man/udev.xml

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   6   SPDX-License-Identifier: LGPL-2.1+
   7   Copyright © 2014 Jason St. John
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   9
  10 <refentry id="udev">
  11   <refentryinfo>
  12     <title>udev</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>udev</refentrytitle>
  18     <manvolnum>7</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>udev</refname>
  23     <refpurpose>Dynamic device management</refpurpose>
  24   </refnamediv>
  25
  26   <refsect1><title>Description</title>
  27     <para>udev supplies the system software with device events, manages permissions
  28     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  29     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  30     device names based on the order of discovery. Meaningful symlinks or network device
  31     names provide a way to reliably identify devices based on their properties or
  32     current configuration.</para>
  33
  34     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  35     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  36     the kernel whenever a device is added or removed from the system, or it changes its
  37     state. When udev receives a device event, it matches its configured set of rules
  38     against various device attributes to identify the device. Rules that match may
  39     provide additional device information to be stored in the udev database or
  40     to be used to create meaningful symlink names.</para>
  41
  42     <para>All device information udev processes is stored in the udev database and
  43     sent out to possible event subscribers. Access to all stored data and the event
  44     sources is provided by the library libudev.</para>
  45   </refsect1>
  46
  47   <refsect1><title>Rules Files</title>
  48       <para>The udev rules are read from the files located in the
  49       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  50       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  51       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  52       All rules files are collectively sorted and processed in lexical order,
  53       regardless of the directories in which they live. However, files with
  54       identical filenames replace each other. Files in <filename>/etc</filename>
  55       have the highest priority, files in <filename>/run</filename> take precedence
  56       over files with the same name in <filename>/usr/lib</filename>. This can be
  57       used to override a system-supplied rules file with a local file if needed;
  58       a symlink in <filename>/etc</filename> with the same name as a rules file in
  59       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
  60       disables the rules file entirely. Rule files must have the extension
  61       <filename>.rules</filename>; other extensions are ignored.</para>
  62
  63       <para>Every line in the rules file contains at least one key-value pair.
  64       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  65       There are two kinds of keys: match and assignment.
  66       If all match keys match against their values, the rule gets applied and the
  67       assignment keys get the specified values assigned.</para>
  68
  69       <para>A matching rule may rename a network interface, add symlinks
  70       pointing to the device node, or run a specified program as part of
  71       the event handling.</para>
  72
  73       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  74       Each key has a distinct operation, depending on the used operator. Valid
  75       operators are:</para>
  76       <variablelist>
  77         <varlistentry>
  78           <term><literal>==</literal></term>
  79           <listitem>
  80             <para>Compare for equality.</para>
  81           </listitem>
  82         </varlistentry>
  83
  84         <varlistentry>
  85           <term><literal>!=</literal></term>
  86           <listitem>
  87             <para>Compare for inequality.</para>
  88           </listitem>
  89         </varlistentry>
  90
  91         <varlistentry>
  92           <term><literal>=</literal></term>
  93           <listitem>
  94             <para>Assign a value to a key. Keys that represent a list are reset
  95             and only this single value is assigned.</para>
  96           </listitem>
  97         </varlistentry>
  98
  99         <varlistentry>
 100           <term><literal>+=</literal></term>
 101           <listitem>
 102             <para>Add the value to a key that holds a list of entries.</para>
 103           </listitem>
 104         </varlistentry>
 105
 106         <varlistentry>
 107           <term><literal>-=</literal></term>
 108           <listitem>
 109             <para>Remove the value from a key that holds a list of entries.</para>
 110           </listitem>
 111         </varlistentry>
 112
 113         <varlistentry>
 114           <term><literal>:=</literal></term>
 115           <listitem>
 116             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 117           </listitem>
 118         </varlistentry>
 119       </variablelist>
 120
 121       <para>The following key names can be used to match against device properties.
 122       Some of the keys also match against properties of the parent devices in sysfs,
 123       not only the device that has generated the event. If multiple keys that match
 124       a parent device are specified in a single rule, all these keys must match at
 125       one and the same parent device.</para>
 126       <variablelist class='udev-directives'>
 127         <varlistentry>
 128           <term><varname>ACTION</varname></term>
 129           <listitem>
 130             <para>Match the name of the event action.</para>
 131           </listitem>
 132         </varlistentry>
 133
 134         <varlistentry>
 135           <term><varname>DEVPATH</varname></term>
 136           <listitem>
 137             <para>Match the devpath of the event device.</para>
 138           </listitem>
 139         </varlistentry>
 140
 141         <varlistentry>
 142           <term><varname>KERNEL</varname></term>
 143           <listitem>
 144             <para>Match the name of the event device.</para>
 145           </listitem>
 146         </varlistentry>
 147
 148         <varlistentry>
 149           <term><varname>NAME</varname></term>
 150           <listitem>
 151             <para>Match the name of a network interface. It can be used once the
 152             NAME key has been set in one of the preceding rules.</para>
 153           </listitem>
 154         </varlistentry>
 155
 156         <varlistentry>
 157           <term><varname>SYMLINK</varname></term>
 158           <listitem>
 159             <para>Match the name of a symlink targeting the node. It can
 160             be used once a SYMLINK key has been set in one of the preceding
 161             rules. There may be multiple symlinks; only one needs to match.
 162             </para>
 163           </listitem>
 164         </varlistentry>
 165
 166         <varlistentry>
 167           <term><varname>SUBSYSTEM</varname></term>
 168           <listitem>
 169             <para>Match the subsystem of the event device.</para>
 170           </listitem>
 171         </varlistentry>
 172         <varlistentry>
 173           <term><varname>DRIVER</varname></term>
 174           <listitem>
 175             <para>Match the driver name of the event device. Only set this key for devices
 176             which are bound to a driver at the time the event is generated.</para>
 177           </listitem>
 178         </varlistentry>
 179         <varlistentry>
 180           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 181           <listitem>
 182             <para>Match sysfs attribute values of the event device. Trailing
 183             whitespace in the attribute values is ignored unless the specified match
 184             value itself contains trailing whitespace.
 185             </para>
 186           </listitem>
 187         </varlistentry>
 188         <varlistentry>
 189           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 190           <listitem>
 191             <para>Match a kernel parameter value.
 192             </para>
 193           </listitem>
 194         </varlistentry>
 195
 196         <varlistentry>
 197           <term><varname>KERNELS</varname></term>
 198           <listitem>
 199             <para>Search the devpath upwards for a matching device name.</para>
 200           </listitem>
 201         </varlistentry>
 202
 203         <varlistentry>
 204           <term><varname>SUBSYSTEMS</varname></term>
 205           <listitem>
 206             <para>Search the devpath upwards for a matching device subsystem name.</para>
 207           </listitem>
 208         </varlistentry>
 209
 210         <varlistentry>
 211           <term><varname>DRIVERS</varname></term>
 212           <listitem>
 213             <para>Search the devpath upwards for a matching device driver name.</para>
 214           </listitem>
 215         </varlistentry>
 216
 217         <varlistentry>
 218           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 219           <listitem>
 220             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 221             If multiple <varname>ATTRS</varname> matches are specified, all of them
 222             must match on the same device. Trailing whitespace in the attribute values is ignored
 223             unless the specified match value itself contains trailing whitespace.</para>
 224           </listitem>
 225         </varlistentry>
 226
 227         <varlistentry>
 228           <term><varname>TAGS</varname></term>
 229           <listitem>
 230             <para>Search the devpath upwards for a device with matching tag.</para>
 231           </listitem>
 232         </varlistentry>
 233
 234         <varlistentry>
 235           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 236           <listitem>
 237             <para>Match against a device property value.</para>
 238           </listitem>
 239         </varlistentry>
 240
 241         <varlistentry>
 242           <term><varname>TAG</varname></term>
 243           <listitem>
 244             <para>Match against a device tag.</para>
 245           </listitem>
 246         </varlistentry>
 247
 248         <varlistentry>
 249           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 250           <listitem>
 251             <para>Test the existence of a file. An octal mode mask can be specified
 252             if needed.</para>
 253           </listitem>
 254         </varlistentry>
 255
 256         <varlistentry>
 257           <term><varname>PROGRAM</varname></term>
 258           <listitem>
 259             <para>Execute a program to determine whether there
 260             is a match; the key is true if the program returns
 261             successfully. The device properties are made available to the
 262             executed program in the environment. The program's standard output
 263             is available in the <varname>RESULT</varname> key.</para>
 264             <para>This can only be used for very short-running foreground tasks. For details,
 265             see <varname>RUN</varname>.</para>
 266           </listitem>
 267         </varlistentry>
 268
 269         <varlistentry>
 270           <term><varname>RESULT</varname></term>
 271           <listitem>
 272             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 273             This key can be used in the same or in any later rule after a
 274             <varname>PROGRAM</varname> call.</para>
 275           </listitem>
 276         </varlistentry>
 277       </variablelist>
 278
 279       <para>Most of the fields support shell glob pattern matching and
 280       alternate patterns. The following special characters are supported:</para>
 281       <variablelist>
 282         <varlistentry>
 283           <term><literal>*</literal></term>
 284           <listitem>
 285             <para>Matches zero or more characters.</para>
 286           </listitem>
 287         </varlistentry>
 288         <varlistentry>
 289           <term><literal>?</literal></term>
 290           <listitem>
 291             <para>Matches any single character.</para>
 292           </listitem>
 293         </varlistentry>
 294         <varlistentry>
 295           <term><literal>[]</literal></term>
 296           <listitem>
 297             <para>Matches any single character specified within the brackets. For
 298             example, the pattern string <literal>tty[SR]</literal>
 299             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 300             Ranges are also supported via the <literal>-</literal> character.
 301             For example, to match on the range of all digits, the pattern
 302             <literal>[0-9]</literal> could be used. If the first character
 303             following the <literal>[</literal> is a <literal>!</literal>,
 304             any characters not enclosed are matched.</para>
 305           </listitem>
 306         </varlistentry>
 307         <varlistentry>
 308           <term><literal>|</literal></term>
 309           <listitem>
 310             <para>Separates alternative patterns. For example, the pattern string
 311             <literal>abc|x*</literal> would match either <literal>abc</literal>
 312             or <literal>x*</literal>.</para>
 313           </listitem>
 314         </varlistentry>
 315       </variablelist>
 316
 317       <para>The following keys can get values assigned:</para>
 318       <variablelist class='udev-directives'>
 319         <varlistentry>
 320           <term><varname>NAME</varname></term>
 321           <listitem>
 322             <para>The name to use for a network interface. See
 323             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 324             for a higher-level mechanism for setting the interface name.
 325             The name of a device node cannot be changed by udev, only additional
 326             symlinks can be created.</para>
 327           </listitem>
 328         </varlistentry>
 329
 330         <varlistentry>
 331           <term><varname>SYMLINK</varname></term>
 332           <listitem>
 333             <para>The name of a symlink targeting the node. Every matching rule adds
 334             this value to the list of symlinks to be created.</para>
 335             <para>The set of characters to name a symlink is limited. Allowed
 336             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 337             sequences, and <literal>\x00</literal> hex encoding. All other
 338             characters are replaced by a <literal>_</literal> character.</para>
 339             <para>Multiple symlinks may be specified by separating the names by the
 340             space character. In case multiple devices claim the same name, the link
 341             always points to the device with the highest link_priority. If the current
 342             device goes away, the links are re-evaluated and the device with the
 343             next highest link_priority becomes the owner of the link. If no
 344             link_priority is specified, the order of the devices (and which one of
 345             them owns the link) is undefined.</para>
 346             <para>Symlink names must never conflict with the kernel's default device
 347             node names, as that would result in unpredictable behavior.
 348             </para>
 349           </listitem>
 350         </varlistentry>
 351
 352         <varlistentry>
 353           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 354           <listitem>
 355             <para>The permissions for the device node. Every specified value overrides
 356             the compiled-in default value.</para>
 357           </listitem>
 358         </varlistentry>
 359
 360         <varlistentry>
 361           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 362           <listitem>
 363             <para>Applies the specified Linux Security Module label to the device node.</para>
 364           </listitem>
 365         </varlistentry>
 366
 367         <varlistentry>
 368           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 369           <listitem>
 370             <para>The value that should be written to a sysfs attribute of the
 371             event device.</para>
 372           </listitem>
 373         </varlistentry>
 374
 375         <varlistentry>
 376           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 377           <listitem>
 378             <para>The value that should be written to kernel parameter.</para>
 379           </listitem>
 380         </varlistentry>
 381
 382         <varlistentry>
 383           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 384           <listitem>
 385             <para>Set a device property value. Property names with a leading <literal>.</literal>
 386             are neither stored in the database nor exported to events or
 387             external tools (run by, for example, the <varname>PROGRAM</varname>
 388             match key).</para>
 389           </listitem>
 390         </varlistentry>
 391
 392         <varlistentry>
 393           <term><varname>TAG</varname></term>
 394           <listitem>
 395             <para>Attach a tag to a device. This is used to filter events for users
 396             of libudev's monitor functionality, or to enumerate a group of tagged
 397             devices. The implementation can only work efficiently if only a few
 398             tags are attached to a device. It is only meant to be used in
 399             contexts with specific device filter requirements, and not as a
 400             general-purpose flag. Excessive use might result in inefficient event
 401             handling.</para>
 402           </listitem>
 403         </varlistentry>
 404
 405         <varlistentry>
 406           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 407           <listitem>
 408             <para>Add a program to the list of programs to be executed after
 409             processing all the rules for a specific event, depending on
 410             <literal>type</literal>:</para>
 411             <variablelist>
 412               <varlistentry>
 413                 <term><literal>program</literal></term>
 414                 <listitem>
 415                   <para>Execute an external program specified as the assigned
 416                   value. If no absolute path is given, the program is expected
 417                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 418                   absolute path must be specified.</para>
 419                   <para>This is the default if no <replaceable>type</replaceable>
 420                   is specified.</para>
 421                 </listitem>
 422               </varlistentry>
 423               <varlistentry>
 424                 <term><literal>builtin</literal></term>
 425                 <listitem>
 426                   <para>As <varname>program</varname>, but use one of the
 427                   built-in programs rather than an external one.</para>
 428                 </listitem>
 429               </varlistentry>
 430             </variablelist>
 431             <para>The program name and following arguments are separated by spaces.
 432             Single quotes can be used to specify arguments with spaces.</para>
 433             <para>This can only be used for very short-running foreground tasks. Running an
 434             event process for a long period of time may block all further events for
 435             this or a dependent device.</para>
 436             <para>Starting daemons or other long-running processes is not appropriate
 437             for udev; the forked processes, detached or not, will be unconditionally
 438             killed after the event handling has finished.</para>
 439             <para>Note that running programs that access the network or mount/unmount
 440             filesystems is not allowed inside of udev rules, due to the default sandbox
 441             that is enforced on <filename>systemd-udevd.service</filename>.</para>
 442           </listitem>
 443         </varlistentry>
 444
 445         <varlistentry>
 446           <term><varname>LABEL</varname></term>
 447           <listitem>
 448             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 449           </listitem>
 450         </varlistentry>
 451
 452         <varlistentry>
 453           <term><varname>GOTO</varname></term>
 454           <listitem>
 455             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 456           </listitem>
 457         </varlistentry>
 458
 459         <varlistentry>
 460           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 461           <listitem>
 462             <para>Import a set of variables as device properties,
 463             depending on <literal>type</literal>:</para>
 464             <variablelist>
 465               <varlistentry>
 466                 <term><literal>program</literal></term>
 467                 <listitem>
 468                   <para>Execute an external program specified as the assigned
 469                   value and, if it returns successfully,
 470                   import its output, which must be in environment key
 471                   format. Path specification, command/argument separation,
 472                   and quoting work like in <varname>RUN</varname>.</para>
 473                 </listitem>
 474               </varlistentry>
 475               <varlistentry>
 476                 <term><literal>builtin</literal></term>
 477                 <listitem>
 478                   <para>Similar to <literal>program</literal>, but use one of the
 479                   built-in programs rather than an external one.</para>
 480                 </listitem>
 481               </varlistentry>
 482              <varlistentry>
 483                 <term><literal>file</literal></term>
 484                 <listitem>
 485                   <para>Import a text file specified as the assigned value, the content
 486                   of which must be in environment key format.</para>
 487                 </listitem>
 488               </varlistentry>
 489               <varlistentry>
 490                 <term><literal>db</literal></term>
 491                 <listitem>
 492                   <para>Import a single property specified as the assigned value from the
 493                   current device database. This works only if the database is already populated
 494                   by an earlier event.</para>
 495                 </listitem>
 496               </varlistentry>
 497               <varlistentry>
 498                 <term><literal>cmdline</literal></term>
 499                 <listitem>
 500                   <para>Import a single property from the kernel command line. For simple flags
 501                   the value of the property is set to <literal>1</literal>.</para>
 502                 </listitem>
 503               </varlistentry>
 504               <varlistentry>
 505                 <term><literal>parent</literal></term>
 506                 <listitem>
 507                   <para>Import the stored keys from the parent device by reading
 508                   the database entry of the parent device. The value assigned to
 509                   <option>IMPORT{parent}</option> is used as a filter of key names
 510                   to import (with the same shell glob pattern matching used for
 511                   comparisons).</para>
 512                 </listitem>
 513               </varlistentry>
 514             </variablelist>
 515             <para>This can only be used for very short-running foreground tasks. For details
 516             see <option>RUN</option>.</para>
 517           </listitem>
 518         </varlistentry>
 519
 520         <varlistentry>
 521           <term><varname>OPTIONS</varname></term>
 522           <listitem>
 523             <para>Rule and device options:</para>
 524             <variablelist class='udev-directives'>
 525               <varlistentry>
 526                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 527                 <listitem>
 528                   <para>Specify the priority of the created symlinks. Devices with higher
 529                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 530                 </listitem>
 531               </varlistentry>
 532               <varlistentry>
 533                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 534                 <listitem>
 535                   <para>Usually, control and other possibly unsafe characters are replaced
 536                   in strings used for device naming. The mode of replacement can be specified
 537                   with this option.</para>
 538                 </listitem>
 539               </varlistentry>
 540               <varlistentry>
 541                 <term><option>static_node=</option></term>
 542                 <listitem>
 543                   <para>Apply the permissions specified in this rule to the
 544                   static device node with the specified name. Also, for every
 545                   tag specified in this rule, create a symlink
 546                   in the directory
 547                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 548                   pointing at the static device node with the specified name.
 549                   Static device node creation is performed by systemd-tmpfiles
 550                   before systemd-udevd is started. The static nodes might not
 551                   have a corresponding kernel device; they are used to trigger
 552                   automatic kernel module loading when they are accessed.</para>
 553                 </listitem>
 554               </varlistentry>
 555               <varlistentry>
 556                 <term><option>watch</option></term>
 557                 <listitem>
 558                   <para>Watch the device node with inotify; when the node is
 559                   closed after being opened for writing, a change uevent is
 560                   synthesized.</para>
 561                 </listitem>
 562               </varlistentry>
 563               <varlistentry>
 564                 <term><option>nowatch</option></term>
 565                 <listitem>
 566                   <para>Disable the watching of a device node with inotify.</para>
 567                 </listitem>
 568               </varlistentry>
 569             </variablelist>
 570           </listitem>
 571         </varlistentry>
 572       </variablelist>
 573
 574       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 575       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 576       <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 577       and <varname>RUN</varname> fields support simple string substitutions.
 578       The <varname>RUN</varname> substitutions are performed after all rules
 579       have been processed, right before the program is executed, allowing for
 580       the use of device properties set by earlier matching rules. For all other
 581       fields, substitutions are performed while the individual rule is being
 582       processed. The available substitutions are:</para>
 583       <variablelist class='udev-directives'>
 584         <varlistentry>
 585           <term><option>$kernel</option>, <option>%k</option></term>
 586           <listitem>
 587             <para>The kernel name for this device.</para>
 588           </listitem>
 589         </varlistentry>
 590
 591         <varlistentry>
 592           <term><option>$number</option>, <option>%n</option></term>
 593           <listitem>
 594             <para>The kernel number for this device. For example,
 595               <literal>sda3</literal> has kernel number <literal>3</literal>.
 596             </para>
 597           </listitem>
 598         </varlistentry>
 599
 600         <varlistentry>
 601           <term><option>$devpath</option>, <option>%p</option></term>
 602           <listitem>
 603             <para>The devpath of the device.</para>
 604           </listitem>
 605         </varlistentry>
 606
 607         <varlistentry>
 608           <term><option>$id</option>, <option>%b</option></term>
 609           <listitem>
 610             <para>The name of the device matched while searching the devpath
 611               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 612               <option>DRIVERS</option>, and <option>ATTRS</option>.
 613             </para>
 614           </listitem>
 615         </varlistentry>
 616
 617         <varlistentry>
 618           <term><option>$driver</option></term>
 619           <listitem>
 620             <para>The driver name of the device matched while searching the
 621               devpath upwards for <option>SUBSYSTEMS</option>,
 622               <option>KERNELS</option>, <option>DRIVERS</option>, and
 623               <option>ATTRS</option>.
 624             </para>
 625           </listitem>
 626         </varlistentry>
 627
 628         <varlistentry>
 629           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 630           <listitem>
 631             <para>The value of a sysfs attribute found at the device where
 632               all keys of the rule have matched. If the matching device does not
 633               have such an attribute, and a previous <option>KERNELS</option>,
 634               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 635               <option>ATTRS</option> test selected a parent device, then the
 636               attribute from that parent device is used.
 637             </para>
 638             <para>If the attribute is a symlink, the last element of the
 639               symlink target is returned as the value.
 640             </para>
 641           </listitem>
 642         </varlistentry>
 643
 644         <varlistentry>
 645           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 646           <listitem>
 647             <para>A device property value.</para>
 648           </listitem>
 649         </varlistentry>
 650
 651         <varlistentry>
 652           <term><option>$major</option>, <option>%M</option></term>
 653           <listitem>
 654             <para>The kernel major number for the device.</para>
 655           </listitem>
 656         </varlistentry>
 657
 658         <varlistentry>
 659           <term><option>$minor</option>, <option>%m</option></term>
 660           <listitem>
 661             <para>The kernel minor number for the device.</para>
 662           </listitem>
 663         </varlistentry>
 664
 665         <varlistentry>
 666           <term><option>$result</option>, <option>%c</option></term>
 667           <listitem>
 668             <para>The string returned by the external program requested with
 669             <varname>PROGRAM</varname>.
 670             A single part of the string, separated by a space character, may be selected
 671             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 672             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 673             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 674           </listitem>
 675         </varlistentry>
 676
 677         <varlistentry>
 678           <term><option>$parent</option>, <option>%P</option></term>
 679           <listitem>
 680             <para>The node name of the parent device.</para>
 681           </listitem>
 682         </varlistentry>
 683
 684         <varlistentry>
 685           <term><option>$name</option></term>
 686           <listitem>
 687             <para>The current name of the device. If not changed by a rule, it is the
 688             name of the kernel device.</para>
 689           </listitem>
 690         </varlistentry>
 691
 692         <varlistentry>
 693           <term><option>$links</option></term>
 694           <listitem>
 695             <para>A space-separated list of the current symlinks. The value is
 696             only set during a remove event or if an earlier rule assigned a value.</para>
 697           </listitem>
 698         </varlistentry>
 699
 700         <varlistentry>
 701           <term><option>$root</option>, <option>%r</option></term>
 702           <listitem>
 703             <para>The udev_root value.</para>
 704           </listitem>
 705         </varlistentry>
 706
 707         <varlistentry>
 708           <term><option>$sys</option>, <option>%S</option></term>
 709           <listitem>
 710             <para>The sysfs mount point.</para>
 711           </listitem>
 712         </varlistentry>
 713
 714         <varlistentry>
 715           <term><option>$devnode</option>, <option>%N</option></term>
 716           <listitem>
 717             <para>The name of the device node.</para>
 718           </listitem>
 719         </varlistentry>
 720
 721         <varlistentry>
 722           <term><option>%%</option></term>
 723           <listitem>
 724           <para>The <literal>%</literal> character itself.</para>
 725           </listitem>
 726         </varlistentry>
 727
 728         <varlistentry>
 729           <term><option>$$</option></term>
 730           <listitem>
 731           <para>The <literal>$</literal> character itself.</para>
 732           </listitem>
 733         </varlistentry>
 734       </variablelist>
 735   </refsect1>
 736
 737   <refsect1>
 738     <title>See Also</title>
 739     <para>
 740       <citerefentry>
 741         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 742       </citerefentry>,
 743       <citerefentry>
 744         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 745       </citerefentry>,
 746       <citerefentry>
 747         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 748       </citerefentry>
 749     </para>
 750   </refsect1>
 751 </refentry>