Introduction to Bash scripting

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Table of contents

1. What is a Bash script?
2. The shebang
3. Creating and running a script
4. Command-line arguments
5. Comments and documentation
6. Practical exercises

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1 - What is a Bash script?

Definition

A Bash script is a text file containing a sequence of commands that the shell executes automatically.

Benefits of scripting

Benefit	Description
Automation	Repeat tasks without intervention
Reproducibility	Same result on every run
Documentation	The code documents the procedure
Efficiency	Faster execution than doing it by hand
Portability	Works on all Unix systems

Use cases

• Automated backups
• Application deployment
• Server configuration
• Monitoring and alerts
• Batch file processing

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2 - The shebang

What is the shebang?

The shebang (or hashbang) is the first line of a script that tells the system which interpreter to use.

#!/bin/bash

Anatomy

#!  /bin/bash
│   │
│   └── Path to the interpreter
└── Magic characters (shebang)

Common variants

Shebang	Interpreter
#!/bin/bash	Bash (fixed path)
#!/usr/bin/env bash	Bash (portable)
#!/bin/sh	POSIX shell
#!/usr/bin/env python3	Python 3
#!/usr/bin/env node	Node.js

Recommendation

Use #!/usr/bin/env bash for better portability. env looks up bash in the PATH.

Why the shebang matters

# Without a shebang
cat script.sh | sh        # Interpreted by sh
./script.sh               # Uses the current shell

# With shebang #!/bin/bash
./script.sh               # Always interpreted by bash

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3 - Creating and running a script

Create a script

# 1. Create the file
nano mon_script.sh

# 2. Add the content
#!/bin/bash
echo "Hello, World!"
echo "Date: $(date)"
echo "Utilisateur: $USER"

Make it executable

# Add execute permission
chmod +x mon_script.sh

# Check
ls -l mon_script.sh
# -rwxr-xr-x 1 user user 100 Jan 15 mon_script.sh

Execution methods

# 1. Direct execution (requires chmod +x)
./mon_script.sh

# 2. Via the interpreter (no chmod +x needed)
bash mon_script.sh

# 3. With source (runs in the current shell)
source mon_script.sh
# or
. mon_script.sh

Difference between ./script.sh and source script.sh

Method	Environment	Variables
./script.sh	New process	Not inherited
source script.sh	Current shell	Inherited

# Example
# script.sh contains: export MA_VAR="test"

./script.sh
echo $MA_VAR    # Empty (variable in subshell)

source script.sh
echo $MA_VAR    # "test" (variable in current shell)

First complete script

#!/bin/bash
# My first script
# Author: Your name
# Date: 2024-01-15

# Display a welcome message
echo "==================================="
echo "   Bienvenue dans mon script !"
echo "==================================="

# System information
echo ""
echo "Informations système :"
echo "  - Utilisateur : $USER"
echo "  - Répertoire  : $(pwd)"
echo "  - Date        : $(date '+%Y-%m-%d %H:%M')"
echo "  - Hostname    : $(hostname)"

echo ""
echo "Script terminé avec succès !"

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4 - Command-line arguments

Special variables

Variable	Meaning
$0	Script name
$1, $2...	Arguments 1, 2...
$#	Number of arguments
$@	All arguments (separate)
$*	All arguments (single string)
$$	Script PID
$?	Return code of the last command

Example

#!/bin/bash
# args_demo.sh

echo "Nom du script : $0"
echo "Premier argument : $1"
echo "Deuxième argument : $2"
echo "Nombre d'arguments : $#"
echo "Tous les arguments : $@"

./args_demo.sh hello world 123
# Nom du script : ./args_demo.sh
# Premier argument : hello
# Deuxième argument : world
# Nombre d'arguments : 3
# Tous les arguments : hello world 123

Check the arguments

#!/bin/bash

# Make sure we have at least one argument
if [ $# -eq 0 ]; then
    echo "Usage: $0 <nom>"
    exit 1
fi

echo "Bonjour, $1 !"

Difference between $@ and $*

#!/bin/bash
# test_args.sh

echo "Avec \$@:"
for arg in "$@"; do
    echo "  - '$arg'"
done

echo "Avec \$*:"
for arg in "$*"; do
    echo "  - '$arg'"
done

./test_args.sh "hello world" "foo bar"
# Avec $@:
#   - 'hello world'
#   - 'foo bar'
# Avec $*:
#   - 'hello world foo bar'

tip

Always use "$@" (with quotes) to preserve spaces within arguments.

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5 - Comments and documentation

Simple comments

# This is a single-line comment

echo "Hello"  # End-of-line comment

Script header

#!/bin/bash
#
# Name: backup.sh
# Description: Automatic backup script
# Author: Jean Dupont
# Date: 2024-01-15
# Version: 1.0
#
# Usage: ./backup.sh [source] [destination]
#
# Examples:
#   ./backup.sh /var/www /backup
#   ./backup.sh ~/Documents /mnt/usb
#

# Script code...

Document the sections

#!/bin/bash

#######################
# Configuration
#######################
BACKUP_DIR="/backup"
LOG_FILE="/var/log/backup.log"

#######################
# Functions
#######################

# Function: log_message
# Description: Writes a message to the log file
# Arguments: $1 - message to log
log_message() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" >> "$LOG_FILE"
}

#######################
# Main script
#######################
log_message "Démarrage du backup"
# ...

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6 - Practical exercises

Exercise 1: First script

# Create a script that displays:
# - The current date
# - The username
# - The current directory
# - The available disk space

Solution

#!/bin/bash
echo "Date: $(date)"
echo "Utilisateur: $USER"
echo "Répertoire: $(pwd)"
echo "Espace disque:"
df -h / | tail -1

Exercise 2: Script with arguments

# Create a script salut.sh that takes a name as an argument
# and displays "Bonjour, [nom] !"
# If no argument is given, display an error message

Solution

#!/bin/bash
if [ $# -eq 0 ]; then
    echo "Erreur: Veuillez fournir un nom"
    echo "Usage: $0 <nom>"
    exit 1
fi

echo "Bonjour, $1 !"

Quiz

Q1. What is the recommended shebang for a portable script?

Answer

#!/usr/bin/env bash

Q2. How do you make a script executable?

Answer

chmod +x script.sh

Q3. What does the variable $# contain?

Answer

The number of arguments passed to the script

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Key takeaways

• Shebang #!/bin/bash specifies the interpreter
• chmod +x makes the script executable
• $1, $2... = arguments, $# = count, $@ = all
• $? = return code of the last command
• Always comment and document your scripts
• Use "$@" with quotes to preserve spaces

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