Conditional Statements – Bash

What You’ll Learn 📚

• Understanding the purpose of conditional statements in Bash
• Key terminology and concepts
• Simple and complex examples
• Common questions and troubleshooting tips

Introduction to Conditional Statements

Conditional statements are like the decision-makers in your Bash scripts. They allow your script to take different actions based on certain conditions. Think of them as the traffic lights of programming: they help control the flow of your script based on what’s happening at any given moment.

Key Terminology

• Condition: A statement that evaluates to true or false.
• If statement: Executes a block of code if a condition is true.
• Else statement: Executes a block of code if the condition is false.
• Elif statement: Short for ‘else if’, allows checking multiple conditions.

Simple Example: The Basics

#!/bin/bash

# Simple if statement example
if [ 1 -eq 1 ]; then
    echo "This is true!"
fi

This is true!

Progressively Complex Examples

Example 1: If-Else Statement

#!/bin/bash

# If-else statement example
number=10
if [ $number -gt 5 ]; then
    echo "The number is greater than 5."
else
    echo "The number is 5 or less."
fi

The number is greater than 5.

Example 2: If-Elif-Else Statement

#!/bin/bash

# If-elif-else statement example
number=3
if [ $number -gt 5 ]; then
    echo "The number is greater than 5."
elif [ $number -eq 5 ]; then
    echo "The number is exactly 5."
else
    echo "The number is less than 5."
fi

The number is less than 5.

Example 3: Nested If Statements

#!/bin/bash

# Nested if statement example
number=7
if [ $number -gt 5 ]; then
    if [ $number -lt 10 ]; then
        echo "The number is between 6 and 9."
    else
        echo "The number is 10 or more."
    fi
else
    echo "The number is 5 or less."
fi

The number is between 6 and 9.

Common Questions and Answers

1. What is the purpose of conditional statements?

   They control the flow of your script by executing different code blocks based on conditions.

2. How do I compare strings in Bash?

   Use == for equality and != for inequality, e.g., [ "$str1" == "$str2" ].

3. Can I use logical operators in conditions?

   Yes, use -a for AND and -o for OR, e.g., [ $a -gt 5 -a $b -lt 10 ].

4. Why isn’t my if statement working?

   Check your syntax, ensure spaces around brackets, and verify your condition logic.

5. How do I handle multiple conditions?

   Use elif for multiple conditions or logical operators for combined conditions.

Troubleshooting Common Issues

Ensure you have spaces around brackets and operators. Bash syntax is sensitive to spaces!

Use set -x at the start of your script to debug and see how your script executes each line.

Practice Exercises

• Write a script that checks if a number is positive, negative, or zero.
• Create a script that prints “Weekend” if the current day is Saturday or Sunday, otherwise “Weekday”.

For more details, check the Bash Manual on Conditional Constructs.