Mastering Bash: If, Else, and Else If Statements

When you start writing Bash scripts you quickly notice that executing commands one by one is not enough Scripts need to make decisions and react to different conditions This is where conditional statements come into play They allow scripts to check conditions handle errors and perform actions based on the situation In this guide we will explore everything from basic to advanced conditional statements with practical examples and tips you can apply immediately We will also cover common mistakes and best practices that will help your scripts run reliably and efficiently.

Why are conditional statements important?

Conditional statements control the flow of your script Without them your script would just run commands blindly With them scripts can check conditions choose the right path and respond to problems You can monitor system resources validate user input automate repetitive tasks and handle errors efficiently Learning these will make your scripts smarter and more reliable Experienced developers often combine multiple conditional statements to handle complex scenarios like system monitoring log analysis or automated deployment

Start simply with if

If is the most basic form of decision making It evaluates a condition and executes a block only if the condition is true Beginners often start with simple numeric or string comparisons but if can also handle file checks process statuses and environment variables

#!/bin/bash
if [ "$disk_usage" -gt 80 ]
then
echo "Warning disk usage is above 80 percent"
fi

For example if your server disk is filling up this script will warn you in time and prevent potential issues You can also extend it to trigger automatic cleanup or alerting emails which is common in real world server management

Use else for the opposite condition

Else ensures that the script handles situations where the condition is false

#!/bin/bash
number=3
if [ $number -gt 5 ]
then
echo "The number is greater than five"
else
echo "The number is five or less"
fi

This way every situation has a defined outcome. It is especially useful when validating user input to prevent scripts from failing. unexpectedly

Use elif for multiple conditions

Elif allows you to check several conditions in sequence Useful when thresholds or multiple levels exist In professional scripts it is common to chain several elif statements to categorize CPU loads memory usage or application states

#!/bin/bash
cpu_load=$(uptime | awk -F'load average:' '{print $2}' | cut -d, -f1)
threshold1=1.0
threshold2=2.0
if (( $(echo "$cpu_load < $threshold1" | bc -l) ))
then
echo "CPU load is normal"
elif (( $(echo "$cpu_load < $threshold2" | bc -l) ))
then
echo "CPU load is medium"
else
echo "CPU load is high"
fi

Using elif improves readability and ensures that scripts remain maintainable even as complexity grows

Nested if for complex decisions

Nested if statements are used when one condition depends on another This allows multi layer decision making Nested structures are common in user authentication systems permission checks and multi step server validations

#!/bin/bash
read -p "Enter your age" age
if [ $age -ge 18 ]
then
echo "You can vote"
if [ $age -ge 21 ]
then
echo "You can also drink alcohol"
else
echo "You cannot drink alcohol"
fi
else
echo "You cannot vote"
fi

Proper indentation and comments make nested statements readable and maintainable. Experienced developers also suggest avoiding overly deep nesting by breaking logic into functions which enhances clarity and reduces errors.

Real world example automatic backup check

Automating backup verification saves time reduces human error and ensures compliance In production servers it is common to integrate conditional checks with logging systems and notifications

#!/bin/bash
backup_dir="/var/backups"
log_file="/var/log/backup_status.log"
if [ -d "$backup_dir" ]
then
if [ "$(ls -A $backup_dir)" ]
then
echo "$(date) backup exists" >> $log_file
else
echo "$(date) backup directory is empty" >> $log_file
fi
else
echo "$(date) backup directory does not exist" >> $log_file
fi

This example demonstrates a practical application of nested if statements combining file existence checks with logging. You can further expand it to send alerts or run automated backup scripts.

Golden tips for conditional statements

• Keep conditions simple and readable
• Quote variables especially if they may contain spaces or special characters
• Test scripts first in safe environments before running on production
• Document complex logic clearly
• Maintain consistent indentation and spacing
• Use functions to simplify complex nested conditions
• Regularly review and refactor scripts to improve efficiency and reliability

Common mistakes

• Forgetting fi at the end of a block
• Using equal sign instead of numeric comparison operators
• Not quoting variables with spaces which can break scripts
• Confusing single and double brackets for test conditions
• Overly deep nesting without using functions which reduces readability

Following these tips Your scripts are correct, reliable and professional while remaining maintainable over time

Conclusion

Conditional statements turn Bash scripts from sequential commands into intelligent tools By mastering if else elif and nested if you can automate tasks monitor systems validate inputs handle errors and create professional scripts Incorporating best practices advanced tips and real world examples ensures your scripts are robust flexible and maintainable These techniques are essential for both beginners and experienced developers who want to write high quality Bash scripts