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 MS-DOS, OS/2, and Windows platforms.

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. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks.

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

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

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

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

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

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

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

3.3 Shell Functions

Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.

Functions are declared using this syntax:

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. compound-command is executed whenever fname is specified as the name of a simple command. When the shell is in POSIX mode (see Bash POSIX Mode), fname must be a valid shell name and may not be the same as one of the special builtins (see Special Builtins). In default 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. A function definition may be 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. Also, when using the braces, the list must be terminated by a semicolon, a ‘&’, or a newline.

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

All other aspects of the shell execution environment are identical between a function and its caller with 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 a numeric argument is given to return, that is the function’s return status; otherwise the function’s return status is the exit status of the last command executed before the return.

Variables local to the function may be declared with the local builtin (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. Consequently, 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 a global variable of the same name: references and assignments refer to the local variable, leaving the global variable 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 will 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 will unset 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 will remain 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 will become visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed will become visible (see below how localvar_unsetshell option changes this behavior).

Function names and definitions may be listed with the -f option to the declare (typeset) builtin command (see Bash Builtin Commands). The -F option to declare or typeset will list the function names only (and optionally the source file and line number, if the extdebug shell option is enabled). Functions may be exported so that 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).

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, no limit is placed on the number of recursive calls.

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).

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command.

A variable may be assigned to by a statement of the form

name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (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 POSIX Mode), these builtins may appear in a command after one or more instances of the command builtin and retain these assignment statement properties.

In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘+=’ operator can be used to append to or add to the variable’s previous value. This includes arguments to builtin commands such as declare that accept assignment statements (declaration commands). When ‘+=’ is applied to a variable for which the integer attribute has been set, value is evaluated as an arithmetic expression and added to the variable’s current value, which is also evaluated. 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 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 will be 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 will be unset.

• Positional Parameters
• Special Parameters

3.5 Shell Expansions

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

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

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; and filename expansion.

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

After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote removal).

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).

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

• 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`

$(( expression ))

<(list)

>(list)

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. Redirection may also be used to 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 will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close. If {varname} is supplied, the redirection persists beyond the scope of the command, allowing the shell programmer to manage the file descriptor’s lifetime manually. 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 expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant. For example, the command

ls > dirlist 2>&1

directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was 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 will use them; otherwise it will emulate them internally with the behavior described below.

/dev/fd/fd

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

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open 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.

Note that the exec builtin command can make redirections take effect in the current shell.

• 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

3.7 Executing Commands

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

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. The shell first searches for the file in the current directory, and looks in the directories in $PATH if not found there.

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

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

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.

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) ¶

. filename [arguments]

Read and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the PATH variable is used to find filename, but 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. 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 is retained when . completes. The return status is the exit status of the last command executed, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source.

break ¶

break [n]

Exit from a for, while, until, or select loop. If n is supplied, the nth enclosing loop is exited. n must be greater than or equal to 1. The return status is zero unless n is not greater than or equal to 1.

cd ¶

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

Change the current working directory to directory. If directory is not supplied, the value of the HOME shell variable is used. If the shell variable CDPATH exists, it is used as a search path: each directory name in CDPATH is searched for directory, with alternative directory names in CDPATH separated by a colon (‘:’). If directory begins with a slash, CDPATH is not used.

The -P option means to not 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, it is processed by removing the immediately preceding pathname component, back to a slash or the beginning of directory.

If the -e option is supplied with -P and the current working directory cannot be successfully determined after a successful directory change, cd will return an unsuccessful 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 the directory change is attempted.

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

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]

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

eval ¶

eval [arguments]

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

exec ¶

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

If command is supplied, it replaces the shell without creating a new process. If the -l option is supplied, the shell places a dash at the beginning of the zeroth 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 failure. An interactive shell returns failure if the file cannot be executed. A subshell exits unconditionally if exec fails. If no command is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.

exit ¶

exit [n]

Exit the shell, returning a status of n to the shell’s parent. If n is omitted, the exit status is that of the last command executed. Any trap on EXIT is executed before the shell terminates.

export ¶

export [-fn] [-p] [name[=value]]

Mark each name to be passed to child processes in the environment. If the -f option is supplied, the names refer to shell functions; otherwise the names refer to shell variables. The -n option means to no longer mark each name for export. If no names are supplied, or if the -p option is given, a list of names of all exported variables is displayed. The -p option displays output in a form that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value.

The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a shell function.

getopts ¶

getopts optstring name [arg …]

getopts is used by shell scripts to parse positional parameters. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by 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 if a new set of parameters is to be used.

When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and name is set to ‘?’.

getopts normally parses the positional parameters, but if more arguments are supplied as arg values, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In normal operation, diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable OPTERR is set to 0, no error messages will be displayed, even if the first character of optstring is not a colon.

If an invalid option is seen, getopts places ‘?’ into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed.

If a required argument is not found, and getopts is not silent, a question mark (‘?’) is placed in name, OPTARG is unset, and a diagnostic message is printed. If getopts is silent, then a colon (‘:’) is placed in name and OPTARG is set to the option character found.

hash ¶

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

Each time hash is invoked, it remembers the full pathnames 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 pathname is discarded. The -p option inhibits the path search, and filename is used as the location of name. The -r option causes the shell to forget all remembered locations. The -d option causes the shell to forget the remembered location of each name. If the -t option is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are supplied with -t, the name is printed before the hashed full pathname. The -l option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only -l is supplied, information about remembered commands is printed. The return status is zero unless a name is not found or an invalid option is supplied.

pwd ¶

pwd [-LP]

Print the absolute pathname of the current working directory. If the -P option is supplied, the pathname printed will not contain symbolic links. If the -L option is supplied, the pathname printed may contain symbolic links. The return status is zero unless an error is encountered while determining the name of the current directory or an invalid option is supplied.

readonly ¶

readonly [-aAf] [-p] [name[=value]] …

Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the -f option is supplied, each name refers to a shell function. The -a option means each name refers to an 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 given, or if the -p option is supplied, a list of all readonly names is printed. The other options may be used to restrict the output to a subset of the set of readonly names. The -p option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =value, the value of the variable is set to value. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the -f option is supplied with a name that is not a shell function.

return ¶

return [n]

Cause a shell function to stop executing 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 in the function. 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. return may also be used to terminate execution of a script being executed with the . (source) builtin, returning 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 zero or greater than $#, the positional parameters are not changed. If n is not supplied, it is assumed to be 1. The return status is zero unless n is greater than $# or less than zero, non-zero otherwise.

test ¶

[

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 the [ form is used, the last argument to the command must be a ].

Expressions may be combined using the following operators, listed in decreasing order of precedence. The evaluation depends on the number of arguments; see below. Operator precedence is used 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 the normal precedence of operators.

expr1 -a expr2

True if both expr1 and expr2 are true.

expr1 -o expr2

True if either expr1 or expr2 is true.

The test and [ builtins evaluate conditional expressions using a set of rules based on the number of arguments.

0 arguments

The expression is false.

1 argument

The expression is true if, and only if, the argument is not null.

2 arguments

If the first argument is ‘!’, the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see 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.

When used with test or ‘[’, the ‘<’ and ‘>’ operators sort lexicographically using ASCII ordering.

times ¶

times

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

trap ¶

trap [-lp] [arg] [sigspec …]

The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent (and there is a single sigspec) or equal to ‘-’, each specified signal’s disposition is reset to the value it had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is not present and -p has been supplied, the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only -p is given, trap prints the list of commands associated with each signal number in a form that may be reused as shell input. The -l option causes the shell to print a list of signal names and their corresponding numbers. Each sigspec is either a signal name or a signal number. Signal names are case insensitive and the SIG prefix is optional.

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

If a sigspec is ERR, the command arg 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 keyword, 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, or if the command’s return status is being inverted using !. These are the same conditions obeyed by the errexit (-e) option.

Signals ignored upon entry to the shell cannot be trapped or reset. 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.

umask ¶

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

Set the shell process’s file creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; if not, it is interpreted as a symbolic mode mask similar to that accepted by the chmod command. If mode is omitted, the current value of the mask is printed. If the -S option is supplied without a mode argument, the mask is printed in a symbolic format. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is zero if the mode is successfully changed or if no mode argument is supplied, and non-zero otherwise.

Note that when the mode is interpreted as an octal number, each number of the umask is subtracted from 7. Thus, a umask of 022 results in permissions of 755.

unset ¶

unset [-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. 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.

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, an alias is defined for each name whose value is given. If no value is given, the name and value of the alias is printed. Aliases are described in Aliases.

bind ¶

bind [-m keymap] [-lpsvPSVX]
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 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 set a Readline variable. Each non-option argument is a 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’.

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 input or in a Readline initialization file.

-P

List current Readline function names and bindings.

-v

Display Readline variable names and values in such a way that they can be used as input or in a Readline initialization file.

-V

List current Readline variable names and values.

-s

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

-S

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

-f filename

Read key bindings from filename.

-q function

Query about which keys invoke the named function.

-u function

Unbind all keys bound to the named function.

-r keyseq

Remove any current binding for keyseq.

-x keyseq:shell-command

Cause shell-command to be executed whenever keyseq is entered. 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 input.

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

builtin ¶

builtin [shell-builtin [args]]

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

caller ¶

caller [expr]

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

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

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

command ¶

command [-pVv] command [arguments …]

Runs command with arguments ignoring any shell function named command. Only shell builtin commands or commands found by searching the PATH are executed. If there is a shell function named ls, running ‘command ls’ within the function will execute the external command ls instead of calling the function recursively. The -p option means to use a default value for PATH that is guaranteed to find all of the standard utilities. The return status in this case is 127 if command cannot be found or an error occurred, and the exit status of command otherwise.

If either the -V or -v option is supplied, a description of command is printed. The -v option causes a single word indicating the command or file name used to invoke command to be displayed; the -V option produces a more verbose description. In this case, the return status is zero if command is found, and non-zero if not.

declare ¶

declare [-aAfFgiIlnrtux] [-p] [name[=value] …]

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

The -p option will display the attributes and values of each name. When -p is used 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 will restrict 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 all other cases.

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

Use function names only.

-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 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 used. 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 suppressed. If the -e option is given, interpretation of the following backslash-escaped characters is enabled. The -E option disables the interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo shell option may be used to dynamically determine whether or not echo expands these escape characters by default. echo 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)

enable ¶

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

Enable and disable builtin shell commands. Disabling a builtin allows a disk command which has the same name as a shell builtin to be executed without specifying a full pathname, even though the shell normally searches for builtins before disk commands. If -n is used, the names become disabled. Otherwise names are enabled. For example, to use the test binary found via $PATH instead of the shell builtin version, type ‘enable -n test’.

If the -p option is supplied, or no name arguments appear, a list of shell builtins is printed. With no other arguments, the list consists of all enabled shell builtins. The -a option means to list each builtin with an indication of whether or not it is enabled.

The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. 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 is system-dependent. The -d option will delete a builtin loaded with -f.

If there are no options, a list of the shell builtins is displayed. The -s option restricts enable to the POSIX special builtins. If -s is used with -f, the new builtin becomes a special builtin (see 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, otherwise a list of the builtins is printed.

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

The return status is zero unless no command matches pattern.

let ¶

let expression [expression …]

The let builtin allows arithmetic to be performed on shell variables. Each expression is evaluated according to the rules given below in Shell Arithmetic. If the last expression evaluates to 0, let returns 1; otherwise 0 is returned.

local ¶

local [option] name[=value] …

For each argument, a local variable named name is created, and assigned value. The option can be any of the options accepted by declare. local can only be used within a function; it makes the variable name have a visible scope restricted to that function and its children. If name is ‘-’, the set of shell options is made local to the function in which local is invoked: shell options changed using the set builtin inside the function are restored to their original values when the function returns. The restore is effected as if a series of set commands were executed to restore the values that were in place before the function. 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 into the indexed array variable array, or from file descriptor fd if the -u option is supplied. The variable MAPFILE is the default array. Options, if supplied, have the following meanings:

-d

The first character of delim is used 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, all lines are copied.

-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 successfully unless an invalid option or option argument is supplied, array is invalid or unassignable, or 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 causes the output to be assigned to the variable var rather than being printed 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(1) formats, printf interprets the following extensions:

%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

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. Two special argument values may be used: -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 given. This is an exception to the usual printf behavior.

The %b, %q, and %T directives 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.

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 ASCII value of the following character.

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

read ¶

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

One line is read from the standard input, or from the file descriptor fd supplied as an argument to the -u option, split into words as described above in Word Splitting, and the first word is assigned 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 ‘\’ may be used to remove any special meaning for the next character read and 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 is used to terminate the input line, rather than newline. If delim is the empty string, read will terminate a line when it reads a NUL character.

-e

Readline (see Command Line Editing) is used 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.

-i text

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

-n nchars

read returns after reading nchars characters rather than waiting for a complete line of input, but honors a delimiter if fewer than nchars characters are read before the delimiter.

-N nchars

read returns after reading exactly nchars characters rather than waiting for a complete line of input, unless EOF is encountered or read times out. Delimiter characters encountered in the input are not treated specially and do not cause read to return until nchars characters are read. 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. The prompt is displayed only if input is coming from a terminal.

-r

If this option is given, backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not 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 a complete line of input (or a specified number of characters) is not read 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, read saves any partial input read into the specified variable name. If timeout is 0, read returns immediately, without trying to read any data. The exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise. The exit status is greater than 128 if the timeout is exceeded.

-u fd

Read input from file descriptor fd.

If no names are supplied, the line read, without the ending delimiter but otherwise unmodified, is assigned 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 filename

A synonym for . (see Bourne Shell Builtins).

type ¶

type [-afptP] [name …]

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

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

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

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

If a command is hashed, -p and -P print the hashed value, 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 an executable named file. This includes aliases and functions, if and only if the -p option is not also used.

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

The return status is zero if 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 processes started by the shell, on systems that allow such control. If an option is given, it is interpreted as follows:

-S

Change and report the soft limit associated with a resource.

-H

Change and report the hard limit associated with a resource.

-a

All current limits are reported; 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 given, 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, the current value of the soft limit for the specified resource is printed, 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, both the hard and soft limits are set. If no option is given, then -f is assumed. Values are in 1024-byte increments, except for -t, which is in seconds; -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 POSIX Mode), -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, all aliases are removed. Aliases are described in Aliases.

4.3 Modifying Shell Behavior

• The Set Builtin
• The Shopt Builtin

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 POSIX Mode.

These are the POSIX special builtins:

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

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.

MAIL ¶

If this parameter 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 last option argument 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.

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.

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.) 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, expands to the last argument to the previous simple command executed in the foreground, after expansion. Also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. When checking mail, this parameter holds 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, each shell option in the list will be enabled before reading any startup files. 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 script, or referencing this variable when extdebug is not set, may result in inconsistent values.

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 parameters supplied are pushed 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.

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). Assignment to BASH_ARGV0 causes the value assigned to also be assigned to $0. 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). Elements added to this array appear in the hash table; however, unsetting array elements currently does not cause command names to be removed from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset.

BASH_COMMAND ¶

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. 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. 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.

BASH_LOADABLES_PATH ¶

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

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]}

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_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., beta1).

BASH_VERSINFO[5]

The value of MACHTYPE.

BASH_VERSION ¶

The version number of the current instance of Bash.

BASH_XTRACEFD ¶

If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor. 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_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 completion attempted 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_KEY ¶

The key (or final key of a key sequence) used to invoke the current completion function.

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

EMACS ¶

If Bash finds this variable in the environment when the shell starts with value ‘t’, it 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 POSIX Mode).

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 EPOCHREALTIME are ignored. If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.

EPOCHSECONDS ¶

Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see the documentation for the C library function time for the definition of Epoch). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently reset.

EUID ¶

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

EXECIGNORE ¶

A colon-separated list of shell patterns (see Pattern Matching) defining the list of filenames to be ignored by command search using PATH. Files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH lookup. This does not affect the behavior of the [, test, and [[ commands. Full pathnames in the command hash table are not subject to EXECIGNORE. Use this variable to ignore shared library files that have the executable bit set, but are not executable files. The pattern matching honors the setting of the extglob shell option.

FCEDIT ¶

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

FIGNORE ¶

A colon-separated list of suffixes to ignore when performing filename completion. A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value is ‘.o:~’

FUNCNAME ¶

An array variable containing the names of all shell functions currently in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is "main". This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

This variable can be used with BASH_LINENO and BASH_SOURCE. Each element of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using this information.

FUNCNEST ¶

If set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed this nesting level will cause the current command to abort.

GLOBIGNORE ¶

A colon-separated list of patterns defining the set of file names to be ignored by filename expansion. If a file name matched by a filename expansion pattern also matches one of the patterns in GLOBIGNORE, it is removed from the list of matches. The pattern matching honors the setting of the extglob shell option.

GROUPS ¶

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

histchars ¶

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

HISTCMD ¶

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

HISTCONTROL ¶

A colon-separated list of values controlling how commands are saved on the history list. If the list of values includes ‘ignorespace’, lines which begin with a space character are not saved in the history list. A value of ‘ignoredups’ causes lines which match the previous history entry to not be saved. A value of ‘ignoreboth’ is shorthand for ‘ignorespace’ and ‘ignoredups’. A value of ‘erasedups’ causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value not in the above list is ignored. If HISTCONTROL is unset, or does not include a valid value, all lines read by the shell parser are saved on the history list, subject to the value of HISTIGNORE. The second and subsequent lines of a multi-line compound command are not tested, and are added to the history regardless of the value of HISTCONTROL.

HISTFILE ¶

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

HISTFILESIZE ¶

The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE after reading any startup files.

HISTIGNORE ¶

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

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

HISTSIZE ¶

The maximum number of commands to remember on the history list. If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files.

HISTTIMEFORMAT ¶

If this variable is set and not null, its value is used as a format string for strftime to print the time stamp associated with each history entry displayed by the history builtin. If this variable is set, time stamps are written to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.

HOSTFILE ¶

Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, Bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, or does not name a readable file, Bash attempts to read /etc/hosts to obtain the list of possible hostname completions. When HOSTFILE is unset, the hostname list is cleared.

HOSTNAME ¶

The name of the current host.

HOSTTYPE ¶

A string describing the machine Bash is running on.

IGNOREEOF ¶

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

INPUTRC ¶

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

INSIDE_EMACS ¶

If Bash finds this variable in the environment when the shell starts, it assumes that the shell is running in an Emacs shell buffer and may disable line editing depending on the value of TERM.

LANG ¶

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

LC_ALL ¶

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

LC_COLLATE ¶

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

LC_CTYPE ¶

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

LC_MESSAGES ¶

This variable determines the locale used to translate double-quoted strings preceded by a ‘$’ (see Locale-Specific Translation).

LC_NUMERIC ¶

This variable determines the locale category used for number formatting.

LC_TIME ¶

This variable determines the locale category used for data and time formatting.

LINENO ¶

The line number in the script or shell function currently executing. If LINENO is unset, it loses its special properties, even if it is subsequently reset.

LINES ¶

Used by the select command to determine the column length for 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.

MACHTYPE ¶

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

MAILCHECK ¶

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

MAPFILE ¶

An array variable created to hold the text read by the mapfile builtin when no variable name is supplied.

OLDPWD ¶

The previous working directory as set by the cd builtin.

OPTERR ¶

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

OSTYPE ¶

A string describing the operating system Bash is running on.

PIPESTATUS ¶

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

POSIXLY_CORRECT ¶

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

set -o posix

had been executed. When the shell enters POSIX mode, it sets this variable if it was not already set.

PPID ¶

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

PROMPT_COMMAND ¶

If this variable is set, and is an array, the value of each set element is interpreted as a command to execute before printing the primary prompt ($PS1). If this is set but not an array variable, its value is used as a command to execute instead.

PROMPT_DIRTRIM ¶

If set to a number greater than zero, the value is used as the number of trailing directory components to retain when expanding the \w and \W prompt string escapes (see Controlling the Prompt). Characters removed are replaced with an ellipsis.

PS0 ¶

The value of this parameter is expanded like PS1 and displayed by interactive shells after reading a command and before the command is executed.

PS3 ¶

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

PS4 ¶

The value of this parameter is expanded like PS1 and the expanded value is the prompt printed before the command line is echoed when the -x option is set (see The Set Builtin). The first character of the expanded value is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is ‘+ ’.

PWD ¶

The current working directory as set by the cd builtin.

RANDOM ¶

Each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to this variable seeds the random number generator. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.

READLINE_ARGUMENT ¶

Any numeric argument given to a Readline command that was defined using ‘bind -x’ (see Bash Builtin Commands when it was invoked.

READLINE_LINE ¶

The contents of the Readline line buffer, for use with ‘bind -x’ (see Bash Builtin Commands).

READLINE_MARK ¶

The position of the mark (saved insertion point) in the Readline line buffer, for use with ‘bind -x’ (see Bash Builtin Commands). The characters between the insertion point and the mark are often called the region.

READLINE_POINT ¶

The position of the insertion point in the Readline line buffer, for use with ‘bind -x’ (see Bash Builtin Commands).

REPLY ¶

The default variable for the read builtin.

SECONDS ¶

This variable expands to the number of seconds since the shell was started. Assignment to this variable resets the count to the value assigned, and the expanded value becomes the value assigned plus the number of seconds since the assignment. The number of seconds at shell invocation and the current time are always determined by querying the system clock. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.

SHELL ¶

This environment variable expands to the full pathname to the shell. If it is not set when the shell starts, Bash assigns to it the full pathname of the current user’s login shell.

SHELLOPTS ¶

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

SHLVL ¶

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

SRANDOM ¶

This variable expands to a 32-bit pseudo-random number each time it is referenced. The random number generator is not linear on systems that support /dev/urandom or arc4random, so each returned number has no relationship to the numbers preceding it. The random number generator cannot be seeded, so assignments to this variable have no effect. If SRANDOM is unset, it loses its special properties, even if it is subsequently reset.

TIMEFORMAT ¶

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

%%

A literal ‘%’.

%[p][l]R

The elapsed time in seconds.

%[p][l]U

The number of CPU seconds spent in user mode.

%[p][l]S

The number of CPU seconds spent in system mode.

%P

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

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

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

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

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

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

TMOUT ¶

If set to a value greater than zero, TMOUT is treated as the default timeout for the read builtin (see Bash Builtin Commands). The select command (see Conditional Constructs) terminates if input does not arrive after TMOUT seconds when input is coming from a terminal.

In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if a complete line of input does not arrive.

TMPDIR ¶

If set, Bash uses its value as the name of a directory in which Bash creates temporary files for the shell’s use.

UID ¶

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

6.1 Invoking Bash

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

All of the single-character options used with the set builtin (see The Set Builtin) can be used as options when the shell is invoked. In addition, there are several multi-character options that you can use. These options must appear on the command line before the single-character options to be recognized.

--debugger

Arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see The Shopt Builtin for a description of the extdebug option to the shopt builtin).

--dump-po-strings

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

--dump-strings

Equivalent to -D.

--help

Display a usage message on standard output and exit successfully.

--init-file filename

--rcfile filename

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

--login

Equivalent to -l.

--noediting

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

--noprofile

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

--norc

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

--posix

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

--restricted

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

--verbose

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

--version

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

There are several single-character options that may be supplied at invocation which are not available with the set builtin.

-c

Read and execute commands from the first non-option argument command_string, then exit. If there are arguments after the command_string, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages.

-i

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

-l

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

-r

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

-s

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

-D

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

[-+]O [shopt_option]

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

--

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

A login shell is one whose first character of argument zero is ‘-’, or one invoked with the --login option.

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

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

6.2 Bash Startup Files

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

Interactive shells are described in Interactive Shells.

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

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

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, Bash reads and executes commands from the file ~/.bash_logout, if it exists.

Invoked as an interactive non-login shell

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

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

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

after (or before) any login-specific initializations.

Invoked non-interactively

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

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

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

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

Invoked with name sh

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

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

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

Invoked in POSIX mode

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

Invoked by remote shell daemon

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the historical remote shell daemon, usually rshd, or the secure shell daemon sshd. If Bash determines it is being run non-interactively in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable. It will not do this if invoked as sh. The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force another file to be read, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified.

Invoked with unequal effective and real UID/GIDs

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

6.3 Interactive Shells

• What is an Interactive Shell?
• Is this Shell Interactive?
• Interactive Shell Behavior

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

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

6.4 Bash Conditional Expressions

Conditional expressions are used by the [[ compound command (see Conditional Constructs) and the test and [ builtin commands (see Bourne Shell Builtins). The test and [ commands determine their behavior based on the number of arguments; see the descriptions of those commands for any other command-specific actions.

Expressions may be unary or binary, and are formed from the following primaries. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. Bash handles several filenames specially when they are used in expressions. If the operating system on which Bash is running provides these special files, Bash will use them; otherwise it will emulate them internally with this behavior: If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.

When used with [[, the ‘<’ and ‘>’ operators sort lexicographically using the current locale. The test command uses ASCII ordering.

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.

See the description of the test builtin command (section see Bash Builtin Commands below) for the handling of parameters (i.e. missing parameters).

-a file

True if file exists.

-b file

True if file exists and is a block special file.

-c file

True if file exists and is a character special file.

-d file

True if file exists and is a directory.

-e file

True if file exists.

-f file

True if file exists and is a regular file.

-g file

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

-h file

True if file exists and is a symbolic link.

-k file

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

-p file

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

-r file

True if file exists and is readable.

-s file

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

-t fd

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

-u file

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

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-G file

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

-L file

True if file exists and is a symbolic link.

-N file

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

-O file

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

-S file

True if file exists and is a socket.

file1 -ef file2

True if file1 and file2 refer to the same device and inode numbers.

file1 -nt file2

True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.

file1 -ot file2

True if file1 is older than file2, or if file2 exists and file1 does not.

-o optname

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

-v varname

True if the shell variable varname is set (has been assigned a value).

-R varname

True if the shell variable varname is set and is a name reference.

-z string

True if the length of string is zero.

-n string

string

True if the length of string is non-zero.

string1 == string2

string1 = string2

True if the strings are equal. When used with the [[ command, this performs pattern matching as described above (see Conditional Constructs).

‘=’ should be used with the test command for POSIX conformance.

string1 != string2

True if the strings are not equal.

string1 < string2

True if string1 sorts before string2 lexicographically.

string1 > string2

True if string1 sorts after string2 lexicographically.

arg1 OP arg2

OP is one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers. When used with the [[ command, Arg1 and Arg2 are evaluated as arithmetic expressions (see Shell Arithmetic).