man/udev.xml

   1 <?xml version='1.0'?>
   2 <?xml-stylesheet type="text/xsl" href="http://docbook.sourceforge.net/release/xsl/current/xhtml/docbook.xsl"?>
   3 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   4   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   5
   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>/usr/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>/usr/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 standard ouput
 259             is available in the <varname>RESULT</varname> 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 <varname>PROGRAM</varname> call.
 269             This key can be used in the same or in any later rule after a
 270             <varname>PROGRAM</varname> call.</para>
 271           </listitem>
 272         </varlistentry>
 273       </variablelist>
 274
 275       <para>Most of the fields support shell glob pattern matching and
 276       alternate patterns. The following special characters are supported:</para>
 277       <variablelist>
 278         <varlistentry>
 279           <term><literal>*</literal></term>
 280           <listitem>
 281             <para>Matches zero or more characters.</para>
 282           </listitem>
 283         </varlistentry>
 284         <varlistentry>
 285           <term><literal>?</literal></term>
 286           <listitem>
 287             <para>Matches any single character.</para>
 288           </listitem>
 289         </varlistentry>
 290         <varlistentry>
 291           <term><literal>[]</literal></term>
 292           <listitem>
 293             <para>Matches any single character specified within the brackets. For
 294             example, the pattern string <literal>tty[SR]</literal>
 295             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 296             Ranges are also supported via the <literal>-</literal> character.
 297             For example, to match on the range of all digits, the pattern
 298             <literal>[0-9]</literal> could be used. If the first character
 299             following the <literal>[</literal> is a <literal>!</literal>,
 300             any characters not enclosed are matched.</para>
 301           </listitem>
 302         </varlistentry>
 303         <varlistentry>
 304           <term><literal>|</literal></term>
 305           <listitem>
 306             <para>Separates alternative patterns. For example, the pattern string
 307             <literal>abc|x*</literal> would match either <literal>abc</literal>
 308             or <literal>x*</literal>.</para>
 309           </listitem>
 310         </varlistentry>
 311       </variablelist>
 312
 313       <para>The following keys can get values assigned:</para>
 314       <variablelist class='udev-directives'>
 315         <varlistentry>
 316           <term><varname>NAME</varname></term>
 317           <listitem>
 318             <para>The name to use for a network interface. See
 319             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 320             for a higher-level mechanism for setting the interface name.
 321             The name of a device node cannot be changed by udev, only additional
 322             symlinks can be created.</para>
 323           </listitem>
 324         </varlistentry>
 325
 326         <varlistentry>
 327           <term><varname>SYMLINK</varname></term>
 328           <listitem>
 329             <para>The name of a symlink targeting the node. Every matching rule adds
 330             this value to the list of symlinks to be created.</para>
 331             <para>The set of characters to name a symlink is limited. Allowed
 332             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 333             sequences, and <literal>\x00</literal> hex encoding. All other
 334             characters are replaced by a <literal>_</literal> character.</para>
 335             <para>Multiple symlinks may be specified by separating the names by the
 336             space character. In case multiple devices claim the same name, the link
 337             always points to the device with the highest link_priority. If the current
 338             device goes away, the links are re-evaluated and the device with the
 339             next highest link_priority becomes the owner of the link. If no
 340             link_priority is specified, the order of the devices (and which one of
 341             them owns the link) is undefined.</para>
 342             <para>Symlink names must never conflict with the kernel's default device
 343             node names, as that would result in unpredictable behavior.
 344             </para>
 345           </listitem>
 346         </varlistentry>
 347
 348         <varlistentry>
 349           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 350           <listitem>
 351             <para>The permissions for the device node. Every specified value overrides
 352             the compiled-in default value.</para>
 353           </listitem>
 354         </varlistentry>
 355
 356         <varlistentry>
 357           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 358           <listitem>
 359             <para>Applies the specified Linux Security Module label to the device node.</para>
 360           </listitem>
 361         </varlistentry>
 362
 363         <varlistentry>
 364           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 365           <listitem>
 366             <para>The value that should be written to a sysfs attribute of the
 367             event device.</para>
 368           </listitem>
 369         </varlistentry>
 370
 371         <varlistentry>
 372           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 373           <listitem>
 374             <para>Set a device property value. Property names with a leading <literal>.</literal>
 375             are neither stored in the database nor exported to events or
 376             external tools (run by, for example, the <varname>PROGRAM</varname>
 377             match key).</para>
 378           </listitem>
 379         </varlistentry>
 380
 381         <varlistentry>
 382           <term><varname>TAG</varname></term>
 383           <listitem>
 384             <para>Attach a tag to a device. This is used to filter events for users
 385             of libudev's monitor functionality, or to enumerate a group of tagged
 386             devices. The implementation can only work efficiently if only a few
 387             tags are attached to a device. It is only meant to be used in
 388             contexts with specific device filter requirements, and not as a
 389             general-purpose flag. Excessive use might result in inefficient event
 390             handling.</para>
 391           </listitem>
 392         </varlistentry>
 393
 394         <varlistentry>
 395           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 396           <listitem>
 397             <para>Add a program to the list of programs to be executed after
 398             processing all the rules for a specific event, depending on
 399             <literal>type</literal>:</para>
 400             <variablelist>
 401               <varlistentry>
 402                 <term><literal>program</literal></term>
 403                 <listitem>
 404                   <para>Execute an external program specified as the assigned
 405                   value. If no absolute path is given, the program is expected
 406                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 407                   absolute path must be specified.</para>
 408                   <para>This is the default if no <replaceable>type</replaceable>
 409                   is specified.</para>
 410                 </listitem>
 411               </varlistentry>
 412               <varlistentry>
 413                 <term><literal>builtin</literal></term>
 414                 <listitem>
 415                   <para>As <varname>program</varname>, but use one of the
 416                   built-in programs rather than an external one.</para>
 417                 </listitem>
 418               </varlistentry>
 419             </variablelist>
 420             <para>The program name and following arguments are separated by spaces.
 421             Single quotes can be used to specify arguments with spaces.</para>
 422             <para>This can only be used for very short-running foreground tasks. Running an
 423             event process for a long period of time may block all further events for
 424             this or a dependent device.</para>
 425             <para>Starting daemons or other long-running processes is not appropriate
 426             for udev; the forked processes, detached or not, will be unconditionally
 427             killed after the event handling has finished.</para>
 428           </listitem>
 429         </varlistentry>
 430
 431         <varlistentry>
 432           <term><varname>LABEL</varname></term>
 433           <listitem>
 434             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 435           </listitem>
 436         </varlistentry>
 437
 438         <varlistentry>
 439           <term><varname>GOTO</varname></term>
 440           <listitem>
 441             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 442           </listitem>
 443         </varlistentry>
 444
 445         <varlistentry>
 446           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 447           <listitem>
 448             <para>Import a set of variables as device properties,
 449             depending on <literal>type</literal>:</para>
 450             <variablelist>
 451               <varlistentry>
 452                 <term><literal>program</literal></term>
 453                 <listitem>
 454                   <para>Execute an external program specified as the assigned value and
 455                   import its output, which must be in environment key
 456                   format. Path specification, command/argument separation,
 457                   and quoting work like in <varname>RUN</varname>.</para>
 458                 </listitem>
 459               </varlistentry>
 460               <varlistentry>
 461                 <term><literal>builtin</literal></term>
 462                 <listitem>
 463                   <para>Similar to <literal>program</literal>, but use one of the
 464                   built-in programs rather than an external one.</para>
 465                 </listitem>
 466               </varlistentry>
 467              <varlistentry>
 468                 <term><literal>file</literal></term>
 469                 <listitem>
 470                   <para>Import a text file specified as the assigned value, the content
 471                   of which must be in environment key format.</para>
 472                 </listitem>
 473               </varlistentry>
 474               <varlistentry>
 475                 <term><literal>db</literal></term>
 476                 <listitem>
 477                   <para>Import a single property specified as the assigned value from the
 478                   current device database. This works only if the database is already populated
 479                   by an earlier event.</para>
 480                 </listitem>
 481               </varlistentry>
 482               <varlistentry>
 483                 <term><literal>cmdline</literal></term>
 484                 <listitem>
 485                   <para>Import a single property from the kernel command line. For simple flags
 486                   the value of the property is set to <literal>1</literal>.</para>
 487                 </listitem>
 488               </varlistentry>
 489               <varlistentry>
 490                 <term><literal>parent</literal></term>
 491                 <listitem>
 492                   <para>Import the stored keys from the parent device by reading
 493                   the database entry of the parent device. The value assigned to
 494                   <option>IMPORT{parent}</option> is used as a filter of key names
 495                   to import (with the same shell glob pattern matching used for
 496                   comparisons).</para>
 497                 </listitem>
 498               </varlistentry>
 499             </variablelist>
 500             <para>This can only be used for very short-running foreground tasks. For details
 501             see <option>RUN</option>.</para>
 502           </listitem>
 503         </varlistentry>
 504
 505         <varlistentry>
 506           <term><varname>WAIT_FOR</varname></term>
 507           <listitem>
 508             <para>Wait for a file to become available or until a timeout of
 509             10 seconds expires. The path is relative to the sysfs device;
 510             if no path is specified, this waits for an attribute to appear.</para>
 511           </listitem>
 512         </varlistentry>
 513
 514         <varlistentry>
 515           <term><varname>OPTIONS</varname></term>
 516           <listitem>
 517             <para>Rule and device options:</para>
 518             <variablelist class='udev-directives'>
 519               <varlistentry>
 520                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 521                 <listitem>
 522                   <para>Specify the priority of the created symlinks. Devices with higher
 523                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 524                 </listitem>
 525               </varlistentry>
 526               <varlistentry>
 527                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 528                 <listitem>
 529                   <para>Usually control and other possibly unsafe characters are replaced
 530                   in strings used for device naming. The mode of replacement can be specified
 531                   with this option.</para>
 532                 </listitem>
 533               </varlistentry>
 534               <varlistentry>
 535                 <term><option>static_node=</option></term>
 536                 <listitem>
 537                   <para>Apply the permissions specified in this rule to the
 538                   static device node with the specified name. Also, for every
 539                   tag specified in this rule, create a symlink
 540                   in the directory
 541                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 542                   pointing at the static device node with the specified name.
 543                   Static device node creation is performed by systemd-tmpfiles
 544                   before systemd-udevd is started. The static nodes might not
 545                   have a corresponding kernel device; they are used to trigger
 546                   automatic kernel module loading when they are accessed.</para>
 547                 </listitem>
 548               </varlistentry>
 549               <varlistentry>
 550                 <term><option>watch</option></term>
 551                 <listitem>
 552                   <para>Watch the device node with inotify; when the node is
 553                   closed after being opened for writing, a change uevent is
 554                   synthesized.</para>
 555                 </listitem>
 556               </varlistentry>
 557               <varlistentry>
 558                 <term><option>nowatch</option></term>
 559                 <listitem>
 560                   <para>Disable the watching of a device node with inotify.</para>
 561                 </listitem>
 562               </varlistentry>
 563             </variablelist>
 564           </listitem>
 565         </varlistentry>
 566       </variablelist>
 567
 568       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 569       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 570       <varname>GROUP</varname>, <varname>MODE</varname>, and
 571       <varname>RUN</varname> fields support simple string substitutions.
 572       The <varname>RUN</varname> substitutions are performed after all rules
 573       have been processed, right before the program is executed, allowing for
 574       the use of device properties set by earlier matching rules. For all other
 575       fields, substitutions are performed while the individual rule is being
 576       processed. The available substitutions are:</para>
 577       <variablelist class='udev-directives'>
 578         <varlistentry>
 579           <term><option>$kernel</option>, <option>%k</option></term>
 580           <listitem>
 581             <para>The kernel name for this device.</para>
 582           </listitem>
 583         </varlistentry>
 584
 585         <varlistentry>
 586           <term><option>$number</option>, <option>%n</option></term>
 587           <listitem>
 588             <para>The kernel number for this device. For example,
 589               <literal>sda3</literal> has kernel number <literal>3</literal>.
 590             </para>
 591           </listitem>
 592         </varlistentry>
 593
 594         <varlistentry>
 595           <term><option>$devpath</option>, <option>%p</option></term>
 596           <listitem>
 597             <para>The devpath of the device.</para>
 598           </listitem>
 599         </varlistentry>
 600
 601         <varlistentry>
 602           <term><option>$id</option>, <option>%b</option></term>
 603           <listitem>
 604             <para>The name of the device matched while searching the devpath
 605               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 606               <option>DRIVERS</option>, and <option>ATTRS</option>.
 607             </para>
 608           </listitem>
 609         </varlistentry>
 610
 611         <varlistentry>
 612           <term><option>$driver</option></term>
 613           <listitem>
 614             <para>The driver name of the device matched while searching the
 615               devpath upwards for <option>SUBSYSTEMS</option>,
 616               <option>KERNELS</option>, <option>DRIVERS</option>, and
 617               <option>ATTRS</option>.
 618             </para>
 619           </listitem>
 620         </varlistentry>
 621
 622         <varlistentry>
 623           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 624           <listitem>
 625             <para>The value of a sysfs attribute found at the device where
 626               all keys of the rule have matched. If the matching device does not
 627               have such an attribute, and a previous <option>KERNELS</option>,
 628               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 629               <option>ATTRS</option> test selected a parent device, then the
 630               attribute from that parent device is used.
 631             </para>
 632             <para>If the attribute is a symlink, the last element of the
 633               symlink target is returned as the value.
 634             </para>
 635           </listitem>
 636         </varlistentry>
 637
 638         <varlistentry>
 639           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 640           <listitem>
 641             <para>A device property value.</para>
 642           </listitem>
 643         </varlistentry>
 644
 645         <varlistentry>
 646           <term><option>$major</option>, <option>%M</option></term>
 647           <listitem>
 648             <para>The kernel major number for the device.</para>
 649           </listitem>
 650         </varlistentry>
 651
 652         <varlistentry>
 653           <term><option>$minor</option>, <option>%m</option></term>
 654           <listitem>
 655             <para>The kernel minor number for the device.</para>
 656           </listitem>
 657         </varlistentry>
 658
 659         <varlistentry>
 660           <term><option>$result</option>, <option>%c</option></term>
 661           <listitem>
 662             <para>The string returned by the external program requested with
 663             <varname>PROGRAM</varname>.
 664             A single part of the string, separated by a space character, may be selected
 665             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 666             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 667             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 668           </listitem>
 669         </varlistentry>
 670
 671         <varlistentry>
 672           <term><option>$parent</option>, <option>%P</option></term>
 673           <listitem>
 674             <para>The node name of the parent device.</para>
 675           </listitem>
 676         </varlistentry>
 677
 678         <varlistentry>
 679           <term><option>$name</option></term>
 680           <listitem>
 681             <para>The current name of the device. If not changed by a rule, it is the
 682             name of the kernel device.</para>
 683           </listitem>
 684         </varlistentry>
 685
 686         <varlistentry>
 687           <term><option>$links</option></term>
 688           <listitem>
 689             <para>A space-separated list of the current symlinks. The value is
 690             only set during a remove event or if an earlier rule assigned a value.</para>
 691           </listitem>
 692         </varlistentry>
 693
 694         <varlistentry>
 695           <term><option>$root</option>, <option>%r</option></term>
 696           <listitem>
 697             <para>The udev_root value.</para>
 698           </listitem>
 699         </varlistentry>
 700
 701         <varlistentry>
 702           <term><option>$sys</option>, <option>%S</option></term>
 703           <listitem>
 704             <para>The sysfs mount point.</para>
 705           </listitem>
 706         </varlistentry>
 707
 708         <varlistentry>
 709           <term><option>$devnode</option>, <option>%N</option></term>
 710           <listitem>
 711             <para>The name of the device node.</para>
 712           </listitem>
 713         </varlistentry>
 714
 715         <varlistentry>
 716           <term><option>%%</option></term>
 717           <listitem>
 718           <para>The <literal>%</literal> character itself.</para>
 719           </listitem>
 720         </varlistentry>
 721
 722         <varlistentry>
 723           <term><option>$$</option></term>
 724           <listitem>
 725           <para>The <literal>$</literal> character itself.</para>
 726           </listitem>
 727         </varlistentry>
 728       </variablelist>
 729   </refsect1>
 730
 731   <refsect1><title>Hardware Database Files</title>
 732       <para>The hwdb files are read from the files located in the
 733       system hwdb directory <filename>/usr/lib/udev/hwdb.d</filename>,
 734       the volatile runtime directory <filename>/run/udev/hwdb.d</filename>
 735       and the local administration directory <filename>/etc/udev/hwdb.d</filename>.
 736       All hwdb files are collectively sorted and processed in lexical order,
 737       regardless of the directories in which they live. However, files with
 738       identical filenames replace each other. Files in <filename>/etc</filename>
 739       have the highest priority, files in <filename>/run</filename> take precedence
 740       over files with the same name in <filename>/usr/lib</filename>. This can be
 741       used to override a system-supplied hwdb file with a local file if needed;
 742       a symlink in <filename>/etc</filename> with the same name as a hwdb file in
 743       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
 744       disables the hwdb file entirely. hwdb files must have the extension
 745       <filename>.hwdb</filename>; other extensions are ignored.</para>
 746
 747       <para>The hwdb file contains data records consisting of matches and
 748       associated key-value pairs. Every record in the hwdb starts with one or
 749       more match string, specifying a shell glob to compare the database
 750       lookup string against. Multiple match lines are specified in additional
 751       consecutive lines. Every match line is compared indivdually, they are
 752       combined by OR. Every match line must start at the first character of
 753       the line.</para>
 754
 755       <para>The match lines are followed by one or more key-value pair lines, which
 756       are recognized by a leading space character. The key name and value are separated
 757       by <literal>=</literal>. An empty line signifies the end
 758       of a record. Lines beginning with <literal>#</literal> are ignored.</para>
 759
 760       <para>The content of all hwdb files is read by
 761       <citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>
 762       and compiled to a binary database located at <filename>/etc/udev/hwdb.bin</filename>.
 763       During runtime only the binary database is used.</para>
 764   </refsect1>
 765
 766   <refsect1>
 767     <title>See Also</title>
 768     <para>
 769       <citerefentry>
 770         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 771       </citerefentry>,
 772       <citerefentry>
 773         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 774       </citerefentry>,
 775       <citerefentry>
 776         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 777       </citerefentry>
 778     </para>
 779   </refsect1>
 780 </refentry>