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man: document alias rules and aliases dropin loading

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Zbigniew Jędrzejewski-Szmek 2020-01-09 12:38:18 +01:00
parent 1bf1558552
commit 2e93770fd8
1 changed files with 29 additions and 5 deletions
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34
man/systemd.unit.xml
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@ -136,11 +136,25 @@
has the alias <filename>dbus-org.freedesktop.network1.service</filename>, created during installation as
a symlink, so when <command>systemd</command> is asked through D-Bus to load
<filename>dbus-org.freedesktop.network1.service</filename>, it'll load
<filename>systemd-networkd.service</filename>. Alias names may be used in commands like
<command>disable</command>, <command>start</command>, <command>stop</command>, <command>status</command>,
and similar, and in all unit dependency directives, including <varname>Wants=</varname>,
<varname>Requires=</varname>, <varname>Before=</varname>, <varname>After=</varname>. Aliases cannot be
used with the <command>preset</command> command.</para>
<filename>systemd-networkd.service</filename>. As another example, <filename>default.target</filename> —
the default system target started at boot — is commonly symlinked (aliased) to either
<filename>multi-user.target</filename> or <filename>graphical.target</filename> to select what is started
by default. Alias names may be used in commands like <command>disable</command>,
<command>start</command>, <command>stop</command>, <command>status</command>, and similar, and in all
unit dependency directives, including <varname>Wants=</varname>, <varname>Requires=</varname>,
<varname>Before=</varname>, <varname>After=</varname>. Aliases cannot be used with the
<command>preset</command> command.</para>
<para>Aliases obey the following restrictions: a unit of a certain type (<literal>.service</literal>,
<literal>.socket</literal>, …) can only be aliased by a name with the same type suffix. A plain unit (not
a template or an instance), may only be aliased by a plain name. A template instance may only be aliased
by another template instance, and the instance part must be identical. A template may be aliased by
another template (in which case the alias applies to all instances of the template). As a special case, a
template instance (e.g. <literal>alias@inst.service</literal>) may be a symlink to different template
(e.g. <literal>template@inst.service</literal>). In that case, just this specific instance is aliased,
while other instances of the template (e.g. <literal>alias@foo.service</literal>,
<literal>alias@bar.service</literal>) are not aliased. Those rule preserve the requirement that the
instance (if any) is always uniquely defined for a given unit and all its aliases.</para>
<para>Unit files may specify aliases through the <varname>Alias=</varname> directive in the [Install]
section. When the unit is enabled, symlinks will be created for those names, and removed when the unit is
@ -184,6 +198,16 @@
i.e. <filename>foo-bar-.service.d/10-override.conf</filename> overrides
<filename>foo-.service.d/10-override.conf</filename>.</para>
<para>In cases of unit aliases (described above), dropins for the aliased name and all aliases are
loaded. In the example of <filename>default.target</filename> aliasing
<filename>graphical.target</filename>, <filename>default.target.d/</filename>,
<filename>default.target.wants/</filename>, <filename>default.target.requires/</filename>,
<filename>graphical.target.d/</filename>, <filename>graphical.target.wants/</filename>,
<filename>graphical.target.requires/</filename> would all be read. For templates, dropins for the
template, any template aliases, the template instance, and all alias instances are read. When just a
specific template instance is aliased, then the dropins for the target template, the target template
instance, and the alias template instance are read.</para>
<para>In addition to <filename>/etc/systemd/system</filename>, the drop-in <literal>.d/</literal>
directories for system services can be placed in <filename>/usr/lib/systemd/system</filename> or
<filename>/run/systemd/system</filename> directories. Drop-in files in <filename>/etc</filename>