File: bash.info, Node: Top, Next: Introduction, Prev: (dir), Up: (dir)
Bash Features
*************
This text is a brief description of the features that are present in
the Bash shell (version 3.0, 27 July 2004)..
This is Edition 3.0, last updated 27 July 2004, of `The GNU Bash
Reference Manual', for `Bash', Version 3.0.
Bash contains features that appear in other popular shells, and some
features that only appear in Bash. Some of the shells that Bash has
borrowed concepts from are the Bourne Shell (`sh'), the Korn Shell
(`ksh'), and the C-shell (`csh' and its successor, `tcsh'). The
following menu breaks the features up into categories based upon which
one of these other shells inspired the feature.
This manual is meant as a brief introduction to features found in
Bash. The Bash manual page should be used as the definitive reference
on shell behavior.
* Menu:
* Introduction:: An introduction to the shell.
* Definitions:: Some definitions used in the rest of this
manual.
* Basic Shell Features:: The shell "building blocks".
* Shell Builtin Commands:: Commands that are a part of the shell.
* Shell Variables:: Variables used or set by Bash.
* Bash Features:: Features found only in Bash.
* Job Control:: A chapter describing what job control is
and how Bash allows you to use it.
* Using History Interactively:: Chapter dealing with history expansion
rules.
* Command Line Editing:: Chapter describing the command line
editing features.
* Installing Bash:: How to build and install Bash on your system.
* Reporting Bugs:: How to report bugs in Bash.
* Major Differences From The Bourne Shell:: A terse list of the differences
between Bash and historical
versions of /bin/sh.
* Copying This Manual:: Copying this manual.
* Builtin Index:: Index of Bash builtin commands.
* Reserved Word Index:: Index of Bash reserved words.
* 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.
File: bash.info, Node: Introduction, Next: Definitions, Prev: Top, Up: Top
1 Introduction
**************
* Menu:
* What is Bash?:: A short description of Bash.
* What is a shell?:: A brief introduction to shells.
File: bash.info, Node: What is Bash?, Next: What is a shell?, Up: Introduction
1.1 What is Bash?
=================
Bash is the shell, or command language interpreter, for the GNU
operating system. The name is an acronym for the `Bourne-Again SHell',
a pun on Stephen Bourne, the author of the direct ancestor of the
current Unix shell `sh', which appeared in the Seventh Edition Bell
Labs Research version of Unix.
Bash is largely compatible with `sh' and incorporates useful
features from the Korn shell `ksh' and the C shell `csh'. It is
intended to be a conformant implementation of the IEEE POSIX Shell and
Tools specification (IEEE Working Group 1003.2). It offers functional
improvements over `sh' for both interactive and programming use.
While the GNU operating system provides other shells, including a
version of `csh', Bash is the default shell. Like other GNU software,
Bash is quite portable. It currently runs on nearly every version of
Unix and a few other operating systems - independently-supported ports
exist for MS-DOS, OS/2, and Windows platforms.
File: bash.info, Node: What is a shell?, Prev: What is Bash?, Up: Introduction
1.2 What is a shell?
====================
At its base, a shell is simply a macro processor that executes
commands. The term macro processor means functionality where text and
symbols are expanded to create larger expressions.
A Unix shell is both a command interpreter and a programming
language. As a command interpreter, the shell provides the user
interface to the rich set of GNU utilities. The programming language
features allow these utilitites to be combined. Files containing
commands can be created, and become commands themselves. These new
commands have the same status as system commands in directories such as
`/bin', allowing users or groups to establish custom environments to
automate their common tasks.
Shells may be used interactively or non-interactively. In
interactive mode, they accept input typed from the keyboard. When
executing non-interactively, shells execute commands read from a file.
A shell allows execution of GNU commands, both synchronously and
asynchronously. The shell waits for synchronous commands to complete
before accepting more input; asynchronous commands continue to execute
in parallel with the shell while it reads and executes additional
commands. The "redirection" constructs permit fine-grained control of
the input and output of those commands. Moreover, the shell allows
control over the contents of commands' environments.
Shells also provide a small set of built-in commands ("builtins")
implementing functionality impossible or inconvenient to obtain via
separate utilities. For example, `cd', `break', `continue', and
`exec') cannot be implemented outside of the shell because they
directly manipulate the shell itself. The `history', `getopts',
`kill', or `pwd' builtins, among others, could be implemented in
separate utilities, but they are more convenient to use as builtin
commands. All of the shell builtins are described in subsequent
sections.
While executing commands is essential, most of the power (and
complexity) of shells is due to their embedded programming languages.
Like any high-level language, the shell provides variables, flow
control constructs, quoting, and functions.
Shells offer features geared specifically for interactive use rather
than to augment the programming language. These interactive features
include job control, command line editing, command history and aliases.
Each of these features is described in this manual.
File: bash.info, Node: Definitions, Next: Basic Shell Features, Prev: Introduction, Up: Top
2 Definitions
*************
These definitions are used throughout the remainder of this manual.
`POSIX'
A family of open system standards based on Unix. Bash is
concerned with POSIX 1003.2, the Shell and Tools Standard.
`blank'
A space or tab character.
`builtin'
A command that is implemented internally by the shell itself,
rather than by an executable program somewhere in the file system.
`control operator'
A `word' that performs a control function. It is a `newline' or
one of the following: `||', `&&', `&', `;', `;;', `|', `(', or `)'.
`exit status'
The value returned by a command to its caller. The value is
restricted to eight bits, so the maximum value is 255.
`field'
A unit of text that is the result of one of the shell expansions.
After expansion, when executing a command, the resulting fields
are used as the command name and arguments.
`filename'
A string of characters used to identify a file.
`job'
A set of processes comprising a pipeline, and any processes
descended from it, that are all in the same process group.
`job control'
A mechanism by which users can selectively stop (suspend) and
restart (resume) execution of processes.
`metacharacter'
A character that, when unquoted, separates words. A metacharacter
is a `blank' or one of the following characters: `|', `&', `;',
`(', `)', `<', or `>'.
`name'
A `word' consisting solely of letters, numbers, and underscores,
and beginning with a letter or underscore. `Name's are used as
shell variable and function names. Also referred to as an
`identifier'.
`operator'
A `control operator' or a `redirection operator'. *Note
Redirections::, for a list of redirection operators.
`process group'
A collection of related processes each having the same process
group ID.
`process group ID'
A unique identifer that represents a `process group' during its
lifetime.
`reserved word'
A `word' that has a special meaning to the shell. Most reserved
words introduce shell flow control constructs, such as `for' and
`while'.
`return status'
A synonym for `exit status'.
`signal'
A mechanism by which a process may be notified by the kernel of an
event occurring in the system.
`special builtin'
A shell builtin command that has been classified as special by the
POSIX 1003.2 standard.
`token'
A sequence of characters considered a single unit by the shell.
It is either a `word' or an `operator'.
`word'
A `token' that is not an `operator'.
File: bash.info, Node: Basic Shell Features, Next: Shell Builtin Commands, Prev: Definitions, Up: Top
3 Basic Shell Features
**********************
Bash is an acronym for `Bourne-Again SHell'. The Bourne shell is the
traditional Unix shell originally written by Stephen Bourne. All of
the Bourne shell builtin commands are available in Bash, The rules for
evaluation and quoting are taken from the POSIX specification for the
`standard' Unix shell.
This chapter briefly summarizes the shell's `building blocks':
commands, control structures, shell functions, shell parameters, shell
expansions, redirections, which are a way to direct input and output
from and to named files, and how the shell executes commands.
* Menu:
* Shell Syntax:: What your input means to the shell.
* Shell Commands:: The types of commands you can use.
* Shell Functions:: Grouping commands by name.
* Shell Parameters:: How the shell stores values.
* Shell Expansions:: How Bash expands parameters and the various
expansions available.
* Redirections:: A way to control where input and output go.
* Executing Commands:: What happens when you run a command.
* Shell Scripts:: Executing files of shell commands.
File: bash.info, Node: Shell Syntax, Next: Shell Commands, Up: Basic Shell Features
3.1 Shell Syntax
================
* Menu:
* Shell Operation:: The basic operation of the shell.
* Quoting:: How to remove the special meaning from characters.
* 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.
File: bash.info, Node: Shell Operation, Next: Quoting, Up: Shell Syntax
3.1.1 Shell Operation
---------------------
The following is a brief description of the shell's operation when it
reads and executes a command. Basically, the shell does the following:
1. Reads its input from a file (*note Shell Scripts::), from a string
supplied as an argument to the `-c' invocation option (*note
Invoking Bash::), or from the user's terminal.
2. Breaks the input into words and operators, obeying the quoting
rules described in *Note Quoting::. These tokens are separated by
`metacharacters'. Alias expansion is performed by this step
(*note Aliases::).
3. Parses the tokens into simple and compound commands (*note Shell
Commands::).
4. Performs the various shell expansions (*note Shell Expansions::),
breaking the expanded tokens into lists of filenames (*note
Filename Expansion::) and commands and arguments.
5. Performs any necessary redirections (*note Redirections::) and
removes the redirection operators and their operands from the
argument list.
6. Executes the command (*note Executing Commands::).
7. Optionally waits for the command to complete and collects its exit
status (*note Exit Status::).
File: bash.info, Node: Quoting, Next: Comments, Prev: Shell Operation, Up: Shell Syntax
3.1.2 Quoting
-------------
* Menu:
* Escape Character:: How to remove the special meaning from a single
character.
* Single Quotes:: How to inhibit all interpretation of a sequence
of characters.
* Double Quotes:: How to suppress most of the interpretation of a
sequence of characters.
* ANSI-C Quoting:: How to expand ANSI-C sequences in quoted strings.
* Locale 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 (*note 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. *Note Bash History Facilities::, for more details
concerning history expansion.
There are three quoting mechanisms: the ESCAPE CHARACTER, single
quotes, and double quotes.
File: bash.info, Node: Escape Character, Next: Single Quotes, Up: Quoting
3.1.2.1 Escape Character
........................
A non-quoted backslash `\' is the Bash escape character. It preserves
the literal value of the next character that follows, with the
exception of `newline'. If a `\newline' pair appears, and the
backslash itself is not quoted, the `\newline' is treated as a line
continuation (that is, it is removed from the input stream and
effectively ignored).
File: bash.info, Node: Single Quotes, Next: Double Quotes, Prev: Escape Character, Up: Quoting
3.1.2.2 Single Quotes
.....................
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.
File: bash.info, Node: Double Quotes, Next: ANSI-C Quoting, Prev: Single Quotes, Up: Quoting
3.1.2.3 Double Quotes
.....................
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 (*note Shell Expansions::). The backslash retains
its special meaning only when followed by one of the following
characters: `$', ``', `"', `\', or `newline'. Within double quotes,
backslashes that are followed by one of these characters are removed.
Backslashes preceding characters without a special meaning are left
unmodified. A double quote may be quoted within double quotes by
preceding it with a backslash. When command history is being used, the
double quote may not be used to quote the history expansion character.
The special parameters `*' and `@' have special meaning when in
double quotes (*note Shell Parameter Expansion::).
File: bash.info, Node: ANSI-C Quoting, Next: Locale Translation, Prev: Double Quotes, Up: Quoting
3.1.2.4 ANSI-C Quoting
......................
Words of the form `$'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'
alert (bell)
`\b'
backspace
`\e'
an escape character (not ANSI C)
`\f'
form feed
`\n'
newline
`\r'
carriage return
`\t'
horizontal tab
`\v'
vertical tab
`\\'
backslash
`\''
single quote
`\NNN'
the eight-bit character whose value is the octal value NNN (one to
three digits)
`\xHH'
the eight-bit character whose value is the hexadecimal value HH
(one or two hex digits)
`\cX'
a control-X character
The expanded result is single-quoted, as if the dollar sign had not
been present.
File: bash.info, Node: Locale Translation, Prev: ANSI-C Quoting, Up: Quoting
3.1.2.5 Locale-Specific Translation
...................................
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.
File: bash.info, Node: Comments, Prev: Quoting, Up: Shell Syntax
3.1.3 Comments
--------------
In a non-interactive shell, or an interactive shell in which the
`interactive_comments' option to the `shopt' builtin is enabled (*note
Bash Builtins::), a word beginning with `#' causes that word and all
remaining characters on that line to be ignored. An interactive shell
without the `interactive_comments' option enabled does not allow
comments. The `interactive_comments' option is on by default in
interactive shells. *Note Interactive Shells::, for a description of
what makes a shell interactive.
File: bash.info, Node: Shell Commands, Next: Shell Functions, Prev: Shell Syntax, Up: Basic Shell Features
3.2 Shell Commands
==================
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.
* Menu:
* Simple Commands:: The most common type of command.
* Pipelines:: Connecting the input and output of several
commands.
* Lists:: How to execute commands sequentially.
* Compound Commands:: Shell commands for control flow.
File: bash.info, Node: Simple Commands, Next: Pipelines, Up: Shell Commands
3.2.1 Simple Commands
---------------------
A simple command is the kind of command encountered most often. It's
just a sequence of words separated by `blank's, terminated by one of
the shell's control operators (*note 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 (*note 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.
File: bash.info, Node: Pipelines, Next: Lists, Prev: Simple Commands, Up: Shell Commands
3.2.2 Pipelines
---------------
A `pipeline' is a sequence of simple commands separated by `|'.
The format for a pipeline is
[`time' [`-p']] [`!'] COMMAND1 [`|' COMMAND2 ...]
The output of each command in the pipeline is connected via a pipe to
the input of the next command. That is, each command reads the
previous command's output.
The reserved word `time' causes timing statistics to be printed for
the pipeline once it finishes. The statistics currently consist of
elapsed (wall-clock) time and user and system time consumed by the
command's execution. The `-p' option changes the output format to that
specified by POSIX. The `TIMEFORMAT' variable may be set to a format
string that specifies how the timing information should be displayed.
*Note 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 (*note Lists::), the
shell waits for all commands in the pipeline to complete.
Each command in a pipeline is executed in its own subshell (*note
Command Execution Environment::). The exit status of a pipeline is the
exit status of the last command in the pipeline, unless the `pipefail'
option is enabled (*note The Set Builtin::). If `pipefail' is enabled,
the pipeline's return status is the value of the last (rightmost)
command to exit with a non-zero status, or zero if all commands exit
successfully. If the reserved word `!' precedes the pipeline, the exit
status is the logical negation of the exit status as described above.
The shell waits for all commands in the pipeline to terminate before
returning a value.
File: bash.info, Node: Lists, Next: Compound Commands, Prev: Pipelines, Up: Shell Commands
3.2.3 Lists of Commands
-----------------------
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 (*note 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.
File: bash.info, Node: Compound Commands, Prev: Lists, Up: Shell Commands
3.2.4 Compound Commands
-----------------------
* Menu:
* Looping Constructs:: Shell commands for iterative action.
* Conditional Constructs:: Shell commands for conditional execution.
* Command Grouping:: Ways to group commands.
Compound commands are the shell programming constructs. Each
construct begins with a reserved word or control operator and is
terminated by a corresponding reserved word or operator. Any
redirections (*note Redirections::) associated with a compound command
apply to all commands within that compound command unless explicitly
overridden.
Bash provides looping constructs, conditional commands, and
mechanisms to group commands and execute them as a unit.
File: bash.info, Node: Looping Constructs, Next: Conditional Constructs, Up: Compound Commands
3.2.4.1 Looping Constructs
..........................
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'
The syntax of the `until' command is:
until TEST-COMMANDS; do CONSEQUENT-COMMANDS; done
Execute CONSEQUENT-COMMANDS as long as TEST-COMMANDS has an exit
status which is not zero. The return status is the exit status of
the last command executed in CONSEQUENT-COMMANDS, or zero if none
was executed.
`while'
The syntax of the `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'
The syntax of the `for' command is:
for NAME [in WORDS ...]; do COMMANDS; done
Expand WORDS, and execute COMMANDS once for each member in the
resultant list, with NAME bound to the current member. If `in
WORDS' is not present, the `for' command executes the COMMANDS
once for each positional parameter that is set, as if `in "$@"'
had been specified (*note Special Parameters::). The return
status is the exit status of the last command that executes. If
there are no items in the expansion of WORDS, no commands are
executed, and the return status is zero.
An alternate form of the `for' command is also supported:
for (( EXPR1 ; EXPR2 ; EXPR3 )) ; do COMMANDS ; done
First, the arithmetic expression EXPR1 is evaluated according to
the rules described below (*note Shell Arithmetic::). The
arithmetic expression EXPR2 is then evaluated repeatedly until it
evaluates to zero. Each time EXPR2 evaluates to a non-zero value,
COMMANDS are executed and the arithmetic expression EXPR3 is
evaluated. If any expression is omitted, it behaves as if it
evaluates to 1. The return value is the exit status of the last
command in LIST that is executed, or false if any of the
expressions is invalid.
The `break' and `continue' builtins (*note Bourne Shell Builtins::)
may be used to control loop execution.
File: bash.info, Node: Conditional Constructs, Next: Command Grouping, Prev: Looping Constructs, Up: Compound Commands
3.2.4.2 Conditional Constructs
..............................
`if'
The syntax of the `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'
The syntax of the `case' command is:
`case WORD in [ [(] PATTERN [| PATTERN]...) COMMAND-LIST ;;]... esac'
`case' will selectively execute the COMMAND-LIST corresponding to
the first PATTERN that matches WORD. The `|' is used to separate
multiple patterns, and the `)' operator terminates a pattern list.
A list of patterns and an associated command-list is known as a
CLAUSE. Each clause must be terminated with `;;'. The WORD
undergoes tilde expansion, parameter expansion, command
substitution, arithmetic expansion, and quote removal before
matching is attempted. Each PATTERN undergoes tilde expansion,
parameter expansion, command substitution, and arithmetic
expansion.
There may be an arbitrary number of `case' clauses, each terminated
by a `;;'. The first pattern that matches determines the
command-list that is executed.
Here is an example using `case' in a script that could be used to
describe one interesting feature of an animal:
echo -n "Enter the name of an animal: "
read ANIMAL
echo -n "The $ANIMAL has "
case $ANIMAL in
horse | dog | cat) echo -n "four";;
man | kangaroo ) echo -n "two";;
*) echo -n "an unknown number of";;
esac
echo " legs."
The return status is zero if no PATTERN is matched. Otherwise, the
return status is the exit status of the COMMAND-LIST executed.
`select'
The `select' construct allows the easy generation of menus. It
has almost the same syntax as the `for' command:
select NAME [in WORDS ...]; do COMMANDS; done
The list of words following `in' is expanded, generating a list of
items. The set of expanded words is printed on the standard error
output stream, each preceded by a number. If the `in WORDS' is
omitted, the positional parameters are printed, as if `in "$@"'
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 (*note 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"
*Note Bash Builtins::, for a full description of the `let' builtin.
`[[...]]'
[[ EXPRESSION ]]
Return a status of 0 or 1 depending on the evaluation of the
conditional expression EXPRESSION. Expressions are composed of
the primaries described below in *Note Bash Conditional
Expressions::. Word splitting and filename expansion are not
performed on the words between the `[[' and `]]'; tilde expansion,
parameter and variable expansion, arithmetic expansion, command
substitution, process substitution, and quote removal are
performed. Conditional operators such as `-f' must be unquoted to
be recognized as primaries.
When the `==' and `!=' operators are used, the string to the right
of the operator is considered a pattern and matched according to
the rules described below in *Note Pattern Matching::. The return
value is 0 if the string matches or does not match the pattern,
respectively, and 1 otherwise. Any part of the pattern may be
quoted to force it to be matched as a string.
An additional binary operator, `=~', is available, with the same
precedence as `==' and `!='. When it is used, the string to the
right of the operator is considered an extended regular expression
and matched accordingly (as in regex3)). The return value is 0 if
the string matches the pattern, and 1 otherwise. If the regular
expression is syntactically incorrect, the conditional
expression's return value is 2. If the shell option `nocaseglob'
(see the description of `shopt' in *Note Bash Builtins::) is
enabled, the match is performed without regard to the case of
alphabetic characters. Substrings matched by parenthesized
subexpressions within the regular expression are saved in the
array variable `BASH_REMATCH'. The element of `BASH_REMATCH' with
index 0 is the portion of the string matching the entire regular
expression. The element of `BASH_REMATCH' with index N is the
portion of the string matching the Nth parenthesized subexpression.
Expressions may be combined using the following operators, listed
in decreasing order of precedence:
`( EXPRESSION )'
Returns the value of EXPRESSION. This may be used to
override the normal precedence of operators.
`! EXPRESSION'
True if EXPRESSION is false.
`EXPRESSION1 && EXPRESSION2'
True if both EXPRESSION1 and EXPRESSION2 are true.
`EXPRESSION1 || EXPRESSION2'
True if either EXPRESSION1 or EXPRESSION2 is true.
The `&&' and `||' operators do not evaluate EXPRESSION2 if the
value of EXPRESSION1 is sufficient to determine the return value
of the entire conditional expression.
File: bash.info, Node: Command Grouping, Prev: Conditional Constructs, Up: Compound Commands
3.2.4.3 Grouping Commands
.........................
Bash provides two ways to group a list of commands to be executed as a
unit. When commands are grouped, redirections may be applied to the
entire command list. For example, the output of all the commands in
the list may be redirected to a single stream.
`()'
( LIST )
Placing a list of commands between parentheses causes a subshell
environment to be created (*note Command Execution Environment::),
and each of the commands in LIST to be executed in that subshell.
Since the LIST is executed in a subshell, variable assignments do
not remain in effect after the subshell completes.
`{}'
{ LIST; }
Placing a list of commands between curly braces causes the list to
be executed in the current shell context. No subshell is created.
The semicolon (or newline) following LIST is required.
In addition to the creation of a subshell, there is a subtle
difference between these two constructs due to historical reasons. The
braces are `reserved words', so they must be separated from the LIST by
`blank's. 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.
File: bash.info, Node: Shell Functions, Next: Shell Parameters, Prev: Shell Commands, Up: Basic Shell Features
3.3 Shell Functions
===================
Shell functions are a way to group commands for later execution using a
single name for the group. They are executed just like a "regular"
command. When the name of a shell function is used as a simple command
name, the list of commands associated with that function name is
executed. Shell functions are executed in the current shell context;
no new process is created to interpret them.
Functions are declared using this syntax:
[ `function' ] NAME () COMPOUND-COMMAND [ REDIRECTIONS ]
This defines a shell function named NAME. The reserved word
`function' is optional. If the `function' reserved word is supplied,
the parentheses are optional. The BODY of the function is the compound
command COMPOUND-COMMAND (*note Compound Commands::). That command is
usually a LIST enclosed between { and }, but may be any compound
command listed above. COMPOUND-COMMAND is executed whenever NAME is
specified as the name of a command. Any redirections (*note
Redirections::) associated with the shell function are performed when
the function is executed.
The exit status of a function definition is zero unless a syntax
error occurs or a readonly function with the same name already exists.
When executed, the exit status of a function is the exit status of the
last command executed in the body.
Note that for historical reasons, in the most common usage the curly
braces that surround the body of the function must be separated from
the body by `blank's or newlines. This is because the braces are
reserved words and are only recognized as such when they are separated
by whitespace. Also, when using the braces, the LIST must be
terminated by a semicolon, a `&', or a newline.
When a function is executed, the arguments to the function become
the positional parameters during its execution (*note Positional
Parameters::). The special parameter `#' that expands to the number of
positional parameters is updated to reflect the change. Special
parameter `0' is unchanged. The first element of the `FUNCNAME'
variable is set to the name of the function while the function is
executing. All other aspects of the shell execution environment are
identical between a function and its caller with the exception that the
`DEBUG' trap below) is not inherited unless the function has been given
the `trace' attribute using the `declare' builtin or the `-o functrace'
option has been enabled with the `set' builtin, (in which case all
functions inherit the `DEBUG' trap). *Note Bourne Shell Builtins::,
for the description of the `trap' builtin.
If the builtin command `return' is executed in a function, the
function completes and execution resumes with the next command after
the function call. Any command associated with the `RETURN' trap is
executed before execution resumes. When a function completes, the
values of the positional parameters and the special parameter `#' are
restored to the values they had prior to the function's execution. If
a numeric argument is given to `return', that is the function's return
status; otherwise the function's return status is the exit status of
the last command executed before the `return'.
Variables local to the function may be declared with the `local'
builtin. These variables are visible only to the function and the
commands it invokes.
Function names and definitions may be listed with the `-f' option to
the `declare' or `typeset' builtin commands (*note Bash Builtins::).
The `-F' option to `declare' or `typeset' will list the function names
only (and optionally the source file and line number, if the `extdebug'
shell option is enabled). Functions may be exported so that subshells
automatically have them defined with the `-f' option to the `export'
builtin (*note Bourne Shell Builtins::). Note that shell functions and
variables with the same name may result in multiple identically-named
entries in the environment passed to the shell's children. Care should
be taken in cases where this may cause a problem.
Functions may be recursive. No limit is placed on the number of
recursive calls.
File: bash.info, Node: Shell Parameters, Next: Shell Expansions, Prev: Shell Functions, Up: Basic Shell Features
3.4 Shell Parameters
====================
* Menu:
* Positional Parameters:: The shell's command-line arguments.
* Special Parameters:: Parameters denoted by special characters.
A PARAMETER is an entity that stores values. It can be a `name', a
number, or one of the special characters listed below. A VARIABLE is a
parameter denoted by a `name'. A variable has a VALUE and zero or more
ATTRIBUTES. Attributes are assigned using the `declare' builtin command
(see the description of the `declare' builtin in *Note Bash Builtins::).
A parameter is set if it has been assigned a value. The null string
is a valid value. Once a variable is set, it may be unset only by using
the `unset' builtin command.
A variable may be assigned to by a statement of the form
NAME=[VALUE]
If VALUE is not given, the variable is assigned the null string. All
VALUEs undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote removal (detailed
below). If the variable has its `integer' attribute set, then VALUE is
evaluated as an arithmetic expression even if the `$((...))' expansion
is not used (*note Arithmetic Expansion::). Word splitting is not
performed, with the exception of `"$@"' as explained below. Filename
expansion is not performed. Assignment statements may also appear as
arguments to the `alias', `declare', `typeset', `export', `readonly',
and `local' builtin commands.
File: bash.info, Node: Positional Parameters, Next: Special Parameters, Up: Shell Parameters
3.4.1 Positional Parameters
---------------------------
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 (*note
Shell Builtin Commands::). The positional parameters are temporarily
replaced when a shell function is executed (*note Shell Functions::).
When a positional parameter consisting of more than a single digit
is expanded, it must be enclosed in braces.
File: bash.info, Node: Special Parameters, Prev: Positional Parameters, Up: Shell Parameters
3.4.2 Special Parameters
------------------------
The shell treats several parameters specially. These parameters may
only be referenced; assignment to them is not allowed.
`*'
Expands to the positional parameters, starting from one. When the
expansion occurs within double quotes, it expands to a single word
with the value of each parameter separated by the first character
of the `IFS' special variable. That is, `"$*"' is equivalent to
`"$1C$2C..."', where C is the first character of the value of the
`IFS' variable. If `IFS' is unset, the parameters are separated
by spaces. If `IFS' is null, the parameters are joined without
intervening separators.
`@'
Expands to the positional parameters, starting from one. When the
expansion occurs within double quotes, each parameter expands to a
separate word. That is, `"$@"' is equivalent to `"$1" "$2" ...'.
When there are no positional parameters, `"$@"' and `$@' expand to
nothing (i.e., they are removed).
`#'
Expands to the number of positional parameters in decimal.
`?'
Expands to the exit status of the most recently executed foreground
pipeline.
`-'
(A hyphen.) Expands to the current option flags as specified upon
invocation, by the `set' builtin command, or those set by the
shell itself (such as the `-i' option).
`$'
Expands to the process ID of the shell. In a `()' subshell, it
expands to the process ID of the invoking shell, not the subshell.
`!'
Expands to the process ID of the most recently executed background
(asynchronous) command.
`0'
Expands to the name of the shell or shell script. This is set at
shell initialization. If Bash is invoked with a file of commands
(*note Shell Scripts::), `$0' is set to the name of that file. If
Bash is started with the `-c' option (*note 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.
`_'
(An underscore.) At shell startup, set to the absolute filename
of the shell or shell script being executed as passed in the
argument list. Subsequently, expands to the last argument to the
previous command, after expansion. Also set to the full pathname
of each command executed and placed in the environment exported to
that command. When checking mail, this parameter holds the name
of the mail file.
File: bash.info, Node: Shell Expansions, Next: Redirections, Prev: Shell Parameters, Up: Basic Shell Features
3.5 Shell Expansions
====================
Expansion is performed on the command line after it has been split into
`token's. There are seven kinds of expansion performed:
* brace expansion
* tilde expansion
* parameter and variable expansion
* command substitution
* arithmetic expansion
* word splitting
* filename expansion
* Menu:
* Brace Expansion:: Expansion of expressions within braces.
* Tilde Expansion:: Expansion of the ~ character.
* Shell Parameter Expansion:: How Bash expands variables to their values.
* Command Substitution:: Using the output of a command as an argument.
* Arithmetic Expansion:: How to use arithmetic in shell expansions.
* Process Substitution:: A way to write and read to and from a
command.
* Word Splitting:: How the results of expansion are split into separate
arguments.
* Filename Expansion:: A shorthand for specifying filenames matching patterns.
* 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 `"$@"' (*note Special Parameters::) and `"${NAME[@]}"'
(*note Arrays::).
After all expansions, `quote removal' (*note Quote Removal::) is
performed.
File: bash.info, Node: Brace Expansion, Next: Tilde Expansion, Up: Shell Expansions
3.5.1 Brace Expansion
---------------------
Brace expansion is a mechanism by which arbitrary strings may be
generated. This mechanism is similar to FILENAME EXPANSION (*note
Filename Expansion::), but the file names generated need not exist.
Patterns to be brace expanded take the form of an optional PREAMBLE,
followed by either a series of comma-separated strings or a sequnce
expression between a pair of braces, followed by an optional POSTSCRIPT.
The preamble is prefixed to each string contained within the braces, and
the postscript is then appended to each resulting string, expanding left
to right.
Brace expansions may be nested. The results of each expanded string
are not sorted; left to right order is preserved. For example,
bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form `{X..Y}', where X and Y are
either integers or single characters. When integers are supplied, the
expression expands to each number between X and Y, inclusive. When
characters are supplied, the expression expands to each character
lexicographically between X and Y, inclusive. Note that both X and Y
must be of the same type.
Brace expansion is performed before any other expansions, and any
characters special to other expansions are preserved in the result. It
is strictly textual. Bash does not apply any syntactic interpretation
to the context of the expansion or the text between the braces. To
avoid conflicts with parameter expansion, the string `${' is not
considered eligible for brace expansion.
A correctly-formed brace expansion must contain unquoted opening and
closing braces, and at least one unquoted comma or a valid sequence
expression. Any incorrectly formed brace expansion is left unchanged.
A { or `,' may be quoted with a backslash to prevent its being
considered part of a brace expression. To avoid conflicts with
parameter expansion, the string `${' is not considered eligible for
brace expansion.
This construct is typically used as shorthand when the common prefix
of the strings to be generated is longer than in the above example:
mkdir /usr/local/src/bash/{old,new,dist,bugs}
or
chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
File: bash.info, Node: Tilde Expansion, Next: Shell Parameter Expansion, Prev: Brace Expansion, Up: Shell Expansions
3.5.2 Tilde Expansion
---------------------
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 (*note 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:
`~'
The value of `$HOME'
`~/foo'
`$HOME/foo'
`~fred/foo'
The subdirectory `foo' of the home directory of the user `fred'
`~+/foo'
`$PWD/foo'
`~-/foo'
`${OLDPWD-'~-'}/foo'
`~N'
The string that would be displayed by `dirs +N'
`~+N'
The string that would be displayed by `dirs +N'
`~-N'
The string that would be displayed by `dirs -N'
File: bash.info, Node: Shell Parameter Expansion, Next: Command Substitution, Prev: Tilde Expansion, Up: Shell Expansions
3.5.3 Shell Parameter Expansion
-------------------------------
The `$' character introduces parameter expansion, command substitution,
or arithmetic expansion. The parameter name or symbol to be expanded
may be enclosed in braces, which are optional but serve to protect the
variable to be expanded from characters immediately following it which
could be interpreted as part of the name.
When braces are used, the matching ending brace is the first `}' not
escaped by a backslash or within a quoted string, and not within an
embedded arithmetic expansion, command substitution, or parameter
expansion.
The basic form of parameter expansion is ${PARAMETER}. The value of
PARAMETER is substituted. The braces are required when PARAMETER is a
positional parameter with more than one digit, or when PARAMETER is
followed by a character that is not to be interpreted as part of its
name.
If the first character of PARAMETER is an exclamation point, a level
of variable indirection is introduced. Bash uses the value of the
variable formed from the rest of PARAMETER as the name of the variable;
this variable is then expanded and that value is used in the rest of
the substitution, rather than the value of PARAMETER itself. This is
known as `indirect expansion'. The exceptions to this are the
expansions of ${!PREFIX*} and ${!NAME[@]} described below. The
exclamation point must immediately follow the left brace in order to
introduce indirection.
In each of the cases below, WORD is subject to tilde expansion,
parameter expansion, command substitution, and arithmetic expansion.
When not performing substring expansion, Bash tests for a parameter
that is unset or null; omitting the colon results in a test only for a
parameter that is unset. Put another way, if the colon is included,
the operator tests for both existence and that the value is not null;
if the colon is omitted, the operator tests only for existence.
`${PARAMETER:-WORD}'
If PARAMETER is unset or null, the expansion of WORD is
substituted. Otherwise, the value of PARAMETER is substituted.
`${PARAMETER:=WORD}'
If PARAMETER is unset or null, the expansion of WORD is assigned
to PARAMETER. The value of PARAMETER is then substituted.
Positional parameters and special parameters may not be assigned to
in this way.
`${PARAMETER:?WORD}'
If PARAMETER is null or unset, the expansion of WORD (or a message
to that effect if WORD is not present) is written to the standard
error and the shell, if it is not interactive, exits. Otherwise,
the value of PARAMETER is substituted.
`${PARAMETER:+WORD}'
If PARAMETER is null or unset, nothing is substituted, otherwise
the expansion of WORD is substituted.
`${PARAMETER:OFFSET}'
`${PARAMETER:OFFSET:LENGTH}'
Expands to up to LENGTH characters of PARAMETER starting at the
character specified by OFFSET. If LENGTH is omitted, expands to
the substring of PARAMETER starting at the character specified by
OFFSET. LENGTH and OFFSET are arithmetic expressions (*note Shell
Arithmetic::). This is referred to as Substring Expansion.
LENGTH must evaluate to a number greater than or equal to zero.
If OFFSET evaluates to a number less than zero, the value is used
as an offset from the end of the value of PARAMETER. If PARAMETER
is `@', the result is LENGTH positional parameters beginning at
OFFSET. If PARAMETER is an array name indexed by `@' or `*', the
result is the LENGTH members of the array beginning with
`${PARAMETER[OFFSET]}'. Substring indexing is zero-based unless
the positional parameters are used, in which case the indexing
starts at 1.
`${!PREFIX*}'
`${!PREFIX@}'
Expands to the names of variables whose names begin with PREFIX,
separated by the first character of the `IFS' special variable.
`${!NAME[@]}'
`${!NAME[*]}'
If NAME is an array variable, expands to the list of array indices
(keys) assigned in NAME. If NAME is not an array, expands to 0 if
NAME is set and null otherwise. When `@' is used and the
expansion appears within double quotes, each key expands to a
separate word.
`${#PARAMETER}'
The length in characters of the expanded value of PARAMETER is
substituted. If PARAMETER is `*' or `@', the value substituted is
the number of positional parameters. If PARAMETER is an array
name subscripted by `*' or `@', the value substituted is the
number of elements in the array.
`${PARAMETER#WORD}'
`${PARAMETER##WORD}'
The WORD is expanded to produce a pattern just as in filename
expansion (*note Filename Expansion::). If the pattern matches
the beginning of the expanded value of PARAMETER, then the result
of the expansion is the expanded value of PARAMETER with the
shortest matching pattern (the `#' case) or the longest matching
pattern (the `##' case) deleted. If PARAMETER is `@' or `*', the
pattern removal operation is applied to each positional parameter
in turn, and the expansion is the resultant list. If PARAMETER is
an array variable subscripted with `@' or `*', the pattern removal
operation is applied to each member of the array in turn, and the
expansion is the resultant list.
`${PARAMETER%WORD}'
`${PARAMETER%%WORD}'
The WORD is expanded to produce a pattern just as in filename
expansion. If the pattern matches a trailing portion of the
expanded value of PARAMETER, then the result of the expansion is
the value of PARAMETER with the shortest matching pattern (the `%'
case) or the longest matching pattern (the `%%' case) deleted. If
PARAMETER is `@' or `*', the pattern removal operation is applied
to each positional parameter in turn, and the expansion is the
resultant list. If PARAMETER is an array variable subscripted
with `@' or `*', the pattern removal operation is applied to each
member of the array in turn, and the expansion is the resultant
list.
`${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.
File: bash.info, Node: Command Substitution, Next: Arithmetic Expansion, Prev: Shell Parameter Expansion, Up: Shell Expansions
3.5.4 Command Substitution
--------------------------
Command substitution allows the output of a command to replace the
command itself. Command substitution occurs when a command is enclosed
as follows:
$(COMMAND)
or
`COMMAND`
Bash performs the expansion by executing COMMAND and replacing the
command substitution with the standard output of the command, with any
trailing newlines deleted. Embedded newlines are not deleted, but they
may be removed during word splitting. The command substitution `$(cat
FILE)' can be replaced by the equivalent but faster `$(< FILE)'.
When the old-style backquote form of substitution is used, backslash
retains its literal meaning except when followed by `$', ``', or `\'.
The first backquote not preceded by a backslash terminates the command
substitution. When using the `$(COMMAND)' form, all characters between
the parentheses make up the command; none are treated specially.
Command substitutions may be nested. To nest when using the
backquoted form, escape the inner backquotes with backslashes.
If the substitution appears within double quotes, word splitting and
filename expansion are not performed on the results.
File: bash.info, Node: Arithmetic Expansion, Next: Process Substitution, Prev: Command Substitution, Up: Shell Expansions
3.5.5 Arithmetic Expansion
--------------------------
Arithmetic expansion allows the evaluation of an arithmetic expression
and the substitution of the result. The format for arithmetic
expansion is:
$(( EXPRESSION ))
The expression is treated as if it were within double quotes, but a
double quote inside the parentheses is not treated specially. All
tokens in the expression undergo parameter expansion, command
substitution, and quote removal. Arithmetic expansions may be nested.
The evaluation is performed according to the rules listed below
(*note Shell Arithmetic::). If the expression is invalid, Bash prints
a message indicating failure to the standard error and no substitution
occurs.
File: bash.info, Node: Process Substitution, Next: Word Splitting, Prev: Arithmetic Expansion, Up: Shell Expansions
3.5.6 Process Substitution
--------------------------
Process substitution is supported on systems that support named pipes
(FIFOs) or the `/dev/fd' method of naming open files. It takes the
form of
<(LIST)
or
>(LIST)
The process LIST is run with its input or output connected to a FIFO
or some file in `/dev/fd'. The name of this file is passed as an
argument to the current command as the result of the expansion. If the
`>(LIST)' form is used, writing to the file will provide input for
LIST. If the `<(LIST)' form is used, the file passed as an argument
should be read to obtain the output of LIST. Note that no space may
appear between the `<' or `>' and the left parenthesis, otherwise the
construct would be interpreted as a redirection.
When available, process substitution is performed simultaneously with
parameter and variable expansion, command substitution, and arithmetic
expansion.
File: bash.info, Node: Word Splitting, Next: Filename Expansion, Prev: Process Substitution, Up: Shell Expansions
3.5.7 Word Splitting
--------------------
The shell scans the results of parameter expansion, command
substitution, and arithmetic expansion that did not occur within double
quotes for word splitting.
The shell treats each character of `$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.
File: bash.info, Node: Filename Expansion, Next: Quote Removal, Prev: Word Splitting, Up: Shell Expansions
3.5.8 Filename Expansion
------------------------
* Menu:
* Pattern Matching:: How the shell matches patterns.
After word splitting, unless the `-f' option has been set (*note The
Set Builtin::), Bash scans each word for the characters `*', `?', and
`['. If one of these characters appears, then the word is regarded as
a PATTERN, and replaced with an alphabetically sorted list of file
names matching the pattern. If no matching file names are found, and
the shell option `nullglob' is disabled, the word is left unchanged.
If the `nullglob' option is set, and no matches are found, the word is
removed. If the `failglob' shell option is set, and no matches are
found, an error message is printed and the command is not executed. If
the shell option `nocaseglob' is enabled, the match is performed
without regard to the case of alphabetic characters.
When a pattern is used for filename generation, the character `.' at
the start of a filename or immediately following a slash must be
matched explicitly, unless the shell option `dotglob' is set. When
matching a file name, the slash character must always be matched
explicitly. In other cases, the `.' character is not treated specially.
See the description of `shopt' in *Note Bash Builtins::, for a
description of the `nocaseglob', `nullglob', `failglob', and `dotglob'
options.
The `GLOBIGNORE' shell variable may be used to restrict the set of
filenames matching a pattern. If `GLOBIGNORE' is set, each matching
filename that also matches one of the patterns in `GLOBIGNORE' is
removed from the list of matches. The filenames `.' and `..' are
always ignored when `GLOBIGNORE' is set and not null. However, setting
`GLOBIGNORE' to a non-null value has the effect of enabling the
`dotglob' shell option, so all other filenames beginning with a `.'
will match. To get the old behavior of ignoring filenames beginning
with a `.', make `.*' one of the patterns in `GLOBIGNORE'. The
`dotglob' option is disabled when `GLOBIGNORE' is unset.
File: bash.info, Node: Pattern Matching, Up: Filename Expansion
3.5.8.1 Pattern Matching
........................
Any character that appears in a pattern, other than the special pattern
characters described below, matches itself. The NUL character may not
occur in a pattern. A backslash escapes the following character; the
escaping backslash is discarded when matching. The special pattern
characters must be quoted if they are to be matched literally.
The special pattern characters have the following meanings:
`*'
Matches any string, including the null string.
`?'
Matches any single character.
`[...]'
Matches any one of the enclosed characters. A pair of characters
separated by a hyphen denotes a RANGE EXPRESSION; any character
that sorts between those two characters, inclusive, using the
current locale's collating sequence and character set, is matched.
If the first character following the `[' is a `!' or a `^' then
any character not enclosed is matched. A `-' may be matched by
including it as the first or last character in the set. A `]' may
be matched by including it as the first character in the set. The
sorting order of characters in range expressions is determined by
the current locale and the value of the `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
A character class matches any character belonging to that class.
The `word' character class matches letters, digits, and the
character `_'.
Within `[' and `]', an EQUIVALENCE CLASS can be specified using
the syntax `[='C`=]', which matches all characters with the same
collation weight (as defined by the current locale) as the
character C.
Within `[' and `]', the syntax `[.'SYMBOL`.]' matches the
collating symbol SYMBOL.
If the `extglob' shell option is enabled using the `shopt' builtin,
several extended pattern matching operators are recognized. In the
following description, a PATTERN-LIST is a list of one or more patterns
separated by a `|'. Composite patterns may be formed using one or more
of the following sub-patterns:
`?(PATTERN-LIST)'
Matches zero or one occurrence of the given patterns.
`*(PATTERN-LIST)'
Matches zero or more occurrences of the given patterns.
`+(PATTERN-LIST)'
Matches one or more occurrences of the given patterns.
`@(PATTERN-LIST)'
Matches exactly one of the given patterns.
`!(PATTERN-LIST)'
Matches anything except one of the given patterns.
File: bash.info, Node: Quote Removal, Prev: Filename Expansion, Up: Shell Expansions
3.5.9 Quote Removal
-------------------
After the preceding expansions, all unquoted occurrences of the
characters `\', `'', and `"' that did not result from one of the above
expansions are removed.
File: bash.info, Node: Redirections, Next: Executing Commands, Prev: Shell Expansions, Up: Basic Shell Features
3.6 Redirections
================
Before a command is executed, its input and output may be REDIRECTED
using a special notation interpreted by the shell. Redirection may
also be used to open and close files for the current shell execution
environment. The following redirection operators may precede or appear
anywhere within a simple command or may follow a command. Redirections
are processed in the order they appear, from left to right.
In the following descriptions, if the file descriptor number is
omitted, and the first character of the redirection operator is `<',
the redirection refers to the standard input (file descriptor 0). If
the first character of the redirection operator is `>', the redirection
refers to the standard output (file descriptor 1).
The word following the redirection operator in the following
descriptions, unless otherwise noted, is subjected to brace expansion,
tilde expansion, parameter expansion, command substitution, arithmetic
expansion, quote removal, filename expansion, and word splitting. If
it expands to more than one word, Bash reports an error.
Note that the order of redirections is significant. For example,
the command
ls > DIRLIST 2>&1
directs both standard output (file descriptor 1) and standard error
(file descriptor 2) to the file DIRLIST, while the command
ls 2>&1 > DIRLIST
directs only the standard output to file DIRLIST, because the
standard error was duplicated as standard output before the standard
output was redirected to DIRLIST.
Bash handles several filenames specially when they are used in
redirections, as described in the following table:
`/dev/fd/FD'
If FD is a valid integer, file descriptor FD is duplicated.
`/dev/stdin'
File descriptor 0 is duplicated.
`/dev/stdout'
File descriptor 1 is duplicated.
`/dev/stderr'
File descriptor 2 is duplicated.
`/dev/tcp/HOST/PORT'
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open a TCP
connection to the corresponding socket.
`/dev/udp/HOST/PORT'
If HOST is a valid hostname or Internet address, and PORT is an
integer port number or service name, Bash attempts to open a UDP
connection to the corresponding socket.
A failure to open or create a file causes the redirection to fail.
3.6.1 Redirecting Input
-----------------------
Redirection of input causes the file whose name results from the
expansion of WORD to be opened for reading on file descriptor `n', or
the standard input (file descriptor 0) if `n' is not specified.
The general format for redirecting input is:
[N]<WORD
3.6.2 Redirecting Output
------------------------
Redirection of output causes the file whose name results from the
expansion of WORD to be opened for writing on file descriptor N, or the
standard output (file descriptor 1) if N is not specified. If the file
does not exist it is created; if it does exist it is truncated to zero
size.
The general format for redirecting output is:
[N]>[|]WORD
If the redirection operator is `>', and the `noclobber' option to
the `set' builtin has been enabled, the redirection will fail if the
file whose name results from the expansion of WORD exists and is a
regular file. If the redirection operator is `>|', or the redirection
operator is `>' and the `noclobber' option is not enabled, the
redirection is attempted even if the file named by WORD exists.
3.6.3 Appending Redirected Output
---------------------------------
Redirection of output in this fashion causes the file whose name
results from the expansion of WORD to be opened for appending on file
descriptor N, or the standard output (file descriptor 1) if N is not
specified. If the file does not exist it is created.
The general format for appending output is:
[N]>>WORD
3.6.4 Redirecting Standard Output and Standard Error
----------------------------------------------------
Bash allows both the standard output (file descriptor 1) and the
standard error output (file descriptor 2) to be redirected to the file
whose name is the expansion of WORD with this construct.
There are two formats for redirecting standard output and standard
error:
&>WORD
and
>&WORD
Of the two forms, the first is preferred. This is semantically
equivalent to
>WORD 2>&1
3.6.5 Here Documents
--------------------
This type of redirection instructs the shell to read input from the
current source until a line containing only WORD (with no trailing
blanks) is seen. All of the lines read up to that point are then used
as the standard input for a command.
The format of here-documents is:
<<[-]WORD
HERE-DOCUMENT
DELIMITER
No parameter expansion, command substitution, arithmetic expansion,
or filename expansion is performed on WORD. If any characters in WORD
are quoted, the DELIMITER is the result of quote removal on WORD, and
the lines in the here-document are not expanded. If WORD is unquoted,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion. In the latter case,
the character sequence `\newline' is ignored, and `\' must be used to
quote the characters `\', `$', and ``'.
If the redirection operator is `<<-', then all leading tab
characters are stripped from input lines and the line containing
DELIMITER. This allows here-documents within shell scripts to be
indented in a natural fashion.
3.6.6 Here Strings
------------------
A variant of here documents, the format is:
<<< WORD
The WORD is expanded and supplied to the command on its standard
input.
3.6.7 Duplicating File Descriptors
----------------------------------
The redirection operator
[N]<&WORD
is used to duplicate input file descriptors. If WORD expands to one
or more digits, the file descriptor denoted by N is made to be a copy
of that file descriptor. If the digits in WORD do not specify a file
descriptor open for input, a redirection error occurs. If WORD
evaluates to `-', file descriptor N is closed. If N is not specified,
the standard input (file descriptor 0) is used.
The operator
[N]>&WORD
is used similarly to duplicate output file descriptors. If N is not
specified, the standard output (file descriptor 1) is used. If the
digits in WORD do not specify a file descriptor open for output, a
redirection error occurs. As a special case, if N is omitted, and WORD
does not expand to one or more digits, the standard output and standard
error are redirected as described previously.
3.6.8 Moving File Descriptors
-----------------------------
The redirection operator
[N]<&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the
standard input (file descriptor 0) if N is not specified. DIGIT is
closed after being duplicated to N.
Similarly, the redirection operator
[N]>&DIGIT-
moves the file descriptor DIGIT to file descriptor N, or the
standard output (file descriptor 1) if N is not specified.
3.6.9 Opening File Descriptors for Reading and Writing
------------------------------------------------------
The redirection operator
[N]<>WORD
causes the file whose name is the expansion of WORD to be opened for
both reading and writing on file descriptor N, or on file descriptor 0
if N is not specified. If the file does not exist, it is created.
File: bash.info, Node: Executing Commands, Next: Shell Scripts, Prev: Redirections, Up: Basic Shell Features
3.7 Executing Commands
======================
* Menu:
* Simple Command Expansion:: How Bash expands simple commands before
executing them.
* Command Search and Execution:: How Bash finds commands and runs them.
* Command Execution Environment:: The environment in which Bash
executes commands that are not
shell builtins.
* Environment:: The environment given to a command.
* Exit Status:: The status returned by commands and how Bash
interprets it.
* Signals:: What happens when Bash or a command it runs
receives a signal.
File: bash.info, Node: Simple Command Expansion, Next: Command Search and Execution, Up: Executing Commands
3.7.1 Simple Command Expansion
------------------------------
When a simple command is executed, the shell performs the following
expansions, assignments, and redirections, from left to right.
1. The words that the parser has marked as variable assignments (those
preceding the command name) and redirections are saved for later
processing.
2. The words that are not variable assignments or redirections are
expanded (*note Shell Expansions::). If any words remain after
expansion, the first word is taken to be the name of the command
and the remaining words are the arguments.
3. Redirections are performed as described above (*note
Redirections::).
4. The text after the `=' in each variable assignment undergoes tilde
expansion, parameter expansion, command substitution, arithmetic
expansion, and quote removal before being assigned to the variable.
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.
File: bash.info, Node: Command Search and Execution, Next: Command Execution Environment, Prev: Simple Command Expansion, Up: Executing Commands
3.7.2 Command Search and Execution
----------------------------------
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.
1. If the command name contains no slashes, the shell attempts to
locate it. If there exists a shell function by that name, that
function is invoked as described in *Note Shell Functions::.
2. If the name does not match a function, the shell searches for it
in the list of shell builtins. If a match is found, that builtin
is invoked.
3. If the name is neither a shell function nor a builtin, and
contains no slashes, Bash searches each element of `$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
*Note 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.
4. If the search is successful, or if the command name contains one
or more slashes, the shell executes the named program in a
separate execution environment. Argument 0 is set to the name
given, and the remaining arguments to the command are set to the
arguments supplied, if any.
5. If this execution fails because the file is not in executable
format, and the file is not a directory, it is assumed to be a
SHELL SCRIPT and the shell executes it as described in *Note Shell
Scripts::.
6. If the command was not begun asynchronously, the shell waits for
the command to complete and collects its exit status.
File: bash.info, Node: Command Execution Environment, Next: Environment, Prev: Command Search and Execution, Up: Executing Commands
3.7.3 Command Execution Environment
-----------------------------------
The shell has an EXECUTION ENVIRONMENT, which consists of the following:
* open files inherited by the shell at invocation, as modified by
redirections supplied to the `exec' builtin
* the current working directory as set by `cd', `pushd', or `popd',
or inherited by the shell at invocation
* the file creation mode mask as set by `umask' or inherited from
the shell's parent
* current traps set by `trap'
* shell parameters that are set by variable assignment or with `set'
or inherited from the shell's parent in the environment
* shell functions defined during execution or inherited from the
shell's parent in the environment
* options enabled at invocation (either by default or with
command-line arguments) or by `set'
* options enabled by `shopt'
* shell aliases defined with `alias' (*note Aliases::)
* various process IDs, including those of background jobs (*note
Lists::), the value of `$$', 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.
* the shell's open files, plus any modifications and additions
specified by redirections to the command
* the current working directory
* the file creation mode mask
* shell variables and functions marked for export, along with
variables exported for the command, passed in the environment
(*note Environment::)
* traps caught by the shell are reset to the values inherited from
the shell's parent, and traps ignored by the shell are ignored
A command invoked in this separate environment cannot affect the
shell's execution environment.
Command substitution, commands grouped with parentheses, and
asynchronous commands are invoked in a subshell environment that is a
duplicate of the shell environment, except that traps caught by the
shell are reset to the values that the shell inherited from its parent
at invocation. Builtin commands that are invoked as part of a pipeline
are also executed in a subshell environment. Changes made to the
subshell environment cannot affect the shell's execution environment.
If a command is followed by a `&' and job control is not active, the
default standard input for the command is the empty file `/dev/null'.
Otherwise, the invoked command inherits the file descriptors of the
calling shell as modified by redirections.
File: bash.info, Node: Environment, Next: Exit Status, Prev: Command Execution Environment, Up: Executing Commands
3.7.4 Environment
-----------------
When a program is invoked it is given an array of strings called the
ENVIRONMENT. This is a list of name-value pairs, of the form
`name=value'.
Bash 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
*Note Shell Parameters::. These assignment statements affect only the
environment seen by that command.
If the `-k' option is set (*note 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.
File: bash.info, Node: Exit Status, Next: Signals, Prev: Environment, Up: Executing Commands
3.7.5 Exit Status
-----------------
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 (*note
Conditional Constructs::) and some of the list constructs (*note
Lists::).
All of the Bash builtins return an exit status of zero if they
succeed and a non-zero status on failure, so they may be used by the
conditional and list constructs. All builtins return an exit status of
2 to indicate incorrect usage.
File: bash.info, Node: Signals, Prev: Exit Status, Up: Executing Commands
3.7.6 Signals
-------------
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 (*note Job Control::), Bash ignores `SIGTTIN', `SIGTTOU',
and `SIGTSTP'.
Non-builtin commands started by Bash have signal handlers set to the
values inherited by the shell from its parent. When job control is not
in effect, asynchronous commands ignore `SIGINT' and `SIGQUIT' in
addition to these inherited handlers. Commands run as a result of
command substitution ignore the keyboard-generated job control signals
`SIGTTIN', `SIGTTOU', and `SIGTSTP'.
The shell exits by default upon receipt of a `SIGHUP'. Before
exiting, an interactive shell resends the `SIGHUP' to all jobs, running
or stopped. Stopped jobs are sent `SIGCONT' to ensure that they receive
the `SIGHUP'. To prevent the shell from sending the `SIGHUP' signal to
a particular job, it should be removed from the jobs table with the
`disown' builtin (*note Job Control Builtins::) or marked to not
receive `SIGHUP' using `disown -h'.
If the `huponexit' shell option has been set with `shopt' (*note
Bash Builtins::), Bash sends a `SIGHUP' to all jobs when an interactive
login shell exits.
If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until the
command completes. When Bash is waiting for an asynchronous command
via the `wait' builtin, the reception of a signal for which a trap has
been set will cause the `wait' builtin to return immediately with an
exit status greater than 128, immediately after which the trap is
executed.
File: bash.info, Node: Shell Scripts, Prev: Executing Commands, Up: Basic Shell Features
3.8 Shell Scripts
=================
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 (*note Invoking Bash::),
Bash reads and executes commands from the file, then exits. This mode
of operation creates a non-interactive shell. The shell first searches
for the file in the current directory, and looks in the directories in
`$PATH' if not found there.
When Bash runs a shell script, it sets the special parameter `0' to
the name of the file, rather than the name of the shell, and the
positional parameters are set to the remaining arguments, if any are
given. If no additional arguments are supplied, the positional
parameters are unset.
A shell script may be made executable by using the `chmod' command
to turn on the execute bit. When Bash finds such a file while
searching the `$PATH' for a command, it spawns a subshell to execute
it. In other words, executing
filename ARGUMENTS
is equivalent to executing
bash filename ARGUMENTS
if `filename' is an executable shell script. This subshell
reinitializes itself, so that the effect is as if a new shell had been
invoked to interpret the script, with the exception that the locations
of commands remembered by the parent (see the description of `hash' in
*Note 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.
File: bash.info, Node: Shell Builtin Commands, Next: Shell Variables, Prev: Basic Shell Features, Up: Top
4 Shell Builtin Commands
************************
* Menu:
* Bourne Shell Builtins:: Builtin commands inherited from the Bourne
Shell.
* Bash Builtins:: Table of builtins specific to Bash.
* The Set Builtin:: This builtin is so overloaded it
deserves its own section.
* 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
(*note 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
(*note Job Control Builtins::), the directory stack (*note Directory
Stack Builtins::), the command history (*note Bash History Builtins::),
and the programmable completion facilities (*note Programmable
Completion Builtins::).
Many of the builtins have been extended by POSIX or Bash.
Unless otherwise noted, each builtin command documented as accepting
options preceded by `-' accepts `--' to signify the end of the options.
File: bash.info, Node: Bourne Shell Builtins, Next: Bash Builtins, Up: Shell Builtin Commands
4.1 Bourne Shell Builtins
=========================
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]
Do nothing beyond expanding ARGUMENTS and performing redirections.
The return status is zero.
`. (a period)'
. FILENAME [ARGUMENTS]
Read and execute commands from the FILENAME argument in the
current shell context. If FILENAME does not contain a slash, the
`PATH' variable is used to find FILENAME. When Bash is not in
POSIX mode, the current directory is searched if FILENAME is not
found in `$PATH'. If any ARGUMENTS are supplied, they become the
positional parameters when FILENAME is executed. Otherwise the
positional parameters are unchanged. The return status is the
exit status of the last command executed, or zero if no commands
are executed. If FILENAME is not found, or cannot be read, the
return status is non-zero. This builtin is equivalent to `source'.
`break'
break [N]
Exit from a `for', `while', `until', or `select' loop. If N is
supplied, the Nth enclosing loop is exited. N must be greater
than or equal to 1. The return status is zero unless N is not
greater than or equal to 1.
`cd'
cd [-L|-P] [DIRECTORY]
Change the current working directory to DIRECTORY. If DIRECTORY
is not given, the value of the `HOME' shell variable is used. If
the shell variable `CDPATH' exists, it is used as a search path.
If DIRECTORY begins with a slash, `CDPATH' is not used.
The `-P' option means to not follow symbolic links; symbolic links
are followed by default or with the `-L' option. If DIRECTORY is
`-', it is equivalent to `$OLDPWD'.
If a non-empty directory name from `CDPATH' is used, or if `-' is
the first argument, and the directory change is successful, the
absolute pathname of the new working directory is written to the
standard output.
The return status is zero if the directory is successfully changed,
non-zero otherwise.
`continue'
continue [N]
Resume the next iteration of an enclosing `for', `while', `until',
or `select' loop. If N is supplied, the execution of the Nth
enclosing loop is resumed. N must be greater than or equal to 1.
The return status is zero unless N is not greater than or equal to
1.
`eval'
eval [ARGUMENTS]
The arguments are concatenated together into a single command,
which is then read and executed, and its exit status returned as
the exit status of `eval'. If there are no arguments or only
empty arguments, the return status is zero.
`exec'
exec [-cl] [-a NAME] [COMMAND [ARGUMENTS]]
If COMMAND is supplied, it replaces the shell without creating a
new process. If the `-l' option is supplied, the shell places a
dash at the beginning of the zeroth arg passed to COMMAND. This
is what the `login' program does. The `-c' option causes COMMAND
to be executed with an empty environment. If `-a' is supplied,
the shell passes NAME as the zeroth argument to COMMAND. If no
COMMAND is specified, redirections may be used to affect the
current shell environment. If there are no redirection errors, the
return status is zero; otherwise the return status is non-zero.
`exit'
exit [N]
Exit the shell, returning a status of N to the shell's parent. If
N is omitted, the exit status is that of the last command executed.
Any trap on `EXIT' is executed before the shell terminates.
`export'
export [-fn] [-p] [NAME[=VALUE]]
Mark each NAME to be passed to child processes in the environment.
If the `-f' option is supplied, the NAMEs refer to shell
functions; otherwise the names refer to shell variables. The `-n'
option means to no longer mark each NAME for export. If no NAMES
are supplied, or if the `-p' option is given, a list of exported
names is displayed. The `-p' option displays output in a form
that may be reused as input. If a variable name is followed by
=VALUE, the value of the variable is set to VALUE.
The return status is zero unless an invalid option is supplied,
one of the names is not a valid shell variable name, or `-f' is
supplied with a name that is not a shell function.
`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]
Remember the full pathnames of commands specified as NAME
arguments, so they need not be searched for on subsequent
invocations. The commands are found by searching through the
directories listed in `$PATH'. The `-p' option inhibits the path
search, and FILENAME is used as the location of NAME. The `-r'
option causes the shell to forget all remembered locations. The
`-d' option causes the shell to forget the remembered location of
each NAME. If the `-t' option is supplied, the full pathname to
which each NAME corresponds is printed. If multiple NAME
arguments are supplied with `-t' the NAME is printed before the
hashed full pathname. The `-l' option causes output to be
displayed in a format that may be reused as input. If no
arguments are given, or if only `-l' is supplied, information
about remembered commands is printed. The return status is zero
unless a NAME is not found or an invalid option is supplied.
`pwd'
pwd [-LP]
Print the absolute pathname of the current working directory. If
the `-P' option is supplied, the pathname printed will not contain
symbolic links. If the `-L' option is supplied, the pathname
printed may contain symbolic links. The return status is zero
unless an error is encountered while determining the name of the
current directory or an invalid option is supplied.
`readonly'
readonly [-apf] [NAME[=VALUE]] ...
Mark each NAME as readonly. The values of these names may not be
changed by subsequent assignment. If the `-f' option is supplied,
each NAME refers to a shell function. The `-a' option means each
NAME refers to an array variable. If no NAME arguments are given,
or if the `-p' option is supplied, a list of all readonly names is
printed. The `-p' option causes output to be displayed in a
format that may be reused as input. If a variable name is
followed by =VALUE, the value of the variable is set to VALUE.
The return status is zero unless an invalid option is supplied,
one of the NAME arguments is not a valid shell variable or
function name, or the `-f' option is supplied with a name that is
not a shell function.
`return'
return [N]
Cause a shell function to exit with the return value N. If N is
not supplied, the return value is the exit status of the last
command executed in the function. This may also be used to
terminate execution of a script being executed with the `.' (or
`source') builtin, returning either N or the exit status of the
last command executed within the script as the exit status of the
script. Any command associated with the `RETURN' trap is executed
before execution resumes after the function or script. The return
status is non-zero if `return' is used outside a function and not
during the execution of a script by `.' or `source'.
`shift'
shift [N]
Shift the positional parameters to the left by N. The positional
parameters from N+1 ... `$#' are renamed to `$1' ... `$#'-N+1.
Parameters represented by the numbers `$#' to N+1 are unset. N
must be a non-negative number less than or equal to `$#'. If N is
zero or greater than `$#', the positional parameters are not
changed. If N is not supplied, it is assumed to be 1. The return
status is zero unless N is greater than `$#' or less than zero,
non-zero otherwise.
`test'
`['
Evaluate a conditional expression EXPR. Each operator and operand
must be a separate argument. Expressions are composed of the
primaries described below in *Note Bash Conditional Expressions::.
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'
True if EXPR is false.
`( EXPR )'
Returns the value of EXPR. This may be used to override the
normal precedence of operators.
`EXPR1 -a EXPR2'
True if both EXPR1 and EXPR2 are true.
`EXPR1 -o EXPR2'
True if either EXPR1 or EXPR2 is true.
The `test' and `[' builtins evaluate conditional expressions using
a set of rules based on the number of arguments.
0 arguments
The expression is false.
1 argument
The expression is true if and only if the argument is not
null.
2 arguments
If the first argument is `!', the expression is true if and
only if the second argument is null. If the first argument
is one of the unary conditional operators (*note Bash
Conditional Expressions::), the expression is true if the
unary test is true. If the first argument is not a valid
unary operator, the expression is false.
3 arguments
If the second argument is one of the binary conditional
operators (*note Bash Conditional Expressions::), the result
of the expression is the result of the binary test using the
first and third arguments as operands. If the first argument
is `!', the value is the negation of the two-argument test
using the second and third arguments. If the first argument
is exactly `(' and the third argument is exactly `)', the
result is the one-argument test of the second argument.
Otherwise, the expression is false. The `-a' and `-o'
operators are considered binary operators in this case.
4 arguments
If the first argument is `!', the result is the negation of
the three-argument expression composed of the remaining
arguments. Otherwise, the expression is parsed and evaluated
according to precedence using the rules listed above.
5 or more arguments
The expression is parsed and evaluated according to precedence
using the rules listed above.
`times'
times
Print out the user and system times used by the shell and its
children. The return status is zero.
`trap'
trap [-lp] [ARG] [SIGSPEC ...]
The commands in ARG are to be read and executed when the shell
receives signal SIGSPEC. If ARG is absent (and there is a single
SIGSPEC) or equal to `-', each specified signal's disposition is
reset to the value it had when the shell was started. If ARG is
the null string, then the signal specified by each SIGSPEC is
ignored by the shell and commands it invokes. If ARG is not
present and `-p' has been supplied, the shell displays the trap
commands associated with each SIGSPEC. If no arguments are
supplied, or only `-p' is given, `trap' prints the list of commands
associated with each signal number in a form that may be reused as
shell input. The `-l' option causes the shell to print a list of
signal names and their corresponding numbers. Each SIGSPEC is
either a signal name or a signal number. Signal names are case
insensitive and the `SIG' prefix is optional. If a SIGSPEC is `0'
or `EXIT', ARG is executed when the shell exits. If a SIGSPEC is
`DEBUG', the command ARG is executed before every simple command,
`for' command, `case' command, `select' command, every arithmetic
`for' command, and before the first command executes in a shell
function. Refer to the description of the `extglob' option to the
`shopt' builtin (*note Bash Builtins::) for details of its effect
on the `DEBUG' trap. If a SIGSPEC is `ERR', the command ARG is
executed whenever a simple command has a non-zero exit status,
subject to the following conditions. The `ERR' trap is not
executed if the failed command is part of the command list
immediately following an `until' or `while' keyword, part of the
test in an `if' statement, part of a `&&' or `||' list, or if the
command's return status is being inverted using `!'. These are
the same conditions obeyed by the `errexit' option. If a SIGSPEC
is `RETURN', the command ARG is executed each time a shell
function or a script executed with the `.' or `source' builtins
finishes executing.
Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child
process when it is created.
The return status is zero unless a SIGSPEC does not specify a
valid signal.
`umask'
umask [-p] [-S] [MODE]
Set the shell process's file creation mask to MODE. If MODE
begins with a digit, it is interpreted as an octal number; if not,
it is interpreted as a symbolic mode mask similar to that accepted
by the `chmod' command. If MODE is omitted, the current value of
the mask is printed. If the `-S' option is supplied without a
MODE argument, the mask is printed in a symbolic format. If the
`-p' option is supplied, and MODE is omitted, the output is in a
form that may be reused as input. The return status is zero if
the mode is successfully changed or if no MODE argument is
supplied, and non-zero otherwise.
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]
Each variable or function NAME is removed. If no options are
supplied, or the `-v' option is given, each NAME refers to a shell
variable. If the `-f' option is given, the NAMEs refer to shell
functions, and the function definition is removed. Readonly
variables and functions may not be unset. The return status is
zero unless a NAME is readonly.
File: bash.info, Node: Bash Builtins, Next: The Set Builtin, Prev: Bourne Shell Builtins, Up: Shell Builtin Commands
4.2 Bash Builtin Commands
=========================
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 [`-p'] [NAME[=VALUE] ...]
Without arguments or with the `-p' option, `alias' prints the list
of aliases on the standard output in a form that allows them to be
reused as input. If arguments are supplied, an alias is defined
for each NAME whose VALUE is given. If no VALUE is given, the name
and value of the alias is printed. Aliases are described in *Note
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 (*note 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
(*note 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'
Use KEYMAP as the keymap to be affected by the subsequent
bindings. Acceptable 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'.
`-l'
List the names of all Readline functions.
`-p'
Display Readline function names and bindings in such a way
that they can be used as input or in a Readline
initialization file.
`-P'
List current Readline function names and bindings.
`-v'
Display Readline variable names and values in such a way that
they can be used as input or in a Readline initialization
file.
`-V'
List current Readline variable names and values.
`-s'
Display Readline key sequences bound to macros and the
strings they output in such a way that they can be used as
input or in a Readline initialization file.
`-S'
Display Readline key sequences bound to macros and the
strings they output.
`-f FILENAME'
Read key bindings from FILENAME.
`-q FUNCTION'
Query about which keys invoke the named FUNCTION.
`-u FUNCTION'
Unbind all keys bound to the named FUNCTION.
`-r KEYSEQ'
Remove any current binding for KEYSEQ.
`-x KEYSEQ:SHELL-COMMAND'
Cause SHELL-COMMAND to be executed whenever KEYSEQ is entered.
The return status is zero unless an invalid option is supplied or
an error occurs.
`builtin'
builtin [SHELL-BUILTIN [ARGS]]
Run a shell builtin, passing it ARGS, and return its exit status.
This is useful when defining a shell function with the same name
as a shell builtin, retaining the functionality of the builtin
within the function. The return status is non-zero if
SHELL-BUILTIN is not a shell builtin command.
`caller'
caller [EXPR]
Returns the context of any active subroutine call (a shell
function or a script executed with the `.' or `source' builtins).
Without EXPR, `caller' displays the line number and source
filename of the current subroutine call. If a non-negative
integer is supplied as EXPR, `caller' displays the line number,
subroutine name, and source file corresponding to that position in
the current execution call stack. This extra information may be
used, for example, to print a stack trace. The current frame is
frame 0.
The return value is 0 unless the shell is not executing a
subroutine call or EXPR does not correspond to a valid position in
the call stack.
`command'
command [-pVv] COMMAND [ARGUMENTS ...]
Runs COMMAND with ARGUMENTS ignoring any shell function named
COMMAND. Only shell builtin commands or commands found by
searching the `PATH' are executed. If there is a shell function
named `ls', running `command ls' within the function will execute
the external command `ls' instead of calling the function
recursively. The `-p' option means to use a default value for
`PATH' that is guaranteed to find all of the standard utilities.
The return status in this case is 127 if COMMAND cannot be found
or an error occurred, and the exit status of COMMAND otherwise.
If either the `-V' or `-v' option is supplied, a description of
COMMAND is printed. The `-v' option causes a single word
indicating the command or file name used to invoke COMMAND to be
displayed; the `-V' option produces a more verbose description.
In this case, the return status is zero if COMMAND is found, and
non-zero if not.
`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. If the `extdebug' shell
option is enabled using `shopt' (*note Bash Builtins::), the
source file name and line number where the function is defined are
displayed as well. `-F' implies `-f'. The following options can
be used to restrict output to variables with the specified
attributes or to give variables attributes:
`-a'
Each NAME is an array variable (*note Arrays::).
`-f'
Use function names only.
`-i'
The variable is to be treated as an integer; arithmetic
evaluation (*note Shell Arithmetic::) is performed when the
variable is assigned a value.
`-r'
Make NAMEs readonly. These names cannot then be assigned
values by subsequent assignment statements or unset.
`-t'
Give each NAME the `trace' attribute. Traced functions
inherit the `DEBUG' trap from the calling shell. The trace
attribute has no special meaning for variables.
`-x'
Mark each NAME for export to subsequent commands via the
environment.
Using `+' instead of `-' turns off the attribute instead. When
used in a function, `declare' makes each NAME local, as with the
`local' command. If a variable name is followed by =VALUE, the
value of the variable is set to VALUE.
The return status is zero unless an invalid option is encountered,
an attempt is made to define a function using `-f foo=bar', an
attempt is made to assign a value to a readonly variable, an
attempt is made to assign a value to an array variable without
using the compound assignment syntax (*note 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 ...]
Output the ARGs, separated by spaces, terminated with a newline.
The return status is always 0. If `-n' is specified, the trailing
newline is suppressed. If the `-e' option is given,
interpretation of the following backslash-escaped characters is
enabled. The `-E' option disables the interpretation of these
escape characters, even on systems where they are interpreted by
default. The `xpg_echo' shell option may be used to dynamically
determine whether or not `echo' expands these escape characters by
default. `echo' interprets the following escape sequences:
`\a'
alert (bell)
`\b'
backspace
`\c'
suppress trailing newline
`\e'
escape
`\f'
form feed
`\n'
new line
`\r'
carriage return
`\t'
horizontal tab
`\v'
vertical tab
`\\'
backslash
`\0NNN'
the eight-bit character whose value is the octal value NNN
(zero to three octal digits)
`\NNN'
the eight-bit character whose value is the octal value NNN
(one to three octal digits)
`\xHH'
the eight-bit character whose value is the hexadecimal value
HH (one or two hex digits)
`enable'
enable [-n] [-p] [-f FILENAME] [-ads] [NAME ...]
Enable and disable builtin shell commands. Disabling a builtin
allows a disk command which has the same name as a shell builtin
to be executed without specifying a full pathname, even though the
shell normally searches for builtins before disk commands. If
`-n' is used, the NAMEs become disabled. Otherwise NAMEs are
enabled. For example, to use the `test' binary found via `$PATH'
instead of the shell builtin version, type `enable -n test'.
If the `-p' option is supplied, or no NAME arguments appear, a
list of shell builtins is printed. With no other arguments, the
list consists of all enabled shell builtins. The `-a' option
means to list each builtin with an indication of whether or not it
is enabled.
The `-f' option means to load the new builtin command NAME from
shared object FILENAME, on systems that support dynamic loading.
The `-d' option will delete a builtin loaded with `-f'.
If there are no options, a list of the shell builtins is displayed.
The `-s' option restricts `enable' to the POSIX special builtins.
If `-s' is used with `-f', the new builtin becomes a special
builtin (*note 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]
Display helpful information about builtin commands. If PATTERN is
specified, `help' gives detailed help on all commands matching
PATTERN, otherwise a list of the builtins is printed. The `-s'
option restricts the information displayed to a short usage
synopsis. The return status is zero unless no command matches
PATTERN.
`let'
let EXPRESSION [EXPRESSION]
The `let' builtin allows arithmetic to be performed on shell
variables. Each EXPRESSION is evaluated according to the rules
given below in *Note Shell Arithmetic::. If the last EXPRESSION
evaluates to 0, `let' returns 1; otherwise 0 is returned.
`local'
local [OPTION] NAME[=VALUE] ...
For each argument, a local variable named NAME is created, and
assigned VALUE. The OPTION can be any of the options accepted by
`declare'. `local' can only be used within a function; it makes
the variable NAME have a visible scope restricted to that function
and its children. The return status is zero unless `local' is
used outside a function, an invalid NAME is supplied, or NAME is a
readonly variable.
`logout'
logout [N]
Exit a login shell, returning a status of N to the shell's parent.
`printf'
`printf' FORMAT [ARGUMENTS]
Write the formatted ARGUMENTS to the standard output under the
control of the FORMAT. The FORMAT is a character string which
contains three types of objects: plain characters, which are
simply copied to standard output, character escape sequences,
which are converted and copied to the standard output, and format
specifications, each of which causes printing of the next
successive ARGUMENT. In addition to the standard `printf(1)'
formats, `%b' causes `printf' to expand backslash escape sequences
in the corresponding ARGUMENT, (except that `\c' terminates
output, backslashes in `\'', `\"', and `\?' are not removed, and
octal escapes beginning with `\0' may contain up to four digits),
and `%q' causes `printf' to output the corresponding ARGUMENT in a
format that can be reused as shell input.
The 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 ...]
One line is read from the standard input, or from the file
descriptor FD supplied as an argument to the `-u' option, and the
first word is assigned to the first NAME, the second word to the
second NAME, and so on, with leftover words and their intervening
separators assigned to the last NAME. If there are fewer words
read from the input stream than names, the remaining names are
assigned empty values. The characters in the value of the `IFS'
variable are used to split the line into words. The backslash
character `\' may be used to remove any special meaning for the
next character read and for line continuation. If no names are
supplied, the line read is assigned to the variable `REPLY'. The
return code is zero, unless end-of-file is encountered, `read'
times out, or an invalid file descriptor is supplied as the
argument to `-u'. Options, if supplied, have the following
meanings:
`-a ANAME'
The words are assigned to sequential indices of the array
variable ANAME, starting at 0. All elements are removed from
ANAME before the assignment. Other NAME arguments are
ignored.
`-d DELIM'
The first character of DELIM is used to terminate the input
line, rather than newline.
`-e'
Readline (*note Command Line Editing::) is used to obtain the
line.
`-n NCHARS'
`read' returns after reading NCHARS characters rather than
waiting for a complete line of input.
`-p PROMPT'
Display PROMPT, without a trailing newline, before attempting
to read any input. The prompt is displayed only if input is
coming from a terminal.
`-r'
If this option is given, backslash does not act as an escape
character. The backslash is considered to be part of the
line. In particular, a backslash-newline pair may not be
used as a line continuation.
`-s'
Silent mode. If input is coming from a terminal, characters
are not echoed.
`-t TIMEOUT'
Cause `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'
Read input from file descriptor FD.
`shopt'
shopt [-pqsu] [-o] [OPTNAME ...]
Toggle the values of variables controlling optional shell behavior.
With no options, or with the `-p' option, a list of all settable
options is displayed, with an indication of whether or not each is
set. The `-p' option causes output to be displayed in a form that
may be reused as input. Other options have the following meanings:
`-s'
Enable (set) each OPTNAME.
`-u'
Disable (unset) each OPTNAME.
`-q'
Suppresses normal output; the return status indicates whether
the OPTNAME is set or unset. If multiple OPTNAME arguments
are given with `-q', the return status is zero if all
OPTNAMES are enabled; non-zero otherwise.
`-o'
Restricts the values of OPTNAME to be those defined for the
`-o' option to the `set' builtin (*note 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'
If this is set, an argument to the `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'
If set, minor errors in the spelling of a directory component
in a `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 co
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