While loop in shell scripts

Last Blog Review →

In the last blog we understood, few examples of shell script where for loop is used.

1. Why while loops are required ?

Used when we have to execute a cmd or a set of cmds multiple times but you are not sure how many times, execute a cmd or set of cmds until a specific condition occurs, to create infinite loop, menu driven programs.

2. Syntax

while
do
    ......
done

3. Shell Script Example-1

   Write a script using while loop to print all the numbers from 1 to 3

    i-002978a646c30dd8b (EC2)
    -------------------------
    ----lp10:~$ vim print-numbers.sh 
    ----lp10:~$ cat print-numbers.sh 
    to_number=$1
    number=1
    while [ $number -lt $to_number ]
    do
         echo $(( number++ ))
    done
    ----lp10:~$ chmod 700 print-numbers.sh
    ----lp10:~$ ./print-numbers.sh 4
    1
    2
    3
   

4. Shell Script Example-2

   Shows a menu driven program with the following options: Add, Subtract, Multiply, Divide, Quit

    i-002978a646c30dd8b (EC2)
    -------------------------
    ----lp10:~$ vim calculator.sh 
    ----lp10:~$ cat calculator.sh 
    #!/bin/bash
   
    while true
    do
      echo "1. Add"
      echo "2. Subtract"
      echo "3. Multiply"
      echo "4. Divide"
      echo "5. Quit"
   
      read -p "Enter your choice: " choice
   
      if [ $choice -eq 1 ]
      then
            read -p "Enter Number1: " number1
            read -p "Enter Number2: " number2
            echo Answer=$(( $number1 + $number2 ))
      elif [ $choice -eq 2 ]
      then
            read -p "Enter Number1: " number1
            read -p "Enter Number2: " number2
            echo Answer=$(( $number1 - $number2 ))
      elif [ $choice -eq 3 ]
      then
            read -p "Enter Number1: " number1
            read -p "Enter Number2: " number2
            echo Answer=$(( $number1 * $number2 ))
      elif [ $choice -eq 4 ]
      then
            read -p "Enter Number1: " number1
            read -p "Enter Number2: " number2
            echo Answer=$(( $number1 / $number2 ))
      elif [ $choice -eq 5 ]
      then
            echo "Thanks for using cal"
            break
      else
            echo "Please enter no. from 1 to 5"
            continue
      fi
   
    done
    ----lp10:~$ chmod 700 calculator.sh
    ----lp10:~$ ./calculator.sh 
    1. Add
    2. Subtract
    3. Multiply
    4. Divide
    5. Quit
    Enter your choice: 1
    Enter Number1: 5
    Enter Number2: 3
    Answer=8
    1. Add
    2. Subtract
    3. Multiply
    4. Divide
    5. Quit
    Enter your choice: 5
    Thanks for using cal
   

   Conclusion →

   In the this blog we understood, why WHILE LOOP is used and it’s syntax along with some examples of using WHILE LOOP.