This section describes features unique to Bash.
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [argument ...] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] -c string [argument ...] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [argument ...]
In addition to the single-character shell command-line options (see section The Set Builtin), there are several multi-character options that you can use. These options must appear on the command line before the single-character options in order for them to be recognized.
--dump-po-strings
gettext
PO (portable object) file format.
--dump-strings
--help
--login
csh. `exec bash --login'
will replace the current shell with a Bash login shell.
--noediting
--noprofile
--norc
sh.
--posix
--rcfile filename
--restricted
--verbose
--version
There are several single-character options that may be supplied at
invocation which are not available with the set builtin.
-c string
$0.
-i
-r
-s
-D
C or POSIX (see section Locale-Specific Translation).
This implies the `-n' option; no commands will be executed.
--
-- signals the end of options and disables further option
processing.
Any arguments after the -- are treated as filenames and arguments.
An interactive shell is one whose input and output are both
connected to terminals (as determined by isatty(3)), or one
started with the `-i' option.
If arguments remain after option processing, and neither the
`-c' nor the `-s'
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands (see section Shell Scripts).
When Bash is invoked in this fashion, $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.
Bash's exit status is the exit status of the last command executed
in the script. If no commands are executed, the exit status is 0.
This section describs how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see section Tilde Expansion).
When Bash is invoked as an interactive login shell, it first reads and executes commands from the file `/etc/profile', if that file exists. After reading that file, it looks for `~/.bash_profile', `~/.bash_login', and `~/.profile', in that order, and reads and executes commands from the first one that exists and is readable. The `--noprofile' option may be used when the shell is started to inhibit this behavior.
When a login shell exits, Bash reads and executes commands from the file `~/.bash_logout', if it exists.
When an interactive shell that is not a login shell is started, Bash reads and executes commands from `~/.bashrc', if that file exists. This may be inhibited by using the `--norc' option. The `--rcfile file' option will force Bash to read and execute commands from file instead of `~/.bashrc'.
So, typically, your `~/.bash_profile' contains the line
if [ -f `~/.bashrc' ]; then . `~/.bashrc'; fi
after (or before) any login-specific initializations.
When Bash is started non-interactively, to run a shell script,
for example, it looks for the variable BASH_ENV in the environment,
expands its value if it appears there, and uses the expanded value as
the name of a file to read and execute. Bash behaves as if the
following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the
file name.
If Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the POSIX standard as well.
When invoked as an interactive login shell, it first attempts to read
and execute commands from `/etc/profile' and `~/.profile', in
that order.
The `--noprofile' option may be used to inhibit this behavior.
When invoked as an interactive shell with the name sh, Bash
looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and 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 startup files.
When invoked as sh, Bash enters POSIX mode after
the startup files are read.
When Bash is started in POSIX mode, as with the
`--posix' command line option, it follows the POSIX standard
for startup files.
In this mode, interactive shells expand the ENV variable
and commands are read and executed from the file whose name is the
expanded value.
No other startup files are read.
Bash attempts to determine when it is being run by the remote shell
daemon, usually rshd. If Bash determines it is being run by
rshd, it reads and executes commands from `~/.bashrc', if that
file exists and is readable.
It will not do this if invoked as sh.
The `--norc' option may be used to inhibit this behavior, and the
`--rcfile' option may be used to force another file to be read, but
rshd does not generally invoke the shell with those options or
allow them to be specified.
As defined in section Invoking Bash, an interactive shell
is one whose input and output are both
connected to terminals (as determined by isatty(3)),
or one started with the `-i' option.
To determine within a startup script whether Bash is
running interactively or not, examine the variable
$PS1; it is unset in non-interactive shells, and set in
interactive shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
Alternatively, startup scripts may test the value of the `-'
special parameter.
It contains i when the shell is interactive. For example:
case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac
This section describes builtin commands which are unique to or have been extended in Bash.
bind
bind [-m keymap] [-lpsvPSV] bind [-m keymap] [-q function] [-u function] [-r keyseq] bind [-m keymap] -f filename bind [-m keymap] keyseq:function-nameDisplay current Readline (see section Command Line Editing) 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' (@xref{Readline Init File}), 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
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
-p
-P
-v
-V
-s
-S
-f filename
-q function
-u function
-r keyseq
builtin
builtin [shell-builtin [args]]Run a shell builtin, passing it args, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if shell-builtin is not a shell builtin command.
command
command [-pVv] command [arguments ...]Runs command with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the
PATH are executed.
If there is a shell function named ls, running `command ls'
within the function will execute the external command ls
instead of calling the function recursively.
The `-p' option means to use a default value for $PATH
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if command cannot be
found or an error occurred, and the exit status of command
otherwise.
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. In this case, the return status is
zero if command is found, and non-zero if not.
declare
declare [-afFrxi] [-p] [name[=value]]Declare variables and give them attributes. If no names are given, then display the values of variables instead. 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 function definitions; only the function name and attributes are printed. `-F' implies `-f'. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:
-a
-f
-i
-r
-x
declare makes each name local,
as with the local command.
The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using -f foo=bar,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see section Arrays),
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 function with `-f'.
echo
echo [-neE] [arg ...]Output the args, separated by spaces, terminated with 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 interprets the following escape sequences:
\a
\b
\c
\e
\f
\n
\r
\t
\v
\\
\nnn
ASCII code is the octal value nnn
(one to three digits)
\xnnn
ASCII code is the hexadecimal value nnn
(one to three digits)
enable
enable [-n] [-p] [-f filename] [-ads] [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, the names become disabled. Otherwise names are enabled. For example, to use the
test binary
found via $PATH instead of the shell builtin version, type
`enable -n test'.
If the `-p' option is supplied, or no name arguments appear,
a list of shell builtins is printed. With no other arguments, the list
consists of all enabled shell builtins.
The `-a' option means to list
each builtin with an indication of whether or not it is enabled.
The `-f' option means to load the new builtin command name
from shared object filename, on systems that support dynamic loading.
The `-d' option will delete a builtin loaded with `-f'.
If there are no options, a list of the shell builtins is displayed.
The `-s' option restricts enable to the POSIX special
builtins. If `-s' is used with `-f', the new builtin becomes
a special builtin.
The return status is zero unless a name is not a shell builtin
or there is an error loading a new builtin from a shared object.
help
help [pattern]Display helpful information about builtin commands. If pattern is specified,
help gives detailed help
on all commands matching pattern, otherwise a list of
the builtins is printed. The return status is zero unless no
command matches pattern.
let
let expression [expression]The
let builtin allows arithmetic to be performed on shell
variables. Each expression is evaluated according to the
rules given below in section Shell Arithmetic. If the
last expression evaluates to 0, let returns 1;
otherwise 0 is returned.
local
local name[=value]For each argument, a local variable named name is created, and assigned value.
local can only be used within a function; it makes the variable
name have a visible scope restricted to that function and its
children. The return status is zero unless local is used outside
a function or an invalid name is supplied.
logout
logout [n]Exit a login shell, returning a status of n to the shell's parent.
printf
printf format [arguments]
Write the formatted arguments to the standard output under the
control of the format.
The format is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
argument.
In addition to the standard printf(1) formats, `%b' causes
printf to expand backslash escape sequences in the corresponding
argument, and `%q' causes printf to output the
corresponding argument in a format that can be reused as shell input.
The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied.
read
read [-a aname] [-p prompt] [-er] [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 the value of the
IFS variable
are used to split the line into words.
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.
Options, if supplied, have the following meanings:
-r
-p prompt
-a aname
-e
shopt
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 following meanings:
-s
-u
-q
-o
set builtin (see section The Set Builtin).
shopt options are disabled (off)
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
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
cd command will be corrected.
The errors checked for are transposed characters,
a missing character, and a character too many.
If a correction is found, the corrected path is printed,
and the command proceeds.
This option is only used by interactive shells.
checkhash
checkwinsize
LINES and COLUMNS.
cmdhist
dotglob
execfail
exec
builtin command. An interactive shell does not exit if exec
fails.
expand_aliases
extglob
histappend
HISTFILE
variable when the shell exits, rather than overwriting the file.
histreedit
histverify
hostcomplete
huponexit
SIGHUP to all jobs when an interactive
login shell exits (see section Signals).
interactive_comments
lithist
cmdhist
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
mailwarn
"The mail in mailfile has been read" is displayed.
nocaseglob
nullglob
promptvars
shift_verbose
shift
builtin prints an error message when the shift count exceeds the
number of positional parameters.
sourcepath
source builtin uses the value of PATH
to find the directory containing the file supplied as an argument.
This option is enabled by default.
source
source filenameA synonym for
. (see section Bourne Shell Builtins).
type
type [-atp] [name ...]For each name, indicate how it would be interpreted if used as a command name. If the `-t' option is used,
type prints a single word
which is one of `alias', `function', `builtin',
`file' or `keyword',
if name is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the name is not found, then nothing is printed, and
type returns a failure status.
If the `-p' option is used, type either returns the name
of the disk file that would be executed, or nothing if `-t'
would not return `file'.
If the `-a' option is used, type returns all of the places
that contain an executable named file.
This includes aliases and functions, if and only if the `-p' option
is not also used.
The return status is zero if any of the names are found, non-zero
if none are found.
typeset
typeset [-afFrxi] [-p] [name[=value]]The
typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the declare
builtin command.
ulimit
ulimit [-acdflmnpstuvSH] [limit]
ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an
option is given, it is interpreted as follows:
-S
-H
-a
-c
-d
-f
-l
-m
-n
-p
-s
-t
-u
-v
unlimited is supplied as a
limit, or an error occurs while setting a new limit.
This builtin is so complicated that it deserves its own section.
set
set [--abefhkmnptuvxBCHP] [-o option] [argument ...]If no options or arguments are supplied,
set displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input.
When options are supplied, they set or unset shell attributes.
Options, if specified, have the following meanings:
-a
-b
-e
until or while loop, part of an if statement,
part of a && or || list, or if the command's return
status is being inverted using !.
-f
-h
-k
-m
-n
-o option-name
allexport
-a.
braceexpand
-B.
emacs
emacs-style line editing interface (see section Command Line Editing).
errexit
-e.
hashall
-h.
histexpand
-H.
history
ignoreeof
keyword
-k.
monitor
-m.
noclobber
-C.
noexec
-n.
noglob
-f.
notify
-b.
nounset
-u.
onecmd
-t.
physical
-P.
posix
privileged
-p.
verbose
-v.
vi
vi-style line editing interface.
xtrace
-x.
-p
$BASH_ENV and $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.
This is enabled automatically
on startup if the effective user (group) id is not equal to the real
user (group) id. Turning this option off causes the effective user
and group ids to be set to the real user and group ids.
-t
-u
-v
-x
-B
-C
-H
-P
cd which change the current directory. The physical directory
is used instead. By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.
For example, if `/usr/sys' is a symbolic link to `/usr/local/sys'
then:
$ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usrIf
set -P is on, then:
$ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local
--
-
$-.
The remaining N arguments are positional parameters and are
assigned, in order, to $1, $2, ... $N.
The special parameter # is set to N.
The return status is always zero unless an invalid option is supplied.
Conditional expressions are used by the [[ compound command
and the test and [ builtin commands.
Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If any file argument to one of the primaries is of the form `/dev/fd/N', then file descriptor N is checked.
-a file
-b file
-c file
-d file
-e file
-f file
-g file
-k file
-p file
-r file
-s file
-t fd
-u file
-w file
-x file
-O file
-G file
-L file
-S file
-N file
file1 -nt file2
file1 -ot file2
file1 -ef file2
-o optname
set builtin (see section The Set Builtin).
-z string
-n string
string
string1 == string2
string1 != string2
string1 < string2
string1 > string2
arg1 OP arg2
OP is one of
`-eq', `-ne', `-lt', `-le', `-gt', or `-ge'.
These arithmetic binary operators return true if arg1
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to arg2,
respectively. Arg1 and arg2
may be positive or negative integers.
These variables are set or used by Bash, but other shells do not normally treat them specially.
BASH
BASH_ENV
BASH_VERSION
BASH_VERSINFO
BASH_VERSINFO[0]
BASH_VERSINFO[1]
BASH_VERSINFO[2]
BASH_VERSINFO[3]
BASH_VERSINFO[4]
BASH_VERSINFO[5]
MACHTYPE.
DIRSTACK
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.
EUID
FCEDIT
fc
builtin command.
FIGNORE
FIGNORE
is excluded from the list of matched file names. A sample
value is `.o:~'
GLOBIGNORE
GLOBIGNORE, it is removed from the list
of matches.
GROUPS
histchars
HISTCMD
HISTCMD is unset, it loses its special properties,
even if it is subsequently reset.
HISTCONTROL
HISTCONTROL.
HISTIGNORE
HISTCONTROL
are applied. In addition to the normal shell pattern matching
characters, `&' matches the previous history line. `&'
may be escaped using a backslash. The backslash is removed
before attempting a match.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTIGNORE.
HISTIGNORE subsumes the function of HISTCONTROL. A
pattern of `&' is identical to ignoredups, and a
pattern of `[ ]*' is identical to ignorespace.
Combining these two patterns, separating them with a colon,
provides the functionality of ignoreboth.
HISTFILE
HISTSIZE
HISTFILESIZE
HOSTFILE
HOSTNAME
HOSTTYPE
IGNOREEOF
EOF character
as the sole input. If set, the value denotes the number
of consecutive EOF characters that can be read as the
first character on an input line
before the shell will exit. If the variable exists but does not
have a numeric value (or has no value) then the default is 10.
If the variable does not exist, then EOF signifies the end of
input to the shell. This is only in effect for interactive shells.
INPUTRC
LANG
LC_.
LC_ALL
LANG and any other
LC_ variable specifying a locale category.
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LINENO
MACHTYPE
MAILCHECK
MAILPATH or MAIL variables.
OLDPWD
cd builtin.
OPTERR
getopts builtin command.
OSTYPE
PIPESTATUS
PPID
PROMPT_COMMAND
$PS1).
PS3
select command. If this variable is not set, the
select command prompts with `#? '
PS4
PS4 is replicated multiple times, as
necessary, to indicate multiple levels of indirection.
The default is `+ '.
PWD
cd builtin.
RANDOM
REPLY
read builtin.
SECONDS
SHELLOPTS
set builtin command (see section The Set Builtin).
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 readonly.
SHLVL
TIMEFORMAT
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.
%%
%[p][l]R
%[p][l]U
%[p][l]S
%P
l specifies a longer format, including minutes, of
the form MMmSS.FFs.
The value of p determines whether or not the fraction is included.
If this variable is not set, Bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.
TMOUT
UID
The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by the let builtin.
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 levels are listed in order of decreasing precedence.
- +
! ~
**
* / %
+ -
<< >>
<= >= < >
== !=
&
^
|
&&
||
expr ? expr : expr
= *= /= %= += -= <<= >>= &= ^= |=
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 override the precedence rules above.
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.
The first word of each simple 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 shell metacharacters, with the exception
that the alias name may not contain `='.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time. This means that one may alias
ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, 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,
as in csh.
If arguments are needed, a shell function should be used
(see section Shell Functions).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see section Bash Builtin Commands).
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 function definition
is itself a compound command. As a consequence, aliases
defined in a function are not available until after that
function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.
For almost every purpose, aliases are superseded by shell functions.
alias
alias [-p] [name[=value] ...]
Without arguments or with the `-p' option, alias prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each name
whose value is given. If no value is given, the name
and value of the alias is printed.
unalias
unalias [-a] [name ... ]Remove each name from the list of aliases. If `-a' is supplied, all aliases are removed.
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 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
The syntax
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. If
the optional subscript is 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 introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell's filename expansion operators. If the
subscript is `@' or `*', the word expands to all members
of the array 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 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 `*'.
${#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 subscript.
unset name, where name is an array, removes the
entire array. A subscript of `*' or `@' also 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, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.
The directory stack is a list of recently-visited directories. The
pushd builtin adds directories to the stack as it changes
the current directory, and the popd builtin removes specified
directories from the stack and changes the current directory to
the directory removed. The dirs builtin displays the contents
of the directory stack.
The contents of the directory stack are also visible
as the value of the DIRSTACK shell variable.
dirs
dirs [+N | -N] [-clvp]Display the list of currently remembered directories. Directories are added to the list with the
pushd command; the
popd command removes directories from the list.
+N
dirs when invoked without options), starting
with zero.
-N
dirs when invoked without options), starting
with zero.
-c
-l
-p
dirs to print the directory stack with one entry per
line.
-v
dirs to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
popd
popd [+N | -N] [-n]Remove the top entry from the directory stack, and
cd
to the new top directory.
When no arguments are given, popd
removes the top directory from the stack and
performs a cd to the new top directory. The
elements are numbered from 0 starting at the first directory listed with
dirs; i.e., popd is equivalent to popd +0.
+N
dirs), starting with zero.
-N
dirs), starting with zero.
-n
pushd
pushd [dir | +N | -N] [-n]Save the current directory on the top of the directory stack and then
cd to dir.
With no arguments, pushd exchanges the top two directories.
+N
dirs, starting with zero) to the top of
the list by rotating the stack.
-N
dirs, starting with zero) to the top of
the list by rotating the stack.
-n
dir
cd dir'.
cds to dir.
The value of the variable PROMPT_COMMAND is examined just before
Bash prints each primary prompt. If it is set and non-null, then the
value is executed just as if it had been typed on the command line.
In addition, the following table describes the special characters which can appear in the prompt variables:
\a
\d
\e
\h
\H
\n
\r
\s
$0 (the portion
following the final slash).
\t
\T
\@
\u
\v
\V
\w
\W
$PWD.
\!
\#
\$
#, otherwise $.
\nnn
\\
\[
\]
If Bash is started with the name rbash, or the
`--restricted'
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash
with the exception that the following are disallowed:
cd builtin.
SHELL or PATH
variables.
.
builtin command.
SHELLOPTS from the shell environment at startup.
exec builtin to replace the shell with another command.
enable builtin.
command builtin.
Starting Bash with the `--posix' command-line option or executing `set -o posix' while Bash is running will cause Bash to conform more closely to the POSIX.2 standard by changing the behavior to match that specified by POSIX.2 in areas where the Bash default differs.
The following list is what's changed when `POSIX mode' is in effect:
$PATH to find the new location. This is also available with
`shopt -s checkhash'.
PS1 and PS2 expansions of `!' to
the history number and `!!' to `!' are enabled,
and parameter expansion is performed on the values of PS1 and
PS2 regardless of the setting of the promptvars option.
$ENV) rather than
the normal Bash files.
$HISTFILE).
. filename
is not found.
names. That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit. Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.
cd builtin finds a directory to change to
using $CDPATH, the
value it assigns to the PWD variable does not contain any
symbolic links, as if `cd -P' had been executed.
$CDPATH is set, the cd builtin will not implicitly
append the current directory to it. This means that cd will
fail if no valid directory name can be constructed from
any of the entries in $CDPATH, even if the a directory with
the same name as the name given as an argument to cd exists
in the current directory.
for statement or the selection variable in a
select statement is a readonly variable.
export and readonly builtin commands display their
output in the format required by POSIX.2.
There is other POSIX.2 behavior that Bash does not implement. Specifically:
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