man/udev.xml

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