Definition
systemctl manages units handled by systemd on many Linux distributions. A unit can describe a service, socket, timer, mount or other managed resource.
How it works
status, start, stop and restart affect current state; enable and disable control automatic activation. Enabling does not necessarily start a service immediately: enable --now performs both actions. Edited unit files generally require systemctl daemon-reload.
Real-world example
If a web service becomes unreachable, systemctl status nginx and journalctl -u nginx help distinguish a configuration failure from a stopped process or a port conflict. Restart interrupts and starts the process again; reload works only when supported by the service.
Limits and best practices
Inspect dependencies with systemctl list-dependencies and verify behavior after reboot. Masking is stronger than disabling: a masked unit cannot be started normally on demand. Schedule disruptive changes during a maintenance window.