while loop in Bash Scripting

Introduction

A while loop lets you repeat a set of commands as long as a condition is true. It helps automate tasks that need to happen multiple times.

When you want to keep asking a user for input until they give a valid answer.

When you want to process lines in a file one by one until the end.

When you want to repeat a task until a certain condition changes, like waiting for a file to appear.

When you want to count or do something a specific number of times using a condition.

When you want to keep checking system status until it meets your needs.

Syntax

Bash Scripting

while [ condition ]
do
  commands
 done

The condition is checked before each loop. If true, commands run; if false, loop stops.

Use square brackets [ ] with spaces around the condition for test expressions.

Examples

This loop prints the count from 1 to 3, increasing count each time.

Bash Scripting

count=1
while [ $count -le 3 ]
do
  echo "Count is $count"
  count=$((count + 1))
done

This loop runs forever until you stop it manually.

Bash Scripting

while true
 do
  echo "Press Ctrl+C to stop"
  sleep 1
done

This loop asks the user to type 'yes' and repeats until they do.

Bash Scripting

input=""
while [ "$input" != "yes" ]
do
  read -p "Type yes to continue: " input
done

Sample Program

This script prints 'Loop number' followed by numbers 1 to 5 using a while loop.

Bash Scripting

#!/bin/bash

count=1
while [ $count -le 5 ]
do
  echo "Loop number $count"
  count=$((count + 1))
done

Important Notes

Always make sure the condition will eventually become false, or the loop will run forever.

Use indentation inside the loop to make your script easier to read.

You can use break to exit the loop early if needed.

Summary

A while loop repeats commands while a condition is true.

It is useful for repeating tasks until something changes.

Remember to update variables inside the loop to avoid infinite loops.