Control Structures: Conditional Statements – in Shell Scripting

What You’ll Learn 📚

• Understanding conditional statements
• Key terminology
• Simple and complex examples
• Common questions and answers
• Troubleshooting tips

Introduction to Conditional Statements

Conditional statements are like the decision-makers in your scripts. They allow your program to choose different paths based on certain conditions. Think of them as traffic lights 🚦 guiding your code on when to stop, go, or take a detour.

Key Terminology

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

Starting Simple: The Basic If Statement

#!/bin/bash
# A simple if statement example
if [ 1 -eq 1 ]; then
    echo "This is true!"
fi

Progressively Complex Examples

Example 1: If-Else Statement

#!/bin/bash
# If-else statement example
if [ 2 -gt 3 ]; then
    echo "2 is greater than 3"
else
    echo "2 is not greater than 3"
fi

Example 2: If-Elif-Else Statement

#!/bin/bash
# If-elif-else statement example
number=5
if [ $number -lt 5 ]; then
    echo "Number is less than 5"
elif [ $number -eq 5 ]; then
    echo "Number is equal to 5"
else
    echo "Number is greater than 5"
fi

Example 3: Nested If Statements

#!/bin/bash
# Nested if statement example
number=10
if [ $number -gt 0 ]; then
    echo "Number is positive"
    if [ $number -lt 20 ]; then
        echo "Number is less than 20"
    fi
fi

Common Questions and Answers

1. What is the purpose of conditional statements?

   They allow your script to make decisions based on conditions, making your programs dynamic and responsive.

2. Can I use multiple conditions in a single if statement?

   Yes, you can use logical operators like && (and) and || (or) to combine conditions.

3. What happens if none of the conditions are true?

   If you have an else block, it will execute. Otherwise, nothing happens.

4. Why isn’t my if statement working?

   Check your syntax, ensure conditions are correctly formatted, and verify variable values.

Troubleshooting Common Issues

Ensure you have spaces around the brackets [ ] in conditions. Missing spaces can cause errors.

Use set -x at the start of your script to debug and see how conditions are evaluated.

Remember, practice makes perfect! Don’t worry if this seems complex at first. With time and practice, it will become second nature. Keep experimenting and have fun! 🚀

Practice Exercises

• Create a script that checks if a file exists and prints a message accordingly.
• Write a script that takes a number as input and checks if it’s positive, negative, or zero.
• Modify the nested if example to include an elif condition.

For more resources, check out the Bash Reference Manual.