In NixOS, all system services are started and monitored using the systemd program. Systemd is the “init” process of the system (i.e. PID 1), the parent of all other processes. It manages a set of so-called “units”, which can be things like system services (programs), but also mount points, swap files, devices, targets (groups of units) and more. Units can have complex dependencies; for instance, one unit can require that another unit must be successfully started before the first unit can be started. When the system boots, it starts a unit named <literal>default.target</literal>; the dependencies of this unit cause all system services to be started, file systems to be mounted, swap files to be activated, and so on.
The command <command>systemctl</command> is the main way to interact with <command>systemd</command>. Without any arguments, it shows the status of active units:
Note that this shows the status of the unit (active and running), all the processes belonging to the service, as well as the most recent log messages from the service.
These operations are synchronous: they wait until the service has finished starting or stopping (or has failed). Starting a unit will cause the dependencies of that unit to be started as well (if necessary).
