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This text is a brief description of the features that are present in the Bash shell.
This is Edition 2.5a, last updated 13 November 2001,
of The GNU Bash Reference Manual,
for Bash, Version 2.05a.
Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc.
Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (`sh'), the Korn Shell (`ksh'), and the C-shell (`csh' and its successor, `tcsh'). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature.
This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.
1. Introduction An introduction to the shell.
2. Definitions Some definitions used in the rest of this manual.
3. Basic Shell Features The shell "building blocks".
4. Shell Builtin Commands Commands that are a part of the shell.
5. Shell Variables Variables used or set by Bash.
6. Bash Features Features found only in Bash.
7. Job Control A chapter describing what job control is and how Bash allows you to use it.
9. Using History Interactively Chapter dealing with history expansion rules.
8. Command Line Editing Chapter describing the command line editing features.
10. Installing Bash How to build and install Bash on your system.
A. Reporting Bugs How to report bugs in Bash.
B. Major Differences From The Bourne Shell A terse list of the differences between Bash and historical versions of /bin/sh.
Index of Shell Builtin Commands Index of Bash builtin commands.
Index of Shell Reserved Words Index of Bash reserved words.
Parameter and Variable Index Quick reference helps you find the variable you want.
Function Index Index of bindable Readline functions.
Concept Index General index for concepts described in this manual.
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1.1 What is Bash? A short description of Bash.
1.2 What is a shell? A brief introduction to shells.
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Bash is the shell, or command language interpreter,
for the GNU operating system.
The name is an acronym for the `Bourne-Again SHell',
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell /bin/sh,
which appeared in the Seventh Edition Bell Labs Research version
of Unix.
Bash is largely compatible with sh and incorporates useful
features from the Korn shell ksh and the C shell csh.
It is intended to be a conformant implementation of the IEEE
POSIX Shell and Tools specification (IEEE Working Group 1003.2).
It offers functional improvements over sh for both interactive and
programming use.
While the GNU operating system provides other shells, including
a version of csh, Bash is the default shell.
Like other GNU software, Bash is quite portable. It currently runs
on nearly every version of Unix and a few other operating systems -
independently-supported ports exist for MS-DOS, OS/2,
Windows 95/98, and Windows NT.
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At its base, a shell is simply a macro processor that executes commands. A Unix shell is both a command interpreter, which provides the user interface to the rich set of GNU utilities, and a programming language, allowing these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as `/bin', allowing users or groups to establish custom environments.
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 may be used interactively or non-interactively: they accept input typed from the keyboard or from a file.
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, history and aliases. Each of these features is described in this manual.
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POSIX
blank
builtin
control operator
word that performs a control function. It is a newline
or one of the following:
`||', `&&', `&', `;', `;;',
`|', `(', or `)'.
exit status
field
filename
job
job control
metacharacter
blank or one of the following characters:
`|', `&', `;', `(', `)', `<', or
`>'.
name
word consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. Names are used as
shell variable and function names.
Also referred to as an identifier.
operator
control operator or a redirection operator.
See section 3.6 Redirections, for a list of redirection operators.
process group
process group ID
process group
during its lifetime.
reserved word
word that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as for and
while.
return status
exit status.
signal
special builtin
token
word or an operator.
word
token that is not an operator.
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Bash is an acronym for `Bourne-Again SHell'. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, and the rules for evaluation and quoting are taken from the POSIX 1003.2 specification for the `standard' Unix shell.
This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
3.1 Shell Syntax What your input means to the shell. 3.2 Shell Commands The types of commands you can use. 3.3 Shell Functions Grouping commands by name. 3.4 Shell Parameters Special shell variables. 3.5 Shell Expansions How Bash expands variables and the various expansions available. 3.6 Redirections A way to control where input and output go. 3.7 Executing Commands What happens when you run a command. 3.8 Shell Scripts Executing files of shell commands.
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3.1.1 Shell Operation The basic operation of the shell.
3.1.2 Quoting How to remove the special meaning from characters.
3.1.3 Comments How to specify comments.
When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (`#'), and the rest of that line. Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.
The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command's exit status, and makes that exit status available for further inspection or processing.
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The following is a brief description of the shell's operation when it reads and executes a command. Basically, the shell does the following:
metacharacters. Alias expansion is performed by this step
(see section 6.6 Aliases).
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3.1.2.1 Escape Character How to remove the special meaning from a single character. 3.1.2.2 Single Quotes How to inhibit all interpretation of a sequence of characters. 3.1.2.3 Double Quotes How to suppress most of the interpretation of a sequence of characters. 3.1.2.4 ANSI-C Quoting How to expand ANSI-C sequences in quoted strings.
3.1.2.5 Locale-Specific Translation How to translate strings into different languages.
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.
Each of the shell metacharacters (see section 2. Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used, the history expansion character, usually `!', must be quoted to prevent history expansion. See section 9.1 Bash History Facilities, for more details concerning history expansion. There are three quoting mechanisms: the escape character, single quotes, and double quotes.
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newline. If a \newline pair
appears, and the backslash itself is not quoted, the \newline
is treated as a line continuation (that is, it is removed from
the input stream and effectively ignored).
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Enclosing characters in single quotes (`'') preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.
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Enclosing characters in double quotes (`"') preserves the literal value
of all characters within the quotes, with the exception of
`$', ``', and `\'.
The characters `$' and ``'
retain their special meaning within double quotes (see section 3.5 Shell Expansions).
The backslash retains its special meaning only when followed by one of
the following characters:
`$', ``', `"', `\', or newline.
Within double quotes, backslashes that are followed by one of these
characters are removed. Backslashes preceding characters without a
special meaning are left unmodified.
A double quote may be quoted within double quotes by preceding it with
a backslash.
The special parameters `*' and `@' have special meaning when in double quotes (see section 3.5.3 Shell Parameter Expansion).
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Words of the form $'string' are treated specially. The
word expands to string, with backslash-escaped characters replaced
as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a
\b
\e
\f
\n
\r
\t
\v
\\
\'
\nnn
\xHH
The expanded result is single-quoted, as if the dollar sign had not been present.
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A double-quoted string preceded by a dollar sign (`$') will cause
the string to be translated according to the current locale.
If the current locale is C or POSIX, the dollar sign
is ignored.
If the string is translated and replaced, the replacement is
double-quoted.
Some systems use the message catalog selected by the LC_MESSAGES
shell variable. Others create the name of the message catalog from the
value of the TEXTDOMAIN shell variable, possibly adding a
suffix of `.mo'. If you use the TEXTDOMAIN variable, you
may need to set the TEXTDOMAINDIR variable to the location of
the message catalog files. Still others use both variables in this
fashion:
TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.
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In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt
builtin is enabled (see section 4.2 Bash Builtin Commands),
a word beginning with `#'
causes that word and all remaining characters on that line to
be ignored. An interactive shell without the interactive_comments
option enabled does not allow comments. The interactive_comments
option is on by default in interactive shells.
See section 6.3 Interactive Shells, for a description of what makes
a shell interactive.
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A simple shell command such as echo a b c consists of the command
itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
3.2.1 Simple Commands The most common type of command. 3.2.2 Pipelines Connecting the input and output of several commands. 3.2.3 Lists of Commands How to execute commands sequentially. 3.2.4 Looping Constructs Shell commands for iterative action. 3.2.5 Conditional Constructs Shell commands for conditional execution. 3.2.6 Grouping Commands Ways to group commands.
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A simple command is the kind of command encountered most often.
It's just a sequence of words separated by blanks, terminated
by one of the shell's control operators (see section 2. Definitions). The
first word generally specifies a command to be executed, with the
rest of the words being that command's arguments.
The return status (see section 3.7.5 Exit Status) of a simple command is
its exit status as provided
by the POSIX 1003.1 waitpid function, or 128+n if
the command was terminated by signal n.
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A pipeline is a sequence of simple commands separated by
`|'.
[ |
The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command's output.
The reserved word time causes timing statistics
to be printed for the pipeline once it finishes.
The statistics currently consist of elapsed (wall-clock) time and
user and system time consumed by the command's execution.
The `-p' option changes the output format to that specified
by POSIX.
The TIMEFORMAT variable may be set to a format string that
specifies how the timing information should be displayed.
See section 5.2 Bash Variables, for a description of the available formats.
The use of time as a reserved word permits the timing of
shell builtins, shell functions, and pipelines. An external
time command cannot time these easily.
If the pipeline is not executed asynchronously (see section 3.2.3 Lists of Commands), the shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell (see section 3.7.3 Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status of the last command.
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A list is a sequence of one or more pipelines separated by one
of the operators `;', `&', `&&', or `||',
and optionally terminated by one of `;', `&', or a
newline.
Of these list operators, `&&' and `||' have equal precedence, followed by `;' and `&', which have equal precedence.
If a command is terminated by the control operator `&',
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the background.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see section 7. Job Control),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from /dev/null.
Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
The control operators `&&' and `||' denote AND lists and OR lists, respectively. An AND list has the form
command1 && command2 |
command2 is executed if, and only if, command1 returns an exit status of zero.
An OR list has the form
command1 || command2 |
command2 is executed if, and only if, command1 returns a non-zero exit status.
The return status of AND and OR lists is the exit status of the last command executed in the list.
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Bash supports the following looping constructs.
Note that wherever a `;' appears in the description of a command's syntax, it may be replaced with one or more newlines.
until
until command is:
until test-commands; do consequent-commands; done |
while
while command is:
while test-commands; do consequent-commands; done |
Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.
for
for command is:
for name [in words ...]; do commands; done |
for command
executes the commands once for each positional parameter that is
set, as if `in "$@"' had been specified
(see section 3.4.2 Special Parameters).
The return status is the exit status of the last command that executes.
If there are no items in the expansion of words, no commands are
executed, and the return status is zero.
An alternate form of the for command is also supported:
for (( expr1 ; expr2 ; expr3 )) ; do commands ; done |
The break and continue builtins (see section 4.1 Bourne Shell Builtins)
may be used to control loop execution.
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if
if command is:
if test-commands; then consequent-commands; [elif more-test-commands; then more-consequents;] [else alternate-consequents;] fi |
The test-commands list is executed, and if its return status is zero,
the consequent-commands list is executed.
If test-commands returns a non-zero status, each elif list
is executed in turn, and if its exit status is zero,
the corresponding more-consequents is executed and the
command completes.
If `else alternate-consequents' is present, and
the final command in the final if or elif clause
has a non-zero exit status, then alternate-consequents is executed.
The return status is the exit status of the last command executed, or
zero if no condition tested true.
case
case command is:
|
case will selectively execute the command-list corresponding to
the first pattern that matches word.
The `|' is used to separate multiple patterns, and the `)'
operator terminates a pattern list.
A list of patterns and an associated command-list is known
as a clause. Each clause must be terminated with `;;'.
The word undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before matching is
attempted. Each pattern undergoes tilde expansion, parameter
expansion, command substitution, and arithmetic expansion.
There may be an arbitrary number of case clauses, each terminated
by a `;;'. The first pattern that matches determines the
command-list that is executed.
Here is an example using case in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: " read ANIMAL echo -n "The $ANIMAL has " case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";; esac echo " legs." |
The return status is zero if no pattern is matched. Otherwise, the return status is the exit status of the command-list executed.
select
The select construct allows the easy generation of menus.
It has almost the same syntax as the for command:
select name [in words ...]; do commands; done |
The list of words following in is expanded, generating a list
of items. The set of expanded words is printed on the standard
error output stream, each preceded by a number. If the
`in words' is omitted, the positional parameters are printed,
as if `in "$@"' had been specifed.
The PS3 prompt is then displayed and a line is read from the
standard input.
If the line consists of a number corresponding to one of the displayed
words, then the value of name is set to that word.
If the line is empty, the words and prompt are displayed again.
If EOF is read, the select command completes.
Any other value read causes name to be set to null.
The line read is saved in the variable REPLY.
The commands are executed after each selection until a
break command is executed, at which
point the select command completes.
Here is an example that allows the user to pick a filename from the current directory, and displays the name and index of the file selected.
select fname in *; do echo you picked $fname \($REPLY\) break; done |
((...))
(( expression )) |
The arithmetic expression is evaluated according to the rules described below (see section 6.5 Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to
let "expression" |
let builtin.
[[...]]
[[ expression ]] |
Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in 6.4 Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the `[[' and `]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed.
When the `==' and `!=' operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in 3.5.8.1 Pattern Matching. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.
Expressions may be combined using the following operators, listed in decreasing order of precedence:
( expression )
! expression
expression1 && expression2
expression1 || expression2
&& and || commands do not execute expression2 if the
value of expression1 is sufficient to determine the return
value of the entire conditional expression.
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Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()
( list ) |
Placing a list of commands between parentheses causes a subshell to be created, and each of the commands in list to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.
{}
{ list; }
|
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are reserved words, so they must be separated from the list
by blanks. The parentheses are operators, and are
recognized as separate tokens by the shell even if they are not separated
from the list by whitespace.
The exit status of both of these constructs is the exit status of list.
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Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax:
[ |
This defines a shell function named name. The reserved
word function is optional.
If the function reserved
word is supplied, the parentheses are optional.
The body of the function is the command-list between { and }.
This list is executed whenever name is specified as the
name of a command. The exit status of a function is
the exit status of the last command executed in the body.
Note that for historical reasons, 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, the command-list must be terminated with a semicolon
or a newline.
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see section 3.4.1 Positional Parameters).
The special parameter `#' that expands to the number of
positional parameters is updated to reflect the change.
Positional parameter 0 is unchanged.
The FUNCNAME variable is set to the name of the function
while the function is executing.
If the builtin command return
is executed in a function, the function completes and
execution resumes with the next command after the function
call. When a function completes, the values of the
positional parameters and the special parameter `#'
are restored to the values they had prior to the function's
execution. If a numeric argument is given to return,
that is the function's return status; otherwise the 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.
Functions may be recursive. No limit is placed on the number of recursive calls.
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3.4.1 Positional Parameters The shell's command-line arguments. 3.4.2 Special Parameters Parameters with special meanings.
A parameter is an entity that stores values.
It can be a name, a number, or one of the special characters
listed below.
For the shell's purposes, a variable is a parameter denoted by a
name.
A variable has a value and zero or more attributes.
Attributes are assigned using the declare builtin command
(see the description of the declare builtin in 4.2 Bash Builtin Commands).
A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
the unset builtin command.
A variable may be assigned to by a statement of the form
name=[value] |
integer
attribute set, then value
is subject to arithmetic expansion even if the $((...))
expansion is not used (see section 3.5.5 Arithmetic Expansion).
Word splitting is not performed, with the exception
of "$@" as explained below.
Filename expansion is not performed.
Assignment statements may also appear as arguments to the
declare, typeset, export, readonly,
and local builtin commands.
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A positional parameter is a parameter denoted by one or more
digits, other than the single digit 0. Positional parameters are
assigned from the shell's arguments when it is invoked,
and may be reassigned using the set builtin command.
Positional parameter N may be referenced as ${N}, or
as $N when N consists of a single digit.
Positional parameters may not be assigned to with assignment statements.
The set and shift builtins are used to set and
unset them (see section 4. Shell Builtin Commands).
The positional parameters are
temporarily replaced when a shell function is executed
(see section 3.3 Shell Functions).
When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.
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The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
*
IFS
special variable. That is, "$*" is equivalent
to "$1c$2c...", where c
is the first character of the value of the IFS
variable.
If IFS is unset, the parameters are separated by spaces.
If IFS is null, the parameters are joined without intervening
separators.
@
"$@" is equivalent to
"$1" "$2" ....
When there are no positional parameters, "$@" and
$@
expand to nothing (i.e., they are removed).
#
?
-
set
builtin command, or those set by the shell itself
(such as the `-i' option).
$
() subshell, it
expands to the process ID of the invoking shell, not the subshell.
!
0
$0 is set to the name of that file.
If Bash is started with the `-c' option (see section 6.1 Invoking Bash),
then $0 is set to the first argument after the string to be
executed, if one is present. Otherwise, it is set
to the filename used to invoke Bash, as given by argument zero.
_
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Expansion is performed on the command line after it has been split into
tokens. There are seven kinds of expansion performed:
3.5.1 Brace Expansion Expansion of expressions within braces. 3.5.2 Tilde Expansion Expansion of the ~ character. 3.5.3 Shell Parameter Expansion How Bash expands variables to their values. 3.5.4 Command Substitution Using the output of a command as an argument. 3.5.5 Arithmetic Expansion How to use arithmetic in shell expansions. 3.5.6 Process Substitution A way to write and read to and from a command. 3.5.7 Word Splitting How the results of expansion are split into separate arguments. 3.5.8 Filename Expansion A shorthand for specifying filenames matching patterns. 3.5.9 Quote Removal How and when quote characters are removed from words.
The order of expansions is: brace expansion, tilde expansion, parameter, variable, and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and filename expansion.
On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution.
Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
"$@" (see section 3.4.2 Special Parameters) and "${name[@]}"
(see section 6.7 Arrays).
After all expansions, quote removal (see section 3.5.9 Quote Removal)
is performed.
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Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see section 3.5.8 Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.
Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe
|
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. Any incorrectly formed brace expansion is left unchanged.
This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
|
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
|
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If a word begins with an unquoted tilde character (`~'), all of the
characters up to the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a tilde-prefix.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible login name.
If this login name is the null string, the tilde is replaced with the
value of the HOME shell variable.
If HOME is unset, the home directory of the user executing the
shell is substituted instead.
Otherwise, the tilde-prefix is replaced with the home directory
associated with the specified login name.
If the tilde-prefix is `~+', the value of
the shell variable PWD replaces the tilde-prefix.
If the tilde-prefix is `~-', the value of the shell variable
OLDPWD, if it is set, is substituted.
If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a `+' or a `-',
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the dirs builtin invoked with the characters following tilde
in the tilde-prefix as an argument (see section 6.8 The Directory Stack).
If the tilde-prefix, sans the tilde, consists of a number without a
leading `+' or `-', `+' is assumed.
If the login name is invalid, or the tilde expansion fails, the word is left unchanged.
Each variable assignment is checked for unquoted tilde-prefixes immediately
following a `:' or `='.
In these cases, tilde expansion is also performed.
Consequently, one may use file names with tildes in assignments to
PATH, MAILPATH, and CDPATH,
and the shell assigns the expanded value.
The following table shows how Bash treats unquoted tilde-prefixes:
~
$HOME
~/foo
~fred/foo
foo of the home directory of the user
fred
~+/foo
~-/foo
~N
~+N
~-N
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The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.
The basic form of parameter expansion is ${parameter}. The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name.
If the first character of parameter is an exclamation point,
a level of variable indirection is introduced.
Bash uses the value of the variable formed from the rest of
parameter as the name of the variable; this variable is then
expanded and that value is used in the rest of the substitution, rather
than the value of parameter itself.
This is known as indirect expansion.
The exception to this is the expansion of ${!prefix*}
described below.
In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both existence and that the value is not null; if the colon is omitted, the operator tests only for existence.
${parameter:-word}
${parameter:=word}
${parameter:?word}
${parameter:+word}
${parameter:offset}
${parameter:offset:length}
length must evaluate to a number greater than or equal to zero.
If offset evaluates to a number less than zero, the value
is used as an offset from the end of the value of parameter.
If parameter is `@', the result is length positional
parameters beginning at offset.
If parameter is an array name indexed by `@' or `*',
the result is the length
members of the array beginning with ${parameter[offset]}.
Substring indexing is zero-based unless the positional parameters
are used, in which case the indexing starts at 1.
${!prefix*}
IFS special variable.
${#parameter}
${parameter#word}
${parameter##word}
${parameter%word}
${parameter%%word}
${parameter/pattern/string}
${parameter//pattern/string}
The pattern is expanded to produce a pattern just as in
filename expansion.
Parameter is expanded and the longest match of pattern
against its value is replaced with string.
In the first form, only the first match is replaced.
The second form causes all matches of pattern to be
replaced with string.
If pattern begins with `#', it must match at the beginning
of the expanded value of parameter.
If pattern begins with `%', it must match at the end
of the expanded value of parameter.
If string is null, matches of pattern are deleted
and the / following pattern may be omitted.
If parameter is `@' or `*',
the substitution operation is applied to each positional
parameter in turn, and the expansion is the resultant list.
If parameter
is an array variable subscripted with `@' or `*',
the substitution operation is applied to each member of the
array in turn, and the expansion is the resultant list.
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Command substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows:
$(command) |
`command` |
Bash performs the expansion by executing command and
replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution $(cat file) can be
replaced by the equivalent but faster $(< file).
When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
`$', ``', or `\'.
The first backquote not preceded by a backslash terminates the
command substitution.
When using the $(command) form, all characters between
the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results.
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Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is:
$(( expression )) |
The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic substitutions may be nested.
The evaluation is performed according to the rules listed below (see section 6.5 Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.
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Process substitution is supported on systems that support named pipes (FIFOs) or the `/dev/fd' method of naming open files. It takes the form of
<(list) |
>(list) |
>(list) form is used, writing to
the file will provide input for list. If the
<(list) form is used, the file passed as an
argument should be read to obtain the output of list.
Note that no space may appear between the < or >
and the left parenthesis, otherwise the construct would be interpreted
as a redirection.
When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.
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The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.
The shell treats each character of $IFS
as a delimiter, and splits the results of the other
expansions into words on these characters. If
IFS is unset, or its value is exactly <space><tab><newline>,
the default, then any sequence of IFS
characters serves to delimit words. If IFS
has a value other than the default, then sequences of
the whitespace characters space and tab
are ignored at the beginning and end of the
word, as long as the whitespace character is in the
value of IFS (an IFS whitespace character).
Any character in IFS that is not IFS
whitespace, along with any adjacent IFS
whitespace characters, delimits a field. A sequence of IFS
whitespace characters is also treated as a delimiter.
If the value of IFS is null, no word splitting occurs.
Explicit null arguments ("" or ") are retained.
Unquoted implicit null arguments, resulting from the expansion of
parameters that have no values, are removed.
If a parameter with no value is expanded within double quotes, a
null argument results and is retained.
Note that if no expansion occurs, no splitting is performed.
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3.5.8.1 Pattern Matching How the shell matches patterns.
After word splitting, unless the `-f' option has been set
(see section 4.3 The Set Builtin), Bash scans each word for the characters
`*', `?', and `['.
If one of these characters appears, then the word is
regarded as a pattern,
and replaced with an alphabetically sorted list of
file names matching the pattern. If no matching file names are found,
and the shell option nullglob is disabled, the word is left
unchanged.
If the nullglob option is set, and no matches are found, the word
is removed.
If the shell option nocaseglob is enabled, the match is performed
without regard to the case of alphabetic characters.
When a pattern is used for filename generation, the character `.'
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option dotglob is set.
When matching a file name, the slash character must always be
matched explicitly.
In other cases, the `.' character is not treated specially.
See the description of shopt in 4.2 Bash Builtin Commands,
for a description of the nocaseglob, nullglob,
and dotglob options.
The GLOBIGNORE
shell variable may be used to restrict the set of filenames matching a
pattern. If GLOBIGNORE
is set, each matching filename that also matches one of the patterns in
GLOBIGNORE is removed from the list of matches. The filenames
`.' and `..'
are always ignored, even when GLOBIGNORE
is set. However, setting GLOBIGNORE has the effect of
enabling the dotglob
shell option, so all other filenames beginning with a
`.' will match.
To get the old behavior of ignoring filenames beginning with a
`.', make `.*' one of the patterns in GLOBIGNORE.
The dotglob option is disabled when GLOBIGNORE
is unset.
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Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. The special pattern characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
*
?
[...]
LC_COLLATE shell variable,
if set.
For example, in the default C locale, `[a-dx-z]' is equivalent to
`[abcdxyz]'. Many locales sort characters in dictionary order, and in
these locales `[a-dx-z]' is typically not equivalent to `[abcdxyz]';
it might be equivalent to `[aBbCcDdxXyYz]', for example. To obtain
the traditional interpretation of ranges in bracket expressions, you can
force the use of the C locale by setting the LC_COLLATE or
LC_ALL environment variable to the value `C'.
Within `[' and `]', character classes can be specified
using the syntax
[:class:], where class is one of the
following classes defined in the POSIX 1003.2 standard:
alnum alpha ascii blank cntrl digit graph lower print punct space upper xdigit |
Within `[' and `]', an equivalence class can be
specified using the syntax [=c=], which
matches all characters with the same collation weight (as defined
by the current locale) as the character c.
Within `[' and `]', the syntax [.symbol.]
matches the collating symbol symbol.
If the extglob shell option is enabled using the shopt
builtin, several extended pattern matching operators are recognized.
In the following description, a pattern-list is a list of one
or more patterns separated by a `|'.
Composite patterns may be formed using one or more of the following
sub-patterns:
?(pattern-list)
*(pattern-list)
+(pattern-list)
@(pattern-list)
!(pattern-list)
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After the preceding expansions, all unquoted occurrences of the characters `\', `'', and `"' that did not result from one of the above expansions are removed.
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Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is `<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is `>', the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1 |
ls 2>&1 > dirlist |
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/port
A failure to open or create a file causes the redirection to fail.
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n,
or the standard input (file descriptor 0) if n
is not specified.
The general format for redirecting input is:
[n]<word |
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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.
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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 |
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There are two formats for redirecting standard output and standard error:
&>word |
>&word |
>word 2>&1 |
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The format of here-documents is as follows:
<<[-]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.
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[n]<&word |
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 |
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.
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[n]<>word |
n, or on file descriptor 0 if n
is not specified. If the file does not exist, it is created.
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3.7.1 Simple Command Expansion How Bash expands simple commands before executing them.
3.7.2 Command Search and Execution How Bash finds commands and runs them.
3.7.3 Command Execution Environment The environment in which Bash executes commands that are not shell builtins.
3.7.4 Environment The environment given to a command.
3.7.5 Exit Status The status returned by commands and how Bash interprets it.
3.7.6 Signals What happens when Bash or a command it runs receives a signal.
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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.
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After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.
$PATH for a directory containing an executable file
by that name. Bash uses a hash table to remember the full
pathnames of executable files to avoid multiple PATH searches
(see the description of hash in 4.1 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.
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The shell has an execution environment, which consists of the following:
exec builtin
cd, pushd, or
popd, or inherited by the shell at invocation
umask or inherited from
the shell's parent
trap
set
or inherited from the shell's parent in the environment
set
shopt
alias (see section 6.6 Aliases)
$$, and the value of
$PPID
When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
A command invoked in this separate environment cannot affect the shell's execution environment.
Command substitution and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.
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.
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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 3.4 Shell Parameters. These assignment statements affect only the environment seen by that command.
If the `-k' option is set (see section 4.3 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.
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For the shell's purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
The exit status is used by the Bash conditional commands (see section 3.2.5 Conditional Constructs) and some of the list constructs (see section 3.2.3 Lists of Commands).
All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage.
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When Bash is interactive, in the absence of any traps, it ignores
SIGTERM (so that `kill 0' does not kill an interactive shell),
and SIGINT
is caught and handled (so that the wait builtin is interruptible).
When Bash receives a SIGINT, it breaks out of any executing loops.
In all cases, Bash ignores SIGQUIT.
If job control is in effect (see section 7. Job Control), Bash
ignores SIGTTIN, SIGTTOU, and SIGTSTP.
Commands started by Bash have signal handlers set to the
values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore SIGINT and SIGQUIT as well.
Commands run as a result of
command substitution ignore the keyboard-generated job control signals
SIGTTIN, SIGTTOU, and SIGTSTP.
The shell exits by default upon receipt of a SIGHUP.
Before exiting, 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 section 7.2 Job Control Builtins) or marked
to not receive SIGHUP using disown -h.
If the huponexit shell option has been set with shopt
(see section 4.2 Bash Builtin Commands), Bash sends a SIGHUP to all jobs when
an interactive login shell exits.
When Bash receives a signal for which a trap has been set while waiting
for a command to complete, the trap will not be executed until the
command completes.
When Bash is waiting for an asynchronous
command via the wait builtin, the reception of a signal for
which a trap has been set will cause the wait builtin to return
immediately with an exit status greater than 128, immediately after
which the trap is executed.
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A shell script is a text file containing shell commands. When such
a file is used as the first non-option argument when invoking Bash,
and neither the `-c' nor `-s' option is supplied
(see section 6.1 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 |
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 4.1 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.
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4.1 Bourne Shell Builtins Builtin commands inherited from the Bourne Shell. 4.2 Bash Builtin Commands Table of builtins specific to Bash. 4.3 The Set Builtin This builtin is so overloaded it deserves its own section. 4.4 Special Builtins Builtin commands classified specially by POSIX.2.
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 section 3.2.1 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 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 section 7.2 Job Control Builtins), the directory stack (see section 6.8.1 Directory Stack Builtins), the command history (see section 9.2 Bash History Builtins), and the programmable completion facilities (see section 8.7 Programmable Completion Builtins).
Many of the builtins have been extended by POSIX or Bash.
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The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.
: (a colon)
: [arguments] |
. (a period)
. filename [arguments] |
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.
break
break [n] |
for, while, until, or select loop.
If n is supplied, the nth enclosing loop is exited.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.
cd
cd [-LP] [directory] |
HOME shell variable is used. If the
shell variable CDPATH exists, it is used as a search path. If
directory begins with a slash, CDPATH is not used.
The `-P' option means
to not follow symbolic links; symbolic links are followed by default
or with the `-L' option.
If directory is `-', it is equivalent to $OLDPWD.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
continue
continue [n] |
for, while,
until, or select loop.
If n is supplied, the execution of the nth enclosing loop
is resumed.
n must be greater than or equal to 1.
The return status is zero unless n is not greater than or equal to 1.
eval
eval [arguments] |
eval.
If there are no arguments or only empty arguments, the return status is
zero.
exec
exec [-cl] [-a name] [command [arguments]] |
login program does.
The `-c' option causes command to be executed with an empty
environment.
If `-a' is supplied, the shell passes name as the zeroth
argument to command.
If no command is specified, redirections may be used to affect
the current shell environment. If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.
exit
exit [n] |
EXIT is executed before the shell terminates.
export
export [-fn] [-p] [name[=value]] |
getopts
getopts 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.
hash
hash [-r] [-p filename] [-t] [name] |
$PATH.
The `-p' option inhibits the path search, and filename is
used as the location of name.
The `-r' option causes the shell to forget all remembered locations.
If 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.
If no arguments are given, information about remembered commands is printed.
The return status is zero unless a name is not found or an invalid
option is supplied.
pwd
pwd [-LP] |
readonly
readonly [-apf] [name] ... |
return
return [n] |
. (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.
The return status is non-zero if return is used outside a function
and not during the execution of a script by . or source.
shift
shift [n] |
$# 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
[
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 )
expr1 -a expr2
expr1 -o expr2
The test and [ builtins evaluate conditional
expressions using a set of rules based on the number of arguments.
times
times |
trap
trap [-lp] [arg] [sigspec ...] |
trap prints the list of commands
associated with each signal number in a form that may be reused as
shell input.
Each sigspec is either a signal name such as SIGINT (with
or without the SIG prefix) or a signal number.
If a sigspec
is 0 or EXIT, arg is executed when the shell exits.
If a sigspec is DEBUG, the command arg is executed
after every simple command.
If a sigspec is ERR, the command arg
is executed whenever a simple command has a non-zero exit status.
The ERR trap is not executed if the failed command is part of an
until or while loop, part of an if statement,
part of a && or || list, or if the command's return
status is being inverted using !.
The `-l' option causes the shell to print a list of signal names
and their corresponding numbers.
Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created.
The return status is zero unless a sigspec does not specify a valid signal.
umask
umask [-p] [-S] [mode] |
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.
unset
unset [-fv] [name] |
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This section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX 1003.2 standard.
alias
alias [ |
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 6.6 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 |
Display current Readline (see section 8. Command Line Editing) key and function bindings, or bind a key sequence to a Readline function or macro. The binding syntax accepted is identical to that of a Readline initialization file (see section 8.3 Readline Init File), but each binding must be passed as a separate argument: e.g., `"\C-x\C-r":re-read-init-file'. Options, if supplied, have the following meanings:
-m keymap
emacs,
emacs-standard,
emacs-meta,
emacs-ctlx,
vi,
vi-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-command
The return status is zero unless an invalid option is supplied or an error occurs.
builtin
builtin [shell-builtin [args]] |
command
command [-pVv] command [arguments ...] |
PATH are executed.
If there is a shell function named ls, running `command ls'
within the function will execute the external command ls
instead of calling the function recursively.
The `-p' option means to use a default value for PATH
that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if command cannot be
found or an error occurred, and the exit status of command
otherwise.
If either the `-V' or `-v' option is supplied, a description of command is printed. The `-v' option causes a single word indicating the command or file name used to invoke command to be displayed; the `-V' option produces a more verbose description. In this case, the return status is zero if command is found, and non-zero if not.
declare
declare [-afFrxi] [-p] [name[=value]] |
Declare variables and give them attributes. If no names are given, then display the values of variables instead.
The `-p' option will display the attributes and values of each name. When `-p' is used, additional options are ignored. The `-F' option inhibits the display of function definitions; only the function name and attributes are printed. `-F' implies `-f'. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:
-a
-f
-i
-r
-x
Using `+' instead of `-' turns off the attribute instead.
When used in a function, declare makes each name local,
as with the local command.
The return status is zero unless an invalid option is encountered, an attempt is made to define a function using `-f foo=bar', an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see section 6.7 Arrays), one of the names is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with `-f'.
echo
echo [-neE] [arg ...] |
xpg_echo shell option may be used to
dynamically determine whether or not echo expands these
escape characters by default.
echo interprets the following escape sequences:
\a
\b
\c
\e
\f
\n
\r
\t
\v
\\
\nnn
\xHH
enable
enable [-n] [-p] [-f filename] [-ads] [name ...] |
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 section 4.4 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.
help
help [-s] [pattern] |
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.
let
let expression [expression] |
let builtin allows arithmetic to be performed on shell
variables. Each expression is evaluated according to the
rules given below in 6.5 Shell Arithmetic. If the
last expression evaluates to 0, let returns 1;
otherwise 0 is returned.
local
local [option] name[=value] |
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.
logout
logout [n] |
printf
|
printf(1) formats, `%b' causes
printf to expand backslash escape sequences in the corresponding
argument, and `%q' causes printf to output the
corresponding argument in a format that can be reused as shell input.
The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure.
read
read [-ers] [-a aname] [-p prompt] [-t timeout] [-n nchars] [-d delim] [name ...] |
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 or read
times out.
Options, if supplied, have the following meanings:
-a aname
-d delim
-e
-n nchars
read returns after reading nchars characters rather than
waiting for a complete line of input.
-p prompt
-r
-s
-t timeout
read 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.
shopt
shopt [-pqsu] [-o] [optname ...] |
-s
-u
-q
-o
set builtin (see section 4.3 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_vars
cd
builtin command that
is not a directory is assumed to be the name of a variable whose
value is the directory to change to.
cdspell
cd command will be corrected.
The errors checked for are transposed characters,
a missing character, and a character too many.
If a correction is found, the corrected path is printed,
and the command proceeds.
This option is only used by interactive shells.
checkhash
checkwinsize
LINES and COLUMNS.
cmdhist
dotglob
execfail
exec
builtin command. An interactive shell does not exit if exec
fails.
expand_aliases
extglob
histappend
HISTFILE
variable when the shell exits, rather than overwriting the file.
histreedit
histverify
hostcomplete
huponexit
SIGHUP to all jobs when an interactive
login shell exits (see section 3.7.6 Signals).
interactive_comments
lithist
cmdhist
option is enabled, multi-line commands are saved to the history with
embedded newlines rather than using semicolon separators where possible.
login_shell
mailwarn
"The mail in mailfile has been read" is displayed.
no_empty_cmd_completion
PATH for possible completions when completion is attempted
on an empty line.
nocaseglob
nullglob
progcomp
promptvars
restricted_shell
shift_verbose
shift
builtin prints an error message when the shift count exceeds the
number of positional parameters.
sourcepath
source builtin uses the value of PATH
to find the directory containing the file supplied as an argument.
This option is enabled by default.
xpg_echo
echo 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.
source
source filename |
. (see section 4.1 Bourne Shell Builtins).
type
type [-atp] [name ...] |
If the `-t' option is used, type prints a single word
which is one of `alias', `function', `builtin',
`file' or `keyword',
if name is an alias, shell function, shell builtin,
disk file, or shell reserved word, respectively.
If the name is not found, then nothing is printed, and
type returns a failure status.
If the `-p' option is used, type either returns the name
of the disk file that would be executed, or nothing if `-t'
would not return `file'.
If the `-a' option is used, type returns all of the places
that contain an executable named file.
This includes aliases and functions, if and only if the `-p' option
is not also used.
The return status is zero if any of the names are found, non-zero if none are found.
typeset
typeset [-afFrxi] [-p] [name[=value]] |
typeset command is supplied for compatibility with the Korn
shell; however, it has been deprecated in favor of the declare
builtin command.
ulimit
ulimit [-acdflmnpstuvSH] [limit] |
ulimit provides control over the resources available to processes
started by the shell, on systems that allow such control. If an
option is given, it is interpreted as follows:
-S
-H
-a
-c
-d
-f
-l
-m
-n
-p
-s
-t
-u
-v
If 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.
unalias
unalias [-a] [name ... ] |
Remove each name from the list of aliases. If `-a' is supplied, all aliases are removed. Aliases are described in 6.6 Aliases.
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This builtin is so complicated that it deserves its own section.
set
set [--abefhkmnptuvxBCHP] [-o option] [argument ...] |
If no options or arguments are supplied, set displays the names
and values of all shell variables and functions, sorted according to the
current locale, in a format that may be reused as input.
When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:
-a
-b
-e
until or while loop, part of an if statement,
part of a && or || list, or if the command's return
status is being inverted using !.
A trap on ERR, if set, is executed before the shell exits.
-f
-h
-k
-m
-n
-o option-name
Set the option corresponding to option-name:
allexport
-a.
braceexpand
-B.
emacs
emacs-style line editing interface (see section 8. Command Line Editing).
errexit
-e.
hashall
-h.
histexpand
-H.
history
ignoreeof
keyword
-k.
monitor
-m.
noclobber
-C.
noexec
-n.
noglob
-f.
nolog
notify
-b.
nounset
-u.
onecmd
-t.
physical
-P.
posix
privileged
-p.
verbose
-v.
vi
vi-style line editing interface.
xtrace
-x.
-p
$BASH_ENV and $ENV files are not
processed, shell functions are not inherited from the environment,
and the SHELLOPTS variable, if it appears in the environment,
is ignored.
If the shell is started with the effective user (group) id not equal to the
real user (group) id, and the -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
-x
-B
-C
-H
-P
cd which change the current directory. The physical directory
is used instead. By default, Bash follows
the logical chain of directories when performing commands
which change the current directory.
For example, if `/usr/sys' is a symbolic link to `/usr/local/sys' then:
$ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usr |
If set -P is on, then:
$ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local |
--
-
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.
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For historical reasons, the POSIX 1003.2 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 6.11 Bash POSIX Mode.
These are the POSIX special builtins:
break : . continue eval exec exit export readonly return set shift trap unset |
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5.1 Bourne Shell Variables Variables which Bash uses in the same way as the Bourne Shell. 5.2 Bash Variables List of variables that exist in Bash.
This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables.
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Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
CDPATH
cd builtin command.
HOME
cd builtin
command.
The value of this variable is also used by tilde expansion
(see section 3.5.2 Tilde Expansion).
IFS
MAIL
MAILPATH variable
is not set, Bash informs the user of the arrival of mail in
the specified file.
MAILPATH
$_ expands to the name of
the current mail file.
OPTARG
getopts builtin.
OPTIND
getopts builtin.
PATH
PS1
PS1 is displayed.
PS2
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These 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 section 7.3 Job Control Variables).
BASH
BASH_ENV
BASH_VERSION
BASH_VERSINFO
BASH_VERSINFO[0]
BASH_VERSINFO[1]
BASH_VERSINFO[2]
BASH_VERSINFO[3]
BASH_VERSINFO[4]
BASH_VERSINFO[5]
MACHTYPE.
COLUMNS
select builtin command to determine the terminal width
when printing selection lists. Automatically set upon receipt of a
SIGWINCH.
COMP_CWORD
${COMP_WORDS} of the word containing the current
cursor position.
This variable is available only in shell functions invoked by the
programmable completion facilities (see section 8.6 Programmable Completion).
COMP_LINE
COMP_POINT
${#COMP_LINE}.
This variable is available only in shell functions and external
commands invoked by the
programmable completion facilities (see section 8.6 Programmable Completion).
COMP_WORDS
COMPREPLY
DIRSTACK
dirs builtin.
Assigning to members of this array variable may be used to modify
directories already in the stack, but the pushd and popd
builtins must be used to add and remove directories.
Assignment to this variable will not change the current directory.
If DIRSTACK is unset, it loses its special properties, even if
it is subsequently reset.
EUID
FCEDIT
fc
builtin command.
FIGNORE
FIGNORE
is excluded from the list of matched file names. A sample
value is `.o:~'
FUNCNAME
FUNCNAME have no effect and return an error status.
If FUNCNAME is unset, it loses its special properties, even if
it is subsequently reset.
GLOBIGNORE
GLOBIGNORE, it is removed from the list
of matches.
GROUPS
GROUPS have no effect and return an error status.
If GROUPS is unset, it loses its special properties, even if it is
subsequently reset.
histchars
HISTCMD
HISTCMD is unset, it loses its special properties,
even if it is subsequently reset.
HISTCONTROL
HISTCONTROL.
HISTFILE
HISTFILESIZE
HISTIGNORE
HISTCONTROL
are applied. In addition to the normal shell pattern matching
characters, `&' matches the previous history line. `&'
may be escaped using a backslash; the backslash is removed
before attempting a match.
The second and subsequent lines of a multi-line compound command are
not tested, and are added to the history regardless of the value of
HISTIGNORE.
HISTIGNORE subsumes the function of HISTCONTROL. A
pattern of `&' is identical to ignoredups, and a
pattern of `[ ]*' is identical to ignorespace.
Combining these two patterns, separating them with a colon,
provides the functionality of ignoreboth.
HISTSIZE
HOSTFILE
HOSTFILE is set, but has no value, Bash attempts to read
`/etc/hosts' to obtain the list of possible hostname completions.
When HOSTFILE is unset, the hostname list is cleared.
HOSTNAME
HOSTTYPE
IGNOREEOF
EOF character
as the sole input. If set, the value denotes the number
of consecutive EOF characters that can be read as the
first character on an input line
before the shell will exit. If the variable exists but does not
have a numeric value (or has no value) then the default is 10.
If the variable does not exist, then EOF signifies the end of
input to the shell. This is only in effect for interactive shells.
INPUTRC
LANG
LC_.
LC_ALL
LANG and any other
LC_ variable specifying a locale category.
LC_COLLATE
LC_CTYPE
LC_MESSAGES
LC_NUMERIC
LINENO
LINES
select builtin command to determine the column length
for printing selection lists. Automatically set upon receipt of a
SIGWINCH.
MACHTYPE
MAILCHECK
MAILPATH or MAIL variables.
The default is 60 seconds. When it is time to check
for mail, the shell does so before displaying the primary prompt.
If this variable is unset, or set to a value that is not a number
greater than or equal to zero, the shell disables mail checking.
OLDPWD
cd builtin.
OPTERR
getopts builtin command.
OSTYPE
PIPESTATUS
POSIXLY_CORRECT
bash starts, the shell
enters POSIX mode (see section 6.11 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
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PPID
PROMPT_COMMAND
$PS1).
PS3
select command. If this variable is not set, the
select command prompts with `#? '
PS4
PS4 is replicated multiple times, as
necessary, to indicate multiple levels of indirection.
The default is `+ '.
PWD
cd builtin.
RANDOM
REPLY
read builtin.
SECONDS
SHELLOPTS
set builtin command (see section 4.3 The Set Builtin).
The options appearing in SHELLOPTS are those reported
as `on' by `set -o'.
If this variable is in the environment when Bash
starts up, each shell option in the list will be enabled before
reading any startup files. This variable is readonly.
SHLVL
TIMEFORMAT
time
reserved word should be displayed.
The `%' character introduces an
escape sequence that is expanded to a time value or other
information.
The escape sequences and their meanings are as
follows; the braces denote optional portions.
%%
%[p][l]R
%[p][l]U
%[p][l]S
%P
The 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
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TMOUT
UID
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This section describes features unique to Bash.
6.1 Invoking Bash Command line options that you can give to Bash. 6.2 Bash Startup Files When and how Bash executes scripts. 6.3 Interactive Shells What an interactive shell is. 6.4 Bash Conditional Expressions Primitives used in composing expressions for the testbuiltin.6.5 Shell Arithmetic Arithmetic on shell variables. 6.6 Aliases Substituting one command for another. 6.7 Arrays Array Variables. 6.8 The Directory Stack History of visited directories. 6.9 Controlling the Prompt Controlling the PS1 string. 6.10 The Restricted Shell A more controlled mode of shell execution. 6.11 Bash POSIX Mode Making Bash behave more closely to what the POSIX standard specifies.
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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 section 4.3 The Set Builtin), there are several multi-character options that you can use. These options must appear on the command line before the single-character options in order for them to be recognized.
--dump-po-strings
gettext PO (portable object) file format.
Equivalent to `-D' except for the output format.
--dump-strings
--help
--init-file filename
--rcfile filename
--login
--noediting
--noprofile
--norc
sh.
--posix
--restricted
--verbose
--version
There are several single-character options that may be supplied at
invocation which are not available with the set builtin.
-c string
$0.
-i
-r
-s
-D
C or POSIX (see section 3.1.2.5 Locale-Specific Translation).
This implies the `-n' option; no commands will be executed.
[-+]O [shopt_option]
shopt builtin (see section 4. 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 section 6.3 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 section 3.8 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.
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This section describs how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see section 3.5.2 Tilde Expansion).
Interactive shells are described in 6.3 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
|
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:
|
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.
sh
If Bash is invoked with the name sh, it tries to mimic the
startup behavior of historical versions of sh as closely as
possible, while conforming to the POSIX standard as well.
When invoked as an interactive login shell, 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.
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6.3.1 What is an Interactive Shell? What determines whether a shell is Interactive. 6.3.2 Is this Shell Interactive? How to tell if a shell is interactive. 6.3.3 Interactive Shell Behavior What changes in a interactive shell?
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An interactive shell
is one started without non-option arguments, unless `-s' is
specified, without specifiying 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.
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.
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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
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When the shell is running interactively, it changes its behavior in several ways.
SIGTTIN, SIGTTOU, and SIGTSTP.
PS1 before reading the first line
of a command, and expands and displays PS2 before reading the
second and subsequent lines of a multi-line command.
PROMPT_COMMAND variable as a command
before printing the primary prompt, $PS1
(see section 5.2 Bash Variables).
ignoreeof option to set -o
instead of exiting immediately when it receives an EOF on its
standard input when reading a command (see section 4.3 The Set Builtin).
$HISTFILE
when an interactive shell exits.
SIGTERM
(see section 3.7.6 Signals).
SIGINT is caught and handled
((see section 3.7.6 Signals).
SIGINT will interrupt some shell builtins.
SIGHUP to all jobs on exit
if the hupoxexit shell option has been enabled (see section 3.7.6 Signals).
MAIL, MAILPATH, and MAILCHECK shell variables
(see section 5.2 Bash Variables).
${var:?word} expansions
(see section 3.5.3 Shell Parameter Expansion).
exec will not cause the shell to exit
(see section 4.1 Bourne Shell Builtins).
cd
builtin is enabled by default (see the description of the cdspell
option to the shopt builtin in 4.2 Bash Builtin Commands).
TMOUT variable and exit
if a command is not read within the specified number of seconds after
printing $PS1 (see section 5.2 Bash Variables).
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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.
-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
file1 -nt file2
file1 -ot file2
file1 -ef file2
-o optname
set builtin (see section 4.3 The Set Builtin).
-z string
-n string
string
string1 == string2
string1 != string2
string1 < string2
string1 > string2
arg1 OP arg2
OP is one of
`-eq', `-ne', `-lt', `-le', `-gt', or `-ge'.
These arithmetic binary operators return true if arg1
is equal to, not equal to, less than, less than or equal to,
greater than, or greater than or equal to arg2,
respectively. Arg1 and arg2
may be positive or negative integers.
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The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by the let builtin.
Evaluation is done in long integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence and associativity 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 --id
- +
! ~
**
* / %
+ -
<< >>
<= >= < >
== !=
&
^
|
&&
||
expr ? expr : expr
= *= /= %= += -= <<= >>= &= ^= |=
expr1 , expr2
Shell 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. The value of a variable is evaluated as an arithmetic expression when it is referenced. A shell variable need not have its integer attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers.
A leading `0x' or `0X' denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where base
is a decimal number between 2 and 64 representing the arithmetic
base, and n is a number in that base. If base# is
omitted, then base 10 is used.
The digits greater than 9 are represented by the lowercase letters,
the uppercase letters, `@', and `_', in that order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangably to represent numbers between 10
and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.
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Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with
the alias and unalias builtin commands.
The first word of each simple command, if unquoted, is checked to see
if it has an alias.
If so, that word is replaced by the text of the alias.
The alias name and the replacement text may contain any valid
shell input, including shell metacharacters, with the exception
that the alias name may not contain `='.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time. This means that one may alias
ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text. If the last character of the alias value is a
space or tab character, then the next command word following the
alias is also checked for alias expansion.
Aliases are created and listed with the alias
command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used
(see section 3.3 Shell Functions).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see section 4.2 Bash Builtin Commands).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash
always reads at least one complete line
of input before executing any
of the commands on that line. Aliases are expanded when a
command is read, not when it is executed. Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a compound command. As a consequence, aliases
defined in a function are not available until after that
function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.
For almost every purpose, shell functions are preferred over aliases.
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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 |
declare -a name[subscript] |
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) |
[[subscript]=]string. If
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
This syntax is also accepted by the declare
builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell's filename expansion operators. If the
subscript is `@' or `*', the word expands to all members
of the array name. These subscripts differ only when the word
appears within double quotes. If the word is double-quoted,
${name[*]} expands to a single word with
the value of each array member separated by the first character of the
IFS variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} expands to nothing. This is analogous to the
expansion of the special parameters `@' and `*'.
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is `@' or
`*', the expansion is the number of elements in the array.
Referencing an array variable without a subscript is equivalent to
referencing element zero.
The unset builtin is used to destroy arrays.
unset name[subscript]
destroys the array element at index subscript.
unset name, where name is an array, removes the
entire array. A subscript of `*' or `@' also removes the
entire array.
The declare, local, and readonly
builtins each accept a `-a'
option to specify an array. The read
builtin accepts a `-a'
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.
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6.8.1 Directory Stack Builtins Bash builtin commands to manipulate the directory stack.
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.
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dirs
dirs [+N | -N] [-clpv] |
pushd command; the
popd command removes directories from the list.
+N
dirs when invoked without options), starting
with zero.
-N
dirs when invoked without options), starting
with zero.
-c
-l
-p
dirs to print the directory stack with one entry per
line.
-v
dirs to print the directory stack with one entry per
line, prefixing each entry with its index in the stack.
popd
popd [+N | -N] [-n] |
Remove the top entry from the directory stack, and cd
to the new top directory.
When no arguments are given, popd
removes the top directory from the stack and
performs a cd to the new top directory. The
elements are numbered from 0 starting at the first directory listed with
dirs; i.e., popd is equivalent to popd +0.
+N
dirs), starting with zero.
-N
dirs), starting with zero.
-n
pushd
pushd [dir | +N | -N] [-n] |
Save the current directory on the top of the directory stack
and then cd to dir.
With no arguments, pushd exchanges the top two directories.
+N
dirs, starting with zero) to the top of
the list by rotating the stack.
-N
dirs, starting with zero) to the top of
the list by rotating the stack.
-n
dir
cd dir'.
cds to dir.
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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
\e
\h
\H
\j
\l
\n
\r
\s
$0 (the portion
following the final slash).
\t
\T
\@
\A
\u
\v
\V
\w
\W
$PWD.
\!
\#
\$
#, 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 section 9.1 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 section 4.2 Bash Builtin Commands).
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If Bash is started with the name rbash, or the
`--restricted'
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash
with the exception that the following are disallowed:
cd builtin.
SHELL, PATH,
ENV, or BASH_ENV variables.
.
builtin command.
hash builtin command.
SHELLOPTS from the shell environment at startup.
exec builtin to replace the shell with another command.
enable builtin.
command builtin.
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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 1003.2 standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.
The following list is what's changed when `POSIX mode' is in effect:
$PATH to find the new location. This is also available with
`shopt -s checkhash'.
SIGTSTP.
PS1 and PS2 expansions of `!' to
the history number and `!!' to `!' are enabled,
and parameter expansion is performed on the values of PS1 and
PS2 regardless of the setting of the promptvars option.
$ENV) rather than
the normal Bash files.
$HISTFILE).
. filename
is not found.
names. That is, they may not
contain characters other than letters, digits, and underscores, and
may not start with a digit. Declaring a function with an invalid name
causes a fatal syntax error in non-interactive shells.
cd builtin finds a directory to change to
using $CDPATH, the
value it assigns to the PWD variable does not contain any
symbolic links, as if `cd -P' had been executed.
CDPATH is set, the cd builtin will not implicitly
append the current directory to it. This means that cd will
fail if no valid directory name can be constructed from
any of the entries in $CDPATH, even if the a directory with
the same name as the name given as an argument to cd exists
in the current directory.
for statement or the selection variable in a
select statement is a readonly variable.
export and readonly builtin commands display their
output in the format required by POSIX 1003.2.
trap builtin displays signal names without the leading
SIG.
. and source builtins do not search the current directory
for the filename argument if it is not found by searching PATH.
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.
There is other POSIX 1003.2 behavior that Bash does not implement. Specifically:
$0 to the full pathname of
the script as found by searching $PATH, rather than the command as
typed by the user.
$PATH, bash
checks execute permission bits rather than read permission bits, just as
if it were searching for a command.
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This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities.
7.1 Job Control Basics How job control works. 7.2 Job Control Builtins Bash builtin commands used to interact with job control. 7.3 Job Control Variables Variables Bash uses to customize job control.
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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 |
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. 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 section 4.3 The Set Builtin).
Any trap on SIGCHLD is executed for each child process
that exits.
If an attempt to exit Bash is 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.
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bg
bg [jobspec] |
fg
fg [jobspec] |
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
-s
If 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] |
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.
wait
wait [jobspec or pid] |
disown
disown [-ar] [-h] [jobspec ...] |
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.
suspend
suspend [-f] |
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.
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auto_resume
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This 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.
8.1 Introduction to Line Editing Notation used in this text. 8.2 Readline Interaction The minimum set of commands for editing a line. 8.3 Readline Init File Customizing Readline from a user's view. 8.4 Bindable Readline Commands A description of most of the Readline commands available for binding 8.5 Readline vi Mode A short description of how to make Readline behave like the vi editor. 8.6 Programmable Completion How to specify the possible completions for a specific command. 8.7 Programmable Completion Builtins Builtin commands to specify how to complete arguments for a particular command.
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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 section 8.3 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.
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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.
8.2.1 Readline Bare Essentials The least you need to know about Readline. 8.2.2 Readline Movement Commands Moving about the input line. 8.2.3 Readline Killing Commands How to delete text, and how to get it back! 8.2.4 Readline Arguments Giving numeric arguments to commands. 8.2.5 Searching for Commands in the History Searching through previous lines.
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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.)
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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.
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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.
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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.
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Readline provides commands for searching through the command history (see section 9.1 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.
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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'.
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.
8.3.1 Readline Init File Syntax Syntax for the commands in the inputrc file.
8.3.2 Conditional Init Constructs Conditional key bindings in the inputrc file.
8.3.3 Sample Init File An example inputrc file.
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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 section 8.3.2 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.
The bind -V command lists the current Readline variable names
and values. See section 4.2 Bash Builtin Commands.
A great deal of run-time behavior is changeable with the following variables.
bell-style
comment-begin
insert-comment command is executed. The default value
is "#".
completion-ignore-case
completion-query-items
100.
convert-meta
disable-completion
self-insert. The default is `off'.
editing-mode
editing-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-keypad
expand-tilde
If set to `on', the history code attempts to place point at the
same location on each history line retrived with previous-history
or next-history.
horizontal-scroll-mode
input-meta
meta-flag is a
synonym for this variable.
isearch-terminators
keymap
keymap 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-directories
mark-modified-lines
match-hidden-files
output-meta
print-completions-horizontally
show-all-if-ambiguous
visible-stats
Once 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. 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 section 4.2 Bash Builtin Commands.
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:
\C-
\M-
\e
\\
\"
\'
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
\xHH
When 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\\": "\\" |
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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.
mode
mode= 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.
term
term= 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 |
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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
|
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8.4.1 Commands For Moving Moving about the line. 8.4.2 Commands For Manipulating The History Getting at previous lines. 8.4.3 Commands For Changing Text Commands for changing text. 8.4.4 Killing And Yanking Commands for killing and yanking. 8.4.5 Specifying Numeric Arguments Specifying numeric arguments, repeat counts. 8.4.6 Letting Readline Type For You Getting Readline to do the typing for you. 8.4.7 Keyboard Macros Saving and re-executing typed characters 8.4.8 Some Miscellaneous Commands Other miscellaneous commands.
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 section 4.2 Bash Builtin Commands.)
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.
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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 ()
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accept-line (Newline or Return)
HISTCONTROL and HISTIGNORE variables.
If this line is a modified history line, then restore the history line
to its original state.
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.
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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)
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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)
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.
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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.
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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-{)
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start-kbd-macro (C-x ()
end-kbd-macro (C-x ))
call-last-kbd-macro (C-x e)
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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,
and 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.
dump-functions ()
dump-variables ()
dump-macros ()
glob-expand-word (C-x *)
glob-list-expansions (C-x g)
glob-expand-word is displayed, and the line is redrawn.
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)
emacs-editing-mode (C-e)
vi editing mode, this causes a switch back to
emacs editing mode, as if the command `set -o emacs' had
been executed.
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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 section 4.3 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.
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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 section 8.7 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 section 8.4.6 Letting Readline Type For You) 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 section 5.2 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, arithmetic expansion, and pathname expansion,
as described above (see section 3.5 Shell Expansions).
The results are split using the rules described above
(see section 3.5.7 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 section 5.2 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 section 8.7 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.
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 default' option was supplied to complete when the
compspec was defined, Readline's default completion will be performed
if the compspec generates no matches.
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Two builtin commands are available to manipulate the programmable completion facilities.
compgen
|
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.
complete
|
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 section 8.6 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-option
default
dirnames
filenames
-A action
alias
arrayvar
binding
builtin
command
directory
disabled
enabled
export
file
function
group
helptopic
help builtin (see section 4.2 Bash Builtin Commands).
hostname
HOSTFILE shell variable (see section 5.2 Bash Variables).
job
keyword
running
setopt
set builtin
(see section 4.3 The Set Builtin).
shopt
shopt builtin
(see section 4.2 Bash Builtin Commands).
signal
stopped
user
variable
-G globpat
-W wordlist
IFS special variable as delimiters, and each resultant word
is expanded.
The possible completions are the members of the resultant list which
match the word being completed.
-C command
-F function
COMPREPLY array variable.
-X filterpat
-P prefix
-S suffix
The 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.
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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.
9.1 Bash History Facilities How Bash lets you manipulate your command history. 9.2 Bash History Builtins The Bash builtin commands that manipulate the command history. 9.3 History Expansion What it feels like using History as a user.
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When the `-o history' option to the set builtin
is enabled (see section 4.3 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 section 4.2 Bash Builtin Commands),
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.
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 section 8.4.2 Commands For Manipulating The 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 section 4.2 Bash Builtin Commands, for a description of shopt.
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Bash provides two builtin commands which manipulate the history list and history file.
fc
|
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 section 6.6 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. Options, if supplied, have the following meanings:
-c
-d offset
-a
-n
-r
-w
-p
-s
When 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.
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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 section 4.2 Bash Builtin Commands) 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.
9.3.1 Event Designators How to specify which history line to use. 9.3.2 Word Designators Specifying which words are of interest. 9.3.3 Modifiers Modifying the results of substitution.
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An event designator is a reference to a command line entry in the history list.
!
!n
!-n
!!
!string
!?string[?]
^string1^string2^
!!:s/string1/string2/.
!#
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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:2
fi.
Here are the word designators:
0 (zero)
0th word. For many applications, this is the command word.
n
^
$
%
x-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.
x*
x-
If a word designator is supplied without an event specification, the previous command is used as the event.
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After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a `:'.
h
t
r
e
p
q
x
s/old/new/
&
g
gs/old/new/,
or with `&'.
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This 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, Windows 95/98, and Windows NT.
10.1 Basic Installation Installation instructions.
10.2 Compilers and Options How to set special options for various systems.
10.3 Compiling For Multiple Architectures How to compile Bash for more than one kind of system from the same source tree.
10.4 Installation Names How to set the various paths used by the installation.
10.5 Specifying the System Type How to configure Bash for a particular system.
10.6 Sharing Defaults How to share default configuration values among GNU programs.
10.7 Operation Controls Options recognized by the configuration program.
10.8 Optional Features How to enable and disable optional features when building Bash.
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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
[email protected] 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'.
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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.
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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.
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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.
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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.
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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.
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configure recognizes the following options to control how it
operates.
--cache-file=file
configure.
--help
configure, and exit.
--quiet
--silent
-q
--srcdir=dir
configure can determine that directory automatically.
--version
configure
script, and exit.
configure also accepts some other, not widely used, boilerplate
options. `configure --help' prints the complete list.
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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-malloc
malloc in `lib/malloc/malloc.c'. This is not the same
malloc that appears in GNU libc, but an older version
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-config
There are several `--enable-' options that alter how Bash is compiled and linked, rather than changing run-time features.
--enable-largefile
--enable-profiling
gprof each time it is executed.
--enable-static-link
gcc 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-alias
alias and unalias
builtins (see section 6.6 Aliases).
--enable-arith-for-command
for command
that behaves like the C language for statement
(see section 3.2.4 Looping Constructs).
--enable-array-variables
--enable-bang-history
csh-like history substitution
(see section 9.3 History Expansion).
--enable-brace-expansion
csh-like brace expansion
( b{a,b}c ==> bac bbc ).
See 3.5.1 Brace Expansion, for a complete description.
--enable-command-timing
time as a reserved word and for
displaying timing statistics for the pipeline following time
(see section 3.2.2 Pipelines).
This allows pipelines as well as shell builtins and functions to be timed.
--enable-cond-command
[[ conditional command
(see section 3.2.5 Conditional Constructs).
--enable-directory-stack
csh-like directory stack and the
pushd, popd, and dirs builtins
(see section 6.8 The Directory Stack).
--enable-disabled-builtins
xxx has been disabled using `enable -n xxx'.
See 4.2 Bash Builtin Commands, for details of the builtin and
enable builtin commands.
--enable-dparen-arithmetic
((...)) command
(see section 3.2.5 Conditional Constructs).
--enable-extended-glob
--enable-help-builtin
help builtin, which displays help on shell builtins and
variables (see section 4.2 Bash Builtin Commands).
--enable-history
fc and history
builtin commands (see section 9.1 Bash History Facilities).
--enable-job-control
--enable-net-redirections
/dev/tcp/host/port and
/dev/udp/host/port
when used in redirections (see section 3.6 Redirections).
--enable-process-substitution
--enable-prompt-string-decoding
$PS1, $PS2, $PS3, and $PS4 prompt
strings. See 6.9 Controlling the Prompt, for a complete list of prompt
string escape sequences.
--enable-progcomp
--enable-readline
--enable-restricted
rbash, enters a restricted mode. See
6.10 The Restricted Shell, for a description of restricted mode.
--enable-select
select builtin, which allows the generation of simple
menus (see section 3.2.5 Conditional Constructs).
--enable-usg-echo-default
--enable-xpg-echo-default.
--enable-xpg-echo-default
echo 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 2.
See section 4.2 Bash Builtin Commands, 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.
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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 that you have.
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 [email protected] 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 [email protected].
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Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell.
Bash uses the POSIX 1003.2 standard as the specification of
how these features are to be implemented. There are some
differences between the traditional Bourne shell and Bash; this
section quickly details the differences of significance. A
number of these differences are explained in greater depth in
previous sections.
This section uses the version of sh included in SVR4.2 as
the baseline reference.
sh behavior (see section 6.11 Bash POSIX Mode).
bind builtin.
complete and compgen, to manipulate it.
history and fc builtins to manipulate it.
csh-like history expansion
(see section 9.3 History Expansion).
$'...' quoting syntax, which expands ANSI-C
backslash-escaped characters in the text between the single quotes,
is supported (see section 3.1.2.4 ANSI-C Quoting).
$"..." quoting syntax to do
locale-specific translation of the characters between the double
quotes. The `-D', `--dump-strings', and `--dump-po-strings'
invocation options list the translatable strings found in a script
(see section 3.1.2.5 Locale-Specific Translation).
! keyword to negate the return value of
a pipeline (see section 3.2.2 Pipelines).
Very useful when an if statement needs to act only if a test fails.
time reserved word and command timing (see section 3.2.2 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 section 3.2.4 Looping Constructs).
select compound command, which allows the
generation of simple menus (see section 3.2.5 Conditional Constructs).
[[ compound command, which makes conditional
testing part of the shell grammar (see section 3.2.5 Conditional Constructs).
alias and unalias
builtins (see section 6.6 Aliases).
(( compound command
(see section 3.2.5 Conditional Constructs),
and arithmetic expansion (see section 6.5 Shell Arithmetic).
export
command.
${#xx}, which returns the length of ${xx},
is supported (see section 3.5.3 Shell Parameter Expansion).
${var:offset[:length]},
which expands to the substring of var's value of length
length, beginning at offset, is present
(see section 3.5.3 Shell Parameter Expansion).
${var/[/]pattern[/replacement]},
which matches pattern and replaces it with replacement in
the value of var, is available (see section 3.5.3 Shell Parameter Expansion).
${!prefix}* expansion, which expands to
the names of all shell variables whose names begin with prefix,
is available (see section 3.5.3 Shell Parameter Expansion).
${!word}
(see section 3.5.3 Shell Parameter Expansion).
$9 using
${num}.
$() form of command substitution
is implemented (see section 3.5.4 Command Substitution),
and preferred to the Bourne shell's " (which
is also implemented for backwards compatibility).
UID, EUID, and GROUPS), the current host
(HOSTTYPE, OSTYPE, MACHTYPE, and HOSTNAME),
and the instance of Bash that is running (BASH,
BASH_VERSION, and BASH_VERSINFO). See section 5.2 Bash Variables,
for details.
IFS variable is used to split only the results of expansion,
not all words (see section 3.5.7 Word Splitting).
This closes a longstanding shell security hole.
extglob
shell option is enabled (see section 3.5.8.1 Pattern Matching).
sh does not separate the two name spaces.
local builtin, and thus useful recursive functions may be written
(see section 4.2 Bash Builtin Commands).
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 section 4.3 The Set Builtin).
The `>|' redirection operator may be used to override noclobber.
cd and pwd builtins (see section 4.1 Bourne Shell Builtins)
each take `-L' and `-P' options to switch between logical and
physical modes.
builtin and command builtins (see section 4.2 Bash Builtin Commands).
command builtin allows selective disabling of functions
when command lookup is performed (see section 4.2 Bash Builtin Commands).
enable
builtin (see section 4.2 Bash Builtin Commands).
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 section 4.1 Bourne Shell Builtins).
export -f (see section 3.3 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 section 4.1 Bourne Shell Builtins).
help builtin for quick reference to shell
facilities (see section 4.2 Bash Builtin Commands).
printf builtin is available to display formatted output
(see section 4.2 Bash Builtin Commands).
read builtin (see section 4.2 Bash Builtin Commands)
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 section 4.1 Bourne Shell Builtins).
shopt builtin, for finer control of shell
optional capabilities (see section 4.2 Bash Builtin Commands), and allows these options
to be set and unset at shell invocation (see section 6.1 Invoking Bash).
set
builtin (see section 4.3 The Set Builtin).
test builtin (see section 4.1 Bourne Shell Builtins)
is slightly different, as it implements the POSIX algorithm,
which specifies the behavior based on the number of arguments.
trap builtin (see section 4.1 Bourne Shell Builtins) allows a
DEBUG pseudo-signal specification, similar to EXIT.
Commands specified with a DEBUG trap are executed after every
simple command.
The DEBUG trap is not inherited by shell functions.
The trap builtin (see section 4.1 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.
type builtin is more extensive and gives more information
about the names it finds (see section 4.2 Bash Builtin Commands).
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 section 4.1 Bourne Shell Builtins).
csh-like directory stack, and provides the
pushd, popd, and dirs builtins to manipulate it
(see section 6.8 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 section 7.2 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.
SHACCT variable or perform shell accounting.
sh uses a TIMEOUT variable like Bash uses
TMOUT.
More features unique to Bash may be found in 6. Bash Features.
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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.
IFS, MAILCHECK,
PATH, PS1, or PS2 variables to be unset.
-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).
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1. Introduction
2. Definitions
3. Basic Shell Features
4. Shell Builtin Commands
5. Shell Variables
6. Bash Features
7. Job Control
8. Command Line Editing
9. Using History Interactively
10. Installing Bash
A. Reporting Bugs
B. Major Differences From The Bourne Shell
Index of Shell Builtin Commands
Index of Shell Reserved Words
Parameter and Variable Index
Function Index
Concept Index
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| Button | Name | Go to | From 1.2.3 go to |
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| [ > ] | Forward | next section in reading order | 1.2.4 |
| [ << ] | FastBack | previous or up-and-previous section | 1.1 |
| [ Up ] | Up | up section | 1.2 |
| [ >> ] | FastForward | next or up-and-next section | 1.3 |
| [Top] | Top | cover (top) of document | |
| [Contents] | Contents | table of contents | |
| [Index] | Index | concept index | |
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