systemd/udev/udev.xml

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<refentry id="udev">
  <refentryinfo>
    <title>udev</title>
    <productname>udev</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>udev</refentrytitle>
    <manvolnum>7</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>udev</refname>
    <refpurpose>Linux dynamic device management</refpurpose>
  </refnamediv>

  <refsect1><title>Description</title>
    <para>udev supplies the system software with device events, manages permissions
    of device nodes and may create additional symlinks in the <filename>/dev</filename>
    directory, or renames network interfaces. The kernel usually just assigns unpredictable
    device names based on the order of discovery. Meaningful symlinks or network device
    names provide a way to reliably identify devices based on their properties or
    current configuration.</para>

    <para>The udev daemon, <citerefentry><refentrytitle>udevd</refentrytitle>
    <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
    the kernel whenever a device is added or removed from the system, or it changes its
    state. When udev receives a device event, it matches its configured set of rules
    against various device attributes to identify the device. Rules that match may
    provide additional device information to be stored in the udev database or
    to be used to create meaningful symlink names.</para>

    <para>All device information udev processes is stored in the udev database and
    sent out to possible event subscribers. Access to all stored data and the event
    sources is provided by the library libudev.</para>
  </refsect1>

  <refsect1><title>Configuration</title>
    <para>udev configuration files are placed in <filename>/etc/udev/</filename>
    and <filename>/lib/udev/</filename>. All empty lines or lines beginning with
    '#' are ignored.</para>

    <refsect2><title>Configuration file</title>
      <para>udev expects its main configuration file at <filename>/etc/udev/udev.conf</filename>.
      It consists of a set of variables allowing the user to override default udev values.
      The following variables can be set:</para>
      <variablelist>
        <varlistentry>
          <term><option>udev_root</option></term>
          <listitem>
            <para>Specifies where to place the device nodes in the filesystem.
            The default value is <filename>/dev</filename>.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>udev_log</option></term>
          <listitem>
            <para>The logging priority. Valid values are the numerical syslog priorities
            or their textual representations: <option>err</option>, <option>info</option>
            and <option>debug</option>.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>

    <refsect2><title>Rules files</title>
      <para>The udev rules are read from the files located in the
      default rules directory <filename>/lib/udev/rules.d/</filename>,
      the custom rules directory <filename>/etc/udev/rules.d/</filename>
      and the temporary rules directory <filename>/run/udev/rules.d/</filename>.
      All rule files are collectively sorted and processed in lexical order,
      regardless of the directories in which they live. However, files in
      <filename>/etc/udev/rules.d/</filename> take precedence over files with
      the same name in <filename>/lib/udev/rules.d/</filename>; this can be
      used to ignore a default rules file if needed.</para>

      <para>Rule files must have the extension <filename>.rules</filename>; other
      extensions are ignored.</para>

      <para>Every line in the rules file contains at least one key-value pair.
      There are two kind of keys: match and assignment.
      If all match keys are matching against its value, the rule gets applied and the
      assignment keys get the specified value assigned.</para>

      <para>A matching rule may rename a network interface, add symlinks
      pointing to the device node, or run a specified program as part of
      the event handling.</para>

      <para>A rule consists of a comma-separated list of one or more key-value pairs.
      Each key has a distinct operation, depending on the used operator. Valid
      operators are:</para>
      <variablelist>
        <varlistentry>
          <term><option>==</option></term>
          <listitem>
            <para>Compare for equality.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>!=</option></term>
          <listitem>
            <para>Compare for inequality.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>=</option></term>
          <listitem>
            <para>Assign a value to a key. Keys that represent a list are reset
            and only this single value is assigned.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>+=</option></term>
          <listitem>
            <para>Add the value to a key that holds a list of entries.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>:=</option></term>
          <listitem>
            <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>The following key names can be used to match against device properties.
      Some of the keys also match against properties of the parent devices in sysfs,
      not only the device that has generated the event. If multiple keys that match
      a parent device are specified in a single rule, all these keys must match at
      one and the same parent device.</para>
      <variablelist>
        <varlistentry>
          <term><option>ACTION</option></term>
          <listitem>
            <para>Match the name of the event action.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>DEVPATH</option></term>
          <listitem>
            <para>Match the devpath of the event device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>KERNEL</option></term>
          <listitem>
            <para>Match the name of the event device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>NAME</option></term>
          <listitem>
            <para>Match the name of the node or network interface. It can
            be used once the NAME key has been set in one of the preceding
            rules.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>SYMLINK</option></term>
          <listitem>
            <para>Match the name of a symlink targeting the node. It can
            be used once a SYMLINK key has been set in one of the preceding
            rules. There may be multiple symlinks; only one needs to match.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>SUBSYSTEM</option></term>
          <listitem>
            <para>Match the subsystem of the event device.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><option>DRIVER</option></term>
          <listitem>
            <para>Match the driver name of the event device. Only set this key for devices
            which are bound to a driver at the time the event is generated.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><option>ATTR{<replaceable>filename</replaceable>}</option></term>
          <listitem>
            <para>Match sysfs attribute values of the event device. Trailing
            whitespace in the attribute values is ignored unless the specified match
            value itself contains trailing whitespace.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>KERNELS</option></term>
          <listitem>
            <para>Search the devpath upwards for a matching device name.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>SUBSYSTEMS</option></term>
          <listitem>
            <para>Search the devpath upwards for a matching device subsystem name.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>DRIVERS</option></term>
          <listitem>
            <para>Search the devpath upwards for a matching device driver name.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>ATTRS{<replaceable>filename</replaceable>}</option></term>
          <listitem>
            <para>Search the devpath upwards for a device with matching sysfs attribute values.
            If multiple <option>ATTRS</option> matches are specified, all of them
            must match on the same device. Trailing whitespace in the attribute values is ignored
            unless the specified match value itself contains trailing whitespace.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>ENV{<replaceable>key</replaceable>}</option></term>
          <listitem>
            <para>Match against a device property value.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>TAG</option></term>
          <listitem>
            <para>Match against a device tag.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>TEST{<replaceable>octal mode mask</replaceable>}</option></term>
          <listitem>
            <para>Test the existence of a file. An octal mode mask can be specified
            if needed.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>PROGRAM</option></term>
          <listitem>
            <para>Execute a program to determine whether there
            is a match; the key is true if the program returns
            successfully. The device properties are made available to the
            executed program in the environment. The program's stdout
            is available in the RESULT key.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>RESULT</option></term>
          <listitem>
            <para>Match the returned string of the last PROGRAM call. This key can
            be used in the same or in any later rule after a PROGRAM call.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>Most of the fields support shell-style pattern matching. The following
      pattern characters are supported:</para>
      <variablelist>
        <varlistentry>
          <term><option>*</option></term>
          <listitem>
            <para>Matches zero or more characters.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><option>?</option></term>
          <listitem>
            <para>Matches any single character.</para>
          </listitem>
        </varlistentry>
        <varlistentry>
          <term><option>[]</option></term>
          <listitem>
            <para>Matches any single character specified within the brackets. For
            example, the pattern string 'tty[SR]' would match either 'ttyS' or 'ttyR'.
            Ranges are also supported via the '-' character.
            For example, to match on the range of all digits, the pattern [0-9] could
            be used. If the first character following the '[' is a '!', any characters
            not enclosed are matched.</para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>The following keys can get values assigned:</para>
      <variablelist>
        <varlistentry>
          <term><option>NAME</option></term>
          <listitem>
            <para>What a network interface should be named.</para>
            <para>Also, as a temporary workaround, this is what a device node
            should be named; usually the kernel provides the defined node name or creates
            and removes the node before udev even receives any event. Changing
            the node name from the kernel's default creates inconsistencies
            and is not supported. If the kernel and NAME specify different names,
            an error is logged. udev is only expected to handle device node
            permissions and to create additional symlinks, not to change
            kernel-provided device node names. Instead of renaming a device node,
            SYMLINK should be used. However, symlink names must never conflict with
            device node names, as that would result in unpredictable behavior.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>SYMLINK</option></term>
          <listitem>
            <para>The name of a symlink targeting the node. Every matching rule adds
            this value to the list of symlinks to be created. Multiple symlinks may be
            specified by separating the names by the space character. In case multiple
            devices claim the same name, the link always points to the device with
            the highest link_priority. If the current device goes away, the links are
            re-evaluated and the device with the next highest link_priority becomes the owner of
            the link. If no link_priority is specified, the order of the devices (and
            which one of them owns the link) is undefined. Also, symlink names must
            never conflict with the kernel's default device node names, as that would
            result in unpredictable behavior.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>OWNER, GROUP, MODE</option></term>
          <listitem>
            <para>The permissions for the device node. Every specified value overwrites
            the compiled-in default value.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>ATTR{<replaceable>key</replaceable>}</option></term>
          <listitem>
            <para>The value that should be written to a sysfs attribute of the
            event device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>ENV{<replaceable>key</replaceable>}</option></term>
          <listitem>
            <para>Set a device property value. Property names with a leading '.'
            are neither stored in the database nor exported to events or
            external tools (run by, say, the PROGRAM match key).</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>TAG</option></term>
          <listitem>
            <para>Attach a tag to a device. This is used to filter events for users
            of libudev's monitor functionality, or to enumerate a group of tagged
            devices. The implementation can only work efficiently if only a few
            tags are attached to a device. It is only meant to be used in
            contexts with specific device filter requirements, and not as a
            general-purpose flag. Excessive use might result in inefficient event
            handling.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>RUN</option></term>
          <listitem>
            <para>Add a program to the list of programs to be executed for a specific
            device. This can only be used for very short running tasks. Running an
            event process for a long period of time may block all further events for
            this or a dependent device. Long running tasks need to be immediately
            detached from the event process itself.</para>
            <para>If no absolute path is given, the program is expected to live in
            the directory provided at compile-time to configure via --libexecdir
            (this is usually <filename>/lib/udev</filename>), otherwise the absolute
            path must be specified. The program name and following arguments are
            separated by spaces. Single quotes can be used to specify arguments with
            spaces.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>LABEL</option></term>
          <listitem>
            <para>A named label to which a GOTO may jump.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>GOTO</option></term>
          <listitem>
            <para>Jumps to the next LABEL with a matching name</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>IMPORT{<replaceable>type</replaceable>}</option></term>
          <listitem>
            <para>Import a set of variables as device properties,
            depending on <replaceable>type</replaceable>:</para>
            <variablelist>
              <varlistentry>
                <term><option>program</option></term>
                <listitem>
                  <para>Execute an external program specified as the assigned value and
                  import its output, which must be in environment key
                  format. Path specification, command/argument separation,
                  and quoting work like in <option>RUN</option>.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>file</option></term>
                <listitem>
                  <para>Import a text file specified as the assigned value, which must be in
                  environment key format.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>db</option></term>
                <listitem>
                  <para>Import a single property specified as the assigned value from the
                  current device database. This works only if the database is already populated
                  by an earlier event.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>cmdline</option></term>
                <listitem>
                  <para>Import a single property from the kernel commandline. For simple flags
                  the value of the property is set to '1'.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>parent</option></term>
                <listitem>
                  <para>Import the stored keys from the parent device by reading
                  the database entry of the parent device. The value assigned to
                  <option>IMPORT{parent}</option> is used as a filter of key names
                  to import (with the same shell-style pattern matching used for
                  comparisons).</para>
                </listitem>
              </varlistentry>
            </variablelist>
            <para>If no option is given, udev chooses between <option>program</option>
            and <option>file</option> based on the executable bit of the file
            permissions.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>WAIT_FOR</option></term>
          <listitem>
            <para>Wait for a file to become available or until a 10
            seconds timeout expires. The path is relative to the sysfs device,
            i. e. if no path is specified this waits for an attribute to appear.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>OPTIONS</option></term>
          <listitem>
            <para>Rule and device options:</para>
            <variablelist>
              <varlistentry>
                <term><option>link_priority=<replaceable>value</replaceable></option></term>
                <listitem>
                  <para>Specify the priority of the created symlinks. Devices with higher
                  priorities overwrite existing symlinks of other devices. The default is 0.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>event_timeout=</option></term>
                <listitem>
                  <para>Number of seconds an event waits for operations to finish before
                  giving up and terminating itself.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
                <listitem>
                  <para>Usually control and other possibly unsafe characters are replaced
                  in strings used for device naming. The mode of replacement can be specified
                  with this option.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>static_node=</option></term>
                <listitem>
                  <para>Apply the permissions specified in this rule to a static device node with
                  the specified name. Static device nodes might be provided by kernel modules
                  or copied from <filename>/lib/udev/devices</filename>. These nodes might not have
                  a corresponding kernel device at the time udevd is started; they can trigger
                  automatic kernel module loading.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>watch</option></term>
                <listitem>
                  <para>Watch the device node with inotify; when closed after being opened for
                  writing, a change uevent is synthesised.</para>
                </listitem>
              </varlistentry>
              <varlistentry>
                <term><option>nowatch</option></term>
                <listitem>
                  <para>Disable the watching of a device node with inotify.</para>
                </listitem>
              </varlistentry>
            </variablelist>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>The <option>NAME</option>, <option>SYMLINK</option>, <option>PROGRAM</option>,
      <option>OWNER</option>, <option>GROUP</option>, <option>MODE</option>  and  <option>RUN</option>
      fields support simple printf-like string substitutions. The <option>RUN</option>
      format chars gets applied after all rules have been processed, right before the program
      is executed. It allows the use of device properties set by earlier matching
      rules. For all other fields, substitutions are applied while the individual rule is
      being processed. The available substitutions are:</para>
      <variablelist>
        <varlistentry>
          <term><option>$kernel</option>, <option>%k</option></term>
          <listitem>
            <para>The kernel name for this device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$number</option>, <option>%n</option></term>
          <listitem>
            <para>The kernel number for this device. For example, 'sda3' has
            kernel number of '3'</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$devpath</option>, <option>%p</option></term>
          <listitem>
            <para>The devpath of the device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$id</option>, <option>%b</option></term>
          <listitem>
            <para>The name of the device matched while searching the devpath upwards for
              <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$driver</option></term>
          <listitem>
            <para>The driver name of the device matched while searching the devpath upwards for
              <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
          <listitem>
            <para>The value of a sysfs attribute found at the device where
            all keys of the rule have matched. If the matching device does not have
            such an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or
            ATTRS test selected a parent device, use the attribute from that
            parent device.
            If the attribute is a symlink, the last element of the symlink target is
            returned as the value.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
          <listitem>
            <para>A device property value.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$major</option>, <option>%M</option></term>
          <listitem>
            <para>The kernel major number for the device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$minor</option>, <option>%m</option></term>
          <listitem>
            <para>The kernel minor number for the device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$result</option>, <option>%c</option></term>
          <listitem>
            <para>The string returned by the external program requested with PROGRAM.
            A single part of the string, separated by a space character may be selected
            by specifying the part number as an attribute: <option>%c{N}</option>.
            If the number is followed by the '+' char this part plus all remaining parts
            of the result string are substituted: <option>%c{N+}</option></para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$parent</option>, <option>%P</option></term>
          <listitem>
            <para>The node name of the parent device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$name</option></term>
          <listitem>
            <para>The current name of the device node. If not changed by a rule, it is the
            name of the kernel device.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$links</option></term>
          <listitem>
            <para>The current list of symlinks, separated by a space character. The value is
            only set if an earlier rule assigned a value, or during a remove events.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$root</option>, <option>%r</option></term>
          <listitem>
            <para>The udev_root value.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$sys</option>, <option>%S</option></term>
          <listitem>
            <para>The sysfs mount point.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$tempnode</option>, <option>%N</option></term>
          <listitem>
            <para>The name of a created temporary device node to provide access to
            the device from a external program before the real node is created.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>%%</option></term>
          <listitem>
          <para>The '%' character itself.</para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><option>$$</option></term>
          <listitem>
          <para>The '$' character itself.</para>
          </listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <refsect1><title>Author</title>
    <para>Written by Greg Kroah-Hartman <email>greg@kroah.com</email> and
    Kay Sievers <email>kay.sievers@vrfy.org</email>. With much help from
    Dan Stekloff and many others.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>
    <para><citerefentry>
        <refentrytitle>udevd</refentrytitle><manvolnum>8</manvolnum>
      </citerefentry>, 
      <citerefentry>
        <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
    </citerefentry></para>
  </refsect1>
</refentry>