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 truly
3418 secure against malicious tampering: although finding a file with a
3419 given MD5 fingerprint, or modifying a file so as to retain its MD5 are
3420 considered infeasible at the moment, it is known how to produce
3421 different files with identical MD5 (a ``collision''), something which
3422 can be a security issue in certain contexts. For more secure hashes,
3423 consider using SHA-1 or SHA-2. @xref{sha1sum invocation}, and
3424 @ref{sha2 utilities}.
3426 If a @var{file} is specified as @samp{-} or if no files are given
3427 @command{md5sum} computes the checksum for the standard input.
3428 @command{md5sum} can also determine whether a file and checksum are
3429 consistent. Synopsis:
3432 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3435 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3436 indicating a binary or text input file, and the file name.
3437 If @var{file} contains a backslash or newline, the
3438 line is started with a backslash, and each problematic character in
3439 the file name is escaped with a backslash, making the output
3440 unambiguous even in the presence of arbitrary file names.
3441 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3443 The program accepts the following options. Also see @ref{Common options}.
3451 @cindex binary input files
3452 Treat each input file as binary, by reading it in binary mode and
3453 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3454 On systems like @acronym{GNU} that do not distinguish between binary
3455 and text files, this option merely flags each input file as binary:
3456 the MD5 checksum is unaffected. This option is the default on systems
3457 like MS-DOS that distinguish between binary and text files, except
3458 for reading standard input when standard input is a terminal.
3462 Read file names and checksum information (not data) from each
3463 @var{file} (or from stdin if no @var{file} was specified) and report
3464 whether the checksums match the contents of the named files.
3465 The input to this mode of @command{md5sum} is usually the output of
3466 a prior, checksum-generating run of @samp{md5sum}.
3467 Each valid line of input consists of an MD5 checksum, a binary/text
3468 flag, and then a file name.
3469 Binary files are marked with @samp{*}, text with @samp{ }.
3470 For each such line, @command{md5sum} reads the named file and computes its
3471 MD5 checksum. Then, if the computed message digest does not match the
3472 one on the line with the file name, the file is noted as having
3473 failed the test. Otherwise, the file passes the test.
3474 By default, for each valid line, one line is written to standard
3475 output indicating whether the named file passed the test.
3476 After all checks have been performed, if there were any failures,
3477 a warning is issued to standard error.
3478 Use the @option{--status} option to inhibit that output.
3479 If any listed file cannot be opened or read, if any valid line has
3480 an MD5 checksum inconsistent with the associated file, or if no valid
3481 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3482 it exits successfully.
3486 @cindex verifying MD5 checksums
3487 This option is useful only when verifying checksums.
3488 When verifying checksums, don't generate an 'OK' message per successfully
3489 checked file. Files that fail the verification are reported in the
3490 default one-line-per-file format. If there is any checksum mismatch,
3491 print a warning summarizing the failures to standard error.
3495 @cindex verifying MD5 checksums
3496 This option is useful only when verifying checksums.
3497 When verifying checksums, don't generate the default one-line-per-file
3498 diagnostic and don't output the warning summarizing any failures.
3499 Failures to open or read a file still evoke individual diagnostics to
3501 If all listed files are readable and are consistent with the associated
3502 MD5 checksums, exit successfully. Otherwise exit with a status code
3503 indicating there was a failure.
3509 @cindex text input files
3510 Treat each input file as text, by reading it in text mode and
3511 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3512 This option is the default on systems like @acronym{GNU} that do not
3513 distinguish between binary and text files. On other systems, it is
3514 the default for reading standard input when standard input is a
3521 @cindex verifying MD5 checksums
3522 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3523 This option is useful only if all but a few lines in the checked input
3531 @node sha1sum invocation
3532 @section @command{sha1sum}: Print or check SHA-1 digests
3536 @cindex 160-bit checksum
3537 @cindex checksum, 160-bit
3538 @cindex fingerprint, 160-bit
3539 @cindex message-digest, 160-bit
3541 @command{sha1sum} computes a 160-bit checksum for each specified
3542 @var{file}. The usage and options of this command are precisely the
3543 same as for @command{md5sum}. @xref{md5sum invocation}.
3545 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3546 it are known (different files having the same fingerprint). However,
3547 it is known that they can be produced with considerable, but not
3548 unreasonable, resources. For this reason, it is generally considered
3549 that SHA-1 should be gradually phased out in favor of the more secure
3550 SHA-2 hash algorithms. @xref{sha2 utilities}.
3553 @node sha2 utilities
3554 @section sha2 utilities: Print or check SHA-2 digests
3561 @cindex 224-bit checksum
3562 @cindex 256-bit checksum
3563 @cindex 384-bit checksum
3564 @cindex 512-bit checksum
3565 @cindex checksum, 224-bit
3566 @cindex checksum, 256-bit
3567 @cindex checksum, 384-bit
3568 @cindex checksum, 512-bit
3569 @cindex fingerprint, 224-bit
3570 @cindex fingerprint, 256-bit
3571 @cindex fingerprint, 384-bit
3572 @cindex fingerprint, 512-bit
3573 @cindex message-digest, 224-bit
3574 @cindex message-digest, 256-bit
3575 @cindex message-digest, 384-bit
3576 @cindex message-digest, 512-bit
3578 The commands @command{sha224sum}, @command{sha256sum},
3579 @command{sha384sum} and @command{sha512sum} compute checksums of
3580 various lengths (respectively 224, 256, 384 and 512 bits),
3581 collectively known as the SHA-2 hashes. The usage and options of
3582 these commands are precisely the same as for @command{md5sum}.
3583 @xref{md5sum invocation}.
3585 Note: The SHA384 and SHA512 digests are considerably slower to
3586 compute, especially on 32-bit computers, than SHA224 or SHA256.
3589 @node Operating on sorted files
3590 @chapter Operating on sorted files
3592 @cindex operating on sorted files
3593 @cindex sorted files, operations on
3595 These commands work with (or produce) sorted files.
3598 * sort invocation:: Sort text files.
3599 * shuf invocation:: Shuffle text files.
3600 * uniq invocation:: Uniquify files.
3601 * comm invocation:: Compare two sorted files line by line.
3602 * ptx invocation:: Produce a permuted index of file contents.
3603 * tsort invocation:: Topological sort.
3607 @node sort invocation
3608 @section @command{sort}: Sort text files
3611 @cindex sorting files
3613 @command{sort} sorts, merges, or compares all the lines from the given
3614 files, or standard input if none are given or for a @var{file} of
3615 @samp{-}. By default, @command{sort} writes the results to standard
3619 sort [@var{option}]@dots{} [@var{file}]@dots{}
3622 @command{sort} has three modes of operation: sort (the default), merge,
3623 and check for sortedness. The following options change the operation
3630 @itemx --check=diagnose-first
3633 @cindex checking for sortedness
3634 Check whether the given file is already sorted: if it is not all
3635 sorted, print a diagnostic containing the first out-of-order line and
3636 exit with a status of 1.
3637 Otherwise, exit successfully.
3638 At most one input file can be given.
3641 @itemx --check=quiet
3642 @itemx --check=silent
3645 @cindex checking for sortedness
3646 Exit successfully if the given file is already sorted, and
3647 exit with status 1 otherwise.
3648 At most one input file can be given.
3649 This is like @option{-c}, except it does not print a diagnostic.
3655 @cindex merging sorted files
3656 Merge the given files by sorting them as a group. Each input file must
3657 always be individually sorted. It always works to sort instead of
3658 merge; merging is provided because it is faster, in the case where it
3663 @cindex sort stability
3664 @cindex sort's last-resort comparison
3665 A pair of lines is compared as follows:
3666 @command{sort} compares each pair of fields, in the
3667 order specified on the command line, according to the associated
3668 ordering options, until a difference is found or no fields are left.
3669 If no key fields are specified, @command{sort} uses a default key of
3670 the entire line. Finally, as a last resort when all keys compare
3671 equal, @command{sort} compares entire lines as if no ordering options
3672 other than @option{--reverse} (@option{-r}) were specified. The
3673 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3674 comparison} so that lines in which all fields compare equal are left
3675 in their original relative order. The @option{--unique}
3676 (@option{-u}) option also disables the last-resort comparison.
3680 Unless otherwise specified, all comparisons use the character collating
3681 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3682 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3683 to @samp{en_US}), then @command{sort} may produce output that is sorted
3684 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3685 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3686 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3687 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3688 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3689 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3690 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3692 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3693 limit on input line length or restrictions on bytes allowed within lines.
3694 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3695 @command{sort} silently supplies one. A line's trailing newline is not
3696 part of the line for comparison purposes.
3698 @cindex exit status of @command{sort}
3702 0 if no error occurred
3703 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3704 2 if an error occurred
3708 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3709 value as the directory for temporary files instead of @file{/tmp}. The
3710 @option{--temporary-directory} (@option{-T}) option in turn overrides
3711 the environment variable.
3713 The following options affect the ordering of output lines. They may be
3714 specified globally or as part of a specific key field. If no key
3715 fields are specified, global options apply to comparison of entire
3716 lines; otherwise the global options are inherited by key fields that do
3717 not specify any special options of their own. In pre-@acronym{POSIX}
3718 versions of @command{sort}, global options affect only later key fields,
3719 so portable shell scripts should specify global options first.
3724 @itemx --ignore-leading-blanks
3726 @opindex --ignore-leading-blanks
3727 @cindex blanks, ignoring leading
3729 Ignore leading blanks when finding sort keys in each line.
3730 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3731 can change this. Note blanks may be ignored by your locale's collating
3732 rules, but without this option they will be significant for character
3733 positions specified in keys with the @option{-k} option.
3736 @itemx --dictionary-order
3738 @opindex --dictionary-order
3739 @cindex dictionary order
3740 @cindex phone directory order
3741 @cindex telephone directory order
3743 Sort in @dfn{phone directory} order: ignore all characters except
3744 letters, digits and blanks when sorting.
3745 By default letters and digits are those of @acronym{ASCII} and a blank
3746 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3749 @itemx --ignore-case
3751 @opindex --ignore-case
3752 @cindex ignoring case
3753 @cindex case folding
3755 Fold lowercase characters into the equivalent uppercase characters when
3756 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3757 The @env{LC_CTYPE} locale determines character types.
3758 When used with @option{--unique} those lower case equivalent lines are
3759 thrown away. (There is currently no way to throw away the upper case
3760 equivalent instead. (Any @option{--reverse} given would only affect
3761 the final result, after the throwing away.))
3764 @itemx --general-numeric-sort
3765 @itemx --sort=general-numeric
3767 @opindex --general-numeric-sort
3769 @cindex general numeric sort
3771 Sort numerically, using the standard C function @code{strtold} to convert
3772 a prefix of each line to a long double-precision floating point number.
3773 This allows floating point numbers to be specified in scientific notation,
3774 like @code{1.0e-34} and @code{10e100}.
3775 The @env{LC_NUMERIC} locale determines the decimal-point character.
3776 Do not report overflow, underflow, or conversion errors.
3777 Use the following collating sequence:
3781 Lines that do not start with numbers (all considered to be equal).
3783 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3784 in a consistent but machine-dependent order.
3788 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3793 Use this option only if there is no alternative; it is much slower than
3794 @option{--numeric-sort} (@option{-n}) and it can lose information when
3795 converting to floating point.
3798 @itemx --human-numeric-sort
3799 @itemx --sort=human-numeric
3801 @opindex --human-numeric-sort
3803 @cindex human numeric sort
3805 Sort numerically, as per the @option{--numeric-sort} option below, and in
3806 addition handle IEC or SI suffixes like MiB, MB etc (@ref{Block size}).
3807 Note a mixture of IEC and SI suffixes is not supported and will
3808 be flagged as an error. Also the numbers must be abbreviated uniformly.
3809 I.E. values with different precisions like 6000K and 5M will be sorted
3813 @itemx --ignore-nonprinting
3815 @opindex --ignore-nonprinting
3816 @cindex nonprinting characters, ignoring
3817 @cindex unprintable characters, ignoring
3819 Ignore nonprinting characters.
3820 The @env{LC_CTYPE} locale determines character types.
3821 This option has no effect if the stronger @option{--dictionary-order}
3822 (@option{-d}) option is also given.
3828 @opindex --month-sort
3830 @cindex months, sorting by
3832 An initial string, consisting of any amount of blanks, followed
3833 by a month name abbreviation, is folded to UPPER case and
3834 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3835 Invalid names compare low to valid names. The @env{LC_TIME} locale
3836 category determines the month spellings.
3837 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3841 @itemx --numeric-sort
3842 @itemx --sort=numeric
3844 @opindex --numeric-sort
3846 @cindex numeric sort
3848 Sort numerically. The number begins each line and consists
3849 of optional blanks, an optional @samp{-} sign, and zero or more
3850 digits possibly separated by thousands separators, optionally followed
3851 by a decimal-point character and zero or more digits. An empty
3852 number is treated as @samp{0}. The @env{LC_NUMERIC}
3853 locale specifies the decimal-point character and thousands separator.
3854 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3857 Comparison is exact; there is no rounding error.
3859 Neither a leading @samp{+} nor exponential notation is recognized.
3860 To compare such strings numerically, use the
3861 @option{--general-numeric-sort} (@option{-g}) option.
3864 @itemx --version-sort
3866 @opindex --version-sort
3867 @cindex version number sort
3868 Sort by version name and number. It behaves like a standard sort,
3869 except that each sequence of decimal digits is treated numerically
3870 as an index/version number. (@xref{Details about version sort}.)
3876 @cindex reverse sorting
3877 Reverse the result of comparison, so that lines with greater key values
3878 appear earlier in the output instead of later.
3881 @itemx --random-sort
3882 @itemx --sort=random
3884 @opindex --random-sort
3887 Sort by hashing the input keys and then sorting the hash values.
3888 Choose the hash function at random, ensuring that it is free of
3889 collisions so that differing keys have differing hash values. This is
3890 like a random permutation of the inputs (@pxref{shuf invocation}),
3891 except that keys with the same value sort together.
3893 If multiple random sort fields are specified, the same random hash
3894 function is used for all fields. To use different random hash
3895 functions for different fields, you can invoke @command{sort} more
3898 The choice of hash function is affected by the
3899 @option{--random-source} option.
3907 @item --compress-program=@var{prog}
3908 Compress any temporary files with the program @var{prog}.
3910 With no arguments, @var{prog} must compress standard input to standard
3911 output, and when given the @option{-d} option it must decompress
3912 standard input to standard output.
3914 Terminate with an error if @var{prog} exits with nonzero status.
3916 White space and the backslash character should not appear in
3917 @var{prog}; they are reserved for future use.
3919 @filesZeroFromOption{sort,,sorted output}
3921 @item -k @var{pos1}[,@var{pos2}]
3922 @itemx --key=@var{pos1}[,@var{pos2}]
3926 Specify a sort field that consists of the part of the line between
3927 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
3928 omitted), @emph{inclusive}.
3930 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
3931 where @var{f} is the number of the field to use, and @var{c} is the number
3932 of the first character from the beginning of the field. Fields and character
3933 positions are numbered starting with 1; a character position of zero in
3934 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
3935 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
3936 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
3937 @var{opts} are ordering options, allowing individual keys to be sorted
3938 according to different rules; see below for details. Keys can span
3941 Example: To sort on the second field, use @option{--key=2,2}
3942 (@option{-k 2,2}). See below for more notes on keys and more examples.
3943 See also the @option{--debug} option to help determine the part
3944 of the line being used in the sort.
3947 Highlight the portion of each line used for sorting.
3948 Also issue warnings about questionable usage to stderr.
3950 @item --batch-size=@var{nmerge}
3951 @opindex --batch-size
3952 @cindex number of inputs to merge, nmerge
3953 Merge at most @var{nmerge} inputs at once.
3955 When @command{sort} has to merge more than @var{nmerge} inputs,
3956 it merges them in groups of @var{nmerge}, saving the result in
3957 a temporary file, which is then used as an input in a subsequent merge.
3959 A large value of @var{nmerge} may improve merge performance and decrease
3960 temporary storage utilization at the expense of increased memory usage
3961 and I/0. Conversely a small value of @var{nmerge} may reduce memory
3962 requirements and I/0 at the expense of temporary storage consumption and
3965 The value of @var{nmerge} must be at least 2. The default value is
3966 currently 16, but this is implementation-dependent and may change in
3969 The value of @var{nmerge} may be bounded by a resource limit for open
3970 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
3971 OPEN_MAX} may display limits for your systems; these limits may be
3972 modified further if your program already has some files open, or if
3973 the operating system has other limits on the number of open files. If
3974 the value of @var{nmerge} exceeds the resource limit, @command{sort}
3975 silently uses a smaller value.
3977 @item -o @var{output-file}
3978 @itemx --output=@var{output-file}
3981 @cindex overwriting of input, allowed
3982 Write output to @var{output-file} instead of standard output.
3983 Normally, @command{sort} reads all input before opening
3984 @var{output-file}, so you can safely sort a file in place by using
3985 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
3986 However, @command{sort} with @option{--merge} (@option{-m}) can open
3987 the output file before reading all input, so a command like @code{cat
3988 F | sort -m -o F - G} is not safe as @command{sort} might start
3989 writing @file{F} before @command{cat} is done reading it.
3991 @vindex POSIXLY_CORRECT
3992 On newer systems, @option{-o} cannot appear after an input file if
3993 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
3994 scripts should specify @option{-o @var{output-file}} before any input
3997 @item --random-source=@var{file}
3998 @opindex --random-source
3999 @cindex random source for sorting
4000 Use @var{file} as a source of random data used to determine which
4001 random hash function to use with the @option{-R} option. @xref{Random
4008 @cindex sort stability
4009 @cindex sort's last-resort comparison
4011 Make @command{sort} stable by disabling its last-resort comparison.
4012 This option has no effect if no fields or global ordering options
4013 other than @option{--reverse} (@option{-r}) are specified.
4016 @itemx --buffer-size=@var{size}
4018 @opindex --buffer-size
4019 @cindex size for main memory sorting
4020 Use a main-memory sort buffer of the given @var{size}. By default,
4021 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4022 @var{size} to be interpreted as a percentage of physical memory.
4023 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4024 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4025 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4026 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4029 This option can improve the performance of @command{sort} by causing it
4030 to start with a larger or smaller sort buffer than the default.
4031 However, this option affects only the initial buffer size. The buffer
4032 grows beyond @var{size} if @command{sort} encounters input lines larger
4035 @item -t @var{separator}
4036 @itemx --field-separator=@var{separator}
4038 @opindex --field-separator
4039 @cindex field separator character
4040 Use character @var{separator} as the field separator when finding the
4041 sort keys in each line. By default, fields are separated by the empty
4042 string between a non-blank character and a blank character.
4043 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4046 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4047 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4048 not considered to be part of either the field preceding or the field
4049 following, so with @samp{sort @w{-t " "}} the same input line has
4050 three fields: an empty field, @samp{foo}, and @samp{bar}.
4051 However, fields that extend to the end of the line,
4052 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4053 retain the field separators present between the endpoints of the range.
4055 To specify @acronym{ASCII} @sc{nul} as the field separator,
4056 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4058 @item -T @var{tempdir}
4059 @itemx --temporary-directory=@var{tempdir}
4061 @opindex --temporary-directory
4062 @cindex temporary directory
4064 Use directory @var{tempdir} to store temporary files, overriding the
4065 @env{TMPDIR} environment variable. If this option is given more than
4066 once, temporary files are stored in all the directories given. If you
4067 have a large sort or merge that is I/O-bound, you can often improve
4068 performance by using this option to specify directories on different
4069 disks and controllers.
4071 @item --parallel=@var{n}
4073 @cindex multithreaded sort
4074 Limit the number of sorts run in parallel to @var{n}. By default,
4075 @var{n} is set to the number of available processors, and values
4076 greater than that are reduced to that limit. Also see
4077 @ref{nproc invocation}.
4083 @cindex uniquifying output
4085 Normally, output only the first of a sequence of lines that compare
4086 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4087 check that no pair of consecutive lines compares equal.
4089 This option also disables the default last-resort comparison.
4091 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4092 this equivalence does not extend to arbitrary @command{sort} options.
4093 For example, @code{sort -n -u} inspects only the value of the initial
4094 numeric string when checking for uniqueness, whereas @code{sort -n |
4095 uniq} inspects the entire line. @xref{uniq invocation}.
4097 @macro zeroTerminatedOption
4099 @itemx --zero-terminated
4101 @opindex --zero-terminated
4102 @cindex process zero-terminated items
4103 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4104 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4105 and terminate output items with @acronym{ASCII} @sc{nul}.
4106 This option can be useful in conjunction with @samp{perl -0} or
4107 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4108 reliably handle arbitrary file names (even those containing blanks
4109 or other special characters).
4111 @zeroTerminatedOption
4115 Historical (BSD and System V) implementations of @command{sort} have
4116 differed in their interpretation of some options, particularly
4117 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4118 behavior, which is usually (but not always!) like the System V behavior.
4119 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4120 consistency, @option{-M} has been changed in the same way. This may
4121 affect the meaning of character positions in field specifications in
4122 obscure cases. The only fix is to add an explicit @option{-b}.
4124 A position in a sort field specified with @option{-k} may have any
4125 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4126 global ordering options are inherited by that particular field. The
4127 @option{-b} option may be independently attached to either or both of
4128 the start and end positions of a field specification, and if it is
4129 inherited from the global options it will be attached to both.
4130 If input lines can contain leading or adjacent blanks and @option{-t}
4131 is not used, then @option{-k} is typically combined with @option{-b} or
4132 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4133 the varying numbers of leading blanks in fields can cause confusing results.
4135 If the start position in a sort field specifier falls after the end of
4136 the line or after the end field, the field is empty. If the @option{-b}
4137 option was specified, the @samp{.@var{c}} part of a field specification
4138 is counted from the first nonblank character of the field.
4140 @vindex _POSIX2_VERSION
4141 @vindex POSIXLY_CORRECT
4142 On older systems, @command{sort} supports an obsolete origin-zero
4143 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4144 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4145 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4146 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4147 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4149 This obsolete behavior can be enabled or disabled with the
4150 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4151 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4152 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4154 Scripts intended for use on standard hosts should avoid obsolete
4155 syntax and should use @option{-k} instead. For example, avoid
4156 @samp{sort +2}, since it might be interpreted as either @samp{sort
4157 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4158 support only the obsolete syntax, it can use a test like @samp{if sort
4159 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4162 Here are some examples to illustrate various combinations of options.
4167 Sort in descending (reverse) numeric order.
4174 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4177 sort --parallel=4 -S 10M
4181 Sort alphabetically, omitting the first and second fields
4182 and the blanks at the start of the third field.
4183 This uses a single key composed of the characters beginning
4184 at the start of the first nonblank character in field three
4185 and extending to the end of each line.
4192 Sort numerically on the second field and resolve ties by sorting
4193 alphabetically on the third and fourth characters of field five.
4194 Use @samp{:} as the field delimiter.
4197 sort -t : -k 2,2n -k 5.3,5.4
4200 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4201 @command{sort} would have used all characters beginning in the second field
4202 and extending to the end of the line as the primary @emph{numeric}
4203 key. For the large majority of applications, treating keys spanning
4204 more than one field as numeric will not do what you expect.
4206 Also note that the @samp{n} modifier was applied to the field-end
4207 specifier for the first key. It would have been equivalent to
4208 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4209 @samp{b} apply to the associated @emph{field}, regardless of whether
4210 the modifier character is attached to the field-start and/or the
4211 field-end part of the key specifier.
4214 Sort the password file on the fifth field and ignore any
4215 leading blanks. Sort lines with equal values in field five
4216 on the numeric user ID in field three. Fields are separated
4220 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4221 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4222 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4225 These three commands have equivalent effect. The first specifies that
4226 the first key's start position ignores leading blanks and the second
4227 key is sorted numerically. The other two commands rely on global
4228 options being inherited by sort keys that lack modifiers. The inheritance
4229 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4230 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4231 character position is not affected by whether initial blanks are
4235 Sort a set of log files, primarily by IPv4 address and secondarily by
4236 time stamp. If two lines' primary and secondary keys are identical,
4237 output the lines in the same order that they were input. The log
4238 files contain lines that look like this:
4241 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4242 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4245 Fields are separated by exactly one space. Sort IPv4 addresses
4246 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4247 because 61 is less than 129.
4250 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4251 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4254 This example cannot be done with a single @command{sort} invocation,
4255 since IPv4 address components are separated by @samp{.} while dates
4256 come just after a space. So it is broken down into two invocations of
4257 @command{sort}: the first sorts by time stamp and the second by IPv4
4258 address. The time stamp is sorted by year, then month, then day, and
4259 finally by hour-minute-second field, using @option{-k} to isolate each
4260 field. Except for hour-minute-second there's no need to specify the
4261 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4262 based on leading prefixes that cannot cross field boundaries. The
4263 IPv4 addresses are sorted lexicographically. The second sort uses
4264 @samp{-s} so that ties in the primary key are broken by the secondary
4265 key; the first sort uses @samp{-s} so that the combination of the two
4269 Generate a tags file in case-insensitive sorted order.
4272 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4275 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4276 that file names that contain blanks or other special characters are
4278 by the sort operation.
4280 @c This example is a bit contrived and needs more explanation.
4282 @c Sort records separated by an arbitrary string by using a pipe to convert
4283 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4284 @c and converting each @samp{\0} back to the original record delimiter.
4287 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4291 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4292 sort lines according to their length.
4295 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4298 In general this technique can be used to sort data that the @command{sort}
4299 command does not support, or is inefficient at, sorting directly.
4302 Shuffle a list of directories, but preserve the order of files within
4303 each directory. For instance, one could use this to generate a music
4304 playlist in which albums are shuffled but the songs of each album are
4308 ls */* | sort -t / -k 1,1R -k 2,2
4314 @node shuf invocation
4315 @section @command{shuf}: Shuffling text
4318 @cindex shuffling files
4320 @command{shuf} shuffles its input by outputting a random permutation
4321 of its input lines. Each output permutation is equally likely.
4325 shuf [@var{option}]@dots{} [@var{file}]
4326 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4327 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4330 @command{shuf} has three modes of operation that affect where it
4331 obtains its input lines. By default, it reads lines from standard
4332 input. The following options change the operation mode:
4340 @cindex command-line operands to shuffle
4341 Treat each command-line operand as an input line.
4343 @item -i @var{lo}-@var{hi}
4344 @itemx --input-range=@var{lo}-@var{hi}
4346 @opindex --input-range
4347 @cindex input range to shuffle
4348 Act as if input came from a file containing the range of unsigned
4349 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4353 @command{shuf}'s other options can affect its behavior in all
4358 @item -n @var{lines}
4359 @itemx --head-count=@var{count}
4361 @opindex --head-count
4362 @cindex head of output
4363 Output at most @var{count} lines. By default, all input lines are
4366 @item -o @var{output-file}
4367 @itemx --output=@var{output-file}
4370 @cindex overwriting of input, allowed
4371 Write output to @var{output-file} instead of standard output.
4372 @command{shuf} reads all input before opening
4373 @var{output-file}, so you can safely shuffle a file in place by using
4374 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4376 @item --random-source=@var{file}
4377 @opindex --random-source
4378 @cindex random source for shuffling
4379 Use @var{file} as a source of random data used to determine which
4380 permutation to generate. @xref{Random sources}.
4382 @zeroTerminatedOption
4398 might produce the output
4408 Similarly, the command:
4411 shuf -e clubs hearts diamonds spades
4425 and the command @samp{shuf -i 1-4} might output:
4435 These examples all have four input lines, so @command{shuf} might
4436 produce any of the twenty-four possible permutations of the input. In
4437 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4438 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4439 output permutations.
4444 @node uniq invocation
4445 @section @command{uniq}: Uniquify files
4448 @cindex uniquify files
4450 @command{uniq} writes the unique lines in the given @file{input}, or
4451 standard input if nothing is given or for an @var{input} name of
4455 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4458 By default, @command{uniq} prints its input lines, except that
4459 it discards all but the first of adjacent repeated lines, so that
4460 no output lines are repeated. Optionally, it can instead discard
4461 lines that are not repeated, or all repeated lines.
4463 The input need not be sorted, but repeated input lines are detected
4464 only if they are adjacent. If you want to discard non-adjacent
4465 duplicate lines, perhaps you want to use @code{sort -u}.
4466 @xref{sort invocation}.
4469 Comparisons honor the rules specified by the @env{LC_COLLATE}
4472 If no @var{output} file is specified, @command{uniq} writes to standard
4475 The program accepts the following options. Also see @ref{Common options}.
4480 @itemx --skip-fields=@var{n}
4482 @opindex --skip-fields
4483 Skip @var{n} fields on each line before checking for uniqueness. Use
4484 a null string for comparison if a line has fewer than @var{n} fields. Fields
4485 are sequences of non-space non-tab characters that are separated from
4486 each other by at least one space or tab.
4488 For compatibility @command{uniq} supports an obsolete option syntax
4489 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4492 @itemx --skip-chars=@var{n}
4494 @opindex --skip-chars
4495 Skip @var{n} characters before checking for uniqueness. Use a null string
4496 for comparison if a line has fewer than @var{n} characters. If you use both
4497 the field and character skipping options, fields are skipped over first.
4499 @vindex _POSIX2_VERSION
4500 On older systems, @command{uniq} supports an obsolete option syntax
4502 This obsolete behavior can be enabled or disabled with the
4503 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4504 conformance}), but portable scripts should avoid commands whose
4505 behavior depends on this variable.
4506 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4507 the ambiguous @samp{uniq +10}.
4513 Print the number of times each line occurred along with the line.
4516 @itemx --ignore-case
4518 @opindex --ignore-case
4519 Ignore differences in case when comparing lines.
4525 @cindex repeated lines, outputting
4526 Discard lines that are not repeated. When used by itself, this option
4527 causes @command{uniq} to print the first copy of each repeated line,
4531 @itemx --all-repeated[=@var{delimit-method}]
4533 @opindex --all-repeated
4534 @cindex all repeated lines, outputting
4535 Do not discard the second and subsequent repeated input lines,
4536 but discard lines that are not repeated.
4537 This option is useful mainly in conjunction with other options e.g.,
4538 to ignore case or to compare only selected fields.
4539 The optional @var{delimit-method} tells how to delimit
4540 groups of repeated lines, and must be one of the following:
4545 Do not delimit groups of repeated lines.
4546 This is equivalent to @option{--all-repeated} (@option{-D}).
4549 Output a newline before each group of repeated lines.
4550 With @option{--zero-terminated} (@option{-z}), use a zero
4551 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4554 Separate groups of repeated lines with a single newline.
4555 With @option{--zero-terminated} (@option{-z}), use a zero
4556 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4557 This is the same as using @samp{prepend}, except that
4558 no delimiter is inserted before the first group, and hence
4559 may be better suited for output direct to users.
4562 Note that when groups are delimited and the input stream contains
4563 two or more consecutive blank lines, then the output is ambiguous.
4564 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4565 each sequence of consecutive newlines with a single newline.
4567 This is a @sc{gnu} extension.
4568 @c FIXME: give an example showing *how* it's useful
4574 @cindex unique lines, outputting
4575 Discard the first repeated line. When used by itself, this option
4576 causes @command{uniq} to print unique lines, and nothing else.
4579 @itemx --check-chars=@var{n}
4581 @opindex --check-chars
4582 Compare at most @var{n} characters on each line (after skipping any specified
4583 fields and characters). By default the entire rest of the lines are
4586 @zeroTerminatedOption
4593 @node comm invocation
4594 @section @command{comm}: Compare two sorted files line by line
4597 @cindex line-by-line comparison
4598 @cindex comparing sorted files
4600 @command{comm} writes to standard output lines that are common, and lines
4601 that are unique, to two input files; a file name of @samp{-} means
4602 standard input. Synopsis:
4605 comm [@var{option}]@dots{} @var{file1} @var{file2}
4609 Before @command{comm} can be used, the input files must be sorted using the
4610 collating sequence specified by the @env{LC_COLLATE} locale.
4611 If an input file ends in a non-newline
4612 character, a newline is silently appended. The @command{sort} command with
4613 no options always outputs a file that is suitable input to @command{comm}.
4615 @cindex differing lines
4616 @cindex common lines
4617 With no options, @command{comm} produces three-column output. Column one
4618 contains lines unique to @var{file1}, column two contains lines unique
4619 to @var{file2}, and column three contains lines common to both files.
4620 Columns are separated by a single TAB character.
4621 @c FIXME: when there's an option to supply an alternative separator
4622 @c string, append `by default' to the above sentence.
4627 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4628 the corresponding columns (and separators). Also see @ref{Common options}.
4630 Unlike some other comparison utilities, @command{comm} has an exit
4631 status that does not depend on the result of the comparison.
4632 Upon normal completion @command{comm} produces an exit code of zero.
4633 If there is an error it exits with nonzero status.
4635 @macro checkOrderOption{cmd}
4636 If the @option{--check-order} option is given, unsorted inputs will
4637 cause a fatal error message. If the option @option{--nocheck-order}
4638 is given, unsorted inputs will never cause an error message. If
4639 neither of these options is given, wrongly sorted inputs are diagnosed
4640 only if an input file is found to contain unpairable lines. If an
4641 input file is diagnosed as being unsorted, the @command{\cmd\} command
4642 will exit with a nonzero status (and the output should not be used).
4644 Forcing @command{\cmd\} to process wrongly sorted input files
4645 containing unpairable lines by specifying @option{--nocheck-order} is
4646 not guaranteed to produce any particular output. The output will
4647 probably not correspond with whatever you hoped it would be.
4649 @checkOrderOption{comm}
4654 Fail with an error message if either input file is wrongly ordered.
4656 @item --nocheck-order
4657 Do not check that both input files are in sorted order.
4661 @item --output-delimiter=@var{str}
4662 Print @var{str} between adjacent output columns,
4663 rather than the default of a single TAB character.
4665 The delimiter @var{str} may not be empty.
4669 @node ptx invocation
4670 @section @command{ptx}: Produce permuted indexes
4674 @command{ptx} reads a text file and essentially produces a permuted index, with
4675 each keyword in its context. The calling sketch is either one of:
4678 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4679 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4682 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4683 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4684 limitations and changing several of the program's default option values.
4685 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4686 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4687 document. For the full list, see @xref{Compatibility in ptx}.
4689 Individual options are explained in the following sections.
4691 When @sc{gnu} extensions are enabled, there may be zero, one or several
4692 @var{file}s after the options. If there is no @var{file}, the program
4693 reads the standard input. If there is one or several @var{file}s, they
4694 give the name of input files which are all read in turn, as if all the
4695 input files were concatenated. However, there is a full contextual
4696 break between each file and, when automatic referencing is requested,
4697 file names and line numbers refer to individual text input files. In
4698 all cases, the program outputs the permuted index to the standard
4701 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4702 operates in traditional mode, there may be zero, one or two parameters
4703 besides the options. If there are no parameters, the program reads the
4704 standard input and outputs the permuted index to the standard output.
4705 If there is only one parameter, it names the text @var{input} to be read
4706 instead of the standard input. If two parameters are given, they give
4707 respectively the name of the @var{input} file to read and the name of
4708 the @var{output} file to produce. @emph{Be very careful} to note that,
4709 in this case, the contents of file given by the second parameter is
4710 destroyed. This behavior is dictated by System V @command{ptx}
4711 compatibility; @sc{gnu} Standards normally discourage output parameters not
4712 introduced by an option.
4714 Note that for @emph{any} file named as the value of an option or as an
4715 input text file, a single dash @kbd{-} may be used, in which case
4716 standard input is assumed. However, it would not make sense to use this
4717 convention more than once per program invocation.
4720 * General options in ptx:: Options which affect general program behavior.
4721 * Charset selection in ptx:: Underlying character set considerations.
4722 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4723 * Output formatting in ptx:: Types of output format, and sizing the fields.
4724 * Compatibility in ptx::
4728 @node General options in ptx
4729 @subsection General options
4734 @itemx --traditional
4735 As already explained, this option disables all @sc{gnu} extensions to
4736 @command{ptx} and switches to traditional mode.
4739 Print a short help on standard output, then exit without further
4743 Print the program version on standard output, then exit without further
4751 @node Charset selection in ptx
4752 @subsection Charset selection
4754 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4755 As it is set up now, the program assumes that the input file is coded
4756 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4757 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4758 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4759 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4760 of characters which are letters is different; this alters the behavior
4761 of regular expression matching. Thus, the default regular expression
4762 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4763 however, is still crude; it obeys the underlying character set ordering
4769 @itemx --ignore-case
4770 Fold lower case letters to upper case for sorting.
4775 @node Input processing in ptx
4776 @subsection Word selection and input processing
4781 @itemx --break-file=@var{file}
4783 This option provides an alternative (to @option{-W}) method of describing
4784 which characters make up words. It introduces the name of a
4785 file which contains a list of characters which can@emph{not} be part of
4786 one word; this file is called the @dfn{Break file}. Any character which
4787 is not part of the Break file is a word constituent. If both options
4788 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4789 @option{-b} is ignored.
4791 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4792 break character is to write all the break characters in the file with no
4793 newline at all, not even at the end of the file. When @sc{gnu} extensions
4794 are disabled, spaces, tabs and newlines are always considered as break
4795 characters even if not included in the Break file.
4798 @itemx --ignore-file=@var{file}
4800 The file associated with this option contains a list of words which will
4801 never be taken as keywords in concordance output. It is called the
4802 @dfn{Ignore file}. The file contains exactly one word in each line; the
4803 end of line separation of words is not subject to the value of the
4807 @itemx --only-file=@var{file}
4809 The file associated with this option contains a list of words which will
4810 be retained in concordance output; any word not mentioned in this file
4811 is ignored. The file is called the @dfn{Only file}. The file contains
4812 exactly one word in each line; the end of line separation of words is
4813 not subject to the value of the @option{-S} option.
4815 There is no default for the Only file. When both an Only file and an
4816 Ignore file are specified, a word is considered a keyword only
4817 if it is listed in the Only file and not in the Ignore file.
4822 On each input line, the leading sequence of non-white space characters will be
4823 taken to be a reference that has the purpose of identifying this input
4824 line in the resulting permuted index. For more information about reference
4825 production, see @xref{Output formatting in ptx}.
4826 Using this option changes the default value for option @option{-S}.
4828 Using this option, the program does not try very hard to remove
4829 references from contexts in output, but it succeeds in doing so
4830 @emph{when} the context ends exactly at the newline. If option
4831 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4832 are disabled, this condition is always met and references are completely
4833 excluded from the output contexts.
4835 @item -S @var{regexp}
4836 @itemx --sentence-regexp=@var{regexp}
4838 This option selects which regular expression will describe the end of a
4839 line or the end of a sentence. In fact, this regular expression is not
4840 the only distinction between end of lines or end of sentences, and input
4841 line boundaries have no special significance outside this option. By
4842 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4843 used, end of sentences are used. In this case, this @var{regex} is
4844 imported from @sc{gnu} Emacs:
4847 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4850 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4851 of lines are used; in this case, the default @var{regexp} is just:
4857 Using an empty @var{regexp} is equivalent to completely disabling end of
4858 line or end of sentence recognition. In this case, the whole file is
4859 considered to be a single big line or sentence. The user might want to
4860 disallow all truncation flag generation as well, through option @option{-F
4861 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4864 When the keywords happen to be near the beginning of the input line or
4865 sentence, this often creates an unused area at the beginning of the
4866 output context line; when the keywords happen to be near the end of the
4867 input line or sentence, this often creates an unused area at the end of
4868 the output context line. The program tries to fill those unused areas
4869 by wrapping around context in them; the tail of the input line or
4870 sentence is used to fill the unused area on the left of the output line;
4871 the head of the input line or sentence is used to fill the unused area
4872 on the right of the output line.
4874 As a matter of convenience to the user, many usual backslashed escape
4875 sequences from the C language are recognized and converted to the
4876 corresponding characters by @command{ptx} itself.
4878 @item -W @var{regexp}
4879 @itemx --word-regexp=@var{regexp}
4881 This option selects which regular expression will describe each keyword.
4882 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
4883 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
4884 disabled, a word is by default anything which ends with a space, a tab
4885 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
4887 An empty @var{regexp} is equivalent to not using this option.
4888 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4891 As a matter of convenience to the user, many usual backslashed escape
4892 sequences, as found in the C language, are recognized and converted to
4893 the corresponding characters by @command{ptx} itself.
4898 @node Output formatting in ptx
4899 @subsection Output formatting
4901 Output format is mainly controlled by the @option{-O} and @option{-T} options
4902 described in the table below. When neither @option{-O} nor @option{-T} are
4903 selected, and if @sc{gnu} extensions are enabled, the program chooses an
4904 output format suitable for a dumb terminal. Each keyword occurrence is
4905 output to the center of one line, surrounded by its left and right
4906 contexts. Each field is properly justified, so the concordance output
4907 can be readily observed. As a special feature, if automatic
4908 references are selected by option @option{-A} and are output before the
4909 left context, that is, if option @option{-R} is @emph{not} selected, then
4910 a colon is added after the reference; this nicely interfaces with @sc{gnu}
4911 Emacs @code{next-error} processing. In this default output format, each
4912 white space character, like newline and tab, is merely changed to
4913 exactly one space, with no special attempt to compress consecutive
4914 spaces. This might change in the future. Except for those white space
4915 characters, every other character of the underlying set of 256
4916 characters is transmitted verbatim.
4918 Output format is further controlled by the following options.
4922 @item -g @var{number}
4923 @itemx --gap-size=@var{number}
4925 Select the size of the minimum white space gap between the fields on the
4928 @item -w @var{number}
4929 @itemx --width=@var{number}
4931 Select the maximum output width of each final line. If references are
4932 used, they are included or excluded from the maximum output width
4933 depending on the value of option @option{-R}. If this option is not
4934 selected, that is, when references are output before the left context,
4935 the maximum output width takes into account the maximum length of all
4936 references. If this option is selected, that is, when references are
4937 output after the right context, the maximum output width does not take
4938 into account the space taken by references, nor the gap that precedes
4942 @itemx --auto-reference
4944 Select automatic references. Each input line will have an automatic
4945 reference made up of the file name and the line ordinal, with a single
4946 colon between them. However, the file name will be empty when standard
4947 input is being read. If both @option{-A} and @option{-r} are selected, then
4948 the input reference is still read and skipped, but the automatic
4949 reference is used at output time, overriding the input reference.
4952 @itemx --right-side-refs
4954 In the default output format, when option @option{-R} is not used, any
4955 references produced by the effect of options @option{-r} or @option{-A} are
4956 placed to the far right of output lines, after the right context. With
4957 default output format, when the @option{-R} option is specified, references
4958 are rather placed at the beginning of each output line, before the left
4959 context. For any other output format, option @option{-R} is
4960 ignored, with one exception: with @option{-R} the width of references
4961 is @emph{not} taken into account in total output width given by @option{-w}.
4963 This option is automatically selected whenever @sc{gnu} extensions are
4966 @item -F @var{string}
4967 @itemx --flac-truncation=@var{string}
4969 This option will request that any truncation in the output be reported
4970 using the string @var{string}. Most output fields theoretically extend
4971 towards the beginning or the end of the current line, or current
4972 sentence, as selected with option @option{-S}. But there is a maximum
4973 allowed output line width, changeable through option @option{-w}, which is
4974 further divided into space for various output fields. When a field has
4975 to be truncated because it cannot extend beyond the beginning or the end of
4976 the current line to fit in, then a truncation occurs. By default,
4977 the string used is a single slash, as in @option{-F /}.
4979 @var{string} may have more than one character, as in @option{-F ...}.
4980 Also, in the particular case when @var{string} is empty (@option{-F ""}),
4981 truncation flagging is disabled, and no truncation marks are appended in
4984 As a matter of convenience to the user, many usual backslashed escape
4985 sequences, as found in the C language, are recognized and converted to
4986 the corresponding characters by @command{ptx} itself.
4988 @item -M @var{string}
4989 @itemx --macro-name=@var{string}
4991 Select another @var{string} to be used instead of @samp{xx}, while
4992 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
4995 @itemx --format=roff
4997 Choose an output format suitable for @command{nroff} or @command{troff}
4998 processing. Each output line will look like:
5001 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5004 so it will be possible to write a @samp{.xx} roff macro to take care of
5005 the output typesetting. This is the default output format when @sc{gnu}
5006 extensions are disabled. Option @option{-M} can be used to change
5007 @samp{xx} to another macro name.
5009 In this output format, each non-graphical character, like newline and
5010 tab, is merely changed to exactly one space, with no special attempt to
5011 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5012 so it will be correctly processed by @command{nroff} or @command{troff}.
5017 Choose an output format suitable for @TeX{} processing. Each output
5018 line will look like:
5021 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5025 so it will be possible to write a @code{\xx} definition to take care of
5026 the output typesetting. Note that when references are not being
5027 produced, that is, neither option @option{-A} nor option @option{-r} is
5028 selected, the last parameter of each @code{\xx} call is inhibited.
5029 Option @option{-M} can be used to change @samp{xx} to another macro
5032 In this output format, some special characters, like @kbd{$}, @kbd{%},
5033 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5034 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5035 backslash and a pair of dollar signs (to force mathematical mode). The
5036 backslash itself produces the sequence @code{\backslash@{@}}.
5037 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5038 @code{~\@{ @}} respectively. Other diacriticized characters of the
5039 underlying character set produce an appropriate @TeX{} sequence as far
5040 as possible. The other non-graphical characters, like newline and tab,
5041 and all other characters which are not part of @acronym{ASCII}, are merely
5042 changed to exactly one space, with no special attempt to compress
5043 consecutive spaces. Let me know how to improve this special character
5044 processing for @TeX{}.
5049 @node Compatibility in ptx
5050 @subsection The @sc{gnu} extensions to @command{ptx}
5052 This version of @command{ptx} contains a few features which do not exist in
5053 System V @command{ptx}. These extra features are suppressed by using the
5054 @option{-G} command line option, unless overridden by other command line
5055 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5056 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5057 Here are the differences between this program and System V @command{ptx}.
5062 This program can read many input files at once, it always writes the
5063 resulting concordance on standard output. On the other hand, System V
5064 @command{ptx} reads only one file and sends the result to standard output
5065 or, if a second @var{file} parameter is given on the command, to that
5068 Having output parameters not introduced by options is a dangerous
5069 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5070 portably between @sc{gnu} and System V, you should always use it with a
5071 single input file, and always expect the result on standard output. You
5072 might also want to automatically configure in a @option{-G} option to
5073 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5074 that the installed @command{ptx} accepts @option{-G}.
5077 The only options available in System V @command{ptx} are options @option{-b},
5078 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5079 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5080 this enumeration. Moreover, some options have a slightly different
5081 meaning when @sc{gnu} extensions are enabled, as explained below.
5084 By default, concordance output is not formatted for @command{troff} or
5085 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5086 or @command{nroff} output may still be selected through option @option{-O}.
5089 Unless @option{-R} option is used, the maximum reference width is
5090 subtracted from the total output line width. With @sc{gnu} extensions
5091 disabled, width of references is not taken into account in the output
5092 line width computations.
5095 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5096 processed from input file with no adverse effect, even if @sc{gnu} extensions
5097 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5098 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5101 Input line length is only limited by available memory, even if @sc{gnu}
5102 extensions are disabled. However, System V @command{ptx} processes only
5103 the first 200 characters in each line.
5106 The break (non-word) characters default to be every character except all
5107 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5108 extensions are disabled, the break characters default to space, tab and
5112 The program makes better use of output line width. If @sc{gnu} extensions
5113 are disabled, the program rather tries to imitate System V @command{ptx},
5114 but still, there are some slight disposition glitches this program does
5115 not completely reproduce.
5118 The user can specify both an Ignore file and an Only file. This is not
5119 allowed with System V @command{ptx}.
5124 @node tsort invocation
5125 @section @command{tsort}: Topological sort
5128 @cindex topological sort
5130 @command{tsort} performs a topological sort on the given @var{file}, or
5131 standard input if no input file is given or for a @var{file} of
5132 @samp{-}. For more details and some history, see @ref{tsort background}.
5136 tsort [@var{option}] [@var{file}]
5139 @command{tsort} reads its input as pairs of strings, separated by blanks,
5140 indicating a partial ordering. The output is a total ordering that
5141 corresponds to the given partial ordering.
5155 will produce the output
5166 Consider a more realistic example.
5167 You have a large set of functions all in one file, and they may all be
5168 declared static except one. Currently that one (say @code{main}) is the
5169 first function defined in the file, and the ones it calls directly follow
5170 it, followed by those they call, etc. Let's say that you are determined
5171 to take advantage of prototypes, so you have to choose between declaring
5172 all of those functions (which means duplicating a lot of information from
5173 the definitions) and rearranging the functions so that as many as possible
5174 are defined before they are used. One way to automate the latter process
5175 is to get a list for each function of the functions it calls directly.
5176 Many programs can generate such lists. They describe a call graph.
5177 Consider the following list, in which a given line indicates that the
5178 function on the left calls the one on the right directly.
5184 tail_file pretty_name
5185 tail_file write_header
5187 tail_forever recheck
5188 tail_forever pretty_name
5189 tail_forever write_header
5190 tail_forever dump_remainder
5193 tail_lines start_lines
5194 tail_lines dump_remainder
5195 tail_lines file_lines
5196 tail_lines pipe_lines
5198 tail_bytes start_bytes
5199 tail_bytes dump_remainder
5200 tail_bytes pipe_bytes
5201 file_lines dump_remainder
5205 then you can use @command{tsort} to produce an ordering of those
5206 functions that satisfies your requirement.
5209 example$ tsort call-graph | tac
5229 @command{tsort} detects any cycles in the input and writes the first cycle
5230 encountered to standard error.
5232 Note that for a given partial ordering, generally there is no unique
5233 total ordering. In the context of the call graph above, the function
5234 @code{parse_options} may be placed anywhere in the list as long as it
5235 precedes @code{main}.
5237 The only options are @option{--help} and @option{--version}. @xref{Common
5243 * tsort background:: Where tsort came from.
5246 @node tsort background
5247 @subsection @command{tsort}: Background
5249 @command{tsort} exists because very early versions of the Unix linker processed
5250 an archive file exactly once, and in order. As @command{ld} read each object
5251 in the archive, it decided whether it was needed in the program based on
5252 whether it defined any symbols which were undefined at that point in
5255 This meant that dependencies within the archive had to be handled
5256 specially. For example, @code{scanf} probably calls @code{read}. That means
5257 that in a single pass through an archive, it was important for @code{scanf.o}
5258 to appear before read.o, because otherwise a program which calls
5259 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5260 reference to @code{read}.
5262 The way to address this problem was to first generate a set of
5263 dependencies of one object file on another. This was done by a shell
5264 script called @command{lorder}. The GNU tools don't provide a version of
5265 lorder, as far as I know, but you can still find it in BSD
5268 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5269 resulting sort to define the order in which you added objects to the archive.
5271 This whole procedure has been obsolete since about 1980, because
5272 Unix archives now contain a symbol table (traditionally built by
5273 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5274 linker uses the symbol table to effectively make multiple passes over
5277 Anyhow, that's where tsort came from. To solve an old problem with
5278 the way the linker handled archive files, which has since been solved
5282 @node Operating on fields
5283 @chapter Operating on fields
5286 * cut invocation:: Print selected parts of lines.
5287 * paste invocation:: Merge lines of files.
5288 * join invocation:: Join lines on a common field.
5292 @node cut invocation
5293 @section @command{cut}: Print selected parts of lines
5296 @command{cut} writes to standard output selected parts of each line of each
5297 input file, or standard input if no files are given or for a file name of
5301 cut @var{option}@dots{} [@var{file}]@dots{}
5304 In the table which follows, the @var{byte-list}, @var{character-list},
5305 and @var{field-list} are one or more numbers or ranges (two numbers
5306 separated by a dash) separated by commas. Bytes, characters, and
5307 fields are numbered starting at 1. Incomplete ranges may be
5308 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5309 @samp{@var{n}} through end of line or last field. The list elements
5310 can be repeated, can overlap, and can be specified in any order; but
5311 the selected input is written in the same order that it is read, and
5312 is written exactly once.
5314 The program accepts the following options. Also see @ref{Common
5319 @item -b @var{byte-list}
5320 @itemx --bytes=@var{byte-list}
5323 Select for printing only the bytes in positions listed in
5324 @var{byte-list}. Tabs and backspaces are treated like any other
5325 character; they take up 1 byte. If an output delimiter is specified,
5326 (see the description of @option{--output-delimiter}), then output that
5327 string between ranges of selected bytes.
5329 @item -c @var{character-list}
5330 @itemx --characters=@var{character-list}
5332 @opindex --characters
5333 Select for printing only the characters in positions listed in
5334 @var{character-list}. The same as @option{-b} for now, but
5335 internationalization will change that. Tabs and backspaces are
5336 treated like any other character; they take up 1 character. If an
5337 output delimiter is specified, (see the description of
5338 @option{--output-delimiter}), then output that string between ranges
5341 @item -f @var{field-list}
5342 @itemx --fields=@var{field-list}
5345 Select for printing only the fields listed in @var{field-list}.
5346 Fields are separated by a TAB character by default. Also print any
5347 line that contains no delimiter character, unless the
5348 @option{--only-delimited} (@option{-s}) option is specified.
5349 Note @command{cut} does not support specifying runs of whitespace as a
5350 delimiter, so to achieve that common functionality one can pre-process
5351 with @command{tr} like:
5353 tr -s '[:blank:]' '\t' | cut -f@dots{}
5356 @item -d @var{input_delim_byte}
5357 @itemx --delimiter=@var{input_delim_byte}
5359 @opindex --delimiter
5360 With @option{-f}, use the first byte of @var{input_delim_byte} as
5361 the input fields separator (default is TAB).
5365 Do not split multi-byte characters (no-op for now).
5368 @itemx --only-delimited
5370 @opindex --only-delimited
5371 For @option{-f}, do not print lines that do not contain the field separator
5372 character. Normally, any line without a field separator is printed verbatim.
5374 @item --output-delimiter=@var{output_delim_string}
5375 @opindex --output-delimiter
5376 With @option{-f}, output fields are separated by @var{output_delim_string}.
5377 The default with @option{-f} is to use the input delimiter.
5378 When using @option{-b} or @option{-c} to select ranges of byte or
5379 character offsets (as opposed to ranges of fields),
5380 output @var{output_delim_string} between non-overlapping
5381 ranges of selected bytes.
5384 @opindex --complement
5385 This option is a @acronym{GNU} extension.
5386 Select for printing the complement of the bytes, characters or fields
5387 selected with the @option{-b}, @option{-c} or @option{-f} options.
5388 In other words, do @emph{not} print the bytes, characters or fields
5389 specified via those options. This option is useful when you have
5390 many fields and want to print all but a few of them.
5397 @node paste invocation
5398 @section @command{paste}: Merge lines of files
5401 @cindex merging files
5403 @command{paste} writes to standard output lines consisting of sequentially
5404 corresponding lines of each given file, separated by a TAB character.
5405 Standard input is used for a file name of @samp{-} or if no input files
5427 paste [@var{option}]@dots{} [@var{file}]@dots{}
5430 The program accepts the following options. Also see @ref{Common options}.
5438 Paste the lines of one file at a time rather than one line from each
5439 file. Using the above example data:
5442 $ paste -s num2 let3
5447 @item -d @var{delim-list}
5448 @itemx --delimiters=@var{delim-list}
5450 @opindex --delimiters
5451 Consecutively use the characters in @var{delim-list} instead of
5452 TAB to separate merged lines. When @var{delim-list} is
5453 exhausted, start again at its beginning. Using the above example data:
5456 $ paste -d '%_' num2 let3 num2
5467 @node join invocation
5468 @section @command{join}: Join lines on a common field
5471 @cindex common field, joining on
5473 @command{join} writes to standard output a line for each pair of input
5474 lines that have identical join fields. Synopsis:
5477 join [@var{option}]@dots{} @var{file1} @var{file2}
5480 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5481 meaning standard input. @var{file1} and @var{file2} should be
5482 sorted on the join fields.
5485 Normally, the sort order is that of the
5486 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5487 the @option{-t} option is given, the sort comparison ignores blanks at
5488 the start of the join field, as in @code{sort -b}. If the
5489 @option{--ignore-case} option is given, the sort comparison ignores
5490 the case of characters in the join field, as in @code{sort -f}.
5492 The @command{sort} and @command{join} commands should use consistent
5493 locales and options if the output of @command{sort} is fed to
5494 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5495 sort a file on its default join field, but if you select a non-default
5496 locale, join field, separator, or comparison options, then you should
5497 do so consistently between @command{join} and @command{sort}.
5498 If @samp{join -t ''} is specified then the whole line is considered which
5499 matches the default operation of sort.
5501 If the input has no unpairable lines, a @acronym{GNU} extension is
5502 available; the sort order can be any order that considers two fields
5503 to be equal if and only if the sort comparison described above
5504 considers them to be equal. For example:
5521 @checkOrderOption{join}
5525 @item the join field is the first field in each line;
5526 @item fields in the input are separated by one or more blanks, with leading
5527 blanks on the line ignored;
5528 @item fields in the output are separated by a space;
5529 @item each output line consists of the join field, the remaining
5530 fields from @var{file1}, then the remaining fields from @var{file2}.
5533 The program accepts the following options. Also see @ref{Common options}.
5537 @item -a @var{file-number}
5539 Print a line for each unpairable line in file @var{file-number} (either
5540 @samp{1} or @samp{2}), in addition to the normal output.
5543 Fail with an error message if either input file is wrongly ordered.
5545 @item --nocheck-order
5546 Do not check that both input files are in sorted order. This is the default.
5548 @item -e @var{string}
5550 Replace those output fields that are missing in the input with
5555 Treat the first line of each input file as a header line. The header lines will
5556 be joined and printed as the first output line. If @option{-o} is used to
5557 specify output format, the header line will be printed according to the
5558 specified format. The header lines will not be checked for ordering even if
5559 @option{--check-order} is specified. Also if the header lines from each file
5560 do not match, the heading fields from the first file will be used.
5563 @itemx --ignore-case
5565 @opindex --ignore-case
5566 Ignore differences in case when comparing keys.
5567 With this option, the lines of the input files must be ordered in the same way.
5568 Use @samp{sort -f} to produce this ordering.
5570 @item -1 @var{field}
5572 Join on field @var{field} (a positive integer) of file 1.
5574 @item -2 @var{field}
5576 Join on field @var{field} (a positive integer) of file 2.
5578 @item -j @var{field}
5579 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5581 @item -o @var{field-list}
5582 Construct each output line according to the format in @var{field-list}.
5583 Each element in @var{field-list} is either the single character @samp{0} or
5584 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5585 @samp{2} and @var{n} is a positive field number.
5587 A field specification of @samp{0} denotes the join field.
5588 In most cases, the functionality of the @samp{0} field spec
5589 may be reproduced using the explicit @var{m.n} that corresponds
5590 to the join field. However, when printing unpairable lines
5591 (using either of the @option{-a} or @option{-v} options), there is no way
5592 to specify the join field using @var{m.n} in @var{field-list}
5593 if there are unpairable lines in both files.
5594 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5595 field specification notation.
5597 The elements in @var{field-list}
5598 are separated by commas or blanks.
5599 Blank separators typically need to be quoted for the shell. For
5600 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5601 2.2'} are equivalent.
5603 All output lines---including those printed because of any -a or -v
5604 option---are subject to the specified @var{field-list}.
5607 Use character @var{char} as the input and output field separator.
5608 Treat as significant each occurrence of @var{char} in the input file.
5609 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5610 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5611 the whole line is considered, matching the default operation of sort.
5612 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5613 character is used to delimit the fields.
5615 @item -v @var{file-number}
5616 Print a line for each unpairable line in file @var{file-number}
5617 (either @samp{1} or @samp{2}), instead of the normal output.
5624 @node Operating on characters
5625 @chapter Operating on characters
5627 @cindex operating on characters
5629 This commands operate on individual characters.
5632 * tr invocation:: Translate, squeeze, and/or delete characters.
5633 * expand invocation:: Convert tabs to spaces.
5634 * unexpand invocation:: Convert spaces to tabs.
5639 @section @command{tr}: Translate, squeeze, and/or delete characters
5646 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5649 @command{tr} copies standard input to standard output, performing
5650 one of the following operations:
5654 translate, and optionally squeeze repeated characters in the result,
5656 squeeze repeated characters,
5660 delete characters, then squeeze repeated characters from the result.
5663 The @var{set1} and (if given) @var{set2} arguments define ordered
5664 sets of characters, referred to below as @var{set1} and @var{set2}. These
5665 sets are the characters of the input that @command{tr} operates on.
5666 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5668 complement (all of the characters that are not in @var{set1}).
5670 Currently @command{tr} fully supports only single-byte characters.
5671 Eventually it will support multibyte characters; when it does, the
5672 @option{-C} option will cause it to complement the set of characters,
5673 whereas @option{-c} will cause it to complement the set of values.
5674 This distinction will matter only when some values are not characters,
5675 and this is possible only in locales using multibyte encodings when
5676 the input contains encoding errors.
5678 The program accepts the @option{--help} and @option{--version}
5679 options. @xref{Common options}. Options must precede operands.
5684 * Character sets:: Specifying sets of characters.
5685 * Translating:: Changing one set of characters to another.
5686 * Squeezing:: Squeezing repeats and deleting.
5690 @node Character sets
5691 @subsection Specifying sets of characters
5693 @cindex specifying sets of characters
5695 The format of the @var{set1} and @var{set2} arguments resembles
5696 the format of regular expressions; however, they are not regular
5697 expressions, only lists of characters. Most characters simply
5698 represent themselves in these strings, but the strings can contain
5699 the shorthands listed below, for convenience. Some of them can be
5700 used only in @var{set1} or @var{set2}, as noted below.
5704 @item Backslash escapes
5705 @cindex backslash escapes
5707 The following backslash escape sequences are recognized:
5725 The character with the value given by @var{ooo}, which is 1 to 3
5731 While a backslash followed by a character not listed above is
5732 interpreted as that character, the backslash also effectively
5733 removes any special significance, so it is useful to escape
5734 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5739 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5740 from @var{m} through @var{n}, in ascending order. @var{m} should
5741 collate before @var{n}; if it doesn't, an error results. As an example,
5742 @samp{0-9} is the same as @samp{0123456789}.
5744 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5745 brackets to enclose ranges. Translations specified in that format
5746 sometimes work as expected, since the brackets are often transliterated
5747 to themselves. However, they should be avoided because they sometimes
5748 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5751 Many historically common and even accepted uses of ranges are not
5752 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5753 range will not do what most would expect because @samp{A} through @samp{Z}
5754 are not contiguous as they are in @acronym{ASCII}.
5755 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5756 the best way to work around this is to use character classes (see below).
5757 Otherwise, it is most portable (and most ugly) to enumerate the members
5760 @item Repeated characters
5761 @cindex repeated characters
5763 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5764 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5765 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5766 to as many copies of @var{c} as are needed to make @var{set2} as long as
5767 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5768 octal, otherwise in decimal.
5770 @item Character classes
5771 @cindex character classes
5773 The notation @samp{[:@var{class}:]} expands to all of the characters in
5774 the (predefined) class @var{class}. The characters expand in no
5775 particular order, except for the @code{upper} and @code{lower} classes,
5776 which expand in ascending order. When the @option{--delete} (@option{-d})
5777 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5778 character class can be used in @var{set2}. Otherwise, only the
5779 character classes @code{lower} and @code{upper} are accepted in
5780 @var{set2}, and then only if the corresponding character class
5781 (@code{upper} and @code{lower}, respectively) is specified in the same
5782 relative position in @var{set1}. Doing this specifies case conversion.
5783 The class names are given below; an error results when an invalid class
5795 Horizontal whitespace.
5804 Printable characters, not including space.
5810 Printable characters, including space.
5813 Punctuation characters.
5816 Horizontal or vertical whitespace.
5825 @item Equivalence classes
5826 @cindex equivalence classes
5828 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5829 equivalent to @var{c}, in no particular order. Equivalence classes are
5830 a relatively recent invention intended to support non-English alphabets.
5831 But there seems to be no standard way to define them or determine their
5832 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5833 each character's equivalence class consists only of that character,
5834 which is of no particular use.
5840 @subsection Translating
5842 @cindex translating characters
5844 @command{tr} performs translation when @var{set1} and @var{set2} are
5845 both given and the @option{--delete} (@option{-d}) option is not given.
5846 @command{tr} translates each character of its input that is in @var{set1}
5847 to the corresponding character in @var{set2}. Characters not in
5848 @var{set1} are passed through unchanged. When a character appears more
5849 than once in @var{set1} and the corresponding characters in @var{set2}
5850 are not all the same, only the final one is used. For example, these
5851 two commands are equivalent:
5858 A common use of @command{tr} is to convert lowercase characters to
5859 uppercase. This can be done in many ways. Here are three of them:
5862 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5864 tr '[:lower:]' '[:upper:]'
5868 But note that using ranges like @code{a-z} above is not portable.
5870 When @command{tr} is performing translation, @var{set1} and @var{set2}
5871 typically have the same length. If @var{set1} is shorter than
5872 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5874 On the other hand, making @var{set1} longer than @var{set2} is not
5875 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5876 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5877 the last character of @var{set2} as many times as necessary. System V
5878 @command{tr} truncates @var{set1} to the length of @var{set2}.
5880 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
5881 When the @option{--truncate-set1} (@option{-t}) option is given,
5882 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
5883 instead. This option is ignored for operations other than translation.
5885 Acting like System V @command{tr} in this case breaks the relatively common
5889 tr -cs A-Za-z0-9 '\012'
5893 because it converts only zero bytes (the first element in the
5894 complement of @var{set1}), rather than all non-alphanumerics, to
5898 By the way, the above idiom is not portable because it uses ranges, and
5899 it assumes that the octal code for newline is 012.
5900 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
5903 tr -cs '[:alnum:]' '[\n*]'
5908 @subsection Squeezing repeats and deleting
5910 @cindex squeezing repeat characters
5911 @cindex deleting characters
5913 When given just the @option{--delete} (@option{-d}) option, @command{tr}
5914 removes any input characters that are in @var{set1}.
5916 When given just the @option{--squeeze-repeats} (@option{-s}) option,
5917 @command{tr} replaces each input sequence of a repeated character that
5918 is in @var{set1} with a single occurrence of that character.
5920 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
5921 first performs any deletions using @var{set1}, then squeezes repeats
5922 from any remaining characters using @var{set2}.
5924 The @option{--squeeze-repeats} option may also be used when translating,
5925 in which case @command{tr} first performs translation, then squeezes
5926 repeats from any remaining characters using @var{set2}.
5928 Here are some examples to illustrate various combinations of options:
5933 Remove all zero bytes:
5940 Put all words on lines by themselves. This converts all
5941 non-alphanumeric characters to newlines, then squeezes each string
5942 of repeated newlines into a single newline:
5945 tr -cs '[:alnum:]' '[\n*]'
5949 Convert each sequence of repeated newlines to a single newline:
5956 Find doubled occurrences of words in a document.
5957 @c Separate the following two "the"s, so typo checkers don't complain.
5958 For example, people often write ``the @w{}the'' with the repeated words
5959 separated by a newline. The Bourne shell script below works first
5960 by converting each sequence of punctuation and blank characters to a
5961 single newline. That puts each ``word'' on a line by itself.
5962 Next it maps all uppercase characters to lower case, and finally it
5963 runs @command{uniq} with the @option{-d} option to print out only the words
5969 | tr -s '[:punct:][:blank:]' '[\n*]' \
5970 | tr '[:upper:]' '[:lower:]' \
5975 Deleting a small set of characters is usually straightforward. For example,
5976 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
5982 However, when @samp{-} is one of those characters, it can be tricky because
5983 @samp{-} has special meanings. Performing the same task as above but also
5984 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
5985 that would fail because @command{tr} would try to interpret @option{-a} as
5986 a command-line option. Alternatively, we could try putting the hyphen
5987 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
5988 it would make @command{tr} interpret @code{a-x} as the range of characters
5989 @samp{a}@dots{}@samp{x} rather than the three.
5990 One way to solve the problem is to put the hyphen at the end of the list
5997 Or you can use @samp{--} to terminate option processing:
6003 More generally, use the character class notation @code{[=c=]}
6004 with @samp{-} (or any other character) in place of the @samp{c}:
6010 Note how single quotes are used in the above example to protect the
6011 square brackets from interpretation by a shell.
6016 @node expand invocation
6017 @section @command{expand}: Convert tabs to spaces
6020 @cindex tabs to spaces, converting
6021 @cindex converting tabs to spaces
6023 @command{expand} writes the contents of each given @var{file}, or standard
6024 input if none are given or for a @var{file} of @samp{-}, to standard
6025 output, with tab characters converted to the appropriate number of
6029 expand [@var{option}]@dots{} [@var{file}]@dots{}
6032 By default, @command{expand} converts all tabs to spaces. It preserves
6033 backspace characters in the output; they decrement the column count for
6034 tab calculations. The default action is equivalent to @option{-t 8} (set
6035 tabs every 8 columns).
6037 The program accepts the following options. Also see @ref{Common options}.
6041 @item -t @var{tab1}[,@var{tab2}]@dots{}
6042 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6045 @cindex tab stops, setting
6046 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6047 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6048 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6049 last tab stop given with single spaces. Tab stops can be separated by
6050 blanks as well as by commas.
6052 For compatibility, GNU @command{expand} also accepts the obsolete
6053 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6054 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6060 @cindex initial tabs, converting
6061 Only convert initial tabs (those that precede all non-space or non-tab
6062 characters) on each line to spaces.
6069 @node unexpand invocation
6070 @section @command{unexpand}: Convert spaces to tabs
6074 @command{unexpand} writes the contents of each given @var{file}, or
6075 standard input if none are given or for a @var{file} of @samp{-}, to
6076 standard output, converting blanks at the beginning of each line into
6077 as many tab characters as needed. In the default @acronym{POSIX}
6078 locale, a @dfn{blank} is a space or a tab; other locales may specify
6079 additional blank characters. Synopsis:
6082 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6085 By default, @command{unexpand} converts only initial blanks (those
6086 that precede all non-blank characters) on each line. It
6087 preserves backspace characters in the output; they decrement the column
6088 count for tab calculations. By default, tabs are set at every 8th
6091 The program accepts the following options. Also see @ref{Common options}.
6095 @item -t @var{tab1}[,@var{tab2}]@dots{}
6096 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6099 If only one tab stop is given, set the tabs @var{tab1} columns apart
6100 instead of the default 8. Otherwise, set the tabs at columns
6101 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6102 beyond the tab stops given unchanged. Tab stops can be separated by
6103 blanks as well as by commas. This option implies the @option{-a} option.
6105 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6106 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6107 separated by commas. (Unlike @option{-t}, this obsolete option does
6108 not imply @option{-a}.) New scripts should use @option{--first-only -t
6109 @var{tab1}[,@var{tab2}]@dots{}} instead.
6115 Also convert all sequences of two or more blanks just before a tab stop,
6116 even if they occur after non-blank characters in a line.
6123 @node Directory listing
6124 @chapter Directory listing
6126 This chapter describes the @command{ls} command and its variants @command{dir}
6127 and @command{vdir}, which list information about files.
6130 * ls invocation:: List directory contents.
6131 * dir invocation:: Briefly ls.
6132 * vdir invocation:: Verbosely ls.
6133 * dircolors invocation:: Color setup for ls, etc.
6138 @section @command{ls}: List directory contents
6141 @cindex directory listing
6143 The @command{ls} program lists information about files (of any type,
6144 including directories). Options and file arguments can be intermixed
6145 arbitrarily, as usual.
6147 For non-option command-line arguments that are directories, by default
6148 @command{ls} lists the contents of directories, not recursively, and
6149 omitting files with names beginning with @samp{.}. For other non-option
6150 arguments, by default @command{ls} lists just the file name. If no
6151 non-option argument is specified, @command{ls} operates on the current
6152 directory, acting as if it had been invoked with a single argument of @samp{.}.
6155 By default, the output is sorted alphabetically, according to the locale
6156 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6157 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6158 produce output that is sorted differently than you're accustomed to.
6159 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6160 If standard output is
6161 a terminal, the output is in columns (sorted vertically) and control
6162 characters are output as question marks; otherwise, the output is listed
6163 one per line and control characters are output as-is.
6165 Because @command{ls} is such a fundamental program, it has accumulated many
6166 options over the years. They are described in the subsections below;
6167 within each section, options are listed alphabetically (ignoring case).
6168 The division of options into the subsections is not absolute, since some
6169 options affect more than one aspect of @command{ls}'s operation.
6171 @cindex exit status of @command{ls}
6176 1 minor problems (e.g., failure to access a file or directory not
6177 specified as a command line argument. This happens when listing a
6178 directory in which entries are actively being removed or renamed.)
6179 2 serious trouble (e.g., memory exhausted, invalid option, failure
6180 to access a file or directory specified as a command line argument
6181 or a directory loop)
6184 Also see @ref{Common options}.
6187 * Which files are listed::
6188 * What information is listed::
6189 * Sorting the output::
6190 * Details about version sort::
6191 * General output formatting::
6192 * Formatting file timestamps::
6193 * Formatting the file names::
6197 @node Which files are listed
6198 @subsection Which files are listed
6200 These options determine which files @command{ls} lists information for.
6201 By default, @command{ls} lists files and the contents of any
6202 directories on the command line, except that in directories it ignores
6203 files whose names start with @samp{.}.
6211 In directories, do not ignore file names that start with @samp{.}.
6216 @opindex --almost-all
6217 In directories, do not ignore all file names that start with @samp{.};
6218 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6219 option overrides this option.
6222 @itemx --ignore-backups
6224 @opindex --ignore-backups
6225 @cindex backup files, ignoring
6226 In directories, ignore files that end with @samp{~}. This option is
6227 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6232 @opindex --directory
6233 List just the names of directories, as with other types of files, rather
6234 than listing their contents.
6235 @c The following sentence is the same as the one for -F.
6236 Do not follow symbolic links listed on the
6237 command line unless the @option{--dereference-command-line} (@option{-H}),
6238 @option{--dereference} (@option{-L}), or
6239 @option{--dereference-command-line-symlink-to-dir} options are specified.
6242 @itemx --dereference-command-line
6244 @opindex --dereference-command-line
6245 @cindex symbolic links, dereferencing
6246 If a command line argument specifies a symbolic link, show information
6247 for the file the link references rather than for the link itself.
6249 @itemx --dereference-command-line-symlink-to-dir
6250 @opindex --dereference-command-line-symlink-to-dir
6251 @cindex symbolic links, dereferencing
6252 Do not dereference symbolic links, with one exception:
6253 if a command line argument specifies a symbolic link that refers to
6254 a directory, show information for that directory rather than for the
6256 This is the default behavior when no other dereferencing-related
6257 option has been specified (@option{--classify} (@option{-F}),
6258 @option{--directory} (@option{-d}),
6260 @option{--dereference} (@option{-L}), or
6261 @option{--dereference-command-line} (@option{-H})).
6263 @item --group-directories-first
6264 @opindex --group-directories-first
6265 Group all the directories before the files and then sort the
6266 directories and the files separately using the selected sort key
6267 (see --sort option).
6268 That is, this option specifies a primary sort key,
6269 and the --sort option specifies a secondary key.
6270 However, any use of @option{--sort=none}
6271 (@option{-U}) disables this option altogether.
6273 @item --hide=PATTERN
6274 @opindex --hide=@var{pattern}
6275 In directories, ignore files whose names match the shell pattern
6276 @var{pattern}, unless the @option{--all} (@option{-a}) or
6277 @option{--almost-all} (@option{-A}) is also given. This
6278 option acts like @option{--ignore=@var{pattern}} except that it has no
6279 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6280 (@option{-A}) is also given.
6282 This option can be useful in shell aliases. For example, if
6283 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6284 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6285 lists the file @file{README~} even though @samp{ly -A} would not.
6287 @item -I @var{pattern}
6288 @itemx --ignore=@var{pattern}
6290 @opindex --ignore=@var{pattern}
6291 In directories, ignore files whose names match the shell pattern
6292 (not regular expression) @var{pattern}. As
6293 in the shell, an initial @samp{.} in a file name does not match a
6294 wildcard at the start of @var{pattern}. Sometimes it is useful
6295 to give this option several times. For example,
6298 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6301 The first option ignores names of length 3 or more that start with @samp{.},
6302 the second ignores all two-character names that start with @samp{.}
6303 except @samp{..}, and the third ignores names that start with @samp{#}.
6306 @itemx --dereference
6308 @opindex --dereference
6309 @cindex symbolic links, dereferencing
6310 When showing file information for a symbolic link, show information
6311 for the file the link references rather than the link itself.
6312 However, even with this option, @command{ls} still prints the name
6313 of the link itself, not the name of the file that the link points to.
6318 @opindex --recursive
6319 @cindex recursive directory listing
6320 @cindex directory listing, recursive
6321 List the contents of all directories recursively.
6326 @node What information is listed
6327 @subsection What information is listed
6329 These options affect the information that @command{ls} displays. By
6330 default, only file names are shown.
6336 @cindex hurd, author, printing
6337 List each file's author when producing long format directory listings.
6338 In GNU/Hurd, file authors can differ from their owners, but in other
6339 operating systems the two are the same.
6345 @cindex dired Emacs mode support
6346 With the long listing (@option{-l}) format, print an additional line after
6350 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6354 The @var{begn} and @var{endn} are unsigned integers that record the
6355 byte position of the beginning and end of each file name in the output.
6356 This makes it easy for Emacs to find the names, even when they contain
6357 unusual characters such as space or newline, without fancy searching.
6359 If directories are being listed recursively (@option{-R}), output a similar
6360 line with offsets for each subdirectory name:
6363 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6366 Finally, output a line of the form:
6369 //DIRED-OPTIONS// --quoting-style=@var{word}
6373 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6375 Here is an actual example:
6378 $ mkdir -p a/sub/deeper a/sub2
6380 $ touch a/sub/deeper/file
6381 $ ls -gloRF --dired a
6384 -rw-r--r-- 1 0 Jun 10 12:27 f1
6385 -rw-r--r-- 1 0 Jun 10 12:27 f2
6386 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6387 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6391 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6395 -rw-r--r-- 1 0 Jun 10 12:27 file
6399 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6400 //SUBDIRED// 2 3 167 172 228 240 290 296
6401 //DIRED-OPTIONS// --quoting-style=literal
6404 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6405 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6407 The offsets on the @samp{//SUBDIRED//} line delimit the following
6408 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6410 Here is an example of how to extract the fifth entry name, @samp{deeper},
6411 corresponding to the pair of offsets, 222 and 228:
6414 $ ls -gloRF --dired a > out
6415 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6419 Note that although the listing above includes a trailing slash
6420 for the @samp{deeper} entry, the offsets select the name without
6421 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6422 along with an option like @option{--escape} (aka @option{-b}) and operate
6423 on a file whose name contains special characters, notice that the backslash
6428 $ ls -blog --dired 'a b'
6429 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6431 //DIRED-OPTIONS// --quoting-style=escape
6434 If you use a quoting style that adds quote marks
6435 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6436 So beware that the user may select the quoting style via the environment
6437 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6438 should either specify an explicit @option{--quoting-style=literal} option
6439 (aka @option{-N} or @option{--literal}) on the command line, or else be
6440 prepared to parse the escaped names.
6443 @opindex --full-time
6444 Produce long format directory listings, and list times in full. It is
6445 equivalent to using @option{--format=long} with
6446 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6450 Produce long format directory listings, but don't display owner information.
6456 Inhibit display of group information in a long format directory listing.
6457 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6458 provide this option for compatibility.)
6466 @cindex inode number, printing
6467 Print the inode number (also called the file serial number and index
6468 number) of each file to the left of the file name. (This number
6469 uniquely identifies each file within a particular file system.)
6472 @itemx --format=long
6473 @itemx --format=verbose
6476 @opindex long ls @r{format}
6477 @opindex verbose ls @r{format}
6478 In addition to the name of each file, print the file type, file mode bits,
6479 number of hard links, owner name, group name, size, and
6480 timestamp (@pxref{Formatting file timestamps}), normally
6481 the modification time. Print question marks for information that
6482 cannot be determined.
6484 Normally the size is printed as a byte count without punctuation, but
6485 this can be overridden (@pxref{Block size}). For example, @option{-h}
6486 prints an abbreviated, human-readable count, and
6487 @samp{--block-size="'1"} prints a byte count with the thousands
6488 separator of the current locale.
6490 For each directory that is listed, preface the files with a line
6491 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6492 for all files in that directory. The block size currently defaults to 1024
6493 bytes, but this can be overridden (@pxref{Block size}).
6494 The @var{blocks} computed counts each hard link separately;
6495 this is arguably a deficiency.
6497 The file type is one of the following characters:
6499 @c The commented-out entries are ones we're not sure about.
6507 character special file
6509 high performance (``contiguous data'') file
6513 door (Solaris 2.5 and up)
6515 @c semaphore, if this is a distinct file type
6519 @c multiplexed file (7th edition Unix; obsolete)
6521 off-line (``migrated'') file (Cray DMF)
6523 network special file (HP-UX)
6527 port (Solaris 10 and up)
6529 @c message queue, if this is a distinct file type
6533 @c shared memory object, if this is a distinct file type
6535 @c typed memory object, if this is a distinct file type
6537 @c whiteout (4.4BSD; not implemented)
6539 some other file type
6542 @cindex permissions, output by @command{ls}
6543 The file mode bits listed are similar to symbolic mode specifications
6544 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6545 third character of each set of permissions as follows:
6549 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6553 If the set-user-ID or set-group-ID bit is set but the corresponding
6554 executable bit is not set.
6557 If the restricted deletion flag or sticky bit, and the
6558 other-executable bit, are both set. The restricted deletion flag is
6559 another name for the sticky bit. @xref{Mode Structure}.
6562 If the restricted deletion flag or sticky bit is set but the
6563 other-executable bit is not set.
6566 If the executable bit is set and none of the above apply.
6572 Following the file mode bits is a single character that specifies
6573 whether an alternate access method such as an access control list
6574 applies to the file. When the character following the file mode bits is a
6575 space, there is no alternate access method. When it is a printing
6576 character, then there is such a method.
6578 GNU @command{ls} uses a @samp{.} character to indicate a file
6579 with an SELinux security context, but no other alternate access method.
6581 A file with any other combination of alternate access methods
6582 is marked with a @samp{+} character.
6585 @itemx --numeric-uid-gid
6587 @opindex --numeric-uid-gid
6588 @cindex numeric uid and gid
6589 @cindex numeric user and group IDs
6590 Produce long format directory listings, but
6591 display numeric user and group IDs instead of the owner and group names.
6595 Produce long format directory listings, but don't display group information.
6596 It is equivalent to using @option{--format=long} with @option{--no-group} .
6602 @cindex disk allocation
6603 @cindex size of files, reporting
6604 Print the disk allocation of each file to the left of the file name.
6605 This is the amount of disk space used by the file, which is usually a
6606 bit more than the file's size, but it can be less if the file has holes.
6608 Normally the disk allocation is printed in units of
6609 1024 bytes, but this can be overridden (@pxref{Block size}).
6611 @cindex NFS mounts from BSD to HP-UX
6612 For files that are NFS-mounted from an HP-UX system to a BSD system,
6613 this option reports sizes that are half the correct values. On HP-UX
6614 systems, it reports sizes that are twice the correct values for files
6615 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6616 it also affects the HP-UX @command{ls} program.
6625 @cindex security context
6626 Display the SELinux security context or @samp{?} if none is found.
6627 When used with the @option{-l} option, print the security context
6628 to the left of the size column.
6633 @node Sorting the output
6634 @subsection Sorting the output
6636 @cindex sorting @command{ls} output
6637 These options change the order in which @command{ls} sorts the information
6638 it outputs. By default, sorting is done by character code
6639 (e.g., @acronym{ASCII} order).
6645 @itemx --time=status
6648 @opindex ctime@r{, printing or sorting by}
6649 @opindex status time@r{, printing or sorting by}
6650 @opindex use time@r{, printing or sorting files by}
6651 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6652 print the status change time (the @samp{ctime} in the inode) instead of
6653 the modification time.
6654 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6655 or when not using a long listing format,
6656 sort according to the status change time.
6660 @cindex unsorted directory listing
6661 @cindex directory order, listing by
6662 Primarily, like @option{-U}---do not sort; list the files in whatever
6663 order they are stored in the directory. But also enable @option{-a} (list
6664 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6665 were specified before the @option{-f}).
6671 @cindex reverse sorting
6672 Reverse whatever the sorting method is---e.g., list files in reverse
6673 alphabetical order, youngest first, smallest first, or whatever.
6679 @opindex size of files@r{, sorting files by}
6680 Sort by file size, largest first.
6686 @opindex modification time@r{, sorting files by}
6687 Sort by modification time (the @samp{mtime} in the inode), newest first.
6691 @itemx --time=access
6695 @opindex use time@r{, printing or sorting files by}
6696 @opindex atime@r{, printing or sorting files by}
6697 @opindex access time@r{, printing or sorting files by}
6698 If the long listing format (e.g., @option{--format=long}) is being used,
6699 print the last access time (the @samp{atime} in the inode).
6700 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6701 or when not using a long listing format, sort according to the access time.
6707 @opindex none@r{, sorting option for @command{ls}}
6708 Do not sort; list the files in whatever order they are
6709 stored in the directory. (Do not do any of the other unrelated things
6710 that @option{-f} does.) This is especially useful when listing very large
6711 directories, since not doing any sorting can be noticeably faster.
6714 @itemx --sort=version
6717 @opindex version@r{, sorting option for @command{ls}}
6718 Sort by version name and number, lowest first. It behaves like a default
6719 sort, except that each sequence of decimal digits is treated numerically
6720 as an index/version number. (@xref{Details about version sort}.)
6723 @itemx --sort=extension
6726 @opindex extension@r{, sorting files by}
6727 Sort directory contents alphabetically by file extension (characters
6728 after the last @samp{.}); files with no extension are sorted first.
6733 @node Details about version sort
6734 @subsection Details about version sort
6736 Version sorting handles the fact that file names frequently include indices or
6737 version numbers. Standard sorting usually does not produce the order that one
6738 expects because comparisons are made on a character-by-character basis.
6739 Version sorting is especially useful when browsing directories that contain
6740 many files with indices/version numbers in their names:
6744 abc.zml-1.gz abc.zml-1.gz
6745 abc.zml-12.gz abc.zml-2.gz
6746 abc.zml-2.gz abc.zml-12.gz
6749 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6750 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6751 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6752 @var{ver1} < @var{ver2} implies that the name composed of
6753 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6754 ``@var{prefix} @var{ver2} @var{suffix}''.
6756 Note also that leading zeros of numeric parts are ignored:
6760 abc-1.007.tgz abc-1.01a.tgz
6761 abc-1.012b.tgz abc-1.007.tgz
6762 abc-1.01a.tgz abc-1.012b.tgz
6765 This functionality is implemented using gnulib's @code{filevercmp} function,
6766 which has some caveats worth noting.
6769 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6770 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6771 was set to @samp{C}.
6772 @item Some suffixes will not be matched by the regular
6773 expression mentioned above. Consequently these examples may
6774 not sort as you expect:
6782 abc-1.2.3.4.x86_64.rpm
6783 abc-1.2.3.x86_64.rpm
6787 @node General output formatting
6788 @subsection General output formatting
6790 These options affect the appearance of the overall output.
6795 @itemx --format=single-column
6798 @opindex single-column @r{output of files}
6799 List one file per line. This is the default for @command{ls} when standard
6800 output is not a terminal.
6803 @itemx --format=vertical
6806 @opindex vertical @r{sorted files in columns}
6807 List files in columns, sorted vertically. This is the default for
6808 @command{ls} if standard output is a terminal. It is always the default
6809 for the @command{dir} program.
6810 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6811 possible in the fewest lines.
6813 @item --color [=@var{when}]
6815 @cindex color, distinguishing file types with
6816 Specify whether to use color for distinguishing file types. @var{when}
6817 may be omitted, or one of:
6820 @vindex none @r{color option}
6821 - Do not use color at all. This is the default.
6823 @vindex auto @r{color option}
6824 @cindex terminal, using color iff
6825 - Only use color if standard output is a terminal.
6827 @vindex always @r{color option}
6830 Specifying @option{--color} and no @var{when} is equivalent to
6831 @option{--color=always}.
6832 Piping a colorized listing through a pager like @command{more} or
6833 @command{less} usually produces unreadable results. However, using
6834 @code{more -f} does seem to work.
6838 @itemx --indicator-style=classify
6841 @opindex --indicator-style
6842 @cindex file type and executables, marking
6843 @cindex executables and file type, marking
6844 Append a character to each file name indicating the file type. Also,
6845 for regular files that are executable, append @samp{*}. The file type
6846 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6847 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6848 and nothing for regular files.
6849 @c The following sentence is the same as the one for -d.
6850 Do not follow symbolic links listed on the
6851 command line unless the @option{--dereference-command-line} (@option{-H}),
6852 @option{--dereference} (@option{-L}), or
6853 @option{--dereference-command-line-symlink-to-dir} options are specified.
6856 @itemx --indicator-style=file-type
6857 @opindex --file-type
6858 @opindex --indicator-style
6859 @cindex file type, marking
6860 Append a character to each file name indicating the file type. This is
6861 like @option{-F}, except that executables are not marked.
6863 @item --indicator-style=@var{word}
6864 @opindex --indicator-style
6865 Append a character indicator with style @var{word} to entry names,
6870 Do not append any character indicator; this is the default.
6872 Append @samp{/} for directories. This is the same as the @option{-p}
6875 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6876 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6877 the same as the @option{--file-type} option.
6879 Append @samp{*} for executable regular files, otherwise behave as for
6880 @samp{file-type}. This is the same as the @option{-F} or
6881 @option{--classify} option.
6886 Print file sizes in 1024-byte blocks, overriding the default block
6887 size (@pxref{Block size}).
6888 This option is equivalent to @option{--block-size=1K}.
6891 @itemx --format=commas
6894 @opindex commas@r{, outputting between files}
6895 List files horizontally, with as many as will fit on each line,
6896 separated by @samp{, } (a comma and a space).
6899 @itemx --indicator-style=slash
6901 @opindex --indicator-style
6902 @cindex file type, marking
6903 Append a @samp{/} to directory names.
6906 @itemx --format=across
6907 @itemx --format=horizontal
6910 @opindex across@r{, listing files}
6911 @opindex horizontal@r{, listing files}
6912 List the files in columns, sorted horizontally.
6915 @itemx --tabsize=@var{cols}
6918 Assume that each tab stop is @var{cols} columns wide. The default is 8.
6919 @command{ls} uses tabs where possible in the output, for efficiency. If
6920 @var{cols} is zero, do not use tabs at all.
6922 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
6923 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
6924 do not properly align columns to the right of a TAB following a
6925 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
6926 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
6927 @command{ls} to align using spaces, not tabs.
6930 @itemx --width=@var{cols}
6934 Assume the screen is @var{cols} columns wide. The default is taken
6935 from the terminal settings if possible; otherwise the environment
6936 variable @env{COLUMNS} is used if it is set; otherwise the default
6942 @node Formatting file timestamps
6943 @subsection Formatting file timestamps
6945 By default, file timestamps are listed in abbreviated form, using
6946 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
6947 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
6948 This format can change depending on the current locale as detailed below.
6950 A timestamp is considered to be @dfn{recent} if it is less than six
6951 months old, and is not dated in the future. If a timestamp dated
6952 today is not listed in recent form, the timestamp is in the future,
6953 which means you probably have clock skew problems which may break
6954 programs like @command{make} that rely on file timestamps.
6957 Time stamps are listed according to the time zone rules specified by
6958 the @env{TZ} environment variable, or by the system default rules if
6959 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
6960 with @env{TZ}, libc, The GNU C Library Reference Manual}.
6962 The following option changes how file timestamps are printed.
6965 @item --time-style=@var{style}
6966 @opindex --time-style
6968 List timestamps in style @var{style}. The @var{style} should
6969 be one of the following:
6974 List timestamps using @var{format}, where @var{format} is interpreted
6975 like the format argument of @command{date} (@pxref{date invocation}).
6976 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
6977 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
6978 with @command{date}, @var{format}'s interpretation is affected by the
6979 @env{LC_TIME} locale category.
6981 If @var{format} contains two format strings separated by a newline,
6982 the former is used for non-recent files and the latter for recent
6983 files; if you want output columns to line up, you may need to insert
6984 spaces in one of the two formats.
6987 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
6988 format with nanosecond precision, e.g., @samp{2002-03-30
6989 23:45:56.477817180 -0700}. This style is equivalent to
6990 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
6992 This is useful because the time output includes all the information that
6993 is available from the operating system. For example, this can help
6994 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
6995 uses the full timestamp to determine whether a file is out of date.
6998 List @acronym{ISO} 8601 date and time in minutes, e.g.,
6999 @samp{2002-03-30 23:45}. These timestamps are shorter than
7000 @samp{full-iso} timestamps, and are usually good enough for everyday
7001 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7004 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7005 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7006 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7007 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7008 nearly the same information in a smaller space and their brevity helps
7009 @command{ls} output fit within traditional 80-column output lines.
7010 The following two @command{ls} invocations are equivalent:
7015 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7016 ls -l --time-style="iso"
7021 List timestamps in a locale-dependent form. For example, a Finnish
7022 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7023 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7024 timestamps typically consume more space than @samp{iso} timestamps and
7025 are harder for programs to parse because locale conventions vary so
7026 widely, but they are easier for many people to read.
7028 The @env{LC_TIME} locale category specifies the timestamp format. The
7029 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7030 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7031 @command{ls} invocations are equivalent:
7036 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7037 ls -l --time-style="locale"
7040 Other locales behave differently. For example, in a German locale,
7041 @option{--time-style="locale"} might be equivalent to
7042 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7043 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7044 @samp{30. M@"ar 23:45}.
7046 @item posix-@var{style}
7048 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7049 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7050 example, the @samp{posix-long-iso} style lists
7051 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7052 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7057 You can specify the default value of the @option{--time-style} option
7058 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7059 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7060 later use the @option{--dired} option and therefore can parse any date
7061 format, but if you are using Emacs 21.1 or 21.2 and specify a
7062 non-@acronym{POSIX} locale you may need to set
7063 @samp{TIME_STYLE="posix-long-iso"}.
7065 To avoid certain denial-of-service attacks, timestamps that would be
7066 longer than 1000 bytes may be treated as errors.
7069 @node Formatting the file names
7070 @subsection Formatting the file names
7072 These options change how file names themselves are printed.
7078 @itemx --quoting-style=escape
7081 @opindex --quoting-style
7082 @cindex backslash sequences for file names
7083 Quote nongraphic characters in file names using alphabetic and octal
7084 backslash sequences like those used in C.
7088 @itemx --quoting-style=literal
7091 @opindex --quoting-style
7092 Do not quote file names. However, with @command{ls} nongraphic
7093 characters are still printed as question marks if the output is a
7094 terminal and you do not specify the @option{--show-control-chars}
7098 @itemx --hide-control-chars
7100 @opindex --hide-control-chars
7101 Print question marks instead of nongraphic characters in file names.
7102 This is the default if the output is a terminal and the program is
7107 @itemx --quoting-style=c
7109 @opindex --quote-name
7110 @opindex --quoting-style
7111 Enclose file names in double quotes and quote nongraphic characters as
7114 @item --quoting-style=@var{word}
7115 @opindex --quoting-style
7116 @cindex quoting style
7117 Use style @var{word} to quote file names and other strings that may
7118 contain arbitrary characters. The @var{word} should
7119 be one of the following:
7123 Output strings as-is; this is the same as the @option{-N} or
7124 @option{--literal} option.
7126 Quote strings for the shell if they contain shell metacharacters or would
7127 cause ambiguous output.
7128 The quoting is suitable for @acronym{POSIX}-compatible shells like
7129 @command{bash}, but it does not always work for incompatible shells
7132 Quote strings for the shell, even if they would normally not require quoting.
7134 Quote strings as for C character string literals, including the
7135 surrounding double-quote characters; this is the same as the
7136 @option{-Q} or @option{--quote-name} option.
7138 Quote strings as for C character string literals, except omit the
7139 surrounding double-quote
7140 characters; this is the same as the @option{-b} or @option{--escape} option.
7142 Quote strings as for C character string literals, except use
7143 surrounding quotation marks appropriate for the
7146 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7147 Quote strings as for C character string literals, except use
7148 surrounding quotation marks appropriate for the locale, and quote
7149 @t{`like this'} instead of @t{"like
7150 this"} in the default C locale. This looks nicer on many displays.
7153 You can specify the default value of the @option{--quoting-style} option
7154 with the environment variable @env{QUOTING_STYLE}. If that environment
7155 variable is not set, the default value is @samp{literal}, but this
7156 default may change to @samp{shell} in a future version of this package.
7158 @item --show-control-chars
7159 @opindex --show-control-chars
7160 Print nongraphic characters as-is in file names.
7161 This is the default unless the output is a terminal and the program is
7167 @node dir invocation
7168 @section @command{dir}: Briefly list directory contents
7171 @cindex directory listing, brief
7173 @command{dir} is equivalent to @code{ls -C
7174 -b}; that is, by default files are listed in columns, sorted vertically,
7175 and special characters are represented by backslash escape sequences.
7177 @xref{ls invocation, @command{ls}}.
7180 @node vdir invocation
7181 @section @command{vdir}: Verbosely list directory contents
7184 @cindex directory listing, verbose
7186 @command{vdir} is equivalent to @code{ls -l
7187 -b}; that is, by default files are listed in long format and special
7188 characters are represented by backslash escape sequences.
7190 @node dircolors invocation
7191 @section @command{dircolors}: Color setup for @command{ls}
7195 @cindex setup for color
7197 @command{dircolors} outputs a sequence of shell commands to set up the
7198 terminal for color output from @command{ls} (and @command{dir}, etc.).
7202 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7205 If @var{file} is specified, @command{dircolors} reads it to determine which
7206 colors to use for which file types and extensions. Otherwise, a
7207 precompiled database is used. For details on the format of these files,
7208 run @samp{dircolors --print-database}.
7210 To make @command{dircolors} read a @file{~/.dircolors} file if it
7211 exists, you can put the following lines in your @file{~/.bashrc} (or
7212 adapt them to your favorite shell):
7216 test -r $d && eval "$(dircolors $d)"
7220 @vindex SHELL @r{environment variable, and color}
7221 The output is a shell command to set the @env{LS_COLORS} environment
7222 variable. You can specify the shell syntax to use on the command line,
7223 or @command{dircolors} will guess it from the value of the @env{SHELL}
7224 environment variable.
7226 The program accepts the following options. Also see @ref{Common options}.
7231 @itemx --bourne-shell
7234 @opindex --bourne-shell
7235 @cindex Bourne shell syntax for color setup
7236 @cindex @command{sh} syntax for color setup
7237 Output Bourne shell commands. This is the default if the @env{SHELL}
7238 environment variable is set and does not end with @samp{csh} or
7247 @cindex C shell syntax for color setup
7248 @cindex @command{csh} syntax for color setup
7249 Output C shell commands. This is the default if @code{SHELL} ends with
7250 @command{csh} or @command{tcsh}.
7253 @itemx --print-database
7255 @opindex --print-database
7256 @cindex color database, printing
7257 @cindex database for color setup, printing
7258 @cindex printing color database
7259 Print the (compiled-in) default color configuration database. This
7260 output is itself a valid configuration file, and is fairly descriptive
7261 of the possibilities.
7268 @node Basic operations
7269 @chapter Basic operations
7271 @cindex manipulating files
7273 This chapter describes the commands for basic file manipulation:
7274 copying, moving (renaming), and deleting (removing).
7277 * cp invocation:: Copy files.
7278 * dd invocation:: Convert and copy a file.
7279 * install invocation:: Copy files and set attributes.
7280 * mv invocation:: Move (rename) files.
7281 * rm invocation:: Remove files or directories.
7282 * shred invocation:: Remove files more securely.
7287 @section @command{cp}: Copy files and directories
7290 @cindex copying files and directories
7291 @cindex files, copying
7292 @cindex directories, copying
7294 @command{cp} copies files (or, optionally, directories). The copy is
7295 completely independent of the original. You can either copy one file to
7296 another, or copy arbitrarily many files to a destination directory.
7300 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7301 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7302 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7307 If two file names are given, @command{cp} copies the first file to the
7311 If the @option{--target-directory} (@option{-t}) option is given, or
7312 failing that if the last file is a directory and the
7313 @option{--no-target-directory} (@option{-T}) option is not given,
7314 @command{cp} copies each @var{source} file to the specified directory,
7315 using the @var{source}s' names.
7318 Generally, files are written just as they are read. For exceptions,
7319 see the @option{--sparse} option below.
7321 By default, @command{cp} does not copy directories. However, the
7322 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7323 copy recursively by descending into source directories and copying files
7324 to corresponding destination directories.
7326 When copying from a symbolic link, @command{cp} normally follows the
7327 link only when not copying
7328 recursively. This default can be overridden with the
7329 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7330 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7331 @option{-H} options. If more than one of these options is specified,
7332 the last one silently overrides the others.
7334 When copying to a symbolic link, @command{cp} follows the
7335 link only when it refers to an existing regular file.
7336 However, when copying to a dangling symbolic link, @command{cp}
7337 refuses by default, and fails with a diagnostic, since the operation
7338 is inherently dangerous. This behavior is contrary to historical
7339 practice and to @acronym{POSIX}.
7340 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7341 the target of a dangling destination symlink, in spite of the possible risk.
7342 Also, when an option like
7343 @option{--backup} or @option{--link} acts to rename or remove the
7344 destination before copying, @command{cp} renames or removes the
7345 symbolic link rather than the file it points to.
7347 By default, @command{cp} copies the contents of special files only
7348 when not copying recursively. This default can be overridden with the
7349 @option{--copy-contents} option.
7351 @cindex self-backups
7352 @cindex backups, making only
7353 @command{cp} generally refuses to copy a file onto itself, with the
7354 following exception: if @option{--force --backup} is specified with
7355 @var{source} and @var{dest} identical, and referring to a regular file,
7356 @command{cp} will make a backup file, either regular or numbered, as
7357 specified in the usual ways (@pxref{Backup options}). This is useful when
7358 you simply want to make a backup of an existing file before changing it.
7360 The program accepts the following options. Also see @ref{Common options}.
7367 Preserve as much as possible of the structure and attributes of the
7368 original files in the copy (but do not attempt to preserve internal
7369 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7370 directory in a different order).
7371 Try to preserve SELinux security context and extended attributes (xattr),
7372 but ignore any failure to do that and print no corresponding diagnostic.
7373 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7375 @itemx --attributes-only
7376 @opindex --attributes-only
7377 Preserve the specified attributes of the original files in the copy,
7378 but do not copy any data. See the @option{--preserve} option for
7379 controlling which attributes to copy.
7382 @itemx @w{@kbd{--backup}[=@var{method}]}
7385 @vindex VERSION_CONTROL
7386 @cindex backups, making
7387 @xref{Backup options}.
7388 Make a backup of each file that would otherwise be overwritten or removed.
7389 As a special case, @command{cp} makes a backup of @var{source} when the force
7390 and backup options are given and @var{source} and @var{dest} are the same
7391 name for an existing, regular file. One useful application of this
7392 combination of options is this tiny Bourne shell script:
7396 # Usage: backup FILE...
7397 # Create a @sc{gnu}-style backup of each listed FILE.
7399 cp --backup --force -- "$i" "$i"
7403 @item --copy-contents
7404 @cindex directories, copying recursively
7405 @cindex copying directories recursively
7406 @cindex recursively copying directories
7407 @cindex non-directories, copying as special files
7408 If copying recursively, copy the contents of any special files (e.g.,
7409 FIFOs and device files) as if they were regular files. This means
7410 trying to read the data in each source file and writing it to the
7411 destination. It is usually a mistake to use this option, as it
7412 normally has undesirable effects on special files like FIFOs and the
7413 ones typically found in the @file{/dev} directory. In most cases,
7414 @code{cp -R --copy-contents} will hang indefinitely trying to read
7415 from FIFOs and special files like @file{/dev/console}, and it will
7416 fill up your destination disk if you use it to copy @file{/dev/zero}.
7417 This option has no effect unless copying recursively, and it does not
7418 affect the copying of symbolic links.
7422 @cindex symbolic links, copying
7423 @cindex hard links, preserving
7424 Copy symbolic links as symbolic links rather than copying the files that
7425 they point to, and preserve hard links between source files in the copies.
7426 Equivalent to @option{--no-dereference --preserve=links}.
7432 When copying without this option and an existing destination file cannot
7433 be opened for writing, the copy fails. However, with @option{--force}),
7434 when a destination file cannot be opened, @command{cp} then removes it and
7435 tries to open it again. Contrast this behavior with that enabled by
7436 @option{--link} and @option{--symbolic-link}, whereby the destination file
7437 is never opened but rather is removed unconditionally. Also see the
7438 description of @option{--remove-destination}.
7440 This option is independent of the @option{--interactive} or
7441 @option{-i} option: neither cancels the effect of the other.
7443 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7448 If a command line argument specifies a symbolic link, then copy the
7449 file it points to rather than the symbolic link itself. However,
7450 copy (preserving its nature) any symbolic link that is encountered
7451 via recursive traversal.
7454 @itemx --interactive
7456 @opindex --interactive
7457 When copying a file other than a directory, prompt whether to
7458 overwrite an existing destination file. The @option{-i} option overrides
7459 a previous @option{-n} option.
7465 Make hard links instead of copies of non-directories.
7468 @itemx --dereference
7470 @opindex --dereference
7471 Follow symbolic links when copying from them.
7472 With this option, @command{cp} cannot create a symbolic link.
7473 For example, a symlink (to regular file) in the source tree will be copied to
7474 a regular file in the destination tree.
7479 @opindex --no-clobber
7480 Do not overwrite an existing file. The @option{-n} option overrides a previous
7481 @option{-i} option. This option is mutually exclusive with @option{-b} or
7482 @option{--backup} option.
7485 @itemx --no-dereference
7487 @opindex --no-dereference
7488 @cindex symbolic links, copying
7489 Copy symbolic links as symbolic links rather than copying the files that
7490 they point to. This option affects only symbolic links in the source;
7491 symbolic links in the destination are always followed if possible.
7494 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7497 @cindex file information, preserving, extended attributes, xattr
7498 Preserve the specified attributes of the original files.
7499 If specified, the @var{attribute_list} must be a comma-separated list
7500 of one or more of the following strings:
7504 Preserve the file mode bits and access control lists.
7506 Preserve the owner and group. On most modern systems,
7507 only users with appropriate privileges may change the owner of a file,
7509 may preserve the group ownership of a file only if they happen to be
7510 a member of the desired group.
7512 Preserve the times of last access and last modification, when possible.
7513 On older systems, it is not possible to preserve these attributes
7514 when the affected file is a symbolic link.
7515 However, many systems now provide the @code{utimensat} function,
7516 which makes it possible even for symbolic links.
7518 Preserve in the destination files
7519 any links between corresponding source files.
7520 Note that with @option{-L} or @option{-H}, this option can convert
7521 symbolic links to hard links. For example,
7523 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7528 Note the inputs: @file{b} is a symlink to regular file @file{a},
7529 yet the files in destination directory, @file{c/}, are hard-linked.
7530 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7531 tells @command{cp} to dereference command line arguments, it sees two files
7532 with the same inode number, and preserves the perceived hard link.
7534 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7536 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7542 Preserve SELinux security context of the file, or fail with full diagnostics.
7544 Preserve extended attributes of the file, or fail with full diagnostics.
7545 If @command{cp} is built without xattr support, ignore this option.
7546 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7547 they are preserved by this option as well.
7549 Preserve all file attributes.
7550 Equivalent to specifying all of the above, but with the difference
7551 that failure to preserve SELinux security context or extended attributes
7552 does not change @command{cp}'s exit status. In contrast to @option{-a},
7553 all but @samp{Operation not supported} warnings are output.
7556 Using @option{--preserve} with no @var{attribute_list} is equivalent
7557 to @option{--preserve=mode,ownership,timestamps}.
7559 In the absence of this option, each destination file is created with the
7560 mode bits of the corresponding source file, minus the bits set in the
7561 umask and minus the set-user-ID and set-group-ID bits.
7562 @xref{File permissions}.
7564 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7565 @cindex file information, preserving
7566 Do not preserve the specified attributes. The @var{attribute_list}
7567 has the same form as for @option{--preserve}.
7571 @cindex parent directories and @command{cp}
7572 Form the name of each destination file by appending to the target
7573 directory a slash and the specified name of the source file. The last
7574 argument given to @command{cp} must be the name of an existing directory.
7575 For example, the command:
7578 cp --parents a/b/c existing_dir
7582 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7583 any missing intermediate directories.
7590 @opindex --recursive
7591 @cindex directories, copying recursively
7592 @cindex copying directories recursively
7593 @cindex recursively copying directories
7594 @cindex non-directories, copying as special files
7595 Copy directories recursively. By default, do not follow symbolic
7596 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7597 @option{--dereference} (@option{-L}), @option{--no-dereference}
7598 (@option{-P}), and @option{-H} options. Special files are copied by
7599 creating a destination file of the same type as the source; see the
7600 @option{--copy-contents} option. It is not portable to use
7601 @option{-r} to copy symbolic links or special files. On some
7602 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7603 @option{-L} and @option{--copy-contents} for historical reasons.
7604 Also, it is not portable to use @option{-R} to copy symbolic links
7605 unless you also specify @option{-P}, as @acronym{POSIX} allows
7606 implementations that dereference symbolic links by default.
7608 @item --reflink[=@var{when}]
7609 @opindex --reflink[=@var{when}]
7612 @cindex copy on write
7613 Perform a lightweight, copy-on-write (COW) copy.
7614 Copying with this option can succeed only on some file systems.
7615 Once it has succeeded, beware that the source and destination files
7616 share the same disk data blocks as long as they remain unmodified.
7617 Thus, if a disk I/O error affects data blocks of one of the files,
7618 the other suffers the exact same fate.
7620 The @var{when} value can be one of the following:
7624 The default behavior: if the copy-on-write operation is not supported
7625 then report the failure for each file and exit with a failure status.
7628 If the copy-on-write operation is not supported then fall back
7629 to the standard copy behaviour.
7633 @item --remove-destination
7634 @opindex --remove-destination
7635 Remove each existing destination file before attempting to open it
7636 (contrast with @option{-f} above).
7638 @item --sparse=@var{when}
7639 @opindex --sparse=@var{when}
7640 @cindex sparse files, copying
7641 @cindex holes, copying files with
7642 @findex read @r{system call, and holes}
7643 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7644 does not occupy any physical disk blocks; the @samp{read} system call
7645 reads these as zeros. This can both save considerable disk space and
7646 increase speed, since many binary files contain lots of consecutive zero
7647 bytes. By default, @command{cp} detects holes in input source files via a crude
7648 heuristic and makes the corresponding output file sparse as well.
7649 Only regular files may be sparse.
7651 The @var{when} value can be one of the following:
7655 The default behavior: if the input file is sparse, attempt to make
7656 the output file sparse, too. However, if an output file exists but
7657 refers to a non-regular file, then do not attempt to make it sparse.
7660 For each sufficiently long sequence of zero bytes in the input file,
7661 attempt to create a corresponding hole in the output file, even if the
7662 input file does not appear to be sparse.
7663 This is useful when the input file resides on a file system
7664 that does not support sparse files
7665 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7666 but the output file is on a type of file system that does support them.
7667 Holes may be created only in regular files, so if the destination file
7668 is of some other type, @command{cp} does not even try to make it sparse.
7671 Never make the output file sparse.
7672 This is useful in creating a file for use with the @command{mkswap} command,
7673 since such a file must not have any holes.
7676 @optStripTrailingSlashes
7679 @itemx --symbolic-link
7681 @opindex --symbolic-link
7682 @cindex symbolic links, copying with
7683 Make symbolic links instead of copies of non-directories. All source
7684 file names must be absolute (starting with @samp{/}) unless the
7685 destination files are in the current directory. This option merely
7686 results in an error message on systems that do not support symbolic links.
7692 @optNoTargetDirectory
7698 @cindex newer files, copying only
7699 Do not copy a non-directory that has an existing destination with the
7700 same or newer modification time. If time stamps are being preserved,
7701 the comparison is to the source time stamp truncated to the
7702 resolutions of the destination file system and of the system calls
7703 used to update time stamps; this avoids duplicate work if several
7704 @samp{cp -pu} commands are executed with the same source and
7711 Print the name of each file before copying it.
7714 @itemx --one-file-system
7716 @opindex --one-file-system
7717 @cindex file systems, omitting copying to different
7718 Skip subdirectories that are on different file systems from the one that
7719 the copy started on.
7720 However, mount point directories @emph{are} copied.
7728 @section @command{dd}: Convert and copy a file
7731 @cindex converting while copying a file
7733 @command{dd} copies a file (from standard input to standard output, by
7734 default) with a changeable I/O block size, while optionally performing
7735 conversions on it. Synopses:
7738 dd [@var{operand}]@dots{}
7742 The only options are @option{--help} and @option{--version}.
7743 @xref{Common options}. @command{dd} accepts the following operands.
7749 Read from @var{file} instead of standard input.
7753 Write to @var{file} instead of standard output. Unless
7754 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7755 bytes (or the size specified with @samp{seek=}).
7757 @item ibs=@var{bytes}
7759 @cindex block size of input
7760 @cindex input block size
7761 Set the input block size to @var{bytes}.
7762 This makes @command{dd} read @var{bytes} per block.
7763 The default is 512 bytes.
7765 @item obs=@var{bytes}
7767 @cindex block size of output
7768 @cindex output block size
7769 Set the output block size to @var{bytes}.
7770 This makes @command{dd} write @var{bytes} per block.
7771 The default is 512 bytes.
7773 @item bs=@var{bytes}
7776 Set both input and output block sizes to @var{bytes}.
7777 This makes @command{dd} read and write @var{bytes} per block,
7778 overriding any @samp{ibs} and @samp{obs} settings.
7779 In addition, if no data-transforming @option{conv} option is specified,
7780 each input block is copied to the output as a single block,
7781 without aggregating short reads.
7783 @item cbs=@var{bytes}
7785 @cindex block size of conversion
7786 @cindex conversion block size
7787 @cindex fixed-length records, converting to variable-length
7788 @cindex variable-length records, converting to fixed-length
7789 Set the conversion block size to @var{bytes}.
7790 When converting variable-length records to fixed-length ones
7791 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7792 use @var{bytes} as the fixed record length.
7794 @item skip=@var{blocks}
7796 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7798 @item seek=@var{blocks}
7800 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7802 @item count=@var{blocks}
7804 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7805 of everything until the end of the file.
7809 Do not print the overall transfer rate and volume statistics
7810 that normally make up the third status line when @command{dd} exits.
7812 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7814 Convert the file as specified by the @var{conversion} argument(s).
7815 (No spaces around any comma(s).)
7822 @opindex ascii@r{, converting to}
7823 Convert @acronym{EBCDIC} to @acronym{ASCII},
7824 using the conversion table specified by @acronym{POSIX}.
7825 This provides a 1:1 translation for all 256 bytes.
7828 @opindex ebcdic@r{, converting to}
7829 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7830 This is the inverse of the @samp{ascii} conversion.
7833 @opindex alternate ebcdic@r{, converting to}
7834 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7835 using the alternate conversion table specified by @acronym{POSIX}.
7836 This is not a 1:1 translation, but reflects common historical practice
7837 for @samp{~}, @samp{[}, and @samp{]}.
7839 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7843 @opindex block @r{(space-padding)}
7844 For each line in the input, output @samp{cbs} bytes, replacing the
7845 input newline with a space and padding with spaces as necessary.
7849 Remove any trailing spaces in each @samp{cbs}-sized input block,
7850 and append a newline.
7852 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7855 @opindex lcase@r{, converting to}
7856 Change uppercase letters to lowercase.
7859 @opindex ucase@r{, converting to}
7860 Change lowercase letters to uppercase.
7862 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7865 @opindex swab @r{(byte-swapping)}
7866 @cindex byte-swapping
7867 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7868 when an odd number of bytes are read---the last byte is simply copied
7869 (since there is nothing to swap it with).
7873 @cindex read errors, ignoring
7874 Continue after read errors.
7878 @cindex creating output file, avoiding
7879 Do not create the output file; the output file must already exist.
7883 @cindex creating output file, requiring
7884 Fail if the output file already exists; @command{dd} must create the
7887 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
7891 @cindex truncating output file, avoiding
7892 Do not truncate the output file.
7895 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
7896 Pad every input block to size of @samp{ibs} with trailing zero bytes.
7897 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
7902 @cindex synchronized data writes, before finishing
7903 Synchronize output data just before finishing. This forces a physical
7904 write of output data.
7908 @cindex synchronized data and metadata writes, before finishing
7909 Synchronize output data and metadata just before finishing. This
7910 forces a physical write of output data and metadata.
7914 @item iflag=@var{flag}[,@var{flag}]@dots{}
7916 Access the input file using the flags specified by the @var{flag}
7917 argument(s). (No spaces around any comma(s).)
7919 @item oflag=@var{flag}[,@var{flag}]@dots{}
7921 Access the output file using the flags specified by the @var{flag}
7922 argument(s). (No spaces around any comma(s).)
7924 Here are the flags. Not every flag is supported on every operating
7931 @cindex appending to the output file
7932 Write in append mode, so that even if some other process is writing to
7933 this file, every @command{dd} write will append to the current
7934 contents of the file. This flag makes sense only for output.
7935 If you combine this flag with the @samp{of=@var{file}} operand,
7936 you should also specify @samp{conv=notrunc} unless you want the
7937 output file to be truncated before being appended to.
7941 @cindex concurrent I/O
7942 Use concurrent I/O mode for data. This mode performs direct I/O
7943 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
7944 A file cannot be opened in CIO mode and with a standard open at the
7950 Use direct I/O for data, avoiding the buffer cache.
7951 Note that the kernel may impose restrictions on read or write buffer sizes.
7952 For example, with an ext4 destination file system and a linux-based kernel,
7953 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
7954 output buffer size is not a multiple of 512.
7958 @cindex directory I/O
7960 Fail unless the file is a directory. Most operating systems do not
7961 allow I/O to a directory, so this flag has limited utility.
7965 @cindex synchronized data reads
7966 Use synchronized I/O for data. For the output file, this forces a
7967 physical write of output data on each write. For the input file,
7968 this flag can matter when reading from a remote file that has been
7969 written to synchronously by some other process. Metadata (e.g.,
7970 last-access and last-modified time) is not necessarily synchronized.
7974 @cindex synchronized data and metadata I/O
7975 Use synchronized I/O for both data and metadata.
7979 @cindex nonblocking I/O
7980 Use non-blocking I/O.
7985 Do not update the file's access time.
7986 Some older file systems silently ignore this flag, so it is a good
7987 idea to test it on your files before relying on it.
7991 @cindex controlling terminal
7992 Do not assign the file to be a controlling terminal for @command{dd}.
7993 This has no effect when the file is not a terminal.
7994 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
7999 @cindex symbolic links, following
8000 Do not follow symbolic links.
8005 Fail if the file has multiple hard links.
8010 Use binary I/O. This option has an effect only on nonstandard
8011 platforms that distinguish binary from text I/O.
8016 Use text I/O. Like @samp{binary}, this option has no effect on
8021 Accumulate full blocks from input. The @code{read} system call
8022 may return early if a full block is not available.
8023 When that happens, continue calling @code{read} to fill the remainder
8025 This flag can be used only with @code{iflag}.
8029 These flags are not supported on all systems, and @samp{dd} rejects
8030 attempts to use them when they are not supported. When reading from
8031 standard input or writing to standard output, the @samp{nofollow} and
8032 @samp{noctty} flags should not be specified, and the other flags
8033 (e.g., @samp{nonblock}) can affect how other processes behave with the
8034 affected file descriptors, even after @command{dd} exits.
8038 @cindex multipliers after numbers
8039 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8040 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8041 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8042 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8044 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8045 should not be too large---values larger than a few megabytes
8046 are generally wasteful or (as in the gigabyte..exabyte case) downright
8047 counterproductive or error-inducing.
8049 Use different @command{dd} invocations to use different block sizes for
8050 skipping and I/O@. For example, the following shell commands copy data
8051 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8052 4 KiB label at the start of the disk:
8055 disk=/dev/rdsk/c0t1d0s2
8058 # Copy all but the label from disk to tape.
8059 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8061 # Copy from tape back to disk, but leave the disk label alone.
8062 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8065 Sending an @samp{INFO} signal to a running @command{dd}
8066 process makes it print I/O statistics to standard error
8067 and then resume copying. In the example below,
8068 @command{dd} is run in the background to copy 10 million blocks.
8069 The @command{kill} command makes it output intermediate I/O statistics,
8070 and when @command{dd} completes normally or is killed by the
8071 @code{SIGINT} signal, it outputs the final statistics.
8074 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8075 $ kill -s INFO $pid; wait $pid
8076 3385223+0 records in
8077 3385223+0 records out
8078 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8079 10000000+0 records in
8080 10000000+0 records out
8081 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8084 @vindex POSIXLY_CORRECT
8085 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8086 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8087 environment variable is set.
8092 @node install invocation
8093 @section @command{install}: Copy files and set attributes
8096 @cindex copying files and setting attributes
8098 @command{install} copies files while setting their file mode bits and, if
8099 possible, their owner and group. Synopses:
8102 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8103 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8104 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8105 install [@var{option}]@dots{} -d @var{directory}@dots{}
8110 If two file names are given, @command{install} copies the first file to the
8114 If the @option{--target-directory} (@option{-t}) option is given, or
8115 failing that if the last file is a directory and the
8116 @option{--no-target-directory} (@option{-T}) option is not given,
8117 @command{install} copies each @var{source} file to the specified
8118 directory, using the @var{source}s' names.
8121 If the @option{--directory} (@option{-d}) option is given,
8122 @command{install} creates each @var{directory} and any missing parent
8123 directories. Parent directories are created with mode
8124 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8125 current umask. @xref{Directory Setuid and Setgid}, for how the
8126 set-user-ID and set-group-ID bits of parent directories are inherited.
8129 @cindex Makefiles, installing programs in
8130 @command{install} is similar to @command{cp}, but allows you to control the
8131 attributes of destination files. It is typically used in Makefiles to
8132 copy programs into their destination directories. It refuses to copy
8133 files onto themselves.
8135 @cindex extended attributes, xattr
8136 @command{install} never preserves extended attributes (xattr).
8138 The program accepts the following options. Also see @ref{Common options}.
8148 Compare each pair of source and destination files, and if the destination has
8149 identical content and any specified owner, group, permissions, and possibly
8150 SELinux context, then do not modify the destination at all.
8154 Ignored; for compatibility with old Unix versions of @command{install}.
8158 Create any missing parent directories of @var{dest},
8159 then copy @var{source} to @var{dest}.
8160 This option is ignored if a destination directory is specified
8161 via @option{--target-directory=DIR}.
8166 @opindex --directory
8167 @cindex directories, creating with given attributes
8168 @cindex parent directories, creating missing
8169 @cindex leading directories, creating missing
8170 Create any missing parent directories, giving them the default
8171 attributes. Then create each given directory, setting their owner,
8172 group and mode as given on the command line or to the defaults.
8174 @item -g @var{group}
8175 @itemx --group=@var{group}
8178 @cindex group ownership of installed files, setting
8179 Set the group ownership of installed files or directories to
8180 @var{group}. The default is the process's current group. @var{group}
8181 may be either a group name or a numeric group ID.
8184 @itemx --mode=@var{mode}
8187 @cindex permissions of installed files, setting
8188 Set the file mode bits for the installed file or directory to @var{mode},
8189 which can be either an octal number, or a symbolic mode as in
8190 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8191 point of departure (@pxref{File permissions}).
8192 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8193 execute for the owner, read and execute for group and other, and with
8194 set-user-ID and set-group-ID disabled.
8195 This default is not quite the same as @samp{755}, since it disables
8196 instead of preserving set-user-ID and set-group-ID on directories.
8197 @xref{Directory Setuid and Setgid}.
8199 @item -o @var{owner}
8200 @itemx --owner=@var{owner}
8203 @cindex ownership of installed files, setting
8204 @cindex appropriate privileges
8205 @vindex root @r{as default owner}
8206 If @command{install} has appropriate privileges (is run as root), set the
8207 ownership of installed files or directories to @var{owner}. The default
8208 is @code{root}. @var{owner} may be either a user name or a numeric user
8211 @item --preserve-context
8212 @opindex --preserve-context
8214 @cindex security context
8215 Preserve the SELinux security context of files and directories.
8216 Failure to preserve the context in all of the files or directories
8217 will result in an exit status of 1. If SELinux is disabled then
8218 print a warning and ignore the option.
8221 @itemx --preserve-timestamps
8223 @opindex --preserve-timestamps
8224 @cindex timestamps of installed files, preserving
8225 Set the time of last access and the time of last modification of each
8226 installed file to match those of each corresponding original file.
8227 When a file is installed without this option, its last access and
8228 last modification times are both set to the time of installation.
8229 This option is useful if you want to use the last modification times
8230 of installed files to keep track of when they were last built as opposed
8231 to when they were last installed.
8237 @cindex symbol table information, stripping
8238 @cindex stripping symbol table information
8239 Strip the symbol tables from installed binary executables.
8241 @itemx --strip-program=@var{program}
8242 @opindex --strip-program
8243 @cindex symbol table information, stripping, program
8244 Program used to strip binaries.
8250 @optNoTargetDirectory
8256 Print the name of each file before copying it.
8258 @item -Z @var{context}
8259 @itemx --context=@var{context}
8263 @cindex security context
8264 Set the default SELinux security context to be used for any
8265 created files and directories. If SELinux is disabled then
8266 print a warning and ignore the option.
8274 @section @command{mv}: Move (rename) files
8278 @command{mv} moves or renames files (or directories). Synopses:
8281 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8282 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8283 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8288 If two file names are given, @command{mv} moves the first file to the
8292 If the @option{--target-directory} (@option{-t}) option is given, or
8293 failing that if the last file is a directory and the
8294 @option{--no-target-directory} (@option{-T}) option is not given,
8295 @command{mv} moves each @var{source} file to the specified
8296 directory, using the @var{source}s' names.
8299 @command{mv} can move any type of file from one file system to another.
8300 Prior to version @code{4.0} of the fileutils,
8301 @command{mv} could move only regular files between file systems.
8302 For example, now @command{mv} can move an entire directory hierarchy
8303 including special device files from one partition to another. It first
8304 uses some of the same code that's used by @code{cp -a} to copy the
8305 requested directories and files, then (assuming the copy succeeded)
8306 it removes the originals. If the copy fails, then the part that was
8307 copied to the destination partition is removed. If you were to copy
8308 three directories from one partition to another and the copy of the first
8309 directory succeeded, but the second didn't, the first would be left on
8310 the destination partition and the second and third would be left on the
8313 @cindex extended attributes, xattr
8314 @command{mv} always tries to copy extended attributes (xattr), which may
8315 include SELinux context, ACLs or Capabilities.
8316 Upon failure all but @samp{Operation not supported} warnings are output.
8318 @cindex prompting, and @command{mv}
8319 If a destination file exists but is normally unwritable, standard input
8320 is a terminal, and the @option{-f} or @option{--force} option is not given,
8321 @command{mv} prompts the user for whether to replace the file. (You might
8322 own the file, or have write permission on its directory.) If the
8323 response is not affirmative, the file is skipped.
8325 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8326 when it might be a symlink to a directory.
8327 Otherwise, @command{mv} may do something very surprising, since
8328 its behavior depends on the underlying rename system call.
8329 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8330 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8331 renames not the symlink but rather the directory referenced by the symlink.
8332 @xref{Trailing slashes}.
8334 The program accepts the following options. Also see @ref{Common options}.
8344 @cindex prompts, omitting
8345 Do not prompt the user before removing a destination file.
8347 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8348 options, only the final one takes effect.
8353 @itemx --interactive
8355 @opindex --interactive
8356 @cindex prompts, forcing
8357 Prompt whether to overwrite each existing destination file, regardless
8359 If the response is not affirmative, the file is skipped.
8365 @opindex --no-clobber
8366 @cindex prompts, omitting
8367 Do not overwrite an existing file.
8369 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8375 @cindex newer files, moving only
8376 Do not move a non-directory that has an existing destination with the
8377 same or newer modification time.
8378 If the move is across file system boundaries, the comparison is to the
8379 source time stamp truncated to the resolutions of the destination file
8380 system and of the system calls used to update time stamps; this avoids
8381 duplicate work if several @samp{mv -u} commands are executed with the
8382 same source and destination.
8388 Print the name of each file before moving it.
8390 @optStripTrailingSlashes
8396 @optNoTargetDirectory
8404 @section @command{rm}: Remove files or directories
8407 @cindex removing files or directories
8409 @command{rm} removes each given @var{file}. By default, it does not remove
8410 directories. Synopsis:
8413 rm [@var{option}]@dots{} [@var{file}]@dots{}
8416 @cindex prompting, and @command{rm}
8417 If the @option{-I} or @option{--interactive=once} option is given,
8418 and there are more than three files or the @option{-r}, @option{-R},
8419 or @option{--recursive} are given, then @command{rm} prompts the user
8420 for whether to proceed with the entire operation. If the response is
8421 not affirmative, the entire command is aborted.
8423 Otherwise, if a file is unwritable, standard input is a terminal, and
8424 the @option{-f} or @option{--force} option is not given, or the
8425 @option{-i} or @option{--interactive=always} option @emph{is} given,
8426 @command{rm} prompts the user for whether to remove the file.
8427 If the response is not affirmative, the file is skipped.
8429 Any attempt to remove a file whose last file name component is
8430 @file{.} or @file{..} is rejected without any prompting.
8432 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8433 possible to recover the contents of that file. If you want more assurance
8434 that the contents are truly unrecoverable, consider using @command{shred}.
8436 The program accepts the following options. Also see @ref{Common options}.
8444 Ignore nonexistent files and never prompt the user.
8445 Ignore any previous @option{--interactive} (@option{-i}) option.
8449 Prompt whether to remove each file.
8450 If the response is not affirmative, the file is skipped.
8451 Ignore any previous @option{--force} (@option{-f}) option.
8452 Equivalent to @option{--interactive=always}.
8456 Prompt once whether to proceed with the command, if more than three
8457 files are named or if a recursive removal is requested. Ignore any
8458 previous @option{--force} (@option{-f}) option. Equivalent to
8459 @option{--interactive=once}.
8461 @itemx --interactive [=@var{when}]
8462 @opindex --interactive
8463 Specify when to issue an interactive prompt. @var{when} may be
8467 @vindex never @r{interactive option}
8468 - Do not prompt at all.
8470 @vindex once @r{interactive option}
8471 - Prompt once if more than three files are named or if a recursive
8472 removal is requested. Equivalent to @option{-I}.
8474 @vindex always @r{interactive option}
8475 - Prompt for every file being removed. Equivalent to @option{-i}.
8477 @option{--interactive} with no @var{when} is equivalent to
8478 @option{--interactive=always}.
8480 @itemx --one-file-system
8481 @opindex --one-file-system
8482 @cindex one file system, restricting @command{rm} to
8483 When removing a hierarchy recursively, skip any directory that is on a
8484 file system different from that of the corresponding command line argument.
8486 This option is useful when removing a build ``chroot'' hierarchy,
8487 which normally contains no valuable data. However, it is not uncommon
8488 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8489 use one's start-up file. The catch is that it's easy to forget to
8490 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8491 your normally throw-away chroot, that command will remove everything
8492 under @file{/home}, too.
8493 Use the @option{--one-file-system} option, and it will
8494 warn about and skip directories on other file systems.
8495 Of course, this will not save your @file{/home} if it and your
8496 chroot happen to be on the same file system.
8498 @itemx --preserve-root
8499 @opindex --preserve-root
8500 @cindex root directory, disallow recursive destruction
8501 Fail upon any attempt to remove the root directory, @file{/},
8502 when used with the @option{--recursive} option.
8503 This is the default behavior.
8504 @xref{Treating / specially}.
8506 @itemx --no-preserve-root
8507 @opindex --no-preserve-root
8508 @cindex root directory, allow recursive destruction
8509 Do not treat @file{/} specially when removing recursively.
8510 This option is not recommended unless you really want to
8511 remove all the files on your computer.
8512 @xref{Treating / specially}.
8519 @opindex --recursive
8520 @cindex directories, removing (recursively)
8521 Remove the listed directories and their contents recursively.
8527 Print the name of each file before removing it.
8531 @cindex files beginning with @samp{-}, removing
8532 @cindex @samp{-}, removing files beginning with
8533 One common question is how to remove files whose names begin with a
8534 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8535 function to parse its arguments, lets you use the @samp{--} option to
8536 indicate that all following arguments are non-options. To remove a file
8537 called @file{-f} in the current directory, you could type either:
8550 @opindex - @r{and Unix @command{rm}}
8551 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8552 predates the development of the getopt standard syntax.
8557 @node shred invocation
8558 @section @command{shred}: Remove files more securely
8561 @cindex data, erasing
8562 @cindex erasing data
8564 @command{shred} overwrites devices or files, to help prevent even
8565 very expensive hardware from recovering the data.
8567 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8568 not actually destroyed. Only the index listing where the file is
8569 stored is destroyed, and the storage is made available for reuse.
8570 There are undelete utilities that will attempt to reconstruct the index
8571 and can bring the file back if the parts were not reused.
8573 On a busy system with a nearly-full drive, space can get reused in a few
8574 seconds. But there is no way to know for sure. If you have sensitive
8575 data, you may want to be sure that recovery is not possible by actually
8576 overwriting the file with non-sensitive data.
8578 However, even after doing that, it is possible to take the disk back
8579 to a laboratory and use a lot of sensitive (and expensive) equipment
8580 to look for the faint ``echoes'' of the original data underneath the
8581 overwritten data. If the data has only been overwritten once, it's not
8584 The best way to remove something irretrievably is to destroy the media
8585 it's on with acid, melt it down, or the like. For cheap removable media
8586 like floppy disks, this is the preferred method. However, hard drives
8587 are expensive and hard to melt, so the @command{shred} utility tries
8588 to achieve a similar effect non-destructively.
8590 This uses many overwrite passes, with the data patterns chosen to
8591 maximize the damage they do to the old data. While this will work on
8592 floppies, the patterns are designed for best effect on hard drives.
8593 For more details, see the source code and Peter Gutmann's paper
8594 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8595 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8596 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8597 California, July 22--25, 1996).
8599 @strong{Please note} that @command{shred} relies on a very important assumption:
8600 that the file system overwrites data in place. This is the traditional
8601 way to do things, but many modern file system designs do not satisfy this
8602 assumption. Exceptions include:
8607 Log-structured or journaled file systems, such as those supplied with
8608 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8609 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8612 File systems that write redundant data and carry on even if some writes
8613 fail, such as RAID-based file systems.
8616 File systems that make snapshots, such as Network Appliance's NFS server.
8619 File systems that cache in temporary locations, such as NFS version 3
8623 Compressed file systems.
8626 In the particular case of ext3 file systems, the above disclaimer applies (and
8627 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8628 mode, which journals file data in addition to just metadata. In both
8629 the @code{data=ordered} (default) and @code{data=writeback} modes,
8630 @command{shred} works as usual. Ext3 journaling modes can be changed
8631 by adding the @code{data=something} option to the mount options for a
8632 particular file system in the @file{/etc/fstab} file, as documented in
8633 the mount man page (man mount).
8635 If you are not sure how your file system operates, then you should assume
8636 that it does not overwrite data in place, which means that shred cannot
8637 reliably operate on regular files in your file system.
8639 Generally speaking, it is more reliable to shred a device than a file,
8640 since this bypasses the problem of file system design mentioned above.
8641 However, even shredding devices is not always completely reliable. For
8642 example, most disks map out bad sectors invisibly to the application; if
8643 the bad sectors contain sensitive data, @command{shred} won't be able to
8646 @command{shred} makes no attempt to detect or report this problem, just as
8647 it makes no attempt to do anything about backups. However, since it is
8648 more reliable to shred devices than files, @command{shred} by default does
8649 not truncate or remove the output file. This default is more suitable
8650 for devices, which typically cannot be truncated and should not be
8653 Finally, consider the risk of backups and mirrors.
8654 File system backups and remote mirrors may contain copies of the
8655 file that cannot be removed, and that will allow a shredded file
8656 to be recovered later. So if you keep any data you may later want
8657 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8660 shred [@var{option}]@dots{} @var{file}[@dots{}]
8663 The program accepts the following options. Also see @ref{Common options}.
8671 @cindex force deletion
8672 Override file permissions if necessary to allow overwriting.
8675 @itemx -n @var{number}
8676 @itemx --iterations=@var{number}
8677 @opindex -n @var{number}
8678 @opindex --iterations=@var{number}
8679 @cindex iterations, selecting the number of
8680 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8681 overwrite. You can reduce this to save time, or increase it if you think it's
8682 appropriate. After 25 passes all of the internal overwrite patterns will have
8683 been used at least once.
8685 @item --random-source=@var{file}
8686 @opindex --random-source
8687 @cindex random source for shredding
8688 Use @var{file} as a source of random data used to overwrite and to
8689 choose pass ordering. @xref{Random sources}.
8691 @item -s @var{bytes}
8692 @itemx --size=@var{bytes}
8693 @opindex -s @var{bytes}
8694 @opindex --size=@var{bytes}
8695 @cindex size of file to shred
8696 Shred the first @var{bytes} bytes of the file. The default is to shred
8697 the whole file. @var{bytes} can be followed by a size specification like
8698 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8704 @cindex removing files after shredding
8705 After shredding a file, truncate it (if possible) and then remove it.
8706 If a file has multiple links, only the named links will be removed.
8712 Display to standard error all status updates as sterilization proceeds.
8718 By default, @command{shred} rounds the size of a regular file up to the next
8719 multiple of the file system block size to fully erase the last block of the file.
8720 Use @option{--exact} to suppress that behavior.
8721 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8722 blocks, the resulting file will be 512 bytes long. With this option,
8723 shred does not increase the apparent size of the file.
8729 Normally, the last pass that @command{shred} writes is made up of
8730 random data. If this would be conspicuous on your hard drive (for
8731 example, because it looks like encrypted data), or you just think
8732 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8733 all zero bits. This is in addition to the number of passes specified
8734 by the @option{--iterations} option.
8738 You might use the following command to erase all trace of the
8739 file system you'd created on the floppy disk in your first drive.
8740 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8744 shred --verbose /dev/fd0
8747 Similarly, to erase all data on a selected partition of
8748 your hard disk, you could give a command like this:
8751 shred --verbose /dev/sda5
8754 A @var{file} of @samp{-} denotes standard output.
8755 The intended use of this is to shred a removed temporary file.
8762 echo "Hello, world" >&3
8767 However, the command @samp{shred - >file} does not shred the contents
8768 of @var{file}, since the shell truncates @var{file} before invoking
8769 @command{shred}. Use the command @samp{shred file} or (if using a
8770 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8775 @node Special file types
8776 @chapter Special file types
8778 @cindex special file types
8779 @cindex file types, special
8781 This chapter describes commands which create special types of files (and
8782 @command{rmdir}, which removes directories, one special file type).
8784 @cindex special file types
8786 Although Unix-like operating systems have markedly fewer special file
8787 types than others, not @emph{everything} can be treated only as the
8788 undifferentiated byte stream of @dfn{normal files}. For example, when a
8789 file is created or removed, the system must record this information,
8790 which it does in a @dfn{directory}---a special type of file. Although
8791 you can read directories as normal files, if you're curious, in order
8792 for the system to do its job it must impose a structure, a certain
8793 order, on the bytes of the file. Thus it is a ``special'' type of file.
8795 Besides directories, other special file types include named pipes
8796 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8799 * link invocation:: Make a hard link via the link syscall
8800 * ln invocation:: Make links between files.
8801 * mkdir invocation:: Make directories.
8802 * mkfifo invocation:: Make FIFOs (named pipes).
8803 * mknod invocation:: Make block or character special files.
8804 * readlink invocation:: Print value of a symlink or canonical file name.
8805 * rmdir invocation:: Remove empty directories.
8806 * unlink invocation:: Remove files via the unlink syscall
8810 @node link invocation
8811 @section @command{link}: Make a hard link via the link syscall
8814 @cindex links, creating
8815 @cindex hard links, creating
8816 @cindex creating links (hard only)
8818 @command{link} creates a single hard link at a time.
8819 It is a minimalist interface to the system-provided
8820 @code{link} function. @xref{Hard Links, , , libc,
8821 The GNU C Library Reference Manual}.
8822 It avoids the bells and whistles of the more commonly-used
8823 @command{ln} command (@pxref{ln invocation}).
8827 link @var{filename} @var{linkname}
8830 @var{filename} must specify an existing file, and @var{linkname}
8831 must specify a nonexistent entry in an existing directory.
8832 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8835 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8836 --no-target-directory @var{filename} @var{linkname}}. However, the
8837 @option{--directory} and @option{--no-target-directory} options are
8838 not specified by @acronym{POSIX}, and the @command{link} command is
8839 more portable in practice.
8841 If @var{filename} is a symbolic link, it is unspecified whether
8842 @var{linkname} will be a hard link to the symbolic link or to the
8843 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8844 to specify which behavior is desired.
8850 @section @command{ln}: Make links between files
8853 @cindex links, creating
8854 @cindex hard links, creating
8855 @cindex symbolic (soft) links, creating
8856 @cindex creating links (hard or soft)
8858 @cindex file systems and hard links
8859 @command{ln} makes links between files. By default, it makes hard links;
8860 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8864 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8865 ln [@var{option}]@dots{} @var{target}
8866 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8867 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
8873 If two file names are given, @command{ln} creates a link to the first
8874 file from the second.
8877 If one @var{target} is given, @command{ln} creates a link to that file
8878 in the current directory.
8881 If the @option{--target-directory} (@option{-t}) option is given, or
8882 failing that if the last file is a directory and the
8883 @option{--no-target-directory} (@option{-T}) option is not given,
8884 @command{ln} creates a link to each @var{target} file in the specified
8885 directory, using the @var{target}s' names.
8889 Normally @command{ln} does not remove existing files. Use the
8890 @option{--force} (@option{-f}) option to remove them unconditionally,
8891 the @option{--interactive} (@option{-i}) option to remove them
8892 conditionally, and the @option{--backup} (@option{-b}) option to
8895 @cindex hard link, defined
8896 @cindex inode, and hard links
8897 A @dfn{hard link} is another name for an existing file; the link and the
8898 original are indistinguishable. Technically speaking, they share the
8899 same inode, and the inode contains all the information about a
8900 file---indeed, it is not incorrect to say that the inode @emph{is} the
8901 file. Most systems prohibit making a hard link to
8902 a directory; on those where it is allowed, only the super-user can do
8903 so (and with caution, since creating a cycle will cause problems to many
8904 other utilities). Hard links cannot cross file system boundaries. (These
8905 restrictions are not mandated by @acronym{POSIX}, however.)
8907 @cindex dereferencing symbolic links
8908 @cindex symbolic link, defined
8909 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
8910 a special file type (which not all kernels support: System V release 3
8911 (and older) systems lack symlinks) in which the link file actually
8912 refers to a different file, by name. When most operations (opening,
8913 reading, writing, and so on) are passed the symbolic link file, the
8914 kernel automatically @dfn{dereferences} the link and operates on the
8915 target of the link. But some operations (e.g., removing) work on the
8916 link file itself, rather than on its target. The owner and group of a
8917 symlink are not significant to file access performed through
8918 the link, but do have implications on deleting a symbolic link from a
8919 directory with the restricted deletion bit set. On the GNU system,
8920 the mode of a symlink has no significance and cannot be changed, but
8921 on some BSD systems, the mode can be changed and will affect whether
8922 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
8923 libc, The GNU C Library Reference Manual}.
8925 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
8926 occurs when the string in the symlink does not resolve to a file.
8927 There are no restrictions against creating dangling symbolic links.
8928 There are trade-offs to using absolute or relative symlinks. An
8929 absolute symlink always points to the same file, even if the directory
8930 containing the link is moved. However, if the symlink is visible from
8931 more than one machine (such as on a networked file system), the file
8932 pointed to might not always be the same. A relative symbolic link is
8933 resolved in relation to the directory that contains the link, and is
8934 often useful in referring to files on the same device without regards
8935 to what name that device is mounted on when accessed via networked
8938 When creating a relative symlink in a different location than the
8939 current directory, the resolution of the symlink will be different
8940 than the resolution of the same string from the current directory.
8941 Therefore, many users prefer to first change directories to the
8942 location where the relative symlink will be created, so that
8943 tab-completion or other file resolution will find the same target as
8944 what will be placed in the symlink.
8946 The program accepts the following options. Also see @ref{Common options}.
8957 @opindex --directory
8958 @cindex hard links to directories
8959 Allow users with appropriate privileges to attempt to make hard links
8961 However, note that this will probably fail due to
8962 system restrictions, even for the super-user.
8968 Remove existing destination files.
8971 @itemx --interactive
8973 @opindex --interactive
8974 @cindex prompting, and @command{ln}
8975 Prompt whether to remove existing destination files.
8981 If @option{-s} is not in effect, and the source file is a symbolic
8982 link, create the hard link to the file referred to by the symbolic
8983 link, rather than the symbolic link itself.
8986 @itemx --no-dereference
8988 @opindex --no-dereference
8989 Do not treat the last operand specially when it is a symbolic link to
8990 a directory. Instead, treat it as if it were a normal file.
8992 When the destination is an actual directory (not a symlink to one),
8993 there is no ambiguity. The link is created in that directory.
8994 But when the specified destination is a symlink to a directory,
8995 there are two ways to treat the user's request. @command{ln} can
8996 treat the destination just as it would a normal directory and create
8997 the link in it. On the other hand, the destination can be viewed as a
8998 non-directory---as the symlink itself. In that case, @command{ln}
8999 must delete or backup that symlink before creating the new link.
9000 The default is to treat a destination that is a symlink to a directory
9001 just like a directory.
9003 This option is weaker than the @option{--no-target-directory}
9004 (@option{-T}) option, so it has no effect if both options are given.
9010 If @option{-s} is not in effect, and the source file is a symbolic
9011 link, create the hard link to the symbolic link itself. On platforms
9012 where this is not supported by the kernel, this option creates a
9013 symbolic link with identical contents; since symbolic link contents
9014 cannot be edited, any file name resolution performed through either
9015 link will be the same as if a hard link had been created.
9021 Make symbolic links instead of hard links. This option merely produces
9022 an error message on systems that do not support symbolic links.
9028 @optNoTargetDirectory
9034 Print the name of each file after linking it successfully.
9038 @cindex hard links to symbolic links
9039 @cindex symbolic links and @command{ln}
9040 If @option{-L} and @option{-P} are both given, the last one takes
9041 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9042 are silently ignored. If neither option is given, then this
9043 implementation defaults to @option{-P} if the system @code{link} supports
9044 hard links to symbolic links (such as the GNU system), and @option{-L}
9045 if @code{link} follows symbolic links (such as on BSD).
9054 # Create link ../a pointing to a in that directory.
9055 # Not really useful because it points to itself.
9060 # Change to the target before creating symlinks to avoid being confused.
9066 # Hard coded file names don't move well.
9067 ln -s $(pwd)/a /some/dir/
9071 # Relative file names survive directory moves and also
9072 # work across networked file systems.
9073 ln -s afile anotherfile
9074 ln -s ../adir/afile yetanotherfile
9078 @node mkdir invocation
9079 @section @command{mkdir}: Make directories
9082 @cindex directories, creating
9083 @cindex creating directories
9085 @command{mkdir} creates directories with the specified names. Synopsis:
9088 mkdir [@var{option}]@dots{} @var{name}@dots{}
9091 @command{mkdir} creates each directory @var{name} in the order given.
9092 It reports an error if @var{name} already exists, unless the
9093 @option{-p} option is given and @var{name} is a directory.
9095 The program accepts the following options. Also see @ref{Common options}.
9100 @itemx --mode=@var{mode}
9103 @cindex modes of created directories, setting
9104 Set the file permission bits of created directories to @var{mode},
9105 which uses the same syntax as
9106 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9107 everyone) for the point of the departure. @xref{File permissions}.
9109 Normally the directory has the desired file mode bits at the moment it
9110 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9111 special mode bits, but in this case there may be a temporary window
9112 during which the directory exists but its special mode bits are
9113 incorrect. @xref{Directory Setuid and Setgid}, for how the
9114 set-user-ID and set-group-ID bits of directories are inherited unless
9115 overridden in this way.
9121 @cindex parent directories, creating
9122 Make any missing parent directories for each argument, setting their
9123 file permission bits to the umask modified by @samp{u+wx}. Ignore
9124 existing parent directories, and do not change their file permission
9127 To set the file permission bits of any newly-created parent
9128 directories to a value that includes @samp{u+wx}, you can set the
9129 umask before invoking @command{mkdir}. For example, if the shell
9130 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9131 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9132 To set a parent's special mode bits as well, you can invoke
9133 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9134 Setgid}, for how the set-user-ID and set-group-ID bits of
9135 newly-created parent directories are inherited.
9141 Print a message for each created directory. This is most useful with
9144 @item -Z @var{context}
9145 @itemx --context=@var{context}
9149 @cindex security context
9150 Set the default SELinux security context to be used for created directories.
9157 @node mkfifo invocation
9158 @section @command{mkfifo}: Make FIFOs (named pipes)
9161 @cindex FIFOs, creating
9162 @cindex named pipes, creating
9163 @cindex creating FIFOs (named pipes)
9165 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9166 specified names. Synopsis:
9169 mkfifo [@var{option}] @var{name}@dots{}
9172 A @dfn{FIFO} is a special file type that permits independent processes
9173 to communicate. One process opens the FIFO file for writing, and
9174 another for reading, after which data can flow as with the usual
9175 anonymous pipe in shells or elsewhere.
9177 The program accepts the following option. Also see @ref{Common options}.
9182 @itemx --mode=@var{mode}
9185 @cindex modes of created FIFOs, setting
9186 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9187 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9188 for the point of departure. @var{mode} should specify only file
9189 permission bits. @xref{File permissions}.
9191 @item -Z @var{context}
9192 @itemx --context=@var{context}
9196 @cindex security context
9197 Set the default SELinux security context to be used for created FIFOs.
9204 @node mknod invocation
9205 @section @command{mknod}: Make block or character special files
9208 @cindex block special files, creating
9209 @cindex character special files, creating
9211 @command{mknod} creates a FIFO, character special file, or block special
9212 file with the specified name. Synopsis:
9215 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9218 @cindex special files
9219 @cindex block special files
9220 @cindex character special files
9221 Unlike the phrase ``special file type'' above, the term @dfn{special
9222 file} has a technical meaning on Unix: something that can generate or
9223 receive data. Usually this corresponds to a physical piece of hardware,
9224 e.g., a printer or a disk. (These files are typically created at
9225 system-configuration time.) The @command{mknod} command is what creates
9226 files of this type. Such devices can be read either a character at a
9227 time or a ``block'' (many characters) at a time, hence we say there are
9228 @dfn{block special} files and @dfn{character special} files.
9230 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9231 @mayConflictWithShellBuiltIn{mknod}
9233 The arguments after @var{name} specify the type of file to make:
9238 @opindex p @r{for FIFO file}
9242 @opindex b @r{for block special file}
9243 for a block special file
9246 @c Don't document the `u' option -- it's just a synonym for `c'.
9247 @c Do *any* versions of mknod still use it?
9249 @opindex c @r{for character special file}
9250 @c @opindex u @r{for character special file}
9251 for a character special file
9255 When making a block or character special file, the major and minor
9256 device numbers must be given after the file type.
9257 If a major or minor device number begins with @samp{0x} or @samp{0X},
9258 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9259 as octal; otherwise, as decimal.
9261 The program accepts the following option. Also see @ref{Common options}.
9266 @itemx --mode=@var{mode}
9269 Set the mode of created files to @var{mode}, which is symbolic as in
9270 @command{chmod} and uses @samp{a=rw} as the point of departure.
9271 @var{mode} should specify only file permission bits.
9272 @xref{File permissions}.
9274 @item -Z @var{context}
9275 @itemx --context=@var{context}
9279 @cindex security context
9280 Set the default SELinux security context to be used for created files.
9287 @node readlink invocation
9288 @section @command{readlink}: Print value of a symlink or canonical file name
9291 @cindex displaying value of a symbolic link
9292 @cindex canonical file name
9293 @cindex canonicalize a file name
9297 @command{readlink} may work in one of two supported modes:
9303 @command{readlink} outputs the value of the given symbolic link.
9304 If @command{readlink} is invoked with an argument other than the name
9305 of a symbolic link, it produces no output and exits with a nonzero exit code.
9307 @item Canonicalize mode
9309 @command{readlink} outputs the absolute name of the given file which contains
9310 no @file{.}, @file{..} components nor any repeated separators
9311 (@file{/}) or symbolic links.
9316 readlink [@var{option}] @var{file}
9319 By default, @command{readlink} operates in readlink mode.
9321 The program accepts the following options. Also see @ref{Common options}.
9326 @itemx --canonicalize
9328 @opindex --canonicalize
9329 Activate canonicalize mode.
9330 If any component of the file name except the last one is missing or unavailable,
9331 @command{readlink} produces no output and exits with a nonzero exit
9332 code. A trailing slash is ignored.
9335 @itemx --canonicalize-existing
9337 @opindex --canonicalize-existing
9338 Activate canonicalize mode.
9339 If any component is missing or unavailable, @command{readlink} produces
9340 no output and exits with a nonzero exit code. A trailing slash
9341 requires that the name resolve to a directory.
9344 @itemx --canonicalize-missing
9346 @opindex --canonicalize-missing
9347 Activate canonicalize mode.
9348 If any component is missing or unavailable, @command{readlink} treats it
9354 @opindex --no-newline
9355 Do not output the trailing newline.
9365 Suppress most error messages.
9371 Report error messages.
9375 The @command{readlink} utility first appeared in OpenBSD 2.1.
9377 There is a @command{realpath} command on some systems
9378 which operates like @command{readlink} in canonicalize mode.
9383 @node rmdir invocation
9384 @section @command{rmdir}: Remove empty directories
9387 @cindex removing empty directories
9388 @cindex directories, removing empty
9390 @command{rmdir} removes empty directories. Synopsis:
9393 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9396 If any @var{directory} argument does not refer to an existing empty
9397 directory, it is an error.
9399 The program accepts the following options. Also see @ref{Common options}.
9403 @item --ignore-fail-on-non-empty
9404 @opindex --ignore-fail-on-non-empty
9405 @cindex directory deletion, ignoring failures
9406 Ignore each failure to remove a directory that is solely because
9407 the directory is non-empty.
9413 @cindex parent directories, removing
9414 Remove @var{directory}, then try to remove each component of @var{directory}.
9415 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9416 As such, it fails if any of those directories turns out not to be empty.
9417 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9418 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9419 exit unsuccessfully.
9425 @cindex directory deletion, reporting
9426 Give a diagnostic for each successful removal.
9427 @var{directory} is removed.
9431 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9436 @node unlink invocation
9437 @section @command{unlink}: Remove files via the unlink syscall
9440 @cindex removing files or directories (via the unlink syscall)
9442 @command{unlink} deletes a single specified file name.
9443 It is a minimalist interface to the system-provided
9444 @code{unlink} function. @xref{Deleting Files, , , libc,
9445 The GNU C Library Reference Manual}. Synopsis:
9446 It avoids the bells and whistles of the more commonly-used
9447 @command{rm} command (@pxref{rm invocation}).
9450 unlink @var{filename}
9453 On some systems @code{unlink} can be used to delete the name of a
9454 directory. On others, it can be used that way only by a privileged user.
9455 In the GNU system @code{unlink} can never delete the name of a directory.
9457 The @command{unlink} command honors the @option{--help} and
9458 @option{--version} options. To remove a file whose name begins with
9459 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9464 @node Changing file attributes
9465 @chapter Changing file attributes
9467 @cindex changing file attributes
9468 @cindex file attributes, changing
9469 @cindex attributes, file
9471 A file is not merely its contents, a name, and a file type
9472 (@pxref{Special file types}). A file also has an owner (a user ID), a
9473 group (a group ID), permissions (what the owner can do with the file,
9474 what people in the group can do, and what everyone else can do), various
9475 timestamps, and other information. Collectively, we call these a file's
9478 These commands change file attributes.
9481 * chgrp invocation:: Change file groups.
9482 * chmod invocation:: Change access permissions.
9483 * chown invocation:: Change file owners and groups.
9484 * touch invocation:: Change file timestamps.
9488 @node chown invocation
9489 @section @command{chown}: Change file owner and group
9492 @cindex file ownership, changing
9493 @cindex group ownership, changing
9494 @cindex changing file ownership
9495 @cindex changing group ownership
9497 @command{chown} changes the user and/or group ownership of each given @var{file}
9498 to @var{new-owner} or to the user and group of an existing reference file.
9502 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9505 If used, @var{new-owner} specifies the new owner and/or group as follows
9506 (with no embedded white space):
9509 [@var{owner}] [ : [@var{group}] ]
9516 If only an @var{owner} (a user name or numeric user ID) is given, that
9517 user is made the owner of each given file, and the files' group is not
9520 @item owner@samp{:}group
9521 If the @var{owner} is followed by a colon and a @var{group} (a
9522 group name or numeric group ID), with no spaces between them, the group
9523 ownership of the files is changed as well (to @var{group}).
9526 If a colon but no group name follows @var{owner}, that user is
9527 made the owner of the files and the group of the files is changed to
9528 @var{owner}'s login group.
9531 If the colon and following @var{group} are given, but the owner
9532 is omitted, only the group of the files is changed; in this case,
9533 @command{chown} performs the same function as @command{chgrp}.
9536 If only a colon is given, or if @var{new-owner} is empty, neither the
9537 owner nor the group is changed.
9541 If @var{owner} or @var{group} is intended to represent a numeric user
9542 or group ID, then you may specify it with a leading @samp{+}.
9543 @xref{Disambiguating names and IDs}.
9545 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9546 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9547 require support for that, but for backward compatibility @acronym{GNU}
9548 @command{chown} supports @samp{.} so long as no ambiguity results.
9549 New scripts should avoid the use of @samp{.} because it is not
9550 portable, and because it has undesirable results if the entire
9551 @var{owner@samp{.}group} happens to identify a user whose name
9554 The @command{chown} command sometimes clears the set-user-ID or
9555 set-group-ID permission bits. This behavior depends on the policy and
9556 functionality of the underlying @code{chown} system call, which may
9557 make system-dependent file mode modifications outside the control of
9558 the @command{chown} command. For example, the @command{chown} command
9559 might not affect those bits when invoked by a user with appropriate
9560 privileges, or when the
9561 bits signify some function other than executable permission (e.g.,
9563 When in doubt, check the underlying system behavior.
9565 The program accepts the following options. Also see @ref{Common options}.
9573 @cindex changed owners, verbosely describing
9574 Verbosely describe the action for each @var{file} whose ownership
9583 @cindex error messages, omitting
9584 Do not print error messages about files whose ownership cannot be
9587 @itemx @w{@kbd{--from}=@var{old-owner}}
9589 @cindex symbolic links, changing owner
9590 Change a @var{file}'s ownership only if it has current attributes specified
9591 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9593 This option is useful primarily from a security standpoint in that
9594 it narrows considerably the window of potential abuse.
9595 For example, to reflect a user ID numbering change for one user's files
9596 without an option like this, @code{root} might run
9599 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9602 But that is dangerous because the interval between when the @command{find}
9603 tests the existing file's owner and when the @command{chown} is actually run
9605 One way to narrow the gap would be to invoke chown for each file
9609 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9612 But that is very slow if there are many affected files.
9613 With this option, it is safer (the gap is narrower still)
9614 though still not perfect:
9617 chown -h -R --from=OLDUSER NEWUSER /
9621 @opindex --dereference
9622 @cindex symbolic links, changing owner
9624 Do not act on symbolic links themselves but rather on what they point to.
9625 This is the default.
9628 @itemx --no-dereference
9630 @opindex --no-dereference
9631 @cindex symbolic links, changing owner
9633 Act on symbolic links themselves instead of what they point to.
9634 This mode relies on the @code{lchown} system call.
9635 On systems that do not provide the @code{lchown} system call,
9636 @command{chown} fails when a file specified on the command line
9638 By default, no diagnostic is issued for symbolic links encountered
9639 during a recursive traversal, but see @option{--verbose}.
9641 @itemx --preserve-root
9642 @opindex --preserve-root
9643 @cindex root directory, disallow recursive modification
9644 Fail upon any attempt to recursively change the root directory, @file{/}.
9645 Without @option{--recursive}, this option has no effect.
9646 @xref{Treating / specially}.
9648 @itemx --no-preserve-root
9649 @opindex --no-preserve-root
9650 @cindex root directory, allow recursive modification
9651 Cancel the effect of any preceding @option{--preserve-root} option.
9652 @xref{Treating / specially}.
9654 @item --reference=@var{ref_file}
9655 @opindex --reference
9656 Change the user and group of each @var{file} to be the same as those of
9657 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9658 user and group of the symbolic link, but rather those of the file it
9665 Output a diagnostic for every file processed.
9666 If a symbolic link is encountered during a recursive traversal
9667 on a system without the @code{lchown} system call, and @option{--no-dereference}
9668 is in effect, then issue a diagnostic saying neither the symbolic link nor
9669 its referent is being changed.
9674 @opindex --recursive
9675 @cindex recursively changing file ownership
9676 Recursively change ownership of directories and their contents.
9679 @xref{Traversing symlinks}.
9682 @xref{Traversing symlinks}.
9685 @xref{Traversing symlinks}.
9694 # Change the owner of /u to "root".
9697 # Likewise, but also change its group to "staff".
9700 # Change the owner of /u and subfiles to "root".
9705 @node chgrp invocation
9706 @section @command{chgrp}: Change group ownership
9709 @cindex group ownership, changing
9710 @cindex changing group ownership
9712 @command{chgrp} changes the group ownership of each given @var{file}
9713 to @var{group} (which can be either a group name or a numeric group ID)
9714 or to the group of an existing reference file. Synopsis:
9717 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9720 If @var{group} is intended to represent a
9721 numeric group ID, then you may specify it with a leading @samp{+}.
9722 @xref{Disambiguating names and IDs}.
9724 The program accepts the following options. Also see @ref{Common options}.
9732 @cindex changed files, verbosely describing
9733 Verbosely describe the action for each @var{file} whose group actually
9742 @cindex error messages, omitting
9743 Do not print error messages about files whose group cannot be
9747 @opindex --dereference
9748 @cindex symbolic links, changing owner
9750 Do not act on symbolic links themselves but rather on what they point to.
9751 This is the default.
9754 @itemx --no-dereference
9756 @opindex --no-dereference
9757 @cindex symbolic links, changing group
9759 Act on symbolic links themselves instead of what they point to.
9760 This mode relies on the @code{lchown} system call.
9761 On systems that do not provide the @code{lchown} system call,
9762 @command{chgrp} fails when a file specified on the command line
9764 By default, no diagnostic is issued for symbolic links encountered
9765 during a recursive traversal, but see @option{--verbose}.
9767 @itemx --preserve-root
9768 @opindex --preserve-root
9769 @cindex root directory, disallow recursive modification
9770 Fail upon any attempt to recursively change the root directory, @file{/}.
9771 Without @option{--recursive}, this option has no effect.
9772 @xref{Treating / specially}.
9774 @itemx --no-preserve-root
9775 @opindex --no-preserve-root
9776 @cindex root directory, allow recursive modification
9777 Cancel the effect of any preceding @option{--preserve-root} option.
9778 @xref{Treating / specially}.
9780 @item --reference=@var{ref_file}
9781 @opindex --reference
9782 Change the group of each @var{file} to be the same as that of
9783 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9784 group of the symbolic link, but rather that of the file it refers to.
9790 Output a diagnostic for every file processed.
9791 If a symbolic link is encountered during a recursive traversal
9792 on a system without the @code{lchown} system call, and @option{--no-dereference}
9793 is in effect, then issue a diagnostic saying neither the symbolic link nor
9794 its referent is being changed.
9799 @opindex --recursive
9800 @cindex recursively changing group ownership
9801 Recursively change the group ownership of directories and their contents.
9804 @xref{Traversing symlinks}.
9807 @xref{Traversing symlinks}.
9810 @xref{Traversing symlinks}.
9819 # Change the group of /u to "staff".
9822 # Change the group of /u and subfiles to "staff".
9827 @node chmod invocation
9828 @section @command{chmod}: Change access permissions
9831 @cindex changing access permissions
9832 @cindex access permissions, changing
9833 @cindex permissions, changing access
9835 @command{chmod} changes the access permissions of the named files. Synopsis:
9838 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9841 @cindex symbolic links, permissions of
9842 @command{chmod} never changes the permissions of symbolic links, since
9843 the @command{chmod} system call cannot change their permissions.
9844 This is not a problem since the permissions of symbolic links are
9845 never used. However, for each symbolic link listed on the command
9846 line, @command{chmod} changes the permissions of the pointed-to file.
9847 In contrast, @command{chmod} ignores symbolic links encountered during
9848 recursive directory traversals.
9850 A successful use of @command{chmod} clears the set-group-ID bit of a
9851 regular file if the file's group ID does not match the user's
9852 effective group ID or one of the user's supplementary group IDs,
9853 unless the user has appropriate privileges. Additional restrictions
9854 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9855 @var{ref_file} to be ignored. This behavior depends on the policy and
9856 functionality of the underlying @code{chmod} system call. When in
9857 doubt, check the underlying system behavior.
9859 If used, @var{mode} specifies the new file mode bits.
9860 For details, see the section on @ref{File permissions}.
9861 If you really want @var{mode} to have a leading @samp{-}, you should
9862 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9863 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9864 file} (without the @option{--}) complains if it behaves differently
9865 from what @samp{chmod a-w file} would do.
9867 The program accepts the following options. Also see @ref{Common options}.
9875 Verbosely describe the action for each @var{file} whose permissions
9884 @cindex error messages, omitting
9885 Do not print error messages about files whose permissions cannot be
9888 @itemx --preserve-root
9889 @opindex --preserve-root
9890 @cindex root directory, disallow recursive modification
9891 Fail upon any attempt to recursively change the root directory, @file{/}.
9892 Without @option{--recursive}, this option has no effect.
9893 @xref{Treating / specially}.
9895 @itemx --no-preserve-root
9896 @opindex --no-preserve-root
9897 @cindex root directory, allow recursive modification
9898 Cancel the effect of any preceding @option{--preserve-root} option.
9899 @xref{Treating / specially}.
9905 Verbosely describe the action or non-action taken for every @var{file}.
9907 @item --reference=@var{ref_file}
9908 @opindex --reference
9909 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
9910 @xref{File permissions}.
9911 If @var{ref_file} is a symbolic link, do not use the mode
9912 of the symbolic link, but rather that of the file it refers to.
9917 @opindex --recursive
9918 @cindex recursively changing access permissions
9919 Recursively change permissions of directories and their contents.
9926 @node touch invocation
9927 @section @command{touch}: Change file timestamps
9930 @cindex changing file timestamps
9931 @cindex file timestamps, changing
9932 @cindex timestamps, changing file
9934 @command{touch} changes the access and/or modification times of the
9935 specified files. Synopsis:
9938 touch [@var{option}]@dots{} @var{file}@dots{}
9941 @cindex empty files, creating
9942 Any @var{file} argument that does not exist is created empty, unless
9943 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
9944 (@option{-h}) was in effect.
9946 A @var{file} argument string of @samp{-} is handled specially and
9947 causes @command{touch} to change the times of the file associated with
9950 @cindex permissions, for changing file timestamps
9951 If changing both the access and modification times to the current
9952 time, @command{touch} can change the timestamps for files that the user
9953 running it does not own but has write permission for. Otherwise, the
9954 user must own the files.
9956 Although @command{touch} provides options for changing two of the times---the
9957 times of last access and modification---of a file, there is actually
9958 a standard third one as well: the inode change time. This is often
9959 referred to as a file's @code{ctime}.
9960 The inode change time represents the time when the file's meta-information
9961 last changed. One common example of this is when the permissions of a
9962 file change. Changing the permissions doesn't access the file, so
9963 the atime doesn't change, nor does it modify the file, so the mtime
9964 doesn't change. Yet, something about the file itself has changed,
9965 and this must be noted somewhere. This is the job of the ctime field.
9966 This is necessary, so that, for example, a backup program can make a
9967 fresh copy of the file, including the new permissions value.
9968 Another operation that modifies a file's ctime without affecting
9969 the others is renaming. In any case, it is not possible, in normal
9970 operations, for a user to change the ctime field to a user-specified value.
9971 Some operating systems and file systems support a fourth time: the
9972 birth time, when the file was first created; by definition, this
9973 timestamp never changes.
9976 Time stamps assume the time zone rules specified by the @env{TZ}
9977 environment variable, or by the system default rules if @env{TZ} is
9978 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
9979 libc, The GNU C Library Reference Manual}.
9980 You can avoid ambiguities during
9981 daylight saving transitions by using @sc{utc} time stamps.
9983 The program accepts the following options. Also see @ref{Common options}.
9989 @itemx --time=access
9993 @opindex atime@r{, changing}
9994 @opindex access @r{time, changing}
9995 @opindex use @r{time, changing}
9996 Change the access time only.
10001 @opindex --no-create
10002 Do not warn about or create files that do not exist.
10005 @itemx --date=@var{time}
10009 Use @var{time} instead of the current time. It can contain month names,
10010 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10011 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10012 specifies the instant of time that is 489,392,193 nanoseconds after
10013 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10014 minutes east of @acronym{UTC}. @xref{Date input formats}.
10015 File systems that do not support high-resolution time stamps
10016 silently ignore any excess precision here.
10020 @cindex BSD @command{touch} compatibility
10021 Ignored; for compatibility with BSD versions of @command{touch}.
10024 @itemx --no-dereference
10026 @opindex --no-dereference
10027 @cindex symbolic links, changing time
10029 Attempt to change the timestamps of a symbolic link, rather than what
10030 the link refers to. When using this option, empty files are not
10031 created, but option @option{-c} must also be used to avoid warning
10032 about files that do not exist. Not all systems support changing the
10033 timestamps of symlinks, since underlying system support for this
10034 action was not required until @acronym{POSIX} 2008. Also, on some
10035 systems, the mere act of examining a symbolic link changes the access
10036 time, such that only changes to the modification time will persist
10037 long enough to be observable. When coupled with option @option{-r}, a
10038 reference timestamp is taken from a symbolic link rather than the file
10042 @itemx --time=mtime
10043 @itemx --time=modify
10046 @opindex mtime@r{, changing}
10047 @opindex modify @r{time, changing}
10048 Change the modification time only.
10050 @item -r @var{file}
10051 @itemx --reference=@var{file}
10053 @opindex --reference
10054 Use the times of the reference @var{file} instead of the current time.
10055 If this option is combined with the @option{--date=@var{time}}
10056 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10057 the origin for any relative @var{time}s given, but is otherwise ignored.
10058 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10059 equal to five seconds before the corresponding time stamp for @file{foo}.
10060 If @var{file} is a symbolic link, the reference timestamp is taken
10061 from the target of the symlink, unless @option{-h} was also in effect.
10063 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10064 Use the argument (optional four-digit or two-digit years, months,
10065 days, hours, minutes, optional seconds) instead of the current time.
10066 If the year is specified with only two digits, then @var{cc}
10067 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10068 69 @dots{} 99. If no digits of the year are specified,
10069 the argument is interpreted as a date in the current year.
10070 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10074 @vindex _POSIX2_VERSION
10075 On older systems, @command{touch} supports an obsolete syntax, as follows.
10076 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10077 @option{-t} options, and if there are two or more @var{file}s and the
10078 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10079 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10080 any, were moved to the front), and if the represented year
10081 is in the range 1969--1999, that argument is interpreted as the time
10082 for the other files instead of as a file name.
10083 This obsolete behavior can be enabled or disabled with the
10084 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10085 conformance}), but portable scripts should avoid commands whose
10086 behavior depends on this variable.
10087 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10088 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10094 @chapter Disk usage
10098 No disk can hold an infinite amount of data. These commands report
10099 how much disk storage is in use or available, report other file and
10100 file status information, and write buffers to disk.
10103 * df invocation:: Report file system disk space usage.
10104 * du invocation:: Estimate file space usage.
10105 * stat invocation:: Report file or file system status.
10106 * sync invocation:: Synchronize memory and disk.
10107 * truncate invocation:: Shrink or extend the size of a file.
10111 @node df invocation
10112 @section @command{df}: Report file system disk space usage
10115 @cindex file system disk usage
10116 @cindex disk usage by file system
10118 @command{df} reports the amount of disk space used and available on
10119 file systems. Synopsis:
10122 df [@var{option}]@dots{} [@var{file}]@dots{}
10125 With no arguments, @command{df} reports the space used and available on all
10126 currently mounted file systems (of all types). Otherwise, @command{df}
10127 reports on the file system containing each argument @var{file}.
10129 Normally the disk space is printed in units of
10130 1024 bytes, but this can be overridden (@pxref{Block size}).
10131 Non-integer quantities are rounded up to the next higher unit.
10133 @cindex disk device file
10134 @cindex device file, disk
10135 If an argument @var{file} is a disk device file containing a mounted
10136 file system, @command{df} shows the space available on that file system
10137 rather than on the file system containing the device node (i.e., the root
10138 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10139 on unmounted file systems, because on most kinds of systems doing so
10140 requires extremely nonportable intimate knowledge of file system
10143 The program accepts the following options. Also see @ref{Common options}.
10151 @cindex automounter file systems
10152 @cindex ignore file systems
10153 Include in the listing dummy file systems, which
10154 are omitted by default. Such file systems are typically special-purpose
10155 pseudo-file-systems, such as automounter entries.
10157 @item -B @var{size}
10158 @itemx --block-size=@var{size}
10160 @opindex --block-size
10161 @cindex file system sizes
10162 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10163 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10167 @cindex grand total of disk size, usage and available space
10168 Print a grand total of all arguments after all arguments have
10169 been processed. This can be used to find out the total disk size, usage
10170 and available space of all listed devices.
10176 Equivalent to @option{--si}.
10182 @cindex inode usage
10183 List inode usage information instead of block usage. An inode (short
10184 for index node) contains information about a file such as its owner,
10185 permissions, timestamps, and location on the disk.
10189 @cindex kibibytes for file system sizes
10190 Print sizes in 1024-byte blocks, overriding the default block size
10191 (@pxref{Block size}).
10192 This option is equivalent to @option{--block-size=1K}.
10198 @cindex file system types, limiting output to certain
10199 Limit the listing to local file systems. By default, remote file systems
10204 @cindex file system space, retrieving old data more quickly
10205 Do not invoke the @code{sync} system call before getting any usage data.
10206 This may make @command{df} run significantly faster on systems with many
10207 disks, but on some systems (notably SunOS) the results may be slightly
10208 out of date. This is the default.
10211 @itemx --portability
10213 @opindex --portability
10214 @cindex one-line output format
10215 @cindex @acronym{POSIX} output format
10216 @cindex portable output format
10217 @cindex output format, portable
10218 Use the @acronym{POSIX} output format. This is like the default format except
10223 The information about each file system is always printed on exactly
10224 one line; a mount device is never put on a line by itself. This means
10225 that if the mount device name is more than 20 characters long (e.g., for
10226 some network mounts), the columns are misaligned.
10229 The labels in the header output line are changed to conform to @acronym{POSIX}.
10232 The default block size and output format are unaffected by the
10233 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10234 variables. However, the default block size is still affected by
10235 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10236 otherwise. @xref{Block size}.
10243 @cindex file system space, retrieving current data more slowly
10244 Invoke the @code{sync} system call before getting any usage data. On
10245 some systems (notably SunOS), doing this yields more up to date results,
10246 but in general this option makes @command{df} much slower, especially when
10247 there are many or very busy file systems.
10249 @item -t @var{fstype}
10250 @itemx --type=@var{fstype}
10253 @cindex file system types, limiting output to certain
10254 Limit the listing to file systems of type @var{fstype}. Multiple
10255 file system types can be specified by giving multiple @option{-t} options.
10256 By default, nothing is omitted.
10259 @itemx --print-type
10261 @opindex --print-type
10262 @cindex file system types, printing
10263 Print each file system's type. The types printed here are the same ones
10264 you can include or exclude with @option{-t} and @option{-x}. The particular
10265 types printed are whatever is supported by the system. Here are some of
10266 the common names (this list is certainly not exhaustive):
10271 @cindex @acronym{NFS} file system type
10272 An @acronym{NFS} file system, i.e., one mounted over a network from another
10273 machine. This is the one type name which seems to be used uniformly by
10276 @item 4.2@r{, }ufs@r{, }efs@dots{}
10277 @cindex Linux file system types
10278 @cindex local file system types
10279 @opindex 4.2 @r{file system type}
10280 @opindex ufs @r{file system type}
10281 @opindex efs @r{file system type}
10282 A file system on a locally-mounted hard disk. (The system might even
10283 support more than one type here; Linux does.)
10285 @item hsfs@r{, }cdfs
10286 @cindex CD-ROM file system type
10287 @cindex High Sierra file system
10288 @opindex hsfs @r{file system type}
10289 @opindex cdfs @r{file system type}
10290 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10291 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10294 @cindex PC file system
10295 @cindex DOS file system
10296 @cindex MS-DOS file system
10297 @cindex diskette file system
10299 An MS-DOS file system, usually on a diskette.
10303 @item -x @var{fstype}
10304 @itemx --exclude-type=@var{fstype}
10306 @opindex --exclude-type
10307 Limit the listing to file systems not of type @var{fstype}.
10308 Multiple file system types can be eliminated by giving multiple
10309 @option{-x} options. By default, no file system types are omitted.
10312 Ignored; for compatibility with System V versions of @command{df}.
10317 Failure includes the case where no output is generated, so you can
10318 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10319 @var{dir}} to test whether @var{dir} is on a file system of type
10320 @samp{ext3} or @samp{reiserfs}.
10323 @node du invocation
10324 @section @command{du}: Estimate file space usage
10327 @cindex file space usage
10328 @cindex disk usage for files
10330 @command{du} reports the amount of disk space used by the specified files
10331 and for each subdirectory (of directory arguments). Synopsis:
10334 du [@var{option}]@dots{} [@var{file}]@dots{}
10337 With no arguments, @command{du} reports the disk space for the current
10338 directory. Normally the disk space is printed in units of
10339 1024 bytes, but this can be overridden (@pxref{Block size}).
10340 Non-integer quantities are rounded up to the next higher unit.
10342 If two or more hard links point to the same file, only one of the hard
10343 links is counted. The @var{file} argument order affects which links
10344 are counted, and changing the argument order may change the numbers
10345 that @command{du} outputs.
10347 The program accepts the following options. Also see @ref{Common options}.
10355 Show counts for all files, not just directories.
10357 @itemx --apparent-size
10358 @opindex --apparent-size
10359 Print apparent sizes, rather than disk usage. The apparent size of a
10360 file is the number of bytes reported by @code{wc -c} on regular files,
10361 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10362 For example, a file containing the word @samp{zoo} with no newline would,
10363 of course, have an apparent size of 3. Such a small file may require
10364 anywhere from 0 to 16 KiB or more of disk space, depending on
10365 the type and configuration of the file system on which the file resides.
10366 However, a sparse file created with this command:
10369 dd bs=1 seek=2GiB if=/dev/null of=big
10373 has an apparent size of 2 GiB, yet on most modern
10374 systems, it actually uses almost no disk space.
10380 Equivalent to @code{--apparent-size --block-size=1}.
10382 @item -B @var{size}
10383 @itemx --block-size=@var{size}
10385 @opindex --block-size
10387 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10388 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10394 @cindex grand total of disk space
10395 Print a grand total of all arguments after all arguments have
10396 been processed. This can be used to find out the total disk usage of
10397 a given set of files or directories.
10400 @itemx --dereference-args
10402 @opindex --dereference-args
10403 Dereference symbolic links that are command line arguments.
10404 Does not affect other symbolic links. This is helpful for finding
10405 out the disk usage of directories, such as @file{/usr/tmp}, which
10406 are often symbolic links.
10408 @c --files0-from=FILE
10409 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10415 Equivalent to @option{--dereference-args} (@option{-D}).
10419 @cindex kibibytes for file sizes
10420 Print sizes in 1024-byte blocks, overriding the default block size
10421 (@pxref{Block size}).
10422 This option is equivalent to @option{--block-size=1K}.
10425 @itemx --count-links
10427 @opindex --count-links
10428 @cindex hard links, counting in @command{du}
10429 Count the size of all files, even if they have appeared already (as a
10433 @itemx --dereference
10435 @opindex --dereference
10436 @cindex symbolic links, dereferencing in @command{du}
10437 Dereference symbolic links (show the disk space used by the file
10438 or directory that the link points to instead of the space used by
10443 @cindex mebibytes for file sizes
10444 Print sizes in 1,048,576-byte blocks, overriding the default block size
10445 (@pxref{Block size}).
10446 This option is equivalent to @option{--block-size=1M}.
10449 @itemx --no-dereference
10451 @opindex --no-dereference
10452 @cindex symbolic links, dereferencing in @command{du}
10453 For each symbolic links encountered by @command{du},
10454 consider the disk space used by the symbolic link.
10456 @item -d @var{depth}
10457 @item --max-depth=@var{depth}
10458 @opindex -d @var{depth}
10459 @opindex --max-depth=@var{depth}
10460 @cindex limiting output of @command{du}
10461 Show the total for each directory (and file if --all) that is at
10462 most MAX_DEPTH levels down from the root of the hierarchy. The root
10463 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10472 @opindex --summarize
10473 Display only a total for each argument.
10476 @itemx --separate-dirs
10478 @opindex --separate-dirs
10479 Normally, in the output of @command{du} (when not using @option{--summarize}),
10480 the size listed next to a directory name, @var{d}, represents the sum
10481 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10482 With @option{--separate-dirs}, the size reported for a directory name,
10483 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10488 @cindex last modified dates, displaying in @command{du}
10489 Show time of the most recent modification of any file in the directory,
10490 or any of its subdirectories.
10492 @itemx --time=ctime
10493 @itemx --time=status
10496 @opindex ctime@r{, show the most recent}
10497 @opindex status time@r{, show the most recent}
10498 @opindex use time@r{, show the most recent}
10499 Show the most recent status change time (the @samp{ctime} in the inode) of
10500 any file in the directory, instead of the modification time.
10502 @itemx --time=atime
10503 @itemx --time=access
10505 @opindex atime@r{, show the most recent}
10506 @opindex access time@r{, show the most recent}
10507 Show the most recent access time (the @samp{atime} in the inode) of
10508 any file in the directory, instead of the modification time.
10510 @item --time-style=@var{style}
10511 @opindex --time-style
10513 List timestamps in style @var{style}. This option has an effect only if
10514 the @option{--time} option is also specified. The @var{style} should
10515 be one of the following:
10518 @item +@var{format}
10520 List timestamps using @var{format}, where @var{format} is interpreted
10521 like the format argument of @command{date} (@pxref{date invocation}).
10522 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10523 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10524 with @command{date}, @var{format}'s interpretation is affected by the
10525 @env{LC_TIME} locale category.
10528 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10529 format with nanosecond precision, e.g., @samp{2002-03-30
10530 23:45:56.477817180 -0700}. This style is equivalent to
10531 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10534 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10535 @samp{2002-03-30 23:45}. These timestamps are shorter than
10536 @samp{full-iso} timestamps, and are usually good enough for everyday
10537 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10540 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10541 This style is equivalent to @samp{+%Y-%m-%d}.
10545 You can specify the default value of the @option{--time-style} option
10546 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10547 the default style is @samp{long-iso}. For compatibility with @command{ls},
10548 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10549 the newline and any later characters are ignored; if @env{TIME_STYLE}
10550 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10551 @env{TIME_STYLE} is @samp{locale} it is ignored.
10554 @itemx --one-file-system
10556 @opindex --one-file-system
10557 @cindex one file system, restricting @command{du} to
10558 Skip directories that are on different file systems from the one that
10559 the argument being processed is on.
10561 @item --exclude=@var{pattern}
10562 @opindex --exclude=@var{pattern}
10563 @cindex excluding files from @command{du}
10564 When recursing, skip subdirectories or files matching @var{pattern}.
10565 For example, @code{du --exclude='*.o'} excludes files whose names
10568 @item -X @var{file}
10569 @itemx --exclude-from=@var{file}
10570 @opindex -X @var{file}
10571 @opindex --exclude-from=@var{file}
10572 @cindex excluding files from @command{du}
10573 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10574 one per line. If @var{file} is @samp{-}, take the patterns from standard
10579 @cindex NFS mounts from BSD to HP-UX
10580 On BSD systems, @command{du} reports sizes that are half the correct
10581 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10582 systems, it reports sizes that are twice the correct values for
10583 files that are NFS-mounted from BSD systems. This is due to a flaw
10584 in HP-UX; it also affects the HP-UX @command{du} program.
10589 @node stat invocation
10590 @section @command{stat}: Report file or file system status
10593 @cindex file status
10594 @cindex file system status
10596 @command{stat} displays information about the specified file(s). Synopsis:
10599 stat [@var{option}]@dots{} [@var{file}]@dots{}
10602 With no option, @command{stat} reports all information about the given files.
10603 But it also can be used to report the information of the file systems the
10604 given files are located on. If the files are links, @command{stat} can
10605 also give information about the files the links point to.
10607 @mayConflictWithShellBuiltIn{stat}
10612 @itemx --dereference
10614 @opindex --dereference
10615 @cindex symbolic links, dereferencing in @command{stat}
10616 Change how @command{stat} treats symbolic links.
10617 With this option, @command{stat} acts on the file referenced
10618 by each symbolic link argument.
10619 Without it, @command{stat} acts on any symbolic link argument directly.
10622 @itemx --file-system
10624 @opindex --file-system
10625 @cindex file systems
10626 Report information about the file systems where the given files are located
10627 instead of information about the files themselves.
10630 @itemx --format=@var{format}
10632 @opindex --format=@var{format}
10633 @cindex output format
10634 Use @var{format} rather than the default format.
10635 @var{format} is automatically newline-terminated, so
10636 running a command like the following with two or more @var{file}
10637 operands produces a line of output for each operand:
10639 $ stat --format=%d:%i / /usr
10644 @itemx --printf=@var{format}
10645 @opindex --printf=@var{format}
10646 @cindex output format
10647 Use @var{format} rather than the default format.
10648 Like @option{--format}, but interpret backslash escapes,
10649 and do not output a mandatory trailing newline.
10650 If you want a newline, include @samp{\n} in the @var{format}.
10651 Here's how you would use @option{--printf} to print the device
10652 and inode numbers of @file{/} and @file{/usr}:
10654 $ stat --printf='%d:%i\n' / /usr
10663 @cindex terse output
10664 Print the information in terse form, suitable for parsing by other programs.
10668 The valid @var{format} directives for files with @option{--format} and
10669 @option{--printf} are:
10672 @item %a - Access rights in octal
10673 @item %A - Access rights in human readable form
10674 @item %b - Number of blocks allocated (see @samp{%B})
10675 @item %B - The size in bytes of each block reported by @samp{%b}
10676 @item %d - Device number in decimal
10677 @item %D - Device number in hex
10678 @item %f - Raw mode in hex
10679 @item %F - File type
10680 @item %g - Group ID of owner
10681 @item %G - Group name of owner
10682 @item %h - Number of hard links
10683 @item %i - Inode number
10684 @item %n - File name
10685 @item %N - Quoted file name with dereference if symbolic link
10686 @item %o - I/O block size
10687 @item %s - Total size, in bytes
10688 @item %t - Major device type in hex
10689 @item %T - Minor device type in hex
10690 @item %u - User ID of owner
10691 @item %U - User name of owner
10692 @item %x - Time of last access
10693 @item %X - Time of last access as seconds since Epoch
10694 @item %y - Time of last modification
10695 @item %Y - Time of last modification as seconds since Epoch
10696 @item %z - Time of last change
10697 @item %Z - Time of last change as seconds since Epoch
10700 When listing file system information (@option{--file-system} (@option{-f})),
10701 you must use a different set of @var{format} directives:
10704 @item %a - Free blocks available to non-super-user
10705 @item %b - Total data blocks in file system
10706 @item %c - Total file nodes in file system
10707 @item %d - Free file nodes in file system
10708 @item %f - Free blocks in file system
10709 @item %i - File System ID in hex
10710 @item %l - Maximum length of file names
10711 @item %n - File name
10712 @item %s - Block size (for faster transfers)
10713 @item %S - Fundamental block size (for block counts)
10714 @item %t - Type in hex
10715 @item %T - Type in human readable form
10719 Time stamps are listed according to the time zone rules specified by
10720 the @env{TZ} environment variable, or by the system default rules if
10721 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10722 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10727 @node sync invocation
10728 @section @command{sync}: Synchronize data on disk with memory
10731 @cindex synchronize disk and memory
10733 @cindex superblock, writing
10734 @cindex inodes, written buffered
10735 @command{sync} writes any data buffered in memory out to disk. This can
10736 include (but is not limited to) modified superblocks, modified inodes,
10737 and delayed reads and writes. This must be implemented by the kernel;
10738 The @command{sync} program does nothing but exercise the @code{sync} system
10741 @cindex crashes and corruption
10742 The kernel keeps data in memory to avoid doing (relatively slow) disk
10743 reads and writes. This improves performance, but if the computer
10744 crashes, data may be lost or the file system corrupted as a
10745 result. The @command{sync} command ensures everything in memory
10746 is written to disk.
10748 Any arguments are ignored, except for a lone @option{--help} or
10749 @option{--version} (@pxref{Common options}).
10754 @node truncate invocation
10755 @section @command{truncate}: Shrink or extend the size of a file
10758 @cindex truncating, file sizes
10760 @command{truncate} shrinks or extends the size of each @var{file} to the
10761 specified size. Synopsis:
10764 truncate @var{option}@dots{} @var{file}@dots{}
10767 @cindex files, creating
10768 Any @var{file} that does not exist is created.
10770 @cindex sparse files, creating
10771 @cindex holes, creating files with
10772 If a @var{file} is larger than the specified size, the extra data is lost.
10773 If a @var{file} is shorter, it is extended and the extended part (or hole)
10774 reads as zero bytes.
10776 The program accepts the following options. Also see @ref{Common options}.
10783 @opindex --no-create
10784 Do not create files that do not exist.
10789 @opindex --io-blocks
10790 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10792 @item -r @var{rfile}
10793 @itemx --reference=@var{rfile}
10795 @opindex --reference
10796 Base the size of each @var{file} on the size of @var{rfile}.
10798 @item -s @var{size}
10799 @itemx --size=@var{size}
10802 Set or adjust the size of each @var{file} according to @var{size}.
10803 @multiplierSuffixesNoBlocks{size}
10805 @var{size} may also be prefixed by one of the following to adjust
10806 the size of each @var{file} based on their current size:
10808 @samp{+} => extend by
10809 @samp{-} => reduce by
10810 @samp{<} => at most
10811 @samp{>} => at least
10812 @samp{/} => round down to multiple of
10813 @samp{%} => round up to multiple of
10821 @node Printing text
10822 @chapter Printing text
10824 @cindex printing text, commands for
10825 @cindex commands for printing text
10827 This section describes commands that display text strings.
10830 * echo invocation:: Print a line of text.
10831 * printf invocation:: Format and print data.
10832 * yes invocation:: Print a string until interrupted.
10836 @node echo invocation
10837 @section @command{echo}: Print a line of text
10840 @cindex displaying text
10841 @cindex printing text
10842 @cindex text, displaying
10843 @cindex arbitrary text, displaying
10845 @command{echo} writes each given @var{string} to standard output, with a
10846 space between each and a newline after the last one. Synopsis:
10849 echo [@var{option}]@dots{} [@var{string}]@dots{}
10852 @mayConflictWithShellBuiltIn{echo}
10854 The program accepts the following options. Also see @ref{Common options}.
10855 Options must precede operands, and the normally-special argument
10856 @samp{--} has no special meaning and is treated like any other
10862 Do not output the trailing newline.
10866 @cindex backslash escapes
10867 Enable interpretation of the following backslash-escaped characters in
10876 produce no further output
10892 the eight-bit value that is the octal number @var{nnn}
10893 (zero to three octal digits)
10895 the eight-bit value that is the octal number @var{nnn}
10896 (one to three octal digits)
10898 the eight-bit value that is the hexadecimal number @var{hh}
10899 (one or two hexadecimal digits)
10904 @cindex backslash escapes
10905 Disable interpretation of backslash escapes in each @var{string}.
10906 This is the default. If @option{-e} and @option{-E} are both
10907 specified, the last one given takes effect.
10911 @vindex POSIXLY_CORRECT
10912 If the @env{POSIXLY_CORRECT} environment variable is set, then when
10913 @command{echo}'s first argument is not @option{-n} it outputs
10914 option-like arguments instead of treating them as options. For
10915 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
10916 plain @samp{hello}.
10918 @acronym{POSIX} does not require support for any options, and says
10919 that the behavior of @command{echo} is implementation-defined if any
10920 @var{string} contains a backslash or if the first argument is
10921 @option{-n}. Portable programs can use the @command{printf} command
10922 if they need to omit trailing newlines or output control characters or
10923 backslashes. @xref{printf invocation}.
10928 @node printf invocation
10929 @section @command{printf}: Format and print data
10932 @command{printf} does formatted printing of text. Synopsis:
10935 printf @var{format} [@var{argument}]@dots{}
10938 @command{printf} prints the @var{format} string, interpreting @samp{%}
10939 directives and @samp{\} escapes to format numeric and string arguments
10940 in a way that is mostly similar to the C @samp{printf} function.
10941 @xref{Output Conversion Syntax,, @command{printf} format directives,
10942 libc, The GNU C Library Reference Manual}, for details.
10943 The differences are listed below.
10945 @mayConflictWithShellBuiltIn{printf}
10950 The @var{format} argument is reused as necessary to convert all the
10951 given @var{argument}s. For example, the command @samp{printf %s a b}
10955 Missing @var{argument}s are treated as null strings or as zeros,
10956 depending on whether the context expects a string or a number. For
10957 example, the command @samp{printf %sx%d} prints @samp{x0}.
10961 An additional escape, @samp{\c}, causes @command{printf} to produce no
10962 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
10963 E} prints @samp{ABC}.
10966 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
10967 digits, as opposed to C where it can have an unlimited number of
10968 digits. For example, the command @samp{printf '\x07e'} prints two
10969 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
10974 @command{printf} has an additional directive, @samp{%b}, which prints its
10975 argument string with @samp{\} escapes interpreted in the same way as in
10976 the @var{format} string, except that octal escapes are of the form
10977 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
10978 If a precision is also given, it limits the number of bytes printed
10979 from the converted string.
10982 Numeric arguments must be single C constants, possibly with leading
10983 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
10987 @vindex POSIXLY_CORRECT
10988 If the leading character of a numeric argument is @samp{"} or @samp{'}
10989 then its value is the numeric value of the immediately following
10990 character. Any remaining characters are silently ignored if the
10991 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
10992 warning is printed. For example, @samp{printf "%d" "'a"} outputs
10993 @samp{97} on hosts that use the @acronym{ASCII} character set, since
10994 @samp{a} has the numeric value 97 in @acronym{ASCII}.
10999 A floating-point argument must use a period before any fractional
11000 digits, but is printed according to the @env{LC_NUMERIC} category of the
11001 current locale. For example, in a locale whose radix character is a
11002 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11003 the command @samp{printf %g 3,14} is an error.
11007 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11008 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11009 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11010 digits) specifying a character to print.
11015 @cindex ISO/IEC 10646
11017 @command{printf} interprets two character syntaxes introduced in
11018 @acronym{ISO} C 99:
11019 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11020 characters, specified as
11021 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11022 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11023 @command{printf} outputs the Unicode characters
11024 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11025 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11026 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11028 The processing of @samp{\u} and @samp{\U} requires a full-featured
11029 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11030 or when @code{libiconv} is installed prior to this package. Otherwise
11031 @samp{\u} and @samp{\U} will print as-is.
11033 The only options are a lone @option{--help} or
11034 @option{--version}. @xref{Common options}.
11035 Options must precede operands.
11037 The Unicode character syntaxes are useful for writing strings in a locale
11038 independent way. For example, a string containing the Euro currency symbol
11041 $ env printf '\u20AC 14.95'
11045 will be output correctly in all locales supporting the Euro symbol
11046 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11049 $ env printf '\u4e2d\u6587'
11053 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11055 Note that in these examples, the @command{printf} command has been
11056 invoked via @command{env} to ensure that we run the program found via
11057 your shell's search path, and not a shell alias or a built-in function.
11059 For larger strings, you don't need to look up the hexadecimal code
11060 values of each character one by one. @acronym{ASCII} characters mixed with \u
11061 escape sequences is also known as the JAVA source file encoding. You can
11062 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11063 is how to convert a piece of text into a shell script which will output
11064 this text in a locale-independent way:
11067 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11068 '\u4e2d\u6587\n' > sample.txt
11069 $ recode BIG5..JAVA < sample.txt \
11070 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11077 @node yes invocation
11078 @section @command{yes}: Print a string until interrupted
11081 @cindex repeated output of a string
11083 @command{yes} prints the command line arguments, separated by spaces and
11084 followed by a newline, forever until it is killed. If no arguments are
11085 given, it prints @samp{y} followed by a newline forever until killed.
11087 Upon a write error, @command{yes} exits with status @samp{1}.
11089 The only options are a lone @option{--help} or @option{--version}.
11090 To output an argument that begins with
11091 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11092 @xref{Common options}.
11096 @chapter Conditions
11099 @cindex commands for exit status
11100 @cindex exit status commands
11102 This section describes commands that are primarily useful for their exit
11103 status, rather than their output. Thus, they are often used as the
11104 condition of shell @code{if} statements, or as the last command in a
11108 * false invocation:: Do nothing, unsuccessfully.
11109 * true invocation:: Do nothing, successfully.
11110 * test invocation:: Check file types and compare values.
11111 * expr invocation:: Evaluate expressions.
11115 @node false invocation
11116 @section @command{false}: Do nothing, unsuccessfully
11119 @cindex do nothing, unsuccessfully
11120 @cindex failure exit status
11121 @cindex exit status of @command{false}
11123 @command{false} does nothing except return an exit status of 1, meaning
11124 @dfn{failure}. It can be used as a place holder in shell scripts
11125 where an unsuccessful command is needed.
11126 In most modern shells, @command{false} is a built-in command, so when
11127 you use @samp{false} in a script, you're probably using the built-in
11128 command, not the one documented here.
11130 @command{false} honors the @option{--help} and @option{--version} options.
11132 This version of @command{false} is implemented as a C program, and is thus
11133 more secure and faster than a shell script implementation, and may safely
11134 be used as a dummy shell for the purpose of disabling accounts.
11136 Note that @command{false} (unlike all other programs documented herein)
11137 exits unsuccessfully, even when invoked with
11138 @option{--help} or @option{--version}.
11140 Portable programs should not assume that the exit status of
11141 @command{false} is 1, as it is greater than 1 on some
11142 non-@acronym{GNU} hosts.
11145 @node true invocation
11146 @section @command{true}: Do nothing, successfully
11149 @cindex do nothing, successfully
11151 @cindex successful exit
11152 @cindex exit status of @command{true}
11154 @command{true} does nothing except return an exit status of 0, meaning
11155 @dfn{success}. It can be used as a place holder in shell scripts
11156 where a successful command is needed, although the shell built-in
11157 command @code{:} (colon) may do the same thing faster.
11158 In most modern shells, @command{true} is a built-in command, so when
11159 you use @samp{true} in a script, you're probably using the built-in
11160 command, not the one documented here.
11162 @command{true} honors the @option{--help} and @option{--version} options.
11164 Note, however, that it is possible to cause @command{true}
11165 to exit with nonzero status: with the @option{--help} or @option{--version}
11166 option, and with standard
11167 output already closed or redirected to a file that evokes an I/O error.
11168 For example, using a Bourne-compatible shell:
11171 $ ./true --version >&-
11172 ./true: write error: Bad file number
11173 $ ./true --version > /dev/full
11174 ./true: write error: No space left on device
11177 This version of @command{true} is implemented as a C program, and is thus
11178 more secure and faster than a shell script implementation, and may safely
11179 be used as a dummy shell for the purpose of disabling accounts.
11181 @node test invocation
11182 @section @command{test}: Check file types and compare values
11185 @cindex check file types
11186 @cindex compare values
11187 @cindex expression evaluation
11189 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11190 evaluation of the conditional expression @var{expr}. Each part of the
11191 expression must be a separate argument.
11193 @command{test} has file status checks, string operators, and numeric
11194 comparison operators.
11196 @command{test} has an alternate form that uses opening and closing
11197 square brackets instead a leading @samp{test}. For example, instead
11198 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11199 brackets must be separate arguments; for example, @samp{[-d /]} does
11200 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11201 @var{expr} ]} have the same meaning, only the former form is discussed
11207 test @var{expression}
11209 [ @var{expression} ]
11214 @mayConflictWithShellBuiltIn{test}
11216 If @var{expression} is omitted, @command{test} returns false.
11217 If @var{expression} is a single argument,
11218 @command{test} returns false if the argument is null and true otherwise. The argument
11219 can be any string, including strings like @samp{-d}, @samp{-1},
11220 @samp{--}, @samp{--help}, and @samp{--version} that most other
11221 programs would treat as options. To get help and version information,
11222 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11223 the usual closing brackets. @xref{Common options}.
11225 @cindex exit status of @command{test}
11229 0 if the expression is true,
11230 1 if the expression is false,
11231 2 if an error occurred.
11235 * File type tests:: -[bcdfhLpSt]
11236 * Access permission tests:: -[gkruwxOG]
11237 * File characteristic tests:: -e -s -nt -ot -ef
11238 * String tests:: -z -n = !=
11239 * Numeric tests:: -eq -ne -lt -le -gt -ge
11240 * Connectives for test:: ! -a -o
11244 @node File type tests
11245 @subsection File type tests
11247 @cindex file type tests
11249 These options test for particular types of files. (Everything's a file,
11250 but not all files are the same!)
11254 @item -b @var{file}
11256 @cindex block special check
11257 True if @var{file} exists and is a block special device.
11259 @item -c @var{file}
11261 @cindex character special check
11262 True if @var{file} exists and is a character special device.
11264 @item -d @var{file}
11266 @cindex directory check
11267 True if @var{file} exists and is a directory.
11269 @item -f @var{file}
11271 @cindex regular file check
11272 True if @var{file} exists and is a regular file.
11274 @item -h @var{file}
11275 @itemx -L @var{file}
11278 @cindex symbolic link check
11279 True if @var{file} exists and is a symbolic link.
11280 Unlike all other file-related tests, this test does not dereference
11281 @var{file} if it is a symbolic link.
11283 @item -p @var{file}
11285 @cindex named pipe check
11286 True if @var{file} exists and is a named pipe.
11288 @item -S @var{file}
11290 @cindex socket check
11291 True if @var{file} exists and is a socket.
11295 @cindex terminal check
11296 True if @var{fd} is a file descriptor that is associated with a
11302 @node Access permission tests
11303 @subsection Access permission tests
11305 @cindex access permission tests
11306 @cindex permission tests
11308 These options test for particular access permissions.
11312 @item -g @var{file}
11314 @cindex set-group-ID check
11315 True if @var{file} exists and has its set-group-ID bit set.
11317 @item -k @var{file}
11319 @cindex sticky bit check
11320 True if @var{file} exists and has its @dfn{sticky} bit set.
11322 @item -r @var{file}
11324 @cindex readable file check
11325 True if @var{file} exists and read permission is granted.
11327 @item -u @var{file}
11329 @cindex set-user-ID check
11330 True if @var{file} exists and has its set-user-ID bit set.
11332 @item -w @var{file}
11334 @cindex writable file check
11335 True if @var{file} exists and write permission is granted.
11337 @item -x @var{file}
11339 @cindex executable file check
11340 True if @var{file} exists and execute permission is granted
11341 (or search permission, if it is a directory).
11343 @item -O @var{file}
11345 @cindex owned by effective user ID check
11346 True if @var{file} exists and is owned by the current effective user ID.
11348 @item -G @var{file}
11350 @cindex owned by effective group ID check
11351 True if @var{file} exists and is owned by the current effective group ID.
11355 @node File characteristic tests
11356 @subsection File characteristic tests
11358 @cindex file characteristic tests
11360 These options test other file characteristics.
11364 @item -e @var{file}
11366 @cindex existence-of-file check
11367 True if @var{file} exists.
11369 @item -s @var{file}
11371 @cindex nonempty file check
11372 True if @var{file} exists and has a size greater than zero.
11374 @item @var{file1} -nt @var{file2}
11376 @cindex newer-than file check
11377 True if @var{file1} is newer (according to modification date) than
11378 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11380 @item @var{file1} -ot @var{file2}
11382 @cindex older-than file check
11383 True if @var{file1} is older (according to modification date) than
11384 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11386 @item @var{file1} -ef @var{file2}
11388 @cindex same file check
11389 @cindex hard link check
11390 True if @var{file1} and @var{file2} have the same device and inode
11391 numbers, i.e., if they are hard links to each other.
11397 @subsection String tests
11399 @cindex string tests
11401 These options test string characteristics. You may need to quote
11402 @var{string} arguments for the shell. For example:
11408 The quotes here prevent the wrong arguments from being passed to
11409 @command{test} if @samp{$V} is empty or contains special characters.
11413 @item -z @var{string}
11415 @cindex zero-length string check
11416 True if the length of @var{string} is zero.
11418 @item -n @var{string}
11419 @itemx @var{string}
11421 @cindex nonzero-length string check
11422 True if the length of @var{string} is nonzero.
11424 @item @var{string1} = @var{string2}
11426 @cindex equal string check
11427 True if the strings are equal.
11429 @item @var{string1} != @var{string2}
11431 @cindex not-equal string check
11432 True if the strings are not equal.
11437 @node Numeric tests
11438 @subsection Numeric tests
11440 @cindex numeric tests
11441 @cindex arithmetic tests
11443 Numeric relational operators. The arguments must be entirely numeric
11444 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11445 which evaluates to the length of @var{string}.
11449 @item @var{arg1} -eq @var{arg2}
11450 @itemx @var{arg1} -ne @var{arg2}
11451 @itemx @var{arg1} -lt @var{arg2}
11452 @itemx @var{arg1} -le @var{arg2}
11453 @itemx @var{arg1} -gt @var{arg2}
11454 @itemx @var{arg1} -ge @var{arg2}
11461 These arithmetic binary operators return true if @var{arg1} is equal,
11462 not-equal, less-than, less-than-or-equal, greater-than, or
11463 greater-than-or-equal than @var{arg2}, respectively.
11470 test -1 -gt -2 && echo yes
11472 test -l abc -gt 1 && echo yes
11475 @error{} test: integer expression expected before -eq
11479 @node Connectives for test
11480 @subsection Connectives for @command{test}
11482 @cindex logical connectives
11483 @cindex connectives, logical
11485 The usual logical connectives.
11491 True if @var{expr} is false.
11493 @item @var{expr1} -a @var{expr2}
11495 @cindex logical and operator
11496 @cindex and operator
11497 True if both @var{expr1} and @var{expr2} are true.
11499 @item @var{expr1} -o @var{expr2}
11501 @cindex logical or operator
11502 @cindex or operator
11503 True if either @var{expr1} or @var{expr2} is true.
11508 @node expr invocation
11509 @section @command{expr}: Evaluate expressions
11512 @cindex expression evaluation
11513 @cindex evaluation of expressions
11515 @command{expr} evaluates an expression and writes the result on standard
11516 output. Each token of the expression must be a separate argument.
11518 Operands are either integers or strings. Integers consist of one or
11519 more decimal digits, with an optional leading @samp{-}.
11520 @command{expr} converts
11521 anything appearing in an operand position to an integer or a string
11522 depending on the operation being applied to it.
11524 Strings are not quoted for @command{expr} itself, though you may need to
11525 quote them to protect characters with special meaning to the shell,
11526 e.g., spaces. However, regardless of whether it is quoted, a string
11527 operand should not be a parenthesis or any of @command{expr}'s
11528 operators like @code{+}, so you cannot safely pass an arbitrary string
11529 @code{$str} to expr merely by quoting it to the shell. One way to
11530 work around this is to use the @sc{gnu} extension @code{+},
11531 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11532 @code{@w{" $str"}} and to adjust the rest of the expression to take
11533 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11535 You should not pass a negative integer or a string with leading
11536 @samp{-} as @command{expr}'s first argument, as it might be
11537 misinterpreted as an option; this can be avoided by parenthesization.
11538 Also, portable scripts should not use a string operand that happens to
11539 take the form of an integer; this can be worked around by inserting
11540 leading spaces as mentioned above.
11542 @cindex parentheses for grouping
11543 Operators may be given as infix symbols or prefix keywords. Parentheses
11544 may be used for grouping in the usual manner. You must quote
11545 parentheses and many operators to avoid the shell evaluating them,
11548 When built with support for the GNU MP library, @command{expr} uses
11549 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11550 types and may fail due to arithmetic overflow.
11552 The only options are @option{--help} and @option{--version}. @xref{Common
11553 options}. Options must precede operands.
11555 @cindex exit status of @command{expr}
11559 0 if the expression is neither null nor 0,
11560 1 if the expression is null or 0,
11561 2 if the expression is invalid,
11562 3 if an internal error occurred (e.g., arithmetic overflow).
11566 * String expressions:: + : match substr index length
11567 * Numeric expressions:: + - * / %
11568 * Relations for expr:: | & < <= = == != >= >
11569 * Examples of expr:: Examples.
11573 @node String expressions
11574 @subsection String expressions
11576 @cindex string expressions
11577 @cindex expressions, string
11579 @command{expr} supports pattern matching and other string operators. These
11580 have higher precedence than both the numeric and relational operators (in
11581 the next sections).
11585 @item @var{string} : @var{regex}
11586 @cindex pattern matching
11587 @cindex regular expression matching
11588 @cindex matching patterns
11589 Perform pattern matching. The arguments are converted to strings and the
11590 second is considered to be a (basic, a la GNU @code{grep}) regular
11591 expression, with a @code{^} implicitly prepended. The first argument is
11592 then matched against this regular expression.
11594 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11595 @code{:} expression returns the part of @var{string} that matched the
11596 subexpression; otherwise, it returns the number of characters matched.
11598 If the match fails, the @code{:} operator returns the null string if
11599 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11601 @kindex \( @r{regexp operator}
11602 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11603 value; additional pairs are meaningful only for grouping the regular
11604 expression operators.
11606 @kindex \+ @r{regexp operator}
11607 @kindex \? @r{regexp operator}
11608 @kindex \| @r{regexp operator}
11609 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11610 operators which respectively match one or more, zero or one, or separate
11611 alternatives. SunOS and other @command{expr}'s treat these as regular
11612 characters. (@acronym{POSIX} allows either behavior.)
11613 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11614 regular expression syntax. Some examples are in @ref{Examples of expr}.
11616 @item match @var{string} @var{regex}
11618 An alternative way to do pattern matching. This is the same as
11619 @w{@samp{@var{string} : @var{regex}}}.
11621 @item substr @var{string} @var{position} @var{length}
11623 Returns the substring of @var{string} beginning at @var{position}
11624 with length at most @var{length}. If either @var{position} or
11625 @var{length} is negative, zero, or non-numeric, returns the null string.
11627 @item index @var{string} @var{charset}
11629 Returns the first position in @var{string} where the first character in
11630 @var{charset} was found. If no character in @var{charset} is found in
11631 @var{string}, return 0.
11633 @item length @var{string}
11635 Returns the length of @var{string}.
11637 @item + @var{token}
11639 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11640 or an operator like @code{/}.
11641 This makes it possible to test @code{expr length + "$x"} or
11642 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11643 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11644 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11645 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11649 To make @command{expr} interpret keywords as strings, you must use the
11650 @code{quote} operator.
11653 @node Numeric expressions
11654 @subsection Numeric expressions
11656 @cindex numeric expressions
11657 @cindex expressions, numeric
11659 @command{expr} supports the usual numeric operators, in order of increasing
11660 precedence. These numeric operators have lower precedence than the
11661 string operators described in the previous section, and higher precedence
11662 than the connectives (next section).
11670 @cindex subtraction
11671 Addition and subtraction. Both arguments are converted to integers;
11672 an error occurs if this cannot be done.
11678 @cindex multiplication
11681 Multiplication, division, remainder. Both arguments are converted to
11682 integers; an error occurs if this cannot be done.
11687 @node Relations for expr
11688 @subsection Relations for @command{expr}
11690 @cindex connectives, logical
11691 @cindex logical connectives
11692 @cindex relations, numeric or string
11694 @command{expr} supports the usual logical connectives and relations. These
11695 have lower precedence than the string and numeric operators
11696 (previous sections). Here is the list, lowest-precedence operator first.
11702 @cindex logical or operator
11703 @cindex or operator
11704 Returns its first argument if that is neither null nor zero, otherwise
11705 its second argument if it is neither null nor zero, otherwise 0. It
11706 does not evaluate its second argument if its first argument is neither
11711 @cindex logical and operator
11712 @cindex and operator
11713 Return its first argument if neither argument is null or zero, otherwise
11714 0. It does not evaluate its second argument if its first argument is
11717 @item < <= = == != >= >
11724 @cindex comparison operators
11726 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11727 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11728 both arguments to integers and do a numeric comparison; if either
11729 conversion fails, it does a lexicographic comparison using the character
11730 collating sequence specified by the @env{LC_COLLATE} locale.
11735 @node Examples of expr
11736 @subsection Examples of using @command{expr}
11738 @cindex examples of @command{expr}
11739 Here are a few examples, including quoting for shell metacharacters.
11741 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11744 foo=`expr $foo + 1`
11747 To print the non-directory part of the file name stored in
11748 @code{$fname}, which need not contain a @code{/}:
11751 expr $fname : '.*/\(.*\)' '|' $fname
11754 An example showing that @code{\+} is an operator:
11762 expr abc : 'a\(.\)c'
11764 expr index abcdef cz
11767 @error{} expr: syntax error
11768 expr index + index a
11774 @chapter Redirection
11776 @cindex redirection
11777 @cindex commands for redirection
11779 Unix shells commonly provide several forms of @dfn{redirection}---ways
11780 to change the input source or output destination of a command. But one
11781 useful redirection is performed by a separate command, not by the shell;
11782 it's described here.
11785 * tee invocation:: Redirect output to multiple files or processes.
11789 @node tee invocation
11790 @section @command{tee}: Redirect output to multiple files or processes
11793 @cindex pipe fitting
11794 @cindex destinations, multiple output
11795 @cindex read from stdin and write to stdout and files
11797 The @command{tee} command copies standard input to standard output and also
11798 to any files given as arguments. This is useful when you want not only
11799 to send some data down a pipe, but also to save a copy. Synopsis:
11802 tee [@var{option}]@dots{} [@var{file}]@dots{}
11805 If a file being written to does not already exist, it is created. If a
11806 file being written to already exists, the data it previously contained
11807 is overwritten unless the @option{-a} option is used.
11809 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11810 input to standard output, but this is typically not that useful as the
11811 copies are interleaved.
11813 The program accepts the following options. Also see @ref{Common options}.
11820 Append standard input to the given files rather than overwriting
11824 @itemx --ignore-interrupts
11826 @opindex --ignore-interrupts
11827 Ignore interrupt signals.
11831 The @command{tee} command is useful when you happen to be transferring a large
11832 amount of data and also want to summarize that data without reading
11833 it a second time. For example, when you are downloading a DVD image,
11834 you often want to verify its signature or checksum right away.
11835 The inefficient way to do it is simply:
11838 wget http://example.com/some.iso && sha1sum some.iso
11841 One problem with the above is that it makes you wait for the
11842 download to complete before starting the time-consuming SHA1 computation.
11843 Perhaps even more importantly, the above requires reading
11844 the DVD image a second time (the first was from the network).
11846 The efficient way to do it is to interleave the download
11847 and SHA1 computation. Then, you'll get the checksum for
11848 free, because the entire process parallelizes so well:
11851 # slightly contrived, to demonstrate process substitution
11852 wget -O - http://example.com/dvd.iso \
11853 | tee >(sha1sum > dvd.sha1) > dvd.iso
11856 That makes @command{tee} write not just to the expected output file,
11857 but also to a pipe running @command{sha1sum} and saving the final
11858 checksum in a file named @file{dvd.sha1}.
11860 Note, however, that this example relies on a feature of modern shells
11861 called @dfn{process substitution}
11862 (the @samp{>(command)} syntax, above;
11863 @xref{Process Substitution,,Process Substitution, bashref,
11864 The Bash Reference Manual}.),
11865 so it works with @command{zsh}, @command{bash}, and @command{ksh},
11866 but not with @command{/bin/sh}. So if you write code like this
11867 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
11869 Since the above example writes to one file and one process,
11870 a more conventional and portable use of @command{tee} is even better:
11873 wget -O - http://example.com/dvd.iso \
11874 | tee dvd.iso | sha1sum > dvd.sha1
11877 You can extend this example to make @command{tee} write to two processes,
11878 computing MD5 and SHA1 checksums in parallel. In this case,
11879 process substitution is required:
11882 wget -O - http://example.com/dvd.iso \
11883 | tee >(sha1sum > dvd.sha1) \
11884 >(md5sum > dvd.md5) \
11888 This technique is also useful when you want to make a @emph{compressed}
11889 copy of the contents of a pipe.
11890 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
11891 For a large hierarchy, @samp{du -ak} can run for a long time,
11892 and can easily produce terabytes of data, so you won't want to
11893 rerun the command unnecessarily. Nor will you want to save
11894 the uncompressed output.
11896 Doing it the inefficient way, you can't even start the GUI
11897 until after you've compressed all of the @command{du} output:
11900 du -ak | gzip -9 > /tmp/du.gz
11901 gzip -d /tmp/du.gz | xdiskusage -a
11904 With @command{tee} and process substitution, you start the GUI
11905 right away and eliminate the decompression completely:
11908 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
11911 Finally, if you regularly create more than one type of
11912 compressed tarball at once, for example when @code{make dist} creates
11913 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
11914 there may be a better way.
11915 Typical @command{automake}-generated @file{Makefile} rules create
11916 the two compressed tar archives with commands in sequence, like this
11917 (slightly simplified):
11920 tardir=your-pkg-M.N
11921 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
11922 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11925 However, if the hierarchy you are archiving and compressing is larger
11926 than a couple megabytes, and especially if you are using a multi-processor
11927 system with plenty of memory, then you can do much better by reading the
11928 directory contents only once and running the compression programs in parallel:
11931 tardir=your-pkg-M.N
11932 tar chof - "$tardir" \
11933 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
11934 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
11940 @node File name manipulation
11941 @chapter File name manipulation
11943 @cindex file name manipulation
11944 @cindex manipulation of file names
11945 @cindex commands for file name manipulation
11947 This section describes commands that manipulate file names.
11950 * basename invocation:: Strip directory and suffix from a file name.
11951 * dirname invocation:: Strip last file name component.
11952 * pathchk invocation:: Check file name validity and portability.
11953 * mktemp invocation:: Create temporary file or directory.
11957 @node basename invocation
11958 @section @command{basename}: Strip directory and suffix from a file name
11961 @cindex strip directory and suffix from file names
11962 @cindex directory, stripping from file names
11963 @cindex suffix, stripping from file names
11964 @cindex file names, stripping directory and suffix
11965 @cindex leading directory components, stripping
11967 @command{basename} removes any leading directory components from
11968 @var{name}. Synopsis:
11971 basename @var{name} [@var{suffix}]
11974 If @var{suffix} is specified and is identical to the end of @var{name},
11975 it is removed from @var{name} as well. Note that since trailing slashes
11976 are removed prior to suffix matching, @var{suffix} will do nothing if it
11977 contains slashes. @command{basename} prints the result on standard
11980 @c This test is used both here and in the section on dirname.
11981 @macro basenameAndDirname
11982 Together, @command{basename} and @command{dirname} are designed such
11983 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
11984 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
11985 for everything except file names containing a trailing newline.
11987 @basenameAndDirname
11989 @acronym{POSIX} allows the implementation to define the results if
11990 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
11991 @command{basename} returns the empty string. In the latter case, the
11992 result is @samp{//} on platforms where @var{//} is distinct from
11993 @var{/}, and @samp{/} on platforms where there is no difference.
11995 The only options are @option{--help} and @option{--version}. @xref{Common
11996 options}. Options must precede operands.
12004 basename /usr/bin/sort
12007 basename include/stdio.h .h
12011 @node dirname invocation
12012 @section @command{dirname}: Strip last file name component
12015 @cindex directory components, printing
12016 @cindex stripping non-directory suffix
12017 @cindex non-directory suffix, stripping
12019 @command{dirname} prints all but the final slash-delimited component of
12020 @var{name}. Slashes on either side of the final component are also
12021 removed. If the string contains no slash, @command{dirname} prints
12022 @samp{.} (meaning the current directory). Synopsis:
12028 @var{name} need not be a file name, but if it is, this operation
12029 effectively lists the directory that contains the final component,
12030 including the case when the final component is itself a directory.
12032 @basenameAndDirname
12034 @acronym{POSIX} allows the implementation to define the results if
12035 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12036 result is @samp{//} on platforms where @var{//} is distinct from
12037 @var{/}, and @samp{/} on platforms where there is no difference.
12039 The only options are @option{--help} and @option{--version}. @xref{Common
12047 # Output "/usr/bin".
12048 dirname /usr/bin/sort
12049 dirname /usr/bin//.//
12056 @node pathchk invocation
12057 @section @command{pathchk}: Check file name validity and portability
12060 @cindex file names, checking validity and portability
12061 @cindex valid file names, checking for
12062 @cindex portable file names, checking for
12064 @command{pathchk} checks validity and portability of file names. Synopsis:
12067 pathchk [@var{option}]@dots{} @var{name}@dots{}
12070 For each @var{name}, @command{pathchk} prints an error message if any of
12071 these conditions is true:
12075 One of the existing directories in @var{name} does not have search
12076 (execute) permission,
12078 The length of @var{name} is larger than the maximum supported by the
12081 The length of one component of @var{name} is longer than
12082 its file system's maximum.
12085 A nonexistent @var{name} is not an error, so long a file with that
12086 name could be created under the above conditions.
12088 The program accepts the following options. Also see @ref{Common options}.
12089 Options must precede operands.
12095 Instead of performing checks based on the underlying file system,
12096 print an error message if any of these conditions is true:
12100 A file name is empty.
12103 A file name contains a character outside the @acronym{POSIX} portable file
12104 name character set, namely, the ASCII letters and digits, @samp{.},
12105 @samp{_}, @samp{-}, and @samp{/}.
12108 The length of a file name or one of its components exceeds the
12109 @acronym{POSIX} minimum limits for portability.
12114 Print an error message if a file name is empty, or if it contains a component
12115 that begins with @samp{-}.
12117 @item --portability
12118 @opindex --portability
12119 Print an error message if a file name is not portable to all @acronym{POSIX}
12120 hosts. This option is equivalent to @samp{-p -P}.
12124 @cindex exit status of @command{pathchk}
12128 0 if all specified file names passed all checks,
12132 @node mktemp invocation
12133 @section @command{mktemp}: Create temporary file or directory
12136 @cindex file names, creating temporary
12137 @cindex directory, creating temporary
12138 @cindex temporary files and directories
12140 @command{mktemp} manages the creation of temporary files and
12141 directories. Synopsis:
12144 mktemp [@var{option}]@dots{} [@var{template}]
12147 Safely create a temporary file or directory based on @var{template},
12148 and print its name. If given, @var{template} must include at least
12149 three consecutive @samp{X}s in the last component. If omitted, the template
12150 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12151 implied. The final run of @samp{X}s in the @var{template} will be replaced
12152 by alpha-numeric characters; thus, on a case-sensitive file system,
12153 and with a @var{template} including a run of @var{n} instances of @samp{X},
12154 there are @samp{62**@var{n}} potential file names.
12156 Older scripts used to create temporary files by simply joining the
12157 name of the program with the process id (@samp{$$}) as a suffix.
12158 However, that naming scheme is easily predictable, and suffers from a
12159 race condition where the attacker can create an appropriately named
12160 symbolic link, such that when the script then opens a handle to what
12161 it thought was an unused file, it is instead modifying an existing
12162 file. Using the same scheme to create a directory is slightly safer,
12163 since the @command{mkdir} will fail if the target already exists, but
12164 it is still inferior because it allows for denial of service attacks.
12165 Therefore, modern scripts should use the @command{mktemp} command to
12166 guarantee that the generated name will be unpredictable, and that
12167 knowledge of the temporary file name implies that the file was created
12168 by the current script and cannot be modified by other users.
12170 When creating a file, the resulting file has read and write
12171 permissions for the current user, but no permissions for the group or
12172 others; these permissions are reduced if the current umask is more
12175 Here are some examples (although note that if you repeat them, you
12176 will most likely get different file names):
12181 Create a temporary file in the current directory.
12188 Create a temporary file with a known suffix.
12190 $ mktemp --suffix=.txt file-XXXX
12192 $ mktemp file-XXXX-XXXX.txt
12197 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12198 but falling back to the current directory rather than @file{/tmp}.
12199 Note that @command{mktemp} does not create fifos, but can create a
12200 secure directory in which the fifo can live. Exit the shell if the
12201 directory or fifo could not be created.
12203 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12205 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12209 Create and use a temporary file if possible, but ignore failure. The
12210 file will reside in the directory named by @env{TMPDIR}, if specified,
12211 or else in @file{/tmp}.
12213 $ file=$(mktemp -q) && @{
12214 > # Safe to use $file only within this block. Use quotes,
12215 > # since $TMPDIR, and thus $file, may contain whitespace.
12216 > echo ... > "$file"
12222 Act as a semi-random character generator (it is not fully random,
12223 since it is impacted by the contents of the current directory). To
12224 avoid security holes, do not use the resulting names to create a file.
12234 The program accepts the following options. Also see @ref{Common options}.
12241 @opindex --directory
12242 Create a directory rather than a file. The directory will have read,
12243 write, and search permissions for the current user, but no permissions
12244 for the group or others; these permissions are reduced if the current
12245 umask is more restrictive.
12251 Suppress diagnostics about failure to create a file or directory. The
12252 exit status will still reflect whether a file was created.
12258 Generate a temporary name that does not name an existing file, without
12259 changing the file system contents. Using the output of this command
12260 to create a new file is inherently unsafe, as there is a window of
12261 time between generating the name and using it where another process
12262 can create an object by the same name.
12265 @itemx --tmpdir[=@var{dir}]
12268 Treat @var{template} relative to the directory @var{dir}. If
12269 @var{dir} is not specified (only possible with the long option
12270 @option{--tmpdir}) or is the empty string, use the value of
12271 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12272 specified, @var{template} must not be absolute. However,
12273 @var{template} can still contain slashes, although intermediate
12274 directories must already exist.
12276 @item --suffix=@var{suffix}
12278 Append @var{suffix} to the @var{template}. @var{suffix} must not
12279 contain slash. If @option{--suffix} is specified, @var{template} must
12280 end in @samp{X}; if it is not specified, then an appropriate
12281 @option{--suffix} is inferred by finding the last @samp{X} in
12282 @var{template}. This option exists for use with the default
12283 @var{template} and for the creation of a @var{suffix} that starts with
12288 Treat @var{template} as a single file relative to the value of
12289 @env{TMPDIR} if available, or to the directory specified by
12290 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12291 contain slashes. This option is deprecated; the use of @option{-p}
12292 without @option{-t} offers better defaults (by favoring the command
12293 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12298 @cindex exit status of @command{mktemp}
12302 0 if the file was created,
12307 @node Working context
12308 @chapter Working context
12310 @cindex working context
12311 @cindex commands for printing the working context
12313 This section describes commands that display or alter the context in
12314 which you are working: the current directory, the terminal settings, and
12315 so forth. See also the user-related commands in the next section.
12318 * pwd invocation:: Print working directory.
12319 * stty invocation:: Print or change terminal characteristics.
12320 * printenv invocation:: Print environment variables.
12321 * tty invocation:: Print file name of terminal on standard input.
12325 @node pwd invocation
12326 @section @command{pwd}: Print working directory
12329 @cindex print name of current directory
12330 @cindex current working directory, printing
12331 @cindex working directory, printing
12334 @command{pwd} prints the name of the current directory. Synopsis:
12337 pwd [@var{option}]@dots{}
12340 The program accepts the following options. Also see @ref{Common options}.
12347 If the contents of the environment variable @env{PWD} provide an
12348 absolute name of the current directory with no @samp{.} or @samp{..}
12349 components, but possibly with symbolic links, then output those
12350 contents. Otherwise, fall back to default @option{-P} handling.
12355 @opindex --physical
12356 Print a fully resolved name for the current directory. That is, all
12357 components of the printed name will be actual directory names---none
12358 will be symbolic links.
12361 @cindex symbolic links and @command{pwd}
12362 If @option{-L} and @option{-P} are both given, the last one takes
12363 precedence. If neither option is given, then this implementation uses
12364 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12365 environment variable is set.
12367 @mayConflictWithShellBuiltIn{pwd}
12372 @node stty invocation
12373 @section @command{stty}: Print or change terminal characteristics
12376 @cindex change or print terminal settings
12377 @cindex terminal settings
12378 @cindex line settings of terminal
12380 @command{stty} prints or changes terminal characteristics, such as baud rate.
12384 stty [@var{option}] [@var{setting}]@dots{}
12385 stty [@var{option}]
12388 If given no line settings, @command{stty} prints the baud rate, line
12389 discipline number (on systems that support it), and line settings
12390 that have been changed from the values set by @samp{stty sane}.
12391 By default, mode reading and setting are performed on the tty line
12392 connected to standard input, although this can be modified by the
12393 @option{--file} option.
12395 @command{stty} accepts many non-option arguments that change aspects of
12396 the terminal line operation, as described below.
12398 The program accepts the following options. Also see @ref{Common options}.
12405 Print all current settings in human-readable form. This option may not
12406 be used in combination with any line settings.
12408 @item -F @var{device}
12409 @itemx --file=@var{device}
12412 Set the line opened by the file name specified in @var{device} instead of
12413 the tty line connected to standard input. This option is necessary
12414 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12415 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12416 the @code{clocal} flag is not set. Hence, it is not always possible
12417 to allow the shell to open the device in the traditional manner.
12423 @cindex machine-readable @command{stty} output
12424 Print all current settings in a form that can be used as an argument to
12425 another @command{stty} command to restore the current settings. This option
12426 may not be used in combination with any line settings.
12430 Many settings can be turned off by preceding them with a @samp{-}.
12431 Such arguments are marked below with ``May be negated'' in their
12432 description. The descriptions themselves refer to the positive
12433 case, that is, when @emph{not} negated (unless stated otherwise,
12436 Some settings are not available on all @acronym{POSIX} systems, since they use
12437 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12438 description. On non-@acronym{POSIX} systems, those or other settings also may not
12439 be available, but it's not feasible to document all the variations: just
12445 * Control:: Control settings
12446 * Input:: Input settings
12447 * Output:: Output settings
12448 * Local:: Local settings
12449 * Combination:: Combination settings
12450 * Characters:: Special characters
12451 * Special:: Special settings
12456 @subsection Control settings
12458 @cindex control settings
12464 @cindex two-way parity
12465 Generate parity bit in output and expect parity bit in input.
12471 @cindex even parity
12472 Set odd parity (even if negated). May be negated.
12479 @cindex character size
12480 @cindex eight-bit characters
12481 Set character size to 5, 6, 7, or 8 bits.
12486 Send a hangup signal when the last process closes the tty. May be
12492 Use two stop bits per character (one if negated). May be negated.
12496 Allow input to be received. May be negated.
12500 @cindex modem control
12501 Disable modem control signals. May be negated.
12505 @cindex hardware flow control
12506 @cindex flow control, hardware
12507 @cindex RTS/CTS flow control
12508 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12513 @subsection Input settings
12515 @cindex input settings
12516 These settings control operations on data received from the terminal.
12521 @cindex breaks, ignoring
12522 Ignore break characters. May be negated.
12526 @cindex breaks, cause interrupts
12527 Make breaks cause an interrupt signal. May be negated.
12531 @cindex parity, ignoring
12532 Ignore characters with parity errors. May be negated.
12536 @cindex parity errors, marking
12537 Mark parity errors (with a 255-0-character sequence). May be negated.
12541 Enable input parity checking. May be negated.
12545 @cindex eight-bit input
12546 Clear high (8th) bit of input characters. May be negated.
12550 @cindex newline, translating to return
12551 Translate newline to carriage return. May be negated.
12555 @cindex return, ignoring
12556 Ignore carriage return. May be negated.
12560 @cindex return, translating to newline
12561 Translate carriage return to newline. May be negated.
12565 @cindex input encoding, UTF-8
12566 Assume input characters are UTF-8 encoded. May be negated.
12570 @kindex C-s/C-q flow control
12571 @cindex XON/XOFF flow control
12572 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12579 @cindex software flow control
12580 @cindex flow control, software
12581 Enable sending of @code{stop} character when the system input buffer
12582 is almost full, and @code{start} character when it becomes almost
12583 empty again. May be negated.
12587 @cindex uppercase, translating to lowercase
12588 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12589 negated. Note ilcuc is not implemented, as one would not be able to issue
12590 almost any (lowercase) Unix command, after invoking it.
12594 Allow any character to restart output (only the start character
12595 if negated). Non-@acronym{POSIX}. May be negated.
12599 @cindex beeping at input buffer full
12600 Enable beeping and not flushing input buffer if a character arrives
12601 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12606 @subsection Output settings
12608 @cindex output settings
12609 These settings control operations on data sent to the terminal.
12614 Postprocess output. May be negated.
12618 @cindex lowercase, translating to output
12619 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12620 negated. (Note ouclc is not currently implemented.)
12624 @cindex return, translating to newline
12625 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12629 @cindex newline, translating to crlf
12630 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12635 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12640 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12644 @cindex pad instead of timing for delaying
12645 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12650 @cindex pad character
12651 Use @acronym{ASCII} @sc{del} characters for fill instead of
12652 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12658 Newline delay style. Non-@acronym{POSIX}.
12665 Carriage return delay style. Non-@acronym{POSIX}.
12671 @opindex tab@var{n}
12672 Horizontal tab delay style. Non-@acronym{POSIX}.
12677 Backspace delay style. Non-@acronym{POSIX}.
12682 Vertical tab delay style. Non-@acronym{POSIX}.
12687 Form feed delay style. Non-@acronym{POSIX}.
12692 @subsection Local settings
12694 @cindex local settings
12699 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12700 characters. May be negated.
12704 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12705 special characters. May be negated.
12709 Enable non-@acronym{POSIX} special characters. May be negated.
12713 Echo input characters. May be negated.
12719 Echo @code{erase} characters as backspace-space-backspace. May be
12724 @cindex newline echoing after @code{kill}
12725 Echo a newline after a @code{kill} character. May be negated.
12729 @cindex newline, echoing
12730 Echo newline even if not echoing other characters. May be negated.
12734 @cindex flushing, disabling
12735 Disable flushing after @code{interrupt} and @code{quit} special
12736 characters. May be negated.
12740 @cindex case translation
12741 Enable input and output of uppercase characters by preceding their
12742 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12743 Non-@acronym{POSIX}. May be negated.
12747 @cindex background jobs, stopping at terminal write
12748 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12755 Echo erased characters backward, between @samp{\} and @samp{/}.
12756 Non-@acronym{POSIX}. May be negated.
12762 @cindex control characters, using @samp{^@var{c}}
12763 @cindex hat notation for control characters
12764 Echo control characters in hat notation (@samp{^@var{c}}) instead
12765 of literally. Non-@acronym{POSIX}. May be negated.
12771 Echo the @code{kill} special character by erasing each character on
12772 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12773 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12779 @subsection Combination settings
12781 @cindex combination settings
12782 Combination settings:
12789 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12790 as @code{-parenb cs8}.
12794 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12795 as @code{-parenb cs8}.
12799 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12800 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12804 Reset the @code{erase} and @code{kill} special characters to their default
12811 @c This is too long to write inline.
12813 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12814 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12815 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12816 ff0 isig icanon iexten echo echoe echok -echonl
12817 -noflsh -xcase -tostop -echoprt echoctl echoke
12821 and also sets all special characters to their default values.
12825 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
12826 sets the @code{eof} and @code{eol} characters to their default values
12827 if they are the same as the @code{min} and @code{time} characters.
12828 May be negated. If negated, same as @code{raw}.
12835 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
12836 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
12837 -imaxbel -opost -isig -icanon -xcase min 1 time 0
12841 May be negated. If negated, same as @code{cooked}.
12845 Same as @option{-icanon}. May be negated. If negated, same as
12850 @cindex eight-bit characters
12851 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
12852 same as @code{parenb istrip cs7}.
12856 Same as @option{-parenb -istrip -opost cs8}. May be negated.
12857 If negated, same as @code{parenb istrip opost cs7}.
12861 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
12865 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
12872 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
12873 (Used for terminals with uppercase characters only.)
12877 Same as @code{echoe echoctl echoke}.
12881 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
12886 @subsection Special characters
12888 @cindex special characters
12889 @cindex characters, special
12891 The special characters' default values vary from system to system.
12892 They are set with the syntax @samp{name value}, where the names are
12893 listed below and the value can be given either literally, in hat
12894 notation (@samp{^@var{c}}), or as an integer which may start with
12895 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
12896 any other digit to indicate decimal.
12898 @cindex disabling special characters
12899 @kindex u@r{, and disabling special characters}
12900 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
12901 special character. (This is incompatible with Ultrix @command{stty},
12902 which uses a value of @samp{u} to disable a special character. GNU
12903 @command{stty} treats a value @samp{u} like any other, namely to set that
12904 special character to @key{U}.)
12910 Send an interrupt signal.
12914 Send a quit signal.
12918 Erase the last character typed.
12922 Erase the current line.
12926 Send an end of file (terminate the input).
12934 Alternate character to end the line. Non-@acronym{POSIX}.
12938 Switch to a different shell layer. Non-@acronym{POSIX}.
12942 Restart the output after stopping it.
12950 Send a terminal stop signal.
12954 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
12958 Redraw the current line. Non-@acronym{POSIX}.
12962 Erase the last word typed. Non-@acronym{POSIX}.
12966 Enter the next character typed literally, even if it is a special
12967 character. Non-@acronym{POSIX}.
12972 @subsection Special settings
12974 @cindex special settings
12979 Set the minimum number of characters that will satisfy a read until
12980 the time value has expired, when @option{-icanon} is set.
12984 Set the number of tenths of a second before reads time out if the minimum
12985 number of characters have not been read, when @option{-icanon} is set.
12987 @item ispeed @var{n}
12989 Set the input speed to @var{n}.
12991 @item ospeed @var{n}
12993 Set the output speed to @var{n}.
12997 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13000 @itemx columns @var{n}
13003 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13009 Print the number of rows and columns that the kernel thinks the
13010 terminal has. (Systems that don't support rows and columns in the kernel
13011 typically use the environment variables @env{LINES} and @env{COLUMNS}
13012 instead; however, GNU @command{stty} does not know anything about them.)
13013 Non-@acronym{POSIX}.
13017 Use line discipline @var{n}. Non-@acronym{POSIX}.
13021 Print the terminal speed.
13024 @cindex baud rate, setting
13025 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13026 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13027 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13028 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13029 support higher speeds. The @command{stty} command includes support
13046 4000000 where the system supports these.
13047 0 hangs up the line if @option{-clocal} is set.
13051 @node printenv invocation
13052 @section @command{printenv}: Print all or some environment variables
13055 @cindex printing all or some environment variables
13056 @cindex environment variables, printing
13058 @command{printenv} prints environment variable values. Synopsis:
13061 printenv [@var{option}] [@var{variable}]@dots{}
13064 If no @var{variable}s are specified, @command{printenv} prints the value of
13065 every environment variable. Otherwise, it prints the value of each
13066 @var{variable} that is set, and nothing for those that are not set.
13068 The program accepts the following option. Also see @ref{Common options}.
13076 @cindex exit status of @command{printenv}
13080 0 if all variables specified were found
13081 1 if at least one specified variable was not found
13082 2 if a write error occurred
13086 @node tty invocation
13087 @section @command{tty}: Print file name of terminal on standard input
13090 @cindex print terminal file name
13091 @cindex terminal file name, printing
13093 @command{tty} prints the file name of the terminal connected to its standard
13094 input. It prints @samp{not a tty} if standard input is not a terminal.
13098 tty [@var{option}]@dots{}
13101 The program accepts the following option. Also see @ref{Common options}.
13111 Print nothing; only return an exit status.
13115 @cindex exit status of @command{tty}
13119 0 if standard input is a terminal
13120 1 if standard input is not a terminal
13121 2 if given incorrect arguments
13122 3 if a write error occurs
13126 @node User information
13127 @chapter User information
13129 @cindex user information, commands for
13130 @cindex commands for printing user information
13132 This section describes commands that print user-related information:
13133 logins, groups, and so forth.
13136 * id invocation:: Print user identity.
13137 * logname invocation:: Print current login name.
13138 * whoami invocation:: Print effective user ID.
13139 * groups invocation:: Print group names a user is in.
13140 * users invocation:: Print login names of users currently logged in.
13141 * who invocation:: Print who is currently logged in.
13145 @node id invocation
13146 @section @command{id}: Print user identity
13149 @cindex real user and group IDs, printing
13150 @cindex effective user and group IDs, printing
13151 @cindex printing real and effective user and group IDs
13153 @command{id} prints information about the given user, or the process
13154 running it if no user is specified. Synopsis:
13157 id [@var{option}]@dots{} [@var{username}]
13160 @vindex POSIXLY_CORRECT
13161 By default, it prints the real user ID, real group ID, effective user ID
13162 if different from the real user ID, effective group ID if different from
13163 the real group ID, and supplemental group IDs.
13164 In addition, if SELinux
13165 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13166 then print @samp{context=@var{c}}, where @var{c} is the security context.
13168 Each of these numeric values is preceded by an identifying string and
13169 followed by the corresponding user or group name in parentheses.
13171 The options cause @command{id} to print only part of the above information.
13172 Also see @ref{Common options}.
13179 Print only the group ID.
13185 Print only the group ID and the supplementary groups.
13191 Print the user or group name instead of the ID number. Requires
13192 @option{-u}, @option{-g}, or @option{-G}.
13198 Print the real, instead of effective, user or group ID. Requires
13199 @option{-u}, @option{-g}, or @option{-G}.
13205 Print only the user ID.
13212 @cindex security context
13213 Print only the security context of the current user.
13214 If SELinux is disabled then print a warning and
13215 set the exit status to 1.
13221 @macro primaryAndSupplementaryGroups{cmd,arg}
13222 Primary and supplementary groups for a process are normally inherited
13223 from its parent and are usually unchanged since login. This means
13224 that if you change the group database after logging in, @command{\cmd\}
13225 will not reflect your changes within your existing login session.
13226 Running @command{\cmd\} with a \arg\ causes the user and group
13227 database to be consulted afresh, and so will give a different result.
13229 @primaryAndSupplementaryGroups{id,user argument}
13231 @node logname invocation
13232 @section @command{logname}: Print current login name
13235 @cindex printing user's login name
13236 @cindex login name, printing
13237 @cindex user name, printing
13240 @command{logname} prints the calling user's name, as found in a
13241 system-maintained file (often @file{/var/run/utmp} or
13242 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13243 for the calling process, @command{logname} prints
13244 an error message and exits with a status of 1.
13246 The only options are @option{--help} and @option{--version}. @xref{Common
13252 @node whoami invocation
13253 @section @command{whoami}: Print effective user ID
13256 @cindex effective user ID, printing
13257 @cindex printing the effective user ID
13259 @command{whoami} prints the user name associated with the current
13260 effective user ID. It is equivalent to the command @samp{id -un}.
13262 The only options are @option{--help} and @option{--version}. @xref{Common
13268 @node groups invocation
13269 @section @command{groups}: Print group names a user is in
13272 @cindex printing groups a user is in
13273 @cindex supplementary groups, printing
13275 @command{groups} prints the names of the primary and any supplementary
13276 groups for each given @var{username}, or the current process if no names
13277 are given. If more than one name is given, the name of each user is
13279 the list of that user's groups and the user name is separated from the
13280 group list by a colon. Synopsis:
13283 groups [@var{username}]@dots{}
13286 The group lists are equivalent to the output of the command @samp{id -Gn}.
13288 @primaryAndSupplementaryGroups{groups,list of users}
13290 The only options are @option{--help} and @option{--version}. @xref{Common
13296 @node users invocation
13297 @section @command{users}: Print login names of users currently logged in
13300 @cindex printing current usernames
13301 @cindex usernames, printing current
13303 @cindex login sessions, printing users with
13304 @command{users} prints on a single line a blank-separated list of user
13305 names of users currently logged in to the current host. Each user name
13306 corresponds to a login session, so if a user has more than one login
13307 session, that user's name will appear the same number of times in the
13316 With no @var{file} argument, @command{users} extracts its information from
13317 a system-maintained file (often @file{/var/run/utmp} or
13318 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13319 that file instead. A common choice is @file{/var/log/wtmp}.
13321 The only options are @option{--help} and @option{--version}. @xref{Common
13327 @node who invocation
13328 @section @command{who}: Print who is currently logged in
13331 @cindex printing current user information
13332 @cindex information, about current users
13334 @command{who} prints information about users who are currently logged on.
13338 @command{who} [@var{option}] [@var{file}] [am i]
13341 @cindex terminal lines, currently used
13343 @cindex remote hostname
13344 If given no non-option arguments, @command{who} prints the following
13345 information for each user currently logged on: login name, terminal
13346 line, login time, and remote hostname or X display.
13350 If given one non-option argument, @command{who} uses that instead of
13351 a default system-maintained file (often @file{/var/run/utmp} or
13352 @file{/etc/utmp}) as the name of the file containing the record of
13353 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13354 to @command{who} to look at who has previously logged on.
13358 If given two non-option arguments, @command{who} prints only the entry
13359 for the user running it (determined from its standard input), preceded
13360 by the hostname. Traditionally, the two arguments given are @samp{am
13361 i}, as in @samp{who am i}.
13364 Time stamps are listed according to the time zone rules specified by
13365 the @env{TZ} environment variable, or by the system default rules if
13366 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13367 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13369 The program accepts the following options. Also see @ref{Common options}.
13377 Same as @samp{-b -d --login -p -r -t -T -u}.
13383 Print the date and time of last system boot.
13389 Print information corresponding to dead processes.
13395 Print a line of column headings.
13401 List only the entries that correspond to processes via which the
13402 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13406 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13407 is not the default because it can cause significant delays on systems with
13408 automatic dial-up internet access.
13412 Same as @samp{who am i}.
13418 List active processes spawned by init.
13424 Print only the login names and the number of users logged on.
13425 Overrides all other options.
13430 @opindex --runlevel
13431 Print the current (and maybe previous) run-level of the init process.
13435 Ignored; for compatibility with other versions of @command{who}.
13441 Print last system clock change.
13446 After the login time, print the number of hours and minutes that the
13447 user has been idle. @samp{.} means the user was active in the last minute.
13448 @samp{old} means the user has been idle for more than 24 hours.
13459 @opindex --writable
13460 @cindex message status
13461 @pindex write@r{, allowed}
13462 After each login name print a character indicating the user's message status:
13465 @samp{+} allowing @code{write} messages
13466 @samp{-} disallowing @code{write} messages
13467 @samp{?} cannot find terminal device
13475 @node System context
13476 @chapter System context
13478 @cindex system context
13479 @cindex context, system
13480 @cindex commands for system context
13482 This section describes commands that print or change system-wide
13486 * date invocation:: Print or set system date and time.
13487 * arch invocation:: Print machine hardware name.
13488 * nproc invocation:: Print the number of processors.
13489 * uname invocation:: Print system information.
13490 * hostname invocation:: Print or set system name.
13491 * hostid invocation:: Print numeric host identifier.
13492 * uptime invocation:: Print system uptime and load.
13495 @node date invocation
13496 @section @command{date}: Print or set system date and time
13499 @cindex time, printing or setting
13500 @cindex printing the current time
13505 date [@var{option}]@dots{} [+@var{format}]
13506 date [-u|--utc|--universal] @c this avoids a newline in the output
13507 [ MMDDhhmm[[CC]YY][.ss] ]
13511 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13512 it with a default format that depends on the @env{LC_TIME} locale category.
13513 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13514 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13517 Normally, @command{date} uses the time zone rules indicated by the
13518 @env{TZ} environment variable, or the system default rules if @env{TZ}
13519 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13520 @env{TZ}, libc, The GNU C Library Reference Manual}.
13522 @findex strftime @r{and @command{date}}
13523 @cindex time formats
13524 @cindex formatting times
13525 If given an argument that starts with a @samp{+}, @command{date} prints the
13526 current date and time (or the date and time specified by the
13527 @option{--date} option, see below) in the format defined by that argument,
13528 which is similar to that of the @code{strftime} function. Except for
13529 conversion specifiers, which start with @samp{%}, characters in the
13530 format string are printed unchanged. The conversion specifiers are
13536 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13537 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13538 * Literal conversion specifiers:: %[%nt]
13539 * Padding and other flags:: Pad with zeros, spaces, etc.
13540 * Setting the time:: Changing the system clock.
13541 * Options for date:: Instead of the current time.
13543 * Date input formats:: Specifying date strings.
13545 * Examples of date:: Examples.
13548 @node Time conversion specifiers
13549 @subsection Time conversion specifiers
13551 @cindex time conversion specifiers
13552 @cindex conversion specifiers, time
13554 @command{date} conversion specifiers related to times.
13558 hour (@samp{00}@dots{}@samp{23})
13560 hour (@samp{01}@dots{}@samp{12})
13562 hour (@samp{ 0}@dots{}@samp{23}).
13563 This is a @acronym{GNU} extension.
13565 hour (@samp{ 1}@dots{}@samp{12}).
13566 This is a @acronym{GNU} extension.
13568 minute (@samp{00}@dots{}@samp{59})
13570 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13571 This is a @acronym{GNU} extension.
13573 locale's equivalent of either @samp{AM} or @samp{PM};
13574 blank in many locales.
13575 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13577 like @samp{%p}, except lower case.
13578 This is a @acronym{GNU} extension.
13580 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13582 24-hour hour and minute. Same as @samp{%H:%M}.
13583 This is a @acronym{GNU} extension.
13585 @cindex epoch, seconds since
13586 @cindex seconds since the epoch
13587 @cindex beginning of time
13588 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13589 Leap seconds are not counted unless leap second support is available.
13590 @xref{%s-examples}, for examples.
13591 This is a @acronym{GNU} extension.
13593 second (@samp{00}@dots{}@samp{60}).
13594 This may be @samp{60} if leap seconds are supported.
13596 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13598 locale's time representation (e.g., @samp{23:13:48})
13600 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13601 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13602 time zone is determinable. This value reflects the numeric time zone
13603 appropriate for the current time, using the time zone rules specified
13604 by the @env{TZ} environment variable.
13605 The time (and optionally, the time zone rules) can be overridden
13606 by the @option{--date} option.
13607 This is a @acronym{GNU} extension.
13609 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13610 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13611 zone is determinable.
13612 This is a @acronym{GNU} extension.
13614 Numeric time zone to the nearest second with @samp{:} (e.g.,
13615 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13617 This is a @acronym{GNU} extension.
13619 Numeric time zone with @samp{:} using the minimum necessary precision
13620 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13621 no time zone is determinable.
13622 This is a @acronym{GNU} extension.
13624 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13625 time zone is determinable. See @samp{%z} for how it is determined.
13629 @node Date conversion specifiers
13630 @subsection Date conversion specifiers
13632 @cindex date conversion specifiers
13633 @cindex conversion specifiers, date
13635 @command{date} conversion specifiers related to dates.
13639 locale's abbreviated weekday name (e.g., @samp{Sun})
13641 locale's full weekday name, variable length (e.g., @samp{Sunday})
13643 locale's abbreviated month name (e.g., @samp{Jan})
13645 locale's full month name, variable length (e.g., @samp{January})
13647 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13649 century. This is like @samp{%Y}, except the last two digits are omitted.
13650 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13651 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13652 It is normally at least two characters, but it may be more.
13654 day of month (e.g., @samp{01})
13656 date; same as @samp{%m/%d/%y}
13658 day of month, space padded; same as @samp{%_d}
13660 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13661 This is a good choice for a date format, as it is standard and
13662 is easy to sort in the usual case where years are in the range
13664 This is a @acronym{GNU} extension.
13666 year corresponding to the @acronym{ISO} week number, but without the century
13667 (range @samp{00} through @samp{99}). This has the same format and value
13668 as @samp{%y}, except that if the @acronym{ISO} week number (see
13670 to the previous or next year, that year is used instead.
13671 This is a @acronym{GNU} extension.
13673 year corresponding to the @acronym{ISO} week number. This has the
13674 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13676 @samp{%V}) belongs to the previous or next year, that year is used
13678 It is normally useful only if @samp{%V} is also used;
13679 for example, the format @samp{%G-%m-%d} is probably a mistake,
13680 since it combines the ISO week number year with the conventional month and day.
13681 This is a @acronym{GNU} extension.
13685 day of year (@samp{001}@dots{}@samp{366})
13687 month (@samp{01}@dots{}@samp{12})
13689 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13691 week number of year, with Sunday as the first day of the week
13692 (@samp{00}@dots{}@samp{53}).
13693 Days in a new year preceding the first Sunday are in week zero.
13695 @acronym{ISO} week number, that is, the
13696 week number of year, with Monday as the first day of the week
13697 (@samp{01}@dots{}@samp{53}).
13698 If the week containing January 1 has four or more days in
13699 the new year, then it is considered week 1; otherwise, it is week 53 of
13700 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13703 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13705 week number of year, with Monday as first day of week
13706 (@samp{00}@dots{}@samp{53}).
13707 Days in a new year preceding the first Monday are in week zero.
13709 locale's date representation (e.g., @samp{12/31/99})
13711 last two digits of year (@samp{00}@dots{}@samp{99})
13713 year. This is normally at least four characters, but it may be more.
13714 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13715 precedes year @samp{0000}.
13719 @node Literal conversion specifiers
13720 @subsection Literal conversion specifiers
13722 @cindex literal conversion specifiers
13723 @cindex conversion specifiers, literal
13725 @command{date} conversion specifiers that produce literal strings.
13737 @node Padding and other flags
13738 @subsection Padding and other flags
13740 @cindex numeric field padding
13741 @cindex padding of numeric fields
13742 @cindex fields, padding numeric
13744 Unless otherwise specified, @command{date} normally pads numeric fields
13745 with zeros, so that, for
13746 example, numeric months are always output as two digits.
13747 Seconds since the epoch are not padded, though,
13748 since there is no natural width for them.
13750 As a @acronym{GNU} extension, @command{date} recognizes any of the
13751 following optional flags after the @samp{%}:
13755 (hyphen) Do not pad the field; useful if the output is intended for
13758 (underscore) Pad with spaces; useful if you need a fixed
13759 number of characters in the output, but zeros are too distracting.
13761 (zero) Pad with zeros even if the conversion specifier
13762 would normally pad with spaces.
13764 Use upper case characters if possible.
13766 Use opposite case characters if possible.
13767 A field that is normally upper case becomes lower case, and vice versa.
13771 Here are some examples of padding:
13774 date +%d/%m -d "Feb 1"
13776 date +%-d/%-m -d "Feb 1"
13778 date +%_d/%_m -d "Feb 1"
13782 As a @acronym{GNU} extension, you can specify the field width
13783 (after any flag, if present) as a decimal number. If the natural size of the
13784 output of the field has less than the specified number of characters,
13785 the result is written right adjusted and padded to the given
13786 size. For example, @samp{%9B} prints the right adjusted month name in
13787 a field of width 9.
13789 An optional modifier can follow the optional flag and width
13790 specification. The modifiers are:
13794 Use the locale's alternate representation for date and time. This
13795 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13796 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13797 example, @samp{%Ex} might yield a date format based on the Japanese
13801 Use the locale's alternate numeric symbols for numbers. This modifier
13802 applies only to numeric conversion specifiers.
13805 If the format supports the modifier but no alternate representation
13806 is available, it is ignored.
13809 @node Setting the time
13810 @subsection Setting the time
13812 @cindex setting the time
13813 @cindex time setting
13814 @cindex appropriate privileges
13816 If given an argument that does not start with @samp{+}, @command{date} sets
13817 the system clock to the date and time specified by that argument (as
13818 described below). You must have appropriate privileges to set the
13819 system clock. The @option{--date} and @option{--set} options may not be
13820 used with such an argument. The @option{--universal} option may be used
13821 with such an argument to indicate that the specified date and time are
13822 relative to Coordinated Universal Time rather than to the local time
13825 The argument must consist entirely of digits, which have the following
13838 first two digits of year (optional)
13840 last two digits of year (optional)
13845 The @option{--set} option also sets the system clock; see the next section.
13848 @node Options for date
13849 @subsection Options for @command{date}
13851 @cindex @command{date} options
13852 @cindex options for @command{date}
13854 The program accepts the following options. Also see @ref{Common options}.
13858 @item -d @var{datestr}
13859 @itemx --date=@var{datestr}
13862 @cindex parsing date strings
13863 @cindex date strings, parsing
13864 @cindex arbitrary date strings, parsing
13867 @opindex next @var{day}
13868 @opindex last @var{day}
13869 Display the date and time specified in @var{datestr} instead of the
13870 current date and time. @var{datestr} can be in almost any common
13871 format. It can contain month names, time zones, @samp{am} and @samp{pm},
13872 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
13873 14:19:13.489392193 +0530"} specifies the instant of time that is
13874 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
13875 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
13876 Note: input currently must be in locale independent format. E.g., the
13877 LC_TIME=C below is needed to print back the correct date in many locales:
13879 date -d "$(LC_TIME=C date)"
13881 @xref{Date input formats}.
13883 @item -f @var{datefile}
13884 @itemx --file=@var{datefile}
13887 Parse each line in @var{datefile} as with @option{-d} and display the
13888 resulting date and time. If @var{datefile} is @samp{-}, use standard
13889 input. This is useful when you have many dates to process, because the
13890 system overhead of starting up the @command{date} executable many times can
13893 @item -r @var{file}
13894 @itemx --reference=@var{file}
13896 @opindex --reference
13897 Display the date and time of the last modification of @var{file},
13898 instead of the current date and time.
13905 @opindex --rfc-2822
13906 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
13907 %z}, evaluated in the C locale so abbreviations are always in English.
13911 Fri, 09 Sep 2005 13:51:39 -0700
13914 This format conforms to
13915 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
13916 @acronym{RFCs} 2822} and
13917 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
13918 current and previous standards for Internet email.
13920 @item --rfc-3339=@var{timespec}
13921 @opindex --rfc-3339=@var{timespec}
13922 Display the date using a format specified by
13923 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
13924 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
13925 format, except that it also permits applications to use a space rather
13926 than a @samp{T} to separate dates from times. Unlike the other
13927 standard formats, @acronym{RFC} 3339 format is always suitable as
13928 input for the @option{--date} (@option{-d}) and @option{--file}
13929 (@option{-f}) options, regardless of the current locale.
13931 The argument @var{timespec} specifies how much of the time to include.
13932 It can be one of the following:
13936 Print just the full-date, e.g., @samp{2005-09-14}.
13937 This is equivalent to the format @samp{%Y-%m-%d}.
13940 Print the full-date and full-time separated by a space, e.g.,
13941 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
13942 time-offset; here the @samp{+05:30} means that local time is five
13943 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
13944 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
13947 Like @samp{seconds}, but also print nanoseconds, e.g.,
13948 @samp{2005-09-14 00:56:06.998458565+05:30}.
13949 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
13953 @item -s @var{datestr}
13954 @itemx --set=@var{datestr}
13957 Set the date and time to @var{datestr}. See @option{-d} above.
13964 @opindex --universal
13965 @cindex Coordinated Universal Time
13967 @cindex Greenwich Mean Time
13970 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
13971 @env{TZ} environment variable were set to the string @samp{UTC0}.
13973 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
13974 historical reasons.
13978 @node Examples of date
13979 @subsection Examples of @command{date}
13981 @cindex examples of @command{date}
13983 Here are a few examples. Also see the documentation for the @option{-d}
13984 option in the previous section.
13989 To print the date of the day before yesterday:
13992 date --date='2 days ago'
13996 To print the date of the day three months and one day hence:
13999 date --date='3 months 1 day'
14003 To print the day of year of Christmas in the current year:
14006 date --date='25 Dec' +%j
14010 To print the current full month name and the day of the month:
14016 But this may not be what you want because for the first nine days of
14017 the month, the @samp{%d} expands to a zero-padded two-digit field,
14018 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14021 To print a date without the leading zero for one-digit days
14022 of the month, you can use the (@acronym{GNU} extension)
14023 @samp{-} flag to suppress
14024 the padding altogether:
14027 date -d 1may '+%B %-d
14031 To print the current date and time in the format required by many
14032 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14035 date +%m%d%H%M%Y.%S
14039 To set the system clock forward by two minutes:
14042 date --set='+2 minutes'
14046 To print the date in @acronym{RFC} 2822 format,
14047 use @samp{date --rfc-2822}. Here is some example output:
14050 Fri, 09 Sep 2005 13:51:39 -0700
14053 @anchor{%s-examples}
14055 To convert a date string to the number of seconds since the epoch
14056 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14057 the @samp{%s} format. That can be useful in sorting and/or graphing
14058 and/or comparing data by date. The following command outputs the
14059 number of the seconds since the epoch for the time two minutes after the
14063 date --date='1970-01-01 00:02:00 +0000' +%s
14067 If you do not specify time zone information in the date string,
14068 @command{date} uses your computer's idea of the time zone when
14069 interpreting the string. For example, if your computer's time zone is
14070 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14071 seconds) behind UTC:
14074 # local time zone used
14075 date --date='1970-01-01 00:02:00' +%s
14080 If you're sorting or graphing dated data, your raw date values may be
14081 represented as seconds since the epoch. But few people can look at
14082 the date @samp{946684800} and casually note ``Oh, that's the first second
14083 of the year 2000 in Greenwich, England.''
14086 date --date='2000-01-01 UTC' +%s
14090 An alternative is to use the @option{--utc} (@option{-u}) option.
14091 Then you may omit @samp{UTC} from the date string. Although this
14092 produces the same result for @samp{%s} and many other format sequences,
14093 with a time zone offset different from zero, it would give a different
14094 result for zone-dependent formats like @samp{%z}.
14097 date -u --date=2000-01-01 +%s
14101 To convert such an unwieldy number of seconds back to
14102 a more readable form, use a command like this:
14105 # local time zone used
14106 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14107 1999-12-31 19:00:00 -0500
14110 Or if you do not mind depending on the @samp{@@} feature present since
14111 coreutils 5.3.0, you could shorten this to:
14114 date -d @@946684800 +"%F %T %z"
14115 1999-12-31 19:00:00 -0500
14118 Often it is better to output UTC-relative date and time:
14121 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14122 2000-01-01 00:00:00 +0000
14128 @node arch invocation
14129 @section @command{arch}: Print machine hardware name
14132 @cindex print machine hardware name
14133 @cindex system information, printing
14135 @command{arch} prints the machine hardware name,
14136 and is equivalent to @samp{uname -m}.
14140 arch [@var{option}]
14143 The program accepts the @ref{Common options} only.
14148 @node nproc invocation
14149 @section @command{nproc}: Print the number of available processors
14152 @cindex Print the number of processors
14153 @cindex system information, printing
14155 Print the number of processing units available to the current process,
14156 which may be less than the number of online processors.
14157 If this information is not accessible, then print the number of
14158 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14159 set, then it will determine the returned value. The result is guaranteed to be
14160 greater than zero. Synopsis:
14163 nproc [@var{option}]
14166 The program accepts the following options. Also see @ref{Common options}.
14172 Print the number of installed processors on the system, which may
14173 be greater than the number online or available to the current process.
14174 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14176 @item --ignore=@var{number}
14178 If possible, exclude this @var{number} of processing units.
14185 @node uname invocation
14186 @section @command{uname}: Print system information
14189 @cindex print system information
14190 @cindex system information, printing
14192 @command{uname} prints information about the machine and operating system
14193 it is run on. If no options are given, @command{uname} acts as if the
14194 @option{-s} option were given. Synopsis:
14197 uname [@var{option}]@dots{}
14200 If multiple options or @option{-a} are given, the selected information is
14201 printed in this order:
14204 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14205 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14208 The information may contain internal spaces, so such output cannot be
14209 parsed reliably. In the following example, @var{release} is
14210 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14214 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14218 The program accepts the following options. Also see @ref{Common options}.
14226 Print all of the below information, except omit the processor type
14227 and the hardware platform name if they are unknown.
14230 @itemx --hardware-platform
14232 @opindex --hardware-platform
14233 @cindex implementation, hardware
14234 @cindex hardware platform
14235 @cindex platform, hardware
14236 Print the hardware platform name
14237 (sometimes called the hardware implementation).
14238 Print @samp{unknown} if the kernel does not make this information
14239 easily available, as is the case with Linux kernels.
14245 @cindex machine type
14246 @cindex hardware class
14247 @cindex hardware type
14248 Print the machine hardware name (sometimes called the hardware class
14254 @opindex --nodename
14257 @cindex network node name
14258 Print the network node hostname.
14263 @opindex --processor
14264 @cindex host processor type
14265 Print the processor type (sometimes called the instruction set
14266 architecture or ISA).
14267 Print @samp{unknown} if the kernel does not make this information
14268 easily available, as is the case with Linux kernels.
14271 @itemx --operating-system
14273 @opindex --operating-system
14274 @cindex operating system name
14275 Print the name of the operating system.
14278 @itemx --kernel-release
14280 @opindex --kernel-release
14281 @cindex kernel release
14282 @cindex release of kernel
14283 Print the kernel release.
14286 @itemx --kernel-name
14288 @opindex --kernel-name
14289 @cindex kernel name
14290 @cindex name of kernel
14291 Print the kernel name.
14292 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14293 ``the implementation of the operating system'', because the
14294 @acronym{POSIX} specification itself has no notion of ``kernel''.
14295 The kernel name might be the same as the operating system name printed
14296 by the @option{-o} or @option{--operating-system} option, but it might
14297 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14298 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14302 @itemx --kernel-version
14304 @opindex --kernel-version
14305 @cindex kernel version
14306 @cindex version of kernel
14307 Print the kernel version.
14314 @node hostname invocation
14315 @section @command{hostname}: Print or set system name
14318 @cindex setting the hostname
14319 @cindex printing the hostname
14320 @cindex system name, printing
14321 @cindex appropriate privileges
14323 With no arguments, @command{hostname} prints the name of the current host
14324 system. With one argument, it sets the current host name to the
14325 specified string. You must have appropriate privileges to set the host
14329 hostname [@var{name}]
14332 The only options are @option{--help} and @option{--version}. @xref{Common
14338 @node hostid invocation
14339 @section @command{hostid}: Print numeric host identifier
14342 @cindex printing the host identifier
14344 @command{hostid} prints the numeric identifier of the current host
14345 in hexadecimal. This command accepts no arguments.
14346 The only options are @option{--help} and @option{--version}.
14347 @xref{Common options}.
14349 For example, here's what it prints on one system I use:
14356 On that system, the 32-bit quantity happens to be closely
14357 related to the system's Internet address, but that isn't always
14362 @node uptime invocation
14363 @section @command{uptime}: Print system uptime and load
14366 @cindex printing the system uptime and load
14368 @command{uptime} prints the current time, the system's uptime, the
14369 number of logged-in users and the current load average.
14371 If an argument is specified, it is used as the file to be read
14372 to discover how many users are logged in. If no argument is
14373 specified, a system default is used (@command{uptime --help} indicates
14374 the default setting).
14376 The only options are @option{--help} and @option{--version}.
14377 @xref{Common options}.
14379 For example, here's what it prints right now on one system I use:
14383 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14386 The precise method of calculation of load average varies somewhat
14387 between systems. Some systems calculate it as the average number of
14388 runnable processes over the last 1, 5 and 15 minutes, but some systems
14389 also include processes in the uninterruptible sleep state (that is,
14390 those processes which are waiting for disk I/O). The Linux kernel
14391 includes uninterruptible processes.
14393 @node SELinux context
14394 @chapter SELinux context
14396 @cindex SELinux context
14397 @cindex SELinux, context
14398 @cindex commands for SELinux context
14400 This section describes commands for operations with SELinux
14404 * chcon invocation:: Change SELinux context of file
14405 * runcon invocation:: Run a command in specified SELinux context
14408 @node chcon invocation
14409 @section @command{chcon}: Change SELinux context of file
14412 @cindex changing security context
14413 @cindex change SELinux context
14415 @command{chcon} changes the SELinux security context of the selected files.
14419 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14420 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14421 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14424 Change the SELinux security context of each @var{file} to @var{context}.
14425 With @option{--reference}, change the security context of each @var{file}
14426 to that of @var{rfile}.
14428 The program accepts the following options. Also see @ref{Common options}.
14433 @itemx --no-dereference
14435 @opindex --no-dereference
14436 @cindex no dereference
14437 Affect symbolic links instead of any referenced file.
14439 @item --reference=@var{rfile}
14440 @opindex --reference
14441 @cindex reference file
14442 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14447 @opindex --recursive
14448 Operate on files and directories recursively.
14451 @xref{Traversing symlinks}.
14454 @xref{Traversing symlinks}.
14457 @xref{Traversing symlinks}.
14464 Output a diagnostic for every file processed.
14466 @item -u @var{user}
14467 @itemx --user=@var{user}
14470 Set user @var{user} in the target security context.
14472 @item -r @var{role}
14473 @itemx --role=@var{role}
14476 Set role @var{role} in the target security context.
14478 @item -t @var{type}
14479 @itemx --type=@var{type}
14482 Set type @var{type} in the target security context.
14484 @item -l @var{range}
14485 @itemx --range=@var{range}
14488 Set range @var{range} in the target security context.
14494 @node runcon invocation
14495 @section @command{runcon}: Run a command in specified SELinux context
14498 @cindex run with security context
14501 @command{runcon} runs file in specified SELinux security context.
14505 runcon @var{context} @var{command} [@var{args}]
14506 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14509 Run @var{command} with completely-specified @var{context}, or with
14510 current or transitioned security context modified by one or more of @var{level},
14511 @var{role}, @var{type} and @var{user}.
14513 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14514 is specified, the first argument is used as the complete context.
14515 Any additional arguments after @var{command}
14516 are interpreted as arguments to the command.
14518 With neither @var{context} nor @var{command}, print the current security context.
14520 The program accepts the following options. Also see @ref{Common options}.
14528 Compute process transition context before modifying.
14530 @item -u @var{user}
14531 @itemx --user=@var{user}
14534 Set user @var{user} in the target security context.
14536 @item -r @var{role}
14537 @itemx --role=@var{role}
14540 Set role @var{role} in the target security context.
14542 @item -t @var{type}
14543 @itemx --type=@var{type}
14546 Set type @var{type} in the target security context.
14548 @item -l @var{range}
14549 @itemx --range=@var{range}
14552 Set range @var{range} in the target security context.
14556 @cindex exit status of @command{runcon}
14560 126 if @var{command} is found but cannot be invoked
14561 127 if @command{runcon} itself fails or if @var{command} cannot be found
14562 the exit status of @var{command} otherwise
14565 @node Modified command invocation
14566 @chapter Modified command invocation
14568 @cindex modified command invocation
14569 @cindex invocation of commands, modified
14570 @cindex commands for invoking other commands
14572 This section describes commands that run other commands in some context
14573 different than the current one: a modified environment, as a different
14577 * chroot invocation:: Modify the root directory.
14578 * env invocation:: Modify environment variables.
14579 * nice invocation:: Modify niceness.
14580 * nohup invocation:: Immunize to hangups.
14581 * stdbuf invocation:: Modify buffering of standard streams.
14582 * su invocation:: Modify user and group ID.
14583 * timeout invocation:: Run with time limit.
14587 @node chroot invocation
14588 @section @command{chroot}: Run a command with a different root directory
14591 @cindex running a program in a specified root directory
14592 @cindex root directory, running a program in a specified
14594 @command{chroot} runs a command with a specified root directory.
14595 On many systems, only the super-user can do this.@footnote{However,
14596 some systems (e.g., FreeBSD) can be configured to allow certain regular
14597 users to use the @code{chroot} system call, and hence to run this program.
14598 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14599 underlying function is non-privileged due to lack of support in MS-Windows.}
14603 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14604 chroot @var{option}
14607 Ordinarily, file names are looked up starting at the root of the
14608 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14609 the directory @var{newroot} (which must exist) and then runs
14610 @var{command} with optional @var{args}. If @var{command} is not
14611 specified, the default is the value of the @env{SHELL} environment
14612 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14613 @var{command} must not be a special built-in utility
14614 (@pxref{Special built-in utilities}).
14616 The program accepts the following options. Also see @ref{Common options}.
14617 Options must precede operands.
14621 @itemx --userspec=@var{user}[:@var{group}]
14622 @opindex --userspec
14623 By default, @var{command} is run with the same credentials
14624 as the invoking process.
14625 Use this option to run it as a different @var{user} and/or with a
14626 different primary @var{group}.
14628 @itemx --groups=@var{groups}
14630 Use this option to specify the supplementary @var{groups} to be
14631 used by the new process.
14632 The items in the list (names or numeric IDs) must be separated by commas.
14636 Here are a few tips to help avoid common problems in using chroot.
14637 To start with a simple example, make @var{command} refer to a statically
14638 linked binary. If you were to use a dynamically linked executable, then
14639 you'd have to arrange to have the shared libraries in the right place under
14640 your new root directory.
14642 For example, if you create a statically linked @command{ls} executable,
14643 and put it in @file{/tmp/empty}, you can run this command as root:
14646 $ chroot /tmp/empty /ls -Rl /
14649 Then you'll see output like this:
14654 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14657 If you want to use a dynamically linked executable, say @command{bash},
14658 then first run @samp{ldd bash} to see what shared objects it needs.
14659 Then, in addition to copying the actual binary, also copy the listed
14660 files to the required positions under your intended new root directory.
14661 Finally, if the executable requires any other files (e.g., data, state,
14662 device files), copy them into place, too.
14664 @cindex exit status of @command{chroot}
14668 125 if @command{chroot} itself fails
14669 126 if @var{command} is found but cannot be invoked
14670 127 if @var{command} cannot be found
14671 the exit status of @var{command} otherwise
14675 @node env invocation
14676 @section @command{env}: Run a command in a modified environment
14679 @cindex environment, running a program in a modified
14680 @cindex modified environment, running a program in a
14681 @cindex running a program in a modified environment
14683 @command{env} runs a command with a modified environment. Synopses:
14686 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14687 [@var{command} [@var{args}]@dots{}]
14691 Operands of the form @samp{@var{variable}=@var{value}} set
14692 the environment variable @var{variable} to value @var{value}.
14693 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14694 to an empty value is different from unsetting it.
14695 These operands are evaluated left-to-right, so if two operands
14696 mention the same variable the earlier is ignored.
14698 Environment variable names can be empty, and can contain any
14699 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14700 However, it is wise to limit yourself to names that
14701 consist solely of underscores, digits, and @acronym{ASCII} letters,
14702 and that begin with a non-digit, as applications like the shell do not
14703 work well with other names.
14706 The first operand that does not contain the character @samp{=}
14707 specifies the program to invoke; it is
14708 searched for according to the @env{PATH} environment variable. Any
14709 remaining arguments are passed as arguments to that program.
14710 The program should not be a special built-in utility
14711 (@pxref{Special built-in utilities}).
14713 Modifications to @env{PATH} take effect prior to searching for
14714 @var{command}. Use caution when reducing @env{PATH}; behavior is
14715 not portable when @env{PATH} is undefined or omits key directories
14716 such as @file{/bin}.
14718 In the rare case that a utility contains a @samp{=} in the name, the
14719 only way to disambiguate it from a variable assignment is to use an
14720 intermediate command for @var{command}, and pass the problematic
14721 program name via @var{args}. For example, if @file{./prog=} is an
14722 executable in the current @env{PATH}:
14725 env prog= true # runs 'true', with prog= in environment
14726 env ./prog= true # runs 'true', with ./prog= in environment
14727 env -- prog= true # runs 'true', with prog= in environment
14728 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14729 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14732 @cindex environment, printing
14734 If no command name is specified following the environment
14735 specifications, the resulting environment is printed. This is like
14736 specifying the @command{printenv} program.
14738 For some examples, suppose the environment passed to @command{env}
14739 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14740 @samp{PATH=.:/gnubin:/hacks}:
14745 Output the current environment.
14747 $ env | LC_ALL=C sort
14750 PATH=.:/gnubin:/hacks
14754 Run @command{foo} with a reduced environment, preserving only the
14755 original @env{PATH} to avoid problems in locating @command{foo}.
14757 env - PATH="$PATH" foo
14761 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14762 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14763 that @command{foo} was found in the file system rather than as a shell
14770 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14771 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14772 @samp{DISPLAY=gnu:0}.
14774 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14778 Attempt to run the program @command{/energy/--} (as that is the only
14779 possible path search result); if the command exists, the environment
14780 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14781 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14783 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14789 The program accepts the following options. Also see @ref{Common options}.
14790 Options must precede operands.
14796 @item -u @var{name}
14797 @itemx --unset=@var{name}
14800 Remove variable @var{name} from the environment, if it was in the
14805 @itemx --ignore-environment
14808 @opindex --ignore-environment
14809 Start with an empty environment, ignoring the inherited environment.
14813 @cindex exit status of @command{env}
14817 0 if no @var{command} is specified and the environment is output
14818 125 if @command{env} itself fails
14819 126 if @var{command} is found but cannot be invoked
14820 127 if @var{command} cannot be found
14821 the exit status of @var{command} otherwise
14825 @node nice invocation
14826 @section @command{nice}: Run a command with modified niceness
14830 @cindex scheduling, affecting
14831 @cindex appropriate privileges
14833 @command{nice} prints or modifies a process's @dfn{niceness},
14834 a parameter that affects whether the process is scheduled favorably.
14838 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
14841 If no arguments are given, @command{nice} prints the current niceness.
14842 Otherwise, @command{nice} runs the given @var{command} with its
14843 niceness adjusted. By default, its niceness is incremented by 10.
14845 Niceness values range at least from @minus{}20 (process has high priority
14846 and gets more resources, thus slowing down other processes) through 19
14847 (process has lower priority and runs slowly itself, but has less impact
14848 on the speed of other running processes). Some systems
14849 may have a wider range of nicenesses; conversely, other systems may
14850 enforce more restrictive limits. An attempt to set the niceness
14851 outside the supported range is treated as an attempt to use the
14852 minimum or maximum supported value.
14854 A niceness should not be confused with a scheduling priority, which
14855 lets applications determine the order in which threads are scheduled
14856 to run. Unlike a priority, a niceness is merely advice to the
14857 scheduler, which the scheduler is free to ignore. Also, as a point of
14858 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
14859 terms of a @dfn{nice value}, which is the nonnegative difference
14860 between a niceness and the minimum niceness. Though @command{nice}
14861 conforms to @acronym{POSIX}, its documentation and diagnostics use the
14862 term ``niceness'' for compatibility with historical practice.
14864 @var{command} must not be a special built-in utility (@pxref{Special
14865 built-in utilities}).
14867 @mayConflictWithShellBuiltIn{nice}
14869 The program accepts the following option. Also see @ref{Common options}.
14870 Options must precede operands.
14873 @item -n @var{adjustment}
14874 @itemx --adjustment=@var{adjustment}
14876 @opindex --adjustment
14877 Add @var{adjustment} instead of 10 to the command's niceness. If
14878 @var{adjustment} is negative and you lack appropriate privileges,
14879 @command{nice} issues a warning but otherwise acts as if you specified
14882 For compatibility @command{nice} also supports an obsolete
14883 option syntax @option{-@var{adjustment}}. New scripts should use
14884 @option{-n @var{adjustment}} instead.
14888 @cindex exit status of @command{nice}
14892 0 if no @var{command} is specified and the niceness is output
14893 125 if @command{nice} itself fails
14894 126 if @var{command} is found but cannot be invoked
14895 127 if @var{command} cannot be found
14896 the exit status of @var{command} otherwise
14899 It is sometimes useful to run a non-interactive program with reduced niceness.
14902 $ nice factor 4611686018427387903
14905 Since @command{nice} prints the current niceness,
14906 you can invoke it through itself to demonstrate how it works.
14908 The default behavior is to increase the niceness by @samp{10}:
14919 The @var{adjustment} is relative to the current niceness. In the
14920 next example, the first @command{nice} invocation runs the second one
14921 with niceness 10, and it in turn runs the final one with a niceness
14925 $ nice nice -n 3 nice
14929 Specifying a niceness larger than the supported range
14930 is the same as specifying the maximum supported value:
14933 $ nice -n 10000000000 nice
14937 Only a privileged user may run a process with lower niceness:
14941 nice: cannot set niceness: Permission denied
14943 $ sudo nice -n -1 nice
14948 @node nohup invocation
14949 @section @command{nohup}: Run a command immune to hangups
14952 @cindex hangups, immunity to
14953 @cindex immunity to hangups
14954 @cindex logging out and continuing to run
14957 @command{nohup} runs the given @var{command} with hangup signals ignored,
14958 so that the command can continue running in the background after you log
14962 nohup @var{command} [@var{arg}]@dots{}
14965 If standard input is a terminal, it is redirected from
14966 @file{/dev/null} so that terminal sessions do not mistakenly consider
14967 the terminal to be used by the command. This is a @acronym{GNU}
14968 extension; programs intended to be portable to non-@acronym{GNU} hosts
14969 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
14973 If standard output is a terminal, the command's standard output is appended
14974 to the file @file{nohup.out}; if that cannot be written to, it is appended
14975 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
14976 command is not run.
14977 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
14978 @command{nohup} is made readable and writable only to the user,
14979 regardless of the current umask settings.
14981 If standard error is a terminal, it is normally redirected to the same file
14982 descriptor as the (possibly-redirected) standard output.
14983 However, if standard output is closed, standard error terminal output
14984 is instead appended to the file @file{nohup.out} or
14985 @file{$HOME/nohup.out} as above.
14987 To capture the command's output to a file other than @file{nohup.out}
14988 you can redirect it. For example, to capture the output of
14992 nohup make > make.log
14995 @command{nohup} does not automatically put the command it runs in the
14996 background; you must do that explicitly, by ending the command line
14997 with an @samp{&}. Also, @command{nohup} does not alter the
14998 niceness of @var{command}; use @command{nice} for that,
14999 e.g., @samp{nohup nice @var{command}}.
15001 @var{command} must not be a special built-in utility (@pxref{Special
15002 built-in utilities}).
15004 The only options are @option{--help} and @option{--version}. @xref{Common
15005 options}. Options must precede operands.
15007 @cindex exit status of @command{nohup}
15011 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15012 126 if @var{command} is found but cannot be invoked
15013 127 if @var{command} cannot be found
15014 the exit status of @var{command} otherwise
15017 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15021 @node stdbuf invocation
15022 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15025 @cindex standard streams, buffering
15026 @cindex line buffered
15028 @command{stdbuf} allows one to modify the buffering operations of the
15029 three standard I/O streams associated with a program. Synopsis:
15032 stdbuf @var{option}@dots{} @var{command}
15035 Any additional @var{arg}s are passed as additional arguments to the
15038 The program accepts the following options. Also see @ref{Common options}.
15042 @item -i @var{mode}
15043 @itemx --input=@var{mode}
15046 Adjust the standard input stream buffering.
15048 @item -o @var{mode}
15049 @itemx --output=@var{mode}
15052 Adjust the standard output stream buffering.
15054 @item -e @var{mode}
15055 @itemx --error=@var{mode}
15058 Adjust the standard error stream buffering.
15062 The @var{mode} can be specified as follows:
15067 Set the stream to line buffered mode.
15068 In this mode data is coalesced until a newline is output or
15069 input is read from any stream attached to a terminal device.
15070 This option is invalid with standard input.
15073 Disable buffering of the selected stream.
15074 In this mode data is output immediately and only the
15075 amount of data requested is read from input.
15078 Specify the size of the buffer to use in fully buffered mode.
15079 @multiplierSuffixesNoBlocks{size}
15083 NOTE: If @var{command} adjusts the buffering of its standard streams
15084 (@command{tee} does for e.g.) then that will override corresponding settings
15085 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15086 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15087 by @command{stdbuf} settings.
15089 @cindex exit status of @command{stdbuf}
15093 125 if @command{stdbuf} itself fails
15094 126 if @var{command} is found but cannot be invoked
15095 127 if @var{command} cannot be found
15096 the exit status of @var{command} otherwise
15100 @node su invocation
15101 @section @command{su}: Run a command with substitute user and group ID
15104 @cindex substitute user and group IDs
15105 @cindex user ID, switching
15106 @cindex super-user, becoming
15107 @cindex root, becoming
15109 @command{su} allows one user to temporarily become another user. It runs a
15110 command (often an interactive shell) with the real and effective user
15111 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15114 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15117 @cindex passwd entry, and @command{su} shell
15119 @flindex /etc/passwd
15120 If no @var{user} is given, the default is @code{root}, the super-user.
15121 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15122 @file{/bin/sh} if none is specified there. If @var{user} has a
15123 password, @command{su} prompts for the password unless run by a user with
15124 effective user ID of zero (the super-user).
15130 @cindex login shell
15131 By default, @command{su} does not change the current directory.
15132 It sets the environment variables @env{HOME} and @env{SHELL}
15133 from the password entry for @var{user}, and if @var{user} is not
15134 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15135 By default, the shell is not a login shell.
15137 Any additional @var{arg}s are passed as additional arguments to the
15140 @cindex @option{-su}
15141 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15142 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15143 to certain shells, etc.).
15146 @command{su} can optionally be compiled to use @code{syslog} to report
15147 failed, and optionally successful, @command{su} attempts. (If the system
15148 supports @code{syslog}.) However, GNU @command{su} does not check if the
15149 user is a member of the @code{wheel} group; see below.
15151 The program accepts the following options. Also see @ref{Common options}.
15154 @item -c @var{command}
15155 @itemx --command=@var{command}
15158 Pass @var{command}, a single command line to run, to the shell with
15159 a @option{-c} option instead of starting an interactive shell.
15166 @cindex file name pattern expansion, disabled
15167 @cindex globbing, disabled
15168 Pass the @option{-f} option to the shell. This probably only makes sense
15169 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15170 option prevents reading the startup file (@file{.cshrc}). With
15171 Bourne-like shells, the @option{-f} option disables file name pattern
15172 expansion (globbing), which is not likely to be useful.
15180 @c other variables already indexed above
15183 @cindex login shell, creating
15184 Make the shell a login shell. This means the following. Unset all
15185 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15186 (which are set as described above), and @env{USER} and @env{LOGNAME}
15187 (which are set, even for the super-user, as described above), and set
15188 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15189 directory. Prepend @samp{-} to the shell's name, intended to make it
15190 read its login startup file(s).
15194 @itemx --preserve-environment
15197 @opindex --preserve-environment
15198 @cindex environment, preserving
15199 @flindex /etc/shells
15200 @cindex restricted shell
15201 Do not change the environment variables @env{HOME}, @env{USER},
15202 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15203 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15204 entry, unless the user running @command{su} is not the super-user and
15205 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15206 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15207 if that file does not exist. Parts of what this option does can be
15208 overridden by @option{--login} and @option{--shell}.
15210 @item -s @var{shell}
15211 @itemx --shell=@var{shell}
15214 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15215 unless the user running @command{su} is not the super-user and @var{user}'s
15216 shell is restricted (see @option{-m} just above).
15220 @cindex exit status of @command{su}
15224 125 if @command{su} itself fails
15225 126 if subshell is found but cannot be invoked
15226 127 if subshell cannot be found
15227 the exit status of the subshell otherwise
15230 @cindex wheel group, not supported
15231 @cindex group wheel, not supported
15233 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15235 (This section is by Richard Stallman.)
15239 Sometimes a few of the users try to hold total power over all the
15240 rest. For example, in 1984, a few users at the MIT AI lab decided to
15241 seize power by changing the operator password on the Twenex system and
15242 keeping it secret from everyone else. (I was able to thwart this coup
15243 and give power back to the users by patching the kernel, but I
15244 wouldn't know how to do that in Unix.)
15246 However, occasionally the rulers do tell someone. Under the usual
15247 @command{su} mechanism, once someone learns the root password who
15248 sympathizes with the ordinary users, he or she can tell the rest. The
15249 ``wheel group'' feature would make this impossible, and thus cement the
15250 power of the rulers.
15252 I'm on the side of the masses, not that of the rulers. If you are
15253 used to supporting the bosses and sysadmins in whatever they do, you
15254 might find this idea strange at first.
15257 @node timeout invocation
15258 @section @command{timeout}: Run a command with a time limit
15262 @cindex run commands with bounded time
15264 @command{timeout} runs the given @var{command} and kills it if it is
15265 still running after the specified time interval. Synopsis:
15268 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15271 @var{command} must not be a special built-in utility (@pxref{Special
15272 built-in utilities}).
15274 The program accepts the following options. Also see @ref{Common options}.
15275 Options must precede operands.
15278 @item -k @var{duration}
15279 @itemx --kill-after=@var{duration}
15281 @opindex --kill-after
15282 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15283 signal, after the specified @var{duration}. Without this option, if the
15284 selected signal proves not to be fatal, @command{timeout} does not kill
15287 @item -s @var{signal}
15288 @itemx --signal=@var{signal}
15291 Send this @var{signal} to @var{command} on timeout, rather than the
15292 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15293 or a number. Also see @xref{Signal specifications}.
15297 @var{duration} is an integer followed by an optional unit:
15299 @samp{s} for seconds (the default)
15300 @samp{m} for minutes
15304 A duration of 0 disables the associated timeout.
15306 @cindex exit status of @command{timeout}
15310 124 if @var{command} times out
15311 125 if @command{timeout} itself fails
15312 126 if @var{command} is found but cannot be invoked
15313 127 if @var{command} cannot be found
15314 the exit status of @var{command} otherwise
15318 @node Process control
15319 @chapter Process control
15321 @cindex processes, commands for controlling
15322 @cindex commands for controlling processes
15325 * kill invocation:: Sending a signal to processes.
15329 @node kill invocation
15330 @section @command{kill}: Send a signal to processes
15333 @cindex send a signal to processes
15335 The @command{kill} command sends a signal to processes, causing them
15336 to terminate or otherwise act upon receiving the signal in some way.
15337 Alternatively, it lists information about signals. Synopses:
15340 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15341 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15344 @mayConflictWithShellBuiltIn{kill}
15346 The first form of the @command{kill} command sends a signal to all
15347 @var{pid} arguments. The default signal to send if none is specified
15348 is @samp{TERM}. The special signal number @samp{0} does not denote a
15349 valid signal, but can be used to test whether the @var{pid} arguments
15350 specify processes to which a signal could be sent.
15352 If @var{pid} is positive, the signal is sent to the process with the
15353 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15354 processes in the process group of the current process. If @var{pid}
15355 is @minus{}1, the signal is sent to all processes for which the user has
15356 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15357 is sent to all processes in the process group that equals the absolute
15358 value of @var{pid}.
15360 If @var{pid} is not positive, a system-dependent set of system
15361 processes is excluded from the list of processes to which the signal
15364 If a negative @var{pid} argument is desired as the first one, it
15365 should be preceded by @option{--}. However, as a common extension to
15366 @acronym{POSIX}, @option{--} is not required with @samp{kill
15367 -@var{signal} -@var{pid}}. The following commands are equivalent:
15376 The first form of the @command{kill} command succeeds if every @var{pid}
15377 argument specifies at least one process that the signal was sent to.
15379 The second form of the @command{kill} command lists signal information.
15380 Either the @option{-l} or @option{--list} option, or the @option{-t}
15381 or @option{--table} option must be specified. Without any
15382 @var{signal} argument, all supported signals are listed. The output
15383 of @option{-l} or @option{--list} is a list of the signal names, one
15384 per line; if @var{signal} is already a name, the signal number is
15385 printed instead. The output of @option{-t} or @option{--table} is a
15386 table of signal numbers, names, and descriptions. This form of the
15387 @command{kill} command succeeds if all @var{signal} arguments are valid
15388 and if there is no output error.
15390 The @command{kill} command also supports the @option{--help} and
15391 @option{--version} options. @xref{Common options}.
15393 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15394 number like @samp{1}, or an exit status of a process terminated by the
15395 signal. A signal name can be given in canonical form or prefixed by
15396 @samp{SIG}. The case of the letters is ignored, except for the
15397 @option{-@var{signal}} option which must use upper case to avoid
15398 ambiguity with lower case option letters. For a list of supported
15399 signal names and numbers see @xref{Signal specifications}.
15404 @cindex delaying commands
15405 @cindex commands for delaying
15407 @c Perhaps @command{wait} or other commands should be described here also?
15410 * sleep invocation:: Delay for a specified time.
15414 @node sleep invocation
15415 @section @command{sleep}: Delay for a specified time
15418 @cindex delay for a specified time
15420 @command{sleep} pauses for an amount of time specified by the sum of
15421 the values of the command line arguments.
15425 sleep @var{number}[smhd]@dots{}
15429 Each argument is a number followed by an optional unit; the default
15430 is seconds. The units are:
15443 Historical implementations of @command{sleep} have required that
15444 @var{number} be an integer, and only accepted a single argument
15445 without a suffix. However, GNU @command{sleep} accepts
15446 arbitrary floating point numbers (using a period before any fractional
15449 The only options are @option{--help} and @option{--version}. @xref{Common
15452 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15453 @mayConflictWithShellBuiltIn{sleep}
15458 @node Numeric operations
15459 @chapter Numeric operations
15461 @cindex numeric operations
15462 These programs do numerically-related operations.
15465 * factor invocation:: Show factors of numbers.
15466 * seq invocation:: Print sequences of numbers.
15470 @node factor invocation
15471 @section @command{factor}: Print prime factors
15474 @cindex prime factors
15476 @command{factor} prints prime factors. Synopses:
15479 factor [@var{number}]@dots{}
15480 factor @var{option}
15483 If no @var{number} is specified on the command line, @command{factor} reads
15484 numbers from standard input, delimited by newlines, tabs, or spaces.
15486 The @command{factor} command supports only a small number of options:
15490 Print a short help on standard output, then exit without further
15494 Print the program version on standard output, then exit without further
15498 Factoring the product of the eighth and ninth Mersenne primes
15499 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15502 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15503 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15504 4951760154835678088235319297: 2147483647 2305843009213693951
15508 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15509 about 20 seconds on the same machine.
15511 Factoring large numbers is, in general, hard. The Pollard Rho
15512 algorithm used by @command{factor} is particularly effective for
15513 numbers with relatively small factors. If you wish to factor large
15514 numbers which do not have small factors (for example, numbers which
15515 are the product of two large primes), other methods are far better.
15517 If @command{factor} is built without using GNU MP, only
15518 single-precision arithmetic is available, and so large numbers
15519 (typically @math{2^{64}} and above) will not be supported. The single-precision
15520 code uses an algorithm which is designed for factoring smaller
15526 @node seq invocation
15527 @section @command{seq}: Print numeric sequences
15530 @cindex numeric sequences
15531 @cindex sequence of numbers
15533 @command{seq} prints a sequence of numbers to standard output. Synopses:
15536 seq [@var{option}]@dots{} @var{last}
15537 seq [@var{option}]@dots{} @var{first} @var{last}
15538 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15541 @command{seq} prints the numbers from @var{first} to @var{last} by
15542 @var{increment}. By default, each number is printed on a separate line.
15543 When @var{increment} is not specified, it defaults to @samp{1},
15544 even when @var{first} is larger than @var{last}.
15545 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15546 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15547 Floating-point numbers
15548 may be specified (using a period before any fractional digits).
15550 The program accepts the following options. Also see @ref{Common options}.
15551 Options must precede operands.
15554 @item -f @var{format}
15555 @itemx --format=@var{format}
15556 @opindex -f @var{format}
15557 @opindex --format=@var{format}
15558 @cindex formatting of numbers in @command{seq}
15559 Print all numbers using @var{format}.
15560 @var{format} must contain exactly one of the @samp{printf}-style
15561 floating point conversion specifications @samp{%a}, @samp{%e},
15562 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15563 The @samp{%} may be followed by zero or more flags taken from the set
15564 @samp{-+#0 '}, then an optional width containing one or more digits,
15565 then an optional precision consisting of a @samp{.} followed by zero
15566 or more digits. @var{format} may also contain any number of @samp{%%}
15567 conversion specifications. All conversion specifications have the
15568 same meaning as with @samp{printf}.
15570 The default format is derived from @var{first}, @var{step}, and
15571 @var{last}. If these all use a fixed point decimal representation,
15572 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15573 precision that can represent the output numbers exactly. Otherwise,
15574 the default format is @samp{%g}.
15576 @item -s @var{string}
15577 @itemx --separator=@var{string}
15578 @cindex separator for numbers in @command{seq}
15579 Separate numbers with @var{string}; default is a newline.
15580 The output always terminates with a newline.
15583 @itemx --equal-width
15584 Print all numbers with the same width, by padding with leading zeros.
15585 @var{first}, @var{step}, and @var{last} should all use a fixed point
15586 decimal representation.
15587 (To have other kinds of padding, use @option{--format}).
15591 You can get finer-grained control over output with @option{-f}:
15594 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15600 If you want hexadecimal integer output, you can use @command{printf}
15601 to perform the conversion:
15604 $ printf '%x\n' `seq 1048575 1024 1050623`
15610 For very long lists of numbers, use xargs to avoid
15611 system limitations on the length of an argument list:
15614 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15620 To generate octal output, use the printf @code{%o} format instead
15623 On most systems, seq can produce whole-number output for values up to
15624 at least @math{2^{53}}. Larger integers are approximated. The details
15625 differ depending on your floating-point implementation, but a common
15626 case is that @command{seq} works with integers through @math{2^{64}},
15627 and larger integers may not be numerically correct:
15630 $ seq 18446744073709551616 1 18446744073709551618
15631 18446744073709551616
15632 18446744073709551616
15633 18446744073709551618
15636 Be careful when using @command{seq} with outlandish values: otherwise
15637 you may see surprising results, as @command{seq} uses floating point
15638 internally. For example, on the x86 platform, where the internal
15639 representation uses a 64-bit fraction, the command:
15642 seq 1 0.0000000000000000001 1.0000000000000000009
15645 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15650 @node File permissions
15651 @chapter File permissions
15654 @include getdate.texi
15658 @node Opening the software toolbox
15659 @chapter Opening the Software Toolbox
15661 An earlier version of this chapter appeared in
15662 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15663 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15664 It was written by Arnold Robbins.
15667 * Toolbox introduction:: Toolbox introduction
15668 * I/O redirection:: I/O redirection
15669 * The who command:: The @command{who} command
15670 * The cut command:: The @command{cut} command
15671 * The sort command:: The @command{sort} command
15672 * The uniq command:: The @command{uniq} command
15673 * Putting the tools together:: Putting the tools together
15677 @node Toolbox introduction
15678 @unnumberedsec Toolbox Introduction
15680 This month's column is only peripherally related to the GNU Project, in
15681 that it describes a number of the GNU tools on your GNU/Linux system and how they
15682 might be used. What it's really about is the ``Software Tools'' philosophy
15683 of program development and usage.
15685 The software tools philosophy was an important and integral concept
15686 in the initial design and development of Unix (of which Linux and GNU are
15687 essentially clones). Unfortunately, in the modern day press of
15688 Internetworking and flashy GUIs, it seems to have fallen by the
15689 wayside. This is a shame, since it provides a powerful mental model
15690 for solving many kinds of problems.
15692 Many people carry a Swiss Army knife around in their pants pockets (or
15693 purse). A Swiss Army knife is a handy tool to have: it has several knife
15694 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15695 a number of other things on it. For the everyday, small miscellaneous jobs
15696 where you need a simple, general purpose tool, it's just the thing.
15698 On the other hand, an experienced carpenter doesn't build a house using
15699 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15700 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15701 exactly when and where to use each tool; you won't catch him hammering nails
15702 with the handle of his screwdriver.
15704 The Unix developers at Bell Labs were all professional programmers and trained
15705 computer scientists. They had found that while a one-size-fits-all program
15706 might appeal to a user because there's only one program to use, in practice
15711 difficult to write,
15714 difficult to maintain and
15718 difficult to extend to meet new situations.
15721 Instead, they felt that programs should be specialized tools. In short, each
15722 program ``should do one thing well.'' No more and no less. Such programs are
15723 simpler to design, write, and get right---they only do one thing.
15725 Furthermore, they found that with the right machinery for hooking programs
15726 together, that the whole was greater than the sum of the parts. By combining
15727 several special purpose programs, you could accomplish a specific task
15728 that none of the programs was designed for, and accomplish it much more
15729 quickly and easily than if you had to write a special purpose program.
15730 We will see some (classic) examples of this further on in the column.
15731 (An important additional point was that, if necessary, take a detour
15732 and build any software tools you may need first, if you don't already
15733 have something appropriate in the toolbox.)
15735 @node I/O redirection
15736 @unnumberedsec I/O Redirection
15738 Hopefully, you are familiar with the basics of I/O redirection in the
15739 shell, in particular the concepts of ``standard input,'' ``standard output,''
15740 and ``standard error''. Briefly, ``standard input'' is a data source, where
15741 data comes from. A program should not need to either know or care if the
15742 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15743 card reader. Similarly, ``standard output'' is a data sink, where data goes
15744 to. The program should neither know nor care where this might be.
15745 Programs that only read their standard input, do something to the data,
15746 and then send it on, are called @dfn{filters}, by analogy to filters in a
15749 With the Unix shell, it's very easy to set up data pipelines:
15752 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15755 We start out by creating the raw data; each filter applies some successive
15756 transformation to the data, until by the time it comes out of the pipeline,
15757 it is in the desired form.
15759 This is fine and good for standard input and standard output. Where does the
15760 standard error come in to play? Well, think about @command{filter1} in
15761 the pipeline above. What happens if it encounters an error in the data it
15762 sees? If it writes an error message to standard output, it will just
15763 disappear down the pipeline into @command{filter2}'s input, and the
15764 user will probably never see it. So programs need a place where they can send
15765 error messages so that the user will notice them. This is standard error,
15766 and it is usually connected to your console or window, even if you have
15767 redirected standard output of your program away from your screen.
15769 For filter programs to work together, the format of the data has to be
15770 agreed upon. The most straightforward and easiest format to use is simply
15771 lines of text. Unix data files are generally just streams of bytes, with
15772 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15773 conventionally called a ``newline'' in the Unix literature. (This is
15774 @code{'\n'} if you're a C programmer.) This is the format used by all
15775 the traditional filtering programs. (Many earlier operating systems
15776 had elaborate facilities and special purpose programs for managing
15777 binary data. Unix has always shied away from such things, under the
15778 philosophy that it's easiest to simply be able to view and edit your
15779 data with a text editor.)
15781 OK, enough introduction. Let's take a look at some of the tools, and then
15782 we'll see how to hook them together in interesting ways. In the following
15783 discussion, we will only present those command line options that interest
15784 us. As you should always do, double check your system documentation
15785 for the full story.
15787 @node The who command
15788 @unnumberedsec The @command{who} Command
15790 The first program is the @command{who} command. By itself, it generates a
15791 list of the users who are currently logged in. Although I'm writing
15792 this on a single-user system, we'll pretend that several people are
15797 @print{} arnold console Jan 22 19:57
15798 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15799 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15800 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15803 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15804 There are three people logged in, and I am logged in twice. On traditional
15805 Unix systems, user names are never more than eight characters long. This
15806 little bit of trivia will be useful later. The output of @command{who} is nice,
15807 but the data is not all that exciting.
15809 @node The cut command
15810 @unnumberedsec The @command{cut} Command
15812 The next program we'll look at is the @command{cut} command. This program
15813 cuts out columns or fields of input data. For example, we can tell it
15814 to print just the login name and full name from the @file{/etc/passwd}
15815 file. The @file{/etc/passwd} file has seven fields, separated by
15819 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15822 To get the first and fifth fields, we would use @command{cut} like this:
15825 $ cut -d: -f1,5 /etc/passwd
15826 @print{} root:Operator
15828 @print{} arnold:Arnold D. Robbins
15829 @print{} miriam:Miriam A. Robbins
15833 With the @option{-c} option, @command{cut} will cut out specific characters
15834 (i.e., columns) in the input lines. This is useful for input data
15835 that has fixed width fields, and does not have a field separator. For
15836 example, list the Monday dates for the current month:
15838 @c Is using cal ok? Looked at gcal, but I don't like it.
15849 @node The sort command
15850 @unnumberedsec The @command{sort} Command
15852 Next we'll look at the @command{sort} command. This is one of the most
15853 powerful commands on a Unix-style system; one that you will often find
15854 yourself using when setting up fancy data plumbing.
15857 command reads and sorts each file named on the command line. It then
15858 merges the sorted data and writes it to standard output. It will read
15859 standard input if no files are given on the command line (thus
15860 making it into a filter). The sort is based on the character collating
15861 sequence or based on user-supplied ordering criteria.
15864 @node The uniq command
15865 @unnumberedsec The @command{uniq} Command
15867 Finally (at least for now), we'll look at the @command{uniq} program. When
15868 sorting data, you will often end up with duplicate lines, lines that
15869 are identical. Usually, all you need is one instance of each line.
15870 This is where @command{uniq} comes in. The @command{uniq} program reads its
15871 standard input. It prints only one
15872 copy of each repeated line. It does have several options. Later on,
15873 we'll use the @option{-c} option, which prints each unique line, preceded
15874 by a count of the number of times that line occurred in the input.
15877 @node Putting the tools together
15878 @unnumberedsec Putting the Tools Together
15880 Now, let's suppose this is a large ISP server system with dozens of users
15881 logged in. The management wants the system administrator to write a program that will
15882 generate a sorted list of logged in users. Furthermore, even if a user
15883 is logged in multiple times, his or her name should only show up in the
15886 The administrator could sit down with the system documentation and write a C
15887 program that did this. It would take perhaps a couple of hundred lines
15888 of code and about two hours to write it, test it, and debug it.
15889 However, knowing the software toolbox, the administrator can instead start out
15890 by generating just a list of logged on users:
15900 Next, sort the list:
15903 $ who | cut -c1-8 | sort
15910 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
15913 $ who | cut -c1-8 | sort | uniq
15919 The @command{sort} command actually has a @option{-u} option that does what
15920 @command{uniq} does. However, @command{uniq} has other uses for which one
15921 cannot substitute @samp{sort -u}.
15923 The administrator puts this pipeline into a shell script, and makes it available for
15924 all the users on the system (@samp{#} is the system administrator,
15925 or @code{root}, prompt):
15928 # cat > /usr/local/bin/listusers
15929 who | cut -c1-8 | sort | uniq
15931 # chmod +x /usr/local/bin/listusers
15934 There are four major points to note here. First, with just four
15935 programs, on one command line, the administrator was able to save about two
15936 hours worth of work. Furthermore, the shell pipeline is just about as
15937 efficient as the C program would be, and it is much more efficient in
15938 terms of programmer time. People time is much more expensive than
15939 computer time, and in our modern ``there's never enough time to do
15940 everything'' society, saving two hours of programmer time is no mean
15943 Second, it is also important to emphasize that with the
15944 @emph{combination} of the tools, it is possible to do a special
15945 purpose job never imagined by the authors of the individual programs.
15947 Third, it is also valuable to build up your pipeline in stages, as we did here.
15948 This allows you to view the data at each stage in the pipeline, which helps
15949 you acquire the confidence that you are indeed using these tools correctly.
15951 Finally, by bundling the pipeline in a shell script, other users can use
15952 your command, without having to remember the fancy plumbing you set up for
15953 them. In terms of how you run them, shell scripts and compiled programs are
15956 After the previous warm-up exercise, we'll look at two additional, more
15957 complicated pipelines. For them, we need to introduce two more tools.
15959 The first is the @command{tr} command, which stands for ``transliterate.''
15960 The @command{tr} command works on a character-by-character basis, changing
15961 characters. Normally it is used for things like mapping upper case to
15965 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
15966 @print{} this example has mixed case!
15969 There are several options of interest:
15973 work on the complement of the listed characters, i.e.,
15974 operations apply to characters not in the given set
15977 delete characters in the first set from the output
15980 squeeze repeated characters in the output into just one character.
15983 We will be using all three options in a moment.
15985 The other command we'll look at is @command{comm}. The @command{comm}
15986 command takes two sorted input files as input data, and prints out the
15987 files' lines in three columns. The output columns are the data lines
15988 unique to the first file, the data lines unique to the second file, and
15989 the data lines that are common to both. The @option{-1}, @option{-2}, and
15990 @option{-3} command line options @emph{omit} the respective columns. (This is
15991 non-intuitive and takes a little getting used to.) For example:
16013 The file name @file{-} tells @command{comm} to read standard input
16014 instead of a regular file.
16016 Now we're ready to build a fancy pipeline. The first application is a word
16017 frequency counter. This helps an author determine if he or she is over-using
16020 The first step is to change the case of all the letters in our input file
16021 to one case. ``The'' and ``the'' are the same word when doing counting.
16024 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16027 The next step is to get rid of punctuation. Quoted words and unquoted words
16028 should be treated identically; it's easiest to just get the punctuation out of
16032 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16035 The second @command{tr} command operates on the complement of the listed
16036 characters, which are all the letters, the digits, the underscore, and
16037 the blank. The @samp{\n} represents the newline character; it has to
16038 be left alone. (The @acronym{ASCII} tab character should also be included for
16039 good measure in a production script.)
16041 At this point, we have data consisting of words separated by blank space.
16042 The words only contain alphanumeric characters (and the underscore). The
16043 next step is break the data apart so that we have one word per line. This
16044 makes the counting operation much easier, as we will see shortly.
16047 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16048 > tr -s ' ' '\n' | ...
16051 This command turns blanks into newlines. The @option{-s} option squeezes
16052 multiple newline characters in the output into just one. This helps us
16053 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16054 This is what the shell prints when it notices you haven't finished
16055 typing in all of a command.)
16057 We now have data consisting of one word per line, no punctuation, all one
16058 case. We're ready to count each word:
16061 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16062 > tr -s ' ' '\n' | sort | uniq -c | ...
16065 At this point, the data might look something like this:
16078 The output is sorted by word, not by count! What we want is the most
16079 frequently used words first. Fortunately, this is easy to accomplish,
16080 with the help of two more @command{sort} options:
16084 do a numeric sort, not a textual one
16087 reverse the order of the sort
16090 The final pipeline looks like this:
16093 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16094 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16103 Whew! That's a lot to digest. Yet, the same principles apply. With six
16104 commands, on two lines (really one long one split for convenience), we've
16105 created a program that does something interesting and useful, in much
16106 less time than we could have written a C program to do the same thing.
16108 A minor modification to the above pipeline can give us a simple spelling
16109 checker! To determine if you've spelled a word correctly, all you have to
16110 do is look it up in a dictionary. If it is not there, then chances are
16111 that your spelling is incorrect. So, we need a dictionary.
16112 The conventional location for a dictionary is @file{/usr/dict/words}.
16113 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16114 revision of this article.}
16115 this is a sorted, 45,402 word dictionary.
16117 Now, how to compare our file with the dictionary? As before, we generate
16118 a sorted list of words, one per line:
16121 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16122 > tr -s ' ' '\n' | sort -u | ...
16125 Now, all we need is a list of words that are @emph{not} in the
16126 dictionary. Here is where the @command{comm} command comes in.
16129 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16130 > tr -s ' ' '\n' | sort -u |
16131 > comm -23 - /usr/dict/words
16134 The @option{-2} and @option{-3} options eliminate lines that are only in the
16135 dictionary (the second file), and lines that are in both files. Lines
16136 only in the first file (standard input, our stream of words), are
16137 words that are not in the dictionary. These are likely candidates for
16138 spelling errors. This pipeline was the first cut at a production
16139 spelling checker on Unix.
16141 There are some other tools that deserve brief mention.
16145 search files for text that matches a regular expression
16148 count lines, words, characters
16151 a T-fitting for data pipes, copies data to files and to standard output
16154 the stream editor, an advanced tool
16157 a data manipulation language, another advanced tool
16160 The software tools philosophy also espoused the following bit of
16161 advice: ``Let someone else do the hard part.'' This means, take
16162 something that gives you most of what you need, and then massage it the
16163 rest of the way until it's in the form that you want.
16169 Each program should do one thing well. No more, no less.
16172 Combining programs with appropriate plumbing leads to results where
16173 the whole is greater than the sum of the parts. It also leads to novel
16174 uses of programs that the authors might never have imagined.
16177 Programs should never print extraneous header or trailer data, since these
16178 could get sent on down a pipeline. (A point we didn't mention earlier.)
16181 Let someone else do the hard part.
16184 Know your toolbox! Use each program appropriately. If you don't have an
16185 appropriate tool, build one.
16188 As of this writing, all the programs we've discussed are available via
16189 anonymous @command{ftp} from: @*
16190 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16191 be more recent versions available now.)
16193 None of what I have presented in this column is new. The Software Tools
16194 philosophy was first introduced in the book @cite{Software Tools}, by
16195 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16196 This book showed how to write and use software tools. It was written in
16197 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16198 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16199 was. The last chapter presented a @command{ratfor} to FORTRAN
16200 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16201 lot like C; if you know C, you won't have any problem following the
16204 In 1981, the book was updated and made available as @cite{Software Tools
16205 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16206 still in print and are well worth
16207 reading if you're a programmer. They certainly made a major change in
16208 how I view programming.
16210 The programs in both books are available from
16211 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16212 For a number of years, there was an active
16213 Software Tools Users Group, whose members had ported the original
16214 @command{ratfor} programs to essentially every computer system with a
16215 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16216 as Unix began to spread beyond universities.
16218 With the current proliferation of GNU code and other clones of Unix programs,
16219 these programs now receive little attention; modern C versions are
16220 much more efficient and do more than these programs do. Nevertheless, as
16221 exposition of good programming style, and evangelism for a still-valuable
16222 philosophy, these books are unparalleled, and I recommend them highly.
16224 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16225 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16227 @node GNU Free Documentation License
16228 @appendix GNU Free Documentation License
16232 @node Concept index
16239 @c Local variables:
16240 @c texinfo-column-for-description: 32