check
11257 True if @var{file} exists and is a socket.
11261 @cindex terminal check
11262 True if @var{fd} is a file descriptor that is associated with a
11268 @node Access permission tests
11269 @subsection Access permission tests
11271 @cindex access permission tests
11272 @cindex permission tests
11274 These options test for particular access permissions.
11278 @item -g @var{file}
11280 @cindex set-group-ID check
11281 True if @var{file} exists and has its set-group-ID bit set.
11283 @item -k @var{file}
11285 @cindex sticky bit check
11286 True if @var{file} exists and has its @dfn{sticky} bit set.
11288 @item -r @var{file}
11290 @cindex readable file check
11291 True if @var{file} exists and read permission is granted.
11293 @item -u @var{file}
11295 @cindex set-user-ID check
11296 True if @var{file} exists and has its set-user-ID bit set.
11298 @item -w @var{file}
11300 @cindex writable file check
11301 True if @var{file} exists and write permission is granted.
11303 @item -x @var{file}
11305 @cindex executable file check
11306 True if @var{file} exists and execute permission is granted
11307 (or search permission, if it is a directory).
11309 @item -O @var{file}
11311 @cindex owned by effective user ID check
11312 True if @var{file} exists and is owned by the current effective user ID.
11314 @item -G @var{file}
11316 @cindex owned by effective group ID check
11317 True if @var{file} exists and is owned by the current effective group ID.
11321 @node File characteristic tests
11322 @subsection File characteristic tests
11324 @cindex file characteristic tests
11326 These options test other file characteristics.
11330 @item -e @var{file}
11332 @cindex existence-of-file check
11333 True if @var{file} exists.
11335 @item -s @var{file}
11337 @cindex nonempty file check
11338 True if @var{file} exists and has a size greater than zero.
11340 @item @var{file1} -nt @var{file2}
11342 @cindex newer-than file check
11343 True if @var{file1} is newer (according to modification date) than
11344 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11346 @item @var{file1} -ot @var{file2}
11348 @cindex older-than file check
11349 True if @var{file1} is older (according to modification date) than
11350 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11352 @item @var{file1} -ef @var{file2}
11354 @cindex same file check
11355 @cindex hard link check
11356 True if @var{file1} and @var{file2} have the same device and inode
11357 numbers, i.e., if they are hard links to each other.
11363 @subsection String tests
11365 @cindex string tests
11367 These options test string characteristics. You may need to quote
11368 @var{string} arguments for the shell. For example:
11374 The quotes here prevent the wrong arguments from being passed to
11375 @command{test} if @samp{$V} is empty or contains special characters.
11379 @item -z @var{string}
11381 @cindex zero-length string check
11382 True if the length of @var{string} is zero.
11384 @item -n @var{string}
11385 @itemx @var{string}
11387 @cindex nonzero-length string check
11388 True if the length of @var{string} is nonzero.
11390 @item @var{string1} = @var{string2}
11392 @cindex equal string check
11393 True if the strings are equal.
11395 @item @var{string1} != @var{string2}
11397 @cindex not-equal string check
11398 True if the strings are not equal.
11403 @node Numeric tests
11404 @subsection Numeric tests
11406 @cindex numeric tests
11407 @cindex arithmetic tests
11409 Numeric relational operators. The arguments must be entirely numeric
11410 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11411 which evaluates to the length of @var{string}.
11415 @item @var{arg1} -eq @var{arg2}
11416 @itemx @var{arg1} -ne @var{arg2}
11417 @itemx @var{arg1} -lt @var{arg2}
11418 @itemx @var{arg1} -le @var{arg2}
11419 @itemx @var{arg1} -gt @var{arg2}
11420 @itemx @var{arg1} -ge @var{arg2}
11427 These arithmetic binary operators return true if @var{arg1} is equal,
11428 not-equal, less-than, less-than-or-equal, greater-than, or
11429 greater-than-or-equal than @var{arg2}, respectively.
11436 test -1 -gt -2 && echo yes
11438 test -l abc -gt 1 && echo yes
11441 @error{} test: integer expression expected before -eq
11445 @node Connectives for test
11446 @subsection Connectives for @command{test}
11448 @cindex logical connectives
11449 @cindex connectives, logical
11451 The usual logical connectives.
11457 True if @var{expr} is false.
11459 @item @var{expr1} -a @var{expr2}
11461 @cindex logical and operator
11462 @cindex and operator
11463 True if both @var{expr1} and @var{expr2} are true.
11465 @item @var{expr1} -o @var{expr2}
11467 @cindex logical or operator
11468 @cindex or operator
11469 True if either @var{expr1} or @var{expr2} is true.
11474 @node expr invocation
11475 @section @command{expr}: Evaluate expressions
11478 @cindex expression evaluation
11479 @cindex evaluation of expressions
11481 @command{expr} evaluates an expression and writes the result on standard
11482 output. Each token of the expression must be a separate argument.
11484 Operands are either integers or strings. Integers consist of one or
11485 more decimal digits, with an optional leading @samp{-}.
11486 @command{expr} converts
11487 anything appearing in an operand position to an integer or a string
11488 depending on the operation being applied to it.
11490 Strings are not quoted for @command{expr} itself, though you may need to
11491 quote them to protect characters with special meaning to the shell,
11492 e.g., spaces. However, regardless of whether it is quoted, a string
11493 operand should not be a parenthesis or any of @command{expr}'s
11494 operators like @code{+}, so you cannot safely pass an arbitrary string
11495 @code{$str} to expr merely by quoting it to the shell. One way to
11496 work around this is to use the @sc{gnu} extension @code{+},
11497 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11498 @code{@w{" $str"}} and to adjust the rest of the expression to take
11499 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11501 You should not pass a negative integer or a string with leading
11502 @samp{-} as @command{expr}'s first argument, as it might be
11503 misinterpreted as an option; this can be avoided by parenthesization.
11504 Also, portable scripts should not use a string operand that happens to
11505 take the form of an integer; this can be worked around by inserting
11506 leading spaces as mentioned above.
11508 @cindex parentheses for grouping
11509 Operators may be given as infix symbols or prefix keywords. Parentheses
11510 may be used for grouping in the usual manner. You must quote
11511 parentheses and many operators to avoid the shell evaluating them,
11514 When built with support for the GNU MP library, @command{expr} uses
11515 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11516 types and may fail due to arithmetic overflow.
11518 The only options are @option{--help} and @option{--version}. @xref{Common
11519 options}. Options must precede operands.
11521 @cindex exit status of @command{expr}
11525 0 if the expression is neither null nor 0,
11526 1 if the expression is null or 0,
11527 2 if the expression is invalid,
11528 3 if an internal error occurred (e.g., arithmetic overflow).
11532 * String expressions:: + : match substr index length
11533 * Numeric expressions:: + - * / %
11534 * Relations for expr:: | & < <= = == != >= >
11535 * Examples of expr:: Examples.
11539 @node String expressions
11540 @subsection String expressions
11542 @cindex string expressions
11543 @cindex expressions, string
11545 @command{expr} supports pattern matching and other string operators. These
11546 have higher precedence than both the numeric and relational operators (in
11547 the next sections).
11551 @item @var{string} : @var{regex}
11552 @cindex pattern matching
11553 @cindex regular expression matching
11554 @cindex matching patterns
11555 Perform pattern matching. The arguments are converted to strings and the
11556 second is considered to be a (basic, a la GNU @code{grep}) regular
11557 expression, with a @code{^} implicitly prepended. The first argument is
11558 then matched against this regular expression.
11560 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11561 @code{:} expression returns the part of @var{string} that matched the
11562 subexpression; otherwise, it returns the number of characters matched.
11564 If the match fails, the @code{:} operator returns the null string if
11565 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11567 @kindex \( @r{regexp operator}
11568 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11569 value; additional pairs are meaningful only for grouping the regular
11570 expression operators.
11572 @kindex \+ @r{regexp operator}
11573 @kindex \? @r{regexp operator}
11574 @kindex \| @r{regexp operator}
11575 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11576 operators which respectively match one or more, zero or one, or separate
11577 alternatives. SunOS and other @command{expr}'s treat these as regular
11578 characters. (@acronym{POSIX} allows either behavior.)
11579 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11580 regular expression syntax. Some examples are in @ref{Examples of expr}.
11582 @item match @var{string} @var{regex}
11584 An alternative way to do pattern matching. This is the same as
11585 @w{@samp{@var{string} : @var{regex}}}.
11587 @item substr @var{string} @var{position} @var{length}
11589 Returns the substring of @var{string} beginning at @var{position}
11590 with length at most @var{length}. If either @var{position} or
11591 @var{length} is negative, zero, or non-numeric, returns the null string.
11593 @item index @var{string} @var{charset}
11595 Returns the first position in @var{string} where the first character in
11596 @var{charset} was found. If no character in @var{charset} is found in
11597 @var{string}, return 0.
11599 @item length @var{string}
11601 Returns the length of @var{string}.
11603 @item + @var{token}
11605 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11606 or an operator like @code{/}.
11607 This makes it possible to test @code{expr length + "$x"} or
11608 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11609 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11610 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11611 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11615 To make @command{expr} interpret keywords as strings, you must use the
11616 @code{quote} operator.
11619 @node Numeric expressions
11620 @subsection Numeric expressions
11622 @cindex numeric expressions
11623 @cindex expressions, numeric
11625 @command{expr} supports the usual numeric operators, in order of increasing
11626 precedence. These numeric operators have lower precedence than the
11627 string operators described in the previous section, and higher precedence
11628 than the connectives (next section).
11636 @cindex subtraction
11637 Addition and subtraction. Both arguments are converted to integers;
11638 an error occurs if this cannot be done.
11644 @cindex multiplication
11647 Multiplication, division, remainder. Both arguments are converted to
11648 integers; an error occurs if this cannot be done.
11653 @node Relations for expr
11654 @subsection Relations for @command{expr}
11656 @cindex connectives, logical
11657 @cindex logical connectives
11658 @cindex relations, numeric or string
11660 @command{expr} supports the usual logical connectives and relations. These
11661 have lower precedence than the string and numeric operators
11662 (previous sections). Here is the list, lowest-precedence operator first.
11668 @cindex logical or operator
11669 @cindex or operator
11670 Returns its first argument if that is neither null nor zero, otherwise
11671 its second argument if it is neither null nor zero, otherwise 0. It
11672 does not evaluate its second argument if its first argument is neither
11677 @cindex logical and operator
11678 @cindex and operator
11679 Return its first argument if neither argument is null or zero, otherwise
11680 0. It does not evaluate its second argument if its first argument is
11683 @item < <= = == != >= >
11690 @cindex comparison operators
11692 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11693 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11694 both arguments to integers and do a numeric comparison; if either
11695 conversion fails, it does a lexicographic comparison using the character
11696 collating sequence specified by the @env{LC_COLLATE} locale.
11701 @node Examples of expr
11702 @subsection Examples of using @command{expr}
11704 @cindex examples of @command{expr}
11705 Here are a few examples, including quoting for shell metacharacters.
11707 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11710 foo=`expr $foo + 1`
11713 To print the non-directory part of the file name stored in
11714 @code{$fname}, which need not contain a @code{/}:
11717 expr $fname : '.*/\(.*\)' '|' $fname
11720 An example showing that @code{\+} is an operator:
11728 expr abc : 'a\(.\)c'
11730 expr index abcdef cz
11733 @error{} expr: syntax error
11734 expr index + index a
11740 @chapter Redirection
11742 @cindex redirection
11743 @cindex commands for redirection
11745 Unix shells commonly provide several forms of @dfn{redirection}---ways
11746 to change the input source or output destination of a command. But one
11747 useful redirection is performed by a separate command, not by the shell;
11748 it's described here.
11751 * tee invocation:: Redirect output to multiple files or processes.
11755 @node tee invocation
11756 @section @command{tee}: Redirect output to multiple files or processes
11759 @cindex pipe fitting
11760 @cindex destinations, multiple output
11761 @cindex read from stdin and write to stdout and files
11763 The @command{tee} command copies standard input to standard output and also
11764 to any files given as arguments. This is useful when you want not only
11765 to send some data down a pipe, but also to save a copy. Synopsis:
11768 tee [@var{option}]@dots{} [@var{file}]@dots{}
11771 If a file being written to does not already exist, it is created. If a
11772 file being written to already exists, the data it previously contained
11773 is overwritten unless the @option{-a} option is used.
11775 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11776 input to standard output, but this is typically not that useful as the
11777 copies are interleaved.
11779 The program accepts the following options. Also see @ref{Common options}.
11786 Append standard input to the given files rather than overwriting
11790 @itemx --ignore-interrupts
11792 @opindex --ignore-interrupts
11793 Ignore interrupt signals.
11797 The @command{tee} command is useful when you happen to be transferring a large
11798 amount of data and also want to summarize that data without reading
11799 it a second time. For example, when you are downloading a DVD image,
11800 you often want to verify its signature or checksum right away.
11801 The inefficient way to do it is simply:
11804 wget http://example.com/some.iso && sha1sum some.iso
11807 One problem with the above is that it makes you wait for the
11808 download to complete before starting the time-consuming SHA1 computation.
11809 Perhaps even more importantly, the above requires reading
11810 the DVD image a second time (the first was from the network).
11812 The efficient way to do it is to interleave the download
11813 and SHA1 computation. Then, you'll get the checksum for
11814 free, because the entire process parallelizes so well:
11817 # slightly contrived, to demonstrate process substitution
11818 wget -O - http://example.com/dvd.iso \
11819 | tee >(sha1sum > dvd.sha1) > dvd.iso
11822 That makes @command{tee} write not just to the expected output file,
11823 but also to a pipe running @command{sha1sum} and saving the final
11824 checksum in a file named @file{dvd.sha1}.
11826 Note, however, that this example relies on a feature of modern shells
11827 called @dfn{process substitution}
11828 (the @samp{>(command)} syntax, above;
11829 @xref{Process Substitution,,Process Substitution, bashref,
11830 The Bash Reference Manual}.),
11831 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11832 but not with @command{/bin/sh}. So if you write code like this
11833 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11835 Since the above example writes to one file and one process,
11836 a more conventional and portable use of @command{tee} is even better:
11839 wget -O - http://example.com/dvd.iso \
11840 | tee dvd.iso | sha1sum > dvd.sha1
11843 You can extend this example to make @command{tee} write to two processes,
11844 computing MD5 and SHA1 checksums in parallel. In this case,
11845 process substitution is required:
11848 wget -O - http://example.com/dvd.iso \
11849 | tee >(sha1sum > dvd.sha1) \
11850 >(md5sum > dvd.md5) \
11854 This technique is also useful when you want to make a @emph{compressed}
11855 copy of the contents of a pipe.
11856 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11857 For a large hierarchy, @samp{du -ak} can run for a long time,
11858 and can easily produce terabytes of data, so you won't want to
11859 rerun the command unnecessarily. Nor will you want to save
11860 the uncompressed output.
11862 Doing it the inefficient way, you can't even start the GUI
11863 until after you've compressed all of the @command{du} output:
11866 du -ak | gzip -9 > /tmp/du.gz
11867 gzip -d /tmp/du.gz | xdiskusage -a
11870 With @command{tee} and process substitution, you start the GUI
11871 right away and eliminate the decompression completely:
11874 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11877 Finally, if you regularly create more than one type of
11878 compressed tarball at once, for example when @code{make dist} creates
11879 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11880 there may be a better way.
11881 Typical @command{automake}-generated @file{Makefile} rules create
11882 the two compressed tar archives with commands in sequence, like this
11883 (slightly simplified):
11886 tardir=your-pkg-M.N
11887 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11888 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11891 However, if the hierarchy you are archiving and compressing is larger
11892 than a couple megabytes, and especially if you are using a multi-processor
11893 system with plenty of memory, then you can do much better by reading the
11894 directory contents only once and running the compression programs in parallel:
11897 tardir=your-pkg-M.N
11898 tar chof - "$tardir" \
11899 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11900 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11906 @node File name manipulation
11907 @chapter File name manipulation
11909 @cindex file name manipulation
11910 @cindex manipulation of file names
11911 @cindex commands for file name manipulation
11913 This section describes commands that manipulate file names.
11916 * basename invocation:: Strip directory and suffix from a file name.
11917 * dirname invocation:: Strip last file name component.
11918 * pathchk invocation:: Check file name validity and portability.
11919 * mktemp invocation:: Create temporary file or directory.
11923 @node basename invocation
11924 @section @command{basename}: Strip directory and suffix from a file name
11927 @cindex strip directory and suffix from file names
11928 @cindex directory, stripping from file names
11929 @cindex suffix, stripping from file names
11930 @cindex file names, stripping directory and suffix
11931 @cindex leading directory components, stripping
11933 @command{basename} removes any leading directory components from
11934 @var{name}. Synopsis:
11937 basename @var{name} [@var{suffix}]
11940 If @var{suffix} is specified and is identical to the end of @var{name},
11941 it is removed from @var{name} as well. Note that since trailing slashes
11942 are removed prior to suffix matching, @var{suffix} will do nothing if it
11943 contains slashes. @command{basename} prints the result on standard
11946 @c This test is used both here and in the section on dirname.
11947 @macro basenameAndDirname
11948 Together, @command{basename} and @command{dirname} are designed such
11949 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
11950 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
11951 for everything except file names containing a trailing newline.
11953 @basenameAndDirname
11955 @acronym{POSIX} allows the implementation to define the results if
11956 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
11957 @command{basename} returns the empty string. In the latter case, the
11958 result is @samp{//} on platforms where @var{//} is distinct from
11959 @var{/}, and @samp{/} on platforms where there is no difference.
11961 The only options are @option{--help} and @option{--version}. @xref{Common
11962 options}. Options must precede operands.
11970 basename /usr/bin/sort
11973 basename include/stdio.h .h
11977 @node dirname invocation
11978 @section @command{dirname}: Strip last file name component
11981 @cindex directory components, printing
11982 @cindex stripping non-directory suffix
11983 @cindex non-directory suffix, stripping
11985 @command{dirname} prints all but the final slash-delimited component of
11986 a string (presumably a file name, but also works on directories). Synopsis:
11992 If @var{name} is a single component, @command{dirname} prints @samp{.}
11993 (meaning the current directory).
11995 @basenameAndDirname
11997 @acronym{POSIX} allows the implementation to define the results if
11998 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
11999 result is @samp{//} on platforms where @var{//} is distinct from
12000 @var{/}, and @samp{/} on platforms where there is no difference.
12002 The only options are @option{--help} and @option{--version}. @xref{Common
12010 # Output "/usr/bin".
12011 dirname /usr/bin/sort
12018 @node pathchk invocation
12019 @section @command{pathchk}: Check file name validity and portability
12022 @cindex file names, checking validity and portability
12023 @cindex valid file names, checking for
12024 @cindex portable file names, checking for
12026 @command{pathchk} checks validity and portability of file names. Synopsis:
12029 pathchk [@var{option}]@dots{} @var{name}@dots{}
12032 For each @var{name}, @command{pathchk} prints an error message if any of
12033 these conditions is true:
12037 One of the existing directories in @var{name} does not have search
12038 (execute) permission,
12040 The length of @var{name} is larger than the maximum supported by the
12043 The length of one component of @var{name} is longer than
12044 its file system's maximum.
12047 A nonexistent @var{name} is not an error, so long a file with that
12048 name could be created under the above conditions.
12050 The program accepts the following options. Also see @ref{Common options}.
12051 Options must precede operands.
12057 Instead of performing checks based on the underlying file system,
12058 print an error message if any of these conditions is true:
12062 A file name is empty.
12065 A file name contains a character outside the @acronym{POSIX} portable file
12066 name character set, namely, the ASCII letters and digits, @samp{.},
12067 @samp{_}, @samp{-}, and @samp{/}.
12070 The length of a file name or one of its components exceeds the
12071 @acronym{POSIX} minimum limits for portability.
12076 Print an error message if a file name is empty, or if it contains a component
12077 that begins with @samp{-}.
12079 @item --portability
12080 @opindex --portability
12081 Print an error message if a file name is not portable to all @acronym{POSIX}
12082 hosts. This option is equivalent to @samp{-p -P}.
12086 @cindex exit status of @command{pathchk}
12090 0 if all specified file names passed all checks,
12094 @node mktemp invocation
12095 @section @command{mktemp}: Create temporary file or directory
12098 @cindex file names, creating temporary
12099 @cindex directory, creating temporary
12100 @cindex temporary files and directories
12102 @command{mktemp} manages the creation of temporary files and
12103 directories. Synopsis:
12106 mktemp [@var{option}]@dots{} [@var{template}]
12109 Safely create a temporary file or directory based on @var{template},
12110 and print its name. If given, @var{template} must include at least
12111 three consecutive @samp{X}s in the last component. If omitted, the template
12112 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12113 implied. The final run of @samp{X}s in the @var{template} will be replaced
12114 by alpha-numeric characters; thus, on a case-sensitive file system,
12115 and with a @var{template} including a run of @var{n} instances of @samp{X},
12116 there are @samp{62**@var{n}} potential file names.
12118 Older scripts used to create temporary files by simply joining the
12119 name of the program with the process id (@samp{$$}) as a suffix.
12120 However, that naming scheme is easily predictable, and suffers from a
12121 race condition where the attacker can create an appropriately named
12122 symbolic link, such that when the script then opens a handle to what
12123 it thought was an unused file, it is instead modifying an existing
12124 file. Using the same scheme to create a directory is slightly safer,
12125 since the @command{mkdir} will fail if the target already exists, but
12126 it is still inferior because it allows for denial of service attacks.
12127 Therefore, modern scripts should use the @command{mktemp} command to
12128 guarantee that the generated name will be unpredictable, and that
12129 knowledge of the temporary file name implies that the file was created
12130 by the current script and cannot be modified by other users.
12132 When creating a file, the resulting file has read and write
12133 permissions for the current user, but no permissions for the group or
12134 others; these permissions are reduced if the current umask is more
12137 Here are some examples (although note that if you repeat them, you
12138 will most likely get different file names):
12143 Create a temporary file in the current directory.
12150 Create a temporary file with a known suffix.
12152 $ mktemp --suffix=.txt file-XXXX
12154 $ mktemp file-XXXX-XXXX.txt
12159 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12160 but falling back to the current directory rather than @file{/tmp}.
12161 Note that @command{mktemp} does not create fifos, but can create a
12162 secure directory in which the fifo can live. Exit the shell if the
12163 directory or fifo could not be created.
12165 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12167 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12171 Create and use a temporary file if possible, but ignore failure. The
12172 file will reside in the directory named by @env{TMPDIR}, if specified,
12173 or else in @file{/tmp}.
12175 $ file=$(mktemp -q) && @{
12176 > # Safe to use $file only within this block. Use quotes,
12177 > # since $TMPDIR, and thus $file, may contain whitespace.
12178 > echo ... > "$file"
12184 Act as a semi-random character generator (it is not fully random,
12185 since it is impacted by the contents of the current directory). To
12186 avoid security holes, do not use the resulting names to create a file.
12196 The program accepts the following options. Also see @ref{Common options}.
12203 @opindex --directory
12204 Create a directory rather than a file. The directory will have read,
12205 write, and search permissions for the current user, but no permissions
12206 for the group or others; these permissions are reduced if the current
12207 umask is more restrictive.
12213 Suppress diagnostics about failure to create a file or directory. The
12214 exit status will still reflect whether a file was created.
12220 Generate a temporary name that does not name an existing file, without
12221 changing the file system contents. Using the output of this command
12222 to create a new file is inherently unsafe, as there is a window of
12223 time between generating the name and using it where another process
12224 can create an object by the same name.
12227 @itemx --tmpdir[=@var{dir}]
12230 Treat @var{template} relative to the directory @var{dir}. If
12231 @var{dir} is not specified (only possible with the long option
12232 @option{--tmpdir}) or is the empty string, use the value of
12233 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12234 specified, @var{template} must not be absolute. However,
12235 @var{template} can still contain slashes, although intermediate
12236 directories must already exist.
12238 @item --suffix=@var{suffix}
12240 Append @var{suffix} to the @var{template}. @var{suffix} must not
12241 contain slash. If @option{--suffix} is specified, @var{template} must
12242 end in @samp{X}; if it is not specified, then an appropriate
12243 @option{--suffix} is inferred by finding the last @samp{X} in
12244 @var{template}. This option exists for use with the default
12245 @var{template} and for the creation of a @var{suffix} that starts with
12250 Treat @var{template} as a single file relative to the value of
12251 @env{TMPDIR} if available, or to the directory specified by
12252 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12253 contain slashes. This option is deprecated; the use of @option{-p}
12254 without @option{-t} offers better defaults (by favoring the command
12255 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12260 @cindex exit status of @command{mktemp}
12264 0 if the file was created,
12269 @node Working context
12270 @chapter Working context
12272 @cindex working context
12273 @cindex commands for printing the working context
12275 This section describes commands that display or alter the context in
12276 which you are working: the current directory, the terminal settings, and
12277 so forth. See also the user-related commands in the next section.
12280 * pwd invocation:: Print working directory.
12281 * stty invocation:: Print or change terminal characteristics.
12282 * printenv invocation:: Print environment variables.
12283 * tty invocation:: Print file name of terminal on standard input.
12287 @node pwd invocation
12288 @section @command{pwd}: Print working directory
12291 @cindex print name of current directory
12292 @cindex current working directory, printing
12293 @cindex working directory, printing
12296 @command{pwd} prints the name of the current directory. Synopsis:
12299 pwd [@var{option}]@dots{}
12302 The program accepts the following options. Also see @ref{Common options}.
12309 If the contents of the environment variable @env{PWD} provide an
12310 absolute name of the current directory with no @samp{.} or @samp{..}
12311 components, but possibly with symbolic links, then output those
12312 contents. Otherwise, fall back to default @option{-P} handling.
12317 @opindex --physical
12318 Print a fully resolved name for the current directory. That is, all
12319 components of the printed name will be actual directory names---none
12320 will be symbolic links.
12323 @cindex symbolic links and @command{pwd}
12324 If @option{-L} and @option{-P} are both given, the last one takes
12325 precedence. If neither option is given, then this implementation uses
12326 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12327 environment variable is set.
12329 @mayConflictWithShellBuiltIn{pwd}
12334 @node stty invocation
12335 @section @command{stty}: Print or change terminal characteristics
12338 @cindex change or print terminal settings
12339 @cindex terminal settings
12340 @cindex line settings of terminal
12342 @command{stty} prints or changes terminal characteristics, such as baud rate.
12346 stty [@var{option}] [@var{setting}]@dots{}
12347 stty [@var{option}]
12350 If given no line settings, @command{stty} prints the baud rate, line
12351 discipline number (on systems that support it), and line settings
12352 that have been changed from the values set by @samp{stty sane}.
12353 By default, mode reading and setting are performed on the tty line
12354 connected to standard input, although this can be modified by the
12355 @option{--file} option.
12357 @command{stty} accepts many non-option arguments that change aspects of
12358 the terminal line operation, as described below.
12360 The program accepts the following options. Also see @ref{Common options}.
12367 Print all current settings in human-readable form. This option may not
12368 be used in combination with any line settings.
12370 @item -F @var{device}
12371 @itemx --file=@var{device}
12374 Set the line opened by the file name specified in @var{device} instead of
12375 the tty line connected to standard input. This option is necessary
12376 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12377 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12378 the @code{clocal} flag is not set. Hence, it is not always possible
12379 to allow the shell to open the device in the traditional manner.
12385 @cindex machine-readable @command{stty} output
12386 Print all current settings in a form that can be used as an argument to
12387 another @command{stty} command to restore the current settings. This option
12388 may not be used in combination with any line settings.
12392 Many settings can be turned off by preceding them with a @samp{-}.
12393 Such arguments are marked below with ``May be negated'' in their
12394 description. The descriptions themselves refer to the positive
12395 case, that is, when @emph{not} negated (unless stated otherwise,
12398 Some settings are not available on all @acronym{POSIX} systems, since they use
12399 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12400 description. On non-@acronym{POSIX} systems, those or other settings also may not
12401 be available, but it's not feasible to document all the variations: just
12407 * Control:: Control settings
12408 * Input:: Input settings
12409 * Output:: Output settings
12410 * Local:: Local settings
12411 * Combination:: Combination settings
12412 * Characters:: Special characters
12413 * Special:: Special settings
12418 @subsection Control settings
12420 @cindex control settings
12426 @cindex two-way parity
12427 Generate parity bit in output and expect parity bit in input.
12433 @cindex even parity
12434 Set odd parity (even if negated). May be negated.
12441 @cindex character size
12442 @cindex eight-bit characters
12443 Set character size to 5, 6, 7, or 8 bits.
12448 Send a hangup signal when the last process closes the tty. May be
12454 Use two stop bits per character (one if negated). May be negated.
12458 Allow input to be received. May be negated.
12462 @cindex modem control
12463 Disable modem control signals. May be negated.
12467 @cindex hardware flow control
12468 @cindex flow control, hardware
12469 @cindex RTS/CTS flow control
12470 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12475 @subsection Input settings
12477 @cindex input settings
12478 These settings control operations on data received from the terminal.
12483 @cindex breaks, ignoring
12484 Ignore break characters. May be negated.
12488 @cindex breaks, cause interrupts
12489 Make breaks cause an interrupt signal. May be negated.
12493 @cindex parity, ignoring
12494 Ignore characters with parity errors. May be negated.
12498 @cindex parity errors, marking
12499 Mark parity errors (with a 255-0-character sequence). May be negated.
12503 Enable input parity checking. May be negated.
12507 @cindex eight-bit input
12508 Clear high (8th) bit of input characters. May be negated.
12512 @cindex newline, translating to return
12513 Translate newline to carriage return. May be negated.
12517 @cindex return, ignoring
12518 Ignore carriage return. May be negated.
12522 @cindex return, translating to newline
12523 Translate carriage return to newline. May be negated.
12527 @cindex input encoding, UTF-8
12528 Assume input characters are UTF-8 encoded. May be negated.
12532 @kindex C-s/C-q flow control
12533 @cindex XON/XOFF flow control
12534 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12541 @cindex software flow control
12542 @cindex flow control, software
12543 Enable sending of @code{stop} character when the system input buffer
12544 is almost full, and @code{start} character when it becomes almost
12545 empty again. May be negated.
12549 @cindex uppercase, translating to lowercase
12550 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12551 negated. Note ilcuc is not implemented, as one would not be able to issue
12552 almost any (lowercase) Unix command, after invoking it.
12556 Allow any character to restart output (only the start character
12557 if negated). Non-@acronym{POSIX}. May be negated.
12561 @cindex beeping at input buffer full
12562 Enable beeping and not flushing input buffer if a character arrives
12563 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12568 @subsection Output settings
12570 @cindex output settings
12571 These settings control operations on data sent to the terminal.
12576 Postprocess output. May be negated.
12580 @cindex lowercase, translating to output
12581 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12582 negated. (Note ouclc is not currently implemented.)
12586 @cindex return, translating to newline
12587 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12591 @cindex newline, translating to crlf
12592 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12597 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12602 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12606 @cindex pad instead of timing for delaying
12607 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12612 @cindex pad character
12613 Use @acronym{ASCII} @sc{del} characters for fill instead of
12614 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12620 Newline delay style. Non-@acronym{POSIX}.
12627 Carriage return delay style. Non-@acronym{POSIX}.
12633 @opindex tab@var{n}
12634 Horizontal tab delay style. Non-@acronym{POSIX}.
12639 Backspace delay style. Non-@acronym{POSIX}.
12644 Vertical tab delay style. Non-@acronym{POSIX}.
12649 Form feed delay style. Non-@acronym{POSIX}.
12654 @subsection Local settings
12656 @cindex local settings
12661 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12662 characters. May be negated.
12666 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12667 special characters. May be negated.
12671 Enable non-@acronym{POSIX} special characters. May be negated.
12675 Echo input characters. May be negated.
12681 Echo @code{erase} characters as backspace-space-backspace. May be
12686 @cindex newline echoing after @code{kill}
12687 Echo a newline after a @code{kill} character. May be negated.
12691 @cindex newline, echoing
12692 Echo newline even if not echoing other characters. May be negated.
12696 @cindex flushing, disabling
12697 Disable flushing after @code{interrupt} and @code{quit} special
12698 characters. May be negated.
12702 @cindex case translation
12703 Enable input and output of uppercase characters by preceding their
12704 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12705 Non-@acronym{POSIX}. May be negated.
12709 @cindex background jobs, stopping at terminal write
12710 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12717 Echo erased characters backward, between @samp{\} and @samp{/}.
12718 Non-@acronym{POSIX}. May be negated.
12724 @cindex control characters, using @samp{^@var{c}}
12725 @cindex hat notation for control characters
12726 Echo control characters in hat notation (@samp{^@var{c}}) instead
12727 of literally. Non-@acronym{POSIX}. May be negated.
12733 Echo the @code{kill} special character by erasing each character on
12734 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12735 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12741 @subsection Combination settings
12743 @cindex combination settings
12744 Combination settings:
12751 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12752 as @code{-parenb cs8}.
12756 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12757 as @code{-parenb cs8}.
12761 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12762 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12766 Reset the @code{erase} and @code{kill} special characters to their default
12773 @c This is too long to write inline.
12775 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12776 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12777 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12778 ff0 isig icanon iexten echo echoe echok -echonl
12779 -noflsh -xcase -tostop -echoprt echoctl echoke
12783 and also sets all special characters to their default values.
12787 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12788 sets the @code{eof} and @code{eol} characters to their default values
12789 if they are the same as the @code{min} and @code{time} characters.
12790 May be negated. If negated, same as @code{raw}.
12797 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12798 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12799 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12803 May be negated. If negated, same as @code{cooked}.
12807 Same as @option{-icanon}. May be negated. If negated, same as
12812 @cindex eight-bit characters
12813 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12814 same as @code{parenb istrip cs7}.
12818 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12819 If negated, same as @code{parenb istrip opost cs7}.
12823 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12827 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12834 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12835 (Used for terminals with uppercase characters only.)
12839 Same as @code{echoe echoctl echoke}.
12843 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12848 @subsection Special characters
12850 @cindex special characters
12851 @cindex characters, special
12853 The special characters' default values vary from system to system.
12854 They are set with the syntax @samp{name value}, where the names are
12855 listed below and the value can be given either literally, in hat
12856 notation (@samp{^@var{c}}), or as an integer which may start with
12857 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12858 any other digit to indicate decimal.
12860 @cindex disabling special characters
12861 @kindex u@r{, and disabling special characters}
12862 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12863 special character. (This is incompatible with Ultrix @command{stty},
12864 which uses a value of @samp{u} to disable a special character. GNU
12865 @command{stty} treats a value @samp{u} like any other, namely to set that
12866 special character to @key{U}.)
12872 Send an interrupt signal.
12876 Send a quit signal.
12880 Erase the last character typed.
12884 Erase the current line.
12888 Send an end of file (terminate the input).
12896 Alternate character to end the line. Non-@acronym{POSIX}.
12900 Switch to a different shell layer. Non-@acronym{POSIX}.
12904 Restart the output after stopping it.
12912 Send a terminal stop signal.
12916 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
12920 Redraw the current line. Non-@acronym{POSIX}.
12924 Erase the last word typed. Non-@acronym{POSIX}.
12928 Enter the next character typed literally, even if it is a special
12929 character. Non-@acronym{POSIX}.
12934 @subsection Special settings
12936 @cindex special settings
12941 Set the minimum number of characters that will satisfy a read until
12942 the time value has expired, when @option{-icanon} is set.
12946 Set the number of tenths of a second before reads time out if the minimum
12947 number of characters have not been read, when @option{-icanon} is set.
12949 @item ispeed @var{n}
12951 Set the input speed to @var{n}.
12953 @item ospeed @var{n}
12955 Set the output speed to @var{n}.
12959 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
12962 @itemx columns @var{n}
12965 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
12971 Print the number of rows and columns that the kernel thinks the
12972 terminal has. (Systems that don't support rows and columns in the kernel
12973 typically use the environment variables @env{LINES} and @env{COLUMNS}
12974 instead; however, GNU @command{stty} does not know anything about them.)
12975 Non-@acronym{POSIX}.
12979 Use line discipline @var{n}. Non-@acronym{POSIX}.
12983 Print the terminal speed.
12986 @cindex baud rate, setting
12987 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
12988 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
12989 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
12990 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
12991 support higher speeds. The @command{stty} command includes support
13008 4000000 where the system supports these.
13009 0 hangs up the line if @option{-clocal} is set.
13013 @node printenv invocation
13014 @section @command{printenv}: Print all or some environment variables
13017 @cindex printing all or some environment variables
13018 @cindex environment variables, printing
13020 @command{printenv} prints environment variable values. Synopsis:
13023 printenv [@var{option}] [@var{variable}]@dots{}
13026 If no @var{variable}s are specified, @command{printenv} prints the value of
13027 every environment variable. Otherwise, it prints the value of each
13028 @var{variable} that is set, and nothing for those that are not set.
13030 The program accepts the following option. Also see @ref{Common options}.
13038 @cindex exit status of @command{printenv}
13042 0 if all variables specified were found
13043 1 if at least one specified variable was not found
13044 2 if a write error occurred
13048 @node tty invocation
13049 @section @command{tty}: Print file name of terminal on standard input
13052 @cindex print terminal file name
13053 @cindex terminal file name, printing
13055 @command{tty} prints the file name of the terminal connected to its standard
13056 input. It prints @samp{not a tty} if standard input is not a terminal.
13060 tty [@var{option}]@dots{}
13063 The program accepts the following option. Also see @ref{Common options}.
13073 Print nothing; only return an exit status.
13077 @cindex exit status of @command{tty}
13081 0 if standard input is a terminal
13082 1 if standard input is not a terminal
13083 2 if given incorrect arguments
13084 3 if a write error occurs
13088 @node User information
13089 @chapter User information
13091 @cindex user information, commands for
13092 @cindex commands for printing user information
13094 This section describes commands that print user-related information:
13095 logins, groups, and so forth.
13098 * id invocation:: Print user identity.
13099 * logname invocation:: Print current login name.
13100 * whoami invocation:: Print effective user ID.
13101 * groups invocation:: Print group names a user is in.
13102 * users invocation:: Print login names of users currently logged in.
13103 * who invocation:: Print who is currently logged in.
13107 @node id invocation
13108 @section @command{id}: Print user identity
13111 @cindex real user and group IDs, printing
13112 @cindex effective user and group IDs, printing
13113 @cindex printing real and effective user and group IDs
13115 @command{id} prints information about the given user, or the process
13116 running it if no user is specified. Synopsis:
13119 id [@var{option}]@dots{} [@var{username}]
13122 @vindex POSIXLY_CORRECT
13123 By default, it prints the real user ID, real group ID, effective user ID
13124 if different from the real user ID, effective group ID if different from
13125 the real group ID, and supplemental group IDs.
13126 In addition, if SELinux
13127 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13128 then print @samp{context=@var{c}}, where @var{c} is the security context.
13130 Each of these numeric values is preceded by an identifying string and
13131 followed by the corresponding user or group name in parentheses.
13133 The options cause @command{id} to print only part of the above information.
13134 Also see @ref{Common options}.
13141 Print only the group ID.
13147 Print only the group ID and the supplementary groups.
13153 Print the user or group name instead of the ID number. Requires
13154 @option{-u}, @option{-g}, or @option{-G}.
13160 Print the real, instead of effective, user or group ID. Requires
13161 @option{-u}, @option{-g}, or @option{-G}.
13167 Print only the user ID.
13174 @cindex security context
13175 Print only the security context of the current user.
13176 If SELinux is disabled then print a warning and
13177 set the exit status to 1.
13183 @macro primaryAndSupplementaryGroups{cmd,arg}
13184 Primary and supplementary groups for a process are normally inherited
13185 from its parent and are usually unchanged since login. This means
13186 that if you change the group database after logging in, @command{\cmd\}
13187 will not reflect your changes within your existing login session.
13188 Running @command{\cmd\} with a \arg\ causes the user and group
13189 database to be consulted afresh, and so will give a different result.
13191 @primaryAndSupplementaryGroups{id,user argument}
13193 @node logname invocation
13194 @section @command{logname}: Print current login name
13197 @cindex printing user's login name
13198 @cindex login name, printing
13199 @cindex user name, printing
13202 @command{logname} prints the calling user's name, as found in a
13203 system-maintained file (often @file{/var/run/utmp} or
13204 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13205 for the calling process, @command{logname} prints
13206 an error message and exits with a status of 1.
13208 The only options are @option{--help} and @option{--version}. @xref{Common
13214 @node whoami invocation
13215 @section @command{whoami}: Print effective user ID
13218 @cindex effective user ID, printing
13219 @cindex printing the effective user ID
13221 @command{whoami} prints the user name associated with the current
13222 effective user ID. It is equivalent to the command @samp{id -un}.
13224 The only options are @option{--help} and @option{--version}. @xref{Common
13230 @node groups invocation
13231 @section @command{groups}: Print group names a user is in
13234 @cindex printing groups a user is in
13235 @cindex supplementary groups, printing
13237 @command{groups} prints the names of the primary and any supplementary
13238 groups for each given @var{username}, or the current process if no names
13239 are given. If more than one name is given, the name of each user is
13241 the list of that user's groups and the user name is separated from the
13242 group list by a colon. Synopsis:
13245 groups [@var{username}]@dots{}
13248 The group lists are equivalent to the output of the command @samp{id -Gn}.
13250 @primaryAndSupplementaryGroups{groups,list of users}
13252 The only options are @option{--help} and @option{--version}. @xref{Common
13258 @node users invocation
13259 @section @command{users}: Print login names of users currently logged in
13262 @cindex printing current usernames
13263 @cindex usernames, printing current
13265 @cindex login sessions, printing users with
13266 @command{users} prints on a single line a blank-separated list of user
13267 names of users currently logged in to the current host. Each user name
13268 corresponds to a login session, so if a user has more than one login
13269 session, that user's name will appear the same number of times in the
13278 With no @var{file} argument, @command{users} extracts its information from
13279 a system-maintained file (often @file{/var/run/utmp} or
13280 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13281 that file instead. A common choice is @file{/var/log/wtmp}.
13283 The only options are @option{--help} and @option{--version}. @xref{Common
13289 @node who invocation
13290 @section @command{who}: Print who is currently logged in
13293 @cindex printing current user information
13294 @cindex information, about current users
13296 @command{who} prints information about users who are currently logged on.
13300 @command{who} [@var{option}] [@var{file}] [am i]
13303 @cindex terminal lines, currently used
13305 @cindex remote hostname
13306 If given no non-option arguments, @command{who} prints the following
13307 information for each user currently logged on: login name, terminal
13308 line, login time, and remote hostname or X display.
13312 If given one non-option argument, @command{who} uses that instead of
13313 a default system-maintained file (often @file{/var/run/utmp} or
13314 @file{/etc/utmp}) as the name of the file containing the record of
13315 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13316 to @command{who} to look at who has previously logged on.
13320 If given two non-option arguments, @command{who} prints only the entry
13321 for the user running it (determined from its standard input), preceded
13322 by the hostname. Traditionally, the two arguments given are @samp{am
13323 i}, as in @samp{who am i}.
13326 Time stamps are listed according to the time zone rules specified by
13327 the @env{TZ} environment variable, or by the system default rules if
13328 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13329 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13331 The program accepts the following options. Also see @ref{Common options}.
13339 Same as @samp{-b -d --login -p -r -t -T -u}.
13345 Print the date and time of last system boot.
13351 Print information corresponding to dead processes.
13357 Print a line of column headings.
13363 List only the entries that correspond to processes via which the
13364 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13368 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13369 is not the default because it can cause significant delays on systems with
13370 automatic dial-up internet access.
13374 Same as @samp{who am i}.
13380 List active processes spawned by init.
13386 Print only the login names and the number of users logged on.
13387 Overrides all other options.
13392 @opindex --runlevel
13393 Print the current (and maybe previous) run-level of the init process.
13397 Ignored; for compatibility with other versions of @command{who}.
13403 Print last system clock change.
13408 After the login time, print the number of hours and minutes that the
13409 user has been idle. @samp{.} means the user was active in the last minute.
13410 @samp{old} means the user has been idle for more than 24 hours.
13421 @opindex --writable
13422 @cindex message status
13423 @pindex write@r{, allowed}
13424 After each login name print a character indicating the user's message status:
13427 @samp{+} allowing @code{write} messages
13428 @samp{-} disallowing @code{write} messages
13429 @samp{?} cannot find terminal device
13437 @node System context
13438 @chapter System context
13440 @cindex system context
13441 @cindex context, system
13442 @cindex commands for system context
13444 This section describes commands that print or change system-wide
13448 * date invocation:: Print or set system date and time.
13449 * arch invocation:: Print machine hardware name.
13450 * nproc invocation:: Print the number of processors.
13451 * uname invocation:: Print system information.
13452 * hostname invocation:: Print or set system name.
13453 * hostid invocation:: Print numeric host identifier.
13454 * uptime invocation:: Print system uptime and load.
13457 @node date invocation
13458 @section @command{date}: Print or set system date and time
13461 @cindex time, printing or setting
13462 @cindex printing the current time
13467 date [@var{option}]@dots{} [+@var{format}]
13468 date [-u|--utc|--universal] @c this avoids a newline in the output
13469 [ MMDDhhmm[[CC]YY][.ss] ]
13473 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13474 it with a default format that depends on the @env{LC_TIME} locale category.
13475 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13476 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13479 Normally, @command{date} uses the time zone rules indicated by the
13480 @env{TZ} environment variable, or the system default rules if @env{TZ}
13481 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13482 @env{TZ}, libc, The GNU C Library Reference Manual}.
13484 @findex strftime @r{and @command{date}}
13485 @cindex time formats
13486 @cindex formatting times
13487 If given an argument that starts with a @samp{+}, @command{date} prints the
13488 current date and time (or the date and time specified by the
13489 @option{--date} option, see below) in the format defined by that argument,
13490 which is similar to that of the @code{strftime} function. Except for
13491 conversion specifiers, which start with @samp{%}, characters in the
13492 format string are printed unchanged. The conversion specifiers are
13498 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13499 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13500 * Literal conversion specifiers:: %[%nt]
13501 * Padding and other flags:: Pad with zeros, spaces, etc.
13502 * Setting the time:: Changing the system clock.
13503 * Options for date:: Instead of the current time.
13505 * Date input formats:: Specifying date strings.
13507 * Examples of date:: Examples.
13510 @node Time conversion specifiers
13511 @subsection Time conversion specifiers
13513 @cindex time conversion specifiers
13514 @cindex conversion specifiers, time
13516 @command{date} conversion specifiers related to times.
13520 hour (@samp{00}@dots{}@samp{23})
13522 hour (@samp{01}@dots{}@samp{12})
13524 hour (@samp{ 0}@dots{}@samp{23}).
13525 This is a @acronym{GNU} extension.
13527 hour (@samp{ 1}@dots{}@samp{12}).
13528 This is a @acronym{GNU} extension.
13530 minute (@samp{00}@dots{}@samp{59})
13532 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13533 This is a @acronym{GNU} extension.
13535 locale's equivalent of either @samp{AM} or @samp{PM};
13536 blank in many locales.
13537 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13539 like @samp{%p}, except lower case.
13540 This is a @acronym{GNU} extension.
13542 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13544 24-hour hour and minute. Same as @samp{%H:%M}.
13545 This is a @acronym{GNU} extension.
13547 @cindex epoch, seconds since
13548 @cindex seconds since the epoch
13549 @cindex beginning of time
13550 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13551 Leap seconds are not counted unless leap second support is available.
13552 @xref{%s-examples}, for examples.
13553 This is a @acronym{GNU} extension.
13555 second (@samp{00}@dots{}@samp{60}).
13556 This may be @samp{60} if leap seconds are supported.
13558 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13560 locale's time representation (e.g., @samp{23:13:48})
13562 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13563 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13564 time zone is determinable. This value reflects the numeric time zone
13565 appropriate for the current time, using the time zone rules specified
13566 by the @env{TZ} environment variable.
13567 The time (and optionally, the time zone rules) can be overridden
13568 by the @option{--date} option.
13569 This is a @acronym{GNU} extension.
13571 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13572 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13573 zone is determinable.
13574 This is a @acronym{GNU} extension.
13576 Numeric time zone to the nearest second with @samp{:} (e.g.,
13577 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13579 This is a @acronym{GNU} extension.
13581 Numeric time zone with @samp{:} using the minimum necessary precision
13582 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13583 no time zone is determinable.
13584 This is a @acronym{GNU} extension.
13586 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13587 time zone is determinable. See @samp{%z} for how it is determined.
13591 @node Date conversion specifiers
13592 @subsection Date conversion specifiers
13594 @cindex date conversion specifiers
13595 @cindex conversion specifiers, date
13597 @command{date} conversion specifiers related to dates.
13601 locale's abbreviated weekday name (e.g., @samp{Sun})
13603 locale's full weekday name, variable length (e.g., @samp{Sunday})
13605 locale's abbreviated month name (e.g., @samp{Jan})
13607 locale's full month name, variable length (e.g., @samp{January})
13609 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13611 century. This is like @samp{%Y}, except the last two digits are omitted.
13612 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13613 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13614 It is normally at least two characters, but it may be more.
13616 day of month (e.g., @samp{01})
13618 date; same as @samp{%m/%d/%y}
13620 day of month, space padded; same as @samp{%_d}
13622 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13623 This is a good choice for a date format, as it is standard and
13624 is easy to sort in the usual case where years are in the range
13626 This is a @acronym{GNU} extension.
13628 year corresponding to the @acronym{ISO} week number, but without the century
13629 (range @samp{00} through @samp{99}). This has the same format and value
13630 as @samp{%y}, except that if the @acronym{ISO} week number (see
13632 to the previous or next year, that year is used instead.
13633 This is a @acronym{GNU} extension.
13635 year corresponding to the @acronym{ISO} week number. This has the
13636 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13638 @samp{%V}) belongs to the previous or next year, that year is used
13640 It is normally useful only if @samp{%V} is also used;
13641 for example, the format @samp{%G-%m-%d} is probably a mistake,
13642 since it combines the ISO week number year with the conventional month and day.
13643 This is a @acronym{GNU} extension.
13647 day of year (@samp{001}@dots{}@samp{366})
13649 month (@samp{01}@dots{}@samp{12})
13651 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13653 week number of year, with Sunday as the first day of the week
13654 (@samp{00}@dots{}@samp{53}).
13655 Days in a new year preceding the first Sunday are in week zero.
13657 @acronym{ISO} week number, that is, the
13658 week number of year, with Monday as the first day of the week
13659 (@samp{01}@dots{}@samp{53}).
13660 If the week containing January 1 has four or more days in
13661 the new year, then it is considered week 1; otherwise, it is week 53 of
13662 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13665 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13667 week number of year, with Monday as first day of week
13668 (@samp{00}@dots{}@samp{53}).
13669 Days in a new year preceding the first Monday are in week zero.
13671 locale's date representation (e.g., @samp{12/31/99})
13673 last two digits of year (@samp{00}@dots{}@samp{99})
13675 year. This is normally at least four characters, but it may be more.
13676 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13677 precedes year @samp{0000}.
13681 @node Literal conversion specifiers
13682 @subsection Literal conversion specifiers
13684 @cindex literal conversion specifiers
13685 @cindex conversion specifiers, literal
13687 @command{date} conversion specifiers that produce literal strings.
13699 @node Padding and other flags
13700 @subsection Padding and other flags
13702 @cindex numeric field padding
13703 @cindex padding of numeric fields
13704 @cindex fields, padding numeric
13706 Unless otherwise specified, @command{date} normally pads numeric fields
13707 with zeros, so that, for
13708 example, numeric months are always output as two digits.
13709 Seconds since the epoch are not padded, though,
13710 since there is no natural width for them.
13712 As a @acronym{GNU} extension, @command{date} recognizes any of the
13713 following optional flags after the @samp{%}:
13717 (hyphen) Do not pad the field; useful if the output is intended for
13720 (underscore) Pad with spaces; useful if you need a fixed
13721 number of characters in the output, but zeros are too distracting.
13723 (zero) Pad with zeros even if the conversion specifier
13724 would normally pad with spaces.
13726 Use upper case characters if possible.
13728 Use opposite case characters if possible.
13729 A field that is normally upper case becomes lower case, and vice versa.
13733 Here are some examples of padding:
13736 date +%d/%m -d "Feb 1"
13738 date +%-d/%-m -d "Feb 1"
13740 date +%_d/%_m -d "Feb 1"
13744 As a @acronym{GNU} extension, you can specify the field width
13745 (after any flag, if present) as a decimal number. If the natural size of the
13746 output of the field has less than the specified number of characters,
13747 the result is written right adjusted and padded to the given
13748 size. For example, @samp{%9B} prints the right adjusted month name in
13749 a field of width 9.
13751 An optional modifier can follow the optional flag and width
13752 specification. The modifiers are:
13756 Use the locale's alternate representation for date and time. This
13757 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13758 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13759 example, @samp{%Ex} might yield a date format based on the Japanese
13763 Use the locale's alternate numeric symbols for numbers. This modifier
13764 applies only to numeric conversion specifiers.
13767 If the format supports the modifier but no alternate representation
13768 is available, it is ignored.
13771 @node Setting the time
13772 @subsection Setting the time
13774 @cindex setting the time
13775 @cindex time setting
13776 @cindex appropriate privileges
13778 If given an argument that does not start with @samp{+}, @command{date} sets
13779 the system clock to the date and time specified by that argument (as
13780 described below). You must have appropriate privileges to set the
13781 system clock. The @option{--date} and @option{--set} options may not be
13782 used with such an argument. The @option{--universal} option may be used
13783 with such an argument to indicate that the specified date and time are
13784 relative to Coordinated Universal Time rather than to the local time
13787 The argument must consist entirely of digits, which have the following
13800 first two digits of year (optional)
13802 last two digits of year (optional)
13807 The @option{--set} option also sets the system clock; see the next section.
13810 @node Options for date
13811 @subsection Options for @command{date}
13813 @cindex @command{date} options
13814 @cindex options for @command{date}
13816 The program accepts the following options. Also see @ref{Common options}.
13820 @item -d @var{datestr}
13821 @itemx --date=@var{datestr}
13824 @cindex parsing date strings
13825 @cindex date strings, parsing
13826 @cindex arbitrary date strings, parsing
13829 @opindex next @var{day}
13830 @opindex last @var{day}
13831 Display the date and time specified in @var{datestr} instead of the
13832 current date and time. @var{datestr} can be in almost any common
13833 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13834 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13835 14:19:13.489392193 +0530"} specifies the instant of time that is
13836 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13837 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13838 Note: input currently must be in locale independent format. E.g., the
13839 LC_TIME=C below is needed to print back the correct date in many locales:
13841 date -d "$(LC_TIME=C date)"
13843 @xref{Date input formats}.
13845 @item -f @var{datefile}
13846 @itemx --file=@var{datefile}
13849 Parse each line in @var{datefile} as with @option{-d} and display the
13850 resulting date and time. If @var{datefile} is @samp{-}, use standard
13851 input. This is useful when you have many dates to process, because the
13852 system overhead of starting up the @command{date} executable many times can
13855 @item -r @var{file}
13856 @itemx --reference=@var{file}
13858 @opindex --reference
13859 Display the date and time of the last modification of @var{file},
13860 instead of the current date and time.
13867 @opindex --rfc-2822
13868 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13869 %z}, evaluated in the C locale so abbreviations are always in English.
13873 Fri, 09 Sep 2005 13:51:39 -0700
13876 This format conforms to
13877 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13878 @acronym{RFCs} 2822} and
13879 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13880 current and previous standards for Internet email.
13882 @item --rfc-3339=@var{timespec}
13883 @opindex --rfc-3339=@var{timespec}
13884 Display the date using a format specified by
13885 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13886 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13887 format, except that it also permits applications to use a space rather
13888 than a @samp{T} to separate dates from times. Unlike the other
13889 standard formats, @acronym{RFC} 3339 format is always suitable as
13890 input for the @option{--date} (@option{-d}) and @option{--file}
13891 (@option{-f}) options, regardless of the current locale.
13893 The argument @var{timespec} specifies how much of the time to include.
13894 It can be one of the following:
13898 Print just the full-date, e.g., @samp{2005-09-14}.
13899 This is equivalent to the format @samp{%Y-%m-%d}.
13902 Print the full-date and full-time separated by a space, e.g.,
13903 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
13904 time-offset; here the @samp{+05:30} means that local time is five
13905 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
13906 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
13909 Like @samp{seconds}, but also print nanoseconds, e.g.,
13910 @samp{2005-09-14 00:56:06.998458565+05:30}.
13911 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
13915 @item -s @var{datestr}
13916 @itemx --set=@var{datestr}
13919 Set the date and time to @var{datestr}. See @option{-d} above.
13926 @opindex --universal
13927 @cindex Coordinated Universal Time
13929 @cindex Greenwich Mean Time
13932 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
13933 @env{TZ} environment variable were set to the string @samp{UTC0}.
13935 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
13936 historical reasons.
13940 @node Examples of date
13941 @subsection Examples of @command{date}
13943 @cindex examples of @command{date}
13945 Here are a few examples. Also see the documentation for the @option{-d}
13946 option in the previous section.
13951 To print the date of the day before yesterday:
13954 date --date='2 days ago'
13958 To print the date of the day three months and one day hence:
13961 date --date='3 months 1 day'
13965 To print the day of year of Christmas in the current year:
13968 date --date='25 Dec' +%j
13972 To print the current full month name and the day of the month:
13978 But this may not be what you want because for the first nine days of
13979 the month, the @samp{%d} expands to a zero-padded two-digit field,
13980 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
13983 To print a date without the leading zero for one-digit days
13984 of the month, you can use the (@acronym{GNU} extension)
13985 @samp{-} flag to suppress
13986 the padding altogether:
13989 date -d 1may '+%B %-d
13993 To print the current date and time in the format required by many
13994 non-@acronym{GNU} versions of @command{date} when setting the system clock:
13997 date +%m%d%H%M%Y.%S
14001 To set the system clock forward by two minutes:
14004 date --set='+2 minutes'
14008 To print the date in @acronym{RFC} 2822 format,
14009 use @samp{date --rfc-2822}. Here is some example output:
14012 Fri, 09 Sep 2005 13:51:39 -0700
14015 @anchor{%s-examples}
14017 To convert a date string to the number of seconds since the epoch
14018 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14019 the @samp{%s} format. That can be useful in sorting and/or graphing
14020 and/or comparing data by date. The following command outputs the
14021 number of the seconds since the epoch for the time two minutes after the
14025 date --date='1970-01-01 00:02:00 +0000' +%s
14029 If you do not specify time zone information in the date string,
14030 @command{date} uses your computer's idea of the time zone when
14031 interpreting the string. For example, if your computer's time zone is
14032 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14033 seconds) behind UTC:
14036 # local time zone used
14037 date --date='1970-01-01 00:02:00' +%s
14042 If you're sorting or graphing dated data, your raw date values may be
14043 represented as seconds since the epoch. But few people can look at
14044 the date @samp{946684800} and casually note ``Oh, that's the first second
14045 of the year 2000 in Greenwich, England.''
14048 date --date='2000-01-01 UTC' +%s
14052 An alternative is to use the @option{--utc} (@option{-u}) option.
14053 Then you may omit @samp{UTC} from the date string. Although this
14054 produces the same result for @samp{%s} and many other format sequences,
14055 with a time zone offset different from zero, it would give a different
14056 result for zone-dependent formats like @samp{%z}.
14059 date -u --date=2000-01-01 +%s
14063 To convert such an unwieldy number of seconds back to
14064 a more readable form, use a command like this:
14067 # local time zone used
14068 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14069 1999-12-31 19:00:00 -0500
14072 Or if you do not mind depending on the @samp{@@} feature present since
14073 coreutils 5.3.0, you could shorten this to:
14076 date -d @@946684800 +"%F %T %z"
14077 1999-12-31 19:00:00 -0500
14080 Often it is better to output UTC-relative date and time:
14083 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14084 2000-01-01 00:00:00 +0000
14090 @node arch invocation
14091 @section @command{arch}: Print machine hardware name
14094 @cindex print machine hardware name
14095 @cindex system information, printing
14097 @command{arch} prints the machine hardware name,
14098 and is equivalent to @samp{uname -m}.
14102 arch [@var{option}]
14105 The program accepts the @ref{Common options} only.
14110 @node nproc invocation
14111 @section @command{nproc}: Print the number of available processors
14114 @cindex Print the number of processors
14115 @cindex system information, printing
14117 Print the number of processing units available to the current process,
14118 which may be less than the number of online processors.
14119 If this information is not accessible, then print the number of
14120 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14121 set, then it will determine the returned value. The result is guaranteed to be
14122 greater than zero. Synopsis:
14125 nproc [@var{option}]
14128 The program accepts the following options. Also see @ref{Common options}.
14134 Print the number of installed processors on the system, which may
14135 be greater than the number online or available to the current process.
14136 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14138 @item --ignore=@var{number}
14140 If possible, exclude this @var{number} of processing units.
14147 @node uname invocation
14148 @section @command{uname}: Print system information
14151 @cindex print system information
14152 @cindex system information, printing
14154 @command{uname} prints information about the machine and operating system
14155 it is run on. If no options are given, @command{uname} acts as if the
14156 @option{-s} option were given. Synopsis:
14159 uname [@var{option}]@dots{}
14162 If multiple options or @option{-a} are given, the selected information is
14163 printed in this order:
14166 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14167 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14170 The information may contain internal spaces, so such output cannot be
14171 parsed reliably. In the following example, @var{release} is
14172 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14176 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14180 The program accepts the following options. Also see @ref{Common options}.
14188 Print all of the below information, except omit the processor type
14189 and the hardware platform name if they are unknown.
14192 @itemx --hardware-platform
14194 @opindex --hardware-platform
14195 @cindex implementation, hardware
14196 @cindex hardware platform
14197 @cindex platform, hardware
14198 Print the hardware platform name
14199 (sometimes called the hardware implementation).
14200 Print @samp{unknown} if the kernel does not make this information
14201 easily available, as is the case with Linux kernels.
14207 @cindex machine type
14208 @cindex hardware class
14209 @cindex hardware type
14210 Print the machine hardware name (sometimes called the hardware class
14216 @opindex --nodename
14219 @cindex network node name
14220 Print the network node hostname.
14225 @opindex --processor
14226 @cindex host processor type
14227 Print the processor type (sometimes called the instruction set
14228 architecture or ISA).
14229 Print @samp{unknown} if the kernel does not make this information
14230 easily available, as is the case with Linux kernels.
14233 @itemx --operating-system
14235 @opindex --operating-system
14236 @cindex operating system name
14237 Print the name of the operating system.
14240 @itemx --kernel-release
14242 @opindex --kernel-release
14243 @cindex kernel release
14244 @cindex release of kernel
14245 Print the kernel release.
14248 @itemx --kernel-name
14250 @opindex --kernel-name
14251 @cindex kernel name
14252 @cindex name of kernel
14253 Print the kernel name.
14254 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14255 ``the implementation of the operating system'', because the
14256 @acronym{POSIX} specification itself has no notion of ``kernel''.
14257 The kernel name might be the same as the operating system name printed
14258 by the @option{-o} or @option{--operating-system} option, but it might
14259 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14260 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14264 @itemx --kernel-version
14266 @opindex --kernel-version
14267 @cindex kernel version
14268 @cindex version of kernel
14269 Print the kernel version.
14276 @node hostname invocation
14277 @section @command{hostname}: Print or set system name
14280 @cindex setting the hostname
14281 @cindex printing the hostname
14282 @cindex system name, printing
14283 @cindex appropriate privileges
14285 With no arguments, @command{hostname} prints the name of the current host
14286 system. With one argument, it sets the current host name to the
14287 specified string. You must have appropriate privileges to set the host
14291 hostname [@var{name}]
14294 The only options are @option{--help} and @option{--version}. @xref{Common
14300 @node hostid invocation
14301 @section @command{hostid}: Print numeric host identifier
14304 @cindex printing the host identifier
14306 @command{hostid} prints the numeric identifier of the current host
14307 in hexadecimal. This command accepts no arguments.
14308 The only options are @option{--help} and @option{--version}.
14309 @xref{Common options}.
14311 For example, here's what it prints on one system I use:
14318 On that system, the 32-bit quantity happens to be closely
14319 related to the system's Internet address, but that isn't always
14324 @node uptime invocation
14325 @section @command{uptime}: Print system uptime and load
14328 @cindex printing the system uptime and load
14330 @command{uptime} prints the current time, the system's uptime, the
14331 number of logged-in users and the current load average.
14333 If an argument is specified, it is used as the file to be read
14334 to discover how many users are logged in. If no argument is
14335 specified, a system default is used (@command{uptime --help} indicates
14336 the default setting).
14338 The only options are @option{--help} and @option{--version}.
14339 @xref{Common options}.
14341 For example, here's what it prints right now on one system I use:
14345 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14348 The precise method of calculation of load average varies somewhat
14349 between systems. Some systems calculate it as the average number of
14350 runnable processes over the last 1, 5 and 15 minutes, but some systems
14351 also include processes in the uninterruptible sleep state (that is,
14352 those processes which are waiting for disk I/O). The Linux kernel
14353 includes uninterruptible processes.
14355 @node SELinux context
14356 @chapter SELinux context
14358 @cindex SELinux context
14359 @cindex SELinux, context
14360 @cindex commands for SELinux context
14362 This section describes commands for operations with SELinux
14366 * chcon invocation:: Change SELinux context of file
14367 * runcon invocation:: Run a command in specified SELinux context
14370 @node chcon invocation
14371 @section @command{chcon}: Change SELinux context of file
14374 @cindex changing security context
14375 @cindex change SELinux context
14377 @command{chcon} changes the SELinux security context of the selected files.
14381 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14382 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14383 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14386 Change the SELinux security context of each @var{file} to @var{context}.
14387 With @option{--reference}, change the security context of each @var{file}
14388 to that of @var{rfile}.
14390 The program accepts the following options. Also see @ref{Common options}.
14395 @itemx --no-dereference
14397 @opindex --no-dereference
14398 @cindex no dereference
14399 Affect symbolic links instead of any referenced file.
14401 @item --reference=@var{rfile}
14402 @opindex --reference
14403 @cindex reference file
14404 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14409 @opindex --recursive
14410 Operate on files and directories recursively.
14413 @xref{Traversing symlinks}.
14416 @xref{Traversing symlinks}.
14419 @xref{Traversing symlinks}.
14426 Output a diagnostic for every file processed.
14428 @item -u @var{user}
14429 @itemx --user=@var{user}
14432 Set user @var{user} in the target security context.
14434 @item -r @var{role}
14435 @itemx --role=@var{role}
14438 Set role @var{role} in the target security context.
14440 @item -t @var{type}
14441 @itemx --type=@var{type}
14444 Set type @var{type} in the target security context.
14446 @item -l @var{range}
14447 @itemx --range=@var{range}
14450 Set range @var{range} in the target security context.
14456 @node runcon invocation
14457 @section @command{runcon}: Run a command in specified SELinux context
14460 @cindex run with security context
14463 @command{runcon} runs file in specified SELinux security context.
14467 runcon @var{context} @var{command} [@var{args}]
14468 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14471 Run @var{command} with completely-specified @var{context}, or with
14472 current or transitioned security context modified by one or more of @var{level},
14473 @var{role}, @var{type} and @var{user}.
14475 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14476 is specified, the first argument is used as the complete context.
14477 Any additional arguments after @var{command}
14478 are interpreted as arguments to the command.
14480 With neither @var{context} nor @var{command}, print the current security context.
14482 The program accepts the following options. Also see @ref{Common options}.
14490 Compute process transition context before modifying.
14492 @item -u @var{user}
14493 @itemx --user=@var{user}
14496 Set user @var{user} in the target security context.
14498 @item -r @var{role}
14499 @itemx --role=@var{role}
14502 Set role @var{role} in the target security context.
14504 @item -t @var{type}
14505 @itemx --type=@var{type}
14508 Set type @var{type} in the target security context.
14510 @item -l @var{range}
14511 @itemx --range=@var{range}
14514 Set range @var{range} in the target security context.
14518 @cindex exit status of @command{runcon}
14522 126 if @var{command} is found but cannot be invoked
14523 127 if @command{runcon} itself fails or if @var{command} cannot be found
14524 the exit status of @var{command} otherwise
14527 @node Modified command invocation
14528 @chapter Modified command invocation
14530 @cindex modified command invocation
14531 @cindex invocation of commands, modified
14532 @cindex commands for invoking other commands
14534 This section describes commands that run other commands in some context
14535 different than the current one: a modified environment, as a different
14539 * chroot invocation:: Modify the root directory.
14540 * env invocation:: Modify environment variables.
14541 * nice invocation:: Modify niceness.
14542 * nohup invocation:: Immunize to hangups.
14543 * stdbuf invocation:: Modify buffering of standard streams.
14544 * su invocation:: Modify user and group ID.
14545 * timeout invocation:: Run with time limit.
14549 @node chroot invocation
14550 @section @command{chroot}: Run a command with a different root directory
14553 @cindex running a program in a specified root directory
14554 @cindex root directory, running a program in a specified
14556 @command{chroot} runs a command with a specified root directory.
14557 On many systems, only the super-user can do this.@footnote{However,
14558 some systems (e.g., FreeBSD) can be configured to allow certain regular
14559 users to use the @code{chroot} system call, and hence to run this program.
14560 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14561 underlying function is non-privileged due to lack of support in MS-Windows.}
14565 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14566 chroot @var{option}
14569 Ordinarily, file names are looked up starting at the root of the
14570 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14571 the directory @var{newroot} (which must exist) and then runs
14572 @var{command} with optional @var{args}. If @var{command} is not
14573 specified, the default is the value of the @env{SHELL} environment
14574 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14575 @var{command} must not be a special built-in utility
14576 (@pxref{Special built-in utilities}).
14578 The program accepts the following options. Also see @ref{Common options}.
14579 Options must precede operands.
14583 @itemx --userspec=@var{user}[:@var{group}]
14584 @opindex --userspec
14585 By default, @var{command} is run with the same credentials
14586 as the invoking process.
14587 Use this option to run it as a different @var{user} and/or with a
14588 different primary @var{group}.
14590 @itemx --groups=@var{groups}
14592 Use this option to specify the supplementary @var{groups} to be
14593 used by the new process.
14594 The items in the list (names or numeric IDs) must be separated by commas.
14598 Here are a few tips to help avoid common problems in using chroot.
14599 To start with a simple example, make @var{command} refer to a statically
14600 linked binary. If you were to use a dynamically linked executable, then
14601 you'd have to arrange to have the shared libraries in the right place under
14602 your new root directory.
14604 For example, if you create a statically linked @command{ls} executable,
14605 and put it in @file{/tmp/empty}, you can run this command as root:
14608 $ chroot /tmp/empty /ls -Rl /
14611 Then you'll see output like this:
14616 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14619 If you want to use a dynamically linked executable, say @command{bash},
14620 then first run @samp{ldd bash} to see what shared objects it needs.
14621 Then, in addition to copying the actual binary, also copy the listed
14622 files to the required positions under your intended new root directory.
14623 Finally, if the executable requires any other files (e.g., data, state,
14624 device files), copy them into place, too.
14626 @cindex exit status of @command{chroot}
14630 125 if @command{chroot} itself fails
14631 126 if @var{command} is found but cannot be invoked
14632 127 if @var{command} cannot be found
14633 the exit status of @var{command} otherwise
14637 @node env invocation
14638 @section @command{env}: Run a command in a modified environment
14641 @cindex environment, running a program in a modified
14642 @cindex modified environment, running a program in a
14643 @cindex running a program in a modified environment
14645 @command{env} runs a command with a modified environment. Synopses:
14648 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14649 [@var{command} [@var{args}]@dots{}]
14653 Operands of the form @samp{@var{variable}=@var{value}} set
14654 the environment variable @var{variable} to value @var{value}.
14655 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14656 to an empty value is different from unsetting it.
14657 These operands are evaluated left-to-right, so if two operands
14658 mention the same variable the earlier is ignored.
14660 Environment variable names can be empty, and can contain any
14661 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14662 However, it is wise to limit yourself to names that
14663 consist solely of underscores, digits, and @acronym{ASCII} letters,
14664 and that begin with a non-digit, as applications like the shell do not
14665 work well with other names.
14668 The first operand that does not contain the character @samp{=}
14669 specifies the program to invoke; it is
14670 searched for according to the @env{PATH} environment variable. Any
14671 remaining arguments are passed as arguments to that program.
14672 The program should not be a special built-in utility
14673 (@pxref{Special built-in utilities}).
14675 Modifications to @env{PATH} take effect prior to searching for
14676 @var{command}. Use caution when reducing @env{PATH}; behavior is
14677 not portable when @env{PATH} is undefined or omits key directories
14678 such as @file{/bin}.
14680 In the rare case that a utility contains a @samp{=} in the name, the
14681 only way to disambiguate it from a variable assignment is to use an
14682 intermediate command for @var{command}, and pass the problematic
14683 program name via @var{args}. For example, if @file{./prog=} is an
14684 executable in the current @env{PATH}:
14687 env prog= true # runs 'true', with prog= in environment
14688 env ./prog= true # runs 'true', with ./prog= in environment
14689 env -- prog= true # runs 'true', with prog= in environment
14690 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14691 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14694 @cindex environment, printing
14696 If no command name is specified following the environment
14697 specifications, the resulting environment is printed. This is like
14698 specifying the @command{printenv} program.
14700 For some examples, suppose the environment passed to @command{env}
14701 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14702 @samp{PATH=.:/gnubin:/hacks}:
14707 Output the current environment.
14709 $ env | LC_ALL=C sort
14712 PATH=.:/gnubin:/hacks
14716 Run @command{foo} with a reduced environment, preserving only the
14717 original @env{PATH} to avoid problems in locating @command{foo}.
14719 env - PATH="$PATH" foo
14723 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14724 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14725 that @command{foo} was found in the file system rather than as a shell
14732 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14733 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14734 @samp{DISPLAY=gnu:0}.
14736 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14740 Attempt to run the program @command{/energy/--} (as that is the only
14741 possible path search result); if the command exists, the environment
14742 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14743 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14745 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14751 The program accepts the following options. Also see @ref{Common options}.
14752 Options must precede operands.
14758 @item -u @var{name}
14759 @itemx --unset=@var{name}
14762 Remove variable @var{name} from the environment, if it was in the
14767 @itemx --ignore-environment
14770 @opindex --ignore-environment
14771 Start with an empty environment, ignoring the inherited environment.
14775 @cindex exit status of @command{env}
14779 0 if no @var{command} is specified and the environment is output
14780 125 if @command{env} itself fails
14781 126 if @var{command} is found but cannot be invoked
14782 127 if @var{command} cannot be found
14783 the exit status of @var{command} otherwise
14787 @node nice invocation
14788 @section @command{nice}: Run a command with modified niceness
14792 @cindex scheduling, affecting
14793 @cindex appropriate privileges
14795 @command{nice} prints or modifies a process's @dfn{niceness},
14796 a parameter that affects whether the process is scheduled favorably.
14800 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14803 If no arguments are given, @command{nice} prints the current niceness.
14804 Otherwise, @command{nice} runs the given @var{command} with its
14805 niceness adjusted. By default, its niceness is incremented by 10.
14807 Niceness values range at least from @minus{}20 (process has high priority
14808 and gets more resources, thus slowing down other processes) through 19
14809 (process has lower priority and runs slowly itself, but has less impact
14810 on the speed of other running processes). Some systems
14811 may have a wider range of nicenesses; conversely, other systems may
14812 enforce more restrictive limits. An attempt to set the niceness
14813 outside the supported range is treated as an attempt to use the
14814 minimum or maximum supported value.
14816 A niceness should not be confused with a scheduling priority, which
14817 lets applications determine the order in which threads are scheduled
14818 to run. Unlike a priority, a niceness is merely advice to the
14819 scheduler, which the scheduler is free to ignore. Also, as a point of
14820 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14821 terms of a @dfn{nice value}, which is the nonnegative difference
14822 between a niceness and the minimum niceness. Though @command{nice}
14823 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14824 term ``niceness'' for compatibility with historical practice.
14826 @var{command} must not be a special built-in utility (@pxref{Special
14827 built-in utilities}).
14829 @mayConflictWithShellBuiltIn{nice}
14831 The program accepts the following option. Also see @ref{Common options}.
14832 Options must precede operands.
14835 @item -n @var{adjustment}
14836 @itemx --adjustment=@var{adjustment}
14838 @opindex --adjustment
14839 Add @var{adjustment} instead of 10 to the command's niceness. If
14840 @var{adjustment} is negative and you lack appropriate privileges,
14841 @command{nice} issues a warning but otherwise acts as if you specified
14844 For compatibility @command{nice} also supports an obsolete
14845 option syntax @option{-@var{adjustment}}. New scripts should use
14846 @option{-n @var{adjustment}} instead.
14850 @cindex exit status of @command{nice}
14854 0 if no @var{command} is specified and the niceness is output
14855 125 if @command{nice} itself fails
14856 126 if @var{command} is found but cannot be invoked
14857 127 if @var{command} cannot be found
14858 the exit status of @var{command} otherwise
14861 It is sometimes useful to run a non-interactive program with reduced niceness.
14864 $ nice factor 4611686018427387903
14867 Since @command{nice} prints the current niceness,
14868 you can invoke it through itself to demonstrate how it works.
14870 The default behavior is to increase the niceness by @samp{10}:
14881 The @var{adjustment} is relative to the current niceness. In the
14882 next example, the first @command{nice} invocation runs the second one
14883 with niceness 10, and it in turn runs the final one with a niceness
14887 $ nice nice -n 3 nice
14891 Specifying a niceness larger than the supported range
14892 is the same as specifying the maximum supported value:
14895 $ nice -n 10000000000 nice
14899 Only a privileged user may run a process with lower niceness:
14903 nice: cannot set niceness: Permission denied
14905 $ sudo nice -n -1 nice
14910 @node nohup invocation
14911 @section @command{nohup}: Run a command immune to hangups
14914 @cindex hangups, immunity to
14915 @cindex immunity to hangups
14916 @cindex logging out and continuing to run
14919 @command{nohup} runs the given @var{command} with hangup signals ignored,
14920 so that the command can continue running in the background after you log
14924 nohup @var{command} [@var{arg}]@dots{}
14927 If standard input is a terminal, it is redirected from
14928 @file{/dev/null} so that terminal sessions do not mistakenly consider
14929 the terminal to be used by the command. This is a @acronym{GNU}
14930 extension; programs intended to be portable to non-@acronym{GNU} hosts
14931 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
14935 If standard output is a terminal, the command's standard output is appended
14936 to the file @file{nohup.out}; if that cannot be written to, it is appended
14937 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
14938 command is not run.
14939 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
14940 @command{nohup} is made readable and writable only to the user,
14941 regardless of the current umask settings.
14943 If standard error is a terminal, it is normally redirected to the same file
14944 descriptor as the (possibly-redirected) standard output.
14945 However, if standard output is closed, standard error terminal output
14946 is instead appended to the file @file{nohup.out} or
14947 @file{$HOME/nohup.out} as above.
14949 To capture the command's output to a file other than @file{nohup.out}
14950 you can redirect it. For example, to capture the output of
14954 nohup make > make.log
14957 @command{nohup} does not automatically put the command it runs in the
14958 background; you must do that explicitly, by ending the command line
14959 with an @samp{&}. Also, @command{nohup} does not alter the
14960 niceness of @var{command}; use @command{nice} for that,
14961 e.g., @samp{nohup nice @var{command}}.
14963 @var{command} must not be a special built-in utility (@pxref{Special
14964 built-in utilities}).
14966 The only options are @option{--help} and @option{--version}. @xref{Common
14967 options}. Options must precede operands.
14969 @cindex exit status of @command{nohup}
14973 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
14974 126 if @var{command} is found but cannot be invoked
14975 127 if @var{command} cannot be found
14976 the exit status of @var{command} otherwise
14979 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
14983 @node stdbuf invocation
14984 @section @command{stdbuf}: Run a command with modified I/O stream buffering
14987 @cindex standard streams, buffering
14988 @cindex line buffered
14990 @command{stdbuf} allows one to modify the buffering operations of the
14991 three standard I/O streams associated with a program. Synopsis:
14994 stdbuf @var{option}@dots{} @var{command}
14997 Any additional @var{arg}s are passed as additional arguments to the
15000 The program accepts the following options. Also see @ref{Common options}.
15004 @item -i @var{mode}
15005 @itemx --input=@var{mode}
15008 Adjust the standard input stream buffering.
15010 @item -o @var{mode}
15011 @itemx --output=@var{mode}
15014 Adjust the standard output stream buffering.
15016 @item -e @var{mode}
15017 @itemx --error=@var{mode}
15020 Adjust the standard error stream buffering.
15024 The @var{mode} can be specified as follows:
15029 Set the stream to line buffered mode.
15030 In this mode data is coalesced until a newline is output or
15031 input is read from any stream attached to a terminal device.
15032 This option is invalid with standard input.
15035 Disable buffering of the selected stream.
15036 In this mode data is output immediately and only the
15037 amount of data requested is read from input.
15040 Specify the size of the buffer to use in fully buffered mode.
15041 @multiplierSuffixesNoBlocks{size}
15045 NOTE: If @var{command} adjusts the buffering of its standard streams
15046 (@command{tee} does for e.g.) then that will override corresponding settings
15047 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15048 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15049 by @command{stdbuf} settings.
15051 @cindex exit status of @command{stdbuf}
15055 125 if @command{stdbuf} itself fails
15056 126 if @var{command} is found but cannot be invoked
15057 127 if @var{command} cannot be found
15058 the exit status of @var{command} otherwise
15062 @node su invocation
15063 @section @command{su}: Run a command with substitute user and group ID
15066 @cindex substitute user and group IDs
15067 @cindex user ID, switching
15068 @cindex super-user, becoming
15069 @cindex root, becoming
15071 @command{su} allows one user to temporarily become another user. It runs a
15072 command (often an interactive shell) with the real and effective user
15073 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15076 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15079 @cindex passwd entry, and @command{su} shell
15081 @flindex /etc/passwd
15082 If no @var{user} is given, the default is @code{root}, the super-user.
15083 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15084 @file{/bin/sh} if none is specified there. If @var{user} has a
15085 password, @command{su} prompts for the password unless run by a user with
15086 effective user ID of zero (the super-user).
15092 @cindex login shell
15093 By default, @command{su} does not change the current directory.
15094 It sets the environment variables @env{HOME} and @env{SHELL}
15095 from the password entry for @var{user}, and if @var{user} is not
15096 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15097 By default, the shell is not a login shell.
15099 Any additional @var{arg}s are passed as additional arguments to the
15102 @cindex @option{-su}
15103 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15104 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15105 to certain shells, etc.).
15108 @command{su} can optionally be compiled to use @code{syslog} to report
15109 failed, and optionally successful, @command{su} attempts. (If the system
15110 supports @code{syslog}.) However, GNU @command{su} does not check if the
15111 user is a member of the @code{wheel} group; see below.
15113 The program accepts the following options. Also see @ref{Common options}.
15116 @item -c @var{command}
15117 @itemx --command=@var{command}
15120 Pass @var{command}, a single command line to run, to the shell with
15121 a @option{-c} option instead of starting an interactive shell.
15128 @cindex file name pattern expansion, disabled
15129 @cindex globbing, disabled
15130 Pass the @option{-f} option to the shell. This probably only makes sense
15131 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15132 option prevents reading the startup file (@file{.cshrc}). With
15133 Bourne-like shells, the @option{-f} option disables file name pattern
15134 expansion (globbing), which is not likely to be useful.
15142 @c other variables already indexed above
15145 @cindex login shell, creating
15146 Make the shell a login shell. This means the following. Unset all
15147 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15148 (which are set as described above), and @env{USER} and @env{LOGNAME}
15149 (which are set, even for the super-user, as described above), and set
15150 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15151 directory. Prepend @samp{-} to the shell's name, intended to make it
15152 read its login startup file(s).
15156 @itemx --preserve-environment
15159 @opindex --preserve-environment
15160 @cindex environment, preserving
15161 @flindex /etc/shells
15162 @cindex restricted shell
15163 Do not change the environment variables @env{HOME}, @env{USER},
15164 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15165 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15166 entry, unless the user running @command{su} is not the super-user and
15167 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15168 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15169 if that file does not exist. Parts of what this option does can be
15170 overridden by @option{--login} and @option{--shell}.
15172 @item -s @var{shell}
15173 @itemx --shell=@var{shell}
15176 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15177 unless the user running @command{su} is not the super-user and @var{user}'s
15178 shell is restricted (see @option{-m} just above).
15182 @cindex exit status of @command{su}
15186 125 if @command{su} itself fails
15187 126 if subshell is found but cannot be invoked
15188 127 if subshell cannot be found
15189 the exit status of the subshell otherwise
15192 @cindex wheel group, not supported
15193 @cindex group wheel, not supported
15195 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15197 (This section is by Richard Stallman.)
15201 Sometimes a few of the users try to hold total power over all the
15202 rest. For example, in 1984, a few users at the MIT AI lab decided to
15203 seize power by changing the operator password on the Twenex system and
15204 keeping it secret from everyone else. (I was able to thwart this coup
15205 and give power back to the users by patching the kernel, but I
15206 wouldn't know how to do that in Unix.)
15208 However, occasionally the rulers do tell someone. Under the usual
15209 @command{su} mechanism, once someone learns the root password who
15210 sympathizes with the ordinary users, he or she can tell the rest. The
15211 ``wheel group'' feature would make this impossible, and thus cement the
15212 power of the rulers.
15214 I'm on the side of the masses, not that of the rulers. If you are
15215 used to supporting the bosses and sysadmins in whatever they do, you
15216 might find this idea strange at first.
15219 @node timeout invocation
15220 @section @command{timeout}: Run a command with a time limit
15224 @cindex run commands with bounded time
15226 @command{timeout} runs the given @var{command} and kills it if it is
15227 still running after the specified time interval. Synopsis:
15230 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15233 @var{command} must not be a special built-in utility (@pxref{Special
15234 built-in utilities}).
15236 The program accepts the following options. Also see @ref{Common options}.
15237 Options must precede operands.
15240 @item -k @var{duration}
15241 @itemx --kill-after=@var{duration}
15243 @opindex --kill-after
15244 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15245 signal, after the specified @var{duration}. Without this option, if the
15246 selected signal proves not to be fatal, @command{timeout} does not kill
15249 @item -s @var{signal}
15250 @itemx --signal=@var{signal}
15253 Send this @var{signal} to @var{command} on timeout, rather than the
15254 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15255 or a number. Also see @xref{Signal specifications}.
15259 @var{duration} is an integer followed by an optional unit:
15261 @samp{s} for seconds (the default)
15262 @samp{m} for minutes
15266 A duration of 0 disables the associated timeout.
15268 @cindex exit status of @command{timeout}
15272 124 if @var{command} times out
15273 125 if @command{timeout} itself fails
15274 126 if @var{command} is found but cannot be invoked
15275 127 if @var{command} cannot be found
15276 the exit status of @var{command} otherwise
15280 @node Process control
15281 @chapter Process control
15283 @cindex processes, commands for controlling
15284 @cindex commands for controlling processes
15287 * kill invocation:: Sending a signal to processes.
15291 @node kill invocation
15292 @section @command{kill}: Send a signal to processes
15295 @cindex send a signal to processes
15297 The @command{kill} command sends a signal to processes, causing them
15298 to terminate or otherwise act upon receiving the signal in some way.
15299 Alternatively, it lists information about signals. Synopses:
15302 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15303 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15306 @mayConflictWithShellBuiltIn{kill}
15308 The first form of the @command{kill} command sends a signal to all
15309 @var{pid} arguments. The default signal to send if none is specified
15310 is @samp{TERM}. The special signal number @samp{0} does not denote a
15311 valid signal, but can be used to test whether the @var{pid} arguments
15312 specify processes to which a signal could be sent.
15314 If @var{pid} is positive, the signal is sent to the process with the
15315 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15316 processes in the process group of the current process. If @var{pid}
15317 is @minus{}1, the signal is sent to all processes for which the user has
15318 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15319 is sent to all processes in the process group that equals the absolute
15320 value of @var{pid}.
15322 If @var{pid} is not positive, a system-dependent set of system
15323 processes is excluded from the list of processes to which the signal
15326 If a negative @var{pid} argument is desired as the first one, it
15327 should be preceded by @option{--}. However, as a common extension to
15328 @acronym{POSIX}, @option{--} is not required with @samp{kill
15329 -@var{signal} -@var{pid}}. The following commands are equivalent:
15338 The first form of the @command{kill} command succeeds if every @var{pid}
15339 argument specifies at least one process that the signal was sent to.
15341 The second form of the @command{kill} command lists signal information.
15342 Either the @option{-l} or @option{--list} option, or the @option{-t}
15343 or @option{--table} option must be specified. Without any
15344 @var{signal} argument, all supported signals are listed. The output
15345 of @option{-l} or @option{--list} is a list of the signal names, one
15346 per line; if @var{signal} is already a name, the signal number is
15347 printed instead. The output of @option{-t} or @option{--table} is a
15348 table of signal numbers, names, and descriptions. This form of the
15349 @command{kill} command succeeds if all @var{signal} arguments are valid
15350 and if there is no output error.
15352 The @command{kill} command also supports the @option{--help} and
15353 @option{--version} options. @xref{Common options}.
15355 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15356 number like @samp{1}, or an exit status of a process terminated by the
15357 signal. A signal name can be given in canonical form or prefixed by
15358 @samp{SIG}. The case of the letters is ignored, except for the
15359 @option{-@var{signal}} option which must use upper case to avoid
15360 ambiguity with lower case option letters. For a list of supported
15361 signal names and numbers see @xref{Signal specifications}.
15366 @cindex delaying commands
15367 @cindex commands for delaying
15369 @c Perhaps @command{wait} or other commands should be described here also?
15372 * sleep invocation:: Delay for a specified time.
15376 @node sleep invocation
15377 @section @command{sleep}: Delay for a specified time
15380 @cindex delay for a specified time
15382 @command{sleep} pauses for an amount of time specified by the sum of
15383 the values of the command line arguments.
15387 sleep @var{number}[smhd]@dots{}
15391 Each argument is a number followed by an optional unit; the default
15392 is seconds. The units are:
15405 Historical implementations of @command{sleep} have required that
15406 @var{number} be an integer, and only accepted a single argument
15407 without a suffix. However, GNU @command{sleep} accepts
15408 arbitrary floating point numbers (using a period before any fractional
15411 The only options are @option{--help} and @option{--version}. @xref{Common
15414 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15415 @mayConflictWithShellBuiltIn{sleep}
15420 @node Numeric operations
15421 @chapter Numeric operations
15423 @cindex numeric operations
15424 These programs do numerically-related operations.
15427 * factor invocation:: Show factors of numbers.
15428 * seq invocation:: Print sequences of numbers.
15432 @node factor invocation
15433 @section @command{factor}: Print prime factors
15436 @cindex prime factors
15438 @command{factor} prints prime factors. Synopses:
15441 factor [@var{number}]@dots{}
15442 factor @var{option}
15445 If no @var{number} is specified on the command line, @command{factor} reads
15446 numbers from standard input, delimited by newlines, tabs, or spaces.
15448 The @command{factor} command supports only a small number of options:
15452 Print a short help on standard output, then exit without further
15456 Print the program version on standard output, then exit without further
15460 Factoring the product of the eighth and ninth Mersenne primes
15461 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15464 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15465 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15466 4951760154835678088235319297: 2147483647 2305843009213693951
15470 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15471 about 20 seconds on the same machine.
15473 Factoring large numbers is, in general, hard. The Pollard Rho
15474 algorithm used by @command{factor} is particularly effective for
15475 numbers with relatively small factors. If you wish to factor large
15476 numbers which do not have small factors (for example, numbers which
15477 are the product of two large primes), other methods are far better.
15479 If @command{factor} is built without using GNU MP, only
15480 single-precision arithmetic is available, and so large numbers
15481 (typically @math{2^{64}} and above) will not be supported. The single-precision
15482 code uses an algorithm which is designed for factoring smaller
15488 @node seq invocation
15489 @section @command{seq}: Print numeric sequences
15492 @cindex numeric sequences
15493 @cindex sequence of numbers
15495 @command{seq} prints a sequence of numbers to standard output. Synopses:
15498 seq [@var{option}]@dots{} @var{last}
15499 seq [@var{option}]@dots{} @var{first} @var{last}
15500 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15503 @command{seq} prints the numbers from @var{first} to @var{last} by
15504 @var{increment}. By default, each number is printed on a separate line.
15505 When @var{increment} is not specified, it defaults to @samp{1},
15506 even when @var{first} is larger than @var{last}.
15507 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15508 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15509 Floating-point numbers
15510 may be specified (using a period before any fractional digits).
15512 The program accepts the following options. Also see @ref{Common options}.
15513 Options must precede operands.
15516 @item -f @var{format}
15517 @itemx --format=@var{format}
15518 @opindex -f @var{format}
15519 @opindex --format=@var{format}
15520 @cindex formatting of numbers in @command{seq}
15521 Print all numbers using @var{format}.
15522 @var{format} must contain exactly one of the @samp{printf}-style
15523 floating point conversion specifications @samp{%a}, @samp{%e},
15524 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15525 The @samp{%} may be followed by zero or more flags taken from the set
15526 @samp{-+#0 '}, then an optional width containing one or more digits,
15527 then an optional precision consisting of a @samp{.} followed by zero
15528 or more digits. @var{format} may also contain any number of @samp{%%}
15529 conversion specifications. All conversion specifications have the
15530 same meaning as with @samp{printf}.
15532 The default format is derived from @var{first}, @var{step}, and
15533 @var{last}. If these all use a fixed point decimal representation,
15534 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15535 precision that can represent the output numbers exactly. Otherwise,
15536 the default format is @samp{%g}.
15538 @item -s @var{string}
15539 @itemx --separator=@var{string}
15540 @cindex separator for numbers in @command{seq}
15541 Separate numbers with @var{string}; default is a newline.
15542 The output always terminates with a newline.
15545 @itemx --equal-width
15546 Print all numbers with the same width, by padding with leading zeros.
15547 @var{first}, @var{step}, and @var{last} should all use a fixed point
15548 decimal representation.
15549 (To have other kinds of padding, use @option{--format}).
15553 You can get finer-grained control over output with @option{-f}:
15556 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15562 If you want hexadecimal integer output, you can use @command{printf}
15563 to perform the conversion:
15566 $ printf '%x\n' `seq 1048575 1024 1050623`
15572 For very long lists of numbers, use xargs to avoid
15573 system limitations on the length of an argument list:
15576 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15582 To generate octal output, use the printf @code{%o} format instead
15585 On most systems, seq can produce whole-number output for values up to
15586 at least @math{2^{53}}. Larger integers are approximated. The details
15587 differ depending on your floating-point implementation, but a common
15588 case is that @command{seq} works with integers through @math{2^{64}},
15589 and larger integers may not be numerically correct:
15592 $ seq 18446744073709551616 1 18446744073709551618
15593 18446744073709551616
15594 18446744073709551616
15595 18446744073709551618
15598 Be careful when using @command{seq} with outlandish values: otherwise
15599 you may see surprising results, as @command{seq} uses floating point
15600 internally. For example, on the x86 platform, where the internal
15601 representation uses a 64-bit fraction, the command:
15604 seq 1 0.0000000000000000001 1.0000000000000000009
15607 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15612 @node File permissions
15613 @chapter File permissions
15616 @include getdate.texi
15620 @node Opening the software toolbox
15621 @chapter Opening the Software Toolbox
15623 An earlier version of this chapter appeared in
15624 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15625 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15626 It was written by Arnold Robbins.
15629 * Toolbox introduction:: Toolbox introduction
15630 * I/O redirection:: I/O redirection
15631 * The who command:: The @command{who} command
15632 * The cut command:: The @command{cut} command
15633 * The sort command:: The @command{sort} command
15634 * The uniq command:: The @command{uniq} command
15635 * Putting the tools together:: Putting the tools together
15639 @node Toolbox introduction
15640 @unnumberedsec Toolbox Introduction
15642 This month's column is only peripherally related to the GNU Project, in
15643 that it describes a number of the GNU tools on your GNU/Linux system and how they
15644 might be used. What it's really about is the ``Software Tools'' philosophy
15645 of program development and usage.
15647 The software tools philosophy was an important and integral concept
15648 in the initial design and development of Unix (of which Linux and GNU are
15649 essentially clones). Unfortunately, in the modern day press of
15650 Internetworking and flashy GUIs, it seems to have fallen by the
15651 wayside. This is a shame, since it provides a powerful mental model
15652 for solving many kinds of problems.
15654 Many people carry a Swiss Army knife around in their pants pockets (or
15655 purse). A Swiss Army knife is a handy tool to have: it has several knife
15656 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15657 a number of other things on it. For the everyday, small miscellaneous jobs
15658 where you need a simple, general purpose tool, it's just the thing.
15660 On the other hand, an experienced carpenter doesn't build a house using
15661 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15662 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15663 exactly when and where to use each tool; you won't catch him hammering nails
15664 with the handle of his screwdriver.
15666 The Unix developers at Bell Labs were all professional programmers and trained
15667 computer scientists. They had found that while a one-size-fits-all program
15668 might appeal to a user because there's only one program to use, in practice
15673 difficult to write,
15676 difficult to maintain and
15680 difficult to extend to meet new situations.
15683 Instead, they felt that programs should be specialized tools. In short, each
15684 program ``should do one thing well.'' No more and no less. Such programs are
15685 simpler to design, write, and get right---they only do one thing.
15687 Furthermore, they found that with the right machinery for hooking programs
15688 together, that the whole was greater than the sum of the parts. By combining
15689 several special purpose programs, you could accomplish a specific task
15690 that none of the programs was designed for, and accomplish it much more
15691 quickly and easily than if you had to write a special purpose program.
15692 We will see some (classic) examples of this further on in the column.
15693 (An important additional point was that, if necessary, take a detour
15694 and build any software tools you may need first, if you don't already
15695 have something appropriate in the toolbox.)
15697 @node I/O redirection
15698 @unnumberedsec I/O Redirection
15700 Hopefully, you are familiar with the basics of I/O redirection in the
15701 shell, in particular the concepts of ``standard input,'' ``standard output,''
15702 and ``standard error''. Briefly, ``standard input'' is a data source, where
15703 data comes from. A program should not need to either know or care if the
15704 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15705 card reader. Similarly, ``standard output'' is a data sink, where data goes
15706 to. The program should neither know nor care where this might be.
15707 Programs that only read their standard input, do something to the data,
15708 and then send it on, are called @dfn{filters}, by analogy to filters in a
15711 With the Unix shell, it's very easy to set up data pipelines:
15714 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15717 We start out by creating the raw data; each filter applies some successive
15718 transformation to the data, until by the time it comes out of the pipeline,
15719 it is in the desired form.
15721 This is fine and good for standard input and standard output. Where does the
15722 standard error come in to play? Well, think about @command{filter1} in
15723 the pipeline above. What happens if it encounters an error in the data it
15724 sees? If it writes an error message to standard output, it will just
15725 disappear down the pipeline into @command{filter2}'s input, and the
15726 user will probably never see it. So programs need a place where they can send
15727 error messages so that the user will notice them. This is standard error,
15728 and it is usually connected to your console or window, even if you have
15729 redirected standard output of your program away from your screen.
15731 For filter programs to work together, the format of the data has to be
15732 agreed upon. The most straightforward and easiest format to use is simply
15733 lines of text. Unix data files are generally just streams of bytes, with
15734 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15735 conventionally called a ``newline'' in the Unix literature. (This is
15736 @code{'\n'} if you're a C programmer.) This is the format used by all
15737 the traditional filtering programs. (Many earlier operating systems
15738 had elaborate facilities and special purpose programs for managing
15739 binary data. Unix has always shied away from such things, under the
15740 philosophy that it's easiest to simply be able to view and edit your
15741 data with a text editor.)
15743 OK, enough introduction. Let's take a look at some of the tools, and then
15744 we'll see how to hook them together in interesting ways. In the following
15745 discussion, we will only present those command line options that interest
15746 us. As you should always do, double check your system documentation
15747 for the full story.
15749 @node The who command
15750 @unnumberedsec The @command{who} Command
15752 The first program is the @command{who} command. By itself, it generates a
15753 list of the users who are currently logged in. Although I'm writing
15754 this on a single-user system, we'll pretend that several people are
15759 @print{} arnold console Jan 22 19:57
15760 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15761 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15762 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15765 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15766 There are three people logged in, and I am logged in twice. On traditional
15767 Unix systems, user names are never more than eight characters long. This
15768 little bit of trivia will be useful later. The output of @command{who} is nice,
15769 but the data is not all that exciting.
15771 @node The cut command
15772 @unnumberedsec The @command{cut} Command
15774 The next program we'll look at is the @command{cut} command. This program
15775 cuts out columns or fields of input data. For example, we can tell it
15776 to print just the login name and full name from the @file{/etc/passwd}
15777 file. The @file{/etc/passwd} file has seven fields, separated by
15781 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15784 To get the first and fifth fields, we would use @command{cut} like this:
15787 $ cut -d: -f1,5 /etc/passwd
15788 @print{} root:Operator
15790 @print{} arnold:Arnold D. Robbins
15791 @print{} miriam:Miriam A. Robbins
15795 With the @option{-c} option, @command{cut} will cut out specific characters
15796 (i.e., columns) in the input lines. This is useful for input data
15797 that has fixed width fields, and does not have a field separator. For
15798 example, list the Monday dates for the current month:
15800 @c Is using cal ok? Looked at gcal, but I don't like it.
15811 @node The sort command
15812 @unnumberedsec The @command{sort} Command
15814 Next we'll look at the @command{sort} command. This is one of the most
15815 powerful commands on a Unix-style system; one that you will often find
15816 yourself using when setting up fancy data plumbing.
15819 command reads and sorts each file named on the command line. It then
15820 merges the sorted data and writes it to standard output. It will read
15821 standard input if no files are given on the command line (thus
15822 making it into a filter). The sort is based on the character collating
15823 sequence or based on user-supplied ordering criteria.
15826 @node The uniq command
15827 @unnumberedsec The @command{uniq} Command
15829 Finally (at least for now), we'll look at the @command{uniq} program. When
15830 sorting data, you will often end up with duplicate lines, lines that
15831 are identical. Usually, all you need is one instance of each line.
15832 This is where @command{uniq} comes in. The @command{uniq} program reads its
15833 standard input. It prints only one
15834 copy of each repeated line. It does have several options. Later on,
15835 we'll use the @option{-c} option, which prints each unique line, preceded
15836 by a count of the number of times that line occurred in the input.
15839 @node Putting the tools together
15840 @unnumberedsec Putting the Tools Together
15842 Now, let's suppose this is a large ISP server system with dozens of users
15843 logged in. The management wants the system administrator to write a program that will
15844 generate a sorted list of logged in users. Furthermore, even if a user
15845 is logged in multiple times, his or her name should only show up in the
15848 The administrator could sit down with the system documentation and write a C
15849 program that did this. It would take perhaps a couple of hundred lines
15850 of code and about two hours to write it, test it, and debug it.
15851 However, knowing the software toolbox, the administrator can instead start out
15852 by generating just a list of logged on users:
15862 Next, sort the list:
15865 $ who | cut -c1-8 | sort
15872 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15875 $ who | cut -c1-8 | sort | uniq
15881 The @command{sort} command actually has a @option{-u} option that does what
15882 @command{uniq} does. However, @command{uniq} has other uses for which one
15883 cannot substitute @samp{sort -u}.
15885 The administrator puts this pipeline into a shell script, and makes it available for
15886 all the users on the system (@samp{#} is the system administrator,
15887 or @code{root}, prompt):
15890 # cat > /usr/local/bin/listusers
15891 who | cut -c1-8 | sort | uniq
15893 # chmod +x /usr/local/bin/listusers
15896 There are four major points to note here. First, with just four
15897 programs, on one command line, the administrator was able to save about two
15898 hours worth of work. Furthermore, the shell pipeline is just about as
15899 efficient as the C program would be, and it is much more efficient in
15900 terms of programmer time. People time is much more expensive than
15901 computer time, and in our modern ``there's never enough time to do
15902 everything'' society, saving two hours of programmer time is no mean
15905 Second, it is also important to emphasize that with the
15906 @emph{combination} of the tools, it is possible to do a special
15907 purpose job never imagined by the authors of the individual programs.
15909 Third, it is also valuable to build up your pipeline in stages, as we did here.
15910 This allows you to view the data at each stage in the pipeline, which helps
15911 you acquire the confidence that you are indeed using these tools correctly.
15913 Finally, by bundling the pipeline in a shell script, other users can use
15914 your command, without having to remember the fancy plumbing you set up for
15915 them. In terms of how you run them, shell scripts and compiled programs are
15918 After the previous warm-up exercise, we'll look at two additional, more
15919 complicated pipelines. For them, we need to introduce two more tools.
15921 The first is the @command{tr} command, which stands for ``transliterate.''
15922 The @command{tr} command works on a character-by-character basis, changing
15923 characters. Normally it is used for things like mapping upper case to
15927 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
15928 @print{} this example has mixed case!
15931 There are several options of interest:
15935 work on the complement of the listed characters, i.e.,
15936 operations apply to characters not in the given set
15939 delete characters in the first set from the output
15942 squeeze repeated characters in the output into just one character.
15945 We will be using all three options in a moment.
15947 The other command we'll look at is @command{comm}. The @command{comm}
15948 command takes two sorted input files as input data, and prints out the
15949 files' lines in three columns. The output columns are the data lines
15950 unique to the first file, the data lines unique to the second file, and
15951 the data lines that are common to both. The @option{-1}, @option{-2}, and
15952 @option{-3} command line options @emph{omit} the respective columns. (This is
15953 non-intuitive and takes a little getting used to.) For example:
15975 The file name @file{-} tells @command{comm} to read standard input
15976 instead of a regular file.
15978 Now we're ready to build a fancy pipeline. The first application is a word
15979 frequency counter. This helps an author determine if he or she is over-using
15982 The first step is to change the case of all the letters in our input file
15983 to one case. ``The'' and ``the'' are the same word when doing counting.
15986 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
15989 The next step is to get rid of punctuation. Quoted words and unquoted words
15990 should be treated identically; it's easiest to just get the punctuation out of
15994 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
15997 The second @command{tr} command operates on the complement of the listed
15998 characters, which are all the letters, the digits, the underscore, and
15999 the blank. The @samp{\n} represents the newline character; it has to
16000 be left alone. (The @acronym{ASCII} tab character should also be included for
16001 good measure in a production script.)
16003 At this point, we have data consisting of words separated by blank space.
16004 The words only contain alphanumeric characters (and the underscore). The
16005 next step is break the data apart so that we have one word per line. This
16006 makes the counting operation much easier, as we will see shortly.
16009 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16010 > tr -s ' ' '\n' | ...
16013 This command turns blanks into newlines. The @option{-s} option squeezes
16014 multiple newline characters in the output into just one. This helps us
16015 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16016 This is what the shell prints when it notices you haven't finished
16017 typing in all of a command.)
16019 We now have data consisting of one word per line, no punctuation, all one
16020 case. We're ready to count each word:
16023 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16024 > tr -s ' ' '\n' | sort | uniq -c | ...
16027 At this point, the data might look something like this:
16040 The output is sorted by word, not by count! What we want is the most
16041 frequently used words first. Fortunately, this is easy to accomplish,
16042 with the help of two more @command{sort} options:
16046 do a numeric sort, not a textual one
16049 reverse the order of the sort
16052 The final pipeline looks like this:
16055 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16056 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16065 Whew! That's a lot to digest. Yet, the same principles apply. With six
16066 commands, on two lines (really one long one split for convenience), we've
16067 created a program that does something interesting and useful, in much
16068 less time than we could have written a C program to do the same thing.
16070 A minor modification to the above pipeline can give us a simple spelling
16071 checker! To determine if you've spelled a word correctly, all you have to
16072 do is look it up in a dictionary. If it is not there, then chances are
16073 that your spelling is incorrect. So, we need a dictionary.
16074 The conventional location for a dictionary is @file{/usr/dict/words}.
16075 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16076 revision of this article.}
16077 this is a sorted, 45,402 word dictionary.
16079 Now, how to compare our file with the dictionary? As before, we generate
16080 a sorted list of words, one per line:
16083 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16084 > tr -s ' ' '\n' | sort -u | ...
16087 Now, all we need is a list of words that are @emph{not} in the
16088 dictionary. Here is where the @command{comm} command comes in.
16091 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16092 > tr -s ' ' '\n' | sort -u |
16093 > comm -23 - /usr/dict/words
16096 The @option{-2} and @option{-3} options eliminate lines that are only in the
16097 dictionary (the second file), and lines that are in both files. Lines
16098 only in the first file (standard input, our stream of words), are
16099 words that are not in the dictionary. These are likely candidates for
16100 spelling errors. This pipeline was the first cut at a production
16101 spelling checker on Unix.
16103 There are some other tools that deserve brief mention.
16107 search files for text that matches a regular expression
16110 count lines, words, characters
16113 a T-fitting for data pipes, copies data to files and to standard output
16116 the stream editor, an advanced tool
16119 a data manipulation language, another advanced tool
16122 The software tools philosophy also espoused the following bit of
16123 advice: ``Let someone else do the hard part.'' This means, take
16124 something that gives you most of what you need, and then massage it the
16125 rest of the way until it's in the form that you want.
16131 Each program should do one thing well. No more, no less.
16134 Combining programs with appropriate plumbing leads to results where
16135 the whole is greater than the sum of the parts. It also leads to novel
16136 uses of programs that the authors might never have imagined.
16139 Programs should never print extraneous header or trailer data, since these
16140 could get sent on down a pipeline. (A point we didn't mention earlier.)
16143 Let someone else do the hard part.
16146 Know your toolbox! Use each program appropriately. If you don't have an
16147 appropriate tool, build one.
16150 As of this writing, all the programs we've discussed are available via
16151 anonymous @command{ftp} from: @*
16152 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16153 be more recent versions available now.)
16155 None of what I have presented in this column is new. The Software Tools
16156 philosophy was first introduced in the book @cite{Software Tools}, by
16157 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16158 This book showed how to write and use software tools. It was written in
16159 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16160 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16161 was. The last chapter presented a @command{ratfor} to FORTRAN
16162 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16163 lot like C; if you know C, you won't have any problem following the
16166 In 1981, the book was updated and made available as @cite{Software Tools
16167 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16168 still in print and are well worth
16169 reading if you're a programmer. They certainly made a major change in
16170 how I view programming.
16172 The programs in both books are available from
16173 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16174 For a number of years, there was an active
16175 Software Tools Users Group, whose members had ported the original
16176 @command{ratfor} programs to essentially every computer system with a
16177 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16178 as Unix began to spread beyond universities.
16180 With the current proliferation of GNU code and other clones of Unix programs,
16181 these programs now receive little attention; modern C versions are
16182 much more efficient and do more than these programs do. Nevertheless, as
16183 exposition of good programming style, and evangelism for a still-valuable
16184 philosophy, these books are unparalleled, and I recommend them highly.
16186 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16187 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16189 @node GNU Free Documentation License
16190 @appendix GNU Free Documentation License
16194 @node Concept index
16201 @c Local variables:
16202 @c texinfo-column-for-description: 32