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.

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Basic Syntax of a While Loop in Bash

The syntax of a while loop is as follows:

while [ condition ]
do
   command1
   command2
   command3
doneCode language: JavaScript (javascript)

• 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; doneCode language: JavaScript (javascript)

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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 ))
doneCode language: PHP (php)

One-liner equivalent:

x=1; while [ $x -le 5 ]; do echo "Welcome $x times"; x=$(( x + 1 )); doneCode language: PHP (php)

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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"Code language: PHP (php)

Usage:

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

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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"Code language: PHP (php)

Usage:

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

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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
doneCode language: PHP (php)

This script allows handling multiple options with flags and arguments.

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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"
doneCode language: PHP (php)

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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"Code language: PHP (php)

Example file (blocked_ips.txt):

192.168.1.50#BLOCK:www-2
180.110.1.6#BLOCK:hacker
99.99.99.99#BLOCK:spammerCode language: CSS (css)

This script parses the file and applies firewall rules accordingly.

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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 ))
doneCode language: PHP (php)

Output:

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

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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"
doneCode language: JavaScript (javascript)

This script only prints odd numbers.

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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.

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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.

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