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11a1589223
Files which are installed as-is (any .service and other unit files, .conf files, .policy files, etc), are left as is. My assumption is that SPDX identifiers are not yet that well known, so it's better to retain the extended header to avoid any doubt. I also kept any copyright lines. We can probably remove them, but it'd nice to obtain explicit acks from all involved authors before doing that.
332 lines
15 KiB
XML
332 lines
15 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
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<!ENTITY % entities SYSTEM "custom-entities.ent" >
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%entities;
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]>
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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This file is part of systemd.
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Copyright 2015 Zbigniew Jędrzejewski-Szmek
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-->
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<refentry id="systemd.generator">
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<refentryinfo>
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<title>systemd.generator</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>systemd.generator</refentrytitle>
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<manvolnum>7</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>systemd.generator</refname>
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<refpurpose>systemd unit generators</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<cmdsynopsis>
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<command>/path/to/generator</command>
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<arg choice="plain"><replaceable>normal-dir</replaceable></arg>
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<arg choice="plain"><replaceable>early-dir</replaceable></arg>
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<arg choice="plain"><replaceable>late-dir</replaceable></arg>
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</cmdsynopsis>
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<para>
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<literallayout><filename>/run/systemd/system-generators/*</filename>
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<filename>/etc/systemd/system-generators/*</filename>
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<filename>/usr/local/lib/systemd/system-generators/*</filename>
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<filename>&systemgeneratordir;/*</filename></literallayout>
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</para>
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<para>
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<literallayout><filename>/run/systemd/user-generators/*</filename>
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<filename>/etc/systemd/user-generators/*</filename>
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<filename>/usr/local/lib/systemd/user-generators/*</filename>
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<filename>&usergeneratordir;/*</filename></literallayout>
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</para>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>Generators are small executables that live in
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<filename>&systemgeneratordir;/</filename> and other directories listed above.
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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will execute those binaries very early at bootup and at configuration reload time
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— before unit files are loaded. Their main purpose is to convert configuration
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that is not native into dynamically generated unit files.</para>
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<para>Each generator is called with three directory paths that are to be used for
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generator output. In these three directories, generators may dynamically generate
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unit files (regular ones, instances, as well as templates), unit file
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<filename>.d/</filename> drop-ins, and create symbolic links to unit files to add
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additional dependencies, create aliases, or instantiate existing templates. Those
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directories are included in the unit load path of
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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allowing generated configuration to extend or override existing
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definitions.</para>
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<para>Directory paths for generator output differ by priority:
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<filename>…/generator.early</filename> has priority higher than the admin
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configuration in <filename>/etc</filename>, while
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<filename>…/generator</filename> has lower priority than
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<filename>/etc</filename> but higher than vendor configuration in
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<filename>/usr</filename>, and <filename>…/generator.late</filename> has priority
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lower than all other configuration. See the next section and the discussion of
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unit load paths and unit overriding in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
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</para>
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<para>Generators are loaded from a set of paths determined during
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compilation, as listed above. System and user generators are loaded
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from directories with names ending in
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<filename>system-generators/</filename> and
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<filename>user-generators/</filename>, respectively. Generators
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found in directories listed earlier override the ones with the
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same name in directories lower in the list. A symlink to
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<filename>/dev/null</filename> or an empty file can be used to
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mask a generator, thereby preventing it from running. Please note
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that the order of the two directories with the highest priority is
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reversed with respect to the unit load path, and generators in
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<filename>/run</filename> overwrite those in
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<filename>/etc</filename>.</para>
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<para>After installing new generators or updating the
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configuration, <command>systemctl daemon-reload</command> may be
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executed. This will delete the previous configuration created by
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generators, re-run all generators, and cause
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<command>systemd</command> to reload units from disk. See
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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for more information.
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</para>
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</refsect1>
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<refsect1>
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<title>Output directories</title>
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<para>Generators are invoked with three arguments: paths to directories where
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generators can place their generated unit files or symlinks. By default those
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paths are runtime directories that are included in the search path of
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<command>systemd</command>, but a generator may be called with different paths
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for debugging purposes.</para>
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<orderedlist>
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<listitem>
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<para><parameter>normal-dir</parameter></para>
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<para>In normal use this is <filename>/run/systemd/generator</filename> in
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case of the system generators and
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<filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user
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generators. Unit files placed in this directory take precedence over vendor
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unit configuration but not over native user/administrator unit configuration.
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</para>
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</listitem>
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<listitem>
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<para><parameter>early-dir</parameter></para>
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<para>In normal use this is <filename>/run/systemd/generator.early</filename>
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in case of the system generators and
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<filename>$XDG_RUNTIME_DIR/generator.early</filename> in case of the user
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generators. Unit files placed in this directory override unit files in
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<filename>/usr</filename>, <filename>/run</filename> and
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<filename>/etc</filename>. This means that unit files placed in this
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directory take precedence over all normal configuration, both vendor and
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user/administrator.</para>
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</listitem>
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<listitem>
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<para><parameter>late-dir</parameter></para>
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<para>In normal use this is <filename>/run/systemd/generator.late</filename>
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in case of the system generators and
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<filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user
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generators. This directory may be used to extend the unit file tree without
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overriding any other unit files. Any native configuration files supplied by
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the vendor or user/administrator take precedence.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>Notes about writing generators</title>
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<itemizedlist>
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<listitem>
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<para>All generators are executed in parallel. That means all executables are
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started at the very same time and need to be able to cope with this
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parallelism.
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</para>
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</listitem>
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<listitem>
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<para>Generators are run very early at boot and cannot rely on any external
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services. They may not talk to any other process. That includes simple things
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such as logging to
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<citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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or <command>systemd</command> itself (this means: no
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!
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Non-essential file systems like <filename>/var</filename> and
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<filename>/home</filename> are mounted after generators have run. Generators
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can however rely on the most basic kernel functionality to be available,
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including a mounted <filename>/sys</filename>, <filename>/proc</filename>,
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<filename>/dev</filename>, <filename>/usr</filename>.
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</para>
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</listitem>
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<listitem>
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<para>Units written by generators are removed when the configuration is
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reloaded. That means the lifetime of the generated units is closely bound to
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the reload cycles of <command>systemd</command> itself.</para>
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</listitem>
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<listitem>
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<para>Generators should only be used to generate unit files and symlinks to
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them, not any other kind of configuration. Due to the lifecycle logic
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mentioned above, generators are not a good fit to generate dynamic
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configuration for other services. If you need to generate dynamic
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configuration for other services, do so in normal services you order before
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the service in question.</para>
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</listitem>
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<listitem>
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<para>Since
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<citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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is not available (see above), log messages have to be written to
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<filename>/dev/kmsg</filename> instead.</para>
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</listitem>
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<listitem>
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<para>It is a good idea to use the <varname>SourcePath=</varname> directive
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in generated unit files to specify the source configuration file you are
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generating the unit from. This makes things more easily understood by the
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user and also has the benefit that systemd can warn the user about
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configuration files that changed on disk but have not been read yet by
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systemd.</para>
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</listitem>
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<listitem>
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<para>Generators may write out dynamic unit files or just hook unit files
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into other units with the usual <filename>.wants/</filename> or
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<filename>.requires/</filename> symlinks. Often, it is nicer to simply
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instantiate a template unit file from <filename>/usr</filename> with a
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generator instead of writing out entirely dynamic unit files. Of course, this
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works only if a single parameter is to be used.</para>
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</listitem>
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<listitem>
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<para>If you are careful, you can implement generators in shell scripts. We
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do recommend C code however, since generators are executed synchronously and
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hence delay the entire boot if they are slow.</para>
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</listitem>
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<listitem>
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<para>Regarding overriding semantics: there are two rules we try to follow
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when thinking about the overriding semantics:</para>
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<orderedlist numeration="lowerroman">
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<listitem>
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<para>User configuration should override vendor configuration. This
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(mostly) means that stuff from <filename>/etc</filename> should override
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stuff from <filename>/usr</filename>.</para>
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</listitem>
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<listitem>
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<para>Native configuration should override non-native configuration. This
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(mostly) means that stuff you generate should never override native unit
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files for the same purpose.</para>
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</listitem>
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</orderedlist>
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<para>Of these two rules the first rule is probably the more important one
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and breaks the second one sometimes. Hence, when deciding whether to use
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argv[1], argv[2], or argv[3], your default choice should probably be
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argv[1].</para>
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</listitem>
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<listitem>
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<para>Instead of heading off now and writing all kind of generators for
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legacy configuration file formats, please think twice! It is often a better
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idea to just deprecate old stuff instead of keeping it artificially alive.
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</para>
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</listitem>
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</itemizedlist>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<example>
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<title>systemd-fstab-generator</title>
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<para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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converts <filename>/etc/fstab</filename> into native mount units. It uses
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argv[1] as location to place the generated unit files in order to allow the
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user to override <filename>/etc/fstab</filename> with her own native unit
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files, but also to ensure that <filename>/etc/fstab</filename> overrides any
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vendor default from <filename>/usr</filename>.</para>
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<para>After editing <filename>/etc/fstab</filename>, the user should invoke
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<command>systemctl daemon-reload</command>. This will re-run all generators and
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cause <command>systemd</command> to reload units from disk. To actually mount
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new directories added to <filename>fstab</filename>, <command>systemctl start
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<replaceable>/path/to/mountpoint</replaceable></command> or <command>systemctl
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start local-fs.target</command> may be used.</para>
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</example>
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<example>
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<title>systemd-system-update-generator</title>
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<para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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temporarily redirects <filename>default.target</filename> to
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<filename>system-update.target</filename>, if a system update is
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scheduled. Since this needs to override the default user configuration for
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<filename>default.target</filename>, it uses argv[2]. For details about this
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logic, see
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<citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
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</para>
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</example>
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<example>
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<title>Debugging a generator</title>
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<programlisting>dir=$(mktemp -d)
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SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
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"$dir" "$dir" "$dir"
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find $dir</programlisting>
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</example>
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</refsect1>
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<refsect1>
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<title>See also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-rc-local-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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