bash Pocket Reference

Functions

A shell function is a grouping of commands within a shell script. Shell functions let you modularize your program by dividing it up into separate tasks. This way, the code for each task is not repeated every time you need to perform the task. The POSIX shell syntax for defining a function follows the Bourne shell:

name () {
    function body's code comes here
} [redirections]

Functions are invoked just as are regular shell built-in commands or external commands. The command-line parameters $1, $2, and so on receive the function’s arguments, temporarily hiding the global values of $1, etc. For example:

# fatal --- print an error message and die:

fatal () {
    # Messages go to standard error.
    echo "$0: fatal error:" "$@" >&2
    exit 1
}
…
if [ $# = 0 ]    # not enough arguments
then
    fatal not enough arguments
fi

A function may use the return command to return an exit value to the calling shell program. Be careful not to use exit from within a function unless you really wish to terminate the entire program.

Per the POSIX standard, any redirections given with the function definition are evaluated when the function executes, not when it is defined.

Bash allows you to define functions using a slightly different syntax, as follows:

function name [()] { body } [redirections]

When using the function keyword, the parentheses following the function name are optional.

Functions share traps with the “parent” shell as described in the following table.

Trap type	Shared/not shared
Signal-based traps	Shared ...