Functions in Bash, Linux Shell Script.

Functions in bash

Syntax

function <name> or <name> ()
Create a function with custom name.
Return <value>
Return value as the return status as you exit the function.
Command <command>
Instead of using the function with the identical name, execute the same command.

Linux shell prompt create functions, unset and declare:

ubuntu:~/Bash_scripts# function f1 { echo "First function"; }

ubuntu:~/Bash_scripts# f2 () { echo "Second function"; }

ubuntu:~/Bash_scripts# f1;f2
First function
Second function

ubuntu:~/Bash_scripts# _user_input () { echo Hello $1;}

ubuntu:~/Bash_scripts# _user_input User
Hello User

ubuntu:~/Bash_scripts# declare -f f1 f2
f1 ()
{
 echo "First function"
}
f2 ()
{
 echo "Second function"
}

ubuntu:~/Bash_scripts# unset -f f1

ubuntu:~/Bash_scripts# declare -F
declare -f _user_input
declare -f f2

Function for user info script:

ubuntu:~$ vim get_user.sh
 1 user_info () {
 2 echo Current user: $USER
 3 echo User UID: $UID
 4 echo Home: $HOME
 5 }
 6
 7 user_info
ubuntu:~$ bash get_user.sh
Current user: chsnv
User UID: 1000
Home: /home/chsnv

Simple function passing arguments:

ubuntu:~/Bash_scripts# vim create_files.sh
 1 #!/bin/bash
 2 f3 () {
 3 touch $*
 4 echo "File(s) created: $*"
 5 }
 6 f3 $*
ubuntu:~/Bash_scripts# bash create_files.sh aa.txt bb.txt dd.txt
File(s) created: aa.txt bb.txt dd.txt

ubuntu:~/Bash_scripts# ls
aa.txt bb.txt dd.txt

Function call, passing arguments another example:

ubuntu:~/Bash_scripts# vim return_values.sh
 1 #!/bin/bash
 2 calc () {
 3 res=$(($1$2$3))
 4 echo $res #return $res
 5 }
 6 n=$(calc $*) #n=$(calc $1 $2 $3)
 7 echo $n
ubuntu:~/Bash_scripts# bash return_values.sh 3*6
18
ubuntu:~/Bash_scripts# bash return_values.sh 3/6
0
ubuntu:~/Bash_scripts# bash return_values.sh 3+6
9
ubuntu:~/Bash_scripts# bash return_values.sh 3–6
-3

Conclusion

A Bash function is a piece of reusable code created to carry out a certain task. The function can be used more than once in a script after it has been declared.