Classic Shell Scripting

Functions

As in other languages, a function is a separate piece of code that performs some well-defined single task. The function can then be used (called) from multiple places within the larger program.

Functions must be defined before they can be used. This is done either at the beginning of a script, or by having them in a separate file and sourcing them with the "dot" (.) command. (The . command is discussed later on in Section 7.9.) They are defined as shown in Example 6-4.

Example 6-4. Wait for a user to log in, function version

# wait_for_user --- wait for a user to log in
#
# usage: wait_for_user user [ sleeptime ]

wait_for_user ( ) {
    until who | grep "$1" > /dev/null
    do
        sleep ${2:-30}
    done
}

Functions are invoked (executed) the same way a command is: by providing its name and any corresponding arguments. The wait_for_user function can be invoked in one of two ways:

wait_for_user tolstoy       Wait for tolstoy, check every 30 seconds

wait_for_user tolstoy 60    Wait for tolstoy, check every 60 seconds

Within a function body, the positional parameters ($1, $2, etc., $#, $*, and $@) refer to the function's arguments. The parent script's arguments are temporarily shadowed, or hidden, by the function's arguments. $0 remains the name of the parent script. When the function finishes, the original command-line arguments are restored.

Within a shell function, the return command serves the same function as exit and works the same way:

answer_the_question ( ) {
    ...
    return 42
}

Note that using ...