BASH Functions – Shell Scripting

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Bash Functions
Bash Functions

A BASH Function is a set of commands used to perform a specific task within a script. This function can be reused anywhere in your script to perform that particular task several times. The function saves our time by the need of writing the same code over and over again with write it once and call it every time.

In this tutorial, we will see the basics of bash functions and learn how to use them in shell scripts.

Simple Function

Creating a function is easy and written in two formats.

Syntax 1

This format begins with the function name followed by parentheses.

function_name () {
    <commands>;
}

function_name

Syntax 2

This format begins with the syntax function followed by the function name.

function function_name () {
    <commands>;
}

function_name

Notes

  1. Both syntaxes should just work fine, and there is no functionality difference.
  2. Semicolon is optional
  3. To execute a function, you must invoke a function using the function name (function_name) whenever required in the shell script

Create a shell script hello_world.sh with the following code.

#!/bin/bash

#Basic Function

#Function Name
func_helloworld () {

    #commands
    echo "Hello World";
}

#Invoking Function
func_helloworld

#Reusing Function
func_helloworld
func_helloworld

Let’s execute this script with the bash shell. Running the above function will produce the following output.

[root@server ~]# chmod +x hello_world.sh
[root@server ~]# ./hello_world.sh
Hello World
Hello World
Hello World
[root@server ~]#

Passing Arguments to Bash Functions

We may not always want a simple static function. Sometimes, we need the function to process the input for us just like we pass argument(s) to bash commands in shell prompt.

In function, we supply the arguments after we call a function in a script.

The first parameter is referred to in the function as $1, the second as $2, etc.

#!/bin/bash

#Passing Arguments to a function

func_welcome () {

    echo Hello $1 Welcome to $2
}

#Calling a Function with Parameters
func_welcome Raj ITzGeek

Let’s execute this script with the bash shell. Running the above function will produce the following output.

[root@server ~]# chmod +x arguments.sh
[root@server ~]# ./arguments.sh
Hello Raj Welcome to ITzGeek
[root@server ~]#

Variable Scope

There are two types of variable

Global Variable

Global variables are variables that can be called from anywhere in the script regardless of where it is defined, inside or outside a function. In Bash, all variables are by default global variable.

var_name=<var_value>

Local Variable

Local variables are are defined within the function {} with the keyword local. Local variables scope is limited and works only within that function.

local var_name=<var_value>

Let’s create a script with global and local variables and see how they work within and outside the function.

#!/bin/bash

#Variable Scope

#Global Variable
var1='X'
var2='Y'

func_varscope () {

    #Local Variable
    local var1='Z'
    
    #Gloabl Variable
    var2='A'

    echo "Inside a Function: var1 is $var1 and var2 is $var2" 

}

echo "Before Calling a Function: var1 is $var1 and var2 is $var2"

func_varscope

echo "After Calling a Function: var1 is $var1 and var2 is $var2"

The above script starts with defining two global variables var1 and var2 and then inside the function a local variable var1 and another global variable with the same name var2.

Let’s execute this script with the bash shell.

Before Calling a Function: var1 is X and var2 is Y
Inside a Function: var1 is Z and var2 is A
After Calling a Function: var1 is X and var2 is A

If you see the above output,

Setting a local variable inside the function with the same as an existing global variable modifies the global variable value and only to the function. Outside the function, the variable value is equal to the global variable.

Global variable within the function can modify the global variable outside the function.

Return Values

While other programming languages return the value of functions, Bash functions don’t return value when called. However, Bash functions allow us to set a return status.

We can use the keyword return to specify the return status.

#!/bin/bash

func_returnvalues () {

    echo "some results"
    return 55
}

func_returnvalues
echo $?

Let’s execute this script with the bash shell.

some results
55

If you want to return the actual value of functions, we need to assign the result of the function to a variable.

Global Variable

#!/bin/bash

func_returnvalues () {

    func_result="some result"
}

func_returnvalues
echo $func_result

Let’s execute this script with the bash shell.

some result

Local Variable

#!/bin/bash

func_returnvalues () {

    local func_result="some result"
    echo $func_result

}

func_result=$(func_returnvalues)
echo $func_result

Let’s execute this script with the bash shell.

some result

Conclusion

Creating functions in a Bash script is easy. Using functions in your script would save you lots of time by reusing them whenever required. Keep your Bash function as simple as possible and break a large function into smaller ones if you found one in your scripts.

Further Reading

BASH Functions – Shell Scripting

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