systemd Fundamentals

systemd is a suite of Linux system components built around a system and service manager. The system manager runs as process ID 1, or PID 1. It starts user space after the kernel boots, supervises services, and coordinates shutdown. Separate per-user managers can run services for logged-in users.

The manager does not treat boot as one long shell script. It loads descriptions of resources called units, builds a dependency transaction, and runs jobs when their ordering constraints permit. A unit can represent a service, socket, timer, path, mount, device, target, scope, slice, or another managed resource.

From request to running process

A request such as starting web.service follows a control path:

systemctl request
      ↓ D-Bus
systemd manager
      ↓ load unit and dependencies
transaction of jobs
      ↓ honor ordering
process starts in a control group
      ↓
state and logs become inspectable

systemctl is a client of the manager. It asks the manager to start, stop, reload, or inspect units. It does not run the service command itself.

systemd Fundamentals

From request to running process

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