This text is a brief description of the features that are present in the Bash shell (version 3.2, 28 September 2006).
This is Edition 3.2, last updated 28 September 2006,
of The GNU Bash Reference Manual,
for Bash, Version 3.2.
Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (sh), the Korn Shell (ksh), and the C-shell (csh and its successor, tcsh). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature.
This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.
Bash is the shell, or command language interpreter,
for the gnu operating system.
The name is an acronym for the ¡®Bourne-Again SHell¡¯,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell sh,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
Bash is largely compatible with sh and incorporates useful
features from the Korn shell ksh and the C shell csh.
It is intended to be a conformant implementation of the ieee
posix Shell and Tools portion of the ieee posix
specification (ieee Standard 1003.1).
It offers functional improvements over sh for both interactive and
programming use.
While the gnu operating system provides other shells, including
a version of csh, Bash is the default shell.
Like other gnu software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems −
independently-supported ports exist for ms-dos, os/2,
and Windows platforms.
At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions.
A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of gnu utilities. The programming language features allow these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.
Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file.
A shell allows execution of gnu commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments.
Shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, cd, break, continue, and
exec) cannot be implemented outside of the shell because
they directly manipulate the shell itself.
The history, getopts, kill, or pwd
builtins, among others, could be implemented in separate utilities,
but they are more convenient to use as builtin commands.
All of the shell builtins are described in
subsequent sections.
While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.
Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual.
These definitions are used throughout the remainder of this manual.
POSIXblankbuiltincontrol operatorword that performs a control function. It is a newline
or one of the following:
¡®||¡¯, ¡®&&¡¯, ¡®&¡¯, ¡®;¡¯, ¡®;;¡¯,
¡®|¡¯, ¡®(¡¯, or ¡®)¡¯.
exit statusfieldfilenamejobjob controlmetacharacterblank or one of the following characters:
¡®|¡¯, ¡®&¡¯, ¡®;¡¯, ¡®(¡¯, ¡®)¡¯, ¡®<¡¯, or
¡®>¡¯.
nameword consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. Names are used as
shell variable and function names.
Also referred to as an identifier.
operatorcontrol operator or a redirection operator.
See Redirections, for a list of redirection operators.
process groupprocess group IDprocess group
during its lifetime.
reserved wordword that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as for and
while.
return statusexit status.
signalspecial builtintokenword or an operator.
wordtoken that is not an operator.
Bash is an acronym for ¡®Bourne-Again SHell¡¯. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the posix specification for the `standard' Unix shell.
This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (¡®#¡¯), and the rest of that line.
Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing.
The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:
metacharacters. Alias expansion is performed by this step
(see Aliases).
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 shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see History Interaction), the history expansion character, usually ¡®!¡¯, must be quoted to prevent history expansion. See Bash History Facilities, for more details concerning history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash ¡®\¡¯ is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of newline. If a \newline pair
appears, and the backslash itself is not quoted, the \newline
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
Enclosing characters in single quotes (¡®'¡¯) preserves the 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 exception of
¡®$¡¯, ¡®`¡¯, ¡®\¡¯,
and, when history expansion is enabled, ¡®!¡¯.
The characters ¡®$¡¯ and ¡®`¡¯
retain their special meaning within double quotes (see Shell Expansions).
The backslash retains its special meaning only when followed by one of
the following characters:
¡®$¡¯, ¡®`¡¯, ¡®"¡¯, ¡®\¡¯, or newline.
Within double quotes, backslashes that are followed by one of these
characters are removed. Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an ¡®!¡¯
appearing in double quotes is escaped using a backslash.
The backslash preceding the ¡®!¡¯ is not removed.
The special parameters ¡®*¡¯ and ¡®@¡¯ have special meaning when in double quotes (see Shell Parameter Expansion).
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a\b\e\f\n\r\t\v\\\'\nnn\xHH\cxThe expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign (¡®$¡¯) 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.
Some systems use the message catalog selected by the LC_MESSAGES shell variable. Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding a suffix of ¡®.mo¡¯. If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files. Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see Bash Builtins),
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 interactive shells.
See Interactive Shells, for a description of what makes
a shell interactive.
A simple shell command such as echo a b c consists of the command
itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
A simple command is the kind of command encountered most often.
It's just a sequence of words separated by blanks, terminated
by one of the shell's control operators (see Definitions). The
first word generally specifies a command to be executed, with the
rest of the words being that command's arguments.
The return status (see Exit Status) of a simple command is
its exit status as provided
by the posix 1003.1 waitpid function, or 128+n if
the command was terminated by signal n.
A pipeline is a sequence of simple commands separated by
¡®|¡¯.
[time[-p]] [!] command1 [|command2 ...]
The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output.
The reserved word time causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
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 Bash Variables, for a description of the available formats.
The use of time as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external
time command cannot time these easily.
If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell
(see Command Execution Environment). The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the pipefail option is enabled
(see The Set Builtin).
If pipefail is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word ¡®!¡¯ precedes the pipeline, the
exit status is the logical negation of the exit status as described
above.
The shell waits for all commands in the pipeline to terminate before
returning a value.
A list is a sequence of one or more pipelines separated by one
of the operators ¡®;¡¯, ¡®&¡¯, ¡®&&¡¯, or ¡®||¡¯,
and optionally terminated by one of ¡®;¡¯, ¡®&¡¯, or a
newline.
Of these list operators, ¡®&&¡¯ and ¡®||¡¯ have equal precedence, followed by ¡®;¡¯ and ¡®&¡¯, which have equal precedence.
A sequence of one or more newlines may appear in a list
to delimit commands, equivalent to a semicolon.
If a command is terminated by the control operator ¡®&¡¯,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the background.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see Job Control),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from /dev/null.
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
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.
An or list has the form
command1 || command2
command2 is executed if, and only if, command1 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 are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.
Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.
Bash supports the following looping constructs.
Note that wherever a ¡®;¡¯ appears in the description of a command's syntax, it may be replaced with one or more newlines.
untiluntil command is:
until test-commands; do consequent-commands; done
Execute consequent-commands as long as
test-commands has an exit status which is not zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.
whilewhile command is:
while test-commands; do consequent-commands; done
Execute consequent-commands as long as
test-commands has an exit status of zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.
forfor command is:
for name [in words ...]; do commands; done
Expand words, and execute commands once for each member
in the resultant list, with name bound to the current member.
If ¡®in words¡¯ is not present, the for command
executes the commands once for each positional parameter that is
set, as if ¡®in "$@"¡¯ had been specified
(see Special Parameters).
The return status is the exit status of the last command that executes.
If there are no items in the expansion of words, no commands are
executed, and the return status is zero.
An alternate form of the for command is also supported:
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below (see Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.
The break and continue builtins (see Bourne Shell Builtins)
may be used to control loop execution.
ifif command is:
if test-commands; then
consequent-commands;
[elif more-test-commands; then
more-consequents;]
[else alternate-consequents;]
fi
The test-commands list is executed, and if its return status is zero,
the consequent-commands list is executed.
If test-commands returns a non-zero status, each elif list
is executed in turn, and if its exit status is zero,
the corresponding more-consequents is executed and the
command completes.
If ¡®else alternate-consequents¡¯ is present, and
the final command in the final if or elif clause
has a non-zero exit status, then alternate-consequents is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.
casecase command is:
casewordin [ [(]pattern[|pattern]...)command-list;;]... esac
case will selectively execute the command-list corresponding to
the first pattern that matches word.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The ¡®|¡¯ is used to separate multiple patterns, and the ¡®)¡¯
operator terminates a pattern list.
A list of patterns and an associated command-list is known
as a clause. Each clause must be terminated with ¡®;;¡¯.
The word undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before matching is
attempted. Each pattern undergoes tilde expansion, parameter
expansion, command substitution, and arithmetic expansion.
There may be an arbitrary number of case clauses, each terminated
by a ¡®;;¡¯. The first pattern that matches determines the
command-list that is executed.
Here is an example using case in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
The return status is zero if no pattern is matched. Otherwise, the
return status is the exit status of the command-list executed.
selectselect construct allows the easy generation of menus.
It has almost the same syntax as the for command:
select name [in words ...]; do commands; done
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
¡®in words¡¯ is omitted, the positional parameters are printed,
as if ¡®in "$@"¡¯ had been specifed.
The PS3 prompt is then displayed and a line is 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 select command completes.
Any other value read causes name to be set to null.
The line read is saved in the variable REPLY.
The commands are executed after each selection until a
break command is executed, at which
point the select command completes.
Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.
select fname in *;
do
echo you picked $fname \($REPLY\)
break;
done
((...))(( expression ))
The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic). 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"
See Bash Builtins, for a full description of the let builtin.
[[...]][[ 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 in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the ¡®[[¡¯ and ¡®]]¡¯; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as ¡®-f¡¯ must be unquoted to be recognized as primaries.
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 in Pattern Matching.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches (¡®==¡¯) or does not
match (¡®!=¡¯)the pattern, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.
An additional binary operator, ¡®=~¡¯, is available, with the same
precedence as ¡®==¡¯ and ¡®!=¡¯.
When it is used, the string to the right of the operator is considered
an extended regular expression and matched accordingly (as in regex3)).
The return value is 0 if the string matches
the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the conditional
expression's return value is 2.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the array variable BASH_REMATCH.
The element of BASH_REMATCH with index 0 is the portion of the string
matching the entire regular expression.
The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )! expression && expression2 || expression2&& and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()( list )
Placing a list of commands between parentheses causes a subshell
environment to be created (see Command Execution Environment), and each
of the commands in list to be executed in that subshell. Since the
list is executed in a subshell, variable assignments do not remain in
effect after the subshell completes.
{} { list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are reserved words, so they must be separated from the list
by blanks. The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from the list by whitespace.
The exit status of both of these constructs is the exit status of list.
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
[ function ] name () compound-command [ redirections ]
This defines a shell function named name. The reserved
word function is optional.
If the function reserved
word is supplied, the parentheses are optional.
The body of the function is the compound command
compound-command (see Compound Commands).
That command is usually a list enclosed between { and }, but
may be any compound command listed above.
compound-command is executed whenever name is specified as the
name of a command.
Any redirections (see Redirections) associated with the shell function
are performed when the function is executed.
A function definition may be deleted using the -f option to the
unset builtin (see Bourne Shell Builtins).
The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.
Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
blanks or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated by whitespace.
Also, when using the braces, the list must be terminated by a semicolon,
a ¡®&¡¯, or a newline.
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see Positional Parameters).
The special parameter ¡®#¡¯ that expands to the number of
positional parameters is updated to reflect the change.
Special parameter 0 is unchanged.
The first element of the FUNCNAME variable is set to the
name of the function while the function is executing.
All other aspects of the shell execution
environment are identical between a function and its caller
with the exception that the DEBUG and RETURN traps
are not inherited unless the function has been given the
trace attribute using the declare builtin or
the -o functrace option has been enabled with
the set builtin,
(in which case all functions inherit the DEBUG and RETURN traps).
See Bourne Shell Builtins, for the description of the
trap builtin.
If the builtin command return
is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the RETURN trap is executed
before execution resumes.
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. If a numeric argument is given to return,
that is the function's return status; otherwise the function's
return status is the exit status of the last command executed
before the return.
Variables local to the function may be declared with the
local builtin. These variables are visible only to
the function and the commands it invokes.
Function names and definitions may be listed with the
-f option to the declare or typeset
builtin commands (see Bash Builtins).
The -F option to declare or typeset
will list the function names only
(and optionally the source file and line number, if the extdebug
shell option is enabled).
Functions may be exported so that subshells
automatically have them defined with the
-f option to the export builtin
(see Bourne Shell Builtins).
Note that shell functions and variables with the same name may result
in multiple identically-named entries in the environment passed to the
shell's children.
Care should be taken in cases where this may cause a problem.
Functions may be recursive. No limit is placed on the number of recursive calls.
A parameter is an entity that stores values.
It can be a name, a number, or one of the special characters
listed below.
A variable is a parameter denoted by a name.
A variable has a value and zero or more attributes.
Attributes are assigned using the declare builtin command
(see the description of the declare builtin in Bash Builtins).
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.
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,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its integer
attribute set, then value
is evaluated as an arithmetic expression even if the $((...))
expansion is not used (see Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
alias,
declare, typeset, export, readonly,
and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ¡®+=¡¯ operator can be used to append to or add to the variable's previous value. When ¡®+=¡¯ is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When ¡®+=¡¯ is applied to an array variable using compound assignment (see Arrays), the variable's value is not unset (as it is when using ¡®=¡¯), and new values are appended to the array beginning at one greater than the array's maximum index. When applied to a string-valued variable, value is expanded and appended to the variable's value.
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the set builtin command.
Positional parameter N may be referenced as ${N}, or
as $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*"$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS
variable.
If IFS is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without intervening
separators.
@"$@" is equivalent to
"$1" "$2" ....
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, "$@" and
$@
expand to nothing (i.e., they are removed).
#?-set
builtin command, or those set by the shell itself
(such as the -i option).
$() subshell, it
expands to the process id of the invoking shell, not the subshell.
!0$0 is set to the name of that file.
If Bash is started with the -c option (see Invoking Bash),
then $0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
Expansion is performed on the command line after it has been split into
tokens. There are seven kinds of expansion performed:
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" (see Special Parameters) and "${name[@]}"
(see Arrays).
After all expansions, quote removal (see Quote Removal)
is performed.
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequnce expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form {x..y},
where x and y are either integers or single characters.
When integers are supplied, the expression expands to each number between
x and y, inclusive.
When characters are supplied, the expression expands to each character
lexicographically between x and y, inclusive. Note that
both x and y must be of the same type.
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. To avoid conflicts with parameter expansion, the string ¡®${¡¯ is not considered eligible for brace expansion.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.
A { or ¡®,¡¯ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ¡®${¡¯ is not considered eligible for brace expansion.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
If a word begins with an unquoted tilde character (¡®~¡¯), all of the characters up to 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 HOME shell variable. If HOME is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.
If the tilde-prefix is ¡®~+¡¯, the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is ¡®~-¡¯, the value of the shell variable OLDPWD, if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a ¡®+¡¯ or a ¡®-¡¯,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the dirs builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see The Directory Stack).
If the tilde-prefix, sans the tilde, consists of a number without a
leading ¡®+¡¯ or ¡®-¡¯, ¡®+¡¯ is assumed.
If the login name is invalid, or the tilde expansion fails, the word is left unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a ¡®:¡¯ or the first ¡®=¡¯. 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.
The following table shows how Bash treats unquoted tilde-prefixes:
~$HOME
~/foo~fred/foofoo of the home directory of the user
fred
~+/foo~-/foo~N~+N~-NThe ¡®$¡¯ 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 parameter expansion.
The basic form of parameter expansion is ${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 that 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 is used in the rest of the substitution, rather
than the value of parameter itself.
This is known as indirect expansion.
The exceptions to this are the expansions of ${!prefix*}
and ${!name[@]}
described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both existence and that the value is not null; if the colon is omitted, the operator tests only for existence.
${parameter:−word}${parameter:=word}${parameter:?word}${parameter:+word}${parameter:offset}${parameter:offset:length}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]}.
A negative offset is taken relative to one greater than the maximum
index of the specified array.
Note that a negative offset must be separated from the colon by at least
one space to avoid being confused with the ¡®:-¡¯ expansion.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1.
${!prefix*}${!prefix@}${!name[@]}${!name[*]}${#parameter}${parameter#word}${parameter##word}${parameter%word}${parameter%%word}${parameter/pattern/string}/ 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 allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command)
or
`command`
Bash performs the expansion by executing command and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
¡®$¡¯, ¡®`¡¯, or ¡®\¡¯.
The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(command) form, all characters between
the 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 filename expansion are not performed on the results.
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, command substitution, and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.
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 process 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.
Note that no space may appear between the < or >
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.
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 character in IFS that is not IFS
whitespace, along with any adjacent 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 argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
nullglob is disabled, the word is left
unchanged.
If the nullglob option is set, and no matches are found, the word
is removed.
If the failglob shell option is set, and no matches are found,
an error message is printed and the command is not executed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters.
When a pattern is used for filename generation, the character ¡®.¡¯
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option dotglob is set.
When matching a file name, the slash character must always be
matched explicitly.
In other cases, the ¡®.¡¯ character is not treated specially.
See the description of shopt in Bash Builtins,
for a description of the nocaseglob, nullglob,
failglob, and dotglob options.
The GLOBIGNORE
shell variable may be used to restrict the set of filenames matching a
pattern. If GLOBIGNORE
is set, each matching filename that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches. The filenames
. and ..
are always ignored when GLOBIGNORE
is set and not null.
However, setting GLOBIGNORE to a non-null value has the effect of
enabling the dotglob
shell option, so all other filenames beginning with a
¡®.¡¯ will match.
To get the old behavior of ignoring filenames beginning with a
¡®.¡¯, make ¡®.*¡¯ one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE
is unset.
Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The nul character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
*?[...]For example, in the default C locale, ¡®[a-dx-z]¡¯ is equivalent to ¡®[abcdxyz]¡¯. Many locales sort characters in dictionary order, and in these locales ¡®[a-dx-z]¡¯ is typically not equivalent to ¡®[abcdxyz]¡¯; it might be equivalent to ¡®[aBbCcDdxXyYz]¡¯, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the LC_COLLATE or LC_ALL environment variable to the value ¡®C¡¯.
Within ¡®[¡¯ and ¡®]¡¯, character classes can be specified
using the syntax
[:class:], where class is one of the
following classes defined in the posix standard:
alnum alpha ascii blank cntrl digit graph lower
print punct space upper word xdigit
A character class matches any character belonging to that class.
The word character class matches letters, digits, and the character
¡®_¡¯.
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 collating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized.
In the following description, a pattern-list is a list of one
or more patterns separated by a ¡®|¡¯.
Composite patterns may be formed using one or more of the following
sub-patterns:
?(pattern-list)*(pattern-list)+(pattern-list)@(pattern-list)!(pattern-list)After the preceding expansions, all unquoted occurrences of the characters ¡®\¡¯, ¡®'¡¯, and ¡®"¡¯ that did not result from one of the above expansions are removed.
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 redirection 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, filename expansion, and word splitting. 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 (file descriptor 1) and standard error (file descriptor 2) 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.
Bash handles several filenames specially when they are used in redirections, as described in the following table:
/dev/fd/fd/dev/stdin/dev/stdout/dev/stderr/dev/tcp/host/port/dev/udp/host/portA failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
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
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 file 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 is not enabled, the redirection
is attempted even if the file named by word exists.
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
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 semantically equivalent to
>word 2>&1
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 command.
The format of here-documents is:
<<[−]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion,
or filename 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 character sequence \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 containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
A variant of here documents, the format is:
<<< word
The word is expanded and supplied to the command on its standard input.
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.
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.
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.
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.