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