# Service Management

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).