Chapter 23. Functions

Table of Contents
23.1. Complex Functions and Function Complexities
23.2. Local Variables
23.3. Recursion Without 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 in procedure, 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...
}

Note

A function may be "compacted" into a single line.

fun () { echo "This is a function"; echo; } # ^ ^

In this case, however, a semicolon must follow the final command in the function.

fun () { echo "This is a function"; echo } # Error! # ^

Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.

Example 23-1. Simple functions

#!/bin/bash JUST_A_SECOND=1 funky () { # This is about as simple as functions get. echo "This is a funky function." echo "Now exiting funky function." } # Function declaration must precede call. fun () { # A somewhat more complex function. i=0 REPEATS=30 echo echo "And now the fun really begins." echo sleep $JUST_A_SECOND # Hey, wait a second! while [ $i -lt $REPEATS ] do echo "----------FUNCTIONS---------->" echo "<------------ARE-------------" echo "<------------FUN------------>" echo let "i+=1" done } # Now, call the functions. funky fun exit 0

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. declare -f f1 # This doesn't help either. f1 # Still an error message. # 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 permissible. # 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. # Even a preceding "declare -f f2" wouldn't help. echo 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.
ls -l | foo() { echo "foo"; } # Permissible, 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 $NO_EXIT is 1, declares "exit ()". # This disables the "exit" builtin by aliasing it to "true". exit # Invokes "exit ()" function, not "exit" builtin. # Or, similarly: filename=file1 [ -f "$filename" ] && foo () { rm -f "$filename"; echo "File "$filename" deleted."; } || foo () { echo "File "$filename" not found."; touch bar; } foo # Thanks, S.C. and Christopher Head

Note

What happens when different versions of the same function appear in a script?
# As Yan Chen points out, # when a function is defined multiple times, # the final version is what is invoked. # This is not, however, particularly useful. func () { echo "First version of func ()." } func () { echo "Second version of func ()." } func # Second version of func (). exit $? # It is even possible to use functions to override #+ or preempt system commands. # Of course, this is authorsnote.html endnotes.html advisable.