Service

Core Idea

A service is a long-running or supervised capability that other programs or users can rely on.

Meanings You Will See

• Network service: a process listening for network requests, such as SSH, HTTP, or SMTP.
• systemd service: a unit managed by systemd, such as site.service.
• System service: a root-managed or administrator-managed service for the whole machine.
• User service: a systemd service managed by your user account with systemctl --user.

Course Examples

PORT="$((10000 + $(id -u)))"
systemctl --user status site.service
journalctl --user -u site.service --no-pager -n 50
curl -I "http://127.0.0.1:$PORT/"

Your personal web service is both a network service and a systemd user service: it listens for HTTP requests on the course-assigned port and is supervised by your per-user systemd instance.

What A Service Needs

• A command to start.
• A user account and permissions.
• Files or directories it needs to read.
• A port if it is a network service.
• Logs for diagnosis.
• A restart or recovery policy if supervised.

Common Confusions

• service does not always mean systemd. A service can be any useful long-running process.
• A systemd service can be a one-shot task, not only a daemon.
• A user service is not a root service. Use systemctl --user.
• A process can listen locally while the public reverse proxy still fails.

Proof Check

For site.service, identify whether it is a network service, systemd service, system service, user service, or more than one of those.

Docs Pointers

• Run man systemd.service and man systemctl.
• Read systemd service units.
• Read Server, systemd User Services, Logging, and Platform Reference.