man/udev.xml

   1 <?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <refentry id="udev">
   6   <refentryinfo>
   7     <title>udev</title>
   8     <productname>systemd</productname>
   9     <authorgroup>
  10       <author>
  11         <contrib>Developer</contrib>
  12         <firstname>Greg</firstname>
  13         <surname>Kroah-Hartmann</surname>
  14         <email>greg@kroah.com</email>
  15       </author>
  16       <author>
  17         <contrib>Developer</contrib>
  18         <firstname>Kay</firstname>
  19         <surname>Sievers</surname>
  20         <email>kay@vrfy.org</email>
  21       </author>
  22     </authorgroup>
  23   </refentryinfo>
  24
  25   <refmeta>
  26     <refentrytitle>udev</refentrytitle>
  27     <manvolnum>7</manvolnum>
  28   </refmeta>
  29
  30   <refnamediv>
  31     <refname>udev</refname>
  32     <refpurpose>Dynamic device management</refpurpose>
  33   </refnamediv>
  34
  35   <refsect1><title>Description</title>
  36     <para>udev supplies the system software with device events, manages permissions
  37     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  38     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  39     device names based on the order of discovery. Meaningful symlinks or network device
  40     names provide a way to reliably identify devices based on their properties or
  41     current configuration.</para>
  42
  43     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  44     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  45     the kernel whenever a device is added or removed from the system, or it changes its
  46     state. When udev receives a device event, it matches its configured set of rules
  47     against various device attributes to identify the device. Rules that match may
  48     provide additional device information to be stored in the udev database or
  49     to be used to create meaningful symlink names.</para>
  50
  51     <para>All device information udev processes is stored in the udev database and
  52     sent out to possible event subscribers. Access to all stored data and the event
  53     sources is provided by the library libudev.</para>
  54   </refsect1>
  55
  56   <refsect1><title>Rules Files</title>
  57       <para>The udev rules are read from the files located in the
  58       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  59       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  60       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  61       All rules files are collectively sorted and processed in lexical order,
  62       regardless of the directories in which they live. However, files with
  63       identical filenames replace each other. Files in <filename>/etc</filename>
  64       have the highest priority, files in <filename>/run</filename> take precedence
  65       over files with the same name in <filename>/usr/lib</filename>. This can be
  66       used to override a system-supplied rules file with a local file if needed;
  67       a symlink in <filename>/etc</filename> with the same name as a rules file in
  68       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
  69       disables the rules file entirely. Rule files must have the extension
  70       <filename>.rules</filename>; other extensions are ignored.</para>
  71
  72       <para>Every line in the rules file contains at least one key-value pair.
  73       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  74       There are two kinds of keys: match and assignment.
  75       If all match keys match against their values, the rule gets applied and the
  76       assignment keys get the specified values assigned.</para>
  77
  78       <para>A matching rule may rename a network interface, add symlinks
  79       pointing to the device node, or run a specified program as part of
  80       the event handling.</para>
  81
  82       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  83       Each key has a distinct operation, depending on the used operator. Valid
  84       operators are:</para>
  85       <variablelist>
  86         <varlistentry>
  87           <term><literal>==</literal></term>
  88           <listitem>
  89             <para>Compare for equality.</para>
  90           </listitem>
  91         </varlistentry>
  92
  93         <varlistentry>
  94           <term><literal>!=</literal></term>
  95           <listitem>
  96             <para>Compare for inequality.</para>
  97           </listitem>
  98         </varlistentry>
  99
 100         <varlistentry>
 101           <term><literal>=</literal></term>
 102           <listitem>
 103             <para>Assign a value to a key. Keys that represent a list are reset
 104             and only this single value is assigned.</para>
 105           </listitem>
 106         </varlistentry>
 107
 108         <varlistentry>
 109           <term><literal>+=</literal></term>
 110           <listitem>
 111             <para>Add the value to a key that holds a list of entries.</para>
 112           </listitem>
 113         </varlistentry>
 114
 115         <varlistentry>
 116           <term><literal>-=</literal></term>
 117           <listitem>
 118             <para>Remove the value from a key that holds a list of entries.</para>
 119           </listitem>
 120         </varlistentry>
 121
 122         <varlistentry>
 123           <term><literal>:=</literal></term>
 124           <listitem>
 125             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 126           </listitem>
 127         </varlistentry>
 128       </variablelist>
 129
 130       <para>The following key names can be used to match against device properties.
 131       Some of the keys also match against properties of the parent devices in sysfs,
 132       not only the device that has generated the event. If multiple keys that match
 133       a parent device are specified in a single rule, all these keys must match at
 134       one and the same parent device.</para>
 135       <variablelist class='udev-directives'>
 136         <varlistentry>
 137           <term><varname>ACTION</varname></term>
 138           <listitem>
 139             <para>Match the name of the event action.</para>
 140           </listitem>
 141         </varlistentry>
 142
 143         <varlistentry>
 144           <term><varname>DEVPATH</varname></term>
 145           <listitem>
 146             <para>Match the devpath of the event device.</para>
 147           </listitem>
 148         </varlistentry>
 149
 150         <varlistentry>
 151           <term><varname>KERNEL</varname></term>
 152           <listitem>
 153             <para>Match the name of the event device.</para>
 154           </listitem>
 155         </varlistentry>
 156
 157         <varlistentry>
 158           <term><varname>NAME</varname></term>
 159           <listitem>
 160             <para>Match the name of a network interface. It can be used once the
 161             NAME key has been set in one of the preceding rules.</para>
 162           </listitem>
 163         </varlistentry>
 164
 165         <varlistentry>
 166           <term><varname>SYMLINK</varname></term>
 167           <listitem>
 168             <para>Match the name of a symlink targeting the node. It can
 169             be used once a SYMLINK key has been set in one of the preceding
 170             rules. There may be multiple symlinks; only one needs to match.
 171             </para>
 172           </listitem>
 173         </varlistentry>
 174
 175         <varlistentry>
 176           <term><varname>SUBSYSTEM</varname></term>
 177           <listitem>
 178             <para>Match the subsystem of the event device.</para>
 179           </listitem>
 180         </varlistentry>
 181         <varlistentry>
 182           <term><varname>DRIVER</varname></term>
 183           <listitem>
 184             <para>Match the driver name of the event device. Only set this key for devices
 185             which are bound to a driver at the time the event is generated.</para>
 186           </listitem>
 187         </varlistentry>
 188         <varlistentry>
 189           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 190           <listitem>
 191             <para>Match sysfs attribute values of the event device. Trailing
 192             whitespace in the attribute values is ignored unless the specified match
 193             value itself contains trailing whitespace.
 194             </para>
 195           </listitem>
 196           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 197           <listitem>
 198             <para>Match a kernel parameter value.
 199             </para>
 200           </listitem>
 201         </varlistentry>
 202
 203         <varlistentry>
 204           <term><varname>KERNELS</varname></term>
 205           <listitem>
 206             <para>Search the devpath upwards for a matching device name.</para>
 207           </listitem>
 208         </varlistentry>
 209
 210         <varlistentry>
 211           <term><varname>SUBSYSTEMS</varname></term>
 212           <listitem>
 213             <para>Search the devpath upwards for a matching device subsystem name.</para>
 214           </listitem>
 215         </varlistentry>
 216
 217         <varlistentry>
 218           <term><varname>DRIVERS</varname></term>
 219           <listitem>
 220             <para>Search the devpath upwards for a matching device driver name.</para>
 221           </listitem>
 222         </varlistentry>
 223
 224         <varlistentry>
 225           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 226           <listitem>
 227             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 228             If multiple <varname>ATTRS</varname> matches are specified, all of them
 229             must match on the same device. Trailing whitespace in the attribute values is ignored
 230             unless the specified match value itself contains trailing whitespace.</para>
 231           </listitem>
 232         </varlistentry>
 233
 234         <varlistentry>
 235           <term><varname>TAGS</varname></term>
 236           <listitem>
 237             <para>Search the devpath upwards for a device with matching tag.</para>
 238           </listitem>
 239         </varlistentry>
 240
 241         <varlistentry>
 242           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 243           <listitem>
 244             <para>Match against a device property value.</para>
 245           </listitem>
 246         </varlistentry>
 247
 248         <varlistentry>
 249           <term><varname>TAG</varname></term>
 250           <listitem>
 251             <para>Match against a device tag.</para>
 252           </listitem>
 253         </varlistentry>
 254
 255         <varlistentry>
 256           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 257           <listitem>
 258             <para>Test the existence of a file. An octal mode mask can be specified
 259             if needed.</para>
 260           </listitem>
 261         </varlistentry>
 262
 263         <varlistentry>
 264           <term><varname>PROGRAM</varname></term>
 265           <listitem>
 266             <para>Execute a program to determine whether there
 267             is a match; the key is true if the program returns
 268             successfully. The device properties are made available to the
 269             executed program in the environment. The program's standard output
 270             is available in the <varname>RESULT</varname> key.</para>
 271             <para>This can only be used for very short-running foreground tasks. For details,
 272             see <varname>RUN</varname>.</para>
 273           </listitem>
 274         </varlistentry>
 275
 276         <varlistentry>
 277           <term><varname>RESULT</varname></term>
 278           <listitem>
 279             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 280             This key can be used in the same or in any later rule after a
 281             <varname>PROGRAM</varname> call.</para>
 282           </listitem>
 283         </varlistentry>
 284       </variablelist>
 285
 286       <para>Most of the fields support shell glob pattern matching and
 287       alternate patterns. The following special characters are supported:</para>
 288       <variablelist>
 289         <varlistentry>
 290           <term><literal>*</literal></term>
 291           <listitem>
 292             <para>Matches zero or more characters.</para>
 293           </listitem>
 294         </varlistentry>
 295         <varlistentry>
 296           <term><literal>?</literal></term>
 297           <listitem>
 298             <para>Matches any single character.</para>
 299           </listitem>
 300         </varlistentry>
 301         <varlistentry>
 302           <term><literal>[]</literal></term>
 303           <listitem>
 304             <para>Matches any single character specified within the brackets. For
 305             example, the pattern string <literal>tty[SR]</literal>
 306             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 307             Ranges are also supported via the <literal>-</literal> character.
 308             For example, to match on the range of all digits, the pattern
 309             <literal>[0-9]</literal> could be used. If the first character
 310             following the <literal>[</literal> is a <literal>!</literal>,
 311             any characters not enclosed are matched.</para>
 312           </listitem>
 313         </varlistentry>
 314         <varlistentry>
 315           <term><literal>|</literal></term>
 316           <listitem>
 317             <para>Separates alternative patterns. For example, the pattern string
 318             <literal>abc|x*</literal> would match either <literal>abc</literal>
 319             or <literal>x*</literal>.</para>
 320           </listitem>
 321         </varlistentry>
 322       </variablelist>
 323
 324       <para>The following keys can get values assigned:</para>
 325       <variablelist class='udev-directives'>
 326         <varlistentry>
 327           <term><varname>NAME</varname></term>
 328           <listitem>
 329             <para>The name to use for a network interface. See
 330             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 331             for a higher-level mechanism for setting the interface name.
 332             The name of a device node cannot be changed by udev, only additional
 333             symlinks can be created.</para>
 334           </listitem>
 335         </varlistentry>
 336
 337         <varlistentry>
 338           <term><varname>SYMLINK</varname></term>
 339           <listitem>
 340             <para>The name of a symlink targeting the node. Every matching rule adds
 341             this value to the list of symlinks to be created.</para>
 342             <para>The set of characters to name a symlink is limited. Allowed
 343             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 344             sequences, and <literal>\x00</literal> hex encoding. All other
 345             characters are replaced by a <literal>_</literal> character.</para>
 346             <para>Multiple symlinks may be specified by separating the names by the
 347             space character. In case multiple devices claim the same name, the link
 348             always points to the device with the highest link_priority. If the current
 349             device goes away, the links are re-evaluated and the device with the
 350             next highest link_priority becomes the owner of the link. If no
 351             link_priority is specified, the order of the devices (and which one of
 352             them owns the link) is undefined.</para>
 353             <para>Symlink names must never conflict with the kernel's default device
 354             node names, as that would result in unpredictable behavior.
 355             </para>
 356           </listitem>
 357         </varlistentry>
 358
 359         <varlistentry>
 360           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 361           <listitem>
 362             <para>The permissions for the device node. Every specified value overrides
 363             the compiled-in default value.</para>
 364           </listitem>
 365         </varlistentry>
 366
 367         <varlistentry>
 368           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 369           <listitem>
 370             <para>Applies the specified Linux Security Module label to the device node.</para>
 371           </listitem>
 372         </varlistentry>
 373
 374         <varlistentry>
 375           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 376           <listitem>
 377             <para>The value that should be written to a sysfs attribute of the
 378             event device.</para>
 379           </listitem>
 380         </varlistentry>
 381
 382         <varlistentry>
 383           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 384           <listitem>
 385             <para>The value that should be written to kernel parameter.</para>
 386           </listitem>
 387         </varlistentry>
 388
 389         <varlistentry>
 390           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 391           <listitem>
 392             <para>Set a device property value. Property names with a leading <literal>.</literal>
 393             are neither stored in the database nor exported to events or
 394             external tools (run by, for example, the <varname>PROGRAM</varname>
 395             match key).</para>
 396           </listitem>
 397         </varlistentry>
 398
 399         <varlistentry>
 400           <term><varname>TAG</varname></term>
 401           <listitem>
 402             <para>Attach a tag to a device. This is used to filter events for users
 403             of libudev's monitor functionality, or to enumerate a group of tagged
 404             devices. The implementation can only work efficiently if only a few
 405             tags are attached to a device. It is only meant to be used in
 406             contexts with specific device filter requirements, and not as a
 407             general-purpose flag. Excessive use might result in inefficient event
 408             handling.</para>
 409           </listitem>
 410         </varlistentry>
 411
 412         <varlistentry>
 413           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 414           <listitem>
 415             <para>Add a program to the list of programs to be executed after
 416             processing all the rules for a specific event, depending on
 417             <literal>type</literal>:</para>
 418             <variablelist>
 419               <varlistentry>
 420                 <term><literal>program</literal></term>
 421                 <listitem>
 422                   <para>Execute an external program specified as the assigned
 423                   value. If no absolute path is given, the program is expected
 424                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 425                   absolute path must be specified.</para>
 426                   <para>This is the default if no <replaceable>type</replaceable>
 427                   is specified.</para>
 428                 </listitem>
 429               </varlistentry>
 430               <varlistentry>
 431                 <term><literal>builtin</literal></term>
 432                 <listitem>
 433                   <para>As <varname>program</varname>, but use one of the
 434                   built-in programs rather than an external one.</para>
 435                 </listitem>
 436               </varlistentry>
 437             </variablelist>
 438             <para>The program name and following arguments are separated by spaces.
 439             Single quotes can be used to specify arguments with spaces.</para>
 440             <para>This can only be used for very short-running foreground tasks. Running an
 441             event process for a long period of time may block all further events for
 442             this or a dependent device.</para>
 443             <para>Starting daemons or other long-running processes is not appropriate
 444             for udev; the forked processes, detached or not, will be unconditionally
 445             killed after the event handling has finished.</para>
 446           </listitem>
 447         </varlistentry>
 448
 449         <varlistentry>
 450           <term><varname>LABEL</varname></term>
 451           <listitem>
 452             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 453           </listitem>
 454         </varlistentry>
 455
 456         <varlistentry>
 457           <term><varname>GOTO</varname></term>
 458           <listitem>
 459             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 460           </listitem>
 461         </varlistentry>
 462
 463         <varlistentry>
 464           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 465           <listitem>
 466             <para>Import a set of variables as device properties,
 467             depending on <literal>type</literal>:</para>
 468             <variablelist>
 469               <varlistentry>
 470                 <term><literal>program</literal></term>
 471                 <listitem>
 472                   <para>Execute an external program specified as the assigned
 473                   value and, if it returns successfully,
 474                   import its output, which must be in environment key
 475                   format. Path specification, command/argument separation,
 476                   and quoting work like in <varname>RUN</varname>.</para>
 477                 </listitem>
 478               </varlistentry>
 479               <varlistentry>
 480                 <term><literal>builtin</literal></term>
 481                 <listitem>
 482                   <para>Similar to <literal>program</literal>, but use one of the
 483                   built-in programs rather than an external one.</para>
 484                 </listitem>
 485               </varlistentry>
 486              <varlistentry>
 487                 <term><literal>file</literal></term>
 488                 <listitem>
 489                   <para>Import a text file specified as the assigned value, the content
 490                   of which must be in environment key format.</para>
 491                 </listitem>
 492               </varlistentry>
 493               <varlistentry>
 494                 <term><literal>db</literal></term>
 495                 <listitem>
 496                   <para>Import a single property specified as the assigned value from the
 497                   current device database. This works only if the database is already populated
 498                   by an earlier event.</para>
 499                 </listitem>
 500               </varlistentry>
 501               <varlistentry>
 502                 <term><literal>cmdline</literal></term>
 503                 <listitem>
 504                   <para>Import a single property from the kernel command line. For simple flags
 505                   the value of the property is set to <literal>1</literal>.</para>
 506                 </listitem>
 507               </varlistentry>
 508               <varlistentry>
 509                 <term><literal>parent</literal></term>
 510                 <listitem>
 511                   <para>Import the stored keys from the parent device by reading
 512                   the database entry of the parent device. The value assigned to
 513                   <option>IMPORT{parent}</option> is used as a filter of key names
 514                   to import (with the same shell glob pattern matching used for
 515                   comparisons).</para>
 516                 </listitem>
 517               </varlistentry>
 518             </variablelist>
 519             <para>This can only be used for very short-running foreground tasks. For details
 520             see <option>RUN</option>.</para>
 521           </listitem>
 522         </varlistentry>
 523
 524         <varlistentry>
 525           <term><varname>OPTIONS</varname></term>
 526           <listitem>
 527             <para>Rule and device options:</para>
 528             <variablelist class='udev-directives'>
 529               <varlistentry>
 530                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 531                 <listitem>
 532                   <para>Specify the priority of the created symlinks. Devices with higher
 533                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 534                 </listitem>
 535               </varlistentry>
 536               <varlistentry>
 537                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 538                 <listitem>
 539                   <para>Usually, control and other possibly unsafe characters are replaced
 540                   in strings used for device naming. The mode of replacement can be specified
 541                   with this option.</para>
 542                 </listitem>
 543               </varlistentry>
 544               <varlistentry>
 545                 <term><option>static_node=</option></term>
 546                 <listitem>
 547                   <para>Apply the permissions specified in this rule to the
 548                   static device node with the specified name. Also, for every
 549                   tag specified in this rule, create a symlink
 550                   in the directory
 551                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 552                   pointing at the static device node with the specified name.
 553                   Static device node creation is performed by systemd-tmpfiles
 554                   before systemd-udevd is started. The static nodes might not
 555                   have a corresponding kernel device; they are used to trigger
 556                   automatic kernel module loading when they are accessed.</para>
 557                 </listitem>
 558               </varlistentry>
 559               <varlistentry>
 560                 <term><option>watch</option></term>
 561                 <listitem>
 562                   <para>Watch the device node with inotify; when the node is
 563                   closed after being opened for writing, a change uevent is
 564                   synthesized.</para>
 565                 </listitem>
 566               </varlistentry>
 567               <varlistentry>
 568                 <term><option>nowatch</option></term>
 569                 <listitem>
 570                   <para>Disable the watching of a device node with inotify.</para>
 571                 </listitem>
 572               </varlistentry>
 573             </variablelist>
 574           </listitem>
 575         </varlistentry>
 576       </variablelist>
 577
 578       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 579       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 580       <varname>GROUP</varname>, <varname>MODE</varname>, and
 581       <varname>RUN</varname> fields support simple string substitutions.
 582       The <varname>RUN</varname> substitutions are performed after all rules
 583       have been processed, right before the program is executed, allowing for
 584       the use of device properties set by earlier matching rules. For all other
 585       fields, substitutions are performed while the individual rule is being
 586       processed. The available substitutions are:</para>
 587       <variablelist class='udev-directives'>
 588         <varlistentry>
 589           <term><option>$kernel</option>, <option>%k</option></term>
 590           <listitem>
 591             <para>The kernel name for this device.</para>
 592           </listitem>
 593         </varlistentry>
 594
 595         <varlistentry>
 596           <term><option>$number</option>, <option>%n</option></term>
 597           <listitem>
 598             <para>The kernel number for this device. For example,
 599               <literal>sda3</literal> has kernel number <literal>3</literal>.
 600             </para>
 601           </listitem>
 602         </varlistentry>
 603
 604         <varlistentry>
 605           <term><option>$devpath</option>, <option>%p</option></term>
 606           <listitem>
 607             <para>The devpath of the device.</para>
 608           </listitem>
 609         </varlistentry>
 610
 611         <varlistentry>
 612           <term><option>$id</option>, <option>%b</option></term>
 613           <listitem>
 614             <para>The name of the device matched while searching the devpath
 615               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 616               <option>DRIVERS</option>, and <option>ATTRS</option>.
 617             </para>
 618           </listitem>
 619         </varlistentry>
 620
 621         <varlistentry>
 622           <term><option>$driver</option></term>
 623           <listitem>
 624             <para>The driver name of the device matched while searching the
 625               devpath upwards for <option>SUBSYSTEMS</option>,
 626               <option>KERNELS</option>, <option>DRIVERS</option>, and
 627               <option>ATTRS</option>.
 628             </para>
 629           </listitem>
 630         </varlistentry>
 631
 632         <varlistentry>
 633           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 634           <listitem>
 635             <para>The value of a sysfs attribute found at the device where
 636               all keys of the rule have matched. If the matching device does not
 637               have such an attribute, and a previous <option>KERNELS</option>,
 638               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 639               <option>ATTRS</option> test selected a parent device, then the
 640               attribute from that parent device is used.
 641             </para>
 642             <para>If the attribute is a symlink, the last element of the
 643               symlink target is returned as the value.
 644             </para>
 645           </listitem>
 646         </varlistentry>
 647
 648         <varlistentry>
 649           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 650           <listitem>
 651             <para>A device property value.</para>
 652           </listitem>
 653         </varlistentry>
 654
 655         <varlistentry>
 656           <term><option>$major</option>, <option>%M</option></term>
 657           <listitem>
 658             <para>The kernel major number for the device.</para>
 659           </listitem>
 660         </varlistentry>
 661
 662         <varlistentry>
 663           <term><option>$minor</option>, <option>%m</option></term>
 664           <listitem>
 665             <para>The kernel minor number for the device.</para>
 666           </listitem>
 667         </varlistentry>
 668
 669         <varlistentry>
 670           <term><option>$result</option>, <option>%c</option></term>
 671           <listitem>
 672             <para>The string returned by the external program requested with
 673             <varname>PROGRAM</varname>.
 674             A single part of the string, separated by a space character, may be selected
 675             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 676             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 677             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 678           </listitem>
 679         </varlistentry>
 680
 681         <varlistentry>
 682           <term><option>$parent</option>, <option>%P</option></term>
 683           <listitem>
 684             <para>The node name of the parent device.</para>
 685           </listitem>
 686         </varlistentry>
 687
 688         <varlistentry>
 689           <term><option>$name</option></term>
 690           <listitem>
 691             <para>The current name of the device. If not changed by a rule, it is the
 692             name of the kernel device.</para>
 693           </listitem>
 694         </varlistentry>
 695
 696         <varlistentry>
 697           <term><option>$links</option></term>
 698           <listitem>
 699             <para>A space-separated list of the current symlinks. The value is
 700             only set during a remove event or if an earlier rule assigned a value.</para>
 701           </listitem>
 702         </varlistentry>
 703
 704         <varlistentry>
 705           <term><option>$root</option>, <option>%r</option></term>
 706           <listitem>
 707             <para>The udev_root value.</para>
 708           </listitem>
 709         </varlistentry>
 710
 711         <varlistentry>
 712           <term><option>$sys</option>, <option>%S</option></term>
 713           <listitem>
 714             <para>The sysfs mount point.</para>
 715           </listitem>
 716         </varlistentry>
 717
 718         <varlistentry>
 719           <term><option>$devnode</option>, <option>%N</option></term>
 720           <listitem>
 721             <para>The name of the device node.</para>
 722           </listitem>
 723         </varlistentry>
 724
 725         <varlistentry>
 726           <term><option>%%</option></term>
 727           <listitem>
 728           <para>The <literal>%</literal> character itself.</para>
 729           </listitem>
 730         </varlistentry>
 731
 732         <varlistentry>
 733           <term><option>$$</option></term>
 734           <listitem>
 735           <para>The <literal>$</literal> character itself.</para>
 736           </listitem>
 737         </varlistentry>
 738       </variablelist>
 739   </refsect1>
 740
 741   <refsect1>
 742     <title>See Also</title>
 743     <para>
 744       <citerefentry>
 745         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 746       </citerefentry>,
 747       <citerefentry>
 748         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 749       </citerefentry>,
 750       <citerefentry>
 751         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 752       </citerefentry>
 753     </para>
 754   </refsect1>
 755 </refentry>