Networking and Shell Scripting

Networking Basics on Linux

Network Configuration

# View network interfaces
ip addr show
ip a                            # Shortcut

# View the routing table
ip route show

# View active connections
ss -tuln                        # Listening ports
ss -tupn                        # With associated processes

# Test connectivity
ping google.com
ping -c 4 192.168.1.1          # 4 pings only

# DNS resolution
nslookup google.com
dig google.com
host google.com

Essential Network Tools

# Download a file
wget https://example.com/fichier.tar.gz
curl -O https://example.com/fichier.tar.gz

# HTTP requests with curl
curl https://api.example.com/users
curl -X POST -H "Content-Type: application/json" \
     -d '{"name": "Jean"}' \
     https://api.example.com/users

# Transfer files via SSH
scp fichier.txt user@serveur:/home/user/
scp -r dossier/ user@serveur:/tmp/

# Synchronize files
rsync -avz ./projet/ user@serveur:/var/www/projet/

SSH — Secure Connection

# Connect to a remote server
ssh user@serveur.example.com
ssh -p 2222 user@serveur.com    # Custom port

# Generate an SSH key pair
ssh-keygen -t ed25519 -C "sacha@example.com"

# Copy the public key to the server
ssh-copy-id user@serveur.com

# SSH configuration (~/.ssh/config)

# ~/.ssh/config
Host prod
    HostName 10.0.1.50
    User deploy
    Port 2222
    IdentityFile ~/.ssh/id_prod

Host staging
    HostName 10.0.2.50
    User deploy
    IdentityFile ~/.ssh/id_staging

# Simplified usage
ssh prod                        # Instead of ssh -p 2222 deploy@10.0.1.50

Firewall with UFW

# Enable the firewall
sudo ufw enable

# Basic rules
sudo ufw allow ssh               # Port 22
sudo ufw allow 80/tcp            # HTTP
sudo ufw allow 443/tcp           # HTTPS
sudo ufw allow from 192.168.1.0/24   # Allow a subnet

# View rules
sudo ufw status verbose

# Delete a rule
sudo ufw delete allow 80/tcp

Bash Scripting

Scripting allows you to automate repetitive tasks. It is an essential skill in system administration.

First Script

#!/bin/bash
# My first script
# Usage: ./hello.sh [name]

nom=${1:-"Monde"}    # First argument, "Monde" by default
echo "Bonjour, $nom !"

# Make executable and run
chmod +x hello.sh
./hello.sh
./hello.sh Sacha

Variables and Types

#!/bin/bash

# Simple variables
nom="Sacha"
age=25
repertoire="/var/log"

# Using variables
echo "Je suis $nom, j'ai $age ans"
echo "Les logs sont dans ${repertoire}"

# Result of a command
date_actuelle=$(date +"%Y-%m-%d")
nb_fichiers=$(ls | wc -l)
echo "Aujourd'hui : $date_actuelle, $nb_fichiers fichiers ici"

# Environment variables
echo "Home : $HOME"
echo "Utilisateur : $USER"
echo "Shell : $SHELL"
echo "PATH : $PATH"

# Export a variable
export API_URL="http://localhost:3000"

Conditions

#!/bin/bash

# Test a file
if [ -f "/etc/nginx/nginx.conf" ]; then
    echo "Nginx is configured"
elif [ -f "/etc/apache2/apache2.conf" ]; then
    echo "Apache is configured"
else
    echo "No web server found"
fi

# Numeric comparisons
cpu_usage=$(top -bn1 | grep "Cpu(s)" | awk '{print int($2)}')
if [ "$cpu_usage" -gt 80 ]; then
    echo "⚠ High CPU usage: ${cpu_usage}%"
fi

# String comparisons
if [ "$USER" = "root" ]; then
    echo "You are root!"
fi

# File tests
# -f : file exists       -d : directory exists
# -r : readable          -w : writable
# -x : executable        -s : size > 0

Loops

#!/bin/bash

# For loop
for serveur in web1 web2 web3 db1; do
    echo "Pinging $serveur..."
    ping -c 1 "$serveur" > /dev/null 2>&1 && echo "  ✓ OK" || echo "  ✗ Failed"
done

# Loop over files
for fichier in /var/log/*.log; do
    taille=$(du -h "$fichier" | cut -f1)
    echo "$fichier : $taille"
done

# While loop
compteur=0
while [ $compteur -lt 5 ]; do
    echo "Iteration $compteur"
    compteur=$((compteur + 1))
done

# Read a file line by line
while IFS= read -r ligne; do
    echo "Processing: $ligne"
done < liste-serveurs.txt

Functions

#!/bin/bash

# Define a function
log() {
    local niveau=$1
    local message=$2
    echo "[$(date +'%Y-%m-%d %H:%M:%S')] [$niveau] $message"
}

verifier_service() {
    local service=$1
    if systemctl is-active --quiet "$service"; then
        log "INFO" "$service is active"
        return 0
    else
        log "ERROR" "$service is inactive!"
        return 1
    fi
}

# Usage
log "INFO" "Starting verification"
verifier_service "nginx"
verifier_service "postgresql"

Practical Script: Automated Backup

#!/bin/bash
# backup.sh — Automated backup of a directory

set -euo pipefail    # Stop on error, undefined variable, error in a pipe

# Configuration
SOURCE="/var/www/html"
DESTINATION="/backups"
DATE=$(date +"%Y%m%d_%H%M%S")
ARCHIVE="backup_${DATE}.tar.gz"
RETENTION=7          # Keep backups from the last 7 days

# Checks
if [ ! -d "$SOURCE" ]; then
    echo "Error: $SOURCE does not exist" >&2
    exit 1
fi

mkdir -p "$DESTINATION"

# Create the backup
echo "Backing up $SOURCE..."
tar -czf "${DESTINATION}/${ARCHIVE}" -C "$(dirname "$SOURCE")" "$(basename "$SOURCE")"

# Check the size
taille=$(du -h "${DESTINATION}/${ARCHIVE}" | cut -f1)
echo "Archive created: ${ARCHIVE} ($taille)"

# Clean up old backups
echo "Deleting backups older than $RETENTION days..."
find "$DESTINATION" -name "backup_*.tar.gz" -mtime +$RETENTION -delete

echo "Backup completed successfully!"

Cron — Scheduling Tasks

# Edit the crontab
crontab -e

# Format: minute hour day month weekday command
# Examples:
# Every day at 2 AM
0 2 * * * /home/sacha/scripts/backup.sh >> /var/log/backup.log 2>&1

# Every 15 minutes
*/15 * * * * /home/sacha/scripts/check-health.sh

# Every Monday at 8 AM
0 8 * * 1 /home/sacha/scripts/rapport-hebdo.sh

# The first of every month
0 0 1 * * /home/sacha/scripts/nettoyage-mensuel.sh

# View scheduled tasks
crontab -l

Scripting Best Practices

Always start with #!/bin/bash — the shebang specifies the interpreter
Use set -euo pipefail to stop at the first error
Quote variables: "$variable" instead of $variable
Use local inside functions to avoid side effects
Test with shellcheck — a linter for Bash scripts
Comment the why, not the what
Always redirect logs in cron tasks