• 1 Introduction
  • 1.1 What is Bash?
  • 1.2 What is a shell?
• 2 Definitions
• 3 Basic Shell Features
  • 3.1 Shell Syntax
    • 3.1.1 Shell Operation
    • 3.1.2 Quoting
      • 3.1.2.1 Escape Character
      • 3.1.2.2 Single Quotes
      • 3.1.2.3 Double Quotes
      • 3.1.2.4 ANSI-C Quoting
      • 3.1.2.5 Locale-Specific Translation
    • 3.1.3 Comments
  • 3.2 Shell Commands
    • 3.2.1 Reserved Words
    • 3.2.2 Simple Commands
    • 3.2.3 Pipelines
    • 3.2.4 Lists of Commands
    • 3.2.5 Compound Commands
      • 3.2.5.1 Looping Constructs
      • 3.2.5.2 Conditional Constructs
      • 3.2.5.3 Grouping Commands
    • 3.2.6 Coprocesses
    • 3.2.7 GNU Parallel
  • 3.3 Shell Functions
  • 3.4 Shell Parameters
    • 3.4.1 Positional Parameters
    • 3.4.2 Special Parameters
  • 3.5 Shell Expansions
    • 3.5.1 Brace Expansion
    • 3.5.2 Tilde Expansion
    • 3.5.3 Shell Parameter Expansion
    • 3.5.4 Command Substitution
    • 3.5.5 Arithmetic Expansion
    • 3.5.6 Process Substitution
    • 3.5.7 Word Splitting
    • 3.5.8 Filename Expansion
      • 3.5.8.1 Pattern Matching
    • 3.5.9 Quote Removal
  • 3.6 Redirections
    • 3.6.1 Redirecting Input
    • 3.6.2 Redirecting Output
    • 3.6.3 Appending Redirected Output
    • 3.6.4 Redirecting Standard Output and Standard Error
    • 3.6.5 Appending Standard Output and Standard Error
    • 3.6.6 Here Documents
    • 3.6.7 Here Strings
    • 3.6.8 Duplicating File Descriptors
    • 3.6.9 Moving File Descriptors
    • 3.6.10 Opening File Descriptors for Reading and Writing
  • 3.7 Executing Commands
    • 3.7.1 Simple Command Expansion
    • 3.7.2 Command Search and Execution
    • 3.7.3 Command Execution Environment
    • 3.7.4 Environment
    • 3.7.5 Exit Status
    • 3.7.6 Signals
  • 3.8 Shell Scripts
• 4 Shell Builtin Commands
  • 4.1 Bourne Shell Builtins
  • 4.2 Bash Builtin Commands
  • 4.3 Modifying Shell Behavior
    • 4.3.1 The Set Builtin
    • 4.3.2 The Shopt Builtin
  • 4.4 Special Builtins
• 5 Shell Variables
  • 5.1 Bourne Shell Variables
  • 5.2 Bash Variables
• 6 Bash Features
  • 6.1 Invoking Bash
  • 6.2 Bash Startup Files
  • 6.3 Interactive Shells
    • 6.3.1 What is an Interactive Shell?
    • 6.3.2 Is this Shell Interactive?
    • 6.3.3 Interactive Shell Behavior
  • 6.4 Bash Conditional Expressions
  • 6.5 Shell Arithmetic
  • 6.6 Aliases
  • 6.7 Arrays
  • 6.8 The Directory Stack
    • 6.8.1 Directory Stack Builtins
  • 6.9 Controlling the Prompt
  • 6.10 The Restricted Shell
  • 6.11 Bash and POSIX
    • 6.11.1 What is POSIX?
    • 6.11.2 Bash POSIX Mode
  • 6.12 Shell Compatibility Mode
• 7 Job Control
  • 7.1 Job Control Basics
  • 7.2 Job Control Builtins
  • 7.3 Job Control Variables
• 8 Command Line Editing
  • 8.1 Introduction to Line Editing
  • 8.2 Readline Interaction
    • 8.2.1 Readline Bare Essentials
    • 8.2.2 Readline Movement Commands
    • 8.2.3 Readline Killing Commands
    • 8.2.4 Readline Arguments
    • 8.2.5 Searching for Commands in the History
  • 8.3 Readline Init File
    • 8.3.1 Readline Init File Syntax
    • 8.3.2 Conditional Init Constructs
    • 8.3.3 Sample Init File
  • 8.4 Bindable Readline Commands
    • 8.4.1 Commands For Moving
    • 8.4.2 Commands For Manipulating The History
    • 8.4.3 Commands For Changing Text
    • 8.4.4 Killing And Yanking
    • 8.4.5 Specifying Numeric Arguments
    • 8.4.6 Letting Readline Type For You
    • 8.4.7 Keyboard Macros
    • 8.4.8 Some Miscellaneous Commands
  • 8.5 Readline vi Mode
  • 8.6 Programmable Completion
  • 8.7 Programmable Completion Builtins
  • 8.8 A Programmable Completion Example
• 9 Using History Interactively
  • 9.1 Bash History Facilities
  • 9.2 Bash History Builtins
  • 9.3 History Expansion
    • 9.3.1 Event Designators
    • 9.3.2 Word Designators
    • 9.3.3 Modifiers
• 10 Installing Bash
  • 10.1 Basic Installation
  • 10.2 Compilers and Options
  • 10.3 Compiling For Multiple Architectures
  • 10.4 Installation Names
  • 10.5 Specifying the System Type
  • 10.6 Sharing Defaults
  • 10.7 Operation Controls
  • 10.8 Optional Features
• Appendix A Reporting Bugs
• Appendix B Major Differences From The Bourne Shell
  • B.1 Implementation Differences From The SVR4.2 Shell
• Appendix C GNU Free Documentation License
• Appendix D Indexes
  • D.1 Index of Shell Builtin Commands
  • D.2 Index of Shell Reserved Words
  • D.3 Parameter and Variable Index
  • D.4 Function Index
  • D.5 Concept Index

Next: What is a shell?, Up: Introduction   [Contents][Index]

1.1 What is Bash? ¶

Bash is the shell, or command language interpreter, for the GNU operating system. The name is an acronym for the ‘Bourne-Again SHell’, a pun on Stephen Bourne, the author of the direct ancestor of the current Unix shell sh, which appeared in the Seventh Edition Bell Labs Research version of Unix.

Bash is largely compatible with sh and incorporates useful features from the Korn shell ksh and the C shell csh. It is intended to be a conformant implementation of the IEEE POSIX Shell and Tools portion of the IEEE POSIX specification (IEEE Standard 1003.1). It offers functional improvements over sh for both interactive and programming use.

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

Previous: What is Bash?, Up: Introduction   [Contents][Index]

1.2 What is a shell? ¶

At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions.

A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilities to be combined. Users can create files containing commands, and these become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.

Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file or a string.

A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands’ environments.

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

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

Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. This manual describes how Bash provides all of these features.

Next: Shell Commands, Up: Basic Shell Features   [Contents][Index]

3.1 Shell Syntax ¶

When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the rest of that line.

Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters.

The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command’s exit status, and makes that exit status available for further inspection or processing.

• Shell Operation
• Quoting
• Comments

bash --dump-po-strings scriptname > domain.pot

cp example.pot es.po

msgfmt -o es.mo es.po

TEXTDOMAIN=example
TEXTDOMAINDIR=/usr/local/share/locale
cp es.mo ${TEXTDOMAINDIR}/es/LC_MESSAGES/${TEXTDOMAIN}.mo
cp eo.mo ${TEXTDOMAINDIR}/eo/LC_MESSAGES/${TEXTDOMAIN}.mo

Next: Shell Functions, Previous: Shell Syntax, Up: Basic Shell Features   [Contents][Index]

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.

• Reserved Words
• Simple Commands
• Pipelines
• Lists of Commands
• Compound Commands
• Coprocesses
• GNU Parallel

[time [-p]] [!] command1 [ | or |& command2 ] ...

command1 && command2

command1 || command2

coproc [NAME] command [redirections]

coproc NAME { command; }

coproc NAME compound-command
coproc compound-command
coproc simple-command

Next: Shell Parameters, Previous: Shell Commands, Up: Basic Shell Features   [Contents][Index]

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" simple command. When the name of a shell function is used as a simple command name, the shell executes the list of commands associated with that function name. Shell functions are executed in the current shell context; there is no new process created to interpret them.

Functions are declared using this syntax:

fname () compound-command [ redirections ]

or

function fname [()] compound-command [ redirections ]

This defines a shell function named fname. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands). That command is usually a list enclosed between { and }, but may be any compound command listed above. If the function reserved word is used, but the parentheses are not supplied, the braces are recommended. When the shell is in POSIX mode (see Bash and POSIX), fname must be a valid shell name and may not be the same as one of the special builtins (see Special Builtins). When not in POSIX mode, a function name can be any unquoted shell word that does not contain ‘$’.

Any redirections (see Redirections) associated with the shell function are performed when the function is executed. Function definitions are deleted using the -f option to the unset builtin (see Bourne Shell Builtins).

The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body.

Note that for historical reasons, in the most common usage the curly braces that surround the body of the function must be separated from the body by blanks or newlines. This is because the braces are reserved words and are only recognized as such when they are separated from the command list by whitespace or another shell metacharacter. When using the braces, the list must be terminated by a semicolon, a ‘&’, or a newline.

compound-command is executed whenever fname is specified as the name of a simple command. Functions are executed in the context of the calling shell; there is no new process created to interpret them (contrast this with the execution of a shell script).

When a function is executed, the arguments to the function become the positional parameters during its execution (see Positional Parameters). The special parameter ‘#’ that expands to the number of positional parameters is updated to reflect the new set of positional parameters. Special parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is executing.

All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the DEBUG and RETURN traps are not inherited unless the function has been given the trace attribute using the declare builtin or the -o functrace option has been enabled with the set builtin, (in which case all functions inherit the DEBUG and RETURN traps), and the ERR trap is not inherited unless the -o errtrace shell option has been enabled. See Bourne Shell Builtins, for the description of the trap builtin.

The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort.

If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. Any command associated with the RETURN trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter ‘#’ are restored to the values they had prior to the function’s execution. If return is supplied a numeric argument, 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 are declared with the local builtin (local variables). Ordinarily, variables and their values are shared between a function and its caller. These variables are visible only to the function and the commands it invokes. This is particularly important when a shell function calls other functions.

In the following description, the current scope is a currently- executing function. Previous scopes consist of that function’s caller and so on, back to the "global" scope, where the shell is not executing any shell function. A local variable at the current local scope is a variable declared using the local or declare builtins in the function that is currently executing.

Local variables "shadow" variables with the same name declared at previous scopes. For instance, a local variable declared in a function hides variables with the same name declared at previous scopes, including global variables: references and assignments refer to the local variable, leaving the variables at previous scopes unmodified. When the function returns, the global variable is once again visible.

The shell uses dynamic scoping to control a variable’s visibility within functions. With dynamic scoping, visible variables and their values are a result of the sequence of function calls that caused execution to reach the current function. The value of a variable that a function sees depends on its value within its caller, if any, whether that caller is the global scope or another shell function. This is also the value that a local variable declaration shadows, and the value that is restored when the function returns.

For example, if a variable var is declared as local in function func1, and func1 calls another function func2, references to var made from within func2 resolve to the local variable var from func1, shadowing any global variable named var.

The following script demonstrates this behavior. When executed, the script displays

In func2, var = func1 local

func1()
{
    local var='func1 local'
    func2
}
func2()
{
    echo "In func2, var = $var"
}
var=global
func1

The unset builtin also acts using the same dynamic scope: if a variable is local to the current scope, unset unsets it; otherwise the unset will refer to the variable found in any calling scope as described above. If a variable at the current local scope is unset, it remains so (appearing as unset) until it is reset in that scope or until the function returns. Once the function returns, any instance of the variable at a previous scope becomes visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed becomes visible (see below how the localvar_unset shell option changes this behavior).

The -f option to the declare (typeset) builtin command (see Bash Builtin Commands) lists function names and definitions. The -F option to declare or typeset lists the function names only (and optionally the source file and line number, if the extdebug shell option is enabled). Functions may be exported so that child shell processes (those created when executing a separate shell invocation) automatically have them defined with the -f option to the export builtin (see Bourne Shell Builtins). The -f option to the unset builtin (see Bourne Shell Builtins) deletes a function definition.

Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, Bash places no limit on the number of recursive calls.

Next: Shell Expansions, Previous: Shell Functions, Up: Basic Shell Features   [Contents][Index]

3.4 Shell Parameters ¶

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see the description of the declare builtin in Bash Builtin Commands). The export and readonly builtins assign specific attributes.

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 is assigned to using 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 (see Shell Parameter Expansion). If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((…)) expansion is not used (see Arithmetic Expansion). Word splitting and filename expansion are not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). When in POSIX mode (see Bash and POSIX), these builtins may appear in a command after one or more instances of the command builtin and retain these assignment statement properties. For example,

command export var=value

In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘+=’ operator appends to or adds to the variable’s previous value. This includes arguments to declaration commands such as declare that accept assignment statements. When ‘+=’ is applied to a variable for which the integer attribute has been set, the variable’s current value and value are each evaluated as arithmetic expressions, and the sum of the results is assigned as the variable’s value. The current value is usually an integer constant, but may be an expression. When ‘+=’ is applied to an array variable using compound assignment (see Arrays), the variable’s value is not unset (as it is when using ‘=’), and new values are appended to the array beginning at one greater than the array’s maximum index (for indexed arrays), or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable’s value.

A variable can be assigned the nameref attribute using the -n option to the declare or local builtin commands (see Bash Builtin Commands) to create a nameref, or a reference to another variable. This allows variables to be manipulated indirectly. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself), the operation is actually performed on the variable specified by the nameref variable’s value. A nameref is commonly used within shell functions to refer to a variable whose name is passed as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running

declare -n ref=$1

inside the function creates a local nameref variable ref whose value is the variable name passed as the first argument. References and assignments to ref, and changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1.

If the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference is established for each word in the list, in turn, when the loop is executed. Array variables cannot be given the nameref attribute. However, nameref variables can reference array variables and subscripted array variables. Namerefs can be unset using the -n option to the unset builtin (see Bourne Shell Builtins). Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable is unset.

When the shell starts, it reads its environment and creates a shell variable from each environment variable that has a valid name, as described below (see Environment).

• Positional Parameters
• Special Parameters

Next: Redirections, Previous: Shell Parameters, Up: Basic Shell Features   [Contents][Index]

3.5 Shell Expansions ¶

Expansion is performed on the command line after it has been split into tokens. Bash performs these expansions:

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

The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; filename expansion; and quote removal.

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

Quote removal is always performed last. It removes quote characters present in the original word, not ones resulting from one of the other expansions, unless they have been quoted themselves. See Quote Removal for more details.

Only brace expansion, word splitting, and filename expansion can increase 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 "$@" and $* (see Special Parameters), and "${name[@]}" and ${name[*]} (see Arrays).

• Brace Expansion
• Tilde Expansion
• Shell Parameter Expansion
• Command Substitution
• Arithmetic Expansion
• Process Substitution
• Word Splitting
• Filename Expansion
• Quote Removal

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

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

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

$(command)

`command`.

${c command; }

${ local X=12345 ; echo $X; }

${| REPLY=12345; }

$(( expression ))

<(list)

>(list)

Next: Executing Commands, Previous: Shell Expansions, Up: Basic Shell Features   [Contents][Index]

3.6 Redirections ¶

Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. When used with the exec builtin, redirections modify file handles in 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.

Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell allocates a file descriptor greater than or equal to 10 and assigns it to {varname}. If {varname} precedes >&- or <&-, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, which allows the shell programmer to manage the file descriptor’s lifetime manually without using the exec builtin. The varredir_close shell option manages this behavior (see The Shopt Builtin).

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 and variable expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

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 made a copy of the 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. If the operating system on which Bash is running provides these special files, Bash uses them; otherwise it emulates them internally with the behavior described below.

/dev/fd/fd

If fd is a valid integer, duplicate file descriptor fd.

/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 the corresponding TCP 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 the corresponding UDP socket.

A failure to open or create a file causes the redirection to fail.

Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally.

• Redirecting Input
• Redirecting Output
• Appending Redirected Output
• Redirecting Standard Output and Standard Error
• Appending Standard Output and Standard Error
• Here Documents
• Here Strings
• Duplicating File Descriptors
• Moving File Descriptors
• Opening File Descriptors for Reading and Writing

[n]<word

[n]>[|]word

[n]>>word

&>word

>&word

>word 2>&1

&>>word

>>word 2>&1

[n]<<[−]word
        here-document
delimiter

[n]<<< word

[n]<&word

[n]>&word

[n]<&digit-

[n]>&digit-

[n]<>word

Next: Shell Scripts, Previous: Redirections, Up: Basic Shell Features   [Contents][Index]

3.7 Executing Commands ¶

• Simple Command Expansion
• Command Search and Execution
• Command Execution Environment
• Environment
• Exit Status
• Signals

Previous: Executing Commands, Up: Basic Shell Features   [Contents][Index]

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 Invoking Bash), Bash reads and executes commands from the file, then exits. This mode of operation creates a non-interactive shell. If the filename does not contain any slashes, the shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.

Bash tries to determine whether the file is a text file or a binary, and will not execute files it determines to be binaries.

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 creates a new instance of itself to execute it. In other words, executing

filename arguments

is equivalent to executing

bash filename arguments

if filename is an executable shell script. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to interpret the script, with the exception that the locations of commands remembered by the parent (see the description of hash in Bourne Shell Builtins) are retained by the child.

The GNU operating system, and 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 and, depending on the operating system, one or more optional arguments for that interpreter. 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 one or more optional arguments 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 supplied to the script. The details of how the interpreter line is split into an interpreter name and a set of arguments vary across systems. 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 a single argument to a maximum of 32 characters, so it’s not portable to assume that using more than one argument will work.

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. It’s a common idiom to use env to find bash even if it’s been installed in another directory: #!/usr/bin/env bash will find the first occurrence of bash in $PATH.

Next: Bash Builtin Commands, Up: Shell Builtin Commands   [Contents][Index]

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 standard.

: (a colon) ¶

: [arguments]

Do nothing beyond expanding arguments and performing redirections. The return status is zero.

. (a period) ¶

. [-p path] filename [arguments]

The . command reads and execute commands from the filename argument in the current shell context.

If filename does not contain a slash, . searches for it. If -p is supplied, . treats path as a colon-separated list of directories in which to find filename; otherwise, . uses the directories in PATH to find filename. filename does not need to be executable. When Bash is not in POSIX mode, it searches the current directory if filename is not found in $PATH, but does not search the current directory if -p is supplied. If the sourcepath option (see The Shopt Builtin) is turned off, . does not search PATH.

If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged.

If the -T option is enabled, . inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and restored around the call to ., and . unsets the DEBUG trap while it executes. If -T is not set, and the sourced file changes the DEBUG trap, the new value persists after . completes. The return status is the exit status of the last command executed from filename, 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, break exits the nth enclosing loop. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

cd ¶

cd [-L] [-@] [directory]
cd -P [-e] [-@] [directory]

Change the current working directory to directory. If directory is not supplied, the value of the HOME shell variable is used as directory. If the shell variable CDPATH exists, and directory does not begin with a slash, cd uses it as a search path: cd searches each directory name in CDPATH for directory, with alternative directory names in CDPATH separated by a colon (‘:’). A null directory name in CDPATH means the same thing as the current directory.

The -P option means not to follow symbolic links: symbolic links are resolved while cd is traversing directory and before processing an instance of .. in directory.

By default, or when the -L option is supplied, symbolic links in directory are resolved after cd processes an instance of .. in directory.

If .. appears in directory, cd processes it by removing the immediately preceding pathname component, back to a slash or the beginning of directory, and verifying that the portion of directory it has processed to that point is still a valid directory name after removing the pathname component. If it is not a valid directory name, cd returns a non-zero status.

If the -e option is supplied with -P and cd cannot successfully determine the current working directory after a successful directory change, it returns a non-zero status.

On systems that support it, the -@ option presents the extended attributes associated with a file as a directory.

If directory is ‘-’, it is converted to $OLDPWD before attempting the directory change.

If cd uses a non-empty directory name from CDPATH, or if ‘-’ is the first argument, and the directory change is successful, cd writes the absolute pathname of the new working directory to the standard output.

If the directory change is successful, cd sets the value of the PWD environment variable to the new directory name, and sets the OLDPWD environment variable to the value of the current working directory before the change.

The return status is zero if the directory is successfully changed, non-zero otherwise.

continue ¶

continue [n]

continue resumes the next iteration of an enclosing for, while, until, or select loop. If n is supplied, Bash resumes the execution of the nth enclosing loop. 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, separated by spaces. Bash then reads and executes this command and returns its exit status 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. command cannot be a shell builtin or function. The arguments become the arguments to command If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument 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 command cannot be executed for some reason, a non-interactive shell exits, unless the execfail shell option is enabled. In that case, it returns a non-zero status. An interactive shell returns a non-zero status if the file cannot be executed. A subshell exits unconditionally if exec fails.

If command is not 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 subsequently executed commands 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 unexport each name: no longer mark it for export. If no names are supplied, or if only the -p option is given, export displays a list of names of all exported variables on the standard output. Using -p and -f together displays exported functions. The -p option displays output in a form that may be reused as input.

export allows the value of a variable to be set at the same time it is exported or unexported by following the variable name with =value. This sets the value of the variable is to value while modifying the export attribute.

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.

false ¶

false

Does nothing; returns a non-zero status.

getopts ¶

getopts optstring name [arg ...]

getopts is used by shell scripts or functions to parse positional parameters and obtain options and their arguments. 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 whitespace. 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 to use a new set of parameters.

When it reaches the end of options, 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 supplied as arg values, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a colon, getopts uses silent error reporting. In normal operation, getopts prints diagnostic messages when it encounters invalid options or missing option arguments. If the variable OPTERR is set to 0, getopts does not display any error messages, even if the first character of optstring is not a colon.

If getopts detects an invalid option, it places ‘?’ into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, it assigns the option character found to OPTARG and does not print a diagnostic message.

If a required argument is not found, and getopts is not silent, it sets the value of name to a question mark (‘?’), unsets OPTARG, and prints a diagnostic message. If getopts is silent, it sets the value of name to a colon (‘:’), and sets OPTARG to the option character found.

getopts returns true if an option, specified or unspecified, is found. It returns false when it encounters the end of options or if an error occurs.

hash ¶

hash [-r] [-p filename] [-dt] [name]

Each time hash is invoked, it remembers the full filenames of the 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. Any previously-remembered filename associated with name is discarded. The -p option inhibits the path search, and hash uses filename as the location of name.

The -r option causes the shell to forget all remembered locations. Assigning to the PATH variable also clears all hashed filenames. The -d option causes the shell to forget the remembered location of each name.

If the -t option is supplied, hash prints the full pathname corresponding to each name. If multiple name arguments are supplied with -t, hash prints each name before the corresponding hashed full path. The -l option displays output in a format that may be reused as input.

If no arguments are given, or if only -l is supplied, hash prints information about remembered commands. The -t, -d, and -p options (the options that act on the name arguments) are mutually exclusive. Only one will be active. If more than one is supplied, -t has higher priority than -p, and both have higher priority than -d.

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, or the -o physical option to the set builtin (see The Set Builtin) is enabled, 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 [-aAf] [-p] [name[=value]] ...

Mark each name as readonly. The values of these names may not be changed by subsequent assignment or unset. If the -f option is supplied, each name refers to a shell function. The -a option means each name refers to an indexed array variable; the -A option means each name refers to an associative array variable. If both options are supplied, -A takes precedence. If no name arguments are supplied, or if the -p option is supplied, print a list of all readonly names. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option displays output in a format that may be reused as input.

readonly allows the value of a variable to be set at the same time the readonly attribute is changed by following the variable name with =value. This sets the value of the variable is to value while modifying the readonly attribute.

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]

Stop executing a shell function or sourced file and return the value n to its caller. If n is not supplied, the return value is the exit status of the last command executed. If return is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. If return is executed during a DEBUG trap, the last command used to determine the status is the last command executed by the trap handler before return was invoked.

When return is used to terminate execution of a script being executed with the . (source) builtin, it returns either n or the exit status of the last command executed within the script as the exit status of the script. If n is supplied, the return value is its least significant 8 bits.

Any command associated with the RETURN trap is executed before execution resumes after the function or script.

The return status is non-zero if return is supplied a non-numeric argument or 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. Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is not supplied, it is assumed to be 1. If n is zero or greater than $#, the positional parameters are not changed. The return status is zero unless n is greater than $# or less than zero, non-zero otherwise.

test ¶

[

test expr

Evaluate a conditional expression expr and return a status of 0 (true) or 1 (false). Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in Bash Conditional Expressions. test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options. When using the [ form, the last argument to the command must be a ].

Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. test uses operator precedence when there are five or more arguments.

! expr

True if expr is false.

( expr )

Returns the value of expr. This may be used to override normal operator precedence.

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 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

The following conditions are applied in the order listed.

1. If the second argument is one of the binary conditional operators (see Bash Conditional Expressions), the result of the expression is the result of the binary test using the first and third arguments as operands. The ‘-a’ and ‘-o’ operators are considered binary operators when there are three arguments.
2. If the first argument is ‘!’, the value is the negation of the two-argument test using the second and third arguments.
3. If the first argument is exactly ‘(’ and the third argument is exactly ‘)’, the result is the one-argument test of the second argument.
4. Otherwise, the expression is false.

4 arguments

The following conditions are applied in the order listed.

1. If the first argument is ‘!’, the result is the negation of the three-argument expression composed of the remaining arguments.
2. If the first argument is exactly ‘(’ and the fourth argument is exactly ‘)’, the result is the two-argument test of the second and third arguments.
3. 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.

If the shell is in POSIX mode, or if the expression is part of the [[ command, the ‘<’ and ‘>’ operators sort using the current locale. If the shell is not in POSIX mode, the test and ‘[’ commands sort lexicographically using ASCII ordering.

The historical operator-precedence parsing with 4 or more arguments can lead to ambiguities when it encounters strings that look like primaries. The POSIX standard has deprecated the -a and -o primaries and enclosing expressions within parentheses. Scripts should no longer use them. It’s much more reliable to restrict test invocations to a single primary, and to replace uses of -a and -o with the shell’s && and || list operators. For example, use

test -n string1 && test -n string2

instead of

test -n string1 -a -n string2

times ¶

times

Print out the user and system times used by the shell and its children. The return status is zero.

trap ¶

trap [-lpP] [action] [sigspec ...]

The action is a command that is read and executed when the shell receives any of the signals sigspec. If action is absent (and there is a single sigspec) or equal to ‘-’, each specified sigspec’s disposition is reset to the value it had when the shell was started. If action is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes.

If no arguments are supplied, trap prints the actions associated with each trapped signal as a set of trap commands that can be reused as shell input to restore the current signal dispositions.

If action is not present and -p has been supplied, trap displays the trap commands associated with each sigspec, or, if no sigspecs are supplied, for all trapped signals, as a set of trap commands that can be reused as shell input to restore the current signal dispositions. The -P option behaves similarly, but displays only the actions associated with each sigspec argument. -P requires at least one sigspec argument. The -P or -p options may be used in a subshell environment (e.g., command substitution) and, as long as they are used before trap is used to change a signal’s handling, will display the state of its parent’s traps.

The -l option prints a list of signal names and their corresponding numbers. Each sigspec is either a signal name or a signal number. Signal names are case insensitive and the SIG prefix is optional. If -l is supplied with no sigspec arguments, it prints a list of valid signal names.

If a sigspec is 0 or EXIT, action is executed when the shell exits. If a sigspec is DEBUG, action is executed before every simple command, for command, case command, select command, (( arithmetic command, [[ conditional command, arithmetic for command, and before the first command executes in a shell function. Refer to the description of the extdebug shell option (see The Shopt Builtin) for details of its effect on the DEBUG trap. If a sigspec is RETURN, action is executed each time a shell function or a script executed with the . or source builtins finishes executing.

If a sigspec is ERR, action is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following an until or while reserved word, part of the test following the if or elif reserved words, part of a command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, (subject to the state of the pipefail shell option), or if the command’s return status is being inverted using !. These are the same conditions obeyed by the errexit (-e) option.

When the shell is not interactive, signals ignored upon entry to a non-interactive shell cannot be trapped or reset. Interactive shells permit trapping signals ignored on entry. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created.

The return status is zero unless a sigspec does not specify a valid signal; non-zero otherwise.

true ¶

true

Does nothing, returns a 0 status.

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, umask prints the current value of the mask. If the -S option is supplied without a mode argument, umask prints the mask in a symbolic format; the default output is an octal number. 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 [-fnv] [name]

Remove each variable or function name. If the -v option is given, each name refers to a shell variable and that variable is removed. If the -f option is given, the names refer to shell functions, and the function definition is removed. If the -n option is supplied, and name is a variable with the nameref attribute, name will be unset rather than the variable it references. -n has no effect if the -f option is supplied. If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset. Readonly variables and functions may not be unset. When variables or functions are removed, they are also removed from the environment passed to subsequent commands. Some shell variables may not be unset. Some shell variables lose their special behavior if they are unset; such behavior is noted in the description of the individual variables. The return status is zero unless a name is readonly or may not be unset.

Next: Modifying Shell Behavior, Previous: Bourne Shell Builtins, Up: Shell Builtin Commands   [Contents][Index]

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 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, define an alias for each name whose value is given. If no value is given, print the name and value of the alias name. A trailing space in value causes the next word to be checked for alias substitution when the alias is expanded during command parsing. alias returns true unless a name is given (without a corresponding =value) for which no alias has been defined. Aliases are described in Aliases.

bind ¶

bind [-m keymap] [-lsvSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq[: ]shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] keyseq:readline-command
bind [-m keymap] -p|-P [readline-command]
bind readline-command-line

Display current Readline (see Command Line Editing) key and function bindings, bind a key sequence to a Readline function or macro or to a shell command, or set a Readline variable. Each non-option argument is a key binding or command as it would appear in a Readline initialization file (see Readline Init File), but each binding or command must be passed as a separate argument; e.g., ‘"\C-x\C-r":re-read-init-file’.

In the following descriptions, options that display output in a form available to be re-read format their output as commands that would appear in a Readline initialization file or that would be supplied as individual arguments to a bind command.

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 (vi-move is also a synonym); 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 an argument to a subsequent bind command or in a Readline initialization file. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.

-P

List current Readline function names and bindings. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.

-s

Display Readline key sequences bound to macros and the strings they output in such a way that they can be used as an argument to a subsequent bind command or in a Readline initialization file.

-S

Display Readline key sequences bound to macros and the strings they output.

-v

Display Readline variable names and values in such a way that they can be used as an argument to a subsequent bind command or in a Readline initialization file.

-V

List current Readline variable names and values.

-f filename

Read key bindings from filename.

-q function

Display key sequences that invoke the named Readline function.

-u function

Unbind all key sequences bound to the named Readline function.

-r keyseq

Remove any current binding for keyseq.

-x keyseq:shell-command

Cause shell-command to be executed whenever keyseq is entered. The separator between keyseq and shell-command is either whitespace or a colon optionally followed by whitespace. If the separator is whitespace, shell-command must be enclosed in double quotes and Readline expands any of its special backslash-escapes in shell-command before saving it. If the separator is a colon, any enclosing double quotes are optional, and Readline does not expand the command string before saving it. Since the entire key binding expression must be a single argument, it should be enclosed in single quotes. When shell-command is executed, the shell sets the READLINE_LINE variable to the contents of the Readline line buffer and the READLINE_POINT and READLINE_MARK variables to the current location of the insertion point and the saved insertion point (the mark), respectively. The shell assigns any numeric argument the user supplied to the READLINE_ARGUMENT variable. If there was no argument, that variable is not set. If the executed command changes the value of any of READLINE_LINE, READLINE_POINT, or READLINE_MARK, those new values will be reflected in the editing state.

-X

List all key sequences bound to shell commands and the associated commands in a format that can be reused as an argument to a subsequent bind command.

The return status is zero unless an invalid option is supplied or an error occurs.

builtin ¶

builtin [shell-builtin [args]]

Execute the specified shell builtin shell-builtin, passing it args, and return its exit status. This is useful when defining a shell function with the same name as a shell builtin, retaining the functionality of the builtin within the function. The return status is non-zero if shell-builtin is not a shell builtin command.

caller ¶

caller [expr]

Returns the context of any active subroutine call (a shell function or a script executed with the . or source builtins).

Without expr, caller displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as expr, caller displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0.

The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack.

command ¶

command [-pVv] command [arguments ...]

The command builtin 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, command prints a description of command. The -v option displays a single word indicating the command or file name used to invoke command; 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 [-aAfFgiIlnrtux] [-p] [name[=value] ...]

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

The -p option will display the attributes and values of each name. When -p is used with name arguments, additional options, other than -f and -F, are ignored.

When -p is supplied without name arguments, declare will display the attributes and values of all variables having the attributes specified by the additional options. If no other options are supplied with -p, declare will display the attributes and values of all shell variables. The -f option restricts the display to shell functions.

The -F option inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug shell option is enabled using shopt (see The Shopt Builtin), the source file name and line number where each name is defined are displayed as well. -F implies -f.

The -g option forces variables to be created or modified at the global scope, even when declare is executed in a shell function. It is ignored in when declare is not executed in a shell function.

The -I option causes local variables to inherit the attributes (except the nameref attribute) and value of any existing variable with the same name at a surrounding scope. If there is no existing variable, the local variable is initially unset.

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 indexed array variable (see Arrays).

-A

Each name is an associative array variable (see Arrays).

-f

Each name refers to a shell function.

-i

The variable is to be treated as an integer; arithmetic evaluation (see Shell Arithmetic) is performed when the variable is assigned a value.

-l

When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.

-n

Give each name the nameref attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name, except for those using or changing the -n attribute itself, are performed on the variable referenced by name’s value. The nameref attribute cannot be applied to array variables.

-r

Make names readonly. These names cannot then be assigned values by subsequent assignment statements or unset.

-t

Give each name the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling shell. The trace attribute has no special meaning for variables.

-u

When the variable is assigned a value, all lower-case characters are converted to upper-case. The lower-case attribute is disabled.

-x

Mark each name for export to subsequent commands via the environment.

Using ‘+’ instead of ‘-’ turns off the specified attribute instead, with the exceptions that ‘+a’ and ‘+A’ may not be used to destroy array variables and ‘+r’ will not remove the readonly attribute.

When used in a function, declare makes each name local, as with the local command, unless the -g option is supplied. If a variable name is followed by =value, the value of the variable is set to value.

When using -a or -A and the compound assignment syntax to create array variables, additional attributes do not take effect until subsequent assignments.

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

echo ¶

echo [-neE] [arg ...]

Output the args, separated by spaces, terminated with a newline. The return status is 0 unless a write error occurs. If -n is specified, the trailing newline is not printed.

If the -e option is given, echo interprets the following backslash-escaped characters. The -E option disables interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo shell option determines whether or not echo interprets any options and expands these escape characters. echo does not interpret -- to mean the end of options.

echo interprets the following escape sequences:

\a

alert (bell)

\b

backspace

\c

suppress further output

\e

\E

escape

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\0nnn

The eight-bit character whose value is the octal value nnn (zero to three octal digits).

\xHH

The eight-bit character whose value is the hexadecimal value HH (one or two hex digits).

\uHHHH

The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits).

\UHHHHHHHH

The Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits).

echo writes any unrecognized backslash-escaped characters unchanged.

enable ¶

enable [-a] [-dnps] [-f filename] [name ...]

Enable and disable builtin shell commands. Disabling a builtin allows an executable file 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 files.

If -n is supplied, the names are disabled. Otherwise names are enabled. For example, to use the test binary found using $PATH instead of the shell builtin version, type ‘enable -n test’.

If the -p option is supplied, or no name arguments are supplied, print a list of shell builtins. With no other arguments, the list consists of all enabled shell builtins. The -n option means to print only disabled builtins. The -a option means to list each builtin with an indication of whether or not it is enabled. The -s option means to restrict enable to the POSIX special builtins.

The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. If filename does not contain a slash. Bash will use the value of the BASH_LOADABLES_PATH variable as a colon-separated list of directories in which to search for filename. The default for BASH_LOADABLES_PATH is system-dependent, and may include "." to force a search of the current directory. The -d option will delete a builtin loaded with -f. If -s is used with -f, the new builtin becomes a POSIX special builtin (see Special Builtins).

If no options are supplied and a name is not a shell builtin, enable will attempt to load name from a shared object named name, as if the command were ‘enable -f name name’.

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 [-dms] [pattern]

Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all commands matching pattern as described below; otherwise it displays a list of all builtins and shell compound commands.

Options, if supplied, have the following meanings:

-d

Display a short description of each pattern

-m

Display the description of each pattern in a manpage-like format

-s

Display only a short usage synopsis for each pattern

If pattern contains pattern matching characters (see Pattern Matching) it’s treated as a shell pattern and help prints the description of each help topic matching pattern.

If not, and pattern exactly matches the name of a help topic, help prints the description associated with that topic. Otherwise, help performs prefix matching and prints the descriptions of all matching help topics.

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 as an arithmetic expression according to the rules given below in Shell Arithmetic. If the last expression evaluates to 0, let returns 1; otherwise let returns 0.

local ¶

local [option] name[=value] ...

For each argument, create a local variable named name, and assign it 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. It is an error to use local when not within a function.

If name is ‘-’, it makes the set of shell options local to the function in which local is invoked: any shell options changed using the set builtin inside the function after the call to local are restored to their original values when the function returns. The restore is performed as if a series of set commands were executed to restore the values that were in place before the function.

With no operands, local writes a list of local variables to the standard output.

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.

mapfile ¶

mapfile [-d delim] [-n count] [-O origin] [-s count]
    [-t] [-u fd] [-C callback] [-c quantum] [array]

Read lines from the standard input, or from file descriptor fd if the -u option is supplied, into the indexed array variable array. The variable MAPFILE is the default array. Options, if supplied, have the following meanings:

-d

Use the first character of delim to terminate each input line, rather than newline. If delim is the empty string, mapfile will terminate a line when it reads a NUL character.

-n

Copy at most count lines. If count is 0, copy all lines.

-O

Begin assigning to array at index origin. The default index is 0.

-s

Discard the first count lines read.

-t

Remove a trailing delim (default newline) from each line read.

-u

Read lines from file descriptor fd instead of the standard input.

-C

Evaluate callback each time quantum lines are read. The -c option specifies quantum.

-c

Specify the number of lines read between each call to callback.

If -C is specified without -c, the default quantum is 5000. When callback is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. callback is evaluated after the line is read but before the array element is assigned.

If not supplied with an explicit origin, mapfile will clear array before assigning to it.

mapfile returns zero unless an invalid option or option argument is supplied, array is invalid or unassignable, or if array is not an indexed array.

printf ¶

printf [-v var] format [arguments]

Write the formatted arguments to the standard output under the control of the format. The -v option assigns the output to the variable var rather than printing it to the standard output.

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(3) format characters cCsSndiouxXeEfFgGaA, printf interprets the following additional format specifiers:

%b

Causes printf to expand backslash escape sequences in the corresponding argument in the same way as echo -e (see Bash Builtin Commands).

%q

Causes printf to output the corresponding argument in a format that can be reused as shell input. %q and %QP use the ANSI-C quoting style (see ANSI-C Quoting) if any characters in the argument string require it, and backslash quoting otherwise. If the format string uses the printf alternate form, these two formats quote the argument string using single quotes.

%Q

like %q, but applies any supplied precision to the argument before quoting it.

%(datefmt)T

Causes printf to output the date-time string resulting from using datefmt as a format string for strftime(3). The corresponding argument is an integer representing the number of seconds since the epoch. This format specifier recognizes Two special argument values: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been supplied. This is an exception to the usual printf behavior.

The %b, %q, and %T format specifiers all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original.

The %n format specifier accepts a corresponding argument that is treated as a shell variable name.

The %s and %c format specifiers accept an l (long) modifier, which forces them to convert the argument string to a wide-character string and apply any supplied field width and precision in terms of characters, not bytes. The %S and %C format specifiers are equivalent to %ls and %lc, respectively.

Arguments to non-string format specifiers are treated as C language constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the numeric value of the following character, using the current locale.

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 if an invalid option is supplied or a write or assignment error occurs.

read ¶

read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars]
    [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Read one line from the standard input, or from the file descriptor fd supplied as an argument to the -u option, split it into words as described above in Word Splitting, and assign the first word to the first name, the second word to the second name, and so on. If there are more words than names, the remaining words and their intervening delimiters are assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in the value of the IFS variable are used to split the line into words using the same rules the shell uses for expansion (described above in Word Splitting). The backslash character ‘\’ removes any special meaning for the next character read and is used for line continuation.

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 terminates the input line, rather than newline. If delim is the empty string, read will terminate a line when it reads a NUL character.

-e

If the standard input is coming from a terminal, read uses Readline (see Command Line Editing) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Readline’s default filename completion.

-E

If the standard input is coming from a terminal, read uses Readline (see Command Line Editing) to obtain the line. Readline uses the current (or default, if line editing was not previously active) editing settings, but uses Bash’s default completion, including programmable completion.

-i text

If Readline is being used to read the line, read places text into the editing buffer before editing begins.

-n nchars

read returns after reading nchars characters rather than waiting for a complete line of input, unless it encounters EOF or read times out, but honors a delimiter if it reads fewer than nchars characters before the delimiter.

-N nchars

read returns after reading exactly nchars characters rather than waiting for a complete line of input, unless it encounters EOF or read times out. Delimiter characters in the input are not treated specially and do not cause read to return until it has read nchars characters. The result is not split on the characters in IFS; the intent is that the variable is assigned exactly the characters read (with the exception of backslash; see the -r option below).

-p prompt

Display prompt, without a trailing newline, before attempting to read any input, but 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 then 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 it does not read a complete line of input (or a specified number of characters) within timeout seconds. timeout may be a decimal number with a fractional portion following the decimal point. This option is only effective if read is reading input from a terminal, pipe, or other special file; it has no effect when reading from regular files. If read times out, it saves any partial input read into the specified variable name, and returns a status greater than 128. If timeout is 0, read returns immediately, without trying to read any data. In this case, the exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise.

-u fd

Read input from file descriptor fd instead of the standard input.

Other than the case where delim is the empty string, read ignores any NUL characters in the input.

If no names are supplied, read assigns the line read, without the ending delimiter but otherwise unmodified, to the variable REPLY.

The exit status is zero, unless end-of-file is encountered, read times out (in which case the status is greater than 128), a variable assignment error (such as assigning to a readonly variable) occurs, or an invalid file descriptor is supplied as the argument to -u.

readarray ¶

readarray [-d delim] [-n count] [-O origin] [-s count]
    [-t] [-u fd] [-C callback] [-c quantum] [array]

Read lines from the standard input into the indexed array variable array, or from file descriptor fd if the -u option is supplied.

A synonym for mapfile.

source ¶

source [-p path] filename [arguments]

A synonym for . (see Bourne Shell Builtins).

type ¶

type [-afptP] [name ...]

Indicate how each name 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’, ‘keyword’, ‘function’, ‘builtin’, or ‘file’, if name is an alias, shell reserved word, shell function, shell builtin, or executable file, respectively. If the name is not found, type prints nothing and returns a failure status.

If the -p option is used, type either returns the name of the executable file that would be found by searching $PATH for name, or nothing if -t would not return ‘file’.

The -P option forces a path search for each name, even if -t would not return ‘file’.

If a name is present in the table of hashed commands, options -p and -P print the hashed value, which is not necessarily the file that appears first in $PATH.

If the -a option is used, type returns all of the places that contain a command named name. This includes aliases, reserved words, functions, and builtins, but the path search options (-p and -P) can be supplied to restrict the output to executable files. If -a is supplied with -p, type does not look in the table of hashed commands, and only performs a PATH search for name.

If the -f option is used, type does not attempt to find shell functions, as with the command builtin.

The return status is zero if all of the names are found, non-zero if any are not found.

typeset ¶

typeset [-afFgrxilnrtux] [-p] [name[=value] ...]

The typeset command is supplied for compatibility with the Korn shell. It is a synonym for the declare builtin command.

ulimit ¶

ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT] [limit]

ulimit provides control over the resources available to the shell and to processes it starts, 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

Report all current limits; no limits are set.

-b

The maximum socket buffer size.

-c

The maximum size of core files created.

-d

The maximum size of a process’s data segment.

-e

The maximum scheduling priority ("nice").

-f

The maximum size of files written by the shell and its children.

-i

The maximum number of pending signals.

-k

The maximum number of kqueues that may be allocated.

-l

The maximum size that may be locked into memory.

-m

The maximum resident set size (many systems do not honor this limit).

-n

The maximum number of open file descriptors (most systems do not allow this value to be set).

-p

The pipe buffer size.

-q

The maximum number of bytes in POSIX message queues.

-r

The maximum real-time scheduling priority.

-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 shell, and, on some systems, to its children.

-x

The maximum number of file locks.

-P

The maximum number of pseudoterminals.

-R

The maximum time a real-time process can run before blocking, in microseconds.

-T

The maximum number of threads.

If limit is supplied, and the -a option is not used, limit 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. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. Otherwise, ulimit prints the current value of the soft limit for the specified resource, unless the -H option is supplied. When more than one resource is specified, the limit name and unit, if appropriate, are printed before the value. When setting new limits, if neither -H nor -S is supplied, ulimit sets both the hard and soft limits. If no option is supplied, then -f is assumed.

Values are in 1024-byte increments, except for -t, which is in seconds; -R, which is in microseconds; -p, which is in units of 512-byte blocks; -P, -T, -b, -k, -n and -u, which are unscaled values; and, when in POSIX mode (see Bash and POSIX), -c and -f, which are in 512-byte increments.

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, remove all aliases. The return value is true unless a supplied name is not a defined alias. Aliases are described in Aliases.

Next: Special Builtins, Previous: Bash Builtin Commands, Up: Shell Builtin Commands   [Contents][Index]

4.3 Modifying Shell Behavior ¶

• The Set Builtin
• The Shopt Builtin

Previous: Modifying Shell Behavior, Up: Shell Builtin Commands   [Contents][Index]

4.4 Special Builtins ¶

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

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.

When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in Bash and POSIX.

These are the POSIX special builtins:

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

Next: Bash Variables, Up: Shell Variables   [Contents][Index]

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 Tilde Expansion).

IFS ¶

A list of characters that separate fields; used when the shell splits words as part of expansion and by the read builtin to split lines into words. See Word Splitting, for a description of word splitting.

MAIL ¶

If the value is set to a filename or directory name and the MAILPATH variable is not set, Bash informs the user of the arrival of mail in the specified file or Maildir-format directory.

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 filename 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 next argument to be processed by the getopts builtin.

PATH ¶

A colon-separated list of directories in which the shell looks for commands. A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. The default path is system-dependent, and is set by the administrator who installs bash. A common value is "/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin".

PS1 ¶

The primary prompt string. The default value is ‘\s-\v\$ ’. See 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 ‘> ’. PS2 is expanded in the same way as PS1 before being displayed.

Previous: Bourne Shell Variables, Up: Shell Variables   [Contents][Index]

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 Job Control Variables).

_ ¶

($_, an underscore.) This has a number of meanings depending on context. At shell startup, $_ set to the pathname used to invoke the shell or shell script being executed as passed in the environment or argument list. Subsequently, it expands to the last argument to the previous simple command executed in the foreground, after expansion. It is also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, $_ expands to the name of the mail file.

BASH ¶

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

BASHOPTS ¶

A colon-separated list of enabled shell options. Each word in the list is a valid argument for the -s option to the shopt builtin command (see The Shopt Builtin). The options appearing in BASHOPTS are those reported as ‘on’ by ‘shopt’. If this variable is in the environment when Bash starts up, the shell enables each option in the list before reading any startup files. If this variable is exported, child shells will enable each option in the list. This variable is readonly.

BASHPID ¶

Expands to the process ID of the current Bash process. This differs from $$ under certain circumstances, such as subshells that do not require Bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset.

BASH_ALIASES ¶

An associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. (see Bourne Shell Builtins). Elements added to this array appear in the alias list; however, unsetting array elements currently does not cause aliases to be removed from the alias list. If BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset.

BASH_ARGC ¶

An array variable whose values are the number of parameters in each frame of the current Bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin). Setting extdebug after the shell has started to execute a subroutine, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGC have no effect, and it may not be unset.

BASH_ARGV ¶

An array variable containing all of the parameters in the current Bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the shell pushes the supplied parameters onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGV have no effect, and it may not be unset.

BASH_ARGV0 ¶

When referenced, this variable expands to the name of the shell or shell script (identical to $0; See Special Parameters, for the description of special parameter 0). Assigning a value to BASH_ARGV0 sets $0 to the same value. If BASH_ARGV0 is unset, it loses its special properties, even if it is subsequently reset.

BASH_CMDS ¶

An associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin (see Bourne Shell Builtins). Adding elements to this array makes them appear in the hash table; however, unsetting array elements currently does not remove command names from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMMAND ¶

Expands to the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMPAT ¶

The value is used to set the shell’s compatibility level. See Shell Compatibility Mode, for a description of the various compatibility levels and their effects. The value may be a decimal number (e.g., 4.2) or an integer (e.g., 42) corresponding to the desired compatibility level. If BASH_COMPAT is unset or set to the empty string, the compatibility level is set to the default for the current version. If BASH_COMPAT is set to a value that is not one of the valid compatibility levels, the shell prints an error message and sets the compatibility level to the default for the current version. A subset of the valid values correspond to the compatibility levels described below (see Shell Compatibility Mode). For example, 4.2 and 42 are valid values that correspond to the compat42 shopt option and set the compatibility level to 42. The current version is also a valid value.

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. Bash does not use PATH to search for the resultant filename. See Bash Startup Files.

BASH_EXECUTION_STRING ¶

The command argument to the -c invocation option.

BASH_LINENO ¶

An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number. Assignments to BASH_LINENO have no effect, and it may not be unset.

BASH_LOADABLES_PATH ¶

A colon-separated list of directories in which the enable command looks for dynamically loadable builtins.

BASH_MONOSECONDS ¶

Each time this variable is referenced, it expands to the value returned by the system’s monotonic clock, if one is available. If there is no monotonic clock, this is equivalent to EPOCHSECONDS. If BASH_MONOSECONDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_REMATCH ¶

An array variable whose members are assigned by the ‘=~’ binary operator to the [[ conditional command (see Conditional Constructs). The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the nth parenthesized subexpression.

BASH_SOURCE ¶

An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]} Assignments to BASH_SOURCE have no effect, and it may not be unset.

BASH_SUBSHELL ¶

Incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset.

BASH_TRAPSIG ¶

Set to the signal number corresponding to the trap action being executed during its execution. See the description of trap (see Bourne Shell Builtins) for information about signal numbers and trap execution.

BASH_VERSINFO ¶

A readonly array variable (see 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., beta).

BASH_VERSINFO[5]

The value of MACHTYPE.

BASH_VERSION ¶

Expands to a string describing the version of this instance of Bash (e.g., 5.2.37(3)-release).

BASH_XTRACEFD ¶

If set to an integer corresponding to a valid file descriptor, Bash writes the trace output generated when ‘set -x’ is enabled to that file descriptor, instead of the standard error. This allows tracing output to be separated from diagnostic and error messages. The file descriptor is closed when BASH_XTRACEFD is unset or assigned a new value. Unsetting BASH_XTRACEFD or assigning it the empty string causes the trace output to be sent to the standard error. Note that setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed.

CHILD_MAX ¶

Set the number of exited child status values for the shell to remember. Bash will not allow this value to be decreased below a POSIX-mandated minimum, and there is a maximum value (currently 8192) that this may not exceed. The minimum value is system-dependent.

COLUMNS ¶

Used by the select command to determine the terminal width when printing selection lists. Automatically set if the checkwinsize option is enabled (see The Shopt Builtin), or in an interactive shell 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 Programmable Completion).

COMP_KEY ¶

The key (or final key of a key sequence) used to invoke the current completion function. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see 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 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 Programmable Completion).

COMP_TYPE ¶

Set to an integer value corresponding to the type of attempted completion that caused a completion function to be called: TAB, for normal completion, ‘?’, for listing completions after successive tabs, ‘!’, for listing alternatives on partial word completion, ‘@’, to list completions if the word is not unmodified, or ‘%’, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion).

COMP_WORDBREAKS ¶

The set of characters that the Readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset.

COMP_WORDS ¶

An array variable consisting of the individual words in the current command line. The line is split into words as Readline would split it, using COMP_WORDBREAKS as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see 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 Programmable Completion). Each array element contains one possible completion.

COPROC ¶

An array variable created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses).

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. Assigning to this variable does not change the current directory. If DIRSTACK is unset, it loses its special properties, even if it is subsequently reset.

EMACS ¶

If Bash finds this variable in the environment when the shell starts, and its value is ‘t’, Bash assumes that the shell is running in an Emacs shell buffer and disables line editing.

ENV ¶

Expanded and executed similarly to BASH_ENV (see Bash Startup Files) when an interactive shell is invoked in POSIX mode (see Bash and POSIX).

EPOCHREALTIME ¶

Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch as a floating-point value with micro-second granularity (see the documentation for the C library function time for the definition of Epoch). Assignments to EPOC