Bash-Beginners-Guide

Chapter 11. Functions
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
11.1. Introduction
11.1.1. 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.
11.1.2. 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.
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.
Chapter 11. Functions 131