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73 lines
3.3 KiB
XML
73 lines
3.3 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-systemctl">
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<title>Service Management</title>
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<para>
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In NixOS, all system services are started and monitored using the systemd
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program. Systemd is the “init” process of the system (i.e. PID 1), the
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parent of all other processes. It manages a set of so-called “units”,
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which can be things like system services (programs), but also mount points,
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swap files, devices, targets (groups of units) and more. Units can have
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complex dependencies; for instance, one unit can require that another unit
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must be successfully started before the first unit can be started. When the
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system boots, it starts a unit named <literal>default.target</literal>; the
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dependencies of this unit cause all system services to be started, file
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systems to be mounted, swap files to be activated, and so on.
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</para>
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<para>
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The command <command>systemctl</command> is the main way to interact with
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<command>systemd</command>. Without any arguments, it shows the status of
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active units:
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<screen>
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$ systemctl
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-.mount loaded active mounted /
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swapfile.swap loaded active active /swapfile
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sshd.service loaded active running SSH Daemon
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graphical.target loaded active active Graphical Interface
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<replaceable>...</replaceable>
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</screen>
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</para>
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<para>
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You can ask for detailed status information about a unit, for instance, the
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PostgreSQL database service:
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<screen>
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$ systemctl status postgresql.service
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postgresql.service - PostgreSQL Server
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Loaded: loaded (/nix/store/pn3q73mvh75gsrl8w7fdlfk3fq5qm5mw-unit/postgresql.service)
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Active: active (running) since Mon, 2013-01-07 15:55:57 CET; 9h ago
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Main PID: 2390 (postgres)
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CGroup: name=systemd:/system/postgresql.service
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├─2390 postgres
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├─2418 postgres: writer process
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├─2419 postgres: wal writer process
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├─2420 postgres: autovacuum launcher process
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├─2421 postgres: stats collector process
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└─2498 postgres: zabbix zabbix [local] idle
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Jan 07 15:55:55 hagbard postgres[2394]: [1-1] LOG: database system was shut down at 2013-01-07 15:55:05 CET
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Jan 07 15:55:57 hagbard postgres[2390]: [1-1] LOG: database system is ready to accept connections
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Jan 07 15:55:57 hagbard postgres[2420]: [1-1] LOG: autovacuum launcher started
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Jan 07 15:55:57 hagbard systemd[1]: Started PostgreSQL Server.
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</screen>
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Note that this shows the status of the unit (active and running), all the
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processes belonging to the service, as well as the most recent log messages
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from the service.
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</para>
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<para>
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Units can be stopped, started or restarted:
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<screen>
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# systemctl stop postgresql.service
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# systemctl start postgresql.service
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# systemctl restart postgresql.service
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</screen>
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These operations are synchronous: they wait until the service has finished
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starting or stopping (or has failed). Starting a unit will cause the
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dependencies of that unit to be started as well (if necessary).
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</para>
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<!-- - cgroups: each service and user session is a cgroup
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- cgroup resource management -->
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</chapter>
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