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man: add details on overriding top level drop-ins

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When using top level drop-ins it isn't immediately obvious that one can
make use of symlinking to disable a top-level drop in for a specific
unit.

Signed-off-by: Peter Morrow <pemorrow@linux.microsoft.com>
This commit is contained in:
Peter Morrow 2021-06-07 13:05:06 +01:00 committed by Zbigniew Jędrzejewski-Szmek
parent d477a094e8
commit 90a404f5d4
1 changed files with 62 additions and 4 deletions
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66
man/systemd.unit.xml
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@ -225,10 +225,12 @@
where <replaceable>type</replaceable> may be e.g. <literal>service</literal> or <literal>socket</literal>,
that allows altering or adding to the settings of all corresponding unit files on the system.
The formatting and precedence of applying drop-in configurations follow what is defined above.
Configurations in <filename><replaceable>type</replaceable>.d/</filename> have the lowest precedence
compared to settings in the name specific override directories. So the contents of
<filename>foo-.service.d/10-override.conf</filename> would override
<filename>service.d/10-override.conf</filename>.</para>
Files in <filename><replaceable>type</replaceable>.d/</filename> have lower precedence compared
to files in name-specific override directories. The usual rules apply: multiple drop-in files
with different names are applied in lexicographic order, regardless of which of the directories
they reside in, so a file in <filename><replaceable>type</replaceable>.d/</filename> applies
to a unit only if there are no drop-ins or masks with that name in directories with higher
precedence. See Examples.</para>
<para>Note that while systemd offers a flexible dependency system
between units it is recommended to use this functionality only
@ -2201,6 +2203,62 @@ PrivateTmp=yes</programlisting>
to override the entire unit.</para>
</example>
<example>
<title>Top level drop-ins with template units</title>
<para>Top level drop-ins can be used to change some aspect of all units
of a particular type. For example by creating the
<filename index='false'>/etc/systemd/system/service.d</filename>
directory with a drop-in file, the contents of the drop-in file can be
applied to all service units. We can take this further by having the
top-level drop-in instantiate a secondary helper unit. Consider for
example the following set of units and drop-in files where we install
an OnFailure dependency for all service units.</para>
<para>
<filename index='false'>/etc/systemd/system/failure-handler@.service</filename>:</para>
<programlisting>[Unit]
Description=My failure handler for %i
[Service]
Type=oneshot
# Perform some special action for when %i exits unexpectedly.
ExecStart=/usr/sbin/myfailurehandler %i
</programlisting>
<para>We can then add an instance of the
<filename index='false'>failure-handler@.service</filename> as an
<varname>OnFailure=</varname> dependency for all service units.</para>
<para>
<filename index='false'>/etc/systemd/system/service.d/10-all.conf</filename>:</para>
<programlisting>[Unit]
OnFailure=failure-handler@%n.service
</programlisting>
<para>Now, after running <command>systemctl daemon-reload</command> all
services will have acquired an <varname>OnFailure=</varname> dependency on
<filename index='false'>failure-handler@%n.service</filename>. The
template instance units will also have gained the dependency which results
in the creation of a recursive dependency chain. We can break the chain by
disabling the drop-in for the template instance units via a symlink to
<filename index='false'>/dev/null</filename>:</para>
<programlisting>
<command>mkdir /etc/systemd/system/failure-handler@.service.d</command>
<command>ln -s /dev/null /etc/systemd/system/failure-handler@.service.d/10-all.conf</command>
<command>systemctl daemon-reload</command>
</programlisting>
<para>This ensures that if a <filename index='false'>failure-handler@.service</filename> instance fails it will not trigger an instance named
<filename index='false'>failure-handler@failure-handler.service.service</filename>.</para>
</example>
</refsect1>
<refsect1>