bash - GNU Bourne-Again SHell

bash [options] [file]

Bash is an sh-compatible command language interpreter that executes commands read from the standard input or from a file. Bash also incorporates useful features from the Korn and C shells (ksh and csh).

Bash is intended to be a conformant implementation of the IEEE POSIX Shell and Tools specification (IEEE Working Group 1003.2). Bash can be configured to be POSIX-conformant by default.

In addition to the single-character shell options documented in the description of the set builtin command, bash interprets the following options when it is invoked:

-c string

If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.

-i

If the -i option is present, the shell is interactive.

-l

Make bash act as if it had been invoked as a login shell (see INVOCATION below).

-r

If the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below).

-s

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

-D

A list of all double-quoted strings preceded by $ is printed on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX. 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 SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets the value of that option; +O unsets it. If shopt_option is not supplied, the names and values of the shell options accepted by shopt are printed on the standard output. If the invocation option is +O, the output is displayed in a format that may be reused as input.

--

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

Bash also interprets a number of multi-character options. 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 description of the extdebug option to the shopt builtin below) and shell function tracing (see the description of the -o functrace option to the set builtin below).

--dump-po-strings

Equivalent to -D, but the output is in the GNU gettext po (portable object) file format.

--dump-strings

Equivalent to -D.

--help

Display a usage message on standard output and exit successfully.

--init-file file

--rcfile file

Execute commands from file instead of the system wide initialization file /etc/bash.bashrc and the standard personal initialization file ~/.bashrc if the shell is interactive (see INVOCATION below).

--login

Equivalent to -l.

--noediting

Do not use the GNU readline library to read command lines when the shell is interactive.

--noprofile

Do not read either the system-wide startup file or any of the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these files when it is invoked as a login shell (see INVOCATION below).

--norc

Do not read and execute the system wide initialization file /etc/bash.bashrc and the personal initialization file ~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.

--posix

Change the behavior of bash where the default operation differs from the POSIX 1003.2 standard to match the standard (posix mode).

--restricted

The shell becomes restricted (see RESTRICTED SHELL below).

--verbose

Equivalent to -v.

--version

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

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. If 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. An attempt is first made to open the file in the current directory, and, if no file is found, then the shell searches the directories in PATH for the script.

A login shell is one whose first character of argument zero is a -, or one started with the --login option.

An interactive shell is one started without non-option arguments and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. PS1 is set and $- includes i if bash is interactive, allowing a shell script or a startup file to test this state.

The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in file names as described below under Tilde Expansion in the EXPANSION section.

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

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

When an interactive shell that is not a login shell is started, bash reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if these files exist. 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 /etc/bash.bashrc and ~/.bashrc.

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

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

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

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

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

The following definitions are used throughout the rest of this document.

blank

A space or tab.

word

A sequence of characters considered as a single unit by the shell. Also known as a token.

name

A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.

metacharacter

A character that, when unquoted, separates words. One of the following:

control operator

A token that performs a control function. It is one of the following symbols:

Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a simple command (see SHELL GRAMMAR below) or the third word of a case or for command:

Simple Commands

Pipelines

Lists

Compound Commands

Shell Function Definitions

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is on by default in interactive shells.

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion.

Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it is to represent itself.

When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually !, must be quoted to prevent history expansion.

There are three quoting mechanisms: the escape character, single quotes, and double quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, with the exception of <newline>. If a \<newline> pair appears, and the backslash is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash.

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).

Words of the form $'string' are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

\a

alert (bell)

\b

backspace

\e

an escape character

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\'

single quote

\nnn

the eight-bit character whose value is the octal value nnn (one to three digits)

\xHH

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

\cx

a control-x character

The expanded result is single-quoted, as if the dollar sign had not been present.

A double-quoted string preceded by a dollar sign ($) will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below under Special Parameters. 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 declare below in SHELL 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 (see SHELL BUILTIN COMMANDS below).

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 EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below). Word splitting is not performed, with the exception of "$@" as explained below under Special Parameters. Pathname expansion is not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands.

In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator can be used to append to or add to the variable's previous value. 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 below), 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. When applied to a string-valued variable, value is expanded and appended to the variable's value.

Positional Parameters

Special Parameters

Shell Variables

Arrays

Expansion is performed on the command line after it has been split into words. There are seven kinds of expansion performed: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, and pathname expansion.

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

On systems that can support it, there is an additional expansion available: process substitution.

Only brace expansion, word splitting, and pathname expansion can change the number of words of the expansion; other expansions expand a single word to a single word. The only exceptions to this are the expansions of "$@" and "${name[@]}" as explained above (see PARAMETERS).

Brace Expansion

Tilde Expansion

Parameter Expansion

Command Substitution

Arithmetic Expansion

Process Substitution

Word Splitting

Pathname Expansion

Quote Removal

Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.

In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is <, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is >, the redirection refers to the standard output (file descriptor 1).

The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, pathname 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 and standard error to the file dirlist, while the command

ls 2>&1 > dirlist

directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

/dev/fd/fd

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

/dev/stdin

File descriptor 0 is duplicated.

/dev/stdout

File descriptor 1 is duplicated.

/dev/stderr

File descriptor 2 is duplicated.

/dev/tcp/host/port

If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open a TCP connection to the corresponding socket.

/dev/udp/host/port

If host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open a UDP connection to the corresponding socket.

NOTE: Bash, as packaged for Debian, does not support using the /dev/tcp and /dev/udp files.

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

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

Redirecting Input

Redirecting Output

Appending Redirected Output

Redirecting Standard Output and Standard Error

Here Documents

<<[-]word
        here-document
delimiter

Here Strings

<<<word

Duplicating File Descriptors

Moving File Descriptors

Opening File Descriptors for Reading and Writing

Aliases allow a string to be substituted for a word when it is used as the first word of a simple command. The shell maintains a list of aliases that may be set and unset with the alias and unalias builtin commands (see SHELL BUILTIN COMMANDS below). The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The characters /, $, `, and = and any of the shell metacharacters or quoting characters listed above may not appear in an alias name. The replacement text may contain any valid shell input, including shell metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias ls to ls -F, for instance, and bash does not try to recursively expand the replacement text. If the last character of the alias value is a blank, then the next command word following the alias is also checked for alias expansion.

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

There is no mechanism for using arguments in the replacement text. If arguments are needed, a shell function should be used (see FUNCTIONS below).

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).

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

For almost every purpose, aliases are superseded by shell functions.

A shell function, defined as described above under SHELL GRAMMAR, stores a series of commands for later execution. 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. Functions are executed in the context of the current shell; no new process is created to interpret them (contrast this with the execution of a shell script). When a function is executed, the arguments to the function become the positional parameters during its execution. The special parameter # is updated to reflect the change. Special parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is executing. All other aspects of the shell execution environment are identical between a function and its caller with the exception that the DEBUG and RETURN traps (see the description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the function has been given the trace attribute (see the description of the declare builtin below) or the -o functrace shell option has been enabled with the set builtin (in which case all functions inherit the DEBUG and RETURN traps).

Variables local to the function may be declared with the local builtin command. Ordinarily, variables and their values are shared between the function and its caller.

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.

Function names and definitions may be listed with the -f option to the declare or typeset 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 subshells automatically have them defined with the -f option to the export builtin. Note that shell functions and variables with the same name may result in multiple identically-named entries in the environment passed to the shell's children. Care should be taken in cases where this may cause a problem.

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

The shell allows arithmetic expressions to be evaluated, under certain circumstances (see the let and declare builtin commands and Arithmetic Expansion). Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.

id++ id--

variable post-increment and post-decrement

++id --id

variable pre-increment and pre-decrement

- +

unary minus and plus

! ~

logical and bitwise negation

**

exponentiation

* / %

multiplication, division, remainder

+ -

addition, subtraction

<< >>

left and right bitwise shifts

<= >= < >

comparison

== !=

equality and inequality

&

bitwise AND

^

bitwise exclusive OR

|

bitwise OR

&&

logical AND

||

logical OR

expr?expr:expr

conditional operator

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

assignment

expr1 , expr2

comma

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.

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

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

Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. Expressions are formed from the following unary or binary primaries. If any 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.

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 SHELL 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 is set-group-id.

-h file

True if file exists and is a symbolic link.

-k file

True if file exists and its ``sticky'' bit is set.

-p file

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

-r file

True if file exists and is readable.

-s file

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

-t fd

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

-u file

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

-w file

True if file exists and is writable.

-x file

True if file exists and is executable.

-O file

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

-G file

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

-L file

True if file exists and is a symbolic link.

-S file

True if file exists and is a socket.

-N file

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

file1 -nt file2

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

file1 -ef file2

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

-o optname

True if shell option optname is enabled. See the list of options under the description of the -o option to the set builtin below.

-z string

True if the length of string is zero.

string

-n string

True if the length of string is non-zero.

string1 == string2

True if the strings are equal. = may be used in place of == for strict POSIX compliance.

string1 != string2

True if the strings are not equal.

string1 < string2

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

string1 > string2

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

arg1 OP arg2

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

When a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.

1.

The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.

2.

The words that are not variable assignments or redirections are expanded. If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.

3.

Redirections are performed as described above under REDIRECTION.

4.

The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.

If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.

After a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.

If the command name contains no slashes, the shell attempts to locate it. If there exists a shell function by that name, that function is invoked as described above in FUNCTIONS. If the name does not match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked.

If the name is neither a shell function nor a builtin, and contains no slashes, bash searches each element of the PATH for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files (see hash under SHELL BUILTIN COMMANDS below). A full search of the directories in PATH is performed only if the command is not found in the hash table. If the search is unsuccessful, the shell prints an error message and returns an exit status of 127.

If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments given, if any.

If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script, a file containing shell commands. A subshell is spawned to execute it. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to handle the script, with the exception that the locations of commands remembered by the parent (see hash below under SHELL BUILTIN COMMANDS) are retained by the child.

If the program is a file beginning with #!, the remainder of the first line specifies an interpreter for the program. The shell executes the specified interpreter on operating systems that do not handle this executable format themselves. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the program, followed by the name of the program, followed by the command arguments, if any.

The shell has an execution environment, which consists of the following:

•

open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin

•

the current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation

•

the file creation mode mask as set by umask or inherited from the shell's parent

•

current traps set by trap

•

shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the environment

•

shell functions defined during execution or inherited from the shell's parent in the environment

•

options enabled at invocation (either by default or with command-line arguments) or by set

•

options enabled by shopt

•

shell aliases defined with alias

•

various process IDs, including those of background jobs, the value of $$, and the value of $PPID

When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.

•

the shell's open files, plus any modifications and additions specified by redirections to the command

•

the current working directory

•

the file creation mode mask

•

shell variables and functions marked for export, along with variables exported for the command, passed in the environment

•

traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored

A command invoked in this separate environment cannot affect the shell's execution environment.

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

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

When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form name=value.

The shell provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for export to child processes. Executed commands inherit the environment. The export and declare -x commands allow parameters and functions to be added to and deleted from the environment. If the value of a parameter in the environment is modified, the new value becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the unset command, plus any additions via the export and declare -x commands.

The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described above in PARAMETERS. These assignment statements affect only the environment seen by that command.

If the -k option is set (see the set builtin command below), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.

When bash invokes an external command, the variable _ is set to the full file name of the command and passed to that command in its environment.

For the shell's purposes, a command which exits with a zero exit status has succeeded. An exit status of zero indicates success. A non-zero exit status indicates failure. When a command terminates on a fatal signal N, bash uses the value of 128+N as the exit status.

If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.

If a command fails because of an error during expansion or redirection, the exit status is greater than zero.

Shell builtin commands return a status of 0 (true) if successful, and non-zero (false) if an error occurs while they execute. All builtins return an exit status of 2 to indicate incorrect usage.

Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with a non-zero value. See also the exit builtin command below.

When bash is interactive, in the absence of any traps, it ignores SIGTERM (so that kill 0 does not kill an interactive shell), and SIGINT is caught and handled (so that the wait builtin is interruptible). In all cases, bash ignores SIGQUIT. If job control is in effect, bash ignores SIGTTIN, SIGTTOU, and SIGTSTP.

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

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

If the huponexit shell option has been set with shopt, bash sends a SIGHUP to all jobs when an interactive login shell exits.

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

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

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

[1] 25647

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

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

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

There are a number of ways to refer to a job in the shell. The character % introduces a job name. Job number n may be referred to as %n. A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, %ce refers to a stopped ce job. If a prefix matches more than one job, bash reports an error. Using %?ce, on the other hand, refers to any job containing the string ce in its command line. If the substring matches more than one job, bash reports an error. The symbols %% and %+ refer to the shell's notion of the current job, which is the last job stopped while it was in the foreground or started in the background. The previous job may be referenced using %-. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a +, and the previous job with a -. A single % (with no accompanying job specification) also refers to the current job.

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

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

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

When executing interactively, bash displays the primary prompt PS1 when it is ready to read a command, and the secondary prompt PS2 when it needs more input to complete a command. Bash allows these prompt strings to be customized by inserting a number of backslash-escaped special characters that are decoded as follows:

\a

an ASCII bell character (07)

\d

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

\D{format}

the format is passed to strftime(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required

\e

an ASCII escape character (033)

\h

the hostname up to the first `.'

\H

the hostname

\j

the number of jobs currently managed by the shell

\l

the basename of the shell's terminal device name

\n

newline

\r

carriage return

\s

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

\t

the current time in 24-hour HH:MM:SS format

\T

the current time in 12-hour HH:MM:SS format

\@

the current time in 12-hour am/pm format

\A

the current time in 24-hour HH:MM format

\u

the username of the current user

\v

the version of bash (e.g., 2.00)

\V

the release of bash, version + patch level (e.g., 2.00.0)

\w

the current working directory, with $HOME abbreviated with a tilde

\W

the basename of the current working directory, with $HOME abbreviated with a tilde

\!

the history number of this command

\#

the command number of this command

\$

if the effective UID is 0, a #, otherwise a $

\nnn

the character corresponding to the octal number nnn

\\

a backslash

\[

begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt

\]

end a sequence of non-printing characters

The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see HISTORY below), while the command number is the position in the sequence of commands executed during the current shell session. After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see the description of the shopt command under SHELL BUILTIN COMMANDS below).

This is the library that handles reading input when using an interactive shell, unless the --noediting option is given at shell invocation. By default, the line editing commands are similar to those of emacs. A vi-style line editing interface is also available. To turn off line editing after the shell is running, use the +o emacs or +o vi options to the set builtin (see SHELL BUILTIN COMMANDS below).

Readline Notation

Readline Initialization

Readline Key Bindings

Readline Variables

Readline Conditional Constructs

Searching

Readline Command Names

Commands for Moving

Commands for Manipulating the History

Commands for Changing Text

Killing and Yanking

Numeric Arguments

Completing

Keyboard Macros

Miscellaneous

Programmable Completion

When the -o history option to the set builtin is enabled, the shell provides access to the command history, the list of commands previously typed. The value of the HISTSIZE variable is used as the number of commands to save in a history list. The text of the last HISTSIZE commands (default 500) is saved. The shell stores each command in the history list prior to parameter and variable expansion (see EXPANSION above) but after history expansion is performed, subject to the values of the shell variables HISTIGNORE and HISTCONTROL.

On startup, the history is initialized from the file named by the variable HISTFILE (default ~/.bash_history). The file named by the value of HISTFILE is truncated, if necessary, to contain no more than the number of lines specified by the value of HISTFILESIZE. When an interactive shell exits, the last $HISTSIZE lines are copied from the history list to $HISTFILE. If the histappend shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), the lines are appended to the history file, otherwise the history file is overwritten. If HISTFILE is unset, or if the history file is unwritable, the history is not saved. After saving the history, the history file is truncated to contain no more than HISTFILESIZE lines. If HISTFILESIZE is not set, no truncation is performed.

The builtin command fc (see SHELL BUILTIN COMMANDS below) may be used to list or edit and re-execute a portion of the history list. The history builtin may be used to display or modify the history list and manipulate the history file. When using command-line editing, search commands are available in each editing mode that provide access to the history list.

The shell allows control over which commands are saved on the history list. The HISTCONTROL and HISTIGNORE variables may be set to cause the shell to save only a subset of the commands entered. The cmdhist shell option, if enabled, causes the shell to attempt to save each line of a multi-line command in the same history entry, adding semicolons where necessary to preserve syntactic correctness. The lithist shell option causes the shell to save the command with embedded newlines instead of semicolons. See the description of the shopt builtin below under SHELL BUILTIN COMMANDS for information on setting and unsetting shell options.

The shell supports a history expansion feature that is similar to the history expansion in csh. This section describes what syntax features are available. This feature is enabled by default for interactive shells, and can be disabled using the +H option to the set builtin command (see SHELL BUILTIN COMMANDS below). Non-interactive shells do not perform history expansion by default.

History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.

History expansion is performed immediately after a complete line is read, before the shell breaks it into words. It takes place in two parts. The first is to determine which line from the history list to use during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the history is the event, and the portions of that line that are acted upon are words. Various modifiers are available to manipulate the selected words. The line is broken into words in the same fashion as when reading input, so that several metacharacter-separated words surrounded by quotes are considered one word. History expansions are introduced by the appearance of the history expansion character, which is ! by default. Only backslash (\) and single quotes can quote the history expansion character.

Several characters inhibit history expansion if found immediately following the history expansion character, even if it is unquoted: space, tab, newline, carriage return, and =. If the extglob shell option is enabled, ( will also inhibit expansion.

Several shell options settable with the shopt builtin may be used to tailor the behavior of history expansion. If the histverify shell option is enabled (see the description of the shopt builtin), and readline is being used, history substitutions are not immediately passed to the shell parser. Instead, the expanded line is reloaded into the readline editing buffer for further modification. If readline is being used, and the histreedit shell option is enabled, a failed history substitution will be reloaded into the readline editing buffer for correction. The -p option to the history builtin command may be used to see what a history expansion will do before using it. The -s option to the history builtin may be used to add commands to the end of the history list without actually executing them, so that they are available for subsequent recall.

The shell allows control of the various characters used by the history expansion mechanism (see the description of histchars above under Shell Variables).

Event Designators

Word Designators

Modifiers

Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the end of the options. For example, the :, true, false, and test builtins do not accept options.

: [arguments]

No effect; the command does nothing beyond expanding arguments and performing any specified redirections. A zero exit code is returned.

. filename [arguments]

source filename [arguments]

Read and execute commands from filename in the current shell environment and return the exit status of the last command executed from filename. If filename does not contain a slash, file names in PATH are used to find the directory containing filename. The file searched for in PATH need not be executable. When bash is not in posix mode, the current directory is searched if no file is found in PATH. If the sourcepath option to the shopt builtin command is turned off, the PATH is not searched. If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged. The return status is the status of the last command exited within the script (0 if no commands are executed), and false if filename is not found or cannot be read.

alias [-p] [name[=value] ...]

Alias with no arguments or with the -p option prints the list of aliases in the form alias name=value on standard output. When arguments are supplied, an alias is defined for each name whose value is given. A trailing space in value causes the next word to be checked for alias substitution when the alias is expanded. For each name in the argument list for which no value is supplied, the name and value of the alias is printed. Alias returns true unless a name is given for which no alias has been defined.

bg [jobspec ...]

Resume each suspended job jobspec in the background, as if it had been started with &. If jobspec is not present, the shell's notion of the current job is used. bg jobspec returns 0 unless run when job control is disabled or, when run with job control enabled, any specified jobspec was not found or was started without job control.

bind [-m keymap] [-lpsvPSV]

bind [-m keymap] [-q function] [-u function] [-r keyseq]

bind [-m keymap] -f filename

bind [-m keymap] -x keyseq:shell-command

bind [-m keymap] keyseq:function-name

bind readline-command

Display current readline 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 .inputrc, but each binding or command must be passed as a separate argument; e.g., '"\C-x\C-r": re-read-init-file'. Options, if supplied, have the following meanings:

-m keymap

Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command; emacs is equivalent to emacs-standard.

-l

List the names of all readline functions.

-p

Display readline function names and bindings in such a way that they can be re-read.

-P

List current readline function names and bindings.

-v

Display readline variable names and values in such a way that they can be re-read.

-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 re-read.

-S

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

-f filename

Read key bindings from filename.

-q function

Query about which keys invoke the named function.

-u function

Unbind all keys bound to the named function.

-r keyseq

Remove any current binding for keyseq.

-x keyseq:shell-command

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

The return value is 0 unless an unrecognized option is given or an error occurred.

break [n]

Exit from within a for, while, until, or select loop. If n is specified, break n levels. n must be 1. If n is greater than the number of enclosing loops, all enclosing loops are exited. The return value is 0 unless the shell is not executing a loop when break is executed.

builtin shell-builtin [arguments]

Execute the specified shell builtin, passing it arguments, and return its exit status. This is useful when defining a function whose name is the same as a shell builtin, retaining the functionality of the builtin within the function. The cd builtin is commonly redefined this way. The return status is false if shell-builtin is not a shell builtin command.

cd [-L|-P] [dir]

Change the current directory to dir. The variable HOME is the default dir. The variable CDPATH defines the search path for the directory containing dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., ``.''. If dir begins with a slash (/), then CDPATH is not used. The -P option says to use the physical directory structure instead of following symbolic links (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed. An argument of - is equivalent to $OLDPWD. If a non-empty directory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return value is true if the directory was successfully changed; false otherwise.

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 [-pVv] command [arg ...]

Run command with args suppressing the normal shell function lookup. Only builtin commands or commands found in the PATH are executed. If the -p option is given, the search for command is performed using a default value for PATH that is guaranteed to find all of the standard utilities. 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. If the -V or -v option is supplied, the exit status is 0 if command was found, and 1 if not. If neither option is supplied and an error occurred or command cannot be found, the exit status is 127. Otherwise, the exit status of the command builtin is the exit status of command.

compgen [option] [word]

Generate possible completion matches for word according to the options, which may be any option accepted by the complete builtin with the exception of -p and -r, and write the matches to the standard output. When using the -F or -C options, the various shell variables set by the programmable completion facilities, while available, will not have useful values. The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed. The return value is true unless an invalid option is supplied, or no matches were generated.

complete [-abcdefgjksuv] [-o comp-option] [-A action] [-G globpat] [-W wordlist] [-P prefix] [-S suffix]

[-X filterpat] [-F function] [-C command] name [name ...]

complete -pr [name ...]

Specify how arguments to each name should be completed. If the -p option is supplied, or if no options are supplied, existing completion specifications are printed in a way that allows them to be reused as input. The -r option removes a completion specification for each name, or, if no names are supplied, all completion specifications. The process of applying these completion specifications when word completion is attempted is described above under Programmable Completion. Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X options (and, if necessary, the -P and -S options) should be quoted to protect them from expansion before the complete builtin is invoked.

-o comp-option

The comp-option controls several aspects of the compspec's behavior beyond the simple generation of completions. comp-option may be one of:

bashdefault

Perform the rest of the default bash completions if the compspec generates no matches.

default

Use readline's default filename completion if the compspec generates no matches.

dirnames

Perform directory name completion if the compspec generates no matches.

filenames

Tell readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names or suppressing trailing spaces). Intended to be used with shell functions.

nospace

Tell readline not to append a space (the default) to words completed at the end of the line.

plusdirs

After any matches defined by the compspec are generated, directory name completion is attempted and any matches are added to the results of the other actions.

-A action

The action may be one of the following to generate a list of possible completions:

alias

Alias names. May also be specified as -a.

arrayvar

Array variable names.

binding

Readline key binding names.

builtin

Names of shell builtin commands. May also be specified as -b.

command

Command names. May also be specified as -c.

directory

Directory names. May also be specified as -d.

disabled

Names of disabled shell builtins.

enabled

Names of enabled shell builtins.

export

Names of exported shell variables. May also be specified as -e.

file

File names. May also be specified as -f.

function

Names of shell functions.

group

Group names. May also be specified as -g.

helptopic

Help topics as accepted by the help builtin.

hostname

Hostnames, as taken from the file specified by the HOSTFILE shell variable.

job

Job names, if job control is active. May also be specified as -j.

keyword

Shell reserved words. May also be specified as -k.

running

Names of running jobs, if job control is active.

service

Service names. May also be specified as -s.

setopt

Valid arguments for the -o option to the set builtin.

shopt

Shell option names as accepted by the shopt builtin.

signal

Signal names.

stopped

Names of stopped jobs, if job control is active.

user

User names. May also be specified as -u.

variable

Names of all shell variables. May also be specified as -v.

-G globpat

The filename expansion pattern globpat is expanded to generate the possible completions.

-W wordlist

The wordlist is split using the characters in the IFS special variable as delimiters, and each resultant word is expanded. The possible completions are the members of the resultant list which match the word being completed.

-C command

command is executed in a subshell environment, and its output is used as the possible completions.

-F function

The shell function function is executed in the current shell environment. When it finishes, the possible completions are retrieved from the value of the COMPREPLY array variable.

-X filterpat

filterpat is a pattern as used for filename expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ! in filterpat negates the pattern; in this case, any completion not matching filterpat is removed.

-P prefix

prefix is added at the beginning of each possible completion after all other options have been applied.

-S suffix

suffix is appended to each possible completion after all other options have been applied.

The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification.

continue [n]

Resume the next iteration of the enclosing for, while, until, or select loop. If n is specified, resume at the nth enclosing loop. n must be 1. If n is greater than the number of enclosing loops, the last enclosing loop (the ``top-level'' loop) is resumed. The return value is 0 unless the shell is not executing a loop when continue is executed.

declare [-afFirtx] [-p] [name[=value] ...]

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

Declare variables and/or give them attributes. If no names are given then display the values of variables. The -p option will display the attributes and values of each name. When -p is used, additional options are ignored. The -F option inhibits the display of function definitions; only the function name and attributes are printed. If the extdebug shell option is enabled using shopt, the source file name and line number where the function is defined are displayed as well. The -F option implies -f. The following options can be used to restrict output to variables with the specified attribute or to give variables attributes:

-a

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

-f

Use function names only.

-i

The variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION ) is performed when the variable is assigned a value.

-r

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

-t

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

-x

Mark names for export to subsequent commands via the environment.

Using `+' instead of `-' turns off the attribute instead, with the exception that +a may not be used to destroy an array variable. When used in a function, makes each name local, as with the local command. If a variable name is followed by =value, the value of the variable is set to value. The return value is 0 unless an invalid option is encountered, an attempt is made to define a function using 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 above), 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.

dirs [-clpv] [+n] [-n]

Without options, displays the list of currently remembered directories. The default display is on a single line with directory names separated by spaces. Directories are added to the list with the pushd command; the popd command removes entries from the list.

+n

Displays the nth entry counting from the left of the list shown by dirs when invoked without options, starting with zero.

-n

Displays the nth entry counting from the right of the list shown by dirs when invoked without options, starting with zero.

-c

Clears the directory stack by deleting all of the entries.

-l

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

-p

Print the directory stack with one entry per line.

-v

Print the directory stack with one entry per line, prefixing each entry with its index in the stack.

The return value is 0 unless an invalid option is supplied or n indexes beyond the end of the directory stack.

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

Without options, each jobspec is removed from the table of active jobs. If the -h option is given, each jobspec is not removed from the table, but is marked so that SIGHUP is not sent to the job if the shell receives a SIGHUP. If no jobspec is present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. The return value is 0 unless a jobspec does not specify a valid job.

echo [-neE] [arg ...]

Output the args, separated by spaces, followed by a newline. The return status is always 0. If -n is specified, the trailing newline is suppressed. If the -e option is given, interpretation of the following backslash-escaped characters is enabled. The -E option disables the interpretation of these escape characters, even on systems where they are interpreted by default. The xpg_echo shell option may be used to dynamically determine whether or not echo expands these escape characters by default. echo does not interpret -- to mean the end of options. echo interprets the following escape sequences:

\a

alert (bell)

\b

backspace

\c

suppress trailing newline

\e

an escape character

\f

form feed

\n

new line

\r

carriage return

\t

horizontal tab

\v

vertical tab

\\

backslash

\0nnn

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

\nnn

the eight-bit character whose value is the octal value nnn (one to three octal digits)

\xHH

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

enable [-adnps] [-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, each name is disabled; otherwise, names are enabled. For example, to use the test binary found via the PATH instead of the shell builtin version, run The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. The -d option will delete a builtin previously loaded with -f. If no name arguments are given, or if the -p option is supplied, a list of shell builtins is printed. With no other option arguments, the list consists of all enabled shell builtins. If -n is supplied, only disabled builtins are printed. If -a is supplied, the list printed includes all builtins, with an indication of whether or not each is enabled. If -s is supplied, the output is restricted to the POSIX special builtins. The return value is 0 unless a name is not a shell builtin or there is an error loading a new builtin from a shared object.

eval [arg ...]

The args are read and concatenated together into a single command. This command is then read and executed by the shell, and its exit status is returned as the value of eval. If there are no args, or only null arguments, eval returns 0.

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

If command is specified, it replaces the shell. No new process is created. The arguments become the arguments to command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth arg passed to command. This is what login(1) 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 the executed command. If command cannot be executed for some reason, a non-interactive shell exits, unless the shell option execfail is enabled, in which case it returns failure. An interactive shell returns failure if the file cannot be executed. If command is not specified, any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the return status is 1.

exit [n]

Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the last command executed. A trap on EXIT is executed before the shell terminates.

export [-fn] [name[=word]] ...

export -p

The supplied names are marked for automatic export to the environment of subsequently executed commands. If the -f option is given, the names refer to functions. If no names are given, or if the -p option is supplied, a list of all names that are exported in this shell is printed. The -n option causes the export property to be removed from each name. If a variable name is followed by =word, the value of the variable is set to word. export returns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

fc [-e ename] [-nlr] [first] [last]

fc -s [pat=rep] [cmd]

Fix Command. In the first form, a range of commands from first to last is selected from the history list. First and last may be specified as a string (to locate the last command beginning with that string) or as a number (an index into the history list, where a negative number is used as an offset from the current command number). If last is not specified it is set to the current command for listing (so that prints the last 10 commands) and to first otherwise. If first is not specified it is set to the previous command for editing and -16 for listing. The -n option suppresses the command numbers when listing. The -r option reverses the order of the commands. If the -l option is given, the commands are listed on standard output. Otherwise, the editor given by ename is invoked on a file containing those commands. If ename is not given, the value of the FCEDIT variable is used, and the value of EDITOR if FCEDIT is not set. If neither variable is set, is used. When editing is complete, the edited commands are echoed and executed. In the second form, command is re-executed after each instance of pat is replaced by rep. A useful alias to use with this is so that typing runs the last command beginning with and typing re-executes the last command. If the first form is used, the return value is 0 unless an invalid option is encountered or first or last specify history lines out of range. If the -e option is supplied, the return value is the value of the last command executed or failure if an error occurs with the temporary file of commands. If the second form is used, the return status is that of the command re-executed, unless cmd does not specify a valid history line, in which case fc returns failure.

fg [jobspec]

Resume jobspec in the foreground, and make it the current job. If jobspec is not present, the shell's notion of the current job is used. The return value is that of the command placed into the foreground, or failure if run when job control is disabled or, when run with job control enabled, if jobspec does not specify a valid job or jobspec specifies a job that was started without job control.

getopts optstring name [args]

getopts is used by shell procedures to parse positional parameters. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. The colon and question mark characters may not be used as option characters. Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used. When the end of options is encountered, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and name is set to ?. getopts normally parses the positional parameters, but if more arguments are given in args, getopts parses those instead. getopts can report errors in two ways. If the first character of optstring is a colon, silent error reporting is used. In normal operation diagnostic messages are printed when invalid options or missing option arguments are encountered. If the variable OPTERR is set to 0, no error messages will be displayed, even if the first character of optstring is not a colon. If an invalid option is seen, getopts places ? into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, the option character found is placed in OPTARG and no diagnostic message is printed. If a required argument is not found, and getopts is not silent, a question mark (?) is placed in name, OPTARG is unset, and a diagnostic message is printed. If getopts is silent, then a colon (:) is placed in name and OPTARG is set to the option character found. getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is encountered or an error occurs.

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

For each name, the full file name of the command is determined by searching the directories in $PATH and remembered. If the -p option is supplied, no path search is performed, and filename is used as the full file name of the command. 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 true unless a name is not found or an invalid option is supplied.

help [-s] [pattern]

Display helpful information about builtin commands. If pattern is specified, help gives detailed help on all commands matching pattern; otherwise help for all the builtins and shell control structures is printed. The -s option restricts the information displayed to a short usage synopsis. The return status is 0 unless no command matches pattern.

history [n]

history -c

history -d offset

history -anrw [filename]

history -p arg [arg ...]

history -s arg [arg ...]

With no options, display the command history list with line numbers. Lines listed with a * have been modified. An argument of n lists only the last n lines. If the shell variable HISTTIMEFORMAT is set and not null, it is used as a format string for strftime(3) to display the time stamp associated with each displayed history entry. No intervening blank is printed between the formatted time stamp and the history line. If filename is supplied, it is used as the name of the history file; if not, the value of HISTFILE is used. Options, if supplied, have the following meanings:

-c

Clear the history list by deleting all the entries.

-d offset

Delete the history entry at position offset.

-a

Append the ``new'' history lines (history lines entered since the beginning of the current bash session) to the history file.

-n

Read the history lines not already read from the history file into the current history list. These are lines appended to the history file since the beginning of the current bash session.

-r

Read the contents of the history file and use them as the current history.

-w

Write the current history to the history file, overwriting the history file's contents.

-p

Perform history substitution on the following args and display the result on the standard output. Does not store the results in the history list. Each arg must be quoted to disable normal history expansion.

-s

Store the args in the history list as a single entry. The last command in the history list is removed before the args are added.

If the HISTTIMEFORMAT is set, the time stamp information associated with each history entry is written to the history file. The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset is supplied as an argument to -d, or the history expansion supplied as an argument to -p fails.

jobs [-lnprs] [ jobspec ... ]

jobs -x command [ args ... ]

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

-l

List process IDs in addition to the normal information.

-p

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

-n

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

-r

Restrict output to running jobs.

-s

Restrict output to stopped jobs.

If jobspec is given, output is restricted to information about that job. The return status is 0 unless an invalid option is encountered or an invalid jobspec is supplied.

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

kill [-s sigspec | -n signum | -sigspec] [pid | jobspec] ...

kill -l [sigspec | exit_status]

Send the signal named by sigspec or signum to the processes named by pid or jobspec. sigspec is either a case-insensitive signal name such as SIGKILL (with or without the SIG prefix) or a signal number; signum is a signal number. If sigspec is not present, then SIGTERM is assumed. An argument of -l lists the signal names. If any arguments are supplied when -l is given, the names of the signals corresponding to the arguments are listed, and the return status is 0. The exit_status argument to -l is a number specifying either a signal number or the exit status of a process terminated by a signal. kill returns true if at least one signal was successfully sent, or false if an error occurs or an invalid option is encountered.

let arg [arg ...]

Each arg is an arithmetic expression to be evaluated (see ARITHMETIC EVALUATION). If the last arg evaluates to 0, let returns 1; 0 is returned otherwise.

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. When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children. With no operands, local writes a list of local variables to the standard output. It is an error to use local when not within a function. The return status is 0 unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.

logout

Exit a login shell.

popd [-n] [+n] [-n]

Removes entries from the directory stack. With no arguments, removes the top directory from the stack, and performs a cd to the new top directory. Arguments, if supplied, have the following meanings:

+n

Removes the nth entry counting from the left of the list shown by dirs, starting with zero. For example: removes the first directory, the second.

-n

Removes the nth entry counting from the right of the list shown by dirs, starting with zero. For example: removes the last directory, the next to last.

-n

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

If the popd command is successful, a dirs is performed as well, and the return status is 0. popd returns false if an invalid option is encountered, the directory stack is empty, a non-existent directory stack entry is specified, or the directory change fails.

printf [-v var] format [arguments]

Write the formatted arguments to the standard output under the control of the format. The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. In addition to the standard printf(1) formats, %b causes printf to expand backslash escape sequences in the corresponding argument (except that \c terminates output, backslashes in \', \", and \? are not removed, and octal escapes beginning with \0 may contain up to four digits), and %q causes printf to output the corresponding argument in a format that can be reused as shell input. The -v option causes the output to be assigned to the variable var rather than being printed to the standard output. 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.

pushd [-n] [dir]

pushd [-n] [+n] [-n]

Adds a directory to the top of the directory stack, or rotates the stack, making the new top of the stack the current working directory. With no arguments, exchanges the top two directories and returns 0, unless the directory stack is empty. Arguments, if supplied, have the following meanings:

+n

Rotates the stack so that the nth directory (counting from the left of the list shown by dirs, starting with zero) is at the top.

-n

Rotates the stack so that the nth directory (counting from the right of the list shown by dirs, starting with zero) is at the top.

-n

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

dir

Adds dir to the directory stack at the top, making it the new current working directory.

If the pushd command is successful, a dirs is performed as well. If the first form is used, pushd returns 0 unless the cd to dir fails. With the second form, pushd returns 0 unless the directory stack is empty, a non-existent directory stack element is specified, or the directory change to the specified new current directory fails.

pwd [-LP]

Print the absolute pathname of the current working directory. The pathname printed contains no symbolic links if the -P option is supplied or the -o physical option to the set builtin command is enabled. If the -L option is used, the pathname printed may contain symbolic links. The return status is 0 unless an error occurs while reading the name of the current directory or an invalid option is supplied.

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

One line is read from the standard input, or from the file descriptor fd supplied as an argument to the -u option, and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. The backslash character (\) may be used to remove any special meaning for the next character read and for line continuation. Options, if supplied, have the following meanings:

-a aname

The words are assigned to sequential indices of the array variable aname, starting at 0. aname is unset before any new values are assigned. Other name arguments are ignored.

-d delim

The first character of delim is used to terminate the input line, rather than newline.

-e

If the standard input is coming from a terminal, readline (see READLINE above) is used to obtain the line.

-n nchars

read returns after reading nchars characters rather than waiting for a complete line of input.

-p prompt

Display prompt on standard error, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.

-r

Backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not be used as a line continuation.

-s

Silent mode. If input is coming from a terminal, characters are not echoed.

-t timeout

Cause read to time out and return failure if a complete line of input is not read within timeout seconds. This option has no effect if read is not reading input from the terminal or a pipe.

-u fd

Read input from file descriptor fd.

If no names are supplied, the line read is assigned to the variable REPLY. The return code is zero, unless end-of-file is encountered, read times out, or an invalid file descriptor is supplied as the argument to -u.

readonly [-apf] [name[=word] ...]

The given names are marked readonly; the values of these names may not be changed by subsequent assignment. If the -f option is supplied, the functions corresponding to the names are so marked. The -a option restricts the variables to arrays. If no name arguments are given, or if the -p option is supplied, a list of all readonly names is printed. The -p option causes output to be displayed in a format that may be reused as input. If a variable name is followed by =word, the value of the variable is set to word. The return status is 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

return [n]

Causes a function to exit with the return value specified by n. If n is omitted, the return status is that of the last command executed in the function body. If used outside a function, but during execution of a script by the . (source) command, it causes the shell to stop executing that script and return either n or the exit status of the last command executed within the script as the exit status of the script. If used outside a function and not during execution of a script by ., the return status is false. Any command associated with the RETURN trap is executed before execution resumes after the function or script.

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

Without options, the name and value of each shell variable are displayed in a format that can be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In posix mode, only shell variables are listed. The output is sorted according to the current locale. When options are specified, they set or unset shell attributes. Any arguments remaining after the options are processed are treated as values for the positional parameters and are assigned, in order, to $1, $2, ... $n. Options, if specified, have the following meanings:

-a

Automatically mark variables and functions which are modified or created for export to the environment of subsequent commands.

-b

Report the status of terminated background jobs immediately, rather than before the next primary prompt. This is effective only when job control is enabled.

-e

Exit immediately if a simple command (see SHELL GRAMMAR above) exits with a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of a && or list, or if the command's return value is being inverted via !. A trap on ERR, if set, is executed before the shell exits.

-f

Disable pathname expansion.

-h

Remember the location of commands as they are looked up for execution. This is enabled by default.

-k

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

-m

Monitor mode. Job control is enabled. This option is on by default for interactive shells on systems that support it (see JOB CONTROL above). Background processes run in a separate process group and a line containing their exit status is printed upon their completion.

-n

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

-o option-name

The option-name can be one of the following:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Use an emacs-style command line editing interface. This is enabled by default when the shell is interactive, unless the shell is started with the --noediting option.

errtrace

Same as -E.

functrace

Same as -T.

errexit

Same as -e.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history, as described above under HISTORY. This option is on by default in interactive shells.

ignoreeof

The effect is as if the shell command had been executed (see Shell Variables above).

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f. nolog Currently ignored.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

pipefail

If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.

posix

Change the behavior of bash where the default operation differs from the POSIX 1003.2 standard to match the standard (posix mode).

privileged

Same as -p.

verbose

Same as -v.

vi

Use a vi-style command line editing interface.

xtrace

Same as -x.

If -o is supplied with no option-name, the values of the current options are printed. If +o is supplied with no option-name, a series of set commands to recreate the current option settings is displayed on the standard output.

-p

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

-t

Exit after reading and executing one command.

-u

Treat unset variables as an error when performing parameter expansion. If expansion is attempted on an unset variable, the shell prints an error message, and, if not interactive, exits with a non-zero status.

-v

Print shell input lines as they are read.

-x

After expanding each simple command, for command, case command, select command, or arithmetic for command, display the expanded value of PS4, followed by the command and its expanded arguments or associated word list.

-B

The shell performs brace expansion (see Brace Expansion above). This is on by default.

-C

If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This may be overridden when creating output files by using the redirection operator >| instead of >.

-E

If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.

-H

Enable ! style history substitution. This option is on by default when the shell is interactive.

-P

If set, the shell does not follow symbolic links when executing commands such as cd that change the current working directory. It uses the physical directory structure instead. By default, bash follows the logical chain of directories when performing commands which change the current directory.

-T

If set, any traps on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The DEBUG and RETURN traps are normally not inherited in such cases.

--

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

-

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

The options are off by default unless otherwise noted. Using + rather than - causes these options to be turned off. The options can also be specified as arguments to an invocation of the shell. The current set of options may be found in $-. The return status is always true unless an invalid option is encountered.

shift [n]

The positional parameters from n+1 ... are renamed to $1 .... 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 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0.

shopt [-pqsu] [-o] [optname ...]

Toggle the values of variables controlling optional shell behavior. With no options, or with the -p option, a list of all settable options is displayed, with an indication of whether or not each is set. The -p option causes output to be displayed in a form that may be reused as input. Other options have the following meanings:

-s

Enable (set) each optname.

-u

Disable (unset) each optname.

-q

Suppresses normal output (quiet mode); the return status indicates whether the optname is set or unset. If multiple optname arguments are given with -q, the return status is zero if all optnames are enabled; non-zero otherwise.

-o

Restricts the values of optname to be those defined for the -o option to the set builtin.

If either -s or -u is used with no optname arguments, the display is limited to those options which are set or unset, respectively. Unless otherwise noted, the shopt options are disabled (unset) by default.

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

The list of shopt options is:

cdable_vars

If set, an argument to the cd builtin command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.

cdspell

If set, minor errors in the spelling of a directory component in a cd command will be corrected. The errors checked for are transposed characters, a missing character, and one character too many. If a correction is found, the corrected file name is printed, and the command proceeds. This option is only used by interactive shells.

checkhash

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

checkwinsize

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

cmdhist

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

dotglob

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

execfail

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

expand_aliases

If set, aliases are expanded as described above under ALIASES. This option is enabled by default for interactive shells.

extdebug

If set, behavior intended for use by debuggers is enabled:

1.

The -F option to the declare builtin displays the source file name and line number corresponding to each function name supplied as an argument.

2.

If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.

3.

If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source builtins), a call to return is simulated.

4.

BASH_ARGC and BASH_ARGV are updated as described in their descriptions above.

5.

Function tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the DEBUG and RETURN traps.

6.

Error tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the ERROR trap.

extglob

If set, the extended pattern matching features described above under Pathname Expansion are enabled.

extquote

If set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in double quotes. This option is enabled by default.

failglob

If set, patterns which fail to match filenames during pathname expansion result in an expansion error.

force_fignore

If set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word completion even if the ignored words are the only possible completions. See SHELL VARIABLES above for a description of FIGNORE. This option is enabled by default.

gnu_errfmt

If set, shell error messages are written in the standard GNU error message format.

histappend

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

histreedit

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

histverify

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

hostcomplete

If set, and readline is being used, bash will attempt to perform hostname completion when a word containing a @ is being completed (see Completing under READLINE above). This is enabled by default.

huponexit

If set, bash will send SIGHUP to all jobs when an interactive login shell exits.

interactive_comments

If set, allow a word beginning with # to cause that word and all remaining characters on that line to be ignored in an interactive shell (see COMMENTS above). This option is enabled by default.

lithist

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

login_shell

The shell sets this option if it is started as a login shell (see INVOCATION above). The value may not be changed.

mailwarn

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

no_empty_cmd_completion

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

nocaseglob

If set, bash matches filenames in a case-insensitive fashion when performing pathname expansion (see Pathname Expansion above).

nocasematch

If set, bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional commands.

nullglob

If set, bash allows patterns which match no files (see Pathname Expansion above) to expand to a null string, rather than themselves.

progcomp

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

promptvars

If set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described in PROMPTING above. This option is enabled by default.

restricted_shell

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

shift_verbose

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

sourcepath

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

xpg_echo

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

suspend [-f]

Suspend the execution of this shell until it receives a SIGCONT signal. The -f option says not to complain if this is a login shell; just suspend anyway. The return status is 0 unless the shell is a login shell and -f is not supplied, or if job control is not enabled.

test expr

[ expr ]

Return a status of 0 or 1 depending on the evaluation of the conditional expression expr. Each operator and operand must be a separate argument. Expressions are composed of the primaries described above under CONDITIONAL EXPRESSIONS. test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options. Expressions may be combined using the following operators, listed in decreasing order of precedence.

! expr

True if expr is false.

( expr )

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

expr1 -a expr2

True if both expr1 and expr2 are true.

expr1 -o expr2

True if either expr1 or expr2 is true.

test and [ 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 listed above under CONDITIONAL EXPRESSIONS, the expression is true if the unary test is true. If the first argument is not a valid unary conditional operator, the expression is false.

3 arguments

If the second argument is one of the binary conditional operators listed above under CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test using the first and third arguments as operands. If the first argument is !, the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly ( and the third argument is exactly ), the result is the one-argument test of the second argument. Otherwise, the expression is false. The -a and -o operators are considered binary operators in this case.

4 arguments

If the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. Otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.

5 or more arguments

The expression is parsed and evaluated according to precedence using the rules listed above.

times

Print the accumulated user and system times for the shell and for processes run from the shell. The return status is 0.

trap [-lp] [[arg] sigspec ...]

The command arg is to be read and executed when the shell receives signal(s) sigspec. If arg is absent (and there is a single sigspec) or -, each specified signal is reset to its original disposition (the value it had upon entrance to the shell). If arg is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes. If arg is not present and -p has been supplied, then the trap commands associated with each sigspec are displayed. If no arguments are supplied or if only -p is given, trap prints the list of commands associated with each signal. The -l option causes the shell to print a list of signal names and their corresponding numbers. Each sigspec is either a signal name defined in <signal.h>, or a signal number. Signal names are case insensitive and the SIG prefix is optional. If a sigspec is EXIT (0) the command arg is executed on exit from the shell. 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 (see SHELL GRAMMAR above). Refer to the description of the extdebug option to the shopt builtin for details of its effect on the DEBUG trap. If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status, subject to the following conditions. The ERR trap is not executed if the failed command is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of a && or list, or if the command's return value is being inverted via !. These are the same conditions obeyed by the errexit option. If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the . or source builtins finishes executing. Signals ignored upon entry to the shell cannot be trapped or reset. Trapped signals are reset to their original values in a child process when it is created. The return status is false if any sigspec is invalid; otherwise trap returns true.

type [-aftpP] name [name ...]

With no options, indicate how each name would be interpreted if used as a command name. If the -t option is used, type prints a string which is one of alias, keyword, function, builtin, or file if name is an alias, shell reserved word, function, builtin, or disk file, respectively. If the name is not found, then nothing is printed, and an exit status of false is returned. If the -p option is used, type either returns the name of the disk file that would be executed if name were specified as a command name, or nothing if would not return file. The -P option forces a PATH search for each name, even if would not return file. If a command is hashed, -p and -P print the hashed value, not necessarily the file that appears first in PATH. If the -a option is used, type prints all of the places that contain an executable named name. This includes aliases and functions, if and only if the -p option is not also used. The table of hashed commands is not consulted when using -a. The -f option suppresses shell function lookup, as with the command builtin. type returns true if any of the arguments are found, false if none are found.

ulimit [-SHacdefilmnpqrstuvx [limit]]

Provides control over the resources available to the shell and to processes started by it, on systems that allow such control. The -H and -S options specify that the hard or soft limit is set for the given resource. A hard limit cannot be increased once it is set; a soft limit may be increased up to the value of the hard limit. If neither -H nor -S is specified, both the soft and hard limits are set. The value of limit can be a number in the unit specified for the resource or one of the special values hard, soft, or unlimited, which stand for the current hard limit, the current soft limit, and no limit, respectively. If limit is omitted, the current value of the soft limit of the resource is printed, unless the -H option is given. When more than one resource is specified, the limit name and unit are printed before the value. Other options are interpreted as follows:

-a

All current limits are reported

-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 created by the shell

-i

The maximum number of pending signals

-l

The maximum size that may be locked into memory

-m

The maximum resident set size

-n

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

-p

The pipe size in 512-byte blocks (this may not be set)

-q

The maximum number of bytes in POSIX message queues

-r

The maximum rt 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

-x

The maximum number of file locks

If limit is given, it is the new value of the specified resource (the -a option is display only). If no option is given, then -f is assumed. Values are in 1024-byte increments, except for -t, which is in seconds, -p, which is in units of 512-byte blocks, and -n and -u, which are unscaled values. The return status is 0 unless an invalid option or argument is supplied, or an error occurs while setting a new limit.

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

The user file-creation mask is set to mode. If mode begins with a digit, it is interpreted as an octal number; otherwise it is interpreted as a symbolic mode mask similar to that accepted by chmod(1). If mode is omitted, the current value of the mask is printed. The -S option causes the mask to be printed in symbolic form; the default output is an octal number. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused as input. The return status is 0 if the mode was successfully changed or if no mode argument was supplied, and false otherwise.

unalias [-a] [name ...]

Remove each name from the list of defined aliases. If -a is supplied, all alias definitions are removed. The return value is true unless a supplied name is not a defined alias.

unset [-fv] [name ...]

For each name, remove the corresponding variable or function. If no options are supplied, or the -v option is given, each name refers to a shell variable. Read-only variables may not be unset. If -f is specified, each name refers to a shell function, and the function definition is removed. Each unset variable or function is removed from the environment passed to subsequent commands. If any of RANDOM, SECONDS, LINENO, HISTCMD, FUNCNAME, GROUPS, or DIRSTACK are unset, they lose their special properties, even if they are subsequently reset. The exit status is true unless a name is readonly.

wait [n ...]

Wait for each specified process and return its termination status. Each n may be a process ID or a job specification; if a job spec is given, all processes in that job's pipeline are waited for. If n is not given, all currently active child processes are waited for, and the return status is zero. If n specifies a non-existent process or job, the return status is 127. Otherwise, the return status is the exit status of the last process or job waited for.

If bash is started with the name rbash, or the -r option is supplied at invocation, the shell becomes restricted. A restricted shell is used to set up an environment more controlled than the standard shell. It behaves identically to bash with the exception that the following are disallowed or not performed:

•

changing directories with cd

•

setting or unsetting the values of SHELL, PATH, ENV, or BASH_ENV

•

specifying command names containing /

•

specifying a file name containing a / as an argument to the . builtin command

•

Specifying a filename containing a slash as an argument to the -p option to the hash builtin command

•

importing function definitions from the shell environment at startup

•

parsing the value of SHELLOPTS from the shell environment at startup

•

redirecting output using the >, >|, <>, >&, &>, and >> redirection operators

•

using the exec builtin command to replace the shell with another command

•

adding or deleting builtin commands with the -f and -d options to the enable builtin command

•

Using the enable builtin command to enable disabled shell builtins

•

specifying the -p option to the command builtin command

•

turning off restricted mode with set +r or set +o restricted.

These restrictions are enforced after any startup files are read.

(see COMMAND EXECUTION above), } rbash turns off any restrictions in the shell spawned to execute the script.

Bash Reference Manual, Brian Fox and Chet Ramey

The Gnu Readline Library, Brian Fox and Chet Ramey

The Gnu History Library, Brian Fox and Chet Ramey

Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE

sh(1), ksh(1), csh(1)

emacs(1), vi(1)

readline(3)

The bash executable

The systemwide initialization file, executed for login shells

The systemwide per-interactive-shell startup file

The systemwide login shell cleanup file, executed when a login shell exits

The personal initialization file, executed for login shells

The individual per-interactive-shell startup file

The individual login shell cleanup file, executed when a login shell exits

Individual readline initialization file

Brian Fox, Free Software Foundation

bfox@gnu.org

Chet Ramey, Case Western Reserve University

chet@po.cwru.edu

If you find a bug in bash, you should report it. But first, you should make sure that it really is a bug, and that it appears in the latest version of bash. The latest version is always available from ftp://ftp.gnu.org/pub/bash/.

Once you have determined that a bug actually exists, use the bashbug command to submit a bug report. If you have a fix, you are encouraged to mail that as well! Suggestions and `philosophical' bug reports may be mailed to bug-bash@gnu.org or posted to the Usenet newsgroup gnu.bash.bug.

ALL bug reports should include:

The version number of bash

The hardware and operating system

The compiler used to compile

A description of the bug behaviour

A short script or `recipe' which exercises the bug

bashbug inserts the first three items automatically into the template it provides for filing a bug report.

Comments and bug reports concerning this manual page should be directed to chet@po.cwru.edu.

It's too big and too slow.

There are some subtle differences between bash and traditional versions of sh, mostly because of the POSIX specification.

Aliases are confusing in some uses.

Shell builtin commands and functions are not stoppable/restartable.

Compound commands and command sequences of the form `a ; b ; c' are not handled gracefully when process suspension is attempted. When a process is stopped, the shell immediately executes the next command in the sequence. It suffices to place the sequence of commands between parentheses to force it into a subshell, which may be stopped as a unit.

Commands inside of $(...) command substitution are not parsed until substitution is attempted. This will delay error reporting until some time after the command is entered. For example, unmatched parentheses, even inside shell comments, will result in error messages while the construct is being read.

Array variables may not (yet) be exported.