Services and systemd

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Table of contents

1. Introduction to systemd
2. systemctl: essential commands
3. Enable/disable at boot
4. Logs with journalctl
5. Creating a simple service
6. Hands-on exercises

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1 - Introduction to systemd

What is systemd?

systemd is the init system and service manager of most modern Linux distributions.

Key concepts

Term	Description
Unit	A resource managed by systemd
Service	A type of unit (daemon)
Target	A group of units (e.g. multi-user.target)
Socket	Inter-process communication
Timer	The modern equivalent of cron

Unit types

Extension	Type
.service	Services/daemons
.socket	Sockets
.target	Groups of units
.timer	Scheduled tasks
.mount	Mount points
.device	Devices

File locations

Path	Description
/lib/systemd/system/	System units (packages)
/etc/systemd/system/	Custom units (admin)
~/.config/systemd/user/	User units

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2 - systemctl: essential commands

Managing services

Command	Description
systemctl start	Start a service
systemctl stop	Stop a service
systemctl restart	Restart
systemctl reload	Reload the configuration
systemctl status	View the state

Practical examples

# Démarrer nginx
sudo systemctl start nginx

# Arrêter nginx
sudo systemctl stop nginx

# Redémarrer (stop + start)
sudo systemctl restart nginx

# Recharger config sans interruption
sudo systemctl reload nginx

# Redémarrer si actif
sudo systemctl try-restart nginx

View the state of a service

sudo systemctl status nginx

Typical output:

● nginx.service - A high performance web server
     Loaded: loaded (/lib/systemd/system/nginx.service; enabled; preset: enabled)
     Active: active (running) since Mon 2024-01-15 10:30:00 UTC; 2h ago
       Docs: man:nginx(8)
    Process: 1234 ExecStartPre=/usr/sbin/nginx -t -q -g daemon on; (code=exited, status=0/SUCCESS)
   Main PID: 1235 (nginx)
      Tasks: 3 (limit: 4096)
     Memory: 8.5M
        CPU: 120ms
     CGroup: /system.slice/nginx.service
             ├─1235 "nginx: master process /usr/sbin/nginx"
             ├─1236 "nginx: worker process"
             └─1237 "nginx: worker process"

List services

# Tous les services
systemctl list-units --type=service

# Services actifs
systemctl list-units --type=service --state=active

# Services en échec
systemctl list-units --type=service --state=failed

# Tous (y compris inactifs)
systemctl list-units --type=service --all

Check if a service is active

# Vérifie si actif
systemctl is-active nginx
# active ou inactive

# Vérifie si activé au démarrage
systemctl is-enabled nginx
# enabled ou disabled

# Vérifie si en échec
systemctl is-failed nginx

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3 - Enable/disable at boot

Concepts

Action	Effect
enable	Starts automatically at boot
disable	Does not start at boot
start	Starts now
stop	Stops now

note

enable ≠ start! Enable configures automatic startup but does not launch the service immediately.

Commands

# Activer au démarrage
sudo systemctl enable nginx

# Désactiver au démarrage
sudo systemctl disable nginx

# Activer ET démarrer
sudo systemctl enable --now nginx

# Désactiver ET arrêter
sudo systemctl disable --now nginx

Mask a service

To completely prevent a service from starting:

# Masquer (impossible à démarrer)
sudo systemctl mask nginx

# Démasquer
sudo systemctl unmask nginx

View services at boot

# Services activés
systemctl list-unit-files --type=service --state=enabled

# Services désactivés
systemctl list-unit-files --type=service --state=disabled

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4 - Logs with journalctl

journalctl displays the logs managed by systemd-journald.

Basic commands

# Tous les logs (peut être très long)
journalctl

# Logs d'un service spécifique
journalctl -u nginx

# Logs depuis le dernier boot
journalctl -b

# Logs en temps réel (comme tail -f)
journalctl -f

# Logs d'un service en temps réel
journalctl -u nginx -f

Filter by time

# Depuis une date
journalctl --since "2024-01-15 10:00:00"

# Jusqu'à une date
journalctl --until "2024-01-15 12:00:00"

# Dernière heure
journalctl --since "1 hour ago"

# Aujourd'hui
journalctl --since today

# Hier
journalctl --since yesterday --until today

Filter by priority

Level	Meaning
0	emerg
1	alert
2	crit
3	err
4	warning
5	notice
6	info
7	debug

# Erreurs et plus grave
journalctl -p err

# Warnings et plus grave
journalctl -p warning

# Seulement les erreurs d'un service
journalctl -u nginx -p err

Useful options

# Dernières N lignes
journalctl -n 50

# Format JSON
journalctl -o json-pretty

# Avec détails du boot
journalctl -b -1  # Boot précédent

# Espace utilisé par les logs
journalctl --disk-usage

# Nettoyer les vieux logs
sudo journalctl --vacuum-time=7d
sudo journalctl --vacuum-size=1G

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5 - Creating a simple service

Structure of a service file

[Unit]
Description=Description du service
After=network.target

[Service]
Type=simple
User=www-data
WorkingDirectory=/app
ExecStart=/usr/bin/python3 /app/server.py
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

Sections of the file

Section	Content
[Unit]	Metadata, dependencies
[Service]	Service configuration
[Install]	When to enable the service

Common options

[Unit]

Option	Description
Description	Description of the service
After	Start after these units
Requires	Mandatory dependencies
Wants	Optional dependencies

[Service]

Option	Description
Type	simple, forking, oneshot...
ExecStart	Start command
ExecStop	Stop command
ExecReload	Reload command
User	Execution user
WorkingDirectory	Working directory
Restart	always, on-failure, no
RestartSec	Delay before restart
Environment	Environment variables

Complete example: Node.js application

# Créer le fichier service
sudo nano /etc/systemd/system/myapp.service

[Unit]
Description=My Node.js Application
Documentation=https://github.com/user/myapp
After=network.target

[Service]
Type=simple
User=deploy
Group=deploy
WorkingDirectory=/app/myapp
ExecStart=/usr/bin/node server.js
Restart=always
RestartSec=10
StandardOutput=journal
StandardError=journal
Environment=NODE_ENV=production
Environment=PORT=3000

[Install]
WantedBy=multi-user.target

Enable the service

# Recharger systemd
sudo systemctl daemon-reload

# Activer et démarrer
sudo systemctl enable --now myapp

# Vérifier
sudo systemctl status myapp

# Voir les logs
journalctl -u myapp -f

Modify an existing service

# Éditer avec override (recommandé)
sudo systemctl edit nginx
# Crée /etc/systemd/system/nginx.service.d/override.conf

# Voir la configuration complète
systemctl cat nginx

# Recharger après modification
sudo systemctl daemon-reload
sudo systemctl restart nginx

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6 - Hands-on exercises

Exercise 1: Manage nginx

# 1. Installer nginx
sudo apt install nginx

# 2. Vérifier l'état
sudo systemctl status nginx

# 3. Arrêter nginx
sudo systemctl stop nginx

# 4. Vérifier qu'il est arrêté
sudo systemctl is-active nginx

# 5. Démarrer nginx
sudo systemctl start nginx

# 6. Voir les logs récents
journalctl -u nginx -n 20

Exercise 2: Create a simple service

# 1. Créer un script
sudo mkdir -p /opt/myservice
cat << 'EOF' | sudo tee /opt/myservice/app.sh
#!/bin/bash
while true; do
    echo "Service running at $(date)"
    sleep 30
done
EOF
sudo chmod +x /opt/myservice/app.sh

# 2. Créer le fichier service
cat << 'EOF' | sudo tee /etc/systemd/system/myservice.service
[Unit]
Description=My Test Service

[Service]
Type=simple
ExecStart=/opt/myservice/app.sh
Restart=always

[Install]
WantedBy=multi-user.target
EOF

# 3. Activer et démarrer
sudo systemctl daemon-reload
sudo systemctl enable --now myservice

# 4. Vérifier
sudo systemctl status myservice
journalctl -u myservice -f

# 5. Nettoyer
sudo systemctl disable --now myservice
sudo rm /etc/systemd/system/myservice.service
sudo rm -rf /opt/myservice
sudo systemctl daemon-reload

Quiz

Q1. How do you enable a service at boot AND launch it immediately?

Answer

sudo systemctl enable --now service

Q2. How do you view a service's logs in real time?

Answer

journalctl -u service -f

Q3. Where do you create a custom service file?

Answer

/etc/systemd/system/

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Key takeaways

• systemctl manages services (start, stop, restart, status)
• enable = at boot, start = now
• journalctl -u service = service logs
• journalctl -f = logs in real time
• Custom services in /etc/systemd/system/
• Always daemon-reload after a change
• systemctl status to diagnose

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