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 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   Copyright 2012 Kay Sievers
   9   Copyright 2014 Jason St. John
  10 -->
  11
  12 <refentry id="udev">
  13   <refentryinfo>
  14     <title>udev</title>
  15     <productname>systemd</productname>
  16     <authorgroup>
  17       <author>
  18         <contrib>Developer</contrib>
  19         <firstname>Greg</firstname>
  20         <surname>Kroah-Hartmann</surname>
  21         <email>greg@kroah.com</email>
  22       </author>
  23       <author>
  24         <contrib>Developer</contrib>
  25         <firstname>Kay</firstname>
  26         <surname>Sievers</surname>
  27         <email>kay@vrfy.org</email>
  28       </author>
  29     </authorgroup>
  30   </refentryinfo>
  31
  32   <refmeta>
  33     <refentrytitle>udev</refentrytitle>
  34     <manvolnum>7</manvolnum>
  35   </refmeta>
  36
  37   <refnamediv>
  38     <refname>udev</refname>
  39     <refpurpose>Dynamic device management</refpurpose>
  40   </refnamediv>
  41
  42   <refsect1><title>Description</title>
  43     <para>udev supplies the system software with device events, manages permissions
  44     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  45     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  46     device names based on the order of discovery. Meaningful symlinks or network device
  47     names provide a way to reliably identify devices based on their properties or
  48     current configuration.</para>
  49
  50     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  51     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  52     the kernel whenever a device is added or removed from the system, or it changes its
  53     state. When udev receives a device event, it matches its configured set of rules
  54     against various device attributes to identify the device. Rules that match may
  55     provide additional device information to be stored in the udev database or
  56     to be used to create meaningful symlink names.</para>
  57
  58     <para>All device information udev processes is stored in the udev database and
  59     sent out to possible event subscribers. Access to all stored data and the event
  60     sources is provided by the library libudev.</para>
  61   </refsect1>
  62
  63   <refsect1><title>Rules Files</title>
  64       <para>The udev rules are read from the files located in the
  65       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  66       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  67       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  68       All rules files are collectively sorted and processed in lexical order,
  69       regardless of the directories in which they live. However, files with
  70       identical filenames replace each other. Files in <filename>/etc</filename>
  71       have the highest priority, files in <filename>/run</filename> take precedence
  72       over files with the same name in <filename>/usr/lib</filename>. This can be
  73       used to override a system-supplied rules file with a local file if needed;
  74       a symlink in <filename>/etc</filename> with the same name as a rules file in
  75       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
  76       disables the rules file entirely. Rule files must have the extension
  77       <filename>.rules</filename>; other extensions are ignored.</para>
  78
  79       <para>Every line in the rules file contains at least one key-value pair.
  80       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  81       There are two kinds of keys: match and assignment.
  82       If all match keys match against their values, the rule gets applied and the
  83       assignment keys get the specified values assigned.</para>
  84
  85       <para>A matching rule may rename a network interface, add symlinks
  86       pointing to the device node, or run a specified program as part of
  87       the event handling.</para>
  88
  89       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  90       Each key has a distinct operation, depending on the used operator. Valid
  91       operators are:</para>
  92       <variablelist>
  93         <varlistentry>
  94           <term><literal>==</literal></term>
  95           <listitem>
  96             <para>Compare for equality.</para>
  97           </listitem>
  98         </varlistentry>
  99
 100         <varlistentry>
 101           <term><literal>!=</literal></term>
 102           <listitem>
 103             <para>Compare for inequality.</para>
 104           </listitem>
 105         </varlistentry>
 106
 107         <varlistentry>
 108           <term><literal>=</literal></term>
 109           <listitem>
 110             <para>Assign a value to a key. Keys that represent a list are reset
 111             and only this single value is assigned.</para>
 112           </listitem>
 113         </varlistentry>
 114
 115         <varlistentry>
 116           <term><literal>+=</literal></term>
 117           <listitem>
 118             <para>Add the value to 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>Remove the value from a key that holds a list of entries.</para>
 126           </listitem>
 127         </varlistentry>
 128
 129         <varlistentry>
 130           <term><literal>:=</literal></term>
 131           <listitem>
 132             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 133           </listitem>
 134         </varlistentry>
 135       </variablelist>
 136
 137       <para>The following key names can be used to match against device properties.
 138       Some of the keys also match against properties of the parent devices in sysfs,
 139       not only the device that has generated the event. If multiple keys that match
 140       a parent device are specified in a single rule, all these keys must match at
 141       one and the same parent device.</para>
 142       <variablelist class='udev-directives'>
 143         <varlistentry>
 144           <term><varname>ACTION</varname></term>
 145           <listitem>
 146             <para>Match the name of the event action.</para>
 147           </listitem>
 148         </varlistentry>
 149
 150         <varlistentry>
 151           <term><varname>DEVPATH</varname></term>
 152           <listitem>
 153             <para>Match the devpath of the event device.</para>
 154           </listitem>
 155         </varlistentry>
 156
 157         <varlistentry>
 158           <term><varname>KERNEL</varname></term>
 159           <listitem>
 160             <para>Match the name of the event device.</para>
 161           </listitem>
 162         </varlistentry>
 163
 164         <varlistentry>
 165           <term><varname>NAME</varname></term>
 166           <listitem>
 167             <para>Match the name of a network interface. It can be used once the
 168             NAME key has been set in one of the preceding rules.</para>
 169           </listitem>
 170         </varlistentry>
 171
 172         <varlistentry>
 173           <term><varname>SYMLINK</varname></term>
 174           <listitem>
 175             <para>Match the name of a symlink targeting the node. It can
 176             be used once a SYMLINK key has been set in one of the preceding
 177             rules. There may be multiple symlinks; only one needs to match.
 178             </para>
 179           </listitem>
 180         </varlistentry>
 181
 182         <varlistentry>
 183           <term><varname>SUBSYSTEM</varname></term>
 184           <listitem>
 185             <para>Match the subsystem of the event device.</para>
 186           </listitem>
 187         </varlistentry>
 188         <varlistentry>
 189           <term><varname>DRIVER</varname></term>
 190           <listitem>
 191             <para>Match the driver name of the event device. Only set this key for devices
 192             which are bound to a driver at the time the event is generated.</para>
 193           </listitem>
 194         </varlistentry>
 195         <varlistentry>
 196           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 197           <listitem>
 198             <para>Match sysfs attribute values of the event device. Trailing
 199             whitespace in the attribute values is ignored unless the specified match
 200             value itself contains trailing whitespace.
 201             </para>
 202           </listitem>
 203           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 204           <listitem>
 205             <para>Match a kernel parameter value.
 206             </para>
 207           </listitem>
 208         </varlistentry>
 209
 210         <varlistentry>
 211           <term><varname>KERNELS</varname></term>
 212           <listitem>
 213             <para>Search the devpath upwards for a matching device name.</para>
 214           </listitem>
 215         </varlistentry>
 216
 217         <varlistentry>
 218           <term><varname>SUBSYSTEMS</varname></term>
 219           <listitem>
 220             <para>Search the devpath upwards for a matching device subsystem name.</para>
 221           </listitem>
 222         </varlistentry>
 223
 224         <varlistentry>
 225           <term><varname>DRIVERS</varname></term>
 226           <listitem>
 227             <para>Search the devpath upwards for a matching device driver name.</para>
 228           </listitem>
 229         </varlistentry>
 230
 231         <varlistentry>
 232           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 233           <listitem>
 234             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 235             If multiple <varname>ATTRS</varname> matches are specified, all of them
 236             must match on the same device. Trailing whitespace in the attribute values is ignored
 237             unless the specified match value itself contains trailing whitespace.</para>
 238           </listitem>
 239         </varlistentry>
 240
 241         <varlistentry>
 242           <term><varname>TAGS</varname></term>
 243           <listitem>
 244             <para>Search the devpath upwards for a device with matching tag.</para>
 245           </listitem>
 246         </varlistentry>
 247
 248         <varlistentry>
 249           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 250           <listitem>
 251             <para>Match against a device property value.</para>
 252           </listitem>
 253         </varlistentry>
 254
 255         <varlistentry>
 256           <term><varname>TAG</varname></term>
 257           <listitem>
 258             <para>Match against a device tag.</para>
 259           </listitem>
 260         </varlistentry>
 261
 262         <varlistentry>
 263           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 264           <listitem>
 265             <para>Test the existence of a file. An octal mode mask can be specified
 266             if needed.</para>
 267           </listitem>
 268         </varlistentry>
 269
 270         <varlistentry>
 271           <term><varname>PROGRAM</varname></term>
 272           <listitem>
 273             <para>Execute a program to determine whether there
 274             is a match; the key is true if the program returns
 275             successfully. The device properties are made available to the
 276             executed program in the environment. The program's standard output
 277             is available in the <varname>RESULT</varname> key.</para>
 278             <para>This can only be used for very short-running foreground tasks. For details,
 279             see <varname>RUN</varname>.</para>
 280           </listitem>
 281         </varlistentry>
 282
 283         <varlistentry>
 284           <term><varname>RESULT</varname></term>
 285           <listitem>
 286             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 287             This key can be used in the same or in any later rule after a
 288             <varname>PROGRAM</varname> call.</para>
 289           </listitem>
 290         </varlistentry>
 291       </variablelist>
 292
 293       <para>Most of the fields support shell glob pattern matching and
 294       alternate patterns. The following special characters are supported:</para>
 295       <variablelist>
 296         <varlistentry>
 297           <term><literal>*</literal></term>
 298           <listitem>
 299             <para>Matches zero or more characters.</para>
 300           </listitem>
 301         </varlistentry>
 302         <varlistentry>
 303           <term><literal>?</literal></term>
 304           <listitem>
 305             <para>Matches any single character.</para>
 306           </listitem>
 307         </varlistentry>
 308         <varlistentry>
 309           <term><literal>[]</literal></term>
 310           <listitem>
 311             <para>Matches any single character specified within the brackets. For
 312             example, the pattern string <literal>tty[SR]</literal>
 313             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 314             Ranges are also supported via the <literal>-</literal> character.
 315             For example, to match on the range of all digits, the pattern
 316             <literal>[0-9]</literal> could be used. If the first character
 317             following the <literal>[</literal> is a <literal>!</literal>,
 318             any characters not enclosed are matched.</para>
 319           </listitem>
 320         </varlistentry>
 321         <varlistentry>
 322           <term><literal>|</literal></term>
 323           <listitem>
 324             <para>Separates alternative patterns. For example, the pattern string
 325             <literal>abc|x*</literal> would match either <literal>abc</literal>
 326             or <literal>x*</literal>.</para>
 327           </listitem>
 328         </varlistentry>
 329       </variablelist>
 330
 331       <para>The following keys can get values assigned:</para>
 332       <variablelist class='udev-directives'>
 333         <varlistentry>
 334           <term><varname>NAME</varname></term>
 335           <listitem>
 336             <para>The name to use for a network interface. See
 337             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 338             for a higher-level mechanism for setting the interface name.
 339             The name of a device node cannot be changed by udev, only additional
 340             symlinks can be created.</para>
 341           </listitem>
 342         </varlistentry>
 343
 344         <varlistentry>
 345           <term><varname>SYMLINK</varname></term>
 346           <listitem>
 347             <para>The name of a symlink targeting the node. Every matching rule adds
 348             this value to the list of symlinks to be created.</para>
 349             <para>The set of characters to name a symlink is limited. Allowed
 350             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 351             sequences, and <literal>\x00</literal> hex encoding. All other
 352             characters are replaced by a <literal>_</literal> character.</para>
 353             <para>Multiple symlinks may be specified by separating the names by the
 354             space character. In case multiple devices claim the same name, the link
 355             always points to the device with the highest link_priority. If the current
 356             device goes away, the links are re-evaluated and the device with the
 357             next highest link_priority becomes the owner of the link. If no
 358             link_priority is specified, the order of the devices (and which one of
 359             them owns the link) is undefined.</para>
 360             <para>Symlink names must never conflict with the kernel's default device
 361             node names, as that would result in unpredictable behavior.
 362             </para>
 363           </listitem>
 364         </varlistentry>
 365
 366         <varlistentry>
 367           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 368           <listitem>
 369             <para>The permissions for the device node. Every specified value overrides
 370             the compiled-in default value.</para>
 371           </listitem>
 372         </varlistentry>
 373
 374         <varlistentry>
 375           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 376           <listitem>
 377             <para>Applies the specified Linux Security Module label to the device node.</para>
 378           </listitem>
 379         </varlistentry>
 380
 381         <varlistentry>
 382           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 383           <listitem>
 384             <para>The value that should be written to a sysfs attribute of the
 385             event device.</para>
 386           </listitem>
 387         </varlistentry>
 388
 389         <varlistentry>
 390           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 391           <listitem>
 392             <para>The value that should be written to kernel parameter.</para>
 393           </listitem>
 394         </varlistentry>
 395
 396         <varlistentry>
 397           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 398           <listitem>
 399             <para>Set a device property value. Property names with a leading <literal>.</literal>
 400             are neither stored in the database nor exported to events or
 401             external tools (run by, for example, the <varname>PROGRAM</varname>
 402             match key).</para>
 403           </listitem>
 404         </varlistentry>
 405
 406         <varlistentry>
 407           <term><varname>TAG</varname></term>
 408           <listitem>
 409             <para>Attach a tag to a device. This is used to filter events for users
 410             of libudev's monitor functionality, or to enumerate a group of tagged
 411             devices. The implementation can only work efficiently if only a few
 412             tags are attached to a device. It is only meant to be used in
 413             contexts with specific device filter requirements, and not as a
 414             general-purpose flag. Excessive use might result in inefficient event
 415             handling.</para>
 416           </listitem>
 417         </varlistentry>
 418
 419         <varlistentry>
 420           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 421           <listitem>
 422             <para>Add a program to the list of programs to be executed after
 423             processing all the rules for a specific event, depending on
 424             <literal>type</literal>:</para>
 425             <variablelist>
 426               <varlistentry>
 427                 <term><literal>program</literal></term>
 428                 <listitem>
 429                   <para>Execute an external program specified as the assigned
 430                   value. If no absolute path is given, the program is expected
 431                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 432                   absolute path must be specified.</para>
 433                   <para>This is the default if no <replaceable>type</replaceable>
 434                   is specified.</para>
 435                 </listitem>
 436               </varlistentry>
 437               <varlistentry>
 438                 <term><literal>builtin</literal></term>
 439                 <listitem>
 440                   <para>As <varname>program</varname>, but use one of the
 441                   built-in programs rather than an external one.</para>
 442                 </listitem>
 443               </varlistentry>
 444             </variablelist>
 445             <para>The program name and following arguments are separated by spaces.
 446             Single quotes can be used to specify arguments with spaces.</para>
 447             <para>This can only be used for very short-running foreground tasks. Running an
 448             event process for a long period of time may block all further events for
 449             this or a dependent device.</para>
 450             <para>Starting daemons or other long-running processes is not appropriate
 451             for udev; the forked processes, detached or not, will be unconditionally
 452             killed after the event handling has finished.</para>
 453             <para>Note that running programs that access the network or mount/unmount
 454             filesystems is not allowed inside of udev rules, due to the default sandbox
 455             that is enforced on <filename>systemd-udevd.service</filename>.</para>
 456           </listitem>
 457         </varlistentry>
 458
 459         <varlistentry>
 460           <term><varname>LABEL</varname></term>
 461           <listitem>
 462             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 463           </listitem>
 464         </varlistentry>
 465
 466         <varlistentry>
 467           <term><varname>GOTO</varname></term>
 468           <listitem>
 469             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 470           </listitem>
 471         </varlistentry>
 472
 473         <varlistentry>
 474           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 475           <listitem>
 476             <para>Import a set of variables as device properties,
 477             depending on <literal>type</literal>:</para>
 478             <variablelist>
 479               <varlistentry>
 480                 <term><literal>program</literal></term>
 481                 <listitem>
 482                   <para>Execute an external program specified as the assigned
 483                   value and, if it returns successfully,
 484                   import its output, which must be in environment key
 485                   format. Path specification, command/argument separation,
 486                   and quoting work like in <varname>RUN</varname>.</para>
 487                 </listitem>
 488               </varlistentry>
 489               <varlistentry>
 490                 <term><literal>builtin</literal></term>
 491                 <listitem>
 492                   <para>Similar to <literal>program</literal>, but use one of the
 493                   built-in programs rather than an external one.</para>
 494                 </listitem>
 495               </varlistentry>
 496              <varlistentry>
 497                 <term><literal>file</literal></term>
 498                 <listitem>
 499                   <para>Import a text file specified as the assigned value, the content
 500                   of which must be in environment key format.</para>
 501                 </listitem>
 502               </varlistentry>
 503               <varlistentry>
 504                 <term><literal>db</literal></term>
 505                 <listitem>
 506                   <para>Import a single property specified as the assigned value from the
 507                   current device database. This works only if the database is already populated
 508                   by an earlier event.</para>
 509                 </listitem>
 510               </varlistentry>
 511               <varlistentry>
 512                 <term><literal>cmdline</literal></term>
 513                 <listitem>
 514                   <para>Import a single property from the kernel command line. For simple flags
 515                   the value of the property is set to <literal>1</literal>.</para>
 516                 </listitem>
 517               </varlistentry>
 518               <varlistentry>
 519                 <term><literal>parent</literal></term>
 520                 <listitem>
 521                   <para>Import the stored keys from the parent device by reading
 522                   the database entry of the parent device. The value assigned to
 523                   <option>IMPORT{parent}</option> is used as a filter of key names
 524                   to import (with the same shell glob pattern matching used for
 525                   comparisons).</para>
 526                 </listitem>
 527               </varlistentry>
 528             </variablelist>
 529             <para>This can only be used for very short-running foreground tasks. For details
 530             see <option>RUN</option>.</para>
 531           </listitem>
 532         </varlistentry>
 533
 534         <varlistentry>
 535           <term><varname>OPTIONS</varname></term>
 536           <listitem>
 537             <para>Rule and device options:</para>
 538             <variablelist class='udev-directives'>
 539               <varlistentry>
 540                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 541                 <listitem>
 542                   <para>Specify the priority of the created symlinks. Devices with higher
 543                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 544                 </listitem>
 545               </varlistentry>
 546               <varlistentry>
 547                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 548                 <listitem>
 549                   <para>Usually, control and other possibly unsafe characters are replaced
 550                   in strings used for device naming. The mode of replacement can be specified
 551                   with this option.</para>
 552                 </listitem>
 553               </varlistentry>
 554               <varlistentry>
 555                 <term><option>static_node=</option></term>
 556                 <listitem>
 557                   <para>Apply the permissions specified in this rule to the
 558                   static device node with the specified name. Also, for every
 559                   tag specified in this rule, create a symlink
 560                   in the directory
 561                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 562                   pointing at the static device node with the specified name.
 563                   Static device node creation is performed by systemd-tmpfiles
 564                   before systemd-udevd is started. The static nodes might not
 565                   have a corresponding kernel device; they are used to trigger
 566                   automatic kernel module loading when they are accessed.</para>
 567                 </listitem>
 568               </varlistentry>
 569               <varlistentry>
 570                 <term><option>watch</option></term>
 571                 <listitem>
 572                   <para>Watch the device node with inotify; when the node is
 573                   closed after being opened for writing, a change uevent is
 574                   synthesized.</para>
 575                 </listitem>
 576               </varlistentry>
 577               <varlistentry>
 578                 <term><option>nowatch</option></term>
 579                 <listitem>
 580                   <para>Disable the watching of a device node with inotify.</para>
 581                 </listitem>
 582               </varlistentry>
 583             </variablelist>
 584           </listitem>
 585         </varlistentry>
 586       </variablelist>
 587
 588       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 589       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 590       <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 591       and <varname>RUN</varname> fields support simple string substitutions.
 592       The <varname>RUN</varname> substitutions are performed after all rules
 593       have been processed, right before the program is executed, allowing for
 594       the use of device properties set by earlier matching rules. For all other
 595       fields, substitutions are performed while the individual rule is being
 596       processed. The available substitutions are:</para>
 597       <variablelist class='udev-directives'>
 598         <varlistentry>
 599           <term><option>$kernel</option>, <option>%k</option></term>
 600           <listitem>
 601             <para>The kernel name for this device.</para>
 602           </listitem>
 603         </varlistentry>
 604
 605         <varlistentry>
 606           <term><option>$number</option>, <option>%n</option></term>
 607           <listitem>
 608             <para>The kernel number for this device. For example,
 609               <literal>sda3</literal> has kernel number <literal>3</literal>.
 610             </para>
 611           </listitem>
 612         </varlistentry>
 613
 614         <varlistentry>
 615           <term><option>$devpath</option>, <option>%p</option></term>
 616           <listitem>
 617             <para>The devpath of the device.</para>
 618           </listitem>
 619         </varlistentry>
 620
 621         <varlistentry>
 622           <term><option>$id</option>, <option>%b</option></term>
 623           <listitem>
 624             <para>The name of the device matched while searching the devpath
 625               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 626               <option>DRIVERS</option>, and <option>ATTRS</option>.
 627             </para>
 628           </listitem>
 629         </varlistentry>
 630
 631         <varlistentry>
 632           <term><option>$driver</option></term>
 633           <listitem>
 634             <para>The driver name of the device matched while searching the
 635               devpath upwards for <option>SUBSYSTEMS</option>,
 636               <option>KERNELS</option>, <option>DRIVERS</option>, and
 637               <option>ATTRS</option>.
 638             </para>
 639           </listitem>
 640         </varlistentry>
 641
 642         <varlistentry>
 643           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 644           <listitem>
 645             <para>The value of a sysfs attribute found at the device where
 646               all keys of the rule have matched. If the matching device does not
 647               have such an attribute, and a previous <option>KERNELS</option>,
 648               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 649               <option>ATTRS</option> test selected a parent device, then the
 650               attribute from that parent device is used.
 651             </para>
 652             <para>If the attribute is a symlink, the last element of the
 653               symlink target is returned as the value.
 654             </para>
 655           </listitem>
 656         </varlistentry>
 657
 658         <varlistentry>
 659           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 660           <listitem>
 661             <para>A device property value.</para>
 662           </listitem>
 663         </varlistentry>
 664
 665         <varlistentry>
 666           <term><option>$major</option>, <option>%M</option></term>
 667           <listitem>
 668             <para>The kernel major number for the device.</para>
 669           </listitem>
 670         </varlistentry>
 671
 672         <varlistentry>
 673           <term><option>$minor</option>, <option>%m</option></term>
 674           <listitem>
 675             <para>The kernel minor number for the device.</para>
 676           </listitem>
 677         </varlistentry>
 678
 679         <varlistentry>
 680           <term><option>$result</option>, <option>%c</option></term>
 681           <listitem>
 682             <para>The string returned by the external program requested with
 683             <varname>PROGRAM</varname>.
 684             A single part of the string, separated by a space character, may be selected
 685             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 686             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 687             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 688           </listitem>
 689         </varlistentry>
 690
 691         <varlistentry>
 692           <term><option>$parent</option>, <option>%P</option></term>
 693           <listitem>
 694             <para>The node name of the parent device.</para>
 695           </listitem>
 696         </varlistentry>
 697
 698         <varlistentry>
 699           <term><option>$name</option></term>
 700           <listitem>
 701             <para>The current name of the device. If not changed by a rule, it is the
 702             name of the kernel device.</para>
 703           </listitem>
 704         </varlistentry>
 705
 706         <varlistentry>
 707           <term><option>$links</option></term>
 708           <listitem>
 709             <para>A space-separated list of the current symlinks. The value is
 710             only set during a remove event or if an earlier rule assigned a value.</para>
 711           </listitem>
 712         </varlistentry>
 713
 714         <varlistentry>
 715           <term><option>$root</option>, <option>%r</option></term>
 716           <listitem>
 717             <para>The udev_root value.</para>
 718           </listitem>
 719         </varlistentry>
 720
 721         <varlistentry>
 722           <term><option>$sys</option>, <option>%S</option></term>
 723           <listitem>
 724             <para>The sysfs mount point.</para>
 725           </listitem>
 726         </varlistentry>
 727
 728         <varlistentry>
 729           <term><option>$devnode</option>, <option>%N</option></term>
 730           <listitem>
 731             <para>The name of the device node.</para>
 732           </listitem>
 733         </varlistentry>
 734
 735         <varlistentry>
 736           <term><option>%%</option></term>
 737           <listitem>
 738           <para>The <literal>%</literal> character itself.</para>
 739           </listitem>
 740         </varlistentry>
 741
 742         <varlistentry>
 743           <term><option>$$</option></term>
 744           <listitem>
 745           <para>The <literal>$</literal> character itself.</para>
 746           </listitem>
 747         </varlistentry>
 748       </variablelist>
 749   </refsect1>
 750
 751   <refsect1>
 752     <title>See Also</title>
 753     <para>
 754       <citerefentry>
 755         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 756       </citerefentry>,
 757       <citerefentry>
 758         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 759       </citerefentry>,
 760       <citerefentry>
 761         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 762       </citerefentry>
 763     </para>
 764   </refsect1>
 765 </refentry>