Adding upstream version 252.22.upstream/252.22upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 359 insertions, 0 deletions

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
+<!ENTITY % entities SYSTEM "custom-entities.ent" >
+%entities;
+]>
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="systemd.generator">
+  <refentryinfo>
+    <title>systemd.generator</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>systemd.generator</refentrytitle>
+    <manvolnum>7</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>systemd.generator</refname>
+    <refpurpose>systemd unit generators</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <cmdsynopsis>
+      <command index='false'>/path/to/generator</command>
+      <arg choice="plain"><replaceable>normal-dir</replaceable></arg>
+      <arg choice="option"><replaceable>early-dir</replaceable></arg>
+      <arg choice="option"><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>&SYSTEM_GENERATOR_DIR;/*</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>&USER_GENERATOR_DIR;/*</filename></literallayout>
+    </para>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+    <para>Generators are small executables placed in <filename>&SYSTEM_GENERATOR_DIR;/</filename> and other
+    directories listed above.
+    <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will execute
+    these binaries very early at bootup and at configuration reload time — before unit files are
+    loaded. Their main purpose is to convert configuration and execution context parameters that are not
+    native to the service manager into dynamically generated unit files, symlinks or unit file drop-ins, so
+    that they can extend the unit file hierarchy the service manager subsequently loads and operates
+    on.</para>
+
+    <para><command>systemd</command> will call each generator with three directory paths that are to be used
+    for generator output. In these three directories, generators may dynamically generate unit files (regular
+    ones, instances, as well as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic
+    links to unit files to add additional dependencies, create aliases, or instantiate existing templates.
+    Those directories are included in the unit load path, allowing generated configuration to extend or
+    override existing definitions. For tests, generators may be called with just one argument; the generator
+    should assume that all three paths are the same in that case.</para>
+
+    <para>Directory paths for generator output differ by priority: <filename>…/generator.early</filename> has
+    priority higher than the admin configuration in <filename>/etc/</filename>, while
+    <filename>…/generator</filename> has lower priority than <filename>/etc/</filename> but higher than
+    vendor configuration in <filename>/usr/</filename>, and <filename>…/generator.late</filename> has
+    priority lower than all other configuration. See the next section and the discussion of unit load paths
+    and unit overriding in
+    <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
+    </para>
+
+    <para>Generators are loaded from a set of paths determined during compilation, as 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>Output directories</title>
+
+    <para>Generators are invoked with three arguments: paths to directories where generators can place their
+    generated unit files or symlinks. By default those paths are runtime directories that are included in the
+    search path of <command>systemd</command>, but a generator may be called with different paths for
+    debugging purposes. If only one argument is provided, the generator should use the same directory as the
+    the three output paths.</para>
+
+    <orderedlist>
+      <listitem>
+        <para><parameter>normal-dir</parameter></para>
+        <para>In normal use this is <filename>/run/systemd/generator</filename> in case of the system
+        generators and <filename>$XDG_RUNTIME_DIR/systemd/generator</filename> in case of the user
+        generators. Unit files placed in this directory take precedence over vendor unit configuration but
+        not over native user/administrator unit configuration.
+        </para>
+      </listitem>
+
+      <listitem>
+        <para><parameter>early-dir</parameter></para>
+        <para>In normal use this is <filename>/run/systemd/generator.early</filename> in case of the system
+        generators and <filename>$XDG_RUNTIME_DIR/systemd/generator.early</filename> in case of the user
+        generators. Unit files placed in this directory override unit files in <filename>/usr/</filename>,
+        <filename>/run/</filename> and <filename>/etc/</filename>. This means that unit files placed in this
+        directory take precedence over all normal configuration, both vendor and user/administrator.</para>
+      </listitem>
+
+      <listitem>
+        <para><parameter>late-dir</parameter></para>
+        <para>In normal use this is <filename>/run/systemd/generator.late</filename> in case of the system
+        generators and <filename>$XDG_RUNTIME_DIR/systemd/generator.late</filename> in case of the user
+        generators. This directory may be used to extend the unit file tree without overriding any other unit
+        files. Any native configuration files supplied by the vendor or user/administrator take
+        precedence.</para>
+      </listitem>
+    </orderedlist>
+  </refsect1>
+
+  <refsect1>
+    <title>Environment</title>
+
+    <para>The service manager sets a number of environment variables when invoking generator
+    executables. They carry information about the execution context of the generator, in order to simplify
+    conditionalizing generators to specific environments. The following environment variables are set:</para>
+
+    <variablelist class='environment-variables'>
+      <varlistentry>
+        <term><varname>$SYSTEMD_SCOPE</varname></term>
+
+        <listitem><para>If the generator is invoked from the system service manager this variable is set to
+        <literal>system</literal>; if invoked from the per-user service manager it is set to
+        <literal>user</literal>.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>$SYSTEMD_IN_INITRD</varname></term>
+
+        <listitem><para>If the generator is run as part of an initrd this is set to <literal>1</literal>. If
+        it is run from the regular host (i.e. after the transition from initrd to host) it is set to
+        <literal>0</literal>. This environment variable is only set for system generators.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>$SYSTEMD_FIRST_BOOT</varname></term>
+
+        <listitem><para>If this boot-up cycle is considered a "first boot", this is set to
+        <literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
+        details see the documentation of <varname>ConditionFirstBoot=</varname> in
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
+        environment variable is only set for system generators.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>
+
+        <listitem><para>If the service manager is run in a virtualized environment,
+        <varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
+        first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
+        of virtualization. The second string identifies the implementation of the virtualization
+        technology. If no virtualization is detected this variable will not be set. This data is identical to
+        what
+        <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+        detects and reports, and uses the same vocabulary of virtualization implementation
+        identifiers.</para></listitem>
+      </varlistentry>
+
+      <varlistentry>
+        <term><varname>$SYSTEMD_ARCHITECTURE</varname></term>
+
+        <listitem><para>This variable is set to a short identifier of the reported architecture of the
+        system. For details about defined values, see documentation of
+        <varname>ConditionArchitecture=</varname> in
+        <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Notes about writing generators</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>)!
+        Non-essential file systems like <filename>/var/</filename> and <filename>/home/</filename> are
+        mounted after generators have run. Generators can however rely on the most basic kernel functionality
+        to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
+        <filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
+        </para>
+      </listitem>
+
+      <listitem>
+        <para>Units written by generators are removed when the 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, <filename>.d/*.conf</filename> drop-ins
+        for them and symlinks to them, not any other kind of non-unit related 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>
+
+        <para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
+        settings of service unit files (see
+        <citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
+        is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
+        definitions, which often might be sufficient to embed data or configuration for other programs into
+        unit files in a native fashion.</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>The generator should always include its own name in a comment at the top of the generated file,
+        so that the user can easily figure out which component created or amended a particular unit.</para>
+
+        <para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
+        source configuration file they are generated 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. The <varname>SourcePath=</varname> value does not have
+        to be a file in a physical filesystem. For example, in the common case of the generator looking at
+        the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</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 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 use 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 is often a better idea to just deprecate old stuff instead of keeping
+        it artificially alive.
+        </para>
+      </listitem>
+    </itemizedlist>
+  </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
+      their 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
+      <citerefentry><refentrytitle>systemd.offline-updates</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
+      </para>
+    </example>
+
+    <example>
+      <title>Debugging a generator</title>
+
+      <programlisting>dir=$(mktemp -d)
+SYSTEMD_LOG_LEVEL=debug &SYSTEM_GENERATOR_DIR;/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-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-rc-local-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-xdg-autostart-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>systemd.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>