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This text is a brief description of the features that are present in the Bash shell (version 3.0, 27 July 2004)..
This is Edition 3.0, last updated 27 July 2004,
of The GNU Bash Reference Manual,
for Bash, Version 3.0.
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.
1. Introduction An introduction to the shell.
2. Definitions Some definitions used in the rest of this manual.
3. Basic Shell Features The shell "building blocks".
4. Shell Builtin Commands Commands that are a part of the shell.
5. Shell Variables Variables used or set by Bash.
6. Bash Features Features found only in Bash.
7. Job Control A chapter describing what job control is and how Bash allows you to use it.
9. Using History Interactively Chapter dealing with history expansion rules.
8. Command Line Editing Chapter describing the command line editing features.
10. Installing Bash How to build and install Bash on your system.
A. Reporting Bugs How to report bugs in Bash.
B. Major Differences From The Bourne Shell A terse list of the differences between Bash and historical versions of /bin/sh.
C. Copying This Manual Copying this manual.
Index of Shell Builtin Commands Index of Bash builtin commands.
Index of Shell Reserved Words Index of Bash reserved words.
Parameter and Variable Index Quick reference helps you find the variable you want.
Function Index Index of bindable Readline functions.
Concept Index General index for concepts described in this manual.
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1.1 What is Bash? A short description of Bash.
1.2 What is a shell? A brief introduction to shells.
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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 specification (IEEE Working Group 1003.2).
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.
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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.
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POSIX
blank
builtin
control operator
word that performs a control function. It is a newline
or one of the following:
`||', `&&', `&', `;', `;;',
`|', `(', or `)'.
exit status
field
filename
job
job control
metacharacter
blank or one of the following characters:
`|', `&', `;', `(', `)', `<', or
`>'.
name
word 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.
operator
control operator or a redirection operator.
See section 3.6 Redirections, for a list of redirection operators.
process group
process group ID
process group
during its lifetime.
reserved word
word that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as for and
while.
return status
exit status.
signal
special builtin
token
word or an operator.
word
token that is not an operator.
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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.
3.1 Shell Syntax What your input means to the shell. 3.2 Shell Commands The types of commands you can use. 3.3 Shell Functions Grouping commands by name. 3.4 Shell Parameters How the shell stores values. 3.5 Shell Expansions How Bash expands parameters and the various expansions available. 3.6 Redirections A way to control where input and output go. 3.7 Executing Commands What happens when you run a command. 3.8 Shell Scripts Executing files of shell commands.
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3.1.1 Shell Operation The basic operation of the shell.
3.1.2 Quoting How to remove the special meaning from characters.
3.1.3 Comments How to specify comments.
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.
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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 section 6.6 Aliases).
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3.1.2.1 Escape Character How to remove the special meaning from a single character. 3.1.2.2 Single Quotes How to inhibit all interpretation of a sequence of characters. 3.1.2.3 Double Quotes How to suppress most of the interpretation of a sequence of characters. 3.1.2.4 ANSI-C Quoting How to expand ANSI-C sequences in quoted strings.
3.1.2.5 Locale-Specific Translation How to translate strings into different languages.
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 section 2. 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, the history expansion character, usually `!', must be quoted to prevent history expansion. See section 9.1 Bash History Facilities, for more details concerning history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
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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).
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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.
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Enclosing characters in double quotes (`"') preserves the literal value
of all characters within the quotes, with the exception of
`$', ``', and `\'.
The characters `$' and ``'
retain their special meaning within double quotes (see section 3.5 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.
When command history is being used, the double quote may not be used to
quote the history expansion character.
The special parameters `*' and `@' have special meaning when in double quotes (see section 3.5.3 Shell Parameter Expansion).
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Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a
\b
\e
\f
\n
\r
\t
\v
\\
\'
\nnn
\xHH
\cx
The expanded result is single-quoted, as if the dollar sign had not been present.
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A double-quoted string preceded by a dollar sign (`$') 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.
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In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see section 4.2 Bash Builtin Commands),
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 section 6.3 Interactive Shells, for a description of what makes
a shell interactive.
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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.
3.2.1 Simple Commands The most common type of command. 3.2.2 Pipelines Connecting the input and output of several commands. 3.2.3 Lists of Commands How to execute commands sequentially. 3.2.4 Compound Commands Shell commands for control flow.
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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 section 2. 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 section 3.7.5 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.
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A pipeline is a sequence of simple commands separated by
`|'.
[ |
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 section 5.2 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 section 3.2.3 Lists of Commands), the shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell
(see section 3.7.3 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 section 4.3 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.
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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 section 7. 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.
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3.2.4.1 Looping Constructs Shell commands for iterative action. 3.2.4.2 Conditional Constructs Shell commands for conditional execution. 3.2.4.3 Grouping Commands Ways to group commands.
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 section 3.6 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.
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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.
until
until command is:
until test-commands; do consequent-commands; done |
while
while 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.
for
for command is:
for name [in words ...]; do commands; done |
for command
executes the commands once for each positional parameter that is
set, as if `in "$@"' had been specified
(see section 3.4.2 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 |
The break and continue builtins (see section 4.1 Bourne Shell Builtins)
may be used to control loop execution.
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if
if 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.
case
case command is:
|
case will selectively execute the command-list corresponding to
the first pattern that matches word.
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 $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs." |
The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.
select
The select construct allows the easy generation of menus.
It has almost the same syntax as the for command:
select name [in words ...]; do commands; done |
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
`in words' is omitted, the positional parameters are printed,
as if `in "$@"' 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 section 6.5 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" |
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 6.4 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 3.5.8.1 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.
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 nocaseglob
(see the description of shopt in 4.2 Bash Builtin Commands)
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
expression1 && expression2
expression1 || expression2
&& and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
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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 section 3.7.3 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.
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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:
[ |
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 section 3.2.4 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 section 3.6 Redirections) associated with the shell function
are performed when the function is executed.
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 section 3.4.1 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 trap
below) is 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 trap).
See section 4.1 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 section 4.2 Bash Builtin Commands).
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 section 4.1 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.
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3.4.1 Positional Parameters The shell's command-line arguments. 3.4.2 Special Parameters Parameters denoted by special characters.
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 4.2 Bash Builtin Commands).
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] |
integer
attribute set, then value
is evaluated as an arithmetic expression even if the $((...))
expansion is not used (see section 3.5.5 Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" 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.
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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 $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see section 4. Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see section 3.3 Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
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The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
IFS
special variable. That is, "$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS
variable.
If IFS is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without intervening
separators.
@
"$@" is equivalent to
"$1" "$2" ....
When there are no positional parameters, "$@" 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 section 6.1 Invoking Bash),
then $0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
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Expansion is performed on the command line after it has been split into
tokens. There are seven kinds of expansion performed:
3.5.1 Brace Expansion Expansion of expressions within braces. 3.5.2 Tilde Expansion Expansion of the ~ character. 3.5.3 Shell Parameter Expansion How Bash expands variables to their values. 3.5.4 Command Substitution Using the output of a command as an argument. 3.5.5 Arithmetic Expansion How to use arithmetic in shell expansions. 3.5.6 Process Substitution A way to write and read to and from a command. 3.5.7 Word Splitting How the results of expansion are split into separate arguments. 3.5.8 Filename Expansion A shorthand for specifying filenames matching patterns. 3.5.9 Quote Removal How and when quote characters are removed from words.
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" (see section 3.4.2 Special Parameters) and "${name[@]}"
(see section 6.7 Arrays).
After all expansions, quote removal (see section 3.5.9 Quote Removal)
is performed.
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Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see section 3.5.8 Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequnce expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ 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}
|
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
|
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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 section 6.8 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 `='.
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:
~
$HOME
~/foo
~fred/foo
foo of the home directory of the user
fred
~+/foo
~-/foo
~N
~+N
~-N
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The `$' 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 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]}.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1.
${!prefix*}
${!prefix@}
IFS special variable.
${!name[@]}
${!name[*]}
${#parameter}
${parameter#word}
${parameter##word}
${parameter%word}
${parameter%%word}
${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in
filename 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.
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Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command) |
`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.
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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 section 6.5 Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.
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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) |
>(list) |
>(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.
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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.
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3.5.8.1 Pattern Matching How the shell matches patterns.
After word splitting, unless the `-f' option has been set
(see section 4.3 The Set Builtin), 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 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 4.2 Bash Builtin Commands,
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.
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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:
*
?
[...]
LC_COLLATE shell variable,
if set.
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 1003.2 standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper word xdigit |
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)
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After the preceding expansions, all unquoted occurrences of the characters `\', `'', and `"' that did not result from one of the above expansions are removed.
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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 |
ls 2>&1 > dirlist |