systemd/man/systemd.generator.xml
Zbigniew Jędrzejewski-Szmek 89fc708c0a man: clarify that generators are run before /var
Just a clarification. At least systemd-openqa-generator from
openqa gets this wrong.
2016-02-06 17:54:48 -05:00

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Copyright 2015 Zbigniew Jędrzejewski-Szmek
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<refentry id="systemd.generator">
<refentryinfo>
<title>systemd.generator</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>systemd.generator</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.generator</refname>
<refpurpose>Systemd unit generators</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>/path/to/generator</command>
<arg choice="plain"><replaceable>normal-dir</replaceable></arg>
<arg choice="plain"><replaceable>early-dir</replaceable></arg>
<arg choice="plain"><replaceable>late-dir</replaceable></arg>
</cmdsynopsis>
<para>
<literallayout><filename>/run/systemd/system-generators/*</filename>
<filename>/etc/systemd/system-generators/*</filename>
<filename>/usr/local/lib/systemd/system-generators/*</filename>
<filename>&systemgeneratordir;/*</filename></literallayout>
</para>
<para>
<literallayout><filename>/run/systemd/user-generators/*</filename>
<filename>/etc/systemd/user-generators/*</filename>
<filename>/usr/local/lib/systemd/user-generators/*</filename>
<filename>&usergeneratordir;/*</filename></literallayout>
</para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>Generators are small binaries that live in
<filename>&usergeneratordir;/</filename> and other directories
listed above.
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
will execute those binaries very early at bootup and at
configuration reload time — before unit files are loaded.
Generators can dynamically generate unit files or create symbolic
links to unit files to add additional dependencies, thus extending
or overriding existing definitions. Their main purpose is to
convert configuration files that are not native unit files
dynamically into native unit files.</para>
<para>Generators are loaded from a set of paths determined during
compilation, 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>Writing generators</title>
<para>Generators are invoked with three arguments: paths to
runtime directories where generators can place their generated
unit files or symlinks.</para>
<orderedlist>
<listitem>
<para><parameter>normal-dir</parameter></para>
<para>argv[1] may be used to override unit files in
<filename>/usr</filename>, but not those in
<filename>/etc</filename>. This means that unit files placed
in this directory take precedence over vendor unit
configuration but not over native user/administrator unit
configuration.</para>
</listitem>
<listitem>
<para><parameter>early-dir</parameter></para>
<para>argv[2] may be used to override unit files in
<filename>/usr</filename> and in
<filename>/etc</filename>. This means that unit files placed
in this directory take precedence over all configuration,
both vendor and user/administrator.</para>
</listitem>
<listitem>
<para><parameter>late-dir</parameter></para>
<para>argv[3] may be used to extend the unit file tree without
overriding any other unit files. Any native configuration
files supplied by the vendor or user/administrator take
precedence over the generated ones placed in this directory.
</para>
</listitem>
</orderedlist>
<refsect2>
<title>Notes</title>
<itemizedlist>
<listitem>
<para>
All generators are executed in parallel. That means all
executables are started at the very same time and need to
be able to cope with this parallelism.
</para>
</listitem>
<listitem>
<para>
Generators are run very early at boot and cannot rely on
any external services. They may not talk to any other
process. That includes simple things such as logging to
<citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
or <command>systemd</command> itself (this means: no
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>)!.
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, including a mounted <filename>/sys</filename>,
<filename>/proc</filename>, <filename>/dev</filename>,
<filename>/usr</filename>.
</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, not
any other kind of configuration. Due to the lifecycle
logic mentioned above, generators are not a good fit to
generate dynamic configuration for other services. If you
need to generate dynamic configuration for other services,
do so in normal services you order before the service in
question.
</para>
</listitem>
<listitem>
<para>
Since
<citerefentry project='man-pages'><refentrytitle>syslog</refentrytitle><manvolnum>3</manvolnum></citerefentry>
is not available (see above), log messages have to be
written to <filename>/dev/kmsg</filename> instead.
</para>
</listitem>
<listitem>
<para>
It is a good idea to use the
<varname>SourcePath=</varname> directive in generated unit
files to specify the source configuration file you are
generating the unit from. This makes things more easily
understood by the user and also has the benefit that
systemd can warn the user about configuration files that
changed on disk but have not been read yet by systemd.
</para>
</listitem>
<listitem>
<para>
Generators may write out dynamic unit files or just hook
unit files into other units with the usual
<filename>.wants/</filename> or
<filename>.requires/</filename> symlinks. Often, it is
nicer to simply instantiate a template unit file from
<filename>/usr</filename> with a generator instead of
writing out entirely dynamic unit files. Of course, this
works only if a single parameter is to be used.
</para>
</listitem>
<listitem>
<para>
If you are careful, you can implement generators in shell
scripts. We do recommend C code however, since generators
are executed synchronously and hence delay the
entire boot if they are slow.
</para>
</listitem>
<listitem>
<para>Regarding overriding semantics: there are two rules we
try to follow when thinking about the overriding semantics:
</para>
<orderedlist numeration="lowerroman">
<listitem>
<para>User configuration should override vendor
configuration. This (mostly) means that stuff from
<filename>/etc</filename> should override stuff from
<filename>/usr</filename>.</para>
</listitem>
<listitem>
<para>Native configuration should override non-native
configuration. This (mostly) means that stuff you
generate should never override native unit files for the
same purpose.</para>
</listitem>
</orderedlist>
<para>Of these two rules the first rule is probably the more
important one and breaks the second one sometimes. Hence,
when deciding whether to user argv[1], argv[2], or argv[3],
your default choice should probably be argv[1].</para>
</listitem>
<listitem>
<para>
Instead of heading off now and writing all kind of
generators for legacy configuration file formats, please
think twice! It is often a better idea to just deprecate
old stuff instead of keeping it artificially alive.
</para>
</listitem>
</itemizedlist>
</refsect2>
</refsect1>
<refsect1>
<title>Examples</title>
<example>
<title>systemd-fstab-generator</title>
<para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
converts <filename>/etc/fstab</filename> into native mount
units. It uses argv[1] as location to place the generated unit
files in order to allow the user to override
<filename>/etc/fstab</filename> with her own native unit files,
but also to ensure that <filename>/etc/fstab</filename>
overrides any vendor default from <filename>/usr</filename>.
</para>
<para>After editing <filename>/etc/fstab</filename>, the user
should invoke <command>systemctl daemon-reload</command>. This
will re-run all generators and cause <command>systemd</command>
to reload units from disk. To actually mount new directories
added to <filename>fstab</filename>, <command>systemctl start
<replaceable>/path/to/mountpoint</replaceable></command> or
<command>systemctl start local-fs.target</command> may be used.
</para>
</example>
<example>
<title>systemd-system-update-generator</title>
<para><citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
temporarily redirects <filename>default.target</filename> to
<filename>system-update.target</filename> if a system update is
scheduled. Since this needs to override the default user
configuration for <filename>default.target</filename>, it uses
argv[2]. For details about this logic, see
<ulink url="http://www.freedesktop.org/wiki/Software/systemd/SystemUpdates">Implementing
Offline System Updates</ulink>.</para>
</example>
<example>
<title>Debugging a generator</title>
<programlisting>dir=$(mktemp -d)
SYSTEMD_LOG_LEVEL=debug &systemgeneratordir;/systemd-fstab-generator \
"$dir" "$dir" "$dir"
find $dir</programlisting>
</example>
</refsect1>
<refsect1>
<title>See also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-cryptsetup-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-debug-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-getty-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-hibernate-resume-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-system-update-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-sysv-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>