3 @setfilename coreutils.info
4 @settitle @sc{gnu} Coreutils
9 @include constants.texi
11 @c Define new indices.
15 @c Put everything in one index (arbitrarily chosen to be the concept index).
25 * Coreutils: (coreutils). Core GNU (file, text, shell) utilities.
26 * Common options: (coreutils)Common options. Common options.
27 * File permissions: (coreutils)File permissions. Access modes.
28 * Date input formats: (coreutils)Date input formats.
31 @c FIXME: the following need documentation
32 @c * [: (coreutils)[ invocation. File/string tests.
33 @c * pinky: (coreutils)pinky invocation. FIXME.
35 @dircategory Individual utilities
37 * arch: (coreutils)arch invocation. Print machine hardware name.
38 * base64: (coreutils)base64 invocation. Base64 encode/decode data.
39 * basename: (coreutils)basename invocation. Strip directory and suffix.
40 * cat: (coreutils)cat invocation. Concatenate and write files.
41 * chcon: (coreutils)chcon invocation. Change SELinux CTX of files.
42 * chgrp: (coreutils)chgrp invocation. Change file groups.
43 * chmod: (coreutils)chmod invocation. Change file permissions.
44 * chown: (coreutils)chown invocation. Change file owners/groups.
45 * chroot: (coreutils)chroot invocation. Specify the root directory.
46 * cksum: (coreutils)cksum invocation. Print POSIX CRC checksum.
47 * comm: (coreutils)comm invocation. Compare sorted files by line.
48 * cp: (coreutils)cp invocation. Copy files.
49 * csplit: (coreutils)csplit invocation. Split by context.
50 * cut: (coreutils)cut invocation. Print selected parts of lines.
51 * date: (coreutils)date invocation. Print/set system date and time.
52 * dd: (coreutils)dd invocation. Copy and convert a file.
53 * df: (coreutils)df invocation. Report file system disk usage.
54 * dir: (coreutils)dir invocation. List directories briefly.
55 * dircolors: (coreutils)dircolors invocation. Color setup for ls.
56 * dirname: (coreutils)dirname invocation. Strip last file name component.
57 * du: (coreutils)du invocation. Report on disk usage.
58 * echo: (coreutils)echo invocation. Print a line of text.
59 * env: (coreutils)env invocation. Modify the environment.
60 * expand: (coreutils)expand invocation. Convert tabs to spaces.
61 * expr: (coreutils)expr invocation. Evaluate expressions.
62 * factor: (coreutils)factor invocation. Print prime factors
63 * false: (coreutils)false invocation. Do nothing, unsuccessfully.
64 * fmt: (coreutils)fmt invocation. Reformat paragraph text.
65 * fold: (coreutils)fold invocation. Wrap long input lines.
66 * groups: (coreutils)groups invocation. Print group names a user is in.
67 * head: (coreutils)head invocation. Output the first part of files.
68 * hostid: (coreutils)hostid invocation. Print numeric host identifier.
69 * hostname: (coreutils)hostname invocation. Print or set system name.
70 * id: (coreutils)id invocation. Print user identity.
71 * install: (coreutils)install invocation. Copy and change attributes.
72 * join: (coreutils)join invocation. Join lines on a common field.
73 * kill: (coreutils)kill invocation. Send a signal to processes.
74 * link: (coreutils)link invocation. Make hard links between files.
75 * ln: (coreutils)ln invocation. Make links between files.
76 * logname: (coreutils)logname invocation. Print current login name.
77 * ls: (coreutils)ls invocation. List directory contents.
78 * md5sum: (coreutils)md5sum invocation. Print or check MD5 digests.
79 * mkdir: (coreutils)mkdir invocation. Create directories.
80 * mkfifo: (coreutils)mkfifo invocation. Create FIFOs (named pipes).
81 * mknod: (coreutils)mknod invocation. Create special files.
82 * mktemp: (coreutils)mktemp invocation. Create temporary files.
83 * mv: (coreutils)mv invocation. Rename files.
84 * nice: (coreutils)nice invocation. Modify niceness.
85 * nl: (coreutils)nl invocation. Number lines and write files.
86 * nohup: (coreutils)nohup invocation. Immunize to hangups.
87 * nproc: (coreutils)nproc invocation. Print the number of processors.
88 * od: (coreutils)od invocation. Dump files in octal, etc.
89 * paste: (coreutils)paste invocation. Merge lines of files.
90 * pathchk: (coreutils)pathchk invocation. Check file name portability.
91 * pr: (coreutils)pr invocation. Paginate or columnate files.
92 * printenv: (coreutils)printenv invocation. Print environment variables.
93 * printf: (coreutils)printf invocation. Format and print data.
94 * ptx: (coreutils)ptx invocation. Produce permuted indexes.
95 * pwd: (coreutils)pwd invocation. Print working directory.
96 * readlink: (coreutils)readlink invocation. Print referent of a symlink.
97 * rm: (coreutils)rm invocation. Remove files.
98 * rmdir: (coreutils)rmdir invocation. Remove empty directories.
99 * runcon: (coreutils)runcon invocation. Run in specified SELinux CTX.
100 * seq: (coreutils)seq invocation. Print numeric sequences
101 * sha1sum: (coreutils)sha1sum invocation. Print or check SHA-1 digests.
102 * sha2: (coreutils)sha2 utilities. Print or check SHA-2 digests.
103 * shred: (coreutils)shred invocation. Remove files more securely.
104 * shuf: (coreutils)shuf invocation. Shuffling text files.
105 * sleep: (coreutils)sleep invocation. Delay for a specified time.
106 * sort: (coreutils)sort invocation. Sort text files.
107 * split: (coreutils)split invocation. Split into pieces.
108 * stat: (coreutils)stat invocation. Report file(system) status.
109 * stdbuf: (coreutils)stdbuf invocation. Modify stdio buffering.
110 * stty: (coreutils)stty invocation. Print/change terminal settings.
111 * su: (coreutils)su invocation. Modify user and group ID.
112 * sum: (coreutils)sum invocation. Print traditional checksum.
113 * sync: (coreutils)sync invocation. Synchronize memory and disk.
114 * tac: (coreutils)tac invocation. Reverse files.
115 * tail: (coreutils)tail invocation. Output the last part of files.
116 * tee: (coreutils)tee invocation. Redirect to multiple files.
117 * test: (coreutils)test invocation. File/string tests.
118 * timeout: (coreutils)timeout invocation. Run with time limit.
119 * touch: (coreutils)touch invocation. Change file timestamps.
120 * tr: (coreutils)tr invocation. Translate characters.
121 * true: (coreutils)true invocation. Do nothing, successfully.
122 * truncate: (coreutils)truncate invocation. Shrink/extend size of a file.
123 * tsort: (coreutils)tsort invocation. Topological sort.
124 * tty: (coreutils)tty invocation. Print terminal name.
125 * uname: (coreutils)uname invocation. Print system information.
126 * unexpand: (coreutils)unexpand invocation. Convert spaces to tabs.
127 * uniq: (coreutils)uniq invocation. Uniquify files.
128 * unlink: (coreutils)unlink invocation. Removal via unlink(2).
129 * uptime: (coreutils)uptime invocation. Print uptime and load.
130 * users: (coreutils)users invocation. Print current user names.
131 * vdir: (coreutils)vdir invocation. List directories verbosely.
132 * wc: (coreutils)wc invocation. Line, word, and byte counts.
133 * who: (coreutils)who invocation. Print who is logged in.
134 * whoami: (coreutils)whoami invocation. Print effective user ID.
135 * yes: (coreutils)yes invocation. Print a string indefinitely.
139 This manual documents version @value{VERSION} of the @sc{gnu} core
140 utilities, including the standard programs for text and file manipulation.
142 Copyright @copyright{} 1994-1996, 2000-2010 Free Software Foundation, Inc.
145 Permission is granted to copy, distribute and/or modify this document
146 under the terms of the GNU Free Documentation License, Version 1.3 or
147 any later version published by the Free Software Foundation; with no
148 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
149 Texts. A copy of the license is included in the section entitled ``GNU
150 Free Documentation License''.
155 @title @sc{gnu} @code{Coreutils}
156 @subtitle Core GNU utilities
157 @subtitle for version @value{VERSION}, @value{UPDATED}
158 @author David MacKenzie et al.
161 @vskip 0pt plus 1filll
174 @cindex core utilities
175 @cindex text utilities
176 @cindex shell utilities
177 @cindex file utilities
180 * Introduction:: Caveats, overview, and authors
181 * Common options:: Common options
182 * Output of entire files:: cat tac nl od base64
183 * Formatting file contents:: fmt pr fold
184 * Output of parts of files:: head tail split csplit
185 * Summarizing files:: wc sum cksum md5sum sha1sum sha2
186 * Operating on sorted files:: sort shuf uniq comm ptx tsort
187 * Operating on fields:: cut paste join
188 * Operating on characters:: tr expand unexpand
189 * Directory listing:: ls dir vdir dircolors
190 * Basic operations:: cp dd install mv rm shred
191 * Special file types:: mkdir rmdir unlink mkfifo mknod ln link readlink
192 * Changing file attributes:: chgrp chmod chown touch
193 * Disk usage:: df du stat sync truncate
194 * Printing text:: echo printf yes
195 * Conditions:: false true test expr
197 * File name manipulation:: dirname basename pathchk mktemp
198 * Working context:: pwd stty printenv tty
199 * User information:: id logname whoami groups users who
200 * System context:: date arch nproc uname hostname hostid uptime
201 * SELinux context:: chcon runcon
202 * Modified command invocation:: chroot env nice nohup stdbuf su timeout
203 * Process control:: kill
205 * Numeric operations:: factor seq
206 * File permissions:: Access modes
207 * Date input formats:: Specifying date strings
208 * Opening the software toolbox:: The software tools philosophy
209 * GNU Free Documentation License:: Copying and sharing this manual
210 * Concept index:: General index
213 --- The Detailed Node Listing ---
217 * Exit status:: Indicating program success or failure
218 * Backup options:: Backup options
219 * Block size:: Block size
220 * Signal specifications:: Specifying signals
221 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
222 * Random sources:: Sources of random data
223 * Target directory:: Target directory
224 * Trailing slashes:: Trailing slashes
225 * Traversing symlinks:: Traversing symlinks to directories
226 * Treating / specially:: Treating / specially
227 * Standards conformance:: Standards conformance
229 Output of entire files
231 * cat invocation:: Concatenate and write files
232 * tac invocation:: Concatenate and write files in reverse
233 * nl invocation:: Number lines and write files
234 * od invocation:: Write files in octal or other formats
235 * base64 invocation:: Transform data into printable data
237 Formatting file contents
239 * fmt invocation:: Reformat paragraph text
240 * pr invocation:: Paginate or columnate files for printing
241 * fold invocation:: Wrap input lines to fit in specified width
243 Output of parts of files
245 * head invocation:: Output the first part of files
246 * tail invocation:: Output the last part of files
247 * split invocation:: Split a file into fixed-size pieces
248 * csplit invocation:: Split a file into context-determined pieces
252 * wc invocation:: Print newline, word, and byte counts
253 * sum invocation:: Print checksum and block counts
254 * cksum invocation:: Print CRC checksum and byte counts
255 * md5sum invocation:: Print or check MD5 digests
256 * sha1sum invocation:: Print or check SHA-1 digests
257 * sha2 utilities:: Print or check SHA-2 digests
259 Operating on sorted files
261 * sort invocation:: Sort text files
262 * shuf invocation:: Shuffle text files
263 * uniq invocation:: Uniquify files
264 * comm invocation:: Compare two sorted files line by line
265 * ptx invocation:: Produce a permuted index of file contents
266 * tsort invocation:: Topological sort
268 @command{ptx}: Produce permuted indexes
270 * General options in ptx:: Options which affect general program behavior
271 * Charset selection in ptx:: Underlying character set considerations
272 * Input processing in ptx:: Input fields, contexts, and keyword selection
273 * Output formatting in ptx:: Types of output format, and sizing the fields
274 * Compatibility in ptx:: The @acronym{GNU} extensions to @command{ptx}
278 * cut invocation:: Print selected parts of lines
279 * paste invocation:: Merge lines of files
280 * join invocation:: Join lines on a common field
282 Operating on characters
284 * tr invocation:: Translate, squeeze, and/or delete characters
285 * expand invocation:: Convert tabs to spaces
286 * unexpand invocation:: Convert spaces to tabs
288 @command{tr}: Translate, squeeze, and/or delete characters
290 * Character sets:: Specifying sets of characters
291 * Translating:: Changing one set of characters to another
292 * Squeezing:: Squeezing repeats and deleting
296 * ls invocation:: List directory contents
297 * dir invocation:: Briefly list directory contents
298 * vdir invocation:: Verbosely list directory contents
299 * dircolors invocation:: Color setup for @command{ls}
301 @command{ls}: List directory contents
303 * Which files are listed:: Which files are listed
304 * What information is listed:: What information is listed
305 * Sorting the output:: Sorting the output
306 * Details about version sort:: More details about version sort
307 * General output formatting:: General output formatting
308 * Formatting the file names:: Formatting the file names
312 * cp invocation:: Copy files and directories
313 * dd invocation:: Convert and copy a file
314 * install invocation:: Copy files and set attributes
315 * mv invocation:: Move (rename) files
316 * rm invocation:: Remove files or directories
317 * shred invocation:: Remove files more securely
321 * link invocation:: Make a hard link via the link syscall
322 * ln invocation:: Make links between files
323 * mkdir invocation:: Make directories
324 * mkfifo invocation:: Make FIFOs (named pipes)
325 * mknod invocation:: Make block or character special files
326 * readlink invocation:: Print value of a symlink or canonical file name
327 * rmdir invocation:: Remove empty directories
328 * unlink invocation:: Remove files via unlink syscall
330 Changing file attributes
332 * chown invocation:: Change file owner and group
333 * chgrp invocation:: Change group ownership
334 * chmod invocation:: Change access permissions
335 * touch invocation:: Change file timestamps
339 * df invocation:: Report file system disk space usage
340 * du invocation:: Estimate file space usage
341 * stat invocation:: Report file or file system status
342 * sync invocation:: Synchronize data on disk with memory
343 * truncate invocation:: Shrink or extend the size of a file
347 * echo invocation:: Print a line of text
348 * printf invocation:: Format and print data
349 * yes invocation:: Print a string until interrupted
353 * false invocation:: Do nothing, unsuccessfully
354 * true invocation:: Do nothing, successfully
355 * test invocation:: Check file types and compare values
356 * expr invocation:: Evaluate expressions
358 @command{test}: Check file types and compare values
360 * File type tests:: File type tests
361 * Access permission tests:: Access permission tests
362 * File characteristic tests:: File characteristic tests
363 * String tests:: String tests
364 * Numeric tests:: Numeric tests
366 @command{expr}: Evaluate expression
368 * String expressions:: + : match substr index length
369 * Numeric expressions:: + - * / %
370 * Relations for expr:: | & < <= = == != >= >
371 * Examples of expr:: Examples of using @command{expr}
375 * tee invocation:: Redirect output to multiple files or processes
377 File name manipulation
379 * basename invocation:: Strip directory and suffix from a file name
380 * dirname invocation:: Strip last file name component
381 * pathchk invocation:: Check file name validity and portability
382 * mktemp invocation:: Create temporary file or directory
386 * pwd invocation:: Print working directory
387 * stty invocation:: Print or change terminal characteristics
388 * printenv invocation:: Print all or some environment variables
389 * tty invocation:: Print file name of terminal on standard input
391 @command{stty}: Print or change terminal characteristics
393 * Control:: Control settings
394 * Input:: Input settings
395 * Output:: Output settings
396 * Local:: Local settings
397 * Combination:: Combination settings
398 * Characters:: Special characters
399 * Special:: Special settings
403 * id invocation:: Print user identity
404 * logname invocation:: Print current login name
405 * whoami invocation:: Print effective user ID
406 * groups invocation:: Print group names a user is in
407 * users invocation:: Print login names of users currently logged in
408 * who invocation:: Print who is currently logged in
412 * arch invocation:: Print machine hardware name
413 * date invocation:: Print or set system date and time
414 * nproc invocation:: Print the number of processors
415 * uname invocation:: Print system information
416 * hostname invocation:: Print or set system name
417 * hostid invocation:: Print numeric host identifier
418 * uptime invocation:: Print system uptime and load
420 @command{date}: Print or set system date and time
422 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
423 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
424 * Literal conversion specifiers:: %[%nt]
425 * Padding and other flags:: Pad with zeros, spaces, etc.
426 * Setting the time:: Changing the system clock
427 * Options for date:: Instead of the current time
428 * Date input formats:: Specifying date strings
429 * Examples of date:: Examples
433 * chcon invocation:: Change SELinux context of file
434 * runcon invocation:: Run a command in specified SELinux context
436 Modified command invocation
438 * chroot invocation:: Run a command with a different root directory
439 * env invocation:: Run a command in a modified environment
440 * nice invocation:: Run a command with modified niceness
441 * nohup invocation:: Run a command immune to hangups
442 * stdbuf invocation:: Run a command with modified I/O buffering
443 * su invocation:: Run a command with substitute user and group ID
444 * timeout invocation:: Run a command with a time limit
448 * kill invocation:: Sending a signal to processes.
452 * sleep invocation:: Delay for a specified time
456 * factor invocation:: Print prime factors
457 * seq invocation:: Print numeric sequences
461 * Mode Structure:: Structure of file mode bits
462 * Symbolic Modes:: Mnemonic representation of file mode bits
463 * Numeric Modes:: File mode bits as octal numbers
464 * Directory Setuid and Setgid:: Set-user-ID and set-group-ID on directories
468 * General date syntax:: Common rules
469 * Calendar date items:: 19 Dec 1994
470 * Time of day items:: 9:20pm
471 * Time zone items:: @sc{est}, @sc{pdt}, @sc{gmt}
472 * Day of week items:: Monday and others
473 * Relative items in date strings:: next tuesday, 2 years ago
474 * Pure numbers in date strings:: 19931219, 1440
475 * Seconds since the Epoch:: @@1078100502
476 * Specifying time zone rules:: TZ="America/New_York", TZ="UTC0"
477 * Authors of parse_datetime:: Bellovin, Eggert, Salz, Berets, et al
479 Opening the software toolbox
481 * Toolbox introduction:: Toolbox introduction
482 * I/O redirection:: I/O redirection
483 * The who command:: The @command{who} command
484 * The cut command:: The @command{cut} command
485 * The sort command:: The @command{sort} command
486 * The uniq command:: The @command{uniq} command
487 * Putting the tools together:: Putting the tools together
491 * GNU Free Documentation License:: Copying and sharing this manual
498 @chapter Introduction
500 This manual is a work in progress: many sections make no attempt to explain
501 basic concepts in a way suitable for novices. Thus, if you are interested,
502 please get involved in improving this manual. The entire @sc{gnu} community
505 @cindex @acronym{POSIX}
506 The @sc{gnu} utilities documented here are mostly compatible with the
507 @acronym{POSIX} standard.
508 @cindex bugs, reporting
509 Please report bugs to @email{bug-coreutils@@gnu.org}. Remember
510 to include the version number, machine architecture, input files, and
511 any other information needed to reproduce the bug: your input, what you
512 expected, what you got, and why it is wrong. Diffs are welcome, but
513 please include a description of the problem as well, since this is
514 sometimes difficult to infer. @xref{Bugs, , , gcc, Using and Porting GNU CC}.
520 @cindex MacKenzie, D.
523 This manual was originally derived from the Unix man pages in the
524 distributions, which were written by David MacKenzie and updated by Jim
525 Meyering. What you are reading now is the authoritative documentation
526 for these utilities; the man pages are no longer being maintained. The
527 original @command{fmt} man page was written by Ross Paterson. Fran@,{c}ois
528 Pinard did the initial conversion to Texinfo format. Karl Berry did the
529 indexing, some reorganization, and editing of the results. Brian
530 Youmans of the Free Software Foundation office staff combined the
531 manuals for textutils, fileutils, and sh-utils to produce the present
532 omnibus manual. Richard Stallman contributed his usual invaluable
533 insights to the overall process.
536 @chapter Common options
540 @itemx @w{@kbd{--backup}[=@var{method}]}
543 @vindex VERSION_CONTROL
544 @cindex backups, making
545 @xref{Backup options}.
546 Make a backup of each file that would otherwise be overwritten or removed.
549 @macro optBackupSuffix
550 @item -S @var{suffix}
551 @itemx --suffix=@var{suffix}
554 Append @var{suffix} to each backup file made with @option{-b}.
555 @xref{Backup options}.
558 @macro optTargetDirectory
559 @item -t @var{directory}
560 @itemx @w{@kbd{--target-directory}=@var{directory}}
562 @opindex --target-directory
563 @cindex target directory
564 @cindex destination directory
565 Specify the destination @var{directory}.
566 @xref{Target directory}.
569 @macro optNoTargetDirectory
571 @itemx --no-target-directory
573 @opindex --no-target-directory
574 @cindex target directory
575 @cindex destination directory
576 Do not treat the last operand specially when it is a directory or a
577 symbolic link to a directory. @xref{Target directory}.
585 @cindex output @sc{nul}-byte-terminated lines
586 Output a zero byte (@acronym{ASCII} @sc{nul}) at the end of each line,
587 rather than a newline. This option enables other programs to parse the
588 output of @command{\cmd\} even when that output would contain data
589 with embedded newlines.
596 Append an SI-style abbreviation to each size, such as @samp{M} for
597 megabytes. Powers of 1000 are used, not 1024; @samp{M} stands for
598 1,000,000 bytes. This option is equivalent to
599 @option{--block-size=si}. Use the @option{-h} or
600 @option{--human-readable} option if
601 you prefer powers of 1024.
604 @macro optHumanReadable
606 @itemx --human-readable
608 @opindex --human-readable
609 @cindex human-readable output
610 Append a size letter to each size, such as @samp{M} for mebibytes.
611 Powers of 1024 are used, not 1000; @samp{M} stands for 1,048,576 bytes.
612 This option is equivalent to @option{--block-size=human-readable}.
613 Use the @option{--si} option if you prefer powers of 1000.
616 @macro optStripTrailingSlashes
617 @itemx @w{@kbd{--strip-trailing-slashes}}
618 @opindex --strip-trailing-slashes
619 @cindex stripping trailing slashes
620 Remove any trailing slashes from each @var{source} argument.
621 @xref{Trailing slashes}.
624 @macro mayConflictWithShellBuiltIn{cmd}
625 @cindex conflicts with shell built-ins
626 @cindex built-in shell commands, conflicts with
627 Due to shell aliases and built-in @command{\cmd\} command, using an
628 unadorned @command{\cmd\} interactively or in a script may get you
629 different functionality than that described here. Invoke it via
630 @command{env} (i.e., @code{env \cmd\ @dots{}}) to avoid interference
635 @macro multiplierSuffixes{varName}
636 @var{\varName\} may be, or may be an integer optionally followed by,
637 one of the following multiplicative suffixes:
639 @samp{b} => 512 ("blocks")
640 @samp{KB} => 1000 (KiloBytes)
641 @samp{K} => 1024 (KibiBytes)
642 @samp{MB} => 1000*1000 (MegaBytes)
643 @samp{M} => 1024*1024 (MebiBytes)
644 @samp{GB} => 1000*1000*1000 (GigaBytes)
645 @samp{G} => 1024*1024*1024 (GibiBytes)
647 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
650 @c FIXME: same as above, but no ``blocks'' line.
651 @macro multiplierSuffixesNoBlocks{varName}
652 @var{\varName\} may be, or may be an integer optionally followed by,
653 one of the following multiplicative suffixes:
655 @samp{KB} => 1000 (KiloBytes)
656 @samp{K} => 1024 (KibiBytes)
657 @samp{MB} => 1000*1000 (MegaBytes)
658 @samp{M} => 1024*1024 (MebiBytes)
659 @samp{GB} => 1000*1000*1000 (GigaBytes)
660 @samp{G} => 1024*1024*1024 (GibiBytes)
662 and so on for @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}.
665 @cindex common options
667 Certain options are available in all of these programs. Rather than
668 writing identical descriptions for each of the programs, they are
669 described here. (In fact, every @sc{gnu} program accepts (or should accept)
672 @vindex POSIXLY_CORRECT
673 Normally options and operands can appear in any order, and programs act
674 as if all the options appear before any operands. For example,
675 @samp{sort -r passwd -t :} acts like @samp{sort -r -t : passwd}, since
676 @samp{:} is an option-argument of @option{-t}. However, if the
677 @env{POSIXLY_CORRECT} environment variable is set, options must appear
678 before operands, unless otherwise specified for a particular command.
680 A few programs can usefully have trailing operands with leading
681 @samp{-}. With such a program, options must precede operands even if
682 @env{POSIXLY_CORRECT} is not set, and this fact is noted in the
683 program description. For example, the @command{env} command's options
684 must appear before its operands, since in some cases the operands
685 specify a command that itself contains options.
687 Most programs that accept long options recognize unambiguous
688 abbreviations of those options. For example, @samp{rmdir
689 --ignore-fail-on-non-empty} can be invoked as @samp{rmdir
690 --ignore-fail} or even @samp{rmdir --i}. Ambiguous options, such as
691 @samp{ls --h}, are identified as such.
693 Some of these programs recognize the @option{--help} and @option{--version}
694 options only when one of them is the sole command line argument. For
695 these programs, abbreviations of the long options are not always recognized.
702 Print a usage message listing all available options, then exit successfully.
706 @cindex version number, finding
707 Print the version number, then exit successfully.
711 @cindex option delimiter
712 Delimit the option list. Later arguments, if any, are treated as
713 operands even if they begin with @samp{-}. For example, @samp{sort --
714 -r} reads from the file named @file{-r}.
718 @cindex standard input
719 @cindex standard output
720 A single @samp{-} operand is not really an option, though it looks like one. It
721 stands for standard input, or for standard output if that is clear from
722 the context. For example, @samp{sort -} reads from standard input,
723 and is equivalent to plain @samp{sort}, and @samp{tee -} writes an
724 extra copy of its input to standard output. Unless otherwise
725 specified, @samp{-} can appear as any operand that requires a file
729 * Exit status:: Indicating program success or failure.
730 * Backup options:: -b -S, in some programs.
731 * Block size:: BLOCK_SIZE and --block-size, in some programs.
732 * Signal specifications:: Specifying signals using the --signal option.
733 * Disambiguating names and IDs:: chgrp and chown owner and group syntax
734 * Random sources:: --random-source, in some programs.
735 * Target directory:: Specifying a target directory, in some programs.
736 * Trailing slashes:: --strip-trailing-slashes, in some programs.
737 * Traversing symlinks:: -H, -L, or -P, in some programs.
738 * Treating / specially:: --preserve-root and --no-preserve-root.
739 * Special built-in utilities:: @command{break}, @command{:}, @command{eval}, @dots{}
740 * Standards conformance:: Conformance to the @acronym{POSIX} standard.
748 An exit status of zero indicates success,
749 and a nonzero value indicates failure.
752 Nearly every command invocation yields an integral @dfn{exit status}
753 that can be used to change how other commands work.
754 For the vast majority of commands, an exit status of zero indicates
755 success. Failure is indicated by a nonzero value---typically
756 @samp{1}, though it may differ on unusual platforms as @acronym{POSIX}
757 requires only that it be nonzero.
759 However, some of the programs documented here do produce
760 other exit status values and a few associate different
761 meanings with the values @samp{0} and @samp{1}.
762 Here are some of the exceptions:
763 @command{chroot}, @command{env}, @command{expr}, @command{nice},
764 @command{nohup}, @command{printenv}, @command{sort}, @command{stdbuf},
765 @command{su}, @command{test}, @command{timeout}, @command{tty}.
769 @section Backup options
771 @cindex backup options
773 Some @sc{gnu} programs (at least @command{cp}, @command{install},
774 @command{ln}, and @command{mv}) optionally make backups of files
775 before writing new versions.
776 These options control the details of these backups. The options are also
777 briefly mentioned in the descriptions of the particular programs.
782 @itemx @w{@kbd{--backup}[=@var{method}]}
785 @vindex VERSION_CONTROL
786 @cindex backups, making
787 Make a backup of each file that would otherwise be overwritten or removed.
788 Without this option, the original versions are destroyed.
789 Use @var{method} to determine the type of backups to make.
790 When this option is used but @var{method} is not specified,
791 then the value of the @env{VERSION_CONTROL}
792 environment variable is used. And if @env{VERSION_CONTROL} is not set,
793 the default backup type is @samp{existing}.
795 Note that the short form of this option, @option{-b} does not accept any
796 argument. Using @option{-b} is equivalent to using @option{--backup=existing}.
798 @vindex version-control @r{Emacs variable}
799 This option corresponds to the Emacs variable @samp{version-control};
800 the values for @var{method} are the same as those used in Emacs.
801 This option also accepts more descriptive names.
802 The valid @var{method}s are (unique abbreviations are accepted):
807 @opindex none @r{backup method}
812 @opindex numbered @r{backup method}
813 Always make numbered backups.
817 @opindex existing @r{backup method}
818 Make numbered backups of files that already have them, simple backups
823 @opindex simple @r{backup method}
824 Always make simple backups. Please note @samp{never} is not to be
825 confused with @samp{none}.
829 @item -S @var{suffix}
830 @itemx --suffix=@var{suffix}
833 @cindex backup suffix
834 @vindex SIMPLE_BACKUP_SUFFIX
835 Append @var{suffix} to each backup file made with @option{-b}. If this
836 option is not specified, the value of the @env{SIMPLE_BACKUP_SUFFIX}
837 environment variable is used. And if @env{SIMPLE_BACKUP_SUFFIX} is not
838 set, the default is @samp{~}, just as in Emacs.
847 Some @sc{gnu} programs (at least @command{df}, @command{du}, and
848 @command{ls}) display sizes in ``blocks''. You can adjust the block size
849 and method of display to make sizes easier to read. The block size
850 used for display is independent of any file system block size.
851 Fractional block counts are rounded up to the nearest integer.
853 @opindex --block-size=@var{size}
856 @vindex DF_BLOCK_SIZE
857 @vindex DU_BLOCK_SIZE
858 @vindex LS_BLOCK_SIZE
859 @vindex POSIXLY_CORRECT@r{, and block size}
861 The default block size is chosen by examining the following environment
862 variables in turn; the first one that is set determines the block size.
867 This specifies the default block size for the @command{df} command.
868 Similarly, @env{DU_BLOCK_SIZE} specifies the default for @command{du} and
869 @env{LS_BLOCK_SIZE} for @command{ls}.
872 This specifies the default block size for all three commands, if the
873 above command-specific environment variables are not set.
876 This specifies the default block size for all values that are normally
877 printed as blocks, if neither @env{BLOCK_SIZE} nor the above
878 command-specific environment variables are set. Unlike the other
879 environment variables, @env{BLOCKSIZE} does not affect values that are
880 normally printed as byte counts, e.g., the file sizes contained in
883 @item POSIXLY_CORRECT
884 If neither @env{@var{command}_BLOCK_SIZE}, nor @env{BLOCK_SIZE}, nor
885 @env{BLOCKSIZE} is set, but this variable is set, the block size
890 If none of the above environment variables are set, the block size
891 currently defaults to 1024 bytes in most contexts, but this number may
892 change in the future. For @command{ls} file sizes, the block size
895 @cindex human-readable output
898 A block size specification can be a positive integer specifying the number
899 of bytes per block, or it can be @code{human-readable} or @code{si} to
900 select a human-readable format. Integers may be followed by suffixes
901 that are upward compatible with the
902 @uref{http://www.bipm.fr/enus/3_SI/si-prefixes.html, SI prefixes}
903 for decimal multiples and with the
904 @uref{http://physics.nist.gov/cuu/Units/binary.html, IEC 60027-2
905 prefixes for binary multiples}.
907 With human-readable formats, output sizes are followed by a size letter
908 such as @samp{M} for megabytes. @code{BLOCK_SIZE=human-readable} uses
909 powers of 1024; @samp{M} stands for 1,048,576 bytes.
910 @code{BLOCK_SIZE=si} is similar, but uses powers of 1000 and appends
911 @samp{B}; @samp{MB} stands for 1,000,000 bytes.
914 A block size specification preceded by @samp{'} causes output sizes to
915 be displayed with thousands separators. The @env{LC_NUMERIC} locale
916 specifies the thousands separator and grouping. For example, in an
917 American English locale, @samp{--block-size="'1kB"} would cause a size
918 of 1234000 bytes to be displayed as @samp{1,234}. In the default C
919 locale, there is no thousands separator so a leading @samp{'} has no
922 An integer block size can be followed by a suffix to specify a
923 multiple of that size. A bare size letter,
924 or one followed by @samp{iB}, specifies
925 a multiple using powers of 1024. A size letter followed by @samp{B}
926 specifies powers of 1000 instead. For example, @samp{1M} and
927 @samp{1MiB} are equivalent to @samp{1048576}, whereas @samp{1MB} is
928 equivalent to @samp{1000000}.
930 A plain suffix without a preceding integer acts as if @samp{1} were
931 prepended, except that it causes a size indication to be appended to
932 the output. For example, @samp{--block-size="kB"} displays 3000 as
935 The following suffixes are defined. Large sizes like @code{1Y}
936 may be rejected by your computer due to limitations of its arithmetic.
940 @cindex kilobyte, definition of
941 kilobyte: @math{10^3 = 1000}.
945 @cindex kibibyte, definition of
946 kibibyte: @math{2^{10} = 1024}. @samp{K} is special: the SI prefix is
947 @samp{k} and the IEC 60027-2 prefix is @samp{Ki}, but tradition and
948 @acronym{POSIX} use @samp{k} to mean @samp{KiB}.
950 @cindex megabyte, definition of
951 megabyte: @math{10^6 = 1,000,000}.
954 @cindex mebibyte, definition of
955 mebibyte: @math{2^{20} = 1,048,576}.
957 @cindex gigabyte, definition of
958 gigabyte: @math{10^9 = 1,000,000,000}.
961 @cindex gibibyte, definition of
962 gibibyte: @math{2^{30} = 1,073,741,824}.
964 @cindex terabyte, definition of
965 terabyte: @math{10^{12} = 1,000,000,000,000}.
968 @cindex tebibyte, definition of
969 tebibyte: @math{2^{40} = 1,099,511,627,776}.
971 @cindex petabyte, definition of
972 petabyte: @math{10^{15} = 1,000,000,000,000,000}.
975 @cindex pebibyte, definition of
976 pebibyte: @math{2^{50} = 1,125,899,906,842,624}.
978 @cindex exabyte, definition of
979 exabyte: @math{10^{18} = 1,000,000,000,000,000,000}.
982 @cindex exbibyte, definition of
983 exbibyte: @math{2^{60} = 1,152,921,504,606,846,976}.
985 @cindex zettabyte, definition of
986 zettabyte: @math{10^{21} = 1,000,000,000,000,000,000,000}
989 @math{2^{70} = 1,180,591,620,717,411,303,424}.
990 (@samp{Zi} is a @acronym{GNU} extension to IEC 60027-2.)
992 @cindex yottabyte, definition of
993 yottabyte: @math{10^{24} = 1,000,000,000,000,000,000,000,000}.
996 @math{2^{80} = 1,208,925,819,614,629,174,706,176}.
997 (@samp{Yi} is a @acronym{GNU} extension to IEC 60027-2.)
1002 @opindex --block-size
1003 @opindex --human-readable
1006 Block size defaults can be overridden by an explicit
1007 @option{--block-size=@var{size}} option. The @option{-k}
1008 option is equivalent to @option{--block-size=1K}, which
1009 is the default unless the @env{POSIXLY_CORRECT} environment variable is
1010 set. The @option{-h} or @option{--human-readable} option is equivalent to
1011 @option{--block-size=human-readable}. The @option{--si} option is
1012 equivalent to @option{--block-size=si}.
1014 @node Signal specifications
1015 @section Signal specifications
1016 @cindex signals, specifying
1018 A @var{signal} may be a signal name like @samp{HUP}, or a signal
1019 number like @samp{1}, or an exit status of a process terminated by the
1020 signal. A signal name can be given in canonical form or prefixed by
1021 @samp{SIG}. The case of the letters is ignored. The following signal names
1022 and numbers are supported on all @acronym{POSIX} compliant systems:
1028 2. Terminal interrupt.
1034 9. Kill (cannot be caught or ignored).
1042 Other supported signal names have system-dependent corresponding
1043 numbers. All systems conforming to @acronym{POSIX} 1003.1-2001 also
1044 support the following signals:
1048 Access to an undefined portion of a memory object.
1050 Child process terminated, stopped, or continued.
1052 Continue executing, if stopped.
1054 Erroneous arithmetic operation.
1056 Illegal Instruction.
1058 Write on a pipe with no one to read it.
1060 Invalid memory reference.
1062 Stop executing (cannot be caught or ignored).
1066 Background process attempting read.
1068 Background process attempting write.
1070 High bandwidth data is available at a socket.
1072 User-defined signal 1.
1074 User-defined signal 2.
1078 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XSI} extension
1079 also support the following signals:
1085 Profiling timer expired.
1089 Trace/breakpoint trap.
1091 Virtual timer expired.
1093 CPU time limit exceeded.
1095 File size limit exceeded.
1099 @acronym{POSIX} 1003.1-2001 systems that support the @acronym{XRT} extension
1100 also support at least eight real-time signals called @samp{RTMIN},
1101 @samp{RTMIN+1}, @dots{}, @samp{RTMAX-1}, @samp{RTMAX}.
1103 @node Disambiguating names and IDs
1104 @section chown and chgrp: Disambiguating user names and IDs
1105 @cindex user names, disambiguating
1106 @cindex user IDs, disambiguating
1107 @cindex group names, disambiguating
1108 @cindex group IDs, disambiguating
1109 @cindex disambiguating group names and IDs
1111 Since the @var{owner} and @var{group} arguments to @command{chown} and
1112 @command{chgrp} may be specified as names or numeric IDs, there is an
1114 What if a user or group @emph{name} is a string of digits?
1115 @footnote{Using a number as a user name is common in some environments.}
1116 Should the command interpret it as a user name or as an ID?
1117 @acronym{POSIX} requires that @command{chown} and @command{chgrp}
1118 first attempt to resolve the specified string as a name, and
1119 only once that fails, then try to interpret it as an ID.
1120 This is troublesome when you want to specify a numeric ID, say 42,
1121 and it must work even in a pathological situation where
1122 @samp{42} is a user name that maps to some other user ID, say 1000.
1123 Simply invoking @code{chown 42 F}, will set @file{F}s owner ID to
1124 1000---not what you intended.
1126 GNU @command{chown} and @command{chgrp} provide a way to work around this,
1127 that at the same time may result in a significant performance improvement
1128 by eliminating a database look-up.
1129 Simply precede each numeric user ID and/or group ID with a @samp{+},
1130 in order to force its interpretation as an integer:
1134 chgrp +$numeric_group_id another-file
1138 GNU @command{chown} and @command{chgrp}
1139 skip the name look-up process for each @samp{+}-prefixed string,
1140 because a string containing @samp{+} is never a valid user or group name.
1141 This syntax is accepted on most common Unix systems, but not on Solaris 10.
1143 @node Random sources
1144 @section Sources of random data
1146 @cindex random sources
1148 The @command{shuf}, @command{shred}, and @command{sort} commands
1149 sometimes need random data to do their work. For example, @samp{sort
1150 -R} must choose a hash function at random, and it needs random data to
1151 make this selection.
1153 By default these commands use an internal pseudorandom generator
1154 initialized by a small amount of entropy, but can be directed to use
1155 an external source with the @option{--random-source=@var{file}} option.
1156 An error is reported if @var{file} does not contain enough bytes.
1158 For example, the device file @file{/dev/urandom} could be used as the
1159 source of random data. Typically, this device gathers environmental
1160 noise from device drivers and other sources into an entropy pool, and
1161 uses the pool to generate random bits. If the pool is short of data,
1162 the device reuses the internal pool to produce more bits, using a
1163 cryptographically secure pseudorandom number generator. But be aware
1164 that this device is not designed for bulk random data generation
1165 and is relatively slow.
1167 @file{/dev/urandom} suffices for most practical uses, but applications
1168 requiring high-value or long-term protection of private data may
1169 require an alternate data source like @file{/dev/random} or
1170 @file{/dev/arandom}. The set of available sources depends on your
1173 To reproduce the results of an earlier invocation of a command, you
1174 can save some random data into a file and then use that file as the
1175 random source in earlier and later invocations of the command.
1177 @node Target directory
1178 @section Target directory
1180 @cindex target directory
1182 The @command{cp}, @command{install}, @command{ln}, and @command{mv}
1183 commands normally treat the last operand specially when it is a
1184 directory or a symbolic link to a directory. For example, @samp{cp
1185 source dest} is equivalent to @samp{cp source dest/source} if
1186 @file{dest} is a directory. Sometimes this behavior is not exactly
1187 what is wanted, so these commands support the following options to
1188 allow more fine-grained control:
1193 @itemx --no-target-directory
1194 @opindex --no-target-directory
1195 @cindex target directory
1196 @cindex destination directory
1197 Do not treat the last operand specially when it is a directory or a
1198 symbolic link to a directory. This can help avoid race conditions in
1199 programs that operate in a shared area. For example, when the command
1200 @samp{mv /tmp/source /tmp/dest} succeeds, there is no guarantee that
1201 @file{/tmp/source} was renamed to @file{/tmp/dest}: it could have been
1202 renamed to @file{/tmp/dest/source} instead, if some other process
1203 created @file{/tmp/dest} as a directory. However, if @file{mv
1204 -T /tmp/source /tmp/dest} succeeds, there is no
1205 question that @file{/tmp/source} was renamed to @file{/tmp/dest}.
1207 In the opposite situation, where you want the last operand to be
1208 treated as a directory and want a diagnostic otherwise, you can use
1209 the @option{--target-directory} (@option{-t}) option.
1211 @item -t @var{directory}
1212 @itemx @w{@kbd{--target-directory}=@var{directory}}
1213 @opindex --target-directory
1214 @cindex target directory
1215 @cindex destination directory
1216 Use @var{directory} as the directory component of each destination
1219 The interface for most programs is that after processing options and a
1220 finite (possibly zero) number of fixed-position arguments, the remaining
1221 argument list is either expected to be empty, or is a list of items
1222 (usually files) that will all be handled identically. The @command{xargs}
1223 program is designed to work well with this convention.
1225 The commands in the @command{mv}-family are unusual in that they take
1226 a variable number of arguments with a special case at the @emph{end}
1227 (namely, the target directory). This makes it nontrivial to perform some
1228 operations, e.g., ``move all files from here to ../d/'', because
1229 @code{mv * ../d/} might exhaust the argument space, and @code{ls | xargs ...}
1230 doesn't have a clean way to specify an extra final argument for each
1231 invocation of the subject command. (It can be done by going through a
1232 shell command, but that requires more human labor and brain power than
1235 The @w{@kbd{--target-directory}} (@option{-t}) option allows the @command{cp},
1236 @command{install}, @command{ln}, and @command{mv} programs to be used
1237 conveniently with @command{xargs}. For example, you can move the files
1238 from the current directory to a sibling directory, @code{d} like this:
1241 ls | xargs mv -t ../d --
1244 However, this doesn't move files whose names begin with @samp{.}.
1245 If you use the @sc{gnu} @command{find} program, you can move those
1246 files too, with this command:
1249 find . -mindepth 1 -maxdepth 1 \
1253 But both of the above approaches fail if there are no files in the
1254 current directory, or if any file has a name containing a blank or
1255 some other special characters.
1256 The following example removes those limitations and requires both
1257 @sc{gnu} @command{find} and @sc{gnu} @command{xargs}:
1260 find . -mindepth 1 -maxdepth 1 -print0 \
1261 | xargs --null --no-run-if-empty \
1268 The @option{--target-directory} (@option{-t}) and
1269 @option{--no-target-directory} (@option{-T})
1270 options cannot be combined.
1272 @node Trailing slashes
1273 @section Trailing slashes
1275 @cindex trailing slashes
1277 Some @sc{gnu} programs (at least @command{cp} and @command{mv}) allow you to
1278 remove any trailing slashes from each @var{source} argument before
1279 operating on it. The @w{@kbd{--strip-trailing-slashes}} option enables
1282 This is useful when a @var{source} argument may have a trailing slash and
1283 @c FIXME: mv's behavior in this case is system-dependent
1284 specify a symbolic link to a directory. This scenario is in fact rather
1285 common because some shells can automatically append a trailing slash when
1286 performing file name completion on such symbolic links. Without this
1287 option, @command{mv}, for example, (via the system's rename function) must
1288 interpret a trailing slash as a request to dereference the symbolic link
1289 and so must rename the indirectly referenced @emph{directory} and not
1290 the symbolic link. Although it may seem surprising that such behavior
1291 be the default, it is required by @acronym{POSIX} and is consistent with
1292 other parts of that standard.
1294 @node Traversing symlinks
1295 @section Traversing symlinks
1297 @cindex symbolic link to directory, controlling traversal of
1299 The following options modify how @command{chown} and @command{chgrp}
1300 @c FIXME: note that `du' has these options, too, but they have slightly
1301 @c different meaning.
1302 traverse a hierarchy when the @option{--recursive} (@option{-R})
1303 option is also specified.
1304 If more than one of the following options is specified, only the final
1306 These options specify whether processing a symbolic link to a directory
1307 entails operating on just the symbolic link or on all files in the
1308 hierarchy rooted at that directory.
1310 These options are independent of @option{--dereference} and
1311 @option{--no-dereference} (@option{-h}), which control whether to modify
1312 a symlink or its referent.
1319 @cindex symbolic link to directory, traverse each that is specified on the command line
1320 If @option{--recursive} (@option{-R}) is specified and
1321 a command line argument is a symbolic link to a directory, traverse it.
1328 @cindex symbolic link to directory, traverse each that is encountered
1329 In a recursive traversal, traverse every symbolic link to a directory
1330 that is encountered.
1337 @cindex symbolic link to directory, never traverse
1338 Do not traverse any symbolic links.
1339 This is the default if none of @option{-H}, @option{-L},
1340 or @option{-P} is specified.
1347 @node Treating / specially
1348 @section Treating @file{/} specially
1350 Certain commands can operate destructively on entire hierarchies.
1351 For example, if a user with appropriate privileges mistakenly runs
1352 @samp{rm -rf / tmp/junk}, that may remove
1353 all files on the entire system. Since there are so few
1354 legitimate uses for such a command,
1355 @sc{gnu} @command{rm} normally declines to operate on any directory
1356 that resolves to @file{/}. If you really want to try to remove all
1357 the files on your system, you can use the @option{--no-preserve-root}
1358 option, but the default behavior, specified by the
1359 @option{--preserve-option}, is safer for most purposes.
1361 The commands @command{chgrp}, @command{chmod} and @command{chown}
1362 can also operate destructively on entire hierarchies, so they too
1363 support these options. Although, unlike @command{rm}, they don't
1364 actually unlink files, these commands are arguably more dangerous
1365 when operating recursively on @file{/}, since they often work much
1366 more quickly, and hence damage more files before an alert user can
1367 interrupt them. Tradition and @acronym{POSIX} require these commands
1368 to operate recursively on @file{/}, so they default to
1369 @option{--no-preserve-root}, but using the @option{--preserve-root}
1370 option makes them safer for most purposes. For convenience you can
1371 specify @option{--preserve-root} in an alias or in a shell function.
1373 Note that the @option{--preserve-root} option also ensures
1374 that @command{chgrp} and @command{chown} do not modify @file{/}
1375 even when dereferencing a symlink pointing to @file{/}.
1377 @node Special built-in utilities
1378 @section Special built-in utilities
1380 Some programs like @command{nice} can invoke other programs; for
1381 example, the command @samp{nice cat file} invokes the program
1382 @command{cat} by executing the command @samp{cat file}. However,
1383 @dfn{special built-in utilities} like @command{exit} cannot be invoked
1384 this way. For example, the command @samp{nice exit} does not have a
1385 well-defined behavior: it may generate an error message instead of
1388 Here is a list of the special built-in utilities that are standardized
1389 by @acronym{POSIX} 1003.1-2004.
1392 @t{.@: : break continue eval exec exit export readonly
1393 return set shift times trap unset}
1396 For example, because @samp{.}, @samp{:}, and @samp{exec} are special,
1397 the commands @samp{nice . foo.sh}, @samp{nice :}, and @samp{nice exec
1398 pwd} do not work as you might expect.
1400 Many shells extend this list. For example, Bash has several extra
1401 special built-in utilities like @command{history}, and
1402 @command{suspend}, and with Bash the command @samp{nice suspend}
1403 generates an error message instead of suspending.
1405 @node Standards conformance
1406 @section Standards conformance
1408 @vindex POSIXLY_CORRECT
1409 In a few cases, the @sc{gnu} utilities' default behavior is
1410 incompatible with the @acronym{POSIX} standard. To suppress these
1411 incompatibilities, define the @env{POSIXLY_CORRECT} environment
1412 variable. Unless you are checking for @acronym{POSIX} conformance, you
1413 probably do not need to define @env{POSIXLY_CORRECT}.
1415 Newer versions of @acronym{POSIX} are occasionally incompatible with older
1416 versions. For example, older versions of @acronym{POSIX} required the
1417 command @samp{sort +1} to sort based on the second and succeeding
1418 fields in each input line, but starting with @acronym{POSIX} 1003.1-2001
1419 the same command is required to sort the file named @file{+1}, and you
1420 must instead use the command @samp{sort -k 2} to get the field-based
1423 @vindex _POSIX2_VERSION
1424 The @sc{gnu} utilities normally conform to the version of @acronym{POSIX}
1425 that is standard for your system. To cause them to conform to a
1426 different version of @acronym{POSIX}, define the @env{_POSIX2_VERSION}
1427 environment variable to a value of the form @var{yyyymm} specifying
1428 the year and month the standard was adopted. Two values are currently
1429 supported for @env{_POSIX2_VERSION}: @samp{199209} stands for
1430 @acronym{POSIX} 1003.2-1992, and @samp{200112} stands for @acronym{POSIX}
1431 1003.1-2001. For example, if you have a newer system but are running software
1432 that assumes an older version of @acronym{POSIX} and uses @samp{sort +1}
1433 or @samp{tail +10}, you can work around any compatibility problems by setting
1434 @samp{_POSIX2_VERSION=199209} in your environment.
1436 @node Output of entire files
1437 @chapter Output of entire files
1439 @cindex output of entire files
1440 @cindex entire files, output of
1442 These commands read and write entire files, possibly transforming them
1446 * cat invocation:: Concatenate and write files.
1447 * tac invocation:: Concatenate and write files in reverse.
1448 * nl invocation:: Number lines and write files.
1449 * od invocation:: Write files in octal or other formats.
1450 * base64 invocation:: Transform data into printable data.
1453 @node cat invocation
1454 @section @command{cat}: Concatenate and write files
1457 @cindex concatenate and write files
1458 @cindex copying files
1460 @command{cat} copies each @var{file} (@samp{-} means standard input), or
1461 standard input if none are given, to standard output. Synopsis:
1464 cat [@var{option}] [@var{file}]@dots{}
1467 The program accepts the following options. Also see @ref{Common options}.
1475 Equivalent to @option{-vET}.
1478 @itemx --number-nonblank
1480 @opindex --number-nonblank
1481 Number all nonempty output lines, starting with 1.
1485 Equivalent to @option{-vE}.
1490 @opindex --show-ends
1491 Display a @samp{$} after the end of each line.
1497 Number all output lines, starting with 1. This option is ignored
1498 if @option{-b} is in effect.
1501 @itemx --squeeze-blank
1503 @opindex --squeeze-blank
1504 @cindex squeezing empty lines
1505 Suppress repeated adjacent empty lines; output just one empty line
1510 Equivalent to @option{-vT}.
1515 @opindex --show-tabs
1516 Display TAB characters as @samp{^I}.
1520 Ignored; for @acronym{POSIX} compatibility.
1523 @itemx --show-nonprinting
1525 @opindex --show-nonprinting
1526 Display control characters except for LFD and TAB using
1527 @samp{^} notation and precede characters that have the high bit set with
1532 On systems like MS-DOS that distinguish between text and binary files,
1533 @command{cat} normally reads and writes in binary mode. However,
1534 @command{cat} reads in text mode if one of the options
1535 @option{-bensAE} is used or if @command{cat} is reading from standard
1536 input and standard input is a terminal. Similarly, @command{cat}
1537 writes in text mode if one of the options @option{-bensAE} is used or
1538 if standard output is a terminal.
1545 # Output f's contents, then standard input, then g's contents.
1548 # Copy standard input to standard output.
1553 @node tac invocation
1554 @section @command{tac}: Concatenate and write files in reverse
1557 @cindex reversing files
1559 @command{tac} copies each @var{file} (@samp{-} means standard input), or
1560 standard input if none are given, to standard output, reversing the
1561 records (lines by default) in each separately. Synopsis:
1564 tac [@var{option}]@dots{} [@var{file}]@dots{}
1567 @dfn{Records} are separated by instances of a string (newline by
1568 default). By default, this separator string is attached to the end of
1569 the record that it follows in the file.
1571 The program accepts the following options. Also see @ref{Common options}.
1579 The separator is attached to the beginning of the record that it
1580 precedes in the file.
1586 Treat the separator string as a regular expression. Users of @command{tac}
1587 on MS-DOS/MS-Windows should note that, since @command{tac} reads files in
1588 binary mode, each line of a text file might end with a CR/LF pair
1589 instead of the Unix-style LF.
1591 @item -s @var{separator}
1592 @itemx --separator=@var{separator}
1594 @opindex --separator
1595 Use @var{separator} as the record separator, instead of newline.
1603 @section @command{nl}: Number lines and write files
1606 @cindex numbering lines
1607 @cindex line numbering
1609 @command{nl} writes each @var{file} (@samp{-} means standard input), or
1610 standard input if none are given, to standard output, with line numbers
1611 added to some or all of the lines. Synopsis:
1614 nl [@var{option}]@dots{} [@var{file}]@dots{}
1617 @cindex logical pages, numbering on
1618 @command{nl} decomposes its input into (logical) pages; by default, the
1619 line number is reset to 1 at the top of each logical page. @command{nl}
1620 treats all of the input files as a single document; it does not reset
1621 line numbers or logical pages between files.
1623 @cindex headers, numbering
1624 @cindex body, numbering
1625 @cindex footers, numbering
1626 A logical page consists of three sections: header, body, and footer.
1627 Any of the sections can be empty. Each can be numbered in a different
1628 style from the others.
1630 The beginnings of the sections of logical pages are indicated in the
1631 input file by a line containing exactly one of these delimiter strings:
1642 The two characters from which these strings are made can be changed from
1643 @samp{\} and @samp{:} via options (see below), but the pattern and
1644 length of each string cannot be changed.
1646 A section delimiter is replaced by an empty line on output. Any text
1647 that comes before the first section delimiter string in the input file
1648 is considered to be part of a body section, so @command{nl} treats a
1649 file that contains no section delimiters as a single body section.
1651 The program accepts the following options. Also see @ref{Common options}.
1655 @item -b @var{style}
1656 @itemx --body-numbering=@var{style}
1658 @opindex --body-numbering
1659 Select the numbering style for lines in the body section of each
1660 logical page. When a line is not numbered, the current line number
1661 is not incremented, but the line number separator character is still
1662 prepended to the line. The styles are:
1668 number only nonempty lines (default for body),
1670 do not number lines (default for header and footer),
1672 number only lines that contain a match for the basic regular
1673 expression @var{bre}.
1674 @xref{Regular Expressions, , Regular Expressions, grep, The GNU Grep Manual}.
1678 @itemx --section-delimiter=@var{cd}
1680 @opindex --section-delimiter
1681 @cindex section delimiters of pages
1682 Set the section delimiter characters to @var{cd}; default is
1683 @samp{\:}. If only @var{c} is given, the second remains @samp{:}.
1684 (Remember to protect @samp{\} or other metacharacters from shell
1685 expansion with quotes or extra backslashes.)
1687 @item -f @var{style}
1688 @itemx --footer-numbering=@var{style}
1690 @opindex --footer-numbering
1691 Analogous to @option{--body-numbering}.
1693 @item -h @var{style}
1694 @itemx --header-numbering=@var{style}
1696 @opindex --header-numbering
1697 Analogous to @option{--body-numbering}.
1699 @item -i @var{number}
1700 @itemx --line-increment=@var{number}
1702 @opindex --line-increment
1703 Increment line numbers by @var{number} (default 1).
1705 @item -l @var{number}
1706 @itemx --join-blank-lines=@var{number}
1708 @opindex --join-blank-lines
1709 @cindex empty lines, numbering
1710 @cindex blank lines, numbering
1711 Consider @var{number} (default 1) consecutive empty lines to be one
1712 logical line for numbering, and only number the last one. Where fewer
1713 than @var{number} consecutive empty lines occur, do not number them.
1714 An empty line is one that contains no characters, not even spaces
1717 @item -n @var{format}
1718 @itemx --number-format=@var{format}
1720 @opindex --number-format
1721 Select the line numbering format (default is @code{rn}):
1725 @opindex ln @r{format for @command{nl}}
1726 left justified, no leading zeros;
1728 @opindex rn @r{format for @command{nl}}
1729 right justified, no leading zeros;
1731 @opindex rz @r{format for @command{nl}}
1732 right justified, leading zeros.
1736 @itemx --no-renumber
1738 @opindex --no-renumber
1739 Do not reset the line number at the start of a logical page.
1741 @item -s @var{string}
1742 @itemx --number-separator=@var{string}
1744 @opindex --number-separator
1745 Separate the line number from the text line in the output with
1746 @var{string} (default is the TAB character).
1748 @item -v @var{number}
1749 @itemx --starting-line-number=@var{number}
1751 @opindex --starting-line-number
1752 Set the initial line number on each logical page to @var{number} (default 1).
1754 @item -w @var{number}
1755 @itemx --number-width=@var{number}
1757 @opindex --number-width
1758 Use @var{number} characters for line numbers (default 6).
1766 @section @command{od}: Write files in octal or other formats
1769 @cindex octal dump of files
1770 @cindex hex dump of files
1771 @cindex ASCII dump of files
1772 @cindex file contents, dumping unambiguously
1774 @command{od} writes an unambiguous representation of each @var{file}
1775 (@samp{-} means standard input), or standard input if none are given.
1779 od [@var{option}]@dots{} [@var{file}]@dots{}
1780 od [-abcdfilosx]@dots{} [@var{file}] [[+]@var{offset}[.][b]]
1781 od [@var{option}]@dots{} --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
1784 Each line of output consists of the offset in the input, followed by
1785 groups of data from the file. By default, @command{od} prints the offset in
1786 octal, and each group of file data is a C @code{short int}'s worth of input
1787 printed as a single octal number.
1789 If @var{offset} is given, it specifies how many input bytes to skip
1790 before formatting and writing. By default, it is interpreted as an
1791 octal number, but the optional trailing decimal point causes it to be
1792 interpreted as decimal. If no decimal is specified and the offset
1793 begins with @samp{0x} or @samp{0X} it is interpreted as a hexadecimal
1794 number. If there is a trailing @samp{b}, the number of bytes skipped
1795 will be @var{offset} multiplied by 512.
1797 If a command is of both the first and second forms, the second form is
1798 assumed if the last operand begins with @samp{+} or (if there are two
1799 operands) a digit. For example, in @samp{od foo 10} and @samp{od +10}
1800 the @samp{10} is an offset, whereas in @samp{od 10} the @samp{10} is a
1803 The program accepts the following options. Also see @ref{Common options}.
1807 @item -A @var{radix}
1808 @itemx --address-radix=@var{radix}
1810 @opindex --address-radix
1811 @cindex radix for file offsets
1812 @cindex file offset radix
1813 Select the base in which file offsets are printed. @var{radix} can
1814 be one of the following:
1824 none (do not print offsets).
1827 The default is octal.
1829 @item -j @var{bytes}
1830 @itemx --skip-bytes=@var{bytes}
1832 @opindex --skip-bytes
1833 Skip @var{bytes} input bytes before formatting and writing. If
1834 @var{bytes} begins with @samp{0x} or @samp{0X}, it is interpreted in
1835 hexadecimal; otherwise, if it begins with @samp{0}, in octal; otherwise,
1837 @multiplierSuffixes{bytes}
1839 @item -N @var{bytes}
1840 @itemx --read-bytes=@var{bytes}
1842 @opindex --read-bytes
1843 Output at most @var{bytes} bytes of the input. Prefixes and suffixes on
1844 @code{bytes} are interpreted as for the @option{-j} option.
1846 @item -S @var{bytes}
1847 @itemx --strings[=@var{bytes}]
1850 @cindex string constants, outputting
1851 Instead of the normal output, output only @dfn{string constants}: at
1852 least @var{bytes} consecutive @acronym{ASCII} graphic characters,
1853 followed by a zero byte (@acronym{ASCII} @sc{nul}).
1854 Prefixes and suffixes on @code{bytes} are interpreted as for the
1857 If @var{n} is omitted with @option{--strings}, the default is 3.
1860 @itemx --format=@var{type}
1863 Select the format in which to output the file data. @var{type} is a
1864 string of one or more of the below type indicator characters. If you
1865 include more than one type indicator character in a single @var{type}
1866 string, or use this option more than once, @command{od} writes one copy
1867 of each output line using each of the data types that you specified,
1868 in the order that you specified.
1870 Adding a trailing ``z'' to any type specification appends a display
1871 of the @acronym{ASCII} character representation of the printable characters
1872 to the output line generated by the type specification.
1876 named character, ignoring high-order bit
1878 @acronym{ASCII} character or backslash escape,
1891 The type @code{a} outputs things like @samp{sp} for space, @samp{nl} for
1892 newline, and @samp{nul} for a zero byte. Only the least significant
1893 seven bits of each byte is used; the high-order bit is ignored.
1894 Type @code{c} outputs
1895 @samp{ }, @samp{\n}, and @code{\0}, respectively.
1898 Except for types @samp{a} and @samp{c}, you can specify the number
1899 of bytes to use in interpreting each number in the given data type
1900 by following the type indicator character with a decimal integer.
1901 Alternately, you can specify the size of one of the C compiler's
1902 built-in data types by following the type indicator character with
1903 one of the following characters. For integers (@samp{d}, @samp{o},
1904 @samp{u}, @samp{x}):
1917 For floating point (@code{f}):
1929 @itemx --output-duplicates
1931 @opindex --output-duplicates
1932 Output consecutive lines that are identical. By default, when two or
1933 more consecutive output lines would be identical, @command{od} outputs only
1934 the first line, and puts just an asterisk on the following line to
1935 indicate the elision.
1938 @itemx --width[=@var{n}]
1941 Dump @code{n} input bytes per output line. This must be a multiple of
1942 the least common multiple of the sizes associated with the specified
1945 If this option is not given at all, the default is 16. If @var{n} is
1946 omitted, the default is 32.
1950 The next several options are shorthands for format specifications.
1951 @sc{gnu} @command{od} accepts any combination of shorthands and format
1952 specification options. These options accumulate.
1958 Output as named characters. Equivalent to @samp{-t a}.
1962 Output as octal bytes. Equivalent to @samp{-t o1}.
1966 Output as @acronym{ASCII} characters or backslash escapes. Equivalent to
1971 Output as unsigned decimal two-byte units. Equivalent to @samp{-t u2}.
1975 Output as floats. Equivalent to @samp{-t fF}.
1979 Output as decimal ints. Equivalent to @samp{-t dI}.
1983 Output as decimal long ints. Equivalent to @samp{-t dL}.
1987 Output as octal two-byte units. Equivalent to @option{-t o2}.
1991 Output as decimal two-byte units. Equivalent to @option{-t d2}.
1995 Output as hexadecimal two-byte units. Equivalent to @samp{-t x2}.
1998 @opindex --traditional
1999 Recognize the non-option label argument that traditional @command{od}
2000 accepted. The following syntax:
2003 od --traditional [@var{file}] [[+]@var{offset}[.][b] [[+]@var{label}[.][b]]]
2007 can be used to specify at most one file and optional arguments
2008 specifying an offset and a pseudo-start address, @var{label}.
2009 The @var{label} argument is interpreted
2010 just like @var{offset}, but it specifies an initial pseudo-address. The
2011 pseudo-addresses are displayed in parentheses following any normal
2018 @node base64 invocation
2019 @section @command{base64}: Transform data into printable data
2022 @cindex base64 encoding
2024 @command{base64} transforms data read from a file, or standard input,
2025 into (or from) base64 encoded form. The base64 encoded form uses
2026 printable @acronym{ASCII} characters to represent binary data.
2030 base64 [@var{option}]@dots{} [@var{file}]
2031 base64 --decode [@var{option}]@dots{} [@var{file}]
2034 The base64 encoding expands data to roughly 133% of the original.
2035 The format conforms to
2036 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc4648.txt, RFC 4648}.
2038 The program accepts the following options. Also see @ref{Common options}.
2043 @itemx --wrap=@var{cols}
2047 @cindex column to wrap data after
2048 During encoding, wrap lines after @var{cols} characters. This must be
2051 The default is to wrap after 76 characters. Use the value 0 to
2052 disable line wrapping altogether.
2058 @cindex Decode base64 data
2059 @cindex Base64 decoding
2060 Change the mode of operation, from the default of encoding data, to
2061 decoding data. Input is expected to be base64 encoded data, and the
2062 output will be the original data.
2065 @itemx --ignore-garbage
2067 @opindex --ignore-garbage
2068 @cindex Ignore garbage in base64 stream
2069 When decoding, newlines are always accepted.
2070 During decoding, ignore unrecognized bytes,
2071 to permit distorted data to be decoded.
2078 @node Formatting file contents
2079 @chapter Formatting file contents
2081 @cindex formatting file contents
2083 These commands reformat the contents of files.
2086 * fmt invocation:: Reformat paragraph text.
2087 * pr invocation:: Paginate or columnate files for printing.
2088 * fold invocation:: Wrap input lines to fit in specified width.
2092 @node fmt invocation
2093 @section @command{fmt}: Reformat paragraph text
2096 @cindex reformatting paragraph text
2097 @cindex paragraphs, reformatting
2098 @cindex text, reformatting
2100 @command{fmt} fills and joins lines to produce output lines of (at most)
2101 a given number of characters (75 by default). Synopsis:
2104 fmt [@var{option}]@dots{} [@var{file}]@dots{}
2107 @command{fmt} reads from the specified @var{file} arguments (or standard
2108 input if none are given), and writes to standard output.
2110 By default, blank lines, spaces between words, and indentation are
2111 preserved in the output; successive input lines with different
2112 indentation are not joined; tabs are expanded on input and introduced on
2115 @cindex line-breaking
2116 @cindex sentences and line-breaking
2117 @cindex Knuth, Donald E.
2118 @cindex Plass, Michael F.
2119 @command{fmt} prefers breaking lines at the end of a sentence, and tries to
2120 avoid line breaks after the first word of a sentence or before the last
2121 word of a sentence. A @dfn{sentence break} is defined as either the end
2122 of a paragraph or a word ending in any of @samp{.?!}, followed by two
2123 spaces or end of line, ignoring any intervening parentheses or quotes.
2124 Like @TeX{}, @command{fmt} reads entire ``paragraphs'' before choosing line
2125 breaks; the algorithm is a variant of that given by Donald E. Knuth
2126 and Michael F. Plass in ``Breaking Paragraphs Into Lines'',
2127 @cite{Software---Practice & Experience} @b{11}, 11 (November 1981),
2130 The program accepts the following options. Also see @ref{Common options}.
2135 @itemx --crown-margin
2137 @opindex --crown-margin
2138 @cindex crown margin
2139 @dfn{Crown margin} mode: preserve the indentation of the first two
2140 lines within a paragraph, and align the left margin of each subsequent
2141 line with that of the second line.
2144 @itemx --tagged-paragraph
2146 @opindex --tagged-paragraph
2147 @cindex tagged paragraphs
2148 @dfn{Tagged paragraph} mode: like crown margin mode, except that if
2149 indentation of the first line of a paragraph is the same as the
2150 indentation of the second, the first line is treated as a one-line
2156 @opindex --split-only
2157 Split lines only. Do not join short lines to form longer ones. This
2158 prevents sample lines of code, and other such ``formatted'' text from
2159 being unduly combined.
2162 @itemx --uniform-spacing
2164 @opindex --uniform-spacing
2165 Uniform spacing. Reduce spacing between words to one space, and spacing
2166 between sentences to two spaces.
2169 @itemx -w @var{width}
2170 @itemx --width=@var{width}
2171 @opindex -@var{width}
2174 Fill output lines up to @var{width} characters (default 75). @command{fmt}
2175 initially tries to make lines about 7% shorter than this, to give it
2176 room to balance line lengths.
2178 @item -p @var{prefix}
2179 @itemx --prefix=@var{prefix}
2180 Only lines beginning with @var{prefix} (possibly preceded by whitespace)
2181 are subject to formatting. The prefix and any preceding whitespace are
2182 stripped for the formatting and then re-attached to each formatted output
2183 line. One use is to format certain kinds of program comments, while
2184 leaving the code unchanged.
2192 @section @command{pr}: Paginate or columnate files for printing
2195 @cindex printing, preparing files for
2196 @cindex multicolumn output, generating
2197 @cindex merging files in parallel
2199 @command{pr} writes each @var{file} (@samp{-} means standard input), or
2200 standard input if none are given, to standard output, paginating and
2201 optionally outputting in multicolumn format; optionally merges all
2202 @var{file}s, printing all in parallel, one per column. Synopsis:
2205 pr [@var{option}]@dots{} [@var{file}]@dots{}
2209 By default, a 5-line header is printed at each page: two blank lines;
2210 a line with the date, the file name, and the page count; and two more
2211 blank lines. A footer of five blank lines is also printed.
2212 The default @var{page_length} is 66
2213 lines. The default number of text lines is therefore 56.
2214 The text line of the header takes the form
2215 @samp{@var{date} @var{string} @var{page}}, with spaces inserted around
2216 @var{string} so that the line takes up the full @var{page_width}. Here,
2217 @var{date} is the date (see the @option{-D} or @option{--date-format}
2218 option for details), @var{string} is the centered header string, and
2219 @var{page} identifies the page number. The @env{LC_MESSAGES} locale
2220 category affects the spelling of @var{page}; in the default C locale, it
2221 is @samp{Page @var{number}} where @var{number} is the decimal page
2224 Form feeds in the input cause page breaks in the output. Multiple form
2225 feeds produce empty pages.
2227 Columns are of equal width, separated by an optional string (default
2228 is @samp{space}). For multicolumn output, lines will always be truncated to
2229 @var{page_width} (default 72), unless you use the @option{-J} option.
2231 column output no line truncation occurs by default. Use @option{-W} option to
2232 truncate lines in that case.
2234 The following changes were made in version 1.22i and apply to later
2235 versions of @command{pr}:
2236 @c FIXME: this whole section here sounds very awkward to me. I
2237 @c made a few small changes, but really it all needs to be redone. - Brian
2238 @c OK, I fixed another sentence or two, but some of it I just don't understand.
2243 Some small @var{letter options} (@option{-s}, @option{-w}) have been
2244 redefined for better @acronym{POSIX} compliance. The output of some further
2245 cases has been adapted to other Unix systems. These changes are not
2246 compatible with earlier versions of the program.
2249 Some @var{new capital letter} options (@option{-J}, @option{-S}, @option{-W})
2250 have been introduced to turn off unexpected interferences of small letter
2251 options. The @option{-N} option and the second argument @var{last_page}
2252 of @samp{+FIRST_PAGE} offer more flexibility. The detailed handling of
2253 form feeds set in the input files requires the @option{-T} option.
2256 Capital letter options override small letter ones.
2259 Some of the option-arguments (compare @option{-s}, @option{-e},
2260 @option{-i}, @option{-n}) cannot be specified as separate arguments from the
2261 preceding option letter (already stated in the @acronym{POSIX} specification).
2264 The program accepts the following options. Also see @ref{Common options}.
2268 @item +@var{first_page}[:@var{last_page}]
2269 @itemx --pages=@var{first_page}[:@var{last_page}]
2270 @c The two following @opindex lines evoke warnings because they contain `:'
2271 @c The `info' spec does not permit that. If we use those lines, we end
2272 @c up with truncated index entries that don't work.
2273 @c @opindex +@var{first_page}[:@var{last_page}]
2274 @c @opindex --pages=@var{first_page}[:@var{last_page}]
2275 @opindex +@var{page_range}
2276 @opindex --pages=@var{page_range}
2277 Begin printing with page @var{first_page} and stop with @var{last_page}.
2278 Missing @samp{:@var{last_page}} implies end of file. While estimating
2279 the number of skipped pages each form feed in the input file results
2280 in a new page. Page counting with and without @samp{+@var{first_page}}
2281 is identical. By default, counting starts with the first page of input
2282 file (not first page printed). Line numbering may be altered by @option{-N}
2286 @itemx --columns=@var{column}
2287 @opindex -@var{column}
2289 @cindex down columns
2290 With each single @var{file}, produce @var{column} columns of output
2291 (default is 1) and print columns down, unless @option{-a} is used. The
2292 column width is automatically decreased as @var{column} increases; unless
2293 you use the @option{-W/-w} option to increase @var{page_width} as well.
2294 This option might well cause some lines to be truncated. The number of
2295 lines in the columns on each page are balanced. The options @option{-e}
2296 and @option{-i} are on for multiple text-column output. Together with
2297 @option{-J} option column alignment and line truncation is turned off.
2298 Lines of full length are joined in a free field format and @option{-S}
2299 option may set field separators. @option{-@var{column}} may not be used
2300 with @option{-m} option.
2306 @cindex across columns
2307 With each single @var{file}, print columns across rather than down. The
2308 @option{-@var{column}} option must be given with @var{column} greater than one.
2309 If a line is too long to fit in a column, it is truncated.
2312 @itemx --show-control-chars
2314 @opindex --show-control-chars
2315 Print control characters using hat notation (e.g., @samp{^G}); print
2316 other nonprinting characters in octal backslash notation. By default,
2317 nonprinting characters are not changed.
2320 @itemx --double-space
2322 @opindex --double-space
2323 @cindex double spacing
2324 Double space the output.
2326 @item -D @var{format}
2327 @itemx --date-format=@var{format}
2328 @cindex time formats
2329 @cindex formatting times
2330 Format header dates using @var{format}, using the same conventions as
2331 for the command @samp{date +@var{format}}; @xref{date invocation}.
2332 Except for directives, which start with
2333 @samp{%}, characters in @var{format} are printed unchanged. You can use
2334 this option to specify an arbitrary string in place of the header date,
2335 e.g., @option{--date-format="Monday morning"}.
2337 @vindex POSIXLY_CORRECT
2339 The default date format is @samp{%Y-%m-%d %H:%M} (for example,
2340 @samp{2001-12-04 23:59});
2341 but if the @env{POSIXLY_CORRECT} environment variable is set
2342 and the @env{LC_TIME} locale category specifies the @acronym{POSIX}
2343 locale, the default is @samp{%b %e %H:%M %Y} (for example,
2344 @samp{Dec@ @ 4 23:59 2001}.
2347 Time stamps are listed according to the time zone rules specified by
2348 the @env{TZ} environment variable, or by the system default rules if
2349 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
2350 with @env{TZ}, libc, The GNU C Library Reference Manual}.
2352 @item -e[@var{in-tabchar}[@var{in-tabwidth}]]
2353 @itemx --expand-tabs[=@var{in-tabchar}[@var{in-tabwidth}]]
2355 @opindex --expand-tabs
2357 Expand @var{tab}s to spaces on input. Optional argument @var{in-tabchar} is
2358 the input tab character (default is the TAB character). Second optional
2359 argument @var{in-tabwidth} is the input tab character's width (default
2367 @opindex --form-feed
2368 Use a form feed instead of newlines to separate output pages. This does
2369 not alter the default page length of 66 lines.
2371 @item -h @var{header}
2372 @itemx --header=@var{header}
2375 Replace the file name in the header with the centered string @var{header}.
2376 When using the shell, @var{header} should be quoted and should be
2377 separated from @option{-h} by a space.
2379 @item -i[@var{out-tabchar}[@var{out-tabwidth}]]
2380 @itemx --output-tabs[=@var{out-tabchar}[@var{out-tabwidth}]]
2382 @opindex --output-tabs
2384 Replace spaces with @var{tab}s on output. Optional argument @var{out-tabchar}
2385 is the output tab character (default is the TAB character). Second optional
2386 argument @var{out-tabwidth} is the output tab character's width (default
2392 @opindex --join-lines
2393 Merge lines of full length. Used together with the column options
2394 @option{-@var{column}}, @option{-a -@var{column}} or @option{-m}. Turns off
2395 @option{-W/-w} line truncation;
2396 no column alignment used; may be used with
2397 @option{--sep-string[=@var{string}]}. @option{-J} has been introduced
2398 (together with @option{-W} and @option{--sep-string})
2399 to disentangle the old (@acronym{POSIX}-compliant) options @option{-w} and
2400 @option{-s} along with the three column options.
2403 @item -l @var{page_length}
2404 @itemx --length=@var{page_length}
2407 Set the page length to @var{page_length} (default 66) lines, including
2408 the lines of the header [and the footer]. If @var{page_length} is less
2409 than or equal to 10, the header and footer are omitted, as if the
2410 @option{-t} option had been given.
2416 Merge and print all @var{file}s in parallel, one in each column. If a
2417 line is too long to fit in a column, it is truncated, unless the @option{-J}
2418 option is used. @option{--sep-string[=@var{string}]} may be used.
2420 some @var{file}s (form feeds set) produce empty columns, still marked
2421 by @var{string}. The result is a continuous line numbering and column
2422 marking throughout the whole merged file. Completely empty merged pages
2423 show no separators or line numbers. The default header becomes
2424 @samp{@var{date} @var{page}} with spaces inserted in the middle; this
2425 may be used with the @option{-h} or @option{--header} option to fill up
2426 the middle blank part.
2428 @item -n[@var{number-separator}[@var{digits}]]
2429 @itemx --number-lines[=@var{number-separator}[@var{digits}]]
2431 @opindex --number-lines
2432 Provide @var{digits} digit line numbering (default for @var{digits} is
2433 5). With multicolumn output the number occupies the first @var{digits}
2434 column positions of each text column or only each line of @option{-m}
2435 output. With single column output the number precedes each line just as
2436 @option{-m} does. Default counting of the line numbers starts with the
2437 first line of the input file (not the first line printed, compare the
2438 @option{--page} option and @option{-N} option).
2439 Optional argument @var{number-separator} is the character appended to
2440 the line number to separate it from the text followed. The default
2441 separator is the TAB character. In a strict sense a TAB is always
2442 printed with single column output only. The TAB width varies
2443 with the TAB position, e.g., with the left @var{margin} specified
2444 by @option{-o} option. With multicolumn output priority is given to
2445 @samp{equal width of output columns} (a @acronym{POSIX} specification).
2446 The TAB width is fixed to the value of the first column and does
2447 not change with different values of left @var{margin}. That means a
2448 fixed number of spaces is always printed in the place of the
2449 @var{number-separator} TAB. The tabification depends upon the output
2452 @item -N @var{line_number}
2453 @itemx --first-line-number=@var{line_number}
2455 @opindex --first-line-number
2456 Start line counting with the number @var{line_number} at first line of
2457 first page printed (in most cases not the first line of the input file).
2459 @item -o @var{margin}
2460 @itemx --indent=@var{margin}
2463 @cindex indenting lines
2465 Indent each line with a margin @var{margin} spaces wide (default is zero).
2466 The total page width is the size of the margin plus the @var{page_width}
2467 set with the @option{-W/-w} option. A limited overflow may occur with
2468 numbered single column output (compare @option{-n} option).
2471 @itemx --no-file-warnings
2473 @opindex --no-file-warnings
2474 Do not print a warning message when an argument @var{file} cannot be
2475 opened. (The exit status will still be nonzero, however.)
2477 @item -s[@var{char}]
2478 @itemx --separator[=@var{char}]
2480 @opindex --separator
2481 Separate columns by a single character @var{char}. The default for
2482 @var{char} is the TAB character without @option{-w} and @samp{no
2483 character} with @option{-w}. Without @option{-s} the default separator
2484 @samp{space} is set. @option{-s[char]} turns off line truncation of all
2485 three column options (@option{-COLUMN}|@option{-a -COLUMN}|@option{-m}) unless
2486 @option{-w} is set. This is a @acronym{POSIX}-compliant formulation.
2489 @item -S@var{string}
2490 @itemx --sep-string[=@var{string}]
2492 @opindex --sep-string
2493 Use @var{string} to separate output columns. The @option{-S} option doesn't
2494 affect the @option{-W/-w} option, unlike the @option{-s} option which does. It
2495 does not affect line truncation or column alignment.
2496 Without @option{-S}, and with @option{-J}, @command{pr} uses the default output
2498 Without @option{-S} or @option{-J}, @command{pr} uses a @samp{space}
2499 (same as @option{-S"@w{ }"}). @option{--sep-string} with no
2500 @samp{=@var{string}} is equivalent to @option{--sep-string=""}.
2503 @itemx --omit-header
2505 @opindex --omit-header
2506 Do not print the usual header [and footer] on each page, and do not fill
2507 out the bottom of pages (with blank lines or a form feed). No page
2508 structure is produced, but form feeds set in the input files are retained.
2509 The predefined pagination is not changed. @option{-t} or @option{-T} may be
2510 useful together with other options; e.g.: @option{-t -e4}, expand TAB characters
2511 in the input file to 4 spaces but don't make any other changes. Use of
2512 @option{-t} overrides @option{-h}.
2515 @itemx --omit-pagination
2517 @opindex --omit-pagination
2518 Do not print header [and footer]. In addition eliminate all form feeds
2519 set in the input files.
2522 @itemx --show-nonprinting
2524 @opindex --show-nonprinting
2525 Print nonprinting characters in octal backslash notation.
2527 @item -w @var{page_width}
2528 @itemx --width=@var{page_width}
2531 Set page width to @var{page_width} characters for multiple text-column
2532 output only (default for @var{page_width} is 72). @option{-s[CHAR]} turns
2533 off the default page width and any line truncation and column alignment.
2534 Lines of full length are merged, regardless of the column options
2535 set. No @var{page_width} setting is possible with single column output.
2536 A @acronym{POSIX}-compliant formulation.
2538 @item -W @var{page_width}
2539 @itemx --page_width=@var{page_width}
2541 @opindex --page_width
2542 Set the page width to @var{page_width} characters. That's valid with and
2543 without a column option. Text lines are truncated, unless @option{-J}
2544 is used. Together with one of the three column options
2545 (@option{-@var{column}}, @option{-a -@var{column}} or @option{-m}) column
2546 alignment is always used. The separator options @option{-S} or @option{-s}
2547 don't affect the @option{-W} option. Default is 72 characters. Without
2548 @option{-W @var{page_width}} and without any of the column options NO line
2549 truncation is used (defined to keep downward compatibility and to meet
2550 most frequent tasks). That's equivalent to @option{-W 72 -J}. The header
2551 line is never truncated.
2558 @node fold invocation
2559 @section @command{fold}: Wrap input lines to fit in specified width
2562 @cindex wrapping long input lines
2563 @cindex folding long input lines
2565 @command{fold} writes each @var{file} (@option{-} means standard input), or
2566 standard input if none are given, to standard output, breaking long
2570 fold [@var{option}]@dots{} [@var{file}]@dots{}
2573 By default, @command{fold} breaks lines wider than 80 columns. The output
2574 is split into as many lines as necessary.
2576 @cindex screen columns
2577 @command{fold} counts screen columns by default; thus, a tab may count more
2578 than one column, backspace decreases the column count, and carriage
2579 return sets the column to zero.
2581 The program accepts the following options. Also see @ref{Common options}.
2589 Count bytes rather than columns, so that tabs, backspaces, and carriage
2590 returns are each counted as taking up one column, just like other
2597 Break at word boundaries: the line is broken after the last blank before
2598 the maximum line length. If the line contains no such blanks, the line
2599 is broken at the maximum line length as usual.
2601 @item -w @var{width}
2602 @itemx --width=@var{width}
2605 Use a maximum line length of @var{width} columns instead of 80.
2607 For compatibility @command{fold} supports an obsolete option syntax
2608 @option{-@var{width}}. New scripts should use @option{-w @var{width}}
2616 @node Output of parts of files
2617 @chapter Output of parts of files
2619 @cindex output of parts of files
2620 @cindex parts of files, output of
2622 These commands output pieces of the input.
2625 * head invocation:: Output the first part of files.
2626 * tail invocation:: Output the last part of files.
2627 * split invocation:: Split a file into pieces.
2628 * csplit invocation:: Split a file into context-determined pieces.
2631 @node head invocation
2632 @section @command{head}: Output the first part of files
2635 @cindex initial part of files, outputting
2636 @cindex first part of files, outputting
2638 @command{head} prints the first part (10 lines by default) of each
2639 @var{file}; it reads from standard input if no files are given or
2640 when given a @var{file} of @option{-}. Synopsis:
2643 head [@var{option}]@dots{} [@var{file}]@dots{}
2646 If more than one @var{file} is specified, @command{head} prints a
2647 one-line header consisting of:
2650 ==> @var{file name} <==
2654 before the output for each @var{file}.
2656 The program accepts the following options. Also see @ref{Common options}.
2661 @itemx --bytes=@var{k}
2664 Print the first @var{k} bytes, instead of initial lines.
2665 However, if @var{k} starts with a @samp{-},
2666 print all but the last @var{k} bytes of each file.
2667 @multiplierSuffixes{k}
2670 @itemx --lines=@var{k}
2673 Output the first @var{k} lines.
2674 However, if @var{k} starts with a @samp{-},
2675 print all but the last @var{k} lines of each file.
2676 Size multiplier suffixes are the same as with the @option{-c} option.
2684 Never print file name headers.
2690 Always print file name headers.
2694 For compatibility @command{head} also supports an obsolete option syntax
2695 @option{-@var{count}@var{options}}, which is recognized only if it is
2696 specified first. @var{count} is a decimal number optionally followed
2697 by a size letter (@samp{b}, @samp{k}, @samp{m}) as in @option{-c}, or
2698 @samp{l} to mean count by lines, or other option letters (@samp{cqv}).
2699 Scripts intended for standard hosts should use @option{-c @var{count}}
2700 or @option{-n @var{count}} instead. If your script must also run on
2701 hosts that support only the obsolete syntax, it is usually simpler to
2702 avoid @command{head}, e.g., by using @samp{sed 5q} instead of
2708 @node tail invocation
2709 @section @command{tail}: Output the last part of files
2712 @cindex last part of files, outputting
2714 @command{tail} prints the last part (10 lines by default) of each
2715 @var{file}; it reads from standard input if no files are given or
2716 when given a @var{file} of @samp{-}. Synopsis:
2719 tail [@var{option}]@dots{} [@var{file}]@dots{}
2722 If more than one @var{file} is specified, @command{tail} prints a
2723 one-line header consisting of:
2726 ==> @var{file name} <==
2730 before the output for each @var{file}.
2732 @cindex BSD @command{tail}
2733 @sc{gnu} @command{tail} can output any amount of data (some other versions of
2734 @command{tail} cannot). It also has no @option{-r} option (print in
2735 reverse), since reversing a file is really a different job from printing
2736 the end of a file; BSD @command{tail} (which is the one with @option{-r}) can
2737 only reverse files that are at most as large as its buffer, which is
2738 typically 32 KiB@. A more reliable and versatile way to reverse files is
2739 the @sc{gnu} @command{tac} command.
2741 The program accepts the following options. Also see @ref{Common options}.
2746 @itemx --bytes=@var{k}
2749 Output the last @var{k} bytes, instead of final lines.
2750 However, if @var{k} starts with a @samp{+}, start printing with the
2751 @var{k}th byte from the start of each file, instead of from the end.
2752 @multiplierSuffixes{k}
2755 @itemx --follow[=@var{how}]
2758 @cindex growing files
2759 @vindex name @r{follow option}
2760 @vindex descriptor @r{follow option}
2761 Loop forever trying to read more characters at the end of the file,
2762 presumably because the file is growing.
2763 If more than one file is given, @command{tail} prints a header whenever it
2764 gets output from a different file, to indicate which file that output is
2767 There are two ways to specify how you'd like to track files with this option,
2768 but that difference is noticeable only when a followed file is removed or
2770 If you'd like to continue to track the end of a growing file even after
2771 it has been unlinked, use @option{--follow=descriptor}. This is the default
2772 behavior, but it is not useful if you're tracking a log file that may be
2773 rotated (removed or renamed, then reopened). In that case, use
2774 @option{--follow=name} to track the named file, perhaps by reopening it
2775 periodically to see if it has been removed and recreated by some other program.
2776 Note that the inotify-based implementation handles this case without
2777 the need for any periodic reopening.
2779 No matter which method you use, if the tracked file is determined to have
2780 shrunk, @command{tail} prints a message saying the file has been truncated
2781 and resumes tracking the end of the file from the newly-determined endpoint.
2783 When a file is removed, @command{tail}'s behavior depends on whether it is
2784 following the name or the descriptor. When following by name, tail can
2785 detect that a file has been removed and gives a message to that effect,
2786 and if @option{--retry} has been specified it will continue checking
2787 periodically to see if the file reappears.
2788 When following a descriptor, tail does not detect that the file has
2789 been unlinked or renamed and issues no message; even though the file
2790 may no longer be accessible via its original name, it may still be
2793 The option values @samp{descriptor} and @samp{name} may be specified only
2794 with the long form of the option, not with @option{-f}.
2796 The @option{-f} option is ignored if
2797 no @var{file} operand is specified and standard input is a FIFO or a pipe.
2798 Likewise, the @option{-f} option has no effect for any
2799 operand specified as @samp{-}, when standard input is a FIFO or a pipe.
2803 This option is the same as @option{--follow=name --retry}. That is, tail
2804 will attempt to reopen a file when it is removed. Should this fail, tail
2805 will keep trying until it becomes accessible again.
2809 This option is useful mainly when following by name (i.e., with
2810 @option{--follow=name}).
2811 Without this option, when tail encounters a file that doesn't
2812 exist or is otherwise inaccessible, it reports that fact and
2813 never checks it again.
2815 @itemx --sleep-interval=@var{number}
2816 @opindex --sleep-interval
2817 Change the number of seconds to wait between iterations (the default is 1.0).
2818 During one iteration, every specified file is checked to see if it has
2820 Historical implementations of @command{tail} have required that
2821 @var{number} be an integer. However, GNU @command{tail} accepts
2822 an arbitrary floating point number (using a period before any
2824 When @command{tail} uses inotify, this polling-related option is ignored.
2826 @itemx --pid=@var{pid}
2828 When following by name or by descriptor, you may specify the process ID,
2829 @var{pid}, of the sole writer of all @var{file} arguments. Then, shortly
2830 after that process terminates, tail will also terminate. This will
2831 work properly only if the writer and the tailing process are running on
2832 the same machine. For example, to save the output of a build in a file
2833 and to watch the file grow, if you invoke @command{make} and @command{tail}
2834 like this then the tail process will stop when your build completes.
2835 Without this option, you would have had to kill the @code{tail -f}
2839 $ make >& makerr & tail --pid=$! -f makerr
2842 If you specify a @var{pid} that is not in use or that does not correspond
2843 to the process that is writing to the tailed files, then @command{tail}
2844 may terminate long before any @var{file}s stop growing or it may not
2845 terminate until long after the real writer has terminated.
2846 Note that @option{--pid} cannot be supported on some systems; @command{tail}
2847 will print a warning if this is the case.
2849 @itemx --max-unchanged-stats=@var{n}
2850 @opindex --max-unchanged-stats
2851 When tailing a file by name, if there have been @var{n} (default
2852 n=@value{DEFAULT_MAX_N_UNCHANGED_STATS_BETWEEN_OPENS}) consecutive
2853 iterations for which the file has not changed, then
2854 @code{open}/@code{fstat} the file to determine if that file name is
2855 still associated with the same device/inode-number pair as before.
2856 When following a log file that is rotated, this is approximately the
2857 number of seconds between when tail prints the last pre-rotation lines
2858 and when it prints the lines that have accumulated in the new log file.
2859 This option is meaningful only when polling (i.e., without inotify)
2860 and when following by name.
2863 @itemx --lines=@var{k}
2866 Output the last @var{k} lines.
2867 However, if @var{k} starts with a @samp{+}, start printing with the
2868 @var{k}th line from the start of each file, instead of from the end.
2869 Size multiplier suffixes are the same as with the @option{-c} option.
2877 Never print file name headers.
2883 Always print file name headers.
2887 For compatibility @command{tail} also supports an obsolete usage
2888 @samp{tail -[@var{count}][bcl][f] [@var{file}]}, which is recognized
2889 only if it does not conflict with the usage described
2890 above. This obsolete form uses exactly one option and at most one
2891 file. In the option, @var{count} is an optional decimal number optionally
2892 followed by a size letter (@samp{b}, @samp{c}, @samp{l}) to mean count
2893 by 512-byte blocks, bytes, or lines, optionally followed by @samp{f}
2894 which has the same meaning as @option{-f}.
2896 @vindex _POSIX2_VERSION
2897 On older systems, the leading @samp{-} can be replaced by @samp{+} in
2898 the obsolete option syntax with the same meaning as in counts, and
2899 obsolete usage overrides normal usage when the two conflict.
2900 This obsolete behavior can be enabled or disabled with the
2901 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
2904 Scripts intended for use on standard hosts should avoid obsolete
2905 syntax and should use @option{-c @var{count}[b]}, @option{-n
2906 @var{count}}, and/or @option{-f} instead. If your script must also
2907 run on hosts that support only the obsolete syntax, you can often
2908 rewrite it to avoid problematic usages, e.g., by using @samp{sed -n
2909 '$p'} rather than @samp{tail -1}. If that's not possible, the script
2910 can use a test like @samp{if tail -c +1 </dev/null >/dev/null 2>&1;
2911 then @dots{}} to decide which syntax to use.
2913 Even if your script assumes the standard behavior, you should still
2914 beware usages whose behaviors differ depending on the @acronym{POSIX}
2915 version. For example, avoid @samp{tail - main.c}, since it might be
2916 interpreted as either @samp{tail main.c} or as @samp{tail -- -
2917 main.c}; avoid @samp{tail -c 4}, since it might mean either @samp{tail
2918 -c4} or @samp{tail -c 10 4}; and avoid @samp{tail +4}, since it might
2919 mean either @samp{tail ./+4} or @samp{tail -n +4}.
2924 @node split invocation
2925 @section @command{split}: Split a file into pieces.
2928 @cindex splitting a file into pieces
2929 @cindex pieces, splitting a file into
2931 @command{split} creates output files containing consecutive or interleaved
2932 sections of @var{input} (standard input if none is given or @var{input}
2933 is @samp{-}). Synopsis:
2936 split [@var{option}] [@var{input} [@var{prefix}]]
2939 By default, @command{split} puts 1000 lines of @var{input} (or whatever is
2940 left over for the last section), into each output file.
2942 @cindex output file name prefix
2943 The output files' names consist of @var{prefix} (@samp{x} by default)
2944 followed by a group of characters (@samp{aa}, @samp{ab}, @dots{} by
2945 default), such that concatenating the output files in traditional
2946 sorted order by file name produces the original input file (except
2947 @option{-r}). If the output file names are exhausted, @command{split}
2948 reports an error without deleting the output files that it did create.
2950 The program accepts the following options. Also see @ref{Common options}.
2954 @item -l @var{lines}
2955 @itemx --lines=@var{lines}
2958 Put @var{lines} lines of @var{input} into each output file.
2960 For compatibility @command{split} also supports an obsolete
2961 option syntax @option{-@var{lines}}. New scripts should use @option{-l
2962 @var{lines}} instead.
2965 @itemx --bytes=@var{size}
2968 Put @var{size} bytes of @var{input} into each output file.
2969 @multiplierSuffixes{size}
2972 @itemx --line-bytes=@var{size}
2974 @opindex --line-bytes
2975 Put into each output file as many complete lines of @var{input} as
2976 possible without exceeding @var{size} bytes. Individual lines longer than
2977 @var{size} bytes are broken into multiple files.
2978 @var{size} has the same format as for the @option{--bytes} option.
2980 @item -n @var{chunks}
2981 @itemx --number=@var{chunks}
2985 Split @var{input} to @var{chunks} output files where @var{chunks} may be:
2988 @var{n} generate @var{n} files based on current size of @var{input}
2989 @var{k}/@var{n} only output @var{k}th of @var{n} to stdout
2990 l/@var{n} generate @var{n} files without splitting lines
2991 l/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2992 r/@var{n} like @samp{l} but use round robin distribution
2993 r/@var{k}/@var{n} likewise but only output @var{k}th of @var{n} to stdout
2996 Any excess bytes remaining after dividing the @var{input}
2997 into @var{n} chunks, are assigned to the last chunk.
2998 Any excess bytes appearing after the initial calculation are discarded
2999 (except when using @samp{r} mode).
3001 All @var{n} files are created even if there are fewer than @var{n} lines,
3002 or the @var{input} is truncated.
3004 For @samp{l} mode, chunks are approximately @var{input} size / @var{n}.
3005 The @var{input} is partitioned into @var{n} equal sized portions, with
3006 the last assigned any excess. If a line @emph{starts} within a partition
3007 it is written completely to the corresponding file. Since lines
3008 are not split even if they overlap a partition, the files written
3009 can be larger or smaller than the partition size, and even empty
3010 if a line is so long as to completely overlap the partition.
3012 For @samp{r} mode, the size of @var{input} is irrelevant,
3013 and so can be a pipe for example.
3015 @item -a @var{length}
3016 @itemx --suffix-length=@var{length}
3018 @opindex --suffix-length
3019 Use suffixes of length @var{length}. The default @var{length} is 2.
3022 @itemx --numeric-suffixes
3024 @opindex --numeric-suffixes
3025 Use digits in suffixes rather than lower-case letters.
3028 @itemx --elide-empty-files
3030 @opindex --elide-empty-files
3031 Suppress the generation of zero-length output files. This can happen
3032 with the @option{--number} option if a file is (truncated to be) shorter
3033 than the number requested, or if a line is so long as to completely
3034 span a chunk. The output file sequence numbers, always run consecutively
3035 even when this option is specified.
3040 @opindex --unbuffered
3041 Immediately copy input to output in @option{--number r/...} mode,
3042 which is a much slower mode of operation.
3046 Write a diagnostic just before each output file is opened.
3052 Here are a few examples to illustrate how the
3053 @option{--number} (@option{-n}) option works:
3055 Notice how, by default, one line may be split onto two or more:
3058 $ seq -w 6 10 > k; split -n3 k; head xa?
3071 Use the "l/" modifier to suppress that:
3074 $ seq -w 6 10 > k; split -nl/3 k; head xa?
3087 Use the "r/" modifier to distribute lines in a round-robin fashion:
3090 $ seq -w 6 10 > k; split -nr/3 k; head xa?
3103 You can also extract just the Kth chunk.
3104 This extracts and prints just the 7th "chunk" of 33:
3107 $ seq 100 > k; split -nl/7/33 k
3114 @node csplit invocation
3115 @section @command{csplit}: Split a file into context-determined pieces
3118 @cindex context splitting
3119 @cindex splitting a file into pieces by context
3121 @command{csplit} creates zero or more output files containing sections of
3122 @var{input} (standard input if @var{input} is @samp{-}). Synopsis:
3125 csplit [@var{option}]@dots{} @var{input} @var{pattern}@dots{}
3128 The contents of the output files are determined by the @var{pattern}
3129 arguments, as detailed below. An error occurs if a @var{pattern}
3130 argument refers to a nonexistent line of the input file (e.g., if no
3131 remaining line matches a given regular expression). After every
3132 @var{pattern} has been matched, any remaining input is copied into one
3135 By default, @command{csplit} prints the number of bytes written to each
3136 output file after it has been created.
3138 The types of pattern arguments are:
3143 Create an output file containing the input up to but not including line
3144 @var{n} (a positive integer). If followed by a repeat count, also
3145 create an output file containing the next @var{n} lines of the input
3146 file once for each repeat.
3148 @item /@var{regexp}/[@var{offset}]
3149 Create an output file containing the current line up to (but not
3150 including) the next line of the input file that contains a match for
3151 @var{regexp}. The optional @var{offset} is an integer.
3152 If it is given, the input up to (but not including) the
3153 matching line plus or minus @var{offset} is put into the output file,
3154 and the line after that begins the next section of input.
3156 @item %@var{regexp}%[@var{offset}]
3157 Like the previous type, except that it does not create an output
3158 file, so that section of the input file is effectively ignored.
3160 @item @{@var{repeat-count}@}
3161 Repeat the previous pattern @var{repeat-count} additional
3162 times. The @var{repeat-count} can either be a positive integer or an
3163 asterisk, meaning repeat as many times as necessary until the input is
3168 The output files' names consist of a prefix (@samp{xx} by default)
3169 followed by a suffix. By default, the suffix is an ascending sequence
3170 of two-digit decimal numbers from @samp{00} to @samp{99}. In any case,
3171 concatenating the output files in sorted order by file name produces the
3172 original input file.
3174 By default, if @command{csplit} encounters an error or receives a hangup,
3175 interrupt, quit, or terminate signal, it removes any output files
3176 that it has created so far before it exits.
3178 The program accepts the following options. Also see @ref{Common options}.
3182 @item -f @var{prefix}
3183 @itemx --prefix=@var{prefix}
3186 @cindex output file name prefix
3187 Use @var{prefix} as the output file name prefix.
3189 @item -b @var{suffix}
3190 @itemx --suffix=@var{suffix}
3193 @cindex output file name suffix
3194 Use @var{suffix} as the output file name suffix. When this option is
3195 specified, the suffix string must include exactly one
3196 @code{printf(3)}-style conversion specification, possibly including
3197 format specification flags, a field width, a precision specifications,
3198 or all of these kinds of modifiers. The format letter must convert a
3199 binary unsigned integer argument to readable form. The format letters
3200 @samp{d} and @samp{i} are aliases for @samp{u}, and the
3201 @samp{u}, @samp{o}, @samp{x}, and @samp{X} conversions are allowed. The
3202 entire @var{suffix} is given (with the current output file number) to
3203 @code{sprintf(3)} to form the file name suffixes for each of the
3204 individual output files in turn. If this option is used, the
3205 @option{--digits} option is ignored.
3207 @item -n @var{digits}
3208 @itemx --digits=@var{digits}
3211 Use output file names containing numbers that are @var{digits} digits
3212 long instead of the default 2.
3217 @opindex --keep-files
3218 Do not remove output files when errors are encountered.
3221 @itemx --elide-empty-files
3223 @opindex --elide-empty-files
3224 Suppress the generation of zero-length output files. (In cases where
3225 the section delimiters of the input file are supposed to mark the first
3226 lines of each of the sections, the first output file will generally be a
3227 zero-length file unless you use this option.) The output file sequence
3228 numbers always run consecutively starting from 0, even when this option
3239 Do not print counts of output file sizes.
3245 Here is an example of its usage.
3246 First, create an empty directory for the exercise,
3253 Now, split the sequence of 1..14 on lines that end with 0 or 5:
3256 $ seq 14 | csplit - '/[05]$/' '@{*@}'
3262 Each number printed above is the size of an output
3263 file that csplit has just created.
3264 List the names of those output files:
3271 Use @command{head} to show their contents:
3296 @node Summarizing files
3297 @chapter Summarizing files
3299 @cindex summarizing files
3301 These commands generate just a few numbers representing entire
3305 * wc invocation:: Print newline, word, and byte counts.
3306 * sum invocation:: Print checksum and block counts.
3307 * cksum invocation:: Print CRC checksum and byte counts.
3308 * md5sum invocation:: Print or check MD5 digests.
3309 * sha1sum invocation:: Print or check SHA-1 digests.
3310 * sha2 utilities:: Print or check SHA-2 digests.
3315 @section @command{wc}: Print newline, word, and byte counts
3319 @cindex character count
3323 @command{wc} counts the number of bytes, characters, whitespace-separated
3324 words, and newlines in each given @var{file}, or standard input if none
3325 are given or for a @var{file} of @samp{-}. Synopsis:
3328 wc [@var{option}]@dots{} [@var{file}]@dots{}
3331 @cindex total counts
3332 @command{wc} prints one line of counts for each file, and if the file was
3333 given as an argument, it prints the file name following the counts. If
3334 more than one @var{file} is given, @command{wc} prints a final line
3335 containing the cumulative counts, with the file name @file{total}. The
3336 counts are printed in this order: newlines, words, characters, bytes,
3337 maximum line length.
3338 Each count is printed right-justified in a field with at least one
3339 space between fields so that the numbers and file names normally line
3340 up nicely in columns. The width of the count fields varies depending
3341 on the inputs, so you should not depend on a particular field width.
3342 However, as a @acronym{GNU} extension, if only one count is printed,
3343 it is guaranteed to be printed without leading spaces.
3345 By default, @command{wc} prints three counts: the newline, words, and byte
3346 counts. Options can specify that only certain counts be printed.
3347 Options do not undo others previously given, so
3354 prints both the byte counts and the word counts.
3356 With the @option{--max-line-length} option, @command{wc} prints the length
3357 of the longest line per file, and if there is more than one file it
3358 prints the maximum (not the sum) of those lengths. The line lengths here
3359 are measured in screen columns, according to the current locale and
3360 assuming tab positions in every 8th column.
3362 The program accepts the following options. Also see @ref{Common options}.
3370 Print only the byte counts.
3376 Print only the character counts.
3382 Print only the word counts.
3388 Print only the newline counts.
3391 @itemx --max-line-length
3393 @opindex --max-line-length
3394 Print only the maximum line lengths.
3396 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
3397 @itemx --files0-from=@var{file}
3398 @opindex --files0-from=@var{file}
3399 @c This is commented out to avoid a texi2dvi failure.
3400 @c texi2dvi (GNU Texinfo 4.11) 1.104
3401 @c @cindex including files from @command{\cmd\}
3402 Disallow processing files named on the command line, and instead process
3403 those named in file @var{file}; each name being terminated by a zero byte
3404 (@acronym{ASCII} @sc{nul}).
3405 This is useful \withTotalOption\
3406 when the list of file names is so long that it may exceed a command line
3408 In such cases, running @command{\cmd\} via @command{xargs} is undesirable
3409 because it splits the list into pieces and makes @command{\cmd\} print
3410 \subListOutput\ for each sublist rather than for the entire list.
3411 One way to produce a list of @acronym{ASCII} @sc{nul} terminated file names is with @sc{gnu}
3412 @command{find}, using its @option{-print0} predicate.
3413 If @var{file} is @samp{-} then the @acronym{ASCII} @sc{nul} terminated file names
3414 are read from standard input.
3416 @filesZeroFromOption{wc,,a total}
3418 For example, to find the length of the longest line in any @file{.c} or
3419 @file{.h} file in the current hierarchy, do this:
3422 find . -name '*.[ch]' -print0 |
3423 wc -L --files0-from=- | tail -n1
3431 @node sum invocation
3432 @section @command{sum}: Print checksum and block counts
3435 @cindex 16-bit checksum
3436 @cindex checksum, 16-bit
3438 @command{sum} computes a 16-bit checksum for each given @var{file}, or
3439 standard input if none are given or for a @var{file} of @samp{-}. Synopsis:
3442 sum [@var{option}]@dots{} [@var{file}]@dots{}
3445 @command{sum} prints the checksum for each @var{file} followed by the
3446 number of blocks in the file (rounded up). If more than one @var{file}
3447 is given, file names are also printed (by default). (With the
3448 @option{--sysv} option, corresponding file names are printed when there is
3449 at least one file argument.)
3451 By default, @sc{gnu} @command{sum} computes checksums using an algorithm
3452 compatible with BSD @command{sum} and prints file sizes in units of
3455 The program accepts the following options. Also see @ref{Common options}.
3461 @cindex BSD @command{sum}
3462 Use the default (BSD compatible) algorithm. This option is included for
3463 compatibility with the System V @command{sum}. Unless @option{-s} was also
3464 given, it has no effect.
3470 @cindex System V @command{sum}
3471 Compute checksums using an algorithm compatible with System V
3472 @command{sum}'s default, and print file sizes in units of 512-byte blocks.
3476 @command{sum} is provided for compatibility; the @command{cksum} program (see
3477 next section) is preferable in new applications.
3482 @node cksum invocation
3483 @section @command{cksum}: Print CRC checksum and byte counts
3486 @cindex cyclic redundancy check
3487 @cindex CRC checksum
3489 @command{cksum} computes a cyclic redundancy check (CRC) checksum for each
3490 given @var{file}, or standard input if none are given or for a
3491 @var{file} of @samp{-}. Synopsis:
3494 cksum [@var{option}]@dots{} [@var{file}]@dots{}
3497 @command{cksum} prints the CRC checksum for each file along with the number
3498 of bytes in the file, and the file name unless no arguments were given.
3500 @command{cksum} is typically used to ensure that files
3501 transferred by unreliable means (e.g., netnews) have not been corrupted,
3502 by comparing the @command{cksum} output for the received files with the
3503 @command{cksum} output for the original files (typically given in the
3506 The CRC algorithm is specified by the @acronym{POSIX} standard. It is not
3507 compatible with the BSD or System V @command{sum} algorithms (see the
3508 previous section); it is more robust.
3510 The only options are @option{--help} and @option{--version}. @xref{Common
3516 @node md5sum invocation
3517 @section @command{md5sum}: Print or check MD5 digests
3521 @cindex 128-bit checksum
3522 @cindex checksum, 128-bit
3523 @cindex fingerprint, 128-bit
3524 @cindex message-digest, 128-bit
3526 @command{md5sum} computes a 128-bit checksum (or @dfn{fingerprint} or
3527 @dfn{message-digest}) for each specified @var{file}.
3529 Note: The MD5 digest is more reliable than a simple CRC (provided by
3530 the @command{cksum} command) for detecting accidental file corruption,
3531 as the chances of accidentally having two files with identical MD5
3532 are vanishingly small. However, it should not be considered secure
3533 against malicious tampering: although finding a file with a given MD5
3534 fingerprint is considered infeasible at the moment, it is known how
3535 to modify certain files, including digital certificates, so that they
3536 appear valid when signed with an MD5 digest.
3537 For more secure hashes, consider using SHA-2. @xref{sha2 utilities}.
3539 If a @var{file} is specified as @samp{-} or if no files are given
3540 @command{md5sum} computes the checksum for the standard input.
3541 @command{md5sum} can also determine whether a file and checksum are
3542 consistent. Synopsis:
3545 md5sum [@var{option}]@dots{} [@var{file}]@dots{}
3548 For each @var{file}, @samp{md5sum} outputs the MD5 checksum, a flag
3549 indicating a binary or text input file, and the file name.
3550 If @var{file} contains a backslash or newline, the
3551 line is started with a backslash, and each problematic character in
3552 the file name is escaped with a backslash, making the output
3553 unambiguous even in the presence of arbitrary file names.
3554 If @var{file} is omitted or specified as @samp{-}, standard input is read.
3556 The program accepts the following options. Also see @ref{Common options}.
3564 @cindex binary input files
3565 Treat each input file as binary, by reading it in binary mode and
3566 outputting a @samp{*} flag. This is the inverse of @option{--text}.
3567 On systems like @acronym{GNU} that do not distinguish between binary
3568 and text files, this option merely flags each input file as binary:
3569 the MD5 checksum is unaffected. This option is the default on systems
3570 like MS-DOS that distinguish between binary and text files, except
3571 for reading standard input when standard input is a terminal.
3575 Read file names and checksum information (not data) from each
3576 @var{file} (or from stdin if no @var{file} was specified) and report
3577 whether the checksums match the contents of the named files.
3578 The input to this mode of @command{md5sum} is usually the output of
3579 a prior, checksum-generating run of @samp{md5sum}.
3580 Each valid line of input consists of an MD5 checksum, a binary/text
3581 flag, and then a file name.
3582 Binary files are marked with @samp{*}, text with @samp{ }.
3583 For each such line, @command{md5sum} reads the named file and computes its
3584 MD5 checksum. Then, if the computed message digest does not match the
3585 one on the line with the file name, the file is noted as having
3586 failed the test. Otherwise, the file passes the test.
3587 By default, for each valid line, one line is written to standard
3588 output indicating whether the named file passed the test.
3589 After all checks have been performed, if there were any failures,
3590 a warning is issued to standard error.
3591 Use the @option{--status} option to inhibit that output.
3592 If any listed file cannot be opened or read, if any valid line has
3593 an MD5 checksum inconsistent with the associated file, or if no valid
3594 line is found, @command{md5sum} exits with nonzero status. Otherwise,
3595 it exits successfully.
3599 @cindex verifying MD5 checksums
3600 This option is useful only when verifying checksums.
3601 When verifying checksums, don't generate an 'OK' message per successfully
3602 checked file. Files that fail the verification are reported in the
3603 default one-line-per-file format. If there is any checksum mismatch,
3604 print a warning summarizing the failures to standard error.
3608 @cindex verifying MD5 checksums
3609 This option is useful only when verifying checksums.
3610 When verifying checksums, don't generate the default one-line-per-file
3611 diagnostic and don't output the warning summarizing any failures.
3612 Failures to open or read a file still evoke individual diagnostics to
3614 If all listed files are readable and are consistent with the associated
3615 MD5 checksums, exit successfully. Otherwise exit with a status code
3616 indicating there was a failure.
3622 @cindex text input files
3623 Treat each input file as text, by reading it in text mode and
3624 outputting a @samp{ } flag. This is the inverse of @option{--binary}.
3625 This option is the default on systems like @acronym{GNU} that do not
3626 distinguish between binary and text files. On other systems, it is
3627 the default for reading standard input when standard input is a
3634 @cindex verifying MD5 checksums
3635 When verifying checksums, warn about improperly formatted MD5 checksum lines.
3636 This option is useful only if all but a few lines in the checked input
3644 @node sha1sum invocation
3645 @section @command{sha1sum}: Print or check SHA-1 digests
3649 @cindex 160-bit checksum
3650 @cindex checksum, 160-bit
3651 @cindex fingerprint, 160-bit
3652 @cindex message-digest, 160-bit
3654 @command{sha1sum} computes a 160-bit checksum for each specified
3655 @var{file}. The usage and options of this command are precisely the
3656 same as for @command{md5sum}. @xref{md5sum invocation}.
3658 Note: The SHA-1 digest is more secure than MD5, and no collisions of
3659 it are known (different files having the same fingerprint). However,
3660 it is known that they can be produced with considerable, but not
3661 unreasonable, resources. For this reason, it is generally considered
3662 that SHA-1 should be gradually phased out in favor of the more secure
3663 SHA-2 hash algorithms. @xref{sha2 utilities}.
3666 @node sha2 utilities
3667 @section sha2 utilities: Print or check SHA-2 digests
3674 @cindex 224-bit checksum
3675 @cindex 256-bit checksum
3676 @cindex 384-bit checksum
3677 @cindex 512-bit checksum
3678 @cindex checksum, 224-bit
3679 @cindex checksum, 256-bit
3680 @cindex checksum, 384-bit
3681 @cindex checksum, 512-bit
3682 @cindex fingerprint, 224-bit
3683 @cindex fingerprint, 256-bit
3684 @cindex fingerprint, 384-bit
3685 @cindex fingerprint, 512-bit
3686 @cindex message-digest, 224-bit
3687 @cindex message-digest, 256-bit
3688 @cindex message-digest, 384-bit
3689 @cindex message-digest, 512-bit
3691 The commands @command{sha224sum}, @command{sha256sum},
3692 @command{sha384sum} and @command{sha512sum} compute checksums of
3693 various lengths (respectively 224, 256, 384 and 512 bits),
3694 collectively known as the SHA-2 hashes. The usage and options of
3695 these commands are precisely the same as for @command{md5sum}.
3696 @xref{md5sum invocation}.
3698 Note: The SHA384 and SHA512 digests are considerably slower to
3699 compute, especially on 32-bit computers, than SHA224 or SHA256.
3702 @node Operating on sorted files
3703 @chapter Operating on sorted files
3705 @cindex operating on sorted files
3706 @cindex sorted files, operations on
3708 These commands work with (or produce) sorted files.
3711 * sort invocation:: Sort text files.
3712 * shuf invocation:: Shuffle text files.
3713 * uniq invocation:: Uniquify files.
3714 * comm invocation:: Compare two sorted files line by line.
3715 * ptx invocation:: Produce a permuted index of file contents.
3716 * tsort invocation:: Topological sort.
3720 @node sort invocation
3721 @section @command{sort}: Sort text files
3724 @cindex sorting files
3726 @command{sort} sorts, merges, or compares all the lines from the given
3727 files, or standard input if none are given or for a @var{file} of
3728 @samp{-}. By default, @command{sort} writes the results to standard
3732 sort [@var{option}]@dots{} [@var{file}]@dots{}
3735 @command{sort} has three modes of operation: sort (the default), merge,
3736 and check for sortedness. The following options change the operation
3743 @itemx --check=diagnose-first
3746 @cindex checking for sortedness
3747 Check whether the given file is already sorted: if it is not all
3748 sorted, print a diagnostic containing the first out-of-order line and
3749 exit with a status of 1.
3750 Otherwise, exit successfully.
3751 At most one input file can be given.
3754 @itemx --check=quiet
3755 @itemx --check=silent
3758 @cindex checking for sortedness
3759 Exit successfully if the given file is already sorted, and
3760 exit with status 1 otherwise.
3761 At most one input file can be given.
3762 This is like @option{-c}, except it does not print a diagnostic.
3768 @cindex merging sorted files
3769 Merge the given files by sorting them as a group. Each input file must
3770 always be individually sorted. It always works to sort instead of
3771 merge; merging is provided because it is faster, in the case where it
3776 @cindex sort stability
3777 @cindex sort's last-resort comparison
3778 A pair of lines is compared as follows:
3779 @command{sort} compares each pair of fields, in the
3780 order specified on the command line, according to the associated
3781 ordering options, until a difference is found or no fields are left.
3782 If no key fields are specified, @command{sort} uses a default key of
3783 the entire line. Finally, as a last resort when all keys compare
3784 equal, @command{sort} compares entire lines as if no ordering options
3785 other than @option{--reverse} (@option{-r}) were specified. The
3786 @option{--stable} (@option{-s}) option disables this @dfn{last-resort
3787 comparison} so that lines in which all fields compare equal are left
3788 in their original relative order. The @option{--unique}
3789 (@option{-u}) option also disables the last-resort comparison.
3793 Unless otherwise specified, all comparisons use the character collating
3794 sequence specified by the @env{LC_COLLATE} locale.@footnote{If you
3795 use a non-@acronym{POSIX} locale (e.g., by setting @env{LC_ALL}
3796 to @samp{en_US}), then @command{sort} may produce output that is sorted
3797 differently than you're accustomed to. In that case, set the @env{LC_ALL}
3798 environment variable to @samp{C}. Note that setting only @env{LC_COLLATE}
3799 has two problems. First, it is ineffective if @env{LC_ALL} is also set.
3800 Second, it has undefined behavior if @env{LC_CTYPE} (or @env{LANG}, if
3801 @env{LC_CTYPE} is unset) is set to an incompatible value. For example,
3802 you get undefined behavior if @env{LC_CTYPE} is @code{ja_JP.PCK} but
3803 @env{LC_COLLATE} is @code{en_US.UTF-8}.}
3805 @sc{gnu} @command{sort} (as specified for all @sc{gnu} utilities) has no
3806 limit on input line length or restrictions on bytes allowed within lines.
3807 In addition, if the final byte of an input file is not a newline, @sc{gnu}
3808 @command{sort} silently supplies one. A line's trailing newline is not
3809 part of the line for comparison purposes.
3811 @cindex exit status of @command{sort}
3815 0 if no error occurred
3816 1 if invoked with @option{-c} or @option{-C} and the input is not sorted
3817 2 if an error occurred
3821 If the environment variable @env{TMPDIR} is set, @command{sort} uses its
3822 value as the directory for temporary files instead of @file{/tmp}. The
3823 @option{--temporary-directory} (@option{-T}) option in turn overrides
3824 the environment variable.
3826 The following options affect the ordering of output lines. They may be
3827 specified globally or as part of a specific key field. If no key
3828 fields are specified, global options apply to comparison of entire
3829 lines; otherwise the global options are inherited by key fields that do
3830 not specify any special options of their own. In pre-@acronym{POSIX}
3831 versions of @command{sort}, global options affect only later key fields,
3832 so portable shell scripts should specify global options first.
3837 @itemx --ignore-leading-blanks
3839 @opindex --ignore-leading-blanks
3840 @cindex blanks, ignoring leading
3842 Ignore leading blanks when finding sort keys in each line.
3843 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3844 can change this. Note blanks may be ignored by your locale's collating
3845 rules, but without this option they will be significant for character
3846 positions specified in keys with the @option{-k} option.
3849 @itemx --dictionary-order
3851 @opindex --dictionary-order
3852 @cindex dictionary order
3853 @cindex phone directory order
3854 @cindex telephone directory order
3856 Sort in @dfn{phone directory} order: ignore all characters except
3857 letters, digits and blanks when sorting.
3858 By default letters and digits are those of @acronym{ASCII} and a blank
3859 is a space or a tab, but the @env{LC_CTYPE} locale can change this.
3862 @itemx --ignore-case
3864 @opindex --ignore-case
3865 @cindex ignoring case
3866 @cindex case folding
3868 Fold lowercase characters into the equivalent uppercase characters when
3869 comparing so that, for example, @samp{b} and @samp{B} sort as equal.
3870 The @env{LC_CTYPE} locale determines character types.
3871 When used with @option{--unique} those lower case equivalent lines are
3872 thrown away. (There is currently no way to throw away the upper case
3873 equivalent instead. (Any @option{--reverse} given would only affect
3874 the final result, after the throwing away.))
3877 @itemx --general-numeric-sort
3878 @itemx --sort=general-numeric
3880 @opindex --general-numeric-sort
3882 @cindex general numeric sort
3884 Sort numerically, using the standard C function @code{strtold} to convert
3885 a prefix of each line to a long double-precision floating point number.
3886 This allows floating point numbers to be specified in scientific notation,
3887 like @code{1.0e-34} and @code{10e100}.
3888 The @env{LC_NUMERIC} locale determines the decimal-point character.
3889 Do not report overflow, underflow, or conversion errors.
3890 Use the following collating sequence:
3894 Lines that do not start with numbers (all considered to be equal).
3896 NaNs (``Not a Number'' values, in IEEE floating point arithmetic)
3897 in a consistent but machine-dependent order.
3901 Finite numbers in ascending numeric order (with @math{-0} and @math{+0} equal).
3906 Use this option only if there is no alternative; it is much slower than
3907 @option{--numeric-sort} (@option{-n}) and it can lose information when
3908 converting to floating point.
3911 @itemx --human-numeric-sort
3912 @itemx --sort=human-numeric
3914 @opindex --human-numeric-sort
3916 @cindex human numeric sort
3918 Sort numerically, first by numeric sign (negative, zero, or positive);
3919 then by @acronym{SI} suffix (either empty, or @samp{k} or @samp{K}, or
3920 one of @samp{MGTPEZY}, in that order; @pxref{Block size}); and finally
3921 by numeric value. For example, @samp{1023M} sorts before @samp{1G}
3922 because @samp{M} (mega) precedes @samp{G} (giga) as an @acronym{SI}
3923 suffix. This option sorts values that are consistently scaled to the
3924 nearest suffix, regardless of whether suffixes denote powers of 1000
3925 or 1024, and it therefore sorts the output of any single invocation of
3926 the @command{df}, @command{du}, or @command{ls} commands that are
3927 invoked with their @option{--human-readable} or @option{--si} options.
3928 The syntax for numbers is the same as for the @option{--numeric-sort}
3929 option; the @acronym{SI} suffix must immediately follow the number.
3932 @itemx --ignore-nonprinting
3934 @opindex --ignore-nonprinting
3935 @cindex nonprinting characters, ignoring
3936 @cindex unprintable characters, ignoring
3938 Ignore nonprinting characters.
3939 The @env{LC_CTYPE} locale determines character types.
3940 This option has no effect if the stronger @option{--dictionary-order}
3941 (@option{-d}) option is also given.
3947 @opindex --month-sort
3949 @cindex months, sorting by
3951 An initial string, consisting of any amount of blanks, followed
3952 by a month name abbreviation, is folded to UPPER case and
3953 compared in the order @samp{JAN} < @samp{FEB} < @dots{} < @samp{DEC}.
3954 Invalid names compare low to valid names. The @env{LC_TIME} locale
3955 category determines the month spellings.
3956 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3960 @itemx --numeric-sort
3961 @itemx --sort=numeric
3963 @opindex --numeric-sort
3965 @cindex numeric sort
3967 Sort numerically. The number begins each line and consists
3968 of optional blanks, an optional @samp{-} sign, and zero or more
3969 digits possibly separated by thousands separators, optionally followed
3970 by a decimal-point character and zero or more digits. An empty
3971 number is treated as @samp{0}. The @env{LC_NUMERIC}
3972 locale specifies the decimal-point character and thousands separator.
3973 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
3976 Comparison is exact; there is no rounding error.
3978 Neither a leading @samp{+} nor exponential notation is recognized.
3979 To compare such strings numerically, use the
3980 @option{--general-numeric-sort} (@option{-g}) option.
3983 @itemx --version-sort
3985 @opindex --version-sort
3986 @cindex version number sort
3987 Sort by version name and number. It behaves like a standard sort,
3988 except that each sequence of decimal digits is treated numerically
3989 as an index/version number. (@xref{Details about version sort}.)
3995 @cindex reverse sorting
3996 Reverse the result of comparison, so that lines with greater key values
3997 appear earlier in the output instead of later.
4000 @itemx --random-sort
4001 @itemx --sort=random
4003 @opindex --random-sort
4006 Sort by hashing the input keys and then sorting the hash values.
4007 Choose the hash function at random, ensuring that it is free of
4008 collisions so that differing keys have differing hash values. This is
4009 like a random permutation of the inputs (@pxref{shuf invocation}),
4010 except that keys with the same value sort together.
4012 If multiple random sort fields are specified, the same random hash
4013 function is used for all fields. To use different random hash
4014 functions for different fields, you can invoke @command{sort} more
4017 The choice of hash function is affected by the
4018 @option{--random-source} option.
4026 @item --compress-program=@var{prog}
4027 Compress any temporary files with the program @var{prog}.
4029 With no arguments, @var{prog} must compress standard input to standard
4030 output, and when given the @option{-d} option it must decompress
4031 standard input to standard output.
4033 Terminate with an error if @var{prog} exits with nonzero status.
4035 White space and the backslash character should not appear in
4036 @var{prog}; they are reserved for future use.
4038 @filesZeroFromOption{sort,,sorted output}
4040 @item -k @var{pos1}[,@var{pos2}]
4041 @itemx --key=@var{pos1}[,@var{pos2}]
4045 Specify a sort field that consists of the part of the line between
4046 @var{pos1} and @var{pos2} (or the end of the line, if @var{pos2} is
4047 omitted), @emph{inclusive}.
4049 Each @var{pos} has the form @samp{@var{f}[.@var{c}][@var{opts}]},
4050 where @var{f} is the number of the field to use, and @var{c} is the number
4051 of the first character from the beginning of the field. Fields and character
4052 positions are numbered starting with 1; a character position of zero in
4053 @var{pos2} indicates the field's last character. If @samp{.@var{c}} is
4054 omitted from @var{pos1}, it defaults to 1 (the beginning of the field);
4055 if omitted from @var{pos2}, it defaults to 0 (the end of the field).
4056 @var{opts} are ordering options, allowing individual keys to be sorted
4057 according to different rules; see below for details. Keys can span
4060 Example: To sort on the second field, use @option{--key=2,2}
4061 (@option{-k 2,2}). See below for more notes on keys and more examples.
4062 See also the @option{--debug} option to help determine the part
4063 of the line being used in the sort.
4066 Highlight the portion of each line used for sorting.
4067 Also issue warnings about questionable usage to stderr.
4069 @item --batch-size=@var{nmerge}
4070 @opindex --batch-size
4071 @cindex number of inputs to merge, nmerge
4072 Merge at most @var{nmerge} inputs at once.
4074 When @command{sort} has to merge more than @var{nmerge} inputs,
4075 it merges them in groups of @var{nmerge}, saving the result in
4076 a temporary file, which is then used as an input in a subsequent merge.
4078 A large value of @var{nmerge} may improve merge performance and decrease
4079 temporary storage utilization at the expense of increased memory usage
4080 and I/0. Conversely a small value of @var{nmerge} may reduce memory
4081 requirements and I/0 at the expense of temporary storage consumption and
4084 The value of @var{nmerge} must be at least 2. The default value is
4085 currently 16, but this is implementation-dependent and may change in
4088 The value of @var{nmerge} may be bounded by a resource limit for open
4089 file descriptors. The commands @samp{ulimit -n} or @samp{getconf
4090 OPEN_MAX} may display limits for your systems; these limits may be
4091 modified further if your program already has some files open, or if
4092 the operating system has other limits on the number of open files. If
4093 the value of @var{nmerge} exceeds the resource limit, @command{sort}
4094 silently uses a smaller value.
4096 @item -o @var{output-file}
4097 @itemx --output=@var{output-file}
4100 @cindex overwriting of input, allowed
4101 Write output to @var{output-file} instead of standard output.
4102 Normally, @command{sort} reads all input before opening
4103 @var{output-file}, so you can safely sort a file in place by using
4104 commands like @code{sort -o F F} and @code{cat F | sort -o F}.
4105 However, @command{sort} with @option{--merge} (@option{-m}) can open
4106 the output file before reading all input, so a command like @code{cat
4107 F | sort -m -o F - G} is not safe as @command{sort} might start
4108 writing @file{F} before @command{cat} is done reading it.
4110 @vindex POSIXLY_CORRECT
4111 On newer systems, @option{-o} cannot appear after an input file if
4112 @env{POSIXLY_CORRECT} is set, e.g., @samp{sort F -o F}. Portable
4113 scripts should specify @option{-o @var{output-file}} before any input
4116 @item --random-source=@var{file}
4117 @opindex --random-source
4118 @cindex random source for sorting
4119 Use @var{file} as a source of random data used to determine which
4120 random hash function to use with the @option{-R} option. @xref{Random
4127 @cindex sort stability
4128 @cindex sort's last-resort comparison
4130 Make @command{sort} stable by disabling its last-resort comparison.
4131 This option has no effect if no fields or global ordering options
4132 other than @option{--reverse} (@option{-r}) are specified.
4135 @itemx --buffer-size=@var{size}
4137 @opindex --buffer-size
4138 @cindex size for main memory sorting
4139 Use a main-memory sort buffer of the given @var{size}. By default,
4140 @var{size} is in units of 1024 bytes. Appending @samp{%} causes
4141 @var{size} to be interpreted as a percentage of physical memory.
4142 Appending @samp{K} multiplies @var{size} by 1024 (the default),
4143 @samp{M} by 1,048,576, @samp{G} by 1,073,741,824, and so on for
4144 @samp{T}, @samp{P}, @samp{E}, @samp{Z}, and @samp{Y}. Appending
4145 @samp{b} causes @var{size} to be interpreted as a byte count, with no
4148 This option can improve the performance of @command{sort} by causing it
4149 to start with a larger or smaller sort buffer than the default.
4150 However, this option affects only the initial buffer size. The buffer
4151 grows beyond @var{size} if @command{sort} encounters input lines larger
4154 @item -t @var{separator}
4155 @itemx --field-separator=@var{separator}
4157 @opindex --field-separator
4158 @cindex field separator character
4159 Use character @var{separator} as the field separator when finding the
4160 sort keys in each line. By default, fields are separated by the empty
4161 string between a non-blank character and a blank character.
4162 By default a blank is a space or a tab, but the @env{LC_CTYPE} locale
4165 That is, given the input line @w{@samp{ foo bar}}, @command{sort} breaks it
4166 into fields @w{@samp{ foo}} and @w{@samp{ bar}}. The field separator is
4167 not considered to be part of either the field preceding or the field
4168 following, so with @samp{sort @w{-t " "}} the same input line has
4169 three fields: an empty field, @samp{foo}, and @samp{bar}.
4170 However, fields that extend to the end of the line,
4171 as @option{-k 2}, or fields consisting of a range, as @option{-k 2,3},
4172 retain the field separators present between the endpoints of the range.
4174 To specify @acronym{ASCII} @sc{nul} as the field separator,
4175 use the two-character string @samp{\0}, e.g., @samp{sort -t '\0'}.
4177 @item -T @var{tempdir}
4178 @itemx --temporary-directory=@var{tempdir}
4180 @opindex --temporary-directory
4181 @cindex temporary directory
4183 Use directory @var{tempdir} to store temporary files, overriding the
4184 @env{TMPDIR} environment variable. If this option is given more than
4185 once, temporary files are stored in all the directories given. If you
4186 have a large sort or merge that is I/O-bound, you can often improve
4187 performance by using this option to specify directories on different
4188 disks and controllers.
4190 @item --parallel=@var{n}
4192 @cindex multithreaded sort
4193 Set the number of sorts run in parallel to @var{n}. By default,
4194 @var{n} is set to the number of available processors, but limited
4195 to 8, as there are diminishing performance gains after that.
4196 Note also that using @var{n} threads increases the memory usage by
4197 a factor of log @var{n}. Also see @ref{nproc invocation}.
4203 @cindex uniquifying output
4205 Normally, output only the first of a sequence of lines that compare
4206 equal. For the @option{--check} (@option{-c} or @option{-C}) option,
4207 check that no pair of consecutive lines compares equal.
4209 This option also disables the default last-resort comparison.
4211 The commands @code{sort -u} and @code{sort | uniq} are equivalent, but
4212 this equivalence does not extend to arbitrary @command{sort} options.
4213 For example, @code{sort -n -u} inspects only the value of the initial
4214 numeric string when checking for uniqueness, whereas @code{sort -n |
4215 uniq} inspects the entire line. @xref{uniq invocation}.
4217 @macro zeroTerminatedOption
4219 @itemx --zero-terminated
4221 @opindex --zero-terminated
4222 @cindex process zero-terminated items
4223 Delimit items with a zero byte rather than a newline (@acronym{ASCII} @sc{lf}).
4224 I.E. treat input as items separated by @acronym{ASCII} @sc{nul}
4225 and terminate output items with @acronym{ASCII} @sc{nul}.
4226 This option can be useful in conjunction with @samp{perl -0} or
4227 @samp{find -print0} and @samp{xargs -0} which do the same in order to
4228 reliably handle arbitrary file names (even those containing blanks
4229 or other special characters).
4231 @zeroTerminatedOption
4235 Historical (BSD and System V) implementations of @command{sort} have
4236 differed in their interpretation of some options, particularly
4237 @option{-b}, @option{-f}, and @option{-n}. @sc{gnu} sort follows the @acronym{POSIX}
4238 behavior, which is usually (but not always!) like the System V behavior.
4239 According to @acronym{POSIX}, @option{-n} no longer implies @option{-b}. For
4240 consistency, @option{-M} has been changed in the same way. This may
4241 affect the meaning of character positions in field specifications in
4242 obscure cases. The only fix is to add an explicit @option{-b}.
4244 A position in a sort field specified with @option{-k} may have any
4245 of the option letters @samp{MbdfghinRrV} appended to it, in which case no
4246 global ordering options are inherited by that particular field. The
4247 @option{-b} option may be independently attached to either or both of
4248 the start and end positions of a field specification, and if it is
4249 inherited from the global options it will be attached to both.
4250 If input lines can contain leading or adjacent blanks and @option{-t}
4251 is not used, then @option{-k} is typically combined with @option{-b} or
4252 an option that implicitly ignores leading blanks (@samp{Mghn}) as otherwise
4253 the varying numbers of leading blanks in fields can cause confusing results.
4255 If the start position in a sort field specifier falls after the end of
4256 the line or after the end field, the field is empty. If the @option{-b}
4257 option was specified, the @samp{.@var{c}} part of a field specification
4258 is counted from the first nonblank character of the field.
4260 @vindex _POSIX2_VERSION
4261 @vindex POSIXLY_CORRECT
4262 On older systems, @command{sort} supports an obsolete origin-zero
4263 syntax @samp{+@var{pos1} [-@var{pos2}]} for specifying sort keys.
4264 The obsolete sequence @samp{sort +@var{a}.@var{x} -@var{b}.@var{y}}
4265 is equivalent to @samp{sort -k @var{a+1}.@var{x+1},@var{b}} if @var{y}
4266 is @samp{0} or absent, otherwise it is equivalent to @samp{sort -k
4267 @var{a+1}.@var{x+1},@var{b+1}.@var{y}}.
4269 This obsolete behavior can be enabled or disabled with the
4270 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4271 conformance}); it can also be enabled when @env{POSIXLY_CORRECT} is
4272 not set by using the obsolete syntax with @samp{-@var{pos2}} present.
4274 Scripts intended for use on standard hosts should avoid obsolete
4275 syntax and should use @option{-k} instead. For example, avoid
4276 @samp{sort +2}, since it might be interpreted as either @samp{sort
4277 ./+2} or @samp{sort -k 3}. If your script must also run on hosts that
4278 support only the obsolete syntax, it can use a test like @samp{if sort
4279 -k 1 </dev/null >/dev/null 2>&1; then @dots{}} to decide which syntax
4282 Here are some examples to illustrate various combinations of options.
4287 Sort in descending (reverse) numeric order.
4294 Run no more that 4 sorts concurrently, using a buffer size of 10M.
4297 sort --parallel=4 -S 10M
4301 Sort alphabetically, omitting the first and second fields
4302 and the blanks at the start of the third field.
4303 This uses a single key composed of the characters beginning
4304 at the start of the first nonblank character in field three
4305 and extending to the end of each line.
4312 Sort numerically on the second field and resolve ties by sorting
4313 alphabetically on the third and fourth characters of field five.
4314 Use @samp{:} as the field delimiter.
4317 sort -t : -k 2,2n -k 5.3,5.4
4320 Note that if you had written @option{-k 2n} instead of @option{-k 2,2n}
4321 @command{sort} would have used all characters beginning in the second field
4322 and extending to the end of the line as the primary @emph{numeric}
4323 key. For the large majority of applications, treating keys spanning
4324 more than one field as numeric will not do what you expect.
4326 Also note that the @samp{n} modifier was applied to the field-end
4327 specifier for the first key. It would have been equivalent to
4328 specify @option{-k 2n,2} or @option{-k 2n,2n}. All modifiers except
4329 @samp{b} apply to the associated @emph{field}, regardless of whether
4330 the modifier character is attached to the field-start and/or the
4331 field-end part of the key specifier.
4334 Sort the password file on the fifth field and ignore any
4335 leading blanks. Sort lines with equal values in field five
4336 on the numeric user ID in field three. Fields are separated
4340 sort -t : -k 5b,5 -k 3,3n /etc/passwd
4341 sort -t : -n -k 5b,5 -k 3,3 /etc/passwd
4342 sort -t : -b -k 5,5 -k 3,3n /etc/passwd
4345 These three commands have equivalent effect. The first specifies that
4346 the first key's start position ignores leading blanks and the second
4347 key is sorted numerically. The other two commands rely on global
4348 options being inherited by sort keys that lack modifiers. The inheritance
4349 works in this case because @option{-k 5b,5b} and @option{-k 5b,5} are
4350 equivalent, as the location of a field-end lacking a @samp{.@var{c}}
4351 character position is not affected by whether initial blanks are
4355 Sort a set of log files, primarily by IPv4 address and secondarily by
4356 time stamp. If two lines' primary and secondary keys are identical,
4357 output the lines in the same order that they were input. The log
4358 files contain lines that look like this:
4361 4.150.156.3 - - [01/Apr/2004:06:31:51 +0000] message 1
4362 211.24.3.231 - - [24/Apr/2004:20:17:39 +0000] message 2
4365 Fields are separated by exactly one space. Sort IPv4 addresses
4366 lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201
4367 because 61 is less than 129.
4370 sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log |
4371 sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n
4374 This example cannot be done with a single @command{sort} invocation,
4375 since IPv4 address components are separated by @samp{.} while dates
4376 come just after a space. So it is broken down into two invocations of
4377 @command{sort}: the first sorts by time stamp and the second by IPv4
4378 address. The time stamp is sorted by year, then month, then day, and
4379 finally by hour-minute-second field, using @option{-k} to isolate each
4380 field. Except for hour-minute-second there's no need to specify the
4381 end of each key field, since the @samp{n} and @samp{M} modifiers sort
4382 based on leading prefixes that cannot cross field boundaries. The
4383 IPv4 addresses are sorted lexicographically. The second sort uses
4384 @samp{-s} so that ties in the primary key are broken by the secondary
4385 key; the first sort uses @samp{-s} so that the combination of the two
4389 Generate a tags file in case-insensitive sorted order.
4392 find src -type f -print0 | sort -z -f | xargs -0 etags --append
4395 The use of @option{-print0}, @option{-z}, and @option{-0} in this case means
4396 that file names that contain blanks or other special characters are
4398 by the sort operation.
4400 @c This example is a bit contrived and needs more explanation.
4402 @c Sort records separated by an arbitrary string by using a pipe to convert
4403 @c each record delimiter string to @samp{\0}, then using sort's -z option,
4404 @c and converting each @samp{\0} back to the original record delimiter.
4407 @c printf 'c\n\nb\n\na\n'|perl -0pe 's/\n\n/\n\0/g'|sort -z|perl -0pe 's/\0/\n/g'
4411 Use the common @acronym{DSU, Decorate Sort Undecorate} idiom to
4412 sort lines according to their length.
4415 awk '@{print length, $0@}' /etc/passwd | sort -n | cut -f2- -d' '
4418 In general this technique can be used to sort data that the @command{sort}
4419 command does not support, or is inefficient at, sorting directly.
4422 Shuffle a list of directories, but preserve the order of files within
4423 each directory. For instance, one could use this to generate a music
4424 playlist in which albums are shuffled but the songs of each album are
4428 ls */* | sort -t / -k 1,1R -k 2,2
4434 @node shuf invocation
4435 @section @command{shuf}: Shuffling text
4438 @cindex shuffling files
4440 @command{shuf} shuffles its input by outputting a random permutation
4441 of its input lines. Each output permutation is equally likely.
4445 shuf [@var{option}]@dots{} [@var{file}]
4446 shuf -e [@var{option}]@dots{} [@var{arg}]@dots{}
4447 shuf -i @var{lo}-@var{hi} [@var{option}]@dots{}
4450 @command{shuf} has three modes of operation that affect where it
4451 obtains its input lines. By default, it reads lines from standard
4452 input. The following options change the operation mode:
4460 @cindex command-line operands to shuffle
4461 Treat each command-line operand as an input line.
4463 @item -i @var{lo}-@var{hi}
4464 @itemx --input-range=@var{lo}-@var{hi}
4466 @opindex --input-range
4467 @cindex input range to shuffle
4468 Act as if input came from a file containing the range of unsigned
4469 decimal integers @var{lo}@dots{}@var{hi}, one per line.
4473 @command{shuf}'s other options can affect its behavior in all
4478 @item -n @var{lines}
4479 @itemx --head-count=@var{count}
4481 @opindex --head-count
4482 @cindex head of output
4483 Output at most @var{count} lines. By default, all input lines are
4486 @item -o @var{output-file}
4487 @itemx --output=@var{output-file}
4490 @cindex overwriting of input, allowed
4491 Write output to @var{output-file} instead of standard output.
4492 @command{shuf} reads all input before opening
4493 @var{output-file}, so you can safely shuffle a file in place by using
4494 commands like @code{shuf -o F <F} and @code{cat F | shuf -o F}.
4496 @item --random-source=@var{file}
4497 @opindex --random-source
4498 @cindex random source for shuffling
4499 Use @var{file} as a source of random data used to determine which
4500 permutation to generate. @xref{Random sources}.
4502 @zeroTerminatedOption
4518 might produce the output
4528 Similarly, the command:
4531 shuf -e clubs hearts diamonds spades
4545 and the command @samp{shuf -i 1-4} might output:
4555 These examples all have four input lines, so @command{shuf} might
4556 produce any of the twenty-four possible permutations of the input. In
4557 general, if there are @var{n} input lines, there are @var{n}! (i.e.,
4558 @var{n} factorial, or @var{n} * (@var{n} - 1) * @dots{} * 1) possible
4559 output permutations.
4564 @node uniq invocation
4565 @section @command{uniq}: Uniquify files
4568 @cindex uniquify files
4570 @command{uniq} writes the unique lines in the given @file{input}, or
4571 standard input if nothing is given or for an @var{input} name of
4575 uniq [@var{option}]@dots{} [@var{input} [@var{output}]]
4578 By default, @command{uniq} prints its input lines, except that
4579 it discards all but the first of adjacent repeated lines, so that
4580 no output lines are repeated. Optionally, it can instead discard
4581 lines that are not repeated, or all repeated lines.
4583 The input need not be sorted, but repeated input lines are detected
4584 only if they are adjacent. If you want to discard non-adjacent
4585 duplicate lines, perhaps you want to use @code{sort -u}.
4586 @xref{sort invocation}.
4589 Comparisons honor the rules specified by the @env{LC_COLLATE}
4592 If no @var{output} file is specified, @command{uniq} writes to standard
4595 The program accepts the following options. Also see @ref{Common options}.
4600 @itemx --skip-fields=@var{n}
4602 @opindex --skip-fields
4603 Skip @var{n} fields on each line before checking for uniqueness. Use
4604 a null string for comparison if a line has fewer than @var{n} fields. Fields
4605 are sequences of non-space non-tab characters that are separated from
4606 each other by at least one space or tab.
4608 For compatibility @command{uniq} supports an obsolete option syntax
4609 @option{-@var{n}}. New scripts should use @option{-f @var{n}} instead.
4612 @itemx --skip-chars=@var{n}
4614 @opindex --skip-chars
4615 Skip @var{n} characters before checking for uniqueness. Use a null string
4616 for comparison if a line has fewer than @var{n} characters. If you use both
4617 the field and character skipping options, fields are skipped over first.
4619 @vindex _POSIX2_VERSION
4620 On older systems, @command{uniq} supports an obsolete option syntax
4622 This obsolete behavior can be enabled or disabled with the
4623 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
4624 conformance}), but portable scripts should avoid commands whose
4625 behavior depends on this variable.
4626 For example, use @samp{uniq ./+10} or @samp{uniq -s 10} rather than
4627 the ambiguous @samp{uniq +10}.
4633 Print the number of times each line occurred along with the line.
4636 @itemx --ignore-case
4638 @opindex --ignore-case
4639 Ignore differences in case when comparing lines.
4645 @cindex repeated lines, outputting
4646 Discard lines that are not repeated. When used by itself, this option
4647 causes @command{uniq} to print the first copy of each repeated line,
4651 @itemx --all-repeated[=@var{delimit-method}]
4653 @opindex --all-repeated
4654 @cindex all repeated lines, outputting
4655 Do not discard the second and subsequent repeated input lines,
4656 but discard lines that are not repeated.
4657 This option is useful mainly in conjunction with other options e.g.,
4658 to ignore case or to compare only selected fields.
4659 The optional @var{delimit-method} tells how to delimit
4660 groups of repeated lines, and must be one of the following:
4665 Do not delimit groups of repeated lines.
4666 This is equivalent to @option{--all-repeated} (@option{-D}).
4669 Output a newline before each group of repeated lines.
4670 With @option{--zero-terminated} (@option{-z}), use a zero
4671 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4674 Separate groups of repeated lines with a single newline.
4675 With @option{--zero-terminated} (@option{-z}), use a zero
4676 byte (@acronym{ASCII} @sc{nul}) instead of a newline.
4677 This is the same as using @samp{prepend}, except that
4678 no delimiter is inserted before the first group, and hence
4679 may be better suited for output direct to users.
4682 Note that when groups are delimited and the input stream contains
4683 two or more consecutive blank lines, then the output is ambiguous.
4684 To avoid that, filter the input through @samp{tr -s '\n'} to replace
4685 each sequence of consecutive newlines with a single newline.
4687 This is a @sc{gnu} extension.
4688 @c FIXME: give an example showing *how* it's useful
4694 @cindex unique lines, outputting
4695 Discard the first repeated line. When used by itself, this option
4696 causes @command{uniq} to print unique lines, and nothing else.
4699 @itemx --check-chars=@var{n}
4701 @opindex --check-chars
4702 Compare at most @var{n} characters on each line (after skipping any specified
4703 fields and characters). By default the entire rest of the lines are
4706 @zeroTerminatedOption
4713 @node comm invocation
4714 @section @command{comm}: Compare two sorted files line by line
4717 @cindex line-by-line comparison
4718 @cindex comparing sorted files
4720 @command{comm} writes to standard output lines that are common, and lines
4721 that are unique, to two input files; a file name of @samp{-} means
4722 standard input. Synopsis:
4725 comm [@var{option}]@dots{} @var{file1} @var{file2}
4729 Before @command{comm} can be used, the input files must be sorted using the
4730 collating sequence specified by the @env{LC_COLLATE} locale.
4731 If an input file ends in a non-newline
4732 character, a newline is silently appended. The @command{sort} command with
4733 no options always outputs a file that is suitable input to @command{comm}.
4735 @cindex differing lines
4736 @cindex common lines
4737 With no options, @command{comm} produces three-column output. Column one
4738 contains lines unique to @var{file1}, column two contains lines unique
4739 to @var{file2}, and column three contains lines common to both files.
4740 Columns are separated by a single TAB character.
4741 @c FIXME: when there's an option to supply an alternative separator
4742 @c string, append `by default' to the above sentence.
4747 The options @option{-1}, @option{-2}, and @option{-3} suppress printing of
4748 the corresponding columns (and separators). Also see @ref{Common options}.
4750 Unlike some other comparison utilities, @command{comm} has an exit
4751 status that does not depend on the result of the comparison.
4752 Upon normal completion @command{comm} produces an exit code of zero.
4753 If there is an error it exits with nonzero status.
4755 @macro checkOrderOption{cmd}
4756 If the @option{--check-order} option is given, unsorted inputs will
4757 cause a fatal error message. If the option @option{--nocheck-order}
4758 is given, unsorted inputs will never cause an error message. If
4759 neither of these options is given, wrongly sorted inputs are diagnosed
4760 only if an input file is found to contain unpairable lines. If an
4761 input file is diagnosed as being unsorted, the @command{\cmd\} command
4762 will exit with a nonzero status (and the output should not be used).
4764 Forcing @command{\cmd\} to process wrongly sorted input files
4765 containing unpairable lines by specifying @option{--nocheck-order} is
4766 not guaranteed to produce any particular output. The output will
4767 probably not correspond with whatever you hoped it would be.
4769 @checkOrderOption{comm}
4774 Fail with an error message if either input file is wrongly ordered.
4776 @item --nocheck-order
4777 Do not check that both input files are in sorted order.
4781 @item --output-delimiter=@var{str}
4782 Print @var{str} between adjacent output columns,
4783 rather than the default of a single TAB character.
4785 The delimiter @var{str} may not be empty.
4789 @node ptx invocation
4790 @section @command{ptx}: Produce permuted indexes
4794 @command{ptx} reads a text file and essentially produces a permuted index, with
4795 each keyword in its context. The calling sketch is either one of:
4798 ptx [@var{option} @dots{}] [@var{file} @dots{}]
4799 ptx -G [@var{option} @dots{}] [@var{input} [@var{output}]]
4802 The @option{-G} (or its equivalent: @option{--traditional}) option disables
4803 all @sc{gnu} extensions and reverts to traditional mode, thus introducing some
4804 limitations and changing several of the program's default option values.
4805 When @option{-G} is not specified, @sc{gnu} extensions are always enabled.
4806 @sc{gnu} extensions to @command{ptx} are documented wherever appropriate in this
4807 document. For the full list, see @xref{Compatibility in ptx}.
4809 Individual options are explained in the following sections.
4811 When @sc{gnu} extensions are enabled, there may be zero, one or several
4812 @var{file}s after the options. If there is no @var{file}, the program
4813 reads the standard input. If there is one or several @var{file}s, they
4814 give the name of input files which are all read in turn, as if all the
4815 input files were concatenated. However, there is a full contextual
4816 break between each file and, when automatic referencing is requested,
4817 file names and line numbers refer to individual text input files. In
4818 all cases, the program outputs the permuted index to the standard
4821 When @sc{gnu} extensions are @emph{not} enabled, that is, when the program
4822 operates in traditional mode, there may be zero, one or two parameters
4823 besides the options. If there are no parameters, the program reads the
4824 standard input and outputs the permuted index to the standard output.
4825 If there is only one parameter, it names the text @var{input} to be read
4826 instead of the standard input. If two parameters are given, they give
4827 respectively the name of the @var{input} file to read and the name of
4828 the @var{output} file to produce. @emph{Be very careful} to note that,
4829 in this case, the contents of file given by the second parameter is
4830 destroyed. This behavior is dictated by System V @command{ptx}
4831 compatibility; @sc{gnu} Standards normally discourage output parameters not
4832 introduced by an option.
4834 Note that for @emph{any} file named as the value of an option or as an
4835 input text file, a single dash @kbd{-} may be used, in which case
4836 standard input is assumed. However, it would not make sense to use this
4837 convention more than once per program invocation.
4840 * General options in ptx:: Options which affect general program behavior.
4841 * Charset selection in ptx:: Underlying character set considerations.
4842 * Input processing in ptx:: Input fields, contexts, and keyword selection.
4843 * Output formatting in ptx:: Types of output format, and sizing the fields.
4844 * Compatibility in ptx::
4848 @node General options in ptx
4849 @subsection General options
4854 @itemx --traditional
4855 As already explained, this option disables all @sc{gnu} extensions to
4856 @command{ptx} and switches to traditional mode.
4859 Print a short help on standard output, then exit without further
4863 Print the program version on standard output, then exit without further
4871 @node Charset selection in ptx
4872 @subsection Charset selection
4874 @c FIXME: People don't necessarily know what an IBM-PC was these days.
4875 As it is set up now, the program assumes that the input file is coded
4876 using 8-bit @acronym{ISO} 8859-1 code, also known as Latin-1 character set,
4877 @emph{unless} it is compiled for MS-DOS, in which case it uses the
4878 character set of the IBM-PC@. (@sc{gnu} @command{ptx} is not known to work on
4879 smaller MS-DOS machines anymore.) Compared to 7-bit @acronym{ASCII}, the set
4880 of characters which are letters is different; this alters the behavior
4881 of regular expression matching. Thus, the default regular expression
4882 for a keyword allows foreign or diacriticized letters. Keyword sorting,
4883 however, is still crude; it obeys the underlying character set ordering
4889 @itemx --ignore-case
4890 Fold lower case letters to upper case for sorting.
4895 @node Input processing in ptx
4896 @subsection Word selection and input processing
4901 @itemx --break-file=@var{file}
4903 This option provides an alternative (to @option{-W}) method of describing
4904 which characters make up words. It introduces the name of a
4905 file which contains a list of characters which can@emph{not} be part of
4906 one word; this file is called the @dfn{Break file}. Any character which
4907 is not part of the Break file is a word constituent. If both options
4908 @option{-b} and @option{-W} are specified, then @option{-W} has precedence and
4909 @option{-b} is ignored.
4911 When @sc{gnu} extensions are enabled, the only way to avoid newline as a
4912 break character is to write all the break characters in the file with no
4913 newline at all, not even at the end of the file. When @sc{gnu} extensions
4914 are disabled, spaces, tabs and newlines are always considered as break
4915 characters even if not included in the Break file.
4918 @itemx --ignore-file=@var{file}
4920 The file associated with this option contains a list of words which will
4921 never be taken as keywords in concordance output. It is called the
4922 @dfn{Ignore file}. The file contains exactly one word in each line; the
4923 end of line separation of words is not subject to the value of the
4927 @itemx --only-file=@var{file}
4929 The file associated with this option contains a list of words which will
4930 be retained in concordance output; any word not mentioned in this file
4931 is ignored. The file is called the @dfn{Only file}. The file contains
4932 exactly one word in each line; the end of line separation of words is
4933 not subject to the value of the @option{-S} option.
4935 There is no default for the Only file. When both an Only file and an
4936 Ignore file are specified, a word is considered a keyword only
4937 if it is listed in the Only file and not in the Ignore file.
4942 On each input line, the leading sequence of non-white space characters will be
4943 taken to be a reference that has the purpose of identifying this input
4944 line in the resulting permuted index. For more information about reference
4945 production, see @xref{Output formatting in ptx}.
4946 Using this option changes the default value for option @option{-S}.
4948 Using this option, the program does not try very hard to remove
4949 references from contexts in output, but it succeeds in doing so
4950 @emph{when} the context ends exactly at the newline. If option
4951 @option{-r} is used with @option{-S} default value, or when @sc{gnu} extensions
4952 are disabled, this condition is always met and references are completely
4953 excluded from the output contexts.
4955 @item -S @var{regexp}
4956 @itemx --sentence-regexp=@var{regexp}
4958 This option selects which regular expression will describe the end of a
4959 line or the end of a sentence. In fact, this regular expression is not
4960 the only distinction between end of lines or end of sentences, and input
4961 line boundaries have no special significance outside this option. By
4962 default, when @sc{gnu} extensions are enabled and if @option{-r} option is not
4963 used, end of sentences are used. In this case, this @var{regex} is
4964 imported from @sc{gnu} Emacs:
4967 [.?!][]\"')@}]*\\($\\|\t\\| \\)[ \t\n]*
4970 Whenever @sc{gnu} extensions are disabled or if @option{-r} option is used, end
4971 of lines are used; in this case, the default @var{regexp} is just:
4977 Using an empty @var{regexp} is equivalent to completely disabling end of
4978 line or end of sentence recognition. In this case, the whole file is
4979 considered to be a single big line or sentence. The user might want to
4980 disallow all truncation flag generation as well, through option @option{-F
4981 ""}. @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
4984 When the keywords happen to be near the beginning of the input line or
4985 sentence, this often creates an unused area at the beginning of the
4986 output context line; when the keywords happen to be near the end of the
4987 input line or sentence, this often creates an unused area at the end of
4988 the output context line. The program tries to fill those unused areas
4989 by wrapping around context in them; the tail of the input line or
4990 sentence is used to fill the unused area on the left of the output line;
4991 the head of the input line or sentence is used to fill the unused area
4992 on the right of the output line.
4994 As a matter of convenience to the user, many usual backslashed escape
4995 sequences from the C language are recognized and converted to the
4996 corresponding characters by @command{ptx} itself.
4998 @item -W @var{regexp}
4999 @itemx --word-regexp=@var{regexp}
5001 This option selects which regular expression will describe each keyword.
5002 By default, if @sc{gnu} extensions are enabled, a word is a sequence of
5003 letters; the @var{regexp} used is @samp{\w+}. When @sc{gnu} extensions are
5004 disabled, a word is by default anything which ends with a space, a tab
5005 or a newline; the @var{regexp} used is @samp{[^ \t\n]+}.
5007 An empty @var{regexp} is equivalent to not using this option.
5008 @xref{Regexps, , Syntax of Regular Expressions, emacs, The GNU Emacs
5011 As a matter of convenience to the user, many usual backslashed escape
5012 sequences, as found in the C language, are recognized and converted to
5013 the corresponding characters by @command{ptx} itself.
5018 @node Output formatting in ptx
5019 @subsection Output formatting
5021 Output format is mainly controlled by the @option{-O} and @option{-T} options
5022 described in the table below. When neither @option{-O} nor @option{-T} are
5023 selected, and if @sc{gnu} extensions are enabled, the program chooses an
5024 output format suitable for a dumb terminal. Each keyword occurrence is
5025 output to the center of one line, surrounded by its left and right
5026 contexts. Each field is properly justified, so the concordance output
5027 can be readily observed. As a special feature, if automatic
5028 references are selected by option @option{-A} and are output before the
5029 left context, that is, if option @option{-R} is @emph{not} selected, then
5030 a colon is added after the reference; this nicely interfaces with @sc{gnu}
5031 Emacs @code{next-error} processing. In this default output format, each
5032 white space character, like newline and tab, is merely changed to
5033 exactly one space, with no special attempt to compress consecutive
5034 spaces. This might change in the future. Except for those white space
5035 characters, every other character of the underlying set of 256
5036 characters is transmitted verbatim.
5038 Output format is further controlled by the following options.
5042 @item -g @var{number}
5043 @itemx --gap-size=@var{number}
5045 Select the size of the minimum white space gap between the fields on the
5048 @item -w @var{number}
5049 @itemx --width=@var{number}
5051 Select the maximum output width of each final line. If references are
5052 used, they are included or excluded from the maximum output width
5053 depending on the value of option @option{-R}. If this option is not
5054 selected, that is, when references are output before the left context,
5055 the maximum output width takes into account the maximum length of all
5056 references. If this option is selected, that is, when references are
5057 output after the right context, the maximum output width does not take
5058 into account the space taken by references, nor the gap that precedes
5062 @itemx --auto-reference
5064 Select automatic references. Each input line will have an automatic
5065 reference made up of the file name and the line ordinal, with a single
5066 colon between them. However, the file name will be empty when standard
5067 input is being read. If both @option{-A} and @option{-r} are selected, then
5068 the input reference is still read and skipped, but the automatic
5069 reference is used at output time, overriding the input reference.
5072 @itemx --right-side-refs
5074 In the default output format, when option @option{-R} is not used, any
5075 references produced by the effect of options @option{-r} or @option{-A} are
5076 placed to the far right of output lines, after the right context. With
5077 default output format, when the @option{-R} option is specified, references
5078 are rather placed at the beginning of each output line, before the left
5079 context. For any other output format, option @option{-R} is
5080 ignored, with one exception: with @option{-R} the width of references
5081 is @emph{not} taken into account in total output width given by @option{-w}.
5083 This option is automatically selected whenever @sc{gnu} extensions are
5086 @item -F @var{string}
5087 @itemx --flac-truncation=@var{string}
5089 This option will request that any truncation in the output be reported
5090 using the string @var{string}. Most output fields theoretically extend
5091 towards the beginning or the end of the current line, or current
5092 sentence, as selected with option @option{-S}. But there is a maximum
5093 allowed output line width, changeable through option @option{-w}, which is
5094 further divided into space for various output fields. When a field has
5095 to be truncated because it cannot extend beyond the beginning or the end of
5096 the current line to fit in, then a truncation occurs. By default,
5097 the string used is a single slash, as in @option{-F /}.
5099 @var{string} may have more than one character, as in @option{-F ...}.
5100 Also, in the particular case when @var{string} is empty (@option{-F ""}),
5101 truncation flagging is disabled, and no truncation marks are appended in
5104 As a matter of convenience to the user, many usual backslashed escape
5105 sequences, as found in the C language, are recognized and converted to
5106 the corresponding characters by @command{ptx} itself.
5108 @item -M @var{string}
5109 @itemx --macro-name=@var{string}
5111 Select another @var{string} to be used instead of @samp{xx}, while
5112 generating output suitable for @command{nroff}, @command{troff} or @TeX{}.
5115 @itemx --format=roff
5117 Choose an output format suitable for @command{nroff} or @command{troff}
5118 processing. Each output line will look like:
5121 .xx "@var{tail}" "@var{before}" "@var{keyword_and_after}" "@var{head}" "@var{ref}"
5124 so it will be possible to write a @samp{.xx} roff macro to take care of
5125 the output typesetting. This is the default output format when @sc{gnu}
5126 extensions are disabled. Option @option{-M} can be used to change
5127 @samp{xx} to another macro name.
5129 In this output format, each non-graphical character, like newline and
5130 tab, is merely changed to exactly one space, with no special attempt to
5131 compress consecutive spaces. Each quote character: @kbd{"} is doubled
5132 so it will be correctly processed by @command{nroff} or @command{troff}.
5137 Choose an output format suitable for @TeX{} processing. Each output
5138 line will look like:
5141 \xx @{@var{tail}@}@{@var{before}@}@{@var{keyword}@}@{@var{after}@}@{@var{head}@}@{@var{ref}@}
5145 so it will be possible to write a @code{\xx} definition to take care of
5146 the output typesetting. Note that when references are not being
5147 produced, that is, neither option @option{-A} nor option @option{-r} is
5148 selected, the last parameter of each @code{\xx} call is inhibited.
5149 Option @option{-M} can be used to change @samp{xx} to another macro
5152 In this output format, some special characters, like @kbd{$}, @kbd{%},
5153 @kbd{&}, @kbd{#} and @kbd{_} are automatically protected with a
5154 backslash. Curly brackets @kbd{@{}, @kbd{@}} are protected with a
5155 backslash and a pair of dollar signs (to force mathematical mode). The
5156 backslash itself produces the sequence @code{\backslash@{@}}.
5157 Circumflex and tilde diacritical marks produce the sequence @code{^\@{ @}} and
5158 @code{~\@{ @}} respectively. Other diacriticized characters of the
5159 underlying character set produce an appropriate @TeX{} sequence as far
5160 as possible. The other non-graphical characters, like newline and tab,
5161 and all other characters which are not part of @acronym{ASCII}, are merely
5162 changed to exactly one space, with no special attempt to compress
5163 consecutive spaces. Let me know how to improve this special character
5164 processing for @TeX{}.
5169 @node Compatibility in ptx
5170 @subsection The @sc{gnu} extensions to @command{ptx}
5172 This version of @command{ptx} contains a few features which do not exist in
5173 System V @command{ptx}. These extra features are suppressed by using the
5174 @option{-G} command line option, unless overridden by other command line
5175 options. Some @sc{gnu} extensions cannot be recovered by overriding, so the
5176 simple rule is to avoid @option{-G} if you care about @sc{gnu} extensions.
5177 Here are the differences between this program and System V @command{ptx}.
5182 This program can read many input files at once, it always writes the
5183 resulting concordance on standard output. On the other hand, System V
5184 @command{ptx} reads only one file and sends the result to standard output
5185 or, if a second @var{file} parameter is given on the command, to that
5188 Having output parameters not introduced by options is a dangerous
5189 practice which @sc{gnu} avoids as far as possible. So, for using @command{ptx}
5190 portably between @sc{gnu} and System V, you should always use it with a
5191 single input file, and always expect the result on standard output. You
5192 might also want to automatically configure in a @option{-G} option to
5193 @command{ptx} calls in products using @command{ptx}, if the configurator finds
5194 that the installed @command{ptx} accepts @option{-G}.
5197 The only options available in System V @command{ptx} are options @option{-b},
5198 @option{-f}, @option{-g}, @option{-i}, @option{-o}, @option{-r}, @option{-t} and
5199 @option{-w}. All other options are @sc{gnu} extensions and are not repeated in
5200 this enumeration. Moreover, some options have a slightly different
5201 meaning when @sc{gnu} extensions are enabled, as explained below.
5204 By default, concordance output is not formatted for @command{troff} or
5205 @command{nroff}. It is rather formatted for a dumb terminal. @command{troff}
5206 or @command{nroff} output may still be selected through option @option{-O}.
5209 Unless @option{-R} option is used, the maximum reference width is
5210 subtracted from the total output line width. With @sc{gnu} extensions
5211 disabled, width of references is not taken into account in the output
5212 line width computations.
5215 All 256 bytes, even @acronym{ASCII} @sc{nul} bytes, are always read and
5216 processed from input file with no adverse effect, even if @sc{gnu} extensions
5217 are disabled. However, System V @command{ptx} does not accept 8-bit characters,
5218 a few control characters are rejected, and the tilde @kbd{~} is also rejected.
5221 Input line length is only limited by available memory, even if @sc{gnu}
5222 extensions are disabled. However, System V @command{ptx} processes only
5223 the first 200 characters in each line.
5226 The break (non-word) characters default to be every character except all
5227 letters of the underlying character set, diacriticized or not. When @sc{gnu}
5228 extensions are disabled, the break characters default to space, tab and
5232 The program makes better use of output line width. If @sc{gnu} extensions
5233 are disabled, the program rather tries to imitate System V @command{ptx},
5234 but still, there are some slight disposition glitches this program does
5235 not completely reproduce.
5238 The user can specify both an Ignore file and an Only file. This is not
5239 allowed with System V @command{ptx}.
5244 @node tsort invocation
5245 @section @command{tsort}: Topological sort
5248 @cindex topological sort
5250 @command{tsort} performs a topological sort on the given @var{file}, or
5251 standard input if no input file is given or for a @var{file} of
5252 @samp{-}. For more details and some history, see @ref{tsort background}.
5256 tsort [@var{option}] [@var{file}]
5259 @command{tsort} reads its input as pairs of strings, separated by blanks,
5260 indicating a partial ordering. The output is a total ordering that
5261 corresponds to the given partial ordering.
5275 will produce the output
5286 Consider a more realistic example.
5287 You have a large set of functions all in one file, and they may all be
5288 declared static except one. Currently that one (say @code{main}) is the
5289 first function defined in the file, and the ones it calls directly follow
5290 it, followed by those they call, etc. Let's say that you are determined
5291 to take advantage of prototypes, so you have to choose between declaring
5292 all of those functions (which means duplicating a lot of information from
5293 the definitions) and rearranging the functions so that as many as possible
5294 are defined before they are used. One way to automate the latter process
5295 is to get a list for each function of the functions it calls directly.
5296 Many programs can generate such lists. They describe a call graph.
5297 Consider the following list, in which a given line indicates that the
5298 function on the left calls the one on the right directly.
5304 tail_file pretty_name
5305 tail_file write_header
5307 tail_forever recheck
5308 tail_forever pretty_name
5309 tail_forever write_header
5310 tail_forever dump_remainder
5313 tail_lines start_lines
5314 tail_lines dump_remainder
5315 tail_lines file_lines
5316 tail_lines pipe_lines
5318 tail_bytes start_bytes
5319 tail_bytes dump_remainder
5320 tail_bytes pipe_bytes
5321 file_lines dump_remainder
5325 then you can use @command{tsort} to produce an ordering of those
5326 functions that satisfies your requirement.
5329 example$ tsort call-graph | tac
5349 @command{tsort} detects any cycles in the input and writes the first cycle
5350 encountered to standard error.
5352 Note that for a given partial ordering, generally there is no unique
5353 total ordering. In the context of the call graph above, the function
5354 @code{parse_options} may be placed anywhere in the list as long as it
5355 precedes @code{main}.
5357 The only options are @option{--help} and @option{--version}. @xref{Common
5363 * tsort background:: Where tsort came from.
5366 @node tsort background
5367 @subsection @command{tsort}: Background
5369 @command{tsort} exists because very early versions of the Unix linker processed
5370 an archive file exactly once, and in order. As @command{ld} read each object
5371 in the archive, it decided whether it was needed in the program based on
5372 whether it defined any symbols which were undefined at that point in
5375 This meant that dependencies within the archive had to be handled
5376 specially. For example, @code{scanf} probably calls @code{read}. That means
5377 that in a single pass through an archive, it was important for @code{scanf.o}
5378 to appear before read.o, because otherwise a program which calls
5379 @code{scanf} but not @code{read} might end up with an unexpected unresolved
5380 reference to @code{read}.
5382 The way to address this problem was to first generate a set of
5383 dependencies of one object file on another. This was done by a shell
5384 script called @command{lorder}. The GNU tools don't provide a version of
5385 lorder, as far as I know, but you can still find it in BSD
5388 Then you ran @command{tsort} over the @command{lorder} output, and you used the
5389 resulting sort to define the order in which you added objects to the archive.
5391 This whole procedure has been obsolete since about 1980, because
5392 Unix archives now contain a symbol table (traditionally built by
5393 @command{ranlib}, now generally built by @command{ar} itself), and the Unix
5394 linker uses the symbol table to effectively make multiple passes over
5397 Anyhow, that's where tsort came from. To solve an old problem with
5398 the way the linker handled archive files, which has since been solved
5402 @node Operating on fields
5403 @chapter Operating on fields
5406 * cut invocation:: Print selected parts of lines.
5407 * paste invocation:: Merge lines of files.
5408 * join invocation:: Join lines on a common field.
5412 @node cut invocation
5413 @section @command{cut}: Print selected parts of lines
5416 @command{cut} writes to standard output selected parts of each line of each
5417 input file, or standard input if no files are given or for a file name of
5421 cut @var{option}@dots{} [@var{file}]@dots{}
5424 In the table which follows, the @var{byte-list}, @var{character-list},
5425 and @var{field-list} are one or more numbers or ranges (two numbers
5426 separated by a dash) separated by commas. Bytes, characters, and
5427 fields are numbered starting at 1. Incomplete ranges may be
5428 given: @option{-@var{m}} means @samp{1-@var{m}}; @samp{@var{n}-} means
5429 @samp{@var{n}} through end of line or last field. The list elements
5430 can be repeated, can overlap, and can be specified in any order; but
5431 the selected input is written in the same order that it is read, and
5432 is written exactly once.
5434 The program accepts the following options. Also see @ref{Common
5439 @item -b @var{byte-list}
5440 @itemx --bytes=@var{byte-list}
5443 Select for printing only the bytes in positions listed in
5444 @var{byte-list}. Tabs and backspaces are treated like any other
5445 character; they take up 1 byte. If an output delimiter is specified,
5446 (see the description of @option{--output-delimiter}), then output that
5447 string between ranges of selected bytes.
5449 @item -c @var{character-list}
5450 @itemx --characters=@var{character-list}
5452 @opindex --characters
5453 Select for printing only the characters in positions listed in
5454 @var{character-list}. The same as @option{-b} for now, but
5455 internationalization will change that. Tabs and backspaces are
5456 treated like any other character; they take up 1 character. If an
5457 output delimiter is specified, (see the description of
5458 @option{--output-delimiter}), then output that string between ranges
5461 @item -f @var{field-list}
5462 @itemx --fields=@var{field-list}
5465 Select for printing only the fields listed in @var{field-list}.
5466 Fields are separated by a TAB character by default. Also print any
5467 line that contains no delimiter character, unless the
5468 @option{--only-delimited} (@option{-s}) option is specified.
5469 Note @command{cut} does not support specifying runs of whitespace as a
5470 delimiter, so to achieve that common functionality one can pre-process
5471 with @command{tr} like:
5473 tr -s '[:blank:]' '\t' | cut -f@dots{}
5476 @item -d @var{input_delim_byte}
5477 @itemx --delimiter=@var{input_delim_byte}
5479 @opindex --delimiter
5480 With @option{-f}, use the first byte of @var{input_delim_byte} as
5481 the input fields separator (default is TAB).
5485 Do not split multi-byte characters (no-op for now).
5488 @itemx --only-delimited
5490 @opindex --only-delimited
5491 For @option{-f}, do not print lines that do not contain the field separator
5492 character. Normally, any line without a field separator is printed verbatim.
5494 @item --output-delimiter=@var{output_delim_string}
5495 @opindex --output-delimiter
5496 With @option{-f}, output fields are separated by @var{output_delim_string}.
5497 The default with @option{-f} is to use the input delimiter.
5498 When using @option{-b} or @option{-c} to select ranges of byte or
5499 character offsets (as opposed to ranges of fields),
5500 output @var{output_delim_string} between non-overlapping
5501 ranges of selected bytes.
5504 @opindex --complement
5505 This option is a @acronym{GNU} extension.
5506 Select for printing the complement of the bytes, characters or fields
5507 selected with the @option{-b}, @option{-c} or @option{-f} options.
5508 In other words, do @emph{not} print the bytes, characters or fields
5509 specified via those options. This option is useful when you have
5510 many fields and want to print all but a few of them.
5517 @node paste invocation
5518 @section @command{paste}: Merge lines of files
5521 @cindex merging files
5523 @command{paste} writes to standard output lines consisting of sequentially
5524 corresponding lines of each given file, separated by a TAB character.
5525 Standard input is used for a file name of @samp{-} or if no input files
5547 paste [@var{option}]@dots{} [@var{file}]@dots{}
5550 The program accepts the following options. Also see @ref{Common options}.
5558 Paste the lines of one file at a time rather than one line from each
5559 file. Using the above example data:
5562 $ paste -s num2 let3
5567 @item -d @var{delim-list}
5568 @itemx --delimiters=@var{delim-list}
5570 @opindex --delimiters
5571 Consecutively use the characters in @var{delim-list} instead of
5572 TAB to separate merged lines. When @var{delim-list} is
5573 exhausted, start again at its beginning. Using the above example data:
5576 $ paste -d '%_' num2 let3 num2
5587 @node join invocation
5588 @section @command{join}: Join lines on a common field
5591 @cindex common field, joining on
5593 @command{join} writes to standard output a line for each pair of input
5594 lines that have identical join fields. Synopsis:
5597 join [@var{option}]@dots{} @var{file1} @var{file2}
5600 Either @var{file1} or @var{file2} (but not both) can be @samp{-},
5601 meaning standard input. @var{file1} and @var{file2} should be
5602 sorted on the join fields.
5605 Normally, the sort order is that of the
5606 collating sequence specified by the @env{LC_COLLATE} locale. Unless
5607 the @option{-t} option is given, the sort comparison ignores blanks at
5608 the start of the join field, as in @code{sort -b}. If the
5609 @option{--ignore-case} option is given, the sort comparison ignores
5610 the case of characters in the join field, as in @code{sort -f}.
5612 The @command{sort} and @command{join} commands should use consistent
5613 locales and options if the output of @command{sort} is fed to
5614 @command{join}. You can use a command like @samp{sort -k 1b,1} to
5615 sort a file on its default join field, but if you select a non-default
5616 locale, join field, separator, or comparison options, then you should
5617 do so consistently between @command{join} and @command{sort}.
5618 If @samp{join -t ''} is specified then the whole line is considered which
5619 matches the default operation of sort.
5621 If the input has no unpairable lines, a @acronym{GNU} extension is
5622 available; the sort order can be any order that considers two fields
5623 to be equal if and only if the sort comparison described above
5624 considers them to be equal. For example:
5641 @checkOrderOption{join}
5645 @item the join field is the first field in each line;
5646 @item fields in the input are separated by one or more blanks, with leading
5647 blanks on the line ignored;
5648 @item fields in the output are separated by a space;
5649 @item each output line consists of the join field, the remaining
5650 fields from @var{file1}, then the remaining fields from @var{file2}.
5653 The program accepts the following options. Also see @ref{Common options}.
5657 @item -a @var{file-number}
5659 Print a line for each unpairable line in file @var{file-number} (either
5660 @samp{1} or @samp{2}), in addition to the normal output.
5663 Fail with an error message if either input file is wrongly ordered.
5665 @item --nocheck-order
5666 Do not check that both input files are in sorted order. This is the default.
5668 @item -e @var{string}
5670 Replace those output fields that are missing in the input with
5675 Treat the first line of each input file as a header line. The header lines will
5676 be joined and printed as the first output line. If @option{-o} is used to
5677 specify output format, the header line will be printed according to the
5678 specified format. The header lines will not be checked for ordering even if
5679 @option{--check-order} is specified. Also if the header lines from each file
5680 do not match, the heading fields from the first file will be used.
5683 @itemx --ignore-case
5685 @opindex --ignore-case
5686 Ignore differences in case when comparing keys.
5687 With this option, the lines of the input files must be ordered in the same way.
5688 Use @samp{sort -f} to produce this ordering.
5690 @item -1 @var{field}
5692 Join on field @var{field} (a positive integer) of file 1.
5694 @item -2 @var{field}
5696 Join on field @var{field} (a positive integer) of file 2.
5698 @item -j @var{field}
5699 Equivalent to @option{-1 @var{field} -2 @var{field}}.
5701 @item -o @var{field-list}
5702 Construct each output line according to the format in @var{field-list}.
5703 Each element in @var{field-list} is either the single character @samp{0} or
5704 has the form @var{m.n} where the file number, @var{m}, is @samp{1} or
5705 @samp{2} and @var{n} is a positive field number.
5707 A field specification of @samp{0} denotes the join field.
5708 In most cases, the functionality of the @samp{0} field spec
5709 may be reproduced using the explicit @var{m.n} that corresponds
5710 to the join field. However, when printing unpairable lines
5711 (using either of the @option{-a} or @option{-v} options), there is no way
5712 to specify the join field using @var{m.n} in @var{field-list}
5713 if there are unpairable lines in both files.
5714 To give @command{join} that functionality, @acronym{POSIX} invented the @samp{0}
5715 field specification notation.
5717 The elements in @var{field-list}
5718 are separated by commas or blanks.
5719 Blank separators typically need to be quoted for the shell. For
5720 example, the commands @samp{join -o 1.2,2.2} and @samp{join -o '1.2
5721 2.2'} are equivalent.
5723 All output lines---including those printed because of any -a or -v
5724 option---are subject to the specified @var{field-list}.
5727 Use character @var{char} as the input and output field separator.
5728 Treat as significant each occurrence of @var{char} in the input file.
5729 Use @samp{sort -t @var{char}}, without the @option{-b} option of
5730 @samp{sort}, to produce this ordering. If @samp{join -t ''} is specified,
5731 the whole line is considered, matching the default operation of sort.
5732 If @samp{-t '\0'} is specified then the @acronym{ASCII} @sc{nul}
5733 character is used to delimit the fields.
5735 @item -v @var{file-number}
5736 Print a line for each unpairable line in file @var{file-number}
5737 (either @samp{1} or @samp{2}), instead of the normal output.
5744 @node Operating on characters
5745 @chapter Operating on characters
5747 @cindex operating on characters
5749 This commands operate on individual characters.
5752 * tr invocation:: Translate, squeeze, and/or delete characters.
5753 * expand invocation:: Convert tabs to spaces.
5754 * unexpand invocation:: Convert spaces to tabs.
5759 @section @command{tr}: Translate, squeeze, and/or delete characters
5766 tr [@var{option}]@dots{} @var{set1} [@var{set2}]
5769 @command{tr} copies standard input to standard output, performing
5770 one of the following operations:
5774 translate, and optionally squeeze repeated characters in the result,
5776 squeeze repeated characters,
5780 delete characters, then squeeze repeated characters from the result.
5783 The @var{set1} and (if given) @var{set2} arguments define ordered
5784 sets of characters, referred to below as @var{set1} and @var{set2}. These
5785 sets are the characters of the input that @command{tr} operates on.
5786 The @option{--complement} (@option{-c}, @option{-C}) option replaces
5788 complement (all of the characters that are not in @var{set1}).
5790 Currently @command{tr} fully supports only single-byte characters.
5791 Eventually it will support multibyte characters; when it does, the
5792 @option{-C} option will cause it to complement the set of characters,
5793 whereas @option{-c} will cause it to complement the set of values.
5794 This distinction will matter only when some values are not characters,
5795 and this is possible only in locales using multibyte encodings when
5796 the input contains encoding errors.
5798 The program accepts the @option{--help} and @option{--version}
5799 options. @xref{Common options}. Options must precede operands.
5804 * Character sets:: Specifying sets of characters.
5805 * Translating:: Changing one set of characters to another.
5806 * Squeezing:: Squeezing repeats and deleting.
5810 @node Character sets
5811 @subsection Specifying sets of characters
5813 @cindex specifying sets of characters
5815 The format of the @var{set1} and @var{set2} arguments resembles
5816 the format of regular expressions; however, they are not regular
5817 expressions, only lists of characters. Most characters simply
5818 represent themselves in these strings, but the strings can contain
5819 the shorthands listed below, for convenience. Some of them can be
5820 used only in @var{set1} or @var{set2}, as noted below.
5824 @item Backslash escapes
5825 @cindex backslash escapes
5827 The following backslash escape sequences are recognized:
5845 The character with the value given by @var{ooo}, which is 1 to 3
5851 While a backslash followed by a character not listed above is
5852 interpreted as that character, the backslash also effectively
5853 removes any special significance, so it is useful to escape
5854 @samp{[}, @samp{]}, @samp{*}, and @samp{-}.
5859 The notation @samp{@var{m}-@var{n}} expands to all of the characters
5860 from @var{m} through @var{n}, in ascending order. @var{m} should
5861 collate before @var{n}; if it doesn't, an error results. As an example,
5862 @samp{0-9} is the same as @samp{0123456789}.
5864 @sc{gnu} @command{tr} does not support the System V syntax that uses square
5865 brackets to enclose ranges. Translations specified in that format
5866 sometimes work as expected, since the brackets are often transliterated
5867 to themselves. However, they should be avoided because they sometimes
5868 behave unexpectedly. For example, @samp{tr -d '[0-9]'} deletes brackets
5871 Many historically common and even accepted uses of ranges are not
5872 portable. For example, on @acronym{EBCDIC} hosts using the @samp{A-Z}
5873 range will not do what most would expect because @samp{A} through @samp{Z}
5874 are not contiguous as they are in @acronym{ASCII}.
5875 If you can rely on a @acronym{POSIX} compliant version of @command{tr}, then
5876 the best way to work around this is to use character classes (see below).
5877 Otherwise, it is most portable (and most ugly) to enumerate the members
5880 @item Repeated characters
5881 @cindex repeated characters
5883 The notation @samp{[@var{c}*@var{n}]} in @var{set2} expands to @var{n}
5884 copies of character @var{c}. Thus, @samp{[y*6]} is the same as
5885 @samp{yyyyyy}. The notation @samp{[@var{c}*]} in @var{string2} expands
5886 to as many copies of @var{c} as are needed to make @var{set2} as long as
5887 @var{set1}. If @var{n} begins with @samp{0}, it is interpreted in
5888 octal, otherwise in decimal.
5890 @item Character classes
5891 @cindex character classes
5893 The notation @samp{[:@var{class}:]} expands to all of the characters in
5894 the (predefined) class @var{class}. The characters expand in no
5895 particular order, except for the @code{upper} and @code{lower} classes,
5896 which expand in ascending order. When the @option{--delete} (@option{-d})
5897 and @option{--squeeze-repeats} (@option{-s}) options are both given, any
5898 character class can be used in @var{set2}. Otherwise, only the
5899 character classes @code{lower} and @code{upper} are accepted in
5900 @var{set2}, and then only if the corresponding character class
5901 (@code{upper} and @code{lower}, respectively) is specified in the same
5902 relative position in @var{set1}. Doing this specifies case conversion.
5903 The class names are given below; an error results when an invalid class
5915 Horizontal whitespace.
5924 Printable characters, not including space.
5930 Printable characters, including space.
5933 Punctuation characters.
5936 Horizontal or vertical whitespace.
5945 @item Equivalence classes
5946 @cindex equivalence classes
5948 The syntax @samp{[=@var{c}=]} expands to all of the characters that are
5949 equivalent to @var{c}, in no particular order. Equivalence classes are
5950 a relatively recent invention intended to support non-English alphabets.
5951 But there seems to be no standard way to define them or determine their
5952 contents. Therefore, they are not fully implemented in @sc{gnu} @command{tr};
5953 each character's equivalence class consists only of that character,
5954 which is of no particular use.
5960 @subsection Translating
5962 @cindex translating characters
5964 @command{tr} performs translation when @var{set1} and @var{set2} are
5965 both given and the @option{--delete} (@option{-d}) option is not given.
5966 @command{tr} translates each character of its input that is in @var{set1}
5967 to the corresponding character in @var{set2}. Characters not in
5968 @var{set1} are passed through unchanged. When a character appears more
5969 than once in @var{set1} and the corresponding characters in @var{set2}
5970 are not all the same, only the final one is used. For example, these
5971 two commands are equivalent:
5978 A common use of @command{tr} is to convert lowercase characters to
5979 uppercase. This can be done in many ways. Here are three of them:
5982 tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
5984 tr '[:lower:]' '[:upper:]'
5988 But note that using ranges like @code{a-z} above is not portable.
5990 When @command{tr} is performing translation, @var{set1} and @var{set2}
5991 typically have the same length. If @var{set1} is shorter than
5992 @var{set2}, the extra characters at the end of @var{set2} are ignored.
5994 On the other hand, making @var{set1} longer than @var{set2} is not
5995 portable; @acronym{POSIX} says that the result is undefined. In this situation,
5996 BSD @command{tr} pads @var{set2} to the length of @var{set1} by repeating
5997 the last character of @var{set2} as many times as necessary. System V
5998 @command{tr} truncates @var{set1} to the length of @var{set2}.
6000 By default, @sc{gnu} @command{tr} handles this case like BSD @command{tr}.
6001 When the @option{--truncate-set1} (@option{-t}) option is given,
6002 @sc{gnu} @command{tr} handles this case like the System V @command{tr}
6003 instead. This option is ignored for operations other than translation.
6005 Acting like System V @command{tr} in this case breaks the relatively common
6009 tr -cs A-Za-z0-9 '\012'
6013 because it converts only zero bytes (the first element in the
6014 complement of @var{set1}), rather than all non-alphanumerics, to
6018 By the way, the above idiom is not portable because it uses ranges, and
6019 it assumes that the octal code for newline is 012.
6020 Assuming a @acronym{POSIX} compliant @command{tr}, here is a better way to write it:
6023 tr -cs '[:alnum:]' '[\n*]'
6028 @subsection Squeezing repeats and deleting
6030 @cindex squeezing repeat characters
6031 @cindex deleting characters
6033 When given just the @option{--delete} (@option{-d}) option, @command{tr}
6034 removes any input characters that are in @var{set1}.
6036 When given just the @option{--squeeze-repeats} (@option{-s}) option,
6037 @command{tr} replaces each input sequence of a repeated character that
6038 is in @var{set1} with a single occurrence of that character.
6040 When given both @option{--delete} and @option{--squeeze-repeats}, @command{tr}
6041 first performs any deletions using @var{set1}, then squeezes repeats
6042 from any remaining characters using @var{set2}.
6044 The @option{--squeeze-repeats} option may also be used when translating,
6045 in which case @command{tr} first performs translation, then squeezes
6046 repeats from any remaining characters using @var{set2}.
6048 Here are some examples to illustrate various combinations of options:
6053 Remove all zero bytes:
6060 Put all words on lines by themselves. This converts all
6061 non-alphanumeric characters to newlines, then squeezes each string
6062 of repeated newlines into a single newline:
6065 tr -cs '[:alnum:]' '[\n*]'
6069 Convert each sequence of repeated newlines to a single newline:
6076 Find doubled occurrences of words in a document.
6077 @c Separate the following two "the"s, so typo checkers don't complain.
6078 For example, people often write ``the @w{}the'' with the repeated words
6079 separated by a newline. The Bourne shell script below works first
6080 by converting each sequence of punctuation and blank characters to a
6081 single newline. That puts each ``word'' on a line by itself.
6082 Next it maps all uppercase characters to lower case, and finally it
6083 runs @command{uniq} with the @option{-d} option to print out only the words
6089 | tr -s '[:punct:][:blank:]' '[\n*]' \
6090 | tr '[:upper:]' '[:lower:]' \
6095 Deleting a small set of characters is usually straightforward. For example,
6096 to remove all @samp{a}s, @samp{x}s, and @samp{M}s you would do this:
6102 However, when @samp{-} is one of those characters, it can be tricky because
6103 @samp{-} has special meanings. Performing the same task as above but also
6104 removing all @samp{-} characters, we might try @code{tr -d -axM}, but
6105 that would fail because @command{tr} would try to interpret @option{-a} as
6106 a command-line option. Alternatively, we could try putting the hyphen
6107 inside the string, @code{tr -d a-xM}, but that wouldn't work either because
6108 it would make @command{tr} interpret @code{a-x} as the range of characters
6109 @samp{a}@dots{}@samp{x} rather than the three.
6110 One way to solve the problem is to put the hyphen at the end of the list
6117 Or you can use @samp{--} to terminate option processing:
6123 More generally, use the character class notation @code{[=c=]}
6124 with @samp{-} (or any other character) in place of the @samp{c}:
6130 Note how single quotes are used in the above example to protect the
6131 square brackets from interpretation by a shell.
6136 @node expand invocation
6137 @section @command{expand}: Convert tabs to spaces
6140 @cindex tabs to spaces, converting
6141 @cindex converting tabs to spaces
6143 @command{expand} writes the contents of each given @var{file}, or standard
6144 input if none are given or for a @var{file} of @samp{-}, to standard
6145 output, with tab characters converted to the appropriate number of
6149 expand [@var{option}]@dots{} [@var{file}]@dots{}
6152 By default, @command{expand} converts all tabs to spaces. It preserves
6153 backspace characters in the output; they decrement the column count for
6154 tab calculations. The default action is equivalent to @option{-t 8} (set
6155 tabs every 8 columns).
6157 The program accepts the following options. Also see @ref{Common options}.
6161 @item -t @var{tab1}[,@var{tab2}]@dots{}
6162 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6165 @cindex tab stops, setting
6166 If only one tab stop is given, set the tabs @var{tab1} spaces apart
6167 (default is 8). Otherwise, set the tabs at columns @var{tab1},
6168 @var{tab2}, @dots{} (numbered from 0), and replace any tabs beyond the
6169 last tab stop given with single spaces. Tab stops can be separated by
6170 blanks as well as by commas.
6172 For compatibility, GNU @command{expand} also accepts the obsolete
6173 option syntax, @option{-@var{t1}[,@var{t2}]@dots{}}. New scripts
6174 should use @option{-t @var{t1}[,@var{t2}]@dots{}} instead.
6180 @cindex initial tabs, converting
6181 Only convert initial tabs (those that precede all non-space or non-tab
6182 characters) on each line to spaces.
6189 @node unexpand invocation
6190 @section @command{unexpand}: Convert spaces to tabs
6194 @command{unexpand} writes the contents of each given @var{file}, or
6195 standard input if none are given or for a @var{file} of @samp{-}, to
6196 standard output, converting blanks at the beginning of each line into
6197 as many tab characters as needed. In the default @acronym{POSIX}
6198 locale, a @dfn{blank} is a space or a tab; other locales may specify
6199 additional blank characters. Synopsis:
6202 unexpand [@var{option}]@dots{} [@var{file}]@dots{}
6205 By default, @command{unexpand} converts only initial blanks (those
6206 that precede all non-blank characters) on each line. It
6207 preserves backspace characters in the output; they decrement the column
6208 count for tab calculations. By default, tabs are set at every 8th
6211 The program accepts the following options. Also see @ref{Common options}.
6215 @item -t @var{tab1}[,@var{tab2}]@dots{}
6216 @itemx --tabs=@var{tab1}[,@var{tab2}]@dots{}
6219 If only one tab stop is given, set the tabs @var{tab1} columns apart
6220 instead of the default 8. Otherwise, set the tabs at columns
6221 @var{tab1}, @var{tab2}, @dots{} (numbered from 0), and leave blanks
6222 beyond the tab stops given unchanged. Tab stops can be separated by
6223 blanks as well as by commas. This option implies the @option{-a} option.
6225 For compatibility, GNU @command{unexpand} supports the obsolete option syntax,
6226 @option{-@var{tab1}[,@var{tab2}]@dots{}}, where tab stops must be
6227 separated by commas. (Unlike @option{-t}, this obsolete option does
6228 not imply @option{-a}.) New scripts should use @option{--first-only -t
6229 @var{tab1}[,@var{tab2}]@dots{}} instead.
6235 Also convert all sequences of two or more blanks just before a tab stop,
6236 even if they occur after non-blank characters in a line.
6243 @node Directory listing
6244 @chapter Directory listing
6246 This chapter describes the @command{ls} command and its variants @command{dir}
6247 and @command{vdir}, which list information about files.
6250 * ls invocation:: List directory contents.
6251 * dir invocation:: Briefly ls.
6252 * vdir invocation:: Verbosely ls.
6253 * dircolors invocation:: Color setup for ls, etc.
6258 @section @command{ls}: List directory contents
6261 @cindex directory listing
6263 The @command{ls} program lists information about files (of any type,
6264 including directories). Options and file arguments can be intermixed
6265 arbitrarily, as usual.
6267 For non-option command-line arguments that are directories, by default
6268 @command{ls} lists the contents of directories, not recursively, and
6269 omitting files with names beginning with @samp{.}. For other non-option
6270 arguments, by default @command{ls} lists just the file name. If no
6271 non-option argument is specified, @command{ls} operates on the current
6272 directory, acting as if it had been invoked with a single argument of @samp{.}.
6275 By default, the output is sorted alphabetically, according to the locale
6276 settings in effect.@footnote{If you use a non-@acronym{POSIX}
6277 locale (e.g., by setting @env{LC_ALL} to @samp{en_US}), then @command{ls} may
6278 produce output that is sorted differently than you're accustomed to.
6279 In that case, set the @env{LC_ALL} environment variable to @samp{C}.}
6280 If standard output is
6281 a terminal, the output is in columns (sorted vertically) and control
6282 characters are output as question marks; otherwise, the output is listed
6283 one per line and control characters are output as-is.
6285 Because @command{ls} is such a fundamental program, it has accumulated many
6286 options over the years. They are described in the subsections below;
6287 within each section, options are listed alphabetically (ignoring case).
6288 The division of options into the subsections is not absolute, since some
6289 options affect more than one aspect of @command{ls}'s operation.
6291 @cindex exit status of @command{ls}
6296 1 minor problems (e.g., failure to access a file or directory not
6297 specified as a command line argument. This happens when listing a
6298 directory in which entries are actively being removed or renamed.)
6299 2 serious trouble (e.g., memory exhausted, invalid option, failure
6300 to access a file or directory specified as a command line argument
6301 or a directory loop)
6304 Also see @ref{Common options}.
6307 * Which files are listed::
6308 * What information is listed::
6309 * Sorting the output::
6310 * Details about version sort::
6311 * General output formatting::
6312 * Formatting file timestamps::
6313 * Formatting the file names::
6317 @node Which files are listed
6318 @subsection Which files are listed
6320 These options determine which files @command{ls} lists information for.
6321 By default, @command{ls} lists files and the contents of any
6322 directories on the command line, except that in directories it ignores
6323 files whose names start with @samp{.}.
6331 In directories, do not ignore file names that start with @samp{.}.
6336 @opindex --almost-all
6337 In directories, do not ignore all file names that start with @samp{.};
6338 ignore only @file{.} and @file{..}. The @option{--all} (@option{-a})
6339 option overrides this option.
6342 @itemx --ignore-backups
6344 @opindex --ignore-backups
6345 @cindex backup files, ignoring
6346 In directories, ignore files that end with @samp{~}. This option is
6347 equivalent to @samp{--ignore='*~' --ignore='.*~'}.
6352 @opindex --directory
6353 List just the names of directories, as with other types of files, rather
6354 than listing their contents.
6355 @c The following sentence is the same as the one for -F.
6356 Do not follow symbolic links listed on the
6357 command line unless the @option{--dereference-command-line} (@option{-H}),
6358 @option{--dereference} (@option{-L}), or
6359 @option{--dereference-command-line-symlink-to-dir} options are specified.
6362 @itemx --dereference-command-line
6364 @opindex --dereference-command-line
6365 @cindex symbolic links, dereferencing
6366 If a command line argument specifies a symbolic link, show information
6367 for the file the link references rather than for the link itself.
6369 @itemx --dereference-command-line-symlink-to-dir
6370 @opindex --dereference-command-line-symlink-to-dir
6371 @cindex symbolic links, dereferencing
6372 Do not dereference symbolic links, with one exception:
6373 if a command line argument specifies a symbolic link that refers to
6374 a directory, show information for that directory rather than for the
6376 This is the default behavior when no other dereferencing-related
6377 option has been specified (@option{--classify} (@option{-F}),
6378 @option{--directory} (@option{-d}),
6380 @option{--dereference} (@option{-L}), or
6381 @option{--dereference-command-line} (@option{-H})).
6383 @item --group-directories-first
6384 @opindex --group-directories-first
6385 Group all the directories before the files and then sort the
6386 directories and the files separately using the selected sort key
6387 (see --sort option).
6388 That is, this option specifies a primary sort key,
6389 and the --sort option specifies a secondary key.
6390 However, any use of @option{--sort=none}
6391 (@option{-U}) disables this option altogether.
6393 @item --hide=PATTERN
6394 @opindex --hide=@var{pattern}
6395 In directories, ignore files whose names match the shell pattern
6396 @var{pattern}, unless the @option{--all} (@option{-a}) or
6397 @option{--almost-all} (@option{-A}) is also given. This
6398 option acts like @option{--ignore=@var{pattern}} except that it has no
6399 effect if @option{--all} (@option{-a}) or @option{--almost-all}
6400 (@option{-A}) is also given.
6402 This option can be useful in shell aliases. For example, if
6403 @command{lx} is an alias for @samp{ls --hide='*~'} and @command{ly} is
6404 an alias for @samp{ls --ignore='*~'}, then the command @samp{lx -A}
6405 lists the file @file{README~} even though @samp{ly -A} would not.
6407 @item -I @var{pattern}
6408 @itemx --ignore=@var{pattern}
6410 @opindex --ignore=@var{pattern}
6411 In directories, ignore files whose names match the shell pattern
6412 (not regular expression) @var{pattern}. As
6413 in the shell, an initial @samp{.} in a file name does not match a
6414 wildcard at the start of @var{pattern}. Sometimes it is useful
6415 to give this option several times. For example,
6418 $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'
6421 The first option ignores names of length 3 or more that start with @samp{.},
6422 the second ignores all two-character names that start with @samp{.}
6423 except @samp{..}, and the third ignores names that start with @samp{#}.
6426 @itemx --dereference
6428 @opindex --dereference
6429 @cindex symbolic links, dereferencing
6430 When showing file information for a symbolic link, show information
6431 for the file the link references rather than the link itself.
6432 However, even with this option, @command{ls} still prints the name
6433 of the link itself, not the name of the file that the link points to.
6438 @opindex --recursive
6439 @cindex recursive directory listing
6440 @cindex directory listing, recursive
6441 List the contents of all directories recursively.
6446 @node What information is listed
6447 @subsection What information is listed
6449 These options affect the information that @command{ls} displays. By
6450 default, only file names are shown.
6456 @cindex hurd, author, printing
6457 List each file's author when producing long format directory listings.
6458 In GNU/Hurd, file authors can differ from their owners, but in other
6459 operating systems the two are the same.
6465 @cindex dired Emacs mode support
6466 With the long listing (@option{-l}) format, print an additional line after
6470 //DIRED// @var{beg1} @var{end1} @var{beg2} @var{end2} @dots{}
6474 The @var{begn} and @var{endn} are unsigned integers that record the
6475 byte position of the beginning and end of each file name in the output.
6476 This makes it easy for Emacs to find the names, even when they contain
6477 unusual characters such as space or newline, without fancy searching.
6479 If directories are being listed recursively (@option{-R}), output a similar
6480 line with offsets for each subdirectory name:
6483 //SUBDIRED// @var{beg1} @var{end1} @dots{}
6486 Finally, output a line of the form:
6489 //DIRED-OPTIONS// --quoting-style=@var{word}
6493 where @var{word} is the quoting style (@pxref{Formatting the file names}).
6495 Here is an actual example:
6498 $ mkdir -p a/sub/deeper a/sub2
6500 $ touch a/sub/deeper/file
6501 $ ls -gloRF --dired a
6504 -rw-r--r-- 1 0 Jun 10 12:27 f1
6505 -rw-r--r-- 1 0 Jun 10 12:27 f2
6506 drwxr-xr-x 3 4096 Jun 10 12:27 sub/
6507 drwxr-xr-x 2 4096 Jun 10 12:27 sub2/
6511 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/
6515 -rw-r--r-- 1 0 Jun 10 12:27 file
6519 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286
6520 //SUBDIRED// 2 3 167 172 228 240 290 296
6521 //DIRED-OPTIONS// --quoting-style=literal
6524 Note that the pairs of offsets on the @samp{//DIRED//} line above delimit
6525 these names: @file{f1}, @file{f2}, @file{sub}, @file{sub2}, @file{deeper},
6527 The offsets on the @samp{//SUBDIRED//} line delimit the following
6528 directory names: @file{a}, @file{a/sub}, @file{a/sub/deeper}, @file{a/sub2}.
6530 Here is an example of how to extract the fifth entry name, @samp{deeper},
6531 corresponding to the pair of offsets, 222 and 228:
6534 $ ls -gloRF --dired a > out
6535 $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo
6539 Note that although the listing above includes a trailing slash
6540 for the @samp{deeper} entry, the offsets select the name without
6541 the trailing slash. However, if you invoke @command{ls} with @option{--dired}
6542 along with an option like @option{--escape} (aka @option{-b}) and operate
6543 on a file whose name contains special characters, notice that the backslash
6548 $ ls -blog --dired 'a b'
6549 -rw-r--r-- 1 0 Jun 10 12:28 a\ b
6551 //DIRED-OPTIONS// --quoting-style=escape
6554 If you use a quoting style that adds quote marks
6555 (e.g., @option{--quoting-style=c}), then the offsets include the quote marks.
6556 So beware that the user may select the quoting style via the environment
6557 variable @env{QUOTING_STYLE}. Hence, applications using @option{--dired}
6558 should either specify an explicit @option{--quoting-style=literal} option
6559 (aka @option{-N} or @option{--literal}) on the command line, or else be
6560 prepared to parse the escaped names.
6563 @opindex --full-time
6564 Produce long format directory listings, and list times in full. It is
6565 equivalent to using @option{--format=long} with
6566 @option{--time-style=full-iso} (@pxref{Formatting file timestamps}).
6570 Produce long format directory listings, but don't display owner information.
6576 Inhibit display of group information in a long format directory listing.
6577 (This is the default in some non-@sc{gnu} versions of @command{ls}, so we
6578 provide this option for compatibility.)
6586 @cindex inode number, printing
6587 Print the inode number (also called the file serial number and index
6588 number) of each file to the left of the file name. (This number
6589 uniquely identifies each file within a particular file system.)
6592 @itemx --format=long
6593 @itemx --format=verbose
6596 @opindex long ls @r{format}
6597 @opindex verbose ls @r{format}
6598 In addition to the name of each file, print the file type, file mode bits,
6599 number of hard links, owner name, group name, size, and
6600 timestamp (@pxref{Formatting file timestamps}), normally
6601 the modification time. Print question marks for information that
6602 cannot be determined.
6604 Normally the size is printed as a byte count without punctuation, but
6605 this can be overridden (@pxref{Block size}). For example, @option{-h}
6606 prints an abbreviated, human-readable count, and
6607 @samp{--block-size="'1"} prints a byte count with the thousands
6608 separator of the current locale.
6610 For each directory that is listed, preface the files with a line
6611 @samp{total @var{blocks}}, where @var{blocks} is the total disk allocation
6612 for all files in that directory. The block size currently defaults to 1024
6613 bytes, but this can be overridden (@pxref{Block size}).
6614 The @var{blocks} computed counts each hard link separately;
6615 this is arguably a deficiency.
6617 The file type is one of the following characters:
6619 @c The commented-out entries are ones we're not sure about.
6627 character special file
6629 high performance (``contiguous data'') file
6633 door (Solaris 2.5 and up)
6635 @c semaphore, if this is a distinct file type
6639 @c multiplexed file (7th edition Unix; obsolete)
6641 off-line (``migrated'') file (Cray DMF)
6643 network special file (HP-UX)
6647 port (Solaris 10 and up)
6649 @c message queue, if this is a distinct file type
6653 @c shared memory object, if this is a distinct file type
6655 @c typed memory object, if this is a distinct file type
6657 @c whiteout (4.4BSD; not implemented)
6659 some other file type
6662 @cindex permissions, output by @command{ls}
6663 The file mode bits listed are similar to symbolic mode specifications
6664 (@pxref{Symbolic Modes}). But @command{ls} combines multiple bits into the
6665 third character of each set of permissions as follows:
6669 If the set-user-ID or set-group-ID bit and the corresponding executable bit
6673 If the set-user-ID or set-group-ID bit is set but the corresponding
6674 executable bit is not set.
6677 If the restricted deletion flag or sticky bit, and the
6678 other-executable bit, are both set. The restricted deletion flag is
6679 another name for the sticky bit. @xref{Mode Structure}.
6682 If the restricted deletion flag or sticky bit is set but the
6683 other-executable bit is not set.
6686 If the executable bit is set and none of the above apply.
6692 Following the file mode bits is a single character that specifies
6693 whether an alternate access method such as an access control list
6694 applies to the file. When the character following the file mode bits is a
6695 space, there is no alternate access method. When it is a printing
6696 character, then there is such a method.
6698 GNU @command{ls} uses a @samp{.} character to indicate a file
6699 with an SELinux security context, but no other alternate access method.
6701 A file with any other combination of alternate access methods
6702 is marked with a @samp{+} character.
6705 @itemx --numeric-uid-gid
6707 @opindex --numeric-uid-gid
6708 @cindex numeric uid and gid
6709 @cindex numeric user and group IDs
6710 Produce long format directory listings, but
6711 display numeric user and group IDs instead of the owner and group names.
6715 Produce long format directory listings, but don't display group information.
6716 It is equivalent to using @option{--format=long} with @option{--no-group} .
6722 @cindex disk allocation
6723 @cindex size of files, reporting
6724 Print the disk allocation of each file to the left of the file name.
6725 This is the amount of disk space used by the file, which is usually a
6726 bit more than the file's size, but it can be less if the file has holes.
6728 Normally the disk allocation is printed in units of
6729 1024 bytes, but this can be overridden (@pxref{Block size}).
6731 @cindex NFS mounts from BSD to HP-UX
6732 For files that are NFS-mounted from an HP-UX system to a BSD system,
6733 this option reports sizes that are half the correct values. On HP-UX
6734 systems, it reports sizes that are twice the correct values for files
6735 that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX;
6736 it also affects the HP-UX @command{ls} program.
6745 @cindex security context
6746 Display the SELinux security context or @samp{?} if none is found.
6747 When used with the @option{-l} option, print the security context
6748 to the left of the size column.
6753 @node Sorting the output
6754 @subsection Sorting the output
6756 @cindex sorting @command{ls} output
6757 These options change the order in which @command{ls} sorts the information
6758 it outputs. By default, sorting is done by character code
6759 (e.g., @acronym{ASCII} order).
6765 @itemx --time=status
6768 @opindex ctime@r{, printing or sorting by}
6769 @opindex status time@r{, printing or sorting by}
6770 @opindex use time@r{, printing or sorting files by}
6771 If the long listing format (e.g., @option{-l}, @option{-o}) is being used,
6772 print the status change time (the @samp{ctime} in the inode) instead of
6773 the modification time.
6774 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6775 or when not using a long listing format,
6776 sort according to the status change time.
6780 @cindex unsorted directory listing
6781 @cindex directory order, listing by
6782 Primarily, like @option{-U}---do not sort; list the files in whatever
6783 order they are stored in the directory. But also enable @option{-a} (list
6784 all files) and disable @option{-l}, @option{--color}, and @option{-s} (if they
6785 were specified before the @option{-f}).
6791 @cindex reverse sorting
6792 Reverse whatever the sorting method is---e.g., list files in reverse
6793 alphabetical order, youngest first, smallest first, or whatever.
6799 @opindex size of files@r{, sorting files by}
6800 Sort by file size, largest first.
6806 @opindex modification time@r{, sorting files by}
6807 Sort by modification time (the @samp{mtime} in the inode), newest first.
6811 @itemx --time=access
6815 @opindex use time@r{, printing or sorting files by}
6816 @opindex atime@r{, printing or sorting files by}
6817 @opindex access time@r{, printing or sorting files by}
6818 If the long listing format (e.g., @option{--format=long}) is being used,
6819 print the last access time (the @samp{atime} in the inode).
6820 When explicitly sorting by time (@option{--sort=time} or @option{-t})
6821 or when not using a long listing format, sort according to the access time.
6827 @opindex none@r{, sorting option for @command{ls}}
6828 Do not sort; list the files in whatever order they are
6829 stored in the directory. (Do not do any of the other unrelated things
6830 that @option{-f} does.) This is especially useful when listing very large
6831 directories, since not doing any sorting can be noticeably faster.
6834 @itemx --sort=version
6837 @opindex version@r{, sorting option for @command{ls}}
6838 Sort by version name and number, lowest first. It behaves like a default
6839 sort, except that each sequence of decimal digits is treated numerically
6840 as an index/version number. (@xref{Details about version sort}.)
6843 @itemx --sort=extension
6846 @opindex extension@r{, sorting files by}
6847 Sort directory contents alphabetically by file extension (characters
6848 after the last @samp{.}); files with no extension are sorted first.
6853 @node Details about version sort
6854 @subsection Details about version sort
6856 Version sorting handles the fact that file names frequently include indices or
6857 version numbers. Standard sorting usually does not produce the order that one
6858 expects because comparisons are made on a character-by-character basis.
6859 Version sorting is especially useful when browsing directories that contain
6860 many files with indices/version numbers in their names:
6864 abc.zml-1.gz abc.zml-1.gz
6865 abc.zml-12.gz abc.zml-2.gz
6866 abc.zml-2.gz abc.zml-12.gz
6869 Version-sorted strings are compared such that if @var{ver1} and @var{ver2}
6870 are version numbers and @var{prefix} and @var{suffix} (@var{suffix} matching
6871 the regular expression @samp{(\.[A-Za-z~][A-Za-z0-9~]*)*}) are strings then
6872 @var{ver1} < @var{ver2} implies that the name composed of
6873 ``@var{prefix} @var{ver1} @var{suffix}'' sorts before
6874 ``@var{prefix} @var{ver2} @var{suffix}''.
6876 Note also that leading zeros of numeric parts are ignored:
6880 abc-1.007.tgz abc-1.01a.tgz
6881 abc-1.012b.tgz abc-1.007.tgz
6882 abc-1.01a.tgz abc-1.012b.tgz
6885 This functionality is implemented using gnulib's @code{filevercmp} function,
6886 which has some caveats worth noting.
6889 @item @env{LC_COLLATE} is ignored, which means @samp{ls -v} and @samp{sort -V}
6890 will sort non-numeric prefixes as if the @env{LC_COLLATE} locale category
6891 was set to @samp{C}.
6892 @item Some suffixes will not be matched by the regular
6893 expression mentioned above. Consequently these examples may
6894 not sort as you expect:
6902 abc-1.2.3.4.x86_64.rpm
6903 abc-1.2.3.x86_64.rpm
6907 @node General output formatting
6908 @subsection General output formatting
6910 These options affect the appearance of the overall output.
6915 @itemx --format=single-column
6918 @opindex single-column @r{output of files}
6919 List one file per line. This is the default for @command{ls} when standard
6920 output is not a terminal.
6923 @itemx --format=vertical
6926 @opindex vertical @r{sorted files in columns}
6927 List files in columns, sorted vertically. This is the default for
6928 @command{ls} if standard output is a terminal. It is always the default
6929 for the @command{dir} program.
6930 @sc{gnu} @command{ls} uses variable width columns to display as many files as
6931 possible in the fewest lines.
6933 @item --color [=@var{when}]
6935 @cindex color, distinguishing file types with
6936 Specify whether to use color for distinguishing file types. @var{when}
6937 may be omitted, or one of:
6940 @vindex none @r{color option}
6941 - Do not use color at all. This is the default.
6943 @vindex auto @r{color option}
6944 @cindex terminal, using color iff
6945 - Only use color if standard output is a terminal.
6947 @vindex always @r{color option}
6950 Specifying @option{--color} and no @var{when} is equivalent to
6951 @option{--color=always}.
6952 Piping a colorized listing through a pager like @command{more} or
6953 @command{less} usually produces unreadable results. However, using
6954 @code{more -f} does seem to work.
6958 @itemx --indicator-style=classify
6961 @opindex --indicator-style
6962 @cindex file type and executables, marking
6963 @cindex executables and file type, marking
6964 Append a character to each file name indicating the file type. Also,
6965 for regular files that are executable, append @samp{*}. The file type
6966 indicators are @samp{/} for directories, @samp{@@} for symbolic links,
6967 @samp{|} for FIFOs, @samp{=} for sockets, @samp{>} for doors,
6968 and nothing for regular files.
6969 @c The following sentence is the same as the one for -d.
6970 Do not follow symbolic links listed on the
6971 command line unless the @option{--dereference-command-line} (@option{-H}),
6972 @option{--dereference} (@option{-L}), or
6973 @option{--dereference-command-line-symlink-to-dir} options are specified.
6976 @itemx --indicator-style=file-type
6977 @opindex --file-type
6978 @opindex --indicator-style
6979 @cindex file type, marking
6980 Append a character to each file name indicating the file type. This is
6981 like @option{-F}, except that executables are not marked.
6983 @item --indicator-style=@var{word}
6984 @opindex --indicator-style
6985 Append a character indicator with style @var{word} to entry names,
6990 Do not append any character indicator; this is the default.
6992 Append @samp{/} for directories. This is the same as the @option{-p}
6995 Append @samp{/} for directories, @samp{@@} for symbolic links, @samp{|}
6996 for FIFOs, @samp{=} for sockets, and nothing for regular files. This is
6997 the same as the @option{--file-type} option.
6999 Append @samp{*} for executable regular files, otherwise behave as for
7000 @samp{file-type}. This is the same as the @option{-F} or
7001 @option{--classify} option.
7006 Print file sizes in 1024-byte blocks, overriding the default block
7007 size (@pxref{Block size}).
7008 This option is equivalent to @option{--block-size=1K}.
7011 @itemx --format=commas
7014 @opindex commas@r{, outputting between files}
7015 List files horizontally, with as many as will fit on each line,
7016 separated by @samp{, } (a comma and a space).
7019 @itemx --indicator-style=slash
7021 @opindex --indicator-style
7022 @cindex file type, marking
7023 Append a @samp{/} to directory names.
7026 @itemx --format=across
7027 @itemx --format=horizontal
7030 @opindex across@r{, listing files}
7031 @opindex horizontal@r{, listing files}
7032 List the files in columns, sorted horizontally.
7035 @itemx --tabsize=@var{cols}
7038 Assume that each tab stop is @var{cols} columns wide. The default is 8.
7039 @command{ls} uses tabs where possible in the output, for efficiency. If
7040 @var{cols} is zero, do not use tabs at all.
7042 @c FIXME: remove in 2009, if Apple Terminal has been fixed for long enough.
7043 Some terminal emulators (at least Apple Terminal 1.5 (133) from Mac OS X 10.4.8)
7044 do not properly align columns to the right of a TAB following a
7045 non-@acronym{ASCII} byte. If you use such a terminal emulator, use the
7046 @option{-T0} option or put @code{TABSIZE=0} in your environment to tell
7047 @command{ls} to align using spaces, not tabs.
7050 @itemx --width=@var{cols}
7054 Assume the screen is @var{cols} columns wide. The default is taken
7055 from the terminal settings if possible; otherwise the environment
7056 variable @env{COLUMNS} is used if it is set; otherwise the default
7062 @node Formatting file timestamps
7063 @subsection Formatting file timestamps
7065 By default, file timestamps are listed in abbreviated form, using
7066 a date like @samp{Mar 30@ @ 2002} for non-recent timestamps, and a
7067 date-without-year and time like @samp{Mar 30 23:45} for recent timestamps.
7068 This format can change depending on the current locale as detailed below.
7070 A timestamp is considered to be @dfn{recent} if it is less than six
7071 months old, and is not dated in the future. If a timestamp dated
7072 today is not listed in recent form, the timestamp is in the future,
7073 which means you probably have clock skew problems which may break
7074 programs like @command{make} that rely on file timestamps.
7077 Time stamps are listed according to the time zone rules specified by
7078 the @env{TZ} environment variable, or by the system default rules if
7079 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
7080 with @env{TZ}, libc, The GNU C Library Reference Manual}.
7082 The following option changes how file timestamps are printed.
7085 @item --time-style=@var{style}
7086 @opindex --time-style
7088 List timestamps in style @var{style}. The @var{style} should
7089 be one of the following:
7094 List timestamps using @var{format}, where @var{format} is interpreted
7095 like the format argument of @command{date} (@pxref{date invocation}).
7096 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
7097 @command{ls} to list timestamps like @samp{2002-03-30 23:45:56}. As
7098 with @command{date}, @var{format}'s interpretation is affected by the
7099 @env{LC_TIME} locale category.
7101 If @var{format} contains two format strings separated by a newline,
7102 the former is used for non-recent files and the latter for recent
7103 files; if you want output columns to line up, you may need to insert
7104 spaces in one of the two formats.
7107 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
7108 format with nanosecond precision, e.g., @samp{2002-03-30
7109 23:45:56.477817180 -0700}. This style is equivalent to
7110 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
7112 This is useful because the time output includes all the information that
7113 is available from the operating system. For example, this can help
7114 explain @command{make}'s behavior, since @acronym{GNU} @command{make}
7115 uses the full timestamp to determine whether a file is out of date.
7118 List @acronym{ISO} 8601 date and time in minutes, e.g.,
7119 @samp{2002-03-30 23:45}. These timestamps are shorter than
7120 @samp{full-iso} timestamps, and are usually good enough for everyday
7121 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
7124 List @acronym{ISO} 8601 dates for non-recent timestamps (e.g.,
7125 @samp{2002-03-30@ }), and @acronym{ISO} 8601 month, day, hour, and
7126 minute for recent timestamps (e.g., @samp{03-30 23:45}). These
7127 timestamps are uglier than @samp{long-iso} timestamps, but they carry
7128 nearly the same information in a smaller space and their brevity helps
7129 @command{ls} output fit within traditional 80-column output lines.
7130 The following two @command{ls} invocations are equivalent:
7135 ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
7136 ls -l --time-style="iso"
7141 List timestamps in a locale-dependent form. For example, a Finnish
7142 locale might list non-recent timestamps like @samp{maalis 30@ @ 2002}
7143 and recent timestamps like @samp{maalis 30 23:45}. Locale-dependent
7144 timestamps typically consume more space than @samp{iso} timestamps and
7145 are harder for programs to parse because locale conventions vary so
7146 widely, but they are easier for many people to read.
7148 The @env{LC_TIME} locale category specifies the timestamp format. The
7149 default @acronym{POSIX} locale uses timestamps like @samp{Mar 30@
7150 @ 2002} and @samp{Mar 30 23:45}; in this locale, the following two
7151 @command{ls} invocations are equivalent:
7156 ls -l --time-style="+%b %e %Y$newline%b %e %H:%M"
7157 ls -l --time-style="locale"
7160 Other locales behave differently. For example, in a German locale,
7161 @option{--time-style="locale"} might be equivalent to
7162 @option{--time-style="+%e. %b %Y $newline%e. %b %H:%M"}
7163 and might generate timestamps like @samp{30. M@"ar 2002@ } and
7164 @samp{30. M@"ar 23:45}.
7166 @item posix-@var{style}
7168 List @acronym{POSIX}-locale timestamps if the @env{LC_TIME} locale
7169 category is @acronym{POSIX}, @var{style} timestamps otherwise. For
7170 example, the @samp{posix-long-iso} style lists
7171 timestamps like @samp{Mar 30@ @ 2002} and @samp{Mar 30 23:45} when in
7172 the @acronym{POSIX} locale, and like @samp{2002-03-30 23:45} otherwise.
7177 You can specify the default value of the @option{--time-style} option
7178 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
7179 the default style is @samp{locale}. @acronym{GNU} Emacs 21.3 and
7180 later use the @option{--dired} option and therefore can parse any date
7181 format, but if you are using Emacs 21.1 or 21.2 and specify a
7182 non-@acronym{POSIX} locale you may need to set
7183 @samp{TIME_STYLE="posix-long-iso"}.
7185 To avoid certain denial-of-service attacks, timestamps that would be
7186 longer than 1000 bytes may be treated as errors.
7189 @node Formatting the file names
7190 @subsection Formatting the file names
7192 These options change how file names themselves are printed.
7198 @itemx --quoting-style=escape
7201 @opindex --quoting-style
7202 @cindex backslash sequences for file names
7203 Quote nongraphic characters in file names using alphabetic and octal
7204 backslash sequences like those used in C.
7208 @itemx --quoting-style=literal
7211 @opindex --quoting-style
7212 Do not quote file names. However, with @command{ls} nongraphic
7213 characters are still printed as question marks if the output is a
7214 terminal and you do not specify the @option{--show-control-chars}
7218 @itemx --hide-control-chars
7220 @opindex --hide-control-chars
7221 Print question marks instead of nongraphic characters in file names.
7222 This is the default if the output is a terminal and the program is
7227 @itemx --quoting-style=c
7229 @opindex --quote-name
7230 @opindex --quoting-style
7231 Enclose file names in double quotes and quote nongraphic characters as
7234 @item --quoting-style=@var{word}
7235 @opindex --quoting-style
7236 @cindex quoting style
7237 Use style @var{word} to quote file names and other strings that may
7238 contain arbitrary characters. The @var{word} should
7239 be one of the following:
7243 Output strings as-is; this is the same as the @option{-N} or
7244 @option{--literal} option.
7246 Quote strings for the shell if they contain shell metacharacters or would
7247 cause ambiguous output.
7248 The quoting is suitable for @acronym{POSIX}-compatible shells like
7249 @command{bash}, but it does not always work for incompatible shells
7252 Quote strings for the shell, even if they would normally not require quoting.
7254 Quote strings as for C character string literals, including the
7255 surrounding double-quote characters; this is the same as the
7256 @option{-Q} or @option{--quote-name} option.
7258 Quote strings as for C character string literals, except omit the
7259 surrounding double-quote
7260 characters; this is the same as the @option{-b} or @option{--escape} option.
7262 Quote strings as for C character string literals, except use
7263 surrounding quotation marks appropriate for the
7266 @c Use @t instead of @samp to avoid duplicate quoting in some output styles.
7267 Quote strings as for C character string literals, except use
7268 surrounding quotation marks appropriate for the locale, and quote
7269 @t{`like this'} instead of @t{"like
7270 this"} in the default C locale. This looks nicer on many displays.
7273 You can specify the default value of the @option{--quoting-style} option
7274 with the environment variable @env{QUOTING_STYLE}. If that environment
7275 variable is not set, the default value is @samp{literal}, but this
7276 default may change to @samp{shell} in a future version of this package.
7278 @item --show-control-chars
7279 @opindex --show-control-chars
7280 Print nongraphic characters as-is in file names.
7281 This is the default unless the output is a terminal and the program is
7287 @node dir invocation
7288 @section @command{dir}: Briefly list directory contents
7291 @cindex directory listing, brief
7293 @command{dir} is equivalent to @code{ls -C
7294 -b}; that is, by default files are listed in columns, sorted vertically,
7295 and special characters are represented by backslash escape sequences.
7297 @xref{ls invocation, @command{ls}}.
7300 @node vdir invocation
7301 @section @command{vdir}: Verbosely list directory contents
7304 @cindex directory listing, verbose
7306 @command{vdir} is equivalent to @code{ls -l
7307 -b}; that is, by default files are listed in long format and special
7308 characters are represented by backslash escape sequences.
7310 @node dircolors invocation
7311 @section @command{dircolors}: Color setup for @command{ls}
7315 @cindex setup for color
7317 @command{dircolors} outputs a sequence of shell commands to set up the
7318 terminal for color output from @command{ls} (and @command{dir}, etc.).
7322 eval "`dircolors [@var{option}]@dots{} [@var{file}]`"
7325 If @var{file} is specified, @command{dircolors} reads it to determine which
7326 colors to use for which file types and extensions. Otherwise, a
7327 precompiled database is used. For details on the format of these files,
7328 run @samp{dircolors --print-database}.
7330 To make @command{dircolors} read a @file{~/.dircolors} file if it
7331 exists, you can put the following lines in your @file{~/.bashrc} (or
7332 adapt them to your favorite shell):
7336 test -r $d && eval "$(dircolors $d)"
7340 @vindex SHELL @r{environment variable, and color}
7341 The output is a shell command to set the @env{LS_COLORS} environment
7342 variable. You can specify the shell syntax to use on the command line,
7343 or @command{dircolors} will guess it from the value of the @env{SHELL}
7344 environment variable.
7346 The program accepts the following options. Also see @ref{Common options}.
7351 @itemx --bourne-shell
7354 @opindex --bourne-shell
7355 @cindex Bourne shell syntax for color setup
7356 @cindex @command{sh} syntax for color setup
7357 Output Bourne shell commands. This is the default if the @env{SHELL}
7358 environment variable is set and does not end with @samp{csh} or
7367 @cindex C shell syntax for color setup
7368 @cindex @command{csh} syntax for color setup
7369 Output C shell commands. This is the default if @code{SHELL} ends with
7370 @command{csh} or @command{tcsh}.
7373 @itemx --print-database
7375 @opindex --print-database
7376 @cindex color database, printing
7377 @cindex database for color setup, printing
7378 @cindex printing color database
7379 Print the (compiled-in) default color configuration database. This
7380 output is itself a valid configuration file, and is fairly descriptive
7381 of the possibilities.
7388 @node Basic operations
7389 @chapter Basic operations
7391 @cindex manipulating files
7393 This chapter describes the commands for basic file manipulation:
7394 copying, moving (renaming), and deleting (removing).
7397 * cp invocation:: Copy files.
7398 * dd invocation:: Convert and copy a file.
7399 * install invocation:: Copy files and set attributes.
7400 * mv invocation:: Move (rename) files.
7401 * rm invocation:: Remove files or directories.
7402 * shred invocation:: Remove files more securely.
7407 @section @command{cp}: Copy files and directories
7410 @cindex copying files and directories
7411 @cindex files, copying
7412 @cindex directories, copying
7414 @command{cp} copies files (or, optionally, directories). The copy is
7415 completely independent of the original. You can either copy one file to
7416 another, or copy arbitrarily many files to a destination directory.
7420 cp [@var{option}]@dots{} [-T] @var{source} @var{dest}
7421 cp [@var{option}]@dots{} @var{source}@dots{} @var{directory}
7422 cp [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
7427 If two file names are given, @command{cp} copies the first file to the
7431 If the @option{--target-directory} (@option{-t}) option is given, or
7432 failing that if the last file is a directory and the
7433 @option{--no-target-directory} (@option{-T}) option is not given,
7434 @command{cp} copies each @var{source} file to the specified directory,
7435 using the @var{source}s' names.
7438 Generally, files are written just as they are read. For exceptions,
7439 see the @option{--sparse} option below.
7441 By default, @command{cp} does not copy directories. However, the
7442 @option{-R}, @option{-a}, and @option{-r} options cause @command{cp} to
7443 copy recursively by descending into source directories and copying files
7444 to corresponding destination directories.
7446 When copying from a symbolic link, @command{cp} normally follows the
7447 link only when not copying
7448 recursively. This default can be overridden with the
7449 @option{--archive} (@option{-a}), @option{-d}, @option{--dereference}
7450 (@option{-L}), @option{--no-dereference} (@option{-P}), and
7451 @option{-H} options. If more than one of these options is specified,
7452 the last one silently overrides the others.
7454 When copying to a symbolic link, @command{cp} follows the
7455 link only when it refers to an existing regular file.
7456 However, when copying to a dangling symbolic link, @command{cp}
7457 refuses by default, and fails with a diagnostic, since the operation
7458 is inherently dangerous. This behavior is contrary to historical
7459 practice and to @acronym{POSIX}.
7460 Set @env{POSIXLY_CORRECT} to make @command{cp} attempt to create
7461 the target of a dangling destination symlink, in spite of the possible risk.
7462 Also, when an option like
7463 @option{--backup} or @option{--link} acts to rename or remove the
7464 destination before copying, @command{cp} renames or removes the
7465 symbolic link rather than the file it points to.
7467 By default, @command{cp} copies the contents of special files only
7468 when not copying recursively. This default can be overridden with the
7469 @option{--copy-contents} option.
7471 @cindex self-backups
7472 @cindex backups, making only
7473 @command{cp} generally refuses to copy a file onto itself, with the
7474 following exception: if @option{--force --backup} is specified with
7475 @var{source} and @var{dest} identical, and referring to a regular file,
7476 @command{cp} will make a backup file, either regular or numbered, as
7477 specified in the usual ways (@pxref{Backup options}). This is useful when
7478 you simply want to make a backup of an existing file before changing it.
7480 The program accepts the following options. Also see @ref{Common options}.
7487 Preserve as much as possible of the structure and attributes of the
7488 original files in the copy (but do not attempt to preserve internal
7489 directory structure; i.e., @samp{ls -U} may list the entries in a copied
7490 directory in a different order).
7491 Try to preserve SELinux security context and extended attributes (xattr),
7492 but ignore any failure to do that and print no corresponding diagnostic.
7493 Equivalent to @option{-dR --preserve=all} with the reduced diagnostics.
7495 @itemx --attributes-only
7496 @opindex --attributes-only
7497 Preserve the specified attributes of the original files in the copy,
7498 but do not copy any data. See the @option{--preserve} option for
7499 controlling which attributes to copy.
7502 @itemx @w{@kbd{--backup}[=@var{method}]}
7505 @vindex VERSION_CONTROL
7506 @cindex backups, making
7507 @xref{Backup options}.
7508 Make a backup of each file that would otherwise be overwritten or removed.
7509 As a special case, @command{cp} makes a backup of @var{source} when the force
7510 and backup options are given and @var{source} and @var{dest} are the same
7511 name for an existing, regular file. One useful application of this
7512 combination of options is this tiny Bourne shell script:
7516 # Usage: backup FILE...
7517 # Create a @sc{gnu}-style backup of each listed FILE.
7519 cp --backup --force -- "$i" "$i"
7523 @item --copy-contents
7524 @cindex directories, copying recursively
7525 @cindex copying directories recursively
7526 @cindex recursively copying directories
7527 @cindex non-directories, copying as special files
7528 If copying recursively, copy the contents of any special files (e.g.,
7529 FIFOs and device files) as if they were regular files. This means
7530 trying to read the data in each source file and writing it to the
7531 destination. It is usually a mistake to use this option, as it
7532 normally has undesirable effects on special files like FIFOs and the
7533 ones typically found in the @file{/dev} directory. In most cases,
7534 @code{cp -R --copy-contents} will hang indefinitely trying to read
7535 from FIFOs and special files like @file{/dev/console}, and it will
7536 fill up your destination disk if you use it to copy @file{/dev/zero}.
7537 This option has no effect unless copying recursively, and it does not
7538 affect the copying of symbolic links.
7542 @cindex symbolic links, copying
7543 @cindex hard links, preserving
7544 Copy symbolic links as symbolic links rather than copying the files that
7545 they point to, and preserve hard links between source files in the copies.
7546 Equivalent to @option{--no-dereference --preserve=links}.
7552 When copying without this option and an existing destination file cannot
7553 be opened for writing, the copy fails. However, with @option{--force}),
7554 when a destination file cannot be opened, @command{cp} then removes it and
7555 tries to open it again. Contrast this behavior with that enabled by
7556 @option{--link} and @option{--symbolic-link}, whereby the destination file
7557 is never opened but rather is removed unconditionally. Also see the
7558 description of @option{--remove-destination}.
7560 This option is independent of the @option{--interactive} or
7561 @option{-i} option: neither cancels the effect of the other.
7563 This option is redundant if the @option{--no-clobber} or @option{-n} option is
7568 If a command line argument specifies a symbolic link, then copy the
7569 file it points to rather than the symbolic link itself. However,
7570 copy (preserving its nature) any symbolic link that is encountered
7571 via recursive traversal.
7574 @itemx --interactive
7576 @opindex --interactive
7577 When copying a file other than a directory, prompt whether to
7578 overwrite an existing destination file. The @option{-i} option overrides
7579 a previous @option{-n} option.
7585 Make hard links instead of copies of non-directories.
7588 @itemx --dereference
7590 @opindex --dereference
7591 Follow symbolic links when copying from them.
7592 With this option, @command{cp} cannot create a symbolic link.
7593 For example, a symlink (to regular file) in the source tree will be copied to
7594 a regular file in the destination tree.
7599 @opindex --no-clobber
7600 Do not overwrite an existing file. The @option{-n} option overrides a previous
7601 @option{-i} option. This option is mutually exclusive with @option{-b} or
7602 @option{--backup} option.
7605 @itemx --no-dereference
7607 @opindex --no-dereference
7608 @cindex symbolic links, copying
7609 Copy symbolic links as symbolic links rather than copying the files that
7610 they point to. This option affects only symbolic links in the source;
7611 symbolic links in the destination are always followed if possible.
7614 @itemx @w{@kbd{--preserve}[=@var{attribute_list}]}
7617 @cindex file information, preserving, extended attributes, xattr
7618 Preserve the specified attributes of the original files.
7619 If specified, the @var{attribute_list} must be a comma-separated list
7620 of one or more of the following strings:
7624 Preserve the file mode bits and access control lists.
7626 Preserve the owner and group. On most modern systems,
7627 only users with appropriate privileges may change the owner of a file,
7629 may preserve the group ownership of a file only if they happen to be
7630 a member of the desired group.
7632 Preserve the times of last access and last modification, when possible.
7633 On older systems, it is not possible to preserve these attributes
7634 when the affected file is a symbolic link.
7635 However, many systems now provide the @code{utimensat} function,
7636 which makes it possible even for symbolic links.
7638 Preserve in the destination files
7639 any links between corresponding source files.
7640 Note that with @option{-L} or @option{-H}, this option can convert
7641 symbolic links to hard links. For example,
7643 $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c
7648 Note the inputs: @file{b} is a symlink to regular file @file{a},
7649 yet the files in destination directory, @file{c/}, are hard-linked.
7650 Since @option{-a} implies @option{--preserve=links}, and since @option{-H}
7651 tells @command{cp} to dereference command line arguments, it sees two files
7652 with the same inode number, and preserves the perceived hard link.
7654 Here is a similar example that exercises @command{cp}'s @option{-L} option:
7656 $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b
7662 Preserve SELinux security context of the file, or fail with full diagnostics.
7664 Preserve extended attributes of the file, or fail with full diagnostics.
7665 If @command{cp} is built without xattr support, ignore this option.
7666 If SELinux context, ACLs or Capabilities are implemented using xattrs,
7667 they are preserved by this option as well.
7669 Preserve all file attributes.
7670 Equivalent to specifying all of the above, but with the difference
7671 that failure to preserve SELinux security context or extended attributes
7672 does not change @command{cp}'s exit status. In contrast to @option{-a},
7673 all but @samp{Operation not supported} warnings are output.
7676 Using @option{--preserve} with no @var{attribute_list} is equivalent
7677 to @option{--preserve=mode,ownership,timestamps}.
7679 In the absence of this option, each destination file is created with the
7680 mode bits of the corresponding source file, minus the bits set in the
7681 umask and minus the set-user-ID and set-group-ID bits.
7682 @xref{File permissions}.
7684 @itemx @w{@kbd{--no-preserve}=@var{attribute_list}}
7685 @cindex file information, preserving
7686 Do not preserve the specified attributes. The @var{attribute_list}
7687 has the same form as for @option{--preserve}.
7691 @cindex parent directories and @command{cp}
7692 Form the name of each destination file by appending to the target
7693 directory a slash and the specified name of the source file. The last
7694 argument given to @command{cp} must be the name of an existing directory.
7695 For example, the command:
7698 cp --parents a/b/c existing_dir
7702 copies the file @file{a/b/c} to @file{existing_dir/a/b/c}, creating
7703 any missing intermediate directories.
7710 @opindex --recursive
7711 @cindex directories, copying recursively
7712 @cindex copying directories recursively
7713 @cindex recursively copying directories
7714 @cindex non-directories, copying as special files
7715 Copy directories recursively. By default, do not follow symbolic
7716 links in the source; see the @option{--archive} (@option{-a}), @option{-d},
7717 @option{--dereference} (@option{-L}), @option{--no-dereference}
7718 (@option{-P}), and @option{-H} options. Special files are copied by
7719 creating a destination file of the same type as the source; see the
7720 @option{--copy-contents} option. It is not portable to use
7721 @option{-r} to copy symbolic links or special files. On some
7722 non-@sc{gnu} systems, @option{-r} implies the equivalent of
7723 @option{-L} and @option{--copy-contents} for historical reasons.
7724 Also, it is not portable to use @option{-R} to copy symbolic links
7725 unless you also specify @option{-P}, as @acronym{POSIX} allows
7726 implementations that dereference symbolic links by default.
7728 @item --reflink[=@var{when}]
7729 @opindex --reflink[=@var{when}]
7732 @cindex copy on write
7733 Perform a lightweight, copy-on-write (COW) copy, if supported by the
7734 file system. Once it has succeeded, beware that the source and destination
7735 files share the same disk data blocks as long as they remain unmodified.
7736 Thus, if a disk I/O error affects data blocks of one of the files,
7737 the other suffers the same fate.
7739 The @var{when} value can be one of the following:
7743 The default behavior: if the copy-on-write operation is not supported
7744 then report the failure for each file and exit with a failure status.
7747 If the copy-on-write operation is not supported then fall back
7748 to the standard copy behaviour.
7751 This option is overridden by the @option{--link}, @option{--symbolic-link}
7752 and @option{--attributes-only} options, thus allowing it to be used
7753 to configure the default data copying behavior for @command{cp}.
7754 For example, with the following alias, @command{cp} will use the
7755 minimum amount of space supported by the file system.
7758 alias cp='cp --reflink=auto --sparse=always'
7761 @item --remove-destination
7762 @opindex --remove-destination
7763 Remove each existing destination file before attempting to open it
7764 (contrast with @option{-f} above).
7766 @item --sparse=@var{when}
7767 @opindex --sparse=@var{when}
7768 @cindex sparse files, copying
7769 @cindex holes, copying files with
7770 @findex read @r{system call, and holes}
7771 A @dfn{sparse file} contains @dfn{holes}---a sequence of zero bytes that
7772 does not occupy any physical disk blocks; the @samp{read} system call
7773 reads these as zeros. This can both save considerable disk space and
7774 increase speed, since many binary files contain lots of consecutive zero
7775 bytes. By default, @command{cp} detects holes in input source files via a crude
7776 heuristic and makes the corresponding output file sparse as well.
7777 Only regular files may be sparse.
7779 The @var{when} value can be one of the following:
7783 The default behavior: if the input file is sparse, attempt to make
7784 the output file sparse, too. However, if an output file exists but
7785 refers to a non-regular file, then do not attempt to make it sparse.
7788 For each sufficiently long sequence of zero bytes in the input file,
7789 attempt to create a corresponding hole in the output file, even if the
7790 input file does not appear to be sparse.
7791 This is useful when the input file resides on a file system
7792 that does not support sparse files
7793 (for example, @samp{efs} file systems in SGI IRIX 5.3 and earlier),
7794 but the output file is on a type of file system that does support them.
7795 Holes may be created only in regular files, so if the destination file
7796 is of some other type, @command{cp} does not even try to make it sparse.
7799 Never make the output file sparse.
7800 This is useful in creating a file for use with the @command{mkswap} command,
7801 since such a file must not have any holes.
7804 @optStripTrailingSlashes
7807 @itemx --symbolic-link
7809 @opindex --symbolic-link
7810 @cindex symbolic links, copying with
7811 Make symbolic links instead of copies of non-directories. All source
7812 file names must be absolute (starting with @samp{/}) unless the
7813 destination files are in the current directory. This option merely
7814 results in an error message on systems that do not support symbolic links.
7820 @optNoTargetDirectory
7826 @cindex newer files, copying only
7827 Do not copy a non-directory that has an existing destination with the
7828 same or newer modification time. If time stamps are being preserved,
7829 the comparison is to the source time stamp truncated to the
7830 resolutions of the destination file system and of the system calls
7831 used to update time stamps; this avoids duplicate work if several
7832 @samp{cp -pu} commands are executed with the same source and
7839 Print the name of each file before copying it.
7842 @itemx --one-file-system
7844 @opindex --one-file-system
7845 @cindex file systems, omitting copying to different
7846 Skip subdirectories that are on different file systems from the one that
7847 the copy started on.
7848 However, mount point directories @emph{are} copied.
7856 @section @command{dd}: Convert and copy a file
7859 @cindex converting while copying a file
7861 @command{dd} copies a file (from standard input to standard output, by
7862 default) with a changeable I/O block size, while optionally performing
7863 conversions on it. Synopses:
7866 dd [@var{operand}]@dots{}
7870 The only options are @option{--help} and @option{--version}.
7871 @xref{Common options}. @command{dd} accepts the following operands.
7877 Read from @var{file} instead of standard input.
7881 Write to @var{file} instead of standard output. Unless
7882 @samp{conv=notrunc} is given, @command{dd} truncates @var{file} to zero
7883 bytes (or the size specified with @samp{seek=}).
7885 @item ibs=@var{bytes}
7887 @cindex block size of input
7888 @cindex input block size
7889 Set the input block size to @var{bytes}.
7890 This makes @command{dd} read @var{bytes} per block.
7891 The default is 512 bytes.
7893 @item obs=@var{bytes}
7895 @cindex block size of output
7896 @cindex output block size
7897 Set the output block size to @var{bytes}.
7898 This makes @command{dd} write @var{bytes} per block.
7899 The default is 512 bytes.
7901 @item bs=@var{bytes}
7904 Set both input and output block sizes to @var{bytes}.
7905 This makes @command{dd} read and write @var{bytes} per block,
7906 overriding any @samp{ibs} and @samp{obs} settings.
7907 In addition, if no data-transforming @option{conv} option is specified,
7908 each input block is copied to the output as a single block,
7909 without aggregating short reads.
7911 @item cbs=@var{bytes}
7913 @cindex block size of conversion
7914 @cindex conversion block size
7915 @cindex fixed-length records, converting to variable-length
7916 @cindex variable-length records, converting to fixed-length
7917 Set the conversion block size to @var{bytes}.
7918 When converting variable-length records to fixed-length ones
7919 (@option{conv=block}) or the reverse (@option{conv=unblock}),
7920 use @var{bytes} as the fixed record length.
7922 @item skip=@var{blocks}
7924 Skip @var{blocks} @samp{ibs}-byte blocks in the input file before copying.
7926 @item seek=@var{blocks}
7928 Skip @var{blocks} @samp{obs}-byte blocks in the output file before copying.
7930 @item count=@var{blocks}
7932 Copy @var{blocks} @samp{ibs}-byte blocks from the input file, instead
7933 of everything until the end of the file.
7937 Do not print the overall transfer rate and volume statistics
7938 that normally make up the third status line when @command{dd} exits.
7940 @item conv=@var{conversion}[,@var{conversion}]@dots{}
7942 Convert the file as specified by the @var{conversion} argument(s).
7943 (No spaces around any comma(s).)
7950 @opindex ascii@r{, converting to}
7951 Convert @acronym{EBCDIC} to @acronym{ASCII},
7952 using the conversion table specified by @acronym{POSIX}.
7953 This provides a 1:1 translation for all 256 bytes.
7956 @opindex ebcdic@r{, converting to}
7957 Convert @acronym{ASCII} to @acronym{EBCDIC}.
7958 This is the inverse of the @samp{ascii} conversion.
7961 @opindex alternate ebcdic@r{, converting to}
7962 Convert @acronym{ASCII} to alternate @acronym{EBCDIC},
7963 using the alternate conversion table specified by @acronym{POSIX}.
7964 This is not a 1:1 translation, but reflects common historical practice
7965 for @samp{~}, @samp{[}, and @samp{]}.
7967 The @samp{ascii}, @samp{ebcdic}, and @samp{ibm} conversions are
7971 @opindex block @r{(space-padding)}
7972 For each line in the input, output @samp{cbs} bytes, replacing the
7973 input newline with a space and padding with spaces as necessary.
7977 Remove any trailing spaces in each @samp{cbs}-sized input block,
7978 and append a newline.
7980 The @samp{block} and @samp{unblock} conversions are mutually exclusive.
7983 @opindex lcase@r{, converting to}
7984 Change uppercase letters to lowercase.
7987 @opindex ucase@r{, converting to}
7988 Change lowercase letters to uppercase.
7990 The @samp{lcase} and @samp{ucase} conversions are mutually exclusive.
7993 @opindex swab @r{(byte-swapping)}
7994 @cindex byte-swapping
7995 Swap every pair of input bytes. @sc{gnu} @command{dd}, unlike others, works
7996 when an odd number of bytes are read---the last byte is simply copied
7997 (since there is nothing to swap it with).
8001 @cindex read errors, ignoring
8002 Continue after read errors.
8006 @cindex creating output file, avoiding
8007 Do not create the output file; the output file must already exist.
8011 @cindex creating output file, requiring
8012 Fail if the output file already exists; @command{dd} must create the
8015 The @samp{excl} and @samp{nocreat} conversions are mutually exclusive.
8019 @cindex truncating output file, avoiding
8020 Do not truncate the output file.
8023 @opindex sync @r{(padding with @acronym{ASCII} @sc{nul}s)}
8024 Pad every input block to size of @samp{ibs} with trailing zero bytes.
8025 When used with @samp{block} or @samp{unblock}, pad with spaces instead of
8030 @cindex synchronized data writes, before finishing
8031 Synchronize output data just before finishing. This forces a physical
8032 write of output data.
8036 @cindex synchronized data and metadata writes, before finishing
8037 Synchronize output data and metadata just before finishing. This
8038 forces a physical write of output data and metadata.
8042 @item iflag=@var{flag}[,@var{flag}]@dots{}
8044 Access the input file using the flags specified by the @var{flag}
8045 argument(s). (No spaces around any comma(s).)
8047 @item oflag=@var{flag}[,@var{flag}]@dots{}
8049 Access the output file using the flags specified by the @var{flag}
8050 argument(s). (No spaces around any comma(s).)
8052 Here are the flags. Not every flag is supported on every operating
8059 @cindex appending to the output file
8060 Write in append mode, so that even if some other process is writing to
8061 this file, every @command{dd} write will append to the current
8062 contents of the file. This flag makes sense only for output.
8063 If you combine this flag with the @samp{of=@var{file}} operand,
8064 you should also specify @samp{conv=notrunc} unless you want the
8065 output file to be truncated before being appended to.
8069 @cindex concurrent I/O
8070 Use concurrent I/O mode for data. This mode performs direct I/O
8071 and drops the @acronym{POSIX} requirement to serialize all I/O to the same file.
8072 A file cannot be opened in CIO mode and with a standard open at the
8078 Use direct I/O for data, avoiding the buffer cache.
8079 Note that the kernel may impose restrictions on read or write buffer sizes.
8080 For example, with an ext4 destination file system and a linux-based kernel,
8081 using @samp{oflag=direct} will cause writes to fail with @code{EINVAL} if the
8082 output buffer size is not a multiple of 512.
8086 @cindex directory I/O
8088 Fail unless the file is a directory. Most operating systems do not
8089 allow I/O to a directory, so this flag has limited utility.
8093 @cindex synchronized data reads
8094 Use synchronized I/O for data. For the output file, this forces a
8095 physical write of output data on each write. For the input file,
8096 this flag can matter when reading from a remote file that has been
8097 written to synchronously by some other process. Metadata (e.g.,
8098 last-access and last-modified time) is not necessarily synchronized.
8102 @cindex synchronized data and metadata I/O
8103 Use synchronized I/O for both data and metadata.
8107 @cindex nonblocking I/O
8108 Use non-blocking I/O.
8113 Do not update the file's access time.
8114 Some older file systems silently ignore this flag, so it is a good
8115 idea to test it on your files before relying on it.
8119 @cindex controlling terminal
8120 Do not assign the file to be a controlling terminal for @command{dd}.
8121 This has no effect when the file is not a terminal.
8122 On many hosts (e.g., @acronym{GNU}/Linux hosts), this option has no effect
8127 @cindex symbolic links, following
8128 Do not follow symbolic links.
8133 Fail if the file has multiple hard links.
8138 Use binary I/O. This option has an effect only on nonstandard
8139 platforms that distinguish binary from text I/O.
8144 Use text I/O. Like @samp{binary}, this option has no effect on
8149 Accumulate full blocks from input. The @code{read} system call
8150 may return early if a full block is not available.
8151 When that happens, continue calling @code{read} to fill the remainder
8153 This flag can be used only with @code{iflag}.
8157 These flags are not supported on all systems, and @samp{dd} rejects
8158 attempts to use them when they are not supported. When reading from
8159 standard input or writing to standard output, the @samp{nofollow} and
8160 @samp{noctty} flags should not be specified, and the other flags
8161 (e.g., @samp{nonblock}) can affect how other processes behave with the
8162 affected file descriptors, even after @command{dd} exits.
8166 @cindex multipliers after numbers
8167 The numeric-valued strings above (@var{bytes} and @var{blocks}) can be
8168 followed by a multiplier: @samp{b}=512, @samp{c}=1,
8169 @samp{w}=2, @samp{x@var{m}}=@var{m}, or any of the
8170 standard block size suffixes like @samp{k}=1024 (@pxref{Block size}).
8172 Any block size you specify via @samp{bs=}, @samp{ibs=}, @samp{obs=}, @samp{cbs=}
8173 should not be too large---values larger than a few megabytes
8174 are generally wasteful or (as in the gigabyte..exabyte case) downright
8175 counterproductive or error-inducing.
8177 Use different @command{dd} invocations to use different block sizes for
8178 skipping and I/O@. For example, the following shell commands copy data
8179 in 512 KiB blocks between a disk and a tape, but do not save or restore a
8180 4 KiB label at the start of the disk:
8183 disk=/dev/rdsk/c0t1d0s2
8186 # Copy all but the label from disk to tape.
8187 (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape
8189 # Copy from tape back to disk, but leave the disk label alone.
8190 (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk
8193 Sending an @samp{INFO} signal to a running @command{dd}
8194 process makes it print I/O statistics to standard error
8195 and then resume copying. In the example below,
8196 @command{dd} is run in the background to copy 10 million blocks.
8197 The @command{kill} command makes it output intermediate I/O statistics,
8198 and when @command{dd} completes normally or is killed by the
8199 @code{SIGINT} signal, it outputs the final statistics.
8202 $ dd if=/dev/zero of=/dev/null count=10MB & pid=$!
8203 $ kill -s INFO $pid; wait $pid
8204 3385223+0 records in
8205 3385223+0 records out
8206 1733234176 bytes (1.7 GB) copied, 6.42173 seconds, 270 MB/s
8207 10000000+0 records in
8208 10000000+0 records out
8209 5120000000 bytes (5.1 GB) copied, 18.913 seconds, 271 MB/s
8212 @vindex POSIXLY_CORRECT
8213 On systems lacking the @samp{INFO} signal @command{dd} responds to the
8214 @samp{USR1} signal instead, unless the @env{POSIXLY_CORRECT}
8215 environment variable is set.
8220 @node install invocation
8221 @section @command{install}: Copy files and set attributes
8224 @cindex copying files and setting attributes
8226 @command{install} copies files while setting their file mode bits and, if
8227 possible, their owner and group. Synopses:
8230 install [@var{option}]@dots{} [-T] @var{source} @var{dest}
8231 install [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8232 install [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8233 install [@var{option}]@dots{} -d @var{directory}@dots{}
8238 If two file names are given, @command{install} copies the first file to the
8242 If the @option{--target-directory} (@option{-t}) option is given, or
8243 failing that if the last file is a directory and the
8244 @option{--no-target-directory} (@option{-T}) option is not given,
8245 @command{install} copies each @var{source} file to the specified
8246 directory, using the @var{source}s' names.
8249 If the @option{--directory} (@option{-d}) option is given,
8250 @command{install} creates each @var{directory} and any missing parent
8251 directories. Parent directories are created with mode
8252 @samp{u=rwx,go=rx} (755), regardless of the @option{-m} option or the
8253 current umask. @xref{Directory Setuid and Setgid}, for how the
8254 set-user-ID and set-group-ID bits of parent directories are inherited.
8257 @cindex Makefiles, installing programs in
8258 @command{install} is similar to @command{cp}, but allows you to control the
8259 attributes of destination files. It is typically used in Makefiles to
8260 copy programs into their destination directories. It refuses to copy
8261 files onto themselves.
8263 @cindex extended attributes, xattr
8264 @command{install} never preserves extended attributes (xattr).
8266 The program accepts the following options. Also see @ref{Common options}.
8276 Compare each pair of source and destination files, and if the destination has
8277 identical content and any specified owner, group, permissions, and possibly
8278 SELinux context, then do not modify the destination at all.
8282 Ignored; for compatibility with old Unix versions of @command{install}.
8286 Create any missing parent directories of @var{dest},
8287 then copy @var{source} to @var{dest}.
8288 This option is ignored if a destination directory is specified
8289 via @option{--target-directory=DIR}.
8294 @opindex --directory
8295 @cindex directories, creating with given attributes
8296 @cindex parent directories, creating missing
8297 @cindex leading directories, creating missing
8298 Create any missing parent directories, giving them the default
8299 attributes. Then create each given directory, setting their owner,
8300 group and mode as given on the command line or to the defaults.
8302 @item -g @var{group}
8303 @itemx --group=@var{group}
8306 @cindex group ownership of installed files, setting
8307 Set the group ownership of installed files or directories to
8308 @var{group}. The default is the process's current group. @var{group}
8309 may be either a group name or a numeric group ID.
8312 @itemx --mode=@var{mode}
8315 @cindex permissions of installed files, setting
8316 Set the file mode bits for the installed file or directory to @var{mode},
8317 which can be either an octal number, or a symbolic mode as in
8318 @command{chmod}, with @samp{a=} (no access allowed to anyone) as the
8319 point of departure (@pxref{File permissions}).
8320 The default mode is @samp{u=rwx,go=rx,a-s}---read, write, and
8321 execute for the owner, read and execute for group and other, and with
8322 set-user-ID and set-group-ID disabled.
8323 This default is not quite the same as @samp{755}, since it disables
8324 instead of preserving set-user-ID and set-group-ID on directories.
8325 @xref{Directory Setuid and Setgid}.
8327 @item -o @var{owner}
8328 @itemx --owner=@var{owner}
8331 @cindex ownership of installed files, setting
8332 @cindex appropriate privileges
8333 @vindex root @r{as default owner}
8334 If @command{install} has appropriate privileges (is run as root), set the
8335 ownership of installed files or directories to @var{owner}. The default
8336 is @code{root}. @var{owner} may be either a user name or a numeric user
8339 @item --preserve-context
8340 @opindex --preserve-context
8342 @cindex security context
8343 Preserve the SELinux security context of files and directories.
8344 Failure to preserve the context in all of the files or directories
8345 will result in an exit status of 1. If SELinux is disabled then
8346 print a warning and ignore the option.
8349 @itemx --preserve-timestamps
8351 @opindex --preserve-timestamps
8352 @cindex timestamps of installed files, preserving
8353 Set the time of last access and the time of last modification of each
8354 installed file to match those of each corresponding original file.
8355 When a file is installed without this option, its last access and
8356 last modification times are both set to the time of installation.
8357 This option is useful if you want to use the last modification times
8358 of installed files to keep track of when they were last built as opposed
8359 to when they were last installed.
8365 @cindex symbol table information, stripping
8366 @cindex stripping symbol table information
8367 Strip the symbol tables from installed binary executables.
8369 @itemx --strip-program=@var{program}
8370 @opindex --strip-program
8371 @cindex symbol table information, stripping, program
8372 Program used to strip binaries.
8378 @optNoTargetDirectory
8384 Print the name of each file before copying it.
8386 @item -Z @var{context}
8387 @itemx --context=@var{context}
8391 @cindex security context
8392 Set the default SELinux security context to be used for any
8393 created files and directories. If SELinux is disabled then
8394 print a warning and ignore the option.
8402 @section @command{mv}: Move (rename) files
8406 @command{mv} moves or renames files (or directories). Synopses:
8409 mv [@var{option}]@dots{} [-T] @var{source} @var{dest}
8410 mv [@var{option}]@dots{} @var{source}@dots{} @var{directory}
8411 mv [@var{option}]@dots{} -t @var{directory} @var{source}@dots{}
8416 If two file names are given, @command{mv} moves the first file to the
8420 If the @option{--target-directory} (@option{-t}) option is given, or
8421 failing that if the last file is a directory and the
8422 @option{--no-target-directory} (@option{-T}) option is not given,
8423 @command{mv} moves each @var{source} file to the specified
8424 directory, using the @var{source}s' names.
8427 @command{mv} can move any type of file from one file system to another.
8428 Prior to version @code{4.0} of the fileutils,
8429 @command{mv} could move only regular files between file systems.
8430 For example, now @command{mv} can move an entire directory hierarchy
8431 including special device files from one partition to another. It first
8432 uses some of the same code that's used by @code{cp -a} to copy the
8433 requested directories and files, then (assuming the copy succeeded)
8434 it removes the originals. If the copy fails, then the part that was
8435 copied to the destination partition is removed. If you were to copy
8436 three directories from one partition to another and the copy of the first
8437 directory succeeded, but the second didn't, the first would be left on
8438 the destination partition and the second and third would be left on the
8441 @cindex extended attributes, xattr
8442 @command{mv} always tries to copy extended attributes (xattr), which may
8443 include SELinux context, ACLs or Capabilities.
8444 Upon failure all but @samp{Operation not supported} warnings are output.
8446 @cindex prompting, and @command{mv}
8447 If a destination file exists but is normally unwritable, standard input
8448 is a terminal, and the @option{-f} or @option{--force} option is not given,
8449 @command{mv} prompts the user for whether to replace the file. (You might
8450 own the file, or have write permission on its directory.) If the
8451 response is not affirmative, the file is skipped.
8453 @emph{Warning}: Avoid specifying a source name with a trailing slash,
8454 when it might be a symlink to a directory.
8455 Otherwise, @command{mv} may do something very surprising, since
8456 its behavior depends on the underlying rename system call.
8457 On a system with a modern Linux-based kernel, it fails with @code{errno=ENOTDIR}.
8458 However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently
8459 renames not the symlink but rather the directory referenced by the symlink.
8460 @xref{Trailing slashes}.
8462 The program accepts the following options. Also see @ref{Common options}.
8472 @cindex prompts, omitting
8473 Do not prompt the user before removing a destination file.
8475 If you specify more than one of the @option{-i}, @option{-f}, @option{-n}
8476 options, only the final one takes effect.
8481 @itemx --interactive
8483 @opindex --interactive
8484 @cindex prompts, forcing
8485 Prompt whether to overwrite each existing destination file, regardless
8487 If the response is not affirmative, the file is skipped.
8493 @opindex --no-clobber
8494 @cindex prompts, omitting
8495 Do not overwrite an existing file.
8497 This option is mutually exclusive with @option{-b} or @option{--backup} option.
8503 @cindex newer files, moving only
8504 Do not move a non-directory that has an existing destination with the
8505 same or newer modification time.
8506 If the move is across file system boundaries, the comparison is to the
8507 source time stamp truncated to the resolutions of the destination file
8508 system and of the system calls used to update time stamps; this avoids
8509 duplicate work if several @samp{mv -u} commands are executed with the
8510 same source and destination.
8516 Print the name of each file before moving it.
8518 @optStripTrailingSlashes
8524 @optNoTargetDirectory
8532 @section @command{rm}: Remove files or directories
8535 @cindex removing files or directories
8537 @command{rm} removes each given @var{file}. By default, it does not remove
8538 directories. Synopsis:
8541 rm [@var{option}]@dots{} [@var{file}]@dots{}
8544 @cindex prompting, and @command{rm}
8545 If the @option{-I} or @option{--interactive=once} option is given,
8546 and there are more than three files or the @option{-r}, @option{-R},
8547 or @option{--recursive} are given, then @command{rm} prompts the user
8548 for whether to proceed with the entire operation. If the response is
8549 not affirmative, the entire command is aborted.
8551 Otherwise, if a file is unwritable, standard input is a terminal, and
8552 the @option{-f} or @option{--force} option is not given, or the
8553 @option{-i} or @option{--interactive=always} option @emph{is} given,
8554 @command{rm} prompts the user for whether to remove the file.
8555 If the response is not affirmative, the file is skipped.
8557 Any attempt to remove a file whose last file name component is
8558 @file{.} or @file{..} is rejected without any prompting.
8560 @emph{Warning}: If you use @command{rm} to remove a file, it is usually
8561 possible to recover the contents of that file. If you want more assurance
8562 that the contents are truly unrecoverable, consider using @command{shred}.
8564 The program accepts the following options. Also see @ref{Common options}.
8572 Ignore nonexistent files and never prompt the user.
8573 Ignore any previous @option{--interactive} (@option{-i}) option.
8577 Prompt whether to remove each file.
8578 If the response is not affirmative, the file is skipped.
8579 Ignore any previous @option{--force} (@option{-f}) option.
8580 Equivalent to @option{--interactive=always}.
8584 Prompt once whether to proceed with the command, if more than three
8585 files are named or if a recursive removal is requested. Ignore any
8586 previous @option{--force} (@option{-f}) option. Equivalent to
8587 @option{--interactive=once}.
8589 @itemx --interactive [=@var{when}]
8590 @opindex --interactive
8591 Specify when to issue an interactive prompt. @var{when} may be
8595 @vindex never @r{interactive option}
8596 - Do not prompt at all.
8598 @vindex once @r{interactive option}
8599 - Prompt once if more than three files are named or if a recursive
8600 removal is requested. Equivalent to @option{-I}.
8602 @vindex always @r{interactive option}
8603 - Prompt for every file being removed. Equivalent to @option{-i}.
8605 @option{--interactive} with no @var{when} is equivalent to
8606 @option{--interactive=always}.
8608 @itemx --one-file-system
8609 @opindex --one-file-system
8610 @cindex one file system, restricting @command{rm} to
8611 When removing a hierarchy recursively, skip any directory that is on a
8612 file system different from that of the corresponding command line argument.
8615 This option is useful when removing a build ``chroot'' hierarchy,
8616 which normally contains no valuable data. However, it is not uncommon
8617 to bind-mount @file{/home} into such a hierarchy, to make it easier to
8618 use one's start-up file. The catch is that it's easy to forget to
8619 unmount @file{/home}. Then, when you use @command{rm -rf} to remove
8620 your normally throw-away chroot, that command will remove everything
8621 under @file{/home}, too.
8622 Use the @option{--one-file-system} option, and it will
8623 warn about and skip directories on other file systems.
8624 Of course, this will not save your @file{/home} if it and your
8625 chroot happen to be on the same file system.
8627 @itemx --preserve-root
8628 @opindex --preserve-root
8629 @cindex root directory, disallow recursive destruction
8630 Fail upon any attempt to remove the root directory, @file{/},
8631 when used with the @option{--recursive} option.
8632 This is the default behavior.
8633 @xref{Treating / specially}.
8635 @itemx --no-preserve-root
8636 @opindex --no-preserve-root
8637 @cindex root directory, allow recursive destruction
8638 Do not treat @file{/} specially when removing recursively.
8639 This option is not recommended unless you really want to
8640 remove all the files on your computer.
8641 @xref{Treating / specially}.
8648 @opindex --recursive
8649 @cindex directories, removing (recursively)
8650 Remove the listed directories and their contents recursively.
8656 Print the name of each file before removing it.
8660 @cindex files beginning with @samp{-}, removing
8661 @cindex @samp{-}, removing files beginning with
8662 One common question is how to remove files whose names begin with a
8663 @samp{-}. @sc{gnu} @command{rm}, like every program that uses the @code{getopt}
8664 function to parse its arguments, lets you use the @samp{--} option to
8665 indicate that all following arguments are non-options. To remove a file
8666 called @file{-f} in the current directory, you could type either:
8679 @opindex - @r{and Unix @command{rm}}
8680 The Unix @command{rm} program's use of a single @samp{-} for this purpose
8681 predates the development of the getopt standard syntax.
8686 @node shred invocation
8687 @section @command{shred}: Remove files more securely
8690 @cindex data, erasing
8691 @cindex erasing data
8693 @command{shred} overwrites devices or files, to help prevent even
8694 very expensive hardware from recovering the data.
8696 Ordinarily when you remove a file (@pxref{rm invocation}), the data is
8697 not actually destroyed. Only the index listing where the file is
8698 stored is destroyed, and the storage is made available for reuse.
8699 There are undelete utilities that will attempt to reconstruct the index
8700 and can bring the file back if the parts were not reused.
8702 On a busy system with a nearly-full drive, space can get reused in a few
8703 seconds. But there is no way to know for sure. If you have sensitive
8704 data, you may want to be sure that recovery is not possible by actually
8705 overwriting the file with non-sensitive data.
8707 However, even after doing that, it is possible to take the disk back
8708 to a laboratory and use a lot of sensitive (and expensive) equipment
8709 to look for the faint ``echoes'' of the original data underneath the
8710 overwritten data. If the data has only been overwritten once, it's not
8713 The best way to remove something irretrievably is to destroy the media
8714 it's on with acid, melt it down, or the like. For cheap removable media
8715 like floppy disks, this is the preferred method. However, hard drives
8716 are expensive and hard to melt, so the @command{shred} utility tries
8717 to achieve a similar effect non-destructively.
8719 This uses many overwrite passes, with the data patterns chosen to
8720 maximize the damage they do to the old data. While this will work on
8721 floppies, the patterns are designed for best effect on hard drives.
8722 For more details, see the source code and Peter Gutmann's paper
8723 @uref{http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
8724 @cite{Secure Deletion of Data from Magnetic and Solid-State Memory}},
8725 from the proceedings of the Sixth @acronym{USENIX} Security Symposium (San Jose,
8726 California, July 22--25, 1996).
8728 @strong{Please note} that @command{shred} relies on a very important assumption:
8729 that the file system overwrites data in place. This is the traditional
8730 way to do things, but many modern file system designs do not satisfy this
8731 assumption. Exceptions include:
8736 Log-structured or journaled file systems, such as those supplied with
8737 AIX and Solaris, and JFS, ReiserFS, XFS, Ext3 (in @code{data=journal} mode),
8738 BFS, NTFS, etc.@: when they are configured to journal @emph{data}.
8741 File systems that write redundant data and carry on even if some writes
8742 fail, such as RAID-based file systems.
8745 File systems that make snapshots, such as Network Appliance's NFS server.
8748 File systems that cache in temporary locations, such as NFS version 3
8752 Compressed file systems.
8755 In the particular case of ext3 file systems, the above disclaimer applies (and
8756 @command{shred} is thus of limited effectiveness) only in @code{data=journal}
8757 mode, which journals file data in addition to just metadata. In both
8758 the @code{data=ordered} (default) and @code{data=writeback} modes,
8759 @command{shred} works as usual. Ext3 journaling modes can be changed
8760 by adding the @code{data=something} option to the mount options for a
8761 particular file system in the @file{/etc/fstab} file, as documented in
8762 the mount man page (man mount).
8764 If you are not sure how your file system operates, then you should assume
8765 that it does not overwrite data in place, which means that shred cannot
8766 reliably operate on regular files in your file system.
8768 Generally speaking, it is more reliable to shred a device than a file,
8769 since this bypasses the problem of file system design mentioned above.
8770 However, even shredding devices is not always completely reliable. For
8771 example, most disks map out bad sectors invisibly to the application; if
8772 the bad sectors contain sensitive data, @command{shred} won't be able to
8775 @command{shred} makes no attempt to detect or report this problem, just as
8776 it makes no attempt to do anything about backups. However, since it is
8777 more reliable to shred devices than files, @command{shred} by default does
8778 not truncate or remove the output file. This default is more suitable
8779 for devices, which typically cannot be truncated and should not be
8782 Finally, consider the risk of backups and mirrors.
8783 File system backups and remote mirrors may contain copies of the
8784 file that cannot be removed, and that will allow a shredded file
8785 to be recovered later. So if you keep any data you may later want
8786 to destroy using @command{shred}, be sure that it is not backed up or mirrored.
8789 shred [@var{option}]@dots{} @var{file}[@dots{}]
8792 The program accepts the following options. Also see @ref{Common options}.
8800 @cindex force deletion
8801 Override file permissions if necessary to allow overwriting.
8804 @itemx -n @var{number}
8805 @itemx --iterations=@var{number}
8806 @opindex -n @var{number}
8807 @opindex --iterations=@var{number}
8808 @cindex iterations, selecting the number of
8809 By default, @command{shred} uses @value{SHRED_DEFAULT_PASSES} passes of
8810 overwrite. You can reduce this to save time, or increase it if you think it's
8811 appropriate. After 25 passes all of the internal overwrite patterns will have
8812 been used at least once.
8814 @item --random-source=@var{file}
8815 @opindex --random-source
8816 @cindex random source for shredding
8817 Use @var{file} as a source of random data used to overwrite and to
8818 choose pass ordering. @xref{Random sources}.
8820 @item -s @var{bytes}
8821 @itemx --size=@var{bytes}
8822 @opindex -s @var{bytes}
8823 @opindex --size=@var{bytes}
8824 @cindex size of file to shred
8825 Shred the first @var{bytes} bytes of the file. The default is to shred
8826 the whole file. @var{bytes} can be followed by a size specification like
8827 @samp{K}, @samp{M}, or @samp{G} to specify a multiple. @xref{Block size}.
8833 @cindex removing files after shredding
8834 After shredding a file, truncate it (if possible) and then remove it.
8835 If a file has multiple links, only the named links will be removed.
8841 Display to standard error all status updates as sterilization proceeds.
8847 By default, @command{shred} rounds the size of a regular file up to the next
8848 multiple of the file system block size to fully erase the last block of the file.
8849 Use @option{--exact} to suppress that behavior.
8850 Thus, by default if you shred a 10-byte regular file on a system with 512-byte
8851 blocks, the resulting file will be 512 bytes long. With this option,
8852 shred does not increase the apparent size of the file.
8858 Normally, the last pass that @command{shred} writes is made up of
8859 random data. If this would be conspicuous on your hard drive (for
8860 example, because it looks like encrypted data), or you just think
8861 it's tidier, the @option{--zero} option adds an additional overwrite pass with
8862 all zero bits. This is in addition to the number of passes specified
8863 by the @option{--iterations} option.
8867 You might use the following command to erase all trace of the
8868 file system you'd created on the floppy disk in your first drive.
8869 That command takes about 20 minutes to erase a ``1.44MB'' (actually
8873 shred --verbose /dev/fd0
8876 Similarly, to erase all data on a selected partition of
8877 your hard disk, you could give a command like this:
8880 shred --verbose /dev/sda5
8883 A @var{file} of @samp{-} denotes standard output.
8884 The intended use of this is to shred a removed temporary file.
8891 echo "Hello, world" >&3
8896 However, the command @samp{shred - >file} does not shred the contents
8897 of @var{file}, since the shell truncates @var{file} before invoking
8898 @command{shred}. Use the command @samp{shred file} or (if using a
8899 Bourne-compatible shell) the command @samp{shred - 1<>file} instead.
8904 @node Special file types
8905 @chapter Special file types
8907 @cindex special file types
8908 @cindex file types, special
8910 This chapter describes commands which create special types of files (and
8911 @command{rmdir}, which removes directories, one special file type).
8913 @cindex special file types
8915 Although Unix-like operating systems have markedly fewer special file
8916 types than others, not @emph{everything} can be treated only as the
8917 undifferentiated byte stream of @dfn{normal files}. For example, when a
8918 file is created or removed, the system must record this information,
8919 which it does in a @dfn{directory}---a special type of file. Although
8920 you can read directories as normal files, if you're curious, in order
8921 for the system to do its job it must impose a structure, a certain
8922 order, on the bytes of the file. Thus it is a ``special'' type of file.
8924 Besides directories, other special file types include named pipes
8925 (FIFOs), symbolic links, sockets, and so-called @dfn{special files}.
8928 * link invocation:: Make a hard link via the link syscall
8929 * ln invocation:: Make links between files.
8930 * mkdir invocation:: Make directories.
8931 * mkfifo invocation:: Make FIFOs (named pipes).
8932 * mknod invocation:: Make block or character special files.
8933 * readlink invocation:: Print value of a symlink or canonical file name.
8934 * rmdir invocation:: Remove empty directories.
8935 * unlink invocation:: Remove files via the unlink syscall
8939 @node link invocation
8940 @section @command{link}: Make a hard link via the link syscall
8943 @cindex links, creating
8944 @cindex hard links, creating
8945 @cindex creating links (hard only)
8947 @command{link} creates a single hard link at a time.
8948 It is a minimalist interface to the system-provided
8949 @code{link} function. @xref{Hard Links, , , libc,
8950 The GNU C Library Reference Manual}.
8951 It avoids the bells and whistles of the more commonly-used
8952 @command{ln} command (@pxref{ln invocation}).
8956 link @var{filename} @var{linkname}
8959 @var{filename} must specify an existing file, and @var{linkname}
8960 must specify a nonexistent entry in an existing directory.
8961 @command{link} simply calls @code{link (@var{filename}, @var{linkname})}
8964 On a @acronym{GNU} system, this command acts like @samp{ln --directory
8965 --no-target-directory @var{filename} @var{linkname}}. However, the
8966 @option{--directory} and @option{--no-target-directory} options are
8967 not specified by @acronym{POSIX}, and the @command{link} command is
8968 more portable in practice.
8970 If @var{filename} is a symbolic link, it is unspecified whether
8971 @var{linkname} will be a hard link to the symbolic link or to the
8972 target of the symbolic link. Use @command{ln -P} or @command{ln -L}
8973 to specify which behavior is desired.
8979 @section @command{ln}: Make links between files
8982 @cindex links, creating
8983 @cindex hard links, creating
8984 @cindex symbolic (soft) links, creating
8985 @cindex creating links (hard or soft)
8987 @cindex file systems and hard links
8988 @command{ln} makes links between files. By default, it makes hard links;
8989 with the @option{-s} option, it makes symbolic (or @dfn{soft}) links.
8993 ln [@var{option}]@dots{} [-T] @var{target} @var{linkname}
8994 ln [@var{option}]@dots{} @var{target}
8995 ln [@var{option}]@dots{} @var{target}@dots{} @var{directory}
8996 ln [@var{option}]@dots{} -t @var{directory} @var{target}@dots{}
9002 If two file names are given, @command{ln} creates a link to the first
9003 file from the second.
9006 If one @var{target} is given, @command{ln} creates a link to that file
9007 in the current directory.
9010 If the @option{--target-directory} (@option{-t}) option is given, or
9011 failing that if the last file is a directory and the
9012 @option{--no-target-directory} (@option{-T}) option is not given,
9013 @command{ln} creates a link to each @var{target} file in the specified
9014 directory, using the @var{target}s' names.
9018 Normally @command{ln} does not remove existing files. Use the
9019 @option{--force} (@option{-f}) option to remove them unconditionally,
9020 the @option{--interactive} (@option{-i}) option to remove them
9021 conditionally, and the @option{--backup} (@option{-b}) option to
9024 @cindex hard link, defined
9025 @cindex inode, and hard links
9026 A @dfn{hard link} is another name for an existing file; the link and the
9027 original are indistinguishable. Technically speaking, they share the
9028 same inode, and the inode contains all the information about a
9029 file---indeed, it is not incorrect to say that the inode @emph{is} the
9030 file. Most systems prohibit making a hard link to
9031 a directory; on those where it is allowed, only the super-user can do
9032 so (and with caution, since creating a cycle will cause problems to many
9033 other utilities). Hard links cannot cross file system boundaries. (These
9034 restrictions are not mandated by @acronym{POSIX}, however.)
9036 @cindex dereferencing symbolic links
9037 @cindex symbolic link, defined
9038 @dfn{Symbolic links} (@dfn{symlinks} for short), on the other hand, are
9039 a special file type (which not all kernels support: System V release 3
9040 (and older) systems lack symlinks) in which the link file actually
9041 refers to a different file, by name. When most operations (opening,
9042 reading, writing, and so on) are passed the symbolic link file, the
9043 kernel automatically @dfn{dereferences} the link and operates on the
9044 target of the link. But some operations (e.g., removing) work on the
9045 link file itself, rather than on its target. The owner and group of a
9046 symlink are not significant to file access performed through
9047 the link, but do have implications on deleting a symbolic link from a
9048 directory with the restricted deletion bit set. On the GNU system,
9049 the mode of a symlink has no significance and cannot be changed, but
9050 on some BSD systems, the mode can be changed and will affect whether
9051 the symlink will be traversed in file name resolution. @xref{Symbolic Links,,,
9052 libc, The GNU C Library Reference Manual}.
9054 Symbolic links can contain arbitrary strings; a @dfn{dangling symlink}
9055 occurs when the string in the symlink does not resolve to a file.
9056 There are no restrictions against creating dangling symbolic links.
9057 There are trade-offs to using absolute or relative symlinks. An
9058 absolute symlink always points to the same file, even if the directory
9059 containing the link is moved. However, if the symlink is visible from
9060 more than one machine (such as on a networked file system), the file
9061 pointed to might not always be the same. A relative symbolic link is
9062 resolved in relation to the directory that contains the link, and is
9063 often useful in referring to files on the same device without regards
9064 to what name that device is mounted on when accessed via networked
9067 When creating a relative symlink in a different location than the
9068 current directory, the resolution of the symlink will be different
9069 than the resolution of the same string from the current directory.
9070 Therefore, many users prefer to first change directories to the
9071 location where the relative symlink will be created, so that
9072 tab-completion or other file resolution will find the same target as
9073 what will be placed in the symlink.
9075 The program accepts the following options. Also see @ref{Common options}.
9086 @opindex --directory
9087 @cindex hard links to directories
9088 Allow users with appropriate privileges to attempt to make hard links
9090 However, note that this will probably fail due to
9091 system restrictions, even for the super-user.
9097 Remove existing destination files.
9100 @itemx --interactive
9102 @opindex --interactive
9103 @cindex prompting, and @command{ln}
9104 Prompt whether to remove existing destination files.
9110 If @option{-s} is not in effect, and the source file is a symbolic
9111 link, create the hard link to the file referred to by the symbolic
9112 link, rather than the symbolic link itself.
9115 @itemx --no-dereference
9117 @opindex --no-dereference
9118 Do not treat the last operand specially when it is a symbolic link to
9119 a directory. Instead, treat it as if it were a normal file.
9121 When the destination is an actual directory (not a symlink to one),
9122 there is no ambiguity. The link is created in that directory.
9123 But when the specified destination is a symlink to a directory,
9124 there are two ways to treat the user's request. @command{ln} can
9125 treat the destination just as it would a normal directory and create
9126 the link in it. On the other hand, the destination can be viewed as a
9127 non-directory---as the symlink itself. In that case, @command{ln}
9128 must delete or backup that symlink before creating the new link.
9129 The default is to treat a destination that is a symlink to a directory
9130 just like a directory.
9132 This option is weaker than the @option{--no-target-directory}
9133 (@option{-T}) option, so it has no effect if both options are given.
9139 If @option{-s} is not in effect, and the source file is a symbolic
9140 link, create the hard link to the symbolic link itself. On platforms
9141 where this is not supported by the kernel, this option creates a
9142 symbolic link with identical contents; since symbolic link contents
9143 cannot be edited, any file name resolution performed through either
9144 link will be the same as if a hard link had been created.
9150 Make symbolic links instead of hard links. This option merely produces
9151 an error message on systems that do not support symbolic links.
9157 @optNoTargetDirectory
9163 Print the name of each file after linking it successfully.
9167 @cindex hard links to symbolic links
9168 @cindex symbolic links and @command{ln}
9169 If @option{-L} and @option{-P} are both given, the last one takes
9170 precedence. If @option{-s} is also given, @option{-L} and @option{-P}
9171 are silently ignored. If neither option is given, then this
9172 implementation defaults to @option{-P} if the system @code{link} supports
9173 hard links to symbolic links (such as the GNU system), and @option{-L}
9174 if @code{link} follows symbolic links (such as on BSD).
9183 # Create link ../a pointing to a in that directory.
9184 # Not really useful because it points to itself.
9189 # Change to the target before creating symlinks to avoid being confused.
9195 # Hard coded file names don't move well.
9196 ln -s $(pwd)/a /some/dir/
9200 # Relative file names survive directory moves and also
9201 # work across networked file systems.
9202 ln -s afile anotherfile
9203 ln -s ../adir/afile yetanotherfile
9207 @node mkdir invocation
9208 @section @command{mkdir}: Make directories
9211 @cindex directories, creating
9212 @cindex creating directories
9214 @command{mkdir} creates directories with the specified names. Synopsis:
9217 mkdir [@var{option}]@dots{} @var{name}@dots{}
9220 @command{mkdir} creates each directory @var{name} in the order given.
9221 It reports an error if @var{name} already exists, unless the
9222 @option{-p} option is given and @var{name} is a directory.
9224 The program accepts the following options. Also see @ref{Common options}.
9229 @itemx --mode=@var{mode}
9232 @cindex modes of created directories, setting
9233 Set the file permission bits of created directories to @var{mode},
9234 which uses the same syntax as
9235 in @command{chmod} and uses @samp{a=rwx} (read, write and execute allowed for
9236 everyone) for the point of the departure. @xref{File permissions}.
9238 Normally the directory has the desired file mode bits at the moment it
9239 is created. As a @acronym{GNU} extension, @var{mode} may also mention
9240 special mode bits, but in this case there may be a temporary window
9241 during which the directory exists but its special mode bits are
9242 incorrect. @xref{Directory Setuid and Setgid}, for how the
9243 set-user-ID and set-group-ID bits of directories are inherited unless
9244 overridden in this way.
9250 @cindex parent directories, creating
9251 Make any missing parent directories for each argument, setting their
9252 file permission bits to the umask modified by @samp{u+wx}. Ignore
9253 existing parent directories, and do not change their file permission
9256 To set the file permission bits of any newly-created parent
9257 directories to a value that includes @samp{u+wx}, you can set the
9258 umask before invoking @command{mkdir}. For example, if the shell
9259 command @samp{(umask u=rwx,go=rx; mkdir -p P/Q)} creates the parent
9260 @file{P} it sets the parent's permission bits to @samp{u=rwx,go=rx}.
9261 To set a parent's special mode bits as well, you can invoke
9262 @command{chmod} after @command{mkdir}. @xref{Directory Setuid and
9263 Setgid}, for how the set-user-ID and set-group-ID bits of
9264 newly-created parent directories are inherited.
9270 Print a message for each created directory. This is most useful with
9273 @item -Z @var{context}
9274 @itemx --context=@var{context}
9278 @cindex security context
9279 Set the default SELinux security context to be used for created directories.
9286 @node mkfifo invocation
9287 @section @command{mkfifo}: Make FIFOs (named pipes)
9290 @cindex FIFOs, creating
9291 @cindex named pipes, creating
9292 @cindex creating FIFOs (named pipes)
9294 @command{mkfifo} creates FIFOs (also called @dfn{named pipes}) with the
9295 specified names. Synopsis:
9298 mkfifo [@var{option}] @var{name}@dots{}
9301 A @dfn{FIFO} is a special file type that permits independent processes
9302 to communicate. One process opens the FIFO file for writing, and
9303 another for reading, after which data can flow as with the usual
9304 anonymous pipe in shells or elsewhere.
9306 The program accepts the following option. Also see @ref{Common options}.
9311 @itemx --mode=@var{mode}
9314 @cindex modes of created FIFOs, setting
9315 Set the mode of created FIFOs to @var{mode}, which is symbolic as in
9316 @command{chmod} and uses @samp{a=rw} (read and write allowed for everyone)
9317 for the point of departure. @var{mode} should specify only file
9318 permission bits. @xref{File permissions}.
9320 @item -Z @var{context}
9321 @itemx --context=@var{context}
9325 @cindex security context
9326 Set the default SELinux security context to be used for created FIFOs.
9333 @node mknod invocation
9334 @section @command{mknod}: Make block or character special files
9337 @cindex block special files, creating
9338 @cindex character special files, creating
9340 @command{mknod} creates a FIFO, character special file, or block special
9341 file with the specified name. Synopsis:
9344 mknod [@var{option}]@dots{} @var{name} @var{type} [@var{major} @var{minor}]
9347 @cindex special files
9348 @cindex block special files
9349 @cindex character special files
9350 Unlike the phrase ``special file type'' above, the term @dfn{special
9351 file} has a technical meaning on Unix: something that can generate or
9352 receive data. Usually this corresponds to a physical piece of hardware,
9353 e.g., a printer or a disk. (These files are typically created at
9354 system-configuration time.) The @command{mknod} command is what creates
9355 files of this type. Such devices can be read either a character at a
9356 time or a ``block'' (many characters) at a time, hence we say there are
9357 @dfn{block special} files and @dfn{character special} files.
9359 @c mknod is a shell built-in at least with OpenBSD's /bin/sh
9360 @mayConflictWithShellBuiltIn{mknod}
9362 The arguments after @var{name} specify the type of file to make:
9367 @opindex p @r{for FIFO file}
9371 @opindex b @r{for block special file}
9372 for a block special file
9375 @c Don't document the `u' option -- it's just a synonym for `c'.
9376 @c Do *any* versions of mknod still use it?
9378 @opindex c @r{for character special file}
9379 @c @opindex u @r{for character special file}
9380 for a character special file
9384 When making a block or character special file, the major and minor
9385 device numbers must be given after the file type.
9386 If a major or minor device number begins with @samp{0x} or @samp{0X},
9387 it is interpreted as hexadecimal; otherwise, if it begins with @samp{0},
9388 as octal; otherwise, as decimal.
9390 The program accepts the following option. Also see @ref{Common options}.
9395 @itemx --mode=@var{mode}
9398 Set the mode of created files to @var{mode}, which is symbolic as in
9399 @command{chmod} and uses @samp{a=rw} as the point of departure.
9400 @var{mode} should specify only file permission bits.
9401 @xref{File permissions}.
9403 @item -Z @var{context}
9404 @itemx --context=@var{context}
9408 @cindex security context
9409 Set the default SELinux security context to be used for created files.
9416 @node readlink invocation
9417 @section @command{readlink}: Print value of a symlink or canonical file name
9420 @cindex displaying value of a symbolic link
9421 @cindex canonical file name
9422 @cindex canonicalize a file name
9426 @command{readlink} may work in one of two supported modes:
9432 @command{readlink} outputs the value of the given symbolic link.
9433 If @command{readlink} is invoked with an argument other than the name
9434 of a symbolic link, it produces no output and exits with a nonzero exit code.
9436 @item Canonicalize mode
9438 @command{readlink} outputs the absolute name of the given file which contains
9439 no @file{.}, @file{..} components nor any repeated separators
9440 (@file{/}) or symbolic links.
9445 readlink [@var{option}] @var{file}
9448 By default, @command{readlink} operates in readlink mode.
9450 The program accepts the following options. Also see @ref{Common options}.
9455 @itemx --canonicalize
9457 @opindex --canonicalize
9458 Activate canonicalize mode.
9459 If any component of the file name except the last one is missing or unavailable,
9460 @command{readlink} produces no output and exits with a nonzero exit
9461 code. A trailing slash is ignored.
9464 @itemx --canonicalize-existing
9466 @opindex --canonicalize-existing
9467 Activate canonicalize mode.
9468 If any component is missing or unavailable, @command{readlink} produces
9469 no output and exits with a nonzero exit code. A trailing slash
9470 requires that the name resolve to a directory.
9473 @itemx --canonicalize-missing
9475 @opindex --canonicalize-missing
9476 Activate canonicalize mode.
9477 If any component is missing or unavailable, @command{readlink} treats it
9483 @opindex --no-newline
9484 Do not output the trailing newline.
9494 Suppress most error messages.
9500 Report error messages.
9504 The @command{readlink} utility first appeared in OpenBSD 2.1.
9506 There is a @command{realpath} command on some systems
9507 which operates like @command{readlink} in canonicalize mode.
9512 @node rmdir invocation
9513 @section @command{rmdir}: Remove empty directories
9516 @cindex removing empty directories
9517 @cindex directories, removing empty
9519 @command{rmdir} removes empty directories. Synopsis:
9522 rmdir [@var{option}]@dots{} @var{directory}@dots{}
9525 If any @var{directory} argument does not refer to an existing empty
9526 directory, it is an error.
9528 The program accepts the following options. Also see @ref{Common options}.
9532 @item --ignore-fail-on-non-empty
9533 @opindex --ignore-fail-on-non-empty
9534 @cindex directory deletion, ignoring failures
9535 Ignore each failure to remove a directory that is solely because
9536 the directory is non-empty.
9542 @cindex parent directories, removing
9543 Remove @var{directory}, then try to remove each component of @var{directory}.
9544 So, for example, @samp{rmdir -p a/b/c} is similar to @samp{rmdir a/b/c a/b a}.
9545 As such, it fails if any of those directories turns out not to be empty.
9546 Use the @option{--ignore-fail-on-non-empty} option to make it so such
9547 a failure does not evoke a diagnostic and does not cause @command{rmdir} to
9548 exit unsuccessfully.
9554 @cindex directory deletion, reporting
9555 Give a diagnostic for each successful removal.
9556 @var{directory} is removed.
9560 @xref{rm invocation}, for how to remove non-empty directories (recursively).
9565 @node unlink invocation
9566 @section @command{unlink}: Remove files via the unlink syscall
9569 @cindex removing files or directories (via the unlink syscall)
9571 @command{unlink} deletes a single specified file name.
9572 It is a minimalist interface to the system-provided
9573 @code{unlink} function. @xref{Deleting Files, , , libc,
9574 The GNU C Library Reference Manual}. Synopsis:
9575 It avoids the bells and whistles of the more commonly-used
9576 @command{rm} command (@pxref{rm invocation}).
9579 unlink @var{filename}
9582 On some systems @code{unlink} can be used to delete the name of a
9583 directory. On others, it can be used that way only by a privileged user.
9584 In the GNU system @code{unlink} can never delete the name of a directory.
9586 The @command{unlink} command honors the @option{--help} and
9587 @option{--version} options. To remove a file whose name begins with
9588 @samp{-}, prefix the name with @samp{./}, e.g., @samp{unlink ./--help}.
9593 @node Changing file attributes
9594 @chapter Changing file attributes
9596 @cindex changing file attributes
9597 @cindex file attributes, changing
9598 @cindex attributes, file
9600 A file is not merely its contents, a name, and a file type
9601 (@pxref{Special file types}). A file also has an owner (a user ID), a
9602 group (a group ID), permissions (what the owner can do with the file,
9603 what people in the group can do, and what everyone else can do), various
9604 timestamps, and other information. Collectively, we call these a file's
9607 These commands change file attributes.
9610 * chgrp invocation:: Change file groups.
9611 * chmod invocation:: Change access permissions.
9612 * chown invocation:: Change file owners and groups.
9613 * touch invocation:: Change file timestamps.
9617 @node chown invocation
9618 @section @command{chown}: Change file owner and group
9621 @cindex file ownership, changing
9622 @cindex group ownership, changing
9623 @cindex changing file ownership
9624 @cindex changing group ownership
9626 @command{chown} changes the user and/or group ownership of each given @var{file}
9627 to @var{new-owner} or to the user and group of an existing reference file.
9631 chown [@var{option}]@dots{} @{@var{new-owner} | --reference=@var{ref_file}@} @var{file}@dots{}
9634 If used, @var{new-owner} specifies the new owner and/or group as follows
9635 (with no embedded white space):
9638 [@var{owner}] [ : [@var{group}] ]
9645 If only an @var{owner} (a user name or numeric user ID) is given, that
9646 user is made the owner of each given file, and the files' group is not
9649 @item owner@samp{:}group
9650 If the @var{owner} is followed by a colon and a @var{group} (a
9651 group name or numeric group ID), with no spaces between them, the group
9652 ownership of the files is changed as well (to @var{group}).
9655 If a colon but no group name follows @var{owner}, that user is
9656 made the owner of the files and the group of the files is changed to
9657 @var{owner}'s login group.
9660 If the colon and following @var{group} are given, but the owner
9661 is omitted, only the group of the files is changed; in this case,
9662 @command{chown} performs the same function as @command{chgrp}.
9665 If only a colon is given, or if @var{new-owner} is empty, neither the
9666 owner nor the group is changed.
9670 If @var{owner} or @var{group} is intended to represent a numeric user
9671 or group ID, then you may specify it with a leading @samp{+}.
9672 @xref{Disambiguating names and IDs}.
9674 Some older scripts may still use @samp{.} in place of the @samp{:} separator.
9675 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) does not
9676 require support for that, but for backward compatibility @acronym{GNU}
9677 @command{chown} supports @samp{.} so long as no ambiguity results.
9678 New scripts should avoid the use of @samp{.} because it is not
9679 portable, and because it has undesirable results if the entire
9680 @var{owner@samp{.}group} happens to identify a user whose name
9683 The @command{chown} command sometimes clears the set-user-ID or
9684 set-group-ID permission bits. This behavior depends on the policy and
9685 functionality of the underlying @code{chown} system call, which may
9686 make system-dependent file mode modifications outside the control of
9687 the @command{chown} command. For example, the @command{chown} command
9688 might not affect those bits when invoked by a user with appropriate
9689 privileges, or when the
9690 bits signify some function other than executable permission (e.g.,
9692 When in doubt, check the underlying system behavior.
9694 The program accepts the following options. Also see @ref{Common options}.
9702 @cindex changed owners, verbosely describing
9703 Verbosely describe the action for each @var{file} whose ownership
9712 @cindex error messages, omitting
9713 Do not print error messages about files whose ownership cannot be
9716 @itemx @w{@kbd{--from}=@var{old-owner}}
9718 @cindex symbolic links, changing owner
9719 Change a @var{file}'s ownership only if it has current attributes specified
9720 by @var{old-owner}. @var{old-owner} has the same form as @var{new-owner}
9722 This option is useful primarily from a security standpoint in that
9723 it narrows considerably the window of potential abuse.
9724 For example, to reflect a user ID numbering change for one user's files
9725 without an option like this, @code{root} might run
9728 find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER
9731 But that is dangerous because the interval between when the @command{find}
9732 tests the existing file's owner and when the @command{chown} is actually run
9734 One way to narrow the gap would be to invoke chown for each file
9738 find / -owner OLDUSER -exec chown -h NEWUSER @{@} \;
9741 But that is very slow if there are many affected files.
9742 With this option, it is safer (the gap is narrower still)
9743 though still not perfect:
9746 chown -h -R --from=OLDUSER NEWUSER /
9750 @opindex --dereference
9751 @cindex symbolic links, changing owner
9753 Do not act on symbolic links themselves but rather on what they point to.
9754 This is the default.
9757 @itemx --no-dereference
9759 @opindex --no-dereference
9760 @cindex symbolic links, changing owner
9762 Act on symbolic links themselves instead of what they point to.
9763 This mode relies on the @code{lchown} system call.
9764 On systems that do not provide the @code{lchown} system call,
9765 @command{chown} fails when a file specified on the command line
9767 By default, no diagnostic is issued for symbolic links encountered
9768 during a recursive traversal, but see @option{--verbose}.
9770 @itemx --preserve-root
9771 @opindex --preserve-root
9772 @cindex root directory, disallow recursive modification
9773 Fail upon any attempt to recursively change the root directory, @file{/}.
9774 Without @option{--recursive}, this option has no effect.
9775 @xref{Treating / specially}.
9777 @itemx --no-preserve-root
9778 @opindex --no-preserve-root
9779 @cindex root directory, allow recursive modification
9780 Cancel the effect of any preceding @option{--preserve-root} option.
9781 @xref{Treating / specially}.
9783 @item --reference=@var{ref_file}
9784 @opindex --reference
9785 Change the user and group of each @var{file} to be the same as those of
9786 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9787 user and group of the symbolic link, but rather those of the file it
9794 Output a diagnostic for every file processed.
9795 If a symbolic link is encountered during a recursive traversal
9796 on a system without the @code{lchown} system call, and @option{--no-dereference}
9797 is in effect, then issue a diagnostic saying neither the symbolic link nor
9798 its referent is being changed.
9803 @opindex --recursive
9804 @cindex recursively changing file ownership
9805 Recursively change ownership of directories and their contents.
9808 @xref{Traversing symlinks}.
9811 @xref{Traversing symlinks}.
9814 @xref{Traversing symlinks}.
9823 # Change the owner of /u to "root".
9826 # Likewise, but also change its group to "staff".
9829 # Change the owner of /u and subfiles to "root".
9834 @node chgrp invocation
9835 @section @command{chgrp}: Change group ownership
9838 @cindex group ownership, changing
9839 @cindex changing group ownership
9841 @command{chgrp} changes the group ownership of each given @var{file}
9842 to @var{group} (which can be either a group name or a numeric group ID)
9843 or to the group of an existing reference file. Synopsis:
9846 chgrp [@var{option}]@dots{} @{@var{group} | --reference=@var{ref_file}@} @var{file}@dots{}
9849 If @var{group} is intended to represent a
9850 numeric group ID, then you may specify it with a leading @samp{+}.
9851 @xref{Disambiguating names and IDs}.
9853 The program accepts the following options. Also see @ref{Common options}.
9861 @cindex changed files, verbosely describing
9862 Verbosely describe the action for each @var{file} whose group actually
9871 @cindex error messages, omitting
9872 Do not print error messages about files whose group cannot be
9876 @opindex --dereference
9877 @cindex symbolic links, changing owner
9879 Do not act on symbolic links themselves but rather on what they point to.
9880 This is the default.
9883 @itemx --no-dereference
9885 @opindex --no-dereference
9886 @cindex symbolic links, changing group
9888 Act on symbolic links themselves instead of what they point to.
9889 This mode relies on the @code{lchown} system call.
9890 On systems that do not provide the @code{lchown} system call,
9891 @command{chgrp} fails when a file specified on the command line
9893 By default, no diagnostic is issued for symbolic links encountered
9894 during a recursive traversal, but see @option{--verbose}.
9896 @itemx --preserve-root
9897 @opindex --preserve-root
9898 @cindex root directory, disallow recursive modification
9899 Fail upon any attempt to recursively change the root directory, @file{/}.
9900 Without @option{--recursive}, this option has no effect.
9901 @xref{Treating / specially}.
9903 @itemx --no-preserve-root
9904 @opindex --no-preserve-root
9905 @cindex root directory, allow recursive modification
9906 Cancel the effect of any preceding @option{--preserve-root} option.
9907 @xref{Treating / specially}.
9909 @item --reference=@var{ref_file}
9910 @opindex --reference
9911 Change the group of each @var{file} to be the same as that of
9912 @var{ref_file}. If @var{ref_file} is a symbolic link, do not use the
9913 group of the symbolic link, but rather that of the file it refers to.
9919 Output a diagnostic for every file processed.
9920 If a symbolic link is encountered during a recursive traversal
9921 on a system without the @code{lchown} system call, and @option{--no-dereference}
9922 is in effect, then issue a diagnostic saying neither the symbolic link nor
9923 its referent is being changed.
9928 @opindex --recursive
9929 @cindex recursively changing group ownership
9930 Recursively change the group ownership of directories and their contents.
9933 @xref{Traversing symlinks}.
9936 @xref{Traversing symlinks}.
9939 @xref{Traversing symlinks}.
9948 # Change the group of /u to "staff".
9951 # Change the group of /u and subfiles to "staff".
9956 @node chmod invocation
9957 @section @command{chmod}: Change access permissions
9960 @cindex changing access permissions
9961 @cindex access permissions, changing
9962 @cindex permissions, changing access
9964 @command{chmod} changes the access permissions of the named files. Synopsis:
9967 chmod [@var{option}]@dots{} @{@var{mode} | --reference=@var{ref_file}@} @var{file}@dots{}
9970 @cindex symbolic links, permissions of
9971 @command{chmod} never changes the permissions of symbolic links, since
9972 the @command{chmod} system call cannot change their permissions.
9973 This is not a problem since the permissions of symbolic links are
9974 never used. However, for each symbolic link listed on the command
9975 line, @command{chmod} changes the permissions of the pointed-to file.
9976 In contrast, @command{chmod} ignores symbolic links encountered during
9977 recursive directory traversals.
9979 A successful use of @command{chmod} clears the set-group-ID bit of a
9980 regular file if the file's group ID does not match the user's
9981 effective group ID or one of the user's supplementary group IDs,
9982 unless the user has appropriate privileges. Additional restrictions
9983 may cause the set-user-ID and set-group-ID bits of @var{mode} or
9984 @var{ref_file} to be ignored. This behavior depends on the policy and
9985 functionality of the underlying @code{chmod} system call. When in
9986 doubt, check the underlying system behavior.
9988 If used, @var{mode} specifies the new file mode bits.
9989 For details, see the section on @ref{File permissions}.
9990 If you really want @var{mode} to have a leading @samp{-}, you should
9991 use @option{--} first, e.g., @samp{chmod -- -w file}. Typically,
9992 though, @samp{chmod a-w file} is preferable, and @command{chmod -w
9993 file} (without the @option{--}) complains if it behaves differently
9994 from what @samp{chmod a-w file} would do.
9996 The program accepts the following options. Also see @ref{Common options}.
10004 Verbosely describe the action for each @var{file} whose permissions
10013 @cindex error messages, omitting
10014 Do not print error messages about files whose permissions cannot be
10017 @itemx --preserve-root
10018 @opindex --preserve-root
10019 @cindex root directory, disallow recursive modification
10020 Fail upon any attempt to recursively change the root directory, @file{/}.
10021 Without @option{--recursive}, this option has no effect.
10022 @xref{Treating / specially}.
10024 @itemx --no-preserve-root
10025 @opindex --no-preserve-root
10026 @cindex root directory, allow recursive modification
10027 Cancel the effect of any preceding @option{--preserve-root} option.
10028 @xref{Treating / specially}.
10034 Verbosely describe the action or non-action taken for every @var{file}.
10036 @item --reference=@var{ref_file}
10037 @opindex --reference
10038 Change the mode of each @var{file} to be the same as that of @var{ref_file}.
10039 @xref{File permissions}.
10040 If @var{ref_file} is a symbolic link, do not use the mode
10041 of the symbolic link, but rather that of the file it refers to.
10046 @opindex --recursive
10047 @cindex recursively changing access permissions
10048 Recursively change permissions of directories and their contents.
10055 @node touch invocation
10056 @section @command{touch}: Change file timestamps
10059 @cindex changing file timestamps
10060 @cindex file timestamps, changing
10061 @cindex timestamps, changing file
10063 @command{touch} changes the access and/or modification times of the
10064 specified files. Synopsis:
10067 touch [@var{option}]@dots{} @var{file}@dots{}
10070 @cindex empty files, creating
10071 Any @var{file} argument that does not exist is created empty, unless
10072 option @option{--no-create} (@option{-c}) or @option{--no-dereference}
10073 (@option{-h}) was in effect.
10075 A @var{file} argument string of @samp{-} is handled specially and
10076 causes @command{touch} to change the times of the file associated with
10079 @cindex permissions, for changing file timestamps
10080 If changing both the access and modification times to the current
10081 time, @command{touch} can change the timestamps for files that the user
10082 running it does not own but has write permission for. Otherwise, the
10083 user must own the files.
10085 Although @command{touch} provides options for changing two of the times---the
10086 times of last access and modification---of a file, there is actually
10087 a standard third one as well: the inode change time. This is often
10088 referred to as a file's @code{ctime}.
10089 The inode change time represents the time when the file's meta-information
10090 last changed. One common example of this is when the permissions of a
10091 file change. Changing the permissions doesn't access the file, so
10092 the atime doesn't change, nor does it modify the file, so the mtime
10093 doesn't change. Yet, something about the file itself has changed,
10094 and this must be noted somewhere. This is the job of the ctime field.
10095 This is necessary, so that, for example, a backup program can make a
10096 fresh copy of the file, including the new permissions value.
10097 Another operation that modifies a file's ctime without affecting
10098 the others is renaming. In any case, it is not possible, in normal
10099 operations, for a user to change the ctime field to a user-specified value.
10100 Some operating systems and file systems support a fourth time: the
10101 birth time, when the file was first created; by definition, this
10102 timestamp never changes.
10105 Time stamps assume the time zone rules specified by the @env{TZ}
10106 environment variable, or by the system default rules if @env{TZ} is
10107 not set. @xref{TZ Variable,, Specifying the Time Zone with @env{TZ},
10108 libc, The GNU C Library Reference Manual}.
10109 You can avoid ambiguities during
10110 daylight saving transitions by using @sc{utc} time stamps.
10112 The program accepts the following options. Also see @ref{Common options}.
10117 @itemx --time=atime
10118 @itemx --time=access
10122 @opindex atime@r{, changing}
10123 @opindex access @r{time, changing}
10124 @opindex use @r{time, changing}
10125 Change the access time only.
10130 @opindex --no-create
10131 Do not warn about or create files that do not exist.
10134 @itemx --date=@var{time}
10138 Use @var{time} instead of the current time. It can contain month names,
10139 time zones, @samp{am} and @samp{pm}, @samp{yesterday}, etc. For
10140 example, @option{--date="2004-02-27 14:19:13.489392193 +0530"}
10141 specifies the instant of time that is 489,392,193 nanoseconds after
10142 February 27, 2004 at 2:19:13 PM in a time zone that is 5 hours and 30
10143 minutes east of @acronym{UTC}. @xref{Date input formats}.
10144 File systems that do not support high-resolution time stamps
10145 silently ignore any excess precision here.
10149 @cindex BSD @command{touch} compatibility
10150 Ignored; for compatibility with BSD versions of @command{touch}.
10153 @itemx --no-dereference
10155 @opindex --no-dereference
10156 @cindex symbolic links, changing time
10158 Attempt to change the timestamps of a symbolic link, rather than what
10159 the link refers to. When using this option, empty files are not
10160 created, but option @option{-c} must also be used to avoid warning
10161 about files that do not exist. Not all systems support changing the
10162 timestamps of symlinks, since underlying system support for this
10163 action was not required until @acronym{POSIX} 2008. Also, on some
10164 systems, the mere act of examining a symbolic link changes the access
10165 time, such that only changes to the modification time will persist
10166 long enough to be observable. When coupled with option @option{-r}, a
10167 reference timestamp is taken from a symbolic link rather than the file
10171 @itemx --time=mtime
10172 @itemx --time=modify
10175 @opindex mtime@r{, changing}
10176 @opindex modify @r{time, changing}
10177 Change the modification time only.
10179 @item -r @var{file}
10180 @itemx --reference=@var{file}
10182 @opindex --reference
10183 Use the times of the reference @var{file} instead of the current time.
10184 If this option is combined with the @option{--date=@var{time}}
10185 (@option{-d @var{time}}) option, the reference @var{file}'s time is
10186 the origin for any relative @var{time}s given, but is otherwise ignored.
10187 For example, @samp{-r foo -d '-5 seconds'} specifies a time stamp
10188 equal to five seconds before the corresponding time stamp for @file{foo}.
10189 If @var{file} is a symbolic link, the reference timestamp is taken
10190 from the target of the symlink, unless @option{-h} was also in effect.
10192 @item -t [[@var{cc}]@var{yy}]@var{mmddhhmm}[.@var{ss}]
10193 Use the argument (optional four-digit or two-digit years, months,
10194 days, hours, minutes, optional seconds) instead of the current time.
10195 If the year is specified with only two digits, then @var{cc}
10196 is 20 for years in the range 0 @dots{} 68, and 19 for years in
10197 69 @dots{} 99. If no digits of the year are specified,
10198 the argument is interpreted as a date in the current year.
10199 Note that @var{ss} may be @samp{60}, to accommodate leap seconds.
10203 @vindex _POSIX2_VERSION
10204 On older systems, @command{touch} supports an obsolete syntax, as follows.
10205 If no timestamp is given with any of the @option{-d}, @option{-r}, or
10206 @option{-t} options, and if there are two or more @var{file}s and the
10207 first @var{file} is of the form @samp{@var{mmddhhmm}[@var{yy}]} and this
10208 would be a valid argument to the @option{-t} option (if the @var{yy}, if
10209 any, were moved to the front), and if the represented year
10210 is in the range 1969--1999, that argument is interpreted as the time
10211 for the other files instead of as a file name.
10212 This obsolete behavior can be enabled or disabled with the
10213 @env{_POSIX2_VERSION} environment variable (@pxref{Standards
10214 conformance}), but portable scripts should avoid commands whose
10215 behavior depends on this variable.
10216 For example, use @samp{touch ./12312359 main.c} or @samp{touch -t
10217 12312359 main.c} rather than the ambiguous @samp{touch 12312359 main.c}.
10223 @chapter Disk usage
10227 No disk can hold an infinite amount of data. These commands report
10228 how much disk storage is in use or available, report other file and
10229 file status information, and write buffers to disk.
10232 * df invocation:: Report file system disk space usage.
10233 * du invocation:: Estimate file space usage.
10234 * stat invocation:: Report file or file system status.
10235 * sync invocation:: Synchronize memory and disk.
10236 * truncate invocation:: Shrink or extend the size of a file.
10240 @node df invocation
10241 @section @command{df}: Report file system disk space usage
10244 @cindex file system disk usage
10245 @cindex disk usage by file system
10247 @command{df} reports the amount of disk space used and available on
10248 file systems. Synopsis:
10251 df [@var{option}]@dots{} [@var{file}]@dots{}
10254 With no arguments, @command{df} reports the space used and available on all
10255 currently mounted file systems (of all types). Otherwise, @command{df}
10256 reports on the file system containing each argument @var{file}.
10258 Normally the disk space is printed in units of
10259 1024 bytes, but this can be overridden (@pxref{Block size}).
10260 Non-integer quantities are rounded up to the next higher unit.
10262 @cindex disk device file
10263 @cindex device file, disk
10264 If an argument @var{file} is a disk device file containing a mounted
10265 file system, @command{df} shows the space available on that file system
10266 rather than on the file system containing the device node (i.e., the root
10267 file system). @sc{gnu} @command{df} does not attempt to determine the disk usage
10268 on unmounted file systems, because on most kinds of systems doing so
10269 requires extremely nonportable intimate knowledge of file system
10272 The program accepts the following options. Also see @ref{Common options}.
10280 @cindex automounter file systems
10281 @cindex ignore file systems
10282 Include in the listing dummy file systems, which
10283 are omitted by default. Such file systems are typically special-purpose
10284 pseudo-file-systems, such as automounter entries.
10286 @item -B @var{size}
10287 @itemx --block-size=@var{size}
10289 @opindex --block-size
10290 @cindex file system sizes
10291 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10292 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10296 @cindex grand total of disk size, usage and available space
10297 Print a grand total of all arguments after all arguments have
10298 been processed. This can be used to find out the total disk size, usage
10299 and available space of all listed devices.
10305 Equivalent to @option{--si}.
10311 @cindex inode usage
10312 List inode usage information instead of block usage. An inode (short
10313 for index node) contains information about a file such as its owner,
10314 permissions, timestamps, and location on the disk.
10318 @cindex kibibytes for file system sizes
10319 Print sizes in 1024-byte blocks, overriding the default block size
10320 (@pxref{Block size}).
10321 This option is equivalent to @option{--block-size=1K}.
10327 @cindex file system types, limiting output to certain
10328 Limit the listing to local file systems. By default, remote file systems
10333 @cindex file system space, retrieving old data more quickly
10334 Do not invoke the @code{sync} system call before getting any usage data.
10335 This may make @command{df} run significantly faster on systems with many
10336 disks, but on some systems (notably SunOS) the results may be slightly
10337 out of date. This is the default.
10340 @itemx --portability
10342 @opindex --portability
10343 @cindex one-line output format
10344 @cindex @acronym{POSIX} output format
10345 @cindex portable output format
10346 @cindex output format, portable
10347 Use the @acronym{POSIX} output format. This is like the default format except
10352 The information about each file system is always printed on exactly
10353 one line; a mount device is never put on a line by itself. This means
10354 that if the mount device name is more than 20 characters long (e.g., for
10355 some network mounts), the columns are misaligned.
10358 The labels in the header output line are changed to conform to @acronym{POSIX}.
10361 The default block size and output format are unaffected by the
10362 @env{DF_BLOCK_SIZE}, @env{BLOCK_SIZE} and @env{BLOCKSIZE} environment
10363 variables. However, the default block size is still affected by
10364 @env{POSIXLY_CORRECT}: it is 512 if @env{POSIXLY_CORRECT} is set, 1024
10365 otherwise. @xref{Block size}.
10372 @cindex file system space, retrieving current data more slowly
10373 Invoke the @code{sync} system call before getting any usage data. On
10374 some systems (notably SunOS), doing this yields more up to date results,
10375 but in general this option makes @command{df} much slower, especially when
10376 there are many or very busy file systems.
10378 @item -t @var{fstype}
10379 @itemx --type=@var{fstype}
10382 @cindex file system types, limiting output to certain
10383 Limit the listing to file systems of type @var{fstype}. Multiple
10384 file system types can be specified by giving multiple @option{-t} options.
10385 By default, nothing is omitted.
10388 @itemx --print-type
10390 @opindex --print-type
10391 @cindex file system types, printing
10392 Print each file system's type. The types printed here are the same ones
10393 you can include or exclude with @option{-t} and @option{-x}. The particular
10394 types printed are whatever is supported by the system. Here are some of
10395 the common names (this list is certainly not exhaustive):
10400 @cindex @acronym{NFS} file system type
10401 An @acronym{NFS} file system, i.e., one mounted over a network from another
10402 machine. This is the one type name which seems to be used uniformly by
10405 @item 4.2@r{, }ufs@r{, }efs@dots{}
10406 @cindex Linux file system types
10407 @cindex local file system types
10408 @opindex 4.2 @r{file system type}
10409 @opindex ufs @r{file system type}
10410 @opindex efs @r{file system type}
10411 A file system on a locally-mounted hard disk. (The system might even
10412 support more than one type here; Linux does.)
10414 @item hsfs@r{, }cdfs
10415 @cindex CD-ROM file system type
10416 @cindex High Sierra file system
10417 @opindex hsfs @r{file system type}
10418 @opindex cdfs @r{file system type}
10419 A file system on a CD-ROM drive. HP-UX uses @samp{cdfs}, most other
10420 systems use @samp{hsfs} (@samp{hs} for ``High Sierra'').
10423 @cindex PC file system
10424 @cindex DOS file system
10425 @cindex MS-DOS file system
10426 @cindex diskette file system
10428 An MS-DOS file system, usually on a diskette.
10432 @item -x @var{fstype}
10433 @itemx --exclude-type=@var{fstype}
10435 @opindex --exclude-type
10436 Limit the listing to file systems not of type @var{fstype}.
10437 Multiple file system types can be eliminated by giving multiple
10438 @option{-x} options. By default, no file system types are omitted.
10441 Ignored; for compatibility with System V versions of @command{df}.
10446 Failure includes the case where no output is generated, so you can
10447 inspect the exit status of a command like @samp{df -t ext3 -t reiserfs
10448 @var{dir}} to test whether @var{dir} is on a file system of type
10449 @samp{ext3} or @samp{reiserfs}.
10452 @node du invocation
10453 @section @command{du}: Estimate file space usage
10456 @cindex file space usage
10457 @cindex disk usage for files
10459 @command{du} reports the amount of disk space used by the specified files
10460 and for each subdirectory (of directory arguments). Synopsis:
10463 du [@var{option}]@dots{} [@var{file}]@dots{}
10466 With no arguments, @command{du} reports the disk space for the current
10467 directory. Normally the disk space is printed in units of
10468 1024 bytes, but this can be overridden (@pxref{Block size}).
10469 Non-integer quantities are rounded up to the next higher unit.
10471 If two or more hard links point to the same file, only one of the hard
10472 links is counted. The @var{file} argument order affects which links
10473 are counted, and changing the argument order may change the numbers
10474 that @command{du} outputs.
10476 The program accepts the following options. Also see @ref{Common options}.
10484 Show counts for all files, not just directories.
10486 @itemx --apparent-size
10487 @opindex --apparent-size
10488 Print apparent sizes, rather than disk usage. The apparent size of a
10489 file is the number of bytes reported by @code{wc -c} on regular files,
10490 or more generally, @code{ls -l --block-size=1} or @code{stat --format=%s}.
10491 For example, a file containing the word @samp{zoo} with no newline would,
10492 of course, have an apparent size of 3. Such a small file may require
10493 anywhere from 0 to 16 KiB or more of disk space, depending on
10494 the type and configuration of the file system on which the file resides.
10495 However, a sparse file created with this command:
10498 dd bs=1 seek=2GiB if=/dev/null of=big
10502 has an apparent size of 2 GiB, yet on most modern
10503 systems, it actually uses almost no disk space.
10509 Equivalent to @code{--apparent-size --block-size=1}.
10511 @item -B @var{size}
10512 @itemx --block-size=@var{size}
10514 @opindex --block-size
10516 Scale sizes by @var{size} before printing them (@pxref{Block size}).
10517 For example, @option{-BG} prints sizes in units of 1,073,741,824 bytes.
10523 @cindex grand total of disk space
10524 Print a grand total of all arguments after all arguments have
10525 been processed. This can be used to find out the total disk usage of
10526 a given set of files or directories.
10529 @itemx --dereference-args
10531 @opindex --dereference-args
10532 Dereference symbolic links that are command line arguments.
10533 Does not affect other symbolic links. This is helpful for finding
10534 out the disk usage of directories, such as @file{/usr/tmp}, which
10535 are often symbolic links.
10537 @c --files0-from=FILE
10538 @filesZeroFromOption{du,, with the @option{--total} (@option{-c}) option}
10544 Equivalent to @option{--dereference-args} (@option{-D}).
10548 @cindex kibibytes for file sizes
10549 Print sizes in 1024-byte blocks, overriding the default block size
10550 (@pxref{Block size}).
10551 This option is equivalent to @option{--block-size=1K}.
10554 @itemx --count-links
10556 @opindex --count-links
10557 @cindex hard links, counting in @command{du}
10558 Count the size of all files, even if they have appeared already (as a
10562 @itemx --dereference
10564 @opindex --dereference
10565 @cindex symbolic links, dereferencing in @command{du}
10566 Dereference symbolic links (show the disk space used by the file
10567 or directory that the link points to instead of the space used by
10572 @cindex mebibytes for file sizes
10573 Print sizes in 1,048,576-byte blocks, overriding the default block size
10574 (@pxref{Block size}).
10575 This option is equivalent to @option{--block-size=1M}.
10578 @itemx --no-dereference
10580 @opindex --no-dereference
10581 @cindex symbolic links, dereferencing in @command{du}
10582 For each symbolic links encountered by @command{du},
10583 consider the disk space used by the symbolic link.
10585 @item -d @var{depth}
10586 @item --max-depth=@var{depth}
10587 @opindex -d @var{depth}
10588 @opindex --max-depth=@var{depth}
10589 @cindex limiting output of @command{du}
10590 Show the total for each directory (and file if --all) that is at
10591 most MAX_DEPTH levels down from the root of the hierarchy. The root
10592 is at level 0, so @code{du --max-depth=0} is equivalent to @code{du -s}.
10601 @opindex --summarize
10602 Display only a total for each argument.
10605 @itemx --separate-dirs
10607 @opindex --separate-dirs
10608 Normally, in the output of @command{du} (when not using @option{--summarize}),
10609 the size listed next to a directory name, @var{d}, represents the sum
10610 of sizes of all entries beneath @var{d} as well as the size of @var{d} itself.
10611 With @option{--separate-dirs}, the size reported for a directory name,
10612 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
10617 @cindex last modified dates, displaying in @command{du}
10618 Show time of the most recent modification of any file in the directory,
10619 or any of its subdirectories.
10621 @itemx --time=ctime
10622 @itemx --time=status
10625 @opindex ctime@r{, show the most recent}
10626 @opindex status time@r{, show the most recent}
10627 @opindex use time@r{, show the most recent}
10628 Show the most recent status change time (the @samp{ctime} in the inode) of
10629 any file in the directory, instead of the modification time.
10631 @itemx --time=atime
10632 @itemx --time=access
10634 @opindex atime@r{, show the most recent}
10635 @opindex access time@r{, show the most recent}
10636 Show the most recent access time (the @samp{atime} in the inode) of
10637 any file in the directory, instead of the modification time.
10639 @item --time-style=@var{style}
10640 @opindex --time-style
10642 List timestamps in style @var{style}. This option has an effect only if
10643 the @option{--time} option is also specified. The @var{style} should
10644 be one of the following:
10647 @item +@var{format}
10649 List timestamps using @var{format}, where @var{format} is interpreted
10650 like the format argument of @command{date} (@pxref{date invocation}).
10651 For example, @option{--time-style="+%Y-%m-%d %H:%M:%S"} causes
10652 @command{du} to list timestamps like @samp{2002-03-30 23:45:56}. As
10653 with @command{date}, @var{format}'s interpretation is affected by the
10654 @env{LC_TIME} locale category.
10657 List timestamps in full using @acronym{ISO} 8601 date, time, and time zone
10658 format with nanosecond precision, e.g., @samp{2002-03-30
10659 23:45:56.477817180 -0700}. This style is equivalent to
10660 @samp{+%Y-%m-%d %H:%M:%S.%N %z}.
10663 List @acronym{ISO} 8601 date and time in minutes, e.g.,
10664 @samp{2002-03-30 23:45}. These timestamps are shorter than
10665 @samp{full-iso} timestamps, and are usually good enough for everyday
10666 work. This style is equivalent to @samp{+%Y-%m-%d %H:%M}.
10669 List @acronym{ISO} 8601 dates for timestamps, e.g., @samp{2002-03-30}.
10670 This style is equivalent to @samp{+%Y-%m-%d}.
10674 You can specify the default value of the @option{--time-style} option
10675 with the environment variable @env{TIME_STYLE}; if @env{TIME_STYLE} is not set
10676 the default style is @samp{long-iso}. For compatibility with @command{ls},
10677 if @env{TIME_STYLE} begins with @samp{+} and contains a newline,
10678 the newline and any later characters are ignored; if @env{TIME_STYLE}
10679 begins with @samp{posix-} the @samp{posix-} is ignored; and if
10680 @env{TIME_STYLE} is @samp{locale} it is ignored.
10683 @itemx --one-file-system
10685 @opindex --one-file-system
10686 @cindex one file system, restricting @command{du} to
10687 Skip directories that are on different file systems from the one that
10688 the argument being processed is on.
10690 @item --exclude=@var{pattern}
10691 @opindex --exclude=@var{pattern}
10692 @cindex excluding files from @command{du}
10693 When recursing, skip subdirectories or files matching @var{pattern}.
10694 For example, @code{du --exclude='*.o'} excludes files whose names
10697 @item -X @var{file}
10698 @itemx --exclude-from=@var{file}
10699 @opindex -X @var{file}
10700 @opindex --exclude-from=@var{file}
10701 @cindex excluding files from @command{du}
10702 Like @option{--exclude}, except take the patterns to exclude from @var{file},
10703 one per line. If @var{file} is @samp{-}, take the patterns from standard
10708 @cindex NFS mounts from BSD to HP-UX
10709 On BSD systems, @command{du} reports sizes that are half the correct
10710 values for files that are NFS-mounted from HP-UX systems. On HP-UX
10711 systems, it reports sizes that are twice the correct values for
10712 files that are NFS-mounted from BSD systems. This is due to a flaw
10713 in HP-UX; it also affects the HP-UX @command{du} program.
10718 @node stat invocation
10719 @section @command{stat}: Report file or file system status
10722 @cindex file status
10723 @cindex file system status
10725 @command{stat} displays information about the specified file(s). Synopsis:
10728 stat [@var{option}]@dots{} [@var{file}]@dots{}
10731 With no option, @command{stat} reports all information about the given files.
10732 But it also can be used to report the information of the file systems the
10733 given files are located on. If the files are links, @command{stat} can
10734 also give information about the files the links point to.
10736 @mayConflictWithShellBuiltIn{stat}
10741 @itemx --dereference
10743 @opindex --dereference
10744 @cindex symbolic links, dereferencing in @command{stat}
10745 Change how @command{stat} treats symbolic links.
10746 With this option, @command{stat} acts on the file referenced
10747 by each symbolic link argument.
10748 Without it, @command{stat} acts on any symbolic link argument directly.
10751 @itemx --file-system
10753 @opindex --file-system
10754 @cindex file systems
10755 Report information about the file systems where the given files are located
10756 instead of information about the files themselves.
10757 This option implies the @option{-L} option.
10760 @itemx --format=@var{format}
10762 @opindex --format=@var{format}
10763 @cindex output format
10764 Use @var{format} rather than the default format.
10765 @var{format} is automatically newline-terminated, so
10766 running a command like the following with two or more @var{file}
10767 operands produces a line of output for each operand:
10769 $ stat --format=%d:%i / /usr
10774 @itemx --printf=@var{format}
10775 @opindex --printf=@var{format}
10776 @cindex output format
10777 Use @var{format} rather than the default format.
10778 Like @option{--format}, but interpret backslash escapes,
10779 and do not output a mandatory trailing newline.
10780 If you want a newline, include @samp{\n} in the @var{format}.
10781 Here's how you would use @option{--printf} to print the device
10782 and inode numbers of @file{/} and @file{/usr}:
10784 $ stat --printf='%d:%i\n' / /usr
10793 @cindex terse output
10794 Print the information in terse form, suitable for parsing by other programs.
10798 The valid @var{format} directives for files with @option{--format} and
10799 @option{--printf} are:
10802 @item %a - Access rights in octal
10803 @item %A - Access rights in human readable form
10804 @item %b - Number of blocks allocated (see @samp{%B})
10805 @item %B - The size in bytes of each block reported by @samp{%b}
10806 @item %C - The SELinux security context of a file, if available
10807 @item %d - Device number in decimal
10808 @item %D - Device number in hex
10809 @item %f - Raw mode in hex
10810 @item %F - File type
10811 @item %g - Group ID of owner
10812 @item %G - Group name of owner
10813 @item %h - Number of hard links
10814 @item %i - Inode number
10815 @item %m - Mount point (See note below)
10816 @item %n - File name
10817 @item %N - Quoted file name with dereference if symbolic link
10818 @item %o - I/O block size
10819 @item %s - Total size, in bytes
10820 @item %t - Major device type in hex
10821 @item %T - Minor device type in hex
10822 @item %u - User ID of owner
10823 @item %U - User name of owner
10824 @item %w - Time of file birth, or @samp{-} if unknown
10825 @item %W - Time of file birth as seconds since Epoch, or @samp{0}
10826 @item %x - Time of last access
10827 @item %X - Time of last access as seconds since Epoch
10828 @item %y - Time of last modification
10829 @item %Y - Time of last modification as seconds since Epoch
10830 @item %z - Time of last change
10831 @item %Z - Time of last change as seconds since Epoch
10834 The @samp{%W}, @samp{%X}, @samp{%Y}, and @samp{%Z} formats accept a
10835 precision preceded by a period to specify the number of digits to
10836 print after the decimal point. For example, @samp{%.3X} outputs the
10837 last access time to millisecond precision. If a period is given but no
10838 precision, @command{stat} uses 9 digits, so @samp{%.X} is equivalent to
10839 @samp{%.9X}. When discarding excess precision, time stamps are truncated
10840 toward minus infinity.
10844 $ stat -c '[%015Y]' /usr
10847 $ stat -c '[%15Y]' /usr
10849 $ stat -c '[%-15Y]' /usr
10852 $ stat -c '[%.3Y]' /usr
10854 $ stat -c '[%.Y]' /usr
10855 [1288929712.114951834]
10858 The mount point printed by @samp{%m} is similar to that output
10859 by @command{df}, except that:
10862 stat does not dereference symlinks by default
10863 (unless @option{-L} is specified)
10865 stat does not search for specified device nodes in the
10866 file system list, instead operating on them directly
10869 stat outputs the alias for a bind mounted file, rather than
10870 the initial mount point of its backing device.
10871 One can recursively call stat until there is no change in output,
10872 to get the current base mount point
10875 When listing file system information (@option{--file-system} (@option{-f})),
10876 you must use a different set of @var{format} directives:
10879 @item %a - Free blocks available to non-super-user
10880 @item %b - Total data blocks in file system
10881 @item %c - Total file nodes in file system
10882 @item %d - Free file nodes in file system
10883 @item %f - Free blocks in file system
10884 @item %i - File System ID in hex
10885 @item %l - Maximum length of file names
10886 @item %n - File name
10887 @item %s - Block size (for faster transfers)
10888 @item %S - Fundamental block size (for block counts)
10889 @item %t - Type in hex
10890 @item %T - Type in human readable form
10894 Time stamps are listed according to the time zone rules specified by
10895 the @env{TZ} environment variable, or by the system default rules if
10896 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
10897 with @env{TZ}, libc, The GNU C Library Reference Manual}.
10902 @node sync invocation
10903 @section @command{sync}: Synchronize data on disk with memory
10906 @cindex synchronize disk and memory
10908 @cindex superblock, writing
10909 @cindex inodes, written buffered
10910 @command{sync} writes any data buffered in memory out to disk. This can
10911 include (but is not limited to) modified superblocks, modified inodes,
10912 and delayed reads and writes. This must be implemented by the kernel;
10913 The @command{sync} program does nothing but exercise the @code{sync} system
10916 @cindex crashes and corruption
10917 The kernel keeps data in memory to avoid doing (relatively slow) disk
10918 reads and writes. This improves performance, but if the computer
10919 crashes, data may be lost or the file system corrupted as a
10920 result. The @command{sync} command ensures everything in memory
10921 is written to disk.
10923 Any arguments are ignored, except for a lone @option{--help} or
10924 @option{--version} (@pxref{Common options}).
10929 @node truncate invocation
10930 @section @command{truncate}: Shrink or extend the size of a file
10933 @cindex truncating, file sizes
10935 @command{truncate} shrinks or extends the size of each @var{file} to the
10936 specified size. Synopsis:
10939 truncate @var{option}@dots{} @var{file}@dots{}
10942 @cindex files, creating
10943 Any @var{file} that does not exist is created.
10945 @cindex sparse files, creating
10946 @cindex holes, creating files with
10947 If a @var{file} is larger than the specified size, the extra data is lost.
10948 If a @var{file} is shorter, it is extended and the extended part (or hole)
10949 reads as zero bytes.
10951 The program accepts the following options. Also see @ref{Common options}.
10958 @opindex --no-create
10959 Do not create files that do not exist.
10964 @opindex --io-blocks
10965 Treat @var{size} as number of I/O blocks of the @var{file} rather than bytes.
10967 @item -r @var{rfile}
10968 @itemx --reference=@var{rfile}
10970 @opindex --reference
10971 Base the size of each @var{file} on the size of @var{rfile}.
10973 @item -s @var{size}
10974 @itemx --size=@var{size}
10977 Set or adjust the size of each @var{file} according to @var{size}.
10978 @multiplierSuffixesNoBlocks{size}
10980 @var{size} may also be prefixed by one of the following to adjust
10981 the size of each @var{file} based on their current size:
10983 @samp{+} => extend by
10984 @samp{-} => reduce by
10985 @samp{<} => at most
10986 @samp{>} => at least
10987 @samp{/} => round down to multiple of
10988 @samp{%} => round up to multiple of
10996 @node Printing text
10997 @chapter Printing text
10999 @cindex printing text, commands for
11000 @cindex commands for printing text
11002 This section describes commands that display text strings.
11005 * echo invocation:: Print a line of text.
11006 * printf invocation:: Format and print data.
11007 * yes invocation:: Print a string until interrupted.
11011 @node echo invocation
11012 @section @command{echo}: Print a line of text
11015 @cindex displaying text
11016 @cindex printing text
11017 @cindex text, displaying
11018 @cindex arbitrary text, displaying
11020 @command{echo} writes each given @var{string} to standard output, with a
11021 space between each and a newline after the last one. Synopsis:
11024 echo [@var{option}]@dots{} [@var{string}]@dots{}
11027 @mayConflictWithShellBuiltIn{echo}
11029 The program accepts the following options. Also see @ref{Common options}.
11030 Options must precede operands, and the normally-special argument
11031 @samp{--} has no special meaning and is treated like any other
11037 Do not output the trailing newline.
11041 @cindex backslash escapes
11042 Enable interpretation of the following backslash-escaped characters in
11051 produce no further output
11067 the eight-bit value that is the octal number @var{nnn}
11068 (zero to three octal digits)
11070 the eight-bit value that is the octal number @var{nnn}
11071 (one to three octal digits)
11073 the eight-bit value that is the hexadecimal number @var{hh}
11074 (one or two hexadecimal digits)
11079 @cindex backslash escapes
11080 Disable interpretation of backslash escapes in each @var{string}.
11081 This is the default. If @option{-e} and @option{-E} are both
11082 specified, the last one given takes effect.
11086 @vindex POSIXLY_CORRECT
11087 If the @env{POSIXLY_CORRECT} environment variable is set, then when
11088 @command{echo}'s first argument is not @option{-n} it outputs
11089 option-like arguments instead of treating them as options. For
11090 example, @code{echo -ne hello} outputs @samp{-ne hello} instead of
11091 plain @samp{hello}.
11093 @acronym{POSIX} does not require support for any options, and says
11094 that the behavior of @command{echo} is implementation-defined if any
11095 @var{string} contains a backslash or if the first argument is
11096 @option{-n}. Portable programs can use the @command{printf} command
11097 if they need to omit trailing newlines or output control characters or
11098 backslashes. @xref{printf invocation}.
11103 @node printf invocation
11104 @section @command{printf}: Format and print data
11107 @command{printf} does formatted printing of text. Synopsis:
11110 printf @var{format} [@var{argument}]@dots{}
11113 @command{printf} prints the @var{format} string, interpreting @samp{%}
11114 directives and @samp{\} escapes to format numeric and string arguments
11115 in a way that is mostly similar to the C @samp{printf} function.
11116 @xref{Output Conversion Syntax,, @command{printf} format directives,
11117 libc, The GNU C Library Reference Manual}, for details.
11118 The differences are listed below.
11120 @mayConflictWithShellBuiltIn{printf}
11125 The @var{format} argument is reused as necessary to convert all the
11126 given @var{argument}s. For example, the command @samp{printf %s a b}
11130 Missing @var{argument}s are treated as null strings or as zeros,
11131 depending on whether the context expects a string or a number. For
11132 example, the command @samp{printf %sx%d} prints @samp{x0}.
11136 An additional escape, @samp{\c}, causes @command{printf} to produce no
11137 further output. For example, the command @samp{printf 'A%sC\cD%sF' B
11138 E} prints @samp{ABC}.
11141 The hexadecimal escape sequence @samp{\x@var{hh}} has at most two
11142 digits, as opposed to C where it can have an unlimited number of
11143 digits. For example, the command @samp{printf '\x07e'} prints two
11144 bytes, whereas the C statement @samp{printf ("\x07e")} prints just
11149 @command{printf} has an additional directive, @samp{%b}, which prints its
11150 argument string with @samp{\} escapes interpreted in the same way as in
11151 the @var{format} string, except that octal escapes are of the form
11152 @samp{\0@var{ooo}} where @var{ooo} is 0 to 3 octal digits.
11153 If a precision is also given, it limits the number of bytes printed
11154 from the converted string.
11157 Numeric arguments must be single C constants, possibly with leading
11158 @samp{+} or @samp{-}. For example, @samp{printf %.4d -3} outputs
11162 @vindex POSIXLY_CORRECT
11163 If the leading character of a numeric argument is @samp{"} or @samp{'}
11164 then its value is the numeric value of the immediately following
11165 character. Any remaining characters are silently ignored if the
11166 @env{POSIXLY_CORRECT} environment variable is set; otherwise, a
11167 warning is printed. For example, @samp{printf "%d" "'a"} outputs
11168 @samp{97} on hosts that use the @acronym{ASCII} character set, since
11169 @samp{a} has the numeric value 97 in @acronym{ASCII}.
11174 A floating-point argument must use a period before any fractional
11175 digits, but is printed according to the @env{LC_NUMERIC} category of the
11176 current locale. For example, in a locale whose radix character is a
11177 comma, the command @samp{printf %g 3.14} outputs @samp{3,14} whereas
11178 the command @samp{printf %g 3,14} is an error.
11182 @command{printf} interprets @samp{\@var{ooo}} in @var{format} as an octal number
11183 (if @var{ooo} is 1 to 3 octal digits) specifying a character to print,
11184 and @samp{\x@var{hh}} as a hexadecimal number (if @var{hh} is 1 to 2 hex
11185 digits) specifying a character to print.
11190 @cindex ISO/IEC 10646
11192 @command{printf} interprets two character syntaxes introduced in
11193 @acronym{ISO} C 99:
11194 @samp{\u} for 16-bit Unicode (@acronym{ISO}/@acronym{IEC} 10646)
11195 characters, specified as
11196 four hexadecimal digits @var{hhhh}, and @samp{\U} for 32-bit Unicode
11197 characters, specified as eight hexadecimal digits @var{hhhhhhhh}.
11198 @command{printf} outputs the Unicode characters
11199 according to the @env{LC_CTYPE} locale. Unicode characters in the ranges
11200 U+0000...U+009F, U+D800...U+DFFF cannot be specified by this syntax, except
11201 for U+0024 ($), U+0040 (@@), and U+0060 (@`).
11203 The processing of @samp{\u} and @samp{\U} requires a full-featured
11204 @code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
11205 or when @code{libiconv} is installed prior to this package. Otherwise
11206 @samp{\u} and @samp{\U} will print as-is.
11208 The only options are a lone @option{--help} or
11209 @option{--version}. @xref{Common options}.
11210 Options must precede operands.
11212 The Unicode character syntaxes are useful for writing strings in a locale
11213 independent way. For example, a string containing the Euro currency symbol
11216 $ env printf '\u20AC 14.95'
11220 will be output correctly in all locales supporting the Euro symbol
11221 (@acronym{ISO}-8859-15, UTF-8, and others). Similarly, a Chinese string
11224 $ env printf '\u4e2d\u6587'
11228 will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).
11230 Note that in these examples, the @command{printf} command has been
11231 invoked via @command{env} to ensure that we run the program found via
11232 your shell's search path, and not a shell alias or a built-in function.
11234 For larger strings, you don't need to look up the hexadecimal code
11235 values of each character one by one. @acronym{ASCII} characters mixed with \u
11236 escape sequences is also known as the JAVA source file encoding. You can
11237 use GNU recode 3.5c (or newer) to convert strings to this encoding. Here
11238 is how to convert a piece of text into a shell script which will output
11239 this text in a locale-independent way:
11242 $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
11243 '\u4e2d\u6587\n' > sample.txt
11244 $ recode BIG5..JAVA < sample.txt \
11245 | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
11252 @node yes invocation
11253 @section @command{yes}: Print a string until interrupted
11256 @cindex repeated output of a string
11258 @command{yes} prints the command line arguments, separated by spaces and
11259 followed by a newline, forever until it is killed. If no arguments are
11260 given, it prints @samp{y} followed by a newline forever until killed.
11262 Upon a write error, @command{yes} exits with status @samp{1}.
11264 The only options are a lone @option{--help} or @option{--version}.
11265 To output an argument that begins with
11266 @samp{-}, precede it with @option{--}, e.g., @samp{yes -- --help}.
11267 @xref{Common options}.
11271 @chapter Conditions
11274 @cindex commands for exit status
11275 @cindex exit status commands
11277 This section describes commands that are primarily useful for their exit
11278 status, rather than their output. Thus, they are often used as the
11279 condition of shell @code{if} statements, or as the last command in a
11283 * false invocation:: Do nothing, unsuccessfully.
11284 * true invocation:: Do nothing, successfully.
11285 * test invocation:: Check file types and compare values.
11286 * expr invocation:: Evaluate expressions.
11290 @node false invocation
11291 @section @command{false}: Do nothing, unsuccessfully
11294 @cindex do nothing, unsuccessfully
11295 @cindex failure exit status
11296 @cindex exit status of @command{false}
11298 @command{false} does nothing except return an exit status of 1, meaning
11299 @dfn{failure}. It can be used as a place holder in shell scripts
11300 where an unsuccessful command is needed.
11301 In most modern shells, @command{false} is a built-in command, so when
11302 you use @samp{false} in a script, you're probably using the built-in
11303 command, not the one documented here.
11305 @command{false} honors the @option{--help} and @option{--version} options.
11307 This version of @command{false} is implemented as a C program, and is thus
11308 more secure and faster than a shell script implementation, and may safely
11309 be used as a dummy shell for the purpose of disabling accounts.
11311 Note that @command{false} (unlike all other programs documented herein)
11312 exits unsuccessfully, even when invoked with
11313 @option{--help} or @option{--version}.
11315 Portable programs should not assume that the exit status of
11316 @command{false} is 1, as it is greater than 1 on some
11317 non-@acronym{GNU} hosts.
11320 @node true invocation
11321 @section @command{true}: Do nothing, successfully
11324 @cindex do nothing, successfully
11326 @cindex successful exit
11327 @cindex exit status of @command{true}
11329 @command{true} does nothing except return an exit status of 0, meaning
11330 @dfn{success}. It can be used as a place holder in shell scripts
11331 where a successful command is needed, although the shell built-in
11332 command @code{:} (colon) may do the same thing faster.
11333 In most modern shells, @command{true} is a built-in command, so when
11334 you use @samp{true} in a script, you're probably using the built-in
11335 command, not the one documented here.
11337 @command{true} honors the @option{--help} and @option{--version} options.
11339 Note, however, that it is possible to cause @command{true}
11340 to exit with nonzero status: with the @option{--help} or @option{--version}
11341 option, and with standard
11342 output already closed or redirected to a file that evokes an I/O error.
11343 For example, using a Bourne-compatible shell:
11346 $ ./true --version >&-
11347 ./true: write error: Bad file number
11348 $ ./true --version > /dev/full
11349 ./true: write error: No space left on device
11352 This version of @command{true} is implemented as a C program, and is thus
11353 more secure and faster than a shell script implementation, and may safely
11354 be used as a dummy shell for the purpose of disabling accounts.
11356 @node test invocation
11357 @section @command{test}: Check file types and compare values
11360 @cindex check file types
11361 @cindex compare values
11362 @cindex expression evaluation
11364 @command{test} returns a status of 0 (true) or 1 (false) depending on the
11365 evaluation of the conditional expression @var{expr}. Each part of the
11366 expression must be a separate argument.
11368 @command{test} has file status checks, string operators, and numeric
11369 comparison operators.
11371 @command{test} has an alternate form that uses opening and closing
11372 square brackets instead a leading @samp{test}. For example, instead
11373 of @samp{test -d /}, you can write @samp{[ -d / ]}. The square
11374 brackets must be separate arguments; for example, @samp{[-d /]} does
11375 not have the desired effect. Since @samp{test @var{expr}} and @samp{[
11376 @var{expr} ]} have the same meaning, only the former form is discussed
11382 test @var{expression}
11384 [ @var{expression} ]
11389 @mayConflictWithShellBuiltIn{test}
11391 If @var{expression} is omitted, @command{test} returns false.
11392 If @var{expression} is a single argument,
11393 @command{test} returns false if the argument is null and true otherwise. The argument
11394 can be any string, including strings like @samp{-d}, @samp{-1},
11395 @samp{--}, @samp{--help}, and @samp{--version} that most other
11396 programs would treat as options. To get help and version information,
11397 invoke the commands @samp{[ --help} and @samp{[ --version}, without
11398 the usual closing brackets. @xref{Common options}.
11400 @cindex exit status of @command{test}
11404 0 if the expression is true,
11405 1 if the expression is false,
11406 2 if an error occurred.
11410 * File type tests:: -[bcdfhLpSt]
11411 * Access permission tests:: -[gkruwxOG]
11412 * File characteristic tests:: -e -s -nt -ot -ef
11413 * String tests:: -z -n = !=
11414 * Numeric tests:: -eq -ne -lt -le -gt -ge
11415 * Connectives for test:: ! -a -o
11419 @node File type tests
11420 @subsection File type tests
11422 @cindex file type tests
11424 These options test for particular types of files. (Everything's a file,
11425 but not all files are the same!)
11429 @item -b @var{file}
11431 @cindex block special check
11432 True if @var{file} exists and is a block special device.
11434 @item -c @var{file}
11436 @cindex character special check
11437 True if @var{file} exists and is a character special device.
11439 @item -d @var{file}
11441 @cindex directory check
11442 True if @var{file} exists and is a directory.
11444 @item -f @var{file}
11446 @cindex regular file check
11447 True if @var{file} exists and is a regular file.
11449 @item -h @var{file}
11450 @itemx -L @var{file}
11453 @cindex symbolic link check
11454 True if @var{file} exists and is a symbolic link.
11455 Unlike all other file-related tests, this test does not dereference
11456 @var{file} if it is a symbolic link.
11458 @item -p @var{file}
11460 @cindex named pipe check
11461 True if @var{file} exists and is a named pipe.
11463 @item -S @var{file}
11465 @cindex socket check
11466 True if @var{file} exists and is a socket.
11470 @cindex terminal check
11471 True if @var{fd} is a file descriptor that is associated with a
11477 @node Access permission tests
11478 @subsection Access permission tests
11480 @cindex access permission tests
11481 @cindex permission tests
11483 These options test for particular access permissions.
11487 @item -g @var{file}
11489 @cindex set-group-ID check
11490 True if @var{file} exists and has its set-group-ID bit set.
11492 @item -k @var{file}
11494 @cindex sticky bit check
11495 True if @var{file} exists and has its @dfn{sticky} bit set.
11497 @item -r @var{file}
11499 @cindex readable file check
11500 True if @var{file} exists and read permission is granted.
11502 @item -u @var{file}
11504 @cindex set-user-ID check
11505 True if @var{file} exists and has its set-user-ID bit set.
11507 @item -w @var{file}
11509 @cindex writable file check
11510 True if @var{file} exists and write permission is granted.
11512 @item -x @var{file}
11514 @cindex executable file check
11515 True if @var{file} exists and execute permission is granted
11516 (or search permission, if it is a directory).
11518 @item -O @var{file}
11520 @cindex owned by effective user ID check
11521 True if @var{file} exists and is owned by the current effective user ID.
11523 @item -G @var{file}
11525 @cindex owned by effective group ID check
11526 True if @var{file} exists and is owned by the current effective group ID.
11530 @node File characteristic tests
11531 @subsection File characteristic tests
11533 @cindex file characteristic tests
11535 These options test other file characteristics.
11539 @item -e @var{file}
11541 @cindex existence-of-file check
11542 True if @var{file} exists.
11544 @item -s @var{file}
11546 @cindex nonempty file check
11547 True if @var{file} exists and has a size greater than zero.
11549 @item @var{file1} -nt @var{file2}
11551 @cindex newer-than file check
11552 True if @var{file1} is newer (according to modification date) than
11553 @var{file2}, or if @var{file1} exists and @var{file2} does not.
11555 @item @var{file1} -ot @var{file2}
11557 @cindex older-than file check
11558 True if @var{file1} is older (according to modification date) than
11559 @var{file2}, or if @var{file2} exists and @var{file1} does not.
11561 @item @var{file1} -ef @var{file2}
11563 @cindex same file check
11564 @cindex hard link check
11565 True if @var{file1} and @var{file2} have the same device and inode
11566 numbers, i.e., if they are hard links to each other.
11572 @subsection String tests
11574 @cindex string tests
11576 These options test string characteristics. You may need to quote
11577 @var{string} arguments for the shell. For example:
11583 The quotes here prevent the wrong arguments from being passed to
11584 @command{test} if @samp{$V} is empty or contains special characters.
11588 @item -z @var{string}
11590 @cindex zero-length string check
11591 True if the length of @var{string} is zero.
11593 @item -n @var{string}
11594 @itemx @var{string}
11596 @cindex nonzero-length string check
11597 True if the length of @var{string} is nonzero.
11599 @item @var{string1} = @var{string2}
11601 @cindex equal string check
11602 True if the strings are equal.
11604 @item @var{string1} != @var{string2}
11606 @cindex not-equal string check
11607 True if the strings are not equal.
11612 @node Numeric tests
11613 @subsection Numeric tests
11615 @cindex numeric tests
11616 @cindex arithmetic tests
11618 Numeric relational operators. The arguments must be entirely numeric
11619 (possibly negative), or the special expression @w{@code{-l @var{string}}},
11620 which evaluates to the length of @var{string}.
11624 @item @var{arg1} -eq @var{arg2}
11625 @itemx @var{arg1} -ne @var{arg2}
11626 @itemx @var{arg1} -lt @var{arg2}
11627 @itemx @var{arg1} -le @var{arg2}
11628 @itemx @var{arg1} -gt @var{arg2}
11629 @itemx @var{arg1} -ge @var{arg2}
11636 These arithmetic binary operators return true if @var{arg1} is equal,
11637 not-equal, less-than, less-than-or-equal, greater-than, or
11638 greater-than-or-equal than @var{arg2}, respectively.
11645 test -1 -gt -2 && echo yes
11647 test -l abc -gt 1 && echo yes
11650 @error{} test: integer expression expected before -eq
11654 @node Connectives for test
11655 @subsection Connectives for @command{test}
11657 @cindex logical connectives
11658 @cindex connectives, logical
11660 The usual logical connectives.
11666 True if @var{expr} is false.
11668 @item @var{expr1} -a @var{expr2}
11670 @cindex logical and operator
11671 @cindex and operator
11672 True if both @var{expr1} and @var{expr2} are true.
11674 @item @var{expr1} -o @var{expr2}
11676 @cindex logical or operator
11677 @cindex or operator
11678 True if either @var{expr1} or @var{expr2} is true.
11683 @node expr invocation
11684 @section @command{expr}: Evaluate expressions
11687 @cindex expression evaluation
11688 @cindex evaluation of expressions
11690 @command{expr} evaluates an expression and writes the result on standard
11691 output. Each token of the expression must be a separate argument.
11693 Operands are either integers or strings. Integers consist of one or
11694 more decimal digits, with an optional leading @samp{-}.
11695 @command{expr} converts
11696 anything appearing in an operand position to an integer or a string
11697 depending on the operation being applied to it.
11699 Strings are not quoted for @command{expr} itself, though you may need to
11700 quote them to protect characters with special meaning to the shell,
11701 e.g., spaces. However, regardless of whether it is quoted, a string
11702 operand should not be a parenthesis or any of @command{expr}'s
11703 operators like @code{+}, so you cannot safely pass an arbitrary string
11704 @code{$str} to expr merely by quoting it to the shell. One way to
11705 work around this is to use the @sc{gnu} extension @code{+},
11706 (e.g., @code{+ "$str" = foo}); a more portable way is to use
11707 @code{@w{" $str"}} and to adjust the rest of the expression to take
11708 the leading space into account (e.g., @code{@w{" $str" = " foo"}}).
11710 You should not pass a negative integer or a string with leading
11711 @samp{-} as @command{expr}'s first argument, as it might be
11712 misinterpreted as an option; this can be avoided by parenthesization.
11713 Also, portable scripts should not use a string operand that happens to
11714 take the form of an integer; this can be worked around by inserting
11715 leading spaces as mentioned above.
11717 @cindex parentheses for grouping
11718 Operators may be given as infix symbols or prefix keywords. Parentheses
11719 may be used for grouping in the usual manner. You must quote
11720 parentheses and many operators to avoid the shell evaluating them,
11723 When built with support for the GNU MP library, @command{expr} uses
11724 arbitrary-precision arithmetic; otherwise, it uses native arithmetic
11725 types and may fail due to arithmetic overflow.
11727 The only options are @option{--help} and @option{--version}. @xref{Common
11728 options}. Options must precede operands.
11730 @cindex exit status of @command{expr}
11734 0 if the expression is neither null nor 0,
11735 1 if the expression is null or 0,
11736 2 if the expression is invalid,
11737 3 if an internal error occurred (e.g., arithmetic overflow).
11741 * String expressions:: + : match substr index length
11742 * Numeric expressions:: + - * / %
11743 * Relations for expr:: | & < <= = == != >= >
11744 * Examples of expr:: Examples.
11748 @node String expressions
11749 @subsection String expressions
11751 @cindex string expressions
11752 @cindex expressions, string
11754 @command{expr} supports pattern matching and other string operators. These
11755 have higher precedence than both the numeric and relational operators (in
11756 the next sections).
11760 @item @var{string} : @var{regex}
11761 @cindex pattern matching
11762 @cindex regular expression matching
11763 @cindex matching patterns
11764 Perform pattern matching. The arguments are converted to strings and the
11765 second is considered to be a (basic, a la GNU @code{grep}) regular
11766 expression, with a @code{^} implicitly prepended. The first argument is
11767 then matched against this regular expression.
11769 If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
11770 @code{:} expression returns the part of @var{string} that matched the
11771 subexpression; otherwise, it returns the number of characters matched.
11773 If the match fails, the @code{:} operator returns the null string if
11774 @samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.
11776 @kindex \( @r{regexp operator}
11777 Only the first @samp{\( @dots{} \)} pair is relevant to the return
11778 value; additional pairs are meaningful only for grouping the regular
11779 expression operators.
11781 @kindex \+ @r{regexp operator}
11782 @kindex \? @r{regexp operator}
11783 @kindex \| @r{regexp operator}
11784 In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
11785 operators which respectively match one or more, zero or one, or separate
11786 alternatives. SunOS and other @command{expr}'s treat these as regular
11787 characters. (@acronym{POSIX} allows either behavior.)
11788 @xref{Top, , Regular Expression Library, regex, Regex}, for details of
11789 regular expression syntax. Some examples are in @ref{Examples of expr}.
11791 @item match @var{string} @var{regex}
11793 An alternative way to do pattern matching. This is the same as
11794 @w{@samp{@var{string} : @var{regex}}}.
11796 @item substr @var{string} @var{position} @var{length}
11798 Returns the substring of @var{string} beginning at @var{position}
11799 with length at most @var{length}. If either @var{position} or
11800 @var{length} is negative, zero, or non-numeric, returns the null string.
11802 @item index @var{string} @var{charset}
11804 Returns the first position in @var{string} where the first character in
11805 @var{charset} was found. If no character in @var{charset} is found in
11806 @var{string}, return 0.
11808 @item length @var{string}
11810 Returns the length of @var{string}.
11812 @item + @var{token}
11814 Interpret @var{token} as a string, even if it is a keyword like @var{match}
11815 or an operator like @code{/}.
11816 This makes it possible to test @code{expr length + "$x"} or
11817 @code{expr + "$x" : '.*/\(.\)'} and have it do the right thing even if
11818 the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
11819 This operator is a @acronym{GNU} extension. Portable shell scripts should use
11820 @code{@w{" $token"} : @w{' \(.*\)'}} instead of @code{+ "$token"}.
11824 To make @command{expr} interpret keywords as strings, you must use the
11825 @code{quote} operator.
11828 @node Numeric expressions
11829 @subsection Numeric expressions
11831 @cindex numeric expressions
11832 @cindex expressions, numeric
11834 @command{expr} supports the usual numeric operators, in order of increasing
11835 precedence. These numeric operators have lower precedence than the
11836 string operators described in the previous section, and higher precedence
11837 than the connectives (next section).
11845 @cindex subtraction
11846 Addition and subtraction. Both arguments are converted to integers;
11847 an error occurs if this cannot be done.
11853 @cindex multiplication
11856 Multiplication, division, remainder. Both arguments are converted to
11857 integers; an error occurs if this cannot be done.
11862 @node Relations for expr
11863 @subsection Relations for @command{expr}
11865 @cindex connectives, logical
11866 @cindex logical connectives
11867 @cindex relations, numeric or string
11869 @command{expr} supports the usual logical connectives and relations. These
11870 have lower precedence than the string and numeric operators
11871 (previous sections). Here is the list, lowest-precedence operator first.
11877 @cindex logical or operator
11878 @cindex or operator
11879 Returns its first argument if that is neither null nor zero, otherwise
11880 its second argument if it is neither null nor zero, otherwise 0. It
11881 does not evaluate its second argument if its first argument is neither
11886 @cindex logical and operator
11887 @cindex and operator
11888 Return its first argument if neither argument is null or zero, otherwise
11889 0. It does not evaluate its second argument if its first argument is
11892 @item < <= = == != >= >
11899 @cindex comparison operators
11901 Compare the arguments and return 1 if the relation is true, 0 otherwise.
11902 @code{==} is a synonym for @code{=}. @command{expr} first tries to convert
11903 both arguments to integers and do a numeric comparison; if either
11904 conversion fails, it does a lexicographic comparison using the character
11905 collating sequence specified by the @env{LC_COLLATE} locale.
11910 @node Examples of expr
11911 @subsection Examples of using @command{expr}
11913 @cindex examples of @command{expr}
11914 Here are a few examples, including quoting for shell metacharacters.
11916 To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
11919 foo=`expr $foo + 1`
11922 To print the non-directory part of the file name stored in
11923 @code{$fname}, which need not contain a @code{/}:
11926 expr $fname : '.*/\(.*\)' '|' $fname
11929 An example showing that @code{\+} is an operator:
11937 expr abc : 'a\(.\)c'
11939 expr index abcdef cz
11942 @error{} expr: syntax error
11943 expr index + index a
11949 @chapter Redirection
11951 @cindex redirection
11952 @cindex commands for redirection
11954 Unix shells commonly provide several forms of @dfn{redirection}---ways
11955 to change the input source or output destination of a command. But one
11956 useful redirection is performed by a separate command, not by the shell;
11957 it's described here.
11960 * tee invocation:: Redirect output to multiple files or processes.
11964 @node tee invocation
11965 @section @command{tee}: Redirect output to multiple files or processes
11968 @cindex pipe fitting
11969 @cindex destinations, multiple output
11970 @cindex read from stdin and write to stdout and files
11972 The @command{tee} command copies standard input to standard output and also
11973 to any files given as arguments. This is useful when you want not only
11974 to send some data down a pipe, but also to save a copy. Synopsis:
11977 tee [@var{option}]@dots{} [@var{file}]@dots{}
11980 If a file being written to does not already exist, it is created. If a
11981 file being written to already exists, the data it previously contained
11982 is overwritten unless the @option{-a} option is used.
11984 A @var{file} of @samp{-} causes @command{tee} to send another copy of
11985 input to standard output, but this is typically not that useful as the
11986 copies are interleaved.
11988 The program accepts the following options. Also see @ref{Common options}.
11995 Append standard input to the given files rather than overwriting
11999 @itemx --ignore-interrupts
12001 @opindex --ignore-interrupts
12002 Ignore interrupt signals.
12006 The @command{tee} command is useful when you happen to be transferring a large
12007 amount of data and also want to summarize that data without reading
12008 it a second time. For example, when you are downloading a DVD image,
12009 you often want to verify its signature or checksum right away.
12010 The inefficient way to do it is simply:
12013 wget http://example.com/some.iso && sha1sum some.iso
12016 One problem with the above is that it makes you wait for the
12017 download to complete before starting the time-consuming SHA1 computation.
12018 Perhaps even more importantly, the above requires reading
12019 the DVD image a second time (the first was from the network).
12021 The efficient way to do it is to interleave the download
12022 and SHA1 computation. Then, you'll get the checksum for
12023 free, because the entire process parallelizes so well:
12026 # slightly contrived, to demonstrate process substitution
12027 wget -O - http://example.com/dvd.iso \
12028 | tee >(sha1sum > dvd.sha1) > dvd.iso
12031 That makes @command{tee} write not just to the expected output file,
12032 but also to a pipe running @command{sha1sum} and saving the final
12033 checksum in a file named @file{dvd.sha1}.
12035 Note, however, that this example relies on a feature of modern shells
12036 called @dfn{process substitution}
12037 (the @samp{>(command)} syntax, above;
12038 @xref{Process Substitution,,Process Substitution, bashref,
12039 The Bash Reference Manual}.),
12040 so it works with @command{zsh}, @command{bash}, and @command{ksh},
12041 but not with @command{/bin/sh}. So if you write code like this
12042 in a shell script, be sure to start the script with @samp{#!/bin/bash}.
12044 Since the above example writes to one file and one process,
12045 a more conventional and portable use of @command{tee} is even better:
12048 wget -O - http://example.com/dvd.iso \
12049 | tee dvd.iso | sha1sum > dvd.sha1
12052 You can extend this example to make @command{tee} write to two processes,
12053 computing MD5 and SHA1 checksums in parallel. In this case,
12054 process substitution is required:
12057 wget -O - http://example.com/dvd.iso \
12058 | tee >(sha1sum > dvd.sha1) \
12059 >(md5sum > dvd.md5) \
12063 This technique is also useful when you want to make a @emph{compressed}
12064 copy of the contents of a pipe.
12065 Consider a tool to graphically summarize disk usage data from @samp{du -ak}.
12066 For a large hierarchy, @samp{du -ak} can run for a long time,
12067 and can easily produce terabytes of data, so you won't want to
12068 rerun the command unnecessarily. Nor will you want to save
12069 the uncompressed output.
12071 Doing it the inefficient way, you can't even start the GUI
12072 until after you've compressed all of the @command{du} output:
12075 du -ak | gzip -9 > /tmp/du.gz
12076 gzip -d /tmp/du.gz | xdiskusage -a
12079 With @command{tee} and process substitution, you start the GUI
12080 right away and eliminate the decompression completely:
12083 du -ak | tee >(gzip -9 > /tmp/du.gz) | xdiskusage -a
12086 Finally, if you regularly create more than one type of
12087 compressed tarball at once, for example when @code{make dist} creates
12088 both @command{gzip}-compressed and @command{bzip2}-compressed tarballs,
12089 there may be a better way.
12090 Typical @command{automake}-generated @file{Makefile} rules create
12091 the two compressed tar archives with commands in sequence, like this
12092 (slightly simplified):
12095 tardir=your-pkg-M.N
12096 tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz
12097 tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12100 However, if the hierarchy you are archiving and compressing is larger
12101 than a couple megabytes, and especially if you are using a multi-processor
12102 system with plenty of memory, then you can do much better by reading the
12103 directory contents only once and running the compression programs in parallel:
12106 tardir=your-pkg-M.N
12107 tar chof - "$tardir" \
12108 | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \
12109 | bzip2 -9 -c > your-pkg-M.N.tar.bz2
12115 @node File name manipulation
12116 @chapter File name manipulation
12118 @cindex file name manipulation
12119 @cindex manipulation of file names
12120 @cindex commands for file name manipulation
12122 This section describes commands that manipulate file names.
12125 * basename invocation:: Strip directory and suffix from a file name.
12126 * dirname invocation:: Strip last file name component.
12127 * pathchk invocation:: Check file name validity and portability.
12128 * mktemp invocation:: Create temporary file or directory.
12132 @node basename invocation
12133 @section @command{basename}: Strip directory and suffix from a file name
12136 @cindex strip directory and suffix from file names
12137 @cindex directory, stripping from file names
12138 @cindex suffix, stripping from file names
12139 @cindex file names, stripping directory and suffix
12140 @cindex leading directory components, stripping
12142 @command{basename} removes any leading directory components from
12143 @var{name}. Synopsis:
12146 basename @var{name} [@var{suffix}]
12149 If @var{suffix} is specified and is identical to the end of @var{name},
12150 it is removed from @var{name} as well. Note that since trailing slashes
12151 are removed prior to suffix matching, @var{suffix} will do nothing if it
12152 contains slashes. @command{basename} prints the result on standard
12155 @c This test is used both here and in the section on dirname.
12156 @macro basenameAndDirname
12157 Together, @command{basename} and @command{dirname} are designed such
12158 that if @samp{ls "$name"} succeeds, then the command sequence @samp{cd
12159 "$(dirname "$name")"; ls "$(basename "$name")"} will, too. This works
12160 for everything except file names containing a trailing newline.
12162 @basenameAndDirname
12164 @acronym{POSIX} allows the implementation to define the results if
12165 @var{name} is empty or @samp{//}. In the former case, @acronym{GNU}
12166 @command{basename} returns the empty string. In the latter case, the
12167 result is @samp{//} on platforms where @var{//} is distinct from
12168 @var{/}, and @samp{/} on platforms where there is no difference.
12170 The only options are @option{--help} and @option{--version}. @xref{Common
12171 options}. Options must precede operands.
12179 basename /usr/bin/sort
12182 basename include/stdio.h .h
12186 @node dirname invocation
12187 @section @command{dirname}: Strip last file name component
12190 @cindex directory components, printing
12191 @cindex stripping non-directory suffix
12192 @cindex non-directory suffix, stripping
12194 @command{dirname} prints all but the final slash-delimited component of
12195 @var{name}. Slashes on either side of the final component are also
12196 removed. If the string contains no slash, @command{dirname} prints
12197 @samp{.} (meaning the current directory). Synopsis:
12203 @var{name} need not be a file name, but if it is, this operation
12204 effectively lists the directory that contains the final component,
12205 including the case when the final component is itself a directory.
12207 @basenameAndDirname
12209 @acronym{POSIX} allows the implementation to define the results if
12210 @var{name} is @samp{//}. With @acronym{GNU} @command{dirname}, the
12211 result is @samp{//} on platforms where @var{//} is distinct from
12212 @var{/}, and @samp{/} on platforms where there is no difference.
12214 The only options are @option{--help} and @option{--version}. @xref{Common
12222 # Output "/usr/bin".
12223 dirname /usr/bin/sort
12224 dirname /usr/bin//.//
12231 @node pathchk invocation
12232 @section @command{pathchk}: Check file name validity and portability
12235 @cindex file names, checking validity and portability
12236 @cindex valid file names, checking for
12237 @cindex portable file names, checking for
12239 @command{pathchk} checks validity and portability of file names. Synopsis:
12242 pathchk [@var{option}]@dots{} @var{name}@dots{}
12245 For each @var{name}, @command{pathchk} prints an error message if any of
12246 these conditions is true:
12250 One of the existing directories in @var{name} does not have search
12251 (execute) permission,
12253 The length of @var{name} is larger than the maximum supported by the
12256 The length of one component of @var{name} is longer than
12257 its file system's maximum.
12260 A nonexistent @var{name} is not an error, so long a file with that
12261 name could be created under the above conditions.
12263 The program accepts the following options. Also see @ref{Common options}.
12264 Options must precede operands.
12270 Instead of performing checks based on the underlying file system,
12271 print an error message if any of these conditions is true:
12275 A file name is empty.
12278 A file name contains a character outside the @acronym{POSIX} portable file
12279 name character set, namely, the ASCII letters and digits, @samp{.},
12280 @samp{_}, @samp{-}, and @samp{/}.
12283 The length of a file name or one of its components exceeds the
12284 @acronym{POSIX} minimum limits for portability.
12289 Print an error message if a file name is empty, or if it contains a component
12290 that begins with @samp{-}.
12292 @item --portability
12293 @opindex --portability
12294 Print an error message if a file name is not portable to all @acronym{POSIX}
12295 hosts. This option is equivalent to @samp{-p -P}.
12299 @cindex exit status of @command{pathchk}
12303 0 if all specified file names passed all checks,
12307 @node mktemp invocation
12308 @section @command{mktemp}: Create temporary file or directory
12311 @cindex file names, creating temporary
12312 @cindex directory, creating temporary
12313 @cindex temporary files and directories
12315 @command{mktemp} manages the creation of temporary files and
12316 directories. Synopsis:
12319 mktemp [@var{option}]@dots{} [@var{template}]
12322 Safely create a temporary file or directory based on @var{template},
12323 and print its name. If given, @var{template} must include at least
12324 three consecutive @samp{X}s in the last component. If omitted, the template
12325 @samp{tmp.XXXXXXXXXX} is used, and option @option{--tmpdir} is
12326 implied. The final run of @samp{X}s in the @var{template} will be replaced
12327 by alpha-numeric characters; thus, on a case-sensitive file system,
12328 and with a @var{template} including a run of @var{n} instances of @samp{X},
12329 there are @samp{62**@var{n}} potential file names.
12331 Older scripts used to create temporary files by simply joining the
12332 name of the program with the process id (@samp{$$}) as a suffix.
12333 However, that naming scheme is easily predictable, and suffers from a
12334 race condition where the attacker can create an appropriately named
12335 symbolic link, such that when the script then opens a handle to what
12336 it thought was an unused file, it is instead modifying an existing
12337 file. Using the same scheme to create a directory is slightly safer,
12338 since the @command{mkdir} will fail if the target already exists, but
12339 it is still inferior because it allows for denial of service attacks.
12340 Therefore, modern scripts should use the @command{mktemp} command to
12341 guarantee that the generated name will be unpredictable, and that
12342 knowledge of the temporary file name implies that the file was created
12343 by the current script and cannot be modified by other users.
12345 When creating a file, the resulting file has read and write
12346 permissions for the current user, but no permissions for the group or
12347 others; these permissions are reduced if the current umask is more
12350 Here are some examples (although note that if you repeat them, you
12351 will most likely get different file names):
12356 Create a temporary file in the current directory.
12363 Create a temporary file with a known suffix.
12365 $ mktemp --suffix=.txt file-XXXX
12367 $ mktemp file-XXXX-XXXX.txt
12372 Create a secure fifo relative to the user's choice of @env{TMPDIR},
12373 but falling back to the current directory rather than @file{/tmp}.
12374 Note that @command{mktemp} does not create fifos, but can create a
12375 secure directory in which the fifo can live. Exit the shell if the
12376 directory or fifo could not be created.
12378 $ dir=$(mktemp -p "$@{TMPDIR:-.@}" -d dir-XXXX) || exit 1
12380 $ mkfifo "$fifo" || @{ rmdir "$dir"; exit 1; @}
12384 Create and use a temporary file if possible, but ignore failure. The
12385 file will reside in the directory named by @env{TMPDIR}, if specified,
12386 or else in @file{/tmp}.
12388 $ file=$(mktemp -q) && @{
12389 > # Safe to use $file only within this block. Use quotes,
12390 > # since $TMPDIR, and thus $file, may contain whitespace.
12391 > echo ... > "$file"
12397 Act as a semi-random character generator (it is not fully random,
12398 since it is impacted by the contents of the current directory). To
12399 avoid security holes, do not use the resulting names to create a file.
12409 The program accepts the following options. Also see @ref{Common options}.
12416 @opindex --directory
12417 Create a directory rather than a file. The directory will have read,
12418 write, and search permissions for the current user, but no permissions
12419 for the group or others; these permissions are reduced if the current
12420 umask is more restrictive.
12426 Suppress diagnostics about failure to create a file or directory. The
12427 exit status will still reflect whether a file was created.
12433 Generate a temporary name that does not name an existing file, without
12434 changing the file system contents. Using the output of this command
12435 to create a new file is inherently unsafe, as there is a window of
12436 time between generating the name and using it where another process
12437 can create an object by the same name.
12440 @itemx --tmpdir[=@var{dir}]
12443 Treat @var{template} relative to the directory @var{dir}. If
12444 @var{dir} is not specified (only possible with the long option
12445 @option{--tmpdir}) or is the empty string, use the value of
12446 @env{TMPDIR} if available, otherwise use @samp{/tmp}. If this is
12447 specified, @var{template} must not be absolute. However,
12448 @var{template} can still contain slashes, although intermediate
12449 directories must already exist.
12451 @item --suffix=@var{suffix}
12453 Append @var{suffix} to the @var{template}. @var{suffix} must not
12454 contain slash. If @option{--suffix} is specified, @var{template} must
12455 end in @samp{X}; if it is not specified, then an appropriate
12456 @option{--suffix} is inferred by finding the last @samp{X} in
12457 @var{template}. This option exists for use with the default
12458 @var{template} and for the creation of a @var{suffix} that starts with
12463 Treat @var{template} as a single file relative to the value of
12464 @env{TMPDIR} if available, or to the directory specified by
12465 @option{-p}, otherwise to @samp{/tmp}. @var{template} must not
12466 contain slashes. This option is deprecated; the use of @option{-p}
12467 without @option{-t} offers better defaults (by favoring the command
12468 line over @env{TMPDIR}) and more flexibility (by allowing intermediate
12473 @cindex exit status of @command{mktemp}
12477 0 if the file was created,
12482 @node Working context
12483 @chapter Working context
12485 @cindex working context
12486 @cindex commands for printing the working context
12488 This section describes commands that display or alter the context in
12489 which you are working: the current directory, the terminal settings, and
12490 so forth. See also the user-related commands in the next section.
12493 * pwd invocation:: Print working directory.
12494 * stty invocation:: Print or change terminal characteristics.
12495 * printenv invocation:: Print environment variables.
12496 * tty invocation:: Print file name of terminal on standard input.
12500 @node pwd invocation
12501 @section @command{pwd}: Print working directory
12504 @cindex print name of current directory
12505 @cindex current working directory, printing
12506 @cindex working directory, printing
12509 @command{pwd} prints the name of the current directory. Synopsis:
12512 pwd [@var{option}]@dots{}
12515 The program accepts the following options. Also see @ref{Common options}.
12522 If the contents of the environment variable @env{PWD} provide an
12523 absolute name of the current directory with no @samp{.} or @samp{..}
12524 components, but possibly with symbolic links, then output those
12525 contents. Otherwise, fall back to default @option{-P} handling.
12530 @opindex --physical
12531 Print a fully resolved name for the current directory. That is, all
12532 components of the printed name will be actual directory names---none
12533 will be symbolic links.
12536 @cindex symbolic links and @command{pwd}
12537 If @option{-L} and @option{-P} are both given, the last one takes
12538 precedence. If neither option is given, then this implementation uses
12539 @option{-P} as the default unless the @env{POSIXLY_CORRECT}
12540 environment variable is set.
12542 @mayConflictWithShellBuiltIn{pwd}
12547 @node stty invocation
12548 @section @command{stty}: Print or change terminal characteristics
12551 @cindex change or print terminal settings
12552 @cindex terminal settings
12553 @cindex line settings of terminal
12555 @command{stty} prints or changes terminal characteristics, such as baud rate.
12559 stty [@var{option}] [@var{setting}]@dots{}
12560 stty [@var{option}]
12563 If given no line settings, @command{stty} prints the baud rate, line
12564 discipline number (on systems that support it), and line settings
12565 that have been changed from the values set by @samp{stty sane}.
12566 By default, mode reading and setting are performed on the tty line
12567 connected to standard input, although this can be modified by the
12568 @option{--file} option.
12570 @command{stty} accepts many non-option arguments that change aspects of
12571 the terminal line operation, as described below.
12573 The program accepts the following options. Also see @ref{Common options}.
12580 Print all current settings in human-readable form. This option may not
12581 be used in combination with any line settings.
12583 @item -F @var{device}
12584 @itemx --file=@var{device}
12587 Set the line opened by the file name specified in @var{device} instead of
12588 the tty line connected to standard input. This option is necessary
12589 because opening a @acronym{POSIX} tty requires use of the @code{O_NONDELAY} flag to
12590 prevent a @acronym{POSIX} tty from blocking until the carrier detect line is high if
12591 the @code{clocal} flag is not set. Hence, it is not always possible
12592 to allow the shell to open the device in the traditional manner.
12598 @cindex machine-readable @command{stty} output
12599 Print all current settings in a form that can be used as an argument to
12600 another @command{stty} command to restore the current settings. This option
12601 may not be used in combination with any line settings.
12605 Many settings can be turned off by preceding them with a @samp{-}.
12606 Such arguments are marked below with ``May be negated'' in their
12607 description. The descriptions themselves refer to the positive
12608 case, that is, when @emph{not} negated (unless stated otherwise,
12611 Some settings are not available on all @acronym{POSIX} systems, since they use
12612 extensions. Such arguments are marked below with ``Non-@acronym{POSIX}'' in their
12613 description. On non-@acronym{POSIX} systems, those or other settings also may not
12614 be available, but it's not feasible to document all the variations: just
12620 * Control:: Control settings
12621 * Input:: Input settings
12622 * Output:: Output settings
12623 * Local:: Local settings
12624 * Combination:: Combination settings
12625 * Characters:: Special characters
12626 * Special:: Special settings
12631 @subsection Control settings
12633 @cindex control settings
12639 @cindex two-way parity
12640 Generate parity bit in output and expect parity bit in input.
12646 @cindex even parity
12647 Set odd parity (even if negated). May be negated.
12654 @cindex character size
12655 @cindex eight-bit characters
12656 Set character size to 5, 6, 7, or 8 bits.
12661 Send a hangup signal when the last process closes the tty. May be
12667 Use two stop bits per character (one if negated). May be negated.
12671 Allow input to be received. May be negated.
12675 @cindex modem control
12676 Disable modem control signals. May be negated.
12680 @cindex hardware flow control
12681 @cindex flow control, hardware
12682 @cindex RTS/CTS flow control
12683 Enable RTS/CTS flow control. Non-@acronym{POSIX}. May be negated.
12688 @subsection Input settings
12690 @cindex input settings
12691 These settings control operations on data received from the terminal.
12696 @cindex breaks, ignoring
12697 Ignore break characters. May be negated.
12701 @cindex breaks, cause interrupts
12702 Make breaks cause an interrupt signal. May be negated.
12706 @cindex parity, ignoring
12707 Ignore characters with parity errors. May be negated.
12711 @cindex parity errors, marking
12712 Mark parity errors (with a 255-0-character sequence). May be negated.
12716 Enable input parity checking. May be negated.
12720 @cindex eight-bit input
12721 Clear high (8th) bit of input characters. May be negated.
12725 @cindex newline, translating to return
12726 Translate newline to carriage return. May be negated.
12730 @cindex return, ignoring
12731 Ignore carriage return. May be negated.
12735 @cindex return, translating to newline
12736 Translate carriage return to newline. May be negated.
12740 @cindex input encoding, UTF-8
12741 Assume input characters are UTF-8 encoded. May be negated.
12745 @kindex C-s/C-q flow control
12746 @cindex XON/XOFF flow control
12747 Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}). May
12754 @cindex software flow control
12755 @cindex flow control, software
12756 Enable sending of @code{stop} character when the system input buffer
12757 is almost full, and @code{start} character when it becomes almost
12758 empty again. May be negated.
12762 @cindex uppercase, translating to lowercase
12763 Translate uppercase characters to lowercase. Non-@acronym{POSIX}. May be
12764 negated. Note ilcuc is not implemented, as one would not be able to issue
12765 almost any (lowercase) Unix command, after invoking it.
12769 Allow any character to restart output (only the start character
12770 if negated). Non-@acronym{POSIX}. May be negated.
12774 @cindex beeping at input buffer full
12775 Enable beeping and not flushing input buffer if a character arrives
12776 when the input buffer is full. Non-@acronym{POSIX}. May be negated.
12781 @subsection Output settings
12783 @cindex output settings
12784 These settings control operations on data sent to the terminal.
12789 Postprocess output. May be negated.
12793 @cindex lowercase, translating to output
12794 Translate lowercase characters to uppercase. Non-@acronym{POSIX}. May be
12795 negated. (Note ouclc is not currently implemented.)
12799 @cindex return, translating to newline
12800 Translate carriage return to newline. Non-@acronym{POSIX}. May be negated.
12804 @cindex newline, translating to crlf
12805 Translate newline to carriage return-newline. Non-@acronym{POSIX}. May be
12810 Do not print carriage returns in the first column. Non-@acronym{POSIX}.
12815 Newline performs a carriage return. Non-@acronym{POSIX}. May be negated.
12819 @cindex pad instead of timing for delaying
12820 Use fill (padding) characters instead of timing for delays. Non-@acronym{POSIX}.
12825 @cindex pad character
12826 Use @acronym{ASCII} @sc{del} characters for fill instead of
12827 @acronym{ASCII} @sc{nul} characters. Non-@acronym{POSIX}.
12833 Newline delay style. Non-@acronym{POSIX}.
12840 Carriage return delay style. Non-@acronym{POSIX}.
12846 @opindex tab@var{n}
12847 Horizontal tab delay style. Non-@acronym{POSIX}.
12852 Backspace delay style. Non-@acronym{POSIX}.
12857 Vertical tab delay style. Non-@acronym{POSIX}.
12862 Form feed delay style. Non-@acronym{POSIX}.
12867 @subsection Local settings
12869 @cindex local settings
12874 Enable @code{interrupt}, @code{quit}, and @code{suspend} special
12875 characters. May be negated.
12879 Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
12880 special characters. May be negated.
12884 Enable non-@acronym{POSIX} special characters. May be negated.
12888 Echo input characters. May be negated.
12894 Echo @code{erase} characters as backspace-space-backspace. May be
12899 @cindex newline echoing after @code{kill}
12900 Echo a newline after a @code{kill} character. May be negated.
12904 @cindex newline, echoing
12905 Echo newline even if not echoing other characters. May be negated.
12909 @cindex flushing, disabling
12910 Disable flushing after @code{interrupt} and @code{quit} special
12911 characters. May be negated.
12915 @cindex case translation
12916 Enable input and output of uppercase characters by preceding their
12917 lowercase equivalents with @samp{\}, when @code{icanon} is set.
12918 Non-@acronym{POSIX}. May be negated.
12922 @cindex background jobs, stopping at terminal write
12923 Stop background jobs that try to write to the terminal. Non-@acronym{POSIX}.
12930 Echo erased characters backward, between @samp{\} and @samp{/}.
12931 Non-@acronym{POSIX}. May be negated.
12937 @cindex control characters, using @samp{^@var{c}}
12938 @cindex hat notation for control characters
12939 Echo control characters in hat notation (@samp{^@var{c}}) instead
12940 of literally. Non-@acronym{POSIX}. May be negated.
12946 Echo the @code{kill} special character by erasing each character on
12947 the line as indicated by the @code{echoprt} and @code{echoe} settings,
12948 instead of by the @code{echoctl} and @code{echok} settings. Non-@acronym{POSIX}.
12954 @subsection Combination settings
12956 @cindex combination settings
12957 Combination settings:
12964 Same as @code{parenb -parodd cs7}. May be negated. If negated, same
12965 as @code{-parenb cs8}.
12969 Same as @code{parenb parodd cs7}. May be negated. If negated, same
12970 as @code{-parenb cs8}.
12974 Same as @code{-icrnl -onlcr}. May be negated. If negated, same as
12975 @code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.
12979 Reset the @code{erase} and @code{kill} special characters to their default
12986 @c This is too long to write inline.
12988 cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
12989 -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
12990 -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
12991 ff0 isig icanon iexten echo echoe echok -echonl
12992 -noflsh -xcase -tostop -echoprt echoctl echoke
12996 and also sets all special characters to their default values.
13000 Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
13001 sets the @code{eof} and @code{eol} characters to their default values
13002 if they are the same as the @code{min} and @code{time} characters.
13003 May be negated. If negated, same as @code{raw}.
13010 -ignbrk -brkint -ignpar -parmrk -inpck -istrip
13011 -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
13012 -imaxbel -opost -isig -icanon -xcase min 1 time 0
13016 May be negated. If negated, same as @code{cooked}.
13020 Same as @option{-icanon}. May be negated. If negated, same as
13025 @cindex eight-bit characters
13026 Same as @code{-parenb -istrip cs8}. May be negated. If negated,
13027 same as @code{parenb istrip cs7}.
13031 Same as @option{-parenb -istrip -opost cs8}. May be negated.
13032 If negated, same as @code{parenb istrip opost cs7}.
13036 Same as @option{-ixany}. Non-@acronym{POSIX}. May be negated.
13040 Same as @code{tab0}. Non-@acronym{POSIX}. May be negated. If negated, same
13047 Same as @code{xcase iuclc olcuc}. Non-@acronym{POSIX}. May be negated.
13048 (Used for terminals with uppercase characters only.)
13052 Same as @code{echoe echoctl echoke}.
13056 Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
13061 @subsection Special characters
13063 @cindex special characters
13064 @cindex characters, special
13066 The special characters' default values vary from system to system.
13067 They are set with the syntax @samp{name value}, where the names are
13068 listed below and the value can be given either literally, in hat
13069 notation (@samp{^@var{c}}), or as an integer which may start with
13070 @samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
13071 any other digit to indicate decimal.
13073 @cindex disabling special characters
13074 @kindex u@r{, and disabling special characters}
13075 For GNU stty, giving a value of @code{^-} or @code{undef} disables that
13076 special character. (This is incompatible with Ultrix @command{stty},
13077 which uses a value of @samp{u} to disable a special character. GNU
13078 @command{stty} treats a value @samp{u} like any other, namely to set that
13079 special character to @key{U}.)
13085 Send an interrupt signal.
13089 Send a quit signal.
13093 Erase the last character typed.
13097 Erase the current line.
13101 Send an end of file (terminate the input).
13109 Alternate character to end the line. Non-@acronym{POSIX}.
13113 Switch to a different shell layer. Non-@acronym{POSIX}.
13117 Restart the output after stopping it.
13125 Send a terminal stop signal.
13129 Send a terminal stop signal after flushing the input. Non-@acronym{POSIX}.
13133 Redraw the current line. Non-@acronym{POSIX}.
13137 Erase the last word typed. Non-@acronym{POSIX}.
13141 Enter the next character typed literally, even if it is a special
13142 character. Non-@acronym{POSIX}.
13147 @subsection Special settings
13149 @cindex special settings
13154 Set the minimum number of characters that will satisfy a read until
13155 the time value has expired, when @option{-icanon} is set.
13159 Set the number of tenths of a second before reads time out if the minimum
13160 number of characters have not been read, when @option{-icanon} is set.
13162 @item ispeed @var{n}
13164 Set the input speed to @var{n}.
13166 @item ospeed @var{n}
13168 Set the output speed to @var{n}.
13172 Tell the tty kernel driver that the terminal has @var{n} rows. Non-@acronym{POSIX}.
13175 @itemx columns @var{n}
13178 Tell the kernel that the terminal has @var{n} columns. Non-@acronym{POSIX}.
13184 Print the number of rows and columns that the kernel thinks the
13185 terminal has. (Systems that don't support rows and columns in the kernel
13186 typically use the environment variables @env{LINES} and @env{COLUMNS}
13187 instead; however, GNU @command{stty} does not know anything about them.)
13188 Non-@acronym{POSIX}.
13192 Use line discipline @var{n}. Non-@acronym{POSIX}.
13196 Print the terminal speed.
13199 @cindex baud rate, setting
13200 Set the input and output speeds to @var{n}. @var{n} can be one of: 0
13201 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200
13202 38400 @code{exta} @code{extb}. @code{exta} is the same as 19200;
13203 @code{extb} is the same as 38400. Many systems, including GNU/Linux,
13204 support higher speeds. The @command{stty} command includes support
13221 4000000 where the system supports these.
13222 0 hangs up the line if @option{-clocal} is set.
13226 @node printenv invocation
13227 @section @command{printenv}: Print all or some environment variables
13230 @cindex printing all or some environment variables
13231 @cindex environment variables, printing
13233 @command{printenv} prints environment variable values. Synopsis:
13236 printenv [@var{option}] [@var{variable}]@dots{}
13239 If no @var{variable}s are specified, @command{printenv} prints the value of
13240 every environment variable. Otherwise, it prints the value of each
13241 @var{variable} that is set, and nothing for those that are not set.
13243 The program accepts the following option. Also see @ref{Common options}.
13251 @cindex exit status of @command{printenv}
13255 0 if all variables specified were found
13256 1 if at least one specified variable was not found
13257 2 if a write error occurred
13261 @node tty invocation
13262 @section @command{tty}: Print file name of terminal on standard input
13265 @cindex print terminal file name
13266 @cindex terminal file name, printing
13268 @command{tty} prints the file name of the terminal connected to its standard
13269 input. It prints @samp{not a tty} if standard input is not a terminal.
13273 tty [@var{option}]@dots{}
13276 The program accepts the following option. Also see @ref{Common options}.
13286 Print nothing; only return an exit status.
13290 @cindex exit status of @command{tty}
13294 0 if standard input is a terminal
13295 1 if standard input is not a terminal
13296 2 if given incorrect arguments
13297 3 if a write error occurs
13301 @node User information
13302 @chapter User information
13304 @cindex user information, commands for
13305 @cindex commands for printing user information
13307 This section describes commands that print user-related information:
13308 logins, groups, and so forth.
13311 * id invocation:: Print user identity.
13312 * logname invocation:: Print current login name.
13313 * whoami invocation:: Print effective user ID.
13314 * groups invocation:: Print group names a user is in.
13315 * users invocation:: Print login names of users currently logged in.
13316 * who invocation:: Print who is currently logged in.
13320 @node id invocation
13321 @section @command{id}: Print user identity
13324 @cindex real user and group IDs, printing
13325 @cindex effective user and group IDs, printing
13326 @cindex printing real and effective user and group IDs
13328 @command{id} prints information about the given user, or the process
13329 running it if no user is specified. Synopsis:
13332 id [@var{option}]@dots{} [@var{username}]
13335 @vindex POSIXLY_CORRECT
13336 By default, it prints the real user ID, real group ID, effective user ID
13337 if different from the real user ID, effective group ID if different from
13338 the real group ID, and supplemental group IDs.
13339 In addition, if SELinux
13340 is enabled and the @env{POSIXLY_CORRECT} environment variable is not set,
13341 then print @samp{context=@var{c}}, where @var{c} is the security context.
13343 Each of these numeric values is preceded by an identifying string and
13344 followed by the corresponding user or group name in parentheses.
13346 The options cause @command{id} to print only part of the above information.
13347 Also see @ref{Common options}.
13354 Print only the group ID.
13360 Print only the group ID and the supplementary groups.
13366 Print the user or group name instead of the ID number. Requires
13367 @option{-u}, @option{-g}, or @option{-G}.
13373 Print the real, instead of effective, user or group ID. Requires
13374 @option{-u}, @option{-g}, or @option{-G}.
13380 Print only the user ID.
13387 @cindex security context
13388 Print only the security context of the current user.
13389 If SELinux is disabled then print a warning and
13390 set the exit status to 1.
13396 @macro primaryAndSupplementaryGroups{cmd,arg}
13397 Primary and supplementary groups for a process are normally inherited
13398 from its parent and are usually unchanged since login. This means
13399 that if you change the group database after logging in, @command{\cmd\}
13400 will not reflect your changes within your existing login session.
13401 Running @command{\cmd\} with a \arg\ causes the user and group
13402 database to be consulted afresh, and so will give a different result.
13404 @primaryAndSupplementaryGroups{id,user argument}
13406 @node logname invocation
13407 @section @command{logname}: Print current login name
13410 @cindex printing user's login name
13411 @cindex login name, printing
13412 @cindex user name, printing
13415 @command{logname} prints the calling user's name, as found in a
13416 system-maintained file (often @file{/var/run/utmp} or
13417 @file{/etc/utmp}), and exits with a status of 0. If there is no entry
13418 for the calling process, @command{logname} prints
13419 an error message and exits with a status of 1.
13421 The only options are @option{--help} and @option{--version}. @xref{Common
13427 @node whoami invocation
13428 @section @command{whoami}: Print effective user ID
13431 @cindex effective user ID, printing
13432 @cindex printing the effective user ID
13434 @command{whoami} prints the user name associated with the current
13435 effective user ID. It is equivalent to the command @samp{id -un}.
13437 The only options are @option{--help} and @option{--version}. @xref{Common
13443 @node groups invocation
13444 @section @command{groups}: Print group names a user is in
13447 @cindex printing groups a user is in
13448 @cindex supplementary groups, printing
13450 @command{groups} prints the names of the primary and any supplementary
13451 groups for each given @var{username}, or the current process if no names
13452 are given. If more than one name is given, the name of each user is
13454 the list of that user's groups and the user name is separated from the
13455 group list by a colon. Synopsis:
13458 groups [@var{username}]@dots{}
13461 The group lists are equivalent to the output of the command @samp{id -Gn}.
13463 @primaryAndSupplementaryGroups{groups,list of users}
13465 The only options are @option{--help} and @option{--version}. @xref{Common
13471 @node users invocation
13472 @section @command{users}: Print login names of users currently logged in
13475 @cindex printing current usernames
13476 @cindex usernames, printing current
13478 @cindex login sessions, printing users with
13479 @command{users} prints on a single line a blank-separated list of user
13480 names of users currently logged in to the current host. Each user name
13481 corresponds to a login session, so if a user has more than one login
13482 session, that user's name will appear the same number of times in the
13491 With no @var{file} argument, @command{users} extracts its information from
13492 a system-maintained file (often @file{/var/run/utmp} or
13493 @file{/etc/utmp}). If a file argument is given, @command{users} uses
13494 that file instead. A common choice is @file{/var/log/wtmp}.
13496 The only options are @option{--help} and @option{--version}. @xref{Common
13502 @node who invocation
13503 @section @command{who}: Print who is currently logged in
13506 @cindex printing current user information
13507 @cindex information, about current users
13509 @command{who} prints information about users who are currently logged on.
13513 @command{who} [@var{option}] [@var{file}] [am i]
13516 @cindex terminal lines, currently used
13518 @cindex remote hostname
13519 If given no non-option arguments, @command{who} prints the following
13520 information for each user currently logged on: login name, terminal
13521 line, login time, and remote hostname or X display.
13525 If given one non-option argument, @command{who} uses that instead of
13526 a default system-maintained file (often @file{/var/run/utmp} or
13527 @file{/etc/utmp}) as the name of the file containing the record of
13528 users logged on. @file{/var/log/wtmp} is commonly given as an argument
13529 to @command{who} to look at who has previously logged on.
13533 If given two non-option arguments, @command{who} prints only the entry
13534 for the user running it (determined from its standard input), preceded
13535 by the hostname. Traditionally, the two arguments given are @samp{am
13536 i}, as in @samp{who am i}.
13539 Time stamps are listed according to the time zone rules specified by
13540 the @env{TZ} environment variable, or by the system default rules if
13541 @env{TZ} is not set. @xref{TZ Variable,, Specifying the Time Zone
13542 with @env{TZ}, libc, The GNU C Library Reference Manual}.
13544 The program accepts the following options. Also see @ref{Common options}.
13552 Same as @samp{-b -d --login -p -r -t -T -u}.
13558 Print the date and time of last system boot.
13564 Print information corresponding to dead processes.
13570 Print a line of column headings.
13576 List only the entries that correspond to processes via which the
13577 system is waiting for a user to login. The user name is always @samp{LOGIN}.
13581 Attempt to canonicalize hostnames found in utmp through a DNS lookup. This
13582 is not the default because it can cause significant delays on systems with
13583 automatic dial-up internet access.
13587 Same as @samp{who am i}.
13593 List active processes spawned by init.
13599 Print only the login names and the number of users logged on.
13600 Overrides all other options.
13605 @opindex --runlevel
13606 Print the current (and maybe previous) run-level of the init process.
13610 Ignored; for compatibility with other versions of @command{who}.
13616 Print last system clock change.
13621 After the login time, print the number of hours and minutes that the
13622 user has been idle. @samp{.} means the user was active in the last minute.
13623 @samp{old} means the user has been idle for more than 24 hours.
13634 @opindex --writable
13635 @cindex message status
13636 @pindex write@r{, allowed}
13637 After each login name print a character indicating the user's message status:
13640 @samp{+} allowing @code{write} messages
13641 @samp{-} disallowing @code{write} messages
13642 @samp{?} cannot find terminal device
13650 @node System context
13651 @chapter System context
13653 @cindex system context
13654 @cindex context, system
13655 @cindex commands for system context
13657 This section describes commands that print or change system-wide
13661 * date invocation:: Print or set system date and time.
13662 * arch invocation:: Print machine hardware name.
13663 * nproc invocation:: Print the number of processors.
13664 * uname invocation:: Print system information.
13665 * hostname invocation:: Print or set system name.
13666 * hostid invocation:: Print numeric host identifier.
13667 * uptime invocation:: Print system uptime and load.
13670 @node date invocation
13671 @section @command{date}: Print or set system date and time
13674 @cindex time, printing or setting
13675 @cindex printing the current time
13680 date [@var{option}]@dots{} [+@var{format}]
13681 date [-u|--utc|--universal] @c this avoids a newline in the output
13682 [ MMDDhhmm[[CC]YY][.ss] ]
13686 Invoking @command{date} with no @var{format} argument is equivalent to invoking
13687 it with a default format that depends on the @env{LC_TIME} locale category.
13688 In the default C locale, this format is @samp{'+%a %b %e %H:%M:%S %Z %Y'},
13689 so the output looks like @samp{Thu Mar @ 3 13:47:51 PST 2005}.
13692 Normally, @command{date} uses the time zone rules indicated by the
13693 @env{TZ} environment variable, or the system default rules if @env{TZ}
13694 is not set. @xref{TZ Variable,, Specifying the Time Zone with
13695 @env{TZ}, libc, The GNU C Library Reference Manual}.
13697 @findex strftime @r{and @command{date}}
13698 @cindex time formats
13699 @cindex formatting times
13700 If given an argument that starts with a @samp{+}, @command{date} prints the
13701 current date and time (or the date and time specified by the
13702 @option{--date} option, see below) in the format defined by that argument,
13703 which is similar to that of the @code{strftime} function. Except for
13704 conversion specifiers, which start with @samp{%}, characters in the
13705 format string are printed unchanged. The conversion specifiers are
13711 * Time conversion specifiers:: %[HIklMNpPrRsSTXzZ]
13712 * Date conversion specifiers:: %[aAbBcCdDeFgGhjmuUVwWxyY]
13713 * Literal conversion specifiers:: %[%nt]
13714 * Padding and other flags:: Pad with zeros, spaces, etc.
13715 * Setting the time:: Changing the system clock.
13716 * Options for date:: Instead of the current time.
13718 * Date input formats:: Specifying date strings.
13720 * Examples of date:: Examples.
13723 @node Time conversion specifiers
13724 @subsection Time conversion specifiers
13726 @cindex time conversion specifiers
13727 @cindex conversion specifiers, time
13729 @command{date} conversion specifiers related to times.
13733 hour (@samp{00}@dots{}@samp{23})
13735 hour (@samp{01}@dots{}@samp{12})
13737 hour (@samp{ 0}@dots{}@samp{23}).
13738 This is a @acronym{GNU} extension.
13740 hour (@samp{ 1}@dots{}@samp{12}).
13741 This is a @acronym{GNU} extension.
13743 minute (@samp{00}@dots{}@samp{59})
13745 nanoseconds (@samp{000000000}@dots{}@samp{999999999}).
13746 This is a @acronym{GNU} extension.
13748 locale's equivalent of either @samp{AM} or @samp{PM};
13749 blank in many locales.
13750 Noon is treated as @samp{PM} and midnight as @samp{AM}.
13752 like @samp{%p}, except lower case.
13753 This is a @acronym{GNU} extension.
13755 locale's 12-hour clock time (e.g., @samp{11:11:04 PM})
13757 24-hour hour and minute. Same as @samp{%H:%M}.
13758 This is a @acronym{GNU} extension.
13760 @cindex epoch, seconds since
13761 @cindex seconds since the epoch
13762 @cindex beginning of time
13763 seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC.
13764 Leap seconds are not counted unless leap second support is available.
13765 @xref{%s-examples}, for examples.
13766 This is a @acronym{GNU} extension.
13768 second (@samp{00}@dots{}@samp{60}).
13769 This may be @samp{60} if leap seconds are supported.
13771 24-hour hour, minute, and second. Same as @samp{%H:%M:%S}.
13773 locale's time representation (e.g., @samp{23:13:48})
13775 @w{@acronym{RFC} 2822/@acronym{ISO} 8601} style numeric time zone
13776 (e.g., @samp{-0600} or @samp{+0530}), or nothing if no
13777 time zone is determinable. This value reflects the numeric time zone
13778 appropriate for the current time, using the time zone rules specified
13779 by the @env{TZ} environment variable.
13780 The time (and optionally, the time zone rules) can be overridden
13781 by the @option{--date} option.
13782 This is a @acronym{GNU} extension.
13784 @w{@acronym{RFC} 3339/@acronym{ISO} 8601} style numeric time zone with
13785 @samp{:} (e.g., @samp{-06:00} or @samp{+05:30}), or nothing if no time
13786 zone is determinable.
13787 This is a @acronym{GNU} extension.
13789 Numeric time zone to the nearest second with @samp{:} (e.g.,
13790 @samp{-06:00:00} or @samp{+05:30:00}), or nothing if no time zone is
13792 This is a @acronym{GNU} extension.
13794 Numeric time zone with @samp{:} using the minimum necessary precision
13795 (e.g., @samp{-06}, @samp{+05:30}, or @samp{-04:56:02}), or nothing if
13796 no time zone is determinable.
13797 This is a @acronym{GNU} extension.
13799 alphabetic time zone abbreviation (e.g., @samp{EDT}), or nothing if no
13800 time zone is determinable. See @samp{%z} for how it is determined.
13804 @node Date conversion specifiers
13805 @subsection Date conversion specifiers
13807 @cindex date conversion specifiers
13808 @cindex conversion specifiers, date
13810 @command{date} conversion specifiers related to dates.
13814 locale's abbreviated weekday name (e.g., @samp{Sun})
13816 locale's full weekday name, variable length (e.g., @samp{Sunday})
13818 locale's abbreviated month name (e.g., @samp{Jan})
13820 locale's full month name, variable length (e.g., @samp{January})
13822 locale's date and time (e.g., @samp{Thu Mar @ 3 23:05:25 2005})
13824 century. This is like @samp{%Y}, except the last two digits are omitted.
13825 For example, it is @samp{20} if @samp{%Y} is @samp{2000},
13826 and is @samp{-0} if @samp{%Y} is @samp{-001}.
13827 It is normally at least two characters, but it may be more.
13829 day of month (e.g., @samp{01})
13831 date; same as @samp{%m/%d/%y}
13833 day of month, space padded; same as @samp{%_d}
13835 full date in @acronym{ISO} 8601 format; same as @samp{%Y-%m-%d}.
13836 This is a good choice for a date format, as it is standard and
13837 is easy to sort in the usual case where years are in the range
13839 This is a @acronym{GNU} extension.
13841 year corresponding to the @acronym{ISO} week number, but without the century
13842 (range @samp{00} through @samp{99}). This has the same format and value
13843 as @samp{%y}, except that if the @acronym{ISO} week number (see
13845 to the previous or next year, that year is used instead.
13846 This is a @acronym{GNU} extension.
13848 year corresponding to the @acronym{ISO} week number. This has the
13849 same format and value as @samp{%Y}, except that if the @acronym{ISO}
13851 @samp{%V}) belongs to the previous or next year, that year is used
13853 It is normally useful only if @samp{%V} is also used;
13854 for example, the format @samp{%G-%m-%d} is probably a mistake,
13855 since it combines the ISO week number year with the conventional month and day.
13856 This is a @acronym{GNU} extension.
13860 day of year (@samp{001}@dots{}@samp{366})
13862 month (@samp{01}@dots{}@samp{12})
13864 day of week (@samp{1}@dots{}@samp{7}) with @samp{1} corresponding to Monday
13866 week number of year, with Sunday as the first day of the week
13867 (@samp{00}@dots{}@samp{53}).
13868 Days in a new year preceding the first Sunday are in week zero.
13870 @acronym{ISO} week number, that is, the
13871 week number of year, with Monday as the first day of the week
13872 (@samp{01}@dots{}@samp{53}).
13873 If the week containing January 1 has four or more days in
13874 the new year, then it is considered week 1; otherwise, it is week 53 of
13875 the previous year, and the next week is week 1. (See the @acronym{ISO} 8601
13878 day of week (@samp{0}@dots{}@samp{6}) with 0 corresponding to Sunday
13880 week number of year, with Monday as first day of week
13881 (@samp{00}@dots{}@samp{53}).
13882 Days in a new year preceding the first Monday are in week zero.
13884 locale's date representation (e.g., @samp{12/31/99})
13886 last two digits of year (@samp{00}@dots{}@samp{99})
13888 year. This is normally at least four characters, but it may be more.
13889 Year @samp{0000} precedes year @samp{0001}, and year @samp{-001}
13890 precedes year @samp{0000}.
13894 @node Literal conversion specifiers
13895 @subsection Literal conversion specifiers
13897 @cindex literal conversion specifiers
13898 @cindex conversion specifiers, literal
13900 @command{date} conversion specifiers that produce literal strings.
13912 @node Padding and other flags
13913 @subsection Padding and other flags
13915 @cindex numeric field padding
13916 @cindex padding of numeric fields
13917 @cindex fields, padding numeric
13919 Unless otherwise specified, @command{date} normally pads numeric fields
13920 with zeros, so that, for
13921 example, numeric months are always output as two digits.
13922 Seconds since the epoch are not padded, though,
13923 since there is no natural width for them.
13925 As a @acronym{GNU} extension, @command{date} recognizes any of the
13926 following optional flags after the @samp{%}:
13930 (hyphen) Do not pad the field; useful if the output is intended for
13933 (underscore) Pad with spaces; useful if you need a fixed
13934 number of characters in the output, but zeros are too distracting.
13936 (zero) Pad with zeros even if the conversion specifier
13937 would normally pad with spaces.
13939 Use upper case characters if possible.
13941 Use opposite case characters if possible.
13942 A field that is normally upper case becomes lower case, and vice versa.
13946 Here are some examples of padding:
13949 date +%d/%m -d "Feb 1"
13951 date +%-d/%-m -d "Feb 1"
13953 date +%_d/%_m -d "Feb 1"
13957 As a @acronym{GNU} extension, you can specify the field width
13958 (after any flag, if present) as a decimal number. If the natural size of the
13959 output of the field has less than the specified number of characters,
13960 the result is written right adjusted and padded to the given
13961 size. For example, @samp{%9B} prints the right adjusted month name in
13962 a field of width 9.
13964 An optional modifier can follow the optional flag and width
13965 specification. The modifiers are:
13969 Use the locale's alternate representation for date and time. This
13970 modifier applies to the @samp{%c}, @samp{%C}, @samp{%x}, @samp{%X},
13971 @samp{%y} and @samp{%Y} conversion specifiers. In a Japanese locale, for
13972 example, @samp{%Ex} might yield a date format based on the Japanese
13976 Use the locale's alternate numeric symbols for numbers. This modifier
13977 applies only to numeric conversion specifiers.
13980 If the format supports the modifier but no alternate representation
13981 is available, it is ignored.
13984 @node Setting the time
13985 @subsection Setting the time
13987 @cindex setting the time
13988 @cindex time setting
13989 @cindex appropriate privileges
13991 If given an argument that does not start with @samp{+}, @command{date} sets
13992 the system clock to the date and time specified by that argument (as
13993 described below). You must have appropriate privileges to set the
13994 system clock. The @option{--date} and @option{--set} options may not be
13995 used with such an argument. The @option{--universal} option may be used
13996 with such an argument to indicate that the specified date and time are
13997 relative to Coordinated Universal Time rather than to the local time
14000 The argument must consist entirely of digits, which have the following
14013 first two digits of year (optional)
14015 last two digits of year (optional)
14020 The @option{--set} option also sets the system clock; see the next section.
14023 @node Options for date
14024 @subsection Options for @command{date}
14026 @cindex @command{date} options
14027 @cindex options for @command{date}
14029 The program accepts the following options. Also see @ref{Common options}.
14033 @item -d @var{datestr}
14034 @itemx --date=@var{datestr}
14037 @cindex parsing date strings
14038 @cindex date strings, parsing
14039 @cindex arbitrary date strings, parsing
14042 @opindex next @var{day}
14043 @opindex last @var{day}
14044 Display the date and time specified in @var{datestr} instead of the
14045 current date and time. @var{datestr} can be in almost any common
14046 format. It can contain month names, time zones, @samp{am} and @samp{pm},
14047 @samp{yesterday}, etc. For example, @option{--date="2004-02-27
14048 14:19:13.489392193 +0530"} specifies the instant of time that is
14049 489,392,193 nanoseconds after February 27, 2004 at 2:19:13 PM in a
14050 time zone that is 5 hours and 30 minutes east of @acronym{UTC}.@*
14051 Note: input currently must be in locale independent format. E.g., the
14052 LC_TIME=C below is needed to print back the correct date in many locales:
14054 date -d "$(LC_TIME=C date)"
14056 @xref{Date input formats}.
14058 @item -f @var{datefile}
14059 @itemx --file=@var{datefile}
14062 Parse each line in @var{datefile} as with @option{-d} and display the
14063 resulting date and time. If @var{datefile} is @samp{-}, use standard
14064 input. This is useful when you have many dates to process, because the
14065 system overhead of starting up the @command{date} executable many times can
14068 @item -r @var{file}
14069 @itemx --reference=@var{file}
14071 @opindex --reference
14072 Display the date and time of the last modification of @var{file},
14073 instead of the current date and time.
14080 @opindex --rfc-2822
14081 Display the date and time using the format @samp{%a, %d %b %Y %H:%M:%S
14082 %z}, evaluated in the C locale so abbreviations are always in English.
14086 Fri, 09 Sep 2005 13:51:39 -0700
14089 This format conforms to
14090 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc2822.txt, Internet
14091 @acronym{RFCs} 2822} and
14092 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc822.txt, 822}, the
14093 current and previous standards for Internet email.
14095 @item --rfc-3339=@var{timespec}
14096 @opindex --rfc-3339=@var{timespec}
14097 Display the date using a format specified by
14098 @uref{ftp://ftp.rfc-editor.org/in-notes/rfc3339.txt, Internet
14099 @acronym{RFC} 3339}. This is a subset of the @acronym{ISO} 8601
14100 format, except that it also permits applications to use a space rather
14101 than a @samp{T} to separate dates from times. Unlike the other
14102 standard formats, @acronym{RFC} 3339 format is always suitable as
14103 input for the @option{--date} (@option{-d}) and @option{--file}
14104 (@option{-f}) options, regardless of the current locale.
14106 The argument @var{timespec} specifies how much of the time to include.
14107 It can be one of the following:
14111 Print just the full-date, e.g., @samp{2005-09-14}.
14112 This is equivalent to the format @samp{%Y-%m-%d}.
14115 Print the full-date and full-time separated by a space, e.g.,
14116 @samp{2005-09-14 00:56:06+05:30}. The output ends with a numeric
14117 time-offset; here the @samp{+05:30} means that local time is five
14118 hours and thirty minutes east of @acronym{UTC}. This is equivalent to
14119 the format @samp{%Y-%m-%d %H:%M:%S%:z}.
14122 Like @samp{seconds}, but also print nanoseconds, e.g.,
14123 @samp{2005-09-14 00:56:06.998458565+05:30}.
14124 This is equivalent to the format @samp{%Y-%m-%d %H:%M:%S.%N%:z}.
14128 @item -s @var{datestr}
14129 @itemx --set=@var{datestr}
14132 Set the date and time to @var{datestr}. See @option{-d} above.
14139 @opindex --universal
14140 @cindex Coordinated Universal Time
14142 @cindex Greenwich Mean Time
14145 Use Coordinated Universal Time (@acronym{UTC}) by operating as if the
14146 @env{TZ} environment variable were set to the string @samp{UTC0}.
14148 Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
14149 historical reasons.
14153 @node Examples of date
14154 @subsection Examples of @command{date}
14156 @cindex examples of @command{date}
14158 Here are a few examples. Also see the documentation for the @option{-d}
14159 option in the previous section.
14164 To print the date of the day before yesterday:
14167 date --date='2 days ago'
14171 To print the date of the day three months and one day hence:
14174 date --date='3 months 1 day'
14178 To print the day of year of Christmas in the current year:
14181 date --date='25 Dec' +%j
14185 To print the current full month name and the day of the month:
14191 But this may not be what you want because for the first nine days of
14192 the month, the @samp{%d} expands to a zero-padded two-digit field,
14193 for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.
14196 To print a date without the leading zero for one-digit days
14197 of the month, you can use the (@acronym{GNU} extension)
14198 @samp{-} flag to suppress
14199 the padding altogether:
14202 date -d 1may '+%B %-d
14206 To print the current date and time in the format required by many
14207 non-@acronym{GNU} versions of @command{date} when setting the system clock:
14210 date +%m%d%H%M%Y.%S
14214 To set the system clock forward by two minutes:
14217 date --set='+2 minutes'
14221 To print the date in @acronym{RFC} 2822 format,
14222 use @samp{date --rfc-2822}. Here is some example output:
14225 Fri, 09 Sep 2005 13:51:39 -0700
14228 @anchor{%s-examples}
14230 To convert a date string to the number of seconds since the epoch
14231 (which is 1970-01-01 00:00:00 UTC), use the @option{--date} option with
14232 the @samp{%s} format. That can be useful in sorting and/or graphing
14233 and/or comparing data by date. The following command outputs the
14234 number of the seconds since the epoch for the time two minutes after the
14238 date --date='1970-01-01 00:02:00 +0000' +%s
14242 If you do not specify time zone information in the date string,
14243 @command{date} uses your computer's idea of the time zone when
14244 interpreting the string. For example, if your computer's time zone is
14245 that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000
14246 seconds) behind UTC:
14249 # local time zone used
14250 date --date='1970-01-01 00:02:00' +%s
14255 If you're sorting or graphing dated data, your raw date values may be
14256 represented as seconds since the epoch. But few people can look at
14257 the date @samp{946684800} and casually note ``Oh, that's the first second
14258 of the year 2000 in Greenwich, England.''
14261 date --date='2000-01-01 UTC' +%s
14265 An alternative is to use the @option{--utc} (@option{-u}) option.
14266 Then you may omit @samp{UTC} from the date string. Although this
14267 produces the same result for @samp{%s} and many other format sequences,
14268 with a time zone offset different from zero, it would give a different
14269 result for zone-dependent formats like @samp{%z}.
14272 date -u --date=2000-01-01 +%s
14276 To convert such an unwieldy number of seconds back to
14277 a more readable form, use a command like this:
14280 # local time zone used
14281 date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
14282 1999-12-31 19:00:00 -0500
14285 Or if you do not mind depending on the @samp{@@} feature present since
14286 coreutils 5.3.0, you could shorten this to:
14289 date -d @@946684800 +"%F %T %z"
14290 1999-12-31 19:00:00 -0500
14293 Often it is better to output UTC-relative date and time:
14296 date -u -d '1970-01-01 946684800 seconds' +"%Y-%m-%d %T %z"
14297 2000-01-01 00:00:00 +0000
14303 @node arch invocation
14304 @section @command{arch}: Print machine hardware name
14307 @cindex print machine hardware name
14308 @cindex system information, printing
14310 @command{arch} prints the machine hardware name,
14311 and is equivalent to @samp{uname -m}.
14315 arch [@var{option}]
14318 The program accepts the @ref{Common options} only.
14323 @node nproc invocation
14324 @section @command{nproc}: Print the number of available processors
14327 @cindex Print the number of processors
14328 @cindex system information, printing
14330 Print the number of processing units available to the current process,
14331 which may be less than the number of online processors.
14332 If this information is not accessible, then print the number of
14333 processors installed. If the @env{OMP_NUM_THREADS} environment variable is
14334 set, then it will determine the returned value. The result is guaranteed to be
14335 greater than zero. Synopsis:
14338 nproc [@var{option}]
14341 The program accepts the following options. Also see @ref{Common options}.
14347 Print the number of installed processors on the system, which may
14348 be greater than the number online or available to the current process.
14349 The @env{OMP_NUM_THREADS} environment variable is not honored in this case.
14351 @item --ignore=@var{number}
14353 If possible, exclude this @var{number} of processing units.
14360 @node uname invocation
14361 @section @command{uname}: Print system information
14364 @cindex print system information
14365 @cindex system information, printing
14367 @command{uname} prints information about the machine and operating system
14368 it is run on. If no options are given, @command{uname} acts as if the
14369 @option{-s} option were given. Synopsis:
14372 uname [@var{option}]@dots{}
14375 If multiple options or @option{-a} are given, the selected information is
14376 printed in this order:
14379 @var{kernel-name} @var{nodename} @var{kernel-release} @var{kernel-version}
14380 @var{machine} @var{processor} @var{hardware-platform} @var{operating-system}
14383 The information may contain internal spaces, so such output cannot be
14384 parsed reliably. In the following example, @var{release} is
14385 @samp{2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001}:
14389 @result{} Linux dum 2.2.18 #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux
14393 The program accepts the following options. Also see @ref{Common options}.
14401 Print all of the below information, except omit the processor type
14402 and the hardware platform name if they are unknown.
14405 @itemx --hardware-platform
14407 @opindex --hardware-platform
14408 @cindex implementation, hardware
14409 @cindex hardware platform
14410 @cindex platform, hardware
14411 Print the hardware platform name
14412 (sometimes called the hardware implementation).
14413 Print @samp{unknown} if the kernel does not make this information
14414 easily available, as is the case with Linux kernels.
14420 @cindex machine type
14421 @cindex hardware class
14422 @cindex hardware type
14423 Print the machine hardware name (sometimes called the hardware class
14429 @opindex --nodename
14432 @cindex network node name
14433 Print the network node hostname.
14438 @opindex --processor
14439 @cindex host processor type
14440 Print the processor type (sometimes called the instruction set
14441 architecture or ISA).
14442 Print @samp{unknown} if the kernel does not make this information
14443 easily available, as is the case with Linux kernels.
14446 @itemx --operating-system
14448 @opindex --operating-system
14449 @cindex operating system name
14450 Print the name of the operating system.
14453 @itemx --kernel-release
14455 @opindex --kernel-release
14456 @cindex kernel release
14457 @cindex release of kernel
14458 Print the kernel release.
14461 @itemx --kernel-name
14463 @opindex --kernel-name
14464 @cindex kernel name
14465 @cindex name of kernel
14466 Print the kernel name.
14467 @acronym{POSIX} 1003.1-2001 (@pxref{Standards conformance}) calls this
14468 ``the implementation of the operating system'', because the
14469 @acronym{POSIX} specification itself has no notion of ``kernel''.
14470 The kernel name might be the same as the operating system name printed
14471 by the @option{-o} or @option{--operating-system} option, but it might
14472 differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same
14473 name as their underlying kernels; others (e.g., GNU/Linux, Solaris)
14477 @itemx --kernel-version
14479 @opindex --kernel-version
14480 @cindex kernel version
14481 @cindex version of kernel
14482 Print the kernel version.
14489 @node hostname invocation
14490 @section @command{hostname}: Print or set system name
14493 @cindex setting the hostname
14494 @cindex printing the hostname
14495 @cindex system name, printing
14496 @cindex appropriate privileges
14498 With no arguments, @command{hostname} prints the name of the current host
14499 system. With one argument, it sets the current host name to the
14500 specified string. You must have appropriate privileges to set the host
14504 hostname [@var{name}]
14507 The only options are @option{--help} and @option{--version}. @xref{Common
14513 @node hostid invocation
14514 @section @command{hostid}: Print numeric host identifier
14517 @cindex printing the host identifier
14519 @command{hostid} prints the numeric identifier of the current host
14520 in hexadecimal. This command accepts no arguments.
14521 The only options are @option{--help} and @option{--version}.
14522 @xref{Common options}.
14524 For example, here's what it prints on one system I use:
14531 On that system, the 32-bit quantity happens to be closely
14532 related to the system's Internet address, but that isn't always
14537 @node uptime invocation
14538 @section @command{uptime}: Print system uptime and load
14541 @cindex printing the system uptime and load
14543 @command{uptime} prints the current time, the system's uptime, the
14544 number of logged-in users and the current load average.
14546 If an argument is specified, it is used as the file to be read
14547 to discover how many users are logged in. If no argument is
14548 specified, a system default is used (@command{uptime --help} indicates
14549 the default setting).
14551 The only options are @option{--help} and @option{--version}.
14552 @xref{Common options}.
14554 For example, here's what it prints right now on one system I use:
14558 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04
14561 The precise method of calculation of load average varies somewhat
14562 between systems. Some systems calculate it as the average number of
14563 runnable processes over the last 1, 5 and 15 minutes, but some systems
14564 also include processes in the uninterruptible sleep state (that is,
14565 those processes which are waiting for disk I/O). The Linux kernel
14566 includes uninterruptible processes.
14568 @node SELinux context
14569 @chapter SELinux context
14571 @cindex SELinux context
14572 @cindex SELinux, context
14573 @cindex commands for SELinux context
14575 This section describes commands for operations with SELinux
14579 * chcon invocation:: Change SELinux context of file
14580 * runcon invocation:: Run a command in specified SELinux context
14583 @node chcon invocation
14584 @section @command{chcon}: Change SELinux context of file
14587 @cindex changing security context
14588 @cindex change SELinux context
14590 @command{chcon} changes the SELinux security context of the selected files.
14594 chcon [@var{option}]@dots{} @var{context} @var{file}@dots{}
14595 chcon [@var{option}]@dots{} [-u @var{user}] [-r @var{role}] [-l @var{range}] [-t @var{type}] @var{file}@dots{}
14596 chcon [@var{option}]@dots{} --reference=@var{rfile} @var{file}@dots{}
14599 Change the SELinux security context of each @var{file} to @var{context}.
14600 With @option{--reference}, change the security context of each @var{file}
14601 to that of @var{rfile}.
14603 The program accepts the following options. Also see @ref{Common options}.
14608 @itemx --no-dereference
14610 @opindex --no-dereference
14611 @cindex no dereference
14612 Affect symbolic links instead of any referenced file.
14614 @item --reference=@var{rfile}
14615 @opindex --reference
14616 @cindex reference file
14617 Use @var{rfile}'s security context rather than specifying a @var{context} value.
14622 @opindex --recursive
14623 Operate on files and directories recursively.
14626 @xref{Traversing symlinks}.
14629 @xref{Traversing symlinks}.
14632 @xref{Traversing symlinks}.
14639 Output a diagnostic for every file processed.
14641 @item -u @var{user}
14642 @itemx --user=@var{user}
14645 Set user @var{user} in the target security context.
14647 @item -r @var{role}
14648 @itemx --role=@var{role}
14651 Set role @var{role} in the target security context.
14653 @item -t @var{type}
14654 @itemx --type=@var{type}
14657 Set type @var{type} in the target security context.
14659 @item -l @var{range}
14660 @itemx --range=@var{range}
14663 Set range @var{range} in the target security context.
14669 @node runcon invocation
14670 @section @command{runcon}: Run a command in specified SELinux context
14673 @cindex run with security context
14676 @command{runcon} runs file in specified SELinux security context.
14680 runcon @var{context} @var{command} [@var{args}]
14681 runcon [ -c ] [-u @var{user}] [-r @var{role}] [-t @var{type}] [-l @var{range}] @var{command} [@var{args}]
14684 Run @var{command} with completely-specified @var{context}, or with
14685 current or transitioned security context modified by one or more of @var{level},
14686 @var{role}, @var{type} and @var{user}.
14688 If none of @option{-c}, @option{-t}, @option{-u}, @option{-r}, or @option{-l}
14689 is specified, the first argument is used as the complete context.
14690 Any additional arguments after @var{command}
14691 are interpreted as arguments to the command.
14693 With neither @var{context} nor @var{command}, print the current security context.
14695 The program accepts the following options. Also see @ref{Common options}.
14703 Compute process transition context before modifying.
14705 @item -u @var{user}
14706 @itemx --user=@var{user}
14709 Set user @var{user} in the target security context.
14711 @item -r @var{role}
14712 @itemx --role=@var{role}
14715 Set role @var{role} in the target security context.
14717 @item -t @var{type}
14718 @itemx --type=@var{type}
14721 Set type @var{type} in the target security context.
14723 @item -l @var{range}
14724 @itemx --range=@var{range}
14727 Set range @var{range} in the target security context.
14731 @cindex exit status of @command{runcon}
14735 126 if @var{command} is found but cannot be invoked
14736 127 if @command{runcon} itself fails or if @var{command} cannot be found
14737 the exit status of @var{command} otherwise
14740 @node Modified command invocation
14741 @chapter Modified command invocation
14743 @cindex modified command invocation
14744 @cindex invocation of commands, modified
14745 @cindex commands for invoking other commands
14747 This section describes commands that run other commands in some context
14748 different than the current one: a modified environment, as a different
14752 * chroot invocation:: Modify the root directory.
14753 * env invocation:: Modify environment variables.
14754 * nice invocation:: Modify niceness.
14755 * nohup invocation:: Immunize to hangups.
14756 * stdbuf invocation:: Modify buffering of standard streams.
14757 * su invocation:: Modify user and group ID.
14758 * timeout invocation:: Run with time limit.
14762 @node chroot invocation
14763 @section @command{chroot}: Run a command with a different root directory
14766 @cindex running a program in a specified root directory
14767 @cindex root directory, running a program in a specified
14769 @command{chroot} runs a command with a specified root directory.
14770 On many systems, only the super-user can do this.@footnote{However,
14771 some systems (e.g., FreeBSD) can be configured to allow certain regular
14772 users to use the @code{chroot} system call, and hence to run this program.
14773 Also, on Cygwin, anyone can run the @command{chroot} command, because the
14774 underlying function is non-privileged due to lack of support in MS-Windows.}
14778 chroot @var{option} @var{newroot} [@var{command} [@var{args}]@dots{}]
14779 chroot @var{option}
14782 Ordinarily, file names are looked up starting at the root of the
14783 directory structure, i.e., @file{/}. @command{chroot} changes the root to
14784 the directory @var{newroot} (which must exist) and then runs
14785 @var{command} with optional @var{args}. If @var{command} is not
14786 specified, the default is the value of the @env{SHELL} environment
14787 variable or @command{/bin/sh} if not set, invoked with the @option{-i} option.
14788 @var{command} must not be a special built-in utility
14789 (@pxref{Special built-in utilities}).
14791 The program accepts the following options. Also see @ref{Common options}.
14792 Options must precede operands.
14796 @itemx --userspec=@var{user}[:@var{group}]
14797 @opindex --userspec
14798 By default, @var{command} is run with the same credentials
14799 as the invoking process.
14800 Use this option to run it as a different @var{user} and/or with a
14801 different primary @var{group}.
14803 @itemx --groups=@var{groups}
14805 Use this option to specify the supplementary @var{groups} to be
14806 used by the new process.
14807 The items in the list (names or numeric IDs) must be separated by commas.
14811 Here are a few tips to help avoid common problems in using chroot.
14812 To start with a simple example, make @var{command} refer to a statically
14813 linked binary. If you were to use a dynamically linked executable, then
14814 you'd have to arrange to have the shared libraries in the right place under
14815 your new root directory.
14817 For example, if you create a statically linked @command{ls} executable,
14818 and put it in @file{/tmp/empty}, you can run this command as root:
14821 $ chroot /tmp/empty /ls -Rl /
14824 Then you'll see output like this:
14829 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls
14832 If you want to use a dynamically linked executable, say @command{bash},
14833 then first run @samp{ldd bash} to see what shared objects it needs.
14834 Then, in addition to copying the actual binary, also copy the listed
14835 files to the required positions under your intended new root directory.
14836 Finally, if the executable requires any other files (e.g., data, state,
14837 device files), copy them into place, too.
14839 @cindex exit status of @command{chroot}
14843 125 if @command{chroot} itself fails
14844 126 if @var{command} is found but cannot be invoked
14845 127 if @var{command} cannot be found
14846 the exit status of @var{command} otherwise
14850 @node env invocation
14851 @section @command{env}: Run a command in a modified environment
14854 @cindex environment, running a program in a modified
14855 @cindex modified environment, running a program in a
14856 @cindex running a program in a modified environment
14858 @command{env} runs a command with a modified environment. Synopses:
14861 env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
14862 [@var{command} [@var{args}]@dots{}]
14866 Operands of the form @samp{@var{variable}=@var{value}} set
14867 the environment variable @var{variable} to value @var{value}.
14868 @var{value} may be empty (@samp{@var{variable}=}). Setting a variable
14869 to an empty value is different from unsetting it.
14870 These operands are evaluated left-to-right, so if two operands
14871 mention the same variable the earlier is ignored.
14873 Environment variable names can be empty, and can contain any
14874 characters other than @samp{=} and @acronym{ASCII} @sc{nul}.
14875 However, it is wise to limit yourself to names that
14876 consist solely of underscores, digits, and @acronym{ASCII} letters,
14877 and that begin with a non-digit, as applications like the shell do not
14878 work well with other names.
14881 The first operand that does not contain the character @samp{=}
14882 specifies the program to invoke; it is
14883 searched for according to the @env{PATH} environment variable. Any
14884 remaining arguments are passed as arguments to that program.
14885 The program should not be a special built-in utility
14886 (@pxref{Special built-in utilities}).
14888 Modifications to @env{PATH} take effect prior to searching for
14889 @var{command}. Use caution when reducing @env{PATH}; behavior is
14890 not portable when @env{PATH} is undefined or omits key directories
14891 such as @file{/bin}.
14893 In the rare case that a utility contains a @samp{=} in the name, the
14894 only way to disambiguate it from a variable assignment is to use an
14895 intermediate command for @var{command}, and pass the problematic
14896 program name via @var{args}. For example, if @file{./prog=} is an
14897 executable in the current @env{PATH}:
14900 env prog= true # runs 'true', with prog= in environment
14901 env ./prog= true # runs 'true', with ./prog= in environment
14902 env -- prog= true # runs 'true', with prog= in environment
14903 env sh -c '\prog= true' # runs 'prog=' with argument 'true'
14904 env sh -c 'exec "$@@"' sh prog= true # also runs 'prog='
14907 @cindex environment, printing
14909 If no command name is specified following the environment
14910 specifications, the resulting environment is printed. This is like
14911 specifying the @command{printenv} program.
14913 For some examples, suppose the environment passed to @command{env}
14914 contains @samp{LOGNAME=rms}, @samp{EDITOR=emacs}, and
14915 @samp{PATH=.:/gnubin:/hacks}:
14920 Output the current environment.
14922 $ env | LC_ALL=C sort
14925 PATH=.:/gnubin:/hacks
14929 Run @command{foo} with a reduced environment, preserving only the
14930 original @env{PATH} to avoid problems in locating @command{foo}.
14932 env - PATH="$PATH" foo
14936 Run @command{foo} with the environment containing @samp{LOGNAME=rms},
14937 @samp{EDITOR=emacs}, and @samp{PATH=.:/gnubin:/hacks}, and guarantees
14938 that @command{foo} was found in the file system rather than as a shell
14945 Run @command{nemacs} with the environment containing @samp{LOGNAME=foo},
14946 @samp{EDITOR=emacs}, @samp{PATH=.:/gnubin:/hacks}, and
14947 @samp{DISPLAY=gnu:0}.
14949 env DISPLAY=gnu:0 LOGNAME=foo nemacs
14953 Attempt to run the program @command{/energy/--} (as that is the only
14954 possible path search result); if the command exists, the environment
14955 will contain @samp{LOGNAME=rms} and @samp{PATH=/energy}, and the
14956 arguments will be @samp{e=mc2}, @samp{bar}, and @samp{baz}.
14958 env -u EDITOR PATH=/energy -- e=mc2 bar baz
14964 The program accepts the following options. Also see @ref{Common options}.
14965 Options must precede operands.
14971 @item -u @var{name}
14972 @itemx --unset=@var{name}
14975 Remove variable @var{name} from the environment, if it was in the
14980 @itemx --ignore-environment
14983 @opindex --ignore-environment
14984 Start with an empty environment, ignoring the inherited environment.
14988 @cindex exit status of @command{env}
14992 0 if no @var{command} is specified and the environment is output
14993 125 if @command{env} itself fails
14994 126 if @var{command} is found but cannot be invoked
14995 127 if @var{command} cannot be found
14996 the exit status of @var{command} otherwise
15000 @node nice invocation
15001 @section @command{nice}: Run a command with modified niceness
15005 @cindex scheduling, affecting
15006 @cindex appropriate privileges
15008 @command{nice} prints or modifies a process's @dfn{niceness},
15009 a parameter that affects whether the process is scheduled favorably.
15013 nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
15016 If no arguments are given, @command{nice} prints the current niceness.
15017 Otherwise, @command{nice} runs the given @var{command} with its
15018 niceness adjusted. By default, its niceness is incremented by 10.
15020 Niceness values range at least from @minus{}20 (process has high priority
15021 and gets more resources, thus slowing down other processes) through 19
15022 (process has lower priority and runs slowly itself, but has less impact
15023 on the speed of other running processes). Some systems
15024 may have a wider range of nicenesses; conversely, other systems may
15025 enforce more restrictive limits. An attempt to set the niceness
15026 outside the supported range is treated as an attempt to use the
15027 minimum or maximum supported value.
15029 A niceness should not be confused with a scheduling priority, which
15030 lets applications determine the order in which threads are scheduled
15031 to run. Unlike a priority, a niceness is merely advice to the
15032 scheduler, which the scheduler is free to ignore. Also, as a point of
15033 terminology, @acronym{POSIX} defines the behavior of @command{nice} in
15034 terms of a @dfn{nice value}, which is the nonnegative difference
15035 between a niceness and the minimum niceness. Though @command{nice}
15036 conforms to @acronym{POSIX}, its documentation and diagnostics use the
15037 term ``niceness'' for compatibility with historical practice.
15039 @var{command} must not be a special built-in utility (@pxref{Special
15040 built-in utilities}).
15042 @mayConflictWithShellBuiltIn{nice}
15044 The program accepts the following option. Also see @ref{Common options}.
15045 Options must precede operands.
15048 @item -n @var{adjustment}
15049 @itemx --adjustment=@var{adjustment}
15051 @opindex --adjustment
15052 Add @var{adjustment} instead of 10 to the command's niceness. If
15053 @var{adjustment} is negative and you lack appropriate privileges,
15054 @command{nice} issues a warning but otherwise acts as if you specified
15057 For compatibility @command{nice} also supports an obsolete
15058 option syntax @option{-@var{adjustment}}. New scripts should use
15059 @option{-n @var{adjustment}} instead.
15063 @cindex exit status of @command{nice}
15067 0 if no @var{command} is specified and the niceness is output
15068 125 if @command{nice} itself fails
15069 126 if @var{command} is found but cannot be invoked
15070 127 if @var{command} cannot be found
15071 the exit status of @var{command} otherwise
15074 It is sometimes useful to run a non-interactive program with reduced niceness.
15077 $ nice factor 4611686018427387903
15080 Since @command{nice} prints the current niceness,
15081 you can invoke it through itself to demonstrate how it works.
15083 The default behavior is to increase the niceness by @samp{10}:
15094 The @var{adjustment} is relative to the current niceness. In the
15095 next example, the first @command{nice} invocation runs the second one
15096 with niceness 10, and it in turn runs the final one with a niceness
15100 $ nice nice -n 3 nice
15104 Specifying a niceness larger than the supported range
15105 is the same as specifying the maximum supported value:
15108 $ nice -n 10000000000 nice
15112 Only a privileged user may run a process with lower niceness:
15116 nice: cannot set niceness: Permission denied
15118 $ sudo nice -n -1 nice
15123 @node nohup invocation
15124 @section @command{nohup}: Run a command immune to hangups
15127 @cindex hangups, immunity to
15128 @cindex immunity to hangups
15129 @cindex logging out and continuing to run
15132 @command{nohup} runs the given @var{command} with hangup signals ignored,
15133 so that the command can continue running in the background after you log
15137 nohup @var{command} [@var{arg}]@dots{}
15140 If standard input is a terminal, it is redirected from
15141 @file{/dev/null} so that terminal sessions do not mistakenly consider
15142 the terminal to be used by the command. This is a @acronym{GNU}
15143 extension; programs intended to be portable to non-@acronym{GNU} hosts
15144 should use @samp{nohup @var{command} [@var{arg}]@dots{} </dev/null}
15148 If standard output is a terminal, the command's standard output is appended
15149 to the file @file{nohup.out}; if that cannot be written to, it is appended
15150 to the file @file{$HOME/nohup.out}; and if that cannot be written to, the
15151 command is not run.
15152 Any @file{nohup.out} or @file{$HOME/nohup.out} file created by
15153 @command{nohup} is made readable and writable only to the user,
15154 regardless of the current umask settings.
15156 If standard error is a terminal, it is normally redirected to the same file
15157 descriptor as the (possibly-redirected) standard output.
15158 However, if standard output is closed, standard error terminal output
15159 is instead appended to the file @file{nohup.out} or
15160 @file{$HOME/nohup.out} as above.
15162 To capture the command's output to a file other than @file{nohup.out}
15163 you can redirect it. For example, to capture the output of
15167 nohup make > make.log
15170 @command{nohup} does not automatically put the command it runs in the
15171 background; you must do that explicitly, by ending the command line
15172 with an @samp{&}. Also, @command{nohup} does not alter the
15173 niceness of @var{command}; use @command{nice} for that,
15174 e.g., @samp{nohup nice @var{command}}.
15176 @var{command} must not be a special built-in utility (@pxref{Special
15177 built-in utilities}).
15179 The only options are @option{--help} and @option{--version}. @xref{Common
15180 options}. Options must precede operands.
15182 @cindex exit status of @command{nohup}
15186 125 if @command{nohup} itself fails, and @env{POSIXLY_CORRECT} is not set
15187 126 if @var{command} is found but cannot be invoked
15188 127 if @var{command} cannot be found
15189 the exit status of @var{command} otherwise
15192 If @env{POSIXLY_CORRECT} is set, internal failures give status 127
15196 @node stdbuf invocation
15197 @section @command{stdbuf}: Run a command with modified I/O stream buffering
15200 @cindex standard streams, buffering
15201 @cindex line buffered
15203 @command{stdbuf} allows one to modify the buffering operations of the
15204 three standard I/O streams associated with a program. Synopsis:
15207 stdbuf @var{option}@dots{} @var{command}
15210 Any additional @var{arg}s are passed as additional arguments to the
15213 The program accepts the following options. Also see @ref{Common options}.
15217 @item -i @var{mode}
15218 @itemx --input=@var{mode}
15221 Adjust the standard input stream buffering.
15223 @item -o @var{mode}
15224 @itemx --output=@var{mode}
15227 Adjust the standard output stream buffering.
15229 @item -e @var{mode}
15230 @itemx --error=@var{mode}
15233 Adjust the standard error stream buffering.
15237 The @var{mode} can be specified as follows:
15242 Set the stream to line buffered mode.
15243 In this mode data is coalesced until a newline is output or
15244 input is read from any stream attached to a terminal device.
15245 This option is invalid with standard input.
15248 Disable buffering of the selected stream.
15249 In this mode data is output immediately and only the
15250 amount of data requested is read from input.
15253 Specify the size of the buffer to use in fully buffered mode.
15254 @multiplierSuffixesNoBlocks{size}
15258 NOTE: If @var{command} adjusts the buffering of its standard streams
15259 (@command{tee} does for e.g.) then that will override corresponding settings
15260 changed by @command{stdbuf}. Also some filters (like @command{dd} and
15261 @command{cat} etc.) don't use streams for I/O, and are thus unaffected
15262 by @command{stdbuf} settings.
15264 @cindex exit status of @command{stdbuf}
15268 125 if @command{stdbuf} itself fails
15269 126 if @var{command} is found but cannot be invoked
15270 127 if @var{command} cannot be found
15271 the exit status of @var{command} otherwise
15275 @node su invocation
15276 @section @command{su}: Run a command with substitute user and group ID
15279 @cindex substitute user and group IDs
15280 @cindex user ID, switching
15281 @cindex super-user, becoming
15282 @cindex root, becoming
15284 @command{su} allows one user to temporarily become another user. It runs a
15285 command (often an interactive shell) with the real and effective user
15286 ID, group ID, and supplemental groups of a given @var{user}. Synopsis:
15289 su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
15292 @cindex passwd entry, and @command{su} shell
15294 @flindex /etc/passwd
15295 If no @var{user} is given, the default is @code{root}, the super-user.
15296 The shell to use is taken from @var{user}'s @code{passwd} entry, or
15297 @file{/bin/sh} if none is specified there. If @var{user} has a
15298 password, @command{su} prompts for the password unless run by a user with
15299 effective user ID of zero (the super-user).
15305 @cindex login shell
15306 By default, @command{su} does not change the current directory.
15307 It sets the environment variables @env{HOME} and @env{SHELL}
15308 from the password entry for @var{user}, and if @var{user} is not
15309 the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
15310 By default, the shell is not a login shell.
15312 Any additional @var{arg}s are passed as additional arguments to the
15315 @cindex @option{-su}
15316 GNU @command{su} does not treat @file{/bin/sh} or any other shells specially
15317 (e.g., by setting @code{argv[0]} to @option{-su}, passing @option{-c} only
15318 to certain shells, etc.).
15321 @command{su} can optionally be compiled to use @code{syslog} to report
15322 failed, and optionally successful, @command{su} attempts. (If the system
15323 supports @code{syslog}.) However, GNU @command{su} does not check if the
15324 user is a member of the @code{wheel} group; see below.
15326 The program accepts the following options. Also see @ref{Common options}.
15329 @item -c @var{command}
15330 @itemx --command=@var{command}
15333 Pass @var{command}, a single command line to run, to the shell with
15334 a @option{-c} option instead of starting an interactive shell.
15341 @cindex file name pattern expansion, disabled
15342 @cindex globbing, disabled
15343 Pass the @option{-f} option to the shell. This probably only makes sense
15344 if the shell run is @command{csh} or @command{tcsh}, for which the @option{-f}
15345 option prevents reading the startup file (@file{.cshrc}). With
15346 Bourne-like shells, the @option{-f} option disables file name pattern
15347 expansion (globbing), which is not likely to be useful.
15355 @c other variables already indexed above
15358 @cindex login shell, creating
15359 Make the shell a login shell. This means the following. Unset all
15360 environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
15361 (which are set as described above), and @env{USER} and @env{LOGNAME}
15362 (which are set, even for the super-user, as described above), and set
15363 @env{PATH} to a compiled-in default value. Change to @var{user}'s home
15364 directory. Prepend @samp{-} to the shell's name, intended to make it
15365 read its login startup file(s).
15369 @itemx --preserve-environment
15372 @opindex --preserve-environment
15373 @cindex environment, preserving
15374 @flindex /etc/shells
15375 @cindex restricted shell
15376 Do not change the environment variables @env{HOME}, @env{USER},
15377 @env{LOGNAME}, or @env{SHELL}. Run the shell given in the environment
15378 variable @env{SHELL} instead of the shell from @var{user}'s passwd
15379 entry, unless the user running @command{su} is not the super-user and
15380 @var{user}'s shell is restricted. A @dfn{restricted shell} is one that
15381 is not listed in the file @file{/etc/shells}, or in a compiled-in list
15382 if that file does not exist. Parts of what this option does can be
15383 overridden by @option{--login} and @option{--shell}.
15385 @item -s @var{shell}
15386 @itemx --shell=@var{shell}
15389 Run @var{shell} instead of the shell from @var{user}'s passwd entry,
15390 unless the user running @command{su} is not the super-user and @var{user}'s
15391 shell is restricted (see @option{-m} just above).
15395 @cindex exit status of @command{su}
15399 125 if @command{su} itself fails
15400 126 if subshell is found but cannot be invoked
15401 127 if subshell cannot be found
15402 the exit status of the subshell otherwise
15405 @cindex wheel group, not supported
15406 @cindex group wheel, not supported
15408 @subsection Why GNU @command{su} does not support the @samp{wheel} group
15410 (This section is by Richard Stallman.)
15414 Sometimes a few of the users try to hold total power over all the
15415 rest. For example, in 1984, a few users at the MIT AI lab decided to
15416 seize power by changing the operator password on the Twenex system and
15417 keeping it secret from everyone else. (I was able to thwart this coup
15418 and give power back to the users by patching the kernel, but I
15419 wouldn't know how to do that in Unix.)
15421 However, occasionally the rulers do tell someone. Under the usual
15422 @command{su} mechanism, once someone learns the root password who
15423 sympathizes with the ordinary users, he or she can tell the rest. The
15424 ``wheel group'' feature would make this impossible, and thus cement the
15425 power of the rulers.
15427 I'm on the side of the masses, not that of the rulers. If you are
15428 used to supporting the bosses and sysadmins in whatever they do, you
15429 might find this idea strange at first.
15432 @node timeout invocation
15433 @section @command{timeout}: Run a command with a time limit
15437 @cindex run commands with bounded time
15439 @command{timeout} runs the given @var{command} and kills it if it is
15440 still running after the specified time interval. Synopsis:
15443 timeout [@var{option}] @var{duration} @var{command} [@var{arg}]@dots{}
15446 @var{command} must not be a special built-in utility (@pxref{Special
15447 built-in utilities}).
15449 The program accepts the following options. Also see @ref{Common options}.
15450 Options must precede operands.
15453 @item -k @var{duration}
15454 @itemx --kill-after=@var{duration}
15456 @opindex --kill-after
15457 Ensure the monitored @var{command} is killed by also sending a @samp{KILL}
15458 signal, after the specified @var{duration}. Without this option, if the
15459 selected signal proves not to be fatal, @command{timeout} does not kill
15462 @item -s @var{signal}
15463 @itemx --signal=@var{signal}
15466 Send this @var{signal} to @var{command} on timeout, rather than the
15467 default @samp{TERM} signal. @var{signal} may be a name like @samp{HUP}
15468 or a number. Also see @xref{Signal specifications}.
15472 @var{duration} is an integer followed by an optional unit:
15474 @samp{s} for seconds (the default)
15475 @samp{m} for minutes
15479 A duration of 0 disables the associated timeout.
15481 @cindex exit status of @command{timeout}
15485 124 if @var{command} times out
15486 125 if @command{timeout} itself fails
15487 126 if @var{command} is found but cannot be invoked
15488 127 if @var{command} cannot be found
15489 the exit status of @var{command} otherwise
15493 @node Process control
15494 @chapter Process control
15496 @cindex processes, commands for controlling
15497 @cindex commands for controlling processes
15500 * kill invocation:: Sending a signal to processes.
15504 @node kill invocation
15505 @section @command{kill}: Send a signal to processes
15508 @cindex send a signal to processes
15510 The @command{kill} command sends a signal to processes, causing them
15511 to terminate or otherwise act upon receiving the signal in some way.
15512 Alternatively, it lists information about signals. Synopses:
15515 kill [-s @var{signal} | --signal @var{signal} | -@var{signal}] @var{pid}@dots{}
15516 kill [-l | --list | -t | --table] [@var{signal}]@dots{}
15519 @mayConflictWithShellBuiltIn{kill}
15521 The first form of the @command{kill} command sends a signal to all
15522 @var{pid} arguments. The default signal to send if none is specified
15523 is @samp{TERM}. The special signal number @samp{0} does not denote a
15524 valid signal, but can be used to test whether the @var{pid} arguments
15525 specify processes to which a signal could be sent.
15527 If @var{pid} is positive, the signal is sent to the process with the
15528 process ID @var{pid}. If @var{pid} is zero, the signal is sent to all
15529 processes in the process group of the current process. If @var{pid}
15530 is @minus{}1, the signal is sent to all processes for which the user has
15531 permission to send a signal. If @var{pid} is less than @minus{}1, the signal
15532 is sent to all processes in the process group that equals the absolute
15533 value of @var{pid}.
15535 If @var{pid} is not positive, a system-dependent set of system
15536 processes is excluded from the list of processes to which the signal
15539 If a negative @var{pid} argument is desired as the first one, it
15540 should be preceded by @option{--}. However, as a common extension to
15541 @acronym{POSIX}, @option{--} is not required with @samp{kill
15542 -@var{signal} -@var{pid}}. The following commands are equivalent:
15551 The first form of the @command{kill} command succeeds if every @var{pid}
15552 argument specifies at least one process that the signal was sent to.
15554 The second form of the @command{kill} command lists signal information.
15555 Either the @option{-l} or @option{--list} option, or the @option{-t}
15556 or @option{--table} option must be specified. Without any
15557 @var{signal} argument, all supported signals are listed. The output
15558 of @option{-l} or @option{--list} is a list of the signal names, one
15559 per line; if @var{signal} is already a name, the signal number is
15560 printed instead. The output of @option{-t} or @option{--table} is a
15561 table of signal numbers, names, and descriptions. This form of the
15562 @command{kill} command succeeds if all @var{signal} arguments are valid
15563 and if there is no output error.
15565 The @command{kill} command also supports the @option{--help} and
15566 @option{--version} options. @xref{Common options}.
15568 A @var{signal} may be a signal name like @samp{HUP}, or a signal
15569 number like @samp{1}, or an exit status of a process terminated by the
15570 signal. A signal name can be given in canonical form or prefixed by
15571 @samp{SIG}. The case of the letters is ignored, except for the
15572 @option{-@var{signal}} option which must use upper case to avoid
15573 ambiguity with lower case option letters. For a list of supported
15574 signal names and numbers see @xref{Signal specifications}.
15579 @cindex delaying commands
15580 @cindex commands for delaying
15582 @c Perhaps @command{wait} or other commands should be described here also?
15585 * sleep invocation:: Delay for a specified time.
15589 @node sleep invocation
15590 @section @command{sleep}: Delay for a specified time
15593 @cindex delay for a specified time
15595 @command{sleep} pauses for an amount of time specified by the sum of
15596 the values of the command line arguments.
15600 sleep @var{number}[smhd]@dots{}
15604 Each argument is a number followed by an optional unit; the default
15605 is seconds. The units are:
15618 Historical implementations of @command{sleep} have required that
15619 @var{number} be an integer, and only accepted a single argument
15620 without a suffix. However, GNU @command{sleep} accepts
15621 arbitrary floating point numbers (using a period before any fractional
15624 The only options are @option{--help} and @option{--version}. @xref{Common
15627 @c sleep is a shell built-in at least with Solaris 11's /bin/sh
15628 @mayConflictWithShellBuiltIn{sleep}
15633 @node Numeric operations
15634 @chapter Numeric operations
15636 @cindex numeric operations
15637 These programs do numerically-related operations.
15640 * factor invocation:: Show factors of numbers.
15641 * seq invocation:: Print sequences of numbers.
15645 @node factor invocation
15646 @section @command{factor}: Print prime factors
15649 @cindex prime factors
15651 @command{factor} prints prime factors. Synopses:
15654 factor [@var{number}]@dots{}
15655 factor @var{option}
15658 If no @var{number} is specified on the command line, @command{factor} reads
15659 numbers from standard input, delimited by newlines, tabs, or spaces.
15661 The @command{factor} command supports only a small number of options:
15665 Print a short help on standard output, then exit without further
15669 Print the program version on standard output, then exit without further
15673 Factoring the product of the eighth and ninth Mersenne primes
15674 takes about 30 milliseconds of CPU time on a 2.2 GHz Athlon.
15677 M8=`echo 2^31-1|bc` ; M9=`echo 2^61-1|bc`
15678 /usr/bin/time -f '%U' factor $(echo "$M8 * $M9" | bc)
15679 4951760154835678088235319297: 2147483647 2305843009213693951
15683 Similarly, factoring the eighth Fermat number @math{2^{256}+1} takes
15684 about 20 seconds on the same machine.
15686 Factoring large numbers is, in general, hard. The Pollard Rho
15687 algorithm used by @command{factor} is particularly effective for
15688 numbers with relatively small factors. If you wish to factor large
15689 numbers which do not have small factors (for example, numbers which
15690 are the product of two large primes), other methods are far better.
15692 If @command{factor} is built without using GNU MP, only
15693 single-precision arithmetic is available, and so large numbers
15694 (typically @math{2^{64}} and above) will not be supported. The single-precision
15695 code uses an algorithm which is designed for factoring smaller
15701 @node seq invocation
15702 @section @command{seq}: Print numeric sequences
15705 @cindex numeric sequences
15706 @cindex sequence of numbers
15708 @command{seq} prints a sequence of numbers to standard output. Synopses:
15711 seq [@var{option}]@dots{} @var{last}
15712 seq [@var{option}]@dots{} @var{first} @var{last}
15713 seq [@var{option}]@dots{} @var{first} @var{increment} @var{last}
15716 @command{seq} prints the numbers from @var{first} to @var{last} by
15717 @var{increment}. By default, each number is printed on a separate line.
15718 When @var{increment} is not specified, it defaults to @samp{1},
15719 even when @var{first} is larger than @var{last}.
15720 @var{first} also defaults to @samp{1}. So @code{seq 1} prints
15721 @samp{1}, but @code{seq 0} and @code{seq 10 5} produce no output.
15722 Floating-point numbers
15723 may be specified (using a period before any fractional digits).
15725 The program accepts the following options. Also see @ref{Common options}.
15726 Options must precede operands.
15729 @item -f @var{format}
15730 @itemx --format=@var{format}
15731 @opindex -f @var{format}
15732 @opindex --format=@var{format}
15733 @cindex formatting of numbers in @command{seq}
15734 Print all numbers using @var{format}.
15735 @var{format} must contain exactly one of the @samp{printf}-style
15736 floating point conversion specifications @samp{%a}, @samp{%e},
15737 @samp{%f}, @samp{%g}, @samp{%A}, @samp{%E}, @samp{%F}, @samp{%G}.
15738 The @samp{%} may be followed by zero or more flags taken from the set
15739 @samp{-+#0 '}, then an optional width containing one or more digits,
15740 then an optional precision consisting of a @samp{.} followed by zero
15741 or more digits. @var{format} may also contain any number of @samp{%%}
15742 conversion specifications. All conversion specifications have the
15743 same meaning as with @samp{printf}.
15745 The default format is derived from @var{first}, @var{step}, and
15746 @var{last}. If these all use a fixed point decimal representation,
15747 the default format is @samp{%.@var{p}f}, where @var{p} is the minimum
15748 precision that can represent the output numbers exactly. Otherwise,
15749 the default format is @samp{%g}.
15751 @item -s @var{string}
15752 @itemx --separator=@var{string}
15753 @cindex separator for numbers in @command{seq}
15754 Separate numbers with @var{string}; default is a newline.
15755 The output always terminates with a newline.
15758 @itemx --equal-width
15759 Print all numbers with the same width, by padding with leading zeros.
15760 @var{first}, @var{step}, and @var{last} should all use a fixed point
15761 decimal representation.
15762 (To have other kinds of padding, use @option{--format}).
15766 You can get finer-grained control over output with @option{-f}:
15769 $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6
15775 If you want hexadecimal integer output, you can use @command{printf}
15776 to perform the conversion:
15779 $ printf '%x\n' `seq 1048575 1024 1050623`
15785 For very long lists of numbers, use xargs to avoid
15786 system limitations on the length of an argument list:
15789 $ seq 1000000 | xargs printf '%x\n' | tail -n 3
15795 To generate octal output, use the printf @code{%o} format instead
15798 On most systems, seq can produce whole-number output for values up to
15799 at least @math{2^{53}}. Larger integers are approximated. The details
15800 differ depending on your floating-point implementation, but a common
15801 case is that @command{seq} works with integers through @math{2^{64}},
15802 and larger integers may not be numerically correct:
15805 $ seq 18446744073709551616 1 18446744073709551618
15806 18446744073709551616
15807 18446744073709551616
15808 18446744073709551618
15811 Be careful when using @command{seq} with outlandish values: otherwise
15812 you may see surprising results, as @command{seq} uses floating point
15813 internally. For example, on the x86 platform, where the internal
15814 representation uses a 64-bit fraction, the command:
15817 seq 1 0.0000000000000000001 1.0000000000000000009
15820 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008.
15825 @node File permissions
15826 @chapter File permissions
15829 @include parse-datetime.texi
15833 @node Opening the software toolbox
15834 @chapter Opening the Software Toolbox
15836 An earlier version of this chapter appeared in
15837 @uref{http://www.linuxjournal.com/article.php?sid=2762, the
15838 @cite{What's GNU?} column of @cite{Linux Journal}, 2 (June, 1994)}.
15839 It was written by Arnold Robbins.
15842 * Toolbox introduction:: Toolbox introduction
15843 * I/O redirection:: I/O redirection
15844 * The who command:: The @command{who} command
15845 * The cut command:: The @command{cut} command
15846 * The sort command:: The @command{sort} command
15847 * The uniq command:: The @command{uniq} command
15848 * Putting the tools together:: Putting the tools together
15852 @node Toolbox introduction
15853 @unnumberedsec Toolbox Introduction
15855 This month's column is only peripherally related to the GNU Project, in
15856 that it describes a number of the GNU tools on your GNU/Linux system and how they
15857 might be used. What it's really about is the ``Software Tools'' philosophy
15858 of program development and usage.
15860 The software tools philosophy was an important and integral concept
15861 in the initial design and development of Unix (of which Linux and GNU are
15862 essentially clones). Unfortunately, in the modern day press of
15863 Internetworking and flashy GUIs, it seems to have fallen by the
15864 wayside. This is a shame, since it provides a powerful mental model
15865 for solving many kinds of problems.
15867 Many people carry a Swiss Army knife around in their pants pockets (or
15868 purse). A Swiss Army knife is a handy tool to have: it has several knife
15869 blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps
15870 a number of other things on it. For the everyday, small miscellaneous jobs
15871 where you need a simple, general purpose tool, it's just the thing.
15873 On the other hand, an experienced carpenter doesn't build a house using
15874 a Swiss Army knife. Instead, he has a toolbox chock full of specialized
15875 tools---a saw, a hammer, a screwdriver, a plane, and so on. And he knows
15876 exactly when and where to use each tool; you won't catch him hammering nails
15877 with the handle of his screwdriver.
15879 The Unix developers at Bell Labs were all professional programmers and trained
15880 computer scientists. They had found that while a one-size-fits-all program
15881 might appeal to a user because there's only one program to use, in practice
15886 difficult to write,
15889 difficult to maintain and
15893 difficult to extend to meet new situations.
15896 Instead, they felt that programs should be specialized tools. In short, each
15897 program ``should do one thing well.'' No more and no less. Such programs are
15898 simpler to design, write, and get right---they only do one thing.
15900 Furthermore, they found that with the right machinery for hooking programs
15901 together, that the whole was greater than the sum of the parts. By combining
15902 several special purpose programs, you could accomplish a specific task
15903 that none of the programs was designed for, and accomplish it much more
15904 quickly and easily than if you had to write a special purpose program.
15905 We will see some (classic) examples of this further on in the column.
15906 (An important additional point was that, if necessary, take a detour
15907 and build any software tools you may need first, if you don't already
15908 have something appropriate in the toolbox.)
15910 @node I/O redirection
15911 @unnumberedsec I/O Redirection
15913 Hopefully, you are familiar with the basics of I/O redirection in the
15914 shell, in particular the concepts of ``standard input,'' ``standard output,''
15915 and ``standard error''. Briefly, ``standard input'' is a data source, where
15916 data comes from. A program should not need to either know or care if the
15917 data source is a disk file, a keyboard, a magnetic tape, or even a punched
15918 card reader. Similarly, ``standard output'' is a data sink, where data goes
15919 to. The program should neither know nor care where this might be.
15920 Programs that only read their standard input, do something to the data,
15921 and then send it on, are called @dfn{filters}, by analogy to filters in a
15924 With the Unix shell, it's very easy to set up data pipelines:
15927 program_to_create_data | filter1 | ... | filterN > final.pretty.data
15930 We start out by creating the raw data; each filter applies some successive
15931 transformation to the data, until by the time it comes out of the pipeline,
15932 it is in the desired form.
15934 This is fine and good for standard input and standard output. Where does the
15935 standard error come in to play? Well, think about @command{filter1} in
15936 the pipeline above. What happens if it encounters an error in the data it
15937 sees? If it writes an error message to standard output, it will just
15938 disappear down the pipeline into @command{filter2}'s input, and the
15939 user will probably never see it. So programs need a place where they can send
15940 error messages so that the user will notice them. This is standard error,
15941 and it is usually connected to your console or window, even if you have
15942 redirected standard output of your program away from your screen.
15944 For filter programs to work together, the format of the data has to be
15945 agreed upon. The most straightforward and easiest format to use is simply
15946 lines of text. Unix data files are generally just streams of bytes, with
15947 lines delimited by the @acronym{ASCII} @sc{lf} (Line Feed) character,
15948 conventionally called a ``newline'' in the Unix literature. (This is
15949 @code{'\n'} if you're a C programmer.) This is the format used by all
15950 the traditional filtering programs. (Many earlier operating systems
15951 had elaborate facilities and special purpose programs for managing
15952 binary data. Unix has always shied away from such things, under the
15953 philosophy that it's easiest to simply be able to view and edit your
15954 data with a text editor.)
15956 OK, enough introduction. Let's take a look at some of the tools, and then
15957 we'll see how to hook them together in interesting ways. In the following
15958 discussion, we will only present those command line options that interest
15959 us. As you should always do, double check your system documentation
15960 for the full story.
15962 @node The who command
15963 @unnumberedsec The @command{who} Command
15965 The first program is the @command{who} command. By itself, it generates a
15966 list of the users who are currently logged in. Although I'm writing
15967 this on a single-user system, we'll pretend that several people are
15972 @print{} arnold console Jan 22 19:57
15973 @print{} miriam ttyp0 Jan 23 14:19(:0.0)
15974 @print{} bill ttyp1 Jan 21 09:32(:0.0)
15975 @print{} arnold ttyp2 Jan 23 20:48(:0.0)
15978 Here, the @samp{$} is the usual shell prompt, at which I typed @samp{who}.
15979 There are three people logged in, and I am logged in twice. On traditional
15980 Unix systems, user names are never more than eight characters long. This
15981 little bit of trivia will be useful later. The output of @command{who} is nice,
15982 but the data is not all that exciting.
15984 @node The cut command
15985 @unnumberedsec The @command{cut} Command
15987 The next program we'll look at is the @command{cut} command. This program
15988 cuts out columns or fields of input data. For example, we can tell it
15989 to print just the login name and full name from the @file{/etc/passwd}
15990 file. The @file{/etc/passwd} file has seven fields, separated by
15994 arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash
15997 To get the first and fifth fields, we would use @command{cut} like this:
16000 $ cut -d: -f1,5 /etc/passwd
16001 @print{} root:Operator
16003 @print{} arnold:Arnold D. Robbins
16004 @print{} miriam:Miriam A. Robbins
16008 With the @option{-c} option, @command{cut} will cut out specific characters
16009 (i.e., columns) in the input lines. This is useful for input data
16010 that has fixed width fields, and does not have a field separator. For
16011 example, list the Monday dates for the current month:
16013 @c Is using cal ok? Looked at gcal, but I don't like it.
16024 @node The sort command
16025 @unnumberedsec The @command{sort} Command
16027 Next we'll look at the @command{sort} command. This is one of the most
16028 powerful commands on a Unix-style system; one that you will often find
16029 yourself using when setting up fancy data plumbing.
16032 command reads and sorts each file named on the command line. It then
16033 merges the sorted data and writes it to standard output. It will read
16034 standard input if no files are given on the command line (thus
16035 making it into a filter). The sort is based on the character collating
16036 sequence or based on user-supplied ordering criteria.
16039 @node The uniq command
16040 @unnumberedsec The @command{uniq} Command
16042 Finally (at least for now), we'll look at the @command{uniq} program. When
16043 sorting data, you will often end up with duplicate lines, lines that
16044 are identical. Usually, all you need is one instance of each line.
16045 This is where @command{uniq} comes in. The @command{uniq} program reads its
16046 standard input. It prints only one
16047 copy of each repeated line. It does have several options. Later on,
16048 we'll use the @option{-c} option, which prints each unique line, preceded
16049 by a count of the number of times that line occurred in the input.
16052 @node Putting the tools together
16053 @unnumberedsec Putting the Tools Together
16055 Now, let's suppose this is a large ISP server system with dozens of users
16056 logged in. The management wants the system administrator to write a program that will
16057 generate a sorted list of logged in users. Furthermore, even if a user
16058 is logged in multiple times, his or her name should only show up in the
16061 The administrator could sit down with the system documentation and write a C
16062 program that did this. It would take perhaps a couple of hundred lines
16063 of code and about two hours to write it, test it, and debug it.
16064 However, knowing the software toolbox, the administrator can instead start out
16065 by generating just a list of logged on users:
16075 Next, sort the list:
16078 $ who | cut -c1-8 | sort
16085 Finally, run the sorted list through @command{uniq}, to weed out duplicates:
16088 $ who | cut -c1-8 | sort | uniq
16094 The @command{sort} command actually has a @option{-u} option that does what
16095 @command{uniq} does. However, @command{uniq} has other uses for which one
16096 cannot substitute @samp{sort -u}.
16098 The administrator puts this pipeline into a shell script, and makes it available for
16099 all the users on the system (@samp{#} is the system administrator,
16100 or @code{root}, prompt):
16103 # cat > /usr/local/bin/listusers
16104 who | cut -c1-8 | sort | uniq
16106 # chmod +x /usr/local/bin/listusers
16109 There are four major points to note here. First, with just four
16110 programs, on one command line, the administrator was able to save about two
16111 hours worth of work. Furthermore, the shell pipeline is just about as
16112 efficient as the C program would be, and it is much more efficient in
16113 terms of programmer time. People time is much more expensive than
16114 computer time, and in our modern ``there's never enough time to do
16115 everything'' society, saving two hours of programmer time is no mean
16118 Second, it is also important to emphasize that with the
16119 @emph{combination} of the tools, it is possible to do a special
16120 purpose job never imagined by the authors of the individual programs.
16122 Third, it is also valuable to build up your pipeline in stages, as we did here.
16123 This allows you to view the data at each stage in the pipeline, which helps
16124 you acquire the confidence that you are indeed using these tools correctly.
16126 Finally, by bundling the pipeline in a shell script, other users can use
16127 your command, without having to remember the fancy plumbing you set up for
16128 them. In terms of how you run them, shell scripts and compiled programs are
16131 After the previous warm-up exercise, we'll look at two additional, more
16132 complicated pipelines. For them, we need to introduce two more tools.
16134 The first is the @command{tr} command, which stands for ``transliterate.''
16135 The @command{tr} command works on a character-by-character basis, changing
16136 characters. Normally it is used for things like mapping upper case to
16140 $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]'
16141 @print{} this example has mixed case!
16144 There are several options of interest:
16148 work on the complement of the listed characters, i.e.,
16149 operations apply to characters not in the given set
16152 delete characters in the first set from the output
16155 squeeze repeated characters in the output into just one character.
16158 We will be using all three options in a moment.
16160 The other command we'll look at is @command{comm}. The @command{comm}
16161 command takes two sorted input files as input data, and prints out the
16162 files' lines in three columns. The output columns are the data lines
16163 unique to the first file, the data lines unique to the second file, and
16164 the data lines that are common to both. The @option{-1}, @option{-2}, and
16165 @option{-3} command line options @emph{omit} the respective columns. (This is
16166 non-intuitive and takes a little getting used to.) For example:
16188 The file name @file{-} tells @command{comm} to read standard input
16189 instead of a regular file.
16191 Now we're ready to build a fancy pipeline. The first application is a word
16192 frequency counter. This helps an author determine if he or she is over-using
16195 The first step is to change the case of all the letters in our input file
16196 to one case. ``The'' and ``the'' are the same word when doing counting.
16199 $ tr '[:upper:]' '[:lower:]' < whats.gnu | ...
16202 The next step is to get rid of punctuation. Quoted words and unquoted words
16203 should be treated identically; it's easiest to just get the punctuation out of
16207 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ...
16210 The second @command{tr} command operates on the complement of the listed
16211 characters, which are all the letters, the digits, the underscore, and
16212 the blank. The @samp{\n} represents the newline character; it has to
16213 be left alone. (The @acronym{ASCII} tab character should also be included for
16214 good measure in a production script.)
16216 At this point, we have data consisting of words separated by blank space.
16217 The words only contain alphanumeric characters (and the underscore). The
16218 next step is break the data apart so that we have one word per line. This
16219 makes the counting operation much easier, as we will see shortly.
16222 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16223 > tr -s ' ' '\n' | ...
16226 This command turns blanks into newlines. The @option{-s} option squeezes
16227 multiple newline characters in the output into just one. This helps us
16228 avoid blank lines. (The @samp{>} is the shell's ``secondary prompt.''
16229 This is what the shell prints when it notices you haven't finished
16230 typing in all of a command.)
16232 We now have data consisting of one word per line, no punctuation, all one
16233 case. We're ready to count each word:
16236 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16237 > tr -s ' ' '\n' | sort | uniq -c | ...
16240 At this point, the data might look something like this:
16253 The output is sorted by word, not by count! What we want is the most
16254 frequently used words first. Fortunately, this is easy to accomplish,
16255 with the help of two more @command{sort} options:
16259 do a numeric sort, not a textual one
16262 reverse the order of the sort
16265 The final pipeline looks like this:
16268 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16269 > tr -s ' ' '\n' | sort | uniq -c | sort -n -r
16278 Whew! That's a lot to digest. Yet, the same principles apply. With six
16279 commands, on two lines (really one long one split for convenience), we've
16280 created a program that does something interesting and useful, in much
16281 less time than we could have written a C program to do the same thing.
16283 A minor modification to the above pipeline can give us a simple spelling
16284 checker! To determine if you've spelled a word correctly, all you have to
16285 do is look it up in a dictionary. If it is not there, then chances are
16286 that your spelling is incorrect. So, we need a dictionary.
16287 The conventional location for a dictionary is @file{/usr/dict/words}.
16288 On my GNU/Linux system,@footnote{Redhat Linux 6.1, for the November 2000
16289 revision of this article.}
16290 this is a sorted, 45,402 word dictionary.
16292 Now, how to compare our file with the dictionary? As before, we generate
16293 a sorted list of words, one per line:
16296 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16297 > tr -s ' ' '\n' | sort -u | ...
16300 Now, all we need is a list of words that are @emph{not} in the
16301 dictionary. Here is where the @command{comm} command comes in.
16304 $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' |
16305 > tr -s ' ' '\n' | sort -u |
16306 > comm -23 - /usr/dict/words
16309 The @option{-2} and @option{-3} options eliminate lines that are only in the
16310 dictionary (the second file), and lines that are in both files. Lines
16311 only in the first file (standard input, our stream of words), are
16312 words that are not in the dictionary. These are likely candidates for
16313 spelling errors. This pipeline was the first cut at a production
16314 spelling checker on Unix.
16316 There are some other tools that deserve brief mention.
16320 search files for text that matches a regular expression
16323 count lines, words, characters
16326 a T-fitting for data pipes, copies data to files and to standard output
16329 the stream editor, an advanced tool
16332 a data manipulation language, another advanced tool
16335 The software tools philosophy also espoused the following bit of
16336 advice: ``Let someone else do the hard part.'' This means, take
16337 something that gives you most of what you need, and then massage it the
16338 rest of the way until it's in the form that you want.
16344 Each program should do one thing well. No more, no less.
16347 Combining programs with appropriate plumbing leads to results where
16348 the whole is greater than the sum of the parts. It also leads to novel
16349 uses of programs that the authors might never have imagined.
16352 Programs should never print extraneous header or trailer data, since these
16353 could get sent on down a pipeline. (A point we didn't mention earlier.)
16356 Let someone else do the hard part.
16359 Know your toolbox! Use each program appropriately. If you don't have an
16360 appropriate tool, build one.
16363 As of this writing, all the programs we've discussed are available via
16364 anonymous @command{ftp} from: @*
16365 @uref{ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz}. (There may
16366 be more recent versions available now.)
16368 None of what I have presented in this column is new. The Software Tools
16369 philosophy was first introduced in the book @cite{Software Tools}, by
16370 Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X).
16371 This book showed how to write and use software tools. It was written in
16372 1976, using a preprocessor for FORTRAN named @command{ratfor} (RATional
16373 FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN
16374 was. The last chapter presented a @command{ratfor} to FORTRAN
16375 processor, written in @command{ratfor}. @command{ratfor} looks an awful
16376 lot like C; if you know C, you won't have any problem following the
16379 In 1981, the book was updated and made available as @cite{Software Tools
16380 in Pascal} (Addison-Wesley, ISBN 0-201-10342-7). Both books are
16381 still in print and are well worth
16382 reading if you're a programmer. They certainly made a major change in
16383 how I view programming.
16385 The programs in both books are available from
16386 @uref{http://cm.bell-labs.com/who/bwk, Brian Kernighan's home page}.
16387 For a number of years, there was an active
16388 Software Tools Users Group, whose members had ported the original
16389 @command{ratfor} programs to essentially every computer system with a
16390 FORTRAN compiler. The popularity of the group waned in the middle 1980s
16391 as Unix began to spread beyond universities.
16393 With the current proliferation of GNU code and other clones of Unix programs,
16394 these programs now receive little attention; modern C versions are
16395 much more efficient and do more than these programs do. Nevertheless, as
16396 exposition of good programming style, and evangelism for a still-valuable
16397 philosophy, these books are unparalleled, and I recommend them highly.
16399 Acknowledgment: I would like to express my gratitude to Brian Kernighan
16400 of Bell Labs, the original Software Toolsmith, for reviewing this column.
16402 @node GNU Free Documentation License
16403 @appendix GNU Free Documentation License
16407 @node Concept index
16414 @c Local variables:
16415 @c texinfo-column-for-description: 32