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Review exercises

Table of contents​

  1. Validation quiz
  2. Exercise 1: Network configuration
  3. Exercise 2: Secure SSH
  4. Mini-project: Secure SSH server
  5. Next steps

1 - Validation quiz​

Questions​

Q1. Which command displays inode usage?

See the answer

df -i

Q2. What is the difference between a symbolic link and a hard link?

See the answer
  • Hard link: same inode, same data
  • Symbolic link: special file pointing to a path

Q3. How do you automatically mount a partition at boot?

See the answer

Add an entry to /etc/fstab with the partition's UUID.

Q4. Which command displays the listening ports?

See the answer

ss -tulnp or netstat -tulnp

Q5. How do you copy an SSH key to a server?

See the answer

ssh-copy-id user@serveur

Q6. Which UFW rule allows SSH from a specific IP?

See the answer

sudo ufw allow from 192.168.1.100 to any port 22

Q7. What does the cron expression 0 */4 * * * mean?

See the answer

Every 4 hours, at minute 0 (0:00, 4:00, 8:00, 12:00, 16:00, 20:00)

Q8. How do you create a .tar.gz archive?

See the answer

tar -czvf archive.tar.gz dossier/

Q9. How do you find all files larger than 100 MB modified in the last 7 days?

See the answer

find / -type f -size +100M -mtime -7

Q10. How do you replace "foo" with "bar" in a file with sed?

See the answer

sed -i 's/foo/bar/g' fichier.txt

Results​

ScoreLevel
10/10Excellent! Ready for Linux 4
7-9/10Good! Review the points you missed
4-6/10Average. Reread the chapters
0-3/10Start the Linux 3 course over

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2 - Exercise 1: Network configuration​

Goal​

Diagnose and document the network configuration of a system.

Instructions​

# 1. Affichez toutes les interfaces réseau
ip a

# 2. Notez :
#    - Nom de l'interface principale
#    - Adresse IP
#    - Masque de sous-réseau

# 3. Affichez la passerelle par défaut
ip route | grep default

# 4. Affichez les serveurs DNS
cat /etc/resolv.conf
# ou
resolvectl status

# 5. Testez la connectivité
ping -c 3 8.8.8.8      # Test IP
ping -c 3 google.com   # Test DNS

# 6. Tracez le chemin vers google.com
traceroute google.com

# 7. Vérifiez les ports en écoute
sudo ss -tulnp

Deliverable​

Create a file with your results:

cat << EOF > ~/network_info.txt
=== Configuration Réseau ===
Date: $(date)
Interface: [VOTRE_INTERFACE]
IP: [VOTRE_IP]
Masque: [VOTRE_MASQUE]
Passerelle: [VOTRE_GATEWAY]
DNS: [VOS_DNS]
=== Tests ===
Ping IP: OK/FAIL
Ping DNS: OK/FAIL
EOF

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3 - Exercise 2: Secure SSH​

Goal​

Configure SSH with key-based authentication.

Instructions​

# 1. Générez une clé ED25519 (si pas déjà fait)
ssh-keygen -t ed25519 -C "exercice-linux3"

# 2. Vérifiez les fichiers créés
ls -la ~/.ssh/

# 3. Vérifiez les permissions
stat ~/.ssh/
stat ~/.ssh/id_ed25519

# 4. Créez un fichier de configuration SSH
cat << EOF > ~/.ssh/config
# Configuration SSH - Exercice Linux 3

# Paramètres par défaut
Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent yes

# Exemple d'alias (à personnaliser)
# Host mon-serveur
#     HostName exemple.com
#     User deploy
#     Port 22
#     IdentityFile ~/.ssh/id_ed25519
EOF

# 5. Sécurisez le fichier
chmod 600 ~/.ssh/config

# 6. Vérifiez
cat ~/.ssh/config

If you have a server available​

# Copiez la clé
ssh-copy-id user@serveur

# Testez la connexion sans mot de passe
ssh user@serveur "echo 'Connexion SSH par clé réussie !'"

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4 - Mini-project: Secure SSH server​

Context​

You need to configure a secure server with:

  • Firewall configured
  • SSH with keys only
  • Automated backup
  • Basic monitoring

Part 1: Firewall​

# Script de configuration pare-feu
cat << 'EOF' > ~/setup_firewall.sh
#!/bin/bash

# Reset UFW
sudo ufw reset

# Politique par défaut
sudo ufw default deny incoming
sudo ufw default allow outgoing

# SSH (limité pour anti-brute-force)
sudo ufw limit ssh

# HTTP/HTTPS (si serveur web)
sudo ufw allow http
sudo ufw allow https

# Activer
sudo ufw enable

# Afficher le statut
sudo ufw status verbose
EOF

chmod +x ~/setup_firewall.sh
echo "Script créé: ~/setup_firewall.sh"

Part 2: Backup script​

cat << 'EOF' > ~/backup_script.sh
#!/bin/bash

# Configuration
BACKUP_DIR="/var/backups/system"
DATE=$(date +%Y%m%d_%H%M%S)
RETENTION_DAYS=7

# Créer le dossier
sudo mkdir -p $BACKUP_DIR

# Backup des configs importantes
sudo tar -czf "$BACKUP_DIR/etc_backup_$DATE.tar.gz" /etc/

# Rotation
sudo find "$BACKUP_DIR" -name "*.tar.gz" -mtime +$RETENTION_DAYS -delete

echo "Backup terminé: etc_backup_$DATE.tar.gz"
echo "Fichiers dans $BACKUP_DIR:"
ls -lh $BACKUP_DIR/
EOF

chmod +x ~/backup_script.sh
echo "Script créé: ~/backup_script.sh"

Part 3: Cron task​

# Ajouter au crontab (simulation)
cat << 'EOF'
# Ajouter cette ligne avec: crontab -e
# Backup quotidien à 3h du matin
0 3 * * * /home/$USER/backup_script.sh >> /var/log/backup.log 2>&1
EOF

Part 4: Monitoring script​

cat << 'EOF' > ~/monitor.sh
#!/bin/bash

echo "=== Monitoring Système ==="
echo "Date: $(date)"
echo ""

echo "=== Espace disque ==="
df -h | grep -E "^/dev|Filesystem"

echo ""
echo "=== Mémoire ==="
free -h

echo ""
echo "=== Charge CPU ==="
uptime

echo ""
echo "=== Top 5 processus CPU ==="
ps aux --sort=-%cpu | head -6

echo ""
echo "=== Services critiques ==="
for service in ssh; do
    if systemctl is-active --quiet $service; then
        echo "$service: ACTIF"
    else
        echo "$service: INACTIF !"
    fi
done

echo ""
echo "=== Dernières connexions ==="
last -n 5
EOF

chmod +x ~/monitor.sh
echo "Script créé: ~/monitor.sh"

Test the monitoring​

./monitor.sh

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5 - Next steps​

Congratulations!​

You have completed the Linux 3 - System and Network course!

What you have learned​

Skills acquired​

SkillMastery
File system and storage✅
Network configuration✅
Network diagnostics✅
Secure SSH✅
Firewall (UFW)✅
Scheduled tasks✅
Archiving and backup✅
Advanced search✅

Next step: Linux 4​

In the Linux 4 - Bash Scripting course, you will learn:

  • Variables and types
  • Conditions and loops
  • Functions
  • Advanced text manipulation
  • Error handling
  • Professional DevOps scripts

Tips for making progress​

  1. Practice the commands regularly
  2. Configure a real server (VPS, Raspberry Pi)
  3. Automate your repetitive tasks
  4. Read the logs to understand problems
  5. Document your configurations

Command summary​

File system​

CommandUsage
df -hSpace per partition
du -shFolder size
lsblkList the disks
mount/umountMount/unmount

Network​

CommandUsage
ip aInterfaces
ip routeRoutes
ss -tulnpListening ports
ping/tracerouteDiagnostics

SSH​

CommandUsage
ssh-keygenGenerate a key
ssh-copy-idCopy a key
scp/rsyncFile transfer

Security​

CommandUsage
ufw enableEnable the firewall
ufw allow/denyRules

Automation​

CommandUsage
crontab -eEdit cron
tar -czvfCreate an archive
CommandUsage
findFile search
grepText search
awk/sedText processing

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