<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-systemctl">

<title>Service Management</title>

<para>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.</para>

<para>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:

<screen>
$ 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
<replaceable>...</replaceable>
</screen>

</para>

<para>You can ask for detailed status information about a unit, for
instance, the PostgreSQL database service:

<screen>
$ 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.
</screen>

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.

</para>

<para>Units can be stopped, started or restarted:

<screen>
# systemctl stop postgresql.service
# systemctl start postgresql.service
# systemctl restart postgresql.service
</screen>

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).</para>

<!-- - cgroups: each service and user session is a cgroup

- cgroup resource management -->

</chapter>