System Administration · #12 of 19

System Services

systemctl & Daemons

What is a Service?

A service (or daemon) is a program that runs in the background:

nginx: Web server
ssh: Remote access
cron: Scheduled tasks
docker: Container runtime
NetworkManager: Network connections

They start automatically at boot and run without user interaction.

systemd & systemctl

systemd is the init system on modern Linux. It manages:

Services (daemons)
Boot process
Logging
Mounts
Timers

systemctl is the command to control systemd.

Service Management

Check Status

systemctl status nginx              # Service status
systemctl status ssh                # Is SSH running?
systemctl is-active nginx           # Just "active" or "inactive"
systemctl is-enabled nginx          # Starts at boot?

Start / Stop / Restart

sudo systemctl start nginx          # Start service
sudo systemctl stop nginx           # Stop service
sudo systemctl restart nginx        # Stop + Start
sudo systemctl reload nginx         # Reload config (no downtime)

Enable / Disable (Boot Behavior)

sudo systemctl enable nginx         # Start at boot
sudo systemctl disable nginx        # Don't start at boot
sudo systemctl enable --now nginx   # Enable AND start now

Understanding Status Output

systemctl status ssh

● ssh.service - OpenBSD Secure Shell server
     Loaded: loaded (/lib/systemd/system/ssh.service; enabled)
     Active: active (running) since Mon 2024-12-30 10:00:00 UTC
   Main PID: 1234 (sshd)
      Tasks: 1 (limit: 4915)
     Memory: 2.1M
        CPU: 32ms
     CGroup: /system.slice/ssh.service
             └─1234 sshd: /usr/sbin/sshd -D

Dec 30 10:00:00 ubuntu sshd[1234]: Server listening on 0.0.0.0 port 22

Key information:

Loaded: Where the service file is, enabled/disabled
Active: Running/stopped and since when
Main PID: Process ID
CGroup: Process hierarchy

Listing Services

systemctl list-units --type=service              # Running services
systemctl list-units --type=service --all        # All services
systemctl list-unit-files --type=service         # All service files
systemctl list-unit-files --state=enabled        # Services enabled at boot

Viewing Logs

journalctl shows systemd logs:

journalctl                          # All logs
journalctl -u ssh                   # Logs for SSH service
journalctl -u nginx -f              # Follow nginx logs (live)
journalctl -u ssh --since "1 hour ago"   # Recent logs
journalctl -u ssh -n 50             # Last 50 lines
journalctl -b                       # Logs since boot
journalctl -p err                   # Only errors

Service Files

Services are defined in .service files:

cat /lib/systemd/system/ssh.service

[Unit]
Description=OpenBSD Secure Shell server
After=network.target

[Service]
ExecStart=/usr/sbin/sshd -D
Restart=on-failure

[Install]
WantedBy=multi-user.target

Creating custom services is beyond this lesson, but now you understand the structure.

Common Services

| Service | Purpose | |---------|---------| | sshd | SSH server (remote login) | | nginx / apache2 | Web server | | docker | Container runtime | | cron | Scheduled tasks | | cups | Printing | | NetworkManager | Network management | | ufw | Firewall |

Try It!

Practice service management in the terminal:

Exercises:

Check SSH status: systemctl status ssh
List running services: systemctl list-units --type=service
Check if a service is enabled: systemctl is-enabled ssh
View logs: journalctl -u ssh -n 10

Try it in the terminal

A sandboxed shell with this lesson’s commands. Type help to see what’s available.

bash — try it

⚙️ System Services