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systemd-stable/man/systemd.unit.xml
Zbigniew Jędrzejewski-Szmek 0f943ae4ea man: add a new page with a general description of common syntax 
We have a common parser, but for the user it might be
completely unobvious that the same general rules apply
to all those files. Let's add a page about the basic syntax
so that the more specific pages don't have to repeat those
details.
2018-04-18 09:11:01 +02:00

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<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY % entities SYSTEM "custom-entities.ent" >
%entities;
]>
<!--
SPDX-License-Identifier: LGPL-2.1+
This file is part of systemd.
Copyright 2010 Lennart Poettering
-->
<refentry id="systemd.unit">
<refentryinfo>
<title>systemd.unit</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.unit</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.unit</refname>
<refpurpose>Unit configuration</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename><replaceable>service</replaceable>.service</filename>,
<filename><replaceable>socket</replaceable>.socket</filename>,
<filename><replaceable>device</replaceable>.device</filename>,
<filename><replaceable>mount</replaceable>.mount</filename>,
<filename><replaceable>automount</replaceable>.automount</filename>,
<filename><replaceable>swap</replaceable>.swap</filename>,
<filename><replaceable>target</replaceable>.target</filename>,
<filename><replaceable>path</replaceable>.path</filename>,
<filename><replaceable>timer</replaceable>.timer</filename>,
<filename><replaceable>slice</replaceable>.slice</filename>,
<filename><replaceable>scope</replaceable>.scope</filename></para>
<para><literallayout><filename>/etc/systemd/system.control/*</filename>
<filename>/run/systemd/system.control/*</filename>
<filename>/run/systemd/transient/*</filename>
<filename>/run/systemd/generator.early/*</filename>
<filename>/etc/systemd/system/*</filename>
<filename>/run/systemd/system/*</filename>
<filename>/run/systemd/generator/*</filename>
<filename>…</filename>
<filename>/usr/lib/systemd/system/*</filename>
<filename>/run/systemd/generator.late/*</filename>
</literallayout></para>
<para><literallayout><filename>~/.config/systemd/user.control/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/user.control/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/transient/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/generator.early/*</filename>
<filename>~/.config/systemd/user/*</filename>
<filename>/etc/systemd/user/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/user/*</filename>
<filename>/run/systemd/user/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/generator/*</filename>
<filename>~/.local/share/systemd/user/*</filename>
<filename>…</filename>
<filename>/usr/lib/systemd/user/*</filename>
<filename>$XDG_RUNTIME_DIR/systemd/generator.late/*</filename>
</literallayout></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>A unit file is a plain text ini-style file that encodes information about a service, a
socket, a device, a mount point, an automount point, a swap file or partition, a start-up
target, a watched file system path, a timer controlled and supervised by
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, a
resource management slice or a group of externally created processes. See
<citerefentry><refentrytitle>systemd.syntax</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for a general description of the syntax.</para>
<para>This man page lists the common configuration options of all
the unit types. These options need to be configured in the [Unit]
or [Install] sections of the unit files.</para>
<para>In addition to the generic [Unit] and [Install] sections
described here, each unit may have a type-specific section, e.g.
[Service] for a service unit. See the respective man pages for
more information:
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>.
</para>
<para>Unit files are loaded from a set of paths determined during
compilation, described in the next section.</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
option or section name is prefixed with <option>X-</option>, it is
ignored completely by systemd. Options within an ignored section
do not need the prefix. Applications may use this to include
additional information in the unit files.</para>
<para>Boolean arguments used in unit files can be written in
various formats. For positive settings the strings
<option>1</option>, <option>yes</option>, <option>true</option>
and <option>on</option> are equivalent. For negative settings, the
strings <option>0</option>, <option>no</option>,
<option>false</option> and <option>off</option> are
equivalent.</para>
<para>Time span values encoded in unit files can be written in various formats. A stand-alone
number specifies a time in seconds. If suffixed with a time unit, the unit is honored. A
concatenation of multiple values with units is supported, in which case the values are added
up. Example: <literal>50</literal> refers to 50 seconds; <literal>2min 200ms</literal> refers to
2 minutes and 200 milliseconds, i.e. 120200 ms. The following time units are understood:
<literal>s</literal>, <literal>min</literal>, <literal>h</literal>, <literal>d</literal>,
<literal>w</literal>, <literal>ms</literal>, <literal>us</literal>. For details see
<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
<para>Units can be aliased (have an alternative name), by creating a symlink from the new name
to the existing name in one of the unit search paths. For example,
<filename>systemd-networkd.service</filename> has the alias
<filename>dbus-org.freedesktop.network1.service</filename>, created during installation as the
symlink <filename>/usr/lib/systemd/system/dbus-org.freedesktop.network1.service</filename>. In
addition, unit files may specify aliases through the <varname>Alias=</varname> directive in the
[Install] section; those aliases are only effective when the unit is enabled. When the unit is
enabled, symlinks will be created for those names, and removed when the unit is disabled. For
example, <filename>reboot.target</filename> specifies
<varname>Alias=ctrl-alt-del.target</varname>, so when enabled it will be invoked whenever
CTRL+ALT+DEL is pressed. Alias names may be used in commands like <command>enable</command>,
<command>disable</command>, <command>start</command>, <command>stop</command>,
<command>status</command>, …, and in unit dependency directives <varname>Wants=</varname>,
<varname>Requires=</varname>, <varname>Before=</varname>, <varname>After=</varname>, …, with the
limitation that aliases specified through <varname>Alias=</varname> are only effective when the
unit is enabled. Aliases cannot be used with the <command>preset</command> command.</para>
<para>Along with a unit file <filename>foo.service</filename>, the directory
<filename>foo.service.wants/</filename> may exist. All unit files symlinked from such a
directory are implicitly added as dependencies of type <varname>Wants=</varname> to the unit.
This is useful to hook units into the start-up of other units, without having to modify their
unit files. For details about the semantics of <varname>Wants=</varname>, see below. The
preferred way to create symlinks in the <filename>.wants/</filename> directory of a unit file is
with the <command>enable</command> command of the
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
tool which reads information from the [Install] section of unit files (see below). A similar
functionality exists for <varname>Requires=</varname> type dependencies as well, the directory
suffix is <filename>.requires/</filename> in this case.</para>
<para>Along with a unit file <filename>foo.service</filename>, a "drop-in" directory
<filename>foo.service.d/</filename> may exist. All files with the suffix <literal>.conf</literal> from this
directory will be parsed after the unit file itself is parsed. This is useful to alter or add configuration
settings for a unit, without having to modify unit files. Drop-in files must contain appropriate section
headers. For instantiated units, this logic will first look for the instance <literal>.d/</literal> subdirectory
(e.g. <literal>foo@bar.service.d/</literal>) and read its <literal>.conf</literal> files, followed by the template
<literal>.d/</literal> subdirectory (e.g. <literal>foo@.service.d/</literal>) and the <literal>.conf</literal>
files there. Moreover for units names containing dashes (<literal>-</literal>), the set of directories generated by
truncating the unit name after all dashes is searched too. Specifically, for a unit name
<filename>foo-bar-baz.service</filename> not only the the regular drop-in directory
<filename>foo-bar-baz.service.d/</filename> is searched but also both <filename>foo-bar-.service.d/</filename> and
<filename>foo-.service.d/</filename>. This is useful for defining common drop-ins for a set of related units, whose
names begin with a common prefix. This scheme is particularly useful for mount, automount and slice units, whose
systematic naming structure is built around dashes as component separators. Note that equally named drop-in files
further down the prefix hierarchy override those further up,
i.e. <filename>foo-bar-.service.d/10-override.conf</filename> overrides
<filename>foo-.service.d/10-override.conf</filename>.</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>
take precedence over those in <filename>/run</filename> which in turn take precedence over those
in <filename>/usr/lib</filename>. Drop-in files under any of these directories take precedence
over unit files wherever located. Multiple drop-in files with different names are applied in
lexicographic order, regardless of which of the directories they reside in.</para>
<!-- Note that we do not document .include here, as we consider it mostly obsolete, and want
people to use .d/ drop-ins instead. -->
<para>Note that while systemd offers a flexible dependency system
between units it is recommended to use this functionality only
sparingly and instead rely on techniques such as bus-based or
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>To refer to the instance string from within the
configuration file you may use the special <literal>%i</literal>
specifier in many of the configuration options. See below for
details.</para>
<para>If a unit file is empty (i.e. has the file size 0) or is
symlinked to <filename>/dev/null</filename>, its configuration
will not be loaded and it appears with a load state of
<literal>masked</literal>, and cannot be activated. Use this as an
effective way to fully disable a unit, making it impossible to
start it even manually.</para>
<para>The unit file format is covered by the
<ulink
url="https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise">Interface
Stability Promise</ulink>.</para>
</refsect1>
<refsect1>
<title>String Escaping for Inclusion in Unit Names</title>
<para>Sometimes it is useful to convert arbitrary strings into unit names. To facilitate this, a method of string
escaping is used, in order to map strings containing arbitrary byte values (except NUL) into valid unit names and
their restricted character set. A common special case are unit names that reflect paths to objects in the file
system hierarchy. Example: a device unit <filename>dev-sda.device</filename> refers to a device with the device
node <filename noindex='true'>/dev/sda</filename> in the file system.</para>
<para>The escaping algorithm operates as follows: given a string, any <literal>/</literal> character is replaced by
<literal>-</literal>, and all other characters which are not ASCII alphanumerics or <literal>_</literal> are
replaced by C-style <literal>\x2d</literal> escapes. In addition, <literal>.</literal> is replaced with such a
C-style escape when it would appear as the first character in the escaped string.</para>
<para>When the input qualifies as absolute file system path, this algorithm is extended slightly: the path to the
root directory <literal>/</literal> is encoded as single dash <literal>-</literal>. In addition, any leading,
trailing or duplicate <literal>/</literal> characters are removed from the string before transformation. Example:
<filename>/foo//bar/baz/</filename> becomes <literal>foo-bar-baz</literal>.</para>
<para>This escaping is fully reversible, as long as it is known whether the escaped string was a path (the
unescaping results are different for paths and non-path strings). The
<citerefentry><refentrytitle>systemd-escape</refentrytitle><manvolnum>1</manvolnum></citerefentry> command may be
used to apply and reverse escaping on arbitrary strings. Use <command>systemd-escape --path</command> to escape
path strings, and <command>systemd-escape</command> without <option>--path</option> otherwise.</para>
</refsect1>
<refsect1>
<title>Implicit Dependencies</title>
<para>A number of unit dependencies are implicitly established,
depending on unit type and unit configuration. These implicit
dependencies can make unit configuration file cleaner. For the
implicit dependencies in each unit type, please refer to
section "Implicit Dependencies" in respective man pages.</para>
<para>For example, service units with <varname>Type=dbus</varname>
automatically acquire dependencies of type <varname>Requires=</varname>
and <varname>After=</varname> on <filename>dbus.socket</filename>. See
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details.</para>
</refsect1>
<refsect1>
<title>Default Dependencies</title>
<para>Default dependencies are similar to implicit dependencies,
but can be turned on and off by setting
<varname>DefaultDependencies=</varname> to <varname>yes</varname>
(the default) and <varname>no</varname>, while implicit dependencies
are always in effect. See section "Default Dependencies" in respective
man pages for the effect of enabling
<varname>DefaultDependencies=</varname> in each unit types.</para>
<para>For example, target units will complement all configured
dependencies of type <varname>Wants=</varname> or
<varname>Requires=</varname> with dependencies of type
<varname>After=</varname> unless <varname>DefaultDependencies=no</varname>
is set in the specified units. See
<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. Note that this behavior can be turned off by setting
<varname>DefaultDependencies=no</varname>.</para>
</refsect1>
<refsect1>
<title>Unit File Load Path</title>
<para>Unit files are loaded from a set of paths determined during
compilation, described in the two tables below. Unit files found
in directories listed earlier override files with the same name in
directories lower in the list.</para>
<para>When the variable <varname>$SYSTEMD_UNIT_PATH</varname> is set,
the contents of this variable overrides the unit load path. If
<varname>$SYSTEMD_UNIT_PATH</varname> ends with an empty component
(<literal>:</literal>), the usual unit load path will be appended
to the contents of the variable.</para>
<table>
<title>
Load path when running in system mode (<option>--system</option>).
</title>
<tgroup cols='2'>
<colspec colname='path' />
<colspec colname='expl' />
<thead>
<row>
<entry>Path</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><filename>/etc/systemd/system.control</filename></entry>
<entry morerows="1">Persistent and transient configuration created using the dbus API</entry>
</row>
<row>
<entry><filename>/run/systemd/system.control</filename></entry>
</row>
<row>
<entry><filename>/run/systemd/transient</filename></entry>
<entry>Dynamic configuration for transient units</entry>
</row>
<row>
<entry><filename>/run/systemd/generator.early</filename></entry>
<entry>Generated units with high priority (see <replaceable>early-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
<row>
<entry><filename>/etc/systemd/system</filename></entry>
<entry>Local configuration</entry>
</row>
<row>
<entry><filename>/run/systemd/system</filename></entry>
<entry>Runtime units</entry>
</row>
<row>
<entry><filename>/run/systemd/generator</filename></entry>
<entry>Generated units with medium priority (see <replaceable>normal-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
<row>
<entry><filename>/usr/local/lib/systemd/system</filename></entry>
<entry morerows="1">Units of installed packages</entry>
</row>
<row>
<entry><filename>/usr/lib/systemd/system</filename></entry>
</row>
<row>
<entry><filename>/run/systemd/generator.late</filename></entry>
<entry>Generated units with low priority (see <replaceable>late-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
</tbody>
</tgroup>
</table>
<table>
<title>
Load path when running in user mode (<option>--user</option>).
</title>
<tgroup cols='2'>
<colspec colname='path' />
<colspec colname='expl' />
<thead>
<row>
<entry>Path</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><filename>$XDG_CONFIG_HOME/systemd/user.control</filename> or <filename
>~/.config/systemd/user.control</filename></entry>
<entry morerows="1">Persistent and transient configuration created using the dbus API (<varname>$XDG_CONFIG_HOME</varname> is used if set, <filename>~/.config</filename> otherwise)</entry>
</row>
<row>
<entry><filename>$XDG_RUNTIME_DIR/systemd/user.control</filename></entry>
</row>
<row>
<entry><filename>/run/systemd/transient</filename></entry>
<entry>Dynamic configuration for transient units</entry>
</row>
<row>
<entry><filename>/run/systemd/generator.early</filename></entry>
<entry>Generated units with high priority (see <replaceable>early-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
<row>
<entry><filename>$XDG_CONFIG_HOME/systemd/user</filename> or <filename>$HOME/.config/systemd/user</filename></entry>
<entry>User configuration (<varname>$XDG_CONFIG_HOME</varname> is used if set, <filename>~/.config</filename> otherwise)</entry>
</row>
<row>
<entry><filename>/etc/systemd/user</filename></entry>
<entry>Local configuration</entry>
</row>
<row>
<entry><filename>$XDG_RUNTIME_DIR/systemd/user</filename></entry>
<entry>Runtime units (only used when $XDG_RUNTIME_DIR is set)</entry>
</row>
<row>
<entry><filename>/run/systemd/user</filename></entry>
<entry>Runtime units</entry>
</row>
<row>
<entry><filename>$XDG_RUNTIME_DIR/systemd/generator</filename></entry>
<entry>Generated units with medium priority (see <replaceable>normal-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
<row>
<entry><filename>$XDG_DATA_HOME/systemd/user</filename> or <filename>$HOME/.local/share/systemd/user</filename></entry>
<entry>Units of packages that have been installed in the home directory (<varname>$XDG_DATA_HOME</varname> is used if set, <filename>~/.local/share</filename> otherwise)</entry>
</row>
<row>
<entry><filename>$dir/systemd/user</filename> for each <varname noindex='true'>$dir</varname> in <varname>$XDG_DATA_DIRS</varname></entry>
<entry>Additional locations for installed user units, one for each entry in <varname>$XDG_DATA_DIRS</varname></entry>
</row>
<row>
<entry><filename>/usr/local/lib/systemd/user</filename></entry>
<entry morerows="1">Units of packages that have been installed system-wide</entry>
</row>
<row>
<entry><filename>/usr/lib/systemd/user</filename></entry>
</row>
<row>
<entry><filename>$XDG_RUNTIME_DIR/systemd/generator.late</filename></entry>
<entry>Generated units with low priority (see <replaceable>late-dir</replaceable> in <citerefentry
><refentrytitle>system.generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>The set of load paths for the user manager instance may be augmented or
changed using various environment variables. And environment variables may in
turn be set using environment generators, see
<citerefentry><refentrytitle>system.environment-generator</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
In particular, <varname>$XDG_DATA_HOME</varname> and
<varname>$XDG_DATA_DIRS</varname> may be easily set using
<citerefentry><refentrytitle>systemd-environment-d-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
Thus, directories listed here are just the defaults. To see the actual list that
would be used based on compilation options and current environment use
<programlisting>systemd-analyze --user unit-paths</programlisting>
</para>
<para>Moreover, additional units might be loaded into systemd ("linked") from
directories not on the unit load path. See the <command>link</command> command
for
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para>
</refsect1>
<refsect1>
<title>Unit Garbage Collection</title>
<para>The system and service manager loads a unit's configuration automatically when a unit is referenced for the
first time. It will automatically unload the unit configuration and state again when the unit is not needed anymore
("garbage collection"). A unit may be referenced through a number of different mechanisms:</para>
<orderedlist>
<listitem><para>Another loaded unit references it with a dependency such as <varname>After=</varname>,
<varname>Wants=</varname>, …</para></listitem>
<listitem><para>The unit is currently starting, running, reloading or stopping.</para></listitem>
<listitem><para>The unit is currently in the <constant>failed</constant> state. (But see below.)</para></listitem>
<listitem><para>A job for the unit is pending.</para></listitem>
<listitem><para>The unit is pinned by an active IPC client program.</para></listitem>
<listitem><para>The unit is a special "perpetual" unit that is always active and loaded. Examples for perpetual
units are the root mount unit <filename>-.mount</filename> or the scope unit <filename>init.scope</filename> that
the service manager itself lives in.</para></listitem>
<listitem><para>The unit has running processes associated with it.</para></listitem>
</orderedlist>
<para>The garbage collection logic may be altered with the <varname>CollectMode=</varname> option, which allows
configuration whether automatic unloading of units that are in <constant>failed</constant> state is permissible,
see below.</para>
<para>Note that when a unit's configuration and state is unloaded, all execution results, such as exit codes, exit
signals, resource consumption and other statistics are lost, except for what is stored in the log subsystem.</para>
<para>Use <command>systemctl daemon-reload</command> or an equivalent command to reload unit configuration while
the unit is already loaded. In this case all configuration settings are flushed out and replaced with the new
configuration (which however might not be in effect immediately), however all runtime state is
saved/restored.</para>
</refsect1>
<refsect1>
<title>[Unit] Section Options</title>
<para>The unit file may include a [Unit] section, which carries
generic information about the unit that is not dependent on the
type of unit:</para>
<variablelist class='unit-directives'>
<varlistentry>
<term><varname>Description=</varname></term>
<listitem><para>A free-form string describing the unit. This
is intended for use in UIs to show descriptive information
along with the unit name. The description should contain a
name that means something to the end user. <literal>Apache2
Web Server</literal> is a good example. Bad examples are
<literal>high-performance light-weight HTTP server</literal>
(too generic) or <literal>Apache2</literal> (too specific and
meaningless for people who do not know
Apache).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Documentation=</varname></term>
<listitem><para>A space-separated list of URIs referencing
documentation for this unit or its configuration. Accepted are
only URIs of the types <literal>http://</literal>,
<literal>https://</literal>, <literal>file:</literal>,
<literal>info:</literal>, <literal>man:</literal>. For more
information about the syntax of these URIs, see <citerefentry
project='man-pages'><refentrytitle>uri</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
The URIs should be listed in order of relevance, starting with
the most relevant. It is a good idea to first reference
documentation that explains what the unit's purpose is,
followed by how it is configured, followed by any other
related documentation. This option may be specified more than
once, in which case the specified list of URIs is merged. If
the empty string is assigned to this option, the list is reset
and all prior assignments will have no
effect.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Requires=</varname></term>
<listitem><para>Configures requirement dependencies on other units. If this unit gets activated, the units
listed here will be activated as well. If one of the other units fails to activate, and an ordering dependency
<varname>After=</varname> on the failing unit is set, this unit will not be started. Besides, with or without
specifying <varname>After=</varname>, this unit will be stopped if one of the other units is explicitly
stopped. This option may be specified more than once or multiple space-separated units may be
specified in one option in which case requirement dependencies for all listed names will be created. Note that
requirement dependencies do not influence the order in which services are started or stopped. This has to be
configured independently with the <varname>After=</varname> or <varname>Before=</varname> options. If a unit
<filename>foo.service</filename> requires a unit <filename>bar.service</filename> as configured with
<varname>Requires=</varname> and no ordering is configured with <varname>After=</varname> or
<varname>Before=</varname>, then both units will be started simultaneously and without any delay between them
if <filename>foo.service</filename> is activated. Often, it is a better choice to use <varname>Wants=</varname>
instead of <varname>Requires=</varname> in order to achieve a system that is more robust when dealing with
failing services.</para>
<para>Note that this dependency type does not imply that the other unit always has to be in active state when
this unit is running. Specifically: failing condition checks (such as <varname>ConditionPathExists=</varname>,
<varname>ConditionPathIsSymbolicLink=</varname>, … — see below) do not cause the start job of a unit with a
<varname>Requires=</varname> dependency on it to fail. Also, some unit types may deactivate on their own (for
example, a service process may decide to exit cleanly, or a device may be unplugged by the user), which is not
propagated to units having a <varname>Requires=</varname> dependency. Use the <varname>BindsTo=</varname>
dependency type together with <varname>After=</varname> to ensure that a unit may never be in active state
without a specific other unit also in active state (see below).</para>
<para>Note that dependencies of this type may also be configured outside of the unit configuration file by
adding a symlink to a <filename>.requires/</filename> directory accompanying the unit file. For details, see
above.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Requisite=</varname></term>
<listitem><para>Similar to <varname>Requires=</varname>.
However, if the units listed here are not started already,
they will not be started and the transaction will fail
immediately.</para>
<para>When <varname>Requisite=b.service</varname> is used on
<filename>a.service</filename>, this dependency will show as
<varname>RequisiteOf=a.service</varname> in property listing of
<filename>b.service</filename>. <varname>RequisiteOf=</varname>
dependency cannot be specified directly.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Wants=</varname></term>
<listitem><para>A weaker version of
<varname>Requires=</varname>. Units listed in this option will
be started if the configuring unit is. However, if the listed
units fail to start or cannot be added to the transaction,
this has no impact on the validity of the transaction as a
whole. This is the recommended way to hook start-up of one
unit to the start-up of another unit.</para>
<para>Note that dependencies of this type may also be
configured outside of the unit configuration file by adding
symlinks to a <filename>.wants/</filename> directory
accompanying the unit file. For details, see
above.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>BindsTo=</varname></term>
<listitem><para>Configures requirement dependencies, very similar in style to
<varname>Requires=</varname>. However, this dependency type is stronger: in addition to the effect of
<varname>Requires=</varname> it declares that if the unit bound to is stopped, this unit will be stopped
too. This means a unit bound to another unit that suddenly enters inactive state will be stopped too.
Units can suddenly, unexpectedly enter inactive state for different reasons: the main process of a service unit
might terminate on its own choice, the backing device of a device unit might be unplugged or the mount point of
a mount unit might be unmounted without involvement of the system and service manager.</para>
<para>When used in conjunction with <varname>After=</varname> on the same unit the behaviour of
<varname>BindsTo=</varname> is even stronger. In this case, the unit bound to strictly has to be in active
state for this unit to also be in active state. This not only means a unit bound to another unit that suddenly
enters inactive state, but also one that is bound to another unit that gets skipped due to a failed condition
check (such as <varname>ConditionPathExists=</varname>, <varname>ConditionPathIsSymbolicLink=</varname>, … —
see below) will be stopped, should it be running. Hence, in many cases it is best to combine
<varname>BindsTo=</varname> with <varname>After=</varname>.</para>
<para>When <varname>BindsTo=b.service</varname> is used on
<filename>a.service</filename>, this dependency will show as
<varname>BoundBy=a.service</varname> in property listing of
<filename>b.service</filename>. <varname>BoundBy=</varname>
dependency cannot be specified directly.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>PartOf=</varname></term>
<listitem><para>Configures dependencies similar to
<varname>Requires=</varname>, but limited to stopping and
restarting of units. When systemd stops or restarts the units
listed here, the action is propagated to this unit. Note that
this is a one-way dependency — changes to this unit do not
affect the listed units.</para>
<para>When <varname>PartOf=b.service</varname> is used on
<filename>a.service</filename>, this dependency will show as
<varname>ConsistsOf=a.service</varname> in property listing of
<filename>b.service</filename>. <varname>ConsistsOf=</varname>
dependency cannot be specified directly.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>Conflicts=</varname></term>
<listitem><para>A space-separated list of unit names.
Configures negative requirement dependencies. If a unit has a
<varname>Conflicts=</varname> setting on another unit,
starting the former will stop the latter and vice versa. Note
that this setting is independent of and orthogonal to the
<varname>After=</varname> and <varname>Before=</varname>
ordering dependencies.</para>
<para>If a unit A that conflicts with a unit B is scheduled to
be started at the same time as B, the transaction will either
fail (in case both are required part of the transaction) or be
modified to be fixed (in case one or both jobs are not a
required part of the transaction). In the latter case, the job
that is not the required will be removed, or in case both are
not required, the unit that conflicts will be started and the
unit that is conflicted is stopped.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Before=</varname></term>
<term><varname>After=</varname></term>
<listitem><para>These two settings expect a space-separated list of unit names. They configure ordering
dependencies between units. If a unit <filename>foo.service</filename> contains a setting
<option>Before=bar.service</option> and both units are being started, <filename>bar.service</filename>'s
start-up is delayed until <filename>foo.service</filename> has finished starting up. Note that this setting is
independent of and orthogonal to the requirement dependencies as configured by <varname>Requires=</varname>,
<varname>Wants=</varname> or <varname>BindsTo=</varname>. It is a common pattern to include a unit name in both
the <varname>After=</varname> and <varname>Requires=</varname> options, in which case the unit listed will be
started before the unit that is configured with these options. This option may be specified more than once, in
which case ordering dependencies for all listed names are created. <varname>After=</varname> is the inverse of
<varname>Before=</varname>, i.e. while <varname>After=</varname> ensures that the configured unit is started
after the listed unit finished starting up, <varname>Before=</varname> ensures the opposite, that the
configured unit is fully started up before the listed unit is started. Note that when two units with an
ordering dependency between them are shut down, the inverse of the start-up order is applied. i.e. if a unit is
configured with <varname>After=</varname> on another unit, the former is stopped before the latter if both are
shut down. Given two units with any ordering dependency between them, if one unit is shut down and the other is
started up, the shutdown is ordered before the start-up. It doesn't matter if the ordering dependency is
<varname>After=</varname> or <varname>Before=</varname>, in this case. It also doesn't matter which of the two
is shut down, as long as one is shut down and the other is started up. The shutdown is ordered before the
start-up in all cases. If two units have no ordering dependencies between them, they are shut down or started
up simultaneously, and no ordering takes place. It depends on the unit type when precisely a unit has finished
starting up. Most importantly, for service units start-up is considered completed for the purpose of
<varname>Before=</varname>/<varname>After=</varname> when all its configured start-up commands have been
invoked and they either failed or reported start-up success.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>OnFailure=</varname></term>
<listitem><para>A space-separated list of one or more units
that are activated when this unit enters the
<literal>failed</literal> state. A service unit using
<varname>Restart=</varname> enters the failed state only after
the start limits are reached.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>PropagatesReloadTo=</varname></term>
<term><varname>ReloadPropagatedFrom=</varname></term>
<listitem><para>A space-separated list of one or more units
where reload requests on this unit will be propagated to, or
reload requests on the other unit will be propagated to this
unit, respectively. Issuing a reload request on a unit will
automatically also enqueue a reload request on all units that
the reload request shall be propagated to via these two
settings.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>JoinsNamespaceOf=</varname></term>
<listitem><para>For units that start processes (such as
service units), lists one or more other units whose network
and/or temporary file namespace to join. This only applies to
unit types which support the
<varname>PrivateNetwork=</varname> and
<varname>PrivateTmp=</varname> directives (see
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details). If a unit that has this setting set is started,
its processes will see the same <filename>/tmp</filename>,
<filename>/var/tmp</filename> and network namespace as one
listed unit that is started. If multiple listed units are
already started, it is not defined which namespace is joined.
Note that this setting only has an effect if
<varname>PrivateNetwork=</varname> and/or
<varname>PrivateTmp=</varname> is enabled for both the unit
that joins the namespace and the unit whose namespace is
joined.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>RequiresMountsFor=</varname></term>
<listitem><para>Takes a space-separated list of absolute
paths. Automatically adds dependencies of type
<varname>Requires=</varname> and <varname>After=</varname> for
all mount units required to access the specified path.</para>
<para>Mount points marked with <option>noauto</option> are not
mounted automatically through <filename>local-fs.target</filename>,
but are still honored for the purposes of this option, i.e. they
will be pulled in by this unit.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>OnFailureJobMode=</varname></term>
<listitem><para>Takes a value of
<literal>fail</literal>,
<literal>replace</literal>,
<literal>replace-irreversibly</literal>,
<literal>isolate</literal>,
<literal>flush</literal>,
<literal>ignore-dependencies</literal> or
<literal>ignore-requirements</literal>. Defaults to
<literal>replace</literal>. Specifies how the units listed in
<varname>OnFailure=</varname> will be enqueued. See
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<option>--job-mode=</option> option for details on the
possible values. If this is set to <literal>isolate</literal>,
only a single unit may be listed in
<varname>OnFailure=</varname>..</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>IgnoreOnIsolate=</varname></term>
<listitem><para>Takes a boolean argument. If <option>true</option>, this unit
will not be stopped when isolating another unit. Defaults to
<option>false</option> for service, target, socket, busname, timer, and path
units, and <option>true</option> for slice, scope, device, swap, mount, and
automount units.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>StopWhenUnneeded=</varname></term>
<listitem><para>Takes a boolean argument. If
<option>true</option>, this unit will be stopped when it is no
longer used. Note that, in order to minimize the work to be
executed, systemd will not stop units by default unless they
are conflicting with other units, or the user explicitly
requested their shut down. If this option is set, a unit will
be automatically cleaned up if no other active unit requires
it. Defaults to <option>false</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>RefuseManualStart=</varname></term>
<term><varname>RefuseManualStop=</varname></term>
<listitem><para>Takes a boolean argument. If
<option>true</option>, this unit can only be activated or
deactivated indirectly. In this case, explicit start-up or
termination requested by the user is denied, however if it is
started or stopped as a dependency of another unit, start-up
or termination will succeed. This is mostly a safety feature
to ensure that the user does not accidentally activate units
that are not intended to be activated explicitly, and not
accidentally deactivate units that are not intended to be
deactivated. These options default to
<option>false</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>AllowIsolate=</varname></term>
<listitem><para>Takes a boolean argument. If
<option>true</option>, this unit may be used with the
<command>systemctl isolate</command> command. Otherwise, this
will be refused. It probably is a good idea to leave this
disabled except for target units that shall be used similar to
runlevels in SysV init systems, just as a precaution to avoid
unusable system states. This option defaults to
<option>false</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DefaultDependencies=</varname></term>
<listitem><para>Takes a boolean argument. If
<option>true</option>, (the default), a few default
dependencies will implicitly be created for the unit. The
actual dependencies created depend on the unit type. For
example, for service units, these dependencies ensure that the
service is started only after basic system initialization is
completed and is properly terminated on system shutdown. See
the respective man pages for details. Generally, only services
involved with early boot or late shutdown should set this
option to <option>false</option>. It is highly recommended to
leave this option enabled for the majority of common units. If
set to <option>false</option>, this option does not disable
all implicit dependencies, just non-essential
ones.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>CollectMode=</varname></term>
<listitem><para>Tweaks the "garbage collection" algorithm for this unit. Takes one of <option>inactive</option>
or <option>inactive-or-failed</option>. If set to <option>inactive</option> the unit will be unloaded if it is
in the <constant>inactive</constant> state and is not referenced by clients, jobs or other units — however it
is not unloaded if it is in the <constant>failed</constant> state. In <option>failed</option> mode, failed
units are not unloaded until the user invoked <command>systemctl reset-failed</command> on them to reset the
<constant>failed</constant> state, or an equivalent command. This behaviour is altered if this option is set to
<option>inactive-or-failed</option>: in this case the unit is unloaded even if the unit is in a
<constant>failed</constant> state, and thus an explicitly resetting of the <constant>failed</constant> state is
not necessary. Note that if this mode is used unit results (such as exit codes, exit signals, consumed
resources, …) are flushed out immediately after the unit completed, except for what is stored in the logging
subsystem. Defaults to <option>inactive</option>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>JobTimeoutSec=</varname></term>
<term><varname>JobRunningTimeoutSec=</varname></term>
<term><varname>JobTimeoutAction=</varname></term>
<term><varname>JobTimeoutRebootArgument=</varname></term>
<listitem><para>When a job for this unit is queued, a time-out <varname>JobTimeoutSec=</varname> may be
configured. Similarly, <varname>JobRunningTimeoutSec=</varname> starts counting when the queued job is actually
started. If either time limit is reached, the job will be cancelled, the unit however will not change state or
even enter the <literal>failed</literal> mode. This value defaults to <literal>infinity</literal> (job timeouts
disabled), except for device units (<varname>JobRunningTimeoutSec=</varname> defaults to
<varname>DefaultTimeoutStartSec=</varname>). NB: this timeout is independent from any unit-specific timeout
(for example, the timeout set with <varname>TimeoutStartSec=</varname> in service units) as the job timeout has
no effect on the unit itself, only on the job that might be pending for it. Or in other words: unit-specific
timeouts are useful to abort unit state changes, and revert them. The job timeout set with this option however
is useful to abort only the job waiting for the unit state to change.</para>
<para><varname>JobTimeoutAction=</varname> optionally configures an additional action to take when the time-out
is hit. It takes the same values as <varname>StartLimitAction=</varname>. Defaults to <option>none</option>.
<varname>JobTimeoutRebootArgument=</varname> configures an optional reboot string to pass to the
<citerefentry><refentrytitle>reboot</refentrytitle><manvolnum>2</manvolnum></citerefentry>
system call.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>StartLimitIntervalSec=<replaceable>interval</replaceable></varname></term>
<term><varname>StartLimitBurst=<replaceable>burst</replaceable></varname></term>
<listitem><para>Configure unit start rate limiting. Units which are started more than
<replaceable>burst</replaceable> times within an <replaceable>interval</replaceable> time interval are not
permitted to start any more. Use <varname>StartLimitIntervalSec=</varname> to configure the checking interval
(defaults to <varname>DefaultStartLimitIntervalSec=</varname> in manager configuration file, set it to 0 to
disable any kind of rate limiting). Use <varname>StartLimitBurst=</varname> to configure how many starts per
interval are allowed (defaults to <varname>DefaultStartLimitBurst=</varname> in manager configuration
file). These configuration options are particularly useful in conjunction with the service setting
<varname>Restart=</varname> (see
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>); however,
they apply to all kinds of starts (including manual), not just those triggered by the
<varname>Restart=</varname> logic. Note that units which are configured for <varname>Restart=</varname> and
which reach the start limit are not attempted to be restarted anymore; however, they may still be restarted
manually at a later point, after the <replaceable>interval</replaceable> has passed. From this point on, the
restart logic is activated again. Note that <command>systemctl reset-failed</command> will cause the restart
rate counter for a service to be flushed, which is useful if the administrator wants to manually start a unit
and the start limit interferes with that. Note that this rate-limiting is enforced after any unit condition
checks are executed, and hence unit activations with failing conditions do not count towards this rate
limit. This setting does not apply to slice, target, device, and scope units, since they are unit types whose
activation may either never fail, or may succeed only a single time.</para>
<para>When a unit is unloaded due to the garbage collection logic (see above) its rate limit counters are
flushed out too. This means that configuring start rate limiting for a unit that is not referenced continously
has no effect.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>StartLimitAction=</varname></term>
<listitem><para>Configure the action to take if the rate limit configured with
<varname>StartLimitIntervalSec=</varname> and <varname>StartLimitBurst=</varname> is hit. Takes one of
<option>none</option>, <option>reboot</option>, <option>reboot-force</option>,
<option>reboot-immediate</option>, <option>poweroff</option>, <option>poweroff-force</option> or
<option>poweroff-immediate</option>. If <option>none</option> is set, hitting the rate limit will trigger no
action besides that the start will not be permitted. <option>reboot</option> causes a reboot following the
normal shutdown procedure (i.e. equivalent to <command>systemctl reboot</command>).
<option>reboot-force</option> causes a forced reboot which will terminate all processes forcibly but should
cause no dirty file systems on reboot (i.e. equivalent to <command>systemctl reboot -f</command>) and
<option>reboot-immediate</option> causes immediate execution of the
<citerefentry><refentrytitle>reboot</refentrytitle><manvolnum>2</manvolnum></citerefentry> system call, which
might result in data loss. Similarly, <option>poweroff</option>, <option>poweroff-force</option>,
<option>poweroff-immediate</option> have the effect of powering down the system with similar
semantics. Defaults to <option>none</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>FailureAction=</varname></term>
<term><varname>SuccessAction=</varname></term>
<listitem><para>Configure the action to take when the unit stops and enters a failed state or inactive
state. Takes the same values as the setting <varname>StartLimitAction=</varname> setting and executes the same
actions. Both options default to <option>none</option>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>RebootArgument=</varname></term>
<listitem><para>Configure the optional argument for the
<citerefentry><refentrytitle>reboot</refentrytitle><manvolnum>2</manvolnum></citerefentry> system call if
<varname>StartLimitAction=</varname> or <varname>FailureAction=</varname> is a reboot action. This
works just like the optional argument to <command>systemctl reboot</command> command.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ConditionArchitecture=</varname></term>
<term><varname>ConditionVirtualization=</varname></term>
<term><varname>ConditionHost=</varname></term>
<term><varname>ConditionKernelCommandLine=</varname></term>
<term><varname>ConditionKernelVersion=</varname></term>
<term><varname>ConditionSecurity=</varname></term>
<term><varname>ConditionCapability=</varname></term>
<term><varname>ConditionACPower=</varname></term>
<term><varname>ConditionNeedsUpdate=</varname></term>
<term><varname>ConditionFirstBoot=</varname></term>
<term><varname>ConditionPathExists=</varname></term>
<term><varname>ConditionPathExistsGlob=</varname></term>
<term><varname>ConditionPathIsDirectory=</varname></term>
<term><varname>ConditionPathIsSymbolicLink=</varname></term>
<term><varname>ConditionPathIsMountPoint=</varname></term>
<term><varname>ConditionPathIsReadWrite=</varname></term>
<term><varname>ConditionDirectoryNotEmpty=</varname></term>
<term><varname>ConditionFileNotEmpty=</varname></term>
<term><varname>ConditionFileIsExecutable=</varname></term>
<term><varname>ConditionUser=</varname></term>
<term><varname>ConditionGroup=</varname></term>
<term><varname>ConditionControlGroupController=</varname></term>
<!-- We do not document ConditionNull=
here, as it is not particularly
useful and probably just
confusing. -->
<listitem><para>Before starting a unit, verify that the specified condition is true. If it is not true, the
starting of the unit will be (mostly silently) skipped, however all ordering dependencies of it are still
respected. A failing condition will not result in the unit being moved into a failure state. The condition is
checked at the time the queued start job is to be executed. Use condition expressions in order to silently skip
units that do not apply to the local running system, for example because the kernel or runtime environment
doesn't require its functionality. Use the various <varname>AssertArchitecture=</varname>,
<varname>AssertVirtualization=</varname>, … options for a similar mechanism that puts the unit in a failure
state and logs about the failed check (see below).</para>
<para><varname>ConditionArchitecture=</varname> may be used to
check whether the system is running on a specific
architecture. Takes one of
<varname>x86</varname>,
<varname>x86-64</varname>,
<varname>ppc</varname>,
<varname>ppc-le</varname>,
<varname>ppc64</varname>,
<varname>ppc64-le</varname>,
<varname>ia64</varname>,
<varname>parisc</varname>,
<varname>parisc64</varname>,
<varname>s390</varname>,
<varname>s390x</varname>,
<varname>sparc</varname>,
<varname>sparc64</varname>,
<varname>mips</varname>,
<varname>mips-le</varname>,
<varname>mips64</varname>,
<varname>mips64-le</varname>,
<varname>alpha</varname>,
<varname>arm</varname>,
<varname>arm-be</varname>,
<varname>arm64</varname>,
<varname>arm64-be</varname>,
<varname>sh</varname>,
<varname>sh64</varname>,
<varname>m68k</varname>,
<varname>tilegx</varname>,
<varname>cris</varname>,
<varname>arc</varname>,
<varname>arc-be</varname> to test
against a specific architecture. The architecture is
determined from the information returned by
<citerefentry project='man-pages'><refentrytitle>uname</refentrytitle><manvolnum>2</manvolnum></citerefentry>
and is thus subject to
<citerefentry><refentrytitle>personality</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
Note that a <varname>Personality=</varname> setting in the
same unit file has no effect on this condition. A special
architecture name <varname>native</varname> is mapped to the
architecture the system manager itself is compiled for. The
test may be negated by prepending an exclamation mark.</para>
<para><varname>ConditionVirtualization=</varname> may be used
to check whether the system is executed in a virtualized
environment and optionally test whether it is a specific
implementation. Takes either boolean value to check if being
executed in any virtualized environment, or one of
<varname>vm</varname> and
<varname>container</varname> to test against a generic type of
virtualization solution, or one of
<varname>qemu</varname>,
<varname>kvm</varname>,
<varname>zvm</varname>,
<varname>vmware</varname>,
<varname>microsoft</varname>,
<varname>oracle</varname>,
<varname>xen</varname>,
<varname>bochs</varname>,
<varname>uml</varname>,
<varname>bhyve</varname>,
<varname>qnx</varname>,
<varname>openvz</varname>,
<varname>lxc</varname>,
<varname>lxc-libvirt</varname>,
<varname>systemd-nspawn</varname>,
<varname>docker</varname>,
<varname>rkt</varname> to test
against a specific implementation, or
<varname>private-users</varname> to check whether we are running in a user namespace. See
<citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
for a full list of known virtualization technologies and their
identifiers. If multiple virtualization technologies are
nested, only the innermost is considered. The test may be
negated by prepending an exclamation mark.</para>
<para><varname>ConditionHost=</varname> may be used to match
against the hostname or machine ID of the host. This either
takes a hostname string (optionally with shell style globs)
which is tested against the locally set hostname as returned
by
<citerefentry><refentrytitle>gethostname</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
or a machine ID formatted as string (see
<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>).
The test may be negated by prepending an exclamation
mark.</para>
<para><varname>ConditionKernelCommandLine=</varname> may be
used to check whether a specific kernel command line option is
set (or if prefixed with the exclamation mark unset). The
argument must either be a single word, or an assignment (i.e.
two words, separated <literal>=</literal>). In the former case
the kernel command line is searched for the word appearing as
is, or as left hand side of an assignment. In the latter case,
the exact assignment is looked for with right and left hand
side matching.</para>
<para><varname>ConditionKernelVersion=</varname> may be used to check whether the kernel version (as reported
by <command>uname -r</command>) matches a certain expression (or if prefixed with the exclamation mark does not
match it). The argument must be a single string. If the string starts with one of <literal>&lt;</literal>,
<literal>&lt;=</literal>, <literal>=</literal>, <literal>&gt;=</literal>, <literal>&gt;</literal> a relative
version comparison is done, otherwise the specified string is matched with shell-style globs.</para>
<para>Note that using the kernel version string is an unreliable way to determine which features are supported
by a kernel, because of the widespread practice of backporting drivers, features, and fixes from newer upstream
kernels into older versions provided by distributions. Hence, this check is inherently unportable and should
not be used for units which may be used on different distributions.</para>
<para><varname>ConditionSecurity=</varname> may be used to
check whether the given security module is enabled on the
system. Currently, the recognized values are
<varname>selinux</varname>,
<varname>apparmor</varname>,
<varname>tomoyo</varname>,
<varname>ima</varname>,
<varname>smack</varname> and
<varname>audit</varname>. The test may be negated by
prepending an exclamation mark.</para>
<para><varname>ConditionCapability=</varname> may be used to
check whether the given capability exists in the capability
bounding set of the service manager (i.e. this does not check
whether capability is actually available in the permitted or
effective sets, see
<citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>
for details). Pass a capability name such as
<literal>CAP_MKNOD</literal>, possibly prefixed with an
exclamation mark to negate the check.</para>
<para><varname>ConditionACPower=</varname> may be used to
check whether the system has AC power, or is exclusively
battery powered at the time of activation of the unit. This
takes a boolean argument. If set to <varname>true</varname>,
the condition will hold only if at least one AC connector of
the system is connected to a power source, or if no AC
connectors are known. Conversely, if set to
<varname>false</varname>, the condition will hold only if
there is at least one AC connector known and all AC connectors
are disconnected from a power source.</para>
<para><varname>ConditionNeedsUpdate=</varname> takes one of
<filename>/var</filename> or <filename>/etc</filename> as
argument, possibly prefixed with a <literal>!</literal> (for
inverting the condition). This condition may be used to
conditionalize units on whether the specified directory
requires an update because <filename>/usr</filename>'s
modification time is newer than the stamp file
<filename>.updated</filename> in the specified directory. This
is useful to implement offline updates of the vendor operating
system resources in <filename>/usr</filename> that require
updating of <filename>/etc</filename> or
<filename>/var</filename> on the next following boot. Units
making use of this condition should order themselves before
<citerefentry><refentrytitle>systemd-update-done.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
to make sure they run before the stamp file's modification
time gets reset indicating a completed update.</para>
<para><varname>ConditionFirstBoot=</varname> takes a boolean argument. This condition may be used to
conditionalize units on whether the system is booting up with an unpopulated <filename>/etc</filename>
directory (specifically: an <filename>/etc</filename> with no <filename>/etc/machine-id</filename>). This may
be used to populate <filename>/etc</filename> on the first boot after factory reset, or when a new system
instance boots up for the first time.</para>
<para>With <varname>ConditionPathExists=</varname> a file
existence condition is checked before a unit is started. If
the specified absolute path name does not exist, the condition
will fail. If the absolute path name passed to
<varname>ConditionPathExists=</varname> is prefixed with an
exclamation mark (<literal>!</literal>), the test is negated,
and the unit is only started if the path does not
exist.</para>
<para><varname>ConditionPathExistsGlob=</varname> is similar
to <varname>ConditionPathExists=</varname>, but checks for the
existence of at least one file or directory matching the
specified globbing pattern.</para>
<para><varname>ConditionPathIsDirectory=</varname> is similar
to <varname>ConditionPathExists=</varname> but verifies
whether a certain path exists and is a directory.</para>
<para><varname>ConditionPathIsSymbolicLink=</varname> is
similar to <varname>ConditionPathExists=</varname> but
verifies whether a certain path exists and is a symbolic
link.</para>
<para><varname>ConditionPathIsMountPoint=</varname> is similar
to <varname>ConditionPathExists=</varname> but verifies
whether a certain path exists and is a mount point.</para>
<para><varname>ConditionPathIsReadWrite=</varname> is similar
to <varname>ConditionPathExists=</varname> but verifies
whether the underlying file system is readable and writable
(i.e. not mounted read-only).</para>
<para><varname>ConditionDirectoryNotEmpty=</varname> is
similar to <varname>ConditionPathExists=</varname> but
verifies whether a certain path exists and is a non-empty
directory.</para>
<para><varname>ConditionFileNotEmpty=</varname> is similar to
<varname>ConditionPathExists=</varname> but verifies whether a
certain path exists and refers to a regular file with a
non-zero size.</para>
<para><varname>ConditionFileIsExecutable=</varname> is similar
to <varname>ConditionPathExists=</varname> but verifies
whether a certain path exists, is a regular file and marked
executable.</para>
<para><varname>ConditionUser=</varname> takes a numeric
<literal>UID</literal>, a UNIX user name, or the special value
<literal>@system</literal>. This condition may be used to check
whether the service manager is running as the given user. The
special value <literal>@system</literal> can be used to check
if the user id is within the system user range. This option is not
useful for system services, as the system manager exclusively
runs as the root user, and thus the test result is constant.</para>
<para><varname>ConditionGroup=</varname> is similar
to <varname>ConditionUser=</varname> but verifies that the
service manager's real or effective group, or any of its
auxiliary groups match the specified group or GID. This setting
does not have a special value <literal>@system</literal>.</para>
<para><varname>ConditionControlGroupController=</varname> takes a
cgroup controller name (eg. <option>cpu</option>), verifying that it is
available for use on the system. For example, a particular controller
may not be available if it was disabled on the kernel command line with
<literal>cgroup_disable=</literal><replaceable>controller</replaceable>.
Multiple controllers may be passed with a space separating them; in
this case the condition will only pass if all listed controllers are
available for use. Controllers unknown to systemd are ignored. Valid
controllers are <option>cpu</option>, <option>cpuacct</option>,
<option>io</option>, <option>blkio</option>, <option>memory</option>,
<option>devices</option>, and <option>pids</option>.</para>
<para>If multiple conditions are specified, the unit will be
executed if all of them apply (i.e. a logical AND is applied).
Condition checks can be prefixed with a pipe symbol (|) in
which case a condition becomes a triggering condition. If at
least one triggering condition is defined for a unit, then the
unit will be executed if at least one of the triggering
conditions apply and all of the non-triggering conditions. If
you prefix an argument with the pipe symbol and an exclamation
mark, the pipe symbol must be passed first, the exclamation
second. Except for
<varname>ConditionPathIsSymbolicLink=</varname>, all path
checks follow symlinks. If any of these options is assigned
the empty string, the list of conditions is reset completely,
all previous condition settings (of any kind) will have no
effect.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>AssertArchitecture=</varname></term>
<term><varname>AssertVirtualization=</varname></term>
<term><varname>AssertHost=</varname></term>
<term><varname>AssertKernelCommandLine=</varname></term>
<term><varname>AssertKernelVersion=</varname></term>
<term><varname>AssertSecurity=</varname></term>
<term><varname>AssertCapability=</varname></term>
<term><varname>AssertACPower=</varname></term>
<term><varname>AssertNeedsUpdate=</varname></term>
<term><varname>AssertFirstBoot=</varname></term>
<term><varname>AssertPathExists=</varname></term>
<term><varname>AssertPathExistsGlob=</varname></term>
<term><varname>AssertPathIsDirectory=</varname></term>
<term><varname>AssertPathIsSymbolicLink=</varname></term>
<term><varname>AssertPathIsMountPoint=</varname></term>
<term><varname>AssertPathIsReadWrite=</varname></term>
<term><varname>AssertDirectoryNotEmpty=</varname></term>
<term><varname>AssertFileNotEmpty=</varname></term>
<term><varname>AssertFileIsExecutable=</varname></term>
<term><varname>AssertUser=</varname></term>
<term><varname>AssertGroup=</varname></term>
<term><varname>AssertControlGroupController=</varname></term>
<listitem><para>Similar to the <varname>ConditionArchitecture=</varname>,
<varname>ConditionVirtualization=</varname>, …, condition settings described above, these settings add
assertion checks to the start-up of the unit. However, unlike the conditions settings, any assertion setting
that is not met results in failure of the start job (which means this is logged loudly). Use assertion
expressions for units that cannot operate when specific requirements are not met, and when this is something
the administrator or user should look into.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>SourcePath=</varname></term>
<listitem><para>A path to a configuration file this unit has
been generated from. This is primarily useful for
implementation of generator tools that convert configuration
from an external configuration file format into native unit
files. This functionality should not be used in normal
units.</para></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Mapping of unit properties to their inverses</title>
<para>Unit settings that create a relationship with a second unit usually show up
in properties of both units, for example in <command>systemctl show</command>
output. In some cases the name of the property is the same as the name of the
configuration setting, but not always. This table lists the pairs of properties
that are shown on two units which are connected through some dependency, and shows
which property on "source" unit corresponds to which property on the "target" unit.
</para>
<table>
<title>
"Forward" and "reverse" unit properties
</title>
<tgroup cols='2'>
<colspec colname='forward' />
<colspec colname='reverse' />
<colspec colname='notes' />
<thead>
<row>
<entry>"Forward" property</entry>
<entry>"Reverse" property</entry>
<entry>Where used</entry>
</row>
</thead>
<tbody>
<row>
<entry><varname>Before=</varname></entry>
<entry><varname>After=</varname></entry>
<entry morerows='1' valign='middle'>Both are unit file options</entry>
</row>
<row>
<entry><varname>After=</varname></entry>
<entry><varname>Before=</varname></entry>
</row>
<row>
<entry><varname>Requires=</varname></entry>
<entry><varname>RequiredBy=</varname></entry>
<entry>A unit file option; an option in the [Install] section</entry>
</row>
<row>
<entry><varname>Wants=</varname></entry>
<entry><varname>WantedBy=</varname></entry>
<entry>A unit file option; an option in the [Install] section</entry>
</row>
<row>
<entry><varname>PartOf=</varname></entry>
<entry><varname>ConsistsOf=</varname></entry>
<entry>A unit file option; an automatic property</entry>
</row>
<row>
<entry><varname>BindsTo=</varname></entry>
<entry><varname>BoundBy=</varname></entry>
<entry>A unit file option; an automatic property</entry>
</row>
<row>
<entry><varname>Requisite=</varname></entry>
<entry><varname>RequisiteOf=</varname></entry>
<entry>A unit file option; an automatic property</entry>
</row>
<row>
<entry><varname>Triggers=</varname></entry>
<entry><varname>TriggeredBy=</varname></entry>
<entry>Automatic properties, see notes below</entry>
</row>
<row>
<entry><varname>Conflicts=</varname></entry>
<entry><varname>ConflictedBy=</varname></entry>
<entry>A unit file option; an automatic property</entry>
</row>
<row>
<entry><varname>PropagatesReloadTo=</varname></entry>
<entry><varname>ReloadPropagatedFrom=</varname></entry>
<entry morerows='1' valign='middle'>Both are unit file options</entry>
</row>
<row>
<entry><varname>ReloadPropagatedFrom=</varname></entry>
<entry><varname>PropagatesReloadTo=</varname></entry>
</row>
</tbody>
</tgroup>
</table>
<para>Note: <varname>WantedBy=</varname> and <varname>RequiredBy=</varname> are
used in the [Install] section to create symlinks in <filename>.wants/</filename>
and <filename>.requires/</filename> directories. They cannot be used directly as a
unit configuration setting.</para>
<para>Note: <varname>ConsistsOf=</varname>, <varname>BoundBy=</varname>,
<varname>RequisiteOf=</varname>, <varname>ConflictedBy=</varname> are created
implicitly along with their reverse and cannot be specified directly.</para>
<para>Note: <varname>Triggers=</varname> is created implicitly between a socket,
path unit, or an automount unit, and the unit they activate. By default a unit
with the same name is triggered, but this can be overriden using
<varname>Sockets=</varname>, <varname>Service=</varname>, and <varname>Unit=</varname>
settings. See
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
and
<citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details. <varname>TriggersBy=</varname> is created implicitly on the
triggered unit.</para>
</refsect1>
<refsect1>
<title>[Install] Section Options</title>
<para>Unit files may include an <literal>[Install]</literal> section, which carries installation information for
the unit. This section is not interpreted by
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> during runtime; it is
used by the <command>enable</command> and <command>disable</command> commands of the
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> tool during
installation of a unit.</para>
<variablelist class='unit-directives'>
<varlistentry>
<term><varname>Alias=</varname></term>
<listitem><para>A space-separated list of additional names this unit shall be installed under. The names listed
here must have the same suffix (i.e. type) as the unit filename. This option may be specified more than once,
in which case all listed names are used. At installation time, <command>systemctl enable</command> will create
symlinks from these names to the unit filename. Note that not all unit types support such alias names, and this
setting is not supported for them. Specifically, mount, slice, swap, and automount units do not support
aliasing.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>WantedBy=</varname></term>
<term><varname>RequiredBy=</varname></term>
<listitem><para>This option may be used more than once, or a
space-separated list of unit names may be given. A symbolic
link is created in the <filename>.wants/</filename> or
<filename>.requires/</filename> directory of each of the
listed units when this unit is installed by <command>systemctl
enable</command>. This has the effect that a dependency of
type <varname>Wants=</varname> or <varname>Requires=</varname>
is added from the listed unit to the current unit. The primary
result is that the current unit will be started when the
listed unit is started. See the description of
<varname>Wants=</varname> and <varname>Requires=</varname> in
the [Unit] section for details.</para>
<para><command>WantedBy=foo.service</command> in a service
<filename>bar.service</filename> is mostly equivalent to
<command>Alias=foo.service.wants/bar.service</command> in the
same file. In case of template units, <command>systemctl
enable</command> must be called with an instance name, and
this instance will be added to the
<filename>.wants/</filename> or
<filename>.requires/</filename> list of the listed unit. E.g.
<command>WantedBy=getty.target</command> in a service
<filename>getty@.service</filename> will result in
<command>systemctl enable getty@tty2.service</command>
creating a
<filename>getty.target.wants/getty@tty2.service</filename>
link to <filename>getty@.service</filename>.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>Also=</varname></term>
<listitem><para>Additional units to install/deinstall when
this unit is installed/deinstalled. If the user requests
installation/deinstallation of a unit with this option
configured, <command>systemctl enable</command> and
<command>systemctl disable</command> will automatically
install/uninstall units listed in this option as well.</para>
<para>This option may be used more than once, or a
space-separated list of unit names may be
given.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>DefaultInstance=</varname></term>
<listitem><para>In template unit files, this specifies for
which instance the unit shall be enabled if the template is
enabled without any explicitly set instance. This option has
no effect in non-template unit files. The specified string
must be usable as instance identifier.</para></listitem>
</varlistentry>
</variablelist>
<para>The following specifiers are interpreted in the Install
section: %n, %N, %p, %i, %U, %u, %m, %H, %b, %v. For their meaning
see the next section.
</para>
</refsect1>
<refsect1>
<title>Specifiers</title>
<para>Many settings resolve specifiers which may be used to write
generic unit files referring to runtime or unit parameters that
are replaced when the unit files are loaded. Specifiers must be known
and resolvable for the setting to be valid. The following
specifiers are understood:</para>
<table>
<title>Specifiers available in unit files</title>
<tgroup cols='3' align='left' colsep='1' rowsep='1'>
<colspec colname="spec" />
<colspec colname="mean" />
<colspec colname="detail" />
<thead>
<row>
<entry>Specifier</entry>
<entry>Meaning</entry>
<entry>Details</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>%n</literal></entry>
<entry>Full unit name</entry>
<entry></entry>
</row>
<row>
<entry><literal>%N</literal></entry>
<entry>Unescaped full unit name</entry>
<entry>Same as <literal>%n</literal>, but with escaping undone. This undoes the escaping used when generating unit names from arbitrary strings (see above). </entry>
</row>
<row>
<entry><literal>%p</literal></entry>
<entry>Prefix name</entry>
<entry>For instantiated units, this refers to the string before the <literal>@</literal> character of the unit name. For non-instantiated units, this refers to the name of the unit with the type suffix removed.</entry>
</row>
<row>
<entry><literal>%P</literal></entry>
<entry>Unescaped prefix name</entry>
<entry>Same as <literal>%p</literal>, but with escaping undone</entry>
</row>
<row>
<entry><literal>%i</literal></entry>
<entry>Instance name</entry>
<entry>For instantiated units: this is the string between the <literal>@</literal> character and the suffix of the unit name.</entry>
</row>
<row>
<entry><literal>%I</literal></entry>
<entry>Unescaped instance name</entry>
<entry>Same as <literal>%i</literal>, but with escaping undone</entry>
</row>
<row>
<entry><literal>%f</literal></entry>
<entry>Unescaped filename</entry>
<entry>This is either the unescaped instance name (if applicable) with <filename>/</filename> prepended (if applicable), or the unescaped prefix name prepended with <filename>/</filename>. This implements unescaping according to the rules for escaping absolute file system paths discussed above.</entry>
</row>
<row>
<entry><literal>%t</literal></entry>
<entry>Runtime directory root</entry>
<entry>This is either <filename>/run</filename> (for the system manager) or the path <literal>$XDG_RUNTIME_DIR</literal> resolves to (for user managers).</entry>
</row>
<row>
<entry><literal>%S</literal></entry>
<entry>State directory root</entry>
<entry>This is either <filename>/var/lib</filename> (for the system manager) or the path <literal>$XDG_CONFIG_HOME</literal> resolves to (for user managers).</entry>
</row>
<row>
<entry><literal>%C</literal></entry>
<entry>Cache directory root</entry>
<entry>This is either <filename>/var/cache</filename> (for the system manager) or the path <literal>$XDG_CACHE_HOME</literal> resolves to (for user managers).</entry>
</row>
<row>
<entry><literal>%L</literal></entry>
<entry>Log directory root</entry>
<entry>This is either <filename>/var/log</filename> (for the system manager) or the path <literal>$XDG_CONFIG_HOME</literal> resolves to with <filename noindex='true'>/log</filename> appended (for user managers).</entry>
</row>
<row>
<entry><literal>%u</literal></entry>
<entry>User name</entry>
<entry>This is the name of the user running the service manager instance. In case of the system manager this resolves to <literal>root</literal>.</entry>
</row>
<row>
<entry><literal>%U</literal></entry>
<entry>User UID</entry>
<entry>This is the numeric UID of the user running the service manager instance. In case of the system manager this resolves to <literal>0</literal>.</entry>
</row>
<row>
<entry><literal>%h</literal></entry>
<entry>User home directory</entry>
<entry>This is the home directory of the user running the service manager instance. In case of the system manager this resolves to <literal>/root</literal>.</entry>
</row>
<row>
<entry><literal>%s</literal></entry>
<entry>User shell</entry>
<entry>This is the shell of the user running the service manager instance. In case of the system manager this resolves to <literal>/bin/sh</literal>.</entry>
</row>
<row>
<entry><literal>%m</literal></entry>
<entry>Machine ID</entry>
<entry>The machine ID of the running system, formatted as string. See <citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry> for more information.</entry>
</row>
<row>
<entry><literal>%b</literal></entry>
<entry>Boot ID</entry>
<entry>The boot ID of the running system, formatted as string. See <citerefentry><refentrytitle>random</refentrytitle><manvolnum>4</manvolnum></citerefentry> for more information.</entry>
</row>
<row>
<entry><literal>%H</literal></entry>
<entry>Host name</entry>
<entry>The hostname of the running system at the point in time the unit configuration is loaded.</entry>
</row>
<row>
<entry><literal>%v</literal></entry>
<entry>Kernel release</entry>
<entry>Identical to <command>uname -r</command> output</entry>
</row>
<row>
<entry><literal>%%</literal></entry>
<entry>Single percent sign</entry>
<entry>Use <literal>%%</literal> in place of <literal>%</literal> to specify a single percent sign.</entry>
</row>
</tbody>
</tgroup>
</table>
</refsect1>
<refsect1>
<title>Examples</title>
<example>
<title>Allowing units to be enabled</title>
<para>The following snippet (highlighted) allows a unit (e.g.
<filename>foo.service</filename>) to be enabled via
<command>systemctl enable</command>:</para>
<programlisting>[Unit]
Description=Foo
[Service]
ExecStart=/usr/sbin/foo-daemon
<emphasis>[Install]</emphasis>
<emphasis>WantedBy=multi-user.target</emphasis></programlisting>
<para>After running <command>systemctl enable</command>, a
symlink
<filename>/etc/systemd/system/multi-user.target.wants/foo.service</filename>
linking to the actual unit will be created. It tells systemd to
pull in the unit when starting
<filename>multi-user.target</filename>. The inverse
<command>systemctl disable</command> will remove that symlink
again.</para>
</example>
<example>
<title>Overriding vendor settings</title>
<para>There are two methods of overriding vendor settings in
unit files: copying the unit file from
<filename>/usr/lib/systemd/system</filename> to
<filename>/etc/systemd/system</filename> and modifying the
chosen settings. Alternatively, one can create a directory named
<filename><replaceable>unit</replaceable>.d/</filename> within
<filename>/etc/systemd/system</filename> and place a drop-in
file <filename><replaceable>name</replaceable>.conf</filename>
there that only changes the specific settings one is interested
in. Note that multiple such drop-in files are read if
present, processed in lexicographic order of their filename.</para>
<para>The advantage of the first method is that one easily
overrides the complete unit, the vendor unit is not parsed at
all anymore. It has the disadvantage that improvements to the
unit file by the vendor are not automatically incorporated on
updates.</para>
<para>The advantage of the second method is that one only
overrides the settings one specifically wants, where updates to
the unit by the vendor automatically apply. This has the
disadvantage that some future updates by the vendor might be
incompatible with the local changes.</para>
<para>This also applies for user instances of systemd, but with
different locations for the unit files. See the section on unit
load paths for further details.</para>
<para>Suppose there is a vendor-supplied unit
<filename>/usr/lib/systemd/system/httpd.service</filename> with
the following contents:</para>
<programlisting>[Unit]
Description=Some HTTP server
After=remote-fs.target sqldb.service
Requires=sqldb.service
AssertPathExists=/srv/webserver
[Service]
Type=notify
ExecStart=/usr/sbin/some-fancy-httpd-server
Nice=5
[Install]
WantedBy=multi-user.target</programlisting>
<para>Now one wants to change some settings as an administrator:
firstly, in the local setup, <filename>/srv/webserver</filename>
might not exist, because the HTTP server is configured to use
<filename>/srv/www</filename> instead. Secondly, the local
configuration makes the HTTP server also depend on a memory
cache service, <filename>memcached.service</filename>, that
should be pulled in (<varname>Requires=</varname>) and also be
ordered appropriately (<varname>After=</varname>). Thirdly, in
order to harden the service a bit more, the administrator would
like to set the <varname>PrivateTmp=</varname> setting (see
<citerefentry><refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for details). And lastly, the administrator would like to reset
the niceness of the service to its default value of 0.</para>
<para>The first possibility is to copy the unit file to
<filename>/etc/systemd/system/httpd.service</filename> and
change the chosen settings:</para>
<programlisting>[Unit]
Description=Some HTTP server
After=remote-fs.target sqldb.service <emphasis>memcached.service</emphasis>
Requires=sqldb.service <emphasis>memcached.service</emphasis>
AssertPathExists=<emphasis>/srv/www</emphasis>
[Service]
Type=notify
ExecStart=/usr/sbin/some-fancy-httpd-server
<emphasis>Nice=0</emphasis>
<emphasis>PrivateTmp=yes</emphasis>
[Install]
WantedBy=multi-user.target</programlisting>
<para>Alternatively, the administrator could create a drop-in
file
<filename>/etc/systemd/system/httpd.service.d/local.conf</filename>
with the following contents:</para>
<programlisting>[Unit]
After=memcached.service
Requires=memcached.service
# Reset all assertions and then re-add the condition we want
AssertPathExists=
AssertPathExists=/srv/www
[Service]
Nice=0
PrivateTmp=yes</programlisting>
<para>Note that for drop-in files, if one wants to remove
entries from a setting that is parsed as a list (and is not a
dependency), such as <varname>AssertPathExists=</varname> (or
e.g. <varname>ExecStart=</varname> in service units), one needs
to first clear the list before re-adding all entries except the
one that is to be removed. Dependencies (<varname>After=</varname>, etc.)
cannot be reset to an empty list, so dependencies can only be
added in drop-ins. If you want to remove dependencies, you have
to override the entire unit.</para>
</example>
</refsect1>
<refsect1>
<title>See Also</title>
<para>
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.special</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.service</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.socket</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.mount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.automount</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.swap</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.target</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.path</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.timer</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.scope</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.slice</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.time</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>capabilities</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry project='man-pages'><refentrytitle>uname</refentrytitle><manvolnum>1</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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