Bash While Loop: Examples and Practical Uses

The Bash while loop is a powerful control structure that allows you to repeatedly execute commands as long as a specified condition remains true. It is widely used for tasks such as automating processes, reading files line by line, handling user input, and evaluating command-line options.

This guide will cover basic and advanced examples of the Bash while loop, including infinite loops, breaking loops, reading files, and handling user input.

Basic Syntax of a While Loop in Bash

The syntax of a while loop is as follows:

while [ condition ]
do
   command1
   command2
   command3
done

• The loop continues executing the commands inside the do block until the condition evaluates to false.
• The condition is typically a comparison statement or a command output.
• If the condition never becomes false, it creates an infinite loop.

One-liner Syntax:

while [ condition ]; do commands; done

Example 1: Printing a Message Multiple Times

The following script prints “Welcome X times” five times:

#!/bin/bash
x=1
while [ $x -le 5 ]
do
  echo "Welcome $x times"
  x=$(( x + 1 ))
done

One-liner equivalent:

x=1; while [ $x -le 5 ]; do echo "Welcome $x times"; x=$(( x + 1 )); done

Example 2: Factorial Calculation Using a While Loop

This script calculates the factorial of a given number:

#!/bin/bash
counter=$1
factorial=1

# Check if the user provided a number
if [ -z "$counter" ]; then
    echo "Syntax: $0 <number>"
    exit 1
fi

# Compute factorial
while [ $counter -gt 0 ]
do
    factorial=$(( factorial * counter ))
    counter=$(( counter - 1 ))
done

echo "Factorial: $factorial"

Usage:

$ chmod +x factorial.sh
$ ./factorial.sh 5

Example 3: Reading a File Line by Line

One of the most common use cases for a while loop is reading a file line by line:

#!/bin/bash
FILE="$1"

if [ -z "$FILE" ]; then
    echo "Syntax: $0 <filename>"
    exit 1
fi

# Read the file line by line
while IFS= read -r line
do
    echo "$line"
done < "$FILE"

Usage:

$ chmod +x read_file.sh
$ ./read_file.sh myfile.txt

Example 4: Handling Command-Line Options with getopts

This example shows how to process command-line options using a while loop and getopts:

while getopts "ae:f:hd:s:qx:" option
do
    case "${option}" in
        a) ALARM="TRUE";;
        e) ADMIN=${OPTARG};;
        d) DOMAIN=${OPTARG};;
        f) SERVERFILE=${OPTARG};;
        s) WHOIS_SERVER=${OPTARG};;
        q) QUIET="TRUE";;
        x) WARNDAYS=${OPTARG};;
        \?) echo "Invalid option"; exit 1;;
    esac
done

This script allows handling multiple options with flags and arguments.

Example 5: Processing User Input in an Infinite Loop

You can use an infinite while loop to keep asking for user input:

#!/bin/bash
while :
do
    echo "Enter two numbers (or -1 to quit):"
    read a b
    if [ "$a" -eq -1 ]; then
        echo "Exiting..."
        break
    fi
    result=$(( a + b ))
    echo "Sum: $result"
done

Example 6: Blocking IP Addresses from a File

The following script reads a list of IP addresses from a file and blocks them using iptables:

#!/bin/bash
IFS='#'  # Set Internal Field Separator to #

INPUT="blocked_ips.txt"

while read -r ip comment
do
   /sbin/iptables -A INPUT -s "$ip" -m comment --comment "$comment" -j DROP
done < "$INPUT"

Example file (blocked_ips.txt):

192.168.1.50#BLOCK:www-2
180.110.1.6#BLOCK:hacker
99.99.99.99#BLOCK:spammer

This script parses the file and applies firewall rules accordingly.

Example 7: Breaking a Loop with break

The break statement terminates a loop when a condition is met:

#!/bin/bash
counter=1
while [ $counter -le 10 ]
do
    if [ $counter -eq 5 ]; then
        echo "Breaking loop at $counter"
        break
    fi
    echo "Counter: $counter"
    counter=$(( counter + 1 ))
done

Output:

Counter: 1
Counter: 2
Counter: 3
Counter: 4
Breaking loop at 5

Example 8: Skipping Iterations with continue

The continue statement skips the current iteration and jumps to the next one:

#!/bin/bash
counter=0
while [ $counter -lt 10 ]
do
    counter=$(( counter + 1 ))
    if [ $(( counter % 2 )) -eq 0 ]; then
        continue
    fi
    echo "Odd number: $counter"
done

This script only prints odd numbers.

Summary: Key Takeaways

• While loops execute commands as long as the condition is true.
• Use break to exit a loop early.
• Use continue to skip an iteration.
• Infinite loops can be created with while : (use CTRL+C to stop).
• While loops are useful for reading files, processing user input, automating tasks, and managing system processes.

Recommended Readings

If you want to dive deeper into Bash scripting, check out these resources:

• Bash Shell Scripting Guide
• Linux Bash For Loop Examples
• Run man bash or help while in your terminal for documentation.

Did you find this guide helpful? Try these examples on your Linux system and let me know how they work for you!