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NAME
bash - GNU Bourne-Again SHell
SYNOPSIS
bash [options] [file]
COPYRIGHT
Bash is Copyright (C) 1989-1999 by the Free Software Founda-
tion, 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 documented
in the description of the set builtin command, bash inter-
prets 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 interac-
tive.
-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 translation
when the current locale is not C or POSIX. This
implies the -n option; no commands will be exe-
cuted.
-- 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 recognized.
--dump-po-strings
Equivalent to -D, but the output is in the GNU gettext
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 ~/.profile.
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 operation
differs from the POSIX 1003.2 standard to match the
standard.
--rcfile file
Execute commands from file instead of the standard per-
sonal initialization file ~/.bashrc if the shell is
interactive (see INVOCATION below).
--restricted
The shell becomes restricted (see RESTRICTED SHELL
below).
--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 com-
mands. 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 executes commands
from this file, then exits. Bash's exit status is the exit
status of the last command executed in 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 output
are both connected to terminals (as determined by
isatty(3)), or one started with the -i option. PS1 is set
and $- includes i if bash is interactive, allowing a shell
script or a startup file to test this state.
The following paragraphs describe how bash executes its
startup files. If any of the files exist but cannot be
read, bash reports an error. Tildes are expanded in file
names as described below under Tilde Expansion in the EXPAN-
SION section.
When bash is invoked as an interactive login shell, or as a
non-interactive shell with the --login option, it first
reads and executes commands from the file /etc/profile, if
that file exists. After reading that file, it looks for
~/.bash_profile, ~/.bash_login, and ~/.profile, in that
order, and reads and executes commands from the first one
that exists and is readable. The --noprofile option may be
used when the shell is started to inhibit this behavior.
When a login shell exits, bash reads and executes commands
from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is
started, bash reads and executes commands from ~/.bashrc, if
that file exists. This may be inhibited by using the --norc
option. The --rcfile file option will force bash to read
and execute commands from file instead of ~/.bashrc.
When bash is started non-interactively, to run a shell
script, for example, it looks for the variable BASH_ENV in
the environment, expands its value if it appears there, and
uses the expanded value as the name of a file to read and
execute. Bash behaves as if the following command were exe-
cuted:
if [ -n "$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 attempts
to read and execute commands from /etc/profile and ~/.pro-
file, 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 execute. 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 com-
mand 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 gen-
erally 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 functions are
not inherited from the environment, the SHELLOPTS variable,
if it appears in the environment, is ignored, and the effec-
tive 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
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 command (see
SHELL GRAMMAR below) or the third word of a case or for com-
mand:
! 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 assign-
ments followed by blank-separated words and redirections,
and terminated by a control operator. The first word speci-
fies 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 standard
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 status
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 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 subshell.
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 assignments
and builtin commands that affect the shell's environ-
ment do not remain in effect after the command com-
pletes. The return status is the exit status of list.
{ list; }
list is simply executed in the current shell environ-
ment. list must be terminated with a newline or semi-
colon. 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 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 evaluation
of the conditional expression expression. Expressions
are composed of the primaries described below under
CONDITIONAL EXPRESSIONS. Word splitting and pathname
expansion are not performed on the words between the [[
and ]]; tilde expansion, parameter and variable expan-
sion, arithmetic expansion, command substitution, pro-
cess substitution, and quote removal are performed.
When the == and != operators are used, the string to
the right of the operator is considered a pattern 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 pattern, 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 opera-
tors, listed in decreasing order of precedence:
( 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, generating
a list of items. The variable name is set to each ele-
ment of this list in turn, and list is executed each
time. If the in word is omitted, the for command exe-
cutes list once for each positional parameter that is
set (see PARAMETERS below). The return status is the
exit status of the last command that executes. If the
expansion of the items 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, generating
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 positional 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 com-
pletes. 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 match-
ing 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 command executed in list.
fi
if list; then list; [ elif list; then list; ] ... [ else list; ]
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 status is zero,
the corresponding then list is executed 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 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 specified 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 characters 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 interac-
tive 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 prevent
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 double
quotes.
A non-quoted backslash (\) is the escape character. It
preserves the literal value of the next character that fol-
lows, with the exception of <newline>. If a \<newline> pair
appears, and the backslash is not itself quoted, the \<new-
line> 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 literal
value of each character within the quotes. A single quote
may not occur between single quotes, even when preceded by a
backslash.
Enclosing characters in double quotes preserves the literal
value of all characters within the quotes, with the excep-
tion 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 follow-
ing characters: $, `, ", \, or <newline>. A double quote
may be quoted within double quotes by preceding it with a
backslash.
The special parameters * and @ have special meaning when in
double quotes (see PARAMETERS below).
Words of the form $'string' are treated specially. The word
expands to string, with backslash-escaped characters
replaced as specifed by the ANSI C standard. Backslash
escape sequences, if present, are decoded as follows:
\a alert (bell)
\b backspace
\e an escape character
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is the octal value
nnn (one to three digits)
\xnnn
the character whose ASCII code is the hexadecimal
value nnn (one to three digits)
The translated result is single-quoted, as if the dollar
sign had not been present.
A double-quoted string preceded by a dollar sign ($) 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 substitution,
arithmetic expansion, and quote removal (see EXPANSION
below). If the variable has its integer attribute set (see
declare below in SHELL BUILTIN COMMANDS) then value is sub-
ject to arithmetic expansion even if the $((...)) expansion
is not used (see Arithmetic Expansion below). Word split-
ting is not performed, with the exception of "$@" as
explained below under Special Parameters. Pathname expan-
sion is not performed.
Positional Parameters
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parame-
ters are assigned from the shell's arguments when it is
invoked, and may be reassigned using the set builtin com-
mand. Positional parameters may not be assigned to with
assignment statements. The positional parameters are tem-
porarily replaced when a shell function is executed (see
FUNCTIONS below).
When a positional parameter consisting of more than a single
digit is expanded, it must be enclosed in braces (see EXPAN-
SION below).
Special Parameters
The shell treats several parameters specially. These param-
eters 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$2c...", where c is the first character of the
value of the IFS variable. If IFS is unset, the param-
eters are separated by spaces. If IFS is null, the
parameters are joined without intervening separators.
@ Expands to the positional parameters, starting from
one. When the expansion occurs within double quotes,
each parameter expands to a separate word. That is,
"$@" 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 () sub-
shell, it expands to the process ID of the current
shell, not the subshell.
! Expands to the process ID of the most recently executed
background (asynchronous) command.
0 Expands to the name of the shell or shell script. 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. Otherwise, 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 argu-
ment to the previous command, after expansion. Also
set to the full file name of each command 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 variable is
readonly.
PWD The current working directory as set by the cd command.
OLDPWD
The previous working directory as set by the cd com-
mand.
REPLY
Set to the line of input read by the read builtin com-
mand when no arguments are supplied.
UID Expands to the user ID of the current user, initialized
at shell startup. This variable is readonly.
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 version
information for this instance of bash. The values
assigned to the array members are as follows:
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.,
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 spe-
cial properties, even if it is subsequently 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 subse-
quent 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 sub-
stitutes a decimal number representing the current
sequential line number (starting with 1) within a
script or function. When not in a script or function,
the value substituted is not guaranteed to be meaning-
ful. If LINENO is unset, it loses its special proper-
ties, even if it is subsequently 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 subsequently reset.
DIRSTACK
An array variable (see Arrays below) containing the
current contents of the directory stack. Directories
appear in the stack in the order they are displayed by
the dirs builtin. Assigning to members of this array
variable may be used to modify directories already in
the stack, but the pushd and popd builtins must be used
to add and remove directories. Assignment to this
variable will not change the current directory. If
DIRSTACK is unset, it loses its special properties,
even if it is subsequently 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 (which may con-
tain only a single command).
OPTARG
The value of the last option argument processed by the
getopts builtin command (see SHELL BUILTIN COMMANDS
below).
OPTIND
The index of the next argument to be processed by the
getopts builtin command (see SHELL BUILTIN COMMANDS
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 executing. 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-separated
list of directories in which the shell looks for com-
mands (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
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 com-
mand. 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 filename contain-
ing commands to initialize the shell, as in ~/.bashrc.
The value of BASH_ENV is subjected to parameter expan-
sion, command substitution, and arithmetic expansion
before being interpreted 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 MAIL-
PATH 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 displaying the pri-
mary prompt. If this variable is unset, the shell dis-
ables 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, $_ expands to the name of the
current mailfile. Example:
MAILPATH='/usr/spool/mail/bfox?"You have
mail":~/shell-mail?"$_ 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 indirection. The default is ``+ ''.
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 specified, the value 3 is
used.
The optional l specifies a longer format, including
minutes, of the form MMmSS.FFs. The value of p deter-
mines 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 displayed. A
trailing newline is added when the format string is
displayed.
HISTSIZE
The number of commands to remember in the command his-
tory (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 history
file. When this variable is assigned a value, the his-
tory 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 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 category
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 deter-
mines the behavior of range expressions, equivalence
classes, and collating sequences within pathname expan-
sion and pattern matching.
LC_CTYPE
This variable determines the interpretation of charac-
ters and the behavior of character classes within path-
name expansion and pattern matching.
LC_MESSAGES
This variable determines the locale used to translate
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 characters 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 terminates
after waiting for that number of seconds if input does
not arrive.
FCEDIT
The default editor for the fc builtin command.
FIGNORE
A colon-separated list of suffixes to ignore when per-
forming 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 pattern also
matches one of the patterns in GLOBIGNORE, it is
removed from the list of matches.
INPUTRC
The filename for the readline startup file, overriding
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 history
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 HISTIGNORE. 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 char-
acters, `&' 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 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-running 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 necessarily
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 interac-
tively; 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 sub-
string value provides functionality analogous 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 analo-
gous to the % job identifier.
Arrays
Bash provides one-dimensional array variables. Any variable
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
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 members 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 elements may be
assigned to using the name[subscript]=value syntax intro-
duced 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 single
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 ${name[subscript]}. If subscript
is * or @, the expansion is the number of elements in the
array. Referencing an array variable without a subscript is
equivalent to referencing element zero.
The unset builtin is used to destroy arrays. unset
name[subscript] destroys the array element at index sub-
script. unset name, where name is an array, or unset
name[subscript], where subscript is * or @, removes the
entire array.
The declare, local, and readonly builtins each accept a -a
option to specify an array. The read builtin accepts a -a
option to assign a list of words read from the standard
input to an array. The set and declare builtins display
array values in a way that allows them to be reused as
assignments.
EXPANSION
Expansion is performed on the command line after it has been
split into words. There are seven kinds of expansion per-
formed: brace expansion, tilde expansion, parameter and
variable expansion, command substitution, arithmetic expan-
sion, word splitting, and pathname expansion.
The order of expansions is: brace expansion, tilde expan-
sion, parameter, variable and arithmetic expansion and com-
mand substitution (done in a left-to-right fashion), word
splitting, and pathname expansion.
On systems that can support it, there is an additional
expansion available: process substitution.
Only brace expansion, word splitting, and pathname expansion
can change the number of words of the expansion; other
expansions expand a single word to a single word. The only
exceptions to this are the expansions of "$@" 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. Pat-
terns 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 preserved
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 com-
mon 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}}
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 expan-
sion by bash. If strict compatibility 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 substi-
tuted 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 vari-
able PWD replaces the tilde-prefix. If the tilde-prefix 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 pre-
fixed 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, MAILPATH,
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
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 parameter with
more than one digit, or when parameter is followed by a
character which is not to be interpreted 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 substitution, rather
than the value of parameter itself. This is known as
indirect expansion.
In each of the cases below, word is subject to tilde expan-
sion, parameter expansion, command substitution, and arith-
metic 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 parameter. 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 stan-
dard error and the shell, if it is not interactive,
exits. Otherwise, the value of parameter is substi-
tuted.
${parameter:+word}
Use Alternate Value. If parameter is null or unset,
nothing is substituted, otherwise the expansion of word
is substituted.
${parameter:offset}
${parameter:offset:length}
Substring Expansion. Expands to up to length charac-
ters of parameter, starting at the characters specified
by offset. If length is omitted, expands to the sub-
string of parameter, starting at the character speci-
fied by offset. length and offset are arithmetic
expressions (see ARITHMETIC EVALUATION 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 ${parameter[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 substi-
tuted 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 begin-
ning 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 long-
est 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 parameter is an
array variable subscripted with @ or *, the pattern
removal operation 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 parameter, then the
result of the expansion is the expanded value of param-
eter 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
parameter is an array variable subscripted with @ or *,
the pattern removal operation is applied to each member
of the array in turn, and the expansion is the resul-
tant 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 / following 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 variable
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 replac-
ing the command substitution with the standard output of the
command, with any trailing newlines deleted. Embedded new-
lines 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 parentheses
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
results.
Arithmetic Expansion
Arithmetic expansion allows the evaluation of an arithmetic
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 substitution,
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 simultane-
ously with parameter and variable expansion, command substi-
tution, and arithmetic expansion.
Word Splitting
The shell scans the results of parameter expansion, command
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 character). Any charac-
ter in IFS that is not IFS whitespace, along with any adja-
cent IFS whitespace characters, delimits a field. A
sequence of IFS whitespace characters is 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 parameter
with no value is expanded within double quotes, a null argu-
ment results and is retained.
Note that if no expansion occurs, no splitting is performed.
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 nullglob is dis-
abled, 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 per-
formed without regard to the case of alphabetic characters.
When a pattern is used for pathname 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 charac-
ter must always be matched explicitly. In other cases, the
``.'' character is not treated specially. See the descrip-
tion 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 pat-
terns 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 patterns in GLOBIGNORE. The dotglob
option is disabled when GLOBIGNORE is unset.
Pattern Matching
Any character that appears in a pattern, other than the spe-
cial 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.
The special pattern characters have the following meanings:
* 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 follow-
ing the [ is a ! or a ^ then any character not enclosed
is matched. A - may be matched by including it as the
first or last character 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 standard:
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 specified
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 col-
lating symbol symbol.
If the extglob shell option is enabled using the shopt buil-
tin, several extended pattern matching operators are recog-
nized. In the following description, a pattern-list is a
list of one or more patterns separated by a |. Composite
patterns may be formed using one or more of the following
sub-patterns:
?(pattern-list)
Matches zero or one occurrence of the given pat-
terns
*(pattern-list)
Matches zero or more occurrences of the given pat-
terns
+(pattern-list)
Matches one or more occurrences of the given pat-
terns
@(pattern-list)
Matches exactly one of the given patterns
!(pattern-list)
Matches anything except one of the given patterns
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 anywhere 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 number
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 redirec-
tion operator is >, the redirection refers to the standard
output (file descriptor 1).
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace
expansion, tilde expansion, parameter expansion, command
substitution, arithmetic expansion, quote removal, and path-
name 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:
[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 semanti-
cally equivalent to
>word 2>&1
Here Documents
This type of redirection instructs the shell to read input
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 com-
mand.
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 descriptor 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
[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 contain any valid shell input,
including the metacharacters listed above, with the excep-
tion 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 replacement
text. If arguments are needed, a shell function should be
used.
Aliases are not expanded when the shell is not interactive,
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 complete
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 definition 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 func-
tion definition is itself a compound command. As a conse-
quence, aliases defined in a function are not available
until after that function is executed. To be safe, always
put alias definitions on a 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 (contrast this
with the execution of a shell script). When a function is
executed, the arguments to the function become the posi-
tional parameters during its execution. The special parame-
ter # is updated to reflect the change. Positional parame-
ter 0 is unchanged. All other aspects of the shell execu-
tion environment are identical between a function 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 com-
mand after the function call. When a function completes,
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 automati-
cally 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 operators. The lev-
els are listed in order of decreasing precedence.
- + unary minus and plus
! ~ logical and bitwise negation
** exponentiation
* / %
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 expansion
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 numbers.
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 represented 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 interchangably to represent
numbers between 10 and 35.
Operators are evaluated in order of precedence. Sub-
expressions in parentheses are evaluated first and may over-
ride the precedence rules above.
CONDITIONAL EXPRESSIONS
Conditional expressions are used by the [[ compound command
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.
-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 ter-
minal.
-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.
-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 lexicographically
in the current locale.
string1 > string2
True if string1 sorts after string2 lexicographically
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 nega-
tive 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 redirec-
tions are expanded. If any words remain after expan-
sion, 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 under-
goes tilde expansion, parameter expansion, command sub-
stitution, arithmetic expansion, and quote removal
before being assigned to the variable.
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 assign-
ments 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, but
do not affect the current shell environment. A redirection
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 substitution,
the exit status of the command is the exit status of the
last command substitution performed. If there were no com-
mand 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 fol-
lowing 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 FUNC-
TIONS. 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 per-
formed 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 contains
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 execut-
able format, and the file is not a directory, it is assumed
to be a shell script, a file containing shell commands. A
subshell is spawned to execute it. This subshell reinitial-
izes 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 program.
The shell executes the specified interpreter on operating
systems that do not handle this executable format them-
selves. The arguments to the interpreter consist of a sin-
gle optional argument following the interpreter name on the
first line of the program, followed by the name of the pro-
gram, followed by the command arguments, if any.
COMMAND EXECUTION ENVIRONMENT
The shell has an execution environment, which consists of
the following:
o open files inherited by the shell at invocation, as
modified by redirections supplied to the exec builtin
o the current working directory as set by cd, pushd, or
popd, or inherited by the shell at invocation
o the file creation mode mask as set by umask or inher-
ited from the shell's parent
o current traps set by trap
o shell parameters that are set by variable assignment or
with set or inherited from the shell's parent in the
environment
o shell functions defined during execution or inherited
from the shell's parent in the environment
o options enabled at invocation (either by default or
with command-line arguments) or by set
o options enabled by shopt
o shell aliases defined with alias
o 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 function
is to be executed, it is invoked in a separate execution
environment that consists of the following. Unless other-
wise noted, the values are inherited from the shell.
o the shell's open files, plus any modifications and
additions specified by redirections to the command
o the current working directory
o the file creation mode mask
o shell variables marked for export, along with variables
exported for the command, passed in the environment
o 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 parent at
invocation. Builtin commands that are invoked as part of a
pipeline are also executed in a subshell environment.
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
several ways. On invocation, the shell scans its own
environment and creates a parameter for each name found,
automatically marking it for export to child processes.
Executed 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 modi-
fied in the shell, less any pairs removed by the unset com-
mand, 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 assign-
ments, 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 com-
mand in its environment.
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 failure. 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 exe-
cute 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 exe-
cuted, 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 interactive
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 SIG-
INT and SIGQUIT as well. Commands run as a result of com-
mand substitution ignore the keyboard-generated 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 sending the
signal to a particular job, it should be removed from the
jobs table with the disown builtin (see SHELL BUILTIN COM-
MANDS 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
be executed until the command completes. When bash is wait-
ing 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 supplied 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 asynchronously (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 pipe-
line 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 process
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 terminal.
Background processes which attempt to read from (write to)
the terminal are sent a SIGTTIN (SIGTTOU) signal by the ter-
minal 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 suspend
character (typically ^Z, Control-Z) while a process is run-
ning causes that process to be stopped and returns you to
bash. Typing the delayed suspend character (typically ^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 foreground, or the kill
command to kill it. A ^Z takes effect 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 substring
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 customized
by inserting a number of backslash-escaped special charac-
ters 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
\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 directory
\! 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
\[ begin a sequence of non-printing characters, which
could be used to embed a terminal control sequence
into the prompt
\] 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 executed
during the current shell session. After the string is
decoded, it is expanded via parameter expansion, command
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 inter-
face 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 prefix. The combination M-C-x means
ESC-Control-x, or press the Escape key then hold the Control
key while pressing the x key.)
Readline commands may be given numeric arguments, which nor-
mally act as a repeat count. Sometimes, however, it is the
sign of the argument that is significant. Passing a nega-
tive argument to a command that acts in the forward direc-
tion (e.g., kill-line) causes that command to act in a back-
ward direction. Commands whose behavior with arguments devi-
ates 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 initializa-
tion file (the inputrc file). The name of this file is
taken from the value of the INPUTRC variable. If that vari-
able is unset, the default is ~/.inputrc. When a program
which uses the readline library starts up, the initializa-
tion file is read, and the key bindings and variables 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 com-
mand 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 two ways: as
a symbolic key name, possibly with Meta- or Control- pre-
fixes, 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
universal-argument, M-DEL is bound to the function
backward-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, keyseq
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
universal-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 hexadecimal
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 text
is assumed to be a function name. In the macro body, the
backslash escapes described above are expanded. Backslash
will quote any other character in the macro text, including
" and '.
Bash allows the current readline key bindings to be
displayed 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 customize
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 audible, 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 command 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 generated 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 variable, 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 prefix).
disable-completion (Off)
If set to On, readline will inhibit word completion.
Completion characters will be inserted into 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 appli-
cation 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 read-
line 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 char-
acters it reads), regardless of what the terminal
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 ter-
minate 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 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 con-
ditional compilation features of the C preprocessor which
allows key bindings and variable settings to be performed 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 key-
map 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 terminal-
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 application name,
and an initialization file can test for a particu-
lar value. This could be used to bind key
sequences to functions useful for a specific pro-
gram. 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
$endif
This command, as seen in the previous example, ter-
minates an $if command.
$else
Commands in this branch of the $if directive are exe-
cuted if the test fails.
$include
This directive takes a single filename as an argument
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 command
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 typ-
ing 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 containing 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 for-
ward 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 execute that
command. For instance, a newline will terminate the search
and accept the line, thereby executing the command 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. Command
names without an accompanying key sequence are unbound by
default.
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 characters (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, mov-
ing back in the list.
next-history (C-n)
Fetch the next command from the history list, moving
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
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 (usu-
ally the second word on the previous line) at point
(the current cursor position). With an argument n,
insert the nth word from the previous command (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 descrip-
tion of history expansion.
history-expand-line (M-^)
Perform history expansion on the current line. See
HISTORY EXPANSION below for a description of history
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.
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 characters 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 cur-
sor is at the end of the line, in which case the char-
acter 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 char-
acter at point. Point moves forward as well. If point
is at the end of the line, then transpose the two
characters before point. Negative arguments 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 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 different 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 accumulating, or
start a new argument. M-- starts a negative 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, exe-
cuting universal-argument again ends the numeric argu-
ment, but is otherwise ignored. As a special case, if
this command is immediately followed 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 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 vari-
able (if the text begins with $), username (if the text
begins with ~), hostname (if the text begins with @),
or command (including aliases and functions) in turn.
If none of these produces a match, filename completion
is attempted.
possible-completions (M-?)
List the possible completions of the text before point.
insert-completions (M-*)
Insert all completions of the text before point that
would have been generated by possible-completions.
menu-complete
Similar to complete, but replaces the word to be com-
pleted with a single match from the list of possible
completions. Repeated execution of menu-complete steps
through the list of possible completions, inserting
each match in turn. At the end of the list of comple-
tions, the bell is rung and the original text is
restored. An argument of n moves n positions forward
in the list of matches; a negative argument may be used
to move backward through the list. This command is
intended to be bound to TAB, but is unbound by default.
delete-char-or-list
Deletes the character under the cursor if not at the
beginning or end of the line (like delete-char). If at
the end of the line, behaves identically to possible-
completions. This command is unbound by default.
complete-filename (M-/)
Attempt filename completion on the text before point.
possible-filename-completions (C-x /)
List the possible completions of the text before point,
treating it as a filename.
complete-username (M-~)
Attempt completion on the text before point, treating
it as a username.
possible-username-completions (C-x ~)
List the possible completions of the text before point,
treating it as a username.
complete-variable (M-$)
Attempt completion on the text before point, treating
it as a shell variable.
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, treating
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, treating
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, comparing
the text against lines from the history list for possi-
ble completion matches.
complete-into-braces (M-{)
Perform filename completion and return the list of pos-
sible 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 key-
board macro.
end-kbd-macro (C-x ))
Stop saving the characters typed into the current key-
board macro and store the definition.
call-last-kbd-macro (C-x e)
Re-execute the last keyboard macro defined, by making
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 incor-
porate any bindings or variable assignments found
there.
abort (C-g)
Abort the current editing command and ring the
terminal'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 com-
mand that is bound to the corresponding uppercase char-
acter.
prefix-meta (ESC)
Metafy the next character typed. ESC f is equivalent
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 exe-
cuting 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 argu-
ment is supplied, the mark is set to that position.
exchange-point-and-mark (C-x C-x)
Swap the point with the mark. The current cursor posi-
tion 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 previous
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 comment.
glob-expand-word (C-x *)
The word before point is treated as a pattern for path-
name 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 generated
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 argument 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 formatted in such a
way that it can be made part of an inputrc file.
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 HISTSIZE
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 values
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 HISTFILE.
If the histappend shell option is enabled (see the descrip-
tion 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 display or
modify the history list and manipulate the history file.
When using the command-line editing, search commands 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 variables
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 correctness. 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.
HISTORY EXPANSION
The shell supports a history expansion feature that is simi-
lar to the history expansion in csh. This section describes
what syntax features are available. This feature 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 his-
tory 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 current
input line, or fix errors in previous commands quickly.
History expansion is performed immediately after a complete
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 substitution. 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 surrounded by quotes are con-
sidered one word. History expansions are introduced by the
appearance of the history expansion character, which is ! by
default. Only backslash (\) and single quotes can quote the
history expansion 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 description 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 read-
line 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
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 begin-
ning 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 command
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 specifica-
tion, 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 `:'.
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 substitu-
tions.
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 char-
acter of the event line. The delimiter may be quoted
in old and new with a single backslash. 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 delim-
iter 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 expanding
arguments and performing any specified redirections. A
zero exit code is returned.
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in the current
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
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 job-
spec was not found or started without 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 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 executing a loop when break is
executed.
builtin shell-builtin [arguments]
Execute the specified shell builtin, passing it argu-
ments, and return its exit status. This is useful when
defining a function whose name is the same as a shell
builtin, retaining the functionality of the builtin
within the function. The cd builtin is commonly rede-
fined 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 containing 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 $OLDPWD. The return value is true if
the directory was successfully changed; false other-
wise.
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 indicating
the command or file name used to invoke command 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. Oth-
erwise, the exit status of the command builtin is the
exit status of command.
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 variables.
The -p option will display the attributes and values of
each name. When -p is used, additional options are
ignored. The -F option inhibits the display of func-
tion definitions; only the function name and attributes
are printed. The -F option implies -f. The following
options can be used to restrict output to variables
with the specified 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 EVALUATION ) is per-
formed when the variable is assigned a value.
-r Make names readonly. These names cannot then be
assigned values by subsequent assignment state-
ments or unset.
-x Mark names for export to subsequent commands via
the environment.
Using `+' instead of `-' turns off the attribute
instead, with the exception that +a may not be used to
destroy an array variable. When used in a function,
makes each name local, as with the local command. The
return value is 0 unless an invalid option is encoun-
tered, 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 variable without using the compound assign-
ment 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 variable,
or an attempt is made to display a non-existent func-
tion with -f.
dirs [-clpv] [+n] [-n]
Without options, displays the list of currently remem-
bered directories. The default display is on a single
line with directory names separated by spaces. Direc-
tories 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 listing
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 sup-
plied or n indexes beyond the end of the directory
stack.
disown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table
of active jobs. If the -h option is given, each job-
spec 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 nei-
ther the -a nor the -r option is supplied, the current
job is used. If no jobspec is supplied, the -a option
means to remove or mark all jobs; the -r option without
a jobspec argument 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 following
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 hexadecimal
value nnn (one to three digits)
enable [-adnps] [-f filename] [name ...]
Enable and disable builtin shell commands. Disabling 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 com-
mand name from shared object filename, on systems that
support dynamic loading. The -d option will delete a
builtin previously loaded with -f. If no name argu-
ments are given, or if the -p option is supplied, 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 buil-
tins 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 problem loading a new builtin from a shared
object.
eval [arg ...]
The args are read and concatenated together into a sin-
gle command. This command is then read and executed 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 supplied, the shell
places a dash in the zeroth arg passed to command.
This is what login(1) does. The -c option causes com-
mand to be executed with an empty environment. If -a
is supplied, the shell passes name as the zeroth argu-
ment 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 commands. 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 encoun-
tered, 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 commands
from first to last is selected from the history list.
First and last may be specified as a string (to locate
the last command beginning with that string) or as a
number (an index into the history list, where a nega-
tive number is used as an offset from the current com-
mand 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 list-
ing. The -r option reverses the order of the commands.
If the -l option is given, the commands are listed on
standard output. Otherwise, the editor 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 typing ``r cc''
runs the last command beginning with ``cc'' and typing
``r'' re-executes the last command.
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 specify 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 control.
getopts optstring name [args]
getopts is used by shell procedures to parse positional
parameters. optstring contains the option letters to
be recognized; if a letter is followed by a colon, the
option is expected to have an 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 argu-
ment 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 report-
ing is used. In normal operation diagnostic 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 optstring 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 char-
acter found is placed in OPTARG and no diagnostic mes-
sage 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 the full
file name of the command. The -r option causes the
shell to forget all remembered locations. If no argu-
ments 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 command 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 modi-
fied. 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 history 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 contents.
-c Clear the history list by deleting all the
entries.
-p Perform history substitution on the following args
and display the result on the standard 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 single
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.
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 infor-
mation.
-p List only the process ID of the job's process
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 informa-
tion 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 corresponding process
group ID, and executes command passing it args, return-
ing 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 argu-
ments 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 status of a process terminated by a signal. kill
returns true if at least one signal was successfully
sent, or false if an error occurs or an invalid option
is encountered.
let arg [arg ...]
Each arg is an arithmetic expression to be evaluated
(see ARITHMETIC EVALUATION). If the last arg evaluates
to 0, let returns 1; 0 is returned otherwise.
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 have a visi-
ble scope restricted to that function and its children.
With no operands, local writes a list of local vari-
ables to the standard output. It is an error to use
local when not within a function. 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 meanings:
+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 performed
as well, and the return status is 0. popd returns
false if an invalid option is encountered, the direc-
tory stack is empty, a non-existent directory stack
entry is specified, or the directory change fails.
printf format [arguments]
Write the formatted arguments to the standard output
under the control of the format. The format is a char-
acter 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 output, and format specifi-
cations, each of which causes printing of the next suc-
cessive 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 arguments than
are supplied, the extra format specifications behave as
if a zero value or null string, 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 arguments,
exchanges the top two directories and returns 0, unless
the directory stack is empty. Arguments, if supplied,
have the following meanings:
+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 performed
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 element 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 symbolic
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 second word to
the second name, and so on, with leftover 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 charac-
ter read and for line continuation. Options, if sup-
plied, have the following meanings:
-r Backslash does not act as an escape character.
The backslash is considered to be part of the
line. In particular, a backslash-newline pair may
not be used as a line continuation.
-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 terminal,
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 assign-
ment. 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 displayed 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 speci-
fied by n. If n is omitted, the return status is that
of the last command executed in the function 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 status of the script. If used out-
side 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 vari-
able are displayed in a format that can be reused as
input. The output is sorted according to the current
locale. When options are specified, 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 modified
or created for export to the environment of
subsequent commands.
-b Report the status of terminated background jobs
immediately, rather than before the next pri-
mary 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 state-
ments are placed in the environment for a com-
mand, not just those that precede the command
name.
-m Monitor mode. Job control is enabled. This
option is on by default for interactive 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 following:
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 interactive, unless
the shell is started with the --noedit-
ing option.
errexit Same as -e.
hashall Same as -h.
histexpand
Same as -H.
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 command
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 current 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 supplied, 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 command.
-u Treat unset variables as an error when perform-
ing parameter expansion. If expansion is
attempted on an unset variable, the shell
prints an error message, and, if not interac-
tive, exits with a non-zero status.
-v Print shell input lines as they are read.
-x After expanding each simple command, display
the expanded value of PS4, followed by the com-
mand 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 opera-
tors. This may be overridden when creating
output files by using the redirection 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 logical chain of
directories when performing commands which
change the current directory.
-- If no arguments follow this option, then the
positional parameters are unset. Otherwise,
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 positional parame-
ters. 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 specified 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-negative 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 $# 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 displayed,
with an indication of whether or not each is set. The
-p option causes output to be displayed in a form that
may be reused as input. Other options have the follow-
ing 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 arguments 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 arguments,
the display is limited to those options which are set
or unset, respectively. Unless otherwise 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 command
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 tran-
sposed characters, a missing character, and one
character too many. If a correction 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 execute it.
If a hashed command no longer exists, a normal
path search is performed.
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 specified 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 features
described above under Pathname Expansion are
enabled.
histappend
If set, the history list is appended to the
file named by the value of the HISTFILE vari-
able 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 his-
tory 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 further modification.
hostcomplete
If set, and readline is being used, bash will
attempt to perform hostname completion when a
word containing a @ is being completed (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 characters on that
line to be ignored in an interactive shell (see
COMMENTS above). This option is enabled by
default.
lithist If set, and the cmdhist option is enabled,
multi-line commands are saved to the history
with embedded newlines rather than using semi-
colon 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 themselves.
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 mes-
sage 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 containing 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 evaluation
of the conditional expression expr. Each operator and
operand must be a separate argument. Expressions are
composed of the primaries described above under CONDI-
TIONAL EXPRESSIONS.
Expressions may be combined using the following opera-
tors, listed in decreasing order of precedence.
! expr
True if expr is false.
( expr )
Returns the value of expr. This may be used to
override the normal precedence of operators.
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 EXPRES-
SIONS, the expression is true if the unary test is
true. If the first argument is not a valid unary
conditional operator, the expression is false.
3 arguments
If the second argument is one of the binary condi-
tional 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 argu-
ment is !, the value is the negation of the two-
argument test using the second and third argu-
ments. If the first argument is exactly ( and the
third argument is exactly ), the result is the
one-argument test of the second argument. Other-
wise, 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 expression composed
of the remaining arguments. Otherwise, the
expression is parsed and evaluated according to
precedence using the 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 signal specified
by each sigspec is ignored by the shell and by the
commands it invokes. If arg is -p then the trap com-
mands 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 command arg is executed on exit from the
shell. If a sigspec is DEBUG, the command arg is exe-
cuted 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; oth-
erwise trap returns true.
type [-atp] name [name ...]
With no options, indicate how each name would be inter-
preted if used as a command name. If the -t option is
used, type prints a string which is one of alias, key-
word, 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 using -a.
type returns true if any of the arguments 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 systems 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 current 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 available to
the shell
If limit is given, it is the new value of the specified
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 octal
number; otherwise it is interpreted as a symbolic 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 argument 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 variable. 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 environment passed to subsequent
commands. If any of RANDOM, SECONDS, LINENO, HISTCMD,
or DIRSTACK are unset, they lose their special proper-
ties, 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 termina-
tion status. n may be a process ID or a job specifica-
tion; if a job spec is given, all processes 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 con-
trolled than the standard shell. It behaves identically to
bash with the exception that the following are disallowed or
not performed:
o changing directories with cd
o setting or unsetting the values of SHELL, PATH, ENV, or
BASH_ENV
o specifying command names containing /
o specifying a file name containing a / as an argument to
the . builtin command
o importing function definitions from the shell environ-
ment at startup
o parsing the value of SHELLOPTS from the shell environ-
ment at startup
o redirecting output using the >, >|, <>, >&, &>, and >>
redirection operators
o using the exec builtin command to replace the shell
with another command
o adding or deleting builtin commands with the -f and -d
options to the enable builtin command
o specifying the -p option to the command builtin command
o turning off restricted mode with set +r or set +o res-
tricted.
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
Utilities, IEEE
Portable Operating System Interface (POSIX) Part 2: Shell and
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)
FILES
/bin/bash
The bash executable
/etc/profile
The systemwide initialization file, executed for login
shells
~/.bash_profile
The personal initialization file, executed for 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 specifi-
cation.
Aliases are confusing in some uses.
Shell builtin commands and functions are not
stoppable/restartable.
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 immediately
executes the next command in the sequence. It suffices to
place the sequence of commands between parentheses 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.
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