man/systemd.device.xml

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  23
  24 <refentry id="systemd.device">
  25   <refentryinfo>
  26     <title>systemd.device</title>
  27     <productname>systemd</productname>
  28
  29     <authorgroup>
  30       <author>
  31         <contrib>Developer</contrib>
  32         <firstname>Lennart</firstname>
  33         <surname>Poettering</surname>
  34         <email>lennart@poettering.net</email>
  35       </author>
  36     </authorgroup>
  37   </refentryinfo>
  38
  39   <refmeta>
  40     <refentrytitle>systemd.device</refentrytitle>
  41     <manvolnum>5</manvolnum>
  42   </refmeta>
  43
  44   <refnamediv>
  45     <refname>systemd.device</refname>
  46     <refpurpose>Device unit configuration</refpurpose>
  47   </refnamediv>
  48
  49   <refsynopsisdiv>
  50     <para><filename><replaceable>device</replaceable>.device</filename></para>
  51   </refsynopsisdiv>
  52
  53   <refsect1>
  54     <title>Description</title>
  55
  56     <para>A unit configuration file whose name ends in
  57     <literal>.device</literal> encodes information about a device unit
  58     as exposed in the
  59     sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  60     device tree.</para>
  61
  62     <para>This unit type has no specific options. See
  63     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  64     for the common options of all unit configuration files. The common
  65     configuration items are configured in the generic
  66     <literal>[Unit]</literal> and <literal>[Install]</literal>
  67     sections. A separate <literal>[Device]</literal> section does not
  68     exist, since no device-specific options may be configured.</para>
  69
  70     <para>systemd will dynamically create device units for all kernel
  71     devices that are marked with the "systemd" udev tag (by default
  72     all block and network devices, and a few others). This may be used
  73     to define dependencies between devices and other units. To tag a
  74     udev device, use <literal>TAG+="systemd"</literal> in the udev
  75     rules file, see
  76     <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  77     for details.</para>
  78
  79     <para>Device units are named after the <filename>/sys</filename>
  80     and <filename>/dev</filename> paths they control. Example: the
  81     device <filename noindex='true'>/dev/sda5</filename> is exposed in
  82     systemd as <filename>dev-sda5.device</filename>. For details about
  83     the escaping logic used to convert a file system path to a unit
  84     name see
  85     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  86
  87     <para>Device units will be reloaded by systemd whenever the
  88     corresponding device generates a <literal>changed</literal> event.
  89     Other units can use <varname>ReloadPropagatedFrom=</varname> to react
  90     to that event</para>
  91
  92   </refsect1>
  93
  94   <refsect1>
  95     <title>Automatic Dependencies</title>
  96
  97     <para>Many unit types automatically acquire dependencies on device
  98     units of devices they require. For example,
  99     <filename>.socket</filename> unit acquire dependencies on the
 100     device units of the network interface specified in
 101     <varname>BindToDevice=</varname>. Similar, swap and mount units
 102     acquire dependencies on the units encapsulating their backing
 103     block devices.</para>
 104   </refsect1>
 105
 106   <refsect1>
 107     <title>The udev Database</title>
 108
 109     <para>The settings of device units may either be configured via
 110     unit files, or directly from the udev database (which is
 111     recommended). The following udev device properties are understood
 112     by systemd:</para>
 113
 114     <variablelist class='udev-directives'>
 115       <varlistentry>
 116         <term><varname>SYSTEMD_WANTS=</varname></term>
 117         <term><varname>SYSTEMD_USER_WANTS=</varname></term>
 118         <listitem><para>Adds dependencies of type
 119         <varname>Wants</varname> from the device unit to all listed
 120         units. The first form is used by the system systemd instance,
 121         the second by user systemd instances. Those settings may be
 122         used to activate arbitrary units when a specific device
 123         becomes available.</para>
 124
 125         <para>Note that this and the other tags are not taken into
 126         account unless the device is tagged with the
 127         <literal>systemd</literal> string in the udev database,
 128         because otherwise the device is not exposed as a systemd unit
 129         (see above).</para>
 130
 131         <para>Note that systemd will only act on
 132         <varname>Wants</varname> dependencies when a device first
 133         becomes active. It will not act on them if they are added to
 134         devices that are already active. Use
 135         <varname>SYSTEMD_READY=</varname> (see below) to influence on
 136         which udev event to trigger the dependencies.
 137         </para></listitem>
 138       </varlistentry>
 139
 140       <varlistentry>
 141         <term><varname>SYSTEMD_ALIAS=</varname></term>
 142         <listitem><para>Adds an additional alias name to the device
 143         unit. This must be an absolute path that is automatically
 144         transformed into a unit name. (See above.)</para></listitem>
 145       </varlistentry>
 146
 147       <varlistentry>
 148         <term><varname>SYSTEMD_READY=</varname></term>
 149         <listitem><para>If set to 0, systemd will consider this device
 150         unplugged even if it shows up in the udev tree. If this
 151         property is unset or set to 1, the device will be considered
 152         plugged if it is visible in the udev tree. This property has
 153         no influence on the behavior when a device disappears from the
 154         udev tree.</para>
 155
 156         <para>This option is useful to support devices that initially
 157         show up in an uninitialized state in the tree, and for which a
 158         <literal>changed</literal> event is generated the moment they
 159         are fully set up. Note that <varname>SYSTEMD_WANTS=</varname>
 160         (see above) is not acted on as long as
 161         <varname>SYSTEMD_READY=0</varname> is set for a
 162         device.</para></listitem>
 163       </varlistentry>
 164
 165       <varlistentry>
 166         <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
 167         <term><varname>ID_MODEL=</varname></term>
 168
 169         <listitem><para>If set, this property is used as description
 170         string for the device unit.</para></listitem>
 171       </varlistentry>
 172
 173     </variablelist>
 174
 175   </refsect1>
 176
 177   <refsect1>
 178       <title>See Also</title>
 179       <para>
 180         <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 181         <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 182         <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 183         <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 184         <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 185       </para>
 186   </refsect1>
 187
 188 </refentry>