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-<?xml version='1.0'?> <!--*-nxml-*-->
-<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
-<!ENTITY % entities SYSTEM "custom-entities.ent" >
-%entities;
-]>
-
-<!--
- This file is part of systemd.
-
- Copyright 2015 Zbigniew Jędrzejewski-Szmek
-
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
-
- systemd is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
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- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
-
-<refentry id="systemd.generator">
- <refentryinfo>
- <title>systemd.generator</title>
- <productname>systemd</productname>
-
- <authorgroup>
- <author>
- <contrib>Developer</contrib>
- <firstname>Lennart</firstname>
- <surname>Poettering</surname>
- <email>lennart@poettering.net</email>
- </author>
- </authorgroup>
- </refentryinfo>
-
- <refmeta>
- <refentrytitle>systemd.generator</refentrytitle>
- <manvolnum>7</manvolnum>
- </refmeta>
-
- <refnamediv>
- <refname>systemd.generator</refname>
- <refpurpose>Systemd unit generators</refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <cmdsynopsis>
- <command>/path/to/generator</command>
- <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
- <arg choice="plain"><replaceable>early-dir</replaceable></arg>
- <arg choice="plain"><replaceable>late-dir</replaceable></arg>
- </cmdsynopsis>
-
- <para>
- <literallayout><filename>/run/systemd/system-generators/*</filename>
-<filename>/etc/systemd/system-generators/*</filename>
-<filename>/usr/local/lib/systemd/system-generators/*</filename>
-<filename>&systemgeneratordir;/*</filename></literallayout>
- </para>
-
- <para>
- <literallayout><filename>/run/systemd/user-generators/*</filename>
-<filename>/etc/systemd/user-generators/*</filename>
-<filename>/usr/local/lib/systemd/user-generators/*</filename>
-<filename>&usergeneratordir;/*</filename></literallayout>
- </para>
- </refsynopsisdiv>
-
- <refsect1>
- <title>Description</title>
- <para>Generators are small binaries that live in
- <filename>&usergeneratordir;/</filename> and other directories
- listed above.
- <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- will execute those binaries very early at bootup and at
- configuration reload time — before unit files are loaded.
- Generators can dynamically generate unit files or create symbolic
- links to unit files to add additional dependencies, thus extending
- or overriding existing definitions. Their main purpose is to
- convert configuration files that are not native unit files
- dynamically into native unit files.</para>
-
- <para>Generators are loaded from a set of paths determined during
- compilation, listed above. System and user generators are loaded
- from directories with names ending in
- <filename>system-generators/</filename> and
- <filename>user-generators/</filename>, respectively. Generators
- found in directories listed earlier override the ones with the
- same name in directories lower in the list. A symlink to
- <filename>/dev/null</filename> or an empty file can be used to
- mask a generator, thereby preventing it from running. Please note
- that the order of the two directories with the highest priority is
- reversed with respect to the unit load path and generators in
- <filename>/run</filename> overwrite those in
- <filename>/etc</filename>.</para>
-
- <para>After installing new generators or updating the
- configuration, <command>systemctl daemon-reload</command> may be
- executed. This will delete the previous configuration created by
- generators, re-run all generators, and cause
- <command>systemd</command> to reload units from disk. See
- <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- for more information.
- </para>
- </refsect1>
-
- <refsect1>
- <title>Writing generators</title>
-
- <para>Generators are invoked with three arguments: paths to
- runtime directories where generators can place their generated
- unit files or symlinks.</para>
-
- <orderedlist>
- <listitem>
- <para><parameter>normal-dir</parameter></para>
- <para>argv[1] may be used to override unit files in
- <filename>/usr</filename>, but not those in
- <filename>/etc</filename>. This means that unit files placed
- in this directory take precedence over vendor unit
- configuration but not over native user/administrator unit
- configuration.</para>
- </listitem>
-
- <listitem>
- <para><parameter>early-dir</parameter></para>
- <para>argv[2] may be used to override unit files in
- <filename>/usr</filename> and in
- <filename>/etc</filename>. This means that unit files placed
- in this directory take precedence over all configuration,
- both vendor and user/administrator.</para>
- </listitem>
-
- <listitem>
- <para><parameter>late-dir</parameter></para>
- <para>argv[3] may be used to extend the unit file tree without
- overridding any other unit files. Any native configuration
- files supplied by the vendor or user/administrator take
- precedence over the generated ones placed in this directory.
- </para>
- </listitem>
- </orderedlist>
-
- <refsect2>
- <title>Notes</title>
-
- <itemizedlist>
- <listitem>
- <para>
- All generators are executed in parallel. That means all
- executables are started at the very same time and need to
- be able to cope with this parallelism.
- </para>
- </listitem>
-
- <listitem>
- <para>
- Generators are run very early at boot and cannot rely on
- any external services. They may not talk to any other
- process. That includes simple things such as logging to
- <citerefentry
- project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
- or <command>systemd</command> itself (this means: no
- <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>!). They
- can however rely on the most basic kernel functionality to
- be available, including mounted <filename>/sys</filename>,
- <filename>/proc</filename>, <filename>/dev</filename>.
- </para>
- </listitem>
-
- <listitem>
- <para>
- Units written by generators are removed when configuration
- is reloaded. That means the lifetime of the generated
- units is closely bound to the reload cycles of
- <command>systemd</command> itself.
- </para>
- </listitem>
-
- <listitem>
- <para>
- Generators should only be used to generate unit files, not
- any other kind of configuration. Due to the lifecycle
- logic mentioned above generators are not a good fit to
- generate dynamic configuration for other services. If you
- need to generate dynamic configuration for other services
- do so in normal services you order before the service in
- question.
- </para>
- </listitem>
-
- <listitem>
- <para>
- Since
- <citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
- is not available (see above) log messages have to be
- written to <filename>/dev/kmsg</filename> instead.
- </para>
- </listitem>
-
- <listitem>
- <para>
- It is a good idea to use the
- <varname>SourcePath=</varname> directive in generated unit
- files to specify the source configuration file you are
- generating the unit from. This makes things more easily
- understood by the user and also has the benefit that
- systemd can warn the user about configuration files that
- changed on disk but have not been read yet by systemd.
- </para>
- </listitem>
-
- <listitem>
- <para>
- Generators may write out dynamic unit files or just hook
- unit files into other units with the usual
- <filename>.wants/</filename> or
- <filename>.requires/</filename> symlinks. Often it is
- nicer to simply instantiate a template unit file from
- <filename>/usr</filename> with a generator instead of
- writing out entirely dynamic unit files. Of course this
- works only if a single parameter is to be used.
- </para>
- </listitem>
-
- <listitem>
- <para>
- If you are careful you can implement generators in shell
- scripts. We do recommend C code however, since generators
- delay are executed synchronously and hence delay the
- entire boot if they are slow.
- </para>
- </listitem>
-
- <listitem>
- <para>Regarding overriding semantics: there are two rules we
- try to follow when thinking about the overriding semantics:
- </para>
-
- <orderedlist numeration="lowerroman">
- <listitem>
- <para>User configuration should override vendor
- configuration. This (mostly) means that stuff from
- <filename>/etc</filename> should override stuff from
- <filename>/usr</filename>.</para>
- </listitem>
-
- <listitem>
- <para>Native configuration should override non-native
- configuration. This (mostly) means that stuff you
- generate should never override native unit files for the
- same purpose.</para>
- </listitem>
- </orderedlist>
-
- <para>Of these two rules the first rule is probably the more
- important one and breaks the second one sometimes. Hence,
- when deciding whether to user argv[1], argv[2], or argv[3],
- your default choice should probably be argv[1].</para>
- </listitem>
-
- <listitem>
- <para>
- Instead of heading off now and writing all kind of
- generators for legacy configuration file formats, please
- think twice! It's often a better idea to just deprecate
- old stuff instead of keeping it artificially alive.
- </para>
- </listitem>
- </itemizedlist>
- </refsect2>
- </refsect1>
-
- <refsect1>
- <title>Examples</title>
- <example>
- <title>systemd-fstab-generator</title>
-
- <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
- converts <filename>/etc/fstab</filename> into native mount
- units. It uses argv[1] as location to place the generated unit
- files in order to allow the user to override
- <filename>/etc/fstab</filename> with her own native unit files,
- but also to ensure that <filename>/etc/fstab</filename>
- overrides any vendor default from <filename>/usr</filename>.
- </para>
-
- <para>After editing <filename>/etc/fstab</filename>, the user
- should invoke <command>systemctl daemon-reload</command>. This
- will re-run all generators and cause <command>systemd</command>
- to reload units from disk. To actually mount new directories
- added to <filename>fstab</filename>, <command>systemctl start
- <replaceable>/path/to/mountpoint</replaceable></command> or
- <command>systemctl start local-fs.target</command> may be used.
- </para>
- </example>
-
- <example>
- <title>systemd-system-update-generator</title>
-
- <para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
- temporarily redirects <filename>default.target</filename> to
- <filename>system-update.target</filename> if a system update is
- scheduled. Since this needs to override the default user
- configuration for <filename>default.target</filename> it uses
- argv[2]. For details about this logic, see
- <ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates">Implementing
- Offline System Updates</ulink>.</para>
- </example>
-
- <example>
- <title>Debuging a generator</title>
-
- <programlisting>dir=$(mktemp -d)
-SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
- "$dir" "$dir" "$dir"
-find $dir</programlisting>
- </example>
- </refsect1>
-
- <refsect1>
- <title>See also</title>
-
- <para>
- <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-efi-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
- <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
- </para>
- </refsect1>
-</refentry>
~ianmdlvl / elogind.git / blobdiff