Debugging Shell Scripts – in Shell Scripting

What You’ll Learn 📚

• Core concepts of debugging shell scripts
• Key terminology and definitions
• Step-by-step examples from simple to complex
• Common questions and answers
• Troubleshooting common issues

Introduction to Debugging

Debugging is like being a detective in the world of programming. It’s the process of finding and fixing errors in your code. In shell scripting, debugging helps you identify where your script is going wrong and how to fix it. Don’t worry if this seems complex at first; we’ll break it down together! 🕵️‍♂️

Core Concepts

• Syntax Errors: Mistakes in the code that prevent it from running.
• Logical Errors: The code runs but doesn’t do what you expect.
• Runtime Errors: Errors that occur when the script is running.

Key Terminology

• Shell: A command-line interface for interacting with the operating system.
• Script: A file containing a series of commands for the shell to execute.
• Debugging: The process of identifying and fixing errors in your code.

Starting Simple: Your First Debugging Experience

#!/bin/bash
# This script prints a welcome message
echo 'Welcome to shell scripting!'

Welcome to shell scripting!

Debugging Step-by-Step

Now, let’s introduce a common mistake and see how we can debug it.

#!/bin/bash
# This script has a syntax error
echo 'Welcome to shell scripting!'

bash: syntax error: unexpected end of file

Tip: Always check for matching quotes and parentheses. They’re common culprits in syntax errors!

Progressively Complex Examples

#!/bin/bash
# This script calculates the sum of two numbers
num1=5
num2=10
sum=num1+num2
echo "The sum is: $sum"

The sum is: num1+num2

Warning: Remember to use $ to access variable values in shell scripts!

Fixing the Logical Error

Let’s fix the logical error by using the correct syntax for arithmetic operations.

#!/bin/bash
# Corrected script for calculating the sum
num1=5
num2=10
sum=$((num1 + num2))
echo "The sum is: $sum"

The sum is: 15

Common Questions and Answers

1. Why is my script not running?

   Check for syntax errors and ensure the script has execute permissions.

2. How do I give execute permissions?

   Use the command chmod +x scriptname.sh.

3. What does ‘unexpected end of file’ mean?

   It usually means there’s a missing quote or parenthesis.

4. How can I debug a script line by line?

   Use the bash -x scriptname.sh command to see each line executed.

5. Why is my variable not displaying?

   Ensure you’re using $ before the variable name to access its value.

Troubleshooting Common Issues

• Permission Denied: Use chmod +x to make your script executable.
• Command Not Found: Check for typos or missing software installations.
• Unexpected Output: Use echo statements to print variable values and debug.

Practice Exercises

Try fixing the following script errors:

1. Fix a script that calculates the product of two numbers but prints the wrong result.
2. Debug a script that should list files in a directory but fails with an error.

Remember, practice makes perfect! Keep experimenting and don’t hesitate to make mistakes—they’re your best teachers. Happy debugging! 🚀

Additional Resources

• Bash Manual
• Shell Scripting Tutorial