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+<?xml version='1.0'?>
+<?xml-stylesheet type="text/xsl" href="http://docbook.sourceforge.net/release/xsl/current/xhtml/docbook.xsl"?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<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>/usr/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
+      system rules directory <filename>/usr/lib/udev/rules.d</filename>,
+      the volatile runtime directory <filename>/run/udev/rules.d</filename>
+      and the local administration directory <filename>/etc/udev/rules.d</filename>.
+      All rules files are collectively sorted and processed in lexical order,
+      regardless of the directories in which they live. However, files with
+      identical file names replace each other. Files in <filename>/etc</filename>
+      have the highest priority, files in <filename>/run</filename> take precedence
+      over files with the same name in <filename>/lib</filename>. This can be
+      used to override a system-supplied rules file with a local file if needed;
+      a symlink in <filename>/etc</filename> with the same name as a rules file in
+      <filename>/lib</filename>, pointing to <filename>/dev/null</filename>,
+      disables the rules file entirely.</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 a 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>TAGS</option></term>
+          <listitem>
+            <para>Search the devpath upwards for a device with matching tag.</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>The name to use for a network interface. The name of a device node
+            can not be changed by udev, only additional symlinks can be created.</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 overrides
+            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.</para>
+            <para>If no absolute path is given, the program is expected to live in
+            /usr/lib/udev, 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>
+            <para>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. Starting daemons or other long running processes
+            is not appropriate for udev.</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, the content
+                  of 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 command line. 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>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><option>WAIT_FOR</option></term>
+          <listitem>
+            <para>Wait for a file to become available or until a timeout of
+            10 seconds expires. The path is relative to the sysfs device;
+            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 the static device node with
+                  the specified name. Static device nodes might be provided by kernel modules
+                  or copied from <filename>/usr/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 the node is closed after being opened for
+                  writing, a change uevent is synthesized.</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 string substitutions. The <option>RUN</option>
+      substitutions are performed after all rules have been processed, right before the program
+      is executed, allowing for the use of device properties set by earlier matching
+      rules. For all other fields, substitutions are performed 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, then the attribute from that
+            parent device is used.</para>
+            <para>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 '+' character, 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. If not changed by a rule, it is the
+            name of the kernel device.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><option>$links</option></term>
+          <listitem>
+            <para>A space-separated list of the current symlinks. The value is
+            only set during a remove event or if an earlier rule assigned a value.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><option>$root</option>, <option>%r</option></term>
+          <listitem>
+            <para>The udev_root value.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><option>$sys</option>, <option>%S</option></term>
+          <listitem>
+            <para>The sysfs mount point.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><option>$devnode</option>, <option>%N</option></term>
+          <listitem>
+            <para>The name of the device node.</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>