3.3 Service Management with Systemd

Systemd is the init system used by almost all modern major distributions. It manages “Units”, most commonly “Service Units” (.service). It replaces older init systems and manages processes (see [[/en/module-2/5-processes|Process Control]]) more robustly.

Systemctl Commands

The main tool for interacting with systemd.

Action	Command	Explanation
Start	sudo systemctl start nginx	Start service now.
Stop	sudo systemctl stop nginx	Stop service now.
Restart	sudo systemctl restart nginx	Stop, then start.
Reload	sudo systemctl reload nginx	Reload config without stopping (zero downtime).
Enable	sudo systemctl enable nginx	Start automatically on boot.
Disable	sudo systemctl disable nginx	Do not start on boot.
Status	systemctl status nginx	Check if running, enabled, and view latest logs.

Cheatsheet: The “Enable and Start” Combo

You often want to do both.

sudo systemctl enable --now nginx

Unit Files

Service definitions live in /lib/systemd/system/ (defaults) and /etc/systemd/system/ (custom overrides).

Practice Exercises

1. Investigate SSH:
   • Check the status of the SSH service: systemctl status ssh (or sshd).
   • Is it active? Is it enabled?
2. Manage a Dummy Service:
   • Install apache2 or nginx if you can, or pick an existing service like cron.
   • Stop the service. Verify it’s stopped.
   • Start it again.
   • Disable it (so it won’t start on boot).
   • Enable it again.
3. Logs:
   • Use journalctl -u ssh (or sshd) to view logs specifically for that service.