man/udev.html

   1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>udev</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><style>
   2     a.headerlink {
   3       color: #c60f0f;
   4       font-size: 0.8em;
   5       padding: 0 4px 0 4px;
   6       text-decoration: none;
   7       visibility: hidden;
   8     }
   9
  10     a.headerlink:hover {
  11       background-color: #c60f0f;
  12       color: white;
  13     }
  14
  15     h1:hover > a.headerlink, h2:hover > a.headerlink, h3:hover > a.headerlink, dt:hover > a.headerlink {
  16       visibility: visible;
  17     }
  18   </style><a href="index.html">Index </a>·
  19   <a href="systemd.directives.html">Directives </a>·
  20   <a href="../python-systemd/index.html">Python </a>·
  21   <a href="../libudev/index.html">libudev </a>·
  22   <a href="../libudev/index.html">gudev </a><span style="float:right">systemd 214</span><hr><div class="refentry"><a name="udev"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>udev — Dynamic device management</p></div><div class="refsect1"><a name="idm214182478784"></a><h2 id="Description">Description<a class="headerlink" title="Permalink to this headline" href="#Description">¶</a></h2><p>udev supplies the system software with device events, manages permissions
  23     of device nodes and may create additional symlinks in the <code class="filename">/dev</code>
  24     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  25     device names based on the order of discovery. Meaningful symlinks or network device
  26     names provide a way to reliably identify devices based on their properties or
  27     current configuration.</p><p>The udev daemon, <a href="systemd-udevd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd-udevd.service</span>(8)</span></a>, receives device uevents directly from
  28     the kernel whenever a device is added or removed from the system, or it changes its
  29     state. When udev receives a device event, it matches its configured set of rules
  30     against various device attributes to identify the device. Rules that match may
  31     provide additional device information to be stored in the udev database or
  32     to be used to create meaningful symlink names.</p><p>All device information udev processes is stored in the udev database and
  33     sent out to possible event subscribers. Access to all stored data and the event
  34     sources is provided by the library libudev.</p></div><div class="refsect1"><a name="idm214182474192"></a><h2 id="Rules Files">Rules Files<a class="headerlink" title="Permalink to this headline" href="#Rules%20Files">¶</a></h2><p>The udev rules are read from the files located in the
  35       system rules directory <code class="filename">/usr/lib/udev/rules.d</code>,
  36       the volatile runtime directory <code class="filename">/run/udev/rules.d</code>
  37       and the local administration directory <code class="filename">/etc/udev/rules.d</code>.
  38       All rules files are collectively sorted and processed in lexical order,
  39       regardless of the directories in which they live. However, files with
  40       identical filenames replace each other. Files in <code class="filename">/etc</code>
  41       have the highest priority, files in <code class="filename">/run</code> take precedence
  42       over files with the same name in <code class="filename">/usr/lib</code>. This can be
  43       used to override a system-supplied rules file with a local file if needed;
  44       a symlink in <code class="filename">/etc</code> with the same name as a rules file in
  45       <code class="filename">/usr/lib</code>, pointing to <code class="filename">/dev/null</code>,
  46       disables the rules file entirely. Rule files must have the extension
  47       <code class="filename">.rules</code>; other extensions are ignored.</p><p>Every line in the rules file contains at least one key-value pair.
  48       Except for empty lines or lines beginning with "<code class="literal">#</code>", which are ignored.
  49       There are two kinds of keys: match and assignment.
  50       If all match keys match against their values, the rule gets applied and the
  51       assignment keys get the specified values assigned.</p><p>A matching rule may rename a network interface, add symlinks
  52       pointing to the device node, or run a specified program as part of
  53       the event handling.</p><p>A rule consists of a comma-separated list of one or more key-value pairs.
  54       Each key has a distinct operation, depending on the used operator. Valid
  55       operators are:</p><div class="variablelist"><dl class="variablelist"><dt id="=="><span class="term">"<code class="literal">==</code>"</span><a class="headerlink" title="Permalink to this term" href="#==">¶</a></dt><dd><p>Compare for equality.</p></dd><dt id="!="><span class="term">"<code class="literal">!=</code>"</span><a class="headerlink" title="Permalink to this term" href="#!=">¶</a></dt><dd><p>Compare for inequality.</p></dd><dt id="="><span class="term">"<code class="literal">=</code>"</span><a class="headerlink" title="Permalink to this term" href="#=">¶</a></dt><dd><p>Assign a value to a key. Keys that represent a list are reset
  56             and only this single value is assigned.</p></dd><dt id="+="><span class="term">"<code class="literal">+=</code>"</span><a class="headerlink" title="Permalink to this term" href="#+=">¶</a></dt><dd><p>Add the value to a key that holds a list of entries.</p></dd><dt id=":="><span class="term">"<code class="literal">:=</code>"</span><a class="headerlink" title="Permalink to this term" href="#:=">¶</a></dt><dd><p>Assign  a  value  to  a key finally; disallow any later changes.</p></dd></dl></div><p>The following key names can be used to match against device properties.
  57       Some of the keys also match against properties of the parent devices in sysfs,
  58       not only the device that has generated the event. If multiple keys that match
  59       a parent device are specified in a single rule, all these keys must match at
  60       one and the same parent device.</p><div class="variablelist"><dl class="variablelist"><dt id="ACTION"><span class="term"><code class="varname">ACTION</code></span><a class="headerlink" title="Permalink to this term" href="#ACTION">¶</a></dt><dd><p>Match the name of the event action.</p></dd><dt id="DEVPATH"><span class="term"><code class="varname">DEVPATH</code></span><a class="headerlink" title="Permalink to this term" href="#DEVPATH">¶</a></dt><dd><p>Match the devpath of the event device.</p></dd><dt id="KERNEL"><span class="term"><code class="varname">KERNEL</code></span><a class="headerlink" title="Permalink to this term" href="#KERNEL">¶</a></dt><dd><p>Match the name of the event device.</p></dd><dt id="NAME"><span class="term"><code class="varname">NAME</code></span><a class="headerlink" title="Permalink to this term" href="#NAME">¶</a></dt><dd><p>Match the name of a network interface. It can be used once the
  61             NAME key has been set in one of the preceding rules.</p></dd><dt id="SYMLINK"><span class="term"><code class="varname">SYMLINK</code></span><a class="headerlink" title="Permalink to this term" href="#SYMLINK">¶</a></dt><dd><p>Match the name of a symlink targeting the node. It can
  62             be used once a SYMLINK key has been set in one of the preceding
  63             rules. There may be multiple symlinks; only one needs to match.
  64             </p></dd><dt id="SUBSYSTEM"><span class="term"><code class="varname">SUBSYSTEM</code></span><a class="headerlink" title="Permalink to this term" href="#SUBSYSTEM">¶</a></dt><dd><p>Match the subsystem of the event device.</p></dd><dt id="DRIVER"><span class="term"><code class="varname">DRIVER</code></span><a class="headerlink" title="Permalink to this term" href="#DRIVER">¶</a></dt><dd><p>Match the driver name of the event device. Only set this key for devices
  65             which are bound to a driver at the time the event is generated.</p></dd><dt id="ATTR{filename}"><span class="term"><code class="varname">ATTR{<em class="replaceable"><code>filename</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#ATTR%7Bfilename%7D">¶</a></dt><dd><p>Match sysfs attribute values of the event device. Trailing
  66             whitespace in the attribute values is ignored unless the specified match
  67             value itself contains trailing whitespace.
  68             </p></dd><dt id="KERNELS"><span class="term"><code class="varname">KERNELS</code></span><a class="headerlink" title="Permalink to this term" href="#KERNELS">¶</a></dt><dd><p>Search the devpath upwards for a matching device name.</p></dd><dt id="SUBSYSTEMS"><span class="term"><code class="varname">SUBSYSTEMS</code></span><a class="headerlink" title="Permalink to this term" href="#SUBSYSTEMS">¶</a></dt><dd><p>Search the devpath upwards for a matching device subsystem name.</p></dd><dt id="DRIVERS"><span class="term"><code class="varname">DRIVERS</code></span><a class="headerlink" title="Permalink to this term" href="#DRIVERS">¶</a></dt><dd><p>Search the devpath upwards for a matching device driver name.</p></dd><dt id="ATTRS{filename}"><span class="term"><code class="varname">ATTRS{<em class="replaceable"><code>filename</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#ATTRS%7Bfilename%7D">¶</a></dt><dd><p>Search the devpath upwards for a device with matching sysfs attribute values.
  69             If multiple <code class="varname">ATTRS</code> matches are specified, all of them
  70             must match on the same device. Trailing whitespace in the attribute values is ignored
  71             unless the specified match value itself contains trailing whitespace.</p></dd><dt id="TAGS"><span class="term"><code class="varname">TAGS</code></span><a class="headerlink" title="Permalink to this term" href="#TAGS">¶</a></dt><dd><p>Search the devpath upwards for a device with matching tag.</p></dd><dt id="ENV{key}"><span class="term"><code class="varname">ENV{<em class="replaceable"><code>key</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#ENV%7Bkey%7D">¶</a></dt><dd><p>Match against a device property value.</p></dd><dt id="TAG"><span class="term"><code class="varname">TAG</code></span><a class="headerlink" title="Permalink to this term" href="#TAG">¶</a></dt><dd><p>Match against a device tag.</p></dd><dt id="TEST{octal mode mask}"><span class="term"><code class="varname">TEST{<em class="replaceable"><code>octal mode mask</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#TEST%7Boctal%20mode%20mask%7D">¶</a></dt><dd><p>Test the existence of a file. An octal mode mask can be specified
  72             if needed.</p></dd><dt id="PROGRAM"><span class="term"><code class="varname">PROGRAM</code></span><a class="headerlink" title="Permalink to this term" href="#PROGRAM">¶</a></dt><dd><p>Execute a program to determine whether there
  73             is a match; the key is true if the program returns
  74             successfully. The device properties are made available to the
  75             executed program in the environment. The program's standard ouput
  76             is available in the <code class="varname">RESULT</code> key.</p><p>This can only be used for very short-running foreground tasks. For details,
  77             see <code class="varname">RUN</code>.</p></dd><dt id="RESULT"><span class="term"><code class="varname">RESULT</code></span><a class="headerlink" title="Permalink to this term" href="#RESULT">¶</a></dt><dd><p>Match the returned string of the last <code class="varname">PROGRAM</code> call.
  78             This key can be used in the same or in any later rule after a
  79             <code class="varname">PROGRAM</code> call.</p></dd></dl></div><p>Most of the fields support shell glob pattern matching. The following
  80       pattern characters are supported:</p><div class="variablelist"><dl class="variablelist"><dt id="*"><span class="term">"<code class="literal">*</code>"</span><a class="headerlink" title="Permalink to this term" href="#*">¶</a></dt><dd><p>Matches zero or more characters.</p></dd><dt id="?"><span class="term">"<code class="literal">?</code>"</span><a class="headerlink" title="Permalink to this term" href="#?">¶</a></dt><dd><p>Matches any single character.</p></dd><dt id="[]"><span class="term">"<code class="literal">[]</code>"</span><a class="headerlink" title="Permalink to this term" href="#%5B%5D">¶</a></dt><dd><p>Matches any single character specified within the brackets. For
  81             example, the pattern string "<code class="literal">tty[SR]</code>"
  82             would match either "<code class="literal">ttyS</code>" or "<code class="literal">ttyR</code>".
  83             Ranges are also supported via the "<code class="literal">-</code>" character.
  84             For example, to match on the range of all digits, the pattern
  85             "<code class="literal">[0-9]</code>" could be used. If the first character
  86             following the "<code class="literal">[</code>" is a "<code class="literal">!</code>",
  87             any characters not enclosed are matched.</p></dd></dl></div><p>The following keys can get values assigned:</p><div class="variablelist"><dl class="variablelist"><dt id="NAME"><span class="term"><code class="varname">NAME</code></span><a class="headerlink" title="Permalink to this term" href="#NAME">¶</a></dt><dd><p>The name to use for a network interface. See
  88             <a href="systemd.link.html"><span class="citerefentry"><span class="refentrytitle">systemd.link</span>(5)</span></a>
  89             for a higher-level mechanism for setting the interface name.
  90             The name of a device node cannot be changed by udev, only additional
  91             symlinks can be created.</p></dd><dt id="SYMLINK"><span class="term"><code class="varname">SYMLINK</code></span><a class="headerlink" title="Permalink to this term" href="#SYMLINK">¶</a></dt><dd><p>The name of a symlink targeting the node. Every matching rule adds
  92             this value to the list of symlinks to be created.</p><p>The set of characters to name a symlink is limited. Allowed
  93             characters are "<code class="literal">0-9A-Za-z#+-.:=@_/</code>", valid UTF-8 character
  94             sequences, and "<code class="literal">\x00</code>" hex encoding. All other
  95             characters are replaced by a "<code class="literal">_</code>" character.</p><p>Multiple symlinks may be specified by separating the names by the
  96             space character. In case multiple devices claim the same name, the link
  97             always points to the device with the highest link_priority. If the current
  98             device goes away, the links are re-evaluated and the device with the
  99             next highest link_priority becomes the owner of the link. If no
 100             link_priority is specified, the order of the devices (and which one of
 101             them owns the link) is undefined.</p><p>Symlink names must never conflict with the kernel's default device
 102             node names, as that would result in unpredictable behavior.
 103             </p></dd><dt id="OWNER, GROUP, MODE"><span class="term"><code class="varname">OWNER</code>, <code class="varname">GROUP</code>, <code class="varname">MODE</code></span><a class="headerlink" title="Permalink to this term" href="#OWNER,%20GROUP,%20MODE">¶</a></dt><dd><p>The permissions for the device node. Every specified value overrides
 104             the compiled-in default value.</p></dd><dt id="SECLABEL{module}"><span class="term"><code class="varname">SECLABEL{<em class="replaceable"><code>module</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#SECLABEL%7Bmodule%7D">¶</a></dt><dd><p>Applies the specified Linux Security Module label to the device node.</p></dd><dt id="ATTR{key}"><span class="term"><code class="varname">ATTR{<em class="replaceable"><code>key</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#ATTR%7Bkey%7D">¶</a></dt><dd><p>The value that should be written to a sysfs attribute of the
 105             event device.</p></dd><dt id="ENV{key}"><span class="term"><code class="varname">ENV{<em class="replaceable"><code>key</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#ENV%7Bkey%7D">¶</a></dt><dd><p>Set a device property value. Property names with a leading "<code class="literal">.</code>"
 106             are neither stored in the database nor exported to events or
 107             external tools (run by, for example, the <code class="varname">PROGRAM</code>
 108             match key).</p></dd><dt id="TAG"><span class="term"><code class="varname">TAG</code></span><a class="headerlink" title="Permalink to this term" href="#TAG">¶</a></dt><dd><p>Attach a tag to a device. This is used to filter events for users
 109             of libudev's monitor functionality, or to enumerate a group of tagged
 110             devices. The implementation can only work efficiently if only a few
 111             tags are attached to a device. It is only meant to be used in
 112             contexts with specific device filter requirements, and not as a
 113             general-purpose flag. Excessive use might result in inefficient event
 114             handling.</p></dd><dt id="RUN{type}"><span class="term"><code class="varname">RUN{<em class="replaceable"><code>type</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#RUN%7Btype%7D">¶</a></dt><dd><p>Add a program to the list of programs to be executed after
 115             processing all the rules for a specific event, depending on
 116             "<code class="literal">type</code>":</p><div class="variablelist"><dl class="variablelist"><dt id="program"><span class="term">"<code class="literal">program</code>"</span><a class="headerlink" title="Permalink to this term" href="#program">¶</a></dt><dd><p>Execute an external program specified as the assigned
 117                   value. If no absolute path is given, the program is expected
 118                   to live in <code class="filename">/usr/lib/udev</code>; otherwise, the
 119                   absolute path must be specified.</p><p>This is the default if no <em class="replaceable"><code>type</code></em>
 120                   is specified.</p></dd><dt id="builtin"><span class="term">"<code class="literal">builtin</code>"</span><a class="headerlink" title="Permalink to this term" href="#builtin">¶</a></dt><dd><p>As <code class="varname">program</code>, but use one of the
 121                   built-in programs rather than an external one.</p></dd></dl></div><p>The program name and following arguments are separated by spaces.
 122             Single quotes can be used to specify arguments with spaces.</p><p>This can only be used for very short-running foreground tasks. Running an
 123             event process for a long period of time may block all further events for
 124             this or a dependent device.</p><p>Starting daemons or other long-running processes is not appropriate
 125             for udev; the forked processes, detached or not, will be unconditionally
 126             killed after the event handling has finished.</p></dd><dt id="LABEL"><span class="term"><code class="varname">LABEL</code></span><a class="headerlink" title="Permalink to this term" href="#LABEL">¶</a></dt><dd><p>A named label to which a <code class="varname">GOTO</code> may jump.</p></dd><dt id="GOTO"><span class="term"><code class="varname">GOTO</code></span><a class="headerlink" title="Permalink to this term" href="#GOTO">¶</a></dt><dd><p>Jumps to the next <code class="varname">LABEL</code> with a matching name.</p></dd><dt id="IMPORT{type}"><span class="term"><code class="varname">IMPORT{<em class="replaceable"><code>type</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#IMPORT%7Btype%7D">¶</a></dt><dd><p>Import a set of variables as device properties,
 127             depending on "<code class="literal">type</code>":</p><div class="variablelist"><dl class="variablelist"><dt id="program"><span class="term">"<code class="literal">program</code>"</span><a class="headerlink" title="Permalink to this term" href="#program">¶</a></dt><dd><p>Execute an external program specified as the assigned value and
 128                   import its output, which must be in environment key
 129                   format. Path specification, command/argument separation,
 130                   and quoting work like in <code class="varname">RUN</code>.</p></dd><dt id="builtin"><span class="term">"<code class="literal">builtin</code>"</span><a class="headerlink" title="Permalink to this term" href="#builtin">¶</a></dt><dd><p>Similar to "<code class="literal">program</code>", but use one of the
 131                   built-in programs rather than an external one.</p></dd><dt id="file"><span class="term">"<code class="literal">file</code>"</span><a class="headerlink" title="Permalink to this term" href="#file">¶</a></dt><dd><p>Import a text file specified as the assigned value, the content
 132                   of which must be in environment key format.</p></dd><dt id="db"><span class="term">"<code class="literal">db</code>"</span><a class="headerlink" title="Permalink to this term" href="#db">¶</a></dt><dd><p>Import a single property specified as the assigned value from the
 133                   current device database. This works only if the database is already populated
 134                   by an earlier event.</p></dd><dt id="cmdline"><span class="term">"<code class="literal">cmdline</code>"</span><a class="headerlink" title="Permalink to this term" href="#cmdline">¶</a></dt><dd><p>Import a single property from the kernel command line. For simple flags
 135                   the value of the property is set to "<code class="literal">1</code>".</p></dd><dt id="parent"><span class="term">"<code class="literal">parent</code>"</span><a class="headerlink" title="Permalink to this term" href="#parent">¶</a></dt><dd><p>Import the stored keys from the parent device by reading
 136                   the database entry of the parent device. The value assigned to
 137                   <code class="option">IMPORT{parent}</code> is used as a filter of key names
 138                   to import (with the same shell glob pattern matching used for
 139                   comparisons).</p></dd></dl></div><p>This can only be used for very short-running foreground tasks. For details
 140             see <code class="option">RUN</code>.</p></dd><dt id="WAIT_FOR"><span class="term"><code class="varname">WAIT_FOR</code></span><a class="headerlink" title="Permalink to this term" href="#WAIT_FOR">¶</a></dt><dd><p>Wait for a file to become available or until a timeout of
 141             10 seconds expires. The path is relative to the sysfs device;
 142             if no path is specified, this waits for an attribute to appear.</p></dd><dt id="OPTIONS"><span class="term"><code class="varname">OPTIONS</code></span><a class="headerlink" title="Permalink to this term" href="#OPTIONS">¶</a></dt><dd><p>Rule and device options:</p><div class="variablelist"><dl class="variablelist"><dt id="link_priority=value"><span class="term"><code class="option">link_priority=<em class="replaceable"><code>value</code></em></code></span><a class="headerlink" title="Permalink to this term" href="#link_priority=value">¶</a></dt><dd><p>Specify the priority of the created symlinks. Devices with higher
 143                   priorities overwrite existing symlinks of other devices. The default is 0.</p></dd><dt id="event_timeout="><span class="term"><code class="option">event_timeout=</code></span><a class="headerlink" title="Permalink to this term" href="#event_timeout=">¶</a></dt><dd><p>Number of seconds an event waits for operations to finish before
 144                   giving up and terminating itself.</p></dd><dt id="string_escape=none|replace"><span class="term"><code class="option">string_escape=<em class="replaceable"><code>none|replace</code></em></code></span><a class="headerlink" title="Permalink to this term" href="#string_escape=none%7Creplace">¶</a></dt><dd><p>Usually control and other possibly unsafe characters are replaced
 145                   in strings used for device naming. The mode of replacement can be specified
 146                   with this option.</p></dd><dt id="static_node="><span class="term"><code class="option">static_node=</code></span><a class="headerlink" title="Permalink to this term" href="#static_node=">¶</a></dt><dd><p>Apply the permissions specified in this rule to the
 147                   static device node with the specified name. Also, for every
 148                   tag specified in this rule, create a symlink
 149                   in the directory
 150                   <code class="filename">/run/udev/static_node-tags/<em class="replaceable"><code>tag</code></em></code>
 151                   pointing at the static device node with the specified name.
 152                   Static device node creation is performed by systemd-tmpfiles
 153                   before systemd-udevd is started. The static nodes might not
 154                   have a corresponding kernel device; they are used to trigger
 155                   automatic kernel module loading when they are accessed.</p></dd><dt id="watch"><span class="term"><code class="option">watch</code></span><a class="headerlink" title="Permalink to this term" href="#watch">¶</a></dt><dd><p>Watch the device node with inotify; when the node is
 156                   closed after being opened for writing, a change uevent is
 157                   synthesized.</p></dd><dt id="nowatch"><span class="term"><code class="option">nowatch</code></span><a class="headerlink" title="Permalink to this term" href="#nowatch">¶</a></dt><dd><p>Disable the watching of a device node with inotify.</p></dd></dl></div></dd></dl></div><p>The <code class="varname">NAME</code>, <code class="varname">SYMLINK</code>,
 158       <code class="varname">PROGRAM</code>, <code class="varname">OWNER</code>,
 159       <code class="varname">GROUP</code>, <code class="varname">MODE</code>, and
 160       <code class="varname">RUN</code> fields support simple string substitutions.
 161       The <code class="varname">RUN</code> substitutions are performed after all rules
 162       have been processed, right before the program is executed, allowing for
 163       the use of device properties set by earlier matching rules. For all other
 164       fields, substitutions are performed while the individual rule is being
 165       processed. The available substitutions are:</p><div class="variablelist"><dl class="variablelist"><dt id="$kernel, %k"><span class="term"><code class="option">$kernel</code>, <code class="option">%k</code></span><a class="headerlink" title="Permalink to this term" href="#%24kernel,%20%k">¶</a></dt><dd><p>The kernel name for this device.</p></dd><dt id="$number, %n"><span class="term"><code class="option">$number</code>, <code class="option">%n</code></span><a class="headerlink" title="Permalink to this term" href="#%24number,%20%n">¶</a></dt><dd><p>The kernel number for this device. For example,
 166               "<code class="literal">sda3</code>" has kernel number "<code class="literal">3</code>".
 167             </p></dd><dt id="$devpath, %p"><span class="term"><code class="option">$devpath</code>, <code class="option">%p</code></span><a class="headerlink" title="Permalink to this term" href="#%24devpath,%20%p">¶</a></dt><dd><p>The devpath of the device.</p></dd><dt id="$id, %b"><span class="term"><code class="option">$id</code>, <code class="option">%b</code></span><a class="headerlink" title="Permalink to this term" href="#%24id,%20%b">¶</a></dt><dd><p>The name of the device matched while searching the devpath
 168               upwards for <code class="option">SUBSYSTEMS</code>, <code class="option">KERNELS</code>,
 169               <code class="option">DRIVERS</code>, and <code class="option">ATTRS</code>.
 170             </p></dd><dt id="$driver"><span class="term"><code class="option">$driver</code></span><a class="headerlink" title="Permalink to this term" href="#%24driver">¶</a></dt><dd><p>The driver name of the device matched while searching the
 171               devpath upwards for <code class="option">SUBSYSTEMS</code>,
 172               <code class="option">KERNELS</code>, <code class="option">DRIVERS</code>, and
 173               <code class="option">ATTRS</code>.
 174             </p></dd><dt id="$attr{file}, %s{file}"><span class="term"><code class="option">$attr{<em class="replaceable"><code>file</code></em>}</code>, <code class="option">%s{<em class="replaceable"><code>file</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#%24attr%7Bfile%7D,%20%s%7Bfile%7D">¶</a></dt><dd><p>The value of a sysfs attribute found at the device where
 175               all keys of the rule have matched. If the matching device does not
 176               have such an attribute, and a previous <code class="option">KERNELS</code>,
 177               <code class="option">SUBSYSTEMS</code>, <code class="option">DRIVERS</code>, or
 178               <code class="option">ATTRS</code> test selected a parent device, then the
 179               attribute from that parent device is used.
 180             </p><p>If the attribute is a symlink, the last element of the
 181               symlink target is returned as the value.
 182             </p></dd><dt id="$env{key}, %E{key}"><span class="term"><code class="option">$env{<em class="replaceable"><code>key</code></em>}</code>, <code class="option">%E{<em class="replaceable"><code>key</code></em>}</code></span><a class="headerlink" title="Permalink to this term" href="#%24env%7Bkey%7D,%20%E%7Bkey%7D">¶</a></dt><dd><p>A device property value.</p></dd><dt id="$major, %M"><span class="term"><code class="option">$major</code>, <code class="option">%M</code></span><a class="headerlink" title="Permalink to this term" href="#%24major,%20%M">¶</a></dt><dd><p>The kernel major number for the device.</p></dd><dt id="$minor, %m"><span class="term"><code class="option">$minor</code>, <code class="option">%m</code></span><a class="headerlink" title="Permalink to this term" href="#%24minor,%20%m">¶</a></dt><dd><p>The kernel minor number for the device.</p></dd><dt id="$result, %c"><span class="term"><code class="option">$result</code>, <code class="option">%c</code></span><a class="headerlink" title="Permalink to this term" href="#%24result,%20%c">¶</a></dt><dd><p>The string returned by the external program requested with
 183             <code class="varname">PROGRAM</code>.
 184             A single part of the string, separated by a space character, may be selected
 185             by specifying the part number as an attribute: "<code class="literal">%c{N}</code>".
 186             If the number is followed by the "<code class="literal">+</code>" character, this part plus all remaining parts
 187             of the result string are substituted: "<code class="literal">%c{N+}</code>".</p></dd><dt id="$parent, %P"><span class="term"><code class="option">$parent</code>, <code class="option">%P</code></span><a class="headerlink" title="Permalink to this term" href="#%24parent,%20%P">¶</a></dt><dd><p>The node name of the parent device.</p></dd><dt id="$name"><span class="term"><code class="option">$name</code></span><a class="headerlink" title="Permalink to this term" href="#%24name">¶</a></dt><dd><p>The current name of the device. If not changed by a rule, it is the
 188             name of the kernel device.</p></dd><dt id="$links"><span class="term"><code class="option">$links</code></span><a class="headerlink" title="Permalink to this term" href="#%24links">¶</a></dt><dd><p>A space-separated list of the current symlinks. The value is
 189             only set during a remove event or if an earlier rule assigned a value.</p></dd><dt id="$root, %r"><span class="term"><code class="option">$root</code>, <code class="option">%r</code></span><a class="headerlink" title="Permalink to this term" href="#%24root,%20%r">¶</a></dt><dd><p>The udev_root value.</p></dd><dt id="$sys, %S"><span class="term"><code class="option">$sys</code>, <code class="option">%S</code></span><a class="headerlink" title="Permalink to this term" href="#%24sys,%20%S">¶</a></dt><dd><p>The sysfs mount point.</p></dd><dt id="$devnode, %N"><span class="term"><code class="option">$devnode</code>, <code class="option">%N</code></span><a class="headerlink" title="Permalink to this term" href="#%24devnode,%20%N">¶</a></dt><dd><p>The name of the device node.</p></dd><dt id="%%"><span class="term"><code class="option">%%</code></span><a class="headerlink" title="Permalink to this term" href="#%%">¶</a></dt><dd><p>The "<code class="literal">%</code>" character itself.</p></dd><dt id="$$"><span class="term"><code class="option">$$</code></span><a class="headerlink" title="Permalink to this term" href="#%24%24">¶</a></dt><dd><p>The "<code class="literal">$</code>" character itself.</p></dd></dl></div></div><div class="refsect1"><a name="idm214181258176"></a><h2 id="Hardware Database Files">Hardware Database Files<a class="headerlink" title="Permalink to this headline" href="#Hardware%20Database%20Files">¶</a></h2><p>The hwdb files are read from the files located in the
 190       system hwdb directory <code class="filename">/usr/lib/udev/hwdb.d</code>,
 191       the volatile runtime directory <code class="filename">/run/udev/hwdb.d</code>
 192       and the local administration directory <code class="filename">/etc/udev/hwdb.d</code>.
 193       All hwdb files are collectively sorted and processed in lexical order,
 194       regardless of the directories in which they live. However, files with
 195       identical filenames replace each other. Files in <code class="filename">/etc</code>
 196       have the highest priority, files in <code class="filename">/run</code> take precedence
 197       over files with the same name in <code class="filename">/usr/lib</code>. This can be
 198       used to override a system-supplied hwdb file with a local file if needed;
 199       a symlink in <code class="filename">/etc</code> with the same name as a hwdb file in
 200       <code class="filename">/usr/lib</code>, pointing to <code class="filename">/dev/null</code>,
 201       disables the hwdb file entirely. hwdb files must have the extension
 202       <code class="filename">.hwdb</code>; other extensions are ignored.</p><p>The hwdb file contains data records consisting of matches and
 203       associated key-value pairs. Every record in the hwdb starts with one or
 204       more match string, specifying a shell glob to compare the database
 205       lookup string against. Multiple match lines are specified in additional
 206       consecutive lines. Every match line is compared indivdually, they are
 207       combined by OR. Every match line must start at the first character of
 208       the line.</p><p>The match lines are followed by one or more key-value pair lines, which
 209       are recognized by a leading space character. The key name and value are separated
 210       by "<code class="literal">=</code>". An empty line signifies the end
 211       of a record. Lines beginning with "<code class="literal">#</code>" are ignored.</p><p>The content of all hwdb files is read by
 212       <a href="udevadm.html"><span class="citerefentry"><span class="refentrytitle">udevadm</span>(8)</span></a>
 213       and compiled to a binary database located at <code class="filename">/etc/udev/hwdb.bin</code>.
 214       During runtime only the binary database is used.</p></div><div class="refsect1"><a name="idm214181245744"></a><h2 id="See Also">See Also<a class="headerlink" title="Permalink to this headline" href="#See%20Also">¶</a></h2><p>
 215       <a href="systemd-udevd.service.html"><span class="citerefentry"><span class="refentrytitle">systemd-udevd.service</span>(8)</span></a>,
 216       <a href="udevadm.html"><span class="citerefentry"><span class="refentrytitle">udevadm</span>(8)</span></a>
 217       <a href="systemd.link.html"><span class="citerefentry"><span class="refentrytitle">systemd.link</span>(5)</span></a>
 218     </p></div></div></body></html>