man/udev.xml

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