     [file version.texi not included]
     [file constants.texi not included]

     START-INFO-DIR-ENTRY
     * GNU Utilities: (coreutils).                   GNU Utilities.
     * Common options: (coreutils)Common options.    Common options.
     * File permissions: (coreutils)File permissions. Access modes.
     * Date input formats: (coreutils)Date input formats.

     * basename: (coreutils)basename invocation.     Strip directory and suffix.
     * cat: (coreutils)cat invocation.               Concatenate and write files.
     * chgrp: (coreutils)chgrp invocation.           Change file groups.
     * chmod: (coreutils)chmod invocation.           Change file permissions.
     * chown: (coreutils)chown invocation.           Change file owners/groups.
     * chroot: (coreutils)chroot invocation.         Specify the root directory.
     * cksum: (coreutils)cksum invocation.           Print POSIX CRC checksum.
     * comm: (coreutils)comm invocation.             Compare sorted files by line.
     * cp: (coreutils)cp invocation.                 Copy files.
     * csplit: (coreutils)csplit invocation.         Split by context.
     * cut: (coreutils)cut invocation.               Print selected parts of lines.
     * date: (coreutils)date invocation.             Print/set system date and time.
     * dd: (coreutils)dd invocation.                 Copy and convert a file.
     * df: (coreutils)df invocation.                 Report filesystem disk usage.
     * dir: (coreutils)dir invocation.               List directories briefly.
     * dircolors: (coreutils)dircolors invocation.   Color setup for ls.
     * dirname: (coreutils)dirname invocation.       Strip non-directory suffix.
     * du: (coreutils)du invocation.                 Report on disk usage.
     * echo: (coreutils)echo invocation.             Print a line of text.
     * env: (coreutils)env invocation.               Modify the environment.
     * expand: (coreutils)expand invocation.         Convert tabs to spaces.
     * expr: (coreutils)expr invocation.             Evaluate expressions.
     * factor: (coreutils)factor invocation.         Print prime factors
     * false: (coreutils)false invocation.           Do nothing, unsuccessfully.
     * fmt: (coreutils)fmt invocation.               Reformat paragraph text.
     * fold: (coreutils)fold invocation.             Wrap long input lines.
     * groups: (coreutils)groups invocation.         Print group names a user is in.
     * head: (coreutils)head invocation.             Output the first part of files.
     * hostid: (coreutils)hostid invocation.         Print numeric host identifier.
     * hostname: (coreutils)hostname invocation.     Print or set system name.
     * id: (coreutils)id invocation.                 Print real/effective uid/gid.
     * install: (coreutils)install invocation.       Copy and change attributes.
     * join: (coreutils)join invocation.             Join lines on a common field.
     * kill: (coreutils)kill invocation.             Send a signal to processes.
     * link: (coreutils)link invocation.             Make hard links between files.
     * ln: (coreutils)ln invocation.                 Make links between files.
     * logname: (coreutils)logname invocation.       Print current login name.
     * ls: (coreutils)ls invocation.                 List directory contents.
     * md5sum: (coreutils)md5sum invocation.         Print or check message-digests.
     * mkdir: (coreutils)mkdir invocation.           Create directories.
     * mkfifo: (coreutils)mkfifo invocation.         Create FIFOs (named pipes).
     * mknod: (coreutils)mknod invocation.           Create special files.
     * mv: (coreutils)mv invocation.                 Rename files.
     * nice: (coreutils)nice invocation.             Modify scheduling priority.
     * nl: (coreutils)nl invocation.                 Number lines and write files.
     * nohup: (coreutils)nohup invocation.           Immunize to hangups.
     * od: (coreutils)od invocation.                 Dump files in octal, etc.
     * paste: (coreutils)paste invocation.           Merge lines of files.
     * pathchk: (coreutils)pathchk invocation.       Check file name portability.
     * pr: (coreutils)pr invocation.                 Paginate or columnate files.
     * printenv: (coreutils)printenv invocation.     Print environment variables.
     * printf: (coreutils)printf invocation.         Format and print data.
     * ptx: (coreutils)ptx invocation.               Produce permuted indexes.
     * pwd: (coreutils)pwd invocation.               Print working directory.
     * rm: (coreutils)rm invocation.                 Remove files.
     * rmdir: (coreutils)rmdir invocation.           Remove empty directories.
     * seq: (coreutils)seq invocation.               Print numeric sequences
     * shred: (coreutils)shred invocation.           Remove files more securely.
     * sleep: (coreutils)sleep invocation.           Delay for a specified time.
     * sort: (coreutils)sort invocation.             Sort text files.
     * split: (coreutils)split invocation.           Split into fixed-size pieces.
     * stat: (coreutils)stat invocation.             Report file or filesystem status.
     * stty: (coreutils)stty invocation.             Print/change terminal settings.
     * su: (coreutils)su invocation.                 Modify user and group id.
     * sum: (coreutils)sum invocation.               Print traditional checksum.
     * sync: (coreutils)sync invocation.             Synchronize memory and disk.
     * tac: (coreutils)tac invocation.               Reverse files.
     * tail: (coreutils)tail invocation.             Output the last part of files.
     * tee: (coreutils)tee invocation.               Redirect to multiple files.
     * test: (coreutils)test invocation.             File/string tests.
     * touch: (coreutils)touch invocation.           Change file timestamps.
     * tr: (coreutils)tr invocation.                 Translate characters.
     * true: (coreutils)true invocation.             Do nothing, successfully.
     * tsort: (coreutils)tsort invocation.           Topological sort.
     * tty: (coreutils)tty invocation.               Print terminal name.
     * uname: (coreutils)uname invocation.           Print system information.
     * unexpand: (coreutils)unexpand invocation.     Convert spaces to tabs.
     * unlink: (coreutils)unlink invocation.         Remove files via unlink syscall.
     * uniq: (coreutils)uniq invocation.             Uniquify files.
     * users: (coreutils)users invocation.           Print current user names.
     * vdir: (coreutils)vdir invocation.             List directories verbosely.
     * wc: (coreutils)wc invocation.                 Byte, word, and line counts.
     * who: (coreutils)who invocation.               Print who is logged in.
     * whoami: (coreutils)whoami invocation.         Print effective user id.
     * yes: (coreutils)yes invocation.               Print a string indefinitely.
     END-INFO-DIR-ENTRY

GNU Core-utils
**************

This manual documents version VERSION of the GNU command line utilities.

Copyright © 1994, 1995, 1996, 2000, 2001, 2002 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts.  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

Introduction
************

This manual is a work in progress: many sections make no attempt to explain basic concepts in a way suitable for novices.  Thus, if you are interested, please get involved in improving this manual.  The entire GNU community will benefit.

The GNU utilities documented here are mostly compatible with the POSIX standard. Please report bugs to bug-coreutils@gnu.org.  Remember to include the version number, machine architecture, input files, and any other information needed to reproduce the bug: your input, what you expected, what you got, and why it is wrong.  Diffs are welcome, but please include a description of the problem as well, since this is sometimes difficult to infer. See Bugs (Using and Porting GNU CC).

This manual was originally derived from the Unix man pages in the distributions, which were written by David MacKenzie and updated by Jim Meyering.  What you are reading now is the authoritative documentation for these utilities; the man pages are no longer being maintained.  The original fmt man page was written by Ross Paterson.  François Pinard did the initial conversion to Texinfo format.  Karl Berry did the indexing, some reorganization, and editing of the results.  Brian Youmans of the Free Software Foundation office staff combined the manuals for textutils, fileutils, and sh-utils to produce the present omnibus manual.  Richard Stallman contributed his usual invaluable insights to the overall process.

Common options
**************

Certain options are available in all of these programs.  Rather than writing identical descriptions for each of the programs, they are described here.  (In fact, every GNU program accepts (or should accept) these options.)

Normally options and operands can appear in any order, and programs act as if all the options appear before any operands.  For example, sort -r passwd -t : acts like sort -r -t : passwd, since : is an option-argument of -t.  However, if the POSIXLY_CORRECT environment variable is set, options must appear before operands, unless otherwise specified for a particular command.

Some of these programs recognize the --help and --version options only when one of them is the sole command line argument.

--help
     Print a usage message listing all available options, then exit successfully.

--version
     Print the version number, then exit successfully.

--
     Delimit the option list.  Later arguments, if any, are treated as operands even if they begin with -.  For example, sort -- -r reads from the file named -r.

A single - is not really an option, though it looks like one.  It stands for standard input, or for standard output if that is clear from the context, and it can be used either as an operand or as an option-argument.  For example, sort -o - - outputs to standard output and reads from standard input, and is equivalent to plain sort.  Unless otherwise specified, - can appear in any context that requires a file name.

Backup options
==============

Some GNU programs (at least cp, install, ln, and mv) optionally make backups of files before writing new versions. These options control the details of these backups.  The options are also briefly mentioned in the descriptions of the particular programs.

-b
--backup[=METHOD]
     Make a backup of each file that would otherwise be overwritten or removed. Without this option, the original versions are destroyed. Use METHOD to determine the type of backups to make. When this option is used but METHOD is not specified, then the value of the VERSION_CONTROL environment variable is used.  And if VERSION_CONTROL is not set, the default backup type is existing.

     Note that the short form of this option, -b does not accept any argument.  Using -b is equivalent to using --backup=existing.

     This option corresponds to the Emacs variable version-control; the values for METHOD are the same as those used in Emacs. This option also accepts more descriptive names. The valid METHODs are (unique abbreviations are accepted):

     none
     off
          Never make backups.

     numbered
     t
          Always make numbered backups.

     existing
     nil
          Make numbered backups of files that already have them, simple backups of the others.

     simple
     never
          Always make simple backups.  Please note never is not to be confused with none.

-S SUFFIX
--suffix=SUFFIX
     Append SUFFIX to each backup file made with -b.  If this option is not specified, the value of the SIMPLE_BACKUP_SUFFIX environment variable is used.  And if SIMPLE_BACKUP_SUFFIX is not set, the default is ~, just as in Emacs.

--version-control=METHOD
     This option is obsolete and will be removed in a future release. It has been replaced with --backup.

Block size
==========

Some GNU programs (at least df, du, and ls) display sizes in ``blocks''.  You can adjust the block size and method of display to make sizes easier to read.  The block size used for display is independent of any filesystem block size. Fractional block counts are rounded up to the nearest integer.

The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size.

DF_BLOCK_SIZE
     This specifies the default block size for the df command. Similarly, DU_BLOCK_SIZE specifies the default for du and LS_BLOCK_SIZE for ls.

BLOCK_SIZE
     This specifies the default block size for all three commands, if the above command-specific environment variables are not set.

POSIXLY_CORRECT
     If neither the COMMAND_BLOCK_SIZE nor the BLOCK_SIZE variables are set, but this variable is set, the block size defaults to 512.

If none of the above environment variables are set, the block size currently defaults to 1024 bytes in most contexts, but this number may change in the future.  For ls file sizes, the block size defaults to 1 byte.

A block size specification can be a positive integer specifying the number of bytes per block, or it can be human-readable or si to select a human-readable format.  Integers may be followed by suffixes that are upward compatible with the SI prefixes (http://www.bipm.fr/enus/3_SI/si-prefixes.html) for decimal multiples and with the IEC 60027-2 prefixes for binary multiples (http://physics.nist.gov/cuu/Units/binary.html).

With human-readable formats, output sizes are followed by a size letter such as M for megabytes.  BLOCK_SIZE=human-readable uses powers of 1024; M stands for 1,048,576 bytes. BLOCK_SIZE=si is similar, but uses powers of 1000 and appends B; MB stands for 1,000,000 bytes.

A block size specification preceded by ' causes output sizes to be displayed with thousands separators.  The LC_NUMERIC locale specifies the thousands separator and grouping.  For example, in an American English locale, --block-size="'1kB" would cause a size of 1234000 bytes to be displayed as 1,234.  In the default C locale, there is no thousands separator so a leading ' has no effect.

An integer block size can be followed by a suffix to specify a multiple of that size.  A bare size letter, or one followed by iB, specifies a multiple using powers of 1024.  A size letter followed by B specifies powers of 1000 instead.  For example, 1M and 1MiB are equivalent to 1048576, whereas 1MB is equivalent to 1000000.

A plain suffix without a preceding integer acts as if 1 were prepended, except that it causes a size indication to be appended to the output.  For example, --block-size="kB" displays 3000 as 3kB.

The following suffixes are defined.  Large sizes like 1Y may be rejected by your computer due to limitations of its arithmetic.

kB
     kilobyte: 10^3 = 1000.

k
K
KiB
     kibibyte: 2^10 = 1024.  K is special: the SI prefix is k and the IEC 60027-2 prefix is Ki, but tradition and POSIX use k to mean KiB.

MB
     megabyte: 10^6 = 1,000,000.

M
MiB
     mebibyte: 2^20 = 1,048,576.

GB
     gigabyte: 10^9 = 1,000,000,000.

G
GiB
     gibibyte: 2^30 = 1,073,741,824.

TB
     terabyte:  10^12 = 1,000,000,000,000.

T
TiB
     tebibyte: 2^40 = 1,099,511,627,776.

PB
     petabyte: 10^15 = 1,000,000,000,000,000.

P
PiB
     pebibyte: 2^50 = 1,125,899,906,842,624.

EB
     exabyte: 10^18 = 1,000,000,000,000,000,000.

E
EiB
     exbibyte: 2^60 = 1,152,921,504,606,846,976.

ZB
     zettabyte: 10^21 = 1,000,000,000,000,000,000,000

Z
ZiB
     2^70 = 1,180,591,620,717,411,303,424. (Zi is a GNU extension to IEC 60027-2.)

YB
     yottabyte: 10^24 = 1,000,000,000,000,000,000,000,000.

Y
YiB
     2^80 = 1,208,925,819,614,629,174,706,176. (Yi is a GNU extension to IEC 60027-2.)

Block size defaults can be overridden by an explicit --block-size=SIZE option.  The -k option is equivalent to --block-size=1K, which is the default unless the POSIXLY_CORRECT environment variable is set.  The -h or --human-readable option is equivalent to --block-size=human-readable.  The --si option is equivalent to --block-size=si.

Target directory
================

Some GNU programs (at least cp, install, ln, and mv) allow you to specify the target directory via this option:

--target-directory=DIRECTORY
     Specify the destination DIRECTORY.

     The interface for most programs is that after processing options and a finite (possibly zero) number of fixed-position arguments, the remaining argument list is either expected to be empty, or is a list of items (usually files) that will all be handled identically.  The xargs program is designed to work well with this convention.

     The commands in the mv-family are unusual in that they take a variable number of arguments with a special case at the end (namely, the target directory).  This makes it nontrivial to perform some operations, e.g., ``move all files from here to ../d/'', because mv * ../d/ might exhaust the argument space, and ls | xargs ... doesn't have a clean way to specify an extra final argument for each invocation of the subject command.  (It can be done by going through a shell command, but that requires more human labor and brain power than it should.)

     The --target-directory option allows the cp, install, ln, and mv programs to be used conveniently with xargs.  For example, you can move the files from the current directory to a sibling directory, d like this: (However, this doesn't move files whose names begin with ..)

          ls |xargs mv --target-directory=../d

     If you use the GNU find program, you can move all files with this command:

          find . -mindepth 1 -maxdepth 1 \
            | xargs mv --target-directory=../d

     But that will fail if there are no files in the current directory or if any file has a name containing a newline character. The following example removes those limitations and requires both GNU find and GNU xargs:

          find . -mindepth 1 -maxdepth 1 -print0 \
            | xargs --null --no-run-if-empty \
                mv --target-directory=../d

Trailing slashes
================

Some GNU programs (at least cp and mv) allow you to remove any trailing slashes from each SOURCE argument before operating on it.  The --strip-trailing-slashes option enables this behavior.

This is useful when a SOURCE argument may have a trailing slash and specify a symbolic link to a directory.  This scenario is in fact rather common because some shells can automatically append a trailing slash when performing file name completion on such symbolic links.  Without this option, mv, for example, (via the system's rename function) must interpret a trailing slash as a request to dereference the symbolic link and so must rename the indirectly referenced directory and not the symbolic link.  Although it may seem surprising that such behavior be the default, it is required by POSIX and is consistent with other parts of that standard.

Standards conformance
=====================

In a few cases, the GNU utilities' default behavior is incompatible with the POSIX standard.  To suppress these incompatibilities, define the POSIXLY_CORRECT environment variable.  Unless you are checking for POSIX conformance, you probably do not need to define POSIXLY_CORRECT.

Newer versions of POSIX are occasionally incompatible with older versions.  For example, older versions of POSIX required the command sort +1 to sort based on the second and succeeding fields in each input line, but starting with POSIX 1003.1-2001 the same command is required to sort the file named +1, and you must instead use the command sort -k 2 to get the field-based sort.

The GNU utilities normally conform to the version of POSIX that is standard for your system.  To cause them to conform to a different version of POSIX, define the _POSIX2_VERSION environment variable to a value of the form YYYYMM specifying the year and month the standard was adopted.  Two values are currently supported for _POSIX2_VERSION: 199209 stands for POSIX 1003.2-1992, and 200112 stands for POSIX 1003.1-2001.  For example, if you are running older software that assumes an older version of POSIX and uses sort +1, you can work around the compatibility problems by setting _POSIX2_VERSION=199209 in your environment.

Output of entire files
**********************

These commands read and write entire files, possibly transforming them in some way.

cat: Concatenate and write files
================================

cat copies each FILE (- means standard input), or standard input if none are given, to standard output.  Synopsis:

     cat [OPTION] [FILE]...

The program accepts the following options.  Also see Common options.

-A
--show-all
     Equivalent to -vET.

-B
--binary
     On MS-DOS and MS-Windows only, read and write the files in binary mode. By default, cat on MS-DOS/MS-Windows uses binary mode only when standard output is redirected to a file or a pipe; this option overrides that.  Binary file I/O is used so that the files retain their format (Unix text as opposed to DOS text and binary), because cat is frequently used as a file-copying program.  Some options (see below) cause cat to read and write files in text mode because in those cases the original file contents aren't important (e.g., when lines are numbered by cat, or when line endings should be marked).  This is so these options work as DOS/Windows users would expect; for example, DOS-style text files have their lines end with the CR-LF pair of characters, which won't be processed as an empty line by -b unless the file is read in text mode.

-b
--number-nonblank
     Number all nonblank output lines, starting with 1.  On MS-DOS and MS-Windows, this option causes cat to read and write files in text mode.

-e
     Equivalent to -vE.

-E
--show-ends
     Display a $ after the end of each line.  On MS-DOS and MS-Windows, this option causes cat to read and write files in text mode.

-n
--number
     Number all output lines, starting with 1.  On MS-DOS and MS-Windows, this option causes cat to read and write files in text mode.

-s
--squeeze-blank
     Replace multiple adjacent blank lines with a single blank line.  On MS-DOS and MS-Windows, this option causes cat to read and write files in text mode.

-t
     Equivalent to -vT.

-T
--show-tabs
     Display TAB characters as ^I.

-u
     Ignored; for Unix compatibility.

-v
--show-nonprinting
     Display control characters except for LFD and TAB using ^ notation and precede characters that have the high bit set with M-.  On MS-DOS and MS-Windows, this option causes cat to read files and standard input in DOS binary mode, so the CR characters at the end of each line are also visible.

tac: Concatenate and write files in reverse
===========================================

tac copies each FILE (- means standard input), or standard input if none are given, to standard output, reversing the records (lines by default) in each separately.  Synopsis:

     tac [OPTION]... [FILE]...

Records are separated by instances of a string (newline by default).  By default, this separator string is attached to the end of the record that it follows in the file.

The program accepts the following options.  Also see Common options.

-b
--before
     The separator is attached to the beginning of the record that it precedes in the file.

-r
--regex
     Treat the separator string as a regular expression.  Users of tac on MS-DOS/MS-Windows should note that, since tac reads files in binary mode, each line of a text file might end with a CR/LF pair instead of the Unix-style LF.

-s SEPARATOR
--separator=SEPARATOR
     Use SEPARATOR as the record separator, instead of newline.

nl: Number lines and write files
================================

nl writes each FILE (- means standard input), or standard input if none are given, to standard output, with line numbers added to some or all of the lines.  Synopsis:

     nl [OPTION]... [FILE]...

nl decomposes its input into (logical) pages; by default, the line number is reset to 1 at the top of each logical page.  nl treats all of the input files as a single document; it does not reset line numbers or logical pages between files.

A logical page consists of three sections: header, body, and footer. Any of the sections can be empty.  Each can be numbered in a different style from the others.

The beginnings of the sections of logical pages are indicated in the input file by a line containing exactly one of these delimiter strings:

\:\:\:
     start of header;

\:\:
     start of body;

\:
     start of footer.

The two characters from which these strings are made can be changed from \ and : via options (see below), but the pattern and length of each string cannot be changed.

A section delimiter is replaced by an empty line on output.  Any text that comes before the first section delimiter string in the input file is considered to be part of a body section, so nl treats a file that contains no section delimiters as a single body section.

The program accepts the following options.  Also see Common options.

-b STYLE
--body-numbering=STYLE
     Select the numbering style for lines in the body section of each logical page.  When a line is not numbered, the current line number is not incremented, but the line number separator character is still prepended to the line.  The styles are:

     a
          number all lines,

     t
          number only nonempty lines (default for body),

     n
          do not number lines (default for header and footer),

     pREGEXP
          number only lines that contain a match for REGEXP.

-d CD
--section-delimiter=CD
     Set the section delimiter characters to CD; default is \:. If only C is given, the second remains :. (Remember to protect \ or other metacharacters from shell expansion with quotes or extra backslashes.)

-f STYLE
--footer-numbering=STYLE
     Analogous to --body-numbering.

-h STYLE
--header-numbering=STYLE
     Analogous to --body-numbering.

-i NUMBER
--page-increment=NUMBER
     Increment line numbers by NUMBER (default 1).

-l NUMBER
--join-blank-lines=NUMBER
     Consider NUMBER (default 1) consecutive empty lines to be one logical line for numbering, and only number the last one.  Where fewer than NUMBER consecutive empty lines occur, do not number them. An empty line is one that contains no characters, not even spaces or tabs.

-n FORMAT
--number-format=FORMAT
     Select the line numbering format (default is rn):

     ln
          left justified, no leading zeros;

     rn
          right justified, no leading zeros;

     rz
          right justified, leading zeros.

-p
--no-renumber
     Do not reset the line number at the start of a logical page.

-s STRING
--number-separator=STRING
     Separate the line number from the text line in the output with STRING (default is the TAB character).

-v NUMBER
--starting-line-number=NUMBER
     Set the initial line number on each logical page to NUMBER (default 1).

-w NUMBER
--number-width=NUMBER
     Use NUMBER characters for line numbers (default 6).

od: Write files in octal or other formats
=========================================

od writes an unambiguous representation of each FILE (- means standard input), or standard input if none are given. Synopses:

     od [OPTION]... [FILE]...
     od --traditional [FILE] [[+]OFFSET [[+]LABEL]]

Each line of output consists of the offset in the input, followed by groups of data from the file. By default, od prints the offset in octal, and each group of file data is two bytes of input printed as a single octal number.

The program accepts the following options.  Also see Common options.

-A RADIX
--address-radix=RADIX
     Select the base in which file offsets are printed.  RADIX can be one of the following:

     d
          decimal;

     o
          octal;

     x
          hexadecimal;

     n
          none (do not print offsets).

     The default is octal.

-j BYTES
--skip-bytes=BYTES
     Skip BYTES input bytes before formatting and writing.  If BYTES begins with 0x or 0X, it is interpreted in hexadecimal; otherwise, if it begins with 0, in octal; otherwise, in decimal.  Appending b multiplies BYTES by 512, k by 1024, and m by 1048576.

-N BYTES
--read-bytes=BYTES
     Output at most BYTES bytes of the input.  Prefixes and suffixes on bytes are interpreted as for the -j option.

-s N
--strings[=N]
     Instead of the normal output, output only string constants: at least N consecutive ASCII graphic characters, followed by a null (zero) byte.

     If N is omitted with --strings, the default is 3.  On older systems, GNU od instead supports an obsolete option -s[N], where N also defaults to 3. POSIX 1003.1-2001 (see Standards conformance) does not allow -s without an argument; use --strings instead.

-t TYPE
--format=TYPE
     Select the format in which to output the file data.  TYPE is a string of one or more of the below type indicator characters.  If you include more than one type indicator character in a single TYPE string, or use this option more than once, od writes one copy of each output line using each of the data types that you specified, in the order that you specified.

     Adding a trailing ``z'' to any type specification appends a display of the ASCII character representation of the printable characters to the output line generated by the type specification.

     a
          named character

     c
          ASCII character or backslash escape,

     d
          signed decimal

     f
          floating point

     o
          octal

     u
          unsigned decimal

     x
          hexadecimal

     The type a outputs things like sp for space, nl for newline, and nul for a null (zero) byte.  Type c outputs , \n, and \0, respectively.

     Except for types a and c, you can specify the number of bytes to use in interpreting each number in the given data type by following the type indicator character with a decimal integer. Alternately, you can specify the size of one of the C compiler's built-in data types by following the type indicator character with one of the following characters.  For integers (d, o, u, x):

     C
          char

     S
          short

     I
          int

     L
          long

     For floating point (f):

     F
          float

     D
          double

     L
          long double

-v
--output-duplicates
     Output consecutive lines that are identical.  By default, when two or more consecutive output lines would be identical, od outputs only the first line, and puts just an asterisk on the following line to indicate the elision.

-w N
--width[=N]
     Dump n input bytes per output line.  This must be a multiple of the least common multiple of the sizes associated with the specified output types.

     If this option is not given at all, the default is 16.  If N is omitted with --width, the default is 32.  On older systems, GNU od instead supports an obsolete option -w[N], where N also defaults to 32.  POSIX 1003.1-2001 (see Standards conformance) does not allow -w without an argument; use --width instead.

The next several options are shorthands for format specifications. GNU od accepts any combination of shorthands and format specification options.  These options accumulate.

-a
     Output as named characters.  Equivalent to -ta.

-b
     Output as octal bytes.  Equivalent to -toC.

-c
     Output as ASCII characters or backslash escapes.  Equivalent to -tc.

-d
     Output as unsigned decimal shorts.  Equivalent to -tu2.

-f
     Output as floats.  Equivalent to -tfF.

-h
     Output as hexadecimal shorts.  Equivalent to -tx2.

-i
     Output as decimal shorts.  Equivalent to -td2.

-l
     Output as decimal longs.  Equivalent to -td4.

-o
     Output as octal shorts.  Equivalent to -to2.

-x
     Output as hexadecimal shorts.  Equivalent to -tx2.

--traditional
     Recognize the non-option arguments that traditional od accepted.  The following syntax:

          od --traditional [FILE] [[+]OFFSET[.][b] [[+]LABEL[.][b]]]

     can be used to specify at most one file and optional arguments specifying an offset and a pseudo-start address, LABEL.  By default, OFFSET is interpreted as an octal number specifying how many input bytes to skip before formatting and writing.  The optional trailing decimal point forces the interpretation of OFFSET as a decimal number.  If no decimal is specified and the offset begins with 0x or 0X it is interpreted as a hexadecimal number.  If there is a trailing b, the number of bytes skipped will be OFFSET multiplied by 512.  The LABEL argument is interpreted just like OFFSET, but it specifies an initial pseudo-address.  The pseudo-addresses are displayed in parentheses following any normal address.

Formatting file contents
************************

These commands reformat the contents of files.

fmt: Reformat paragraph text
============================

fmt fills and joins lines to produce output lines of (at most) a given number of characters (75 by default).  Synopsis:

     fmt [OPTION]... [FILE]...

fmt reads from the specified FILE arguments (or standard input if none are given), and writes to standard output.

By default, blank lines, spaces between words, and indentation are preserved in the output; successive input lines with different indentation are not joined; tabs are expanded on input and introduced on output.

fmt prefers breaking lines at the end of a sentence, and tries to avoid line breaks after the first word of a sentence or before the last word of a sentence.  A sentence break is defined as either the end of a paragraph or a word ending in any of .?!, followed by two spaces or end of line, ignoring any intervening parentheses or quotes. Like TeX, fmt reads entire ``paragraphs'' before choosing line breaks; the algorithm is a variant of that in ``Breaking Paragraphs Into Lines'' (Donald E. Knuth and Michael F. Plass, Software---Practice and Experience, 11 (1981), 1119--1184).

The program accepts the following options.  Also see Common options.

-c
--crown-margin
     Crown margin mode: preserve the indentation of the first two lines within a paragraph, and align the left margin of each subsequent line with that of the second line.

-t
--tagged-paragraph
     Tagged paragraph mode: like crown margin mode, except that if indentation of the first line of a paragraph is the same as the indentation of the second, the first line is treated as a one-line paragraph.

-s
--split-only
     Split lines only.  Do not join short lines to form longer ones.  This prevents sample lines of code, and other such ``formatted'' text from being unduly combined.

-u
--uniform-spacing
     Uniform spacing.  Reduce spacing between words to one space, and spacing between sentences to two spaces.

-WIDTH
-w WIDTH
--width=WIDTH
     Fill output lines up to WIDTH characters (default 75).  fmt initially tries to make lines about 7% shorter than this, to give it room to balance line lengths.

-p PREFIX
--prefix=PREFIX
     Only lines beginning with PREFIX (possibly preceded by whitespace) are subject to formatting. The prefix and any preceding whitespace are stripped for the formatting and then re-attached to each formatted output line.  One use is to format certain kinds of program comments, while leaving the code unchanged.

pr: Paginate or columnate files for printing
============================================

pr writes each FILE (- means standard input), or standard input if none are given, to standard output, paginating and optionally outputting in multicolumn format; optionally merges all FILEs, printing all in parallel, one per column.  Synopsis:

     pr [OPTION]... [FILE]...

By default, a 5-line header is printed at each page: two blank lines; a line with the date, the filename, and the page count; and two more blank lines.  A footer of five blank lines is also printed. With the -F option, a 3-line header is printed: the leading two blank lines are omitted; no footer is used.  The default PAGE_LENGTH in both cases is 66 lines.  The default number of text lines changes from 56 (without -F) to 63 (with -F).  The text line of the header takes the form DATE STRING PAGE, with spaces inserted around STRING so that the line takes up the full PAGE_WIDTH.  Here, DATE is the date (see the -D or --date-format option for details), STRING is the centered header string, and PAGE identifies the page number.  The LC_MESSAGES locale category affects the spelling of PAGE; in the default C locale, it is Page NUMBER where NUMBER is the decimal page number.

Form feeds in the input cause page breaks in the output.  Multiple form feeds produce empty pages.

Columns are of equal width, separated by an optional string (default is space).  For multicolumn output, lines will always be truncated to PAGE_WIDTH (default 72), unless you use the -J option. For single column output no line truncation occurs by default.  Use -W option to truncate lines in that case.

The following changes were made in version 1.22i and apply to later versions of pr: - Brian

  * Some small LETTER OPTIONS (-s, -w) have been redefined for better POSIX compliance.  The output of some further cases has been adapted to other Unix systems.  These changes are not compatible with earlier versions of the program.

  * Some NEW CAPITAL LETTER options (-J, -S, -W) have been introduced to turn off unexpected interferences of small letter options.  The -N option and the second argument LAST_PAGE of +FIRST_PAGE offer more flexibility.  The detailed handling of form feeds set in the input files requires the -T option.

  * Capital letter options override small letter ones.

  * Some of the option-arguments (compare -s, -e, -i, -n) cannot be specified as separate arguments from the preceding option letter (already stated in the POSIX specification).

The program accepts the following options.  Also see Common options.

+FIRST_PAGE[:LAST_PAGE]
--pages=FIRST_PAGE[:LAST_PAGE]
     Begin printing with page FIRST_PAGE and stop with LAST_PAGE. Missing :LAST_PAGE implies end of file.  While estimating the number of skipped pages each form feed in the input file results in a new page.  Page counting with and without +FIRST_PAGE is identical.  By default, counting starts with the first page of input file (not first page printed).  Line numbering may be altered by -N option.

-COLUMN
--columns=COLUMN
     With each single FILE, produce COLUMN columns of output (default is 1) and print columns down, unless -a is used.  The column width is automatically decreased as COLUMN increases; unless you use the -W/-w option to increase PAGE_WIDTH as well. This option might well cause some lines to be truncated.  The number of lines in the columns on each page are balanced.  The options -e and -i are on for multiple text-column output.  Together with -J option column alignment and line truncation is turned off. Lines of full length are joined in a free field format and -S option may set field separators.  -COLUMN may not be used with -m option.

-a
--across
     With each single FILE, print columns across rather than down.  The -COLUMN option must be given with COLUMN greater than one. If a line is too long to fit in a column, it is truncated.

-c
--show-control-chars
     Print control characters using hat notation (e.g., ^G); print other nonprinting characters in octal backslash notation.  By default, nonprinting characters are not changed.

-d
--double-space
     Double space the output.

-D FORMAT
--date-format=FORMAT
     Format header dates using FORMAT, using the same conventions as for the the command date +FORMAT; see date invocation. Except for directives, which start with %, characters in FORMAT are printed unchanged.  You can use this option to specify an arbitrary string in place of the header date, e.g., --date-format="Monday morning".

     If the POSIXLY_CORRECT environment variable is not set, the date format defaults to %Y-%m-%d %H:%M (for example, 2001-12-04 23:59); otherwise, the format depends on the LC_TIME locale category, with the default being %b %e %H:%M %Y (for example, Dec  4 23:59 2001.

-e[IN-TABCHAR[IN-TABWIDTH]]
--expand-tabs[=IN-TABCHAR[IN-TABWIDTH]]
     Expand TABs to spaces on input.  Optional argument IN-TABCHAR is the input tab character (default is the TAB character).  Second optional argument IN-TABWIDTH is the input tab character's width (default is 8).

-f
-F
--form-feed
     Use a form feed instead of newlines to separate output pages.  The default page length of 66 lines is not altered.  But the number of lines of text per page changes from default 56 to 63 lines.

-h HEADER
--header=HEADER
     Replace the filename in the header with the centered string HEADER. When using the shell, HEADER should be quoted and should be separated from -h by a space.

-i[OUT-TABCHAR[OUT-TABWIDTH]]
--output-tabs[=OUT-TABCHAR[OUT-TABWIDTH]]
     Replace spaces with TABs on output.  Optional argument OUT-TABCHAR is the output tab character (default is the TAB character).  Second optional argument OUT-TABWIDTH is the output tab character's width (default is 8).

-J
--join-lines
     Merge lines of full length.  Used together with the column options -COLUMN, -a -COLUMN or -m.  Turns off -W/-w line truncation; no column alignment used; may be used with --sep-string[=STRING].  -J has been introduced (together with -W and --sep-string) to disentangle the old (POSIX-compliant) options -w and -s along with the three column options.

-l PAGE_LENGTH
--length=PAGE_LENGTH
     Set the page length to PAGE_LENGTH (default 66) lines, including the lines of the header [and the footer].  If PAGE_LENGTH is less than or equal to 10 (or <= 3 with -F), the header and footer are omitted, and all form feeds set in input files are eliminated, as if the -T option had been given.

-m
--merge
     Merge and print all FILEs in parallel, one in each column.  If a line is too long to fit in a column, it is truncated, unless the -J option is used.  --sep-string[=STRING] may be used. Empty pages in some FILEs (form feeds set) produce empty columns, still marked by STRING.  The result is a continuous line numbering and column marking throughout the whole merged file.  Completely empty merged pages show no separators or line numbers.  The default header becomes DATE PAGE with spaces inserted in the middle; this may be used with the -h or --header option to fill up the middle blank part.

-n[NUMBER-SEPARATOR[DIGITS]]
--number-lines[=NUMBER-SEPARATOR[DIGITS]]
     Provide DIGITS digit line numbering (default for DIGITS is 5).  With multicolumn output the number occupies the first DIGITS column positions of each text column or only each line of -m output.  With single column output the number precedes each line just as -m does.  Default counting of the line numbers starts with the first line of the input file (not the first line printed, compare the --page option and -N option). Optional argument NUMBER-SEPARATOR is the character appended to the line number to separate it from the text followed.  The default separator is the TAB character.  In a strict sense a TAB is always printed with single column output only.  The TAB-width varies with the TAB-position, e.g. with the left MARGIN specified by -o option.  With multicolumn output priority is given to equal width of output columns (a POSIX specification). The TAB-width is fixed to the value of the first column and does not change with different values of left MARGIN.  That means a fixed number of spaces is always printed in the place of the NUMBER-SEPARATOR TAB.  The tabification depends upon the output position.

-N LINE_NUMBER
--first-line-number=LINE_NUMBER
     Start line counting with the number LINE_NUMBER at first line of first page printed (in most cases not the first line of the input file).

-o MARGIN
--indent=MARGIN
     Indent each line with a margin MARGIN spaces wide (default is zero). The total page width is the size of the margin plus the PAGE_WIDTH set with the -W/-w option.  A limited overflow may occur with numbered single column output (compare -n option).

-r
--no-file-warnings
     Do not print a warning message when an argument FILE cannot be opened.  (The exit status will still be nonzero, however.)

-s[CHAR]
--separator[=CHAR]
     Separate columns by a single character CHAR.  The default for CHAR is the TAB character without -w and no character with -w.  Without -s the default separator space is set.  -s[char] turns off line truncation of all three column options (-COLUMN|-a -COLUMN|-m) unless -w is set.  This is a POSIX-compliant formulation.

-S STRING
--sep-string[=STRING]
     Use STRING to separate output columns.  The -S option doesn't affect the -W/-w option, unlike the -s option which does.  It does not affect line truncation or column alignment. Without -S, and with -J, pr uses the default output separator, TAB. Without -S or -J, pr uses a space (same as -S" ").  With -SSTRING, STRING must be nonempty; --sep-string with no STRING is equivalent to --sep-string="".

     On older systems, pr instead supports an obsolete option -S[STRING], where STRING is optional.  POSIX 1003.1-2001 (see Standards conformance) does not allow this older usage.  To specify an empty STRING portably, use --sep-string.

-t
--omit-header
     Do not print the usual header [and footer] on each page, and do not fill out the bottom of pages (with blank lines or a form feed).  No page structure is produced, but form feeds set in the input files are retained. The predefined pagination is not changed.  -t or -T may be useful together with other options; e.g.: -t -e4, expand TAB characters in the input file to 4 spaces but don't make any other changes.  Use of -t overrides -h.

-T
--omit-pagination
     Do not print header [and footer].  In addition eliminate all form feeds set in the input files.

-v
--show-nonprinting
     Print nonprinting characters in octal backslash notation.

-w PAGE_WIDTH
--width=PAGE_WIDTH
     Set page width to PAGE_WIDTH characters for multiple text-column output only (default for PAGE_WIDTH is 72).  -s[CHAR] turns off the default page width and any line truncation and column alignment. Lines of full length are merged, regardless of the column options set.  No PAGE_WIDTH setting is possible with single column output. A POSIX-compliant formulation.

-W PAGE_WIDTH
--page_width=PAGE_WIDTH
     Set the page width to PAGE_WIDTH characters.  That's valid with and without a column option.  Text lines are truncated, unless -J is used.  Together with one of the three column options (-COLUMN, -a -COLUMN or -m) column alignment is always used.  The separator options -S or -s don't affect the -W option.  Default is 72 characters.  Without -W PAGE_WIDTH and without any of the column options NO line truncation is used (defined to keep downward compatibility and to meet most frequent tasks).  That's equivalent to -W 72 -J.  The header line is never truncated.

fold: Wrap input lines to fit in specified width
================================================

fold writes each FILE (- means standard input), or standard input if none are given, to standard output, breaking long lines.  Synopsis:

     fold [OPTION]... [FILE]...

By default, fold breaks lines wider than 80 columns.  The output is split into as many lines as necessary.

fold counts screen columns by default; thus, a tab may count more than one column, backspace decreases the column count, and carriage return sets the column to zero.

The program accepts the following options.  Also see Common options.

-b
--bytes
     Count bytes rather than columns, so that tabs, backspaces, and carriage returns are each counted as taking up one column, just like other characters.

-s
--spaces
     Break at word boundaries: the line is broken after the last blank before the maximum line length.  If the line contains no such blanks, the line is broken at the maximum line length as usual.

-w WIDTH
--width=WIDTH
     Use a maximum line length of WIDTH columns instead of 80.

     On older systems, fold supports an obsolete option -WIDTH.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -w WIDTH instead.

Output of parts of files
************************

These commands output pieces of the input.

head: Output the first part of files
====================================

head prints the first part (10 lines by default) of each FILE; it reads from standard input if no files are given or when given a FILE of -.  Synopsis:

     head [OPTION]... [FILE]...

If more than one FILE is specified, head prints a one-line header consisting of

     ==> FILE NAME <==

before the output for each FILE.

The program accepts the following options.  Also see Common options.

-c BYTES
--bytes=BYTES
     Print the first BYTES bytes, instead of initial lines.  Appending b multiplies BYTES by 512, k by 1024, and m by 1048576.

-n N
--lines=N
     Output the first N lines.

-q
--quiet
--silent
     Never print file name headers.

-v
--verbose
     Always print file name headers.

On older systems, head supports an obsolete option -COUNTOPTIONS, which is recognized only if it is specified first.  COUNT is a decimal number optionally followed by a size letter (b, k, m) as in -c, or l to mean count by lines, or other option letters (cqv). POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -c COUNT or -n COUNT instead.

tail: Output the last part of files
===================================

tail prints the last part (10 lines by default) of each FILE; it reads from standard input if no files are given or when given a FILE of -.  Synopsis:

     tail [OPTION]... [FILE]...

If more than one FILE is specified, tail prints a one-line header consisting of

     ==> FILE NAME <==

before the output for each FILE.

GNU tail can output any amount of data (some other versions of tail cannot).  It also has no -r option (print in reverse), since reversing a file is really a different job from printing the end of a file; BSD tail (which is the one with -r) can only reverse files that are at most as large as its buffer, which is typically 32 KiB.  A more reliable and versatile way to reverse files is the GNU tac command.

If any option-argument is a number N starting with a +, tail begins printing with the Nth item from the start of each file, instead of from the end.

The program accepts the following options.  Also see Common options.

-c BYTES
--bytes=BYTES
     Output the last BYTES bytes, instead of final lines.  Appending b multiplies BYTES by 512, k by 1024, and m by 1048576.

-f
--follow[=HOW]
     Loop forever trying to read more characters at the end of the file, presumably because the file is growing.  This option is ignored when reading from a pipe. If more than one file is given, tail prints a header whenever it gets output from a different file, to indicate which file that output is from.

     There are two ways to specify how you'd like to track files with this option, but that difference is noticeable only when a followed file is removed or renamed. If you'd like to continue to track the end of a growing file even after it has been unlinked, use --follow=descriptor.  This is the default behavior, but it is not useful if you're tracking a log file that may be rotated (removed or renamed, then reopened).  In that case, use --follow=name to track the named file by reopening it periodically to see if it has been removed and recreated by some other program.

     No matter which method you use, if the tracked file is determined to have shrunk, tail prints a message saying the file has been truncated and resumes tracking the end of the file from the newly-determined endpoint.

     When a file is removed, tail's behavior depends on whether it is following the name or the descriptor.  When following by name, tail can detect that a file has been removed and gives a message to that effect, and if --retry has been specified it will continue checking periodically to see if the file reappears. When following a descriptor, tail does not detect that the file has been unlinked or renamed and issues no message;  even though the file may no longer be accessible via its original name, it may still be growing.

     The option values descriptor and name may be specified only with the long form of the option, not with -f.

-F
     This option is the same as --follow=name --retry.  That is, tail will attempt to reopen a file when it is removed.  Should this fail, tail will keep trying until it becomes accessible again.

--retry
     This option is meaningful only when following by name. Without this option, when tail encounters a file that doesn't exist or is otherwise inaccessible, it reports that fact and never checks it again.

--sleep-interval=NUMBER
     Change the number of seconds to wait between iterations (the default is 1.0). During one iteration, every specified file is checked to see if it has Historical implementations of tail have required that NUMBER be an integer.  However, GNU tail accepts an arbitrary floating point number.

--pid=PID
     When following by name or by descriptor, you may specify the process ID, PID, of the sole writer of all FILE arguments.  Then, shortly after that process terminates, tail will also terminate.  This will work properly only if the writer and the tailing process are running on the same machine.  For example, to save the output of a build in a file and to watch the file grow, if you invoke make and tail like this then the tail process will stop when your build completes. Without this option, you would have had to kill the tail -f process yourself.

          $ make >& makerr & tail --pid=$! -f makerr

     If you specify a PID that is not in use or that does not correspond to the process that is writing to the tailed files, then tail may terminate long before any FILEs stop growing or it may not terminate until long after the real writer has terminated. Note that --pid cannot be supported on some systems; tail will print a warning if this is the case.

--max-unchanged-stats=N
     When tailing a file by name, if there have been N (default n=) consecutive iterations for which the size has remained the same, then open/fstat the file to determine if that file name is still associated with the same device/inode-number pair as before. When following a log file that is rotated, this is approximately the number of seconds between when tail prints the last pre-rotation lines and when it prints the lines that have accumulated in the new log file. This option is meaningful only when following by name.

-n N
--lines=N
     Output the last N lines.

-q
--quiet
--silent
     Never print file name headers.

-v
--verbose
     Always print file name headers.

On older systems, tail supports an obsolete option -COUNTOPTIONS, which is recognized only if it is specified first.  COUNT is a decimal number optionally followed by a size letter (b, k, m) as in -c, or l to mean count by lines, or other option letters (cfqv).  Some older tail implementations also support an obsolete option +COUNT with the same meaning as -+COUNT.  POSIX 1003.1-2001 (see Standards conformance) does not allow these options; use -c COUNT or -n COUNT instead.

split: Split a file into fixed-size pieces
==========================================

split creates output files containing consecutive sections of INPUT (standard input if none is given or INPUT is -).  Synopsis:

     split [OPTION] [INPUT [PREFIX]]

By default, split puts 1000 lines of INPUT (or whatever is left over for the last section), into each output file.

The output files' names consist of PREFIX (x by default) followed by a group of letters (aa, ab, ... by default), such that concatenating the output files in sorted order by file name produces the original input file.  If the output file names are exhausted, split reports an error without deleting the output files that it did create.

The program accepts the following options.  Also see Common options.

-a LENGTH
--suffix-length=LENGTH
     Use suffixes of length LENGTH.  The default LENGTH is 2.

-l LINES
--lines=LINES
     Put LINES lines of INPUT into each output file.

     On older systems, split supports an obsolete option -LINES.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -l LINES instead.

-b BYTES
--bytes=BYTES
     Put the first BYTES bytes of INPUT into each output file. Appending b multiplies BYTES by 512, k by 1024, and m by 1048576.

-C BYTES
--line-bytes=BYTES
     Put into each output file as many complete lines of INPUT as possible without exceeding BYTES bytes.  For lines longer than BYTES bytes, put BYTES bytes into each output file until less than BYTES bytes of the line are left, then continue normally.  BYTES has the same format as for the --bytes option.

--verbose
     Write a diagnostic to standard error just before each output file is opened.

csplit: Split a file into context-determined pieces
===================================================

csplit creates zero or more output files containing sections of INPUT (standard input if INPUT is -).  Synopsis:

     csplit [OPTION]... INPUT PATTERN...

The contents of the output files are determined by the PATTERN arguments, as detailed below.  An error occurs if a PATTERN argument refers to a nonexistent line of the input file (e.g., if no remaining line matches a given regular expression).  After every PATTERN has been matched, any remaining input is copied into one last output file.

By default, csplit prints the number of bytes written to each output file after it has been created.

The types of pattern arguments are:

N
     Create an output file containing the input up to but not including line N (a positive integer).  If followed by a repeat count, also create an output file containing the next LINE lines of the input file once for each repeat.

/REGEXP/[OFFSET]
     Create an output file containing the current line up to (but not including) the next line of the input file that contains a match for REGEXP.  The optional OFFSET is a + or - followed by a positive integer.  If it is given, the input up to the matching line plus or minus OFFSET is put into the output file, and the line after that begins the next section of input.

%REGEXP%[OFFSET]
     Like the previous type, except that it does not create an output file, so that section of the input file is effectively ignored.

{REPEAT-COUNT}
     Repeat the previous pattern REPEAT-COUNT additional times. REPEAT-COUNT can either be a positive integer or an asterisk, meaning repeat as many times as necessary until the input is exhausted.

The output files' names consist of a prefix (xx by default) followed by a suffix.  By default, the suffix is an ascending sequence of two-digit decimal numbers from 00 to 99.  In any case, concatenating the output files in sorted order by filename produces the original input file.

By default, if csplit encounters an error or receives a hangup, interrupt, quit, or terminate signal, it removes any output files that it has created so far before it exits.

The program accepts the following options.  Also see Common options.

-f PREFIX
--prefix=PREFIX
     Use PREFIX as the output file name prefix.

-b SUFFIX
--suffix=SUFFIX
     Use SUFFIX as the output file name suffix.  When this option is specified, the suffix string must include exactly one printf(3)-style conversion specification, possibly including format specification flags, a field width, a precision specifications, or all of these kinds of modifiers.  The format letter must convert a binary integer argument to readable form; thus, only d, i, u, o, x, and X conversions are allowed.  The entire SUFFIX is given (with the current output file number) to sprintf(3) to form the file name suffixes for each of the individual output files in turn.  If this option is used, the --digits option is ignored.

-n DIGITS
--digits=DIGITS
     Use output file names containing numbers that are DIGITS digits long instead of the default 2.

-k
--keep-files
     Do not remove output files when errors are encountered.

-z
--elide-empty-files
     Suppress the generation of zero-length output files.  (In cases where the section delimiters of the input file are supposed to mark the first lines of each of the sections, the first output file will generally be a zero-length file unless you use this option.)  The output file sequence numbers always run consecutively starting from 0, even when this option is specified.

-s
-q
--silent
--quiet
     Do not print counts of output file sizes.

Summarizing files
*****************

These commands generate just a few numbers representing entire contents of files.

wc: Print byte, word, and line counts
=====================================

wc counts the number of bytes, characters, whitespace-separated words, and newlines in each given FILE, or standard input if none are given or for a FILE of -.  Synopsis:

     wc [OPTION]... [FILE]...

wc prints one line of counts for each file, and if the file was given as an argument, it prints the file name following the counts.  If more than one FILE is given, wc prints a final line containing the cumulative counts, with the file name total.  The counts are printed in this order: newlines, words, characters, bytes. By default, each count is output right-justified in a 7-byte field with one space between fields so that the numbers and file names line up nicely in columns.  However, POSIX requires that there be exactly one space separating columns.  You can make wc use the POSIX-mandated output format by setting the POSIXLY_CORRECT environment variable.

By default, wc prints three counts: the newline, words, and byte counts.  Options can specify that only certain counts be printed. Options do not undo others previously given, so

     wc --bytes --words

prints both the byte counts and the word counts.

With the --max-line-length option, wc prints the length of the longest line per file, and if there is more than one file it prints the maximum (not the sum) of those lengths.

The program accepts the following options.  Also see Common options.

-c
--bytes
     Print only the byte counts.

-m
--chars
     Print only the character counts.

-w
--words
     Print only the word counts.

-l
--lines
     Print only the newline counts.

-L
--max-line-length
     Print only the maximum line lengths.

sum: Print checksum and block counts
====================================

sum computes a 16-bit checksum for each given FILE, or standard input if none are given or for a FILE of -.  Synopsis:

     sum [OPTION]... [FILE]...

sum prints the checksum for each FILE followed by the number of blocks in the file (rounded up).  If more than one FILE is given, file names are also printed (by default).  (With the --sysv option, corresponding file names are printed when there is at least one file argument.)

By default, GNU sum computes checksums using an algorithm compatible with BSD sum and prints file sizes in units of 1024-byte blocks.

The program accepts the following options.  Also see Common options.

-r
     Use the default (BSD compatible) algorithm.  This option is included for compatibility with the System V sum.  Unless -s was also given, it has no effect.

-s
--sysv
     Compute checksums using an algorithm compatible with System V sum's default, and print file sizes in units of 512-byte blocks.

sum is provided for compatibility; the cksum program (see next section) is preferable in new applications.

cksum: Print CRC checksum and byte counts
=========================================

cksum computes a cyclic redundancy check (CRC) checksum for each given FILE, or standard input if none are given or for a FILE of -.  Synopsis:

     cksum [OPTION]... [FILE]...

cksum prints the CRC checksum for each file along with the number of bytes in the file, and the filename unless no arguments were given.

cksum is typically used to ensure that files transferred by unreliable means (e.g., netnews) have not been corrupted, by comparing the cksum output for the received files with the cksum output for the original files (typically given in the distribution).

The CRC algorithm is specified by the POSIX standard.  It is not compatible with the BSD or System V sum algorithms (see the previous section); it is more robust.

The only options are --help and --version.  See Common options.

md5sum: Print or check message-digests
======================================

md5sum computes a 128-bit checksum (or fingerprint or message-digest) for each specified FILE. If a FILE is specified as - or if no files are given md5sum computes the checksum for the standard input. md5sum can also determine whether a file and checksum are consistent. Synopses:

     md5sum [OPTION]... [FILE]...
     md5sum [OPTION]... --check [FILE]

For each FILE, md5sum outputs the MD5 checksum, a flag indicating a binary or text input file, and the filename. If FILE is omitted or specified as -, standard input is read.

The program accepts the following options.  Also see Common options.

-b
--binary
     Treat all input files as binary.  This option has no effect on Unix systems, since they don't distinguish between binary and text files. This option is useful on systems that have different internal and external character representations.  On MS-DOS and MS-Windows, this is the default.

-c
--check
     Read filenames and checksum information from the single FILE (or from stdin if no FILE was specified) and report whether each named file and the corresponding checksum data are consistent. The input to this mode of md5sum is usually the output of a prior, checksum-generating run of md5sum. Each valid line of input consists of an MD5 checksum, a binary/text flag, and then a filename. Binary files are marked with *, text with  . For each such line, md5sum reads the named file and computes its MD5 checksum.  Then, if the computed message digest does not match the one on the line with the filename, the file is noted as having failed the test.  Otherwise, the file passes the test. By default, for each valid line, one line is written to standard output indicating whether the named file passed the test. After all checks have been performed, if there were any failures, a warning is issued to standard error. Use the --status option to inhibit that output. If any listed file cannot be opened or read, if any valid line has an MD5 checksum inconsistent with the associated file, or if no valid line is found, md5sum exits with nonzero status.  Otherwise, it exits successfully.

--status
     This option is useful only when verifying checksums. When verifying checksums, don't generate the default one-line-per-file diagnostic and don't output the warning summarizing any failures. Failures to open or read a file still evoke individual diagnostics to standard error. If all listed files are readable and are consistent with the associated MD5 checksums, exit successfully.  Otherwise exit with a status code indicating there was a failure.

-t
--text
     Treat all input files as text files.  This is the reverse of --binary.

-w
--warn
     When verifying checksums, warn about improperly formatted MD5 checksum lines. This option is useful only if all but a few lines in the checked input are valid.

Operating on sorted files
*************************

These commands work with (or produce) sorted files.

sort: Sort text files
=====================

sort sorts, merges, or compares all the lines from the given files, or standard input if none are given or for a FILE of -.  By default, sort writes the results to standard output.  Synopsis:

     sort [OPTION]... [FILE]...

sort has three modes of operation: sort (the default), merge, and check for sortedness.  The following options change the operation mode:

-c
--check
     Check whether the given files are already sorted: if they are not all sorted, print an error message and exit with a status of 1. Otherwise, exit successfully.

-m
--merge
     Merge the given files by sorting them as a group.  Each input file must always be individually sorted.  It always works to sort instead of merge; merging is provided because it is faster, in the case where it works.

A pair of lines is compared as follows: if any key fields have been specified, sort compares each pair of fields, in the order specified on the command line, according to the associated ordering options, until a difference is found or no fields are left. Unless otherwise specified, all comparisons use the character collating sequence specified by the LC_COLLATE locale.   (If you use a non-POSIX locale (e.g., by setting LC_ALL to en_US), then sort may produce output that is sorted differently than you're accustomed to.  In that case, set the LC_ALL environment variable to C.  Note that setting only LC_COLLATE has two problems.  First, it is ineffective if LC_ALL is also set. Second, it has undefined behavior if LC_CTYPE (or LANG, if LC_CTYPE is unset) is set to an incompatible value.  For example, you get undefined behavior if LC_CTYPE is ja_JP.PCK but LC_COLLATE is en_US.UTF-8.)

If any of the global options bdfgiMnr are given but no key fields are specified, sort compares the entire lines according to the global options.

Finally, as a last resort when all keys compare equal (or if no ordering options were specified at all), sort compares the entire lines. The last resort comparison honors the --reverse (-r) global option.  The --stable (-s) option disables this last-resort comparison so that lines in which all fields compare equal are left in their original relative order.  If no fields or global options are specified, --stable (-s) has no effect.

GNU sort (as specified for all GNU utilities) has no limits on input line length or restrictions on bytes allowed within lines.  In addition, if the final byte of an input file is not a newline, GNU sort silently supplies one.  A line's trailing newline is not part of the line for comparison purposes.

Upon any error, sort exits with a status of 2.

If the environment variable TMPDIR is set, sort uses its value as the directory for temporary files instead of /tmp.  The --temporary-directory (-T) option in turn overrides the environment variable.

The following options affect the ordering of output lines.  They may be specified globally or as part of a specific key field.  If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own.  In pre-POSIX versions of sort, global options affect only later key fields, so portable shell scripts should specify global options first.

-b
--ignore-leading-blanks
     Ignore leading blanks when finding sort keys in each line. The LC_CTYPE locale determines character types.

-d
--dictionary-order
     Sort in phone directory order: ignore all characters except letters, digits and blanks when sorting. The LC_CTYPE locale determines character types.

-f
--ignore-case
     Fold lowercase characters into the equivalent uppercase characters when comparing so that, for example, b and B sort as equal. The LC_CTYPE locale determines character types.

-g
--general-numeric-sort
     Sort numerically, using the standard C function strtod to convert a prefix of each line to a double-precision floating point number. This allows floating point numbers to be specified in scientific notation, like 1.0e-34 and 10e100. The LC_NUMERIC locale determines the decimal-point character. Do not report overflow, underflow, or conversion errors. Use the following collating sequence:

       * Lines that do not start with numbers (all considered to be equal).

       * NaNs (``Not a Number'' values, in IEEE floating point arithmetic) in a consistent but machine-dependent order.

       * Minus infinity.

       * Finite numbers in ascending numeric order (with -0 and +0 equal).

       * Plus infinity.

     Use this option only if there is no alternative; it is much slower than --numeric-sort (-n) and it can lose information when converting to floating point.

-i
--ignore-nonprinting
     Ignore nonprinting characters. The LC_CTYPE locale determines character types.

-M
--month-sort
     An initial string, consisting of any amount of whitespace, followed by a month name abbreviation, is folded to UPPER case and compared in the order JAN < FEB < ... < DEC. Invalid names compare low to valid names.  The LC_TIME locale category determines the month spellings.

-n
--numeric-sort
     Sort numerically: the number begins each line; specifically, it consists of optional whitespace, an optional - sign, and zero or more digits possibly separated by thousands separators, optionally followed by a decimal-point character and zero or more digits.  The LC_NUMERIC locale specifies the decimal-point character and thousands separator.

     Numeric sort uses what might be considered an unconventional method to compare strings representing floating point numbers.  Rather than first converting each string to the C double type and then comparing those values, sort aligns the decimal-point characters in the two strings and compares the strings a character at a time.  One benefit of using this approach is its speed.  In practice this is much more efficient than performing the two corresponding string-to-double (or even string-to-integer) conversions and then comparing doubles.  In addition, there is no corresponding loss of precision.  Converting each string to double before comparison would limit precision to about 16 digits on most systems.

     Neither a leading + nor exponential notation is recognized. To compare such strings numerically, use the --general-numeric-sort (-g) option.

-r
--reverse
     Reverse the result of comparison, so that lines with greater key values appear earlier in the output instead of later.

Other options are:

-o OUTPUT-FILE
--output=OUTPUT-FILE
     Write output to OUTPUT-FILE instead of standard output. If necessary, sort reads input before opening OUTPUT-FILE, so you can safely sort a file in place by using commands like sort -o F F and cat F | sort -o F.

     On newer systems, -o cannot appear after an input file if POSIXLY_CORRECT is set, e.g., sort F -o F.  Portable scripts should specify -o OUTPUT-FILE before any input files.

-S SIZE
--buffer-size=SIZE
     Use a main-memory sort buffer of the given SIZE.  By default, SIZE is in units of 1024 bytes.  Appending % causes SIZE to be interpreted as a percentage of physical memory. Appending K multiplies SIZE by 1024 (the default), M by 1,048,576, G by 1,073,741,824, and so on for T, P, E, Z, and Y.  Appending b causes SIZE to be interpreted as a byte count, with no multiplication.

     This option can improve the performance of sort by causing it to start with a larger or smaller sort buffer than the default. However, this option affects only the initial buffer size.  The buffer grows beyond SIZE if sort encounters input lines larger than SIZE.

-t SEPARATOR
--field-separator=SEPARATOR
     Use character SEPARATOR as the field separator when finding the sort keys in each line.  By default, fields are separated by the empty string between a non-whitespace character and a whitespace character. That is, given the input line  foo bar, sort breaks it into fields  foo and  bar.  The field separator is not considered to be part of either the field preceding or the field following.  But note that sort fields that extend to the end of the line, as -k 2, or sort fields consisting of a range, as -k 2,3, retain the field separators present between the endpoints of the range.

-T TEMPDIR
--temporary-directory=TEMPDIR
     Use directory TEMPDIR to store temporary files, overriding the TMPDIR environment variable.  If this option is given more than once, temporary files are stored in all the directories given.  If you have a large sort or merge that is I/O-bound, you can often improve performance by using this option to specify directories on different disks and controllers.

-u
--unique

     Normally, output only the first of a sequence of lines that compare equal.  For the --check (-c) option, check that no pair of consecutive lines compares equal.

-k POS1[,POS2]
--key=POS1[,POS2]
     Specify a sort field that consists of the part of the line between POS1 and POS2 (or the end of the line, if POS2 is omitted), inclusive.  Fields and character positions are numbered starting with 1.  So to sort on the second field, you'd use --key=2,2 (-k 2,2).  See below for more examples.

-z
--zero-terminated
     Treat the input as a set of lines, each terminated by a zero byte (ASCII NUL (Null) character) instead of an ASCII LF (Line Feed). This option can be useful in conjunction with perl -0 or find -print0 and xargs -0 which do the same in order to reliably handle arbitrary pathnames (even those which contain Line Feed characters.)

Historical (BSD and System V) implementations of sort have differed in their interpretation of some options, particularly -b, -f, and -n.  GNU sort follows the POSIX behavior, which is usually (but not always!) like the System V behavior. According to POSIX, -n no longer implies -b.  For consistency, -M has been changed in the same way.  This may affect the meaning of character positions in field specifications in obscure cases.  The only fix is to add an explicit -b.

A position in a sort field specified with the -k option has the form F.C, where F is the number of the field to use and C is the number of the first character from the beginning of the field.  In a start position, an omitted .C stands for the field's first character.  In an end position, an omitted or zero .C stands for the field's last character.  If the -b option was specified, the .C part of a field specification is counted from the first nonblank character of the field.

A sort key position may also have any of the option letters Mbdfinr appended to it, in which case the global ordering options are not used for that particular field.  The -b option may be independently attached to either or both of the start and end positions of a field specification, and if it is inherited from the global options it will be attached to both. Keys may span multiple fields.

On older systems, sort supports an obsolete origin-zero syntax +POS1 [-POS2] for specifying sort keys. POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -k instead.

Here are some examples to illustrate various combinations of options.

  * Sort in descending (reverse) numeric order.

          sort -nr

  * Sort alphabetically, omitting the first and second fields. This uses a single key composed of the characters beginning at the start of field three and extending to the end of each line.

          sort -k 3

  * Sort numerically on the second field and resolve ties by sorting alphabetically on the third and fourth characters of field five. Use : as the field delimiter.

          sort -t : -k 2,2n -k 5.3,5.4

     Note that if you had written -k 2 instead of -k 2,2 sort would have used all characters beginning in the second field and extending to the end of the line as the primary numeric key.  For the large majority of applications, treating keys spanning more than one field as numeric will not do what you expect.

     Also note that the n modifier was applied to the field-end specifier for the first key.  It would have been equivalent to specify -k 2n,2 or -k 2n,2n.  All modifiers except b apply to the associated field, regardless of whether the modifier character is attached to the field-start and/or the field-end part of the key specifier.

  * Sort the password file on the fifth field and ignore any leading white space.  Sort lines with equal values in field five on the numeric user ID in field three.

          sort -t : -k 5b,5 -k 3,3n /etc/passwd

     An alternative is to use the global numeric modifier -n.

          sort -t : -n -k 5b,5 -k 3,3 /etc/passwd

  * Generate a tags file in case-insensitive sorted order.

          find src -type f -print0 | sort -t / -z -f | xargs -0 etags --append

     The use of -print0, -z, and -0 in this case means that pathnames that contain Line Feed characters will not get broken up by the sort operation.

     Finally, to ignore both leading and trailing white space, you could have applied the b modifier to the field-end specifier for the first key,

          sort -t : -n -k 5b,5b -k 3,3 /etc/passwd

     or by using the global -b modifier instead of -n and an explicit n with the second key specifier.

          sort -t : -b -k 5,5 -k 3,3n /etc/passwd

uniq: Uniquify files
====================

uniq writes the unique lines in the given input, or standard input if nothing is given or for an INPUT name of -.  Synopsis:

     uniq [OPTION]... [INPUT [OUTPUT]]

By default, uniq prints the unique lines in a sorted file, i.e., discards all but one of identical successive lines.  Optionally, it can instead show only lines that appear exactly once, or lines that appear more than once.

The input need not be sorted, but duplicate input lines are detected only if they are adjacent.  If you want to discard non-adjacent duplicate lines, perhaps you want to use sort -u.

Comparisons use the character collating sequence specified by the LC_COLLATE locale category.

If no OUTPUT file is specified, uniq writes to standard output.

The program accepts the following options.  Also see Common options.

-f N
--skip-fields=N
     Skip N fields on each line before checking for uniqueness.  Fields are sequences of non-space non-tab characters that are separated from each other by at least one space or tab.

     On older systems, uniq supports an obsolete option -N.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -f N instead.

-s N
--skip-chars=N
     Skip N characters before checking for uniqueness.  If you use both the field and character skipping options, fields are skipped over first.

     On older systems, uniq supports an obsolete option +N.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -s N instead.

-c
--count
     Print the number of times each line occurred along with the line.

-i
--ignore-case
     Ignore differences in case when comparing lines.

-d
--repeated
     Print one copy of each duplicate line.

-D
--all-repeated[=DELIMIT-METHOD]
     Print all copies of each duplicate line. This option is useful mainly in conjunction with other options e.g., to ignore case or to compare only selected fields. The optional DELIMIT-METHOD tells how to delimit groups of duplicate lines, and must be one of the following:

     none
          Do not delimit groups of duplicate lines. This is equivalent to --all-repeated (-D).

     prepend
          Output a newline before each group of duplicate lines.

     separate
          Separate groups of duplicate lines with a single newline. This is the same as using prepend, except that there is no newline before the first group, and hence may be better suited for output direct to users.

     Note that when groups are delimited and the input stream contains two or more consecutive blank lines, then the output is ambiguous. To avoid that, filter the input through tr -s '\n' to replace each sequence of consecutive newlines with a single newline.

     This is a GNU extension.

-u
--unique
     Print non-duplicate lines.

-w N
--check-chars=N
     Compare N characters on each line (after skipping any specified fields and characters).  By default the entire rest of the lines are compared.

comm: Compare two sorted files line by line
===========================================

comm writes to standard output lines that are common, and lines that are unique, to two input files; a file name of - means standard input.  Synopsis:

     comm [OPTION]... FILE1 FILE2

Before comm can be used, the input files must be sorted using the collating sequence specified by the LC_COLLATE locale. If an input file ends in a non-newline character, a newline is silently appended.  The sort command with no options always outputs a file that is suitable input to comm.

With no options, comm produces three column output.  Column one contains lines unique to FILE1, column two contains lines unique to FILE2, and column three contains lines common to both files. Columns are separated by a single TAB character.

The options -1, -2, and -3 suppress printing of the corresponding columns.  Also see Common options.

Unlike some other comparison utilities, comm has an exit status that does not depend on the result of the comparison. Upon normal completion comm produces an exit code of zero. If there is an error it exits with nonzero status.

tsort: Topological sort
=======================

tsort performs a topological sort on the given FILE, or standard input if no input file is given or for a FILE of -.  For more details and some history, see tsort background. Synopsis:

     tsort [OPTION] [FILE]

tsort reads its input as pairs of strings, separated by blanks, indicating a partial ordering.  The output is a total ordering that corresponds to the given partial ordering.

For example

     tsort <<EOF
     a b c
     d
     e f
     b c d e
     EOF

will produce the output

     a
     b
     c
     d
     e
     f

Consider a more realistic example. You have a large set of functions all in one file, and they may all be declared static except one.  Currently that one (say main) is the first function defined in the file, and the ones it calls directly follow it, followed by those they call, etc.  Let's say that you are determined to take advantage of prototypes, so you have to choose between declaring all of those functions (which means duplicating a lot of information from the definitions) and rearranging the functions so that as many as possible are defined before they are used.  One way to automate the latter process is to get a list for each function of the functions it calls directly. Many programs can generate such lists.  They describe a call graph. Consider the following list, in which a given line indicates that the function on the left calls the one on the right directly.

     main parse_options
     main tail_file
     main tail_forever
     tail_file pretty_name
     tail_file write_header
     tail_file tail
     tail_forever recheck
     tail_forever pretty_name
     tail_forever write_header
     tail_forever dump_remainder
     tail tail_lines
     tail tail_bytes
     tail_lines start_lines
     tail_lines dump_remainder
     tail_lines file_lines
     tail_lines pipe_lines
     tail_bytes xlseek
     tail_bytes start_bytes
     tail_bytes dump_remainder
     tail_bytes pipe_bytes
     file_lines dump_remainder
     recheck pretty_name

then you can use tsort to produce an ordering of those functions that satisfies your requirement.

     example$ tsort call-graph | tac
     dump_remainder
     start_lines
     file_lines
     pipe_lines
     xlseek
     start_bytes
     pipe_bytes
     tail_lines
     tail_bytes
     pretty_name
     write_header
     tail
     recheck
     parse_options
     tail_file
     tail_forever
     main

tsort detects any cycles in the input and writes the first cycle encountered to standard error.

Note that for a given partial ordering, generally there is no unique total ordering.  In the context of the call graph above, the function parse_options may be placed anywhere in the list as long as it precedes main.

The only options are --help and --version.  See Common options.

tsort: Background
=================

tsort exists because very early versions of the Unix linker processed an archive file exactly once, and in order.  As ld read each object in the archive, it decided whether it was needed in the program based on whether it defined any symbols which were undefined at that point in the link.

This meant that dependencies within the archive had to be handled specially.  For example, scanf probably calls read.  That means that in a single pass through an archive, it was important for scanf.o to appear before read.o, because otherwise a program which calls scanf but not read might end up with an unexpected unresolved reference to read.

The way to address this problem was to first generate a set of dependencies of one object file on another.  This was done by a shell script called lorder.  The GNU tools don't provide a version of lorder, as far as I know, but you can still find it in BSD distributions.

Then you ran tsort over the lorder output, and you used the resulting sort to define the order in which you added objects to the archive.

This whole procedure has been obsolete since about 1980, because Unix archives now contain a symbol table (traditionally built by ranlib, now generally built by ar itself), and the Unix linker uses the symbol table to effectively make multiple passes over an archive file.

Anyhow, that's where tsort came from.  To solve an old problem with the way the linker handled archive files, which has since been solved in different ways.

ptx: Produce permuted indexes
=============================

ptx reads a text file and essentially produces a permuted index, with each keyword in its context.  The calling sketch is either one of:

     ptx [OPTION ...] [FILE ...]
     ptx -G [OPTION ...] [INPUT [OUTPUT]]

The -G (or its equivalent: --traditional) option disables all GNU extensions and reverts to traditional mode, thus introducing some limitations and changing several of the program's default option values. When -G is not specified, GNU extensions are always enabled. GNU extensions to ptx are documented wherever appropriate in this document.  For the full list, see Compatibility in ptx.

Individual options are explained in the following sections.

When GNU extensions are enabled, there may be zero, one or several FILEs after the options.  If there is no FILE, the program reads the standard input.  If there is one or several FILEs, they give the name of input files which are all read in turn, as if all the input files were concatenated.  However, there is a full contextual break between each file and, when automatic referencing is requested, file names and line numbers refer to individual text input files.  In all cases, the program outputs the permuted index to the standard output.

When GNU extensions are not enabled, that is, when the program operates in traditional mode, there may be zero, one or two parameters besides the options.  If there are no parameters, the program reads the standard input and outputs the permuted index to the standard output. If there is only one parameter, it names the text INPUT to be read instead of the standard input.  If two parameters are given, they give respectively the name of the INPUT file to read and the name of the OUTPUT file to produce.  Be very careful to note that, in this case, the contents of file given by the second parameter is destroyed.  This behavior is dictated by System V ptx compatibility; GNU Standards normally discourage output parameters not introduced by an option.

Note that for any file named as the value of an option or as an input text file, a single dash - may be used, in which case standard input is assumed.  However, it would not make sense to use this convention more than once per program invocation.

General options
---------------

-C
--copyright
     Print a short note about the copyright and copying conditions, then exit without further processing.

-G
--traditional
     As already explained, this option disables all GNU extensions to ptx and switches to traditional mode.

--help
     Print a short help on standard output, then exit without further processing.

--version
     Print the program version on standard output, then exit without further processing.

Charset selection
-----------------

As it is set up now, the program assumes that the input file is coded using 8-bit ISO 8859-1 code, also known as Latin-1 character set, unless it is compiled for MS-DOS, in which case it uses the character set of the IBM-PC.  (GNU ptx is not known to work on smaller MS-DOS machines anymore.)  Compared to 7-bit ASCII, the set of characters which are letters is different; this alters the behavior of regular expression matching.  Thus, the default regular expression for a keyword allows foreign or diacriticized letters.  Keyword sorting, however, is still crude; it obeys the underlying character set ordering quite blindly.

-f
--ignore-case
     Fold lower case letters to upper case for sorting.

Word selection and input processing
-----------------------------------

-b FILE

--break-file=FILE

     This option provides an alternative (to -W) method of describing which characters make up words.  It introduces the name of a file which contains a list of characters which cannot be part of one word; this file is called the Break file.  Any character which is not part of the Break file is a word constituent.  If both options -b and -W are specified, then -W has precedence and -b is ignored.

     When GNU extensions are enabled, the only way to avoid newline as a break character is to write all the break characters in the file with no newline at all, not even at the end of the file.  When GNU extensions are disabled, spaces, tabs and newlines are always considered as break characters even if not included in the Break file.

-i FILE
--ignore-file=FILE

     The file associated with this option contains a list of words which will never be taken as keywords in concordance output.  It is called the Ignore file.  The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the -S option.

     There is a default Ignore file used by ptx when this option is not specified, usually found in /usr/local/lib/eign if this has not been changed at installation time.  If you want to deactivate the default Ignore file, specify /dev/null instead.

-o FILE
--only-file=FILE

     The file associated with this option contains a list of words which will be retained in concordance output; any word not mentioned in this file is ignored.  The file is called the Only file.  The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the -S option.

     There is no default for the Only file.  When both an Only file and an Ignore file are specified, a word is considered a keyword only if it is listed in the Only file and not in the Ignore file.

-r
--references

     On each input line, the leading sequence of non-white space characters will be taken to be a reference that has the purpose of identifying this input line in the resulting permuted index.  For more information about reference production, see Output formatting in ptx. Using this option changes the default value for option -S.

     Using this option, the program does not try very hard to remove references from contexts in output, but it succeeds in doing so when the context ends exactly at the newline.  If option -r is used with -S default value, or when GNU extensions are disabled, this condition is always met and references are completely excluded from the output contexts.

-S REGEXP
--sentence-regexp=REGEXP

     This option selects which regular expression will describe the end of a line or the end of a sentence.  In fact, this regular expression is not the only distinction between end of lines or end of sentences, and input line boundaries have no special significance outside this option.  By default, when GNU extensions are enabled and if -r option is not used, end of sentences are used.  In this case, this REGEX is imported from GNU Emacs:

          [.?!][]\"')}]*\\($\\|\t\\|  \\)[ \t\n]*

     Whenever GNU extensions are disabled or if -r option is used, end of lines are used; in this case, the default REGEXP is just:

          \n

     Using an empty REGEXP is equivalent to completely disabling end of line or end of sentence recognition.  In this case, the whole file is considered to be a single big line or sentence.  The user might want to disallow all truncation flag generation as well, through option -F "".  See Syntax of Regular Expressions (The GNU Emacs Manual).

     When the keywords happen to be near the beginning of the input line or sentence, this often creates an unused area at the beginning of the output context line; when the keywords happen to be near the end of the input line or sentence, this often creates an unused area at the end of the output context line.  The program tries to fill those unused areas by wrapping around context in them; the tail of the input line or sentence is used to fill the unused area on the left of the output line; the head of the input line or sentence is used to fill the unused area on the right of the output line.

     As a matter of convenience to the user, many usual backslashed escape sequences from the C language are recognized and converted to the corresponding characters by ptx itself.

-W REGEXP
--word-regexp=REGEXP

     This option selects which regular expression will describe each keyword. By default, if GNU extensions are enabled, a word is a sequence of letters; the REGEXP used is \w+.  When GNU extensions are disabled, a word is by default anything which ends with a space, a tab or a newline; the REGEXP used is [^ \t\n]+.

     An empty REGEXP is equivalent to not using this option. See Syntax of Regular Expressions (The GNU Emacs Manual).

     As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ptx itself.

Output formatting
-----------------

Output format is mainly controlled by the -O and -T options described in the table below.  When neither -O nor -T are selected, and if GNU extensions are enabled, the program chooses an output format suitable for a dumb terminal.  Each keyword occurrence is output to the center of one line, surrounded by its left and right contexts.  Each field is properly justified, so the concordance output can be readily observed.  As a special feature, if automatic references are selected by option -A and are output before the left context, that is, if option -R is not selected, then a colon is added after the reference; this nicely interfaces with GNU Emacs next-error processing.  In this default output format, each white space character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces.  This might change in the future.  Except for those white space characters, every other character of the underlying set of 256 characters is transmitted verbatim.

Output format is further controlled by the following options.

-g NUMBER
--gap-size=NUMBER

     Select the size of the minimum white space gap between the fields on the output line.

-w NUMBER
--width=NUMBER

     Select the maximum output width of each final line.  If references are used, they are included or excluded from the maximum output width depending on the value of option -R.  If this option is not selected, that is, when references are output before the left context, the maximum output width takes into account the maximum length of all references.  If this option is selected, that is, when references are output after the right context, the maximum output width does not take into account the space taken by references, nor the gap that precedes them.

-A
--auto-reference

     Select automatic references.  Each input line will have an automatic reference made up of the file name and the line ordinal, with a single colon between them.  However, the file name will be empty when standard input is being read.  If both -A and -r are selected, then the input reference is still read and skipped, but the automatic reference is used at output time, overriding the input reference.

-R
--right-side-refs

     In the default output format, when option -R is not used, any references produced by the effect of options -r or -A are placed to the far right of output lines, after the right context.  With default output format, when the -R option is specified, references are rather placed at the beginning of each output line, before the left context.  For any other output format, option -R is ignored, with one exception:  with -R the width of references is not taken into account in total output width given by -w.

     This option is automatically selected whenever GNU extensions are disabled.

-F STRING
--flac-truncation=STRING

     This option will request that any truncation in the output be reported using the string STRING.  Most output fields theoretically extend towards the beginning or the end of the current line, or current sentence, as selected with option -S.  But there is a maximum allowed output line width, changeable through option -w, which is further divided into space for various output fields.  When a field has to be truncated because it cannot extend beyond the beginning or the end of the current line to fit in, then a truncation occurs.  By default, the string used is a single slash, as in -F /.

     STRING may have more than one character, as in -F .... Also, in the particular case when STRING is empty (-F ""), truncation flagging is disabled, and no truncation marks are appended in this case.

     As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ptx itself.

-M STRING
--macro-name=STRING

     Select another STRING to be used instead of xx, while generating output suitable for nroff, troff or TeX.

-O
--format=roff

     Choose an output format suitable for nroff or troff processing.  Each output line will look like:

          .xx "TAIL" "BEFORE" "KEYWORD_AND_AFTER" "HEAD" "REF"

     so it will be possible to write a .xx roff macro to take care of the output typesetting.  This is the default output format when GNU extensions are disabled.  Option -M can be used to change xx to another macro name.

     In this output format, each non-graphical character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces.  Each quote character: " is doubled so it will be correctly processed by nroff or troff.

-T
--format=tex

     Choose an output format suitable for TeX processing.  Each output line will look like:

          \xx {TAIL}{BEFORE}{KEYWORD}{AFTER}{HEAD}{REF}

     so it will be possible to write a \xx definition to take care of the output typesetting.  Note that when references are not being produced, that is, neither option -A nor option -r is selected, the last parameter of each \xx call is inhibited. Option -M can be used to change xx to another macro name.

     In this output format, some special characters, like $, %, &, # and _ are automatically protected with a backslash.  Curly brackets {, } are protected with a backslash and a pair of dollar signs (to force mathematical mode).  The backslash itself produces the sequence \backslash{}. Circumflex and tilde diacritical marks produce the sequence ^\{ } and ~\{ } respectively.  Other diacriticized characters of the underlying character set produce an appropriate TeX sequence as far as possible.  The other non-graphical characters, like newline and tab, and all other characters which are not part of ASCII, are merely changed to exactly one space, with no special attempt to compress consecutive spaces.  Let me know how to improve this special character processing for TeX.

The GNU extensions to ptx
-------------------------

This version of ptx contains a few features which do not exist in System V ptx.  These extra features are suppressed by using the -G command line option, unless overridden by other command line options.  Some GNU extensions cannot be recovered by overriding, so the simple rule is to avoid -G if you care about GNU extensions. Here are the differences between this program and System V ptx.

  * This program can read many input files at once, it always writes the resulting concordance on standard output.  On the other hand, System V ptx reads only one file and sends the result to standard output or, if a second FILE parameter is given on the command, to that FILE.

     Having output parameters not introduced by options is a dangerous practice which GNU avoids as far as possible.  So, for using ptx portably between GNU and System V, you should always use it with a single input file, and always expect the result on standard output.  You might also want to automatically configure in a -G option to ptx calls in products using ptx, if the configurator finds that the installed ptx accepts -G.

  * The only options available in System V ptx are options -b, -f, -g, -i, -o, -r, -t and -w.  All other options are GNU extensions and are not repeated in this enumeration.  Moreover, some options have a slightly different meaning when GNU extensions are enabled, as explained below.

  * By default, concordance output is not formatted for troff or nroff.  It is rather formatted for a dumb terminal.  troff or nroff output may still be selected through option -O.

  * Unless -R option is used, the maximum reference width is subtracted from the total output line width.  With GNU extensions disabled, width of references is not taken into account in the output line width computations.

  * All 256 characters, even NULs, are always read and processed from input file with no adverse effect, even if GNU extensions are disabled. However, System V ptx does not accept 8-bit characters, a few control characters are rejected, and the tilde ~ is also rejected.

  * Input line length is only limited by available memory, even if GNU extensions are disabled.  However, System V ptx processes only the first 200 characters in each line.

  * The break (non-word) characters default to be every character except all letters of the underlying character set, diacriticized or not.  When GNU extensions are disabled, the break characters default to space, tab and newline only.

  * The program makes better use of output line width.  If GNU extensions are disabled, the program rather tries to imitate System V ptx, but still, there are some slight disposition glitches this program does not completely reproduce.

  * The user can specify both an Ignore file and an Only file.  This is not allowed with System V ptx.

Operating on fields within a line
*********************************

cut: Print selected parts of lines
==================================

cut writes to standard output selected parts of each line of each input file, or standard input if no files are given or for a file name of -.  Synopsis:

     cut [OPTION]... [FILE]...

In the table which follows, the BYTE-LIST, CHARACTER-LIST, and FIELD-LIST are one or more numbers or ranges (two numbers separated by a dash) separated by commas.  Bytes, characters, and fields are numbered starting at 1.  Incomplete ranges may be given: -M means 1-M; N- means N through end of line or last field.  The list elements can be repeated, can overlap, and can be specified in any order; but the selected input is written in the same order that it is read, and is written exactly once.

The program accepts the following options.  Also see Common options.

-b BYTE-LIST
--bytes=BYTE-LIST
     Print only the bytes in positions listed in BYTE-LIST.  Tabs and backspaces are treated like any other character; they take up 1 byte.

-c CHARACTER-LIST
--characters=CHARACTER-LIST
     Print only characters in positions listed in CHARACTER-LIST. The same as -b for now, but internationalization will change that.  Tabs and backspaces are treated like any other character; they take up 1 character.

-f FIELD-LIST
--fields=FIELD-LIST
     Print only the fields listed in FIELD-LIST.  Fields are separated by a TAB character by default. Also print any line that contains no delimiter character, unless the --only-delimited (-s) option is specified

-d INPUT_DELIM_BYTE
--delimiter=INPUT_DELIM_BYTE
     For -f, fields are separated in the input by the first character in INPUT_DELIM_BYTE (default is TAB).

-n
     Do not split multi-byte characters (no-op for now).

-s
--only-delimited
     For -f, do not print lines that do not contain the field separator character.  Normally, any line without a field separator is printed verbatim.

--output-delimiter=OUTPUT_DELIM_STRING
     For -f, output fields are separated by OUTPUT_DELIM_STRING. The default is to use the input delimiter.

paste: Merge lines of files
===========================

paste writes to standard output lines consisting of sequentially corresponding lines of each given file, separated by a TAB character. Standard input is used for a file name of - or if no input files are given.

For example:

     $ cat num2
     1
     2
     $ cat let3
     a
     b
     c
     $ paste num2 let3
     1       a
     2       b
             c

Synopsis:

     paste [OPTION]... [FILE]...

The program accepts the following options.  Also see Common options.

-s
--serial
     Paste the lines of one file at a time rather than one line from each file.  Using the above example data:

          $ paste -s num2 let3
          1       2
          a       b       c

-d DELIM-LIST
--delimiters=DELIM-LIST
     Consecutively use the characters in DELIM-LIST instead of TAB to separate merged lines.  When DELIM-LIST is exhausted, start again at its beginning.  Using the above example data:

          $ paste -d '%_' num2 let3 num2
          1%a_1
          2%b_2
          %c_

join: Join lines on a common field
==================================

join writes to standard output a line for each pair of input lines that have identical join fields.  Synopsis:

     join [OPTION]... FILE1 FILE2

Either FILE1 or FILE2 (but not both) can be -, meaning standard input.  FILE1 and FILE2 should be sorted on the join fields.

Normally, the sort order is that of the collating sequence specified by the LC_COLLATE locale.  Unless the -t option is given, the sort comparison ignores blanks at the start of the join field, as in sort -b.  If the --ignore-case option is given, the sort comparison ignores the case of characters in the join field, as in sort -f.

However, as a GNU extension, if the input has no unpairable lines the sort order can be any order that considers two fields to be equal if and only if the sort comparison described above considers them to be equal. For example:

     $ cat file1
     a a1
     c c1
     b b1
     $ cat file2
     a a2
     c c2
     b b2
     $ join file1 file2
     a a1 a2
     c c1 c2
     b b1 b2

The defaults are: the join field is the first field in each line; fields in the input are separated by one or more blanks, with leading blanks on the line ignored; fields in the output are separated by a space; each output line consists of the join field, the remaining fields from FILE1, then the remaining fields from FILE2.

The program accepts the following options.  Also see Common options.

-a FILE-NUMBER
     Print a line for each unpairable line in file FILE-NUMBER (either 1 or 2), in addition to the normal output.

-e STRING
     Replace those output fields that are missing in the input with STRING.

-i
--ignore-case
     Ignore differences in case when comparing keys. With this option, the lines of the input files must be ordered in the same way. Use sort -f to produce this ordering.

-1 FIELD
-j1 FIELD
     Join on field FIELD (a positive integer) of file 1.

-2 FIELD
-j2 FIELD
     Join on field FIELD (a positive integer) of file 2.

-j FIELD
     Equivalent to -1 FIELD -2 FIELD.

-o FIELD-LIST...
     Construct each output line according to the format in FIELD-LIST. Each element in FIELD-LIST is either the single character 0 or has the form M.N where the file number, M, is 1 or 2 and N is a positive field number.

     A field specification of 0 denotes the join field. In most cases, the functionality of the 0 field spec may be reproduced using the explicit M.N that corresponds to the join field.  However, when printing unpairable lines (using either of the -a or -v options), there is no way to specify the join field using M.N in FIELD-LIST if there are unpairable lines in both files. To give join that functionality, POSIX invented the 0 field specification notation.

     The elements in FIELD-LIST are separated by commas or blanks.  Multiple FIELD-LIST arguments can be given after a single -o option; the values of all lists given with -o are concatenated together. All output lines -- including those printed because of any -a or -v option -- are subject to the specified FIELD-LIST.

-t CHAR
     Use character CHAR as the input and output field separator.

-v FILE-NUMBER
     Print a line for each unpairable line in file FILE-NUMBER (either 1 or 2), instead of the normal output.

In addition, when GNU join is invoked with exactly one argument, the --help and --version options are recognized. See Common options.

Operating on characters
***********************

This commands operate on individual characters.

tr: Translate, squeeze, and/or delete characters
================================================

Synopsis:

     tr [OPTION]... SET1 [SET2]

tr copies standard input to standard output, performing one of the following operations:

  * translate, and optionally squeeze repeated characters in the result,

  * squeeze repeated characters,

  * delete characters,

  * delete characters, then squeeze repeated characters from the result.

The SET1 and (if given) SET2 arguments define ordered sets of characters, referred to below as SET1 and SET2.  These sets are the characters of the input that tr operates on. The --complement (-c) option replaces SET1 with its complement (all of the characters that are not in SET1).

Specifying sets of characters
-----------------------------

The format of the SET1 and SET2 arguments resembles the format of regular expressions; however, they are not regular expressions, only lists of characters.  Most characters simply represent themselves in these strings, but the strings can contain the shorthands listed below, for convenience.  Some of them can be used only in SET1 or SET2, as noted below.

Backslash escapes

     A backslash followed by a character not listed below causes an error message.

     \a
          Control-G.

     \b
          Control-H.

     \f
          Control-L.

     \n
          Control-J.

     \r
          Control-M.

     \t
          Control-I.

     \v
          Control-K.

     \OOO
          The character with the value given by OOO, which is 1 to 3 octal digits,

     \\
          A backslash.

Ranges

     The notation M-N expands to all of the characters from M through N, in ascending order.  M should collate before N; if it doesn't, an error results.  As an example, 0-9 is the same as 0123456789.

     GNU tr does not support the System V syntax that uses square brackets to enclose ranges.  Translations specified in that format sometimes work as expected, since the brackets are often transliterated to themselves.  However, they should be avoided because they sometimes behave unexpectedly.  For example, tr -d '[0-9]' deletes brackets as well as digits.

     Many historically common and even accepted uses of ranges are not portable.  For example, on EBCDIC hosts using the A-Z range will not do what most would expect because A through Z are not contiguous as they are in ASCII. If you can rely on a POSIX compliant version of tr, then the best way to work around this is to use character classes (see below). Otherwise, it is most portable (and most ugly) to enumerate the members of the ranges.

Repeated characters

     The notation [C*N] in SET2 expands to N copies of character C.  Thus, [y*6] is the same as yyyyyy.  The notation [C*] in STRING2 expands to as many copies of C as are needed to make SET2 as long as SET1.  If N begins with 0, it is interpreted in octal, otherwise in decimal.

Character classes

     The notation [:CLASS:] expands to all of the characters in the (predefined) class CLASS.  The characters expand in no particular order, except for the upper and lower classes, which expand in ascending order.  When the --delete (-d) and --squeeze-repeats (-s) options are both given, any character class can be used in SET2.  Otherwise, only the character classes lower and upper are accepted in SET2, and then only if the corresponding character class (upper and lower, respectively) is specified in the same relative position in SET1.  Doing this specifies case conversion. The class names are given below; an error results when an invalid class name is given.

     alnum
          Letters and digits.

     alpha
          Letters.

     blank
          Horizontal whitespace.

     cntrl
          Control characters.

     digit
          Digits.

     graph
          Printable characters, not including space.

     lower
          Lowercase letters.

     print
          Printable characters, including space.

     punct
          Punctuation characters.

     space
          Horizontal or vertical whitespace.

     upper
          Uppercase letters.

     xdigit
          Hexadecimal digits.

Equivalence classes

     The syntax [=C=] expands to all of the characters that are equivalent to C, in no particular order.  Equivalence classes are a relatively recent invention intended to support non-English alphabets. But there seems to be no standard way to define them or determine their contents.  Therefore, they are not fully implemented in GNU tr; each character's equivalence class consists only of that character, which is of no particular use.

Translating
-----------

tr performs translation when SET1 and SET2 are both given and the --delete (-d) option is not given. tr translates each character of its input that is in SET1 to the corresponding character in SET2.  Characters not in SET1 are passed through unchanged.  When a character appears more than once in SET1 and the corresponding characters in SET2 are not all the same, only the final one is used.  For example, these two commands are equivalent:

     tr aaa xyz
     tr a z

A common use of tr is to convert lowercase characters to uppercase.  This can be done in many ways.  Here are three of them:

     tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
     tr a-z A-Z
     tr '[:lower:]' '[:upper:]'

But note that using ranges like a-z above is not portable.

When tr is performing translation, SET1 and SET2 typically have the same length.  If SET1 is shorter than SET2, the extra characters at the end of SET2 are ignored.

On the other hand, making SET1 longer than SET2 is not portable; POSIX says that the result is undefined.  In this situation, BSD tr pads SET2 to the length of SET1 by repeating the last character of SET2 as many times as necessary.  System V tr truncates SET1 to the length of SET2.

By default, GNU tr handles this case like BSD tr.  When the --truncate-set1 (-t) option is given, GNU tr handles this case like the System V tr instead.  This option is ignored for operations other than translation.

Acting like System V tr in this case breaks the relatively common BSD idiom:

     tr -cs A-Za-z0-9 '\012'

because it converts only zero bytes (the first element in the complement of SET1), rather than all non-alphanumerics, to newlines.

By the way, the above idiom is not portable because it uses ranges. Assuming a POSIX compliant tr, here is a better way to write it:

     tr -cs '[:alnum:]' '[\n*]'

Squeezing repeats and deleting
------------------------------

When given just the --delete (-d) option, tr removes any input characters that are in SET1.

When given just the --squeeze-repeats (-s) option, tr replaces each input sequence of a repeated character that is in SET1 with a single occurrence of that character.

When given both --delete and --squeeze-repeats, tr first performs any deletions using SET1, then squeezes repeats from any remaining characters using SET2.

The --squeeze-repeats option may also be used when translating, in which case tr first performs translation, then squeezes repeats from any remaining characters using SET2.

Here are some examples to illustrate various combinations of options:

  * Remove all zero bytes:

          tr -d '\000'

  * Put all words on lines by themselves.  This converts all non-alphanumeric characters to newlines, then squeezes each string of repeated newlines into a single newline:

          tr -cs '[:alnum:]' '[\n*]'

  * Convert each sequence of repeated newlines to a single newline:

          tr -s '\n'

  * Find doubled occurrences of words in a document. For example, people often write ``the the'' with the duplicated words separated by a newline.  The bourne shell script below works first by converting each sequence of punctuation and blank characters to a single newline.  That puts each ``word'' on a line by itself. Next it maps all uppercase characters to lower case, and finally it runs uniq with the -d option to print out only the words that were adjacent duplicates.

          #!/bin/sh
          cat "$@" \
            | tr -s '[:punct:][:blank:]' '\n' \
            | tr '[:upper:]' '[:lower:]' \
            | uniq -d

  * Deleting a small set of characters is usually straightforward.  For example, to remove all as, xs, and Ms you would do this:

          tr -d axM

     However, when - is one of those characters, it can be tricky because - has special meanings.  Performing the same task as above but also removing all - characters, we might try tr -d -axM, but that would fail because tr would try to interpret -a as a command-line option.  Alternatively, we could try putting the hyphen inside the string, tr -d a-xM, but that wouldn't work either because it would make tr interpret a-x as the range of characters a...x rather than the three. One way to solve the problem is to put the hyphen at the end of the list of characters:

          tr -d axM-

     More generally, use the character class notation [=c=] with - (or any other character) in place of the c:

          tr -d '[=-=]axM'

     Note how single quotes are used in the above example to protect the square brackets from interpretation by a shell.

Warning messages
----------------

Setting the environment variable POSIXLY_CORRECT turns off the following warning and error messages, for strict compliance with POSIX.  Otherwise, the following diagnostics are issued:

 1. When the --delete option is given but --squeeze-repeats is not, and SET2 is given, GNU tr by default prints a usage message and exits, because SET2 would not be used. The POSIX specification says that SET2 must be ignored in this case. Silently ignoring arguments is a bad idea.

 2. When an ambiguous octal escape is given.  For example, \400 is actually \40 followed by the digit 0, because the value 400 octal does not fit into a single byte.

GNU tr does not provide complete BSD or System V compatibility. For example, it is impossible to disable interpretation of the POSIX constructs [:alpha:], [=c=], and [c*10].  Also, GNU tr does not delete zero bytes automatically, unlike traditional Unix versions, which provide no way to preserve zero bytes.

expand: Convert tabs to spaces
==============================

expand writes the contents of each given FILE, or standard input if none are given or for a FILE of -, to standard output, with tab characters converted to the appropriate number of spaces.  Synopsis:

     expand [OPTION]... [FILE]...

By default, expand converts all tabs to spaces.  It preserves backspace characters in the output; they decrement the column count for tab calculations.  The default action is equivalent to -t 8 (set tabs every 8 columns).

The program accepts the following options.  Also see Common options.

-t TAB1[,TAB2]...
--tabs=TAB1[,TAB2]...
     If only one tab stop is given, set the tabs TAB1 spaces apart (default is 8).  Otherwise, set the tabs at columns TAB1, TAB2, ... (numbered from 0), and replace any tabs beyond the last tabstop given with single spaces.  Tabstops can be separated by blanks as well as by commas.

     On older systems, expand supports an obsolete option -TAB1[,TAB2]..., where tabstops must be separated by commas.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -t TAB1[,TAB2]... instead.

-i
--initial
     Only convert initial tabs (those that precede all non-space or non-tab characters) on each line to spaces.

unexpand: Convert spaces to tabs
================================

unexpand writes the contents of each given FILE, or standard input if none are given or for a FILE of -, to standard output, with strings of two or more space or tab characters converted to as many tabs as possible followed by as many spaces as are needed.  Synopsis:

     unexpand [OPTION]... [FILE]...

By default, unexpand converts only initial spaces and tabs (those that precede all non space or tab characters) on each line.  It preserves backspace characters in the output; they decrement the column count for tab calculations.  By default, tabs are set at every 8th column.

The program accepts the following options.  Also see Common options.

-t TAB1[,TAB2]...
--tabs=TAB1[,TAB2]...
     If only one tab stop is given, set the tabs TAB1 spaces apart instead of the default 8.  Otherwise, set the tabs at columns TAB1, TAB2, ... (numbered from 0), and leave spaces and tabs beyond the tabstops given unchanged.  Tabstops can be separated by blanks as well as by commas.  This option implies the -a option.

     On older systems, unexpand supports an obsolete option -TAB1[,TAB2]..., where tabstops must be separated by commas.  (Unlike -t, this obsolete option does not imply -a.)  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use --first-only -t TAB1[,TAB2]... instead.

-a
--all
     Convert all strings of two or more spaces or tabs, not just initial ones, to tabs.

Directory listing
*****************

This chapter describes the ls command and its variants dir and vdir, which list information about files.

ls: List directory contents
===========================

The ls program lists information about files (of any type, including directories).  Options and file arguments can be intermixed arbitrarily, as usual.

For non-option command-line arguments that are directories, by default ls lists the contents of directories, not recursively, and omitting files with names beginning with ..  For other non-option arguments, by default ls lists just the file name.  If no non-option argument is specified, ls operates on the current directory, acting as if it had been invoked with a single argument of ..

By default, the output is sorted alphabetically, according to the locale settings in effect.  (If you use a non-POSIX locale (e.g., by setting LC_ALL to en_US), then ls may produce output that is sorted differently than you're accustomed to. In that case, set the LC_ALL environment variable to C.) If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is listed one per line and control characters are output as-is.

Because ls is such a fundamental program, it has accumulated many options over the years.  They are described in the subsections below; within each section, options are listed alphabetically (ignoring case). The division of options into the subsections is not absolute, since some options affect more than one aspect of ls's operation.

Also see Common options.

Which files are listed
----------------------

These options determine which files ls lists information for. By default, any files and the contents of any directories on the command line are shown.

-a
--all
     List all files in directories, including files that start with ..

-A
--almost-all
     List all files in directories except for . and ...

-B
--ignore-backups
     Do not list files that end with ~, unless they are given on the command line.

-d
--directory
     List just the names of directories, as with other types of files, rather than listing their contents. Do not follow symbolic links listed on the command line unless the --dereference-command-line (-H) or --dereference (-L) options are specified.

-H
--dereference-command-line
     If a command line argument specifies a symbolic link, show information for the file the link references rather than for the link itself.

-I PATTERN
--ignore=PATTERN
     Do not list files whose names match the shell pattern (not regular expression) PATTERN unless they are given on the command line.  As in the shell, an initial . in a file name does not match a wildcard at the start of PATTERN.  Sometimes it is useful to give this option several times.  For example,

          $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*'

     The first option ignores names of length 3 or more that start with ., the second ignores all two-character names that start with . except .., and the third ignores names that start with #.

-L
--dereference
     When showing file information for a symbolic link, show information for the file the link references rather than the link itself.

-R
--recursive
     List the contents of all directories recursively.

What information is listed
--------------------------

These options affect the information that ls displays.  By default, only file names are shown.

--author
     List each file's author when producing long format directory listings. In GNU/Hurd, file authors can differ from their owners, but in other operating systems the two are the same.

-D
--dired
     With the long listing (-l) format, print an additional line after the main output:

          //DIRED// BEG1 END1 BEG2 END2 ...

     The BEGN and ENDN are unsigned integers that record the byte position of the beginning and end of each file name in the output. This makes it easy for Emacs to find the names, even when they contain unusual characters such as space or newline, without fancy searching.

     If directories are being listed recursively (-R), output a similar line with offsets for each subdirectory name:

          //SUBDIRED// BEG1 END1 ...

     Finally, output a line of the form:

          //DIRED-OPTIONS// --quoting-style=WORD

     where WORD is the quoting style (see Formatting the file names).

     Here is an actual example:

          $ mkdir -p a/sub/deeper a/sub2
          $ touch a/f1 a/f2
          $ touch a/sub/deeper/file
          $ ls -gloRF --dired a
            a:
            total 8
            -rw-r--r--    1        0 Nov  9 18:30 f1
            -rw-r--r--    1        0 Nov  9 18:30 f2
            drwxr-xr-x    3     4096 Nov  9 18:30 sub/
            drwxr-xr-x    2     4096 Nov  9 18:30 sub2/

            a/sub:
            total 4
            drwxr-xr-x    2     4096 Nov  9 18:30 deeper/

            a/sub/deeper:
            total 0
            -rw-r--r--    1        0 Nov  9 18:30 file

            a/sub2:
            total 0
          //DIRED// 55 57 98 100 141 144 186 190 252 258 327 331
          //SUBDIRED// 2 3 195 200 263 275 335 341
          //DIRED-OPTIONS// --quoting-style=literal

     Note that the pairs of offsets on the //DIRED// line above delimit these names: f1, f2, sub, sub2, deeper, file. The offsets on the //SUBDIRED// line delimit the following directory names: a, a/sub, a/sub/deeper, a/sub2.

     Here is an example of how to extract the fifth entry name, deeper, corresponding to the pair of offsets, 252 and 258:

          $ ls -gloRF --dired a > out
          $ dd bs=1 skip=252 count=6 < out 2>/dev/null; echo
          deeper

     Note that although the listing above includes a trailing slash for the deeper entry, the offsets select the name without the trailing slash.  However, if you invoke ls with --dired along with an option like --escape (aka -b) and operate on a file whose name contains special characters, notice that the backslash is included:

          $ touch 'a b'
          $ ls -blog --dired 'a b'
            -rw-r--r--    1        0 Nov  9 18:41 a\ b
          //DIRED// 40 44
          //DIRED-OPTIONS// --quoting-style=escape

     If you use a quoting style that adds quote marks (e.g., --quoting-style=c), then the offsets include the quote marks. So beware that the user may select the quoting style via the environment variable QUOTING_STYLE.  Hence, applications using --dired should either specify an explicit --quoting-style=literal option (aka -N or --literal) on the command line, or else be prepared to parse the escaped names.

--full-time
     Produce long format directory listings, and list times in full.  It is equivalent to using --format=long with --time-style=full-iso (see Formatting file timestamps).

-g
     Produce long format directory listings, but don't display owner information.

-G
--no-group
     Inhibit display of group information in a long format directory listing. (This is the default in some non-GNU versions of ls, so we provide this option for compatibility.)

-h
--human-readable
     Append a size letter to each size, such as M for mebibytes. Powers of 1024 are used, not 1000; M stands for 1,048,576 bytes. This option is equivalent to --block-size=human (see Block size). Use the --si option if you prefer powers of 1000.

-i
--inode
     Print the inode number (also called the file serial number and index number) of each file to the left of the file name.  (This number uniquely identifies each file within a particular filesystem.)

-l
--format=long
--format=verbose
     In addition to the name of each file, print the file type, permissions, number of hard links, owner name, group name, size, and timestamp (see Formatting file timestamps), normally the modification time.

     Normally the size is printed as a byte count without punctuation, but this can be overridden (see Block size).  For example, -h prints an abbreviated, human-readable count, and --block-size="'1" prints a byte count with the thousands separator of the current locale.

     For each directory that is listed, preface the files with a line total BLOCKS, where BLOCKS is the total disk allocation for all files in that directory.  The block size currently defaults to 1024 bytes, but this can be overridden (see Block size). The BLOCKS computed counts each hard link separately; this is arguably a deficiency.

     The permissions listed are similar to symbolic mode specifications (see Symbolic Modes).  But ls combines multiple bits into the third character of each set of permissions as follows:

     s
          If the setuid or setgid bit and the corresponding executable bit are both set.

     S
          If the setuid or setgid bit is set but the corresponding executable bit is not set.

     t
          If the sticky bit and the other-executable bit are both set.

     T
          If the sticky bit is set but the other-executable bit is not set.

     x
          If the executable bit is set and none of the above apply.

     -
          Otherwise.

     Following the permission bits is a single character that specifies whether an alternate access method applies to the file.  When that character is a space, there is no alternate access method.  When it is a printing character (e.g., +), then there is such a method.

-n
--numeric-uid-gid
     Produce long format directory listings, but display numeric UIDs and GIDs instead of the owner and group names.

-o
     Produce long format directory listings, but don't display group information. It is equivalent to using --format=long with --no-group .

-s
--size
     Print the disk allocation of each file to the left of the file name. This is the amount of disk space used by the file, which is usually a bit more than the file's size, but it can be less if the file has holes.

     Normally the disk allocation is printed in units of 1024 bytes, but this can be overridden (see Block size).

     For files that are NFS-mounted from an HP-UX system to a BSD system, this option reports sizes that are half the correct values.  On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems.  This is due to a flaw in HP-UX; it also affects the HP-UX ls program.

--si
     Append an SI-style abbreviation to each size, such as MB for megabytes.  Powers of 1000 are used, not 1024; MB stands for 1,000,000 bytes.  This option is equivalent to --block-size=si.  Use the -h or --human-readable option if you prefer powers of 1024.

Sorting the output
------------------

These options change the order in which ls sorts the information it outputs.  By default, sorting is done by character code (e.g., ASCII order).

-c
--time=ctime
--time=status
--time=use
     If the long listing format (e.g., -l, -o) is being used, print the status change time (the ctime in the inode) instead of the modification time. When explicitly sorting by time (--sort=time or -t) or when not using a long listing format, sort according to the status change time.

-f
     Primarily, like -U---do not sort; list the files in whatever order they are stored in the directory.  But also enable -a (list all files) and disable -l, --color, and -s (if they were specified before the -f).

-r
--reverse
     Reverse whatever the sorting method is---e.g., list files in reverse alphabetical order, youngest first, smallest first, or whatever.

-S
--sort=size
     Sort by file size, largest first.

-t
--sort=time
     Sort by modification time (the mtime in the inode), newest first.

-u
--time=atime
--time=access
     If the long listing format (e.g., --format=long) is being used, print the last access time (the atime in the inode). When explicitly sorting by time (--sort=time or -t) or when not using a long listing format, sort according to the access time.

-U
--sort=none
     Do not sort; list the files in whatever order they are stored in the directory.  (Do not do any of the other unrelated things that -f does.)  This is especially useful when listing very large directories, since not doing any sorting can be noticeably faster.

-v
--sort=version
     Sort by version name and number, lowest first.  It behaves like a default sort, except that each sequence of decimal digits is treated numerically as an index/version number.  (see More details about version sort.)

-X
--sort=extension
     Sort directory contents alphabetically by file extension (characters after the last .); files with no extension are sorted first.

More details about version sort
-------------------------------

The version sort takes into account the fact that file names frequently include indices or version numbers.  Standard sorting functions usually do not produce the ordering that people expect because comparisons are made on a character-by-character basis.  The version sort addresses this problem, and is especially useful when browsing directories that contain many files with indices/version numbers in their names:

           > ls -1            > ls -1v
           foo.zml-1.gz       foo.zml-1.gz
           foo.zml-100.gz     foo.zml-2.gz
           foo.zml-12.gz      foo.zml-6.gz
           foo.zml-13.gz      foo.zml-12.gz
           foo.zml-2.gz       foo.zml-13.gz
           foo.zml-25.gz      foo.zml-25.gz
           foo.zml-6.gz       foo.zml-100.gz

Note also that numeric parts with leading zeroes are considered as fractional one:

           > ls -1            > ls -1v
           abc-1.007.tgz      abc-1.007.tgz
           abc-1.012b.tgz     abc-1.01a.tgz
           abc-1.01a.tgz      abc-1.012b.tgz

General output formatting
-------------------------

These options affect the appearance of the overall output.

-1
--format=single-column
     List one file per line.  This is the default for ls when standard output is not a terminal.

-C
--format=vertical
     List files in columns, sorted vertically.  This is the default for ls if standard output is a terminal.  It is always the default for the dir and d programs. GNU ls uses variable width columns to display as many files as possible in the fewest lines.

--color [=WHEN]
     Specify whether to use color for distinguishing file types.  WHEN may be omitted, or one of:

       * none - Do not use color at all.  This is the default.

       * auto - Only use color if standard output is a terminal.

       * always - Always use color.

     Specifying --color and no WHEN is equivalent to --color=always. Piping a colorized listing through a pager like more or less usually produces unreadable results.  However, using more -f does seem to work.

-F
--classify
--indicator-style=classify
     Append a character to each file name indicating the file type.  Also, for regular files that are executable, append *.  The file type indicators are / for directories, @ for symbolic links, | for FIFOs, = for sockets, and nothing for regular files. Do not follow symbolic links listed on the command line unless the --dereference-command-line (-H) or --dereference (-L) options are specified.

--indicator-style=WORD
     Append a character indicator with style WORD to entry names, as follows:

     none
          Do not append any character indicator; this is the default.

     file-type
          Append / for directories, @ for symbolic links, | for FIFOs, = for sockets, and nothing for regular files.  This is the same as the -p or --file-type option.

     classify
          Append * for executable regular files, otherwise behave as for file-type.  This is the same as the -F or --classify option.

-k
     Print file sizes in 1024-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1K.

-m
--format=commas
     List files horizontally, with as many as will fit on each line, separated by ,  (a comma and a space).

-p
--file-type
--indicator-style=file-type
     Append a character to each file name indicating the file type. This is like -F, except that executables are not marked.

-x FORMAT
--format=across
--format=horizontal
     List the files in columns, sorted horizontally.

-T COLS
--tabsize=COLS
     Assume that each tabstop is COLS columns wide.  The default is 8. ls uses tabs where possible in the output, for efficiency.  If COLS is zero, do not use tabs at all.

-w
--width=COLS
     Assume the screen is COLS columns wide.  The default is taken from the terminal settings if possible; otherwise the environment variable COLUMNS is used if it is set; otherwise the default is 80.

Formatting file timestamps
--------------------------

By default, file timestamps are listed in abbreviated form.  Most locales use a timestamp like 2002-03-30 23:45.  However, the default POSIX locale uses a date like Mar 30  2002 for non-recent timestamps, and a date-without-year and time like Mar 30 23:45 for recent timestamps.

A timestamp is considered to be recent if it is less than six months old, and is not dated in the future.  If a timestamp dated today is not listed in recent form, the timestamp is in the future, which means you probably have clock skew problems which may break programs like make that rely on file timestamps.

The following option changes how file timestamps are printed.

--time-style=STYLE
     List timestamps in style STYLE.  The STYLE should be one of the following:

     +FORMAT
          List timestamps using FORMAT, where FORMAT is interpreted like the format argument of date (see date invocation). For example, --time-style="+%Y-%m-%d %H:%M:S" causes ls to list timestamps like 2002-03-30 23:45:56.  As with date, FORMAT's interpretation is affected by the LC_TIME locale category.

          If FORMAT contains two format strings separated by a newline, the former is used for non-recent files and the latter for recent files; if you want output columns to line up, you may need to insert spaces in one of the two formats.

     full-iso
          List timestamps in full using ISO 8601 date, time, and time zone format with nanosecond precision, e.g., 2002-03-30 23:45:56.477817180 -0700.  This style is equivalent to +%Y-%m-%d %H:%M:%S.%N %z.

          This is useful because the time output includes all the information that is available from the operating system.  For example, this can help explain make's behavior, since GNU make uses the full timestamp to determine whether a file is out of date.

     long-iso
          List ISO 8601 date and time in minutes, e.g., 2002-03-30 23:45.  These timestamps are shorter than full-iso timestamps, and are usually good enough for everyday work.  This style is equivalent to %Y-%m-%d %H:%M.

     iso
          List ISO 8601 dates for non-recent timestamps (e.g., 2002-03-30 ), and ISO 8601 month, day, hour, and minute for recent timestamps (e.g., 03-30 23:45).  These timestamps are uglier than long-iso timestamps, but they carry nearly the same information in a smaller space and their brevity helps ls output fit within traditional 80-column output lines. The following two ls invocations are equivalent:

               newline='
               '
               ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M"
               ls -l --time-style="iso"

     locale
          List timestamps in a locale-dependent form.  For example, a Finnish locale might list non-recent timestamps like maalis 30  2002 and recent timestamps like maalis 30 23:45.  Locale-dependent timestamps typically consume more space than iso timestamps and are harder for programs to parse because locale conventions vary so widely, but they are easier for many people to read.

          The LC_TIME locale category specifies the timestamp format.  The default POSIX locale uses timestamps like Mar 30 2002 and Mar 30 23:45; in this locale, the following two ls invocations are equivalent:

               newline='
               '
               ls -l --time-style="+%b %e  %Y$newline%b %e %H:%M"
               ls -l --time-style="locale"

          Other locales behave differently.  For example, in a German locale, --time-style="locale" might be equivalent to --time-style="+%e. %b %Y $newline%e. %b %H:%M" and might generate timestamps like 30. Mär 2002  and 30. Mär 23:45.

     posix-STYLE
          List POSIX-locale timestamps if the LC_TIME locale category is POSIX, STYLE timestamps otherwise.  For example, the default style, which is posix-long-iso, lists timestamps like Mar 30  2002 and Mar 30 23:45 when in the POSIX locale, and like 2002-03-30 23:45 otherwise.

You can specify the default value of the --time-style option with the environment variable TIME_STYLE; if TIME_STYLE is not set the default style is posix-long-iso.  GNU Emacs 21 and later can parse ISO dates, but older Emacs versions do not, so if you are using an older version of Emacs and specify a non-POSIX locale, you may need to set TIME_STYLE="locale".

Formatting the file names
-------------------------

These options change how file names themselves are printed.

-b
--escape
--quoting-style=escape
     Quote nongraphic characters in file names using alphabetic and octal backslash sequences like those used in C.

-N
--literal
--quoting-style=literal
     Do not quote file names.

-q
--hide-control-chars
     Print question marks instead of nongraphic characters in file names. This is the default if the output is a terminal and the program is ls.

-Q
--quote-name
--quoting-style=c
     Enclose file names in double quotes and quote nongraphic characters as in C.

--quoting-style=WORD
     Use style WORD to quote output names.  The WORD should be one of the following:

     literal
          Output names as-is; this is the same as the -N or --literal option.

     shell
          Quote names for the shell if they contain shell metacharacters or would cause ambiguous output.

     shell-always
          Quote names for the shell, even if they would normally not require quoting.

     c
          Quote names as for a C language string; this is the same as the -Q or --quote-name option.

     escape
          Quote as with c except omit the surrounding double-quote characters; this is the same as the -b or --escape option.

     clocale
          Quote as with c except use quotation marks appropriate for the locale.

     locale
          Like clocale, but quote `like this' instead of "like this" in the default C locale.  This looks nicer on many displays.

     You can specify the default value of the --quoting-style option with the environment variable QUOTING_STYLE.  If that environment variable is not set, the default value is literal, but this default may change to shell in a future version of this package.

--show-control-chars
     Print nongraphic characters as-is in file names. This is the default unless the output is a terminal and the program is ls.

dir: Briefly list directory contents
====================================

dir (also installed as d) is equivalent to ls -C -b; that is, by default files are listed in columns, sorted vertically, and special characters are represented by backslash escape sequences.

See ls.

vdir: Verbosely list directory contents
=======================================

vdir (also installed as v) is equivalent to ls -l -b; that is, by default files are listed in long format and special characters are represented by backslash escape sequences.

dircolors: Color setup for ls
=============================

dircolors outputs a sequence of shell commands to set up the terminal for color output from ls (and dir, etc.). Typical usage:

     eval `dircolors [OPTION]... [FILE]`

If FILE is specified, dircolors reads it to determine which colors to use for which file types and extensions.  Otherwise, a precompiled database is used.  For details on the format of these files, run dircolors --print-database.

The output is a shell command to set the LS_COLORS environment variable.  You can specify the shell syntax to use on the command line, or dircolors will guess it from the value of the SHELL environment variable.

The program accepts the following options.  Also see Common options.

-b
--sh
--bourne-shell
     Output Bourne shell commands.  This is the default if the SHELL environment variable is set and does not end with csh or tcsh.

-c
--csh
--c-shell
     Output C shell commands.  This is the default if SHELL ends with csh or tcsh.

-p
--print-database
     Print the (compiled-in) default color configuration database.  This output is itself a valid configuration file, and is fairly descriptive of the possibilities.

Basic operations
****************

This chapter describes the commands for basic file manipulation: copying, moving (renaming), and deleting (removing).

cp: Copy files and directories
==============================

cp copies files (or, optionally, directories).  The copy is completely independent of the original.  You can either copy one file to another, or copy arbitrarily many files to a destination directory. Synopsis:

     cp [OPTION]... SOURCE DEST
     cp [OPTION]... SOURCE... DIRECTORY

If the last argument names an existing directory, cp copies each SOURCE file into that directory (retaining the same name). Otherwise, if only two files are given, it copies the first onto the second.  It is an error if the last argument is not a directory and more than two non-option arguments are given.

Generally, files are written just as they are read.  For exceptions, see the --sparse option below.

By default, cp does not copy directories.  However, the -R, -a, and -r options cause cp to copy recursively by descending into source directories and copying files to corresponding destination directories.

By default, cp follows symbolic links only when not copying recursively.  This default can be overridden with the --archive (-a), -d, --dereference (-L), --no-dereference (-P), and -H options.  If more than one of these options is specified, the last one silently overrides the others.

By default, cp copies the contents of special files only when not copying recursively.  This default can be overridden with the --copy-contents option.

cp generally refuses to copy a file onto itself, with the following exception: if --force --backup is specified with SOURCE and DEST identical, and referring to a regular file, cp will make a backup file, either regular or numbered, as specified in the usual ways (see Backup options).  This is useful when you simply want to make a backup of an existing file before changing it.

The program accepts the following options.  Also see Common options.

-a
--archive
     Preserve as much as possible of the structure and attributes of the original files in the copy (but do not attempt to preserve internal directory structure; i.e., ls -U may list the entries in a copied directory in a different order). Equivalent to -dpPR.

-b
--backup[=METHOD]
     See Backup options. Make a backup of each file that would otherwise be overwritten or removed. As a special case, cp makes a backup of SOURCE when the force and backup options are given and SOURCE and DEST are the same name for an existing, regular file.  One useful application of this combination of options is this tiny Bourne shell script:

          #!/bin/sh
          # Usage: backup FILE...
          # Create a GNU-style backup of each listed FILE.
          for i; do
            cp --backup --force "$i" "$i"
          done

--copy-contents
     If copying recursively, copy the contents of any special files (e.g., FIFOs and device files) as if they were regular files.  This means trying to read the data in each source file and writing it to the destination.  It is usually a mistake to use this option, as it normally has undesirable effects on special files like FIFOs and the ones typically found in the /dev directory.  In most cases, cp -R --copy-contents will hang indefinitely trying to read from FIFOs and special files like /dev/console, and it will fill up your destination disk if you use it to copy /dev/zero. This option has no effect unless copying recursively, and it does not affect the copying of symbolic links.

-d
     Copy symbolic links as symbolic links rather than copying the files that they point to, and preserve hard links between source files in the copies. Equivalent to --no-dereference --preserve=links.

-f
--force
     When copying without this option and an existing destination file cannot be opened for writing, the copy fails.  However, with --force), when a destination file cannot be opened, cp then unlinks it and tries to open it again.  Contrast this behavior with that enabled by --link and --symbolic-link, whereby the destination file is never opened but rather is unlinked unconditionally.  Also see the description of --remove-destination.

-H
     If a command line argument specifies a symbolic link, then copy the file it points to rather than the symbolic link itself.  However, copy (preserving its nature) any symbolic link that is encountered via recursive traversal.

-i
--interactive
     Prompt whether to overwrite existing regular destination files.

-l
--link
     Make hard links instead of copies of non-directories.

-L
--dereference
     Always follow symbolic links.

-P
--no-dereference
     Copy symbolic links as symbolic links rather than copying the files that they point to.

-p
--preserve[=ATTRIBUTE_LIST]
     Preserve the specified attributes of the original files. If specified, the ATTRIBUTE_LIST must be a comma-separated list of one or more of the following strings:

     mode
          Preserve the permission attributes.
     ownership
          Preserve the owner and group.  On most modern systems, only the super-user may change the owner of a file, and regular users may preserve the group ownership of a file only if they happen to be a member of the desired group.
     timestamps
          Preserve the times of last access and last modification.
     links
          Preserve in the destination files any links between corresponding source files.
     all
          Preserve all file attributes. Equivalent to specifying all of the above.

     Using --preserve with no ATTRIBUTE_LIST is equivalent to --preserve=mode,ownership,timestamps.

     In the absence of this option, each destination file is created with the permissions of the corresponding source file, minus the bits set in the umask and minus the set-user-id and set-group-id bits. See File permissions.

--no-preserve=ATTRIBUTE_LIST
     Do not preserve the specified attributes.  The ATTRIBUTE_LIST has the same form as for --preserve.

--parents
     Form the name of each destination file by appending to the target directory a slash and the specified name of the source file.  The last argument given to cp must be the name of an existing directory. For example, the command:

          cp --parents a/b/c existing_dir

     copies the file a/b/c to existing_dir/a/b/c, creating any missing intermediate directories.

--reply[=HOW]
     Using --reply=yes makes cp act as if yes were given as a response to every prompt about a destination file.  That effectively cancels any preceding --interactive or -i option. Specify --reply=no to make cp act as if no were given as a response to every prompt about a destination file. Specify --reply=query to make cp prompt the user about each existing destination file.

-R
-r
--recursive
     Copy directories recursively.  Symbolic links are not followed by default; see the --archive (-a), -d, --dereference (-L), --no-dereference (-P), and -H options.  Special files are copied by creating a destination file of the same type as the source; see the --copy-contents option.  It is not portable to use -r to copy symbolic links or special files.  On some non-GNU systems, -r implies the equivalent of -L and --copy-contents for historical reasons. Also, it is not portable to use -R to copy symbolic links unless you also specify -P, as POSIX allows implementations that dereference symbolic links by default.

--remove-destination
     Remove each existing destination file before attempting to open it (contrast with -f above).

--sparse=WHEN
     A sparse file contains holes---a sequence of zero bytes that does not occupy any physical disk blocks; the read system call reads these as zeroes.  This can both save considerable disk space and increase speed, since many binary files contain lots of consecutive zero bytes.  By default, cp detects holes in input source files via a crude heuristic and makes the corresponding output file sparse as well.

     The WHEN value can be one of the following:

     auto
          The default behavior: the output file is sparse if the input file is sparse.

     always
          Always make the output file sparse.  This is useful when the input file resides on a filesystem that does not support sparse files (the most notable example is efs filesystems in SGI IRIX 5.3 and earlier), but the output file is on another type of filesystem.

     never
          Never make the output file sparse. This is useful in creating a file for use with the mkswap command, since such a file must not have any holes.

--strip-trailing-slashes
     Remove any trailing slashes from each SOURCE argument. See Trailing slashes.

-s
--symbolic-link
     Make symbolic links instead of copies of non-directories.  All source file names must be absolute (starting with /) unless the destination files are in the current directory.  This option merely results in an error message on systems that do not support symbolic links.

-S SUFFIX
--suffix=SUFFIX
     Append SUFFIX to each backup file made with -b. See Backup options.

--target-directory=DIRECTORY
     Specify the destination DIRECTORY. See Target directory.

-v
--verbose
     Print the name of each file before copying it.

-V METHOD
--version-control=METHOD
     Change the type of backups made with -b.  The METHOD argument can be none (or off), numbered (or t), existing (or nil), or never (or simple).  See Backup options.

-x
--one-file-system
     Skip subdirectories that are on different filesystems from the one that the copy started on. However, mount point directories are copied.

dd: Convert and copy a file
===========================

dd copies a file (from standard input to standard output, by default) with a changeable I/O block size, while optionally performing conversions on it.  Synopsis:

     dd [OPTION]...

The program accepts the following options.  Also see Common options.

The numeric-valued options below (BYTES and BLOCKS) can be followed by a multiplier: b=512, c=1, w=2, xM=M, or any of the standard block size suffixes like k=1024 (see Block size).

Use different dd invocations to use different block sizes for skipping and I/O.  For example, the following shell commands copy data in 512 KiB blocks between a disk and a tape, but do not save or restore a 4 KiB label at the start of the disk:

     disk=/dev/rdsk/c0t1d0s2
     tape=/dev/rmt/0

     # Copy all but the label from disk to tape.
     (dd bs=4k skip=1 count=0 && dd bs=512k) <$disk >$tape

     # Copy from tape back to disk, but leave the disk label alone.
     (dd bs=4k seek=1 count=0 && dd bs=512k) <$tape >$disk

if=FILE
     Read from FILE instead of standard input.

of=FILE
     Write to FILE instead of standard output.  Unless conv=notrunc is given, dd truncates FILE to zero bytes (or the size specified with seek=).

ibs=BYTES
     Read BYTES bytes at a time.

obs=BYTES
     Write BYTES bytes at a time.

bs=BYTES
     Both read and write BYTES bytes at a time.  This overrides ibs and obs.

cbs=BYTES
     Convert BYTES bytes at a time.

skip=BLOCKS
     Skip BLOCKS ibs-byte blocks in the input file before copying.

seek=BLOCKS
     Skip BLOCKS obs-byte blocks in the output file before copying.

count=BLOCKS
     Copy BLOCKS ibs-byte blocks from the input file, instead of everything until the end of the file.

conv=CONVERSION[,CONVERSION]...
     Convert the file as specified by the CONVERSION argument(s). (No spaces around any comma(s).)

     Conversions:

     ascii
          Convert EBCDIC to ASCII.

     ebcdic
          Convert ASCII to EBCDIC.

     ibm
          Convert ASCII to alternate EBCDIC.

     block
          For each line in the input, output cbs bytes, replacing the input newline with a space and padding with spaces as necessary.

     unblock
          Replace trailing spaces in each cbs-sized input block with a newline.

     lcase
          Change uppercase letters to lowercase.

     ucase
          Change lowercase letters to uppercase.

     swab
          Swap every pair of input bytes.  GNU dd, unlike others, works when an odd number of bytes are read---the last byte is simply copied (since there is nothing to swap it with).

     noerror
          Continue after read errors.

     notrunc
          Do not truncate the output file.

     sync
          Pad every input block to size of ibs with trailing zero bytes. When used with block or unblock, pad with spaces instead of zero bytes.

install: Copy files and set attributes
======================================

install copies files while setting their permission modes and, if possible, their owner and group.  Synopses:

     install [OPTION]... SOURCE DEST
     install [OPTION]... SOURCE... DIRECTORY
     install -d [OPTION]... DIRECTORY...

In the first of these, the SOURCE file is copied to the DEST target file.  In the second, each of the SOURCE files are copied to the destination DIRECTORY.  In the last, each DIRECTORY (and any missing parent directories) is created.

install is similar to cp, but allows you to control the attributes of destination files.  It is typically used in Makefiles to copy programs into their destination directories.  It refuses to copy files onto themselves.

The program accepts the following options.  Also see Common options.

-b
--backup[=METHOD]
     See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-c
     Ignored; for compatibility with old Unix versions of install.

-d
--directory
     Create each given directory and any missing parent directories, setting the owner, group and mode as given on the command line or to the defaults.  It also gives any parent directories it creates those attributes.  (This is different from the SunOS 4.x install, which gives directories that it creates the default attributes.)

-g GROUP
--group=GROUP
     Set the group ownership of installed files or directories to GROUP. The default is the process' current group.  GROUP may be either a group name or a numeric group id.

-m MODE
--mode=MODE
     Set the permissions for the installed file or directory to MODE, which can be either an octal number, or a symbolic mode as in chmod, with 0 as the point of departure (see File permissions).  The default mode is u=rwx,go=rx---read, write, and execute for the owner, and read and execute for group and other.

-o OWNER
--owner=OWNER
     If install has appropriate privileges (is run as root), set the ownership of installed files or directories to OWNER. The default is root.  OWNER may be either a user name or a numeric user ID.

-p
--preserve-timestamps
     Set the time of last access and the time of last modification of each installed file to match those of each corresponding original file. When a file is installed without this option, its last access and last modification times are both set to the time of installation. This option is useful if you want to use the last modification times of installed files to keep track of when they were last built as opposed to when they were last installed.

-s
--strip
     Strip the symbol tables from installed binary executables.

-S SUFFIX
--suffix=SUFFIX
     Append SUFFIX to each backup file made with -b. See Backup options.

--target-directory=DIRECTORY
     Specify the destination DIRECTORY. See Target directory.

-v
--verbose
     Print the name of each file before copying it.

-V METHOD
--version-control=METHOD
     Change the type of backups made with -b.  The METHOD argument can be none (or off), numbered (or t), existing (or nil), or never (or simple).  See Backup options.

mv: Move (rename) files
=======================

mv moves or renames files (or directories).  Synopsis:

     mv [OPTION]... SOURCE DEST
     mv [OPTION]... SOURCE... DIRECTORY

If the last argument names an existing directory, mv moves each other given file into a file with the same name in that directory. Otherwise, if only two files are given, it renames the first as the second.  It is an error if the last argument is not a directory and more than two files are given.

mv can move any type of file from one filesystem to another. Prior to version 4.0 of the fileutils, mv could move only regular files between filesystems. For example, now mv can move an entire directory hierarchy including special device files from one partition to another.  It first uses some of the same code that's used by cp -a to copy the requested directories and files, then (assuming the copy succeeded) it removes the originals.  If the copy fails, then the part that was copied to the destination partition is removed.  If you were to copy three directories from one partition to another and the copy of the first directory succeeded, but the second didn't, the first would be left on the destination partition and the second and third would be left on the original partition.

If a destination file exists but is normally unwritable, standard input is a terminal, and the -f or --force option is not given, mv prompts the user for whether to replace the file.  (You might own the file, or have write permission on its directory.)  If the response does not begin with y or Y, the file is skipped.

Warning: If you try to move a symlink that points to a directory, and you specify the symlink with a trailing slash, then mv doesn't move the symlink but instead moves the directory referenced by the symlink.  See Trailing slashes.

The program accepts the following options.  Also see Common options.

-b
--backup[=METHOD]
     See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-f
--force
     Do not prompt the user before removing a destination file.

-i
--interactive
     Prompt whether to overwrite each existing destination file, regardless of its permissions.  If the response does not begin with y or Y, the file is skipped.

--reply[=HOW]
     Specifying --reply=yes is equivalent to using --force. Specify --reply=no to make mv act as if no were given as a response to every prompt about a destination file. Specify --reply=query to make mv prompt the user about each existing destination file.

-u
--update
     Do not move a non-directory that has an existing destination with the same or newer modification time.

-v
--verbose
     Print the name of each file before moving it.

--strip-trailing-slashes
     Remove any trailing slashes from each SOURCE argument. See Trailing slashes.

-S SUFFIX
--suffix=SUFFIX
     Append SUFFIX to each backup file made with -b. See Backup options.

--target-directory=DIRECTORY
     Specify the destination DIRECTORY. See Target directory.

-V METHOD
--version-control=METHOD
     Change the type of backups made with -b.  The METHOD argument can be none (or off), numbered (or t), existing (or nil), or never (or simple).  See Backup options.

rm: Remove files or directories
===============================

rm removes each given FILE.  By default, it does not remove directories.  Synopsis:

     rm [OPTION]... [FILE]...

If a file is unwritable, standard input is a terminal, and the -f or --force option is not given, or the -i or --interactive option is given, rm prompts the user for whether to remove the file.  If the response does not begin with y or Y, the file is skipped.

Warning: If you use rm to remove a file, it is usually possible to recover the contents of that file.  If you want more assurance that the contents are truly unrecoverable, consider using shred.

The program accepts the following options.  Also see Common options.

-d
--directory
     Attempt to remove directories using the unlink function rather than the rmdir function, and don't require a directory to be empty before trying to unlink it.  This works only if you have appropriate privileges and if your operating system supports unlink for directories.  Because unlinking a directory causes any files in the deleted directory to become unreferenced, it is wise to fsck the filesystem after doing this.

-f
--force
     Ignore nonexistent files and never prompt the user. Ignore any previous --interactive (-i) option.

-i
--interactive
     Prompt whether to remove each file.  If the response does not begin with y or Y, the file is skipped. Ignore any previous --force (-f) option.

-r
-R
--recursive
     Remove the contents of directories recursively.

-v
--verbose
     Print the name of each file before removing it.

One common question is how to remove files whose names begin with a -.  GNU rm, like every program that uses the getopt function to parse its arguments, lets you use the -- option to indicate that all following arguments are non-options.  To remove a file called -f in the current directory, you could type either:

     rm -- -f

or:

     rm ./-f

The Unix rm program's use of a single - for this purpose predates the development of the getopt standard syntax.

shred: Remove files more securely
=================================

shred overwrites devices or files, to help prevent even very expensive hardware from recovering the data.

Ordinarily when you remove a file (see rm invocation), the data is not actually destroyed.  Only the index listing where the file is stored is destroyed, and the storage is made available for reuse. There are undelete utilities that will attempt to reconstruct the index and can bring the file back if the parts were not reused.

On a busy system with a nearly-full drive, space can get reused in a few seconds.  But there is no way to know for sure.  If you have sensitive data, you may want to be sure that recovery is not possible by actually overwriting the file with non-sensitive data.

However, even after doing that, it is possible to take the disk back to a laboratory and use a lot of sensitive (and expensive) equipment to look for the faint ``echoes'' of the original data underneath the overwritten data.  If the data has only been overwritten once, it's not even that hard.

The best way to remove something irretrievably is to destroy the media it's on with acid, melt it down, or the like.  For cheap removable media like floppy disks, this is the preferred method.  However, hard drives are expensive and hard to melt, so the shred utility tries to achieve a similar effect non-destructively.

This uses many overwrite passes, with the data patterns chosen to maximize the damage they do to the old data.  While this will work on floppies, the patterns are designed for best effect on hard drives. For more details, see the source code and Peter Gutmann's paper Secure Deletion of Data from Magnetic and Solid-State Memory, from the proceedings of the Sixth USENIX Security Symposium (San Jose, California, 22--25 July, 1996).  The paper is also available online http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html.

Please note that shred relies on a very important assumption: that the filesystem overwrites data in place.  This is the traditional way to do things, but many modern filesystem designs do not satisfy this assumption.  Exceptions include:

  * Log-structured or journaled filesystems, such as those supplied with AIX and Solaris, and JFS, ReiserFS, XFS, Ext3, etc.

  * Filesystems that write redundant data and carry on even if some writes fail, such as RAID-based filesystems.

  * Filesystems that make snapshots, such as Network Appliance's NFS server.

  * Filesystems that cache in temporary locations, such as NFS version 3 clients.

  * Compressed filesystems.

If you are not sure how your filesystem operates, then you should assume that it does not overwrite data in place, which means that shred cannot reliably operate on regular files in your filesystem.

Generally speaking, it is more reliable to shred a device than a file, since this bypasses the problem of filesystem design mentioned above. However, even shredding devices is not always completely reliable.  For example, most disks map out bad sectors invisibly to the application; if the bad sectors contain sensitive data, shred won't be able to destroy it.

shred makes no attempt to detect or report this problem, just as it makes no attempt to do anything about backups.  However, since it is more reliable to shred devices than files, shred by default does not truncate or remove the output file.  This default is more suitable for devices, which typically cannot be truncated and should not be removed.

Finally, consider the risk of backups and mirrors. File system backups and remote mirrors may contain copies of the file that cannot be removed, and that will allow a shredded file to be recovered later.  So if you keep any data you may later want to destroy using shred, be sure that it is not backed up or mirrored.

     shred [OPTION]... FILE[...]

The program accepts the following options.  Also see Common options.

-f
--force
     Override file permissions if necessary to allow overwriting.

-NUMBER
-n NUMBER
--iterations=NUMBER
     By default, shred uses 25 passes of overwrite.  This is enough for all of the useful overwrite patterns to be used at least once. You can reduce this to save time, or increase it if you have a lot of time to waste.

-s BYTES
--size=BYTES
     Shred the first BYTES bytes of the file. The default is to shred the whole file.  BYTES can be followed by a size specification like K, M, or G to specify a multiple. See Block size.

-u
--remove
     After shredding a file, truncate it (if possible) and then remove it. If a file has multiple links, only the named links will be removed.

-v
--verbose
     Display status updates as sterilization proceeds.

-x
--exact
     Normally, shred rounds the file size up to the next multiple of the filesystem block size to fully erase the last block of the file. This option suppresses that behavior. Thus, by default if you shred a 10-byte file on a system with 512-byte blocks, the resulting file will be 512 bytes long.  With this option, shred does not increase the size of the file.

-z
--zero
     Normally, the last pass that shred writes is made up of random data.  If this would be conspicuous on your hard drive (for example, because it looks like encrypted data), or you just think it's tidier, the --zero option adds an additional overwrite pass with all zero bits.  This is in addition to the number of passes specified by the --iterations option.

-
     Shred standard output.

     This argument is considered an option.  If the common -- option has been used to indicate the end of options on the command line, then - will be interpreted as an ordinary file name.

     The intended use of this is to shred a removed temporary file. For example

          i=`tempfile -m 0600`
          exec 3<>"$i"
          rm -- "$i"
          echo "Hello, world" >&3
          shred - >&3
          exec 3>-

     Note that the shell command shred - >file does not shred the contents of FILE, since it truncates FILE before invoking shred.  Use the command shred file or (if using a Bourne-compatible shell) the command shred - 1<>file instead.

You might use the following command to erase all trace of the filesystem you'd created on the floppy disk in your first drive. That command takes about 20 minutes to erase a ``1.44MB'' (actually 1440 KiB) floppy.

     shred --verbose /dev/fd0

Similarly, to erase all data on a selected partition of your hard disk, you could give a command like this:

     shred --verbose /dev/sda5

Special file types
******************

This chapter describes commands which create special types of files (and rmdir, which removes directories, one special file type).

Although Unix-like operating systems have markedly fewer special file types than others, not everything can be treated only as the undifferentiated byte stream of normal files.  For example, when a file is created or removed, the system must record this information, which it does in a directory---a special type of file.  Although you can read directories as normal files, if you're curious, in order for the system to do its job it must impose a structure, a certain order, on the bytes of the file.  Thus it is a ``special'' type of file.

Besides directories, other special file types include named pipes (FIFOs), symbolic links, sockets, and so-called special files.

link: Make a hard link via the link syscall
===========================================

link creates a single hard link at a time. It is a minimalist interface to the system-provided link function.  See Hard Links (The GNU C Library Reference Manual). Synopsis:

     link FILENAME LINKNAME

FILENAME must specify an existing file, and LINKNAME must specify a nonexistent entry in an existing directory. link simply calls link (FILENAME, LINKNAME) to create the link.

ln: Make links between files
============================

ln makes links between files.  By default, it makes hard links; with the -s option, it makes symbolic (or soft) links. Synopses:

     ln [OPTION]... TARGET [LINKNAME]
     ln [OPTION]... TARGET... DIRECTORY

  * If the last argument names an existing directory, ln creates a link to each TARGET file in that directory, using the TARGETs' names.  (But see the description of the --no-dereference option below.)

  * If two filenames are given, ln creates a link from the second to the first.

  * If one TARGET is given, ln creates a link to that file in the current directory.

  * It is an error if the last argument is not a directory and more than two files are given.  Without -f or -i (see below), ln will not remove an existing file.  Use the --backup option to make ln rename existing files.

A hard link is another name for an existing file; the link and the original are indistinguishable.  Technically speaking, they share the same inode, and the inode contains all the information about a file---indeed, it is not incorrect to say that the inode is the file.  On all existing implementations, you cannot make a hard link to a directory, and hard links cannot cross filesystem boundaries.  (These restrictions are not mandated by POSIX, however.)

Symbolic links (symlinks for short), on the other hand, are a special file type (which not all kernels support: System V release 3 (and older) systems lack symlinks) in which the link file actually refers to a different file, by name.  When most operations (opening, reading, writing, and so on) are passed the symbolic link file, the kernel automatically dereferences the link and operates on the target of the link.  But some operations (e.g., removing) work on the link file itself, rather than on its target.  See Symbolic Links (The GNU C Library Reference Manual).

The program accepts the following options.  Also see Common options.

-b
--backup[=METHOD]
     See Backup options. Make a backup of each file that would otherwise be overwritten or removed.

-d
-F
--directory
     Allow the super-user to make hard links to directories.

-f
--force
     Remove existing destination files.

-i
--interactive
     Prompt whether to remove existing destination files.

-n
--no-dereference
     When given an explicit destination that is a symlink to a directory, treat that destination as if it were a normal file.

     When the destination is an actual directory (not a symlink to one), there is no ambiguity.  The link is created in that directory. But when the specified destination is a symlink to a directory, there are two ways to treat the user's request.  ln can treat the destination just as it would a normal directory and create the link in it.  On the other hand, the destination can be viewed as a non-directory---as the symlink itself.  In that case, ln must delete or backup that symlink before creating the new link. The default is to treat a destination that is a symlink to a directory just like a directory.

-s
--symbolic
     Make symbolic links instead of hard links.  This option merely produces an error message on systems that do not support symbolic links.

-S SUFFIX
--suffix=SUFFIX
     Append SUFFIX to each backup file made with -b. See Backup options.

--target-directory=DIRECTORY
     Specify the destination DIRECTORY. See Target directory.

-v
--verbose
     Print the name of each file before linking it.

-V METHOD
--version-control=METHOD
     Change the type of backups made with -b.  The METHOD argument can be none (or off), numbered (or t), existing (or nil), or never (or simple).  See Backup options.

Examples:

     ln -s /some/name  # creates link ./name pointing to /some/name
     ln -s /some/name myname  # creates link ./myname pointing to /some/name
     ln -s a b ..      # creates links ../a and ../b pointing to ./a and ./b

mkdir: Make directories
=======================

mkdir creates directories with the specified names.  Synopsis:

     mkdir [OPTION]... NAME...

If a NAME is an existing file but not a directory, mkdir prints a warning message on stderr and will exit with a status of 1 after processing any remaining NAMEs.  The same is done when a NAME is an existing directory and the -p option is not given.  If a NAME is an existing directory and the -p option is given, mkdir will ignore it. That is, mkdir will not print a warning, raise an error, or change the mode of the directory (even if the -m option is given), and will move on to processing any remaining NAMEs.

The program accepts the following options.  Also see Common options.

-m MODE
--mode=MODE
     Set the mode of created directories to MODE, which is symbolic as in chmod and uses a=rwx (read, write and execute allowed for everyone) minus the bits set in the umask for the point of the departure.  See File permissions.

-p
--parents
     Make any missing parent directories for each argument.  The mode for parent directories is set to the umask modified by u+wx. Ignore arguments corresponding to existing directories.

-v

--verbose
     Print a message for each created directory.  This is most useful with --parents.

mkfifo: Make FIFOs (named pipes)
================================

mkfifo creates FIFOs (also called named pipes) with the specified names.  Synopsis:

     mkfifo [OPTION] NAME...

A FIFO is a special file type that permits independent processes to communicate.  One process opens the FIFO file for writing, and another for reading, after which data can flow as with the usual anonymous pipe in shells or elsewhere.

The program accepts the following option.  Also see Common options.

-m MODE
--mode=MODE
     Set the mode of created FIFOs to MODE, which is symbolic as in chmod and uses a=rw (read and write allowed for everyone) minus the bits set in the umask for the point of departure.  See File permissions.

mknod: Make block or character special files
============================================

mknod creates a FIFO, character special file, or block special file with the specified name.  Synopsis:

     mknod [OPTION]... NAME TYPE [MAJOR MINOR]

Unlike the phrase ``special file type'' above, the term special file has a technical meaning on Unix: something that can generate or receive data.  Usually this corresponds to a physical piece of hardware, e.g., a printer or a disk.  (These files are typically created at system-configuration time.)  The mknod command is what creates files of this type.  Such devices can be read either a character at a time or a ``block'' (many characters) at a time, hence we say there are block special files and character special files.

The arguments after NAME specify the type of file to make:

p
     for a FIFO

b
     for a block special file

c
     for a character special file

When making a block or character special file, the major and minor device numbers must be given after the file type.

The program accepts the following option.  Also see Common options.

-m MODE
--mode=MODE
     Set the mode of created files to MODE, which is symbolic as in chmod and uses a=rw minus the bits set in the umask as the point of departure.  See File permissions.

rmdir: Remove empty directories
===============================

rmdir removes empty directories.  Synopsis:

     rmdir [OPTION]... DIRECTORY...

If any DIRECTORY argument does not refer to an existing empty directory, it is an error.

The program accepts the following option.  Also see Common options.

--ignore-fail-on-non-empty
     Ignore each failure to remove a directory that is solely because the directory is non-empty.

-p
--parents
     Remove DIRECTORY, then try to remove each component of DIRECTORY. So, for example, rmdir -p a/b/c is similar to rmdir a/b/c a/b a. As such, it fails if any of those directories turns out not to be empty. Use the --ignore-fail-on-non-empty option to make it so such a failure does not evoke a diagnostic and does not cause rmdir to exit unsuccessfully.

-v

--verbose
     Give a diagnostic for each successful removal. DIRECTORY is removed.

See rm invocation, for how to remove non-empty directories (recursively).

unlink: Remove files via the unlink syscall
===========================================

unlink deletes a single specified file name. It is a minimalist interface to the system-provided unlink function.  See Deleting Files (The GNU C Library Reference Manual).  Synopsis:

     unlink FILENAME

On some systems unlink can be used to delete the name of a directory.  On others, it can be used that way only by a privileged user. In the GNU system unlink can never delete the name of a directory.

By default, unlink honors the --help and --version options.  That makes it a little harder to remove files named --help and --version, so when the environment variable POSIXLY_CORRECT is set, unlink treats such a command line arguments not as an option, but as an operand.

Changing file attributes
************************

A file is not merely its contents, a name, and a file type (see Special file types).  A file also has an owner (a userid), a group (a group id), permissions (what the owner can do with the file, what people in the group can do, and what everyone else can do), various timestamps, and other information.  Collectively, we call these a file's attributes.

These commands change file attributes.

chown: Change file owner and group
==================================

chown changes the user and/or group ownership of each given FILE to NEW-OWNER or to the user and group of an existing reference file. Synopsis:

     chown [OPTION]... {NEW-OWNER | --reference=REF_FILE} FILE...

If used, NEW-OWNER specifies the new owner and/or group as follows (with no embedded white space):

     [OWNER] [ [:] [GROUP] ]

Specifically:

owner
     If only an OWNER (a user name or numeric user id) is given, that user is made the owner of each given file, and the files' group is not changed.

owner:group
     If the OWNER is followed by a colon and a GROUP (a group name or numeric group id), with no spaces between them, the group ownership of the files is changed as well (to GROUP).

owner:
     If a colon but no group name follows OWNER, that user is made the owner of the files and the group of the files is changed to OWNER's login group.

:group
     If the colon and following GROUP are given, but the owner is omitted, only the group of the files is changed; in this case, chown performs the same function as chgrp.

You may use . in place of the : separator.  This is a GNU extension for compatibility with older scripts. New scripts should avoid the use of . because GNU chown may fail if OWNER contains . characters.

The program accepts the following options.  Also see Common options.

-c
--changes
     Verbosely describe the action for each FILE whose ownership actually changes.

-f
--silent
--quiet
     Do not print error messages about files whose ownership cannot be changed.

--from=OLD-OWNER
     Change a FILE's ownership only if it has current attributes specified by OLD-OWNER.  OLD-OWNER has the same form as NEW-OWNER described above. This option is useful primarily from a security standpoint in that it narrows considerably the window of potential abuse. For example, to reflect a UID numbering change for one user's files without an option like this, root might run

          find / -owner OLDUSER -print0 | xargs -0 chown NEWUSER

     But that is dangerous because the interval between when the find tests the existing file's owner and when the chown is actually run may be quite large. One way to narrow the gap would be to invoke chown for each file as it is found:

          find / -owner OLDUSER -exec chown NEWUSER {} \;

     But that is very slow if there are many affected files. With this option, it is safer (the gap is narrower still) though still not perfect:

          chown -R --from=OLDUSER NEWUSER /

--dereference
     Do not act on symbolic links themselves but rather on what they point to.

-h
--no-dereference
     Act on symbolic links themselves instead of what they point to. This is the default. This mode relies on the lchown system call. On systems that do not provide the lchown system call, chown fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see --verbose.

--reference=REF_FILE
     Change the user and group of each FILE to be the same as those of REF_FILE.  If REF_FILE is a symbolic link, do not use the user and group of the symbolic link, but rather those of the file it refers to.

-v
--verbose
     Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and --no-dereference is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

-R
--recursive
     Recursively change ownership of directories and their contents.

chgrp: Change group ownership
=============================

chgrp changes the group ownership of each given FILE to GROUP (which can be either a group name or a numeric group id) or to the group of an existing reference file.  Synopsis:

     chgrp [OPTION]... {GROUP | --reference=REF_FILE} FILE...

The program accepts the following options.  Also see Common options.

-c
--changes
     Verbosely describe the action for each FILE whose group actually changes.

-f
--silent
--quiet
     Do not print error messages about files whose group cannot be changed.

--dereference
     Do not act on symbolic links themselves but rather on what they point to.

-h
--no-dereference
     Act on symbolic links themselves instead of what they point to. This is the default. This mode relies on the lchown system call. On systems that do not provide the lchown system call, chgrp fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see --verbose.

--reference=REF_FILE
     Change the group of each FILE to be the same as that of REF_FILE.  If REF_FILE is a symbolic link, do not use the group of the symbolic link, but rather that of the file it refers to.

-v
--verbose
     Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the lchown system call, and --no-dereference is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed.

-R
--recursive
     Recursively change the group ownership of directories and their contents.

chmod: Change access permissions
================================

chmod changes the access permissions of the named files.  Synopsis:

     chmod [OPTION]... {MODE | --reference=REF_FILE} FILE...

chmod never changes the permissions of symbolic links, since the chmod system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used.  However, for each symbolic link listed on the command line, chmod changes the permissions of the pointed-to file. In contrast, chmod ignores symbolic links encountered during recursive directory traversals.

If used, MODE specifies the new permissions. For details, see the section on File permissions.

The program accepts the following options.  Also see Common options.

-c
--changes
     Verbosely describe the action for each FILE whose permissions actually changes.

-f
--silent
--quiet
     Do not print error messages about files whose permissions cannot be changed.

-v
--verbose
     Verbosely describe the action or non-action taken for every FILE.

--reference=REF_FILE
     Change the mode of each FILE to be the same as that of REF_FILE. See File permissions. If REF_FILE is a symbolic link, do not use the mode of the symbolic link, but rather that of the file it refers to.

-R
--recursive
     Recursively change permissions of directories and their contents.

touch: Change file timestamps
=============================

touch changes the access and/or modification times of the specified files.  Synopsis:

     touch [OPTION]... FILE...

On older systems, touch supports an obsolete syntax, as follows. If the first FILE would be a valid argument to the -t option and no timestamp is given with any of the -d, -r, or -t options and the -- argument is not given, that argument is interpreted as the time for the other files instead of as a file name.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -t instead.

Any FILE that does not exist is created empty.

If changing both the access and modification times to the current time, touch can change the timestamps for files that the user running it does not own but has write permission for.  Otherwise, the user must own the files.

Although touch provides options for changing two of the times -- the times of last access and modification -- of a file, there is actually a third one as well: the inode change time.  This is often referred to as a file's ctime. The inode change time represents the time when the file's meta-information last changed.  One common example of this is when the permissions of a file change.  Changing the permissions doesn't access the file, so the atime doesn't change, nor does it modify the file, so the mtime doesn't change.  Yet, something about the file itself has changed, and this must be noted somewhere.  This is the job of the ctime field. This is necessary, so that, for example, a backup program can make a fresh copy of the file, including the new permissions value. Another operation that modifies a file's ctime without affecting the others is renaming.  In any case, it is not possible, in normal operations, for a user to change the ctime field to a user-specified value.

The program accepts the following options.  Also see Common options.

-a
--time=atime
--time=access
--time=use
     Change the access time only.

-c
--no-create
     Do not create files that do not exist.

-d
--date=time
     Use TIME instead of the current time.  It can contain month names, time zones, am and pm, etc.  See Date input formats.

-f
     Ignored; for compatibility with BSD versions of touch.

-m
--time=mtime
--time=modify
     Change the modification time only.

-r FILE
--reference=FILE
     Use the times of the reference FILE instead of the current time.

-t [[CC]YY]MMDDhhmm[.ss]
     Use the argument (optional four-digit or two-digit years, months, days, hours, minutes, optional seconds) instead of the current time. If the year is specified with only two digits, then CC is 20 for years in the range 0 ... 68, and 19 for years in 69 ... 99.  If no digits of the year are specified, the argument is interpreted as a date in the current year.

Disk usage
**********

No disk can hold an infinite amount of data.  These commands report on how much disk storage is in use or available.  (This has nothing much to do with how much main memory, i.e., RAM, a program is using when it runs; for that, you want ps or pstat or swap or some such command.)

df: Report filesystem disk space usage
======================================

df reports the amount of disk space used and available on filesystems.  Synopsis:

     df [OPTION]... [FILE]...

With no arguments, df reports the space used and available on all currently mounted filesystems (of all types).  Otherwise, df reports on the filesystem containing each argument FILE.

Normally the disk space is printed in units of 1024 bytes, but this can be overridden (see Block size). Non-integer quantities are rounded up to the next higher unit.

If an argument FILE is a disk device file containing a mounted filesystem, df shows the space available on that filesystem rather than on the filesystem containing the device node (i.e., the root filesystem).  GNU df does not attempt to determine the disk usage on unmounted filesystems, because on most kinds of systems doing so requires extremely nonportable intimate knowledge of filesystem structures.

The program accepts the following options.  Also see Common options.

-a
--all
     Include in the listing filesystems that have a size of 0 blocks, which are omitted by default.  Such filesystems are typically special-purpose pseudo-filesystems, such as automounter entries.  Also, filesystems of type ``ignore'' or ``auto'', supported by some operating systems, are only included if this option is specified.

-B SIZE
--block-size=SIZE
     Scale sizes by SIZE before printing them (see Block size). For example, -BG prints sizes in units of 1,073,741,824 bytes.

-h
--human-readable
     Append a size letter to each size, such as M for mebibytes. Powers of 1024 are used, not 1000; M stands for 1,048,576 bytes. Use the -H or --si option if you prefer powers of 1000.

-H
--si
     Append an SI-style abbreviation to each size, such as MB for megabytes.  Powers of 1000 are used, not 1024; MB stands for 1,000,000 bytes.  Use the -h or --human-readable option if you prefer powers of 1024.

-i
--inodes
     List inode usage information instead of block usage.  An inode (short for index node) contains information about a file such as its owner, permissions, timestamps, and location on the disk.

-k
     Print sizes in 1024-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1K.

-l
--local
     Limit the listing to local filesystems.  By default, remote filesystems are also listed.

--no-sync
     Do not invoke the sync system call before getting any usage data. This may make df run significantly faster on systems with many disks, but on some systems (notably SunOS) the results may be slightly out of date.  This is the default.

-P
--portability
     Use the POSIX output format.  This is like the default format except for the following:

      1. The information about each filesystem is always printed on exactly one line; a mount device is never put on a line by itself.  This means that if the mount device name is more than 20 characters long (e.g., for some network mounts), the columns are misaligned.

      2. The labels in the header output line are changed to conform to POSIX.

--sync
     Invoke the sync system call before getting any usage data.  On some systems (notably SunOS), doing this yields more up to date results, but in general this option makes df much slower, especially when there are many or very busy filesystems.

-t FSTYPE
--type=FSTYPE
     Limit the listing to filesystems of type FSTYPE.  Multiple filesystem types can be specified by giving multiple -t options. By default, nothing is omitted.

-T
--print-type
     Print each filesystem's type.  The types printed here are the same ones you can include or exclude with -t and -x.  The particular types printed are whatever is supported by the system.  Here are some of the common names (this list is certainly not exhaustive):

     nfs
          An NFS filesystem, i.e., one mounted over a network from another machine.  This is the one type name which seems to be used uniformly by all systems.

     4.2, ufs, efs...
          A filesystem on a locally-mounted hard disk.  (The system might even support more than one type here; Linux does.)

     hsfs, cdfs
          A filesystem on a CD-ROM drive.  HP-UX uses cdfs, most other systems use hsfs (hs for ``High Sierra'').

     pcfs
          An MS-DOS filesystem, usually on a diskette.

-x FSTYPE
--exclude-type=FSTYPE
     Limit the listing to filesystems not of type FSTYPE. Multiple filesystem types can be eliminated by giving multiple -x options.  By default, no filesystem types are omitted.

-v
     Ignored; for compatibility with System V versions of df.

du: Estimate file space usage
=============================

du reports the amount of disk space used by the specified files and for each subdirectory (of directory arguments).  Synopsis:

     du [OPTION]... [FILE]...

With no arguments, du reports the disk space for the current directory.  Normally the disk space is printed in units of 1024 bytes, but this can be overridden (see Block size). Non-integer quantities are rounded up to the next higher unit.

The program accepts the following options.  Also see Common options.

-a
--all
     Show counts for all files, not just directories.

-b
--bytes
     Print sizes in bytes, overriding the default block size (see Block size).

-B SIZE
--block-size=SIZE
     Scale sizes by SIZE before printing them (see Block size). For example, -BG prints sizes in units of 1,073,741,824 bytes.

-c
--total
     Print a grand total of all arguments after all arguments have been processed.  This can be used to find out the total disk usage of a given set of files or directories.

-D
--dereference-args
     Dereference symbolic links that are command line arguments. Does not affect other symbolic links.  This is helpful for finding out the disk usage of directories, such as /usr/tmp, which are often symbolic links.

-h
--human-readable
     Append a size letter to each size, such as M for mebibytes. Powers of 1024 are used, not 1000; M stands for 1,048,576 bytes. Use the -H or --si option if you prefer powers of 1000.

-H
--si
     Append an SI-style abbreviation to each size, such as MB for megabytes.  Powers of 1000 are used, not 1024; MB stands for 1,000,000 bytes.  Use the -h or --human-readable option if you prefer powers of 1024.

-k
     Print sizes in 1024-byte blocks, overriding the default block size (see Block size). This option is equivalent to --block-size=1K.

-l
--count-links
     Count the size of all files, even if they have appeared already (as a hard link).

-L
--dereference
     Dereference symbolic links (show the disk space used by the file or directory that the link points to instead of the space used by the link).

--max-depth=DEPTH
     Show the total for each directory (and file if --all) that is at most MAX_DEPTH levels down from the root of the hierarchy.  The root is at level 0, so du --max-depth=0 is equivalent to du -s.

-s
--summarize
     Display only a total for each argument.

-S
--separate-dirs
     Report the size of each directory separately, not including the sizes of subdirectories.

-x
--one-file-system
     Skip directories that are on different filesystems from the one that the argument being processed is on.

--exclude=PATTERN
     When recursing, skip subdirectories or files matching PATTERN. For example, du --exclude='*.o' excludes files whose names end in .o.

-X FILE
--exclude-from=FILE
     Like --exclude, except take the patterns to exclude from FILE, one per line.  If FILE is -, take the patterns from standard input.

On BSD systems, du reports sizes that are half the correct values for files that are NFS-mounted from HP-UX systems.  On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems.  This is due to a flaw in HP-UX; it also affects the HP-UX du program.

stat: Report file or filesystem status
======================================

stat displays information about the specified file(s).  Synopsis:

     stat [OPTION]... [FILE]...

With no option, stat reports all information about the given files. But it also can be used to report the information of the filesystems the given files are located on. If the files are links, stat can also give information about the files the links point to.

-f
--filesystem
     Report information about the filesystems where the given files are located instead of information about the files themselves.

-L
--dereference
     Change how stat treats symbolic links. With this option, stat acts on the file referenced by each symbolic link argument. Without it, stat acts on any symbolic link argument directly.

-t
--terse
     Print the information in terse form, suitable for parsing by other programs.

-c
--format
     Allow user to specify the output format.

     Interpreted sequences for file stat are:

       * %n - File name

       * %N - Quoted File name with dereference if symbolic link

       * %d - Device number in decimal

       * %D - Device number in hex

       * %i - Inode number

       * %a - Access rights in octal

       * %A - Access rights in human readable form

       * %f - raw mode in hex

       * %F - File type

       * %h - Number of hard links

       * %u - User Id of owner

       * %U - User name of owner

       * %g - Group Id of owner

       * %G - Group name of owner

       * %t - Major device type in hex

       * %T - Minor device type in hex

       * %s - Total size, in bytes

       * %b - Number of blocks allocated

       * %o - IO block size

       * %x - Time of last access

       * %X - Time of last access as seconds since Epoch

       * %y - Time of last modification

       * %Y - Time of last modification as seconds since Epoch

       * %z - Time of last change

       * %Z - Time of last change as seconds since Epoch

     Interpreted sequences for filesystem stat are:

       * %n - File name

       * %i - File System id in hex

       * %l - Maximum length of filenames

       * %t - Type in hex

       * %T - Type in human readable form

       * %b - Total data blocks in file system

       * %f - Free blocks in file system

       * %a - Free blocks available to non-superuser

       * %s - Optimal transfer block size

       * %c - Total file nodes in file system

sync: Synchronize data on disk with memory
==========================================

sync writes any data buffered in memory out to disk.  This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes.  This must be implemented by the kernel; The sync program does nothing but exercise the sync system call.

The kernel keeps data in memory to avoid doing (relatively slow) disk reads and writes.  This improves performance, but if the computer crashes, data may be lost or the filesystem corrupted as a result. sync ensures everything in memory is written to disk.

Any arguments are ignored, except for a lone --help or --version (see Common options).

Printing text
*************

This section describes commands that display text strings.

echo: Print a line of text
==========================

echo writes each given STRING to standard output, with a space between each and a newline after the last one.  Synopsis:

     echo [OPTION]... [STRING]...

The program accepts the following options.  Also see Common options.

-n
     Do not output the trailing newline.

-e
     Enable interpretation of the following backslash-escaped characters in each STRING:

     \a
          alert (bell)

     \b
          backspace

     \c
          suppress trailing newline

     \f
          form feed

     \n
          new line

     \r
          carriage return

     \t
          horizontal tab

     \v
          vertical tab

     \\
          backslash

     \NNN
          the character whose ASCII code is NNN (octal); if NNN is not a valid octal number, it is printed literally.

printf: Format and print data
=============================

printf does formatted printing of text. Synopsis:

     printf FORMAT [ARGUMENT]...

printf prints the FORMAT string, interpreting % directives and \ escapes in the same way as the C printf function.  The FORMAT argument is re-used as necessary to convert all of the given ARGUMENTs.

printf has one additional directive, %b, which prints its argument string with \ escapes interpreted in the same way as in the FORMAT string.

printf interprets \0ooo in FORMAT as an octal number (if OOO is 0 to 3 octal digits) specifying a character to print, and \xhh as a hexadecimal number (if HH is 1 to 2 hex digits) specifying a character to print.

printf interprets two character syntaxes introduced in ISO C 99: \u for 16-bit Unicode characters, specified as 4 hex digits HHHH, and \U for 32-bit Unicode characters, specified as 8 hex digits HHHHHHHH. printf outputs the Unicode characters according to the LC_CTYPE part of the current locale, i.e. depending on the values of the environment variables LC_ALL, LC_CTYPE, LANG.

The processing of \u and \U requires a full-featured iconv facility. It is activated on systems with glibc 2.2 (or newer), or when libiconv is installed prior to this package. Otherwise the use of \u and \U will give an error message.

An additional escape, \c, causes printf to produce no further output.

The only options are a lone --help or --version.  See Common options.

The Unicode character syntaxes are useful for writing strings in a locale independent way. For example, a string containing the Euro currency symbol

     $ /usr/local/bin/printf '\u20AC 14.95'

will be output correctly in all locales supporting the Euro symbol (ISO-8859-15, UTF-8, and others). Similarly, a Chinese string

     $ /usr/local/bin/printf '\u4e2d\u6587'

will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).

Note that in these examples, the full pathname of printf has been given, to distinguish it from the GNU bash builtin function printf.

For larger strings, you don't need to look up the hexadecimal code values of each character one by one. ASCII characters mixed with \u escape sequences is also known as the JAVA source file encoding. You can use GNU recode 3.5c (or newer) to convert strings to this encoding. Here is how to convert a piece of text into a shell script which will output this text in a locale-independent way:

     $ LC_CTYPE=zh_CN.big5 /usr/local/bin/printf \
         '\u4e2d\u6587\n' > sample.txt
     $ recode BIG5..JAVA < sample.txt \
         | sed -e "s|^|/usr/local/bin/printf '|" -e "s|$|\\\\n'|" \
         > sample.sh

yes: Print a string until interrupted
=====================================

yes prints the command line arguments, separated by spaces and followed by a newline, forever until it is killed.  If no arguments are given, it prints y followed by a newline forever until killed.

The only options are a lone --help or --version. See Common options.

Conditions
**********

This section describes commands that are primarily useful for their exit status, rather than their output.  Thus, they are often used as the condition of shell if statements, or as the last command in a pipeline.

false: Do nothing, unsuccessfully
=================================

false does nothing except return an exit status of 1, meaning failure.  It can be used as a place holder in shell scripts where an unsuccessful command is needed.

By default, false honors the --help and --version options.  However, that is contrary to POSIX, so when the environment variable POSIXLY_CORRECT is set, false ignores all command line arguments, including --help and --version.

This version of false is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts.

true: Do nothing, successfully
==============================

true does nothing except return an exit status of 0, meaning success.  It can be used as a place holder in shell scripts where a successful command is needed, although the shell built-in command : (colon) may do the same thing faster. In most modern shells, true is a built-in command, so when you use true in a script, you're probably using the built-in command, not the one documented here.

By default, true honors the --help and --version options.  However, that is contrary to POSIX, so when the environment variable POSIXLY_CORRECT is set, true ignores all command line arguments, including --help and --version.

This version of true is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts.

test: Check file types and compare values
=========================================

test returns a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression EXPR.  Each part of the expression must be a separate argument.

test has file status checks, string operators, and numeric comparison operators.

Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.

Besides the options below, test accepts a lone --help or --version.  See Common options.  A single non-option argument is also allowed: test returns true if the argument is not null.

File type tests
---------------

These options test for particular types of files.  (Everything's a file, but not all files are the same!)

-b FILE
     True if FILE exists and is a block special device.

-c FILE
     True if FILE exists and is a character special device.

-d FILE
     True if FILE exists and is a directory.

-f FILE
     True if FILE exists and is a regular file.

-h FILE
-L FILE
     True if FILE exists and is a symbolic link.

-p FILE
     True if FILE exists and is a named pipe.

-S FILE
     True if FILE exists and is a socket.

-t [FD]
     True if FD is opened on a terminal.  If FD is omitted, it defaults to 1 (standard output).

Access permission tests
-----------------------

These options test for particular access permissions.

-g FILE
     True if FILE exists and has its set-group-id bit set.

-k FILE
     True if FILE has its sticky bit set.

-r FILE
     True if FILE exists and is readable.

-u FILE
     True if FILE exists and has its set-user-id bit set.

-w FILE
     True if FILE exists and is writable.

-x FILE
     True if FILE exists and is executable.

-O FILE
     True if FILE exists and is owned by the current effective user id.

-G FILE
     True if FILE exists and is owned by the current effective group id.

File characteristic tests
-------------------------

These options test other file characteristics.

-e FILE
     True if FILE exists.

-s FILE
     True if FILE exists and has a size greater than zero.

FILE1 -nt FILE2
     True if FILE1 is newer (according to modification date) than FILE2, or if FILE1 exists and FILE2 does not.

FILE1 -ot FILE2
     True if FILE1 is older (according to modification date) than FILE2, or if FILE2 exists and FILE1 does not.

FILE1 -ef FILE2
     True if FILE1 and FILE2 have the same device and inode numbers, i.e., if they are hard links to each other.

String tests
------------

These options test string characteristics.  Strings are not quoted for test, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces.

-z STRING
     True if the length of STRING is zero.

-n STRING
STRING
     True if the length of STRING is nonzero.

STRING1 = STRING2
     True if the strings are equal.

STRING1 != STRING2
     True if the strings are not equal.

Numeric tests
-------------

Numeric relationals.  The arguments must be entirely numeric (possibly negative), or the special expression -l STRING, which evaluates to the length of STRING.

ARG1 -eq ARG2
ARG1 -ne ARG2
ARG1 -lt ARG2
ARG1 -le ARG2
ARG1 -gt ARG2
ARG1 -ge ARG2
     These arithmetic binary operators return true if ARG1 is equal, not-equal, less-than, less-than-or-equal, greater-than, or greater-than-or-equal than ARG2, respectively.

For example:

     test -1 -gt -2 && echo yes
     => yes
     test -l abc -gt 1 && echo yes
     => yes
     test 0x100 -eq 1
     error--> test: integer expression expected before -eq

Connectives for test
--------------------

The usual logical connectives.

! EXPR
     True if EXPR is false.

EXPR1 -a EXPR2
     True if both EXPR1 and EXPR2 are true.

EXPR1 -o EXPR2
     True if either EXPR1 or EXPR2 is true.

expr: Evaluate expressions
==========================

expr evaluates an expression and writes the result on standard output.  Each token of the expression must be a separate argument.

Operands are either numbers or strings.  expr converts anything appearing in an operand position to an integer or a string depending on the operation being applied to it.

Strings are not quoted for expr itself, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces.

Operators may be given as infix symbols or prefix keywords.  Parentheses may be used for grouping in the usual manner (you must quote parentheses to avoid the shell evaluating them, however).

Exit status:

     0 if the expression is neither null nor 0,
     1 if the expression is null or 0,
     2 for invalid expressions.

String expressions
------------------

expr supports pattern matching and other string operators.  These have lower precedence than both the numeric and relational operators (in the next sections).

STRING : REGEX
     Perform pattern matching.  The arguments are converted to strings and the second is considered to be a (basic, a la GNU grep) regular expression, with a ^ implicitly prepended.  The first argument is then matched against this regular expression.

     If the match succeeds and REGEX uses \( and \), the : expression returns the part of STRING that matched the subexpression; otherwise, it returns the number of characters matched.

     If the match fails, the : operator returns the null string if \( and \) are used in REGEX, otherwise 0.

     Only the first \( ... \) pair is relevant to the return value; additional pairs are meaningful only for grouping the regular expression operators.

     In the regular expression, \+, \?, and \| are operators which respectively match one or more, zero or one, or separate alternatives.  SunOS and other expr's treat these as regular characters.  (POSIX allows either behavior.) See Regular Expression Library (Regex), for details of regular expression syntax.  Some examples are in Examples of expr.

match STRING REGEX
     An alternative way to do pattern matching.  This is the same as STRING : REGEX.

substr STRING POSITION LENGTH
     Returns the substring of STRING beginning at POSITION with length at most LENGTH.  If either POSITION or LENGTH is negative, zero, or non-numeric, returns the null string.

index STRING CHARSET
     Returns the first position in STRING where the first character in CHARSET was found.  If no character in CHARSET is found in STRING, return 0.

length STRING
     Returns the length of STRING.

+ TOKEN
     Interpret TOKEN as a string, even if it is a keyword like MATCH or an operator like /. This makes it possible to test expr length + "$x" or expr + "$x" : '.*/\(.\)' and have it do the right thing even if the value of $X happens to be (for example) / or index. This operator is a GNU extension.  Portable shell scripts should use " $token" : ' \(.*\)' instead of + "$token".

To make expr interpret keywords as strings, you must use the quote operator.

Numeric expressions
-------------------

expr supports the usual numeric operators, in order of increasing precedence.  The string operators (previous section) have lower precedence, the connectives (next section) have higher.

+ -
     Addition and subtraction.  Both arguments are converted to numbers; an error occurs if this cannot be done.

* / %
     Multiplication, division, remainder.  Both arguments are converted to numbers; an error occurs if this cannot be done.

Relations for expr
------------------

expr supports the usual logical connectives and relations.  These are higher precedence than either the string or numeric operators (previous sections).  Here is the list, lowest-precedence operator first.

|
     Returns its first argument if that is neither null nor 0, otherwise its second argument.

&
     Return its first argument if neither argument is null or 0, otherwise 0.

< <= = == != >= >
     Compare the arguments and return 1 if the relation is true, 0 otherwise. == is a synonym for =.  expr first tries to convert both arguments to numbers and do a numeric comparison; if either conversion fails, it does a lexicographic comparison using the character collating sequence specified by the LC_COLLATE locale.

Examples of using expr
----------------------

Here are a few examples, including quoting for shell metacharacters.

To add 1 to the shell variable foo, in Bourne-compatible shells:

     foo=`expr $foo + 1`

To print the non-directory part of the file name stored in $fname, which need not contain a /.

     expr $fname : '.*/\(.*\)' '|' $fname

An example showing that \+ is an operator:

     expr aaa : 'a\+'
     => 3

     expr abc : 'a\(.\)c'
     => b
     expr index abcdef cz
     => 3
     expr index index a
     error--> expr: syntax error
     expr index quote index a
     => 0

Redirection
***********

Unix shells commonly provide several forms of redirection---ways to change the input source or output destination of a command.  But one useful redirection is performed by a separate command, not by the shell; it's described here.

tee: Redirect output to multiple files
======================================

The tee command copies standard input to standard output and also to any files given as arguments.  This is useful when you want not only to send some data down a pipe, but also to save a copy.  Synopsis:

     tee [OPTION]... [FILE]...

If a file being written to does not already exist, it is created.  If a file being written to already exists, the data it previously contained is overwritten unless the -a option is used.

The program accepts the following options.  Also see Common options.

-a
--append
     Append standard input to the given files rather than overwriting them.

-i
--ignore-interrupts
     Ignore interrupt signals.

File name manipulation
**********************

This section describes commands that manipulate file names.

basename: Strip directory and suffix from a file name
=====================================================

basename removes any leading directory components from NAME.  Synopsis:

     basename NAME [SUFFIX]

If SUFFIX is specified and is identical to the end of NAME, it is removed from NAME as well.  basename prints the result on standard output.

The only options are --help and --version.  See Common options.

dirname: Strip non-directory suffix from a file name
====================================================

dirname prints all but the final slash-delimited component of a string (presumably a filename).  Synopsis:

     dirname NAME

If NAME is a single component, dirname prints . (meaning the current directory).

The only options are --help and --version.  See Common options.

pathchk: Check file name portability
====================================

pathchk checks portability of filenames.  Synopsis:

     pathchk [OPTION]... NAME...

For each NAME, pathchk prints a message if any of these conditions is true:

 1. one of the existing directories in NAME does not have search (execute) permission,

 2. the length of NAME is larger than its filesystem's maximum file name length,

 3. the length of one component of NAME, corresponding to an existing directory name, is larger than its filesystem's maximum length for a file name component.

The program accepts the following option.  Also see Common options.

-p
--portability
     Instead of performing length checks on the underlying filesystem, test the length of each file name and its components against the POSIX minimum limits for portability.  Also check that the file name contains no characters not in the portable file name character set.

Exit status:

     0 if all specified file names passed all of the tests,
     1 otherwise.

Working context
***************

This section describes commands that display or alter the context in which you are working: the current directory, the terminal settings, and so forth.  See also the user-related commands in the next section.

pwd: Print working directory
============================

pwd prints the fully resolved name of the current directory. That is, all components of the printed name will be actual directory names---none will be symbolic links.

Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.

The only options are a lone --help or --version.  See Common options.

stty: Print or change terminal characteristics
==============================================

stty prints or changes terminal characteristics, such as baud rate. Synopses:

     stty [OPTION] [SETTING]...
     stty [OPTION]

If given no line settings, stty prints the baud rate, line discipline number (on systems that support it), and line settings that have been changed from the values set by stty sane. By default, mode reading and setting are performed on the tty line connected to standard input, although this can be modified by the --file option.

stty accepts many non-option arguments that change aspects of the terminal line operation, as described below.

The program accepts the following options.  Also see Common options.

-a
--all
     Print all current settings in human-readable form.  This option may not be used in combination with any line settings.

-F DEVICE
--file=DEVICE
     Set the line opened by the filename specified in DEVICE instead of the tty line connected to standard input.  This option is necessary because opening a POSIX tty requires use of the O_NONDELAY flag to prevent a POSIX tty from blocking until the carrier detect line is high if the clocal flag is not set.  Hence, it is not always possible to allow the shell to open the device in the traditional manner.

-g
--save
     Print all current settings in a form that can be used as an argument to another stty command to restore the current settings.  This option may not be used in combination with any line settings.

Many settings can be turned off by preceding them with a -. Such arguments are marked below with ``May be negated'' in their description.  The descriptions themselves refer to the positive case, that is, when not negated (unless stated otherwise, of course).

Some settings are not available on all POSIX systems, since they use extensions.  Such arguments are marked below with ``Non-POSIX'' in their description.  On non-POSIX systems, those or other settings also may not be available, but it's not feasible to document all the variations: just try it and see.

Control settings
----------------

Control settings:

parenb
     Generate parity bit in output and expect parity bit in input. May be negated.

parodd
     Set odd parity (even if negated).  May be negated.

cs5
cs6
cs7
cs8
     Set character size to 5, 6, 7, or 8 bits.

hup
hupcl
     Send a hangup signal when the last process closes the tty.  May be negated.

cstopb
     Use two stop bits per character (one if negated).  May be negated.

cread
     Allow input to be received.  May be negated.

clocal
     Disable modem control signals.  May be negated.

crtscts
     Enable RTS/CTS flow control.  Non-POSIX.  May be negated.

Input settings
--------------

ignbrk
     Ignore break characters.  May be negated.

brkint
     Make breaks cause an interrupt signal.  May be negated.

ignpar
     Ignore characters with parity errors.  May be negated.

parmrk
     Mark parity errors (with a 255-0-character sequence).  May be negated.

inpck
     Enable input parity checking.  May be negated.

istrip
     Clear high (8th) bit of input characters.  May be negated.

inlcr
     Translate newline to carriage return.  May be negated.

igncr
     Ignore carriage return.  May be negated.

icrnl
     Translate carriage return to newline.  May be negated.

ixon
     Enable XON/XOFF flow control (that is, CTRL-S/CTRL-Q).  May be negated.

ixoff
tandem
     Enable sending of stop character when the system input buffer is almost full, and start character when it becomes almost empty again.  May be negated.

iuclc
     Translate uppercase characters to lowercase.  Non-POSIX.  May be negated.

ixany
     Allow any character to restart output (only the start character if negated).  Non-POSIX.  May be negated.

imaxbel
     Enable beeping and not flushing input buffer if a character arrives when the input buffer is full.  Non-POSIX.  May be negated.

Output settings
---------------

These arguments specify output-related operations.

opost
     Postprocess output.  May be negated.

olcuc
     Translate lowercase characters to uppercase.  Non-POSIX.  May be negated.

ocrnl
     Translate carriage return to newline.  Non-POSIX.  May be negated.

onlcr
     Translate newline to carriage return-newline.  Non-POSIX.  May be negated.

onocr
     Do not print carriage returns in the first column.  Non-POSIX. May be negated.

onlret
     Newline performs a carriage return.  Non-POSIX.  May be negated.

ofill
     Use fill (padding) characters instead of timing for delays.  Non-POSIX. May be negated.

ofdel
     Use delete characters for fill instead of null characters.  Non-POSIX. May be negated.

nl1
nl0
     Newline delay style.  Non-POSIX.

cr3
cr2
cr1
cr0
     Carriage return delay style.  Non-POSIX.

tab3
tab2
tab1
tab0
     Horizontal tab delay style.  Non-POSIX.

bs1
bs0
     Backspace delay style.  Non-POSIX.

vt1
vt0
     Vertical tab delay style.  Non-POSIX.

ff1
ff0
     Form feed delay style.  Non-POSIX.

Local settings
--------------

isig
     Enable interrupt, quit, and suspend special characters.  May be negated.

icanon
     Enable erase, kill, werase, and rprnt special characters.  May be negated.

iexten
     Enable non-POSIX special characters.  May be negated.

echo
     Echo input characters.  May be negated.

echoe
crterase
     Echo erase characters as backspace-space-backspace.  May be negated.

echok
     Echo a newline after a kill character.  May be negated.

echonl
     Echo newline even if not echoing other characters.  May be negated.

noflsh
     Disable flushing after interrupt and quit special characters.  May be negated.

xcase
     Enable input and output of uppercase characters by preceding their lowercase equivalents with \, when icanon is set. Non-POSIX.  May be negated.

tostop
     Stop background jobs that try to write to the terminal.  Non-POSIX. May be negated.

echoprt
prterase
     Echo erased characters backward, between \ and /. Non-POSIX.  May be negated.

echoctl
ctlecho
     Echo control characters in hat notation (^C) instead of literally.  Non-POSIX.  May be negated.

echoke
crtkill
     Echo the kill special character by erasing each character on the line as indicated by the echoprt and echoe settings, instead of by the echoctl and echok settings.  Non-POSIX. May be negated.

Combination settings
--------------------

Combination settings:

evenp
parity
     Same as parenb -parodd cs7.  May be negated.  If negated, same as -parenb cs8.

oddp
     Same as parenb parodd cs7.  May be negated.  If negated, same as -parenb cs8.

nl
     Same as -icrnl -onlcr.  May be negated.  If negated, same as icrnl -inlcr -igncr onlcr -ocrnl -onlret.

ek
     Reset the erase and kill special characters to their default values.

sane
     Same as:

          cread -ignbrk brkint -inlcr -igncr icrnl -ixoff
          -iuclc -ixany imaxbel opost -olcuc -ocrnl onlcr
          -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0
          ff0 isig icanon iexten echo echoe echok -echonl
          -noflsh -xcase -tostop -echoprt echoctl echoke

     and also sets all special characters to their default values.

cooked
     Same as brkint ignpar istrip icrnl ixon opost isig icanon, plus sets the eof and eol characters to their default values if they are the same as the min and time characters. May be negated.  If negated, same as raw.

raw
     Same as:

          -ignbrk -brkint -ignpar -parmrk -inpck -istrip
          -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
          -imaxbel -opost -isig -icanon -xcase min 1 time 0

     May be negated.  If negated, same as cooked.

cbreak
     Same as -icanon.  May be negated.  If negated, same as icanon.

pass8
     Same as -parenb -istrip cs8.  May be negated.  If negated, same as parenb istrip cs7.

litout
     Same as -parenb -istrip -opost cs8.  May be negated. If negated, same as parenb istrip opost cs7.

decctlq
     Same as -ixany.  Non-POSIX.  May be negated.

tabs
     Same as tab0.  Non-POSIX.  May be negated.  If negated, same as tab3.

lcase
LCASE
     Same as xcase iuclc olcuc.  Non-POSIX.  May be negated.

crt
     Same as echoe echoctl echoke.

dec
     Same as echoe echoctl echoke -ixany intr ^C erase ^? kill C-u.

Special characters
------------------

The special characters' default values vary from system to system. They are set with the syntax name value, where the names are listed below and the value can be given either literally, in hat notation (^C), or as an integer which may start with 0x to indicate hexadecimal, 0 to indicate octal, or any other digit to indicate decimal.

For GNU stty, giving a value of ^- or undef disables that special character.  (This is incompatible with Ultrix stty, which uses  a value of u to disable a special character.  GNU stty treats a value u like any other, namely to set that special character to U.)

intr
     Send an interrupt signal.

quit
     Send a quit signal.

erase
     Erase the last character typed.

kill
     Erase the current line.

eof
     Send an end of file (terminate the input).

eol
     End the line.

eol2
     Alternate character to end the line.  Non-POSIX.

swtch
     Switch to a different shell layer.  Non-POSIX.

start
     Restart the output after stopping it.

stop
     Stop the output.

susp
     Send a terminal stop signal.

dsusp
     Send a terminal stop signal after flushing the input.  Non-POSIX.

rprnt
     Redraw the current line.  Non-POSIX.

werase
     Erase the last word typed.  Non-POSIX.

lnext
     Enter the next character typed literally, even if it is a special character.  Non-POSIX.

Special settings
----------------

min N
     Set the minimum number of characters that will satisfy a read until the time value has expired, when -icanon is set.

time N
     Set the number of tenths of a second before reads time out if the minimum number of characters have not been read, when -icanon is set.

ispeed N
     Set the input speed to N.

ospeed N
     Set the output speed to N.

rows N
     Tell the tty kernel driver that the terminal has N rows.  Non-POSIX.

cols N
columns N
     Tell the kernel that the terminal has N columns.  Non-POSIX.

size
     Print the number of rows and columns that the kernel thinks the terminal has.  (Systems that don't support rows and columns in the kernel typically use the environment variables LINES and COLUMNS instead; however, GNU stty does not know anything about them.) Non-POSIX.

line N
     Use line discipline N.  Non-POSIX.

speed
     Print the terminal speed.

N
     Set the input and output speeds to N.  N can be one of: 0 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200 38400 exta extb.  exta is the same as 19200; extb is the same as 38400.  0 hangs up the line if -clocal is set.

printenv: Print all or some environment variables
=================================================

printenv prints environment variable values.  Synopsis:

     printenv [OPTION] [VARIABLE]...

If no VARIABLEs are specified, printenv prints the value of every environment variable.  Otherwise, it prints the value of each VARIABLE that is set, and nothing for those that are not set.

The only options are a lone --help or --version. See Common options.

Exit status:

     0 if all variables specified were found
     1 if at least one specified variable was not found
     2 if a write error occurred

tty: Print file name of terminal on standard input
==================================================

tty prints the file name of the terminal connected to its standard input.  It prints not a tty if standard input is not a terminal. Synopsis:

     tty [OPTION]...

The program accepts the following option.  Also see Common options.

-s
--silent
--quiet
     Print nothing; only return an exit status.

Exit status:

     0 if standard input is a terminal
     1 if standard input is not a terminal
     2 if given incorrect arguments
     3 if a write error occurs

User information
****************

This section describes commands that print user-related information: logins, groups, and so forth.

id: Print real and effective uid and gid
========================================

id prints information about the given user, or the process running it if no user is specified.  Synopsis:

     id [OPTION]... [USERNAME]

By default, it prints the real user id, real group id, effective user id if different from the real user id, effective group id if different from the real group id, and supplemental group ids.

Each of these numeric values is preceded by an identifying string and followed by the corresponding user or group name in parentheses.

The options cause id to print only part of the above information. Also see Common options.

-g
--group
     Print only the group id.

-G
--groups
     Print only the supplementary groups.

-n
--name
     Print the user or group name instead of the ID number.  Requires -u, -g, or -G.

-r
--real
     Print the real, instead of effective, user or group id.  Requires -u, -g, or -G.

-u
--user
     Print only the user id.

logname: Print current login name
=================================

logname prints the calling user's name, as found in the file /etc/utmp, and exits with a status of 0.  If there is no /etc/utmp entry for the calling process, logname prints an error message and exits with a status of 1.

The only options are --help and --version.  See Common options.

whoami: Print effective user id
===============================

whoami prints the user name associated with the current effective user id.  It is equivalent to the command id -un.

The only options are --help and --version.  See Common options.

groups: Print group names a user is in
======================================

groups prints the names of the primary and any supplementary groups for each given USERNAME, or the current process if no names are given.  If names are given, the name of each user is printed before the list of that user's groups.  Synopsis:

     groups [USERNAME]...

The group lists are equivalent to the output of the command id -Gn.

The only options are --help and --version.  See Common options.

users: Print login names of users currently logged in
=====================================================

users prints on a single line a blank-separated list of user names of users currently logged in to the current host.  Each user name corresponds to a login session, so if a user has more than one login session, that user's name will appear the same number of times in the output. Synopsis:

     users [FILE]

With no FILE argument, users extracts its information from the file /etc/utmp.  If a file argument is given, users uses that file instead.  A common choice is /etc/wtmp.

The only options are --help and --version.  See Common options.

who: Print who is currently logged in
=====================================

who prints information about users who are currently logged on. Synopsis:

     who [OPTION] [FILE] [am i]

If given no non-option arguments, who prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display.

If given one non-option argument, who uses that instead of /etc/utmp as the name of the file containing the record of users logged on.  /etc/wtmp is commonly given as an argument to who to look at who has previously logged on.

If given two non-option arguments, who prints only the entry for the user running it (determined from its standard input), preceded by the hostname.  Traditionally, the two arguments given are am i, as in who am i.

The program accepts the following options.  Also see Common options.

-m
     Same as who am i.

-q
--count
     Print only the login names and the number of users logged on. Overrides all other options.

-s
     Ignored; for compatibility with other versions of who.

-i
-u
--idle
     After the login time, print the number of hours and minutes that the user has been idle.  . means the user was active in last minute. old means the user was idle for more than 24 hours.

-l
--lookup
     Attempt to canonicalize hostnames found in utmp through a DNS lookup.  This is not the default because it can cause significant delays on systems with automatic dial-up internet access.

-H
--heading
     Print a line of column headings.

-w
-T
--mesg
--message
--writable
     After each login name print a character indicating the user's message status:

          + allowing write messages
          - disallowing write messages
          ? cannot find terminal device

System context
**************

This section describes commands that print or change system-wide information.

date: Print or set system date and time
=======================================

Synopses:

     date [OPTION]... [+FORMAT]
     date [-u|--utc|--universal]
     [ MMDDhhmm[[CC]YY][.ss] ]

Invoking date with no FORMAT argument is equivalent to invoking date '+%a %b %e %H:%M:%S %Z %Y'.

If given an argument that starts with a +, date prints the current time and date (or the time and date specified by the --date option, see below) in the format defined by that argument, which is the same as in the strftime function.  Except for directives, which start with %, characters in the format string are printed unchanged.  The directives are described below.

Time directives
---------------

date directives related to times.

%H
     hour (00...23)

%I
     hour (01...12)

%k
     hour ( 0...23)

%l
     hour ( 1...12)

%M
     minute (00...59)

%N
     nanoseconds (000000000...999999999)

%p
     locale's upper case AM or PM (blank in many locales)

%P
     locale's lower case am or pm (blank in many locales)

%r
     time, 12-hour (hh:mm:ss [AP]M)

%R
     time, 24-hour (hh:mm).  Same as %H:%M.

%s
     seconds since the epoch, i.e., 1 January 1970 00:00:00 UTC (a GNU extension). Note that this value is the number of seconds between the epoch and the current date as defined by the localtime system call. It isn't changed by the --date option.

%S
     second (00...60).  The range is [00...60], and not [00...59], in order to accommodate the occasional positive leap second.

%T
     time, 24-hour (hh:mm:ss)

%X
     locale's time representation (%H:%M:%S)

%z
     RFC-822 style numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable.  This value reflects the current time zone.  It isn't changed by the --date option.

%Z
     time zone (e.g., EDT), or nothing if no time zone is determinable. Note that this value reflects the current time zone. It isn't changed by the --date option.

Date directives
---------------

date directives related to dates.

%a
     locale's abbreviated weekday name (Sun...Sat)

%A
     locale's full weekday name, variable length (Sunday...Saturday)

%b
     locale's abbreviated month name (Jan...Dec)

%B
     locale's full month name, variable length (January...December)

%c
     locale's date and time (Sat Nov 04 12:02:33 EST 1989)

%C
     century (year divided by 100 and truncated to an integer) (00...99)

%d
     day of month (01...31)

%D
     date (mm/dd/yy)

%e
     blank-padded day of month (1...31)

%F
     the ISO 8601 standard date format: %Y-%m-%d. This is the preferred form for all uses.

%g
     The year corresponding to the ISO week number, but without the century (range 00 through 99).  This has the same format and value as %y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.

%G
     The year corresponding to the ISO week number.  This has the same format and value as %Y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.

%h
     same as %b

%j
     day of year (001...366)

%m
     month (01...12)

%u
     day of week (1...7) with 1 corresponding to Monday

%U
     week number of year with Sunday as first day of week (00...53). Days in a new year preceding the first Sunday are in week zero.

%V
     week number of year with Monday as first day of the week as a decimal (01...53). If the week containing January 1 has four or more days in the new year, then it is considered week 1; otherwise, it is week 53 of the previous year, and the next week is week 1. (See the ISO 8601 standard.)

%w
     day of week (0...6) with 0 corresponding to Sunday

%W
     week number of year with Monday as first day of week (00...53). Days in a new year preceding the first Monday are in week zero.

%x
     locale's date representation (mm/dd/yy)

%y
     last two digits of year (00...99)

%Y
     year (1970....)

Literal directives
------------------

date directives that produce literal strings.

%%
     a literal %

%n
     a newline

%t
     a horizontal tab

Padding
-------

By default, date pads numeric fields with zeroes, so that, for example, numeric months are always output as two digits. GNU date recognizes the following numeric modifiers between the % and the directive.

-
     (hyphen) do not pad the field; useful if the output is intended for human consumption.

_
     (underscore) pad the field with spaces; useful if you need a fixed number of characters in the output, but zeroes are too distracting.

These are GNU extensions.

Here is an example illustrating the differences:

     date +%d/%m -d "Feb 1"
     => 01/02
     date +%-d/%-m -d "Feb 1"
     => 1/2
     date +%_d/%_m -d "Feb 1"
     =>  1/ 2

Setting the time
----------------

If given an argument that does not start with +, date sets the system clock to the time and date specified by that argument (as described below).  You must have appropriate privileges to set the system clock.  The --date and --set options may not be used with such an argument.  The --universal option may be used with such an argument to indicate that the specified time and date are relative to Coordinated Universal Time rather than to the local time zone.

The argument must consist entirely of digits, which have the following meaning:

MM
     month

DD
     day within month

hh
     hour

mm
     minute

CC
     first two digits of year (optional)

YY
     last two digits of year (optional)

ss
     second (optional)

The --set option also sets the system clock; see the next section.

Options for date
----------------

The program accepts the following options.  Also see Common options.

-d DATESTR
--date=DATESTR
     Display the time and date specified in DATESTR instead of the current time and date.  DATESTR can be in almost any common format.  It can contain month names, time zones, am and pm, yesterday, ago, next, etc.  See Date input formats.

-f DATEFILE
--file=DATEFILE
     Parse each line in DATEFILE as with -d and display the resulting time and date.  If DATEFILE is -, use standard input.  This is useful when you have many dates to process, because the system overhead of starting up the date executable many times can be considerable.

-I TIMESPEC
--iso-8601[=TIMESPEC]
     Display the date using the ISO 8601 format, %Y-%m-%d.

     The argument TIMESPEC specifies the number of additional terms of the time to include.  It can be one of the following:

     auto
          The default behavior: print just the date.

     hours
          Append the hour of the day to the date.

     minutes
          Append the hours and minutes.

     seconds
          Append the hours, minutes, and seconds.

     If showing any time terms, then include the time zone using the format %z.

     If TIMESPEC is omitted with --iso-8601, the default is auto.  On older systems, GNU date instead supports an obsolete option -I[TIMESPEC], where TIMESPEC defaults to auto.  POSIX 1003.1-2001 (see Standards conformance) does not allow -I without an argument; use --iso-8601 instead.

-R
--rfc-822
     Display the time and date using the RFC-822-conforming format, %a, %_d %b %Y %H:%M:%S %z.

-r FILE
--reference=FILE
     Display the time and date reference according to the last modification time of FILE, instead of the current time and date.

-s DATESTR
--set=DATESTR
     Set the time and date to DATESTR.  See -d above.

-u
--utc
--universal
     Use Coordinated Universal Time (UTC) by operating as if the TZ environment variable were set to the string UTC0. Normally, date operates in the time zone indicated by TZ, or the system default if TZ is not set.  Coordinated Universal Time is often called ``Greenwich Mean Time'' (GMT) for historical reasons.

Examples of date
----------------

Here are a few examples.  Also see the documentation for the -d option in the previous section.

  * To print the date of the day before yesterday:

          date --date='2 days ago'

  * To print the date of the day three months and one day hence:

          date --date='3 months 1 day'

  * To print the day of year of Christmas in the current year:

          date --date='25 Dec' +%j

  * To print the current full month name and the day of the month:

          date '+%B %d'

     But this may not be what you want because for the first nine days of the month, the %d expands to a zero-padded two-digit field, for example date -d 1may '+%B %d' will print May 01.

  * To print a date without the leading zero for one-digit days of the month, you can use the (GNU extension) - modifier to suppress the padding altogether.

          date -d 1may '+%B %-d

  * To print the current date and time in the format required by many non-GNU versions of date when setting the system clock:

          date +%m%d%H%M%Y.%S

  * To set the system clock forward by two minutes:

          date --set='+2 minutes'

  * To print the date in the format specified by RFC-822, use date --rfc.  I just did and saw this:

          Mon, 25 Mar 1996 23:34:17 -0600

  * To convert a date string to the number of seconds since the epoch (which is 1970-01-01 00:00:00 UTC), use the --date option with the %s format.  That can be useful in sorting and/or graphing and/or comparing data by date.  The following command outputs the number of the seconds since the epoch for the time two minutes after the epoch:

          date --date='1970-01-01 00:02:00 +0000' +%s
          120

     If you do not specify time zone information in the date string, date uses your computer's idea of the time zone when interpreting the string.  For example, if your computer's time zone is that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000 seconds) behind UTC:

          # local time zone used
          date --date='1970-01-01 00:02:00' +%s
          18120

  * If you're sorting or graphing dated data, your raw date values may be represented as seconds since the epoch.  But few people can look at the date 946684800 and casually note ``Oh, that's the first second of the year 2000 in Greenwich, England.''

          date --date='2000-01-01 UTC' +%s
          946684800

     To convert such an unwieldy number of seconds back to a more readable form, use a command like this:

          # local time zone used
          date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
          1999-12-31 19:00:00 -0500

uname: Print system information
===============================

uname prints information about the machine and operating system it is run on.  If no options are given, uname acts as if the -s option were given. Synopsis:

     uname [OPTION]...

If multiple options or -a are given, the selected information is printed in this order:

     KERNEL-NAME NODENAME KERNEL-RELEASE KERNEL-VERSION MACHINE PROCESSOR HARDWARE-PLATFORM OPERATING-SYSTEM

The information may contain internal spaces, so such output cannot be parsed reliably.  In the following example, RELEASE is 2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001:

     uname -a
     => Linux dum 2.2.18ss.e820-bda652a #4 SMP Tue Jun 5 11:24:08 PDT 2001 i686 unknown unknown GNU/Linux

The program accepts the following options.  Also see Common options.

-a
--all
     Print all of the below information.

-i
--hardware-platform
     Print the hardware platform name (sometimes called the hardware implementation).

-m
--machine
     Print the machine hardware name (sometimes called the hardware class).

-n
--nodename
     Print the network node hostname.

-p
--processor
     Print the processor type (sometimes called the instruction set architecture or ISA).

-o
--operating-system
     Print the name of the operating system.

-r
--kernel-release
     Print the kernel release.

-s
--kernel-name
     Print the kernel name.

-v
--kernel-version
     Print the kernel version.

hostname: Print or set system name
==================================

With no arguments, hostname prints the name of the current host system.  With one argument, it sets the current host name to the specified string.  You must have appropriate privileges to set the host name. Synopsis:

     hostname [NAME]

The only options are --help and --version.  See Common options.

hostid: Print numeric host identifier.
======================================

hostid prints the numeric identifier of the current host in hexadecimal.  This command accepts no arguments. The only options are --help and --version. See Common options.

For example, here's what it prints on one system I use:

     $ hostid
     1bac013d

On that system, the 32-bit quantity happens to be closely related to the system's Internet address, but that isn't always the case.

Modified command invocation
***************************

This section describes commands that run other commands in some context different than the current one: a modified environment, as a different user, etc.

chroot: Run a command with a different root directory
=====================================================

chroot runs a command with a specified root directory. On many systems, only the super-user can do this. Synopses:

     chroot NEWROOT [COMMAND [ARGS]...]
     chroot OPTION

Ordinarily, filenames are looked up starting at the root of the directory structure, i.e., /.  chroot changes the root to the directory NEWROOT (which must exist) and then runs COMMAND with optional ARGS.  If COMMAND is not specified, the default is the value of the SHELL environment variable or /bin/sh if not set, invoked with the -i option.

The only options are --help and --version.  See Common options.

Here are a few tips to help avoid common problems in using chroot. To start with a simple example, make COMMAND refer to a statically linked binary.  If you were to use a dynamically linked executable, then you'd have to arrange to have the shared libraries in the right place under your new root directory.

For example, if you create a statically linked `ls' executable, and put it in /tmp/empty, you can run this command as root:

     $ chroot /tmp/empty /ls -Rl /

Then you'll see output like this:

     /:
     total 1023
     -rwxr-xr-x    1 0        0         1041745 Aug 16 11:17 ls

If you want to use a dynamically linked executable, say bash, then first run ldd bash to see what shared objects it needs. Then, in addition to copying the actual binary, also copy the listed files to the required positions under your intended new root directory. Finally, if the executable requires any other files (e.g., data, state, device files), copy them into place, too.

env: Run a command in a modified environment
============================================

env runs a command with a modified environment.  Synopses:

     env [OPTION]... [NAME=VALUE]...
     [COMMAND [ARGS]...]
     env

Arguments of the form VARIABLE=VALUE set the environment variable VARIABLE to value VALUE. VALUE may be empty (VARIABLE=).  Setting a variable to an empty value is different from unsetting it.

The first remaining argument specifies the program name to invoke; it is searched for according to the PATH environment variable.  Any remaining arguments are passed as arguments to that program.

If no command name is specified following the environment specifications, the resulting environment is printed.  This is like specifying a command name of printenv.

The program accepts the following options.  Also see Common options.

-u NAME
--unset=NAME
     Remove variable NAME from the environment, if it was in the environment.

-
-i
--ignore-environment
     Start with an empty environment, ignoring the inherited environment.

nice: Run a command with modified scheduling priority
=====================================================

nice prints or modifies the scheduling priority of a job. Synopsis:

     nice [OPTION]... [COMMAND [ARG]...]

If no arguments are given, nice prints the current scheduling priority, which it inherited.  Otherwise, nice runs the given COMMAND with its scheduling priority adjusted.  If no ADJUSTMENT is given, the priority of the command is incremented by 10.  You must have appropriate privileges to specify a negative adjustment.  The priority can be adjusted by nice over the range of -20 (the highest priority) to 19 (the lowest).

Because most shells have a built-in command by the same name, using the unadorned command name in a script or interactively may get you different functionality than that described here.

The program accepts the following option.  Also see Common options.

-n ADJUSTMENT
--adjustment=ADJUSTMENT
     Add ADJUSTMENT instead of 10 to the command's priority.

     On older systems, nice supports an obsolete option -ADJUSTMENT.  POSIX 1003.1-2001 (see Standards conformance) does not allow this; use -n ADJUSTMENT instead.

nohup: Run a command immune to hangups
======================================

nohup runs the given COMMAND with hangup signals ignored, so that the command can continue running in the background after you log out. Synopsis:

     nohup COMMAND [ARG]...

If standard output is a terminal, it is redirected so that it is appended to the file nohup.out; if that cannot be written to, it is appended to the file $HOME/nohup.out.  If that cannot be written to, the command is not run.

If nohup creates either nohup.out or $HOME/nohup.out, it creates it with no ``group'' or ``other'' access permissions.  It does not change the permissions if the output file already existed.

If standard error is a terminal, it is redirected to the same file descriptor as the standard output.

nohup does not automatically put the command it runs in the background; you must do that explicitly, by ending the command line with an &.  Also, nohup does not change the scheduling priority of COMMAND; use nice for that, e.g., nohup nice COMMAND.

The only options are --help and --version.  See Common options.

Exit status:

     126 if COMMAND was found but could not be invoked
     127 if nohup itself failed or if COMMAND could not be found
     the exit status of COMMAND otherwise

su: Run a command with substitute user and group id
===================================================

su allows one user to temporarily become another user.  It runs a command (often an interactive shell) with the real and effective user id, group id, and supplemental groups of a given USER. Synopsis:

     su [OPTION]... [USER [ARG]...]

If no USER is given, the default is root, the super-user. The shell to use is taken from USER's passwd entry, or /bin/sh if none is specified there.  If USER has a password, su prompts for the password unless run by a user with effective user id of zero (the super-user).

By default, su does not change the current directory. It sets the environment variables HOME and SHELL from the password entry for USER, and if USER is not the super-user, sets USER and LOGNAME to USER. By default, the shell is not a login shell.

Any additional ARGs are passed as additional arguments to the shell.

GNU su does not treat /bin/sh or any other shells specially (e.g., by setting argv[0] to -su, passing -c only to certain shells, etc.).

su can optionally be compiled to use syslog to report failed, and optionally successful, su attempts.  (If the system supports syslog.)  However, GNU su does not check if the user is a member of the wheel group; see below.

The program accepts the following options.  Also see Common options.

-c COMMAND
--command=COMMAND
     Pass COMMAND, a single command line to run, to the shell with a -c option instead of starting an interactive shell.

-f
--fast
     Pass the -f option to the shell.  This probably only makes sense if the shell run is csh or tcsh, for which the -f option prevents reading the startup file (.cshrc).  With Bourne-like shells, the -f option disables file name pattern expansion (globbing), which is not likely to be useful.

-
-l
--login
     Make the shell a login shell.  This means the following.  Unset all environment variables except TERM, HOME, and SHELL (which are set as described above), and USER and LOGNAME (which are set, even for the super-user, as described above), and set PATH to a compiled-in default value.  Change to USER's home directory.  Prepend - to the shell's name, intended to make it read its login startup file(s).

-m
-p
--preserve-environment
     Do not change the environment variables HOME, USER, LOGNAME, or SHELL.  Run the shell given in the environment variable SHELL instead of the shell from USER's passwd entry, unless the user running su is not the superuser and USER's shell is restricted.  A restricted shell is one that is not listed in the file /etc/shells, or in a compiled-in list if that file does not exist.  Parts of what this option does can be overridden by --login and --shell.

-s SHELL
--shell=SHELL
     Run SHELL instead of the shell from USER's passwd entry, unless the user running su is not the superuser and USER's shell is restricted (see -m just above).

Why GNU su does not support the wheel group
===========================================

(This section is by Richard Stallman.)

Sometimes a few of the users try to hold total power over all the rest.  For example, in 1984, a few users at the MIT AI lab decided to seize power by changing the operator password on the Twenex system and keeping it secret from everyone else.  (I was able to thwart this coup and give power back to the users by patching the kernel, but I wouldn't know how to do that in Unix.)

However, occasionally the rulers do tell someone.  Under the usual su mechanism, once someone learns the root password who sympathizes with the ordinary users, he or she can tell the rest.  The ``wheel group'' feature would make this impossible, and thus cement the power of the rulers.

I'm on the side of the masses, not that of the rulers.  If you are used to supporting the bosses and sysadmins in whatever they do, you might find this idea strange at first.

Process control
***************

kill: Send a signal to processes
================================

The kill command sends a signal to processes, causing them to terminate or otherwise act upon receiving the signal in some way. Alternatively, it lists information about signals.  Synopses:

     kill [-s SIGNAL | --signal SIGNAL | -SIGNAL] PID...
     kill [-l | --list | -t | --table] [SIGNAL]...

The first form of the kill command sends a signal to all PID arguments.  The default signal to send if none is specified is TERM.  The special signal number 0 does not denote a valid signal, but can be used to test whether the PID arguments specify processes to which a signal could be sent.

If PID is positive, the signal is sent to the process with the process id PID.  If PID is zero, the signal is sent to all processes in the process group of the current process.  If PID is -1, the signal is sent to all processes for which the user has permission to send a signal.  If PID is less than -1, the signal is sent to all processes in the process group that equals the absolute value of PID.

If PID is not positive, a system-dependent set of system processes is excluded from the list of processes to which the signal is sent.

If a negative PID argument is desired as the first one, either a signal must be specified as well, or the option parsing must be interrupted with `--' before the first PID argument. The following three commands are equivalent:

     kill -15 -1
     kill -TERM -1
     kill -- -1

The first form of the kill command succeeds if every PID argument specifies at least one process that the signal was sent to.

The second form of the kill command lists signal information. Either the -l or --list option, or the -t or --table option must be specified.  Without any SIGNAL argument, all supported signals are listed.  The output of -l or --list is a list of the signal names, one per line; if SIGNAL is already a name, the signal number is printed instead.  The output of -t or --table is a table of signal numbers, names, and descriptions.  This form of the kill command succeeds if all SIGNAL arguments are valid and if there is no output error.

The kill command also supports the --help and --version options.  See Common options.

A SIGNAL may be a signal name like HUP, or a signal number like 1, or an exit status of a process terminated by the signal.  A signal name can be given in canonical form or prefixed by SIG.  The case of the letters is ignored, except for the -SIGNAL option which must use upper case to avoid ambiguity with lower case option letters.  The following signal names and numbers are supported on all POSIX compliant systems:

HUP
     1.  Hangup.

INT
     2.  Terminal interrupt.

QUIT
     3.  Terminal quit.

ABRT
     6.  Process abort.

KILL
     9.  Kill (cannot be caught or ignored).

ALRM
     14.  Alarm Clock.

TERM
     15.  Termination.

Other supported signal names have system-dependent corresponding numbers.  All systems conforming to POSIX 1003.1-2001 also support the following signals:

BUS
     Access to an undefined portion of a memory object.

CHLD
     Child process terminated, stopped, or continued.

CONT
     Continue executing, if stopped.

FPE
     Erroneous arithmetic operation.

ILL
     Illegal Instruction.

PIPE
     Write on a pipe with no one to read it.

SEGV
     Invalid memory reference.

STOP
     Stop executing (cannot be caught or ignored).

TSTP
     Terminal stop.

TTIN
     Background process attempting read.

TTOU
     Background process attempting write.

URG
     High bandwidth data is available at a socket.

USR1
     User-defined signal 1.

USR2
     User-defined signal 2.

POSIX 1003.1-2001 systems that support the XSI extension also support the following signals:

POLL
     Pollable event.

PROF
     Profiling timer expired.

SYS
     Bad system call.

TRAP
     Trace/breakpoint trap.

VTALRM
     Virtual timer expired.

XCPU
     CPU time limit exceeded.

XFSZ
     File size limit exceeded.

POSIX 1003.1-2001 systems that support the XRT extension also support at least eight real-time signals called RTMIN, RTMIN+1, ..., RTMAX-1, RTMAX.

Delaying
********

sleep: Delay for a specified time
=================================

sleep pauses for an amount of time specified by the sum of the values of the command line arguments. Synopsis:

     sleep NUMBER[smhd]...

Each argument is a number followed by an optional unit; the default is seconds.  The units are:

s
     seconds

m
     minutes

h
     hours

d
     days

Historical implementations of sleep have required that NUMBER be an integer.  However, GNU sleep accepts arbitrary floating point numbers.

The only options are --help and --version.  See Common options.

Numeric operations
******************

These programs do numerically-related operations.

factor: Print prime factors
===========================

factor prints prime factors.  Synopses:

     factor [NUMBER]...
     factor OPTION

If no NUMBER is specified on the command line, factor reads numbers from standard input, delimited by newlines, tabs, or spaces.

The only options are --help and --version.  See Common options.

The algorithm it uses is not very sophisticated, so for some inputs factor runs for a long time.  The hardest numbers to factor are the products of large primes.  Factoring the product of the two largest 32-bit prime numbers takes over 10 minutes of CPU time on a 400MHz Pentium II.

     $ p=`echo '4294967279 * 4294967291'|bc`
     $ factor $p
     18446743979220271189: 4294967279 4294967291

In contrast, factor factors the largest 64-bit number in just over a tenth of a second:

     $ factor `echo '2^64-1'|bc`
     18446744073709551615: 3 5 17 257 641 65537 6700417

seq: Print numeric sequences
============================

seq prints a sequence of numbers to standard output.  Synopses:

     seq [OPTION]... [FIRST [INCREMENT]] LAST...

seq prints the numbers from FIRST to LAST by INCREMENT.  By default, FIRST and INCREMENT are both 1, and each number is printed on its own line.  All numbers can be reals, not just integers.

The program accepts the following options.  Also see Common options.

-f FORMAT
--format=FORMAT
     Print all numbers using FORMAT; default %g. FORMAT must contain exactly one of the floating point output formats %e, %f, or %g.

-s STRING
--separator=STRING
     Separate numbers with STRING; default is a newline. The output always terminates with a newline.

-w
--equal-width
     Print all numbers with the same width, by padding with leading zeroes. (To have other kinds of padding, use --format).

If you want to use seq to print sequences of large integer values, don't use the default %g format since it can result in loss of precision:

     $ seq 1000000 1000001
     1e+06
     1e+06

Instead, you can use the format, %1.f, to print large decimal numbers with no exponent and no decimal point.

     $ seq --format=%1.f 1000000 1000001
     1000000
     1000001

If you want hexadecimal output, you can use printf to perform the conversion:

     $ printf %x'\n' `seq -f %1.f 1048575 1024 1050623`
     fffff
     1003ff
     1007ff

For very long lists of numbers, use xargs to avoid system limitations on the length of an argument list:

     $ seq -f %1.f 1000000 | xargs printf %x'\n' | tail -n 3
     f423e
     f423f
     f4240

To generate octal output, use the printf %o format instead of %x.  Note however that using printf works only for numbers smaller than 2^32:

     $ printf "%x\n" `seq -f %1.f 4294967295 4294967296`
     ffffffff
     bash: printf: 4294967296: Numerical result out of range

On most systems, seq can produce whole-number output for values up to 2^53, so here's a more general approach to base conversion that also happens to be more robust for such large numbers.  It works by using bc and setting its output radix variable, OBASE, to 16 in this case to produce hexadecimal output.

     $ (echo obase=16; seq -f %1.f 4294967295 4294967296)|bc
     FFFFFFFF
     100000000

Be careful when using seq with a fractional INCREMENT, otherwise you may see surprising results.  Most people would expect to see 0.3 printed as the last number in this example:

     $ seq -s' ' 0 .1 .3
     0 0.1 0.2

But that doesn't happen on most systems because seq is implemented using binary floating point arithmetic (via the C double type) -- which means some decimal numbers like .1 cannot be represented exactly.  That in turn means some nonintuitive conditions like .1 * 3 > .3 will end up being true.

To work around that in the above example, use a slightly larger number as the LAST value:

     $ seq -s' ' 0 .1 .31
     0 0.1 0.2 0.3

In general, when using an INCREMENT with a fractional part, where (LAST - FIRST) / INCREMENT is (mathematically) a whole number, specify a slightly larger (or smaller, if INCREMENT is negative) value for LAST to ensure that LAST is the final value printed by seq.

File permissions
****************

     [file perm.texi not included]

     [file getdate.texi not included]

Opening the Software Toolbox
****************************

This chapter originally appeared in Linux Journal, volume 1, number 2, in the What's GNU? column. It was written by Arnold Robbins.

Toolbox Introduction
====================

This month's column is only peripherally related to the GNU Project, in that it describes a number of the GNU tools on your GNU/Linux system and how they might be used.  What it's really about is the ``Software Tools'' philosophy of program development and usage.

The software tools philosophy was an important and integral concept in the initial design and development of Unix (of which Linux and GNU are essentially clones).  Unfortunately, in the modern day press of Internetworking and flashy GUIs, it seems to have fallen by the wayside.  This is a shame, since it provides a powerful mental model for solving many kinds of problems.

Many people carry a Swiss Army knife around in their pants pockets (or purse).  A Swiss Army knife is a handy tool to have: it has several knife blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps a number of other things on it.  For the everyday, small miscellaneous jobs where you need a simple, general purpose tool, it's just the thing.

On the other hand, an experienced carpenter doesn't build a house using a Swiss Army knife.  Instead, he has a toolbox chock full of specialized tools---a saw, a hammer, a screwdriver, a plane, and so on.  And he knows exactly when and where to use each tool; you won't catch him hammering nails with the handle of his screwdriver.

The Unix developers at Bell Labs were all professional programmers and trained computer scientists.  They had found that while a one-size-fits-all program might appeal to a user because there's only one program to use, in practice such programs are

 a. difficult to write,

 b. difficult to maintain and debug, and

 c. difficult to extend to meet new situations.

Instead, they felt that programs should be specialized tools.  In short, each program ``should do one thing well.''  No more and no less.  Such programs are simpler to design, write, and get right---they only do one thing.

Furthermore, they found that with the right machinery for hooking programs together, that the whole was greater than the sum of the parts.  By combining several special purpose programs, you could accomplish a specific task that none of the programs was designed for, and accomplish it much more quickly and easily than if you had to write a special purpose program. We will see some (classic) examples of this further on in the column. (An important additional point was that, if necessary, take a detour and build any software tools you may need first, if you don't already have something appropriate in the toolbox.)

I/O Redirection
===============

Hopefully, you are familiar with the basics of I/O redirection in the shell, in particular the concepts of ``standard input,'' ``standard output,'' and ``standard error''.  Briefly, ``standard input'' is a data source, where data comes from.  A program should not need to either know or care if the data source is a disk file, a keyboard, a magnetic tape, or even a punched card reader.  Similarly, ``standard output'' is a data sink, where data goes to.  The program should neither know nor care where this might be. Programs that only read their standard input, do something to the data, and then send it on, are called filters, by analogy to filters in a water pipeline.

With the Unix shell, it's very easy to set up data pipelines:

     program_to_create_data | filter1 | .... | filterN > final.pretty.data

We start out by creating the raw data; each filter applies some successive transformation to the data, until by the time it comes out of the pipeline, it is in the desired form.

This is fine and good for standard input and standard output.  Where does the standard error come in to play?  Well, think about filter1 in the pipeline above.  What happens if it encounters an error in the data it sees?  If it writes an error message to standard output, it will just disappear down the pipeline into filter2's input, and the user will probably never see it.  So programs need a place where they can send error messages so that the user will notice them.  This is standard error, and it is usually connected to your console or window, even if you have redirected standard output of your program away from your screen.

For filter programs to work together, the format of the data has to be agreed upon.  The most straightforward and easiest format to use is simply lines of text.  Unix data files are generally just streams of bytes, with lines delimited by the ASCII LF (Line Feed) character, conventionally called a ``newline'' in the Unix literature. (This is '\n' if you're a C programmer.)  This is the format used by all the traditional filtering programs.  (Many earlier operating systems had elaborate facilities and special purpose programs for managing binary data.  Unix has always shied away from such things, under the philosophy that it's easiest to simply be able to view and edit your data with a text editor.)

OK, enough introduction. Let's take a look at some of the tools, and then we'll see how to hook them together in interesting ways.   In the following discussion, we will only present those command line options that interest us.  As you should always do, double check your system documentation for the full story.

The who Command
===============

The first program is the who command.  By itself, it generates a list of the users who are currently logged in.  Although I'm writing this on a single-user system, we'll pretend that several people are logged in:

     $ who
     -| arnold   console Jan 22 19:57
     -| miriam   ttyp0   Jan 23 14:19(:0.0)
     -| bill     ttyp1   Jan 21 09:32(:0.0)
     -| arnold   ttyp2   Jan 23 20:48(:0.0)

Here, the $ is the usual shell prompt, at which I typed who. There are three people logged in, and I am logged in twice.  On traditional Unix systems, user names are never more than eight characters long.  This little bit of trivia will be useful later.  The output of who is nice, but the data is not all that exciting.

The cut Command
===============

The next program we'll look at is the cut command.  This program cuts out columns or fields of input data.  For example, we can tell it to print just the login name and full name from the /etc/passwd file.  The /etc/passwd file has seven fields, separated by colons:

     arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash

To get the first and fifth fields, we would use cut like this:

     $ cut -d: -f1,5 /etc/passwd
     -| root:Operator
     ...
     -| arnold:Arnold D. Robbins
     -| miriam:Miriam A. Robbins
     ...

With the -c option, cut will cut out specific characters (i.e., columns) in the input lines.  This command looks like it might be useful for data filtering.

The sort Command
================

Next we'll look at the sort command.  This is one of the most powerful commands on a Unix-style system; one that you will often find yourself using when setting up fancy data plumbing.

The sort command reads and sorts each file named on the command line.  It then merges the sorted data and writes it to standard output.  It will read standard input if no files are given on the command line (thus making it into a filter).  The sort is based on the character collating sequence or based on user-supplied ordering criteria.

The uniq Command
================

Finally (at least for now), we'll look at the uniq program.  When sorting data, you will often end up with duplicate lines, lines that are identical.  Usually, all you need is one instance of each line. This is where uniq comes in. The uniq program reads its standard input, which it expects to be sorted.  It only prints out one copy of each duplicated line.  It does have several options.  Later on, we'll use the -c option, which prints each unique line, preceded by a count of the number of times that line occurred in the input.

Putting the Tools Together
==========================

Now, let's suppose this is a large ISP server system with dozens of users logged in.  The management wants the system administrator to write a program that will generate a sorted list of logged in users.  Furthermore, even if a user is logged in multiple times, his or her name should only show up in the output once.

The administrator could sit down with the system documentation and write a C program that did this. It would take perhaps a couple of hundred lines of code and about two hours to write it, test it, and debug it. However, knowing the software toolbox, the administrator can instead start out by generating just a list of logged on users:

     $ who | cut -c1-8
     -| arnold
     -| miriam
     -| bill
     -| arnold

Next, sort the list:

     $ who | cut -c1-8 | sort
     -| arnold
     -| arnold
     -| bill
     -| miriam

Finally, run the sorted list through uniq, to weed out duplicates:

     $ who | cut -c1-8 | sort | uniq
     -| arnold
     -| bill
     -| miriam

The sort command actually has a -u option that does what uniq does. However, uniq has other uses for which one cannot substitute sort -u.

The administrator puts this pipeline into a shell script, and makes it available for all the users on the system (# is the system administrator, or root, prompt):

     # cat > /usr/local/bin/listusers
     who | cut -c1-8 | sort | uniq
     ^D
     # chmod +x /usr/local/bin/listusers

There are four major points to note here.  First, with just four programs, on one command line, the administrator was able to save about two hours worth of work.  Furthermore, the shell pipeline is just about as efficient as the C program would be, and it is much more efficient in terms of programmer time.  People time is much more expensive than computer time, and in our modern ``there's never enough time to do everything'' society, saving two hours of programmer time is no mean feat.

Second, it is also important to emphasize that with the combination of the tools, it is possible to do a special purpose job never imagined by the authors of the individual programs.

Third, it is also valuable to build up your pipeline in stages, as we did here. This allows you to view the data at each stage in the pipeline, which helps you acquire the confidence that you are indeed using these tools correctly.

Finally, by bundling the pipeline in a shell script, other users can use your command, without having to remember the fancy plumbing you set up for them. In terms of how you run them, shell scripts and compiled programs are indistinguishable.

After the previous warm-up exercise, we'll look at two additional, more complicated pipelines.  For them, we need to introduce two more tools.

The first is the tr command, which stands for ``transliterate.'' The tr command works on a character-by-character basis, changing characters. Normally it is used for things like mapping upper case to lower case:

     $ echo ThIs ExAmPlE HaS MIXED case! | tr '[A-Z]' '[a-z]'
     -| this example has mixed case!

There are several options of interest:

-c
     work on the complement of the listed characters, i.e., operations apply to characters not in the given set

-d
     delete characters in the first set from the output

-s
     squeeze repeated characters in the output into just one character.

We will be using all three options in a moment.

The other command we'll look at is comm.  The comm command takes two sorted input files as input data, and prints out the files' lines in three columns.  The output columns are the data lines unique to the first file, the data lines unique to the second file, and the data lines that are common to both.  The -1, -2, and -3 command line options omit the respective columns. (This is non-intuitive and takes a little getting used to.)  For example:

     $ cat f1
     -| 11111
     -| 22222
     -| 33333
     -| 44444
     $ cat f2
     -| 00000
     -| 22222
     -| 33333
     -| 55555
     $ comm f1 f2
     -|         00000
     -| 11111
     -|                 22222
     -|                 33333
     -| 44444
     -|         55555

The single dash as a filename tells comm to read standard input instead of a regular file.

Now we're ready to build a fancy pipeline.  The first application is a word frequency counter.  This helps an author determine if he or she is over-using certain words.

The first step is to change the case of all the letters in our input file to one case.  ``The'' and ``the'' are the same word when doing counting.

     $ tr '[A-Z]' '[a-z]' < whats.gnu | ...

The next step is to get rid of punctuation.  Quoted words and unquoted words should be treated identically; it's easiest to just get the punctuation out of the way.

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' | ...

The second tr command operates on the complement of the listed characters, which are all the letters, the digits, the underscore, and the blank.  The \012 represents the newline character; it has to be left alone.  (The ASCII tab character should also be included for good measure in a production script.)

At this point, we have data consisting of words separated by blank space. The words only contain alphanumeric characters (and the underscore).  The next step is break the data apart so that we have one word per line. This makes the counting operation much easier, as we will see shortly.

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' |
     > tr -s '[ ]' '\012' | ...

This command turns blanks into newlines.  The -s option squeezes multiple newline characters in the output into just one.  This helps us avoid blank lines. (The > is the shell's ``secondary prompt.'' This is what the shell prints when it notices you haven't finished typing in all of a command.)

We now have data consisting of one word per line, no punctuation, all one case.  We're ready to count each word:

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' |
     > tr -s '[ ]' '\012' | sort | uniq -c | ...

At this point, the data might look something like this:

       60 a
        2 able
        6 about
        1 above
        2 accomplish
        1 acquire
        1 actually
        2 additional

The output is sorted by word, not by count!  What we want is the most frequently used words first.  Fortunately, this is easy to accomplish, with the help of two more sort options:

-n
     do a numeric sort, not a textual one

-r
     reverse the order of the sort

The final pipeline looks like this:

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' |
     > tr -s '[ ]' '\012' | sort | uniq -c | sort -nr
     -|  156 the
     -|   60 a
     -|   58 to
     -|   51 of
     -|   51 and
     ...

Whew!  That's a lot to digest.  Yet, the same principles apply. With six commands, on two lines (really one long one split for convenience), we've created a program that does something interesting and useful, in much less time than we could have written a C program to do the same thing.

A minor modification to the above pipeline can give us a simple spelling checker!  To determine if you've spelled a word correctly, all you have to do is look it up in a dictionary.  If it is not there, then chances are that your spelling is incorrect.  So, we need a dictionary. The conventional location for a dictionary is /usr/dict/words. On my GNU/Linux system, (Redhat Linux 6.1, for the November 2000 revision of this article.) this is a is a sorted, 45,402 word dictionary.

Now, how to compare our file with the dictionary?  As before, we generate a sorted list of words, one per line:

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' |
     > tr -s '[ ]' '\012' | sort -u | ...

Now, all we need is a list of words that are not in the dictionary.  Here is where the comm command comes in.

     $ tr '[A-Z]' '[a-z]' < whats.gnu | tr -cd '[A-Za-z0-9_ \012]' |
     > tr -s '[ ]' '\012' | sort -u |
     > comm -23 - /usr/dict/words

The -2 and -3 options eliminate lines that are only in the dictionary (the second file), and lines that are in both files.  Lines only in the first file (standard input, our stream of words), are words that are not in the dictionary.  These are likely candidates for spelling errors.  This pipeline was the first cut at a production spelling checker on Unix.

There are some other tools that deserve brief mention.

grep
     search files for text that matches a regular expression

wc
     count lines, words, characters

tee
     a T-fitting for data pipes, copies data to files and to standard output

sed
     the stream editor, an advanced tool

awk
     a data manipulation language, another advanced tool

The software tools philosophy also espoused the following bit of advice: ``Let someone else do the hard part.''  This means, take something that gives you most of what you need, and then massage it the rest of the way until it's in the form that you want.

To summarize:

 1. Each program should do one thing well.  No more, no less.

 2. Combining programs with appropriate plumbing leads to results where the whole is greater than the sum of the parts.  It also leads to novel uses of programs that the authors might never have imagined.

 3. Programs should never print extraneous header or trailer data, since these could get sent on down a pipeline. (A point we didn't mention earlier.)

 4. Let someone else do the hard part.

 5. Know your toolbox!  Use each program appropriately. If you don't have an appropriate tool, build one.

As of this writing, all the programs we've discussed are available via anonymous ftp from:

ftp://gnudist.gnu.org/textutils/textutils-1.22.tar.gz.  (There may be more recent versions available now.)

None of what I have presented in this column is new. The Software Tools philosophy was first introduced in the book Software Tools, by Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X). This book showed how to write and use software tools.  It was written in 1976, using a preprocessor for FORTRAN named ratfor (RATional FORtran).  At the time, C was not as ubiquitous as it is now; FORTRAN was.  The last chapter presented a ratfor to FORTRAN processor, written in ratfor. ratfor looks an awful lot like C; if you know C, you won't have any problem following the code.

In 1981, the book was updated and made available as Software Tools in Pascal (Addison-Wesley, ISBN 0-201-10342-7).  The first book is still in print; the second, alas, is not.  Both books are well worth reading if you're a programmer.  They certainly made a major change in how I view programming.

Initially, the programs in both books were available (on 9-track tape) from Addison-Wesley.  Unfortunately, this is no longer the case, although the ratfor versions are available from Brian Kernighan's home page (http://cm.bell-labs.come/who/bwk), and you might be able to find copies of the Pascal versions floating around the Internet.  For a number of years, there was an active Software Tools Users Group, whose members had ported the original ratfor programs to essentially every computer system with a FORTRAN compiler.  The popularity of the group waned in the middle 1980s as Unix began to spread beyond universities.

With the current proliferation of GNU code and other clones of Unix programs, these programs now receive little attention; modern C versions are much more efficient and do more than these programs do.  Nevertheless, as exposition of good programming style, and evangelism for a still-valuable philosophy, these books are unparalleled, and I recommend them highly.

Acknowledgment: I would like to express my gratitude to Brian Kernighan of Bell Labs, the original Software Toolsmith, for reviewing this column.

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