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 file names 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 '#', 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><option>ACTION</option></term>
 134           <listitem>
 135             <para>Match the name of the event action.</para>
 136           </listitem>
 137         </varlistentry>
 138
 139         <varlistentry>
 140           <term><option>DEVPATH</option></term>
 141           <listitem>
 142             <para>Match the devpath of the event device.</para>
 143           </listitem>
 144         </varlistentry>
 145
 146         <varlistentry>
 147           <term><option>KERNEL</option></term>
 148           <listitem>
 149             <para>Match the name of the event device.</para>
 150           </listitem>
 151         </varlistentry>
 152
 153         <varlistentry>
 154           <term><option>NAME</option></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><option>SYMLINK</option></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><option>SUBSYSTEM</option></term>
 173           <listitem>
 174             <para>Match the subsystem of the event device.</para>
 175           </listitem>
 176         </varlistentry>
 177         <varlistentry>
 178           <term><option>DRIVER</option></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><option>ATTR{<replaceable>filename</replaceable>}</option></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><option>KERNELS</option></term>
 196           <listitem>
 197             <para>Search the devpath upwards for a matching device name.</para>
 198           </listitem>
 199         </varlistentry>
 200
 201         <varlistentry>
 202           <term><option>SUBSYSTEMS</option></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><option>DRIVERS</option></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><option>ATTRS{<replaceable>filename</replaceable>}</option></term>
 217           <listitem>
 218             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 219             If multiple <option>ATTRS</option> 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><option>TAGS</option></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><option>ENV{<replaceable>key</replaceable>}</option></term>
 234           <listitem>
 235             <para>Match against a device property value.</para>
 236           </listitem>
 237         </varlistentry>
 238
 239         <varlistentry>
 240           <term><option>TAG</option></term>
 241           <listitem>
 242             <para>Match against a device tag.</para>
 243           </listitem>
 244         </varlistentry>
 245
 246         <varlistentry>
 247           <term><option>TEST{<replaceable>octal mode mask</replaceable>}</option></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><option>PROGRAM</option></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 <option>RUN</option>.</para>
 264           </listitem>
 265         </varlistentry>
 266
 267         <varlistentry>
 268           <term><option>RESULT</option></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 'tty[SR]' would match either 'ttyS' or 'ttyR'.
 296             Ranges are also supported via the '-' character.
 297             For example, to match on the range of all digits, the pattern [0-9] could
 298             be used. If the first character following the '[' is a '!', any characters
 299             not enclosed are matched.</para>
 300           </listitem>
 301         </varlistentry>
 302       </variablelist>
 303
 304       <para>The following keys can get values assigned:</para>
 305       <variablelist class='udev-directives'>
 306         <varlistentry>
 307           <term><option>NAME</option></term>
 308           <listitem>
 309             <para>The name to use for a network interface. The name of a device node
 310             cannot be changed by udev, only additional symlinks can be created.</para>
 311           </listitem>
 312         </varlistentry>
 313
 314         <varlistentry>
 315           <term><option>SYMLINK</option></term>
 316           <listitem>
 317             <para>The name of a symlink targeting the node. Every matching rule adds
 318             this value to the list of symlinks to be created.</para>
 319             <para>The set of characters to name a symlink is limited. Allowed
 320             characters are [0-9A-Za-z#+-.:=@_/], valid utf8 character sequences,
 321             and "\x00" hex encoding. All other characters are replaced by
 322             a '_' character.</para>
 323             <para>Multiple symlinks may be specified by separating the names by the
 324             space character. In case multiple devices claim the same name, the link
 325             always points to the device with the highest link_priority. If the current
 326             device goes away, the links are re-evaluated and the device with the
 327             next highest link_priority becomes the owner of the link. If no
 328             link_priority is specified, the order of the devices (and which one of
 329             them owns the link) is undefined.</para>
 330             <para>Symlink names must never conflict with the kernel's default device
 331             node names, as that would result in unpredictable behavior.
 332             </para>
 333           </listitem>
 334         </varlistentry>
 335
 336         <varlistentry>
 337           <term><option>OWNER</option>, <option>GROUP</option>, <option>MODE</option></term>
 338           <listitem>
 339             <para>The permissions for the device node. Every specified value overrides
 340             the compiled-in default value.</para>
 341           </listitem>
 342         </varlistentry>
 343
 344         <varlistentry>
 345           <term><option>ATTR{<replaceable>key</replaceable>}</option></term>
 346           <listitem>
 347             <para>The value that should be written to a sysfs attribute of the
 348             event device.</para>
 349           </listitem>
 350         </varlistentry>
 351
 352         <varlistentry>
 353           <term><option>ENV{<replaceable>key</replaceable>}</option></term>
 354           <listitem>
 355             <para>Set a device property value. Property names with a leading '.'
 356             are neither stored in the database nor exported to events or
 357             external tools (run by, say, the PROGRAM match key).</para>
 358           </listitem>
 359         </varlistentry>
 360
 361         <varlistentry>
 362           <term><option>TAG</option></term>
 363           <listitem>
 364             <para>Attach a tag to a device. This is used to filter events for users
 365             of libudev's monitor functionality, or to enumerate a group of tagged
 366             devices. The implementation can only work efficiently if only a few
 367             tags are attached to a device. It is only meant to be used in
 368             contexts with specific device filter requirements, and not as a
 369             general-purpose flag. Excessive use might result in inefficient event
 370             handling.</para>
 371           </listitem>
 372         </varlistentry>
 373
 374         <varlistentry>
 375           <term><option>RUN{<replaceable>type</replaceable>}</option></term>
 376           <listitem>
 377             <para>Add a program to the list of programs to be executed after processing all the
 378             rules for a specific event, depending on <literal>type</literal>:</para>
 379             <variablelist>
 380               <varlistentry>
 381                 <term><literal>program</literal></term>
 382                 <listitem>
 383                   <para>Execute an external program specified as the assigned
 384                   value. If no absolute path is given, the program is expected to live in
 385                   /usr/lib/udev, otherwise the absolute path must be specified.</para>
 386                   <para>This is the default if no <replaceable>type</replaceable> is
 387                   specified.</para>
 388                 </listitem>
 389               </varlistentry>
 390               <varlistentry>
 391                 <term><literal>builtin</literal></term>
 392                 <listitem>
 393                   <para>As <option>program</option>, but use one of the built-in programs rather
 394                   than an external one.</para>
 395                 </listitem>
 396               </varlistentry>
 397             </variablelist>
 398             <para>The program name and following arguments are separated by spaces.
 399             Single quotes can be used to specify arguments with spaces.</para>
 400             <para>This can only be used for very short-running foreground tasks. Running an
 401             event process for a long period of time may block all further events for
 402             this or a dependent device.</para>
 403             <para>Starting daemons or other long running processes is not appropriate
 404             for udev; the forked processes, detached or not, will be unconditionally
 405             killed after the event handling has finished.</para>
 406           </listitem>
 407         </varlistentry>
 408
 409         <varlistentry>
 410           <term><option>LABEL</option></term>
 411           <listitem>
 412             <para>A named label to which a GOTO may jump.</para>
 413           </listitem>
 414         </varlistentry>
 415
 416         <varlistentry>
 417           <term><option>GOTO</option></term>
 418           <listitem>
 419             <para>Jumps to the next LABEL with a matching name.</para>
 420           </listitem>
 421         </varlistentry>
 422
 423         <varlistentry>
 424           <term><option>IMPORT{<replaceable>type</replaceable>}</option></term>
 425           <listitem>
 426             <para>Import a set of variables as device properties,
 427             depending on <literal>type</literal>:</para>
 428             <variablelist>
 429               <varlistentry>
 430                 <term><literal>program</literal></term>
 431                 <listitem>
 432                   <para>Execute an external program specified as the assigned value and
 433                   import its output, which must be in environment key
 434                   format. Path specification, command/argument separation,
 435                   and quoting work like in <option>RUN</option>.</para>
 436                 </listitem>
 437               </varlistentry>
 438               <varlistentry>
 439                 <term><literal>builtin</literal></term>
 440                 <listitem>
 441                   <para>As <option>program</option>, but use one of the built-in programs rather
 442                   than an external one.</para>
 443                 </listitem>
 444               </varlistentry>
 445              <varlistentry>
 446                 <term><literal>file</literal></term>
 447                 <listitem>
 448                   <para>Import a text file specified as the assigned value, the content
 449                   of which must be in environment key format.</para>
 450                 </listitem>
 451               </varlistentry>
 452               <varlistentry>
 453                 <term><literal>db</literal></term>
 454                 <listitem>
 455                   <para>Import a single property specified as the assigned value from the
 456                   current device database. This works only if the database is already populated
 457                   by an earlier event.</para>
 458                 </listitem>
 459               </varlistentry>
 460               <varlistentry>
 461                 <term><literal>cmdline</literal></term>
 462                 <listitem>
 463                   <para>Import a single property from the kernel command line. For simple flags
 464                   the value of the property is set to '1'.</para>
 465                 </listitem>
 466               </varlistentry>
 467               <varlistentry>
 468                 <term><literal>parent</literal></term>
 469                 <listitem>
 470                   <para>Import the stored keys from the parent device by reading
 471                   the database entry of the parent device. The value assigned to
 472                   <option>IMPORT{parent}</option> is used as a filter of key names
 473                   to import (with the same shell-style pattern matching used for
 474                   comparisons).</para>
 475                 </listitem>
 476               </varlistentry>
 477             </variablelist>
 478             <para>This can only be used for very short-running foreground tasks. For details
 479             see <option>RUN</option>.</para>
 480           </listitem>
 481         </varlistentry>
 482
 483         <varlistentry>
 484           <term><option>WAIT_FOR</option></term>
 485           <listitem>
 486             <para>Wait for a file to become available or until a timeout of
 487             10 seconds expires. The path is relative to the sysfs device;
 488             if no path is specified, this waits for an attribute to appear.</para>
 489           </listitem>
 490         </varlistentry>
 491
 492         <varlistentry>
 493           <term><option>OPTIONS</option></term>
 494           <listitem>
 495             <para>Rule and device options:</para>
 496             <variablelist class='udev-directives'>
 497               <varlistentry>
 498                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 499                 <listitem>
 500                   <para>Specify the priority of the created symlinks. Devices with higher
 501                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 502                 </listitem>
 503               </varlistentry>
 504               <varlistentry>
 505                 <term><option>event_timeout=</option></term>
 506                 <listitem>
 507                   <para>Number of seconds an event waits for operations to finish before
 508                   giving up and terminating itself.</para>
 509                 </listitem>
 510               </varlistentry>
 511               <varlistentry>
 512                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 513                 <listitem>
 514                   <para>Usually control and other possibly unsafe characters are replaced
 515                   in strings used for device naming. The mode of replacement can be specified
 516                   with this option.</para>
 517                 </listitem>
 518               </varlistentry>
 519               <varlistentry>
 520                 <term><option>static_node=</option></term>
 521                 <listitem>
 522                   <para>Apply the permissions specified in this rule to the static device node with
 523                   the specified name. Static device node creation can be requested by kernel modules.
 524                   These nodes might not have a corresponding kernel device at the time systemd-udevd is
 525                   started; they can trigger automatic kernel module loading.</para>
 526                 </listitem>
 527               </varlistentry>
 528               <varlistentry>
 529                 <term><option>watch</option></term>
 530                 <listitem>
 531                   <para>Watch the device node with inotify; when the node is closed after being opened for
 532                   writing, a change uevent is synthesized.</para>
 533                 </listitem>
 534               </varlistentry>
 535               <varlistentry>
 536                 <term><option>nowatch</option></term>
 537                 <listitem>
 538                   <para>Disable the watching of a device node with inotify.</para>
 539                 </listitem>
 540               </varlistentry>
 541             </variablelist>
 542           </listitem>
 543         </varlistentry>
 544       </variablelist>
 545
 546       <para>The <option>NAME</option>, <option>SYMLINK</option>, <option>PROGRAM</option>,
 547       <option>OWNER</option>, <option>GROUP</option>, <option>MODE</option>  and  <option>RUN</option>
 548       fields support simple string substitutions. The <option>RUN</option>
 549       substitutions are performed after all rules have been processed, right before the program
 550       is executed, allowing for the use of device properties set by earlier matching
 551       rules. For all other fields, substitutions are performed while the individual rule is
 552       being processed. The available substitutions are:</para>
 553       <variablelist class='udev-directives'>
 554         <varlistentry>
 555           <term><option>$kernel</option>, <option>%k</option></term>
 556           <listitem>
 557             <para>The kernel name for this device.</para>
 558           </listitem>
 559         </varlistentry>
 560
 561         <varlistentry>
 562           <term><option>$number</option>, <option>%n</option></term>
 563           <listitem>
 564             <para>The kernel number for this device. For example, 'sda3' has
 565             kernel number of '3'</para>
 566           </listitem>
 567         </varlistentry>
 568
 569         <varlistentry>
 570           <term><option>$devpath</option>, <option>%p</option></term>
 571           <listitem>
 572             <para>The devpath of the device.</para>
 573           </listitem>
 574         </varlistentry>
 575
 576         <varlistentry>
 577           <term><option>$id</option>, <option>%b</option></term>
 578           <listitem>
 579             <para>The name of the device matched while searching the devpath upwards for
 580               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 581             </para>
 582           </listitem>
 583         </varlistentry>
 584
 585         <varlistentry>
 586           <term><option>$driver</option></term>
 587           <listitem>
 588             <para>The driver name of the device matched while searching the devpath upwards for
 589               <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
 590             </para>
 591           </listitem>
 592         </varlistentry>
 593
 594         <varlistentry>
 595           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 596           <listitem>
 597             <para>The value of a sysfs attribute found at the device where
 598             all keys of the rule have matched. If the matching device does not have
 599             such an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or
 600             ATTRS test selected a parent device, then the attribute from that
 601             parent device is used.</para>
 602             <para>If the attribute is a symlink, the last element of the symlink target is
 603             returned as the value.</para>
 604           </listitem>
 605         </varlistentry>
 606
 607         <varlistentry>
 608           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 609           <listitem>
 610             <para>A device property value.</para>
 611           </listitem>
 612         </varlistentry>
 613
 614         <varlistentry>
 615           <term><option>$major</option>, <option>%M</option></term>
 616           <listitem>
 617             <para>The kernel major number for the device.</para>
 618           </listitem>
 619         </varlistentry>
 620
 621         <varlistentry>
 622           <term><option>$minor</option>, <option>%m</option></term>
 623           <listitem>
 624             <para>The kernel minor number for the device.</para>
 625           </listitem>
 626         </varlistentry>
 627
 628         <varlistentry>
 629           <term><option>$result</option>, <option>%c</option></term>
 630           <listitem>
 631             <para>The string returned by the external program requested with PROGRAM.
 632             A single part of the string, separated by a space character, may be selected
 633             by specifying the part number as an attribute: <option>%c{N}</option>.
 634             If the number is followed by the '+' character, this part plus all remaining parts
 635             of the result string are substituted: <option>%c{N+}</option></para>
 636           </listitem>
 637         </varlistentry>
 638
 639         <varlistentry>
 640           <term><option>$parent</option>, <option>%P</option></term>
 641           <listitem>
 642             <para>The node name of the parent device.</para>
 643           </listitem>
 644         </varlistentry>
 645
 646         <varlistentry>
 647           <term><option>$name</option></term>
 648           <listitem>
 649             <para>The current name of the device. If not changed by a rule, it is the
 650             name of the kernel device.</para>
 651           </listitem>
 652         </varlistentry>
 653
 654         <varlistentry>
 655           <term><option>$links</option></term>
 656           <listitem>
 657             <para>A space-separated list of the current symlinks. The value is
 658             only set during a remove event or if an earlier rule assigned a value.</para>
 659           </listitem>
 660         </varlistentry>
 661
 662         <varlistentry>
 663           <term><option>$root</option>, <option>%r</option></term>
 664           <listitem>
 665             <para>The udev_root value.</para>
 666           </listitem>
 667         </varlistentry>
 668
 669         <varlistentry>
 670           <term><option>$sys</option>, <option>%S</option></term>
 671           <listitem>
 672             <para>The sysfs mount point.</para>
 673           </listitem>
 674         </varlistentry>
 675
 676         <varlistentry>
 677           <term><option>$devnode</option>, <option>%N</option></term>
 678           <listitem>
 679             <para>The name of the device node.</para>
 680           </listitem>
 681         </varlistentry>
 682
 683         <varlistentry>
 684           <term><literal>%%</literal></term>
 685           <listitem>
 686           <para>The '%' character itself.</para>
 687           </listitem>
 688         </varlistentry>
 689
 690         <varlistentry>
 691           <term><literal>$$</literal></term>
 692           <listitem>
 693           <para>The '$' character itself.</para>
 694           </listitem>
 695         </varlistentry>
 696       </variablelist>
 697   </refsect1>
 698
 699   <refsect1>
 700     <title>See Also</title>
 701     <para><citerefentry>
 702         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 703       </citerefentry>,
 704       <citerefentry>
 705         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 706     </citerefentry></para>
 707   </refsect1>
 708 </refentry>