Bash Reference Manual

This is Edition 2.5a, last updated 13 November 2001, of The GNU Bash Reference Manual, for Bash, Version 2.05a.

Copyright (C) 1991, 1993, 1996 Free Software Foundation, Inc.

Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (`sh'), the Korn Shell (`ksh'), and the C-shell (`csh' and its successor, `tcsh'). The following menu breaks the features up into categories based upon which one of these other shells inspired the feature.

This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.

 

1. Introduction

An introduction to the shell.

2. Definitions

Some definitions used in the rest of this manual.

3. Basic Shell Features

The shell "building blocks".

4. Shell Builtin Commands

Commands that are a part of the shell.

5. Shell Variables

Variables used or set by Bash.

6. Bash Features

Features found only in Bash.

7. Job Control

A chapter describing what job control is and how Bash allows you to use it.

9. Using History Interactively

Chapter dealing with history expansion rules.

8. Command Line Editing

Chapter describing the command line editing features.

10. Installing Bash

How to build and install Bash on your system.

A. Reporting Bugs

How to report bugs in Bash.

B. Major Differences From The Bourne Shell

A terse list of the differences between Bash and historical versions of /bin/sh.

Index of Shell Builtin Commands

Index of Bash builtin commands.

Index of Shell Reserved Words

Index of Bash reserved words.

Parameter and Variable Index

Quick reference helps you find the variable you want.

Function Index

Index of bindable Readline functions.

Concept Index

General index for concepts described in this manual.

1. Introduction

 

1.1 What is Bash?

A short description of Bash.

1.2 What is a shell?

A brief introduction to shells.

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 /bin/sh, which appeared in the Seventh Edition Bell Labs Research version of Unix.

Bash is largely compatible with sh and incorporates useful features from the Korn shell ksh and the C shell csh. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). It offers functional improvements over sh for both interactive and programming use.

While the GNU operating system provides other shells, including a version of csh, Bash is the default shell. Like other GNU software, Bash is quite portable. It currently runs on nearly every version of Unix and a few other operating systems - independently-supported ports exist for MS-DOS, OS/2, Windows 95/98, and Windows NT.

1.2 What is a shell?

At its base, a shell is simply a macro processor that executes commands. A Unix shell is both a command interpreter, which provides the user interface to the rich set of GNU utilities, and a programming language, allowing these utilitites to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as `/bin', allowing users or groups to establish custom environments.

A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands' environments. Shells may be used interactively or non-interactively: they accept input typed from the keyboard or from a file.

Shells also provide a small set of built-in commands (builtins) implementing functionality impossible or inconvenient to obtain via separate utilities. For example, cd, break, continue, and exec) cannot be implemented outside of the shell because they directly manipulate the shell itself. The history, getopts, kill, or pwd builtins, among others, could be implemented in separate utilities, but they are more convenient to use as builtin commands. All of the shell builtins are described in subsequent sections.

While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions.

Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, history and aliases. Each of these features is described in this manual.

2. Definitions

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. Names are used as shell variable and function names. Also referred to as an identifier.

operator

A control operator or a redirection operator. See section 3.6 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.

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, and the rules for evaluation and quoting are taken from the POSIX 1003.2 specification for the `standard' Unix shell.

This chapter briefly summarizes the shell's `building blocks': commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.

 

3.1 Shell Syntax		What your input means to the shell.
3.2 Shell Commands		The types of commands you can use.
3.3 Shell Functions		Grouping commands by name.
3.4 Shell Parameters		Special shell variables.
3.5 Shell Expansions		How Bash expands variables and the various expansions available.
3.6 Redirections		A way to control where input and output go.
3.7 Executing Commands		What happens when you run a command.
3.8 Shell Scripts		Executing files of shell commands.

3.1 Shell Syntax

 

3.1.1 Shell Operation

The basic operation of the shell.

3.1.2 Quoting

How to remove the special meaning from characters.

3.1.3 Comments

How to specify comments.

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.

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 (see section 3.8 Shell Scripts), from a string supplied as an argument to the `-c' invocation option (see section 6.1 Invoking Bash), or from the user's terminal.
2. Breaks the input into words and operators, obeying the quoting rules described in 3.1.2 Quoting. These tokens are separated by metacharacters. Alias expansion is performed by this step (see section 6.6 Aliases).
3. Parses the tokens into simple and compound commands (see section 3.2 Shell Commands).
4. Performs the various shell expansions (see section 3.5 Shell Expansions), breaking the expanded tokens into lists of filenames (see section 3.5.8 Filename Expansion) and commands and arguments.
5. Performs any necessary redirections (see section 3.6 Redirections) and removes the redirection operators and their operands from the argument list.
6. Executes the command (see section 3.7 Executing Commands).
7. Optionally waits for the command to complete and collects its exit status (see section 3.7.5 Exit Status).

 

 

3.1.2 Quoting

 

3.1.2.1 Escape Character		How to remove the special meaning from a single character.
3.1.2.2 Single Quotes		How to inhibit all interpretation of a sequence of characters.
3.1.2.3 Double Quotes		How to suppress most of the interpretation of a sequence of characters.
3.1.2.4 ANSI-C Quoting		How to expand ANSI-C sequences in quoted strings.

3.1.2.5 Locale-Specific Translation

How to translate strings into different languages.

Each of the shell metacharacters (see section 2. Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used, the history expansion character, usually `!', must be quoted to prevent history expansion. See section 9.1 Bash History Facilities, for more details concerning history expansion. There are three quoting mechanisms: the escape character, single quotes, and double quotes.

3.1.2.1 Escape Character

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.

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 (see section 3.5 Shell Expansions). The backslash retains its special meaning only when followed by one of the following characters: `$', ``', `"', `\', or newline. Within double quotes, backslashes that are followed by one of these characters are removed. Backslashes preceding characters without a special meaning are left unmodified. A double quote may be quoted within double quotes by preceding it with a backslash.

The special parameters `*' and `@' have special meaning when in double quotes (see section 3.5.3 Shell Parameter Expansion).

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)

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.

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 (see section 4.2 Bash Builtin Commands), a word beginning with `#' causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells. See section 6.3 Interactive Shells, for a description of what makes a shell interactive.

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.

 

3.2.1 Simple Commands		The most common type of command.
3.2.2 Pipelines		Connecting the input and output of several commands.
3.2.3 Lists of Commands		How to execute commands sequentially.
3.2.4 Looping Constructs		Shell commands for iterative action.
3.2.5 Conditional Constructs		Shell commands for conditional execution.
3.2.6 Grouping Commands		Ways to group commands.

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 blanks, terminated by one of the shell's control operators (see section 2. Definitions). The first word generally specifies a command to be executed, with the rest of the words being that command's arguments.

The return status (see section 3.7.5 Exit Status) of a simple command is its exit status as provided by the POSIX 1003.1 waitpid function, or 128+n if the command was terminated by signal n.

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. See section 5.2 Bash Variables, for a description of the available formats. The use of time as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external time command cannot time these easily.

If the pipeline is not executed asynchronously (see section 3.2.3 Lists of Commands), the shell waits for all commands in the pipeline to complete.

Each command in a pipeline is executed in its own subshell (see section 3.7.3 Command Execution Environment). The exit status of a pipeline is the exit status of the last command in the pipeline. If the reserved word `!' precedes the pipeline, the exit status is the logical negation of the exit status of the last command.

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.

If a command is terminated by the control operator `&', the shell executes the command asynchronously in a subshell. This is known as executing the command in the background. The shell does not wait for the command to finish, and the return status is 0 (true). When job control is not active (see section 7. Job Control), the standard input for asynchronous commands, in the absence of any explicit redirections, is redirected from /dev/null.

Commands separated by a `;' are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators `&&' and `||' denote AND lists and OR lists, respectively. An AND list has the form

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status.

The return status of AND and OR lists is the exit status of the last command executed in the list.

3.2.4 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 (see section 3.4.2 Special Parameters). The return status is the exit status of the last command that executes. If there are no items in the expansion of words, no commands are executed, and the return status is zero.

An alternate form of the for command is also supported:

for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

First, the arithmetic expression expr1 is evaluated according to the rules described below (see section 6.5 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.
 
 

3.2.5 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 (see section 6.5 Shell Arithmetic). If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to

let "expression"

See section 4.2 Bash Builtin Commands, for a full description of the let builtin.

[[...]]

[[ expression ]]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in 6.4 Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the `[[' and `]]'; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed.

When the `==' and `!=' operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below in 3.5.8.1 Pattern Matching. The return value is 0 if the string matches or does not match the pattern, respectively, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.

Expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression )

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 || commands do not execute expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.
 
 

3.2.6 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 to be created, and each of the commands in list to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.

{}

{ list; }

Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.

The exit status of both of these constructs is the exit status of list.

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 () { command-list; }

This defines a shell function named name. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the command-list between { and }. This list is executed whenever name is specified as the name of a command. The exit status of a function is the exit status of the last command executed in the body.

Note that for historical reasons, the curly braces that surround the body of the function must be separated from the body by blanks or newlines. This is because the braces are reserved words and are only recognized as such when they are separated by whitespace. Also, the command-list must be terminated with a semicolon or a newline.

When a function is executed, the arguments to the function become the positional parameters during its execution (see section 3.4.1 Positional Parameters). The special parameter `#' that expands to the number of positional parameters is updated to reflect the change. Positional parameter 0 is unchanged. The FUNCNAME variable is set to the name of the function while the function is executing.

If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter `#' are restored to the values they had prior to the function's execution. If a numeric argument is given to return, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the return.

Variables local to the function may be declared with the local builtin. These variables are visible only to the function and the commands it invokes.

Functions may be recursive. No limit is placed on the number of recursive calls.

3.4 Shell Parameters

 

3.4.1 Positional Parameters		The shell's command-line arguments.
3.4.2 Special Parameters		Parameters with special meanings.

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 subject to arithmetic expansion even if the $((...)) expansion is not used (see section 3.5.5 Arithmetic Expansion). Word splitting is not performed, with the exception of "$@" as explained below. Filename expansion is not performed. Assignment statements may also appear as arguments to the declare, typeset, export, readonly, and local builtin commands.

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 (see section 4. Shell Builtin Commands). The positional parameters are temporarily replaced when a shell function is executed (see section 3.3 Shell Functions).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces.

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 (see section 3.8 Shell Scripts), $0 is set to the name of that file. If Bash is started with the `-c' option (see section 6.1 Invoking Bash), then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke Bash, as given by argument zero.

_

(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.

3.5 Shell Expansions

Expansion is performed on the command line after it has been split into tokens. There are seven kinds of expansion performed:

• brace expansion
• tilde expansion
• parameter and variable expansion
• command substitution
• arithmetic expansion
• word splitting
• filename expansion

 

3.5.1 Brace Expansion		Expansion of expressions within braces.
3.5.2 Tilde Expansion		Expansion of the ~ character.
3.5.3 Shell Parameter Expansion		How Bash expands variables to their values.
3.5.4 Command Substitution		Using the output of a command as an argument.
3.5.5 Arithmetic Expansion		How to use arithmetic in shell expansions.
3.5.6 Process Substitution		A way to write and read to and from a command.
3.5.7 Word Splitting		How the results of expansion are split into separate arguments.
3.5.8 Filename Expansion		A shorthand for specifying filenames matching patterns.
3.5.9 Quote Removal		How and when quote characters are removed from words.

On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as parameter, variable, and arithmetic expansion and command substitution.

Only brace expansion, word splitting, and filename expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@" (see section 3.4.2 Special Parameters) and "${name[@]}" (see section 6.7 Arrays).

After all expansions, quote removal (see section 3.5.9 Quote Removal) is performed.

3.5.1 Brace Expansion

Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see section 3.5.8 Filename Expansion), but the file names generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right.

Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example,

bash$ echo a{d,c,b}e ade ace abe

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string `${' is not considered eligible for brace expansion.

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma. Any incorrectly formed brace expansion is left unchanged.

This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example:

mkdir /usr/local/src/bash/{old,new,dist,bugs}

or

chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

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 (see section 6.8 The Directory Stack). If the tilde-prefix, sans the tilde, consists of a number without a leading `+' or `-', `+' is assumed.

If the login name is invalid, or the tilde expansion fails, the word is left unchanged.

Each variable assignment is checked for unquoted tilde-prefixes immediately following a `:' or `='. In these cases, tilde expansion is also performed. Consequently, one may use file names with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

The following table shows how Bash treats unquoted tilde-prefixes:

~

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'

 

 

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 exception to this is the expansion of ${!prefix*} described below.

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both existence and that the value is not null; if the colon is omitted, the operator tests only for existence.

${parameter:-word}

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 (see section 6.5 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*}

Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable.

${#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 (see section 3.5.8 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.
 
 

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.

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 substitutions may be nested.

The evaluation is performed according to the rules listed below (see section 6.5 Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs.

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.

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.

3.5.8 Filename Expansion

 

3.5.8.1 Pattern Matching

How the shell matches patterns.

After word splitting, unless the `-f' option has been set (see section 4.3 The Set Builtin), Bash scans each word for the characters `*', `?', and `['. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found, and the shell option nullglob is disabled, the word is left unchanged. If the nullglob option is set, and no matches are found, the word is removed. If the shell option nocaseglob is enabled, the match is performed without regard to the case of alphabetic characters.

When a pattern is used for filename generation, the character `.' at the start of a filename or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. When matching a file name, the slash character must always be matched explicitly. In other cases, the `.' character is not treated specially.

See the description of shopt in 4.2 Bash Builtin Commands, for a description of the nocaseglob, nullglob, and dotglob options.

The GLOBIGNORE shell variable may be used to restrict the set of filenames matching a pattern. If GLOBIGNORE is set, each matching filename that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. The filenames `.' and `..' are always ignored, even when GLOBIGNORE is set. However, setting GLOBIGNORE has the effect of enabling the dotglob shell option, so all other filenames beginning with a `.' will match. To get the old behavior of ignoring filenames beginning with a `.', make `.*' one of the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is unset.

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. 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   xdigit

A character class matches any character belonging to that class.

Within `[' and `]', an equivalence class can be specified using the syntax [=c=], which matches all characters with the same collation weight (as defined by the current locale) as the character c.

Within `[' and `]', the syntax [.symbol.] matches the collating symbol symbol.

?(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.

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.

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.

 

 

3.6.1 Redirecting Input

The general format for redirecting input is:

[n]<word

3.6.2 Redirecting Output

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

The general format for appending output is:

[n]>>word

3.6.4 Redirecting Standard Output and Standard Error

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

The format of here-documents is as follows:

<<[-]word         here-document delimiter

No parameter expansion, command substitution, arithmetic expansion, or filename expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence \newline is ignored, and `\' must be used to quote the characters `\', `$', and ``'.

If the redirection operator is `<<-', then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

3.6.6 Duplicating File Descriptors

[n]<&word

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.

3.6.7 Opening File Descriptors for Reading and Writing

[n]<>word

3.7 Executing Commands

 

3.7.1 Simple Command Expansion

How Bash expands simple commands before executing them.

3.7.2 Command Search and Execution

How Bash finds commands and runs them.

3.7.3 Command Execution Environment

The environment in which Bash executes commands that are not shell builtins.

3.7.4 Environment

The environment given to a command.

3.7.5 Exit Status

The status returned by commands and how Bash interprets it.

3.7.6 Signals

What happens when Bash or a command it runs receives a signal.

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 (see section 3.5 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 (see section 3.6 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, 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.

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 3.3 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 4.1 Bourne Shell Builtins). A full search of the directories in $PATH is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127.
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 3.8 Shell Scripts.
6. If the command was not begun asynchronously, the shell waits for the command to complete and collects its exit status.

 

 

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 (see section 6.6 Aliases)
• various process IDs, including those of background jobs (see section 3.2.3 Lists of Commands), the value of $$, and the value of $PPID

 

 

• 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 marked for export, along with variables exported for the command, passed in the environment (see section 3.7.4 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

 

 

Command substitution and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment.

If a command is followed by a `&' and job control is not active, the default standard input for the command is the empty file `/dev/null'. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections.

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 3.4 Shell Parameters. These assignment statements affect only the environment seen by that command.

If the `-k' option is set (see section 4.3 The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.

When Bash invokes an external command, the variable `$_' is set to the full path name of the command and passed to that command in its environment.

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 (see section 3.2.5 Conditional Constructs) and some of the list constructs (see section 3.2.3 Lists of Commands).

All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage.

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 (see section 7. Job Control), Bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

Commands started by Bash have signal handlers set to the values inherited by the shell from its parent. When job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT as well. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.

The shell exits by default upon receipt of a SIGHUP. Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped. Stopped jobs are sent SIGCONT to ensure that they receive the SIGHUP. To prevent the shell from sending the SIGHUP signal to a particular job, it should be removed from the jobs table with the disown builtin (see section 7.2 Job Control Builtins) or marked to not receive SIGHUP using disown -h.

If the huponexit shell option has been set with shopt (see section 4.2 Bash Builtin Commands), Bash sends a SIGHUP to all jobs when an interactive login shell exits.

When Bash receives a signal for which a trap has been set while waiting for a command to complete, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait builtin, the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.

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 (see section 6.1 Invoking Bash), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.

When Bash runs a shell script, it sets the special parameter 0 to the name of the file, rather than the name of the shell, and the positional parameters are set to the remaining arguments, if any are given. If no additional arguments are supplied, the positional parameters are unset.

A shell script may be made executable by using the chmod command to turn on the execute bit. When Bash finds such a file while searching the $PATH for a command, it spawns a subshell to execute it. In other words, executing

filename arguments

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 4.1 Bourne Shell Builtins) are retained by the child.

Most versions of Unix make this a part of the operating system's command execution mechanism. If the first line of a script begins with the two characters `#!', the remainder of the line specifies an interpreter for the program. Thus, you can specify Bash, awk, Perl, or some other interpreter and write the rest of the script file in that language.

The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters.

Bash scripts often begin with #! /bin/bash (assuming that Bash has been installed in `/bin'), since this ensures that Bash will be used to interpret the script, even if it is executed under another shell.

4. Shell Builtin Commands

 

4.1 Bourne Shell Builtins		Builtin commands inherited from the Bourne Shell.
4.2 Bash Builtin Commands		Table of builtins specific to Bash.
4.3 The Set Builtin		This builtin is so overloaded it deserves its own section.
4.4 Special Builtins		Builtin commands classified specially by POSIX.2.

This section briefly the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash.

Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see section 7.2 Job Control Builtins), the directory stack (see section 6.8.1 Directory Stack Builtins), the command history (see section 9.2 Bash History Builtins), and the programmable completion facilities (see section 8.7 Programmable Completion Builtins).

Many of the builtins have been extended by POSIX or Bash.

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 [-LP] [directory]

Change the current working directory to directory. If directory is not given, the value of the HOME shell variable is used. If the shell variable CDPATH exists, it is used as a search path. If directory begins with a slash, CDPATH is not used. The `-P' option means to not follow symbolic links; symbolic links are followed by default or with the `-L' option. If directory is `-', it is equivalent to $OLDPWD. The return status is zero if the directory is successfully changed, non-zero otherwise.

continue

continue [n]

Resume the next iteration of an enclosing for, while, until, or select loop. If n is supplied, the execution of the nth enclosing loop is resumed. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

eval

eval [arguments]

The arguments are concatenated together into a single command, which is then read and executed, and its exit status returned as the exit status of eval. If there are no arguments or only empty arguments, the return status is zero.

exec

exec [-cl] [-a name] [command [arguments]]

If command is supplied, it replaces the shell without creating a new process. If the `-l' option is supplied, the shell places a dash 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. 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] [-t] [name]

Remember the full pathnames of commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in $PATH. The `-p' option inhibits the path search, and filename is used as the location of name. The `-r' option causes the shell to forget all remembered locations. If the `-t' option is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are supplied with `-t' the name is printed before the hashed full pathname. If no arguments are given, information about remembered commands is printed. The return status is zero unless a name is not found or an invalid option is supplied.

pwd

pwd [-LP]

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] ...

Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each name refers to a shell function. The `-a' option means each name refers to an array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the `-f' option is supplied with a name that is not a shell function.

return

return [n]

Cause a shell function to exit with the return value n. 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. 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 6.4 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 (see section 6.4 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 (see section 6.4 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 or equal to `-', all specified signals are reset to the values they had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is 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. Each sigspec is either a signal name such as SIGINT (with or without the SIG prefix) or a signal number. If a sigspec is 0 or EXIT, arg is executed when the shell exits. If a sigspec is DEBUG, the command arg is executed after every simple command. If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status. The ERR trap is not executed if the failed command is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return status is being inverted using !. The `-l' option causes the shell to print a list of signal names and their corresponding numbers.

Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created.

The return status is zero unless a sigspec does not specify a valid signal.

umask

umask [-p] [-S] [mode]

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 does not exist or is readonly.

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 6.6 Aliases.

bind

bind [-m keymap] [-lpsvPSV] bind [-m keymap] [-q function] [-u function] [-r keyseq] bind [-m keymap] -f filename bind [-m keymap] -x keyseq:shell-command bind [-m keymap] keyseq:function-name

Display current Readline (see section 8. Command Line Editing) key and function bindings, or bind a key sequence to a Readline function or macro. The binding syntax accepted is identical to that of a Readline initialization file (see section 8.3 Readline Init File), but each binding must be passed as a separate argument: e.g., `"\C-x\C-r":re-read-init-file'. Options, if supplied, have the following meanings:

-m keymap

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.

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 [-afFrxi] [-p] [name[=value]]

Declare variables and give them attributes. If no names are given, then display the values of variables instead.

The `-p' option will display the attributes and values of each name. When `-p' is used, additional options are ignored. The `-F' option inhibits the display of function definitions; only the function name and attributes are printed. `-F' implies `-f'. The following options can be used to restrict output to variables with the specified attributes or to give variables attributes:

-a

Each name is an array variable (see section 6.7 Arrays).

-f

Use function names only.

-i

The variable is to be treated as an integer; arithmetic evaluation (see section 6.5 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.

-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.

The return status is zero unless an invalid option is encountered, an attempt is made to define a function using `-f foo=bar', an attempt is made to assign a value to a readonly variable, an attempt is made to assign a value to an array variable without using the compound assignment syntax (see section 6.7 Arrays), one of the names is not a valid shell variable name, an attempt is made to turn off readonly status for a readonly variable, an attempt is made to turn off array status for an array variable, or an attempt is made to display a non-existent function with `-f'.

echo

echo [-neE] [arg ...]

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

\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)

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 (see section 4.4 Special Builtins).

The return status is zero unless a name is not a shell builtin or there is an error loading a new builtin from a shared object.

help

help [-s] [pattern]

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 6.5 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, and `%q' causes printf to output the corresponding argument in a format that can be reused as shell input.

The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero on failure.

read

read [-ers] [-a aname] [-p prompt] [-t timeout] [-n nchars] [-d delim] [name ...]

One line is read from the standard input, 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 standard input 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 or read times out. 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 (see section 8. 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.

 

 

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 (see section 4.3 The Set Builtin).

If either `-s' or `-u' is used with no optname arguments, the display is limited to those options which are set or unset, respectively.

Unless otherwise noted, the shopt options are disabled (off) by default.

The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.

The list of shopt options is:

cdable_vars

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 command proceeds. This option is only used by interactive shells.

checkhash

If this is set, Bash checks that a command found in the hash table exists before trying to execute it. If a hashed command no longer exists, a normal path search is performed.

checkwinsize

If set, Bash checks the window size after each command and, if necessary, updates the values of LINES and COLUMNS.

cmdhist

If set, Bash attempts to save all lines of a multiple-line command in the same history entry. This allows easy re-editing of multi-line commands.

dotglob

If set, Bash includes filenames beginning with a `.' in the results of filename expansion.

execfail

If this is set, a non-interactive shell will not exit if it cannot execute the file specified as an argument to the exec builtin command. An interactive shell does not exit if exec fails.

expand_aliases

If set, aliases are expanded as described below under Aliases, 6.6 Aliases. This option is enabled by default for interactive shells.

extglob

If set, the extended pattern matching features described above (see section 3.5.8.1 Pattern Matching) are enabled.

histappend

If set, the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file.

histreedit

If set, and Readline is being used, a user is given the opportunity to re-edit a failed history substitution.

histverify

If set, and Readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the Readline editing buffer, allowing further modification.

hostcomplete

If set, and Readline is being used, Bash will attempt to perform hostname completion when a word containing a `@' is being completed (see section 8.4.6 Letting Readline Type For You). This option is enabled by default.

huponexit

If set, Bash will send SIGHUP to all jobs when an interactive login shell exits (see section 3.7.6 Signals).

interactive_comments

Allow a word beginning with `#' to cause that word and all remaining characters on that line to be ignored in an interactive shell. This option is enabled by default.

lithist

If enabled, and the cmdhist option is enabled, multi-line commands are saved to the history with embedded newlines rather than using semicolon separators where possible.

login_shell

The shell sets this option if it is started as a login shell (see section 6.1 Invoking Bash). The value may not be changed.

mailwarn

If set, and a file that Bash is checking for mail has been accessed since the last time it was checked, the message "The mail in mailfile has been read" is displayed.

no_empty_cmd_completion

If set, and Readline is being used, Bash will not attempt to search the PATH for possible completions when completion is attempted on an empty line.

nocaseglob

If set, Bash matches filenames in a case-insensitive fashion when performing filename expansion.

nullglob

If set, Bash allows filename patterns which match no files to expand to a null string, rather than themselves.

progcomp

If set, the programmable completion facilities (see section 8.6 Programmable Completion) are enabled. This option is enabled by default.

promptvars

If set, prompt strings undergo variable and parameter expansion after being expanded (see section 6.9 Controlling the Prompt). This option is enabled by default.

restricted_shell

The shell sets this option if it is started in restricted mode (see section 6.10 The Restricted Shell). The value may not be changed. This is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted.

shift_verbose

If this is set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters.

sourcepath

If set, the source builtin uses the value of PATH to find the directory containing the file supplied as an argument. This option is enabled by default.

xpg_echo

If set, the echo builtin expands backslash-escape sequences by default.

 

 

The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.

source

source filename

A synonym for . (see section 4.1 Bourne Shell Builtins).

type

type [-atp] [name ...]

For each name, indicate how it would be interpreted if used as a command name.

If the `-t' option is used, type prints a single word which is one of `alias', `function', `builtin', `file' or `keyword', if name is an alias, shell function, shell builtin, disk file, or shell reserved word, respectively. If the name is not found, then nothing is printed, and type returns a failure status.

If the `-p' option is used, type either returns the name of the disk file that would be executed, or nothing if `-t' would not return `file'.

If the `-a' option is used, type returns all of the places that contain an executable named file. This includes aliases and functions, if and only if the `-p' option is not also used.

The return status is zero if any of the names are found, non-zero if none are found.

typeset

typeset [-afFrxi] [-p] [name[=value]]

The typeset command is supplied for compatibility with the Korn shell; however, it has been deprecated in favor of the declare builtin command.

ulimit

ulimit [-acdflmnpstuvSH] [limit]

ulimit provides control over the resources available to processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows:

-S

Change and report the soft limit associated with a resource.

-H

Change and report the hard limit associated with a resource.

-a

All current limits are reported.

-c

The maximum size of core files created.

-d

The maximum size of a process's data segment.

-f

The maximum size of files created by the shell.

-l

The maximum size that may be locked into memory.

-m

The maximum resident set size.

-n

The maximum number of open file descriptors.

-p

The pipe buffer size.

-s

The maximum stack size.

-t

The maximum amount of cpu time in seconds.

-u

The maximum number of processes available to a single user.

-v

The maximum amount of virtual memory available to the process.

 

 

If limit is given, it is the new value of the specified resource; the special limit values hard, soft, and unlimited stand for the current hard limit, the current soft limit, and no limit, respectively. Otherwise, the current value of the soft limit for the specified resource is printed, unless the `-H' option is supplied. When setting new limits, if neither `-H' nor `-S' is supplied, both the hard and soft limits are set. If no option is given, then `-f' is assumed. Values are in 1024-byte increments, except for `-t', which is in seconds, `-p', which is in units of 512-byte blocks, and `-n' and `-u', which are unscaled values.

The return status is zero unless an invalid option or argument is supplied, or an error occurs while setting a new limit.

unalias

unalias [-a] [name ... ]

Remove each name from the list of aliases. If `-a' is supplied, all aliases are removed. Aliases are described in 6.6 Aliases.
 
 

4.3 The Set Builtin

This builtin is so complicated that it deserves its own section.

set

set [--abefhkmnptuvxBCHP] [-o option] [argument ...]

If no options or arguments are supplied, set displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input.

When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:

-a

Mark variables and function which are modified or created for export to the environment of subsequent commands.

-b

Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.

-e

Exit immediately if a simple command (see section 3.2.1 Simple Commands) exits with a non-zero status, unless the command that fails is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return status is being inverted using !. A trap on ERR, if set, is executed before the shell exits.

-f

Disable file name generation (globbing).

-h

Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.

-k

All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.

-m

Job control is enabled (see section 7. Job Control).

-n

Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells.

-o option-name

Set the option corresponding to option-name:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Use an emacs-style line editing interface (see section 8. Command Line Editing).

errexit

Same as -e.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history, as described in 9.1 Bash History Facilities. This option is on by default in interactive shells.

ignoreeof

An interactive shell will not exit upon reading EOF.

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f.

nolog

Currently ignored.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

posix

Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard (see section 6.11 Bash POSIX Mode). This is intended to make Bash behave as a strict superset of that standard.

privileged

Same as -p.

verbose

Same as -v.

vi

Use a vi-style line editing interface.

xtrace

Same as -x.

-p

Turn on privileged mode. In this mode, the $BASH_ENV and $ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS variable, if it appears in the environment, is ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.

-t

Exit after reading and executing one command.

-u

Treat unset variables as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.

-v

Print shell input lines as they are read.

-x

Print a trace of simple commands and their arguments after they are expanded and before they are executed.

-B

The shell will perform brace expansion (see section 3.5.1 Brace Expansion). This option is on by default.

-C

Prevent output redirection using `>', `>&', and `<>' from overwriting existing files.

-H

Enable `!' style history substitution (see section 9.3 History Expansion). This option is on by default for interactive shells.

-P

If set, do not follow symbolic links when performing commands such as cd which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory.

For example, if `/usr/sys' is a symbolic link to `/usr/local/sys' then:

$ cd /usr/sys; echo $PWD /usr/sys $ cd ..; pwd /usr

If set -P is on, then:

$ cd /usr/sys; echo $PWD /usr/local/sys $ cd ..; pwd /usr/local

--

If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the arguments, even if some of them begin with a `-'.

-

Signal the end of options, cause all remaining arguments to be assigned to the positional parameters. The `-x' and `-v' options are turned off. If there are no arguments, the positional parameters remain unchanged.

Using `+' rather than `-' causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in $-.

The remaining N arguments are positional parameters and are assigned, in order, to $1, $2, ... $N. The special parameter # is set to N.

The return status is always zero unless an invalid option is supplied.

4.4 Special Builtins

For historical reasons, the POSIX 1003.2 standard has classified several builtin commands as special. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects:

1. Special builtins are found before shell functions during command lookup.
2. If a special builtin returns an error status, a non-interactive shell exits.
3. Assignment statements preceding the command stay in effect in the shell environment after the command completes.

These are the POSIX special builtins:

break : . continue eval exec exit export readonly return set shift trap unset

5. Shell Variables

 

5.1 Bourne Shell Variables		Variables which Bash uses in the same way as the Bourne Shell.
5.2 Bash Variables		List of variables that exist in Bash.

5.1 Bourne Shell Variables

Bash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.

CDPATH

A colon-separated list of directories used as a search path for the cd builtin command.

HOME

The current user's home directory; the default for the cd builtin command. The value of this variable is also used by tilde expansion (see section 3.5.2 Tilde Expansion).

IFS

A list of characters that separate fields; used when the shell splits words as part of expansion.

MAIL

If this parameter is set to a filename and the MAILPATH variable is not set, Bash informs the user of the arrival of mail in the specified file.

MAILPATH

A colon-separated list of filenames which the shell periodically checks for new mail. Each list entry can specify the message that is printed when new mail arrives in the mail file by separating the file name from the message with a `?'. When used in the text of the message, $_ expands to the name of the current mail file.

OPTARG

The value of the last option argument processed by the getopts builtin.

OPTIND

The index of the last option argument processed by the getopts builtin.

PATH

A colon-separated list of directories in which the shell looks for commands.

PS1

The primary prompt string. The default value is `\s-\v\$ '. See section 6.9 Controlling the Prompt, for the complete list of escape sequences that are expanded before PS1 is displayed.

PS2

The secondary prompt string. The default value is `> '.

 

 

5.2 Bash Variables

These variables are set or used by Bash, but other shells do not normally treat them specially.

A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see section 7.3 Job Control Variables).

BASH

The full pathname used to execute the current instance of Bash.

BASH_ENV

If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. See section 6.2 Bash Startup Files.

BASH_VERSION

The version number of the current instance of Bash.

BASH_VERSINFO

A readonly array variable (see section 6.7 Arrays) whose members hold version information for this instance of Bash. The values assigned to the array members are as follows:

BASH_VERSINFO[0]

The major version number (the release).

BASH_VERSINFO[1]

The minor version number (the version).

BASH_VERSINFO[2]

The patch level.

BASH_VERSINFO[3]

The build version.

BASH_VERSINFO[4]

The release status (e.g., beta1).

BASH_VERSINFO[5]

The value of MACHTYPE.

 

 

COLUMNS

Used by the select builtin command to determine the terminal width when printing selection lists. Automatically set upon receipt of a SIGWINCH.

COMP_CWORD

An index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see section 8.6 Programmable Completion).

COMP_LINE

The current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see section 8.6 Programmable Completion).

COMP_POINT

The index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see section 8.6 Programmable Completion).

COMP_WORDS

An array variable consisting of the individual words in the current command line. This variable is available only in shell functions invoked by the programmable completion facilities (see section 8.6 Programmable Completion).

COMPREPLY

An array variable from which Bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see section 8.6 Programmable Completion).

DIRSTACK

An array variable containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assignment to this variable will not change the current directory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.

EUID

The numeric effective user id of the current user. This variable is readonly.

FCEDIT

The editor used as a default by the `-e' option to the fc builtin command.

FIGNORE

A colon-separated list of suffixes to ignore when performing filename completion. A file name whose suffix matches one of the entries in FIGNORE is excluded from the list of matched file names. A sample value is `.o:~'

FUNCNAME

The name of any currently-executing shell function. This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect and return an error status. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

GLOBIGNORE

A colon-separated list of patterns defining the set of filenames to be ignored by filename expansion. If a filename matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches.

GROUPS

An array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect and return an error status. If GROUPS is unset, it loses its special properties, even if it is subsequently reset.

histchars

Up to three characters which control history expansion, quick substitution, and tokenization (see section 9.3 History Expansion). The first character is the history expansion character, that is, the character which signifies the start of a history expansion, normally `!'. The second character is the character which signifies `quick substitution' when seen as the first character on a line, normally `^'. The optional third character is the character which indicates that the remainder of the line is a comment when found as the first character of a word, usually `#'. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.

HISTCMD

The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.

HISTCONTROL

A value of `ignorespace' means to not enter lines which begin with a space or tab into the history list. A value of `ignoredups' means to not enter lines which match the last entered line. A value of `ignoreboth' combines the two options. Unset, or set to any other value than those above, means to save all lines on the history list. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.

HISTFILE

The name of the file to which the command history is saved. The default value is `~/.bash_history'.

HISTFILESIZE

The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines. The history file is also truncated to this size after writing it when an interactive shell exits. The default value is 500.

HISTIGNORE

A colon-separated list of patterns used to decide which command lines should be saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (no implicit `*' is appended). Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. In addition to the normal shell pattern matching characters, `&' matches the previous history line. `&' may be escaped using a backslash; the backslash is removed before attempting a match. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTIGNORE.

HISTIGNORE subsumes the function of HISTCONTROL. A pattern of `&' is identical to ignoredups, and a pattern of `[ ]*' is identical to ignorespace. Combining these two patterns, separating them with a colon, provides the functionality of ignoreboth.

HISTSIZE

The maximum number of commands to remember on the history list. The default value is 500.

HOSTFILE

Contains the name of a file in the same format as `/etc/hosts' that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, Bash attempts to read `/etc/hosts' to obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared.

HOSTNAME

The name of the current host.

HOSTTYPE

A string describing the machine Bash is running on.

IGNOREEOF

Controls the action of the shell on receipt of an EOF character as the sole input. If set, the value denotes the number of consecutive EOF characters that can be read as the first character on an input line before the shell will exit. If the variable exists but does not have a numeric value (or has no value) then the default is 10. If the variable does not exist, then EOF signifies the end of input to the shell. This is only in effect for interactive shells.

INPUTRC

The name of the Readline initialization file, overriding the default of `~/.inputrc'.

LANG

Used to determine the locale category for any category not specifically selected with a variable starting with LC_.

LC_ALL

This variable overrides the value of LANG and any other LC_ variable specifying a locale category.

LC_COLLATE

This variable determines the collation order used when sorting the results of filename expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within filename expansion and pattern matching (see section 3.5.8 Filename Expansion).

LC_CTYPE

This variable determines the interpretation of characters and the behavior of character classes within filename expansion and pattern matching (see section 3.5.8 Filename Expansion).

LC_MESSAGES

This variable determines the locale used to translate double-quoted strings preceded by a `$' (see section 3.1.2.5 Locale-Specific Translation).

LC_NUMERIC

This variable determines the locale category used for number formatting.

LINENO

The line number in the script or shell function currently executing.

LINES

Used by the select builtin command to determine the column length for printing selection lists. Automatically set upon receipt of a SIGWINCH.

MACHTYPE

A string that fully describes the system type on which Bash is executing, in the standard GNU cpu-company-system format.

MAILCHECK

How often (in seconds) that the shell should check for mail in the files specified in the MAILPATH or MAIL variables. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.

OLDPWD

The previous working directory as set by the cd builtin.

OPTERR

If set to the value 1, Bash displays error messages generated by the getopts builtin command.

OSTYPE

A string describing the operating system Bash is running on.

PIPESTATUS

An array variable (see section 6.7 Arrays) containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command).

POSIXLY_CORRECT

If this variable is in the environment when bash starts, the shell enters POSIX mode (see section 6.11 Bash POSIX Mode) before reading the startup files, as if the `--posix' invocation option had been supplied. If it is set while the shell is running, bash enables POSIX mode, as if the command

set -o posix

had been executed.

PPID

The process ID of the shell's parent process. This variable is readonly.

PROMPT_COMMAND

If set, the value is interpreted as a command to execute before the printing of each primary prompt ($PS1).

PS3

The value of this variable is used as the prompt for the select command. If this variable is not set, the select command prompts with `#? '

PS4

The value is the prompt printed before the command line is echoed when the `-x' option is set (see section 4.3 The Set Builtin). The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is `+ '.

PWD

The current working directory as set by the cd builtin.

RANDOM

Each time this parameter is referenced, a random integer between 0 and 32767 is generated. Assigning a value to this variable seeds the random number generator.

REPLY

The default variable for the read builtin.

SECONDS

This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment.

SHELLOPTS

A colon-separated list of enabled shell options. Each word in the list is a valid argument for the `-o' option to the set builtin command (see section 4.3 The Set Builtin). The options appearing in SHELLOPTS are those reported as `on' by `set -o'. If this variable is in the environment when Bash starts up, each shell option in the list will be enabled before reading any startup files. This variable is readonly.

SHLVL

Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.

TIMEFORMAT

The value of this parameter is used as a format string specifying how the timing information for pipelines prefixed with the time reserved word should be displayed. The `%' character introduces an escape sequence that is expanded to a time value or other information. The escape sequences and their meanings are as follows; the braces denote optional portions.

%%

A literal `%'.

%[p][l]R

The elapsed time in seconds.

%[p][l]U

The number of CPU seconds spent in user mode.

%[p][l]S

The number of CPU seconds spent in system mode.

%P

The CPU percentage, computed as (%U + %S) / %R.

The optional p is a digit specifying the precision, the number of fractional digits after a decimal point. A value of 0 causes no decimal point or fraction to be output. At most three places after the decimal point may be specified; values of p greater than 3 are changed to 3. If p is not specified, the value 3 is used.

The optional l specifies a longer format, including minutes, of the form MMmSS.FFs. The value of p determines whether or not the fraction is included.

If this variable is not set, Bash acts as if it had the value

$'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'

If the value is null, no timing information is displayed. A trailing newline is added when the format string is displayed.

TMOUT

If set to a value greater than zero, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt when the shell is interactive. Bash terminates after that number of seconds if input does not arrive.

UID

The numeric real user id of the current user. This variable is readonly.

 

 

6. Bash Features

This section describes features unique to Bash.

 

6.1 Invoking Bash		Command line options that you can give to Bash.
6.2 Bash Startup Files		When and how Bash executes scripts.
6.3 Interactive Shells		What an interactive shell is.
6.4 Bash Conditional Expressions		Primitives used in composing expressions for the test builtin.
6.5 Shell Arithmetic		Arithmetic on shell variables.
6.6 Aliases		Substituting one command for another.
6.7 Arrays		Array Variables.
6.8 The Directory Stack		History of visited directories.
6.9 Controlling the Prompt		Controlling the PS1 string.
6.10 The Restricted Shell		A more controlled mode of shell execution.
6.11 Bash POSIX Mode		Making Bash behave more closely to what the POSIX standard specifies.

6.1 Invoking Bash

bash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]
bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument ...]
bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument ...]

In addition to the single-character shell command-line options (see section 4.3 The Set Builtin), there are several multi-character options that you can use. These options must appear on the command line before the single-character options in order for them to be recognized.

--dump-po-strings

A list of all double-quoted strings preceded by `$' is printed on the standard ouput in the GNU gettext PO (portable object) file format. Equivalent to `-D' except for the output format.

--dump-strings

Equivalent to `-D'.

--help

Display a usage message on standard output and exit sucessfully.

--init-file filename

--rcfile filename

Execute commands from filename (instead of `~/.bashrc') in an interactive shell.

--login

Make this shell act as if it had been directly invoked by login. When the shell is interactive, this is equivalent to starting a login shell with `exec -l bash'. When the shell is not interactive, the login shell startup files will be executed. `exec bash --login' will replace the current shell with a Bash login shell. See section 6.2 Bash Startup Files, for a description of the special behavior of a login shell.

--noediting

Do not use the GNU Readline library (see section 8. Command Line Editing) to read command lines when the shell is interactive.

--noprofile

Don't load the system-wide startup file `/etc/profile' or any of the personal initialization files `~/.bash_profile', `~/.bash_login', or `~/.profile' when Bash is invoked as a login shell.

--norc

Don't read the `~/.bashrc' initialization file in an interactive shell. This is on by default if the shell is invoked as sh.

--posix

Change the behavior of Bash where the default operation differs from the POSIX 1003.2 standard to match the standard. This is intended to make Bash behave as a strict superset of that standard. See section 6.11 Bash POSIX Mode, for a description of the Bash POSIX mode.

--restricted

Make the shell a restricted shell (see section 6.10 The Restricted Shell).

--verbose

Equivalent to `-v'. Print shell input lines as they're read.

--version

Show version information for this instance of Bash on the standard output and exit successfully.

 

 

-c string

Read and execute commands from string after processing the options, then exit. Any remaining arguments are assigned to the positional parameters, starting with $0.

-i

Force the shell to run interactively. Interactive shells are described in 6.3 Interactive Shells.

-r

Make the shell a restricted shell (see section 6.10 The Restricted Shell).

-s

If this option is present, or if no arguments remain after option processing, then commands are read from the standard input. This option allows the positional parameters to be set when invoking an interactive shell.

-D

A list of all double-quoted strings preceded by `$' is printed on the standard ouput. These are the strings that are subject to language translation when the current locale is not C or POSIX (see section 3.1.2.5 Locale-Specific Translation). This implies the `-n' option; no commands will be executed.

[-+]O [shopt_option]

shopt_option is one of the shell options accepted by the shopt builtin (see section 4. Shell Builtin Commands). If shopt_option is present, `-O' sets the value of that option; `+O' unsets it. If shopt_option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is `+O', the output is displayed in a format that may be reused as input.

--

A -- signals the end of options and disables further option processing. Any arguments after the -- are treated as filenames and arguments.

 

 

An interactive shell is one started without non-option arguments, unless `-s' is specified, without specifying the `-c' option, and whose input and output are both connected to terminals (as determined by isatty(3)), or one started with the `-i' option. See section 6.3 Interactive Shells, for more information.

If arguments remain after option processing, and neither the `-c' nor the `-s' option has been supplied, the first argument is assumed to be the name of a file containing shell commands (see section 3.8 Shell Scripts). When Bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits. Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0.

6.2 Bash Startup Files

This section describs how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in file names as described above under Tilde Expansion (see section 3.5.2 Tilde Expansion).

Interactive shells are described in 6.3 Interactive Shells.

Invoked as an interactive login shell, or with `--login'

When Bash is invoked as an interactive login shell, or as a non-interactive shell with the `--login' option, it first reads and executes commands from the file `/etc/profile', if that file exists. After reading that file, it looks for `~/.bash_profile', `~/.bash_login', and `~/.profile', in that order, and reads and executes commands from the first one that exists and is readable. The `--noprofile' option may be used when the shell is started to inhibit this behavior.

When a login shell exits, Bash reads and executes commands from the file `~/.bash_logout', if it exists.

Invoked as an interactive non-login shell

When an interactive shell that is not a login shell is started, Bash reads and executes commands from `~/.bashrc', if that file exists. This may be inhibited by using the `--norc' option. The `--rcfile file' option will force Bash to read and execute commands from file instead of `~/.bashrc'.

So, typically, your `~/.bash_profile' contains the line

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

after (or before) any login-specific initializations.

Invoked non-interactively

When Bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but the value of the PATH variable is not used to search for the file name.

As noted above, if a non-interactive shell is invoked with the `--login' option, Bash attempts to read and execute commands from the login shell startup files.

Invoked with name sh

If Bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.

When invoked as an interactive login shell, or as a non-interactive shell with the `--login' option, it first attempts to read and execute commands from `/etc/profile' and `~/.profile', in that order. The `--noprofile' option may be used to inhibit this behavior. When invoked as an interactive shell with the name sh, Bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the `--rcfile' option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files.

When invoked as sh, Bash enters POSIX mode after the startup files are read.

Invoked in POSIX mode

When Bash is started in POSIX mode, as with the `--posix' command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value. No other startup files are read.

Invoked by remote shell daemon

Bash attempts to determine when it is being run by the remote shell daemon, usually rshd. If Bash determines it is being run by rshd, it reads and executes commands from `~/.bashrc', if that file exists and is readable. It will not do this if invoked as sh. The `--norc' option may be used to inhibit this behavior, and the `--rcfile' option may be used to force another file to be read, but rshd does not generally invoke the shell with those options or allow them to be specified.

Invoked with unequal effective and real UID/GIDs

If Bash is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS variable, if it appears in the environment, is ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.

6.3 Interactive Shells

 

6.3.1 What is an Interactive Shell?		What determines whether a shell is Interactive.
6.3.2 Is this Shell Interactive?		How to tell if a shell is interactive.
6.3.3 Interactive Shell Behavior		What changes in a interactive shell?

6.3.1 What is an Interactive Shell?

An interactive shell is one started without non-option arguments, unless `-s' is specified, without specifiying the `-c' option, and whose input and output are both connected to terminals (as determined by isatty(3)), or one started with the `-i' option.

An interactive shell generally reads from and writes to a user's terminal.

The `-s' invocation option may be used to set the positional parameters when an interactive shell is started.

6.3.2 Is this Shell Interactive?

To determine within a startup script whether or not Bash is running interactively, test the value of the `-' special parameter. It contains i when the shell is interactive. For example:

case "$-" in *i*)    echo This shell is interactive ;; *)      echo This shell is not interactive ;; esac

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:

if [ -z "$PS1" ]; then         echo This shell is not interactive else         echo This shell is interactive fi

6.3.3 Interactive Shell Behavior

When the shell is running interactively, it changes its behavior in several ways.

1. Startup files are read and executed as described in 6.2 Bash Startup Files.
2. Job Control (see section 7. Job Control) is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.
3. Bash expands and displays PS1 before reading the first line of a command, and expands and displays PS2 before reading the second and subsequent lines of a multi-line command.
4. Bash executes the value of the PROMPT_COMMAND variable as a command before printing the primary prompt, $PS1 (see section 5.2 Bash Variables).
5. Readline (see section 8. Command Line Editing) is used to read commands from the user's terminal.
6. Bash inspects the value of the ignoreeof option to set -o instead of exiting immediately when it receives an EOF on its standard input when reading a command (see section 4.3 The Set Builtin).
7. Command history (see section 9.1 Bash History Facilities) and history expansion (see section 9.3 History Expansion) are enabled by default. Bash will save the command history to the file named by $HISTFILE when an interactive shell exits.
8. Alias expansion (see section 6.6 Aliases) is performed by default.
9. In the absence of any traps, Bash ignores SIGTERM (see section 3.7.6 Signals).
10. In the absence of any traps, SIGINT is caught and handled ((see section 3.7.6 Signals). SIGINT will interrupt some shell builtins.
11. An interactive login shell sends a SIGHUP to all jobs on exit if the hupoxexit shell option has been enabled (see section 3.7.6 Signals).
12. The `-n' invocation option is ignored, and `set -n' has no effect (see section 4.3 The Set Builtin).
13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH, and MAILCHECK shell variables (see section 5.2 Bash Variables).
14. Expansion errors due to references to unbound shell variables after `set -u' has been enabled will not cause the shell to exit (see section 4.3 The Set Builtin).
15. The shell will not exit on expansion errors caused by var being unset or null in ${var:?word} expansions (see section 3.5.3 Shell Parameter Expansion).
16. Redirection errors encountered by shell builtins will not cause the shell to exit.
17. When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see section 6.11 Bash POSIX Mode).
18. A failed exec will not cause the shell to exit (see section 4.1 Bourne Shell Builtins).
19. Parser syntax errors will not cause the shell to exit.
20. Simple spelling correction for directory arguments to the cd builtin is enabled by default (see the description of the cdspell option to the shopt builtin in 4.2 Bash Builtin Commands).
21. The shell will check the value of the TMOUT variable and exit if a command is not read within the specified number of seconds after printing $PS1 (see section 5.2 Bash Variables).

 

 

6.4 Bash Conditional Expressions

Conditional expressions are used by the [[ compound command and the test and [ builtin commands.

Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. If the file argument to one of the primaries is of the form `/dev/fd/N', then file descriptor N is checked. If the file argument to one of the primaries is one of `/dev/stdin', `/dev/stdout', or `/dev/stderr', file descriptor 0, 1, or 2, respectively, is checked.

-a file

True if file exists.

-b file

True if file exists and is a block special file.

-c file

True if file exists and is a character special file.

-d file

True if file exists and is a directory.

-e file

True if file exists.

-f file

True if file exists and is a regular file.

-g file

True if file exists and its set-group-id bit is set.

-h file

True if file exists and is a symbolic link.

-k file

True if file exists and its "sticky" bit is set.

-p file

True if file exists and is a named pipe (FIFO).

-r file

True if file exists and is readable.

-s file

True if file exists and has a size greater than zero.

-t fd

True if file descriptor fd is open and refers to a terminal.

-u file

True if file exists and its set-user-id bit is set.

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-O file

True if file exists and is owned by the effective user id.

-G file

True if file exists and is owned by the effective group id.

-L file

True if file exists and is a symbolic link.

-S file

True if file exists and is a socket.

-N file

True if file exists and has been modified since it was last read.

file1 -nt file2

True if file1 is newer (according to modification date) than file2.

file1 -ot file2

True if file1 is older than file2.

file1 -ef file2

True if file1 and file2 have the same device and inode numbers.

-o optname

True if shell option optname is enabled. The list of options appears in the description of the `-o' option to the set builtin (see section 4.3 The Set Builtin).

-z string

True if the length of string is zero.

-n string

string

True if the length of string is non-zero.

string1 == string2

True if the strings are equal. `=' may be used in place of `=='.

string1 != string2

True if the strings are not equal.

string1 < string2

True if string1 sorts before string2 lexicographically in the current locale.

string1 > string2

True if string1 sorts after string2 lexicographically in the current locale.

arg1 OP arg2

OP is one of `-eq', `-ne', `-lt', `-le', `-gt', or `-ge'. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.

 

 

6.5 Shell Arithmetic

The shell allows arithmetic expressions to be evaluated, as one of the shell expansions or by the let builtin.

Evaluation is done in long integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence and associativity are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

id++ id--

variable post-increment and post-decrement

++id --id

variable pre-increment and pre-decrement

- +

unary minus and plus

! ~

logical and bitwise negation

**

exponentiation

* / %

multiplication, division, remainder

+ -

addition, subtraction

<< >>

left and right bitwise shifts

<= >= < >

comparison

== !=

equality and inequality

&

bitwise AND

^

bitwise exclusive OR

|

bitwise OR

&&

logical AND

||

logical OR

expr ? expr : expr

conditional evaluation

= *= /= %= += -= <<= >>= &= ^= |=

assignment

expr1 , expr2

comma

Constants with a leading 0 are interpreted as octal numbers. A leading `0x' or `0X' denotes hexadecimal. Otherwise, numbers take the form [base#]n, where base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base. If base# is omitted, then base 10 is used. The digits greater than 9 are represented by the lowercase letters, the uppercase letters, `@', and `_', in that order. If base is less than or equal to 36, lowercase and uppercase letters may be used interchangably to represent numbers between 10 and 35.

Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.

6.6 Aliases

Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands.

The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The alias name and the replacement text may contain any valid shell input, including shell metacharacters, with the exception that the alias name may not contain `='. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias ls to "ls -F", for instance, and Bash does not try to recursively expand the replacement text. If the last character of the alias value is a space or tab character, then the next command word following the alias is also checked for alias expansion.

Aliases are created and listed with the alias command, and removed with the unalias command.

There is no mechanism for using arguments in the replacement text, as in csh. If arguments are needed, a shell function should be used (see section 3.3 Shell Functions).

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see section 4.2 Bash Builtin Commands).

The rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete line of input before executing any of the commands on that line. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the next line of input is read. The commands following the alias definition on that line are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a compound command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use alias in compound commands.

For almost every purpose, shell functions are preferred over aliases.

6.7 Arrays

Bash provides one-dimensional array variables. Any variable may be used as an array; the declare builtin will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement that members be indexed or assigned contiguously. Arrays are zero-based.

An array is created automatically if any variable is assigned to using the syntax

name[subscript]=value

The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal to zero. To explicitly declare an array, use

declare -a name

The syntax

declare -a name[subscript]

is also accepted; the subscript is ignored. Attributes may be specified for an array variable using the declare and readonly builtins. Each attribute applies to all members of an array.

Arrays are assigned to using compound assignments of the form

name=(value1 ... valuen)

where each value is of the form [[subscript]=]string. If the optional subscript is supplied, that index is assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero. This syntax is also accepted by the declare builtin. Individual array elements may be assigned to using the name[subscript]=value syntax introduced above.

Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid conflicts with the shell's filename expansion operators. If the subscript is `@' or `*', the word expands to all members of the array name. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS variable, and ${name[@]} expands each element of name to a separate word. When there are no array members, ${name[@]} expands to nothing. This is analogous to the expansion of the special parameters `@' and `*'. ${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is `@' or `*', the expansion is the number of elements in the array. Referencing an array variable without a subscript is equivalent to referencing element zero.

The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index subscript. unset name, where name is an array, removes the entire array. A subscript of `*' or `@' also removes the entire array.

The declare, local, and readonly builtins each accept a `-a' option to specify an array. The read builtin accepts a `-a' option to assign a list of words read from the standard input to an array, and can read values from the standard input into individual array elements. The set and declare builtins display array values in a way that allows them to be reused as input.

6.8 The Directory Stack

 

6.8.1 Directory Stack Builtins

Bash builtin commands to manipulate the directory stack.

The contents of the directory stack are also visible as the value of the DIRSTACK shell variable.

6.8.1 Directory Stack Builtins

dirs

dirs [+N | -N] [-clpv]

Display the list of currently remembered directories. Directories are added to the list with the pushd command; the popd command removes directories from the list.

+N

Displays the Nth directory (counting from the left of the list printed by dirs when invoked without options), starting with zero.

-N

Displays the Nth directory (counting from the right of the list printed by dirs when invoked without options), starting with zero.

-c

Clears the directory stack by deleting all of the elements.

-l

Produces a longer listing; the default listing format uses a tilde to denote the home directory.

-p

Causes dirs to print the directory stack with one entry per line.

-v

Causes dirs to print the directory stack with one entry per line, prefixing each entry with its index in the stack.

popd

popd [+N | -N] [-n]

Remove the top entry from the directory stack, and cd to the new top directory. When no arguments are given, popd removes the top directory from the stack and performs a cd to the new top directory. The elements are numbered from 0 starting at the first directory listed with dirs; i.e., popd is equivalent to popd +0.

+N

Removes the Nth directory (counting from the left of the list printed by dirs), starting with zero.

-N

Removes the Nth directory (counting from the right of the list printed by dirs), starting with zero.

-n

Suppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated.

pushd

pushd [dir | +N | -N] [-n]

Save the current directory on the top of the directory stack and then cd to dir. With no arguments, pushd exchanges the top two directories.

+N

Brings the Nth directory (counting from the left of the list printed by dirs, starting with zero) to the top of the list by rotating the stack.

-N

Brings the Nth directory (counting from the right of the list printed by dirs, starting with zero) to the top of the list by rotating the stack.

-n

Suppresses the normal change of directory when adding directories to the stack, so that only the stack is manipulated.

dir

Makes the current working directory be the top of the stack, and then executes the equivalent of `cd dir'. cds to dir.

6.9 Controlling the Prompt

The value of the variable PROMPT_COMMAND is examined just before Bash prints each primary prompt. If PROMPT_COMMAND is set and has a non-null value, then the value is executed just as if it had been typed on the command line.

In addition, the following table describes the special characters which can appear in the prompt variables:

\a

A bell character.

\d

The date, in "Weekday Month Date" format (e.g., "Tue May 26").

\e

An escape character.

\h

The hostname, up to the first `.'.

\H

The hostname.

\j

The number of jobs currently managed by the shell.

\l

The basename of the shell's terminal device name.

\n

A newline.

\r

A carriage return.

\s

The name of the shell, the basename of $0 (the portion following the final slash).

\t

The time, in 24-hour HH:MM:SS format.

\T

The time, in 12-hour HH:MM:SS format.

\@

The time, in 12-hour am/pm format.

\A

The time, in 24-hour HH:MM format.

\u

The username of the current user.

\v

The version of Bash (e.g., 2.00)

\V

The release of Bash, version + patchlevel (e.g., 2.00.0)

\w

The current working directory.

\W

The basename of $PWD.

\!

The history number of this command.

\#

The command number of this command.

\$

If the effective uid is 0, #, otherwise $.

\nnn

The character whose ASCII code is the octal value nnn.

\\

A backslash.

\[

Begin a sequence of non-printing characters. This could be used to embed a terminal control sequence into the prompt.

\]

End a sequence of non-printing characters.

After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see section 4.2 Bash Builtin Commands).

6.10 The Restricted Shell

If Bash is started with the name rbash, or the `--restricted' option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. A restricted shell behaves identically to bash with the exception that the following are disallowed:

• Changing directories with the cd builtin.
• Setting or unsetting the values of the SHELL, PATH, ENV, or BASH_ENV variables.
• Specifying command names containing slashes.
• Specifying a filename containing a slash as an argument to the . builtin command.
• Specifying a filename containing a slash as an argument to the `-p' option to the hash builtin command.
• Importing function definitions from the shell environment at startup.
• Parsing the value of SHELLOPTS from the shell environment at startup.
• Redirecting output using the `>', `>|', `<>', `>&', `&>', and `>>' redirection operators.
• Using the exec builtin to replace the shell with another command.
• Adding or deleting builtin commands with the `-f' and `-d' options to the enable builtin.
• Specifying the `-p' option to the command builtin.
• Turning off restricted mode with `set +r' or `set +o restricted'.

6.11 Bash POSIX Mode

Starting Bash with the `--posix' command-line option or executing `set -o posix' while Bash is running will cause Bash to conform more closely to the POSIX 1003.2 standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.

The following list is what's changed when `POSIX mode' is in effect:

1. When a command in the hash table no longer exists, Bash will re-search $PATH to find the new location. This is also available with `shopt -s checkhash'.
2. The message printed by the job control code and builtins when a job exits with a non-zero status is `Done(status)'.
3. The message printed by the job control code and builtins when a job is stopped is `Stopped(signame)', where signame is, for example, SIGTSTP.
4. Reserved words may not be aliased.
5. The POSIX 1003.2 PS1 and PS2 expansions of `!' to the history number and `!!' to `!' are enabled, and parameter expansion is performed on the values of PS1 and PS2 regardless of the setting of the promptvars option.
6. Interactive comments are enabled by default. (Bash has them on by default anyway.)
7. The POSIX 1003.2 startup files are executed ($ENV) rather than the normal Bash files.
8. Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
9. The default history file is `~/.sh_history' (this is the default value of $HISTFILE).
10. The output of `kill -l' prints all the signal names on a single line, separated by spaces.
11. Non-interactive shells exit if filename in . filename is not found.
12. Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
13. Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.
14. Redirection operators do not perform word splitting on the word in the redirection.
15. Function names must be valid shell names. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells.
16. POSIX 1003.2 `special' builtins are found before shell functions during command lookup.
17. If a POSIX 1003.2 special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX.2 standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.
18. If the cd builtin finds a directory to change to using $CDPATH, the value it assigns to the PWD variable does not contain any symbolic links, as if `cd -P' had been executed.
19. If CDPATH is set, the cd builtin will not implicitly append the current directory to it. This means that cd will fail if no valid directory name can be constructed from any of the entries in $CDPATH, even if the a directory with the same name as the name given as an argument to cd exists in the current directory.
20. A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.
21. A non-interactive shell exits with an error status if the iteration variable in a for statement or the selection variable in a select statement is a readonly variable.
22. Process substitution is not available.
23. Assignment statements preceding POSIX 1003.2 special builtins persist in the shell environment after the builtin completes.
24. Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a POSIX special builtin command had been executed.
25. The export and readonly builtin commands display their output in the format required by POSIX 1003.2.
26. The trap builtin displays signal names without the leading SIG.
27. The . and source builtins do not search the current directory for the filename argument if it is not found by searching PATH.
28. Subshells spawned to execute command substitutions inherit the value of the `-e' option from the parent shell. When not in POSIX mode, Bash clears the `-e' option in such subshells.
29. Alias expansion is always enabled, even in non-interactive shells.
30. When the set builtin is invoked without options, it does not display shell function names and definitions.
31. When the set builtin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters.

1. Assignment statements affect the execution environment of all builtins, not just special ones.
2. When a subshell is created to execute a shell script with execute permission, but without a leading `#!', Bash sets $0 to the full pathname of the script as found by searching $PATH, rather than the command as typed by the user.
3. When using `.' to source a shell script found in $PATH, bash checks execute permission bits rather than read permission bits, just as if it were searching for a command.

 

 

7. Job Control

This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities.

 

7.1 Job Control Basics		How job control works.
7.2 Job Control Builtins		Bash builtin commands used to interact with job control.
7.3 Job Control Variables		Variables Bash uses to customize job control.

7.1 Job Control Basics

Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the system's terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which may be listed with the jobs command. When Bash starts a job asynchronously, it prints a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the operating system maintains the notion of a current terminal process group ID. Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the foreground. Background processes are those whose process group ID differs from the terminal's; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or write to the terminal. Background processes which attempt to read from (write to) the terminal are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which, unless caught, suspends the process.

If the operating system on which Bash is running supports job control, Bash contains facilities to use it. Typing the suspend character (typically `^Z', Control-Z) while a process is running causes that process to be stopped and returns control to Bash. Typing the delayed suspend character (typically `^Y', Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash. The user then manipulates the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it. A `^Z' takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The character `%' introduces a job name.

Job number n may be referred to as `%n'. The symbols `%%' and `%+' refer to the shell's notion of the current job, which is the last job stopped while it was in the foreground or started in the background. The previous job may be referenced using `%-'. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a `+', and the previous job with a `-'.

A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, `%ce' refers to a stopped ce job. Using `%?ce', on the other hand, refers to any job containing the string `ce' in its command line. If the prefix or substring matches more than one job, Bash reports an error.

Simply naming a job can be used to bring it into the foreground: `%1' is a synonym for `fg %1', bringing job 1 from the background into the foreground. Similarly, `%1 &' resumes job 1 in the background, equivalent to `bg %1'

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job's status so as to not interrupt any other output. If the `-b' option to the set builtin is enabled, Bash reports such changes immediately (see section 4.3 The Set Builtin). Any trap on SIGCHLD is executed for each child process that exits.

If an attempt to exit Bash is while jobs are stopped, the shell prints a message warning that there are stopped jobs. The jobs command may then be used to inspect their status. If a second attempt to exit is made without an intervening command, Bash does not print another warning, and the stopped jobs are terminated.

7.2 Job Control Builtins

bg

bg [jobspec]

Resume the suspended job jobspec in the background, as if it had been started with `&'. If jobspec is not supplied, the current job is used. The return status is zero unless it is run when job control is not enabled, or, when run with job control enabled, if jobspec was not found or jobspec specifies a job that was started without job control.

fg

fg [jobspec]

Resume the job jobspec in the foreground and make it the current job. If jobspec is not supplied, the current job is used. The return status is that of the command placed into the foreground, or non-zero if run when job control is disabled or, when run with job control enabled, jobspec does not specify a valid job or jobspec specifies a job that was started without job control.

jobs

jobs [-lnprs] [jobspec] jobs -x command [arguments]

The first form lists the active jobs. The options have the following meanings:

-l

List process IDs in addition to the normal information.

-n

Display information only about jobs that have changed status since the user was last notified of their status.

-p

List only the process ID of the job's process group leader.

-r

Restrict output to running jobs.

-s

Restrict output to stopped jobs.

If jobspec is given, output is restricted to information about that job. If jobspec is not supplied, the status of all jobs is listed.

If the `-x' option is supplied, jobs replaces any jobspec found in command or arguments with the corresponding process group ID, and executes command, passing it arguments, returning its exit status.

kill

kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid kill -l [exit_status]

Send a signal specified by sigspec or signum to the process named by job specification jobspec or process ID pid. sigspec is either a signal name such as SIGINT (with or without the SIG prefix) or a signal number; signum is a signal number. If sigspec and signum are not present, SIGTERM is used. The `-l' option lists the signal names. If any arguments are supplied when `-l' is given, the names of the signals corresponding to the arguments are listed, and the return status is zero. exit_status is a number specifying a signal number or the exit status of a process terminated by a signal. The return status is zero if at least one signal was successfully sent, or non-zero if an error occurs or an invalid option is encountered.

wait

wait [jobspec or pid]

Wait until the child process specified by process ID pid or job specification jobspec exits and return the exit status of the last command waited for. If a job spec is given, all processes in the job are waited for. If no arguments are given, all currently active child processes are waited for, and the return status is zero. If neither jobspec nor pid specifies an active child process of the shell, the return status is 127.

disown

disown [-ar] [-h] [jobspec ...]

Without options, each jobspec is removed from the table of active jobs. If the `-h' option is given, the job is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If jobspec is not present, and neither the `-a' nor `-r' option is supplied, the current job is used. If no jobspec is supplied, the `-a' option means to remove or mark all jobs; the `-r' option without a jobspec argument restricts operation to running jobs.

suspend

suspend [-f]

Suspend the execution of this shell until it receives a SIGCONT signal. The `-f' option means to suspend even if the shell is a login shell.