Functions

Advanced Bash-Scripting Guide:

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Chapter 23. Functions

Table of Contents

23.1.Complex Functions and Function Complexities

23.2.Local Variables

Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A function is a subroutine, a code block that implements a set of operations, a "black box" that performs a specified task. Wherever there is repetitive code, when a task repeats with only slight variations, then consider using a function.

function function_name { command...

}

or

function_name () { command...

}

This second form will cheer the hearts of C programmers (and is more portable).

As in C, the function's opening bracket may optionally appear on the second line.

function_name ()

{

command...

}

Functions are called, triggered, simply by invoking their names.

Example 23-1. Simple function

#!/bin/bash

funky ()

{

echo "This is a funky function." echo "Now exiting funky function."

}# Function declaration must precede call.

# Now, call the function.

funky

exit 0

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Functions

The function definition must precede the first call to it. There is no method of "declaring" the function, as, for example, in C.

#f1

#Will give an error message, since function "f1" not yet defined.

#However...

f1 ()

{

echo "Calling function \"f2\" from within function \"f1\"." f2

}

f2 ()

{

echo "Function \"f2\"."

}

f1 # Function "f2" is not actually called until this point,

#although it is referenced before its definition.

#This is permissable.

#Thanks, S.C.

It is even possible to nest a function within another function, although this is not very useful.

f1 ()

{

f2 () # nested

{

echo "Function \"f2\", inside \"f1\"."

}

}

#f2

#Gives an error message.

f1 # Does nothing, since calling "f1" does not automatically call "f2". f2 # Now, it's all right to call "f2",

#since its definition has been made visible by calling "f1".

#Thanks, S.C.

Function declarations can appear in unlikely places, even where a command would otherwise go.

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Functions

ls -l | foo() { echo "foo"; } # Permissable, but useless.

if [ "$USER" = bozo ] then

bozo_greet () # Function definition embedded in an if/then construct.

{

echo "Hello, Bozo."

}

fi
bozo_greet	# Works	only	for	Bozo, and other users get an error.
# Something	like this might be		useful in some		contexts.
NO_EXIT=1	# Will enable	function		definition	below.
[[ $NO_EXIT	-eq 1 ]] && exit()		{ true; }		# Function definition in an "and-list".

#If $NO_EXIT is 1, declares "exit ()".

#This disables the "exit" builtin by aliasing it to "true".

exit # Invokes "exit ()" function, not "exit" builtin.

# Thanks, S.C.

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Process Substitution	Up	Complex Functions and Function
		Complexities

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