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This text is a brief description of the features that are present in the Bash shell.
This is Edition 2.5b, last updated 15 July 2002, of The GNU Bash
Reference Manual, for Bash, Version 2.05b.
Copyright (C) 1991-2002 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.
8. Command Line Editing Chapter describing the command line editing features.
9. Using History Interactively Chapter dealing with history expansion rules.
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
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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 $'string' are treated specially.
The word expands to string, with backslash-escaped characters
replaced as specified by the ANSI C standard. Backslash escape sequences, if
present, are decoded as follows:
\a
\b
\e
\f
\n
\r
\t
\v
\\
\'
\nnn
\xHH
\cx
The expanded result is single-quoted, as if the dollar sign had not been present.
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A double-quoted string preceded by a dollar sign (`$') will
cause the string to be translated according to the current locale. If the
current locale is C or POSIX, the dollar sign is
ignored. If the string is translated and replaced, the replacement is
double-quoted.
Some systems use the
message catalog selected by the LC_MESSAGES shell variable. Others
create the name of the message catalog from the value of the
TEXTDOMAIN shell variable, possibly adding a suffix of
`.mo'. If you use the TEXTDOMAIN variable, you may
need to set the TEXTDOMAINDIR variable to the location of the
message catalog files. Still others use both variables in this fashion:
TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.
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In a non-interactive shell, or an interactive shell in which the
interactive_comments option to the shopt builtin is
enabled (see section 4.2 Bash
Builtin Commands), a word beginning with `#' causes that word
and all remaining characters on that line to be ignored. An interactive shell
without the interactive_comments option enabled does not allow
comments. The interactive_comments option is on by default in
interactive shells. See section 6.3
Interactive Shells, for a description of what makes a shell interactive.
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A simple shell command such as echo a b c consists of the
command itself followed by arguments, separated by spaces.
More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
3.2.1 Simple Commands The most common type of command. 3.2.2 Pipelines Connecting the input and output of several commands. 3.2.3 Lists of Commands How to execute commands sequentially. 3.2.4 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.
A sequence of one or more newlines may appear in a list to
delimit commands, equivalent to a semicolon.
If a command is terminated by the control operator `&', the
shell executes the command asynchronously in a subshell. This is known as
executing the command in the background. The shell does not wait for
the command to finish, and the return status is 0 (true). When job control is
not active (see section 7. Job
Control), the standard input for asynchronous commands, in the absence of
any explicit redirections, is redirected from /dev/null.
Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.
The control operators `&&' and `||' denote AND lists and OR lists, respectively. An AND list has the form
command1 && command2 |
command2 is executed if, and only if, command1 returns an exit status of zero.
An OR list has the form
command1 || command2 |
command2 is executed if, and only if, command1 returns a non-zero exit status.
The return status of AND and OR lists is the exit status of the last command executed in the list.
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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 || operators do not
evaluate expression2 if the value of expression1 is
sufficient to determine the return value of the entire conditional expression.
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Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()
( list ) |
Placing a list of commands between parentheses causes a subshell 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 word xdigit |
word character class matches
letters, digits, and the character `_'.
Within `[' and `]', an equivalence
class can be specified using the syntax
[=c=], which matches all characters with
the same collation weight (as defined by the current locale) as the character
c.
Within `[' and `]', the syntax
[.symbol.] matches the collating symbol
symbol.
If the extglob shell option is enabled using the
shopt builtin, several extended pattern matching operators are
recognized. In the following description, a pattern-list is a list of
one or more patterns separated by a `|'. Composite patterns may be
formed using one or more of the following sub-patterns:
?(pattern-list)
*(pattern-list)
+(pattern-list)
@(pattern-list)
!(pattern-list)
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After the preceding expansions, all unquoted occurrences of the characters `\', `'', and `"' that did not result from one of the above expansions are removed.
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Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is `<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is `>', the redirection refers to the standard output (file descriptor 1).
The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example, the command
ls > dirlist 2>&1 |
ls 2>&1 > dirlist |
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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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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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:
<<[-]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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<<< word |
The word is expanded and supplied to the command on its standard input.
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[n]<&word |
The operator
[n]>&word |
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[n]<&digit- |
Similarly, the redirection operator
[n]>&digit- |
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[n]<>word |
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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 [-L|-P] [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] [-dt] [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. The `-d' option causes the shell to forget
the remembered location of each name. If the `-t'
option is supplied, the full pathname to which each name
corresponds is printed. If multiple name arguments are supplied
with `-t' the name is printed before the hashed full
pathname. The `-l' option causes output to be displayed in a
format that may be reused as input. If no arguments are given, or if only
`-l' is supplied, information about remembered commands is
printed. The return status is zero unless a name is not found or an
invalid option is supplied.
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 bind readline-command |
Display current Readline (see section 8. Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro, or set a Readline variable. Each non-option argument is a command as it would appear in a a Readline initialization file (see section 8.3 Readline Init File), but each binding or command must be passed as a separate argument; e.g., `"\C-x\C-r":re-read-init-file'. Options, if supplied, have the following meanings:
-m 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 [-afFirtx] [-p] [name[=value]] |
Declare variables and give them attributes. If no names are given, then display the values of variables instead.
The `-p' option will display the attributes and values of each name. When `-p' is used, additional options are ignored. The `-F' option inhibits the display of function definitions; only the function name and attributes are printed. `-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
-t
trace attribute. Traced
functions inherit the DEBUG trap from the calling shell. The
trace attribute has no special meaning for variables.
-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
\\
\0nnn
\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] [-d delim] [-n nchars] [-p prompt] [-t timeout] [-u fd] [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, read
times out, or an invalid file descriptor is supplied as the argument to
`-u'. Options, if supplied, have the following meanings:
-a aname
-d delim
-e
-n 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.
-u fd
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 [-afptP] [name ...] |
If the `-t' option is used, type prints a single
word which is one of `alias', `function',
`builtin', `file' or `keyword', if
name is an alias, shell function, shell builtin, disk file, or
shell reserved word, respectively. If the name is not found, then
nothing is printed, and type returns a failure status.
If the `-p' option is used, type either returns
the name of the disk file that would be executed, or nothing if
`-t' would not return `file'.
The `-P' option forces a path search for each name, even if `-t' would not return `file'.
If a command is hashed, `-p' and `-P' print the
hashed value, not necessarily the file that appears first in
$PATH.
If the `-a' option is used, type returns all of
the places that contain an executable named file. This includes
aliases and functions, if and only if the `-p' option is not also
used.
If the `-f' option is used, type does not attempt
to find shell functions, as with the command builtin.
The return status is zero if any of the names are found, non-zero if none are found.
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
|
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
|
TMOUT
TMOUT
is treated as the default timeout for the read builtin (see
section 4.2 Bash
Builtin Commands). The select command (see section 3.2.5
Conditional Constructs) terminates if input does not arrive after
TMOUT seconds when input is coming from a terminal.
In an interative shell, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive.
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 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
-l
-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 fixed-width 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. | [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
The value of the variable PROMPT_COMMAND is examined just before
Bash prints each primary prompt. If PROMPT_COMMAND is set and has a
non-null value, then the value is executed just as if it had been typed on the
command line.
In addition, the following table describes the special characters which can appear in the prompt variables:
\a
\d
\D{format}
strftime(3) and the result
is inserted into the prompt string; an empty format results in a
locale-specific time representation. The braces are required.
\e
\h
\H
\j
\l
\n
\r
\s
$0 (the portion
following the final slash).
\t
\T
\@
\A
\u
\v
\V
\w
\W
$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' or `-r' option is supplied at
invocation, the shell becomes restricted. A restricted shell is used to set up
an environment more controlled than the standard shell. A restricted shell
behaves identically to bash with the exception that the following
are disallowed or not performed:
cd builtin.
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.
enable builtin command to enable disabled shell
builtins.
command
builtin.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed (see section 3.8 Shell
Scripts), rbash turns off any restrictions in the shell spawned
to execute the script.
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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.
When invoked as sh, Bash enters POSIX mode after reading the
startup files.
The following list is what's changed when `POSIX mode' is in effect:
$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.
cd builtin is invoked in logical mode,
and the pathname constructed from $PWD and the directory name
supplied as an argument does not refer to an existing directory,
cd will fail instead of falling back to physical mode.
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
mark-symlinked-directories
mark-directories). The default is `off'.
match-hidden-files
output-meta
page-completions
more-like pager to display a screenful of possible
completions at a time. This variable is `on' by default.
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)
overwrite-mode ()
emacs mode; vi mode does overwrite differently. Each
call to readline() starts in insert mode.
In overwrite mode, characters bound to self-insert replace the
text at point rather than pushing the text to the right. Characters bound to
backward-delete-char replace the character before point with a
space.
By default, this command is unbound.
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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. | [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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. | [ < ] | [ > ] | [ << ] | [ Up ] | [ >> ] | [Top] | [Contents] | [Index] | [ ? ] |
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. If a numeric argument is supplied, this command acts as a
toggle: if the characters at the beginning of the line do not match the value
of comment-begin, the value is inserted, otherwise the characters
in comment-begin are deleted from the beginning of the line. In
either case, the line is accepted as if a newline had been typed. The default
value of comment-begin causes this command to make the current
line a shell comment. If a numeric argument causes the comment character to be
removed, the line will be executed by the shell.
dump-functions ()
dump-variables ()
dump-macros ()
glob-complete-word (M-g)
glob-expand-word (C-x *)
glob-list-expansions (C-x g)
glob-expand-word is displayed, and the line is redrawn. If a
numeric argument is supplied, a `*' is appended before pathname
expansion.
display-shell-version (C-x C-v)
shell-expand-line (M-C-e)
history-expand-line (M-^)
magic-space ()
alias-expand-line ()
history-and-alias-expand-line ()
insert-last-argument (M-. or M-_)
yank-last-arg.
operate-and-get-next (C-o)
edit-and-execute-command (C-xC-e)
$FCEDIT, $EDITOR, and emacs as the
editor, in that order.
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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