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141 lines
6.8 KiB
XML
141 lines
6.8 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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<section xml:id="sect-nixos-systemd-general">
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<title>Interacting with a running systemd</title>
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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>. The following paragraphs demonstrate ways to
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interact with any OS running systemd as init system. NixOS is of no
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exception. The <link xlink:href="#sect-nixos-systemd-nixos">next section
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</link> explains NixOS specific things worth knowing.
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</para>
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<para>
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Without any arguments, <literal>systmctl</literal> the status of active units:
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<screen>
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<prompt>$ </prompt>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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<prompt>$ </prompt>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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<prompt># </prompt>systemctl stop postgresql.service
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<prompt># </prompt>systemctl start postgresql.service
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<prompt># </prompt>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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<!-- TODO: document cgroups, draft:
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each service and user session is a cgroup
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- cgroup resource management -->
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</section>
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<section xml:id="sect-nixos-systemd-nixos">
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<title>systemd in NixOS</title>
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<para>
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Packages in Nixpkgs sometimes provide systemd units with them, usually in
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e.g <literal>#pkg-out#/lib/systemd/</literal>. Putting such a package in
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<literal>environment.systemPackages</literal> doesn't make the service
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available to users or the system.
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</para>
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<para>
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In order to enable a systemd <emphasis>system</emphasis> service with
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provided upstream package, use (e.g):
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<programlisting>
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<xref linkend="opt-systemd.packages"/> = [ pkgs.packagekit ];
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</programlisting>
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</para>
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<para>
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Usually NixOS modules written by the community do the above, plus take care of
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other details. If a module was written for a service you are interested in,
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you'd probably need only to use
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<literal>services.#name#.enable = true;</literal>. These services are defined
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in Nixpkgs'
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<link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/modules">
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<literal>nixos/modules/</literal> directory </link>. In case the service is
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simple enough, the above method should work, and start the service on boot.
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</para>
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<para>
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<emphasis>User</emphasis> systemd services on the other hand, should be
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treated differently. Given a package that has a systemd unit file at
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<literal>#pkg-out#/lib/systemd/user/</literal>, using
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<xref linkend="opt-systemd.packages"/> will make you able to start the service via
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<literal>systemctl --user start</literal>, but it won't start automatically on login.
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<!-- TODO: Document why systemd.packages doesn't work for user services or fix this.
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https://github.com/NixOS/nixpkgs/blob/2cd6594a8710a801038af2b72348658f732ce84a/nixos/modules/system/boot/systemd-lib.nix#L177-L198
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This has been talked over at https://discourse.nixos.org/t/how-to-enable-upstream-systemd-user-services-declaratively/7649/5
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-->
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However, You can imperatively enable it by adding the package's attribute to
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<link linkend="opt-environment.systemPackages">
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<literal>systemd.packages</literal></link> and then do this (e.g):
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<screen>
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<prompt>$ </prompt>mkdir -p ~/.config/systemd/user/default.target.wants
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<prompt>$ </prompt>ln -s /run/current-system/sw/lib/systemd/user/syncthing.service ~/.config/systemd/user/default.target.wants/
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<prompt>$ </prompt>systemctl --user daemon-reload
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<prompt>$ </prompt>systemctl --user enable syncthing.service
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</screen>
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If you are interested in a timer file, use <literal>timers.target.wants</literal>
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instead of <literal>default.target.wants</literal> in the 1st and 2nd command.
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</para>
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<para>
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Using <literal>systemctl --user enable syncthing.service</literal> instead of
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the above, will work, but it'll use the absolute path of
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<literal>syncthing.service</literal> for the symlink, and this path is in
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<literal>/nix/store/.../lib/systemd/user/</literal>. Hence
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<link xlink:href="#sec-nix-gc">garbage collection</link> will remove that file
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and you will wind up with a broken symlink in your systemd configuration, which
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in turn will not make the service / timer start on login.
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</para>
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</section>
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</chapter>
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