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   This file is part of systemd.
   9
  10   Copyright 2012 Kay Sievers
  11   Copyright 2014 Jason St. John
  12 -->
  13
  14 <refentry id="udev">
  15   <refentryinfo>
  16     <title>udev</title>
  17     <productname>systemd</productname>
  18     <authorgroup>
  19       <author>
  20         <contrib>Developer</contrib>
  21         <firstname>Greg</firstname>
  22         <surname>Kroah-Hartmann</surname>
  23         <email>greg@kroah.com</email>
  24       </author>
  25       <author>
  26         <contrib>Developer</contrib>
  27         <firstname>Kay</firstname>
  28         <surname>Sievers</surname>
  29         <email>kay@vrfy.org</email>
  30       </author>
  31     </authorgroup>
  32   </refentryinfo>
  33
  34   <refmeta>
  35     <refentrytitle>udev</refentrytitle>
  36     <manvolnum>7</manvolnum>
  37   </refmeta>
  38
  39   <refnamediv>
  40     <refname>udev</refname>
  41     <refpurpose>Dynamic device management</refpurpose>
  42   </refnamediv>
  43
  44   <refsect1><title>Description</title>
  45     <para>udev supplies the system software with device events, manages permissions
  46     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  47     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  48     device names based on the order of discovery. Meaningful symlinks or network device
  49     names provide a way to reliably identify devices based on their properties or
  50     current configuration.</para>
  51
  52     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  53     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  54     the kernel whenever a device is added or removed from the system, or it changes its
  55     state. When udev receives a device event, it matches its configured set of rules
  56     against various device attributes to identify the device. Rules that match may
  57     provide additional device information to be stored in the udev database or
  58     to be used to create meaningful symlink names.</para>
  59
  60     <para>All device information udev processes is stored in the udev database and
  61     sent out to possible event subscribers. Access to all stored data and the event
  62     sources is provided by the library libudev.</para>
  63   </refsect1>
  64
  65   <refsect1><title>Rules Files</title>
  66       <para>The udev rules are read from the files located in the
  67       system rules directory <filename>/usr/lib/udev/rules.d</filename>,
  68       the volatile runtime directory <filename>/run/udev/rules.d</filename>
  69       and the local administration directory <filename>/etc/udev/rules.d</filename>.
  70       All rules files are collectively sorted and processed in lexical order,
  71       regardless of the directories in which they live. However, files with
  72       identical filenames replace each other. Files in <filename>/etc</filename>
  73       have the highest priority, files in <filename>/run</filename> take precedence
  74       over files with the same name in <filename>/usr/lib</filename>. This can be
  75       used to override a system-supplied rules file with a local file if needed;
  76       a symlink in <filename>/etc</filename> with the same name as a rules file in
  77       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
  78       disables the rules file entirely. Rule files must have the extension
  79       <filename>.rules</filename>; other extensions are ignored.</para>
  80
  81       <para>Every line in the rules file contains at least one key-value pair.
  82       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  83       There are two kinds of keys: match and assignment.
  84       If all match keys match against their values, the rule gets applied and the
  85       assignment keys get the specified values assigned.</para>
  86
  87       <para>A matching rule may rename a network interface, add symlinks
  88       pointing to the device node, or run a specified program as part of
  89       the event handling.</para>
  90
  91       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  92       Each key has a distinct operation, depending on the used operator. Valid
  93       operators are:</para>
  94       <variablelist>
  95         <varlistentry>
  96           <term><literal>==</literal></term>
  97           <listitem>
  98             <para>Compare for equality.</para>
  99           </listitem>
 100         </varlistentry>
 101
 102         <varlistentry>
 103           <term><literal>!=</literal></term>
 104           <listitem>
 105             <para>Compare for inequality.</para>
 106           </listitem>
 107         </varlistentry>
 108
 109         <varlistentry>
 110           <term><literal>=</literal></term>
 111           <listitem>
 112             <para>Assign a value to a key. Keys that represent a list are reset
 113             and only this single value is assigned.</para>
 114           </listitem>
 115         </varlistentry>
 116
 117         <varlistentry>
 118           <term><literal>+=</literal></term>
 119           <listitem>
 120             <para>Add the value to a key that holds a list of entries.</para>
 121           </listitem>
 122         </varlistentry>
 123
 124         <varlistentry>
 125           <term><literal>-=</literal></term>
 126           <listitem>
 127             <para>Remove the value from a key that holds a list of entries.</para>
 128           </listitem>
 129         </varlistentry>
 130
 131         <varlistentry>
 132           <term><literal>:=</literal></term>
 133           <listitem>
 134             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 135           </listitem>
 136         </varlistentry>
 137       </variablelist>
 138
 139       <para>The following key names can be used to match against device properties.
 140       Some of the keys also match against properties of the parent devices in sysfs,
 141       not only the device that has generated the event. If multiple keys that match
 142       a parent device are specified in a single rule, all these keys must match at
 143       one and the same parent device.</para>
 144       <variablelist class='udev-directives'>
 145         <varlistentry>
 146           <term><varname>ACTION</varname></term>
 147           <listitem>
 148             <para>Match the name of the event action.</para>
 149           </listitem>
 150         </varlistentry>
 151
 152         <varlistentry>
 153           <term><varname>DEVPATH</varname></term>
 154           <listitem>
 155             <para>Match the devpath of the event device.</para>
 156           </listitem>
 157         </varlistentry>
 158
 159         <varlistentry>
 160           <term><varname>KERNEL</varname></term>
 161           <listitem>
 162             <para>Match the name of the event device.</para>
 163           </listitem>
 164         </varlistentry>
 165
 166         <varlistentry>
 167           <term><varname>NAME</varname></term>
 168           <listitem>
 169             <para>Match the name of a network interface. It can be used once the
 170             NAME key has been set in one of the preceding rules.</para>
 171           </listitem>
 172         </varlistentry>
 173
 174         <varlistentry>
 175           <term><varname>SYMLINK</varname></term>
 176           <listitem>
 177             <para>Match the name of a symlink targeting the node. It can
 178             be used once a SYMLINK key has been set in one of the preceding
 179             rules. There may be multiple symlinks; only one needs to match.
 180             </para>
 181           </listitem>
 182         </varlistentry>
 183
 184         <varlistentry>
 185           <term><varname>SUBSYSTEM</varname></term>
 186           <listitem>
 187             <para>Match the subsystem of the event device.</para>
 188           </listitem>
 189         </varlistentry>
 190         <varlistentry>
 191           <term><varname>DRIVER</varname></term>
 192           <listitem>
 193             <para>Match the driver name of the event device. Only set this key for devices
 194             which are bound to a driver at the time the event is generated.</para>
 195           </listitem>
 196         </varlistentry>
 197         <varlistentry>
 198           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 199           <listitem>
 200             <para>Match sysfs attribute values of the event device. Trailing
 201             whitespace in the attribute values is ignored unless the specified match
 202             value itself contains trailing whitespace.
 203             </para>
 204           </listitem>
 205           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 206           <listitem>
 207             <para>Match a kernel parameter value.
 208             </para>
 209           </listitem>
 210         </varlistentry>
 211
 212         <varlistentry>
 213           <term><varname>KERNELS</varname></term>
 214           <listitem>
 215             <para>Search the devpath upwards for a matching device name.</para>
 216           </listitem>
 217         </varlistentry>
 218
 219         <varlistentry>
 220           <term><varname>SUBSYSTEMS</varname></term>
 221           <listitem>
 222             <para>Search the devpath upwards for a matching device subsystem name.</para>
 223           </listitem>
 224         </varlistentry>
 225
 226         <varlistentry>
 227           <term><varname>DRIVERS</varname></term>
 228           <listitem>
 229             <para>Search the devpath upwards for a matching device driver name.</para>
 230           </listitem>
 231         </varlistentry>
 232
 233         <varlistentry>
 234           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 235           <listitem>
 236             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 237             If multiple <varname>ATTRS</varname> matches are specified, all of them
 238             must match on the same device. Trailing whitespace in the attribute values is ignored
 239             unless the specified match value itself contains trailing whitespace.</para>
 240           </listitem>
 241         </varlistentry>
 242
 243         <varlistentry>
 244           <term><varname>TAGS</varname></term>
 245           <listitem>
 246             <para>Search the devpath upwards for a device with matching tag.</para>
 247           </listitem>
 248         </varlistentry>
 249
 250         <varlistentry>
 251           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 252           <listitem>
 253             <para>Match against a device property value.</para>
 254           </listitem>
 255         </varlistentry>
 256
 257         <varlistentry>
 258           <term><varname>TAG</varname></term>
 259           <listitem>
 260             <para>Match against a device tag.</para>
 261           </listitem>
 262         </varlistentry>
 263
 264         <varlistentry>
 265           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 266           <listitem>
 267             <para>Test the existence of a file. An octal mode mask can be specified
 268             if needed.</para>
 269           </listitem>
 270         </varlistentry>
 271
 272         <varlistentry>
 273           <term><varname>PROGRAM</varname></term>
 274           <listitem>
 275             <para>Execute a program to determine whether there
 276             is a match; the key is true if the program returns
 277             successfully. The device properties are made available to the
 278             executed program in the environment. The program's standard output
 279             is available in the <varname>RESULT</varname> key.</para>
 280             <para>This can only be used for very short-running foreground tasks. For details,
 281             see <varname>RUN</varname>.</para>
 282           </listitem>
 283         </varlistentry>
 284
 285         <varlistentry>
 286           <term><varname>RESULT</varname></term>
 287           <listitem>
 288             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 289             This key can be used in the same or in any later rule after a
 290             <varname>PROGRAM</varname> call.</para>
 291           </listitem>
 292         </varlistentry>
 293       </variablelist>
 294
 295       <para>Most of the fields support shell glob pattern matching and
 296       alternate patterns. The following special characters are supported:</para>
 297       <variablelist>
 298         <varlistentry>
 299           <term><literal>*</literal></term>
 300           <listitem>
 301             <para>Matches zero or more characters.</para>
 302           </listitem>
 303         </varlistentry>
 304         <varlistentry>
 305           <term><literal>?</literal></term>
 306           <listitem>
 307             <para>Matches any single character.</para>
 308           </listitem>
 309         </varlistentry>
 310         <varlistentry>
 311           <term><literal>[]</literal></term>
 312           <listitem>
 313             <para>Matches any single character specified within the brackets. For
 314             example, the pattern string <literal>tty[SR]</literal>
 315             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 316             Ranges are also supported via the <literal>-</literal> character.
 317             For example, to match on the range of all digits, the pattern
 318             <literal>[0-9]</literal> could be used. If the first character
 319             following the <literal>[</literal> is a <literal>!</literal>,
 320             any characters not enclosed are matched.</para>
 321           </listitem>
 322         </varlistentry>
 323         <varlistentry>
 324           <term><literal>|</literal></term>
 325           <listitem>
 326             <para>Separates alternative patterns. For example, the pattern string
 327             <literal>abc|x*</literal> would match either <literal>abc</literal>
 328             or <literal>x*</literal>.</para>
 329           </listitem>
 330         </varlistentry>
 331       </variablelist>
 332
 333       <para>The following keys can get values assigned:</para>
 334       <variablelist class='udev-directives'>
 335         <varlistentry>
 336           <term><varname>NAME</varname></term>
 337           <listitem>
 338             <para>The name to use for a network interface. See
 339             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 340             for a higher-level mechanism for setting the interface name.
 341             The name of a device node cannot be changed by udev, only additional
 342             symlinks can be created.</para>
 343           </listitem>
 344         </varlistentry>
 345
 346         <varlistentry>
 347           <term><varname>SYMLINK</varname></term>
 348           <listitem>
 349             <para>The name of a symlink targeting the node. Every matching rule adds
 350             this value to the list of symlinks to be created.</para>
 351             <para>The set of characters to name a symlink is limited. Allowed
 352             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 353             sequences, and <literal>\x00</literal> hex encoding. All other
 354             characters are replaced by a <literal>_</literal> character.</para>
 355             <para>Multiple symlinks may be specified by separating the names by the
 356             space character. In case multiple devices claim the same name, the link
 357             always points to the device with the highest link_priority. If the current
 358             device goes away, the links are re-evaluated and the device with the
 359             next highest link_priority becomes the owner of the link. If no
 360             link_priority is specified, the order of the devices (and which one of
 361             them owns the link) is undefined.</para>
 362             <para>Symlink names must never conflict with the kernel's default device
 363             node names, as that would result in unpredictable behavior.
 364             </para>
 365           </listitem>
 366         </varlistentry>
 367
 368         <varlistentry>
 369           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 370           <listitem>
 371             <para>The permissions for the device node. Every specified value overrides
 372             the compiled-in default value.</para>
 373           </listitem>
 374         </varlistentry>
 375
 376         <varlistentry>
 377           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 378           <listitem>
 379             <para>Applies the specified Linux Security Module label to the device node.</para>
 380           </listitem>
 381         </varlistentry>
 382
 383         <varlistentry>
 384           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 385           <listitem>
 386             <para>The value that should be written to a sysfs attribute of the
 387             event device.</para>
 388           </listitem>
 389         </varlistentry>
 390
 391         <varlistentry>
 392           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 393           <listitem>
 394             <para>The value that should be written to kernel parameter.</para>
 395           </listitem>
 396         </varlistentry>
 397
 398         <varlistentry>
 399           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 400           <listitem>
 401             <para>Set a device property value. Property names with a leading <literal>.</literal>
 402             are neither stored in the database nor exported to events or
 403             external tools (run by, for example, the <varname>PROGRAM</varname>
 404             match key).</para>
 405           </listitem>
 406         </varlistentry>
 407
 408         <varlistentry>
 409           <term><varname>TAG</varname></term>
 410           <listitem>
 411             <para>Attach a tag to a device. This is used to filter events for users
 412             of libudev's monitor functionality, or to enumerate a group of tagged
 413             devices. The implementation can only work efficiently if only a few
 414             tags are attached to a device. It is only meant to be used in
 415             contexts with specific device filter requirements, and not as a
 416             general-purpose flag. Excessive use might result in inefficient event
 417             handling.</para>
 418           </listitem>
 419         </varlistentry>
 420
 421         <varlistentry>
 422           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 423           <listitem>
 424             <para>Add a program to the list of programs to be executed after
 425             processing all the rules for a specific event, depending on
 426             <literal>type</literal>:</para>
 427             <variablelist>
 428               <varlistentry>
 429                 <term><literal>program</literal></term>
 430                 <listitem>
 431                   <para>Execute an external program specified as the assigned
 432                   value. If no absolute path is given, the program is expected
 433                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 434                   absolute path must be specified.</para>
 435                   <para>This is the default if no <replaceable>type</replaceable>
 436                   is specified.</para>
 437                 </listitem>
 438               </varlistentry>
 439               <varlistentry>
 440                 <term><literal>builtin</literal></term>
 441                 <listitem>
 442                   <para>As <varname>program</varname>, but use one of the
 443                   built-in programs rather than an external one.</para>
 444                 </listitem>
 445               </varlistentry>
 446             </variablelist>
 447             <para>The program name and following arguments are separated by spaces.
 448             Single quotes can be used to specify arguments with spaces.</para>
 449             <para>This can only be used for very short-running foreground tasks. Running an
 450             event process for a long period of time may block all further events for
 451             this or a dependent device.</para>
 452             <para>Starting daemons or other long-running processes is not appropriate
 453             for udev; the forked processes, detached or not, will be unconditionally
 454             killed after the event handling has finished.</para>
 455             <para>Note that running programs that access the network or mount/unmount
 456             filesystems is not allowed inside of udev rules, due to the default sandbox
 457             that is enforced on <filename>systemd-udevd.service</filename>.</para>
 458           </listitem>
 459         </varlistentry>
 460
 461         <varlistentry>
 462           <term><varname>LABEL</varname></term>
 463           <listitem>
 464             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 465           </listitem>
 466         </varlistentry>
 467
 468         <varlistentry>
 469           <term><varname>GOTO</varname></term>
 470           <listitem>
 471             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 472           </listitem>
 473         </varlistentry>
 474
 475         <varlistentry>
 476           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 477           <listitem>
 478             <para>Import a set of variables as device properties,
 479             depending on <literal>type</literal>:</para>
 480             <variablelist>
 481               <varlistentry>
 482                 <term><literal>program</literal></term>
 483                 <listitem>
 484                   <para>Execute an external program specified as the assigned
 485                   value and, if it returns successfully,
 486                   import its output, which must be in environment key
 487                   format. Path specification, command/argument separation,
 488                   and quoting work like in <varname>RUN</varname>.</para>
 489                 </listitem>
 490               </varlistentry>
 491               <varlistentry>
 492                 <term><literal>builtin</literal></term>
 493                 <listitem>
 494                   <para>Similar to <literal>program</literal>, but use one of the
 495                   built-in programs rather than an external one.</para>
 496                 </listitem>
 497               </varlistentry>
 498              <varlistentry>
 499                 <term><literal>file</literal></term>
 500                 <listitem>
 501                   <para>Import a text file specified as the assigned value, the content
 502                   of which must be in environment key format.</para>
 503                 </listitem>
 504               </varlistentry>
 505               <varlistentry>
 506                 <term><literal>db</literal></term>
 507                 <listitem>
 508                   <para>Import a single property specified as the assigned value from the
 509                   current device database. This works only if the database is already populated
 510                   by an earlier event.</para>
 511                 </listitem>
 512               </varlistentry>
 513               <varlistentry>
 514                 <term><literal>cmdline</literal></term>
 515                 <listitem>
 516                   <para>Import a single property from the kernel command line. For simple flags
 517                   the value of the property is set to <literal>1</literal>.</para>
 518                 </listitem>
 519               </varlistentry>
 520               <varlistentry>
 521                 <term><literal>parent</literal></term>
 522                 <listitem>
 523                   <para>Import the stored keys from the parent device by reading
 524                   the database entry of the parent device. The value assigned to
 525                   <option>IMPORT{parent}</option> is used as a filter of key names
 526                   to import (with the same shell glob pattern matching used for
 527                   comparisons).</para>
 528                 </listitem>
 529               </varlistentry>
 530             </variablelist>
 531             <para>This can only be used for very short-running foreground tasks. For details
 532             see <option>RUN</option>.</para>
 533           </listitem>
 534         </varlistentry>
 535
 536         <varlistentry>
 537           <term><varname>OPTIONS</varname></term>
 538           <listitem>
 539             <para>Rule and device options:</para>
 540             <variablelist class='udev-directives'>
 541               <varlistentry>
 542                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 543                 <listitem>
 544                   <para>Specify the priority of the created symlinks. Devices with higher
 545                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 546                 </listitem>
 547               </varlistentry>
 548               <varlistentry>
 549                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 550                 <listitem>
 551                   <para>Usually, control and other possibly unsafe characters are replaced
 552                   in strings used for device naming. The mode of replacement can be specified
 553                   with this option.</para>
 554                 </listitem>
 555               </varlistentry>
 556               <varlistentry>
 557                 <term><option>static_node=</option></term>
 558                 <listitem>
 559                   <para>Apply the permissions specified in this rule to the
 560                   static device node with the specified name. Also, for every
 561                   tag specified in this rule, create a symlink
 562                   in the directory
 563                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 564                   pointing at the static device node with the specified name.
 565                   Static device node creation is performed by systemd-tmpfiles
 566                   before systemd-udevd is started. The static nodes might not
 567                   have a corresponding kernel device; they are used to trigger
 568                   automatic kernel module loading when they are accessed.</para>
 569                 </listitem>
 570               </varlistentry>
 571               <varlistentry>
 572                 <term><option>watch</option></term>
 573                 <listitem>
 574                   <para>Watch the device node with inotify; when the node is
 575                   closed after being opened for writing, a change uevent is
 576                   synthesized.</para>
 577                 </listitem>
 578               </varlistentry>
 579               <varlistentry>
 580                 <term><option>nowatch</option></term>
 581                 <listitem>
 582                   <para>Disable the watching of a device node with inotify.</para>
 583                 </listitem>
 584               </varlistentry>
 585             </variablelist>
 586           </listitem>
 587         </varlistentry>
 588       </variablelist>
 589
 590       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 591       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 592       <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 593       and <varname>RUN</varname> fields support simple string substitutions.
 594       The <varname>RUN</varname> substitutions are performed after all rules
 595       have been processed, right before the program is executed, allowing for
 596       the use of device properties set by earlier matching rules. For all other
 597       fields, substitutions are performed while the individual rule is being
 598       processed. The available substitutions are:</para>
 599       <variablelist class='udev-directives'>
 600         <varlistentry>
 601           <term><option>$kernel</option>, <option>%k</option></term>
 602           <listitem>
 603             <para>The kernel name for this device.</para>
 604           </listitem>
 605         </varlistentry>
 606
 607         <varlistentry>
 608           <term><option>$number</option>, <option>%n</option></term>
 609           <listitem>
 610             <para>The kernel number for this device. For example,
 611               <literal>sda3</literal> has kernel number <literal>3</literal>.
 612             </para>
 613           </listitem>
 614         </varlistentry>
 615
 616         <varlistentry>
 617           <term><option>$devpath</option>, <option>%p</option></term>
 618           <listitem>
 619             <para>The devpath of the device.</para>
 620           </listitem>
 621         </varlistentry>
 622
 623         <varlistentry>
 624           <term><option>$id</option>, <option>%b</option></term>
 625           <listitem>
 626             <para>The name of the device matched while searching the devpath
 627               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 628               <option>DRIVERS</option>, and <option>ATTRS</option>.
 629             </para>
 630           </listitem>
 631         </varlistentry>
 632
 633         <varlistentry>
 634           <term><option>$driver</option></term>
 635           <listitem>
 636             <para>The driver name of the device matched while searching the
 637               devpath upwards for <option>SUBSYSTEMS</option>,
 638               <option>KERNELS</option>, <option>DRIVERS</option>, and
 639               <option>ATTRS</option>.
 640             </para>
 641           </listitem>
 642         </varlistentry>
 643
 644         <varlistentry>
 645           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 646           <listitem>
 647             <para>The value of a sysfs attribute found at the device where
 648               all keys of the rule have matched. If the matching device does not
 649               have such an attribute, and a previous <option>KERNELS</option>,
 650               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 651               <option>ATTRS</option> test selected a parent device, then the
 652               attribute from that parent device is used.
 653             </para>
 654             <para>If the attribute is a symlink, the last element of the
 655               symlink target is returned as the value.
 656             </para>
 657           </listitem>
 658         </varlistentry>
 659
 660         <varlistentry>
 661           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 662           <listitem>
 663             <para>A device property value.</para>
 664           </listitem>
 665         </varlistentry>
 666
 667         <varlistentry>
 668           <term><option>$major</option>, <option>%M</option></term>
 669           <listitem>
 670             <para>The kernel major number for the device.</para>
 671           </listitem>
 672         </varlistentry>
 673
 674         <varlistentry>
 675           <term><option>$minor</option>, <option>%m</option></term>
 676           <listitem>
 677             <para>The kernel minor number for the device.</para>
 678           </listitem>
 679         </varlistentry>
 680
 681         <varlistentry>
 682           <term><option>$result</option>, <option>%c</option></term>
 683           <listitem>
 684             <para>The string returned by the external program requested with
 685             <varname>PROGRAM</varname>.
 686             A single part of the string, separated by a space character, may be selected
 687             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 688             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 689             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 690           </listitem>
 691         </varlistentry>
 692
 693         <varlistentry>
 694           <term><option>$parent</option>, <option>%P</option></term>
 695           <listitem>
 696             <para>The node name of the parent device.</para>
 697           </listitem>
 698         </varlistentry>
 699
 700         <varlistentry>
 701           <term><option>$name</option></term>
 702           <listitem>
 703             <para>The current name of the device. If not changed by a rule, it is the
 704             name of the kernel device.</para>
 705           </listitem>
 706         </varlistentry>
 707
 708         <varlistentry>
 709           <term><option>$links</option></term>
 710           <listitem>
 711             <para>A space-separated list of the current symlinks. The value is
 712             only set during a remove event or if an earlier rule assigned a value.</para>
 713           </listitem>
 714         </varlistentry>
 715
 716         <varlistentry>
 717           <term><option>$root</option>, <option>%r</option></term>
 718           <listitem>
 719             <para>The udev_root value.</para>
 720           </listitem>
 721         </varlistentry>
 722
 723         <varlistentry>
 724           <term><option>$sys</option>, <option>%S</option></term>
 725           <listitem>
 726             <para>The sysfs mount point.</para>
 727           </listitem>
 728         </varlistentry>
 729
 730         <varlistentry>
 731           <term><option>$devnode</option>, <option>%N</option></term>
 732           <listitem>
 733             <para>The name of the device node.</para>
 734           </listitem>
 735         </varlistentry>
 736
 737         <varlistentry>
 738           <term><option>%%</option></term>
 739           <listitem>
 740           <para>The <literal>%</literal> character itself.</para>
 741           </listitem>
 742         </varlistentry>
 743
 744         <varlistentry>
 745           <term><option>$$</option></term>
 746           <listitem>
 747           <para>The <literal>$</literal> character itself.</para>
 748           </listitem>
 749         </varlistentry>
 750       </variablelist>
 751   </refsect1>
 752
 753   <refsect1>
 754     <title>See Also</title>
 755     <para>
 756       <citerefentry>
 757         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 758       </citerefentry>,
 759       <citerefentry>
 760         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 761       </citerefentry>,
 762       <citerefentry>
 763         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 764       </citerefentry>
 765     </para>
 766   </refsect1>
 767 </refentry>