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17021368fc
Since the general generator logic was established in the rewrite in
07719a21b6, generators would always write to /tmp
by default. I think this not a good default at all, because generators write a
bunch of files and would create a mess in /tmp. And for debugging, one
generally needs to remove all the files in the output directory, because
generators will complain in the output paths are already present. Thus the
approach of disabling console logging and writing many files to /tmp when
invoked with no arguments is not nice, so let's disallow operation with no
args.
But when debugging, one generally does not care about the separate output dirs
(most generators use only one). Thus the general pattern I use is something
like:
rm -rf /tmp/x && mkdir /tmp/x && build/some-generator /tmp/{x,x,x}
This commit allows only one directory to be specified and simplifies this to:
rm -rf /tmp/x && mkdir /tmp/x && build/some-generator /tmp/x
361 lines
19 KiB
XML
361 lines
19 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//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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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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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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</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 index='false'>/path/to/generator</command>
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<arg choice="plain"><replaceable>normal-dir</replaceable></arg>
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<arg choice="option"><replaceable>early-dir</replaceable></arg>
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<arg choice="option"><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>&SYSTEM_GENERATOR_DIR;/*</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>&USER_GENERATOR_DIR;/*</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 placed in <filename>&SYSTEM_GENERATOR_DIR;/</filename> and other
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directories listed above.
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> will execute
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these binaries very early at bootup and at configuration reload time — before unit files are
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loaded. Their main purpose is to convert configuration and execution context parameters that are not
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native to the service manager into dynamically generated unit files, symlinks or unit file drop-ins, so
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that they can extend the unit file hierarchy the service manager subsequently loads and operates
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on.</para>
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<para><command>systemd</command> will call each generator with three directory paths that are to be used
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for generator output. In these three directories, generators may dynamically generate unit files (regular
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ones, instances, as well as templates), unit file <filename>.d/</filename> drop-ins, and create symbolic
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links to unit files to add additional dependencies, create aliases, or instantiate existing templates.
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Those directories are included in the unit load path, allowing generated configuration to extend or
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override existing definitions. For tests, generators may be called with just one argument; the generator
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should assume that all three paths are the same in that case.</para>
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<para>Directory paths for generator output differ by priority: <filename>…/generator.early</filename> has
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priority higher than the admin configuration in <filename>/etc/</filename>, while
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<filename>…/generator</filename> has lower priority than <filename>/etc/</filename> but higher than
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vendor configuration in <filename>/usr/</filename>, and <filename>…/generator.late</filename> has
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priority lower than all other configuration. See the next section and the discussion of unit load paths
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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 compilation, as listed above. System
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and user generators are loaded from directories with names ending in
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<filename>system-generators/</filename> and <filename>user-generators/</filename>,
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respectively. Generators found in directories listed earlier override the ones with the same name in
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directories lower in the list. A symlink to <filename>/dev/null</filename> or an empty file can be used
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to mask a generator, thereby preventing it from running. Please note that the order of the two
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directories with the highest priority is reversed with respect to the unit load path, and generators in
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<filename>/run/</filename> overwrite those in <filename>/etc/</filename>.</para>
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<para>After installing new generators or updating the configuration, <command>systemctl
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daemon-reload</command> may be executed. This will delete the previous configuration created by
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generators, re-run all generators, and cause <command>systemd</command> to reload units from disk. See
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<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for more
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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 generators can place their
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generated unit files or symlinks. By default those paths are runtime directories that are included in the
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search path of <command>systemd</command>, but a generator may be called with different paths for
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debugging purposes. If only one argument is provided, the generator should use the same directory as the
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the three output paths.</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 case of the system
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generators and <filename>$XDG_RUNTIME_DIR/generator</filename> in case of the user generators. Unit
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files placed in this directory take precedence over vendor unit configuration but not over native
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user/administrator unit configuration.
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</para>
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</listitem>
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<listitem>
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<para><parameter>early-dir</parameter></para>
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<para>In normal use this is <filename>/run/systemd/generator.early</filename> in case of the system
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generators and <filename>$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 <filename>/usr/</filename>,
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<filename>/run/</filename> and <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 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> in case of the system
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generators and <filename>$XDG_RUNTIME_DIR/generator.late</filename> in case of the user
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generators. This directory may be used to extend the unit file tree without overriding any other unit
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files. Any native configuration files supplied by the vendor or user/administrator take
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precedence.</para>
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</listitem>
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</orderedlist>
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</refsect1>
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<refsect1>
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<title>Environment</title>
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<para>The service manager sets a number of environment variables when invoking generator
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executables. They carry information about the execution context of the generator, in order to simplify
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conditionalizing generators to specific environments. The following environment variables are set:</para>
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<variablelist class='environment-variables'>
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<varlistentry>
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<term><varname>$SYSTEMD_SCOPE</varname></term>
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<listitem><para>If the generator is invoked from the system service manager this variable is set to
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<literal>system</literal>; if invoked from the per-user service manager it is set to
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<literal>user</literal>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>$SYSTEMD_IN_INITRD</varname></term>
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<listitem><para>If the generator is run as part of an initial RAM file system (initrd) this is set to
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<literal>1</literal>. If it is run from the regular host (i.e. after the transition from initrd to
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host) it is set to <literal>0</literal>. This environment variable is only set for system
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generators.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>$SYSTEMD_FIRST_BOOT</varname></term>
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<listitem><para>If this boot-up cycle is considered a "first boot", this is set to
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<literal>1</literal>; if it is a subsequent, regular boot it is set to <literal>0</literal>. For
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details see the documentation of <varname>ConditionFirstBoot=</varname> in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>. This
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environment variable is only set for system generators.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>$SYSTEMD_VIRTUALIZATION</varname></term>
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<listitem><para>If the service manager is run in a virtualized environment,
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<varname>$SYSTEMD_VIRTUALIZATION</varname> is set to a pair of strings, separated by a colon. The
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first string is either <literal>vm</literal> or <literal>container</literal>, categorizing the type
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of virtualization. The second string identifies the implementation of the virtualization
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technology. If no virtualization is detected this variable will not be set. This data is identical to
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what
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<citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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detects and reports, and uses the same vocabulary of virtualization implementation
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identifiers.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><varname>$SYSTEMD_ARCHITECTURE</varname></term>
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<listitem><para>This variable is set to a short identifier of the reported architecture of the
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system. For details about defined values, see documentation of
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<varname>ConditionArchitecture=</varname> in
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<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para></listitem>
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</varlistentry>
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</variablelist>
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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 started at the very
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same time and need to 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>Generators are run very early at boot and cannot rely on any external services. They may not
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talk to any other process. That includes simple things such as logging to <citerefentry
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project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>, or
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<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 <filename>/home/</filename> are
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mounted after generators have run. Generators can however rely on the most basic kernel functionality
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to be available, as well as mounted <filename>/sys/</filename>, <filename>/proc/</filename>,
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<filename>/dev/</filename>, <filename>/usr/</filename> and <filename>/run/</filename> file systems.
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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 reloaded. That means the
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lifetime of the generated units is closely bound to the reload cycles of <command>systemd</command>
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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, <filename>.d/*.conf</filename> drop-ins
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for them and symlinks to them, not any other kind of non-unit related configuration. Due to the
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lifecycle logic mentioned above, generators are not a good fit to generate dynamic configuration for
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other services. If you need to generate dynamic configuration for other services, do so in normal
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services you order before the service in question.</para>
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<para>Note that using the <varname>StandardInputData=</varname>/<varname>StandardInputText=</varname>
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settings of service unit files (see
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<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>), it
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is possible to make arbitrary input data (including daemon-specific configuration) part of the unit
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definitions, which often might be sufficient to embed data or configuration for other programs into
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unit files in a native fashion.</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 <filename>/dev/kmsg</filename>
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instead.</para>
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</listitem>
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<listitem>
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<para>The generator should always include its own name in a comment at the top of the generated file,
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so that the user can easily figure out which component created or amended a particular unit.</para>
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<para>The <varname>SourcePath=</varname> directive should be used in generated files to specify the
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source configuration file they are generated 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 configuration files that changed
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on disk but have not been read yet by systemd. The <varname>SourcePath=</varname> value does not have
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to be a file in a physical filesystem. For example, in the common case of the generator looking at
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the kernel command line, <option>SourcePath=/proc/cmdline</option> should be used.</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 into other units with the
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usual <filename>.wants/</filename> or <filename>.requires/</filename> symlinks. Often, it is nicer to
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simply instantiate a template unit file from <filename>/usr/</filename> with a generator instead of
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writing out entirely dynamic unit files. Of course, this works only if a single parameter is to be
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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 do recommend C code
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however, since generators are executed synchronously and hence delay the entire boot if they are
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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 when thinking about the
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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 (mostly) means that stuff
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from <filename>/etc/</filename> should override 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 (mostly) means that
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stuff you generate should never override native unit 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 and breaks the second one
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sometimes. Hence, when deciding whether to use argv[1], argv[2], or argv[3], your default choice
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should probably be 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 legacy configuration file
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formats, please think twice! It is often a better idea to just deprecate old stuff instead of keeping
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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 argv[1] as location to place
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the generated unit files in order to allow the user to override <filename>/etc/fstab</filename> with
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their own native unit 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 <command>systemctl
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daemon-reload</command>. This will re-run all generators and cause <command>systemd</command> to reload
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units from disk. To actually mount new directories added to <filename>fstab</filename>,
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<command>systemctl start <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 <filename>system-update.target</filename>,
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if a system update is 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 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 &SYSTEM_GENERATOR_DIR;/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-xdg-autostart-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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