This text is a brief description of the features that are present in the Bash shell (version 3.2, 28 September 2006).
This is Edition 3.2, last updated 28 September 2006,
of The GNU Bash Reference Manual,
for Bash, Version 3.2.
Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (sh), the Korn Shell (ksh), and the C-shell (csh and its successor, tcsh). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature.
This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.
Bash is the shell, or command language interpreter,
for the gnu operating system.
The name is an acronym for the ‘Bourne-Again SHell’,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell sh,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
Bash is largely compatible with sh and incorporates useful
features from the Korn shell ksh and the C shell csh.
It is intended to be a conformant implementation of the ieee
posix Shell and Tools portion of the ieee posix
specification (ieee Standard 1003.1).
It offers functional improvements over sh for both interactive and
programming use.
While the gnu operating system provides other shells, including
a version of csh, Bash is the default shell.
Like other gnu software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems −
independently-supported ports exist for ms-dos, os/2,
and Windows platforms.
At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions.
A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of gnu utilities. The programming language features allow these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.
Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file.
A shell allows execution of gnu commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments.
Shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, cd, break, continue, and
exec) cannot be implemented outside of the shell because
they directly manipulate the shell itself.
The history, getopts, kill, or pwd
builtins, among others, could be implemented in separate utilities,
but they are more convenient to use as builtin commands.
All of the shell builtins are described in
subsequent sections.
While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.
Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual.
These definitions are used throughout the remainder of this manual.
POSIXblankbuiltincontrol operatorword that performs a control function. It is a newline
or one of the following:
‘||’, ‘&&’, ‘&’, ‘;’, ‘;;’,
‘|’, ‘(’, or ‘)’.
exit statusfieldfilenamejobjob controlmetacharacterblank or one of the following characters:
‘|’, ‘&’, ‘;’, ‘(’, ‘)’, ‘<’, or
‘>’.
nameword consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. Names are used as
shell variable and function names.
Also referred to as an identifier.
operatorcontrol operator or a redirection operator.
See Redirections, for a list of redirection operators.
process groupprocess group IDprocess group
during its lifetime.
reserved wordword that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as for and
while.
return statusexit status.
signalspecial builtintokenword or an operator.
wordtoken that is not an operator.
Bash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the posix specification for the `standard' Unix shell.
This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the rest of that line.
Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing.
The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:
metacharacters. Alias expansion is performed by this step
(see Aliases).
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
Each of the shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see History Interaction), the history expansion character, usually ‘!’, must be quoted to prevent history expansion. See Bash History Facilities, for more details concerning history expansion.
There are three quoting mechanisms: the escape character, single quotes, and double quotes.
A non-quoted backslash ‘\’ is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of newline. If a \newline pair
appears, and the backslash itself is not quoted, the \newline
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
Enclosing characters in single quotes (‘'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
Enclosing characters in double quotes (‘"’) preserves the literal value
of all characters within the quotes, with the exception of
‘$’, ‘`’, ‘\’,
and, when history expansion is enabled, ‘!’.
The characters ‘$’ and ‘`’
retain their special meaning within double quotes (see Shell Expansions).
The backslash retains its special meaning only when followed by one of
the following characters:
‘$’, ‘`’, ‘"’, ‘\’, or newline.
Within double quotes, backslashes that are followed by one of these
characters are removed. Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
If enabled, history expansion will be performed unless an ‘!’
appearing in double quotes is escaped using a backslash.
The backslash preceding the ‘!’ is not removed.
The special parameters ‘*’ and ‘@’ have special meaning when in double quotes (see Shell Parameter Expansion).
Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a\b\e\f\n\r\t\v\\\'\nnn\xHH\cxThe expanded result is single-quoted, as if the dollar sign had not been present.
A double-quoted string preceded by a dollar sign (‘$’) will cause
the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
Some systems use the message catalog selected by the LC_MESSAGES shell variable. Others create the name of the message catalog from the value of the TEXTDOMAIN shell variable, possibly adding a suffix of ‘.mo’. If you use the TEXTDOMAIN variable, you may need to set the TEXTDOMAINDIR variable to the location of the message catalog files. Still others use both variables in this fashion: TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.
In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see Bash Builtins),
a word beginning with ‘#’
causes that word and all remaining characters on that line to
be ignored. An interactive shell without the interactive_comments
option enabled does not allow comments. The interactive_comments
option is on by default in interactive shells.
See Interactive Shells, for a description of what makes
a shell interactive.
A simple shell command such as echo a b c consists of the command
itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
A simple command is the kind of command encountered most often.
It's just a sequence of words separated by blanks, terminated
by one of the shell's control operators (see Definitions). The
first word generally specifies a command to be executed, with the
rest of the words being that command's arguments.
The return status (see Exit Status) of a simple command is
its exit status as provided
by the posix 1003.1 waitpid function, or 128+n if
the command was terminated by signal n.
A pipeline is a sequence of simple commands separated by
‘|’.
[time[-p]] [!] command1 [|command2 ...]
The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output.
The reserved word time causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
The -p option changes the output format to that specified
by posix.
The TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed.
See Bash Variables, for a description of the available formats.
The use of time as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external
time command cannot time these easily.
If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell
(see Command Execution Environment). The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the pipefail option is enabled
(see The Set Builtin).
If pipefail is enabled, the pipeline's return status is the
value of the last (rightmost) command to exit with a non-zero status,
or zero if all commands exit successfully.
If the reserved word ‘!’ precedes the pipeline, the
exit status is the logical negation of the exit status as described
above.
The shell waits for all commands in the pipeline to terminate before
returning a value.
A list is a sequence of one or more pipelines separated by one
of the operators ‘;’, ‘&’, ‘&&’, or ‘||’,
and optionally terminated by one of ‘;’, ‘&’, or a
newline.
Of these list operators, ‘&&’ and ‘||’ have equal precedence, followed by ‘;’ and ‘&’, which have equal precedence.
A sequence of one or more newlines may appear in a list
to delimit commands, equivalent to a semicolon.
If a command is terminated by the control operator ‘&’,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the background.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see Job Control),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from /dev/null.
Commands separated by a ‘;’ are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
The control operators ‘&&’ and ‘||’ denote and lists and or lists, respectively. An and list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.
An or list has the form
command1 || command2
command2 is executed if, and only if, command1 returns a non-zero exit status.
The return status of and and or lists is the exit status of the last command executed in the list.
Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden.
Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit.
Bash supports the following looping constructs.
Note that wherever a ‘;’ appears in the description of a command's syntax, it may be replaced with one or more newlines.
untiluntil command is:
until test-commands; do consequent-commands; done
Execute consequent-commands as long as
test-commands has an exit status which is not zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.
whilewhile command is:
while test-commands; do consequent-commands; done
Execute consequent-commands as long as
test-commands has an exit status of zero.
The return status is the exit status of the last command executed
in consequent-commands, or zero if none was executed.
forfor command is:
for name [in words ...]; do commands; done
Expand words, and execute commands once for each member
in the resultant list, with name bound to the current member.
If ‘in words’ is not present, the for command
executes the commands once for each positional parameter that is
set, as if ‘in "$@"’ had been specified
(see Special Parameters).
The return status is the exit status of the last command that executes.
If there are no items in the expansion of words, no commands are
executed, and the return status is zero.
An alternate form of the for command is also supported:
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done
First, the arithmetic expression expr1 is evaluated according to the rules described below (see Shell Arithmetic). The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, commands are executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.
The break and continue builtins (see Bourne Shell Builtins)
may be used to control loop execution.
ifif command is:
if test-commands; then
consequent-commands;
[elif more-test-commands; then
more-consequents;]
[else alternate-consequents;]
fi
The test-commands list is executed, and if its return status is zero,
the consequent-commands list is executed.
If test-commands returns a non-zero status, each elif list
is executed in turn, and if its exit status is zero,
the corresponding more-consequents is executed and the
command completes.
If ‘else alternate-consequents’ is present, and
the final command in the final if or elif clause
has a non-zero exit status, then alternate-consequents is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.
casecase command is:
casewordin [ [(]pattern[|pattern]...)command-list;;]... esac
case will selectively execute the command-list corresponding to
the first pattern that matches word.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The ‘|’ is used to separate multiple patterns, and the ‘)’
operator terminates a pattern list.
A list of patterns and an associated command-list is known
as a clause. Each clause must be terminated with ‘;;’.
The word undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before matching is
attempted. Each pattern undergoes tilde expansion, parameter
expansion, command substitution, and arithmetic expansion.
There may be an arbitrary number of case clauses, each terminated
by a ‘;;’. The first pattern that matches determines the
command-list that is executed.
Here is an example using case in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
The return status is zero if no pattern is matched. Otherwise, the
return status is the exit status of the command-list executed.
selectselect construct allows the easy generation of menus.
It has almost the same syntax as the for command:
select name [in words ...]; do commands; done
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
‘in words’ is omitted, the positional parameters are printed,
as if ‘in "$@"’ had been specifed.
The PS3 prompt is then displayed and a line is read from the
standard input.
If the line consists of a number corresponding to one of the displayed
words, then the value of name is set to that word.
If the line is empty, the words and prompt are displayed again.
If EOF is read, the select command completes.
Any other value read causes name to be set to null.
The line read is saved in the variable REPLY.
The commands are executed after each selection until a
break command is executed, at which
point the select command completes.
Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.
select fname in *;
do
echo you picked $fname \($REPLY\)
break;
done
((...))(( expression ))
The arithmetic expression is evaluated according to the rules described below (see Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to
let "expression"
See Bash Builtins, for a full description of the let builtin.
[[...]][[ expression ]]
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the ‘[[’ and ‘]]’; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as ‘-f’ must be unquoted to be recognized as primaries.
When the ‘==’ and ‘!=’ operators are used, the string to the
right of the operator is considered a pattern and matched according
to the rules described below in Pattern Matching.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
The return value is 0 if the string matches (‘==’) or does not
match (‘!=’)the pattern, and 1 otherwise.
Any part of the pattern may be quoted to force it to be matched as a
string.
An additional binary operator, ‘=~’, is available, with the same
precedence as ‘==’ and ‘!=’.
When it is used, the string to the right of the operator is considered
an extended regular expression and matched accordingly (as in regex3)).
The return value is 0 if the string matches
the pattern, and 1 otherwise.
If the regular expression is syntactically incorrect, the conditional
expression's return value is 2.
If the shell option nocasematch
(see the description of shopt in Bash Builtins)
is enabled, the match is performed without regard to the case
of alphabetic characters.
Substrings matched by parenthesized subexpressions within the regular
expression are saved in the array variable BASH_REMATCH.
The element of BASH_REMATCH with index 0 is the portion of the string
matching the entire regular expression.
The element of BASH_REMATCH with index n is the portion of the
string matching the nth parenthesized subexpression.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )! expression && expression2 || expression2&& and || operators do not evaluate expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()( list )
Placing a list of commands between parentheses causes a subshell
environment to be created (see Command Execution Environment), and each
of the commands in list to be executed in that subshell. Since the
list is executed in a subshell, variable assignments do not remain in
effect after the subshell completes.
{} { list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are reserved words, so they must be separated from the list
by blanks. The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from the list by whitespace.
The exit status of both of these constructs is the exit status of list.
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
[ function ] name () compound-command [ redirections ]
This defines a shell function named name. The reserved
word function is optional.
If the function reserved
word is supplied, the parentheses are optional.
The body of the function is the compound command
compound-command (see Compound Commands).
That command is usually a list enclosed between { and }, but
may be any compound command listed above.
compound-command is executed whenever name is specified as the
name of a command.
Any redirections (see Redirections) associated with the shell function
are performed when the function is executed.
A function definition may be deleted using the -f option to the
unset builtin (see Bourne Shell Builtins).
The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.
Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
blanks or newlines.
This is because the braces are reserved words and are only recognized
as such when they are separated by whitespace.
Also, when using the braces, the list must be terminated by a semicolon,
a ‘&’, or a newline.
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see Positional Parameters).
The special parameter ‘#’ that expands to the number of
positional parameters is updated to reflect the change.
Special parameter 0 is unchanged.
The first element of the FUNCNAME variable is set to the
name of the function while the function is executing.
All other aspects of the shell execution
environment are identical between a function and its caller
with the exception that the DEBUG and RETURN traps
are not inherited unless the function has been given the
trace attribute using the declare builtin or
the -o functrace option has been enabled with
the set builtin,
(in which case all functions inherit the DEBUG and RETURN traps).
See Bourne Shell Builtins, for the description of the
trap builtin.
If the builtin command return
is executed in a function, the function completes and
execution resumes with the next command after the function
call.
Any command associated with the RETURN trap is executed
before execution resumes.
When a function completes, the values of the
positional parameters and the special parameter ‘#’
are restored to the values they had prior to the function's
execution. If a numeric argument is given to return,
that is the function's return status; otherwise the function's
return status is the exit status of the last command executed
before the return.
Variables local to the function may be declared with the
local builtin. These variables are visible only to
the function and the commands it invokes.
Function names and definitions may be listed with the
-f option to the declare or typeset
builtin commands (see Bash Builtins).
The -F option to declare or typeset
will list the function names only
(and optionally the source file and line number, if the extdebug
shell option is enabled).
Functions may be exported so that subshells
automatically have them defined with the
-f option to the export builtin
(see Bourne Shell Builtins).
Note that shell functions and variables with the same name may result
in multiple identically-named entries in the environment passed to the
shell's children.
Care should be taken in cases where this may cause a problem.
Functions may be recursive. No limit is placed on the number of recursive calls.
A parameter is an entity that stores values.
It can be a name, a number, or one of the special characters
listed below.
A variable is a parameter denoted by a name.
A variable has a value and zero or more attributes.
Attributes are assigned using the declare builtin command
(see the description of the declare builtin in Bash Builtins).
A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
the unset builtin command.
A variable may be assigned to by a statement of the form
name=[value]
If value
is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its integer
attribute set, then value
is evaluated as an arithmetic expression even if the $((...))
expansion is not used (see Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
alias,
declare, typeset, export, readonly,
and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘+=’ operator can be used to append to or add to the variable's previous value. When ‘+=’ is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable's current value, which is also evaluated. When ‘+=’ is applied to an array variable using compound assignment (see Arrays), the variable's value is not unset (as it is when using ‘=’), and new values are appended to the array beginning at one greater than the array's maximum index. When applied to a string-valued variable, value is expanded and appended to the variable's value.
A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the set builtin command.
Positional parameter N may be referenced as ${N}, or
as $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*"$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS
variable.
If IFS is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without intervening
separators.
@"$@" is equivalent to
"$1" "$2" ....
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
When there are no positional parameters, "$@" and
$@
expand to nothing (i.e., they are removed).
#?-set
builtin command, or those set by the shell itself
(such as the -i option).
$() subshell, it
expands to the process id of the invoking shell, not the subshell.
!0$0 is set to the name of that file.
If Bash is started with the -c option (see Invoking Bash),
then $0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
Expansion is performed on the command line after it has been split into
tokens. There are seven kinds of expansion performed:
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" (see Special Parameters) and "${name[@]}"
(see Arrays).
After all expansions, quote removal (see Quote Removal)
is performed.
Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequnce expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form {x..y},
where x and y are either integers or single characters.
When integers are supplied, the expression expands to each number between
x and y, inclusive.
When characters are supplied, the expression expands to each character
lexicographically between x and y, inclusive. Note that
both x and y must be of the same type.
Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion.
A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged.
A { or ‘,’ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
If a word begins with an unquoted tilde character (‘~’), all of the characters up to the first unquoted slash (or all characters, if there is no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the HOME shell variable. If HOME is unset, the home directory of the user executing the shell is substituted instead. Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.
If the tilde-prefix is ‘~+’, the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is ‘~-’, the value of the shell variable OLDPWD, if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a ‘+’ or a ‘-’,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the dirs builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see The Directory Stack).
If the tilde-prefix, sans the tilde, consists of a number without a
leading ‘+’ or ‘-’, ‘+’ is assumed.
If the login name is invalid, or the tilde expansion fails, the word is left unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately following a ‘:’ or the first ‘=’. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.
The following table shows how Bash treats unquoted tilde-prefixes:
~$HOME
~/foo~fred/foofoo of the home directory of the user
fred
~+/foo~-/foo~N~+N~-NThe ‘$’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first ‘}’ not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.
The basic form of parameter expansion is ${parameter}. The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point,
a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest of
parameter as the name of the variable; this variable is then
expanded and that value is used in the rest of the substitution, rather
than the value of parameter itself.
This is known as indirect expansion.
The exceptions to this are the expansions of ${!prefix*}
and ${!name[@]}
described below.
The exclamation point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both existence and that the value is not null; if the colon is omitted, the operator tests only for existence.
${parameter:−word}${parameter:=word}${parameter:?word}${parameter:+word}${parameter:offset}${parameter:offset:length}length must evaluate to a number greater than or equal to zero.
If offset evaluates to a number less than zero, the value
is used as an offset from the end of the value of parameter.
If parameter is ‘@’, the result is length positional
parameters beginning at offset.
If parameter is an array name indexed by ‘@’ or ‘*’,
the result is the length
members of the array beginning with ${parameter[offset]}.
A negative offset is taken relative to one greater than the maximum
index of the specified array.
Note that a negative offset must be separated from the colon by at least
one space to avoid being confused with the ‘:-’ expansion.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1.
${!prefix*}${!prefix@}${!name[@]}${!name[*]}${#parameter}${parameter#word}${parameter##word}${parameter%word}${parameter%%word}${parameter/pattern/string}/ following pattern may be omitted.
If parameter is ‘@’ or ‘*’,
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with ‘@’ or ‘*’,
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command)
or
`command`
Bash performs the expansion by executing command and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
‘$’, ‘`’, or ‘\’.
The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(command) form, all characters between
the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.
Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$(( expression ))
The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.
Process substitution is supported on systems that support named pipes (fifos) or the /dev/fd method of naming open files. It takes the form of
<(list)
or
>(list)
The process list is run with its input or output connected to a
fifo or some file in /dev/fd. The name of this file is
passed as an argument to the current command as the result of the
expansion. If the >(list) form is used, writing to
the file will provide input for list. If the
<(list) form is used, the file passed as an
argument should be read to obtain the output of list.
Note that no space may appear between the < or >
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.
The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of $IFS
as a delimiter, and splits the results of the other
expansions into words on these characters. If
IFS is unset, or its value is exactly <space><tab><newline>,
the default, then any sequence of IFS
characters serves to delimit words. If IFS
has a value other than the default, then sequences of
the whitespace characters space and tab
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or '') are retained.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
nullglob is disabled, the word is left
unchanged.
If the nullglob option is set, and no matches are found, the word
is removed.
If the failglob shell option is set, and no matches are found,
an error message is printed and the command is not executed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters.
When a pattern is used for filename generation, the character ‘.’
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option dotglob is set.
When matching a file name, the slash character must always be
matched explicitly.
In other cases, the ‘.’ character is not treated specially.
See the description of shopt in Bash Builtins,
for a description of the nocaseglob, nullglob,
failglob, and dotglob options.
The GLOBIGNORE
shell variable may be used to restrict the set of filenames matching a
pattern. If GLOBIGNORE
is set, each matching filename that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches. The filenames
. and ..
are always ignored when GLOBIGNORE
is set and not null.
However, setting GLOBIGNORE to a non-null value has the effect of
enabling the dotglob
shell option, so all other filenames beginning with a
‘.’ will match.
To get the old behavior of ignoring filenames beginning with a
‘.’, make ‘.*’ one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE
is unset.
Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The nul character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
*?[...]For example, in the default C locale, ‘[a-dx-z]’ is equivalent to ‘[abcdxyz]’. Many locales sort characters in dictionary order, and in these locales ‘[a-dx-z]’ is typically not equivalent to ‘[abcdxyz]’; it might be equivalent to ‘[aBbCcDdxXyYz]’, for example. To obtain the traditional interpretation of ranges in bracket expressions, you can force the use of the C locale by setting the LC_COLLATE or LC_ALL environment variable to the value ‘C’.
Within ‘[’ and ‘]’, character classes can be specified
using the syntax
[:class:], where class is one of the
following classes defined in the posix standard:
alnum alpha ascii blank cntrl digit graph lower
print punct space upper word xdigit
A character class matches any character belonging to that class.
The word character class matches letters, digits, and the character
‘_’.
Within ‘[’ and ‘]’, an equivalence class can be
specified using the syntax [=c=], which
matches all characters with the same collation weight (as defined
by the current locale) as the character c.
Within ‘[’ and ‘]’, the syntax [.symbol.]
matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized.
In the following description, a pattern-list is a list of one
or more patterns separated by a ‘|’.
Composite patterns may be formed using one or more of the following
sub-patterns:
?(pattern-list)*(pattern-list)+(pattern-list)@(pattern-list)!(pattern-list)After the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘'’, and ‘"’ that did not result from one of the above expansions are removed.
Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is ‘<’, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is ‘>’, the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.
Bash handles several filenames specially when they are used in redirections, as described in the following table:
/dev/fd/fd/dev/stdin/dev/stdout/dev/stderr/dev/tcp/host/port/dev/udp/host/portA failure to open or create a file causes the redirection to fail.
Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.
Redirection of input causes the file whose name results from
the expansion of word
to be opened for reading on file descriptor n,
or the standard input (file descriptor 0) if n
is not specified.
The general format for redirecting input is:
[n]<word
Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
[n]>[|]word
If the redirection operator is ‘>’, and the noclobber
option to the set builtin has been enabled, the redirection
will fail if the file whose name results from the expansion of
word exists and is a regular file.
If the redirection operator is ‘>|’, or the redirection operator is
‘>’ and the noclobber option is not enabled, the redirection
is attempted even if the file named by word exists.
Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created.
The general format for appending output is:
[n]>>word
Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word with this construct.
There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.
The format of here-documents is:
<<[−]word
here-document
delimiter
No parameter expansion, command substitution, arithmetic expansion,
or filename expansion is performed on
word. If any characters in word are quoted, the
delimiter is the result of quote removal on word,
and the lines in the here-document are not expanded.
If word is unquoted,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the latter
case, the character sequence \newline is ignored, and ‘\’
must be used to quote the characters
‘\’, ‘$’, and ‘`’.
If the redirection operator is ‘<<-’, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
A variant of here documents, the format is:
<<< word
The word is expanded and supplied to the command on its standard input.
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. As a special case, if n is omitted, and word does not expand to one or more digits, the standard output and standard error are redirected as described previously.
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified.
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.
When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.
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.
hash in 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 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, commands grouped with parentheses, 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.
If a command is followed by a ‘&’ and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.
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 provides several ways to manipulate the environment.
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 Shell Parameters. These assignment statements affect only the environment seen by that command.
If the -k option is set (see 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 Conditional Constructs) and some of the list constructs (see Lists).
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 Job Control), Bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Non-builtin 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 in addition to these inherited
handlers.
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, an interactive shell 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 Job Control Builtins) or marked
to not receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt
(see Bash Builtins), Bash sends a SIGHUP to all jobs when
an interactive login shell exits.
If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, 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 Invoking Bash), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.
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 Bourne Shell Builtins)
are retained by the child.
Most versions of Unix make this a part of the operating system'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.
Thus, you can specify Bash, awk, Perl, or some other
interpreter and write the rest of the script file in that language.
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.
Bash scripts often begin with #! /bin/bash (assuming that
Bash has been installed in /bin), since this ensures that
Bash will be used to interpret the script, even if it is executed
under another shell.
Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (see Simple Commands), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities.
This section briefly describes the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash.
Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Job Control Builtins), the directory stack (see Directory Stack Builtins), the command history (see Bash History Builtins), and the programmable completion facilities (see Programmable Completion Builtins).
Many of the builtins have been extended by posix or Bash.
Unless otherwise noted, each builtin command documented as accepting
options preceded by ‘-’ accepts ‘--’
to signify the end of the options.
For example, the :, true, false, and test
builtins do not accept options.
The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the posix standard.
: (a colon): [arguments]
Do nothing beyond expanding arguments and performing redirections.
The return status is zero.
. (a period). filename [arguments]
Read 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.
When Bash is not in posix mode, the current directory is searched
if filename is not found in $PATH.
If any arguments are supplied, they become the positional
parameters when filename is executed. Otherwise the positional
parameters are unchanged.
The return status is the 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.
This builtin is equivalent to source.
breakbreak [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.
cdcd [-L|-P] [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.
If a non-empty directory name from CDPATH is used, or if ‘-’ is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
continuecontinue [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.
evaleval [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.
execexec [-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 at the
beginning of 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.
exitexit [n]
Exit the shell, returning a status of n to the shell's parent.
If n is omitted, the exit status is that of the last command executed.
Any trap on EXIT is executed before the shell terminates.
exportexport [-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. If a variable name is followed by =value, the value of the variable is set to value.
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.
getoptsgetopts optstring name [args]
getopts is used by shell scripts to parse positional parameters.
optstring contains the option characters to be recognized; if a
character is followed by a colon, the option is expected to have an
argument, which should be separated from it by white space.
The colon (‘:’) and question mark (‘?’) may not be
used as option characters.
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.
hashhash [-r] [-p filename] [-dt] [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.
The -d option causes the shell to forget the remembered location
of each name.
If the -t option is supplied, the full pathname to which each
name corresponds is printed. If multiple name arguments are
supplied with -t the name is printed before the hashed
full pathname.
The -l option causes output to be displayed in a format
that may be reused as input.
If no arguments are given, or if only -l is supplied,
information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.
pwdpwd [-LP]
Print the absolute pathname of the current working directory.
If the -P option is supplied, the pathname printed will not
contain symbolic links.
If the -L option is supplied, the pathname 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.
readonlyreadonly [-apf] [name[=value]] ...
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.
If a variable name is followed by =value, the value of
the variable is set to value.
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.
returnreturn [n]
Cause a shell function to exit with the return value n.
If n is not supplied, the return value is the exit status of the
last command executed in the function.
This may also be used to terminate execution of a script being executed
with the . (or source) builtin, returning either n or
the exit status of the last command executed within the script as the exit
status of the script.
Any command associated with the RETURN trap is executed
before execution resumes after the function or script.
The return status is non-zero if return is used outside a function
and not during the execution of a script by . or source.
shiftshift [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.
If n is not supplied, it is assumed to be 1.
The return status is zero unless n is greater than $# or
less than zero, non-zero otherwise.
test[test does not accept any options, nor does it accept and ignore
an argument of -- as signifying the end of options.
When the [ form is used, the last argument to the command must
be a ].
Expressions may be combined using the following operators, listed in decreasing order of precedence.
! expr( expr ) -a expr2 -o expr2The test and [ builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
timestimes
Print out the user and system times used by the shell and its children.
The return status is zero.
traptrap [-lp] [arg] [sigspec ...]
The commands in arg are to be read and executed when the
shell receives signal sigspec. If arg is absent (and
there is a single sigspec) or
equal to ‘-’, each specified signal's disposition is reset
to the value it 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 not present and -p has been supplied,
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.
The -l option causes the shell to print a list of signal names
and their corresponding numbers.
Each sigspec is either a signal name or a signal number.
Signal names are case insensitive and the SIG prefix is optional.
If a sigspec
is 0 or EXIT, arg is executed when the shell exits.
If a sigspec is DEBUG, the command arg is executed
before every simple command, for command, case command,
select command, every arithmetic for command, and before
the first command executes in a shell function.
Refer to the description of the extglob option to the
shopt builtin (see Bash Builtins) for details of its
effect on the DEBUG trap.
If a sigspec is ERR, the command arg
is executed whenever a simple command has a non-zero exit status,
subject to the following conditions.
The ERR trap is not executed if the failed command is part of the
command list immediately following an until or while keyword,
part of the test in an if statement,
part of a && or || list, or if the command's return
status is being inverted using !.
These are the same conditions obeyed by the errexit option.
If a sigspec is RETURN, the command arg is executed
each time a shell function or a script executed with the . or
source builtins finishes executing.
Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals that are not being ignored 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.
umaskumask [-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.
Note that when the mode is interpreted as an octal number, each number
of the umask is subtracted from 7. Thus, a umask of 022
results in permissions of 755.
unsetunset [-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 is readonly.
This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the posix standard.
alias alias [-p] [name[=value] ...]
Without arguments or with the -p option, alias prints
the list of aliases on the standard output in a form that allows
them to be reused as input.
If arguments are supplied, an alias is defined for each name
whose value is given. If no value is given, the name
and value of the alias is printed.
Aliases are described in Aliases.
bind bind [-m keymap] [-lpsvPSV]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq:shell-command
bind [-m keymap] keyseq:function-name
bind readline-command
Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘"\C-x\C-r":re-read-init-file’. Options, if supplied, have the following meanings:
-m keymapemacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-move,
vi-command, and
vi-insert.
vi is equivalent to vi-command;
emacs is equivalent to emacs-standard.
-l-p-P-v-V-s-S-f filename-q function-u function-r keyseq-x keyseq:shell-commandThe return status is zero unless an invalid option is supplied or an
error occurs.
builtinbuiltin [shell-builtin [args]]
Run a shell builtin, passing it args, and return its exit status.
This is useful when defining a shell function with the same
name as a shell builtin, retaining the functionality of the builtin within
the function.
The return status is non-zero if shell-builtin is not a shell
builtin command.
callercaller [expr]
Returns the context of any active subroutine call (a shell function or
a script executed with the . or source builtins).
Without expr, caller displays the line number and source
filename of the current subroutine call.
If a non-negative integer is supplied as expr, caller
displays the line number, subroutine name, and source file corresponding
to that position in the current execution call stack. This extra
information may be used, for example, to print a stack trace. The
current frame is frame 0.
The return value is 0 unless the shell is not executing a subroutine
call or expr does not correspond to a valid position in the
call stack.
commandcommand [-pVv] command [arguments ...]
Runs command with arguments ignoring any shell function
named command.
Only shell builtin commands or commands found by searching the
PATH are executed.
If there is a shell function named ls, running ‘command ls’
within the function will execute the external command ls
instead of calling the function recursively.
The -p option means to use a default value for PATH
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if command cannot be
found or an error occurred, and the exit status of command
otherwise.
If either the -V or -v option is supplied, a
description of command is printed. The -v option
causes a single word indicating the command or file name used to
invoke command to be displayed; the -V option produces
a more verbose description. In this case, the return status is
zero if command is found, and non-zero if not.
declaredeclare [-afFirtx] [-p] [name[=value] ...]
Declare variables and give them attributes. If no names are given, then display the values of variables instead.
The -p option will display the attributes and values of each
name.
When -p is used, additional options are ignored.
The -F option inhibits the display of function definitions;
only the function name and attributes are printed.
If the extdebug shell option is enabled using shopt
(see Bash Builtins), the source file name and line number where
the function is defined are displayed as well.
-F implies -f.
The following options can be used to restrict output to variables with
the specified attributes or to give variables attributes:
-a-f-i-r-ttrace attribute.
Traced functions inherit the DEBUG and RETURN traps from
the calling shell.
The trace attribute has no special meaning for variables.
-xUsing ‘+’ instead of ‘-’ turns off the attribute instead.
When used in a function, declare makes each name local,
as with the local command. If a variable name is followed by
=value, the value of the variable is set to value.
The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using ‘-f foo=bar’,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without
using the compound assignment syntax (see Arrays),
one of the names is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with -f.
echoecho [-neE] [arg ...]
Output the args, separated by spaces, terminated with a
newline.
The return status is always 0.
If -n is specified, the trailing newline is suppressed.
If the -e option is given, interpretation of the following
backslash-escaped characters is enabled.
The -E option disables the interpretation of these escape characters,
even on systems where they are interpreted by default.
The xpg_echo shell option may be used to
dynamically determine whether or not echo expands these
escape characters by default.
echo does not interpret -- to mean the end of options.
echo interprets the following escape sequences:
\a\b\c\e\f\n\r\t\v\\\0nnn\xHHenableenable [-n] [-p] [-f filename] [-ads] [name ...]
Enable and disable builtin shell commands.
Disabling a builtin allows a disk command which has the same name
as a shell builtin to be executed without specifying a full pathname,
even though the shell normally searches for builtins before disk commands.
If -n is used, the names become disabled. Otherwise
names are enabled. For example, to use the test binary
found via $PATH instead of the shell builtin version, type
‘enable -n test’.
If the -p option is supplied, or no name arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The -a option means to list each builtin with an indication of whether or not it is enabled.
The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. The -d option will delete a builtin loaded with -f.
If there are no options, a list of the shell builtins is displayed.
The -s option restricts enable to the posix special
builtins. If -s is used with -f, the new builtin becomes
a special builtin (see Special Builtins).
The return status is zero unless a name is not a shell builtin
or there is an error loading a new builtin from a shared object.
helphelp [-s] [pattern]
Display helpful information about builtin commands.
If pattern is specified, help gives detailed help
on all commands matching pattern, otherwise a list of
the builtins is printed.
The -s option restricts the information displayed to a short
usage synopsis.
The return status is zero unless no command matches pattern.
letlet expression [expression]
The let builtin allows arithmetic to be performed on shell
variables. Each expression is evaluated according to the
rules given below in Shell Arithmetic. If the
last expression evaluates to 0, let returns 1;
otherwise 0 is returned.
locallocal [option] name[=value] ...
For each argument, a local variable named name is created,
and assigned value.
The option can be any of the options accepted by declare.
local can only be used within a function; it makes the variable
name have a visible scope restricted to that function and its
children. The return status is zero unless local is used outside
a function, an invalid name is supplied, or name is a
readonly variable.
logoutlogout [n]
Exit a login shell, returning a status of n to the shell's
parent.
printf printf [-v var] format [arguments]
Write the formatted arguments to the standard output under the
control of the format.
The format is a character string which contains three types of objects:
plain characters, which are simply copied to standard output, character
escape sequences, which are converted and copied to the standard output, and
format specifications, each of which causes printing of the next successive
argument.
In addition to the standard printf(1) formats, ‘%b’ causes
printf to expand backslash escape sequences in the corresponding
argument,
(except that ‘\c’ terminates output, backslashes in
‘\'’, ‘\"’, and ‘\?’ are not removed, and octal escapes
beginning with ‘\0’ may contain up to four digits),
and ‘%q’ causes printf to output the
corresponding argument in a format that can be reused as shell input.
The -v option causes the output to be assigned to the variable var rather than being printed to the standard output.
The format is reused as necessary to consume all of the arguments.
If the format requires more arguments than are supplied, the
extra format specifications behave as if a zero value or null string, as
appropriate, had been supplied. The return value is zero on success,
non-zero on failure.
readread [-ers] [-a aname] [-d delim] [-n nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
One line is read from the standard input, or from the file descriptor
fd supplied as an argument to the -u option, and the first word
is assigned to the first name, the second word to the second name,
and so on, with leftover words and their intervening separators assigned
to the last name.
If there are fewer words read from the input stream than names,
the remaining names are assigned empty values.
The characters in the value of the IFS variable
are used to split the line into words.
The backslash character ‘\’ may be used to remove any special
meaning for the next character read and for line continuation.
If no names are supplied, the line read is assigned to the
variable REPLY.
The return code is zero, unless end-of-file is encountered, read
times out, or an invalid file descriptor is supplied as the argument to
-u.
Options, if supplied, have the following meanings:
-a aname-d delim-e-n ncharsread returns after reading nchars characters rather than
waiting for a complete line of input.
-p prompt-r-s-t timeoutread to time out and return failure if a complete line of
input is not read within timeout seconds.
This option has no effect if read is not reading input from the
terminal or a pipe.
-u fdshoptshopt [-pqsu] [-o] [optname ...]
Toggle the values of variables controlling optional shell behavior. With no options, or with the -p option, a list of all settable options is displayed, with an indication of whether or not each is set. The -p option causes output to be displayed in a form that may be reused as input. Other options have the following meanings:
-s-u-q-oset builtin (see The Set Builtin).
If either -s or -u is used with no optname arguments, the display is limited to those options which are set or unset, respectively.
Unless otherwise noted, the shopt options are disabled (off)
by default.
The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.
The list of shopt options is:
cdable_varscd
builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.
cdspellcd command will be corrected.
The errors checked for are transposed characters,
a missing character, and a character too many.
If a correction is found, the corrected path is printed,
and the command proceeds.
This option is only used by interactive shells.
checkhashcheckwinsizecmdhistdotglobexecfailexec
builtin command. An interactive shell does not exit if exec
fails.
expand_aliasesextdebugdeclare builtin (see Bash Builtins)
displays the source file name and line number corresponding to each function
name supplied as an argument.
DEBUG trap returns a non-zero value, the
next command is skipped and not executed.
DEBUG trap returns a value of 2, and the
shell is executing in a subroutine (a shell function or a shell script
executed by the . or source builtins), a call to
return is simulated.
BASH_ARGC and BASH_ARGV are updated as described in their
descriptions (see Bash Variables).
( command ) inherit the
DEBUG and RETURN traps.
( command ) inherit the
ERROR trap.
extglobextquote$'string' and $"string" quoting is
performed within ${parameter} expansions
enclosed in double quotes. This option is enabled by default.
failglobforce_fignoregnu_errfmthistappendhistreedithistverifyhostcompletehuponexitSIGHUP to all jobs when an interactive
login shell exits (see Signals).
interactive_commentslithistcmdhist
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
login_shellmailwarn"The mail in mailfile has been read" is displayed.
no_empty_cmd_completionnocaseglobnocasematchcase or [[
conditional commands.
nullglobprogcomppromptvarsrestricted_shellshift_verboseshift
builtin prints an error message when the shift count exceeds the
number of positional parameters.
sourcepathsource builtin uses the value of PATH
to find the directory containing the file supplied as an argument.
This option is enabled by default.
xpg_echoecho builtin expands backslash-escape sequences
by default.
The return status when listing options is zero if all optnames
are enabled, non-zero otherwise.
When setting or unsetting options, the return status is zero unless an
optname is not a valid shell option.
sourcesource filename
A synonym for . (see Bourne Shell Builtins).
typetype [-afptP] [name ...]
For each name, indicate how it would be interpreted if used as a command name.
If the -t option is used, type prints a single word
which is one of ‘alias’, ‘function’, ‘builtin’,
‘file’ or ‘keyword’,
if name is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the name is not found, then nothing is printed, and
type returns a failure status.
If the -p option is used, type either returns the name
of the disk file that would be executed, or nothing if -t
would not return ‘file’.
The -P option forces a path search for each name, even if -t would not return ‘file’.
If a command is hashed, -p and -P print the hashed value,
not necessarily the file that appears first in $PATH.
If the -a option is used, type returns all of the places
that contain an executable named file.
This includes aliases and functions, if and only if the -p option
is not also used.
If the -f option is used, type does not attempt to find
shell functions, as with the command builtin.
The return status is zero if any of the names are found, non-zero
if none are found.
typesettypeset [-afFrxi] [-p] [name[=value] ...]
The typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the declare
builtin command.
ulimitulimit [-acdefilmnpqrstuvxSH] [limit]
ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an
option is given, it is interpreted as follows:
-S-H-a-c-d-e-f-i-l-m-n-p-q-r-s-t-u-v-xIf limit is given, it is the new value of the specified resource;
the special limit values hard, soft, and
unlimited stand for the current hard limit, the current soft limit,
and no limit, respectively.
Otherwise, the current value of the soft limit for the specified resource
is printed, unless the -H option is supplied.
When setting new limits, if neither -H nor -S is supplied,
both the hard and soft limits are set.
If no option is given, then -f is assumed. Values are in 1024-byte
increments, except for -t, which is in seconds, -p,
which is in units of 512-byte blocks, and -n and -u, which
are unscaled values.
The return status is zero unless an invalid option or argument is supplied,
or an error occurs while setting a new limit.
unaliasunalias [-a] [name ... ]
Remove each name from the list of aliases. If -a is supplied, all aliases are removed. Aliases are described in Aliases.
This builtin is so complicated that it deserves its own section.
setset [--abefhkmnptuvxBCHP] [-o option] [argument ...]
If no options or arguments are supplied, set displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input
for setting or resetting the currently-set variables.
Read-only variables cannot be reset.
In posix mode, only shell variables are listed.
When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:
-a-b-ewhile or until
keyword, part of the test in an if statement,
part of a && or || list, or if the command's return
status is being inverted using !.
A trap on ERR, if set, is executed before the shell exits.
-f-h-k-m-n-o option-nameallexport-a.
braceexpand-B.
emacsemacs-style line editing interface (see Command Line Editing).
errexit-e.
errtrace-E.
functrace-T.
hashall-h.
histexpand-H.
historyignoreeofkeyword-k.
monitor-m.
noclobber-C.
noexec-n.
noglob-f.
nolognotify-b.
nounset-u.
onecmd-t.
physical-P.
pipefailposixprivileged-p.
verbose-v.
vivi-style line editing interface.
xtrace-x.
-p-p option is not supplied, these actions
are taken and the effective user id is set to the real user id.
If the -p option is supplied at startup, the effective user id is
not reset.
Turning this option off causes the effective user
and group ids to be set to the real user and group ids.
-t-u-v-xfor commands, case
commands, select commands, and arithmetic for commands
and their arguments or associated word lists after they are
expanded and before they are executed. The value of the PS4
variable is expanded and the resultant value is printed before
the command and its expanded arguments.
-B-C-EERR is inherited by shell functions, command
substitutions, and commands executed in a subshell environment.
The ERR trap is normally not inherited in such cases.
-H-Pcd which change the current directory. The physical directory
is used instead. By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.
For example, if /usr/sys is a symbolic link to /usr/local/sys then:
$ cd /usr/sys; echo $PWD
/usr/sys
$ cd ..; pwd
/usr
If set -P is on, then:
$ cd /usr/sys; echo $PWD
/usr/local/sys
$ cd ..; pwd
/usr/local
-TDEBUG and RETURN are inherited by
shell functions, command substitutions, and commands executed
in a subshell environment.
The DEBUG and RETURN traps are normally not inherited
in such cases.
---Using ‘+’ rather than ‘-’ causes these options to be
turned off. The options can also be used upon invocation of the
shell. The current set of options may be found in $-.
The remaining N arguments are positional parameters and are
assigned, in order, to $1, $2, ... $N.
The special parameter # is set to N.
The return status is always zero unless an invalid option is supplied.
For historical reasons, the posix standard has classified several builtin commands as special. When Bash is executing in posix mode, the special builtins differ from other builtin commands in three respects:
When Bash is not executing in posix mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash posix mode is described in Bash POSIX Mode.
These are the posix special builtins:
break : . continue eval exec exit export readonly return set
shift trap unset
This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables.
Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
CDPATHcd builtin command.
HOMEcd builtin
command.
The value of this variable is also used by tilde expansion
(see Tilde Expansion).
IFSMAILMAILPATH$_ expands to the name of
the current mail file.
OPTARGgetopts builtin.
OPTINDgetopts builtin.
PATHPATH indicates the
current directory.
A null directory name may appear as two adjacent colons, or as an initial
or trailing colon.
PS1PS2These variables are set or used by Bash, but other shells do not normally treat them specially.
A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Job Control Variables).
BASHBASH_ARGC. or source) is at the top of the stack. When a
subroutine is executed, the number of parameters passed is pushed onto
BASH_ARGC.
The shell sets BASH_ARGC only when in extended debugging mode
(see Bash Builtins
for a description of the extdebug option to the shopt
builtin).
BASH_ARGVBASH_ARGV.
The shell sets BASH_ARGV only when in extended debugging mode
(see Bash Builtins
for a description of the extdebug option to the shopt
builtin).
BASH_COMMANDBASH_ENVBASH_EXECUTION_STRINGBASH_LINENO${BASH_LINENO[$i]} is the line number in the source file where
${FUNCNAME[$i]} was called.
The corresponding source file name is ${BASH_SOURCE[$i]}.
Use LINENO to obtain the current line number.
BASH_REMATCH[[ conditional command
(see Conditional Constructs).
The element with index 0 is the portion of the string
matching the entire regular expression.
The element with index n is the portion of the
string matching the nth parenthesized subexpression.
This variable is read-only.
BASH_SOURCEFUNCNAME array variable.
BASH_SUBSHELLBASH_VERSINFOBASH_VERSINFO[0]BASH_VERSINFO[1]BASH_VERSINFO[2]BASH_VERSINFO[3]BASH_VERSINFO[4]BASH_VERSINFO[5]BASH_VERSIONCOLUMNSselect builtin command to determine the terminal width
when printing selection lists. Automatically set upon receipt of a
SIGWINCH.
COMP_CWORDCOMP_LINECOMP_POINT${#COMP_LINE}.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see Programmable Completion).
COMP_WORDBREAKSCOMP_WORDBREAKS is unset, it loses its special properties,
even if it is subsequently reset.
COMP_WORDSCOMPREPLYDIRSTACKdirs builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If DIRSTACK is unset, it loses its special properties, even if
it is subsequently reset.
EMACSEUIDFCEDITfc
builtin command.
FIGNOREFUNCNAMEGLOBIGNOREGROUPShistcharsHISTCMDHISTCONTROLHISTFILEHISTFILESIZEHISTIGNOREHISTIGNORE subsumes the function of HISTCONTROL. A
pattern of ‘&’ is identical to ignoredups, and a
pattern of ‘[ ]*’ is identical to ignorespace.
Combining these two patterns, separating them with a colon,
provides the functionality of ignoreboth.
HISTSIZEHISTTIMEFORMAThistory builtin.
If this variable is set, time stamps are written to the history file so
they may be preserved across shell sessions.
HOSTFILEHOSTNAMEHOSTTYPEIGNOREEOFEOF character
as the sole input. If set, the value denotes the number
of consecutive EOF characters that can be read as the
first character on an input line
before the shell will exit. If the variable exists but does not
have a numeric value (or has no value) then the default is 10.
If the variable does not exist, then EOF signifies the end of
input to the shell. This is only in effect for interactive shells.
INPUTRCLANGLC_.
LC_ALLLC_ variable specifying a locale category.
LC_COLLATELC_CTYPELC_MESSAGESLC_NUMERICLINENOLINESselect builtin command to determine the column length
for printing selection lists. Automatically set upon receipt of a
SIGWINCH.
MACHTYPEMAILCHECKOLDPWDcd builtin.
OPTERRgetopts builtin command.
OSTYPEPIPESTATUSPOSIXLY_CORRECTbash starts, the shell
enters posix mode (see Bash POSIX Mode) before reading the
startup files, as if the --posix invocation option had been supplied.
If it is set while the shell is running, bash enables posix mode,
as if the command
set -o posix
had been executed.
PPIDPROMPT_COMMANDPS3select command. If this variable is not set, the
select command prompts with ‘#? ’
PS4PWDcd builtin.
RANDOMREPLYread builtin.
SECONDSSHELLSHELLOPTSset builtin command (see The Set Builtin).
The options appearing in SHELLOPTS are those reported
as ‘on’ by ‘set -o’.
If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before
reading any startup files. This variable is readonly.
SHLVLTIMEFORMATtime
reserved word should be displayed.
The ‘%’ character introduces an
escape sequence that is expanded to a time value or other
information.
The escape sequences and their meanings are as
follows; the braces denote optional portions.
%%%[p][l]R%[p][l]U%[p][l]S%PThe optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.
The optional l specifies a longer format, including minutes, of
the form MMmSS.FFs.
The value of p determines whether or not the fraction is included.
If this variable is not set, Bash acts as if it had the value
$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'
If the value is null, no timing information is displayed.
A trailing newline is added when the format string is displayed.
TMOUTTMOUT is treated as the
default timeout for the read builtin (see Bash Builtins).
The select command (see Conditional Constructs) terminates
if input does not arrive after TMOUT seconds when input is coming
from a terminal.
In an interative shell, the value is interpreted as
the number of seconds to wait for input after issuing the primary
prompt when the shell is interactive.
Bash terminates after that number of seconds if input does
not arrive.
TMPDIRUIDThis section describes features unique to Bash.
bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument ...]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]
In addition to the single-character shell command-line options (see The Set Builtin), there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized.
--debuggerextdebug option to the shopt
builtin) and shell function tracing
(see The Set Builtin for a description of the -o functrace
option).
--dump-po-stringsgettext PO (portable object) file format.
Equivalent to -D except for the output format.
--dump-strings--help--init-file filename--rcfile filename--login--noediting--noprofile--norcsh.
--posix--restricted--verbose--versionThere are several single-character options that may be supplied at
invocation which are not available with the set builtin.
-c string$0.
-i-l-r-s-DC or POSIX (see Locale Translation).
This implies the -n option; no commands will be executed.
[-+]O [shopt_option]shopt builtin (see Shell Builtin Commands).
If shopt_option is present, -O sets the value of that option;
+O unsets it.
If shopt_option is not supplied, the names and values of the shell
options accepted by shopt are printed on the standard output.
If the invocation option is +O, the output is displayed in a format
that may be reused as input.
---- signals the end of options and disables further option
processing.
Any arguments after the -- are treated as filenames and arguments.
A login shell is one whose first character of argument zero is ‘-’, or one invoked with the --login option.
An interactive shell is one started without non-option arguments,
unless -s is specified,
without specifying the -c option, and whose input and output are both
connected to terminals (as determined by isatty(3)), or one
started with the -i option. See Interactive Shells, for more
information.
If arguments remain after option processing, and neither the
-c nor the -s
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands (see Shell Scripts).
When Bash is invoked in this fashion, $0
is set to the name of the file, and the positional parameters
are set to the remaining arguments.
Bash reads and executes commands from this file, then exits.
Bash's exit status is the exit status of the last command executed
in the script. If no commands are executed, the exit status is 0.
This section describs how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see Tilde Expansion).
Interactive shells are described in Interactive Shells.
When Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.
When a login shell exits, Bash reads and executes commands from the file ~/.bash_logout, if it exists.
When an interactive shell that is not a login shell is started, Bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force Bash to read and execute commands from file instead of ~/.bashrc.
So, typically, your ~/.bash_profile contains the line
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi
after (or before) any login-specific initializations.
When Bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:
if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi
but the value of the PATH variable is not used to search for the file name.
As noted above, if a non-interactive shell is invoked with the --login option, Bash attempts to read and execute commands from the login shell startup files.
shIf Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the posix standard as well.
When invoked as an interactive login shell, or as a non-interactive
shell with the --login option, it first attempts to read
and execute commands from /etc/profile and ~/.profile, in
that order.
The --noprofile option may be used to inhibit this behavior.
When invoked as an interactive shell with the name sh, Bash
looks for the variable ENV, expands its value if it is defined,
and uses the expanded value as the name of a file to read and execute.
Since a shell invoked as sh does not attempt to read and execute
commands from any other startup files, the --rcfile option has
no effect.
A non-interactive shell invoked with the name sh does not attempt
to read any other startup files.
When invoked as sh, Bash enters posix mode after
the startup files are read.
When Bash is started in posix mode, as with the --posix command line option, it follows the posix standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.
Bash attempts to determine when it is being run by the remote shell
daemon, usually rshd. If Bash determines it is being run by
rshd, it reads and executes commands from ~/.bashrc, if that
file exists and is readable.
It will not do this if invoked as sh.
The --norc option may be used to inhibit this behavior, and the
--rcfile option may be used to force another file to be read, but
rshd does not generally invoke the shell with those options or
allow them to be specified.
If Bash is started with the effective user (group) id not equal to the
real user (group) id, and the -p option is not supplied, no startup
files are read, shell functions are not inherited from the environment,
the SHELLOPTS variable, if it appears in the environment, is ignored,
and the effective user id is set to the real user id.
If the -p option is supplied at invocation, the startup behavior is
the same, but the effective user id is not reset.
An interactive shell
is one started without non-option arguments, unless -s is
specified, without specifiying the -c option, and
whose input and error output are both
connected to terminals (as determined by isatty(3)),
or one started with the -i option.
An interactive shell generally reads from and writes to a user's terminal.
The -s invocation option may be used to set the positional parameters when an interactive shell is started.
To determine within a startup script whether or not Bash is
running interactively,
test the value of the ‘-’ special parameter.
It contains i when the shell is interactive. For example:
case "$-" in
*i*) echo This shell is interactive ;;
*) echo This shell is not interactive ;;
esac
Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
When the shell is running interactively, it changes its behavior in several ways.
SIGTTIN, SIGTTOU, and SIGTSTP.
ignoreeof option to set -o
instead of exiting immediately when it receives an EOF on its
standard input when reading a command (see The Set Builtin).
SIGTERM
(see Signals).
SIGINT is caught and handled
((see Signals).
SIGINT will interrupt some shell builtins.
SIGHUP to all jobs on exit
if the hupoxexit shell option has been enabled (see Signals).
${var:?word} expansions
(see Shell Parameter Expansion).
exec will not cause the shell to exit
(see Bourne Shell Builtins).
cd
builtin is enabled by default (see the description of the cdspell
option to the shopt builtin in Bash Builtins).
Conditional expressions are used by the [[ compound command
and the test and [ builtin commands.
Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.
-a file-b file-c file-d file-e file-f file-g file-h file-k file-p file-r file-s file-t fd-u file-w file-x file-O file-G file-L file-S file-N file -nt file2 -ot file2 -ef file2-o optnameset builtin (see The Set Builtin).
-z string-n string == string2 != string2 < string2 > string2 OP arg2OP is one of
‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’.
These arithmetic binary operators return true if arg1
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to arg2,
respectively. Arg1 and arg2
may be positive or negative integers.
The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by the let and the -i option
to the declare builtins.
Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.
++ id--++id --id- +! ~*** / %+ -<< >><= >= < >== !=&^|&&||expr ? expr : expr= *= /= %= += -= <<= >>= &= ^= |=expr1 , expr2Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using ‘declare -i’ is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers.
A leading ‘0x’ or ‘0X’ denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where base
is a decimal number between 2 and 64 representing the arithmetic
base, and n is a number in that base. If base# is
omitted, then base 10 is used.
The digits greater than 9 are represented by the lowercase letters,
the uppercase letters, ‘@’, and ‘_’, in that order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between 10
and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.
Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with
the alias and unalias builtin commands.
The first word of each simple command, if unquoted, is checked to see
if it has an alias.
If so, that word is replaced by the text of the alias.
The characters ‘/’, ‘$’, ‘`’, ‘=’ and any of the
shell metacharacters or quoting characters listed above may not appear
in an alias name.
The replacement text may contain any valid
shell input, including shell metacharacters.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time.
This means that one may alias ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, then the next command word following the
alias is also checked for alias expansion.
Aliases are created and listed with the alias
command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used
(see Shell Functions).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see Bash Builtins).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash
always reads at least one complete line
of input before executing any
of the commands on that line. Aliases are expanded when a
command is read, not when it is executed. Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a compound command. As a consequence, aliases
defined in a function are not available until after that
function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.
For almost every purpose, shell functions are preferred over aliases.
Bash provides one-dimensional array variables. Any variable may be used as
an array; the declare builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously. Arrays are zero-based.
An array is created automatically if any variable is assigned to using the syntax
name[subscript]=value
The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. To explicitly declare an array, use
declare -a name
The syntax
declare -a name[subscript]
is also accepted; the subscript is ignored. Attributes may be
specified for an array variable using the declare and
readonly builtins. Each attribute applies to all members of
an array.
Arrays are assigned to using compound assignments of the form
name=(value1 ... valuen)
where each
value is of the form [[subscript]=]string. If
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
This syntax is also accepted by the declare
builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell's filename expansion operators. If the
subscript is ‘@’ or ‘*’, the word expands to all members
of the array name. These subscripts differ only when the word
appears within double quotes.
If the word is double-quoted,
${name[*]} expands to a single word with
the value of each array member separated by the first character of the
IFS variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} expands to nothing.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
This is analogous to the
expansion of the special parameters ‘@’ and ‘*’.
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is ‘@’ or
‘*’, the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equivalent to
referencing element zero.
The unset builtin is used to destroy arrays.
unset name[subscript]
destroys the array element at index subscript.
Care must be taken to avoid unwanted side effects caused by filename
generation.
unset name, where name is an array, removes the
entire array. A subscript of ‘*’ or ‘@’ also removes the
entire array.
The declare, local, and readonly
builtins each accept a -a
option to specify an array. The read
builtin accepts a -a
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.
The directory stack is a list of recently-visited directories. The
pushd builtin adds directories to the stack as it changes
the current directory, and the popd builtin removes specified
directories from the stack and changes the current directory to
the directory removed. The dirs builtin displays the contents
of the directory stack.
The contents of the directory stack are also visible as the value of the DIRSTACK shell variable.
dirsdirs [+N | -N] [-clpv]
Display the list of currently remembered directories. Directories
are added to the list with the pushd command; the
popd command removes directories from the list.
+Ndirs when invoked without options), starting
with zero.
-Ndirs when invoked without options), starting
with zero.
-c-l-pdirs to print the directory stack with one entry per
line.
-vdirs to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
popdpopd [+N | -N] [-n]
Remove the top entry from the directory stack, and cd
to the new top directory.
When no arguments are given, popd
removes the top directory from the stack and
performs a cd to the new top directory. The
elements are numbered from 0 starting at the first directory listed with
dirs; i.e., popd is equivalent to popd +0.
+Ndirs), starting with zero.
-Ndirs), starting with zero.
-npushdpushd [dir | +N | -N] [-n]
Save the current directory on the top of the directory stack
and then cd to dir.
With no arguments, pushd exchanges the top two directories.
+Ndirs, starting with zero) to the top of
the list by rotating the stack.
-Ndirs, starting with zero) to the top of
the list by rotating the stack.
-ncd dir'.
cds to dir.
The value of the variable PROMPT_COMMAND is examined just before Bash prints each primary prompt. If PROMPT_COMMAND is set and has a non-null value, then the value is executed just as if it had been typed on the command line.
In addition, the following table describes the special characters which can appear in the prompt variables:
\a\d\D{format}strftime(3) and the result is inserted
into the prompt string; an empty format results in a locale-specific
time representation. The braces are required.
\e\h\H\j\l\n\r\s$0 (the portion
following the final slash).
\t\T\@\A\u\v\V\w\W\!\#\$#, otherwise $.
\nnn\\\[\]The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see Bash History Facilities), while the command number is the position in the sequence of commands executed during the current shell session.
After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
promptvars shell option (see Bash Builtins).
If Bash is started with the name rbash, or the
--restricted
or
-r
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash
with the exception that the following are disallowed or not performed:
cd builtin.
.
builtin command.
hash builtin command.
exec builtin to replace the shell with another command.
enable builtin.
enable builtin command to enable disabled shell builtins.
command builtin.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed
(see Shell Scripts), rbash turns off any restrictions in
the shell spawned to execute the script.
Starting Bash with the --posix command-line option or executing ‘set -o posix’ while Bash is running will cause Bash to conform more closely to the posix standard by changing the behavior to match that specified by posix in areas where the Bash default differs.
When invoked as sh, Bash enters posix mode after reading the
startup files.
The following list is what's changed when `posix mode' is in effect:
SIGTSTP.
bg builtin uses the required format to describe each job placed
in the background, which does not include an indication of whether the job
is the current or previous job.
promptvars option.
kill builtin does not accept signal names with a ‘SIG’
prefix.
. filename
is not found.
names. That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit. Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.
cd builtin will not implicitly
append the current directory to it. This means that cd will
fail if no valid directory name can be constructed from
any of the entries in $CDPATH, even if the a directory with
the same name as the name given as an argument to cd exists
in the current directory.
for statement or the selection variable in a
select statement is a readonly variable.
export and readonly builtin commands display their
output in the format required by posix.
trap builtin displays signal names without the leading
SIG.
trap builtin doesn't check the first argument for a possible
signal specification and revert the signal handling to the original
disposition if it is, unless that argument consists solely of digits and
is a valid signal number. If users want to reset the handler for a given
signal to the original disposition, they should use ‘-’ as the
first argument.
. and source builtins do not search the current directory
for the filename argument if it is not found by searching PATH.
alias builtin displays alias definitions, it does not
display them with a leading ‘alias ’ unless the -p option
is supplied.
set builtin is invoked without options, it does not display
shell function names and definitions.
set builtin is invoked without options, it displays
variable values without quotes, unless they contain shell metacharacters,
even if the result contains nonprinting characters.
cd builtin is invoked in logical mode, and the pathname
constructed from $PWD and the directory name supplied as an argument
does not refer to an existing directory, cd will fail instead of
falling back to physical mode.
pwd builtin is supplied the -P option, it resets
$PWD to a pathname containing no symlinks.
pwd builtin verifies that the value it prints is the same as the
current directory, even if it is not asked to check the file system with the
-P option.
fc builtin does not include an
indication of whether or not a history entry has been modified.
fc is ed.
type and command builtins will not report a non-executable
file as having been found, though the shell will attempt to execute such a
file if it is the only so-named file found in $PATH.
vi editing mode will invoke the vi editor directly when
the ‘v’ command is run, instead of checking $FCEDIT and
$EDITOR.
xpg_echo option is enabled, Bash does not attempt to interpret
any arguments to echo as options. Each argument is displayed, after
escape characters are converted.
There is other posix behavior that Bash does not implement by default even when in posix mode. Specifically:
fc builtin checks $EDITOR as a program to edit history
entries if FCEDIT is unset, rather than defaulting directly to
ed. fc uses ed if EDITOR is unset.
xpg_echo option to be enabled for
the echo builtin to be fully conformant.
Bash can be configured to be posix-conformant by default, by specifying
the --enable-strict-posix-default to configure when building
(see Optional Features).
This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities.
Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the system's terminal driver and Bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with the
jobs command. When Bash starts a job
asynchronously, it prints a line that looks
like:
[1] 25647
indicating that this job is job number 1 and that the process id of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group id. Members of this process group (processes whose
process group id is equal to the current terminal process group
id) receive keyboard-generated signals such as SIGINT.
These processes are said to be in the foreground. Background
processes are those whose process group id differs from the
terminal's; such processes are immune to keyboard-generated
signals. Only foreground processes are allowed to read from or
write to the terminal. Background processes which attempt to
read from (write to) the terminal are sent a SIGTTIN
(SIGTTOU) signal by the terminal driver, which, unless
caught, suspends the process.
If the operating system on which Bash is running supports
job control, Bash contains facilities to use it. Typing the
suspend character (typically ‘^Z’, Control-Z) while a
process is running causes that process to be stopped and returns
control to Bash. Typing the delayed suspend character
(typically ‘^Y’, Control-Y) causes the process to be stopped
when it attempts to read input from the terminal, and control to
be returned to Bash. The user then manipulates the state of
this job, using the bg command to continue it in the
background, the fg command to continue it in the
foreground, or the kill command to kill it. A ‘^Z’
takes effect immediately, and has the additional side effect of
causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The character ‘%’ introduces a job name.
Job number n may be referred to as ‘%n’.
The symbols ‘%%’ and ‘%+’ refer to the shell's notion of the
current job, which is the last job stopped while it was in the foreground
or started in the background.
A single ‘%’ (with no accompanying job specification) also refers
to the current job.
The previous job may be referenced using ‘%-’. In output
pertaining to jobs (e.g., the output of the jobs command),
the current job is always flagged with a ‘+’, and the
previous job with a ‘-’.
A job may also be referred to
using a prefix of the name used to start it, or using a substring
that appears in its command line. For example, ‘%ce’ refers
to a stopped ce job. Using ‘%?ce’, on the
other hand, refers to any job containing the string ‘ce’ in
its command line. If the prefix or substring matches more than one job,
Bash reports an error.
Simply naming a job can be used to bring it into the foreground: ‘%1’ is a synonym for ‘fg %1’, bringing job 1 from the background into the foreground. Similarly, ‘%1 &’ resumes job 1 in the background, equivalent to ‘bg %1’
The shell learns immediately whenever a job changes state.
Normally, Bash waits until it is about to print a prompt
before reporting changes in a job's status so as to not interrupt
any other output.
If the -b option to the set builtin is enabled,
Bash reports such changes immediately (see The Set Builtin).
Any trap on SIGCHLD is executed for each child process
that exits.
If an attempt to exit Bash is made while jobs are stopped, the
shell prints a message warning that there are stopped jobs.
The jobs command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
Bash does not print another warning, and the stopped jobs are terminated.
bgbg [jobspec ...]
Resume each suspended job jobspec in the background, as if it
had been started with ‘&’.
If jobspec is not supplied, the current job is used.
The return status is zero unless it is run when job control is not
enabled, or, when run with job control enabled, any
jobspec was not found or specifies a job
that was started without job control.
fgfg [jobspec]
Resume the job jobspec in the foreground and make it the current job.
If jobspec is not supplied, the current job is used.
The return status is that of the command placed into the foreground,
or non-zero if run when job control is disabled or, when run with
job control enabled, jobspec does not specify a valid job or
jobspec specifies a job that was started without job control.
jobs jobs [-lnprs] [jobspec]
jobs -x command [arguments]
The first form lists the active jobs. The options have the following meanings:
-l-n-p-r-sIf jobspec is given, output is restricted to information about that job. If jobspec is not supplied, the status of all jobs is listed.
If the -x option is supplied, jobs replaces any
jobspec found in command or arguments with the
corresponding process group id, and executes command,
passing it arguments, returning its exit status.
kill kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid
kill -l [exit_status]
Send a signal specified by sigspec or signum to the process
named by job specification jobspec or process id pid.
sigspec is either a case-insensitive signal name such as
SIGINT (with or without the SIG prefix)
or a signal number; signum is a signal number.
If sigspec and signum are not present, SIGTERM is used.
The -l option lists the signal names.
If any arguments are supplied when -l is given, the names of the
signals corresponding to the arguments are listed, and the return status
is zero.
exit_status is a number specifying a signal number or the exit
status of a process terminated by a signal.
The return status is zero if at least one signal was successfully sent,
or non-zero if an error occurs or an invalid option is encountered.
waitwait [jobspec or pid ...]
Wait until the child process specified by each process id pid
or job specification jobspec exits and return the exit status of the
last command waited for.
If a job spec is given, all processes in the job are waited for.
If no arguments are given, all currently active child processes are
waited for, and the return status is zero.
If neither jobspec nor pid specifies an active child process
of the shell, the return status is 127.
disowndisown [-ar] [-h] [jobspec ...]
Without options, each jobspec is removed from the table of
active jobs.
If the -h option is given, the job is not removed from the table,
but is marked so that SIGHUP is not sent to the job if the shell
receives a SIGHUP.
If jobspec is not present, and neither the -a nor -r
option is supplied, the current job is used.
If no jobspec is supplied, the -a option means to remove or
mark all jobs; the -r option without a jobspec
argument restricts operation to running jobs.
suspendsuspend [-f]
Suspend the execution of this shell until it receives a
SIGCONT signal. The -f option means to suspend
even if the shell is a login shell.
When job control is not active, the kill and wait
builtins do not accept jobspec arguments. They must be
supplied process ids.
auto_resumeThis chapter describes the basic features of the gnu command line editing interface. Command line editing is provided by the Readline library, which is used by several different programs, including Bash.
The following paragraphs describe the notation used to represent keystrokes.
The text C-k is read as `Control-K' and describes the character produced when the <k> key is pressed while the Control key is depressed.
The text M-k is read as `Meta-K' and describes the character produced when the Meta key (if you have one) is depressed, and the <k> key is pressed. The Meta key is labeled <ALT> on many keyboards. On keyboards with two keys labeled <ALT> (usually to either side of the space bar), the <ALT> on the left side is generally set to work as a Meta key. The <ALT> key on the right may also be configured to work as a Meta key or may be configured as some other modifier, such as a Compose key for typing accented characters.
If you do not have a Meta or <ALT> key, or another key working as a Meta key, the identical keystroke can be generated by typing <ESC> first, and then typing <k>. Either process is known as metafying the <k> key.
The text M-C-k is read as `Meta-Control-k' and describes the character produced by metafying C-k.
In addition, several keys have their own names. Specifically, <DEL>, <ESC>, <LFD>, <SPC>, <RET>, and <TAB> all stand for themselves when seen in this text, or in an init file (see Readline Init File). If your keyboard lacks a <LFD> key, typing <C-j> will produce the desired character. The <RET> key may be labeled <Return> or <Enter> on some keyboards.
Often during an interactive session you type in a long line of text, only to notice that the first word on the line is misspelled. The Readline library gives you a set of commands for manipulating the text as you type it in, allowing you to just fix your typo, and not forcing you to retype the majority of the line. Using these editing commands, you move the cursor to the place that needs correction, and delete or insert the text of the corrections. Then, when you are satisfied with the line, you simply press <RET>. You do not have to be at the end of the line to press <RET>; the entire line is accepted regardless of the location of the cursor within the line.
In order to enter characters into the line, simply type them. The typed character appears where the cursor was, and then the cursor moves one space to the right. If you mistype a character, you can use your erase character to back up and delete the mistyped character.
Sometimes you may mistype a character, and not notice the error until you have typed several other characters. In that case, you can type C-b to move the cursor to the left, and then correct your mistake. Afterwards, you can move the cursor to the right with C-f.
When you add text in the middle of a line, you will notice that characters to the right of the cursor are `pushed over' to make room for the text that you have inserted. Likewise, when you delete text behind the cursor, characters to the right of the cursor are `pulled back' to fill in the blank space created by the removal of the text. A list of the bare essentials for editing the text of an input line follows.
(Depending on your configuration, the <Backspace> key be set to delete the character to the left of the cursor and the <DEL> key set to delete the character underneath the cursor, like C-d, rather than the character to the left of the cursor.)
The above table describes the most basic keystrokes that you need in order to do editing of the input line. For your convenience, many other commands have been added in addition to C-b, C-f, C-d, and <DEL>. Here are some commands for moving more rapidly about the line.
Notice how C-f moves forward a character, while M-f moves forward a word. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words.
Killing text means to delete the text from the line, but to save it away for later use, usually by yanking (re-inserting) it back into the line. (`Cut' and `paste' are more recent jargon for `kill' and `yank'.)
If the description for a command says that it `kills' text, then you can be sure that you can get the text back in a different (or the same) place later.
When you use a kill command, the text is saved in a kill-ring. Any number of consecutive kills save all of the killed text together, so that when you yank it back, you get it all. The kill ring is not line specific; the text that you killed on a previously typed line is available to be yanked back later, when you are typing another line. Here is the list of commands for killing text.
Here is how to yank the text back into the line. Yanking means to copy the most-recently-killed text from the kill buffer.
You can pass numeric arguments to Readline commands. Sometimes the argument acts as a repeat count, other times it is the sign of the argument that is significant. If you pass a negative argument to a command which normally acts in a forward direction, that command will act in a backward direction. For example, to kill text back to the start of the line, you might type ‘M-- C-k’.
The general way to pass numeric arguments to a command is to type meta digits before the command. If the first `digit' typed is a minus sign (‘-’), then the sign of the argument will be negative. Once you have typed one meta digit to get the argument started, you can type the remainder of the digits, and then the command. For example, to give the C-d command an argument of 10, you could type ‘M-1 0 C-d’, which will delete the next ten characters on the input line.
Readline provides commands for searching through the command history (see Bash History Facilities) for lines containing a specified string. There are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, Readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
To search backward in the history for a particular string, type
C-r. Typing C-s searches forward through the history.
The characters present in the value of the isearch-terminators variable
are used to terminate an incremental search.
If that variable has not been assigned a value, the <ESC> and
C-J characters will terminate an incremental search.
C-g will abort an incremental search and restore the original line.
When the search is terminated, the history entry containing the
search string becomes the current line.
To find other matching entries in the history list, type C-r or C-s as appropriate. This will search backward or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a Readline command will terminate the search and execute that command. For instance, a <RET> will terminate the search and accept the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.
Readline remembers the last incremental search string. If two C-rs are typed without any intervening characters defining a new search string, any remembered search string is used.
Non-incremental searches read the entire search string before starting to search for matching history lines. The search string may be typed by the user or be part of the contents of the current line.
Although the Readline library comes with a set of Emacs-like keybindings installed by default, it is possible to use a different set of keybindings. Any user can customize programs that use Readline by putting commands in an inputrc file, conventionally in his home directory. The name of this file is taken from the value of the shell variable INPUTRC. If that variable is unset, the default is ~/.inputrc. If that file does not exist or cannot be read, the ultimate default is /etc/inputrc.
When a program which uses the Readline library starts up, the init file is read, and the key bindings are set.
In addition, the C-x C-r command re-reads this init file, thus
incorporating any changes that you might have made to it.
There are only a few basic constructs allowed in the Readline init file. Blank lines are ignored. Lines beginning with a ‘#’ are comments. Lines beginning with a ‘$’ indicate conditional constructs (see Conditional Init Constructs). Other lines denote variable settings and key bindings.
set command within the init file.
The syntax is simple:
set variable value
Here, for example, is how to
change from the default Emacs-like key binding to use
vi line editing commands:
set editing-mode vi
Variable names and values, where appropriate, are recognized without regard to case. Unrecognized variable names are ignored.
Boolean variables (those that can be set to on or off) are set to on if the value is null or empty, on (case-insensitive), or 1. Any other value results in the variable being set to off.
The bind -V command lists the current Readline variable names
and values. See Bash Builtins.
A great deal of run-time behavior is changeable with the following variables.
bell-stylebind-tty-special-charscomment-begininsert-comment command is executed. The default value
is "#".
completion-ignore-casecompletion-query-items100.
convert-metadisable-completionself-insert. The default is ‘off’.
editing-modeediting-mode variable controls which default set of
key bindings is used. By default, Readline starts up in Emacs editing
mode, where the keystrokes are most similar to Emacs. This variable can be
set to either ‘emacs’ or ‘vi’.
enable-keypadexpand-tildehistory-preserve-pointprevious-history
or next-history. The default is ‘off’.
horizontal-scroll-modeinput-metameta-flag is a
synonym for this variable.
isearch-terminatorskeymapkeymap names are
emacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-move,
vi-command, and
vi-insert.
vi is equivalent to vi-command; emacs is
equivalent to emacs-standard. The default value is emacs.
The value of the editing-mode variable also affects the
default keymap.
mark-directoriesmark-modified-linesmark-symlinked-directoriesmark-directories).
The default is ‘off’.
match-hidden-filesoutput-metapage-completionsmore-like pager
to display a screenful of possible completions at a time.
This variable is ‘on’ by default.
print-completions-horizontallyshow-all-if-ambiguousshow-all-if-unmodifiedvisible-statsOnce you know the name of the command, simply place on a line in the init file the name of the key you wish to bind the command to, a colon, and then the name of the command. There can be no space between the key name and the colon – that will be interpreted as part of the key name. The name of the key can be expressed in different ways, depending on what you find most comfortable.
In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro).
The bind -p command displays Readline function names and
bindings in a format that can put directly into an initialization file.
See Bash Builtins.
Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"
In the above example, C-u is bound to the function
universal-argument,
M-DEL is bound to the function backward-kill-word, and
C-o is bound to run the macro
expressed on the right hand side (that is, to insert the text
‘> output’ into the line).
A number of symbolic character names are recognized while
processing this key binding syntax:
DEL,
ESC,
ESCAPE,
LFD,
NEWLINE,
RET,
RETURN,
RUBOUT,
SPACE,
SPC,
and
TAB.
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"
In the above example, C-u is again bound to the function
universal-argument (just as it was in the first example),
‘C-x C-r’ is bound to the function re-read-init-file,
and ‘<ESC> <[> <1> <1> <~>’ is bound to insert
the text ‘Function Key 1’.
The following gnu Emacs style escape sequences are available when specifying key sequences:
In addition to the gnu Emacs style escape sequences, a second set of backslash escapes is available:
\a\b\d\f\n\r\t\v\nnn\xHHWhen entering the text of a macro, single or double quotes must be used to indicate a macro definition. Unquoted text is assumed to be a function name. In the macro body, the backslash escapes described above are expanded. Backslash will quote any other character in the macro text, including ‘"’ and ‘'’. For example, the following binding will make ‘C-x \’ insert a single ‘\’ into the line:
"\C-x\\": "\\"
Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are four parser directives used.
$if$if construct allows bindings to be made based on the
editing mode, the terminal being used, or the application using
Readline. The text of the test extends to the end of the line;
no characters are required to isolate it.
modemode= form of the $if directive is used to test
whether Readline is in emacs or vi mode.
This may be used in conjunction
with the ‘set keymap’ command, for instance, to set bindings in
the emacs-standard and emacs-ctlx keymaps only if
Readline is starting out in emacs mode.
termterm= form may be used to include terminal-specific
key bindings, perhaps to bind the key sequences output by the
terminal's function keys. The word on the right side of the
‘=’ is tested against both the full name of the terminal and
the portion of the terminal name before the first ‘-’. This
allows sun to match both sun and sun-cmd,
for instance.
application $if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif
$endif$if command.
$else$if directive are executed if
the test fails.
$include$include /etc/inputrc
Here is an example of an inputrc file. This illustrates key binding, variable assignment, and conditional syntax.
# This file controls the behaviour of line input editing for
# programs that use the GNU Readline library. Existing
# programs include FTP, Bash, and GDB.
#
# You can re-read the inputrc file with C-x C-r.
# Lines beginning with '#' are comments.
#
# First, include any systemwide bindings and variable
# assignments from /etc/Inputrc
$include /etc/Inputrc
#
# Set various bindings for emacs mode.
set editing-mode emacs
$if mode=emacs
Meta-Control-h: backward-kill-word Text after the function name is ignored
#
# Arrow keys in keypad mode
#
#"\M-OD": backward-char
#"\M-OC": forward-char
#"\M-OA": previous-history
#"\M-OB": next-history
#
# Arrow keys in ANSI mode
#
"\M-[D": backward-char
"\M-[C": forward-char
"\M-[A": previous-history
"\M-[B": next-history
#
# Arrow keys in 8 bit keypad mode
#
#"\M-\C-OD": backward-char
#"\M-\C-OC": forward-char
#"\M-\C-OA": previous-history
#"\M-\C-OB": next-history
#
# Arrow keys in 8 bit ANSI mode
#
#"\M-\C-[D": backward-char
#"\M-\C-[C": forward-char
#"\M-\C-[A": previous-history
#"\M-\C-[B": next-history
C-q: quoted-insert
$endif
# An old-style binding. This happens to be the default.
TAB: complete
# Macros that are convenient for shell interaction
$if Bash
# edit the path
"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
# prepare to type a quoted word --
# insert open and close double quotes
# and move to just after the open quote
"\C-x\"": "\"\"\C-b"
# insert a backslash (testing backslash escapes
# in sequences and macros)
"\C-x\\": "\\"
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
# Add a binding to refresh the line, which is unbound
"\C-xr": redraw-current-line
# Edit variable on current line.
"\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
$endif
# use a visible bell if one is available
set bell-style visible
# don't strip characters to 7 bits when reading
set input-meta on
# allow iso-latin1 characters to be inserted rather
# than converted to prefix-meta sequences
set convert-meta off
# display characters with the eighth bit set directly
# rather than as meta-prefixed characters
set output-meta on
# if there are more than 150 possible completions for
# a word, ask the user if he wants to see all of them
set completion-query-items 150
# For FTP
$if Ftp
"\C-xg": "get \M-?"
"\C-xt": "put \M-?"
"\M-.": yank-last-arg
$endif
This section describes Readline commands that may be bound to key
sequences.
You can list your key bindings by executing
bind -P or, for a more terse format, suitable for an
inputrc file, bind -p. (See Bash Builtins.)
Command names without an accompanying key sequence are unbound by default.
In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark command.
The text between the point and mark is referred to as the region.
beginning-of-line (C-a)end-of-line (C-e)forward-char (C-f)backward-char (C-b)forward-word (M-f)backward-word (M-b)clear-screen (C-l)redraw-current-line ()accept-line (Newline or Return)previous-history (C-p)next-history (C-n)beginning-of-history (M-<)end-of-history (M->)reverse-search-history (C-r)forward-search-history (C-s)non-incremental-reverse-search-history (M-p)non-incremental-forward-search-history (M-n)history-search-forward ()history-search-backward ()yank-nth-arg (M-C-y)yank-last-arg (M-. or M-_)yank-nth-arg.
Successive calls to yank-last-arg move back through the history
list, inserting the last argument of each line in turn.
The history expansion facilities are used to extract the last argument,
as if the ‘!$’ history expansion had been specified.
delete-char (C-d)delete-char, then
return eof.
backward-delete-char (Rubout)forward-backward-delete-char ()quoted-insert (C-q or C-v)self-insert (a, b, A, 1, !, ...)transpose-chars (C-t)transpose-words (M-t)upcase-word (M-u)downcase-word (M-l)capitalize-word (M-c)overwrite-mode ()emacs mode; vi mode does overwrite differently.
Each call to readline() starts in insert mode.
In overwrite mode, characters bound to self-insert replace
the text at point rather than pushing the text to the right.
Characters bound to backward-delete-char replace the character
before point with a space.
By default, this command is unbound.
kill-line (C-k)backward-kill-line (C-x Rubout)unix-line-discard (C-u)kill-whole-line ()kill-word (M-d)forward-word.
backward-kill-word (M-<DEL>)backward-word.
unix-word-rubout (C-w)unix-filename-rubout ()delete-horizontal-space ()kill-region ()copy-region-as-kill ()copy-backward-word ()backward-word.
By default, this command is unbound.
copy-forward-word ()forward-word.
By default, this command is unbound.
yank (C-y)yank-pop (M-y)yank or yank-pop.
digit-argument (M-0, M-1, ... M--)universal-argument ()universal-argument
again ends the numeric argument, but is otherwise ignored.
As a special case, if this command is immediately followed by a
character that is neither a digit or minus sign, the argument count
for the next command is multiplied by four.
The argument count is initially one, so executing this function the
first time makes the argument count four, a second time makes the
argument count sixteen, and so on.
By default, this is not bound to a key.
complete (<TAB>)possible-completions (M-?)insert-completions (M-*)possible-completions.
menu-complete ()complete, but replaces the word to be completed
with a single match from the list of possible completions.
Repeated execution of menu-complete steps through the list
of possible completions, inserting each match in turn.
At the end of the list of completions, the bell is rung
(subject to the setting of bell-style)
and the original text is restored.
An argument of n moves n positions forward in the list
of matches; a negative argument may be used to move backward
through the list.
This command is intended to be bound to <TAB>, but is unbound
by default.
delete-char-or-list ()delete-char).
If at the end of the line, behaves identically to
possible-completions.
This command is unbound by default.
complete-filename (M-/)possible-filename-completions (C-x /)complete-username (M-~)possible-username-completions (C-x ~)complete-variable (M-$)possible-variable-completions (C-x $)complete-hostname (M-@)possible-hostname-completions (C-x @)complete-command (M-!)possible-command-completions (C-x !)dynamic-complete-history (M-<TAB>)complete-into-braces (M-{)start-kbd-macro (C-x ()end-kbd-macro (C-x ))call-last-kbd-macro (C-x e)re-read-init-file (C-x C-r)abort (C-g)bell-style).
do-uppercase-version (M-a, M-b, M-x, ...)prefix-meta (<ESC>)undo (C-_ or C-x C-u)revert-line (M-r)undo
command enough times to get back to the beginning.
tilde-expand (M-&)set-mark (C-@)exchange-point-and-mark (C-x C-x)character-search (C-])character-search-backward (M-C-])insert-comment (M-#)comment-begin
variable is inserted at the beginning of the current line.
If a numeric argument is supplied, this command acts as a toggle: if
the characters at the beginning of the line do not match the value
of comment-begin, the value is inserted, otherwise
the characters in comment-begin are deleted from the beginning of
the line.
In either case, the line is accepted as if a newline had been typed.
The default value of comment-begin causes this command
to make the current line a shell comment.
If a numeric argument causes the comment character to be removed, the line
will be executed by the shell.
dump-functions ()dump-variables ()dump-macros ()glob-complete-word (M-g)glob-expand-word (C-x *)glob-list-expansions (C-x g)glob-expand-word is displayed, and the line is redrawn.
If a numeric argument is supplied, a ‘*’ is appended before
pathname expansion.
display-shell-version (C-x C-v)shell-expand-line (M-C-e)history-expand-line (M-^)magic-space ()alias-expand-line ()history-and-alias-expand-line ()insert-last-argument (M-. or M-_)yank-last-arg.
operate-and-get-next (C-o)edit-and-execute-command (C-xC-e)$VISUAL, $EDITOR, and emacs
as the editor, in that order.
While the Readline library does not have a full set of vi
editing functions, it does contain enough to allow simple editing
of the line. The Readline vi mode behaves as specified in
the posix 1003.2 standard.
In order to switch interactively between emacs and vi
editing modes, use the ‘set -o emacs’ and ‘set -o vi’
commands (see The Set Builtin).
The Readline default is emacs mode.
When you enter a line in vi mode, you are already placed in
`insertion' mode, as if you had typed an ‘i’. Pressing <ESC>
switches you into `command' mode, where you can edit the text of the
line with the standard vi movement keys, move to previous
history lines with ‘k’ and subsequent lines with ‘j’, and
so forth.
When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined
using the complete builtin (see Programmable Completion Builtins),
the programmable completion facilities are invoked.
First, the command name is identified. If a compspec has been defined for that command, the compspec is used to generate the list of possible completions for the word. If the command word is a full pathname, a compspec for the full pathname is searched for first. If no compspec is found for the full pathname, an attempt is made to find a compspec for the portion following the final slash.
Once a compspec has been found, it is used to generate the list of matching words. If a compspec is not found, the default Bash completion described above (see Commands For Completion) is performed.
First, the actions specified by the compspec are used. Only matches which are prefixed by the word being completed are returned. When the -f or -d option is used for filename or directory name completion, the shell variable FIGNORE is used to filter the matches. See Bash Variables, for a description of FIGNORE.
Any completions specified by a filename expansion pattern to the -G option are generated next. The words generated by the pattern need not match the word being completed. The GLOBIGNORE shell variable is not used to filter the matches, but the FIGNORE shell variable is used.
Next, the string specified as the argument to the -W option is considered. The string is first split using the characters in the IFS special variable as delimiters. Shell quoting is honored. Each word is then expanded using brace expansion, tilde expansion, parameter and variable expansion, command substitution, and arithmetic expansion, as described above (see Shell Expansions). The results are split using the rules described above (see Word Splitting). The results of the expansion are prefix-matched against the word being completed, and the matching words become the possible completions.
After these matches have been generated, any shell function or command specified with the -F and -C options is invoked. When the command or function is invoked, the COMP_LINE and COMP_POINT variables are assigned values as described above (see Bash Variables). If a shell function is being invoked, the COMP_WORDS and COMP_CWORD variables are also set. When the function or command is invoked, the first argument is the name of the command whose arguments are being completed, the second argument is the word being completed, and the third argument is the word preceding the word being completed on the current command line. No filtering of the generated completions against the word being completed is performed; the function or command has complete freedom in generating the matches.
Any function specified with -F is invoked first.
The function may use any of the shell facilities, including the
compgen builtin described below
(see Programmable Completion Builtins), to generate the matches.
It must put the possible completions in the COMPREPLY array
variable.
Next, any command specified with the -C option is invoked in an environment equivalent to command substitution. It should print a list of completions, one per line, to the standard output. Backslash may be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter specified with the -X option is applied to the list. The filter is a pattern as used for pathname expansion; a ‘&’ in the pattern is replaced with the text of the word being completed. A literal ‘&’ may be escaped with a backslash; the backslash is removed before attempting a match. Any completion that matches the pattern will be removed from the list. A leading ‘!’ negates the pattern; in this case any completion not matching the pattern will be removed.
Finally, any prefix and suffix specified with the -P and -S options are added to each member of the completion list, and the result is returned to the Readline completion code as the list of possible completions.
If the previously-applied actions do not generate any matches, and the
-o dirnames option was supplied to complete when the
compspec was defined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when
the compspec was defined, directory name completion is attempted and any
matches are added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions.
The default Bash completions are not attempted, and the Readline default
of filename completion is disabled.
If the -o bashdefault option was supplied to complete when
the compspec was defined, the default Bash completions are attempted
if the compspec generates no matches.
If the -o default option was supplied to complete when the
compspec was defined, Readline's default completion will be performed
if the compspec (and, if attempted, the default Bash completions)
generate no matches.
When a compspec indicates that directory name completion is desired, the programmable completion functions force Readline to append a slash to completed names which are symbolic links to directories, subject to the value of the mark-directories Readline variable, regardless of the setting of the mark-symlinked-directories Readline variable.
Two builtin commands are available to manipulate the programmable completion facilities.
compgencompgen [option] [word]
Generate possible completion matches for word according to
the options, which may be any option accepted by the
complete
builtin with the exception of -p and -r, and write
the matches to the standard output.
When using the -F or -C options, the various shell variables
set by the programmable completion facilities, while available, will not
have useful values.
The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or no
matches were generated.
completecomplete [-abcdefgjksuv] [-ocomp-option] [-Aaction] [-Gglobpat] [-Wwordlist] [-Pprefix] [-Ssuffix] [-Xfilterpat] [-Ffunction] [-Ccommand]name[name...]complete -pr [name...]
Specify how arguments to each name should be completed. If the -p option is supplied, or if no options are supplied, existing completion specifications are printed in a way that allows them to be reused as input. The -r option removes a completion specification for each name, or, if no names are supplied, all completion specifications.
The process of applying these completion specifications when word completion is attempted is described above (see Programmable Completion).
Other options, if specified, have the following meanings.
The arguments to the -G, -W, and -X options
(and, if necessary, the -P and -S options)
should be quoted to protect them from expansion before the
complete builtin is invoked.
-o comp-optionbashdefaultdefaultdirnamesfilenamesnospaceplusdirs-A actionaliasarrayvarbindingbuiltincommanddirectorydisabledenabledexportfilefunctiongrouphelptopichelp builtin (see Bash Builtins).
hostnamejobkeywordrunningservicesetoptset builtin
(see The Set Builtin).
shoptshopt builtin
(see Bash Builtins).
signalstoppeduservariable-G globpat-W wordlist-C command-F function-X filterpat-P prefix-S suffixThe return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification.
This chapter describes how to use the gnu History Library interactively, from a user's standpoint. It should be considered a user's guide. For information on using the gnu History Library in other programs, see the gnu Readline Library Manual.
When the -o history option to the set builtin
is enabled (see The Set Builtin),
the shell provides access to the command history,
the list of commands previously typed.
The value of the HISTSIZE shell variable is used as the
number of commands to save in a history list.
The text of the last $HISTSIZE
commands (default 500) is saved.
The shell stores each command in the history list prior to
parameter and variable expansion
but after history expansion is performed, subject to the
values of the shell variables
HISTIGNORE and HISTCONTROL.
When the shell starts up, the history is initialized from the
file named by the HISTFILE variable (default ~/.bash_history).
The file named by the value of HISTFILE is truncated, if
necessary, to contain no more than the number of lines specified by
the value of the HISTFILESIZE variable.
When an interactive shell exits, the last
$HISTSIZE lines are copied from the history list to the file
named by $HISTFILE.
If the histappend shell option is set (see Bash Builtins),
the lines are appended to the history file,
otherwise the history file is overwritten.
If HISTFILE
is unset, or if the history file is unwritable, the history is
not saved. After saving the history, the history file is truncated
to contain no more than $HISTFILESIZE
lines. If HISTFILESIZE is not set, no truncation is performed.
If the HISTTIMEFORMAT is set, the time stamp information associated with each history entry is written to the history file.
The builtin command fc may be used to list or edit and re-execute
a portion of the history list.
The history builtin may be used to display or modify the history
list and manipulate the history file.
When using command-line editing, search commands
are available in each editing mode that provide access to the
history list (see Commands For History).
The shell allows control over which commands are saved on the history
list. The HISTCONTROL and HISTIGNORE
variables may be set to cause the shell to save only a subset of the
commands entered.
The cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The lithist
shell option causes the shell to save the command with embedded newlines
instead of semicolons.
The shopt builtin is used to set these options.
See Bash Builtins, for a description of shopt.
Bash provides two builtin commands which manipulate the history list and history file.
fcfc [-eename] [-nlr] [first] [last]fc -s [pat=rep] [command]
Fix Command. In the first form, a range of commands from first to
last is selected from the history list. Both first and
last may be specified as a string (to locate the most recent
command beginning with that string) or as a number (an index into the
history list, where a negative number is used as an offset from the
current command number). If last is not specified it is set to
first. If first is not specified it is set to the previous
command for editing and −16 for listing. If the -l flag is
given, the commands are listed on standard output. The -n flag
suppresses the command numbers when listing. The -r flag
reverses the order of the listing. Otherwise, the editor given by
ename is invoked on a file containing those commands. If
ename is not given, the value of the following variable expansion
is used: ${FCEDIT:-${EDITOR:-vi}}. This says to use the
value of the FCEDIT variable if set, or the value of the
EDITOR variable if that is set, or vi if neither is set.
When editing is complete, the edited commands are echoed and executed.
In the second form, command is re-executed after each instance of pat in the selected command is replaced by rep.
A useful alias to use with the fc command is r='fc -s', so
that typing ‘r cc’ runs the last command beginning with cc
and typing ‘r’ re-executes the last command (see Aliases).
history history [n]
history -c
history -d offset
history [-anrw] [filename]
history -ps arg
With no options, display the history list with line numbers. Lines prefixed with a ‘*’ have been modified. An argument of n lists only the last n lines. If the shell variable HISTTIMEFORMAT is set and not null, it is used as a format string for strftime to display the time stamp associated with each displayed history entry. No intervening blank is printed between the formatted time stamp and the history line.
Options, if supplied, have the following meanings:
-c-d offset-a-n-r-w-p-sWhen any of the -w, -r, -a, or -n options is used, if filename is given, then it is used as the history file. If not, then the value of the HISTFILE variable is used.
The History library provides a history expansion feature that is similar
to the history expansion provided by csh. This section
describes the syntax used to manipulate the history information.
History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.
History expansion takes place in two parts. The first is to determine which line from the history list should be used during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the history is called the event, and the portions of that line that are acted upon are called words. Various modifiers are available to manipulate the selected words. The line is broken into words in the same fashion that Bash does, so that several words surrounded by quotes are considered one word. History expansions are introduced by the appearance of the history expansion character, which is ‘!’ by default. Only ‘\’ and ‘'’ may be used to escape the history expansion character.
Several shell options settable with the shopt
builtin (see Bash Builtins) may be used to tailor
the behavior of history expansion. If the
histverify shell option is enabled, and Readline
is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the Readline
editing buffer for further modification.
If Readline is being used, and the histreedit
shell option is enabled, a failed history expansion will be
reloaded into the Readline editing buffer for correction.
The -p option to the history builtin command
may be used to see what a history expansion will do before using it.
The -s option to the history builtin may be used to
add commands to the end of the history list without actually executing
them, so that they are available for subsequent recall.
This is most useful in conjunction with Readline.
The shell allows control of the various characters used by the
history expansion mechanism with the histchars variable.
An event designator is a reference to a command line entry in the history list.
!extglob shell option is enabled using the shopt builtin).
!n!-n!!!string!?string[?]^string1^string2^!!:s/string1/string2/.
!#Word designators are used to select desired words from the event. A ‘:’ separates the event specification from the word designator. It may be omitted if the word designator begins with a ‘^’, ‘$’, ‘*’, ‘-’, or ‘%’. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces.
For example,
!!!!:$!$.
!fi:2fi.
Here are the word designators:
0 (zero)0th word. For many applications, this is the command word.
^$%-y*0th. This is a synonym for ‘1-$’.
It is not an error to use ‘*’ if there is just one word in the event;
the empty string is returned in that case.
*-If a word designator is supplied without an event specification, the previous command is used as the event.
After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a ‘:’.
htrepqxs/old/new/&gags/old/new/,
or with ‘&’.
GThis chapter provides basic instructions for installing Bash on the various supported platforms. The distribution supports the gnu operating systems, nearly every version of Unix, and several non-Unix systems such as BeOS and Interix. Other independent ports exist for ms-dos, os/2, and Windows platforms.
These are installation instructions for Bash.
The simplest way to compile Bash is:
cd to the directory containing the source code and type
‘./configure’ to configure Bash for your system. If you're
using csh on an old version of System V, you might need to
type ‘sh ./configure’ instead to prevent csh from trying
to execute configure itself.
Running configure takes some time.
While running, it prints messages telling which features it is
checking for.
bashbug bug
reporting script.
bash and bashbug.
This will also install the manual pages and Info file.
The configure shell script attempts to guess correct
values for various system-dependent variables used during
compilation. It uses those values to create a Makefile in
each directory of the package (the top directory, the
builtins, doc, and support directories,
each directory under lib, and several others). It also creates a
config.h file containing system-dependent definitions.
Finally, it creates a shell script named config.status that you
can run in the future to recreate the current configuration, a
file config.cache that saves the results of its tests to
speed up reconfiguring, and a file config.log containing
compiler output (useful mainly for debugging configure).
If at some point
config.cache contains results you don't want to keep, you
may remove or edit it.
To find out more about the options and arguments that the
configure script understands, type
bash-2.04$ ./configure --help
at the Bash prompt in your Bash source directory.
If you need to do unusual things to compile Bash, please
try to figure out how configure could check whether or not
to do them, and mail diffs or instructions to
bash-maintainers@gnu.org so they can be
considered for the next release.
The file configure.in is used to create configure
by a program called Autoconf. You only need
configure.in if you want to change it or regenerate
configure using a newer version of Autoconf. If
you do this, make sure you are using Autoconf version 2.50 or
newer.
You can remove the program binaries and object files from the
source code directory by typing ‘make clean’. To also remove the
files that configure created (so you can compile Bash for
a different kind of computer), type ‘make distclean’.
Some systems require unusual options for compilation or linking
that the configure script does not know about. You can
give configure initial values for variables by setting
them in the environment. Using a Bourne-compatible shell, you
can do that on the command line like this:
CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
On systems that have the env program, you can do it like this:
env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
The configuration process uses GCC to build Bash if it is available.
You can compile Bash for more than one kind of computer at the
same time, by placing the object files for each architecture in their
own directory. To do this, you must use a version of make that
supports the VPATH variable, such as GNU make.
cd to the
directory where you want the object files and executables to go and run
the configure script from the source directory. You may need to
supply the --srcdir=PATH argument to tell configure where the
source files are. configure automatically checks for the
source code in the directory that configure is in and in `..'.
If you have to use a make that does not supports the VPATH
variable, you can compile Bash for one architecture at a
time in the source code directory. After you have installed
Bash for one architecture, use ‘make distclean’ before
reconfiguring for another architecture.
Alternatively, if your system supports symbolic links, you can use the support/mkclone script to create a build tree which has symbolic links back to each file in the source directory. Here's an example that creates a build directory in the current directory from a source directory /usr/gnu/src/bash-2.0:
bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
The mkclone script requires Bash, so you must have already built
Bash for at least one architecture before you can create build
directories for other architectures.
By default, ‘make install’ will install into
/usr/local/bin, /usr/local/man, etc. You can
specify an installation prefix other than /usr/local by
giving configure the option --prefix=PATH,
or by specifying a value for the DESTDIR ‘make’
variable when running ‘make install’.
You can specify separate installation prefixes for
architecture-specific files and architecture-independent files.
If you give configure the option
--exec-prefix=PATH, ‘make install’ will use
PATH as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.
There may be some features configure can not figure out
automatically, but need to determine by the type of host Bash
will run on. Usually configure can figure that
out, but if it prints a message saying it can not guess the host
type, give it the --host=TYPE option. ‘TYPE’ can
either be a short name for the system type, such as ‘sun4’,
or a canonical name with three fields: ‘CPU-COMPANY-SYSTEM’
(e.g., ‘i386-unknown-freebsd4.2’).
See the file support/config.sub for the possible values of each field.
If you want to set default values for configure scripts to
share, you can create a site shell script called
config.site that gives default values for variables like
CC, cache_file, and prefix. configure
looks for PREFIX/share/config.site if it exists, then
PREFIX/etc/config.site if it exists. Or, you can set the
CONFIG_SITE environment variable to the location of the site
script. A warning: the Bash configure looks for a site script,
but not all configure scripts do.
configure recognizes the following options to control how it
operates.
--cache-file=fileconfigure.
--helpconfigure, and exit.
--quiet--silent-q--srcdir=dirconfigure can determine that directory automatically.
--versionconfigure
script, and exit.
configure also accepts some other, not widely used, boilerplate
options. ‘configure --help’ prints the complete list.
The Bash configure has a number of --enable-feature
options, where feature indicates an optional part of Bash.
There are also several --with-package options,
where package is something like ‘bash-malloc’ or ‘purify’.
To turn off the default use of a package, use
--without-package. To configure Bash without a feature
that is enabled by default, use --disable-feature.
Here is a complete list of the --enable- and
--with- options that the Bash configure recognizes.
--with-afs--with-bash-mallocmalloc in the directory lib/malloc. This is not the same
malloc that appears in gnu libc, but an older version
originally derived from the 4.2 bsd malloc. This malloc
is very fast, but wastes some space on each allocation.
This option is enabled by default.
The NOTES file contains a list of systems for
which this should be turned off, and configure disables this
option automatically for a number of systems.
--with-curses--with-gnu-malloc--with-bash-malloc.
--with-installed-readline[=PREFIX]yes or not
supplied, configure uses the values of the make variables
includedir and libdir, which are subdirectories of prefix
by default, to find the installed version of Readline if it is not in
the standard system include and library directories.
If PREFIX is no, Bash links with the version in
lib/readline.
If PREFIX is set to any other value, configure treats it as
a directory pathname and looks for
the installed version of Readline in subdirectories of that directory
(include files in PREFIX/include and the library in
PREFIX/lib).
--with-purify--enable-minimal-configThere are several --enable- options that alter how Bash is compiled and linked, rather than changing run-time features.
--enable-largefile--enable-profilinggprof each time it is executed.
--enable-static-linkgcc is being used.
This could be used to build a version to use as root's shell.
The ‘minimal-config’ option can be used to disable all of the following options, but it is processed first, so individual options may be enabled using ‘enable-feature’.
All of the following options except for ‘disabled-builtins’ and ‘xpg-echo-default’ are enabled by default, unless the operating system does not provide the necessary support.
--enable-aliasalias and unalias
builtins (see Aliases).
--enable-arith-for-commandfor command
that behaves like the C language for statement
(see Looping Constructs).
--enable-array-variables--enable-bang-historycsh-like history substitution
(see History Interaction).
--enable-brace-expansioncsh-like brace expansion
( b{a,b}c ==> bac bbc ).
See Brace Expansion, for a complete description.
--enable-command-timingtime as a reserved word and for
displaying timing statistics for the pipeline following time
(see Pipelines).
This allows pipelines as well as shell builtins and functions to be timed.
--enable-cond-command[[ conditional command.
(see Conditional Constructs).
--enable-cond-regexp[[ conditional command.
(see Conditional Constructs).
--enable-debugger--enable-directory-stackcsh-like directory stack and the
pushd, popd, and dirs builtins
(see The Directory Stack).
--enable-disabled-builtinsxxx has been disabled using ‘enable -n xxx’.
See Bash Builtins, for details of the builtin and
enable builtin commands.
--enable-dparen-arithmetic((...)) command
(see Conditional Constructs).
--enable-extended-glob--enable-help-builtinhelp builtin, which displays help on shell builtins and
variables (see Bash Builtins).
--enable-historyfc and history
builtin commands (see Bash History Facilities).
--enable-job-control--enable-multibyte--enable-net-redirections/dev/tcp/host/port and
/dev/udp/host/port
when used in redirections (see Redirections).
--enable-process-substitution--enable-progcomp--enable-prompt-string-decoding--enable-readline--enable-restrictedrbash, enters a restricted mode. See
The Restricted Shell, for a description of restricted mode.
--enable-selectselect builtin, which allows the generation of simple
menus (see Conditional Constructs).
--enable-separate-helpfileshelp builtin
instead of storing the text internally.
--enable-single-help-stringshelp builtin as a single string for
each help topic. This aids in translating the text to different languages.
You may need to disable this if your compiler cannot handle very long string
literals.
--enable-strict-posix-default--enable-usg-echo-default--enable-xpg-echo-default.
--enable-xpg-echo-defaultecho builtin expand backslash-escaped characters by default,
without requiring the -e option.
This sets the default value of the xpg_echo shell option to on,
which makes the Bash echo behave more like the version specified in
the Single Unix Specification, version 3.
See Bash Builtins, for a description of the escape sequences that
echo recognizes.
The file config-top.h contains C Preprocessor
‘#define’ statements for options which are not settable from
configure.
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.
Please report all bugs you find in Bash. But first, you should make sure that it really is a bug, and that it appears in the latest version of Bash. The latest version of Bash is always available for FTP from ftp://ftp.gnu.org/pub/bash/.
Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report.
If you have a fix, you are encouraged to mail that as well!
Suggestions and `philosophical' bug reports may be mailed
to bug-bash@gnu.org or posted to the Usenet
newsgroup gnu.bash.bug.
All bug reports should include:
bashbug inserts the first three items automatically into
the template it provides for filing a bug report.
Please send all reports concerning this manual to chet@po.CWRU.Edu.
Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell.
Bash uses the posix 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
previous sections.
This section uses the version of sh included in SVR4.2 (the
last version of the historical Bourne shell) as the baseline reference.
sh behavior (see Bash POSIX Mode).
bind builtin.
complete and compgen, to manipulate it.
history and fc builtins to manipulate it.
The Bash history list maintains timestamp information and uses the
value of the HISTTIMEFORMAT variable to display it.
csh-like history expansion
(see History Interaction).
$'...' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see 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 Locale Translation).
! keyword to negate the return value of
a pipeline (see Pipelines).
Very useful when an if statement needs to act only if a test fails.
The Bash ‘-o pipefail’ option to set will cause a pipeline to
return a failure status if any command fails.
time reserved word and command timing (see Pipelines).
The display of the timing statistics may be controlled with the
TIMEFORMAT variable.
for (( expr1 ; expr2 ; expr3 ))
arithmetic for command, similar to the C language (see Looping Constructs).
select compound command, which allows the
generation of simple menus (see Conditional Constructs).
[[ compound command, which makes conditional
testing part of the shell grammar (see Conditional Constructs), including
optional regular expression matching.
case and
[[ constructs.
alias and unalias
builtins (see Aliases).
(( compound command
(see Conditional Constructs),
and arithmetic expansion (see Shell Arithmetic).
export
command.
${#xx}, which returns the length of ${xx},
is supported (see Shell Parameter Expansion).
${var:offset[:length]},
which expands to the substring of var's value of length
length, beginning at offset, is present
(see Shell Parameter Expansion).
${var/[/]pattern[/replacement]},
which matches pattern and replaces it with replacement in
the value of var, is available (see Shell Parameter Expansion).
${!prefix}* expansion, which expands to
the names of all shell variables whose names begin with prefix,
is available (see Shell Parameter Expansion).
${!word}
(see Shell Parameter Expansion).
$9 using
${num}.
$() form of command substitution
is implemented (see Command Substitution),
and preferred to the Bourne shell's `` (which
is also implemented for backwards compatibility).
extglob
shell option is enabled (see Pattern Matching).
sh does not separate the two name spaces.
local builtin, and thus useful recursive functions may be written
(see Bash Builtins).
sh, all variable assignments
preceding commands are global unless the command is executed from the
file system.
noclobber option is available to avoid overwriting existing
files with output redirection (see The Set Builtin).
The ‘>|’ redirection operator may be used to override noclobber.
cd and pwd builtins (see Bourne Shell Builtins)
each take -L and -P options to switch between logical and
physical modes.
builtin and command builtins (see Bash Builtins).
command builtin allows selective disabling of functions
when command lookup is performed (see Bash Builtins).
enable
builtin (see Bash Builtins).
exec builtin takes additional options that allow users
to control the contents of the environment passed to the executed
command, and what the zeroth argument to the command is to be
(see Bourne Shell Builtins).
export -f (see Shell Functions).
export, readonly, and declare builtins can
take a -f option to act on shell functions, a -p option to
display variables with various attributes set in a format that can be
used as shell input, a -n option to remove various variable
attributes, and ‘name=value’ arguments to set variable attributes
and values simultaneously.
hash builtin allows a name to be associated with
an arbitrary filename, even when that filename cannot be found by
searching the $PATH, using ‘hash -p’
(see Bourne Shell Builtins).
help builtin for quick reference to shell
facilities (see Bash Builtins).
printf builtin is available to display formatted output
(see Bash Builtins).
read builtin (see Bash Builtins)
will read a line ending in ‘\’ with
the -r option, and will use the REPLY variable as a
default if no non-option arguments are supplied.
The Bash read builtin
also accepts a prompt string with the -p option and will use
Readline to obtain the line when given the -e option.
The read builtin also has additional options to control input:
the -s option will turn off echoing of input characters as
they are read, the -t option will allow read to time out
if input does not arrive within a specified number of seconds, the
-n option will allow reading only a specified number of
characters rather than a full line, and the -d option will read
until a particular character rather than newline.
return builtin may be used to abort execution of scripts
executed with the . or source builtins
(see Bourne Shell Builtins).
shopt builtin, for finer control of shell
optional capabilities (see Bash Builtins), and allows these options
to be set and unset at shell invocation (see Invoking Bash).
set
builtin (see The Set Builtin).
xtrace) option displays commands other than
simple commands when performing an execution trace
(see The Set Builtin).
test builtin (see Bourne Shell Builtins)
is slightly different, as it implements the posix algorithm,
which specifies the behavior based on the number of arguments.
caller builtin, which displays the context of
any active subroutine call (a shell function or a script executed with
the . or source builtins). This supports the bash
debugger.
trap builtin (see Bourne Shell Builtins) allows a
DEBUG pseudo-signal specification, similar to EXIT.
Commands specified with a DEBUG trap are executed before every
simple command, for command, case command,
select command, every arithmetic for command, and before
the first command executes in a shell function.
The DEBUG trap is not inherited by shell functions unless the
function has been given the trace attribute or the
functrace option has been enabled using the shopt builtin.
The extdebug shell option has additional effects on the
DEBUG trap.
The trap builtin (see Bourne Shell Builtins) allows an
ERR pseudo-signal specification, similar to EXIT and DEBUG.
Commands specified with an ERR trap are executed after a simple
command fails, with a few exceptions.
The ERR trap is not inherited by shell functions unless the
-o errtrace option to the set builtin is enabled.
The trap builtin (see Bourne Shell Builtins) allows a
RETURN pseudo-signal specification, similar to
EXIT and DEBUG.
Commands specified with an RETURN trap are executed before
execution resumes after a shell function or a shell script executed with
. or source returns.
The RETURN trap is not inherited by shell functions unless the
function has been given the trace attribute or the
functrace option has been enabled using the shopt builtin.
type builtin is more extensive and gives more information
about the names it finds (see Bash Builtins).
umask builtin permits a -p option to cause
the output to be displayed in the form of a umask command
that may be reused as input (see Bourne Shell Builtins).
csh-like directory stack, and provides the
pushd, popd, and dirs builtins to manipulate it
(see The Directory Stack).
Bash also makes the directory stack visible as the value of the
DIRSTACK shell variable.
disown builtin can remove a job from the internal shell
job table (see Job Control Builtins) or suppress the sending
of SIGHUP to a job when the shell exits as the result of a
SIGHUP.
mldmode and priv) not present in Bash.
stop or newgrp builtins.
sh uses a TIMEOUT variable like Bash uses
TMOUT.
More features unique to Bash may be found in Bash Features.
Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:
if or while
statement.
EOF under certain circumstances.
This can be the cause of some hard-to-find errors.
SIGSEGV. If the shell is started from a process with
SIGSEGV blocked (e.g., by using the system() C library
function call), it misbehaves badly.
SIGSEGV,
SIGALRM, or SIGCHLD.
-x -v);
the SVR4.2 shell allows only one option argument (-xv). In
fact, some versions of the shell dump core if the second argument begins
with a ‘-’.
jsh
(it turns on job control).
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
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.: Bourne Shell Builtins:: Bourne Shell Builtins[: Bourne Shell Builtinsalias: Bash Builtinsbg: Job Control Builtinsbind: Bash Builtinsbreak: Bourne Shell Builtinsbuiltin: Bash Builtinscaller: Bash Builtinscd: Bourne Shell Builtinscommand: Bash Builtinscompgen: Programmable Completion Builtinscomplete: Programmable Completion Builtinscontinue: Bourne Shell Builtinsdeclare: Bash Builtinsdirs: Directory Stack Builtinsdisown: Job Control Builtinsecho: Bash Builtinsenable: Bash Builtinseval: Bourne Shell Builtinsexec: Bourne Shell Builtinsexit: Bourne Shell Builtinsexport: Bourne Shell Builtinsfc: Bash History Builtinsfg: Job Control Builtinsgetopts: Bourne Shell Builtinshash: Bourne Shell Builtinshelp: Bash Builtinshistory: Bash History Builtinsjobs: Job Control Builtinskill: Job Control Builtinslet: Bash Builtinslocal: Bash Builtinslogout: Bash Builtinspopd: Directory Stack Builtinsprintf: Bash Builtinspushd: Directory Stack Builtinspwd: Bourne Shell Builtinsread: Bash Builtinsreadonly: Bourne Shell Builtinsreturn: Bourne Shell Builtinsset: The Set Builtinshift: Bourne Shell Builtinsshopt: Bash Builtinssource: Bash Builtinssuspend: Job Control Builtinstest: Bourne Shell Builtinstimes: Bourne Shell Builtinstrap: Bourne Shell Builtinstype: Bash Builtinstypeset: Bash Builtinsulimit: Bash Builtinsumask: Bourne Shell Builtinsunalias: Bash Builtinsunset: Bourne Shell Builtinswait: Job Control Builtins!: Pipelines[[: Conditional Constructs]]: Conditional Constructscase: Conditional Constructsdo: Looping Constructsdone: Looping Constructselif: Conditional Constructselse: Conditional Constructsesac: Conditional Constructsfi: Conditional Constructsfor: Looping Constructsfunction: Shell Functionsif: Conditional Constructsin: Conditional Constructsselect: Conditional Constructsthen: Conditional Constructstime: Pipelinesuntil: Looping Constructswhile: Looping Constructs{: Command Grouping}: Command Grouping!: Special Parameters#: Special Parameters$: Special Parameters*: Special Parameters-: Special Parameters0: Special Parameters?: Special Parameters@: Special Parameters_: Special Parametersauto_resume: Job Control VariablesBASH: Bash VariablesBASH_ARGC: Bash VariablesBASH_ARGV: Bash VariablesBASH_COMMAND: Bash VariablesBASH_ENV: Bash VariablesBASH_EXECUTION_STRING: Bash VariablesBASH_LINENO: Bash VariablesBASH_REMATCH: Bash VariablesBASH_SOURCE: Bash VariablesBASH_SUBSHELL: Bash VariablesBASH_VERSINFO: Bash VariablesBASH_VERSION: Bash Variablesbell-style: Readline Init File Syntaxbind-tty-special-chars: Readline Init File SyntaxCDPATH: Bourne Shell VariablesCOLUMNS: Bash Variablescomment-begin: Readline Init File SyntaxCOMP_CWORD: Bash VariablesCOMP_LINE: Bash VariablesCOMP_POINT: Bash VariablesCOMP_WORDBREAKS: Bash VariablesCOMP_WORDS: Bash Variablescompletion-query-items: Readline Init File SyntaxCOMPREPLY: Bash Variablesconvert-meta: Readline Init File SyntaxDIRSTACK: Bash Variablesdisable-completion: Readline Init File Syntaxediting-mode: Readline Init File SyntaxEMACS: Bash Variablesenable-keypad: Readline Init File SyntaxEUID: Bash Variablesexpand-tilde: Readline Init File SyntaxFCEDIT: Bash VariablesFIGNORE: Bash VariablesFUNCNAME: Bash VariablesGLOBIGNORE: Bash VariablesGROUPS: Bash Variableshistchars: Bash VariablesHISTCMD: Bash VariablesHISTCONTROL: Bash VariablesHISTFILE: Bash VariablesHISTFILESIZE: Bash VariablesHISTIGNORE: Bash Variableshistory-preserve-point: Readline Init File SyntaxHISTSIZE: Bash VariablesHISTTIMEFORMAT: Bash VariablesHOME: Bourne Shell Variableshorizontal-scroll-mode: Readline Init File SyntaxHOSTFILE: Bash VariablesHOSTNAME: Bash VariablesHOSTTYPE: Bash VariablesIFS: Bourne Shell VariablesIGNOREEOF: Bash Variablesinput-meta: Readline Init File SyntaxINPUTRC: Bash Variablesisearch-terminators: Readline Init File Syntaxkeymap: Readline Init File SyntaxLANG: Bash VariablesLC_ALL: Bash VariablesLC_COLLATE: Bash VariablesLC_CTYPE: Bash VariablesLC_MESSAGES: Bash VariablesLC_MESSAGES: Locale TranslationLC_NUMERIC: Bash VariablesLINENO: Bash VariablesLINES: Bash VariablesMACHTYPE: Bash VariablesMAIL: Bourne Shell VariablesMAILCHECK: Bash VariablesMAILPATH: Bourne Shell Variablesmark-modified-lines: Readline Init File Syntaxmark-symlinked-directories: Readline Init File Syntaxmatch-hidden-files: Readline Init File Syntaxmeta-flag: Readline Init File SyntaxOLDPWD: Bash VariablesOPTARG: Bourne Shell VariablesOPTERR: Bash VariablesOPTIND: Bourne Shell VariablesOSTYPE: Bash Variablesoutput-meta: Readline Init File Syntaxpage-completions: Readline Init File SyntaxPATH: Bourne Shell VariablesPIPESTATUS: Bash VariablesPOSIXLY_CORRECT: Bash VariablesPPID: Bash VariablesPROMPT_COMMAND: Bash VariablesPS1: Bourne Shell VariablesPS2: Bourne Shell VariablesPS3: Bash VariablesPS4: Bash VariablesPWD: Bash VariablesRANDOM: Bash VariablesREPLY: Bash VariablesSECONDS: Bash VariablesSHELL: Bash VariablesSHELLOPTS: Bash VariablesSHLVL: Bash Variablesshow-all-if-ambiguous: Readline Init File Syntaxshow-all-if-unmodified: Readline Init File SyntaxTEXTDOMAIN: Locale TranslationTEXTDOMAINDIR: Locale TranslationTIMEFORMAT: Bash VariablesTMOUT: Bash VariablesTMPDIR: Bash VariablesUID: Bash Variablesvisible-stats: Readline Init File Syntaxabort (C-g): Miscellaneous Commandsaccept-line (Newline or Return): Commands For Historybackward-char (C-b): Commands For Movingbackward-delete-char (Rubout): Commands For Textbackward-kill-line (C-x Rubout): Commands For Killingbackward-kill-word (M-<DEL>): Commands For Killingbackward-word (M-b): Commands For Movingbeginning-of-history (M-<): Commands For Historybeginning-of-line (C-a): Commands For Movingcall-last-kbd-macro (C-x e): Keyboard Macroscapitalize-word (M-c): Commands For Textcharacter-search (C-]): Miscellaneous Commandscharacter-search-backward (M-C-]): Miscellaneous Commandsclear-screen (C-l): Commands For Movingcomplete (<TAB>): Commands For Completioncopy-backward-word (): Commands For Killingcopy-forward-word (): Commands For Killingcopy-region-as-kill (): Commands For Killingdelete-char (C-d): Commands For Textdelete-char-or-list (): Commands For Completiondelete-horizontal-space (): Commands For Killingdigit-argument (M-0, M-1, ... M--): Numeric Argumentsdo-uppercase-version (M-a, M-b, M-x, ...): Miscellaneous Commandsdowncase-word (M-l): Commands For Textdump-functions (): Miscellaneous Commandsdump-macros (): Miscellaneous Commandsdump-variables (): Miscellaneous Commandsend-kbd-macro (C-x )): Keyboard Macrosend-of-history (M->): Commands For Historyend-of-line (C-e): Commands For Movingexchange-point-and-mark (C-x C-x): Miscellaneous Commandsforward-backward-delete-char (): Commands For Textforward-char (C-f): Commands For Movingforward-search-history (C-s): Commands For Historyforward-word (M-f): Commands For Movinghistory-search-backward (): Commands For Historyhistory-search-forward (): Commands For Historyinsert-comment (M-#): Miscellaneous Commandsinsert-completions (M-*): Commands For Completionkill-line (C-k): Commands For Killingkill-region (): Commands For Killingkill-whole-line (): Commands For Killingkill-word (M-d): Commands For Killingmenu-complete (): Commands For Completionnext-history (C-n): Commands For Historynon-incremental-forward-search-history (M-n): Commands For Historynon-incremental-reverse-search-history (M-p): Commands For Historyoverwrite-mode (): Commands For Textpossible-completions (M-?): Commands For Completionprefix-meta (<ESC>): Miscellaneous Commandsprevious-history (C-p): Commands For Historyquoted-insert (C-q or C-v): Commands For Textre-read-init-file (C-x C-r): Miscellaneous Commandsredraw-current-line (): Commands For Movingreverse-search-history (C-r): Commands For Historyrevert-line (M-r): Miscellaneous Commandsself-insert (a, b, A, 1, !, ...): Commands For Textset-mark (C-@): Miscellaneous Commandsstart-kbd-macro (C-x (): Keyboard Macrostranspose-chars (C-t): Commands For Texttranspose-words (M-t): Commands For Textundo (C-_ or C-x C-u): Miscellaneous Commandsuniversal-argument (): Numeric Argumentsunix-filename-rubout (): Commands For Killingunix-line-discard (C-u): Commands For Killingunix-word-rubout (C-w): Commands For Killingupcase-word (M-u): Commands For Textyank (C-y): Commands For Killingyank-last-arg (M-. or M-_): Commands For Historyyank-nth-arg (M-C-y): Commands For Historyyank-pop (M-y): Commands For Killing