Linux Step By Steps

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:

-c string Commands are read from string. If there are arguments after string, they are assigned starting with $0.
-i Shell is interactive
-s 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.
- A single - signals the end of options and disables further option processing. Any arguments after - are treated as filenames and arguments. An argument of -- is equivalent to an argument -.

Bash also interprets a number of multi-character options. These options must appear on the command line before the single character options to be recognised.

-norc Do not read and execute the personal initialisation file ~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.
-noprofile Do not read either the system-wide startup file /etc/profile or any of the personal initialisation files~/.bash_profile,~/.bash_login,~/.profile.

By default, bash normally reads these files when it is invoked as a login shell (see INVOCATION below).

-rcfile file Execute commands from file instead of the standard personal initialisation file ~/.bashrc, if the shell is interactive (see INVOCATION).
-version Show the version number of this instance of bash when starting.
-quiet Do not be verbose when starting up (do not show the shell version or any other information). This is the default.
-login Make bash act as if it had been invoked as a login shell.
-nobraceexpansion Do not perform curly brace expansion (see Brace expansion).
-nolineediting Do not use the GNU readline library to read command lines if interactive.
-posix Change the behaviour of bash where the default operation differs from the Posix 1003.2 standard to match the standard

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

blank A space or tab character.
word or token A sequence of characters considered as a single unit by the shell
name or identifier A word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore.
metacharacters | & ; ( ) < > space tab
control operator | & ; ( ) || && ;; <newline>

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:

\ escape 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 quoted, the \<newline> is treated as a line continuation (that is, it is effectively ignored).
' single quote 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.
" double quote Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, and \. The characters 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. The special parameters * and @ have special meaning when in double quotes.

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.

* Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS special variable. That is, ``$*'' is equivalent to ``$1 $2 c...'', where c is the first character of the value of the IFS variable. If IFS is null or unset, the parameters are separated by spaces.
@ Expands to the positional parameters, starting from one. When the expansion occurs within double quotes, each parameter expands as a separate word. That is, `` $@'' is equivalent to ``$1'' ``$2'' ...

When there are no positional parameters, ``$@'' and $@ expand to nothing.

# Expands to the number of positional parameters in decimal.
? Expands to the status of the most recently executed foreground pipeline.
- Expands to the current option flags as specified upon invocation, by the set command, or those set by the shell itself (such as the -i flag).
$ Expands to the process ID of the shell. In a () subshell, it expands to the process ID of the current shell, not the subshell.
! Expands to the process ID of the most recently executed background (asynchronous) command.
0 Expands to the name of the shell or shell script. This is set at shell initialisation. If bash is invoked with a file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the pathname used to invoke bash, as given by argument zero.
_ Expands to the last argument to the previous command, after expansion. Also set to the full pathname of each command executed and placed in the environment exported to that command.

Shell Variables

The following variables are set by the shell:

BASH Expands to the full pathname used to invoke this instance of bash.
BASH_VERSION Expands to the version number of this instance of bash.
EUID Expands to the effective user ID of the current user, initialised at shell startup.
HOSTTYPE Automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.
HISTCMD The history number, or index in the history list, of the current command. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
OLDPWD The previous working directory as set by the cd command.
OPTARG The value of the last option argument processed by the getopts command
OPTIND The index of the next argument to be processed by the getopts command.
OSTYPE Automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent.
PPID The process ID of the shell's parent.
PWD The current working directory as set by the cd command.
LINENO Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function. When not in a script or function, the value substituted is not guaranteed to be meaningful. When in a function, the value is not the number of the source line that the command appears on (that information has been lost by the time the function is executed), but is an approximation of the number of simple commands executed in the current function. If LINENO is unset, it loses its special properties, even if it is subsequently reset.
REPLY Set to the line of input read by the read command when no arguments are supplied.
RANDOM Each time this parameter is referenced, a random integer is generated. The sequence of random numbers may be initialised by assigning a value to RANDOM. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.
SECONDS Each time this parameter is referenced, the number of seconds since shell invocation is returned. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. If SECONDS is unset, it loses its special properties, even if it is subsequently reset.
SHLVL Incremented by one each time an instance of bash is started.
UID Expands to the user ID of the current user, initialised at shell startup.

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

IFS The Internal File ld Separator that is used for word splitting after expansion and to split lines into words with the read command. The default value is ``<space><tab><newline>''.
PATH The search path for commands. It is a colon-separated list of directories in which the shell looks for commands (see COMMAND EXECUTION). The default path is system-dependent, and is set by the administrator who installs bash. A common value is ``/usr/gnu/bin:/usr/local/bin:/usr/ucb:/bin:/usr/bin:.''.
HOME The home directory of the current user; the default argument for the cd command.
CDPATH The search path for the cd command. This is a colon-separated list of directories in which the shell looks for destination directories specified by the cd command. A sample value is ``.:~:/usr''.
ENV If this parameter is set when bash is executing a shell script, its value is interpreted as a filename containing commands to initialise the shell, as in .bashrc. The value of ENV is subjected to parameter expansion, command substitution, and arithmetic expansion before being interpreted as a pathname. PATH is not used to search for the resultant pathname.
MAIL If this parameter is set to a filename and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file.
MAILCHECK Specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so before prompting. If this variable is unset, the shell disables mail checking.
MAILPATH A colon-separated list of pathnames to be checked for mail. The message to be printed may be specified by separating the pathname from the message with a `?'. $_ stands for the name of the current mailfile.

Example:

MAILPATH='/usr/spool/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'

Bash supplies a default value for this variable, but the location of the user mail files that it uses is system dependent (/usr/spool/mail/$USER).

MAIL_WARNING 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 printed.
PS1 The value of this parameter is expanded (see PROMPTING) and used as the primary prompt string. The default value is ``bash\$ ''.
PS2 The value of this parameter is expanded and used as the secondary prompt string. The default is ``> ''.
PS3 The value of this parameter is used as the prompt for the select command (see SHELL GRAMMAR).
PS4 The value of this parameter is expanded and the value is printed before each command bash displays during an execution trace. The first character of PS4 is replicated multiple times, as necessary, to indicate multiple levels of indirection. The default is ``+ ''.
HISTSIZE The number of commands to remember in the command history (see HISTORY). The default value is 500.
HISTFILE The name of the file in which command history is saved. (See HISTORY) The default value is ~/.bash_history. If unset, the command history is not saved when an interactive shell exits.
HISTFILESIZE The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines. The default value is 500.
OPTERR If set to the value 1, bash displays error messages generated by the getopts inbuilt command (see SHELL COMMANDS). OPTERR is initialised to 1 each time the shell is invoked or a shell script is executed.
PROMPT_COMMAND If set, the value is executed as a command prior to issuing each primary prompt.
IGNOREEOF Controls the action of the shell on receipt of an EOF character as the sole input. If set, the value is the number of consecutive EOF characters typed as the first characters on an input line before bash exits. If the variable exists but does not have a numeric value, or has no value, the default value is 10. If it does not exist, EOF signifies the end of input to the shell. This is only in effect for interactive shells.
TMOUT If set to a value greater than zero, the value is interpreted as the number of seconds to wait for input after issuing the primary prompt. Bash terminates after waiting for that number of seconds if input does not arrive.
FCEDIT The default editor for the fc command.
FIGNORE A colon-separated list of suffixes to ignore when performing filename completion (see READLINE). A filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. A sample value is ``.o:~''.
INPUTRC The filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE). Notify If set, bash reports terminated background jobs immediately, rather than waiting until before printing the next primary prompt (see also the -b option to the set command). history_control
HISTCONTROL If set to a value of ignorespace, lines which begin with a space character are not entered on the history list. If set to a value of ignoredups, lines matching the last history line are not entered. A value of ignoreboth combines the two options. If unset, or if set to any other value than those above, all lines read by the parser are saved on the history list.

 

command_oriented_history 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.
glob_dot_filenames If set, bash includes filenames beginning with a `.' in the results ofpathname expansion.
allow_null_glob_expansion If set, bash allows pathname patterns which match no files (see Pathname Expansion) to expand to a null string, rather than themselves.
histchars The two or three characters which control history expansion and tokenization (see HISTORY EXPANSION below). The first character is the history expansion character, that is, the character which signals the start of a history expansion, normally `!'. The second character is the quick substitution character, which is used as shorthand for rerunning the previous command entered, substituting one string for another in the command. The default is `^'. The optional third character is the character which signifies that the remainder of the line is a comment, when found as the first character of a word, normally `#'. The history comment character causes history substitution to be skipped for the remaining words on the line. It does not necessarily cause the shell parser to treat the rest of the line as a comment.
nolinks If set, the shell does not follow symbolic links when executing commands 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, such as cd. See also the description of the -P option to the set command.
hostname_completion_file HOSTFILE Contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The file may be changed interactively; the next time hostname completion is attempted bash adds the contents of the new file to the already existing database.
noclobber If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. This variable may be overridden when creating output files by using the redirection operator >| instead of > (see also the -C option to the set command).
auto_resume This variable controls how the shell interacts with the user and job control. If this variable is set, single word simple commands without redirections are treated as candidates for resumption of an existing stopped job. There is no ambiguity allowed; if there is more than one job beginning with the string typed, the job most recently accessed is selected. The name of a stopped job, in this context, is the command line used to start it. If set to the value exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied needs to match a substring of the name of a stopped job.

The substring value provides functionality analogous to the %? job id (see JOB CONTROL). If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the % job id.

no_exit_on_failed_exec If this variable exists, a non-interactive shell will not exit if it cannot execute the file specified in the exec inbuilt command. An interactive shell does not exit if exec fails
cdable_vars If this is set, an argument to the cd command that is not a directory is assumed to be the name of a variable whose value is the directory to change to.

EXPANSION

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

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.

${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.
${parameter:?word} Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.
${parameter:+word} Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.
${#parameter} The length in characters of the value of parameter is substituted. If parameter is * or @, the length substituted is the length of * expanded within double quotes.
${parameter#word}

${parameter##word}

The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches the beginning of the value of parameter, then the expansion is the value of parameter with the shortest matching pattern deleted (the ``#'' case) or the longest matching pattern deleted (the ``##'' case).
${parameter%word}

${parameter%%word}

The word is expanded to produce a pattern just as in pathname expansion. If the pattern matches a trailing portion of the value of parameter, then the expansion is the value of parameter with the shortest matching pattern deleted (the ``%'' case) or the longest matching pattern deleted (the ``%%'' case).

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:

0 Matches any string, including the null string.
? Matches any single character.
[...] Matches any one of the enclosed characters. A pair of characters separated by a minus sign denotes a range; any character lexically between those two characters, inclusive, is matched. If the first character following the [ is a ! or a ^ then any character not enclosed is matched. A - or ] may be matched by including it as the first or last character in the set.

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:

\t the current time in HH:MM:SS format
\d the date in "Weekday Month Date" format (e.g., "Tue May 26")
\n newline
\s the name of the shell, the basename of $0 (the portion following the final slash)
\w the current working directory
\W the basename of the current working directory
\u the username of the current user
\h the hostname
\# the command number of this command
\! the history 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), 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

\C- control prefix
\M- meta prefix
\e an escape character
\\ backslash
\" literal "
\' literal '

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:

horizontal-scroll-mode (Off) When set to On, makes readline use a single line for display, scrolling the input horizontally on a single screen line when it becomes longer than the screen width rather than wrapping to a new line.
editing-mode (emacs) Controls whether readline begins with a set of key bindings similar to emacs or vi. editing-mode can be set to either emacs or vi.
mark-modified-lines (Off) If set to On, history lines that have been modified are displayed with a preceding asterisk (*).
bell-style (audible) If set to none, readline never rings the bell. If set to visible, readline uses a visible bell if one is available. If set to audible, readline attempts to ring the terminal's bell.
comment-begin (``#'') The string that is inserted in vi mode when the vi-comment command is executed.
meta-flag (Off) If set to On, readline will enable eight-bit input (that is, it will not strip the high bit from the characters it reads), regardless of what the terminal claims it can support.
Convert-meta (On) If set to On, readline will convert characters with the eighth bit set to an ASCII key sequence by stripping the eighth bit and prepending an escape character (in effect, using escape as the meta prefix).
output-meta (Off) If set to On, readline will display characters with the eighth bit set directly rather than as a meta prefixed escape sequence.
completion-query-items (100) This determines when the user is queried about viewing the number of possible completions generated by the possible-completions command. It may be set to any integer value greater than or equal to zero. If the number of possible completions is greater than or equal to the value of this variable, the user is asked whether or not he wishes to view them; otherwise they are simply listed on the terminal.
keymap (emacs) Set the current readline keymap. The set of legal keymap names is

emacs, (or emacs-standard),

emacs-meta,

emacs-ctlx,

vi, (or vi-command)

vi-move, and

vi-insert.

the value of editing-mode also affects the default keymap.

show-all-if-ambiguous (Off) This alters the default behaviour of the completion functions. If set to on, words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell.
expand-tilde (Off) If set to on, tilde expansion is performed when readline attempts word completion.

Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are three parser directives used.

$if The $if construct allows bindings to be made based on the editing mode, the terminal being used, or the application using readline. The text of the test extends to the end of the line; no characters are required to isolate it.
$if mode= is used to test whether readline is in emacs or vi mode. This may be used in conjunction with the set keymap command, for instance, to set bindings in the emacs-standard and emacs-ctlx keymaps only if readline is starting out in emacs mode.
$if term= may be used to include terminal-specific key bindings, perhaps to bind the key sequences output by the terminal's function keys. The word on the right side of the = is tested against the full name of the terminal and the portion of the terminal name before the first -. This allows sun to match both sun and sun-cmd, for instance.
$if application construct is used to include application-specific settings. Each program using the readline library sets the application name, and an initialisation file can test for a particular value. This could be used to bind key sequences to functions useful for a specific program. For instance, the following command adds a key sequence that quotes the current or previous word in Bash:

$if Bash

# Quote the current or previous word

"\C-xq": "\eb\"\ef\""

$endif

$else Commands in this branch of the $if directive are executed if the test fails.
$endif This command, as you saw in the previous example, terminates an $if command.

Readline commands may be given numeric arguments, which normally act as a repeat count. Sometimes, however, it is the sign of the argument that is significant. Passing a negative argument to a command that acts in the forward direction (e.g., kill-line) causes that command to act in a backward direction. Commands whose behaviour with arguments deviates from this are noted.

When a command is described as killing text, the text deleted is saved for possible future retrieval (yanking). The killed text is saved in a kill-ring. Consecutive kills cause the text to be accumulated into one unit, which can be yanked all at once. Commands which do not kill text separate the chunks of text on the kill-ring. The following is a list of the names of the commands and the default key sequences to which they are bound.

Commands for Moving
C-a beginning-of-line Move to the start of the current line.
C-e end-of-line Move to the end of the line.
C-f forward-char Move forward a character.
C-b backward-char Move back a character.
M-f forward-word Move forward to the end of the next word. Words are composed of alphanumeric characters (letters and digits).
M-b backward-word Move back to the start of this, or the previous, word. Words are composed of alphanumeric characters (letters and digits).
C-l clear-screen Clear the screen leaving the current line at the top of the screen. With an argument, refresh the current line without clearing the screen
redraw-current-line Refresh the current line. By default, this is unbound.
Manipulating History
Newline Return accept-line Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list according to the state of the HISTCONTROL variable. If the line is a modified history line, then restore the history line to its original state.
C-p previous-history Fetch the previous command from the history list, moving back in the list.
C-n next-history Fetch the next command from the history list, moving forward in the list.
M-< beginning-of-history Move to the first line in the history.
M-> end-of-history Move to the end of the input history, i.e., the line currently being entered.
C-r reverse-search-history Search backward starting at the current line and moving `up' through the history as necessary. This is an incremental search.
C-s forward-search-history Search forward starting at the current line and moving `down' through the history as necessary. This is an incremental search.
M-p non-incremental-reverse-search-history Search backward through the history starting at the current line using a non-incremental search for a string supplied by the user.
M-n no-incremental-forward-search-history Search forward through the history using a non-incremental search for a string supplied by the user.
history-search-forward Search forward through the history for the string of characters between the start of the current line and the current point. This is a non-incremental search. By default, this command is unbound.
history-search-backward Search backward through the history for the string of characters between the start of the current line and the current point. This is a non-incremental search. By default, this command is unbound.
M-C-y yank-nth-ar Insert the first argument to the previous command (usually the second word on the previous line) at point (the current cursor position). With an argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A negative argument inserts the nth word from the end of the previous command.
M-., M-_ yank-last-arg Insert the last argument to the previous command (the last word on the previous line). With an argument, behave exactly like yank-nth-arg.
M-C-e shell-expand-line Expand the line the way the shell does when it reads it. This performs alias and history expansion as well as all of the shell word expansions. See HISTORY EXPANSION below for a description of history expansion.
M-^ history-expand-line Perform history expansion on the current line. See HISTORY EXPANSION below for a description of history expansion.
M-., M-_ insert-last-argument A synonym for yank-last-arg.
C-o operate-and-get-next Accept the current line for execution and fetch the next line relative to the current line from the history for editing. Any argument is ignored.
Changing Text
C-d delete-char Delete the character under the cursor. If point is at the beginning of the line, there are no characters in the line, and the last character typed was not C-d, then return EOF.
Rubout backward-delete-char Delete the character behind the cursor. When given a numeric argument, save the deleted text on the kill-ring.
C-q, C-v quoted-insert Add the next character that you type to the line verbatim. This is how to insert characters like C-q, for example.
C-v TAB tab-insert Insert a tab character.
a, b, A, 1, !, ... self-insert Insert the character typed.
C-t transpose-chars Drag the character before point forward over the character at point. Point moves forward as well. If point is at the end of the line, then transpose the two characters before point. Negative arguments don't work.
M-t transpose-words Drag the word behind the cursor past the word in front of the cursor moving the cursor over that word as well.
M-u upcase-word Uppercase the current (or following) word. With a negative argument, do the previous word, but do not move point.
M-l downcase-word Lowercase the current (or following) word. With a negative argument, do the previous word, but do not move point.
M-c capitalise-word Capitalise the current (or following) word. With a negative argument, do the previous word, but do not move point.
Killing and Yanking
C-k kill-line Kill the text from the current cursor position to the end of the line.
C-x C-Rubout backward-kill-line Kill backward to the beginning of the line.
C-u unix-line-discard Kill backward from point to the beginning of the line.
kill-whole-line Kill all characters on the current line, no matter where the cursor is. By default, this is unbound.
M-d kill-word Kill from the cursor to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as those used by forward-word.
M-Rubout backward-kill-word Kill the word behind the cursor. Word boundaries are the same as those used by backward-word.
C-w unix-word-rubout Kill the word behind the cursor, using white space as a word boundary. The word boundaries are different from backward-kill-word.
delete-horizontal-space Delete all spaces and tabs around point. By default, this is unbound.
C-y yank Yank the top of the kill ring into the buffer at the cursor
M-y yank-pop Rotate the kill-ring, and yank the new top. Only works following yank or yank-pop.
Numeric Arguments
M-0, M-1, ..., M-- digit-argument Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument.
universal-argument Each time this is executed, the argument count is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four. By default, this is not bound to a key.
Completing
TAB complete Attempt to perform completion on the text before point. Bash attempts completion treating the text as a variable (if the text begins with $), username (if the text begins with ~), hostname (if the text begins with @), or command (including aliases and functions) in turn. If none of these produces a match, filename completion is attempted
M-? possible-completions List the possible completions of the text before point.
insert-completions Insert all completions of the text before point that would have been generated by possible-completions. By default, this is not bound to a key.(1)
M-/ complete-filename Attempt filename completion on the text before point.
C-x / possible-filename-completions List the possible completions of the text before point, treating it as a filename.
M-~ complete-username Attempt completion on the text before point, treating it as a username.
C-x ~ possible-username-completions List the possible completions of the text before point, treating it as a username.
M-$ complete-variable Attempt completion on the text before point, treating it as a shell variable.
C-x $ possible-variable-completions List the possible completions of the text before point, treating it as a shell variable.
M-@ complete-hostname Attempt completion on the text before point, treating it as a hostname.
C-x @ possible-hostname-completions List the possible completions of the text before point, treating it as a hostname.
M-! complete-command Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, inbuilts, and finally executable filenames, in that order.
C-x ! possible-command-completions List the possible completions of the text before point, treating it as a command name.
M-TAB dynamic-complete-history Attempt completion on the text before point, comparing the text against lines from the history list for possible completion matches.
M-{ complete-into-braces Perform filename completion and return the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion).
Keyboard Macros
C-x ( start-kbd-macro Begin saving the characters typed into the current keyboard macro.
C-x ) end-kbd-macro Stop saving the characters typed into the current keyboard macro and save the definition.
C-x e call-last-kbd-macro Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.
Miscellaneous
C-x C-r re-read-init-file Read in the contents of your init file, and incorporate any bindings or variable assignments found there.
C-g abort Abort the current editing command and ring the terminal's bell (subject to the setting of bell-style).
M-a, M-b, ... do-upperccase-version Run the command that is bound to the corresponding uppercase character.
ESC prefix-meta Metafy the next character typed. ESC f is equivalent to Meta-f.
C-_, C-x C-u undo Incremental undo, separately remembered for each line.
M-r revert-line Undo all changes made to this line. This is like typing the undo command enough times to return the line to its initial state.
M-~ tilde-expand Perform tilde expansion on the current word.
dump-functions Print all of the functions and their key bindings to the readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file.
C-x C-v display-shell-version Display version information about the current instance of bash.

HISTORY

When interactive, the shell provides access to the command history, the list of commands previously typed. The text of the last HISTSIZE commands (default 500) is saved in a history list. The shell stores each command in the history list prior to parameter and variable expansion (see EXPANSION) but after history expansion is performed, subject to the values of the shell variables command_oriented_history and HISTCONTROL. On startup, the history is initialised from the file named by the variable HISTFILE (default ~/.bash_history). HISTFILE is truncated, if necessary, to contain no more than HISTFILESIZE lines. The command fc may be used to list or edit and re-execute a portion of the history list. The history inbuilt can be used to display the history list and manipulate the history file. When using the command-line editing, search commands are available in each editing mode that provide access to the history list. When an interactive shell exits, the last HISTSIZE lines are copied from the history list to HISTFILE. If HISTFILE is unset, or if the history file is unwritable, the history is not saved.

HISTORY EXPANSION

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 command. Non-interactive shells do not perform history expansion.

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 previous history to use during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the previous history is the event, and the portions of that line that are acted upon are 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 as one word. Only backslash (\) and single quotes can quote the history escape character, which is ! by default.

The shell allows control of the various characters used by the history expansion mechanism (see histchars).

Event Designators

An event designator is a reference to a command line entry in the history list.

! Start a history substitution, except when followed by a blank, newline, = or (.
!! Refer to the previous command. This is a synonym for `!-1'.
!_ Refer to command line n.
!-_ Refer to the current command line minus n.
!string Refer to the most recent command starting with string.
!?string[?] Refer to the most recent command containing string.
^string1^string2^ Quick substitution. Repeat the last command, replacing string1 with string2. Equivalent to ``!!:s/string1/string2/'' (see Modifiers below).
!# The entire command line typed so far.

Word Designators

A : separates the event specification from the word designator. It can be omitted if the word designator begins with a ^, $, *, or %. Words are numbered from the beginning of the line, with the first word being denoted by a 0 (zero).

0 (zero) The zeroth word. For the shell, this is the command word.
n The nth word.
^ The first argument. That is, word 1.
$ The last argument.
% The word matched by the most recent `?string?' search.
x-y A range of words; `-y' abbreviates `0-y'.
0 All of the words but the zeroth. This is a synonym for `1-$'. It is not an error to use * if there is just one word in the event; the empty string is returned in that case.
x* Abbreviates x-$.
x- Abbreviates x-$ like x*, but omits the last word.

Modifiers

After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a `:'.

h Remove a trailing pathname component, leaving only the head.
r Remove a trailing suffix of the form .xxx, leaving the basename.
e Remove all but the trailing suffix.
t Remove all leading pathname components, leaving the tail.
p Print the new command but do not execute it.
q Quote the substituted words, escaping further substitutions.
x Quote the substituted words as with q, but break into words at blanks and newlines.
s/old/new/ Substitute new for the first occurrence of old in the event line. Any delimiter can be used in place of /. The final delimiter is optional if it is the last character of the event line. The delimiter may be quoted in old and new with a single backslash. If & appears in new, it is replaced by old. A single backslash will quote the &.
& Repeat the previous substitution.
g Cause changes to be applied over the entire event line. This is used in conjunction with `:s' (e.g., `:gs/old/new/') or `:&'. If used with `:s', any delimiter can be used in place of /, and the final delimiter is optional if it is the last character of the event line.

ARITHMETIC EVALUATION

The shell allows arithmetic expressions to be evaluated, under certain circumstances (see let and Arithmetic Expansion). Evaluation is done in long integers with no check for overflow, though division by 0 is trapped and flagged as an error. The following list of operators is grouped into levels of equal precedence operators. The levels are listed in order of decreasing precedence.

- + unary minus and plus
! ~ logical and bitwise negation
* / % 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
= *= /= %= += -= <<= >>= &= ^= |= assignment

Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. The value of a parameter is coerced to a long integer within an expression. 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 36 representing the arithmetic base, and n is a number in that base. If base is omitted, then base 10 is used.

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

Inbuilt SHELL COMMANDS

: [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, pathnames in PATH are used to find the directory containing filename. The file searched for in PATH need not be executable. The current directory is searched if no file is found in PATH. If any arguments are supplied, they become the positional parameters when file 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.

alias [name[=value] ...]

Alias with no arguments prints the list of aliases in the form 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]

Place 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, if jobspec was not found or started without job control.

bind [-m keymap] [-lvd] [-q name]

bind [-m keymap] -f filename

bind [-m keymap] keyseq:function-name

Display current readline key and function bindings, or bind a key sequence to a readline function or macro. The binding syntax accepted is identical to that of .inputrc, but each binding must be passed as a separate argument; e.g.,

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

Options, if supplied, have the following meanings:

-m keymap Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are

emacs (or emacs-standard),

emacs-meta,

emacs-ctlx,

vi (or vi-command),

vi-move, and

vi-insert.

-l List the names of all readline functions
-v List current function names and bindings
-d Dump function names and bindings in such a way that they can be re-read
-f filename Read key bindings from filename
-q function Query about which keys invoke the named function

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

break [n]

Exit from within a for, while, or until 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 command [arguments]

Execute the specified command, passing it arguments, and return its exit status.

This is useful when you wish to define a function whose name is the same as a shell inbuilt, but also need the functionality of the inbuilt within the function itself. The cd command is commonly redefined this way. The return status is false if shell-inbuilt is not a shell inbuilt command.

cd [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 are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, (ie .). If dir begins with a slash (/), then CDPATH is not used. An argument of - is equivalent to $OLDPWD. The return value is true if the directory was successfully changed; false otherwise.

command [-pVv] command [arg ...]

Run command with args suppressing the normal shell function lookup. Only inbuilt 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 pathname used to invoke command to be printed; the -V option produces a more verbose description. An argument of -- disables option checking for the rest of the arguments. 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 inbuilt is the exit status of command.

continue [n]

Resume the next iteration of the enclosing for, while, or until 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 [-frxi] [name[=value]]

typeset [-frxi] [name[=value]]

Declare variables and/or give them attributes. If no names are given, then display the values of variables instead. The options can be used to restrict output to variables with the specified attribute.

-f Use function names only
-r Make names readonly. These names cannot then be assigned values by subsequent assignment statements.
-x Mark names for export to subsequent commands via the environment.
-i The variable is treated as an integer; arithmetic evaluation is performed when the variable is assigned a value.

Using `+' instead of `-' turns off the attribute instead. When used in a function, makes names local, as with the local command. The return value is 0 unless an illegal option is encountered, an attempt is made to define a function using

"-f foo=bar",

one of the names is not a legal shell variable name, an attempt is made to turn off readonly status for a readonly variable, or an attempt is made to display a non-existent function with -f.

dirs [-l] [+/-n]

Display the list of currently remembered directories. Directories are added to the list with the pushd command; the popd command moves back up through 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.
-l produces a longer listing; the default listing format uses a tilde to denote the home directory.

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

echo [-neE] [arg ...]

Output the args, separated by spaces. 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.

\a alert (bell)
\b backspace
\c suppress trailing newline
\f form feed
\n new line
\r carriage return
\t horizontal tab
\v vertical tab
\\ backslash
\nnn the character whose ASCII code is nnn (octal)

enable [-n] [-all] [name ...]

Enable and disable inbuilt shell commands. This allows the execution of a disk command which has the same name as a shell inbuilt without specifying a full pathname. 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 inbuilt version, type ``enable -n test''. If no arguments are given, a list of all enabled shell inbuilts is printed. If only -n is supplied, a list of all disabled inbuilts is printed. If only -all is supplied, the list printed includes all inbuilts, with an indication of whether or not each is enabled. enable accepts -a as a synonym for -all. The return value is 0 unless a name is not a shell inbuilt.

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 the eval command. If there are no args, or only null arguments, eval returns true.

exec [[-] command [arguments]]

If command is specified, it replaces the shell. No new process is created. The arguments become the arguments to command. If the first argument is -, the shell places a dash in the zeroth arg passed to command. This is what login does. If the file cannot be executed for some reason, a non-interactive shell exits, unless the shell variable no_exit_on_failed_exec exists, 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.

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 [-nf] [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 the named variables. An argument of -- disables option checking for the rest of the arguments. export returns an exit status of 0 unless an illegal option is encountered, one of the names is not a legal 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 fc -l -10 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 flag suppresses the command numbers when listing. The -r flag reverses the order of the commands. If the -l flag 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, vi 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 ``r=fc -s'', so that typing ``r cc'' runs the last command beginning with ``cc'' and typing ``r'' re-executes the last command.

If the first form is used, the return value is 0 unless an illegal 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]

Place 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 letters to be recognised; if a letter is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. Each time it is invoked, getopts places the next option in the shell variable name, initialising name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialised 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.

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 illegal options or missing option arguments are encountered. If the variable OPTERR is set to 0, no error message will be displayed, even if the first character of optstring is not a colon.

If an illegal 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 normally parses the positional parameters, but if more arguments are given in args, getopts parses those instead. 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 [-r] [name]

For each name, the full pathname of the command is determined and remembered. The -r option causes the shell to forget all remembered locations. If no arguments are given, information about remembered commands is printed. An argument of -- disables option checking for the rest of the arguments. The return status is true unless a name is not found or an illegal option is supplied.

help [pattern]

Display helpful information about inbuilt commands. If pattern is specified, help gives detailed help on all commands matching pattern; otherwise a list of the inbuilts is printed. The return status is 0 unless no command matches pattern.

history [n]

history -rwan [filename]

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 a non-option argument 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:

-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.

The return value is 0 unless an illegal option is encountered or an error occurs while reading or writing the history file.

jobs [-lnp] [ jobspec ... ]

jobs -x command [ args ... ]

The first form lists the active jobs. The -l option lists process IDs in addition to the normal information; the -p option lists only the process ID of the job's process group leader. The -n option displays only jobs that have changed status since last notified. If jobspec is given, output is restricted to information about that job. The return status is 0 unless an illegal option is encountered or an illegal 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 | -sigspec] [pid | jobspec] ...

kill -l [signum]

Send the signal named by sigspec to the processes named by pid or jobspec. sigspec is either a signal name such as SIGKILL or a signal number. If sigspec is a signal name, the name is case insensitive and may be given with or without the SIG prefix. 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 specified signals are listed, and the return status is 0. An argument of -- disables option checking for the rest of the arguments. kill returns true if at least one signal was successfully sent, or false if an error occurs or an illegal option is encountered.

let arg [arg ...]

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

local [name[=value] ...]

For each argument, create a local variable named name, and assign it value. 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, or an illegal name is supplied.

logout Exit a login shell.

popd [+/-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.

+n removes the nth entry counting from the left of the list shown by dirs, starting with zero. For example: ``popd +0'' removes the first directory, ``popd +1'' the second.

-n removes the nth entry counting from the right of the list shown by dirs, starting with zero. For example: ``popd -0'' removes the last directory, ``popd -1'' the next to last.

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

pushd [dir]

pushd +/-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. +n Rotates the stack so that the nth directory (counting from the left of the list shown by dirs) is at the top.

-n Rotates the stack so that the nth directory (counting from the right) is at the top. 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

Print the absolute pathname of the current working directory. The path printed contains no symbolic links if the -P option to the set command is set. See nolinks). The return status is 0 unless an error occurs while reading the pathname of the current directory.

read [-r] [name ...]

One line is read from the standard input, and the first word is assigned to the first name, the second word to the second name, and so on, with left over words assigned to the last name. Only the characters in IFS are recognised as word delimiters. 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. If the -r option is given, a backslash-newline pair is not ignored, and the backslash is considered to be part of the line.

readonly [-f] [name ...]

readonly -p

The given names are marked readonly and 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. If no arguments are given, or if the -p option is supplied, a list of all readonly names is printed.

An argument of -- disables option checking for the rest of the arguments. The return status is 0 unless an illegal option is encountered, one of the names is not a legal 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.

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

-a Automatically mark variables which are modified or created for export to the environment of subsequent commands.
-b Cause the status of terminated background jobs to be reported immediately, rather than before the next primary prompt. (Also see notify under Shell Variables above).
-e Exit immediately if a simple-command (see SHELL GRAMMAR) exits with a non-zero status. The shell does not exit if the command that fails is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return value is being inverted via !.
-f Disable pathname expansion.
-h Locate and remember function commands as functions are defined. Function commands are normally looked up when the function is executed.
-k All keyword arguments are placed in the environment for a command, not just those that precede the command name.
-m Monitor mode. Job control is enabled.

This flag is on by default for interactive shells on systems that support it (see JOB CONTROL). 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 for interactive shells.

-o option-name The option-name can be one of the following:
allexport Same as -a.
braceexpand The shell performs brace expansion (see Brace Expansion). This is on by default.
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 -nolineediting option.
errexit Same as -e.
histexpand Same as -H.
ignoreeeof The effect is as if the shell command `IGNOREEOF=10' had been executed.
interactive-comments Allow a word beginning with # to cause that word and all remaining characters on that line to be ignored in an interactive shell (see COMMENTS).
monitor Same as -m.
noclobber Same as -C.
noexec Same as -n.
noglob Same as -f.
nohash Same as -d.
notify Same as -b.
nounset Same as -u.
physical Same as -P. posix Change the behaviour of bash where the default operation differs from the Posix 1003.2 standard to match the standard.
privileged Same as -p.
verbose Same as -v.
vi Use a vi-style command line editing interface.
xtrace Same as -x.
If no option-name is supplied, the values of the current options are printed.
-p Turn on privileged mode. In this mode, the $ENV file is not processed, and shell functions are not inherited from the environment. This is enabled automatically on startup if the effective user (group) id is not equal to the real user (group) id. 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, bash displays the expanded value of PS4, followed by the command and its expanded arguments.
-l Save and restore the binding of name in a for name [in word] command (see SHELL GRAMMAR).
-d Disable the hashing of commands that are looked up for execution. Normally, commands are remembered in a hash table, and once found, do not have to be looked up again.
-C The effect is as if the shell command `noclobber=' had been executed.
-H Enable ! style history substitution. This flag is on by default when the shell is interactive.
-P If set, do not follow symbolic links when performing commands such as cd which change the current directory. The physical directory is used instead.
-- If no arguments follow this flag, 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 flags are off by default unless otherwise noted. Using + rather than - causes these flags to be turned off. The flags can also be specified as options to an invocation of the shell. The current set of flags may be found in $-. After the option arguments are processed, the remaining n args are treated as values for the positional parameters and are assigned, in order, to $1, $2, ... $n. If no options or args are supplied, all shell variables are printed. The return status is always true unless an illegal 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. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. n must be a non-negative number less than or equal to $#. If n is greater than $#, the positional parameters are not changed. The return status is greater than 0 if n is greater than $# or less than 0; otherwise 0.

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 (true) or 1 (false) depending on the evaluation of the conditional expression expr. Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. Each operator and operand must be a separate argument. If file is of the form /dev/fd/n, then file descriptor n is checked.

-b file True if file exists and is block special.
-c file True if file exists and is character special.
-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.
-k file True if file has its ``sticky'' bit set.
-L file True if file exists and is a symbolic link.
-p file True if file exists and is a named pipe.
-r file True if file exists and is readable.
-s file True if file exists and has a size greater than zero.
-S file True if file exists and is a socket.
-t fd True if fd is opened on 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.
file1 -nt file2 True if file1 is newer (according to modification date) than file2.
file1 -ot file2 True if file1 is older than file2.
file1 -ef file True if file1 and file2 have the same device and inode numbers.
-z string True if the length of string is zero.
-n string True if the length of string is non-zero.
string1 = string2 True if the strings are equal.
string1 != string2 True if the strings are not equal.
! expr True if expr is false.
expr1 -a expr2 True if both expr1 AND expr2 are true.
expr1 -o expr2 True if either expr1 OR expr2 is true.
arg1 OP arg2 OP is one of -eq, -ne, -lt, -le, -gt, or -ge.
These arithmetic binary operators return true if arg1 is equal, not-equal, less-than, less-than-or-equal, greater-than,or greater-than-or-equal than arg2, respectively. Arg1 and arg2 may be positive integers, negative integers, or the special expression -l string, which evaluates to the length of string

times

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

trap [-l] [arg] [sigspec]

The command arg is to be read and executed when the shell receives signal(s) sigspec. If arg is absent or -, all specified signals are reset to their original values (the values they had upon entrance to the shell). If arg is the null string this signal is ignored by the shell and by the commands it invokes. sigspec is either a signal name defined in <signal.h>, or a signal number. If sigspec is EXIT (0) the command arg is executed on exit from the shell. With no arguments, trap prints the list of commands associated with each signal number. The -l option causes the shell to print a list of signal names and their corresponding numbers. An argument of -- disables option checking for the rest of the arguments. 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 either the trap name or number is invalid; otherwise trap returns true.

type [-all] [-type | -path] name [name ...]

With no options, indicate how each name would be interpreted if used as a command name. If the -type flag is used, type prints a phrase which is one of alias, keyword, function, inbuilt, or file if name is an alias, shell reserved word, function, inbuilt, or disk file, respectively. If the name is not found, then nothing is printed, and an exit status of false is returned. If the -path flag 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 -type would not return file. If a command is hashed, -path prints the hashed value, not necessarily the file that appears first in PATH. If the -all flag is used, type prints all of the places that contain an executable named name. This includes aliases and functions, if and only if the -path flag is not also used.

The table of hashed commands is not consulted when using -all. type accepts -a, -t, and -p in place of -all, -type, and -path, respectively. An argument of -- disables option checking for the rest of the arguments. type returns true if any of the arguments are found, false if none are found.

ulimit [-SHacdfmstpnuv [limit]]

Ulimit provides control over the resources available to the shell and to processes started by it, on systems that allow such control. The value of limit can be a number in the unit specified for the resource, or the value unlimited. 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, the command applies to the soft limit. 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 is 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
-f the maximum size of files created by the shell
-m the maximum resident set size
-s the maximum stack size
-t the maximum amount of cpu time in seconds
-p the pipe size in 512-byte blocks (this may not be set)
-n the maximum number of open file descriptors (most systems do not allow this value to be set, only displayed)
-u the maximum number of processes available to a single user
-v The maximum amount of virtual memory available to the shell
-- disables option checking for the rest of the arguments. 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 illegal option is encountered, a non-numeric argument other than unlimited is supplied as limit, or an error occurs while setting a new limit.

umask [-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, or if the -S option is supplied, 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. An argument of -- disables option checking for the rest of the arguments. 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 names 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, given the -f option, function. An argument of -- disables option checking for the rest of the arguments. Note that PATH, IFS, PPID, PS1, PS2, UID, and EUID cannot be unset. If any of RANDOM, SECONDS, LINENO, or HISTCMD are unset, they lose their special properties, even if they are subsequently reset. The exit status is true unless a name does not exist or is non-unlettable.

wait [n]

Wait for the specified process and return its termination status. 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.

INVOCATION

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

An interactive shell is one whose standard input and output 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.

Login shells:

On login (subject to the -noprofile option):

if /etc/profile exists, source it.

if ~/.bash_profile exists, source it,

else if ~/.bash_login exists, source it,

else if ~/.profile exists, source it.

On exit:

if ~/.bash_logout exists, source it.

Non-login interactive shells:

On startup (subject to the -norc and -rcfile options):

if ~/.bashrc exists, source it.

Non-interactive shells:

On startup:

if the environment variable ENV is non-null, expand it and source the file it names, as if the command if [ "$ENV" ]; then . $ENV; fi had been executed, but do not use PATH to search for the pathname. When not started in Posix mode, bash looks for BASH_ENV before ENV.

If Bash is invoked as sh, it tries to mimic the behaviour of sh as closely as possible. For a login shell, it attempts to source only /etc/profile and ~/.profile, in that order. The -noprofile option may still be used to disable this behaviour. A shell invoked as sh does not attempt to source any other startup files.

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, the ENV variable is expanded and that file sourced; no other startup files are read.

FILES

/bin/bash The bash executable
/etc/profile The systemwide initialisation file, executed for login shells
~/.bash_profile The personal initialisation file, executed for login shells
~/.bashrc The individual per-interactive-shell startup file
~/.inputrc Individual readline initialisation file