Functions in shell Script

Last Blog Review →

In the last blog we understood, the use of case statements to use instead of using multiple if else statements along with its syntax and shell scripts.

1. What are functions in shell script ?

In shell script always the execution of code starts from top to bottom. So, when using the function in shell script always write the Function Def first then Main Function. If the Main Function is written before the Function Definition then error is generated as the shell script will not understand the Main Function without the Function Definition is encountered. And don't use exit statement instead use return statement in Function. The return statement doesn't return any value as in C/Java, the return statement is similar to the exit statement so it can only return a number and not a text.

2. Syntax

function launch () {  ----Function Definition---
..
..
}                     ----Function Definition---

launch parameters     ----Main Function----

3. Function without return and with return

a. without return statement

function add () {
        result $(( $1 + $2 ))
        echo $result
}

sum=$( add 3 5 )

b. with return statement

function add () {
        return echo $(( $1 + $2 ))
}

add 3 5 
sum=$?

4. Lets improve the calculator program using functions.

.:~$ vim /home/bob/calculator.sh
:~$ cat /home/bob/calculator.sh
#!/bin/bash

function read_numbers(){
  read -p "Enter Number1: " number1
  read -p "Enter Number2: " number2
}

while true
do
  echo "1. Add"
  echo "2. Subtract"
  echo "3. Multiply"
  echo "4. Divide"
  echo "5. Quit"

  read -p "Enter your choice: " choice

  case $choice in
    1)  read_numbers
        echo $(( $number1 + $number2 )) ;;
    2)
        read_numbers
        echo $(( $number1 - $number2 )) ;;
    3)
        read_numbers
        echo $(( $number1 * $number2 )) ;;    
    4)
        read_numbers
        echo $(( $number1 / $number2 )) ;;
    5)  break
  esac

done

.:~$ chmod 700 calculator.sh 
.:~$ ./calculator.sh 
1. Add
2. Subtract
3. Multiply
4. Divide
5. Quit
Enter your choice: 1
Enter Number1: 5
Enter Number2: 2
7
1. Add
2. Subtract
3. Multiply
4. Divide
5. Quit
Enter your choice: 5
.:~$

Conclusion

In the this blog we understood, what are functions in shell script. The syntax of the functions and the use of the return statement in the functions. And we saw the example using function that improved the cal. script.