Understanding Services

Understanding Services

A systemd service unit is a configuration file that describes how to start, stop, and supervise a process. Before you can manage services effectively, you need to understand what a unit file contains and what each directive does. This knowledge is also essential for writing your own service units for custom applications.

What is a service unit?

# View an existing service unit file
systemctl cat nginx.service

nginx.service unit file

[Unit]
Description=A high performance web server and a reverse proxy server
Documentation=man:nginx(8)
After=network.target nss-lookup.target

[Service]
Type=forking
PIDFile=/run/nginx.pid
ExecStartPre=/usr/sbin/nginx -t -q -g 'daemon on; master_process on;'
ExecStart=/usr/sbin/nginx -g 'daemon on; master_process on;'
ExecReload=/bin/kill -s HUP $MAINPID
KillSignal=SIGQUIT
TimeoutStopSec=5
KillMode=mixed
PrivateTmp=true

[Install]
WantedBy=multi-user.target

Service unit file structure

# Show all properties of a service (including inherited defaults)
systemctl show nginx.service | grep -E "^(Type|ExecStart|Restart|User|WorkingDirectory)"

Service types

The Type= directive tells systemd how the process behaves at startup — specifically, when systemd should consider the service "ready."

# Check what type a service uses
systemctl show sshd.service -p Type

Output

Type=notify

Where unit files live

# Unit file locations (in priority order, highest to lowest):
# /etc/systemd/system/          — Local overrides (your changes go here)
# /run/systemd/system/          — Runtime units (transient, cleared on reboot)
# /lib/systemd/system/          — Package-installed units (don't edit these)

# NEVER edit /lib/systemd/system/ — apt will overwrite your changes on upgrade
# ALWAYS put overrides in /etc/systemd/system/

# For partial overrides (change one directive without replacing the whole file):
systemctl edit nginx.service    # Creates /etc/systemd/system/nginx.service.d/override.conf

# For complete replacement:
systemctl edit --full nginx.service  # Copies to /etc/systemd/system/nginx.service

System vs user services

# System services: run as root or a specified user, managed by PID 1
# User services: run in user session context (no root)

# System service (requires root or sudo)
sudo systemctl start nginx
sudo systemctl status nginx

# User service (for your own user session)
# Unit files in ~/.config/systemd/user/
systemctl --user start myapp
systemctl --user status myapp
systemctl --user enable myapp

# Enable user services to run after logout (for CI runners, etc.)
sudo loginctl enable-linger username

Conclusion

Every systemd service is described by a unit file with three sections: [Unit] for metadata and dependencies, [Service] for how to run the process, and [Install] for enable/disable behavior. The Type= directive is critical — using the wrong type (e.g., simple for a forking daemon) causes systemd to report success before the service is actually ready. Always put your custom unit files and overrides in /etc/systemd/system/ — never edit files in /lib/systemd/system/ because package upgrades will overwrite them.