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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2010 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="systemd.device">
<refentryinfo>
<title>systemd.device</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.device</refentrytitle>
<manvolnum>5</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd.device</refname>
<refpurpose>Device unit configuration</refpurpose>
</refnamediv>
<refsynopsisdiv>
<para><filename><replaceable>device</replaceable>.device</filename></para>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>A unit configuration file whose name ends in
<literal>.device</literal> encodes information about a device unit
as exposed in the
sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
device tree.</para>
<para>This unit type has no specific options. See
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
for the common options of all unit configuration files. The common
configuration items are configured in the generic
<literal>[Unit]</literal> and <literal>[Install]</literal>
sections. A separate <literal>[Device]</literal> section does not
exist, since no device-specific options may be configured.</para>
<para>systemd will dynamically create device units for all kernel
devices that are marked with the "systemd" udev tag (by default
all block and network devices, and a few others). This may be used
to define dependencies between devices and other units. To tag a
udev device, use <literal>TAG+="systemd"</literal> in the udev
rules file, see
<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
for details.</para>
<para>Device units are named after the <filename>/sys</filename>
and <filename>/dev</filename> paths they control. Example: the
device <filename noindex='true'>/dev/sda5</filename> is exposed in
systemd as <filename>dev-sda5.device</filename>. For details about
the escaping logic used to convert a file system path to a unit
name see
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
</refsect1>
<refsect1>
<title>Automatic Dependencies</title>
<para>Many unit types automatically acquire dependencies on device
units of devices they require. For example,
<filename>.socket</filename> unit acquire dependencies on the
device units of the network interface specified in
<varname>BindToDevice=</varname>. Similar, swap and mount units
acquire dependencies on the units encapsulating their backing
block devices.</para>
</refsect1>
<refsect1>
<title>The udev Database</title>
<para>The settings of device units may either be configured via
unit files, or directly from the udev database (which is
recommended). The following udev device properties are understood
by systemd:</para>
<variablelist class='udev-directives'>
<varlistentry>
<term><varname>SYSTEMD_WANTS=</varname></term>
<term><varname>SYSTEMD_USER_WANTS=</varname></term>
<listitem><para>Adds dependencies of type
<varname>Wants</varname> from the device unit to all listed
units. The first form is used by the system systemd instance,
the second by user systemd instances. Those settings may be
used to activate arbitrary units when a specific device
becomes available.</para>
<para>Note that this and the other tags are not taken into
account unless the device is tagged with the
<literal>systemd</literal> string in the udev database,
because otherwise the device is not exposed as a systemd unit
(see above).</para>
<para>Note that systemd will only act on
<varname>Wants</varname> dependencies when a device first
becomes active. It will not act on them if they are added to
devices that are already active. Use
<varname>SYSTEMD_READY=</varname> (see below) to influence on
which udev event to trigger the dependencies.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>SYSTEMD_ALIAS=</varname></term>
<listitem><para>Adds an additional alias name to the device
unit. This must be an absolute path that is automatically
transformed into a unit name. (See above.)</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>SYSTEMD_READY=</varname></term>
<listitem><para>If set to 0, systemd will consider this device
unplugged even if it shows up in the udev tree. If this
property is unset or set to 1, the device will be considered
plugged if it is visible in the udev tree. This property has
no influence on the behavior when a device disappears from the
udev tree.</para>
<para>This option is useful to support devices that initially
show up in an uninitialized state in the tree, and for which a
<literal>changed</literal> event is generated the moment they
are fully set up. Note that <varname>SYSTEMD_WANTS=</varname>
(see above) is not acted on as long as
<varname>SYSTEMD_READY=0</varname> is set for a
device.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
<term><varname>ID_MODEL=</varname></term>
<listitem><para>If set, this property is used as description
string for the device unit.</para></listitem>
</varlistentry>
</variablelist>
</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.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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