3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2011 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
479 Opening the software toolbox
481 * Toolbox introduction:: Toolbox introduction
482 * I/O redirection:: I/O redirection
483 * The who command:: The @command{who} command
484 * The cut command:: The @command{cut} command
485 * The sort command:: The @command{sort} command
486 * The uniq command:: The @command{uniq} command
487 * Putting the tools together:: Putting the tools together
491 * GNU Free Documentation License:: Copying and sharing this manual
498 @chapter Introduction
500 This manual is a work in progress: many sections make no attempt to explain
501 basic concepts in a way suitable for novices. Thus, if you are interested,
502 please get involved in improving this manual. The entire @sc{gnu} community
505 @cindex @acronym{POSIX}
506 The @sc{gnu} utilities documented here are mostly compatible with the
507 @acronym{POSIX} standard.
508 @cindex bugs, reporting
509 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
510 to include the version number, machine architecture, input files, and
511 any other information needed to reproduce the bug: your input, what you
512 expected, what you got, and why it is wrong. Diffs are welcome, but
513 please include a description of the problem as well, since this is
514 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
520 @cindex MacKenzie, D.
523 This manual was originally derived from the Unix man pages in the
524 distributions, which were written by David MacKenzie and updated by Jim
525 Meyering. What you are reading now is the authoritative documentation
526 for these utilities; the man pages are no longer being maintained. The
527 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
528 Pinard did the initial conversion to Texinfo format. Karl Berry did the
529 indexing, some reorganization, and editing of the results. Brian
530 Youmans of the Free Software Foundation office staff combined the
531 manuals for textutils, fileutils, and sh-utils to produce the present
532 omnibus manual. Richard Stallman contributed his usual invaluable
533 insights to the overall process.
536 @chapter Common options
540 @itemx @w{@kbd{--backup}[=@var{method}]}
543 @vindex VERSION_CONTROL
544 @cindex backups, making
545 @xref{Backup options}.
546 Make a backup of each file that would otherwise be overwritten or removed.
549 @macro optBackupSuffix
550 @item -S @var{suffix}
551 @itemx --suffix=@var{suffix}
554 Append @var{suffix} to each backup file made with @option{-b}.
555 @xref{Backup options}.
558 @macro optTargetDirectory
559 @item -t @var{directory}
560 @itemx @w{@kbd{--target-directory}=@var{directory}}
562 @opindex --target-directory
563 @cindex target directory
564 @cindex destination directory
565 Specify the destination @var{directory}.
566 @xref{Target directory}.
569 @macro optNoTargetDirectory
571 @itemx --no-target-directory
573 @opindex --no-target-directory
574 @cindex target directory
575 @cindex destination directory
576 Do not treat the last operand specially when it is a directory or a
577 symbolic link to a directory. @xref{Target directory}.
585 @cindex output @sc{nul}-byte-terminated lines
586 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
587 rather than a newline. This option enables other programs to parse the
588 output of @command{\cmd\} even when that output would contain data
589 with embedded newlines.
596 Append an SI-style abbreviation to each size, such as @samp{M} for
597 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
598 1,000,000 bytes. This option is equivalent to
599 @option{--block-size=si}. Use the @option{-h} or
600 @option{--human-readable} option if
601 you prefer powers of 1024.
604 @macro optHumanReadable
606 @itemx --human-readable
608 @opindex --human-readable
609 @cindex human-readable output
610 Append a size letter to each size, such as @samp{M} for mebibytes.
611 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
612 This option is equivalent to @option{--block-size=human-readable}.
613 Use the @option{--si} option if you prefer powers of 1000.
616 @macro optStripTrailingSlashes
617 @itemx @w{@kbd{--strip-trailing-slashes}}
618 @opindex --strip-trailing-slashes
619 @cindex stripping trailing slashes
620 Remove any trailing slashes from each @var{source} argument.
621 @xref{Trailing slashes}.
624 @macro mayConflictWithShellBuiltIn{cmd}
625 @cindex conflicts with shell built-ins
626 @cindex built-in shell commands, conflicts with
627 Due to shell aliases and built-in @command{\cmd\} command, using an
628 unadorned @command{\cmd\} interactively or in a script may get you
629 different functionality than that described here. Invoke it via
630 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
635 @macro multiplierSuffixes{varName}
636 @var{\varName\} may be, or may be an integer optionally followed by,
637 one of the following multiplicative suffixes:
639 @samp{b} => 512 ("blocks")
640 @samp{KB} => 1000 (KiloBytes)
641 @samp{K} => 1024 (KibiBytes)
642 @samp{MB} => 1000*1000 (MegaBytes)
643 @samp{M} => 1024*1024 (MebiBytes)
644 @samp{GB} => 1000*1000*1000 (GigaBytes)
645 @samp{G} => 1024*1024*1024 (GibiBytes)
647 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
650 @c FIXME: same as above, but no ``blocks'' line.
651 @macro multiplierSuffixesNoBlocks{varName}
652 @var{\varName\} may be, or may be an integer optionally followed by,
653 one of the following multiplicative suffixes:
655 @samp{KB} => 1000 (KiloBytes)
656 @samp{K} => 1024 (KibiBytes)
657 @samp{MB} => 1000*1000 (MegaBytes)
658 @samp{M} => 1024*1024 (MebiBytes)
659 @samp{GB} => 1000*1000*1000 (GigaBytes)
660 @samp{G} => 1024*1024*1024 (GibiBytes)
662 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
665 @cindex common options
667 Certain options are available in all of these programs. Rather than
668 writing identical descriptions for each of the programs, they are
669 described here. (In fact, every @sc{gnu} program accepts (or should accept)
672 @vindex POSIXLY_CORRECT
673 Normally options and operands can appear in any order, and programs act
674 as if all the options appear before any operands. For example,
675 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
676 @samp{:} is an option-argument of @option{-t}. However, if the
677 @env{POSIXLY_CORRECT} environment variable is set, options must appear
678 before operands, unless otherwise specified for a particular command.
680 A few programs can usefully have trailing operands with leading
681 @samp{-}. With such a program, options must precede operands even if
682 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
683 program description. For example, the @command{env} command's options
684 must appear before its operands, since in some cases the operands
685 specify a command that itself contains options.
687 Most programs that accept long options recognize unambiguous
688 abbreviations of those options. For example, @samp{rmdir
689 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
690 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
691 @samp{ls --h}, are identified as such.
693 Some of these programs recognize the @option{--help} and @option{--version}
694 options only when one of them is the sole command line argument. For
695 these programs, abbreviations of the long options are not always recognized.
702 Print a usage message listing all available options, then exit successfully.
706 @cindex version number, finding
707 Print the version number, then exit successfully.
711 @cindex option delimiter
712 Delimit the option list. Later arguments, if any, are treated as
713 operands even if they begin with @samp{-}. For example, @samp{sort --
714 -r} reads from the file named @file{-r}.
718 @cindex standard input
719 @cindex standard output
720 A single @samp{-} operand is not really an option, though it looks like one. It
721 stands for standard input, or for standard output if that is clear from
722 the context. For example, @samp{sort -} reads from standard input,
723 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
724 extra copy of its input to standard output. Unless otherwise
725 specified, @samp{-} can appear as any operand that requires a file
729 * Exit status:: Indicating program success or failure.
730 * Backup options:: -b -S, in some programs.
731 * Block size:: BLOCK_SIZE and --block-size, in some programs.
732 * Signal specifications:: Specifying signals using the --signal option.
733 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
734 * Random sources:: --random-source, in some programs.
735 * Target directory:: Specifying a target directory, in some programs.
736 * Trailing slashes:: --strip-trailing-slashes, in some programs.
737 * Traversing symlinks:: -H, -L, or -P, in some programs.
738 * Treating / specially:: --preserve-root and --no-preserve-root.
739 * Special built-in utilities:: @command{break}, @command{:}, @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 if 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}]@c
1782 [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1785 Each line of output consists of the offset in the input, followed by
1786 groups of data from the file. By default, @command{od} prints the offset in
1787 octal, and each group of file data is a C @code{short int}'s worth of input
1788 printed as a single octal number.
1790 If @var{offset} is given, it specifies how many input bytes to skip
1791 before formatting and writing. By default, it is interpreted as an
1792 octal number, but the optional trailing decimal point causes it to be
1793 interpreted as decimal. If no decimal is specified and the offset
1794 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1795 number. If there is a trailing @samp{b}, the number of bytes skipped
1796 will be @var{offset} multiplied by 512.
1798 If a command is of both the first and second forms, the second form is
1799 assumed if the last operand begins with @samp{+} or (if there are two
1800 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1801 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1804 The program accepts the following options. Also see @ref{Common options}.
1808 @item -A @var{radix}
1809 @itemx --address-radix=@var{radix}
1811 @opindex --address-radix
1812 @cindex radix for file offsets
1813 @cindex file offset radix
1814 Select the base in which file offsets are printed. @var{radix} can
1815 be one of the following:
1825 none (do not print offsets).
1828 The default is octal.
1830 @item -j @var{bytes}
1831 @itemx --skip-bytes=@var{bytes}
1833 @opindex --skip-bytes
1834 Skip @var{bytes} input bytes before formatting and writing. If
1835 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1836 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1838 @multiplierSuffixes{bytes}
1840 @item -N @var{bytes}
1841 @itemx --read-bytes=@var{bytes}
1843 @opindex --read-bytes
1844 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1845 @code{bytes} are interpreted as for the @option{-j} option.
1847 @item -S @var{bytes}
1848 @itemx --strings[=@var{bytes}]
1851 @cindex string constants, outputting
1852 Instead of the normal output, output only @dfn{string constants}: at
1853 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1854 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1855 Prefixes and suffixes on @code{bytes} are interpreted as for the
1858 If @var{n} is omitted with @option{--strings}, the default is 3.
1861 @itemx --format=@var{type}
1864 Select the format in which to output the file data. @var{type} is a
1865 string of one or more of the below type indicator characters. If you
1866 include more than one type indicator character in a single @var{type}
1867 string, or use this option more than once, @command{od} writes one copy
1868 of each output line using each of the data types that you specified,
1869 in the order that you specified.
1871 Adding a trailing ``z'' to any type specification appends a display
1872 of the @acronym{ASCII} character representation of the printable characters
1873 to the output line generated by the type specification.
1877 named character, ignoring high-order bit
1879 @acronym{ASCII} character or backslash escape,
1892 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1893 newline, and @samp{nul} for a zero byte. Only the least significant
1894 seven bits of each byte is used; the high-order bit is ignored.
1895 Type @code{c} outputs
1896 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1899 Except for types @samp{a} and @samp{c}, you can specify the number
1900 of bytes to use in interpreting each number in the given data type
1901 by following the type indicator character with a decimal integer.
1902 Alternately, you can specify the size of one of the C compiler's
1903 built-in data types by following the type indicator character with
1904 one of the following characters. For integers (@samp{d}, @samp{o},
1905 @samp{u}, @samp{x}):
1918 For floating point (@code{f}):
1930 @itemx --output-duplicates
1932 @opindex --output-duplicates
1933 Output consecutive lines that are identical. By default, when two or
1934 more consecutive output lines would be identical, @command{od} outputs only
1935 the first line, and puts just an asterisk on the following line to
1936 indicate the elision.
1939 @itemx --width[=@var{n}]
1942 Dump @code{n} input bytes per output line. This must be a multiple of
1943 the least common multiple of the sizes associated with the specified
1946 If this option is not given at all, the default is 16. If @var{n} is
1947 omitted, the default is 32.
1951 The next several options are shorthands for format specifications.
1952 @sc{gnu} @command{od} accepts any combination of shorthands and format
1953 specification options. These options accumulate.
1959 Output as named characters. Equivalent to @samp{-t a}.
1963 Output as octal bytes. Equivalent to @samp{-t o1}.
1967 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1972 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1976 Output as floats. Equivalent to @samp{-t fF}.
1980 Output as decimal ints. Equivalent to @samp{-t dI}.
1984 Output as decimal long ints. Equivalent to @samp{-t dL}.
1988 Output as octal two-byte units. Equivalent to @option{-t o2}.
1992 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1996 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1999 @opindex --traditional
2000 Recognize the non-option label argument that traditional @command{od}
2001 accepted. The following syntax:
2004 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2008 can be used to specify at most one file and optional arguments
2009 specifying an offset and a pseudo-start address, @var{label}.
2010 The @var{label} argument is interpreted
2011 just like @var{offset}, but it specifies an initial pseudo-address. The
2012 pseudo-addresses are displayed in parentheses following any normal
2019 @node base64 invocation
2020 @section @command{base64}: Transform data into printable data
2023 @cindex base64 encoding
2025 @command{base64} transforms data read from a file, or standard input,
2026 into (or from) base64 encoded form. The base64 encoded form uses
2027 printable @acronym{ASCII} characters to represent binary data.
2031 base64 [@var{option}]@dots{} [@var{file}]
2032 base64 --decode [@var{option}]@dots{} [@var{file}]
2035 The base64 encoding expands data to roughly 133% of the original.
2036 The format conforms to
2037 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2039 The program accepts the following options. Also see @ref{Common options}.
2044 @itemx --wrap=@var{cols}
2048 @cindex column to wrap data after
2049 During encoding, wrap lines after @var{cols} characters. This must be
2052 The default is to wrap after 76 characters. Use the value 0 to
2053 disable line wrapping altogether.
2059 @cindex Decode base64 data
2060 @cindex Base64 decoding
2061 Change the mode of operation, from the default of encoding data, to
2062 decoding data. Input is expected to be base64 encoded data, and the
2063 output will be the original data.
2066 @itemx --ignore-garbage
2068 @opindex --ignore-garbage
2069 @cindex Ignore garbage in base64 stream
2070 When decoding, newlines are always accepted.
2071 During decoding, ignore unrecognized bytes,
2072 to permit distorted data to be decoded.
2079 @node Formatting file contents
2080 @chapter Formatting file contents
2082 @cindex formatting file contents
2084 These commands reformat the contents of files.
2087 * fmt invocation:: Reformat paragraph text.
2088 * pr invocation:: Paginate or columnate files for printing.
2089 * fold invocation:: Wrap input lines to fit in specified width.
2093 @node fmt invocation
2094 @section @command{fmt}: Reformat paragraph text
2097 @cindex reformatting paragraph text
2098 @cindex paragraphs, reformatting
2099 @cindex text, reformatting
2101 @command{fmt} fills and joins lines to produce output lines of (at most)
2102 a given number of characters (75 by default). Synopsis:
2105 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2108 @command{fmt} reads from the specified @var{file} arguments (or standard
2109 input if none are given), and writes to standard output.
2111 By default, blank lines, spaces between words, and indentation are
2112 preserved in the output; successive input lines with different
2113 indentation are not joined; tabs are expanded on input and introduced on
2116 @cindex line-breaking
2117 @cindex sentences and line-breaking
2118 @cindex Knuth, Donald E.
2119 @cindex Plass, Michael F.
2120 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2121 avoid line breaks after the first word of a sentence or before the last
2122 word of a sentence. A @dfn{sentence break} is defined as either the end
2123 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2124 spaces or end of line, ignoring any intervening parentheses or quotes.
2125 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2126 breaks; the algorithm is a variant of that given by Donald E. Knuth
2127 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2128 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2131 The program accepts the following options. Also see @ref{Common options}.
2136 @itemx --crown-margin
2138 @opindex --crown-margin
2139 @cindex crown margin
2140 @dfn{Crown margin} mode: preserve the indentation of the first two
2141 lines within a paragraph, and align the left margin of each subsequent
2142 line with that of the second line.
2145 @itemx --tagged-paragraph
2147 @opindex --tagged-paragraph
2148 @cindex tagged paragraphs
2149 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2150 indentation of the first line of a paragraph is the same as the
2151 indentation of the second, the first line is treated as a one-line
2157 @opindex --split-only
2158 Split lines only. Do not join short lines to form longer ones. This
2159 prevents sample lines of code, and other such ``formatted'' text from
2160 being unduly combined.
2163 @itemx --uniform-spacing
2165 @opindex --uniform-spacing
2166 Uniform spacing. Reduce spacing between words to one space, and spacing
2167 between sentences to two spaces.
2170 @itemx -w @var{width}
2171 @itemx --width=@var{width}
2172 @opindex -@var{width}
2175 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2176 initially tries to make lines about 7% shorter than this, to give it
2177 room to balance line lengths.
2179 @item -p @var{prefix}
2180 @itemx --prefix=@var{prefix}
2181 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2182 are subject to formatting. The prefix and any preceding whitespace are
2183 stripped for the formatting and then re-attached to each formatted output
2184 line. One use is to format certain kinds of program comments, while
2185 leaving the code unchanged.
2193 @section @command{pr}: Paginate or columnate files for printing
2196 @cindex printing, preparing files for
2197 @cindex multicolumn output, generating
2198 @cindex merging files in parallel
2200 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2201 standard input if none are given, to standard output, paginating and
2202 optionally outputting in multicolumn format; optionally merges all
2203 @var{file}s, printing all in parallel, one per column. Synopsis:
2206 pr [@var{option}]@dots{} [@var{file}]@dots{}
2210 By default, a 5-line header is printed at each page: two blank lines;
2211 a line with the date, the file name, and the page count; and two more
2212 blank lines. A footer of five blank lines is also printed.
2213 The default @var{page_length} is 66
2214 lines. The default number of text lines is therefore 56.
2215 The text line of the header takes the form
2216 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2217 @var{string} so that the line takes up the full @var{page_width}. Here,
2218 @var{date} is the date (see the @option{-D} or @option{--date-format}
2219 option for details), @var{string} is the centered header string, and
2220 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2221 category affects the spelling of @var{page}; in the default C locale, it
2222 is @samp{Page @var{number}} where @var{number} is the decimal page
2225 Form feeds in the input cause page breaks in the output. Multiple form
2226 feeds produce empty pages.
2228 Columns are of equal width, separated by an optional string (default
2229 is @samp{space}). For multicolumn output, lines will always be truncated to
2230 @var{page_width} (default 72), unless you use the @option{-J} option.
2232 column output no line truncation occurs by default. Use @option{-W} option to
2233 truncate lines in that case.
2235 The following changes were made in version 1.22i and apply to later
2236 versions of @command{pr}:
2237 @c FIXME: this whole section here sounds very awkward to me. I
2238 @c made a few small changes, but really it all needs to be redone. - Brian
2239 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2244 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2245 redefined for better @acronym{POSIX} compliance. The output of some further
2246 cases has been adapted to other Unix systems. These changes are not
2247 compatible with earlier versions of the program.
2250 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2251 have been introduced to turn off unexpected interferences of small letter
2252 options. The @option{-N} option and the second argument @var{last_page}
2253 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2254 form feeds set in the input files requires the @option{-T} option.
2257 Capital letter options override small letter ones.
2260 Some of the option-arguments (compare @option{-s}, @option{-e},
2261 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2262 preceding option letter (already stated in the @acronym{POSIX} specification).
2265 The program accepts the following options. Also see @ref{Common options}.
2269 @item +@var{first_page}[:@var{last_page}]
2270 @itemx --pages=@var{first_page}[:@var{last_page}]
2271 @c The two following @opindex lines evoke warnings because they contain `:'
2272 @c The `info' spec does not permit that. If we use those lines, we end
2273 @c up with truncated index entries that don't work.
2274 @c @opindex +@var{first_page}[:@var{last_page}]
2275 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2276 @opindex +@var{page_range}
2277 @opindex --pages=@var{page_range}
2278 Begin printing with page @var{first_page} and stop with @var{last_page}.
2279 Missing @samp{:@var{last_page}} implies end of file. While estimating
2280 the number of skipped pages each form feed in the input file results
2281 in a new page. Page counting with and without @samp{+@var{first_page}}
2282 is identical. By default, counting starts with the first page of input
2283 file (not first page printed). Line numbering may be altered by @option{-N}
2287 @itemx --columns=@var{column}
2288 @opindex -@var{column}
2290 @cindex down columns
2291 With each single @var{file}, produce @var{column} columns of output
2292 (default is 1) and print columns down, unless @option{-a} is used. The
2293 column width is automatically decreased as @var{column} increases; unless
2294 you use the @option{-W/-w} option to increase @var{page_width} as well.
2295 This option might well cause some lines to be truncated. The number of
2296 lines in the columns on each page are balanced. The options @option{-e}
2297 and @option{-i} are on for multiple text-column output. Together with
2298 @option{-J} option column alignment and line truncation is turned off.
2299 Lines of full length are joined in a free field format and @option{-S}
2300 option may set field separators. @option{-@var{column}} may not be used
2301 with @option{-m} option.
2307 @cindex across columns
2308 With each single @var{file}, print columns across rather than down. The
2309 @option{-@var{column}} option must be given with @var{column} greater than one.
2310 If a line is too long to fit in a column, it is truncated.
2313 @itemx --show-control-chars
2315 @opindex --show-control-chars
2316 Print control characters using hat notation (e.g., @samp{^G}); print
2317 other nonprinting characters in octal backslash notation. By default,
2318 nonprinting characters are not changed.
2321 @itemx --double-space
2323 @opindex --double-space
2324 @cindex double spacing
2325 Double space the output.
2327 @item -D @var{format}
2328 @itemx --date-format=@var{format}
2329 @cindex time formats
2330 @cindex formatting times
2331 Format header dates using @var{format}, using the same conventions as
2332 for the command @samp{date +@var{format}}; @xref{date invocation}.
2333 Except for directives, which start with
2334 @samp{%}, characters in @var{format} are printed unchanged. You can use
2335 this option to specify an arbitrary string in place of the header date,
2336 e.g., @option{--date-format="Monday morning"}.
2338 @vindex POSIXLY_CORRECT
2340 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2341 @samp{2001-12-04 23:59});
2342 but if the @env{POSIXLY_CORRECT} environment variable is set
2343 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2344 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2345 @samp{Dec@ @ 4 23:59 2001}.
2348 Time stamps are listed according to the time zone rules specified by
2349 the @env{TZ} environment variable, or by the system default rules if
2350 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2351 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2353 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2354 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2356 @opindex --expand-tabs
2358 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2359 the input tab character (default is the TAB character). Second optional
2360 argument @var{in-tabwidth} is the input tab character's width (default
2368 @opindex --form-feed
2369 Use a form feed instead of newlines to separate output pages. This does
2370 not alter the default page length of 66 lines.
2372 @item -h @var{header}
2373 @itemx --header=@var{header}
2376 Replace the file name in the header with the centered string @var{header}.
2377 When using the shell, @var{header} should be quoted and should be
2378 separated from @option{-h} by a space.
2380 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2381 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2383 @opindex --output-tabs
2385 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2386 is the output tab character (default is the TAB character). Second optional
2387 argument @var{out-tabwidth} is the output tab character's width (default
2393 @opindex --join-lines
2394 Merge lines of full length. Used together with the column options
2395 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2396 @option{-W/-w} line truncation;
2397 no column alignment used; may be used with
2398 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2399 (together with @option{-W} and @option{--sep-string})
2400 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2401 @option{-s} along with the three column options.
2404 @item -l @var{page_length}
2405 @itemx --length=@var{page_length}
2408 Set the page length to @var{page_length} (default 66) lines, including
2409 the lines of the header [and the footer]. If @var{page_length} is less
2410 than or equal to 10, the header and footer are omitted, as if the
2411 @option{-t} option had been given.
2417 Merge and print all @var{file}s in parallel, one in each column. If a
2418 line is too long to fit in a column, it is truncated, unless the @option{-J}
2419 option is used. @option{--sep-string[=@var{string}]} may be used.
2421 some @var{file}s (form feeds set) produce empty columns, still marked
2422 by @var{string}. The result is a continuous line numbering and column
2423 marking throughout the whole merged file. Completely empty merged pages
2424 show no separators or line numbers. The default header becomes
2425 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2426 may be used with the @option{-h} or @option{--header} option to fill up
2427 the middle blank part.
2429 @item -n[@var{number-separator}[@var{digits}]]
2430 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2432 @opindex --number-lines
2433 Provide @var{digits} digit line numbering (default for @var{digits} is
2434 5). With multicolumn output the number occupies the first @var{digits}
2435 column positions of each text column or only each line of @option{-m}
2436 output. With single column output the number precedes each line just as
2437 @option{-m} does. Default counting of the line numbers starts with the
2438 first line of the input file (not the first line printed, compare the
2439 @option{--page} option and @option{-N} option).
2440 Optional argument @var{number-separator} is the character appended to
2441 the line number to separate it from the text followed. The default
2442 separator is the TAB character. In a strict sense a TAB is always
2443 printed with single column output only. The TAB width varies
2444 with the TAB position, e.g., with the left @var{margin} specified
2445 by @option{-o} option. With multicolumn output priority is given to
2446 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2447 The TAB width is fixed to the value of the first column and does
2448 not change with different values of left @var{margin}. That means a
2449 fixed number of spaces is always printed in the place of the
2450 @var{number-separator} TAB. The tabification depends upon the output
2453 @item -N @var{line_number}
2454 @itemx --first-line-number=@var{line_number}
2456 @opindex --first-line-number
2457 Start line counting with the number @var{line_number} at first line of
2458 first page printed (in most cases not the first line of the input file).
2460 @item -o @var{margin}
2461 @itemx --indent=@var{margin}
2464 @cindex indenting lines
2466 Indent each line with a margin @var{margin} spaces wide (default is zero).
2467 The total page width is the size of the margin plus the @var{page_width}
2468 set with the @option{-W/-w} option. A limited overflow may occur with
2469 numbered single column output (compare @option{-n} option).
2472 @itemx --no-file-warnings
2474 @opindex --no-file-warnings
2475 Do not print a warning message when an argument @var{file} cannot be
2476 opened. (The exit status will still be nonzero, however.)
2478 @item -s[@var{char}]
2479 @itemx --separator[=@var{char}]
2481 @opindex --separator
2482 Separate columns by a single character @var{char}. The default for
2483 @var{char} is the TAB character without @option{-w} and @samp{no
2484 character} with @option{-w}. Without @option{-s} the default separator
2485 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2486 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2487 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2490 @item -S@var{string}
2491 @itemx --sep-string[=@var{string}]
2493 @opindex --sep-string
2494 Use @var{string} to separate output columns. The @option{-S} option doesn't
2495 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2496 does not affect line truncation or column alignment.
2497 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2499 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2500 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2501 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2504 @itemx --omit-header
2506 @opindex --omit-header
2507 Do not print the usual header [and footer] on each page, and do not fill
2508 out the bottom of pages (with blank lines or a form feed). No page
2509 structure is produced, but form feeds set in the input files are retained.
2510 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2511 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2512 in the input file to 4 spaces but don't make any other changes. Use of
2513 @option{-t} overrides @option{-h}.
2516 @itemx --omit-pagination
2518 @opindex --omit-pagination
2519 Do not print header [and footer]. In addition eliminate all form feeds
2520 set in the input files.
2523 @itemx --show-nonprinting
2525 @opindex --show-nonprinting
2526 Print nonprinting characters in octal backslash notation.
2528 @item -w @var{page_width}
2529 @itemx --width=@var{page_width}
2532 Set page width to @var{page_width} characters for multiple text-column
2533 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2534 off the default page width and any line truncation and column alignment.
2535 Lines of full length are merged, regardless of the column options
2536 set. No @var{page_width} setting is possible with single column output.
2537 A @acronym{POSIX}-compliant formulation.
2539 @item -W @var{page_width}
2540 @itemx --page_width=@var{page_width}
2542 @opindex --page_width
2543 Set the page width to @var{page_width} characters. That's valid with and
2544 without a column option. Text lines are truncated, unless @option{-J}
2545 is used. Together with one of the three column options
2546 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2547 alignment is always used. The separator options @option{-S} or @option{-s}
2548 don't affect the @option{-W} option. Default is 72 characters. Without
2549 @option{-W @var{page_width}} and without any of the column options NO line
2550 truncation is used (defined to keep downward compatibility and to meet
2551 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2552 line is never truncated.
2559 @node fold invocation
2560 @section @command{fold}: Wrap input lines to fit in specified width
2563 @cindex wrapping long input lines
2564 @cindex folding long input lines
2566 @command{fold} writes each @var{file} (@option{-} means standard input), or
2567 standard input if none are given, to standard output, breaking long
2571 fold [@var{option}]@dots{} [@var{file}]@dots{}
2574 By default, @command{fold} breaks lines wider than 80 columns. The output
2575 is split into as many lines as necessary.
2577 @cindex screen columns
2578 @command{fold} counts screen columns by default; thus, a tab may count more
2579 than one column, backspace decreases the column count, and carriage
2580 return sets the column to zero.
2582 The program accepts the following options. Also see @ref{Common options}.
2590 Count bytes rather than columns, so that tabs, backspaces, and carriage
2591 returns are each counted as taking up one column, just like other
2598 Break at word boundaries: the line is broken after the last blank before
2599 the maximum line length. If the line contains no such blanks, the line
2600 is broken at the maximum line length as usual.
2602 @item -w @var{width}
2603 @itemx --width=@var{width}
2606 Use a maximum line length of @var{width} columns instead of 80.
2608 For compatibility @command{fold} supports an obsolete option syntax
2609 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2617 @node Output of parts of files
2618 @chapter Output of parts of files
2620 @cindex output of parts of files
2621 @cindex parts of files, output of
2623 These commands output pieces of the input.
2626 * head invocation:: Output the first part of files.
2627 * tail invocation:: Output the last part of files.
2628 * split invocation:: Split a file into pieces.
2629 * csplit invocation:: Split a file into context-determined pieces.
2632 @node head invocation
2633 @section @command{head}: Output the first part of files
2636 @cindex initial part of files, outputting
2637 @cindex first part of files, outputting
2639 @command{head} prints the first part (10 lines by default) of each
2640 @var{file}; it reads from standard input if no files are given or
2641 when given a @var{file} of @option{-}. Synopsis:
2644 head [@var{option}]@dots{} [@var{file}]@dots{}
2647 If more than one @var{file} is specified, @command{head} prints a
2648 one-line header consisting of:
2651 ==> @var{file name} <==
2655 before the output for each @var{file}.
2657 The program accepts the following options. Also see @ref{Common options}.
2662 @itemx --bytes=@var{k}
2665 Print the first @var{k} bytes, instead of initial lines.
2666 However, if @var{k} starts with a @samp{-},
2667 print all but the last @var{k} bytes of each file.
2668 @multiplierSuffixes{k}
2671 @itemx --lines=@var{k}
2674 Output the first @var{k} lines.
2675 However, if @var{k} starts with a @samp{-},
2676 print all but the last @var{k} lines of each file.
2677 Size multiplier suffixes are the same as with the @option{-c} option.
2685 Never print file name headers.
2691 Always print file name headers.
2695 For compatibility @command{head} also supports an obsolete option syntax
2696 @option{-@var{count}@var{options}}, which is recognized only if it is
2697 specified first. @var{count} is a decimal number optionally followed
2698 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2699 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2700 Scripts intended for standard hosts should use @option{-c @var{count}}
2701 or @option{-n @var{count}} instead. If your script must also run on
2702 hosts that support only the obsolete syntax, it is usually simpler to
2703 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2709 @node tail invocation
2710 @section @command{tail}: Output the last part of files
2713 @cindex last part of files, outputting
2715 @command{tail} prints the last part (10 lines by default) of each
2716 @var{file}; it reads from standard input if no files are given or
2717 when given a @var{file} of @samp{-}. Synopsis:
2720 tail [@var{option}]@dots{} [@var{file}]@dots{}
2723 If more than one @var{file} is specified, @command{tail} prints a
2724 one-line header consisting of:
2727 ==> @var{file name} <==
2731 before the output for each @var{file}.
2733 @cindex BSD @command{tail}
2734 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2735 @command{tail} cannot). It also has no @option{-r} option (print in
2736 reverse), since reversing a file is really a different job from printing
2737 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2738 only reverse files that are at most as large as its buffer, which is
2739 typically 32 KiB@. A more reliable and versatile way to reverse files is
2740 the @sc{gnu} @command{tac} command.
2742 The program accepts the following options. Also see @ref{Common options}.
2747 @itemx --bytes=@var{k}
2750 Output the last @var{k} bytes, instead of final lines.
2751 However, if @var{k} starts with a @samp{+}, start printing with the
2752 @var{k}th byte from the start of each file, instead of from the end.
2753 @multiplierSuffixes{k}
2756 @itemx --follow[=@var{how}]
2759 @cindex growing files
2760 @vindex name @r{follow option}
2761 @vindex descriptor @r{follow option}
2762 Loop forever trying to read more characters at the end of the file,
2763 presumably because the file is growing.
2764 If more than one file is given, @command{tail} prints a header whenever it
2765 gets output from a different file, to indicate which file that output is
2768 There are two ways to specify how you'd like to track files with this option,
2769 but that difference is noticeable only when a followed file is removed or
2771 If you'd like to continue to track the end of a growing file even after
2772 it has been unlinked, use @option{--follow=descriptor}. This is the default
2773 behavior, but it is not useful if you're tracking a log file that may be
2774 rotated (removed or renamed, then reopened). In that case, use
2775 @option{--follow=name} to track the named file, perhaps by reopening it
2776 periodically to see if it has been removed and recreated by some other program.
2777 Note that the inotify-based implementation handles this case without
2778 the need for any periodic reopening.
2780 No matter which method you use, if the tracked file is determined to have
2781 shrunk, @command{tail} prints a message saying the file has been truncated
2782 and resumes tracking the end of the file from the newly-determined endpoint.
2784 When a file is removed, @command{tail}'s behavior depends on whether it is
2785 following the name or the descriptor. When following by name, tail can
2786 detect that a file has been removed and gives a message to that effect,
2787 and if @option{--retry} has been specified it will continue checking
2788 periodically to see if the file reappears.
2789 When following a descriptor, tail does not detect that the file has
2790 been unlinked or renamed and issues no message; even though the file
2791 may no longer be accessible via its original name, it may still be
2794 The option values @samp{descriptor} and @samp{name} may be specified only
2795 with the long form of the option, not with @option{-f}.
2797 The @option{-f} option is ignored if
2798 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2799 Likewise, the @option{-f} option has no effect for any
2800 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2804 This option is the same as @option{--follow=name --retry}. That is, tail
2805 will attempt to reopen a file when it is removed. Should this fail, tail
2806 will keep trying until it becomes accessible again.
2810 This option is useful mainly when following by name (i.e., with
2811 @option{--follow=name}).
2812 Without this option, when tail encounters a file that doesn't
2813 exist or is otherwise inaccessible, it reports that fact and
2814 never checks it again.
2816 @itemx --sleep-interval=@var{number}
2817 @opindex --sleep-interval
2818 Change the number of seconds to wait between iterations (the default is 1.0).
2819 During one iteration, every specified file is checked to see if it has
2821 Historical implementations of @command{tail} have required that
2822 @var{number} be an integer. However, GNU @command{tail} accepts
2823 an arbitrary floating point number (using a period before any
2825 When @command{tail} uses inotify, this polling-related option is ignored.
2827 @itemx --pid=@var{pid}
2829 When following by name or by descriptor, you may specify the process ID,
2830 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2831 after that process terminates, tail will also terminate. This will
2832 work properly only if the writer and the tailing process are running on
2833 the same machine. For example, to save the output of a build in a file
2834 and to watch the file grow, if you invoke @command{make} and @command{tail}
2835 like this then the tail process will stop when your build completes.
2836 Without this option, you would have had to kill the @code{tail -f}
2840 $ make >& makerr & tail --pid=$! -f makerr
2843 If you specify a @var{pid} that is not in use or that does not correspond
2844 to the process that is writing to the tailed files, then @command{tail}
2845 may terminate long before any @var{file}s stop growing or it may not
2846 terminate until long after the real writer has terminated.
2847 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2848 will print a warning if this is the case.
2850 @itemx --max-unchanged-stats=@var{n}
2851 @opindex --max-unchanged-stats
2852 When tailing a file by name, if there have been @var{n} (default
2853 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2854 iterations for which the file has not changed, then
2855 @code{open}/@code{fstat} the file to determine if that file name is
2856 still associated with the same device/inode-number pair as before.
2857 When following a log file that is rotated, this is approximately the
2858 number of seconds between when tail prints the last pre-rotation lines
2859 and when it prints the lines that have accumulated in the new log file.
2860 This option is meaningful only when polling (i.e., without inotify)
2861 and when following by name.
2864 @itemx --lines=@var{k}
2867 Output the last @var{k} lines.
2868 However, if @var{k} starts with a @samp{+}, start printing with the
2869 @var{k}th line from the start of each file, instead of from the end.
2870 Size multiplier suffixes are the same as with the @option{-c} option.
2878 Never print file name headers.
2884 Always print file name headers.
2888 For compatibility @command{tail} also supports an obsolete usage
2889 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2890 only if it does not conflict with the usage described
2891 above. This obsolete form uses exactly one option and at most one
2892 file. In the option, @var{count} is an optional decimal number optionally
2893 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2894 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2895 which has the same meaning as @option{-f}.
2897 @vindex _POSIX2_VERSION
2898 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2899 the obsolete option syntax with the same meaning as in counts, and
2900 obsolete usage overrides normal usage when the two conflict.
2901 This obsolete behavior can be enabled or disabled with the
2902 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2905 Scripts intended for use on standard hosts should avoid obsolete
2906 syntax and should use @option{-c @var{count}[b]}, @option{-n
2907 @var{count}}, and/or @option{-f} instead. If your script must also
2908 run on hosts that support only the obsolete syntax, you can often
2909 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2910 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2911 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2912 then @dots{}} to decide which syntax to use.
2914 Even if your script assumes the standard behavior, you should still
2915 beware usages whose behaviors differ depending on the @acronym{POSIX}
2916 version. For example, avoid @samp{tail - main.c}, since it might be
2917 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2918 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2919 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2920 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2925 @node split invocation
2926 @section @command{split}: Split a file into pieces.
2929 @cindex splitting a file into pieces
2930 @cindex pieces, splitting a file into
2932 @command{split} creates output files containing consecutive or interleaved
2933 sections of @var{input} (standard input if none is given or @var{input}
2934 is @samp{-}). Synopsis:
2937 split [@var{option}] [@var{input} [@var{prefix}]]
2940 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2941 left over for the last section), into each output file.
2943 @cindex output file name prefix
2944 The output files' names consist of @var{prefix} (@samp{x} by default)
2945 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2946 default), such that concatenating the output files in traditional
2947 sorted order by file name produces the original input file (except
2948 @option{-r}). If the output file names are exhausted, @command{split}
2949 reports an error without deleting the output files that it did create.
2951 The program accepts the following options. Also see @ref{Common options}.
2955 @item -l @var{lines}
2956 @itemx --lines=@var{lines}
2959 Put @var{lines} lines of @var{input} into each output file.
2961 For compatibility @command{split} also supports an obsolete
2962 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2963 @var{lines}} instead.
2966 @itemx --bytes=@var{size}
2969 Put @var{size} bytes of @var{input} into each output file.
2970 @multiplierSuffixes{size}
2973 @itemx --line-bytes=@var{size}
2975 @opindex --line-bytes
2976 Put into each output file as many complete lines of @var{input} as
2977 possible without exceeding @var{size} bytes. Individual lines longer than
2978 @var{size} bytes are broken into multiple files.
2979 @var{size} has the same format as for the @option{--bytes} option.
2981 @item -n @var{chunks}
2982 @itemx --number=@var{chunks}
2986 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
2989 @var{n} generate @var{n} files based on current size of @var{input}
2990 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
2991 l/@var{n} generate @var{n} files without splitting lines
2992 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2993 r/@var{n} like @samp{l} but use round robin distribution
2994 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2997 Any excess bytes remaining after dividing the @var{input}
2998 into @var{n} chunks, are assigned to the last chunk.
2999 Any excess bytes appearing after the initial calculation are discarded
3000 (except when using @samp{r} mode).
3002 All @var{n} files are created even if there are fewer than @var{n} lines,
3003 or the @var{input} is truncated.
3005 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3006 The @var{input} is partitioned into @var{n} equal sized portions, with
3007 the last assigned any excess. If a line @emph{starts} within a partition
3008 it is written completely to the corresponding file. Since lines
3009 are not split even if they overlap a partition, the files written
3010 can be larger or smaller than the partition size, and even empty
3011 if a line is so long as to completely overlap the partition.
3013 For @samp{r} mode, the size of @var{input} is irrelevant,
3014 and so can be a pipe for example.
3016 @item -a @var{length}
3017 @itemx --suffix-length=@var{length}
3019 @opindex --suffix-length
3020 Use suffixes of length @var{length}. The default @var{length} is 2.
3023 @itemx --numeric-suffixes
3025 @opindex --numeric-suffixes
3026 Use digits in suffixes rather than lower-case letters.
3029 @itemx --elide-empty-files
3031 @opindex --elide-empty-files
3032 Suppress the generation of zero-length output files. This can happen
3033 with the @option{--number} option if a file is (truncated to be) shorter
3034 than the number requested, or if a line is so long as to completely
3035 span a chunk. The output file sequence numbers, always run consecutively
3036 even when this option is specified.
3041 @opindex --unbuffered
3042 Immediately copy input to output in @option{--number r/...} mode,
3043 which is a much slower mode of operation.
3047 Write a diagnostic just before each output file is opened.
3053 Here are a few examples to illustrate how the
3054 @option{--number} (@option{-n}) option works:
3056 Notice how, by default, one line may be split onto two or more:
3059 $ seq -w 6 10 > k; split -n3 k; head xa?
3072 Use the "l/" modifier to suppress that:
3075 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3088 Use the "r/" modifier to distribute lines in a round-robin fashion:
3091 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3104 You can also extract just the Kth chunk.
3105 This extracts and prints just the 7th "chunk" of 33:
3108 $ seq 100 > k; split -nl/7/33 k
3115 @node csplit invocation
3116 @section @command{csplit}: Split a file into context-determined pieces
3119 @cindex context splitting
3120 @cindex splitting a file into pieces by context
3122 @command{csplit} creates zero or more output files containing sections of
3123 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3126 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3129 The contents of the output files are determined by the @var{pattern}
3130 arguments, as detailed below. An error occurs if a @var{pattern}
3131 argument refers to a nonexistent line of the input file (e.g., if no
3132 remaining line matches a given regular expression). After every
3133 @var{pattern} has been matched, any remaining input is copied into one
3136 By default, @command{csplit} prints the number of bytes written to each
3137 output file after it has been created.
3139 The types of pattern arguments are:
3144 Create an output file containing the input up to but not including line
3145 @var{n} (a positive integer). If followed by a repeat count, also
3146 create an output file containing the next @var{n} lines of the input
3147 file once for each repeat.
3149 @item /@var{regexp}/[@var{offset}]
3150 Create an output file containing the current line up to (but not
3151 including) the next line of the input file that contains a match for
3152 @var{regexp}. The optional @var{offset} is an integer.
3153 If it is given, the input up to (but not including) the
3154 matching line plus or minus @var{offset} is put into the output file,
3155 and the line after that begins the next section of input.
3157 @item %@var{regexp}%[@var{offset}]
3158 Like the previous type, except that it does not create an output
3159 file, so that section of the input file is effectively ignored.
3161 @item @{@var{repeat-count}@}
3162 Repeat the previous pattern @var{repeat-count} additional
3163 times. The @var{repeat-count} can either be a positive integer or an
3164 asterisk, meaning repeat as many times as necessary until the input is
3169 The output files' names consist of a prefix (@samp{xx} by default)
3170 followed by a suffix. By default, the suffix is an ascending sequence
3171 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3172 concatenating the output files in sorted order by file name produces the
3173 original input file.
3175 By default, if @command{csplit} encounters an error or receives a hangup,
3176 interrupt, quit, or terminate signal, it removes any output files
3177 that it has created so far before it exits.
3179 The program accepts the following options. Also see @ref{Common options}.
3183 @item -f @var{prefix}
3184 @itemx --prefix=@var{prefix}
3187 @cindex output file name prefix
3188 Use @var{prefix} as the output file name prefix.
3190 @item -b @var{suffix}
3191 @itemx --suffix=@var{suffix}
3194 @cindex output file name suffix
3195 Use @var{suffix} as the output file name suffix. When this option is
3196 specified, the suffix string must include exactly one
3197 @code{printf(3)}-style conversion specification, possibly including
3198 format specification flags, a field width, a precision specifications,
3199 or all of these kinds of modifiers. The format letter must convert a
3200 binary unsigned integer argument to readable form. The format letters
3201 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3202 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3203 entire @var{suffix} is given (with the current output file number) to
3204 @code{sprintf(3)} to form the file name suffixes for each of the
3205 individual output files in turn. If this option is used, the
3206 @option{--digits} option is ignored.
3208 @item -n @var{digits}
3209 @itemx --digits=@var{digits}
3212 Use output file names containing numbers that are @var{digits} digits
3213 long instead of the default 2.
3218 @opindex --keep-files
3219 Do not remove output files when errors are encountered.
3222 @itemx --elide-empty-files
3224 @opindex --elide-empty-files
3225 Suppress the generation of zero-length output files. (In cases where
3226 the section delimiters of the input file are supposed to mark the first
3227 lines of each of the sections, the first output file will generally be a
3228 zero-length file unless you use this option.) The output file sequence
3229 numbers always run consecutively starting from 0, even when this option
3240 Do not print counts of output file sizes.
3246 Here is an example of its usage.
3247 First, create an empty directory for the exercise,
3254 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3257 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3263 Each number printed above is the size of an output
3264 file that csplit has just created.
3265 List the names of those output files:
3272 Use @command{head} to show their contents:
3297 @node Summarizing files
3298 @chapter Summarizing files
3300 @cindex summarizing files
3302 These commands generate just a few numbers representing entire
3306 * wc invocation:: Print newline, word, and byte counts.
3307 * sum invocation:: Print checksum and block counts.
3308 * cksum invocation:: Print CRC checksum and byte counts.
3309 * md5sum invocation:: Print or check MD5 digests.
3310 * sha1sum invocation:: Print or check SHA-1 digests.
3311 * sha2 utilities:: Print or check SHA-2 digests.
3316 @section @command{wc}: Print newline, word, and byte counts
3320 @cindex character count
3324 @command{wc} counts the number of bytes, characters, whitespace-separated
3325 words, and newlines in each given @var{file}, or standard input if none
3326 are given or for a @var{file} of @samp{-}. Synopsis:
3329 wc [@var{option}]@dots{} [@var{file}]@dots{}
3332 @cindex total counts
3333 @command{wc} prints one line of counts for each file, and if the file was
3334 given as an argument, it prints the file name following the counts. If
3335 more than one @var{file} is given, @command{wc} prints a final line
3336 containing the cumulative counts, with the file name @file{total}. The
3337 counts are printed in this order: newlines, words, characters, bytes,
3338 maximum line length.
3339 Each count is printed right-justified in a field with at least one
3340 space between fields so that the numbers and file names normally line
3341 up nicely in columns. The width of the count fields varies depending
3342 on the inputs, so you should not depend on a particular field width.
3343 However, as a @acronym{GNU} extension, if only one count is printed,
3344 it is guaranteed to be printed without leading spaces.
3346 By default, @command{wc} prints three counts: the newline, words, and byte
3347 counts. Options can specify that only certain counts be printed.
3348 Options do not undo others previously given, so
3355 prints both the byte counts and the word counts.
3357 With the @option{--max-line-length} option, @command{wc} prints the length
3358 of the longest line per file, and if there is more than one file it
3359 prints the maximum (not the sum) of those lengths. The line lengths here
3360 are measured in screen columns, according to the current locale and
3361 assuming tab positions in every 8th column.
3363 The program accepts the following options. Also see @ref{Common options}.
3371 Print only the byte counts.
3377 Print only the character counts.
3383 Print only the word counts.
3389 Print only the newline counts.
3392 @itemx --max-line-length
3394 @opindex --max-line-length
3395 Print only the maximum line lengths.
3397 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3398 @itemx --files0-from=@var{file}
3399 @opindex --files0-from=@var{file}
3400 @c This is commented out to avoid a texi2dvi failure.
3401 @c texi2dvi (GNU Texinfo 4.11) 1.104
3402 @c @cindex including files from @command{\cmd\}
3403 Disallow processing files named on the command line, and instead process
3404 those named in file @var{file}; each name being terminated by a zero byte
3405 (@acronym{ASCII} @sc{nul}).
3406 This is useful \withTotalOption\
3407 when the list of file names is so long that it may exceed a command line
3409 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3410 because it splits the list into pieces and makes @command{\cmd\} print
3411 \subListOutput\ for each sublist rather than for the entire list.
3412 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file
3413 names is with @sc{gnu}
3414 @command{find}, using its @option{-print0} predicate.
3415 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated
3416 file names are read from standard input.
3418 @filesZeroFromOption{wc,,a total}
3420 For example, to find the length of the longest line in any @file{.c} or
3421 @file{.h} file in the current hierarchy, do this:
3424 find . -name '*.[ch]' -print0 |
3425 wc -L --files0-from=- | tail -n1
3433 @node sum invocation
3434 @section @command{sum}: Print checksum and block counts
3437 @cindex 16-bit checksum
3438 @cindex checksum, 16-bit
3440 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3441 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3444 sum [@var{option}]@dots{} [@var{file}]@dots{}
3447 @command{sum} prints the checksum for each @var{file} followed by the
3448 number of blocks in the file (rounded up). If more than one @var{file}
3449 is given, file names are also printed (by default). (With the
3450 @option{--sysv} option, corresponding file names are printed when there is
3451 at least one file argument.)
3453 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3454 compatible with BSD @command{sum} and prints file sizes in units of
3457 The program accepts the following options. Also see @ref{Common options}.
3463 @cindex BSD @command{sum}
3464 Use the default (BSD compatible) algorithm. This option is included for
3465 compatibility with the System V @command{sum}. Unless @option{-s} was also
3466 given, it has no effect.
3472 @cindex System V @command{sum}
3473 Compute checksums using an algorithm compatible with System V
3474 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3478 @command{sum} is provided for compatibility; the @command{cksum} program (see
3479 next section) is preferable in new applications.
3484 @node cksum invocation
3485 @section @command{cksum}: Print CRC checksum and byte counts
3488 @cindex cyclic redundancy check
3489 @cindex CRC checksum
3491 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3492 given @var{file}, or standard input if none are given or for a
3493 @var{file} of @samp{-}. Synopsis:
3496 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3499 @command{cksum} prints the CRC checksum for each file along with the number
3500 of bytes in the file, and the file name unless no arguments were given.
3502 @command{cksum} is typically used to ensure that files
3503 transferred by unreliable means (e.g., netnews) have not been corrupted,
3504 by comparing the @command{cksum} output for the received files with the
3505 @command{cksum} output for the original files (typically given in the
3508 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3509 compatible with the BSD or System V @command{sum} algorithms (see the
3510 previous section); it is more robust.
3512 The only options are @option{--help} and @option{--version}. @xref{Common
3518 @node md5sum invocation
3519 @section @command{md5sum}: Print or check MD5 digests
3523 @cindex 128-bit checksum
3524 @cindex checksum, 128-bit
3525 @cindex fingerprint, 128-bit
3526 @cindex message-digest, 128-bit
3528 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3529 @dfn{message-digest}) for each specified @var{file}.
3531 Note: The MD5 digest is more reliable than a simple CRC (provided by
3532 the @command{cksum} command) for detecting accidental file corruption,
3533 as the chances of accidentally having two files with identical MD5
3534 are vanishingly small. However, it should not be considered secure
3535 against malicious tampering: although finding a file with a given MD5
3536 fingerprint is considered infeasible at the moment, it is known how
3537 to modify certain files, including digital certificates, so that they
3538 appear valid when signed with an MD5 digest.
3539 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3541 If a @var{file} is specified as @samp{-} or if no files are given
3542 @command{md5sum} computes the checksum for the standard input.
3543 @command{md5sum} can also determine whether a file and checksum are
3544 consistent. Synopsis:
3547 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3550 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3551 indicating a binary or text input file, and the file name.
3552 If @var{file} contains a backslash or newline, the
3553 line is started with a backslash, and each problematic character in
3554 the file name is escaped with a backslash, making the output
3555 unambiguous even in the presence of arbitrary file names.
3556 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3558 The program accepts the following options. Also see @ref{Common options}.
3566 @cindex binary input files
3567 Treat each input file as binary, by reading it in binary mode and
3568 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3569 On systems like @acronym{GNU} that do not distinguish between binary
3570 and text files, this option merely flags each input file as binary:
3571 the MD5 checksum is unaffected. This option is the default on systems
3572 like MS-DOS that distinguish between binary and text files, except
3573 for reading standard input when standard input is a terminal.
3577 Read file names and checksum information (not data) from each
3578 @var{file} (or from stdin if no @var{file} was specified) and report
3579 whether the checksums match the contents of the named files.
3580 The input to this mode of @command{md5sum} is usually the output of
3581 a prior, checksum-generating run of @samp{md5sum}.
3582 Each valid line of input consists of an MD5 checksum, a binary/text
3583 flag, and then a file name.
3584 Binary files are marked with @samp{*}, text with @samp{ }.
3585 For each such line, @command{md5sum} reads the named file and computes its
3586 MD5 checksum. Then, if the computed message digest does not match the
3587 one on the line with the file name, the file is noted as having
3588 failed the test. Otherwise, the file passes the test.
3589 By default, for each valid line, one line is written to standard
3590 output indicating whether the named file passed the test.
3591 After all checks have been performed, if there were any failures,
3592 a warning is issued to standard error.
3593 Use the @option{--status} option to inhibit that output.
3594 If any listed file cannot be opened or read, if any valid line has
3595 an MD5 checksum inconsistent with the associated file, or if no valid
3596 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3597 it exits successfully.
3601 @cindex verifying MD5 checksums
3602 This option is useful only when verifying checksums.
3603 When verifying checksums, don't generate an 'OK' message per successfully
3604 checked file. Files that fail the verification are reported in the
3605 default one-line-per-file format. If there is any checksum mismatch,
3606 print a warning summarizing the failures to standard error.
3610 @cindex verifying MD5 checksums
3611 This option is useful only when verifying checksums.
3612 When verifying checksums, don't generate the default one-line-per-file
3613 diagnostic and don't output the warning summarizing any failures.
3614 Failures to open or read a file still evoke individual diagnostics to
3616 If all listed files are readable and are consistent with the associated
3617 MD5 checksums, exit successfully. Otherwise exit with a status code
3618 indicating there was a failure.
3624 @cindex text input files
3625 Treat each input file as text, by reading it in text mode and
3626 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3627 This option is the default on systems like @acronym{GNU} that do not
3628 distinguish between binary and text files. On other systems, it is
3629 the default for reading standard input when standard input is a
3636 @cindex verifying MD5 checksums
3637 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3638 This option is useful only if all but a few lines in the checked input
3646 @node sha1sum invocation
3647 @section @command{sha1sum}: Print or check SHA-1 digests
3651 @cindex 160-bit checksum
3652 @cindex checksum, 160-bit
3653 @cindex fingerprint, 160-bit
3654 @cindex message-digest, 160-bit
3656 @command{sha1sum} computes a 160-bit checksum for each specified
3657 @var{file}. The usage and options of this command are precisely the
3658 same as for @command{md5sum}. @xref{md5sum invocation}.
3660 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3661 it are known (different files having the same fingerprint). However,
3662 it is known that they can be produced with considerable, but not
3663 unreasonable, resources. For this reason, it is generally considered
3664 that SHA-1 should be gradually phased out in favor of the more secure
3665 SHA-2 hash algorithms. @xref{sha2 utilities}.
3668 @node sha2 utilities
3669 @section sha2 utilities: Print or check SHA-2 digests
3676 @cindex 224-bit checksum
3677 @cindex 256-bit checksum
3678 @cindex 384-bit checksum
3679 @cindex 512-bit checksum
3680 @cindex checksum, 224-bit
3681 @cindex checksum, 256-bit
3682 @cindex checksum, 384-bit
3683 @cindex checksum, 512-bit
3684 @cindex fingerprint, 224-bit
3685 @cindex fingerprint, 256-bit
3686 @cindex fingerprint, 384-bit
3687 @cindex fingerprint, 512-bit
3688 @cindex message-digest, 224-bit
3689 @cindex message-digest, 256-bit
3690 @cindex message-digest, 384-bit
3691 @cindex message-digest, 512-bit
3693 The commands @command{sha224sum}, @command{sha256sum},
3694 @command{sha384sum} and @command{sha512sum} compute checksums of
3695 various lengths (respectively 224, 256, 384 and 512 bits),
3696 collectively known as the SHA-2 hashes. The usage and options of
3697 these commands are precisely the same as for @command{md5sum}.
3698 @xref{md5sum invocation}.
3700 Note: The SHA384 and SHA512 digests are considerably slower to
3701 compute, especially on 32-bit computers, than SHA224 or SHA256.
3704 @node Operating on sorted files
3705 @chapter Operating on sorted files
3707 @cindex operating on sorted files
3708 @cindex sorted files, operations on
3710 These commands work with (or produce) sorted files.
3713 * sort invocation:: Sort text files.
3714 * shuf invocation:: Shuffle text files.
3715 * uniq invocation:: Uniquify files.
3716 * comm invocation:: Compare two sorted files line by line.
3717 * ptx invocation:: Produce a permuted index of file contents.
3718 * tsort invocation:: Topological sort.
3722 @node sort invocation
3723 @section @command{sort}: Sort text files
3726 @cindex sorting files
3728 @command{sort} sorts, merges, or compares all the lines from the given
3729 files, or standard input if none are given or for a @var{file} of
3730 @samp{-}. By default, @command{sort} writes the results to standard
3734 sort [@var{option}]@dots{} [@var{file}]@dots{}
3737 @command{sort} has three modes of operation: sort (the default), merge,
3738 and check for sortedness. The following options change the operation
3745 @itemx --check=diagnose-first
3748 @cindex checking for sortedness
3749 Check whether the given file is already sorted: if it is not all
3750 sorted, print a diagnostic containing the first out-of-order line and
3751 exit with a status of 1.
3752 Otherwise, exit successfully.
3753 At most one input file can be given.
3756 @itemx --check=quiet
3757 @itemx --check=silent
3760 @cindex checking for sortedness
3761 Exit successfully if the given file is already sorted, and
3762 exit with status 1 otherwise.
3763 At most one input file can be given.
3764 This is like @option{-c}, except it does not print a diagnostic.
3770 @cindex merging sorted files
3771 Merge the given files by sorting them as a group. Each input file must
3772 always be individually sorted. It always works to sort instead of
3773 merge; merging is provided because it is faster, in the case where it
3778 @cindex sort stability
3779 @cindex sort's last-resort comparison
3780 A pair of lines is compared as follows:
3781 @command{sort} compares each pair of fields, in the
3782 order specified on the command line, according to the associated
3783 ordering options, until a difference is found or no fields are left.
3784 If no key fields are specified, @command{sort} uses a default key of
3785 the entire line. Finally, as a last resort when all keys compare
3786 equal, @command{sort} compares entire lines as if no ordering options
3787 other than @option{--reverse} (@option{-r}) were specified. The
3788 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3789 comparison} so that lines in which all fields compare equal are left
3790 in their original relative order. The @option{--unique}
3791 (@option{-u}) option also disables the last-resort comparison.
3795 Unless otherwise specified, all comparisons use the character collating
3796 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3797 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3798 to @samp{en_US}), then @command{sort} may produce output that is sorted
3799 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3800 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3801 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3802 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3803 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3804 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3805 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3807 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3808 limit on input line length or restrictions on bytes allowed within lines.
3809 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3810 @command{sort} silently supplies one. A line's trailing newline is not
3811 part of the line for comparison purposes.
3813 @cindex exit status of @command{sort}
3817 0 if no error occurred
3818 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3819 2 if an error occurred
3823 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3824 value as the directory for temporary files instead of @file{/tmp}. The
3825 @option{--temporary-directory} (@option{-T}) option in turn overrides
3826 the environment variable.
3828 The following options affect the ordering of output lines. They may be
3829 specified globally or as part of a specific key field. If no key
3830 fields are specified, global options apply to comparison of entire
3831 lines; otherwise the global options are inherited by key fields that do
3832 not specify any special options of their own. In pre-@acronym{POSIX}
3833 versions of @command{sort}, global options affect only later key fields,
3834 so portable shell scripts should specify global options first.
3839 @itemx --ignore-leading-blanks
3841 @opindex --ignore-leading-blanks
3842 @cindex blanks, ignoring leading
3844 Ignore leading blanks when finding sort keys in each line.
3845 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3846 can change this. Note blanks may be ignored by your locale's collating
3847 rules, but without this option they will be significant for character
3848 positions specified in keys with the @option{-k} option.
3851 @itemx --dictionary-order
3853 @opindex --dictionary-order
3854 @cindex dictionary order
3855 @cindex phone directory order
3856 @cindex telephone directory order
3858 Sort in @dfn{phone directory} order: ignore all characters except
3859 letters, digits and blanks when sorting.
3860 By default letters and digits are those of @acronym{ASCII} and a blank
3861 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3864 @itemx --ignore-case
3866 @opindex --ignore-case
3867 @cindex ignoring case
3868 @cindex case folding
3870 Fold lowercase characters into the equivalent uppercase characters when
3871 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3872 The @env{LC_CTYPE} locale determines character types.
3873 When used with @option{--unique} those lower case equivalent lines are
3874 thrown away. (There is currently no way to throw away the upper case
3875 equivalent instead. (Any @option{--reverse} given would only affect
3876 the final result, after the throwing away.))
3879 @itemx --general-numeric-sort
3880 @itemx --sort=general-numeric
3882 @opindex --general-numeric-sort
3884 @cindex general numeric sort
3886 Sort numerically, using the standard C function @code{strtold} to convert
3887 a prefix of each line to a long double-precision floating point number.
3888 This allows floating point numbers to be specified in scientific notation,
3889 like @code{1.0e-34} and @code{10e100}.
3890 The @env{LC_NUMERIC} locale determines the decimal-point character.
3891 Do not report overflow, underflow, or conversion errors.
3892 Use the following collating sequence:
3896 Lines that do not start with numbers (all considered to be equal).
3898 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3899 in a consistent but machine-dependent order.
3903 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3908 Use this option only if there is no alternative; it is much slower than
3909 @option{--numeric-sort} (@option{-n}) and it can lose information when
3910 converting to floating point.
3913 @itemx --human-numeric-sort
3914 @itemx --sort=human-numeric
3916 @opindex --human-numeric-sort
3918 @cindex human numeric sort
3920 Sort numerically, first by numeric sign (negative, zero, or positive);
3921 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3922 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3923 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3924 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3925 suffix. This option sorts values that are consistently scaled to the
3926 nearest suffix, regardless of whether suffixes denote powers of 1000
3927 or 1024, and it therefore sorts the output of any single invocation of
3928 the @command{df}, @command{du}, or @command{ls} commands that are
3929 invoked with their @option{--human-readable} or @option{--si} options.
3930 The syntax for numbers is the same as for the @option{--numeric-sort}
3931 option; the @acronym{SI} suffix must immediately follow the number.
3934 @itemx --ignore-nonprinting
3936 @opindex --ignore-nonprinting
3937 @cindex nonprinting characters, ignoring
3938 @cindex unprintable characters, ignoring
3940 Ignore nonprinting characters.
3941 The @env{LC_CTYPE} locale determines character types.
3942 This option has no effect if the stronger @option{--dictionary-order}
3943 (@option{-d}) option is also given.
3949 @opindex --month-sort
3951 @cindex months, sorting by
3953 An initial string, consisting of any amount of blanks, followed
3954 by a month name abbreviation, is folded to UPPER case and
3955 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3956 Invalid names compare low to valid names. The @env{LC_TIME} locale
3957 category determines the month spellings.
3958 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3962 @itemx --numeric-sort
3963 @itemx --sort=numeric
3965 @opindex --numeric-sort
3967 @cindex numeric sort
3969 Sort numerically. The number begins each line and consists
3970 of optional blanks, an optional @samp{-} sign, and zero or more
3971 digits possibly separated by thousands separators, optionally followed
3972 by a decimal-point character and zero or more digits. An empty
3973 number is treated as @samp{0}. The @env{LC_NUMERIC}
3974 locale specifies the decimal-point character and thousands separator.
3975 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3978 Comparison is exact; there is no rounding error.
3980 Neither a leading @samp{+} nor exponential notation is recognized.
3981 To compare such strings numerically, use the
3982 @option{--general-numeric-sort} (@option{-g}) option.
3985 @itemx --version-sort
3987 @opindex --version-sort
3988 @cindex version number sort
3989 Sort by version name and number. It behaves like a standard sort,
3990 except that each sequence of decimal digits is treated numerically
3991 as an index/version number. (@xref{Details about version sort}.)
3997 @cindex reverse sorting
3998 Reverse the result of comparison, so that lines with greater key values
3999 appear earlier in the output instead of later.
4002 @itemx --random-sort
4003 @itemx --sort=random
4005 @opindex --random-sort
4008 Sort by hashing the input keys and then sorting the hash values.
4009 Choose the hash function at random, ensuring that it is free of
4010 collisions so that differing keys have differing hash values. This is
4011 like a random permutation of the inputs (@pxref{shuf invocation}),
4012 except that keys with the same value sort together.
4014 If multiple random sort fields are specified, the same random hash
4015 function is used for all fields. To use different random hash
4016 functions for different fields, you can invoke @command{sort} more
4019 The choice of hash function is affected by the
4020 @option{--random-source} option.
4028 @item --compress-program=@var{prog}
4029 Compress any temporary files with the program @var{prog}.
4031 With no arguments, @var{prog} must compress standard input to standard
4032 output, and when given the @option{-d} option it must decompress
4033 standard input to standard output.
4035 Terminate with an error if @var{prog} exits with nonzero status.
4037 White space and the backslash character should not appear in
4038 @var{prog}; they are reserved for future use.
4040 @filesZeroFromOption{sort,,sorted output}
4042 @item -k @var{pos1}[,@var{pos2}]
4043 @itemx --key=@var{pos1}[,@var{pos2}]
4047 Specify a sort field that consists of the part of the line between
4048 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4049 omitted), @emph{inclusive}.
4051 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4052 where @var{f} is the number of the field to use, and @var{c} is the number
4053 of the first character from the beginning of the field. Fields and character
4054 positions are numbered starting with 1; a character position of zero in
4055 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4056 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4057 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4058 @var{opts} are ordering options, allowing individual keys to be sorted
4059 according to different rules; see below for details. Keys can span
4062 Example: To sort on the second field, use @option{--key=2,2}
4063 (@option{-k 2,2}). See below for more notes on keys and more examples.
4064 See also the @option{--debug} option to help determine the part
4065 of the line being used in the sort.
4068 Highlight the portion of each line used for sorting.
4069 Also issue warnings about questionable usage to stderr.
4071 @item --batch-size=@var{nmerge}
4072 @opindex --batch-size
4073 @cindex number of inputs to merge, nmerge
4074 Merge at most @var{nmerge} inputs at once.
4076 When @command{sort} has to merge more than @var{nmerge} inputs,
4077 it merges them in groups of @var{nmerge}, saving the result in
4078 a temporary file, which is then used as an input in a subsequent merge.
4080 A large value of @var{nmerge} may improve merge performance and decrease
4081 temporary storage utilization at the expense of increased memory usage
4082 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4083 requirements and I/0 at the expense of temporary storage consumption and
4086 The value of @var{nmerge} must be at least 2. The default value is
4087 currently 16, but this is implementation-dependent and may change in
4090 The value of @var{nmerge} may be bounded by a resource limit for open
4091 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4092 OPEN_MAX} may display limits for your systems; these limits may be
4093 modified further if your program already has some files open, or if
4094 the operating system has other limits on the number of open files. If
4095 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4096 silently uses a smaller value.
4098 @item -o @var{output-file}
4099 @itemx --output=@var{output-file}
4102 @cindex overwriting of input, allowed
4103 Write output to @var{output-file} instead of standard output.
4104 Normally, @command{sort} reads all input before opening
4105 @var{output-file}, so you can safely sort a file in place by using
4106 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4107 However, @command{sort} with @option{--merge} (@option{-m}) can open
4108 the output file before reading all input, so a command like @code{cat
4109 F | sort -m -o F - G} is not safe as @command{sort} might start
4110 writing @file{F} before @command{cat} is done reading it.
4112 @vindex POSIXLY_CORRECT
4113 On newer systems, @option{-o} cannot appear after an input file if
4114 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4115 scripts should specify @option{-o @var{output-file}} before any input
4118 @item --random-source=@var{file}
4119 @opindex --random-source
4120 @cindex random source for sorting
4121 Use @var{file} as a source of random data used to determine which
4122 random hash function to use with the @option{-R} option. @xref{Random
4129 @cindex sort stability
4130 @cindex sort's last-resort comparison
4132 Make @command{sort} stable by disabling its last-resort comparison.
4133 This option has no effect if no fields or global ordering options
4134 other than @option{--reverse} (@option{-r}) are specified.
4137 @itemx --buffer-size=@var{size}
4139 @opindex --buffer-size
4140 @cindex size for main memory sorting
4141 Use a main-memory sort buffer of the given @var{size}. By default,
4142 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4143 @var{size} to be interpreted as a percentage of physical memory.
4144 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4145 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4146 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4147 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4150 This option can improve the performance of @command{sort} by causing it
4151 to start with a larger or smaller sort buffer than the default.
4152 However, this option affects only the initial buffer size. The buffer
4153 grows beyond @var{size} if @command{sort} encounters input lines larger
4156 @item -t @var{separator}
4157 @itemx --field-separator=@var{separator}
4159 @opindex --field-separator
4160 @cindex field separator character
4161 Use character @var{separator} as the field separator when finding the
4162 sort keys in each line. By default, fields are separated by the empty
4163 string between a non-blank character and a blank character.
4164 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4167 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4168 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4169 not considered to be part of either the field preceding or the field
4170 following, so with @samp{sort @w{-t " "}} the same input line has
4171 three fields: an empty field, @samp{foo}, and @samp{bar}.
4172 However, fields that extend to the end of the line,
4173 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4174 retain the field separators present between the endpoints of the range.
4176 To specify @acronym{ASCII} @sc{nul} as the field separator,
4177 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4179 @item -T @var{tempdir}
4180 @itemx --temporary-directory=@var{tempdir}
4182 @opindex --temporary-directory
4183 @cindex temporary directory
4185 Use directory @var{tempdir} to store temporary files, overriding the
4186 @env{TMPDIR} environment variable. If this option is given more than
4187 once, temporary files are stored in all the directories given. If you
4188 have a large sort or merge that is I/O-bound, you can often improve
4189 performance by using this option to specify directories on different
4190 disks and controllers.
4192 @item --parallel=@var{n}
4194 @cindex multithreaded sort
4195 Set the number of sorts run in parallel to @var{n}. By default,
4196 @var{n} is set to the number of available processors, but limited
4197 to 8, as there are diminishing performance gains after that.
4198 Note also that using @var{n} threads increases the memory usage by
4199 a factor of log @var{n}. Also see @ref{nproc invocation}.
4205 @cindex uniquifying output
4207 Normally, output only the first of a sequence of lines that compare
4208 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4209 check that no pair of consecutive lines compares equal.
4211 This option also disables the default last-resort comparison.
4213 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4214 this equivalence does not extend to arbitrary @command{sort} options.
4215 For example, @code{sort -n -u} inspects only the value of the initial
4216 numeric string when checking for uniqueness, whereas @code{sort -n |
4217 uniq} inspects the entire line. @xref{uniq invocation}.
4219 @macro zeroTerminatedOption
4221 @itemx --zero-terminated
4223 @opindex --zero-terminated
4224 @cindex process zero-terminated items
4225 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4226 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4227 and terminate output items with @acronym{ASCII} @sc{nul}.
4228 This option can be useful in conjunction with @samp{perl -0} or
4229 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4230 reliably handle arbitrary file names (even those containing blanks
4231 or other special characters).
4233 @zeroTerminatedOption
4237 Historical (BSD and System V) implementations of @command{sort} have
4238 differed in their interpretation of some options, particularly
4239 @option{-b}, @option{-f}, and @option{-n}.
4240 @sc{gnu} sort follows the @acronym{POSIX}
4241 behavior, which is usually (but not always!) like the System V behavior.
4242 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4243 consistency, @option{-M} has been changed in the same way. This may
4244 affect the meaning of character positions in field specifications in
4245 obscure cases. The only fix is to add an explicit @option{-b}.
4247 A position in a sort field specified with @option{-k} may have any
4248 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4249 global ordering options are inherited by that particular field. The
4250 @option{-b} option may be independently attached to either or both of
4251 the start and end positions of a field specification, and if it is
4252 inherited from the global options it will be attached to both.
4253 If input lines can contain leading or adjacent blanks and @option{-t}
4254 is not used, then @option{-k} is typically combined with @option{-b} or
4255 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4256 the varying numbers of leading blanks in fields can cause confusing results.
4258 If the start position in a sort field specifier falls after the end of
4259 the line or after the end field, the field is empty. If the @option{-b}
4260 option was specified, the @samp{.@var{c}} part of a field specification
4261 is counted from the first nonblank character of the field.
4263 @vindex _POSIX2_VERSION
4264 @vindex POSIXLY_CORRECT
4265 On older systems, @command{sort} supports an obsolete origin-zero
4266 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4267 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4268 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4269 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4270 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4272 This obsolete behavior can be enabled or disabled with the
4273 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4274 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4275 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4277 Scripts intended for use on standard hosts should avoid obsolete
4278 syntax and should use @option{-k} instead. For example, avoid
4279 @samp{sort +2}, since it might be interpreted as either @samp{sort
4280 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4281 support only the obsolete syntax, it can use a test like @samp{if sort
4282 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4285 Here are some examples to illustrate various combinations of options.
4290 Sort in descending (reverse) numeric order.
4297 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4300 sort --parallel=4 -S 10M
4304 Sort alphabetically, omitting the first and second fields
4305 and the blanks at the start of the third field.
4306 This uses a single key composed of the characters beginning
4307 at the start of the first nonblank character in field three
4308 and extending to the end of each line.
4315 Sort numerically on the second field and resolve ties by sorting
4316 alphabetically on the third and fourth characters of field five.
4317 Use @samp{:} as the field delimiter.
4320 sort -t : -k 2,2n -k 5.3,5.4
4323 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4324 @command{sort} would have used all characters beginning in the second field
4325 and extending to the end of the line as the primary @emph{numeric}
4326 key. For the large majority of applications, treating keys spanning
4327 more than one field as numeric will not do what you expect.
4329 Also note that the @samp{n} modifier was applied to the field-end
4330 specifier for the first key. It would have been equivalent to
4331 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4332 @samp{b} apply to the associated @emph{field}, regardless of whether
4333 the modifier character is attached to the field-start and/or the
4334 field-end part of the key specifier.
4337 Sort the password file on the fifth field and ignore any
4338 leading blanks. Sort lines with equal values in field five
4339 on the numeric user ID in field three. Fields are separated
4343 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4344 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4345 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4348 These three commands have equivalent effect. The first specifies that
4349 the first key's start position ignores leading blanks and the second
4350 key is sorted numerically. The other two commands rely on global
4351 options being inherited by sort keys that lack modifiers. The inheritance
4352 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4353 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4354 character position is not affected by whether initial blanks are
4358 Sort a set of log files, primarily by IPv4 address and secondarily by
4359 time stamp. If two lines' primary and secondary keys are identical,
4360 output the lines in the same order that they were input. The log
4361 files contain lines that look like this:
4364 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4365 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4368 Fields are separated by exactly one space. Sort IPv4 addresses
4369 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4370 because 61 is less than 129.
4373 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4374 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4377 This example cannot be done with a single @command{sort} invocation,
4378 since IPv4 address components are separated by @samp{.} while dates
4379 come just after a space. So it is broken down into two invocations of
4380 @command{sort}: the first sorts by time stamp and the second by IPv4
4381 address. The time stamp is sorted by year, then month, then day, and
4382 finally by hour-minute-second field, using @option{-k} to isolate each
4383 field. Except for hour-minute-second there's no need to specify the
4384 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4385 based on leading prefixes that cannot cross field boundaries. The
4386 IPv4 addresses are sorted lexicographically. The second sort uses
4387 @samp{-s} so that ties in the primary key are broken by the secondary
4388 key; the first sort uses @samp{-s} so that the combination of the two
4392 Generate a tags file in case-insensitive sorted order.
4395 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4398 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4399 that file names that contain blanks or other special characters are
4401 by the sort operation.
4403 @c This example is a bit contrived and needs more explanation.
4405 @c Sort records separated by an arbitrary string by using a pipe to convert
4406 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4407 @c and converting each @samp{\0} back to the original record delimiter.
4410 @c printf 'c\n\nb\n\na\n' |
4411 @c perl -0pe 's/\n\n/\n\0/g' |
4413 @c perl -0pe 's/\0/\n/g'
4417 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4418 sort lines according to their length.
4421 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4424 In general this technique can be used to sort data that the @command{sort}
4425 command does not support, or is inefficient at, sorting directly.
4428 Shuffle a list of directories, but preserve the order of files within
4429 each directory. For instance, one could use this to generate a music
4430 playlist in which albums are shuffled but the songs of each album are
4434 ls */* | sort -t / -k 1,1R -k 2,2
4440 @node shuf invocation
4441 @section @command{shuf}: Shuffling text
4444 @cindex shuffling files
4446 @command{shuf} shuffles its input by outputting a random permutation
4447 of its input lines. Each output permutation is equally likely.
4451 shuf [@var{option}]@dots{} [@var{file}]
4452 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4453 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4456 @command{shuf} has three modes of operation that affect where it
4457 obtains its input lines. By default, it reads lines from standard
4458 input. The following options change the operation mode:
4466 @cindex command-line operands to shuffle
4467 Treat each command-line operand as an input line.
4469 @item -i @var{lo}-@var{hi}
4470 @itemx --input-range=@var{lo}-@var{hi}
4472 @opindex --input-range
4473 @cindex input range to shuffle
4474 Act as if input came from a file containing the range of unsigned
4475 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4479 @command{shuf}'s other options can affect its behavior in all
4484 @item -n @var{lines}
4485 @itemx --head-count=@var{count}
4487 @opindex --head-count
4488 @cindex head of output
4489 Output at most @var{count} lines. By default, all input lines are
4492 @item -o @var{output-file}
4493 @itemx --output=@var{output-file}
4496 @cindex overwriting of input, allowed
4497 Write output to @var{output-file} instead of standard output.
4498 @command{shuf} reads all input before opening
4499 @var{output-file}, so you can safely shuffle a file in place by using
4500 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4502 @item --random-source=@var{file}
4503 @opindex --random-source
4504 @cindex random source for shuffling
4505 Use @var{file} as a source of random data used to determine which
4506 permutation to generate. @xref{Random sources}.
4508 @zeroTerminatedOption
4524 might produce the output
4534 Similarly, the command:
4537 shuf -e clubs hearts diamonds spades
4551 and the command @samp{shuf -i 1-4} might output:
4561 These examples all have four input lines, so @command{shuf} might
4562 produce any of the twenty-four possible permutations of the input. In
4563 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4564 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4565 output permutations.
4570 @node uniq invocation
4571 @section @command{uniq}: Uniquify files
4574 @cindex uniquify files
4576 @command{uniq} writes the unique lines in the given @file{input}, or
4577 standard input if nothing is given or for an @var{input} name of
4581 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4584 By default, @command{uniq} prints its input lines, except that
4585 it discards all but the first of adjacent repeated lines, so that
4586 no output lines are repeated. Optionally, it can instead discard
4587 lines that are not repeated, or all repeated lines.
4589 The input need not be sorted, but repeated input lines are detected
4590 only if they are adjacent. If you want to discard non-adjacent
4591 duplicate lines, perhaps you want to use @code{sort -u}.
4592 @xref{sort invocation}.
4595 Comparisons honor the rules specified by the @env{LC_COLLATE}
4598 If no @var{output} file is specified, @command{uniq} writes to standard
4601 The program accepts the following options. Also see @ref{Common options}.
4606 @itemx --skip-fields=@var{n}
4608 @opindex --skip-fields
4609 Skip @var{n} fields on each line before checking for uniqueness. Use
4610 a null string for comparison if a line has fewer than @var{n} fields. Fields
4611 are sequences of non-space non-tab characters that are separated from
4612 each other by at least one space or tab.
4614 For compatibility @command{uniq} supports an obsolete option syntax
4615 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4618 @itemx --skip-chars=@var{n}
4620 @opindex --skip-chars
4621 Skip @var{n} characters before checking for uniqueness. Use a null string
4622 for comparison if a line has fewer than @var{n} characters. If you use both
4623 the field and character skipping options, fields are skipped over first.
4625 @vindex _POSIX2_VERSION
4626 On older systems, @command{uniq} supports an obsolete option syntax
4628 This obsolete behavior can be enabled or disabled with the
4629 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4630 conformance}), but portable scripts should avoid commands whose
4631 behavior depends on this variable.
4632 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4633 the ambiguous @samp{uniq +10}.
4639 Print the number of times each line occurred along with the line.
4642 @itemx --ignore-case
4644 @opindex --ignore-case
4645 Ignore differences in case when comparing lines.
4651 @cindex repeated lines, outputting
4652 Discard lines that are not repeated. When used by itself, this option
4653 causes @command{uniq} to print the first copy of each repeated line,
4657 @itemx --all-repeated[=@var{delimit-method}]
4659 @opindex --all-repeated
4660 @cindex all repeated lines, outputting
4661 Do not discard the second and subsequent repeated input lines,
4662 but discard lines that are not repeated.
4663 This option is useful mainly in conjunction with other options e.g.,
4664 to ignore case or to compare only selected fields.
4665 The optional @var{delimit-method} tells how to delimit
4666 groups of repeated lines, and must be one of the following:
4671 Do not delimit groups of repeated lines.
4672 This is equivalent to @option{--all-repeated} (@option{-D}).
4675 Output a newline before each group of repeated lines.
4676 With @option{--zero-terminated} (@option{-z}), use a zero
4677 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4680 Separate groups of repeated lines with a single newline.
4681 With @option{--zero-terminated} (@option{-z}), use a zero
4682 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4683 This is the same as using @samp{prepend}, except that
4684 no delimiter is inserted before the first group, and hence
4685 may be better suited for output direct to users.
4688 Note that when groups are delimited and the input stream contains
4689 two or more consecutive blank lines, then the output is ambiguous.
4690 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4691 each sequence of consecutive newlines with a single newline.
4693 This is a @sc{gnu} extension.
4694 @c FIXME: give an example showing *how* it's useful
4700 @cindex unique lines, outputting
4701 Discard the first repeated line. When used by itself, this option
4702 causes @command{uniq} to print unique lines, and nothing else.
4705 @itemx --check-chars=@var{n}
4707 @opindex --check-chars
4708 Compare at most @var{n} characters on each line (after skipping any specified
4709 fields and characters). By default the entire rest of the lines are
4712 @zeroTerminatedOption
4719 @node comm invocation
4720 @section @command{comm}: Compare two sorted files line by line
4723 @cindex line-by-line comparison
4724 @cindex comparing sorted files
4726 @command{comm} writes to standard output lines that are common, and lines
4727 that are unique, to two input files; a file name of @samp{-} means
4728 standard input. Synopsis:
4731 comm [@var{option}]@dots{} @var{file1} @var{file2}
4735 Before @command{comm} can be used, the input files must be sorted using the
4736 collating sequence specified by the @env{LC_COLLATE} locale.
4737 If an input file ends in a non-newline
4738 character, a newline is silently appended. The @command{sort} command with
4739 no options always outputs a file that is suitable input to @command{comm}.
4741 @cindex differing lines
4742 @cindex common lines
4743 With no options, @command{comm} produces three-column output. Column one
4744 contains lines unique to @var{file1}, column two contains lines unique
4745 to @var{file2}, and column three contains lines common to both files.
4746 Columns are separated by a single TAB character.
4747 @c FIXME: when there's an option to supply an alternative separator
4748 @c string, append `by default' to the above sentence.
4753 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4754 the corresponding columns (and separators). Also see @ref{Common options}.
4756 Unlike some other comparison utilities, @command{comm} has an exit
4757 status that does not depend on the result of the comparison.
4758 Upon normal completion @command{comm} produces an exit code of zero.
4759 If there is an error it exits with nonzero status.
4761 @macro checkOrderOption{cmd}
4762 If the @option{--check-order} option is given, unsorted inputs will
4763 cause a fatal error message. If the option @option{--nocheck-order}
4764 is given, unsorted inputs will never cause an error message. If
4765 neither of these options is given, wrongly sorted inputs are diagnosed
4766 only if an input file is found to contain unpairable lines. If an
4767 input file is diagnosed as being unsorted, the @command{\cmd\} command
4768 will exit with a nonzero status (and the output should not be used).
4770 Forcing @command{\cmd\} to process wrongly sorted input files
4771 containing unpairable lines by specifying @option{--nocheck-order} is
4772 not guaranteed to produce any particular output. The output will
4773 probably not correspond with whatever you hoped it would be.
4775 @checkOrderOption{comm}
4780 Fail with an error message if either input file is wrongly ordered.
4782 @item --nocheck-order
4783 Do not check that both input files are in sorted order.
4787 @item --output-delimiter=@var{str}
4788 Print @var{str} between adjacent output columns,
4789 rather than the default of a single TAB character.
4791 The delimiter @var{str} may not be empty.
4795 @node ptx invocation
4796 @section @command{ptx}: Produce permuted indexes
4800 @command{ptx} reads a text file and essentially produces a permuted index, with
4801 each keyword in its context. The calling sketch is either one of:
4804 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4805 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4808 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4809 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4810 limitations and changing several of the program's default option values.
4811 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4812 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4813 document. For the full list, see @xref{Compatibility in ptx}.
4815 Individual options are explained in the following sections.
4817 When @sc{gnu} extensions are enabled, there may be zero, one or several
4818 @var{file}s after the options. If there is no @var{file}, the program
4819 reads the standard input. If there is one or several @var{file}s, they
4820 give the name of input files which are all read in turn, as if all the
4821 input files were concatenated. However, there is a full contextual
4822 break between each file and, when automatic referencing is requested,
4823 file names and line numbers refer to individual text input files. In
4824 all cases, the program outputs the permuted index to the standard
4827 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4828 operates in traditional mode, there may be zero, one or two parameters
4829 besides the options. If there are no parameters, the program reads the
4830 standard input and outputs the permuted index to the standard output.
4831 If there is only one parameter, it names the text @var{input} to be read
4832 instead of the standard input. If two parameters are given, they give
4833 respectively the name of the @var{input} file to read and the name of
4834 the @var{output} file to produce. @emph{Be very careful} to note that,
4835 in this case, the contents of file given by the second parameter is
4836 destroyed. This behavior is dictated by System V @command{ptx}
4837 compatibility; @sc{gnu} Standards normally discourage output parameters not
4838 introduced by an option.
4840 Note that for @emph{any} file named as the value of an option or as an
4841 input text file, a single dash @kbd{-} may be used, in which case
4842 standard input is assumed. However, it would not make sense to use this
4843 convention more than once per program invocation.
4846 * General options in ptx:: Options which affect general program behavior.
4847 * Charset selection in ptx:: Underlying character set considerations.
4848 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4849 * Output formatting in ptx:: Types of output format, and sizing the fields.
4850 * Compatibility in ptx::
4854 @node General options in ptx
4855 @subsection General options
4860 @itemx --traditional
4861 As already explained, this option disables all @sc{gnu} extensions to
4862 @command{ptx} and switches to traditional mode.
4865 Print a short help on standard output, then exit without further
4869 Print the program version on standard output, then exit without further
4877 @node Charset selection in ptx
4878 @subsection Charset selection
4880 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4881 As it is set up now, the program assumes that the input file is coded
4882 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4883 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4884 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4885 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4886 of characters which are letters is different; this alters the behavior
4887 of regular expression matching. Thus, the default regular expression
4888 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4889 however, is still crude; it obeys the underlying character set ordering
4895 @itemx --ignore-case
4896 Fold lower case letters to upper case for sorting.
4901 @node Input processing in ptx
4902 @subsection Word selection and input processing
4907 @itemx --break-file=@var{file}
4909 This option provides an alternative (to @option{-W}) method of describing
4910 which characters make up words. It introduces the name of a
4911 file which contains a list of characters which can@emph{not} be part of
4912 one word; this file is called the @dfn{Break file}. Any character which
4913 is not part of the Break file is a word constituent. If both options
4914 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4915 @option{-b} is ignored.
4917 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4918 break character is to write all the break characters in the file with no
4919 newline at all, not even at the end of the file. When @sc{gnu} extensions
4920 are disabled, spaces, tabs and newlines are always considered as break
4921 characters even if not included in the Break file.
4924 @itemx --ignore-file=@var{file}
4926 The file associated with this option contains a list of words which will
4927 never be taken as keywords in concordance output. It is called the
4928 @dfn{Ignore file}. The file contains exactly one word in each line; the
4929 end of line separation of words is not subject to the value of the
4933 @itemx --only-file=@var{file}
4935 The file associated with this option contains a list of words which will
4936 be retained in concordance output; any word not mentioned in this file
4937 is ignored. The file is called the @dfn{Only file}. The file contains
4938 exactly one word in each line; the end of line separation of words is
4939 not subject to the value of the @option{-S} option.
4941 There is no default for the Only file. When both an Only file and an
4942 Ignore file are specified, a word is considered a keyword only
4943 if it is listed in the Only file and not in the Ignore file.
4948 On each input line, the leading sequence of non-white space characters will be
4949 taken to be a reference that has the purpose of identifying this input
4950 line in the resulting permuted index. For more information about reference
4951 production, see @xref{Output formatting in ptx}.
4952 Using this option changes the default value for option @option{-S}.
4954 Using this option, the program does not try very hard to remove
4955 references from contexts in output, but it succeeds in doing so
4956 @emph{when} the context ends exactly at the newline. If option
4957 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4958 are disabled, this condition is always met and references are completely
4959 excluded from the output contexts.
4961 @item -S @var{regexp}
4962 @itemx --sentence-regexp=@var{regexp}
4964 This option selects which regular expression will describe the end of a
4965 line or the end of a sentence. In fact, this regular expression is not
4966 the only distinction between end of lines or end of sentences, and input
4967 line boundaries have no special significance outside this option. By
4968 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4969 used, end of sentences are used. In this case, this @var{regex} is
4970 imported from @sc{gnu} Emacs:
4973 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4976 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4977 of lines are used; in this case, the default @var{regexp} is just:
4983 Using an empty @var{regexp} is equivalent to completely disabling end of
4984 line or end of sentence recognition. In this case, the whole file is
4985 considered to be a single big line or sentence. The user might want to
4986 disallow all truncation flag generation as well, through option @option{-F
4987 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4990 When the keywords happen to be near the beginning of the input line or
4991 sentence, this often creates an unused area at the beginning of the
4992 output context line; when the keywords happen to be near the end of the
4993 input line or sentence, this often creates an unused area at the end of
4994 the output context line. The program tries to fill those unused areas
4995 by wrapping around context in them; the tail of the input line or
4996 sentence is used to fill the unused area on the left of the output line;
4997 the head of the input line or sentence is used to fill the unused area
4998 on the right of the output line.
5000 As a matter of convenience to the user, many usual backslashed escape
5001 sequences from the C language are recognized and converted to the
5002 corresponding characters by @command{ptx} itself.
5004 @item -W @var{regexp}
5005 @itemx --word-regexp=@var{regexp}
5007 This option selects which regular expression will describe each keyword.
5008 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5009 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5010 disabled, a word is by default anything which ends with a space, a tab
5011 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5013 An empty @var{regexp} is equivalent to not using this option.
5014 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5017 As a matter of convenience to the user, many usual backslashed escape
5018 sequences, as found in the C language, are recognized and converted to
5019 the corresponding characters by @command{ptx} itself.
5024 @node Output formatting in ptx
5025 @subsection Output formatting
5027 Output format is mainly controlled by the @option{-O} and @option{-T} options
5028 described in the table below. When neither @option{-O} nor @option{-T} are
5029 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5030 output format suitable for a dumb terminal. Each keyword occurrence is
5031 output to the center of one line, surrounded by its left and right
5032 contexts. Each field is properly justified, so the concordance output
5033 can be readily observed. As a special feature, if automatic
5034 references are selected by option @option{-A} and are output before the
5035 left context, that is, if option @option{-R} is @emph{not} selected, then
5036 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5037 Emacs @code{next-error} processing. In this default output format, each
5038 white space character, like newline and tab, is merely changed to
5039 exactly one space, with no special attempt to compress consecutive
5040 spaces. This might change in the future. Except for those white space
5041 characters, every other character of the underlying set of 256
5042 characters is transmitted verbatim.
5044 Output format is further controlled by the following options.
5048 @item -g @var{number}
5049 @itemx --gap-size=@var{number}
5051 Select the size of the minimum white space gap between the fields on the
5054 @item -w @var{number}
5055 @itemx --width=@var{number}
5057 Select the maximum output width of each final line. If references are
5058 used, they are included or excluded from the maximum output width
5059 depending on the value of option @option{-R}. If this option is not
5060 selected, that is, when references are output before the left context,
5061 the maximum output width takes into account the maximum length of all
5062 references. If this option is selected, that is, when references are
5063 output after the right context, the maximum output width does not take
5064 into account the space taken by references, nor the gap that precedes
5068 @itemx --auto-reference
5070 Select automatic references. Each input line will have an automatic
5071 reference made up of the file name and the line ordinal, with a single
5072 colon between them. However, the file name will be empty when standard
5073 input is being read. If both @option{-A} and @option{-r} are selected, then
5074 the input reference is still read and skipped, but the automatic
5075 reference is used at output time, overriding the input reference.
5078 @itemx --right-side-refs
5080 In the default output format, when option @option{-R} is not used, any
5081 references produced by the effect of options @option{-r} or @option{-A} are
5082 placed to the far right of output lines, after the right context. With
5083 default output format, when the @option{-R} option is specified, references
5084 are rather placed at the beginning of each output line, before the left
5085 context. For any other output format, option @option{-R} is
5086 ignored, with one exception: with @option{-R} the width of references
5087 is @emph{not} taken into account in total output width given by @option{-w}.
5089 This option is automatically selected whenever @sc{gnu} extensions are
5092 @item -F @var{string}
5093 @itemx --flac-truncation=@var{string}
5095 This option will request that any truncation in the output be reported
5096 using the string @var{string}. Most output fields theoretically extend
5097 towards the beginning or the end of the current line, or current
5098 sentence, as selected with option @option{-S}. But there is a maximum
5099 allowed output line width, changeable through option @option{-w}, which is
5100 further divided into space for various output fields. When a field has
5101 to be truncated because it cannot extend beyond the beginning or the end of
5102 the current line to fit in, then a truncation occurs. By default,
5103 the string used is a single slash, as in @option{-F /}.
5105 @var{string} may have more than one character, as in @option{-F ...}.
5106 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5107 truncation flagging is disabled, and no truncation marks are appended in
5110 As a matter of convenience to the user, many usual backslashed escape
5111 sequences, as found in the C language, are recognized and converted to
5112 the corresponding characters by @command{ptx} itself.
5114 @item -M @var{string}
5115 @itemx --macro-name=@var{string}
5117 Select another @var{string} to be used instead of @samp{xx}, while
5118 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5121 @itemx --format=roff
5123 Choose an output format suitable for @command{nroff} or @command{troff}
5124 processing. Each output line will look like:
5127 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}"@c
5128 "@var{head}" "@var{ref}"
5131 so it will be possible to write a @samp{.xx} roff macro to take care of
5132 the output typesetting. This is the default output format when @sc{gnu}
5133 extensions are disabled. Option @option{-M} can be used to change
5134 @samp{xx} to another macro name.
5136 In this output format, each non-graphical character, like newline and
5137 tab, is merely changed to exactly one space, with no special attempt to
5138 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5139 so it will be correctly processed by @command{nroff} or @command{troff}.
5144 Choose an output format suitable for @TeX{} processing. Each output
5145 line will look like:
5148 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@c
5149 @{@var{after}@}@{@var{head}@}@{@var{ref}@}
5153 so it will be possible to write a @code{\xx} definition to take care of
5154 the output typesetting. Note that when references are not being
5155 produced, that is, neither option @option{-A} nor option @option{-r} is
5156 selected, the last parameter of each @code{\xx} call is inhibited.
5157 Option @option{-M} can be used to change @samp{xx} to another macro
5160 In this output format, some special characters, like @kbd{$}, @kbd{%},
5161 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5162 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5163 backslash and a pair of dollar signs (to force mathematical mode). The
5164 backslash itself produces the sequence @code{\backslash@{@}}.
5165 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5166 @code{~\@{ @}} respectively. Other diacriticized characters of the
5167 underlying character set produce an appropriate @TeX{} sequence as far
5168 as possible. The other non-graphical characters, like newline and tab,
5169 and all other characters which are not part of @acronym{ASCII}, are merely
5170 changed to exactly one space, with no special attempt to compress
5171 consecutive spaces. Let me know how to improve this special character
5172 processing for @TeX{}.
5177 @node Compatibility in ptx
5178 @subsection The @sc{gnu} extensions to @command{ptx}
5180 This version of @command{ptx} contains a few features which do not exist in
5181 System V @command{ptx}. These extra features are suppressed by using the
5182 @option{-G} command line option, unless overridden by other command line
5183 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5184 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5185 Here are the differences between this program and System V @command{ptx}.
5190 This program can read many input files at once, it always writes the
5191 resulting concordance on standard output. On the other hand, System V
5192 @command{ptx} reads only one file and sends the result to standard output
5193 or, if a second @var{file} parameter is given on the command, to that
5196 Having output parameters not introduced by options is a dangerous
5197 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5198 portably between @sc{gnu} and System V, you should always use it with a
5199 single input file, and always expect the result on standard output. You
5200 might also want to automatically configure in a @option{-G} option to
5201 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5202 that the installed @command{ptx} accepts @option{-G}.
5205 The only options available in System V @command{ptx} are options @option{-b},
5206 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5207 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5208 this enumeration. Moreover, some options have a slightly different
5209 meaning when @sc{gnu} extensions are enabled, as explained below.
5212 By default, concordance output is not formatted for @command{troff} or
5213 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5214 or @command{nroff} output may still be selected through option @option{-O}.
5217 Unless @option{-R} option is used, the maximum reference width is
5218 subtracted from the total output line width. With @sc{gnu} extensions
5219 disabled, width of references is not taken into account in the output
5220 line width computations.
5223 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5224 processed from input file with no adverse effect, even if @sc{gnu} extensions
5225 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5226 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5229 Input line length is only limited by available memory, even if @sc{gnu}
5230 extensions are disabled. However, System V @command{ptx} processes only
5231 the first 200 characters in each line.
5234 The break (non-word) characters default to be every character except all
5235 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5236 extensions are disabled, the break characters default to space, tab and
5240 The program makes better use of output line width. If @sc{gnu} extensions
5241 are disabled, the program rather tries to imitate System V @command{ptx},
5242 but still, there are some slight disposition glitches this program does
5243 not completely reproduce.
5246 The user can specify both an Ignore file and an Only file. This is not
5247 allowed with System V @command{ptx}.
5252 @node tsort invocation
5253 @section @command{tsort}: Topological sort
5256 @cindex topological sort
5258 @command{tsort} performs a topological sort on the given @var{file}, or
5259 standard input if no input file is given or for a @var{file} of
5260 @samp{-}. For more details and some history, see @ref{tsort background}.
5264 tsort [@var{option}] [@var{file}]
5267 @command{tsort} reads its input as pairs of strings, separated by blanks,
5268 indicating a partial ordering. The output is a total ordering that
5269 corresponds to the given partial ordering.
5283 will produce the output
5294 Consider a more realistic example.
5295 You have a large set of functions all in one file, and they may all be
5296 declared static except one. Currently that one (say @code{main}) is the
5297 first function defined in the file, and the ones it calls directly follow
5298 it, followed by those they call, etc. Let's say that you are determined
5299 to take advantage of prototypes, so you have to choose between declaring
5300 all of those functions (which means duplicating a lot of information from
5301 the definitions) and rearranging the functions so that as many as possible
5302 are defined before they are used. One way to automate the latter process
5303 is to get a list for each function of the functions it calls directly.
5304 Many programs can generate such lists. They describe a call graph.
5305 Consider the following list, in which a given line indicates that the
5306 function on the left calls the one on the right directly.
5312 tail_file pretty_name
5313 tail_file write_header
5315 tail_forever recheck
5316 tail_forever pretty_name
5317 tail_forever write_header
5318 tail_forever dump_remainder
5321 tail_lines start_lines
5322 tail_lines dump_remainder
5323 tail_lines file_lines
5324 tail_lines pipe_lines
5326 tail_bytes start_bytes
5327 tail_bytes dump_remainder
5328 tail_bytes pipe_bytes
5329 file_lines dump_remainder
5333 then you can use @command{tsort} to produce an ordering of those
5334 functions that satisfies your requirement.
5337 example$ tsort call-graph | tac
5357 @command{tsort} detects any cycles in the input and writes the first cycle
5358 encountered to standard error.
5360 Note that for a given partial ordering, generally there is no unique
5361 total ordering. In the context of the call graph above, the function
5362 @code{parse_options} may be placed anywhere in the list as long as it
5363 precedes @code{main}.
5365 The only options are @option{--help} and @option{--version}. @xref{Common
5371 * tsort background:: Where tsort came from.
5374 @node tsort background
5375 @subsection @command{tsort}: Background
5377 @command{tsort} exists because very early versions of the Unix linker processed
5378 an archive file exactly once, and in order. As @command{ld} read each object
5379 in the archive, it decided whether it was needed in the program based on
5380 whether it defined any symbols which were undefined at that point in
5383 This meant that dependencies within the archive had to be handled
5384 specially. For example, @code{scanf} probably calls @code{read}. That means
5385 that in a single pass through an archive, it was important for @code{scanf.o}
5386 to appear before read.o, because otherwise a program which calls
5387 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5388 reference to @code{read}.
5390 The way to address this problem was to first generate a set of
5391 dependencies of one object file on another. This was done by a shell
5392 script called @command{lorder}. The GNU tools don't provide a version of
5393 lorder, as far as I know, but you can still find it in BSD
5396 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5397 resulting sort to define the order in which you added objects to the archive.
5399 This whole procedure has been obsolete since about 1980, because
5400 Unix archives now contain a symbol table (traditionally built by
5401 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5402 linker uses the symbol table to effectively make multiple passes over
5405 Anyhow, that's where tsort came from. To solve an old problem with
5406 the way the linker handled archive files, which has since been solved
5410 @node Operating on fields
5411 @chapter Operating on fields
5414 * cut invocation:: Print selected parts of lines.
5415 * paste invocation:: Merge lines of files.
5416 * join invocation:: Join lines on a common field.
5420 @node cut invocation
5421 @section @command{cut}: Print selected parts of lines
5424 @command{cut} writes to standard output selected parts of each line of each
5425 input file, or standard input if no files are given or for a file name of
5429 cut @var{option}@dots{} [@var{file}]@dots{}
5432 In the table which follows, the @var{byte-list}, @var{character-list},
5433 and @var{field-list} are one or more numbers or ranges (two numbers
5434 separated by a dash) separated by commas. Bytes, characters, and
5435 fields are numbered starting at 1. Incomplete ranges may be
5436 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5437 @samp{@var{n}} through end of line or last field. The list elements
5438 can be repeated, can overlap, and can be specified in any order; but
5439 the selected input is written in the same order that it is read, and
5440 is written exactly once.
5442 The program accepts the following options. Also see @ref{Common
5447 @item -b @var{byte-list}
5448 @itemx --bytes=@var{byte-list}
5451 Select for printing only the bytes in positions listed in
5452 @var{byte-list}. Tabs and backspaces are treated like any other
5453 character; they take up 1 byte. If an output delimiter is specified,
5454 (see the description of @option{--output-delimiter}), then output that
5455 string between ranges of selected bytes.
5457 @item -c @var{character-list}
5458 @itemx --characters=@var{character-list}
5460 @opindex --characters
5461 Select for printing only the characters in positions listed in
5462 @var{character-list}. The same as @option{-b} for now, but
5463 internationalization will change that. Tabs and backspaces are
5464 treated like any other character; they take up 1 character. If an
5465 output delimiter is specified, (see the description of
5466 @option{--output-delimiter}), then output that string between ranges
5469 @item -f @var{field-list}
5470 @itemx --fields=@var{field-list}
5473 Select for printing only the fields listed in @var{field-list}.
5474 Fields are separated by a TAB character by default. Also print any
5475 line that contains no delimiter character, unless the
5476 @option{--only-delimited} (@option{-s}) option is specified.
5477 Note @command{cut} does not support specifying runs of whitespace as a
5478 delimiter, so to achieve that common functionality one can pre-process
5479 with @command{tr} like:
5481 tr -s '[:blank:]' '\t' | cut -f@dots{}
5484 @item -d @var{input_delim_byte}
5485 @itemx --delimiter=@var{input_delim_byte}
5487 @opindex --delimiter
5488 With @option{-f}, use the first byte of @var{input_delim_byte} as
5489 the input fields separator (default is TAB).
5493 Do not split multi-byte characters (no-op for now).
5496 @itemx --only-delimited
5498 @opindex --only-delimited
5499 For @option{-f}, do not print lines that do not contain the field separator
5500 character. Normally, any line without a field separator is printed verbatim.
5502 @item --output-delimiter=@var{output_delim_string}
5503 @opindex --output-delimiter
5504 With @option{-f}, output fields are separated by @var{output_delim_string}.
5505 The default with @option{-f} is to use the input delimiter.
5506 When using @option{-b} or @option{-c} to select ranges of byte or
5507 character offsets (as opposed to ranges of fields),
5508 output @var{output_delim_string} between non-overlapping
5509 ranges of selected bytes.
5512 @opindex --complement
5513 This option is a @acronym{GNU} extension.
5514 Select for printing the complement of the bytes, characters or fields
5515 selected with the @option{-b}, @option{-c} or @option{-f} options.
5516 In other words, do @emph{not} print the bytes, characters or fields
5517 specified via those options. This option is useful when you have
5518 many fields and want to print all but a few of them.
5525 @node paste invocation
5526 @section @command{paste}: Merge lines of files
5529 @cindex merging files
5531 @command{paste} writes to standard output lines consisting of sequentially
5532 corresponding lines of each given file, separated by a TAB character.
5533 Standard input is used for a file name of @samp{-} or if no input files
5555 paste [@var{option}]@dots{} [@var{file}]@dots{}
5558 The program accepts the following options. Also see @ref{Common options}.
5566 Paste the lines of one file at a time rather than one line from each
5567 file. Using the above example data:
5570 $ paste -s num2 let3
5575 @item -d @var{delim-list}
5576 @itemx --delimiters=@var{delim-list}
5578 @opindex --delimiters
5579 Consecutively use the characters in @var{delim-list} instead of
5580 TAB to separate merged lines. When @var{delim-list} is
5581 exhausted, start again at its beginning. Using the above example data:
5584 $ paste -d '%_' num2 let3 num2
5595 @node join invocation
5596 @section @command{join}: Join lines on a common field
5599 @cindex common field, joining on
5601 @command{join} writes to standard output a line for each pair of input
5602 lines that have identical join fields. Synopsis:
5605 join [@var{option}]@dots{} @var{file1} @var{file2}
5608 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5609 meaning standard input. @var{file1} and @var{file2} should be
5610 sorted on the join fields.
5613 Normally, the sort order is that of the
5614 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5615 the @option{-t} option is given, the sort comparison ignores blanks at
5616 the start of the join field, as in @code{sort -b}. If the
5617 @option{--ignore-case} option is given, the sort comparison ignores
5618 the case of characters in the join field, as in @code{sort -f}.
5620 The @command{sort} and @command{join} commands should use consistent
5621 locales and options if the output of @command{sort} is fed to
5622 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5623 sort a file on its default join field, but if you select a non-default
5624 locale, join field, separator, or comparison options, then you should
5625 do so consistently between @command{join} and @command{sort}.
5626 If @samp{join -t ''} is specified then the whole line is considered which
5627 matches the default operation of sort.
5629 If the input has no unpairable lines, a @acronym{GNU} extension is
5630 available; the sort order can be any order that considers two fields
5631 to be equal if and only if the sort comparison described above
5632 considers them to be equal. For example:
5649 @checkOrderOption{join}
5653 @item the join field is the first field in each line;
5654 @item fields in the input are separated by one or more blanks, with leading
5655 blanks on the line ignored;
5656 @item fields in the output are separated by a space;
5657 @item each output line consists of the join field, the remaining
5658 fields from @var{file1}, then the remaining fields from @var{file2}.
5661 The program accepts the following options. Also see @ref{Common options}.
5665 @item -a @var{file-number}
5667 Print a line for each unpairable line in file @var{file-number} (either
5668 @samp{1} or @samp{2}), in addition to the normal output.
5671 Fail with an error message if either input file is wrongly ordered.
5673 @item --nocheck-order
5674 Do not check that both input files are in sorted order. This is the default.
5676 @item -e @var{string}
5678 Replace those output fields that are missing in the input with
5683 Treat the first line of each input file as a header line. The header lines will
5684 be joined and printed as the first output line. If @option{-o} is used to
5685 specify output format, the header line will be printed according to the
5686 specified format. The header lines will not be checked for ordering even if
5687 @option{--check-order} is specified. Also if the header lines from each file
5688 do not match, the heading fields from the first file will be used.
5691 @itemx --ignore-case
5693 @opindex --ignore-case
5694 Ignore differences in case when comparing keys.
5695 With this option, the lines of the input files must be ordered in the same way.
5696 Use @samp{sort -f} to produce this ordering.
5698 @item -1 @var{field}
5700 Join on field @var{field} (a positive integer) of file 1.
5702 @item -2 @var{field}
5704 Join on field @var{field} (a positive integer) of file 2.
5706 @item -j @var{field}
5707 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5709 @item -o @var{field-list}
5710 Construct each output line according to the format in @var{field-list}.
5711 Each element in @var{field-list} is either the single character @samp{0} or
5712 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5713 @samp{2} and @var{n} is a positive field number.
5715 A field specification of @samp{0} denotes the join field.
5716 In most cases, the functionality of the @samp{0} field spec
5717 may be reproduced using the explicit @var{m.n} that corresponds
5718 to the join field. However, when printing unpairable lines
5719 (using either of the @option{-a} or @option{-v} options), there is no way
5720 to specify the join field using @var{m.n} in @var{field-list}
5721 if there are unpairable lines in both files.
5722 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5723 field specification notation.
5725 The elements in @var{field-list}
5726 are separated by commas or blanks.
5727 Blank separators typically need to be quoted for the shell. For
5728 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5729 2.2'} are equivalent.
5731 All output lines---including those printed because of any -a or -v
5732 option---are subject to the specified @var{field-list}.
5735 Use character @var{char} as the input and output field separator.
5736 Treat as significant each occurrence of @var{char} in the input file.
5737 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5738 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5739 the whole line is considered, matching the default operation of sort.
5740 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5741 character is used to delimit the fields.
5743 @item -v @var{file-number}
5744 Print a line for each unpairable line in file @var{file-number}
5745 (either @samp{1} or @samp{2}), instead of the normal output.
5752 @node Operating on characters
5753 @chapter Operating on characters
5755 @cindex operating on characters
5757 This commands operate on individual characters.
5760 * tr invocation:: Translate, squeeze, and/or delete characters.
5761 * expand invocation:: Convert tabs to spaces.
5762 * unexpand invocation:: Convert spaces to tabs.
5767 @section @command{tr}: Translate, squeeze, and/or delete characters
5774 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5777 @command{tr} copies standard input to standard output, performing
5778 one of the following operations:
5782 translate, and optionally squeeze repeated characters in the result,
5784 squeeze repeated characters,
5788 delete characters, then squeeze repeated characters from the result.
5791 The @var{set1} and (if given) @var{set2} arguments define ordered
5792 sets of characters, referred to below as @var{set1} and @var{set2}. These
5793 sets are the characters of the input that @command{tr} operates on.
5794 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5796 complement (all of the characters that are not in @var{set1}).
5798 Currently @command{tr} fully supports only single-byte characters.
5799 Eventually it will support multibyte characters; when it does, the
5800 @option{-C} option will cause it to complement the set of characters,
5801 whereas @option{-c} will cause it to complement the set of values.
5802 This distinction will matter only when some values are not characters,
5803 and this is possible only in locales using multibyte encodings when
5804 the input contains encoding errors.
5806 The program accepts the @option{--help} and @option{--version}
5807 options. @xref{Common options}. Options must precede operands.
5812 * Character sets:: Specifying sets of characters.
5813 * Translating:: Changing one set of characters to another.
5814 * Squeezing:: Squeezing repeats and deleting.
5818 @node Character sets
5819 @subsection Specifying sets of characters
5821 @cindex specifying sets of characters
5823 The format of the @var{set1} and @var{set2} arguments resembles
5824 the format of regular expressions; however, they are not regular
5825 expressions, only lists of characters. Most characters simply
5826 represent themselves in these strings, but the strings can contain
5827 the shorthands listed below, for convenience. Some of them can be
5828 used only in @var{set1} or @var{set2}, as noted below.
5832 @item Backslash escapes
5833 @cindex backslash escapes
5835 The following backslash escape sequences are recognized:
5853 The character with the value given by @var{ooo}, which is 1 to 3
5859 While a backslash followed by a character not listed above is
5860 interpreted as that character, the backslash also effectively
5861 removes any special significance, so it is useful to escape
5862 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5867 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5868 from @var{m} through @var{n}, in ascending order. @var{m} should
5869 collate before @var{n}; if it doesn't, an error results. As an example,
5870 @samp{0-9} is the same as @samp{0123456789}.
5872 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5873 brackets to enclose ranges. Translations specified in that format
5874 sometimes work as expected, since the brackets are often transliterated
5875 to themselves. However, they should be avoided because they sometimes
5876 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5879 Many historically common and even accepted uses of ranges are not
5880 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5881 range will not do what most would expect because @samp{A} through @samp{Z}
5882 are not contiguous as they are in @acronym{ASCII}.
5883 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5884 the best way to work around this is to use character classes (see below).
5885 Otherwise, it is most portable (and most ugly) to enumerate the members
5888 @item Repeated characters
5889 @cindex repeated characters
5891 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5892 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5893 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5894 to as many copies of @var{c} as are needed to make @var{set2} as long as
5895 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5896 octal, otherwise in decimal.
5898 @item Character classes
5899 @cindex character classes
5901 The notation @samp{[:@var{class}:]} expands to all of the characters in
5902 the (predefined) class @var{class}. The characters expand in no
5903 particular order, except for the @code{upper} and @code{lower} classes,
5904 which expand in ascending order. When the @option{--delete} (@option{-d})
5905 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5906 character class can be used in @var{set2}. Otherwise, only the
5907 character classes @code{lower} and @code{upper} are accepted in
5908 @var{set2}, and then only if the corresponding character class
5909 (@code{upper} and @code{lower}, respectively) is specified in the same
5910 relative position in @var{set1}. Doing this specifies case conversion.
5911 The class names are given below; an error results when an invalid class
5923 Horizontal whitespace.
5932 Printable characters, not including space.
5938 Printable characters, including space.
5941 Punctuation characters.
5944 Horizontal or vertical whitespace.
5953 @item Equivalence classes
5954 @cindex equivalence classes
5956 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5957 equivalent to @var{c}, in no particular order. Equivalence classes are
5958 a relatively recent invention intended to support non-English alphabets.
5959 But there seems to be no standard way to define them or determine their
5960 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5961 each character's equivalence class consists only of that character,
5962 which is of no particular use.
5968 @subsection Translating
5970 @cindex translating characters
5972 @command{tr} performs translation when @var{set1} and @var{set2} are
5973 both given and the @option{--delete} (@option{-d}) option is not given.
5974 @command{tr} translates each character of its input that is in @var{set1}
5975 to the corresponding character in @var{set2}. Characters not in
5976 @var{set1} are passed through unchanged. When a character appears more
5977 than once in @var{set1} and the corresponding characters in @var{set2}
5978 are not all the same, only the final one is used. For example, these
5979 two commands are equivalent:
5986 A common use of @command{tr} is to convert lowercase characters to
5987 uppercase. This can be done in many ways. Here are three of them:
5990 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5992 tr '[:lower:]' '[:upper:]'
5996 But note that using ranges like @code{a-z} above is not portable.
5998 When @command{tr} is performing translation, @var{set1} and @var{set2}
5999 typically have the same length. If @var{set1} is shorter than
6000 @var{set2}, the extra characters at the end of @var{set2} are ignored.
6002 On the other hand, making @var{set1} longer than @var{set2} is not
6003 portable; @acronym{POSIX} says that the result is undefined. In this situation,
6004 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
6005 the last character of @var{set2} as many times as necessary. System V
6006 @command{tr} truncates @var{set1} to the length of @var{set2}.
6008 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6009 When the @option{--truncate-set1} (@option{-t}) option is given,
6010 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6011 instead. This option is ignored for operations other than translation.
6013 Acting like System V @command{tr} in this case breaks the relatively common
6017 tr -cs A-Za-z0-9 '\012'
6021 because it converts only zero bytes (the first element in the
6022 complement of @var{set1}), rather than all non-alphanumerics, to
6026 By the way, the above idiom is not portable because it uses ranges, and
6027 it assumes that the octal code for newline is 012.
6028 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better
6032 tr -cs '[:alnum:]' '[\n*]'
6037 @subsection Squeezing repeats and deleting
6039 @cindex squeezing repeat characters
6040 @cindex deleting characters
6042 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6043 removes any input characters that are in @var{set1}.
6045 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6046 @command{tr} replaces each input sequence of a repeated character that
6047 is in @var{set1} with a single occurrence of that character.
6049 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6050 first performs any deletions using @var{set1}, then squeezes repeats
6051 from any remaining characters using @var{set2}.
6053 The @option{--squeeze-repeats} option may also be used when translating,
6054 in which case @command{tr} first performs translation, then squeezes
6055 repeats from any remaining characters using @var{set2}.
6057 Here are some examples to illustrate various combinations of options:
6062 Remove all zero bytes:
6069 Put all words on lines by themselves. This converts all
6070 non-alphanumeric characters to newlines, then squeezes each string
6071 of repeated newlines into a single newline:
6074 tr -cs '[:alnum:]' '[\n*]'
6078 Convert each sequence of repeated newlines to a single newline:
6085 Find doubled occurrences of words in a document.
6086 @c Separate the following two "the"s, so typo checkers don't complain.
6087 For example, people often write ``the @w{}the'' with the repeated words
6088 separated by a newline. The Bourne shell script below works first
6089 by converting each sequence of punctuation and blank characters to a
6090 single newline. That puts each ``word'' on a line by itself.
6091 Next it maps all uppercase characters to lower case, and finally it
6092 runs @command{uniq} with the @option{-d} option to print out only the words
6098 | tr -s '[:punct:][:blank:]' '[\n*]' \
6099 | tr '[:upper:]' '[:lower:]' \
6104 Deleting a small set of characters is usually straightforward. For example,
6105 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6111 However, when @samp{-} is one of those characters, it can be tricky because
6112 @samp{-} has special meanings. Performing the same task as above but also
6113 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6114 that would fail because @command{tr} would try to interpret @option{-a} as
6115 a command-line option. Alternatively, we could try putting the hyphen
6116 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6117 it would make @command{tr} interpret @code{a-x} as the range of characters
6118 @samp{a}@dots{}@samp{x} rather than the three.
6119 One way to solve the problem is to put the hyphen at the end of the list
6126 Or you can use @samp{--} to terminate option processing:
6132 More generally, use the character class notation @code{[=c=]}
6133 with @samp{-} (or any other character) in place of the @samp{c}:
6139 Note how single quotes are used in the above example to protect the
6140 square brackets from interpretation by a shell.
6145 @node expand invocation
6146 @section @command{expand}: Convert tabs to spaces
6149 @cindex tabs to spaces, converting
6150 @cindex converting tabs to spaces
6152 @command{expand} writes the contents of each given @var{file}, or standard
6153 input if none are given or for a @var{file} of @samp{-}, to standard
6154 output, with tab characters converted to the appropriate number of
6158 expand [@var{option}]@dots{} [@var{file}]@dots{}
6161 By default, @command{expand} converts all tabs to spaces. It preserves
6162 backspace characters in the output; they decrement the column count for
6163 tab calculations. The default action is equivalent to @option{-t 8} (set
6164 tabs every 8 columns).
6166 The program accepts the following options. Also see @ref{Common options}.
6170 @item -t @var{tab1}[,@var{tab2}]@dots{}
6171 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6174 @cindex tab stops, setting
6175 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6176 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6177 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6178 last tab stop given with single spaces. Tab stops can be separated by
6179 blanks as well as by commas.
6181 For compatibility, GNU @command{expand} also accepts the obsolete
6182 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6183 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6189 @cindex initial tabs, converting
6190 Only convert initial tabs (those that precede all non-space or non-tab
6191 characters) on each line to spaces.
6198 @node unexpand invocation
6199 @section @command{unexpand}: Convert spaces to tabs
6203 @command{unexpand} writes the contents of each given @var{file}, or
6204 standard input if none are given or for a @var{file} of @samp{-}, to
6205 standard output, converting blanks at the beginning of each line into
6206 as many tab characters as needed. In the default @acronym{POSIX}
6207 locale, a @dfn{blank} is a space or a tab; other locales may specify
6208 additional blank characters. Synopsis:
6211 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6214 By default, @command{unexpand} converts only initial blanks (those
6215 that precede all non-blank characters) on each line. It
6216 preserves backspace characters in the output; they decrement the column
6217 count for tab calculations. By default, tabs are set at every 8th
6220 The program accepts the following options. Also see @ref{Common options}.
6224 @item -t @var{tab1}[,@var{tab2}]@dots{}
6225 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6228 If only one tab stop is given, set the tabs @var{tab1} columns apart
6229 instead of the default 8. Otherwise, set the tabs at columns
6230 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6231 beyond the tab stops given unchanged. Tab stops can be separated by
6232 blanks as well as by commas. This option implies the @option{-a} option.
6234 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6235 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6236 separated by commas. (Unlike @option{-t}, this obsolete option does
6237 not imply @option{-a}.) New scripts should use @option{--first-only -t
6238 @var{tab1}[,@var{tab2}]@dots{}} instead.
6244 Also convert all sequences of two or more blanks just before a tab stop,
6245 even if they occur after non-blank characters in a line.
6252 @node Directory listing
6253 @chapter Directory listing
6255 This chapter describes the @command{ls} command and its variants @command{dir}
6256 and @command{vdir}, which list information about files.
6259 * ls invocation:: List directory contents.
6260 * dir invocation:: Briefly ls.
6261 * vdir invocation:: Verbosely ls.
6262 * dircolors invocation:: Color setup for ls, etc.
6267 @section @command{ls}: List directory contents
6270 @cindex directory listing
6272 The @command{ls} program lists information about files (of any type,
6273 including directories). Options and file arguments can be intermixed
6274 arbitrarily, as usual.
6276 For non-option command-line arguments that are directories, by default
6277 @command{ls} lists the contents of directories, not recursively, and
6278 omitting files with names beginning with @samp{.}. For other non-option
6279 arguments, by default @command{ls} lists just the file name. If no
6280 non-option argument is specified, @command{ls} operates on the current
6281 directory, acting as if it had been invoked with a single argument of @samp{.}.
6284 By default, the output is sorted alphabetically, according to the locale
6285 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6286 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6287 produce output that is sorted differently than you're accustomed to.
6288 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6289 If standard output is
6290 a terminal, the output is in columns (sorted vertically) and control
6291 characters are output as question marks; otherwise, the output is listed
6292 one per line and control characters are output as-is.
6294 Because @command{ls} is such a fundamental program, it has accumulated many
6295 options over the years. They are described in the subsections below;
6296 within each section, options are listed alphabetically (ignoring case).
6297 The division of options into the subsections is not absolute, since some
6298 options affect more than one aspect of @command{ls}'s operation.
6300 @cindex exit status of @command{ls}
6305 1 minor problems (e.g., failure to access a file or directory not
6306 specified as a command line argument. This happens when listing a
6307 directory in which entries are actively being removed or renamed.)
6308 2 serious trouble (e.g., memory exhausted, invalid option, failure
6309 to access a file or directory specified as a command line argument
6310 or a directory loop)
6313 Also see @ref{Common options}.
6316 * Which files are listed::
6317 * What information is listed::
6318 * Sorting the output::
6319 * Details about version sort::
6320 * General output formatting::
6321 * Formatting file timestamps::
6322 * Formatting the file names::
6326 @node Which files are listed
6327 @subsection Which files are listed
6329 These options determine which files @command{ls} lists information for.
6330 By default, @command{ls} lists files and the contents of any
6331 directories on the command line, except that in directories it ignores
6332 files whose names start with @samp{.}.
6340 In directories, do not ignore file names that start with @samp{.}.
6345 @opindex --almost-all
6346 In directories, do not ignore all file names that start with @samp{.};
6347 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6348 option overrides this option.
6351 @itemx --ignore-backups
6353 @opindex --ignore-backups
6354 @cindex backup files, ignoring
6355 In directories, ignore files that end with @samp{~}. This option is
6356 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6361 @opindex --directory
6362 List just the names of directories, as with other types of files, rather
6363 than listing their contents.
6364 @c The following sentence is the same as the one for -F.
6365 Do not follow symbolic links listed on the
6366 command line unless the @option{--dereference-command-line} (@option{-H}),
6367 @option{--dereference} (@option{-L}), or
6368 @option{--dereference-command-line-symlink-to-dir} options are specified.
6371 @itemx --dereference-command-line
6373 @opindex --dereference-command-line
6374 @cindex symbolic links, dereferencing
6375 If a command line argument specifies a symbolic link, show information
6376 for the file the link references rather than for the link itself.
6378 @itemx --dereference-command-line-symlink-to-dir
6379 @opindex --dereference-command-line-symlink-to-dir
6380 @cindex symbolic links, dereferencing
6381 Do not dereference symbolic links, with one exception:
6382 if a command line argument specifies a symbolic link that refers to
6383 a directory, show information for that directory rather than for the
6385 This is the default behavior when no other dereferencing-related
6386 option has been specified (@option{--classify} (@option{-F}),
6387 @option{--directory} (@option{-d}),
6389 @option{--dereference} (@option{-L}), or
6390 @option{--dereference-command-line} (@option{-H})).
6392 @item --group-directories-first
6393 @opindex --group-directories-first
6394 Group all the directories before the files and then sort the
6395 directories and the files separately using the selected sort key
6396 (see --sort option).
6397 That is, this option specifies a primary sort key,
6398 and the --sort option specifies a secondary key.
6399 However, any use of @option{--sort=none}
6400 (@option{-U}) disables this option altogether.
6402 @item --hide=PATTERN
6403 @opindex --hide=@var{pattern}
6404 In directories, ignore files whose names match the shell pattern
6405 @var{pattern}, unless the @option{--all} (@option{-a}) or
6406 @option{--almost-all} (@option{-A}) is also given. This
6407 option acts like @option{--ignore=@var{pattern}} except that it has no
6408 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6409 (@option{-A}) is also given.
6411 This option can be useful in shell aliases. For example, if
6412 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6413 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6414 lists the file @file{README~} even though @samp{ly -A} would not.
6416 @item -I @var{pattern}
6417 @itemx --ignore=@var{pattern}
6419 @opindex --ignore=@var{pattern}
6420 In directories, ignore files whose names match the shell pattern
6421 (not regular expression) @var{pattern}. As
6422 in the shell, an initial @samp{.} in a file name does not match a
6423 wildcard at the start of @var{pattern}. Sometimes it is useful
6424 to give this option several times. For example,
6427 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6430 The first option ignores names of length 3 or more that start with @samp{.},
6431 the second ignores all two-character names that start with @samp{.}
6432 except @samp{..}, and the third ignores names that start with @samp{#}.
6435 @itemx --dereference
6437 @opindex --dereference
6438 @cindex symbolic links, dereferencing
6439 When showing file information for a symbolic link, show information
6440 for the file the link references rather than the link itself.
6441 However, even with this option, @command{ls} still prints the name
6442 of the link itself, not the name of the file that the link points to.
6447 @opindex --recursive
6448 @cindex recursive directory listing
6449 @cindex directory listing, recursive
6450 List the contents of all directories recursively.
6455 @node What information is listed
6456 @subsection What information is listed
6458 These options affect the information that @command{ls} displays. By
6459 default, only file names are shown.
6465 @cindex hurd, author, printing
6466 List each file's author when producing long format directory listings.
6467 In GNU/Hurd, file authors can differ from their owners, but in other
6468 operating systems the two are the same.
6474 @cindex dired Emacs mode support
6475 With the long listing (@option{-l}) format, print an additional line after
6479 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6483 The @var{begn} and @var{endn} are unsigned integers that record the
6484 byte position of the beginning and end of each file name in the output.
6485 This makes it easy for Emacs to find the names, even when they contain
6486 unusual characters such as space or newline, without fancy searching.
6488 If directories are being listed recursively (@option{-R}), output a similar
6489 line with offsets for each subdirectory name:
6492 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6495 Finally, output a line of the form:
6498 //DIRED-OPTIONS// --quoting-style=@var{word}
6502 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6504 Here is an actual example:
6507 $ mkdir -p a/sub/deeper a/sub2
6509 $ touch a/sub/deeper/file
6510 $ ls -gloRF --dired a
6513 -rw-r--r-- 1 0 Jun 10 12:27 f1
6514 -rw-r--r-- 1 0 Jun 10 12:27 f2
6515 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6516 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6520 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6524 -rw-r--r-- 1 0 Jun 10 12:27 file
6528 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6529 //SUBDIRED// 2 3 167 172 228 240 290 296
6530 //DIRED-OPTIONS// --quoting-style=literal
6533 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6534 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6536 The offsets on the @samp{//SUBDIRED//} line delimit the following
6537 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6539 Here is an example of how to extract the fifth entry name, @samp{deeper},
6540 corresponding to the pair of offsets, 222 and 228:
6543 $ ls -gloRF --dired a > out
6544 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6548 Note that although the listing above includes a trailing slash
6549 for the @samp{deeper} entry, the offsets select the name without
6550 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6551 along with an option like @option{--escape} (aka @option{-b}) and operate
6552 on a file whose name contains special characters, notice that the backslash
6557 $ ls -blog --dired 'a b'
6558 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6560 //DIRED-OPTIONS// --quoting-style=escape
6563 If you use a quoting style that adds quote marks
6564 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6565 So beware that the user may select the quoting style via the environment
6566 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6567 should either specify an explicit @option{--quoting-style=literal} option
6568 (aka @option{-N} or @option{--literal}) on the command line, or else be
6569 prepared to parse the escaped names.
6572 @opindex --full-time
6573 Produce long format directory listings, and list times in full. It is
6574 equivalent to using @option{--format=long} with
6575 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6579 Produce long format directory listings, but don't display owner information.
6585 Inhibit display of group information in a long format directory listing.
6586 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6587 provide this option for compatibility.)
6595 @cindex inode number, printing
6596 Print the inode number (also called the file serial number and index
6597 number) of each file to the left of the file name. (This number
6598 uniquely identifies each file within a particular file system.)
6601 @itemx --format=long
6602 @itemx --format=verbose
6605 @opindex long ls @r{format}
6606 @opindex verbose ls @r{format}
6607 In addition to the name of each file, print the file type, file mode bits,
6608 number of hard links, owner name, group name, size, and
6609 timestamp (@pxref{Formatting file timestamps}), normally
6610 the modification time. Print question marks for information that
6611 cannot be determined.
6613 Normally the size is printed as a byte count without punctuation, but
6614 this can be overridden (@pxref{Block size}). For example, @option{-h}
6615 prints an abbreviated, human-readable count, and
6616 @samp{--block-size="'1"} prints a byte count with the thousands
6617 separator of the current locale.
6619 For each directory that is listed, preface the files with a line
6620 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6621 for all files in that directory. The block size currently defaults to 1024
6622 bytes, but this can be overridden (@pxref{Block size}).
6623 The @var{blocks} computed counts each hard link separately;
6624 this is arguably a deficiency.
6626 The file type is one of the following characters:
6628 @c The commented-out entries are ones we're not sure about.
6636 character special file
6638 high performance (``contiguous data'') file
6642 door (Solaris 2.5 and up)
6644 @c semaphore, if this is a distinct file type
6648 @c multiplexed file (7th edition Unix; obsolete)
6650 off-line (``migrated'') file (Cray DMF)
6652 network special file (HP-UX)
6656 port (Solaris 10 and up)
6658 @c message queue, if this is a distinct file type
6662 @c shared memory object, if this is a distinct file type
6664 @c typed memory object, if this is a distinct file type
6666 @c whiteout (4.4BSD; not implemented)
6668 some other file type
6671 @cindex permissions, output by @command{ls}
6672 The file mode bits listed are similar to symbolic mode specifications
6673 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6674 third character of each set of permissions as follows:
6678 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6682 If the set-user-ID or set-group-ID bit is set but the corresponding
6683 executable bit is not set.
6686 If the restricted deletion flag or sticky bit, and the
6687 other-executable bit, are both set. The restricted deletion flag is
6688 another name for the sticky bit. @xref{Mode Structure}.
6691 If the restricted deletion flag or sticky bit is set but the
6692 other-executable bit is not set.
6695 If the executable bit is set and none of the above apply.
6701 Following the file mode bits is a single character that specifies
6702 whether an alternate access method such as an access control list
6703 applies to the file. When the character following the file mode bits is a
6704 space, there is no alternate access method. When it is a printing
6705 character, then there is such a method.
6707 GNU @command{ls} uses a @samp{.} character to indicate a file
6708 with an SELinux security context, but no other alternate access method.
6710 A file with any other combination of alternate access methods
6711 is marked with a @samp{+} character.
6714 @itemx --numeric-uid-gid
6716 @opindex --numeric-uid-gid
6717 @cindex numeric uid and gid
6718 @cindex numeric user and group IDs
6719 Produce long format directory listings, but
6720 display numeric user and group IDs instead of the owner and group names.
6724 Produce long format directory listings, but don't display group information.
6725 It is equivalent to using @option{--format=long} with @option{--no-group} .
6731 @cindex disk allocation
6732 @cindex size of files, reporting
6733 Print the disk allocation of each file to the left of the file name.
6734 This is the amount of disk space used by the file, which is usually a
6735 bit more than the file's size, but it can be less if the file has holes.
6737 Normally the disk allocation is printed in units of
6738 1024 bytes, but this can be overridden (@pxref{Block size}).
6740 @cindex NFS mounts from BSD to HP-UX
6741 For files that are NFS-mounted from an HP-UX system to a BSD system,
6742 this option reports sizes that are half the correct values. On HP-UX
6743 systems, it reports sizes that are twice the correct values for files
6744 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6745 it also affects the HP-UX @command{ls} program.
6754 @cindex security context
6755 Display the SELinux security context or @samp{?} if none is found.
6756 When used with the @option{-l} option, print the security context
6757 to the left of the size column.
6762 @node Sorting the output
6763 @subsection Sorting the output
6765 @cindex sorting @command{ls} output
6766 These options change the order in which @command{ls} sorts the information
6767 it outputs. By default, sorting is done by character code
6768 (e.g., @acronym{ASCII} order).
6774 @itemx --time=status
6777 @opindex ctime@r{, printing or sorting by}
6778 @opindex status time@r{, printing or sorting by}
6779 @opindex use time@r{, printing or sorting files by}
6780 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6781 print the status change time (the @samp{ctime} in the inode) instead of
6782 the modification time.
6783 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6784 or when not using a long listing format,
6785 sort according to the status change time.
6789 @cindex unsorted directory listing
6790 @cindex directory order, listing by
6791 Primarily, like @option{-U}---do not sort; list the files in whatever
6792 order they are stored in the directory. But also enable @option{-a} (list
6793 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6794 were specified before the @option{-f}).
6800 @cindex reverse sorting
6801 Reverse whatever the sorting method is---e.g., list files in reverse
6802 alphabetical order, youngest first, smallest first, or whatever.
6808 @opindex size of files@r{, sorting files by}
6809 Sort by file size, largest first.
6815 @opindex modification time@r{, sorting files by}
6816 Sort by modification time (the @samp{mtime} in the inode), newest first.
6820 @itemx --time=access
6824 @opindex use time@r{, printing or sorting files by}
6825 @opindex atime@r{, printing or sorting files by}
6826 @opindex access time@r{, printing or sorting files by}
6827 If the long listing format (e.g., @option{--format=long}) is being used,
6828 print the last access time (the @samp{atime} in the inode).
6829 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6830 or when not using a long listing format, sort according to the access time.
6836 @opindex none@r{, sorting option for @command{ls}}
6837 Do not sort; list the files in whatever order they are
6838 stored in the directory. (Do not do any of the other unrelated things
6839 that @option{-f} does.) This is especially useful when listing very large
6840 directories, since not doing any sorting can be noticeably faster.
6843 @itemx --sort=version
6846 @opindex version@r{, sorting option for @command{ls}}
6847 Sort by version name and number, lowest first. It behaves like a default
6848 sort, except that each sequence of decimal digits is treated numerically
6849 as an index/version number. (@xref{Details about version sort}.)
6852 @itemx --sort=extension
6855 @opindex extension@r{, sorting files by}
6856 Sort directory contents alphabetically by file extension (characters
6857 after the last @samp{.}); files with no extension are sorted first.
6862 @node Details about version sort
6863 @subsection Details about version sort
6865 Version sorting handles the fact that file names frequently include indices or
6866 version numbers. Standard sorting usually does not produce the order that one
6867 expects because comparisons are made on a character-by-character basis.
6868 Version sorting is especially useful when browsing directories that contain
6869 many files with indices/version numbers in their names:
6873 abc.zml-1.gz abc.zml-1.gz
6874 abc.zml-12.gz abc.zml-2.gz
6875 abc.zml-2.gz abc.zml-12.gz
6878 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6879 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6880 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6881 @var{ver1} < @var{ver2} implies that the name composed of
6882 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6883 ``@var{prefix} @var{ver2} @var{suffix}''.
6885 Note also that leading zeros of numeric parts are ignored:
6889 abc-1.007.tgz abc-1.01a.tgz
6890 abc-1.012b.tgz abc-1.007.tgz
6891 abc-1.01a.tgz abc-1.012b.tgz
6894 This functionality is implemented using gnulib's @code{filevercmp} function,
6895 which has some caveats worth noting.
6898 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6899 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6900 was set to @samp{C}.
6901 @item Some suffixes will not be matched by the regular
6902 expression mentioned above. Consequently these examples may
6903 not sort as you expect:
6911 abc-1.2.3.4.x86_64.rpm
6912 abc-1.2.3.x86_64.rpm
6916 @node General output formatting
6917 @subsection General output formatting
6919 These options affect the appearance of the overall output.
6924 @itemx --format=single-column
6927 @opindex single-column @r{output of files}
6928 List one file per line. This is the default for @command{ls} when standard
6929 output is not a terminal.
6932 @itemx --format=vertical
6935 @opindex vertical @r{sorted files in columns}
6936 List files in columns, sorted vertically. This is the default for
6937 @command{ls} if standard output is a terminal. It is always the default
6938 for the @command{dir} program.
6939 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6940 possible in the fewest lines.
6942 @item --color [=@var{when}]
6944 @cindex color, distinguishing file types with
6945 Specify whether to use color for distinguishing file types. @var{when}
6946 may be omitted, or one of:
6949 @vindex none @r{color option}
6950 - Do not use color at all. This is the default.
6952 @vindex auto @r{color option}
6953 @cindex terminal, using color iff
6954 - Only use color if standard output is a terminal.
6956 @vindex always @r{color option}
6959 Specifying @option{--color} and no @var{when} is equivalent to
6960 @option{--color=always}.
6961 Piping a colorized listing through a pager like @command{more} or
6962 @command{less} usually produces unreadable results. However, using
6963 @code{more -f} does seem to work.
6967 @itemx --indicator-style=classify
6970 @opindex --indicator-style
6971 @cindex file type and executables, marking
6972 @cindex executables and file type, marking
6973 Append a character to each file name indicating the file type. Also,
6974 for regular files that are executable, append @samp{*}. The file type
6975 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6976 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6977 and nothing for regular files.
6978 @c The following sentence is the same as the one for -d.
6979 Do not follow symbolic links listed on the
6980 command line unless the @option{--dereference-command-line} (@option{-H}),
6981 @option{--dereference} (@option{-L}), or
6982 @option{--dereference-command-line-symlink-to-dir} options are specified.
6985 @itemx --indicator-style=file-type
6986 @opindex --file-type
6987 @opindex --indicator-style
6988 @cindex file type, marking
6989 Append a character to each file name indicating the file type. This is
6990 like @option{-F}, except that executables are not marked.
6992 @item --indicator-style=@var{word}
6993 @opindex --indicator-style
6994 Append a character indicator with style @var{word} to entry names,
6999 Do not append any character indicator; this is the default.
7001 Append @samp{/} for directories. This is the same as the @option{-p}
7004 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
7005 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
7006 the same as the @option{--file-type} option.
7008 Append @samp{*} for executable regular files, otherwise behave as for
7009 @samp{file-type}. This is the same as the @option{-F} or
7010 @option{--classify} option.
7015 Print file sizes in 1024-byte blocks, overriding the default block
7016 size (@pxref{Block size}).
7017 This option is equivalent to @option{--block-size=1K}.
7020 @itemx --format=commas
7023 @opindex commas@r{, outputting between files}
7024 List files horizontally, with as many as will fit on each line,
7025 separated by @samp{, } (a comma and a space).
7028 @itemx --indicator-style=slash
7030 @opindex --indicator-style
7031 @cindex file type, marking
7032 Append a @samp{/} to directory names.
7035 @itemx --format=across
7036 @itemx --format=horizontal
7039 @opindex across@r{, listing files}
7040 @opindex horizontal@r{, listing files}
7041 List the files in columns, sorted horizontally.
7044 @itemx --tabsize=@var{cols}
7047 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7048 @command{ls} uses tabs where possible in the output, for efficiency. If
7049 @var{cols} is zero, do not use tabs at all.
7051 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7052 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7053 do not properly align columns to the right of a TAB following a
7054 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7055 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7056 @command{ls} to align using spaces, not tabs.
7059 @itemx --width=@var{cols}
7063 Assume the screen is @var{cols} columns wide. The default is taken
7064 from the terminal settings if possible; otherwise the environment
7065 variable @env{COLUMNS} is used if it is set; otherwise the default
7071 @node Formatting file timestamps
7072 @subsection Formatting file timestamps
7074 By default, file timestamps are listed in abbreviated form, using
7075 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7076 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7077 This format can change depending on the current locale as detailed below.
7079 A timestamp is considered to be @dfn{recent} if it is less than six
7080 months old, and is not dated in the future. If a timestamp dated
7081 today is not listed in recent form, the timestamp is in the future,
7082 which means you probably have clock skew problems which may break
7083 programs like @command{make} that rely on file timestamps.
7086 Time stamps are listed according to the time zone rules specified by
7087 the @env{TZ} environment variable, or by the system default rules if
7088 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7089 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7091 The following option changes how file timestamps are printed.
7094 @item --time-style=@var{style}
7095 @opindex --time-style
7097 List timestamps in style @var{style}. The @var{style} should
7098 be one of the following:
7103 List timestamps using @var{format}, where @var{format} is interpreted
7104 like the format argument of @command{date} (@pxref{date invocation}).
7105 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7106 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7107 with @command{date}, @var{format}'s interpretation is affected by the
7108 @env{LC_TIME} locale category.
7110 If @var{format} contains two format strings separated by a newline,
7111 the former is used for non-recent files and the latter for recent
7112 files; if you want output columns to line up, you may need to insert
7113 spaces in one of the two formats.
7116 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7117 format with nanosecond precision, e.g., @samp{2002-03-30
7118 23:45:56.477817180 -0700}. This style is equivalent to
7119 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7121 This is useful because the time output includes all the information that
7122 is available from the operating system. For example, this can help
7123 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7124 uses the full timestamp to determine whether a file is out of date.
7127 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7128 @samp{2002-03-30 23:45}. These timestamps are shorter than
7129 @samp{full-iso} timestamps, and are usually good enough for everyday
7130 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7133 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7134 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7135 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7136 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7137 nearly the same information in a smaller space and their brevity helps
7138 @command{ls} output fit within traditional 80-column output lines.
7139 The following two @command{ls} invocations are equivalent:
7144 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7145 ls -l --time-style="iso"
7150 List timestamps in a locale-dependent form. For example, a Finnish
7151 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7152 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7153 timestamps typically consume more space than @samp{iso} timestamps and
7154 are harder for programs to parse because locale conventions vary so
7155 widely, but they are easier for many people to read.
7157 The @env{LC_TIME} locale category specifies the timestamp format. The
7158 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7159 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7160 @command{ls} invocations are equivalent:
7165 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7166 ls -l --time-style="locale"
7169 Other locales behave differently. For example, in a German locale,
7170 @option{--time-style="locale"} might be equivalent to
7171 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7172 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7173 @samp{30. M@"ar 23:45}.
7175 @item posix-@var{style}
7177 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7178 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7179 example, the @samp{posix-long-iso} style lists
7180 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7181 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7186 You can specify the default value of the @option{--time-style} option
7187 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7188 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7189 later use the @option{--dired} option and therefore can parse any date
7190 format, but if you are using Emacs 21.1 or 21.2 and specify a
7191 non-@acronym{POSIX} locale you may need to set
7192 @samp{TIME_STYLE="posix-long-iso"}.
7194 To avoid certain denial-of-service attacks, timestamps that would be
7195 longer than 1000 bytes may be treated as errors.
7198 @node Formatting the file names
7199 @subsection Formatting the file names
7201 These options change how file names themselves are printed.
7207 @itemx --quoting-style=escape
7210 @opindex --quoting-style
7211 @cindex backslash sequences for file names
7212 Quote nongraphic characters in file names using alphabetic and octal
7213 backslash sequences like those used in C.
7217 @itemx --quoting-style=literal
7220 @opindex --quoting-style
7221 Do not quote file names. However, with @command{ls} nongraphic
7222 characters are still printed as question marks if the output is a
7223 terminal and you do not specify the @option{--show-control-chars}
7227 @itemx --hide-control-chars
7229 @opindex --hide-control-chars
7230 Print question marks instead of nongraphic characters in file names.
7231 This is the default if the output is a terminal and the program is
7236 @itemx --quoting-style=c
7238 @opindex --quote-name
7239 @opindex --quoting-style
7240 Enclose file names in double quotes and quote nongraphic characters as
7243 @item --quoting-style=@var{word}
7244 @opindex --quoting-style
7245 @cindex quoting style
7246 Use style @var{word} to quote file names and other strings that may
7247 contain arbitrary characters. The @var{word} should
7248 be one of the following:
7252 Output strings as-is; this is the same as the @option{-N} or
7253 @option{--literal} option.
7255 Quote strings for the shell if they contain shell metacharacters or would
7256 cause ambiguous output.
7257 The quoting is suitable for @acronym{POSIX}-compatible shells like
7258 @command{bash}, but it does not always work for incompatible shells
7261 Quote strings for the shell, even if they would normally not require quoting.
7263 Quote strings as for C character string literals, including the
7264 surrounding double-quote characters; this is the same as the
7265 @option{-Q} or @option{--quote-name} option.
7267 Quote strings as for C character string literals, except omit the
7268 surrounding double-quote
7269 characters; this is the same as the @option{-b} or @option{--escape} option.
7271 Quote strings as for C character string literals, except use
7272 surrounding quotation marks appropriate for the
7275 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7276 Quote strings as for C character string literals, except use
7277 surrounding quotation marks appropriate for the locale, and quote
7278 @t{`like this'} instead of @t{"like
7279 this"} in the default C locale. This looks nicer on many displays.
7282 You can specify the default value of the @option{--quoting-style} option
7283 with the environment variable @env{QUOTING_STYLE}. If that environment
7284 variable is not set, the default value is @samp{literal}, but this
7285 default may change to @samp{shell} in a future version of this package.
7287 @item --show-control-chars
7288 @opindex --show-control-chars
7289 Print nongraphic characters as-is in file names.
7290 This is the default unless the output is a terminal and the program is
7296 @node dir invocation
7297 @section @command{dir}: Briefly list directory contents
7300 @cindex directory listing, brief
7302 @command{dir} is equivalent to @code{ls -C
7303 -b}; that is, by default files are listed in columns, sorted vertically,
7304 and special characters are represented by backslash escape sequences.
7306 @xref{ls invocation, @command{ls}}.
7309 @node vdir invocation
7310 @section @command{vdir}: Verbosely list directory contents
7313 @cindex directory listing, verbose
7315 @command{vdir} is equivalent to @code{ls -l
7316 -b}; that is, by default files are listed in long format and special
7317 characters are represented by backslash escape sequences.
7319 @node dircolors invocation
7320 @section @command{dircolors}: Color setup for @command{ls}
7324 @cindex setup for color
7326 @command{dircolors} outputs a sequence of shell commands to set up the
7327 terminal for color output from @command{ls} (and @command{dir}, etc.).
7331 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7334 If @var{file} is specified, @command{dircolors} reads it to determine which
7335 colors to use for which file types and extensions. Otherwise, a
7336 precompiled database is used. For details on the format of these files,
7337 run @samp{dircolors --print-database}.
7339 To make @command{dircolors} read a @file{~/.dircolors} file if it
7340 exists, you can put the following lines in your @file{~/.bashrc} (or
7341 adapt them to your favorite shell):
7345 test -r $d && eval "$(dircolors $d)"
7349 @vindex SHELL @r{environment variable, and color}
7350 The output is a shell command to set the @env{LS_COLORS} environment
7351 variable. You can specify the shell syntax to use on the command line,
7352 or @command{dircolors} will guess it from the value of the @env{SHELL}
7353 environment variable.
7355 The program accepts the following options. Also see @ref{Common options}.
7360 @itemx --bourne-shell
7363 @opindex --bourne-shell
7364 @cindex Bourne shell syntax for color setup
7365 @cindex @command{sh} syntax for color setup
7366 Output Bourne shell commands. This is the default if the @env{SHELL}
7367 environment variable is set and does not end with @samp{csh} or
7376 @cindex C shell syntax for color setup
7377 @cindex @command{csh} syntax for color setup
7378 Output C shell commands. This is the default if @code{SHELL} ends with
7379 @command{csh} or @command{tcsh}.
7382 @itemx --print-database
7384 @opindex --print-database
7385 @cindex color database, printing
7386 @cindex database for color setup, printing
7387 @cindex printing color database
7388 Print the (compiled-in) default color configuration database. This
7389 output is itself a valid configuration file, and is fairly descriptive
7390 of the possibilities.
7397 @node Basic operations
7398 @chapter Basic operations
7400 @cindex manipulating files
7402 This chapter describes the commands for basic file manipulation:
7403 copying, moving (renaming), and deleting (removing).
7406 * cp invocation:: Copy files.
7407 * dd invocation:: Convert and copy a file.
7408 * install invocation:: Copy files and set attributes.
7409 * mv invocation:: Move (rename) files.
7410 * rm invocation:: Remove files or directories.
7411 * shred invocation:: Remove files more securely.
7416 @section @command{cp}: Copy files and directories
7419 @cindex copying files and directories
7420 @cindex files, copying
7421 @cindex directories, copying
7423 @command{cp} copies files (or, optionally, directories). The copy is
7424 completely independent of the original. You can either copy one file to
7425 another, or copy arbitrarily many files to a destination directory.
7429 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7430 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7431 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7436 If two file names are given, @command{cp} copies the first file to the
7440 If the @option{--target-directory} (@option{-t}) option is given, or
7441 failing that if the last file is a directory and the
7442 @option{--no-target-directory} (@option{-T}) option is not given,
7443 @command{cp} copies each @var{source} file to the specified directory,
7444 using the @var{source}s' names.
7447 Generally, files are written just as they are read. For exceptions,
7448 see the @option{--sparse} option below.
7450 By default, @command{cp} does not copy directories. However, the
7451 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7452 copy recursively by descending into source directories and copying files
7453 to corresponding destination directories.
7455 When copying from a symbolic link, @command{cp} normally follows the
7456 link only when not copying
7457 recursively. This default can be overridden with the
7458 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7459 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7460 @option{-H} options. If more than one of these options is specified,
7461 the last one silently overrides the others.
7463 When copying to a symbolic link, @command{cp} follows the
7464 link only when it refers to an existing regular file.
7465 However, when copying to a dangling symbolic link, @command{cp}
7466 refuses by default, and fails with a diagnostic, since the operation
7467 is inherently dangerous. This behavior is contrary to historical
7468 practice and to @acronym{POSIX}.
7469 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7470 the target of a dangling destination symlink, in spite of the possible risk.
7471 Also, when an option like
7472 @option{--backup} or @option{--link} acts to rename or remove the
7473 destination before copying, @command{cp} renames or removes the
7474 symbolic link rather than the file it points to.
7476 By default, @command{cp} copies the contents of special files only
7477 when not copying recursively. This default can be overridden with the
7478 @option{--copy-contents} option.
7480 @cindex self-backups
7481 @cindex backups, making only
7482 @command{cp} generally refuses to copy a file onto itself, with the
7483 following exception: if @option{--force --backup} is specified with
7484 @var{source} and @var{dest} identical, and referring to a regular file,
7485 @command{cp} will make a backup file, either regular or numbered, as
7486 specified in the usual ways (@pxref{Backup options}). This is useful when
7487 you simply want to make a backup of an existing file before changing it.
7489 The program accepts the following options. Also see @ref{Common options}.
7496 Preserve as much as possible of the structure and attributes of the
7497 original files in the copy (but do not attempt to preserve internal
7498 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7499 directory in a different order).
7500 Try to preserve SELinux security context and extended attributes (xattr),
7501 but ignore any failure to do that and print no corresponding diagnostic.
7502 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7504 @itemx --attributes-only
7505 @opindex --attributes-only
7506 Preserve the specified attributes of the original files in the copy,
7507 but do not copy any data. See the @option{--preserve} option for
7508 controlling which attributes to copy.
7511 @itemx @w{@kbd{--backup}[=@var{method}]}
7514 @vindex VERSION_CONTROL
7515 @cindex backups, making
7516 @xref{Backup options}.
7517 Make a backup of each file that would otherwise be overwritten or removed.
7518 As a special case, @command{cp} makes a backup of @var{source} when the force
7519 and backup options are given and @var{source} and @var{dest} are the same
7520 name for an existing, regular file. One useful application of this
7521 combination of options is this tiny Bourne shell script:
7525 # Usage: backup FILE...
7526 # Create a @sc{gnu}-style backup of each listed FILE.
7528 cp --backup --force -- "$i" "$i"
7532 @item --copy-contents
7533 @cindex directories, copying recursively
7534 @cindex copying directories recursively
7535 @cindex recursively copying directories
7536 @cindex non-directories, copying as special files
7537 If copying recursively, copy the contents of any special files (e.g.,
7538 FIFOs and device files) as if they were regular files. This means
7539 trying to read the data in each source file and writing it to the
7540 destination. It is usually a mistake to use this option, as it
7541 normally has undesirable effects on special files like FIFOs and the
7542 ones typically found in the @file{/dev} directory. In most cases,
7543 @code{cp -R --copy-contents} will hang indefinitely trying to read
7544 from FIFOs and special files like @file{/dev/console}, and it will
7545 fill up your destination disk if you use it to copy @file{/dev/zero}.
7546 This option has no effect unless copying recursively, and it does not
7547 affect the copying of symbolic links.
7551 @cindex symbolic links, copying
7552 @cindex hard links, preserving
7553 Copy symbolic links as symbolic links rather than copying the files that
7554 they point to, and preserve hard links between source files in the copies.
7555 Equivalent to @option{--no-dereference --preserve=links}.
7561 When copying without this option and an existing destination file cannot
7562 be opened for writing, the copy fails. However, with @option{--force}),
7563 when a destination file cannot be opened, @command{cp} then removes it and
7564 tries to open it again. Contrast this behavior with that enabled by
7565 @option{--link} and @option{--symbolic-link}, whereby the destination file
7566 is never opened but rather is removed unconditionally. Also see the
7567 description of @option{--remove-destination}.
7569 This option is independent of the @option{--interactive} or
7570 @option{-i} option: neither cancels the effect of the other.
7572 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7577 If a command line argument specifies a symbolic link, then copy the
7578 file it points to rather than the symbolic link itself. However,
7579 copy (preserving its nature) any symbolic link that is encountered
7580 via recursive traversal.
7583 @itemx --interactive
7585 @opindex --interactive
7586 When copying a file other than a directory, prompt whether to
7587 overwrite an existing destination file. The @option{-i} option overrides
7588 a previous @option{-n} option.
7594 Make hard links instead of copies of non-directories.
7597 @itemx --dereference
7599 @opindex --dereference
7600 Follow symbolic links when copying from them.
7601 With this option, @command{cp} cannot create a symbolic link.
7602 For example, a symlink (to regular file) in the source tree will be copied to
7603 a regular file in the destination tree.
7608 @opindex --no-clobber
7609 Do not overwrite an existing file. The @option{-n} option overrides a previous
7610 @option{-i} option. This option is mutually exclusive with @option{-b} or
7611 @option{--backup} option.
7614 @itemx --no-dereference
7616 @opindex --no-dereference
7617 @cindex symbolic links, copying
7618 Copy symbolic links as symbolic links rather than copying the files that
7619 they point to. This option affects only symbolic links in the source;
7620 symbolic links in the destination are always followed if possible.
7623 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7626 @cindex file information, preserving, extended attributes, xattr
7627 Preserve the specified attributes of the original files.
7628 If specified, the @var{attribute_list} must be a comma-separated list
7629 of one or more of the following strings:
7633 Preserve the file mode bits and access control lists.
7635 Preserve the owner and group. On most modern systems,
7636 only users with appropriate privileges may change the owner of a file,
7638 may preserve the group ownership of a file only if they happen to be
7639 a member of the desired group.
7641 Preserve the times of last access and last modification, when possible.
7642 On older systems, it is not possible to preserve these attributes
7643 when the affected file is a symbolic link.
7644 However, many systems now provide the @code{utimensat} function,
7645 which makes it possible even for symbolic links.
7647 Preserve in the destination files
7648 any links between corresponding source files.
7649 Note that with @option{-L} or @option{-H}, this option can convert
7650 symbolic links to hard links. For example,
7652 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7657 Note the inputs: @file{b} is a symlink to regular file @file{a},
7658 yet the files in destination directory, @file{c/}, are hard-linked.
7659 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7660 tells @command{cp} to dereference command line arguments, it sees two files
7661 with the same inode number, and preserves the perceived hard link.
7663 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7665 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7671 Preserve SELinux security context of the file, or fail with full diagnostics.
7673 Preserve extended attributes of the file, or fail with full diagnostics.
7674 If @command{cp} is built without xattr support, ignore this option.
7675 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7676 they are preserved by this option as well.
7678 Preserve all file attributes.
7679 Equivalent to specifying all of the above, but with the difference
7680 that failure to preserve SELinux security context or extended attributes
7681 does not change @command{cp}'s exit status. In contrast to @option{-a},
7682 all but @samp{Operation not supported} warnings are output.
7685 Using @option{--preserve} with no @var{attribute_list} is equivalent
7686 to @option{--preserve=mode,ownership,timestamps}.
7688 In the absence of this option, each destination file is created with the
7689 mode bits of the corresponding source file, minus the bits set in the
7690 umask and minus the set-user-ID and set-group-ID bits.
7691 @xref{File permissions}.
7693 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7694 @cindex file information, preserving
7695 Do not preserve the specified attributes. The @var{attribute_list}
7696 has the same form as for @option{--preserve}.
7700 @cindex parent directories and @command{cp}
7701 Form the name of each destination file by appending to the target
7702 directory a slash and the specified name of the source file. The last
7703 argument given to @command{cp} must be the name of an existing directory.
7704 For example, the command:
7707 cp --parents a/b/c existing_dir
7711 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7712 any missing intermediate directories.
7719 @opindex --recursive
7720 @cindex directories, copying recursively
7721 @cindex copying directories recursively
7722 @cindex recursively copying directories
7723 @cindex non-directories, copying as special files
7724 Copy directories recursively. By default, do not follow symbolic
7725 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7726 @option{--dereference} (@option{-L}), @option{--no-dereference}
7727 (@option{-P}), and @option{-H} options. Special files are copied by
7728 creating a destination file of the same type as the source; see the
7729 @option{--copy-contents} option. It is not portable to use
7730 @option{-r} to copy symbolic links or special files. On some
7731 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7732 @option{-L} and @option{--copy-contents} for historical reasons.
7733 Also, it is not portable to use @option{-R} to copy symbolic links
7734 unless you also specify @option{-P}, as @acronym{POSIX} allows
7735 implementations that dereference symbolic links by default.
7737 @item --reflink[=@var{when}]
7738 @opindex --reflink[=@var{when}]
7741 @cindex copy on write
7742 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7743 file system. Once it has succeeded, beware that the source and destination
7744 files share the same disk data blocks as long as they remain unmodified.
7745 Thus, if a disk I/O error affects data blocks of one of the files,
7746 the other suffers the same fate.
7748 The @var{when} value can be one of the following:
7752 The default behavior: if the copy-on-write operation is not supported
7753 then report the failure for each file and exit with a failure status.
7756 If the copy-on-write operation is not supported then fall back
7757 to the standard copy behaviour.
7760 This option is overridden by the @option{--link}, @option{--symbolic-link}
7761 and @option{--attributes-only} options, thus allowing it to be used
7762 to configure the default data copying behavior for @command{cp}.
7763 For example, with the following alias, @command{cp} will use the
7764 minimum amount of space supported by the file system.
7767 alias cp='cp --reflink=auto --sparse=always'
7770 @item --remove-destination
7771 @opindex --remove-destination
7772 Remove each existing destination file before attempting to open it
7773 (contrast with @option{-f} above).
7775 @item --sparse=@var{when}
7776 @opindex --sparse=@var{when}
7777 @cindex sparse files, copying
7778 @cindex holes, copying files with
7779 @findex read @r{system call, and holes}
7780 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7781 does not occupy any physical disk blocks; the @samp{read} system call
7782 reads these as zeros. This can both save considerable disk space and
7783 increase speed, since many binary files contain lots of consecutive zero
7784 bytes. By default, @command{cp} detects holes in input source files via a crude
7785 heuristic and makes the corresponding output file sparse as well.
7786 Only regular files may be sparse.
7788 The @var{when} value can be one of the following:
7792 The default behavior: if the input file is sparse, attempt to make
7793 the output file sparse, too. However, if an output file exists but
7794 refers to a non-regular file, then do not attempt to make it sparse.
7797 For each sufficiently long sequence of zero bytes in the input file,
7798 attempt to create a corresponding hole in the output file, even if the
7799 input file does not appear to be sparse.
7800 This is useful when the input file resides on a file system
7801 that does not support sparse files
7802 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7803 but the output file is on a type of file system that does support them.
7804 Holes may be created only in regular files, so if the destination file
7805 is of some other type, @command{cp} does not even try to make it sparse.
7808 Never make the output file sparse.
7809 This is useful in creating a file for use with the @command{mkswap} command,
7810 since such a file must not have any holes.
7813 @optStripTrailingSlashes
7816 @itemx --symbolic-link
7818 @opindex --symbolic-link
7819 @cindex symbolic links, copying with
7820 Make symbolic links instead of copies of non-directories. All source
7821 file names must be absolute (starting with @samp{/}) unless the
7822 destination files are in the current directory. This option merely
7823 results in an error message on systems that do not support symbolic links.
7829 @optNoTargetDirectory
7835 @cindex newer files, copying only
7836 Do not copy a non-directory that has an existing destination with the
7837 same or newer modification time. If time stamps are being preserved,
7838 the comparison is to the source time stamp truncated to the
7839 resolutions of the destination file system and of the system calls
7840 used to update time stamps; this avoids duplicate work if several
7841 @samp{cp -pu} commands are executed with the same source and
7848 Print the name of each file before copying it.
7851 @itemx --one-file-system
7853 @opindex --one-file-system
7854 @cindex file systems, omitting copying to different
7855 Skip subdirectories that are on different file systems from the one that
7856 the copy started on.
7857 However, mount point directories @emph{are} copied.
7865 @section @command{dd}: Convert and copy a file
7868 @cindex converting while copying a file
7870 @command{dd} copies a file (from standard input to standard output, by
7871 default) with a changeable I/O block size, while optionally performing
7872 conversions on it. Synopses:
7875 dd [@var{operand}]@dots{}
7879 The only options are @option{--help} and @option{--version}.
7880 @xref{Common options}. @command{dd} accepts the following operands.
7886 Read from @var{file} instead of standard input.
7890 Write to @var{file} instead of standard output. Unless
7891 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7892 bytes (or the size specified with @samp{seek=}).
7894 @item ibs=@var{bytes}
7896 @cindex block size of input
7897 @cindex input block size
7898 Set the input block size to @var{bytes}.
7899 This makes @command{dd} read @var{bytes} per block.
7900 The default is 512 bytes.
7902 @item obs=@var{bytes}
7904 @cindex block size of output
7905 @cindex output block size
7906 Set the output block size to @var{bytes}.
7907 This makes @command{dd} write @var{bytes} per block.
7908 The default is 512 bytes.
7910 @item bs=@var{bytes}
7913 Set both input and output block sizes to @var{bytes}.
7914 This makes @command{dd} read and write @var{bytes} per block,
7915 overriding any @samp{ibs} and @samp{obs} settings.
7916 In addition, if no data-transforming @option{conv} option is specified,
7917 each input block is copied to the output as a single block,
7918 without aggregating short reads.
7920 @item cbs=@var{bytes}
7922 @cindex block size of conversion
7923 @cindex conversion block size
7924 @cindex fixed-length records, converting to variable-length
7925 @cindex variable-length records, converting to fixed-length
7926 Set the conversion block size to @var{bytes}.
7927 When converting variable-length records to fixed-length ones
7928 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7929 use @var{bytes} as the fixed record length.
7931 @item skip=@var{blocks}
7933 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7935 @item seek=@var{blocks}
7937 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7939 @item count=@var{blocks}
7941 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7942 of everything until the end of the file.
7946 Do not print the overall transfer rate and volume statistics
7947 that normally make up the third status line when @command{dd} exits.
7949 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7951 Convert the file as specified by the @var{conversion} argument(s).
7952 (No spaces around any comma(s).)
7959 @opindex ascii@r{, converting to}
7960 Convert @acronym{EBCDIC} to @acronym{ASCII},
7961 using the conversion table specified by @acronym{POSIX}.
7962 This provides a 1:1 translation for all 256 bytes.
7965 @opindex ebcdic@r{, converting to}
7966 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7967 This is the inverse of the @samp{ascii} conversion.
7970 @opindex alternate ebcdic@r{, converting to}
7971 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7972 using the alternate conversion table specified by @acronym{POSIX}.
7973 This is not a 1:1 translation, but reflects common historical practice
7974 for @samp{~}, @samp{[}, and @samp{]}.
7976 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7980 @opindex block @r{(space-padding)}
7981 For each line in the input, output @samp{cbs} bytes, replacing the
7982 input newline with a space and padding with spaces as necessary.
7986 Remove any trailing spaces in each @samp{cbs}-sized input block,
7987 and append a newline.
7989 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7992 @opindex lcase@r{, converting to}
7993 Change uppercase letters to lowercase.
7996 @opindex ucase@r{, converting to}
7997 Change lowercase letters to uppercase.
7999 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
8002 @opindex swab @r{(byte-swapping)}
8003 @cindex byte-swapping
8004 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
8005 when an odd number of bytes are read---the last byte is simply copied
8006 (since there is nothing to swap it with).
8010 @cindex read errors, ignoring
8011 Continue after read errors.
8015 @cindex creating output file, avoiding
8016 Do not create the output file; the output file must already exist.
8020 @cindex creating output file, requiring
8021 Fail if the output file already exists; @command{dd} must create the
8024 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8028 @cindex truncating output file, avoiding
8029 Do not truncate the output file.
8032 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8033 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8034 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8039 @cindex synchronized data writes, before finishing
8040 Synchronize output data just before finishing. This forces a physical
8041 write of output data.
8045 @cindex synchronized data and metadata writes, before finishing
8046 Synchronize output data and metadata just before finishing. This
8047 forces a physical write of output data and metadata.
8051 @item iflag=@var{flag}[,@var{flag}]@dots{}
8053 Access the input file using the flags specified by the @var{flag}
8054 argument(s). (No spaces around any comma(s).)
8056 @item oflag=@var{flag}[,@var{flag}]@dots{}
8058 Access the output file using the flags specified by the @var{flag}
8059 argument(s). (No spaces around any comma(s).)
8061 Here are the flags. Not every flag is supported on every operating
8068 @cindex appending to the output file
8069 Write in append mode, so that even if some other process is writing to
8070 this file, every @command{dd} write will append to the current
8071 contents of the file. This flag makes sense only for output.
8072 If you combine this flag with the @samp{of=@var{file}} operand,
8073 you should also specify @samp{conv=notrunc} unless you want the
8074 output file to be truncated before being appended to.
8078 @cindex concurrent I/O
8079 Use concurrent I/O mode for data. This mode performs direct I/O
8080 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8081 A file cannot be opened in CIO mode and with a standard open at the
8087 Use direct I/O for data, avoiding the buffer cache.
8088 Note that the kernel may impose restrictions on read or write buffer sizes.
8089 For example, with an ext4 destination file system and a linux-based kernel,
8090 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8091 output buffer size is not a multiple of 512.
8095 @cindex directory I/O
8097 Fail unless the file is a directory. Most operating systems do not
8098 allow I/O to a directory, so this flag has limited utility.
8102 @cindex synchronized data reads
8103 Use synchronized I/O for data. For the output file, this forces a
8104 physical write of output data on each write. For the input file,
8105 this flag can matter when reading from a remote file that has been
8106 written to synchronously by some other process. Metadata (e.g.,
8107 last-access and last-modified time) is not necessarily synchronized.
8111 @cindex synchronized data and metadata I/O
8112 Use synchronized I/O for both data and metadata.
8116 @cindex nonblocking I/O
8117 Use non-blocking I/O.
8122 Do not update the file's access time.
8123 Some older file systems silently ignore this flag, so it is a good
8124 idea to test it on your files before relying on it.
8128 @cindex controlling terminal
8129 Do not assign the file to be a controlling terminal for @command{dd}.
8130 This has no effect when the file is not a terminal.
8131 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8136 @cindex symbolic links, following
8137 Do not follow symbolic links.
8142 Fail if the file has multiple hard links.
8147 Use binary I/O. This option has an effect only on nonstandard
8148 platforms that distinguish binary from text I/O.
8153 Use text I/O. Like @samp{binary}, this option has no effect on
8158 Accumulate full blocks from input. The @code{read} system call
8159 may return early if a full block is not available.
8160 When that happens, continue calling @code{read} to fill the remainder
8162 This flag can be used only with @code{iflag}.
8166 These flags are not supported on all systems, and @samp{dd} rejects
8167 attempts to use them when they are not supported. When reading from
8168 standard input or writing to standard output, the @samp{nofollow} and
8169 @samp{noctty} flags should not be specified, and the other flags
8170 (e.g., @samp{nonblock}) can affect how other processes behave with the
8171 affected file descriptors, even after @command{dd} exits.
8175 @cindex multipliers after numbers
8176 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8177 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8178 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8179 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8181 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8182 should not be too large---values larger than a few megabytes
8183 are generally wasteful or (as in the gigabyte..exabyte case) downright
8184 counterproductive or error-inducing.
8186 Use different @command{dd} invocations to use different block sizes for
8187 skipping and I/O@. For example, the following shell commands copy data
8188 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8189 4 KiB label at the start of the disk:
8192 disk=/dev/rdsk/c0t1d0s2
8195 # Copy all but the label from disk to tape.
8196 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8198 # Copy from tape back to disk, but leave the disk label alone.
8199 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8202 Sending an @samp{INFO} signal to a running @command{dd}
8203 process makes it print I/O statistics to standard error
8204 and then resume copying. In the example below,
8205 @command{dd} is run in the background to copy 10 million blocks.
8206 The @command{kill} command makes it output intermediate I/O statistics,
8207 and when @command{dd} completes normally or is killed by the
8208 @code{SIGINT} signal, it outputs the final statistics.
8211 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8212 $ kill -s INFO $pid; wait $pid
8213 3385223+0 records in
8214 3385223+0 records out
8215 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8216 10000000+0 records in
8217 10000000+0 records out
8218 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8221 @vindex POSIXLY_CORRECT
8222 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8223 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8224 environment variable is set.
8229 @node install invocation
8230 @section @command{install}: Copy files and set attributes
8233 @cindex copying files and setting attributes
8235 @command{install} copies files while setting their file mode bits and, if
8236 possible, their owner and group. Synopses:
8239 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8240 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8241 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8242 install [@var{option}]@dots{} -d @var{directory}@dots{}
8247 If two file names are given, @command{install} copies the first file to the
8251 If the @option{--target-directory} (@option{-t}) option is given, or
8252 failing that if the last file is a directory and the
8253 @option{--no-target-directory} (@option{-T}) option is not given,
8254 @command{install} copies each @var{source} file to the specified
8255 directory, using the @var{source}s' names.
8258 If the @option{--directory} (@option{-d}) option is given,
8259 @command{install} creates each @var{directory} and any missing parent
8260 directories. Parent directories are created with mode
8261 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8262 current umask. @xref{Directory Setuid and Setgid}, for how the
8263 set-user-ID and set-group-ID bits of parent directories are inherited.
8266 @cindex Makefiles, installing programs in
8267 @command{install} is similar to @command{cp}, but allows you to control the
8268 attributes of destination files. It is typically used in Makefiles to
8269 copy programs into their destination directories. It refuses to copy
8270 files onto themselves.
8272 @cindex extended attributes, xattr
8273 @command{install} never preserves extended attributes (xattr).
8275 The program accepts the following options. Also see @ref{Common options}.
8285 Compare each pair of source and destination files, and if the destination has
8286 identical content and any specified owner, group, permissions, and possibly
8287 SELinux context, then do not modify the destination at all.
8291 Ignored; for compatibility with old Unix versions of @command{install}.
8295 Create any missing parent directories of @var{dest},
8296 then copy @var{source} to @var{dest}.
8297 This option is ignored if a destination directory is specified
8298 via @option{--target-directory=DIR}.
8303 @opindex --directory
8304 @cindex directories, creating with given attributes
8305 @cindex parent directories, creating missing
8306 @cindex leading directories, creating missing
8307 Create any missing parent directories, giving them the default
8308 attributes. Then create each given directory, setting their owner,
8309 group and mode as given on the command line or to the defaults.
8311 @item -g @var{group}
8312 @itemx --group=@var{group}
8315 @cindex group ownership of installed files, setting
8316 Set the group ownership of installed files or directories to
8317 @var{group}. The default is the process's current group. @var{group}
8318 may be either a group name or a numeric group ID.
8321 @itemx --mode=@var{mode}
8324 @cindex permissions of installed files, setting
8325 Set the file mode bits for the installed file or directory to @var{mode},
8326 which can be either an octal number, or a symbolic mode as in
8327 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8328 point of departure (@pxref{File permissions}).
8329 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8330 execute for the owner, read and execute for group and other, and with
8331 set-user-ID and set-group-ID disabled.
8332 This default is not quite the same as @samp{755}, since it disables
8333 instead of preserving set-user-ID and set-group-ID on directories.
8334 @xref{Directory Setuid and Setgid}.
8336 @item -o @var{owner}
8337 @itemx --owner=@var{owner}
8340 @cindex ownership of installed files, setting
8341 @cindex appropriate privileges
8342 @vindex root @r{as default owner}
8343 If @command{install} has appropriate privileges (is run as root), set the
8344 ownership of installed files or directories to @var{owner}. The default
8345 is @code{root}. @var{owner} may be either a user name or a numeric user
8348 @item --preserve-context
8349 @opindex --preserve-context
8351 @cindex security context
8352 Preserve the SELinux security context of files and directories.
8353 Failure to preserve the context in all of the files or directories
8354 will result in an exit status of 1. If SELinux is disabled then
8355 print a warning and ignore the option.
8358 @itemx --preserve-timestamps
8360 @opindex --preserve-timestamps
8361 @cindex timestamps of installed files, preserving
8362 Set the time of last access and the time of last modification of each
8363 installed file to match those of each corresponding original file.
8364 When a file is installed without this option, its last access and
8365 last modification times are both set to the time of installation.
8366 This option is useful if you want to use the last modification times
8367 of installed files to keep track of when they were last built as opposed
8368 to when they were last installed.
8374 @cindex symbol table information, stripping
8375 @cindex stripping symbol table information
8376 Strip the symbol tables from installed binary executables.
8378 @itemx --strip-program=@var{program}
8379 @opindex --strip-program
8380 @cindex symbol table information, stripping, program
8381 Program used to strip binaries.
8387 @optNoTargetDirectory
8393 Print the name of each file before copying it.
8395 @item -Z @var{context}
8396 @itemx --context=@var{context}
8400 @cindex security context
8401 Set the default SELinux security context to be used for any
8402 created files and directories. If SELinux is disabled then
8403 print a warning and ignore the option.
8411 @section @command{mv}: Move (rename) files
8415 @command{mv} moves or renames files (or directories). Synopses:
8418 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8419 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8420 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8425 If two file names are given, @command{mv} moves the first file to the
8429 If the @option{--target-directory} (@option{-t}) option is given, or
8430 failing that if the last file is a directory and the
8431 @option{--no-target-directory} (@option{-T}) option is not given,
8432 @command{mv} moves each @var{source} file to the specified
8433 directory, using the @var{source}s' names.
8436 @command{mv} can move any type of file from one file system to another.
8437 Prior to version @code{4.0} of the fileutils,
8438 @command{mv} could move only regular files between file systems.
8439 For example, now @command{mv} can move an entire directory hierarchy
8440 including special device files from one partition to another. It first
8441 uses some of the same code that's used by @code{cp -a} to copy the
8442 requested directories and files, then (assuming the copy succeeded)
8443 it removes the originals. If the copy fails, then the part that was
8444 copied to the destination partition is removed. If you were to copy
8445 three directories from one partition to another and the copy of the first
8446 directory succeeded, but the second didn't, the first would be left on
8447 the destination partition and the second and third would be left on the
8450 @cindex extended attributes, xattr
8451 @command{mv} always tries to copy extended attributes (xattr), which may
8452 include SELinux context, ACLs or Capabilities.
8453 Upon failure all but @samp{Operation not supported} warnings are output.
8455 @cindex prompting, and @command{mv}
8456 If a destination file exists but is normally unwritable, standard input
8457 is a terminal, and the @option{-f} or @option{--force} option is not given,
8458 @command{mv} prompts the user for whether to replace the file. (You might
8459 own the file, or have write permission on its directory.) If the
8460 response is not affirmative, the file is skipped.
8462 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8463 when it might be a symlink to a directory.
8464 Otherwise, @command{mv} may do something very surprising, since
8465 its behavior depends on the underlying rename system call.
8466 On a system with a modern Linux-based kernel, it fails with
8467 @code{errno=ENOTDIR}.
8468 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8469 renames not the symlink but rather the directory referenced by the symlink.
8470 @xref{Trailing slashes}.
8472 The program accepts the following options. Also see @ref{Common options}.
8482 @cindex prompts, omitting
8483 Do not prompt the user before removing a destination file.
8485 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8486 options, only the final one takes effect.
8491 @itemx --interactive
8493 @opindex --interactive
8494 @cindex prompts, forcing
8495 Prompt whether to overwrite each existing destination file, regardless
8497 If the response is not affirmative, the file is skipped.
8503 @opindex --no-clobber
8504 @cindex prompts, omitting
8505 Do not overwrite an existing file.
8507 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8513 @cindex newer files, moving only
8514 Do not move a non-directory that has an existing destination with the
8515 same or newer modification time.
8516 If the move is across file system boundaries, the comparison is to the
8517 source time stamp truncated to the resolutions of the destination file
8518 system and of the system calls used to update time stamps; this avoids
8519 duplicate work if several @samp{mv -u} commands are executed with the
8520 same source and destination.
8526 Print the name of each file before moving it.
8528 @optStripTrailingSlashes
8534 @optNoTargetDirectory
8542 @section @command{rm}: Remove files or directories
8545 @cindex removing files or directories
8547 @command{rm} removes each given @var{file}. By default, it does not remove
8548 directories. Synopsis:
8551 rm [@var{option}]@dots{} [@var{file}]@dots{}
8554 @cindex prompting, and @command{rm}
8555 If the @option{-I} or @option{--interactive=once} option is given,
8556 and there are more than three files or the @option{-r}, @option{-R},
8557 or @option{--recursive} are given, then @command{rm} prompts the user
8558 for whether to proceed with the entire operation. If the response is
8559 not affirmative, the entire command is aborted.
8561 Otherwise, if a file is unwritable, standard input is a terminal, and
8562 the @option{-f} or @option{--force} option is not given, or the
8563 @option{-i} or @option{--interactive=always} option @emph{is} given,
8564 @command{rm} prompts the user for whether to remove the file.
8565 If the response is not affirmative, the file is skipped.
8567 Any attempt to remove a file whose last file name component is
8568 @file{.} or @file{..} is rejected without any prompting.
8570 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8571 possible to recover the contents of that file. If you want more assurance
8572 that the contents are truly unrecoverable, consider using @command{shred}.
8574 The program accepts the following options. Also see @ref{Common options}.
8582 Ignore nonexistent files and never prompt the user.
8583 Ignore any previous @option{--interactive} (@option{-i}) option.
8587 Prompt whether to remove each file.
8588 If the response is not affirmative, the file is skipped.
8589 Ignore any previous @option{--force} (@option{-f}) option.
8590 Equivalent to @option{--interactive=always}.
8594 Prompt once whether to proceed with the command, if more than three
8595 files are named or if a recursive removal is requested. Ignore any
8596 previous @option{--force} (@option{-f}) option. Equivalent to
8597 @option{--interactive=once}.
8599 @itemx --interactive [=@var{when}]
8600 @opindex --interactive
8601 Specify when to issue an interactive prompt. @var{when} may be
8605 @vindex never @r{interactive option}
8606 - Do not prompt at all.
8608 @vindex once @r{interactive option}
8609 - Prompt once if more than three files are named or if a recursive
8610 removal is requested. Equivalent to @option{-I}.
8612 @vindex always @r{interactive option}
8613 - Prompt for every file being removed. Equivalent to @option{-i}.
8615 @option{--interactive} with no @var{when} is equivalent to
8616 @option{--interactive=always}.
8618 @itemx --one-file-system
8619 @opindex --one-file-system
8620 @cindex one file system, restricting @command{rm} to
8621 When removing a hierarchy recursively, skip any directory that is on a
8622 file system different from that of the corresponding command line argument.
8625 This option is useful when removing a build ``chroot'' hierarchy,
8626 which normally contains no valuable data. However, it is not uncommon
8627 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8628 use one's start-up file. The catch is that it's easy to forget to
8629 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8630 your normally throw-away chroot, that command will remove everything
8631 under @file{/home}, too.
8632 Use the @option{--one-file-system} option, and it will
8633 warn about and skip directories on other file systems.
8634 Of course, this will not save your @file{/home} if it and your
8635 chroot happen to be on the same file system.
8637 @itemx --preserve-root
8638 @opindex --preserve-root
8639 @cindex root directory, disallow recursive destruction
8640 Fail upon any attempt to remove the root directory, @file{/},
8641 when used with the @option{--recursive} option.
8642 This is the default behavior.
8643 @xref{Treating / specially}.
8645 @itemx --no-preserve-root
8646 @opindex --no-preserve-root
8647 @cindex root directory, allow recursive destruction
8648 Do not treat @file{/} specially when removing recursively.
8649 This option is not recommended unless you really want to
8650 remove all the files on your computer.
8651 @xref{Treating / specially}.
8658 @opindex --recursive
8659 @cindex directories, removing (recursively)
8660 Remove the listed directories and their contents recursively.
8666 Print the name of each file before removing it.
8670 @cindex files beginning with @samp{-}, removing
8671 @cindex @samp{-}, removing files beginning with
8672 One common question is how to remove files whose names begin with a
8673 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8674 function to parse its arguments, lets you use the @samp{--} option to
8675 indicate that all following arguments are non-options. To remove a file
8676 called @file{-f} in the current directory, you could type either:
8689 @opindex - @r{and Unix @command{rm}}
8690 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8691 predates the development of the getopt standard syntax.
8696 @node shred invocation
8697 @section @command{shred}: Remove files more securely
8700 @cindex data, erasing
8701 @cindex erasing data
8703 @command{shred} overwrites devices or files, to help prevent even
8704 very expensive hardware from recovering the data.
8706 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8707 not actually destroyed. Only the index listing where the file is
8708 stored is destroyed, and the storage is made available for reuse.
8709 There are undelete utilities that will attempt to reconstruct the index
8710 and can bring the file back if the parts were not reused.
8712 On a busy system with a nearly-full drive, space can get reused in a few
8713 seconds. But there is no way to know for sure. If you have sensitive
8714 data, you may want to be sure that recovery is not possible by actually
8715 overwriting the file with non-sensitive data.
8717 However, even after doing that, it is possible to take the disk back
8718 to a laboratory and use a lot of sensitive (and expensive) equipment
8719 to look for the faint ``echoes'' of the original data underneath the
8720 overwritten data. If the data has only been overwritten once, it's not
8723 The best way to remove something irretrievably is to destroy the media
8724 it's on with acid, melt it down, or the like. For cheap removable media
8725 like floppy disks, this is the preferred method. However, hard drives
8726 are expensive and hard to melt, so the @command{shred} utility tries
8727 to achieve a similar effect non-destructively.
8729 This uses many overwrite passes, with the data patterns chosen to
8730 maximize the damage they do to the old data. While this will work on
8731 floppies, the patterns are designed for best effect on hard drives.
8732 For more details, see the source code and Peter Gutmann's paper
8733 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8734 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8735 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8736 California, July 22--25, 1996).
8738 @strong{Please note} that @command{shred} relies on a very important assumption:
8739 that the file system overwrites data in place. This is the traditional
8740 way to do things, but many modern file system designs do not satisfy this
8741 assumption. Exceptions include:
8746 Log-structured or journaled file systems, such as those supplied with
8747 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8748 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8751 File systems that write redundant data and carry on even if some writes
8752 fail, such as RAID-based file systems.
8755 File systems that make snapshots, such as Network Appliance's NFS server.
8758 File systems that cache in temporary locations, such as NFS version 3
8762 Compressed file systems.
8765 In the particular case of ext3 file systems, the above disclaimer applies (and
8766 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8767 mode, which journals file data in addition to just metadata. In both
8768 the @code{data=ordered} (default) and @code{data=writeback} modes,
8769 @command{shred} works as usual. Ext3 journaling modes can be changed
8770 by adding the @code{data=something} option to the mount options for a
8771 particular file system in the @file{/etc/fstab} file, as documented in
8772 the mount man page (man mount).
8774 If you are not sure how your file system operates, then you should assume
8775 that it does not overwrite data in place, which means that shred cannot
8776 reliably operate on regular files in your file system.
8778 Generally speaking, it is more reliable to shred a device than a file,
8779 since this bypasses the problem of file system design mentioned above.
8780 However, even shredding devices is not always completely reliable. For
8781 example, most disks map out bad sectors invisibly to the application; if
8782 the bad sectors contain sensitive data, @command{shred} won't be able to
8785 @command{shred} makes no attempt to detect or report this problem, just as
8786 it makes no attempt to do anything about backups. However, since it is
8787 more reliable to shred devices than files, @command{shred} by default does
8788 not truncate or remove the output file. This default is more suitable
8789 for devices, which typically cannot be truncated and should not be
8792 Finally, consider the risk of backups and mirrors.
8793 File system backups and remote mirrors may contain copies of the
8794 file that cannot be removed, and that will allow a shredded file
8795 to be recovered later. So if you keep any data you may later want
8796 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8799 shred [@var{option}]@dots{} @var{file}[@dots{}]
8802 The program accepts the following options. Also see @ref{Common options}.
8810 @cindex force deletion
8811 Override file permissions if necessary to allow overwriting.
8814 @itemx -n @var{number}
8815 @itemx --iterations=@var{number}
8816 @opindex -n @var{number}
8817 @opindex --iterations=@var{number}
8818 @cindex iterations, selecting the number of
8819 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8820 overwrite. You can reduce this to save time, or increase it if you think it's
8821 appropriate. After 25 passes all of the internal overwrite patterns will have
8822 been used at least once.
8824 @item --random-source=@var{file}
8825 @opindex --random-source
8826 @cindex random source for shredding
8827 Use @var{file} as a source of random data used to overwrite and to
8828 choose pass ordering. @xref{Random sources}.
8830 @item -s @var{bytes}
8831 @itemx --size=@var{bytes}
8832 @opindex -s @var{bytes}
8833 @opindex --size=@var{bytes}
8834 @cindex size of file to shred
8835 Shred the first @var{bytes} bytes of the file. The default is to shred
8836 the whole file. @var{bytes} can be followed by a size specification like
8837 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8843 @cindex removing files after shredding
8844 After shredding a file, truncate it (if possible) and then remove it.
8845 If a file has multiple links, only the named links will be removed.
8851 Display to standard error all status updates as sterilization proceeds.
8857 By default, @command{shred} rounds the size of a regular file up to the next
8858 multiple of the file system block size to fully erase the last block
8860 Use @option{--exact} to suppress that behavior.
8861 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8862 blocks, the resulting file will be 512 bytes long. With this option,
8863 shred does not increase the apparent size of the file.
8869 Normally, the last pass that @command{shred} writes is made up of
8870 random data. If this would be conspicuous on your hard drive (for
8871 example, because it looks like encrypted data), or you just think
8872 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8873 all zero bits. This is in addition to the number of passes specified
8874 by the @option{--iterations} option.
8878 You might use the following command to erase all trace of the
8879 file system you'd created on the floppy disk in your first drive.
8880 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8884 shred --verbose /dev/fd0
8887 Similarly, to erase all data on a selected partition of
8888 your hard disk, you could give a command like this:
8891 shred --verbose /dev/sda5
8894 A @var{file} of @samp{-} denotes standard output.
8895 The intended use of this is to shred a removed temporary file.
8902 echo "Hello, world" >&3
8907 However, the command @samp{shred - >file} does not shred the contents
8908 of @var{file}, since the shell truncates @var{file} before invoking
8909 @command{shred}. Use the command @samp{shred file} or (if using a
8910 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8915 @node Special file types
8916 @chapter Special file types
8918 @cindex special file types
8919 @cindex file types, special
8921 This chapter describes commands which create special types of files (and
8922 @command{rmdir}, which removes directories, one special file type).
8924 @cindex special file types
8926 Although Unix-like operating systems have markedly fewer special file
8927 types than others, not @emph{everything} can be treated only as the
8928 undifferentiated byte stream of @dfn{normal files}. For example, when a
8929 file is created or removed, the system must record this information,
8930 which it does in a @dfn{directory}---a special type of file. Although
8931 you can read directories as normal files, if you're curious, in order
8932 for the system to do its job it must impose a structure, a certain
8933 order, on the bytes of the file. Thus it is a ``special'' type of file.
8935 Besides directories, other special file types include named pipes
8936 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8939 * link invocation:: Make a hard link via the link syscall
8940 * ln invocation:: Make links between files.
8941 * mkdir invocation:: Make directories.
8942 * mkfifo invocation:: Make FIFOs (named pipes).
8943 * mknod invocation:: Make block or character special files.
8944 * readlink invocation:: Print value of a symlink or canonical file name.
8945 * rmdir invocation:: Remove empty directories.
8946 * unlink invocation:: Remove files via the unlink syscall
8950 @node link invocation
8951 @section @command{link}: Make a hard link via the link syscall
8954 @cindex links, creating
8955 @cindex hard links, creating
8956 @cindex creating links (hard only)
8958 @command{link} creates a single hard link at a time.
8959 It is a minimalist interface to the system-provided
8960 @code{link} function. @xref{Hard Links, , , libc,
8961 The GNU C Library Reference Manual}.
8962 It avoids the bells and whistles of the more commonly-used
8963 @command{ln} command (@pxref{ln invocation}).
8967 link @var{filename} @var{linkname}
8970 @var{filename} must specify an existing file, and @var{linkname}
8971 must specify a nonexistent entry in an existing directory.
8972 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8975 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8976 --no-target-directory @var{filename} @var{linkname}}. However, the
8977 @option{--directory} and @option{--no-target-directory} options are
8978 not specified by @acronym{POSIX}, and the @command{link} command is
8979 more portable in practice.
8981 If @var{filename} is a symbolic link, it is unspecified whether
8982 @var{linkname} will be a hard link to the symbolic link or to the
8983 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8984 to specify which behavior is desired.
8990 @section @command{ln}: Make links between files
8993 @cindex links, creating
8994 @cindex hard links, creating
8995 @cindex symbolic (soft) links, creating
8996 @cindex creating links (hard or soft)
8998 @cindex file systems and hard links
8999 @command{ln} makes links between files. By default, it makes hard links;
9000 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
9004 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
9005 ln [@var{option}]@dots{} @var{target}
9006 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
9007 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9013 If two file names are given, @command{ln} creates a link to the first
9014 file from the second.
9017 If one @var{target} is given, @command{ln} creates a link to that file
9018 in the current directory.
9021 If the @option{--target-directory} (@option{-t}) option is given, or
9022 failing that if the last file is a directory and the
9023 @option{--no-target-directory} (@option{-T}) option is not given,
9024 @command{ln} creates a link to each @var{target} file in the specified
9025 directory, using the @var{target}s' names.
9029 Normally @command{ln} does not remove existing files. Use the
9030 @option{--force} (@option{-f}) option to remove them unconditionally,
9031 the @option{--interactive} (@option{-i}) option to remove them
9032 conditionally, and the @option{--backup} (@option{-b}) option to
9035 @cindex hard link, defined
9036 @cindex inode, and hard links
9037 A @dfn{hard link} is another name for an existing file; the link and the
9038 original are indistinguishable. Technically speaking, they share the
9039 same inode, and the inode contains all the information about a
9040 file---indeed, it is not incorrect to say that the inode @emph{is} the
9041 file. Most systems prohibit making a hard link to
9042 a directory; on those where it is allowed, only the super-user can do
9043 so (and with caution, since creating a cycle will cause problems to many
9044 other utilities). Hard links cannot cross file system boundaries. (These
9045 restrictions are not mandated by @acronym{POSIX}, however.)
9047 @cindex dereferencing symbolic links
9048 @cindex symbolic link, defined
9049 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9050 a special file type (which not all kernels support: System V release 3
9051 (and older) systems lack symlinks) in which the link file actually
9052 refers to a different file, by name. When most operations (opening,
9053 reading, writing, and so on) are passed the symbolic link file, the
9054 kernel automatically @dfn{dereferences} the link and operates on the
9055 target of the link. But some operations (e.g., removing) work on the
9056 link file itself, rather than on its target. The owner and group of a
9057 symlink are not significant to file access performed through
9058 the link, but do have implications on deleting a symbolic link from a
9059 directory with the restricted deletion bit set. On the GNU system,
9060 the mode of a symlink has no significance and cannot be changed, but
9061 on some BSD systems, the mode can be changed and will affect whether
9062 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9063 libc, The GNU C Library Reference Manual}.
9065 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9066 occurs when the string in the symlink does not resolve to a file.
9067 There are no restrictions against creating dangling symbolic links.
9068 There are trade-offs to using absolute or relative symlinks. An
9069 absolute symlink always points to the same file, even if the directory
9070 containing the link is moved. However, if the symlink is visible from
9071 more than one machine (such as on a networked file system), the file
9072 pointed to might not always be the same. A relative symbolic link is
9073 resolved in relation to the directory that contains the link, and is
9074 often useful in referring to files on the same device without regards
9075 to what name that device is mounted on when accessed via networked
9078 When creating a relative symlink in a different location than the
9079 current directory, the resolution of the symlink will be different
9080 than the resolution of the same string from the current directory.
9081 Therefore, many users prefer to first change directories to the
9082 location where the relative symlink will be created, so that
9083 tab-completion or other file resolution will find the same target as
9084 what will be placed in the symlink.
9086 The program accepts the following options. Also see @ref{Common options}.
9097 @opindex --directory
9098 @cindex hard links to directories
9099 Allow users with appropriate privileges to attempt to make hard links
9101 However, note that this will probably fail due to
9102 system restrictions, even for the super-user.
9108 Remove existing destination files.
9111 @itemx --interactive
9113 @opindex --interactive
9114 @cindex prompting, and @command{ln}
9115 Prompt whether to remove existing destination files.
9121 If @option{-s} is not in effect, and the source file is a symbolic
9122 link, create the hard link to the file referred to by the symbolic
9123 link, rather than the symbolic link itself.
9126 @itemx --no-dereference
9128 @opindex --no-dereference
9129 Do not treat the last operand specially when it is a symbolic link to
9130 a directory. Instead, treat it as if it were a normal file.
9132 When the destination is an actual directory (not a symlink to one),
9133 there is no ambiguity. The link is created in that directory.
9134 But when the specified destination is a symlink to a directory,
9135 there are two ways to treat the user's request. @command{ln} can
9136 treat the destination just as it would a normal directory and create
9137 the link in it. On the other hand, the destination can be viewed as a
9138 non-directory---as the symlink itself. In that case, @command{ln}
9139 must delete or backup that symlink before creating the new link.
9140 The default is to treat a destination that is a symlink to a directory
9141 just like a directory.
9143 This option is weaker than the @option{--no-target-directory}
9144 (@option{-T}) option, so it has no effect if both options are given.
9150 If @option{-s} is not in effect, and the source file is a symbolic
9151 link, create the hard link to the symbolic link itself. On platforms
9152 where this is not supported by the kernel, this option creates a
9153 symbolic link with identical contents; since symbolic link contents
9154 cannot be edited, any file name resolution performed through either
9155 link will be the same as if a hard link had been created.
9161 Make symbolic links instead of hard links. This option merely produces
9162 an error message on systems that do not support symbolic links.
9168 @optNoTargetDirectory
9174 Print the name of each file after linking it successfully.
9178 @cindex hard links to symbolic links
9179 @cindex symbolic links and @command{ln}
9180 If @option{-L} and @option{-P} are both given, the last one takes
9181 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9182 are silently ignored. If neither option is given, then this
9183 implementation defaults to @option{-P} if the system @code{link} supports
9184 hard links to symbolic links (such as the GNU system), and @option{-L}
9185 if @code{link} follows symbolic links (such as on BSD).
9194 # Create link ../a pointing to a in that directory.
9195 # Not really useful because it points to itself.
9200 # Change to the target before creating symlinks to avoid being confused.
9206 # Hard coded file names don't move well.
9207 ln -s $(pwd)/a /some/dir/
9211 # Relative file names survive directory moves and also
9212 # work across networked file systems.
9213 ln -s afile anotherfile
9214 ln -s ../adir/afile yetanotherfile
9218 @node mkdir invocation
9219 @section @command{mkdir}: Make directories
9222 @cindex directories, creating
9223 @cindex creating directories
9225 @command{mkdir} creates directories with the specified names. Synopsis:
9228 mkdir [@var{option}]@dots{} @var{name}@dots{}
9231 @command{mkdir} creates each directory @var{name} in the order given.
9232 It reports an error if @var{name} already exists, unless the
9233 @option{-p} option is given and @var{name} is a directory.
9235 The program accepts the following options. Also see @ref{Common options}.
9240 @itemx --mode=@var{mode}
9243 @cindex modes of created directories, setting
9244 Set the file permission bits of created directories to @var{mode},
9245 which uses the same syntax as
9246 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9247 everyone) for the point of the departure. @xref{File permissions}.
9249 Normally the directory has the desired file mode bits at the moment it
9250 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9251 special mode bits, but in this case there may be a temporary window
9252 during which the directory exists but its special mode bits are
9253 incorrect. @xref{Directory Setuid and Setgid}, for how the
9254 set-user-ID and set-group-ID bits of directories are inherited unless
9255 overridden in this way.
9261 @cindex parent directories, creating
9262 Make any missing parent directories for each argument, setting their
9263 file permission bits to the umask modified by @samp{u+wx}. Ignore
9264 existing parent directories, and do not change their file permission
9267 To set the file permission bits of any newly-created parent
9268 directories to a value that includes @samp{u+wx}, you can set the
9269 umask before invoking @command{mkdir}. For example, if the shell
9270 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9271 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9272 To set a parent's special mode bits as well, you can invoke
9273 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9274 Setgid}, for how the set-user-ID and set-group-ID bits of
9275 newly-created parent directories are inherited.
9281 Print a message for each created directory. This is most useful with
9284 @item -Z @var{context}
9285 @itemx --context=@var{context}
9289 @cindex security context
9290 Set the default SELinux security context to be used for created directories.
9297 @node mkfifo invocation
9298 @section @command{mkfifo}: Make FIFOs (named pipes)
9301 @cindex FIFOs, creating
9302 @cindex named pipes, creating
9303 @cindex creating FIFOs (named pipes)
9305 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9306 specified names. Synopsis:
9309 mkfifo [@var{option}] @var{name}@dots{}
9312 A @dfn{FIFO} is a special file type that permits independent processes
9313 to communicate. One process opens the FIFO file for writing, and
9314 another for reading, after which data can flow as with the usual
9315 anonymous pipe in shells or elsewhere.
9317 The program accepts the following option. Also see @ref{Common options}.
9322 @itemx --mode=@var{mode}
9325 @cindex modes of created FIFOs, setting
9326 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9327 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9328 for the point of departure. @var{mode} should specify only file
9329 permission bits. @xref{File permissions}.
9331 @item -Z @var{context}
9332 @itemx --context=@var{context}
9336 @cindex security context
9337 Set the default SELinux security context to be used for created FIFOs.
9344 @node mknod invocation
9345 @section @command{mknod}: Make block or character special files
9348 @cindex block special files, creating
9349 @cindex character special files, creating
9351 @command{mknod} creates a FIFO, character special file, or block special
9352 file with the specified name. Synopsis:
9355 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9358 @cindex special files
9359 @cindex block special files
9360 @cindex character special files
9361 Unlike the phrase ``special file type'' above, the term @dfn{special
9362 file} has a technical meaning on Unix: something that can generate or
9363 receive data. Usually this corresponds to a physical piece of hardware,
9364 e.g., a printer or a disk. (These files are typically created at
9365 system-configuration time.) The @command{mknod} command is what creates
9366 files of this type. Such devices can be read either a character at a
9367 time or a ``block'' (many characters) at a time, hence we say there are
9368 @dfn{block special} files and @dfn{character special} files.
9370 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9371 @mayConflictWithShellBuiltIn{mknod}
9373 The arguments after @var{name} specify the type of file to make:
9378 @opindex p @r{for FIFO file}
9382 @opindex b @r{for block special file}
9383 for a block special file
9386 @c Don't document the `u' option -- it's just a synonym for `c'.
9387 @c Do *any* versions of mknod still use it?
9389 @opindex c @r{for character special file}
9390 @c @opindex u @r{for character special file}
9391 for a character special file
9395 When making a block or character special file, the major and minor
9396 device numbers must be given after the file type.
9397 If a major or minor device number begins with @samp{0x} or @samp{0X},
9398 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9399 as octal; otherwise, as decimal.
9401 The program accepts the following option. Also see @ref{Common options}.
9406 @itemx --mode=@var{mode}
9409 Set the mode of created files to @var{mode}, which is symbolic as in
9410 @command{chmod} and uses @samp{a=rw} as the point of departure.
9411 @var{mode} should specify only file permission bits.
9412 @xref{File permissions}.
9414 @item -Z @var{context}
9415 @itemx --context=@var{context}
9419 @cindex security context
9420 Set the default SELinux security context to be used for created files.
9427 @node readlink invocation
9428 @section @command{readlink}: Print value of a symlink or canonical file name
9431 @cindex displaying value of a symbolic link
9432 @cindex canonical file name
9433 @cindex canonicalize a file name
9437 @command{readlink} may work in one of two supported modes:
9443 @command{readlink} outputs the value of the given symbolic link.
9444 If @command{readlink} is invoked with an argument other than the name
9445 of a symbolic link, it produces no output and exits with a nonzero exit code.
9447 @item Canonicalize mode
9449 @command{readlink} outputs the absolute name of the given file which contains
9450 no @file{.}, @file{..} components nor any repeated separators
9451 (@file{/}) or symbolic links.
9456 readlink [@var{option}] @var{file}
9459 By default, @command{readlink} operates in readlink mode.
9461 The program accepts the following options. Also see @ref{Common options}.
9466 @itemx --canonicalize
9468 @opindex --canonicalize
9469 Activate canonicalize mode.
9470 If any component of the file name except the last one is missing or unavailable,
9471 @command{readlink} produces no output and exits with a nonzero exit
9472 code. A trailing slash is ignored.
9475 @itemx --canonicalize-existing
9477 @opindex --canonicalize-existing
9478 Activate canonicalize mode.
9479 If any component is missing or unavailable, @command{readlink} produces
9480 no output and exits with a nonzero exit code. A trailing slash
9481 requires that the name resolve to a directory.
9484 @itemx --canonicalize-missing
9486 @opindex --canonicalize-missing
9487 Activate canonicalize mode.
9488 If any component is missing or unavailable, @command{readlink} treats it
9494 @opindex --no-newline
9495 Do not output the trailing newline.
9505 Suppress most error messages.
9511 Report error messages.
9515 The @command{readlink} utility first appeared in OpenBSD 2.1.
9517 There is a @command{realpath} command on some systems
9518 which operates like @command{readlink} in canonicalize mode.
9523 @node rmdir invocation
9524 @section @command{rmdir}: Remove empty directories
9527 @cindex removing empty directories
9528 @cindex directories, removing empty
9530 @command{rmdir} removes empty directories. Synopsis:
9533 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9536 If any @var{directory} argument does not refer to an existing empty
9537 directory, it is an error.
9539 The program accepts the following options. Also see @ref{Common options}.
9543 @item --ignore-fail-on-non-empty
9544 @opindex --ignore-fail-on-non-empty
9545 @cindex directory deletion, ignoring failures
9546 Ignore each failure to remove a directory that is solely because
9547 the directory is non-empty.
9553 @cindex parent directories, removing
9554 Remove @var{directory}, then try to remove each component of @var{directory}.
9555 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9556 As such, it fails if any of those directories turns out not to be empty.
9557 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9558 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9559 exit unsuccessfully.
9565 @cindex directory deletion, reporting
9566 Give a diagnostic for each successful removal.
9567 @var{directory} is removed.
9571 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9576 @node unlink invocation
9577 @section @command{unlink}: Remove files via the unlink syscall
9580 @cindex removing files or directories (via the unlink syscall)
9582 @command{unlink} deletes a single specified file name.
9583 It is a minimalist interface to the system-provided
9584 @code{unlink} function. @xref{Deleting Files, , , libc,
9585 The GNU C Library Reference Manual}. Synopsis:
9586 It avoids the bells and whistles of the more commonly-used
9587 @command{rm} command (@pxref{rm invocation}).
9590 unlink @var{filename}
9593 On some systems @code{unlink} can be used to delete the name of a
9594 directory. On others, it can be used that way only by a privileged user.
9595 In the GNU system @code{unlink} can never delete the name of a directory.
9597 The @command{unlink} command honors the @option{--help} and
9598 @option{--version} options. To remove a file whose name begins with
9599 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9604 @node Changing file attributes
9605 @chapter Changing file attributes
9607 @cindex changing file attributes
9608 @cindex file attributes, changing
9609 @cindex attributes, file
9611 A file is not merely its contents, a name, and a file type
9612 (@pxref{Special file types}). A file also has an owner (a user ID), a
9613 group (a group ID), permissions (what the owner can do with the file,
9614 what people in the group can do, and what everyone else can do), various
9615 timestamps, and other information. Collectively, we call these a file's
9618 These commands change file attributes.
9621 * chgrp invocation:: Change file groups.
9622 * chmod invocation:: Change access permissions.
9623 * chown invocation:: Change file owners and groups.
9624 * touch invocation:: Change file timestamps.
9628 @node chown invocation
9629 @section @command{chown}: Change file owner and group
9632 @cindex file ownership, changing
9633 @cindex group ownership, changing
9634 @cindex changing file ownership
9635 @cindex changing group ownership
9637 @command{chown} changes the user and/or group ownership of each given @var{file}
9638 to @var{new-owner} or to the user and group of an existing reference file.
9642 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@}@c
9646 If used, @var{new-owner} specifies the new owner and/or group as follows
9647 (with no embedded white space):
9650 [@var{owner}] [ : [@var{group}] ]
9657 If only an @var{owner} (a user name or numeric user ID) is given, that
9658 user is made the owner of each given file, and the files' group is not
9661 @item owner@samp{:}group
9662 If the @var{owner} is followed by a colon and a @var{group} (a
9663 group name or numeric group ID), with no spaces between them, the group
9664 ownership of the files is changed as well (to @var{group}).
9667 If a colon but no group name follows @var{owner}, that user is
9668 made the owner of the files and the group of the files is changed to
9669 @var{owner}'s login group.
9672 If the colon and following @var{group} are given, but the owner
9673 is omitted, only the group of the files is changed; in this case,
9674 @command{chown} performs the same function as @command{chgrp}.
9677 If only a colon is given, or if @var{new-owner} is empty, neither the
9678 owner nor the group is changed.
9682 If @var{owner} or @var{group} is intended to represent a numeric user
9683 or group ID, then you may specify it with a leading @samp{+}.
9684 @xref{Disambiguating names and IDs}.
9686 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9687 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9688 require support for that, but for backward compatibility @acronym{GNU}
9689 @command{chown} supports @samp{.} so long as no ambiguity results.
9690 New scripts should avoid the use of @samp{.} because it is not
9691 portable, and because it has undesirable results if the entire
9692 @var{owner@samp{.}group} happens to identify a user whose name
9695 The @command{chown} command sometimes clears the set-user-ID or
9696 set-group-ID permission bits. This behavior depends on the policy and
9697 functionality of the underlying @code{chown} system call, which may
9698 make system-dependent file mode modifications outside the control of
9699 the @command{chown} command. For example, the @command{chown} command
9700 might not affect those bits when invoked by a user with appropriate
9701 privileges, or when the
9702 bits signify some function other than executable permission (e.g.,
9704 When in doubt, check the underlying system behavior.
9706 The program accepts the following options. Also see @ref{Common options}.
9714 @cindex changed owners, verbosely describing
9715 Verbosely describe the action for each @var{file} whose ownership
9724 @cindex error messages, omitting
9725 Do not print error messages about files whose ownership cannot be
9728 @itemx @w{@kbd{--from}=@var{old-owner}}
9730 @cindex symbolic links, changing owner
9731 Change a @var{file}'s ownership only if it has current attributes specified
9732 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9734 This option is useful primarily from a security standpoint in that
9735 it narrows considerably the window of potential abuse.
9736 For example, to reflect a user ID numbering change for one user's files
9737 without an option like this, @code{root} might run
9740 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9743 But that is dangerous because the interval between when the @command{find}
9744 tests the existing file's owner and when the @command{chown} is actually run
9746 One way to narrow the gap would be to invoke chown for each file
9750 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9753 But that is very slow if there are many affected files.
9754 With this option, it is safer (the gap is narrower still)
9755 though still not perfect:
9758 chown -h -R --from=OLDUSER NEWUSER /
9762 @opindex --dereference
9763 @cindex symbolic links, changing owner
9765 Do not act on symbolic links themselves but rather on what they point to.
9766 This is the default.
9769 @itemx --no-dereference
9771 @opindex --no-dereference
9772 @cindex symbolic links, changing owner
9774 Act on symbolic links themselves instead of what they point to.
9775 This mode relies on the @code{lchown} system call.
9776 On systems that do not provide the @code{lchown} system call,
9777 @command{chown} fails when a file specified on the command line
9779 By default, no diagnostic is issued for symbolic links encountered
9780 during a recursive traversal, but see @option{--verbose}.
9782 @itemx --preserve-root
9783 @opindex --preserve-root
9784 @cindex root directory, disallow recursive modification
9785 Fail upon any attempt to recursively change the root directory, @file{/}.
9786 Without @option{--recursive}, this option has no effect.
9787 @xref{Treating / specially}.
9789 @itemx --no-preserve-root
9790 @opindex --no-preserve-root
9791 @cindex root directory, allow recursive modification
9792 Cancel the effect of any preceding @option{--preserve-root} option.
9793 @xref{Treating / specially}.
9795 @item --reference=@var{ref_file}
9796 @opindex --reference
9797 Change the user and group of each @var{file} to be the same as those of
9798 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9799 user and group of the symbolic link, but rather those of the file it
9806 Output a diagnostic for every file processed.
9807 If a symbolic link is encountered during a recursive traversal
9808 on a system without the @code{lchown} system call, and @option{--no-dereference}
9809 is in effect, then issue a diagnostic saying neither the symbolic link nor
9810 its referent is being changed.
9815 @opindex --recursive
9816 @cindex recursively changing file ownership
9817 Recursively change ownership of directories and their contents.
9820 @xref{Traversing symlinks}.
9823 @xref{Traversing symlinks}.
9826 @xref{Traversing symlinks}.
9835 # Change the owner of /u to "root".
9838 # Likewise, but also change its group to "staff".
9841 # Change the owner of /u and subfiles to "root".
9846 @node chgrp invocation
9847 @section @command{chgrp}: Change group ownership
9850 @cindex group ownership, changing
9851 @cindex changing group ownership
9853 @command{chgrp} changes the group ownership of each given @var{file}
9854 to @var{group} (which can be either a group name or a numeric group ID)
9855 or to the group of an existing reference file. Synopsis:
9858 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@}@c
9862 If @var{group} is intended to represent a
9863 numeric group ID, then you may specify it with a leading @samp{+}.
9864 @xref{Disambiguating names and IDs}.
9866 The program accepts the following options. Also see @ref{Common options}.
9874 @cindex changed files, verbosely describing
9875 Verbosely describe the action for each @var{file} whose group actually
9884 @cindex error messages, omitting
9885 Do not print error messages about files whose group cannot be
9889 @opindex --dereference
9890 @cindex symbolic links, changing owner
9892 Do not act on symbolic links themselves but rather on what they point to.
9893 This is the default.
9896 @itemx --no-dereference
9898 @opindex --no-dereference
9899 @cindex symbolic links, changing group
9901 Act on symbolic links themselves instead of what they point to.
9902 This mode relies on the @code{lchown} system call.
9903 On systems that do not provide the @code{lchown} system call,
9904 @command{chgrp} fails when a file specified on the command line
9906 By default, no diagnostic is issued for symbolic links encountered
9907 during a recursive traversal, but see @option{--verbose}.
9909 @itemx --preserve-root
9910 @opindex --preserve-root
9911 @cindex root directory, disallow recursive modification
9912 Fail upon any attempt to recursively change the root directory, @file{/}.
9913 Without @option{--recursive}, this option has no effect.
9914 @xref{Treating / specially}.
9916 @itemx --no-preserve-root
9917 @opindex --no-preserve-root
9918 @cindex root directory, allow recursive modification
9919 Cancel the effect of any preceding @option{--preserve-root} option.
9920 @xref{Treating / specially}.
9922 @item --reference=@var{ref_file}
9923 @opindex --reference
9924 Change the group of each @var{file} to be the same as that of
9925 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9926 group of the symbolic link, but rather that of the file it refers to.
9932 Output a diagnostic for every file processed.
9933 If a symbolic link is encountered during a recursive traversal
9934 on a system without the @code{lchown} system call, and @option{--no-dereference}
9935 is in effect, then issue a diagnostic saying neither the symbolic link nor
9936 its referent is being changed.
9941 @opindex --recursive
9942 @cindex recursively changing group ownership
9943 Recursively change the group ownership of directories and their contents.
9946 @xref{Traversing symlinks}.
9949 @xref{Traversing symlinks}.
9952 @xref{Traversing symlinks}.
9961 # Change the group of /u to "staff".
9964 # Change the group of /u and subfiles to "staff".
9969 @node chmod invocation
9970 @section @command{chmod}: Change access permissions
9973 @cindex changing access permissions
9974 @cindex access permissions, changing
9975 @cindex permissions, changing access
9977 @command{chmod} changes the access permissions of the named files. Synopsis:
9980 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@}@c
9984 @cindex symbolic links, permissions of
9985 @command{chmod} never changes the permissions of symbolic links, since
9986 the @command{chmod} system call cannot change their permissions.
9987 This is not a problem since the permissions of symbolic links are
9988 never used. However, for each symbolic link listed on the command
9989 line, @command{chmod} changes the permissions of the pointed-to file.
9990 In contrast, @command{chmod} ignores symbolic links encountered during
9991 recursive directory traversals.
9993 A successful use of @command{chmod} clears the set-group-ID bit of a
9994 regular file if the file's group ID does not match the user's
9995 effective group ID or one of the user's supplementary group IDs,
9996 unless the user has appropriate privileges. Additional restrictions
9997 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9998 @var{ref_file} to be ignored. This behavior depends on the policy and
9999 functionality of the underlying @code{chmod} system call. When in
10000 doubt, check the underlying system behavior.
10002 If used, @var{mode} specifies the new file mode bits.
10003 For details, see the section on @ref{File permissions}.
10004 If you really want @var{mode} to have a leading @samp{-}, you should
10005 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
10006 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
10007 file} (without the @option{--}) complains if it behaves differently
10008 from what @samp{chmod a-w file} would do.
10010 The program accepts the following options. Also see @ref{Common options}.
10018 Verbosely describe the action for each @var{file} whose permissions
10027 @cindex error messages, omitting
10028 Do not print error messages about files whose permissions cannot be
10031 @itemx --preserve-root
10032 @opindex --preserve-root
10033 @cindex root directory, disallow recursive modification
10034 Fail upon any attempt to recursively change the root directory, @file{/}.
10035 Without @option{--recursive}, this option has no effect.
10036 @xref{Treating / specially}.
10038 @itemx --no-preserve-root
10039 @opindex --no-preserve-root
10040 @cindex root directory, allow recursive modification
10041 Cancel the effect of any preceding @option{--preserve-root} option.
10042 @xref{Treating / specially}.
10048 Verbosely describe the action or non-action taken for every @var{file}.
10050 @item --reference=@var{ref_file}
10051 @opindex --reference
10052 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10053 @xref{File permissions}.
10054 If @var{ref_file} is a symbolic link, do not use the mode
10055 of the symbolic link, but rather that of the file it refers to.
10060 @opindex --recursive
10061 @cindex recursively changing access permissions
10062 Recursively change permissions of directories and their contents.
10069 @node touch invocation
10070 @section @command{touch}: Change file timestamps
10073 @cindex changing file timestamps
10074 @cindex file timestamps, changing
10075 @cindex timestamps, changing file
10077 @command{touch} changes the access and/or modification times of the
10078 specified files. Synopsis:
10081 touch [@var{option}]@dots{} @var{file}@dots{}
10084 @cindex empty files, creating
10085 Any @var{file} argument that does not exist is created empty, unless
10086 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10087 (@option{-h}) was in effect.
10089 A @var{file} argument string of @samp{-} is handled specially and
10090 causes @command{touch} to change the times of the file associated with
10093 @cindex permissions, for changing file timestamps
10094 If changing both the access and modification times to the current
10095 time, @command{touch} can change the timestamps for files that the user
10096 running it does not own but has write permission for. Otherwise, the
10097 user must own the files.
10099 Although @command{touch} provides options for changing two of the times---the
10100 times of last access and modification---of a file, there is actually
10101 a standard third one as well: the inode change time. This is often
10102 referred to as a file's @code{ctime}.
10103 The inode change time represents the time when the file's meta-information
10104 last changed. One common example of this is when the permissions of a
10105 file change. Changing the permissions doesn't access the file, so
10106 the atime doesn't change, nor does it modify the file, so the mtime
10107 doesn't change. Yet, something about the file itself has changed,
10108 and this must be noted somewhere. This is the job of the ctime field.
10109 This is necessary, so that, for example, a backup program can make a
10110 fresh copy of the file, including the new permissions value.
10111 Another operation that modifies a file's ctime without affecting
10112 the others is renaming. In any case, it is not possible, in normal
10113 operations, for a user to change the ctime field to a user-specified value.
10114 Some operating systems and file systems support a fourth time: the
10115 birth time, when the file was first created; by definition, this
10116 timestamp never changes.
10119 Time stamps assume the time zone rules specified by the @env{TZ}
10120 environment variable, or by the system default rules if @env{TZ} is
10121 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10122 libc, The GNU C Library Reference Manual}.
10123 You can avoid ambiguities during
10124 daylight saving transitions by using @sc{utc} time stamps.
10126 The program accepts the following options. Also see @ref{Common options}.
10131 @itemx --time=atime
10132 @itemx --time=access
10136 @opindex atime@r{, changing}
10137 @opindex access @r{time, changing}
10138 @opindex use @r{time, changing}
10139 Change the access time only.
10144 @opindex --no-create
10145 Do not warn about or create files that do not exist.
10148 @itemx --date=@var{time}
10152 Use @var{time} instead of the current time. It can contain month names,
10153 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10154 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10155 specifies the instant of time that is 489,392,193 nanoseconds after
10156 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10157 minutes east of @acronym{UTC}. @xref{Date input formats}.
10158 File systems that do not support high-resolution time stamps
10159 silently ignore any excess precision here.
10163 @cindex BSD @command{touch} compatibility
10164 Ignored; for compatibility with BSD versions of @command{touch}.
10167 @itemx --no-dereference
10169 @opindex --no-dereference
10170 @cindex symbolic links, changing time
10172 Attempt to change the timestamps of a symbolic link, rather than what
10173 the link refers to. When using this option, empty files are not
10174 created, but option @option{-c} must also be used to avoid warning
10175 about files that do not exist. Not all systems support changing the
10176 timestamps of symlinks, since underlying system support for this
10177 action was not required until @acronym{POSIX} 2008. Also, on some
10178 systems, the mere act of examining a symbolic link changes the access
10179 time, such that only changes to the modification time will persist
10180 long enough to be observable. When coupled with option @option{-r}, a
10181 reference timestamp is taken from a symbolic link rather than the file
10185 @itemx --time=mtime
10186 @itemx --time=modify
10189 @opindex mtime@r{, changing}
10190 @opindex modify @r{time, changing}
10191 Change the modification time only.
10193 @item -r @var{file}
10194 @itemx --reference=@var{file}
10196 @opindex --reference
10197 Use the times of the reference @var{file} instead of the current time.
10198 If this option is combined with the @option{--date=@var{time}}
10199 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10200 the origin for any relative @var{time}s given, but is otherwise ignored.
10201 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10202 equal to five seconds before the corresponding time stamp for @file{foo}.
10203 If @var{file} is a symbolic link, the reference timestamp is taken
10204 from the target of the symlink, unless @option{-h} was also in effect.
10206 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10207 Use the argument (optional four-digit or two-digit years, months,
10208 days, hours, minutes, optional seconds) instead of the current time.
10209 If the year is specified with only two digits, then @var{cc}
10210 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10211 69 @dots{} 99. If no digits of the year are specified,
10212 the argument is interpreted as a date in the current year.
10213 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10217 @vindex _POSIX2_VERSION
10218 On older systems, @command{touch} supports an obsolete syntax, as follows.
10219 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10220 @option{-t} options, and if there are two or more @var{file}s and the
10221 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10222 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10223 any, were moved to the front), and if the represented year
10224 is in the range 1969--1999, that argument is interpreted as the time
10225 for the other files instead of as a file name.
10226 This obsolete behavior can be enabled or disabled with the
10227 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10228 conformance}), but portable scripts should avoid commands whose
10229 behavior depends on this variable.
10230 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10231 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10237 @chapter Disk usage
10241 No disk can hold an infinite amount of data. These commands report
10242 how much disk storage is in use or available, report other file and
10243 file status information, and write buffers to disk.
10246 * df invocation:: Report file system disk space usage.
10247 * du invocation:: Estimate file space usage.
10248 * stat invocation:: Report file or file system status.
10249 * sync invocation:: Synchronize memory and disk.
10250 * truncate invocation:: Shrink or extend the size of a file.
10254 @node df invocation
10255 @section @command{df}: Report file system disk space usage
10258 @cindex file system disk usage
10259 @cindex disk usage by file system
10261 @command{df} reports the amount of disk space used and available on
10262 file systems. Synopsis:
10265 df [@var{option}]@dots{} [@var{file}]@dots{}
10268 With no arguments, @command{df} reports the space used and available on all
10269 currently mounted file systems (of all types). Otherwise, @command{df}
10270 reports on the file system containing each argument @var{file}.
10272 Normally the disk space is printed in units of
10273 1024 bytes, but this can be overridden (@pxref{Block size}).
10274 Non-integer quantities are rounded up to the next higher unit.
10276 @cindex disk device file
10277 @cindex device file, disk
10278 If an argument @var{file} is a disk device file containing a mounted
10279 file system, @command{df} shows the space available on that file system
10280 rather than on the file system containing the device node (i.e., the root
10281 file system). @sc{gnu} @command{df} does not attempt to determine the
10283 on unmounted file systems, because on most kinds of systems doing so
10284 requires extremely nonportable intimate knowledge of file system
10287 The program accepts the following options. Also see @ref{Common options}.
10295 @cindex automounter file systems
10296 @cindex ignore file systems
10297 Include in the listing dummy file systems, which
10298 are omitted by default. Such file systems are typically special-purpose
10299 pseudo-file-systems, such as automounter entries.
10301 @item -B @var{size}
10302 @itemx --block-size=@var{size}
10304 @opindex --block-size
10305 @cindex file system sizes
10306 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10307 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10311 @cindex grand total of disk size, usage and available space
10312 Print a grand total of all arguments after all arguments have
10313 been processed. This can be used to find out the total disk size, usage
10314 and available space of all listed devices.
10320 Equivalent to @option{--si}.
10326 @cindex inode usage
10327 List inode usage information instead of block usage. An inode (short
10328 for index node) contains information about a file such as its owner,
10329 permissions, timestamps, and location on the disk.
10333 @cindex kibibytes for file system sizes
10334 Print sizes in 1024-byte blocks, overriding the default block size
10335 (@pxref{Block size}).
10336 This option is equivalent to @option{--block-size=1K}.
10342 @cindex file system types, limiting output to certain
10343 Limit the listing to local file systems. By default, remote file systems
10348 @cindex file system space, retrieving old data more quickly
10349 Do not invoke the @code{sync} system call before getting any usage data.
10350 This may make @command{df} run significantly faster on systems with many
10351 disks, but on some systems (notably SunOS) the results may be slightly
10352 out of date. This is the default.
10355 @itemx --portability
10357 @opindex --portability
10358 @cindex one-line output format
10359 @cindex @acronym{POSIX} output format
10360 @cindex portable output format
10361 @cindex output format, portable
10362 Use the @acronym{POSIX} output format. This is like the default format except
10367 The information about each file system is always printed on exactly
10368 one line; a mount device is never put on a line by itself. This means
10369 that if the mount device name is more than 20 characters long (e.g., for
10370 some network mounts), the columns are misaligned.
10373 The labels in the header output line are changed to conform to @acronym{POSIX}.
10376 The default block size and output format are unaffected by the
10377 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10378 variables. However, the default block size is still affected by
10379 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10380 otherwise. @xref{Block size}.
10387 @cindex file system space, retrieving current data more slowly
10388 Invoke the @code{sync} system call before getting any usage data. On
10389 some systems (notably SunOS), doing this yields more up to date results,
10390 but in general this option makes @command{df} much slower, especially when
10391 there are many or very busy file systems.
10393 @item -t @var{fstype}
10394 @itemx --type=@var{fstype}
10397 @cindex file system types, limiting output to certain
10398 Limit the listing to file systems of type @var{fstype}. Multiple
10399 file system types can be specified by giving multiple @option{-t} options.
10400 By default, nothing is omitted.
10403 @itemx --print-type
10405 @opindex --print-type
10406 @cindex file system types, printing
10407 Print each file system's type. The types printed here are the same ones
10408 you can include or exclude with @option{-t} and @option{-x}. The particular
10409 types printed are whatever is supported by the system. Here are some of
10410 the common names (this list is certainly not exhaustive):
10415 @cindex @acronym{NFS} file system type
10416 An @acronym{NFS} file system, i.e., one mounted over a network from another
10417 machine. This is the one type name which seems to be used uniformly by
10420 @item 4.2@r{, }ufs@r{, }efs@dots{}
10421 @cindex Linux file system types
10422 @cindex local file system types
10423 @opindex 4.2 @r{file system type}
10424 @opindex ufs @r{file system type}
10425 @opindex efs @r{file system type}
10426 A file system on a locally-mounted hard disk. (The system might even
10427 support more than one type here; Linux does.)
10429 @item hsfs@r{, }cdfs
10430 @cindex CD-ROM file system type
10431 @cindex High Sierra file system
10432 @opindex hsfs @r{file system type}
10433 @opindex cdfs @r{file system type}
10434 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10435 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10438 @cindex PC file system
10439 @cindex DOS file system
10440 @cindex MS-DOS file system
10441 @cindex diskette file system
10443 An MS-DOS file system, usually on a diskette.
10447 @item -x @var{fstype}
10448 @itemx --exclude-type=@var{fstype}
10450 @opindex --exclude-type
10451 Limit the listing to file systems not of type @var{fstype}.
10452 Multiple file system types can be eliminated by giving multiple
10453 @option{-x} options. By default, no file system types are omitted.
10456 Ignored; for compatibility with System V versions of @command{df}.
10461 Failure includes the case where no output is generated, so you can
10462 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10463 @var{dir}} to test whether @var{dir} is on a file system of type
10464 @samp{ext3} or @samp{reiserfs}.
10467 @node du invocation
10468 @section @command{du}: Estimate file space usage
10471 @cindex file space usage
10472 @cindex disk usage for files
10474 @command{du} reports the amount of disk space used by the specified files
10475 and for each subdirectory (of directory arguments). Synopsis:
10478 du [@var{option}]@dots{} [@var{file}]@dots{}
10481 With no arguments, @command{du} reports the disk space for the current
10482 directory. Normally the disk space is printed in units of
10483 1024 bytes, but this can be overridden (@pxref{Block size}).
10484 Non-integer quantities are rounded up to the next higher unit.
10486 If two or more hard links point to the same file, only one of the hard
10487 links is counted. The @var{file} argument order affects which links
10488 are counted, and changing the argument order may change the numbers
10489 that @command{du} outputs.
10491 The program accepts the following options. Also see @ref{Common options}.
10499 Show counts for all files, not just directories.
10501 @itemx --apparent-size
10502 @opindex --apparent-size
10503 Print apparent sizes, rather than disk usage. The apparent size of a
10504 file is the number of bytes reported by @code{wc -c} on regular files,
10505 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10506 For example, a file containing the word @samp{zoo} with no newline would,
10507 of course, have an apparent size of 3. Such a small file may require
10508 anywhere from 0 to 16 KiB or more of disk space, depending on
10509 the type and configuration of the file system on which the file resides.
10510 However, a sparse file created with this command:
10513 dd bs=1 seek=2GiB if=/dev/null of=big
10517 has an apparent size of 2 GiB, yet on most modern
10518 systems, it actually uses almost no disk space.
10524 Equivalent to @code{--apparent-size --block-size=1}.
10526 @item -B @var{size}
10527 @itemx --block-size=@var{size}
10529 @opindex --block-size
10531 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10532 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10538 @cindex grand total of disk space
10539 Print a grand total of all arguments after all arguments have
10540 been processed. This can be used to find out the total disk usage of
10541 a given set of files or directories.
10544 @itemx --dereference-args
10546 @opindex --dereference-args
10547 Dereference symbolic links that are command line arguments.
10548 Does not affect other symbolic links. This is helpful for finding
10549 out the disk usage of directories, such as @file{/usr/tmp}, which
10550 are often symbolic links.
10552 @c --files0-from=FILE
10553 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10559 Equivalent to @option{--dereference-args} (@option{-D}).
10563 @cindex kibibytes for file sizes
10564 Print sizes in 1024-byte blocks, overriding the default block size
10565 (@pxref{Block size}).
10566 This option is equivalent to @option{--block-size=1K}.
10569 @itemx --count-links
10571 @opindex --count-links
10572 @cindex hard links, counting in @command{du}
10573 Count the size of all files, even if they have appeared already (as a
10577 @itemx --dereference
10579 @opindex --dereference
10580 @cindex symbolic links, dereferencing in @command{du}
10581 Dereference symbolic links (show the disk space used by the file
10582 or directory that the link points to instead of the space used by
10587 @cindex mebibytes for file sizes
10588 Print sizes in 1,048,576-byte blocks, overriding the default block size
10589 (@pxref{Block size}).
10590 This option is equivalent to @option{--block-size=1M}.
10593 @itemx --no-dereference
10595 @opindex --no-dereference
10596 @cindex symbolic links, dereferencing in @command{du}
10597 For each symbolic links encountered by @command{du},
10598 consider the disk space used by the symbolic link.
10600 @item -d @var{depth}
10601 @item --max-depth=@var{depth}
10602 @opindex -d @var{depth}
10603 @opindex --max-depth=@var{depth}
10604 @cindex limiting output of @command{du}
10605 Show the total for each directory (and file if --all) that is at
10606 most MAX_DEPTH levels down from the root of the hierarchy. The root
10607 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10616 @opindex --summarize
10617 Display only a total for each argument.
10620 @itemx --separate-dirs
10622 @opindex --separate-dirs
10623 Normally, in the output of @command{du} (when not using @option{--summarize}),
10624 the size listed next to a directory name, @var{d}, represents the sum
10625 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10626 With @option{--separate-dirs}, the size reported for a directory name,
10627 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10632 @cindex last modified dates, displaying in @command{du}
10633 Show time of the most recent modification of any file in the directory,
10634 or any of its subdirectories.
10636 @itemx --time=ctime
10637 @itemx --time=status
10640 @opindex ctime@r{, show the most recent}
10641 @opindex status time@r{, show the most recent}
10642 @opindex use time@r{, show the most recent}
10643 Show the most recent status change time (the @samp{ctime} in the inode) of
10644 any file in the directory, instead of the modification time.
10646 @itemx --time=atime
10647 @itemx --time=access
10649 @opindex atime@r{, show the most recent}
10650 @opindex access time@r{, show the most recent}
10651 Show the most recent access time (the @samp{atime} in the inode) of
10652 any file in the directory, instead of the modification time.
10654 @item --time-style=@var{style}
10655 @opindex --time-style
10657 List timestamps in style @var{style}. This option has an effect only if
10658 the @option{--time} option is also specified. The @var{style} should
10659 be one of the following:
10662 @item +@var{format}
10664 List timestamps using @var{format}, where @var{format} is interpreted
10665 like the format argument of @command{date} (@pxref{date invocation}).
10666 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10667 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10668 with @command{date}, @var{format}'s interpretation is affected by the
10669 @env{LC_TIME} locale category.
10672 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10673 format with nanosecond precision, e.g., @samp{2002-03-30
10674 23:45:56.477817180 -0700}. This style is equivalent to
10675 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10678 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10679 @samp{2002-03-30 23:45}. These timestamps are shorter than
10680 @samp{full-iso} timestamps, and are usually good enough for everyday
10681 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10684 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10685 This style is equivalent to @samp{+%Y-%m-%d}.
10689 You can specify the default value of the @option{--time-style} option
10690 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10691 the default style is @samp{long-iso}. For compatibility with @command{ls},
10692 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10693 the newline and any later characters are ignored; if @env{TIME_STYLE}
10694 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10695 @env{TIME_STYLE} is @samp{locale} it is ignored.
10698 @itemx --one-file-system
10700 @opindex --one-file-system
10701 @cindex one file system, restricting @command{du} to
10702 Skip directories that are on different file systems from the one that
10703 the argument being processed is on.
10705 @item --exclude=@var{pattern}
10706 @opindex --exclude=@var{pattern}
10707 @cindex excluding files from @command{du}
10708 When recursing, skip subdirectories or files matching @var{pattern}.
10709 For example, @code{du --exclude='*.o'} excludes files whose names
10712 @item -X @var{file}
10713 @itemx --exclude-from=@var{file}
10714 @opindex -X @var{file}
10715 @opindex --exclude-from=@var{file}
10716 @cindex excluding files from @command{du}
10717 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10718 one per line. If @var{file} is @samp{-}, take the patterns from standard
10723 @cindex NFS mounts from BSD to HP-UX
10724 On BSD systems, @command{du} reports sizes that are half the correct
10725 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10726 systems, it reports sizes that are twice the correct values for
10727 files that are NFS-mounted from BSD systems. This is due to a flaw
10728 in HP-UX; it also affects the HP-UX @command{du} program.
10733 @node stat invocation
10734 @section @command{stat}: Report file or file system status
10737 @cindex file status
10738 @cindex file system status
10740 @command{stat} displays information about the specified file(s). Synopsis:
10743 stat [@var{option}]@dots{} [@var{file}]@dots{}
10746 With no option, @command{stat} reports all information about the given files.
10747 But it also can be used to report the information of the file systems the
10748 given files are located on. If the files are links, @command{stat} can
10749 also give information about the files the links point to.
10751 @mayConflictWithShellBuiltIn{stat}
10756 @itemx --dereference
10758 @opindex --dereference
10759 @cindex symbolic links, dereferencing in @command{stat}
10760 Change how @command{stat} treats symbolic links.
10761 With this option, @command{stat} acts on the file referenced
10762 by each symbolic link argument.
10763 Without it, @command{stat} acts on any symbolic link argument directly.
10766 @itemx --file-system
10768 @opindex --file-system
10769 @cindex file systems
10770 Report information about the file systems where the given files are located
10771 instead of information about the files themselves.
10772 This option implies the @option{-L} option.
10775 @itemx --format=@var{format}
10777 @opindex --format=@var{format}
10778 @cindex output format
10779 Use @var{format} rather than the default format.
10780 @var{format} is automatically newline-terminated, so
10781 running a command like the following with two or more @var{file}
10782 operands produces a line of output for each operand:
10784 $ stat --format=%d:%i / /usr
10789 @itemx --printf=@var{format}
10790 @opindex --printf=@var{format}
10791 @cindex output format
10792 Use @var{format} rather than the default format.
10793 Like @option{--format}, but interpret backslash escapes,
10794 and do not output a mandatory trailing newline.
10795 If you want a newline, include @samp{\n} in the @var{format}.
10796 Here's how you would use @option{--printf} to print the device
10797 and inode numbers of @file{/} and @file{/usr}:
10799 $ stat --printf='%d:%i\n' / /usr
10808 @cindex terse output
10809 Print the information in terse form, suitable for parsing by other programs.
10813 The valid @var{format} directives for files with @option{--format} and
10814 @option{--printf} are:
10817 @item %a - Access rights in octal
10818 @item %A - Access rights in human readable form
10819 @item %b - Number of blocks allocated (see @samp{%B})
10820 @item %B - The size in bytes of each block reported by @samp{%b}
10821 @item %C - The SELinux security context of a file, if available
10822 @item %d - Device number in decimal
10823 @item %D - Device number in hex
10824 @item %f - Raw mode in hex
10825 @item %F - File type
10826 @item %g - Group ID of owner
10827 @item %G - Group name of owner
10828 @item %h - Number of hard links
10829 @item %i - Inode number
10830 @item %m - Mount point (See note below)
10831 @item %n - File name
10832 @item %N - Quoted file name with dereference if symbolic link
10833 @item %o - I/O block size
10834 @item %s - Total size, in bytes
10835 @item %t - Major device type in hex
10836 @item %T - Minor device type in hex
10837 @item %u - User ID of owner
10838 @item %U - User name of owner
10839 @item %w - Time of file birth, or @samp{-} if unknown
10840 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10841 @item %x - Time of last access
10842 @item %X - Time of last access as seconds since Epoch
10843 @item %y - Time of last modification
10844 @item %Y - Time of last modification as seconds since Epoch
10845 @item %z - Time of last change
10846 @item %Z - Time of last change as seconds since Epoch
10849 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10850 precision preceded by a period to specify the number of digits to
10851 print after the decimal point. For example, @samp{%.3X} outputs the
10852 last access time to millisecond precision. If a period is given but no
10853 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10854 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10855 toward minus infinity.
10859 $ stat -c '[%015Y]' /usr
10862 $ stat -c '[%15Y]' /usr
10864 $ stat -c '[%-15Y]' /usr
10867 $ stat -c '[%.3Y]' /usr
10869 $ stat -c '[%.Y]' /usr
10870 [1288929712.114951834]
10873 The mount point printed by @samp{%m} is similar to that output
10874 by @command{df}, except that:
10877 stat does not dereference symlinks by default
10878 (unless @option{-L} is specified)
10880 stat does not search for specified device nodes in the
10881 file system list, instead operating on them directly
10884 stat outputs the alias for a bind mounted file, rather than
10885 the initial mount point of its backing device.
10886 One can recursively call stat until there is no change in output,
10887 to get the current base mount point
10890 When listing file system information (@option{--file-system} (@option{-f})),
10891 you must use a different set of @var{format} directives:
10894 @item %a - Free blocks available to non-super-user
10895 @item %b - Total data blocks in file system
10896 @item %c - Total file nodes in file system
10897 @item %d - Free file nodes in file system
10898 @item %f - Free blocks in file system
10899 @item %i - File System ID in hex
10900 @item %l - Maximum length of file names
10901 @item %n - File name
10902 @item %s - Block size (for faster transfers)
10903 @item %S - Fundamental block size (for block counts)
10904 @item %t - Type in hex
10905 @item %T - Type in human readable form
10909 Time stamps are listed according to the time zone rules specified by
10910 the @env{TZ} environment variable, or by the system default rules if
10911 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10912 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10917 @node sync invocation
10918 @section @command{sync}: Synchronize data on disk with memory
10921 @cindex synchronize disk and memory
10923 @cindex superblock, writing
10924 @cindex inodes, written buffered
10925 @command{sync} writes any data buffered in memory out to disk. This can
10926 include (but is not limited to) modified superblocks, modified inodes,
10927 and delayed reads and writes. This must be implemented by the kernel;
10928 The @command{sync} program does nothing but exercise the @code{sync} system
10931 @cindex crashes and corruption
10932 The kernel keeps data in memory to avoid doing (relatively slow) disk
10933 reads and writes. This improves performance, but if the computer
10934 crashes, data may be lost or the file system corrupted as a
10935 result. The @command{sync} command ensures everything in memory
10936 is written to disk.
10938 Any arguments are ignored, except for a lone @option{--help} or
10939 @option{--version} (@pxref{Common options}).
10944 @node truncate invocation
10945 @section @command{truncate}: Shrink or extend the size of a file
10948 @cindex truncating, file sizes
10950 @command{truncate} shrinks or extends the size of each @var{file} to the
10951 specified size. Synopsis:
10954 truncate @var{option}@dots{} @var{file}@dots{}
10957 @cindex files, creating
10958 Any @var{file} that does not exist is created.
10960 @cindex sparse files, creating
10961 @cindex holes, creating files with
10962 If a @var{file} is larger than the specified size, the extra data is lost.
10963 If a @var{file} is shorter, it is extended and the extended part (or hole)
10964 reads as zero bytes.
10966 The program accepts the following options. Also see @ref{Common options}.
10973 @opindex --no-create
10974 Do not create files that do not exist.
10979 @opindex --io-blocks
10980 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10982 @item -r @var{rfile}
10983 @itemx --reference=@var{rfile}
10985 @opindex --reference
10986 Base the size of each @var{file} on the size of @var{rfile}.
10988 @item -s @var{size}
10989 @itemx --size=@var{size}
10992 Set or adjust the size of each @var{file} according to @var{size}.
10993 @multiplierSuffixesNoBlocks{size}
10995 @var{size} may also be prefixed by one of the following to adjust
10996 the size of each @var{file} based on their current size:
10998 @samp{+} => extend by
10999 @samp{-} => reduce by
11000 @samp{<} => at most
11001 @samp{>} => at least
11002 @samp{/} => round down to multiple of
11003 @samp{%} => round up to multiple of
11011 @node Printing text
11012 @chapter Printing text
11014 @cindex printing text, commands for
11015 @cindex commands for printing text
11017 This section describes commands that display text strings.
11020 * echo invocation:: Print a line of text.
11021 * printf invocation:: Format and print data.
11022 * yes invocation:: Print a string until interrupted.
11026 @node echo invocation
11027 @section @command{echo}: Print a line of text
11030 @cindex displaying text
11031 @cindex printing text
11032 @cindex text, displaying
11033 @cindex arbitrary text, displaying
11035 @command{echo} writes each given @var{string} to standard output, with a
11036 space between each and a newline after the last one. Synopsis:
11039 echo [@var{option}]@dots{} [@var{string}]@dots{}
11042 @mayConflictWithShellBuiltIn{echo}
11044 The program accepts the following options. Also see @ref{Common options}.
11045 Options must precede operands, and the normally-special argument
11046 @samp{--} has no special meaning and is treated like any other
11052 Do not output the trailing newline.
11056 @cindex backslash escapes
11057 Enable interpretation of the following backslash-escaped characters in
11066 produce no further output
11082 the eight-bit value that is the octal number @var{nnn}
11083 (zero to three octal digits)
11085 the eight-bit value that is the octal number @var{nnn}
11086 (one to three octal digits)
11088 the eight-bit value that is the hexadecimal number @var{hh}
11089 (one or two hexadecimal digits)
11094 @cindex backslash escapes
11095 Disable interpretation of backslash escapes in each @var{string}.
11096 This is the default. If @option{-e} and @option{-E} are both
11097 specified, the last one given takes effect.
11101 @vindex POSIXLY_CORRECT
11102 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11103 @command{echo}'s first argument is not @option{-n} it outputs
11104 option-like arguments instead of treating them as options. For
11105 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11106 plain @samp{hello}.
11108 @acronym{POSIX} does not require support for any options, and says
11109 that the behavior of @command{echo} is implementation-defined if any
11110 @var{string} contains a backslash or if the first argument is
11111 @option{-n}. Portable programs can use the @command{printf} command
11112 if they need to omit trailing newlines or output control characters or
11113 backslashes. @xref{printf invocation}.
11118 @node printf invocation
11119 @section @command{printf}: Format and print data
11122 @command{printf} does formatted printing of text. Synopsis:
11125 printf @var{format} [@var{argument}]@dots{}
11128 @command{printf} prints the @var{format} string, interpreting @samp{%}
11129 directives and @samp{\} escapes to format numeric and string arguments
11130 in a way that is mostly similar to the C @samp{printf} function.
11131 @xref{Output Conversion Syntax,, @command{printf} format directives,
11132 libc, The GNU C Library Reference Manual}, for details.
11133 The differences are listed below.
11135 @mayConflictWithShellBuiltIn{printf}
11140 The @var{format} argument is reused as necessary to convert all the
11141 given @var{argument}s. For example, the command @samp{printf %s a b}
11145 Missing @var{argument}s are treated as null strings or as zeros,
11146 depending on whether the context expects a string or a number. For
11147 example, the command @samp{printf %sx%d} prints @samp{x0}.
11151 An additional escape, @samp{\c}, causes @command{printf} to produce no
11152 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11153 E} prints @samp{ABC}.
11156 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11157 digits, as opposed to C where it can have an unlimited number of
11158 digits. For example, the command @samp{printf '\x07e'} prints two
11159 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11164 @command{printf} has an additional directive, @samp{%b}, which prints its
11165 argument string with @samp{\} escapes interpreted in the same way as in
11166 the @var{format} string, except that octal escapes are of the form
11167 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11168 If a precision is also given, it limits the number of bytes printed
11169 from the converted string.
11172 Numeric arguments must be single C constants, possibly with leading
11173 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11177 @vindex POSIXLY_CORRECT
11178 If the leading character of a numeric argument is @samp{"} or @samp{'}
11179 then its value is the numeric value of the immediately following
11180 character. Any remaining characters are silently ignored if the
11181 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11182 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11183 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11184 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11189 A floating-point argument must use a period before any fractional
11190 digits, but is printed according to the @env{LC_NUMERIC} category of the
11191 current locale. For example, in a locale whose radix character is a
11192 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11193 the command @samp{printf %g 3,14} is an error.
11197 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11198 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11199 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11200 digits) specifying a character to print.
11205 @cindex ISO/IEC 10646
11207 @command{printf} interprets two character syntaxes introduced in
11208 @acronym{ISO} C 99:
11209 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11210 characters, specified as
11211 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11212 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11213 @command{printf} outputs the Unicode characters
11214 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11215 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11216 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11218 The processing of @samp{\u} and @samp{\U} requires a full-featured
11219 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11220 or when @code{libiconv} is installed prior to this package. Otherwise
11221 @samp{\u} and @samp{\U} will print as-is.
11223 The only options are a lone @option{--help} or
11224 @option{--version}. @xref{Common options}.
11225 Options must precede operands.
11227 The Unicode character syntaxes are useful for writing strings in a locale
11228 independent way. For example, a string containing the Euro currency symbol
11231 $ env printf '\u20AC 14.95'
11235 will be output correctly in all locales supporting the Euro symbol
11236 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11239 $ env printf '\u4e2d\u6587'
11243 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11245 Note that in these examples, the @command{printf} command has been
11246 invoked via @command{env} to ensure that we run the program found via
11247 your shell's search path, and not a shell alias or a built-in function.
11249 For larger strings, you don't need to look up the hexadecimal code
11250 values of each character one by one. @acronym{ASCII} characters mixed with \u
11251 escape sequences is also known as the JAVA source file encoding. You can
11252 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11253 is how to convert a piece of text into a shell script which will output
11254 this text in a locale-independent way:
11257 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11258 '\u4e2d\u6587\n' > sample.txt
11259 $ recode BIG5..JAVA < sample.txt \
11260 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11267 @node yes invocation
11268 @section @command{yes}: Print a string until interrupted
11271 @cindex repeated output of a string
11273 @command{yes} prints the command line arguments, separated by spaces and
11274 followed by a newline, forever until it is killed. If no arguments are
11275 given, it prints @samp{y} followed by a newline forever until killed.
11277 Upon a write error, @command{yes} exits with status @samp{1}.
11279 The only options are a lone @option{--help} or @option{--version}.
11280 To output an argument that begins with
11281 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11282 @xref{Common options}.
11286 @chapter Conditions
11289 @cindex commands for exit status
11290 @cindex exit status commands
11292 This section describes commands that are primarily useful for their exit
11293 status, rather than their output. Thus, they are often used as the
11294 condition of shell @code{if} statements, or as the last command in a
11298 * false invocation:: Do nothing, unsuccessfully.
11299 * true invocation:: Do nothing, successfully.
11300 * test invocation:: Check file types and compare values.
11301 * expr invocation:: Evaluate expressions.
11305 @node false invocation
11306 @section @command{false}: Do nothing, unsuccessfully
11309 @cindex do nothing, unsuccessfully
11310 @cindex failure exit status
11311 @cindex exit status of @command{false}
11313 @command{false} does nothing except return an exit status of 1, meaning
11314 @dfn{failure}. It can be used as a place holder in shell scripts
11315 where an unsuccessful command is needed.
11316 In most modern shells, @command{false} is a built-in command, so when
11317 you use @samp{false} in a script, you're probably using the built-in
11318 command, not the one documented here.
11320 @command{false} honors the @option{--help} and @option{--version} options.
11322 This version of @command{false} is implemented as a C program, and is thus
11323 more secure and faster than a shell script implementation, and may safely
11324 be used as a dummy shell for the purpose of disabling accounts.
11326 Note that @command{false} (unlike all other programs documented herein)
11327 exits unsuccessfully, even when invoked with
11328 @option{--help} or @option{--version}.
11330 Portable programs should not assume that the exit status of
11331 @command{false} is 1, as it is greater than 1 on some
11332 non-@acronym{GNU} hosts.
11335 @node true invocation
11336 @section @command{true}: Do nothing, successfully
11339 @cindex do nothing, successfully
11341 @cindex successful exit
11342 @cindex exit status of @command{true}
11344 @command{true} does nothing except return an exit status of 0, meaning
11345 @dfn{success}. It can be used as a place holder in shell scripts
11346 where a successful command is needed, although the shell built-in
11347 command @code{:} (colon) may do the same thing faster.
11348 In most modern shells, @command{true} is a built-in command, so when
11349 you use @samp{true} in a script, you're probably using the built-in
11350 command, not the one documented here.
11352 @command{true} honors the @option{--help} and @option{--version} options.
11354 Note, however, that it is possible to cause @command{true}
11355 to exit with nonzero status: with the @option{--help} or @option{--version}
11356 option, and with standard
11357 output already closed or redirected to a file that evokes an I/O error.
11358 For example, using a Bourne-compatible shell:
11361 $ ./true --version >&-
11362 ./true: write error: Bad file number
11363 $ ./true --version > /dev/full
11364 ./true: write error: No space left on device
11367 This version of @command{true} is implemented as a C program, and is thus
11368 more secure and faster than a shell script implementation, and may safely
11369 be used as a dummy shell for the purpose of disabling accounts.
11371 @node test invocation
11372 @section @command{test}: Check file types and compare values
11375 @cindex check file types
11376 @cindex compare values
11377 @cindex expression evaluation
11379 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11380 evaluation of the conditional expression @var{expr}. Each part of the
11381 expression must be a separate argument.
11383 @command{test} has file status checks, string operators, and numeric
11384 comparison operators.
11386 @command{test} has an alternate form that uses opening and closing
11387 square brackets instead a leading @samp{test}. For example, instead
11388 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11389 brackets must be separate arguments; for example, @samp{[-d /]} does
11390 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11391 @var{expr} ]} have the same meaning, only the former form is discussed
11397 test @var{expression}
11399 [ @var{expression} ]
11404 @mayConflictWithShellBuiltIn{test}
11406 If @var{expression} is omitted, @command{test} returns false.
11407 If @var{expression} is a single argument,
11408 @command{test} returns false if the argument is null and true
11409 otherwise. The argument
11410 can be any string, including strings like @samp{-d}, @samp{-1},
11411 @samp{--}, @samp{--help}, and @samp{--version} that most other
11412 programs would treat as options. To get help and version information,
11413 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11414 the usual closing brackets. @xref{Common options}.
11416 @cindex exit status of @command{test}
11420 0 if the expression is true,
11421 1 if the expression is false,
11422 2 if an error occurred.
11426 * File type tests:: -[bcdfhLpSt]
11427 * Access permission tests:: -[gkruwxOG]
11428 * File characteristic tests:: -e -s -nt -ot -ef
11429 * String tests:: -z -n = !=
11430 * Numeric tests:: -eq -ne -lt -le -gt -ge
11431 * Connectives for test:: ! -a -o
11435 @node File type tests
11436 @subsection File type tests
11438 @cindex file type tests
11440 These options test for particular types of files. (Everything's a file,
11441 but not all files are the same!)
11445 @item -b @var{file}
11447 @cindex block special check
11448 True if @var{file} exists and is a block special device.
11450 @item -c @var{file}
11452 @cindex character special check
11453 True if @var{file} exists and is a character special device.
11455 @item -d @var{file}
11457 @cindex directory check
11458 True if @var{file} exists and is a directory.
11460 @item -f @var{file}
11462 @cindex regular file check
11463 True if @var{file} exists and is a regular file.
11465 @item -h @var{file}
11466 @itemx -L @var{file}
11469 @cindex symbolic link check
11470 True if @var{file} exists and is a symbolic link.
11471 Unlike all other file-related tests, this test does not dereference
11472 @var{file} if it is a symbolic link.
11474 @item -p @var{file}
11476 @cindex named pipe check
11477 True if @var{file} exists and is a named pipe.
11479 @item -S @var{file}
11481 @cindex socket check
11482 True if @var{file} exists and is a socket.
11486 @cindex terminal check
11487 True if @var{fd} is a file descriptor that is associated with a
11493 @node Access permission tests
11494 @subsection Access permission tests
11496 @cindex access permission tests
11497 @cindex permission tests
11499 These options test for particular access permissions.
11503 @item -g @var{file}
11505 @cindex set-group-ID check
11506 True if @var{file} exists and has its set-group-ID bit set.
11508 @item -k @var{file}
11510 @cindex sticky bit check
11511 True if @var{file} exists and has its @dfn{sticky} bit set.
11513 @item -r @var{file}
11515 @cindex readable file check
11516 True if @var{file} exists and read permission is granted.
11518 @item -u @var{file}
11520 @cindex set-user-ID check
11521 True if @var{file} exists and has its set-user-ID bit set.
11523 @item -w @var{file}
11525 @cindex writable file check
11526 True if @var{file} exists and write permission is granted.
11528 @item -x @var{file}
11530 @cindex executable file check
11531 True if @var{file} exists and execute permission is granted
11532 (or search permission, if it is a directory).
11534 @item -O @var{file}
11536 @cindex owned by effective user ID check
11537 True if @var{file} exists and is owned by the current effective user ID.
11539 @item -G @var{file}
11541 @cindex owned by effective group ID check
11542 True if @var{file} exists and is owned by the current effective group ID.
11546 @node File characteristic tests
11547 @subsection File characteristic tests
11549 @cindex file characteristic tests
11551 These options test other file characteristics.
11555 @item -e @var{file}
11557 @cindex existence-of-file check
11558 True if @var{file} exists.
11560 @item -s @var{file}
11562 @cindex nonempty file check
11563 True if @var{file} exists and has a size greater than zero.
11565 @item @var{file1} -nt @var{file2}
11567 @cindex newer-than file check
11568 True if @var{file1} is newer (according to modification date) than
11569 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11571 @item @var{file1} -ot @var{file2}
11573 @cindex older-than file check
11574 True if @var{file1} is older (according to modification date) than
11575 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11577 @item @var{file1} -ef @var{file2}
11579 @cindex same file check
11580 @cindex hard link check
11581 True if @var{file1} and @var{file2} have the same device and inode
11582 numbers, i.e., if they are hard links to each other.
11588 @subsection String tests
11590 @cindex string tests
11592 These options test string characteristics. You may need to quote
11593 @var{string} arguments for the shell. For example:
11599 The quotes here prevent the wrong arguments from being passed to
11600 @command{test} if @samp{$V} is empty or contains special characters.
11604 @item -z @var{string}
11606 @cindex zero-length string check
11607 True if the length of @var{string} is zero.
11609 @item -n @var{string}
11610 @itemx @var{string}
11612 @cindex nonzero-length string check
11613 True if the length of @var{string} is nonzero.
11615 @item @var{string1} = @var{string2}
11617 @cindex equal string check
11618 True if the strings are equal.
11620 @item @var{string1} != @var{string2}
11622 @cindex not-equal string check
11623 True if the strings are not equal.
11628 @node Numeric tests
11629 @subsection Numeric tests
11631 @cindex numeric tests
11632 @cindex arithmetic tests
11634 Numeric relational operators. The arguments must be entirely numeric
11635 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11636 which evaluates to the length of @var{string}.
11640 @item @var{arg1} -eq @var{arg2}
11641 @itemx @var{arg1} -ne @var{arg2}
11642 @itemx @var{arg1} -lt @var{arg2}
11643 @itemx @var{arg1} -le @var{arg2}
11644 @itemx @var{arg1} -gt @var{arg2}
11645 @itemx @var{arg1} -ge @var{arg2}
11652 These arithmetic binary operators return true if @var{arg1} is equal,
11653 not-equal, less-than, less-than-or-equal, greater-than, or
11654 greater-than-or-equal than @var{arg2}, respectively.
11661 test -1 -gt -2 && echo yes
11663 test -l abc -gt 1 && echo yes
11666 @error{} test: integer expression expected before -eq
11670 @node Connectives for test
11671 @subsection Connectives for @command{test}
11673 @cindex logical connectives
11674 @cindex connectives, logical
11676 The usual logical connectives.
11682 True if @var{expr} is false.
11684 @item @var{expr1} -a @var{expr2}
11686 @cindex logical and operator
11687 @cindex and operator
11688 True if both @var{expr1} and @var{expr2} are true.
11690 @item @var{expr1} -o @var{expr2}
11692 @cindex logical or operator
11693 @cindex or operator
11694 True if either @var{expr1} or @var{expr2} is true.
11699 @node expr invocation
11700 @section @command{expr}: Evaluate expressions
11703 @cindex expression evaluation
11704 @cindex evaluation of expressions
11706 @command{expr} evaluates an expression and writes the result on standard
11707 output. Each token of the expression must be a separate argument.
11709 Operands are either integers or strings. Integers consist of one or
11710 more decimal digits, with an optional leading @samp{-}.
11711 @command{expr} converts
11712 anything appearing in an operand position to an integer or a string
11713 depending on the operation being applied to it.
11715 Strings are not quoted for @command{expr} itself, though you may need to
11716 quote them to protect characters with special meaning to the shell,
11717 e.g., spaces. However, regardless of whether it is quoted, a string
11718 operand should not be a parenthesis or any of @command{expr}'s
11719 operators like @code{+}, so you cannot safely pass an arbitrary string
11720 @code{$str} to expr merely by quoting it to the shell. One way to
11721 work around this is to use the @sc{gnu} extension @code{+},
11722 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11723 @code{@w{" $str"}} and to adjust the rest of the expression to take
11724 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11726 You should not pass a negative integer or a string with leading
11727 @samp{-} as @command{expr}'s first argument, as it might be
11728 misinterpreted as an option; this can be avoided by parenthesization.
11729 Also, portable scripts should not use a string operand that happens to
11730 take the form of an integer; this can be worked around by inserting
11731 leading spaces as mentioned above.
11733 @cindex parentheses for grouping
11734 Operators may be given as infix symbols or prefix keywords. Parentheses
11735 may be used for grouping in the usual manner. You must quote
11736 parentheses and many operators to avoid the shell evaluating them,
11739 When built with support for the GNU MP library, @command{expr} uses
11740 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11741 types and may fail due to arithmetic overflow.
11743 The only options are @option{--help} and @option{--version}. @xref{Common
11744 options}. Options must precede operands.
11746 @cindex exit status of @command{expr}
11750 0 if the expression is neither null nor 0,
11751 1 if the expression is null or 0,
11752 2 if the expression is invalid,
11753 3 if an internal error occurred (e.g., arithmetic overflow).
11757 * String expressions:: + : match substr index length
11758 * Numeric expressions:: + - * / %
11759 * Relations for expr:: | & < <= = == != >= >
11760 * Examples of expr:: Examples.
11764 @node String expressions
11765 @subsection String expressions
11767 @cindex string expressions
11768 @cindex expressions, string
11770 @command{expr} supports pattern matching and other string operators. These
11771 have higher precedence than both the numeric and relational operators (in
11772 the next sections).
11776 @item @var{string} : @var{regex}
11777 @cindex pattern matching
11778 @cindex regular expression matching
11779 @cindex matching patterns
11780 Perform pattern matching. The arguments are converted to strings and the
11781 second is considered to be a (basic, a la GNU @code{grep}) regular
11782 expression, with a @code{^} implicitly prepended. The first argument is
11783 then matched against this regular expression.
11785 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11786 @code{:} expression returns the part of @var{string} that matched the
11787 subexpression; otherwise, it returns the number of characters matched.
11789 If the match fails, the @code{:} operator returns the null string if
11790 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11792 @kindex \( @r{regexp operator}
11793 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11794 value; additional pairs are meaningful only for grouping the regular
11795 expression operators.
11797 @kindex \+ @r{regexp operator}
11798 @kindex \? @r{regexp operator}
11799 @kindex \| @r{regexp operator}
11800 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11801 operators which respectively match one or more, zero or one, or separate
11802 alternatives. SunOS and other @command{expr}'s treat these as regular
11803 characters. (@acronym{POSIX} allows either behavior.)
11804 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11805 regular expression syntax. Some examples are in @ref{Examples of expr}.
11807 @item match @var{string} @var{regex}
11809 An alternative way to do pattern matching. This is the same as
11810 @w{@samp{@var{string} : @var{regex}}}.
11812 @item substr @var{string} @var{position} @var{length}
11814 Returns the substring of @var{string} beginning at @var{position}
11815 with length at most @var{length}. If either @var{position} or
11816 @var{length} is negative, zero, or non-numeric, returns the null string.
11818 @item index @var{string} @var{charset}
11820 Returns the first position in @var{string} where the first character in
11821 @var{charset} was found. If no character in @var{charset} is found in
11822 @var{string}, return 0.
11824 @item length @var{string}
11826 Returns the length of @var{string}.
11828 @item + @var{token}
11830 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11831 or an operator like @code{/}.
11832 This makes it possible to test @code{expr length + "$x"} or
11833 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11834 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11835 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11836 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11840 To make @command{expr} interpret keywords as strings, you must use the
11841 @code{quote} operator.
11844 @node Numeric expressions
11845 @subsection Numeric expressions
11847 @cindex numeric expressions
11848 @cindex expressions, numeric
11850 @command{expr} supports the usual numeric operators, in order of increasing
11851 precedence. These numeric operators have lower precedence than the
11852 string operators described in the previous section, and higher precedence
11853 than the connectives (next section).
11861 @cindex subtraction
11862 Addition and subtraction. Both arguments are converted to integers;
11863 an error occurs if this cannot be done.
11869 @cindex multiplication
11872 Multiplication, division, remainder. Both arguments are converted to
11873 integers; an error occurs if this cannot be done.
11878 @node Relations for expr
11879 @subsection Relations for @command{expr}
11881 @cindex connectives, logical
11882 @cindex logical connectives
11883 @cindex relations, numeric or string
11885 @command{expr} supports the usual logical connectives and relations. These
11886 have lower precedence than the string and numeric operators
11887 (previous sections). Here is the list, lowest-precedence operator first.
11893 @cindex logical or operator
11894 @cindex or operator
11895 Returns its first argument if that is neither null nor zero, otherwise
11896 its second argument if it is neither null nor zero, otherwise 0. It
11897 does not evaluate its second argument if its first argument is neither
11902 @cindex logical and operator
11903 @cindex and operator
11904 Return its first argument if neither argument is null or zero, otherwise
11905 0. It does not evaluate its second argument if its first argument is
11908 @item < <= = == != >= >
11915 @cindex comparison operators
11917 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11918 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11919 both arguments to integers and do a numeric comparison; if either
11920 conversion fails, it does a lexicographic comparison using the character
11921 collating sequence specified by the @env{LC_COLLATE} locale.
11926 @node Examples of expr
11927 @subsection Examples of using @command{expr}
11929 @cindex examples of @command{expr}
11930 Here are a few examples, including quoting for shell metacharacters.
11932 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11935 foo=`expr $foo + 1`
11938 To print the non-directory part of the file name stored in
11939 @code{$fname}, which need not contain a @code{/}:
11942 expr $fname : '.*/\(.*\)' '|' $fname
11945 An example showing that @code{\+} is an operator:
11953 expr abc : 'a\(.\)c'
11955 expr index abcdef cz
11958 @error{} expr: syntax error
11959 expr index + index a
11965 @chapter Redirection
11967 @cindex redirection
11968 @cindex commands for redirection
11970 Unix shells commonly provide several forms of @dfn{redirection}---ways
11971 to change the input source or output destination of a command. But one
11972 useful redirection is performed by a separate command, not by the shell;
11973 it's described here.
11976 * tee invocation:: Redirect output to multiple files or processes.
11980 @node tee invocation
11981 @section @command{tee}: Redirect output to multiple files or processes
11984 @cindex pipe fitting
11985 @cindex destinations, multiple output
11986 @cindex read from stdin and write to stdout and files
11988 The @command{tee} command copies standard input to standard output and also
11989 to any files given as arguments. This is useful when you want not only
11990 to send some data down a pipe, but also to save a copy. Synopsis:
11993 tee [@var{option}]@dots{} [@var{file}]@dots{}
11996 If a file being written to does not already exist, it is created. If a
11997 file being written to already exists, the data it previously contained
11998 is overwritten unless the @option{-a} option is used.
12000 A @var{file} of @samp{-} causes @command{tee} to send another copy of
12001 input to standard output, but this is typically not that useful as the
12002 copies are interleaved.
12004 The program accepts the following options. Also see @ref{Common options}.
12011 Append standard input to the given files rather than overwriting
12015 @itemx --ignore-interrupts
12017 @opindex --ignore-interrupts
12018 Ignore interrupt signals.
12022 The @command{tee} command is useful when you happen to be transferring a large
12023 amount of data and also want to summarize that data without reading
12024 it a second time. For example, when you are downloading a DVD image,
12025 you often want to verify its signature or checksum right away.
12026 The inefficient way to do it is simply:
12029 wget http://example.com/some.iso && sha1sum some.iso
12032 One problem with the above is that it makes you wait for the
12033 download to complete before starting the time-consuming SHA1 computation.
12034 Perhaps even more importantly, the above requires reading
12035 the DVD image a second time (the first was from the network).
12037 The efficient way to do it is to interleave the download
12038 and SHA1 computation. Then, you'll get the checksum for
12039 free, because the entire process parallelizes so well:
12042 # slightly contrived, to demonstrate process substitution
12043 wget -O - http://example.com/dvd.iso \
12044 | tee >(sha1sum > dvd.sha1) > dvd.iso
12047 That makes @command{tee} write not just to the expected output file,
12048 but also to a pipe running @command{sha1sum} and saving the final
12049 checksum in a file named @file{dvd.sha1}.
12051 Note, however, that this example relies on a feature of modern shells
12052 called @dfn{process substitution}
12053 (the @samp{>(command)} syntax, above;
12054 @xref{Process Substitution,,Process Substitution, bashref,
12055 The Bash Reference Manual}.),
12056 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12057 but not with @command{/bin/sh}. So if you write code like this
12058 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12060 Since the above example writes to one file and one process,
12061 a more conventional and portable use of @command{tee} is even better:
12064 wget -O - http://example.com/dvd.iso \
12065 | tee dvd.iso | sha1sum > dvd.sha1
12068 You can extend this example to make @command{tee} write to two processes,
12069 computing MD5 and SHA1 checksums in parallel. In this case,
12070 process substitution is required:
12073 wget -O - http://example.com/dvd.iso \
12074 | tee >(sha1sum > dvd.sha1) \
12075 >(md5sum > dvd.md5) \
12079 This technique is also useful when you want to make a @emph{compressed}
12080 copy of the contents of a pipe.
12081 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12082 For a large hierarchy, @samp{du -ak} can run for a long time,
12083 and can easily produce terabytes of data, so you won't want to
12084 rerun the command unnecessarily. Nor will you want to save
12085 the uncompressed output.
12087 Doing it the inefficient way, you can't even start the GUI
12088 until after you've compressed all of the @command{du} output:
12091 du -ak | gzip -9 > /tmp/du.gz
12092 gzip -d /tmp/du.gz | xdiskusage -a
12095 With @command{tee} and process substitution, you start the GUI
12096 right away and eliminate the decompression completely:
12099 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12102 Finally, if you regularly create more than one type of
12103 compressed tarball at once, for example when @code{make dist} creates
12104 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12105 there may be a better way.
12106 Typical @command{automake}-generated @file{Makefile} rules create
12107 the two compressed tar archives with commands in sequence, like this
12108 (slightly simplified):
12111 tardir=your-pkg-M.N
12112 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12113 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12116 However, if the hierarchy you are archiving and compressing is larger
12117 than a couple megabytes, and especially if you are using a multi-processor
12118 system with plenty of memory, then you can do much better by reading the
12119 directory contents only once and running the compression programs in parallel:
12122 tardir=your-pkg-M.N
12123 tar chof - "$tardir" \
12124 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12125 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12131 @node File name manipulation
12132 @chapter File name manipulation
12134 @cindex file name manipulation
12135 @cindex manipulation of file names
12136 @cindex commands for file name manipulation
12138 This section describes commands that manipulate file names.
12141 * basename invocation:: Strip directory and suffix from a file name.
12142 * dirname invocation:: Strip last file name component.
12143 * pathchk invocation:: Check file name validity and portability.
12144 * mktemp invocation:: Create temporary file or directory.
12148 @node basename invocation
12149 @section @command{basename}: Strip directory and suffix from a file name
12152 @cindex strip directory and suffix from file names
12153 @cindex directory, stripping from file names
12154 @cindex suffix, stripping from file names
12155 @cindex file names, stripping directory and suffix
12156 @cindex leading directory components, stripping
12158 @command{basename} removes any leading directory components from
12159 @var{name}. Synopsis:
12162 basename @var{name} [@var{suffix}]
12165 If @var{suffix} is specified and is identical to the end of @var{name},
12166 it is removed from @var{name} as well. Note that since trailing slashes
12167 are removed prior to suffix matching, @var{suffix} will do nothing if it
12168 contains slashes. @command{basename} prints the result on standard
12171 @c This test is used both here and in the section on dirname.
12172 @macro basenameAndDirname
12173 Together, @command{basename} and @command{dirname} are designed such
12174 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12175 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12176 for everything except file names containing a trailing newline.
12178 @basenameAndDirname
12180 @acronym{POSIX} allows the implementation to define the results if
12181 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12182 @command{basename} returns the empty string. In the latter case, the
12183 result is @samp{//} on platforms where @var{//} is distinct from
12184 @var{/}, and @samp{/} on platforms where there is no difference.
12186 The only options are @option{--help} and @option{--version}. @xref{Common
12187 options}. Options must precede operands.
12195 basename /usr/bin/sort
12198 basename include/stdio.h .h
12202 @node dirname invocation
12203 @section @command{dirname}: Strip last file name component
12206 @cindex directory components, printing
12207 @cindex stripping non-directory suffix
12208 @cindex non-directory suffix, stripping
12210 @command{dirname} prints all but the final slash-delimited component of
12211 @var{name}. Slashes on either side of the final component are also
12212 removed. If the string contains no slash, @command{dirname} prints
12213 @samp{.} (meaning the current directory). Synopsis:
12219 @var{name} need not be a file name, but if it is, this operation
12220 effectively lists the directory that contains the final component,
12221 including the case when the final component is itself a directory.
12223 @basenameAndDirname
12225 @acronym{POSIX} allows the implementation to define the results if
12226 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12227 result is @samp{//} on platforms where @var{//} is distinct from
12228 @var{/}, and @samp{/} on platforms where there is no difference.
12230 The only options are @option{--help} and @option{--version}. @xref{Common
12238 # Output "/usr/bin".
12239 dirname /usr/bin/sort
12240 dirname /usr/bin//.//
12247 @node pathchk invocation
12248 @section @command{pathchk}: Check file name validity and portability
12251 @cindex file names, checking validity and portability
12252 @cindex valid file names, checking for
12253 @cindex portable file names, checking for
12255 @command{pathchk} checks validity and portability of file names. Synopsis:
12258 pathchk [@var{option}]@dots{} @var{name}@dots{}
12261 For each @var{name}, @command{pathchk} prints an error message if any of
12262 these conditions is true:
12266 One of the existing directories in @var{name} does not have search
12267 (execute) permission,
12269 The length of @var{name} is larger than the maximum supported by the
12272 The length of one component of @var{name} is longer than
12273 its file system's maximum.
12276 A nonexistent @var{name} is not an error, so long a file with that
12277 name could be created under the above conditions.
12279 The program accepts the following options. Also see @ref{Common options}.
12280 Options must precede operands.
12286 Instead of performing checks based on the underlying file system,
12287 print an error message if any of these conditions is true:
12291 A file name is empty.
12294 A file name contains a character outside the @acronym{POSIX} portable file
12295 name character set, namely, the ASCII letters and digits, @samp{.},
12296 @samp{_}, @samp{-}, and @samp{/}.
12299 The length of a file name or one of its components exceeds the
12300 @acronym{POSIX} minimum limits for portability.
12305 Print an error message if a file name is empty, or if it contains a component
12306 that begins with @samp{-}.
12308 @item --portability
12309 @opindex --portability
12310 Print an error message if a file name is not portable to all @acronym{POSIX}
12311 hosts. This option is equivalent to @samp{-p -P}.
12315 @cindex exit status of @command{pathchk}
12319 0 if all specified file names passed all checks,
12323 @node mktemp invocation
12324 @section @command{mktemp}: Create temporary file or directory
12327 @cindex file names, creating temporary
12328 @cindex directory, creating temporary
12329 @cindex temporary files and directories
12331 @command{mktemp} manages the creation of temporary files and
12332 directories. Synopsis:
12335 mktemp [@var{option}]@dots{} [@var{template}]
12338 Safely create a temporary file or directory based on @var{template},
12339 and print its name. If given, @var{template} must include at least
12340 three consecutive @samp{X}s in the last component. If omitted, the template
12341 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12342 implied. The final run of @samp{X}s in the @var{template} will be replaced
12343 by alpha-numeric characters; thus, on a case-sensitive file system,
12344 and with a @var{template} including a run of @var{n} instances of @samp{X},
12345 there are @samp{62**@var{n}} potential file names.
12347 Older scripts used to create temporary files by simply joining the
12348 name of the program with the process id (@samp{$$}) as a suffix.
12349 However, that naming scheme is easily predictable, and suffers from a
12350 race condition where the attacker can create an appropriately named
12351 symbolic link, such that when the script then opens a handle to what
12352 it thought was an unused file, it is instead modifying an existing
12353 file. Using the same scheme to create a directory is slightly safer,
12354 since the @command{mkdir} will fail if the target already exists, but
12355 it is still inferior because it allows for denial of service attacks.
12356 Therefore, modern scripts should use the @command{mktemp} command to
12357 guarantee that the generated name will be unpredictable, and that
12358 knowledge of the temporary file name implies that the file was created
12359 by the current script and cannot be modified by other users.
12361 When creating a file, the resulting file has read and write
12362 permissions for the current user, but no permissions for the group or
12363 others; these permissions are reduced if the current umask is more
12366 Here are some examples (although note that if you repeat them, you
12367 will most likely get different file names):
12372 Create a temporary file in the current directory.
12379 Create a temporary file with a known suffix.
12381 $ mktemp --suffix=.txt file-XXXX
12383 $ mktemp file-XXXX-XXXX.txt
12388 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12389 but falling back to the current directory rather than @file{/tmp}.
12390 Note that @command{mktemp} does not create fifos, but can create a
12391 secure directory in which the fifo can live. Exit the shell if the
12392 directory or fifo could not be created.
12394 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12396 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12400 Create and use a temporary file if possible, but ignore failure. The
12401 file will reside in the directory named by @env{TMPDIR}, if specified,
12402 or else in @file{/tmp}.
12404 $ file=$(mktemp -q) && @{
12405 > # Safe to use $file only within this block. Use quotes,
12406 > # since $TMPDIR, and thus $file, may contain whitespace.
12407 > echo ... > "$file"
12413 Act as a semi-random character generator (it is not fully random,
12414 since it is impacted by the contents of the current directory). To
12415 avoid security holes, do not use the resulting names to create a file.
12425 The program accepts the following options. Also see @ref{Common options}.
12432 @opindex --directory
12433 Create a directory rather than a file. The directory will have read,
12434 write, and search permissions for the current user, but no permissions
12435 for the group or others; these permissions are reduced if the current
12436 umask is more restrictive.
12442 Suppress diagnostics about failure to create a file or directory. The
12443 exit status will still reflect whether a file was created.
12449 Generate a temporary name that does not name an existing file, without
12450 changing the file system contents. Using the output of this command
12451 to create a new file is inherently unsafe, as there is a window of
12452 time between generating the name and using it where another process
12453 can create an object by the same name.
12456 @itemx --tmpdir[=@var{dir}]
12459 Treat @var{template} relative to the directory @var{dir}. If
12460 @var{dir} is not specified (only possible with the long option
12461 @option{--tmpdir}) or is the empty string, use the value of
12462 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12463 specified, @var{template} must not be absolute. However,
12464 @var{template} can still contain slashes, although intermediate
12465 directories must already exist.
12467 @item --suffix=@var{suffix}
12469 Append @var{suffix} to the @var{template}. @var{suffix} must not
12470 contain slash. If @option{--suffix} is specified, @var{template} must
12471 end in @samp{X}; if it is not specified, then an appropriate
12472 @option{--suffix} is inferred by finding the last @samp{X} in
12473 @var{template}. This option exists for use with the default
12474 @var{template} and for the creation of a @var{suffix} that starts with
12479 Treat @var{template} as a single file relative to the value of
12480 @env{TMPDIR} if available, or to the directory specified by
12481 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12482 contain slashes. This option is deprecated; the use of @option{-p}
12483 without @option{-t} offers better defaults (by favoring the command
12484 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12489 @cindex exit status of @command{mktemp}
12493 0 if the file was created,
12498 @node Working context
12499 @chapter Working context
12501 @cindex working context
12502 @cindex commands for printing the working context
12504 This section describes commands that display or alter the context in
12505 which you are working: the current directory, the terminal settings, and
12506 so forth. See also the user-related commands in the next section.
12509 * pwd invocation:: Print working directory.
12510 * stty invocation:: Print or change terminal characteristics.
12511 * printenv invocation:: Print environment variables.
12512 * tty invocation:: Print file name of terminal on standard input.
12516 @node pwd invocation
12517 @section @command{pwd}: Print working directory
12520 @cindex print name of current directory
12521 @cindex current working directory, printing
12522 @cindex working directory, printing
12525 @command{pwd} prints the name of the current directory. Synopsis:
12528 pwd [@var{option}]@dots{}
12531 The program accepts the following options. Also see @ref{Common options}.
12538 If the contents of the environment variable @env{PWD} provide an
12539 absolute name of the current directory with no @samp{.} or @samp{..}
12540 components, but possibly with symbolic links, then output those
12541 contents. Otherwise, fall back to default @option{-P} handling.
12546 @opindex --physical
12547 Print a fully resolved name for the current directory. That is, all
12548 components of the printed name will be actual directory names---none
12549 will be symbolic links.
12552 @cindex symbolic links and @command{pwd}
12553 If @option{-L} and @option{-P} are both given, the last one takes
12554 precedence. If neither option is given, then this implementation uses
12555 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12556 environment variable is set.
12558 @mayConflictWithShellBuiltIn{pwd}
12563 @node stty invocation
12564 @section @command{stty}: Print or change terminal characteristics
12567 @cindex change or print terminal settings
12568 @cindex terminal settings
12569 @cindex line settings of terminal
12571 @command{stty} prints or changes terminal characteristics, such as baud rate.
12575 stty [@var{option}] [@var{setting}]@dots{}
12576 stty [@var{option}]
12579 If given no line settings, @command{stty} prints the baud rate, line
12580 discipline number (on systems that support it), and line settings
12581 that have been changed from the values set by @samp{stty sane}.
12582 By default, mode reading and setting are performed on the tty line
12583 connected to standard input, although this can be modified by the
12584 @option{--file} option.
12586 @command{stty} accepts many non-option arguments that change aspects of
12587 the terminal line operation, as described below.
12589 The program accepts the following options. Also see @ref{Common options}.
12596 Print all current settings in human-readable form. This option may not
12597 be used in combination with any line settings.
12599 @item -F @var{device}
12600 @itemx --file=@var{device}
12603 Set the line opened by the file name specified in @var{device} instead of
12604 the tty line connected to standard input. This option is necessary
12605 because opening a @acronym{POSIX} tty requires use of the
12606 @code{O_NONDELAY} flag to prevent a @acronym{POSIX} tty from blocking
12607 until the carrier detect line is high if
12608 the @code{clocal} flag is not set. Hence, it is not always possible
12609 to allow the shell to open the device in the traditional manner.
12615 @cindex machine-readable @command{stty} output
12616 Print all current settings in a form that can be used as an argument to
12617 another @command{stty} command to restore the current settings. This option
12618 may not be used in combination with any line settings.
12622 Many settings can be turned off by preceding them with a @samp{-}.
12623 Such arguments are marked below with ``May be negated'' in their
12624 description. The descriptions themselves refer to the positive
12625 case, that is, when @emph{not} negated (unless stated otherwise,
12628 Some settings are not available on all @acronym{POSIX} systems, since they use
12629 extensions. Such arguments are marked below with
12630 ``Non-@acronym{POSIX}'' in their description. On non-@acronym{POSIX}
12631 systems, those or other settings also may not
12632 be available, but it's not feasible to document all the variations: just
12638 * Control:: Control settings
12639 * Input:: Input settings
12640 * Output:: Output settings
12641 * Local:: Local settings
12642 * Combination:: Combination settings
12643 * Characters:: Special characters
12644 * Special:: Special settings
12649 @subsection Control settings
12651 @cindex control settings
12657 @cindex two-way parity
12658 Generate parity bit in output and expect parity bit in input.
12664 @cindex even parity
12665 Set odd parity (even if negated). May be negated.
12672 @cindex character size
12673 @cindex eight-bit characters
12674 Set character size to 5, 6, 7, or 8 bits.
12679 Send a hangup signal when the last process closes the tty. May be
12685 Use two stop bits per character (one if negated). May be negated.
12689 Allow input to be received. May be negated.
12693 @cindex modem control
12694 Disable modem control signals. May be negated.
12698 @cindex hardware flow control
12699 @cindex flow control, hardware
12700 @cindex RTS/CTS flow control
12701 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12706 @subsection Input settings
12708 @cindex input settings
12709 These settings control operations on data received from the terminal.
12714 @cindex breaks, ignoring
12715 Ignore break characters. May be negated.
12719 @cindex breaks, cause interrupts
12720 Make breaks cause an interrupt signal. May be negated.
12724 @cindex parity, ignoring
12725 Ignore characters with parity errors. May be negated.
12729 @cindex parity errors, marking
12730 Mark parity errors (with a 255-0-character sequence). May be negated.
12734 Enable input parity checking. May be negated.
12738 @cindex eight-bit input
12739 Clear high (8th) bit of input characters. May be negated.
12743 @cindex newline, translating to return
12744 Translate newline to carriage return. May be negated.
12748 @cindex return, ignoring
12749 Ignore carriage return. May be negated.
12753 @cindex return, translating to newline
12754 Translate carriage return to newline. May be negated.
12758 @cindex input encoding, UTF-8
12759 Assume input characters are UTF-8 encoded. May be negated.
12763 @kindex C-s/C-q flow control
12764 @cindex XON/XOFF flow control
12765 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12772 @cindex software flow control
12773 @cindex flow control, software
12774 Enable sending of @code{stop} character when the system input buffer
12775 is almost full, and @code{start} character when it becomes almost
12776 empty again. May be negated.
12780 @cindex uppercase, translating to lowercase
12781 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12782 negated. Note ilcuc is not implemented, as one would not be able to issue
12783 almost any (lowercase) Unix command, after invoking it.
12787 Allow any character to restart output (only the start character
12788 if negated). Non-@acronym{POSIX}. May be negated.
12792 @cindex beeping at input buffer full
12793 Enable beeping and not flushing input buffer if a character arrives
12794 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12799 @subsection Output settings
12801 @cindex output settings
12802 These settings control operations on data sent to the terminal.
12807 Postprocess output. May be negated.
12811 @cindex lowercase, translating to output
12812 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12813 negated. (Note ouclc is not currently implemented.)
12817 @cindex return, translating to newline
12818 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12822 @cindex newline, translating to crlf
12823 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12828 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12833 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12837 @cindex pad instead of timing for delaying
12838 Use fill (padding) characters instead of timing for delays.
12839 Non-@acronym{POSIX}.
12844 @cindex pad character
12845 Use @acronym{ASCII} @sc{del} characters for fill instead of
12846 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12852 Newline delay style. Non-@acronym{POSIX}.
12859 Carriage return delay style. Non-@acronym{POSIX}.
12865 @opindex tab@var{n}
12866 Horizontal tab delay style. Non-@acronym{POSIX}.
12871 Backspace delay style. Non-@acronym{POSIX}.
12876 Vertical tab delay style. Non-@acronym{POSIX}.
12881 Form feed delay style. Non-@acronym{POSIX}.
12886 @subsection Local settings
12888 @cindex local settings
12893 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12894 characters. May be negated.
12898 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12899 special characters. May be negated.
12903 Enable non-@acronym{POSIX} special characters. May be negated.
12907 Echo input characters. May be negated.
12913 Echo @code{erase} characters as backspace-space-backspace. May be
12918 @cindex newline echoing after @code{kill}
12919 Echo a newline after a @code{kill} character. May be negated.
12923 @cindex newline, echoing
12924 Echo newline even if not echoing other characters. May be negated.
12928 @cindex flushing, disabling
12929 Disable flushing after @code{interrupt} and @code{quit} special
12930 characters. May be negated.
12934 @cindex case translation
12935 Enable input and output of uppercase characters by preceding their
12936 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12937 Non-@acronym{POSIX}. May be negated.
12941 @cindex background jobs, stopping at terminal write
12942 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12949 Echo erased characters backward, between @samp{\} and @samp{/}.
12950 Non-@acronym{POSIX}. May be negated.
12956 @cindex control characters, using @samp{^@var{c}}
12957 @cindex hat notation for control characters
12958 Echo control characters in hat notation (@samp{^@var{c}}) instead
12959 of literally. Non-@acronym{POSIX}. May be negated.
12965 Echo the @code{kill} special character by erasing each character on
12966 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12967 instead of by the @code{echoctl} and @code{echok} settings.
12968 Non-@acronym{POSIX}.
12974 @subsection Combination settings
12976 @cindex combination settings
12977 Combination settings:
12984 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12985 as @code{-parenb cs8}.
12989 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12990 as @code{-parenb cs8}.
12994 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12995 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12999 Reset the @code{erase} and @code{kill} special characters to their default
13006 @c This is too long to write inline.
13008 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
13009 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
13010 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
13011 ff0 isig icanon iexten echo echoe echok -echonl
13012 -noflsh -xcase -tostop -echoprt echoctl echoke
13016 and also sets all special characters to their default values.
13020 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13021 sets the @code{eof} and @code{eol} characters to their default values
13022 if they are the same as the @code{min} and @code{time} characters.
13023 May be negated. If negated, same as @code{raw}.
13030 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13031 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13032 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13036 May be negated. If negated, same as @code{cooked}.
13040 Same as @option{-icanon}. May be negated. If negated, same as
13045 @cindex eight-bit characters
13046 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13047 same as @code{parenb istrip cs7}.
13051 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13052 If negated, same as @code{parenb istrip opost cs7}.
13056 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13060 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13067 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13068 (Used for terminals with uppercase characters only.)
13072 Same as @code{echoe echoctl echoke}.
13076 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13081 @subsection Special characters
13083 @cindex special characters
13084 @cindex characters, special
13086 The special characters' default values vary from system to system.
13087 They are set with the syntax @samp{name value}, where the names are
13088 listed below and the value can be given either literally, in hat
13089 notation (@samp{^@var{c}}), or as an integer which may start with
13090 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13091 any other digit to indicate decimal.
13093 @cindex disabling special characters
13094 @kindex u@r{, and disabling special characters}
13095 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13096 special character. (This is incompatible with Ultrix @command{stty},
13097 which uses a value of @samp{u} to disable a special character. GNU
13098 @command{stty} treats a value @samp{u} like any other, namely to set that
13099 special character to @key{U}.)
13105 Send an interrupt signal.
13109 Send a quit signal.
13113 Erase the last character typed.
13117 Erase the current line.
13121 Send an end of file (terminate the input).
13129 Alternate character to end the line. Non-@acronym{POSIX}.
13133 Switch to a different shell layer. Non-@acronym{POSIX}.
13137 Restart the output after stopping it.
13145 Send a terminal stop signal.
13149 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13153 Redraw the current line. Non-@acronym{POSIX}.
13157 Erase the last word typed. Non-@acronym{POSIX}.
13161 Enter the next character typed literally, even if it is a special
13162 character. Non-@acronym{POSIX}.
13167 @subsection Special settings
13169 @cindex special settings
13174 Set the minimum number of characters that will satisfy a read until
13175 the time value has expired, when @option{-icanon} is set.
13179 Set the number of tenths of a second before reads time out if the minimum
13180 number of characters have not been read, when @option{-icanon} is set.
13182 @item ispeed @var{n}
13184 Set the input speed to @var{n}.
13186 @item ospeed @var{n}
13188 Set the output speed to @var{n}.
13192 Tell the tty kernel driver that the terminal has @var{n} rows.
13193 Non-@acronym{POSIX}.
13196 @itemx columns @var{n}
13199 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13205 Print the number of rows and columns that the kernel thinks the
13206 terminal has. (Systems that don't support rows and columns in the kernel
13207 typically use the environment variables @env{LINES} and @env{COLUMNS}
13208 instead; however, GNU @command{stty} does not know anything about them.)
13209 Non-@acronym{POSIX}.
13213 Use line discipline @var{n}. Non-@acronym{POSIX}.
13217 Print the terminal speed.
13220 @cindex baud rate, setting
13221 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13222 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13223 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13224 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13225 support higher speeds. The @command{stty} command includes support
13242 4000000 where the system supports these.
13243 0 hangs up the line if @option{-clocal} is set.
13247 @node printenv invocation
13248 @section @command{printenv}: Print all or some environment variables
13251 @cindex printing all or some environment variables
13252 @cindex environment variables, printing
13254 @command{printenv} prints environment variable values. Synopsis:
13257 printenv [@var{option}] [@var{variable}]@dots{}
13260 If no @var{variable}s are specified, @command{printenv} prints the value of
13261 every environment variable. Otherwise, it prints the value of each
13262 @var{variable} that is set, and nothing for those that are not set.
13264 The program accepts the following option. Also see @ref{Common options}.
13272 @cindex exit status of @command{printenv}
13276 0 if all variables specified were found
13277 1 if at least one specified variable was not found
13278 2 if a write error occurred
13282 @node tty invocation
13283 @section @command{tty}: Print file name of terminal on standard input
13286 @cindex print terminal file name
13287 @cindex terminal file name, printing
13289 @command{tty} prints the file name of the terminal connected to its standard
13290 input. It prints @samp{not a tty} if standard input is not a terminal.
13294 tty [@var{option}]@dots{}
13297 The program accepts the following option. Also see @ref{Common options}.
13307 Print nothing; only return an exit status.
13311 @cindex exit status of @command{tty}
13315 0 if standard input is a terminal
13316 1 if standard input is not a terminal
13317 2 if given incorrect arguments
13318 3 if a write error occurs
13322 @node User information
13323 @chapter User information
13325 @cindex user information, commands for
13326 @cindex commands for printing user information
13328 This section describes commands that print user-related information:
13329 logins, groups, and so forth.
13332 * id invocation:: Print user identity.
13333 * logname invocation:: Print current login name.
13334 * whoami invocation:: Print effective user ID.
13335 * groups invocation:: Print group names a user is in.
13336 * users invocation:: Print login names of users currently logged in.
13337 * who invocation:: Print who is currently logged in.
13341 @node id invocation
13342 @section @command{id}: Print user identity
13345 @cindex real user and group IDs, printing
13346 @cindex effective user and group IDs, printing
13347 @cindex printing real and effective user and group IDs
13349 @command{id} prints information about the given user, or the process
13350 running it if no user is specified. Synopsis:
13353 id [@var{option}]@dots{} [@var{username}]
13356 @vindex POSIXLY_CORRECT
13357 By default, it prints the real user ID, real group ID, effective user ID
13358 if different from the real user ID, effective group ID if different from
13359 the real group ID, and supplemental group IDs.
13360 In addition, if SELinux
13361 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13362 then print @samp{context=@var{c}}, where @var{c} is the security context.
13364 Each of these numeric values is preceded by an identifying string and
13365 followed by the corresponding user or group name in parentheses.
13367 The options cause @command{id} to print only part of the above information.
13368 Also see @ref{Common options}.
13375 Print only the group ID.
13381 Print only the group ID and the supplementary groups.
13387 Print the user or group name instead of the ID number. Requires
13388 @option{-u}, @option{-g}, or @option{-G}.
13394 Print the real, instead of effective, user or group ID. Requires
13395 @option{-u}, @option{-g}, or @option{-G}.
13401 Print only the user ID.
13408 @cindex security context
13409 Print only the security context of the current user.
13410 If SELinux is disabled then print a warning and
13411 set the exit status to 1.
13417 @macro primaryAndSupplementaryGroups{cmd,arg}
13418 Primary and supplementary groups for a process are normally inherited
13419 from its parent and are usually unchanged since login. This means
13420 that if you change the group database after logging in, @command{\cmd\}
13421 will not reflect your changes within your existing login session.
13422 Running @command{\cmd\} with a \arg\ causes the user and group
13423 database to be consulted afresh, and so will give a different result.
13425 @primaryAndSupplementaryGroups{id,user argument}
13427 @node logname invocation
13428 @section @command{logname}: Print current login name
13431 @cindex printing user's login name
13432 @cindex login name, printing
13433 @cindex user name, printing
13436 @command{logname} prints the calling user's name, as found in a
13437 system-maintained file (often @file{/var/run/utmp} or
13438 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13439 for the calling process, @command{logname} prints
13440 an error message and exits with a status of 1.
13442 The only options are @option{--help} and @option{--version}. @xref{Common
13448 @node whoami invocation
13449 @section @command{whoami}: Print effective user ID
13452 @cindex effective user ID, printing
13453 @cindex printing the effective user ID
13455 @command{whoami} prints the user name associated with the current
13456 effective user ID. It is equivalent to the command @samp{id -un}.
13458 The only options are @option{--help} and @option{--version}. @xref{Common
13464 @node groups invocation
13465 @section @command{groups}: Print group names a user is in
13468 @cindex printing groups a user is in
13469 @cindex supplementary groups, printing
13471 @command{groups} prints the names of the primary and any supplementary
13472 groups for each given @var{username}, or the current process if no names
13473 are given. If more than one name is given, the name of each user is
13475 the list of that user's groups and the user name is separated from the
13476 group list by a colon. Synopsis:
13479 groups [@var{username}]@dots{}
13482 The group lists are equivalent to the output of the command @samp{id -Gn}.
13484 @primaryAndSupplementaryGroups{groups,list of users}
13486 The only options are @option{--help} and @option{--version}. @xref{Common
13492 @node users invocation
13493 @section @command{users}: Print login names of users currently logged in
13496 @cindex printing current usernames
13497 @cindex usernames, printing current
13499 @cindex login sessions, printing users with
13500 @command{users} prints on a single line a blank-separated list of user
13501 names of users currently logged in to the current host. Each user name
13502 corresponds to a login session, so if a user has more than one login
13503 session, that user's name will appear the same number of times in the
13512 With no @var{file} argument, @command{users} extracts its information from
13513 a system-maintained file (often @file{/var/run/utmp} or
13514 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13515 that file instead. A common choice is @file{/var/log/wtmp}.
13517 The only options are @option{--help} and @option{--version}. @xref{Common
13523 @node who invocation
13524 @section @command{who}: Print who is currently logged in
13527 @cindex printing current user information
13528 @cindex information, about current users
13530 @command{who} prints information about users who are currently logged on.
13534 @command{who} [@var{option}] [@var{file}] [am i]
13537 @cindex terminal lines, currently used
13539 @cindex remote hostname
13540 If given no non-option arguments, @command{who} prints the following
13541 information for each user currently logged on: login name, terminal
13542 line, login time, and remote hostname or X display.
13546 If given one non-option argument, @command{who} uses that instead of
13547 a default system-maintained file (often @file{/var/run/utmp} or
13548 @file{/etc/utmp}) as the name of the file containing the record of
13549 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13550 to @command{who} to look at who has previously logged on.
13554 If given two non-option arguments, @command{who} prints only the entry
13555 for the user running it (determined from its standard input), preceded
13556 by the hostname. Traditionally, the two arguments given are @samp{am
13557 i}, as in @samp{who am i}.
13560 Time stamps are listed according to the time zone rules specified by
13561 the @env{TZ} environment variable, or by the system default rules if
13562 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13563 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13565 The program accepts the following options. Also see @ref{Common options}.
13573 Same as @samp{-b -d --login -p -r -t -T -u}.
13579 Print the date and time of last system boot.
13585 Print information corresponding to dead processes.
13591 Print a line of column headings.
13597 List only the entries that correspond to processes via which the
13598 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13602 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13603 is not the default because it can cause significant delays on systems with
13604 automatic dial-up internet access.
13608 Same as @samp{who am i}.
13614 List active processes spawned by init.
13620 Print only the login names and the number of users logged on.
13621 Overrides all other options.
13626 @opindex --runlevel
13627 Print the current (and maybe previous) run-level of the init process.
13631 Ignored; for compatibility with other versions of @command{who}.
13637 Print last system clock change.
13642 After the login time, print the number of hours and minutes that the
13643 user has been idle. @samp{.} means the user was active in the last minute.
13644 @samp{old} means the user has been idle for more than 24 hours.
13655 @opindex --writable
13656 @cindex message status
13657 @pindex write@r{, allowed}
13658 After each login name print a character indicating the user's message status:
13661 @samp{+} allowing @code{write} messages
13662 @samp{-} disallowing @code{write} messages
13663 @samp{?} cannot find terminal device
13671 @node System context
13672 @chapter System context
13674 @cindex system context
13675 @cindex context, system
13676 @cindex commands for system context
13678 This section describes commands that print or change system-wide
13682 * date invocation:: Print or set system date and time.
13683 * arch invocation:: Print machine hardware name.
13684 * nproc invocation:: Print the number of processors.
13685 * uname invocation:: Print system information.
13686 * hostname invocation:: Print or set system name.
13687 * hostid invocation:: Print numeric host identifier.
13688 * uptime invocation:: Print system uptime and load.
13691 @node date invocation
13692 @section @command{date}: Print or set system date and time
13695 @cindex time, printing or setting
13696 @cindex printing the current time
13701 date [@var{option}]@dots{} [+@var{format}]
13702 date [-u|--utc|--universal] @c this avoids a newline in the output
13703 [ MMDDhhmm[[CC]YY][.ss] ]
13707 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13708 it with a default format that depends on the @env{LC_TIME} locale category.
13709 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13710 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13713 Normally, @command{date} uses the time zone rules indicated by the
13714 @env{TZ} environment variable, or the system default rules if @env{TZ}
13715 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13716 @env{TZ}, libc, The GNU C Library Reference Manual}.
13718 @findex strftime @r{and @command{date}}
13719 @cindex time formats
13720 @cindex formatting times
13721 If given an argument that starts with a @samp{+}, @command{date} prints the
13722 current date and time (or the date and time specified by the
13723 @option{--date} option, see below) in the format defined by that argument,
13724 which is similar to that of the @code{strftime} function. Except for
13725 conversion specifiers, which start with @samp{%}, characters in the
13726 format string are printed unchanged. The conversion specifiers are
13732 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13733 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13734 * Literal conversion specifiers:: %[%nt]
13735 * Padding and other flags:: Pad with zeros, spaces, etc.
13736 * Setting the time:: Changing the system clock.
13737 * Options for date:: Instead of the current time.
13739 * Date input formats:: Specifying date strings.
13741 * Examples of date:: Examples.
13744 @node Time conversion specifiers
13745 @subsection Time conversion specifiers
13747 @cindex time conversion specifiers
13748 @cindex conversion specifiers, time
13750 @command{date} conversion specifiers related to times.
13754 hour (@samp{00}@dots{}@samp{23})
13756 hour (@samp{01}@dots{}@samp{12})
13758 hour (@samp{ 0}@dots{}@samp{23}).
13759 This is a @acronym{GNU} extension.
13761 hour (@samp{ 1}@dots{}@samp{12}).
13762 This is a @acronym{GNU} extension.
13764 minute (@samp{00}@dots{}@samp{59})
13766 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13767 This is a @acronym{GNU} extension.
13769 locale's equivalent of either @samp{AM} or @samp{PM};
13770 blank in many locales.
13771 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13773 like @samp{%p}, except lower case.
13774 This is a @acronym{GNU} extension.
13776 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13778 24-hour hour and minute. Same as @samp{%H:%M}.
13779 This is a @acronym{GNU} extension.
13781 @cindex epoch, seconds since
13782 @cindex seconds since the epoch
13783 @cindex beginning of time
13784 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13785 Leap seconds are not counted unless leap second support is available.
13786 @xref{%s-examples}, for examples.
13787 This is a @acronym{GNU} extension.
13789 second (@samp{00}@dots{}@samp{60}).
13790 This may be @samp{60} if leap seconds are supported.
13792 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13794 locale's time representation (e.g., @samp{23:13:48})
13796 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13797 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13798 time zone is determinable. This value reflects the numeric time zone
13799 appropriate for the current time, using the time zone rules specified
13800 by the @env{TZ} environment variable.
13801 The time (and optionally, the time zone rules) can be overridden
13802 by the @option{--date} option.
13803 This is a @acronym{GNU} extension.
13805 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13806 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13807 zone is determinable.
13808 This is a @acronym{GNU} extension.
13810 Numeric time zone to the nearest second with @samp{:} (e.g.,
13811 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13813 This is a @acronym{GNU} extension.
13815 Numeric time zone with @samp{:} using the minimum necessary precision
13816 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13817 no time zone is determinable.
13818 This is a @acronym{GNU} extension.
13820 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13821 time zone is determinable. See @samp{%z} for how it is determined.
13825 @node Date conversion specifiers
13826 @subsection Date conversion specifiers
13828 @cindex date conversion specifiers
13829 @cindex conversion specifiers, date
13831 @command{date} conversion specifiers related to dates.
13835 locale's abbreviated weekday name (e.g., @samp{Sun})
13837 locale's full weekday name, variable length (e.g., @samp{Sunday})
13839 locale's abbreviated month name (e.g., @samp{Jan})
13841 locale's full month name, variable length (e.g., @samp{January})
13843 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13845 century. This is like @samp{%Y}, except the last two digits are omitted.
13846 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13847 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13848 It is normally at least two characters, but it may be more.
13850 day of month (e.g., @samp{01})
13852 date; same as @samp{%m/%d/%y}
13854 day of month, space padded; same as @samp{%_d}
13856 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13857 This is a good choice for a date format, as it is standard and
13858 is easy to sort in the usual case where years are in the range
13860 This is a @acronym{GNU} extension.
13862 year corresponding to the @acronym{ISO} week number, but without the century
13863 (range @samp{00} through @samp{99}). This has the same format and value
13864 as @samp{%y}, except that if the @acronym{ISO} week number (see
13866 to the previous or next year, that year is used instead.
13867 This is a @acronym{GNU} extension.
13869 year corresponding to the @acronym{ISO} week number. This has the
13870 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13872 @samp{%V}) belongs to the previous or next year, that year is used
13874 It is normally useful only if @samp{%V} is also used;
13875 for example, the format @samp{%G-%m-%d} is probably a mistake,
13876 since it combines the ISO week number year with the conventional month and day.
13877 This is a @acronym{GNU} extension.
13881 day of year (@samp{001}@dots{}@samp{366})
13883 month (@samp{01}@dots{}@samp{12})
13885 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13887 week number of year, with Sunday as the first day of the week
13888 (@samp{00}@dots{}@samp{53}).
13889 Days in a new year preceding the first Sunday are in week zero.
13891 @acronym{ISO} week number, that is, the
13892 week number of year, with Monday as the first day of the week
13893 (@samp{01}@dots{}@samp{53}).
13894 If the week containing January 1 has four or more days in
13895 the new year, then it is considered week 1; otherwise, it is week 53 of
13896 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13899 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13901 week number of year, with Monday as first day of week
13902 (@samp{00}@dots{}@samp{53}).
13903 Days in a new year preceding the first Monday are in week zero.
13905 locale's date representation (e.g., @samp{12/31/99})
13907 last two digits of year (@samp{00}@dots{}@samp{99})
13909 year. This is normally at least four characters, but it may be more.
13910 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13911 precedes year @samp{0000}.
13915 @node Literal conversion specifiers
13916 @subsection Literal conversion specifiers
13918 @cindex literal conversion specifiers
13919 @cindex conversion specifiers, literal
13921 @command{date} conversion specifiers that produce literal strings.
13933 @node Padding and other flags
13934 @subsection Padding and other flags
13936 @cindex numeric field padding
13937 @cindex padding of numeric fields
13938 @cindex fields, padding numeric
13940 Unless otherwise specified, @command{date} normally pads numeric fields
13941 with zeros, so that, for
13942 example, numeric months are always output as two digits.
13943 Seconds since the epoch are not padded, though,
13944 since there is no natural width for them.
13946 As a @acronym{GNU} extension, @command{date} recognizes any of the
13947 following optional flags after the @samp{%}:
13951 (hyphen) Do not pad the field; useful if the output is intended for
13954 (underscore) Pad with spaces; useful if you need a fixed
13955 number of characters in the output, but zeros are too distracting.
13957 (zero) Pad with zeros even if the conversion specifier
13958 would normally pad with spaces.
13960 Use upper case characters if possible.
13962 Use opposite case characters if possible.
13963 A field that is normally upper case becomes lower case, and vice versa.
13967 Here are some examples of padding:
13970 date +%d/%m -d "Feb 1"
13972 date +%-d/%-m -d "Feb 1"
13974 date +%_d/%_m -d "Feb 1"
13978 As a @acronym{GNU} extension, you can specify the field width
13979 (after any flag, if present) as a decimal number. If the natural size of the
13980 output of the field has less than the specified number of characters,
13981 the result is written right adjusted and padded to the given
13982 size. For example, @samp{%9B} prints the right adjusted month name in
13983 a field of width 9.
13985 An optional modifier can follow the optional flag and width
13986 specification. The modifiers are:
13990 Use the locale's alternate representation for date and time. This
13991 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13992 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13993 example, @samp{%Ex} might yield a date format based on the Japanese
13997 Use the locale's alternate numeric symbols for numbers. This modifier
13998 applies only to numeric conversion specifiers.
14001 If the format supports the modifier but no alternate representation
14002 is available, it is ignored.
14005 @node Setting the time
14006 @subsection Setting the time
14008 @cindex setting the time
14009 @cindex time setting
14010 @cindex appropriate privileges
14012 If given an argument that does not start with @samp{+}, @command{date} sets
14013 the system clock to the date and time specified by that argument (as
14014 described below). You must have appropriate privileges to set the
14015 system clock. The @option{--date} and @option{--set} options may not be
14016 used with such an argument. The @option{--universal} option may be used
14017 with such an argument to indicate that the specified date and time are
14018 relative to Coordinated Universal Time rather than to the local time
14021 The argument must consist entirely of digits, which have the following
14034 first two digits of year (optional)
14036 last two digits of year (optional)
14041 The @option{--set} option also sets the system clock; see the next section.
14044 @node Options for date
14045 @subsection Options for @command{date}
14047 @cindex @command{date} options
14048 @cindex options for @command{date}
14050 The program accepts the following options. Also see @ref{Common options}.
14054 @item -d @var{datestr}
14055 @itemx --date=@var{datestr}
14058 @cindex parsing date strings
14059 @cindex date strings, parsing
14060 @cindex arbitrary date strings, parsing
14063 @opindex next @var{day}
14064 @opindex last @var{day}
14065 Display the date and time specified in @var{datestr} instead of the
14066 current date and time. @var{datestr} can be in almost any common
14067 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14068 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14069 14:19:13.489392193 +0530"} specifies the instant of time that is
14070 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14071 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14072 Note: input currently must be in locale independent format. E.g., the
14073 LC_TIME=C below is needed to print back the correct date in many locales:
14075 date -d "$(LC_TIME=C date)"
14077 @xref{Date input formats}.
14079 @item -f @var{datefile}
14080 @itemx --file=@var{datefile}
14083 Parse each line in @var{datefile} as with @option{-d} and display the
14084 resulting date and time. If @var{datefile} is @samp{-}, use standard
14085 input. This is useful when you have many dates to process, because the
14086 system overhead of starting up the @command{date} executable many times can
14089 @item -r @var{file}
14090 @itemx --reference=@var{file}
14092 @opindex --reference
14093 Display the date and time of the last modification of @var{file},
14094 instead of the current date and time.
14101 @opindex --rfc-2822
14102 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14103 %z}, evaluated in the C locale so abbreviations are always in English.
14107 Fri, 09 Sep 2005 13:51:39 -0700
14110 This format conforms to
14111 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14112 @acronym{RFCs} 2822} and
14113 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14114 current and previous standards for Internet email.
14116 @item --rfc-3339=@var{timespec}
14117 @opindex --rfc-3339=@var{timespec}
14118 Display the date using a format specified by
14119 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14120 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14121 format, except that it also permits applications to use a space rather
14122 than a @samp{T} to separate dates from times. Unlike the other
14123 standard formats, @acronym{RFC} 3339 format is always suitable as
14124 input for the @option{--date} (@option{-d}) and @option{--file}
14125 (@option{-f}) options, regardless of the current locale.
14127 The argument @var{timespec} specifies how much of the time to include.
14128 It can be one of the following:
14132 Print just the full-date, e.g., @samp{2005-09-14}.
14133 This is equivalent to the format @samp{%Y-%m-%d}.
14136 Print the full-date and full-time separated by a space, e.g.,
14137 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14138 time-offset; here the @samp{+05:30} means that local time is five
14139 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14140 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14143 Like @samp{seconds}, but also print nanoseconds, e.g.,
14144 @samp{2005-09-14 00:56:06.998458565+05:30}.
14145 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14149 @item -s @var{datestr}
14150 @itemx --set=@var{datestr}
14153 Set the date and time to @var{datestr}. See @option{-d} above.
14160 @opindex --universal
14161 @cindex Coordinated Universal Time
14163 @cindex Greenwich Mean Time
14166 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14167 @env{TZ} environment variable were set to the string @samp{UTC0}.
14169 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14170 historical reasons.
14174 @node Examples of date
14175 @subsection Examples of @command{date}
14177 @cindex examples of @command{date}
14179 Here are a few examples. Also see the documentation for the @option{-d}
14180 option in the previous section.
14185 To print the date of the day before yesterday:
14188 date --date='2 days ago'
14192 To print the date of the day three months and one day hence:
14195 date --date='3 months 1 day'
14199 To print the day of year of Christmas in the current year:
14202 date --date='25 Dec' +%j
14206 To print the current full month name and the day of the month:
14212 But this may not be what you want because for the first nine days of
14213 the month, the @samp{%d} expands to a zero-padded two-digit field,
14214 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14217 To print a date without the leading zero for one-digit days
14218 of the month, you can use the (@acronym{GNU} extension)
14219 @samp{-} flag to suppress
14220 the padding altogether:
14223 date -d 1may '+%B %-d
14227 To print the current date and time in the format required by many
14228 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14231 date +%m%d%H%M%Y.%S
14235 To set the system clock forward by two minutes:
14238 date --set='+2 minutes'
14242 To print the date in @acronym{RFC} 2822 format,
14243 use @samp{date --rfc-2822}. Here is some example output:
14246 Fri, 09 Sep 2005 13:51:39 -0700
14249 @anchor{%s-examples}
14251 To convert a date string to the number of seconds since the epoch
14252 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14253 the @samp{%s} format. That can be useful in sorting and/or graphing
14254 and/or comparing data by date. The following command outputs the
14255 number of the seconds since the epoch for the time two minutes after the
14259 date --date='1970-01-01 00:02:00 +0000' +%s
14263 If you do not specify time zone information in the date string,
14264 @command{date} uses your computer's idea of the time zone when
14265 interpreting the string. For example, if your computer's time zone is
14266 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14267 seconds) behind UTC:
14270 # local time zone used
14271 date --date='1970-01-01 00:02:00' +%s
14276 If you're sorting or graphing dated data, your raw date values may be
14277 represented as seconds since the epoch. But few people can look at
14278 the date @samp{946684800} and casually note ``Oh, that's the first second
14279 of the year 2000 in Greenwich, England.''
14282 date --date='2000-01-01 UTC' +%s
14286 An alternative is to use the @option{--utc} (@option{-u}) option.
14287 Then you may omit @samp{UTC} from the date string. Although this
14288 produces the same result for @samp{%s} and many other format sequences,
14289 with a time zone offset different from zero, it would give a different
14290 result for zone-dependent formats like @samp{%z}.
14293 date -u --date=2000-01-01 +%s
14297 To convert such an unwieldy number of seconds back to
14298 a more readable form, use a command like this:
14301 # local time zone used
14302 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14303 1999-12-31 19:00:00 -0500
14306 Or if you do not mind depending on the @samp{@@} feature present since
14307 coreutils 5.3.0, you could shorten this to:
14310 date -d @@946684800 +"%F %T %z"
14311 1999-12-31 19:00:00 -0500
14314 Often it is better to output UTC-relative date and time:
14317 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14318 2000-01-01 00:00:00 +0000
14324 @node arch invocation
14325 @section @command{arch}: Print machine hardware name
14328 @cindex print machine hardware name
14329 @cindex system information, printing
14331 @command{arch} prints the machine hardware name,
14332 and is equivalent to @samp{uname -m}.
14336 arch [@var{option}]
14339 The program accepts the @ref{Common options} only.
14344 @node nproc invocation
14345 @section @command{nproc}: Print the number of available processors
14348 @cindex Print the number of processors
14349 @cindex system information, printing
14351 Print the number of processing units available to the current process,
14352 which may be less than the number of online processors.
14353 If this information is not accessible, then print the number of
14354 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14355 set, then it will determine the returned value. The result is guaranteed to be
14356 greater than zero. Synopsis:
14359 nproc [@var{option}]
14362 The program accepts the following options. Also see @ref{Common options}.
14368 Print the number of installed processors on the system, which may
14369 be greater than the number online or available to the current process.
14370 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14372 @item --ignore=@var{number}
14374 If possible, exclude this @var{number} of processing units.
14381 @node uname invocation
14382 @section @command{uname}: Print system information
14385 @cindex print system information
14386 @cindex system information, printing
14388 @command{uname} prints information about the machine and operating system
14389 it is run on. If no options are given, @command{uname} acts as if the
14390 @option{-s} option were given. Synopsis:
14393 uname [@var{option}]@dots{}
14396 If multiple options or @option{-a} are given, the selected information is
14397 printed in this order:
14400 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14401 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14404 The information may contain internal spaces, so such output cannot be
14405 parsed reliably. In the following example, @var{release} is
14406 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14410 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686@c
14411 unknown unknown GNU/Linux
14415 The program accepts the following options. Also see @ref{Common options}.
14423 Print all of the below information, except omit the processor type
14424 and the hardware platform name if they are unknown.
14427 @itemx --hardware-platform
14429 @opindex --hardware-platform
14430 @cindex implementation, hardware
14431 @cindex hardware platform
14432 @cindex platform, hardware
14433 Print the hardware platform name
14434 (sometimes called the hardware implementation).
14435 Print @samp{unknown} if the kernel does not make this information
14436 easily available, as is the case with Linux kernels.
14442 @cindex machine type
14443 @cindex hardware class
14444 @cindex hardware type
14445 Print the machine hardware name (sometimes called the hardware class
14451 @opindex --nodename
14454 @cindex network node name
14455 Print the network node hostname.
14460 @opindex --processor
14461 @cindex host processor type
14462 Print the processor type (sometimes called the instruction set
14463 architecture or ISA).
14464 Print @samp{unknown} if the kernel does not make this information
14465 easily available, as is the case with Linux kernels.
14468 @itemx --operating-system
14470 @opindex --operating-system
14471 @cindex operating system name
14472 Print the name of the operating system.
14475 @itemx --kernel-release
14477 @opindex --kernel-release
14478 @cindex kernel release
14479 @cindex release of kernel
14480 Print the kernel release.
14483 @itemx --kernel-name
14485 @opindex --kernel-name
14486 @cindex kernel name
14487 @cindex name of kernel
14488 Print the kernel name.
14489 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14490 ``the implementation of the operating system'', because the
14491 @acronym{POSIX} specification itself has no notion of ``kernel''.
14492 The kernel name might be the same as the operating system name printed
14493 by the @option{-o} or @option{--operating-system} option, but it might
14494 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14495 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14499 @itemx --kernel-version
14501 @opindex --kernel-version
14502 @cindex kernel version
14503 @cindex version of kernel
14504 Print the kernel version.
14511 @node hostname invocation
14512 @section @command{hostname}: Print or set system name
14515 @cindex setting the hostname
14516 @cindex printing the hostname
14517 @cindex system name, printing
14518 @cindex appropriate privileges
14520 With no arguments, @command{hostname} prints the name of the current host
14521 system. With one argument, it sets the current host name to the
14522 specified string. You must have appropriate privileges to set the host
14526 hostname [@var{name}]
14529 The only options are @option{--help} and @option{--version}. @xref{Common
14535 @node hostid invocation
14536 @section @command{hostid}: Print numeric host identifier
14539 @cindex printing the host identifier
14541 @command{hostid} prints the numeric identifier of the current host
14542 in hexadecimal. This command accepts no arguments.
14543 The only options are @option{--help} and @option{--version}.
14544 @xref{Common options}.
14546 For example, here's what it prints on one system I use:
14553 On that system, the 32-bit quantity happens to be closely
14554 related to the system's Internet address, but that isn't always
14559 @node uptime invocation
14560 @section @command{uptime}: Print system uptime and load
14563 @cindex printing the system uptime and load
14565 @command{uptime} prints the current time, the system's uptime, the
14566 number of logged-in users and the current load average.
14568 If an argument is specified, it is used as the file to be read
14569 to discover how many users are logged in. If no argument is
14570 specified, a system default is used (@command{uptime --help} indicates
14571 the default setting).
14573 The only options are @option{--help} and @option{--version}.
14574 @xref{Common options}.
14576 For example, here's what it prints right now on one system I use:
14580 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14583 The precise method of calculation of load average varies somewhat
14584 between systems. Some systems calculate it as the average number of
14585 runnable processes over the last 1, 5 and 15 minutes, but some systems
14586 also include processes in the uninterruptible sleep state (that is,
14587 those processes which are waiting for disk I/O). The Linux kernel
14588 includes uninterruptible processes.
14590 @node SELinux context
14591 @chapter SELinux context
14593 @cindex SELinux context
14594 @cindex SELinux, context
14595 @cindex commands for SELinux context
14597 This section describes commands for operations with SELinux
14601 * chcon invocation:: Change SELinux context of file
14602 * runcon invocation:: Run a command in specified SELinux context
14605 @node chcon invocation
14606 @section @command{chcon}: Change SELinux context of file
14609 @cindex changing security context
14610 @cindex change SELinux context
14612 @command{chcon} changes the SELinux security context of the selected files.
14616 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14617 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}]@c
14618 [-t @var{type}] @var{file}@dots{}
14619 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14622 Change the SELinux security context of each @var{file} to @var{context}.
14623 With @option{--reference}, change the security context of each @var{file}
14624 to that of @var{rfile}.
14626 The program accepts the following options. Also see @ref{Common options}.
14631 @itemx --no-dereference
14633 @opindex --no-dereference
14634 @cindex no dereference
14635 Affect symbolic links instead of any referenced file.
14637 @item --reference=@var{rfile}
14638 @opindex --reference
14639 @cindex reference file
14640 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14645 @opindex --recursive
14646 Operate on files and directories recursively.
14649 @xref{Traversing symlinks}.
14652 @xref{Traversing symlinks}.
14655 @xref{Traversing symlinks}.
14662 Output a diagnostic for every file processed.
14664 @item -u @var{user}
14665 @itemx --user=@var{user}
14668 Set user @var{user} in the target security context.
14670 @item -r @var{role}
14671 @itemx --role=@var{role}
14674 Set role @var{role} in the target security context.
14676 @item -t @var{type}
14677 @itemx --type=@var{type}
14680 Set type @var{type} in the target security context.
14682 @item -l @var{range}
14683 @itemx --range=@var{range}
14686 Set range @var{range} in the target security context.
14692 @node runcon invocation
14693 @section @command{runcon}: Run a command in specified SELinux context
14696 @cindex run with security context
14699 @command{runcon} runs file in specified SELinux security context.
14703 runcon @var{context} @var{command} [@var{args}]
14704 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}]@c
14705 [-l @var{range}] @var{command} [@var{args}]
14708 Run @var{command} with completely-specified @var{context}, or with
14709 current or transitioned security context modified by one or more of @var{level},
14710 @var{role}, @var{type} and @var{user}.
14712 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14713 is specified, the first argument is used as the complete context.
14714 Any additional arguments after @var{command}
14715 are interpreted as arguments to the command.
14717 With neither @var{context} nor @var{command}, print the current
14720 The program accepts the following options. Also see @ref{Common options}.
14728 Compute process transition context before modifying.
14730 @item -u @var{user}
14731 @itemx --user=@var{user}
14734 Set user @var{user} in the target security context.
14736 @item -r @var{role}
14737 @itemx --role=@var{role}
14740 Set role @var{role} in the target security context.
14742 @item -t @var{type}
14743 @itemx --type=@var{type}
14746 Set type @var{type} in the target security context.
14748 @item -l @var{range}
14749 @itemx --range=@var{range}
14752 Set range @var{range} in the target security context.
14756 @cindex exit status of @command{runcon}
14760 126 if @var{command} is found but cannot be invoked
14761 127 if @command{runcon} itself fails or if @var{command} cannot be found
14762 the exit status of @var{command} otherwise
14765 @node Modified command invocation
14766 @chapter Modified command invocation
14768 @cindex modified command invocation
14769 @cindex invocation of commands, modified
14770 @cindex commands for invoking other commands
14772 This section describes commands that run other commands in some context
14773 different than the current one: a modified environment, as a different
14777 * chroot invocation:: Modify the root directory.
14778 * env invocation:: Modify environment variables.
14779 * nice invocation:: Modify niceness.
14780 * nohup invocation:: Immunize to hangups.
14781 * stdbuf invocation:: Modify buffering of standard streams.
14782 * su invocation:: Modify user and group ID.
14783 * timeout invocation:: Run with time limit.
14787 @node chroot invocation
14788 @section @command{chroot}: Run a command with a different root directory
14791 @cindex running a program in a specified root directory
14792 @cindex root directory, running a program in a specified
14794 @command{chroot} runs a command with a specified root directory.
14795 On many systems, only the super-user can do this.@footnote{However,
14796 some systems (e.g., FreeBSD) can be configured to allow certain regular
14797 users to use the @code{chroot} system call, and hence to run this program.
14798 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14799 underlying function is non-privileged due to lack of support in MS-Windows.}
14803 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14804 chroot @var{option}
14807 Ordinarily, file names are looked up starting at the root of the
14808 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14809 the directory @var{newroot} (which must exist) and then runs
14810 @var{command} with optional @var{args}. If @var{command} is not
14811 specified, the default is the value of the @env{SHELL} environment
14812 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14813 @var{command} must not be a special built-in utility
14814 (@pxref{Special built-in utilities}).
14816 The program accepts the following options. Also see @ref{Common options}.
14817 Options must precede operands.
14821 @itemx --userspec=@var{user}[:@var{group}]
14822 @opindex --userspec
14823 By default, @var{command} is run with the same credentials
14824 as the invoking process.
14825 Use this option to run it as a different @var{user} and/or with a
14826 different primary @var{group}.
14828 @itemx --groups=@var{groups}
14830 Use this option to specify the supplementary @var{groups} to be
14831 used by the new process.
14832 The items in the list (names or numeric IDs) must be separated by commas.
14836 Here are a few tips to help avoid common problems in using chroot.
14837 To start with a simple example, make @var{command} refer to a statically
14838 linked binary. If you were to use a dynamically linked executable, then
14839 you'd have to arrange to have the shared libraries in the right place under
14840 your new root directory.
14842 For example, if you create a statically linked @command{ls} executable,
14843 and put it in @file{/tmp/empty}, you can run this command as root:
14846 $ chroot /tmp/empty /ls -Rl /
14849 Then you'll see output like this:
14854 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14857 If you want to use a dynamically linked executable, say @command{bash},
14858 then first run @samp{ldd bash} to see what shared objects it needs.
14859 Then, in addition to copying the actual binary, also copy the listed
14860 files to the required positions under your intended new root directory.
14861 Finally, if the executable requires any other files (e.g., data, state,
14862 device files), copy them into place, too.
14864 @cindex exit status of @command{chroot}
14868 125 if @command{chroot} itself fails
14869 126 if @var{command} is found but cannot be invoked
14870 127 if @var{command} cannot be found
14871 the exit status of @var{command} otherwise
14875 @node env invocation
14876 @section @command{env}: Run a command in a modified environment
14879 @cindex environment, running a program in a modified
14880 @cindex modified environment, running a program in a
14881 @cindex running a program in a modified environment
14883 @command{env} runs a command with a modified environment. Synopses:
14886 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14887 [@var{command} [@var{args}]@dots{}]
14891 Operands of the form @samp{@var{variable}=@var{value}} set
14892 the environment variable @var{variable} to value @var{value}.
14893 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14894 to an empty value is different from unsetting it.
14895 These operands are evaluated left-to-right, so if two operands
14896 mention the same variable the earlier is ignored.
14898 Environment variable names can be empty, and can contain any
14899 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14900 However, it is wise to limit yourself to names that
14901 consist solely of underscores, digits, and @acronym{ASCII} letters,
14902 and that begin with a non-digit, as applications like the shell do not
14903 work well with other names.
14906 The first operand that does not contain the character @samp{=}
14907 specifies the program to invoke; it is
14908 searched for according to the @env{PATH} environment variable. Any
14909 remaining arguments are passed as arguments to that program.
14910 The program should not be a special built-in utility
14911 (@pxref{Special built-in utilities}).
14913 Modifications to @env{PATH} take effect prior to searching for
14914 @var{command}. Use caution when reducing @env{PATH}; behavior is
14915 not portable when @env{PATH} is undefined or omits key directories
14916 such as @file{/bin}.
14918 In the rare case that a utility contains a @samp{=} in the name, the
14919 only way to disambiguate it from a variable assignment is to use an
14920 intermediate command for @var{command}, and pass the problematic
14921 program name via @var{args}. For example, if @file{./prog=} is an
14922 executable in the current @env{PATH}:
14925 env prog= true # runs 'true', with prog= in environment
14926 env ./prog= true # runs 'true', with ./prog= in environment
14927 env -- prog= true # runs 'true', with prog= in environment
14928 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14929 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14932 @cindex environment, printing
14934 If no command name is specified following the environment
14935 specifications, the resulting environment is printed. This is like
14936 specifying the @command{printenv} program.
14938 For some examples, suppose the environment passed to @command{env}
14939 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14940 @samp{PATH=.:/gnubin:/hacks}:
14945 Output the current environment.
14947 $ env | LC_ALL=C sort
14950 PATH=.:/gnubin:/hacks
14954 Run @command{foo} with a reduced environment, preserving only the
14955 original @env{PATH} to avoid problems in locating @command{foo}.
14957 env - PATH="$PATH" foo
14961 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14962 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14963 that @command{foo} was found in the file system rather than as a shell
14970 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14971 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14972 @samp{DISPLAY=gnu:0}.
14974 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14978 Attempt to run the program @command{/energy/--} (as that is the only
14979 possible path search result); if the command exists, the environment
14980 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14981 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14983 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14989 The program accepts the following options. Also see @ref{Common options}.
14990 Options must precede operands.
14996 @item -u @var{name}
14997 @itemx --unset=@var{name}
15000 Remove variable @var{name} from the environment, if it was in the
15005 @itemx --ignore-environment
15008 @opindex --ignore-environment
15009 Start with an empty environment, ignoring the inherited environment.
15013 @cindex exit status of @command{env}
15017 0 if no @var{command} is specified and the environment is output
15018 125 if @command{env} itself fails
15019 126 if @var{command} is found but cannot be invoked
15020 127 if @var{command} cannot be found
15021 the exit status of @var{command} otherwise
15025 @node nice invocation
15026 @section @command{nice}: Run a command with modified niceness
15030 @cindex scheduling, affecting
15031 @cindex appropriate privileges
15033 @command{nice} prints or modifies a process's @dfn{niceness},
15034 a parameter that affects whether the process is scheduled favorably.
15038 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15041 If no arguments are given, @command{nice} prints the current niceness.
15042 Otherwise, @command{nice} runs the given @var{command} with its
15043 niceness adjusted. By default, its niceness is incremented by 10.
15045 Niceness values range at least from @minus{}20 (process has high priority
15046 and gets more resources, thus slowing down other processes) through 19
15047 (process has lower priority and runs slowly itself, but has less impact
15048 on the speed of other running processes). Some systems
15049 may have a wider range of nicenesses; conversely, other systems may
15050 enforce more restrictive limits. An attempt to set the niceness
15051 outside the supported range is treated as an attempt to use the
15052 minimum or maximum supported value.
15054 A niceness should not be confused with a scheduling priority, which
15055 lets applications determine the order in which threads are scheduled
15056 to run. Unlike a priority, a niceness is merely advice to the
15057 scheduler, which the scheduler is free to ignore. Also, as a point of
15058 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15059 terms of a @dfn{nice value}, which is the nonnegative difference
15060 between a niceness and the minimum niceness. Though @command{nice}
15061 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15062 term ``niceness'' for compatibility with historical practice.
15064 @var{command} must not be a special built-in utility (@pxref{Special
15065 built-in utilities}).
15067 @mayConflictWithShellBuiltIn{nice}
15069 The program accepts the following option. Also see @ref{Common options}.
15070 Options must precede operands.
15073 @item -n @var{adjustment}
15074 @itemx --adjustment=@var{adjustment}
15076 @opindex --adjustment
15077 Add @var{adjustment} instead of 10 to the command's niceness. If
15078 @var{adjustment} is negative and you lack appropriate privileges,
15079 @command{nice} issues a warning but otherwise acts as if you specified
15082 For compatibility @command{nice} also supports an obsolete
15083 option syntax @option{-@var{adjustment}}. New scripts should use
15084 @option{-n @var{adjustment}} instead.
15088 @cindex exit status of @command{nice}
15092 0 if no @var{command} is specified and the niceness is output
15093 125 if @command{nice} 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
15099 It is sometimes useful to run a non-interactive program with reduced niceness.
15102 $ nice factor 4611686018427387903
15105 Since @command{nice} prints the current niceness,
15106 you can invoke it through itself to demonstrate how it works.
15108 The default behavior is to increase the niceness by @samp{10}:
15119 The @var{adjustment} is relative to the current niceness. In the
15120 next example, the first @command{nice} invocation runs the second one
15121 with niceness 10, and it in turn runs the final one with a niceness
15125 $ nice nice -n 3 nice
15129 Specifying a niceness larger than the supported range
15130 is the same as specifying the maximum supported value:
15133 $ nice -n 10000000000 nice
15137 Only a privileged user may run a process with lower niceness:
15141 nice: cannot set niceness: Permission denied
15143 $ sudo nice -n -1 nice
15148 @node nohup invocation
15149 @section @command{nohup}: Run a command immune to hangups
15152 @cindex hangups, immunity to
15153 @cindex immunity to hangups
15154 @cindex logging out and continuing to run
15157 @command{nohup} runs the given @var{command} with hangup signals ignored,
15158 so that the command can continue running in the background after you log
15162 nohup @var{command} [@var{arg}]@dots{}
15165 If standard input is a terminal, it is redirected from
15166 @file{/dev/null} so that terminal sessions do not mistakenly consider
15167 the terminal to be used by the command. This is a @acronym{GNU}
15168 extension; programs intended to be portable to non-@acronym{GNU} hosts
15169 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15173 If standard output is a terminal, the command's standard output is appended
15174 to the file @file{nohup.out}; if that cannot be written to, it is appended
15175 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15176 command is not run.
15177 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15178 @command{nohup} is made readable and writable only to the user,
15179 regardless of the current umask settings.
15181 If standard error is a terminal, it is normally redirected to the same file
15182 descriptor as the (possibly-redirected) standard output.
15183 However, if standard output is closed, standard error terminal output
15184 is instead appended to the file @file{nohup.out} or
15185 @file{$HOME/nohup.out} as above.
15187 To capture the command's output to a file other than @file{nohup.out}
15188 you can redirect it. For example, to capture the output of
15192 nohup make > make.log
15195 @command{nohup} does not automatically put the command it runs in the
15196 background; you must do that explicitly, by ending the command line
15197 with an @samp{&}. Also, @command{nohup} does not alter the
15198 niceness of @var{command}; use @command{nice} for that,
15199 e.g., @samp{nohup nice @var{command}}.
15201 @var{command} must not be a special built-in utility (@pxref{Special
15202 built-in utilities}).
15204 The only options are @option{--help} and @option{--version}. @xref{Common
15205 options}. Options must precede operands.
15207 @cindex exit status of @command{nohup}
15211 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15212 126 if @var{command} is found but cannot be invoked
15213 127 if @var{command} cannot be found
15214 the exit status of @var{command} otherwise
15217 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15221 @node stdbuf invocation
15222 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15225 @cindex standard streams, buffering
15226 @cindex line buffered
15228 @command{stdbuf} allows one to modify the buffering operations of the
15229 three standard I/O streams associated with a program. Synopsis:
15232 stdbuf @var{option}@dots{} @var{command}
15235 Any additional @var{arg}s are passed as additional arguments to the
15238 The program accepts the following options. Also see @ref{Common options}.
15242 @item -i @var{mode}
15243 @itemx --input=@var{mode}
15246 Adjust the standard input stream buffering.
15248 @item -o @var{mode}
15249 @itemx --output=@var{mode}
15252 Adjust the standard output stream buffering.
15254 @item -e @var{mode}
15255 @itemx --error=@var{mode}
15258 Adjust the standard error stream buffering.
15262 The @var{mode} can be specified as follows:
15267 Set the stream to line buffered mode.
15268 In this mode data is coalesced until a newline is output or
15269 input is read from any stream attached to a terminal device.
15270 This option is invalid with standard input.
15273 Disable buffering of the selected stream.
15274 In this mode data is output immediately and only the
15275 amount of data requested is read from input.
15278 Specify the size of the buffer to use in fully buffered mode.
15279 @multiplierSuffixesNoBlocks{size}
15283 NOTE: If @var{command} adjusts the buffering of its standard streams
15284 (@command{tee} does for e.g.) then that will override corresponding settings
15285 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15286 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15287 by @command{stdbuf} settings.
15289 @cindex exit status of @command{stdbuf}
15293 125 if @command{stdbuf} itself fails
15294 126 if @var{command} is found but cannot be invoked
15295 127 if @var{command} cannot be found
15296 the exit status of @var{command} otherwise
15300 @node su invocation
15301 @section @command{su}: Run a command with substitute user and group ID
15304 @cindex substitute user and group IDs
15305 @cindex user ID, switching
15306 @cindex super-user, becoming
15307 @cindex root, becoming
15309 @command{su} allows one user to temporarily become another user. It runs a
15310 command (often an interactive shell) with the real and effective user
15311 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15314 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15317 @cindex passwd entry, and @command{su} shell
15319 @flindex /etc/passwd
15320 If no @var{user} is given, the default is @code{root}, the super-user.
15321 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15322 @file{/bin/sh} if none is specified there. If @var{user} has a
15323 password, @command{su} prompts for the password unless run by a user with
15324 effective user ID of zero (the super-user).
15330 @cindex login shell
15331 By default, @command{su} does not change the current directory.
15332 It sets the environment variables @env{HOME} and @env{SHELL}
15333 from the password entry for @var{user}, and if @var{user} is not
15334 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15335 By default, the shell is not a login shell.
15337 Any additional @var{arg}s are passed as additional arguments to the
15340 @cindex @option{-su}
15341 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15342 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15343 to certain shells, etc.).
15346 @command{su} can optionally be compiled to use @code{syslog} to report
15347 failed, and optionally successful, @command{su} attempts. (If the system
15348 supports @code{syslog}.) However, GNU @command{su} does not check if the
15349 user is a member of the @code{wheel} group; see below.
15351 The program accepts the following options. Also see @ref{Common options}.
15354 @item -c @var{command}
15355 @itemx --command=@var{command}
15358 Pass @var{command}, a single command line to run, to the shell with
15359 a @option{-c} option instead of starting an interactive shell.
15366 @cindex file name pattern expansion, disabled
15367 @cindex globbing, disabled
15368 Pass the @option{-f} option to the shell. This probably only makes sense
15369 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15370 option prevents reading the startup file (@file{.cshrc}). With
15371 Bourne-like shells, the @option{-f} option disables file name pattern
15372 expansion (globbing), which is not likely to be useful.
15380 @c other variables already indexed above
15383 @cindex login shell, creating
15384 Make the shell a login shell. This means the following. Unset all
15385 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15386 (which are set as described above), and @env{USER} and @env{LOGNAME}
15387 (which are set, even for the super-user, as described above), and set
15388 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15389 directory. Prepend @samp{-} to the shell's name, intended to make it
15390 read its login startup file(s).
15394 @itemx --preserve-environment
15397 @opindex --preserve-environment
15398 @cindex environment, preserving
15399 @flindex /etc/shells
15400 @cindex restricted shell
15401 Do not change the environment variables @env{HOME}, @env{USER},
15402 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15403 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15404 entry, unless the user running @command{su} is not the super-user and
15405 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15406 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15407 if that file does not exist. Parts of what this option does can be
15408 overridden by @option{--login} and @option{--shell}.
15410 @item -s @var{shell}
15411 @itemx --shell=@var{shell}
15414 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15415 unless the user running @command{su} is not the super-user and @var{user}'s
15416 shell is restricted (see @option{-m} just above).
15420 @cindex exit status of @command{su}
15424 125 if @command{su} itself fails
15425 126 if subshell is found but cannot be invoked
15426 127 if subshell cannot be found
15427 the exit status of the subshell otherwise
15430 @cindex wheel group, not supported
15431 @cindex group wheel, not supported
15433 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15435 (This section is by Richard Stallman.)
15439 Sometimes a few of the users try to hold total power over all the
15440 rest. For example, in 1984, a few users at the MIT AI lab decided to
15441 seize power by changing the operator password on the Twenex system and
15442 keeping it secret from everyone else. (I was able to thwart this coup
15443 and give power back to the users by patching the kernel, but I
15444 wouldn't know how to do that in Unix.)
15446 However, occasionally the rulers do tell someone. Under the usual
15447 @command{su} mechanism, once someone learns the root password who
15448 sympathizes with the ordinary users, he or she can tell the rest. The
15449 ``wheel group'' feature would make this impossible, and thus cement the
15450 power of the rulers.
15452 I'm on the side of the masses, not that of the rulers. If you are
15453 used to supporting the bosses and sysadmins in whatever they do, you
15454 might find this idea strange at first.
15457 @node timeout invocation
15458 @section @command{timeout}: Run a command with a time limit
15462 @cindex run commands with bounded time
15464 @command{timeout} runs the given @var{command} and kills it if it is
15465 still running after the specified time interval. Synopsis:
15468 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15471 @var{command} must not be a special built-in utility (@pxref{Special
15472 built-in utilities}).
15474 The program accepts the following options. Also see @ref{Common options}.
15475 Options must precede operands.
15478 @item -k @var{duration}
15479 @itemx --kill-after=@var{duration}
15481 @opindex --kill-after
15482 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15483 signal, after the specified @var{duration}. Without this option, if the
15484 selected signal proves not to be fatal, @command{timeout} does not kill
15487 @item -s @var{signal}
15488 @itemx --signal=@var{signal}
15491 Send this @var{signal} to @var{command} on timeout, rather than the
15492 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15493 or a number. Also see @xref{Signal specifications}.
15497 @var{duration} is an integer followed by an optional unit:
15499 @samp{s} for seconds (the default)
15500 @samp{m} for minutes
15504 A duration of 0 disables the associated timeout.
15506 @cindex exit status of @command{timeout}
15510 124 if @var{command} times out
15511 125 if @command{timeout} itself fails
15512 126 if @var{command} is found but cannot be invoked
15513 127 if @var{command} cannot be found
15514 the exit status of @var{command} otherwise
15518 @node Process control
15519 @chapter Process control
15521 @cindex processes, commands for controlling
15522 @cindex commands for controlling processes
15525 * kill invocation:: Sending a signal to processes.
15529 @node kill invocation
15530 @section @command{kill}: Send a signal to processes
15533 @cindex send a signal to processes
15535 The @command{kill} command sends a signal to processes, causing them
15536 to terminate or otherwise act upon receiving the signal in some way.
15537 Alternatively, it lists information about signals. Synopses:
15540 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15541 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15544 @mayConflictWithShellBuiltIn{kill}
15546 The first form of the @command{kill} command sends a signal to all
15547 @var{pid} arguments. The default signal to send if none is specified
15548 is @samp{TERM}. The special signal number @samp{0} does not denote a
15549 valid signal, but can be used to test whether the @var{pid} arguments
15550 specify processes to which a signal could be sent.
15552 If @var{pid} is positive, the signal is sent to the process with the
15553 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15554 processes in the process group of the current process. If @var{pid}
15555 is @minus{}1, the signal is sent to all processes for which the user has
15556 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15557 is sent to all processes in the process group that equals the absolute
15558 value of @var{pid}.
15560 If @var{pid} is not positive, a system-dependent set of system
15561 processes is excluded from the list of processes to which the signal
15564 If a negative @var{pid} argument is desired as the first one, it
15565 should be preceded by @option{--}. However, as a common extension to
15566 @acronym{POSIX}, @option{--} is not required with @samp{kill
15567 -@var{signal} -@var{pid}}. The following commands are equivalent:
15576 The first form of the @command{kill} command succeeds if every @var{pid}
15577 argument specifies at least one process that the signal was sent to.
15579 The second form of the @command{kill} command lists signal information.
15580 Either the @option{-l} or @option{--list} option, or the @option{-t}
15581 or @option{--table} option must be specified. Without any
15582 @var{signal} argument, all supported signals are listed. The output
15583 of @option{-l} or @option{--list} is a list of the signal names, one
15584 per line; if @var{signal} is already a name, the signal number is
15585 printed instead. The output of @option{-t} or @option{--table} is a
15586 table of signal numbers, names, and descriptions. This form of the
15587 @command{kill} command succeeds if all @var{signal} arguments are valid
15588 and if there is no output error.
15590 The @command{kill} command also supports the @option{--help} and
15591 @option{--version} options. @xref{Common options}.
15593 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15594 number like @samp{1}, or an exit status of a process terminated by the
15595 signal. A signal name can be given in canonical form or prefixed by
15596 @samp{SIG}. The case of the letters is ignored, except for the
15597 @option{-@var{signal}} option which must use upper case to avoid
15598 ambiguity with lower case option letters. For a list of supported
15599 signal names and numbers see @xref{Signal specifications}.
15604 @cindex delaying commands
15605 @cindex commands for delaying
15607 @c Perhaps @command{wait} or other commands should be described here also?
15610 * sleep invocation:: Delay for a specified time.
15614 @node sleep invocation
15615 @section @command{sleep}: Delay for a specified time
15618 @cindex delay for a specified time
15620 @command{sleep} pauses for an amount of time specified by the sum of
15621 the values of the command line arguments.
15625 sleep @var{number}[smhd]@dots{}
15629 Each argument is a number followed by an optional unit; the default
15630 is seconds. The units are:
15643 Historical implementations of @command{sleep} have required that
15644 @var{number} be an integer, and only accepted a single argument
15645 without a suffix. However, GNU @command{sleep} accepts
15646 arbitrary floating point numbers (using a period before any fractional
15649 The only options are @option{--help} and @option{--version}. @xref{Common
15652 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15653 @mayConflictWithShellBuiltIn{sleep}
15658 @node Numeric operations
15659 @chapter Numeric operations
15661 @cindex numeric operations
15662 These programs do numerically-related operations.
15665 * factor invocation:: Show factors of numbers.
15666 * seq invocation:: Print sequences of numbers.
15670 @node factor invocation
15671 @section @command{factor}: Print prime factors
15674 @cindex prime factors
15676 @command{factor} prints prime factors. Synopses:
15679 factor [@var{number}]@dots{}
15680 factor @var{option}
15683 If no @var{number} is specified on the command line, @command{factor} reads
15684 numbers from standard input, delimited by newlines, tabs, or spaces.
15686 The @command{factor} command supports only a small number of options:
15690 Print a short help on standard output, then exit without further
15694 Print the program version on standard output, then exit without further
15698 Factoring the product of the eighth and ninth Mersenne primes
15699 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15702 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15703 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15704 4951760154835678088235319297: 2147483647 2305843009213693951
15708 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15709 about 20 seconds on the same machine.
15711 Factoring large numbers is, in general, hard. The Pollard Rho
15712 algorithm used by @command{factor} is particularly effective for
15713 numbers with relatively small factors. If you wish to factor large
15714 numbers which do not have small factors (for example, numbers which
15715 are the product of two large primes), other methods are far better.
15717 If @command{factor} is built without using GNU MP, only
15718 single-precision arithmetic is available, and so large numbers
15719 (typically @math{2^{64}} and above) will not be supported. The single-precision
15720 code uses an algorithm which is designed for factoring smaller
15726 @node seq invocation
15727 @section @command{seq}: Print numeric sequences
15730 @cindex numeric sequences
15731 @cindex sequence of numbers
15733 @command{seq} prints a sequence of numbers to standard output. Synopses:
15736 seq [@var{option}]@dots{} @var{last}
15737 seq [@var{option}]@dots{} @var{first} @var{last}
15738 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15741 @command{seq} prints the numbers from @var{first} to @var{last} by
15742 @var{increment}. By default, each number is printed on a separate line.
15743 When @var{increment} is not specified, it defaults to @samp{1},
15744 even when @var{first} is larger than @var{last}.
15745 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15746 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15747 Floating-point numbers
15748 may be specified (using a period before any fractional digits).
15750 The program accepts the following options. Also see @ref{Common options}.
15751 Options must precede operands.
15754 @item -f @var{format}
15755 @itemx --format=@var{format}
15756 @opindex -f @var{format}
15757 @opindex --format=@var{format}
15758 @cindex formatting of numbers in @command{seq}
15759 Print all numbers using @var{format}.
15760 @var{format} must contain exactly one of the @samp{printf}-style
15761 floating point conversion specifications @samp{%a}, @samp{%e},
15762 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15763 The @samp{%} may be followed by zero or more flags taken from the set
15764 @samp{-+#0 '}, then an optional width containing one or more digits,
15765 then an optional precision consisting of a @samp{.} followed by zero
15766 or more digits. @var{format} may also contain any number of @samp{%%}
15767 conversion specifications. All conversion specifications have the
15768 same meaning as with @samp{printf}.
15770 The default format is derived from @var{first}, @var{step}, and
15771 @var{last}. If these all use a fixed point decimal representation,
15772 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15773 precision that can represent the output numbers exactly. Otherwise,
15774 the default format is @samp{%g}.
15776 @item -s @var{string}
15777 @itemx --separator=@var{string}
15778 @cindex separator for numbers in @command{seq}
15779 Separate numbers with @var{string}; default is a newline.
15780 The output always terminates with a newline.
15783 @itemx --equal-width
15784 Print all numbers with the same width, by padding with leading zeros.
15785 @var{first}, @var{step}, and @var{last} should all use a fixed point
15786 decimal representation.
15787 (To have other kinds of padding, use @option{--format}).
15791 You can get finer-grained control over output with @option{-f}:
15794 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15800 If you want hexadecimal integer output, you can use @command{printf}
15801 to perform the conversion:
15804 $ printf '%x\n' `seq 1048575 1024 1050623`
15810 For very long lists of numbers, use xargs to avoid
15811 system limitations on the length of an argument list:
15814 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15820 To generate octal output, use the printf @code{%o} format instead
15823 On most systems, seq can produce whole-number output for values up to
15824 at least @math{2^{53}}. Larger integers are approximated. The details
15825 differ depending on your floating-point implementation, but a common
15826 case is that @command{seq} works with integers through @math{2^{64}},
15827 and larger integers may not be numerically correct:
15830 $ seq 18446744073709551616 1 18446744073709551618
15831 18446744073709551616
15832 18446744073709551616
15833 18446744073709551618
15836 Be careful when using @command{seq} with outlandish values: otherwise
15837 you may see surprising results, as @command{seq} uses floating point
15838 internally. For example, on the x86 platform, where the internal
15839 representation uses a 64-bit fraction, the command:
15842 seq 1 0.0000000000000000001 1.0000000000000000009
15845 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15850 @node File permissions
15851 @chapter File permissions
15854 @include parse-datetime.texi
15858 @node Opening the software toolbox
15859 @chapter Opening the Software Toolbox
15861 An earlier version of this chapter appeared in
15862 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15863 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15864 It was written by Arnold Robbins.
15867 * Toolbox introduction:: Toolbox introduction
15868 * I/O redirection:: I/O redirection
15869 * The who command:: The @command{who} command
15870 * The cut command:: The @command{cut} command
15871 * The sort command:: The @command{sort} command
15872 * The uniq command:: The @command{uniq} command
15873 * Putting the tools together:: Putting the tools together
15877 @node Toolbox introduction
15878 @unnumberedsec Toolbox Introduction
15880 This month's column is only peripherally related to the GNU Project, in
15881 that it describes a number of the GNU tools on your GNU/Linux system
15883 might be used. What it's really about is the ``Software Tools'' philosophy
15884 of program development and usage.
15886 The software tools philosophy was an important and integral concept
15887 in the initial design and development of Unix (of which Linux and GNU are
15888 essentially clones). Unfortunately, in the modern day press of
15889 Internetworking and flashy GUIs, it seems to have fallen by the
15890 wayside. This is a shame, since it provides a powerful mental model
15891 for solving many kinds of problems.
15893 Many people carry a Swiss Army knife around in their pants pockets (or
15894 purse). A Swiss Army knife is a handy tool to have: it has several knife
15895 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15896 a number of other things on it. For the everyday, small miscellaneous jobs
15897 where you need a simple, general purpose tool, it's just the thing.
15899 On the other hand, an experienced carpenter doesn't build a house using
15900 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15901 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15902 exactly when and where to use each tool; you won't catch him hammering nails
15903 with the handle of his screwdriver.
15905 The Unix developers at Bell Labs were all professional programmers and trained
15906 computer scientists. They had found that while a one-size-fits-all program
15907 might appeal to a user because there's only one program to use, in practice
15912 difficult to write,
15915 difficult to maintain and
15919 difficult to extend to meet new situations.
15922 Instead, they felt that programs should be specialized tools. In short, each
15923 program ``should do one thing well.'' No more and no less. Such programs are
15924 simpler to design, write, and get right---they only do one thing.
15926 Furthermore, they found that with the right machinery for hooking programs
15927 together, that the whole was greater than the sum of the parts. By combining
15928 several special purpose programs, you could accomplish a specific task
15929 that none of the programs was designed for, and accomplish it much more
15930 quickly and easily than if you had to write a special purpose program.
15931 We will see some (classic) examples of this further on in the column.
15932 (An important additional point was that, if necessary, take a detour
15933 and build any software tools you may need first, if you don't already
15934 have something appropriate in the toolbox.)
15936 @node I/O redirection
15937 @unnumberedsec I/O Redirection
15939 Hopefully, you are familiar with the basics of I/O redirection in the
15940 shell, in particular the concepts of ``standard input,'' ``standard output,''
15941 and ``standard error''. Briefly, ``standard input'' is a data source, where
15942 data comes from. A program should not need to either know or care if the
15943 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15944 card reader. Similarly, ``standard output'' is a data sink, where data goes
15945 to. The program should neither know nor care where this might be.
15946 Programs that only read their standard input, do something to the data,
15947 and then send it on, are called @dfn{filters}, by analogy to filters in a
15950 With the Unix shell, it's very easy to set up data pipelines:
15953 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15956 We start out by creating the raw data; each filter applies some successive
15957 transformation to the data, until by the time it comes out of the pipeline,
15958 it is in the desired form.
15960 This is fine and good for standard input and standard output. Where does the
15961 standard error come in to play? Well, think about @command{filter1} in
15962 the pipeline above. What happens if it encounters an error in the data it
15963 sees? If it writes an error message to standard output, it will just
15964 disappear down the pipeline into @command{filter2}'s input, and the
15965 user will probably never see it. So programs need a place where they can send
15966 error messages so that the user will notice them. This is standard error,
15967 and it is usually connected to your console or window, even if you have
15968 redirected standard output of your program away from your screen.
15970 For filter programs to work together, the format of the data has to be
15971 agreed upon. The most straightforward and easiest format to use is simply
15972 lines of text. Unix data files are generally just streams of bytes, with
15973 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15974 conventionally called a ``newline'' in the Unix literature. (This is
15975 @code{'\n'} if you're a C programmer.) This is the format used by all
15976 the traditional filtering programs. (Many earlier operating systems
15977 had elaborate facilities and special purpose programs for managing
15978 binary data. Unix has always shied away from such things, under the
15979 philosophy that it's easiest to simply be able to view and edit your
15980 data with a text editor.)
15982 OK, enough introduction. Let's take a look at some of the tools, and then
15983 we'll see how to hook them together in interesting ways. In the following
15984 discussion, we will only present those command line options that interest
15985 us. As you should always do, double check your system documentation
15986 for the full story.
15988 @node The who command
15989 @unnumberedsec The @command{who} Command
15991 The first program is the @command{who} command. By itself, it generates a
15992 list of the users who are currently logged in. Although I'm writing
15993 this on a single-user system, we'll pretend that several people are
15998 @print{} arnold console Jan 22 19:57
15999 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
16000 @print{} bill ttyp1 Jan 21 09:32(:0.0)
16001 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
16004 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
16005 There are three people logged in, and I am logged in twice. On traditional
16006 Unix systems, user names are never more than eight characters long. This
16007 little bit of trivia will be useful later. The output of @command{who} is nice,
16008 but the data is not all that exciting.
16010 @node The cut command
16011 @unnumberedsec The @command{cut} Command
16013 The next program we'll look at is the @command{cut} command. This program
16014 cuts out columns or fields of input data. For example, we can tell it
16015 to print just the login name and full name from the @file{/etc/passwd}
16016 file. The @file{/etc/passwd} file has seven fields, separated by
16020 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
16023 To get the first and fifth fields, we would use @command{cut} like this:
16026 $ cut -d: -f1,5 /etc/passwd
16027 @print{} root:Operator
16029 @print{} arnold:Arnold D. Robbins
16030 @print{} miriam:Miriam A. Robbins
16034 With the @option{-c} option, @command{cut} will cut out specific characters
16035 (i.e., columns) in the input lines. This is useful for input data
16036 that has fixed width fields, and does not have a field separator. For
16037 example, list the Monday dates for the current month:
16039 @c Is using cal ok? Looked at gcal, but I don't like it.
16050 @node The sort command
16051 @unnumberedsec The @command{sort} Command
16053 Next we'll look at the @command{sort} command. This is one of the most
16054 powerful commands on a Unix-style system; one that you will often find
16055 yourself using when setting up fancy data plumbing.
16058 command reads and sorts each file named on the command line. It then
16059 merges the sorted data and writes it to standard output. It will read
16060 standard input if no files are given on the command line (thus
16061 making it into a filter). The sort is based on the character collating
16062 sequence or based on user-supplied ordering criteria.
16065 @node The uniq command
16066 @unnumberedsec The @command{uniq} Command
16068 Finally (at least for now), we'll look at the @command{uniq} program. When
16069 sorting data, you will often end up with duplicate lines, lines that
16070 are identical. Usually, all you need is one instance of each line.
16071 This is where @command{uniq} comes in. The @command{uniq} program reads its
16072 standard input. It prints only one
16073 copy of each repeated line. It does have several options. Later on,
16074 we'll use the @option{-c} option, which prints each unique line, preceded
16075 by a count of the number of times that line occurred in the input.
16078 @node Putting the tools together
16079 @unnumberedsec Putting the Tools Together
16081 Now, let's suppose this is a large ISP server system with dozens of users
16082 logged in. The management wants the system administrator to write a
16084 generate a sorted list of logged in users. Furthermore, even if a user
16085 is logged in multiple times, his or her name should only show up in the
16088 The administrator could sit down with the system documentation and write a C
16089 program that did this. It would take perhaps a couple of hundred lines
16090 of code and about two hours to write it, test it, and debug it.
16091 However, knowing the software toolbox, the administrator can instead start out
16092 by generating just a list of logged on users:
16102 Next, sort the list:
16105 $ who | cut -c1-8 | sort
16112 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16115 $ who | cut -c1-8 | sort | uniq
16121 The @command{sort} command actually has a @option{-u} option that does what
16122 @command{uniq} does. However, @command{uniq} has other uses for which one
16123 cannot substitute @samp{sort -u}.
16125 The administrator puts this pipeline into a shell script, and makes it
16127 all the users on the system (@samp{#} is the system administrator,
16128 or @code{root}, prompt):
16131 # cat > /usr/local/bin/listusers
16132 who | cut -c1-8 | sort | uniq
16134 # chmod +x /usr/local/bin/listusers
16137 There are four major points to note here. First, with just four
16138 programs, on one command line, the administrator was able to save about two
16139 hours worth of work. Furthermore, the shell pipeline is just about as
16140 efficient as the C program would be, and it is much more efficient in
16141 terms of programmer time. People time is much more expensive than
16142 computer time, and in our modern ``there's never enough time to do
16143 everything'' society, saving two hours of programmer time is no mean
16146 Second, it is also important to emphasize that with the
16147 @emph{combination} of the tools, it is possible to do a special
16148 purpose job never imagined by the authors of the individual programs.
16150 Third, it is also valuable to build up your pipeline in stages, as we did here.
16151 This allows you to view the data at each stage in the pipeline, which helps
16152 you acquire the confidence that you are indeed using these tools correctly.
16154 Finally, by bundling the pipeline in a shell script, other users can use
16155 your command, without having to remember the fancy plumbing you set up for
16156 them. In terms of how you run them, shell scripts and compiled programs are
16159 After the previous warm-up exercise, we'll look at two additional, more
16160 complicated pipelines. For them, we need to introduce two more tools.
16162 The first is the @command{tr} command, which stands for ``transliterate.''
16163 The @command{tr} command works on a character-by-character basis, changing
16164 characters. Normally it is used for things like mapping upper case to
16168 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16169 @print{} this example has mixed case!
16172 There are several options of interest:
16176 work on the complement of the listed characters, i.e.,
16177 operations apply to characters not in the given set
16180 delete characters in the first set from the output
16183 squeeze repeated characters in the output into just one character.
16186 We will be using all three options in a moment.
16188 The other command we'll look at is @command{comm}. The @command{comm}
16189 command takes two sorted input files as input data, and prints out the
16190 files' lines in three columns. The output columns are the data lines
16191 unique to the first file, the data lines unique to the second file, and
16192 the data lines that are common to both. The @option{-1}, @option{-2}, and
16193 @option{-3} command line options @emph{omit} the respective columns. (This is
16194 non-intuitive and takes a little getting used to.) For example:
16216 The file name @file{-} tells @command{comm} to read standard input
16217 instead of a regular file.
16219 Now we're ready to build a fancy pipeline. The first application is a word
16220 frequency counter. This helps an author determine if he or she is over-using
16223 The first step is to change the case of all the letters in our input file
16224 to one case. ``The'' and ``the'' are the same word when doing counting.
16227 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16230 The next step is to get rid of punctuation. Quoted words and unquoted words
16231 should be treated identically; it's easiest to just get the punctuation out of
16235 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16238 The second @command{tr} command operates on the complement of the listed
16239 characters, which are all the letters, the digits, the underscore, and
16240 the blank. The @samp{\n} represents the newline character; it has to
16241 be left alone. (The @acronym{ASCII} tab character should also be included for
16242 good measure in a production script.)
16244 At this point, we have data consisting of words separated by blank space.
16245 The words only contain alphanumeric characters (and the underscore). The
16246 next step is break the data apart so that we have one word per line. This
16247 makes the counting operation much easier, as we will see shortly.
16250 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16251 > tr -s ' ' '\n' | ...
16254 This command turns blanks into newlines. The @option{-s} option squeezes
16255 multiple newline characters in the output into just one. This helps us
16256 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16257 This is what the shell prints when it notices you haven't finished
16258 typing in all of a command.)
16260 We now have data consisting of one word per line, no punctuation, all one
16261 case. We're ready to count each word:
16264 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16265 > tr -s ' ' '\n' | sort | uniq -c | ...
16268 At this point, the data might look something like this:
16281 The output is sorted by word, not by count! What we want is the most
16282 frequently used words first. Fortunately, this is easy to accomplish,
16283 with the help of two more @command{sort} options:
16287 do a numeric sort, not a textual one
16290 reverse the order of the sort
16293 The final pipeline looks like this:
16296 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16297 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16306 Whew! That's a lot to digest. Yet, the same principles apply. With six
16307 commands, on two lines (really one long one split for convenience), we've
16308 created a program that does something interesting and useful, in much
16309 less time than we could have written a C program to do the same thing.
16311 A minor modification to the above pipeline can give us a simple spelling
16312 checker! To determine if you've spelled a word correctly, all you have to
16313 do is look it up in a dictionary. If it is not there, then chances are
16314 that your spelling is incorrect. So, we need a dictionary.
16315 The conventional location for a dictionary is @file{/usr/dict/words}.
16316 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16317 revision of this article.}
16318 this is a sorted, 45,402 word dictionary.
16320 Now, how to compare our file with the dictionary? As before, we generate
16321 a sorted list of words, one per line:
16324 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16325 > tr -s ' ' '\n' | sort -u | ...
16328 Now, all we need is a list of words that are @emph{not} in the
16329 dictionary. Here is where the @command{comm} command comes in.
16332 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16333 > tr -s ' ' '\n' | sort -u |
16334 > comm -23 - /usr/dict/words
16337 The @option{-2} and @option{-3} options eliminate lines that are only in the
16338 dictionary (the second file), and lines that are in both files. Lines
16339 only in the first file (standard input, our stream of words), are
16340 words that are not in the dictionary. These are likely candidates for
16341 spelling errors. This pipeline was the first cut at a production
16342 spelling checker on Unix.
16344 There are some other tools that deserve brief mention.
16348 search files for text that matches a regular expression
16351 count lines, words, characters
16354 a T-fitting for data pipes, copies data to files and to standard output
16357 the stream editor, an advanced tool
16360 a data manipulation language, another advanced tool
16363 The software tools philosophy also espoused the following bit of
16364 advice: ``Let someone else do the hard part.'' This means, take
16365 something that gives you most of what you need, and then massage it the
16366 rest of the way until it's in the form that you want.
16372 Each program should do one thing well. No more, no less.
16375 Combining programs with appropriate plumbing leads to results where
16376 the whole is greater than the sum of the parts. It also leads to novel
16377 uses of programs that the authors might never have imagined.
16380 Programs should never print extraneous header or trailer data, since these
16381 could get sent on down a pipeline. (A point we didn't mention earlier.)
16384 Let someone else do the hard part.
16387 Know your toolbox! Use each program appropriately. If you don't have an
16388 appropriate tool, build one.
16391 As of this writing, all the programs we've discussed are available via
16392 anonymous @command{ftp} from: @*
16393 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16394 be more recent versions available now.)
16396 None of what I have presented in this column is new. The Software Tools
16397 philosophy was first introduced in the book @cite{Software Tools}, by
16398 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16399 This book showed how to write and use software tools. It was written in
16400 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16401 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16402 was. The last chapter presented a @command{ratfor} to FORTRAN
16403 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16404 lot like C; if you know C, you won't have any problem following the
16407 In 1981, the book was updated and made available as @cite{Software Tools
16408 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16409 still in print and are well worth
16410 reading if you're a programmer. They certainly made a major change in
16411 how I view programming.
16413 The programs in both books are available from
16414 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16415 For a number of years, there was an active
16416 Software Tools Users Group, whose members had ported the original
16417 @command{ratfor} programs to essentially every computer system with a
16418 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16419 as Unix began to spread beyond universities.
16421 With the current proliferation of GNU code and other clones of Unix programs,
16422 these programs now receive little attention; modern C versions are
16423 much more efficient and do more than these programs do. Nevertheless, as
16424 exposition of good programming style, and evangelism for a still-valuable
16425 philosophy, these books are unparalleled, and I recommend them highly.
16427 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16428 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16430 @node GNU Free Documentation License
16431 @appendix GNU Free Documentation License
16435 @node Concept index
16442 @c Local variables:
16443 @c texinfo-column-for-description: 32