bash Version 2.02.
Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc.
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Bash is the shell, or command language interpreter, that will appear in the
GNU operating system. The name is an acronym for the `Bourne-Again
SHell', a pun on Steve Bourne, the author of the direct ancestor of the
current Unix shell /bin/sh, which appeared in the Seventh Edition
Bell Labs Research version of Unix.
Bash is an sh-compatible shell that 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 will include a version of csh,
Bash will be 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, Windows 95, and
Windows NT.
At its base, a shell is simply a macro processor that executes commands. A Unix shell is both a command interpreter, which provides the user interface to the rich set of Unix utilities, and a programming language, allowing 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 like `/bin', allowing users or groups to establish custom environments.
A shell allows execution of Unix 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, and the shell allows control over the contents of
their environment. Unix shells also provide a small set of built-in commands
(builtins) implementing functionality impossible (e.g.,
cd, break, continue, and
exec), or inconvenient (history, getopts,
kill, or pwd, for example) to obtain via separate
utilities. Shells may be used interactively or non-interactively: they accept
input typed from the keyboard or from a file. 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 have begun offering features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, history and aliases. Each of these features is described in this manual.
These definitions are used throughout the remainder of this manual.
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 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. 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, and the rules for evaluation and quoting are taken from the POSIX 1003.2 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.
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 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 section Definitions) has special meaning to the shell and must be quoted if it is to represent itself. 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 `$',
``', and `\'. The characters `$' and
``' retain their special meaning within double quotes (see section
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.
The special parameters `*' and `@' have special meaning when in double quotes (see section Shell Parameter Expansion).
Words of the form $'string' are treated specially.
The word expands to string, with backslash-escaped characters
replaced as specifed by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a
\b
\e
\f
\n
\r
\t
\v
\\
\nnn
ASCII code is the octal value
nnn (one to three digits)
\xnnn
ASCII code is the hexadecimal value
nnn (one to three digits) The result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign (`$') 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.
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is
enabled (see section 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 Is This Shell
Interactive?, for a description of what makes a shell interactive.
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 Definitions).
The first word generally specifies a command to be executed.
The return status (see section Exit
Status) of a simple command is its exit status as provided by the POSIX.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 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 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 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 Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status of the last command.
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.
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). 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
command && command2
command2 is executed if, and only if, command returns an exit status of zero.
An OR list has the form
command || command2
command2 is executed if, and only if, command returns a non-zero exit status.
The return status of AND and OR lists is the exit status of the last command executed in the list.
Bash supports the following looping constructs.
Note that wherever you see a `;' 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; doneExecute 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.
while
while command is: while test-commands; do consequent-commands; doneExecute 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; doneExpand 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, `in "$@"' is assumed. 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.
The break and continue builtins (see section Bourne Shell
Builtins) may be used to control loop execution.
if
if command is: if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fiThe 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 word in [ [(] pattern [| pattern]...) command-list ;;]... esac
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 $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs."The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.
select
select construct allows the easy
generation of menus. It has almost the same syntax as the for
command: select name [in words ...]; do commands; doneThe list of words following
in is expanded, generating a
list of items. The set of expanded words is printed on the standard error
output stream, each preceded by a number. If the `in
words' is omitted, the positional parameters are printed, as
if `in "$@"' 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 or return 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 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 section Bash Builtin Commands, 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 section 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. 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 section Pattern Matching. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string. Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )
! expression
expression1 && expression2
expression1 || expression2
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 to be created, 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. 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 () { command-list; }
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 command-list between { and }. This list is executed whenever
name is specified as the name of a command. The exit status of a
function is the exit status of the last command executed in the body.
When a function is executed, the arguments to the function become the
positional parameters during its execution (see section Positional
Parameters). The special parameter `#' that expands to the
number of positional parameters is updated to reflect the change. Positional
parameter 0 is unchanged.
If the builtin command return is executed in a function, the
function completes and execution resumes with the next command after the
function call. When a function 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
functions'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.
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. For
the shell's purposes, a variable is a parameter denoted by a
name.
A parameter is set if it has been assigned a value. The null string is a
valid value. Once a variable is set, it may be unset only by using the
unset builtin command.
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 (see the description
of the declare builtin in section Bash Builtin
Commands), then value is subject to arithmetic expansion even if
the $((...)) expansion is not used (see section Arithmetic
Expansion). Word splitting is not performed, with the exception of
"$@" as explained below. Filename expansion is not performed.
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 ${N}. Positional parameters may not be
assigned to with assignment statements. The positional parameters are
temporarily replaced when a shell function is executed (see section Shell
Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
IFS special variable. That is, "$*" 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 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 section Special
Parameters) and "${name[@]}" (see section Arrays).
After all expansions, quote removal (see section Quote
Removal) is performed.
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see section Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional postscript. The preamble is prepended to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma. Any incorrectly formed brace expansion is left unchanged.
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 section
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:
~
$HOME
~/foo
~fred/foo
foo of the home directory of the user
fred
~+/foo
~-/foo
~N
~+N
~-N
The `$' 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.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset.
${parameter:-word}
${parameter:=word}
${parameter:?word}
${parameter:+word}
${parameter:offset}
${parameter:offset:length}
${parameter[offset]}. Substring indexing is
zero-based unless the positional parameters are used, in which case the
indexing starts at 1.
${#parameter}
${parameter#word}
${parameter##word}
${parameter%word}
${parameter%%word}
${parameter/pattern/string}
${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 name. There are two forms:
$(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 substitutions may be nested.
The evaluation is performed according to the rules listed below (see section 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.
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.
After word splitting, unless the `-f' option has been set (see
section 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 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 section Bash Builtin
Commands, for a description of the nocaseglob,
nullglob, 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, even when GLOBIGNORE is set. However, setting
GLOBIGNORE 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. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
*
?
[...]
[:class:],
where class is one of the following classes defined in the POSIX.2
standard: alnum alpha ascii blank cntrl digit graph lower print punct space upper xdigitA character class matches any character belonging to that class. Within `[' and `]', an equivalence class can be specified using the syntax
[=c=], which
matches all characters with the same collation weight (as defined by the
current locale) as the character c. Within `[' and
`]', the syntax [.symbol.]
matches the 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, and filename expansion. If it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output and standard error to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.
A failure to open or create a file causes the redirection to fail.
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 filename whose name results from the expansion
of word exists and is a regular file. If the redirection operator is
`>|', or the redirection operator is `>' and the
noclobber option 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 as follows:
<<[-]word
here-document
delimiter
No parameter expansion, command substitution, filename expansion, or
arithmetic expansion is performed on word. If any characters in
word are quoted, the delimiter is the result of quote
removal on word, and the lines in the here-document are not expanded.
If word is unquoted, all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion. In the
latter case, the pair \newline is ignored, and `\'
must be used to quote the characters `\', `$', and
``'.
If the redirection operator is `<<-', then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
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]<>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.
If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.
If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.
After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.
$PATH for a directory
containing an executable file by that name. Bash uses a hash table to remember
the full pathnames of executable files to avoid multiple PATH
searches (see the description of hash in section Bourne
Shell Builtins). A full search of the directories in $PATH is
performed only if the command is not found in the hash table. If the search is
unsuccessful, the shell prints an error message and returns an exit status of
127.
The shell has an execution environment, which consists of the following:
exec builtin
cd,
pushd, or popd, or inherited by the shell at
invocation
umask or inherited from
the shell's parent
trap
set or inherited from the shell's parent in the environment
set
shopt
alias (see section Aliases)
$$, and the value of
$PPID When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
A command invoked in this separate environment cannot affect the shell's execution environment.
Command substitution and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.
When a program is invoked it is given an array of strings called the
environment. This is a list of name-value pairs, of the form
name=value.
Bash allows you to manipulate the environment in several ways. On invocation,
the shell scans its own environment and creates a parameter for each name found,
automatically marking it for export to child processes. Executed
commands inherit the environment. The export and `declare
-x' commands allow parameters and functions to be added to and deleted
from the environment. If the value of a parameter in the environment is
modified, the new value becomes part of the environment, replacing the old. The
environment inherited by any executed command consists of the shell's initial
environment, whose values may be modified in the shell, less any pairs removed
by the unset and `export -n' commands, plus any
additions via the export and `declare -x' commands.
The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in section Shell Parameters. These assignment statements affect only the environment seen by that command.
If the `-k' option is set (see section The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.
When Bash invokes an external command, the variable `$_' is set to the full path name of the command and passed to that command in its environment.
For the shell's purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is n, Bash uses the value 128+n as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
The exit status is used by the Bash conditional commands (see section Conditional Constructs) and some of the list constructs (see section Lists of Commands).
All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage.
When Bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that `kill 0' does not kill an interactive
shell), and SIGINT is caught and handled (so that the
wait builtin is interruptible). When Bash receives a
SIGINT, it breaks out of any executing loops. In all cases, Bash
ignores SIGQUIT. If job control is in effect (see section Job
Control), Bash ignores SIGTTIN, SIGTTOU, and
SIGTSTP.
Commands started by Bash have signal handlers set to the values inherited by
the shell from its parent. When job control is not in effect, asynchronous
commands ignore SIGINT and SIGQUIT as well. Commands
run as a result of command substitution ignore the keyboard-generated job
control signals SIGTTIN, SIGTTOU, and
SIGTSTP.
The shell exits by default upon receipt of a SIGHUP. Before
exiting, it resends the SIGHUP to all jobs, running or stopped.
Stopped jobs are sent SIGCONT to ensure that they receive the
SIGHUP. To prevent the shell from sending the SIGHUP
signal to a particular job, it should be removed from the jobs table with the
disown builtin (see section Job Control
Builtins) or marked to not receive SIGHUP using disown
-h.
If the huponexit shell option has been set with
shopt (see section Bash Builtin
Commands), Bash sends a SIGHUP to all jobs when an interactive
login shell exits.
When Bash receives a signal for which a trap has been set while waiting for a
command to complete, the trap will not be executed until the command completes.
When Bash is waiting for an asynchronous command via the wait
builtin, the reception of a signal for which a trap has been set will cause the
wait builtin to return immediately with an exit status greater than
128, immediately after which the trap is executed.
A shell script is a text file containing shell commands. When such a file is
used as the first non-option argument when invoking Bash, and neither the
`-c' nor `-s' option is supplied (see section Invoking
Bash), Bash reads and executes commands from the file, then exits. This mode
of operation creates a non-interactive shell. When Bash runs a shell script, it
sets the special parameter 0 to the name of the file, rather than
the name of the shell, and the positional parameters are set to the remaining
arguments, if any are given. If no additional arguments are supplied, the
positional parameters are unset.
A shell script may be made executable by using the chmod command
to turn on the execute bit. When Bash finds such a file while searching the
$PATH for a command, it spawns a subshell to execute it. In other
words, executing
filename arguments
is equivalent to executing
bash filename arguments
if filename is an executable shell script. This subshell
reinitializes itself, so that the effect is as if a new shell had been invoked
to interpret the script, with the exception that the locations of commands
remembered by the parent (see the description of hash in section Bourne Shell
Builtins) are retained by the child.
Most versions of Unix make this a part of the kernel's command execution mechanism. If the first line of a script begins with the two characters `#!', the remainder of the line specifies an interpreter for the program. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters.
This section briefly summarizes things which Bash inherits from the Bourne Shell: builtins, variables, and other features. It also lists the significant differences between Bash and the Bourne Shell. Many of the builtins have been extended by POSIX or Bash.
The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.
:
: [arguments]Do nothing beyond expanding arguments and performing redirections. The return status is zero.
.
. filenameRead and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the
$PATH variable is used to find filename. The current
directory is searched if filename is not found in
$PATH. The return status is the exit status of the last command
executed, or zero if no commands are executed. If filename is not
found, or cannot be read, the return status is non-zero.
break
break [n]Exit from a
for, while, until, or
select loop. If n is supplied, the nth
enclosing loop is exited. n must be greater than or equal to 1. The
return status is zero unless n is not greater than or equal to 1.
cd
cd [-LP] [directory]Change the current working directory to directory. If directory is not given, the value of the
HOME shell
variable is used. If the shell variable CDPATH exists, it is used
as a search path. If directory begins with a slash,
CDPATH is not used. The `-P' option means to not
follow symbolic links; symbolic links are followed by default or with the
`-L' option. If directory is `-', it is
equivalent to $OLDPWD. The return status is zero if the directory
is successfully changed, non-zero otherwise.
continue
continue [n]Resume the next iteration of an enclosing
for,
while, until, or select loop. If
n is supplied, the execution of the nth enclosing loop
is resumed. n must be greater than or equal to 1. The return status
is zero unless n is not greater than or equal to 1.
eval
eval [arguments]The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of
eval. If there are no arguments or only empty arguments, the
return status is zero.
exec
exec [-cl] [-a name] [command [arguments]]If command is supplied, it replaces the shell without creating a new process. If the `-l' option is supplied, the shell places a dash in the zeroth arg passed to command. This is what the
login program does. The `-c' option causes
command to be executed with an empty environment. If
`-a' is supplied, the shell passes name as the zeroth
argument to command. If no command is specified,
redirections may be used to affect the current shell environment. If there are
no redirection errors, the return status is zero; otherwise the return status
is non-zero.
exit
exit [n]Exit the shell, returning a status of n to the shell's parent. Any trap on
EXIT is executed before the shell terminates.
export
export [-fn] [-p] [name[=value]]Mark each name to be passed to child processes in the environment. If the `-f' option is supplied, the names refer to shell functions; otherwise the names refer to shell variables. The `-n' option means to no longer mark each name for export. If no names are supplied, or if the `-p' option is given, a list of exported names is displayed. The `-p' option displays output in a form that may be reused as input. The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or `-f' is supplied with a name that is not a shell function.
getopts
getopts optstring name [args]
getopts is used by shell scripts to parse positional
parameters. optstring contains the option letters to be recognized;
if a letter is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space. Each time it is
invoked, getopts places the next option in the shell variable
name, initializing name if it does not exist, and the
index of the next argument to be processed into the variable
OPTIND. OPTIND is initialized to 1 each time the
shell or a shell script is invoked. When an option requires an argument,
getopts places that argument into the variable
OPTARG. The shell does not reset OPTIND
automatically; it must be manually reset between multiple calls to
getopts within the same shell invocation if a new set of
parameters is to be used. When the end of options is encountered,
getopts exits with a return value greater than zero.
OPTIND is set to the index of the first non-option argument, and
name is set to `?'. getopts normally
parses the positional parameters, but if more arguments are given in
args, getopts parses those instead.
getopts can report errors in two ways. If the first character of
optstring is a colon, silent error reporting is used. In
normal operation diagnostic messages are printed when invalid options or
missing option arguments are encountered. If the variable OPTERR
is set to 0, no error messages will be displayed, even if the first character
of optstring is not a colon. If an invalid option is seen,
getopts places `?' into name and, if not
silent, prints an error message and unsets OPTARG. If
getopts is silent, the option character found is placed in
OPTARG and no diagnostic message is printed. If a required
argument is not found, and getopts is not silent, a question mark
(`?') is placed in name, OPTARG is unset,
and a diagnostic message is printed. If getopts is silent, then a
colon (`:') is placed in name and OPTARG
is set to the option character found.
hash
hash [-r] [-p filename] [name]Remember the full pathnames of commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in
$PATH. The `-p' option inhibits the path search, and
filename is used as the location of name. The
`-r' option causes the shell to forget all remembered locations.
If no arguments are given, information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.
pwd
pwd [-LP]Print the current working directory. If the `-P' option is supplied, the path printed will not contain symbolic links. If the `-L' option is supplied, the path printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.
readonly
readonly [-apf] [name] ...Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each name refers to a shell function. The `-a' option means each name refers to an array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the `-f' option is supplied with a name that is not a shell function.
return
return [n]Cause a shell function to exit with the return value n. This may also be used to terminate execution of a script being executed with the
. builtin, returning either n or the exit status of
the last command executed within the script as the exit status of the script.
The return status is false if return is used outside a function
and not during the execution of a script by `.'.
shift
shift [n]Shift the positional parameters to the left by n. The positional parameters from n+1 ...
$# are renamed to
$1 ... $#-n+1. Parameters represented by
the numbers $# to n+1 are unset. n must be
a non-negative number less than or equal to $#. If n
is zero or greater than $#, the positional parameters are not
changed. The return status is zero unless n is greater than
$# or less than zero, non-zero otherwise.
test
[
! expr
( expr )
expr1 -a expr2
expr1 -o expr2
test and [ builtins evaluate conditional expressions
using a set of rules based on the number of arguments.
times
timesPrint out the user and system times used by the shell and its children. The return status is zero.
trap
trap [-lp] [arg] [sigspec ...]The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent or equal to `-', all specified signals are reset to the values they had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is `-p', the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only `-p' is given,
trap
prints the list of commands associated with each signal number in a form that
may be reused as shell input. Each sigspec is either a signal name
such as SIGINT (with or without the SIG prefix) or a
signal number. If a sigspec is 0 or EXIT,
arg is executed when the shell exits. If a sigspec is
DEBUG, the command arg is executed after every simple
command. The `-l' option causes the shell to print a list of
signal names and their corresponding numbers. Signals ignored upon entry to
the shell cannot be trapped or reset. Trapped signals are reset to their
original values in a child process when it is created. The return status is
zero unless a sigspec does not specify a valid signal.
umask
umask [-p] [-S] [mode]Set the shell process's file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the
chmod command. If mode is omitted, the current value
of the mask is printed. If the `-S' option is supplied without a
mode argument, the mask is printed in a symbolic format. If the
`-p' option is supplied, and mode is omitted, the
output is in a form that may be reused as input. The return status is zero if
the mode is successfully changed or if no mode argument is
supplied, and non-zero otherwise.
unset
unset [-fv] [name]Each variable or function name is removed. If no options are supplied, or the `-v' option is given, each name refers to a shell variable. If the `-f' option is given, the names refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a name does not exist or is readonly.
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
CDPATH
cd builtin command.
HOME
cd builtin command. The value of this variable is also used by
tilde expansion (see section Tilde
Expansion).
IFS
MAIL
MAILPATH variable is not set, Bash informs the user of the
arrival of mail in the specified file.
MAILPATH
$_ expands to the name of the current mail file.
OPTARG
getopts builtin.
OPTIND
getopts builtin.
PATH
PS1
PS2
Bash implements essentially the same grammar, parameter and variable expansion, redirection, and quoting as the Bourne Shell. Bash uses the POSIX 1003.2 standard as the specification of how these features are to be implemented. There are some differences between the traditional Bourne shell and Bash; this section quickly details the differences of significance. A number of these differences are explained in greater depth in subsequent sections.
sh behavior.
bind builtin.
history and fc
builtins to manipulate it.
csh-like history expansion (see section History
Expansion).
$'...' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes, is
supported (see section ANSI-C
Quoting).
$"..." quoting syntax to do locale-specific
translation of the characters between the double quotes. The
`-D', `--dump-strings', and
`--dump-po-strings' invocation options list the translatable
strings found in a script (see section Locale-Specific
Translation).
! keyword to negate the return value of a
pipeline (see section Pipelines).
Very useful when an if statement needs to act only if a test
fails.
time reserved word and command timing (see
section Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT variable.
select compound command, which allows the
generation of simple menus (see section Conditional
Constructs).
[[ compound command, which makes
conditional testing part of the shell grammar (see section Conditional
Constructs).
alias and
unalias builtins (see section Aliases).
(( compound command (see
section Conditional
Constructs), and arithmetic expansion (see section