Skip to main content

Chapter 6.1 - Introduction to Shell Scripting

Learning Objectives​

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

  • Understand what a shell script is
  • Create your first Bash script
  • Understand the shebang
  • Make a script executable
  • Use basic variables

What is a Shell Script?​

A shell script is a file containing a series of shell commands that are executed sequentially. It is a powerful way to automate tasks.

Advantages:

  • Automation of repetitive tasks
  • Combination of complex commands
  • Reusability
  • Scheduling with cron

First Script​

Create a Script​

#!/bin/bash
# My first script
echo "Hello Linux!"
echo "Date: $(date)"
echo "User: $(whoami)"

Steps:

  1. Create the file: nano my_script.sh
  2. Add the content above
  3. Make it executable: chmod +x my_script.sh
  4. Execute: ./my_script.sh

Shebang​

The first line #!/bin/bash is the shebang:

  • Indicates which interpreter to use
  • Necessary for direct execution
  • Alternatives: #!/bin/sh, #!/usr/bin/env bash

Variables​

Declaration​

#!/bin/bash
# Simple variables
NAME="Linux"
VERSION=6.1
ACTIVE=true

# Usage
echo "Name: $NAME"
echo "Version: $VERSION"
echo "Active: $ACTIVE"

Rules:

  • No spaces around the =
  • Quotes for strings with spaces
  • $ to use the variable

Environment Variables​

#!/bin/bash
# System variables
echo "Home: $HOME"
echo "User: $USER"
echo "Path: $PATH"
echo "PWD: $PWD"

Special Variables​

#!/bin/bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"
echo "Number of arguments: $#"
echo "All arguments: $@"
echo "Return code: $?"

Reading Input​

read​

#!/bin/bash
# Read input
echo "Enter your name:"
read NAME
echo "Hello $NAME!"

# With integrated prompt
read -p "Enter your age: " AGE
echo "You are $AGE years old"

# Silent read (password)
read -s -p "Password: " PASS
echo

Comments​

#!/bin/bash
# Single line comment

# Multi-line
# comment

# End of line comment
COMMAND  # Explanation

Complete Example​

#!/bin/bash
# Example script
# Author: Your name
# Date: $(date +%Y-%m-%d)

# Variables
SCRIPT_NAME="my_script.sh"
VERSION="1.0"

# Main function
main() {
    echo "=== $SCRIPT_NAME v$VERSION ==="
    echo "Date: $(date)"
    echo "User: $USER"
    echo "Directory: $PWD"
    echo "Arguments: $#"
    
    if [ $# -gt 0 ]; then
        echo "First argument: $1"
    fi
}

# Execution
main "$@"

Execution​

Methods​

# 1. Direct execution (requires shebang and chmod +x)
./my_script.sh

# 2. With explicit bash
bash my_script.sh

# 3. With sh
sh my_script.sh

# 4. Source (execute in current shell)
source my_script.sh
. my_script.sh  # Shortcut

Best Practices​

Structure​

#!/bin/bash
# 1. Shebang
# 2. Header comments
# 3. Global variables
# 4. Functions
# 5. Main code

Useful Options​

#!/bin/bash
set -e      # Stop on error
set -u      # Error if undefined variable
set -o pipefail  # Error in pipeline

# Or combine
set -euo pipefail

Summary​

In this chapter, you learned:

Shell script: File of executable commands
Shebang: #!/bin/bash to indicate the interpreter
Variables: Declaration and usage with $
Reading: read for user input
Execution: ./script.sh, bash script.sh, source script.sh
Best practices: Structure, set -e, comments

Next Steps​

Chapter 6.2: Variables and Parameters
Chapter 6.3: Control Structures

Chapter created on: December 2024