man/systemd.device.xml

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  24
  25 <refentry id="systemd.device">
  26         <refentryinfo>
  27                 <title>systemd.device</title>
  28                 <productname>systemd</productname>
  29
  30                 <authorgroup>
  31                         <author>
  32                                 <contrib>Developer</contrib>
  33                                 <firstname>Lennart</firstname>
  34                                 <surname>Poettering</surname>
  35                                 <email>lennart@poettering.net</email>
  36                         </author>
  37                 </authorgroup>
  38         </refentryinfo>
  39
  40         <refmeta>
  41                 <refentrytitle>systemd.device</refentrytitle>
  42                 <manvolnum>5</manvolnum>
  43         </refmeta>
  44
  45         <refnamediv>
  46                 <refname>systemd.device</refname>
  47                 <refpurpose>Device unit configuration</refpurpose>
  48         </refnamediv>
  49
  50         <refsynopsisdiv>
  51                 <para><filename><replaceable>device</replaceable>.device</filename></para>
  52         </refsynopsisdiv>
  53
  54         <refsect1>
  55                 <title>Description</title>
  56
  57                 <para>A unit configuration file whose name ends in
  58                 <literal>.device</literal> encodes information about
  59                 a device unit as exposed in the
  60                 sysfs/<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  61                 device tree.</para>
  62
  63                 <para>This unit type has no specific options. See
  64                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
  65                 for the common options of all unit configuration
  66                 files. The common configuration items are configured
  67                 in the generic <literal>[Unit]</literal> and
  68                 <literal>[Install]</literal> sections. A separate
  69                 <literal>[Device]</literal> section does not exist,
  70                 since no device-specific options may be
  71                 configured.</para>
  72
  73                 <para>systemd will dynamically create device units for
  74                 all kernel devices that are marked with the "systemd"
  75                 udev tag (by default all block and network devices,
  76                 and a few others). This may be used to define
  77                 dependencies between devices and other units. To tag a
  78                 udev device, use <literal>TAG+="systemd"</literal> in
  79                 the udev rules file, see
  80                 <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>
  81                 for details.</para>
  82
  83                 <para>Device units are named after the
  84                 <filename>/sys</filename> and
  85                 <filename>/dev</filename> paths they control. Example:
  86                 the device <filename noindex='true'>/dev/sda5</filename> is exposed
  87                 in systemd as <filename>dev-sda5.device</filename>. For
  88                 details about the escaping logic used to convert a
  89                 file system path to a unit name see
  90                 <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
  91
  92         </refsect1>
  93
  94         <refsect1>
  95                 <title>The udev Database</title>
  96
  97                 <para>The settings of device units may either be
  98                 configured via unit files, or directly from the udev
  99                 database (which is recommended). The following udev device
 100                 properties are understood by systemd:</para>
 101
 102                 <variablelist class='udev-directives'>
 103                         <varlistentry>
 104                                 <term><varname>SYSTEMD_WANTS=</varname></term>
 105                                 <term><varname>SYSTEMD_USER_WANTS=</varname></term>
 106                                 <listitem><para>Adds dependencies of
 107                                 type <varname>Wants</varname> from the
 108                                 device unit to all listed units. The
 109                                 first form is used by the system
 110                                 systemd instance, the second by user
 111                                 systemd instances. Those settings may
 112                                 be used to activate arbitrary units
 113                                 when a specific device becomes
 114                                 available.</para>
 115
 116                                 <para>Note that this and the
 117                                 other tags are not taken into account
 118                                 unless the device is tagged with the
 119                                 <literal>systemd</literal> string in
 120                                 the udev database, because otherwise
 121                                 the device is not exposed as a systemd
 122                                 unit (see above).</para>
 123
 124                                 <para>Note that systemd will only act
 125                                 on <varname>Wants</varname>
 126                                 dependencies when a device first
 127                                 becomes active. It will not act on
 128                                 them if they are added to devices that
 129                                 are already active. Use
 130                                 <varname>SYSTEMD_READY=</varname> (see
 131                                 below) to influence on which udev
 132                                 event to trigger the dependencies.
 133                                 </para></listitem>
 134                         </varlistentry>
 135
 136                         <varlistentry>
 137                                 <term><varname>SYSTEMD_ALIAS=</varname></term>
 138                                 <listitem><para>Adds an additional
 139                                 alias name to the device unit. This
 140                                 must be an absolute path that is
 141                                 automatically transformed into a unit
 142                                 name. (See above.)</para></listitem>
 143                         </varlistentry>
 144
 145                         <varlistentry>
 146                                 <term><varname>SYSTEMD_READY=</varname></term>
 147                                 <listitem><para>If set to 0, systemd
 148                                 will consider this device unplugged
 149                                 even if it shows up in the udev
 150                                 tree. If this property is unset or set
 151                                 to 1, the device will be considered
 152                                 plugged if it is visible in the
 153                                 udev tree. This property has no
 154                                 influence on the behavior when a
 155                                 device disappears from the udev
 156                                 tree.</para>
 157
 158                                 <para>This option is useful to support
 159                                 devices that initially show up in an
 160                                 uninitialized state in the tree, and
 161                                 for which a <literal>changed</literal>
 162                                 event is generated the moment they are
 163                                 fully set up. Note that
 164                                 <varname>SYSTEMD_WANTS=</varname> (see
 165                                 above) is not acted on as long as
 166                                 <varname>SYSTEMD_READY=0</varname> is
 167                                 set for a device.</para></listitem>
 168                         </varlistentry>
 169
 170                         <varlistentry>
 171                                 <term><varname>ID_MODEL_FROM_DATABASE=</varname></term>
 172                                 <term><varname>ID_MODEL=</varname></term>
 173
 174                                 <listitem><para>If set, this property is
 175                                 used as description string for the
 176                                 device unit.</para></listitem>
 177                         </varlistentry>
 178
 179                 </variablelist>
 180
 181
 182         </refsect1>
 183
 184         <refsect1>
 185                   <title>See Also</title>
 186                   <para>
 187                           <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 188                           <citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 189                           <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
 190                           <citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
 191                           <citerefentry><refentrytitle>systemd.directives</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 192                   </para>
 193         </refsect1>
 194
 195 </refentry>