Chapter 11. Functions

Table of Contents

Introduction
What are functions?
Function syntax
Positional parameters in functions
Displaying functions
Examples of functions in scripts
Recycling
Setting the path
Remote backups
Summary
Exercises

Abstract

In this chapter, we will discuss

  • What functions are

  • Creation and displaying of functions from the command line

  • Functions in scripts

  • Passing arguments to functions

  • When to use functions

Introduction

What are functions?

Shell functions are a way to group commands for later execution, using a single name for this group, or routine. The name of the routine must be unique within the shell or script. All the commands that make up a function are executed like regular commands. When calling on a function as a simple command name, the list of commands associated with that function name is executed. A function is executed within the shell in which it has been declared: no new process is created to interpret the commands.

Special built-in commands are found before shell functions during command lookup. The special built-ins are: break, :, ., continue, eval, exec, exit, export, readonly, return, set, shift, trap and unset.

Function syntax

Functions either use the syntax

function FUNCTION { COMMANDS; }

or

FUNCTION () { COMMANDS; }

Both define a shell function FUNCTION. The use of the built-in command function is optional; however, if it is not used, parentheses are needed.

The commands listed between curly braces make up the body of the function. These commands are executed whenever FUNCTION is specified as the name of a command. The exit status is the exit status of the last command executed in the body.

[Note]Common mistakes

The curly braces must be separated from the body by spaces, otherwise they are interpreted in the wrong way.

The body of a function should end in a semicolon or a newline.

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
	<start the daemon>

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

The commands after && are only executed when the test proves to be true; this is a shorter way to represent an if/then/fi structure.

The return code of the function is often used as exit code of the entire script. You'll see a lot of initscripts ending in something like exit $RETVAL.

Displaying functions

All functions known by the current shell can be displayed using the set built-in without options. Functions are retained after they are used, unless they are unset after use. The which command also displays functions:

[lydia@cointreau ~] which zless
zless is a function
zless ()
{
    zcat "$@" | "$PAGER"
}

[lydia@cointreau ~] echo $PAGER
less

This is the sort of function that is typically configured in the user's shell resource configuration files. Functions are more flexible than aliases and provide a simple and easy way of adapting the user environment.

Here's one for DOS users:

dir ()
{
    ls -F --color=auto -lF --color=always "$@" | less -r
}