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man: describe unit templating explicitly 

This patch is heavily based on the text suggested by
archenemies in #3791.

Fixes #3791.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2018-04-16 17:57:37 +02:00
parent 2116134b04
commit 75695fb798
2 changed files with 31 additions and 15 deletions
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14
man/systemd.service.xml
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@ -77,6 +77,20 @@
with SysV</ulink> document.</para>
</refsect1>
<refsect1>
<title>Service Templates</title>
<para>It is possible for <command>systemd</command> services to take a single argument via the
<literal><replaceable>service</replaceable>@<replaceable>argument</replaceable>.service</literal>
syntax. Such services are called "instantiated" services, while the unit definition without the
<replaceable>argument</replaceable> parameter is called a "template". An example could be a
<filename>dhcpcd@.service</filename> service template which takes a network interface as a
parameter to form an instantiated service. Within the service file, this parameter or "instance
name" can be accessed with %-specifiers. See
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details.</para>
</refsect1>
<refsect1>
<title>Implicit Dependencies</title>

32
man/systemd.unit.xml
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@ -115,6 +115,14 @@
<para>Unit files are loaded from a set of paths determined during
compilation, described in the next section.</para>
<para>Unit files can be parameterized by a single argument called the "instance name". The unit
is then constructed based on a "template file" which serves as the definition of multiple
services or other units. A template unit must have a single <literal>@</literal> at the end of
the name (right before the type suffix). The name of the full unit is formed by inserting the
instance name between <literal>@</literal> and the unit type suffix. In the unit file itself,
the instance parameter may be referred to using <literal>%i</literal> and other specifiers, see
below.</para>
<para>Unit files may contain additional options on top of those
listed here. If systemd encounters an unknown option, it will
write a warning log message but continue loading the unit. If an
@ -204,21 +212,15 @@
socket-based activation which make dependencies implicit,
resulting in a both simpler and more flexible system.</para>
<para>Optionally, units may be instantiated from a
template file at runtime. This allows creation of
multiple units from a single configuration file. If
systemd looks for a unit configuration file, it will
first search for the literal unit name in the
file system. If that yields no success and the unit
name contains an <literal>@</literal> character, systemd will look for a
unit template that shares the same name but with the
instance string (i.e. the part between the <literal>@</literal> character
and the suffix) removed. Example: if a service
<filename>getty@tty3.service</filename> is requested
and no file by that name is found, systemd will look
for <filename>getty@.service</filename> and
instantiate a service from that configuration file if
it is found.</para>
<para>As mentioned above, a unit may be instantiated from a template file. This allows creation
of multiple units from a single configuration file. If systemd looks for a unit configuration
file, it will first search for the literal unit name in the file system. If that yields no
success and the unit name contains an <literal>@</literal> character, systemd will look for a
unit template that shares the same name but with the instance string (i.e. the part between the
<literal>@</literal> character and the suffix) removed. Example: if a service
<filename>getty@tty3.service</filename> is requested and no file by that name is found, systemd
will look for <filename>getty@.service</filename> and instantiate a service from that
configuration file if it is found.</para>
<para>To refer to the instance string from within the
configuration file you may use the special <literal>%i</literal>