Unix - Systemd Unit File Structure

A systemd unit file defines how a service, device, mount point, socket, or other component should be started, stopped, and managed.
These files are typically located in:

 
/etc/systemd/system/ (administrator overrides and custom units) /usr/lib/systemd/system/ (distribution-provided units) /run/systemd/system/ (runtime units)

A unit file is a simple text file divided into standardized sections.

1. [Unit] Section

This section describes the unit at a high level and defines its relationships with other units.

Common directives:

 
[Unit] Description=Short description of the service Documentation=URL or man pages After=network.target Before=multi-user.target Requires=network-online.target Wants=mysqld.service

Meaning:

  • Description: human-readable name.

  • Documentation: where to find its docs.

  • After/Before: ordering — when to start relative to others.

  • Requires: hard dependency; failure stops this unit.

  • Wants: soft dependency; failure doesn't stop the unit.

2. [Service] Section

Used only for .service units.
Defines how the service runs, what command starts it, and how systemd supervises it.

Common directives:

 
[Service] Type=simple ExecStart=/usr/bin/myapp --option ExecStop=/usr/bin/myapp --stop ExecReload=/usr/bin/kill -HUP $MAINPID Restart=on-failure RestartSec=3 User=myuser Group=mygroup WorkingDirectory=/opt/myapp/ Environment="ENV=prod"

Key parameters:

  • Type=

    • simple: default, runs and stays running.

    • forking: service daemonizes (like old SysV daemons).

    • oneshot: runs a single task and exits.

    • notify: service notifies systemd when ready.

  • ExecStart: required; the command to start the service.

  • ExecStop: optional; command to stop it gracefully.

  • Restart: auto-restart options (always, on-failure, etc.).

  • User/Group: run service as non-root.

  • Environment: environment variables.

3. [Install] Section

Defines how the unit integrates into the system when using systemctl enable.

 
[Install] WantedBy=multi-user.target Alias=myapp.service RequiredBy=network.target

Meaning:

  • WantedBy: which target this service should start with.

  • RequiredBy: hard dependency from another target.

  • Alias: alternative name.

When you run:

 
systemctl enable myapp.service

systemd creates symlinks based on the WantedBy or RequiredBy values.

Complete Example Unit File

 
[Unit] Description=My Custom Web Service Documentation=https://docs.example.com/myservice After=network.target Wants=network-online.target [Service] Type=simple ExecStart=/usr/bin/python3 /opt/webapp/server.py Restart=always RestartSec=5 User=webuser WorkingDirectory=/opt/webapp Environment="ENV=production" [Install] WantedBy=multi-user.target

Summary of Unit File Sections

 

Section Purpose
[Unit] Metadata and dependencies
[Service] How the service starts, runs, and restarts (services only)
[Install] Integration with boot targets (systemctl enable)