BASH2(1) BASH2(1)
NAME
bash2 - GNU Bourne-Again SHell
SYNOPSIS
bash2 [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-1999 by the Free Software Foun
dation, Inc.
DESCRIPTION
Bash is an sh-compatible command language interpreter that
executes commands read from the standard input or from a
file. Bash also incorporates useful features from the
Korn and C shells (ksh and csh).
Bash is intended to be a conformant implementation of the
IEEE POSIX Shell and Tools specification (IEEE Working
Group 1003.2).
OPTIONS
In addition to the single-character shell options docu
mented in the description of the set builtin command, bash
interprets the following options when it is invoked:
-c string If the -c option is present, then commands are
read from string. If there are arguments after
the string, they are assigned to the positional
parameters, starting with $0.
-r If the -r option is present, the shell becomes
restricted (see RESTRICTED SHELL below).
-i If the -i option is present, the shell is inter
active.
-s If the -s option is present, or if no arguments
remain after option processing, then commands
are read from the standard input. This option
allows the positional parameters to be set when
invoking an interactive shell.
-D A list of all double-quoted strings preceded by
$ is printed on the standard ouput. These are
the strings that are subject to language trans
lation when the current locale is not C or
POSIX. This implies the -n option; no commands
will be executed.
-- A -- signals the end of options and disables
further option processing. Any arguments after
the -- are treated as filenames and arguments.
An argument of - is equivalent to --.
Bash also interprets a number of multi-character options.
These options must appear on the command line before the
single-character options in order for them to be recog
nized.
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--dump-po-strings
Equivalent to -D, but the output is in the GNU get
text po (portable object) file format.
--dump-strings
Equivalent to -D.
--help Display a usage message on standard output and exit
successfully.
--login
Make bash act as if it had been invoked as a login
shell (see INVOCATION below).
--noediting
Do not use the GNU readline library to read command
lines if interactive.
--noprofile
Do not read either the system-wide startup file
/etc/profile or any of the personal initialization
files ~/.bash_profile, ~/.bash_login, or ~/.pro
file. By default, bash reads these files when it
is invoked as a login shell (see INVOCATION below).
--norc Do not read and execute the personal initialization
file ~/.bashrc if the shell is interactive. This
option is on by default if the shell is invoked as
sh.
--posix
Change the behavior of bash where the default oper
ation differs from the POSIX 1003.2 standard to
match the standard.
--rcfile file
Execute commands from file instead of the standard
personal initialization file ~/.bashrc if the shell
is interactive (see INVOCATION below).
--restricted
The shell becomes restricted (see RESTRICTED SHELL
below).
--rpm-requires
Produce the list of files that are required for the
shell script to run. This implies '-n' and is sub
ject to the same limitations as compile time error
checking checking; Backticks, [] tests, and evals
are not parsed so some dependencies may be missed.
--verbose Equivalent to -v.
--version
Show version information for this instance of bash
on the standard output and exit successfully.
ARGUMENTS
If arguments remain after option processing, and neither
the -c nor the -s option has been supplied, the first
argument is assumed to be the name of a file containing
shell commands. If bash is invoked in this fashion, $0 is
set to the name of the file, and the positional parameters
are set to the remaining arguments. Bash reads and exe
cutes commands from this file, then exits. Bash's exit
status is the exit status of the last command executed in
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the script. If no commands are executed, the exit status
is 0.
INVOCATION
A login shell is one whose first character of argument
zero is a -, or one started with the --login option.
An interactive shell is one whose standard input and out
put are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. PS1 is set
and $- includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
The following paragraphs describe how bash executes its
startup files. If any of the files exist but cannot be
read, bash reports an error. Tildes are expanded in file
names as described below under Tilde Expansion in the
EXPANSION section.
When bash is invoked as an interactive login shell, or as
a non-interactive shell with the --login option, it first
reads and executes commands from the file /etc/profile, if
that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and ~/.profile, in that
order, and reads and executes commands from the first one
that exists and is readable. The --noprofile option may
be used when the shell is started to inhibit this behav
ior.
When a login shell exits, bash reads and executes commands
from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is
started, bash reads and executes commands from ~/.bashrc,
if that file exists. This may be inhibited by using the
--norc option. The --rcfile file option will force bash
to read and execute commands from file instead of
~/.bashrc.
When bash is started non-interactively, to run a shell
script, for example, it looks for the variable BASH_ENV in
the environment, expands its value if it appears there,
and uses the expanded value as the name of a file to read
and execute. Bash behaves as if the following command
were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search
for the file name.
If bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely
as possible, while conforming to the POSIX standard as
well. When invoked as an interactive login shell, or a
non-interactive shell with the --login option, it first
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attempts to read and execute commands from /etc/profile
and ~/.profile, in that order. The --noprofile option may
be used to inhibit this behavior. When invoked as an
interactive shell with the name sh, bash looks for the
variable ENV, expands its value if it is defined, and uses
the expanded value as the name of a file to read and exe
cute. Since a shell invoked as sh does not attempt to
read and execute commands from any other startup files,
the --rcfile option has no effect. A non-interactive
shell invoked with the name sh does not attempt to read
any other startup files. When invoked as sh, bash enters
posix mode after the startup files are read.
When bash is started in posix mode, as with the --posix
command line option, it follows the POSIX standard for
startup files. In this mode, interactive shells expand
the ENV variable and commands are read and executed from
the file whose name is the expanded value. No other
startup files are read.
Bash attempts to determine when it is being run by the
remote shell daemon, usually rshd. If bash determines it
is being run by rshd, it reads and executes commands from
~/.bashrc, if that file exists and is readable. It will
not do this if invoked as sh. The --norc option may be
used to inhibit this behavior, and the --rcfile option may
be used to force another file to be read, but rshd does
not generally invoke the shell with those options or allow
them to be specified.
If the shell is started with the effective user (group) id
not equal to the real user (group) id, and the -p option
is not supplied, no startup files are read, shell func
tions are not inherited from the environment, the SHEL
LOPTS variable, if it appears in the environment, is
ignored, and the effective user id is set to the real user
id. If the -p option is supplied at invocation, the
startup behavior is the same, but the effective user id is
not reset.
DEFINITIONS
The following definitions are used throughout the rest of
this document.
blank A space or tab.
word A sequence of characters considered as a single
unit by the shell. Also known as a token.
name A word consisting only of alphanumeric characters
and underscores, and beginning with an alphabetic
character or an underscore. Also referred to as an
identifier.
metacharacter
A character that, when unquoted, separates words.
One of the following:
| & ; ( ) < > space tab
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control operator
A token that performs a control function. It is
one of the following symbols:
|| & && ; ;; ( ) | <newline>
RESERVED WORDS
Reserved words are words that have a special meaning to
the shell. The following words are recognized as reserved
when unquoted and either the first word of a simple com
mand (see SHELL GRAMMAR below) or the third word of a case
or for command:
! case do done elif else esac fi for function if in
select then until while { } time [[ ]]
SHELL GRAMMAR
Simple Commands
A simple command is a sequence of optional variable
assignments followed by blank-separated words and redirec
tions, and terminated by a control operator. The first
word specifies the command to be executed. The remaining
words are passed as arguments to the invoked command.
The return value of a simple command is its exit status,
or 128+n if the command is terminated by signal n.
Pipelines
A pipeline is a sequence of one or more commands separated
by the character |. The format for a pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected to the stan
dard input of command2. This connection is performed
before any redirections specified by the command (see
REDIRECTION below).
If the reserved word ! precedes a pipeline, the exit sta
tus of that pipeline is the logical NOT of the exit status
of the last command. Otherwise, the status of the
pipeline is the exit status of the last command. The
shell waits for all commands in the pipeline to terminate
before returning a value.
If the time reserved word precedes a pipeline, the elapsed
as well as user and system time consumed by its execution
are reported when the pipeline terminates. The -p option
changes the output format to that specified by POSIX. The
TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed;
see the description of TIMEFORMAT under Shell Variables
below.
Each command in a pipeline is executed as a separate
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process (i.e., in a subshell).
Lists
A list is a sequence of one or more pipelines separated by
one of the operators ;, &, &&, or ||, and optionally ter
minated by one of ;, &, or <newline>.
Of these list operators, && and || have equal precedence,
followed by ; and &, which have equal precedence.
If a command is terminated by the control operator &, the
shell executes the command in the background in a sub
shell. The shell does not wait for the command to finish,
and the return status is 0. Commands separated by a ; are
executed sequentially; the shell waits for each command to
terminate in turn. The return status is the exit status
of the last command executed.
The control operators && and || denote AND lists and OR
lists, respectively. An AND list has the form
command && command2
command2 is executed if, and only if, command returns an
exit status of zero.
An OR list has the form
command || command2
command2 is executed if and only if command returns a non-
zero exit status. The return status of AND and OR lists
is the exit status of the last command executed in the
list.
Compound Commands
A compound command is one of the following:
(list) list is executed in a subshell. Variable assign
ments and builtin commands that affect the shell's
environment do not remain in effect after the com
mand completes. The return status is the exit sta
tus of list.
{ list; }
list is simply executed in the current shell envi
ronment. list must be terminated with a newline or
semicolon. This is known as a group command. The
return status is the exit status of list.
((expression))
The expression is evaluated according to the rules
described below under ARITHMETIC EVALUATION. If
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the value of the expression is non-zero, the return
status is 0; otherwise the return status is 1.
This is exactly equivalent to let "expression".
[[ expression ]]
Return a status of 0 or 1 depending on the evalua
tion of the conditional expression expression.
Expressions are composed of the primaries described
below under CONDITIONAL EXPRESSIONS. Word split
ting and pathname expansion are not performed on
the words between the [[ and ]]; tilde expansion,
parameter and variable expansion, arithmetic expan
sion, command substitution, process substitution,
and quote removal are performed.
When the == and != operators are used, the string
to the right of the operator is considered a pat
tern and matched according to the rules described
below under Pattern Matching. The return value is
0 if the string matches or does not match the pat
tern, respectively, and 1 otherwise. Any part of
the pattern may be quoted to force it to be matched
as a string.
Expressions may be combined using the following
operators, listed in decreasing order of prece
dence:
( expression )
Returns the value of expression. This may
be used to override the normal precedence of
operators.
! expression
True if expression is false.
expression1 && expression2
True if both expression1 and expression2 are
true.
expression1 || expression2
True if either expression1 or expression2 is
true.
The && and || operators do not execute expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
for name [ in word ] ; do list ; done
The list of words following in is expanded, gener
ating a list of items. The variable name is set to
each element of this list in turn, and list is exe
cuted each time. If the in word is omitted, the
for command executes list once for each positional
parameter that is set (see PARAMETERS below). The
return status is the exit status of the last com
mand that executes. If the expansion of the items
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following in results in an empty list, no commands
are executed, and the return status is 0.
select name [ in word ] ; do list ; done
The list of words following in is expanded, gener
ating a list of items. The set of expanded words
is printed on the standard error, each preceded by
a number. If the in word is omitted, the posi
tional parameters are printed (see PARAMETERS
below). The PS3 prompt is then displayed and a
line read from the standard input. If the line
consists of a number corresponding to one of the
displayed words, then the value of name is set to
that word. If the line is empty, the words and
prompt are displayed again. If EOF is read, the
command completes. Any other value read causes
name to be set to null. The line read is saved in
the variable REPLY. The list is executed after
each selection until a break or return command is
executed. The exit status of select is the exit
status of the last command executed in list, or
zero if no commands were executed.
case word in [ ( pattern [ | pattern ] ... ) list ;; ] ...
esac
A case command first expands word, and tries to
match it against each pattern in turn, using the
same matching rules as for pathname expansion (see
Pathname Expansion below). When a match is found,
the corresponding list is executed. After the
first match, no subsequent matches are attempted.
The exit status is zero if no pattern matches.
Otherwise, it is the exit status of the last com
mand executed in list.
if list; then list; [ elif list; then list; ] ... [ else
list; ] fi
The if list is executed. If its exit status is
zero, the then list is executed. Otherwise, each
elif list is executed in turn, and if its exit sta
tus is zero, the corresponding then list is exe
cuted and the command completes. Otherwise, the
else list is executed, if present. The exit status
is the exit status of the last command executed, or
zero if no condition tested true.
while list; do list; done
until list; do list; done
The while command continuously executes the do list
as long as the last command in list returns an exit
status of zero. The until command is identical to
the while command, except that the test is negated;
the do list is executed as long as the last command
in list returns a non-zero exit status. The exit
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status of the while and until commands is the exit
status of the last do list command executed, or
zero if none was executed.
[ function ] name () { list; }
This defines a function named name. The body of
the function is the list of commands between { and
}. This list is executed whenever name is speci
fied as the name of a simple command. The exit
status of a function is the exit status of the last
command executed in the body. (See FUNCTIONS
below.)
COMMENTS
In a non-interactive shell, or an interactive shell in
which the interactive_comments option to the shopt builtin
is enabled (see SHELL BUILTIN COMMANDS below), a word
beginning with # causes that word and all remaining char
acters on that line to be ignored. An interactive shell
without the interactive_comments option enabled does not
allow comments. The interactive_comments option is on by
default in interactive shells.
QUOTING
Quoting is used to remove the special meaning of certain
characters or words to the shell. Quoting can be used to
disable special treatment for special characters, to pre
vent reserved words from being recognized as such, and to
prevent parameter expansion.
Each of the metacharacters listed above under DEFINITIONS
has special meaning to the shell and must be quoted if
they are to represent themselves. There are three quoting
mechanisms: the escape character, single quotes, and dou
ble quotes.
A non-quoted backslash (\) is the escape character. It
preserves the literal value of the next character that
follows, with the exception of <newline>. If a \<newline>
pair appears, and the backslash is not itself quoted, the
\<newline> is treated as a line continuation (that is, it
is removed from the input stream and effectively ignored).
Enclosing characters in single quotes preserves the lit
eral value of each character within the quotes. A single
quote may not occur between single quotes, even when pre
ceded by a backslash.
Enclosing characters in double quotes preserves the lit
eral value of all characters within the quotes, with the
exception of $, `, and \. The characters $ and ` retain
their special meaning within double quotes. The backslash
retains its special meaning only when followed by one of
the following characters: $, `, ", \, or <newline>. A
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double quote may be quoted within double quotes by preced
ing it with a backslash.
The special parameters * and @ have special meaning when
in double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters
replaced as specifed by the ANSI C standard. Backslash
escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is the octal
value nnn (one to three digits)
\xnnn the character whose ASCII code is the hex
adecimal value nnn (one to three digits)
The translated result is single-quoted, as if the dollar
sign had not been present.
A double-quoted string preceded by a dollar sign ($) will
cause the string to be translated according to the current
locale. If the current locale is C or POSIX, the dollar
sign is ignored. If the string is translated and
replaced, the replacement is double-quoted.
PARAMETERS
A parameter is an entity that stores values. It can be a
name, a number, or one of the special characters listed
below under Special Parameters. For the shell's purposes,
a variable is a parameter denoted by a name.
A parameter is set if it has been assigned a value. The
null string is a valid value. Once a variable is set, it
may be unset only by using the unset builtin command (see
SHELL BUILTIN COMMANDS below).
A variable may be assigned to by a statement of the form
name=[value]
If value is not given, the variable is assigned the null
string. All values undergo tilde expansion, parameter and
variable expansion, string expansion, command substitu
tion, arithmetic expansion, and quote removal (see EXPAN
SION below). If the variable has its integer attribute
set (see declare below in SHELL BUILTIN COMMANDS) then
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value is subject to arithmetic expansion even if the
$((...)) expansion is not used (see Arithmetic Expansion
below). Word splitting is not performed, with the excep
tion of "$@" as explained below under Special Parameters.
Pathname expansion is not performed.
Positional Parameters
A positional parameter is a parameter denoted by one or
more digits, other than the single digit 0. Positional
parameters are assigned from the shell's arguments when it
is invoked, and may be reassigned using the set builtin
command. Positional parameters may not be assigned to
with assignment statements. The positional parameters are
temporarily replaced when a shell function is executed
(see FUNCTIONS below).
When a positional parameter consisting of more than a sin
gle digit is expanded, it must be enclosed in braces (see
EXPANSION below).
Special Parameters
The shell treats several parameters specially. These
parameters may only be referenced; assignment to them is
not allowed.
* Expands to the positional parameters, starting from
one. When the expansion occurs within double
quotes, it expands to a single word with the value
of each parameter separated by the first character
of the IFS special variable. That is, "$*" is
equivalent to "$1c$2c...", where c is the first
character of the value of the IFS variable. If IFS
is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without
intervening separators.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double
quotes, each parameter expands to a separate word.
That is, "$@" is equivalent to "$1" "$2" ... When
there are no positional parameters, "$@" and $@
expand to nothing (i.e., they are removed).
# Expands to the number of positional parameters in
decimal.
? Expands to the status of the most recently executed
foreground pipeline.
- Expands to the current option flags as specified
upon invocation, by the set builtin command, or
those set by the shell itself (such as the -i
option).
$ Expands to the process ID of the shell. In a ()
subshell, it expands to the process ID of the cur
rent shell, not the subshell.
! Expands to the process ID of the most recently exe
cuted background (asynchronous) command.
0 Expands to the name of the shell or shell script.
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This is set at shell initialization. If bash is
invoked with a file of commands, $0 is set to the
name of that file. If bash is started with the -c
option, then $0 is set to the first argument after
the string to be executed, if one is present. Oth
erwise, it is set to the file name used to invoke
bash, as given by argument zero.
_ At shell startup, set to the absolute file name of
the shell or shell script being executed as passed
in the argument list. Subsequently, expands to the
last argument to the previous command, after expan
sion. Also set to the full file name of each com
mand executed and placed in the environment
exported to that command. When checking mail, this
parameter holds the name of the mail file currently
being checked.
Shell Variables
The following variables are set by the shell:
PPID The process ID of the shell's parent. This vari
able is readonly.
PWD The current working directory as set by the cd com
mand.
OLDPWD The previous working directory as set by the cd
command.
REPLY Set to the line of input read by the read builtin
command when no arguments are supplied.
UID Expands to the user ID of the current user, ini
tialized at shell startup. This variable is read
only.
EUID Expands to the effective user ID of the current
user, initialized at shell startup. This variable
is readonly.
GROUPS An array variable containing the list of groups of
which the current user is a member. This variable
is readonly.
BASH Expands to the full file name used to invoke this
instance of bash.
BASH_VERSION
Expands to a string describing the version of this
instance of bash.
BASH_VERSINFO
A readonly array variable whose members hold ver
sion information for this instance of bash. The
values assigned to the array members are as fol
lows:
BASH_VERSINFO[0] The major version number
(the release).
BASH_VERSINFO[1] The minor version number
(the version).
BASH_VERSINFO[2] The patch level.
BASH_VERSINFO[3] The build version.
BASH_VERSINFO[4] The release status (e.g.,
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beta1).
BASH_VERSINFO[5] The value of MACHTYPE.
SHLVL Incremented by one each time an instance of bash is
started.
RANDOM Each time this parameter is referenced, a random
integer between 0 and 32767 is generated. The
sequence of random numbers may be initialized by
assigning a value to RANDOM. If RANDOM is unset,
it loses its special properties, even if it is sub
sequently reset.
SECONDS
Each time this parameter is referenced, the number
of seconds since shell invocation is returned. If
a value is assigned to SECONDS, the value returned
upon subsequent references is the number of seconds
since the assignment plus the value assigned. If
SECONDS is unset, it loses its special properties,
even if it is subsequently reset.
LINENO Each time this parameter is referenced, the shell
substitutes a decimal number representing the cur
rent sequential line number (starting with 1)
within a script or function. When not in a script
or function, the value substituted is not guaran
teed to be meaningful. If LINENO is unset, it
loses its special properties, even if it is subse
quently reset.
HISTCMD
The history number, or index in the history list,
of the current command. If HISTCMD is unset, it
loses its special properties, even if it is subse
quently reset.
DIRSTACK
An array variable (see Arrays below) containing the
current contents of the directory stack. Directo
ries appear in the stack in the order they are dis
played by the dirs builtin. Assigning to members
of this array variable may be used to modify direc
tories already in the stack, but the pushd and popd
builtins must be used to add and remove directo
ries. Assignment to this variable will not change
the current directory. If DIRSTACK is unset, it
loses its special properties, even if it is subse
quently reset.
PIPESTATUS
An array variable (see Arrays below) containing a
list of exit status values from the processes in
the most-recently-executed foreground pipeline
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(which may contain only a single command).
OPTARG The value of the last option argument processed by
the getopts builtin command (see SHELL BUILTIN COM
MANDS below).
OPTIND The index of the next argument to be processed by
the getopts builtin command (see SHELL BUILTIN COM
MANDS below).
HOSTNAME
Automatically set to the name of the current host.
HOSTTYPE
Automatically set to a string that uniquely
describes the type of machine on which bash is exe
cuting. The default is system-dependent.
OSTYPE Automatically set to a string that describes the
operating system on which bash is executing. The
default is system-dependent.
MACHTYPE
Automatically set to a string that fully describes
the system type on which bash is executing, in the
standard GNU cpu-company-system format. The
default is system-dependent.
SHELLOPTS
A colon-separated list of enabled shell options.
Each word in the list is a valid argument for the
-o option to the set builtin command (see SHELL
BUILTIN COMMANDS below). The options appearing in
SHELLOPTS are those reported as on by set -o. If
this variable is in the environment when bash
starts up, each shell option in the list will be
enabled before reading any startup files. This
variable is read-only.
The following variables are used by the shell. In some
cases, bash assigns a default value to a variable; these
cases are noted below.
IFS The Internal Field Separator that is used for word
splitting after expansion and to split lines into
words with the read builtin command. The default
value is ``<space><tab><newline>''.
PATH The search path for commands. It is a colon-sepa
rated list of directories in which the shell looks
for commands (see COMMAND EXECUTION below). The
default path is system-dependent, and is set by the
administrator who installs bash. A common value is
``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.
HOME The home directory of the current user; the default
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argument for the cd builtin command. The value of
this variable is also used when performing tilde
expansion.
CDPATH The search path for the cd command. This is a
colon-separated list of directories in which the
shell looks for destination directories specified
by the cd command. A sample value is ``.:~:/usr''.
BASH_ENV
If this parameter is set when bash is executing a
shell script, its value is interpreted as a file
name containing commands to initialize the shell,
as in ~/.bashrc. The value of BASH_ENV is sub
jected to parameter expansion, command substitu
tion, and arithmetic expansion before being inter
preted as a file name. PATH is not used to search
for the resultant file name.
MAIL If this parameter is set to a file name and the
MAILPATH variable is not set, bash informs the user
of the arrival of mail in the specified file.
MAILCHECK
Specifies how often (in seconds) bash checks for
mail. The default is 60 seconds. When it is time
to check for mail, the shell does so before dis
playing the primary prompt. If this variable is
unset, the shell disables mail checking.
MAILPATH
A colon-separated list of file names to be checked
for mail. The message to be printed when mail
arrives in a particular file may be specified by
separating the file name from the message with a
`?'. When used in the text of the message, $_
expands to the name of the current mailfile. Exam
ple:
MAILPATH='/usr/spool/mail/bfox?"You have
mail":~/shell-mail?"$_ has mail!"'
Bash supplies a default value for this variable,
but the location of the user mail files that it
uses is system dependent (e.g.,
/usr/spool/mail/$USER).
PS1 The value of this parameter is expanded (see
PROMPTING below) and used as the primary prompt
string. The default value is ``\s-\v\$ ''.
PS2 The value of this parameter is expanded as with PS1
and used as the secondary prompt string. The
default is ``> ''.
PS3 The value of this parameter is used as the prompt
for the select command (see SHELL GRAMMAR above).
PS4 The value of this parameter is expanded as with PS1
and the value is printed before each command bash
displays during an execution trace. The first
character of PS4 is replicated multiple times, as
necessary, to indicate multiple levels of indirec
tion. The default is ``+ ''.
GNU 1999 Jan 20 15
BASH2(1) BASH2(1)
TIMEFORMAT
The value of this parameter is used as a format
string specifying how the timing information for
pipelines prefixed with the time reserved word
should be displayed. The % character introduces an
escape sequence that is expanded to a time value or
other information. The escape sequences and their
meanings are as follows; the braces denote optional
portions.
%% A literal %.
%[p][l]R The elapsed time in seconds.
%[p][l]U The number of CPU seconds spent in user
mode.
%[p][l]S The number of CPU seconds spent in system
mode.
%P The CPU percentage, computed as (%U + %S)
/ %R.
The optional p is a digit specifying the precision,
the number of fractional digits after a decimal
point. A value of 0 causes no decimal point or
fraction to be output. At most three places after
the decimal point may be specified; values of p
greater than 3 are changed to 3. If p is not spec
ified, the value 3 is used.
The optional l specifies a longer format, including
minutes, of the form MMmSS.FFs. The value of p
determines whether or not the fraction is included.
If this variable is not set, bash acts as if it had
the value $'\nreal\t%3lR\nuser\t%3lU\nsys%3lS'. If
the value is null, no timing information is dis
played. A trailing newline is added when the for
mat string is displayed.
HISTSIZE
The number of commands to remember in the command
history (see HISTORY below). The default value is
500.
HISTFILE
The name of the file in which command history is
saved (see HISTORY below). The default value is
~/.bash_history. If unset, the command history is
not saved when an interactive shell exits.
HISTFILESIZE
The maximum number of lines contained in the his
tory file. When this variable is assigned a value,
the history file is truncated, if necessary, to
contain no more than that number of lines. The
default value is 500. The history file is also
truncated to this size after writing it when an
GNU 1999 Jan 20 16
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interactive shell exits.
OPTERR If set to the value 1, bash displays error messages
generated by the getopts builtin command (see SHELL
BUILTIN COMMANDS below). OPTERR is initialized to
1 each time the shell is invoked or a shell script
is executed.
LANG Used to determine the locale category for any cate
gory not specifically selected with a variable
starting with LC_.
LC_ALL This variable overrides the value of LANG and any
other LC_ variable specifying a locale category.
LC_COLLATE
This variable determines the collation order used
when sorting the results of pathname expansion, and
determines the behavior of range expressions,
equivalence classes, and collating sequences within
pathname expansion and pattern matching.
LC_CTYPE
This variable determines the interpretation of
characters and the behavior of character classes
within pathname expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to trans
late double-quoted strings preceded by a $.
PROMPT_COMMAND
If set, the value is executed as a command prior to
issuing each primary prompt.
IGNOREEOF
Controls the action of an interactive shell on
receipt of an EOF character as the sole input. If
set, the value is the number of consecutive EOF
characters which must be typed as the first charac
ters on an input line before bash exits. If the
variable exists but does not have a numeric value,
or has no value, the default value is 10. If it
does not exist, EOF signifies the end of input to
the shell.
TMOUT If set to a value greater than zero, the value is
interpreted as the number of seconds to wait for
input after issuing the primary prompt. Bash ter
minates after waiting for that number of seconds if
input does not arrive.
FCEDIT The default editor for the fc builtin command.
GNU 1999 Jan 20 17
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FIGNORE
A colon-separated list of suffixes to ignore when
performing filename completion (see READLINE
below). A filename whose suffix matches one of the
entries in FIGNORE is excluded from the list of
matched filenames. A sample value is ``.o:~''.
GLOBIGNORE
A colon-separated list of patterns defining the set
of filenames to be ignored by pathname expansion.
If a filename matched by a pathname expansion pat
tern also matches one of the patterns in GLOBIG
NORE, it is removed from the list of matches.
INPUTRC
The filename for the readline startup file, over
riding the default of ~/.inputrc (see READLINE
below).
HISTCONTROL
If set to a value of ignorespace, lines which begin
with a space character are not entered on the his
tory list. If set to a value of ignoredups, lines
matching the last history line are not entered. A
value of ignoreboth combines the two options. If
unset, or if set to any other value than those
above, all lines read by the parser are saved on
the history list, subject to the value of HISTIG
NORE. This variable's function is superseded by
HISTIGNORE. The second and subsequent lines of a
multi-line compound command are not tested, and are
added to the history regardless of the value of
HISTCONTROL.
HISTIGNORE
A colon-separated list of patterns used to decide
which command lines should be saved on the history
list. Each pattern is anchored at the beginning of
the line and must fully specify the line (no
implicit `*' is appended). Each pattern is tested
against the line after the checks specified by
HISTCONTROL are applied. In addition to the normal
shell pattern matching characters, `&' matches the
previous history line. `&' may be escaped using a
backslash. The backslash is removed before
attempting a match. The second and subsequent
lines of a multi-line compound command are not
tested, and are added to the history regardless of
the value of HISTIGNORE.
histchars
The two or three characters which control history
expansion and tokenization (see HISTORY EXPANSION
below). The first character is the history
GNU 1999 Jan 20 18
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expansion character, the character which signals
the start of a history expansion, normally `!'.
The second character is the quick substitution
character, which is used as shorthand for re-run
ning the previous command entered, substituting one
string for another in the command. The default is
`^'. The optional third character is the character
which indicates that the remainder of the line is a
comment when found as the first character of a
word, normally `#'. The history comment character
causes history substitution to be skipped for the
remaining words on the line. It does not necessar
ily cause the shell parser to treat the rest of the
line as a comment.
HOSTFILE
Contains the name of a file in the same format as
/etc/hosts that should be read when the shell needs
to complete a hostname. The file may be changed
interactively; the next time hostname completion is
attempted bash adds the contents of the new file to
the already existing database.
auto_resume
This variable controls how the shell interacts with
the user and job control. If this variable is set,
single word simple commands without redirections
are treated as candidates for resumption of an
existing stopped job. There is no ambiguity
allowed; if there is more than one job beginning
with the string typed, the job most recently
accessed is selected. The name of a stopped job,
in this context, is the command line used to start
it. If set to the value exact, the string supplied
must match the name of a stopped job exactly; if
set to substring, the string supplied needs to
match a substring of the name of a stopped job.
The substring value provides functionality analo
gous to the %? job identifier (see JOB CONTROL
below). If set to any other value, the supplied
string must be a prefix of a stopped job's name;
this provides functionality analogous to the % job
identifier.
Arrays
Bash provides one-dimensional array variables. Any vari
able may be used as an array; the declare builtin will
explicitly declare an array. There is no maximum limit on
the size of an array, nor any requirement that members be
indexed or assigned contiguously. Arrays are indexed
using integers and are zero-based.
An array is created automatically if any variable is
assigned to using the syntax name[subscript]=value. The
GNU 1999 Jan 20 19
BASH2(1) BASH2(1)
subscript is treated as an arithmetic expression that must
evaluate to a number greater than or equal to zero. To
explicitly declare an array, use declare -a name (see
SHELL BUILTIN COMMANDS below). declare -a name[subscript]
is also accepted; the subscript is ignored. Attributes
may be specified for an array variable using the declare
and readonly builtins. Each attribute applies to all mem
bers of an array.
Arrays are assigned to using compound assignments of the
form name=(value1 ... valuen), where each value is of the
form [subscript]=string. Only string is required. If the
optional brackets and subscript are supplied, that index
is assigned to; otherwise the index of the element
assigned is the last index assigned to by the statement
plus one. Indexing starts at zero. This syntax is also
accepted by the declare builtin. Individual array ele
ments may be assigned to using the name[subscript]=value
syntax introduced above.
Any element of an array may be referenced using
${name[subscript]}. The braces are required to avoid con
flicts with pathname expansion. If subscript is @ or *,
the word expands to all members of name. These subscripts
differ only when the word appears within double quotes.
If the word is double-quoted, ${name[*]} expands to a sin
gle word with the value of each array member separated by
the first character of the IFS special variable, and
${name[@]} expands each element of name to a separate
word. When there are no array members, ${name[@]} expands
to nothing. This is analogous to the expansion of the
special parameters * and @ (see Special Parameters above).
${#name[subscript]} expands to the length of ${name[sub
script]}. If subscript is * or @, the expansion is the
number of elements in the array. Referencing an array
variable without a subscript is equivalent to referencing
element zero.
The unset builtin is used to destroy arrays. unset
name[subscript] destroys the array element at index sub
script. unset name, where name is an array, or unset
name[subscript], where subscript is * or @, removes the
entire array.
The declare, local, and readonly builtins each accept a -a
option to specify an array. The read builtin accepts a -a
option to assign a list of words read from the standard
input to an array. The set and declare builtins display
array values in a way that allows them to be reused as
assignments.
EXPANSION
Expansion is performed on the command line after it has
been split into words. There are seven kinds of expansion
GNU 1999 Jan 20 20
BASH2(1) BASH2(1)
performed: brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic
expansion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expan
sion, parameter, variable and arithmetic expansion and
command substitution (done in a left-to-right fashion),
word splitting, and pathname expansion.
On systems that can support it, there is an additional
expansion available: process substitution.
Only brace expansion, word splitting, and pathname expan
sion can change the number of words of the expansion;
other expansions expand a single word to a single word.
The only exceptions to this are the expansions of "$@" and
"${name[@]}" as explained above (see PARAMETERS).
Brace Expansion
Brace expansion is a mechanism by which arbitrary strings
may be generated. This mechanism is similar to pathname
expansion, but the filenames generated need not exist.
Patterns to be brace expanded take the form of an optional
preamble, followed by a series of comma-separated strings
between a pair of braces, followed by an optional
postscript. The preamble is prefixed to each string con
tained within the braces, and the postscript is then
appended to each resulting string, expanding left to
right.
Brace expansions may be nested. The results of each
expanded string are not sorted; left to right order is
preserved. For example, a{d,c,b}e expands into `ade ace
abe'.
Brace expansion is performed before any other expansions,
and any characters special to other expansions are pre
served in the result. It is strictly textual. Bash does
not apply any syntactic interpretation to the context of
the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted
opening and closing braces, and at least one unquoted
comma. Any incorrectly formed brace expansion is left
unchanged. A { or , may be quoted with a backslash to
prevent its being considered part of a brace expression.
This construct is typically used as shorthand when the
common prefix of the strings to be generated is longer
than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
GNU 1999 Jan 20 21
BASH2(1) BASH2(1)
Brace expansion introduces a slight incompatibility with
historical versions of sh. sh does not treat opening or
closing braces specially when they appear as part of a
word, and preserves them in the output. Bash removes
braces from words as a consequence of brace expansion.
For example, a word entered to sh as file{1,2} appears
identically in the output. The same word is output as
file1 file2 after expansion by bash. If strict compati
bility with sh is desired, start bash with the +B option
or disable brace expansion with the +B option to the set
command (see SHELL BUILTIN COMMANDS below).
Tilde Expansion
If a word begins with an unquoted tilde character (`~'),
all of the characters preceding the first unquoted slash
(or all characters, if there is no unquoted slash) are
considered a tilde-prefix. If none of the characters in
the tilde-prefix are quoted, the characters in the tilde-
prefix following the tilde are treated as a possible login
name. If this login name is the null string, the tilde is
replaced with the value of the shell parameter HOME. If
HOME is unset, the home directory of the user executing
the shell is substituted instead. Otherwise, the tilde-
prefix is replaced with the home directory associated with
the specified login name.
If the tilde-prefix is a `~+', the value of the shell
variable PWD replaces the tilde-prefix. If the tilde-pre
fix is a `~-', the value of the shell variable OLDPWD, if
it is set, is substituted. If the characters following
the tilde in the tilde-prefix consist of a number N,
optionally prefixed by a `+' or a `-', the tilde-prefix is
replaced with the corresponding element from the directory
stack, as it would be displayed by the dirs builtin
invoked with the tilde-prefix as an argument. If the
characters following the tilde in the tilde-prefix consist
of a number without a leading `+' or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion
fails, the word is unchanged.
Each variable assignment is checked for unquoted tilde-
prefixes immediately following a : or =. In these cases,
tilde expansion is also performed. Consequently, one may
use file names with tildes in assignments to PATH, MAIL
PATH, and CDPATH, and the shell assigns the expanded
value.
Parameter Expansion
The `$' character introduces parameter expansion, command
substitution, or arithmetic expansion. The parameter name
or symbol to be expanded may be enclosed in braces, which
are optional but serve to protect the variable to be
expanded from characters immediately following it which
GNU 1999 Jan 20 22
BASH2(1) BASH2(1)
could be interpreted as part of the name.
When braces are used, the matching ending brace is the
first `}' not escaped by a backslash or within a quoted
string, and not within an embedded arithmetic expansion,
command substitution, or paramter expansion.
${parameter}
The value of parameter is substituted. The braces
are required when parameter is a positional parame
ter with more than one digit, or when parameter is
followed by a character which is not to be inter
preted as part of its name.
If the first character of parameter is an exclamation
point, a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest
of parameter as the name of the variable; this variable is
then expanded and that value used in the rest of the sub
stitution, rather than the value of parameter itself.
This is known as indirect expansion.
In each of the cases below, word is subject to tilde
expansion, parameter expansion, command substitution, and
arithmetic expansion. When not performing substring
expansion, bash tests for a parameter that is unset or
null; omitting the colon results in a test only for a
parameter that is unset.
${parameter:-word}
Use Default Values. If parameter is unset or null,
the expansion of word is substituted. Otherwise,
the value of parameter is substituted.
${parameter:=word}
Assign Default Values. If parameter is unset or
null, the expansion of word is assigned to parame
ter. The value of parameter is then substituted.
Positional parameters and special parameters may
not be assigned to in this way.
${parameter:?word}
Display Error if Null or Unset. If parameter is
null or unset, the expansion of word (or a message
to that effect if word is not present) is written
to the standard error and the shell, if it is not
interactive, exits. Otherwise, the value of param
eter is substituted.
${parameter:+word}
Use Alternate Value. If parameter is null or
unset, nothing is substituted, otherwise the expan
sion of word is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length char
acters of parameter, starting at the characters
GNU 1999 Jan 20 23
BASH2(1) BASH2(1)
specified by offset. If length is omitted, expands
to the substring of parameter, starting at the
character specified by offset. length and offset
are arithmetic expressions (see ARITHMETIC EVALUA
TION below). length must evaluate to a number
greater than or equal to zero. If offset evaluates
to a number less than zero, the value is used as an
offset from the end of the value of parameter. If
parameter is @, the result is length positional
parameters beginning at offset. If parameter is an
array name indexed by @ or *, the result is the
length members of the array beginning with ${param
eter[offset]}. Substring indexing is zero-based
unless the positional parameters are used, in which
case the indexing starts at 1.
${#parameter}
The length in characters of the value of parameter
is substituted. If parameter is * or @, the value
substituted is the number of positional parameters.
If parameter is an array name subscripted by * or
@, the value substituted is the number of elements
in the array.
${parameter#word}
${parameter##word}
The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches the
beginning of the value of parameter, then the
result of the expansion is the expanded value of
parameter with the shortest matching pattern (the
``#'' case) or the longest matching pattern (the
``##'' case) deleted. If parameter is @ or *, the
pattern removal operation is applied to each posi
tional parameter in turn, and the expansion is the
resultant list. If parameter is an array variable
subscripted with @ or *, the pattern removal opera
tion is applied to each member of the array in
turn, and the expansion is the resultant list.
${parameter%word}
${parameter%%word}
The word is expanded to produce a pattern just as
in pathname expansion. If the pattern matches a
trailing portion of the expanded value of parame
ter, then the result of the expansion is the
expanded value of parameter with the shortest
matching pattern (the ``%'' case) or the longest
matching pattern (the ``%%'' case) deleted. If
parameter is @ or *, the pattern removal operation
is applied to each positional parameter in turn,
and the expansion is the resultant list. If param
eter is an array variable subscripted with @ or *,
the pattern removal operation is applied to each
GNU 1999 Jan 20 24
BASH2(1) BASH2(1)
member of the array in turn, and the expansion is
the resultant list.
${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just
as in pathname expansion. Parameter is expanded
and the longest match of pattern against its value
is replaced with string. In the first form, only
the first match is replaced. The second form
causes all matches of pattern to be replaced with
string. If pattern begins with #, it must match at
the beginning of the expanded value of parameter.
If pattern begins with %, it must match at the end
of the expanded value of parameter. If string is
null, matches of pattern are deleted and the / fol
lowing pattern may be omitted. If parameter is @
or *, the substitution operation is applied to each
positional parameter in turn, and the expansion is
the resultant list. If parameter is an array vari
able subscripted with @ or *, the substitution
operation is applied to each member of the array in
turn, and the expansion is the resultant list.
Command Substitution
Command substitution allows the output of a command to
replace the command name. There are two forms:
$(command)
or
`command`
Bash performs the expansion by executing command and
replacing the command substitution with the standard out
put of the command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed
during word splitting. The command substitution $(cat
file) can be replaced by the equivalent but faster $(<
file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed
by $, `, or \. The first backquote not preceded by a
backslash terminates the command substitution. When using
the $(command) form, all characters between the parenthe
ses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using
the backquoted form, escape the inner backquotes with
backslashes.
If the substitution appears within double quotes, word
splitting and pathname expansion are not performed on the
GNU 1999 Jan 20 25
BASH2(1) BASH2(1)
results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arith
metic expression and the substitution of the result. The
format for arithmetic expansion is:
$((expression))
The expression is treated as if it were within double
quotes, but a double quote inside the parentheses is not
treated specially. All tokens in the expression undergo
parameter expansion, string expansion, command substitu
tion, and quote removal. Arithmetic substitutions may be
nested.
The evaluation is performed according to the rules listed
below under ARITHMETIC EVALUATION. If expression is
invalid, bash prints a message indicating failure and no
substitution occurs.
Process Substitution
Process substitution is supported on systems that support
named pipes (FIFOs) or the /dev/fd method of naming open
files. It takes the form of <(list) or >(list). The pro
cess list is run with its input or output connected to a
FIFO or some file in /dev/fd. The name of this file is
passed as an argument to the current command as the result
of the expansion. If the >(list) form is used, writing to
the file will provide input for list. If the <(list) form
is used, the file passed as an argument should be read to
obtain the output of list.
When available, process substitution is performed simulta
neously with parameter and variable expansion, command
substitution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, com
mand substitution, and arithmetic expansion that did not
occur within double quotes for word splitting.
The shell treats each character of IFS as a delimiter, and
splits the results of the other expansions into words on
these characters. If IFS is unset, or its value is
exactly <space><tab><newline>, the default, then any
sequence of IFS characters serves to delimit words. If
IFS has a value other than the default, then sequences of
the whitespace characters space and tab are ignored at the
beginning and end of the word, as long as the whitespace
character is in the value of IFS (an IFS whitespace char
acter). Any character in IFS that is not IFS whitespace,
along with any adjacent IFS whitespace characters, delim
its a field. A sequence of IFS whitespace characters is
GNU 1999 Jan 20 26
BASH2(1) BASH2(1)
also treated as a delimiter. If the value of IFS is null,
no word splitting occurs.
Explicit null arguments ("" or '') are retained. Unquoted
implicit null arguments, resulting from the expansion of
parameters that have no values, are removed. If a parame
ter with no value is expanded within double quotes, a null
argument results and is retained.
Note that if no expansion occurs, no splitting is per
formed.
Pathname Expansion
After word splitting, unless the -f option has been set,
bash scans each word for the characters *, ?, (, and [.
If one of these characters appears, then the word is
regarded as a pattern, and replaced with an alphabetically
sorted list of file names matching the pattern. If no
matching file names are found, and the shell option null
glob is disabled, the word is left unchanged. If the
nullglob option is set, and no matches are found, the word
is removed. If the shell option nocaseglob is enabled,
the match is performed without regard to the case of
alphabetic characters. When a pattern is used for path
name expansion, the character ``.'' at the start of a
name or immediately following a slash must be matched
explicitly, unless the shell option dotglob is set. When
matching a pathname, the slash character must always be
matched explicitly. In other cases, the ``.'' character
is not treated specially. See the description of shopt
below under SHELL BUILTIN COMMANDS for a description of
the nocaseglob, nullglob, and dotglob shell options.
The GLOBIGNORE shell variable may be used to restrict the
set of file names matching a pattern. If GLOBIGNORE is
set, each matching file name that also matches one of the
patterns in GLOBIGNORE is removed from the list of
matches. The file names ``.'' and ``..'' are always
ignored, even when GLOBIGNORE is set. However, setting
GLOBIGNORE has the effect of enabling the dotglob shell
option, so all other file names beginning with a ``.''
will match. To get the old behavior of ignoring file
names beginning with a ``.'', make ``.*'' one of the pat
terns in GLOBIGNORE. The dotglob option is disabled when
GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the
special pattern characters described below, matches
itself. The NUL character may not occur in a pattern.
The special pattern characters must be quoted if they are
to be matched literally.
GNU 1999 Jan 20 27
BASH2(1) BASH2(1)
The special pattern characters have the following mean
ings:
* Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair
of characters separated by a minus sign denotes a
range; any character lexically between those two
characters, inclusive, is matched. If the first
character following the [ is a ! or a ^ then any
character not enclosed is matched. A - may be
matched by including it as the first or last char
acter in the set. A ] may be matched by including
it as the first character in the set.
Within [ and ], character classes can be specified
using the syntax [:class:], where class is one of
the following classes defined in the POSIX.2 stan
dard:
alnum alpha ascii blank cntrl digit graph lower
print punct space upper xdigit
A character class matches any character belonging
to that class.
Within [ and ], an equivalence class can be speci
fied using the syntax [=c=], which matches all
characters with the same collation weight (as
defined by the current locale) as the character c.
Within [ and ], the syntax [.symbol.] matches the
collating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are
recognized. In the following description, a pattern-list
is a list of one or more patterns separated by a |. Com
posite patterns may be formed using one or more of the
following sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given
patterns
*(pattern-list)
Matches zero or more occurrences of the
given patterns
+(pattern-list)
Matches one or more occurrences of the given
patterns
@(pattern-list)
Matches exactly one of the given patterns
!(pattern-list)
Matches anything except one of the given
patterns
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Quote Removal
After the preceding expansions, all unquoted occurrences
of the characters \, ', and " that did not result from one
of the above expansions are removed.
REDIRECTION
Before a command is executed, its input and output may be
redirected using a special notation interpreted by the
shell. Redirection may also be used to open and close
files for the current shell execution environment. The
following redirection operators may precede or appear any
where within a simple command or may follow a command.
Redirections are processed in the order they appear, from
left to right.
In the following descriptions, if the file descriptor num
ber is omitted, and the first character of the redirection
operator is <, the redirection refers to the standard
input (file descriptor 0). If the first character of the
redirection operator is >, the redirection refers to the
standard output (file descriptor 1).
The word following the redirection operator in the follow
ing descriptions, unless otherwise noted, is subjected to
brace expansion, tilde expansion, parameter expansion,
command substitution, arithmetic expansion, quote removal,
and pathname expansion. If it expands to more than one
word, bash reports an error.
Note that the order of redirections is significant. For
example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the
file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because
the standard error was duplicated as standard output
before the standard output was redirected to dirlist.
A failure to open or create a file causes the redirection
to fail.
Redirecting Input
Redirection of input causes the file whose name results
from the expansion of word to be opened for reading on
file descriptor n, or the standard input (file descriptor
0) if n is not specified.
The general format for redirecting input is:
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[n]<word
Redirecting Output
Redirection of output causes the file whose name results
from the expansion of word to be opened for writing on
file descriptor n, or the standard output (file descriptor
1) if n is not specified. If the file does not exist it
is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>word
If the redirection operator is >, and the noclobber option
to the set builtin has been enabled, the redirection will
fail if the filename whose name results from the expansion
of word exists and is a regular file. If the redirection
operator is >|, or the redirection operator is > and the
noclobber option to the set builtin command is not
enabled, the redirection is attempted even if the file
named by word exists.
Appending Redirected Output
Redirection of output in this fashion causes the file
whose name results from the expansion of word to be opened
for appending on file descriptor n, or the standard output
(file descriptor 1) if n is not specified. If the file
does not exist it is created.
The general format for appending output is:
[n]>>word
Redirecting Standard Output and Standard Error
Bash allows both the standard output (file descriptor 1)
and the standard error output (file descriptor 2) to be
redirected to the file whose name is the expansion of word
with this construct.
There are two formats for redirecting standard output and
standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is seman
tically equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input
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from the current source until a line containing only word
(with no trailing blanks) is seen. All of the lines read
up to that point are then used as the standard input for a
command.
The format of here-documents is as follows:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, pathname
expansion, or arithmetic expansion is performed on word.
If any characters in word are quoted, the delimiter is the
result of quote removal on word, and the lines in the
here-document are not expanded. If word is unquoted, all
lines of the here-document are subjected to parameter
expansion, command substitution, and arithmetic expansion.
In the latter case, the pair \<newline> is ignored, and \
must be used to quote the characters \, $, and `.
If the redirection operator is <<-, then all leading tab
characters are stripped from input lines and the line con
taining delimiter. This allows here-documents within
shell scripts to be indented in a natural fashion.
Duplicating File Descriptors
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word
expands to one or more digits, the file descriptor denoted
by n is made to be a copy of that file descriptor. If the
digits in word do not specify a file descriptor open for
input, a redirection error occurs. If word evaluates to
-, file descriptor n is closed. If n is not specified,
the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors.
If n is not specified, the standard output (file descrip
tor 1) is used. If the digits in word do not specify a
file descriptor open for output, a redirection error
occurs. As a special case, if n is omitted, and word does
not expand to one or more digits, the standard output and
standard error are redirected as described previously.
Opening File Descriptors for Reading and Writing
The redirection operator
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[n]<>word
causes the file whose name is the expansion of word to be
opened for both reading and writing on file descriptor n,
or on file descriptor 0 if n is not specified. If the
file does not exist, it is created.
ALIASES
Aliases allow a string to be substituted for a word when
it is used as the first word of a simple command. The
shell maintains a list of aliases that may be set and
unset with the alias and unalias builtin commands (see
SHELL BUILTIN COMMANDS below). The first word of each
command, if unquoted, is checked to see if it has an
alias. If so, that word is replaced by the text of the
alias. The alias name and the replacement text may con
tain any valid shell input, including the metacharacters
listed above, with the exception that the alias name may
not contain =. The first word of the replacement text is
tested for aliases, but a word that is identical to an
alias being expanded is not expanded a second time. This
means that one may alias ls to ls -F, for instance, and
bash does not try to recursively expand the replacement
text. If the last character of the alias value is a
blank, then the next command word following the alias is
also checked for alias expansion.
Aliases are created and listed with the alias command, and
removed with the unalias command.
There is no mechanism for using arguments in the replace
ment text. If arguments are needed, a shell function
should be used.
Aliases are not expanded when the shell is not interac
tive, unless the expand_aliases shell option is set using
shopt (see the description of shopt under SHELL BUILTIN
COMMANDS below).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash always reads at least one com
plete line of input before executing any of the commands
on that line. Aliases are expanded when a command is
read, not when it is executed. Therefore, an alias defi
nition appearing on the same line as another command does
not take effect until the next line of input is read. The
commands following the alias definition on that line are
not affected by the new alias. This behavior is also an
issue when functions are executed. Aliases are expanded
when a function definition is read, not when the function
is executed, because a function definition is itself a
compound command. As a consequence, aliases defined in a
function are not available until after that function is
executed. To be safe, always put alias definitions on a
GNU 1999 Jan 20 32
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separate line, and do not use alias in compound commands.
For almost every purpose, aliases are superseded by shell
functions.
FUNCTIONS
A shell function, defined as described above under SHELL
GRAMMAR, stores a series of commands for later execution.
Functions are executed in the context of the current
shell; no new process is created to interpret them (con
trast this with the execution of a shell script). When a
function is executed, the arguments to the function become
the positional parameters during its execution. The spe
cial parameter # is updated to reflect the change. Posi
tional parameter 0 is unchanged. All other aspects of the
shell execution environment are identical between a func
tion and its caller with the exception that the DEBUG trap
(see the description of the trap builtin under SHELL
BUILTIN COMMANDS below) is not inherited.
Variables local to the function may be declared with the
local builtin command. Ordinarily, variables and their
values are shared between the function and its caller.
If the builtin command return is executed in a function,
the function completes and execution resumes with the next
command after the function call. When a function com
pletes, the values of the positional parameters and the
special parameter # are restored to the values they had
prior to the function's execution.
Function names and definitions may be listed with the -f
option to the declare or typeset builtin commands. The -F
option to declare or typeset will list the function names
only. Functions may be exported so that subshells auto
matically have them defined with the -f option to the
export builtin.
Functions may be recursive. No limit is imposed on the
number of recursive calls.
ARITHMETIC EVALUATION
The shell allows arithmetic expressions to be evaluated,
under certain circumstances (see the let builtin command
and Arithmetic Expansion). Evaluation is done in long
integers with no check for overflow, though division by 0
is trapped and flagged as an error. The following list of
operators is grouped into levels of equal-precedence oper
ators. The levels are listed in order of decreasing
precedence.
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
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* / % multiplication, division, remainder
+ - addition, subtraction
<< >> left and right bitwise shifts
<= >= < >
comparison
== != equality and inequality
& bitwise AND
^ bitwise exclusive OR
| bitwise OR
&& logical AND
|| logical OR
expr?expr:expr
conditional evaluation
= *= /= %= += -= <<= >>= &= ^= |=
assignment
Shell variables are allowed as operands; parameter expan
sion is performed before the expression is evaluated. The
value of a parameter is coerced to a long integer within
an expression. A shell variable need not have its integer
attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal num
bers. A leading 0x or 0X denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where base is a decimal
number between 2 and 64 representing the arithmetic base,
and n is a number in that base. If base is omitted, then
base 10 is used. The digits greater than 9 are repre
sented by the lowercase letters, the uppercase letters, _,
and @, in that order. If base is less than or equal to
36, lowercase and uppercase letters may be used inter
changably to represent numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-
expressions in parentheses are evaluated first and may
override the precedence rules above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound com
mand and the test and [ builtin commands to test file
attributes and perform string and arithmetic comparisons.
Expressions are formed from the following unary or binary
primaries. If any file argument to one of the primaries
is of the form /dev/fd/n, then file descriptor n is
checked.
-a file
True if file exists.
-b file
True if file exists and is a block special file.
-c file
True if file exists and is a character special
file.
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-d file
True if file exists and is a directory.
-e file
True if file exists.
-f file
True if file exists and is a regular file.
-g file
True if file exists and is set-group-id.
-h file
True if file exists and is a symbolic link.
-k file
True if file exists and its ``sticky'' bit is set.
-p file
True if file exists and is a named pipe (FIFO).
-r file
True if file exists and is readable.
-s file
True if file exists and has a size greater than
zero.
-t fd True if file descriptor fd is open and refers to a
terminal.
-u file
True if file exists and its set-user-id bit is set.
-w file
True if file exists and is writable.
-x file
True if file exists and is executable.
-O file
True if file exists and is owned by the effective
user id.
-G file
True if file exists and is owned by the effective
group id.
-L file
True if file exists and is a symbolic link.
-S file
True if file exists and is a socket.
-N file
True if file exists and has been modified since it
was last read.
file1 -nt file2
True if file1 is newer (according to modification
date) than file2.
file1 -ot file2
True if file1 is older than file2.
file1 -ef file2
True if file1 and file2 have the same device and
inode numbers.
-o optname
True if shell option optname is enabled. See the
list of options under the description of the -o
option to the set builtin below.
-z string
True if the length of string is zero.
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-n string
string True if the length of string is non-zero.
string1 == string2
True if the strings are equal. = may be used in
place of ==.
string1 != string2
True if the strings are not equal.
string1 < string2
True if string1 sorts before string2 lexicographi
cally in the current locale.
string1 > string2
True if string1 sorts after string2 lexicographi
cally in the current locale.
arg1 OP arg2
OP is one of -eq, -ne, -lt, -le, -gt, or -ge.
These arithmetic binary operators return true if
arg1 is equal to, not equal to, less than, less
than or equal to, greater than, or greater than or
equal to arg2, respectively. Arg1 and arg2 may be
positive or negative integers.
SIMPLE COMMAND EXPANSION
When a simple command is executed, the shell performs the
following expansions, assignments, and redirections, from
left to right.
1. The words that the parser has marked as variable
assignments (those preceding the command name) and
redirections are saved for later processing.
2. The words that are not variable assignments or
redirections are expanded. If any words remain
after expansion, the first word is taken to be the
name of the command and the remaining words are the
arguments.
3. Redirections are performed as described above under
REDIRECTION.
4. The text after the = in each variable assignment
undergoes tilde expansion, parameter expansion,
command substitution, arithmetic expansion, and
quote removal before being assigned to the vari
able.
If no command name results, the variable assignments
affect the current shell environment. Otherwise, the
variables are added to the environment of the executed
command and do not affect the current shell environment.
If any of the assignments attempts to assign a value to a
readonly variable, an error occurs, and the command exits
with a non-zero status.
If no command name results, redirections are performed,
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but do not affect the current shell environment. A redi
rection error causes the command to exit with a non-zero
status.
If there is a command name left after expansion, execution
proceeds as described below. Otherwise, the command
exits. If one of the expansions contained a command sub
stitution, the exit status of the command is the exit sta
tus of the last command substitution performed. If there
were no command substitutions, the command exits with a
status of zero.
COMMAND EXECUTION
After a command has been split into words, if it results
in a simple command and an optional list of arguments, the
following actions are taken.
If the command name contains no slashes, the shell
attempts to locate it. If there exists a shell function
by that name, that function is invoked as described above
in FUNCTIONS. If the name does not match a function, the
shell searches for it in the list of shell builtins. If a
match is found, that builtin is invoked.
If the name is neither a shell function nor a builtin, and
contains no slashes, bash searches each element of the
PATH for a directory containing an executable file by that
name. Bash uses a hash table to remember the full file
names of executable files (see hash under SHELL BUILTIN
COMMANDS below). A full search of the directories in PATH
is performed only if the command is not found in the hash
table. If the search is unsuccessful, the shell prints an
error message and returns an exit status of 127.
If the search is successful, or if the command name con
tains one or more slashes, the shell executes the named
program in a separate execution environment. Argument 0
is set to the name given, and the remaining arguments to
the command are set to the arguments given, if any.
If this execution fails because the file is not in exe
cutable format, and the file is not a directory, it is
assumed to be a shell script, a file containing shell com
mands. A subshell is spawned to execute it. This sub
shell reinitializes itself, so that the effect is as if a
new shell had been invoked to handle the script, with the
exception that the locations of commands remembered by the
parent (see hash below under SHELL BUILTIN COMMANDS) are
retained by the child.
If the program is a file beginning with #!, the remainder
of the first line specifies an interpreter for the pro
gram. The shell executes the specified interpreter on
operating systems that do not handle this executable
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format themselves. The arguments to the interpreter con
sist of a single optional argument following the inter
preter name on the first line of the program, followed by
the name of the program, followed by the command argu
ments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of
the following:
· open files inherited by the shell at invocation, as
modified by redirections supplied to the exec
builtin
· the current working directory as set by cd, pushd,
or popd, or inherited by the shell at invocation
· the file creation mode mask as set by umask or
inherited from the shell's parent
· current traps set by trap
· shell parameters that are set by variable assign
ment or with set or inherited from the shell's par
ent in the environment
· shell functions defined during execution or inher
ited from the shell's parent in the environment
· options enabled at invocation (either by default or
with command-line arguments) or by set
· options enabled by shopt
· shell aliases defined with alias
· various process IDs, including those of background
jobs, the value of $$, and the value of $PPID
When a simple command other than a builtin or shell func
tion is to be executed, it is invoked in a separate execu
tion environment that consists of the following. Unless
otherwise noted, the values are inherited from the shell.
· the shell's open files, plus any modifications and
additions specified by redirections to the command
· the current working directory
· the file creation mode mask
· shell variables marked for export, along with
GNU 1999 Jan 20 38
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variables exported for the command, passed in the
environment
· traps caught by the shell are reset to the values
the inherited from the shell's parent, and traps
ignored by the shell are ignored
A command invoked in this separate environment cannot
affect the shell's execution environment.
Command substitution and asynchronous commands are invoked
in a subshell environment that is a duplicate of the shell
environment, except that traps caught by the shell are
reset to the values that the shell inherited from its par
ent at invocation. Builtin commands that are invoked as
part of a pipeline are also executed in a subshell envi
ronment. Changes made to the subshell environment cannot
affect the shell's execution environment.
ENVIRONMENT
When a program is invoked it is given an array of strings
called the environment. This is a list of name-value
pairs, of the form name=value.
The shell allows you to manipulate the environment in sev
eral ways. On invocation, the shell scans its own envi
ronment and creates a parameter for each name found, auto
matically marking it for export to child processes. Exe
cuted commands inherit the environment. The export and
declare -x commands allow parameters and functions to be
added to and deleted from the environment. If the value
of a parameter in the environment is modified, the new
value becomes part of the environment, replacing the old.
The environment inherited by any executed command consists
of the shell's initial environment, whose values may be
modified in the shell, less any pairs removed by the unset
command, plus any additions via the export and declare -x
commands.
The environment for any simple command or function may be
augmented temporarily by prefixing it with parameter
assignments, as described above in PARAMETERS. These
assignment statements affect only the environment seen by
that command.
If the -k option is set (see the set builtin command
below), then all parameter assignments are placed in the
environment for a command, not just those that precede the
command name.
When bash invokes an external command, the variable _ is
set to the full file name of the command and passed to
that command in its environment.
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EXIT STATUS
For the shell's purposes, a command which exits with a
zero exit status has succeeded. An exit status of zero
indicates success. A non-zero exit status indicates fail
ure. When a command terminates on a fatal signal, bash
uses the value of 128+signal as the exit status.
If a command is not found, the child process created to
execute it returns a status of 127. If a command is found
but is not executable, the return status is 126.
If a command fails because of an error during expansion or
redirection, the exit status is greater than zero.
Shell builtin commands return a status of 0 (true) if suc
cessful, and non-zero (false) if an error occurs while
they execute. All builtins return an exit status of 2 to
indicate incorrect usage.
Bash itself returns the exit status of the last command
executed, unless a syntax error occurs, in which case it
exits with a non-zero value. See also the exit builtin
command below.
SIGNALS
When bash is interactive, in the absence of any traps, it
ignores SIGTERM (so that kill 0 does not kill an interac
tive shell), and SIGINT is caught and handled (so that the
wait builtin is interruptible). In all cases, bash
ignores SIGQUIT. If job control is in effect, bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Synchronous jobs started by bash have signal handlers set
to the values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore SIGINT and SIGQUIT as well. Commands run as a
result of command substitution ignore the keyboard-gener
ated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP.
Before exiting, it resends the SIGHUP to all jobs, running
or stopped. Stopped jobs are sent SIGCONT to ensure that
they receive the SIGHUP. To prevent the shell from send
ing the signal to a particular job, it should be removed
from the jobs table with the disown builtin (see SHELL
BUILTIN COMMANDS below) or marked to not receive SIGHUP
using disown -h.
If the huponexit shell option has been set with shopt,
bash sends a SIGHUP to all jobs when an interactive login
shell exits.
When bash receives a signal for which a trap has been set
while waiting for a command to complete, the trap will not
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be executed until the command completes. When bash is
waiting for an asynchronous command via the wait builtin,
the reception of a signal for which a trap has been set
will cause the wait builtin to return immediately with an
exit status greater than 128, immediately after which the
trap is executed.
JOB CONTROL
Job control refers to the ability to selectively stop
(suspend) the execution of processes and continue (resume)
their execution at a later point. A user typically
employs this facility via an interactive interface sup
plied jointly by the system's terminal driver and bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed
with the jobs command. When bash starts a job asyn
chronously (in the background), it prints a line that
looks like:
[1] 25647
indicating that this job is job number 1 and that the pro
cess ID of the last process in the pipeline associated
with this job is 25647. All of the processes in a single
pipeline are members of the same job. Bash uses the job
abstraction as the basis for job control.
To facilitate the implementation of the user interface to
job control, the system maintains the notion of a current
terminal process group ID. Members of this process group
(processes whose process group ID is equal to the current
terminal process group ID) receive keyboard-generated sig
nals such as SIGINT. These processes are said to be in
the foreground. Background processes are those whose pro
cess group ID differs from the terminal's; such processes
are immune to keyboard-generated signals. Only foreground
processes are allowed to read from or write to the termi
nal. Background processes which attempt to read from
(write to) the terminal are sent a SIGTTIN (SIGTTOU) sig
nal by the terminal driver, which, unless caught, suspends
the process.
If the operating system on which bash is running supports
job control, bash allows you to use it. Typing the sus
pend character (typically ^Z, Control-Z) while a process
is running causes that process to be stopped and returns
you to bash. Typing the delayed suspend character (typi
cally ^Y, Control-Y) causes the process to be stopped when
it attempts to read input from the terminal, and control
to be returned to bash. The user may then manipulate the
state of this job, using the bg command to continue it in
the background, the fg command to continue it in the fore
ground, or the kill command to kill it. A ^Z takes effect
GNU 1999 Jan 20 41
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immediately, and has the additional side effect of causing
pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell.
The character % introduces a job name. Job number n may
be referred to as %n. A job may also be referred to using
a prefix of the name used to start it, or using a sub
string that appears in its command line. For example, %ce
refers to a stopped ce job. If a prefix matches more than
one job, bash reports an error. Using %?ce, on the other
hand, refers to any job containing the string ce in its
command line. If the substring matches more than one job,
bash reports an error. The symbols %% and %+ refer to the
shell's notion of the current job, which is the last job
stopped while it was in the foreground or started in the
background. The previous job may be referenced using %-.
In output pertaining to jobs (e.g., the output of the jobs
command), the current job is always flagged with a +, and
the previous job with a -.
Simply naming a job can be used to bring it into the fore
ground: %1 is a synonym for ``fg %1'', bringing job 1 from
the background into the foreground. Similarly, ``%1 &''
resumes job 1 in the background, equivalent to ``bg %1''.
The shell learns immediately whenever a job changes state.
Normally, bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not
interrupt any other output. If the -b option to the set
builtin command is enabled, bash reports such changes
immediately.
If an attempt to exit bash is made while jobs are stopped,
the shell prints a warning message. The jobs command may
then be used to inspect their status. If a second attempt
to exit is made without an intervening command, the shell
does not print another warning, and the stopped jobs are
terminated.
PROMPTING
When executing interactively, bash displays the primary
prompt PS1 when it is ready to read a command, and the
secondary prompt PS2 when it needs more input to complete
a command. Bash allows these prompt strings to be cus
tomized by inserting a number of backslash-escaped special
characters that are decoded as follows:
\a an ASCII bell character (07)
\d the date in "Weekday Month Date" format
(e.g., "Tue May 26")
\e an ASCII escape character (033)
\h the hostname up to the first `.'
\H the hostname
\n newline
\r carriage return
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\s the name of the shell, the basename of $0
(the portion following the final slash)
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\u the username of the current user
\v the version of bash (e.g., 2.00)
\V the release of bash, version + patchlevel
(e.g., 2.00.0)
\w the current working directory
\W the basename of the current working direc
tory
\! the history number of this command
\# the command number of this command
\$ if the effective UID is 0, a #, otherwise a
$
\nnn the character corresponding to the octal
number nnn
\\ a backslash
\[ begin a sequence of non-printing characters,
which could be used to embed a terminal con
trol sequence into the prompt
\] end a sequence of non-printing characters
The command number and the history number are usually dif
ferent: the history number of a command is its position in
the history list, which may include commands restored from
the history file (see HISTORY below), while the command
number is the position in the sequence of commands exe
cuted during the current shell session. After the string
is decoded, it is expanded via parameter expansion, com
mand substitution, arithmetic expansion, string expansion,
and quote removal, subject to the value of the promptvars
shell option (see the description of the shopt command
under SHELL BUILTIN COMMANDS below).
READLINE
This is the library that handles reading input when using
an interactive shell, unless the --noediting option is
given at shell invocation. By default, the line editing
commands are similar to those of emacs. A vi-style line
editing interface is also available. To turn off line
editing after the shell is running, use the +o emacs or +o
vi options to the set builtin (see SHELL BUILTIN COMMANDS
below).
Readline Notation
In this section, the emacs-style notation is used to
denote keystrokes. Control keys are denoted by C-key,
e.g., C-n means Control-N. Similarly, meta keys are
denoted by M-key, so M-x means Meta-X. (On keyboards
without a meta key, M-x means ESC x, i.e., press the
Escape key then the x key. This makes ESC the meta pre
fix. The combination M-C-x means ESC-Control-x, or press
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the Escape key then hold the Control key while pressing
the x key.)
Readline commands may be given numeric arguments, which
normally act as a repeat count. Sometimes, however, it is
the sign of the argument that is significant. Passing a
negative argument to a command that acts in the forward
direction (e.g., kill-line) causes that command to act in
a backward direction. Commands whose behavior with argu
ments deviates from this are noted below.
When a command is described as killing text, the text
deleted is saved for possible future retrieval (yanking).
The killed text is saved in a kill ring. Consecutive
kills cause the text to be accumulated into one unit,
which can be yanked all at once. Commands which do not
kill text separate the chunks of text on the kill ring.
Readline Initialization
Readline is customized by putting commands in an initial
ization file (the inputrc file). The name of this file is
taken from the value of the INPUTRC variable. If that
variable is unset, the default is ~/.inputrc. When a pro
gram which uses the readline library starts up, the ini
tialization file is read, and the key bindings and vari
ables are set. There are only a few basic constructs
allowed in the readline initialization file. Blank lines
are ignored. Lines beginning with a # are comments.
Lines beginning with a $ indicate conditional constructs.
Other lines denote key bindings and variable settings.
The default key-bindings may be changed with an inputrc
file. Other programs that use this library may add their
own commands and bindings.
For example, placing
M-Control-u: universal-argument
or
C-Meta-u: universal-argument
into the inputrc would make M-C-u execute the readline
command universal-argument.
The following symbolic character names are recognized:
RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE,
and TAB. In addition to command names, readline allows
keys to be bound to a string that is inserted when the key
is pressed (a macro).
Readline Key Bindings
The syntax for controlling key bindings in the inputrc
file is simple. All that is required is the name of the
command or the text of a macro and a key sequence to which
it should be bound. The name may be specified in one of
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two ways: as a symbolic key name, possibly with Meta- or
Control- prefixes, or as a key sequence. When using the
form keyname:function-name or macro, keyname is the name
of a key spelled out in English. For example:
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function univer
sal-argument, M-DEL is bound to the function back
ward-kill-word, and C-o is bound to run the macro
expressed on the right hand side (that is, to insert the
text > output into the line).
In the second form, "keyseq":function-name or macro, key
seq differs from keyname above in that strings denoting an
entire key sequence may be specified by placing the
sequence within double quotes. Some GNU Emacs style key
escapes can be used, as in the following example.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In this example, C-u is again bound to the function uni
versal-argument. C-x C-r is bound to the function
re-read-init-file, and ESC [ 1 1 ~ is bound to insert the
text Function Key 1. The full set of GNU Emacs style
escape sequences is
\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '
In addition to the GNU Emacs style escape sequences, a
second set of backslash escapes is available:
\a alert (bell)
\b backspace
\d delete
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\nnn the character whose ASCII code is the octal
value nnn (one to three digits)
\xnnn the character whose ASCII code is the hex
adecimal value nnn (one to three digits)
When entering the text of a macro, single or double quotes
must be used to indicate a macro definition. Unquoted
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text is assumed to be a function name. In the macro body,
the backslash escapes described above are expanded. Back
slash will quote any other character in the macro text,
including " and '.
Bash allows the current readline key bindings to be dis
played or modified with the bind builtin command. The
editing mode may be switched during interactive use by
using the -o option to the set builtin command (see SHELL
BUILTIN COMMANDS below).
Readline Variables
Readline has variables that can be used to further cus
tomize its behavior. A variable may be set in the inputrc
file with a statement of the form
set variable-name value
Except where noted, readline variables can take the values
On or Off. The variables and their default values are:
bell-style (audible)
Controls what happens when readline wants to ring
the terminal bell. If set to none, readline never
rings the bell. If set to visible, readline uses a
visible bell if one is available. If set to audi
ble, readline attempts to ring the terminal's bell.
comment-begin (``#'')
The string that is inserted when the readline
insert-comment command is executed. This command
is bound to M-# in emacs mode and to # in vi com
mand mode.
completion-ignore-case (Off)
If set to On, readline performs filename matching
and completion in a case-insensitive fashion.
completion-query-items (100)
This determines when the user is queried about
viewing the number of possible completions gener
ated by the possible-completions command. It may
be set to any integer value greater than or equal
to zero. If the number of possible completions is
greater than or equal to the value of this vari
able, the user is asked whether or not he wishes to
view them; otherwise they are simply listed on the
terminal.
convert-meta (On)
If set to On, readline will convert characters with
the eighth bit set to an ASCII key sequence by
stripping the eighth bit and prepending an escape
character (in effect, using escape as the meta pre
fix).
disable-completion (Off)
If set to On, readline will inhibit word comple
tion. Completion characters will be inserted into
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the line as if they had been mapped to self-insert.
editing-mode (emacs)
Controls whether readline begins with a set of key
bindings similar to emacs or vi. editing-mode can
be set to either emacs or vi.
enable-keypad (Off)
When set to On, readline will try to enable the
application keypad when it is called. Some systems
need this to enable the arrow keys.
expand-tilde (Off)
If set to on, tilde expansion is performed when
readline attempts word completion.
horizontal-scroll-mode (Off)
When set to On, makes readline use a single line
for display, scrolling the input horizontally on a
single screen line when it becomes longer than the
screen width rather than wrapping to a new line.
input-meta (Off)
If set to On, readline will enable eight-bit input
(that is, it will not strip the high bit from the
characters it reads), regardless of what the termi
nal claims it can support. The name meta-flag is a
synonym for this variable.
isearch-terminators (``C-[C-J'')
The string of characters that should terminate an
incremental search without subsequently executing
the character as a command. If this variable has
not been given a value, the characters ESC and C-J
will terminate an incremental search.
keymap (emacs)
Set the current readline keymap. The set of valid
keymap names is emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-command, and vi-insert. vi is
equivalent to vi-command; emacs is equivalent to
emacs-standard. The default value is emacs; the
value of editing-mode also affects the default
keymap.
mark-directories (On)
If set to On, completed directory names have a
slash appended.
mark-modified-lines (Off)
If set to On, history lines that have been modified
are displayed with a preceding asterisk (*).
output-meta (Off)
If set to On, readline will display characters with
the eighth bit set directly rather than as a meta-
prefixed escape sequence.
print-completions-horizontally (Off)
If set to On, readline will display completions
with matches sorted horizontally in alphabetical
order, rather than down the screen.
show-all-if-ambiguous (Off)
This alters the default behavior of the completion
functions. If set to on, words which have more
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than one possible completion cause the matches to
be listed immediately instead of ringing the bell.
visible-stats (Off)
If set to On, a character denoting a file's type as
reported by stat(2) is appended to the filename
when listing possible completions.
Readline Conditional Constructs
Readline implements a facility similar in spirit to the
conditional compilation features of the C preprocessor
which allows key bindings and variable settings to be per
formed as the result of tests. There are four parser
directives used.
$if The $if construct allows bindings to be made based
on the editing mode, the terminal being used, or
the application using readline. The text of the
test extends to the end of the line; no characters
are required to isolate it.
mode The mode= form of the $if directive is used
to test whether readline is in emacs or vi
mode. This may be used in conjunction with
the set keymap command, for instance, to set
bindings in the emacs-standard and
emacs-ctlx keymaps only if readline is
starting out in emacs mode.
term The term= form may be used to include termi
nal-specific key bindings, perhaps to bind
the key sequences output by the terminal's
function keys. The word on the right side
of the = is tested against the both full
name of the terminal and the portion of the
terminal name before the first -. This
allows sun to match both sun and sun-cmd,
for instance.
application
The application construct is used to include
application-specific settings. Each program
using the readline library sets the applica
tion name, and an initialization file can
test for a particular value. This could be
used to bind key sequences to functions use
ful for a specific program. For instance,
the following command adds a key sequence
that quotes the current or previous word in
Bash:
$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
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$endif This command, as seen in the previous example, ter
minates an $if command.
$else Commands in this branch of the $if directive are
executed if the test fails.
$include
This directive takes a single filename as an argu
ment and reads commands and bindings from that
file. For example, the following directive would
read /etc/inputrc:
$include /etc/inputrc
Searching
Readline provides commands for searching through the com
mand history (see HISTORY below) for lines containing a
specified string. There are two search modes: incremental
and non-incremental.
Incremental searches begin before the user has finished
typing the search string. As each character of the search
string is typed, readline displays the next entry from the
history matching the string typed so far. An incremental
search requires only as many characters as needed to find
the desired history entry. The characters present in the
value of the isearch-terminators variable are used to ter
minate an incremental search. If that variable has not
been assigned a value the Escape and Control-J characters
will terminate an incremental search. Control-G will
abort an incremental search and restore the original line.
When the search is terminated, the history entry contain
ing the search string becomes the current line. To find
other matching entries in the history list, type Control-S
or Control-R as appropriate. This will search backward or
forward in the history for the next entry matching the
search string typed so far. Any other key sequence bound
to a readline command will terminate the search and exe
cute that command. For instance, a newline will terminate
the search and accept the line, thereby executing the com
mand from the history list.
Non-incremental searches read the entire search string
before starting to search for matching history lines. The
search string may be typed by the user or be part of the
contents of the current line.
Readline Command Names
The following is a list of the names of the commands and
the default key sequences to which they are bound. Com
mand names without an accompanying key sequence are
unbound by default.
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Commands for Moving
beginning-of-line (C-a)
Move to the start of the current line.
end-of-line (C-e)
Move to the end of the line.
forward-char (C-f)
Move forward a character.
backward-char (C-b)
Move back a character.
forward-word (M-f)
Move forward to the end of the next word. Words
are composed of alphanumeric characters (letters
and digits).
backward-word (M-b)
Move back to the start of this, or the previous,
word. Words are composed of alphanumeric charac
ters (letters and digits).
clear-screen (C-l)
Clear the screen leaving the current line at the
top of the screen. With an argument, refresh the
current line without clearing the screen.
redraw-current-line
Refresh the current line.
Commands for Manipulating the History
accept-line (Newline, Return)
Accept the line regardless of where the cursor is.
If this line is non-empty, add it to the history
list according to the state of the HISTCONTROL
variable. If the line is a modified history line,
then restore the history line to its original
state.
previous-history (C-p)
Fetch the previous command from the history list,
moving back in the list.
next-history (C-n)
Fetch the next command from the history list, mov
ing forward in the list.
beginning-of-history (M-<)
Move to the first line in the history.
end-of-history (M->)
Move to the end of the input history, i.e., the
line currently being entered.
reverse-search-history (C-r)
Search backward starting at the current line and
moving `up' through the history as necessary. This
is an incremental search.
forward-search-history (C-s)
Search forward starting at the current line and
moving `down' through the history as necessary.
This is an incremental search.
non-incremental-reverse-search-history (M-p)
Search backward through the history starting at the
current line using a non-incremental search for a
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string supplied by the user.
non-incremental-forward-search-history (M-n)
Search forward through the history using a non-
incremental search for a string supplied by the
user.
history-search-forward
Search forward through the history for the string
of characters between the start of the current line
and the current cursor position (the point). This
is a non-incremental search.
history-search-backward
Search backward through the history for the string
of characters between the start of the current line
and the point. This is a non-incremental search.
yank-nth-arg (M-C-y)
Insert the first argument to the previous command
(usually the second word on the previous line) at
point (the current cursor position). With an argu
ment n, insert the nth word from the previous com
mand (the words in the previous command begin with
word 0). A negative argument inserts the nth word
from the end of the previous command.
yank-last-arg (M-., M-_)
Insert the last argument to the previous command
(the last word of the previous history entry).
With an argument, behave exactly like yank-nth-arg.
Successive calls to yank-last-arg move back through
the history list, inserting the last argument of
each line in turn.
shell-expand-line (M-C-e)
Expand the line as the shell does. This performs
alias and history expansion as well as all of the
shell word expansions. See HISTORY EXPANSION below
for a description of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description of his
tory expansion.
magic-space
Perform history expansion on the current line and
insert a space. See HISTORY EXPANSION below for a
description of history expansion.
alias-expand-line
Perform alias expansion on the current line. See
ALIASES above for a description of alias expansion.
history-and-alias-expand-line
Perform history and alias expansion on the current
line.
insert-last-argument (M-., M-_)
A synonym for yank-last-arg.
operate-and-get-next (C-o)
Accept the current line for execution and fetch the
next line relative to the current line from the
history for editing. Any argument is ignored.
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Commands for Changing Text
delete-char (C-d)
Delete the character under the cursor. If point is
at the beginning of the line, there are no charac
ters in the line, and the last character typed was
not bound to delete-char, then return EOF.
backward-delete-char (Rubout)
Delete the character behind the cursor. When given
a numeric argument, save the deleted text on the
kill ring.
forward-backward-delete-char
Delete the character under the cursor, unless the
cursor is at the end of the line, in which case the
character behind the cursor is deleted. By
default, this is not bound to a key.
quoted-insert (C-q, C-v)
Add the next character typed to the line verbatim.
This is how to insert characters like C-q, for
example.
tab-insert (C-v TAB)
Insert a tab character.
self-insert (a, b, A, 1, !, ...)
Insert the character typed.
transpose-chars (C-t)
Drag the character before point forward over the
character at point. Point moves forward as well.
If point is at the end of the line, then transpose
the two characters before point. Negative argu
ments don't work.
transpose-words (M-t)
Drag the word behind the cursor past the word in
front of the cursor moving the cursor over that
word as well.
upcase-word (M-u)
Uppercase the current (or following) word. With a
negative argument, uppercase the previous word, but
do not move point.
downcase-word (M-l)
Lowercase the current (or following) word. With a
negative argument, lowercase the previous word, but
do not move point.
capitalize-word (M-c)
Capitalize the current (or following) word. With a
negative argument, capitalize the previous word,
but do not move point.
Killing and Yanking
kill-line (C-k)
Kill the text from the current cursor position to
the end of the line.
backward-kill-line (C-x Rubout)
Kill backward to the beginning of the line.
unix-line-discard (C-u)
Kill backward from point to the beginning of the
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line. The killed text is saved on the kill-ring.
kill-whole-line
Kill all characters on the current line, no matter
where the cursor is.
kill-word (M-d)
Kill from the cursor to the end of the current
word, or if between words, to the end of the next
word. Word boundaries are the same as those used
by forward-word.
backward-kill-word (M-Rubout)
Kill the word behind the cursor. Word boundaries
are the same as those used by backward-word.
unix-word-rubout (C-w)
Kill the word behind the cursor, using white space
as a word boundary. The word boundaries are dif
ferent from backward-kill-word.
delete-horizontal-space (M-\)
Delete all spaces and tabs around point.
kill-region
Kill the text between the point and mark (saved
cursor position). This text is referred to as the
region.
copy-region-as-kill
Copy the text in the region to the kill buffer.
copy-backward-word
Copy the word before point to the kill buffer. The
word boundaries are the same as backward-word.
copy-forward-word
Copy the word following point to the kill buffer.
The word boundaries are the same as forward-word.
yank (C-y)
Yank the top of the kill ring into the buffer at
the cursor.
yank-pop (M-y)
Rotate the kill ring, and yank the new top. Only
works following yank or yank-pop.
Numeric Arguments
digit-argument (M-0, M-1, ..., M--)
Add this digit to the argument already accumulat
ing, or start a new argument. M-- starts a nega
tive argument.
universal-argument
This is another way to specify an argument. If
this command is followed by one or more digits,
optionally with a leading minus sign, those digits
define the argument. If the command is followed by
digits, executing universal-argument again ends the
numeric argument, but is otherwise ignored. As a
special case, if this command is immediately fol
lowed by a character that is neither a digit or
minus sign, the argument count for the next command
is multiplied by four. The argument count is
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initially one, so executing this function the first
time makes the argument count four, a second time
makes the argument count sixteen, and so on.
Completing
complete (TAB)
Attempt to perform completion on the text before
point. Bash attempts completion treating the text
as a variable (if the text begins with $), username
(if the text begins with ~), hostname (if the text
begins with @), or command (including aliases and
functions) in turn. If none of these produces a
match, filename completion is attempted.
possible-completions (M-?)
List the possible completions of the text before
point.
insert-completions (M-*)
Insert all completions of the text before point
that would have been generated by possible-comple
tions.
menu-complete
Similar to complete, but replaces the word to be
completed with a single match from the list of pos
sible completions. Repeated execution of menu-com
plete steps through the list of possible comple
tions, inserting each match in turn. At the end of
the list of completions, the bell is rung and the
original text is restored. An argument of n moves
n positions forward in the list of matches; a nega
tive argument may be used to move backward through
the list. This command is intended to be bound to
TAB, but is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at
the beginning or end of the line (like delete-
char). If at the end of the line, behaves identi
cally to possible-completions. This command is
unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before
point.
possible-filename-completions (C-x /)
List the possible completions of the text before
point, treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treat
ing it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before
point, treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treat
ing it as a shell variable.
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possible-variable-completions (C-x $)
List the possible completions of the text before
point, treating it as a shell variable.
complete-hostname (M-@)
Attempt completion on the text before point, treat
ing it as a hostname.
possible-hostname-completions (C-x @)
List the possible completions of the text before
point, treating it as a hostname.
complete-command (M-!)
Attempt completion on the text before point, treat
ing it as a command name. Command completion
attempts to match the text against aliases,
reserved words, shell functions, shell builtins,
and finally executable filenames, in that order.
possible-command-completions (C-x !)
List the possible completions of the text before
point, treating it as a command name.
dynamic-complete-history (M-TAB)
Attempt completion on the text before point, com
paring the text against lines from the history list
for possible completion matches.
complete-into-braces (M-{)
Perform filename completion and return the list of
possible completions enclosed within braces so the
list is available to the shell (see Brace Expansion
above).
Keyboard Macros
start-kbd-macro (C-x ()
Begin saving the characters typed into the current
keyboard macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current
keyboard macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by mak
ing the characters in the macro appear as if typed
at the keyboard.
Miscellaneous
re-read-init-file (C-x C-r)
Read in the contents of the inputrc file, and
incorporate any bindings or variable assignments
found there.
abort (C-g)
Abort the current editing command and ring the ter
minal's bell (subject to the setting of
bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)
If the metafied character x is lowercase, run the
command that is bound to the corresponding upper
case character.
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prefix-meta (ESC)
Metafy the next character typed. ESC f is equiva
lent to Meta-f.
undo (C-_, C-x C-u)
Incremental undo, separately remembered for each
line.
revert-line (M-r)
Undo all changes made to this line. This is like
executing the undo command enough times to return
the line to its initial state.
tilde-expand (M-&)
Perform tilde expansion on the current word.
set-mark (C-@, M-<space>)
Set the mark to the current point. If a numeric
argument is supplied, the mark is set to that posi
tion.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor
position is set to the saved position, and the old
cursor position is saved as the mark.
character-search (C-])
A character is read and point is moved to the next
occurrence of that character. A negative count
searches for previous occurrences.
character-search-backward (M-C-])
A character is read and point is moved to the pre
vious occurrence of that character. A negative
count searches for subsequent occurrences.
insert-comment (M-#)
The value of the readline comment-begin variable is
inserted at the beginning of the current line, and
the line is accepted as if a newline had been
typed. This makes the current line a shell com
ment.
glob-expand-word (C-x *)
The word before point is treated as a pattern for
pathname expansion, and the list of matching file
names is inserted, replacing the word.
glob-list-expansions (C-x g)
The list of expansions that would have been gener
ated by glob-expand-word is displayed, and the line
is redrawn.
dump-functions
Print all of the functions and their key bindings
to the readline output stream. If a numeric argu
ment is supplied, the output is formatted in such a
way that it can be made part of an inputrc file.
dump-variables
Print all of the settable readline variables and
their values to the readline output stream. If a
numeric argument is supplied, the output is format
ted in such a way that it can be made part of an
inputrc file.
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dump-macros
Print all of the readline key sequences bound to
macros and the strings they ouput. If a numeric
argument is supplied, the output is formatted in
such a way that it can be made part of an inputrc
file.
display-shell-version (C-x C-v)
Display version information about the current
instance of bash.
HISTORY
When the -o history option to the set builtin is enabled,
the shell provides access to the command history, the list
of commands previously typed. The text of the last HIST
SIZE commands (default 500) is saved in a history list.
The shell stores each command in the history list prior to
parameter and variable expansion (see EXPANSION above) but
after history expansion is performed, subject to the val
ues of the shell variables HISTIGNORE and HISTCONTROL. On
startup, the history is initialized from the file named by
the variable HISTFILE (default ~/.bash_history). HISTFILE
is truncated, if necessary, to contain no more than HIST
FILESIZE lines. When an interactive shell exits, the last
HISTSIZE lines are copied from the history list to HIST
FILE. If the histappend shell option is enabled (see the
description of shopt under SHELL BUILTIN COMMANDS below),
the lines are appended to the history file, otherwise the
history file is overwritten. If HISTFILE is unset, or if
the history file is unwritable, the history is not saved.
After saving the history, the history file is truncated to
contain no more than HISTFILESIZE lines. If HISTFILESIZE
is not set, no truncation is performed.
The builtin command fc (see SHELL BUILTIN COMMANDS below)
may be used to list or edit and re-execute a portion of
the history list. The history builtin can be used to dis
play or modify the history list and manipulate the history
file. When using the command-line editing, search com
mands are available in each editing mode that provide
access to the history list.
The shell allows control over which commands are saved on
the history list. The HISTCONTROL and HISTIGNORE vari
ables may be set to cause the shell to save only a subset
of the commands entered. The cmdhist shell option, if
enabled, causes the shell to attempt to save each line of
a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correct
ness. The lithist shell option causes the shell to save
the command with embedded newlines instead of semicolons.
See the description of the shopt builtin below under SHELL
BUILTIN COMMANDS for information on setting and unsetting
shell options.
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HISTORY EXPANSION
The shell supports a history expansion feature that is
similar to the history expansion in csh. This section
describes what syntax features are available. This fea
ture is enabled by default for interactive shells, and can
be disabled using the +H option to the set builtin command
(see SHELL BUILTIN COMMANDS below). Non-interactive
shells do not perform history expansion by default.
History expansions introduce words from the history list
into the input stream, making it easy to repeat commands,
insert the arguments to a previous command into the cur
rent input line, or fix errors in previous commands
quickly.
History expansion is performed immediately after a com
plete line is read, before the shell breaks it into words.
It takes place in two parts. The first is to determine
which line from the history list to use during substitu
tion. The second is to select portions of that line for
inclusion into the current one. The line selected from
the history is the event, and the portions of that line
that are acted upon are words. Various modifiers are
available to manipulate the selected words. The line is
broken into words in the same fashion as when reading
input, so that several metacharacter-separated words sur
rounded by quotes are considered one word. History expan
sions are introduced by the appearance of the history
expansion character, which is ! by default. Only back
slash (\) and single quotes can quote the history expan
sion character.
Several shell options settable with the shopt builtin may
be used to tailor the behavior of history expansion. If
the histverify shell option is enabled (see the descrip
tion of the shopt builtin), and readline is being used,
history substitutions are not immediately passed to the
shell parser. Instead, the expanded line is reloaded into
the readline editing buffer for further modification. If
readline is being used, and the histreedit shell option is
enabled, a failed history substitution will be reloaded
into the readline editing buffer for correction. The -p
option to the history builtin command may be used to see
what a history expansion will do before using it. The -s
option to the history builtin may be used to add commands
to the end of the history list without actually executing
them, so that they are available for subsequent recall.
The shell allows control of the various characters used by
the history expansion mechanism (see the description of
histchars above under Shell Variables).
Event Designators
An event designator is a reference to a command line entry
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in the history list.
! Start a history substitution, except when followed
by a blank, newline, = or (.
!n Refer to command line n.
!-n Refer to the current command line minus n.
!! Refer to the previous command. This is a synonym
for `!-1'.
!string
Refer to the most recent command starting with
string.
!?string[?]
Refer to the most recent command containing string.
The trailing ? may be omitted if string is followed
immediately by a newline.
^string1^string2^
Quick substitution. Repeat the last command,
replacing string1 with string2. Equivalent to
``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.
Word Designators
Word designators are used to select desired words from the
event. A : separates the event specification from the
word designator. It may be omitted if the word designator
begins with a ^, $, *, -, or %. Words are numbered from
the beginning of the line, with the first word being
denoted by 0 (zero). Words are inserted into the current
line separated by single spaces.
0 (zero)
The zeroth word. For the shell, this is the com
mand word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?'
search.
x-y A range of words; `-y' abbreviates `0-y'.
* All of the words but the zeroth. This is a synonym
for `1-$'. It is not an error to use * if there is
just one word in the event; the empty string is
returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.
If a word designator is supplied without an event specifi
cation, the previous command is used as the event.
Modifiers
After the optional word designator, there may appear a
sequence of one or more of the following modifiers, each
preceded by a `:'.
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h Remove a trailing file name component, leaving only
the head.
t Remove all leading file name components, leaving
the tail.
r Remove a trailing suffix of the form .xxx, leaving
the basename.
e Remove all but the trailing suffix.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further sub
stitutions.
x Quote the substituted words as with q, but break
into words at blanks and newlines.
s/old/new/
Substitute new for the first occurrence of old in
the event line. Any delimiter can be used in place
of /. The final delimiter is optional if it is the
last character of the event line. The delimiter
may be quoted in old and new with a single back
slash. If & appears in new, it is replaced by old.
A single backslash will quote the &. If old is
null, it is set to the last old substituted, or, if
no previous history substitutions took place, the
last string in a !?string[?] search.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event
line. This is used in conjunction with `:s' (e.g.,
`:gs/old/new/') or `:&'. If used with `:s', any
delimiter can be used in place of /, and the final
delimiter is optional if it is the last character
of the event line.
SHELL BUILTIN COMMANDS
Unless otherwise noted, each builtin command documented in
this section as accepting options preceded by - accepts --
to signify the end of the options.
: [arguments]
No effect; the command does nothing beyond expand
ing arguments and performing any specified redirec
tions. A zero exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the cur
rent shell environment and return the exit status
of the last command executed from filename. If
filename does not contain a slash, file names in
PATH are used to find the directory containing
filename. The file searched for in PATH need not
be executable. The current directory is searched
if no file is found in PATH. If the sourcepath
option to the shopt builtin command is turned off,
the PATH is not searched. If any arguments are
supplied, they become the positional parameters
when filename is executed. Otherwise the
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positional parameters are unchanged. The return
status is the status of the last command exited
within the script (0 if no commands are executed),
and false if filename is not found or cannot be
read.
alias [-p] [name[=value] ...]
Alias with no arguments or with the -p option
prints the list of aliases in the form alias
name=value on standard output. When arguments are
supplied, an alias is defined for each name whose
value is given. A trailing space in value causes
the next word to be checked for alias substitution
when the alias is expanded. For each name in the
argument list for which no value is supplied, the
name and value of the alias is printed. Alias
returns true unless a name is given for which no
alias has been defined.
bg [jobspec]
Resume the suspended job jobspec in the background,
as if it had been started with &. If jobspec is
not present, the shell's notion of the current job
is used. bg jobspec returns 0 unless run when job
control is disabled or, when run with job control
enabled, if jobspec was not found or started with
out job control.
bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] keyseq:function-name
Display current readline key and function bindings,
or bind a key sequence to a readline function or
macro. The binding syntax accepted is identical to
that of .inputrc, but each binding must be passed
as a separate argument; e.g., '"\C-x\C-r":
re-read-init-file'. Options, if supplied, have the
following meanings:
-m keymap
Use keymap as the keymap to be affected by
the subsequent bindings. Acceptable keymap
names are emacs, emacs-standard, emacs-meta,
emacs-ctlx, vi, vi-command, and vi-insert.
vi is equivalent to vi-command; emacs is
equivalent to emacs-standard.
-l List the names of all readline functions.
-p Display readline function names and bindings
in such a way that they can be re-read.
-P List current readline function names and
bindings.
-v Display readline variable names and values
in such a way that they can be re-read.
-V List current readline variable names and
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values.
-s Display readline key sequences bound to
macros and the strings they output in such a
way that they can be re-read.
-S Display readline key sequences bound to
macros and the strings they output.
-f filename
Read key bindings from filename.
-q function
Query about which keys invoke the named
function.
-u function
Unbind all keys bound to the named function.
-r keyseq
Remove any current binding for keyseq.
The return value is 0 unless an unrecognized option
is given or an error occurred.
break [n]
Exit from within a for, while, until, or select
loop. If n is specified, break n levels. n must
be >= 1. If n is greater than the number of
enclosing loops, all enclosing loops are exited.
The return value is 0 unless the shell is not exe
cuting a loop when break is executed.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it
arguments, and return its exit status. This is
useful when defining a function whose name is the
same as a shell builtin, retaining the functional
ity of the builtin within the function. The cd
builtin is commonly redefined this way. The return
status is false if shell-builtin is not a shell
builtin command.
cd [-LP] [dir]
Change the current directory to dir. The variable
HOME is the default dir. The variable CDPATH
defines the search path for the directory contain
ing dir. Alternative directory names in CDPATH are
separated by a colon (:). A null directory name in
CDPATH is the same as the current directory, i.e.,
``.''. If dir begins with a slash (/), then CDPATH
is not used. The -P option says to use the physical
directory structure instead of following symbolic
links (see also the -P option to the set builtin
command); the -L option forces symbolic links to be
followed. An argument of - is equivalent to $OLD
PWD. The return value is true if the directory was
successfully changed; false otherwise.
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command [-pVv] command [arg ...]
Run command with args suppressing the normal shell
function lookup. Only builtin commands or commands
found in the PATH are executed. If the -p option
is given, the search for command is performed using
a default value for PATH that is guaranteed to find
all of the standard utilities. If either the -V or
-v option is supplied, a description of command is
printed. The -v option causes a single word indi
cating the command or file name used to invoke com
mand to be displayed; the -V option produces a more
verbose description. If the -V or -v option is
supplied, the exit status is 0 if command was
found, and 1 if not. If neither option is supplied
and an error occurred or command cannot be found,
the exit status is 127. Otherwise, the exit status
of the command builtin is the exit status of com
mand.
continue [n]
Resume the next iteration of the enclosing for,
while, until, or select loop. If n is specified,
resume at the nth enclosing loop. n must be >= 1.
If n is greater than the number of enclosing loops,
the last enclosing loop (the ``top-level'' loop) is
resumed. The return value is 0 unless the shell is
not executing a loop when continue is executed.
declare [-afFirx] [-p] [name[=value]]
typeset [-afFirx] [-p] [name[=value]]
Declare variables and/or give them attributes. If
no names are given then display the values of vari
ables. The -p option will display the attributes
and values of each name. When -p is used, addi
tional options are ignored. The -F option inhibits
the display of function definitions; only the func
tion name and attributes are printed. The -F
option implies -f. The following options can be
used to restrict output to variables with the spec
ified attribute or to give variables attributes:
-a Each name is an array variable (see Arrays
above).
-f Use function names only.
-i The variable is treated as an integer;
arithmetic evaluation (see ARITHMETIC EVALU
ATION ) is performed when the variable is
assigned a value.
-r Make names readonly. These names cannot
then be assigned values by subsequent
assignment statements or unset.
-x Mark names for export to subsequent commands
via the environment.
Using `+' instead of `-' turns off the attribute
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instead, with the exception that +a may not be used
to destroy an array variable. When used in a func
tion, makes each name local, as with the local com
mand. The return value is 0 unless an invalid
option is encountered, an attempt is made to define
a function using "-f foo=bar", an attempt is made
to assign a value to a readonly variable, an
attempt is made to assign a value to an array vari
able without using the compound assignment syntax
(see Arrays above), one of the names is not a valid
shell variable name, an attempt is made to turn off
readonly status for a readonly variable, an attempt
is made to turn off array status for an array vari
able, or an attempt is made to display a non-exis
tent function with -f.
dirs [-clpv] [+n] [-n]
Without options, displays the list of currently
remembered directories. The default display is on
a single line with directory names separated by
spaces. Directories are added to the list with the
pushd command; the popd command removes entries
from the list.
+n Displays the nth entry counting from the
left of the list shown by dirs when invoked
without options, starting with zero.
-n Displays the nth entry counting from the
right of the list shown by dirs when invoked
without options, starting with zero.
-c Clears the directory stack by deleting all
of the entries.
-l Produces a longer listing; the default list
ing format uses a tilde to denote the home
directory.
-p Print the directory stack with one entry per
line.
-v Print the directory stack with one entry per
line, prefixing each entry with its index in
the stack.
The return value is 0 unless an invalid option is
supplied or n indexes beyond the end of the direc
tory stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the
table of active jobs. If the -h option is given,
each jobspec is not removed from the table, but is
marked so that SIGHUP is not sent to the job if the
shell receives a SIGHUP. If no jobspec is present,
and neither the -a nor the -r option is supplied,
the current job is used. If no jobspec is sup
plied, the -a option means to remove or mark all
jobs; the -r option without a jobspec argument
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restricts operation to running jobs. The return
value is 0 unless a jobspec does not specify a
valid job.
echo [-neE] [arg ...]
Output the args, separated by spaces, followed by a
newline. The return status is always 0. If -n is
specified, the trailing newline is suppressed. If
the -e option is given, interpretation of the fol
lowing backslash-escaped characters is enabled.
The -E option disables the interpretation of these
escape characters, even on systems where they are
interpreted by default. echo does not interpret --
to mean the end of options. echo interprets the
following escape sequences:
\a alert (bell)
\b backspace
\c suppress trailing newline
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is the octal
value nnn (one to three digits)
\xnnn the character whose ASCII code is the hex
adecimal value nnn (one to three digits)
enable [-adnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Dis
abling a builtin allows a disk command which has
the same name as a shell builtin to be executed
with specifying a full pathname, even though the
shell normally searches for builtins before disk
commands. If -n is used, each name is disabled;
otherwise, names are enabled. For example, to use
the test binary found via the PATH instead of the
shell builtin version, run enable -n test. The -f
option means to load the new builtin command name
from shared object filename, on systems that sup
port dynamic loading. The -d option will delete a
builtin previously loaded with -f. If no name
arguments are given, or if the -p option is sup
plied, a list of shell builtins is printed. With
no other option arguments, the list consists of all
enabled shell builtins. If -n is supplied, only
disabled builtins are printed. If -a is supplied,
the list printed includes all builtins, with an
indication of whether or not each is enabled. If
-s is supplied, the output is restricted to the
POSIX special builtins. The return value is 0
unless a name is not a shell builtin or there is a
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problem loading a new builtin from a shared object.
eval [arg ...]
The args are read and concatenated together into a
single command. This command is then read and exe
cuted by the shell, and its exit status is returned
as the value of eval. If there are no args, or
only null arguments, eval returns 0.
exec [-cl] [-a name] [command [arguments]]
If command is specified, it replaces the shell. No
new process is created. The arguments become the
arguments to command. If the -l option is sup
plied, the shell places a dash in the zeroth arg
passed to command. This is what login(1) does.
The -c option causes command to be executed with an
empty environment. If -a is supplied, the shell
passes name as the zeroth argument to the executed
command. If command cannot be executed for some
reason, a non-interactive shell exits, unless the
shell option execfail is enabled, in which case it
returns failure. An interactive shell returns
failure if the file cannot be executed. If command
is not specified, any redirections take effect in
the current shell, and the return status is 0. If
there is a redirection error, the return status is
1.
exit [n]
Cause the shell to exit with a status of n. If n
is omitted, the exit status is that of the last
command executed. A trap on EXIT is executed
before the shell terminates.
export [-fn] [name[=word]] ...
export -p
The supplied names are marked for automatic export
to the environment of subsequently executed com
mands. If the -f option is given, the names refer
to functions. If no names are given, or if the -p
option is supplied, a list of all names that are
exported in this shell is printed. The -n option
causes the export property to be removed from the
named variables. export returns an exit status of
0 unless an invalid option is encountered, one of
the names is not a valid shell variable name, or -f
is supplied with a name that is not a function.
fc [-e ename] [-nlr] [first] [last]
fc -s [pat=rep] [cmd]
Fix Command. In the first form, a range of com
mands from first to last is selected from the his
tory list. First and last may be specified as a
string (to locate the last command beginning with
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that string) or as a number (an index into the his
tory list, where a negative number is used as an
offset from the current command number). If last
is not specified it is set to the current command
for listing (so that fc -l -10 prints the last 10
commands) and to first otherwise. If first is not
specified it is set to the previous command for
editing and -16 for listing.
The -n option suppresses the command numbers when
listing. The -r option reverses the order of the
commands. If the -l option is given, the commands
are listed on standard output. Otherwise, the edi
tor given by ename is invoked on a file containing
those commands. If ename is not given, the value
of the FCEDIT variable is used, and the value of
EDITOR if FCEDIT is not set. If neither variable
is set, vi is used. When editing is complete, the
edited commands are echoed and executed.
In the second form, command is re-executed after
each instance of pat is replaced by rep. A useful
alias to use with this is ``r=fc -s'', so that typ
ing ``r cc'' runs the last command beginning with
``cc'' and typing ``r'' re-executes the last com
mand.
If the first form is used, the return value is 0
unless an invalid option is encountered or first or
last specify history lines out of range. If the -e
option is supplied, the return value is the value
of the last command executed or failure if an error
occurs with the temporary file of commands. If the
second form is used, the return status is that of
the command re-executed, unless cmd does not spec
ify a valid history line, in which case fc returns
failure.
fg [jobspec]
Resume jobspec in the foreground, and make it the
current job. If jobspec is not present, the
shell's notion of the current job is used. The
return value is that of the command placed into the
foreground, or failure if run when job control is
disabled or, when run with job control enabled, if
jobspec does not specify a valid job or jobspec
specifies a job that was started without job con
trol.
getopts optstring name [args]
getopts is used by shell procedures to parse posi
tional parameters. optstring contains the option
letters to be recognized; if a letter is followed
by a colon, the option is expected to have an
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argument, which should be separated from it by
white space. Each time it is invoked, getopts
places the next option in the shell variable name,
initializing name if it does not exist, and the
index of the next argument to be processed into the
variable OPTIND. OPTIND is initialized to 1 each
time the shell or a shell script is invoked. When
an option requires an argument, getopts places that
argument into the variable OPTARG. The shell does
not reset OPTIND automatically; it must be manually
reset between multiple calls to getopts within the
same shell invocation if a new set of parameters is
to be used.
When the end of options is encountered, getopts
exits with a return value greater than zero.
OPTIND is set to the index of the first non-option
argument, and name is set to ?.
getopts normally parses the positional parameters,
but if more arguments are given in args, getopts
parses those instead.
getopts can report errors in two ways. If the
first character of optstring is a colon, silent
error reporting is used. In normal operation diag
nostic messages are printed when invalid options or
missing option arguments are encountered. If the
variable OPTERR is set to 0, no error messages will
be displayed, even if the first character of opt
string is not a colon.
If an invalid option is seen, getopts places ? into
name and, if not silent, prints an error message
and unsets OPTARG. If getopts is silent, the
option character found is placed in OPTARG and no
diagnostic message is printed.
If a required argument is not found, and getopts is
not silent, a question mark (?) is placed in name,
OPTARG is unset, and a diagnostic message is
printed. If getopts is silent, then a colon (:) is
placed in name and OPTARG is set to the option
character found.
getopts returns true if an option, specified or
unspecified, is found. It returns false if the end
of options is encountered or an error occurs.
hash [-r] [-p filename] [name]
For each name, the full file name of the command is
determined by searching the directories in $PATH
and remembered. If the -p option is supplied, no
path search is performed, and filename is used as
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the full file name of the command. The -r option
causes the shell to forget all remembered loca
tions. If no arguments are given, information
about remembered commands is printed. The return
status is true unless a name is not found or an
invalid option is supplied.
help [pattern]
Display helpful information about builtin commands.
If pattern is specified, help gives detailed help
on all commands matching pattern; otherwise help
for all the builtins and shell control structures
is printed. The return status is 0 unless no com
mand matches pattern.
history [-c] [n]
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]
With no options, display the command history list
with line numbers. Lines listed with a * have been
modified. An argument of n lists only the last n
lines. If filename is supplied, it is used as the
name of the history file; if not, the value of
HISTFILE is used. Options, if supplied, have the
following meanings:
-a Append the ``new'' history lines (history
lines entered since the beginning of the
current bash session) to the history file.
-n Read the history lines not already read from
the history file into the current history
list. These are lines appended to the his
tory file since the beginning of the current
bash session.
-r Read the contents of the history file and
use them as the current history.
-w Write the current history to the history
file, overwriting the history file's con
tents.
-c Clear the history list by deleting all the
entries.
-p Perform history substitution on the follow
ing args and display the result on the stan
dard output. Does not store the results in
the history list. Each arg must be quoted
to disable normal history expansion.
-s Store the args in the history list as a sin
gle entry. The last command in the history
list is removed before the args are added.
The return value is 0 unless an invalid option is
encountered or an error occurs while reading or
writing the history file.
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jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]
The first form lists the active jobs. The options
have the following meanings:
-l List process IDs in addition to the normal
information.
-p List only the process ID of the job's pro
cess group leader.
-n Display information only about jobs that
have changed status since the user was last
notified of their status.
-r Restrict output to running jobs.
-s Restrict output to stopped jobs.
If jobspec is given, output is restricted to infor
mation about that job. The return status is 0
unless an invalid option is encountered or an
invalid jobspec is supplied.
If the -x option is supplied, jobs replaces any
jobspec found in command or args with the corre
sponding process group ID, and executes command
passing it args, returning its exit status.
kill [-s sigspec | -n signum | -sigspec] [pid | jobspec]
...
kill -l [sigspec | exit_status]
Send the signal named by sigspec or signum to the
processes named by pid or jobspec. sigspec is
either a signal name such as SIGKILL or a signal
number; signum is a signal number. If sigspec is a
signal name, the name may be given with or without
the SIG prefix. If sigspec is not present, then
SIGTERM is assumed. An argument of -l lists the
signal names. If any arguments are supplied when
-l is given, the names of the signals corresponding
to the arguments are listed, and the return status
is 0. The exit_status argument to -l is a number
specifying either a signal number or the exit sta
tus of a process terminated by a signal. kill
returns true if at least one signal was success
fully sent, or false if an error occurs or an
invalid option is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evalu
ated (see ARITHMETIC EVALUATION). If the last arg
evaluates to 0, let returns 1; 0 is returned other
wise.
local [name[=value] ...]
For each argument, a local variable named name is
created, and assigned value. When local is used
within a function, it causes the variable name to
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have a visible scope restricted to that function
and its children. With no operands, local writes a
list of local variables to the standard output. It
is an error to use local when not within a func
tion. The return status is 0 unless local is used
outside a function, or an invalid name is supplied.
logout Exit a login shell.
popd [-n] [+n] [-n]
Removes entries from the directory stack. With no
arguments, removes the top directory from the
stack, and performs a cd to the new top directory.
Arguments, if supplied, have the following mean
ings:
+n Removes the nth entry counting from the left
of the list shown by dirs, starting with
zero. For example: ``popd +0'' removes the
first directory, ``popd +1'' the second.
-n Removes the nth entry counting from the
right of the list shown by dirs, starting
with zero. For example: ``popd -0'' removes
the last directory, ``popd -1'' the next to
last.
-n Suppresses the normal change of directory
when removing directories from the stack, so
that only the stack is manipulated.
If the popd command is successful, a dirs is per
formed as well, and the return status is 0. popd
returns false if an invalid option is encountered,
the directory stack is empty, a non-existent direc
tory stack entry is specified, or the directory
change fails.
printf format [arguments]
Write the formatted arguments to the standard out
put under the control of the format. The format is
a character string which contains three types of
objects: plain characters, which are simply copied
to standard output, character escape sequences,
which are converted and copied to the standard out
put, and format specifications, each of which
causes printing of the next successive argument.
In addition to the standard printf(1) formats, %b
causes printf to expand backslash escape sequences
in the corresponding argument, and %q causes printf
to output the corresponding argument in a format
that can be reused as shell input.
The format is reused as necessary to consume all of
the arguments. If the format requires more argu
ments than are supplied, the extra format specifi
cations behave as if a zero value or null string,
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as appropriate, had been supplied.
pushd [-n] [dir]
pushd [-n] [+n] [-n]
Adds a directory to the top of the directory stack,
or rotates the stack, making the new top of the
stack the current working directory. With no argu
ments, exchanges the top two directories and
returns 0, unless the directory stack is empty.
Arguments, if supplied, have the following mean
ings:
+n Rotates the stack so that the nth directory
(counting from the left of the list shown by
dirs, starting with zero) is at the top.
-n Rotates the stack so that the nth directory
(counting from the right of the list shown
by dirs, starting with zero) is at the top.
-n Suppresses the normal change of directory
when adding directories to the stack, so
that only the stack is manipulated.
dir Adds dir to the directory stack at the top,
making it the new current working directory.
If the pushd command is successful, a dirs is per
formed as well. If the first form is used, pushd
returns 0 unless the cd to dir fails. With the
second form, pushd returns 0 unless the directory
stack is empty, a non-existent directory stack ele
ment is specified, or the directory change to the
specified new current directory fails.
pwd [-LP]
Print the absolute file name of the current working
directory. The file name printed contains no sym
bolic links if the -P option is supplied or the -o
physical option to the set builtin command is
enabled. If the -L option is used, symbolic links
are followed. The return status is 0 unless an
error occurs while reading the name of the current
directory or an invalid option is supplied.
read [-er] [-a aname] [-p prompt] [name ...]
One line is read from the standard input, and the
first word is assigned to the first name, the sec
ond word to the second name, and so on, with left
over words and their intervening separators
assigned to the last name. If there are fewer
words read from the standard input than names, the
remaining names are assigned empty values. The
characters in IFS are used to split the line into
words. The backslash character (\) may be used to
remove any special meaning for the next character
read and for line continuation. Options, if sup
plied, have the following meanings:
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-r Backslash does not act as an escape charac
ter. The backslash is considered to be part
of the line. In particular, a backslash-
newline pair may not be used as a line con
tinuation.
-p Display prompt, without a trailing newline,
before attempting to read any input. The
prompt is displayed only if input is coming
from a terminal.
-a The words are assigned to sequential indices
of the array variable aname, starting at 0.
aname is unset before any new values are
assigned. Other name arguments are ignored.
-e If the standard input is coming from a ter
minal, readline (see READLINE above) is used
to obtain the line.
If no names are supplied, the line read is assigned
to the variable REPLY. The return code is zero,
unless end-of-file is encountered.
readonly [-apf] [name ...]
The given names are marked readonly; the values of
these names may not be changed by subsequent
assignment. If the -f option is supplied, the
functions corresponding to the names are so marked.
The -a option restricts the variables to arrays.
If no name arguments are given, or if the -p option
is supplied, a list of all readonly names is
printed. The -p option causes output to be dis
played in a format thatmay be reused as input. The
return status is 0 unless an invalid option is
encountered, one of the names is not a valid shell
variable name, or -f is supplied with a name that
is not a function.
return [n]
Causes a function to exit with the return value
specified by n. If n is omitted, the return status
is that of the last command executed in the func
tion body. If used outside a function, but during
execution of a script by the . (source) command,
it causes the shell to stop executing that script
and return either n or the exit status of the last
command executed within the script as the exit sta
tus of the script. If used outside a function and
not during execution of a script by ., the return
status is false.
set [--abefhkmnptuvxBCHP] [-o option] [arg ...]
Without options, the name and value of each shell
variable are displayed in a format that can be
reused as input. The output is sorted according to
the current locale. When options are specified,
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they set or unset shell attributes. Any arguments
remaining after the options are processed are
treated as values for the positional parameters and
are assigned, in order, to $1, $2, ... $n.
Options, if specified, have the following meanings:
-a Automatically mark variables which are mod
ified or created for export to the environ
ment of subsequent commands.
-b Report the status of terminated background
jobs immediately, rather than before the
next primary prompt. This is effective
only when job control is enabled.
-e Exit immediately if a simple command (see
SHELL GRAMMAR above) exits with a non-zero
status. The shell does not exit if the
command that fails is part of an until or
while loop, part of an if statement, part
of a && or || list, or if the command's
return value is being inverted via !.
-f Disable pathname expansion.
-h Remember the location of commands as they
are looked up for execution. This is
enabled by default.
-k All arguments in the form of assignment
statements are placed in the environment
for a command, not just those that precede
the command name.
-m Monitor mode. Job control is enabled.
This option is on by default for interac
tive shells on systems that support it (see
JOB CONTROL above). Background processes
run in a separate process group and a line
containing their exit status is printed
upon their completion.
-n Read commands but do not execute them.
This may be used to check a shell script
for syntax errors. This is ignored by
interactive shells.
-o option-name
The option-name can be one of the follow
ing:
allexport
Same as -a.
braceexpand
Same as -B.
emacs Use an emacs-style command line
editing interface. This is enabled
by default when the shell is inter
active, unless the shell is started
with the --noediting option.
errexit Same as -e.
hashall Same as -h.
histexpand
Same as -H.
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history Enable command history, as
described above under HISTORY.
This option is on by default in
interactive shells.
ignoreeof
The effect is as if the shell com
mand IGNOREEOF=10 had been executed
(see Shell Variables above).
keyword Same as -k.
monitor Same as -m.
noclobber
Same as -C.
noexec Same as -n.
noglob Same as -f.
notify Same as -b.
nounset Same as -u.
onecmd Same as -t.
physical
Same as -P.
posix Change the behavior of bash where
the default operation differs from
the POSIX 1003.2 standard to match
the standard.
privileged
Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing
interface.
xtrace Same as -x.
If -o is supplied with no option-name, the
values of the current options are printed.
If +o is supplied with no option-name, a
series of set commands to recreate the cur
rent option settings is displayed on the
standard output.
-p Turn on privileged mode. In this mode, the
$ENV and $BASH_ENV files are not processed,
shell functions are not inherited from the
environment, and the SHELLOPTS variable, if
it appears in the environment, is ignored.
If the shell is started with the effective
user (group) id not equal to the real user
(group) id, and the -p option is not sup
plied, these actions are taken and the
effective user id is set to the real user
id. If the -p option is supplied at
startup, the effective user id is not
reset. Turning this option off causes the
effective user and group ids to be set to
the real user and group ids.
-t Exit after reading and executing one com
mand.
-u Treat unset variables as an error when per
forming parameter expansion. If expansion
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is attempted on an unset variable, the
shell prints an error message, and, if not
interactive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, dis
play the expanded value of PS4, followed by
the command and its expanded arguments.
-B The shell performs brace expansion (see
Brace Expansion above). This is on by
default.
-C If set, bash does not overwrite an existing
file with the >, >&, and <> redirection
operators. This may be overridden when
creating output files by using the redirec
tion operator >| instead of >.
-H Enable ! style history substitution. This
option is on by default when the shell is
interactive.
-P If set, the shell does not follow symbolic
links when executing commands such as cd
that change the current working directory.
It uses the physical directory structure
instead. By default, bash follows the log
ical chain of directories when performing
commands which change the current direc
tory.
-- If no arguments follow this option, then
the positional parameters are unset. Oth
erwise, the positional parameters are set
to the args, even if some of them begin
with a -.
- Signal the end of options, cause all
remaining args to be assigned to the posi
tional parameters. The -x and -v options
are turned off. If there are no args, the
positional parameters remain unchanged.
The options are off by default unless otherwise
noted. Using + rather than - causes these options
to be turned off. The options can also be speci
fied as arguments to an invocation of the shell.
The current set of options may be found in $-. The
return status is always true unless an invalid
option is encountered.
shift [n]
The positional parameters from n+1 ... are renamed
to $1 .... Parameters represented by the numbers
$# down to $#-n+1 are unset. n must be a non-nega
tive number less than or equal to $#. If n is 0,
no parameters are changed. If n is not given, it
is assumed to be 1. If n is greater than $#, the
positional parameters are not changed. The return
status is greater than zero if n is greater than $#
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or less than zero; otherwise 0.
shopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional
shell behavior. With no options, or with the -p
option, a list of all settable options is dis
played, with an indication of whether or not each
is set. The -p option causes output to be dis
played in a form that may be reused as input.
Other options have the following meanings:
-s Enable (set) each optname.
-u Disable (unset) each optname.
-q Suppresses normal output (quiet mode); the
return status indicates whether the optname
is set or unset. If multiple optname argu
ments are given with -q, the return status
is zero if all optnames are enabled; non-
zero otherwise.
-o Restricts the values of optname to be those
defined for the -o option to the set
builtin.
If either -s or -u is used with no optname argu
ments, the display is limited to those options
which are set or unset, respectively. Unless oth
erwise noted, the shopt options are disabled
(unset) by default.
The return status when listing options is zero if
all optnames are enabled, non-zero otherwise. When
setting or unsetting options, the return status is
zero unless an optname is not a valid shell option.
The list of shopt options is:
cdable_vars
If set, an argument to the cd builtin com
mand that is not a directory is assumed to
be the name of a variable whose value is
the directory to change to.
cdspell If set, minor errors in the spelling of a
directory component in a cd command will be
corrected. The errors checked for are
transposed characters, a missing character,
and one character too many. If a correc
tion is found, the corrected file name is
printed, and the command proceeds. This
option is only used by interactive shells.
checkhash
If set, bash checks that a command found in
the hash table exists before trying to exe
cute it. If a hashed command no longer
exists, a normal path search is performed.
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checkwinsize
If set, bash checks the window size after
each command and, if necessary, updates the
values of LINES and COLUMNS.
cmdhist If set, bash attempts to save all lines of
a multiple-line command in the same history
entry. This allows easy re-editing of
multi-line commands.
dotglob If set, bash includes filenames beginning
with a `.' in the results of pathname
expansion.
execfail
If set, a non-interactive shell will not
exit if it cannot execute the file speci
fied as an argument to the exec builtin
command. An interactive shell does not
exit if exec fails.
expand_aliases
If set, aliases are expanded as described
above under ALIASES. This option is
enabled by default for interactive shells.
extglob If set, the extended pattern matching fea
tures described above under Pathname Expan
sion are enabled.
histappend
If set, the history list is appended to the
file named by the value of the HISTFILE
variable when the shell exits, rather than
overwriting the file.
histreedit
If set, and readline is being used, a user
is given the opportunity to re-edit a
failed history substitution.
histverify
If set, and readline is being used, the
results of history substitution are not
immediately passed to the shell parser.
Instead, the resulting line is loaded into
the readline editing buffer, allowing fur
ther modification.
hostcomplete
If set, and readline is being used, bash
will attempt to perform hostname completion
when a word containing a @ is being com
pleted (see Completing under READLINE
above). This is enabled by default.
huponexit
If set, bash will send SIGHUP to all jobs
when an interactive login shell exits.
interactive_comments
If set, allow a word beginning with # to
cause that word and all remaining charac
ters on that line to be ignored in an
interactive shell (see COMMENTS above).
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This option is enabled by default.
lithist If set, and the cmdhist option is enabled,
multi-line commands are saved to the his
tory with embedded newlines rather than
using semicolon separators where possible.
mailwarn
If set, and a file that bash is checking
for mail has been accessed since the last
time it was checked, the message ``The mail
in mailfile has been read'' is displayed.
nocaseglob
If set, bash matches filenames in a
case-insensitive fashion when performing
pathname expansion (see Pathname Expansion
above).
nullglob
If set, bash allows patterns which match no
files (see Pathname Expansion above) to
expand to a null string, rather than them
selves.
promptvars
If set, prompt strings undergo variable and
parameter expansion after being expanded as
described in PROMPTING above. This option
is enabled by default.
restricted_shell
The shell sets this option if it is started
in restricted mode (see RESTRICTED SHELL
below). The value may not be changed.
This is not reset when the startup files
are executed, allowing the startup files to
discover whether or not a shell is
restricted.
shift_verbose
If set, the shift builtin prints an error
message when the shift count exceeds the
number of positional parameters.
sourcepath
If set, the source (.) builtin uses the
value of PATH to find the directory con
taining the file supplied as an argument.
This option is enabled by default.
suspend [-f]
Suspend the execution of this shell until it
receives a SIGCONT signal. The -f option says not
to complain if this is a login shell; just suspend
anyway. The return status is 0 unless the shell is
a login shell and -f is not supplied, or if job
control is not enabled.
test expr
[ expr ]
Return a status of 0 or 1 depending on the evalua
tion of the conditional expression expr. Each
operator and operand must be a separate argument.
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Expressions are composed of the primaries described
above under CONDITIONAL EXPRESSIONS.
Expressions may be combined using the following
operators, listed in decreasing order of prece
dence.
! expr True if expr is false.
( expr )
Returns the value of expr. This may be used
to override the normal precedence of opera
tors.
expr1 -a expr2
True if both expr1 and expr2 are true.
expr1 -o expr2
True if either expr1 or expr2 is true.
test and [ evaluate conditional expressions using a
set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the
argument is not null.
2 arguments
If the first argument is !, the expression
is true if and only if the second argument
is null. If the first argument is one of
the unary conditional operators listed above
under CONDITIONAL EXPRESSIONS, the expres
sion is true if the unary test is true. If
the first argument is not a valid unary con
ditional operator, the expression is false.
3 arguments
If the second argument is one of the binary
conditional operators listed above under
CONDITIONAL EXPRESSIONS, the result of the
expression is the result of the binary test
using the first and third arguments as
operands. If the first argument is !, the
value is the negation of the two-argument
test using the second and third arguments.
If the first argument is exactly ( and the
third argument is exactly ), the result is
the one-argument test of the second argu
ment. Otherwise, the expression is false.
The -a and -o operators are considered
binary operators in this case.
4 arguments
If the first argument is !, the result is
the negation of the three-argument expres
sion composed of the remaining arguments.
Otherwise, the expression is parsed and
evaluated according to precedence using the
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rules listed above.
5 or more arguments
The expression is parsed and evaluated
according to precedence using the rules
listed above.
times Print the accumulated user and system times for the
shell and for processes run from the shell. The
return status is 0.
trap [-lp] [arg] [sigspec ...]
The command arg is to be read and executed when the
shell receives signal(s) sigspec. If arg is absent
or -, all specified signals are reset to their
original values (the values they had upon entrance
to the shell). If arg is the null string the sig
nal specified by each sigspec is ignored by the
shell and by the commands it invokes. If arg is -p
then the trap commands associated with each sigspec
are displayed. If no arguments are supplied or if
only -p is given, trap prints the list of commands
associated with each signal number. Each sigspec
is either a signal name defined in <signal.h>, or a
signal number. If a sigspec is EXIT (0) the com
mand arg is executed on exit from the shell. If a
sigspec is DEBUG, the command arg is executed after
every simple command (see SHELL GRAMMAR above).
The -l option causes the shell to print a list of
signal names and their corresponding numbers. Sig
nals ignored upon entry to the shell cannot be
trapped or reset. Trapped signals are reset to
their original values in a child process when it is
created. The return status is false if any sigspec
is invalid; otherwise trap returns true.
type [-atp] name [name ...]
With no options, indicate how each name would be
interpreted if used as a command name. If the -t
option is used, type prints a string which is one
of alias, keyword, function, builtin, or file if
name is an alias, shell reserved word, function,
builtin, or disk file, respectively. If the name
is not found, then nothing is printed, and an exit
status of false is returned. If the -p option is
used, type either returns the name of the disk file
that would be executed if name were specified as a
command name, or nothing if type -t name would not
return file. If a command is hashed, -p prints the
hashed value, not necessarily the file that appears
first in PATH. If the -a option is used, type
prints all of the places that contain an executable
named name. This includes aliases and functions,
if and only if the -p option is not also used. The
table of hashed commands is not consulted when
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using -a. type returns true if any of the argu
ments are found, false if none are found.
ulimit [-SHacdflmnpstuv [limit]]
Provides control over the resources available to
the shell and to processes started by it, on sys
tems that allow such control. The value of limit
can be a number in the unit specified for the
resource, or the value unlimited. The -H and -S
options specify that the hard or soft limit is set
for the given resource. A hard limit cannot be
increased once it is set; a soft limit may be
increased up to the value of the hard limit. If
neither -H nor -S is specified, both the soft and
hard limits are set. If limit is omitted, the cur
rent value of the soft limit of the resource is
printed, unless the -H option is given. When more
than one resource is specified, the limit name and
unit are printed before the value. Other options
are interpreted as follows:
-a All current limits are reported
-c The maximum size of core files created
-d The maximum size of a process's data segment
-f The maximum size of files created by the
shell
-l The maximum size that may be locked into
memory
-m The maximum resident set size
-n The maximum number of open file descriptors
(most systems do not allow this value to be
set)
-p The pipe size in 512-byte blocks (this may
not be set)
-s The maximum stack size
-t The maximum amount of cpu time in seconds
-u The maximum number of processes available to
a single user
-v The maximum amount of virtual memory avail
able to the shell
If limit is given, it is the new value of the spec
ified resource (the -a option is display only). If
no option is given, then -f is assumed. Values are
in 1024-byte increments, except for -t, which is in
seconds, -p, which is in units of 512-byte blocks,
and -n and -u, which are unscaled values. The
return status is 0 unless an invalid option is
encountered, a non-numeric argument other than
unlimited is supplied as limit, or an error occurs
while setting a new limit.
umask [-p] [-S] [mode]
The user file-creation mask is set to mode. If
mode begins with a digit, it is interpreted as an
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octal number; otherwise it is interpreted as a sym
bolic mode mask similar to that accepted by
chmod(1). If mode is omitted, or if the -S option
is supplied, the current value of the mask is
printed. The -S option causes the mask to be
printed in symbolic form; the default output is an
octal number. If the -p option is supplied, and
mode is omitted, the output is in a form that may
be reused as input. The return status is 0 if the
mode was successfully changed or if no mode argu
ment was supplied, and false otherwise.
unalias [-a] [name ...]
Remove names from the list of defined aliases. If
-a is supplied, all alias definitions are removed.
The return value is true unless a supplied name is
not a defined alias.
unset [-fv] [name ...]
For each name, remove the corresponding variable or
function. If no options are supplied, or the -v
option is given, each name refers to a shell vari
able. Read-only variables may not be unset. If -f
is specifed, each name refers to a shell function,
and the function definition is removed. Each unset
variable or function is removed from the environ
ment passed to subsequent commands. If any of RAN
DOM, SECONDS, LINENO, HISTCMD, or DIRSTACK are
unset, they lose their special properties, even if
they are subsequently reset. The exit status is
true unless a name does not exist or is readonly.
wait [n]
Wait for the specified process and return its ter
mination status. n may be a process ID or a job
specification; if a job spec is given, all pro
cesses in that job's pipeline are waited for. If n
is not given, all currently active child processes
are waited for, and the return status is zero. If
n specifies a non-existent process or job, the
return status is 127. Otherwise, the return status
is the exit status of the last process or job
waited for.
RESTRICTED SHELL
If bash is started with the name rbash, or the -r option
is supplied at invocation, the shell becomes restricted.
A restricted shell is used to set up an environment more
controlled than the standard shell. It behaves identi
cally to bash with the exception that the following are
disallowed or not performed:
· changing directories with cd
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· setting or unsetting the values of SHELL, PATH,
ENV, or BASH_ENV
· specifying command names containing /
· specifying a file name containing a / as an argu
ment to the . builtin command
· importing function definitions from the shell envi
ronment at startup
· parsing the value of SHELLOPTS from the shell envi
ronment at startup
· redirecting output using the >, >|, <>, >&, &>, and
>> redirection operators
· using the exec builtin command to replace the shell
with another command
· adding or deleting builtin commands with the -f and
-d options to the enable builtin command
· specifying the -p option to the command builtin
command
· turning off restricted mode with set +r or set +o
restricted.
These restrictions are enforced after any startup files
are read.
When a command that is found to be a shell script is exe
cuted (see COMMAND EXECUTION above), rbash turns off any
restrictions in the shell spawned to execute the script.
SEE ALSO
Bash Features, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2:
Shell and Utilities, IEEE
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash2
The bash executable
/etc/profile
The systemwide initialization file, executed for
login shells
~/.bash_profile
The personal initialization file, executed for
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BASH2(1) BASH2(1)
login shells
~/.bashrc
The individual per-interactive-shell startup file
~/.bash_logout
The individual login shell cleanup file, executed
when a login shell exits
~/.inputrc
Individual readline initialization file
AUTHORS
Brian Fox, Free Software Foundation
bfox@gnu.ai.MIT.Edu
Chet Ramey, Case Western Reserve University
chet@ins.CWRU.Edu
BUG REPORTS
If you find a bug in bash, you should report it. But
first, you should make sure that it really is a bug, and
that it appears in the latest version of bash that you
have.
Once you have determined that a bug actually exists, use
the bashbug command to submit a bug report. If you have a
fix, you are encouraged to mail that as well! Suggestions
and `philosophical' bug reports may be mailed to bug-
bash@gnu.org or posted to the Usenet newsgroup
gnu.bash.bug.
ALL bug reports should include:
The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behaviour
A short script or `recipe' which exercises the bug
bashbug inserts the first three items automatically into
the template it provides for filing a bug report.
Comments and bug reports concerning this manual page
should be directed to chet@ins.CWRU.Edu.
BUGS
It's too big and too slow.
There are some subtle differences between bash and tradi
tional versions of sh, mostly because of the POSIX speci
fication.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stop
pable/restartable.
GNU 1999 Jan 20 85
BASH2(1) BASH2(1)
Compound commands and command sequences of the form `a ; b
; c' are not handled gracefully when process suspension is
attempted. When a process is stopped, the shell immedi
ately executes the next command in the sequence. It suf
fices to place the sequence of commands between parenthe
ses to force it into a subshell, which may be stopped as a
unit.
Commands inside of $(...) command substitution are not
parsed until substitution is attempted. This will delay
error reporting until some time after the command is
entered.
Array variables may not (yet) be exported.
GNU 1999 Jan 20 86
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