man/udev.xml

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   6 <refentry id="udev">
   7   <refentryinfo>
   8     <title>udev</title>
   9     <productname>systemd</productname>
  10     <authorgroup>
  11       <author>
  12         <contrib>Developer</contrib>
  13         <firstname>Greg</firstname>
  14         <surname>Kroah-Hartmann</surname>
  15         <email>greg@kroah.com</email>
  16       </author>
  17       <author>
  18         <contrib>Developer</contrib>
  19         <firstname>Kay</firstname>
  20         <surname>Sievers</surname>
  21         <email>kay@vrfy.org</email>
  22       </author>
  23     </authorgroup>
  24   </refentryinfo>
  25
  26   <refmeta>
  27     <refentrytitle>udev</refentrytitle>
  28     <manvolnum>7</manvolnum>
  29   </refmeta>
  30
  31   <refnamediv>
  32     <refname>udev</refname>
  33     <refpurpose>Dynamic device management</refpurpose>
  34   </refnamediv>
  35
  36   <refsect1><title>Description</title>
  37     <para>udev supplies the system software with device events, manages permissions
  38     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  39     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  40     device names based on the order of discovery. Meaningful symlinks or network device
  41     names provide a way to reliably identify devices based on their properties or
  42     current configuration.</para>
  43
  44     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  45     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  46     the kernel whenever a device is added or removed from the system, or it changes its
  47     state. When udev receives a device event, it matches its configured set of rules
  48     against various device attributes to identify the device. Rules that match may
  49     provide additional device information to be stored in the udev database or
  50     to be used to create meaningful symlink names.</para>
  51
  52     <para>All device information udev processes is stored in the udev database and
  53     sent out to possible event subscribers. Access to all stored data and the event
  54     sources is provided by the library libudev.</para>
  55   </refsect1>
  56
  57   <refsect1><title>Rules Files</title>
  58       <para>The udev rules are read from the files located in the
  59       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  60       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  61       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  62       All rules files are collectively sorted and processed in lexical order,
  63       regardless of the directories in which they live. However, files with
  64       identical filenames replace each other. Files in <filename>/etc</filename>
  65       have the highest priority, files in <filename>/run</filename> take precedence
  66       over files with the same name in <filename>/lib</filename>. This can be
  67       used to override a system-supplied rules file with a local file if needed;
  68       a symlink in <filename>/etc</filename> with the same name as a rules file in
  69       <filename>/lib</filename>, pointing to <filename>/dev/null</filename>,
  70       disables the rules file entirely. Rule files must have the extension
  71       <filename>.rules</filename>; other extensions are ignored.</para>
  72
  73       <para>Every line in the rules file contains at least one key-value pair.
  74       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  75       There are two kinds of keys: match and assignment.
  76       If all match keys match against their values, the rule gets applied and the
  77       assignment keys get the specified values assigned.</para>
  78
  79       <para>A matching rule may rename a network interface, add symlinks
  80       pointing to the device node, or run a specified program as part of
  81       the event handling.</para>
  82
  83       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  84       Each key has a distinct operation, depending on the used operator. Valid
  85       operators are:</para>
  86       <variablelist>
  87         <varlistentry>
  88           <term><literal>==</literal></term>
  89           <listitem>
  90             <para>Compare for equality.</para>
  91           </listitem>
  92         </varlistentry>
  93
  94         <varlistentry>
  95           <term><literal>!=</literal></term>
  96           <listitem>
  97             <para>Compare for inequality.</para>
  98           </listitem>
  99         </varlistentry>
 100
 101         <varlistentry>
 102           <term><literal>=</literal></term>
 103           <listitem>
 104             <para>Assign a value to a key. Keys that represent a list are reset
 105             and only this single value is assigned.</para>
 106           </listitem>
 107         </varlistentry>
 108
 109         <varlistentry>
 110           <term><literal>+=</literal></term>
 111           <listitem>
 112             <para>Add the value to a key that holds a list of entries.</para>
 113           </listitem>
 114         </varlistentry>
 115
 116         <varlistentry>
 117           <term><literal>:=</literal></term>
 118           <listitem>
 119             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 120           </listitem>
 121         </varlistentry>
 122       </variablelist>
 123
 124       <para>The following key names can be used to match against device properties.
 125       Some of the keys also match against properties of the parent devices in sysfs,
 126       not only the device that has generated the event. If multiple keys that match
 127       a parent device are specified in a single rule, all these keys must match at
 128       one and the same parent device.</para>
 129       <variablelist class='udev-directives'>
 130         <varlistentry>
 131           <term><varname>ACTION</varname></term>
 132           <listitem>
 133             <para>Match the name of the event action.</para>
 134           </listitem>
 135         </varlistentry>
 136
 137         <varlistentry>
 138           <term><varname>DEVPATH</varname></term>
 139           <listitem>
 140             <para>Match the devpath of the event device.</para>
 141           </listitem>
 142         </varlistentry>
 143
 144         <varlistentry>
 145           <term><varname>KERNEL</varname></term>
 146           <listitem>
 147             <para>Match the name of the event device.</para>
 148           </listitem>
 149         </varlistentry>
 150
 151         <varlistentry>
 152           <term><varname>NAME</varname></term>
 153           <listitem>
 154             <para>Match the name of a network interface. It can be used once the
 155             NAME key has been set in one of the preceding rules.</para>
 156           </listitem>
 157         </varlistentry>
 158
 159         <varlistentry>
 160           <term><varname>SYMLINK</varname></term>
 161           <listitem>
 162             <para>Match the name of a symlink targeting the node. It can
 163             be used once a SYMLINK key has been set in one of the preceding
 164             rules. There may be multiple symlinks; only one needs to match.
 165             </para>
 166           </listitem>
 167         </varlistentry>
 168
 169         <varlistentry>
 170           <term><varname>SUBSYSTEM</varname></term>
 171           <listitem>
 172             <para>Match the subsystem of the event device.</para>
 173           </listitem>
 174         </varlistentry>
 175         <varlistentry>
 176           <term><varname>DRIVER</varname></term>
 177           <listitem>
 178             <para>Match the driver name of the event device. Only set this key for devices
 179             which are bound to a driver at the time the event is generated.</para>
 180           </listitem>
 181         </varlistentry>
 182         <varlistentry>
 183           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 184           <listitem>
 185             <para>Match sysfs attribute values of the event device. Trailing
 186             whitespace in the attribute values is ignored unless the specified match
 187             value itself contains trailing whitespace.
 188             </para>
 189           </listitem>
 190         </varlistentry>
 191
 192         <varlistentry>
 193           <term><varname>KERNELS</varname></term>
 194           <listitem>
 195             <para>Search the devpath upwards for a matching device name.</para>
 196           </listitem>
 197         </varlistentry>
 198
 199         <varlistentry>
 200           <term><varname>SUBSYSTEMS</varname></term>
 201           <listitem>
 202             <para>Search the devpath upwards for a matching device subsystem name.</para>
 203           </listitem>
 204         </varlistentry>
 205
 206         <varlistentry>
 207           <term><varname>DRIVERS</varname></term>
 208           <listitem>
 209             <para>Search the devpath upwards for a matching device driver name.</para>
 210           </listitem>
 211         </varlistentry>
 212
 213         <varlistentry>
 214           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 215           <listitem>
 216             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 217             If multiple <varname>ATTRS</varname> matches are specified, all of them
 218             must match on the same device. Trailing whitespace in the attribute values is ignored
 219             unless the specified match value itself contains trailing whitespace.</para>
 220           </listitem>
 221         </varlistentry>
 222
 223         <varlistentry>
 224           <term><varname>TAGS</varname></term>
 225           <listitem>
 226             <para>Search the devpath upwards for a device with matching tag.</para>
 227           </listitem>
 228         </varlistentry>
 229
 230         <varlistentry>
 231           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 232           <listitem>
 233             <para>Match against a device property value.</para>
 234           </listitem>
 235         </varlistentry>
 236
 237         <varlistentry>
 238           <term><varname>TAG</varname></term>
 239           <listitem>
 240             <para>Match against a device tag.</para>
 241           </listitem>
 242         </varlistentry>
 243
 244         <varlistentry>
 245           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 246           <listitem>
 247             <para>Test the existence of a file. An octal mode mask can be specified
 248             if needed.</para>
 249           </listitem>
 250         </varlistentry>
 251
 252         <varlistentry>
 253           <term><varname>PROGRAM</varname></term>
 254           <listitem>
 255             <para>Execute a program to determine whether there
 256             is a match; the key is true if the program returns
 257             successfully. The device properties are made available to the
 258             executed program in the environment. The program's stdout
 259             is available in the RESULT key.</para>
 260             <para>This can only be used for very short-running foreground tasks. For details
 261             see <varname>RUN</varname>.</para>
 262           </listitem>
 263         </varlistentry>
 264
 265         <varlistentry>
 266           <term><varname>RESULT</varname></term>
 267           <listitem>
 268             <para>Match the returned string of the last PROGRAM call. This key can
 269             be used in the same or in any later rule after a PROGRAM call.</para>
 270           </listitem>
 271         </varlistentry>
 272       </variablelist>
 273
 274       <para>Most of the fields support shell glob pattern matching. The following
 275       pattern characters are supported:</para>
 276       <variablelist>
 277         <varlistentry>
 278           <term><literal>*</literal></term>
 279           <listitem>
 280             <para>Matches zero or more characters.</para>
 281           </listitem>
 282         </varlistentry>
 283         <varlistentry>
 284           <term><literal>?</literal></term>
 285           <listitem>
 286             <para>Matches any single character.</para>
 287           </listitem>
 288         </varlistentry>
 289         <varlistentry>
 290           <term><literal>[]</literal></term>
 291           <listitem>
 292             <para>Matches any single character specified within the brackets. For
 293             example, the pattern string <literal>tty[SR]</literal>
 294             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 295             Ranges are also supported via the <literal>-</literal> character.
 296             For example, to match on the range of all digits, the pattern [0-9] could
 297             be used. If the first character following the <literal>[</literal> is a
 298             <literal>!</literal>, any characters not enclosed are matched.</para>
 299           </listitem>
 300         </varlistentry>
 301       </variablelist>
 302
 303       <para>The following keys can get values assigned:</para>
 304       <variablelist class='udev-directives'>
 305         <varlistentry>
 306           <term><varname>NAME</varname></term>
 307           <listitem>
 308             <para>The name to use for a network interface. The name of a device node
 309             cannot be changed by udev, only additional symlinks can be created.</para>
 310           </listitem>
 311         </varlistentry>
 312
 313         <varlistentry>
 314           <term><varname>SYMLINK</varname></term>
 315           <listitem>
 316             <para>The name of a symlink targeting the node. Every matching rule adds
 317             this value to the list of symlinks to be created.</para>
 318             <para>The set of characters to name a symlink is limited. Allowed
 319             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 320             sequences, and <literal>\x00</literal> hex encoding. All other
 321             characters are replaced by a <literal>_</literal> character.</para>
 322             <para>Multiple symlinks may be specified by separating the names by the
 323             space character. In case multiple devices claim the same name, the link
 324             always points to the device with the highest link_priority. If the current
 325             device goes away, the links are re-evaluated and the device with the
 326             next highest link_priority becomes the owner of the link. If no
 327             link_priority is specified, the order of the devices (and which one of
 328             them owns the link) is undefined.</para>
 329             <para>Symlink names must never conflict with the kernel's default device
 330             node names, as that would result in unpredictable behavior.
 331             </para>
 332           </listitem>
 333         </varlistentry>
 334
 335         <varlistentry>
 336           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 337           <listitem>
 338             <para>The permissions for the device node. Every specified value overrides
 339             the compiled-in default value.</para>
 340           </listitem>
 341         </varlistentry>
 342
 343         <varlistentry>
 344           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 345           <listitem>
 346             <para>The value that should be written to a sysfs attribute of the
 347             event device.</para>
 348           </listitem>
 349         </varlistentry>
 350
 351         <varlistentry>
 352           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 353           <listitem>
 354             <para>Set a device property value. Property names with a leading <literal>.</literal>
 355             are neither stored in the database nor exported to events or
 356             external tools (run by, say, the PROGRAM match key).</para>
 357           </listitem>
 358         </varlistentry>
 359
 360         <varlistentry>
 361           <term><varname>TAG</varname></term>
 362           <listitem>
 363             <para>Attach a tag to a device. This is used to filter events for users
 364             of libudev's monitor functionality, or to enumerate a group of tagged
 365             devices. The implementation can only work efficiently if only a few
 366             tags are attached to a device. It is only meant to be used in
 367             contexts with specific device filter requirements, and not as a
 368             general-purpose flag. Excessive use might result in inefficient event
 369             handling.</para>
 370           </listitem>
 371         </varlistentry>
 372
 373         <varlistentry>
 374           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 375           <listitem>
 376             <para>Add a program to the list of programs to be executed after processing all the
 377             rules for a specific event, depending on <literal>type</literal>:</para>
 378             <variablelist>
 379               <varlistentry>
 380                 <term><literal>program</literal></term>
 381                 <listitem>
 382                   <para>Execute an external program specified as the assigned
 383                   value. If no absolute path is given, the program is expected to live in
 384                   /usr/lib/udev, otherwise the absolute path must be specified.</para>
 385                   <para>This is the default if no <replaceable>type</replaceable> is
 386                   specified.</para>
 387                 </listitem>
 388               </varlistentry>
 389               <varlistentry>
 390                 <term><literal>builtin</literal></term>
 391                 <listitem>
 392                   <para>As <varname>program</varname>, but use one of the built-in programs rather
 393                   than an external one.</para>
 394                 </listitem>
 395               </varlistentry>
 396             </variablelist>
 397             <para>The program name and following arguments are separated by spaces.
 398             Single quotes can be used to specify arguments with spaces.</para>
 399             <para>This can only be used for very short-running foreground tasks. Running an
 400             event process for a long period of time may block all further events for
 401             this or a dependent device.</para>
 402             <para>Starting daemons or other long running processes is not appropriate
 403             for udev; the forked processes, detached or not, will be unconditionally
 404             killed after the event handling has finished.</para>
 405           </listitem>
 406         </varlistentry>
 407
 408         <varlistentry>
 409           <term><varname>LABEL</varname></term>
 410           <listitem>
 411             <para>A named label to which a GOTO may jump.</para>
 412           </listitem>
 413         </varlistentry>
 414
 415         <varlistentry>
 416           <term><varname>GOTO</varname></term>
 417           <listitem>
 418             <para>Jumps to the next LABEL with a matching name.</para>
 419           </listitem>
 420         </varlistentry>
 421
 422         <varlistentry>
 423           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 424           <listitem>
 425             <para>Import a set of variables as device properties,
 426             depending on <literal>type</literal>:</para>
 427             <variablelist>
 428               <varlistentry>
 429                 <term><literal>program</literal></term>
 430                 <listitem>
 431                   <para>Execute an external program specified as the assigned value and
 432                   import its output, which must be in environment key
 433                   format. Path specification, command/argument separation,
 434                   and quoting work like in <varname>RUN</varname>.</para>
 435                 </listitem>
 436               </varlistentry>
 437               <varlistentry>
 438                 <term><literal>builtin</literal></term>
 439                 <listitem>
 440                   <para>Similar to <literal>program</literal>, but use one of the
 441                   built-in programs rather than an external one.</para>
 442                 </listitem>
 443               </varlistentry>
 444              <varlistentry>
 445                 <term><literal>file</literal></term>
 446                 <listitem>
 447                   <para>Import a text file specified as the assigned value, the content
 448                   of which must be in environment key format.</para>
 449                 </listitem>
 450               </varlistentry>
 451               <varlistentry>
 452                 <term><literal>db</literal></term>
 453                 <listitem>
 454                   <para>Import a single property specified as the assigned value from the
 455                   current device database. This works only if the database is already populated
 456                   by an earlier event.</para>
 457                 </listitem>
 458               </varlistentry>
 459               <varlistentry>
 460                 <term><literal>cmdline</literal></term>
 461                 <listitem>
 462                   <para>Import a single property from the kernel command line. For simple flags
 463                   the value of the property is set to <literal>1</literal>.</para>
 464                 </listitem>
 465               </varlistentry>
 466               <varlistentry>
 467                 <term><literal>parent</literal></term>
 468                 <listitem>
 469                   <para>Import the stored keys from the parent device by reading
 470                   the database entry of the parent device. The value assigned to
 471                   <option>IMPORT{parent}</option> is used as a filter of key names
 472                   to import (with the same shell glob pattern matching used for
 473                   comparisons).</para>
 474                 </listitem>
 475               </varlistentry>
 476             </variablelist>
 477             <para>This can only be used for very short-running foreground tasks. For details
 478             see <option>RUN</option>.</para>
 479           </listitem>
 480         </varlistentry>
 481
 482         <varlistentry>
 483           <term><varname>WAIT_FOR</varname></term>
 484           <listitem>
 485             <para>Wait for a file to become available or until a timeout of
 486             10 seconds expires. The path is relative to the sysfs device;
 487             if no path is specified, this waits for an attribute to appear.</para>
 488           </listitem>
 489         </varlistentry>
 490
 491         <varlistentry>
 492           <term><varname>OPTIONS</varname></term>
 493           <listitem>
 494             <para>Rule and device options:</para>
 495             <variablelist class='udev-directives'>
 496               <varlistentry>
 497                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 498                 <listitem>
 499                   <para>Specify the priority of the created symlinks. Devices with higher
 500                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 501                 </listitem>
 502               </varlistentry>
 503               <varlistentry>
 504                 <term><option>event_timeout=</option></term>
 505                 <listitem>
 506                   <para>Number of seconds an event waits for operations to finish before
 507                   giving up and terminating itself.</para>
 508                 </listitem>
 509               </varlistentry>
 510               <varlistentry>
 511                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 512                 <listitem>
 513                   <para>Usually control and other possibly unsafe characters are replaced
 514                   in strings used for device naming. The mode of replacement can be specified
 515                   with this option.</para>
 516                 </listitem>
 517               </varlistentry>
 518               <varlistentry>
 519                 <term><option>static_node=</option></term>
 520                 <listitem>
 521                   <para>Apply the permissions specified in this rule to the static device node with
 522                   the specified name. Also, for every tag specified in this rule, create a symlink
 523                   in the directory
 524                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 525                   pointing at the static device node with the specified name. Static device node
 526                   creation is performed by systemd-tmpfiles before systemd-udevd is started. The
 527                   static nodes might not have a corresponding kernel device; they are used to
 528                   trigger automatic kernel module loading when they are accessed.</para>
 529                 </listitem>
 530               </varlistentry>
 531               <varlistentry>
 532                 <term><option>watch</option></term>
 533                 <listitem>
 534                   <para>Watch the device node with inotify; when the node is closed after being opened for
 535                   writing, a change uevent is synthesized.</para>
 536                 </listitem>
 537               </varlistentry>
 538               <varlistentry>
 539                 <term><option>nowatch</option></term>
 540                 <listitem>
 541                   <para>Disable the watching of a device node with inotify.</para>
 542                 </listitem>
 543               </varlistentry>
 544             </variablelist>
 545           </listitem>
 546         </varlistentry>
 547       </variablelist>
 548
 549       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>, <varname>PROGRAM</varname>,
 550       <varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname>  and  <varname>RUN</varname>
 551       fields support simple string substitutions. The <varname>RUN</varname>
 552       substitutions are performed after all rules have been processed, right before the program
 553       is executed, allowing for the use of device properties set by earlier matching
 554       rules. For all other fields, substitutions are performed while the individual rule is
 555       being processed. The available substitutions are:</para>
 556       <variablelist class='udev-directives'>
 557         <varlistentry>
 558           <term><option>$kernel</option>, <option>%k</option></term>
 559           <listitem>
 560             <para>The kernel name for this device.</para>
 561           </listitem>
 562         </varlistentry>
 563
 564         <varlistentry>
 565           <term><option>$number</option>, <option>%n</option></term>
 566           <listitem>
 567             <para>The kernel number for this device. For example,
 568             <literal>sda3</literal> has kernel number <literal>3</literal>.</para>
 569           </listitem>
 570         </varlistentry>
 571
 572         <varlistentry>
 573           <term><option>$devpath</option>, <option>%p</option></term>
 574           <listitem>
 575             <para>The devpath of the device.</para>
 576           </listitem>
 577         </varlistentry>
 578
 579         <varlistentry>
 580           <term><option>$id</option>, <option>%b</option></term>
 581           <listitem>
 582             <para>The name of the device matched while searching the devpath upwards for
 583               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 584             </para>
 585           </listitem>
 586         </varlistentry>
 587
 588         <varlistentry>
 589           <term><option>$driver</option></term>
 590           <listitem>
 591             <para>The driver name of the device matched while searching the devpath upwards for
 592               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 593             </para>
 594           </listitem>
 595         </varlistentry>
 596
 597         <varlistentry>
 598           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 599           <listitem>
 600             <para>The value of a sysfs attribute found at the device where
 601             all keys of the rule have matched. If the matching device does not have
 602             such an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or
 603             ATTRS test selected a parent device, then the attribute from that
 604             parent device is used.</para>
 605             <para>If the attribute is a symlink, the last element of the symlink target is
 606             returned as the value.</para>
 607           </listitem>
 608         </varlistentry>
 609
 610         <varlistentry>
 611           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 612           <listitem>
 613             <para>A device property value.</para>
 614           </listitem>
 615         </varlistentry>
 616
 617         <varlistentry>
 618           <term><option>$major</option>, <option>%M</option></term>
 619           <listitem>
 620             <para>The kernel major number for the device.</para>
 621           </listitem>
 622         </varlistentry>
 623
 624         <varlistentry>
 625           <term><option>$minor</option>, <option>%m</option></term>
 626           <listitem>
 627             <para>The kernel minor number for the device.</para>
 628           </listitem>
 629         </varlistentry>
 630
 631         <varlistentry>
 632           <term><option>$result</option>, <option>%c</option></term>
 633           <listitem>
 634             <para>The string returned by the external program requested with PROGRAM.
 635             A single part of the string, separated by a space character, may be selected
 636             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 637             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 638             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 639           </listitem>
 640         </varlistentry>
 641
 642         <varlistentry>
 643           <term><option>$parent</option>, <option>%P</option></term>
 644           <listitem>
 645             <para>The node name of the parent device.</para>
 646           </listitem>
 647         </varlistentry>
 648
 649         <varlistentry>
 650           <term><option>$name</option></term>
 651           <listitem>
 652             <para>The current name of the device. If not changed by a rule, it is the
 653             name of the kernel device.</para>
 654           </listitem>
 655         </varlistentry>
 656
 657         <varlistentry>
 658           <term><option>$links</option></term>
 659           <listitem>
 660             <para>A space-separated list of the current symlinks. The value is
 661             only set during a remove event or if an earlier rule assigned a value.</para>
 662           </listitem>
 663         </varlistentry>
 664
 665         <varlistentry>
 666           <term><option>$root</option>, <option>%r</option></term>
 667           <listitem>
 668             <para>The udev_root value.</para>
 669           </listitem>
 670         </varlistentry>
 671
 672         <varlistentry>
 673           <term><option>$sys</option>, <option>%S</option></term>
 674           <listitem>
 675             <para>The sysfs mount point.</para>
 676           </listitem>
 677         </varlistentry>
 678
 679         <varlistentry>
 680           <term><option>$devnode</option>, <option>%N</option></term>
 681           <listitem>
 682             <para>The name of the device node.</para>
 683           </listitem>
 684         </varlistentry>
 685
 686         <varlistentry>
 687           <term><option>%%</option></term>
 688           <listitem>
 689           <para>The <literal>%</literal> character itself.</para>
 690           </listitem>
 691         </varlistentry>
 692
 693         <varlistentry>
 694           <term><option>$$</option></term>
 695           <listitem>
 696           <para>The <literal>$</literal> character itself.</para>
 697           </listitem>
 698         </varlistentry>
 699       </variablelist>
 700   </refsect1>
 701
 702   <refsect1><title>Hardware Database Files</title>
 703       <para>The hwdb files are read from the files located in the
 704       system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename>,
 705       the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
 706       and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
 707       All rules files are collectively sorted and processed in lexical order,
 708       regardless of the directories in which they live. However, files with
 709       identical filenames replace each other. Files in <filename>/etc</filename>
 710       have the highest priority, files in <filename>/run</filename> take precedence
 711       over files with the same name in <filename>/lib</filename>. This can be
 712       used to override a system-supplied hwdb file with a local file if needed;
 713       a symlink in <filename>/etc</filename> with the same name as a rules file in
 714       <filename>/lib</filename>, pointing to <filename>/dev/null</filename>,
 715       disables the rules file entirely. Hwdb files must have the extension
 716       <filename>.hwdb</filename>; other extensions are ignored.</para>
 717
 718       <para>The hwdb file contains data records consisting of matches and
 719       associated key-value pairs. Every record in the hwdb starts with one or
 720       more match string, specifying a shell glob to compare the database
 721       lookup string against. Multiple match lines are specified in additional
 722       consecutive lines. Every match line is compared indivdually, they are
 723       combined by OR. Every match line must start at the first character of
 724       the line.</para>
 725
 726       <para>The match lines are followed by one or more key-value pair lines, which
 727       are recognized by a leading space character. The key name and value are separated
 728       by <literal>=</literal>. An empty line signifies the end
 729       of a record. Lines beginning with <literal>#</literal> are ignored.</para>
 730
 731       <para>The content of all hwdb files is read by
 732       <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 733       and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
 734       During runtime only the binary database is used.</para>
 735   </refsect1>
 736
 737   <refsect1>
 738     <title>See Also</title>
 739     <para><citerefentry>
 740         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 741       </citerefentry>,
 742       <citerefentry>
 743         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 744     </citerefentry></para>
 745   </refsect1>
 746 </refentry>