man/udev.xml

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