Bash-Beginners-Guide

Bash Guide for Beginners
11.1.3. Positional parameters in functions
Functions are like mini-scripts: they can accept parameters, they can use variables only known within the
function (using the local shell built-in) and they can return values to the calling shell.
A function also has a system for interpreting positional parameters. However, the positional parameters passed
to a function are not the same as the ones passed to a command or script.
When a function is executed, the arguments to the function become the positional parameters during its
execution. The special parameter # that expands to the number of positional parameters is updated to reflect
the change. Positional parameter 0 is unchanged. The Bash variable FUNCNAME is set to the name of the
function, while it is executing.
If the return built-in 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 the function's execution. If a numeric argument
is given to return, that status is returned. A simple example:
[lydia@cointreau ~/test] cat showparams.sh
#!/bin/bash
echo "This script demonstrates function arguments."
echo
echo "Positional parameter 1 for the script is $1."
echo
test ()
{
echo "Positional parameter 1 in the function is $1."
RETURN_VALUE=$?
echo "The exit code of this function is $RETURN_VALUE."
}
test other_param
[lydia@cointreau ~/test] ./showparams.sh parameter1
This script demonstrates function arguments.
Positional parameter 1 for the script is parameter1.
Positional parameter 1 in the function is other_param.
The exit code of this function is 0.
[lydia@cointreau ~/test]
Note that the return value or exit code of the function is often storen in a variable, so that it can be probed at a
later point. The init scripts on your system often use the technique of probing the RETVAL variable in a
conditional test, like this one:
if [ $RETVAL -eq 0 ]; then
Or like this example from the /etc/init.d/amd script, where Bash's optimization features are used:
[ $RETVAL = 0 ] && touch /var/lock/subsys/amd
Chapter 11. Functions 132