3. Services and User Management (20%)
Services and User Management is the operational heartbeat of Linux administration. Every running application is a process; every person and automated task is an account; every deployed service needs a package manager to install it and systemd to run it. Getting this domain wrong produces subtle failures: a user with incorrect group membership, a zombie process consuming table entries, a package installed without verifying GPG signatures. Unlike storage failures (which are loud), user and service management failures often manifest as intermittent permission errors and inexplicable service restarts.
The mental model for this phase is a factory floor: processes are workers, systemd is the supervisor tracking their state, package managers are the supply chain, and user accounts are the access badges. Every piece must be in order for the factory to run reliably.
⚠️ Common Misconception (M1): The single most-tested trap in this domain is confusing systemctl enable with systemctl start. enable configures a service to start at boot — it does nothing to the currently running system. start runs the service now — it has no effect on boot behavior. You almost always need both.
