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 default.target; 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 systemctl is the main way to interact with systemd. Without any arguments, it shows the status of active units: $ systemctl -.mount loaded active mounted / swapfile.swap loaded active active /swapfile sshd.service loaded active running SSH Daemon graphical.target loaded active active Graphical Interface ... You can ask for detailed status information about a unit, for instance, the PostgreSQL database service: $ systemctl status postgresql.service postgresql.service - PostgreSQL Server Loaded: loaded (/nix/store/pn3q73mvh75gsrl8w7fdlfk3fq5qm5mw-unit/postgresql.service) Active: active (running) since Mon, 2013-01-07 15:55:57 CET; 9h ago Main PID: 2390 (postgres) CGroup: name=systemd:/system/postgresql.service ├─2390 postgres ├─2418 postgres: writer process ├─2419 postgres: wal writer process ├─2420 postgres: autovacuum launcher process ├─2421 postgres: stats collector process └─2498 postgres: zabbix zabbix [local] idle Jan 07 15:55:55 hagbard postgres[2394]: [1-1] LOG: database system was shut down at 2013-01-07 15:55:05 CET Jan 07 15:55:57 hagbard postgres[2390]: [1-1] LOG: database system is ready to accept connections Jan 07 15:55:57 hagbard postgres[2420]: [1-1] LOG: autovacuum launcher started Jan 07 15:55:57 hagbard systemd[1]: Started PostgreSQL Server. 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. Units can be stopped, started or restarted: $ systemctl stop postgresql.service $ systemctl start postgresql.service $ systemctl restart postgresql.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).