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[coreutils.git] / doc / sh-utils.texi

\input texinfo
@c %**start of header
@setfilename sh-utils.info
@settitle GNU shell utilities
@c %**end of header

@include version.texi

@c Define new indices for file names and options.
@defcodeindex fl
@defcodeindex op

@c Put everything in one index (arbitrarily chosen to be the concept index).
@syncodeindex fl cp
@syncodeindex fn cp
@syncodeindex ky cp
@syncodeindex op cp
@syncodeindex pg cp
@syncodeindex vr cp

@set Francois Fran@,{c}ois

@ifinfo
@format
START-INFO-DIR-ENTRY
* Shell utilities: (sh-utils).          GNU shell utilities.
* basename: (sh-utils)basename invocation.      Strip directory and suffix.
* chroot: (sh-utils)chroot invocation.          Specify the root directory.
* date: (sh-utils)date invocation.              Print/set system date and time.
* dirname: (sh-utils)dirname invocation.        Strip non-directory suffix.
* echo: (sh-utils)echo invocation.              Print a line of text.
* env: (sh-utils)env invocation.                Modify the environment.
* expr: (sh-utils)expr invocation.              Evaluate expressions.
* factor: (sh-utils)factor invocation.          Print prime factors
* false: (sh-utils)false invocation.            Do nothing, unsuccessfully.
* groups: (sh-utils)groups invocation.          Print group names a user is in.
* hostid: (sh-utils)hostid invocation.          Print numeric host identifier.
* hostname: (sh-utils)hostname invocation.      Print or set system name.
* id: (sh-utils)id invocation.                  Print real/effective uid/gid.
* logname: (sh-utils)logname invocation.        Print current login name.
* nice: (sh-utils)nice invocation.              Modify scheduling priority.
* nohup: (sh-utils)nohup invocation.            Immunize to hangups.
* pathchk: (sh-utils)pathchk invocation.        Check file name portability.
* printenv: (sh-utils)printenv invocation.      Print environment variables.
* printf: (sh-utils)printf invocation.          Format and print data.
* pwd: (sh-utils)pwd invocation.                Print working directory.
* seq: (sh-utils)seq invocation.                Print numeric sequences
* sleep: (sh-utils)sleep invocation.            Delay for a specified time.
* stty: (sh-utils)stty invocation.              Print/change terminal settings.
* su: (sh-utils)su invocation.                  Modify user and group id.
* tee: (sh-utils)tee invocation.                Redirect to multiple files.
* test: (sh-utils)test invocation.              File/string tests.
* true: (sh-utils)true invocation.              Do nothing, successfully.
* tty: (sh-utils)tty invocation.                Print terminal name.
* uname: (sh-utils)uname invocation.            Print system information.
* users: (sh-utils)users invocation.            Print current user names.
* who: (sh-utils)who invocation.                Print who is logged in.
* whoami: (sh-utils)whoami invocation.          Print effective user id.
* yes: (sh-utils)yes invocation.                Print a string indefinitely.
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@ifinfo
This file documents the GNU shell utilities.

Copyright (C) 1994, 1995, 1996, 2000 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end ifinfo

@titlepage
@title GNU @code{sh-utils}
@subtitle A set of shell utilities
@subtitle for version @value{VERSION}, @value{UPDATED}
@author David MacKenzie et al.

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1994, 1995, 1996, 2000 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end titlepage


@ifnottex
@node Top
@top GNU shell utilities

@cindex shell utilities
@cindex utilities for shell programming

This manual documents version @value{VERSION} of the GNU shell utilities.

@menu
* Introduction::                Caveats, overview, and authors.
* Common options::              Common options.

* Date input formats::          Specifying date strings.
* Printing text::               echo printf yes
* Conditions::                  false true test expr
* Redirection::                 tee
* File name manipulation::      dirname basename pathchk
* Working context::             pwd stty printenv tty
* User information::            id logname whoami groups users who
* System context::              date uname hostname
* Modified command invocation:: chroot env nice nohup su
* Delaying::                    sleep
* Numeric operations::          factor seq

* Index::                       General index.
@end menu
@end ifnottex


@node Introduction
@chapter Introduction

@cindex introduction

First of all, this manual is incomplete.  The @code{stty} section, in
particular, needs substantial reorganization and additional explanatory
text before it will be up to the standard of other GNU manuals.
Explanatory text in general is lacking; the manual presently assumes you
pretty much know what to do, and just need to be reminded of how.  Thus,
if you are interested, please get involved in improving this manual.
The entire GNU community will benefit.

Some of these programs are useful only when writing shell scripts;
utilities like these are, in fact, the ``language'' of shell scripts (to
a great extent).  Others are occasionally useful interactively.

@cindex POSIX.2
The GNU shell utilities are mostly compatible with the POSIX.2 standard.

@c This paragraph appears in all of fileutils.texi, textutils.texi, and
@c sh-utils.texi too -- so be sure to keep them consistent.
@cindex bugs, reporting
Please report bugs to @samp{bug-sh-utils@@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. @xref{Bugs, , , gcc, GNU CC}.

@cindex history
@cindex MacKenzie, David
@cindex Meyering, Jim
@c Sorry, but the @value trick doesn't work with TeX in indexing
@c commands, and I don't want to fix it right now. --karl.
@cindex Pinard, @value{Francois}
@cindex Berry, Karl
@cindex Stallman, Richard
This manual was originally derived from the Unix man pages in the
distribution, 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.
@value{Francois} Pinard did the initial conversion to Texinfo format.
Karl Berry did the indexing, some reorganization, and editing of the results.
Richard Stallman contributed his usual invaluable insights to the
overall process.


@node Common options
@chapter Common options

@cindex common options

Certain options are available in all 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.)

Many of these programs take arbitrary strings as arguments.  In those
cases, @samp{--help} and @samp{--version} are taken as these options
only if there is one and exactly one command line argument.

@table @samp

@item --help
@opindex --help
@cindex help, online
Print a usage message listing all available options, then exit successfully.

@item --version
@opindex --version
@cindex version number, finding
Print the version number, then exit successfully.

@end table


@include getdate.texi


@node Printing text
@chapter Printing text

@cindex printing text, commands for
@cindex commands for printing text

This section describes commands that display text strings.

@menu
* echo invocation::             Print a line of text.
* printf invocation::           Format and print data.
* yes invocation::              Print a string until interrupted.
@end menu


@node echo invocation
@section @code{echo}: Print a line of text

@pindex echo
@cindex displaying text
@cindex printing text
@cindex text, displaying
@cindex arbitrary text, displaying

@code{echo} writes each given @var{string} to standard output, with a
space between each and a newline after the last one.  Synopsis:

@example
echo [@var{option}]@dots{} [@var{string}]@dots{}
@end example

The program accepts the following options.  Also see @ref{Common options}.

@table @samp
@item -n
@opindex -n
Do not output the trailing newline.

@item -e
@opindex -e
@cindex backslash escapes
Enable interpretation of the following backslash-escaped characters in
each @var{string}:

@table @samp
@item \a
alert (bell)
@item \b
backspace
@item \c
suppress trailing newline
@item \f
form feed
@item \n
new line
@item \r
carriage return
@item \t
horizontal tab
@item \v
vertical tab
@item \\
backslash
@item \@var{nnn}
the character whose ASCII code is @var{nnn} (octal); if @var{nnn} is not
a valid octal number, it is printed literally.
@end table

@end table


@node printf invocation
@section @code{printf}: Format and print data

@pindex printf
@code{printf} does formatted printing of text. Synopsis:

@example
printf @var{format} [@var{argument}]@dots{}
@end example

@code{printf} prints the @var{format} string, interpreting @samp{%}
directives and @samp{\} escapes in the same way as the C @code{printf}
function.  The @var{format} argument is re-used as necessary to convert
all of the given @var{argument}s.

@code{printf} has one additional directive, @samp{%b}, which prints its
argument string with @samp{\} escapes interpreted in the same way as in
the @var{format} string.

@kindex \0ooo
@kindex \0xhhh
@code{printf} interprets @samp{\0ooo} in @var{format} as an octal number
(if @var{ooo} is 0 to 3 octal digits) specifying a character to print,
and @samp{\xhhh} as a hexadecimal number (if @var{hhh} is 1 to 3 hex
digits) specifying a character to print.

@kindex \uhhhh
@kindex \Uhhhhhhhh
@code{printf} interprets two character syntaxes introduced in ISO C 99:
@samp{\u} for 16-bit Unicode characters, specified as 4 hex digits
@var{hhhh}, and @samp{\U} for 32-bit Unicode characters, specified as 8 hex
digits @var{hhhhhhhh}. @code{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 @code{LC_ALL}, @code{LC_CTYPE},
@code{LANG}.

The processing of @samp{\u} and @samp{\U} requires a full-featured
@code{iconv} facility. It is activated on systems with glibc 2.2 (or newer),
or when @code{libiconv} is installed prior to the sh-utils. Otherwise the
use of @samp{\u} and @samp{\U} will give an error message.

@kindex \c
An additional escape, @samp{\c}, causes @code{printf} to produce no
further output.

The only options are a lone @samp{--help} or
@samp{--version}.  @xref{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

@example
$ /usr/local/bin/printf '\u20AC 14.95'
@end example

@noindent
will be output correctly in all locales supporting the Euro symbol
(ISO-8859-15, UTF-8, and others). Similarly, a Chinese string

@example
$ /usr/local/bin/printf '\u4e2d\u6587'
@end example

@noindent
will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc).

Note that in these examples, the full pathname of @code{printf} has been
given, to distinguish it from the GNU @code{bash} builtin function
@code{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:

@smallexample
$ 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
@end smallexample


@node yes invocation
@section @code{yes}: Print a string until interrupted

@pindex yes
@cindex repeated output of a string

@code{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 @samp{y} followed by a newline forever until killed.

The only options are a lone @samp{--help} or @samp{--version}.
@xref{Common options}.


@node Conditions
@chapter Conditions

@cindex conditions
@cindex commands for exit status
@cindex exit status commands

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 @code{if} statements, or as the last command in a
pipeline.

@menu
* false invocation::            Do nothing, unsuccessfully.
* true invocation::             Do nothing, successfully.
* test invocation::             Check file types and compare values.
* expr invocation::             Evaluate expressions.
@end menu


@node false invocation
@section @code{false}: Do nothing, unsuccessfully

@pindex false
@cindex do nothing, unsuccessfully
@cindex failure exit status
@cindex exit status of @code{false}

@code{false} does nothing except return an exit status of 1, meaning
@dfn{failure}.  It can be used as a place holder in shell scripts
where an unsuccessful command is needed.

By default, @code{false} honors the @samp{--help} and @samp{--version}
options.  However, that is contrary to @sc{POSIX}, so when the environment
variable @env{POSIXLY_CORRECT} is set, @code{false} ignores @emph{all}
command line arguments, including @samp{--help} and @samp{--version}.

This version of @code{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.


@node true invocation
@section @code{true}: Do nothing, successfully

@pindex true
@cindex do nothing, successfully
@cindex no-op
@cindex successful exit
@cindex exit status of @code{true}

@code{true} does nothing except return an exit status of 0, meaning
@dfn{success}.  It can be used as a place holder in shell scripts
where a successful command is needed, although the shell built-in
command @code{:} (colon) may do the same thing faster.
In most modern shells, @code{true} is built-in command, so when
you use @samp{true} in a script, you're probably using the built-in
command, not the one documented here.

By default, @code{true} honors the @samp{--help} and @samp{--version}
options.  However, that is contrary to @sc{POSIX}, so when the environment
variable @env{POSIXLY_CORRECT} is set, @code{true} ignores @emph{all}
command line arguments, including @samp{--help} and @samp{--version}.

This version of @code{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.


@node test invocation
@section @code{test}: Check file types and compare values

@pindex test
@cindex check file types
@cindex compare values
@cindex expression evaluation

@code{test} returns a status of 0 (true) or 1 (false) depending on the
evaluation of the conditional expression @var{expr}.  Each part of the
expression must be a separate argument.

@code{test} has file status checks, string operators, and numeric
comparison operators.

@cindex conflicts with shell built-ins
@cindex built-in shell commands, conflicts with
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, @code{test} accepts a lone @samp{--help} or
@samp{--version}.  @xref{Common options}.  A single non-option argument
is also allowed: @code{test} returns true if the argument is not null.

@menu
* File type tests::             -[bcdfhLpSt]
* Access permission tests::     -[gkruwxOG]
* File characteristics tests::  -e -s -nt -ot -ef
* String tests::                -z -n = !=
* Numeric tests::               -eq -ne -lt -le -gt -ge
* Connectives for test::        ! -a -o
@end menu


@node File type tests
@subsection File type tests

@cindex file type tests

These options test for particular types of files.  (Everything's a file,
but not all files are the same!)

@table @samp

@item -b @var{file}
@opindex -b
@cindex block special check
True if @var{file} exists and is a block special device.

@item -c @var{file}
@opindex -c
@cindex character special check
True if @var{file} exists and is a character special device.

@item -d @var{file}
@opindex -d
@cindex directory check
True if @var{file} exists and is a directory.

@item -f @var{file}
@opindex -f
@cindex regular file check
True if @var{file} exists and is a regular file.

@item -h @var{file}
@itemx -L @var{file}
@opindex -L
@opindex -h
@cindex symbolic link check
True if @var{file} exists and is a symbolic link.

@item -p @var{file}
@opindex -p
@cindex named pipe check
True if @var{file} exists and is a named pipe.

@item -S @var{file}
@opindex -S
@cindex socket check
True if @var{file} exists and is a socket.

@item -t [@var{fd}]
@opindex -t
@cindex terminal check
True if @var{fd} is opened on a terminal.  If @var{fd} is omitted, it
defaults to 1 (standard output).

@end table


@node Access permission tests
@subsection Access permission tests

@cindex access permission tests
@cindex permission tests

These options test for particular access permissions.

@table @samp

@item -g @var{file}
@opindex -g
@cindex set-group-id check
True if @var{file} exists and has its set-group-id bit set.

@item -k @var{file}
@opindex -k
@cindex sticky bit check
True if @var{file} has its @dfn{sticky} bit set.

@item -r @var{file}
@opindex -r
@cindex readable file check
True if @var{file} exists and is readable.

@item -u @var{file}
@opindex -u
@cindex set-user-id check
True if @var{file} exists and has its set-user-id bit set.

@item -w @var{file}
@opindex -w
@cindex writable file check
True if @var{file} exists and is writable.

@item -x @var{file}
@opindex -x
@cindex executable file check
True if @var{file} exists and is executable.

@item -O @var{file}
@opindex -O
@cindex owned by effective uid check
True if @var{file} exists and is owned by the current effective user id.

@item -G @var{file}
@opindex -G
@cindex owned by effective gid check
True if @var{file} exists and is owned by the current effective group id.

@end table

@node File characteristics tests
@subsection File characteristics tests

@cindex file characteristics tests

These options test other file characteristics.

@table @samp

@item -e @var{file}
@opindex -e
@cindex existence-of-file check
True if @var{file} exists.

@item -s @var{file}
@opindex -s
@cindex nonempty file check
True if @var{file} exists and has a size greater than zero.

@item @var{file1} -nt @var{file2}
@opindex -nt
@cindex newer-than file check
True if @var{file1} is newer (according to modification date) than
@var{file2}.

@item @var{file1} -ot @var{file2}
@opindex -ot
@cindex older-than file check
True if @var{file1} is older (according to modification date) than
@var{file2}.

@item @var{file1} -ef @var{file2}
@opindex -ef
@cindex same file check
@cindex hard link check
True if @var{file1} and @var{file2} have the same device and inode
numbers, i.e., if they are hard links to each other.

@end table


@node String tests
@subsection String tests

@cindex string tests

These options test string characteristics.  Strings are not quoted for
@code{test}, though you may need to quote them to protect characters
with special meaning to the shell, e.g., spaces.

@table @samp

@item -z @var{string}
@opindex -z
@cindex zero-length string check
True if the length of @var{string} is zero.

@item -n @var{string}
@itemx @var{string}
@opindex -n
@cindex nonzero-length string check
True if the length of @var{string} is nonzero.

@item @var{string1} = @var{string2}
@opindex =
@cindex equal string check
True if the strings are equal.

@item @var{string1} != @var{string2}
@opindex !=
@cindex not-equal string check
True if the strings are not equal.

@end table


@node Numeric tests
@subsection Numeric tests

@cindex numeric tests
@cindex arithmetic tests

Numeric relationals.  The arguments must be entirely numeric (possibly
negative), or the special expression @w{@code{-l @var{string}}}, which
evaluates to the length of @var{string}.

@table @samp

@item @var{arg1} -eq @var{arg2}
@itemx @var{arg1} -ne @var{arg2}
@itemx @var{arg1} -lt @var{arg2}
@itemx @var{arg1} -le @var{arg2}
@itemx @var{arg1} -gt @var{arg2}
@itemx @var{arg1} -ge @var{arg2}
@opindex -eq
@opindex -ne
@opindex -lt
@opindex -le
@opindex -gt
@opindex -ge
These arithmetic binary operators return true if @var{arg1} is equal,
not-equal, less-than, less-than-or-equal, greater-than, or
greater-than-or-equal than @var{arg2}, respectively.

@end table

For example:

@example
test -1 -gt -2 && echo yes
@result{} yes
test -l abc -gt 1 && echo yes
@result{} yes
test 0x100 -eq 1
@error{} test: integer expression expected before -eq
@end example


@node Connectives for test
@subsection Connectives for @code{test}

@cindex logical connectives
@cindex connectives, logical

The usual logical connectives.

@table @samp

@item ! @var{expr}
@opindex !
True if @var{expr} is false.

@item @var{expr1} -a @var{expr2}
@opindex -a
@cindex logical and operator
@cindex and operator
True if both @var{expr1} and @var{expr2} are true.

@item @var{expr1} -o @var{expr2}
@opindex -o
@cindex logical or operator
@cindex or operator
True if either @var{expr1} or @var{expr2} is true.

@end table


@node expr invocation
@section @code{expr}: Evaluate expressions

@pindex expr
@cindex expression evaluation
@cindex evaluation of expressions

@code{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.  @code{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 @code{expr} itself, though you may need to
quote them to protect characters with special meaning to the shell,
e.g., spaces.

@cindex parentheses for grouping
Operators may 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).

@cindex exit status of @code{expr}
Exit status:

@display
0 if the expression is neither null nor 0,
1 if the expression is null or 0,
2 for invalid expressions.
@end display

@menu
* String expressions::          <colon> match substr index length quote
* Numeric expressions::         + - * / %
* Relations for expr::          | & < <= = == != >= >
* Examples of expr::            Examples.
@end menu


@node String expressions
@subsection String expressions

@cindex string expressions
@cindex expressions, string

@code{expr} supports pattern matching and other string operators.  These
have lower precedence than both the numeric and relational operators (in
the next sections).

@table @samp

@item @var{string} : @var{regex}
@cindex pattern matching
@cindex regular expression matching
@cindex matching patterns
Perform pattern matching.  The arguments are converted to strings and the
second is considered to be a (basic, a la GNU @code{grep}) regular
expression, with a @code{^} implicitly prepended.  The first argument is
then matched against this regular expression.

If the match succeeds and @var{regex} uses @samp{\(} and @samp{\)}, the
@code{:} expression returns the part of @var{string} that matched the
subexpression; otherwise, it returns the number of characters matched.

If the match fails, the @code{:} operator returns the null string if
@samp{\(} and @samp{\)} are used in @var{regex}, otherwise 0.

@kindex \( @r{regexp operator}
Only the first @samp{\( @dots{} \)} pair is relevant to the return
value; additional pairs are meaningful only for grouping the regular
expression operators.

@kindex \+ @r{regexp operator}
@kindex \? @r{regexp operator}
@kindex \| @r{regexp operator}
In the regular expression, @code{\+}, @code{\?}, and @code{\|} are
operators which respectively match one or more, zero or one, or separate
alternatives.  SunOS and other @code{expr}'s treat these as regular
characters.  (POSIX allows either behavior.)
@xref{Top, , Regular Expression Library, regex, Regex}, for details of
regular expression syntax.  Some examples are in @ref{Examples of expr}.

@item match @var{string} @var{regex}
@findex match
An alternative way to do pattern matching.  This is the same as
@w{@samp{@var{string} : @var{regex}}}.

@item substr @var{string} @var{position} @var{length}
@findex substr
Returns the substring of @var{string} beginning at @var{position}
with length at most @var{length}.  If either @var{position} or
@var{length} is negative, zero, or non-numeric, returns the null string.

@item index @var{string} @var{charset}
@findex index
Returns the first position in @var{string} where the first character in
@var{charset} was found.  If no character in @var{charset} is found in
@var{string}, return 0.

@item length @var{string}
@findex length
Returns the length of @var{string}.

@item quote @var{token}
@findex quote
Interpret @var{token} as a string, even if it is a keyword like @var{match}
or an operator like @code{/}.
This makes it possible to test @code{expr length quote "$x"} or
@code{expr quote "$x" : '.*/\(.\)'} and have it do the right thing even if
the value of @var{$x} happens to be (for example) @code{/} or @code{index}.
This operator is a GNU extension.  It is disabled when
the environment variable @env{POSIXLY_CORRECT} is set.

@end table

To make @code{expr} interpret keywords as strings, you must use the
@code{quote} operator.


@node Numeric expressions
@subsection Numeric expressions

@cindex numeric expressions
@cindex expressions, numeric

@code{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.

@table @samp

@item + -
@kindex +
@kindex -
@cindex addition
@cindex subtraction
Addition and subtraction.  Both arguments are converted to numbers;
an error occurs if this cannot be done.

@item * / %
@kindex *
@kindex /
@kindex %
@cindex multiplication
@cindex division
@cindex remainder
Multiplication, division, remainder.  Both arguments are converted to
numbers; an error occurs if this cannot be done.

@end table


@node Relations for expr
@subsection Relations for @code{expr}

@cindex connectives, logical
@cindex logical connectives
@cindex relations, numeric or string

@code{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.

@table @samp

@item |
@kindex |
@cindex logical or operator
@cindex or operator
Returns its first argument if that is neither null nor 0, otherwise its
second argument.

@item &
@kindex &
@cindex logical and operator
@cindex and operator
Return its first argument if neither argument is null or 0, otherwise
0.

@item < <= = == != >= >
@kindex <
@kindex <=
@kindex =
@kindex ==
@kindex >
@kindex >=
@cindex comparison operators
Compare the arguments and return 1 if the relation is true, 0 otherwise.
@code{==} is a synonym for @code{=}.  @code{expr} first tries to convert
both arguments to numbers and do a numeric comparison; if either
conversion fails, it does a lexicographic comparison.

@end table


@node Examples of expr
@subsection Examples of using @code{expr}

@cindex examples of @code{expr}
Here are a few examples, including quoting for shell metacharacters.

To add 1 to the shell variable @code{foo}, in Bourne-compatible shells:
@example
foo=`expr $foo + 1`
@end example

To print the non-directory part of the file name stored in
@code{$fname}, which need not contain a @code{/}.
@example
expr $fname : '.*/\(^.*\)' '^|' $fname
@end example

An example showing that @code{\+} is an operator:
@example
expr aaa : 'a\+'
@result{} 3
@end example

@example
expr abc : 'a\(.\)c'
@result{} b
expr index abcdef cz
@result{} 3
expr index index a
@error{} expr: syntax error
expr index quote index a
@result{} 0
@end example


@node Redirection
@chapter Redirection

@cindex redirection
@cindex commands for redirection

Unix shells commonly provide several forms of @dfn{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.

@menu
* tee invocation::              Redirect output to multiple files.
@end menu


@node tee invocation
@section @code{tee}: Redirect output to multiple files

@pindex tee
@cindex pipe fitting
@cindex destinations, multiple output
@cindex read from stdin and write to stdout and files

The @code{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:

@example
tee [@var{option}]@dots{} [@var{file}]@dots{}
@end example

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 @code{-a} option is used.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp
@item -a
@itemx --append
@opindex -a
@opindex --append
Append standard input to the given files rather than overwriting
them.

@item -i
@itemx --ignore-interrupts
@opindex -i
@opindex --ignore-interrupts
Ignore interrupt signals.

@end table


@node File name manipulation
@chapter File name manipulation

@cindex file name manipulation
@cindex manipulation of file names
@cindex commands for file name manipulation

This section describes commands that manipulate file names.

@menu
* basename invocation::         Strip directory and suffix from a file name.
* dirname invocation::          Strip non-directory suffix from a file name.
* pathchk invocation::          Check file name portability.
@end menu


@node basename invocation
@section @code{basename}: Strip directory and suffix from a file name

@pindex basename
@cindex strip directory and suffix from file names
@cindex directory, stripping from file names
@cindex suffix, stripping from file names
@cindex file names, stripping directory and suffix
@cindex leading directory components, stripping

@code{basename} removes any leading directory components from
@var{name}.  Synopsis:

@example
basename @var{name} [@var{suffix}]
@end example

If @var{suffix} is specified and is identical to the end of @var{name},
it is removed from @var{name} as well.  @code{basename} prints the
result on standard output.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node dirname invocation
@section @code{dirname}: Strip non-directory suffix from a file name

@pindex dirname
@cindex directory components, printing
@cindex stripping non-directory suffix
@cindex non-directory suffix, stripping

@code{dirname} prints all but the final slash-delimited component of
a string (presumably a filename).  Synopsis:

@example
dirname @var{name}
@end example

If @var{name} is a single component, @code{dirname} prints @samp{.}
(meaning the current directory).

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node pathchk invocation
@section @code{pathchk}: Check file name portability

@pindex pathchk
@cindex file names, checking validity and portability
@cindex valid file names, checking for
@cindex portable file names, checking for

@code{pathchk} checks portability of filenames.  Synopsis:

@example
pathchk [@var{option}]@dots{} @var{name}@dots{}
@end example

For each @var{name}, @code{pathchk} prints a message if any of
these conditions is true:
@enumerate
@item
one of the existing directories in @var{name} does not have search
(execute) permission,
@item
the length of @var{name} is larger than its filesystem's maximum
file name length,
@item
the length of one component of @var{name}, corresponding to an
existing directory name, is larger than its filesystem's maximum
length for a file name component.
@end enumerate

The program accepts the following option.  Also see @ref{Common options}.

@table @samp

@item -p
@itemx --portability
@opindex -p
@opindex --portability
Instead of performing length checks on the underlying filesystem,
test the length of each file name and its components against the
POSIX.1 minimum limits for portability.  Also check that the file
name contains no characters not in the portable file name character set.

@end table

@cindex exit status of @code{pathchk}
Exit status:

@display
0 if all specified file names passed all of the tests,
1 otherwise.
@end display


@node Working context
@chapter Working context

@cindex working context
@cindex commands for printing the 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.

@menu
* pwd invocation::              Print working directory.
* stty invocation::             Print or change terminal characteristics.
* printenv invocation::         Print environment variables.
* tty invocation::              Print file name of terminal on standard input.
@end menu


@node pwd invocation
@section @code{pwd}: Print working directory

@pindex pwd
@cindex print name of current directory
@cindex current working directory, printing
@cindex working directory, printing

@cindex symbolic links and @code{pwd}
@code{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.

@cindex conflicts with shell built-ins
@cindex built-in shell commands, conflicts with
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 @samp{--help} or
@samp{--version}.  @xref{Common options}.


@node stty invocation
@section @code{stty}: Print or change terminal characteristics

@pindex stty
@cindex change or print terminal settings
@cindex terminal settings
@cindex line settings of terminal

@code{stty} prints or changes terminal characteristics, such as baud rate.
Synopses:

@example
stty [@var{option}] [@var{setting}]@dots{}
stty [@var{option}]
@end example

If given no line settings, @code{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 @samp{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
@samp{--file} option.

@code{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 @ref{Common options}.

@table @samp
@item -a
@itemx --all
@opindex -a
@opindex --all
Print all current settings in human-readable form.  This option may not
be used in combination with any line settings.

@item -F @var{device}
@itemx --file=@var{device}
@opindex -F
@opindex --file
Set the line opened by the filename specified in @var{device} instead of
the tty line connected to standard input.  This option is necessary
because opening a POSIX tty requires use of the @code{O_NONDELAY} flag to
prevent a POSIX tty from blocking until the carrier detect line is high if
the @code{clocal} flag is not set.  Hence, it is not always possible
to allow the shell to open the device in the traditional manner.

@item -g
@itemx --save
@opindex -g
@opindex --save
@cindex machine-readable @code{stty} output
Print all current settings in a form that can be used as an argument to
another @code{stty} command to restore the current settings.  This option
may not be used in combination with any line settings.

@end table

Many settings can be turned off by preceding them with a @samp{-}.
Such arguments are marked below with ``May be negated'' in their
description.  The descriptions themselves refer to the positive
case, that is, when @emph{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.

@menu
* Control::                     Control settings
* Input::                       Input settings
* Output::                      Output settings
* Local::                       Local settings
* Combination::                 Combination settings
* Characters::                  Special characters
* Special::                     Special settings
@end menu


@node Control
@subsection Control settings

@cindex control settings
Control settings:

@table @samp
@item parenb
@opindex parenb
@cindex two-way parity
Generate parity bit in output and expect parity bit in input.
May be negated.

@item parodd
@opindex parodd
@cindex odd parity
@cindex even parity
Set odd parity (even if negated).  May be negated.

@item cs5
@itemx cs6
@itemx cs7
@itemx cs8
@opindex cs@var{n}
@cindex character size
@cindex eight-bit characters
Set character size to 5, 6, 7, or 8 bits.

@item hup
@itemx hupcl
@opindex hup[cl]
Send a hangup signal when the last process closes the tty.  May be
negated.

@item cstopb
@opindex cstopb
@cindex stop bits
Use two stop bits per character (one if negated).  May be negated.

@item cread
@opindex cread
Allow input to be received.  May be negated.

@item clocal
@opindex clocal
@cindex modem control
Disable modem control signals.  May be negated.

@item crtscts
@opindex crtscts
@cindex hardware flow control
@cindex flow control, hardware
@cindex RTS/CTS flow control
Enable RTS/CTS flow control.  Non-POSIX.  May be negated.
@end table


@node Input
@subsection Input settings

@cindex input settings

@table @samp
@item ignbrk
@opindex ignbrk
@cindex breaks, ignoring
Ignore break characters.  May be negated.

@item brkint
@opindex brkint
@cindex breaks, cause interrupts
Make breaks cause an interrupt signal.  May be negated.

@item ignpar
@opindex ignpar
@cindex parity, ignoring
Ignore characters with parity errors.  May be negated.

@item parmrk
@opindex parmrk
@cindex parity errors, marking
Mark parity errors (with a 255-0-character sequence).  May be negated.

@item inpck
@opindex inpck
Enable input parity checking.  May be negated.

@item istrip
@opindex istrip
@cindex eight-bit input
Clear high (8th) bit of input characters.  May be negated.

@item inlcr
@opindex inlcr
@cindex newline, translating to return
Translate newline to carriage return.  May be negated.

@item igncr
@opindex igncr
@cindex return, ignoring
Ignore carriage return.  May be negated.

@item icrnl
@opindex icrnl
@cindex return, translating to newline
Translate carriage return to newline.  May be negated.

@item ixon
@opindex ixon
@kindex C-s/C-q flow control
@cindex XON/XOFF flow control
Enable XON/XOFF flow control (that is, @kbd{CTRL-S}/@kbd{CTRL-Q}).  May
be negated.

@item ixoff
@itemx tandem
@opindex ixoff
@opindex tandem
@cindex software flow control
@cindex flow control, software
Enable sending of @code{stop} character when the system input buffer
is almost full, and @code{start} character when it becomes almost
empty again.  May be negated.

@item iuclc
@opindex iuclc
@cindex uppercase, translating to lowercase
Translate uppercase characters to lowercase.  Non-POSIX.  May be
negated.

@item ixany
@opindex ixany
Allow any character to restart output (only the start character
if negated).  Non-POSIX.  May be negated.

@item imaxbel
@opindex imaxbel
@cindex beeping at input buffer full
Enable beeping and not flushing input buffer if a character arrives
when the input buffer is full.  Non-POSIX.  May be negated.
@end table


@node Output
@subsection Output settings

@cindex output settings
These arguments specify output-related operations.

@table @samp
@item opost
@opindex opost
Postprocess output.  May be negated.

@item olcuc
@opindex olcuc
@cindex lowercase, translating to output
Translate lowercase characters to uppercase.  Non-POSIX.  May be
negated.

@item ocrnl
@opindex ocrnl
@cindex return, translating to newline
Translate carriage return to newline.  Non-POSIX.  May be negated.

@item onlcr
@opindex onlcr
@cindex newline, translating to crlf
Translate newline to carriage return-newline.  Non-POSIX.  May be
negated.

@item onocr
@opindex onocr
Do not print carriage returns in the first column.  Non-POSIX.
May be negated.

@item onlret
@opindex onlret
Newline performs a carriage return.  Non-POSIX.  May be negated.

@item ofill
@opindex ofill
@cindex pad instead of timing for delaying
Use fill (padding) characters instead of timing for delays.  Non-POSIX.
May be negated.

@item ofdel
@opindex ofdel
@cindex pad character
Use delete characters for fill instead of null characters.  Non-POSIX.
May be negated.

@item nl1
@itemx nl0
@opindex nl@var{n}
Newline delay style.  Non-POSIX.

@item cr3
@itemx cr2
@itemx cr1
@itemx cr0
@opindex cr@var{n}
Carriage return delay style.  Non-POSIX.

@item tab3
@itemx tab2
@itemx tab1
@itemx tab0
@opindex tab@var{n}
Horizontal tab delay style.  Non-POSIX.

@item bs1
@itemx bs0
@opindex bs@var{n}
Backspace delay style.  Non-POSIX.

@item vt1
@itemx vt0
@opindex vt@var{n}
Vertical tab delay style.  Non-POSIX.

@item ff1
@itemx ff0
@opindex ff@var{n}
Form feed delay style.  Non-POSIX.
@end table


@node Local
@subsection Local settings

@cindex local settings

@table @samp
@item isig
@opindex isig
Enable @code{interrupt}, @code{quit}, and @code{suspend} special
characters.  May be negated.

@item icanon
@opindex icanon
Enable @code{erase}, @code{kill}, @code{werase}, and @code{rprnt}
special characters.  May be negated.

@item iexten
@opindex iexten
Enable non-POSIX special characters.  May be negated.

@item echo
@opindex echo
Echo input characters.  May be negated.

@item echoe
@itemx crterase
@opindex echoe
@opindex crterase
Echo @code{erase} characters as backspace-space-backspace.  May be
negated.

@item echok
@opindex echok
@cindex newline echoing after @code{kill}
Echo a newline after a @code{kill} character.  May be negated.

@item echonl
@opindex echonl
@cindex newline, echoing
Echo newline even if not echoing other characters.  May be negated.

@item noflsh
@opindex noflsh
@cindex flushing, disabling
Disable flushing after @code{interrupt} and @code{quit} special
characters.  May be negated.

@item xcase
@opindex xcase
@cindex case translation
Enable input and output of uppercase characters by preceding their
lowercase equivalents with @samp{\}, when @code{icanon} is set.
Non-POSIX.  May be negated.

@item tostop
@opindex tostop
@cindex background jobs, stopping at terminal write
Stop background jobs that try to write to the terminal.  Non-POSIX.
May be negated.

@item echoprt
@itemx prterase
@opindex echoprt
@opindex prterase
Echo erased characters backward, between @samp{\} and @samp{/}.
Non-POSIX.  May be negated.

@item echoctl
@itemx ctlecho
@opindex echoctl
@opindex ctlecho
@cindex control characters, using @samp{^@var{c}}
@cindex hat notation for control characters
Echo control characters in hat notation (@samp{^@var{c}}) instead
of literally.  Non-POSIX.  May be negated.

@item echoke
@itemx crtkill
@opindex echoke
@opindex crtkill
Echo the @code{kill} special character by erasing each character on
the line as indicated by the @code{echoprt} and @code{echoe} settings,
instead of by the @code{echoctl} and @code{echok} settings.  Non-POSIX.
May be negated.
@end table


@node Combination
@subsection Combination settings

@cindex combination settings
Combination settings:

@table @samp
@item evenp
@opindex evenp
@itemx parity
@opindex parity
Same as @code{parenb -parodd cs7}.  May be negated.  If negated, same
as @code{-parenb cs8}.

@item oddp
@opindex oddp
Same as @code{parenb parodd cs7}.  May be negated.  If negated, same
as @code{-parenb cs8}.

@item nl
@opindex nl
Same as @code{-icrnl -onlcr}.  May be negated.  If negated, same as
@code{icrnl -inlcr -igncr onlcr -ocrnl -onlret}.

@item ek
@opindex ek
Reset the @code{erase} and @code{kill} special characters to their default
values.

@item sane
@opindex sane
Same as:
@c This is too long to write inline.
@example
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
@end example
@noindent and also sets all special characters to their default values.

@item cooked
@opindex cooked
Same as @code{brkint ignpar istrip icrnl ixon opost isig icanon}, plus
sets the @code{eof} and @code{eol} characters to their default values
if they are the same as the @code{min} and @code{time} characters.
May be negated.  If negated, same as @code{raw}.

@item raw
@opindex raw
Same as:
@example
-ignbrk -brkint -ignpar -parmrk -inpck -istrip
-inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany
-imaxbel -opost -isig -icanon -xcase min 1 time 0
@end example
@noindent May be negated.  If negated, same as @code{cooked}.

@item cbreak
@opindex cbreak
Same as @code{-icanon}.  May be negated.  If negated, same as
@code{icanon}.

@item pass8
@opindex pass8
@cindex eight-bit characters
Same as @code{-parenb -istrip cs8}.  May be negated.  If negated,
same as @code{parenb istrip cs7}.

@item litout
@opindex litout
Same as @code{-parenb -istrip -opost cs8}.  May be negated.
If negated, same as @code{parenb istrip opost cs7}.

@item decctlq
@opindex decctlq
Same as @code{-ixany}.  Non-POSIX.  May be negated.

@item tabs
@opindex tabs
Same as @code{tab0}.  Non-POSIX.  May be negated.  If negated, same
as @code{tab3}.

@item lcase
@itemx LCASE
@opindex lcase
@opindex LCASE
Same as @code{xcase iuclc olcuc}.  Non-POSIX.  May be negated.

@item crt
@opindex crt
Same as @code{echoe echoctl echoke}.

@item dec
@opindex dec
Same as @code{echoe echoctl echoke -ixany intr ^C erase ^? kill C-u}.
@end table


@node Characters
@subsection Special characters

@cindex special characters
@cindex characters, special

The special characters' default values vary from system to system.
They are set with the syntax @samp{name value}, where the names are
listed below and the value can be given either literally, in hat
notation (@samp{^@var{c}}), or as an integer which may start with
@samp{0x} to indicate hexadecimal, @samp{0} to indicate octal, or
any other digit to indicate decimal.

@cindex disabling special characters
@kindex u@r{, and disabling special characters}
For GNU stty, giving a value of @code{^-} or @code{undef} disables that
special character.  (This is incompatible with Ultrix @code{stty},
which uses  a value of @samp{u} to disable a special character.  GNU
@code{stty} treats a value @samp{u} like any other, namely to set that
special character to @key{U}.)

@table @samp

@item intr
@opindex intr
Send an interrupt signal.

@item quit
@opindex quit
Send a quit signal.

@item erase
@opindex erase
Erase the last character typed.

@item kill
@opindex kill
Erase the current line.

@item eof
@opindex eof
Send an end of file (terminate the input).

@item eol
@opindex eol
End the line.

@item eol2
@opindex eol2
Alternate character to end the line.  Non-POSIX.

@item swtch
@opindex swtch
Switch to a different shell layer.  Non-POSIX.

@item start
@opindex start
Restart the output after stopping it.

@item stop
@opindex stop
Stop the output.

@item susp
@opindex susp
Send a terminal stop signal.

@item dsusp
@opindex dsusp
Send a terminal stop signal after flushing the input.  Non-POSIX.

@item rprnt
@opindex rprnt
Redraw the current line.  Non-POSIX.

@item werase
@opindex werase
Erase the last word typed.  Non-POSIX.

@item lnext
@opindex lnext
Enter the next character typed literally, even if it is a special
character.  Non-POSIX.
@end table


@node Special
@subsection Special settings

@cindex special settings

@table @samp
@item min @var{n}
@opindex min
Set the minimum number of characters that will satisfy a read until
the time value has expired, when @code{-icanon} is set.

@item time @var{n}
@opindex time
Set the number of tenths of a second before reads time out if the minimum
number of characters have not been read, when @code{-icanon} is set.

@item ispeed @var{n}
@opindex ispeed
Set the input speed to @var{n}.

@item ospeed @var{n}
@opindex ospeed
Set the output speed to @var{n}.

@item rows @var{n}
@opindex rows
Tell the tty kernel driver that the terminal has @var{n} rows.  Non-POSIX.

@item cols @var{n}
@itemx columns @var{n}
@opindex cols
@opindex columns
Tell the kernel that the terminal has @var{n} columns.  Non-POSIX.

@item size
@opindex size
@vindex LINES
@vindex COLUMNS
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 @env{LINES} and @env{COLUMNS}
instead; however, GNU @code{stty} does not know anything about them.)
Non-POSIX.

@item line @var{n}
@opindex line
Use line discipline @var{n}.  Non-POSIX.

@item speed
@opindex speed
Print the terminal speed.

@item @var{n}
@cindex baud rate, setting
@c FIXME: Is this still true that the baud rate can't be set
@c higher than 38400?
Set the input and output speeds to @var{n}.  @var{n} can be one
of: 0 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600
19200 38400 @code{exta} @code{extb}.  @code{exta} is the same as
19200; @code{extb} is the same as 38400.  0 hangs up the line if
@code{-clocal} is set.
@end table


@node printenv invocation
@section @code{printenv}: Print all or some environment variables

@pindex printenv
@cindex printing all or some environment variables
@cindex environment variables, printing

@code{printenv} prints environment variable values.  Synopsis:

@example
printenv [@var{option}] [@var{variable}]@dots{}
@end example

If no @var{variable}s are specified, @code{printenv} prints the value of
every environment variable.  Otherwise, it prints the value of each
@var{variable} that is set, and nothing for those that are not set.

The only options are a lone @samp{--help} or @samp{--version}.
@xref{Common options}.

@cindex exit status of @code{printenv}
Exit status:

@display
0 if all variables specified were found
1 if at least one specified variable was not found
2 if a write error occurred
@end display


@node tty invocation
@section @code{tty}: Print file name of terminal on standard input

@pindex tty
@cindex print terminal file name
@cindex terminal file name, printing

@code{tty} prints the file name of the terminal connected to its standard
input.  It prints @samp{not a tty} if standard input is not a terminal.
Synopsis:

@example
tty [@var{option}]@dots{}
@end example

The program accepts the following option.  Also see @ref{Common options}.

@table @samp

@item -s
@itemx --silent
@itemx --quiet
@opindex -s
@opindex --silent
@opindex --quiet
Print nothing; only return an exit status.

@end table

@cindex exit status of @code{tty}
Exit status:

@display
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
@end display


@node User information
@chapter User information

@cindex user information, commands for
@cindex commands for printing user information

This section describes commands that print user-related information:
logins, groups, and so forth.

@menu
* id invocation::               Print real and effective uid and gid.
* logname invocation::          Print current login name.
* whoami invocation::           Print effective user id.
* groups invocation::           Print group names a user is in.
* users invocation::            Print login names of users currently logged in.
* who invocation::              Print who is currently logged in.
@end menu


@node id invocation
@section @code{id}: Print real and effective uid and gid

@pindex id
@cindex real uid and gid, printing
@cindex effective uid and gid, printing
@cindex printing real and effective uid and gid

@code{id} prints information about the given user, or the process
running it if no user is specified.  Synopsis:

@example
id [@var{option}]@dots{} [@var{username}]
@end example

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 @code{id} to print only part of the above information.
Also see @ref{Common options}.

@table @samp
@item -g
@itemx --group
@opindex -g
@opindex --group
Print only the group id.

@item -G
@itemx --groups
@opindex -G
@opindex --groups
Print only the supplementary groups.

@item -n
@itemx --name
@opindex -n
@opindex --name
Print the user or group name instead of the ID number.  Requires
@code{-u}, @code{-g}, or @code{-G}.

@item -r
@itemx --real
@opindex -r
@opindex --real
Print the real, instead of effective, user or group id.  Requires
@code{-u}, @code{-g}, or @code{-G}.

@item -u
@itemx --user
@opindex -u
@opindex --user
Print only the user id.

@end table


@node logname invocation
@section @code{logname}: Print current login name

@pindex logname
@cindex printing user's login name
@cindex login name, printing
@cindex user name, printing

@flindex /etc/utmp
@flindex utmp

@code{logname} prints the calling user's name, as found in the file
@file{/etc/utmp}, and exits with a status of 0.  If there is no
@file{/etc/utmp} entry for the calling process, @code{logname} prints
an error message and exits with a status of 1.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node whoami invocation
@section @code{whoami}: Print effective user id

@pindex whoami
@cindex effective UID, printing
@cindex printing the effective UID

@code{whoami} prints the user name associated with the current
effective user id.  It is equivalent to the command @samp{id -un}.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node groups invocation
@section @code{groups}: Print group names a user is in

@pindex groups
@cindex printing groups a user is in
@cindex supplementary groups, printing

@code{groups} prints the names of the primary and any supplementary
groups for each given @var{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:

@example
groups [@var{username}]@dots{}
@end example

The group lists are equivalent to the output of the command @samp{id -Gn}.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node users invocation
@section @code{users}: Print login names of users currently logged in

@pindex users
@cindex printing current usernames
@cindex usernames, printing current

@cindex login sessions, printing users with
@code{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:

@example
users [@var{file}]
@end example

@flindex /etc/utmp
@flindex /etc/wtmp
With no @var{file} argument, @code{users} extracts its information from
the file @file{/etc/utmp}.  If a file argument is given, @code{users}
uses that file instead.  A common choice is @file{/etc/wtmp}.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node who invocation
@section @code{who}: Print who is currently logged in

@pindex who
@cindex printing current user information
@cindex information, about current users

@code{who} prints information about users who are currently logged on.
Synopsis:

@example
@code{who} [@var{option}] [@var{file}] [am i]
@end example

@cindex terminal lines, currently used
@cindex login time
@cindex remote hostname
If given no non-option arguments, @code{who} prints the following
information for each user currently logged on: login name, terminal
line, login time, and remote hostname or X display.

@flindex /etc/utmp
@flindex /etc/wtmp
If given one non-option argument, @code{who} uses that instead of
@file{/etc/utmp} as the name of the file containing the record of
users logged on.  @file{/etc/wtmp} is commonly given as an argument
to @code{who} to look at who has previously logged on.

@opindex am i
@opindex who am i
If given two non-option arguments, @code{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 @samp{am
i}, as in @samp{who am i}.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp
@item -m
@opindex -m
Same as @samp{who am i}.

@item -q
@itemx --count
@opindex -q
@opindex --count
Print only the login names and the number of users logged on.
Overrides all other options.

@item -s
@opindex -s
Ignored; for compatibility with other versions of @code{who}.

@item -i
@itemx -u
@itemx --idle
@opindex -i
@opindex -u
@opindex --idle
@cindex idle time
After the login time, print the number of hours and minutes that the
user has been idle.  @samp{.} means the user was active in last minute.
@samp{old} means the user was idle for more than 24 hours.

@item -l
@itemx --lookup
@opindex -l
@opindex --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.

@item -H
@itemx --heading
@opindex -H
@opindex --heading
Print a line of column headings.

@item -w
@itemx -T
@itemx --mesg
@itemx --message
@itemx --writable
@opindex -w
@opindex -T
@opindex --mesg
@opindex --message
@opindex --writable
@cindex message status
@pindex write@r{, allowed}
After each login name print a character indicating the user's message status:

@display
@samp{+} allowing @code{write} messages
@samp{-} disallowing @code{write} messages
@samp{?} cannot find terminal device
@end display

@end table


@node System context
@chapter System context

@cindex system context
@cindex context, system
@cindex commands for system context

This section describes commands that print or change system-wide
information.

@menu
* date invocation::             Print or set system date and time.
* uname invocation::            Print system information.
* hostname invocation::         Print or set system name.
* hostid invocation::           Print numeric host identifier.
@end menu


@node date invocation
@section @code{date}: Print or set system date and time

@pindex date
@cindex time, printing or setting
@cindex printing the current time

Synopses:

@example
date [@var{option}]@dots{} [+@var{format}]
date [-u|--utc|--universal] @c this avoids a newline in the output
[ MMDDhhmm[[CC]YY][.ss] ]
@end example

Invoking @code{date} with no @var{format} argument is equivalent to invoking
@samp{date '+%a %b %e %H:%M:%S %Z %Y'}.

@findex strftime @r{and @code{date}}
@cindex time formats
@cindex formatting times
If given an argument that starts with a @samp{+}, @code{date} prints the
current time and date (or the time and date specified by the
@code{--date} option, see below) in the format defined by that argument,
which is the same as in the @code{strftime} function.  Except for
directives, which start with @samp{%}, characters in the format string
are printed unchanged.  The directives are described below.

@menu
* Time directives::             %[HIklMprsSTXzZ]
* Date directives::             %[aAbBcdDhjmUwWxyY]
* Literal directives::          %[%nt]
* Padding::                     Pad with zeroes, spaces (%_), or nothing (%-).
* Setting the time::            Changing the system clock.
* Options for date::            Instead of the current time.
* Examples of date::            Examples.
@end menu

@node Time directives
@subsection Time directives

@cindex time directives
@cindex directives, time

@code{date} directives related to times.

@table @samp
@item %H
hour (00@dots{}23)
@item %I
hour (01@dots{}12)
@item %k
hour ( 0@dots{}23)
@item %l
hour ( 1@dots{}12)
@item %M
minute (00@dots{}59)
@item %p
locale's AM or PM
@item %r
time, 12-hour (hh:mm:ss [AP]M)
@item %s
@cindex epoch, seconds since
@cindex seconds since the epoch
@cindex beginning of time
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 @samp{--date} option.
@item %S
second (00@dots{}60)
@item %T
time, 24-hour (hh:mm:ss)
@item %X
locale's time representation (%H:%M:%S)
@item %z
RFC-822 style numeric time zone (e.g., -0600 or +0100), or nothing if no
time zone is determinable.  This value reflects the @emph{current} time
zone.  It isn't changed by the @samp{--date} option.
@item %Z
time zone (e.g., EDT), or nothing if no timezone is
determinable.
Note that this value reflects the @emph{current} time zone.
It isn't changed by the @samp{--date} option.
@end table


@node Date directives
@subsection Date directives

@cindex date directives
@cindex directives, date

@code{date} directives related to dates.

@table @samp
@item %a
locale's abbreviated weekday name (Sun@dots{}Sat)
@item %A
locale's full weekday name, variable length (Sunday@dots{}Saturday)
@item %b
locale's abbreviated month name (Jan@dots{}Dec)
@item %B
locale's full month name, variable length (January@dots{}December)
@item %c
locale's date and time (Sat Nov 04 12:02:33 EST 1989)
@item %C
century (year divided by 100 and truncated to an integer) (00@dots{}99)
@item %d
day of month (01@dots{}31)
@item %D
date (mm/dd/yy)
@item %h
same as %b
@item %j
day of year (001@dots{}366)
@item %m
month (01@dots{}12)
@item %U
week number of year with Sunday as first day of week (00@dots{}53).
Days in a new year preceding the first Sunday are in week zero.
@item %V
week number of year with Monday as first day of the week as a decimal
(01@dots{}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: 1988
standard.)
@item %w
day of week (0@dots{}6) with 0 corresponding to Sunday
@item %W
week number of year with Monday as first day of week (00@dots{}53).
Days in a new year preceding the first Monday are in week zero.
@item %x
locale's date representation (mm/dd/yy)
@item %y
last two digits of year (00@dots{}99)
@item %Y
year (1970@dots{}.)
@end table


@node Literal directives
@subsection Literal directives

@cindex literal directives
@cindex directives, literal

@code{date} directives that produce literal strings.

@table @samp
@item %%
a literal %
@item %n
a newline
@item %t
a horizontal tab
@end table


@node Padding
@subsection Padding

@cindex numeric field padding
@cindex padding of numeric fields
@cindex fields, padding numeric

By default, @code{date} pads numeric fields with zeroes, so that, for
example, numeric months are always output as two digits. GNU @code{date}
recognizes the following numeric modifiers between the @samp{%} and the
directive.

@table @samp
@item -
(hyphen) do not pad the field; useful if the output is intended for
human consumption.
@item _
(underscore) pad the field with spaces; useful if you need a fixed
number of characters in the output, but zeroes are too distracting.
@end table

@noindent
These are GNU extensions.

Here is an example illustrating the differences:

@example
date +%d/%m -d "Feb 1"
@result{} 01/02
date +%-d/%-m -d "Feb 1"
@result{} 1/2
date +%_d/%_m -d "Feb 1"
@result{}  1/ 2
@end example


@node Setting the time
@subsection Setting the time

@cindex setting the time
@cindex time setting
@cindex appropriate privileges

If given an argument that does not start with @samp{+}, @code{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 @samp{--date} and @samp{--set} options may not be
used with such an argument.  The @samp{--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:

@table @samp
@item MM
month
@item DD
day within month
@item hh
hour
@item mm
minute
@item CC
first two digits of year (optional)
@item YY
last two digits of year (optional)
@item ss
second (optional)
@end table

The @samp{--set} option also sets the system clock; see the next section.


@node Options for date
@subsection Options for @code{date}

@cindex @code{date} options
@cindex options for @code{date}

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -d @var{datestr}
@itemx --date=@var{datestr}
@opindex -d
@opindex --date
@cindex parsing date strings
@cindex date strings, parsing
@cindex arbitrary date strings, parsing
@opindex yesterday
@opindex tomorrow
@opindex next @var{day}
@opindex last @var{day}
Display the time and date specified in @var{datestr} instead of the
current time and date.  @var{datestr} can be in almost any common
format.  It can contain month names, timezones, @samp{am} and @samp{pm},
@samp{yesterday}, @samp{ago}, @samp{next}, etc.  @xref{Date input formats}.

@item -f @var{datefile}
@itemx --file=@var{datefile}
@opindex -f
@opindex --file
Parse each line in @var{datefile} as with @samp{-d} and display the
resulting time and date.  If @var{datefile} is @samp{-}, use standard
input.  This is useful when you have many dates to process, because the
system overhead of starting up the @code{date} executable many times can
be considerable.

@item -I[@var{timespec}]
@itemx --iso-8601[=@var{timespec}]
@opindex -I[@var{timespec}]
@opindex --iso-8601[=@var{timespec}]
Display the date using the ISO 8601 format, @samp{%Y-%m-%d}.

The optional argument @var{timespec} specifies the number of additional
terms of the time to include.  It can be one of the following:
@table @samp
@item auto
The default behavior: print just the date.

@item hours
Append the hour of the day to the date.

@item minutes
Append the hours and minutes.

@item seconds
Append the hours, minutes, and seconds.
@end table

If showing any time terms, then include the time zone using the format
@samp{%z}.

@item -R
@itemx --rfc-822
@opindex -R
@opindex --rfc-822
Display the time and date using the RFC-822-conforming
format, @samp{%a, %_d %b %Y %H:%M:%S %z}.

@item -r @var{file}
@itemx --reference=@var{file}
@opindex -r
@opindex --reference
Display the time and date reference according to the last modification
time of @var{file}, instead of the current time and date.

@item -s @var{datestr}
@itemx --set=@var{datestr}
@opindex -s
@opindex --set
Set the time and date to @var{datestr}.  See @samp{-d} above.

@item -u
@itemx --utc
@itemx --universal
@opindex -u
@opindex --utc
@opindex --universal
@cindex Coordinated Universal Time
@cindex UTC
@cindex Greenwich Mean Time
@cindex GMT
Use Coordinated Universal Time (@sc{utc}) by operating as if the
@env{TZ} environment variable was set to the string @samp{UTC0}.
Normally, @command{date} operates in the time zone indicated by
@env{TZ}, or the system default if @env{TZ} is not set.  Coordinated
Universal Time is often called ``Greenwich Mean Time'' (@sc{gmt}) for
historical reasons.
@end table


@node Examples of date
@subsection Examples of @code{date}

@cindex examples of @code{date}

Here are a few examples.  Also see the documentation for the @samp{-d}
option in the previous section.

@itemize @bullet

@item
To print the date of the day before yesterday:

@example
date --date='2 days ago'
@end example

@item
To print the date of the day three months and one day hence:
@example
date --date='3 months 1 day'
@end example

@item
To print the day of year of Christmas in the current year:
@example
date --date='25 Dec' +%j
@end example

@item
To print the current full month name and the day of the month:
@example
date '+%B %d'
@end example

But this may not be what you want because for the first nine days of
the month, the @samp{%d} expands to a zero-padded two-digit field,
for example @samp{date -d 1may '+%B %d'} will print @samp{May 01}.

@item
To print a date without the leading zero for one-digit days
of the month, you can use the (GNU extension) @code{-} modifier to suppress
the padding altogether.
@example
date -d 1may '+%B %-d'
@end example

@item
To print the current date and time in the format required by many
non-GNU versions of @code{date} when setting the system clock:
@example
date +%m%d%H%M%Y.%S
@end example

@item
To set the system clock forward by two minutes:
@example
date --set='+2 minutes'
@end example

@item
To print the date in the format specified by RFC-822,
use @samp{date --rfc}.  I just did and saw this:

@example
Mon, 25 Mar 1996 23:34:17 -0600
@end example

@item
To convert a date string to the number of seconds since the epoch
(which is 1970-01-01 00:00:00 UTC), use the @samp{--date} option with
the @samp{%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:

@example
date --date='1970-01-01 00:02:00 +0000' +%s
120
@end example

If you do not specify time zone information in the date string,
@command{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:

@example
# local time zone used
date --date='1970-01-01 00:02:00' +%s
18120
@end example

@item
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 @samp{946684800} and casually note ``Oh, that's the first second
of the year 2000 in Greenwich, England.''

@example
date --date='2000-01-01 UTC' +%s
946684800
@end example

To convert such an unwieldy number of seconds back to
a more readable form, use a command like this:

@smallexample
# local time zone used
date -d '1970-01-01 UTC 946684800 seconds' +"%Y-%m-%d %T %z"
1999-12-31 19:00:00 -0500
@end smallexample

@end itemize


@node uname invocation
@section @code{uname}: Print system information

@pindex uname
@cindex print system information
@cindex system information, printing

@code{uname} prints information about the machine and operating system
it is run on.  If no options are given, @code{uname} acts as if the
@code{-s} option were given. Synopsis:

@example
uname [@var{option}]@dots{}
@end example

If multiple options or @code{-a} are given, the selected information is
printed in this order:

@example
@var{sysname} @var{nodename} @var{release} @var{osversion} @var{machine}
@end example

The @var{osversion}, at least, may well be multiple words.  For example:

@example
uname -a
@result{} Linux hayley 1.0.4 #3 Thu May 12 18:06:34 1994 i486
@end example


The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -a
@itemx --all
@opindex -a
@opindex --all
Print all of the below information.

@item -m
@itemx --machine
@opindex -m
@opindex --machine
@cindex machine type
@cindex hardware type
Print the machine (hardware) type.

@item -n
@itemx --nodename
@opindex -n
@opindex --nodename
@cindex hostname
@cindex node name
@cindex network node name
Print the machine's network node hostname.

@item -p
@itemx --processor
@opindex -p
@opindex --processor
@cindex host processor type
Print the machine's processor type

@item -r
@itemx --release
@opindex -r
@opindex --release
@cindex operating system release
@cindex release of operating system
Print the operating system release.

@item -s
@itemx --sysname
@opindex -s
@opindex --sysname
@cindex operating system name
@cindex name of operating system
Print the operating system name.

@item -v
@opindex -v
@cindex operating system version
@cindex version of operating system
Print the operating system version.

@end table

@node hostname invocation
@section @code{hostname}: Print or set system name

@pindex hostname
@cindex setting the hostname
@cindex printing the hostname
@cindex system name, printing
@cindex appropriate privileges

With no arguments, @code{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:

@example
hostname [@var{name}]
@end example

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node hostid invocation
@section @code{hostid}: Print numeric host identifier.

@pindex hostid
@cindex printing the host identifier

@code{hostid} prints the numeric identifier of the current host
in hexadecimal.  This command accepts no arguments.
The only options are @samp{--help} and @samp{--version}.
@xref{Common options}.

For example, here's what it prints on one system I use:

@example
$ hostid
1bac013d
@end example

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.


@node Modified command invocation
@chapter Modified command invocation

@cindex modified command invocation
@cindex invocation of commands, modified
@cindex commands for invoking other commands

This section describes commands that run other commands in some context
different than the current one: a modified environment, as a different
user, etc.

@menu
* chroot invocation::           Modify the root directory.
* env invocation::              Modify environment variables.
* nice invocation::             Modify scheduling priority.
* nohup invocation::            Immunize to hangups.
* su invocation::               Modify user and group id.
@end menu


@node chroot invocation
@section @code{chroot}: Run a command with a different root directory

@pindex chroot
@cindex running a program in a specified root directory
@cindex root directory, running a program in a specified

@code{chroot} runs a command with a specified root directory.
On many systems, only the super-user can do this.
Synopses:

@example
chroot @var{newroot} [@var{command} [@var{args}]@dots{}]
chroot @var{option}
@end example

Ordinarily, filenames are looked up starting at the root of the
directory structure, i.e., @file{/}.  @code{chroot} changes the root to
the directory @var{newroot} (which must exist) and then runs
@var{command} with optional @var{args}.  If @var{command} is not
specified, the default is the value of the @env{SHELL} environment
variable or @code{/bin/sh} if not set, invoked with the @samp{-i} option.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.

Here are a few tips to help avoid common problems in using chroot.
To start with a simple example, make @var{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:

@example
$ chroot /tmp/empty /ls -Rl /
@end example

Then you'll see output like this:

@example
/:
total 1023
-rwxr-xr-x    1 0        0         1041745 Aug 16 11:17 ls
@end example

If you want to use a dynamically linked executable, say @code{bash},
then first run @samp{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.


@node env invocation
@section @code{env}: Run a command in a modified environment

@pindex env
@cindex environment, running a program in a modified
@cindex modified environment, running a program in a
@cindex running a program in a modified environment

@code{env} runs a command with a modified environment.  Synopses:

@example
env [@var{option}]@dots{} [@var{name}=@var{value}]@dots{} @c
[@var{command} [@var{args}]@dots{}]
env
@end example

Arguments of the form @samp{@var{variable}=@var{value}} set
the environment variable @var{variable} to value @var{value}.
@var{value} may be empty (@samp{@var{variable}=}).  Setting a variable
to an empty value is different from unsetting it.

@vindex PATH
The first remaining argument specifies the program name to invoke; it is
searched for according to the @env{PATH} environment variable.  Any
remaining arguments are passed as arguments to that program.

@cindex environment, printing

If no command name is specified following the environment
specifications, the resulting environment is printed.  This is like
specifying a command name of @code{printenv}.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp

@item -u @var{name}
@itemx --unset=@var{name}
@opindex -u
@opindex -unset
Remove variable @var{name} from the environment, if it was in the
environment.

@item -
@itemx -i
@itemx --ignore-environment
@opindex -
@opindex -i
@opindex --ignore-environment
Start with an empty environment, ignoring the inherited environment.

@end table


@node nice invocation
@section @code{nice}: Run a command with modified scheduling priority

@pindex nice
@cindex modifying scheduling priority
@cindex scheduling priority, modifying
@cindex priority, modifying
@cindex appropriate privileges

@code{nice} prints or modifies the scheduling priority of a job.
Synopsis:

@example
nice [@var{option}]@dots{} [@var{command} [@var{arg}]@dots{}]
@end example

If no arguments are given, @code{nice} prints the current scheduling
priority, which it inherited.  Otherwise, @code{nice} runs the given
@var{command} with its scheduling priority adjusted.  If no
@var{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 @code{nice} over the range
of -20 (the highest priority) to 19 (the lowest).

@cindex conflicts with shell built-ins
@cindex built-in shell commands, conflicts with
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 @ref{Common options}.

@table @samp
@item -n @var{adjustment}
@itemx -@var{adjustment}
@itemx --adjustment=@var{adjustment}
@opindex -n
@opindex --adjustment
@opindex -@var{adjustment}
Add @var{adjustment} instead of 10 to the command's priority.
@end table


@node nohup invocation
@section @code{nohup}: Run a command immune to hangups

@pindex nohup
@cindex hangups, immunity to
@cindex immunity to hangups
@cindex logging out and continuing to run

@flindex nohup.out
@code{nohup} runs the given @var{command} with hangup signals ignored,
so that the command can continue running in the background after you log
out. Synopsis:

@example
nohup @var{command} [@var{arg}]@dots{}
@end example

@flindex nohup.out
@code{nohup} increases the scheduling priority of @var{command} by 5, so
it has a slightly smaller chance to run.  If standard output is a terminal,
it and standard error are redirected so that they are appended to the
file @file{nohup.out}; if that cannot be written to, they are appended
to the file @file{$HOME/nohup.out}.  If that cannot be written to, the
command is not run.

If @code{nohup} creates either @file{nohup.out} or
@file{$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.

@code{nohup} does not automatically put the command it runs in the
background; you must do that explicitly, by ending the command line
with an @samp{&}.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node su invocation
@section @code{su}: Run a command with substitute user and group id

@pindex su
@cindex substitute user and group ids
@cindex user id, switching
@cindex super-user, becoming
@cindex root, becoming

@code{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 @var{user}. Synopsis:

@example
su [@var{option}]@dots{} [@var{user} [@var{arg}]@dots{}]
@end example

@cindex passwd entry, and @code{su} shell
@flindex /bin/sh
@flindex /etc/passwd
If no @var{user} is given, the default is @code{root}, the super-user.
The shell to use is taken from @var{user}'s @code{passwd} entry, or
@file{/bin/sh} if none is specified there.  If @var{user} has a
password, @code{su} prompts for the password unless run by a user with
effective user id of zero (the super-user).

@vindex HOME
@vindex SHELL
@vindex USER
@vindex LOGNAME
@cindex login shell
By default, @code{su} does not change the current directory.
It sets the environment variables @env{HOME} and @env{SHELL}
from the password entry for @var{user}, and if @var{user} is not
the super-user, sets @env{USER} and @env{LOGNAME} to @var{user}.
By default, the shell is not a login shell.

Any additional @var{arg}s are passed as additional arguments to the
shell.

@cindex @samp{-su}
GNU @code{su} does not treat @file{/bin/sh} or any other shells specially
(e.g., by setting @code{argv[0]} to @samp{-su}, passing @code{-c} only
to certain shells, etc.).

@findex syslog
@code{su} can optionally be compiled to use @code{syslog} to report
failed, and optionally successful, @code{su} attempts.  (If the system
supports @code{syslog}.)  However, GNU @code{su} does not check if the
user is a member of the @code{wheel} group; see below.

The program accepts the following options.  Also see @ref{Common options}.

@table @samp
@item -c @var{command}
@itemx --command=@var{command}
@opindex -c
@opindex --command
Pass @var{command}, a single command line to run, to the shell with
a @code{-c} option instead of starting an interactive shell.

@item -f
@itemx --fast
@opindex -f
@opindex --fast
@flindex .cshrc
@cindex file name pattern expansion, disabled
@cindex globbing, disabled
Pass the @code{-f} option to the shell.  This probably only makes sense
if the shell run is @code{csh} or @code{tcsh}, for which the @code{-f}
option prevents reading the startup file (@file{.cshrc}).  With
Bourne-like shells, the @code{-f} option disables file name pattern
expansion (globbing), which is not likely to be useful.

@item -
@itemx -l
@itemx --login
@opindex -
@opindex -l
@opindex --login
@c other variables already indexed above
@vindex TERM
@vindex PATH
@cindex login shell, creating
Make the shell a login shell.  This means the following.  Unset all
environment variables except @env{TERM}, @env{HOME}, and @env{SHELL}
(which are set as described above), and @env{USER} and @env{LOGNAME}
(which are set, even for the super-user, as described above), and set
@env{PATH} to a compiled-in default value.  Change to @var{user}'s home
directory.  Prepend @samp{-} to the shell's name, intended to make it
read its login startup file(s).

@item -m
@itemx -p
@itemx --preserve-environment
@opindex -m
@opindex -p
@opindex --preserve-environment
@cindex environment, preserving
@flindex /etc/shells
@cindex restricted shell
Do not change the environment variables @env{HOME}, @env{USER},
@env{LOGNAME}, or @env{SHELL}.  Run the shell given in the environment
variable @env{SHELL} instead of the shell from @var{user}'s passwd
entry, unless the user running @code{su} is not the superuser and
@var{user}'s shell is restricted.  A @dfn{restricted shell} is one that
is not listed in the file @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 @code{--login} and @code{--shell}.

@item -s @var{shell}
@itemx --shell=@var{shell}
@opindex -s
@opindex --shell
Run @var{shell} instead of the shell from @var{user}'s passwd entry,
unless the user running @code{su} is not the superuser and @var{user}'s
shell is restricted (see @samp{-m} just above).

@end table

@cindex wheel group, not supported
@cindex group wheel, not supported
@cindex fascism
@heading Why GNU @code{su} does not support the @samp{wheel} group

(This section is by Richard Stallman.)

@cindex Twenex
@cindex MIT AI lab
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
@code{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.


@node Delaying
@chapter Delaying

@cindex delaying commands
@cindex commands for delaying

@c Perhaps @code{wait} or other commands should be described here also?

@menu
* sleep invocation::            Delay for a specified time.
@end menu


@node sleep invocation
@section @code{sleep}: Delay for a specified time

@pindex sleep
@cindex delay for a specified time

@code{sleep} pauses for an amount of time specified by the sum of
the values of the command line arguments.
Synopsis:

@example
sleep @var{number}[smhd]@dots{}
@end example

@cindex time units
Each argument is a number followed by an optional unit; the default
is seconds.  The units are:

@table @samp
@item s
seconds
@item m
minutes
@item h
hours
@item d
days
@end table

Historical implementations of @code{sleep} have required that
@var{number} be an integer.  However, GNU @code{sleep} accepts
arbitrary floating point numbers.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.


@node Numeric operations
@chapter Numeric operations

@cindex numeric operations
These programs do numerically-related operations.

@menu
* factor invocation::              Show factors of numbers.
* seq invocation::                 Print sequences of numbers.
@end menu


@node factor invocation
@section @code{factor}: Print prime factors

@pindex factor
@cindex prime factors

@code{factor} prints prime factors.  Synopses:

@example
factor [@var{number}]@dots{}
factor @var{option}
@end example

If no @var{number} is specified on the command line, @code{factor} reads
numbers from standard input, delimited by newlines, tabs, or spaces.

The only options are @samp{--help} and @samp{--version}.  @xref{Common
options}.

The algorithm it uses is not very sophisticated, so for some inputs
@code{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.

@example
$ p=`echo '4294967279 * 4294967291'|bc`
$ factor $p
18446743979220271189: 4294967279 4294967291
@end example

In contrast, @code{factor} factors the largest 64-bit number in just
over a tenth of a second:

@example
$ factor `echo '2^64-1'|bc`
18446744073709551615: 3 5 17 257 641 65537 6700417
@end example

@node seq invocation
@section @code{seq}: Print numeric sequences

@pindex seq
@cindex numeric sequences
@cindex sequence of numbers

@code{seq} prints a sequence of numbers to standard output.  Synopses:

@example
seq [@var{option}]@dots{} [@var{first} [@var{increment}]] @var{last}@dots{}
@end example

@code{seq} prints the numbers from @var{first} to @var{last} by
@var{increment}.  By default, @var{first} and @var{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 @ref{Common options}.

@table @samp
@item -f @var{format}
@itemx --format=@var{format}
@opindex -f @var{format}
@opindex --format=@var{format}
@cindex formatting of numbers in @code{seq}
Print all numbers using @var{format}; default @samp{%g}.
@var{format} must contain exactly one of the floating point
output formats @samp{%e}, @samp{%f}, or @samp{%g}.

@item -s @var{string}
@itemx --separator=@var{string}
@cindex separator for numbers in @code{seq}
Separate numbers with @var{string}; default is a newline.
The output always terminates with a newline.

@item -w
@itemx --equal-width
Print all numbers with the same width, by padding with leading zeroes.
(To have other kinds of padding, use @samp{--format}).

@end table

If you want to use @code{seq} to print sequences of large integer values,
don't use the default @samp{%g} format since it can result in
loss of precision:

@example
$ seq 1000000 1000001
1e+06
1e+06
@end example

Instead, you can use the format, @samp{%1.f},
to print large decimal numbers with no exponent and no decimal point.

@example
$ seq --format=%1.f 1000000 1000001
1000000
1000001
@end example

If you want hexadecimal output, you can use @code{printf}
to perform the conversion:

@example
$ printf %x'\n' `seq -f %1.f 1048575 1024 1050623`
fffff
1003ff
1007ff
@end example

For very long lists of numbers, use xargs to avoid
system limitations on the length of an argument list:

@example
$ seq -f %1.f 1000000 | xargs printf %x'\n' |tail -3
f423e
f423f
f4240
@end example

To generate octal output, use the printf @code{%o} format instead
of @code{%x}.  Note however that using printf works only for numbers
smaller than @code{2^32}:

@example
$ printf "%x\n" `seq -f %1.f 4294967295 4294967296`
ffffffff
bash: printf: 4294967296: Numerical result out of range
@end example

On most systems, seq can produce whole-number output for values up to
@code{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 @code{bc} and setting its output radix variable, @var{obase},
to @samp{16} in this case to produce hexadecimal output.

@example
$ (echo obase=16; seq -f %1.f 4294967295 4294967296)|bc
FFFFFFFF
100000000
@end example

Be careful when using @code{seq} with a fractional @var{increment},
otherwise you may see surprising results.  Most people would expect to
see @code{0.3} printed as the last number in this example:

@example
$ seq -s' ' 0 .1 .3
0 0.1 0.2
@end example

But that doesn't happen on most systems because @code{seq} is
implemented using binary floating point arithmetic (via the C
@code{double} type) -- which means some decimal numbers like @code{.1}
cannot be represented exactly.  That in turn means some nonintuitive
conditions like @code{.1 * 3 > .3} will end up being true.

To work around that in the above example, use a slightly larger number as
the @var{last} value:

@example
$ seq -s' ' 0 .1 .31
0 0.1 0.2 0.3
@end example

In general, when using an @var{increment} with a fractional part, where
(@var{last} - @var{first}) / @var{increment} is (mathematically) a whole
number, specify a slightly larger (or smaller, if @var{increment} is negative)
value for @var{last} to ensure that @var{last} is the final value printed
by seq.

@node Index
@unnumbered Index

@printindex cp

@contents
@bye

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