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12b42c7667
This did not really work out as we had hoped. Trying to do this upstream introduced several problems that probably makes it better suited as a downstream patch after all. At any rate, it is not releaseable in the current state, so we at least need to revert this before the release. * by adjusting the path to binaries, but not do the same thing to the search path we end up with inconsistent man-pages. Adjusting the search path too would be quite messy, and it is not at all obvious that this is worth the effort, but at any rate it would have to be done before we could ship this. * this means that distributed man-pages does not make sense as they depend on config options, and for better or worse we are still distributing man pages, so that is something that definitely needs sorting out before we could ship with this patch. * we have long held that split-usr is only minimally supported in order to boot, and something we hope will eventually go away. So before we start adding even more magic/effort in order to make this work nicely, we should probably question if it makes sense at all.
347 lines
14 KiB
XML
347 lines
14 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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This file is part of systemd.
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Copyright 2015 Zbigniew Jędrzejewski-Szmek
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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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 binaries that live in
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<filename>&usergeneratordir;/</filename> and other directories
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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
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configuration reload time — before unit files are loaded.
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Generators can dynamically generate unit files or create symbolic
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links to unit files to add additional dependencies, thus extending
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or overriding existing definitions. Their main purpose is to
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convert configuration files that are not native unit files
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dynamically into native unit files.</para>
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<para>Generators are loaded from a set of paths determined during
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compilation, 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>Writing generators</title>
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<para>Generators are invoked with three arguments: paths to
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runtime directories where generators can place their generated
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unit files or symlinks.</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>argv[1] may be used to override unit files in
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<filename>/usr</filename>, but not those in
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<filename>/etc</filename>. This means that unit files placed
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in this directory take precedence over vendor unit
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configuration but not over native user/administrator unit
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configuration.</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>argv[2] may be used to override unit files in
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<filename>/usr</filename> and in
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<filename>/etc</filename>. This means that unit files placed
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in this directory take precedence over all configuration,
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both vendor and 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>argv[3] may be used to extend the unit file tree without
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overriding any other unit files. Any native configuration
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files supplied by the vendor or user/administrator take
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precedence over the generated ones placed in this directory.
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</para>
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</listitem>
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</orderedlist>
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<refsect2>
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<title>Notes</title>
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<itemizedlist>
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<listitem>
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<para>
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All generators are executed in parallel. That means all
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executables are started at the very same time and need to
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be able to cope with this parallelism.
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</para>
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</listitem>
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<listitem>
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<para>
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Generators are run very early at boot and cannot rely on
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any external services. They may not talk to any other
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process. That includes simple things such as logging to
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<citerefentry
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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>!). They
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can however rely on the most basic kernel functionality to
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be available, including mounted <filename>/sys</filename>,
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<filename>/proc</filename>, <filename>/dev</filename>.
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</para>
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</listitem>
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<listitem>
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<para>
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Units written by generators are removed when configuration
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is reloaded. That means the lifetime of the generated
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units is closely bound to the reload cycles of
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<command>systemd</command> itself.
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</para>
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</listitem>
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<listitem>
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<para>
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Generators should only be used to generate unit files, not
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any other kind of configuration. Due to the lifecycle
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logic mentioned above generators are not a good fit to
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generate dynamic configuration for other services. If you
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need to generate dynamic configuration for other services
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do so in normal services you order before the service in
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question.
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</para>
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</listitem>
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<listitem>
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<para>
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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
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written to <filename>/dev/kmsg</filename> instead.
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</para>
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</listitem>
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<listitem>
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<para>
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It is a good idea to use the
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<varname>SourcePath=</varname> directive in generated unit
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files to specify the source configuration file you are
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generating the unit from. This makes things more easily
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understood by the user and also has the benefit that
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systemd can warn the user about configuration files that
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changed on disk but have not been read yet by systemd.
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</para>
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</listitem>
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<listitem>
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<para>
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Generators may write out dynamic unit files or just hook
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unit files into other units with the usual
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<filename>.wants/</filename> or
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<filename>.requires/</filename> symlinks. Often it is
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nicer to simply instantiate a template unit file from
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<filename>/usr</filename> with a generator instead of
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writing out entirely dynamic unit files. Of course this
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works only if a single parameter is to be used.
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</para>
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</listitem>
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<listitem>
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<para>
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If you are careful you can implement generators in shell
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scripts. We do recommend C code however, since generators
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delay are executed synchronously and hence delay the
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entire boot if they are slow.
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</para>
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</listitem>
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<listitem>
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<para>Regarding overriding semantics: there are two rules we
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try to follow when thinking about the overriding semantics:
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</para>
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<orderedlist numeration="lowerroman">
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<listitem>
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<para>User configuration should override vendor
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configuration. This (mostly) means that stuff from
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<filename>/etc</filename> should override stuff from
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<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
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configuration. This (mostly) means that stuff you
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generate should never override native unit files for the
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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
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important one and breaks the second one sometimes. Hence,
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when deciding whether to user argv[1], argv[2], or argv[3],
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your default choice should probably be argv[1].</para>
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</listitem>
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<listitem>
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<para>
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Instead of heading off now and writing all kind of
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generators for legacy configuration file formats, please
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think twice! It's often a better idea to just deprecate
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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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</refsect2>
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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
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units. It uses argv[1] as location to place the generated unit
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files in order to allow the user to override
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<filename>/etc/fstab</filename> with her own native unit files,
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but also to ensure that <filename>/etc/fstab</filename>
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overrides any vendor default from <filename>/usr</filename>.
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</para>
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<para>After editing <filename>/etc/fstab</filename>, the user
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should invoke <command>systemctl daemon-reload</command>. This
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will re-run all generators and cause <command>systemd</command>
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to reload units from disk. To actually mount new directories
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added to <filename>fstab</filename>, <command>systemctl start
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<replaceable>/path/to/mountpoint</replaceable></command> or
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<command>systemctl start local-fs.target</command> may be used.
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</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
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configuration for <filename>default.target</filename> it uses
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argv[2]. For details about this logic, see
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<ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates">Implementing
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Offline System Updates</ulink>.</para>
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</example>
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<example>
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<title>Debuging 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-efi-boot-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-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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</para>
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</refsect1>
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</refentry>
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