BASH

Command Line Options

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

ARGUMENTS

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.

DEFINITIONS

RESERVED WORDS

The following words are recognised as reserved when unquoted and either the first word of a simple command or the third word of a case or for command:

! case do done elif else esac fi for function if in select then until while { }

SHELL GRAMMAR

Simple commands

A simple command is a sequence of optional variable assignments followed by blank separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed. The remaining words are passed as arguments to the invoked command. The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n.

Pipelines

A pipeline is a sequence of commands each separated by the character |. The format for a pipeline is:

[ ! ] command [ | command2 ... ]

The standard output of command is connected to the standard input of command2. This connection is performed before any redirections specified by the command.

If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical NOT of the exit status of the last command. Otherwise, the status of the pipeline is the exit status of the last command. The shell waits for all commands in the pipeline to terminate before returning a value.

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

LISTS

A list is a sequence of one or more pipelines separated by one of the control operators ; & && or ||, and terminated by one of ; & or <newline>.

Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.

If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish,and the return status is 0. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed.

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form command && command2

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

An OR list has the form

command || command2

command2 is executed if and only if command returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands

a compound command is one of the following:

(list) list is executed in a subshell. Variable assignments and inbuilt commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.

{ list; }

list is simply executed in the current shell environment. This is known as a group command. The return status is the exit status of list.

for name [ in word; ] do list ; done

The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set.

select name [ in word; ] do list ; done

The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the standard error, each preceded by a number. If the in word is omitted, the positional parameters are printed. The PS3 prompt is then displayed and a line read from the standard input. If the line consists of the number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY. The list is executed after each selection until a break or return command is executed. The exit status of select is the exit status of the last command executed in list, or zero if no commands were executed.

case word in [ pattern [ | pattern ] ... ) list ;; ] ... esac

A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules as for pathname expansion. When a match is found, the corresponding list is executed. After the first match, no subsequent matches are attempted.

The exit status is zero if no patterns are matches. Otherwise, it is the exit status of the last command executed in list.

if list then list [ elif list then list ] ... [ else list ] fi

The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.

while list do list done

until list do list done

The while command continuously executes the do list as long as the last command in list returns an exit status of zero. The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in list returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.

[ function ] name () { list; }

This defines a function named name. The body of the function is the list of commands between { and }. This list is executed whenever name is specified as the name of a simple command. The exit status of a function is the exit status of the last command executed in the body.

COMMENTS

In a non-interactive shell, or an interactive shell in which the -o interactive comments option to the set command is enabled, a word beginning with # causes that word and all remaining characters on that line to be ignored. An interactive shell without the -o interactive-comments option enabled does not allow comments.

QUOTING

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 recognised 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 they are to represent themselves. There are three quoting mechanisms:

PARAMETERS

A parameter is an entity that stores values, somewhat like a variable in a conventional programming language. It can be a name, a number, or one of the special characters listed below under Special Parameters. For the shell's purposes, a variable is a parameter denoted by a name.

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 command.

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

name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. If the variable has its -i attribute set (see declare) the value is subject to arithmetic expansion even if the $[...] syntax does not appear. Word splitting is not performed, with the exception of "$@" as explained below under Special Parameters. Pathname expansion is not performed.

Positional Parameters

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. Positional parameters are assigned from the shell's arguments when it is invoked, and may be reassigned using the set command. Positional parameters may not be assigned to with assignment statements. The positional parameters are temporarily replaced when a shell function is executed (see FUNCTIONS).

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

Special Parameters

The shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.

Shell Variables

The following variables are set by the shell:

The following variables are used by the shell. In some cases, bash assigns a default value to a variable; these cases are noted below.

EXPANSION

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

• brace expansion,
• tilde expansion,
• parameter and variable expansion,
• command substitution,
• arithmetic expansion,
• 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 single exception to this is the expansion of ``$@'' as explained above (see PARAMETERS).

Brace Expansion

Brace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to pathname expansion, but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by a series of comma-separated strings between a pair of braces, followed by an optional postamble. The preamble is prepended to each string contained within the braces, and the postamble is then appended to each resulting string, expanding left to right.

Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, a{d,c,b}e expands into `ade ace abe'.

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces.

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

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

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

or

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

Brace expansion introduces a slight incompatibility with traditional versions of sh, the Bourne shell. sh does not treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as file{1,2} appears identically in the output. The same word is output as file1 file2 after expansion by bash. If strict compatibility with sh is desired, start bash with the -nobraceexpansion flag (see OPTIONS) or disable brace expansion with the +o braceexpand option to the set command.

Tilde Expansion

If a word begins with a tilde character (`~'), all of the characters preceding the first slash (or all characters, if there is no slash) are treated as a possible login name. If this login name is the null string, the tilde is replaced with the value of the parameter HOME. If HOME is unset, the home directory of the user executing the shell is substituted instead.

If a `+' follows the tilde, the value of PWD replaces the tilde and `+'. If a `-' follows, the value of OLDPWD is substituted. If the value following the tilde is a valid login name, the tilde and login name are replaced with the home directory associated with that name. If the name is invalid, or the tilde expansion fails, the word is unchanged.

Each variable assignment is checked for unquoted instances of tildes following a : or =. In these cases, tilde substitution is also performed. Consequently, one may use pathnames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value.

Parameter Expansion

The `$' character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.

${parameter}

The value of parameter is substituted. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character which is not to be interpreted as part of its name.

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. Bash tests for a parameter that is unset or null; omitting the colon results in a test only for a parameter that is unset.

Command Substitution

Command substitution allows the output of a command to replace the command name. There are two forms:

$(command) or

`command`

Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.

When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or \. When using the $(command) form, all characters between the parentheses make up the command; none are treated specially.

Command substitutions may be nested. To nest when using the old form, escape the inner backquotes with back slashes.

If the substitution appears within double quotes, word splitting and pathname expansion are not performed on the results.

Arithmetic Expansion

Arithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. There are two formats for arithmetic expansion:

$[expression]

$((expression))

The expression is treated as if it were within double quotes, but a double quote inside the braces or parentheses is not treated specially. All tokens in the expression undergo parameter expansion, command substitution, and quote removal. Arithmetic substitutions may be nested.

The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If expression is invalid, bash prints a message indicating failure and no substitution occurs.

Process Substitution

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files. It takes the form of <(list) or >(list). The process list is run with its input or output connected to a FIFO or some file in /dev/fd. The name of this file is passed as an argument to the current command as the result of the expansion. If the >(list) form is used, writing to the file will provide input for list. If the <(list) form is used, the file passed as an argument should be read to obtain the output of list.

On systems that support it, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

Word Splitting

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting.

The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words on these characters. If the value of IFS is exactly <space><tab><newline>, the default, then any sequence of IFS characters serves to delimit words. If IFS has a value other than the default, then sequences of the whitespace characters space and tab are ignored at the beginning and end of the word, as long as the whitespace character is in the value of IFS (an IFS whitespace character). Any character in IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. A sequence of IFS whitespace characters is also treated as a delimiter. If the value of IFS is null, no word splitting occurs. IFS cannot be unset.

Explicit null arguments ("" or '') are retained. Implicit null arguments, resulting from the expansion of parameters that have no values, are removed.

Note that if no expansion occurs, no splitting is performed.

Pathname Expansion

After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of pathnames matching the pattern. If no matching pathnames are found, and the shell variable allow_null_glob_expansion is unset, the word is left unchanged. If the variable is set, and no matches are found, the word is removed. When a pattern is used for pathname generation, the character ``.'' at the start of a name or immediately following a slash must be matched explicitly, unless the shell variable glob_dot_filenames is set. The slash character must always be matched explicitly. In other cases, the ``.'' character is not treated specially.

The special pattern characters have the following meanings:

Quote Removal

After the preceding expansions, all unquoted occurrences of the characters \, `, and " are removed.

REDIRECTION

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 that follows the redirection operator in the following descriptions is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, and pathname expansion. 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.

Redirecting Input

Redirection of input causes the file whose name results from the expansion of word to be opened for reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified. The general format for redirecting input is:

[n]<word

Redirecting Output

Redirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. The general format for redirecting output is:

[n]>word

If the redirection operator is >|, then the value of the -C option to the set command is not tested, and file creation is attempted. (See also the description of noclobber)

Appending Redirected Output

Redirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created. The general format for appending output is:

[n]>>word

Redirecting Standard Output and Standard Error

Bash allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word with this construct. There are two formats for redirecting standard output and standard error:

&>word and

>&word

Of the two forms, the first is preferred. This is semantically equivalent to

>word 2>&1

Here Documents

This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command.

The format of here-documents is as follows:

<<[-]word

here-document

delimiter

No parameter expansion, command substitution, pathname expansion, or arithmetic expansion is performed on word.

If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. Otherwise, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the pair \<newline> is ignored, and \ must be used to quote the characters \, $, and `.

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

Duplicating File Descriptors

The redirection operator

[n]<&word

is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If word evaluates to -, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used.

The operator

[n]>&word

is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. As a special case, if n is omitted, and word does not expand to one or more digits, the standard output and standard error are redirected as described previously.

Opening File Descriptors for Reading and Writing

The redirection operator

[n]<>word

causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or as the standard input and standard output if n is not specified. If the file does not exist, it is created.

FUNCTIONS

A shell function, defined as described under SHELL GRAMMAR, stores a series of commands for later execution.

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.

Positional parameter 0 is unchanged.

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

If the inbuilt command return is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter # are restored to the values they had prior to function execution.

Function names and definitions may be listed with the -f option to the declare or typeset commands. Functions may be exported so that subshells automatically have them defined with the -f option to the export inbuilt.

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

ALIASES

The shell maintains a list of aliases that may be set and unset with the alias and unalias commands. The first word of each command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The alias name and the replacement text may contain any valid shell input, including the metacharacters listed above, with the exception that the alias name may not contain =. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means that one may alias ls to ls -F, for instance, and bash does not try to recursively expand the replacement text. If the last character of the alias value is a 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, as in csh. If arguments are needed, a shell function should be used.

Aliases are not expanded when the shell is not interactive.

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.

This means that the commands following the alias definition on that line are not affected by the new alias. This behaviour is also an issue when functions are executed. Aliases are expanded when the 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.

Note that for almost every purpose, aliases are superseded by shell functions.

JOB CONTROL

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 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 allows you to use it. Typing the suspend character (typically ^Z, Control-Z) while a process is running causes that process to be stopped and returns you 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. You 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. 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 -.

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 command is set, bash reports such changes immediately. (See also the description of notify variable under Shell Variables above.)

If you attempt to exit bash while jobs are stopped, the shell prints a message warning you. You may then use the jobs command to inspect their status. If you do this, or try to exit again immediately, you are not warned again, and the stopped jobs are terminated.

SIGNALS

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

Synchronous jobs started by bash have signals set to the alues inherited by the shell from its parent. When job control is not in effect, background jobs (jobs started with &) ignore SIGINT and SIGQUIT. Commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU and SIGTSTP.

COMMAND EXECUTION

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

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 inbuilts. If a match is found, that inbuilt is invoked.

If the name is neither a shell function nor a inbuilt, and contains no slashes, bash searches each element of the PATH for a directory containing an executable file by that name. If the search is unsuccessful, the shell prints an error message and returns a nonzero exit status.

If the search is successful, or if the command name contains one or more slashes, the shell executes the named program. 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) 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.

ENVIRONMENT

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

The shell allows you to manipulate the environment in several ways. 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 flag is set (see set), then all parameter assignments are placed in the environment for a command, not just those that precede the command name.

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

EXIT STATUS

For the purposes of the shell, 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, bash uses the value of 128+signal 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.

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 exit.

PROMPTING

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 customised by inserting a number of backslash-escaped special characters that are decoded as follows:

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), 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 word splitting.

READLINE

This is the library that handles reading input when using an interactive shell, unless the -nolineediting option is given. By default, the line editing commands are similar to those of emacs. A vi-style line editing interface is also available.

In this section, the emacs-style notation is used to denote keystrokes. Control keys are denoted by C-key, e.g., C-n means Control-N. Similarly, meta keys are denoted by M-key, so M-x means Meta-X. (On keyboards without a meta key, M-x means ESC x, i.e., press the Escape key then the x key. This makes ESC the meta prefix. The combination M-C-x means ESC-Control-x, or press the Escape key then hold the Control key while pressing the x key.)

The default key-bindings may be changed with an ~/.inputrc file. The value of the shell variable INPUTRC, if set, is used instead of ~/.inputrc. Other programs that use this library may add their own commands and bindings.

For example, placing

M-Control-u: universal-argument or

C-Meta-u: universal-argument

into the ~/.inputrc would make M-C-u execute the readline command universal-argument.

The following symbolic character names are recognised:

RUBOUT, DEL, ESC, LFD, NEWLINE, RET, RETURN, SPC, SPACE, and TAB.

In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro).

Readline is customised by putting commands in an initialisation file. The name of this file is taken from the value of the INPUTRC variable. If that variable is unset, the default is ~/.inputrc. When a program which uses the readline library starts up, the init file is read, and the key bindings and variables are set. There are only a few basic constructs allowed in the readline init file. Blank lines are ignored. Lines beginning with a # are comments. Lines beginning with a $ indicate conditional constructs. Other lines denote key bindings and variable settings.

The syntax for controlling key bindings in the ~/.inputrc file is simple. All that is required is the name of the command or the text of a macro and a key sequence to which it should be bound. The name may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- prefixes, or as a key sequence. When using the form keyname:function-name or macro, keyname is the name of a key spelled out in English. For example:

Control-u: universal-argument

Meta-Rubout: backward-kill-word

Control-o: ">&output"

In the above example, C-u is bound to the function universal-argument, M-DEL is bound to the function backward-kill-word, and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text >&output into the line).

In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in that strings denoting an entire key sequence may be specified by placing the sequence within double quotes. Some GNU Emacs style key escapes can be used, as in the following example.

"\C-u": universal-argument

"\C-x\C-r": re-read-init-file

"\e[11~": "Function Key 1"

In this example, C-u is again bound to the function universal-argument. C-x C-r is bound to the function re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text Function Key 1. The full set of escape sequences is

When entering the text of a macro, single or double quotes should be used to indicate a macro definition. Unquoted text is assumed to be a function name. Backslash will quote any character in the macro text, including " and '.

Bash allows the current readline key bindings to be displayed or modified with the bind inbuilt command. The editing mode may be switched during interactive use by using the -o option to the set command.

Readline has variables that can be used to further customise its behaviour. A variable may be set in the inputrc file with a statement of the form

set variable value

Except where noted, readline variables can take the values On or Off. The variables and their default values are: