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Bash Reference Manual

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4.1 Bourne Shell Builtins

The following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX 1003.2 standard.

: (a colon)
 
: [arguments]
Do nothing beyond expanding arguments and performing redirections. The return status is zero.

. (a period)
 
. filename [arguments]
Read and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the PATH variable is used to find filename. When Bash is not in POSIX mode, the current directory is searched if filename is not found in $PATH. If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged. The return status is the exit status of the last command executed, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source.

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

cd
 
cd [-L|-P] [directory]
Change the current working directory to directory. If directory is not given, the value of the HOME shell variable is used. If the shell variable CDPATH exists, it is used as a search path. If directory begins with a slash, CDPATH is not used. The `-P' option means to not follow symbolic links; symbolic links are followed by default or with the `-L' option. If directory is `-', it is equivalent to $OLDPWD. The return status is zero if the directory is successfully changed, non-zero otherwise.

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

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

exec
 
exec [-cl] [-a name] [command [arguments]]
If command is supplied, it replaces the shell without creating a new process. If the `-l' option is supplied, the shell places a dash at the beginning of the zeroth arg passed to command. This is what the login program does. The `-c' option causes command to be executed with an empty environment. If `-a' is supplied, the shell passes name as the zeroth argument to command. If no command is specified, redirections may be used to affect the current shell environment. If there are no redirection errors, the return status is zero; otherwise the return status is non-zero.

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

export
 
export [-fn] [-p] [name[=value]]
Mark each name to be passed to child processes in the environment. If the `-f' option is supplied, the names refer to shell functions; otherwise the names refer to shell variables. The `-n' option means to no longer mark each name for export. If no names are supplied, or if the `-p' option is given, a list of exported names is displayed. The `-p' option displays output in a form that may be reused as input. The return status is zero unless an invalid option is supplied, one of the names is not a valid shell variable name, or `-f' is supplied with a name that is not a shell function.

getopts
 
getopts optstring name [args]
getopts is used by shell scripts to parse positional parameters. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. The colon (`:') and question mark (`?') may not be used as option characters. Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used.

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

getopts normally parses the positional parameters, but if more arguments are given in args, getopts parses those instead.

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

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

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

hash
 
hash [-'r] [-p filename] [-dt] [name]
Remember the full pathnames of commands specified as name arguments, so they need not be searched for on subsequent invocations. The commands are found by searching through the directories listed in $PATH. The `-p' option inhibits the path search, and filename is used as the location of name. The `-r' option causes the shell to forget all remembered locations. The `-d' option causes the shell to forget the remembered location of each name. If the `-t' option is supplied, the full pathname to which each name corresponds is printed. If multiple name arguments are supplied with `-t' the name is printed before the hashed full pathname. The `-l' option causes output to be displayed in a format that may be reused as input. If no arguments are given, or if only `-l' is supplied, information about remembered commands is printed. The return status is zero unless a name is not found or an invalid option is supplied.

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

readonly
 
readonly [-apf] [name] ...
Mark each name as readonly. The values of these names may not be changed by subsequent assignment. If the `-f' option is supplied, each name refers to a shell function. The `-a' option means each name refers to an array variable. If no name arguments are given, or if the `-p' option is supplied, a list of all readonly names is printed. The `-p' option causes output to be displayed in a format that may be reused as input. The return status is zero unless an invalid option is supplied, one of the name arguments is not a valid shell variable or function name, or the `-f' option is supplied with a name that is not a shell function.

return
 
return [n]
Cause a shell function to exit with the return value n. If n is not supplied, the return value is the exit status of the last command executed in the function. This may also be used to terminate execution of a script being executed with the . (or source) builtin, returning either n or the exit status of the last command executed within the script as the exit status of the script. The return status is non-zero if return is used outside a function and not during the execution of a script by . or source.

shift
 
shift [n]
Shift the positional parameters to the left by n. The positional parameters from n+1 ... $# are renamed to $1 ... $#-n+1. Parameters represented by the numbers $# to n+1 are unset. n must be a non-negative number less than or equal to $#. If n is zero or greater than $#, the positional parameters are not changed. If n is not supplied, it is assumed to be 1. The return status is zero unless n is greater than $# or less than zero, non-zero otherwise.

test
[
Evaluate a conditional expression expr. Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in 6.4 Bash Conditional Expressions.

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

Expressions may be combined using the following operators, listed in decreasing order of precedence.

! expr
True if expr is false.

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

expr1 -a expr2
True if both expr1 and expr2 are true.

expr1 -o expr2
True if either expr1 or expr2 is true.

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

0 arguments
The expression is false.

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

2 arguments
If the first argument is `!', the expression is true if and only if the second argument is null. If the first argument is one of the unary conditional operators (see section 6.4 Bash Conditional Expressions), the expression is true if the unary test is true. If the first argument is not a valid unary operator, the expression is false.

3 arguments
If the second argument is one of the binary conditional operators (see section 6.4 Bash Conditional Expressions), the result of the expression is the result of the binary test using the first and third arguments as operands. If the first argument is `!', the value is the negation of the two-argument test using the second and third arguments. If the first argument is exactly `(' and the third argument is exactly `)', the result is the one-argument test of the second argument. Otherwise, the expression is false. The `-a' and `-o' operators are considered binary operators in this case.

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

5 or more arguments
The expression is parsed and evaluated according to precedence using the rules listed above.

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

trap
 
trap [-lp] [arg] [sigspec ...]
The commands in arg are to be read and executed when the shell receives signal sigspec. If arg is absent or equal to `-', all specified signals are reset to the values they had when the shell was started. If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If arg is not present and `-p' has been supplied, the shell displays the trap commands associated with each sigspec. If no arguments are supplied, or only `-p' is given, trap prints the list of commands associated with each signal number in a form that may be reused as shell input. Each sigspec is either a signal name such as SIGINT (with or without the SIG prefix) or a signal number. If a sigspec is 0 or EXIT, arg is executed when the shell exits. If a sigspec is DEBUG, the command arg is executed after every simple command. If a sigspec is ERR, the command arg is executed whenever a simple command has a non-zero exit status. The ERR trap is not executed if the failed command is part of an until or while loop, part of an if statement, part of a && or || list, or if the command's return status is being inverted using !. The `-l' option causes the shell to print a list of signal names and their corresponding numbers.

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 zero unless a sigspec does not specify a valid signal.

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

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

unset
 
unset [-fv] [name]
Each variable or function name is removed. If no options are supplied, or the `-v' option is given, each name refers to a shell variable. If the `-f' option is given, the names refer to shell functions, and the function definition is removed. Readonly variables and functions may not be unset. The return status is zero unless a name does not exist or is readonly.


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