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Chapter 8: Bash Scripting

Overview

Bash scripting is a powerful tool for automating tasks, managing systems, and creating efficient workflows in Linux. This chapter introduces the fundamentals of Bash scripting, common use cases, and advanced techniques to write robust scripts.

1. Introduction to Bash Scripting

What is a Bash Script?

  • A Bash script is a file containing a series of commands that are executed sequentially by the Bash shell.
  • Scripts are used to automate repetitive tasks and manage system operations.

Creating a Bash Script

  1. Open a text editor (e.g., nano, vim).
  2. Write your script.
  3. Save the file with a .sh extension (e.g., script.sh).
  4. Make the script executable using chmod: 
    chmod +x script.sh
  5. Run the script: 
    ./script.sh

Example:

#!/bin/bash
# Simple Bash Script

echo "Hello, World!"

2. Script Components

Shebang (#!)

  • The shebang specifies the interpreter for the script.
  • Common example: 
    #!/bin/bash

Comments

  • Use # for single-line comments.
  • Use comments to document the purpose of your script and its sections.

Variables

  • Store and reuse data within the script.

Syntax:

VARIABLE_NAME=value

Example:

name="Linux User"
echo "Hello, $name!"

3. Control Structures

Conditional Statements

if-else

if [ condition ]; then
  # Commands
else
  # Commands
fi

Example:

#!/bin/bash
number=10
if [ $number -gt 5 ]; then
  echo "Number is greater than 5"
else
  echo "Number is 5 or less"
fi

Loops

for Loop

for var in list; do
  # Commands
done

Example:

for i in 1 2 3 4 5; do
  echo "Number: $i"
done

while Loop

while [ condition ]; do
  # Commands
done

Example:

count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

4. Functions

Defining and Calling Functions

  • Functions allow you to organize reusable blocks of code.

Syntax:

function_name() {
  # Commands
}

Example:

#!/bin/bash
hello() {
  echo "Hello, $1!"
}
hello "World"

5. Input and Output

Reading Input

  • Use read to get input from the user.

Example:

#!/bin/bash
echo "Enter your name:"
read name
echo "Hello, $name!"

Redirecting Output

  • Redirect standard output to a file: 
    command > file
  • Append output to a file: 
    command >> file
  • Redirect standard error: 
    command 2> error.log

6. Debugging Scripts

Enable Debug Mode

  • Use -x to debug a script: 
    bash -x script.sh

Add Debug Statements

  • Use set -x to enable debug mode within a script and set +x to disable it.

Example:

#!/bin/bash
set -x
echo "Debugging this script"
set +x
echo "Debugging disabled"

7. Advanced Techniques

Arrays

  • Store multiple values in a single variable.

Example:

fruits=("apple" "banana" "cherry")
echo ${fruits[1]}

Case Statements

  • Handle multiple conditions elegantly.

Example:

#!/bin/bash
echo "Enter a number:"
read number
case $number in
  1)
    echo "You entered one" ;;
  2)
    echo "You entered two" ;;
  *)
    echo "Invalid choice" ;;
esac

Summary

By the end of this chapter, you should be able to:

  • Write and execute basic Bash scripts.
  • Use variables, control structures, and functions in scripts.
  • Handle user input and output effectively.
  • Debug scripts and implement advanced scripting techniques.

Next Steps:

Exercises

  1. Write a script that calculates the factorial of a number using a while loop.
  2. Create a script that backs up a directory to a specified location.
  3. Write a script to check if a file exists and display an appropriate message.
  4. Use a for loop to iterate through all .txt files in a directory and display their content.
  5. Debug a script using bash -x and identify potential issues.