3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2010 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
479 Opening the software toolbox
481 * Toolbox introduction:: Toolbox introduction
482 * I/O redirection:: I/O redirection
483 * The who command:: The @command{who} command
484 * The cut command:: The @command{cut} command
485 * The sort command:: The @command{sort} command
486 * The uniq command:: The @command{uniq} command
487 * Putting the tools together:: Putting the tools together
491 * GNU Free Documentation License:: Copying and sharing this manual
498 @chapter Introduction
500 This manual is a work in progress: many sections make no attempt to explain
501 basic concepts in a way suitable for novices. Thus, if you are interested,
502 please get involved in improving this manual. The entire @sc{gnu} community
505 @cindex @acronym{POSIX}
506 The @sc{gnu} utilities documented here are mostly compatible with the
507 @acronym{POSIX} standard.
508 @cindex bugs, reporting
509 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
510 to include the version number, machine architecture, input files, and
511 any other information needed to reproduce the bug: your input, what you
512 expected, what you got, and why it is wrong. Diffs are welcome, but
513 please include a description of the problem as well, since this is
514 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
520 @cindex MacKenzie, D.
523 This manual was originally derived from the Unix man pages in the
524 distributions, which were written by David MacKenzie and updated by Jim
525 Meyering. What you are reading now is the authoritative documentation
526 for these utilities; the man pages are no longer being maintained. The
527 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
528 Pinard did the initial conversion to Texinfo format. Karl Berry did the
529 indexing, some reorganization, and editing of the results. Brian
530 Youmans of the Free Software Foundation office staff combined the
531 manuals for textutils, fileutils, and sh-utils to produce the present
532 omnibus manual. Richard Stallman contributed his usual invaluable
533 insights to the overall process.
536 @chapter Common options
540 @itemx @w{@kbd{--backup}[=@var{method}]}
543 @vindex VERSION_CONTROL
544 @cindex backups, making
545 @xref{Backup options}.
546 Make a backup of each file that would otherwise be overwritten or removed.
549 @macro optBackupSuffix
550 @item -S @var{suffix}
551 @itemx --suffix=@var{suffix}
554 Append @var{suffix} to each backup file made with @option{-b}.
555 @xref{Backup options}.
558 @macro optTargetDirectory
559 @item -t @var{directory}
560 @itemx @w{@kbd{--target-directory}=@var{directory}}
562 @opindex --target-directory
563 @cindex target directory
564 @cindex destination directory
565 Specify the destination @var{directory}.
566 @xref{Target directory}.
569 @macro optNoTargetDirectory
571 @itemx --no-target-directory
573 @opindex --no-target-directory
574 @cindex target directory
575 @cindex destination directory
576 Do not treat the last operand specially when it is a directory or a
577 symbolic link to a directory. @xref{Target directory}.
585 @cindex output @sc{nul}-byte-terminated lines
586 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
587 rather than a newline. This option enables other programs to parse the
588 output of @command{\cmd\} even when that output would contain data
589 with embedded newlines.
596 Append an SI-style abbreviation to each size, such as @samp{M} for
597 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
598 1,000,000 bytes. This option is equivalent to
599 @option{--block-size=si}. Use the @option{-h} or
600 @option{--human-readable} option if
601 you prefer powers of 1024.
604 @macro optHumanReadable
606 @itemx --human-readable
608 @opindex --human-readable
609 @cindex human-readable output
610 Append a size letter to each size, such as @samp{M} for mebibytes.
611 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
612 This option is equivalent to @option{--block-size=human-readable}.
613 Use the @option{--si} option if you prefer powers of 1000.
616 @macro optStripTrailingSlashes
617 @itemx @w{@kbd{--strip-trailing-slashes}}
618 @opindex --strip-trailing-slashes
619 @cindex stripping trailing slashes
620 Remove any trailing slashes from each @var{source} argument.
621 @xref{Trailing slashes}.
624 @macro mayConflictWithShellBuiltIn{cmd}
625 @cindex conflicts with shell built-ins
626 @cindex built-in shell commands, conflicts with
627 Due to shell aliases and built-in @command{\cmd\} command, using an
628 unadorned @command{\cmd\} interactively or in a script may get you
629 different functionality than that described here. Invoke it via
630 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
635 @macro multiplierSuffixes{varName}
636 @var{\varName\} may be, or may be an integer optionally followed by,
637 one of the following multiplicative suffixes:
639 @samp{b} => 512 ("blocks")
640 @samp{KB} => 1000 (KiloBytes)
641 @samp{K} => 1024 (KibiBytes)
642 @samp{MB} => 1000*1000 (MegaBytes)
643 @samp{M} => 1024*1024 (MebiBytes)
644 @samp{GB} => 1000*1000*1000 (GigaBytes)
645 @samp{G} => 1024*1024*1024 (GibiBytes)
647 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
650 @c FIXME: same as above, but no ``blocks'' line.
651 @macro multiplierSuffixesNoBlocks{varName}
652 @var{\varName\} may be, or may be an integer optionally followed by,
653 one of the following multiplicative suffixes:
655 @samp{KB} => 1000 (KiloBytes)
656 @samp{K} => 1024 (KibiBytes)
657 @samp{MB} => 1000*1000 (MegaBytes)
658 @samp{M} => 1024*1024 (MebiBytes)
659 @samp{GB} => 1000*1000*1000 (GigaBytes)
660 @samp{G} => 1024*1024*1024 (GibiBytes)
662 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
665 @cindex common options
667 Certain options are available in all of these programs. Rather than
668 writing identical descriptions for each of the programs, they are
669 described here. (In fact, every @sc{gnu} program accepts (or should accept)
672 @vindex POSIXLY_CORRECT
673 Normally options and operands can appear in any order, and programs act
674 as if all the options appear before any operands. For example,
675 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
676 @samp{:} is an option-argument of @option{-t}. However, if the
677 @env{POSIXLY_CORRECT} environment variable is set, options must appear
678 before operands, unless otherwise specified for a particular command.
680 A few programs can usefully have trailing operands with leading
681 @samp{-}. With such a program, options must precede operands even if
682 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
683 program description. For example, the @command{env} command's options
684 must appear before its operands, since in some cases the operands
685 specify a command that itself contains options.
687 Most programs that accept long options recognize unambiguous
688 abbreviations of those options. For example, @samp{rmdir
689 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
690 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
691 @samp{ls --h}, are identified as such.
693 Some of these programs recognize the @option{--help} and @option{--version}
694 options only when one of them is the sole command line argument. For
695 these programs, abbreviations of the long options are not always recognized.
702 Print a usage message listing all available options, then exit successfully.
706 @cindex version number, finding
707 Print the version number, then exit successfully.
711 @cindex option delimiter
712 Delimit the option list. Later arguments, if any, are treated as
713 operands even if they begin with @samp{-}. For example, @samp{sort --
714 -r} reads from the file named @file{-r}.
718 @cindex standard input
719 @cindex standard output
720 A single @samp{-} operand is not really an option, though it looks like one. It
721 stands for standard input, or for standard output if that is clear from
722 the context. For example, @samp{sort -} reads from standard input,
723 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
724 extra copy of its input to standard output. Unless otherwise
725 specified, @samp{-} can appear as any operand that requires a file
729 * Exit status:: Indicating program success or failure.
730 * Backup options:: -b -S, in some programs.
731 * Block size:: BLOCK_SIZE and --block-size, in some programs.
732 * Signal specifications:: Specifying signals using the --signal option.
733 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
734 * Random sources:: --random-source, in some programs.
735 * Target directory:: Specifying a target directory, in some programs.
736 * Trailing slashes:: --strip-trailing-slashes, in some programs.
737 * Traversing symlinks:: -H, -L, or -P, in some programs.
738 * Treating / specially:: --preserve-root and --no-preserve-root.
739 * Special built-in utilities:: @command{break}, @command{:}, @command{eval}, @dots{}
740 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
748 An exit status of zero indicates success,
749 and a nonzero value indicates failure.
752 Nearly every command invocation yields an integral @dfn{exit status}
753 that can be used to change how other commands work.
754 For the vast majority of commands, an exit status of zero indicates
755 success. Failure is indicated by a nonzero value---typically
756 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
757 requires only that it be nonzero.
759 However, some of the programs documented here do produce
760 other exit status values and a few associate different
761 meanings with the values @samp{0} and @samp{1}.
762 Here are some of the exceptions:
763 @command{chroot}, @command{env}, @command{expr}, @command{nice},
764 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
765 @command{su}, @command{test}, @command{timeout}, @command{tty}.
769 @section Backup options
771 @cindex backup options
773 Some @sc{gnu} programs (at least @command{cp}, @command{install},
774 @command{ln}, and @command{mv}) optionally make backups of files
775 before writing new versions.
776 These options control the details of these backups. The options are also
777 briefly mentioned in the descriptions of the particular programs.
782 @itemx @w{@kbd{--backup}[=@var{method}]}
785 @vindex VERSION_CONTROL
786 @cindex backups, making
787 Make a backup of each file that would otherwise be overwritten or removed.
788 Without this option, the original versions are destroyed.
789 Use @var{method} to determine the type of backups to make.
790 When this option is used but @var{method} is not specified,
791 then the value of the @env{VERSION_CONTROL}
792 environment variable is used. And if @env{VERSION_CONTROL} is not set,
793 the default backup type is @samp{existing}.
795 Note that the short form of this option, @option{-b} does not accept any
796 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
798 @vindex version-control @r{Emacs variable}
799 This option corresponds to the Emacs variable @samp{version-control};
800 the values for @var{method} are the same as those used in Emacs.
801 This option also accepts more descriptive names.
802 The valid @var{method}s are (unique abbreviations are accepted):
807 @opindex none @r{backup method}
812 @opindex numbered @r{backup method}
813 Always make numbered backups.
817 @opindex existing @r{backup method}
818 Make numbered backups of files that already have them, simple backups
823 @opindex simple @r{backup method}
824 Always make simple backups. Please note @samp{never} is not to be
825 confused with @samp{none}.
829 @item -S @var{suffix}
830 @itemx --suffix=@var{suffix}
833 @cindex backup suffix
834 @vindex SIMPLE_BACKUP_SUFFIX
835 Append @var{suffix} to each backup file made with @option{-b}. If this
836 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
837 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
838 set, the default is @samp{~}, just as in Emacs.
847 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
848 @command{ls}) display sizes in ``blocks''. You can adjust the block size
849 and method of display to make sizes easier to read. The block size
850 used for display is independent of any file system block size.
851 Fractional block counts are rounded up to the nearest integer.
853 @opindex --block-size=@var{size}
856 @vindex DF_BLOCK_SIZE
857 @vindex DU_BLOCK_SIZE
858 @vindex LS_BLOCK_SIZE
859 @vindex POSIXLY_CORRECT@r{, and block size}
861 The default block size is chosen by examining the following environment
862 variables in turn; the first one that is set determines the block size.
867 This specifies the default block size for the @command{df} command.
868 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
869 @env{LS_BLOCK_SIZE} for @command{ls}.
872 This specifies the default block size for all three commands, if the
873 above command-specific environment variables are not set.
876 This specifies the default block size for all values that are normally
877 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
878 command-specific environment variables are set. Unlike the other
879 environment variables, @env{BLOCKSIZE} does not affect values that are
880 normally printed as byte counts, e.g., the file sizes contained in
883 @item POSIXLY_CORRECT
884 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
885 @env{BLOCKSIZE} is set, but this variable is set, the block size
890 If none of the above environment variables are set, the block size
891 currently defaults to 1024 bytes in most contexts, but this number may
892 change in the future. For @command{ls} file sizes, the block size
895 @cindex human-readable output
898 A block size specification can be a positive integer specifying the number
899 of bytes per block, or it can be @code{human-readable} or @code{si} to
900 select a human-readable format. Integers may be followed by suffixes
901 that are upward compatible with the
902 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
903 for decimal multiples and with the
904 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
905 prefixes for binary multiples}.
907 With human-readable formats, output sizes are followed by a size letter
908 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
909 powers of 1024; @samp{M} stands for 1,048,576 bytes.
910 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
911 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
914 A block size specification preceded by @samp{'} causes output sizes to
915 be displayed with thousands separators. The @env{LC_NUMERIC} locale
916 specifies the thousands separator and grouping. For example, in an
917 American English locale, @samp{--block-size="'1kB"} would cause a size
918 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
919 locale, there is no thousands separator so a leading @samp{'} has no
922 An integer block size can be followed by a suffix to specify a
923 multiple of that size. A bare size letter,
924 or one followed by @samp{iB}, specifies
925 a multiple using powers of 1024. A size letter followed by @samp{B}
926 specifies powers of 1000 instead. For example, @samp{1M} and
927 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
928 equivalent to @samp{1000000}.
930 A plain suffix without a preceding integer acts as if @samp{1} were
931 prepended, except that it causes a size indication to be appended to
932 the output. For example, @samp{--block-size="kB"} displays 3000 as
935 The following suffixes are defined. Large sizes like @code{1Y}
936 may be rejected by your computer due to limitations of its arithmetic.
940 @cindex kilobyte, definition of
941 kilobyte: @math{10^3 = 1000}.
945 @cindex kibibyte, definition of
946 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
947 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
948 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
950 @cindex megabyte, definition of
951 megabyte: @math{10^6 = 1,000,000}.
954 @cindex mebibyte, definition of
955 mebibyte: @math{2^{20} = 1,048,576}.
957 @cindex gigabyte, definition of
958 gigabyte: @math{10^9 = 1,000,000,000}.
961 @cindex gibibyte, definition of
962 gibibyte: @math{2^{30} = 1,073,741,824}.
964 @cindex terabyte, definition of
965 terabyte: @math{10^{12} = 1,000,000,000,000}.
968 @cindex tebibyte, definition of
969 tebibyte: @math{2^{40} = 1,099,511,627,776}.
971 @cindex petabyte, definition of
972 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
975 @cindex pebibyte, definition of
976 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
978 @cindex exabyte, definition of
979 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
982 @cindex exbibyte, definition of
983 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
985 @cindex zettabyte, definition of
986 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
989 @math{2^{70} = 1,180,591,620,717,411,303,424}.
990 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
992 @cindex yottabyte, definition of
993 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
996 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
997 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1002 @opindex --block-size
1003 @opindex --human-readable
1006 Block size defaults can be overridden by an explicit
1007 @option{--block-size=@var{size}} option. The @option{-k}
1008 option is equivalent to @option{--block-size=1K}, which
1009 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1010 set. The @option{-h} or @option{--human-readable} option is equivalent to
1011 @option{--block-size=human-readable}. The @option{--si} option is
1012 equivalent to @option{--block-size=si}.
1014 @node Signal specifications
1015 @section Signal specifications
1016 @cindex signals, specifying
1018 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1019 number like @samp{1}, or an exit status of a process terminated by the
1020 signal. A signal name can be given in canonical form or prefixed by
1021 @samp{SIG}. The case of the letters is ignored. The following signal names
1022 and numbers are supported on all @acronym{POSIX} compliant systems:
1028 2. Terminal interrupt.
1034 9. Kill (cannot be caught or ignored).
1042 Other supported signal names have system-dependent corresponding
1043 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1044 support the following signals:
1048 Access to an undefined portion of a memory object.
1050 Child process terminated, stopped, or continued.
1052 Continue executing, if stopped.
1054 Erroneous arithmetic operation.
1056 Illegal Instruction.
1058 Write on a pipe with no one to read it.
1060 Invalid memory reference.
1062 Stop executing (cannot be caught or ignored).
1066 Background process attempting read.
1068 Background process attempting write.
1070 High bandwidth data is available at a socket.
1072 User-defined signal 1.
1074 User-defined signal 2.
1078 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1079 also support the following signals:
1085 Profiling timer expired.
1089 Trace/breakpoint trap.
1091 Virtual timer expired.
1093 CPU time limit exceeded.
1095 File size limit exceeded.
1099 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1100 also support at least eight real-time signals called @samp{RTMIN},
1101 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1103 @node Disambiguating names and IDs
1104 @section chown and chgrp: Disambiguating user names and IDs
1105 @cindex user names, disambiguating
1106 @cindex user IDs, disambiguating
1107 @cindex group names, disambiguating
1108 @cindex group IDs, disambiguating
1109 @cindex disambiguating group names and IDs
1111 Since the @var{owner} and @var{group} arguments to @command{chown} and
1112 @command{chgrp} may be specified as names or numeric IDs, there is an
1114 What if a user or group @emph{name} is a string of digits?
1115 @footnote{Using a number as a user name is common in some environments.}
1116 Should the command interpret it as a user name or as an ID?
1117 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1118 first attempt to resolve the specified string as a name, and
1119 only once that fails, then try to interpret it as an ID.
1120 This is troublesome when you want to specify a numeric ID, say 42,
1121 and it must work even in a pathological situation where
1122 @samp{42} is a user name that maps to some other user ID, say 1000.
1123 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1124 1000---not what you intended.
1126 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1127 that at the same time may result in a significant performance improvement
1128 by eliminating a database look-up.
1129 Simply precede each numeric user ID and/or group ID with a @samp{+},
1130 in order to force its interpretation as an integer:
1134 chgrp +$numeric_group_id another-file
1138 GNU @command{chown} and @command{chgrp}
1139 skip the name look-up process for each @samp{+}-prefixed string,
1140 because a string containing @samp{+} is never a valid user or group name.
1141 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1143 @node Random sources
1144 @section Sources of random data
1146 @cindex random sources
1148 The @command{shuf}, @command{shred}, and @command{sort} commands
1149 sometimes need random data to do their work. For example, @samp{sort
1150 -R} must choose a hash function at random, and it needs random data to
1151 make this selection.
1153 By default these commands use an internal pseudorandom generator
1154 initialized by a small amount of entropy, but can be directed to use
1155 an external source with the @option{--random-source=@var{file}} option.
1156 An error is reported if @var{file} does not contain enough bytes.
1158 For example, the device file @file{/dev/urandom} could be used as the
1159 source of random data. Typically, this device gathers environmental
1160 noise from device drivers and other sources into an entropy pool, and
1161 uses the pool to generate random bits. If the pool is short of data,
1162 the device reuses the internal pool to produce more bits, using a
1163 cryptographically secure pseudorandom number generator. But be aware
1164 that this device is not designed for bulk random data generation
1165 and is relatively slow.
1167 @file{/dev/urandom} suffices for most practical uses, but applications
1168 requiring high-value or long-term protection of private data may
1169 require an alternate data source like @file{/dev/random} or
1170 @file{/dev/arandom}. The set of available sources depends on your
1173 To reproduce the results of an earlier invocation of a command, you
1174 can save some random data into a file and then use that file as the
1175 random source in earlier and later invocations of the command.
1177 @node Target directory
1178 @section Target directory
1180 @cindex target directory
1182 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1183 commands normally treat the last operand specially when it is a
1184 directory or a symbolic link to a directory. For example, @samp{cp
1185 source dest} is equivalent to @samp{cp source dest/source} if
1186 @file{dest} is a directory. Sometimes this behavior is not exactly
1187 what is wanted, so these commands support the following options to
1188 allow more fine-grained control:
1193 @itemx --no-target-directory
1194 @opindex --no-target-directory
1195 @cindex target directory
1196 @cindex destination directory
1197 Do not treat the last operand specially when it is a directory or a
1198 symbolic link to a directory. This can help avoid race conditions in
1199 programs that operate in a shared area. For example, when the command
1200 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1201 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1202 renamed to @file{/tmp/dest/source} instead, if some other process
1203 created @file{/tmp/dest} as a directory. However, if @file{mv
1204 -T /tmp/source /tmp/dest} succeeds, there is no
1205 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1207 In the opposite situation, where you want the last operand to be
1208 treated as a directory and want a diagnostic otherwise, you can use
1209 the @option{--target-directory} (@option{-t}) option.
1211 @item -t @var{directory}
1212 @itemx @w{@kbd{--target-directory}=@var{directory}}
1213 @opindex --target-directory
1214 @cindex target directory
1215 @cindex destination directory
1216 Use @var{directory} as the directory component of each destination
1219 The interface for most programs is that after processing options and a
1220 finite (possibly zero) number of fixed-position arguments, the remaining
1221 argument list is either expected to be empty, or is a list of items
1222 (usually files) that will all be handled identically. The @command{xargs}
1223 program is designed to work well with this convention.
1225 The commands in the @command{mv}-family are unusual in that they take
1226 a variable number of arguments with a special case at the @emph{end}
1227 (namely, the target directory). This makes it nontrivial to perform some
1228 operations, e.g., ``move all files from here to ../d/'', because
1229 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1230 doesn't have a clean way to specify an extra final argument for each
1231 invocation of the subject command. (It can be done by going through a
1232 shell command, but that requires more human labor and brain power than
1235 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1236 @command{install}, @command{ln}, and @command{mv} programs to be used
1237 conveniently with @command{xargs}. For example, you can move the files
1238 from the current directory to a sibling directory, @code{d} like this:
1241 ls | xargs mv -t ../d --
1244 However, this doesn't move files whose names begin with @samp{.}.
1245 If you use the @sc{gnu} @command{find} program, you can move those
1246 files too, with this command:
1249 find . -mindepth 1 -maxdepth 1 \
1253 But both of the above approaches fail if there are no files in the
1254 current directory, or if any file has a name containing a blank or
1255 some other special characters.
1256 The following example removes those limitations and requires both
1257 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1260 find . -mindepth 1 -maxdepth 1 -print0 \
1261 | xargs --null --no-run-if-empty \
1268 The @option{--target-directory} (@option{-t}) and
1269 @option{--no-target-directory} (@option{-T})
1270 options cannot be combined.
1272 @node Trailing slashes
1273 @section Trailing slashes
1275 @cindex trailing slashes
1277 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1278 remove any trailing slashes from each @var{source} argument before
1279 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1282 This is useful when a @var{source} argument may have a trailing slash and
1283 @c FIXME: mv's behavior in this case is system-dependent
1284 specify a symbolic link to a directory. This scenario is in fact rather
1285 common because some shells can automatically append a trailing slash when
1286 performing file name completion on such symbolic links. Without this
1287 option, @command{mv}, for example, (via the system's rename function) must
1288 interpret a trailing slash as a request to dereference the symbolic link
1289 and so must rename the indirectly referenced @emph{directory} and not
1290 the symbolic link. Although it may seem surprising that such behavior
1291 be the default, it is required by @acronym{POSIX} and is consistent with
1292 other parts of that standard.
1294 @node Traversing symlinks
1295 @section Traversing symlinks
1297 @cindex symbolic link to directory, controlling traversal of
1299 The following options modify how @command{chown} and @command{chgrp}
1300 @c FIXME: note that `du' has these options, too, but they have slightly
1301 @c different meaning.
1302 traverse a hierarchy when the @option{--recursive} (@option{-R})
1303 option is also specified.
1304 If more than one of the following options is specified, only the final
1306 These options specify whether processing a symbolic link to a directory
1307 entails operating on just the symbolic link or on all files in the
1308 hierarchy rooted at that directory.
1310 These options are independent of @option{--dereference} and
1311 @option{--no-dereference} (@option{-h}), which control whether to modify
1312 a symlink or its referent.
1319 @cindex symbolic link to directory, traverse each that is specified on the command line
1320 If @option{--recursive} (@option{-R}) is specified and
1321 a command line argument is a symbolic link to a directory, traverse it.
1328 @cindex symbolic link to directory, traverse each that is encountered
1329 In a recursive traversal, traverse every symbolic link to a directory
1330 that is encountered.
1337 @cindex symbolic link to directory, never traverse
1338 Do not traverse any symbolic links.
1339 This is the default if none of @option{-H}, @option{-L},
1340 or @option{-P} is specified.
1347 @node Treating / specially
1348 @section Treating @file{/} specially
1350 Certain commands can operate destructively on entire hierarchies.
1351 For example, if a user with appropriate privileges mistakenly runs
1352 @samp{rm -rf / tmp/junk}, that may remove
1353 all files on the entire system. Since there are so few
1354 legitimate uses for such a command,
1355 @sc{gnu} @command{rm} normally declines to operate on any directory
1356 that resolves to @file{/}. If you really want to try to remove all
1357 the files on your system, you can use the @option{--no-preserve-root}
1358 option, but the default behavior, specified by the
1359 @option{--preserve-option}, is safer for most purposes.
1361 The commands @command{chgrp}, @command{chmod} and @command{chown}
1362 can also operate destructively on entire hierarchies, so they too
1363 support these options. Although, unlike @command{rm}, they don't
1364 actually unlink files, these commands are arguably more dangerous
1365 when operating recursively on @file{/}, since they often work much
1366 more quickly, and hence damage more files before an alert user can
1367 interrupt them. Tradition and @acronym{POSIX} require these commands
1368 to operate recursively on @file{/}, so they default to
1369 @option{--no-preserve-root}, but using the @option{--preserve-root}
1370 option makes them safer for most purposes. For convenience you can
1371 specify @option{--preserve-root} in an alias or in a shell function.
1373 Note that the @option{--preserve-root} option also ensures
1374 that @command{chgrp} and @command{chown} do not modify @file{/}
1375 even when dereferencing a symlink pointing to @file{/}.
1377 @node Special built-in utilities
1378 @section Special built-in utilities
1380 Some programs like @command{nice} can invoke other programs; for
1381 example, the command @samp{nice cat file} invokes the program
1382 @command{cat} by executing the command @samp{cat file}. However,
1383 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1384 this way. For example, the command @samp{nice exit} does not have a
1385 well-defined behavior: it may generate an error message instead of
1388 Here is a list of the special built-in utilities that are standardized
1389 by @acronym{POSIX} 1003.1-2004.
1392 @t{.@: : break continue eval exec exit export readonly
1393 return set shift times trap unset}
1396 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1397 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1398 pwd} do not work as you might expect.
1400 Many shells extend this list. For example, Bash has several extra
1401 special built-in utilities like @command{history}, and
1402 @command{suspend}, and with Bash the command @samp{nice suspend}
1403 generates an error message instead of suspending.
1405 @node Standards conformance
1406 @section Standards conformance
1408 @vindex POSIXLY_CORRECT
1409 In a few cases, the @sc{gnu} utilities' default behavior is
1410 incompatible with the @acronym{POSIX} standard. To suppress these
1411 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1412 variable. Unless you are checking for @acronym{POSIX} conformance, you
1413 probably do not need to define @env{POSIXLY_CORRECT}.
1415 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1416 versions. For example, older versions of @acronym{POSIX} required the
1417 command @samp{sort +1} to sort based on the second and succeeding
1418 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1419 the same command is required to sort the file named @file{+1}, and you
1420 must instead use the command @samp{sort -k 2} to get the field-based
1423 @vindex _POSIX2_VERSION
1424 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1425 that is standard for your system. To cause them to conform to a
1426 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1427 environment variable to a value of the form @var{yyyymm} specifying
1428 the year and month the standard was adopted. Two values are currently
1429 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1430 @acronym{POSIX} 1003.2-1992, and @samp{200112} stands for @acronym{POSIX}
1431 1003.1-2001. For example, if you have a newer system but are running software
1432 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1433 or @samp{tail +10}, you can work around any compatibility problems by setting
1434 @samp{_POSIX2_VERSION=199209} in your environment.
1436 @node Output of entire files
1437 @chapter Output of entire files
1439 @cindex output of entire files
1440 @cindex entire files, output of
1442 These commands read and write entire files, possibly transforming them
1446 * cat invocation:: Concatenate and write files.
1447 * tac invocation:: Concatenate and write files in reverse.
1448 * nl invocation:: Number lines and write files.
1449 * od invocation:: Write files in octal or other formats.
1450 * base64 invocation:: Transform data into printable data.
1453 @node cat invocation
1454 @section @command{cat}: Concatenate and write files
1457 @cindex concatenate and write files
1458 @cindex copying files
1460 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1461 standard input if none are given, to standard output. Synopsis:
1464 cat [@var{option}] [@var{file}]@dots{}
1467 The program accepts the following options. Also see @ref{Common options}.
1475 Equivalent to @option{-vET}.
1478 @itemx --number-nonblank
1480 @opindex --number-nonblank
1481 Number all nonempty output lines, starting with 1.
1485 Equivalent to @option{-vE}.
1490 @opindex --show-ends
1491 Display a @samp{$} after the end of each line.
1497 Number all output lines, starting with 1. This option is ignored
1498 if @option{-b} is in effect.
1501 @itemx --squeeze-blank
1503 @opindex --squeeze-blank
1504 @cindex squeezing empty lines
1505 Suppress repeated adjacent empty lines; output just one empty line
1510 Equivalent to @option{-vT}.
1515 @opindex --show-tabs
1516 Display TAB characters as @samp{^I}.
1520 Ignored; for @acronym{POSIX} compatibility.
1523 @itemx --show-nonprinting
1525 @opindex --show-nonprinting
1526 Display control characters except for LFD and TAB using
1527 @samp{^} notation and precede characters that have the high bit set with
1532 On systems like MS-DOS that distinguish between text and binary files,
1533 @command{cat} normally reads and writes in binary mode. However,
1534 @command{cat} reads in text mode if one of the options
1535 @option{-bensAE} is used or if @command{cat} is reading from standard
1536 input and standard input is a terminal. Similarly, @command{cat}
1537 writes in text mode if one of the options @option{-bensAE} is used or
1538 if standard output is a terminal.
1545 # Output f's contents, then standard input, then g's contents.
1548 # Copy standard input to standard output.
1553 @node tac invocation
1554 @section @command{tac}: Concatenate and write files in reverse
1557 @cindex reversing files
1559 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1560 standard input if none are given, to standard output, reversing the
1561 records (lines by default) in each separately. Synopsis:
1564 tac [@var{option}]@dots{} [@var{file}]@dots{}
1567 @dfn{Records} are separated by instances of a string (newline by
1568 default). By default, this separator string is attached to the end of
1569 the record that it follows in the file.
1571 The program accepts the following options. Also see @ref{Common options}.
1579 The separator is attached to the beginning of the record that it
1580 precedes in the file.
1586 Treat the separator string as a regular expression. Users of @command{tac}
1587 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1588 binary mode, each line of a text file might end with a CR/LF pair
1589 instead of the Unix-style LF.
1591 @item -s @var{separator}
1592 @itemx --separator=@var{separator}
1594 @opindex --separator
1595 Use @var{separator} as the record separator, instead of newline.
1603 @section @command{nl}: Number lines and write files
1606 @cindex numbering lines
1607 @cindex line numbering
1609 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1610 standard input if none are given, to standard output, with line numbers
1611 added to some or all of the lines. Synopsis:
1614 nl [@var{option}]@dots{} [@var{file}]@dots{}
1617 @cindex logical pages, numbering on
1618 @command{nl} decomposes its input into (logical) pages; by default, the
1619 line number is reset to 1 at the top of each logical page. @command{nl}
1620 treats all of the input files as a single document; it does not reset
1621 line numbers or logical pages between files.
1623 @cindex headers, numbering
1624 @cindex body, numbering
1625 @cindex footers, numbering
1626 A logical page consists of three sections: header, body, and footer.
1627 Any of the sections can be empty. Each can be numbered in a different
1628 style from the others.
1630 The beginnings of the sections of logical pages are indicated in the
1631 input file by a line containing exactly one of these delimiter strings:
1642 The two characters from which these strings are made can be changed from
1643 @samp{\} and @samp{:} via options (see below), but the pattern and
1644 length of each string cannot be changed.
1646 A section delimiter is replaced by an empty line on output. Any text
1647 that comes before the first section delimiter string in the input file
1648 is considered to be part of a body section, so @command{nl} treats a
1649 file that contains no section delimiters as a single body section.
1651 The program accepts the following options. Also see @ref{Common options}.
1655 @item -b @var{style}
1656 @itemx --body-numbering=@var{style}
1658 @opindex --body-numbering
1659 Select the numbering style for lines in the body section of each
1660 logical page. When a line is not numbered, the current line number
1661 is not incremented, but the line number separator character is still
1662 prepended to the line. The styles are:
1668 number only nonempty lines (default for body),
1670 do not number lines (default for header and footer),
1672 number only lines that contain a match for the basic regular
1673 expression @var{bre}.
1674 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1678 @itemx --section-delimiter=@var{cd}
1680 @opindex --section-delimiter
1681 @cindex section delimiters of pages
1682 Set the section delimiter characters to @var{cd}; default is
1683 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1684 (Remember to protect @samp{\} or other metacharacters from shell
1685 expansion with quotes or extra backslashes.)
1687 @item -f @var{style}
1688 @itemx --footer-numbering=@var{style}
1690 @opindex --footer-numbering
1691 Analogous to @option{--body-numbering}.
1693 @item -h @var{style}
1694 @itemx --header-numbering=@var{style}
1696 @opindex --header-numbering
1697 Analogous to @option{--body-numbering}.
1699 @item -i @var{number}
1700 @itemx --line-increment=@var{number}
1702 @opindex --line-increment
1703 Increment line numbers by @var{number} (default 1).
1705 @item -l @var{number}
1706 @itemx --join-blank-lines=@var{number}
1708 @opindex --join-blank-lines
1709 @cindex empty lines, numbering
1710 @cindex blank lines, numbering
1711 Consider @var{number} (default 1) consecutive empty lines to be one
1712 logical line for numbering, and only number the last one. Where fewer
1713 than @var{number} consecutive empty lines occur, do not number them.
1714 An empty line is one that contains no characters, not even spaces
1717 @item -n @var{format}
1718 @itemx --number-format=@var{format}
1720 @opindex --number-format
1721 Select the line numbering format (default is @code{rn}):
1725 @opindex ln @r{format for @command{nl}}
1726 left justified, no leading zeros;
1728 @opindex rn @r{format for @command{nl}}
1729 right justified, no leading zeros;
1731 @opindex rz @r{format for @command{nl}}
1732 right justified, leading zeros.
1736 @itemx --no-renumber
1738 @opindex --no-renumber
1739 Do not reset the line number at the start of a logical page.
1741 @item -s @var{string}
1742 @itemx --number-separator=@var{string}
1744 @opindex --number-separator
1745 Separate the line number from the text line in the output with
1746 @var{string} (default is the TAB character).
1748 @item -v @var{number}
1749 @itemx --starting-line-number=@var{number}
1751 @opindex --starting-line-number
1752 Set the initial line number on each logical page to @var{number} (default 1).
1754 @item -w @var{number}
1755 @itemx --number-width=@var{number}
1757 @opindex --number-width
1758 Use @var{number} characters for line numbers (default 6).
1766 @section @command{od}: Write files in octal or other formats
1769 @cindex octal dump of files
1770 @cindex hex dump of files
1771 @cindex ASCII dump of files
1772 @cindex file contents, dumping unambiguously
1774 @command{od} writes an unambiguous representation of each @var{file}
1775 (@samp{-} means standard input), or standard input if none are given.
1779 od [@var{option}]@dots{} [@var{file}]@dots{}
1780 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1781 od [@var{option}]@dots{} --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1784 Each line of output consists of the offset in the input, followed by
1785 groups of data from the file. By default, @command{od} prints the offset in
1786 octal, and each group of file data is a C @code{short int}'s worth of input
1787 printed as a single octal number.
1789 If @var{offset} is given, it specifies how many input bytes to skip
1790 before formatting and writing. By default, it is interpreted as an
1791 octal number, but the optional trailing decimal point causes it to be
1792 interpreted as decimal. If no decimal is specified and the offset
1793 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1794 number. If there is a trailing @samp{b}, the number of bytes skipped
1795 will be @var{offset} multiplied by 512.
1797 If a command is of both the first and second forms, the second form is
1798 assumed if the last operand begins with @samp{+} or (if there are two
1799 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1800 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1803 The program accepts the following options. Also see @ref{Common options}.
1807 @item -A @var{radix}
1808 @itemx --address-radix=@var{radix}
1810 @opindex --address-radix
1811 @cindex radix for file offsets
1812 @cindex file offset radix
1813 Select the base in which file offsets are printed. @var{radix} can
1814 be one of the following:
1824 none (do not print offsets).
1827 The default is octal.
1829 @item -j @var{bytes}
1830 @itemx --skip-bytes=@var{bytes}
1832 @opindex --skip-bytes
1833 Skip @var{bytes} input bytes before formatting and writing. If
1834 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1835 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1837 @multiplierSuffixes{bytes}
1839 @item -N @var{bytes}
1840 @itemx --read-bytes=@var{bytes}
1842 @opindex --read-bytes
1843 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1844 @code{bytes} are interpreted as for the @option{-j} option.
1846 @item -S @var{bytes}
1847 @itemx --strings[=@var{bytes}]
1850 @cindex string constants, outputting
1851 Instead of the normal output, output only @dfn{string constants}: at
1852 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1853 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1854 Prefixes and suffixes on @code{bytes} are interpreted as for the
1857 If @var{n} is omitted with @option{--strings}, the default is 3.
1860 @itemx --format=@var{type}
1863 Select the format in which to output the file data. @var{type} is a
1864 string of one or more of the below type indicator characters. If you
1865 include more than one type indicator character in a single @var{type}
1866 string, or use this option more than once, @command{od} writes one copy
1867 of each output line using each of the data types that you specified,
1868 in the order that you specified.
1870 Adding a trailing ``z'' to any type specification appends a display
1871 of the @acronym{ASCII} character representation of the printable characters
1872 to the output line generated by the type specification.
1876 named character, ignoring high-order bit
1878 @acronym{ASCII} character or backslash escape,
1891 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1892 newline, and @samp{nul} for a zero byte. Only the least significant
1893 seven bits of each byte is used; the high-order bit is ignored.
1894 Type @code{c} outputs
1895 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1898 Except for types @samp{a} and @samp{c}, you can specify the number
1899 of bytes to use in interpreting each number in the given data type
1900 by following the type indicator character with a decimal integer.
1901 Alternately, you can specify the size of one of the C compiler's
1902 built-in data types by following the type indicator character with
1903 one of the following characters. For integers (@samp{d}, @samp{o},
1904 @samp{u}, @samp{x}):
1917 For floating point (@code{f}):
1929 @itemx --output-duplicates
1931 @opindex --output-duplicates
1932 Output consecutive lines that are identical. By default, when two or
1933 more consecutive output lines would be identical, @command{od} outputs only
1934 the first line, and puts just an asterisk on the following line to
1935 indicate the elision.
1938 @itemx --width[=@var{n}]
1941 Dump @code{n} input bytes per output line. This must be a multiple of
1942 the least common multiple of the sizes associated with the specified
1945 If this option is not given at all, the default is 16. If @var{n} is
1946 omitted, the default is 32.
1950 The next several options are shorthands for format specifications.
1951 @sc{gnu} @command{od} accepts any combination of shorthands and format
1952 specification options. These options accumulate.
1958 Output as named characters. Equivalent to @samp{-t a}.
1962 Output as octal bytes. Equivalent to @samp{-t o1}.
1966 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1971 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1975 Output as floats. Equivalent to @samp{-t fF}.
1979 Output as decimal ints. Equivalent to @samp{-t dI}.
1983 Output as decimal long ints. Equivalent to @samp{-t dL}.
1987 Output as octal two-byte units. Equivalent to @option{-t o2}.
1991 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1995 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1998 @opindex --traditional
1999 Recognize the non-option label argument that traditional @command{od}
2000 accepted. The following syntax:
2003 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2007 can be used to specify at most one file and optional arguments
2008 specifying an offset and a pseudo-start address, @var{label}.
2009 The @var{label} argument is interpreted
2010 just like @var{offset}, but it specifies an initial pseudo-address. The
2011 pseudo-addresses are displayed in parentheses following any normal
2018 @node base64 invocation
2019 @section @command{base64}: Transform data into printable data
2022 @cindex base64 encoding
2024 @command{base64} transforms data read from a file, or standard input,
2025 into (or from) base64 encoded form. The base64 encoded form uses
2026 printable @acronym{ASCII} characters to represent binary data.
2030 base64 [@var{option}]@dots{} [@var{file}]
2031 base64 --decode [@var{option}]@dots{} [@var{file}]
2034 The base64 encoding expands data to roughly 133% of the original.
2035 The format conforms to
2036 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2038 The program accepts the following options. Also see @ref{Common options}.
2043 @itemx --wrap=@var{cols}
2047 @cindex column to wrap data after
2048 During encoding, wrap lines after @var{cols} characters. This must be
2051 The default is to wrap after 76 characters. Use the value 0 to
2052 disable line wrapping altogether.
2058 @cindex Decode base64 data
2059 @cindex Base64 decoding
2060 Change the mode of operation, from the default of encoding data, to
2061 decoding data. Input is expected to be base64 encoded data, and the
2062 output will be the original data.
2065 @itemx --ignore-garbage
2067 @opindex --ignore-garbage
2068 @cindex Ignore garbage in base64 stream
2069 When decoding, newlines are always accepted.
2070 During decoding, ignore unrecognized bytes,
2071 to permit distorted data to be decoded.
2078 @node Formatting file contents
2079 @chapter Formatting file contents
2081 @cindex formatting file contents
2083 These commands reformat the contents of files.
2086 * fmt invocation:: Reformat paragraph text.
2087 * pr invocation:: Paginate or columnate files for printing.
2088 * fold invocation:: Wrap input lines to fit in specified width.
2092 @node fmt invocation
2093 @section @command{fmt}: Reformat paragraph text
2096 @cindex reformatting paragraph text
2097 @cindex paragraphs, reformatting
2098 @cindex text, reformatting
2100 @command{fmt} fills and joins lines to produce output lines of (at most)
2101 a given number of characters (75 by default). Synopsis:
2104 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2107 @command{fmt} reads from the specified @var{file} arguments (or standard
2108 input if none are given), and writes to standard output.
2110 By default, blank lines, spaces between words, and indentation are
2111 preserved in the output; successive input lines with different
2112 indentation are not joined; tabs are expanded on input and introduced on
2115 @cindex line-breaking
2116 @cindex sentences and line-breaking
2117 @cindex Knuth, Donald E.
2118 @cindex Plass, Michael F.
2119 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2120 avoid line breaks after the first word of a sentence or before the last
2121 word of a sentence. A @dfn{sentence break} is defined as either the end
2122 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2123 spaces or end of line, ignoring any intervening parentheses or quotes.
2124 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2125 breaks; the algorithm is a variant of that given by Donald E. Knuth
2126 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2127 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2130 The program accepts the following options. Also see @ref{Common options}.
2135 @itemx --crown-margin
2137 @opindex --crown-margin
2138 @cindex crown margin
2139 @dfn{Crown margin} mode: preserve the indentation of the first two
2140 lines within a paragraph, and align the left margin of each subsequent
2141 line with that of the second line.
2144 @itemx --tagged-paragraph
2146 @opindex --tagged-paragraph
2147 @cindex tagged paragraphs
2148 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2149 indentation of the first line of a paragraph is the same as the
2150 indentation of the second, the first line is treated as a one-line
2156 @opindex --split-only
2157 Split lines only. Do not join short lines to form longer ones. This
2158 prevents sample lines of code, and other such ``formatted'' text from
2159 being unduly combined.
2162 @itemx --uniform-spacing
2164 @opindex --uniform-spacing
2165 Uniform spacing. Reduce spacing between words to one space, and spacing
2166 between sentences to two spaces.
2169 @itemx -w @var{width}
2170 @itemx --width=@var{width}
2171 @opindex -@var{width}
2174 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2175 initially tries to make lines about 7% shorter than this, to give it
2176 room to balance line lengths.
2178 @item -p @var{prefix}
2179 @itemx --prefix=@var{prefix}
2180 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2181 are subject to formatting. The prefix and any preceding whitespace are
2182 stripped for the formatting and then re-attached to each formatted output
2183 line. One use is to format certain kinds of program comments, while
2184 leaving the code unchanged.
2192 @section @command{pr}: Paginate or columnate files for printing
2195 @cindex printing, preparing files for
2196 @cindex multicolumn output, generating
2197 @cindex merging files in parallel
2199 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2200 standard input if none are given, to standard output, paginating and
2201 optionally outputting in multicolumn format; optionally merges all
2202 @var{file}s, printing all in parallel, one per column. Synopsis:
2205 pr [@var{option}]@dots{} [@var{file}]@dots{}
2209 By default, a 5-line header is printed at each page: two blank lines;
2210 a line with the date, the file name, and the page count; and two more
2211 blank lines. A footer of five blank lines is also printed.
2212 The default @var{page_length} is 66
2213 lines. The default number of text lines is therefore 56.
2214 The text line of the header takes the form
2215 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2216 @var{string} so that the line takes up the full @var{page_width}. Here,
2217 @var{date} is the date (see the @option{-D} or @option{--date-format}
2218 option for details), @var{string} is the centered header string, and
2219 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2220 category affects the spelling of @var{page}; in the default C locale, it
2221 is @samp{Page @var{number}} where @var{number} is the decimal page
2224 Form feeds in the input cause page breaks in the output. Multiple form
2225 feeds produce empty pages.
2227 Columns are of equal width, separated by an optional string (default
2228 is @samp{space}). For multicolumn output, lines will always be truncated to
2229 @var{page_width} (default 72), unless you use the @option{-J} option.
2231 column output no line truncation occurs by default. Use @option{-W} option to
2232 truncate lines in that case.
2234 The following changes were made in version 1.22i and apply to later
2235 versions of @command{pr}:
2236 @c FIXME: this whole section here sounds very awkward to me. I
2237 @c made a few small changes, but really it all needs to be redone. - Brian
2238 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2243 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2244 redefined for better @acronym{POSIX} compliance. The output of some further
2245 cases has been adapted to other Unix systems. These changes are not
2246 compatible with earlier versions of the program.
2249 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2250 have been introduced to turn off unexpected interferences of small letter
2251 options. The @option{-N} option and the second argument @var{last_page}
2252 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2253 form feeds set in the input files requires the @option{-T} option.
2256 Capital letter options override small letter ones.
2259 Some of the option-arguments (compare @option{-s}, @option{-e},
2260 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2261 preceding option letter (already stated in the @acronym{POSIX} specification).
2264 The program accepts the following options. Also see @ref{Common options}.
2268 @item +@var{first_page}[:@var{last_page}]
2269 @itemx --pages=@var{first_page}[:@var{last_page}]
2270 @c The two following @opindex lines evoke warnings because they contain `:'
2271 @c The `info' spec does not permit that. If we use those lines, we end
2272 @c up with truncated index entries that don't work.
2273 @c @opindex +@var{first_page}[:@var{last_page}]
2274 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2275 @opindex +@var{page_range}
2276 @opindex --pages=@var{page_range}
2277 Begin printing with page @var{first_page} and stop with @var{last_page}.
2278 Missing @samp{:@var{last_page}} implies end of file. While estimating
2279 the number of skipped pages each form feed in the input file results
2280 in a new page. Page counting with and without @samp{+@var{first_page}}
2281 is identical. By default, counting starts with the first page of input
2282 file (not first page printed). Line numbering may be altered by @option{-N}
2286 @itemx --columns=@var{column}
2287 @opindex -@var{column}
2289 @cindex down columns
2290 With each single @var{file}, produce @var{column} columns of output
2291 (default is 1) and print columns down, unless @option{-a} is used. The
2292 column width is automatically decreased as @var{column} increases; unless
2293 you use the @option{-W/-w} option to increase @var{page_width} as well.
2294 This option might well cause some lines to be truncated. The number of
2295 lines in the columns on each page are balanced. The options @option{-e}
2296 and @option{-i} are on for multiple text-column output. Together with
2297 @option{-J} option column alignment and line truncation is turned off.
2298 Lines of full length are joined in a free field format and @option{-S}
2299 option may set field separators. @option{-@var{column}} may not be used
2300 with @option{-m} option.
2306 @cindex across columns
2307 With each single @var{file}, print columns across rather than down. The
2308 @option{-@var{column}} option must be given with @var{column} greater than one.
2309 If a line is too long to fit in a column, it is truncated.
2312 @itemx --show-control-chars
2314 @opindex --show-control-chars
2315 Print control characters using hat notation (e.g., @samp{^G}); print
2316 other nonprinting characters in octal backslash notation. By default,
2317 nonprinting characters are not changed.
2320 @itemx --double-space
2322 @opindex --double-space
2323 @cindex double spacing
2324 Double space the output.
2326 @item -D @var{format}
2327 @itemx --date-format=@var{format}
2328 @cindex time formats
2329 @cindex formatting times
2330 Format header dates using @var{format}, using the same conventions as
2331 for the command @samp{date +@var{format}}; @xref{date invocation}.
2332 Except for directives, which start with
2333 @samp{%}, characters in @var{format} are printed unchanged. You can use
2334 this option to specify an arbitrary string in place of the header date,
2335 e.g., @option{--date-format="Monday morning"}.
2337 @vindex POSIXLY_CORRECT
2339 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2340 @samp{2001-12-04 23:59});
2341 but if the @env{POSIXLY_CORRECT} environment variable is set
2342 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2343 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2344 @samp{Dec@ @ 4 23:59 2001}.
2347 Time stamps are listed according to the time zone rules specified by
2348 the @env{TZ} environment variable, or by the system default rules if
2349 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2350 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2352 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2353 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2355 @opindex --expand-tabs
2357 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2358 the input tab character (default is the TAB character). Second optional
2359 argument @var{in-tabwidth} is the input tab character's width (default
2367 @opindex --form-feed
2368 Use a form feed instead of newlines to separate output pages. This does
2369 not alter the default page length of 66 lines.
2371 @item -h @var{header}
2372 @itemx --header=@var{header}
2375 Replace the file name in the header with the centered string @var{header}.
2376 When using the shell, @var{header} should be quoted and should be
2377 separated from @option{-h} by a space.
2379 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2380 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2382 @opindex --output-tabs
2384 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2385 is the output tab character (default is the TAB character). Second optional
2386 argument @var{out-tabwidth} is the output tab character's width (default
2392 @opindex --join-lines
2393 Merge lines of full length. Used together with the column options
2394 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2395 @option{-W/-w} line truncation;
2396 no column alignment used; may be used with
2397 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2398 (together with @option{-W} and @option{--sep-string})
2399 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2400 @option{-s} along with the three column options.
2403 @item -l @var{page_length}
2404 @itemx --length=@var{page_length}
2407 Set the page length to @var{page_length} (default 66) lines, including
2408 the lines of the header [and the footer]. If @var{page_length} is less
2409 than or equal to 10, the header and footer are omitted, as if the
2410 @option{-t} option had been given.
2416 Merge and print all @var{file}s in parallel, one in each column. If a
2417 line is too long to fit in a column, it is truncated, unless the @option{-J}
2418 option is used. @option{--sep-string[=@var{string}]} may be used.
2420 some @var{file}s (form feeds set) produce empty columns, still marked
2421 by @var{string}. The result is a continuous line numbering and column
2422 marking throughout the whole merged file. Completely empty merged pages
2423 show no separators or line numbers. The default header becomes
2424 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2425 may be used with the @option{-h} or @option{--header} option to fill up
2426 the middle blank part.
2428 @item -n[@var{number-separator}[@var{digits}]]
2429 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2431 @opindex --number-lines
2432 Provide @var{digits} digit line numbering (default for @var{digits} is
2433 5). With multicolumn output the number occupies the first @var{digits}
2434 column positions of each text column or only each line of @option{-m}
2435 output. With single column output the number precedes each line just as
2436 @option{-m} does. Default counting of the line numbers starts with the
2437 first line of the input file (not the first line printed, compare the
2438 @option{--page} option and @option{-N} option).
2439 Optional argument @var{number-separator} is the character appended to
2440 the line number to separate it from the text followed. The default
2441 separator is the TAB character. In a strict sense a TAB is always
2442 printed with single column output only. The TAB width varies
2443 with the TAB position, e.g., with the left @var{margin} specified
2444 by @option{-o} option. With multicolumn output priority is given to
2445 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2446 The TAB width is fixed to the value of the first column and does
2447 not change with different values of left @var{margin}. That means a
2448 fixed number of spaces is always printed in the place of the
2449 @var{number-separator} TAB. The tabification depends upon the output
2452 @item -N @var{line_number}
2453 @itemx --first-line-number=@var{line_number}
2455 @opindex --first-line-number
2456 Start line counting with the number @var{line_number} at first line of
2457 first page printed (in most cases not the first line of the input file).
2459 @item -o @var{margin}
2460 @itemx --indent=@var{margin}
2463 @cindex indenting lines
2465 Indent each line with a margin @var{margin} spaces wide (default is zero).
2466 The total page width is the size of the margin plus the @var{page_width}
2467 set with the @option{-W/-w} option. A limited overflow may occur with
2468 numbered single column output (compare @option{-n} option).
2471 @itemx --no-file-warnings
2473 @opindex --no-file-warnings
2474 Do not print a warning message when an argument @var{file} cannot be
2475 opened. (The exit status will still be nonzero, however.)
2477 @item -s[@var{char}]
2478 @itemx --separator[=@var{char}]
2480 @opindex --separator
2481 Separate columns by a single character @var{char}. The default for
2482 @var{char} is the TAB character without @option{-w} and @samp{no
2483 character} with @option{-w}. Without @option{-s} the default separator
2484 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2485 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2486 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2489 @item -S@var{string}
2490 @itemx --sep-string[=@var{string}]
2492 @opindex --sep-string
2493 Use @var{string} to separate output columns. The @option{-S} option doesn't
2494 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2495 does not affect line truncation or column alignment.
2496 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2498 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2499 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2500 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2503 @itemx --omit-header
2505 @opindex --omit-header
2506 Do not print the usual header [and footer] on each page, and do not fill
2507 out the bottom of pages (with blank lines or a form feed). No page
2508 structure is produced, but form feeds set in the input files are retained.
2509 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2510 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2511 in the input file to 4 spaces but don't make any other changes. Use of
2512 @option{-t} overrides @option{-h}.
2515 @itemx --omit-pagination
2517 @opindex --omit-pagination
2518 Do not print header [and footer]. In addition eliminate all form feeds
2519 set in the input files.
2522 @itemx --show-nonprinting
2524 @opindex --show-nonprinting
2525 Print nonprinting characters in octal backslash notation.
2527 @item -w @var{page_width}
2528 @itemx --width=@var{page_width}
2531 Set page width to @var{page_width} characters for multiple text-column
2532 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2533 off the default page width and any line truncation and column alignment.
2534 Lines of full length are merged, regardless of the column options
2535 set. No @var{page_width} setting is possible with single column output.
2536 A @acronym{POSIX}-compliant formulation.
2538 @item -W @var{page_width}
2539 @itemx --page_width=@var{page_width}
2541 @opindex --page_width
2542 Set the page width to @var{page_width} characters. That's valid with and
2543 without a column option. Text lines are truncated, unless @option{-J}
2544 is used. Together with one of the three column options
2545 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2546 alignment is always used. The separator options @option{-S} or @option{-s}
2547 don't affect the @option{-W} option. Default is 72 characters. Without
2548 @option{-W @var{page_width}} and without any of the column options NO line
2549 truncation is used (defined to keep downward compatibility and to meet
2550 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2551 line is never truncated.
2558 @node fold invocation
2559 @section @command{fold}: Wrap input lines to fit in specified width
2562 @cindex wrapping long input lines
2563 @cindex folding long input lines
2565 @command{fold} writes each @var{file} (@option{-} means standard input), or
2566 standard input if none are given, to standard output, breaking long
2570 fold [@var{option}]@dots{} [@var{file}]@dots{}
2573 By default, @command{fold} breaks lines wider than 80 columns. The output
2574 is split into as many lines as necessary.
2576 @cindex screen columns
2577 @command{fold} counts screen columns by default; thus, a tab may count more
2578 than one column, backspace decreases the column count, and carriage
2579 return sets the column to zero.
2581 The program accepts the following options. Also see @ref{Common options}.
2589 Count bytes rather than columns, so that tabs, backspaces, and carriage
2590 returns are each counted as taking up one column, just like other
2597 Break at word boundaries: the line is broken after the last blank before
2598 the maximum line length. If the line contains no such blanks, the line
2599 is broken at the maximum line length as usual.
2601 @item -w @var{width}
2602 @itemx --width=@var{width}
2605 Use a maximum line length of @var{width} columns instead of 80.
2607 For compatibility @command{fold} supports an obsolete option syntax
2608 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2616 @node Output of parts of files
2617 @chapter Output of parts of files
2619 @cindex output of parts of files
2620 @cindex parts of files, output of
2622 These commands output pieces of the input.
2625 * head invocation:: Output the first part of files.
2626 * tail invocation:: Output the last part of files.
2627 * split invocation:: Split a file into pieces.
2628 * csplit invocation:: Split a file into context-determined pieces.
2631 @node head invocation
2632 @section @command{head}: Output the first part of files
2635 @cindex initial part of files, outputting
2636 @cindex first part of files, outputting
2638 @command{head} prints the first part (10 lines by default) of each
2639 @var{file}; it reads from standard input if no files are given or
2640 when given a @var{file} of @option{-}. Synopsis:
2643 head [@var{option}]@dots{} [@var{file}]@dots{}
2646 If more than one @var{file} is specified, @command{head} prints a
2647 one-line header consisting of:
2650 ==> @var{file name} <==
2654 before the output for each @var{file}.
2656 The program accepts the following options. Also see @ref{Common options}.
2661 @itemx --bytes=@var{k}
2664 Print the first @var{k} bytes, instead of initial lines.
2665 However, if @var{k} starts with a @samp{-},
2666 print all but the last @var{k} bytes of each file.
2667 @multiplierSuffixes{k}
2670 @itemx --lines=@var{k}
2673 Output the first @var{k} lines.
2674 However, if @var{k} starts with a @samp{-},
2675 print all but the last @var{k} lines of each file.
2676 Size multiplier suffixes are the same as with the @option{-c} option.
2684 Never print file name headers.
2690 Always print file name headers.
2694 For compatibility @command{head} also supports an obsolete option syntax
2695 @option{-@var{count}@var{options}}, which is recognized only if it is
2696 specified first. @var{count} is a decimal number optionally followed
2697 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2698 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2699 Scripts intended for standard hosts should use @option{-c @var{count}}
2700 or @option{-n @var{count}} instead. If your script must also run on
2701 hosts that support only the obsolete syntax, it is usually simpler to
2702 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2708 @node tail invocation
2709 @section @command{tail}: Output the last part of files
2712 @cindex last part of files, outputting
2714 @command{tail} prints the last part (10 lines by default) of each
2715 @var{file}; it reads from standard input if no files are given or
2716 when given a @var{file} of @samp{-}. Synopsis:
2719 tail [@var{option}]@dots{} [@var{file}]@dots{}
2722 If more than one @var{file} is specified, @command{tail} prints a
2723 one-line header consisting of:
2726 ==> @var{file name} <==
2730 before the output for each @var{file}.
2732 @cindex BSD @command{tail}
2733 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2734 @command{tail} cannot). It also has no @option{-r} option (print in
2735 reverse), since reversing a file is really a different job from printing
2736 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2737 only reverse files that are at most as large as its buffer, which is
2738 typically 32 KiB@. A more reliable and versatile way to reverse files is
2739 the @sc{gnu} @command{tac} command.
2741 The program accepts the following options. Also see @ref{Common options}.
2746 @itemx --bytes=@var{k}
2749 Output the last @var{k} bytes, instead of final lines.
2750 However, if @var{k} starts with a @samp{+}, start printing with the
2751 @var{k}th byte from the start of each file, instead of from the end.
2752 @multiplierSuffixes{k}
2755 @itemx --follow[=@var{how}]
2758 @cindex growing files
2759 @vindex name @r{follow option}
2760 @vindex descriptor @r{follow option}
2761 Loop forever trying to read more characters at the end of the file,
2762 presumably because the file is growing.
2763 If more than one file is given, @command{tail} prints a header whenever it
2764 gets output from a different file, to indicate which file that output is
2767 There are two ways to specify how you'd like to track files with this option,
2768 but that difference is noticeable only when a followed file is removed or
2770 If you'd like to continue to track the end of a growing file even after
2771 it has been unlinked, use @option{--follow=descriptor}. This is the default
2772 behavior, but it is not useful if you're tracking a log file that may be
2773 rotated (removed or renamed, then reopened). In that case, use
2774 @option{--follow=name} to track the named file, perhaps by reopening it
2775 periodically to see if it has been removed and recreated by some other program.
2776 Note that the inotify-based implementation handles this case without
2777 the need for any periodic reopening.
2779 No matter which method you use, if the tracked file is determined to have
2780 shrunk, @command{tail} prints a message saying the file has been truncated
2781 and resumes tracking the end of the file from the newly-determined endpoint.
2783 When a file is removed, @command{tail}'s behavior depends on whether it is
2784 following the name or the descriptor. When following by name, tail can
2785 detect that a file has been removed and gives a message to that effect,
2786 and if @option{--retry} has been specified it will continue checking
2787 periodically to see if the file reappears.
2788 When following a descriptor, tail does not detect that the file has
2789 been unlinked or renamed and issues no message; even though the file
2790 may no longer be accessible via its original name, it may still be
2793 The option values @samp{descriptor} and @samp{name} may be specified only
2794 with the long form of the option, not with @option{-f}.
2796 The @option{-f} option is ignored if
2797 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2798 Likewise, the @option{-f} option has no effect for any
2799 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2803 This option is the same as @option{--follow=name --retry}. That is, tail
2804 will attempt to reopen a file when it is removed. Should this fail, tail
2805 will keep trying until it becomes accessible again.
2809 This option is useful mainly when following by name (i.e., with
2810 @option{--follow=name}).
2811 Without this option, when tail encounters a file that doesn't
2812 exist or is otherwise inaccessible, it reports that fact and
2813 never checks it again.
2815 @itemx --sleep-interval=@var{number}
2816 @opindex --sleep-interval
2817 Change the number of seconds to wait between iterations (the default is 1.0).
2818 During one iteration, every specified file is checked to see if it has
2820 Historical implementations of @command{tail} have required that
2821 @var{number} be an integer. However, GNU @command{tail} accepts
2822 an arbitrary floating point number (using a period before any
2825 @itemx --pid=@var{pid}
2827 When following by name or by descriptor, you may specify the process ID,
2828 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2829 after that process terminates, tail will also terminate. This will
2830 work properly only if the writer and the tailing process are running on
2831 the same machine. For example, to save the output of a build in a file
2832 and to watch the file grow, if you invoke @command{make} and @command{tail}
2833 like this then the tail process will stop when your build completes.
2834 Without this option, you would have had to kill the @code{tail -f}
2838 $ make >& makerr & tail --pid=$! -f makerr
2841 If you specify a @var{pid} that is not in use or that does not correspond
2842 to the process that is writing to the tailed files, then @command{tail}
2843 may terminate long before any @var{file}s stop growing or it may not
2844 terminate until long after the real writer has terminated.
2845 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2846 will print a warning if this is the case.
2848 @itemx --max-unchanged-stats=@var{n}
2849 @opindex --max-unchanged-stats
2850 When tailing a file by name, if there have been @var{n} (default
2851 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2852 iterations for which the file has not changed, then
2853 @code{open}/@code{fstat} the file to determine if that file name is
2854 still associated with the same device/inode-number pair as before.
2855 When following a log file that is rotated, this is approximately the
2856 number of seconds between when tail prints the last pre-rotation lines
2857 and when it prints the lines that have accumulated in the new log file.
2858 This option is meaningful only when following by name.
2861 @itemx --lines=@var{k}
2864 Output the last @var{k} lines.
2865 However, if @var{k} starts with a @samp{+}, start printing with the
2866 @var{k}th line from the start of each file, instead of from the end.
2867 Size multiplier suffixes are the same as with the @option{-c} option.
2875 Never print file name headers.
2881 Always print file name headers.
2885 For compatibility @command{tail} also supports an obsolete usage
2886 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2887 only if it does not conflict with the usage described
2888 above. This obsolete form uses exactly one option and at most one
2889 file. In the option, @var{count} is an optional decimal number optionally
2890 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2891 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2892 which has the same meaning as @option{-f}.
2894 @vindex _POSIX2_VERSION
2895 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2896 the obsolete option syntax with the same meaning as in counts, and
2897 obsolete usage overrides normal usage when the two conflict.
2898 This obsolete behavior can be enabled or disabled with the
2899 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2902 Scripts intended for use on standard hosts should avoid obsolete
2903 syntax and should use @option{-c @var{count}[b]}, @option{-n
2904 @var{count}}, and/or @option{-f} instead. If your script must also
2905 run on hosts that support only the obsolete syntax, you can often
2906 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2907 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2908 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2909 then @dots{}} to decide which syntax to use.
2911 Even if your script assumes the standard behavior, you should still
2912 beware usages whose behaviors differ depending on the @acronym{POSIX}
2913 version. For example, avoid @samp{tail - main.c}, since it might be
2914 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2915 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2916 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2917 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2922 @node split invocation
2923 @section @command{split}: Split a file into pieces.
2926 @cindex splitting a file into pieces
2927 @cindex pieces, splitting a file into
2929 @command{split} creates output files containing consecutive or interleaved
2930 sections of @var{input} (standard input if none is given or @var{input}
2931 is @samp{-}). Synopsis:
2934 split [@var{option}] [@var{input} [@var{prefix}]]
2937 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2938 left over for the last section), into each output file.
2940 @cindex output file name prefix
2941 The output files' names consist of @var{prefix} (@samp{x} by default)
2942 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2943 default), such that concatenating the output files in traditional
2944 sorted order by file name produces the original input file (except
2945 @option{-r}). If the output file names are exhausted, @command{split}
2946 reports an error without deleting the output files that it did create.
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 -n @var{chunks}
2979 @itemx --number=@var{chunks}
2983 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
2986 @var{n} generate @var{n} files based on current size of @var{input}
2987 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
2988 l/@var{n} generate @var{n} files without splitting lines
2989 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2990 r/@var{n} like @samp{l} but use round robin distribution
2991 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2994 Any excess bytes remaining after dividing the @var{input}
2995 into @var{n} chunks, are assigned to the last chunk.
2996 Any excess bytes appearing after the initial calculation are discarded
2997 (except when using @samp{r} mode).
2999 All @var{n} files are created even if there are fewer than @var{n} lines,
3000 or the @var{input} is truncated.
3002 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3003 The @var{input} is partitioned into @var{n} equal sized portions, with
3004 the last assigned any excess. If a line @emph{starts} within a partition
3005 it is written completely to the corresponding file. Since lines
3006 are not split even if they overlap a partition, the files written
3007 can be larger or smaller than the partition size, and even empty
3008 if a line is so long as to completely overlap the partition.
3010 For @samp{r} mode, the size of @var{input} is irrelevant,
3011 and so can be a pipe for example.
3013 @item -a @var{length}
3014 @itemx --suffix-length=@var{length}
3016 @opindex --suffix-length
3017 Use suffixes of length @var{length}. The default @var{length} is 2.
3020 @itemx --numeric-suffixes
3022 @opindex --numeric-suffixes
3023 Use digits in suffixes rather than lower-case letters.
3026 @itemx --elide-empty-files
3028 @opindex --elide-empty-files
3029 Suppress the generation of zero-length output files. This can happen
3030 with the @option{--number} option if a file is (truncated to be) shorter
3031 than the number requested, or if a line is so long as to completely
3032 span a chunk. The output file sequence numbers, always run consecutively
3033 even when this option is specified.
3038 @opindex --unbuffered
3039 Immediately copy input to output in @option{--number r/...} mode,
3040 which is a much slower mode of operation.
3044 Write a diagnostic just before each output file is opened.
3051 @node csplit invocation
3052 @section @command{csplit}: Split a file into context-determined pieces
3055 @cindex context splitting
3056 @cindex splitting a file into pieces by context
3058 @command{csplit} creates zero or more output files containing sections of
3059 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3062 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3065 The contents of the output files are determined by the @var{pattern}
3066 arguments, as detailed below. An error occurs if a @var{pattern}
3067 argument refers to a nonexistent line of the input file (e.g., if no
3068 remaining line matches a given regular expression). After every
3069 @var{pattern} has been matched, any remaining input is copied into one
3072 By default, @command{csplit} prints the number of bytes written to each
3073 output file after it has been created.
3075 The types of pattern arguments are:
3080 Create an output file containing the input up to but not including line
3081 @var{n} (a positive integer). If followed by a repeat count, also
3082 create an output file containing the next @var{n} lines of the input
3083 file once for each repeat.
3085 @item /@var{regexp}/[@var{offset}]
3086 Create an output file containing the current line up to (but not
3087 including) the next line of the input file that contains a match for
3088 @var{regexp}. The optional @var{offset} is an integer.
3089 If it is given, the input up to (but not including) the
3090 matching line plus or minus @var{offset} is put into the output file,
3091 and the line after that begins the next section of input.
3093 @item %@var{regexp}%[@var{offset}]
3094 Like the previous type, except that it does not create an output
3095 file, so that section of the input file is effectively ignored.
3097 @item @{@var{repeat-count}@}
3098 Repeat the previous pattern @var{repeat-count} additional
3099 times. The @var{repeat-count} can either be a positive integer or an
3100 asterisk, meaning repeat as many times as necessary until the input is
3105 The output files' names consist of a prefix (@samp{xx} by default)
3106 followed by a suffix. By default, the suffix is an ascending sequence
3107 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3108 concatenating the output files in sorted order by file name produces the
3109 original input file.
3111 By default, if @command{csplit} encounters an error or receives a hangup,
3112 interrupt, quit, or terminate signal, it removes any output files
3113 that it has created so far before it exits.
3115 The program accepts the following options. Also see @ref{Common options}.
3119 @item -f @var{prefix}
3120 @itemx --prefix=@var{prefix}
3123 @cindex output file name prefix
3124 Use @var{prefix} as the output file name prefix.
3126 @item -b @var{suffix}
3127 @itemx --suffix=@var{suffix}
3130 @cindex output file name suffix
3131 Use @var{suffix} as the output file name suffix. When this option is
3132 specified, the suffix string must include exactly one
3133 @code{printf(3)}-style conversion specification, possibly including
3134 format specification flags, a field width, a precision specifications,
3135 or all of these kinds of modifiers. The format letter must convert a
3136 binary unsigned integer argument to readable form. The format letters
3137 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3138 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3139 entire @var{suffix} is given (with the current output file number) to
3140 @code{sprintf(3)} to form the file name suffixes for each of the
3141 individual output files in turn. If this option is used, the
3142 @option{--digits} option is ignored.
3144 @item -n @var{digits}
3145 @itemx --digits=@var{digits}
3148 Use output file names containing numbers that are @var{digits} digits
3149 long instead of the default 2.
3154 @opindex --keep-files
3155 Do not remove output files when errors are encountered.
3158 @itemx --elide-empty-files
3160 @opindex --elide-empty-files
3161 Suppress the generation of zero-length output files. (In cases where
3162 the section delimiters of the input file are supposed to mark the first
3163 lines of each of the sections, the first output file will generally be a
3164 zero-length file unless you use this option.) The output file sequence
3165 numbers always run consecutively starting from 0, even when this option
3176 Do not print counts of output file sizes.
3182 Here is an example of its usage.
3183 First, create an empty directory for the exercise,
3190 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3193 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3199 Each number printed above is the size of an output
3200 file that csplit has just created.
3201 List the names of those output files:
3208 Use @command{head} to show their contents:
3233 @node Summarizing files
3234 @chapter Summarizing files
3236 @cindex summarizing files
3238 These commands generate just a few numbers representing entire
3242 * wc invocation:: Print newline, word, and byte counts.
3243 * sum invocation:: Print checksum and block counts.
3244 * cksum invocation:: Print CRC checksum and byte counts.
3245 * md5sum invocation:: Print or check MD5 digests.
3246 * sha1sum invocation:: Print or check SHA-1 digests.
3247 * sha2 utilities:: Print or check SHA-2 digests.
3252 @section @command{wc}: Print newline, word, and byte counts
3256 @cindex character count
3260 @command{wc} counts the number of bytes, characters, whitespace-separated
3261 words, and newlines in each given @var{file}, or standard input if none
3262 are given or for a @var{file} of @samp{-}. Synopsis:
3265 wc [@var{option}]@dots{} [@var{file}]@dots{}
3268 @cindex total counts
3269 @command{wc} prints one line of counts for each file, and if the file was
3270 given as an argument, it prints the file name following the counts. If
3271 more than one @var{file} is given, @command{wc} prints a final line
3272 containing the cumulative counts, with the file name @file{total}. The
3273 counts are printed in this order: newlines, words, characters, bytes,
3274 maximum line length.
3275 Each count is printed right-justified in a field with at least one
3276 space between fields so that the numbers and file names normally line
3277 up nicely in columns. The width of the count fields varies depending
3278 on the inputs, so you should not depend on a particular field width.
3279 However, as a @acronym{GNU} extension, if only one count is printed,
3280 it is guaranteed to be printed without leading spaces.
3282 By default, @command{wc} prints three counts: the newline, words, and byte
3283 counts. Options can specify that only certain counts be printed.
3284 Options do not undo others previously given, so
3291 prints both the byte counts and the word counts.
3293 With the @option{--max-line-length} option, @command{wc} prints the length
3294 of the longest line per file, and if there is more than one file it
3295 prints the maximum (not the sum) of those lengths. The line lengths here
3296 are measured in screen columns, according to the current locale and
3297 assuming tab positions in every 8th column.
3299 The program accepts the following options. Also see @ref{Common options}.
3307 Print only the byte counts.
3313 Print only the character counts.
3319 Print only the word counts.
3325 Print only the newline counts.
3328 @itemx --max-line-length
3330 @opindex --max-line-length
3331 Print only the maximum line lengths.
3333 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3334 @itemx --files0-from=@var{file}
3335 @opindex --files0-from=@var{file}
3336 @c This is commented out to avoid a texi2dvi failure.
3337 @c texi2dvi (GNU Texinfo 4.11) 1.104
3338 @c @cindex including files from @command{\cmd\}
3339 Disallow processing files named on the command line, and instead process
3340 those named in file @var{file}; each name being terminated by a zero byte
3341 (@acronym{ASCII} @sc{nul}).
3342 This is useful \withTotalOption\
3343 when the list of file names is so long that it may exceed a command line
3345 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3346 because it splits the list into pieces and makes @command{\cmd\} print
3347 \subListOutput\ for each sublist rather than for the entire list.
3348 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3349 @command{find}, using its @option{-print0} predicate.
3350 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3351 are read from standard input.
3353 @filesZeroFromOption{wc,,a total}
3355 For example, to find the length of the longest line in any @file{.c} or
3356 @file{.h} file in the current hierarchy, do this:
3359 find . -name '*.[ch]' -print0 |
3360 wc -L --files0-from=- | tail -n1
3368 @node sum invocation
3369 @section @command{sum}: Print checksum and block counts
3372 @cindex 16-bit checksum
3373 @cindex checksum, 16-bit
3375 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3376 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3379 sum [@var{option}]@dots{} [@var{file}]@dots{}
3382 @command{sum} prints the checksum for each @var{file} followed by the
3383 number of blocks in the file (rounded up). If more than one @var{file}
3384 is given, file names are also printed (by default). (With the
3385 @option{--sysv} option, corresponding file names are printed when there is
3386 at least one file argument.)
3388 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3389 compatible with BSD @command{sum} and prints file sizes in units of
3392 The program accepts the following options. Also see @ref{Common options}.
3398 @cindex BSD @command{sum}
3399 Use the default (BSD compatible) algorithm. This option is included for
3400 compatibility with the System V @command{sum}. Unless @option{-s} was also
3401 given, it has no effect.
3407 @cindex System V @command{sum}
3408 Compute checksums using an algorithm compatible with System V
3409 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3413 @command{sum} is provided for compatibility; the @command{cksum} program (see
3414 next section) is preferable in new applications.
3419 @node cksum invocation
3420 @section @command{cksum}: Print CRC checksum and byte counts
3423 @cindex cyclic redundancy check
3424 @cindex CRC checksum
3426 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3427 given @var{file}, or standard input if none are given or for a
3428 @var{file} of @samp{-}. Synopsis:
3431 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3434 @command{cksum} prints the CRC checksum for each file along with the number
3435 of bytes in the file, and the file name unless no arguments were given.
3437 @command{cksum} is typically used to ensure that files
3438 transferred by unreliable means (e.g., netnews) have not been corrupted,
3439 by comparing the @command{cksum} output for the received files with the
3440 @command{cksum} output for the original files (typically given in the
3443 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3444 compatible with the BSD or System V @command{sum} algorithms (see the
3445 previous section); it is more robust.
3447 The only options are @option{--help} and @option{--version}. @xref{Common
3453 @node md5sum invocation
3454 @section @command{md5sum}: Print or check MD5 digests
3458 @cindex 128-bit checksum
3459 @cindex checksum, 128-bit
3460 @cindex fingerprint, 128-bit
3461 @cindex message-digest, 128-bit
3463 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3464 @dfn{message-digest}) for each specified @var{file}.
3466 Note: The MD5 digest is more reliable than a simple CRC (provided by
3467 the @command{cksum} command) for detecting accidental file corruption,
3468 as the chances of accidentally having two files with identical MD5
3469 are vanishingly small. However, it should not be considered secure
3470 against malicious tampering: although finding a file with a given MD5
3471 fingerprint is considered infeasible at the moment, it is known how
3472 to modify certain files, including digital certificates, so that they
3473 appear valid when signed with an MD5 digest.
3474 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3476 If a @var{file} is specified as @samp{-} or if no files are given
3477 @command{md5sum} computes the checksum for the standard input.
3478 @command{md5sum} can also determine whether a file and checksum are
3479 consistent. Synopsis:
3482 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3485 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3486 indicating a binary or text input file, and the file name.
3487 If @var{file} contains a backslash or newline, the
3488 line is started with a backslash, and each problematic character in
3489 the file name is escaped with a backslash, making the output
3490 unambiguous even in the presence of arbitrary file names.
3491 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3493 The program accepts the following options. Also see @ref{Common options}.
3501 @cindex binary input files
3502 Treat each input file as binary, by reading it in binary mode and
3503 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3504 On systems like @acronym{GNU} that do not distinguish between binary
3505 and text files, this option merely flags each input file as binary:
3506 the MD5 checksum is unaffected. This option is the default on systems
3507 like MS-DOS that distinguish between binary and text files, except
3508 for reading standard input when standard input is a terminal.
3512 Read file names and checksum information (not data) from each
3513 @var{file} (or from stdin if no @var{file} was specified) and report
3514 whether the checksums match the contents of the named files.
3515 The input to this mode of @command{md5sum} is usually the output of
3516 a prior, checksum-generating run of @samp{md5sum}.
3517 Each valid line of input consists of an MD5 checksum, a binary/text
3518 flag, and then a file name.
3519 Binary files are marked with @samp{*}, text with @samp{ }.
3520 For each such line, @command{md5sum} reads the named file and computes its
3521 MD5 checksum. Then, if the computed message digest does not match the
3522 one on the line with the file name, the file is noted as having
3523 failed the test. Otherwise, the file passes the test.
3524 By default, for each valid line, one line is written to standard
3525 output indicating whether the named file passed the test.
3526 After all checks have been performed, if there were any failures,
3527 a warning is issued to standard error.
3528 Use the @option{--status} option to inhibit that output.
3529 If any listed file cannot be opened or read, if any valid line has
3530 an MD5 checksum inconsistent with the associated file, or if no valid
3531 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3532 it exits successfully.
3536 @cindex verifying MD5 checksums
3537 This option is useful only when verifying checksums.
3538 When verifying checksums, don't generate an 'OK' message per successfully
3539 checked file. Files that fail the verification are reported in the
3540 default one-line-per-file format. If there is any checksum mismatch,
3541 print a warning summarizing the failures to standard error.
3545 @cindex verifying MD5 checksums
3546 This option is useful only when verifying checksums.
3547 When verifying checksums, don't generate the default one-line-per-file
3548 diagnostic and don't output the warning summarizing any failures.
3549 Failures to open or read a file still evoke individual diagnostics to
3551 If all listed files are readable and are consistent with the associated
3552 MD5 checksums, exit successfully. Otherwise exit with a status code
3553 indicating there was a failure.
3559 @cindex text input files
3560 Treat each input file as text, by reading it in text mode and
3561 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3562 This option is the default on systems like @acronym{GNU} that do not
3563 distinguish between binary and text files. On other systems, it is
3564 the default for reading standard input when standard input is a
3571 @cindex verifying MD5 checksums
3572 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3573 This option is useful only if all but a few lines in the checked input
3581 @node sha1sum invocation
3582 @section @command{sha1sum}: Print or check SHA-1 digests
3586 @cindex 160-bit checksum
3587 @cindex checksum, 160-bit
3588 @cindex fingerprint, 160-bit
3589 @cindex message-digest, 160-bit
3591 @command{sha1sum} computes a 160-bit checksum for each specified
3592 @var{file}. The usage and options of this command are precisely the
3593 same as for @command{md5sum}. @xref{md5sum invocation}.
3595 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3596 it are known (different files having the same fingerprint). However,
3597 it is known that they can be produced with considerable, but not
3598 unreasonable, resources. For this reason, it is generally considered
3599 that SHA-1 should be gradually phased out in favor of the more secure
3600 SHA-2 hash algorithms. @xref{sha2 utilities}.
3603 @node sha2 utilities
3604 @section sha2 utilities: Print or check SHA-2 digests
3611 @cindex 224-bit checksum
3612 @cindex 256-bit checksum
3613 @cindex 384-bit checksum
3614 @cindex 512-bit checksum
3615 @cindex checksum, 224-bit
3616 @cindex checksum, 256-bit
3617 @cindex checksum, 384-bit
3618 @cindex checksum, 512-bit
3619 @cindex fingerprint, 224-bit
3620 @cindex fingerprint, 256-bit
3621 @cindex fingerprint, 384-bit
3622 @cindex fingerprint, 512-bit
3623 @cindex message-digest, 224-bit
3624 @cindex message-digest, 256-bit
3625 @cindex message-digest, 384-bit
3626 @cindex message-digest, 512-bit
3628 The commands @command{sha224sum}, @command{sha256sum},
3629 @command{sha384sum} and @command{sha512sum} compute checksums of
3630 various lengths (respectively 224, 256, 384 and 512 bits),
3631 collectively known as the SHA-2 hashes. The usage and options of
3632 these commands are precisely the same as for @command{md5sum}.
3633 @xref{md5sum invocation}.
3635 Note: The SHA384 and SHA512 digests are considerably slower to
3636 compute, especially on 32-bit computers, than SHA224 or SHA256.
3639 @node Operating on sorted files
3640 @chapter Operating on sorted files
3642 @cindex operating on sorted files
3643 @cindex sorted files, operations on
3645 These commands work with (or produce) sorted files.
3648 * sort invocation:: Sort text files.
3649 * shuf invocation:: Shuffle text files.
3650 * uniq invocation:: Uniquify files.
3651 * comm invocation:: Compare two sorted files line by line.
3652 * ptx invocation:: Produce a permuted index of file contents.
3653 * tsort invocation:: Topological sort.
3657 @node sort invocation
3658 @section @command{sort}: Sort text files
3661 @cindex sorting files
3663 @command{sort} sorts, merges, or compares all the lines from the given
3664 files, or standard input if none are given or for a @var{file} of
3665 @samp{-}. By default, @command{sort} writes the results to standard
3669 sort [@var{option}]@dots{} [@var{file}]@dots{}
3672 @command{sort} has three modes of operation: sort (the default), merge,
3673 and check for sortedness. The following options change the operation
3680 @itemx --check=diagnose-first
3683 @cindex checking for sortedness
3684 Check whether the given file is already sorted: if it is not all
3685 sorted, print a diagnostic containing the first out-of-order line and
3686 exit with a status of 1.
3687 Otherwise, exit successfully.
3688 At most one input file can be given.
3691 @itemx --check=quiet
3692 @itemx --check=silent
3695 @cindex checking for sortedness
3696 Exit successfully if the given file is already sorted, and
3697 exit with status 1 otherwise.
3698 At most one input file can be given.
3699 This is like @option{-c}, except it does not print a diagnostic.
3705 @cindex merging sorted files
3706 Merge the given files by sorting them as a group. Each input file must
3707 always be individually sorted. It always works to sort instead of
3708 merge; merging is provided because it is faster, in the case where it
3713 @cindex sort stability
3714 @cindex sort's last-resort comparison
3715 A pair of lines is compared as follows:
3716 @command{sort} compares each pair of fields, in the
3717 order specified on the command line, according to the associated
3718 ordering options, until a difference is found or no fields are left.
3719 If no key fields are specified, @command{sort} uses a default key of
3720 the entire line. Finally, as a last resort when all keys compare
3721 equal, @command{sort} compares entire lines as if no ordering options
3722 other than @option{--reverse} (@option{-r}) were specified. The
3723 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3724 comparison} so that lines in which all fields compare equal are left
3725 in their original relative order. The @option{--unique}
3726 (@option{-u}) option also disables the last-resort comparison.
3730 Unless otherwise specified, all comparisons use the character collating
3731 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3732 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3733 to @samp{en_US}), then @command{sort} may produce output that is sorted
3734 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3735 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3736 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3737 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3738 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3739 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3740 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3742 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3743 limit on input line length or restrictions on bytes allowed within lines.
3744 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3745 @command{sort} silently supplies one. A line's trailing newline is not
3746 part of the line for comparison purposes.
3748 @cindex exit status of @command{sort}
3752 0 if no error occurred
3753 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3754 2 if an error occurred
3758 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3759 value as the directory for temporary files instead of @file{/tmp}. The
3760 @option{--temporary-directory} (@option{-T}) option in turn overrides
3761 the environment variable.
3763 The following options affect the ordering of output lines. They may be
3764 specified globally or as part of a specific key field. If no key
3765 fields are specified, global options apply to comparison of entire
3766 lines; otherwise the global options are inherited by key fields that do
3767 not specify any special options of their own. In pre-@acronym{POSIX}
3768 versions of @command{sort}, global options affect only later key fields,
3769 so portable shell scripts should specify global options first.
3774 @itemx --ignore-leading-blanks
3776 @opindex --ignore-leading-blanks
3777 @cindex blanks, ignoring leading
3779 Ignore leading blanks when finding sort keys in each line.
3780 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3781 can change this. Note blanks may be ignored by your locale's collating
3782 rules, but without this option they will be significant for character
3783 positions specified in keys with the @option{-k} option.
3786 @itemx --dictionary-order
3788 @opindex --dictionary-order
3789 @cindex dictionary order
3790 @cindex phone directory order
3791 @cindex telephone directory order
3793 Sort in @dfn{phone directory} order: ignore all characters except
3794 letters, digits and blanks when sorting.
3795 By default letters and digits are those of @acronym{ASCII} and a blank
3796 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3799 @itemx --ignore-case
3801 @opindex --ignore-case
3802 @cindex ignoring case
3803 @cindex case folding
3805 Fold lowercase characters into the equivalent uppercase characters when
3806 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3807 The @env{LC_CTYPE} locale determines character types.
3808 When used with @option{--unique} those lower case equivalent lines are
3809 thrown away. (There is currently no way to throw away the upper case
3810 equivalent instead. (Any @option{--reverse} given would only affect
3811 the final result, after the throwing away.))
3814 @itemx --general-numeric-sort
3815 @itemx --sort=general-numeric
3817 @opindex --general-numeric-sort
3819 @cindex general numeric sort
3821 Sort numerically, using the standard C function @code{strtold} to convert
3822 a prefix of each line to a long double-precision floating point number.
3823 This allows floating point numbers to be specified in scientific notation,
3824 like @code{1.0e-34} and @code{10e100}.
3825 The @env{LC_NUMERIC} locale determines the decimal-point character.
3826 Do not report overflow, underflow, or conversion errors.
3827 Use the following collating sequence:
3831 Lines that do not start with numbers (all considered to be equal).
3833 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3834 in a consistent but machine-dependent order.
3838 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3843 Use this option only if there is no alternative; it is much slower than
3844 @option{--numeric-sort} (@option{-n}) and it can lose information when
3845 converting to floating point.
3848 @itemx --human-numeric-sort
3849 @itemx --sort=human-numeric
3851 @opindex --human-numeric-sort
3853 @cindex human numeric sort
3855 Sort numerically, first by numeric sign (negative, zero, or positive);
3856 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3857 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3858 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3859 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3860 suffix. This option sorts values that are consistently scaled to the
3861 nearest suffix, regardless of whether suffixes denote powers of 1000
3862 or 1024, and it therefore sorts the output of any single invocation of
3863 the @command{df}, @command{du}, or @command{ls} commands that are
3864 invoked with their @option{--human-readable} or @option{--si} options.
3865 The syntax for numbers is the same as for the @option{--numeric-sort}
3866 option; the @acronym{SI} suffix must immediately follow the number.
3869 @itemx --ignore-nonprinting
3871 @opindex --ignore-nonprinting
3872 @cindex nonprinting characters, ignoring
3873 @cindex unprintable characters, ignoring
3875 Ignore nonprinting characters.
3876 The @env{LC_CTYPE} locale determines character types.
3877 This option has no effect if the stronger @option{--dictionary-order}
3878 (@option{-d}) option is also given.
3884 @opindex --month-sort
3886 @cindex months, sorting by
3888 An initial string, consisting of any amount of blanks, followed
3889 by a month name abbreviation, is folded to UPPER case and
3890 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3891 Invalid names compare low to valid names. The @env{LC_TIME} locale
3892 category determines the month spellings.
3893 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3897 @itemx --numeric-sort
3898 @itemx --sort=numeric
3900 @opindex --numeric-sort
3902 @cindex numeric sort
3904 Sort numerically. The number begins each line and consists
3905 of optional blanks, an optional @samp{-} sign, and zero or more
3906 digits possibly separated by thousands separators, optionally followed
3907 by a decimal-point character and zero or more digits. An empty
3908 number is treated as @samp{0}. The @env{LC_NUMERIC}
3909 locale specifies the decimal-point character and thousands separator.
3910 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3913 Comparison is exact; there is no rounding error.
3915 Neither a leading @samp{+} nor exponential notation is recognized.
3916 To compare such strings numerically, use the
3917 @option{--general-numeric-sort} (@option{-g}) option.
3920 @itemx --version-sort
3922 @opindex --version-sort
3923 @cindex version number sort
3924 Sort by version name and number. It behaves like a standard sort,
3925 except that each sequence of decimal digits is treated numerically
3926 as an index/version number. (@xref{Details about version sort}.)
3932 @cindex reverse sorting
3933 Reverse the result of comparison, so that lines with greater key values
3934 appear earlier in the output instead of later.
3937 @itemx --random-sort
3938 @itemx --sort=random
3940 @opindex --random-sort
3943 Sort by hashing the input keys and then sorting the hash values.
3944 Choose the hash function at random, ensuring that it is free of
3945 collisions so that differing keys have differing hash values. This is
3946 like a random permutation of the inputs (@pxref{shuf invocation}),
3947 except that keys with the same value sort together.
3949 If multiple random sort fields are specified, the same random hash
3950 function is used for all fields. To use different random hash
3951 functions for different fields, you can invoke @command{sort} more
3954 The choice of hash function is affected by the
3955 @option{--random-source} option.
3963 @item --compress-program=@var{prog}
3964 Compress any temporary files with the program @var{prog}.
3966 With no arguments, @var{prog} must compress standard input to standard
3967 output, and when given the @option{-d} option it must decompress
3968 standard input to standard output.
3970 Terminate with an error if @var{prog} exits with nonzero status.
3972 White space and the backslash character should not appear in
3973 @var{prog}; they are reserved for future use.
3975 @filesZeroFromOption{sort,,sorted output}
3977 @item -k @var{pos1}[,@var{pos2}]
3978 @itemx --key=@var{pos1}[,@var{pos2}]
3982 Specify a sort field that consists of the part of the line between
3983 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3984 omitted), @emph{inclusive}.
3986 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3987 where @var{f} is the number of the field to use, and @var{c} is the number
3988 of the first character from the beginning of the field. Fields and character
3989 positions are numbered starting with 1; a character position of zero in
3990 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3991 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3992 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3993 @var{opts} are ordering options, allowing individual keys to be sorted
3994 according to different rules; see below for details. Keys can span
3997 Example: To sort on the second field, use @option{--key=2,2}
3998 (@option{-k 2,2}). See below for more notes on keys and more examples.
3999 See also the @option{--debug} option to help determine the part
4000 of the line being used in the sort.
4003 Highlight the portion of each line used for sorting.
4004 Also issue warnings about questionable usage to stderr.
4006 @item --batch-size=@var{nmerge}
4007 @opindex --batch-size
4008 @cindex number of inputs to merge, nmerge
4009 Merge at most @var{nmerge} inputs at once.
4011 When @command{sort} has to merge more than @var{nmerge} inputs,
4012 it merges them in groups of @var{nmerge}, saving the result in
4013 a temporary file, which is then used as an input in a subsequent merge.
4015 A large value of @var{nmerge} may improve merge performance and decrease
4016 temporary storage utilization at the expense of increased memory usage
4017 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4018 requirements and I/0 at the expense of temporary storage consumption and
4021 The value of @var{nmerge} must be at least 2. The default value is
4022 currently 16, but this is implementation-dependent and may change in
4025 The value of @var{nmerge} may be bounded by a resource limit for open
4026 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4027 OPEN_MAX} may display limits for your systems; these limits may be
4028 modified further if your program already has some files open, or if
4029 the operating system has other limits on the number of open files. If
4030 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4031 silently uses a smaller value.
4033 @item -o @var{output-file}
4034 @itemx --output=@var{output-file}
4037 @cindex overwriting of input, allowed
4038 Write output to @var{output-file} instead of standard output.
4039 Normally, @command{sort} reads all input before opening
4040 @var{output-file}, so you can safely sort a file in place by using
4041 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4042 However, @command{sort} with @option{--merge} (@option{-m}) can open
4043 the output file before reading all input, so a command like @code{cat
4044 F | sort -m -o F - G} is not safe as @command{sort} might start
4045 writing @file{F} before @command{cat} is done reading it.
4047 @vindex POSIXLY_CORRECT
4048 On newer systems, @option{-o} cannot appear after an input file if
4049 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4050 scripts should specify @option{-o @var{output-file}} before any input
4053 @item --random-source=@var{file}
4054 @opindex --random-source
4055 @cindex random source for sorting
4056 Use @var{file} as a source of random data used to determine which
4057 random hash function to use with the @option{-R} option. @xref{Random
4064 @cindex sort stability
4065 @cindex sort's last-resort comparison
4067 Make @command{sort} stable by disabling its last-resort comparison.
4068 This option has no effect if no fields or global ordering options
4069 other than @option{--reverse} (@option{-r}) are specified.
4072 @itemx --buffer-size=@var{size}
4074 @opindex --buffer-size
4075 @cindex size for main memory sorting
4076 Use a main-memory sort buffer of the given @var{size}. By default,
4077 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4078 @var{size} to be interpreted as a percentage of physical memory.
4079 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4080 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4081 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4082 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4085 This option can improve the performance of @command{sort} by causing it
4086 to start with a larger or smaller sort buffer than the default.
4087 However, this option affects only the initial buffer size. The buffer
4088 grows beyond @var{size} if @command{sort} encounters input lines larger
4091 @item -t @var{separator}
4092 @itemx --field-separator=@var{separator}
4094 @opindex --field-separator
4095 @cindex field separator character
4096 Use character @var{separator} as the field separator when finding the
4097 sort keys in each line. By default, fields are separated by the empty
4098 string between a non-blank character and a blank character.
4099 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4102 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4103 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4104 not considered to be part of either the field preceding or the field
4105 following, so with @samp{sort @w{-t " "}} the same input line has
4106 three fields: an empty field, @samp{foo}, and @samp{bar}.
4107 However, fields that extend to the end of the line,
4108 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4109 retain the field separators present between the endpoints of the range.
4111 To specify @acronym{ASCII} @sc{nul} as the field separator,
4112 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4114 @item -T @var{tempdir}
4115 @itemx --temporary-directory=@var{tempdir}
4117 @opindex --temporary-directory
4118 @cindex temporary directory
4120 Use directory @var{tempdir} to store temporary files, overriding the
4121 @env{TMPDIR} environment variable. If this option is given more than
4122 once, temporary files are stored in all the directories given. If you
4123 have a large sort or merge that is I/O-bound, you can often improve
4124 performance by using this option to specify directories on different
4125 disks and controllers.
4127 @item --parallel=@var{n}
4129 @cindex multithreaded sort
4130 Limit the number of sorts run in parallel to @var{n}. By default,
4131 @var{n} is set to the number of available processors, and values
4132 greater than that are reduced to that limit. Also see
4133 @ref{nproc invocation}.
4139 @cindex uniquifying output
4141 Normally, output only the first of a sequence of lines that compare
4142 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4143 check that no pair of consecutive lines compares equal.
4145 This option also disables the default last-resort comparison.
4147 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4148 this equivalence does not extend to arbitrary @command{sort} options.
4149 For example, @code{sort -n -u} inspects only the value of the initial
4150 numeric string when checking for uniqueness, whereas @code{sort -n |
4151 uniq} inspects the entire line. @xref{uniq invocation}.
4153 @macro zeroTerminatedOption
4155 @itemx --zero-terminated
4157 @opindex --zero-terminated
4158 @cindex process zero-terminated items
4159 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4160 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4161 and terminate output items with @acronym{ASCII} @sc{nul}.
4162 This option can be useful in conjunction with @samp{perl -0} or
4163 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4164 reliably handle arbitrary file names (even those containing blanks
4165 or other special characters).
4167 @zeroTerminatedOption
4171 Historical (BSD and System V) implementations of @command{sort} have
4172 differed in their interpretation of some options, particularly
4173 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4174 behavior, which is usually (but not always!) like the System V behavior.
4175 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4176 consistency, @option{-M} has been changed in the same way. This may
4177 affect the meaning of character positions in field specifications in
4178 obscure cases. The only fix is to add an explicit @option{-b}.
4180 A position in a sort field specified with @option{-k} may have any
4181 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4182 global ordering options are inherited by that particular field. The
4183 @option{-b} option may be independently attached to either or both of
4184 the start and end positions of a field specification, and if it is
4185 inherited from the global options it will be attached to both.
4186 If input lines can contain leading or adjacent blanks and @option{-t}
4187 is not used, then @option{-k} is typically combined with @option{-b} or
4188 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4189 the varying numbers of leading blanks in fields can cause confusing results.
4191 If the start position in a sort field specifier falls after the end of
4192 the line or after the end field, the field is empty. If the @option{-b}
4193 option was specified, the @samp{.@var{c}} part of a field specification
4194 is counted from the first nonblank character of the field.
4196 @vindex _POSIX2_VERSION
4197 @vindex POSIXLY_CORRECT
4198 On older systems, @command{sort} supports an obsolete origin-zero
4199 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4200 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4201 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4202 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4203 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4205 This obsolete behavior can be enabled or disabled with the
4206 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4207 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4208 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4210 Scripts intended for use on standard hosts should avoid obsolete
4211 syntax and should use @option{-k} instead. For example, avoid
4212 @samp{sort +2}, since it might be interpreted as either @samp{sort
4213 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4214 support only the obsolete syntax, it can use a test like @samp{if sort
4215 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4218 Here are some examples to illustrate various combinations of options.
4223 Sort in descending (reverse) numeric order.
4230 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4233 sort --parallel=4 -S 10M
4237 Sort alphabetically, omitting the first and second fields
4238 and the blanks at the start of the third field.
4239 This uses a single key composed of the characters beginning
4240 at the start of the first nonblank character in field three
4241 and extending to the end of each line.
4248 Sort numerically on the second field and resolve ties by sorting
4249 alphabetically on the third and fourth characters of field five.
4250 Use @samp{:} as the field delimiter.
4253 sort -t : -k 2,2n -k 5.3,5.4
4256 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4257 @command{sort} would have used all characters beginning in the second field
4258 and extending to the end of the line as the primary @emph{numeric}
4259 key. For the large majority of applications, treating keys spanning
4260 more than one field as numeric will not do what you expect.
4262 Also note that the @samp{n} modifier was applied to the field-end
4263 specifier for the first key. It would have been equivalent to
4264 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4265 @samp{b} apply to the associated @emph{field}, regardless of whether
4266 the modifier character is attached to the field-start and/or the
4267 field-end part of the key specifier.
4270 Sort the password file on the fifth field and ignore any
4271 leading blanks. Sort lines with equal values in field five
4272 on the numeric user ID in field three. Fields are separated
4276 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4277 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4278 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4281 These three commands have equivalent effect. The first specifies that
4282 the first key's start position ignores leading blanks and the second
4283 key is sorted numerically. The other two commands rely on global
4284 options being inherited by sort keys that lack modifiers. The inheritance
4285 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4286 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4287 character position is not affected by whether initial blanks are
4291 Sort a set of log files, primarily by IPv4 address and secondarily by
4292 time stamp. If two lines' primary and secondary keys are identical,
4293 output the lines in the same order that they were input. The log
4294 files contain lines that look like this:
4297 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4298 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4301 Fields are separated by exactly one space. Sort IPv4 addresses
4302 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4303 because 61 is less than 129.
4306 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4307 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4310 This example cannot be done with a single @command{sort} invocation,
4311 since IPv4 address components are separated by @samp{.} while dates
4312 come just after a space. So it is broken down into two invocations of
4313 @command{sort}: the first sorts by time stamp and the second by IPv4
4314 address. The time stamp is sorted by year, then month, then day, and
4315 finally by hour-minute-second field, using @option{-k} to isolate each
4316 field. Except for hour-minute-second there's no need to specify the
4317 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4318 based on leading prefixes that cannot cross field boundaries. The
4319 IPv4 addresses are sorted lexicographically. The second sort uses
4320 @samp{-s} so that ties in the primary key are broken by the secondary
4321 key; the first sort uses @samp{-s} so that the combination of the two
4325 Generate a tags file in case-insensitive sorted order.
4328 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4331 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4332 that file names that contain blanks or other special characters are
4334 by the sort operation.
4336 @c This example is a bit contrived and needs more explanation.
4338 @c Sort records separated by an arbitrary string by using a pipe to convert
4339 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4340 @c and converting each @samp{\0} back to the original record delimiter.
4343 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4347 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4348 sort lines according to their length.
4351 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4354 In general this technique can be used to sort data that the @command{sort}
4355 command does not support, or is inefficient at, sorting directly.
4358 Shuffle a list of directories, but preserve the order of files within
4359 each directory. For instance, one could use this to generate a music
4360 playlist in which albums are shuffled but the songs of each album are
4364 ls */* | sort -t / -k 1,1R -k 2,2
4370 @node shuf invocation
4371 @section @command{shuf}: Shuffling text
4374 @cindex shuffling files
4376 @command{shuf} shuffles its input by outputting a random permutation
4377 of its input lines. Each output permutation is equally likely.
4381 shuf [@var{option}]@dots{} [@var{file}]
4382 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4383 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4386 @command{shuf} has three modes of operation that affect where it
4387 obtains its input lines. By default, it reads lines from standard
4388 input. The following options change the operation mode:
4396 @cindex command-line operands to shuffle
4397 Treat each command-line operand as an input line.
4399 @item -i @var{lo}-@var{hi}
4400 @itemx --input-range=@var{lo}-@var{hi}
4402 @opindex --input-range
4403 @cindex input range to shuffle
4404 Act as if input came from a file containing the range of unsigned
4405 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4409 @command{shuf}'s other options can affect its behavior in all
4414 @item -n @var{lines}
4415 @itemx --head-count=@var{count}
4417 @opindex --head-count
4418 @cindex head of output
4419 Output at most @var{count} lines. By default, all input lines are
4422 @item -o @var{output-file}
4423 @itemx --output=@var{output-file}
4426 @cindex overwriting of input, allowed
4427 Write output to @var{output-file} instead of standard output.
4428 @command{shuf} reads all input before opening
4429 @var{output-file}, so you can safely shuffle a file in place by using
4430 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4432 @item --random-source=@var{file}
4433 @opindex --random-source
4434 @cindex random source for shuffling
4435 Use @var{file} as a source of random data used to determine which
4436 permutation to generate. @xref{Random sources}.
4438 @zeroTerminatedOption
4454 might produce the output
4464 Similarly, the command:
4467 shuf -e clubs hearts diamonds spades
4481 and the command @samp{shuf -i 1-4} might output:
4491 These examples all have four input lines, so @command{shuf} might
4492 produce any of the twenty-four possible permutations of the input. In
4493 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4494 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4495 output permutations.
4500 @node uniq invocation
4501 @section @command{uniq}: Uniquify files
4504 @cindex uniquify files
4506 @command{uniq} writes the unique lines in the given @file{input}, or
4507 standard input if nothing is given or for an @var{input} name of
4511 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4514 By default, @command{uniq} prints its input lines, except that
4515 it discards all but the first of adjacent repeated lines, so that
4516 no output lines are repeated. Optionally, it can instead discard
4517 lines that are not repeated, or all repeated lines.
4519 The input need not be sorted, but repeated input lines are detected
4520 only if they are adjacent. If you want to discard non-adjacent
4521 duplicate lines, perhaps you want to use @code{sort -u}.
4522 @xref{sort invocation}.
4525 Comparisons honor the rules specified by the @env{LC_COLLATE}
4528 If no @var{output} file is specified, @command{uniq} writes to standard
4531 The program accepts the following options. Also see @ref{Common options}.
4536 @itemx --skip-fields=@var{n}
4538 @opindex --skip-fields
4539 Skip @var{n} fields on each line before checking for uniqueness. Use
4540 a null string for comparison if a line has fewer than @var{n} fields. Fields
4541 are sequences of non-space non-tab characters that are separated from
4542 each other by at least one space or tab.
4544 For compatibility @command{uniq} supports an obsolete option syntax
4545 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4548 @itemx --skip-chars=@var{n}
4550 @opindex --skip-chars
4551 Skip @var{n} characters before checking for uniqueness. Use a null string
4552 for comparison if a line has fewer than @var{n} characters. If you use both
4553 the field and character skipping options, fields are skipped over first.
4555 @vindex _POSIX2_VERSION
4556 On older systems, @command{uniq} supports an obsolete option syntax
4558 This obsolete behavior can be enabled or disabled with the
4559 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4560 conformance}), but portable scripts should avoid commands whose
4561 behavior depends on this variable.
4562 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4563 the ambiguous @samp{uniq +10}.
4569 Print the number of times each line occurred along with the line.
4572 @itemx --ignore-case
4574 @opindex --ignore-case
4575 Ignore differences in case when comparing lines.
4581 @cindex repeated lines, outputting
4582 Discard lines that are not repeated. When used by itself, this option
4583 causes @command{uniq} to print the first copy of each repeated line,
4587 @itemx --all-repeated[=@var{delimit-method}]
4589 @opindex --all-repeated
4590 @cindex all repeated lines, outputting
4591 Do not discard the second and subsequent repeated input lines,
4592 but discard lines that are not repeated.
4593 This option is useful mainly in conjunction with other options e.g.,
4594 to ignore case or to compare only selected fields.
4595 The optional @var{delimit-method} tells how to delimit
4596 groups of repeated lines, and must be one of the following:
4601 Do not delimit groups of repeated lines.
4602 This is equivalent to @option{--all-repeated} (@option{-D}).
4605 Output a newline before each group of repeated lines.
4606 With @option{--zero-terminated} (@option{-z}), use a zero
4607 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4610 Separate groups of repeated lines with a single newline.
4611 With @option{--zero-terminated} (@option{-z}), use a zero
4612 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4613 This is the same as using @samp{prepend}, except that
4614 no delimiter is inserted before the first group, and hence
4615 may be better suited for output direct to users.
4618 Note that when groups are delimited and the input stream contains
4619 two or more consecutive blank lines, then the output is ambiguous.
4620 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4621 each sequence of consecutive newlines with a single newline.
4623 This is a @sc{gnu} extension.
4624 @c FIXME: give an example showing *how* it's useful
4630 @cindex unique lines, outputting
4631 Discard the first repeated line. When used by itself, this option
4632 causes @command{uniq} to print unique lines, and nothing else.
4635 @itemx --check-chars=@var{n}
4637 @opindex --check-chars
4638 Compare at most @var{n} characters on each line (after skipping any specified
4639 fields and characters). By default the entire rest of the lines are
4642 @zeroTerminatedOption
4649 @node comm invocation
4650 @section @command{comm}: Compare two sorted files line by line
4653 @cindex line-by-line comparison
4654 @cindex comparing sorted files
4656 @command{comm} writes to standard output lines that are common, and lines
4657 that are unique, to two input files; a file name of @samp{-} means
4658 standard input. Synopsis:
4661 comm [@var{option}]@dots{} @var{file1} @var{file2}
4665 Before @command{comm} can be used, the input files must be sorted using the
4666 collating sequence specified by the @env{LC_COLLATE} locale.
4667 If an input file ends in a non-newline
4668 character, a newline is silently appended. The @command{sort} command with
4669 no options always outputs a file that is suitable input to @command{comm}.
4671 @cindex differing lines
4672 @cindex common lines
4673 With no options, @command{comm} produces three-column output. Column one
4674 contains lines unique to @var{file1}, column two contains lines unique
4675 to @var{file2}, and column three contains lines common to both files.
4676 Columns are separated by a single TAB character.
4677 @c FIXME: when there's an option to supply an alternative separator
4678 @c string, append `by default' to the above sentence.
4683 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4684 the corresponding columns (and separators). Also see @ref{Common options}.
4686 Unlike some other comparison utilities, @command{comm} has an exit
4687 status that does not depend on the result of the comparison.
4688 Upon normal completion @command{comm} produces an exit code of zero.
4689 If there is an error it exits with nonzero status.
4691 @macro checkOrderOption{cmd}
4692 If the @option{--check-order} option is given, unsorted inputs will
4693 cause a fatal error message. If the option @option{--nocheck-order}
4694 is given, unsorted inputs will never cause an error message. If
4695 neither of these options is given, wrongly sorted inputs are diagnosed
4696 only if an input file is found to contain unpairable lines. If an
4697 input file is diagnosed as being unsorted, the @command{\cmd\} command
4698 will exit with a nonzero status (and the output should not be used).
4700 Forcing @command{\cmd\} to process wrongly sorted input files
4701 containing unpairable lines by specifying @option{--nocheck-order} is
4702 not guaranteed to produce any particular output. The output will
4703 probably not correspond with whatever you hoped it would be.
4705 @checkOrderOption{comm}
4710 Fail with an error message if either input file is wrongly ordered.
4712 @item --nocheck-order
4713 Do not check that both input files are in sorted order.
4717 @item --output-delimiter=@var{str}
4718 Print @var{str} between adjacent output columns,
4719 rather than the default of a single TAB character.
4721 The delimiter @var{str} may not be empty.
4725 @node ptx invocation
4726 @section @command{ptx}: Produce permuted indexes
4730 @command{ptx} reads a text file and essentially produces a permuted index, with
4731 each keyword in its context. The calling sketch is either one of:
4734 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4735 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4738 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4739 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4740 limitations and changing several of the program's default option values.
4741 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4742 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4743 document. For the full list, see @xref{Compatibility in ptx}.
4745 Individual options are explained in the following sections.
4747 When @sc{gnu} extensions are enabled, there may be zero, one or several
4748 @var{file}s after the options. If there is no @var{file}, the program
4749 reads the standard input. If there is one or several @var{file}s, they
4750 give the name of input files which are all read in turn, as if all the
4751 input files were concatenated. However, there is a full contextual
4752 break between each file and, when automatic referencing is requested,
4753 file names and line numbers refer to individual text input files. In
4754 all cases, the program outputs the permuted index to the standard
4757 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4758 operates in traditional mode, there may be zero, one or two parameters
4759 besides the options. If there are no parameters, the program reads the
4760 standard input and outputs the permuted index to the standard output.
4761 If there is only one parameter, it names the text @var{input} to be read
4762 instead of the standard input. If two parameters are given, they give
4763 respectively the name of the @var{input} file to read and the name of
4764 the @var{output} file to produce. @emph{Be very careful} to note that,
4765 in this case, the contents of file given by the second parameter is
4766 destroyed. This behavior is dictated by System V @command{ptx}
4767 compatibility; @sc{gnu} Standards normally discourage output parameters not
4768 introduced by an option.
4770 Note that for @emph{any} file named as the value of an option or as an
4771 input text file, a single dash @kbd{-} may be used, in which case
4772 standard input is assumed. However, it would not make sense to use this
4773 convention more than once per program invocation.
4776 * General options in ptx:: Options which affect general program behavior.
4777 * Charset selection in ptx:: Underlying character set considerations.
4778 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4779 * Output formatting in ptx:: Types of output format, and sizing the fields.
4780 * Compatibility in ptx::
4784 @node General options in ptx
4785 @subsection General options
4790 @itemx --traditional
4791 As already explained, this option disables all @sc{gnu} extensions to
4792 @command{ptx} and switches to traditional mode.
4795 Print a short help on standard output, then exit without further
4799 Print the program version on standard output, then exit without further
4807 @node Charset selection in ptx
4808 @subsection Charset selection
4810 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4811 As it is set up now, the program assumes that the input file is coded
4812 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4813 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4814 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4815 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4816 of characters which are letters is different; this alters the behavior
4817 of regular expression matching. Thus, the default regular expression
4818 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4819 however, is still crude; it obeys the underlying character set ordering
4825 @itemx --ignore-case
4826 Fold lower case letters to upper case for sorting.
4831 @node Input processing in ptx
4832 @subsection Word selection and input processing
4837 @itemx --break-file=@var{file}
4839 This option provides an alternative (to @option{-W}) method of describing
4840 which characters make up words. It introduces the name of a
4841 file which contains a list of characters which can@emph{not} be part of
4842 one word; this file is called the @dfn{Break file}. Any character which
4843 is not part of the Break file is a word constituent. If both options
4844 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4845 @option{-b} is ignored.
4847 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4848 break character is to write all the break characters in the file with no
4849 newline at all, not even at the end of the file. When @sc{gnu} extensions
4850 are disabled, spaces, tabs and newlines are always considered as break
4851 characters even if not included in the Break file.
4854 @itemx --ignore-file=@var{file}
4856 The file associated with this option contains a list of words which will
4857 never be taken as keywords in concordance output. It is called the
4858 @dfn{Ignore file}. The file contains exactly one word in each line; the
4859 end of line separation of words is not subject to the value of the
4863 @itemx --only-file=@var{file}
4865 The file associated with this option contains a list of words which will
4866 be retained in concordance output; any word not mentioned in this file
4867 is ignored. The file is called the @dfn{Only file}. The file contains
4868 exactly one word in each line; the end of line separation of words is
4869 not subject to the value of the @option{-S} option.
4871 There is no default for the Only file. When both an Only file and an
4872 Ignore file are specified, a word is considered a keyword only
4873 if it is listed in the Only file and not in the Ignore file.
4878 On each input line, the leading sequence of non-white space characters will be
4879 taken to be a reference that has the purpose of identifying this input
4880 line in the resulting permuted index. For more information about reference
4881 production, see @xref{Output formatting in ptx}.
4882 Using this option changes the default value for option @option{-S}.
4884 Using this option, the program does not try very hard to remove
4885 references from contexts in output, but it succeeds in doing so
4886 @emph{when} the context ends exactly at the newline. If option
4887 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4888 are disabled, this condition is always met and references are completely
4889 excluded from the output contexts.
4891 @item -S @var{regexp}
4892 @itemx --sentence-regexp=@var{regexp}
4894 This option selects which regular expression will describe the end of a
4895 line or the end of a sentence. In fact, this regular expression is not
4896 the only distinction between end of lines or end of sentences, and input
4897 line boundaries have no special significance outside this option. By
4898 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4899 used, end of sentences are used. In this case, this @var{regex} is
4900 imported from @sc{gnu} Emacs:
4903 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4906 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4907 of lines are used; in this case, the default @var{regexp} is just:
4913 Using an empty @var{regexp} is equivalent to completely disabling end of
4914 line or end of sentence recognition. In this case, the whole file is
4915 considered to be a single big line or sentence. The user might want to
4916 disallow all truncation flag generation as well, through option @option{-F
4917 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4920 When the keywords happen to be near the beginning of the input line or
4921 sentence, this often creates an unused area at the beginning of the
4922 output context line; when the keywords happen to be near the end of the
4923 input line or sentence, this often creates an unused area at the end of
4924 the output context line. The program tries to fill those unused areas
4925 by wrapping around context in them; the tail of the input line or
4926 sentence is used to fill the unused area on the left of the output line;
4927 the head of the input line or sentence is used to fill the unused area
4928 on the right of the output line.
4930 As a matter of convenience to the user, many usual backslashed escape
4931 sequences from the C language are recognized and converted to the
4932 corresponding characters by @command{ptx} itself.
4934 @item -W @var{regexp}
4935 @itemx --word-regexp=@var{regexp}
4937 This option selects which regular expression will describe each keyword.
4938 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4939 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4940 disabled, a word is by default anything which ends with a space, a tab
4941 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4943 An empty @var{regexp} is equivalent to not using this option.
4944 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4947 As a matter of convenience to the user, many usual backslashed escape
4948 sequences, as found in the C language, are recognized and converted to
4949 the corresponding characters by @command{ptx} itself.
4954 @node Output formatting in ptx
4955 @subsection Output formatting
4957 Output format is mainly controlled by the @option{-O} and @option{-T} options
4958 described in the table below. When neither @option{-O} nor @option{-T} are
4959 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4960 output format suitable for a dumb terminal. Each keyword occurrence is
4961 output to the center of one line, surrounded by its left and right
4962 contexts. Each field is properly justified, so the concordance output
4963 can be readily observed. As a special feature, if automatic
4964 references are selected by option @option{-A} and are output before the
4965 left context, that is, if option @option{-R} is @emph{not} selected, then
4966 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4967 Emacs @code{next-error} processing. In this default output format, each
4968 white space character, like newline and tab, is merely changed to
4969 exactly one space, with no special attempt to compress consecutive
4970 spaces. This might change in the future. Except for those white space
4971 characters, every other character of the underlying set of 256
4972 characters is transmitted verbatim.
4974 Output format is further controlled by the following options.
4978 @item -g @var{number}
4979 @itemx --gap-size=@var{number}
4981 Select the size of the minimum white space gap between the fields on the
4984 @item -w @var{number}
4985 @itemx --width=@var{number}
4987 Select the maximum output width of each final line. If references are
4988 used, they are included or excluded from the maximum output width
4989 depending on the value of option @option{-R}. If this option is not
4990 selected, that is, when references are output before the left context,
4991 the maximum output width takes into account the maximum length of all
4992 references. If this option is selected, that is, when references are
4993 output after the right context, the maximum output width does not take
4994 into account the space taken by references, nor the gap that precedes
4998 @itemx --auto-reference
5000 Select automatic references. Each input line will have an automatic
5001 reference made up of the file name and the line ordinal, with a single
5002 colon between them. However, the file name will be empty when standard
5003 input is being read. If both @option{-A} and @option{-r} are selected, then
5004 the input reference is still read and skipped, but the automatic
5005 reference is used at output time, overriding the input reference.
5008 @itemx --right-side-refs
5010 In the default output format, when option @option{-R} is not used, any
5011 references produced by the effect of options @option{-r} or @option{-A} are
5012 placed to the far right of output lines, after the right context. With
5013 default output format, when the @option{-R} option is specified, references
5014 are rather placed at the beginning of each output line, before the left
5015 context. For any other output format, option @option{-R} is
5016 ignored, with one exception: with @option{-R} the width of references
5017 is @emph{not} taken into account in total output width given by @option{-w}.
5019 This option is automatically selected whenever @sc{gnu} extensions are
5022 @item -F @var{string}
5023 @itemx --flac-truncation=@var{string}
5025 This option will request that any truncation in the output be reported
5026 using the string @var{string}. Most output fields theoretically extend
5027 towards the beginning or the end of the current line, or current
5028 sentence, as selected with option @option{-S}. But there is a maximum
5029 allowed output line width, changeable through option @option{-w}, which is
5030 further divided into space for various output fields. When a field has
5031 to be truncated because it cannot extend beyond the beginning or the end of
5032 the current line to fit in, then a truncation occurs. By default,
5033 the string used is a single slash, as in @option{-F /}.
5035 @var{string} may have more than one character, as in @option{-F ...}.
5036 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5037 truncation flagging is disabled, and no truncation marks are appended in
5040 As a matter of convenience to the user, many usual backslashed escape
5041 sequences, as found in the C language, are recognized and converted to
5042 the corresponding characters by @command{ptx} itself.
5044 @item -M @var{string}
5045 @itemx --macro-name=@var{string}
5047 Select another @var{string} to be used instead of @samp{xx}, while
5048 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5051 @itemx --format=roff
5053 Choose an output format suitable for @command{nroff} or @command{troff}
5054 processing. Each output line will look like:
5057 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5060 so it will be possible to write a @samp{.xx} roff macro to take care of
5061 the output typesetting. This is the default output format when @sc{gnu}
5062 extensions are disabled. Option @option{-M} can be used to change
5063 @samp{xx} to another macro name.
5065 In this output format, each non-graphical character, like newline and
5066 tab, is merely changed to exactly one space, with no special attempt to
5067 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5068 so it will be correctly processed by @command{nroff} or @command{troff}.
5073 Choose an output format suitable for @TeX{} processing. Each output
5074 line will look like:
5077 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5081 so it will be possible to write a @code{\xx} definition to take care of
5082 the output typesetting. Note that when references are not being
5083 produced, that is, neither option @option{-A} nor option @option{-r} is
5084 selected, the last parameter of each @code{\xx} call is inhibited.
5085 Option @option{-M} can be used to change @samp{xx} to another macro
5088 In this output format, some special characters, like @kbd{$}, @kbd{%},
5089 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5090 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5091 backslash and a pair of dollar signs (to force mathematical mode). The
5092 backslash itself produces the sequence @code{\backslash@{@}}.
5093 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5094 @code{~\@{ @}} respectively. Other diacriticized characters of the
5095 underlying character set produce an appropriate @TeX{} sequence as far
5096 as possible. The other non-graphical characters, like newline and tab,
5097 and all other characters which are not part of @acronym{ASCII}, are merely
5098 changed to exactly one space, with no special attempt to compress
5099 consecutive spaces. Let me know how to improve this special character
5100 processing for @TeX{}.
5105 @node Compatibility in ptx
5106 @subsection The @sc{gnu} extensions to @command{ptx}
5108 This version of @command{ptx} contains a few features which do not exist in
5109 System V @command{ptx}. These extra features are suppressed by using the
5110 @option{-G} command line option, unless overridden by other command line
5111 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5112 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5113 Here are the differences between this program and System V @command{ptx}.
5118 This program can read many input files at once, it always writes the
5119 resulting concordance on standard output. On the other hand, System V
5120 @command{ptx} reads only one file and sends the result to standard output
5121 or, if a second @var{file} parameter is given on the command, to that
5124 Having output parameters not introduced by options is a dangerous
5125 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5126 portably between @sc{gnu} and System V, you should always use it with a
5127 single input file, and always expect the result on standard output. You
5128 might also want to automatically configure in a @option{-G} option to
5129 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5130 that the installed @command{ptx} accepts @option{-G}.
5133 The only options available in System V @command{ptx} are options @option{-b},
5134 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5135 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5136 this enumeration. Moreover, some options have a slightly different
5137 meaning when @sc{gnu} extensions are enabled, as explained below.
5140 By default, concordance output is not formatted for @command{troff} or
5141 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5142 or @command{nroff} output may still be selected through option @option{-O}.
5145 Unless @option{-R} option is used, the maximum reference width is
5146 subtracted from the total output line width. With @sc{gnu} extensions
5147 disabled, width of references is not taken into account in the output
5148 line width computations.
5151 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5152 processed from input file with no adverse effect, even if @sc{gnu} extensions
5153 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5154 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5157 Input line length is only limited by available memory, even if @sc{gnu}
5158 extensions are disabled. However, System V @command{ptx} processes only
5159 the first 200 characters in each line.
5162 The break (non-word) characters default to be every character except all
5163 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5164 extensions are disabled, the break characters default to space, tab and
5168 The program makes better use of output line width. If @sc{gnu} extensions
5169 are disabled, the program rather tries to imitate System V @command{ptx},
5170 but still, there are some slight disposition glitches this program does
5171 not completely reproduce.
5174 The user can specify both an Ignore file and an Only file. This is not
5175 allowed with System V @command{ptx}.
5180 @node tsort invocation
5181 @section @command{tsort}: Topological sort
5184 @cindex topological sort
5186 @command{tsort} performs a topological sort on the given @var{file}, or
5187 standard input if no input file is given or for a @var{file} of
5188 @samp{-}. For more details and some history, see @ref{tsort background}.
5192 tsort [@var{option}] [@var{file}]
5195 @command{tsort} reads its input as pairs of strings, separated by blanks,
5196 indicating a partial ordering. The output is a total ordering that
5197 corresponds to the given partial ordering.
5211 will produce the output
5222 Consider a more realistic example.
5223 You have a large set of functions all in one file, and they may all be
5224 declared static except one. Currently that one (say @code{main}) is the
5225 first function defined in the file, and the ones it calls directly follow
5226 it, followed by those they call, etc. Let's say that you are determined
5227 to take advantage of prototypes, so you have to choose between declaring
5228 all of those functions (which means duplicating a lot of information from
5229 the definitions) and rearranging the functions so that as many as possible
5230 are defined before they are used. One way to automate the latter process
5231 is to get a list for each function of the functions it calls directly.
5232 Many programs can generate such lists. They describe a call graph.
5233 Consider the following list, in which a given line indicates that the
5234 function on the left calls the one on the right directly.
5240 tail_file pretty_name
5241 tail_file write_header
5243 tail_forever recheck
5244 tail_forever pretty_name
5245 tail_forever write_header
5246 tail_forever dump_remainder
5249 tail_lines start_lines
5250 tail_lines dump_remainder
5251 tail_lines file_lines
5252 tail_lines pipe_lines
5254 tail_bytes start_bytes
5255 tail_bytes dump_remainder
5256 tail_bytes pipe_bytes
5257 file_lines dump_remainder
5261 then you can use @command{tsort} to produce an ordering of those
5262 functions that satisfies your requirement.
5265 example$ tsort call-graph | tac
5285 @command{tsort} detects any cycles in the input and writes the first cycle
5286 encountered to standard error.
5288 Note that for a given partial ordering, generally there is no unique
5289 total ordering. In the context of the call graph above, the function
5290 @code{parse_options} may be placed anywhere in the list as long as it
5291 precedes @code{main}.
5293 The only options are @option{--help} and @option{--version}. @xref{Common
5299 * tsort background:: Where tsort came from.
5302 @node tsort background
5303 @subsection @command{tsort}: Background
5305 @command{tsort} exists because very early versions of the Unix linker processed
5306 an archive file exactly once, and in order. As @command{ld} read each object
5307 in the archive, it decided whether it was needed in the program based on
5308 whether it defined any symbols which were undefined at that point in
5311 This meant that dependencies within the archive had to be handled
5312 specially. For example, @code{scanf} probably calls @code{read}. That means
5313 that in a single pass through an archive, it was important for @code{scanf.o}
5314 to appear before read.o, because otherwise a program which calls
5315 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5316 reference to @code{read}.
5318 The way to address this problem was to first generate a set of
5319 dependencies of one object file on another. This was done by a shell
5320 script called @command{lorder}. The GNU tools don't provide a version of
5321 lorder, as far as I know, but you can still find it in BSD
5324 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5325 resulting sort to define the order in which you added objects to the archive.
5327 This whole procedure has been obsolete since about 1980, because
5328 Unix archives now contain a symbol table (traditionally built by
5329 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5330 linker uses the symbol table to effectively make multiple passes over
5333 Anyhow, that's where tsort came from. To solve an old problem with
5334 the way the linker handled archive files, which has since been solved
5338 @node Operating on fields
5339 @chapter Operating on fields
5342 * cut invocation:: Print selected parts of lines.
5343 * paste invocation:: Merge lines of files.
5344 * join invocation:: Join lines on a common field.
5348 @node cut invocation
5349 @section @command{cut}: Print selected parts of lines
5352 @command{cut} writes to standard output selected parts of each line of each
5353 input file, or standard input if no files are given or for a file name of
5357 cut @var{option}@dots{} [@var{file}]@dots{}
5360 In the table which follows, the @var{byte-list}, @var{character-list},
5361 and @var{field-list} are one or more numbers or ranges (two numbers
5362 separated by a dash) separated by commas. Bytes, characters, and
5363 fields are numbered starting at 1. Incomplete ranges may be
5364 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5365 @samp{@var{n}} through end of line or last field. The list elements
5366 can be repeated, can overlap, and can be specified in any order; but
5367 the selected input is written in the same order that it is read, and
5368 is written exactly once.
5370 The program accepts the following options. Also see @ref{Common
5375 @item -b @var{byte-list}
5376 @itemx --bytes=@var{byte-list}
5379 Select for printing only the bytes in positions listed in
5380 @var{byte-list}. Tabs and backspaces are treated like any other
5381 character; they take up 1 byte. If an output delimiter is specified,
5382 (see the description of @option{--output-delimiter}), then output that
5383 string between ranges of selected bytes.
5385 @item -c @var{character-list}
5386 @itemx --characters=@var{character-list}
5388 @opindex --characters
5389 Select for printing only the characters in positions listed in
5390 @var{character-list}. The same as @option{-b} for now, but
5391 internationalization will change that. Tabs and backspaces are
5392 treated like any other character; they take up 1 character. If an
5393 output delimiter is specified, (see the description of
5394 @option{--output-delimiter}), then output that string between ranges
5397 @item -f @var{field-list}
5398 @itemx --fields=@var{field-list}
5401 Select for printing only the fields listed in @var{field-list}.
5402 Fields are separated by a TAB character by default. Also print any
5403 line that contains no delimiter character, unless the
5404 @option{--only-delimited} (@option{-s}) option is specified.
5405 Note @command{cut} does not support specifying runs of whitespace as a
5406 delimiter, so to achieve that common functionality one can pre-process
5407 with @command{tr} like:
5409 tr -s '[:blank:]' '\t' | cut -f@dots{}
5412 @item -d @var{input_delim_byte}
5413 @itemx --delimiter=@var{input_delim_byte}
5415 @opindex --delimiter
5416 With @option{-f}, use the first byte of @var{input_delim_byte} as
5417 the input fields separator (default is TAB).
5421 Do not split multi-byte characters (no-op for now).
5424 @itemx --only-delimited
5426 @opindex --only-delimited
5427 For @option{-f}, do not print lines that do not contain the field separator
5428 character. Normally, any line without a field separator is printed verbatim.
5430 @item --output-delimiter=@var{output_delim_string}
5431 @opindex --output-delimiter
5432 With @option{-f}, output fields are separated by @var{output_delim_string}.
5433 The default with @option{-f} is to use the input delimiter.
5434 When using @option{-b} or @option{-c} to select ranges of byte or
5435 character offsets (as opposed to ranges of fields),
5436 output @var{output_delim_string} between non-overlapping
5437 ranges of selected bytes.
5440 @opindex --complement
5441 This option is a @acronym{GNU} extension.
5442 Select for printing the complement of the bytes, characters or fields
5443 selected with the @option{-b}, @option{-c} or @option{-f} options.
5444 In other words, do @emph{not} print the bytes, characters or fields
5445 specified via those options. This option is useful when you have
5446 many fields and want to print all but a few of them.
5453 @node paste invocation
5454 @section @command{paste}: Merge lines of files
5457 @cindex merging files
5459 @command{paste} writes to standard output lines consisting of sequentially
5460 corresponding lines of each given file, separated by a TAB character.
5461 Standard input is used for a file name of @samp{-} or if no input files
5483 paste [@var{option}]@dots{} [@var{file}]@dots{}
5486 The program accepts the following options. Also see @ref{Common options}.
5494 Paste the lines of one file at a time rather than one line from each
5495 file. Using the above example data:
5498 $ paste -s num2 let3
5503 @item -d @var{delim-list}
5504 @itemx --delimiters=@var{delim-list}
5506 @opindex --delimiters
5507 Consecutively use the characters in @var{delim-list} instead of
5508 TAB to separate merged lines. When @var{delim-list} is
5509 exhausted, start again at its beginning. Using the above example data:
5512 $ paste -d '%_' num2 let3 num2
5523 @node join invocation
5524 @section @command{join}: Join lines on a common field
5527 @cindex common field, joining on
5529 @command{join} writes to standard output a line for each pair of input
5530 lines that have identical join fields. Synopsis:
5533 join [@var{option}]@dots{} @var{file1} @var{file2}
5536 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5537 meaning standard input. @var{file1} and @var{file2} should be
5538 sorted on the join fields.
5541 Normally, the sort order is that of the
5542 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5543 the @option{-t} option is given, the sort comparison ignores blanks at
5544 the start of the join field, as in @code{sort -b}. If the
5545 @option{--ignore-case} option is given, the sort comparison ignores
5546 the case of characters in the join field, as in @code{sort -f}.
5548 The @command{sort} and @command{join} commands should use consistent
5549 locales and options if the output of @command{sort} is fed to
5550 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5551 sort a file on its default join field, but if you select a non-default
5552 locale, join field, separator, or comparison options, then you should
5553 do so consistently between @command{join} and @command{sort}.
5554 If @samp{join -t ''} is specified then the whole line is considered which
5555 matches the default operation of sort.
5557 If the input has no unpairable lines, a @acronym{GNU} extension is
5558 available; the sort order can be any order that considers two fields
5559 to be equal if and only if the sort comparison described above
5560 considers them to be equal. For example:
5577 @checkOrderOption{join}
5581 @item the join field is the first field in each line;
5582 @item fields in the input are separated by one or more blanks, with leading
5583 blanks on the line ignored;
5584 @item fields in the output are separated by a space;
5585 @item each output line consists of the join field, the remaining
5586 fields from @var{file1}, then the remaining fields from @var{file2}.
5589 The program accepts the following options. Also see @ref{Common options}.
5593 @item -a @var{file-number}
5595 Print a line for each unpairable line in file @var{file-number} (either
5596 @samp{1} or @samp{2}), in addition to the normal output.
5599 Fail with an error message if either input file is wrongly ordered.
5601 @item --nocheck-order
5602 Do not check that both input files are in sorted order. This is the default.
5604 @item -e @var{string}
5606 Replace those output fields that are missing in the input with
5611 Treat the first line of each input file as a header line. The header lines will
5612 be joined and printed as the first output line. If @option{-o} is used to
5613 specify output format, the header line will be printed according to the
5614 specified format. The header lines will not be checked for ordering even if
5615 @option{--check-order} is specified. Also if the header lines from each file
5616 do not match, the heading fields from the first file will be used.
5619 @itemx --ignore-case
5621 @opindex --ignore-case
5622 Ignore differences in case when comparing keys.
5623 With this option, the lines of the input files must be ordered in the same way.
5624 Use @samp{sort -f} to produce this ordering.
5626 @item -1 @var{field}
5628 Join on field @var{field} (a positive integer) of file 1.
5630 @item -2 @var{field}
5632 Join on field @var{field} (a positive integer) of file 2.
5634 @item -j @var{field}
5635 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5637 @item -o @var{field-list}
5638 Construct each output line according to the format in @var{field-list}.
5639 Each element in @var{field-list} is either the single character @samp{0} or
5640 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5641 @samp{2} and @var{n} is a positive field number.
5643 A field specification of @samp{0} denotes the join field.
5644 In most cases, the functionality of the @samp{0} field spec
5645 may be reproduced using the explicit @var{m.n} that corresponds
5646 to the join field. However, when printing unpairable lines
5647 (using either of the @option{-a} or @option{-v} options), there is no way
5648 to specify the join field using @var{m.n} in @var{field-list}
5649 if there are unpairable lines in both files.
5650 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5651 field specification notation.
5653 The elements in @var{field-list}
5654 are separated by commas or blanks.
5655 Blank separators typically need to be quoted for the shell. For
5656 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5657 2.2'} are equivalent.
5659 All output lines---including those printed because of any -a or -v
5660 option---are subject to the specified @var{field-list}.
5663 Use character @var{char} as the input and output field separator.
5664 Treat as significant each occurrence of @var{char} in the input file.
5665 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5666 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5667 the whole line is considered, matching the default operation of sort.
5668 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5669 character is used to delimit the fields.
5671 @item -v @var{file-number}
5672 Print a line for each unpairable line in file @var{file-number}
5673 (either @samp{1} or @samp{2}), instead of the normal output.
5680 @node Operating on characters
5681 @chapter Operating on characters
5683 @cindex operating on characters
5685 This commands operate on individual characters.
5688 * tr invocation:: Translate, squeeze, and/or delete characters.
5689 * expand invocation:: Convert tabs to spaces.
5690 * unexpand invocation:: Convert spaces to tabs.
5695 @section @command{tr}: Translate, squeeze, and/or delete characters
5702 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5705 @command{tr} copies standard input to standard output, performing
5706 one of the following operations:
5710 translate, and optionally squeeze repeated characters in the result,
5712 squeeze repeated characters,
5716 delete characters, then squeeze repeated characters from the result.
5719 The @var{set1} and (if given) @var{set2} arguments define ordered
5720 sets of characters, referred to below as @var{set1} and @var{set2}. These
5721 sets are the characters of the input that @command{tr} operates on.
5722 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5724 complement (all of the characters that are not in @var{set1}).
5726 Currently @command{tr} fully supports only single-byte characters.
5727 Eventually it will support multibyte characters; when it does, the
5728 @option{-C} option will cause it to complement the set of characters,
5729 whereas @option{-c} will cause it to complement the set of values.
5730 This distinction will matter only when some values are not characters,
5731 and this is possible only in locales using multibyte encodings when
5732 the input contains encoding errors.
5734 The program accepts the @option{--help} and @option{--version}
5735 options. @xref{Common options}. Options must precede operands.
5740 * Character sets:: Specifying sets of characters.
5741 * Translating:: Changing one set of characters to another.
5742 * Squeezing:: Squeezing repeats and deleting.
5746 @node Character sets
5747 @subsection Specifying sets of characters
5749 @cindex specifying sets of characters
5751 The format of the @var{set1} and @var{set2} arguments resembles
5752 the format of regular expressions; however, they are not regular
5753 expressions, only lists of characters. Most characters simply
5754 represent themselves in these strings, but the strings can contain
5755 the shorthands listed below, for convenience. Some of them can be
5756 used only in @var{set1} or @var{set2}, as noted below.
5760 @item Backslash escapes
5761 @cindex backslash escapes
5763 The following backslash escape sequences are recognized:
5781 The character with the value given by @var{ooo}, which is 1 to 3
5787 While a backslash followed by a character not listed above is
5788 interpreted as that character, the backslash also effectively
5789 removes any special significance, so it is useful to escape
5790 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5795 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5796 from @var{m} through @var{n}, in ascending order. @var{m} should
5797 collate before @var{n}; if it doesn't, an error results. As an example,
5798 @samp{0-9} is the same as @samp{0123456789}.
5800 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5801 brackets to enclose ranges. Translations specified in that format
5802 sometimes work as expected, since the brackets are often transliterated
5803 to themselves. However, they should be avoided because they sometimes
5804 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5807 Many historically common and even accepted uses of ranges are not
5808 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5809 range will not do what most would expect because @samp{A} through @samp{Z}
5810 are not contiguous as they are in @acronym{ASCII}.
5811 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5812 the best way to work around this is to use character classes (see below).
5813 Otherwise, it is most portable (and most ugly) to enumerate the members
5816 @item Repeated characters
5817 @cindex repeated characters
5819 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5820 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5821 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5822 to as many copies of @var{c} as are needed to make @var{set2} as long as
5823 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5824 octal, otherwise in decimal.
5826 @item Character classes
5827 @cindex character classes
5829 The notation @samp{[:@var{class}:]} expands to all of the characters in
5830 the (predefined) class @var{class}. The characters expand in no
5831 particular order, except for the @code{upper} and @code{lower} classes,
5832 which expand in ascending order. When the @option{--delete} (@option{-d})
5833 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5834 character class can be used in @var{set2}. Otherwise, only the
5835 character classes @code{lower} and @code{upper} are accepted in
5836 @var{set2}, and then only if the corresponding character class
5837 (@code{upper} and @code{lower}, respectively) is specified in the same
5838 relative position in @var{set1}. Doing this specifies case conversion.
5839 The class names are given below; an error results when an invalid class
5851 Horizontal whitespace.
5860 Printable characters, not including space.
5866 Printable characters, including space.
5869 Punctuation characters.
5872 Horizontal or vertical whitespace.
5881 @item Equivalence classes
5882 @cindex equivalence classes
5884 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5885 equivalent to @var{c}, in no particular order. Equivalence classes are
5886 a relatively recent invention intended to support non-English alphabets.
5887 But there seems to be no standard way to define them or determine their
5888 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5889 each character's equivalence class consists only of that character,
5890 which is of no particular use.
5896 @subsection Translating
5898 @cindex translating characters
5900 @command{tr} performs translation when @var{set1} and @var{set2} are
5901 both given and the @option{--delete} (@option{-d}) option is not given.
5902 @command{tr} translates each character of its input that is in @var{set1}
5903 to the corresponding character in @var{set2}. Characters not in
5904 @var{set1} are passed through unchanged. When a character appears more
5905 than once in @var{set1} and the corresponding characters in @var{set2}
5906 are not all the same, only the final one is used. For example, these
5907 two commands are equivalent:
5914 A common use of @command{tr} is to convert lowercase characters to
5915 uppercase. This can be done in many ways. Here are three of them:
5918 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5920 tr '[:lower:]' '[:upper:]'
5924 But note that using ranges like @code{a-z} above is not portable.
5926 When @command{tr} is performing translation, @var{set1} and @var{set2}
5927 typically have the same length. If @var{set1} is shorter than
5928 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5930 On the other hand, making @var{set1} longer than @var{set2} is not
5931 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5932 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5933 the last character of @var{set2} as many times as necessary. System V
5934 @command{tr} truncates @var{set1} to the length of @var{set2}.
5936 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5937 When the @option{--truncate-set1} (@option{-t}) option is given,
5938 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5939 instead. This option is ignored for operations other than translation.
5941 Acting like System V @command{tr} in this case breaks the relatively common
5945 tr -cs A-Za-z0-9 '\012'
5949 because it converts only zero bytes (the first element in the
5950 complement of @var{set1}), rather than all non-alphanumerics, to
5954 By the way, the above idiom is not portable because it uses ranges, and
5955 it assumes that the octal code for newline is 012.
5956 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5959 tr -cs '[:alnum:]' '[\n*]'
5964 @subsection Squeezing repeats and deleting
5966 @cindex squeezing repeat characters
5967 @cindex deleting characters
5969 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5970 removes any input characters that are in @var{set1}.
5972 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5973 @command{tr} replaces each input sequence of a repeated character that
5974 is in @var{set1} with a single occurrence of that character.
5976 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5977 first performs any deletions using @var{set1}, then squeezes repeats
5978 from any remaining characters using @var{set2}.
5980 The @option{--squeeze-repeats} option may also be used when translating,
5981 in which case @command{tr} first performs translation, then squeezes
5982 repeats from any remaining characters using @var{set2}.
5984 Here are some examples to illustrate various combinations of options:
5989 Remove all zero bytes:
5996 Put all words on lines by themselves. This converts all
5997 non-alphanumeric characters to newlines, then squeezes each string
5998 of repeated newlines into a single newline:
6001 tr -cs '[:alnum:]' '[\n*]'
6005 Convert each sequence of repeated newlines to a single newline:
6012 Find doubled occurrences of words in a document.
6013 @c Separate the following two "the"s, so typo checkers don't complain.
6014 For example, people often write ``the @w{}the'' with the repeated words
6015 separated by a newline. The Bourne shell script below works first
6016 by converting each sequence of punctuation and blank characters to a
6017 single newline. That puts each ``word'' on a line by itself.
6018 Next it maps all uppercase characters to lower case, and finally it
6019 runs @command{uniq} with the @option{-d} option to print out only the words
6025 | tr -s '[:punct:][:blank:]' '[\n*]' \
6026 | tr '[:upper:]' '[:lower:]' \
6031 Deleting a small set of characters is usually straightforward. For example,
6032 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6038 However, when @samp{-} is one of those characters, it can be tricky because
6039 @samp{-} has special meanings. Performing the same task as above but also
6040 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6041 that would fail because @command{tr} would try to interpret @option{-a} as
6042 a command-line option. Alternatively, we could try putting the hyphen
6043 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6044 it would make @command{tr} interpret @code{a-x} as the range of characters
6045 @samp{a}@dots{}@samp{x} rather than the three.
6046 One way to solve the problem is to put the hyphen at the end of the list
6053 Or you can use @samp{--} to terminate option processing:
6059 More generally, use the character class notation @code{[=c=]}
6060 with @samp{-} (or any other character) in place of the @samp{c}:
6066 Note how single quotes are used in the above example to protect the
6067 square brackets from interpretation by a shell.
6072 @node expand invocation
6073 @section @command{expand}: Convert tabs to spaces
6076 @cindex tabs to spaces, converting
6077 @cindex converting tabs to spaces
6079 @command{expand} writes the contents of each given @var{file}, or standard
6080 input if none are given or for a @var{file} of @samp{-}, to standard
6081 output, with tab characters converted to the appropriate number of
6085 expand [@var{option}]@dots{} [@var{file}]@dots{}
6088 By default, @command{expand} converts all tabs to spaces. It preserves
6089 backspace characters in the output; they decrement the column count for
6090 tab calculations. The default action is equivalent to @option{-t 8} (set
6091 tabs every 8 columns).
6093 The program accepts the following options. Also see @ref{Common options}.
6097 @item -t @var{tab1}[,@var{tab2}]@dots{}
6098 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6101 @cindex tab stops, setting
6102 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6103 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6104 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6105 last tab stop given with single spaces. Tab stops can be separated by
6106 blanks as well as by commas.
6108 For compatibility, GNU @command{expand} also accepts the obsolete
6109 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6110 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6116 @cindex initial tabs, converting
6117 Only convert initial tabs (those that precede all non-space or non-tab
6118 characters) on each line to spaces.
6125 @node unexpand invocation
6126 @section @command{unexpand}: Convert spaces to tabs
6130 @command{unexpand} writes the contents of each given @var{file}, or
6131 standard input if none are given or for a @var{file} of @samp{-}, to
6132 standard output, converting blanks at the beginning of each line into
6133 as many tab characters as needed. In the default @acronym{POSIX}
6134 locale, a @dfn{blank} is a space or a tab; other locales may specify
6135 additional blank characters. Synopsis:
6138 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6141 By default, @command{unexpand} converts only initial blanks (those
6142 that precede all non-blank characters) on each line. It
6143 preserves backspace characters in the output; they decrement the column
6144 count for tab calculations. By default, tabs are set at every 8th
6147 The program accepts the following options. Also see @ref{Common options}.
6151 @item -t @var{tab1}[,@var{tab2}]@dots{}
6152 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6155 If only one tab stop is given, set the tabs @var{tab1} columns apart
6156 instead of the default 8. Otherwise, set the tabs at columns
6157 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6158 beyond the tab stops given unchanged. Tab stops can be separated by
6159 blanks as well as by commas. This option implies the @option{-a} option.
6161 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6162 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6163 separated by commas. (Unlike @option{-t}, this obsolete option does
6164 not imply @option{-a}.) New scripts should use @option{--first-only -t
6165 @var{tab1}[,@var{tab2}]@dots{}} instead.
6171 Also convert all sequences of two or more blanks just before a tab stop,
6172 even if they occur after non-blank characters in a line.
6179 @node Directory listing
6180 @chapter Directory listing
6182 This chapter describes the @command{ls} command and its variants @command{dir}
6183 and @command{vdir}, which list information about files.
6186 * ls invocation:: List directory contents.
6187 * dir invocation:: Briefly ls.
6188 * vdir invocation:: Verbosely ls.
6189 * dircolors invocation:: Color setup for ls, etc.
6194 @section @command{ls}: List directory contents
6197 @cindex directory listing
6199 The @command{ls} program lists information about files (of any type,
6200 including directories). Options and file arguments can be intermixed
6201 arbitrarily, as usual.
6203 For non-option command-line arguments that are directories, by default
6204 @command{ls} lists the contents of directories, not recursively, and
6205 omitting files with names beginning with @samp{.}. For other non-option
6206 arguments, by default @command{ls} lists just the file name. If no
6207 non-option argument is specified, @command{ls} operates on the current
6208 directory, acting as if it had been invoked with a single argument of @samp{.}.
6211 By default, the output is sorted alphabetically, according to the locale
6212 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6213 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6214 produce output that is sorted differently than you're accustomed to.
6215 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6216 If standard output is
6217 a terminal, the output is in columns (sorted vertically) and control
6218 characters are output as question marks; otherwise, the output is listed
6219 one per line and control characters are output as-is.
6221 Because @command{ls} is such a fundamental program, it has accumulated many
6222 options over the years. They are described in the subsections below;
6223 within each section, options are listed alphabetically (ignoring case).
6224 The division of options into the subsections is not absolute, since some
6225 options affect more than one aspect of @command{ls}'s operation.
6227 @cindex exit status of @command{ls}
6232 1 minor problems (e.g., failure to access a file or directory not
6233 specified as a command line argument. This happens when listing a
6234 directory in which entries are actively being removed or renamed.)
6235 2 serious trouble (e.g., memory exhausted, invalid option, failure
6236 to access a file or directory specified as a command line argument
6237 or a directory loop)
6240 Also see @ref{Common options}.
6243 * Which files are listed::
6244 * What information is listed::
6245 * Sorting the output::
6246 * Details about version sort::
6247 * General output formatting::
6248 * Formatting file timestamps::
6249 * Formatting the file names::
6253 @node Which files are listed
6254 @subsection Which files are listed
6256 These options determine which files @command{ls} lists information for.
6257 By default, @command{ls} lists files and the contents of any
6258 directories on the command line, except that in directories it ignores
6259 files whose names start with @samp{.}.
6267 In directories, do not ignore file names that start with @samp{.}.
6272 @opindex --almost-all
6273 In directories, do not ignore all file names that start with @samp{.};
6274 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6275 option overrides this option.
6278 @itemx --ignore-backups
6280 @opindex --ignore-backups
6281 @cindex backup files, ignoring
6282 In directories, ignore files that end with @samp{~}. This option is
6283 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6288 @opindex --directory
6289 List just the names of directories, as with other types of files, rather
6290 than listing their contents.
6291 @c The following sentence is the same as the one for -F.
6292 Do not follow symbolic links listed on the
6293 command line unless the @option{--dereference-command-line} (@option{-H}),
6294 @option{--dereference} (@option{-L}), or
6295 @option{--dereference-command-line-symlink-to-dir} options are specified.
6298 @itemx --dereference-command-line
6300 @opindex --dereference-command-line
6301 @cindex symbolic links, dereferencing
6302 If a command line argument specifies a symbolic link, show information
6303 for the file the link references rather than for the link itself.
6305 @itemx --dereference-command-line-symlink-to-dir
6306 @opindex --dereference-command-line-symlink-to-dir
6307 @cindex symbolic links, dereferencing
6308 Do not dereference symbolic links, with one exception:
6309 if a command line argument specifies a symbolic link that refers to
6310 a directory, show information for that directory rather than for the
6312 This is the default behavior when no other dereferencing-related
6313 option has been specified (@option{--classify} (@option{-F}),
6314 @option{--directory} (@option{-d}),
6316 @option{--dereference} (@option{-L}), or
6317 @option{--dereference-command-line} (@option{-H})).
6319 @item --group-directories-first
6320 @opindex --group-directories-first
6321 Group all the directories before the files and then sort the
6322 directories and the files separately using the selected sort key
6323 (see --sort option).
6324 That is, this option specifies a primary sort key,
6325 and the --sort option specifies a secondary key.
6326 However, any use of @option{--sort=none}
6327 (@option{-U}) disables this option altogether.
6329 @item --hide=PATTERN
6330 @opindex --hide=@var{pattern}
6331 In directories, ignore files whose names match the shell pattern
6332 @var{pattern}, unless the @option{--all} (@option{-a}) or
6333 @option{--almost-all} (@option{-A}) is also given. This
6334 option acts like @option{--ignore=@var{pattern}} except that it has no
6335 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6336 (@option{-A}) is also given.
6338 This option can be useful in shell aliases. For example, if
6339 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6340 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6341 lists the file @file{README~} even though @samp{ly -A} would not.
6343 @item -I @var{pattern}
6344 @itemx --ignore=@var{pattern}
6346 @opindex --ignore=@var{pattern}
6347 In directories, ignore files whose names match the shell pattern
6348 (not regular expression) @var{pattern}. As
6349 in the shell, an initial @samp{.} in a file name does not match a
6350 wildcard at the start of @var{pattern}. Sometimes it is useful
6351 to give this option several times. For example,
6354 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6357 The first option ignores names of length 3 or more that start with @samp{.},
6358 the second ignores all two-character names that start with @samp{.}
6359 except @samp{..}, and the third ignores names that start with @samp{#}.
6362 @itemx --dereference
6364 @opindex --dereference
6365 @cindex symbolic links, dereferencing
6366 When showing file information for a symbolic link, show information
6367 for the file the link references rather than the link itself.
6368 However, even with this option, @command{ls} still prints the name
6369 of the link itself, not the name of the file that the link points to.
6374 @opindex --recursive
6375 @cindex recursive directory listing
6376 @cindex directory listing, recursive
6377 List the contents of all directories recursively.
6382 @node What information is listed
6383 @subsection What information is listed
6385 These options affect the information that @command{ls} displays. By
6386 default, only file names are shown.
6392 @cindex hurd, author, printing
6393 List each file's author when producing long format directory listings.
6394 In GNU/Hurd, file authors can differ from their owners, but in other
6395 operating systems the two are the same.
6401 @cindex dired Emacs mode support
6402 With the long listing (@option{-l}) format, print an additional line after
6406 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6410 The @var{begn} and @var{endn} are unsigned integers that record the
6411 byte position of the beginning and end of each file name in the output.
6412 This makes it easy for Emacs to find the names, even when they contain
6413 unusual characters such as space or newline, without fancy searching.
6415 If directories are being listed recursively (@option{-R}), output a similar
6416 line with offsets for each subdirectory name:
6419 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6422 Finally, output a line of the form:
6425 //DIRED-OPTIONS// --quoting-style=@var{word}
6429 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6431 Here is an actual example:
6434 $ mkdir -p a/sub/deeper a/sub2
6436 $ touch a/sub/deeper/file
6437 $ ls -gloRF --dired a
6440 -rw-r--r-- 1 0 Jun 10 12:27 f1
6441 -rw-r--r-- 1 0 Jun 10 12:27 f2
6442 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6443 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6447 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6451 -rw-r--r-- 1 0 Jun 10 12:27 file
6455 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6456 //SUBDIRED// 2 3 167 172 228 240 290 296
6457 //DIRED-OPTIONS// --quoting-style=literal
6460 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6461 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6463 The offsets on the @samp{//SUBDIRED//} line delimit the following
6464 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6466 Here is an example of how to extract the fifth entry name, @samp{deeper},
6467 corresponding to the pair of offsets, 222 and 228:
6470 $ ls -gloRF --dired a > out
6471 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6475 Note that although the listing above includes a trailing slash
6476 for the @samp{deeper} entry, the offsets select the name without
6477 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6478 along with an option like @option{--escape} (aka @option{-b}) and operate
6479 on a file whose name contains special characters, notice that the backslash
6484 $ ls -blog --dired 'a b'
6485 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6487 //DIRED-OPTIONS// --quoting-style=escape
6490 If you use a quoting style that adds quote marks
6491 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6492 So beware that the user may select the quoting style via the environment
6493 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6494 should either specify an explicit @option{--quoting-style=literal} option
6495 (aka @option{-N} or @option{--literal}) on the command line, or else be
6496 prepared to parse the escaped names.
6499 @opindex --full-time
6500 Produce long format directory listings, and list times in full. It is
6501 equivalent to using @option{--format=long} with
6502 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6506 Produce long format directory listings, but don't display owner information.
6512 Inhibit display of group information in a long format directory listing.
6513 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6514 provide this option for compatibility.)
6522 @cindex inode number, printing
6523 Print the inode number (also called the file serial number and index
6524 number) of each file to the left of the file name. (This number
6525 uniquely identifies each file within a particular file system.)
6528 @itemx --format=long
6529 @itemx --format=verbose
6532 @opindex long ls @r{format}
6533 @opindex verbose ls @r{format}
6534 In addition to the name of each file, print the file type, file mode bits,
6535 number of hard links, owner name, group name, size, and
6536 timestamp (@pxref{Formatting file timestamps}), normally
6537 the modification time. Print question marks for information that
6538 cannot be determined.
6540 Normally the size is printed as a byte count without punctuation, but
6541 this can be overridden (@pxref{Block size}). For example, @option{-h}
6542 prints an abbreviated, human-readable count, and
6543 @samp{--block-size="'1"} prints a byte count with the thousands
6544 separator of the current locale.
6546 For each directory that is listed, preface the files with a line
6547 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6548 for all files in that directory. The block size currently defaults to 1024
6549 bytes, but this can be overridden (@pxref{Block size}).
6550 The @var{blocks} computed counts each hard link separately;
6551 this is arguably a deficiency.
6553 The file type is one of the following characters:
6555 @c The commented-out entries are ones we're not sure about.
6563 character special file
6565 high performance (``contiguous data'') file
6569 door (Solaris 2.5 and up)
6571 @c semaphore, if this is a distinct file type
6575 @c multiplexed file (7th edition Unix; obsolete)
6577 off-line (``migrated'') file (Cray DMF)
6579 network special file (HP-UX)
6583 port (Solaris 10 and up)
6585 @c message queue, if this is a distinct file type
6589 @c shared memory object, if this is a distinct file type
6591 @c typed memory object, if this is a distinct file type
6593 @c whiteout (4.4BSD; not implemented)
6595 some other file type
6598 @cindex permissions, output by @command{ls}
6599 The file mode bits listed are similar to symbolic mode specifications
6600 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6601 third character of each set of permissions as follows:
6605 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6609 If the set-user-ID or set-group-ID bit is set but the corresponding
6610 executable bit is not set.
6613 If the restricted deletion flag or sticky bit, and the
6614 other-executable bit, are both set. The restricted deletion flag is
6615 another name for the sticky bit. @xref{Mode Structure}.
6618 If the restricted deletion flag or sticky bit is set but the
6619 other-executable bit is not set.
6622 If the executable bit is set and none of the above apply.
6628 Following the file mode bits is a single character that specifies
6629 whether an alternate access method such as an access control list
6630 applies to the file. When the character following the file mode bits is a
6631 space, there is no alternate access method. When it is a printing
6632 character, then there is such a method.
6634 GNU @command{ls} uses a @samp{.} character to indicate a file
6635 with an SELinux security context, but no other alternate access method.
6637 A file with any other combination of alternate access methods
6638 is marked with a @samp{+} character.
6641 @itemx --numeric-uid-gid
6643 @opindex --numeric-uid-gid
6644 @cindex numeric uid and gid
6645 @cindex numeric user and group IDs
6646 Produce long format directory listings, but
6647 display numeric user and group IDs instead of the owner and group names.
6651 Produce long format directory listings, but don't display group information.
6652 It is equivalent to using @option{--format=long} with @option{--no-group} .
6658 @cindex disk allocation
6659 @cindex size of files, reporting
6660 Print the disk allocation of each file to the left of the file name.
6661 This is the amount of disk space used by the file, which is usually a
6662 bit more than the file's size, but it can be less if the file has holes.
6664 Normally the disk allocation is printed in units of
6665 1024 bytes, but this can be overridden (@pxref{Block size}).
6667 @cindex NFS mounts from BSD to HP-UX
6668 For files that are NFS-mounted from an HP-UX system to a BSD system,
6669 this option reports sizes that are half the correct values. On HP-UX
6670 systems, it reports sizes that are twice the correct values for files
6671 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6672 it also affects the HP-UX @command{ls} program.
6681 @cindex security context
6682 Display the SELinux security context or @samp{?} if none is found.
6683 When used with the @option{-l} option, print the security context
6684 to the left of the size column.
6689 @node Sorting the output
6690 @subsection Sorting the output
6692 @cindex sorting @command{ls} output
6693 These options change the order in which @command{ls} sorts the information
6694 it outputs. By default, sorting is done by character code
6695 (e.g., @acronym{ASCII} order).
6701 @itemx --time=status
6704 @opindex ctime@r{, printing or sorting by}
6705 @opindex status time@r{, printing or sorting by}
6706 @opindex use time@r{, printing or sorting files by}
6707 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6708 print the status change time (the @samp{ctime} in the inode) instead of
6709 the modification time.
6710 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6711 or when not using a long listing format,
6712 sort according to the status change time.
6716 @cindex unsorted directory listing
6717 @cindex directory order, listing by
6718 Primarily, like @option{-U}---do not sort; list the files in whatever
6719 order they are stored in the directory. But also enable @option{-a} (list
6720 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6721 were specified before the @option{-f}).
6727 @cindex reverse sorting
6728 Reverse whatever the sorting method is---e.g., list files in reverse
6729 alphabetical order, youngest first, smallest first, or whatever.
6735 @opindex size of files@r{, sorting files by}
6736 Sort by file size, largest first.
6742 @opindex modification time@r{, sorting files by}
6743 Sort by modification time (the @samp{mtime} in the inode), newest first.
6747 @itemx --time=access
6751 @opindex use time@r{, printing or sorting files by}
6752 @opindex atime@r{, printing or sorting files by}
6753 @opindex access time@r{, printing or sorting files by}
6754 If the long listing format (e.g., @option{--format=long}) is being used,
6755 print the last access time (the @samp{atime} in the inode).
6756 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6757 or when not using a long listing format, sort according to the access time.
6763 @opindex none@r{, sorting option for @command{ls}}
6764 Do not sort; list the files in whatever order they are
6765 stored in the directory. (Do not do any of the other unrelated things
6766 that @option{-f} does.) This is especially useful when listing very large
6767 directories, since not doing any sorting can be noticeably faster.
6770 @itemx --sort=version
6773 @opindex version@r{, sorting option for @command{ls}}
6774 Sort by version name and number, lowest first. It behaves like a default
6775 sort, except that each sequence of decimal digits is treated numerically
6776 as an index/version number. (@xref{Details about version sort}.)
6779 @itemx --sort=extension
6782 @opindex extension@r{, sorting files by}
6783 Sort directory contents alphabetically by file extension (characters
6784 after the last @samp{.}); files with no extension are sorted first.
6789 @node Details about version sort
6790 @subsection Details about version sort
6792 Version sorting handles the fact that file names frequently include indices or
6793 version numbers. Standard sorting usually does not produce the order that one
6794 expects because comparisons are made on a character-by-character basis.
6795 Version sorting is especially useful when browsing directories that contain
6796 many files with indices/version numbers in their names:
6800 abc.zml-1.gz abc.zml-1.gz
6801 abc.zml-12.gz abc.zml-2.gz
6802 abc.zml-2.gz abc.zml-12.gz
6805 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6806 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6807 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6808 @var{ver1} < @var{ver2} implies that the name composed of
6809 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6810 ``@var{prefix} @var{ver2} @var{suffix}''.
6812 Note also that leading zeros of numeric parts are ignored:
6816 abc-1.007.tgz abc-1.01a.tgz
6817 abc-1.012b.tgz abc-1.007.tgz
6818 abc-1.01a.tgz abc-1.012b.tgz
6821 This functionality is implemented using gnulib's @code{filevercmp} function,
6822 which has some caveats worth noting.
6825 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6826 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6827 was set to @samp{C}.
6828 @item Some suffixes will not be matched by the regular
6829 expression mentioned above. Consequently these examples may
6830 not sort as you expect:
6838 abc-1.2.3.4.x86_64.rpm
6839 abc-1.2.3.x86_64.rpm
6843 @node General output formatting
6844 @subsection General output formatting
6846 These options affect the appearance of the overall output.
6851 @itemx --format=single-column
6854 @opindex single-column @r{output of files}
6855 List one file per line. This is the default for @command{ls} when standard
6856 output is not a terminal.
6859 @itemx --format=vertical
6862 @opindex vertical @r{sorted files in columns}
6863 List files in columns, sorted vertically. This is the default for
6864 @command{ls} if standard output is a terminal. It is always the default
6865 for the @command{dir} program.
6866 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6867 possible in the fewest lines.
6869 @item --color [=@var{when}]
6871 @cindex color, distinguishing file types with
6872 Specify whether to use color for distinguishing file types. @var{when}
6873 may be omitted, or one of:
6876 @vindex none @r{color option}
6877 - Do not use color at all. This is the default.
6879 @vindex auto @r{color option}
6880 @cindex terminal, using color iff
6881 - Only use color if standard output is a terminal.
6883 @vindex always @r{color option}
6886 Specifying @option{--color} and no @var{when} is equivalent to
6887 @option{--color=always}.
6888 Piping a colorized listing through a pager like @command{more} or
6889 @command{less} usually produces unreadable results. However, using
6890 @code{more -f} does seem to work.
6894 @itemx --indicator-style=classify
6897 @opindex --indicator-style
6898 @cindex file type and executables, marking
6899 @cindex executables and file type, marking
6900 Append a character to each file name indicating the file type. Also,
6901 for regular files that are executable, append @samp{*}. The file type
6902 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6903 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6904 and nothing for regular files.
6905 @c The following sentence is the same as the one for -d.
6906 Do not follow symbolic links listed on the
6907 command line unless the @option{--dereference-command-line} (@option{-H}),
6908 @option{--dereference} (@option{-L}), or
6909 @option{--dereference-command-line-symlink-to-dir} options are specified.
6912 @itemx --indicator-style=file-type
6913 @opindex --file-type
6914 @opindex --indicator-style
6915 @cindex file type, marking
6916 Append a character to each file name indicating the file type. This is
6917 like @option{-F}, except that executables are not marked.
6919 @item --indicator-style=@var{word}
6920 @opindex --indicator-style
6921 Append a character indicator with style @var{word} to entry names,
6926 Do not append any character indicator; this is the default.
6928 Append @samp{/} for directories. This is the same as the @option{-p}
6931 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6932 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6933 the same as the @option{--file-type} option.
6935 Append @samp{*} for executable regular files, otherwise behave as for
6936 @samp{file-type}. This is the same as the @option{-F} or
6937 @option{--classify} option.
6942 Print file sizes in 1024-byte blocks, overriding the default block
6943 size (@pxref{Block size}).
6944 This option is equivalent to @option{--block-size=1K}.
6947 @itemx --format=commas
6950 @opindex commas@r{, outputting between files}
6951 List files horizontally, with as many as will fit on each line,
6952 separated by @samp{, } (a comma and a space).
6955 @itemx --indicator-style=slash
6957 @opindex --indicator-style
6958 @cindex file type, marking
6959 Append a @samp{/} to directory names.
6962 @itemx --format=across
6963 @itemx --format=horizontal
6966 @opindex across@r{, listing files}
6967 @opindex horizontal@r{, listing files}
6968 List the files in columns, sorted horizontally.
6971 @itemx --tabsize=@var{cols}
6974 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6975 @command{ls} uses tabs where possible in the output, for efficiency. If
6976 @var{cols} is zero, do not use tabs at all.
6978 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6979 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6980 do not properly align columns to the right of a TAB following a
6981 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6982 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6983 @command{ls} to align using spaces, not tabs.
6986 @itemx --width=@var{cols}
6990 Assume the screen is @var{cols} columns wide. The default is taken
6991 from the terminal settings if possible; otherwise the environment
6992 variable @env{COLUMNS} is used if it is set; otherwise the default
6998 @node Formatting file timestamps
6999 @subsection Formatting file timestamps
7001 By default, file timestamps are listed in abbreviated form, using
7002 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7003 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7004 This format can change depending on the current locale as detailed below.
7006 A timestamp is considered to be @dfn{recent} if it is less than six
7007 months old, and is not dated in the future. If a timestamp dated
7008 today is not listed in recent form, the timestamp is in the future,
7009 which means you probably have clock skew problems which may break
7010 programs like @command{make} that rely on file timestamps.
7013 Time stamps are listed according to the time zone rules specified by
7014 the @env{TZ} environment variable, or by the system default rules if
7015 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7016 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7018 The following option changes how file timestamps are printed.
7021 @item --time-style=@var{style}
7022 @opindex --time-style
7024 List timestamps in style @var{style}. The @var{style} should
7025 be one of the following:
7030 List timestamps using @var{format}, where @var{format} is interpreted
7031 like the format argument of @command{date} (@pxref{date invocation}).
7032 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7033 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7034 with @command{date}, @var{format}'s interpretation is affected by the
7035 @env{LC_TIME} locale category.
7037 If @var{format} contains two format strings separated by a newline,
7038 the former is used for non-recent files and the latter for recent
7039 files; if you want output columns to line up, you may need to insert
7040 spaces in one of the two formats.
7043 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7044 format with nanosecond precision, e.g., @samp{2002-03-30
7045 23:45:56.477817180 -0700}. This style is equivalent to
7046 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7048 This is useful because the time output includes all the information that
7049 is available from the operating system. For example, this can help
7050 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7051 uses the full timestamp to determine whether a file is out of date.
7054 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7055 @samp{2002-03-30 23:45}. These timestamps are shorter than
7056 @samp{full-iso} timestamps, and are usually good enough for everyday
7057 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7060 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7061 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7062 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7063 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7064 nearly the same information in a smaller space and their brevity helps
7065 @command{ls} output fit within traditional 80-column output lines.
7066 The following two @command{ls} invocations are equivalent:
7071 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7072 ls -l --time-style="iso"
7077 List timestamps in a locale-dependent form. For example, a Finnish
7078 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7079 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7080 timestamps typically consume more space than @samp{iso} timestamps and
7081 are harder for programs to parse because locale conventions vary so
7082 widely, but they are easier for many people to read.
7084 The @env{LC_TIME} locale category specifies the timestamp format. The
7085 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7086 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7087 @command{ls} invocations are equivalent:
7092 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7093 ls -l --time-style="locale"
7096 Other locales behave differently. For example, in a German locale,
7097 @option{--time-style="locale"} might be equivalent to
7098 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7099 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7100 @samp{30. M@"ar 23:45}.
7102 @item posix-@var{style}
7104 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7105 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7106 example, the @samp{posix-long-iso} style lists
7107 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7108 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7113 You can specify the default value of the @option{--time-style} option
7114 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7115 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7116 later use the @option{--dired} option and therefore can parse any date
7117 format, but if you are using Emacs 21.1 or 21.2 and specify a
7118 non-@acronym{POSIX} locale you may need to set
7119 @samp{TIME_STYLE="posix-long-iso"}.
7121 To avoid certain denial-of-service attacks, timestamps that would be
7122 longer than 1000 bytes may be treated as errors.
7125 @node Formatting the file names
7126 @subsection Formatting the file names
7128 These options change how file names themselves are printed.
7134 @itemx --quoting-style=escape
7137 @opindex --quoting-style
7138 @cindex backslash sequences for file names
7139 Quote nongraphic characters in file names using alphabetic and octal
7140 backslash sequences like those used in C.
7144 @itemx --quoting-style=literal
7147 @opindex --quoting-style
7148 Do not quote file names. However, with @command{ls} nongraphic
7149 characters are still printed as question marks if the output is a
7150 terminal and you do not specify the @option{--show-control-chars}
7154 @itemx --hide-control-chars
7156 @opindex --hide-control-chars
7157 Print question marks instead of nongraphic characters in file names.
7158 This is the default if the output is a terminal and the program is
7163 @itemx --quoting-style=c
7165 @opindex --quote-name
7166 @opindex --quoting-style
7167 Enclose file names in double quotes and quote nongraphic characters as
7170 @item --quoting-style=@var{word}
7171 @opindex --quoting-style
7172 @cindex quoting style
7173 Use style @var{word} to quote file names and other strings that may
7174 contain arbitrary characters. The @var{word} should
7175 be one of the following:
7179 Output strings as-is; this is the same as the @option{-N} or
7180 @option{--literal} option.
7182 Quote strings for the shell if they contain shell metacharacters or would
7183 cause ambiguous output.
7184 The quoting is suitable for @acronym{POSIX}-compatible shells like
7185 @command{bash}, but it does not always work for incompatible shells
7188 Quote strings for the shell, even if they would normally not require quoting.
7190 Quote strings as for C character string literals, including the
7191 surrounding double-quote characters; this is the same as the
7192 @option{-Q} or @option{--quote-name} option.
7194 Quote strings as for C character string literals, except omit the
7195 surrounding double-quote
7196 characters; this is the same as the @option{-b} or @option{--escape} option.
7198 Quote strings as for C character string literals, except use
7199 surrounding quotation marks appropriate for the
7202 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7203 Quote strings as for C character string literals, except use
7204 surrounding quotation marks appropriate for the locale, and quote
7205 @t{`like this'} instead of @t{"like
7206 this"} in the default C locale. This looks nicer on many displays.
7209 You can specify the default value of the @option{--quoting-style} option
7210 with the environment variable @env{QUOTING_STYLE}. If that environment
7211 variable is not set, the default value is @samp{literal}, but this
7212 default may change to @samp{shell} in a future version of this package.
7214 @item --show-control-chars
7215 @opindex --show-control-chars
7216 Print nongraphic characters as-is in file names.
7217 This is the default unless the output is a terminal and the program is
7223 @node dir invocation
7224 @section @command{dir}: Briefly list directory contents
7227 @cindex directory listing, brief
7229 @command{dir} is equivalent to @code{ls -C
7230 -b}; that is, by default files are listed in columns, sorted vertically,
7231 and special characters are represented by backslash escape sequences.
7233 @xref{ls invocation, @command{ls}}.
7236 @node vdir invocation
7237 @section @command{vdir}: Verbosely list directory contents
7240 @cindex directory listing, verbose
7242 @command{vdir} is equivalent to @code{ls -l
7243 -b}; that is, by default files are listed in long format and special
7244 characters are represented by backslash escape sequences.
7246 @node dircolors invocation
7247 @section @command{dircolors}: Color setup for @command{ls}
7251 @cindex setup for color
7253 @command{dircolors} outputs a sequence of shell commands to set up the
7254 terminal for color output from @command{ls} (and @command{dir}, etc.).
7258 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7261 If @var{file} is specified, @command{dircolors} reads it to determine which
7262 colors to use for which file types and extensions. Otherwise, a
7263 precompiled database is used. For details on the format of these files,
7264 run @samp{dircolors --print-database}.
7266 To make @command{dircolors} read a @file{~/.dircolors} file if it
7267 exists, you can put the following lines in your @file{~/.bashrc} (or
7268 adapt them to your favorite shell):
7272 test -r $d && eval "$(dircolors $d)"
7276 @vindex SHELL @r{environment variable, and color}
7277 The output is a shell command to set the @env{LS_COLORS} environment
7278 variable. You can specify the shell syntax to use on the command line,
7279 or @command{dircolors} will guess it from the value of the @env{SHELL}
7280 environment variable.
7282 The program accepts the following options. Also see @ref{Common options}.
7287 @itemx --bourne-shell
7290 @opindex --bourne-shell
7291 @cindex Bourne shell syntax for color setup
7292 @cindex @command{sh} syntax for color setup
7293 Output Bourne shell commands. This is the default if the @env{SHELL}
7294 environment variable is set and does not end with @samp{csh} or
7303 @cindex C shell syntax for color setup
7304 @cindex @command{csh} syntax for color setup
7305 Output C shell commands. This is the default if @code{SHELL} ends with
7306 @command{csh} or @command{tcsh}.
7309 @itemx --print-database
7311 @opindex --print-database
7312 @cindex color database, printing
7313 @cindex database for color setup, printing
7314 @cindex printing color database
7315 Print the (compiled-in) default color configuration database. This
7316 output is itself a valid configuration file, and is fairly descriptive
7317 of the possibilities.
7324 @node Basic operations
7325 @chapter Basic operations
7327 @cindex manipulating files
7329 This chapter describes the commands for basic file manipulation:
7330 copying, moving (renaming), and deleting (removing).
7333 * cp invocation:: Copy files.
7334 * dd invocation:: Convert and copy a file.
7335 * install invocation:: Copy files and set attributes.
7336 * mv invocation:: Move (rename) files.
7337 * rm invocation:: Remove files or directories.
7338 * shred invocation:: Remove files more securely.
7343 @section @command{cp}: Copy files and directories
7346 @cindex copying files and directories
7347 @cindex files, copying
7348 @cindex directories, copying
7350 @command{cp} copies files (or, optionally, directories). The copy is
7351 completely independent of the original. You can either copy one file to
7352 another, or copy arbitrarily many files to a destination directory.
7356 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7357 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7358 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7363 If two file names are given, @command{cp} copies the first file to the
7367 If the @option{--target-directory} (@option{-t}) option is given, or
7368 failing that if the last file is a directory and the
7369 @option{--no-target-directory} (@option{-T}) option is not given,
7370 @command{cp} copies each @var{source} file to the specified directory,
7371 using the @var{source}s' names.
7374 Generally, files are written just as they are read. For exceptions,
7375 see the @option{--sparse} option below.
7377 By default, @command{cp} does not copy directories. However, the
7378 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7379 copy recursively by descending into source directories and copying files
7380 to corresponding destination directories.
7382 When copying from a symbolic link, @command{cp} normally follows the
7383 link only when not copying
7384 recursively. This default can be overridden with the
7385 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7386 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7387 @option{-H} options. If more than one of these options is specified,
7388 the last one silently overrides the others.
7390 When copying to a symbolic link, @command{cp} follows the
7391 link only when it refers to an existing regular file.
7392 However, when copying to a dangling symbolic link, @command{cp}
7393 refuses by default, and fails with a diagnostic, since the operation
7394 is inherently dangerous. This behavior is contrary to historical
7395 practice and to @acronym{POSIX}.
7396 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7397 the target of a dangling destination symlink, in spite of the possible risk.
7398 Also, when an option like
7399 @option{--backup} or @option{--link} acts to rename or remove the
7400 destination before copying, @command{cp} renames or removes the
7401 symbolic link rather than the file it points to.
7403 By default, @command{cp} copies the contents of special files only
7404 when not copying recursively. This default can be overridden with the
7405 @option{--copy-contents} option.
7407 @cindex self-backups
7408 @cindex backups, making only
7409 @command{cp} generally refuses to copy a file onto itself, with the
7410 following exception: if @option{--force --backup} is specified with
7411 @var{source} and @var{dest} identical, and referring to a regular file,
7412 @command{cp} will make a backup file, either regular or numbered, as
7413 specified in the usual ways (@pxref{Backup options}). This is useful when
7414 you simply want to make a backup of an existing file before changing it.
7416 The program accepts the following options. Also see @ref{Common options}.
7423 Preserve as much as possible of the structure and attributes of the
7424 original files in the copy (but do not attempt to preserve internal
7425 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7426 directory in a different order).
7427 Try to preserve SELinux security context and extended attributes (xattr),
7428 but ignore any failure to do that and print no corresponding diagnostic.
7429 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7431 @itemx --attributes-only
7432 @opindex --attributes-only
7433 Preserve the specified attributes of the original files in the copy,
7434 but do not copy any data. See the @option{--preserve} option for
7435 controlling which attributes to copy.
7438 @itemx @w{@kbd{--backup}[=@var{method}]}
7441 @vindex VERSION_CONTROL
7442 @cindex backups, making
7443 @xref{Backup options}.
7444 Make a backup of each file that would otherwise be overwritten or removed.
7445 As a special case, @command{cp} makes a backup of @var{source} when the force
7446 and backup options are given and @var{source} and @var{dest} are the same
7447 name for an existing, regular file. One useful application of this
7448 combination of options is this tiny Bourne shell script:
7452 # Usage: backup FILE...
7453 # Create a @sc{gnu}-style backup of each listed FILE.
7455 cp --backup --force -- "$i" "$i"
7459 @item --copy-contents
7460 @cindex directories, copying recursively
7461 @cindex copying directories recursively
7462 @cindex recursively copying directories
7463 @cindex non-directories, copying as special files
7464 If copying recursively, copy the contents of any special files (e.g.,
7465 FIFOs and device files) as if they were regular files. This means
7466 trying to read the data in each source file and writing it to the
7467 destination. It is usually a mistake to use this option, as it
7468 normally has undesirable effects on special files like FIFOs and the
7469 ones typically found in the @file{/dev} directory. In most cases,
7470 @code{cp -R --copy-contents} will hang indefinitely trying to read
7471 from FIFOs and special files like @file{/dev/console}, and it will
7472 fill up your destination disk if you use it to copy @file{/dev/zero}.
7473 This option has no effect unless copying recursively, and it does not
7474 affect the copying of symbolic links.
7478 @cindex symbolic links, copying
7479 @cindex hard links, preserving
7480 Copy symbolic links as symbolic links rather than copying the files that
7481 they point to, and preserve hard links between source files in the copies.
7482 Equivalent to @option{--no-dereference --preserve=links}.
7488 When copying without this option and an existing destination file cannot
7489 be opened for writing, the copy fails. However, with @option{--force}),
7490 when a destination file cannot be opened, @command{cp} then removes it and
7491 tries to open it again. Contrast this behavior with that enabled by
7492 @option{--link} and @option{--symbolic-link}, whereby the destination file
7493 is never opened but rather is removed unconditionally. Also see the
7494 description of @option{--remove-destination}.
7496 This option is independent of the @option{--interactive} or
7497 @option{-i} option: neither cancels the effect of the other.
7499 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7504 If a command line argument specifies a symbolic link, then copy the
7505 file it points to rather than the symbolic link itself. However,
7506 copy (preserving its nature) any symbolic link that is encountered
7507 via recursive traversal.
7510 @itemx --interactive
7512 @opindex --interactive
7513 When copying a file other than a directory, prompt whether to
7514 overwrite an existing destination file. The @option{-i} option overrides
7515 a previous @option{-n} option.
7521 Make hard links instead of copies of non-directories.
7524 @itemx --dereference
7526 @opindex --dereference
7527 Follow symbolic links when copying from them.
7528 With this option, @command{cp} cannot create a symbolic link.
7529 For example, a symlink (to regular file) in the source tree will be copied to
7530 a regular file in the destination tree.
7535 @opindex --no-clobber
7536 Do not overwrite an existing file. The @option{-n} option overrides a previous
7537 @option{-i} option. This option is mutually exclusive with @option{-b} or
7538 @option{--backup} option.
7541 @itemx --no-dereference
7543 @opindex --no-dereference
7544 @cindex symbolic links, copying
7545 Copy symbolic links as symbolic links rather than copying the files that
7546 they point to. This option affects only symbolic links in the source;
7547 symbolic links in the destination are always followed if possible.
7550 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7553 @cindex file information, preserving, extended attributes, xattr
7554 Preserve the specified attributes of the original files.
7555 If specified, the @var{attribute_list} must be a comma-separated list
7556 of one or more of the following strings:
7560 Preserve the file mode bits and access control lists.
7562 Preserve the owner and group. On most modern systems,
7563 only users with appropriate privileges may change the owner of a file,
7565 may preserve the group ownership of a file only if they happen to be
7566 a member of the desired group.
7568 Preserve the times of last access and last modification, when possible.
7569 On older systems, it is not possible to preserve these attributes
7570 when the affected file is a symbolic link.
7571 However, many systems now provide the @code{utimensat} function,
7572 which makes it possible even for symbolic links.
7574 Preserve in the destination files
7575 any links between corresponding source files.
7576 Note that with @option{-L} or @option{-H}, this option can convert
7577 symbolic links to hard links. For example,
7579 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7584 Note the inputs: @file{b} is a symlink to regular file @file{a},
7585 yet the files in destination directory, @file{c/}, are hard-linked.
7586 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7587 tells @command{cp} to dereference command line arguments, it sees two files
7588 with the same inode number, and preserves the perceived hard link.
7590 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7592 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7598 Preserve SELinux security context of the file, or fail with full diagnostics.
7600 Preserve extended attributes of the file, or fail with full diagnostics.
7601 If @command{cp} is built without xattr support, ignore this option.
7602 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7603 they are preserved by this option as well.
7605 Preserve all file attributes.
7606 Equivalent to specifying all of the above, but with the difference
7607 that failure to preserve SELinux security context or extended attributes
7608 does not change @command{cp}'s exit status. In contrast to @option{-a},
7609 all but @samp{Operation not supported} warnings are output.
7612 Using @option{--preserve} with no @var{attribute_list} is equivalent
7613 to @option{--preserve=mode,ownership,timestamps}.
7615 In the absence of this option, each destination file is created with the
7616 mode bits of the corresponding source file, minus the bits set in the
7617 umask and minus the set-user-ID and set-group-ID bits.
7618 @xref{File permissions}.
7620 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7621 @cindex file information, preserving
7622 Do not preserve the specified attributes. The @var{attribute_list}
7623 has the same form as for @option{--preserve}.
7627 @cindex parent directories and @command{cp}
7628 Form the name of each destination file by appending to the target
7629 directory a slash and the specified name of the source file. The last
7630 argument given to @command{cp} must be the name of an existing directory.
7631 For example, the command:
7634 cp --parents a/b/c existing_dir
7638 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7639 any missing intermediate directories.
7646 @opindex --recursive
7647 @cindex directories, copying recursively
7648 @cindex copying directories recursively
7649 @cindex recursively copying directories
7650 @cindex non-directories, copying as special files
7651 Copy directories recursively. By default, do not follow symbolic
7652 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7653 @option{--dereference} (@option{-L}), @option{--no-dereference}
7654 (@option{-P}), and @option{-H} options. Special files are copied by
7655 creating a destination file of the same type as the source; see the
7656 @option{--copy-contents} option. It is not portable to use
7657 @option{-r} to copy symbolic links or special files. On some
7658 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7659 @option{-L} and @option{--copy-contents} for historical reasons.
7660 Also, it is not portable to use @option{-R} to copy symbolic links
7661 unless you also specify @option{-P}, as @acronym{POSIX} allows
7662 implementations that dereference symbolic links by default.
7664 @item --reflink[=@var{when}]
7665 @opindex --reflink[=@var{when}]
7668 @cindex copy on write
7669 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7670 file system. Once it has succeeded, beware that the source and destination
7671 files share the same disk data blocks as long as they remain unmodified.
7672 Thus, if a disk I/O error affects data blocks of one of the files,
7673 the other suffers the same fate.
7675 The @var{when} value can be one of the following:
7679 The default behavior: if the copy-on-write operation is not supported
7680 then report the failure for each file and exit with a failure status.
7683 If the copy-on-write operation is not supported then fall back
7684 to the standard copy behaviour.
7687 This option is overridden by the @option{--link}, @option{--symbolic-link}
7688 and @option{--attributes-only} options, thus allowing it to be used
7689 to configure the default data copying behavior for @command{cp}.
7690 For example, with the following alias, @command{cp} will use the
7691 minimum amount of space supported by the file system.
7694 alias cp='cp --reflink=auto --sparse=always'
7697 @item --remove-destination
7698 @opindex --remove-destination
7699 Remove each existing destination file before attempting to open it
7700 (contrast with @option{-f} above).
7702 @item --sparse=@var{when}
7703 @opindex --sparse=@var{when}
7704 @cindex sparse files, copying
7705 @cindex holes, copying files with
7706 @findex read @r{system call, and holes}
7707 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7708 does not occupy any physical disk blocks; the @samp{read} system call
7709 reads these as zeros. This can both save considerable disk space and
7710 increase speed, since many binary files contain lots of consecutive zero
7711 bytes. By default, @command{cp} detects holes in input source files via a crude
7712 heuristic and makes the corresponding output file sparse as well.
7713 Only regular files may be sparse.
7715 The @var{when} value can be one of the following:
7719 The default behavior: if the input file is sparse, attempt to make
7720 the output file sparse, too. However, if an output file exists but
7721 refers to a non-regular file, then do not attempt to make it sparse.
7724 For each sufficiently long sequence of zero bytes in the input file,
7725 attempt to create a corresponding hole in the output file, even if the
7726 input file does not appear to be sparse.
7727 This is useful when the input file resides on a file system
7728 that does not support sparse files
7729 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7730 but the output file is on a type of file system that does support them.
7731 Holes may be created only in regular files, so if the destination file
7732 is of some other type, @command{cp} does not even try to make it sparse.
7735 Never make the output file sparse.
7736 This is useful in creating a file for use with the @command{mkswap} command,
7737 since such a file must not have any holes.
7740 @optStripTrailingSlashes
7743 @itemx --symbolic-link
7745 @opindex --symbolic-link
7746 @cindex symbolic links, copying with
7747 Make symbolic links instead of copies of non-directories. All source
7748 file names must be absolute (starting with @samp{/}) unless the
7749 destination files are in the current directory. This option merely
7750 results in an error message on systems that do not support symbolic links.
7756 @optNoTargetDirectory
7762 @cindex newer files, copying only
7763 Do not copy a non-directory that has an existing destination with the
7764 same or newer modification time. If time stamps are being preserved,
7765 the comparison is to the source time stamp truncated to the
7766 resolutions of the destination file system and of the system calls
7767 used to update time stamps; this avoids duplicate work if several
7768 @samp{cp -pu} commands are executed with the same source and
7775 Print the name of each file before copying it.
7778 @itemx --one-file-system
7780 @opindex --one-file-system
7781 @cindex file systems, omitting copying to different
7782 Skip subdirectories that are on different file systems from the one that
7783 the copy started on.
7784 However, mount point directories @emph{are} copied.
7792 @section @command{dd}: Convert and copy a file
7795 @cindex converting while copying a file
7797 @command{dd} copies a file (from standard input to standard output, by
7798 default) with a changeable I/O block size, while optionally performing
7799 conversions on it. Synopses:
7802 dd [@var{operand}]@dots{}
7806 The only options are @option{--help} and @option{--version}.
7807 @xref{Common options}. @command{dd} accepts the following operands.
7813 Read from @var{file} instead of standard input.
7817 Write to @var{file} instead of standard output. Unless
7818 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7819 bytes (or the size specified with @samp{seek=}).
7821 @item ibs=@var{bytes}
7823 @cindex block size of input
7824 @cindex input block size
7825 Set the input block size to @var{bytes}.
7826 This makes @command{dd} read @var{bytes} per block.
7827 The default is 512 bytes.
7829 @item obs=@var{bytes}
7831 @cindex block size of output
7832 @cindex output block size
7833 Set the output block size to @var{bytes}.
7834 This makes @command{dd} write @var{bytes} per block.
7835 The default is 512 bytes.
7837 @item bs=@var{bytes}
7840 Set both input and output block sizes to @var{bytes}.
7841 This makes @command{dd} read and write @var{bytes} per block,
7842 overriding any @samp{ibs} and @samp{obs} settings.
7843 In addition, if no data-transforming @option{conv} option is specified,
7844 each input block is copied to the output as a single block,
7845 without aggregating short reads.
7847 @item cbs=@var{bytes}
7849 @cindex block size of conversion
7850 @cindex conversion block size
7851 @cindex fixed-length records, converting to variable-length
7852 @cindex variable-length records, converting to fixed-length
7853 Set the conversion block size to @var{bytes}.
7854 When converting variable-length records to fixed-length ones
7855 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7856 use @var{bytes} as the fixed record length.
7858 @item skip=@var{blocks}
7860 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7862 @item seek=@var{blocks}
7864 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7866 @item count=@var{blocks}
7868 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7869 of everything until the end of the file.
7873 Do not print the overall transfer rate and volume statistics
7874 that normally make up the third status line when @command{dd} exits.
7876 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7878 Convert the file as specified by the @var{conversion} argument(s).
7879 (No spaces around any comma(s).)
7886 @opindex ascii@r{, converting to}
7887 Convert @acronym{EBCDIC} to @acronym{ASCII},
7888 using the conversion table specified by @acronym{POSIX}.
7889 This provides a 1:1 translation for all 256 bytes.
7892 @opindex ebcdic@r{, converting to}
7893 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7894 This is the inverse of the @samp{ascii} conversion.
7897 @opindex alternate ebcdic@r{, converting to}
7898 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7899 using the alternate conversion table specified by @acronym{POSIX}.
7900 This is not a 1:1 translation, but reflects common historical practice
7901 for @samp{~}, @samp{[}, and @samp{]}.
7903 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7907 @opindex block @r{(space-padding)}
7908 For each line in the input, output @samp{cbs} bytes, replacing the
7909 input newline with a space and padding with spaces as necessary.
7913 Remove any trailing spaces in each @samp{cbs}-sized input block,
7914 and append a newline.
7916 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7919 @opindex lcase@r{, converting to}
7920 Change uppercase letters to lowercase.
7923 @opindex ucase@r{, converting to}
7924 Change lowercase letters to uppercase.
7926 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7929 @opindex swab @r{(byte-swapping)}
7930 @cindex byte-swapping
7931 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7932 when an odd number of bytes are read---the last byte is simply copied
7933 (since there is nothing to swap it with).
7937 @cindex read errors, ignoring
7938 Continue after read errors.
7942 @cindex creating output file, avoiding
7943 Do not create the output file; the output file must already exist.
7947 @cindex creating output file, requiring
7948 Fail if the output file already exists; @command{dd} must create the
7951 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7955 @cindex truncating output file, avoiding
7956 Do not truncate the output file.
7959 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7960 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7961 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7966 @cindex synchronized data writes, before finishing
7967 Synchronize output data just before finishing. This forces a physical
7968 write of output data.
7972 @cindex synchronized data and metadata writes, before finishing
7973 Synchronize output data and metadata just before finishing. This
7974 forces a physical write of output data and metadata.
7978 @item iflag=@var{flag}[,@var{flag}]@dots{}
7980 Access the input file using the flags specified by the @var{flag}
7981 argument(s). (No spaces around any comma(s).)
7983 @item oflag=@var{flag}[,@var{flag}]@dots{}
7985 Access the output file using the flags specified by the @var{flag}
7986 argument(s). (No spaces around any comma(s).)
7988 Here are the flags. Not every flag is supported on every operating
7995 @cindex appending to the output file
7996 Write in append mode, so that even if some other process is writing to
7997 this file, every @command{dd} write will append to the current
7998 contents of the file. This flag makes sense only for output.
7999 If you combine this flag with the @samp{of=@var{file}} operand,
8000 you should also specify @samp{conv=notrunc} unless you want the
8001 output file to be truncated before being appended to.
8005 @cindex concurrent I/O
8006 Use concurrent I/O mode for data. This mode performs direct I/O
8007 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8008 A file cannot be opened in CIO mode and with a standard open at the
8014 Use direct I/O for data, avoiding the buffer cache.
8015 Note that the kernel may impose restrictions on read or write buffer sizes.
8016 For example, with an ext4 destination file system and a linux-based kernel,
8017 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8018 output buffer size is not a multiple of 512.
8022 @cindex directory I/O
8024 Fail unless the file is a directory. Most operating systems do not
8025 allow I/O to a directory, so this flag has limited utility.
8029 @cindex synchronized data reads
8030 Use synchronized I/O for data. For the output file, this forces a
8031 physical write of output data on each write. For the input file,
8032 this flag can matter when reading from a remote file that has been
8033 written to synchronously by some other process. Metadata (e.g.,
8034 last-access and last-modified time) is not necessarily synchronized.
8038 @cindex synchronized data and metadata I/O
8039 Use synchronized I/O for both data and metadata.
8043 @cindex nonblocking I/O
8044 Use non-blocking I/O.
8049 Do not update the file's access time.
8050 Some older file systems silently ignore this flag, so it is a good
8051 idea to test it on your files before relying on it.
8055 @cindex controlling terminal
8056 Do not assign the file to be a controlling terminal for @command{dd}.
8057 This has no effect when the file is not a terminal.
8058 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8063 @cindex symbolic links, following
8064 Do not follow symbolic links.
8069 Fail if the file has multiple hard links.
8074 Use binary I/O. This option has an effect only on nonstandard
8075 platforms that distinguish binary from text I/O.
8080 Use text I/O. Like @samp{binary}, this option has no effect on
8085 Accumulate full blocks from input. The @code{read} system call
8086 may return early if a full block is not available.
8087 When that happens, continue calling @code{read} to fill the remainder
8089 This flag can be used only with @code{iflag}.
8093 These flags are not supported on all systems, and @samp{dd} rejects
8094 attempts to use them when they are not supported. When reading from
8095 standard input or writing to standard output, the @samp{nofollow} and
8096 @samp{noctty} flags should not be specified, and the other flags
8097 (e.g., @samp{nonblock}) can affect how other processes behave with the
8098 affected file descriptors, even after @command{dd} exits.
8102 @cindex multipliers after numbers
8103 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8104 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8105 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8106 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8108 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8109 should not be too large---values larger than a few megabytes
8110 are generally wasteful or (as in the gigabyte..exabyte case) downright
8111 counterproductive or error-inducing.
8113 Use different @command{dd} invocations to use different block sizes for
8114 skipping and I/O@. For example, the following shell commands copy data
8115 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8116 4 KiB label at the start of the disk:
8119 disk=/dev/rdsk/c0t1d0s2
8122 # Copy all but the label from disk to tape.
8123 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8125 # Copy from tape back to disk, but leave the disk label alone.
8126 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8129 Sending an @samp{INFO} signal to a running @command{dd}
8130 process makes it print I/O statistics to standard error
8131 and then resume copying. In the example below,
8132 @command{dd} is run in the background to copy 10 million blocks.
8133 The @command{kill} command makes it output intermediate I/O statistics,
8134 and when @command{dd} completes normally or is killed by the
8135 @code{SIGINT} signal, it outputs the final statistics.
8138 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8139 $ kill -s INFO $pid; wait $pid
8140 3385223+0 records in
8141 3385223+0 records out
8142 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8143 10000000+0 records in
8144 10000000+0 records out
8145 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8148 @vindex POSIXLY_CORRECT
8149 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8150 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8151 environment variable is set.
8156 @node install invocation
8157 @section @command{install}: Copy files and set attributes
8160 @cindex copying files and setting attributes
8162 @command{install} copies files while setting their file mode bits and, if
8163 possible, their owner and group. Synopses:
8166 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8167 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8168 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8169 install [@var{option}]@dots{} -d @var{directory}@dots{}
8174 If two file names are given, @command{install} copies the first file to the
8178 If the @option{--target-directory} (@option{-t}) option is given, or
8179 failing that if the last file is a directory and the
8180 @option{--no-target-directory} (@option{-T}) option is not given,
8181 @command{install} copies each @var{source} file to the specified
8182 directory, using the @var{source}s' names.
8185 If the @option{--directory} (@option{-d}) option is given,
8186 @command{install} creates each @var{directory} and any missing parent
8187 directories. Parent directories are created with mode
8188 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8189 current umask. @xref{Directory Setuid and Setgid}, for how the
8190 set-user-ID and set-group-ID bits of parent directories are inherited.
8193 @cindex Makefiles, installing programs in
8194 @command{install} is similar to @command{cp}, but allows you to control the
8195 attributes of destination files. It is typically used in Makefiles to
8196 copy programs into their destination directories. It refuses to copy
8197 files onto themselves.
8199 @cindex extended attributes, xattr
8200 @command{install} never preserves extended attributes (xattr).
8202 The program accepts the following options. Also see @ref{Common options}.
8212 Compare each pair of source and destination files, and if the destination has
8213 identical content and any specified owner, group, permissions, and possibly
8214 SELinux context, then do not modify the destination at all.
8218 Ignored; for compatibility with old Unix versions of @command{install}.
8222 Create any missing parent directories of @var{dest},
8223 then copy @var{source} to @var{dest}.
8224 This option is ignored if a destination directory is specified
8225 via @option{--target-directory=DIR}.
8230 @opindex --directory
8231 @cindex directories, creating with given attributes
8232 @cindex parent directories, creating missing
8233 @cindex leading directories, creating missing
8234 Create any missing parent directories, giving them the default
8235 attributes. Then create each given directory, setting their owner,
8236 group and mode as given on the command line or to the defaults.
8238 @item -g @var{group}
8239 @itemx --group=@var{group}
8242 @cindex group ownership of installed files, setting
8243 Set the group ownership of installed files or directories to
8244 @var{group}. The default is the process's current group. @var{group}
8245 may be either a group name or a numeric group ID.
8248 @itemx --mode=@var{mode}
8251 @cindex permissions of installed files, setting
8252 Set the file mode bits for the installed file or directory to @var{mode},
8253 which can be either an octal number, or a symbolic mode as in
8254 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8255 point of departure (@pxref{File permissions}).
8256 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8257 execute for the owner, read and execute for group and other, and with
8258 set-user-ID and set-group-ID disabled.
8259 This default is not quite the same as @samp{755}, since it disables
8260 instead of preserving set-user-ID and set-group-ID on directories.
8261 @xref{Directory Setuid and Setgid}.
8263 @item -o @var{owner}
8264 @itemx --owner=@var{owner}
8267 @cindex ownership of installed files, setting
8268 @cindex appropriate privileges
8269 @vindex root @r{as default owner}
8270 If @command{install} has appropriate privileges (is run as root), set the
8271 ownership of installed files or directories to @var{owner}. The default
8272 is @code{root}. @var{owner} may be either a user name or a numeric user
8275 @item --preserve-context
8276 @opindex --preserve-context
8278 @cindex security context
8279 Preserve the SELinux security context of files and directories.
8280 Failure to preserve the context in all of the files or directories
8281 will result in an exit status of 1. If SELinux is disabled then
8282 print a warning and ignore the option.
8285 @itemx --preserve-timestamps
8287 @opindex --preserve-timestamps
8288 @cindex timestamps of installed files, preserving
8289 Set the time of last access and the time of last modification of each
8290 installed file to match those of each corresponding original file.
8291 When a file is installed without this option, its last access and
8292 last modification times are both set to the time of installation.
8293 This option is useful if you want to use the last modification times
8294 of installed files to keep track of when they were last built as opposed
8295 to when they were last installed.
8301 @cindex symbol table information, stripping
8302 @cindex stripping symbol table information
8303 Strip the symbol tables from installed binary executables.
8305 @itemx --strip-program=@var{program}
8306 @opindex --strip-program
8307 @cindex symbol table information, stripping, program
8308 Program used to strip binaries.
8314 @optNoTargetDirectory
8320 Print the name of each file before copying it.
8322 @item -Z @var{context}
8323 @itemx --context=@var{context}
8327 @cindex security context
8328 Set the default SELinux security context to be used for any
8329 created files and directories. If SELinux is disabled then
8330 print a warning and ignore the option.
8338 @section @command{mv}: Move (rename) files
8342 @command{mv} moves or renames files (or directories). Synopses:
8345 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8346 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8347 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8352 If two file names are given, @command{mv} moves the first file to the
8356 If the @option{--target-directory} (@option{-t}) option is given, or
8357 failing that if the last file is a directory and the
8358 @option{--no-target-directory} (@option{-T}) option is not given,
8359 @command{mv} moves each @var{source} file to the specified
8360 directory, using the @var{source}s' names.
8363 @command{mv} can move any type of file from one file system to another.
8364 Prior to version @code{4.0} of the fileutils,
8365 @command{mv} could move only regular files between file systems.
8366 For example, now @command{mv} can move an entire directory hierarchy
8367 including special device files from one partition to another. It first
8368 uses some of the same code that's used by @code{cp -a} to copy the
8369 requested directories and files, then (assuming the copy succeeded)
8370 it removes the originals. If the copy fails, then the part that was
8371 copied to the destination partition is removed. If you were to copy
8372 three directories from one partition to another and the copy of the first
8373 directory succeeded, but the second didn't, the first would be left on
8374 the destination partition and the second and third would be left on the
8377 @cindex extended attributes, xattr
8378 @command{mv} always tries to copy extended attributes (xattr), which may
8379 include SELinux context, ACLs or Capabilities.
8380 Upon failure all but @samp{Operation not supported} warnings are output.
8382 @cindex prompting, and @command{mv}
8383 If a destination file exists but is normally unwritable, standard input
8384 is a terminal, and the @option{-f} or @option{--force} option is not given,
8385 @command{mv} prompts the user for whether to replace the file. (You might
8386 own the file, or have write permission on its directory.) If the
8387 response is not affirmative, the file is skipped.
8389 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8390 when it might be a symlink to a directory.
8391 Otherwise, @command{mv} may do something very surprising, since
8392 its behavior depends on the underlying rename system call.
8393 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8394 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8395 renames not the symlink but rather the directory referenced by the symlink.
8396 @xref{Trailing slashes}.
8398 The program accepts the following options. Also see @ref{Common options}.
8408 @cindex prompts, omitting
8409 Do not prompt the user before removing a destination file.
8411 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8412 options, only the final one takes effect.
8417 @itemx --interactive
8419 @opindex --interactive
8420 @cindex prompts, forcing
8421 Prompt whether to overwrite each existing destination file, regardless
8423 If the response is not affirmative, the file is skipped.
8429 @opindex --no-clobber
8430 @cindex prompts, omitting
8431 Do not overwrite an existing file.
8433 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8439 @cindex newer files, moving only
8440 Do not move a non-directory that has an existing destination with the
8441 same or newer modification time.
8442 If the move is across file system boundaries, the comparison is to the
8443 source time stamp truncated to the resolutions of the destination file
8444 system and of the system calls used to update time stamps; this avoids
8445 duplicate work if several @samp{mv -u} commands are executed with the
8446 same source and destination.
8452 Print the name of each file before moving it.
8454 @optStripTrailingSlashes
8460 @optNoTargetDirectory
8468 @section @command{rm}: Remove files or directories
8471 @cindex removing files or directories
8473 @command{rm} removes each given @var{file}. By default, it does not remove
8474 directories. Synopsis:
8477 rm [@var{option}]@dots{} [@var{file}]@dots{}
8480 @cindex prompting, and @command{rm}
8481 If the @option{-I} or @option{--interactive=once} option is given,
8482 and there are more than three files or the @option{-r}, @option{-R},
8483 or @option{--recursive} are given, then @command{rm} prompts the user
8484 for whether to proceed with the entire operation. If the response is
8485 not affirmative, the entire command is aborted.
8487 Otherwise, if a file is unwritable, standard input is a terminal, and
8488 the @option{-f} or @option{--force} option is not given, or the
8489 @option{-i} or @option{--interactive=always} option @emph{is} given,
8490 @command{rm} prompts the user for whether to remove the file.
8491 If the response is not affirmative, the file is skipped.
8493 Any attempt to remove a file whose last file name component is
8494 @file{.} or @file{..} is rejected without any prompting.
8496 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8497 possible to recover the contents of that file. If you want more assurance
8498 that the contents are truly unrecoverable, consider using @command{shred}.
8500 The program accepts the following options. Also see @ref{Common options}.
8508 Ignore nonexistent files and never prompt the user.
8509 Ignore any previous @option{--interactive} (@option{-i}) option.
8513 Prompt whether to remove each file.
8514 If the response is not affirmative, the file is skipped.
8515 Ignore any previous @option{--force} (@option{-f}) option.
8516 Equivalent to @option{--interactive=always}.
8520 Prompt once whether to proceed with the command, if more than three
8521 files are named or if a recursive removal is requested. Ignore any
8522 previous @option{--force} (@option{-f}) option. Equivalent to
8523 @option{--interactive=once}.
8525 @itemx --interactive [=@var{when}]
8526 @opindex --interactive
8527 Specify when to issue an interactive prompt. @var{when} may be
8531 @vindex never @r{interactive option}
8532 - Do not prompt at all.
8534 @vindex once @r{interactive option}
8535 - Prompt once if more than three files are named or if a recursive
8536 removal is requested. Equivalent to @option{-I}.
8538 @vindex always @r{interactive option}
8539 - Prompt for every file being removed. Equivalent to @option{-i}.
8541 @option{--interactive} with no @var{when} is equivalent to
8542 @option{--interactive=always}.
8544 @itemx --one-file-system
8545 @opindex --one-file-system
8546 @cindex one file system, restricting @command{rm} to
8547 When removing a hierarchy recursively, skip any directory that is on a
8548 file system different from that of the corresponding command line argument.
8551 This option is useful when removing a build ``chroot'' hierarchy,
8552 which normally contains no valuable data. However, it is not uncommon
8553 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8554 use one's start-up file. The catch is that it's easy to forget to
8555 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8556 your normally throw-away chroot, that command will remove everything
8557 under @file{/home}, too.
8558 Use the @option{--one-file-system} option, and it will
8559 warn about and skip directories on other file systems.
8560 Of course, this will not save your @file{/home} if it and your
8561 chroot happen to be on the same file system.
8563 @itemx --preserve-root
8564 @opindex --preserve-root
8565 @cindex root directory, disallow recursive destruction
8566 Fail upon any attempt to remove the root directory, @file{/},
8567 when used with the @option{--recursive} option.
8568 This is the default behavior.
8569 @xref{Treating / specially}.
8571 @itemx --no-preserve-root
8572 @opindex --no-preserve-root
8573 @cindex root directory, allow recursive destruction
8574 Do not treat @file{/} specially when removing recursively.
8575 This option is not recommended unless you really want to
8576 remove all the files on your computer.
8577 @xref{Treating / specially}.
8584 @opindex --recursive
8585 @cindex directories, removing (recursively)
8586 Remove the listed directories and their contents recursively.
8592 Print the name of each file before removing it.
8596 @cindex files beginning with @samp{-}, removing
8597 @cindex @samp{-}, removing files beginning with
8598 One common question is how to remove files whose names begin with a
8599 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8600 function to parse its arguments, lets you use the @samp{--} option to
8601 indicate that all following arguments are non-options. To remove a file
8602 called @file{-f} in the current directory, you could type either:
8615 @opindex - @r{and Unix @command{rm}}
8616 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8617 predates the development of the getopt standard syntax.
8622 @node shred invocation
8623 @section @command{shred}: Remove files more securely
8626 @cindex data, erasing
8627 @cindex erasing data
8629 @command{shred} overwrites devices or files, to help prevent even
8630 very expensive hardware from recovering the data.
8632 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8633 not actually destroyed. Only the index listing where the file is
8634 stored is destroyed, and the storage is made available for reuse.
8635 There are undelete utilities that will attempt to reconstruct the index
8636 and can bring the file back if the parts were not reused.
8638 On a busy system with a nearly-full drive, space can get reused in a few
8639 seconds. But there is no way to know for sure. If you have sensitive
8640 data, you may want to be sure that recovery is not possible by actually
8641 overwriting the file with non-sensitive data.
8643 However, even after doing that, it is possible to take the disk back
8644 to a laboratory and use a lot of sensitive (and expensive) equipment
8645 to look for the faint ``echoes'' of the original data underneath the
8646 overwritten data. If the data has only been overwritten once, it's not
8649 The best way to remove something irretrievably is to destroy the media
8650 it's on with acid, melt it down, or the like. For cheap removable media
8651 like floppy disks, this is the preferred method. However, hard drives
8652 are expensive and hard to melt, so the @command{shred} utility tries
8653 to achieve a similar effect non-destructively.
8655 This uses many overwrite passes, with the data patterns chosen to
8656 maximize the damage they do to the old data. While this will work on
8657 floppies, the patterns are designed for best effect on hard drives.
8658 For more details, see the source code and Peter Gutmann's paper
8659 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8660 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8661 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8662 California, July 22--25, 1996).
8664 @strong{Please note} that @command{shred} relies on a very important assumption:
8665 that the file system overwrites data in place. This is the traditional
8666 way to do things, but many modern file system designs do not satisfy this
8667 assumption. Exceptions include:
8672 Log-structured or journaled file systems, such as those supplied with
8673 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8674 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8677 File systems that write redundant data and carry on even if some writes
8678 fail, such as RAID-based file systems.
8681 File systems that make snapshots, such as Network Appliance's NFS server.
8684 File systems that cache in temporary locations, such as NFS version 3
8688 Compressed file systems.
8691 In the particular case of ext3 file systems, the above disclaimer applies (and
8692 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8693 mode, which journals file data in addition to just metadata. In both
8694 the @code{data=ordered} (default) and @code{data=writeback} modes,
8695 @command{shred} works as usual. Ext3 journaling modes can be changed
8696 by adding the @code{data=something} option to the mount options for a
8697 particular file system in the @file{/etc/fstab} file, as documented in
8698 the mount man page (man mount).
8700 If you are not sure how your file system operates, then you should assume
8701 that it does not overwrite data in place, which means that shred cannot
8702 reliably operate on regular files in your file system.
8704 Generally speaking, it is more reliable to shred a device than a file,
8705 since this bypasses the problem of file system design mentioned above.
8706 However, even shredding devices is not always completely reliable. For
8707 example, most disks map out bad sectors invisibly to the application; if
8708 the bad sectors contain sensitive data, @command{shred} won't be able to
8711 @command{shred} makes no attempt to detect or report this problem, just as
8712 it makes no attempt to do anything about backups. However, since it is
8713 more reliable to shred devices than files, @command{shred} by default does
8714 not truncate or remove the output file. This default is more suitable
8715 for devices, which typically cannot be truncated and should not be
8718 Finally, consider the risk of backups and mirrors.
8719 File system backups and remote mirrors may contain copies of the
8720 file that cannot be removed, and that will allow a shredded file
8721 to be recovered later. So if you keep any data you may later want
8722 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8725 shred [@var{option}]@dots{} @var{file}[@dots{}]
8728 The program accepts the following options. Also see @ref{Common options}.
8736 @cindex force deletion
8737 Override file permissions if necessary to allow overwriting.
8740 @itemx -n @var{number}
8741 @itemx --iterations=@var{number}
8742 @opindex -n @var{number}
8743 @opindex --iterations=@var{number}
8744 @cindex iterations, selecting the number of
8745 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8746 overwrite. You can reduce this to save time, or increase it if you think it's
8747 appropriate. After 25 passes all of the internal overwrite patterns will have
8748 been used at least once.
8750 @item --random-source=@var{file}
8751 @opindex --random-source
8752 @cindex random source for shredding
8753 Use @var{file} as a source of random data used to overwrite and to
8754 choose pass ordering. @xref{Random sources}.
8756 @item -s @var{bytes}
8757 @itemx --size=@var{bytes}
8758 @opindex -s @var{bytes}
8759 @opindex --size=@var{bytes}
8760 @cindex size of file to shred
8761 Shred the first @var{bytes} bytes of the file. The default is to shred
8762 the whole file. @var{bytes} can be followed by a size specification like
8763 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8769 @cindex removing files after shredding
8770 After shredding a file, truncate it (if possible) and then remove it.
8771 If a file has multiple links, only the named links will be removed.
8777 Display to standard error all status updates as sterilization proceeds.
8783 By default, @command{shred} rounds the size of a regular file up to the next
8784 multiple of the file system block size to fully erase the last block of the file.
8785 Use @option{--exact} to suppress that behavior.
8786 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8787 blocks, the resulting file will be 512 bytes long. With this option,
8788 shred does not increase the apparent size of the file.
8794 Normally, the last pass that @command{shred} writes is made up of
8795 random data. If this would be conspicuous on your hard drive (for
8796 example, because it looks like encrypted data), or you just think
8797 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8798 all zero bits. This is in addition to the number of passes specified
8799 by the @option{--iterations} option.
8803 You might use the following command to erase all trace of the
8804 file system you'd created on the floppy disk in your first drive.
8805 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8809 shred --verbose /dev/fd0
8812 Similarly, to erase all data on a selected partition of
8813 your hard disk, you could give a command like this:
8816 shred --verbose /dev/sda5
8819 A @var{file} of @samp{-} denotes standard output.
8820 The intended use of this is to shred a removed temporary file.
8827 echo "Hello, world" >&3
8832 However, the command @samp{shred - >file} does not shred the contents
8833 of @var{file}, since the shell truncates @var{file} before invoking
8834 @command{shred}. Use the command @samp{shred file} or (if using a
8835 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8840 @node Special file types
8841 @chapter Special file types
8843 @cindex special file types
8844 @cindex file types, special
8846 This chapter describes commands which create special types of files (and
8847 @command{rmdir}, which removes directories, one special file type).
8849 @cindex special file types
8851 Although Unix-like operating systems have markedly fewer special file
8852 types than others, not @emph{everything} can be treated only as the
8853 undifferentiated byte stream of @dfn{normal files}. For example, when a
8854 file is created or removed, the system must record this information,
8855 which it does in a @dfn{directory}---a special type of file. Although
8856 you can read directories as normal files, if you're curious, in order
8857 for the system to do its job it must impose a structure, a certain
8858 order, on the bytes of the file. Thus it is a ``special'' type of file.
8860 Besides directories, other special file types include named pipes
8861 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8864 * link invocation:: Make a hard link via the link syscall
8865 * ln invocation:: Make links between files.
8866 * mkdir invocation:: Make directories.
8867 * mkfifo invocation:: Make FIFOs (named pipes).
8868 * mknod invocation:: Make block or character special files.
8869 * readlink invocation:: Print value of a symlink or canonical file name.
8870 * rmdir invocation:: Remove empty directories.
8871 * unlink invocation:: Remove files via the unlink syscall
8875 @node link invocation
8876 @section @command{link}: Make a hard link via the link syscall
8879 @cindex links, creating
8880 @cindex hard links, creating
8881 @cindex creating links (hard only)
8883 @command{link} creates a single hard link at a time.
8884 It is a minimalist interface to the system-provided
8885 @code{link} function. @xref{Hard Links, , , libc,
8886 The GNU C Library Reference Manual}.
8887 It avoids the bells and whistles of the more commonly-used
8888 @command{ln} command (@pxref{ln invocation}).
8892 link @var{filename} @var{linkname}
8895 @var{filename} must specify an existing file, and @var{linkname}
8896 must specify a nonexistent entry in an existing directory.
8897 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8900 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8901 --no-target-directory @var{filename} @var{linkname}}. However, the
8902 @option{--directory} and @option{--no-target-directory} options are
8903 not specified by @acronym{POSIX}, and the @command{link} command is
8904 more portable in practice.
8906 If @var{filename} is a symbolic link, it is unspecified whether
8907 @var{linkname} will be a hard link to the symbolic link or to the
8908 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8909 to specify which behavior is desired.
8915 @section @command{ln}: Make links between files
8918 @cindex links, creating
8919 @cindex hard links, creating
8920 @cindex symbolic (soft) links, creating
8921 @cindex creating links (hard or soft)
8923 @cindex file systems and hard links
8924 @command{ln} makes links between files. By default, it makes hard links;
8925 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8929 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8930 ln [@var{option}]@dots{} @var{target}
8931 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8932 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8938 If two file names are given, @command{ln} creates a link to the first
8939 file from the second.
8942 If one @var{target} is given, @command{ln} creates a link to that file
8943 in the current directory.
8946 If the @option{--target-directory} (@option{-t}) option is given, or
8947 failing that if the last file is a directory and the
8948 @option{--no-target-directory} (@option{-T}) option is not given,
8949 @command{ln} creates a link to each @var{target} file in the specified
8950 directory, using the @var{target}s' names.
8954 Normally @command{ln} does not remove existing files. Use the
8955 @option{--force} (@option{-f}) option to remove them unconditionally,
8956 the @option{--interactive} (@option{-i}) option to remove them
8957 conditionally, and the @option{--backup} (@option{-b}) option to
8960 @cindex hard link, defined
8961 @cindex inode, and hard links
8962 A @dfn{hard link} is another name for an existing file; the link and the
8963 original are indistinguishable. Technically speaking, they share the
8964 same inode, and the inode contains all the information about a
8965 file---indeed, it is not incorrect to say that the inode @emph{is} the
8966 file. Most systems prohibit making a hard link to
8967 a directory; on those where it is allowed, only the super-user can do
8968 so (and with caution, since creating a cycle will cause problems to many
8969 other utilities). Hard links cannot cross file system boundaries. (These
8970 restrictions are not mandated by @acronym{POSIX}, however.)
8972 @cindex dereferencing symbolic links
8973 @cindex symbolic link, defined
8974 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8975 a special file type (which not all kernels support: System V release 3
8976 (and older) systems lack symlinks) in which the link file actually
8977 refers to a different file, by name. When most operations (opening,
8978 reading, writing, and so on) are passed the symbolic link file, the
8979 kernel automatically @dfn{dereferences} the link and operates on the
8980 target of the link. But some operations (e.g., removing) work on the
8981 link file itself, rather than on its target. The owner and group of a
8982 symlink are not significant to file access performed through
8983 the link, but do have implications on deleting a symbolic link from a
8984 directory with the restricted deletion bit set. On the GNU system,
8985 the mode of a symlink has no significance and cannot be changed, but
8986 on some BSD systems, the mode can be changed and will affect whether
8987 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8988 libc, The GNU C Library Reference Manual}.
8990 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8991 occurs when the string in the symlink does not resolve to a file.
8992 There are no restrictions against creating dangling symbolic links.
8993 There are trade-offs to using absolute or relative symlinks. An
8994 absolute symlink always points to the same file, even if the directory
8995 containing the link is moved. However, if the symlink is visible from
8996 more than one machine (such as on a networked file system), the file
8997 pointed to might not always be the same. A relative symbolic link is
8998 resolved in relation to the directory that contains the link, and is
8999 often useful in referring to files on the same device without regards
9000 to what name that device is mounted on when accessed via networked
9003 When creating a relative symlink in a different location than the
9004 current directory, the resolution of the symlink will be different
9005 than the resolution of the same string from the current directory.
9006 Therefore, many users prefer to first change directories to the
9007 location where the relative symlink will be created, so that
9008 tab-completion or other file resolution will find the same target as
9009 what will be placed in the symlink.
9011 The program accepts the following options. Also see @ref{Common options}.
9022 @opindex --directory
9023 @cindex hard links to directories
9024 Allow users with appropriate privileges to attempt to make hard links
9026 However, note that this will probably fail due to
9027 system restrictions, even for the super-user.
9033 Remove existing destination files.
9036 @itemx --interactive
9038 @opindex --interactive
9039 @cindex prompting, and @command{ln}
9040 Prompt whether to remove existing destination files.
9046 If @option{-s} is not in effect, and the source file is a symbolic
9047 link, create the hard link to the file referred to by the symbolic
9048 link, rather than the symbolic link itself.
9051 @itemx --no-dereference
9053 @opindex --no-dereference
9054 Do not treat the last operand specially when it is a symbolic link to
9055 a directory. Instead, treat it as if it were a normal file.
9057 When the destination is an actual directory (not a symlink to one),
9058 there is no ambiguity. The link is created in that directory.
9059 But when the specified destination is a symlink to a directory,
9060 there are two ways to treat the user's request. @command{ln} can
9061 treat the destination just as it would a normal directory and create
9062 the link in it. On the other hand, the destination can be viewed as a
9063 non-directory---as the symlink itself. In that case, @command{ln}
9064 must delete or backup that symlink before creating the new link.
9065 The default is to treat a destination that is a symlink to a directory
9066 just like a directory.
9068 This option is weaker than the @option{--no-target-directory}
9069 (@option{-T}) option, so it has no effect if both options are given.
9075 If @option{-s} is not in effect, and the source file is a symbolic
9076 link, create the hard link to the symbolic link itself. On platforms
9077 where this is not supported by the kernel, this option creates a
9078 symbolic link with identical contents; since symbolic link contents
9079 cannot be edited, any file name resolution performed through either
9080 link will be the same as if a hard link had been created.
9086 Make symbolic links instead of hard links. This option merely produces
9087 an error message on systems that do not support symbolic links.
9093 @optNoTargetDirectory
9099 Print the name of each file after linking it successfully.
9103 @cindex hard links to symbolic links
9104 @cindex symbolic links and @command{ln}
9105 If @option{-L} and @option{-P} are both given, the last one takes
9106 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9107 are silently ignored. If neither option is given, then this
9108 implementation defaults to @option{-P} if the system @code{link} supports
9109 hard links to symbolic links (such as the GNU system), and @option{-L}
9110 if @code{link} follows symbolic links (such as on BSD).
9119 # Create link ../a pointing to a in that directory.
9120 # Not really useful because it points to itself.
9125 # Change to the target before creating symlinks to avoid being confused.
9131 # Hard coded file names don't move well.
9132 ln -s $(pwd)/a /some/dir/
9136 # Relative file names survive directory moves and also
9137 # work across networked file systems.
9138 ln -s afile anotherfile
9139 ln -s ../adir/afile yetanotherfile
9143 @node mkdir invocation
9144 @section @command{mkdir}: Make directories
9147 @cindex directories, creating
9148 @cindex creating directories
9150 @command{mkdir} creates directories with the specified names. Synopsis:
9153 mkdir [@var{option}]@dots{} @var{name}@dots{}
9156 @command{mkdir} creates each directory @var{name} in the order given.
9157 It reports an error if @var{name} already exists, unless the
9158 @option{-p} option is given and @var{name} is a directory.
9160 The program accepts the following options. Also see @ref{Common options}.
9165 @itemx --mode=@var{mode}
9168 @cindex modes of created directories, setting
9169 Set the file permission bits of created directories to @var{mode},
9170 which uses the same syntax as
9171 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9172 everyone) for the point of the departure. @xref{File permissions}.
9174 Normally the directory has the desired file mode bits at the moment it
9175 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9176 special mode bits, but in this case there may be a temporary window
9177 during which the directory exists but its special mode bits are
9178 incorrect. @xref{Directory Setuid and Setgid}, for how the
9179 set-user-ID and set-group-ID bits of directories are inherited unless
9180 overridden in this way.
9186 @cindex parent directories, creating
9187 Make any missing parent directories for each argument, setting their
9188 file permission bits to the umask modified by @samp{u+wx}. Ignore
9189 existing parent directories, and do not change their file permission
9192 To set the file permission bits of any newly-created parent
9193 directories to a value that includes @samp{u+wx}, you can set the
9194 umask before invoking @command{mkdir}. For example, if the shell
9195 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9196 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9197 To set a parent's special mode bits as well, you can invoke
9198 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9199 Setgid}, for how the set-user-ID and set-group-ID bits of
9200 newly-created parent directories are inherited.
9206 Print a message for each created directory. This is most useful with
9209 @item -Z @var{context}
9210 @itemx --context=@var{context}
9214 @cindex security context
9215 Set the default SELinux security context to be used for created directories.
9222 @node mkfifo invocation
9223 @section @command{mkfifo}: Make FIFOs (named pipes)
9226 @cindex FIFOs, creating
9227 @cindex named pipes, creating
9228 @cindex creating FIFOs (named pipes)
9230 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9231 specified names. Synopsis:
9234 mkfifo [@var{option}] @var{name}@dots{}
9237 A @dfn{FIFO} is a special file type that permits independent processes
9238 to communicate. One process opens the FIFO file for writing, and
9239 another for reading, after which data can flow as with the usual
9240 anonymous pipe in shells or elsewhere.
9242 The program accepts the following option. Also see @ref{Common options}.
9247 @itemx --mode=@var{mode}
9250 @cindex modes of created FIFOs, setting
9251 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9252 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9253 for the point of departure. @var{mode} should specify only file
9254 permission bits. @xref{File permissions}.
9256 @item -Z @var{context}
9257 @itemx --context=@var{context}
9261 @cindex security context
9262 Set the default SELinux security context to be used for created FIFOs.
9269 @node mknod invocation
9270 @section @command{mknod}: Make block or character special files
9273 @cindex block special files, creating
9274 @cindex character special files, creating
9276 @command{mknod} creates a FIFO, character special file, or block special
9277 file with the specified name. Synopsis:
9280 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9283 @cindex special files
9284 @cindex block special files
9285 @cindex character special files
9286 Unlike the phrase ``special file type'' above, the term @dfn{special
9287 file} has a technical meaning on Unix: something that can generate or
9288 receive data. Usually this corresponds to a physical piece of hardware,
9289 e.g., a printer or a disk. (These files are typically created at
9290 system-configuration time.) The @command{mknod} command is what creates
9291 files of this type. Such devices can be read either a character at a
9292 time or a ``block'' (many characters) at a time, hence we say there are
9293 @dfn{block special} files and @dfn{character special} files.
9295 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9296 @mayConflictWithShellBuiltIn{mknod}
9298 The arguments after @var{name} specify the type of file to make:
9303 @opindex p @r{for FIFO file}
9307 @opindex b @r{for block special file}
9308 for a block special file
9311 @c Don't document the `u' option -- it's just a synonym for `c'.
9312 @c Do *any* versions of mknod still use it?
9314 @opindex c @r{for character special file}
9315 @c @opindex u @r{for character special file}
9316 for a character special file
9320 When making a block or character special file, the major and minor
9321 device numbers must be given after the file type.
9322 If a major or minor device number begins with @samp{0x} or @samp{0X},
9323 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9324 as octal; otherwise, as decimal.
9326 The program accepts the following option. Also see @ref{Common options}.
9331 @itemx --mode=@var{mode}
9334 Set the mode of created files to @var{mode}, which is symbolic as in
9335 @command{chmod} and uses @samp{a=rw} as the point of departure.
9336 @var{mode} should specify only file permission bits.
9337 @xref{File permissions}.
9339 @item -Z @var{context}
9340 @itemx --context=@var{context}
9344 @cindex security context
9345 Set the default SELinux security context to be used for created files.
9352 @node readlink invocation
9353 @section @command{readlink}: Print value of a symlink or canonical file name
9356 @cindex displaying value of a symbolic link
9357 @cindex canonical file name
9358 @cindex canonicalize a file name
9362 @command{readlink} may work in one of two supported modes:
9368 @command{readlink} outputs the value of the given symbolic link.
9369 If @command{readlink} is invoked with an argument other than the name
9370 of a symbolic link, it produces no output and exits with a nonzero exit code.
9372 @item Canonicalize mode
9374 @command{readlink} outputs the absolute name of the given file which contains
9375 no @file{.}, @file{..} components nor any repeated separators
9376 (@file{/}) or symbolic links.
9381 readlink [@var{option}] @var{file}
9384 By default, @command{readlink} operates in readlink mode.
9386 The program accepts the following options. Also see @ref{Common options}.
9391 @itemx --canonicalize
9393 @opindex --canonicalize
9394 Activate canonicalize mode.
9395 If any component of the file name except the last one is missing or unavailable,
9396 @command{readlink} produces no output and exits with a nonzero exit
9397 code. A trailing slash is ignored.
9400 @itemx --canonicalize-existing
9402 @opindex --canonicalize-existing
9403 Activate canonicalize mode.
9404 If any component is missing or unavailable, @command{readlink} produces
9405 no output and exits with a nonzero exit code. A trailing slash
9406 requires that the name resolve to a directory.
9409 @itemx --canonicalize-missing
9411 @opindex --canonicalize-missing
9412 Activate canonicalize mode.
9413 If any component is missing or unavailable, @command{readlink} treats it
9419 @opindex --no-newline
9420 Do not output the trailing newline.
9430 Suppress most error messages.
9436 Report error messages.
9440 The @command{readlink} utility first appeared in OpenBSD 2.1.
9442 There is a @command{realpath} command on some systems
9443 which operates like @command{readlink} in canonicalize mode.
9448 @node rmdir invocation
9449 @section @command{rmdir}: Remove empty directories
9452 @cindex removing empty directories
9453 @cindex directories, removing empty
9455 @command{rmdir} removes empty directories. Synopsis:
9458 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9461 If any @var{directory} argument does not refer to an existing empty
9462 directory, it is an error.
9464 The program accepts the following options. Also see @ref{Common options}.
9468 @item --ignore-fail-on-non-empty
9469 @opindex --ignore-fail-on-non-empty
9470 @cindex directory deletion, ignoring failures
9471 Ignore each failure to remove a directory that is solely because
9472 the directory is non-empty.
9478 @cindex parent directories, removing
9479 Remove @var{directory}, then try to remove each component of @var{directory}.
9480 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9481 As such, it fails if any of those directories turns out not to be empty.
9482 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9483 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9484 exit unsuccessfully.
9490 @cindex directory deletion, reporting
9491 Give a diagnostic for each successful removal.
9492 @var{directory} is removed.
9496 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9501 @node unlink invocation
9502 @section @command{unlink}: Remove files via the unlink syscall
9505 @cindex removing files or directories (via the unlink syscall)
9507 @command{unlink} deletes a single specified file name.
9508 It is a minimalist interface to the system-provided
9509 @code{unlink} function. @xref{Deleting Files, , , libc,
9510 The GNU C Library Reference Manual}. Synopsis:
9511 It avoids the bells and whistles of the more commonly-used
9512 @command{rm} command (@pxref{rm invocation}).
9515 unlink @var{filename}
9518 On some systems @code{unlink} can be used to delete the name of a
9519 directory. On others, it can be used that way only by a privileged user.
9520 In the GNU system @code{unlink} can never delete the name of a directory.
9522 The @command{unlink} command honors the @option{--help} and
9523 @option{--version} options. To remove a file whose name begins with
9524 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9529 @node Changing file attributes
9530 @chapter Changing file attributes
9532 @cindex changing file attributes
9533 @cindex file attributes, changing
9534 @cindex attributes, file
9536 A file is not merely its contents, a name, and a file type
9537 (@pxref{Special file types}). A file also has an owner (a user ID), a
9538 group (a group ID), permissions (what the owner can do with the file,
9539 what people in the group can do, and what everyone else can do), various
9540 timestamps, and other information. Collectively, we call these a file's
9543 These commands change file attributes.
9546 * chgrp invocation:: Change file groups.
9547 * chmod invocation:: Change access permissions.
9548 * chown invocation:: Change file owners and groups.
9549 * touch invocation:: Change file timestamps.
9553 @node chown invocation
9554 @section @command{chown}: Change file owner and group
9557 @cindex file ownership, changing
9558 @cindex group ownership, changing
9559 @cindex changing file ownership
9560 @cindex changing group ownership
9562 @command{chown} changes the user and/or group ownership of each given @var{file}
9563 to @var{new-owner} or to the user and group of an existing reference file.
9567 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9570 If used, @var{new-owner} specifies the new owner and/or group as follows
9571 (with no embedded white space):
9574 [@var{owner}] [ : [@var{group}] ]
9581 If only an @var{owner} (a user name or numeric user ID) is given, that
9582 user is made the owner of each given file, and the files' group is not
9585 @item owner@samp{:}group
9586 If the @var{owner} is followed by a colon and a @var{group} (a
9587 group name or numeric group ID), with no spaces between them, the group
9588 ownership of the files is changed as well (to @var{group}).
9591 If a colon but no group name follows @var{owner}, that user is
9592 made the owner of the files and the group of the files is changed to
9593 @var{owner}'s login group.
9596 If the colon and following @var{group} are given, but the owner
9597 is omitted, only the group of the files is changed; in this case,
9598 @command{chown} performs the same function as @command{chgrp}.
9601 If only a colon is given, or if @var{new-owner} is empty, neither the
9602 owner nor the group is changed.
9606 If @var{owner} or @var{group} is intended to represent a numeric user
9607 or group ID, then you may specify it with a leading @samp{+}.
9608 @xref{Disambiguating names and IDs}.
9610 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9611 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9612 require support for that, but for backward compatibility @acronym{GNU}
9613 @command{chown} supports @samp{.} so long as no ambiguity results.
9614 New scripts should avoid the use of @samp{.} because it is not
9615 portable, and because it has undesirable results if the entire
9616 @var{owner@samp{.}group} happens to identify a user whose name
9619 The @command{chown} command sometimes clears the set-user-ID or
9620 set-group-ID permission bits. This behavior depends on the policy and
9621 functionality of the underlying @code{chown} system call, which may
9622 make system-dependent file mode modifications outside the control of
9623 the @command{chown} command. For example, the @command{chown} command
9624 might not affect those bits when invoked by a user with appropriate
9625 privileges, or when the
9626 bits signify some function other than executable permission (e.g.,
9628 When in doubt, check the underlying system behavior.
9630 The program accepts the following options. Also see @ref{Common options}.
9638 @cindex changed owners, verbosely describing
9639 Verbosely describe the action for each @var{file} whose ownership
9648 @cindex error messages, omitting
9649 Do not print error messages about files whose ownership cannot be
9652 @itemx @w{@kbd{--from}=@var{old-owner}}
9654 @cindex symbolic links, changing owner
9655 Change a @var{file}'s ownership only if it has current attributes specified
9656 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9658 This option is useful primarily from a security standpoint in that
9659 it narrows considerably the window of potential abuse.
9660 For example, to reflect a user ID numbering change for one user's files
9661 without an option like this, @code{root} might run
9664 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9667 But that is dangerous because the interval between when the @command{find}
9668 tests the existing file's owner and when the @command{chown} is actually run
9670 One way to narrow the gap would be to invoke chown for each file
9674 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9677 But that is very slow if there are many affected files.
9678 With this option, it is safer (the gap is narrower still)
9679 though still not perfect:
9682 chown -h -R --from=OLDUSER NEWUSER /
9686 @opindex --dereference
9687 @cindex symbolic links, changing owner
9689 Do not act on symbolic links themselves but rather on what they point to.
9690 This is the default.
9693 @itemx --no-dereference
9695 @opindex --no-dereference
9696 @cindex symbolic links, changing owner
9698 Act on symbolic links themselves instead of what they point to.
9699 This mode relies on the @code{lchown} system call.
9700 On systems that do not provide the @code{lchown} system call,
9701 @command{chown} fails when a file specified on the command line
9703 By default, no diagnostic is issued for symbolic links encountered
9704 during a recursive traversal, but see @option{--verbose}.
9706 @itemx --preserve-root
9707 @opindex --preserve-root
9708 @cindex root directory, disallow recursive modification
9709 Fail upon any attempt to recursively change the root directory, @file{/}.
9710 Without @option{--recursive}, this option has no effect.
9711 @xref{Treating / specially}.
9713 @itemx --no-preserve-root
9714 @opindex --no-preserve-root
9715 @cindex root directory, allow recursive modification
9716 Cancel the effect of any preceding @option{--preserve-root} option.
9717 @xref{Treating / specially}.
9719 @item --reference=@var{ref_file}
9720 @opindex --reference
9721 Change the user and group of each @var{file} to be the same as those of
9722 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9723 user and group of the symbolic link, but rather those of the file it
9730 Output a diagnostic for every file processed.
9731 If a symbolic link is encountered during a recursive traversal
9732 on a system without the @code{lchown} system call, and @option{--no-dereference}
9733 is in effect, then issue a diagnostic saying neither the symbolic link nor
9734 its referent is being changed.
9739 @opindex --recursive
9740 @cindex recursively changing file ownership
9741 Recursively change ownership of directories and their contents.
9744 @xref{Traversing symlinks}.
9747 @xref{Traversing symlinks}.
9750 @xref{Traversing symlinks}.
9759 # Change the owner of /u to "root".
9762 # Likewise, but also change its group to "staff".
9765 # Change the owner of /u and subfiles to "root".
9770 @node chgrp invocation
9771 @section @command{chgrp}: Change group ownership
9774 @cindex group ownership, changing
9775 @cindex changing group ownership
9777 @command{chgrp} changes the group ownership of each given @var{file}
9778 to @var{group} (which can be either a group name or a numeric group ID)
9779 or to the group of an existing reference file. Synopsis:
9782 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9785 If @var{group} is intended to represent a
9786 numeric group ID, then you may specify it with a leading @samp{+}.
9787 @xref{Disambiguating names and IDs}.
9789 The program accepts the following options. Also see @ref{Common options}.
9797 @cindex changed files, verbosely describing
9798 Verbosely describe the action for each @var{file} whose group actually
9807 @cindex error messages, omitting
9808 Do not print error messages about files whose group cannot be
9812 @opindex --dereference
9813 @cindex symbolic links, changing owner
9815 Do not act on symbolic links themselves but rather on what they point to.
9816 This is the default.
9819 @itemx --no-dereference
9821 @opindex --no-dereference
9822 @cindex symbolic links, changing group
9824 Act on symbolic links themselves instead of what they point to.
9825 This mode relies on the @code{lchown} system call.
9826 On systems that do not provide the @code{lchown} system call,
9827 @command{chgrp} fails when a file specified on the command line
9829 By default, no diagnostic is issued for symbolic links encountered
9830 during a recursive traversal, but see @option{--verbose}.
9832 @itemx --preserve-root
9833 @opindex --preserve-root
9834 @cindex root directory, disallow recursive modification
9835 Fail upon any attempt to recursively change the root directory, @file{/}.
9836 Without @option{--recursive}, this option has no effect.
9837 @xref{Treating / specially}.
9839 @itemx --no-preserve-root
9840 @opindex --no-preserve-root
9841 @cindex root directory, allow recursive modification
9842 Cancel the effect of any preceding @option{--preserve-root} option.
9843 @xref{Treating / specially}.
9845 @item --reference=@var{ref_file}
9846 @opindex --reference
9847 Change the group of each @var{file} to be the same as that of
9848 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9849 group of the symbolic link, but rather that of the file it refers to.
9855 Output a diagnostic for every file processed.
9856 If a symbolic link is encountered during a recursive traversal
9857 on a system without the @code{lchown} system call, and @option{--no-dereference}
9858 is in effect, then issue a diagnostic saying neither the symbolic link nor
9859 its referent is being changed.
9864 @opindex --recursive
9865 @cindex recursively changing group ownership
9866 Recursively change the group ownership of directories and their contents.
9869 @xref{Traversing symlinks}.
9872 @xref{Traversing symlinks}.
9875 @xref{Traversing symlinks}.
9884 # Change the group of /u to "staff".
9887 # Change the group of /u and subfiles to "staff".
9892 @node chmod invocation
9893 @section @command{chmod}: Change access permissions
9896 @cindex changing access permissions
9897 @cindex access permissions, changing
9898 @cindex permissions, changing access
9900 @command{chmod} changes the access permissions of the named files. Synopsis:
9903 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9906 @cindex symbolic links, permissions of
9907 @command{chmod} never changes the permissions of symbolic links, since
9908 the @command{chmod} system call cannot change their permissions.
9909 This is not a problem since the permissions of symbolic links are
9910 never used. However, for each symbolic link listed on the command
9911 line, @command{chmod} changes the permissions of the pointed-to file.
9912 In contrast, @command{chmod} ignores symbolic links encountered during
9913 recursive directory traversals.
9915 A successful use of @command{chmod} clears the set-group-ID bit of a
9916 regular file if the file's group ID does not match the user's
9917 effective group ID or one of the user's supplementary group IDs,
9918 unless the user has appropriate privileges. Additional restrictions
9919 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9920 @var{ref_file} to be ignored. This behavior depends on the policy and
9921 functionality of the underlying @code{chmod} system call. When in
9922 doubt, check the underlying system behavior.
9924 If used, @var{mode} specifies the new file mode bits.
9925 For details, see the section on @ref{File permissions}.
9926 If you really want @var{mode} to have a leading @samp{-}, you should
9927 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9928 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9929 file} (without the @option{--}) complains if it behaves differently
9930 from what @samp{chmod a-w file} would do.
9932 The program accepts the following options. Also see @ref{Common options}.
9940 Verbosely describe the action for each @var{file} whose permissions
9949 @cindex error messages, omitting
9950 Do not print error messages about files whose permissions cannot be
9953 @itemx --preserve-root
9954 @opindex --preserve-root
9955 @cindex root directory, disallow recursive modification
9956 Fail upon any attempt to recursively change the root directory, @file{/}.
9957 Without @option{--recursive}, this option has no effect.
9958 @xref{Treating / specially}.
9960 @itemx --no-preserve-root
9961 @opindex --no-preserve-root
9962 @cindex root directory, allow recursive modification
9963 Cancel the effect of any preceding @option{--preserve-root} option.
9964 @xref{Treating / specially}.
9970 Verbosely describe the action or non-action taken for every @var{file}.
9972 @item --reference=@var{ref_file}
9973 @opindex --reference
9974 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9975 @xref{File permissions}.
9976 If @var{ref_file} is a symbolic link, do not use the mode
9977 of the symbolic link, but rather that of the file it refers to.
9982 @opindex --recursive
9983 @cindex recursively changing access permissions
9984 Recursively change permissions of directories and their contents.
9991 @node touch invocation
9992 @section @command{touch}: Change file timestamps
9995 @cindex changing file timestamps
9996 @cindex file timestamps, changing
9997 @cindex timestamps, changing file
9999 @command{touch} changes the access and/or modification times of the
10000 specified files. Synopsis:
10003 touch [@var{option}]@dots{} @var{file}@dots{}
10006 @cindex empty files, creating
10007 Any @var{file} argument that does not exist is created empty, unless
10008 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10009 (@option{-h}) was in effect.
10011 A @var{file} argument string of @samp{-} is handled specially and
10012 causes @command{touch} to change the times of the file associated with
10015 @cindex permissions, for changing file timestamps
10016 If changing both the access and modification times to the current
10017 time, @command{touch} can change the timestamps for files that the user
10018 running it does not own but has write permission for. Otherwise, the
10019 user must own the files.
10021 Although @command{touch} provides options for changing two of the times---the
10022 times of last access and modification---of a file, there is actually
10023 a standard third one as well: the inode change time. This is often
10024 referred to as a file's @code{ctime}.
10025 The inode change time represents the time when the file's meta-information
10026 last changed. One common example of this is when the permissions of a
10027 file change. Changing the permissions doesn't access the file, so
10028 the atime doesn't change, nor does it modify the file, so the mtime
10029 doesn't change. Yet, something about the file itself has changed,
10030 and this must be noted somewhere. This is the job of the ctime field.
10031 This is necessary, so that, for example, a backup program can make a
10032 fresh copy of the file, including the new permissions value.
10033 Another operation that modifies a file's ctime without affecting
10034 the others is renaming. In any case, it is not possible, in normal
10035 operations, for a user to change the ctime field to a user-specified value.
10036 Some operating systems and file systems support a fourth time: the
10037 birth time, when the file was first created; by definition, this
10038 timestamp never changes.
10041 Time stamps assume the time zone rules specified by the @env{TZ}
10042 environment variable, or by the system default rules if @env{TZ} is
10043 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10044 libc, The GNU C Library Reference Manual}.
10045 You can avoid ambiguities during
10046 daylight saving transitions by using @sc{utc} time stamps.
10048 The program accepts the following options. Also see @ref{Common options}.
10053 @itemx --time=atime
10054 @itemx --time=access
10058 @opindex atime@r{, changing}
10059 @opindex access @r{time, changing}
10060 @opindex use @r{time, changing}
10061 Change the access time only.
10066 @opindex --no-create
10067 Do not warn about or create files that do not exist.
10070 @itemx --date=@var{time}
10074 Use @var{time} instead of the current time. It can contain month names,
10075 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10076 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10077 specifies the instant of time that is 489,392,193 nanoseconds after
10078 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10079 minutes east of @acronym{UTC}. @xref{Date input formats}.
10080 File systems that do not support high-resolution time stamps
10081 silently ignore any excess precision here.
10085 @cindex BSD @command{touch} compatibility
10086 Ignored; for compatibility with BSD versions of @command{touch}.
10089 @itemx --no-dereference
10091 @opindex --no-dereference
10092 @cindex symbolic links, changing time
10094 Attempt to change the timestamps of a symbolic link, rather than what
10095 the link refers to. When using this option, empty files are not
10096 created, but option @option{-c} must also be used to avoid warning
10097 about files that do not exist. Not all systems support changing the
10098 timestamps of symlinks, since underlying system support for this
10099 action was not required until @acronym{POSIX} 2008. Also, on some
10100 systems, the mere act of examining a symbolic link changes the access
10101 time, such that only changes to the modification time will persist
10102 long enough to be observable. When coupled with option @option{-r}, a
10103 reference timestamp is taken from a symbolic link rather than the file
10107 @itemx --time=mtime
10108 @itemx --time=modify
10111 @opindex mtime@r{, changing}
10112 @opindex modify @r{time, changing}
10113 Change the modification time only.
10115 @item -r @var{file}
10116 @itemx --reference=@var{file}
10118 @opindex --reference
10119 Use the times of the reference @var{file} instead of the current time.
10120 If this option is combined with the @option{--date=@var{time}}
10121 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10122 the origin for any relative @var{time}s given, but is otherwise ignored.
10123 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10124 equal to five seconds before the corresponding time stamp for @file{foo}.
10125 If @var{file} is a symbolic link, the reference timestamp is taken
10126 from the target of the symlink, unless @option{-h} was also in effect.
10128 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10129 Use the argument (optional four-digit or two-digit years, months,
10130 days, hours, minutes, optional seconds) instead of the current time.
10131 If the year is specified with only two digits, then @var{cc}
10132 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10133 69 @dots{} 99. If no digits of the year are specified,
10134 the argument is interpreted as a date in the current year.
10135 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10139 @vindex _POSIX2_VERSION
10140 On older systems, @command{touch} supports an obsolete syntax, as follows.
10141 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10142 @option{-t} options, and if there are two or more @var{file}s and the
10143 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10144 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10145 any, were moved to the front), and if the represented year
10146 is in the range 1969--1999, that argument is interpreted as the time
10147 for the other files instead of as a file name.
10148 This obsolete behavior can be enabled or disabled with the
10149 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10150 conformance}), but portable scripts should avoid commands whose
10151 behavior depends on this variable.
10152 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10153 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10159 @chapter Disk usage
10163 No disk can hold an infinite amount of data. These commands report
10164 how much disk storage is in use or available, report other file and
10165 file status information, and write buffers to disk.
10168 * df invocation:: Report file system disk space usage.
10169 * du invocation:: Estimate file space usage.
10170 * stat invocation:: Report file or file system status.
10171 * sync invocation:: Synchronize memory and disk.
10172 * truncate invocation:: Shrink or extend the size of a file.
10176 @node df invocation
10177 @section @command{df}: Report file system disk space usage
10180 @cindex file system disk usage
10181 @cindex disk usage by file system
10183 @command{df} reports the amount of disk space used and available on
10184 file systems. Synopsis:
10187 df [@var{option}]@dots{} [@var{file}]@dots{}
10190 With no arguments, @command{df} reports the space used and available on all
10191 currently mounted file systems (of all types). Otherwise, @command{df}
10192 reports on the file system containing each argument @var{file}.
10194 Normally the disk space is printed in units of
10195 1024 bytes, but this can be overridden (@pxref{Block size}).
10196 Non-integer quantities are rounded up to the next higher unit.
10198 @cindex disk device file
10199 @cindex device file, disk
10200 If an argument @var{file} is a disk device file containing a mounted
10201 file system, @command{df} shows the space available on that file system
10202 rather than on the file system containing the device node (i.e., the root
10203 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10204 on unmounted file systems, because on most kinds of systems doing so
10205 requires extremely nonportable intimate knowledge of file system
10208 The program accepts the following options. Also see @ref{Common options}.
10216 @cindex automounter file systems
10217 @cindex ignore file systems
10218 Include in the listing dummy file systems, which
10219 are omitted by default. Such file systems are typically special-purpose
10220 pseudo-file-systems, such as automounter entries.
10222 @item -B @var{size}
10223 @itemx --block-size=@var{size}
10225 @opindex --block-size
10226 @cindex file system sizes
10227 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10228 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10232 @cindex grand total of disk size, usage and available space
10233 Print a grand total of all arguments after all arguments have
10234 been processed. This can be used to find out the total disk size, usage
10235 and available space of all listed devices.
10241 Equivalent to @option{--si}.
10247 @cindex inode usage
10248 List inode usage information instead of block usage. An inode (short
10249 for index node) contains information about a file such as its owner,
10250 permissions, timestamps, and location on the disk.
10254 @cindex kibibytes for file system sizes
10255 Print sizes in 1024-byte blocks, overriding the default block size
10256 (@pxref{Block size}).
10257 This option is equivalent to @option{--block-size=1K}.
10263 @cindex file system types, limiting output to certain
10264 Limit the listing to local file systems. By default, remote file systems
10269 @cindex file system space, retrieving old data more quickly
10270 Do not invoke the @code{sync} system call before getting any usage data.
10271 This may make @command{df} run significantly faster on systems with many
10272 disks, but on some systems (notably SunOS) the results may be slightly
10273 out of date. This is the default.
10276 @itemx --portability
10278 @opindex --portability
10279 @cindex one-line output format
10280 @cindex @acronym{POSIX} output format
10281 @cindex portable output format
10282 @cindex output format, portable
10283 Use the @acronym{POSIX} output format. This is like the default format except
10288 The information about each file system is always printed on exactly
10289 one line; a mount device is never put on a line by itself. This means
10290 that if the mount device name is more than 20 characters long (e.g., for
10291 some network mounts), the columns are misaligned.
10294 The labels in the header output line are changed to conform to @acronym{POSIX}.
10297 The default block size and output format are unaffected by the
10298 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10299 variables. However, the default block size is still affected by
10300 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10301 otherwise. @xref{Block size}.
10308 @cindex file system space, retrieving current data more slowly
10309 Invoke the @code{sync} system call before getting any usage data. On
10310 some systems (notably SunOS), doing this yields more up to date results,
10311 but in general this option makes @command{df} much slower, especially when
10312 there are many or very busy file systems.
10314 @item -t @var{fstype}
10315 @itemx --type=@var{fstype}
10318 @cindex file system types, limiting output to certain
10319 Limit the listing to file systems of type @var{fstype}. Multiple
10320 file system types can be specified by giving multiple @option{-t} options.
10321 By default, nothing is omitted.
10324 @itemx --print-type
10326 @opindex --print-type
10327 @cindex file system types, printing
10328 Print each file system's type. The types printed here are the same ones
10329 you can include or exclude with @option{-t} and @option{-x}. The particular
10330 types printed are whatever is supported by the system. Here are some of
10331 the common names (this list is certainly not exhaustive):
10336 @cindex @acronym{NFS} file system type
10337 An @acronym{NFS} file system, i.e., one mounted over a network from another
10338 machine. This is the one type name which seems to be used uniformly by
10341 @item 4.2@r{, }ufs@r{, }efs@dots{}
10342 @cindex Linux file system types
10343 @cindex local file system types
10344 @opindex 4.2 @r{file system type}
10345 @opindex ufs @r{file system type}
10346 @opindex efs @r{file system type}
10347 A file system on a locally-mounted hard disk. (The system might even
10348 support more than one type here; Linux does.)
10350 @item hsfs@r{, }cdfs
10351 @cindex CD-ROM file system type
10352 @cindex High Sierra file system
10353 @opindex hsfs @r{file system type}
10354 @opindex cdfs @r{file system type}
10355 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10356 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10359 @cindex PC file system
10360 @cindex DOS file system
10361 @cindex MS-DOS file system
10362 @cindex diskette file system
10364 An MS-DOS file system, usually on a diskette.
10368 @item -x @var{fstype}
10369 @itemx --exclude-type=@var{fstype}
10371 @opindex --exclude-type
10372 Limit the listing to file systems not of type @var{fstype}.
10373 Multiple file system types can be eliminated by giving multiple
10374 @option{-x} options. By default, no file system types are omitted.
10377 Ignored; for compatibility with System V versions of @command{df}.
10382 Failure includes the case where no output is generated, so you can
10383 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10384 @var{dir}} to test whether @var{dir} is on a file system of type
10385 @samp{ext3} or @samp{reiserfs}.
10388 @node du invocation
10389 @section @command{du}: Estimate file space usage
10392 @cindex file space usage
10393 @cindex disk usage for files
10395 @command{du} reports the amount of disk space used by the specified files
10396 and for each subdirectory (of directory arguments). Synopsis:
10399 du [@var{option}]@dots{} [@var{file}]@dots{}
10402 With no arguments, @command{du} reports the disk space for the current
10403 directory. Normally the disk space is printed in units of
10404 1024 bytes, but this can be overridden (@pxref{Block size}).
10405 Non-integer quantities are rounded up to the next higher unit.
10407 If two or more hard links point to the same file, only one of the hard
10408 links is counted. The @var{file} argument order affects which links
10409 are counted, and changing the argument order may change the numbers
10410 that @command{du} outputs.
10412 The program accepts the following options. Also see @ref{Common options}.
10420 Show counts for all files, not just directories.
10422 @itemx --apparent-size
10423 @opindex --apparent-size
10424 Print apparent sizes, rather than disk usage. The apparent size of a
10425 file is the number of bytes reported by @code{wc -c} on regular files,
10426 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10427 For example, a file containing the word @samp{zoo} with no newline would,
10428 of course, have an apparent size of 3. Such a small file may require
10429 anywhere from 0 to 16 KiB or more of disk space, depending on
10430 the type and configuration of the file system on which the file resides.
10431 However, a sparse file created with this command:
10434 dd bs=1 seek=2GiB if=/dev/null of=big
10438 has an apparent size of 2 GiB, yet on most modern
10439 systems, it actually uses almost no disk space.
10445 Equivalent to @code{--apparent-size --block-size=1}.
10447 @item -B @var{size}
10448 @itemx --block-size=@var{size}
10450 @opindex --block-size
10452 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10453 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10459 @cindex grand total of disk space
10460 Print a grand total of all arguments after all arguments have
10461 been processed. This can be used to find out the total disk usage of
10462 a given set of files or directories.
10465 @itemx --dereference-args
10467 @opindex --dereference-args
10468 Dereference symbolic links that are command line arguments.
10469 Does not affect other symbolic links. This is helpful for finding
10470 out the disk usage of directories, such as @file{/usr/tmp}, which
10471 are often symbolic links.
10473 @c --files0-from=FILE
10474 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10480 Equivalent to @option{--dereference-args} (@option{-D}).
10484 @cindex kibibytes for file sizes
10485 Print sizes in 1024-byte blocks, overriding the default block size
10486 (@pxref{Block size}).
10487 This option is equivalent to @option{--block-size=1K}.
10490 @itemx --count-links
10492 @opindex --count-links
10493 @cindex hard links, counting in @command{du}
10494 Count the size of all files, even if they have appeared already (as a
10498 @itemx --dereference
10500 @opindex --dereference
10501 @cindex symbolic links, dereferencing in @command{du}
10502 Dereference symbolic links (show the disk space used by the file
10503 or directory that the link points to instead of the space used by
10508 @cindex mebibytes for file sizes
10509 Print sizes in 1,048,576-byte blocks, overriding the default block size
10510 (@pxref{Block size}).
10511 This option is equivalent to @option{--block-size=1M}.
10514 @itemx --no-dereference
10516 @opindex --no-dereference
10517 @cindex symbolic links, dereferencing in @command{du}
10518 For each symbolic links encountered by @command{du},
10519 consider the disk space used by the symbolic link.
10521 @item -d @var{depth}
10522 @item --max-depth=@var{depth}
10523 @opindex -d @var{depth}
10524 @opindex --max-depth=@var{depth}
10525 @cindex limiting output of @command{du}
10526 Show the total for each directory (and file if --all) that is at
10527 most MAX_DEPTH levels down from the root of the hierarchy. The root
10528 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10537 @opindex --summarize
10538 Display only a total for each argument.
10541 @itemx --separate-dirs
10543 @opindex --separate-dirs
10544 Normally, in the output of @command{du} (when not using @option{--summarize}),
10545 the size listed next to a directory name, @var{d}, represents the sum
10546 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10547 With @option{--separate-dirs}, the size reported for a directory name,
10548 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10553 @cindex last modified dates, displaying in @command{du}
10554 Show time of the most recent modification of any file in the directory,
10555 or any of its subdirectories.
10557 @itemx --time=ctime
10558 @itemx --time=status
10561 @opindex ctime@r{, show the most recent}
10562 @opindex status time@r{, show the most recent}
10563 @opindex use time@r{, show the most recent}
10564 Show the most recent status change time (the @samp{ctime} in the inode) of
10565 any file in the directory, instead of the modification time.
10567 @itemx --time=atime
10568 @itemx --time=access
10570 @opindex atime@r{, show the most recent}
10571 @opindex access time@r{, show the most recent}
10572 Show the most recent access time (the @samp{atime} in the inode) of
10573 any file in the directory, instead of the modification time.
10575 @item --time-style=@var{style}
10576 @opindex --time-style
10578 List timestamps in style @var{style}. This option has an effect only if
10579 the @option{--time} option is also specified. The @var{style} should
10580 be one of the following:
10583 @item +@var{format}
10585 List timestamps using @var{format}, where @var{format} is interpreted
10586 like the format argument of @command{date} (@pxref{date invocation}).
10587 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10588 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10589 with @command{date}, @var{format}'s interpretation is affected by the
10590 @env{LC_TIME} locale category.
10593 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10594 format with nanosecond precision, e.g., @samp{2002-03-30
10595 23:45:56.477817180 -0700}. This style is equivalent to
10596 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10599 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10600 @samp{2002-03-30 23:45}. These timestamps are shorter than
10601 @samp{full-iso} timestamps, and are usually good enough for everyday
10602 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10605 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10606 This style is equivalent to @samp{+%Y-%m-%d}.
10610 You can specify the default value of the @option{--time-style} option
10611 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10612 the default style is @samp{long-iso}. For compatibility with @command{ls},
10613 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10614 the newline and any later characters are ignored; if @env{TIME_STYLE}
10615 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10616 @env{TIME_STYLE} is @samp{locale} it is ignored.
10619 @itemx --one-file-system
10621 @opindex --one-file-system
10622 @cindex one file system, restricting @command{du} to
10623 Skip directories that are on different file systems from the one that
10624 the argument being processed is on.
10626 @item --exclude=@var{pattern}
10627 @opindex --exclude=@var{pattern}
10628 @cindex excluding files from @command{du}
10629 When recursing, skip subdirectories or files matching @var{pattern}.
10630 For example, @code{du --exclude='*.o'} excludes files whose names
10633 @item -X @var{file}
10634 @itemx --exclude-from=@var{file}
10635 @opindex -X @var{file}
10636 @opindex --exclude-from=@var{file}
10637 @cindex excluding files from @command{du}
10638 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10639 one per line. If @var{file} is @samp{-}, take the patterns from standard
10644 @cindex NFS mounts from BSD to HP-UX
10645 On BSD systems, @command{du} reports sizes that are half the correct
10646 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10647 systems, it reports sizes that are twice the correct values for
10648 files that are NFS-mounted from BSD systems. This is due to a flaw
10649 in HP-UX; it also affects the HP-UX @command{du} program.
10654 @node stat invocation
10655 @section @command{stat}: Report file or file system status
10658 @cindex file status
10659 @cindex file system status
10661 @command{stat} displays information about the specified file(s). Synopsis:
10664 stat [@var{option}]@dots{} [@var{file}]@dots{}
10667 With no option, @command{stat} reports all information about the given files.
10668 But it also can be used to report the information of the file systems the
10669 given files are located on. If the files are links, @command{stat} can
10670 also give information about the files the links point to.
10672 @mayConflictWithShellBuiltIn{stat}
10677 @itemx --dereference
10679 @opindex --dereference
10680 @cindex symbolic links, dereferencing in @command{stat}
10681 Change how @command{stat} treats symbolic links.
10682 With this option, @command{stat} acts on the file referenced
10683 by each symbolic link argument.
10684 Without it, @command{stat} acts on any symbolic link argument directly.
10687 @itemx --file-system
10689 @opindex --file-system
10690 @cindex file systems
10691 Report information about the file systems where the given files are located
10692 instead of information about the files themselves.
10693 This option implies the @option{-L} option.
10696 @itemx --format=@var{format}
10698 @opindex --format=@var{format}
10699 @cindex output format
10700 Use @var{format} rather than the default format.
10701 @var{format} is automatically newline-terminated, so
10702 running a command like the following with two or more @var{file}
10703 operands produces a line of output for each operand:
10705 $ stat --format=%d:%i / /usr
10710 @itemx --printf=@var{format}
10711 @opindex --printf=@var{format}
10712 @cindex output format
10713 Use @var{format} rather than the default format.
10714 Like @option{--format}, but interpret backslash escapes,
10715 and do not output a mandatory trailing newline.
10716 If you want a newline, include @samp{\n} in the @var{format}.
10717 Here's how you would use @option{--printf} to print the device
10718 and inode numbers of @file{/} and @file{/usr}:
10720 $ stat --printf='%d:%i\n' / /usr
10729 @cindex terse output
10730 Print the information in terse form, suitable for parsing by other programs.
10734 The valid @var{format} directives for files with @option{--format} and
10735 @option{--printf} are:
10738 @item %a - Access rights in octal
10739 @item %A - Access rights in human readable form
10740 @item %b - Number of blocks allocated (see @samp{%B})
10741 @item %B - The size in bytes of each block reported by @samp{%b}
10742 @item %C - The SELinux security context of a file, if available
10743 @item %d - Device number in decimal
10744 @item %D - Device number in hex
10745 @item %f - Raw mode in hex
10746 @item %F - File type
10747 @item %g - Group ID of owner
10748 @item %G - Group name of owner
10749 @item %h - Number of hard links
10750 @item %i - Inode number
10751 @item %m - Mount point (See note below)
10752 @item %n - File name
10753 @item %N - Quoted file name with dereference if symbolic link
10754 @item %o - I/O block size
10755 @item %s - Total size, in bytes
10756 @item %t - Major device type in hex
10757 @item %T - Minor device type in hex
10758 @item %u - User ID of owner
10759 @item %U - User name of owner
10760 @item %w - Time of file birth, or @samp{-} if unknown
10761 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10762 @item %x - Time of last access
10763 @item %X - Time of last access as seconds since Epoch
10764 @item %y - Time of last modification
10765 @item %Y - Time of last modification as seconds since Epoch
10766 @item %z - Time of last change
10767 @item %Z - Time of last change as seconds since Epoch
10770 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10771 precision preceded by a period to specify the number of digits to
10772 print after the decimal point. For example, @samp{%.3X} outputs the
10773 last access time to millisecond precision. If a period is given but no
10774 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10775 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10776 toward minus infinity.
10780 $ stat -c '[%015Y]' /usr
10783 $ stat -c '[%15Y]' /usr
10785 $ stat -c '[%-15Y]' /usr
10788 $ stat -c '[%.3Y]' /usr
10790 $ stat -c '[%.Y]' /usr
10791 [1288929712.114951834]
10794 The mount point printed by @samp{%m} is similar to that output
10795 by @command{df}, except that:
10798 stat does not dereference symlinks by default
10799 (unless @option{-L} is specified)
10801 stat does not search for specified device nodes in the
10802 file system list, instead operating on them directly
10805 stat outputs the alias for a bind mounted file, rather than
10806 the initial mount point of its backing device.
10807 One can recursively call stat until there is no change in output,
10808 to get the current base mount point
10811 When listing file system information (@option{--file-system} (@option{-f})),
10812 you must use a different set of @var{format} directives:
10815 @item %a - Free blocks available to non-super-user
10816 @item %b - Total data blocks in file system
10817 @item %c - Total file nodes in file system
10818 @item %d - Free file nodes in file system
10819 @item %f - Free blocks in file system
10820 @item %i - File System ID in hex
10821 @item %l - Maximum length of file names
10822 @item %n - File name
10823 @item %s - Block size (for faster transfers)
10824 @item %S - Fundamental block size (for block counts)
10825 @item %t - Type in hex
10826 @item %T - Type in human readable form
10830 Time stamps are listed according to the time zone rules specified by
10831 the @env{TZ} environment variable, or by the system default rules if
10832 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10833 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10838 @node sync invocation
10839 @section @command{sync}: Synchronize data on disk with memory
10842 @cindex synchronize disk and memory
10844 @cindex superblock, writing
10845 @cindex inodes, written buffered
10846 @command{sync} writes any data buffered in memory out to disk. This can
10847 include (but is not limited to) modified superblocks, modified inodes,
10848 and delayed reads and writes. This must be implemented by the kernel;
10849 The @command{sync} program does nothing but exercise the @code{sync} system
10852 @cindex crashes and corruption
10853 The kernel keeps data in memory to avoid doing (relatively slow) disk
10854 reads and writes. This improves performance, but if the computer
10855 crashes, data may be lost or the file system corrupted as a
10856 result. The @command{sync} command ensures everything in memory
10857 is written to disk.
10859 Any arguments are ignored, except for a lone @option{--help} or
10860 @option{--version} (@pxref{Common options}).
10865 @node truncate invocation
10866 @section @command{truncate}: Shrink or extend the size of a file
10869 @cindex truncating, file sizes
10871 @command{truncate} shrinks or extends the size of each @var{file} to the
10872 specified size. Synopsis:
10875 truncate @var{option}@dots{} @var{file}@dots{}
10878 @cindex files, creating
10879 Any @var{file} that does not exist is created.
10881 @cindex sparse files, creating
10882 @cindex holes, creating files with
10883 If a @var{file} is larger than the specified size, the extra data is lost.
10884 If a @var{file} is shorter, it is extended and the extended part (or hole)
10885 reads as zero bytes.
10887 The program accepts the following options. Also see @ref{Common options}.
10894 @opindex --no-create
10895 Do not create files that do not exist.
10900 @opindex --io-blocks
10901 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10903 @item -r @var{rfile}
10904 @itemx --reference=@var{rfile}
10906 @opindex --reference
10907 Base the size of each @var{file} on the size of @var{rfile}.
10909 @item -s @var{size}
10910 @itemx --size=@var{size}
10913 Set or adjust the size of each @var{file} according to @var{size}.
10914 @multiplierSuffixesNoBlocks{size}
10916 @var{size} may also be prefixed by one of the following to adjust
10917 the size of each @var{file} based on their current size:
10919 @samp{+} => extend by
10920 @samp{-} => reduce by
10921 @samp{<} => at most
10922 @samp{>} => at least
10923 @samp{/} => round down to multiple of
10924 @samp{%} => round up to multiple of
10932 @node Printing text
10933 @chapter Printing text
10935 @cindex printing text, commands for
10936 @cindex commands for printing text
10938 This section describes commands that display text strings.
10941 * echo invocation:: Print a line of text.
10942 * printf invocation:: Format and print data.
10943 * yes invocation:: Print a string until interrupted.
10947 @node echo invocation
10948 @section @command{echo}: Print a line of text
10951 @cindex displaying text
10952 @cindex printing text
10953 @cindex text, displaying
10954 @cindex arbitrary text, displaying
10956 @command{echo} writes each given @var{string} to standard output, with a
10957 space between each and a newline after the last one. Synopsis:
10960 echo [@var{option}]@dots{} [@var{string}]@dots{}
10963 @mayConflictWithShellBuiltIn{echo}
10965 The program accepts the following options. Also see @ref{Common options}.
10966 Options must precede operands, and the normally-special argument
10967 @samp{--} has no special meaning and is treated like any other
10973 Do not output the trailing newline.
10977 @cindex backslash escapes
10978 Enable interpretation of the following backslash-escaped characters in
10987 produce no further output
11003 the eight-bit value that is the octal number @var{nnn}
11004 (zero to three octal digits)
11006 the eight-bit value that is the octal number @var{nnn}
11007 (one to three octal digits)
11009 the eight-bit value that is the hexadecimal number @var{hh}
11010 (one or two hexadecimal digits)
11015 @cindex backslash escapes
11016 Disable interpretation of backslash escapes in each @var{string}.
11017 This is the default. If @option{-e} and @option{-E} are both
11018 specified, the last one given takes effect.
11022 @vindex POSIXLY_CORRECT
11023 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11024 @command{echo}'s first argument is not @option{-n} it outputs
11025 option-like arguments instead of treating them as options. For
11026 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11027 plain @samp{hello}.
11029 @acronym{POSIX} does not require support for any options, and says
11030 that the behavior of @command{echo} is implementation-defined if any
11031 @var{string} contains a backslash or if the first argument is
11032 @option{-n}. Portable programs can use the @command{printf} command
11033 if they need to omit trailing newlines or output control characters or
11034 backslashes. @xref{printf invocation}.
11039 @node printf invocation
11040 @section @command{printf}: Format and print data
11043 @command{printf} does formatted printing of text. Synopsis:
11046 printf @var{format} [@var{argument}]@dots{}
11049 @command{printf} prints the @var{format} string, interpreting @samp{%}
11050 directives and @samp{\} escapes to format numeric and string arguments
11051 in a way that is mostly similar to the C @samp{printf} function.
11052 @xref{Output Conversion Syntax,, @command{printf} format directives,
11053 libc, The GNU C Library Reference Manual}, for details.
11054 The differences are listed below.
11056 @mayConflictWithShellBuiltIn{printf}
11061 The @var{format} argument is reused as necessary to convert all the
11062 given @var{argument}s. For example, the command @samp{printf %s a b}
11066 Missing @var{argument}s are treated as null strings or as zeros,
11067 depending on whether the context expects a string or a number. For
11068 example, the command @samp{printf %sx%d} prints @samp{x0}.
11072 An additional escape, @samp{\c}, causes @command{printf} to produce no
11073 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11074 E} prints @samp{ABC}.
11077 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11078 digits, as opposed to C where it can have an unlimited number of
11079 digits. For example, the command @samp{printf '\x07e'} prints two
11080 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11085 @command{printf} has an additional directive, @samp{%b}, which prints its
11086 argument string with @samp{\} escapes interpreted in the same way as in
11087 the @var{format} string, except that octal escapes are of the form
11088 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11089 If a precision is also given, it limits the number of bytes printed
11090 from the converted string.
11093 Numeric arguments must be single C constants, possibly with leading
11094 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11098 @vindex POSIXLY_CORRECT
11099 If the leading character of a numeric argument is @samp{"} or @samp{'}
11100 then its value is the numeric value of the immediately following
11101 character. Any remaining characters are silently ignored if the
11102 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11103 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11104 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11105 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11110 A floating-point argument must use a period before any fractional
11111 digits, but is printed according to the @env{LC_NUMERIC} category of the
11112 current locale. For example, in a locale whose radix character is a
11113 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11114 the command @samp{printf %g 3,14} is an error.
11118 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11119 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11120 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11121 digits) specifying a character to print.
11126 @cindex ISO/IEC 10646
11128 @command{printf} interprets two character syntaxes introduced in
11129 @acronym{ISO} C 99:
11130 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11131 characters, specified as
11132 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11133 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11134 @command{printf} outputs the Unicode characters
11135 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11136 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11137 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11139 The processing of @samp{\u} and @samp{\U} requires a full-featured
11140 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11141 or when @code{libiconv} is installed prior to this package. Otherwise
11142 @samp{\u} and @samp{\U} will print as-is.
11144 The only options are a lone @option{--help} or
11145 @option{--version}. @xref{Common options}.
11146 Options must precede operands.
11148 The Unicode character syntaxes are useful for writing strings in a locale
11149 independent way. For example, a string containing the Euro currency symbol
11152 $ env printf '\u20AC 14.95'
11156 will be output correctly in all locales supporting the Euro symbol
11157 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11160 $ env printf '\u4e2d\u6587'
11164 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11166 Note that in these examples, the @command{printf} command has been
11167 invoked via @command{env} to ensure that we run the program found via
11168 your shell's search path, and not a shell alias or a built-in function.
11170 For larger strings, you don't need to look up the hexadecimal code
11171 values of each character one by one. @acronym{ASCII} characters mixed with \u
11172 escape sequences is also known as the JAVA source file encoding. You can
11173 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11174 is how to convert a piece of text into a shell script which will output
11175 this text in a locale-independent way:
11178 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11179 '\u4e2d\u6587\n' > sample.txt
11180 $ recode BIG5..JAVA < sample.txt \
11181 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11188 @node yes invocation
11189 @section @command{yes}: Print a string until interrupted
11192 @cindex repeated output of a string
11194 @command{yes} prints the command line arguments, separated by spaces and
11195 followed by a newline, forever until it is killed. If no arguments are
11196 given, it prints @samp{y} followed by a newline forever until killed.
11198 Upon a write error, @command{yes} exits with status @samp{1}.
11200 The only options are a lone @option{--help} or @option{--version}.
11201 To output an argument that begins with
11202 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11203 @xref{Common options}.
11207 @chapter Conditions
11210 @cindex commands for exit status
11211 @cindex exit status commands
11213 This section describes commands that are primarily useful for their exit
11214 status, rather than their output. Thus, they are often used as the
11215 condition of shell @code{if} statements, or as the last command in a
11219 * false invocation:: Do nothing, unsuccessfully.
11220 * true invocation:: Do nothing, successfully.
11221 * test invocation:: Check file types and compare values.
11222 * expr invocation:: Evaluate expressions.
11226 @node false invocation
11227 @section @command{false}: Do nothing, unsuccessfully
11230 @cindex do nothing, unsuccessfully
11231 @cindex failure exit status
11232 @cindex exit status of @command{false}
11234 @command{false} does nothing except return an exit status of 1, meaning
11235 @dfn{failure}. It can be used as a place holder in shell scripts
11236 where an unsuccessful command is needed.
11237 In most modern shells, @command{false} is a built-in command, so when
11238 you use @samp{false} in a script, you're probably using the built-in
11239 command, not the one documented here.
11241 @command{false} honors the @option{--help} and @option{--version} options.
11243 This version of @command{false} is implemented as a C program, and is thus
11244 more secure and faster than a shell script implementation, and may safely
11245 be used as a dummy shell for the purpose of disabling accounts.
11247 Note that @command{false} (unlike all other programs documented herein)
11248 exits unsuccessfully, even when invoked with
11249 @option{--help} or @option{--version}.
11251 Portable programs should not assume that the exit status of
11252 @command{false} is 1, as it is greater than 1 on some
11253 non-@acronym{GNU} hosts.
11256 @node true invocation
11257 @section @command{true}: Do nothing, successfully
11260 @cindex do nothing, successfully
11262 @cindex successful exit
11263 @cindex exit status of @command{true}
11265 @command{true} does nothing except return an exit status of 0, meaning
11266 @dfn{success}. It can be used as a place holder in shell scripts
11267 where a successful command is needed, although the shell built-in
11268 command @code{:} (colon) may do the same thing faster.
11269 In most modern shells, @command{true} is a built-in command, so when
11270 you use @samp{true} in a script, you're probably using the built-in
11271 command, not the one documented here.
11273 @command{true} honors the @option{--help} and @option{--version} options.
11275 Note, however, that it is possible to cause @command{true}
11276 to exit with nonzero status: with the @option{--help} or @option{--version}
11277 option, and with standard
11278 output already closed or redirected to a file that evokes an I/O error.
11279 For example, using a Bourne-compatible shell:
11282 $ ./true --version >&-
11283 ./true: write error: Bad file number
11284 $ ./true --version > /dev/full
11285 ./true: write error: No space left on device
11288 This version of @command{true} is implemented as a C program, and is thus
11289 more secure and faster than a shell script implementation, and may safely
11290 be used as a dummy shell for the purpose of disabling accounts.
11292 @node test invocation
11293 @section @command{test}: Check file types and compare values
11296 @cindex check file types
11297 @cindex compare values
11298 @cindex expression evaluation
11300 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11301 evaluation of the conditional expression @var{expr}. Each part of the
11302 expression must be a separate argument.
11304 @command{test} has file status checks, string operators, and numeric
11305 comparison operators.
11307 @command{test} has an alternate form that uses opening and closing
11308 square brackets instead a leading @samp{test}. For example, instead
11309 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11310 brackets must be separate arguments; for example, @samp{[-d /]} does
11311 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11312 @var{expr} ]} have the same meaning, only the former form is discussed
11318 test @var{expression}
11320 [ @var{expression} ]
11325 @mayConflictWithShellBuiltIn{test}
11327 If @var{expression} is omitted, @command{test} returns false.
11328 If @var{expression} is a single argument,
11329 @command{test} returns false if the argument is null and true otherwise. The argument
11330 can be any string, including strings like @samp{-d}, @samp{-1},
11331 @samp{--}, @samp{--help}, and @samp{--version} that most other
11332 programs would treat as options. To get help and version information,
11333 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11334 the usual closing brackets. @xref{Common options}.
11336 @cindex exit status of @command{test}
11340 0 if the expression is true,
11341 1 if the expression is false,
11342 2 if an error occurred.
11346 * File type tests:: -[bcdfhLpSt]
11347 * Access permission tests:: -[gkruwxOG]
11348 * File characteristic tests:: -e -s -nt -ot -ef
11349 * String tests:: -z -n = !=
11350 * Numeric tests:: -eq -ne -lt -le -gt -ge
11351 * Connectives for test:: ! -a -o
11355 @node File type tests
11356 @subsection File type tests
11358 @cindex file type tests
11360 These options test for particular types of files. (Everything's a file,
11361 but not all files are the same!)
11365 @item -b @var{file}
11367 @cindex block special check
11368 True if @var{file} exists and is a block special device.
11370 @item -c @var{file}
11372 @cindex character special check
11373 True if @var{file} exists and is a character special device.
11375 @item -d @var{file}
11377 @cindex directory check
11378 True if @var{file} exists and is a directory.
11380 @item -f @var{file}
11382 @cindex regular file check
11383 True if @var{file} exists and is a regular file.
11385 @item -h @var{file}
11386 @itemx -L @var{file}
11389 @cindex symbolic link check
11390 True if @var{file} exists and is a symbolic link.
11391 Unlike all other file-related tests, this test does not dereference
11392 @var{file} if it is a symbolic link.
11394 @item -p @var{file}
11396 @cindex named pipe check
11397 True if @var{file} exists and is a named pipe.
11399 @item -S @var{file}
11401 @cindex socket check
11402 True if @var{file} exists and is a socket.
11406 @cindex terminal check
11407 True if @var{fd} is a file descriptor that is associated with a
11413 @node Access permission tests
11414 @subsection Access permission tests
11416 @cindex access permission tests
11417 @cindex permission tests
11419 These options test for particular access permissions.
11423 @item -g @var{file}
11425 @cindex set-group-ID check
11426 True if @var{file} exists and has its set-group-ID bit set.
11428 @item -k @var{file}
11430 @cindex sticky bit check
11431 True if @var{file} exists and has its @dfn{sticky} bit set.
11433 @item -r @var{file}
11435 @cindex readable file check
11436 True if @var{file} exists and read permission is granted.
11438 @item -u @var{file}
11440 @cindex set-user-ID check
11441 True if @var{file} exists and has its set-user-ID bit set.
11443 @item -w @var{file}
11445 @cindex writable file check
11446 True if @var{file} exists and write permission is granted.
11448 @item -x @var{file}
11450 @cindex executable file check
11451 True if @var{file} exists and execute permission is granted
11452 (or search permission, if it is a directory).
11454 @item -O @var{file}
11456 @cindex owned by effective user ID check
11457 True if @var{file} exists and is owned by the current effective user ID.
11459 @item -G @var{file}
11461 @cindex owned by effective group ID check
11462 True if @var{file} exists and is owned by the current effective group ID.
11466 @node File characteristic tests
11467 @subsection File characteristic tests
11469 @cindex file characteristic tests
11471 These options test other file characteristics.
11475 @item -e @var{file}
11477 @cindex existence-of-file check
11478 True if @var{file} exists.
11480 @item -s @var{file}
11482 @cindex nonempty file check
11483 True if @var{file} exists and has a size greater than zero.
11485 @item @var{file1} -nt @var{file2}
11487 @cindex newer-than file check
11488 True if @var{file1} is newer (according to modification date) than
11489 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11491 @item @var{file1} -ot @var{file2}
11493 @cindex older-than file check
11494 True if @var{file1} is older (according to modification date) than
11495 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11497 @item @var{file1} -ef @var{file2}
11499 @cindex same file check
11500 @cindex hard link check
11501 True if @var{file1} and @var{file2} have the same device and inode
11502 numbers, i.e., if they are hard links to each other.
11508 @subsection String tests
11510 @cindex string tests
11512 These options test string characteristics. You may need to quote
11513 @var{string} arguments for the shell. For example:
11519 The quotes here prevent the wrong arguments from being passed to
11520 @command{test} if @samp{$V} is empty or contains special characters.
11524 @item -z @var{string}
11526 @cindex zero-length string check
11527 True if the length of @var{string} is zero.
11529 @item -n @var{string}
11530 @itemx @var{string}
11532 @cindex nonzero-length string check
11533 True if the length of @var{string} is nonzero.
11535 @item @var{string1} = @var{string2}
11537 @cindex equal string check
11538 True if the strings are equal.
11540 @item @var{string1} != @var{string2}
11542 @cindex not-equal string check
11543 True if the strings are not equal.
11548 @node Numeric tests
11549 @subsection Numeric tests
11551 @cindex numeric tests
11552 @cindex arithmetic tests
11554 Numeric relational operators. The arguments must be entirely numeric
11555 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11556 which evaluates to the length of @var{string}.
11560 @item @var{arg1} -eq @var{arg2}
11561 @itemx @var{arg1} -ne @var{arg2}
11562 @itemx @var{arg1} -lt @var{arg2}
11563 @itemx @var{arg1} -le @var{arg2}
11564 @itemx @var{arg1} -gt @var{arg2}
11565 @itemx @var{arg1} -ge @var{arg2}
11572 These arithmetic binary operators return true if @var{arg1} is equal,
11573 not-equal, less-than, less-than-or-equal, greater-than, or
11574 greater-than-or-equal than @var{arg2}, respectively.
11581 test -1 -gt -2 && echo yes
11583 test -l abc -gt 1 && echo yes
11586 @error{} test: integer expression expected before -eq
11590 @node Connectives for test
11591 @subsection Connectives for @command{test}
11593 @cindex logical connectives
11594 @cindex connectives, logical
11596 The usual logical connectives.
11602 True if @var{expr} is false.
11604 @item @var{expr1} -a @var{expr2}
11606 @cindex logical and operator
11607 @cindex and operator
11608 True if both @var{expr1} and @var{expr2} are true.
11610 @item @var{expr1} -o @var{expr2}
11612 @cindex logical or operator
11613 @cindex or operator
11614 True if either @var{expr1} or @var{expr2} is true.
11619 @node expr invocation
11620 @section @command{expr}: Evaluate expressions
11623 @cindex expression evaluation
11624 @cindex evaluation of expressions
11626 @command{expr} evaluates an expression and writes the result on standard
11627 output. Each token of the expression must be a separate argument.
11629 Operands are either integers or strings. Integers consist of one or
11630 more decimal digits, with an optional leading @samp{-}.
11631 @command{expr} converts
11632 anything appearing in an operand position to an integer or a string
11633 depending on the operation being applied to it.
11635 Strings are not quoted for @command{expr} itself, though you may need to
11636 quote them to protect characters with special meaning to the shell,
11637 e.g., spaces. However, regardless of whether it is quoted, a string
11638 operand should not be a parenthesis or any of @command{expr}'s
11639 operators like @code{+}, so you cannot safely pass an arbitrary string
11640 @code{$str} to expr merely by quoting it to the shell. One way to
11641 work around this is to use the @sc{gnu} extension @code{+},
11642 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11643 @code{@w{" $str"}} and to adjust the rest of the expression to take
11644 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11646 You should not pass a negative integer or a string with leading
11647 @samp{-} as @command{expr}'s first argument, as it might be
11648 misinterpreted as an option; this can be avoided by parenthesization.
11649 Also, portable scripts should not use a string operand that happens to
11650 take the form of an integer; this can be worked around by inserting
11651 leading spaces as mentioned above.
11653 @cindex parentheses for grouping
11654 Operators may be given as infix symbols or prefix keywords. Parentheses
11655 may be used for grouping in the usual manner. You must quote
11656 parentheses and many operators to avoid the shell evaluating them,
11659 When built with support for the GNU MP library, @command{expr} uses
11660 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11661 types and may fail due to arithmetic overflow.
11663 The only options are @option{--help} and @option{--version}. @xref{Common
11664 options}. Options must precede operands.
11666 @cindex exit status of @command{expr}
11670 0 if the expression is neither null nor 0,
11671 1 if the expression is null or 0,
11672 2 if the expression is invalid,
11673 3 if an internal error occurred (e.g., arithmetic overflow).
11677 * String expressions:: + : match substr index length
11678 * Numeric expressions:: + - * / %
11679 * Relations for expr:: | & < <= = == != >= >
11680 * Examples of expr:: Examples.
11684 @node String expressions
11685 @subsection String expressions
11687 @cindex string expressions
11688 @cindex expressions, string
11690 @command{expr} supports pattern matching and other string operators. These
11691 have higher precedence than both the numeric and relational operators (in
11692 the next sections).
11696 @item @var{string} : @var{regex}
11697 @cindex pattern matching
11698 @cindex regular expression matching
11699 @cindex matching patterns
11700 Perform pattern matching. The arguments are converted to strings and the
11701 second is considered to be a (basic, a la GNU @code{grep}) regular
11702 expression, with a @code{^} implicitly prepended. The first argument is
11703 then matched against this regular expression.
11705 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11706 @code{:} expression returns the part of @var{string} that matched the
11707 subexpression; otherwise, it returns the number of characters matched.
11709 If the match fails, the @code{:} operator returns the null string if
11710 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11712 @kindex \( @r{regexp operator}
11713 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11714 value; additional pairs are meaningful only for grouping the regular
11715 expression operators.
11717 @kindex \+ @r{regexp operator}
11718 @kindex \? @r{regexp operator}
11719 @kindex \| @r{regexp operator}
11720 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11721 operators which respectively match one or more, zero or one, or separate
11722 alternatives. SunOS and other @command{expr}'s treat these as regular
11723 characters. (@acronym{POSIX} allows either behavior.)
11724 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11725 regular expression syntax. Some examples are in @ref{Examples of expr}.
11727 @item match @var{string} @var{regex}
11729 An alternative way to do pattern matching. This is the same as
11730 @w{@samp{@var{string} : @var{regex}}}.
11732 @item substr @var{string} @var{position} @var{length}
11734 Returns the substring of @var{string} beginning at @var{position}
11735 with length at most @var{length}. If either @var{position} or
11736 @var{length} is negative, zero, or non-numeric, returns the null string.
11738 @item index @var{string} @var{charset}
11740 Returns the first position in @var{string} where the first character in
11741 @var{charset} was found. If no character in @var{charset} is found in
11742 @var{string}, return 0.
11744 @item length @var{string}
11746 Returns the length of @var{string}.
11748 @item + @var{token}
11750 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11751 or an operator like @code{/}.
11752 This makes it possible to test @code{expr length + "$x"} or
11753 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11754 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11755 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11756 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11760 To make @command{expr} interpret keywords as strings, you must use the
11761 @code{quote} operator.
11764 @node Numeric expressions
11765 @subsection Numeric expressions
11767 @cindex numeric expressions
11768 @cindex expressions, numeric
11770 @command{expr} supports the usual numeric operators, in order of increasing
11771 precedence. These numeric operators have lower precedence than the
11772 string operators described in the previous section, and higher precedence
11773 than the connectives (next section).
11781 @cindex subtraction
11782 Addition and subtraction. Both arguments are converted to integers;
11783 an error occurs if this cannot be done.
11789 @cindex multiplication
11792 Multiplication, division, remainder. Both arguments are converted to
11793 integers; an error occurs if this cannot be done.
11798 @node Relations for expr
11799 @subsection Relations for @command{expr}
11801 @cindex connectives, logical
11802 @cindex logical connectives
11803 @cindex relations, numeric or string
11805 @command{expr} supports the usual logical connectives and relations. These
11806 have lower precedence than the string and numeric operators
11807 (previous sections). Here is the list, lowest-precedence operator first.
11813 @cindex logical or operator
11814 @cindex or operator
11815 Returns its first argument if that is neither null nor zero, otherwise
11816 its second argument if it is neither null nor zero, otherwise 0. It
11817 does not evaluate its second argument if its first argument is neither
11822 @cindex logical and operator
11823 @cindex and operator
11824 Return its first argument if neither argument is null or zero, otherwise
11825 0. It does not evaluate its second argument if its first argument is
11828 @item < <= = == != >= >
11835 @cindex comparison operators
11837 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11838 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11839 both arguments to integers and do a numeric comparison; if either
11840 conversion fails, it does a lexicographic comparison using the character
11841 collating sequence specified by the @env{LC_COLLATE} locale.
11846 @node Examples of expr
11847 @subsection Examples of using @command{expr}
11849 @cindex examples of @command{expr}
11850 Here are a few examples, including quoting for shell metacharacters.
11852 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11855 foo=`expr $foo + 1`
11858 To print the non-directory part of the file name stored in
11859 @code{$fname}, which need not contain a @code{/}:
11862 expr $fname : '.*/\(.*\)' '|' $fname
11865 An example showing that @code{\+} is an operator:
11873 expr abc : 'a\(.\)c'
11875 expr index abcdef cz
11878 @error{} expr: syntax error
11879 expr index + index a
11885 @chapter Redirection
11887 @cindex redirection
11888 @cindex commands for redirection
11890 Unix shells commonly provide several forms of @dfn{redirection}---ways
11891 to change the input source or output destination of a command. But one
11892 useful redirection is performed by a separate command, not by the shell;
11893 it's described here.
11896 * tee invocation:: Redirect output to multiple files or processes.
11900 @node tee invocation
11901 @section @command{tee}: Redirect output to multiple files or processes
11904 @cindex pipe fitting
11905 @cindex destinations, multiple output
11906 @cindex read from stdin and write to stdout and files
11908 The @command{tee} command copies standard input to standard output and also
11909 to any files given as arguments. This is useful when you want not only
11910 to send some data down a pipe, but also to save a copy. Synopsis:
11913 tee [@var{option}]@dots{} [@var{file}]@dots{}
11916 If a file being written to does not already exist, it is created. If a
11917 file being written to already exists, the data it previously contained
11918 is overwritten unless the @option{-a} option is used.
11920 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11921 input to standard output, but this is typically not that useful as the
11922 copies are interleaved.
11924 The program accepts the following options. Also see @ref{Common options}.
11931 Append standard input to the given files rather than overwriting
11935 @itemx --ignore-interrupts
11937 @opindex --ignore-interrupts
11938 Ignore interrupt signals.
11942 The @command{tee} command is useful when you happen to be transferring a large
11943 amount of data and also want to summarize that data without reading
11944 it a second time. For example, when you are downloading a DVD image,
11945 you often want to verify its signature or checksum right away.
11946 The inefficient way to do it is simply:
11949 wget http://example.com/some.iso && sha1sum some.iso
11952 One problem with the above is that it makes you wait for the
11953 download to complete before starting the time-consuming SHA1 computation.
11954 Perhaps even more importantly, the above requires reading
11955 the DVD image a second time (the first was from the network).
11957 The efficient way to do it is to interleave the download
11958 and SHA1 computation. Then, you'll get the checksum for
11959 free, because the entire process parallelizes so well:
11962 # slightly contrived, to demonstrate process substitution
11963 wget -O - http://example.com/dvd.iso \
11964 | tee >(sha1sum > dvd.sha1) > dvd.iso
11967 That makes @command{tee} write not just to the expected output file,
11968 but also to a pipe running @command{sha1sum} and saving the final
11969 checksum in a file named @file{dvd.sha1}.
11971 Note, however, that this example relies on a feature of modern shells
11972 called @dfn{process substitution}
11973 (the @samp{>(command)} syntax, above;
11974 @xref{Process Substitution,,Process Substitution, bashref,
11975 The Bash Reference Manual}.),
11976 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11977 but not with @command{/bin/sh}. So if you write code like this
11978 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11980 Since the above example writes to one file and one process,
11981 a more conventional and portable use of @command{tee} is even better:
11984 wget -O - http://example.com/dvd.iso \
11985 | tee dvd.iso | sha1sum > dvd.sha1
11988 You can extend this example to make @command{tee} write to two processes,
11989 computing MD5 and SHA1 checksums in parallel. In this case,
11990 process substitution is required:
11993 wget -O - http://example.com/dvd.iso \
11994 | tee >(sha1sum > dvd.sha1) \
11995 >(md5sum > dvd.md5) \
11999 This technique is also useful when you want to make a @emph{compressed}
12000 copy of the contents of a pipe.
12001 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12002 For a large hierarchy, @samp{du -ak} can run for a long time,
12003 and can easily produce terabytes of data, so you won't want to
12004 rerun the command unnecessarily. Nor will you want to save
12005 the uncompressed output.
12007 Doing it the inefficient way, you can't even start the GUI
12008 until after you've compressed all of the @command{du} output:
12011 du -ak | gzip -9 > /tmp/du.gz
12012 gzip -d /tmp/du.gz | xdiskusage -a
12015 With @command{tee} and process substitution, you start the GUI
12016 right away and eliminate the decompression completely:
12019 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12022 Finally, if you regularly create more than one type of
12023 compressed tarball at once, for example when @code{make dist} creates
12024 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12025 there may be a better way.
12026 Typical @command{automake}-generated @file{Makefile} rules create
12027 the two compressed tar archives with commands in sequence, like this
12028 (slightly simplified):
12031 tardir=your-pkg-M.N
12032 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12033 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12036 However, if the hierarchy you are archiving and compressing is larger
12037 than a couple megabytes, and especially if you are using a multi-processor
12038 system with plenty of memory, then you can do much better by reading the
12039 directory contents only once and running the compression programs in parallel:
12042 tardir=your-pkg-M.N
12043 tar chof - "$tardir" \
12044 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12045 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12051 @node File name manipulation
12052 @chapter File name manipulation
12054 @cindex file name manipulation
12055 @cindex manipulation of file names
12056 @cindex commands for file name manipulation
12058 This section describes commands that manipulate file names.
12061 * basename invocation:: Strip directory and suffix from a file name.
12062 * dirname invocation:: Strip last file name component.
12063 * pathchk invocation:: Check file name validity and portability.
12064 * mktemp invocation:: Create temporary file or directory.
12068 @node basename invocation
12069 @section @command{basename}: Strip directory and suffix from a file name
12072 @cindex strip directory and suffix from file names
12073 @cindex directory, stripping from file names
12074 @cindex suffix, stripping from file names
12075 @cindex file names, stripping directory and suffix
12076 @cindex leading directory components, stripping
12078 @command{basename} removes any leading directory components from
12079 @var{name}. Synopsis:
12082 basename @var{name} [@var{suffix}]
12085 If @var{suffix} is specified and is identical to the end of @var{name},
12086 it is removed from @var{name} as well. Note that since trailing slashes
12087 are removed prior to suffix matching, @var{suffix} will do nothing if it
12088 contains slashes. @command{basename} prints the result on standard
12091 @c This test is used both here and in the section on dirname.
12092 @macro basenameAndDirname
12093 Together, @command{basename} and @command{dirname} are designed such
12094 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12095 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12096 for everything except file names containing a trailing newline.
12098 @basenameAndDirname
12100 @acronym{POSIX} allows the implementation to define the results if
12101 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12102 @command{basename} returns the empty string. In the latter case, the
12103 result is @samp{//} on platforms where @var{//} is distinct from
12104 @var{/}, and @samp{/} on platforms where there is no difference.
12106 The only options are @option{--help} and @option{--version}. @xref{Common
12107 options}. Options must precede operands.
12115 basename /usr/bin/sort
12118 basename include/stdio.h .h
12122 @node dirname invocation
12123 @section @command{dirname}: Strip last file name component
12126 @cindex directory components, printing
12127 @cindex stripping non-directory suffix
12128 @cindex non-directory suffix, stripping
12130 @command{dirname} prints all but the final slash-delimited component of
12131 @var{name}. Slashes on either side of the final component are also
12132 removed. If the string contains no slash, @command{dirname} prints
12133 @samp{.} (meaning the current directory). Synopsis:
12139 @var{name} need not be a file name, but if it is, this operation
12140 effectively lists the directory that contains the final component,
12141 including the case when the final component is itself a directory.
12143 @basenameAndDirname
12145 @acronym{POSIX} allows the implementation to define the results if
12146 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12147 result is @samp{//} on platforms where @var{//} is distinct from
12148 @var{/}, and @samp{/} on platforms where there is no difference.
12150 The only options are @option{--help} and @option{--version}. @xref{Common
12158 # Output "/usr/bin".
12159 dirname /usr/bin/sort
12160 dirname /usr/bin//.//
12167 @node pathchk invocation
12168 @section @command{pathchk}: Check file name validity and portability
12171 @cindex file names, checking validity and portability
12172 @cindex valid file names, checking for
12173 @cindex portable file names, checking for
12175 @command{pathchk} checks validity and portability of file names. Synopsis:
12178 pathchk [@var{option}]@dots{} @var{name}@dots{}
12181 For each @var{name}, @command{pathchk} prints an error message if any of
12182 these conditions is true:
12186 One of the existing directories in @var{name} does not have search
12187 (execute) permission,
12189 The length of @var{name} is larger than the maximum supported by the
12192 The length of one component of @var{name} is longer than
12193 its file system's maximum.
12196 A nonexistent @var{name} is not an error, so long a file with that
12197 name could be created under the above conditions.
12199 The program accepts the following options. Also see @ref{Common options}.
12200 Options must precede operands.
12206 Instead of performing checks based on the underlying file system,
12207 print an error message if any of these conditions is true:
12211 A file name is empty.
12214 A file name contains a character outside the @acronym{POSIX} portable file
12215 name character set, namely, the ASCII letters and digits, @samp{.},
12216 @samp{_}, @samp{-}, and @samp{/}.
12219 The length of a file name or one of its components exceeds the
12220 @acronym{POSIX} minimum limits for portability.
12225 Print an error message if a file name is empty, or if it contains a component
12226 that begins with @samp{-}.
12228 @item --portability
12229 @opindex --portability
12230 Print an error message if a file name is not portable to all @acronym{POSIX}
12231 hosts. This option is equivalent to @samp{-p -P}.
12235 @cindex exit status of @command{pathchk}
12239 0 if all specified file names passed all checks,
12243 @node mktemp invocation
12244 @section @command{mktemp}: Create temporary file or directory
12247 @cindex file names, creating temporary
12248 @cindex directory, creating temporary
12249 @cindex temporary files and directories
12251 @command{mktemp} manages the creation of temporary files and
12252 directories. Synopsis:
12255 mktemp [@var{option}]@dots{} [@var{template}]
12258 Safely create a temporary file or directory based on @var{template},
12259 and print its name. If given, @var{template} must include at least
12260 three consecutive @samp{X}s in the last component. If omitted, the template
12261 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12262 implied. The final run of @samp{X}s in the @var{template} will be replaced
12263 by alpha-numeric characters; thus, on a case-sensitive file system,
12264 and with a @var{template} including a run of @var{n} instances of @samp{X},
12265 there are @samp{62**@var{n}} potential file names.
12267 Older scripts used to create temporary files by simply joining the
12268 name of the program with the process id (@samp{$$}) as a suffix.
12269 However, that naming scheme is easily predictable, and suffers from a
12270 race condition where the attacker can create an appropriately named
12271 symbolic link, such that when the script then opens a handle to what
12272 it thought was an unused file, it is instead modifying an existing
12273 file. Using the same scheme to create a directory is slightly safer,
12274 since the @command{mkdir} will fail if the target already exists, but
12275 it is still inferior because it allows for denial of service attacks.
12276 Therefore, modern scripts should use the @command{mktemp} command to
12277 guarantee that the generated name will be unpredictable, and that
12278 knowledge of the temporary file name implies that the file was created
12279 by the current script and cannot be modified by other users.
12281 When creating a file, the resulting file has read and write
12282 permissions for the current user, but no permissions for the group or
12283 others; these permissions are reduced if the current umask is more
12286 Here are some examples (although note that if you repeat them, you
12287 will most likely get different file names):
12292 Create a temporary file in the current directory.
12299 Create a temporary file with a known suffix.
12301 $ mktemp --suffix=.txt file-XXXX
12303 $ mktemp file-XXXX-XXXX.txt
12308 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12309 but falling back to the current directory rather than @file{/tmp}.
12310 Note that @command{mktemp} does not create fifos, but can create a
12311 secure directory in which the fifo can live. Exit the shell if the
12312 directory or fifo could not be created.
12314 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12316 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12320 Create and use a temporary file if possible, but ignore failure. The
12321 file will reside in the directory named by @env{TMPDIR}, if specified,
12322 or else in @file{/tmp}.
12324 $ file=$(mktemp -q) && @{
12325 > # Safe to use $file only within this block. Use quotes,
12326 > # since $TMPDIR, and thus $file, may contain whitespace.
12327 > echo ... > "$file"
12333 Act as a semi-random character generator (it is not fully random,
12334 since it is impacted by the contents of the current directory). To
12335 avoid security holes, do not use the resulting names to create a file.
12345 The program accepts the following options. Also see @ref{Common options}.
12352 @opindex --directory
12353 Create a directory rather than a file. The directory will have read,
12354 write, and search permissions for the current user, but no permissions
12355 for the group or others; these permissions are reduced if the current
12356 umask is more restrictive.
12362 Suppress diagnostics about failure to create a file or directory. The
12363 exit status will still reflect whether a file was created.
12369 Generate a temporary name that does not name an existing file, without
12370 changing the file system contents. Using the output of this command
12371 to create a new file is inherently unsafe, as there is a window of
12372 time between generating the name and using it where another process
12373 can create an object by the same name.
12376 @itemx --tmpdir[=@var{dir}]
12379 Treat @var{template} relative to the directory @var{dir}. If
12380 @var{dir} is not specified (only possible with the long option
12381 @option{--tmpdir}) or is the empty string, use the value of
12382 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12383 specified, @var{template} must not be absolute. However,
12384 @var{template} can still contain slashes, although intermediate
12385 directories must already exist.
12387 @item --suffix=@var{suffix}
12389 Append @var{suffix} to the @var{template}. @var{suffix} must not
12390 contain slash. If @option{--suffix} is specified, @var{template} must
12391 end in @samp{X}; if it is not specified, then an appropriate
12392 @option{--suffix} is inferred by finding the last @samp{X} in
12393 @var{template}. This option exists for use with the default
12394 @var{template} and for the creation of a @var{suffix} that starts with
12399 Treat @var{template} as a single file relative to the value of
12400 @env{TMPDIR} if available, or to the directory specified by
12401 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12402 contain slashes. This option is deprecated; the use of @option{-p}
12403 without @option{-t} offers better defaults (by favoring the command
12404 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12409 @cindex exit status of @command{mktemp}
12413 0 if the file was created,
12418 @node Working context
12419 @chapter Working context
12421 @cindex working context
12422 @cindex commands for printing the working context
12424 This section describes commands that display or alter the context in
12425 which you are working: the current directory, the terminal settings, and
12426 so forth. See also the user-related commands in the next section.
12429 * pwd invocation:: Print working directory.
12430 * stty invocation:: Print or change terminal characteristics.
12431 * printenv invocation:: Print environment variables.
12432 * tty invocation:: Print file name of terminal on standard input.
12436 @node pwd invocation
12437 @section @command{pwd}: Print working directory
12440 @cindex print name of current directory
12441 @cindex current working directory, printing
12442 @cindex working directory, printing
12445 @command{pwd} prints the name of the current directory. Synopsis:
12448 pwd [@var{option}]@dots{}
12451 The program accepts the following options. Also see @ref{Common options}.
12458 If the contents of the environment variable @env{PWD} provide an
12459 absolute name of the current directory with no @samp{.} or @samp{..}
12460 components, but possibly with symbolic links, then output those
12461 contents. Otherwise, fall back to default @option{-P} handling.
12466 @opindex --physical
12467 Print a fully resolved name for the current directory. That is, all
12468 components of the printed name will be actual directory names---none
12469 will be symbolic links.
12472 @cindex symbolic links and @command{pwd}
12473 If @option{-L} and @option{-P} are both given, the last one takes
12474 precedence. If neither option is given, then this implementation uses
12475 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12476 environment variable is set.
12478 @mayConflictWithShellBuiltIn{pwd}
12483 @node stty invocation
12484 @section @command{stty}: Print or change terminal characteristics
12487 @cindex change or print terminal settings
12488 @cindex terminal settings
12489 @cindex line settings of terminal
12491 @command{stty} prints or changes terminal characteristics, such as baud rate.
12495 stty [@var{option}] [@var{setting}]@dots{}
12496 stty [@var{option}]
12499 If given no line settings, @command{stty} prints the baud rate, line
12500 discipline number (on systems that support it), and line settings
12501 that have been changed from the values set by @samp{stty sane}.
12502 By default, mode reading and setting are performed on the tty line
12503 connected to standard input, although this can be modified by the
12504 @option{--file} option.
12506 @command{stty} accepts many non-option arguments that change aspects of
12507 the terminal line operation, as described below.
12509 The program accepts the following options. Also see @ref{Common options}.
12516 Print all current settings in human-readable form. This option may not
12517 be used in combination with any line settings.
12519 @item -F @var{device}
12520 @itemx --file=@var{device}
12523 Set the line opened by the file name specified in @var{device} instead of
12524 the tty line connected to standard input. This option is necessary
12525 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12526 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12527 the @code{clocal} flag is not set. Hence, it is not always possible
12528 to allow the shell to open the device in the traditional manner.
12534 @cindex machine-readable @command{stty} output
12535 Print all current settings in a form that can be used as an argument to
12536 another @command{stty} command to restore the current settings. This option
12537 may not be used in combination with any line settings.
12541 Many settings can be turned off by preceding them with a @samp{-}.
12542 Such arguments are marked below with ``May be negated'' in their
12543 description. The descriptions themselves refer to the positive
12544 case, that is, when @emph{not} negated (unless stated otherwise,
12547 Some settings are not available on all @acronym{POSIX} systems, since they use
12548 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12549 description. On non-@acronym{POSIX} systems, those or other settings also may not
12550 be available, but it's not feasible to document all the variations: just
12556 * Control:: Control settings
12557 * Input:: Input settings
12558 * Output:: Output settings
12559 * Local:: Local settings
12560 * Combination:: Combination settings
12561 * Characters:: Special characters
12562 * Special:: Special settings
12567 @subsection Control settings
12569 @cindex control settings
12575 @cindex two-way parity
12576 Generate parity bit in output and expect parity bit in input.
12582 @cindex even parity
12583 Set odd parity (even if negated). May be negated.
12590 @cindex character size
12591 @cindex eight-bit characters
12592 Set character size to 5, 6, 7, or 8 bits.
12597 Send a hangup signal when the last process closes the tty. May be
12603 Use two stop bits per character (one if negated). May be negated.
12607 Allow input to be received. May be negated.
12611 @cindex modem control
12612 Disable modem control signals. May be negated.
12616 @cindex hardware flow control
12617 @cindex flow control, hardware
12618 @cindex RTS/CTS flow control
12619 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12624 @subsection Input settings
12626 @cindex input settings
12627 These settings control operations on data received from the terminal.
12632 @cindex breaks, ignoring
12633 Ignore break characters. May be negated.
12637 @cindex breaks, cause interrupts
12638 Make breaks cause an interrupt signal. May be negated.
12642 @cindex parity, ignoring
12643 Ignore characters with parity errors. May be negated.
12647 @cindex parity errors, marking
12648 Mark parity errors (with a 255-0-character sequence). May be negated.
12652 Enable input parity checking. May be negated.
12656 @cindex eight-bit input
12657 Clear high (8th) bit of input characters. May be negated.
12661 @cindex newline, translating to return
12662 Translate newline to carriage return. May be negated.
12666 @cindex return, ignoring
12667 Ignore carriage return. May be negated.
12671 @cindex return, translating to newline
12672 Translate carriage return to newline. May be negated.
12676 @cindex input encoding, UTF-8
12677 Assume input characters are UTF-8 encoded. May be negated.
12681 @kindex C-s/C-q flow control
12682 @cindex XON/XOFF flow control
12683 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12690 @cindex software flow control
12691 @cindex flow control, software
12692 Enable sending of @code{stop} character when the system input buffer
12693 is almost full, and @code{start} character when it becomes almost
12694 empty again. May be negated.
12698 @cindex uppercase, translating to lowercase
12699 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12700 negated. Note ilcuc is not implemented, as one would not be able to issue
12701 almost any (lowercase) Unix command, after invoking it.
12705 Allow any character to restart output (only the start character
12706 if negated). Non-@acronym{POSIX}. May be negated.
12710 @cindex beeping at input buffer full
12711 Enable beeping and not flushing input buffer if a character arrives
12712 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12717 @subsection Output settings
12719 @cindex output settings
12720 These settings control operations on data sent to the terminal.
12725 Postprocess output. May be negated.
12729 @cindex lowercase, translating to output
12730 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12731 negated. (Note ouclc is not currently implemented.)
12735 @cindex return, translating to newline
12736 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12740 @cindex newline, translating to crlf
12741 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12746 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12751 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12755 @cindex pad instead of timing for delaying
12756 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12761 @cindex pad character
12762 Use @acronym{ASCII} @sc{del} characters for fill instead of
12763 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12769 Newline delay style. Non-@acronym{POSIX}.
12776 Carriage return delay style. Non-@acronym{POSIX}.
12782 @opindex tab@var{n}
12783 Horizontal tab delay style. Non-@acronym{POSIX}.
12788 Backspace delay style. Non-@acronym{POSIX}.
12793 Vertical tab delay style. Non-@acronym{POSIX}.
12798 Form feed delay style. Non-@acronym{POSIX}.
12803 @subsection Local settings
12805 @cindex local settings
12810 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12811 characters. May be negated.
12815 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12816 special characters. May be negated.
12820 Enable non-@acronym{POSIX} special characters. May be negated.
12824 Echo input characters. May be negated.
12830 Echo @code{erase} characters as backspace-space-backspace. May be
12835 @cindex newline echoing after @code{kill}
12836 Echo a newline after a @code{kill} character. May be negated.
12840 @cindex newline, echoing
12841 Echo newline even if not echoing other characters. May be negated.
12845 @cindex flushing, disabling
12846 Disable flushing after @code{interrupt} and @code{quit} special
12847 characters. May be negated.
12851 @cindex case translation
12852 Enable input and output of uppercase characters by preceding their
12853 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12854 Non-@acronym{POSIX}. May be negated.
12858 @cindex background jobs, stopping at terminal write
12859 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12866 Echo erased characters backward, between @samp{\} and @samp{/}.
12867 Non-@acronym{POSIX}. May be negated.
12873 @cindex control characters, using @samp{^@var{c}}
12874 @cindex hat notation for control characters
12875 Echo control characters in hat notation (@samp{^@var{c}}) instead
12876 of literally. Non-@acronym{POSIX}. May be negated.
12882 Echo the @code{kill} special character by erasing each character on
12883 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12884 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12890 @subsection Combination settings
12892 @cindex combination settings
12893 Combination settings:
12900 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12901 as @code{-parenb cs8}.
12905 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12906 as @code{-parenb cs8}.
12910 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12911 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12915 Reset the @code{erase} and @code{kill} special characters to their default
12922 @c This is too long to write inline.
12924 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12925 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12926 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12927 ff0 isig icanon iexten echo echoe echok -echonl
12928 -noflsh -xcase -tostop -echoprt echoctl echoke
12932 and also sets all special characters to their default values.
12936 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12937 sets the @code{eof} and @code{eol} characters to their default values
12938 if they are the same as the @code{min} and @code{time} characters.
12939 May be negated. If negated, same as @code{raw}.
12946 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12947 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12948 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12952 May be negated. If negated, same as @code{cooked}.
12956 Same as @option{-icanon}. May be negated. If negated, same as
12961 @cindex eight-bit characters
12962 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12963 same as @code{parenb istrip cs7}.
12967 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12968 If negated, same as @code{parenb istrip opost cs7}.
12972 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12976 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12983 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12984 (Used for terminals with uppercase characters only.)
12988 Same as @code{echoe echoctl echoke}.
12992 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12997 @subsection Special characters
12999 @cindex special characters
13000 @cindex characters, special
13002 The special characters' default values vary from system to system.
13003 They are set with the syntax @samp{name value}, where the names are
13004 listed below and the value can be given either literally, in hat
13005 notation (@samp{^@var{c}}), or as an integer which may start with
13006 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13007 any other digit to indicate decimal.
13009 @cindex disabling special characters
13010 @kindex u@r{, and disabling special characters}
13011 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13012 special character. (This is incompatible with Ultrix @command{stty},
13013 which uses a value of @samp{u} to disable a special character. GNU
13014 @command{stty} treats a value @samp{u} like any other, namely to set that
13015 special character to @key{U}.)
13021 Send an interrupt signal.
13025 Send a quit signal.
13029 Erase the last character typed.
13033 Erase the current line.
13037 Send an end of file (terminate the input).
13045 Alternate character to end the line. Non-@acronym{POSIX}.
13049 Switch to a different shell layer. Non-@acronym{POSIX}.
13053 Restart the output after stopping it.
13061 Send a terminal stop signal.
13065 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13069 Redraw the current line. Non-@acronym{POSIX}.
13073 Erase the last word typed. Non-@acronym{POSIX}.
13077 Enter the next character typed literally, even if it is a special
13078 character. Non-@acronym{POSIX}.
13083 @subsection Special settings
13085 @cindex special settings
13090 Set the minimum number of characters that will satisfy a read until
13091 the time value has expired, when @option{-icanon} is set.
13095 Set the number of tenths of a second before reads time out if the minimum
13096 number of characters have not been read, when @option{-icanon} is set.
13098 @item ispeed @var{n}
13100 Set the input speed to @var{n}.
13102 @item ospeed @var{n}
13104 Set the output speed to @var{n}.
13108 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13111 @itemx columns @var{n}
13114 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13120 Print the number of rows and columns that the kernel thinks the
13121 terminal has. (Systems that don't support rows and columns in the kernel
13122 typically use the environment variables @env{LINES} and @env{COLUMNS}
13123 instead; however, GNU @command{stty} does not know anything about them.)
13124 Non-@acronym{POSIX}.
13128 Use line discipline @var{n}. Non-@acronym{POSIX}.
13132 Print the terminal speed.
13135 @cindex baud rate, setting
13136 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13137 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13138 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13139 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13140 support higher speeds. The @command{stty} command includes support
13157 4000000 where the system supports these.
13158 0 hangs up the line if @option{-clocal} is set.
13162 @node printenv invocation
13163 @section @command{printenv}: Print all or some environment variables
13166 @cindex printing all or some environment variables
13167 @cindex environment variables, printing
13169 @command{printenv} prints environment variable values. Synopsis:
13172 printenv [@var{option}] [@var{variable}]@dots{}
13175 If no @var{variable}s are specified, @command{printenv} prints the value of
13176 every environment variable. Otherwise, it prints the value of each
13177 @var{variable} that is set, and nothing for those that are not set.
13179 The program accepts the following option. Also see @ref{Common options}.
13187 @cindex exit status of @command{printenv}
13191 0 if all variables specified were found
13192 1 if at least one specified variable was not found
13193 2 if a write error occurred
13197 @node tty invocation
13198 @section @command{tty}: Print file name of terminal on standard input
13201 @cindex print terminal file name
13202 @cindex terminal file name, printing
13204 @command{tty} prints the file name of the terminal connected to its standard
13205 input. It prints @samp{not a tty} if standard input is not a terminal.
13209 tty [@var{option}]@dots{}
13212 The program accepts the following option. Also see @ref{Common options}.
13222 Print nothing; only return an exit status.
13226 @cindex exit status of @command{tty}
13230 0 if standard input is a terminal
13231 1 if standard input is not a terminal
13232 2 if given incorrect arguments
13233 3 if a write error occurs
13237 @node User information
13238 @chapter User information
13240 @cindex user information, commands for
13241 @cindex commands for printing user information
13243 This section describes commands that print user-related information:
13244 logins, groups, and so forth.
13247 * id invocation:: Print user identity.
13248 * logname invocation:: Print current login name.
13249 * whoami invocation:: Print effective user ID.
13250 * groups invocation:: Print group names a user is in.
13251 * users invocation:: Print login names of users currently logged in.
13252 * who invocation:: Print who is currently logged in.
13256 @node id invocation
13257 @section @command{id}: Print user identity
13260 @cindex real user and group IDs, printing
13261 @cindex effective user and group IDs, printing
13262 @cindex printing real and effective user and group IDs
13264 @command{id} prints information about the given user, or the process
13265 running it if no user is specified. Synopsis:
13268 id [@var{option}]@dots{} [@var{username}]
13271 @vindex POSIXLY_CORRECT
13272 By default, it prints the real user ID, real group ID, effective user ID
13273 if different from the real user ID, effective group ID if different from
13274 the real group ID, and supplemental group IDs.
13275 In addition, if SELinux
13276 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13277 then print @samp{context=@var{c}}, where @var{c} is the security context.
13279 Each of these numeric values is preceded by an identifying string and
13280 followed by the corresponding user or group name in parentheses.
13282 The options cause @command{id} to print only part of the above information.
13283 Also see @ref{Common options}.
13290 Print only the group ID.
13296 Print only the group ID and the supplementary groups.
13302 Print the user or group name instead of the ID number. Requires
13303 @option{-u}, @option{-g}, or @option{-G}.
13309 Print the real, instead of effective, user or group ID. Requires
13310 @option{-u}, @option{-g}, or @option{-G}.
13316 Print only the user ID.
13323 @cindex security context
13324 Print only the security context of the current user.
13325 If SELinux is disabled then print a warning and
13326 set the exit status to 1.
13332 @macro primaryAndSupplementaryGroups{cmd,arg}
13333 Primary and supplementary groups for a process are normally inherited
13334 from its parent and are usually unchanged since login. This means
13335 that if you change the group database after logging in, @command{\cmd\}
13336 will not reflect your changes within your existing login session.
13337 Running @command{\cmd\} with a \arg\ causes the user and group
13338 database to be consulted afresh, and so will give a different result.
13340 @primaryAndSupplementaryGroups{id,user argument}
13342 @node logname invocation
13343 @section @command{logname}: Print current login name
13346 @cindex printing user's login name
13347 @cindex login name, printing
13348 @cindex user name, printing
13351 @command{logname} prints the calling user's name, as found in a
13352 system-maintained file (often @file{/var/run/utmp} or
13353 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13354 for the calling process, @command{logname} prints
13355 an error message and exits with a status of 1.
13357 The only options are @option{--help} and @option{--version}. @xref{Common
13363 @node whoami invocation
13364 @section @command{whoami}: Print effective user ID
13367 @cindex effective user ID, printing
13368 @cindex printing the effective user ID
13370 @command{whoami} prints the user name associated with the current
13371 effective user ID. It is equivalent to the command @samp{id -un}.
13373 The only options are @option{--help} and @option{--version}. @xref{Common
13379 @node groups invocation
13380 @section @command{groups}: Print group names a user is in
13383 @cindex printing groups a user is in
13384 @cindex supplementary groups, printing
13386 @command{groups} prints the names of the primary and any supplementary
13387 groups for each given @var{username}, or the current process if no names
13388 are given. If more than one name is given, the name of each user is
13390 the list of that user's groups and the user name is separated from the
13391 group list by a colon. Synopsis:
13394 groups [@var{username}]@dots{}
13397 The group lists are equivalent to the output of the command @samp{id -Gn}.
13399 @primaryAndSupplementaryGroups{groups,list of users}
13401 The only options are @option{--help} and @option{--version}. @xref{Common
13407 @node users invocation
13408 @section @command{users}: Print login names of users currently logged in
13411 @cindex printing current usernames
13412 @cindex usernames, printing current
13414 @cindex login sessions, printing users with
13415 @command{users} prints on a single line a blank-separated list of user
13416 names of users currently logged in to the current host. Each user name
13417 corresponds to a login session, so if a user has more than one login
13418 session, that user's name will appear the same number of times in the
13427 With no @var{file} argument, @command{users} extracts its information from
13428 a system-maintained file (often @file{/var/run/utmp} or
13429 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13430 that file instead. A common choice is @file{/var/log/wtmp}.
13432 The only options are @option{--help} and @option{--version}. @xref{Common
13438 @node who invocation
13439 @section @command{who}: Print who is currently logged in
13442 @cindex printing current user information
13443 @cindex information, about current users
13445 @command{who} prints information about users who are currently logged on.
13449 @command{who} [@var{option}] [@var{file}] [am i]
13452 @cindex terminal lines, currently used
13454 @cindex remote hostname
13455 If given no non-option arguments, @command{who} prints the following
13456 information for each user currently logged on: login name, terminal
13457 line, login time, and remote hostname or X display.
13461 If given one non-option argument, @command{who} uses that instead of
13462 a default system-maintained file (often @file{/var/run/utmp} or
13463 @file{/etc/utmp}) as the name of the file containing the record of
13464 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13465 to @command{who} to look at who has previously logged on.
13469 If given two non-option arguments, @command{who} prints only the entry
13470 for the user running it (determined from its standard input), preceded
13471 by the hostname. Traditionally, the two arguments given are @samp{am
13472 i}, as in @samp{who am i}.
13475 Time stamps are listed according to the time zone rules specified by
13476 the @env{TZ} environment variable, or by the system default rules if
13477 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13478 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13480 The program accepts the following options. Also see @ref{Common options}.
13488 Same as @samp{-b -d --login -p -r -t -T -u}.
13494 Print the date and time of last system boot.
13500 Print information corresponding to dead processes.
13506 Print a line of column headings.
13512 List only the entries that correspond to processes via which the
13513 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13517 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13518 is not the default because it can cause significant delays on systems with
13519 automatic dial-up internet access.
13523 Same as @samp{who am i}.
13529 List active processes spawned by init.
13535 Print only the login names and the number of users logged on.
13536 Overrides all other options.
13541 @opindex --runlevel
13542 Print the current (and maybe previous) run-level of the init process.
13546 Ignored; for compatibility with other versions of @command{who}.
13552 Print last system clock change.
13557 After the login time, print the number of hours and minutes that the
13558 user has been idle. @samp{.} means the user was active in the last minute.
13559 @samp{old} means the user has been idle for more than 24 hours.
13570 @opindex --writable
13571 @cindex message status
13572 @pindex write@r{, allowed}
13573 After each login name print a character indicating the user's message status:
13576 @samp{+} allowing @code{write} messages
13577 @samp{-} disallowing @code{write} messages
13578 @samp{?} cannot find terminal device
13586 @node System context
13587 @chapter System context
13589 @cindex system context
13590 @cindex context, system
13591 @cindex commands for system context
13593 This section describes commands that print or change system-wide
13597 * date invocation:: Print or set system date and time.
13598 * arch invocation:: Print machine hardware name.
13599 * nproc invocation:: Print the number of processors.
13600 * uname invocation:: Print system information.
13601 * hostname invocation:: Print or set system name.
13602 * hostid invocation:: Print numeric host identifier.
13603 * uptime invocation:: Print system uptime and load.
13606 @node date invocation
13607 @section @command{date}: Print or set system date and time
13610 @cindex time, printing or setting
13611 @cindex printing the current time
13616 date [@var{option}]@dots{} [+@var{format}]
13617 date [-u|--utc|--universal] @c this avoids a newline in the output
13618 [ MMDDhhmm[[CC]YY][.ss] ]
13622 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13623 it with a default format that depends on the @env{LC_TIME} locale category.
13624 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13625 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13628 Normally, @command{date} uses the time zone rules indicated by the
13629 @env{TZ} environment variable, or the system default rules if @env{TZ}
13630 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13631 @env{TZ}, libc, The GNU C Library Reference Manual}.
13633 @findex strftime @r{and @command{date}}
13634 @cindex time formats
13635 @cindex formatting times
13636 If given an argument that starts with a @samp{+}, @command{date} prints the
13637 current date and time (or the date and time specified by the
13638 @option{--date} option, see below) in the format defined by that argument,
13639 which is similar to that of the @code{strftime} function. Except for
13640 conversion specifiers, which start with @samp{%}, characters in the
13641 format string are printed unchanged. The conversion specifiers are
13647 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13648 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13649 * Literal conversion specifiers:: %[%nt]
13650 * Padding and other flags:: Pad with zeros, spaces, etc.
13651 * Setting the time:: Changing the system clock.
13652 * Options for date:: Instead of the current time.
13654 * Date input formats:: Specifying date strings.
13656 * Examples of date:: Examples.
13659 @node Time conversion specifiers
13660 @subsection Time conversion specifiers
13662 @cindex time conversion specifiers
13663 @cindex conversion specifiers, time
13665 @command{date} conversion specifiers related to times.
13669 hour (@samp{00}@dots{}@samp{23})
13671 hour (@samp{01}@dots{}@samp{12})
13673 hour (@samp{ 0}@dots{}@samp{23}).
13674 This is a @acronym{GNU} extension.
13676 hour (@samp{ 1}@dots{}@samp{12}).
13677 This is a @acronym{GNU} extension.
13679 minute (@samp{00}@dots{}@samp{59})
13681 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13682 This is a @acronym{GNU} extension.
13684 locale's equivalent of either @samp{AM} or @samp{PM};
13685 blank in many locales.
13686 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13688 like @samp{%p}, except lower case.
13689 This is a @acronym{GNU} extension.
13691 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13693 24-hour hour and minute. Same as @samp{%H:%M}.
13694 This is a @acronym{GNU} extension.
13696 @cindex epoch, seconds since
13697 @cindex seconds since the epoch
13698 @cindex beginning of time
13699 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13700 Leap seconds are not counted unless leap second support is available.
13701 @xref{%s-examples}, for examples.
13702 This is a @acronym{GNU} extension.
13704 second (@samp{00}@dots{}@samp{60}).
13705 This may be @samp{60} if leap seconds are supported.
13707 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13709 locale's time representation (e.g., @samp{23:13:48})
13711 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13712 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13713 time zone is determinable. This value reflects the numeric time zone
13714 appropriate for the current time, using the time zone rules specified
13715 by the @env{TZ} environment variable.
13716 The time (and optionally, the time zone rules) can be overridden
13717 by the @option{--date} option.
13718 This is a @acronym{GNU} extension.
13720 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13721 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13722 zone is determinable.
13723 This is a @acronym{GNU} extension.
13725 Numeric time zone to the nearest second with @samp{:} (e.g.,
13726 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13728 This is a @acronym{GNU} extension.
13730 Numeric time zone with @samp{:} using the minimum necessary precision
13731 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13732 no time zone is determinable.
13733 This is a @acronym{GNU} extension.
13735 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13736 time zone is determinable. See @samp{%z} for how it is determined.
13740 @node Date conversion specifiers
13741 @subsection Date conversion specifiers
13743 @cindex date conversion specifiers
13744 @cindex conversion specifiers, date
13746 @command{date} conversion specifiers related to dates.
13750 locale's abbreviated weekday name (e.g., @samp{Sun})
13752 locale's full weekday name, variable length (e.g., @samp{Sunday})
13754 locale's abbreviated month name (e.g., @samp{Jan})
13756 locale's full month name, variable length (e.g., @samp{January})
13758 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13760 century. This is like @samp{%Y}, except the last two digits are omitted.
13761 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13762 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13763 It is normally at least two characters, but it may be more.
13765 day of month (e.g., @samp{01})
13767 date; same as @samp{%m/%d/%y}
13769 day of month, space padded; same as @samp{%_d}
13771 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13772 This is a good choice for a date format, as it is standard and
13773 is easy to sort in the usual case where years are in the range
13775 This is a @acronym{GNU} extension.
13777 year corresponding to the @acronym{ISO} week number, but without the century
13778 (range @samp{00} through @samp{99}). This has the same format and value
13779 as @samp{%y}, except that if the @acronym{ISO} week number (see
13781 to the previous or next year, that year is used instead.
13782 This is a @acronym{GNU} extension.
13784 year corresponding to the @acronym{ISO} week number. This has the
13785 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13787 @samp{%V}) belongs to the previous or next year, that year is used
13789 It is normally useful only if @samp{%V} is also used;
13790 for example, the format @samp{%G-%m-%d} is probably a mistake,
13791 since it combines the ISO week number year with the conventional month and day.
13792 This is a @acronym{GNU} extension.
13796 day of year (@samp{001}@dots{}@samp{366})
13798 month (@samp{01}@dots{}@samp{12})
13800 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13802 week number of year, with Sunday as the first day of the week
13803 (@samp{00}@dots{}@samp{53}).
13804 Days in a new year preceding the first Sunday are in week zero.
13806 @acronym{ISO} week number, that is, the
13807 week number of year, with Monday as the first day of the week
13808 (@samp{01}@dots{}@samp{53}).
13809 If the week containing January 1 has four or more days in
13810 the new year, then it is considered week 1; otherwise, it is week 53 of
13811 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13814 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13816 week number of year, with Monday as first day of week
13817 (@samp{00}@dots{}@samp{53}).
13818 Days in a new year preceding the first Monday are in week zero.
13820 locale's date representation (e.g., @samp{12/31/99})
13822 last two digits of year (@samp{00}@dots{}@samp{99})
13824 year. This is normally at least four characters, but it may be more.
13825 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13826 precedes year @samp{0000}.
13830 @node Literal conversion specifiers
13831 @subsection Literal conversion specifiers
13833 @cindex literal conversion specifiers
13834 @cindex conversion specifiers, literal
13836 @command{date} conversion specifiers that produce literal strings.
13848 @node Padding and other flags
13849 @subsection Padding and other flags
13851 @cindex numeric field padding
13852 @cindex padding of numeric fields
13853 @cindex fields, padding numeric
13855 Unless otherwise specified, @command{date} normally pads numeric fields
13856 with zeros, so that, for
13857 example, numeric months are always output as two digits.
13858 Seconds since the epoch are not padded, though,
13859 since there is no natural width for them.
13861 As a @acronym{GNU} extension, @command{date} recognizes any of the
13862 following optional flags after the @samp{%}:
13866 (hyphen) Do not pad the field; useful if the output is intended for
13869 (underscore) Pad with spaces; useful if you need a fixed
13870 number of characters in the output, but zeros are too distracting.
13872 (zero) Pad with zeros even if the conversion specifier
13873 would normally pad with spaces.
13875 Use upper case characters if possible.
13877 Use opposite case characters if possible.
13878 A field that is normally upper case becomes lower case, and vice versa.
13882 Here are some examples of padding:
13885 date +%d/%m -d "Feb 1"
13887 date +%-d/%-m -d "Feb 1"
13889 date +%_d/%_m -d "Feb 1"
13893 As a @acronym{GNU} extension, you can specify the field width
13894 (after any flag, if present) as a decimal number. If the natural size of the
13895 output of the field has less than the specified number of characters,
13896 the result is written right adjusted and padded to the given
13897 size. For example, @samp{%9B} prints the right adjusted month name in
13898 a field of width 9.
13900 An optional modifier can follow the optional flag and width
13901 specification. The modifiers are:
13905 Use the locale's alternate representation for date and time. This
13906 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13907 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13908 example, @samp{%Ex} might yield a date format based on the Japanese
13912 Use the locale's alternate numeric symbols for numbers. This modifier
13913 applies only to numeric conversion specifiers.
13916 If the format supports the modifier but no alternate representation
13917 is available, it is ignored.
13920 @node Setting the time
13921 @subsection Setting the time
13923 @cindex setting the time
13924 @cindex time setting
13925 @cindex appropriate privileges
13927 If given an argument that does not start with @samp{+}, @command{date} sets
13928 the system clock to the date and time specified by that argument (as
13929 described below). You must have appropriate privileges to set the
13930 system clock. The @option{--date} and @option{--set} options may not be
13931 used with such an argument. The @option{--universal} option may be used
13932 with such an argument to indicate that the specified date and time are
13933 relative to Coordinated Universal Time rather than to the local time
13936 The argument must consist entirely of digits, which have the following
13949 first two digits of year (optional)
13951 last two digits of year (optional)
13956 The @option{--set} option also sets the system clock; see the next section.
13959 @node Options for date
13960 @subsection Options for @command{date}
13962 @cindex @command{date} options
13963 @cindex options for @command{date}
13965 The program accepts the following options. Also see @ref{Common options}.
13969 @item -d @var{datestr}
13970 @itemx --date=@var{datestr}
13973 @cindex parsing date strings
13974 @cindex date strings, parsing
13975 @cindex arbitrary date strings, parsing
13978 @opindex next @var{day}
13979 @opindex last @var{day}
13980 Display the date and time specified in @var{datestr} instead of the
13981 current date and time. @var{datestr} can be in almost any common
13982 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13983 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13984 14:19:13.489392193 +0530"} specifies the instant of time that is
13985 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13986 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13987 Note: input currently must be in locale independent format. E.g., the
13988 LC_TIME=C below is needed to print back the correct date in many locales:
13990 date -d "$(LC_TIME=C date)"
13992 @xref{Date input formats}.
13994 @item -f @var{datefile}
13995 @itemx --file=@var{datefile}
13998 Parse each line in @var{datefile} as with @option{-d} and display the
13999 resulting date and time. If @var{datefile} is @samp{-}, use standard
14000 input. This is useful when you have many dates to process, because the
14001 system overhead of starting up the @command{date} executable many times can
14004 @item -r @var{file}
14005 @itemx --reference=@var{file}
14007 @opindex --reference
14008 Display the date and time of the last modification of @var{file},
14009 instead of the current date and time.
14016 @opindex --rfc-2822
14017 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14018 %z}, evaluated in the C locale so abbreviations are always in English.
14022 Fri, 09 Sep 2005 13:51:39 -0700
14025 This format conforms to
14026 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14027 @acronym{RFCs} 2822} and
14028 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14029 current and previous standards for Internet email.
14031 @item --rfc-3339=@var{timespec}
14032 @opindex --rfc-3339=@var{timespec}
14033 Display the date using a format specified by
14034 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14035 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14036 format, except that it also permits applications to use a space rather
14037 than a @samp{T} to separate dates from times. Unlike the other
14038 standard formats, @acronym{RFC} 3339 format is always suitable as
14039 input for the @option{--date} (@option{-d}) and @option{--file}
14040 (@option{-f}) options, regardless of the current locale.
14042 The argument @var{timespec} specifies how much of the time to include.
14043 It can be one of the following:
14047 Print just the full-date, e.g., @samp{2005-09-14}.
14048 This is equivalent to the format @samp{%Y-%m-%d}.
14051 Print the full-date and full-time separated by a space, e.g.,
14052 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14053 time-offset; here the @samp{+05:30} means that local time is five
14054 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14055 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14058 Like @samp{seconds}, but also print nanoseconds, e.g.,
14059 @samp{2005-09-14 00:56:06.998458565+05:30}.
14060 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14064 @item -s @var{datestr}
14065 @itemx --set=@var{datestr}
14068 Set the date and time to @var{datestr}. See @option{-d} above.
14075 @opindex --universal
14076 @cindex Coordinated Universal Time
14078 @cindex Greenwich Mean Time
14081 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14082 @env{TZ} environment variable were set to the string @samp{UTC0}.
14084 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14085 historical reasons.
14089 @node Examples of date
14090 @subsection Examples of @command{date}
14092 @cindex examples of @command{date}
14094 Here are a few examples. Also see the documentation for the @option{-d}
14095 option in the previous section.
14100 To print the date of the day before yesterday:
14103 date --date='2 days ago'
14107 To print the date of the day three months and one day hence:
14110 date --date='3 months 1 day'
14114 To print the day of year of Christmas in the current year:
14117 date --date='25 Dec' +%j
14121 To print the current full month name and the day of the month:
14127 But this may not be what you want because for the first nine days of
14128 the month, the @samp{%d} expands to a zero-padded two-digit field,
14129 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14132 To print a date without the leading zero for one-digit days
14133 of the month, you can use the (@acronym{GNU} extension)
14134 @samp{-} flag to suppress
14135 the padding altogether:
14138 date -d 1may '+%B %-d
14142 To print the current date and time in the format required by many
14143 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14146 date +%m%d%H%M%Y.%S
14150 To set the system clock forward by two minutes:
14153 date --set='+2 minutes'
14157 To print the date in @acronym{RFC} 2822 format,
14158 use @samp{date --rfc-2822}. Here is some example output:
14161 Fri, 09 Sep 2005 13:51:39 -0700
14164 @anchor{%s-examples}
14166 To convert a date string to the number of seconds since the epoch
14167 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14168 the @samp{%s} format. That can be useful in sorting and/or graphing
14169 and/or comparing data by date. The following command outputs the
14170 number of the seconds since the epoch for the time two minutes after the
14174 date --date='1970-01-01 00:02:00 +0000' +%s
14178 If you do not specify time zone information in the date string,
14179 @command{date} uses your computer's idea of the time zone when
14180 interpreting the string. For example, if your computer's time zone is
14181 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14182 seconds) behind UTC:
14185 # local time zone used
14186 date --date='1970-01-01 00:02:00' +%s
14191 If you're sorting or graphing dated data, your raw date values may be
14192 represented as seconds since the epoch. But few people can look at
14193 the date @samp{946684800} and casually note ``Oh, that's the first second
14194 of the year 2000 in Greenwich, England.''
14197 date --date='2000-01-01 UTC' +%s
14201 An alternative is to use the @option{--utc} (@option{-u}) option.
14202 Then you may omit @samp{UTC} from the date string. Although this
14203 produces the same result for @samp{%s} and many other format sequences,
14204 with a time zone offset different from zero, it would give a different
14205 result for zone-dependent formats like @samp{%z}.
14208 date -u --date=2000-01-01 +%s
14212 To convert such an unwieldy number of seconds back to
14213 a more readable form, use a command like this:
14216 # local time zone used
14217 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14218 1999-12-31 19:00:00 -0500
14221 Or if you do not mind depending on the @samp{@@} feature present since
14222 coreutils 5.3.0, you could shorten this to:
14225 date -d @@946684800 +"%F %T %z"
14226 1999-12-31 19:00:00 -0500
14229 Often it is better to output UTC-relative date and time:
14232 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14233 2000-01-01 00:00:00 +0000
14239 @node arch invocation
14240 @section @command{arch}: Print machine hardware name
14243 @cindex print machine hardware name
14244 @cindex system information, printing
14246 @command{arch} prints the machine hardware name,
14247 and is equivalent to @samp{uname -m}.
14251 arch [@var{option}]
14254 The program accepts the @ref{Common options} only.
14259 @node nproc invocation
14260 @section @command{nproc}: Print the number of available processors
14263 @cindex Print the number of processors
14264 @cindex system information, printing
14266 Print the number of processing units available to the current process,
14267 which may be less than the number of online processors.
14268 If this information is not accessible, then print the number of
14269 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14270 set, then it will determine the returned value. The result is guaranteed to be
14271 greater than zero. Synopsis:
14274 nproc [@var{option}]
14277 The program accepts the following options. Also see @ref{Common options}.
14283 Print the number of installed processors on the system, which may
14284 be greater than the number online or available to the current process.
14285 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14287 @item --ignore=@var{number}
14289 If possible, exclude this @var{number} of processing units.
14296 @node uname invocation
14297 @section @command{uname}: Print system information
14300 @cindex print system information
14301 @cindex system information, printing
14303 @command{uname} prints information about the machine and operating system
14304 it is run on. If no options are given, @command{uname} acts as if the
14305 @option{-s} option were given. Synopsis:
14308 uname [@var{option}]@dots{}
14311 If multiple options or @option{-a} are given, the selected information is
14312 printed in this order:
14315 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14316 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14319 The information may contain internal spaces, so such output cannot be
14320 parsed reliably. In the following example, @var{release} is
14321 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14325 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14329 The program accepts the following options. Also see @ref{Common options}.
14337 Print all of the below information, except omit the processor type
14338 and the hardware platform name if they are unknown.
14341 @itemx --hardware-platform
14343 @opindex --hardware-platform
14344 @cindex implementation, hardware
14345 @cindex hardware platform
14346 @cindex platform, hardware
14347 Print the hardware platform name
14348 (sometimes called the hardware implementation).
14349 Print @samp{unknown} if the kernel does not make this information
14350 easily available, as is the case with Linux kernels.
14356 @cindex machine type
14357 @cindex hardware class
14358 @cindex hardware type
14359 Print the machine hardware name (sometimes called the hardware class
14365 @opindex --nodename
14368 @cindex network node name
14369 Print the network node hostname.
14374 @opindex --processor
14375 @cindex host processor type
14376 Print the processor type (sometimes called the instruction set
14377 architecture or ISA).
14378 Print @samp{unknown} if the kernel does not make this information
14379 easily available, as is the case with Linux kernels.
14382 @itemx --operating-system
14384 @opindex --operating-system
14385 @cindex operating system name
14386 Print the name of the operating system.
14389 @itemx --kernel-release
14391 @opindex --kernel-release
14392 @cindex kernel release
14393 @cindex release of kernel
14394 Print the kernel release.
14397 @itemx --kernel-name
14399 @opindex --kernel-name
14400 @cindex kernel name
14401 @cindex name of kernel
14402 Print the kernel name.
14403 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14404 ``the implementation of the operating system'', because the
14405 @acronym{POSIX} specification itself has no notion of ``kernel''.
14406 The kernel name might be the same as the operating system name printed
14407 by the @option{-o} or @option{--operating-system} option, but it might
14408 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14409 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14413 @itemx --kernel-version
14415 @opindex --kernel-version
14416 @cindex kernel version
14417 @cindex version of kernel
14418 Print the kernel version.
14425 @node hostname invocation
14426 @section @command{hostname}: Print or set system name
14429 @cindex setting the hostname
14430 @cindex printing the hostname
14431 @cindex system name, printing
14432 @cindex appropriate privileges
14434 With no arguments, @command{hostname} prints the name of the current host
14435 system. With one argument, it sets the current host name to the
14436 specified string. You must have appropriate privileges to set the host
14440 hostname [@var{name}]
14443 The only options are @option{--help} and @option{--version}. @xref{Common
14449 @node hostid invocation
14450 @section @command{hostid}: Print numeric host identifier
14453 @cindex printing the host identifier
14455 @command{hostid} prints the numeric identifier of the current host
14456 in hexadecimal. This command accepts no arguments.
14457 The only options are @option{--help} and @option{--version}.
14458 @xref{Common options}.
14460 For example, here's what it prints on one system I use:
14467 On that system, the 32-bit quantity happens to be closely
14468 related to the system's Internet address, but that isn't always
14473 @node uptime invocation
14474 @section @command{uptime}: Print system uptime and load
14477 @cindex printing the system uptime and load
14479 @command{uptime} prints the current time, the system's uptime, the
14480 number of logged-in users and the current load average.
14482 If an argument is specified, it is used as the file to be read
14483 to discover how many users are logged in. If no argument is
14484 specified, a system default is used (@command{uptime --help} indicates
14485 the default setting).
14487 The only options are @option{--help} and @option{--version}.
14488 @xref{Common options}.
14490 For example, here's what it prints right now on one system I use:
14494 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14497 The precise method of calculation of load average varies somewhat
14498 between systems. Some systems calculate it as the average number of
14499 runnable processes over the last 1, 5 and 15 minutes, but some systems
14500 also include processes in the uninterruptible sleep state (that is,
14501 those processes which are waiting for disk I/O). The Linux kernel
14502 includes uninterruptible processes.
14504 @node SELinux context
14505 @chapter SELinux context
14507 @cindex SELinux context
14508 @cindex SELinux, context
14509 @cindex commands for SELinux context
14511 This section describes commands for operations with SELinux
14515 * chcon invocation:: Change SELinux context of file
14516 * runcon invocation:: Run a command in specified SELinux context
14519 @node chcon invocation
14520 @section @command{chcon}: Change SELinux context of file
14523 @cindex changing security context
14524 @cindex change SELinux context
14526 @command{chcon} changes the SELinux security context of the selected files.
14530 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14531 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14532 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14535 Change the SELinux security context of each @var{file} to @var{context}.
14536 With @option{--reference}, change the security context of each @var{file}
14537 to that of @var{rfile}.
14539 The program accepts the following options. Also see @ref{Common options}.
14544 @itemx --no-dereference
14546 @opindex --no-dereference
14547 @cindex no dereference
14548 Affect symbolic links instead of any referenced file.
14550 @item --reference=@var{rfile}
14551 @opindex --reference
14552 @cindex reference file
14553 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14558 @opindex --recursive
14559 Operate on files and directories recursively.
14562 @xref{Traversing symlinks}.
14565 @xref{Traversing symlinks}.
14568 @xref{Traversing symlinks}.
14575 Output a diagnostic for every file processed.
14577 @item -u @var{user}
14578 @itemx --user=@var{user}
14581 Set user @var{user} in the target security context.
14583 @item -r @var{role}
14584 @itemx --role=@var{role}
14587 Set role @var{role} in the target security context.
14589 @item -t @var{type}
14590 @itemx --type=@var{type}
14593 Set type @var{type} in the target security context.
14595 @item -l @var{range}
14596 @itemx --range=@var{range}
14599 Set range @var{range} in the target security context.
14605 @node runcon invocation
14606 @section @command{runcon}: Run a command in specified SELinux context
14609 @cindex run with security context
14612 @command{runcon} runs file in specified SELinux security context.
14616 runcon @var{context} @var{command} [@var{args}]
14617 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14620 Run @var{command} with completely-specified @var{context}, or with
14621 current or transitioned security context modified by one or more of @var{level},
14622 @var{role}, @var{type} and @var{user}.
14624 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14625 is specified, the first argument is used as the complete context.
14626 Any additional arguments after @var{command}
14627 are interpreted as arguments to the command.
14629 With neither @var{context} nor @var{command}, print the current security context.
14631 The program accepts the following options. Also see @ref{Common options}.
14639 Compute process transition context before modifying.
14641 @item -u @var{user}
14642 @itemx --user=@var{user}
14645 Set user @var{user} in the target security context.
14647 @item -r @var{role}
14648 @itemx --role=@var{role}
14651 Set role @var{role} in the target security context.
14653 @item -t @var{type}
14654 @itemx --type=@var{type}
14657 Set type @var{type} in the target security context.
14659 @item -l @var{range}
14660 @itemx --range=@var{range}
14663 Set range @var{range} in the target security context.
14667 @cindex exit status of @command{runcon}
14671 126 if @var{command} is found but cannot be invoked
14672 127 if @command{runcon} itself fails or if @var{command} cannot be found
14673 the exit status of @var{command} otherwise
14676 @node Modified command invocation
14677 @chapter Modified command invocation
14679 @cindex modified command invocation
14680 @cindex invocation of commands, modified
14681 @cindex commands for invoking other commands
14683 This section describes commands that run other commands in some context
14684 different than the current one: a modified environment, as a different
14688 * chroot invocation:: Modify the root directory.
14689 * env invocation:: Modify environment variables.
14690 * nice invocation:: Modify niceness.
14691 * nohup invocation:: Immunize to hangups.
14692 * stdbuf invocation:: Modify buffering of standard streams.
14693 * su invocation:: Modify user and group ID.
14694 * timeout invocation:: Run with time limit.
14698 @node chroot invocation
14699 @section @command{chroot}: Run a command with a different root directory
14702 @cindex running a program in a specified root directory
14703 @cindex root directory, running a program in a specified
14705 @command{chroot} runs a command with a specified root directory.
14706 On many systems, only the super-user can do this.@footnote{However,
14707 some systems (e.g., FreeBSD) can be configured to allow certain regular
14708 users to use the @code{chroot} system call, and hence to run this program.
14709 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14710 underlying function is non-privileged due to lack of support in MS-Windows.}
14714 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14715 chroot @var{option}
14718 Ordinarily, file names are looked up starting at the root of the
14719 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14720 the directory @var{newroot} (which must exist) and then runs
14721 @var{command} with optional @var{args}. If @var{command} is not
14722 specified, the default is the value of the @env{SHELL} environment
14723 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14724 @var{command} must not be a special built-in utility
14725 (@pxref{Special built-in utilities}).
14727 The program accepts the following options. Also see @ref{Common options}.
14728 Options must precede operands.
14732 @itemx --userspec=@var{user}[:@var{group}]
14733 @opindex --userspec
14734 By default, @var{command} is run with the same credentials
14735 as the invoking process.
14736 Use this option to run it as a different @var{user} and/or with a
14737 different primary @var{group}.
14739 @itemx --groups=@var{groups}
14741 Use this option to specify the supplementary @var{groups} to be
14742 used by the new process.
14743 The items in the list (names or numeric IDs) must be separated by commas.
14747 Here are a few tips to help avoid common problems in using chroot.
14748 To start with a simple example, make @var{command} refer to a statically
14749 linked binary. If you were to use a dynamically linked executable, then
14750 you'd have to arrange to have the shared libraries in the right place under
14751 your new root directory.
14753 For example, if you create a statically linked @command{ls} executable,
14754 and put it in @file{/tmp/empty}, you can run this command as root:
14757 $ chroot /tmp/empty /ls -Rl /
14760 Then you'll see output like this:
14765 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14768 If you want to use a dynamically linked executable, say @command{bash},
14769 then first run @samp{ldd bash} to see what shared objects it needs.
14770 Then, in addition to copying the actual binary, also copy the listed
14771 files to the required positions under your intended new root directory.
14772 Finally, if the executable requires any other files (e.g., data, state,
14773 device files), copy them into place, too.
14775 @cindex exit status of @command{chroot}
14779 125 if @command{chroot} itself fails
14780 126 if @var{command} is found but cannot be invoked
14781 127 if @var{command} cannot be found
14782 the exit status of @var{command} otherwise
14786 @node env invocation
14787 @section @command{env}: Run a command in a modified environment
14790 @cindex environment, running a program in a modified
14791 @cindex modified environment, running a program in a
14792 @cindex running a program in a modified environment
14794 @command{env} runs a command with a modified environment. Synopses:
14797 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14798 [@var{command} [@var{args}]@dots{}]
14802 Operands of the form @samp{@var{variable}=@var{value}} set
14803 the environment variable @var{variable} to value @var{value}.
14804 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14805 to an empty value is different from unsetting it.
14806 These operands are evaluated left-to-right, so if two operands
14807 mention the same variable the earlier is ignored.
14809 Environment variable names can be empty, and can contain any
14810 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14811 However, it is wise to limit yourself to names that
14812 consist solely of underscores, digits, and @acronym{ASCII} letters,
14813 and that begin with a non-digit, as applications like the shell do not
14814 work well with other names.
14817 The first operand that does not contain the character @samp{=}
14818 specifies the program to invoke; it is
14819 searched for according to the @env{PATH} environment variable. Any
14820 remaining arguments are passed as arguments to that program.
14821 The program should not be a special built-in utility
14822 (@pxref{Special built-in utilities}).
14824 Modifications to @env{PATH} take effect prior to searching for
14825 @var{command}. Use caution when reducing @env{PATH}; behavior is
14826 not portable when @env{PATH} is undefined or omits key directories
14827 such as @file{/bin}.
14829 In the rare case that a utility contains a @samp{=} in the name, the
14830 only way to disambiguate it from a variable assignment is to use an
14831 intermediate command for @var{command}, and pass the problematic
14832 program name via @var{args}. For example, if @file{./prog=} is an
14833 executable in the current @env{PATH}:
14836 env prog= true # runs 'true', with prog= in environment
14837 env ./prog= true # runs 'true', with ./prog= in environment
14838 env -- prog= true # runs 'true', with prog= in environment
14839 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14840 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14843 @cindex environment, printing
14845 If no command name is specified following the environment
14846 specifications, the resulting environment is printed. This is like
14847 specifying the @command{printenv} program.
14849 For some examples, suppose the environment passed to @command{env}
14850 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14851 @samp{PATH=.:/gnubin:/hacks}:
14856 Output the current environment.
14858 $ env | LC_ALL=C sort
14861 PATH=.:/gnubin:/hacks
14865 Run @command{foo} with a reduced environment, preserving only the
14866 original @env{PATH} to avoid problems in locating @command{foo}.
14868 env - PATH="$PATH" foo
14872 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14873 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14874 that @command{foo} was found in the file system rather than as a shell
14881 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14882 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14883 @samp{DISPLAY=gnu:0}.
14885 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14889 Attempt to run the program @command{/energy/--} (as that is the only
14890 possible path search result); if the command exists, the environment
14891 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14892 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14894 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14900 The program accepts the following options. Also see @ref{Common options}.
14901 Options must precede operands.
14907 @item -u @var{name}
14908 @itemx --unset=@var{name}
14911 Remove variable @var{name} from the environment, if it was in the
14916 @itemx --ignore-environment
14919 @opindex --ignore-environment
14920 Start with an empty environment, ignoring the inherited environment.
14924 @cindex exit status of @command{env}
14928 0 if no @var{command} is specified and the environment is output
14929 125 if @command{env} itself fails
14930 126 if @var{command} is found but cannot be invoked
14931 127 if @var{command} cannot be found
14932 the exit status of @var{command} otherwise
14936 @node nice invocation
14937 @section @command{nice}: Run a command with modified niceness
14941 @cindex scheduling, affecting
14942 @cindex appropriate privileges
14944 @command{nice} prints or modifies a process's @dfn{niceness},
14945 a parameter that affects whether the process is scheduled favorably.
14949 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14952 If no arguments are given, @command{nice} prints the current niceness.
14953 Otherwise, @command{nice} runs the given @var{command} with its
14954 niceness adjusted. By default, its niceness is incremented by 10.
14956 Niceness values range at least from @minus{}20 (process has high priority
14957 and gets more resources, thus slowing down other processes) through 19
14958 (process has lower priority and runs slowly itself, but has less impact
14959 on the speed of other running processes). Some systems
14960 may have a wider range of nicenesses; conversely, other systems may
14961 enforce more restrictive limits. An attempt to set the niceness
14962 outside the supported range is treated as an attempt to use the
14963 minimum or maximum supported value.
14965 A niceness should not be confused with a scheduling priority, which
14966 lets applications determine the order in which threads are scheduled
14967 to run. Unlike a priority, a niceness is merely advice to the
14968 scheduler, which the scheduler is free to ignore. Also, as a point of
14969 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14970 terms of a @dfn{nice value}, which is the nonnegative difference
14971 between a niceness and the minimum niceness. Though @command{nice}
14972 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14973 term ``niceness'' for compatibility with historical practice.
14975 @var{command} must not be a special built-in utility (@pxref{Special
14976 built-in utilities}).
14978 @mayConflictWithShellBuiltIn{nice}
14980 The program accepts the following option. Also see @ref{Common options}.
14981 Options must precede operands.
14984 @item -n @var{adjustment}
14985 @itemx --adjustment=@var{adjustment}
14987 @opindex --adjustment
14988 Add @var{adjustment} instead of 10 to the command's niceness. If
14989 @var{adjustment} is negative and you lack appropriate privileges,
14990 @command{nice} issues a warning but otherwise acts as if you specified
14993 For compatibility @command{nice} also supports an obsolete
14994 option syntax @option{-@var{adjustment}}. New scripts should use
14995 @option{-n @var{adjustment}} instead.
14999 @cindex exit status of @command{nice}
15003 0 if no @var{command} is specified and the niceness is output
15004 125 if @command{nice} itself fails
15005 126 if @var{command} is found but cannot be invoked
15006 127 if @var{command} cannot be found
15007 the exit status of @var{command} otherwise
15010 It is sometimes useful to run a non-interactive program with reduced niceness.
15013 $ nice factor 4611686018427387903
15016 Since @command{nice} prints the current niceness,
15017 you can invoke it through itself to demonstrate how it works.
15019 The default behavior is to increase the niceness by @samp{10}:
15030 The @var{adjustment} is relative to the current niceness. In the
15031 next example, the first @command{nice} invocation runs the second one
15032 with niceness 10, and it in turn runs the final one with a niceness
15036 $ nice nice -n 3 nice
15040 Specifying a niceness larger than the supported range
15041 is the same as specifying the maximum supported value:
15044 $ nice -n 10000000000 nice
15048 Only a privileged user may run a process with lower niceness:
15052 nice: cannot set niceness: Permission denied
15054 $ sudo nice -n -1 nice
15059 @node nohup invocation
15060 @section @command{nohup}: Run a command immune to hangups
15063 @cindex hangups, immunity to
15064 @cindex immunity to hangups
15065 @cindex logging out and continuing to run
15068 @command{nohup} runs the given @var{command} with hangup signals ignored,
15069 so that the command can continue running in the background after you log
15073 nohup @var{command} [@var{arg}]@dots{}
15076 If standard input is a terminal, it is redirected from
15077 @file{/dev/null} so that terminal sessions do not mistakenly consider
15078 the terminal to be used by the command. This is a @acronym{GNU}
15079 extension; programs intended to be portable to non-@acronym{GNU} hosts
15080 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15084 If standard output is a terminal, the command's standard output is appended
15085 to the file @file{nohup.out}; if that cannot be written to, it is appended
15086 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15087 command is not run.
15088 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15089 @command{nohup} is made readable and writable only to the user,
15090 regardless of the current umask settings.
15092 If standard error is a terminal, it is normally redirected to the same file
15093 descriptor as the (possibly-redirected) standard output.
15094 However, if standard output is closed, standard error terminal output
15095 is instead appended to the file @file{nohup.out} or
15096 @file{$HOME/nohup.out} as above.
15098 To capture the command's output to a file other than @file{nohup.out}
15099 you can redirect it. For example, to capture the output of
15103 nohup make > make.log
15106 @command{nohup} does not automatically put the command it runs in the
15107 background; you must do that explicitly, by ending the command line
15108 with an @samp{&}. Also, @command{nohup} does not alter the
15109 niceness of @var{command}; use @command{nice} for that,
15110 e.g., @samp{nohup nice @var{command}}.
15112 @var{command} must not be a special built-in utility (@pxref{Special
15113 built-in utilities}).
15115 The only options are @option{--help} and @option{--version}. @xref{Common
15116 options}. Options must precede operands.
15118 @cindex exit status of @command{nohup}
15122 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15123 126 if @var{command} is found but cannot be invoked
15124 127 if @var{command} cannot be found
15125 the exit status of @var{command} otherwise
15128 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15132 @node stdbuf invocation
15133 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15136 @cindex standard streams, buffering
15137 @cindex line buffered
15139 @command{stdbuf} allows one to modify the buffering operations of the
15140 three standard I/O streams associated with a program. Synopsis:
15143 stdbuf @var{option}@dots{} @var{command}
15146 Any additional @var{arg}s are passed as additional arguments to the
15149 The program accepts the following options. Also see @ref{Common options}.
15153 @item -i @var{mode}
15154 @itemx --input=@var{mode}
15157 Adjust the standard input stream buffering.
15159 @item -o @var{mode}
15160 @itemx --output=@var{mode}
15163 Adjust the standard output stream buffering.
15165 @item -e @var{mode}
15166 @itemx --error=@var{mode}
15169 Adjust the standard error stream buffering.
15173 The @var{mode} can be specified as follows:
15178 Set the stream to line buffered mode.
15179 In this mode data is coalesced until a newline is output or
15180 input is read from any stream attached to a terminal device.
15181 This option is invalid with standard input.
15184 Disable buffering of the selected stream.
15185 In this mode data is output immediately and only the
15186 amount of data requested is read from input.
15189 Specify the size of the buffer to use in fully buffered mode.
15190 @multiplierSuffixesNoBlocks{size}
15194 NOTE: If @var{command} adjusts the buffering of its standard streams
15195 (@command{tee} does for e.g.) then that will override corresponding settings
15196 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15197 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15198 by @command{stdbuf} settings.
15200 @cindex exit status of @command{stdbuf}
15204 125 if @command{stdbuf} itself fails
15205 126 if @var{command} is found but cannot be invoked
15206 127 if @var{command} cannot be found
15207 the exit status of @var{command} otherwise
15211 @node su invocation
15212 @section @command{su}: Run a command with substitute user and group ID
15215 @cindex substitute user and group IDs
15216 @cindex user ID, switching
15217 @cindex super-user, becoming
15218 @cindex root, becoming
15220 @command{su} allows one user to temporarily become another user. It runs a
15221 command (often an interactive shell) with the real and effective user
15222 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15225 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15228 @cindex passwd entry, and @command{su} shell
15230 @flindex /etc/passwd
15231 If no @var{user} is given, the default is @code{root}, the super-user.
15232 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15233 @file{/bin/sh} if none is specified there. If @var{user} has a
15234 password, @command{su} prompts for the password unless run by a user with
15235 effective user ID of zero (the super-user).
15241 @cindex login shell
15242 By default, @command{su} does not change the current directory.
15243 It sets the environment variables @env{HOME} and @env{SHELL}
15244 from the password entry for @var{user}, and if @var{user} is not
15245 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15246 By default, the shell is not a login shell.
15248 Any additional @var{arg}s are passed as additional arguments to the
15251 @cindex @option{-su}
15252 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15253 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15254 to certain shells, etc.).
15257 @command{su} can optionally be compiled to use @code{syslog} to report
15258 failed, and optionally successful, @command{su} attempts. (If the system
15259 supports @code{syslog}.) However, GNU @command{su} does not check if the
15260 user is a member of the @code{wheel} group; see below.
15262 The program accepts the following options. Also see @ref{Common options}.
15265 @item -c @var{command}
15266 @itemx --command=@var{command}
15269 Pass @var{command}, a single command line to run, to the shell with
15270 a @option{-c} option instead of starting an interactive shell.
15277 @cindex file name pattern expansion, disabled
15278 @cindex globbing, disabled
15279 Pass the @option{-f} option to the shell. This probably only makes sense
15280 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15281 option prevents reading the startup file (@file{.cshrc}). With
15282 Bourne-like shells, the @option{-f} option disables file name pattern
15283 expansion (globbing), which is not likely to be useful.
15291 @c other variables already indexed above
15294 @cindex login shell, creating
15295 Make the shell a login shell. This means the following. Unset all
15296 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15297 (which are set as described above), and @env{USER} and @env{LOGNAME}
15298 (which are set, even for the super-user, as described above), and set
15299 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15300 directory. Prepend @samp{-} to the shell's name, intended to make it
15301 read its login startup file(s).
15305 @itemx --preserve-environment
15308 @opindex --preserve-environment
15309 @cindex environment, preserving
15310 @flindex /etc/shells
15311 @cindex restricted shell
15312 Do not change the environment variables @env{HOME}, @env{USER},
15313 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15314 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15315 entry, unless the user running @command{su} is not the super-user and
15316 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15317 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15318 if that file does not exist. Parts of what this option does can be
15319 overridden by @option{--login} and @option{--shell}.
15321 @item -s @var{shell}
15322 @itemx --shell=@var{shell}
15325 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15326 unless the user running @command{su} is not the super-user and @var{user}'s
15327 shell is restricted (see @option{-m} just above).
15331 @cindex exit status of @command{su}
15335 125 if @command{su} itself fails
15336 126 if subshell is found but cannot be invoked
15337 127 if subshell cannot be found
15338 the exit status of the subshell otherwise
15341 @cindex wheel group, not supported
15342 @cindex group wheel, not supported
15344 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15346 (This section is by Richard Stallman.)
15350 Sometimes a few of the users try to hold total power over all the
15351 rest. For example, in 1984, a few users at the MIT AI lab decided to
15352 seize power by changing the operator password on the Twenex system and
15353 keeping it secret from everyone else. (I was able to thwart this coup
15354 and give power back to the users by patching the kernel, but I
15355 wouldn't know how to do that in Unix.)
15357 However, occasionally the rulers do tell someone. Under the usual
15358 @command{su} mechanism, once someone learns the root password who
15359 sympathizes with the ordinary users, he or she can tell the rest. The
15360 ``wheel group'' feature would make this impossible, and thus cement the
15361 power of the rulers.
15363 I'm on the side of the masses, not that of the rulers. If you are
15364 used to supporting the bosses and sysadmins in whatever they do, you
15365 might find this idea strange at first.
15368 @node timeout invocation
15369 @section @command{timeout}: Run a command with a time limit
15373 @cindex run commands with bounded time
15375 @command{timeout} runs the given @var{command} and kills it if it is
15376 still running after the specified time interval. Synopsis:
15379 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15382 @var{command} must not be a special built-in utility (@pxref{Special
15383 built-in utilities}).
15385 The program accepts the following options. Also see @ref{Common options}.
15386 Options must precede operands.
15389 @item -k @var{duration}
15390 @itemx --kill-after=@var{duration}
15392 @opindex --kill-after
15393 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15394 signal, after the specified @var{duration}. Without this option, if the
15395 selected signal proves not to be fatal, @command{timeout} does not kill
15398 @item -s @var{signal}
15399 @itemx --signal=@var{signal}
15402 Send this @var{signal} to @var{command} on timeout, rather than the
15403 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15404 or a number. Also see @xref{Signal specifications}.
15408 @var{duration} is an integer followed by an optional unit:
15410 @samp{s} for seconds (the default)
15411 @samp{m} for minutes
15415 A duration of 0 disables the associated timeout.
15417 @cindex exit status of @command{timeout}
15421 124 if @var{command} times out
15422 125 if @command{timeout} itself fails
15423 126 if @var{command} is found but cannot be invoked
15424 127 if @var{command} cannot be found
15425 the exit status of @var{command} otherwise
15429 @node Process control
15430 @chapter Process control
15432 @cindex processes, commands for controlling
15433 @cindex commands for controlling processes
15436 * kill invocation:: Sending a signal to processes.
15440 @node kill invocation
15441 @section @command{kill}: Send a signal to processes
15444 @cindex send a signal to processes
15446 The @command{kill} command sends a signal to processes, causing them
15447 to terminate or otherwise act upon receiving the signal in some way.
15448 Alternatively, it lists information about signals. Synopses:
15451 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15452 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15455 @mayConflictWithShellBuiltIn{kill}
15457 The first form of the @command{kill} command sends a signal to all
15458 @var{pid} arguments. The default signal to send if none is specified
15459 is @samp{TERM}. The special signal number @samp{0} does not denote a
15460 valid signal, but can be used to test whether the @var{pid} arguments
15461 specify processes to which a signal could be sent.
15463 If @var{pid} is positive, the signal is sent to the process with the
15464 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15465 processes in the process group of the current process. If @var{pid}
15466 is @minus{}1, the signal is sent to all processes for which the user has
15467 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15468 is sent to all processes in the process group that equals the absolute
15469 value of @var{pid}.
15471 If @var{pid} is not positive, a system-dependent set of system
15472 processes is excluded from the list of processes to which the signal
15475 If a negative @var{pid} argument is desired as the first one, it
15476 should be preceded by @option{--}. However, as a common extension to
15477 @acronym{POSIX}, @option{--} is not required with @samp{kill
15478 -@var{signal} -@var{pid}}. The following commands are equivalent:
15487 The first form of the @command{kill} command succeeds if every @var{pid}
15488 argument specifies at least one process that the signal was sent to.
15490 The second form of the @command{kill} command lists signal information.
15491 Either the @option{-l} or @option{--list} option, or the @option{-t}
15492 or @option{--table} option must be specified. Without any
15493 @var{signal} argument, all supported signals are listed. The output
15494 of @option{-l} or @option{--list} is a list of the signal names, one
15495 per line; if @var{signal} is already a name, the signal number is
15496 printed instead. The output of @option{-t} or @option{--table} is a
15497 table of signal numbers, names, and descriptions. This form of the
15498 @command{kill} command succeeds if all @var{signal} arguments are valid
15499 and if there is no output error.
15501 The @command{kill} command also supports the @option{--help} and
15502 @option{--version} options. @xref{Common options}.
15504 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15505 number like @samp{1}, or an exit status of a process terminated by the
15506 signal. A signal name can be given in canonical form or prefixed by
15507 @samp{SIG}. The case of the letters is ignored, except for the
15508 @option{-@var{signal}} option which must use upper case to avoid
15509 ambiguity with lower case option letters. For a list of supported
15510 signal names and numbers see @xref{Signal specifications}.
15515 @cindex delaying commands
15516 @cindex commands for delaying
15518 @c Perhaps @command{wait} or other commands should be described here also?
15521 * sleep invocation:: Delay for a specified time.
15525 @node sleep invocation
15526 @section @command{sleep}: Delay for a specified time
15529 @cindex delay for a specified time
15531 @command{sleep} pauses for an amount of time specified by the sum of
15532 the values of the command line arguments.
15536 sleep @var{number}[smhd]@dots{}
15540 Each argument is a number followed by an optional unit; the default
15541 is seconds. The units are:
15554 Historical implementations of @command{sleep} have required that
15555 @var{number} be an integer, and only accepted a single argument
15556 without a suffix. However, GNU @command{sleep} accepts
15557 arbitrary floating point numbers (using a period before any fractional
15560 The only options are @option{--help} and @option{--version}. @xref{Common
15563 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15564 @mayConflictWithShellBuiltIn{sleep}
15569 @node Numeric operations
15570 @chapter Numeric operations
15572 @cindex numeric operations
15573 These programs do numerically-related operations.
15576 * factor invocation:: Show factors of numbers.
15577 * seq invocation:: Print sequences of numbers.
15581 @node factor invocation
15582 @section @command{factor}: Print prime factors
15585 @cindex prime factors
15587 @command{factor} prints prime factors. Synopses:
15590 factor [@var{number}]@dots{}
15591 factor @var{option}
15594 If no @var{number} is specified on the command line, @command{factor} reads
15595 numbers from standard input, delimited by newlines, tabs, or spaces.
15597 The @command{factor} command supports only a small number of options:
15601 Print a short help on standard output, then exit without further
15605 Print the program version on standard output, then exit without further
15609 Factoring the product of the eighth and ninth Mersenne primes
15610 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15613 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15614 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15615 4951760154835678088235319297: 2147483647 2305843009213693951
15619 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15620 about 20 seconds on the same machine.
15622 Factoring large numbers is, in general, hard. The Pollard Rho
15623 algorithm used by @command{factor} is particularly effective for
15624 numbers with relatively small factors. If you wish to factor large
15625 numbers which do not have small factors (for example, numbers which
15626 are the product of two large primes), other methods are far better.
15628 If @command{factor} is built without using GNU MP, only
15629 single-precision arithmetic is available, and so large numbers
15630 (typically @math{2^{64}} and above) will not be supported. The single-precision
15631 code uses an algorithm which is designed for factoring smaller
15637 @node seq invocation
15638 @section @command{seq}: Print numeric sequences
15641 @cindex numeric sequences
15642 @cindex sequence of numbers
15644 @command{seq} prints a sequence of numbers to standard output. Synopses:
15647 seq [@var{option}]@dots{} @var{last}
15648 seq [@var{option}]@dots{} @var{first} @var{last}
15649 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15652 @command{seq} prints the numbers from @var{first} to @var{last} by
15653 @var{increment}. By default, each number is printed on a separate line.
15654 When @var{increment} is not specified, it defaults to @samp{1},
15655 even when @var{first} is larger than @var{last}.
15656 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15657 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15658 Floating-point numbers
15659 may be specified (using a period before any fractional digits).
15661 The program accepts the following options. Also see @ref{Common options}.
15662 Options must precede operands.
15665 @item -f @var{format}
15666 @itemx --format=@var{format}
15667 @opindex -f @var{format}
15668 @opindex --format=@var{format}
15669 @cindex formatting of numbers in @command{seq}
15670 Print all numbers using @var{format}.
15671 @var{format} must contain exactly one of the @samp{printf}-style
15672 floating point conversion specifications @samp{%a}, @samp{%e},
15673 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15674 The @samp{%} may be followed by zero or more flags taken from the set
15675 @samp{-+#0 '}, then an optional width containing one or more digits,
15676 then an optional precision consisting of a @samp{.} followed by zero
15677 or more digits. @var{format} may also contain any number of @samp{%%}
15678 conversion specifications. All conversion specifications have the
15679 same meaning as with @samp{printf}.
15681 The default format is derived from @var{first}, @var{step}, and
15682 @var{last}. If these all use a fixed point decimal representation,
15683 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15684 precision that can represent the output numbers exactly. Otherwise,
15685 the default format is @samp{%g}.
15687 @item -s @var{string}
15688 @itemx --separator=@var{string}
15689 @cindex separator for numbers in @command{seq}
15690 Separate numbers with @var{string}; default is a newline.
15691 The output always terminates with a newline.
15694 @itemx --equal-width
15695 Print all numbers with the same width, by padding with leading zeros.
15696 @var{first}, @var{step}, and @var{last} should all use a fixed point
15697 decimal representation.
15698 (To have other kinds of padding, use @option{--format}).
15702 You can get finer-grained control over output with @option{-f}:
15705 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15711 If you want hexadecimal integer output, you can use @command{printf}
15712 to perform the conversion:
15715 $ printf '%x\n' `seq 1048575 1024 1050623`
15721 For very long lists of numbers, use xargs to avoid
15722 system limitations on the length of an argument list:
15725 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15731 To generate octal output, use the printf @code{%o} format instead
15734 On most systems, seq can produce whole-number output for values up to
15735 at least @math{2^{53}}. Larger integers are approximated. The details
15736 differ depending on your floating-point implementation, but a common
15737 case is that @command{seq} works with integers through @math{2^{64}},
15738 and larger integers may not be numerically correct:
15741 $ seq 18446744073709551616 1 18446744073709551618
15742 18446744073709551616
15743 18446744073709551616
15744 18446744073709551618
15747 Be careful when using @command{seq} with outlandish values: otherwise
15748 you may see surprising results, as @command{seq} uses floating point
15749 internally. For example, on the x86 platform, where the internal
15750 representation uses a 64-bit fraction, the command:
15753 seq 1 0.0000000000000000001 1.0000000000000000009
15756 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15761 @node File permissions
15762 @chapter File permissions
15765 @include parse-datetime.texi
15769 @node Opening the software toolbox
15770 @chapter Opening the Software Toolbox
15772 An earlier version of this chapter appeared in
15773 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15774 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15775 It was written by Arnold Robbins.
15778 * Toolbox introduction:: Toolbox introduction
15779 * I/O redirection:: I/O redirection
15780 * The who command:: The @command{who} command
15781 * The cut command:: The @command{cut} command
15782 * The sort command:: The @command{sort} command
15783 * The uniq command:: The @command{uniq} command
15784 * Putting the tools together:: Putting the tools together
15788 @node Toolbox introduction
15789 @unnumberedsec Toolbox Introduction
15791 This month's column is only peripherally related to the GNU Project, in
15792 that it describes a number of the GNU tools on your GNU/Linux system and how they
15793 might be used. What it's really about is the ``Software Tools'' philosophy
15794 of program development and usage.
15796 The software tools philosophy was an important and integral concept
15797 in the initial design and development of Unix (of which Linux and GNU are
15798 essentially clones). Unfortunately, in the modern day press of
15799 Internetworking and flashy GUIs, it seems to have fallen by the
15800 wayside. This is a shame, since it provides a powerful mental model
15801 for solving many kinds of problems.
15803 Many people carry a Swiss Army knife around in their pants pockets (or
15804 purse). A Swiss Army knife is a handy tool to have: it has several knife
15805 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15806 a number of other things on it. For the everyday, small miscellaneous jobs
15807 where you need a simple, general purpose tool, it's just the thing.
15809 On the other hand, an experienced carpenter doesn't build a house using
15810 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15811 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15812 exactly when and where to use each tool; you won't catch him hammering nails
15813 with the handle of his screwdriver.
15815 The Unix developers at Bell Labs were all professional programmers and trained
15816 computer scientists. They had found that while a one-size-fits-all program
15817 might appeal to a user because there's only one program to use, in practice
15822 difficult to write,
15825 difficult to maintain and
15829 difficult to extend to meet new situations.
15832 Instead, they felt that programs should be specialized tools. In short, each
15833 program ``should do one thing well.'' No more and no less. Such programs are
15834 simpler to design, write, and get right---they only do one thing.
15836 Furthermore, they found that with the right machinery for hooking programs
15837 together, that the whole was greater than the sum of the parts. By combining
15838 several special purpose programs, you could accomplish a specific task
15839 that none of the programs was designed for, and accomplish it much more
15840 quickly and easily than if you had to write a special purpose program.
15841 We will see some (classic) examples of this further on in the column.
15842 (An important additional point was that, if necessary, take a detour
15843 and build any software tools you may need first, if you don't already
15844 have something appropriate in the toolbox.)
15846 @node I/O redirection
15847 @unnumberedsec I/O Redirection
15849 Hopefully, you are familiar with the basics of I/O redirection in the
15850 shell, in particular the concepts of ``standard input,'' ``standard output,''
15851 and ``standard error''. Briefly, ``standard input'' is a data source, where
15852 data comes from. A program should not need to either know or care if the
15853 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15854 card reader. Similarly, ``standard output'' is a data sink, where data goes
15855 to. The program should neither know nor care where this might be.
15856 Programs that only read their standard input, do something to the data,
15857 and then send it on, are called @dfn{filters}, by analogy to filters in a
15860 With the Unix shell, it's very easy to set up data pipelines:
15863 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15866 We start out by creating the raw data; each filter applies some successive
15867 transformation to the data, until by the time it comes out of the pipeline,
15868 it is in the desired form.
15870 This is fine and good for standard input and standard output. Where does the
15871 standard error come in to play? Well, think about @command{filter1} in
15872 the pipeline above. What happens if it encounters an error in the data it
15873 sees? If it writes an error message to standard output, it will just
15874 disappear down the pipeline into @command{filter2}'s input, and the
15875 user will probably never see it. So programs need a place where they can send
15876 error messages so that the user will notice them. This is standard error,
15877 and it is usually connected to your console or window, even if you have
15878 redirected standard output of your program away from your screen.
15880 For filter programs to work together, the format of the data has to be
15881 agreed upon. The most straightforward and easiest format to use is simply
15882 lines of text. Unix data files are generally just streams of bytes, with
15883 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15884 conventionally called a ``newline'' in the Unix literature. (This is
15885 @code{'\n'} if you're a C programmer.) This is the format used by all
15886 the traditional filtering programs. (Many earlier operating systems
15887 had elaborate facilities and special purpose programs for managing
15888 binary data. Unix has always shied away from such things, under the
15889 philosophy that it's easiest to simply be able to view and edit your
15890 data with a text editor.)
15892 OK, enough introduction. Let's take a look at some of the tools, and then
15893 we'll see how to hook them together in interesting ways. In the following
15894 discussion, we will only present those command line options that interest
15895 us. As you should always do, double check your system documentation
15896 for the full story.
15898 @node The who command
15899 @unnumberedsec The @command{who} Command
15901 The first program is the @command{who} command. By itself, it generates a
15902 list of the users who are currently logged in. Although I'm writing
15903 this on a single-user system, we'll pretend that several people are
15908 @print{} arnold console Jan 22 19:57
15909 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15910 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15911 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15914 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15915 There are three people logged in, and I am logged in twice. On traditional
15916 Unix systems, user names are never more than eight characters long. This
15917 little bit of trivia will be useful later. The output of @command{who} is nice,
15918 but the data is not all that exciting.
15920 @node The cut command
15921 @unnumberedsec The @command{cut} Command
15923 The next program we'll look at is the @command{cut} command. This program
15924 cuts out columns or fields of input data. For example, we can tell it
15925 to print just the login name and full name from the @file{/etc/passwd}
15926 file. The @file{/etc/passwd} file has seven fields, separated by
15930 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15933 To get the first and fifth fields, we would use @command{cut} like this:
15936 $ cut -d: -f1,5 /etc/passwd
15937 @print{} root:Operator
15939 @print{} arnold:Arnold D. Robbins
15940 @print{} miriam:Miriam A. Robbins
15944 With the @option{-c} option, @command{cut} will cut out specific characters
15945 (i.e., columns) in the input lines. This is useful for input data
15946 that has fixed width fields, and does not have a field separator. For
15947 example, list the Monday dates for the current month:
15949 @c Is using cal ok? Looked at gcal, but I don't like it.
15960 @node The sort command
15961 @unnumberedsec The @command{sort} Command
15963 Next we'll look at the @command{sort} command. This is one of the most
15964 powerful commands on a Unix-style system; one that you will often find
15965 yourself using when setting up fancy data plumbing.
15968 command reads and sorts each file named on the command line. It then
15969 merges the sorted data and writes it to standard output. It will read
15970 standard input if no files are given on the command line (thus
15971 making it into a filter). The sort is based on the character collating
15972 sequence or based on user-supplied ordering criteria.
15975 @node The uniq command
15976 @unnumberedsec The @command{uniq} Command
15978 Finally (at least for now), we'll look at the @command{uniq} program. When
15979 sorting data, you will often end up with duplicate lines, lines that
15980 are identical. Usually, all you need is one instance of each line.
15981 This is where @command{uniq} comes in. The @command{uniq} program reads its
15982 standard input. It prints only one
15983 copy of each repeated line. It does have several options. Later on,
15984 we'll use the @option{-c} option, which prints each unique line, preceded
15985 by a count of the number of times that line occurred in the input.
15988 @node Putting the tools together
15989 @unnumberedsec Putting the Tools Together
15991 Now, let's suppose this is a large ISP server system with dozens of users
15992 logged in. The management wants the system administrator to write a program that will
15993 generate a sorted list of logged in users. Furthermore, even if a user
15994 is logged in multiple times, his or her name should only show up in the
15997 The administrator could sit down with the system documentation and write a C
15998 program that did this. It would take perhaps a couple of hundred lines
15999 of code and about two hours to write it, test it, and debug it.
16000 However, knowing the software toolbox, the administrator can instead start out
16001 by generating just a list of logged on users:
16011 Next, sort the list:
16014 $ who | cut -c1-8 | sort
16021 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16024 $ who | cut -c1-8 | sort | uniq
16030 The @command{sort} command actually has a @option{-u} option that does what
16031 @command{uniq} does. However, @command{uniq} has other uses for which one
16032 cannot substitute @samp{sort -u}.
16034 The administrator puts this pipeline into a shell script, and makes it available for
16035 all the users on the system (@samp{#} is the system administrator,
16036 or @code{root}, prompt):
16039 # cat > /usr/local/bin/listusers
16040 who | cut -c1-8 | sort | uniq
16042 # chmod +x /usr/local/bin/listusers
16045 There are four major points to note here. First, with just four
16046 programs, on one command line, the administrator was able to save about two
16047 hours worth of work. Furthermore, the shell pipeline is just about as
16048 efficient as the C program would be, and it is much more efficient in
16049 terms of programmer time. People time is much more expensive than
16050 computer time, and in our modern ``there's never enough time to do
16051 everything'' society, saving two hours of programmer time is no mean
16054 Second, it is also important to emphasize that with the
16055 @emph{combination} of the tools, it is possible to do a special
16056 purpose job never imagined by the authors of the individual programs.
16058 Third, it is also valuable to build up your pipeline in stages, as we did here.
16059 This allows you to view the data at each stage in the pipeline, which helps
16060 you acquire the confidence that you are indeed using these tools correctly.
16062 Finally, by bundling the pipeline in a shell script, other users can use
16063 your command, without having to remember the fancy plumbing you set up for
16064 them. In terms of how you run them, shell scripts and compiled programs are
16067 After the previous warm-up exercise, we'll look at two additional, more
16068 complicated pipelines. For them, we need to introduce two more tools.
16070 The first is the @command{tr} command, which stands for ``transliterate.''
16071 The @command{tr} command works on a character-by-character basis, changing
16072 characters. Normally it is used for things like mapping upper case to
16076 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16077 @print{} this example has mixed case!
16080 There are several options of interest:
16084 work on the complement of the listed characters, i.e.,
16085 operations apply to characters not in the given set
16088 delete characters in the first set from the output
16091 squeeze repeated characters in the output into just one character.
16094 We will be using all three options in a moment.
16096 The other command we'll look at is @command{comm}. The @command{comm}
16097 command takes two sorted input files as input data, and prints out the
16098 files' lines in three columns. The output columns are the data lines
16099 unique to the first file, the data lines unique to the second file, and
16100 the data lines that are common to both. The @option{-1}, @option{-2}, and
16101 @option{-3} command line options @emph{omit} the respective columns. (This is
16102 non-intuitive and takes a little getting used to.) For example:
16124 The file name @file{-} tells @command{comm} to read standard input
16125 instead of a regular file.
16127 Now we're ready to build a fancy pipeline. The first application is a word
16128 frequency counter. This helps an author determine if he or she is over-using
16131 The first step is to change the case of all the letters in our input file
16132 to one case. ``The'' and ``the'' are the same word when doing counting.
16135 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16138 The next step is to get rid of punctuation. Quoted words and unquoted words
16139 should be treated identically; it's easiest to just get the punctuation out of
16143 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16146 The second @command{tr} command operates on the complement of the listed
16147 characters, which are all the letters, the digits, the underscore, and
16148 the blank. The @samp{\n} represents the newline character; it has to
16149 be left alone. (The @acronym{ASCII} tab character should also be included for
16150 good measure in a production script.)
16152 At this point, we have data consisting of words separated by blank space.
16153 The words only contain alphanumeric characters (and the underscore). The
16154 next step is break the data apart so that we have one word per line. This
16155 makes the counting operation much easier, as we will see shortly.
16158 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16159 > tr -s ' ' '\n' | ...
16162 This command turns blanks into newlines. The @option{-s} option squeezes
16163 multiple newline characters in the output into just one. This helps us
16164 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16165 This is what the shell prints when it notices you haven't finished
16166 typing in all of a command.)
16168 We now have data consisting of one word per line, no punctuation, all one
16169 case. We're ready to count each word:
16172 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16173 > tr -s ' ' '\n' | sort | uniq -c | ...
16176 At this point, the data might look something like this:
16189 The output is sorted by word, not by count! What we want is the most
16190 frequently used words first. Fortunately, this is easy to accomplish,
16191 with the help of two more @command{sort} options:
16195 do a numeric sort, not a textual one
16198 reverse the order of the sort
16201 The final pipeline looks like this:
16204 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16205 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16214 Whew! That's a lot to digest. Yet, the same principles apply. With six
16215 commands, on two lines (really one long one split for convenience), we've
16216 created a program that does something interesting and useful, in much
16217 less time than we could have written a C program to do the same thing.
16219 A minor modification to the above pipeline can give us a simple spelling
16220 checker! To determine if you've spelled a word correctly, all you have to
16221 do is look it up in a dictionary. If it is not there, then chances are
16222 that your spelling is incorrect. So, we need a dictionary.
16223 The conventional location for a dictionary is @file{/usr/dict/words}.
16224 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16225 revision of this article.}
16226 this is a sorted, 45,402 word dictionary.
16228 Now, how to compare our file with the dictionary? As before, we generate
16229 a sorted list of words, one per line:
16232 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16233 > tr -s ' ' '\n' | sort -u | ...
16236 Now, all we need is a list of words that are @emph{not} in the
16237 dictionary. Here is where the @command{comm} command comes in.
16240 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16241 > tr -s ' ' '\n' | sort -u |
16242 > comm -23 - /usr/dict/words
16245 The @option{-2} and @option{-3} options eliminate lines that are only in the
16246 dictionary (the second file), and lines that are in both files. Lines
16247 only in the first file (standard input, our stream of words), are
16248 words that are not in the dictionary. These are likely candidates for
16249 spelling errors. This pipeline was the first cut at a production
16250 spelling checker on Unix.
16252 There are some other tools that deserve brief mention.
16256 search files for text that matches a regular expression
16259 count lines, words, characters
16262 a T-fitting for data pipes, copies data to files and to standard output
16265 the stream editor, an advanced tool
16268 a data manipulation language, another advanced tool
16271 The software tools philosophy also espoused the following bit of
16272 advice: ``Let someone else do the hard part.'' This means, take
16273 something that gives you most of what you need, and then massage it the
16274 rest of the way until it's in the form that you want.
16280 Each program should do one thing well. No more, no less.
16283 Combining programs with appropriate plumbing leads to results where
16284 the whole is greater than the sum of the parts. It also leads to novel
16285 uses of programs that the authors might never have imagined.
16288 Programs should never print extraneous header or trailer data, since these
16289 could get sent on down a pipeline. (A point we didn't mention earlier.)
16292 Let someone else do the hard part.
16295 Know your toolbox! Use each program appropriately. If you don't have an
16296 appropriate tool, build one.
16299 As of this writing, all the programs we've discussed are available via
16300 anonymous @command{ftp} from: @*
16301 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16302 be more recent versions available now.)
16304 None of what I have presented in this column is new. The Software Tools
16305 philosophy was first introduced in the book @cite{Software Tools}, by
16306 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16307 This book showed how to write and use software tools. It was written in
16308 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16309 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16310 was. The last chapter presented a @command{ratfor} to FORTRAN
16311 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16312 lot like C; if you know C, you won't have any problem following the
16315 In 1981, the book was updated and made available as @cite{Software Tools
16316 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16317 still in print and are well worth
16318 reading if you're a programmer. They certainly made a major change in
16319 how I view programming.
16321 The programs in both books are available from
16322 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16323 For a number of years, there was an active
16324 Software Tools Users Group, whose members had ported the original
16325 @command{ratfor} programs to essentially every computer system with a
16326 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16327 as Unix began to spread beyond universities.
16329 With the current proliferation of GNU code and other clones of Unix programs,
16330 these programs now receive little attention; modern C versions are
16331 much more efficient and do more than these programs do. Nevertheless, as
16332 exposition of good programming style, and evangelism for a still-valuable
16333 philosophy, these books are unparalleled, and I recommend them highly.
16335 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16336 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16338 @node GNU Free Documentation License
16339 @appendix GNU Free Documentation License
16343 @node Concept index
16350 @c Local variables:
16351 @c texinfo-column-for-description: 32