3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into fixed-size pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2010 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of get_date:: Bellovin, Eggert, Salz, Berets, et al
479 Opening the software toolbox
481 * Toolbox introduction:: Toolbox introduction
482 * I/O redirection:: I/O redirection
483 * The who command:: The @command{who} command
484 * The cut command:: The @command{cut} command
485 * The sort command:: The @command{sort} command
486 * The uniq command:: The @command{uniq} command
487 * Putting the tools together:: Putting the tools together
491 * GNU Free Documentation License:: Copying and sharing this manual
498 @chapter Introduction
500 This manual is a work in progress: many sections make no attempt to explain
501 basic concepts in a way suitable for novices. Thus, if you are interested,
502 please get involved in improving this manual. The entire @sc{gnu} community
505 @cindex @acronym{POSIX}
506 The @sc{gnu} utilities documented here are mostly compatible with the
507 @acronym{POSIX} standard.
508 @cindex bugs, reporting
509 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
510 to include the version number, machine architecture, input files, and
511 any other information needed to reproduce the bug: your input, what you
512 expected, what you got, and why it is wrong. Diffs are welcome, but
513 please include a description of the problem as well, since this is
514 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
520 @cindex MacKenzie, D.
523 This manual was originally derived from the Unix man pages in the
524 distributions, which were written by David MacKenzie and updated by Jim
525 Meyering. What you are reading now is the authoritative documentation
526 for these utilities; the man pages are no longer being maintained. The
527 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
528 Pinard did the initial conversion to Texinfo format. Karl Berry did the
529 indexing, some reorganization, and editing of the results. Brian
530 Youmans of the Free Software Foundation office staff combined the
531 manuals for textutils, fileutils, and sh-utils to produce the present
532 omnibus manual. Richard Stallman contributed his usual invaluable
533 insights to the overall process.
536 @chapter Common options
540 @itemx @w{@kbd{--backup}[=@var{method}]}
543 @vindex VERSION_CONTROL
544 @cindex backups, making
545 @xref{Backup options}.
546 Make a backup of each file that would otherwise be overwritten or removed.
549 @macro optBackupSuffix
550 @item -S @var{suffix}
551 @itemx --suffix=@var{suffix}
554 Append @var{suffix} to each backup file made with @option{-b}.
555 @xref{Backup options}.
558 @macro optTargetDirectory
559 @item -t @var{directory}
560 @itemx @w{@kbd{--target-directory}=@var{directory}}
562 @opindex --target-directory
563 @cindex target directory
564 @cindex destination directory
565 Specify the destination @var{directory}.
566 @xref{Target directory}.
569 @macro optNoTargetDirectory
571 @itemx --no-target-directory
573 @opindex --no-target-directory
574 @cindex target directory
575 @cindex destination directory
576 Do not treat the last operand specially when it is a directory or a
577 symbolic link to a directory. @xref{Target directory}.
585 @cindex output @sc{nul}-byte-terminated lines
586 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
587 rather than a newline. This option enables other programs to parse the
588 output of @command{\cmd\} even when that output would contain data
589 with embedded newlines.
596 Append an SI-style abbreviation to each size, such as @samp{M} for
597 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
598 1,000,000 bytes. This option is equivalent to
599 @option{--block-size=si}. Use the @option{-h} or
600 @option{--human-readable} option if
601 you prefer powers of 1024.
604 @macro optHumanReadable
606 @itemx --human-readable
608 @opindex --human-readable
609 @cindex human-readable output
610 Append a size letter to each size, such as @samp{M} for mebibytes.
611 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
612 This option is equivalent to @option{--block-size=human-readable}.
613 Use the @option{--si} option if you prefer powers of 1000.
616 @macro optStripTrailingSlashes
617 @itemx @w{@kbd{--strip-trailing-slashes}}
618 @opindex --strip-trailing-slashes
619 @cindex stripping trailing slashes
620 Remove any trailing slashes from each @var{source} argument.
621 @xref{Trailing slashes}.
624 @macro mayConflictWithShellBuiltIn{cmd}
625 @cindex conflicts with shell built-ins
626 @cindex built-in shell commands, conflicts with
627 Due to shell aliases and built-in @command{\cmd\} command, using an
628 unadorned @command{\cmd\} interactively or in a script may get you
629 different functionality than that described here. Invoke it via
630 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
635 @macro multiplierSuffixes{varName}
636 @var{\varName\} may be, or may be an integer optionally followed by,
637 one of the following multiplicative suffixes:
639 @samp{b} => 512 ("blocks")
640 @samp{KB} => 1000 (KiloBytes)
641 @samp{K} => 1024 (KibiBytes)
642 @samp{MB} => 1000*1000 (MegaBytes)
643 @samp{M} => 1024*1024 (MebiBytes)
644 @samp{GB} => 1000*1000*1000 (GigaBytes)
645 @samp{G} => 1024*1024*1024 (GibiBytes)
647 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
650 @c FIXME: same as above, but no ``blocks'' line.
651 @macro multiplierSuffixesNoBlocks{varName}
652 @var{\varName\} may be, or may be an integer optionally followed by,
653 one of the following multiplicative suffixes:
655 @samp{KB} => 1000 (KiloBytes)
656 @samp{K} => 1024 (KibiBytes)
657 @samp{MB} => 1000*1000 (MegaBytes)
658 @samp{M} => 1024*1024 (MebiBytes)
659 @samp{GB} => 1000*1000*1000 (GigaBytes)
660 @samp{G} => 1024*1024*1024 (GibiBytes)
662 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
665 @cindex common options
667 Certain options are available in all of these programs. Rather than
668 writing identical descriptions for each of the programs, they are
669 described here. (In fact, every @sc{gnu} program accepts (or should accept)
672 @vindex POSIXLY_CORRECT
673 Normally options and operands can appear in any order, and programs act
674 as if all the options appear before any operands. For example,
675 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
676 @samp{:} is an option-argument of @option{-t}. However, if the
677 @env{POSIXLY_CORRECT} environment variable is set, options must appear
678 before operands, unless otherwise specified for a particular command.
680 A few programs can usefully have trailing operands with leading
681 @samp{-}. With such a program, options must precede operands even if
682 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
683 program description. For example, the @command{env} command's options
684 must appear before its operands, since in some cases the operands
685 specify a command that itself contains options.
687 Most programs that accept long options recognize unambiguous
688 abbreviations of those options. For example, @samp{rmdir
689 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
690 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
691 @samp{ls --h}, are identified as such.
693 Some of these programs recognize the @option{--help} and @option{--version}
694 options only when one of them is the sole command line argument. For
695 these programs, abbreviations of the long options are not always recognized.
702 Print a usage message listing all available options, then exit successfully.
706 @cindex version number, finding
707 Print the version number, then exit successfully.
711 @cindex option delimiter
712 Delimit the option list. Later arguments, if any, are treated as
713 operands even if they begin with @samp{-}. For example, @samp{sort --
714 -r} reads from the file named @file{-r}.
718 @cindex standard input
719 @cindex standard output
720 A single @samp{-} operand is not really an option, though it looks like one. It
721 stands for standard input, or for standard output if that is clear from
722 the context. For example, @samp{sort -} reads from standard input,
723 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
724 extra copy of its input to standard output. Unless otherwise
725 specified, @samp{-} can appear as any operand that requires a file
729 * Exit status:: Indicating program success or failure.
730 * Backup options:: -b -S, in some programs.
731 * Block size:: BLOCK_SIZE and --block-size, in some programs.
732 * Signal specifications:: Specifying signals using the --signal option.
733 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
734 * Random sources:: --random-source, in some programs.
735 * Target directory:: Specifying a target directory, in some programs.
736 * Trailing slashes:: --strip-trailing-slashes, in some programs.
737 * Traversing symlinks:: -H, -L, or -P, in some programs.
738 * Treating / specially:: --preserve-root and --no-preserve-root.
739 * Special built-in utilities:: @command{break}, @command{:}, @command{eval}, @dots{}
740 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
748 An exit status of zero indicates success,
749 and a nonzero value indicates failure.
752 Nearly every command invocation yields an integral @dfn{exit status}
753 that can be used to change how other commands work.
754 For the vast majority of commands, an exit status of zero indicates
755 success. Failure is indicated by a nonzero value---typically
756 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
757 requires only that it be nonzero.
759 However, some of the programs documented here do produce
760 other exit status values and a few associate different
761 meanings with the values @samp{0} and @samp{1}.
762 Here are some of the exceptions:
763 @command{chroot}, @command{env}, @command{expr}, @command{nice},
764 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
765 @command{su}, @command{test}, @command{timeout}, @command{tty}.
769 @section Backup options
771 @cindex backup options
773 Some @sc{gnu} programs (at least @command{cp}, @command{install},
774 @command{ln}, and @command{mv}) optionally make backups of files
775 before writing new versions.
776 These options control the details of these backups. The options are also
777 briefly mentioned in the descriptions of the particular programs.
782 @itemx @w{@kbd{--backup}[=@var{method}]}
785 @vindex VERSION_CONTROL
786 @cindex backups, making
787 Make a backup of each file that would otherwise be overwritten or removed.
788 Without this option, the original versions are destroyed.
789 Use @var{method} to determine the type of backups to make.
790 When this option is used but @var{method} is not specified,
791 then the value of the @env{VERSION_CONTROL}
792 environment variable is used. And if @env{VERSION_CONTROL} is not set,
793 the default backup type is @samp{existing}.
795 Note that the short form of this option, @option{-b} does not accept any
796 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
798 @vindex version-control @r{Emacs variable}
799 This option corresponds to the Emacs variable @samp{version-control};
800 the values for @var{method} are the same as those used in Emacs.
801 This option also accepts more descriptive names.
802 The valid @var{method}s are (unique abbreviations are accepted):
807 @opindex none @r{backup method}
812 @opindex numbered @r{backup method}
813 Always make numbered backups.
817 @opindex existing @r{backup method}
818 Make numbered backups of files that already have them, simple backups
823 @opindex simple @r{backup method}
824 Always make simple backups. Please note @samp{never} is not to be
825 confused with @samp{none}.
829 @item -S @var{suffix}
830 @itemx --suffix=@var{suffix}
833 @cindex backup suffix
834 @vindex SIMPLE_BACKUP_SUFFIX
835 Append @var{suffix} to each backup file made with @option{-b}. If this
836 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
837 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
838 set, the default is @samp{~}, just as in Emacs.
847 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
848 @command{ls}) display sizes in ``blocks''. You can adjust the block size
849 and method of display to make sizes easier to read. The block size
850 used for display is independent of any file system block size.
851 Fractional block counts are rounded up to the nearest integer.
853 @opindex --block-size=@var{size}
856 @vindex DF_BLOCK_SIZE
857 @vindex DU_BLOCK_SIZE
858 @vindex LS_BLOCK_SIZE
859 @vindex POSIXLY_CORRECT@r{, and block size}
861 The default block size is chosen by examining the following environment
862 variables in turn; the first one that is set determines the block size.
867 This specifies the default block size for the @command{df} command.
868 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
869 @env{LS_BLOCK_SIZE} for @command{ls}.
872 This specifies the default block size for all three commands, if the
873 above command-specific environment variables are not set.
876 This specifies the default block size for all values that are normally
877 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
878 command-specific environment variables are set. Unlike the other
879 environment variables, @env{BLOCKSIZE} does not affect values that are
880 normally printed as byte counts, e.g., the file sizes contained in
883 @item POSIXLY_CORRECT
884 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
885 @env{BLOCKSIZE} is set, but this variable is set, the block size
890 If none of the above environment variables are set, the block size
891 currently defaults to 1024 bytes in most contexts, but this number may
892 change in the future. For @command{ls} file sizes, the block size
895 @cindex human-readable output
898 A block size specification can be a positive integer specifying the number
899 of bytes per block, or it can be @code{human-readable} or @code{si} to
900 select a human-readable format. Integers may be followed by suffixes
901 that are upward compatible with the
902 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
903 for decimal multiples and with the
904 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
905 prefixes for binary multiples}.
907 With human-readable formats, output sizes are followed by a size letter
908 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
909 powers of 1024; @samp{M} stands for 1,048,576 bytes.
910 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
911 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
914 A block size specification preceded by @samp{'} causes output sizes to
915 be displayed with thousands separators. The @env{LC_NUMERIC} locale
916 specifies the thousands separator and grouping. For example, in an
917 American English locale, @samp{--block-size="'1kB"} would cause a size
918 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
919 locale, there is no thousands separator so a leading @samp{'} has no
922 An integer block size can be followed by a suffix to specify a
923 multiple of that size. A bare size letter,
924 or one followed by @samp{iB}, specifies
925 a multiple using powers of 1024. A size letter followed by @samp{B}
926 specifies powers of 1000 instead. For example, @samp{1M} and
927 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
928 equivalent to @samp{1000000}.
930 A plain suffix without a preceding integer acts as if @samp{1} were
931 prepended, except that it causes a size indication to be appended to
932 the output. For example, @samp{--block-size="kB"} displays 3000 as
935 The following suffixes are defined. Large sizes like @code{1Y}
936 may be rejected by your computer due to limitations of its arithmetic.
940 @cindex kilobyte, definition of
941 kilobyte: @math{10^3 = 1000}.
945 @cindex kibibyte, definition of
946 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
947 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
948 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
950 @cindex megabyte, definition of
951 megabyte: @math{10^6 = 1,000,000}.
954 @cindex mebibyte, definition of
955 mebibyte: @math{2^{20} = 1,048,576}.
957 @cindex gigabyte, definition of
958 gigabyte: @math{10^9 = 1,000,000,000}.
961 @cindex gibibyte, definition of
962 gibibyte: @math{2^{30} = 1,073,741,824}.
964 @cindex terabyte, definition of
965 terabyte: @math{10^{12} = 1,000,000,000,000}.
968 @cindex tebibyte, definition of
969 tebibyte: @math{2^{40} = 1,099,511,627,776}.
971 @cindex petabyte, definition of
972 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
975 @cindex pebibyte, definition of
976 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
978 @cindex exabyte, definition of
979 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
982 @cindex exbibyte, definition of
983 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
985 @cindex zettabyte, definition of
986 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
989 @math{2^{70} = 1,180,591,620,717,411,303,424}.
990 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
992 @cindex yottabyte, definition of
993 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
996 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
997 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1002 @opindex --block-size
1003 @opindex --human-readable
1006 Block size defaults can be overridden by an explicit
1007 @option{--block-size=@var{size}} option. The @option{-k}
1008 option is equivalent to @option{--block-size=1K}, which
1009 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1010 set. The @option{-h} or @option{--human-readable} option is equivalent to
1011 @option{--block-size=human-readable}. The @option{--si} option is
1012 equivalent to @option{--block-size=si}.
1014 @node Signal specifications
1015 @section Signal specifications
1016 @cindex signals, specifying
1018 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1019 number like @samp{1}, or an exit status of a process terminated by the
1020 signal. A signal name can be given in canonical form or prefixed by
1021 @samp{SIG}. The case of the letters is ignored. The following signal names
1022 and numbers are supported on all @acronym{POSIX} compliant systems:
1028 2. Terminal interrupt.
1034 9. Kill (cannot be caught or ignored).
1042 Other supported signal names have system-dependent corresponding
1043 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1044 support the following signals:
1048 Access to an undefined portion of a memory object.
1050 Child process terminated, stopped, or continued.
1052 Continue executing, if stopped.
1054 Erroneous arithmetic operation.
1056 Illegal Instruction.
1058 Write on a pipe with no one to read it.
1060 Invalid memory reference.
1062 Stop executing (cannot be caught or ignored).
1066 Background process attempting read.
1068 Background process attempting write.
1070 High bandwidth data is available at a socket.
1072 User-defined signal 1.
1074 User-defined signal 2.
1078 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1079 also support the following signals:
1085 Profiling timer expired.
1089 Trace/breakpoint trap.
1091 Virtual timer expired.
1093 CPU time limit exceeded.
1095 File size limit exceeded.
1099 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1100 also support at least eight real-time signals called @samp{RTMIN},
1101 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1103 @node Disambiguating names and IDs
1104 @section chown and chgrp: Disambiguating user names and IDs
1105 @cindex user names, disambiguating
1106 @cindex user IDs, disambiguating
1107 @cindex group names, disambiguating
1108 @cindex group IDs, disambiguating
1109 @cindex disambiguating group names and IDs
1111 Since the @var{owner} and @var{group} arguments to @command{chown} and
1112 @command{chgrp} may be specified as names or numeric IDs, there is an
1114 What if a user or group @emph{name} is a string of digits?
1115 @footnote{Using a number as a user name is common in some environments.}
1116 Should the command interpret it as a user name or as an ID?
1117 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1118 first attempt to resolve the specified string as a name, and
1119 only once that fails, then try to interpret it as an ID.
1120 This is troublesome when you want to specify a numeric ID, say 42,
1121 and it must work even in a pathological situation where
1122 @samp{42} is a user name that maps to some other user ID, say 1000.
1123 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1124 1000---not what you intended.
1126 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1127 that at the same time may result in a significant performance improvement
1128 by eliminating a database look-up.
1129 Simply precede each numeric user ID and/or group ID with a @samp{+},
1130 in order to force its interpretation as an integer:
1134 chgrp +$numeric_group_id another-file
1138 GNU @command{chown} and @command{chgrp}
1139 skip the name look-up process for each @samp{+}-prefixed string,
1140 because a string containing @samp{+} is never a valid user or group name.
1141 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1143 @node Random sources
1144 @section Sources of random data
1146 @cindex random sources
1148 The @command{shuf}, @command{shred}, and @command{sort} commands
1149 sometimes need random data to do their work. For example, @samp{sort
1150 -R} must choose a hash function at random, and it needs random data to
1151 make this selection.
1153 By default these commands use an internal pseudorandom generator
1154 initialized by a small amount of entropy, but can be directed to use
1155 an external source with the @option{--random-source=@var{file}} option.
1156 An error is reported if @var{file} does not contain enough bytes.
1158 For example, the device file @file{/dev/urandom} could be used as the
1159 source of random data. Typically, this device gathers environmental
1160 noise from device drivers and other sources into an entropy pool, and
1161 uses the pool to generate random bits. If the pool is short of data,
1162 the device reuses the internal pool to produce more bits, using a
1163 cryptographically secure pseudorandom number generator. But be aware
1164 that this device is not designed for bulk random data generation
1165 and is relatively slow.
1167 @file{/dev/urandom} suffices for most practical uses, but applications
1168 requiring high-value or long-term protection of private data may
1169 require an alternate data source like @file{/dev/random} or
1170 @file{/dev/arandom}. The set of available sources depends on your
1173 To reproduce the results of an earlier invocation of a command, you
1174 can save some random data into a file and then use that file as the
1175 random source in earlier and later invocations of the command.
1177 @node Target directory
1178 @section Target directory
1180 @cindex target directory
1182 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1183 commands normally treat the last operand specially when it is a
1184 directory or a symbolic link to a directory. For example, @samp{cp
1185 source dest} is equivalent to @samp{cp source dest/source} if
1186 @file{dest} is a directory. Sometimes this behavior is not exactly
1187 what is wanted, so these commands support the following options to
1188 allow more fine-grained control:
1193 @itemx --no-target-directory
1194 @opindex --no-target-directory
1195 @cindex target directory
1196 @cindex destination directory
1197 Do not treat the last operand specially when it is a directory or a
1198 symbolic link to a directory. This can help avoid race conditions in
1199 programs that operate in a shared area. For example, when the command
1200 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1201 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1202 renamed to @file{/tmp/dest/source} instead, if some other process
1203 created @file{/tmp/dest} as a directory. However, if @file{mv
1204 -T /tmp/source /tmp/dest} succeeds, there is no
1205 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1207 In the opposite situation, where you want the last operand to be
1208 treated as a directory and want a diagnostic otherwise, you can use
1209 the @option{--target-directory} (@option{-t}) option.
1211 @item -t @var{directory}
1212 @itemx @w{@kbd{--target-directory}=@var{directory}}
1213 @opindex --target-directory
1214 @cindex target directory
1215 @cindex destination directory
1216 Use @var{directory} as the directory component of each destination
1219 The interface for most programs is that after processing options and a
1220 finite (possibly zero) number of fixed-position arguments, the remaining
1221 argument list is either expected to be empty, or is a list of items
1222 (usually files) that will all be handled identically. The @command{xargs}
1223 program is designed to work well with this convention.
1225 The commands in the @command{mv}-family are unusual in that they take
1226 a variable number of arguments with a special case at the @emph{end}
1227 (namely, the target directory). This makes it nontrivial to perform some
1228 operations, e.g., ``move all files from here to ../d/'', because
1229 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1230 doesn't have a clean way to specify an extra final argument for each
1231 invocation of the subject command. (It can be done by going through a
1232 shell command, but that requires more human labor and brain power than
1235 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1236 @command{install}, @command{ln}, and @command{mv} programs to be used
1237 conveniently with @command{xargs}. For example, you can move the files
1238 from the current directory to a sibling directory, @code{d} like this:
1241 ls | xargs mv -t ../d --
1244 However, this doesn't move files whose names begin with @samp{.}.
1245 If you use the @sc{gnu} @command{find} program, you can move those
1246 files too, with this command:
1249 find . -mindepth 1 -maxdepth 1 \
1253 But both of the above approaches fail if there are no files in the
1254 current directory, or if any file has a name containing a blank or
1255 some other special characters.
1256 The following example removes those limitations and requires both
1257 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1260 find . -mindepth 1 -maxdepth 1 -print0 \
1261 | xargs --null --no-run-if-empty \
1268 The @option{--target-directory} (@option{-t}) and
1269 @option{--no-target-directory} (@option{-T})
1270 options cannot be combined.
1272 @node Trailing slashes
1273 @section Trailing slashes
1275 @cindex trailing slashes
1277 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1278 remove any trailing slashes from each @var{source} argument before
1279 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1282 This is useful when a @var{source} argument may have a trailing slash and
1283 @c FIXME: mv's behavior in this case is system-dependent
1284 specify a symbolic link to a directory. This scenario is in fact rather
1285 common because some shells can automatically append a trailing slash when
1286 performing file name completion on such symbolic links. Without this
1287 option, @command{mv}, for example, (via the system's rename function) must
1288 interpret a trailing slash as a request to dereference the symbolic link
1289 and so must rename the indirectly referenced @emph{directory} and not
1290 the symbolic link. Although it may seem surprising that such behavior
1291 be the default, it is required by @acronym{POSIX} and is consistent with
1292 other parts of that standard.
1294 @node Traversing symlinks
1295 @section Traversing symlinks
1297 @cindex symbolic link to directory, controlling traversal of
1299 The following options modify how @command{chown} and @command{chgrp}
1300 @c FIXME: note that `du' has these options, too, but they have slightly
1301 @c different meaning.
1302 traverse a hierarchy when the @option{--recursive} (@option{-R})
1303 option is also specified.
1304 If more than one of the following options is specified, only the final
1306 These options specify whether processing a symbolic link to a directory
1307 entails operating on just the symbolic link or on all files in the
1308 hierarchy rooted at that directory.
1310 These options are independent of @option{--dereference} and
1311 @option{--no-dereference} (@option{-h}), which control whether to modify
1312 a symlink or its referent.
1319 @cindex symbolic link to directory, traverse each that is specified on the command line
1320 If @option{--recursive} (@option{-R}) is specified and
1321 a command line argument is a symbolic link to a directory, traverse it.
1328 @cindex symbolic link to directory, traverse each that is encountered
1329 In a recursive traversal, traverse every symbolic link to a directory
1330 that is encountered.
1337 @cindex symbolic link to directory, never traverse
1338 Do not traverse any symbolic links.
1339 This is the default if none of @option{-H}, @option{-L},
1340 or @option{-P} is specified.
1347 @node Treating / specially
1348 @section Treating @file{/} specially
1350 Certain commands can operate destructively on entire hierarchies.
1351 For example, if a user with appropriate privileges mistakenly runs
1352 @samp{rm -rf / tmp/junk}, that may remove
1353 all files on the entire system. Since there are so few
1354 legitimate uses for such a command,
1355 @sc{gnu} @command{rm} normally declines to operate on any directory
1356 that resolves to @file{/}. If you really want to try to remove all
1357 the files on your system, you can use the @option{--no-preserve-root}
1358 option, but the default behavior, specified by the
1359 @option{--preserve-option}, is safer for most purposes.
1361 The commands @command{chgrp}, @command{chmod} and @command{chown}
1362 can also operate destructively on entire hierarchies, so they too
1363 support these options. Although, unlike @command{rm}, they don't
1364 actually unlink files, these commands are arguably more dangerous
1365 when operating recursively on @file{/}, since they often work much
1366 more quickly, and hence damage more files before an alert user can
1367 interrupt them. Tradition and @acronym{POSIX} require these commands
1368 to operate recursively on @file{/}, so they default to
1369 @option{--no-preserve-root}, but using the @option{--preserve-root}
1370 option makes them safer for most purposes. For convenience you can
1371 specify @option{--preserve-root} in an alias or in a shell function.
1373 Note that the @option{--preserve-root} option also ensures
1374 that @command{chgrp} and @command{chown} do not modify @file{/}
1375 even when dereferencing a symlink pointing to @file{/}.
1377 @node Special built-in utilities
1378 @section Special built-in utilities
1380 Some programs like @command{nice} can invoke other programs; for
1381 example, the command @samp{nice cat file} invokes the program
1382 @command{cat} by executing the command @samp{cat file}. However,
1383 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1384 this way. For example, the command @samp{nice exit} does not have a
1385 well-defined behavior: it may generate an error message instead of
1388 Here is a list of the special built-in utilities that are standardized
1389 by @acronym{POSIX} 1003.1-2004.
1392 @t{.@: : break continue eval exec exit export readonly
1393 return set shift times trap unset}
1396 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1397 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1398 pwd} do not work as you might expect.
1400 Many shells extend this list. For example, Bash has several extra
1401 special built-in utilities like @command{history}, and
1402 @command{suspend}, and with Bash the command @samp{nice suspend}
1403 generates an error message instead of suspending.
1405 @node Standards conformance
1406 @section Standards conformance
1408 @vindex POSIXLY_CORRECT
1409 In a few cases, the @sc{gnu} utilities' default behavior is
1410 incompatible with the @acronym{POSIX} standard. To suppress these
1411 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1412 variable. Unless you are checking for @acronym{POSIX} conformance, you
1413 probably do not need to define @env{POSIXLY_CORRECT}.
1415 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1416 versions. For example, older versions of @acronym{POSIX} required the
1417 command @samp{sort +1} to sort based on the second and succeeding
1418 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1419 the same command is required to sort the file named @file{+1}, and you
1420 must instead use the command @samp{sort -k 2} to get the field-based
1423 @vindex _POSIX2_VERSION
1424 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1425 that is standard for your system. To cause them to conform to a
1426 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1427 environment variable to a value of the form @var{yyyymm} specifying
1428 the year and month the standard was adopted. Two values are currently
1429 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1430 @acronym{POSIX} 1003.2-1992, and @samp{200112} stands for @acronym{POSIX}
1431 1003.1-2001. For example, if you have a newer system but are running software
1432 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1433 or @samp{tail +10}, you can work around any compatibility problems by setting
1434 @samp{_POSIX2_VERSION=199209} in your environment.
1436 @node Output of entire files
1437 @chapter Output of entire files
1439 @cindex output of entire files
1440 @cindex entire files, output of
1442 These commands read and write entire files, possibly transforming them
1446 * cat invocation:: Concatenate and write files.
1447 * tac invocation:: Concatenate and write files in reverse.
1448 * nl invocation:: Number lines and write files.
1449 * od invocation:: Write files in octal or other formats.
1450 * base64 invocation:: Transform data into printable data.
1453 @node cat invocation
1454 @section @command{cat}: Concatenate and write files
1457 @cindex concatenate and write files
1458 @cindex copying files
1460 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1461 standard input if none are given, to standard output. Synopsis:
1464 cat [@var{option}] [@var{file}]@dots{}
1467 The program accepts the following options. Also see @ref{Common options}.
1475 Equivalent to @option{-vET}.
1478 @itemx --number-nonblank
1480 @opindex --number-nonblank
1481 Number all nonempty output lines, starting with 1.
1485 Equivalent to @option{-vE}.
1490 @opindex --show-ends
1491 Display a @samp{$} after the end of each line.
1497 Number all output lines, starting with 1. This option is ignored
1498 if @option{-b} is in effect.
1501 @itemx --squeeze-blank
1503 @opindex --squeeze-blank
1504 @cindex squeezing empty lines
1505 Suppress repeated adjacent empty lines; output just one empty line
1510 Equivalent to @option{-vT}.
1515 @opindex --show-tabs
1516 Display TAB characters as @samp{^I}.
1520 Ignored; for @acronym{POSIX} compatibility.
1523 @itemx --show-nonprinting
1525 @opindex --show-nonprinting
1526 Display control characters except for LFD and TAB using
1527 @samp{^} notation and precede characters that have the high bit set with
1532 On systems like MS-DOS that distinguish between text and binary files,
1533 @command{cat} normally reads and writes in binary mode. However,
1534 @command{cat} reads in text mode if one of the options
1535 @option{-bensAE} is used or if @command{cat} is reading from standard
1536 input and standard input is a terminal. Similarly, @command{cat}
1537 writes in text mode if one of the options @option{-bensAE} is used or
1538 if standard output is a terminal.
1545 # Output f's contents, then standard input, then g's contents.
1548 # Copy standard input to standard output.
1553 @node tac invocation
1554 @section @command{tac}: Concatenate and write files in reverse
1557 @cindex reversing files
1559 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1560 standard input if none are given, to standard output, reversing the
1561 records (lines by default) in each separately. Synopsis:
1564 tac [@var{option}]@dots{} [@var{file}]@dots{}
1567 @dfn{Records} are separated by instances of a string (newline by
1568 default). By default, this separator string is attached to the end of
1569 the record that it follows in the file.
1571 The program accepts the following options. Also see @ref{Common options}.
1579 The separator is attached to the beginning of the record that it
1580 precedes in the file.
1586 Treat the separator string as a regular expression. Users of @command{tac}
1587 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1588 binary mode, each line of a text file might end with a CR/LF pair
1589 instead of the Unix-style LF.
1591 @item -s @var{separator}
1592 @itemx --separator=@var{separator}
1594 @opindex --separator
1595 Use @var{separator} as the record separator, instead of newline.
1603 @section @command{nl}: Number lines and write files
1606 @cindex numbering lines
1607 @cindex line numbering
1609 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1610 standard input if none are given, to standard output, with line numbers
1611 added to some or all of the lines. Synopsis:
1614 nl [@var{option}]@dots{} [@var{file}]@dots{}
1617 @cindex logical pages, numbering on
1618 @command{nl} decomposes its input into (logical) pages; by default, the
1619 line number is reset to 1 at the top of each logical page. @command{nl}
1620 treats all of the input files as a single document; it does not reset
1621 line numbers or logical pages between files.
1623 @cindex headers, numbering
1624 @cindex body, numbering
1625 @cindex footers, numbering
1626 A logical page consists of three sections: header, body, and footer.
1627 Any of the sections can be empty. Each can be numbered in a different
1628 style from the others.
1630 The beginnings of the sections of logical pages are indicated in the
1631 input file by a line containing exactly one of these delimiter strings:
1642 The two characters from which these strings are made can be changed from
1643 @samp{\} and @samp{:} via options (see below), but the pattern and
1644 length of each string cannot be changed.
1646 A section delimiter is replaced by an empty line on output. Any text
1647 that comes before the first section delimiter string in the input file
1648 is considered to be part of a body section, so @command{nl} treats a
1649 file that contains no section delimiters as a single body section.
1651 The program accepts the following options. Also see @ref{Common options}.
1655 @item -b @var{style}
1656 @itemx --body-numbering=@var{style}
1658 @opindex --body-numbering
1659 Select the numbering style for lines in the body section of each
1660 logical page. When a line is not numbered, the current line number
1661 is not incremented, but the line number separator character is still
1662 prepended to the line. The styles are:
1668 number only nonempty lines (default for body),
1670 do not number lines (default for header and footer),
1672 number only lines that contain a match for the basic regular
1673 expression @var{bre}.
1674 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1678 @itemx --section-delimiter=@var{cd}
1680 @opindex --section-delimiter
1681 @cindex section delimiters of pages
1682 Set the section delimiter characters to @var{cd}; default is
1683 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1684 (Remember to protect @samp{\} or other metacharacters from shell
1685 expansion with quotes or extra backslashes.)
1687 @item -f @var{style}
1688 @itemx --footer-numbering=@var{style}
1690 @opindex --footer-numbering
1691 Analogous to @option{--body-numbering}.
1693 @item -h @var{style}
1694 @itemx --header-numbering=@var{style}
1696 @opindex --header-numbering
1697 Analogous to @option{--body-numbering}.
1699 @item -i @var{number}
1700 @itemx --line-increment=@var{number}
1702 @opindex --line-increment
1703 Increment line numbers by @var{number} (default 1).
1705 @item -l @var{number}
1706 @itemx --join-blank-lines=@var{number}
1708 @opindex --join-blank-lines
1709 @cindex empty lines, numbering
1710 @cindex blank lines, numbering
1711 Consider @var{number} (default 1) consecutive empty lines to be one
1712 logical line for numbering, and only number the last one. Where fewer
1713 than @var{number} consecutive empty lines occur, do not number them.
1714 An empty line is one that contains no characters, not even spaces
1717 @item -n @var{format}
1718 @itemx --number-format=@var{format}
1720 @opindex --number-format
1721 Select the line numbering format (default is @code{rn}):
1725 @opindex ln @r{format for @command{nl}}
1726 left justified, no leading zeros;
1728 @opindex rn @r{format for @command{nl}}
1729 right justified, no leading zeros;
1731 @opindex rz @r{format for @command{nl}}
1732 right justified, leading zeros.
1736 @itemx --no-renumber
1738 @opindex --no-renumber
1739 Do not reset the line number at the start of a logical page.
1741 @item -s @var{string}
1742 @itemx --number-separator=@var{string}
1744 @opindex --number-separator
1745 Separate the line number from the text line in the output with
1746 @var{string} (default is the TAB character).
1748 @item -v @var{number}
1749 @itemx --starting-line-number=@var{number}
1751 @opindex --starting-line-number
1752 Set the initial line number on each logical page to @var{number} (default 1).
1754 @item -w @var{number}
1755 @itemx --number-width=@var{number}
1757 @opindex --number-width
1758 Use @var{number} characters for line numbers (default 6).
1766 @section @command{od}: Write files in octal or other formats
1769 @cindex octal dump of files
1770 @cindex hex dump of files
1771 @cindex ASCII dump of files
1772 @cindex file contents, dumping unambiguously
1774 @command{od} writes an unambiguous representation of each @var{file}
1775 (@samp{-} means standard input), or standard input if none are given.
1779 od [@var{option}]@dots{} [@var{file}]@dots{}
1780 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1781 od [@var{option}]@dots{} --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1784 Each line of output consists of the offset in the input, followed by
1785 groups of data from the file. By default, @command{od} prints the offset in
1786 octal, and each group of file data is a C @code{short int}'s worth of input
1787 printed as a single octal number.
1789 If @var{offset} is given, it specifies how many input bytes to skip
1790 before formatting and writing. By default, it is interpreted as an
1791 octal number, but the optional trailing decimal point causes it to be
1792 interpreted as decimal. If no decimal is specified and the offset
1793 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1794 number. If there is a trailing @samp{b}, the number of bytes skipped
1795 will be @var{offset} multiplied by 512.
1797 If a command is of both the first and second forms, the second form is
1798 assumed if the last operand begins with @samp{+} or (if there are two
1799 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1800 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1803 The program accepts the following options. Also see @ref{Common options}.
1807 @item -A @var{radix}
1808 @itemx --address-radix=@var{radix}
1810 @opindex --address-radix
1811 @cindex radix for file offsets
1812 @cindex file offset radix
1813 Select the base in which file offsets are printed. @var{radix} can
1814 be one of the following:
1824 none (do not print offsets).
1827 The default is octal.
1829 @item -j @var{bytes}
1830 @itemx --skip-bytes=@var{bytes}
1832 @opindex --skip-bytes
1833 Skip @var{bytes} input bytes before formatting and writing. If
1834 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1835 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1837 @multiplierSuffixes{bytes}
1839 @item -N @var{bytes}
1840 @itemx --read-bytes=@var{bytes}
1842 @opindex --read-bytes
1843 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1844 @code{bytes} are interpreted as for the @option{-j} option.
1846 @item -S @var{bytes}
1847 @itemx --strings[=@var{bytes}]
1850 @cindex string constants, outputting
1851 Instead of the normal output, output only @dfn{string constants}: at
1852 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1853 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1854 Prefixes and suffixes on @code{bytes} are interpreted as for the
1857 If @var{n} is omitted with @option{--strings}, the default is 3.
1860 @itemx --format=@var{type}
1863 Select the format in which to output the file data. @var{type} is a
1864 string of one or more of the below type indicator characters. If you
1865 include more than one type indicator character in a single @var{type}
1866 string, or use this option more than once, @command{od} writes one copy
1867 of each output line using each of the data types that you specified,
1868 in the order that you specified.
1870 Adding a trailing ``z'' to any type specification appends a display
1871 of the @acronym{ASCII} character representation of the printable characters
1872 to the output line generated by the type specification.
1876 named character, ignoring high-order bit
1878 @acronym{ASCII} character or backslash escape,
1891 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1892 newline, and @samp{nul} for a zero byte. Only the least significant
1893 seven bits of each byte is used; the high-order bit is ignored.
1894 Type @code{c} outputs
1895 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1898 Except for types @samp{a} and @samp{c}, you can specify the number
1899 of bytes to use in interpreting each number in the given data type
1900 by following the type indicator character with a decimal integer.
1901 Alternately, you can specify the size of one of the C compiler's
1902 built-in data types by following the type indicator character with
1903 one of the following characters. For integers (@samp{d}, @samp{o},
1904 @samp{u}, @samp{x}):
1917 For floating point (@code{f}):
1929 @itemx --output-duplicates
1931 @opindex --output-duplicates
1932 Output consecutive lines that are identical. By default, when two or
1933 more consecutive output lines would be identical, @command{od} outputs only
1934 the first line, and puts just an asterisk on the following line to
1935 indicate the elision.
1938 @itemx --width[=@var{n}]
1941 Dump @code{n} input bytes per output line. This must be a multiple of
1942 the least common multiple of the sizes associated with the specified
1945 If this option is not given at all, the default is 16. If @var{n} is
1946 omitted, the default is 32.
1950 The next several options are shorthands for format specifications.
1951 @sc{gnu} @command{od} accepts any combination of shorthands and format
1952 specification options. These options accumulate.
1958 Output as named characters. Equivalent to @samp{-t a}.
1962 Output as octal bytes. Equivalent to @samp{-t o1}.
1966 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1971 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1975 Output as floats. Equivalent to @samp{-t fF}.
1979 Output as decimal ints. Equivalent to @samp{-t dI}.
1983 Output as decimal long ints. Equivalent to @samp{-t dL}.
1987 Output as octal two-byte units. Equivalent to @option{-t o2}.
1991 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1995 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1998 @opindex --traditional
1999 Recognize the non-option label argument that traditional @command{od}
2000 accepted. The following syntax:
2003 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2007 can be used to specify at most one file and optional arguments
2008 specifying an offset and a pseudo-start address, @var{label}.
2009 The @var{label} argument is interpreted
2010 just like @var{offset}, but it specifies an initial pseudo-address. The
2011 pseudo-addresses are displayed in parentheses following any normal
2018 @node base64 invocation
2019 @section @command{base64}: Transform data into printable data
2022 @cindex base64 encoding
2024 @command{base64} transforms data read from a file, or standard input,
2025 into (or from) base64 encoded form. The base64 encoded form uses
2026 printable @acronym{ASCII} characters to represent binary data.
2030 base64 [@var{option}]@dots{} [@var{file}]
2031 base64 --decode [@var{option}]@dots{} [@var{file}]
2034 The base64 encoding expands data to roughly 133% of the original.
2035 The format conforms to
2036 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2038 The program accepts the following options. Also see @ref{Common options}.
2043 @itemx --wrap=@var{cols}
2047 @cindex column to wrap data after
2048 During encoding, wrap lines after @var{cols} characters. This must be
2051 The default is to wrap after 76 characters. Use the value 0 to
2052 disable line wrapping altogether.
2058 @cindex Decode base64 data
2059 @cindex Base64 decoding
2060 Change the mode of operation, from the default of encoding data, to
2061 decoding data. Input is expected to be base64 encoded data, and the
2062 output will be the original data.
2065 @itemx --ignore-garbage
2067 @opindex --ignore-garbage
2068 @cindex Ignore garbage in base64 stream
2069 When decoding, newlines are always accepted.
2070 During decoding, ignore unrecognized bytes,
2071 to permit distorted data to be decoded.
2078 @node Formatting file contents
2079 @chapter Formatting file contents
2081 @cindex formatting file contents
2083 These commands reformat the contents of files.
2086 * fmt invocation:: Reformat paragraph text.
2087 * pr invocation:: Paginate or columnate files for printing.
2088 * fold invocation:: Wrap input lines to fit in specified width.
2092 @node fmt invocation
2093 @section @command{fmt}: Reformat paragraph text
2096 @cindex reformatting paragraph text
2097 @cindex paragraphs, reformatting
2098 @cindex text, reformatting
2100 @command{fmt} fills and joins lines to produce output lines of (at most)
2101 a given number of characters (75 by default). Synopsis:
2104 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2107 @command{fmt} reads from the specified @var{file} arguments (or standard
2108 input if none are given), and writes to standard output.
2110 By default, blank lines, spaces between words, and indentation are
2111 preserved in the output; successive input lines with different
2112 indentation are not joined; tabs are expanded on input and introduced on
2115 @cindex line-breaking
2116 @cindex sentences and line-breaking
2117 @cindex Knuth, Donald E.
2118 @cindex Plass, Michael F.
2119 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2120 avoid line breaks after the first word of a sentence or before the last
2121 word of a sentence. A @dfn{sentence break} is defined as either the end
2122 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2123 spaces or end of line, ignoring any intervening parentheses or quotes.
2124 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2125 breaks; the algorithm is a variant of that given by Donald E. Knuth
2126 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2127 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2130 The program accepts the following options. Also see @ref{Common options}.
2135 @itemx --crown-margin
2137 @opindex --crown-margin
2138 @cindex crown margin
2139 @dfn{Crown margin} mode: preserve the indentation of the first two
2140 lines within a paragraph, and align the left margin of each subsequent
2141 line with that of the second line.
2144 @itemx --tagged-paragraph
2146 @opindex --tagged-paragraph
2147 @cindex tagged paragraphs
2148 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2149 indentation of the first line of a paragraph is the same as the
2150 indentation of the second, the first line is treated as a one-line
2156 @opindex --split-only
2157 Split lines only. Do not join short lines to form longer ones. This
2158 prevents sample lines of code, and other such ``formatted'' text from
2159 being unduly combined.
2162 @itemx --uniform-spacing
2164 @opindex --uniform-spacing
2165 Uniform spacing. Reduce spacing between words to one space, and spacing
2166 between sentences to two spaces.
2169 @itemx -w @var{width}
2170 @itemx --width=@var{width}
2171 @opindex -@var{width}
2174 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2175 initially tries to make lines about 7% shorter than this, to give it
2176 room to balance line lengths.
2178 @item -p @var{prefix}
2179 @itemx --prefix=@var{prefix}
2180 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2181 are subject to formatting. The prefix and any preceding whitespace are
2182 stripped for the formatting and then re-attached to each formatted output
2183 line. One use is to format certain kinds of program comments, while
2184 leaving the code unchanged.
2192 @section @command{pr}: Paginate or columnate files for printing
2195 @cindex printing, preparing files for
2196 @cindex multicolumn output, generating
2197 @cindex merging files in parallel
2199 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2200 standard input if none are given, to standard output, paginating and
2201 optionally outputting in multicolumn format; optionally merges all
2202 @var{file}s, printing all in parallel, one per column. Synopsis:
2205 pr [@var{option}]@dots{} [@var{file}]@dots{}
2209 By default, a 5-line header is printed at each page: two blank lines;
2210 a line with the date, the file name, and the page count; and two more
2211 blank lines. A footer of five blank lines is also printed.
2212 The default @var{page_length} is 66
2213 lines. The default number of text lines is therefore 56.
2214 The text line of the header takes the form
2215 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2216 @var{string} so that the line takes up the full @var{page_width}. Here,
2217 @var{date} is the date (see the @option{-D} or @option{--date-format}
2218 option for details), @var{string} is the centered header string, and
2219 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2220 category affects the spelling of @var{page}; in the default C locale, it
2221 is @samp{Page @var{number}} where @var{number} is the decimal page
2224 Form feeds in the input cause page breaks in the output. Multiple form
2225 feeds produce empty pages.
2227 Columns are of equal width, separated by an optional string (default
2228 is @samp{space}). For multicolumn output, lines will always be truncated to
2229 @var{page_width} (default 72), unless you use the @option{-J} option.
2231 column output no line truncation occurs by default. Use @option{-W} option to
2232 truncate lines in that case.
2234 The following changes were made in version 1.22i and apply to later
2235 versions of @command{pr}:
2236 @c FIXME: this whole section here sounds very awkward to me. I
2237 @c made a few small changes, but really it all needs to be redone. - Brian
2238 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2243 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2244 redefined for better @acronym{POSIX} compliance. The output of some further
2245 cases has been adapted to other Unix systems. These changes are not
2246 compatible with earlier versions of the program.
2249 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2250 have been introduced to turn off unexpected interferences of small letter
2251 options. The @option{-N} option and the second argument @var{last_page}
2252 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2253 form feeds set in the input files requires the @option{-T} option.
2256 Capital letter options override small letter ones.
2259 Some of the option-arguments (compare @option{-s}, @option{-e},
2260 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2261 preceding option letter (already stated in the @acronym{POSIX} specification).
2264 The program accepts the following options. Also see @ref{Common options}.
2268 @item +@var{first_page}[:@var{last_page}]
2269 @itemx --pages=@var{first_page}[:@var{last_page}]
2270 @c The two following @opindex lines evoke warnings because they contain `:'
2271 @c The `info' spec does not permit that. If we use those lines, we end
2272 @c up with truncated index entries that don't work.
2273 @c @opindex +@var{first_page}[:@var{last_page}]
2274 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2275 @opindex +@var{page_range}
2276 @opindex --pages=@var{page_range}
2277 Begin printing with page @var{first_page} and stop with @var{last_page}.
2278 Missing @samp{:@var{last_page}} implies end of file. While estimating
2279 the number of skipped pages each form feed in the input file results
2280 in a new page. Page counting with and without @samp{+@var{first_page}}
2281 is identical. By default, counting starts with the first page of input
2282 file (not first page printed). Line numbering may be altered by @option{-N}
2286 @itemx --columns=@var{column}
2287 @opindex -@var{column}
2289 @cindex down columns
2290 With each single @var{file}, produce @var{column} columns of output
2291 (default is 1) and print columns down, unless @option{-a} is used. The
2292 column width is automatically decreased as @var{column} increases; unless
2293 you use the @option{-W/-w} option to increase @var{page_width} as well.
2294 This option might well cause some lines to be truncated. The number of
2295 lines in the columns on each page are balanced. The options @option{-e}
2296 and @option{-i} are on for multiple text-column output. Together with
2297 @option{-J} option column alignment and line truncation is turned off.
2298 Lines of full length are joined in a free field format and @option{-S}
2299 option may set field separators. @option{-@var{column}} may not be used
2300 with @option{-m} option.
2306 @cindex across columns
2307 With each single @var{file}, print columns across rather than down. The
2308 @option{-@var{column}} option must be given with @var{column} greater than one.
2309 If a line is too long to fit in a column, it is truncated.
2312 @itemx --show-control-chars
2314 @opindex --show-control-chars
2315 Print control characters using hat notation (e.g., @samp{^G}); print
2316 other nonprinting characters in octal backslash notation. By default,
2317 nonprinting characters are not changed.
2320 @itemx --double-space
2322 @opindex --double-space
2323 @cindex double spacing
2324 Double space the output.
2326 @item -D @var{format}
2327 @itemx --date-format=@var{format}
2328 @cindex time formats
2329 @cindex formatting times
2330 Format header dates using @var{format}, using the same conventions as
2331 for the command @samp{date +@var{format}}; @xref{date invocation}.
2332 Except for directives, which start with
2333 @samp{%}, characters in @var{format} are printed unchanged. You can use
2334 this option to specify an arbitrary string in place of the header date,
2335 e.g., @option{--date-format="Monday morning"}.
2337 @vindex POSIXLY_CORRECT
2339 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2340 @samp{2001-12-04 23:59});
2341 but if the @env{POSIXLY_CORRECT} environment variable is set
2342 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2343 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2344 @samp{Dec@ @ 4 23:59 2001}.
2347 Time stamps are listed according to the time zone rules specified by
2348 the @env{TZ} environment variable, or by the system default rules if
2349 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2350 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2352 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2353 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2355 @opindex --expand-tabs
2357 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2358 the input tab character (default is the TAB character). Second optional
2359 argument @var{in-tabwidth} is the input tab character's width (default
2367 @opindex --form-feed
2368 Use a form feed instead of newlines to separate output pages. This does
2369 not alter the default page length of 66 lines.
2371 @item -h @var{header}
2372 @itemx --header=@var{header}
2375 Replace the file name in the header with the centered string @var{header}.
2376 When using the shell, @var{header} should be quoted and should be
2377 separated from @option{-h} by a space.
2379 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2380 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2382 @opindex --output-tabs
2384 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2385 is the output tab character (default is the TAB character). Second optional
2386 argument @var{out-tabwidth} is the output tab character's width (default
2392 @opindex --join-lines
2393 Merge lines of full length. Used together with the column options
2394 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2395 @option{-W/-w} line truncation;
2396 no column alignment used; may be used with
2397 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2398 (together with @option{-W} and @option{--sep-string})
2399 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2400 @option{-s} along with the three column options.
2403 @item -l @var{page_length}
2404 @itemx --length=@var{page_length}
2407 Set the page length to @var{page_length} (default 66) lines, including
2408 the lines of the header [and the footer]. If @var{page_length} is less
2409 than or equal to 10, the header and footer are omitted, as if the
2410 @option{-t} option had been given.
2416 Merge and print all @var{file}s in parallel, one in each column. If a
2417 line is too long to fit in a column, it is truncated, unless the @option{-J}
2418 option is used. @option{--sep-string[=@var{string}]} may be used.
2420 some @var{file}s (form feeds set) produce empty columns, still marked
2421 by @var{string}. The result is a continuous line numbering and column
2422 marking throughout the whole merged file. Completely empty merged pages
2423 show no separators or line numbers. The default header becomes
2424 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2425 may be used with the @option{-h} or @option{--header} option to fill up
2426 the middle blank part.
2428 @item -n[@var{number-separator}[@var{digits}]]
2429 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2431 @opindex --number-lines
2432 Provide @var{digits} digit line numbering (default for @var{digits} is
2433 5). With multicolumn output the number occupies the first @var{digits}
2434 column positions of each text column or only each line of @option{-m}
2435 output. With single column output the number precedes each line just as
2436 @option{-m} does. Default counting of the line numbers starts with the
2437 first line of the input file (not the first line printed, compare the
2438 @option{--page} option and @option{-N} option).
2439 Optional argument @var{number-separator} is the character appended to
2440 the line number to separate it from the text followed. The default
2441 separator is the TAB character. In a strict sense a TAB is always
2442 printed with single column output only. The TAB width varies
2443 with the TAB position, e.g., with the left @var{margin} specified
2444 by @option{-o} option. With multicolumn output priority is given to
2445 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2446 The TAB width is fixed to the value of the first column and does
2447 not change with different values of left @var{margin}. That means a
2448 fixed number of spaces is always printed in the place of the
2449 @var{number-separator} TAB. The tabification depends upon the output
2452 @item -N @var{line_number}
2453 @itemx --first-line-number=@var{line_number}
2455 @opindex --first-line-number
2456 Start line counting with the number @var{line_number} at first line of
2457 first page printed (in most cases not the first line of the input file).
2459 @item -o @var{margin}
2460 @itemx --indent=@var{margin}
2463 @cindex indenting lines
2465 Indent each line with a margin @var{margin} spaces wide (default is zero).
2466 The total page width is the size of the margin plus the @var{page_width}
2467 set with the @option{-W/-w} option. A limited overflow may occur with
2468 numbered single column output (compare @option{-n} option).
2471 @itemx --no-file-warnings
2473 @opindex --no-file-warnings
2474 Do not print a warning message when an argument @var{file} cannot be
2475 opened. (The exit status will still be nonzero, however.)
2477 @item -s[@var{char}]
2478 @itemx --separator[=@var{char}]
2480 @opindex --separator
2481 Separate columns by a single character @var{char}. The default for
2482 @var{char} is the TAB character without @option{-w} and @samp{no
2483 character} with @option{-w}. Without @option{-s} the default separator
2484 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2485 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2486 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2489 @item -S@var{string}
2490 @itemx --sep-string[=@var{string}]
2492 @opindex --sep-string
2493 Use @var{string} to separate output columns. The @option{-S} option doesn't
2494 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2495 does not affect line truncation or column alignment.
2496 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2498 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2499 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2500 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2503 @itemx --omit-header
2505 @opindex --omit-header
2506 Do not print the usual header [and footer] on each page, and do not fill
2507 out the bottom of pages (with blank lines or a form feed). No page
2508 structure is produced, but form feeds set in the input files are retained.
2509 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2510 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2511 in the input file to 4 spaces but don't make any other changes. Use of
2512 @option{-t} overrides @option{-h}.
2515 @itemx --omit-pagination
2517 @opindex --omit-pagination
2518 Do not print header [and footer]. In addition eliminate all form feeds
2519 set in the input files.
2522 @itemx --show-nonprinting
2524 @opindex --show-nonprinting
2525 Print nonprinting characters in octal backslash notation.
2527 @item -w @var{page_width}
2528 @itemx --width=@var{page_width}
2531 Set page width to @var{page_width} characters for multiple text-column
2532 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2533 off the default page width and any line truncation and column alignment.
2534 Lines of full length are merged, regardless of the column options
2535 set. No @var{page_width} setting is possible with single column output.
2536 A @acronym{POSIX}-compliant formulation.
2538 @item -W @var{page_width}
2539 @itemx --page_width=@var{page_width}
2541 @opindex --page_width
2542 Set the page width to @var{page_width} characters. That's valid with and
2543 without a column option. Text lines are truncated, unless @option{-J}
2544 is used. Together with one of the three column options
2545 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2546 alignment is always used. The separator options @option{-S} or @option{-s}
2547 don't affect the @option{-W} option. Default is 72 characters. Without
2548 @option{-W @var{page_width}} and without any of the column options NO line
2549 truncation is used (defined to keep downward compatibility and to meet
2550 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2551 line is never truncated.
2558 @node fold invocation
2559 @section @command{fold}: Wrap input lines to fit in specified width
2562 @cindex wrapping long input lines
2563 @cindex folding long input lines
2565 @command{fold} writes each @var{file} (@option{-} means standard input), or
2566 standard input if none are given, to standard output, breaking long
2570 fold [@var{option}]@dots{} [@var{file}]@dots{}
2573 By default, @command{fold} breaks lines wider than 80 columns. The output
2574 is split into as many lines as necessary.
2576 @cindex screen columns
2577 @command{fold} counts screen columns by default; thus, a tab may count more
2578 than one column, backspace decreases the column count, and carriage
2579 return sets the column to zero.
2581 The program accepts the following options. Also see @ref{Common options}.
2589 Count bytes rather than columns, so that tabs, backspaces, and carriage
2590 returns are each counted as taking up one column, just like other
2597 Break at word boundaries: the line is broken after the last blank before
2598 the maximum line length. If the line contains no such blanks, the line
2599 is broken at the maximum line length as usual.
2601 @item -w @var{width}
2602 @itemx --width=@var{width}
2605 Use a maximum line length of @var{width} columns instead of 80.
2607 For compatibility @command{fold} supports an obsolete option syntax
2608 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2616 @node Output of parts of files
2617 @chapter Output of parts of files
2619 @cindex output of parts of files
2620 @cindex parts of files, output of
2622 These commands output pieces of the input.
2625 * head invocation:: Output the first part of files.
2626 * tail invocation:: Output the last part of files.
2627 * split invocation:: Split a file into fixed-size pieces.
2628 * csplit invocation:: Split a file into context-determined pieces.
2631 @node head invocation
2632 @section @command{head}: Output the first part of files
2635 @cindex initial part of files, outputting
2636 @cindex first part of files, outputting
2638 @command{head} prints the first part (10 lines by default) of each
2639 @var{file}; it reads from standard input if no files are given or
2640 when given a @var{file} of @option{-}. Synopsis:
2643 head [@var{option}]@dots{} [@var{file}]@dots{}
2646 If more than one @var{file} is specified, @command{head} prints a
2647 one-line header consisting of:
2650 ==> @var{file name} <==
2654 before the output for each @var{file}.
2656 The program accepts the following options. Also see @ref{Common options}.
2661 @itemx --bytes=@var{k}
2664 Print the first @var{k} bytes, instead of initial lines.
2665 However, if @var{k} starts with a @samp{-},
2666 print all but the last @var{k} bytes of each file.
2667 @multiplierSuffixes{k}
2670 @itemx --lines=@var{k}
2673 Output the first @var{k} lines.
2674 However, if @var{k} starts with a @samp{-},
2675 print all but the last @var{k} lines of each file.
2676 Size multiplier suffixes are the same as with the @option{-c} option.
2684 Never print file name headers.
2690 Always print file name headers.
2694 For compatibility @command{head} also supports an obsolete option syntax
2695 @option{-@var{count}@var{options}}, which is recognized only if it is
2696 specified first. @var{count} is a decimal number optionally followed
2697 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2698 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2699 Scripts intended for standard hosts should use @option{-c @var{count}}
2700 or @option{-n @var{count}} instead. If your script must also run on
2701 hosts that support only the obsolete syntax, it is usually simpler to
2702 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2708 @node tail invocation
2709 @section @command{tail}: Output the last part of files
2712 @cindex last part of files, outputting
2714 @command{tail} prints the last part (10 lines by default) of each
2715 @var{file}; it reads from standard input if no files are given or
2716 when given a @var{file} of @samp{-}. Synopsis:
2719 tail [@var{option}]@dots{} [@var{file}]@dots{}
2722 If more than one @var{file} is specified, @command{tail} prints a
2723 one-line header consisting of:
2726 ==> @var{file name} <==
2730 before the output for each @var{file}.
2732 @cindex BSD @command{tail}
2733 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2734 @command{tail} cannot). It also has no @option{-r} option (print in
2735 reverse), since reversing a file is really a different job from printing
2736 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2737 only reverse files that are at most as large as its buffer, which is
2738 typically 32 KiB@. A more reliable and versatile way to reverse files is
2739 the @sc{gnu} @command{tac} command.
2741 The program accepts the following options. Also see @ref{Common options}.
2746 @itemx --bytes=@var{k}
2749 Output the last @var{k} bytes, instead of final lines.
2750 However, if @var{k} starts with a @samp{+}, start printing with the
2751 @var{k}th byte from the start of each file, instead of from the end.
2752 @multiplierSuffixes{k}
2755 @itemx --follow[=@var{how}]
2758 @cindex growing files
2759 @vindex name @r{follow option}
2760 @vindex descriptor @r{follow option}
2761 Loop forever trying to read more characters at the end of the file,
2762 presumably because the file is growing.
2763 If more than one file is given, @command{tail} prints a header whenever it
2764 gets output from a different file, to indicate which file that output is
2767 There are two ways to specify how you'd like to track files with this option,
2768 but that difference is noticeable only when a followed file is removed or
2770 If you'd like to continue to track the end of a growing file even after
2771 it has been unlinked, use @option{--follow=descriptor}. This is the default
2772 behavior, but it is not useful if you're tracking a log file that may be
2773 rotated (removed or renamed, then reopened). In that case, use
2774 @option{--follow=name} to track the named file, perhaps by reopening it
2775 periodically to see if it has been removed and recreated by some other program.
2776 Note that the inotify-based implementation handles this case without
2777 the need for any periodic reopening.
2779 No matter which method you use, if the tracked file is determined to have
2780 shrunk, @command{tail} prints a message saying the file has been truncated
2781 and resumes tracking the end of the file from the newly-determined endpoint.
2783 When a file is removed, @command{tail}'s behavior depends on whether it is
2784 following the name or the descriptor. When following by name, tail can
2785 detect that a file has been removed and gives a message to that effect,
2786 and if @option{--retry} has been specified it will continue checking
2787 periodically to see if the file reappears.
2788 When following a descriptor, tail does not detect that the file has
2789 been unlinked or renamed and issues no message; even though the file
2790 may no longer be accessible via its original name, it may still be
2793 The option values @samp{descriptor} and @samp{name} may be specified only
2794 with the long form of the option, not with @option{-f}.
2796 The @option{-f} option is ignored if
2797 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2798 Likewise, the @option{-f} option has no effect for any
2799 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2803 This option is the same as @option{--follow=name --retry}. That is, tail
2804 will attempt to reopen a file when it is removed. Should this fail, tail
2805 will keep trying until it becomes accessible again.
2809 This option is useful mainly when following by name (i.e., with
2810 @option{--follow=name}).
2811 Without this option, when tail encounters a file that doesn't
2812 exist or is otherwise inaccessible, it reports that fact and
2813 never checks it again.
2815 @itemx --sleep-interval=@var{number}
2816 @opindex --sleep-interval
2817 Change the number of seconds to wait between iterations (the default is 1.0).
2818 During one iteration, every specified file is checked to see if it has
2820 Historical implementations of @command{tail} have required that
2821 @var{number} be an integer. However, GNU @command{tail} accepts
2822 an arbitrary floating point number (using a period before any
2825 @itemx --pid=@var{pid}
2827 When following by name or by descriptor, you may specify the process ID,
2828 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2829 after that process terminates, tail will also terminate. This will
2830 work properly only if the writer and the tailing process are running on
2831 the same machine. For example, to save the output of a build in a file
2832 and to watch the file grow, if you invoke @command{make} and @command{tail}
2833 like this then the tail process will stop when your build completes.
2834 Without this option, you would have had to kill the @code{tail -f}
2838 $ make >& makerr & tail --pid=$! -f makerr
2841 If you specify a @var{pid} that is not in use or that does not correspond
2842 to the process that is writing to the tailed files, then @command{tail}
2843 may terminate long before any @var{file}s stop growing or it may not
2844 terminate until long after the real writer has terminated.
2845 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2846 will print a warning if this is the case.
2848 @itemx --max-unchanged-stats=@var{n}
2849 @opindex --max-unchanged-stats
2850 When tailing a file by name, if there have been @var{n} (default
2851 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2852 iterations for which the file has not changed, then
2853 @code{open}/@code{fstat} the file to determine if that file name is
2854 still associated with the same device/inode-number pair as before.
2855 When following a log file that is rotated, this is approximately the
2856 number of seconds between when tail prints the last pre-rotation lines
2857 and when it prints the lines that have accumulated in the new log file.
2858 This option is meaningful only when following by name.
2861 @itemx --lines=@var{k}
2864 Output the last @var{k} lines.
2865 However, if @var{k} starts with a @samp{+}, start printing with the
2866 @var{k}th line from the start of each file, instead of from the end.
2867 Size multiplier suffixes are the same as with the @option{-c} option.
2875 Never print file name headers.
2881 Always print file name headers.
2885 For compatibility @command{tail} also supports an obsolete usage
2886 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2887 only if it does not conflict with the usage described
2888 above. This obsolete form uses exactly one option and at most one
2889 file. In the option, @var{count} is an optional decimal number optionally
2890 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2891 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2892 which has the same meaning as @option{-f}.
2894 @vindex _POSIX2_VERSION
2895 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2896 the obsolete option syntax with the same meaning as in counts, and
2897 obsolete usage overrides normal usage when the two conflict.
2898 This obsolete behavior can be enabled or disabled with the
2899 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2902 Scripts intended for use on standard hosts should avoid obsolete
2903 syntax and should use @option{-c @var{count}[b]}, @option{-n
2904 @var{count}}, and/or @option{-f} instead. If your script must also
2905 run on hosts that support only the obsolete syntax, you can often
2906 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2907 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2908 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2909 then @dots{}} to decide which syntax to use.
2911 Even if your script assumes the standard behavior, you should still
2912 beware usages whose behaviors differ depending on the @acronym{POSIX}
2913 version. For example, avoid @samp{tail - main.c}, since it might be
2914 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2915 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2916 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2917 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2922 @node split invocation
2923 @section @command{split}: Split a file into fixed-size pieces
2926 @cindex splitting a file into pieces
2927 @cindex pieces, splitting a file into
2929 @command{split} creates output files containing consecutive sections of
2930 @var{input} (standard input if none is given or @var{input} is
2931 @samp{-}). Synopsis:
2934 split [@var{option}] [@var{input} [@var{prefix}]]
2937 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2938 left over for the last section), into each output file.
2940 @cindex output file name prefix
2941 The output files' names consist of @var{prefix} (@samp{x} by default)
2942 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2943 default), such that concatenating the output files in traditional
2944 sorted order by file name produces
2945 the original input file. If the output file names are exhausted,
2946 @command{split} reports an error without deleting the output files
2949 The program accepts the following options. Also see @ref{Common options}.
2953 @item -l @var{lines}
2954 @itemx --lines=@var{lines}
2957 Put @var{lines} lines of @var{input} into each output file.
2959 For compatibility @command{split} also supports an obsolete
2960 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2961 @var{lines}} instead.
2964 @itemx --bytes=@var{size}
2967 Put @var{size} bytes of @var{input} into each output file.
2968 @multiplierSuffixes{size}
2971 @itemx --line-bytes=@var{size}
2973 @opindex --line-bytes
2974 Put into each output file as many complete lines of @var{input} as
2975 possible without exceeding @var{size} bytes. Individual lines longer than
2976 @var{size} bytes are broken into multiple files.
2977 @var{size} has the same format as for the @option{--bytes} option.
2979 @item -a @var{length}
2980 @itemx --suffix-length=@var{length}
2982 @opindex --suffix-length
2983 Use suffixes of length @var{length}. The default @var{length} is 2.
2986 @itemx --numeric-suffixes
2988 @opindex --numeric-suffixes
2989 Use digits in suffixes rather than lower-case letters.
2993 Write a diagnostic just before each output file is opened.
3000 @node csplit invocation
3001 @section @command{csplit}: Split a file into context-determined pieces
3004 @cindex context splitting
3005 @cindex splitting a file into pieces by context
3007 @command{csplit} creates zero or more output files containing sections of
3008 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3011 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3014 The contents of the output files are determined by the @var{pattern}
3015 arguments, as detailed below. An error occurs if a @var{pattern}
3016 argument refers to a nonexistent line of the input file (e.g., if no
3017 remaining line matches a given regular expression). After every
3018 @var{pattern} has been matched, any remaining input is copied into one
3021 By default, @command{csplit} prints the number of bytes written to each
3022 output file after it has been created.
3024 The types of pattern arguments are:
3029 Create an output file containing the input up to but not including line
3030 @var{n} (a positive integer). If followed by a repeat count, also
3031 create an output file containing the next @var{n} lines of the input
3032 file once for each repeat.
3034 @item /@var{regexp}/[@var{offset}]
3035 Create an output file containing the current line up to (but not
3036 including) the next line of the input file that contains a match for
3037 @var{regexp}. The optional @var{offset} is an integer.
3038 If it is given, the input up to (but not including) the
3039 matching line plus or minus @var{offset} is put into the output file,
3040 and the line after that begins the next section of input.
3042 @item %@var{regexp}%[@var{offset}]
3043 Like the previous type, except that it does not create an output
3044 file, so that section of the input file is effectively ignored.
3046 @item @{@var{repeat-count}@}
3047 Repeat the previous pattern @var{repeat-count} additional
3048 times. The @var{repeat-count} can either be a positive integer or an
3049 asterisk, meaning repeat as many times as necessary until the input is
3054 The output files' names consist of a prefix (@samp{xx} by default)
3055 followed by a suffix. By default, the suffix is an ascending sequence
3056 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3057 concatenating the output files in sorted order by file name produces the
3058 original input file.
3060 By default, if @command{csplit} encounters an error or receives a hangup,
3061 interrupt, quit, or terminate signal, it removes any output files
3062 that it has created so far before it exits.
3064 The program accepts the following options. Also see @ref{Common options}.
3068 @item -f @var{prefix}
3069 @itemx --prefix=@var{prefix}
3072 @cindex output file name prefix
3073 Use @var{prefix} as the output file name prefix.
3075 @item -b @var{suffix}
3076 @itemx --suffix=@var{suffix}
3079 @cindex output file name suffix
3080 Use @var{suffix} as the output file name suffix. When this option is
3081 specified, the suffix string must include exactly one
3082 @code{printf(3)}-style conversion specification, possibly including
3083 format specification flags, a field width, a precision specifications,
3084 or all of these kinds of modifiers. The format letter must convert a
3085 binary integer argument to readable form; thus, only @samp{d}, @samp{i},
3086 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3087 entire @var{suffix} is given (with the current output file number) to
3088 @code{sprintf(3)} to form the file name suffixes for each of the
3089 individual output files in turn. If this option is used, the
3090 @option{--digits} option is ignored.
3092 @item -n @var{digits}
3093 @itemx --digits=@var{digits}
3096 Use output file names containing numbers that are @var{digits} digits
3097 long instead of the default 2.
3102 @opindex --keep-files
3103 Do not remove output files when errors are encountered.
3106 @itemx --elide-empty-files
3108 @opindex --elide-empty-files
3109 Suppress the generation of zero-length output files. (In cases where
3110 the section delimiters of the input file are supposed to mark the first
3111 lines of each of the sections, the first output file will generally be a
3112 zero-length file unless you use this option.) The output file sequence
3113 numbers always run consecutively starting from 0, even when this option
3124 Do not print counts of output file sizes.
3130 Here is an example of its usage.
3131 First, create an empty directory for the exercise,
3138 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3141 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3147 Each number printed above is the size of an output
3148 file that csplit has just created.
3149 List the names of those output files:
3156 Use @command{head} to show their contents:
3181 @node Summarizing files
3182 @chapter Summarizing files
3184 @cindex summarizing files
3186 These commands generate just a few numbers representing entire
3190 * wc invocation:: Print newline, word, and byte counts.
3191 * sum invocation:: Print checksum and block counts.
3192 * cksum invocation:: Print CRC checksum and byte counts.
3193 * md5sum invocation:: Print or check MD5 digests.
3194 * sha1sum invocation:: Print or check SHA-1 digests.
3195 * sha2 utilities:: Print or check SHA-2 digests.
3200 @section @command{wc}: Print newline, word, and byte counts
3204 @cindex character count
3208 @command{wc} counts the number of bytes, characters, whitespace-separated
3209 words, and newlines in each given @var{file}, or standard input if none
3210 are given or for a @var{file} of @samp{-}. Synopsis:
3213 wc [@var{option}]@dots{} [@var{file}]@dots{}
3216 @cindex total counts
3217 @command{wc} prints one line of counts for each file, and if the file was
3218 given as an argument, it prints the file name following the counts. If
3219 more than one @var{file} is given, @command{wc} prints a final line
3220 containing the cumulative counts, with the file name @file{total}. The
3221 counts are printed in this order: newlines, words, characters, bytes,
3222 maximum line length.
3223 Each count is printed right-justified in a field with at least one
3224 space between fields so that the numbers and file names normally line
3225 up nicely in columns. The width of the count fields varies depending
3226 on the inputs, so you should not depend on a particular field width.
3227 However, as a @acronym{GNU} extension, if only one count is printed,
3228 it is guaranteed to be printed without leading spaces.
3230 By default, @command{wc} prints three counts: the newline, words, and byte
3231 counts. Options can specify that only certain counts be printed.
3232 Options do not undo others previously given, so
3239 prints both the byte counts and the word counts.
3241 With the @option{--max-line-length} option, @command{wc} prints the length
3242 of the longest line per file, and if there is more than one file it
3243 prints the maximum (not the sum) of those lengths. The line lengths here
3244 are measured in screen columns, according to the current locale and
3245 assuming tab positions in every 8th column.
3247 The program accepts the following options. Also see @ref{Common options}.
3255 Print only the byte counts.
3261 Print only the character counts.
3267 Print only the word counts.
3273 Print only the newline counts.
3276 @itemx --max-line-length
3278 @opindex --max-line-length
3279 Print only the maximum line lengths.
3281 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3282 @itemx --files0-from=@var{file}
3283 @opindex --files0-from=@var{file}
3284 @c This is commented out to avoid a texi2dvi failure.
3285 @c texi2dvi (GNU Texinfo 4.11) 1.104
3286 @c @cindex including files from @command{\cmd\}
3287 Disallow processing files named on the command line, and instead process
3288 those named in file @var{file}; each name being terminated by a zero byte
3289 (@acronym{ASCII} @sc{nul}).
3290 This is useful \withTotalOption\
3291 when the list of file names is so long that it may exceed a command line
3293 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3294 because it splits the list into pieces and makes @command{\cmd\} print
3295 \subListOutput\ for each sublist rather than for the entire list.
3296 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3297 @command{find}, using its @option{-print0} predicate.
3298 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3299 are read from standard input.
3301 @filesZeroFromOption{wc,,a total}
3303 For example, to find the length of the longest line in any @file{.c} or
3304 @file{.h} file in the current hierarchy, do this:
3307 find . -name '*.[ch]' -print0 |
3308 wc -L --files0-from=- | tail -n1
3316 @node sum invocation
3317 @section @command{sum}: Print checksum and block counts
3320 @cindex 16-bit checksum
3321 @cindex checksum, 16-bit
3323 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3324 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3327 sum [@var{option}]@dots{} [@var{file}]@dots{}
3330 @command{sum} prints the checksum for each @var{file} followed by the
3331 number of blocks in the file (rounded up). If more than one @var{file}
3332 is given, file names are also printed (by default). (With the
3333 @option{--sysv} option, corresponding file names are printed when there is
3334 at least one file argument.)
3336 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3337 compatible with BSD @command{sum} and prints file sizes in units of
3340 The program accepts the following options. Also see @ref{Common options}.
3346 @cindex BSD @command{sum}
3347 Use the default (BSD compatible) algorithm. This option is included for
3348 compatibility with the System V @command{sum}. Unless @option{-s} was also
3349 given, it has no effect.
3355 @cindex System V @command{sum}
3356 Compute checksums using an algorithm compatible with System V
3357 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3361 @command{sum} is provided for compatibility; the @command{cksum} program (see
3362 next section) is preferable in new applications.
3367 @node cksum invocation
3368 @section @command{cksum}: Print CRC checksum and byte counts
3371 @cindex cyclic redundancy check
3372 @cindex CRC checksum
3374 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3375 given @var{file}, or standard input if none are given or for a
3376 @var{file} of @samp{-}. Synopsis:
3379 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3382 @command{cksum} prints the CRC checksum for each file along with the number
3383 of bytes in the file, and the file name unless no arguments were given.
3385 @command{cksum} is typically used to ensure that files
3386 transferred by unreliable means (e.g., netnews) have not been corrupted,
3387 by comparing the @command{cksum} output for the received files with the
3388 @command{cksum} output for the original files (typically given in the
3391 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3392 compatible with the BSD or System V @command{sum} algorithms (see the
3393 previous section); it is more robust.
3395 The only options are @option{--help} and @option{--version}. @xref{Common
3401 @node md5sum invocation
3402 @section @command{md5sum}: Print or check MD5 digests
3406 @cindex 128-bit checksum
3407 @cindex checksum, 128-bit
3408 @cindex fingerprint, 128-bit
3409 @cindex message-digest, 128-bit
3411 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3412 @dfn{message-digest}) for each specified @var{file}.
3414 Note: The MD5 digest is more reliable than a simple CRC (provided by
3415 the @command{cksum} command) for detecting accidental file corruption,
3416 as the chances of accidentally having two files with identical MD5
3417 are vanishingly small. However, it should not be considered secure
3418 against malicious tampering: although finding a file with a given MD5
3419 fingerprint is considered infeasible at the moment, it is known how
3420 to modify certain files, including digital certificates, so that they
3421 appear valid when signed with an MD5 digest.
3422 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3424 If a @var{file} is specified as @samp{-} or if no files are given
3425 @command{md5sum} computes the checksum for the standard input.
3426 @command{md5sum} can also determine whether a file and checksum are
3427 consistent. Synopsis:
3430 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3433 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3434 indicating a binary or text input file, and the file name.
3435 If @var{file} contains a backslash or newline, the
3436 line is started with a backslash, and each problematic character in
3437 the file name is escaped with a backslash, making the output
3438 unambiguous even in the presence of arbitrary file names.
3439 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3441 The program accepts the following options. Also see @ref{Common options}.
3449 @cindex binary input files
3450 Treat each input file as binary, by reading it in binary mode and
3451 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3452 On systems like @acronym{GNU} that do not distinguish between binary
3453 and text files, this option merely flags each input file as binary:
3454 the MD5 checksum is unaffected. This option is the default on systems
3455 like MS-DOS that distinguish between binary and text files, except
3456 for reading standard input when standard input is a terminal.
3460 Read file names and checksum information (not data) from each
3461 @var{file} (or from stdin if no @var{file} was specified) and report
3462 whether the checksums match the contents of the named files.
3463 The input to this mode of @command{md5sum} is usually the output of
3464 a prior, checksum-generating run of @samp{md5sum}.
3465 Each valid line of input consists of an MD5 checksum, a binary/text
3466 flag, and then a file name.
3467 Binary files are marked with @samp{*}, text with @samp{ }.
3468 For each such line, @command{md5sum} reads the named file and computes its
3469 MD5 checksum. Then, if the computed message digest does not match the
3470 one on the line with the file name, the file is noted as having
3471 failed the test. Otherwise, the file passes the test.
3472 By default, for each valid line, one line is written to standard
3473 output indicating whether the named file passed the test.
3474 After all checks have been performed, if there were any failures,
3475 a warning is issued to standard error.
3476 Use the @option{--status} option to inhibit that output.
3477 If any listed file cannot be opened or read, if any valid line has
3478 an MD5 checksum inconsistent with the associated file, or if no valid
3479 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3480 it exits successfully.
3484 @cindex verifying MD5 checksums
3485 This option is useful only when verifying checksums.
3486 When verifying checksums, don't generate an 'OK' message per successfully
3487 checked file. Files that fail the verification are reported in the
3488 default one-line-per-file format. If there is any checksum mismatch,
3489 print a warning summarizing the failures to standard error.
3493 @cindex verifying MD5 checksums
3494 This option is useful only when verifying checksums.
3495 When verifying checksums, don't generate the default one-line-per-file
3496 diagnostic and don't output the warning summarizing any failures.
3497 Failures to open or read a file still evoke individual diagnostics to
3499 If all listed files are readable and are consistent with the associated
3500 MD5 checksums, exit successfully. Otherwise exit with a status code
3501 indicating there was a failure.
3507 @cindex text input files
3508 Treat each input file as text, by reading it in text mode and
3509 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3510 This option is the default on systems like @acronym{GNU} that do not
3511 distinguish between binary and text files. On other systems, it is
3512 the default for reading standard input when standard input is a
3519 @cindex verifying MD5 checksums
3520 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3521 This option is useful only if all but a few lines in the checked input
3529 @node sha1sum invocation
3530 @section @command{sha1sum}: Print or check SHA-1 digests
3534 @cindex 160-bit checksum
3535 @cindex checksum, 160-bit
3536 @cindex fingerprint, 160-bit
3537 @cindex message-digest, 160-bit
3539 @command{sha1sum} computes a 160-bit checksum for each specified
3540 @var{file}. The usage and options of this command are precisely the
3541 same as for @command{md5sum}. @xref{md5sum invocation}.
3543 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3544 it are known (different files having the same fingerprint). However,
3545 it is known that they can be produced with considerable, but not
3546 unreasonable, resources. For this reason, it is generally considered
3547 that SHA-1 should be gradually phased out in favor of the more secure
3548 SHA-2 hash algorithms. @xref{sha2 utilities}.
3551 @node sha2 utilities
3552 @section sha2 utilities: Print or check SHA-2 digests
3559 @cindex 224-bit checksum
3560 @cindex 256-bit checksum
3561 @cindex 384-bit checksum
3562 @cindex 512-bit checksum
3563 @cindex checksum, 224-bit
3564 @cindex checksum, 256-bit
3565 @cindex checksum, 384-bit
3566 @cindex checksum, 512-bit
3567 @cindex fingerprint, 224-bit
3568 @cindex fingerprint, 256-bit
3569 @cindex fingerprint, 384-bit
3570 @cindex fingerprint, 512-bit
3571 @cindex message-digest, 224-bit
3572 @cindex message-digest, 256-bit
3573 @cindex message-digest, 384-bit
3574 @cindex message-digest, 512-bit
3576 The commands @command{sha224sum}, @command{sha256sum},
3577 @command{sha384sum} and @command{sha512sum} compute checksums of
3578 various lengths (respectively 224, 256, 384 and 512 bits),
3579 collectively known as the SHA-2 hashes. The usage and options of
3580 these commands are precisely the same as for @command{md5sum}.
3581 @xref{md5sum invocation}.
3583 Note: The SHA384 and SHA512 digests are considerably slower to
3584 compute, especially on 32-bit computers, than SHA224 or SHA256.
3587 @node Operating on sorted files
3588 @chapter Operating on sorted files
3590 @cindex operating on sorted files
3591 @cindex sorted files, operations on
3593 These commands work with (or produce) sorted files.
3596 * sort invocation:: Sort text files.
3597 * shuf invocation:: Shuffle text files.
3598 * uniq invocation:: Uniquify files.
3599 * comm invocation:: Compare two sorted files line by line.
3600 * ptx invocation:: Produce a permuted index of file contents.
3601 * tsort invocation:: Topological sort.
3605 @node sort invocation
3606 @section @command{sort}: Sort text files
3609 @cindex sorting files
3611 @command{sort} sorts, merges, or compares all the lines from the given
3612 files, or standard input if none are given or for a @var{file} of
3613 @samp{-}. By default, @command{sort} writes the results to standard
3617 sort [@var{option}]@dots{} [@var{file}]@dots{}
3620 @command{sort} has three modes of operation: sort (the default), merge,
3621 and check for sortedness. The following options change the operation
3628 @itemx --check=diagnose-first
3631 @cindex checking for sortedness
3632 Check whether the given file is already sorted: if it is not all
3633 sorted, print a diagnostic containing the first out-of-order line and
3634 exit with a status of 1.
3635 Otherwise, exit successfully.
3636 At most one input file can be given.
3639 @itemx --check=quiet
3640 @itemx --check=silent
3643 @cindex checking for sortedness
3644 Exit successfully if the given file is already sorted, and
3645 exit with status 1 otherwise.
3646 At most one input file can be given.
3647 This is like @option{-c}, except it does not print a diagnostic.
3653 @cindex merging sorted files
3654 Merge the given files by sorting them as a group. Each input file must
3655 always be individually sorted. It always works to sort instead of
3656 merge; merging is provided because it is faster, in the case where it
3661 @cindex sort stability
3662 @cindex sort's last-resort comparison
3663 A pair of lines is compared as follows:
3664 @command{sort} compares each pair of fields, in the
3665 order specified on the command line, according to the associated
3666 ordering options, until a difference is found or no fields are left.
3667 If no key fields are specified, @command{sort} uses a default key of
3668 the entire line. Finally, as a last resort when all keys compare
3669 equal, @command{sort} compares entire lines as if no ordering options
3670 other than @option{--reverse} (@option{-r}) were specified. The
3671 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3672 comparison} so that lines in which all fields compare equal are left
3673 in their original relative order. The @option{--unique}
3674 (@option{-u}) option also disables the last-resort comparison.
3678 Unless otherwise specified, all comparisons use the character collating
3679 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3680 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3681 to @samp{en_US}), then @command{sort} may produce output that is sorted
3682 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3683 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3684 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3685 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3686 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3687 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3688 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3690 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3691 limit on input line length or restrictions on bytes allowed within lines.
3692 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3693 @command{sort} silently supplies one. A line's trailing newline is not
3694 part of the line for comparison purposes.
3696 @cindex exit status of @command{sort}
3700 0 if no error occurred
3701 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3702 2 if an error occurred
3706 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3707 value as the directory for temporary files instead of @file{/tmp}. The
3708 @option{--temporary-directory} (@option{-T}) option in turn overrides
3709 the environment variable.
3711 The following options affect the ordering of output lines. They may be
3712 specified globally or as part of a specific key field. If no key
3713 fields are specified, global options apply to comparison of entire
3714 lines; otherwise the global options are inherited by key fields that do
3715 not specify any special options of their own. In pre-@acronym{POSIX}
3716 versions of @command{sort}, global options affect only later key fields,
3717 so portable shell scripts should specify global options first.
3722 @itemx --ignore-leading-blanks
3724 @opindex --ignore-leading-blanks
3725 @cindex blanks, ignoring leading
3727 Ignore leading blanks when finding sort keys in each line.
3728 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3729 can change this. Note blanks may be ignored by your locale's collating
3730 rules, but without this option they will be significant for character
3731 positions specified in keys with the @option{-k} option.
3734 @itemx --dictionary-order
3736 @opindex --dictionary-order
3737 @cindex dictionary order
3738 @cindex phone directory order
3739 @cindex telephone directory order
3741 Sort in @dfn{phone directory} order: ignore all characters except
3742 letters, digits and blanks when sorting.
3743 By default letters and digits are those of @acronym{ASCII} and a blank
3744 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3747 @itemx --ignore-case
3749 @opindex --ignore-case
3750 @cindex ignoring case
3751 @cindex case folding
3753 Fold lowercase characters into the equivalent uppercase characters when
3754 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3755 The @env{LC_CTYPE} locale determines character types.
3756 When used with @option{--unique} those lower case equivalent lines are
3757 thrown away. (There is currently no way to throw away the upper case
3758 equivalent instead. (Any @option{--reverse} given would only affect
3759 the final result, after the throwing away.))
3762 @itemx --general-numeric-sort
3763 @itemx --sort=general-numeric
3765 @opindex --general-numeric-sort
3767 @cindex general numeric sort
3769 Sort numerically, using the standard C function @code{strtold} to convert
3770 a prefix of each line to a long double-precision floating point number.
3771 This allows floating point numbers to be specified in scientific notation,
3772 like @code{1.0e-34} and @code{10e100}.
3773 The @env{LC_NUMERIC} locale determines the decimal-point character.
3774 Do not report overflow, underflow, or conversion errors.
3775 Use the following collating sequence:
3779 Lines that do not start with numbers (all considered to be equal).
3781 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3782 in a consistent but machine-dependent order.
3786 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3791 Use this option only if there is no alternative; it is much slower than
3792 @option{--numeric-sort} (@option{-n}) and it can lose information when
3793 converting to floating point.
3796 @itemx --human-numeric-sort
3797 @itemx --sort=human-numeric
3799 @opindex --human-numeric-sort
3801 @cindex human numeric sort
3803 Sort numerically, first by numeric sign (negative, zero, or positive);
3804 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3805 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3806 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3807 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3808 suffix. This option sorts values that are consistently scaled to the
3809 nearest suffix, regardless of whether suffixes denote powers of 1000
3810 or 1024, and it therefore sorts the output of any single invocation of
3811 the @command{df}, @command{du}, or @command{ls} commands that are
3812 invoked with their @option{--human-readable} or @option{--si} options.
3813 The syntax for numbers is the same as for the @option{--numeric-sort}
3814 option; the @acronym{SI} suffix must immediately follow the number.
3817 @itemx --ignore-nonprinting
3819 @opindex --ignore-nonprinting
3820 @cindex nonprinting characters, ignoring
3821 @cindex unprintable characters, ignoring
3823 Ignore nonprinting characters.
3824 The @env{LC_CTYPE} locale determines character types.
3825 This option has no effect if the stronger @option{--dictionary-order}
3826 (@option{-d}) option is also given.
3832 @opindex --month-sort
3834 @cindex months, sorting by
3836 An initial string, consisting of any amount of blanks, followed
3837 by a month name abbreviation, is folded to UPPER case and
3838 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3839 Invalid names compare low to valid names. The @env{LC_TIME} locale
3840 category determines the month spellings.
3841 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3845 @itemx --numeric-sort
3846 @itemx --sort=numeric
3848 @opindex --numeric-sort
3850 @cindex numeric sort
3852 Sort numerically. The number begins each line and consists
3853 of optional blanks, an optional @samp{-} sign, and zero or more
3854 digits possibly separated by thousands separators, optionally followed
3855 by a decimal-point character and zero or more digits. An empty
3856 number is treated as @samp{0}. The @env{LC_NUMERIC}
3857 locale specifies the decimal-point character and thousands separator.
3858 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3861 Comparison is exact; there is no rounding error.
3863 Neither a leading @samp{+} nor exponential notation is recognized.
3864 To compare such strings numerically, use the
3865 @option{--general-numeric-sort} (@option{-g}) option.
3868 @itemx --version-sort
3870 @opindex --version-sort
3871 @cindex version number sort
3872 Sort by version name and number. It behaves like a standard sort,
3873 except that each sequence of decimal digits is treated numerically
3874 as an index/version number. (@xref{Details about version sort}.)
3880 @cindex reverse sorting
3881 Reverse the result of comparison, so that lines with greater key values
3882 appear earlier in the output instead of later.
3885 @itemx --random-sort
3886 @itemx --sort=random
3888 @opindex --random-sort
3891 Sort by hashing the input keys and then sorting the hash values.
3892 Choose the hash function at random, ensuring that it is free of
3893 collisions so that differing keys have differing hash values. This is
3894 like a random permutation of the inputs (@pxref{shuf invocation}),
3895 except that keys with the same value sort together.
3897 If multiple random sort fields are specified, the same random hash
3898 function is used for all fields. To use different random hash
3899 functions for different fields, you can invoke @command{sort} more
3902 The choice of hash function is affected by the
3903 @option{--random-source} option.
3911 @item --compress-program=@var{prog}
3912 Compress any temporary files with the program @var{prog}.
3914 With no arguments, @var{prog} must compress standard input to standard
3915 output, and when given the @option{-d} option it must decompress
3916 standard input to standard output.
3918 Terminate with an error if @var{prog} exits with nonzero status.
3920 White space and the backslash character should not appear in
3921 @var{prog}; they are reserved for future use.
3923 @filesZeroFromOption{sort,,sorted output}
3925 @item -k @var{pos1}[,@var{pos2}]
3926 @itemx --key=@var{pos1}[,@var{pos2}]
3930 Specify a sort field that consists of the part of the line between
3931 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3932 omitted), @emph{inclusive}.
3934 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3935 where @var{f} is the number of the field to use, and @var{c} is the number
3936 of the first character from the beginning of the field. Fields and character
3937 positions are numbered starting with 1; a character position of zero in
3938 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3939 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3940 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3941 @var{opts} are ordering options, allowing individual keys to be sorted
3942 according to different rules; see below for details. Keys can span
3945 Example: To sort on the second field, use @option{--key=2,2}
3946 (@option{-k 2,2}). See below for more notes on keys and more examples.
3947 See also the @option{--debug} option to help determine the part
3948 of the line being used in the sort.
3951 Highlight the portion of each line used for sorting.
3952 Also issue warnings about questionable usage to stderr.
3954 @item --batch-size=@var{nmerge}
3955 @opindex --batch-size
3956 @cindex number of inputs to merge, nmerge
3957 Merge at most @var{nmerge} inputs at once.
3959 When @command{sort} has to merge more than @var{nmerge} inputs,
3960 it merges them in groups of @var{nmerge}, saving the result in
3961 a temporary file, which is then used as an input in a subsequent merge.
3963 A large value of @var{nmerge} may improve merge performance and decrease
3964 temporary storage utilization at the expense of increased memory usage
3965 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3966 requirements and I/0 at the expense of temporary storage consumption and
3969 The value of @var{nmerge} must be at least 2. The default value is
3970 currently 16, but this is implementation-dependent and may change in
3973 The value of @var{nmerge} may be bounded by a resource limit for open
3974 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3975 OPEN_MAX} may display limits for your systems; these limits may be
3976 modified further if your program already has some files open, or if
3977 the operating system has other limits on the number of open files. If
3978 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3979 silently uses a smaller value.
3981 @item -o @var{output-file}
3982 @itemx --output=@var{output-file}
3985 @cindex overwriting of input, allowed
3986 Write output to @var{output-file} instead of standard output.
3987 Normally, @command{sort} reads all input before opening
3988 @var{output-file}, so you can safely sort a file in place by using
3989 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3990 However, @command{sort} with @option{--merge} (@option{-m}) can open
3991 the output file before reading all input, so a command like @code{cat
3992 F | sort -m -o F - G} is not safe as @command{sort} might start
3993 writing @file{F} before @command{cat} is done reading it.
3995 @vindex POSIXLY_CORRECT
3996 On newer systems, @option{-o} cannot appear after an input file if
3997 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3998 scripts should specify @option{-o @var{output-file}} before any input
4001 @item --random-source=@var{file}
4002 @opindex --random-source
4003 @cindex random source for sorting
4004 Use @var{file} as a source of random data used to determine which
4005 random hash function to use with the @option{-R} option. @xref{Random
4012 @cindex sort stability
4013 @cindex sort's last-resort comparison
4015 Make @command{sort} stable by disabling its last-resort comparison.
4016 This option has no effect if no fields or global ordering options
4017 other than @option{--reverse} (@option{-r}) are specified.
4020 @itemx --buffer-size=@var{size}
4022 @opindex --buffer-size
4023 @cindex size for main memory sorting
4024 Use a main-memory sort buffer of the given @var{size}. By default,
4025 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4026 @var{size} to be interpreted as a percentage of physical memory.
4027 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4028 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4029 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4030 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4033 This option can improve the performance of @command{sort} by causing it
4034 to start with a larger or smaller sort buffer than the default.
4035 However, this option affects only the initial buffer size. The buffer
4036 grows beyond @var{size} if @command{sort} encounters input lines larger
4039 @item -t @var{separator}
4040 @itemx --field-separator=@var{separator}
4042 @opindex --field-separator
4043 @cindex field separator character
4044 Use character @var{separator} as the field separator when finding the
4045 sort keys in each line. By default, fields are separated by the empty
4046 string between a non-blank character and a blank character.
4047 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4050 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4051 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4052 not considered to be part of either the field preceding or the field
4053 following, so with @samp{sort @w{-t " "}} the same input line has
4054 three fields: an empty field, @samp{foo}, and @samp{bar}.
4055 However, fields that extend to the end of the line,
4056 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4057 retain the field separators present between the endpoints of the range.
4059 To specify @acronym{ASCII} @sc{nul} as the field separator,
4060 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4062 @item -T @var{tempdir}
4063 @itemx --temporary-directory=@var{tempdir}
4065 @opindex --temporary-directory
4066 @cindex temporary directory
4068 Use directory @var{tempdir} to store temporary files, overriding the
4069 @env{TMPDIR} environment variable. If this option is given more than
4070 once, temporary files are stored in all the directories given. If you
4071 have a large sort or merge that is I/O-bound, you can often improve
4072 performance by using this option to specify directories on different
4073 disks and controllers.
4075 @item --parallel=@var{n}
4077 @cindex multithreaded sort
4078 Limit the number of sorts run in parallel to @var{n}. By default,
4079 @var{n} is set to the number of available processors, and values
4080 greater than that are reduced to that limit. Also see
4081 @ref{nproc invocation}.
4087 @cindex uniquifying output
4089 Normally, output only the first of a sequence of lines that compare
4090 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4091 check that no pair of consecutive lines compares equal.
4093 This option also disables the default last-resort comparison.
4095 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4096 this equivalence does not extend to arbitrary @command{sort} options.
4097 For example, @code{sort -n -u} inspects only the value of the initial
4098 numeric string when checking for uniqueness, whereas @code{sort -n |
4099 uniq} inspects the entire line. @xref{uniq invocation}.
4101 @macro zeroTerminatedOption
4103 @itemx --zero-terminated
4105 @opindex --zero-terminated
4106 @cindex process zero-terminated items
4107 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4108 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4109 and terminate output items with @acronym{ASCII} @sc{nul}.
4110 This option can be useful in conjunction with @samp{perl -0} or
4111 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4112 reliably handle arbitrary file names (even those containing blanks
4113 or other special characters).
4115 @zeroTerminatedOption
4119 Historical (BSD and System V) implementations of @command{sort} have
4120 differed in their interpretation of some options, particularly
4121 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4122 behavior, which is usually (but not always!) like the System V behavior.
4123 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4124 consistency, @option{-M} has been changed in the same way. This may
4125 affect the meaning of character positions in field specifications in
4126 obscure cases. The only fix is to add an explicit @option{-b}.
4128 A position in a sort field specified with @option{-k} may have any
4129 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4130 global ordering options are inherited by that particular field. The
4131 @option{-b} option may be independently attached to either or both of
4132 the start and end positions of a field specification, and if it is
4133 inherited from the global options it will be attached to both.
4134 If input lines can contain leading or adjacent blanks and @option{-t}
4135 is not used, then @option{-k} is typically combined with @option{-b} or
4136 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4137 the varying numbers of leading blanks in fields can cause confusing results.
4139 If the start position in a sort field specifier falls after the end of
4140 the line or after the end field, the field is empty. If the @option{-b}
4141 option was specified, the @samp{.@var{c}} part of a field specification
4142 is counted from the first nonblank character of the field.
4144 @vindex _POSIX2_VERSION
4145 @vindex POSIXLY_CORRECT
4146 On older systems, @command{sort} supports an obsolete origin-zero
4147 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4148 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4149 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4150 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4151 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4153 This obsolete behavior can be enabled or disabled with the
4154 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4155 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4156 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4158 Scripts intended for use on standard hosts should avoid obsolete
4159 syntax and should use @option{-k} instead. For example, avoid
4160 @samp{sort +2}, since it might be interpreted as either @samp{sort
4161 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4162 support only the obsolete syntax, it can use a test like @samp{if sort
4163 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4166 Here are some examples to illustrate various combinations of options.
4171 Sort in descending (reverse) numeric order.
4178 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4181 sort --parallel=4 -S 10M
4185 Sort alphabetically, omitting the first and second fields
4186 and the blanks at the start of the third field.
4187 This uses a single key composed of the characters beginning
4188 at the start of the first nonblank character in field three
4189 and extending to the end of each line.
4196 Sort numerically on the second field and resolve ties by sorting
4197 alphabetically on the third and fourth characters of field five.
4198 Use @samp{:} as the field delimiter.
4201 sort -t : -k 2,2n -k 5.3,5.4
4204 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4205 @command{sort} would have used all characters beginning in the second field
4206 and extending to the end of the line as the primary @emph{numeric}
4207 key. For the large majority of applications, treating keys spanning
4208 more than one field as numeric will not do what you expect.
4210 Also note that the @samp{n} modifier was applied to the field-end
4211 specifier for the first key. It would have been equivalent to
4212 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4213 @samp{b} apply to the associated @emph{field}, regardless of whether
4214 the modifier character is attached to the field-start and/or the
4215 field-end part of the key specifier.
4218 Sort the password file on the fifth field and ignore any
4219 leading blanks. Sort lines with equal values in field five
4220 on the numeric user ID in field three. Fields are separated
4224 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4225 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4226 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4229 These three commands have equivalent effect. The first specifies that
4230 the first key's start position ignores leading blanks and the second
4231 key is sorted numerically. The other two commands rely on global
4232 options being inherited by sort keys that lack modifiers. The inheritance
4233 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4234 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4235 character position is not affected by whether initial blanks are
4239 Sort a set of log files, primarily by IPv4 address and secondarily by
4240 time stamp. If two lines' primary and secondary keys are identical,
4241 output the lines in the same order that they were input. The log
4242 files contain lines that look like this:
4245 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4246 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4249 Fields are separated by exactly one space. Sort IPv4 addresses
4250 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4251 because 61 is less than 129.
4254 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4255 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4258 This example cannot be done with a single @command{sort} invocation,
4259 since IPv4 address components are separated by @samp{.} while dates
4260 come just after a space. So it is broken down into two invocations of
4261 @command{sort}: the first sorts by time stamp and the second by IPv4
4262 address. The time stamp is sorted by year, then month, then day, and
4263 finally by hour-minute-second field, using @option{-k} to isolate each
4264 field. Except for hour-minute-second there's no need to specify the
4265 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4266 based on leading prefixes that cannot cross field boundaries. The
4267 IPv4 addresses are sorted lexicographically. The second sort uses
4268 @samp{-s} so that ties in the primary key are broken by the secondary
4269 key; the first sort uses @samp{-s} so that the combination of the two
4273 Generate a tags file in case-insensitive sorted order.
4276 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4279 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4280 that file names that contain blanks or other special characters are
4282 by the sort operation.
4284 @c This example is a bit contrived and needs more explanation.
4286 @c Sort records separated by an arbitrary string by using a pipe to convert
4287 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4288 @c and converting each @samp{\0} back to the original record delimiter.
4291 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4295 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4296 sort lines according to their length.
4299 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4302 In general this technique can be used to sort data that the @command{sort}
4303 command does not support, or is inefficient at, sorting directly.
4306 Shuffle a list of directories, but preserve the order of files within
4307 each directory. For instance, one could use this to generate a music
4308 playlist in which albums are shuffled but the songs of each album are
4312 ls */* | sort -t / -k 1,1R -k 2,2
4318 @node shuf invocation
4319 @section @command{shuf}: Shuffling text
4322 @cindex shuffling files
4324 @command{shuf} shuffles its input by outputting a random permutation
4325 of its input lines. Each output permutation is equally likely.
4329 shuf [@var{option}]@dots{} [@var{file}]
4330 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4331 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4334 @command{shuf} has three modes of operation that affect where it
4335 obtains its input lines. By default, it reads lines from standard
4336 input. The following options change the operation mode:
4344 @cindex command-line operands to shuffle
4345 Treat each command-line operand as an input line.
4347 @item -i @var{lo}-@var{hi}
4348 @itemx --input-range=@var{lo}-@var{hi}
4350 @opindex --input-range
4351 @cindex input range to shuffle
4352 Act as if input came from a file containing the range of unsigned
4353 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4357 @command{shuf}'s other options can affect its behavior in all
4362 @item -n @var{lines}
4363 @itemx --head-count=@var{count}
4365 @opindex --head-count
4366 @cindex head of output
4367 Output at most @var{count} lines. By default, all input lines are
4370 @item -o @var{output-file}
4371 @itemx --output=@var{output-file}
4374 @cindex overwriting of input, allowed
4375 Write output to @var{output-file} instead of standard output.
4376 @command{shuf} reads all input before opening
4377 @var{output-file}, so you can safely shuffle a file in place by using
4378 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4380 @item --random-source=@var{file}
4381 @opindex --random-source
4382 @cindex random source for shuffling
4383 Use @var{file} as a source of random data used to determine which
4384 permutation to generate. @xref{Random sources}.
4386 @zeroTerminatedOption
4402 might produce the output
4412 Similarly, the command:
4415 shuf -e clubs hearts diamonds spades
4429 and the command @samp{shuf -i 1-4} might output:
4439 These examples all have four input lines, so @command{shuf} might
4440 produce any of the twenty-four possible permutations of the input. In
4441 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4442 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4443 output permutations.
4448 @node uniq invocation
4449 @section @command{uniq}: Uniquify files
4452 @cindex uniquify files
4454 @command{uniq} writes the unique lines in the given @file{input}, or
4455 standard input if nothing is given or for an @var{input} name of
4459 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4462 By default, @command{uniq} prints its input lines, except that
4463 it discards all but the first of adjacent repeated lines, so that
4464 no output lines are repeated. Optionally, it can instead discard
4465 lines that are not repeated, or all repeated lines.
4467 The input need not be sorted, but repeated input lines are detected
4468 only if they are adjacent. If you want to discard non-adjacent
4469 duplicate lines, perhaps you want to use @code{sort -u}.
4470 @xref{sort invocation}.
4473 Comparisons honor the rules specified by the @env{LC_COLLATE}
4476 If no @var{output} file is specified, @command{uniq} writes to standard
4479 The program accepts the following options. Also see @ref{Common options}.
4484 @itemx --skip-fields=@var{n}
4486 @opindex --skip-fields
4487 Skip @var{n} fields on each line before checking for uniqueness. Use
4488 a null string for comparison if a line has fewer than @var{n} fields. Fields
4489 are sequences of non-space non-tab characters that are separated from
4490 each other by at least one space or tab.
4492 For compatibility @command{uniq} supports an obsolete option syntax
4493 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4496 @itemx --skip-chars=@var{n}
4498 @opindex --skip-chars
4499 Skip @var{n} characters before checking for uniqueness. Use a null string
4500 for comparison if a line has fewer than @var{n} characters. If you use both
4501 the field and character skipping options, fields are skipped over first.
4503 @vindex _POSIX2_VERSION
4504 On older systems, @command{uniq} supports an obsolete option syntax
4506 This obsolete behavior can be enabled or disabled with the
4507 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4508 conformance}), but portable scripts should avoid commands whose
4509 behavior depends on this variable.
4510 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4511 the ambiguous @samp{uniq +10}.
4517 Print the number of times each line occurred along with the line.
4520 @itemx --ignore-case
4522 @opindex --ignore-case
4523 Ignore differences in case when comparing lines.
4529 @cindex repeated lines, outputting
4530 Discard lines that are not repeated. When used by itself, this option
4531 causes @command{uniq} to print the first copy of each repeated line,
4535 @itemx --all-repeated[=@var{delimit-method}]
4537 @opindex --all-repeated
4538 @cindex all repeated lines, outputting
4539 Do not discard the second and subsequent repeated input lines,
4540 but discard lines that are not repeated.
4541 This option is useful mainly in conjunction with other options e.g.,
4542 to ignore case or to compare only selected fields.
4543 The optional @var{delimit-method} tells how to delimit
4544 groups of repeated lines, and must be one of the following:
4549 Do not delimit groups of repeated lines.
4550 This is equivalent to @option{--all-repeated} (@option{-D}).
4553 Output a newline before each group of repeated lines.
4554 With @option{--zero-terminated} (@option{-z}), use a zero
4555 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4558 Separate groups of repeated lines with a single newline.
4559 With @option{--zero-terminated} (@option{-z}), use a zero
4560 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4561 This is the same as using @samp{prepend}, except that
4562 no delimiter is inserted before the first group, and hence
4563 may be better suited for output direct to users.
4566 Note that when groups are delimited and the input stream contains
4567 two or more consecutive blank lines, then the output is ambiguous.
4568 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4569 each sequence of consecutive newlines with a single newline.
4571 This is a @sc{gnu} extension.
4572 @c FIXME: give an example showing *how* it's useful
4578 @cindex unique lines, outputting
4579 Discard the first repeated line. When used by itself, this option
4580 causes @command{uniq} to print unique lines, and nothing else.
4583 @itemx --check-chars=@var{n}
4585 @opindex --check-chars
4586 Compare at most @var{n} characters on each line (after skipping any specified
4587 fields and characters). By default the entire rest of the lines are
4590 @zeroTerminatedOption
4597 @node comm invocation
4598 @section @command{comm}: Compare two sorted files line by line
4601 @cindex line-by-line comparison
4602 @cindex comparing sorted files
4604 @command{comm} writes to standard output lines that are common, and lines
4605 that are unique, to two input files; a file name of @samp{-} means
4606 standard input. Synopsis:
4609 comm [@var{option}]@dots{} @var{file1} @var{file2}
4613 Before @command{comm} can be used, the input files must be sorted using the
4614 collating sequence specified by the @env{LC_COLLATE} locale.
4615 If an input file ends in a non-newline
4616 character, a newline is silently appended. The @command{sort} command with
4617 no options always outputs a file that is suitable input to @command{comm}.
4619 @cindex differing lines
4620 @cindex common lines
4621 With no options, @command{comm} produces three-column output. Column one
4622 contains lines unique to @var{file1}, column two contains lines unique
4623 to @var{file2}, and column three contains lines common to both files.
4624 Columns are separated by a single TAB character.
4625 @c FIXME: when there's an option to supply an alternative separator
4626 @c string, append `by default' to the above sentence.
4631 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4632 the corresponding columns (and separators). Also see @ref{Common options}.
4634 Unlike some other comparison utilities, @command{comm} has an exit
4635 status that does not depend on the result of the comparison.
4636 Upon normal completion @command{comm} produces an exit code of zero.
4637 If there is an error it exits with nonzero status.
4639 @macro checkOrderOption{cmd}
4640 If the @option{--check-order} option is given, unsorted inputs will
4641 cause a fatal error message. If the option @option{--nocheck-order}
4642 is given, unsorted inputs will never cause an error message. If
4643 neither of these options is given, wrongly sorted inputs are diagnosed
4644 only if an input file is found to contain unpairable lines. If an
4645 input file is diagnosed as being unsorted, the @command{\cmd\} command
4646 will exit with a nonzero status (and the output should not be used).
4648 Forcing @command{\cmd\} to process wrongly sorted input files
4649 containing unpairable lines by specifying @option{--nocheck-order} is
4650 not guaranteed to produce any particular output. The output will
4651 probably not correspond with whatever you hoped it would be.
4653 @checkOrderOption{comm}
4658 Fail with an error message if either input file is wrongly ordered.
4660 @item --nocheck-order
4661 Do not check that both input files are in sorted order.
4665 @item --output-delimiter=@var{str}
4666 Print @var{str} between adjacent output columns,
4667 rather than the default of a single TAB character.
4669 The delimiter @var{str} may not be empty.
4673 @node ptx invocation
4674 @section @command{ptx}: Produce permuted indexes
4678 @command{ptx} reads a text file and essentially produces a permuted index, with
4679 each keyword in its context. The calling sketch is either one of:
4682 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4683 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4686 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4687 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4688 limitations and changing several of the program's default option values.
4689 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4690 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4691 document. For the full list, see @xref{Compatibility in ptx}.
4693 Individual options are explained in the following sections.
4695 When @sc{gnu} extensions are enabled, there may be zero, one or several
4696 @var{file}s after the options. If there is no @var{file}, the program
4697 reads the standard input. If there is one or several @var{file}s, they
4698 give the name of input files which are all read in turn, as if all the
4699 input files were concatenated. However, there is a full contextual
4700 break between each file and, when automatic referencing is requested,
4701 file names and line numbers refer to individual text input files. In
4702 all cases, the program outputs the permuted index to the standard
4705 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4706 operates in traditional mode, there may be zero, one or two parameters
4707 besides the options. If there are no parameters, the program reads the
4708 standard input and outputs the permuted index to the standard output.
4709 If there is only one parameter, it names the text @var{input} to be read
4710 instead of the standard input. If two parameters are given, they give
4711 respectively the name of the @var{input} file to read and the name of
4712 the @var{output} file to produce. @emph{Be very careful} to note that,
4713 in this case, the contents of file given by the second parameter is
4714 destroyed. This behavior is dictated by System V @command{ptx}
4715 compatibility; @sc{gnu} Standards normally discourage output parameters not
4716 introduced by an option.
4718 Note that for @emph{any} file named as the value of an option or as an
4719 input text file, a single dash @kbd{-} may be used, in which case
4720 standard input is assumed. However, it would not make sense to use this
4721 convention more than once per program invocation.
4724 * General options in ptx:: Options which affect general program behavior.
4725 * Charset selection in ptx:: Underlying character set considerations.
4726 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4727 * Output formatting in ptx:: Types of output format, and sizing the fields.
4728 * Compatibility in ptx::
4732 @node General options in ptx
4733 @subsection General options
4738 @itemx --traditional
4739 As already explained, this option disables all @sc{gnu} extensions to
4740 @command{ptx} and switches to traditional mode.
4743 Print a short help on standard output, then exit without further
4747 Print the program version on standard output, then exit without further
4755 @node Charset selection in ptx
4756 @subsection Charset selection
4758 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4759 As it is set up now, the program assumes that the input file is coded
4760 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4761 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4762 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4763 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4764 of characters which are letters is different; this alters the behavior
4765 of regular expression matching. Thus, the default regular expression
4766 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4767 however, is still crude; it obeys the underlying character set ordering
4773 @itemx --ignore-case
4774 Fold lower case letters to upper case for sorting.
4779 @node Input processing in ptx
4780 @subsection Word selection and input processing
4785 @itemx --break-file=@var{file}
4787 This option provides an alternative (to @option{-W}) method of describing
4788 which characters make up words. It introduces the name of a
4789 file which contains a list of characters which can@emph{not} be part of
4790 one word; this file is called the @dfn{Break file}. Any character which
4791 is not part of the Break file is a word constituent. If both options
4792 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4793 @option{-b} is ignored.
4795 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4796 break character is to write all the break characters in the file with no
4797 newline at all, not even at the end of the file. When @sc{gnu} extensions
4798 are disabled, spaces, tabs and newlines are always considered as break
4799 characters even if not included in the Break file.
4802 @itemx --ignore-file=@var{file}
4804 The file associated with this option contains a list of words which will
4805 never be taken as keywords in concordance output. It is called the
4806 @dfn{Ignore file}. The file contains exactly one word in each line; the
4807 end of line separation of words is not subject to the value of the
4811 @itemx --only-file=@var{file}
4813 The file associated with this option contains a list of words which will
4814 be retained in concordance output; any word not mentioned in this file
4815 is ignored. The file is called the @dfn{Only file}. The file contains
4816 exactly one word in each line; the end of line separation of words is
4817 not subject to the value of the @option{-S} option.
4819 There is no default for the Only file. When both an Only file and an
4820 Ignore file are specified, a word is considered a keyword only
4821 if it is listed in the Only file and not in the Ignore file.
4826 On each input line, the leading sequence of non-white space characters will be
4827 taken to be a reference that has the purpose of identifying this input
4828 line in the resulting permuted index. For more information about reference
4829 production, see @xref{Output formatting in ptx}.
4830 Using this option changes the default value for option @option{-S}.
4832 Using this option, the program does not try very hard to remove
4833 references from contexts in output, but it succeeds in doing so
4834 @emph{when} the context ends exactly at the newline. If option
4835 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4836 are disabled, this condition is always met and references are completely
4837 excluded from the output contexts.
4839 @item -S @var{regexp}
4840 @itemx --sentence-regexp=@var{regexp}
4842 This option selects which regular expression will describe the end of a
4843 line or the end of a sentence. In fact, this regular expression is not
4844 the only distinction between end of lines or end of sentences, and input
4845 line boundaries have no special significance outside this option. By
4846 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4847 used, end of sentences are used. In this case, this @var{regex} is
4848 imported from @sc{gnu} Emacs:
4851 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4854 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4855 of lines are used; in this case, the default @var{regexp} is just:
4861 Using an empty @var{regexp} is equivalent to completely disabling end of
4862 line or end of sentence recognition. In this case, the whole file is
4863 considered to be a single big line or sentence. The user might want to
4864 disallow all truncation flag generation as well, through option @option{-F
4865 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4868 When the keywords happen to be near the beginning of the input line or
4869 sentence, this often creates an unused area at the beginning of the
4870 output context line; when the keywords happen to be near the end of the
4871 input line or sentence, this often creates an unused area at the end of
4872 the output context line. The program tries to fill those unused areas
4873 by wrapping around context in them; the tail of the input line or
4874 sentence is used to fill the unused area on the left of the output line;
4875 the head of the input line or sentence is used to fill the unused area
4876 on the right of the output line.
4878 As a matter of convenience to the user, many usual backslashed escape
4879 sequences from the C language are recognized and converted to the
4880 corresponding characters by @command{ptx} itself.
4882 @item -W @var{regexp}
4883 @itemx --word-regexp=@var{regexp}
4885 This option selects which regular expression will describe each keyword.
4886 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4887 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4888 disabled, a word is by default anything which ends with a space, a tab
4889 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4891 An empty @var{regexp} is equivalent to not using this option.
4892 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4895 As a matter of convenience to the user, many usual backslashed escape
4896 sequences, as found in the C language, are recognized and converted to
4897 the corresponding characters by @command{ptx} itself.
4902 @node Output formatting in ptx
4903 @subsection Output formatting
4905 Output format is mainly controlled by the @option{-O} and @option{-T} options
4906 described in the table below. When neither @option{-O} nor @option{-T} are
4907 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4908 output format suitable for a dumb terminal. Each keyword occurrence is
4909 output to the center of one line, surrounded by its left and right
4910 contexts. Each field is properly justified, so the concordance output
4911 can be readily observed. As a special feature, if automatic
4912 references are selected by option @option{-A} and are output before the
4913 left context, that is, if option @option{-R} is @emph{not} selected, then
4914 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4915 Emacs @code{next-error} processing. In this default output format, each
4916 white space character, like newline and tab, is merely changed to
4917 exactly one space, with no special attempt to compress consecutive
4918 spaces. This might change in the future. Except for those white space
4919 characters, every other character of the underlying set of 256
4920 characters is transmitted verbatim.
4922 Output format is further controlled by the following options.
4926 @item -g @var{number}
4927 @itemx --gap-size=@var{number}
4929 Select the size of the minimum white space gap between the fields on the
4932 @item -w @var{number}
4933 @itemx --width=@var{number}
4935 Select the maximum output width of each final line. If references are
4936 used, they are included or excluded from the maximum output width
4937 depending on the value of option @option{-R}. If this option is not
4938 selected, that is, when references are output before the left context,
4939 the maximum output width takes into account the maximum length of all
4940 references. If this option is selected, that is, when references are
4941 output after the right context, the maximum output width does not take
4942 into account the space taken by references, nor the gap that precedes
4946 @itemx --auto-reference
4948 Select automatic references. Each input line will have an automatic
4949 reference made up of the file name and the line ordinal, with a single
4950 colon between them. However, the file name will be empty when standard
4951 input is being read. If both @option{-A} and @option{-r} are selected, then
4952 the input reference is still read and skipped, but the automatic
4953 reference is used at output time, overriding the input reference.
4956 @itemx --right-side-refs
4958 In the default output format, when option @option{-R} is not used, any
4959 references produced by the effect of options @option{-r} or @option{-A} are
4960 placed to the far right of output lines, after the right context. With
4961 default output format, when the @option{-R} option is specified, references
4962 are rather placed at the beginning of each output line, before the left
4963 context. For any other output format, option @option{-R} is
4964 ignored, with one exception: with @option{-R} the width of references
4965 is @emph{not} taken into account in total output width given by @option{-w}.
4967 This option is automatically selected whenever @sc{gnu} extensions are
4970 @item -F @var{string}
4971 @itemx --flac-truncation=@var{string}
4973 This option will request that any truncation in the output be reported
4974 using the string @var{string}. Most output fields theoretically extend
4975 towards the beginning or the end of the current line, or current
4976 sentence, as selected with option @option{-S}. But there is a maximum
4977 allowed output line width, changeable through option @option{-w}, which is
4978 further divided into space for various output fields. When a field has
4979 to be truncated because it cannot extend beyond the beginning or the end of
4980 the current line to fit in, then a truncation occurs. By default,
4981 the string used is a single slash, as in @option{-F /}.
4983 @var{string} may have more than one character, as in @option{-F ...}.
4984 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4985 truncation flagging is disabled, and no truncation marks are appended in
4988 As a matter of convenience to the user, many usual backslashed escape
4989 sequences, as found in the C language, are recognized and converted to
4990 the corresponding characters by @command{ptx} itself.
4992 @item -M @var{string}
4993 @itemx --macro-name=@var{string}
4995 Select another @var{string} to be used instead of @samp{xx}, while
4996 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
4999 @itemx --format=roff
5001 Choose an output format suitable for @command{nroff} or @command{troff}
5002 processing. Each output line will look like:
5005 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5008 so it will be possible to write a @samp{.xx} roff macro to take care of
5009 the output typesetting. This is the default output format when @sc{gnu}
5010 extensions are disabled. Option @option{-M} can be used to change
5011 @samp{xx} to another macro name.
5013 In this output format, each non-graphical character, like newline and
5014 tab, is merely changed to exactly one space, with no special attempt to
5015 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5016 so it will be correctly processed by @command{nroff} or @command{troff}.
5021 Choose an output format suitable for @TeX{} processing. Each output
5022 line will look like:
5025 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5029 so it will be possible to write a @code{\xx} definition to take care of
5030 the output typesetting. Note that when references are not being
5031 produced, that is, neither option @option{-A} nor option @option{-r} is
5032 selected, the last parameter of each @code{\xx} call is inhibited.
5033 Option @option{-M} can be used to change @samp{xx} to another macro
5036 In this output format, some special characters, like @kbd{$}, @kbd{%},
5037 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5038 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5039 backslash and a pair of dollar signs (to force mathematical mode). The
5040 backslash itself produces the sequence @code{\backslash@{@}}.
5041 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5042 @code{~\@{ @}} respectively. Other diacriticized characters of the
5043 underlying character set produce an appropriate @TeX{} sequence as far
5044 as possible. The other non-graphical characters, like newline and tab,
5045 and all other characters which are not part of @acronym{ASCII}, are merely
5046 changed to exactly one space, with no special attempt to compress
5047 consecutive spaces. Let me know how to improve this special character
5048 processing for @TeX{}.
5053 @node Compatibility in ptx
5054 @subsection The @sc{gnu} extensions to @command{ptx}
5056 This version of @command{ptx} contains a few features which do not exist in
5057 System V @command{ptx}. These extra features are suppressed by using the
5058 @option{-G} command line option, unless overridden by other command line
5059 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5060 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5061 Here are the differences between this program and System V @command{ptx}.
5066 This program can read many input files at once, it always writes the
5067 resulting concordance on standard output. On the other hand, System V
5068 @command{ptx} reads only one file and sends the result to standard output
5069 or, if a second @var{file} parameter is given on the command, to that
5072 Having output parameters not introduced by options is a dangerous
5073 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5074 portably between @sc{gnu} and System V, you should always use it with a
5075 single input file, and always expect the result on standard output. You
5076 might also want to automatically configure in a @option{-G} option to
5077 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5078 that the installed @command{ptx} accepts @option{-G}.
5081 The only options available in System V @command{ptx} are options @option{-b},
5082 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5083 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5084 this enumeration. Moreover, some options have a slightly different
5085 meaning when @sc{gnu} extensions are enabled, as explained below.
5088 By default, concordance output is not formatted for @command{troff} or
5089 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5090 or @command{nroff} output may still be selected through option @option{-O}.
5093 Unless @option{-R} option is used, the maximum reference width is
5094 subtracted from the total output line width. With @sc{gnu} extensions
5095 disabled, width of references is not taken into account in the output
5096 line width computations.
5099 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5100 processed from input file with no adverse effect, even if @sc{gnu} extensions
5101 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5102 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5105 Input line length is only limited by available memory, even if @sc{gnu}
5106 extensions are disabled. However, System V @command{ptx} processes only
5107 the first 200 characters in each line.
5110 The break (non-word) characters default to be every character except all
5111 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5112 extensions are disabled, the break characters default to space, tab and
5116 The program makes better use of output line width. If @sc{gnu} extensions
5117 are disabled, the program rather tries to imitate System V @command{ptx},
5118 but still, there are some slight disposition glitches this program does
5119 not completely reproduce.
5122 The user can specify both an Ignore file and an Only file. This is not
5123 allowed with System V @command{ptx}.
5128 @node tsort invocation
5129 @section @command{tsort}: Topological sort
5132 @cindex topological sort
5134 @command{tsort} performs a topological sort on the given @var{file}, or
5135 standard input if no input file is given or for a @var{file} of
5136 @samp{-}. For more details and some history, see @ref{tsort background}.
5140 tsort [@var{option}] [@var{file}]
5143 @command{tsort} reads its input as pairs of strings, separated by blanks,
5144 indicating a partial ordering. The output is a total ordering that
5145 corresponds to the given partial ordering.
5159 will produce the output
5170 Consider a more realistic example.
5171 You have a large set of functions all in one file, and they may all be
5172 declared static except one. Currently that one (say @code{main}) is the
5173 first function defined in the file, and the ones it calls directly follow
5174 it, followed by those they call, etc. Let's say that you are determined
5175 to take advantage of prototypes, so you have to choose between declaring
5176 all of those functions (which means duplicating a lot of information from
5177 the definitions) and rearranging the functions so that as many as possible
5178 are defined before they are used. One way to automate the latter process
5179 is to get a list for each function of the functions it calls directly.
5180 Many programs can generate such lists. They describe a call graph.
5181 Consider the following list, in which a given line indicates that the
5182 function on the left calls the one on the right directly.
5188 tail_file pretty_name
5189 tail_file write_header
5191 tail_forever recheck
5192 tail_forever pretty_name
5193 tail_forever write_header
5194 tail_forever dump_remainder
5197 tail_lines start_lines
5198 tail_lines dump_remainder
5199 tail_lines file_lines
5200 tail_lines pipe_lines
5202 tail_bytes start_bytes
5203 tail_bytes dump_remainder
5204 tail_bytes pipe_bytes
5205 file_lines dump_remainder
5209 then you can use @command{tsort} to produce an ordering of those
5210 functions that satisfies your requirement.
5213 example$ tsort call-graph | tac
5233 @command{tsort} detects any cycles in the input and writes the first cycle
5234 encountered to standard error.
5236 Note that for a given partial ordering, generally there is no unique
5237 total ordering. In the context of the call graph above, the function
5238 @code{parse_options} may be placed anywhere in the list as long as it
5239 precedes @code{main}.
5241 The only options are @option{--help} and @option{--version}. @xref{Common
5247 * tsort background:: Where tsort came from.
5250 @node tsort background
5251 @subsection @command{tsort}: Background
5253 @command{tsort} exists because very early versions of the Unix linker processed
5254 an archive file exactly once, and in order. As @command{ld} read each object
5255 in the archive, it decided whether it was needed in the program based on
5256 whether it defined any symbols which were undefined at that point in
5259 This meant that dependencies within the archive had to be handled
5260 specially. For example, @code{scanf} probably calls @code{read}. That means
5261 that in a single pass through an archive, it was important for @code{scanf.o}
5262 to appear before read.o, because otherwise a program which calls
5263 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5264 reference to @code{read}.
5266 The way to address this problem was to first generate a set of
5267 dependencies of one object file on another. This was done by a shell
5268 script called @command{lorder}. The GNU tools don't provide a version of
5269 lorder, as far as I know, but you can still find it in BSD
5272 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5273 resulting sort to define the order in which you added objects to the archive.
5275 This whole procedure has been obsolete since about 1980, because
5276 Unix archives now contain a symbol table (traditionally built by
5277 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5278 linker uses the symbol table to effectively make multiple passes over
5281 Anyhow, that's where tsort came from. To solve an old problem with
5282 the way the linker handled archive files, which has since been solved
5286 @node Operating on fields
5287 @chapter Operating on fields
5290 * cut invocation:: Print selected parts of lines.
5291 * paste invocation:: Merge lines of files.
5292 * join invocation:: Join lines on a common field.
5296 @node cut invocation
5297 @section @command{cut}: Print selected parts of lines
5300 @command{cut} writes to standard output selected parts of each line of each
5301 input file, or standard input if no files are given or for a file name of
5305 cut @var{option}@dots{} [@var{file}]@dots{}
5308 In the table which follows, the @var{byte-list}, @var{character-list},
5309 and @var{field-list} are one or more numbers or ranges (two numbers
5310 separated by a dash) separated by commas. Bytes, characters, and
5311 fields are numbered starting at 1. Incomplete ranges may be
5312 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5313 @samp{@var{n}} through end of line or last field. The list elements
5314 can be repeated, can overlap, and can be specified in any order; but
5315 the selected input is written in the same order that it is read, and
5316 is written exactly once.
5318 The program accepts the following options. Also see @ref{Common
5323 @item -b @var{byte-list}
5324 @itemx --bytes=@var{byte-list}
5327 Select for printing only the bytes in positions listed in
5328 @var{byte-list}. Tabs and backspaces are treated like any other
5329 character; they take up 1 byte. If an output delimiter is specified,
5330 (see the description of @option{--output-delimiter}), then output that
5331 string between ranges of selected bytes.
5333 @item -c @var{character-list}
5334 @itemx --characters=@var{character-list}
5336 @opindex --characters
5337 Select for printing only the characters in positions listed in
5338 @var{character-list}. The same as @option{-b} for now, but
5339 internationalization will change that. Tabs and backspaces are
5340 treated like any other character; they take up 1 character. If an
5341 output delimiter is specified, (see the description of
5342 @option{--output-delimiter}), then output that string between ranges
5345 @item -f @var{field-list}
5346 @itemx --fields=@var{field-list}
5349 Select for printing only the fields listed in @var{field-list}.
5350 Fields are separated by a TAB character by default. Also print any
5351 line that contains no delimiter character, unless the
5352 @option{--only-delimited} (@option{-s}) option is specified.
5353 Note @command{cut} does not support specifying runs of whitespace as a
5354 delimiter, so to achieve that common functionality one can pre-process
5355 with @command{tr} like:
5357 tr -s '[:blank:]' '\t' | cut -f@dots{}
5360 @item -d @var{input_delim_byte}
5361 @itemx --delimiter=@var{input_delim_byte}
5363 @opindex --delimiter
5364 With @option{-f}, use the first byte of @var{input_delim_byte} as
5365 the input fields separator (default is TAB).
5369 Do not split multi-byte characters (no-op for now).
5372 @itemx --only-delimited
5374 @opindex --only-delimited
5375 For @option{-f}, do not print lines that do not contain the field separator
5376 character. Normally, any line without a field separator is printed verbatim.
5378 @item --output-delimiter=@var{output_delim_string}
5379 @opindex --output-delimiter
5380 With @option{-f}, output fields are separated by @var{output_delim_string}.
5381 The default with @option{-f} is to use the input delimiter.
5382 When using @option{-b} or @option{-c} to select ranges of byte or
5383 character offsets (as opposed to ranges of fields),
5384 output @var{output_delim_string} between non-overlapping
5385 ranges of selected bytes.
5388 @opindex --complement
5389 This option is a @acronym{GNU} extension.
5390 Select for printing the complement of the bytes, characters or fields
5391 selected with the @option{-b}, @option{-c} or @option{-f} options.
5392 In other words, do @emph{not} print the bytes, characters or fields
5393 specified via those options. This option is useful when you have
5394 many fields and want to print all but a few of them.
5401 @node paste invocation
5402 @section @command{paste}: Merge lines of files
5405 @cindex merging files
5407 @command{paste} writes to standard output lines consisting of sequentially
5408 corresponding lines of each given file, separated by a TAB character.
5409 Standard input is used for a file name of @samp{-} or if no input files
5431 paste [@var{option}]@dots{} [@var{file}]@dots{}
5434 The program accepts the following options. Also see @ref{Common options}.
5442 Paste the lines of one file at a time rather than one line from each
5443 file. Using the above example data:
5446 $ paste -s num2 let3
5451 @item -d @var{delim-list}
5452 @itemx --delimiters=@var{delim-list}
5454 @opindex --delimiters
5455 Consecutively use the characters in @var{delim-list} instead of
5456 TAB to separate merged lines. When @var{delim-list} is
5457 exhausted, start again at its beginning. Using the above example data:
5460 $ paste -d '%_' num2 let3 num2
5471 @node join invocation
5472 @section @command{join}: Join lines on a common field
5475 @cindex common field, joining on
5477 @command{join} writes to standard output a line for each pair of input
5478 lines that have identical join fields. Synopsis:
5481 join [@var{option}]@dots{} @var{file1} @var{file2}
5484 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5485 meaning standard input. @var{file1} and @var{file2} should be
5486 sorted on the join fields.
5489 Normally, the sort order is that of the
5490 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5491 the @option{-t} option is given, the sort comparison ignores blanks at
5492 the start of the join field, as in @code{sort -b}. If the
5493 @option{--ignore-case} option is given, the sort comparison ignores
5494 the case of characters in the join field, as in @code{sort -f}.
5496 The @command{sort} and @command{join} commands should use consistent
5497 locales and options if the output of @command{sort} is fed to
5498 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5499 sort a file on its default join field, but if you select a non-default
5500 locale, join field, separator, or comparison options, then you should
5501 do so consistently between @command{join} and @command{sort}.
5502 If @samp{join -t ''} is specified then the whole line is considered which
5503 matches the default operation of sort.
5505 If the input has no unpairable lines, a @acronym{GNU} extension is
5506 available; the sort order can be any order that considers two fields
5507 to be equal if and only if the sort comparison described above
5508 considers them to be equal. For example:
5525 @checkOrderOption{join}
5529 @item the join field is the first field in each line;
5530 @item fields in the input are separated by one or more blanks, with leading
5531 blanks on the line ignored;
5532 @item fields in the output are separated by a space;
5533 @item each output line consists of the join field, the remaining
5534 fields from @var{file1}, then the remaining fields from @var{file2}.
5537 The program accepts the following options. Also see @ref{Common options}.
5541 @item -a @var{file-number}
5543 Print a line for each unpairable line in file @var{file-number} (either
5544 @samp{1} or @samp{2}), in addition to the normal output.
5547 Fail with an error message if either input file is wrongly ordered.
5549 @item --nocheck-order
5550 Do not check that both input files are in sorted order. This is the default.
5552 @item -e @var{string}
5554 Replace those output fields that are missing in the input with
5559 Treat the first line of each input file as a header line. The header lines will
5560 be joined and printed as the first output line. If @option{-o} is used to
5561 specify output format, the header line will be printed according to the
5562 specified format. The header lines will not be checked for ordering even if
5563 @option{--check-order} is specified. Also if the header lines from each file
5564 do not match, the heading fields from the first file will be used.
5567 @itemx --ignore-case
5569 @opindex --ignore-case
5570 Ignore differences in case when comparing keys.
5571 With this option, the lines of the input files must be ordered in the same way.
5572 Use @samp{sort -f} to produce this ordering.
5574 @item -1 @var{field}
5576 Join on field @var{field} (a positive integer) of file 1.
5578 @item -2 @var{field}
5580 Join on field @var{field} (a positive integer) of file 2.
5582 @item -j @var{field}
5583 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5585 @item -o @var{field-list}
5586 Construct each output line according to the format in @var{field-list}.
5587 Each element in @var{field-list} is either the single character @samp{0} or
5588 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5589 @samp{2} and @var{n} is a positive field number.
5591 A field specification of @samp{0} denotes the join field.
5592 In most cases, the functionality of the @samp{0} field spec
5593 may be reproduced using the explicit @var{m.n} that corresponds
5594 to the join field. However, when printing unpairable lines
5595 (using either of the @option{-a} or @option{-v} options), there is no way
5596 to specify the join field using @var{m.n} in @var{field-list}
5597 if there are unpairable lines in both files.
5598 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5599 field specification notation.
5601 The elements in @var{field-list}
5602 are separated by commas or blanks.
5603 Blank separators typically need to be quoted for the shell. For
5604 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5605 2.2'} are equivalent.
5607 All output lines---including those printed because of any -a or -v
5608 option---are subject to the specified @var{field-list}.
5611 Use character @var{char} as the input and output field separator.
5612 Treat as significant each occurrence of @var{char} in the input file.
5613 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5614 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5615 the whole line is considered, matching the default operation of sort.
5616 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5617 character is used to delimit the fields.
5619 @item -v @var{file-number}
5620 Print a line for each unpairable line in file @var{file-number}
5621 (either @samp{1} or @samp{2}), instead of the normal output.
5628 @node Operating on characters
5629 @chapter Operating on characters
5631 @cindex operating on characters
5633 This commands operate on individual characters.
5636 * tr invocation:: Translate, squeeze, and/or delete characters.
5637 * expand invocation:: Convert tabs to spaces.
5638 * unexpand invocation:: Convert spaces to tabs.
5643 @section @command{tr}: Translate, squeeze, and/or delete characters
5650 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5653 @command{tr} copies standard input to standard output, performing
5654 one of the following operations:
5658 translate, and optionally squeeze repeated characters in the result,
5660 squeeze repeated characters,
5664 delete characters, then squeeze repeated characters from the result.
5667 The @var{set1} and (if given) @var{set2} arguments define ordered
5668 sets of characters, referred to below as @var{set1} and @var{set2}. These
5669 sets are the characters of the input that @command{tr} operates on.
5670 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5672 complement (all of the characters that are not in @var{set1}).
5674 Currently @command{tr} fully supports only single-byte characters.
5675 Eventually it will support multibyte characters; when it does, the
5676 @option{-C} option will cause it to complement the set of characters,
5677 whereas @option{-c} will cause it to complement the set of values.
5678 This distinction will matter only when some values are not characters,
5679 and this is possible only in locales using multibyte encodings when
5680 the input contains encoding errors.
5682 The program accepts the @option{--help} and @option{--version}
5683 options. @xref{Common options}. Options must precede operands.
5688 * Character sets:: Specifying sets of characters.
5689 * Translating:: Changing one set of characters to another.
5690 * Squeezing:: Squeezing repeats and deleting.
5694 @node Character sets
5695 @subsection Specifying sets of characters
5697 @cindex specifying sets of characters
5699 The format of the @var{set1} and @var{set2} arguments resembles
5700 the format of regular expressions; however, they are not regular
5701 expressions, only lists of characters. Most characters simply
5702 represent themselves in these strings, but the strings can contain
5703 the shorthands listed below, for convenience. Some of them can be
5704 used only in @var{set1} or @var{set2}, as noted below.
5708 @item Backslash escapes
5709 @cindex backslash escapes
5711 The following backslash escape sequences are recognized:
5729 The character with the value given by @var{ooo}, which is 1 to 3
5735 While a backslash followed by a character not listed above is
5736 interpreted as that character, the backslash also effectively
5737 removes any special significance, so it is useful to escape
5738 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5743 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5744 from @var{m} through @var{n}, in ascending order. @var{m} should
5745 collate before @var{n}; if it doesn't, an error results. As an example,
5746 @samp{0-9} is the same as @samp{0123456789}.
5748 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5749 brackets to enclose ranges. Translations specified in that format
5750 sometimes work as expected, since the brackets are often transliterated
5751 to themselves. However, they should be avoided because they sometimes
5752 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5755 Many historically common and even accepted uses of ranges are not
5756 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5757 range will not do what most would expect because @samp{A} through @samp{Z}
5758 are not contiguous as they are in @acronym{ASCII}.
5759 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5760 the best way to work around this is to use character classes (see below).
5761 Otherwise, it is most portable (and most ugly) to enumerate the members
5764 @item Repeated characters
5765 @cindex repeated characters
5767 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5768 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5769 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5770 to as many copies of @var{c} as are needed to make @var{set2} as long as
5771 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5772 octal, otherwise in decimal.
5774 @item Character classes
5775 @cindex character classes
5777 The notation @samp{[:@var{class}:]} expands to all of the characters in
5778 the (predefined) class @var{class}. The characters expand in no
5779 particular order, except for the @code{upper} and @code{lower} classes,
5780 which expand in ascending order. When the @option{--delete} (@option{-d})
5781 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5782 character class can be used in @var{set2}. Otherwise, only the
5783 character classes @code{lower} and @code{upper} are accepted in
5784 @var{set2}, and then only if the corresponding character class
5785 (@code{upper} and @code{lower}, respectively) is specified in the same
5786 relative position in @var{set1}. Doing this specifies case conversion.
5787 The class names are given below; an error results when an invalid class
5799 Horizontal whitespace.
5808 Printable characters, not including space.
5814 Printable characters, including space.
5817 Punctuation characters.
5820 Horizontal or vertical whitespace.
5829 @item Equivalence classes
5830 @cindex equivalence classes
5832 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5833 equivalent to @var{c}, in no particular order. Equivalence classes are
5834 a relatively recent invention intended to support non-English alphabets.
5835 But there seems to be no standard way to define them or determine their
5836 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5837 each character's equivalence class consists only of that character,
5838 which is of no particular use.
5844 @subsection Translating
5846 @cindex translating characters
5848 @command{tr} performs translation when @var{set1} and @var{set2} are
5849 both given and the @option{--delete} (@option{-d}) option is not given.
5850 @command{tr} translates each character of its input that is in @var{set1}
5851 to the corresponding character in @var{set2}. Characters not in
5852 @var{set1} are passed through unchanged. When a character appears more
5853 than once in @var{set1} and the corresponding characters in @var{set2}
5854 are not all the same, only the final one is used. For example, these
5855 two commands are equivalent:
5862 A common use of @command{tr} is to convert lowercase characters to
5863 uppercase. This can be done in many ways. Here are three of them:
5866 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5868 tr '[:lower:]' '[:upper:]'
5872 But note that using ranges like @code{a-z} above is not portable.
5874 When @command{tr} is performing translation, @var{set1} and @var{set2}
5875 typically have the same length. If @var{set1} is shorter than
5876 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5878 On the other hand, making @var{set1} longer than @var{set2} is not
5879 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5880 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5881 the last character of @var{set2} as many times as necessary. System V
5882 @command{tr} truncates @var{set1} to the length of @var{set2}.
5884 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5885 When the @option{--truncate-set1} (@option{-t}) option is given,
5886 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5887 instead. This option is ignored for operations other than translation.
5889 Acting like System V @command{tr} in this case breaks the relatively common
5893 tr -cs A-Za-z0-9 '\012'
5897 because it converts only zero bytes (the first element in the
5898 complement of @var{set1}), rather than all non-alphanumerics, to
5902 By the way, the above idiom is not portable because it uses ranges, and
5903 it assumes that the octal code for newline is 012.
5904 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5907 tr -cs '[:alnum:]' '[\n*]'
5912 @subsection Squeezing repeats and deleting
5914 @cindex squeezing repeat characters
5915 @cindex deleting characters
5917 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5918 removes any input characters that are in @var{set1}.
5920 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5921 @command{tr} replaces each input sequence of a repeated character that
5922 is in @var{set1} with a single occurrence of that character.
5924 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5925 first performs any deletions using @var{set1}, then squeezes repeats
5926 from any remaining characters using @var{set2}.
5928 The @option{--squeeze-repeats} option may also be used when translating,
5929 in which case @command{tr} first performs translation, then squeezes
5930 repeats from any remaining characters using @var{set2}.
5932 Here are some examples to illustrate various combinations of options:
5937 Remove all zero bytes:
5944 Put all words on lines by themselves. This converts all
5945 non-alphanumeric characters to newlines, then squeezes each string
5946 of repeated newlines into a single newline:
5949 tr -cs '[:alnum:]' '[\n*]'
5953 Convert each sequence of repeated newlines to a single newline:
5960 Find doubled occurrences of words in a document.
5961 @c Separate the following two "the"s, so typo checkers don't complain.
5962 For example, people often write ``the @w{}the'' with the repeated words
5963 separated by a newline. The Bourne shell script below works first
5964 by converting each sequence of punctuation and blank characters to a
5965 single newline. That puts each ``word'' on a line by itself.
5966 Next it maps all uppercase characters to lower case, and finally it
5967 runs @command{uniq} with the @option{-d} option to print out only the words
5973 | tr -s '[:punct:][:blank:]' '[\n*]' \
5974 | tr '[:upper:]' '[:lower:]' \
5979 Deleting a small set of characters is usually straightforward. For example,
5980 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5986 However, when @samp{-} is one of those characters, it can be tricky because
5987 @samp{-} has special meanings. Performing the same task as above but also
5988 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5989 that would fail because @command{tr} would try to interpret @option{-a} as
5990 a command-line option. Alternatively, we could try putting the hyphen
5991 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5992 it would make @command{tr} interpret @code{a-x} as the range of characters
5993 @samp{a}@dots{}@samp{x} rather than the three.
5994 One way to solve the problem is to put the hyphen at the end of the list
6001 Or you can use @samp{--} to terminate option processing:
6007 More generally, use the character class notation @code{[=c=]}
6008 with @samp{-} (or any other character) in place of the @samp{c}:
6014 Note how single quotes are used in the above example to protect the
6015 square brackets from interpretation by a shell.
6020 @node expand invocation
6021 @section @command{expand}: Convert tabs to spaces
6024 @cindex tabs to spaces, converting
6025 @cindex converting tabs to spaces
6027 @command{expand} writes the contents of each given @var{file}, or standard
6028 input if none are given or for a @var{file} of @samp{-}, to standard
6029 output, with tab characters converted to the appropriate number of
6033 expand [@var{option}]@dots{} [@var{file}]@dots{}
6036 By default, @command{expand} converts all tabs to spaces. It preserves
6037 backspace characters in the output; they decrement the column count for
6038 tab calculations. The default action is equivalent to @option{-t 8} (set
6039 tabs every 8 columns).
6041 The program accepts the following options. Also see @ref{Common options}.
6045 @item -t @var{tab1}[,@var{tab2}]@dots{}
6046 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6049 @cindex tab stops, setting
6050 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6051 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6052 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6053 last tab stop given with single spaces. Tab stops can be separated by
6054 blanks as well as by commas.
6056 For compatibility, GNU @command{expand} also accepts the obsolete
6057 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6058 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6064 @cindex initial tabs, converting
6065 Only convert initial tabs (those that precede all non-space or non-tab
6066 characters) on each line to spaces.
6073 @node unexpand invocation
6074 @section @command{unexpand}: Convert spaces to tabs
6078 @command{unexpand} writes the contents of each given @var{file}, or
6079 standard input if none are given or for a @var{file} of @samp{-}, to
6080 standard output, converting blanks at the beginning of each line into
6081 as many tab characters as needed. In the default @acronym{POSIX}
6082 locale, a @dfn{blank} is a space or a tab; other locales may specify
6083 additional blank characters. Synopsis:
6086 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6089 By default, @command{unexpand} converts only initial blanks (those
6090 that precede all non-blank characters) on each line. It
6091 preserves backspace characters in the output; they decrement the column
6092 count for tab calculations. By default, tabs are set at every 8th
6095 The program accepts the following options. Also see @ref{Common options}.
6099 @item -t @var{tab1}[,@var{tab2}]@dots{}
6100 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6103 If only one tab stop is given, set the tabs @var{tab1} columns apart
6104 instead of the default 8. Otherwise, set the tabs at columns
6105 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6106 beyond the tab stops given unchanged. Tab stops can be separated by
6107 blanks as well as by commas. This option implies the @option{-a} option.
6109 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6110 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6111 separated by commas. (Unlike @option{-t}, this obsolete option does
6112 not imply @option{-a}.) New scripts should use @option{--first-only -t
6113 @var{tab1}[,@var{tab2}]@dots{}} instead.
6119 Also convert all sequences of two or more blanks just before a tab stop,
6120 even if they occur after non-blank characters in a line.
6127 @node Directory listing
6128 @chapter Directory listing
6130 This chapter describes the @command{ls} command and its variants @command{dir}
6131 and @command{vdir}, which list information about files.
6134 * ls invocation:: List directory contents.
6135 * dir invocation:: Briefly ls.
6136 * vdir invocation:: Verbosely ls.
6137 * dircolors invocation:: Color setup for ls, etc.
6142 @section @command{ls}: List directory contents
6145 @cindex directory listing
6147 The @command{ls} program lists information about files (of any type,
6148 including directories). Options and file arguments can be intermixed
6149 arbitrarily, as usual.
6151 For non-option command-line arguments that are directories, by default
6152 @command{ls} lists the contents of directories, not recursively, and
6153 omitting files with names beginning with @samp{.}. For other non-option
6154 arguments, by default @command{ls} lists just the file name. If no
6155 non-option argument is specified, @command{ls} operates on the current
6156 directory, acting as if it had been invoked with a single argument of @samp{.}.
6159 By default, the output is sorted alphabetically, according to the locale
6160 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6161 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6162 produce output that is sorted differently than you're accustomed to.
6163 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6164 If standard output is
6165 a terminal, the output is in columns (sorted vertically) and control
6166 characters are output as question marks; otherwise, the output is listed
6167 one per line and control characters are output as-is.
6169 Because @command{ls} is such a fundamental program, it has accumulated many
6170 options over the years. They are described in the subsections below;
6171 within each section, options are listed alphabetically (ignoring case).
6172 The division of options into the subsections is not absolute, since some
6173 options affect more than one aspect of @command{ls}'s operation.
6175 @cindex exit status of @command{ls}
6180 1 minor problems (e.g., failure to access a file or directory not
6181 specified as a command line argument. This happens when listing a
6182 directory in which entries are actively being removed or renamed.)
6183 2 serious trouble (e.g., memory exhausted, invalid option, failure
6184 to access a file or directory specified as a command line argument
6185 or a directory loop)
6188 Also see @ref{Common options}.
6191 * Which files are listed::
6192 * What information is listed::
6193 * Sorting the output::
6194 * Details about version sort::
6195 * General output formatting::
6196 * Formatting file timestamps::
6197 * Formatting the file names::
6201 @node Which files are listed
6202 @subsection Which files are listed
6204 These options determine which files @command{ls} lists information for.
6205 By default, @command{ls} lists files and the contents of any
6206 directories on the command line, except that in directories it ignores
6207 files whose names start with @samp{.}.
6215 In directories, do not ignore file names that start with @samp{.}.
6220 @opindex --almost-all
6221 In directories, do not ignore all file names that start with @samp{.};
6222 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6223 option overrides this option.
6226 @itemx --ignore-backups
6228 @opindex --ignore-backups
6229 @cindex backup files, ignoring
6230 In directories, ignore files that end with @samp{~}. This option is
6231 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6236 @opindex --directory
6237 List just the names of directories, as with other types of files, rather
6238 than listing their contents.
6239 @c The following sentence is the same as the one for -F.
6240 Do not follow symbolic links listed on the
6241 command line unless the @option{--dereference-command-line} (@option{-H}),
6242 @option{--dereference} (@option{-L}), or
6243 @option{--dereference-command-line-symlink-to-dir} options are specified.
6246 @itemx --dereference-command-line
6248 @opindex --dereference-command-line
6249 @cindex symbolic links, dereferencing
6250 If a command line argument specifies a symbolic link, show information
6251 for the file the link references rather than for the link itself.
6253 @itemx --dereference-command-line-symlink-to-dir
6254 @opindex --dereference-command-line-symlink-to-dir
6255 @cindex symbolic links, dereferencing
6256 Do not dereference symbolic links, with one exception:
6257 if a command line argument specifies a symbolic link that refers to
6258 a directory, show information for that directory rather than for the
6260 This is the default behavior when no other dereferencing-related
6261 option has been specified (@option{--classify} (@option{-F}),
6262 @option{--directory} (@option{-d}),
6264 @option{--dereference} (@option{-L}), or
6265 @option{--dereference-command-line} (@option{-H})).
6267 @item --group-directories-first
6268 @opindex --group-directories-first
6269 Group all the directories before the files and then sort the
6270 directories and the files separately using the selected sort key
6271 (see --sort option).
6272 That is, this option specifies a primary sort key,
6273 and the --sort option specifies a secondary key.
6274 However, any use of @option{--sort=none}
6275 (@option{-U}) disables this option altogether.
6277 @item --hide=PATTERN
6278 @opindex --hide=@var{pattern}
6279 In directories, ignore files whose names match the shell pattern
6280 @var{pattern}, unless the @option{--all} (@option{-a}) or
6281 @option{--almost-all} (@option{-A}) is also given. This
6282 option acts like @option{--ignore=@var{pattern}} except that it has no
6283 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6284 (@option{-A}) is also given.
6286 This option can be useful in shell aliases. For example, if
6287 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6288 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6289 lists the file @file{README~} even though @samp{ly -A} would not.
6291 @item -I @var{pattern}
6292 @itemx --ignore=@var{pattern}
6294 @opindex --ignore=@var{pattern}
6295 In directories, ignore files whose names match the shell pattern
6296 (not regular expression) @var{pattern}. As
6297 in the shell, an initial @samp{.} in a file name does not match a
6298 wildcard at the start of @var{pattern}. Sometimes it is useful
6299 to give this option several times. For example,
6302 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6305 The first option ignores names of length 3 or more that start with @samp{.},
6306 the second ignores all two-character names that start with @samp{.}
6307 except @samp{..}, and the third ignores names that start with @samp{#}.
6310 @itemx --dereference
6312 @opindex --dereference
6313 @cindex symbolic links, dereferencing
6314 When showing file information for a symbolic link, show information
6315 for the file the link references rather than the link itself.
6316 However, even with this option, @command{ls} still prints the name
6317 of the link itself, not the name of the file that the link points to.
6322 @opindex --recursive
6323 @cindex recursive directory listing
6324 @cindex directory listing, recursive
6325 List the contents of all directories recursively.
6330 @node What information is listed
6331 @subsection What information is listed
6333 These options affect the information that @command{ls} displays. By
6334 default, only file names are shown.
6340 @cindex hurd, author, printing
6341 List each file's author when producing long format directory listings.
6342 In GNU/Hurd, file authors can differ from their owners, but in other
6343 operating systems the two are the same.
6349 @cindex dired Emacs mode support
6350 With the long listing (@option{-l}) format, print an additional line after
6354 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6358 The @var{begn} and @var{endn} are unsigned integers that record the
6359 byte position of the beginning and end of each file name in the output.
6360 This makes it easy for Emacs to find the names, even when they contain
6361 unusual characters such as space or newline, without fancy searching.
6363 If directories are being listed recursively (@option{-R}), output a similar
6364 line with offsets for each subdirectory name:
6367 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6370 Finally, output a line of the form:
6373 //DIRED-OPTIONS// --quoting-style=@var{word}
6377 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6379 Here is an actual example:
6382 $ mkdir -p a/sub/deeper a/sub2
6384 $ touch a/sub/deeper/file
6385 $ ls -gloRF --dired a
6388 -rw-r--r-- 1 0 Jun 10 12:27 f1
6389 -rw-r--r-- 1 0 Jun 10 12:27 f2
6390 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6391 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6395 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6399 -rw-r--r-- 1 0 Jun 10 12:27 file
6403 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6404 //SUBDIRED// 2 3 167 172 228 240 290 296
6405 //DIRED-OPTIONS// --quoting-style=literal
6408 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6409 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6411 The offsets on the @samp{//SUBDIRED//} line delimit the following
6412 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6414 Here is an example of how to extract the fifth entry name, @samp{deeper},
6415 corresponding to the pair of offsets, 222 and 228:
6418 $ ls -gloRF --dired a > out
6419 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6423 Note that although the listing above includes a trailing slash
6424 for the @samp{deeper} entry, the offsets select the name without
6425 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6426 along with an option like @option{--escape} (aka @option{-b}) and operate
6427 on a file whose name contains special characters, notice that the backslash
6432 $ ls -blog --dired 'a b'
6433 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6435 //DIRED-OPTIONS// --quoting-style=escape
6438 If you use a quoting style that adds quote marks
6439 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6440 So beware that the user may select the quoting style via the environment
6441 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6442 should either specify an explicit @option{--quoting-style=literal} option
6443 (aka @option{-N} or @option{--literal}) on the command line, or else be
6444 prepared to parse the escaped names.
6447 @opindex --full-time
6448 Produce long format directory listings, and list times in full. It is
6449 equivalent to using @option{--format=long} with
6450 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6454 Produce long format directory listings, but don't display owner information.
6460 Inhibit display of group information in a long format directory listing.
6461 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6462 provide this option for compatibility.)
6470 @cindex inode number, printing
6471 Print the inode number (also called the file serial number and index
6472 number) of each file to the left of the file name. (This number
6473 uniquely identifies each file within a particular file system.)
6476 @itemx --format=long
6477 @itemx --format=verbose
6480 @opindex long ls @r{format}
6481 @opindex verbose ls @r{format}
6482 In addition to the name of each file, print the file type, file mode bits,
6483 number of hard links, owner name, group name, size, and
6484 timestamp (@pxref{Formatting file timestamps}), normally
6485 the modification time. Print question marks for information that
6486 cannot be determined.
6488 Normally the size is printed as a byte count without punctuation, but
6489 this can be overridden (@pxref{Block size}). For example, @option{-h}
6490 prints an abbreviated, human-readable count, and
6491 @samp{--block-size="'1"} prints a byte count with the thousands
6492 separator of the current locale.
6494 For each directory that is listed, preface the files with a line
6495 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6496 for all files in that directory. The block size currently defaults to 1024
6497 bytes, but this can be overridden (@pxref{Block size}).
6498 The @var{blocks} computed counts each hard link separately;
6499 this is arguably a deficiency.
6501 The file type is one of the following characters:
6503 @c The commented-out entries are ones we're not sure about.
6511 character special file
6513 high performance (``contiguous data'') file
6517 door (Solaris 2.5 and up)
6519 @c semaphore, if this is a distinct file type
6523 @c multiplexed file (7th edition Unix; obsolete)
6525 off-line (``migrated'') file (Cray DMF)
6527 network special file (HP-UX)
6531 port (Solaris 10 and up)
6533 @c message queue, if this is a distinct file type
6537 @c shared memory object, if this is a distinct file type
6539 @c typed memory object, if this is a distinct file type
6541 @c whiteout (4.4BSD; not implemented)
6543 some other file type
6546 @cindex permissions, output by @command{ls}
6547 The file mode bits listed are similar to symbolic mode specifications
6548 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6549 third character of each set of permissions as follows:
6553 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6557 If the set-user-ID or set-group-ID bit is set but the corresponding
6558 executable bit is not set.
6561 If the restricted deletion flag or sticky bit, and the
6562 other-executable bit, are both set. The restricted deletion flag is
6563 another name for the sticky bit. @xref{Mode Structure}.
6566 If the restricted deletion flag or sticky bit is set but the
6567 other-executable bit is not set.
6570 If the executable bit is set and none of the above apply.
6576 Following the file mode bits is a single character that specifies
6577 whether an alternate access method such as an access control list
6578 applies to the file. When the character following the file mode bits is a
6579 space, there is no alternate access method. When it is a printing
6580 character, then there is such a method.
6582 GNU @command{ls} uses a @samp{.} character to indicate a file
6583 with an SELinux security context, but no other alternate access method.
6585 A file with any other combination of alternate access methods
6586 is marked with a @samp{+} character.
6589 @itemx --numeric-uid-gid
6591 @opindex --numeric-uid-gid
6592 @cindex numeric uid and gid
6593 @cindex numeric user and group IDs
6594 Produce long format directory listings, but
6595 display numeric user and group IDs instead of the owner and group names.
6599 Produce long format directory listings, but don't display group information.
6600 It is equivalent to using @option{--format=long} with @option{--no-group} .
6606 @cindex disk allocation
6607 @cindex size of files, reporting
6608 Print the disk allocation of each file to the left of the file name.
6609 This is the amount of disk space used by the file, which is usually a
6610 bit more than the file's size, but it can be less if the file has holes.
6612 Normally the disk allocation is printed in units of
6613 1024 bytes, but this can be overridden (@pxref{Block size}).
6615 @cindex NFS mounts from BSD to HP-UX
6616 For files that are NFS-mounted from an HP-UX system to a BSD system,
6617 this option reports sizes that are half the correct values. On HP-UX
6618 systems, it reports sizes that are twice the correct values for files
6619 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6620 it also affects the HP-UX @command{ls} program.
6629 @cindex security context
6630 Display the SELinux security context or @samp{?} if none is found.
6631 When used with the @option{-l} option, print the security context
6632 to the left of the size column.
6637 @node Sorting the output
6638 @subsection Sorting the output
6640 @cindex sorting @command{ls} output
6641 These options change the order in which @command{ls} sorts the information
6642 it outputs. By default, sorting is done by character code
6643 (e.g., @acronym{ASCII} order).
6649 @itemx --time=status
6652 @opindex ctime@r{, printing or sorting by}
6653 @opindex status time@r{, printing or sorting by}
6654 @opindex use time@r{, printing or sorting files by}
6655 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6656 print the status change time (the @samp{ctime} in the inode) instead of
6657 the modification time.
6658 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6659 or when not using a long listing format,
6660 sort according to the status change time.
6664 @cindex unsorted directory listing
6665 @cindex directory order, listing by
6666 Primarily, like @option{-U}---do not sort; list the files in whatever
6667 order they are stored in the directory. But also enable @option{-a} (list
6668 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6669 were specified before the @option{-f}).
6675 @cindex reverse sorting
6676 Reverse whatever the sorting method is---e.g., list files in reverse
6677 alphabetical order, youngest first, smallest first, or whatever.
6683 @opindex size of files@r{, sorting files by}
6684 Sort by file size, largest first.
6690 @opindex modification time@r{, sorting files by}
6691 Sort by modification time (the @samp{mtime} in the inode), newest first.
6695 @itemx --time=access
6699 @opindex use time@r{, printing or sorting files by}
6700 @opindex atime@r{, printing or sorting files by}
6701 @opindex access time@r{, printing or sorting files by}
6702 If the long listing format (e.g., @option{--format=long}) is being used,
6703 print the last access time (the @samp{atime} in the inode).
6704 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6705 or when not using a long listing format, sort according to the access time.
6711 @opindex none@r{, sorting option for @command{ls}}
6712 Do not sort; list the files in whatever order they are
6713 stored in the directory. (Do not do any of the other unrelated things
6714 that @option{-f} does.) This is especially useful when listing very large
6715 directories, since not doing any sorting can be noticeably faster.
6718 @itemx --sort=version
6721 @opindex version@r{, sorting option for @command{ls}}
6722 Sort by version name and number, lowest first. It behaves like a default
6723 sort, except that each sequence of decimal digits is treated numerically
6724 as an index/version number. (@xref{Details about version sort}.)
6727 @itemx --sort=extension
6730 @opindex extension@r{, sorting files by}
6731 Sort directory contents alphabetically by file extension (characters
6732 after the last @samp{.}); files with no extension are sorted first.
6737 @node Details about version sort
6738 @subsection Details about version sort
6740 Version sorting handles the fact that file names frequently include indices or
6741 version numbers. Standard sorting usually does not produce the order that one
6742 expects because comparisons are made on a character-by-character basis.
6743 Version sorting is especially useful when browsing directories that contain
6744 many files with indices/version numbers in their names:
6748 abc.zml-1.gz abc.zml-1.gz
6749 abc.zml-12.gz abc.zml-2.gz
6750 abc.zml-2.gz abc.zml-12.gz
6753 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6754 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6755 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6756 @var{ver1} < @var{ver2} implies that the name composed of
6757 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6758 ``@var{prefix} @var{ver2} @var{suffix}''.
6760 Note also that leading zeros of numeric parts are ignored:
6764 abc-1.007.tgz abc-1.01a.tgz
6765 abc-1.012b.tgz abc-1.007.tgz
6766 abc-1.01a.tgz abc-1.012b.tgz
6769 This functionality is implemented using gnulib's @code{filevercmp} function,
6770 which has some caveats worth noting.
6773 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6774 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6775 was set to @samp{C}.
6776 @item Some suffixes will not be matched by the regular
6777 expression mentioned above. Consequently these examples may
6778 not sort as you expect:
6786 abc-1.2.3.4.x86_64.rpm
6787 abc-1.2.3.x86_64.rpm
6791 @node General output formatting
6792 @subsection General output formatting
6794 These options affect the appearance of the overall output.
6799 @itemx --format=single-column
6802 @opindex single-column @r{output of files}
6803 List one file per line. This is the default for @command{ls} when standard
6804 output is not a terminal.
6807 @itemx --format=vertical
6810 @opindex vertical @r{sorted files in columns}
6811 List files in columns, sorted vertically. This is the default for
6812 @command{ls} if standard output is a terminal. It is always the default
6813 for the @command{dir} program.
6814 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6815 possible in the fewest lines.
6817 @item --color [=@var{when}]
6819 @cindex color, distinguishing file types with
6820 Specify whether to use color for distinguishing file types. @var{when}
6821 may be omitted, or one of:
6824 @vindex none @r{color option}
6825 - Do not use color at all. This is the default.
6827 @vindex auto @r{color option}
6828 @cindex terminal, using color iff
6829 - Only use color if standard output is a terminal.
6831 @vindex always @r{color option}
6834 Specifying @option{--color} and no @var{when} is equivalent to
6835 @option{--color=always}.
6836 Piping a colorized listing through a pager like @command{more} or
6837 @command{less} usually produces unreadable results. However, using
6838 @code{more -f} does seem to work.
6842 @itemx --indicator-style=classify
6845 @opindex --indicator-style
6846 @cindex file type and executables, marking
6847 @cindex executables and file type, marking
6848 Append a character to each file name indicating the file type. Also,
6849 for regular files that are executable, append @samp{*}. The file type
6850 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6851 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6852 and nothing for regular files.
6853 @c The following sentence is the same as the one for -d.
6854 Do not follow symbolic links listed on the
6855 command line unless the @option{--dereference-command-line} (@option{-H}),
6856 @option{--dereference} (@option{-L}), or
6857 @option{--dereference-command-line-symlink-to-dir} options are specified.
6860 @itemx --indicator-style=file-type
6861 @opindex --file-type
6862 @opindex --indicator-style
6863 @cindex file type, marking
6864 Append a character to each file name indicating the file type. This is
6865 like @option{-F}, except that executables are not marked.
6867 @item --indicator-style=@var{word}
6868 @opindex --indicator-style
6869 Append a character indicator with style @var{word} to entry names,
6874 Do not append any character indicator; this is the default.
6876 Append @samp{/} for directories. This is the same as the @option{-p}
6879 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6880 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6881 the same as the @option{--file-type} option.
6883 Append @samp{*} for executable regular files, otherwise behave as for
6884 @samp{file-type}. This is the same as the @option{-F} or
6885 @option{--classify} option.
6890 Print file sizes in 1024-byte blocks, overriding the default block
6891 size (@pxref{Block size}).
6892 This option is equivalent to @option{--block-size=1K}.
6895 @itemx --format=commas
6898 @opindex commas@r{, outputting between files}
6899 List files horizontally, with as many as will fit on each line,
6900 separated by @samp{, } (a comma and a space).
6903 @itemx --indicator-style=slash
6905 @opindex --indicator-style
6906 @cindex file type, marking
6907 Append a @samp{/} to directory names.
6910 @itemx --format=across
6911 @itemx --format=horizontal
6914 @opindex across@r{, listing files}
6915 @opindex horizontal@r{, listing files}
6916 List the files in columns, sorted horizontally.
6919 @itemx --tabsize=@var{cols}
6922 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6923 @command{ls} uses tabs where possible in the output, for efficiency. If
6924 @var{cols} is zero, do not use tabs at all.
6926 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6927 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6928 do not properly align columns to the right of a TAB following a
6929 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6930 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6931 @command{ls} to align using spaces, not tabs.
6934 @itemx --width=@var{cols}
6938 Assume the screen is @var{cols} columns wide. The default is taken
6939 from the terminal settings if possible; otherwise the environment
6940 variable @env{COLUMNS} is used if it is set; otherwise the default
6946 @node Formatting file timestamps
6947 @subsection Formatting file timestamps
6949 By default, file timestamps are listed in abbreviated form, using
6950 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
6951 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
6952 This format can change depending on the current locale as detailed below.
6954 A timestamp is considered to be @dfn{recent} if it is less than six
6955 months old, and is not dated in the future. If a timestamp dated
6956 today is not listed in recent form, the timestamp is in the future,
6957 which means you probably have clock skew problems which may break
6958 programs like @command{make} that rely on file timestamps.
6961 Time stamps are listed according to the time zone rules specified by
6962 the @env{TZ} environment variable, or by the system default rules if
6963 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6964 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6966 The following option changes how file timestamps are printed.
6969 @item --time-style=@var{style}
6970 @opindex --time-style
6972 List timestamps in style @var{style}. The @var{style} should
6973 be one of the following:
6978 List timestamps using @var{format}, where @var{format} is interpreted
6979 like the format argument of @command{date} (@pxref{date invocation}).
6980 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6981 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6982 with @command{date}, @var{format}'s interpretation is affected by the
6983 @env{LC_TIME} locale category.
6985 If @var{format} contains two format strings separated by a newline,
6986 the former is used for non-recent files and the latter for recent
6987 files; if you want output columns to line up, you may need to insert
6988 spaces in one of the two formats.
6991 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6992 format with nanosecond precision, e.g., @samp{2002-03-30
6993 23:45:56.477817180 -0700}. This style is equivalent to
6994 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6996 This is useful because the time output includes all the information that
6997 is available from the operating system. For example, this can help
6998 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
6999 uses the full timestamp to determine whether a file is out of date.
7002 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7003 @samp{2002-03-30 23:45}. These timestamps are shorter than
7004 @samp{full-iso} timestamps, and are usually good enough for everyday
7005 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7008 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7009 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7010 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7011 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7012 nearly the same information in a smaller space and their brevity helps
7013 @command{ls} output fit within traditional 80-column output lines.
7014 The following two @command{ls} invocations are equivalent:
7019 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7020 ls -l --time-style="iso"
7025 List timestamps in a locale-dependent form. For example, a Finnish
7026 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7027 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7028 timestamps typically consume more space than @samp{iso} timestamps and
7029 are harder for programs to parse because locale conventions vary so
7030 widely, but they are easier for many people to read.
7032 The @env{LC_TIME} locale category specifies the timestamp format. The
7033 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7034 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7035 @command{ls} invocations are equivalent:
7040 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7041 ls -l --time-style="locale"
7044 Other locales behave differently. For example, in a German locale,
7045 @option{--time-style="locale"} might be equivalent to
7046 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7047 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7048 @samp{30. M@"ar 23:45}.
7050 @item posix-@var{style}
7052 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7053 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7054 example, the @samp{posix-long-iso} style lists
7055 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7056 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7061 You can specify the default value of the @option{--time-style} option
7062 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7063 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7064 later use the @option{--dired} option and therefore can parse any date
7065 format, but if you are using Emacs 21.1 or 21.2 and specify a
7066 non-@acronym{POSIX} locale you may need to set
7067 @samp{TIME_STYLE="posix-long-iso"}.
7069 To avoid certain denial-of-service attacks, timestamps that would be
7070 longer than 1000 bytes may be treated as errors.
7073 @node Formatting the file names
7074 @subsection Formatting the file names
7076 These options change how file names themselves are printed.
7082 @itemx --quoting-style=escape
7085 @opindex --quoting-style
7086 @cindex backslash sequences for file names
7087 Quote nongraphic characters in file names using alphabetic and octal
7088 backslash sequences like those used in C.
7092 @itemx --quoting-style=literal
7095 @opindex --quoting-style
7096 Do not quote file names. However, with @command{ls} nongraphic
7097 characters are still printed as question marks if the output is a
7098 terminal and you do not specify the @option{--show-control-chars}
7102 @itemx --hide-control-chars
7104 @opindex --hide-control-chars
7105 Print question marks instead of nongraphic characters in file names.
7106 This is the default if the output is a terminal and the program is
7111 @itemx --quoting-style=c
7113 @opindex --quote-name
7114 @opindex --quoting-style
7115 Enclose file names in double quotes and quote nongraphic characters as
7118 @item --quoting-style=@var{word}
7119 @opindex --quoting-style
7120 @cindex quoting style
7121 Use style @var{word} to quote file names and other strings that may
7122 contain arbitrary characters. The @var{word} should
7123 be one of the following:
7127 Output strings as-is; this is the same as the @option{-N} or
7128 @option{--literal} option.
7130 Quote strings for the shell if they contain shell metacharacters or would
7131 cause ambiguous output.
7132 The quoting is suitable for @acronym{POSIX}-compatible shells like
7133 @command{bash}, but it does not always work for incompatible shells
7136 Quote strings for the shell, even if they would normally not require quoting.
7138 Quote strings as for C character string literals, including the
7139 surrounding double-quote characters; this is the same as the
7140 @option{-Q} or @option{--quote-name} option.
7142 Quote strings as for C character string literals, except omit the
7143 surrounding double-quote
7144 characters; this is the same as the @option{-b} or @option{--escape} option.
7146 Quote strings as for C character string literals, except use
7147 surrounding quotation marks appropriate for the
7150 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7151 Quote strings as for C character string literals, except use
7152 surrounding quotation marks appropriate for the locale, and quote
7153 @t{`like this'} instead of @t{"like
7154 this"} in the default C locale. This looks nicer on many displays.
7157 You can specify the default value of the @option{--quoting-style} option
7158 with the environment variable @env{QUOTING_STYLE}. If that environment
7159 variable is not set, the default value is @samp{literal}, but this
7160 default may change to @samp{shell} in a future version of this package.
7162 @item --show-control-chars
7163 @opindex --show-control-chars
7164 Print nongraphic characters as-is in file names.
7165 This is the default unless the output is a terminal and the program is
7171 @node dir invocation
7172 @section @command{dir}: Briefly list directory contents
7175 @cindex directory listing, brief
7177 @command{dir} is equivalent to @code{ls -C
7178 -b}; that is, by default files are listed in columns, sorted vertically,
7179 and special characters are represented by backslash escape sequences.
7181 @xref{ls invocation, @command{ls}}.
7184 @node vdir invocation
7185 @section @command{vdir}: Verbosely list directory contents
7188 @cindex directory listing, verbose
7190 @command{vdir} is equivalent to @code{ls -l
7191 -b}; that is, by default files are listed in long format and special
7192 characters are represented by backslash escape sequences.
7194 @node dircolors invocation
7195 @section @command{dircolors}: Color setup for @command{ls}
7199 @cindex setup for color
7201 @command{dircolors} outputs a sequence of shell commands to set up the
7202 terminal for color output from @command{ls} (and @command{dir}, etc.).
7206 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7209 If @var{file} is specified, @command{dircolors} reads it to determine which
7210 colors to use for which file types and extensions. Otherwise, a
7211 precompiled database is used. For details on the format of these files,
7212 run @samp{dircolors --print-database}.
7214 To make @command{dircolors} read a @file{~/.dircolors} file if it
7215 exists, you can put the following lines in your @file{~/.bashrc} (or
7216 adapt them to your favorite shell):
7220 test -r $d && eval "$(dircolors $d)"
7224 @vindex SHELL @r{environment variable, and color}
7225 The output is a shell command to set the @env{LS_COLORS} environment
7226 variable. You can specify the shell syntax to use on the command line,
7227 or @command{dircolors} will guess it from the value of the @env{SHELL}
7228 environment variable.
7230 The program accepts the following options. Also see @ref{Common options}.
7235 @itemx --bourne-shell
7238 @opindex --bourne-shell
7239 @cindex Bourne shell syntax for color setup
7240 @cindex @command{sh} syntax for color setup
7241 Output Bourne shell commands. This is the default if the @env{SHELL}
7242 environment variable is set and does not end with @samp{csh} or
7251 @cindex C shell syntax for color setup
7252 @cindex @command{csh} syntax for color setup
7253 Output C shell commands. This is the default if @code{SHELL} ends with
7254 @command{csh} or @command{tcsh}.
7257 @itemx --print-database
7259 @opindex --print-database
7260 @cindex color database, printing
7261 @cindex database for color setup, printing
7262 @cindex printing color database
7263 Print the (compiled-in) default color configuration database. This
7264 output is itself a valid configuration file, and is fairly descriptive
7265 of the possibilities.
7272 @node Basic operations
7273 @chapter Basic operations
7275 @cindex manipulating files
7277 This chapter describes the commands for basic file manipulation:
7278 copying, moving (renaming), and deleting (removing).
7281 * cp invocation:: Copy files.
7282 * dd invocation:: Convert and copy a file.
7283 * install invocation:: Copy files and set attributes.
7284 * mv invocation:: Move (rename) files.
7285 * rm invocation:: Remove files or directories.
7286 * shred invocation:: Remove files more securely.
7291 @section @command{cp}: Copy files and directories
7294 @cindex copying files and directories
7295 @cindex files, copying
7296 @cindex directories, copying
7298 @command{cp} copies files (or, optionally, directories). The copy is
7299 completely independent of the original. You can either copy one file to
7300 another, or copy arbitrarily many files to a destination directory.
7304 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7305 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7306 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7311 If two file names are given, @command{cp} copies the first file to the
7315 If the @option{--target-directory} (@option{-t}) option is given, or
7316 failing that if the last file is a directory and the
7317 @option{--no-target-directory} (@option{-T}) option is not given,
7318 @command{cp} copies each @var{source} file to the specified directory,
7319 using the @var{source}s' names.
7322 Generally, files are written just as they are read. For exceptions,
7323 see the @option{--sparse} option below.
7325 By default, @command{cp} does not copy directories. However, the
7326 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7327 copy recursively by descending into source directories and copying files
7328 to corresponding destination directories.
7330 When copying from a symbolic link, @command{cp} normally follows the
7331 link only when not copying
7332 recursively. This default can be overridden with the
7333 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7334 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7335 @option{-H} options. If more than one of these options is specified,
7336 the last one silently overrides the others.
7338 When copying to a symbolic link, @command{cp} follows the
7339 link only when it refers to an existing regular file.
7340 However, when copying to a dangling symbolic link, @command{cp}
7341 refuses by default, and fails with a diagnostic, since the operation
7342 is inherently dangerous. This behavior is contrary to historical
7343 practice and to @acronym{POSIX}.
7344 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7345 the target of a dangling destination symlink, in spite of the possible risk.
7346 Also, when an option like
7347 @option{--backup} or @option{--link} acts to rename or remove the
7348 destination before copying, @command{cp} renames or removes the
7349 symbolic link rather than the file it points to.
7351 By default, @command{cp} copies the contents of special files only
7352 when not copying recursively. This default can be overridden with the
7353 @option{--copy-contents} option.
7355 @cindex self-backups
7356 @cindex backups, making only
7357 @command{cp} generally refuses to copy a file onto itself, with the
7358 following exception: if @option{--force --backup} is specified with
7359 @var{source} and @var{dest} identical, and referring to a regular file,
7360 @command{cp} will make a backup file, either regular or numbered, as
7361 specified in the usual ways (@pxref{Backup options}). This is useful when
7362 you simply want to make a backup of an existing file before changing it.
7364 The program accepts the following options. Also see @ref{Common options}.
7371 Preserve as much as possible of the structure and attributes of the
7372 original files in the copy (but do not attempt to preserve internal
7373 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7374 directory in a different order).
7375 Try to preserve SELinux security context and extended attributes (xattr),
7376 but ignore any failure to do that and print no corresponding diagnostic.
7377 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7379 @itemx --attributes-only
7380 @opindex --attributes-only
7381 Preserve the specified attributes of the original files in the copy,
7382 but do not copy any data. See the @option{--preserve} option for
7383 controlling which attributes to copy.
7386 @itemx @w{@kbd{--backup}[=@var{method}]}
7389 @vindex VERSION_CONTROL
7390 @cindex backups, making
7391 @xref{Backup options}.
7392 Make a backup of each file that would otherwise be overwritten or removed.
7393 As a special case, @command{cp} makes a backup of @var{source} when the force
7394 and backup options are given and @var{source} and @var{dest} are the same
7395 name for an existing, regular file. One useful application of this
7396 combination of options is this tiny Bourne shell script:
7400 # Usage: backup FILE...
7401 # Create a @sc{gnu}-style backup of each listed FILE.
7403 cp --backup --force -- "$i" "$i"
7407 @item --copy-contents
7408 @cindex directories, copying recursively
7409 @cindex copying directories recursively
7410 @cindex recursively copying directories
7411 @cindex non-directories, copying as special files
7412 If copying recursively, copy the contents of any special files (e.g.,
7413 FIFOs and device files) as if they were regular files. This means
7414 trying to read the data in each source file and writing it to the
7415 destination. It is usually a mistake to use this option, as it
7416 normally has undesirable effects on special files like FIFOs and the
7417 ones typically found in the @file{/dev} directory. In most cases,
7418 @code{cp -R --copy-contents} will hang indefinitely trying to read
7419 from FIFOs and special files like @file{/dev/console}, and it will
7420 fill up your destination disk if you use it to copy @file{/dev/zero}.
7421 This option has no effect unless copying recursively, and it does not
7422 affect the copying of symbolic links.
7426 @cindex symbolic links, copying
7427 @cindex hard links, preserving
7428 Copy symbolic links as symbolic links rather than copying the files that
7429 they point to, and preserve hard links between source files in the copies.
7430 Equivalent to @option{--no-dereference --preserve=links}.
7436 When copying without this option and an existing destination file cannot
7437 be opened for writing, the copy fails. However, with @option{--force}),
7438 when a destination file cannot be opened, @command{cp} then removes it and
7439 tries to open it again. Contrast this behavior with that enabled by
7440 @option{--link} and @option{--symbolic-link}, whereby the destination file
7441 is never opened but rather is removed unconditionally. Also see the
7442 description of @option{--remove-destination}.
7444 This option is independent of the @option{--interactive} or
7445 @option{-i} option: neither cancels the effect of the other.
7447 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7452 If a command line argument specifies a symbolic link, then copy the
7453 file it points to rather than the symbolic link itself. However,
7454 copy (preserving its nature) any symbolic link that is encountered
7455 via recursive traversal.
7458 @itemx --interactive
7460 @opindex --interactive
7461 When copying a file other than a directory, prompt whether to
7462 overwrite an existing destination file. The @option{-i} option overrides
7463 a previous @option{-n} option.
7469 Make hard links instead of copies of non-directories.
7472 @itemx --dereference
7474 @opindex --dereference
7475 Follow symbolic links when copying from them.
7476 With this option, @command{cp} cannot create a symbolic link.
7477 For example, a symlink (to regular file) in the source tree will be copied to
7478 a regular file in the destination tree.
7483 @opindex --no-clobber
7484 Do not overwrite an existing file. The @option{-n} option overrides a previous
7485 @option{-i} option. This option is mutually exclusive with @option{-b} or
7486 @option{--backup} option.
7489 @itemx --no-dereference
7491 @opindex --no-dereference
7492 @cindex symbolic links, copying
7493 Copy symbolic links as symbolic links rather than copying the files that
7494 they point to. This option affects only symbolic links in the source;
7495 symbolic links in the destination are always followed if possible.
7498 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7501 @cindex file information, preserving, extended attributes, xattr
7502 Preserve the specified attributes of the original files.
7503 If specified, the @var{attribute_list} must be a comma-separated list
7504 of one or more of the following strings:
7508 Preserve the file mode bits and access control lists.
7510 Preserve the owner and group. On most modern systems,
7511 only users with appropriate privileges may change the owner of a file,
7513 may preserve the group ownership of a file only if they happen to be
7514 a member of the desired group.
7516 Preserve the times of last access and last modification, when possible.
7517 On older systems, it is not possible to preserve these attributes
7518 when the affected file is a symbolic link.
7519 However, many systems now provide the @code{utimensat} function,
7520 which makes it possible even for symbolic links.
7522 Preserve in the destination files
7523 any links between corresponding source files.
7524 Note that with @option{-L} or @option{-H}, this option can convert
7525 symbolic links to hard links. For example,
7527 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7532 Note the inputs: @file{b} is a symlink to regular file @file{a},
7533 yet the files in destination directory, @file{c/}, are hard-linked.
7534 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7535 tells @command{cp} to dereference command line arguments, it sees two files
7536 with the same inode number, and preserves the perceived hard link.
7538 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7540 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7546 Preserve SELinux security context of the file, or fail with full diagnostics.
7548 Preserve extended attributes of the file, or fail with full diagnostics.
7549 If @command{cp} is built without xattr support, ignore this option.
7550 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7551 they are preserved by this option as well.
7553 Preserve all file attributes.
7554 Equivalent to specifying all of the above, but with the difference
7555 that failure to preserve SELinux security context or extended attributes
7556 does not change @command{cp}'s exit status. In contrast to @option{-a},
7557 all but @samp{Operation not supported} warnings are output.
7560 Using @option{--preserve} with no @var{attribute_list} is equivalent
7561 to @option{--preserve=mode,ownership,timestamps}.
7563 In the absence of this option, each destination file is created with the
7564 mode bits of the corresponding source file, minus the bits set in the
7565 umask and minus the set-user-ID and set-group-ID bits.
7566 @xref{File permissions}.
7568 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7569 @cindex file information, preserving
7570 Do not preserve the specified attributes. The @var{attribute_list}
7571 has the same form as for @option{--preserve}.
7575 @cindex parent directories and @command{cp}
7576 Form the name of each destination file by appending to the target
7577 directory a slash and the specified name of the source file. The last
7578 argument given to @command{cp} must be the name of an existing directory.
7579 For example, the command:
7582 cp --parents a/b/c existing_dir
7586 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7587 any missing intermediate directories.
7594 @opindex --recursive
7595 @cindex directories, copying recursively
7596 @cindex copying directories recursively
7597 @cindex recursively copying directories
7598 @cindex non-directories, copying as special files
7599 Copy directories recursively. By default, do not follow symbolic
7600 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7601 @option{--dereference} (@option{-L}), @option{--no-dereference}
7602 (@option{-P}), and @option{-H} options. Special files are copied by
7603 creating a destination file of the same type as the source; see the
7604 @option{--copy-contents} option. It is not portable to use
7605 @option{-r} to copy symbolic links or special files. On some
7606 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7607 @option{-L} and @option{--copy-contents} for historical reasons.
7608 Also, it is not portable to use @option{-R} to copy symbolic links
7609 unless you also specify @option{-P}, as @acronym{POSIX} allows
7610 implementations that dereference symbolic links by default.
7612 @item --reflink[=@var{when}]
7613 @opindex --reflink[=@var{when}]
7616 @cindex copy on write
7617 Perform a lightweight, copy-on-write (COW) copy.
7618 Copying with this option can succeed only on some file systems.
7619 Once it has succeeded, beware that the source and destination files
7620 share the same disk data blocks as long as they remain unmodified.
7621 Thus, if a disk I/O error affects data blocks of one of the files,
7622 the other suffers the exact same fate.
7624 The @var{when} value can be one of the following:
7628 The default behavior: if the copy-on-write operation is not supported
7629 then report the failure for each file and exit with a failure status.
7632 If the copy-on-write operation is not supported then fall back
7633 to the standard copy behaviour.
7637 @item --remove-destination
7638 @opindex --remove-destination
7639 Remove each existing destination file before attempting to open it
7640 (contrast with @option{-f} above).
7642 @item --sparse=@var{when}
7643 @opindex --sparse=@var{when}
7644 @cindex sparse files, copying
7645 @cindex holes, copying files with
7646 @findex read @r{system call, and holes}
7647 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7648 does not occupy any physical disk blocks; the @samp{read} system call
7649 reads these as zeros. This can both save considerable disk space and
7650 increase speed, since many binary files contain lots of consecutive zero
7651 bytes. By default, @command{cp} detects holes in input source files via a crude
7652 heuristic and makes the corresponding output file sparse as well.
7653 Only regular files may be sparse.
7655 The @var{when} value can be one of the following:
7659 The default behavior: if the input file is sparse, attempt to make
7660 the output file sparse, too. However, if an output file exists but
7661 refers to a non-regular file, then do not attempt to make it sparse.
7664 For each sufficiently long sequence of zero bytes in the input file,
7665 attempt to create a corresponding hole in the output file, even if the
7666 input file does not appear to be sparse.
7667 This is useful when the input file resides on a file system
7668 that does not support sparse files
7669 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7670 but the output file is on a type of file system that does support them.
7671 Holes may be created only in regular files, so if the destination file
7672 is of some other type, @command{cp} does not even try to make it sparse.
7675 Never make the output file sparse.
7676 This is useful in creating a file for use with the @command{mkswap} command,
7677 since such a file must not have any holes.
7680 @optStripTrailingSlashes
7683 @itemx --symbolic-link
7685 @opindex --symbolic-link
7686 @cindex symbolic links, copying with
7687 Make symbolic links instead of copies of non-directories. All source
7688 file names must be absolute (starting with @samp{/}) unless the
7689 destination files are in the current directory. This option merely
7690 results in an error message on systems that do not support symbolic links.
7696 @optNoTargetDirectory
7702 @cindex newer files, copying only
7703 Do not copy a non-directory that has an existing destination with the
7704 same or newer modification time. If time stamps are being preserved,
7705 the comparison is to the source time stamp truncated to the
7706 resolutions of the destination file system and of the system calls
7707 used to update time stamps; this avoids duplicate work if several
7708 @samp{cp -pu} commands are executed with the same source and
7715 Print the name of each file before copying it.
7718 @itemx --one-file-system
7720 @opindex --one-file-system
7721 @cindex file systems, omitting copying to different
7722 Skip subdirectories that are on different file systems from the one that
7723 the copy started on.
7724 However, mount point directories @emph{are} copied.
7732 @section @command{dd}: Convert and copy a file
7735 @cindex converting while copying a file
7737 @command{dd} copies a file (from standard input to standard output, by
7738 default) with a changeable I/O block size, while optionally performing
7739 conversions on it. Synopses:
7742 dd [@var{operand}]@dots{}
7746 The only options are @option{--help} and @option{--version}.
7747 @xref{Common options}. @command{dd} accepts the following operands.
7753 Read from @var{file} instead of standard input.
7757 Write to @var{file} instead of standard output. Unless
7758 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7759 bytes (or the size specified with @samp{seek=}).
7761 @item ibs=@var{bytes}
7763 @cindex block size of input
7764 @cindex input block size
7765 Set the input block size to @var{bytes}.
7766 This makes @command{dd} read @var{bytes} per block.
7767 The default is 512 bytes.
7769 @item obs=@var{bytes}
7771 @cindex block size of output
7772 @cindex output block size
7773 Set the output block size to @var{bytes}.
7774 This makes @command{dd} write @var{bytes} per block.
7775 The default is 512 bytes.
7777 @item bs=@var{bytes}
7780 Set both input and output block sizes to @var{bytes}.
7781 This makes @command{dd} read and write @var{bytes} per block,
7782 overriding any @samp{ibs} and @samp{obs} settings.
7783 In addition, if no data-transforming @option{conv} option is specified,
7784 each input block is copied to the output as a single block,
7785 without aggregating short reads.
7787 @item cbs=@var{bytes}
7789 @cindex block size of conversion
7790 @cindex conversion block size
7791 @cindex fixed-length records, converting to variable-length
7792 @cindex variable-length records, converting to fixed-length
7793 Set the conversion block size to @var{bytes}.
7794 When converting variable-length records to fixed-length ones
7795 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7796 use @var{bytes} as the fixed record length.
7798 @item skip=@var{blocks}
7800 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7802 @item seek=@var{blocks}
7804 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7806 @item count=@var{blocks}
7808 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7809 of everything until the end of the file.
7813 Do not print the overall transfer rate and volume statistics
7814 that normally make up the third status line when @command{dd} exits.
7816 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7818 Convert the file as specified by the @var{conversion} argument(s).
7819 (No spaces around any comma(s).)
7826 @opindex ascii@r{, converting to}
7827 Convert @acronym{EBCDIC} to @acronym{ASCII},
7828 using the conversion table specified by @acronym{POSIX}.
7829 This provides a 1:1 translation for all 256 bytes.
7832 @opindex ebcdic@r{, converting to}
7833 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7834 This is the inverse of the @samp{ascii} conversion.
7837 @opindex alternate ebcdic@r{, converting to}
7838 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7839 using the alternate conversion table specified by @acronym{POSIX}.
7840 This is not a 1:1 translation, but reflects common historical practice
7841 for @samp{~}, @samp{[}, and @samp{]}.
7843 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7847 @opindex block @r{(space-padding)}
7848 For each line in the input, output @samp{cbs} bytes, replacing the
7849 input newline with a space and padding with spaces as necessary.
7853 Remove any trailing spaces in each @samp{cbs}-sized input block,
7854 and append a newline.
7856 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7859 @opindex lcase@r{, converting to}
7860 Change uppercase letters to lowercase.
7863 @opindex ucase@r{, converting to}
7864 Change lowercase letters to uppercase.
7866 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7869 @opindex swab @r{(byte-swapping)}
7870 @cindex byte-swapping
7871 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7872 when an odd number of bytes are read---the last byte is simply copied
7873 (since there is nothing to swap it with).
7877 @cindex read errors, ignoring
7878 Continue after read errors.
7882 @cindex creating output file, avoiding
7883 Do not create the output file; the output file must already exist.
7887 @cindex creating output file, requiring
7888 Fail if the output file already exists; @command{dd} must create the
7891 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7895 @cindex truncating output file, avoiding
7896 Do not truncate the output file.
7899 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7900 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7901 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7906 @cindex synchronized data writes, before finishing
7907 Synchronize output data just before finishing. This forces a physical
7908 write of output data.
7912 @cindex synchronized data and metadata writes, before finishing
7913 Synchronize output data and metadata just before finishing. This
7914 forces a physical write of output data and metadata.
7918 @item iflag=@var{flag}[,@var{flag}]@dots{}
7920 Access the input file using the flags specified by the @var{flag}
7921 argument(s). (No spaces around any comma(s).)
7923 @item oflag=@var{flag}[,@var{flag}]@dots{}
7925 Access the output file using the flags specified by the @var{flag}
7926 argument(s). (No spaces around any comma(s).)
7928 Here are the flags. Not every flag is supported on every operating
7935 @cindex appending to the output file
7936 Write in append mode, so that even if some other process is writing to
7937 this file, every @command{dd} write will append to the current
7938 contents of the file. This flag makes sense only for output.
7939 If you combine this flag with the @samp{of=@var{file}} operand,
7940 you should also specify @samp{conv=notrunc} unless you want the
7941 output file to be truncated before being appended to.
7945 @cindex concurrent I/O
7946 Use concurrent I/O mode for data. This mode performs direct I/O
7947 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7948 A file cannot be opened in CIO mode and with a standard open at the
7954 Use direct I/O for data, avoiding the buffer cache.
7955 Note that the kernel may impose restrictions on read or write buffer sizes.
7956 For example, with an ext4 destination file system and a linux-based kernel,
7957 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7958 output buffer size is not a multiple of 512.
7962 @cindex directory I/O
7964 Fail unless the file is a directory. Most operating systems do not
7965 allow I/O to a directory, so this flag has limited utility.
7969 @cindex synchronized data reads
7970 Use synchronized I/O for data. For the output file, this forces a
7971 physical write of output data on each write. For the input file,
7972 this flag can matter when reading from a remote file that has been
7973 written to synchronously by some other process. Metadata (e.g.,
7974 last-access and last-modified time) is not necessarily synchronized.
7978 @cindex synchronized data and metadata I/O
7979 Use synchronized I/O for both data and metadata.
7983 @cindex nonblocking I/O
7984 Use non-blocking I/O.
7989 Do not update the file's access time.
7990 Some older file systems silently ignore this flag, so it is a good
7991 idea to test it on your files before relying on it.
7995 @cindex controlling terminal
7996 Do not assign the file to be a controlling terminal for @command{dd}.
7997 This has no effect when the file is not a terminal.
7998 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8003 @cindex symbolic links, following
8004 Do not follow symbolic links.
8009 Fail if the file has multiple hard links.
8014 Use binary I/O. This option has an effect only on nonstandard
8015 platforms that distinguish binary from text I/O.
8020 Use text I/O. Like @samp{binary}, this option has no effect on
8025 Accumulate full blocks from input. The @code{read} system call
8026 may return early if a full block is not available.
8027 When that happens, continue calling @code{read} to fill the remainder
8029 This flag can be used only with @code{iflag}.
8033 These flags are not supported on all systems, and @samp{dd} rejects
8034 attempts to use them when they are not supported. When reading from
8035 standard input or writing to standard output, the @samp{nofollow} and
8036 @samp{noctty} flags should not be specified, and the other flags
8037 (e.g., @samp{nonblock}) can affect how other processes behave with the
8038 affected file descriptors, even after @command{dd} exits.
8042 @cindex multipliers after numbers
8043 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8044 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8045 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8046 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8048 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8049 should not be too large---values larger than a few megabytes
8050 are generally wasteful or (as in the gigabyte..exabyte case) downright
8051 counterproductive or error-inducing.
8053 Use different @command{dd} invocations to use different block sizes for
8054 skipping and I/O@. For example, the following shell commands copy data
8055 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8056 4 KiB label at the start of the disk:
8059 disk=/dev/rdsk/c0t1d0s2
8062 # Copy all but the label from disk to tape.
8063 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8065 # Copy from tape back to disk, but leave the disk label alone.
8066 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8069 Sending an @samp{INFO} signal to a running @command{dd}
8070 process makes it print I/O statistics to standard error
8071 and then resume copying. In the example below,
8072 @command{dd} is run in the background to copy 10 million blocks.
8073 The @command{kill} command makes it output intermediate I/O statistics,
8074 and when @command{dd} completes normally or is killed by the
8075 @code{SIGINT} signal, it outputs the final statistics.
8078 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8079 $ kill -s INFO $pid; wait $pid
8080 3385223+0 records in
8081 3385223+0 records out
8082 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8083 10000000+0 records in
8084 10000000+0 records out
8085 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8088 @vindex POSIXLY_CORRECT
8089 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8090 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8091 environment variable is set.
8096 @node install invocation
8097 @section @command{install}: Copy files and set attributes
8100 @cindex copying files and setting attributes
8102 @command{install} copies files while setting their file mode bits and, if
8103 possible, their owner and group. Synopses:
8106 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8107 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8108 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8109 install [@var{option}]@dots{} -d @var{directory}@dots{}
8114 If two file names are given, @command{install} copies the first file to the
8118 If the @option{--target-directory} (@option{-t}) option is given, or
8119 failing that if the last file is a directory and the
8120 @option{--no-target-directory} (@option{-T}) option is not given,
8121 @command{install} copies each @var{source} file to the specified
8122 directory, using the @var{source}s' names.
8125 If the @option{--directory} (@option{-d}) option is given,
8126 @command{install} creates each @var{directory} and any missing parent
8127 directories. Parent directories are created with mode
8128 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8129 current umask. @xref{Directory Setuid and Setgid}, for how the
8130 set-user-ID and set-group-ID bits of parent directories are inherited.
8133 @cindex Makefiles, installing programs in
8134 @command{install} is similar to @command{cp}, but allows you to control the
8135 attributes of destination files. It is typically used in Makefiles to
8136 copy programs into their destination directories. It refuses to copy
8137 files onto themselves.
8139 @cindex extended attributes, xattr
8140 @command{install} never preserves extended attributes (xattr).
8142 The program accepts the following options. Also see @ref{Common options}.
8152 Compare each pair of source and destination files, and if the destination has
8153 identical content and any specified owner, group, permissions, and possibly
8154 SELinux context, then do not modify the destination at all.
8158 Ignored; for compatibility with old Unix versions of @command{install}.
8162 Create any missing parent directories of @var{dest},
8163 then copy @var{source} to @var{dest}.
8164 This option is ignored if a destination directory is specified
8165 via @option{--target-directory=DIR}.
8170 @opindex --directory
8171 @cindex directories, creating with given attributes
8172 @cindex parent directories, creating missing
8173 @cindex leading directories, creating missing
8174 Create any missing parent directories, giving them the default
8175 attributes. Then create each given directory, setting their owner,
8176 group and mode as given on the command line or to the defaults.
8178 @item -g @var{group}
8179 @itemx --group=@var{group}
8182 @cindex group ownership of installed files, setting
8183 Set the group ownership of installed files or directories to
8184 @var{group}. The default is the process's current group. @var{group}
8185 may be either a group name or a numeric group ID.
8188 @itemx --mode=@var{mode}
8191 @cindex permissions of installed files, setting
8192 Set the file mode bits for the installed file or directory to @var{mode},
8193 which can be either an octal number, or a symbolic mode as in
8194 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8195 point of departure (@pxref{File permissions}).
8196 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8197 execute for the owner, read and execute for group and other, and with
8198 set-user-ID and set-group-ID disabled.
8199 This default is not quite the same as @samp{755}, since it disables
8200 instead of preserving set-user-ID and set-group-ID on directories.
8201 @xref{Directory Setuid and Setgid}.
8203 @item -o @var{owner}
8204 @itemx --owner=@var{owner}
8207 @cindex ownership of installed files, setting
8208 @cindex appropriate privileges
8209 @vindex root @r{as default owner}
8210 If @command{install} has appropriate privileges (is run as root), set the
8211 ownership of installed files or directories to @var{owner}. The default
8212 is @code{root}. @var{owner} may be either a user name or a numeric user
8215 @item --preserve-context
8216 @opindex --preserve-context
8218 @cindex security context
8219 Preserve the SELinux security context of files and directories.
8220 Failure to preserve the context in all of the files or directories
8221 will result in an exit status of 1. If SELinux is disabled then
8222 print a warning and ignore the option.
8225 @itemx --preserve-timestamps
8227 @opindex --preserve-timestamps
8228 @cindex timestamps of installed files, preserving
8229 Set the time of last access and the time of last modification of each
8230 installed file to match those of each corresponding original file.
8231 When a file is installed without this option, its last access and
8232 last modification times are both set to the time of installation.
8233 This option is useful if you want to use the last modification times
8234 of installed files to keep track of when they were last built as opposed
8235 to when they were last installed.
8241 @cindex symbol table information, stripping
8242 @cindex stripping symbol table information
8243 Strip the symbol tables from installed binary executables.
8245 @itemx --strip-program=@var{program}
8246 @opindex --strip-program
8247 @cindex symbol table information, stripping, program
8248 Program used to strip binaries.
8254 @optNoTargetDirectory
8260 Print the name of each file before copying it.
8262 @item -Z @var{context}
8263 @itemx --context=@var{context}
8267 @cindex security context
8268 Set the default SELinux security context to be used for any
8269 created files and directories. If SELinux is disabled then
8270 print a warning and ignore the option.
8278 @section @command{mv}: Move (rename) files
8282 @command{mv} moves or renames files (or directories). Synopses:
8285 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8286 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8287 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8292 If two file names are given, @command{mv} moves the first file to the
8296 If the @option{--target-directory} (@option{-t}) option is given, or
8297 failing that if the last file is a directory and the
8298 @option{--no-target-directory} (@option{-T}) option is not given,
8299 @command{mv} moves each @var{source} file to the specified
8300 directory, using the @var{source}s' names.
8303 @command{mv} can move any type of file from one file system to another.
8304 Prior to version @code{4.0} of the fileutils,
8305 @command{mv} could move only regular files between file systems.
8306 For example, now @command{mv} can move an entire directory hierarchy
8307 including special device files from one partition to another. It first
8308 uses some of the same code that's used by @code{cp -a} to copy the
8309 requested directories and files, then (assuming the copy succeeded)
8310 it removes the originals. If the copy fails, then the part that was
8311 copied to the destination partition is removed. If you were to copy
8312 three directories from one partition to another and the copy of the first
8313 directory succeeded, but the second didn't, the first would be left on
8314 the destination partition and the second and third would be left on the
8317 @cindex extended attributes, xattr
8318 @command{mv} always tries to copy extended attributes (xattr), which may
8319 include SELinux context, ACLs or Capabilities.
8320 Upon failure all but @samp{Operation not supported} warnings are output.
8322 @cindex prompting, and @command{mv}
8323 If a destination file exists but is normally unwritable, standard input
8324 is a terminal, and the @option{-f} or @option{--force} option is not given,
8325 @command{mv} prompts the user for whether to replace the file. (You might
8326 own the file, or have write permission on its directory.) If the
8327 response is not affirmative, the file is skipped.
8329 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8330 when it might be a symlink to a directory.
8331 Otherwise, @command{mv} may do something very surprising, since
8332 its behavior depends on the underlying rename system call.
8333 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8334 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8335 renames not the symlink but rather the directory referenced by the symlink.
8336 @xref{Trailing slashes}.
8338 The program accepts the following options. Also see @ref{Common options}.
8348 @cindex prompts, omitting
8349 Do not prompt the user before removing a destination file.
8351 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8352 options, only the final one takes effect.
8357 @itemx --interactive
8359 @opindex --interactive
8360 @cindex prompts, forcing
8361 Prompt whether to overwrite each existing destination file, regardless
8363 If the response is not affirmative, the file is skipped.
8369 @opindex --no-clobber
8370 @cindex prompts, omitting
8371 Do not overwrite an existing file.
8373 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8379 @cindex newer files, moving only
8380 Do not move a non-directory that has an existing destination with the
8381 same or newer modification time.
8382 If the move is across file system boundaries, the comparison is to the
8383 source time stamp truncated to the resolutions of the destination file
8384 system and of the system calls used to update time stamps; this avoids
8385 duplicate work if several @samp{mv -u} commands are executed with the
8386 same source and destination.
8392 Print the name of each file before moving it.
8394 @optStripTrailingSlashes
8400 @optNoTargetDirectory
8408 @section @command{rm}: Remove files or directories
8411 @cindex removing files or directories
8413 @command{rm} removes each given @var{file}. By default, it does not remove
8414 directories. Synopsis:
8417 rm [@var{option}]@dots{} [@var{file}]@dots{}
8420 @cindex prompting, and @command{rm}
8421 If the @option{-I} or @option{--interactive=once} option is given,
8422 and there are more than three files or the @option{-r}, @option{-R},
8423 or @option{--recursive} are given, then @command{rm} prompts the user
8424 for whether to proceed with the entire operation. If the response is
8425 not affirmative, the entire command is aborted.
8427 Otherwise, if a file is unwritable, standard input is a terminal, and
8428 the @option{-f} or @option{--force} option is not given, or the
8429 @option{-i} or @option{--interactive=always} option @emph{is} given,
8430 @command{rm} prompts the user for whether to remove the file.
8431 If the response is not affirmative, the file is skipped.
8433 Any attempt to remove a file whose last file name component is
8434 @file{.} or @file{..} is rejected without any prompting.
8436 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8437 possible to recover the contents of that file. If you want more assurance
8438 that the contents are truly unrecoverable, consider using @command{shred}.
8440 The program accepts the following options. Also see @ref{Common options}.
8448 Ignore nonexistent files and never prompt the user.
8449 Ignore any previous @option{--interactive} (@option{-i}) option.
8453 Prompt whether to remove each file.
8454 If the response is not affirmative, the file is skipped.
8455 Ignore any previous @option{--force} (@option{-f}) option.
8456 Equivalent to @option{--interactive=always}.
8460 Prompt once whether to proceed with the command, if more than three
8461 files are named or if a recursive removal is requested. Ignore any
8462 previous @option{--force} (@option{-f}) option. Equivalent to
8463 @option{--interactive=once}.
8465 @itemx --interactive [=@var{when}]
8466 @opindex --interactive
8467 Specify when to issue an interactive prompt. @var{when} may be
8471 @vindex never @r{interactive option}
8472 - Do not prompt at all.
8474 @vindex once @r{interactive option}
8475 - Prompt once if more than three files are named or if a recursive
8476 removal is requested. Equivalent to @option{-I}.
8478 @vindex always @r{interactive option}
8479 - Prompt for every file being removed. Equivalent to @option{-i}.
8481 @option{--interactive} with no @var{when} is equivalent to
8482 @option{--interactive=always}.
8484 @itemx --one-file-system
8485 @opindex --one-file-system
8486 @cindex one file system, restricting @command{rm} to
8487 When removing a hierarchy recursively, skip any directory that is on a
8488 file system different from that of the corresponding command line argument.
8491 This option is useful when removing a build ``chroot'' hierarchy,
8492 which normally contains no valuable data. However, it is not uncommon
8493 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8494 use one's start-up file. The catch is that it's easy to forget to
8495 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8496 your normally throw-away chroot, that command will remove everything
8497 under @file{/home}, too.
8498 Use the @option{--one-file-system} option, and it will
8499 warn about and skip directories on other file systems.
8500 Of course, this will not save your @file{/home} if it and your
8501 chroot happen to be on the same file system.
8503 @itemx --preserve-root
8504 @opindex --preserve-root
8505 @cindex root directory, disallow recursive destruction
8506 Fail upon any attempt to remove the root directory, @file{/},
8507 when used with the @option{--recursive} option.
8508 This is the default behavior.
8509 @xref{Treating / specially}.
8511 @itemx --no-preserve-root
8512 @opindex --no-preserve-root
8513 @cindex root directory, allow recursive destruction
8514 Do not treat @file{/} specially when removing recursively.
8515 This option is not recommended unless you really want to
8516 remove all the files on your computer.
8517 @xref{Treating / specially}.
8524 @opindex --recursive
8525 @cindex directories, removing (recursively)
8526 Remove the listed directories and their contents recursively.
8532 Print the name of each file before removing it.
8536 @cindex files beginning with @samp{-}, removing
8537 @cindex @samp{-}, removing files beginning with
8538 One common question is how to remove files whose names begin with a
8539 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8540 function to parse its arguments, lets you use the @samp{--} option to
8541 indicate that all following arguments are non-options. To remove a file
8542 called @file{-f} in the current directory, you could type either:
8555 @opindex - @r{and Unix @command{rm}}
8556 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8557 predates the development of the getopt standard syntax.
8562 @node shred invocation
8563 @section @command{shred}: Remove files more securely
8566 @cindex data, erasing
8567 @cindex erasing data
8569 @command{shred} overwrites devices or files, to help prevent even
8570 very expensive hardware from recovering the data.
8572 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8573 not actually destroyed. Only the index listing where the file is
8574 stored is destroyed, and the storage is made available for reuse.
8575 There are undelete utilities that will attempt to reconstruct the index
8576 and can bring the file back if the parts were not reused.
8578 On a busy system with a nearly-full drive, space can get reused in a few
8579 seconds. But there is no way to know for sure. If you have sensitive
8580 data, you may want to be sure that recovery is not possible by actually
8581 overwriting the file with non-sensitive data.
8583 However, even after doing that, it is possible to take the disk back
8584 to a laboratory and use a lot of sensitive (and expensive) equipment
8585 to look for the faint ``echoes'' of the original data underneath the
8586 overwritten data. If the data has only been overwritten once, it's not
8589 The best way to remove something irretrievably is to destroy the media
8590 it's on with acid, melt it down, or the like. For cheap removable media
8591 like floppy disks, this is the preferred method. However, hard drives
8592 are expensive and hard to melt, so the @command{shred} utility tries
8593 to achieve a similar effect non-destructively.
8595 This uses many overwrite passes, with the data patterns chosen to
8596 maximize the damage they do to the old data. While this will work on
8597 floppies, the patterns are designed for best effect on hard drives.
8598 For more details, see the source code and Peter Gutmann's paper
8599 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8600 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8601 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8602 California, July 22--25, 1996).
8604 @strong{Please note} that @command{shred} relies on a very important assumption:
8605 that the file system overwrites data in place. This is the traditional
8606 way to do things, but many modern file system designs do not satisfy this
8607 assumption. Exceptions include:
8612 Log-structured or journaled file systems, such as those supplied with
8613 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8614 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8617 File systems that write redundant data and carry on even if some writes
8618 fail, such as RAID-based file systems.
8621 File systems that make snapshots, such as Network Appliance's NFS server.
8624 File systems that cache in temporary locations, such as NFS version 3
8628 Compressed file systems.
8631 In the particular case of ext3 file systems, the above disclaimer applies (and
8632 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8633 mode, which journals file data in addition to just metadata. In both
8634 the @code{data=ordered} (default) and @code{data=writeback} modes,
8635 @command{shred} works as usual. Ext3 journaling modes can be changed
8636 by adding the @code{data=something} option to the mount options for a
8637 particular file system in the @file{/etc/fstab} file, as documented in
8638 the mount man page (man mount).
8640 If you are not sure how your file system operates, then you should assume
8641 that it does not overwrite data in place, which means that shred cannot
8642 reliably operate on regular files in your file system.
8644 Generally speaking, it is more reliable to shred a device than a file,
8645 since this bypasses the problem of file system design mentioned above.
8646 However, even shredding devices is not always completely reliable. For
8647 example, most disks map out bad sectors invisibly to the application; if
8648 the bad sectors contain sensitive data, @command{shred} won't be able to
8651 @command{shred} makes no attempt to detect or report this problem, just as
8652 it makes no attempt to do anything about backups. However, since it is
8653 more reliable to shred devices than files, @command{shred} by default does
8654 not truncate or remove the output file. This default is more suitable
8655 for devices, which typically cannot be truncated and should not be
8658 Finally, consider the risk of backups and mirrors.
8659 File system backups and remote mirrors may contain copies of the
8660 file that cannot be removed, and that will allow a shredded file
8661 to be recovered later. So if you keep any data you may later want
8662 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8665 shred [@var{option}]@dots{} @var{file}[@dots{}]
8668 The program accepts the following options. Also see @ref{Common options}.
8676 @cindex force deletion
8677 Override file permissions if necessary to allow overwriting.
8680 @itemx -n @var{number}
8681 @itemx --iterations=@var{number}
8682 @opindex -n @var{number}
8683 @opindex --iterations=@var{number}
8684 @cindex iterations, selecting the number of
8685 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8686 overwrite. You can reduce this to save time, or increase it if you think it's
8687 appropriate. After 25 passes all of the internal overwrite patterns will have
8688 been used at least once.
8690 @item --random-source=@var{file}
8691 @opindex --random-source
8692 @cindex random source for shredding
8693 Use @var{file} as a source of random data used to overwrite and to
8694 choose pass ordering. @xref{Random sources}.
8696 @item -s @var{bytes}
8697 @itemx --size=@var{bytes}
8698 @opindex -s @var{bytes}
8699 @opindex --size=@var{bytes}
8700 @cindex size of file to shred
8701 Shred the first @var{bytes} bytes of the file. The default is to shred
8702 the whole file. @var{bytes} can be followed by a size specification like
8703 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8709 @cindex removing files after shredding
8710 After shredding a file, truncate it (if possible) and then remove it.
8711 If a file has multiple links, only the named links will be removed.
8717 Display to standard error all status updates as sterilization proceeds.
8723 By default, @command{shred} rounds the size of a regular file up to the next
8724 multiple of the file system block size to fully erase the last block of the file.
8725 Use @option{--exact} to suppress that behavior.
8726 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8727 blocks, the resulting file will be 512 bytes long. With this option,
8728 shred does not increase the apparent size of the file.
8734 Normally, the last pass that @command{shred} writes is made up of
8735 random data. If this would be conspicuous on your hard drive (for
8736 example, because it looks like encrypted data), or you just think
8737 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8738 all zero bits. This is in addition to the number of passes specified
8739 by the @option{--iterations} option.
8743 You might use the following command to erase all trace of the
8744 file system you'd created on the floppy disk in your first drive.
8745 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8749 shred --verbose /dev/fd0
8752 Similarly, to erase all data on a selected partition of
8753 your hard disk, you could give a command like this:
8756 shred --verbose /dev/sda5
8759 A @var{file} of @samp{-} denotes standard output.
8760 The intended use of this is to shred a removed temporary file.
8767 echo "Hello, world" >&3
8772 However, the command @samp{shred - >file} does not shred the contents
8773 of @var{file}, since the shell truncates @var{file} before invoking
8774 @command{shred}. Use the command @samp{shred file} or (if using a
8775 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8780 @node Special file types
8781 @chapter Special file types
8783 @cindex special file types
8784 @cindex file types, special
8786 This chapter describes commands which create special types of files (and
8787 @command{rmdir}, which removes directories, one special file type).
8789 @cindex special file types
8791 Although Unix-like operating systems have markedly fewer special file
8792 types than others, not @emph{everything} can be treated only as the
8793 undifferentiated byte stream of @dfn{normal files}. For example, when a
8794 file is created or removed, the system must record this information,
8795 which it does in a @dfn{directory}---a special type of file. Although
8796 you can read directories as normal files, if you're curious, in order
8797 for the system to do its job it must impose a structure, a certain
8798 order, on the bytes of the file. Thus it is a ``special'' type of file.
8800 Besides directories, other special file types include named pipes
8801 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8804 * link invocation:: Make a hard link via the link syscall
8805 * ln invocation:: Make links between files.
8806 * mkdir invocation:: Make directories.
8807 * mkfifo invocation:: Make FIFOs (named pipes).
8808 * mknod invocation:: Make block or character special files.
8809 * readlink invocation:: Print value of a symlink or canonical file name.
8810 * rmdir invocation:: Remove empty directories.
8811 * unlink invocation:: Remove files via the unlink syscall
8815 @node link invocation
8816 @section @command{link}: Make a hard link via the link syscall
8819 @cindex links, creating
8820 @cindex hard links, creating
8821 @cindex creating links (hard only)
8823 @command{link} creates a single hard link at a time.
8824 It is a minimalist interface to the system-provided
8825 @code{link} function. @xref{Hard Links, , , libc,
8826 The GNU C Library Reference Manual}.
8827 It avoids the bells and whistles of the more commonly-used
8828 @command{ln} command (@pxref{ln invocation}).
8832 link @var{filename} @var{linkname}
8835 @var{filename} must specify an existing file, and @var{linkname}
8836 must specify a nonexistent entry in an existing directory.
8837 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8840 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8841 --no-target-directory @var{filename} @var{linkname}}. However, the
8842 @option{--directory} and @option{--no-target-directory} options are
8843 not specified by @acronym{POSIX}, and the @command{link} command is
8844 more portable in practice.
8846 If @var{filename} is a symbolic link, it is unspecified whether
8847 @var{linkname} will be a hard link to the symbolic link or to the
8848 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8849 to specify which behavior is desired.
8855 @section @command{ln}: Make links between files
8858 @cindex links, creating
8859 @cindex hard links, creating
8860 @cindex symbolic (soft) links, creating
8861 @cindex creating links (hard or soft)
8863 @cindex file systems and hard links
8864 @command{ln} makes links between files. By default, it makes hard links;
8865 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8869 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8870 ln [@var{option}]@dots{} @var{target}
8871 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8872 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8878 If two file names are given, @command{ln} creates a link to the first
8879 file from the second.
8882 If one @var{target} is given, @command{ln} creates a link to that file
8883 in the current directory.
8886 If the @option{--target-directory} (@option{-t}) option is given, or
8887 failing that if the last file is a directory and the
8888 @option{--no-target-directory} (@option{-T}) option is not given,
8889 @command{ln} creates a link to each @var{target} file in the specified
8890 directory, using the @var{target}s' names.
8894 Normally @command{ln} does not remove existing files. Use the
8895 @option{--force} (@option{-f}) option to remove them unconditionally,
8896 the @option{--interactive} (@option{-i}) option to remove them
8897 conditionally, and the @option{--backup} (@option{-b}) option to
8900 @cindex hard link, defined
8901 @cindex inode, and hard links
8902 A @dfn{hard link} is another name for an existing file; the link and the
8903 original are indistinguishable. Technically speaking, they share the
8904 same inode, and the inode contains all the information about a
8905 file---indeed, it is not incorrect to say that the inode @emph{is} the
8906 file. Most systems prohibit making a hard link to
8907 a directory; on those where it is allowed, only the super-user can do
8908 so (and with caution, since creating a cycle will cause problems to many
8909 other utilities). Hard links cannot cross file system boundaries. (These
8910 restrictions are not mandated by @acronym{POSIX}, however.)
8912 @cindex dereferencing symbolic links
8913 @cindex symbolic link, defined
8914 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8915 a special file type (which not all kernels support: System V release 3
8916 (and older) systems lack symlinks) in which the link file actually
8917 refers to a different file, by name. When most operations (opening,
8918 reading, writing, and so on) are passed the symbolic link file, the
8919 kernel automatically @dfn{dereferences} the link and operates on the
8920 target of the link. But some operations (e.g., removing) work on the
8921 link file itself, rather than on its target. The owner and group of a
8922 symlink are not significant to file access performed through
8923 the link, but do have implications on deleting a symbolic link from a
8924 directory with the restricted deletion bit set. On the GNU system,
8925 the mode of a symlink has no significance and cannot be changed, but
8926 on some BSD systems, the mode can be changed and will affect whether
8927 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8928 libc, The GNU C Library Reference Manual}.
8930 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8931 occurs when the string in the symlink does not resolve to a file.
8932 There are no restrictions against creating dangling symbolic links.
8933 There are trade-offs to using absolute or relative symlinks. An
8934 absolute symlink always points to the same file, even if the directory
8935 containing the link is moved. However, if the symlink is visible from
8936 more than one machine (such as on a networked file system), the file
8937 pointed to might not always be the same. A relative symbolic link is
8938 resolved in relation to the directory that contains the link, and is
8939 often useful in referring to files on the same device without regards
8940 to what name that device is mounted on when accessed via networked
8943 When creating a relative symlink in a different location than the
8944 current directory, the resolution of the symlink will be different
8945 than the resolution of the same string from the current directory.
8946 Therefore, many users prefer to first change directories to the
8947 location where the relative symlink will be created, so that
8948 tab-completion or other file resolution will find the same target as
8949 what will be placed in the symlink.
8951 The program accepts the following options. Also see @ref{Common options}.
8962 @opindex --directory
8963 @cindex hard links to directories
8964 Allow users with appropriate privileges to attempt to make hard links
8966 However, note that this will probably fail due to
8967 system restrictions, even for the super-user.
8973 Remove existing destination files.
8976 @itemx --interactive
8978 @opindex --interactive
8979 @cindex prompting, and @command{ln}
8980 Prompt whether to remove existing destination files.
8986 If @option{-s} is not in effect, and the source file is a symbolic
8987 link, create the hard link to the file referred to by the symbolic
8988 link, rather than the symbolic link itself.
8991 @itemx --no-dereference
8993 @opindex --no-dereference
8994 Do not treat the last operand specially when it is a symbolic link to
8995 a directory. Instead, treat it as if it were a normal file.
8997 When the destination is an actual directory (not a symlink to one),
8998 there is no ambiguity. The link is created in that directory.
8999 But when the specified destination is a symlink to a directory,
9000 there are two ways to treat the user's request. @command{ln} can
9001 treat the destination just as it would a normal directory and create
9002 the link in it. On the other hand, the destination can be viewed as a
9003 non-directory---as the symlink itself. In that case, @command{ln}
9004 must delete or backup that symlink before creating the new link.
9005 The default is to treat a destination that is a symlink to a directory
9006 just like a directory.
9008 This option is weaker than the @option{--no-target-directory}
9009 (@option{-T}) option, so it has no effect if both options are given.
9015 If @option{-s} is not in effect, and the source file is a symbolic
9016 link, create the hard link to the symbolic link itself. On platforms
9017 where this is not supported by the kernel, this option creates a
9018 symbolic link with identical contents; since symbolic link contents
9019 cannot be edited, any file name resolution performed through either
9020 link will be the same as if a hard link had been created.
9026 Make symbolic links instead of hard links. This option merely produces
9027 an error message on systems that do not support symbolic links.
9033 @optNoTargetDirectory
9039 Print the name of each file after linking it successfully.
9043 @cindex hard links to symbolic links
9044 @cindex symbolic links and @command{ln}
9045 If @option{-L} and @option{-P} are both given, the last one takes
9046 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9047 are silently ignored. If neither option is given, then this
9048 implementation defaults to @option{-P} if the system @code{link} supports
9049 hard links to symbolic links (such as the GNU system), and @option{-L}
9050 if @code{link} follows symbolic links (such as on BSD).
9059 # Create link ../a pointing to a in that directory.
9060 # Not really useful because it points to itself.
9065 # Change to the target before creating symlinks to avoid being confused.
9071 # Hard coded file names don't move well.
9072 ln -s $(pwd)/a /some/dir/
9076 # Relative file names survive directory moves and also
9077 # work across networked file systems.
9078 ln -s afile anotherfile
9079 ln -s ../adir/afile yetanotherfile
9083 @node mkdir invocation
9084 @section @command{mkdir}: Make directories
9087 @cindex directories, creating
9088 @cindex creating directories
9090 @command{mkdir} creates directories with the specified names. Synopsis:
9093 mkdir [@var{option}]@dots{} @var{name}@dots{}
9096 @command{mkdir} creates each directory @var{name} in the order given.
9097 It reports an error if @var{name} already exists, unless the
9098 @option{-p} option is given and @var{name} is a directory.
9100 The program accepts the following options. Also see @ref{Common options}.
9105 @itemx --mode=@var{mode}
9108 @cindex modes of created directories, setting
9109 Set the file permission bits of created directories to @var{mode},
9110 which uses the same syntax as
9111 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9112 everyone) for the point of the departure. @xref{File permissions}.
9114 Normally the directory has the desired file mode bits at the moment it
9115 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9116 special mode bits, but in this case there may be a temporary window
9117 during which the directory exists but its special mode bits are
9118 incorrect. @xref{Directory Setuid and Setgid}, for how the
9119 set-user-ID and set-group-ID bits of directories are inherited unless
9120 overridden in this way.
9126 @cindex parent directories, creating
9127 Make any missing parent directories for each argument, setting their
9128 file permission bits to the umask modified by @samp{u+wx}. Ignore
9129 existing parent directories, and do not change their file permission
9132 To set the file permission bits of any newly-created parent
9133 directories to a value that includes @samp{u+wx}, you can set the
9134 umask before invoking @command{mkdir}. For example, if the shell
9135 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9136 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9137 To set a parent's special mode bits as well, you can invoke
9138 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9139 Setgid}, for how the set-user-ID and set-group-ID bits of
9140 newly-created parent directories are inherited.
9146 Print a message for each created directory. This is most useful with
9149 @item -Z @var{context}
9150 @itemx --context=@var{context}
9154 @cindex security context
9155 Set the default SELinux security context to be used for created directories.
9162 @node mkfifo invocation
9163 @section @command{mkfifo}: Make FIFOs (named pipes)
9166 @cindex FIFOs, creating
9167 @cindex named pipes, creating
9168 @cindex creating FIFOs (named pipes)
9170 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9171 specified names. Synopsis:
9174 mkfifo [@var{option}] @var{name}@dots{}
9177 A @dfn{FIFO} is a special file type that permits independent processes
9178 to communicate. One process opens the FIFO file for writing, and
9179 another for reading, after which data can flow as with the usual
9180 anonymous pipe in shells or elsewhere.
9182 The program accepts the following option. Also see @ref{Common options}.
9187 @itemx --mode=@var{mode}
9190 @cindex modes of created FIFOs, setting
9191 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9192 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9193 for the point of departure. @var{mode} should specify only file
9194 permission bits. @xref{File permissions}.
9196 @item -Z @var{context}
9197 @itemx --context=@var{context}
9201 @cindex security context
9202 Set the default SELinux security context to be used for created FIFOs.
9209 @node mknod invocation
9210 @section @command{mknod}: Make block or character special files
9213 @cindex block special files, creating
9214 @cindex character special files, creating
9216 @command{mknod} creates a FIFO, character special file, or block special
9217 file with the specified name. Synopsis:
9220 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9223 @cindex special files
9224 @cindex block special files
9225 @cindex character special files
9226 Unlike the phrase ``special file type'' above, the term @dfn{special
9227 file} has a technical meaning on Unix: something that can generate or
9228 receive data. Usually this corresponds to a physical piece of hardware,
9229 e.g., a printer or a disk. (These files are typically created at
9230 system-configuration time.) The @command{mknod} command is what creates
9231 files of this type. Such devices can be read either a character at a
9232 time or a ``block'' (many characters) at a time, hence we say there are
9233 @dfn{block special} files and @dfn{character special} files.
9235 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9236 @mayConflictWithShellBuiltIn{mknod}
9238 The arguments after @var{name} specify the type of file to make:
9243 @opindex p @r{for FIFO file}
9247 @opindex b @r{for block special file}
9248 for a block special file
9251 @c Don't document the `u' option -- it's just a synonym for `c'.
9252 @c Do *any* versions of mknod still use it?
9254 @opindex c @r{for character special file}
9255 @c @opindex u @r{for character special file}
9256 for a character special file
9260 When making a block or character special file, the major and minor
9261 device numbers must be given after the file type.
9262 If a major or minor device number begins with @samp{0x} or @samp{0X},
9263 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9264 as octal; otherwise, as decimal.
9266 The program accepts the following option. Also see @ref{Common options}.
9271 @itemx --mode=@var{mode}
9274 Set the mode of created files to @var{mode}, which is symbolic as in
9275 @command{chmod} and uses @samp{a=rw} as the point of departure.
9276 @var{mode} should specify only file permission bits.
9277 @xref{File permissions}.
9279 @item -Z @var{context}
9280 @itemx --context=@var{context}
9284 @cindex security context
9285 Set the default SELinux security context to be used for created files.
9292 @node readlink invocation
9293 @section @command{readlink}: Print value of a symlink or canonical file name
9296 @cindex displaying value of a symbolic link
9297 @cindex canonical file name
9298 @cindex canonicalize a file name
9302 @command{readlink} may work in one of two supported modes:
9308 @command{readlink} outputs the value of the given symbolic link.
9309 If @command{readlink} is invoked with an argument other than the name
9310 of a symbolic link, it produces no output and exits with a nonzero exit code.
9312 @item Canonicalize mode
9314 @command{readlink} outputs the absolute name of the given file which contains
9315 no @file{.}, @file{..} components nor any repeated separators
9316 (@file{/}) or symbolic links.
9321 readlink [@var{option}] @var{file}
9324 By default, @command{readlink} operates in readlink mode.
9326 The program accepts the following options. Also see @ref{Common options}.
9331 @itemx --canonicalize
9333 @opindex --canonicalize
9334 Activate canonicalize mode.
9335 If any component of the file name except the last one is missing or unavailable,
9336 @command{readlink} produces no output and exits with a nonzero exit
9337 code. A trailing slash is ignored.
9340 @itemx --canonicalize-existing
9342 @opindex --canonicalize-existing
9343 Activate canonicalize mode.
9344 If any component is missing or unavailable, @command{readlink} produces
9345 no output and exits with a nonzero exit code. A trailing slash
9346 requires that the name resolve to a directory.
9349 @itemx --canonicalize-missing
9351 @opindex --canonicalize-missing
9352 Activate canonicalize mode.
9353 If any component is missing or unavailable, @command{readlink} treats it
9359 @opindex --no-newline
9360 Do not output the trailing newline.
9370 Suppress most error messages.
9376 Report error messages.
9380 The @command{readlink} utility first appeared in OpenBSD 2.1.
9382 There is a @command{realpath} command on some systems
9383 which operates like @command{readlink} in canonicalize mode.
9388 @node rmdir invocation
9389 @section @command{rmdir}: Remove empty directories
9392 @cindex removing empty directories
9393 @cindex directories, removing empty
9395 @command{rmdir} removes empty directories. Synopsis:
9398 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9401 If any @var{directory} argument does not refer to an existing empty
9402 directory, it is an error.
9404 The program accepts the following options. Also see @ref{Common options}.
9408 @item --ignore-fail-on-non-empty
9409 @opindex --ignore-fail-on-non-empty
9410 @cindex directory deletion, ignoring failures
9411 Ignore each failure to remove a directory that is solely because
9412 the directory is non-empty.
9418 @cindex parent directories, removing
9419 Remove @var{directory}, then try to remove each component of @var{directory}.
9420 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9421 As such, it fails if any of those directories turns out not to be empty.
9422 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9423 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9424 exit unsuccessfully.
9430 @cindex directory deletion, reporting
9431 Give a diagnostic for each successful removal.
9432 @var{directory} is removed.
9436 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9441 @node unlink invocation
9442 @section @command{unlink}: Remove files via the unlink syscall
9445 @cindex removing files or directories (via the unlink syscall)
9447 @command{unlink} deletes a single specified file name.
9448 It is a minimalist interface to the system-provided
9449 @code{unlink} function. @xref{Deleting Files, , , libc,
9450 The GNU C Library Reference Manual}. Synopsis:
9451 It avoids the bells and whistles of the more commonly-used
9452 @command{rm} command (@pxref{rm invocation}).
9455 unlink @var{filename}
9458 On some systems @code{unlink} can be used to delete the name of a
9459 directory. On others, it can be used that way only by a privileged user.
9460 In the GNU system @code{unlink} can never delete the name of a directory.
9462 The @command{unlink} command honors the @option{--help} and
9463 @option{--version} options. To remove a file whose name begins with
9464 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9469 @node Changing file attributes
9470 @chapter Changing file attributes
9472 @cindex changing file attributes
9473 @cindex file attributes, changing
9474 @cindex attributes, file
9476 A file is not merely its contents, a name, and a file type
9477 (@pxref{Special file types}). A file also has an owner (a user ID), a
9478 group (a group ID), permissions (what the owner can do with the file,
9479 what people in the group can do, and what everyone else can do), various
9480 timestamps, and other information. Collectively, we call these a file's
9483 These commands change file attributes.
9486 * chgrp invocation:: Change file groups.
9487 * chmod invocation:: Change access permissions.
9488 * chown invocation:: Change file owners and groups.
9489 * touch invocation:: Change file timestamps.
9493 @node chown invocation
9494 @section @command{chown}: Change file owner and group
9497 @cindex file ownership, changing
9498 @cindex group ownership, changing
9499 @cindex changing file ownership
9500 @cindex changing group ownership
9502 @command{chown} changes the user and/or group ownership of each given @var{file}
9503 to @var{new-owner} or to the user and group of an existing reference file.
9507 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9510 If used, @var{new-owner} specifies the new owner and/or group as follows
9511 (with no embedded white space):
9514 [@var{owner}] [ : [@var{group}] ]
9521 If only an @var{owner} (a user name or numeric user ID) is given, that
9522 user is made the owner of each given file, and the files' group is not
9525 @item owner@samp{:}group
9526 If the @var{owner} is followed by a colon and a @var{group} (a
9527 group name or numeric group ID), with no spaces between them, the group
9528 ownership of the files is changed as well (to @var{group}).
9531 If a colon but no group name follows @var{owner}, that user is
9532 made the owner of the files and the group of the files is changed to
9533 @var{owner}'s login group.
9536 If the colon and following @var{group} are given, but the owner
9537 is omitted, only the group of the files is changed; in this case,
9538 @command{chown} performs the same function as @command{chgrp}.
9541 If only a colon is given, or if @var{new-owner} is empty, neither the
9542 owner nor the group is changed.
9546 If @var{owner} or @var{group} is intended to represent a numeric user
9547 or group ID, then you may specify it with a leading @samp{+}.
9548 @xref{Disambiguating names and IDs}.
9550 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9551 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9552 require support for that, but for backward compatibility @acronym{GNU}
9553 @command{chown} supports @samp{.} so long as no ambiguity results.
9554 New scripts should avoid the use of @samp{.} because it is not
9555 portable, and because it has undesirable results if the entire
9556 @var{owner@samp{.}group} happens to identify a user whose name
9559 The @command{chown} command sometimes clears the set-user-ID or
9560 set-group-ID permission bits. This behavior depends on the policy and
9561 functionality of the underlying @code{chown} system call, which may
9562 make system-dependent file mode modifications outside the control of
9563 the @command{chown} command. For example, the @command{chown} command
9564 might not affect those bits when invoked by a user with appropriate
9565 privileges, or when the
9566 bits signify some function other than executable permission (e.g.,
9568 When in doubt, check the underlying system behavior.
9570 The program accepts the following options. Also see @ref{Common options}.
9578 @cindex changed owners, verbosely describing
9579 Verbosely describe the action for each @var{file} whose ownership
9588 @cindex error messages, omitting
9589 Do not print error messages about files whose ownership cannot be
9592 @itemx @w{@kbd{--from}=@var{old-owner}}
9594 @cindex symbolic links, changing owner
9595 Change a @var{file}'s ownership only if it has current attributes specified
9596 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9598 This option is useful primarily from a security standpoint in that
9599 it narrows considerably the window of potential abuse.
9600 For example, to reflect a user ID numbering change for one user's files
9601 without an option like this, @code{root} might run
9604 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9607 But that is dangerous because the interval between when the @command{find}
9608 tests the existing file's owner and when the @command{chown} is actually run
9610 One way to narrow the gap would be to invoke chown for each file
9614 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9617 But that is very slow if there are many affected files.
9618 With this option, it is safer (the gap is narrower still)
9619 though still not perfect:
9622 chown -h -R --from=OLDUSER NEWUSER /
9626 @opindex --dereference
9627 @cindex symbolic links, changing owner
9629 Do not act on symbolic links themselves but rather on what they point to.
9630 This is the default.
9633 @itemx --no-dereference
9635 @opindex --no-dereference
9636 @cindex symbolic links, changing owner
9638 Act on symbolic links themselves instead of what they point to.
9639 This mode relies on the @code{lchown} system call.
9640 On systems that do not provide the @code{lchown} system call,
9641 @command{chown} fails when a file specified on the command line
9643 By default, no diagnostic is issued for symbolic links encountered
9644 during a recursive traversal, but see @option{--verbose}.
9646 @itemx --preserve-root
9647 @opindex --preserve-root
9648 @cindex root directory, disallow recursive modification
9649 Fail upon any attempt to recursively change the root directory, @file{/}.
9650 Without @option{--recursive}, this option has no effect.
9651 @xref{Treating / specially}.
9653 @itemx --no-preserve-root
9654 @opindex --no-preserve-root
9655 @cindex root directory, allow recursive modification
9656 Cancel the effect of any preceding @option{--preserve-root} option.
9657 @xref{Treating / specially}.
9659 @item --reference=@var{ref_file}
9660 @opindex --reference
9661 Change the user and group of each @var{file} to be the same as those of
9662 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9663 user and group of the symbolic link, but rather those of the file it
9670 Output a diagnostic for every file processed.
9671 If a symbolic link is encountered during a recursive traversal
9672 on a system without the @code{lchown} system call, and @option{--no-dereference}
9673 is in effect, then issue a diagnostic saying neither the symbolic link nor
9674 its referent is being changed.
9679 @opindex --recursive
9680 @cindex recursively changing file ownership
9681 Recursively change ownership of directories and their contents.
9684 @xref{Traversing symlinks}.
9687 @xref{Traversing symlinks}.
9690 @xref{Traversing symlinks}.
9699 # Change the owner of /u to "root".
9702 # Likewise, but also change its group to "staff".
9705 # Change the owner of /u and subfiles to "root".
9710 @node chgrp invocation
9711 @section @command{chgrp}: Change group ownership
9714 @cindex group ownership, changing
9715 @cindex changing group ownership
9717 @command{chgrp} changes the group ownership of each given @var{file}
9718 to @var{group} (which can be either a group name or a numeric group ID)
9719 or to the group of an existing reference file. Synopsis:
9722 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9725 If @var{group} is intended to represent a
9726 numeric group ID, then you may specify it with a leading @samp{+}.
9727 @xref{Disambiguating names and IDs}.
9729 The program accepts the following options. Also see @ref{Common options}.
9737 @cindex changed files, verbosely describing
9738 Verbosely describe the action for each @var{file} whose group actually
9747 @cindex error messages, omitting
9748 Do not print error messages about files whose group cannot be
9752 @opindex --dereference
9753 @cindex symbolic links, changing owner
9755 Do not act on symbolic links themselves but rather on what they point to.
9756 This is the default.
9759 @itemx --no-dereference
9761 @opindex --no-dereference
9762 @cindex symbolic links, changing group
9764 Act on symbolic links themselves instead of what they point to.
9765 This mode relies on the @code{lchown} system call.
9766 On systems that do not provide the @code{lchown} system call,
9767 @command{chgrp} fails when a file specified on the command line
9769 By default, no diagnostic is issued for symbolic links encountered
9770 during a recursive traversal, but see @option{--verbose}.
9772 @itemx --preserve-root
9773 @opindex --preserve-root
9774 @cindex root directory, disallow recursive modification
9775 Fail upon any attempt to recursively change the root directory, @file{/}.
9776 Without @option{--recursive}, this option has no effect.
9777 @xref{Treating / specially}.
9779 @itemx --no-preserve-root
9780 @opindex --no-preserve-root
9781 @cindex root directory, allow recursive modification
9782 Cancel the effect of any preceding @option{--preserve-root} option.
9783 @xref{Treating / specially}.
9785 @item --reference=@var{ref_file}
9786 @opindex --reference
9787 Change the group of each @var{file} to be the same as that of
9788 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9789 group of the symbolic link, but rather that of the file it refers to.
9795 Output a diagnostic for every file processed.
9796 If a symbolic link is encountered during a recursive traversal
9797 on a system without the @code{lchown} system call, and @option{--no-dereference}
9798 is in effect, then issue a diagnostic saying neither the symbolic link nor
9799 its referent is being changed.
9804 @opindex --recursive
9805 @cindex recursively changing group ownership
9806 Recursively change the group ownership of directories and their contents.
9809 @xref{Traversing symlinks}.
9812 @xref{Traversing symlinks}.
9815 @xref{Traversing symlinks}.
9824 # Change the group of /u to "staff".
9827 # Change the group of /u and subfiles to "staff".
9832 @node chmod invocation
9833 @section @command{chmod}: Change access permissions
9836 @cindex changing access permissions
9837 @cindex access permissions, changing
9838 @cindex permissions, changing access
9840 @command{chmod} changes the access permissions of the named files. Synopsis:
9843 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9846 @cindex symbolic links, permissions of
9847 @command{chmod} never changes the permissions of symbolic links, since
9848 the @command{chmod} system call cannot change their permissions.
9849 This is not a problem since the permissions of symbolic links are
9850 never used. However, for each symbolic link listed on the command
9851 line, @command{chmod} changes the permissions of the pointed-to file.
9852 In contrast, @command{chmod} ignores symbolic links encountered during
9853 recursive directory traversals.
9855 A successful use of @command{chmod} clears the set-group-ID bit of a
9856 regular file if the file's group ID does not match the user's
9857 effective group ID or one of the user's supplementary group IDs,
9858 unless the user has appropriate privileges. Additional restrictions
9859 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9860 @var{ref_file} to be ignored. This behavior depends on the policy and
9861 functionality of the underlying @code{chmod} system call. When in
9862 doubt, check the underlying system behavior.
9864 If used, @var{mode} specifies the new file mode bits.
9865 For details, see the section on @ref{File permissions}.
9866 If you really want @var{mode} to have a leading @samp{-}, you should
9867 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9868 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9869 file} (without the @option{--}) complains if it behaves differently
9870 from what @samp{chmod a-w file} would do.
9872 The program accepts the following options. Also see @ref{Common options}.
9880 Verbosely describe the action for each @var{file} whose permissions
9889 @cindex error messages, omitting
9890 Do not print error messages about files whose permissions cannot be
9893 @itemx --preserve-root
9894 @opindex --preserve-root
9895 @cindex root directory, disallow recursive modification
9896 Fail upon any attempt to recursively change the root directory, @file{/}.
9897 Without @option{--recursive}, this option has no effect.
9898 @xref{Treating / specially}.
9900 @itemx --no-preserve-root
9901 @opindex --no-preserve-root
9902 @cindex root directory, allow recursive modification
9903 Cancel the effect of any preceding @option{--preserve-root} option.
9904 @xref{Treating / specially}.
9910 Verbosely describe the action or non-action taken for every @var{file}.
9912 @item --reference=@var{ref_file}
9913 @opindex --reference
9914 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9915 @xref{File permissions}.
9916 If @var{ref_file} is a symbolic link, do not use the mode
9917 of the symbolic link, but rather that of the file it refers to.
9922 @opindex --recursive
9923 @cindex recursively changing access permissions
9924 Recursively change permissions of directories and their contents.
9931 @node touch invocation
9932 @section @command{touch}: Change file timestamps
9935 @cindex changing file timestamps
9936 @cindex file timestamps, changing
9937 @cindex timestamps, changing file
9939 @command{touch} changes the access and/or modification times of the
9940 specified files. Synopsis:
9943 touch [@var{option}]@dots{} @var{file}@dots{}
9946 @cindex empty files, creating
9947 Any @var{file} argument that does not exist is created empty, unless
9948 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9949 (@option{-h}) was in effect.
9951 A @var{file} argument string of @samp{-} is handled specially and
9952 causes @command{touch} to change the times of the file associated with
9955 @cindex permissions, for changing file timestamps
9956 If changing both the access and modification times to the current
9957 time, @command{touch} can change the timestamps for files that the user
9958 running it does not own but has write permission for. Otherwise, the
9959 user must own the files.
9961 Although @command{touch} provides options for changing two of the times---the
9962 times of last access and modification---of a file, there is actually
9963 a standard third one as well: the inode change time. This is often
9964 referred to as a file's @code{ctime}.
9965 The inode change time represents the time when the file's meta-information
9966 last changed. One common example of this is when the permissions of a
9967 file change. Changing the permissions doesn't access the file, so
9968 the atime doesn't change, nor does it modify the file, so the mtime
9969 doesn't change. Yet, something about the file itself has changed,
9970 and this must be noted somewhere. This is the job of the ctime field.
9971 This is necessary, so that, for example, a backup program can make a
9972 fresh copy of the file, including the new permissions value.
9973 Another operation that modifies a file's ctime without affecting
9974 the others is renaming. In any case, it is not possible, in normal
9975 operations, for a user to change the ctime field to a user-specified value.
9976 Some operating systems and file systems support a fourth time: the
9977 birth time, when the file was first created; by definition, this
9978 timestamp never changes.
9981 Time stamps assume the time zone rules specified by the @env{TZ}
9982 environment variable, or by the system default rules if @env{TZ} is
9983 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9984 libc, The GNU C Library Reference Manual}.
9985 You can avoid ambiguities during
9986 daylight saving transitions by using @sc{utc} time stamps.
9988 The program accepts the following options. Also see @ref{Common options}.
9994 @itemx --time=access
9998 @opindex atime@r{, changing}
9999 @opindex access @r{time, changing}
10000 @opindex use @r{time, changing}
10001 Change the access time only.
10006 @opindex --no-create
10007 Do not warn about or create files that do not exist.
10010 @itemx --date=@var{time}
10014 Use @var{time} instead of the current time. It can contain month names,
10015 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10016 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10017 specifies the instant of time that is 489,392,193 nanoseconds after
10018 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10019 minutes east of @acronym{UTC}. @xref{Date input formats}.
10020 File systems that do not support high-resolution time stamps
10021 silently ignore any excess precision here.
10025 @cindex BSD @command{touch} compatibility
10026 Ignored; for compatibility with BSD versions of @command{touch}.
10029 @itemx --no-dereference
10031 @opindex --no-dereference
10032 @cindex symbolic links, changing time
10034 Attempt to change the timestamps of a symbolic link, rather than what
10035 the link refers to. When using this option, empty files are not
10036 created, but option @option{-c} must also be used to avoid warning
10037 about files that do not exist. Not all systems support changing the
10038 timestamps of symlinks, since underlying system support for this
10039 action was not required until @acronym{POSIX} 2008. Also, on some
10040 systems, the mere act of examining a symbolic link changes the access
10041 time, such that only changes to the modification time will persist
10042 long enough to be observable. When coupled with option @option{-r}, a
10043 reference timestamp is taken from a symbolic link rather than the file
10047 @itemx --time=mtime
10048 @itemx --time=modify
10051 @opindex mtime@r{, changing}
10052 @opindex modify @r{time, changing}
10053 Change the modification time only.
10055 @item -r @var{file}
10056 @itemx --reference=@var{file}
10058 @opindex --reference
10059 Use the times of the reference @var{file} instead of the current time.
10060 If this option is combined with the @option{--date=@var{time}}
10061 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10062 the origin for any relative @var{time}s given, but is otherwise ignored.
10063 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10064 equal to five seconds before the corresponding time stamp for @file{foo}.
10065 If @var{file} is a symbolic link, the reference timestamp is taken
10066 from the target of the symlink, unless @option{-h} was also in effect.
10068 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10069 Use the argument (optional four-digit or two-digit years, months,
10070 days, hours, minutes, optional seconds) instead of the current time.
10071 If the year is specified with only two digits, then @var{cc}
10072 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10073 69 @dots{} 99. If no digits of the year are specified,
10074 the argument is interpreted as a date in the current year.
10075 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10079 @vindex _POSIX2_VERSION
10080 On older systems, @command{touch} supports an obsolete syntax, as follows.
10081 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10082 @option{-t} options, and if there are two or more @var{file}s and the
10083 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10084 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10085 any, were moved to the front), and if the represented year
10086 is in the range 1969--1999, that argument is interpreted as the time
10087 for the other files instead of as a file name.
10088 This obsolete behavior can be enabled or disabled with the
10089 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10090 conformance}), but portable scripts should avoid commands whose
10091 behavior depends on this variable.
10092 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10093 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10099 @chapter Disk usage
10103 No disk can hold an infinite amount of data. These commands report
10104 how much disk storage is in use or available, report other file and
10105 file status information, and write buffers to disk.
10108 * df invocation:: Report file system disk space usage.
10109 * du invocation:: Estimate file space usage.
10110 * stat invocation:: Report file or file system status.
10111 * sync invocation:: Synchronize memory and disk.
10112 * truncate invocation:: Shrink or extend the size of a file.
10116 @node df invocation
10117 @section @command{df}: Report file system disk space usage
10120 @cindex file system disk usage
10121 @cindex disk usage by file system
10123 @command{df} reports the amount of disk space used and available on
10124 file systems. Synopsis:
10127 df [@var{option}]@dots{} [@var{file}]@dots{}
10130 With no arguments, @command{df} reports the space used and available on all
10131 currently mounted file systems (of all types). Otherwise, @command{df}
10132 reports on the file system containing each argument @var{file}.
10134 Normally the disk space is printed in units of
10135 1024 bytes, but this can be overridden (@pxref{Block size}).
10136 Non-integer quantities are rounded up to the next higher unit.
10138 @cindex disk device file
10139 @cindex device file, disk
10140 If an argument @var{file} is a disk device file containing a mounted
10141 file system, @command{df} shows the space available on that file system
10142 rather than on the file system containing the device node (i.e., the root
10143 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10144 on unmounted file systems, because on most kinds of systems doing so
10145 requires extremely nonportable intimate knowledge of file system
10148 The program accepts the following options. Also see @ref{Common options}.
10156 @cindex automounter file systems
10157 @cindex ignore file systems
10158 Include in the listing dummy file systems, which
10159 are omitted by default. Such file systems are typically special-purpose
10160 pseudo-file-systems, such as automounter entries.
10162 @item -B @var{size}
10163 @itemx --block-size=@var{size}
10165 @opindex --block-size
10166 @cindex file system sizes
10167 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10168 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10172 @cindex grand total of disk size, usage and available space
10173 Print a grand total of all arguments after all arguments have
10174 been processed. This can be used to find out the total disk size, usage
10175 and available space of all listed devices.
10181 Equivalent to @option{--si}.
10187 @cindex inode usage
10188 List inode usage information instead of block usage. An inode (short
10189 for index node) contains information about a file such as its owner,
10190 permissions, timestamps, and location on the disk.
10194 @cindex kibibytes for file system sizes
10195 Print sizes in 1024-byte blocks, overriding the default block size
10196 (@pxref{Block size}).
10197 This option is equivalent to @option{--block-size=1K}.
10203 @cindex file system types, limiting output to certain
10204 Limit the listing to local file systems. By default, remote file systems
10209 @cindex file system space, retrieving old data more quickly
10210 Do not invoke the @code{sync} system call before getting any usage data.
10211 This may make @command{df} run significantly faster on systems with many
10212 disks, but on some systems (notably SunOS) the results may be slightly
10213 out of date. This is the default.
10216 @itemx --portability
10218 @opindex --portability
10219 @cindex one-line output format
10220 @cindex @acronym{POSIX} output format
10221 @cindex portable output format
10222 @cindex output format, portable
10223 Use the @acronym{POSIX} output format. This is like the default format except
10228 The information about each file system is always printed on exactly
10229 one line; a mount device is never put on a line by itself. This means
10230 that if the mount device name is more than 20 characters long (e.g., for
10231 some network mounts), the columns are misaligned.
10234 The labels in the header output line are changed to conform to @acronym{POSIX}.
10237 The default block size and output format are unaffected by the
10238 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10239 variables. However, the default block size is still affected by
10240 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10241 otherwise. @xref{Block size}.
10248 @cindex file system space, retrieving current data more slowly
10249 Invoke the @code{sync} system call before getting any usage data. On
10250 some systems (notably SunOS), doing this yields more up to date results,
10251 but in general this option makes @command{df} much slower, especially when
10252 there are many or very busy file systems.
10254 @item -t @var{fstype}
10255 @itemx --type=@var{fstype}
10258 @cindex file system types, limiting output to certain
10259 Limit the listing to file systems of type @var{fstype}. Multiple
10260 file system types can be specified by giving multiple @option{-t} options.
10261 By default, nothing is omitted.
10264 @itemx --print-type
10266 @opindex --print-type
10267 @cindex file system types, printing
10268 Print each file system's type. The types printed here are the same ones
10269 you can include or exclude with @option{-t} and @option{-x}. The particular
10270 types printed are whatever is supported by the system. Here are some of
10271 the common names (this list is certainly not exhaustive):
10276 @cindex @acronym{NFS} file system type
10277 An @acronym{NFS} file system, i.e., one mounted over a network from another
10278 machine. This is the one type name which seems to be used uniformly by
10281 @item 4.2@r{, }ufs@r{, }efs@dots{}
10282 @cindex Linux file system types
10283 @cindex local file system types
10284 @opindex 4.2 @r{file system type}
10285 @opindex ufs @r{file system type}
10286 @opindex efs @r{file system type}
10287 A file system on a locally-mounted hard disk. (The system might even
10288 support more than one type here; Linux does.)
10290 @item hsfs@r{, }cdfs
10291 @cindex CD-ROM file system type
10292 @cindex High Sierra file system
10293 @opindex hsfs @r{file system type}
10294 @opindex cdfs @r{file system type}
10295 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10296 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10299 @cindex PC file system
10300 @cindex DOS file system
10301 @cindex MS-DOS file system
10302 @cindex diskette file system
10304 An MS-DOS file system, usually on a diskette.
10308 @item -x @var{fstype}
10309 @itemx --exclude-type=@var{fstype}
10311 @opindex --exclude-type
10312 Limit the listing to file systems not of type @var{fstype}.
10313 Multiple file system types can be eliminated by giving multiple
10314 @option{-x} options. By default, no file system types are omitted.
10317 Ignored; for compatibility with System V versions of @command{df}.
10322 Failure includes the case where no output is generated, so you can
10323 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10324 @var{dir}} to test whether @var{dir} is on a file system of type
10325 @samp{ext3} or @samp{reiserfs}.
10328 @node du invocation
10329 @section @command{du}: Estimate file space usage
10332 @cindex file space usage
10333 @cindex disk usage for files
10335 @command{du} reports the amount of disk space used by the specified files
10336 and for each subdirectory (of directory arguments). Synopsis:
10339 du [@var{option}]@dots{} [@var{file}]@dots{}
10342 With no arguments, @command{du} reports the disk space for the current
10343 directory. Normally the disk space is printed in units of
10344 1024 bytes, but this can be overridden (@pxref{Block size}).
10345 Non-integer quantities are rounded up to the next higher unit.
10347 If two or more hard links point to the same file, only one of the hard
10348 links is counted. The @var{file} argument order affects which links
10349 are counted, and changing the argument order may change the numbers
10350 that @command{du} outputs.
10352 The program accepts the following options. Also see @ref{Common options}.
10360 Show counts for all files, not just directories.
10362 @itemx --apparent-size
10363 @opindex --apparent-size
10364 Print apparent sizes, rather than disk usage. The apparent size of a
10365 file is the number of bytes reported by @code{wc -c} on regular files,
10366 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10367 For example, a file containing the word @samp{zoo} with no newline would,
10368 of course, have an apparent size of 3. Such a small file may require
10369 anywhere from 0 to 16 KiB or more of disk space, depending on
10370 the type and configuration of the file system on which the file resides.
10371 However, a sparse file created with this command:
10374 dd bs=1 seek=2GiB if=/dev/null of=big
10378 has an apparent size of 2 GiB, yet on most modern
10379 systems, it actually uses almost no disk space.
10385 Equivalent to @code{--apparent-size --block-size=1}.
10387 @item -B @var{size}
10388 @itemx --block-size=@var{size}
10390 @opindex --block-size
10392 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10393 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10399 @cindex grand total of disk space
10400 Print a grand total of all arguments after all arguments have
10401 been processed. This can be used to find out the total disk usage of
10402 a given set of files or directories.
10405 @itemx --dereference-args
10407 @opindex --dereference-args
10408 Dereference symbolic links that are command line arguments.
10409 Does not affect other symbolic links. This is helpful for finding
10410 out the disk usage of directories, such as @file{/usr/tmp}, which
10411 are often symbolic links.
10413 @c --files0-from=FILE
10414 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10420 Equivalent to @option{--dereference-args} (@option{-D}).
10424 @cindex kibibytes for file sizes
10425 Print sizes in 1024-byte blocks, overriding the default block size
10426 (@pxref{Block size}).
10427 This option is equivalent to @option{--block-size=1K}.
10430 @itemx --count-links
10432 @opindex --count-links
10433 @cindex hard links, counting in @command{du}
10434 Count the size of all files, even if they have appeared already (as a
10438 @itemx --dereference
10440 @opindex --dereference
10441 @cindex symbolic links, dereferencing in @command{du}
10442 Dereference symbolic links (show the disk space used by the file
10443 or directory that the link points to instead of the space used by
10448 @cindex mebibytes for file sizes
10449 Print sizes in 1,048,576-byte blocks, overriding the default block size
10450 (@pxref{Block size}).
10451 This option is equivalent to @option{--block-size=1M}.
10454 @itemx --no-dereference
10456 @opindex --no-dereference
10457 @cindex symbolic links, dereferencing in @command{du}
10458 For each symbolic links encountered by @command{du},
10459 consider the disk space used by the symbolic link.
10461 @item -d @var{depth}
10462 @item --max-depth=@var{depth}
10463 @opindex -d @var{depth}
10464 @opindex --max-depth=@var{depth}
10465 @cindex limiting output of @command{du}
10466 Show the total for each directory (and file if --all) that is at
10467 most MAX_DEPTH levels down from the root of the hierarchy. The root
10468 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10477 @opindex --summarize
10478 Display only a total for each argument.
10481 @itemx --separate-dirs
10483 @opindex --separate-dirs
10484 Normally, in the output of @command{du} (when not using @option{--summarize}),
10485 the size listed next to a directory name, @var{d}, represents the sum
10486 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10487 With @option{--separate-dirs}, the size reported for a directory name,
10488 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10493 @cindex last modified dates, displaying in @command{du}
10494 Show time of the most recent modification of any file in the directory,
10495 or any of its subdirectories.
10497 @itemx --time=ctime
10498 @itemx --time=status
10501 @opindex ctime@r{, show the most recent}
10502 @opindex status time@r{, show the most recent}
10503 @opindex use time@r{, show the most recent}
10504 Show the most recent status change time (the @samp{ctime} in the inode) of
10505 any file in the directory, instead of the modification time.
10507 @itemx --time=atime
10508 @itemx --time=access
10510 @opindex atime@r{, show the most recent}
10511 @opindex access time@r{, show the most recent}
10512 Show the most recent access time (the @samp{atime} in the inode) of
10513 any file in the directory, instead of the modification time.
10515 @item --time-style=@var{style}
10516 @opindex --time-style
10518 List timestamps in style @var{style}. This option has an effect only if
10519 the @option{--time} option is also specified. The @var{style} should
10520 be one of the following:
10523 @item +@var{format}
10525 List timestamps using @var{format}, where @var{format} is interpreted
10526 like the format argument of @command{date} (@pxref{date invocation}).
10527 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10528 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10529 with @command{date}, @var{format}'s interpretation is affected by the
10530 @env{LC_TIME} locale category.
10533 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10534 format with nanosecond precision, e.g., @samp{2002-03-30
10535 23:45:56.477817180 -0700}. This style is equivalent to
10536 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10539 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10540 @samp{2002-03-30 23:45}. These timestamps are shorter than
10541 @samp{full-iso} timestamps, and are usually good enough for everyday
10542 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10545 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10546 This style is equivalent to @samp{+%Y-%m-%d}.
10550 You can specify the default value of the @option{--time-style} option
10551 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10552 the default style is @samp{long-iso}. For compatibility with @command{ls},
10553 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10554 the newline and any later characters are ignored; if @env{TIME_STYLE}
10555 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10556 @env{TIME_STYLE} is @samp{locale} it is ignored.
10559 @itemx --one-file-system
10561 @opindex --one-file-system
10562 @cindex one file system, restricting @command{du} to
10563 Skip directories that are on different file systems from the one that
10564 the argument being processed is on.
10566 @item --exclude=@var{pattern}
10567 @opindex --exclude=@var{pattern}
10568 @cindex excluding files from @command{du}
10569 When recursing, skip subdirectories or files matching @var{pattern}.
10570 For example, @code{du --exclude='*.o'} excludes files whose names
10573 @item -X @var{file}
10574 @itemx --exclude-from=@var{file}
10575 @opindex -X @var{file}
10576 @opindex --exclude-from=@var{file}
10577 @cindex excluding files from @command{du}
10578 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10579 one per line. If @var{file} is @samp{-}, take the patterns from standard
10584 @cindex NFS mounts from BSD to HP-UX
10585 On BSD systems, @command{du} reports sizes that are half the correct
10586 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10587 systems, it reports sizes that are twice the correct values for
10588 files that are NFS-mounted from BSD systems. This is due to a flaw
10589 in HP-UX; it also affects the HP-UX @command{du} program.
10594 @node stat invocation
10595 @section @command{stat}: Report file or file system status
10598 @cindex file status
10599 @cindex file system status
10601 @command{stat} displays information about the specified file(s). Synopsis:
10604 stat [@var{option}]@dots{} [@var{file}]@dots{}
10607 With no option, @command{stat} reports all information about the given files.
10608 But it also can be used to report the information of the file systems the
10609 given files are located on. If the files are links, @command{stat} can
10610 also give information about the files the links point to.
10612 @mayConflictWithShellBuiltIn{stat}
10617 @itemx --dereference
10619 @opindex --dereference
10620 @cindex symbolic links, dereferencing in @command{stat}
10621 Change how @command{stat} treats symbolic links.
10622 With this option, @command{stat} acts on the file referenced
10623 by each symbolic link argument.
10624 Without it, @command{stat} acts on any symbolic link argument directly.
10627 @itemx --file-system
10629 @opindex --file-system
10630 @cindex file systems
10631 Report information about the file systems where the given files are located
10632 instead of information about the files themselves.
10633 This option implies the @option{-L} option.
10636 @itemx --format=@var{format}
10638 @opindex --format=@var{format}
10639 @cindex output format
10640 Use @var{format} rather than the default format.
10641 @var{format} is automatically newline-terminated, so
10642 running a command like the following with two or more @var{file}
10643 operands produces a line of output for each operand:
10645 $ stat --format=%d:%i / /usr
10650 @itemx --printf=@var{format}
10651 @opindex --printf=@var{format}
10652 @cindex output format
10653 Use @var{format} rather than the default format.
10654 Like @option{--format}, but interpret backslash escapes,
10655 and do not output a mandatory trailing newline.
10656 If you want a newline, include @samp{\n} in the @var{format}.
10657 Here's how you would use @option{--printf} to print the device
10658 and inode numbers of @file{/} and @file{/usr}:
10660 $ stat --printf='%d:%i\n' / /usr
10669 @cindex terse output
10670 Print the information in terse form, suitable for parsing by other programs.
10674 The valid @var{format} directives for files with @option{--format} and
10675 @option{--printf} are:
10678 @item %a - Access rights in octal
10679 @item %A - Access rights in human readable form
10680 @item %b - Number of blocks allocated (see @samp{%B})
10681 @item %B - The size in bytes of each block reported by @samp{%b}
10682 @item %C - The SELinux security context of a file, if available
10683 @item %d - Device number in decimal
10684 @item %D - Device number in hex
10685 @item %f - Raw mode in hex
10686 @item %F - File type
10687 @item %g - Group ID of owner
10688 @item %G - Group name of owner
10689 @item %h - Number of hard links
10690 @item %i - Inode number
10691 @item %m - Mount point (See note below)
10692 @item %n - File name
10693 @item %N - Quoted file name with dereference if symbolic link
10694 @item %o - I/O block size
10695 @item %s - Total size, in bytes
10696 @item %t - Major device type in hex
10697 @item %T - Minor device type in hex
10698 @item %u - User ID of owner
10699 @item %U - User name of owner
10700 @item %w - Time of file birth, or @samp{-} if unknown
10701 @item %W - Time of file birth as seconds since Epoch, or @samp{-}
10702 @item %x - Time of last access
10703 @item %X - Time of last access as seconds since Epoch
10704 @item %y - Time of last modification
10705 @item %Y - Time of last modification as seconds since Epoch
10706 @item %z - Time of last change
10707 @item %Z - Time of last change as seconds since Epoch
10710 The mount point printed by @samp{%m} is similar to that output
10711 by @command{df}, except that:
10714 stat does not dereference symlinks by default
10715 (unless @option{-L} is specified)
10717 stat does not search for specified device nodes in the
10718 file system list, instead operating on them directly
10721 stat outputs the alias for a bind mounted file, rather than
10722 the initial mount point of its backing device.
10723 One can recursively call stat until there is no change in output,
10724 to get the current base mount point
10727 When listing file system information (@option{--file-system} (@option{-f})),
10728 you must use a different set of @var{format} directives:
10731 @item %a - Free blocks available to non-super-user
10732 @item %b - Total data blocks in file system
10733 @item %c - Total file nodes in file system
10734 @item %d - Free file nodes in file system
10735 @item %f - Free blocks in file system
10736 @item %i - File System ID in hex
10737 @item %l - Maximum length of file names
10738 @item %n - File name
10739 @item %s - Block size (for faster transfers)
10740 @item %S - Fundamental block size (for block counts)
10741 @item %t - Type in hex
10742 @item %T - Type in human readable form
10746 Time stamps are listed according to the time zone rules specified by
10747 the @env{TZ} environment variable, or by the system default rules if
10748 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10749 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10754 @node sync invocation
10755 @section @command{sync}: Synchronize data on disk with memory
10758 @cindex synchronize disk and memory
10760 @cindex superblock, writing
10761 @cindex inodes, written buffered
10762 @command{sync} writes any data buffered in memory out to disk. This can
10763 include (but is not limited to) modified superblocks, modified inodes,
10764 and delayed reads and writes. This must be implemented by the kernel;
10765 The @command{sync} program does nothing but exercise the @code{sync} system
10768 @cindex crashes and corruption
10769 The kernel keeps data in memory to avoid doing (relatively slow) disk
10770 reads and writes. This improves performance, but if the computer
10771 crashes, data may be lost or the file system corrupted as a
10772 result. The @command{sync} command ensures everything in memory
10773 is written to disk.
10775 Any arguments are ignored, except for a lone @option{--help} or
10776 @option{--version} (@pxref{Common options}).
10781 @node truncate invocation
10782 @section @command{truncate}: Shrink or extend the size of a file
10785 @cindex truncating, file sizes
10787 @command{truncate} shrinks or extends the size of each @var{file} to the
10788 specified size. Synopsis:
10791 truncate @var{option}@dots{} @var{file}@dots{}
10794 @cindex files, creating
10795 Any @var{file} that does not exist is created.
10797 @cindex sparse files, creating
10798 @cindex holes, creating files with
10799 If a @var{file} is larger than the specified size, the extra data is lost.
10800 If a @var{file} is shorter, it is extended and the extended part (or hole)
10801 reads as zero bytes.
10803 The program accepts the following options. Also see @ref{Common options}.
10810 @opindex --no-create
10811 Do not create files that do not exist.
10816 @opindex --io-blocks
10817 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10819 @item -r @var{rfile}
10820 @itemx --reference=@var{rfile}
10822 @opindex --reference
10823 Base the size of each @var{file} on the size of @var{rfile}.
10825 @item -s @var{size}
10826 @itemx --size=@var{size}
10829 Set or adjust the size of each @var{file} according to @var{size}.
10830 @multiplierSuffixesNoBlocks{size}
10832 @var{size} may also be prefixed by one of the following to adjust
10833 the size of each @var{file} based on their current size:
10835 @samp{+} => extend by
10836 @samp{-} => reduce by
10837 @samp{<} => at most
10838 @samp{>} => at least
10839 @samp{/} => round down to multiple of
10840 @samp{%} => round up to multiple of
10848 @node Printing text
10849 @chapter Printing text
10851 @cindex printing text, commands for
10852 @cindex commands for printing text
10854 This section describes commands that display text strings.
10857 * echo invocation:: Print a line of text.
10858 * printf invocation:: Format and print data.
10859 * yes invocation:: Print a string until interrupted.
10863 @node echo invocation
10864 @section @command{echo}: Print a line of text
10867 @cindex displaying text
10868 @cindex printing text
10869 @cindex text, displaying
10870 @cindex arbitrary text, displaying
10872 @command{echo} writes each given @var{string} to standard output, with a
10873 space between each and a newline after the last one. Synopsis:
10876 echo [@var{option}]@dots{} [@var{string}]@dots{}
10879 @mayConflictWithShellBuiltIn{echo}
10881 The program accepts the following options. Also see @ref{Common options}.
10882 Options must precede operands, and the normally-special argument
10883 @samp{--} has no special meaning and is treated like any other
10889 Do not output the trailing newline.
10893 @cindex backslash escapes
10894 Enable interpretation of the following backslash-escaped characters in
10903 produce no further output
10919 the eight-bit value that is the octal number @var{nnn}
10920 (zero to three octal digits)
10922 the eight-bit value that is the octal number @var{nnn}
10923 (one to three octal digits)
10925 the eight-bit value that is the hexadecimal number @var{hh}
10926 (one or two hexadecimal digits)
10931 @cindex backslash escapes
10932 Disable interpretation of backslash escapes in each @var{string}.
10933 This is the default. If @option{-e} and @option{-E} are both
10934 specified, the last one given takes effect.
10938 @vindex POSIXLY_CORRECT
10939 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10940 @command{echo}'s first argument is not @option{-n} it outputs
10941 option-like arguments instead of treating them as options. For
10942 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10943 plain @samp{hello}.
10945 @acronym{POSIX} does not require support for any options, and says
10946 that the behavior of @command{echo} is implementation-defined if any
10947 @var{string} contains a backslash or if the first argument is
10948 @option{-n}. Portable programs can use the @command{printf} command
10949 if they need to omit trailing newlines or output control characters or
10950 backslashes. @xref{printf invocation}.
10955 @node printf invocation
10956 @section @command{printf}: Format and print data
10959 @command{printf} does formatted printing of text. Synopsis:
10962 printf @var{format} [@var{argument}]@dots{}
10965 @command{printf} prints the @var{format} string, interpreting @samp{%}
10966 directives and @samp{\} escapes to format numeric and string arguments
10967 in a way that is mostly similar to the C @samp{printf} function.
10968 @xref{Output Conversion Syntax,, @command{printf} format directives,
10969 libc, The GNU C Library Reference Manual}, for details.
10970 The differences are listed below.
10972 @mayConflictWithShellBuiltIn{printf}
10977 The @var{format} argument is reused as necessary to convert all the
10978 given @var{argument}s. For example, the command @samp{printf %s a b}
10982 Missing @var{argument}s are treated as null strings or as zeros,
10983 depending on whether the context expects a string or a number. For
10984 example, the command @samp{printf %sx%d} prints @samp{x0}.
10988 An additional escape, @samp{\c}, causes @command{printf} to produce no
10989 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
10990 E} prints @samp{ABC}.
10993 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
10994 digits, as opposed to C where it can have an unlimited number of
10995 digits. For example, the command @samp{printf '\x07e'} prints two
10996 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11001 @command{printf} has an additional directive, @samp{%b}, which prints its
11002 argument string with @samp{\} escapes interpreted in the same way as in
11003 the @var{format} string, except that octal escapes are of the form
11004 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11005 If a precision is also given, it limits the number of bytes printed
11006 from the converted string.
11009 Numeric arguments must be single C constants, possibly with leading
11010 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11014 @vindex POSIXLY_CORRECT
11015 If the leading character of a numeric argument is @samp{"} or @samp{'}
11016 then its value is the numeric value of the immediately following
11017 character. Any remaining characters are silently ignored if the
11018 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11019 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11020 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11021 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11026 A floating-point argument must use a period before any fractional
11027 digits, but is printed according to the @env{LC_NUMERIC} category of the
11028 current locale. For example, in a locale whose radix character is a
11029 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11030 the command @samp{printf %g 3,14} is an error.
11034 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11035 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11036 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11037 digits) specifying a character to print.
11042 @cindex ISO/IEC 10646
11044 @command{printf} interprets two character syntaxes introduced in
11045 @acronym{ISO} C 99:
11046 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11047 characters, specified as
11048 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11049 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11050 @command{printf} outputs the Unicode characters
11051 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11052 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11053 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11055 The processing of @samp{\u} and @samp{\U} requires a full-featured
11056 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11057 or when @code{libiconv} is installed prior to this package. Otherwise
11058 @samp{\u} and @samp{\U} will print as-is.
11060 The only options are a lone @option{--help} or
11061 @option{--version}. @xref{Common options}.
11062 Options must precede operands.
11064 The Unicode character syntaxes are useful for writing strings in a locale
11065 independent way. For example, a string containing the Euro currency symbol
11068 $ env printf '\u20AC 14.95'
11072 will be output correctly in all locales supporting the Euro symbol
11073 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11076 $ env printf '\u4e2d\u6587'
11080 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11082 Note that in these examples, the @command{printf} command has been
11083 invoked via @command{env} to ensure that we run the program found via
11084 your shell's search path, and not a shell alias or a built-in function.
11086 For larger strings, you don't need to look up the hexadecimal code
11087 values of each character one by one. @acronym{ASCII} characters mixed with \u
11088 escape sequences is also known as the JAVA source file encoding. You can
11089 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11090 is how to convert a piece of text into a shell script which will output
11091 this text in a locale-independent way:
11094 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11095 '\u4e2d\u6587\n' > sample.txt
11096 $ recode BIG5..JAVA < sample.txt \
11097 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11104 @node yes invocation
11105 @section @command{yes}: Print a string until interrupted
11108 @cindex repeated output of a string
11110 @command{yes} prints the command line arguments, separated by spaces and
11111 followed by a newline, forever until it is killed. If no arguments are
11112 given, it prints @samp{y} followed by a newline forever until killed.
11114 Upon a write error, @command{yes} exits with status @samp{1}.
11116 The only options are a lone @option{--help} or @option{--version}.
11117 To output an argument that begins with
11118 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11119 @xref{Common options}.
11123 @chapter Conditions
11126 @cindex commands for exit status
11127 @cindex exit status commands
11129 This section describes commands that are primarily useful for their exit
11130 status, rather than their output. Thus, they are often used as the
11131 condition of shell @code{if} statements, or as the last command in a
11135 * false invocation:: Do nothing, unsuccessfully.
11136 * true invocation:: Do nothing, successfully.
11137 * test invocation:: Check file types and compare values.
11138 * expr invocation:: Evaluate expressions.
11142 @node false invocation
11143 @section @command{false}: Do nothing, unsuccessfully
11146 @cindex do nothing, unsuccessfully
11147 @cindex failure exit status
11148 @cindex exit status of @command{false}
11150 @command{false} does nothing except return an exit status of 1, meaning
11151 @dfn{failure}. It can be used as a place holder in shell scripts
11152 where an unsuccessful command is needed.
11153 In most modern shells, @command{false} is a built-in command, so when
11154 you use @samp{false} in a script, you're probably using the built-in
11155 command, not the one documented here.
11157 @command{false} honors the @option{--help} and @option{--version} options.
11159 This version of @command{false} is implemented as a C program, and is thus
11160 more secure and faster than a shell script implementation, and may safely
11161 be used as a dummy shell for the purpose of disabling accounts.
11163 Note that @command{false} (unlike all other programs documented herein)
11164 exits unsuccessfully, even when invoked with
11165 @option{--help} or @option{--version}.
11167 Portable programs should not assume that the exit status of
11168 @command{false} is 1, as it is greater than 1 on some
11169 non-@acronym{GNU} hosts.
11172 @node true invocation
11173 @section @command{true}: Do nothing, successfully
11176 @cindex do nothing, successfully
11178 @cindex successful exit
11179 @cindex exit status of @command{true}
11181 @command{true} does nothing except return an exit status of 0, meaning
11182 @dfn{success}. It can be used as a place holder in shell scripts
11183 where a successful command is needed, although the shell built-in
11184 command @code{:} (colon) may do the same thing faster.
11185 In most modern shells, @command{true} is a built-in command, so when
11186 you use @samp{true} in a script, you're probably using the built-in
11187 command, not the one documented here.
11189 @command{true} honors the @option{--help} and @option{--version} options.
11191 Note, however, that it is possible to cause @command{true}
11192 to exit with nonzero status: with the @option{--help} or @option{--version}
11193 option, and with standard
11194 output already closed or redirected to a file that evokes an I/O error.
11195 For example, using a Bourne-compatible shell:
11198 $ ./true --version >&-
11199 ./true: write error: Bad file number
11200 $ ./true --version > /dev/full
11201 ./true: write error: No space left on device
11204 This version of @command{true} is implemented as a C program, and is thus
11205 more secure and faster than a shell script implementation, and may safely
11206 be used as a dummy shell for the purpose of disabling accounts.
11208 @node test invocation
11209 @section @command{test}: Check file types and compare values
11212 @cindex check file types
11213 @cindex compare values
11214 @cindex expression evaluation
11216 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11217 evaluation of the conditional expression @var{expr}. Each part of the
11218 expression must be a separate argument.
11220 @command{test} has file status checks, string operators, and numeric
11221 comparison operators.
11223 @command{test} has an alternate form that uses opening and closing
11224 square brackets instead a leading @samp{test}. For example, instead
11225 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11226 brackets must be separate arguments; for example, @samp{[-d /]} does
11227 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11228 @var{expr} ]} have the same meaning, only the former form is discussed
11234 test @var{expression}
11236 [ @var{expression} ]
11241 @mayConflictWithShellBuiltIn{test}
11243 If @var{expression} is omitted, @command{test} returns false.
11244 If @var{expression} is a single argument,
11245 @command{test} returns false if the argument is null and true otherwise. The argument
11246 can be any string, including strings like @samp{-d}, @samp{-1},
11247 @samp{--}, @samp{--help}, and @samp{--version} that most other
11248 programs would treat as options. To get help and version information,
11249 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11250 the usual closing brackets. @xref{Common options}.
11252 @cindex exit status of @command{test}
11256 0 if the expression is true,
11257 1 if the expression is false,
11258 2 if an error occurred.
11262 * File type tests:: -[bcdfhLpSt]
11263 * Access permission tests:: -[gkruwxOG]
11264 * File characteristic tests:: -e -s -nt -ot -ef
11265 * String tests:: -z -n = !=
11266 * Numeric tests:: -eq -ne -lt -le -gt -ge
11267 * Connectives for test:: ! -a -o
11271 @node File type tests
11272 @subsection File type tests
11274 @cindex file type tests
11276 These options test for particular types of files. (Everything's a file,
11277 but not all files are the same!)
11281 @item -b @var{file}
11283 @cindex block special check
11284 True if @var{file} exists and is a block special device.
11286 @item -c @var{file}
11288 @cindex character special check
11289 True if @var{file} exists and is a character special device.
11291 @item -d @var{file}
11293 @cindex directory check
11294 True if @var{file} exists and is a directory.
11296 @item -f @var{file}
11298 @cindex regular file check
11299 True if @var{file} exists and is a regular file.
11301 @item -h @var{file}
11302 @itemx -L @var{file}
11305 @cindex symbolic link check
11306 True if @var{file} exists and is a symbolic link.
11307 Unlike all other file-related tests, this test does not dereference
11308 @var{file} if it is a symbolic link.
11310 @item -p @var{file}
11312 @cindex named pipe check
11313 True if @var{file} exists and is a named pipe.
11315 @item -S @var{file}
11317 @cindex socket check
11318 True if @var{file} exists and is a socket.
11322 @cindex terminal check
11323 True if @var{fd} is a file descriptor that is associated with a
11329 @node Access permission tests
11330 @subsection Access permission tests
11332 @cindex access permission tests
11333 @cindex permission tests
11335 These options test for particular access permissions.
11339 @item -g @var{file}
11341 @cindex set-group-ID check
11342 True if @var{file} exists and has its set-group-ID bit set.
11344 @item -k @var{file}
11346 @cindex sticky bit check
11347 True if @var{file} exists and has its @dfn{sticky} bit set.
11349 @item -r @var{file}
11351 @cindex readable file check
11352 True if @var{file} exists and read permission is granted.
11354 @item -u @var{file}
11356 @cindex set-user-ID check
11357 True if @var{file} exists and has its set-user-ID bit set.
11359 @item -w @var{file}
11361 @cindex writable file check
11362 True if @var{file} exists and write permission is granted.
11364 @item -x @var{file}
11366 @cindex executable file check
11367 True if @var{file} exists and execute permission is granted
11368 (or search permission, if it is a directory).
11370 @item -O @var{file}
11372 @cindex owned by effective user ID check
11373 True if @var{file} exists and is owned by the current effective user ID.
11375 @item -G @var{file}
11377 @cindex owned by effective group ID check
11378 True if @var{file} exists and is owned by the current effective group ID.
11382 @node File characteristic tests
11383 @subsection File characteristic tests
11385 @cindex file characteristic tests
11387 These options test other file characteristics.
11391 @item -e @var{file}
11393 @cindex existence-of-file check
11394 True if @var{file} exists.
11396 @item -s @var{file}
11398 @cindex nonempty file check
11399 True if @var{file} exists and has a size greater than zero.
11401 @item @var{file1} -nt @var{file2}
11403 @cindex newer-than file check
11404 True if @var{file1} is newer (according to modification date) than
11405 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11407 @item @var{file1} -ot @var{file2}
11409 @cindex older-than file check
11410 True if @var{file1} is older (according to modification date) than
11411 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11413 @item @var{file1} -ef @var{file2}
11415 @cindex same file check
11416 @cindex hard link check
11417 True if @var{file1} and @var{file2} have the same device and inode
11418 numbers, i.e., if they are hard links to each other.
11424 @subsection String tests
11426 @cindex string tests
11428 These options test string characteristics. You may need to quote
11429 @var{string} arguments for the shell. For example:
11435 The quotes here prevent the wrong arguments from being passed to
11436 @command{test} if @samp{$V} is empty or contains special characters.
11440 @item -z @var{string}
11442 @cindex zero-length string check
11443 True if the length of @var{string} is zero.
11445 @item -n @var{string}
11446 @itemx @var{string}
11448 @cindex nonzero-length string check
11449 True if the length of @var{string} is nonzero.
11451 @item @var{string1} = @var{string2}
11453 @cindex equal string check
11454 True if the strings are equal.
11456 @item @var{string1} != @var{string2}
11458 @cindex not-equal string check
11459 True if the strings are not equal.
11464 @node Numeric tests
11465 @subsection Numeric tests
11467 @cindex numeric tests
11468 @cindex arithmetic tests
11470 Numeric relational operators. The arguments must be entirely numeric
11471 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11472 which evaluates to the length of @var{string}.
11476 @item @var{arg1} -eq @var{arg2}
11477 @itemx @var{arg1} -ne @var{arg2}
11478 @itemx @var{arg1} -lt @var{arg2}
11479 @itemx @var{arg1} -le @var{arg2}
11480 @itemx @var{arg1} -gt @var{arg2}
11481 @itemx @var{arg1} -ge @var{arg2}
11488 These arithmetic binary operators return true if @var{arg1} is equal,
11489 not-equal, less-than, less-than-or-equal, greater-than, or
11490 greater-than-or-equal than @var{arg2}, respectively.
11497 test -1 -gt -2 && echo yes
11499 test -l abc -gt 1 && echo yes
11502 @error{} test: integer expression expected before -eq
11506 @node Connectives for test
11507 @subsection Connectives for @command{test}
11509 @cindex logical connectives
11510 @cindex connectives, logical
11512 The usual logical connectives.
11518 True if @var{expr} is false.
11520 @item @var{expr1} -a @var{expr2}
11522 @cindex logical and operator
11523 @cindex and operator
11524 True if both @var{expr1} and @var{expr2} are true.
11526 @item @var{expr1} -o @var{expr2}
11528 @cindex logical or operator
11529 @cindex or operator
11530 True if either @var{expr1} or @var{expr2} is true.
11535 @node expr invocation
11536 @section @command{expr}: Evaluate expressions
11539 @cindex expression evaluation
11540 @cindex evaluation of expressions
11542 @command{expr} evaluates an expression and writes the result on standard
11543 output. Each token of the expression must be a separate argument.
11545 Operands are either integers or strings. Integers consist of one or
11546 more decimal digits, with an optional leading @samp{-}.
11547 @command{expr} converts
11548 anything appearing in an operand position to an integer or a string
11549 depending on the operation being applied to it.
11551 Strings are not quoted for @command{expr} itself, though you may need to
11552 quote them to protect characters with special meaning to the shell,
11553 e.g., spaces. However, regardless of whether it is quoted, a string
11554 operand should not be a parenthesis or any of @command{expr}'s
11555 operators like @code{+}, so you cannot safely pass an arbitrary string
11556 @code{$str} to expr merely by quoting it to the shell. One way to
11557 work around this is to use the @sc{gnu} extension @code{+},
11558 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11559 @code{@w{" $str"}} and to adjust the rest of the expression to take
11560 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11562 You should not pass a negative integer or a string with leading
11563 @samp{-} as @command{expr}'s first argument, as it might be
11564 misinterpreted as an option; this can be avoided by parenthesization.
11565 Also, portable scripts should not use a string operand that happens to
11566 take the form of an integer; this can be worked around by inserting
11567 leading spaces as mentioned above.
11569 @cindex parentheses for grouping
11570 Operators may be given as infix symbols or prefix keywords. Parentheses
11571 may be used for grouping in the usual manner. You must quote
11572 parentheses and many operators to avoid the shell evaluating them,
11575 When built with support for the GNU MP library, @command{expr} uses
11576 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11577 types and may fail due to arithmetic overflow.
11579 The only options are @option{--help} and @option{--version}. @xref{Common
11580 options}. Options must precede operands.
11582 @cindex exit status of @command{expr}
11586 0 if the expression is neither null nor 0,
11587 1 if the expression is null or 0,
11588 2 if the expression is invalid,
11589 3 if an internal error occurred (e.g., arithmetic overflow).
11593 * String expressions:: + : match substr index length
11594 * Numeric expressions:: + - * / %
11595 * Relations for expr:: | & < <= = == != >= >
11596 * Examples of expr:: Examples.
11600 @node String expressions
11601 @subsection String expressions
11603 @cindex string expressions
11604 @cindex expressions, string
11606 @command{expr} supports pattern matching and other string operators. These
11607 have higher precedence than both the numeric and relational operators (in
11608 the next sections).
11612 @item @var{string} : @var{regex}
11613 @cindex pattern matching
11614 @cindex regular expression matching
11615 @cindex matching patterns
11616 Perform pattern matching. The arguments are converted to strings and the
11617 second is considered to be a (basic, a la GNU @code{grep}) regular
11618 expression, with a @code{^} implicitly prepended. The first argument is
11619 then matched against this regular expression.
11621 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11622 @code{:} expression returns the part of @var{string} that matched the
11623 subexpression; otherwise, it returns the number of characters matched.
11625 If the match fails, the @code{:} operator returns the null string if
11626 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11628 @kindex \( @r{regexp operator}
11629 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11630 value; additional pairs are meaningful only for grouping the regular
11631 expression operators.
11633 @kindex \+ @r{regexp operator}
11634 @kindex \? @r{regexp operator}
11635 @kindex \| @r{regexp operator}
11636 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11637 operators which respectively match one or more, zero or one, or separate
11638 alternatives. SunOS and other @command{expr}'s treat these as regular
11639 characters. (@acronym{POSIX} allows either behavior.)
11640 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11641 regular expression syntax. Some examples are in @ref{Examples of expr}.
11643 @item match @var{string} @var{regex}
11645 An alternative way to do pattern matching. This is the same as
11646 @w{@samp{@var{string} : @var{regex}}}.
11648 @item substr @var{string} @var{position} @var{length}
11650 Returns the substring of @var{string} beginning at @var{position}
11651 with length at most @var{length}. If either @var{position} or
11652 @var{length} is negative, zero, or non-numeric, returns the null string.
11654 @item index @var{string} @var{charset}
11656 Returns the first position in @var{string} where the first character in
11657 @var{charset} was found. If no character in @var{charset} is found in
11658 @var{string}, return 0.
11660 @item length @var{string}
11662 Returns the length of @var{string}.
11664 @item + @var{token}
11666 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11667 or an operator like @code{/}.
11668 This makes it possible to test @code{expr length + "$x"} or
11669 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11670 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11671 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11672 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11676 To make @command{expr} interpret keywords as strings, you must use the
11677 @code{quote} operator.
11680 @node Numeric expressions
11681 @subsection Numeric expressions
11683 @cindex numeric expressions
11684 @cindex expressions, numeric
11686 @command{expr} supports the usual numeric operators, in order of increasing
11687 precedence. These numeric operators have lower precedence than the
11688 string operators described in the previous section, and higher precedence
11689 than the connectives (next section).
11697 @cindex subtraction
11698 Addition and subtraction. Both arguments are converted to integers;
11699 an error occurs if this cannot be done.
11705 @cindex multiplication
11708 Multiplication, division, remainder. Both arguments are converted to
11709 integers; an error occurs if this cannot be done.
11714 @node Relations for expr
11715 @subsection Relations for @command{expr}
11717 @cindex connectives, logical
11718 @cindex logical connectives
11719 @cindex relations, numeric or string
11721 @command{expr} supports the usual logical connectives and relations. These
11722 have lower precedence than the string and numeric operators
11723 (previous sections). Here is the list, lowest-precedence operator first.
11729 @cindex logical or operator
11730 @cindex or operator
11731 Returns its first argument if that is neither null nor zero, otherwise
11732 its second argument if it is neither null nor zero, otherwise 0. It
11733 does not evaluate its second argument if its first argument is neither
11738 @cindex logical and operator
11739 @cindex and operator
11740 Return its first argument if neither argument is null or zero, otherwise
11741 0. It does not evaluate its second argument if its first argument is
11744 @item < <= = == != >= >
11751 @cindex comparison operators
11753 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11754 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11755 both arguments to integers and do a numeric comparison; if either
11756 conversion fails, it does a lexicographic comparison using the character
11757 collating sequence specified by the @env{LC_COLLATE} locale.
11762 @node Examples of expr
11763 @subsection Examples of using @command{expr}
11765 @cindex examples of @command{expr}
11766 Here are a few examples, including quoting for shell metacharacters.
11768 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11771 foo=`expr $foo + 1`
11774 To print the non-directory part of the file name stored in
11775 @code{$fname}, which need not contain a @code{/}:
11778 expr $fname : '.*/\(.*\)' '|' $fname
11781 An example showing that @code{\+} is an operator:
11789 expr abc : 'a\(.\)c'
11791 expr index abcdef cz
11794 @error{} expr: syntax error
11795 expr index + index a
11801 @chapter Redirection
11803 @cindex redirection
11804 @cindex commands for redirection
11806 Unix shells commonly provide several forms of @dfn{redirection}---ways
11807 to change the input source or output destination of a command. But one
11808 useful redirection is performed by a separate command, not by the shell;
11809 it's described here.
11812 * tee invocation:: Redirect output to multiple files or processes.
11816 @node tee invocation
11817 @section @command{tee}: Redirect output to multiple files or processes
11820 @cindex pipe fitting
11821 @cindex destinations, multiple output
11822 @cindex read from stdin and write to stdout and files
11824 The @command{tee} command copies standard input to standard output and also
11825 to any files given as arguments. This is useful when you want not only
11826 to send some data down a pipe, but also to save a copy. Synopsis:
11829 tee [@var{option}]@dots{} [@var{file}]@dots{}
11832 If a file being written to does not already exist, it is created. If a
11833 file being written to already exists, the data it previously contained
11834 is overwritten unless the @option{-a} option is used.
11836 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11837 input to standard output, but this is typically not that useful as the
11838 copies are interleaved.
11840 The program accepts the following options. Also see @ref{Common options}.
11847 Append standard input to the given files rather than overwriting
11851 @itemx --ignore-interrupts
11853 @opindex --ignore-interrupts
11854 Ignore interrupt signals.
11858 The @command{tee} command is useful when you happen to be transferring a large
11859 amount of data and also want to summarize that data without reading
11860 it a second time. For example, when you are downloading a DVD image,
11861 you often want to verify its signature or checksum right away.
11862 The inefficient way to do it is simply:
11865 wget http://example.com/some.iso && sha1sum some.iso
11868 One problem with the above is that it makes you wait for the
11869 download to complete before starting the time-consuming SHA1 computation.
11870 Perhaps even more importantly, the above requires reading
11871 the DVD image a second time (the first was from the network).
11873 The efficient way to do it is to interleave the download
11874 and SHA1 computation. Then, you'll get the checksum for
11875 free, because the entire process parallelizes so well:
11878 # slightly contrived, to demonstrate process substitution
11879 wget -O - http://example.com/dvd.iso \
11880 | tee >(sha1sum > dvd.sha1) > dvd.iso
11883 That makes @command{tee} write not just to the expected output file,
11884 but also to a pipe running @command{sha1sum} and saving the final
11885 checksum in a file named @file{dvd.sha1}.
11887 Note, however, that this example relies on a feature of modern shells
11888 called @dfn{process substitution}
11889 (the @samp{>(command)} syntax, above;
11890 @xref{Process Substitution,,Process Substitution, bashref,
11891 The Bash Reference Manual}.),
11892 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11893 but not with @command{/bin/sh}. So if you write code like this
11894 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11896 Since the above example writes to one file and one process,
11897 a more conventional and portable use of @command{tee} is even better:
11900 wget -O - http://example.com/dvd.iso \
11901 | tee dvd.iso | sha1sum > dvd.sha1
11904 You can extend this example to make @command{tee} write to two processes,
11905 computing MD5 and SHA1 checksums in parallel. In this case,
11906 process substitution is required:
11909 wget -O - http://example.com/dvd.iso \
11910 | tee >(sha1sum > dvd.sha1) \
11911 >(md5sum > dvd.md5) \
11915 This technique is also useful when you want to make a @emph{compressed}
11916 copy of the contents of a pipe.
11917 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11918 For a large hierarchy, @samp{du -ak} can run for a long time,
11919 and can easily produce terabytes of data, so you won't want to
11920 rerun the command unnecessarily. Nor will you want to save
11921 the uncompressed output.
11923 Doing it the inefficient way, you can't even start the GUI
11924 until after you've compressed all of the @command{du} output:
11927 du -ak | gzip -9 > /tmp/du.gz
11928 gzip -d /tmp/du.gz | xdiskusage -a
11931 With @command{tee} and process substitution, you start the GUI
11932 right away and eliminate the decompression completely:
11935 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11938 Finally, if you regularly create more than one type of
11939 compressed tarball at once, for example when @code{make dist} creates
11940 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11941 there may be a better way.
11942 Typical @command{automake}-generated @file{Makefile} rules create
11943 the two compressed tar archives with commands in sequence, like this
11944 (slightly simplified):
11947 tardir=your-pkg-M.N
11948 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11949 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11952 However, if the hierarchy you are archiving and compressing is larger
11953 than a couple megabytes, and especially if you are using a multi-processor
11954 system with plenty of memory, then you can do much better by reading the
11955 directory contents only once and running the compression programs in parallel:
11958 tardir=your-pkg-M.N
11959 tar chof - "$tardir" \
11960 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11961 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11967 @node File name manipulation
11968 @chapter File name manipulation
11970 @cindex file name manipulation
11971 @cindex manipulation of file names
11972 @cindex commands for file name manipulation
11974 This section describes commands that manipulate file names.
11977 * basename invocation:: Strip directory and suffix from a file name.
11978 * dirname invocation:: Strip last file name component.
11979 * pathchk invocation:: Check file name validity and portability.
11980 * mktemp invocation:: Create temporary file or directory.
11984 @node basename invocation
11985 @section @command{basename}: Strip directory and suffix from a file name
11988 @cindex strip directory and suffix from file names
11989 @cindex directory, stripping from file names
11990 @cindex suffix, stripping from file names
11991 @cindex file names, stripping directory and suffix
11992 @cindex leading directory components, stripping
11994 @command{basename} removes any leading directory components from
11995 @var{name}. Synopsis:
11998 basename @var{name} [@var{suffix}]
12001 If @var{suffix} is specified and is identical to the end of @var{name},
12002 it is removed from @var{name} as well. Note that since trailing slashes
12003 are removed prior to suffix matching, @var{suffix} will do nothing if it
12004 contains slashes. @command{basename} prints the result on standard
12007 @c This test is used both here and in the section on dirname.
12008 @macro basenameAndDirname
12009 Together, @command{basename} and @command{dirname} are designed such
12010 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12011 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12012 for everything except file names containing a trailing newline.
12014 @basenameAndDirname
12016 @acronym{POSIX} allows the implementation to define the results if
12017 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12018 @command{basename} returns the empty string. In the latter case, the
12019 result is @samp{//} on platforms where @var{//} is distinct from
12020 @var{/}, and @samp{/} on platforms where there is no difference.
12022 The only options are @option{--help} and @option{--version}. @xref{Common
12023 options}. Options must precede operands.
12031 basename /usr/bin/sort
12034 basename include/stdio.h .h
12038 @node dirname invocation
12039 @section @command{dirname}: Strip last file name component
12042 @cindex directory components, printing
12043 @cindex stripping non-directory suffix
12044 @cindex non-directory suffix, stripping
12046 @command{dirname} prints all but the final slash-delimited component of
12047 @var{name}. Slashes on either side of the final component are also
12048 removed. If the string contains no slash, @command{dirname} prints
12049 @samp{.} (meaning the current directory). Synopsis:
12055 @var{name} need not be a file name, but if it is, this operation
12056 effectively lists the directory that contains the final component,
12057 including the case when the final component is itself a directory.
12059 @basenameAndDirname
12061 @acronym{POSIX} allows the implementation to define the results if
12062 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12063 result is @samp{//} on platforms where @var{//} is distinct from
12064 @var{/}, and @samp{/} on platforms where there is no difference.
12066 The only options are @option{--help} and @option{--version}. @xref{Common
12074 # Output "/usr/bin".
12075 dirname /usr/bin/sort
12076 dirname /usr/bin//.//
12083 @node pathchk invocation
12084 @section @command{pathchk}: Check file name validity and portability
12087 @cindex file names, checking validity and portability
12088 @cindex valid file names, checking for
12089 @cindex portable file names, checking for
12091 @command{pathchk} checks validity and portability of file names. Synopsis:
12094 pathchk [@var{option}]@dots{} @var{name}@dots{}
12097 For each @var{name}, @command{pathchk} prints an error message if any of
12098 these conditions is true:
12102 One of the existing directories in @var{name} does not have search
12103 (execute) permission,
12105 The length of @var{name} is larger than the maximum supported by the
12108 The length of one component of @var{name} is longer than
12109 its file system's maximum.
12112 A nonexistent @var{name} is not an error, so long a file with that
12113 name could be created under the above conditions.
12115 The program accepts the following options. Also see @ref{Common options}.
12116 Options must precede operands.
12122 Instead of performing checks based on the underlying file system,
12123 print an error message if any of these conditions is true:
12127 A file name is empty.
12130 A file name contains a character outside the @acronym{POSIX} portable file
12131 name character set, namely, the ASCII letters and digits, @samp{.},
12132 @samp{_}, @samp{-}, and @samp{/}.
12135 The length of a file name or one of its components exceeds the
12136 @acronym{POSIX} minimum limits for portability.
12141 Print an error message if a file name is empty, or if it contains a component
12142 that begins with @samp{-}.
12144 @item --portability
12145 @opindex --portability
12146 Print an error message if a file name is not portable to all @acronym{POSIX}
12147 hosts. This option is equivalent to @samp{-p -P}.
12151 @cindex exit status of @command{pathchk}
12155 0 if all specified file names passed all checks,
12159 @node mktemp invocation
12160 @section @command{mktemp}: Create temporary file or directory
12163 @cindex file names, creating temporary
12164 @cindex directory, creating temporary
12165 @cindex temporary files and directories
12167 @command{mktemp} manages the creation of temporary files and
12168 directories. Synopsis:
12171 mktemp [@var{option}]@dots{} [@var{template}]
12174 Safely create a temporary file or directory based on @var{template},
12175 and print its name. If given, @var{template} must include at least
12176 three consecutive @samp{X}s in the last component. If omitted, the template
12177 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12178 implied. The final run of @samp{X}s in the @var{template} will be replaced
12179 by alpha-numeric characters; thus, on a case-sensitive file system,
12180 and with a @var{template} including a run of @var{n} instances of @samp{X},
12181 there are @samp{62**@var{n}} potential file names.
12183 Older scripts used to create temporary files by simply joining the
12184 name of the program with the process id (@samp{$$}) as a suffix.
12185 However, that naming scheme is easily predictable, and suffers from a
12186 race condition where the attacker can create an appropriately named
12187 symbolic link, such that when the script then opens a handle to what
12188 it thought was an unused file, it is instead modifying an existing
12189 file. Using the same scheme to create a directory is slightly safer,
12190 since the @command{mkdir} will fail if the target already exists, but
12191 it is still inferior because it allows for denial of service attacks.
12192 Therefore, modern scripts should use the @command{mktemp} command to
12193 guarantee that the generated name will be unpredictable, and that
12194 knowledge of the temporary file name implies that the file was created
12195 by the current script and cannot be modified by other users.
12197 When creating a file, the resulting file has read and write
12198 permissions for the current user, but no permissions for the group or
12199 others; these permissions are reduced if the current umask is more
12202 Here are some examples (although note that if you repeat them, you
12203 will most likely get different file names):
12208 Create a temporary file in the current directory.
12215 Create a temporary file with a known suffix.
12217 $ mktemp --suffix=.txt file-XXXX
12219 $ mktemp file-XXXX-XXXX.txt
12224 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12225 but falling back to the current directory rather than @file{/tmp}.
12226 Note that @command{mktemp} does not create fifos, but can create a
12227 secure directory in which the fifo can live. Exit the shell if the
12228 directory or fifo could not be created.
12230 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12232 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12236 Create and use a temporary file if possible, but ignore failure. The
12237 file will reside in the directory named by @env{TMPDIR}, if specified,
12238 or else in @file{/tmp}.
12240 $ file=$(mktemp -q) && @{
12241 > # Safe to use $file only within this block. Use quotes,
12242 > # since $TMPDIR, and thus $file, may contain whitespace.
12243 > echo ... > "$file"
12249 Act as a semi-random character generator (it is not fully random,
12250 since it is impacted by the contents of the current directory). To
12251 avoid security holes, do not use the resulting names to create a file.
12261 The program accepts the following options. Also see @ref{Common options}.
12268 @opindex --directory
12269 Create a directory rather than a file. The directory will have read,
12270 write, and search permissions for the current user, but no permissions
12271 for the group or others; these permissions are reduced if the current
12272 umask is more restrictive.
12278 Suppress diagnostics about failure to create a file or directory. The
12279 exit status will still reflect whether a file was created.
12285 Generate a temporary name that does not name an existing file, without
12286 changing the file system contents. Using the output of this command
12287 to create a new file is inherently unsafe, as there is a window of
12288 time between generating the name and using it where another process
12289 can create an object by the same name.
12292 @itemx --tmpdir[=@var{dir}]
12295 Treat @var{template} relative to the directory @var{dir}. If
12296 @var{dir} is not specified (only possible with the long option
12297 @option{--tmpdir}) or is the empty string, use the value of
12298 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12299 specified, @var{template} must not be absolute. However,
12300 @var{template} can still contain slashes, although intermediate
12301 directories must already exist.
12303 @item --suffix=@var{suffix}
12305 Append @var{suffix} to the @var{template}. @var{suffix} must not
12306 contain slash. If @option{--suffix} is specified, @var{template} must
12307 end in @samp{X}; if it is not specified, then an appropriate
12308 @option{--suffix} is inferred by finding the last @samp{X} in
12309 @var{template}. This option exists for use with the default
12310 @var{template} and for the creation of a @var{suffix} that starts with
12315 Treat @var{template} as a single file relative to the value of
12316 @env{TMPDIR} if available, or to the directory specified by
12317 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12318 contain slashes. This option is deprecated; the use of @option{-p}
12319 without @option{-t} offers better defaults (by favoring the command
12320 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12325 @cindex exit status of @command{mktemp}
12329 0 if the file was created,
12334 @node Working context
12335 @chapter Working context
12337 @cindex working context
12338 @cindex commands for printing the working context
12340 This section describes commands that display or alter the context in
12341 which you are working: the current directory, the terminal settings, and
12342 so forth. See also the user-related commands in the next section.
12345 * pwd invocation:: Print working directory.
12346 * stty invocation:: Print or change terminal characteristics.
12347 * printenv invocation:: Print environment variables.
12348 * tty invocation:: Print file name of terminal on standard input.
12352 @node pwd invocation
12353 @section @command{pwd}: Print working directory
12356 @cindex print name of current directory
12357 @cindex current working directory, printing
12358 @cindex working directory, printing
12361 @command{pwd} prints the name of the current directory. Synopsis:
12364 pwd [@var{option}]@dots{}
12367 The program accepts the following options. Also see @ref{Common options}.
12374 If the contents of the environment variable @env{PWD} provide an
12375 absolute name of the current directory with no @samp{.} or @samp{..}
12376 components, but possibly with symbolic links, then output those
12377 contents. Otherwise, fall back to default @option{-P} handling.
12382 @opindex --physical
12383 Print a fully resolved name for the current directory. That is, all
12384 components of the printed name will be actual directory names---none
12385 will be symbolic links.
12388 @cindex symbolic links and @command{pwd}
12389 If @option{-L} and @option{-P} are both given, the last one takes
12390 precedence. If neither option is given, then this implementation uses
12391 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12392 environment variable is set.
12394 @mayConflictWithShellBuiltIn{pwd}
12399 @node stty invocation
12400 @section @command{stty}: Print or change terminal characteristics
12403 @cindex change or print terminal settings
12404 @cindex terminal settings
12405 @cindex line settings of terminal
12407 @command{stty} prints or changes terminal characteristics, such as baud rate.
12411 stty [@var{option}] [@var{setting}]@dots{}
12412 stty [@var{option}]
12415 If given no line settings, @command{stty} prints the baud rate, line
12416 discipline number (on systems that support it), and line settings
12417 that have been changed from the values set by @samp{stty sane}.
12418 By default, mode reading and setting are performed on the tty line
12419 connected to standard input, although this can be modified by the
12420 @option{--file} option.
12422 @command{stty} accepts many non-option arguments that change aspects of
12423 the terminal line operation, as described below.
12425 The program accepts the following options. Also see @ref{Common options}.
12432 Print all current settings in human-readable form. This option may not
12433 be used in combination with any line settings.
12435 @item -F @var{device}
12436 @itemx --file=@var{device}
12439 Set the line opened by the file name specified in @var{device} instead of
12440 the tty line connected to standard input. This option is necessary
12441 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12442 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12443 the @code{clocal} flag is not set. Hence, it is not always possible
12444 to allow the shell to open the device in the traditional manner.
12450 @cindex machine-readable @command{stty} output
12451 Print all current settings in a form that can be used as an argument to
12452 another @command{stty} command to restore the current settings. This option
12453 may not be used in combination with any line settings.
12457 Many settings can be turned off by preceding them with a @samp{-}.
12458 Such arguments are marked below with ``May be negated'' in their
12459 description. The descriptions themselves refer to the positive
12460 case, that is, when @emph{not} negated (unless stated otherwise,
12463 Some settings are not available on all @acronym{POSIX} systems, since they use
12464 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12465 description. On non-@acronym{POSIX} systems, those or other settings also may not
12466 be available, but it's not feasible to document all the variations: just
12472 * Control:: Control settings
12473 * Input:: Input settings
12474 * Output:: Output settings
12475 * Local:: Local settings
12476 * Combination:: Combination settings
12477 * Characters:: Special characters
12478 * Special:: Special settings
12483 @subsection Control settings
12485 @cindex control settings
12491 @cindex two-way parity
12492 Generate parity bit in output and expect parity bit in input.
12498 @cindex even parity
12499 Set odd parity (even if negated). May be negated.
12506 @cindex character size
12507 @cindex eight-bit characters
12508 Set character size to 5, 6, 7, or 8 bits.
12513 Send a hangup signal when the last process closes the tty. May be
12519 Use two stop bits per character (one if negated). May be negated.
12523 Allow input to be received. May be negated.
12527 @cindex modem control
12528 Disable modem control signals. May be negated.
12532 @cindex hardware flow control
12533 @cindex flow control, hardware
12534 @cindex RTS/CTS flow control
12535 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12540 @subsection Input settings
12542 @cindex input settings
12543 These settings control operations on data received from the terminal.
12548 @cindex breaks, ignoring
12549 Ignore break characters. May be negated.
12553 @cindex breaks, cause interrupts
12554 Make breaks cause an interrupt signal. May be negated.
12558 @cindex parity, ignoring
12559 Ignore characters with parity errors. May be negated.
12563 @cindex parity errors, marking
12564 Mark parity errors (with a 255-0-character sequence). May be negated.
12568 Enable input parity checking. May be negated.
12572 @cindex eight-bit input
12573 Clear high (8th) bit of input characters. May be negated.
12577 @cindex newline, translating to return
12578 Translate newline to carriage return. May be negated.
12582 @cindex return, ignoring
12583 Ignore carriage return. May be negated.
12587 @cindex return, translating to newline
12588 Translate carriage return to newline. May be negated.
12592 @cindex input encoding, UTF-8
12593 Assume input characters are UTF-8 encoded. May be negated.
12597 @kindex C-s/C-q flow control
12598 @cindex XON/XOFF flow control
12599 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12606 @cindex software flow control
12607 @cindex flow control, software
12608 Enable sending of @code{stop} character when the system input buffer
12609 is almost full, and @code{start} character when it becomes almost
12610 empty again. May be negated.
12614 @cindex uppercase, translating to lowercase
12615 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12616 negated. Note ilcuc is not implemented, as one would not be able to issue
12617 almost any (lowercase) Unix command, after invoking it.
12621 Allow any character to restart output (only the start character
12622 if negated). Non-@acronym{POSIX}. May be negated.
12626 @cindex beeping at input buffer full
12627 Enable beeping and not flushing input buffer if a character arrives
12628 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12633 @subsection Output settings
12635 @cindex output settings
12636 These settings control operations on data sent to the terminal.
12641 Postprocess output. May be negated.
12645 @cindex lowercase, translating to output
12646 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12647 negated. (Note ouclc is not currently implemented.)
12651 @cindex return, translating to newline
12652 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12656 @cindex newline, translating to crlf
12657 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12662 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12667 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12671 @cindex pad instead of timing for delaying
12672 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12677 @cindex pad character
12678 Use @acronym{ASCII} @sc{del} characters for fill instead of
12679 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12685 Newline delay style. Non-@acronym{POSIX}.
12692 Carriage return delay style. Non-@acronym{POSIX}.
12698 @opindex tab@var{n}
12699 Horizontal tab delay style. Non-@acronym{POSIX}.
12704 Backspace delay style. Non-@acronym{POSIX}.
12709 Vertical tab delay style. Non-@acronym{POSIX}.
12714 Form feed delay style. Non-@acronym{POSIX}.
12719 @subsection Local settings
12721 @cindex local settings
12726 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12727 characters. May be negated.
12731 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12732 special characters. May be negated.
12736 Enable non-@acronym{POSIX} special characters. May be negated.
12740 Echo input characters. May be negated.
12746 Echo @code{erase} characters as backspace-space-backspace. May be
12751 @cindex newline echoing after @code{kill}
12752 Echo a newline after a @code{kill} character. May be negated.
12756 @cindex newline, echoing
12757 Echo newline even if not echoing other characters. May be negated.
12761 @cindex flushing, disabling
12762 Disable flushing after @code{interrupt} and @code{quit} special
12763 characters. May be negated.
12767 @cindex case translation
12768 Enable input and output of uppercase characters by preceding their
12769 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12770 Non-@acronym{POSIX}. May be negated.
12774 @cindex background jobs, stopping at terminal write
12775 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12782 Echo erased characters backward, between @samp{\} and @samp{/}.
12783 Non-@acronym{POSIX}. May be negated.
12789 @cindex control characters, using @samp{^@var{c}}
12790 @cindex hat notation for control characters
12791 Echo control characters in hat notation (@samp{^@var{c}}) instead
12792 of literally. Non-@acronym{POSIX}. May be negated.
12798 Echo the @code{kill} special character by erasing each character on
12799 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12800 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12806 @subsection Combination settings
12808 @cindex combination settings
12809 Combination settings:
12816 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12817 as @code{-parenb cs8}.
12821 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12822 as @code{-parenb cs8}.
12826 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12827 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12831 Reset the @code{erase} and @code{kill} special characters to their default
12838 @c This is too long to write inline.
12840 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12841 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12842 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12843 ff0 isig icanon iexten echo echoe echok -echonl
12844 -noflsh -xcase -tostop -echoprt echoctl echoke
12848 and also sets all special characters to their default values.
12852 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12853 sets the @code{eof} and @code{eol} characters to their default values
12854 if they are the same as the @code{min} and @code{time} characters.
12855 May be negated. If negated, same as @code{raw}.
12862 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12863 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12864 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12868 May be negated. If negated, same as @code{cooked}.
12872 Same as @option{-icanon}. May be negated. If negated, same as
12877 @cindex eight-bit characters
12878 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12879 same as @code{parenb istrip cs7}.
12883 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12884 If negated, same as @code{parenb istrip opost cs7}.
12888 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12892 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12899 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12900 (Used for terminals with uppercase characters only.)
12904 Same as @code{echoe echoctl echoke}.
12908 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12913 @subsection Special characters
12915 @cindex special characters
12916 @cindex characters, special
12918 The special characters' default values vary from system to system.
12919 They are set with the syntax @samp{name value}, where the names are
12920 listed below and the value can be given either literally, in hat
12921 notation (@samp{^@var{c}}), or as an integer which may start with
12922 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12923 any other digit to indicate decimal.
12925 @cindex disabling special characters
12926 @kindex u@r{, and disabling special characters}
12927 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12928 special character. (This is incompatible with Ultrix @command{stty},
12929 which uses a value of @samp{u} to disable a special character. GNU
12930 @command{stty} treats a value @samp{u} like any other, namely to set that
12931 special character to @key{U}.)
12937 Send an interrupt signal.
12941 Send a quit signal.
12945 Erase the last character typed.
12949 Erase the current line.
12953 Send an end of file (terminate the input).
12961 Alternate character to end the line. Non-@acronym{POSIX}.
12965 Switch to a different shell layer. Non-@acronym{POSIX}.
12969 Restart the output after stopping it.
12977 Send a terminal stop signal.
12981 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
12985 Redraw the current line. Non-@acronym{POSIX}.
12989 Erase the last word typed. Non-@acronym{POSIX}.
12993 Enter the next character typed literally, even if it is a special
12994 character. Non-@acronym{POSIX}.
12999 @subsection Special settings
13001 @cindex special settings
13006 Set the minimum number of characters that will satisfy a read until
13007 the time value has expired, when @option{-icanon} is set.
13011 Set the number of tenths of a second before reads time out if the minimum
13012 number of characters have not been read, when @option{-icanon} is set.
13014 @item ispeed @var{n}
13016 Set the input speed to @var{n}.
13018 @item ospeed @var{n}
13020 Set the output speed to @var{n}.
13024 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13027 @itemx columns @var{n}
13030 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13036 Print the number of rows and columns that the kernel thinks the
13037 terminal has. (Systems that don't support rows and columns in the kernel
13038 typically use the environment variables @env{LINES} and @env{COLUMNS}
13039 instead; however, GNU @command{stty} does not know anything about them.)
13040 Non-@acronym{POSIX}.
13044 Use line discipline @var{n}. Non-@acronym{POSIX}.
13048 Print the terminal speed.
13051 @cindex baud rate, setting
13052 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13053 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13054 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13055 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13056 support higher speeds. The @command{stty} command includes support
13073 4000000 where the system supports these.
13074 0 hangs up the line if @option{-clocal} is set.
13078 @node printenv invocation
13079 @section @command{printenv}: Print all or some environment variables
13082 @cindex printing all or some environment variables
13083 @cindex environment variables, printing
13085 @command{printenv} prints environment variable values. Synopsis:
13088 printenv [@var{option}] [@var{variable}]@dots{}
13091 If no @var{variable}s are specified, @command{printenv} prints the value of
13092 every environment variable. Otherwise, it prints the value of each
13093 @var{variable} that is set, and nothing for those that are not set.
13095 The program accepts the following option. Also see @ref{Common options}.
13103 @cindex exit status of @command{printenv}
13107 0 if all variables specified were found
13108 1 if at least one specified variable was not found
13109 2 if a write error occurred
13113 @node tty invocation
13114 @section @command{tty}: Print file name of terminal on standard input
13117 @cindex print terminal file name
13118 @cindex terminal file name, printing
13120 @command{tty} prints the file name of the terminal connected to its standard
13121 input. It prints @samp{not a tty} if standard input is not a terminal.
13125 tty [@var{option}]@dots{}
13128 The program accepts the following option. Also see @ref{Common options}.
13138 Print nothing; only return an exit status.
13142 @cindex exit status of @command{tty}
13146 0 if standard input is a terminal
13147 1 if standard input is not a terminal
13148 2 if given incorrect arguments
13149 3 if a write error occurs
13153 @node User information
13154 @chapter User information
13156 @cindex user information, commands for
13157 @cindex commands for printing user information
13159 This section describes commands that print user-related information:
13160 logins, groups, and so forth.
13163 * id invocation:: Print user identity.
13164 * logname invocation:: Print current login name.
13165 * whoami invocation:: Print effective user ID.
13166 * groups invocation:: Print group names a user is in.
13167 * users invocation:: Print login names of users currently logged in.
13168 * who invocation:: Print who is currently logged in.
13172 @node id invocation
13173 @section @command{id}: Print user identity
13176 @cindex real user and group IDs, printing
13177 @cindex effective user and group IDs, printing
13178 @cindex printing real and effective user and group IDs
13180 @command{id} prints information about the given user, or the process
13181 running it if no user is specified. Synopsis:
13184 id [@var{option}]@dots{} [@var{username}]
13187 @vindex POSIXLY_CORRECT
13188 By default, it prints the real user ID, real group ID, effective user ID
13189 if different from the real user ID, effective group ID if different from
13190 the real group ID, and supplemental group IDs.
13191 In addition, if SELinux
13192 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13193 then print @samp{context=@var{c}}, where @var{c} is the security context.
13195 Each of these numeric values is preceded by an identifying string and
13196 followed by the corresponding user or group name in parentheses.
13198 The options cause @command{id} to print only part of the above information.
13199 Also see @ref{Common options}.
13206 Print only the group ID.
13212 Print only the group ID and the supplementary groups.
13218 Print the user or group name instead of the ID number. Requires
13219 @option{-u}, @option{-g}, or @option{-G}.
13225 Print the real, instead of effective, user or group ID. Requires
13226 @option{-u}, @option{-g}, or @option{-G}.
13232 Print only the user ID.
13239 @cindex security context
13240 Print only the security context of the current user.
13241 If SELinux is disabled then print a warning and
13242 set the exit status to 1.
13248 @macro primaryAndSupplementaryGroups{cmd,arg}
13249 Primary and supplementary groups for a process are normally inherited
13250 from its parent and are usually unchanged since login. This means
13251 that if you change the group database after logging in, @command{\cmd\}
13252 will not reflect your changes within your existing login session.
13253 Running @command{\cmd\} with a \arg\ causes the user and group
13254 database to be consulted afresh, and so will give a different result.
13256 @primaryAndSupplementaryGroups{id,user argument}
13258 @node logname invocation
13259 @section @command{logname}: Print current login name
13262 @cindex printing user's login name
13263 @cindex login name, printing
13264 @cindex user name, printing
13267 @command{logname} prints the calling user's name, as found in a
13268 system-maintained file (often @file{/var/run/utmp} or
13269 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13270 for the calling process, @command{logname} prints
13271 an error message and exits with a status of 1.
13273 The only options are @option{--help} and @option{--version}. @xref{Common
13279 @node whoami invocation
13280 @section @command{whoami}: Print effective user ID
13283 @cindex effective user ID, printing
13284 @cindex printing the effective user ID
13286 @command{whoami} prints the user name associated with the current
13287 effective user ID. It is equivalent to the command @samp{id -un}.
13289 The only options are @option{--help} and @option{--version}. @xref{Common
13295 @node groups invocation
13296 @section @command{groups}: Print group names a user is in
13299 @cindex printing groups a user is in
13300 @cindex supplementary groups, printing
13302 @command{groups} prints the names of the primary and any supplementary
13303 groups for each given @var{username}, or the current process if no names
13304 are given. If more than one name is given, the name of each user is
13306 the list of that user's groups and the user name is separated from the
13307 group list by a colon. Synopsis:
13310 groups [@var{username}]@dots{}
13313 The group lists are equivalent to the output of the command @samp{id -Gn}.
13315 @primaryAndSupplementaryGroups{groups,list of users}
13317 The only options are @option{--help} and @option{--version}. @xref{Common
13323 @node users invocation
13324 @section @command{users}: Print login names of users currently logged in
13327 @cindex printing current usernames
13328 @cindex usernames, printing current
13330 @cindex login sessions, printing users with
13331 @command{users} prints on a single line a blank-separated list of user
13332 names of users currently logged in to the current host. Each user name
13333 corresponds to a login session, so if a user has more than one login
13334 session, that user's name will appear the same number of times in the
13343 With no @var{file} argument, @command{users} extracts its information from
13344 a system-maintained file (often @file{/var/run/utmp} or
13345 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13346 that file instead. A common choice is @file{/var/log/wtmp}.
13348 The only options are @option{--help} and @option{--version}. @xref{Common
13354 @node who invocation
13355 @section @command{who}: Print who is currently logged in
13358 @cindex printing current user information
13359 @cindex information, about current users
13361 @command{who} prints information about users who are currently logged on.
13365 @command{who} [@var{option}] [@var{file}] [am i]
13368 @cindex terminal lines, currently used
13370 @cindex remote hostname
13371 If given no non-option arguments, @command{who} prints the following
13372 information for each user currently logged on: login name, terminal
13373 line, login time, and remote hostname or X display.
13377 If given one non-option argument, @command{who} uses that instead of
13378 a default system-maintained file (often @file{/var/run/utmp} or
13379 @file{/etc/utmp}) as the name of the file containing the record of
13380 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13381 to @command{who} to look at who has previously logged on.
13385 If given two non-option arguments, @command{who} prints only the entry
13386 for the user running it (determined from its standard input), preceded
13387 by the hostname. Traditionally, the two arguments given are @samp{am
13388 i}, as in @samp{who am i}.
13391 Time stamps are listed according to the time zone rules specified by
13392 the @env{TZ} environment variable, or by the system default rules if
13393 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13394 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13396 The program accepts the following options. Also see @ref{Common options}.
13404 Same as @samp{-b -d --login -p -r -t -T -u}.
13410 Print the date and time of last system boot.
13416 Print information corresponding to dead processes.
13422 Print a line of column headings.
13428 List only the entries that correspond to processes via which the
13429 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13433 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13434 is not the default because it can cause significant delays on systems with
13435 automatic dial-up internet access.
13439 Same as @samp{who am i}.
13445 List active processes spawned by init.
13451 Print only the login names and the number of users logged on.
13452 Overrides all other options.
13457 @opindex --runlevel
13458 Print the current (and maybe previous) run-level of the init process.
13462 Ignored; for compatibility with other versions of @command{who}.
13468 Print last system clock change.
13473 After the login time, print the number of hours and minutes that the
13474 user has been idle. @samp{.} means the user was active in the last minute.
13475 @samp{old} means the user has been idle for more than 24 hours.
13486 @opindex --writable
13487 @cindex message status
13488 @pindex write@r{, allowed}
13489 After each login name print a character indicating the user's message status:
13492 @samp{+} allowing @code{write} messages
13493 @samp{-} disallowing @code{write} messages
13494 @samp{?} cannot find terminal device
13502 @node System context
13503 @chapter System context
13505 @cindex system context
13506 @cindex context, system
13507 @cindex commands for system context
13509 This section describes commands that print or change system-wide
13513 * date invocation:: Print or set system date and time.
13514 * arch invocation:: Print machine hardware name.
13515 * nproc invocation:: Print the number of processors.
13516 * uname invocation:: Print system information.
13517 * hostname invocation:: Print or set system name.
13518 * hostid invocation:: Print numeric host identifier.
13519 * uptime invocation:: Print system uptime and load.
13522 @node date invocation
13523 @section @command{date}: Print or set system date and time
13526 @cindex time, printing or setting
13527 @cindex printing the current time
13532 date [@var{option}]@dots{} [+@var{format}]
13533 date [-u|--utc|--universal] @c this avoids a newline in the output
13534 [ MMDDhhmm[[CC]YY][.ss] ]
13538 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13539 it with a default format that depends on the @env{LC_TIME} locale category.
13540 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13541 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13544 Normally, @command{date} uses the time zone rules indicated by the
13545 @env{TZ} environment variable, or the system default rules if @env{TZ}
13546 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13547 @env{TZ}, libc, The GNU C Library Reference Manual}.
13549 @findex strftime @r{and @command{date}}
13550 @cindex time formats
13551 @cindex formatting times
13552 If given an argument that starts with a @samp{+}, @command{date} prints the
13553 current date and time (or the date and time specified by the
13554 @option{--date} option, see below) in the format defined by that argument,
13555 which is similar to that of the @code{strftime} function. Except for
13556 conversion specifiers, which start with @samp{%}, characters in the
13557 format string are printed unchanged. The conversion specifiers are
13563 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13564 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13565 * Literal conversion specifiers:: %[%nt]
13566 * Padding and other flags:: Pad with zeros, spaces, etc.
13567 * Setting the time:: Changing the system clock.
13568 * Options for date:: Instead of the current time.
13570 * Date input formats:: Specifying date strings.
13572 * Examples of date:: Examples.
13575 @node Time conversion specifiers
13576 @subsection Time conversion specifiers
13578 @cindex time conversion specifiers
13579 @cindex conversion specifiers, time
13581 @command{date} conversion specifiers related to times.
13585 hour (@samp{00}@dots{}@samp{23})
13587 hour (@samp{01}@dots{}@samp{12})
13589 hour (@samp{ 0}@dots{}@samp{23}).
13590 This is a @acronym{GNU} extension.
13592 hour (@samp{ 1}@dots{}@samp{12}).
13593 This is a @acronym{GNU} extension.
13595 minute (@samp{00}@dots{}@samp{59})
13597 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13598 This is a @acronym{GNU} extension.
13600 locale's equivalent of either @samp{AM} or @samp{PM};
13601 blank in many locales.
13602 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13604 like @samp{%p}, except lower case.
13605 This is a @acronym{GNU} extension.
13607 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13609 24-hour hour and minute. Same as @samp{%H:%M}.
13610 This is a @acronym{GNU} extension.
13612 @cindex epoch, seconds since
13613 @cindex seconds since the epoch
13614 @cindex beginning of time
13615 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13616 Leap seconds are not counted unless leap second support is available.
13617 @xref{%s-examples}, for examples.
13618 This is a @acronym{GNU} extension.
13620 second (@samp{00}@dots{}@samp{60}).
13621 This may be @samp{60} if leap seconds are supported.
13623 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13625 locale's time representation (e.g., @samp{23:13:48})
13627 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13628 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13629 time zone is determinable. This value reflects the numeric time zone
13630 appropriate for the current time, using the time zone rules specified
13631 by the @env{TZ} environment variable.
13632 The time (and optionally, the time zone rules) can be overridden
13633 by the @option{--date} option.
13634 This is a @acronym{GNU} extension.
13636 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13637 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13638 zone is determinable.
13639 This is a @acronym{GNU} extension.
13641 Numeric time zone to the nearest second with @samp{:} (e.g.,
13642 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13644 This is a @acronym{GNU} extension.
13646 Numeric time zone with @samp{:} using the minimum necessary precision
13647 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13648 no time zone is determinable.
13649 This is a @acronym{GNU} extension.
13651 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13652 time zone is determinable. See @samp{%z} for how it is determined.
13656 @node Date conversion specifiers
13657 @subsection Date conversion specifiers
13659 @cindex date conversion specifiers
13660 @cindex conversion specifiers, date
13662 @command{date} conversion specifiers related to dates.
13666 locale's abbreviated weekday name (e.g., @samp{Sun})
13668 locale's full weekday name, variable length (e.g., @samp{Sunday})
13670 locale's abbreviated month name (e.g., @samp{Jan})
13672 locale's full month name, variable length (e.g., @samp{January})
13674 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13676 century. This is like @samp{%Y}, except the last two digits are omitted.
13677 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13678 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13679 It is normally at least two characters, but it may be more.
13681 day of month (e.g., @samp{01})
13683 date; same as @samp{%m/%d/%y}
13685 day of month, space padded; same as @samp{%_d}
13687 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13688 This is a good choice for a date format, as it is standard and
13689 is easy to sort in the usual case where years are in the range
13691 This is a @acronym{GNU} extension.
13693 year corresponding to the @acronym{ISO} week number, but without the century
13694 (range @samp{00} through @samp{99}). This has the same format and value
13695 as @samp{%y}, except that if the @acronym{ISO} week number (see
13697 to the previous or next year, that year is used instead.
13698 This is a @acronym{GNU} extension.
13700 year corresponding to the @acronym{ISO} week number. This has the
13701 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13703 @samp{%V}) belongs to the previous or next year, that year is used
13705 It is normally useful only if @samp{%V} is also used;
13706 for example, the format @samp{%G-%m-%d} is probably a mistake,
13707 since it combines the ISO week number year with the conventional month and day.
13708 This is a @acronym{GNU} extension.
13712 day of year (@samp{001}@dots{}@samp{366})
13714 month (@samp{01}@dots{}@samp{12})
13716 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13718 week number of year, with Sunday as the first day of the week
13719 (@samp{00}@dots{}@samp{53}).
13720 Days in a new year preceding the first Sunday are in week zero.
13722 @acronym{ISO} week number, that is, the
13723 week number of year, with Monday as the first day of the week
13724 (@samp{01}@dots{}@samp{53}).
13725 If the week containing January 1 has four or more days in
13726 the new year, then it is considered week 1; otherwise, it is week 53 of
13727 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13730 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13732 week number of year, with Monday as first day of week
13733 (@samp{00}@dots{}@samp{53}).
13734 Days in a new year preceding the first Monday are in week zero.
13736 locale's date representation (e.g., @samp{12/31/99})
13738 last two digits of year (@samp{00}@dots{}@samp{99})
13740 year. This is normally at least four characters, but it may be more.
13741 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13742 precedes year @samp{0000}.
13746 @node Literal conversion specifiers
13747 @subsection Literal conversion specifiers
13749 @cindex literal conversion specifiers
13750 @cindex conversion specifiers, literal
13752 @command{date} conversion specifiers that produce literal strings.
13764 @node Padding and other flags
13765 @subsection Padding and other flags
13767 @cindex numeric field padding
13768 @cindex padding of numeric fields
13769 @cindex fields, padding numeric
13771 Unless otherwise specified, @command{date} normally pads numeric fields
13772 with zeros, so that, for
13773 example, numeric months are always output as two digits.
13774 Seconds since the epoch are not padded, though,
13775 since there is no natural width for them.
13777 As a @acronym{GNU} extension, @command{date} recognizes any of the
13778 following optional flags after the @samp{%}:
13782 (hyphen) Do not pad the field; useful if the output is intended for
13785 (underscore) Pad with spaces; useful if you need a fixed
13786 number of characters in the output, but zeros are too distracting.
13788 (zero) Pad with zeros even if the conversion specifier
13789 would normally pad with spaces.
13791 Use upper case characters if possible.
13793 Use opposite case characters if possible.
13794 A field that is normally upper case becomes lower case, and vice versa.
13798 Here are some examples of padding:
13801 date +%d/%m -d "Feb 1"
13803 date +%-d/%-m -d "Feb 1"
13805 date +%_d/%_m -d "Feb 1"
13809 As a @acronym{GNU} extension, you can specify the field width
13810 (after any flag, if present) as a decimal number. If the natural size of the
13811 output of the field has less than the specified number of characters,
13812 the result is written right adjusted and padded to the given
13813 size. For example, @samp{%9B} prints the right adjusted month name in
13814 a field of width 9.
13816 An optional modifier can follow the optional flag and width
13817 specification. The modifiers are:
13821 Use the locale's alternate representation for date and time. This
13822 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13823 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13824 example, @samp{%Ex} might yield a date format based on the Japanese
13828 Use the locale's alternate numeric symbols for numbers. This modifier
13829 applies only to numeric conversion specifiers.
13832 If the format supports the modifier but no alternate representation
13833 is available, it is ignored.
13836 @node Setting the time
13837 @subsection Setting the time
13839 @cindex setting the time
13840 @cindex time setting
13841 @cindex appropriate privileges
13843 If given an argument that does not start with @samp{+}, @command{date} sets
13844 the system clock to the date and time specified by that argument (as
13845 described below). You must have appropriate privileges to set the
13846 system clock. The @option{--date} and @option{--set} options may not be
13847 used with such an argument. The @option{--universal} option may be used
13848 with such an argument to indicate that the specified date and time are
13849 relative to Coordinated Universal Time rather than to the local time
13852 The argument must consist entirely of digits, which have the following
13865 first two digits of year (optional)
13867 last two digits of year (optional)
13872 The @option{--set} option also sets the system clock; see the next section.
13875 @node Options for date
13876 @subsection Options for @command{date}
13878 @cindex @command{date} options
13879 @cindex options for @command{date}
13881 The program accepts the following options. Also see @ref{Common options}.
13885 @item -d @var{datestr}
13886 @itemx --date=@var{datestr}
13889 @cindex parsing date strings
13890 @cindex date strings, parsing
13891 @cindex arbitrary date strings, parsing
13894 @opindex next @var{day}
13895 @opindex last @var{day}
13896 Display the date and time specified in @var{datestr} instead of the
13897 current date and time. @var{datestr} can be in almost any common
13898 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13899 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13900 14:19:13.489392193 +0530"} specifies the instant of time that is
13901 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13902 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13903 Note: input currently must be in locale independent format. E.g., the
13904 LC_TIME=C below is needed to print back the correct date in many locales:
13906 date -d "$(LC_TIME=C date)"
13908 @xref{Date input formats}.
13910 @item -f @var{datefile}
13911 @itemx --file=@var{datefile}
13914 Parse each line in @var{datefile} as with @option{-d} and display the
13915 resulting date and time. If @var{datefile} is @samp{-}, use standard
13916 input. This is useful when you have many dates to process, because the
13917 system overhead of starting up the @command{date} executable many times can
13920 @item -r @var{file}
13921 @itemx --reference=@var{file}
13923 @opindex --reference
13924 Display the date and time of the last modification of @var{file},
13925 instead of the current date and time.
13932 @opindex --rfc-2822
13933 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13934 %z}, evaluated in the C locale so abbreviations are always in English.
13938 Fri, 09 Sep 2005 13:51:39 -0700
13941 This format conforms to
13942 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13943 @acronym{RFCs} 2822} and
13944 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13945 current and previous standards for Internet email.
13947 @item --rfc-3339=@var{timespec}
13948 @opindex --rfc-3339=@var{timespec}
13949 Display the date using a format specified by
13950 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13951 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13952 format, except that it also permits applications to use a space rather
13953 than a @samp{T} to separate dates from times. Unlike the other
13954 standard formats, @acronym{RFC} 3339 format is always suitable as
13955 input for the @option{--date} (@option{-d}) and @option{--file}
13956 (@option{-f}) options, regardless of the current locale.
13958 The argument @var{timespec} specifies how much of the time to include.
13959 It can be one of the following:
13963 Print just the full-date, e.g., @samp{2005-09-14}.
13964 This is equivalent to the format @samp{%Y-%m-%d}.
13967 Print the full-date and full-time separated by a space, e.g.,
13968 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
13969 time-offset; here the @samp{+05:30} means that local time is five
13970 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
13971 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
13974 Like @samp{seconds}, but also print nanoseconds, e.g.,
13975 @samp{2005-09-14 00:56:06.998458565+05:30}.
13976 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
13980 @item -s @var{datestr}
13981 @itemx --set=@var{datestr}
13984 Set the date and time to @var{datestr}. See @option{-d} above.
13991 @opindex --universal
13992 @cindex Coordinated Universal Time
13994 @cindex Greenwich Mean Time
13997 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
13998 @env{TZ} environment variable were set to the string @samp{UTC0}.
14000 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14001 historical reasons.
14005 @node Examples of date
14006 @subsection Examples of @command{date}
14008 @cindex examples of @command{date}
14010 Here are a few examples. Also see the documentation for the @option{-d}
14011 option in the previous section.
14016 To print the date of the day before yesterday:
14019 date --date='2 days ago'
14023 To print the date of the day three months and one day hence:
14026 date --date='3 months 1 day'
14030 To print the day of year of Christmas in the current year:
14033 date --date='25 Dec' +%j
14037 To print the current full month name and the day of the month:
14043 But this may not be what you want because for the first nine days of
14044 the month, the @samp{%d} expands to a zero-padded two-digit field,
14045 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14048 To print a date without the leading zero for one-digit days
14049 of the month, you can use the (@acronym{GNU} extension)
14050 @samp{-} flag to suppress
14051 the padding altogether:
14054 date -d 1may '+%B %-d
14058 To print the current date and time in the format required by many
14059 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14062 date +%m%d%H%M%Y.%S
14066 To set the system clock forward by two minutes:
14069 date --set='+2 minutes'
14073 To print the date in @acronym{RFC} 2822 format,
14074 use @samp{date --rfc-2822}. Here is some example output:
14077 Fri, 09 Sep 2005 13:51:39 -0700
14080 @anchor{%s-examples}
14082 To convert a date string to the number of seconds since the epoch
14083 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14084 the @samp{%s} format. That can be useful in sorting and/or graphing
14085 and/or comparing data by date. The following command outputs the
14086 number of the seconds since the epoch for the time two minutes after the
14090 date --date='1970-01-01 00:02:00 +0000' +%s
14094 If you do not specify time zone information in the date string,
14095 @command{date} uses your computer's idea of the time zone when
14096 interpreting the string. For example, if your computer's time zone is
14097 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14098 seconds) behind UTC:
14101 # local time zone used
14102 date --date='1970-01-01 00:02:00' +%s
14107 If you're sorting or graphing dated data, your raw date values may be
14108 represented as seconds since the epoch. But few people can look at
14109 the date @samp{946684800} and casually note ``Oh, that's the first second
14110 of the year 2000 in Greenwich, England.''
14113 date --date='2000-01-01 UTC' +%s
14117 An alternative is to use the @option{--utc} (@option{-u}) option.
14118 Then you may omit @samp{UTC} from the date string. Although this
14119 produces the same result for @samp{%s} and many other format sequences,
14120 with a time zone offset different from zero, it would give a different
14121 result for zone-dependent formats like @samp{%z}.
14124 date -u --date=2000-01-01 +%s
14128 To convert such an unwieldy number of seconds back to
14129 a more readable form, use a command like this:
14132 # local time zone used
14133 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14134 1999-12-31 19:00:00 -0500
14137 Or if you do not mind depending on the @samp{@@} feature present since
14138 coreutils 5.3.0, you could shorten this to:
14141 date -d @@946684800 +"%F %T %z"
14142 1999-12-31 19:00:00 -0500
14145 Often it is better to output UTC-relative date and time:
14148 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14149 2000-01-01 00:00:00 +0000
14155 @node arch invocation
14156 @section @command{arch}: Print machine hardware name
14159 @cindex print machine hardware name
14160 @cindex system information, printing
14162 @command{arch} prints the machine hardware name,
14163 and is equivalent to @samp{uname -m}.
14167 arch [@var{option}]
14170 The program accepts the @ref{Common options} only.
14175 @node nproc invocation
14176 @section @command{nproc}: Print the number of available processors
14179 @cindex Print the number of processors
14180 @cindex system information, printing
14182 Print the number of processing units available to the current process,
14183 which may be less than the number of online processors.
14184 If this information is not accessible, then print the number of
14185 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14186 set, then it will determine the returned value. The result is guaranteed to be
14187 greater than zero. Synopsis:
14190 nproc [@var{option}]
14193 The program accepts the following options. Also see @ref{Common options}.
14199 Print the number of installed processors on the system, which may
14200 be greater than the number online or available to the current process.
14201 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14203 @item --ignore=@var{number}
14205 If possible, exclude this @var{number} of processing units.
14212 @node uname invocation
14213 @section @command{uname}: Print system information
14216 @cindex print system information
14217 @cindex system information, printing
14219 @command{uname} prints information about the machine and operating system
14220 it is run on. If no options are given, @command{uname} acts as if the
14221 @option{-s} option were given. Synopsis:
14224 uname [@var{option}]@dots{}
14227 If multiple options or @option{-a} are given, the selected information is
14228 printed in this order:
14231 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14232 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14235 The information may contain internal spaces, so such output cannot be
14236 parsed reliably. In the following example, @var{release} is
14237 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14241 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14245 The program accepts the following options. Also see @ref{Common options}.
14253 Print all of the below information, except omit the processor type
14254 and the hardware platform name if they are unknown.
14257 @itemx --hardware-platform
14259 @opindex --hardware-platform
14260 @cindex implementation, hardware
14261 @cindex hardware platform
14262 @cindex platform, hardware
14263 Print the hardware platform name
14264 (sometimes called the hardware implementation).
14265 Print @samp{unknown} if the kernel does not make this information
14266 easily available, as is the case with Linux kernels.
14272 @cindex machine type
14273 @cindex hardware class
14274 @cindex hardware type
14275 Print the machine hardware name (sometimes called the hardware class
14281 @opindex --nodename
14284 @cindex network node name
14285 Print the network node hostname.
14290 @opindex --processor
14291 @cindex host processor type
14292 Print the processor type (sometimes called the instruction set
14293 architecture or ISA).
14294 Print @samp{unknown} if the kernel does not make this information
14295 easily available, as is the case with Linux kernels.
14298 @itemx --operating-system
14300 @opindex --operating-system
14301 @cindex operating system name
14302 Print the name of the operating system.
14305 @itemx --kernel-release
14307 @opindex --kernel-release
14308 @cindex kernel release
14309 @cindex release of kernel
14310 Print the kernel release.
14313 @itemx --kernel-name
14315 @opindex --kernel-name
14316 @cindex kernel name
14317 @cindex name of kernel
14318 Print the kernel name.
14319 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14320 ``the implementation of the operating system'', because the
14321 @acronym{POSIX} specification itself has no notion of ``kernel''.
14322 The kernel name might be the same as the operating system name printed
14323 by the @option{-o} or @option{--operating-system} option, but it might
14324 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14325 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14329 @itemx --kernel-version
14331 @opindex --kernel-version
14332 @cindex kernel version
14333 @cindex version of kernel
14334 Print the kernel version.
14341 @node hostname invocation
14342 @section @command{hostname}: Print or set system name
14345 @cindex setting the hostname
14346 @cindex printing the hostname
14347 @cindex system name, printing
14348 @cindex appropriate privileges
14350 With no arguments, @command{hostname} prints the name of the current host
14351 system. With one argument, it sets the current host name to the
14352 specified string. You must have appropriate privileges to set the host
14356 hostname [@var{name}]
14359 The only options are @option{--help} and @option{--version}. @xref{Common
14365 @node hostid invocation
14366 @section @command{hostid}: Print numeric host identifier
14369 @cindex printing the host identifier
14371 @command{hostid} prints the numeric identifier of the current host
14372 in hexadecimal. This command accepts no arguments.
14373 The only options are @option{--help} and @option{--version}.
14374 @xref{Common options}.
14376 For example, here's what it prints on one system I use:
14383 On that system, the 32-bit quantity happens to be closely
14384 related to the system's Internet address, but that isn't always
14389 @node uptime invocation
14390 @section @command{uptime}: Print system uptime and load
14393 @cindex printing the system uptime and load
14395 @command{uptime} prints the current time, the system's uptime, the
14396 number of logged-in users and the current load average.
14398 If an argument is specified, it is used as the file to be read
14399 to discover how many users are logged in. If no argument is
14400 specified, a system default is used (@command{uptime --help} indicates
14401 the default setting).
14403 The only options are @option{--help} and @option{--version}.
14404 @xref{Common options}.
14406 For example, here's what it prints right now on one system I use:
14410 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14413 The precise method of calculation of load average varies somewhat
14414 between systems. Some systems calculate it as the average number of
14415 runnable processes over the last 1, 5 and 15 minutes, but some systems
14416 also include processes in the uninterruptible sleep state (that is,
14417 those processes which are waiting for disk I/O). The Linux kernel
14418 includes uninterruptible processes.
14420 @node SELinux context
14421 @chapter SELinux context
14423 @cindex SELinux context
14424 @cindex SELinux, context
14425 @cindex commands for SELinux context
14427 This section describes commands for operations with SELinux
14431 * chcon invocation:: Change SELinux context of file
14432 * runcon invocation:: Run a command in specified SELinux context
14435 @node chcon invocation
14436 @section @command{chcon}: Change SELinux context of file
14439 @cindex changing security context
14440 @cindex change SELinux context
14442 @command{chcon} changes the SELinux security context of the selected files.
14446 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14447 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14448 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14451 Change the SELinux security context of each @var{file} to @var{context}.
14452 With @option{--reference}, change the security context of each @var{file}
14453 to that of @var{rfile}.
14455 The program accepts the following options. Also see @ref{Common options}.
14460 @itemx --no-dereference
14462 @opindex --no-dereference
14463 @cindex no dereference
14464 Affect symbolic links instead of any referenced file.
14466 @item --reference=@var{rfile}
14467 @opindex --reference
14468 @cindex reference file
14469 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14474 @opindex --recursive
14475 Operate on files and directories recursively.
14478 @xref{Traversing symlinks}.
14481 @xref{Traversing symlinks}.
14484 @xref{Traversing symlinks}.
14491 Output a diagnostic for every file processed.
14493 @item -u @var{user}
14494 @itemx --user=@var{user}
14497 Set user @var{user} in the target security context.
14499 @item -r @var{role}
14500 @itemx --role=@var{role}
14503 Set role @var{role} in the target security context.
14505 @item -t @var{type}
14506 @itemx --type=@var{type}
14509 Set type @var{type} in the target security context.
14511 @item -l @var{range}
14512 @itemx --range=@var{range}
14515 Set range @var{range} in the target security context.
14521 @node runcon invocation
14522 @section @command{runcon}: Run a command in specified SELinux context
14525 @cindex run with security context
14528 @command{runcon} runs file in specified SELinux security context.
14532 runcon @var{context} @var{command} [@var{args}]
14533 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14536 Run @var{command} with completely-specified @var{context}, or with
14537 current or transitioned security context modified by one or more of @var{level},
14538 @var{role}, @var{type} and @var{user}.
14540 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14541 is specified, the first argument is used as the complete context.
14542 Any additional arguments after @var{command}
14543 are interpreted as arguments to the command.
14545 With neither @var{context} nor @var{command}, print the current security context.
14547 The program accepts the following options. Also see @ref{Common options}.
14555 Compute process transition context before modifying.
14557 @item -u @var{user}
14558 @itemx --user=@var{user}
14561 Set user @var{user} in the target security context.
14563 @item -r @var{role}
14564 @itemx --role=@var{role}
14567 Set role @var{role} in the target security context.
14569 @item -t @var{type}
14570 @itemx --type=@var{type}
14573 Set type @var{type} in the target security context.
14575 @item -l @var{range}
14576 @itemx --range=@var{range}
14579 Set range @var{range} in the target security context.
14583 @cindex exit status of @command{runcon}
14587 126 if @var{command} is found but cannot be invoked
14588 127 if @command{runcon} itself fails or if @var{command} cannot be found
14589 the exit status of @var{command} otherwise
14592 @node Modified command invocation
14593 @chapter Modified command invocation
14595 @cindex modified command invocation
14596 @cindex invocation of commands, modified
14597 @cindex commands for invoking other commands
14599 This section describes commands that run other commands in some context
14600 different than the current one: a modified environment, as a different
14604 * chroot invocation:: Modify the root directory.
14605 * env invocation:: Modify environment variables.
14606 * nice invocation:: Modify niceness.
14607 * nohup invocation:: Immunize to hangups.
14608 * stdbuf invocation:: Modify buffering of standard streams.
14609 * su invocation:: Modify user and group ID.
14610 * timeout invocation:: Run with time limit.
14614 @node chroot invocation
14615 @section @command{chroot}: Run a command with a different root directory
14618 @cindex running a program in a specified root directory
14619 @cindex root directory, running a program in a specified
14621 @command{chroot} runs a command with a specified root directory.
14622 On many systems, only the super-user can do this.@footnote{However,
14623 some systems (e.g., FreeBSD) can be configured to allow certain regular
14624 users to use the @code{chroot} system call, and hence to run this program.
14625 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14626 underlying function is non-privileged due to lack of support in MS-Windows.}
14630 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14631 chroot @var{option}
14634 Ordinarily, file names are looked up starting at the root of the
14635 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14636 the directory @var{newroot} (which must exist) and then runs
14637 @var{command} with optional @var{args}. If @var{command} is not
14638 specified, the default is the value of the @env{SHELL} environment
14639 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14640 @var{command} must not be a special built-in utility
14641 (@pxref{Special built-in utilities}).
14643 The program accepts the following options. Also see @ref{Common options}.
14644 Options must precede operands.
14648 @itemx --userspec=@var{user}[:@var{group}]
14649 @opindex --userspec
14650 By default, @var{command} is run with the same credentials
14651 as the invoking process.
14652 Use this option to run it as a different @var{user} and/or with a
14653 different primary @var{group}.
14655 @itemx --groups=@var{groups}
14657 Use this option to specify the supplementary @var{groups} to be
14658 used by the new process.
14659 The items in the list (names or numeric IDs) must be separated by commas.
14663 Here are a few tips to help avoid common problems in using chroot.
14664 To start with a simple example, make @var{command} refer to a statically
14665 linked binary. If you were to use a dynamically linked executable, then
14666 you'd have to arrange to have the shared libraries in the right place under
14667 your new root directory.
14669 For example, if you create a statically linked @command{ls} executable,
14670 and put it in @file{/tmp/empty}, you can run this command as root:
14673 $ chroot /tmp/empty /ls -Rl /
14676 Then you'll see output like this:
14681 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14684 If you want to use a dynamically linked executable, say @command{bash},
14685 then first run @samp{ldd bash} to see what shared objects it needs.
14686 Then, in addition to copying the actual binary, also copy the listed
14687 files to the required positions under your intended new root directory.
14688 Finally, if the executable requires any other files (e.g., data, state,
14689 device files), copy them into place, too.
14691 @cindex exit status of @command{chroot}
14695 125 if @command{chroot} itself fails
14696 126 if @var{command} is found but cannot be invoked
14697 127 if @var{command} cannot be found
14698 the exit status of @var{command} otherwise
14702 @node env invocation
14703 @section @command{env}: Run a command in a modified environment
14706 @cindex environment, running a program in a modified
14707 @cindex modified environment, running a program in a
14708 @cindex running a program in a modified environment
14710 @command{env} runs a command with a modified environment. Synopses:
14713 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14714 [@var{command} [@var{args}]@dots{}]
14718 Operands of the form @samp{@var{variable}=@var{value}} set
14719 the environment variable @var{variable} to value @var{value}.
14720 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14721 to an empty value is different from unsetting it.
14722 These operands are evaluated left-to-right, so if two operands
14723 mention the same variable the earlier is ignored.
14725 Environment variable names can be empty, and can contain any
14726 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14727 However, it is wise to limit yourself to names that
14728 consist solely of underscores, digits, and @acronym{ASCII} letters,
14729 and that begin with a non-digit, as applications like the shell do not
14730 work well with other names.
14733 The first operand that does not contain the character @samp{=}
14734 specifies the program to invoke; it is
14735 searched for according to the @env{PATH} environment variable. Any
14736 remaining arguments are passed as arguments to that program.
14737 The program should not be a special built-in utility
14738 (@pxref{Special built-in utilities}).
14740 Modifications to @env{PATH} take effect prior to searching for
14741 @var{command}. Use caution when reducing @env{PATH}; behavior is
14742 not portable when @env{PATH} is undefined or omits key directories
14743 such as @file{/bin}.
14745 In the rare case that a utility contains a @samp{=} in the name, the
14746 only way to disambiguate it from a variable assignment is to use an
14747 intermediate command for @var{command}, and pass the problematic
14748 program name via @var{args}. For example, if @file{./prog=} is an
14749 executable in the current @env{PATH}:
14752 env prog= true # runs 'true', with prog= in environment
14753 env ./prog= true # runs 'true', with ./prog= in environment
14754 env -- prog= true # runs 'true', with prog= in environment
14755 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14756 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14759 @cindex environment, printing
14761 If no command name is specified following the environment
14762 specifications, the resulting environment is printed. This is like
14763 specifying the @command{printenv} program.
14765 For some examples, suppose the environment passed to @command{env}
14766 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14767 @samp{PATH=.:/gnubin:/hacks}:
14772 Output the current environment.
14774 $ env | LC_ALL=C sort
14777 PATH=.:/gnubin:/hacks
14781 Run @command{foo} with a reduced environment, preserving only the
14782 original @env{PATH} to avoid problems in locating @command{foo}.
14784 env - PATH="$PATH" foo
14788 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14789 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14790 that @command{foo} was found in the file system rather than as a shell
14797 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14798 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14799 @samp{DISPLAY=gnu:0}.
14801 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14805 Attempt to run the program @command{/energy/--} (as that is the only
14806 possible path search result); if the command exists, the environment
14807 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14808 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14810 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14816 The program accepts the following options. Also see @ref{Common options}.
14817 Options must precede operands.
14823 @item -u @var{name}
14824 @itemx --unset=@var{name}
14827 Remove variable @var{name} from the environment, if it was in the
14832 @itemx --ignore-environment
14835 @opindex --ignore-environment
14836 Start with an empty environment, ignoring the inherited environment.
14840 @cindex exit status of @command{env}
14844 0 if no @var{command} is specified and the environment is output
14845 125 if @command{env} itself fails
14846 126 if @var{command} is found but cannot be invoked
14847 127 if @var{command} cannot be found
14848 the exit status of @var{command} otherwise
14852 @node nice invocation
14853 @section @command{nice}: Run a command with modified niceness
14857 @cindex scheduling, affecting
14858 @cindex appropriate privileges
14860 @command{nice} prints or modifies a process's @dfn{niceness},
14861 a parameter that affects whether the process is scheduled favorably.
14865 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14868 If no arguments are given, @command{nice} prints the current niceness.
14869 Otherwise, @command{nice} runs the given @var{command} with its
14870 niceness adjusted. By default, its niceness is incremented by 10.
14872 Niceness values range at least from @minus{}20 (process has high priority
14873 and gets more resources, thus slowing down other processes) through 19
14874 (process has lower priority and runs slowly itself, but has less impact
14875 on the speed of other running processes). Some systems
14876 may have a wider range of nicenesses; conversely, other systems may
14877 enforce more restrictive limits. An attempt to set the niceness
14878 outside the supported range is treated as an attempt to use the
14879 minimum or maximum supported value.
14881 A niceness should not be confused with a scheduling priority, which
14882 lets applications determine the order in which threads are scheduled
14883 to run. Unlike a priority, a niceness is merely advice to the
14884 scheduler, which the scheduler is free to ignore. Also, as a point of
14885 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14886 terms of a @dfn{nice value}, which is the nonnegative difference
14887 between a niceness and the minimum niceness. Though @command{nice}
14888 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14889 term ``niceness'' for compatibility with historical practice.
14891 @var{command} must not be a special built-in utility (@pxref{Special
14892 built-in utilities}).
14894 @mayConflictWithShellBuiltIn{nice}
14896 The program accepts the following option. Also see @ref{Common options}.
14897 Options must precede operands.
14900 @item -n @var{adjustment}
14901 @itemx --adjustment=@var{adjustment}
14903 @opindex --adjustment
14904 Add @var{adjustment} instead of 10 to the command's niceness. If
14905 @var{adjustment} is negative and you lack appropriate privileges,
14906 @command{nice} issues a warning but otherwise acts as if you specified
14909 For compatibility @command{nice} also supports an obsolete
14910 option syntax @option{-@var{adjustment}}. New scripts should use
14911 @option{-n @var{adjustment}} instead.
14915 @cindex exit status of @command{nice}
14919 0 if no @var{command} is specified and the niceness is output
14920 125 if @command{nice} itself fails
14921 126 if @var{command} is found but cannot be invoked
14922 127 if @var{command} cannot be found
14923 the exit status of @var{command} otherwise
14926 It is sometimes useful to run a non-interactive program with reduced niceness.
14929 $ nice factor 4611686018427387903
14932 Since @command{nice} prints the current niceness,
14933 you can invoke it through itself to demonstrate how it works.
14935 The default behavior is to increase the niceness by @samp{10}:
14946 The @var{adjustment} is relative to the current niceness. In the
14947 next example, the first @command{nice} invocation runs the second one
14948 with niceness 10, and it in turn runs the final one with a niceness
14952 $ nice nice -n 3 nice
14956 Specifying a niceness larger than the supported range
14957 is the same as specifying the maximum supported value:
14960 $ nice -n 10000000000 nice
14964 Only a privileged user may run a process with lower niceness:
14968 nice: cannot set niceness: Permission denied
14970 $ sudo nice -n -1 nice
14975 @node nohup invocation
14976 @section @command{nohup}: Run a command immune to hangups
14979 @cindex hangups, immunity to
14980 @cindex immunity to hangups
14981 @cindex logging out and continuing to run
14984 @command{nohup} runs the given @var{command} with hangup signals ignored,
14985 so that the command can continue running in the background after you log
14989 nohup @var{command} [@var{arg}]@dots{}
14992 If standard input is a terminal, it is redirected from
14993 @file{/dev/null} so that terminal sessions do not mistakenly consider
14994 the terminal to be used by the command. This is a @acronym{GNU}
14995 extension; programs intended to be portable to non-@acronym{GNU} hosts
14996 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15000 If standard output is a terminal, the command's standard output is appended
15001 to the file @file{nohup.out}; if that cannot be written to, it is appended
15002 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15003 command is not run.
15004 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15005 @command{nohup} is made readable and writable only to the user,
15006 regardless of the current umask settings.
15008 If standard error is a terminal, it is normally redirected to the same file
15009 descriptor as the (possibly-redirected) standard output.
15010 However, if standard output is closed, standard error terminal output
15011 is instead appended to the file @file{nohup.out} or
15012 @file{$HOME/nohup.out} as above.
15014 To capture the command's output to a file other than @file{nohup.out}
15015 you can redirect it. For example, to capture the output of
15019 nohup make > make.log
15022 @command{nohup} does not automatically put the command it runs in the
15023 background; you must do that explicitly, by ending the command line
15024 with an @samp{&}. Also, @command{nohup} does not alter the
15025 niceness of @var{command}; use @command{nice} for that,
15026 e.g., @samp{nohup nice @var{command}}.
15028 @var{command} must not be a special built-in utility (@pxref{Special
15029 built-in utilities}).
15031 The only options are @option{--help} and @option{--version}. @xref{Common
15032 options}. Options must precede operands.
15034 @cindex exit status of @command{nohup}
15038 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15039 126 if @var{command} is found but cannot be invoked
15040 127 if @var{command} cannot be found
15041 the exit status of @var{command} otherwise
15044 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15048 @node stdbuf invocation
15049 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15052 @cindex standard streams, buffering
15053 @cindex line buffered
15055 @command{stdbuf} allows one to modify the buffering operations of the
15056 three standard I/O streams associated with a program. Synopsis:
15059 stdbuf @var{option}@dots{} @var{command}
15062 Any additional @var{arg}s are passed as additional arguments to the
15065 The program accepts the following options. Also see @ref{Common options}.
15069 @item -i @var{mode}
15070 @itemx --input=@var{mode}
15073 Adjust the standard input stream buffering.
15075 @item -o @var{mode}
15076 @itemx --output=@var{mode}
15079 Adjust the standard output stream buffering.
15081 @item -e @var{mode}
15082 @itemx --error=@var{mode}
15085 Adjust the standard error stream buffering.
15089 The @var{mode} can be specified as follows:
15094 Set the stream to line buffered mode.
15095 In this mode data is coalesced until a newline is output or
15096 input is read from any stream attached to a terminal device.
15097 This option is invalid with standard input.
15100 Disable buffering of the selected stream.
15101 In this mode data is output immediately and only the
15102 amount of data requested is read from input.
15105 Specify the size of the buffer to use in fully buffered mode.
15106 @multiplierSuffixesNoBlocks{size}
15110 NOTE: If @var{command} adjusts the buffering of its standard streams
15111 (@command{tee} does for e.g.) then that will override corresponding settings
15112 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15113 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15114 by @command{stdbuf} settings.
15116 @cindex exit status of @command{stdbuf}
15120 125 if @command{stdbuf} itself fails
15121 126 if @var{command} is found but cannot be invoked
15122 127 if @var{command} cannot be found
15123 the exit status of @var{command} otherwise
15127 @node su invocation
15128 @section @command{su}: Run a command with substitute user and group ID
15131 @cindex substitute user and group IDs
15132 @cindex user ID, switching
15133 @cindex super-user, becoming
15134 @cindex root, becoming
15136 @command{su} allows one user to temporarily become another user. It runs a
15137 command (often an interactive shell) with the real and effective user
15138 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15141 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15144 @cindex passwd entry, and @command{su} shell
15146 @flindex /etc/passwd
15147 If no @var{user} is given, the default is @code{root}, the super-user.
15148 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15149 @file{/bin/sh} if none is specified there. If @var{user} has a
15150 password, @command{su} prompts for the password unless run by a user with
15151 effective user ID of zero (the super-user).
15157 @cindex login shell
15158 By default, @command{su} does not change the current directory.
15159 It sets the environment variables @env{HOME} and @env{SHELL}
15160 from the password entry for @var{user}, and if @var{user} is not
15161 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15162 By default, the shell is not a login shell.
15164 Any additional @var{arg}s are passed as additional arguments to the
15167 @cindex @option{-su}
15168 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15169 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15170 to certain shells, etc.).
15173 @command{su} can optionally be compiled to use @code{syslog} to report
15174 failed, and optionally successful, @command{su} attempts. (If the system
15175 supports @code{syslog}.) However, GNU @command{su} does not check if the
15176 user is a member of the @code{wheel} group; see below.
15178 The program accepts the following options. Also see @ref{Common options}.
15181 @item -c @var{command}
15182 @itemx --command=@var{command}
15185 Pass @var{command}, a single command line to run, to the shell with
15186 a @option{-c} option instead of starting an interactive shell.
15193 @cindex file name pattern expansion, disabled
15194 @cindex globbing, disabled
15195 Pass the @option{-f} option to the shell. This probably only makes sense
15196 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15197 option prevents reading the startup file (@file{.cshrc}). With
15198 Bourne-like shells, the @option{-f} option disables file name pattern
15199 expansion (globbing), which is not likely to be useful.
15207 @c other variables already indexed above
15210 @cindex login shell, creating
15211 Make the shell a login shell. This means the following. Unset all
15212 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15213 (which are set as described above), and @env{USER} and @env{LOGNAME}
15214 (which are set, even for the super-user, as described above), and set
15215 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15216 directory. Prepend @samp{-} to the shell's name, intended to make it
15217 read its login startup file(s).
15221 @itemx --preserve-environment
15224 @opindex --preserve-environment
15225 @cindex environment, preserving
15226 @flindex /etc/shells
15227 @cindex restricted shell
15228 Do not change the environment variables @env{HOME}, @env{USER},
15229 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15230 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15231 entry, unless the user running @command{su} is not the super-user and
15232 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15233 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15234 if that file does not exist. Parts of what this option does can be
15235 overridden by @option{--login} and @option{--shell}.
15237 @item -s @var{shell}
15238 @itemx --shell=@var{shell}
15241 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15242 unless the user running @command{su} is not the super-user and @var{user}'s
15243 shell is restricted (see @option{-m} just above).
15247 @cindex exit status of @command{su}
15251 125 if @command{su} itself fails
15252 126 if subshell is found but cannot be invoked
15253 127 if subshell cannot be found
15254 the exit status of the subshell otherwise
15257 @cindex wheel group, not supported
15258 @cindex group wheel, not supported
15260 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15262 (This section is by Richard Stallman.)
15266 Sometimes a few of the users try to hold total power over all the
15267 rest. For example, in 1984, a few users at the MIT AI lab decided to
15268 seize power by changing the operator password on the Twenex system and
15269 keeping it secret from everyone else. (I was able to thwart this coup
15270 and give power back to the users by patching the kernel, but I
15271 wouldn't know how to do that in Unix.)
15273 However, occasionally the rulers do tell someone. Under the usual
15274 @command{su} mechanism, once someone learns the root password who
15275 sympathizes with the ordinary users, he or she can tell the rest. The
15276 ``wheel group'' feature would make this impossible, and thus cement the
15277 power of the rulers.
15279 I'm on the side of the masses, not that of the rulers. If you are
15280 used to supporting the bosses and sysadmins in whatever they do, you
15281 might find this idea strange at first.
15284 @node timeout invocation
15285 @section @command{timeout}: Run a command with a time limit
15289 @cindex run commands with bounded time
15291 @command{timeout} runs the given @var{command} and kills it if it is
15292 still running after the specified time interval. Synopsis:
15295 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15298 @var{command} must not be a special built-in utility (@pxref{Special
15299 built-in utilities}).
15301 The program accepts the following options. Also see @ref{Common options}.
15302 Options must precede operands.
15305 @item -k @var{duration}
15306 @itemx --kill-after=@var{duration}
15308 @opindex --kill-after
15309 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15310 signal, after the specified @var{duration}. Without this option, if the
15311 selected signal proves not to be fatal, @command{timeout} does not kill
15314 @item -s @var{signal}
15315 @itemx --signal=@var{signal}
15318 Send this @var{signal} to @var{command} on timeout, rather than the
15319 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15320 or a number. Also see @xref{Signal specifications}.
15324 @var{duration} is an integer followed by an optional unit:
15326 @samp{s} for seconds (the default)
15327 @samp{m} for minutes
15331 A duration of 0 disables the associated timeout.
15333 @cindex exit status of @command{timeout}
15337 124 if @var{command} times out
15338 125 if @command{timeout} itself fails
15339 126 if @var{command} is found but cannot be invoked
15340 127 if @var{command} cannot be found
15341 the exit status of @var{command} otherwise
15345 @node Process control
15346 @chapter Process control
15348 @cindex processes, commands for controlling
15349 @cindex commands for controlling processes
15352 * kill invocation:: Sending a signal to processes.
15356 @node kill invocation
15357 @section @command{kill}: Send a signal to processes
15360 @cindex send a signal to processes
15362 The @command{kill} command sends a signal to processes, causing them
15363 to terminate or otherwise act upon receiving the signal in some way.
15364 Alternatively, it lists information about signals. Synopses:
15367 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15368 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15371 @mayConflictWithShellBuiltIn{kill}
15373 The first form of the @command{kill} command sends a signal to all
15374 @var{pid} arguments. The default signal to send if none is specified
15375 is @samp{TERM}. The special signal number @samp{0} does not denote a
15376 valid signal, but can be used to test whether the @var{pid} arguments
15377 specify processes to which a signal could be sent.
15379 If @var{pid} is positive, the signal is sent to the process with the
15380 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15381 processes in the process group of the current process. If @var{pid}
15382 is @minus{}1, the signal is sent to all processes for which the user has
15383 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15384 is sent to all processes in the process group that equals the absolute
15385 value of @var{pid}.
15387 If @var{pid} is not positive, a system-dependent set of system
15388 processes is excluded from the list of processes to which the signal
15391 If a negative @var{pid} argument is desired as the first one, it
15392 should be preceded by @option{--}. However, as a common extension to
15393 @acronym{POSIX}, @option{--} is not required with @samp{kill
15394 -@var{signal} -@var{pid}}. The following commands are equivalent:
15403 The first form of the @command{kill} command succeeds if every @var{pid}
15404 argument specifies at least one process that the signal was sent to.
15406 The second form of the @command{kill} command lists signal information.
15407 Either the @option{-l} or @option{--list} option, or the @option{-t}
15408 or @option{--table} option must be specified. Without any
15409 @var{signal} argument, all supported signals are listed. The output
15410 of @option{-l} or @option{--list} is a list of the signal names, one
15411 per line; if @var{signal} is already a name, the signal number is
15412 printed instead. The output of @option{-t} or @option{--table} is a
15413 table of signal numbers, names, and descriptions. This form of the
15414 @command{kill} command succeeds if all @var{signal} arguments are valid
15415 and if there is no output error.
15417 The @command{kill} command also supports the @option{--help} and
15418 @option{--version} options. @xref{Common options}.
15420 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15421 number like @samp{1}, or an exit status of a process terminated by the
15422 signal. A signal name can be given in canonical form or prefixed by
15423 @samp{SIG}. The case of the letters is ignored, except for the
15424 @option{-@var{signal}} option which must use upper case to avoid
15425 ambiguity with lower case option letters. For a list of supported
15426 signal names and numbers see @xref{Signal specifications}.
15431 @cindex delaying commands
15432 @cindex commands for delaying
15434 @c Perhaps @command{wait} or other commands should be described here also?
15437 * sleep invocation:: Delay for a specified time.
15441 @node sleep invocation
15442 @section @command{sleep}: Delay for a specified time
15445 @cindex delay for a specified time
15447 @command{sleep} pauses for an amount of time specified by the sum of
15448 the values of the command line arguments.
15452 sleep @var{number}[smhd]@dots{}
15456 Each argument is a number followed by an optional unit; the default
15457 is seconds. The units are:
15470 Historical implementations of @command{sleep} have required that
15471 @var{number} be an integer, and only accepted a single argument
15472 without a suffix. However, GNU @command{sleep} accepts
15473 arbitrary floating point numbers (using a period before any fractional
15476 The only options are @option{--help} and @option{--version}. @xref{Common
15479 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15480 @mayConflictWithShellBuiltIn{sleep}
15485 @node Numeric operations
15486 @chapter Numeric operations
15488 @cindex numeric operations
15489 These programs do numerically-related operations.
15492 * factor invocation:: Show factors of numbers.
15493 * seq invocation:: Print sequences of numbers.
15497 @node factor invocation
15498 @section @command{factor}: Print prime factors
15501 @cindex prime factors
15503 @command{factor} prints prime factors. Synopses:
15506 factor [@var{number}]@dots{}
15507 factor @var{option}
15510 If no @var{number} is specified on the command line, @command{factor} reads
15511 numbers from standard input, delimited by newlines, tabs, or spaces.
15513 The @command{factor} command supports only a small number of options:
15517 Print a short help on standard output, then exit without further
15521 Print the program version on standard output, then exit without further
15525 Factoring the product of the eighth and ninth Mersenne primes
15526 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15529 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15530 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15531 4951760154835678088235319297: 2147483647 2305843009213693951
15535 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15536 about 20 seconds on the same machine.
15538 Factoring large numbers is, in general, hard. The Pollard Rho
15539 algorithm used by @command{factor} is particularly effective for
15540 numbers with relatively small factors. If you wish to factor large
15541 numbers which do not have small factors (for example, numbers which
15542 are the product of two large primes), other methods are far better.
15544 If @command{factor} is built without using GNU MP, only
15545 single-precision arithmetic is available, and so large numbers
15546 (typically @math{2^{64}} and above) will not be supported. The single-precision
15547 code uses an algorithm which is designed for factoring smaller
15553 @node seq invocation
15554 @section @command{seq}: Print numeric sequences
15557 @cindex numeric sequences
15558 @cindex sequence of numbers
15560 @command{seq} prints a sequence of numbers to standard output. Synopses:
15563 seq [@var{option}]@dots{} @var{last}
15564 seq [@var{option}]@dots{} @var{first} @var{last}
15565 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15568 @command{seq} prints the numbers from @var{first} to @var{last} by
15569 @var{increment}. By default, each number is printed on a separate line.
15570 When @var{increment} is not specified, it defaults to @samp{1},
15571 even when @var{first} is larger than @var{last}.
15572 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15573 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15574 Floating-point numbers
15575 may be specified (using a period before any fractional digits).
15577 The program accepts the following options. Also see @ref{Common options}.
15578 Options must precede operands.
15581 @item -f @var{format}
15582 @itemx --format=@var{format}
15583 @opindex -f @var{format}
15584 @opindex --format=@var{format}
15585 @cindex formatting of numbers in @command{seq}
15586 Print all numbers using @var{format}.
15587 @var{format} must contain exactly one of the @samp{printf}-style
15588 floating point conversion specifications @samp{%a}, @samp{%e},
15589 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15590 The @samp{%} may be followed by zero or more flags taken from the set
15591 @samp{-+#0 '}, then an optional width containing one or more digits,
15592 then an optional precision consisting of a @samp{.} followed by zero
15593 or more digits. @var{format} may also contain any number of @samp{%%}
15594 conversion specifications. All conversion specifications have the
15595 same meaning as with @samp{printf}.
15597 The default format is derived from @var{first}, @var{step}, and
15598 @var{last}. If these all use a fixed point decimal representation,
15599 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15600 precision that can represent the output numbers exactly. Otherwise,
15601 the default format is @samp{%g}.
15603 @item -s @var{string}
15604 @itemx --separator=@var{string}
15605 @cindex separator for numbers in @command{seq}
15606 Separate numbers with @var{string}; default is a newline.
15607 The output always terminates with a newline.
15610 @itemx --equal-width
15611 Print all numbers with the same width, by padding with leading zeros.
15612 @var{first}, @var{step}, and @var{last} should all use a fixed point
15613 decimal representation.
15614 (To have other kinds of padding, use @option{--format}).
15618 You can get finer-grained control over output with @option{-f}:
15621 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15627 If you want hexadecimal integer output, you can use @command{printf}
15628 to perform the conversion:
15631 $ printf '%x\n' `seq 1048575 1024 1050623`
15637 For very long lists of numbers, use xargs to avoid
15638 system limitations on the length of an argument list:
15641 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15647 To generate octal output, use the printf @code{%o} format instead
15650 On most systems, seq can produce whole-number output for values up to
15651 at least @math{2^{53}}. Larger integers are approximated. The details
15652 differ depending on your floating-point implementation, but a common
15653 case is that @command{seq} works with integers through @math{2^{64}},
15654 and larger integers may not be numerically correct:
15657 $ seq 18446744073709551616 1 18446744073709551618
15658 18446744073709551616
15659 18446744073709551616
15660 18446744073709551618
15663 Be careful when using @command{seq} with outlandish values: otherwise
15664 you may see surprising results, as @command{seq} uses floating point
15665 internally. For example, on the x86 platform, where the internal
15666 representation uses a 64-bit fraction, the command:
15669 seq 1 0.0000000000000000001 1.0000000000000000009
15672 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15677 @node File permissions
15678 @chapter File permissions
15681 @include get_date.texi
15685 @node Opening the software toolbox
15686 @chapter Opening the Software Toolbox
15688 An earlier version of this chapter appeared in
15689 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15690 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15691 It was written by Arnold Robbins.
15694 * Toolbox introduction:: Toolbox introduction
15695 * I/O redirection:: I/O redirection
15696 * The who command:: The @command{who} command
15697 * The cut command:: The @command{cut} command
15698 * The sort command:: The @command{sort} command
15699 * The uniq command:: The @command{uniq} command
15700 * Putting the tools together:: Putting the tools together
15704 @node Toolbox introduction
15705 @unnumberedsec Toolbox Introduction
15707 This month's column is only peripherally related to the GNU Project, in
15708 that it describes a number of the GNU tools on your GNU/Linux system and how they
15709 might be used. What it's really about is the ``Software Tools'' philosophy
15710 of program development and usage.
15712 The software tools philosophy was an important and integral concept
15713 in the initial design and development of Unix (of which Linux and GNU are
15714 essentially clones). Unfortunately, in the modern day press of
15715 Internetworking and flashy GUIs, it seems to have fallen by the
15716 wayside. This is a shame, since it provides a powerful mental model
15717 for solving many kinds of problems.
15719 Many people carry a Swiss Army knife around in their pants pockets (or
15720 purse). A Swiss Army knife is a handy tool to have: it has several knife
15721 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15722 a number of other things on it. For the everyday, small miscellaneous jobs
15723 where you need a simple, general purpose tool, it's just the thing.
15725 On the other hand, an experienced carpenter doesn't build a house using
15726 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15727 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15728 exactly when and where to use each tool; you won't catch him hammering nails
15729 with the handle of his screwdriver.
15731 The Unix developers at Bell Labs were all professional programmers and trained
15732 computer scientists. They had found that while a one-size-fits-all program
15733 might appeal to a user because there's only one program to use, in practice
15738 difficult to write,
15741 difficult to maintain and
15745 difficult to extend to meet new situations.
15748 Instead, they felt that programs should be specialized tools. In short, each
15749 program ``should do one thing well.'' No more and no less. Such programs are
15750 simpler to design, write, and get right---they only do one thing.
15752 Furthermore, they found that with the right machinery for hooking programs
15753 together, that the whole was greater than the sum of the parts. By combining
15754 several special purpose programs, you could accomplish a specific task
15755 that none of the programs was designed for, and accomplish it much more
15756 quickly and easily than if you had to write a special purpose program.
15757 We will see some (classic) examples of this further on in the column.
15758 (An important additional point was that, if necessary, take a detour
15759 and build any software tools you may need first, if you don't already
15760 have something appropriate in the toolbox.)
15762 @node I/O redirection
15763 @unnumberedsec I/O Redirection
15765 Hopefully, you are familiar with the basics of I/O redirection in the
15766 shell, in particular the concepts of ``standard input,'' ``standard output,''
15767 and ``standard error''. Briefly, ``standard input'' is a data source, where
15768 data comes from. A program should not need to either know or care if the
15769 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15770 card reader. Similarly, ``standard output'' is a data sink, where data goes
15771 to. The program should neither know nor care where this might be.
15772 Programs that only read their standard input, do something to the data,
15773 and then send it on, are called @dfn{filters}, by analogy to filters in a
15776 With the Unix shell, it's very easy to set up data pipelines:
15779 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15782 We start out by creating the raw data; each filter applies some successive
15783 transformation to the data, until by the time it comes out of the pipeline,
15784 it is in the desired form.
15786 This is fine and good for standard input and standard output. Where does the
15787 standard error come in to play? Well, think about @command{filter1} in
15788 the pipeline above. What happens if it encounters an error in the data it
15789 sees? If it writes an error message to standard output, it will just
15790 disappear down the pipeline into @command{filter2}'s input, and the
15791 user will probably never see it. So programs need a place where they can send
15792 error messages so that the user will notice them. This is standard error,
15793 and it is usually connected to your console or window, even if you have
15794 redirected standard output of your program away from your screen.
15796 For filter programs to work together, the format of the data has to be
15797 agreed upon. The most straightforward and easiest format to use is simply
15798 lines of text. Unix data files are generally just streams of bytes, with
15799 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15800 conventionally called a ``newline'' in the Unix literature. (This is
15801 @code{'\n'} if you're a C programmer.) This is the format used by all
15802 the traditional filtering programs. (Many earlier operating systems
15803 had elaborate facilities and special purpose programs for managing
15804 binary data. Unix has always shied away from such things, under the
15805 philosophy that it's easiest to simply be able to view and edit your
15806 data with a text editor.)
15808 OK, enough introduction. Let's take a look at some of the tools, and then
15809 we'll see how to hook them together in interesting ways. In the following
15810 discussion, we will only present those command line options that interest
15811 us. As you should always do, double check your system documentation
15812 for the full story.
15814 @node The who command
15815 @unnumberedsec The @command{who} Command
15817 The first program is the @command{who} command. By itself, it generates a
15818 list of the users who are currently logged in. Although I'm writing
15819 this on a single-user system, we'll pretend that several people are
15824 @print{} arnold console Jan 22 19:57
15825 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15826 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15827 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15830 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15831 There are three people logged in, and I am logged in twice. On traditional
15832 Unix systems, user names are never more than eight characters long. This
15833 little bit of trivia will be useful later. The output of @command{who} is nice,
15834 but the data is not all that exciting.
15836 @node The cut command
15837 @unnumberedsec The @command{cut} Command
15839 The next program we'll look at is the @command{cut} command. This program
15840 cuts out columns or fields of input data. For example, we can tell it
15841 to print just the login name and full name from the @file{/etc/passwd}
15842 file. The @file{/etc/passwd} file has seven fields, separated by
15846 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15849 To get the first and fifth fields, we would use @command{cut} like this:
15852 $ cut -d: -f1,5 /etc/passwd
15853 @print{} root:Operator
15855 @print{} arnold:Arnold D. Robbins
15856 @print{} miriam:Miriam A. Robbins
15860 With the @option{-c} option, @command{cut} will cut out specific characters
15861 (i.e., columns) in the input lines. This is useful for input data
15862 that has fixed width fields, and does not have a field separator. For
15863 example, list the Monday dates for the current month:
15865 @c Is using cal ok? Looked at gcal, but I don't like it.
15876 @node The sort command
15877 @unnumberedsec The @command{sort} Command
15879 Next we'll look at the @command{sort} command. This is one of the most
15880 powerful commands on a Unix-style system; one that you will often find
15881 yourself using when setting up fancy data plumbing.
15884 command reads and sorts each file named on the command line. It then
15885 merges the sorted data and writes it to standard output. It will read
15886 standard input if no files are given on the command line (thus
15887 making it into a filter). The sort is based on the character collating
15888 sequence or based on user-supplied ordering criteria.
15891 @node The uniq command
15892 @unnumberedsec The @command{uniq} Command
15894 Finally (at least for now), we'll look at the @command{uniq} program. When
15895 sorting data, you will often end up with duplicate lines, lines that
15896 are identical. Usually, all you need is one instance of each line.
15897 This is where @command{uniq} comes in. The @command{uniq} program reads its
15898 standard input. It prints only one
15899 copy of each repeated line. It does have several options. Later on,
15900 we'll use the @option{-c} option, which prints each unique line, preceded
15901 by a count of the number of times that line occurred in the input.
15904 @node Putting the tools together
15905 @unnumberedsec Putting the Tools Together
15907 Now, let's suppose this is a large ISP server system with dozens of users
15908 logged in. The management wants the system administrator to write a program that will
15909 generate a sorted list of logged in users. Furthermore, even if a user
15910 is logged in multiple times, his or her name should only show up in the
15913 The administrator could sit down with the system documentation and write a C
15914 program that did this. It would take perhaps a couple of hundred lines
15915 of code and about two hours to write it, test it, and debug it.
15916 However, knowing the software toolbox, the administrator can instead start out
15917 by generating just a list of logged on users:
15927 Next, sort the list:
15930 $ who | cut -c1-8 | sort
15937 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15940 $ who | cut -c1-8 | sort | uniq
15946 The @command{sort} command actually has a @option{-u} option that does what
15947 @command{uniq} does. However, @command{uniq} has other uses for which one
15948 cannot substitute @samp{sort -u}.
15950 The administrator puts this pipeline into a shell script, and makes it available for
15951 all the users on the system (@samp{#} is the system administrator,
15952 or @code{root}, prompt):
15955 # cat > /usr/local/bin/listusers
15956 who | cut -c1-8 | sort | uniq
15958 # chmod +x /usr/local/bin/listusers
15961 There are four major points to note here. First, with just four
15962 programs, on one command line, the administrator was able to save about two
15963 hours worth of work. Furthermore, the shell pipeline is just about as
15964 efficient as the C program would be, and it is much more efficient in
15965 terms of programmer time. People time is much more expensive than
15966 computer time, and in our modern ``there's never enough time to do
15967 everything'' society, saving two hours of programmer time is no mean
15970 Second, it is also important to emphasize that with the
15971 @emph{combination} of the tools, it is possible to do a special
15972 purpose job never imagined by the authors of the individual programs.
15974 Third, it is also valuable to build up your pipeline in stages, as we did here.
15975 This allows you to view the data at each stage in the pipeline, which helps
15976 you acquire the confidence that you are indeed using these tools correctly.
15978 Finally, by bundling the pipeline in a shell script, other users can use
15979 your command, without having to remember the fancy plumbing you set up for
15980 them. In terms of how you run them, shell scripts and compiled programs are
15983 After the previous warm-up exercise, we'll look at two additional, more
15984 complicated pipelines. For them, we need to introduce two more tools.
15986 The first is the @command{tr} command, which stands for ``transliterate.''
15987 The @command{tr} command works on a character-by-character basis, changing
15988 characters. Normally it is used for things like mapping upper case to
15992 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
15993 @print{} this example has mixed case!
15996 There are several options of interest:
16000 work on the complement of the listed characters, i.e.,
16001 operations apply to characters not in the given set
16004 delete characters in the first set from the output
16007 squeeze repeated characters in the output into just one character.
16010 We will be using all three options in a moment.
16012 The other command we'll look at is @command{comm}. The @command{comm}
16013 command takes two sorted input files as input data, and prints out the
16014 files' lines in three columns. The output columns are the data lines
16015 unique to the first file, the data lines unique to the second file, and
16016 the data lines that are common to both. The @option{-1}, @option{-2}, and
16017 @option{-3} command line options @emph{omit} the respective columns. (This is
16018 non-intuitive and takes a little getting used to.) For example:
16040 The file name @file{-} tells @command{comm} to read standard input
16041 instead of a regular file.
16043 Now we're ready to build a fancy pipeline. The first application is a word
16044 frequency counter. This helps an author determine if he or she is over-using
16047 The first step is to change the case of all the letters in our input file
16048 to one case. ``The'' and ``the'' are the same word when doing counting.
16051 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16054 The next step is to get rid of punctuation. Quoted words and unquoted words
16055 should be treated identically; it's easiest to just get the punctuation out of
16059 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16062 The second @command{tr} command operates on the complement of the listed
16063 characters, which are all the letters, the digits, the underscore, and
16064 the blank. The @samp{\n} represents the newline character; it has to
16065 be left alone. (The @acronym{ASCII} tab character should also be included for
16066 good measure in a production script.)
16068 At this point, we have data consisting of words separated by blank space.
16069 The words only contain alphanumeric characters (and the underscore). The
16070 next step is break the data apart so that we have one word per line. This
16071 makes the counting operation much easier, as we will see shortly.
16074 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16075 > tr -s ' ' '\n' | ...
16078 This command turns blanks into newlines. The @option{-s} option squeezes
16079 multiple newline characters in the output into just one. This helps us
16080 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16081 This is what the shell prints when it notices you haven't finished
16082 typing in all of a command.)
16084 We now have data consisting of one word per line, no punctuation, all one
16085 case. We're ready to count each word:
16088 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16089 > tr -s ' ' '\n' | sort | uniq -c | ...
16092 At this point, the data might look something like this:
16105 The output is sorted by word, not by count! What we want is the most
16106 frequently used words first. Fortunately, this is easy to accomplish,
16107 with the help of two more @command{sort} options:
16111 do a numeric sort, not a textual one
16114 reverse the order of the sort
16117 The final pipeline looks like this:
16120 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16121 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16130 Whew! That's a lot to digest. Yet, the same principles apply. With six
16131 commands, on two lines (really one long one split for convenience), we've
16132 created a program that does something interesting and useful, in much
16133 less time than we could have written a C program to do the same thing.
16135 A minor modification to the above pipeline can give us a simple spelling
16136 checker! To determine if you've spelled a word correctly, all you have to
16137 do is look it up in a dictionary. If it is not there, then chances are
16138 that your spelling is incorrect. So, we need a dictionary.
16139 The conventional location for a dictionary is @file{/usr/dict/words}.
16140 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16141 revision of this article.}
16142 this is a sorted, 45,402 word dictionary.
16144 Now, how to compare our file with the dictionary? As before, we generate
16145 a sorted list of words, one per line:
16148 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16149 > tr -s ' ' '\n' | sort -u | ...
16152 Now, all we need is a list of words that are @emph{not} in the
16153 dictionary. Here is where the @command{comm} command comes in.
16156 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16157 > tr -s ' ' '\n' | sort -u |
16158 > comm -23 - /usr/dict/words
16161 The @option{-2} and @option{-3} options eliminate lines that are only in the
16162 dictionary (the second file), and lines that are in both files. Lines
16163 only in the first file (standard input, our stream of words), are
16164 words that are not in the dictionary. These are likely candidates for
16165 spelling errors. This pipeline was the first cut at a production
16166 spelling checker on Unix.
16168 There are some other tools that deserve brief mention.
16172 search files for text that matches a regular expression
16175 count lines, words, characters
16178 a T-fitting for data pipes, copies data to files and to standard output
16181 the stream editor, an advanced tool
16184 a data manipulation language, another advanced tool
16187 The software tools philosophy also espoused the following bit of
16188 advice: ``Let someone else do the hard part.'' This means, take
16189 something that gives you most of what you need, and then massage it the
16190 rest of the way until it's in the form that you want.
16196 Each program should do one thing well. No more, no less.
16199 Combining programs with appropriate plumbing leads to results where
16200 the whole is greater than the sum of the parts. It also leads to novel
16201 uses of programs that the authors might never have imagined.
16204 Programs should never print extraneous header or trailer data, since these
16205 could get sent on down a pipeline. (A point we didn't mention earlier.)
16208 Let someone else do the hard part.
16211 Know your toolbox! Use each program appropriately. If you don't have an
16212 appropriate tool, build one.
16215 As of this writing, all the programs we've discussed are available via
16216 anonymous @command{ftp} from: @*
16217 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16218 be more recent versions available now.)
16220 None of what I have presented in this column is new. The Software Tools
16221 philosophy was first introduced in the book @cite{Software Tools}, by
16222 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16223 This book showed how to write and use software tools. It was written in
16224 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16225 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16226 was. The last chapter presented a @command{ratfor} to FORTRAN
16227 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16228 lot like C; if you know C, you won't have any problem following the
16231 In 1981, the book was updated and made available as @cite{Software Tools
16232 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16233 still in print and are well worth
16234 reading if you're a programmer. They certainly made a major change in
16235 how I view programming.
16237 The programs in both books are available from
16238 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16239 For a number of years, there was an active
16240 Software Tools Users Group, whose members had ported the original
16241 @command{ratfor} programs to essentially every computer system with a
16242 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16243 as Unix began to spread beyond universities.
16245 With the current proliferation of GNU code and other clones of Unix programs,
16246 these programs now receive little attention; modern C versions are
16247 much more efficient and do more than these programs do. Nevertheless, as
16248 exposition of good programming style, and evangelism for a still-valuable
16249 philosophy, these books are unparalleled, and I recommend them highly.
16251 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16252 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16254 @node GNU Free Documentation License
16255 @appendix GNU Free Documentation License
16259 @node Concept index
16266 @c Local variables:
16267 @c texinfo-column-for-description: 32