man/udev.xml

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   6   SPDX-License-Identifier: LGPL-2.1+
   7   Copyright © 2014 Jason St. John
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   9
  10 <refentry id="udev">
  11   <refentryinfo>
  12     <title>udev</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>udev</refentrytitle>
  18     <manvolnum>7</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>udev</refname>
  23     <refpurpose>Dynamic device management</refpurpose>
  24   </refnamediv>
  25
  26   <refsect1><title>Description</title>
  27     <para>udev supplies the system software with device events, manages permissions
  28     of device nodes and may create additional symlinks in the <filename>/dev</filename>
  29     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  30     device names based on the order of discovery. Meaningful symlinks or network device
  31     names provide a way to reliably identify devices based on their properties or
  32     current configuration.</para>
  33
  34     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  35     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  36     the kernel whenever a device is added or removed from the system, or it changes its
  37     state. When udev receives a device event, it matches its configured set of rules
  38     against various device attributes to identify the device. Rules that match may
  39     provide additional device information to be stored in the udev database or
  40     to be used to create meaningful symlink names.</para>
  41
  42     <para>All device information udev processes is stored in the udev database and
  43     sent out to possible event subscribers. Access to all stored data and the event
  44     sources is provided by the library libudev.</para>
  45   </refsect1>
  46
  47   <refsect1><title>Rules Files</title>
  48       <para>The udev rules are read from the files located in the system rules directories
  49       <filename>/usr/lib/udev/rules.d</filename> and <filename>/usr/local/lib/udev/rules.d</filename>, the
  50       volatile runtime directory <filename>/run/udev/rules.d</filename> and the local administration
  51       directory <filename>/etc/udev/rules.d</filename>.  All rules files are collectively sorted and
  52       processed in lexical order, regardless of the directories in which they live. However, files with
  53       identical filenames replace each other. Files in <filename>/etc</filename> have the highest priority,
  54       files in <filename>/run</filename> take precedence over files with the same name under
  55       <filename>/usr</filename>. This can be used to override a system-supplied rules file with a local
  56       file if needed; a symlink in <filename>/etc</filename> with the same name as a rules file in
  57       <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>, disables the rules file
  58       entirely. Rule files must have the extension <filename>.rules</filename>; other extensions are
  59       ignored.</para>
  60
  61       <para>Every line in the rules file contains at least one key-value pair.
  62       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  63       There are two kinds of keys: match and assignment.
  64       If all match keys match against their values, the rule gets applied and the
  65       assignment keys get the specified values assigned.</para>
  66
  67       <para>A matching rule may rename a network interface, add symlinks
  68       pointing to the device node, or run a specified program as part of
  69       the event handling.</para>
  70
  71       <para>A rule consists of a comma-separated list of one or more key-value pairs.
  72       Each key has a distinct operation, depending on the used operator. Valid
  73       operators are:</para>
  74       <variablelist>
  75         <varlistentry>
  76           <term><literal>==</literal></term>
  77           <listitem>
  78             <para>Compare for equality.</para>
  79           </listitem>
  80         </varlistentry>
  81
  82         <varlistentry>
  83           <term><literal>!=</literal></term>
  84           <listitem>
  85             <para>Compare for inequality.</para>
  86           </listitem>
  87         </varlistentry>
  88
  89         <varlistentry>
  90           <term><literal>=</literal></term>
  91           <listitem>
  92             <para>Assign a value to a key. Keys that represent a list are reset
  93             and only this single value is assigned.</para>
  94           </listitem>
  95         </varlistentry>
  96
  97         <varlistentry>
  98           <term><literal>+=</literal></term>
  99           <listitem>
 100             <para>Add the value to a key that holds a list of entries.</para>
 101           </listitem>
 102         </varlistentry>
 103
 104         <varlistentry>
 105           <term><literal>-=</literal></term>
 106           <listitem>
 107             <para>Remove the value from a key that holds a list of entries.</para>
 108           </listitem>
 109         </varlistentry>
 110
 111         <varlistentry>
 112           <term><literal>:=</literal></term>
 113           <listitem>
 114             <para>Assign  a  value  to  a key finally; disallow any later changes.</para>
 115           </listitem>
 116         </varlistentry>
 117       </variablelist>
 118
 119       <para>The following key names can be used to match against device properties.
 120       Some of the keys also match against properties of the parent devices in sysfs,
 121       not only the device that has generated the event. If multiple keys that match
 122       a parent device are specified in a single rule, all these keys must match at
 123       one and the same parent device.</para>
 124       <variablelist class='udev-directives'>
 125         <varlistentry>
 126           <term><varname>ACTION</varname></term>
 127           <listitem>
 128             <para>Match the name of the event action.</para>
 129           </listitem>
 130         </varlistentry>
 131
 132         <varlistentry>
 133           <term><varname>DEVPATH</varname></term>
 134           <listitem>
 135             <para>Match the devpath of the event device.</para>
 136           </listitem>
 137         </varlistentry>
 138
 139         <varlistentry>
 140           <term><varname>KERNEL</varname></term>
 141           <listitem>
 142             <para>Match the name of the event device.</para>
 143           </listitem>
 144         </varlistentry>
 145
 146         <varlistentry>
 147           <term><varname>NAME</varname></term>
 148           <listitem>
 149             <para>Match the name of a network interface. It can be used once the
 150             NAME key has been set in one of the preceding rules.</para>
 151           </listitem>
 152         </varlistentry>
 153
 154         <varlistentry>
 155           <term><varname>SYMLINK</varname></term>
 156           <listitem>
 157             <para>Match the name of a symlink targeting the node. It can
 158             be used once a SYMLINK key has been set in one of the preceding
 159             rules. There may be multiple symlinks; only one needs to match.
 160             </para>
 161           </listitem>
 162         </varlistentry>
 163
 164         <varlistentry>
 165           <term><varname>SUBSYSTEM</varname></term>
 166           <listitem>
 167             <para>Match the subsystem of the event device.</para>
 168           </listitem>
 169         </varlistentry>
 170         <varlistentry>
 171           <term><varname>DRIVER</varname></term>
 172           <listitem>
 173             <para>Match the driver name of the event device. Only set this key for devices
 174             which are bound to a driver at the time the event is generated.</para>
 175           </listitem>
 176         </varlistentry>
 177         <varlistentry>
 178           <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 179           <listitem>
 180             <para>Match sysfs attribute values of the event device. Trailing
 181             whitespace in the attribute values is ignored unless the specified match
 182             value itself contains trailing whitespace.
 183             </para>
 184           </listitem>
 185         </varlistentry>
 186         <varlistentry>
 187           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 188           <listitem>
 189             <para>Match a kernel parameter value.
 190             </para>
 191           </listitem>
 192         </varlistentry>
 193
 194         <varlistentry>
 195           <term><varname>KERNELS</varname></term>
 196           <listitem>
 197             <para>Search the devpath upwards for a matching device name.</para>
 198           </listitem>
 199         </varlistentry>
 200
 201         <varlistentry>
 202           <term><varname>SUBSYSTEMS</varname></term>
 203           <listitem>
 204             <para>Search the devpath upwards for a matching device subsystem name.</para>
 205           </listitem>
 206         </varlistentry>
 207
 208         <varlistentry>
 209           <term><varname>DRIVERS</varname></term>
 210           <listitem>
 211             <para>Search the devpath upwards for a matching device driver name.</para>
 212           </listitem>
 213         </varlistentry>
 214
 215         <varlistentry>
 216           <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 217           <listitem>
 218             <para>Search the devpath upwards for a device with matching sysfs attribute values.
 219             If multiple <varname>ATTRS</varname> matches are specified, all of them
 220             must match on the same device. Trailing whitespace in the attribute values is ignored
 221             unless the specified match value itself contains trailing whitespace.</para>
 222           </listitem>
 223         </varlistentry>
 224
 225         <varlistentry>
 226           <term><varname>TAGS</varname></term>
 227           <listitem>
 228             <para>Search the devpath upwards for a device with matching tag.</para>
 229           </listitem>
 230         </varlistentry>
 231
 232         <varlistentry>
 233           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 234           <listitem>
 235             <para>Match against a device property value.</para>
 236           </listitem>
 237         </varlistentry>
 238
 239         <varlistentry>
 240           <term><varname>TAG</varname></term>
 241           <listitem>
 242             <para>Match against a device tag.</para>
 243           </listitem>
 244         </varlistentry>
 245
 246         <varlistentry>
 247           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 248           <listitem>
 249             <para>Test the existence of a file. An octal mode mask can be specified
 250             if needed.</para>
 251           </listitem>
 252         </varlistentry>
 253
 254         <varlistentry>
 255           <term><varname>PROGRAM</varname></term>
 256           <listitem>
 257             <para>Execute a program to determine whether there
 258             is a match; the key is true if the program returns
 259             successfully. The device properties are made available to the
 260             executed program in the environment. The program's standard output
 261             is available in the <varname>RESULT</varname> key.</para>
 262             <para>This can only be used for very short-running foreground tasks. For details,
 263             see <varname>RUN</varname>.</para>
 264           </listitem>
 265         </varlistentry>
 266
 267         <varlistentry>
 268           <term><varname>RESULT</varname></term>
 269           <listitem>
 270             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 271             This key can be used in the same or in any later rule after a
 272             <varname>PROGRAM</varname> call.</para>
 273           </listitem>
 274         </varlistentry>
 275       </variablelist>
 276
 277       <para>Most of the fields support shell glob pattern matching and
 278       alternate patterns. The following special characters are supported:</para>
 279       <variablelist>
 280         <varlistentry>
 281           <term><literal>*</literal></term>
 282           <listitem>
 283             <para>Matches zero or more characters.</para>
 284           </listitem>
 285         </varlistentry>
 286         <varlistentry>
 287           <term><literal>?</literal></term>
 288           <listitem>
 289             <para>Matches any single character.</para>
 290           </listitem>
 291         </varlistentry>
 292         <varlistentry>
 293           <term><literal>[]</literal></term>
 294           <listitem>
 295             <para>Matches any single character specified within the brackets. For
 296             example, the pattern string <literal>tty[SR]</literal>
 297             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 298             Ranges are also supported via the <literal>-</literal> character.
 299             For example, to match on the range of all digits, the pattern
 300             <literal>[0-9]</literal> could be used. If the first character
 301             following the <literal>[</literal> is a <literal>!</literal>,
 302             any characters not enclosed are matched.</para>
 303           </listitem>
 304         </varlistentry>
 305         <varlistentry>
 306           <term><literal>|</literal></term>
 307           <listitem>
 308             <para>Separates alternative patterns. For example, the pattern string
 309             <literal>abc|x*</literal> would match either <literal>abc</literal>
 310             or <literal>x*</literal>.</para>
 311           </listitem>
 312         </varlistentry>
 313       </variablelist>
 314
 315       <para>The following keys can get values assigned:</para>
 316       <variablelist class='udev-directives'>
 317         <varlistentry>
 318           <term><varname>NAME</varname></term>
 319           <listitem>
 320             <para>The name to use for a network interface. See
 321             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 322             for a higher-level mechanism for setting the interface name.
 323             The name of a device node cannot be changed by udev, only additional
 324             symlinks can be created.</para>
 325           </listitem>
 326         </varlistentry>
 327
 328         <varlistentry>
 329           <term><varname>SYMLINK</varname></term>
 330           <listitem>
 331             <para>The name of a symlink targeting the node. Every matching rule adds
 332             this value to the list of symlinks to be created.</para>
 333             <para>The set of characters to name a symlink is limited. Allowed
 334             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 335             sequences, and <literal>\x00</literal> hex encoding. All other
 336             characters are replaced by a <literal>_</literal> character.</para>
 337             <para>Multiple symlinks may be specified by separating the names by the
 338             space character. In case multiple devices claim the same name, the link
 339             always points to the device with the highest link_priority. If the current
 340             device goes away, the links are re-evaluated and the device with the
 341             next highest link_priority becomes the owner of the link. If no
 342             link_priority is specified, the order of the devices (and which one of
 343             them owns the link) is undefined.</para>
 344             <para>Symlink names must never conflict with the kernel's default device
 345             node names, as that would result in unpredictable behavior.
 346             </para>
 347           </listitem>
 348         </varlistentry>
 349
 350         <varlistentry>
 351           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 352           <listitem>
 353             <para>The permissions for the device node. Every specified value overrides
 354             the compiled-in default value.</para>
 355           </listitem>
 356         </varlistentry>
 357
 358         <varlistentry>
 359           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 360           <listitem>
 361             <para>Applies the specified Linux Security Module label to the device node.</para>
 362           </listitem>
 363         </varlistentry>
 364
 365         <varlistentry>
 366           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 367           <listitem>
 368             <para>The value that should be written to a sysfs attribute of the
 369             event device.</para>
 370           </listitem>
 371         </varlistentry>
 372
 373         <varlistentry>
 374           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 375           <listitem>
 376             <para>The value that should be written to kernel parameter.</para>
 377           </listitem>
 378         </varlistentry>
 379
 380         <varlistentry>
 381           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 382           <listitem>
 383             <para>Set a device property value. Property names with a leading <literal>.</literal>
 384             are neither stored in the database nor exported to events or
 385             external tools (run by, for example, the <varname>PROGRAM</varname>
 386             match key).</para>
 387           </listitem>
 388         </varlistentry>
 389
 390         <varlistentry>
 391           <term><varname>TAG</varname></term>
 392           <listitem>
 393             <para>Attach a tag to a device. This is used to filter events for users
 394             of libudev's monitor functionality, or to enumerate a group of tagged
 395             devices. The implementation can only work efficiently if only a few
 396             tags are attached to a device. It is only meant to be used in
 397             contexts with specific device filter requirements, and not as a
 398             general-purpose flag. Excessive use might result in inefficient event
 399             handling.</para>
 400           </listitem>
 401         </varlistentry>
 402
 403         <varlistentry>
 404           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 405           <listitem>
 406             <para>Add a program to the list of programs to be executed after
 407             processing all the rules for a specific event, depending on
 408             <literal>type</literal>:</para>
 409             <variablelist>
 410               <varlistentry>
 411                 <term><literal>program</literal></term>
 412                 <listitem>
 413                   <para>Execute an external program specified as the assigned
 414                   value. If no absolute path is given, the program is expected
 415                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 416                   absolute path must be specified.</para>
 417                   <para>This is the default if no <replaceable>type</replaceable>
 418                   is specified.</para>
 419                 </listitem>
 420               </varlistentry>
 421               <varlistentry>
 422                 <term><literal>builtin</literal></term>
 423                 <listitem>
 424                   <para>As <varname>program</varname>, but use one of the
 425                   built-in programs rather than an external one.</para>
 426                 </listitem>
 427               </varlistentry>
 428             </variablelist>
 429             <para>The program name and following arguments are separated by spaces.
 430             Single quotes can be used to specify arguments with spaces.</para>
 431             <para>This can only be used for very short-running foreground tasks. Running an
 432             event process for a long period of time may block all further events for
 433             this or a dependent device.</para>
 434             <para>Starting daemons or other long-running processes is not appropriate
 435             for udev; the forked processes, detached or not, will be unconditionally
 436             killed after the event handling has finished.</para>
 437             <para>Note that running programs that access the network or mount/unmount
 438             filesystems is not allowed inside of udev rules, due to the default sandbox
 439             that is enforced on <filename>systemd-udevd.service</filename>.</para>
 440             <para>Please also note that <literal>:=</literal> and <literal>=</literal> are clearing
 441             both, program and builtin commands.</para>
 442           </listitem>
 443         </varlistentry>
 444
 445         <varlistentry>
 446           <term><varname>LABEL</varname></term>
 447           <listitem>
 448             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 449           </listitem>
 450         </varlistentry>
 451
 452         <varlistentry>
 453           <term><varname>GOTO</varname></term>
 454           <listitem>
 455             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 456           </listitem>
 457         </varlistentry>
 458
 459         <varlistentry>
 460           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 461           <listitem>
 462             <para>Import a set of variables as device properties,
 463             depending on <literal>type</literal>:</para>
 464             <variablelist>
 465               <varlistentry>
 466                 <term><literal>program</literal></term>
 467                 <listitem>
 468                   <para>Execute an external program specified as the assigned
 469                   value and, if it returns successfully,
 470                   import its output, which must be in environment key
 471                   format. Path specification, command/argument separation,
 472                   and quoting work like in <varname>RUN</varname>.</para>
 473                 </listitem>
 474               </varlistentry>
 475               <varlistentry>
 476                 <term><literal>builtin</literal></term>
 477                 <listitem>
 478                   <para>Similar to <literal>program</literal>, but use one of the
 479                   built-in programs rather than an external one.</para>
 480                 </listitem>
 481               </varlistentry>
 482              <varlistentry>
 483                 <term><literal>file</literal></term>
 484                 <listitem>
 485                   <para>Import a text file specified as the assigned value, the content
 486                   of which must be in environment key format.</para>
 487                 </listitem>
 488               </varlistentry>
 489               <varlistentry>
 490                 <term><literal>db</literal></term>
 491                 <listitem>
 492                   <para>Import a single property specified as the assigned value from the
 493                   current device database. This works only if the database is already populated
 494                   by an earlier event.</para>
 495                 </listitem>
 496               </varlistentry>
 497               <varlistentry>
 498                 <term><literal>cmdline</literal></term>
 499                 <listitem>
 500                   <para>Import a single property from the kernel command line. For simple flags
 501                   the value of the property is set to <literal>1</literal>.</para>
 502                 </listitem>
 503               </varlistentry>
 504               <varlistentry>
 505                 <term><literal>parent</literal></term>
 506                 <listitem>
 507                   <para>Import the stored keys from the parent device by reading
 508                   the database entry of the parent device. The value assigned to
 509                   <option>IMPORT{parent}</option> is used as a filter of key names
 510                   to import (with the same shell glob pattern matching used for
 511                   comparisons).</para>
 512                 </listitem>
 513               </varlistentry>
 514             </variablelist>
 515             <para>This can only be used for very short-running foreground tasks. For details
 516             see <option>RUN</option>.</para>
 517           </listitem>
 518         </varlistentry>
 519
 520         <varlistentry>
 521           <term><varname>OPTIONS</varname></term>
 522           <listitem>
 523             <para>Rule and device options:</para>
 524             <variablelist class='udev-directives'>
 525               <varlistentry>
 526                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 527                 <listitem>
 528                   <para>Specify the priority of the created symlinks. Devices with higher
 529                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 530                 </listitem>
 531               </varlistentry>
 532               <varlistentry>
 533                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 534                 <listitem>
 535                   <para>Usually, control and other possibly unsafe characters are replaced
 536                   in strings used for device naming. The mode of replacement can be specified
 537                   with this option.</para>
 538                 </listitem>
 539               </varlistentry>
 540               <varlistentry>
 541                 <term><option>static_node=</option></term>
 542                 <listitem>
 543                   <para>Apply the permissions specified in this rule to the
 544                   static device node with the specified name. Also, for every
 545                   tag specified in this rule, create a symlink
 546                   in the directory
 547                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 548                   pointing at the static device node with the specified name.
 549                   Static device node creation is performed by systemd-tmpfiles
 550                   before systemd-udevd is started. The static nodes might not
 551                   have a corresponding kernel device; they are used to trigger
 552                   automatic kernel module loading when they are accessed.</para>
 553                 </listitem>
 554               </varlistentry>
 555               <varlistentry>
 556                 <term><option>watch</option></term>
 557                 <listitem>
 558                   <para>Watch the device node with inotify; when the node is
 559                   closed after being opened for writing, a change uevent is
 560                   synthesized.</para>
 561                 </listitem>
 562               </varlistentry>
 563               <varlistentry>
 564                 <term><option>nowatch</option></term>
 565                 <listitem>
 566                   <para>Disable the watching of a device node with inotify.</para>
 567                 </listitem>
 568               </varlistentry>
 569               <varlistentry>
 570                 <term><option>db_persist</option></term>
 571                 <listitem>
 572                   <para>Set the flag (sticky bit) on the udev database entry
 573                   of the event device. Device properties are then kept in the
 574                   database even when
 575                   <command>udevadm info --cleanup-db</command> is called.
 576                   This option can be useful in certain cases
 577                   (e.g. Device Mapper devices) for persisting device state
 578                   on the transition from initramfs.</para>
 579                 </listitem>
 580               </varlistentry>
 581             </variablelist>
 582           </listitem>
 583         </varlistentry>
 584       </variablelist>
 585
 586       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 587       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 588       <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 589       and <varname>RUN</varname> fields support simple string substitutions.
 590       The <varname>RUN</varname> substitutions are performed after all rules
 591       have been processed, right before the program is executed, allowing for
 592       the use of device properties set by earlier matching rules. For all other
 593       fields, substitutions are performed while the individual rule is being
 594       processed. The available substitutions are:</para>
 595       <variablelist class='udev-directives'>
 596         <varlistentry>
 597           <term><option>$kernel</option>, <option>%k</option></term>
 598           <listitem>
 599             <para>The kernel name for this device.</para>
 600           </listitem>
 601         </varlistentry>
 602
 603         <varlistentry>
 604           <term><option>$number</option>, <option>%n</option></term>
 605           <listitem>
 606             <para>The kernel number for this device. For example,
 607               <literal>sda3</literal> has kernel number <literal>3</literal>.
 608             </para>
 609           </listitem>
 610         </varlistentry>
 611
 612         <varlistentry>
 613           <term><option>$devpath</option>, <option>%p</option></term>
 614           <listitem>
 615             <para>The devpath of the device.</para>
 616           </listitem>
 617         </varlistentry>
 618
 619         <varlistentry>
 620           <term><option>$id</option>, <option>%b</option></term>
 621           <listitem>
 622             <para>The name of the device matched while searching the devpath
 623               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 624               <option>DRIVERS</option>, and <option>ATTRS</option>.
 625             </para>
 626           </listitem>
 627         </varlistentry>
 628
 629         <varlistentry>
 630           <term><option>$driver</option></term>
 631           <listitem>
 632             <para>The driver name of the device matched while searching the
 633               devpath upwards for <option>SUBSYSTEMS</option>,
 634               <option>KERNELS</option>, <option>DRIVERS</option>, and
 635               <option>ATTRS</option>.
 636             </para>
 637           </listitem>
 638         </varlistentry>
 639
 640         <varlistentry>
 641           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 642           <listitem>
 643             <para>The value of a sysfs attribute found at the device where
 644               all keys of the rule have matched. If the matching device does not
 645               have such an attribute, and a previous <option>KERNELS</option>,
 646               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 647               <option>ATTRS</option> test selected a parent device, then the
 648               attribute from that parent device is used.
 649             </para>
 650             <para>If the attribute is a symlink, the last element of the
 651               symlink target is returned as the value.
 652             </para>
 653           </listitem>
 654         </varlistentry>
 655
 656         <varlistentry>
 657           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 658           <listitem>
 659             <para>A device property value.</para>
 660           </listitem>
 661         </varlistentry>
 662
 663         <varlistentry>
 664           <term><option>$major</option>, <option>%M</option></term>
 665           <listitem>
 666             <para>The kernel major number for the device.</para>
 667           </listitem>
 668         </varlistentry>
 669
 670         <varlistentry>
 671           <term><option>$minor</option>, <option>%m</option></term>
 672           <listitem>
 673             <para>The kernel minor number for the device.</para>
 674           </listitem>
 675         </varlistentry>
 676
 677         <varlistentry>
 678           <term><option>$result</option>, <option>%c</option></term>
 679           <listitem>
 680             <para>The string returned by the external program requested with
 681             <varname>PROGRAM</varname>.
 682             A single part of the string, separated by a space character, may be selected
 683             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 684             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 685             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 686           </listitem>
 687         </varlistentry>
 688
 689         <varlistentry>
 690           <term><option>$parent</option>, <option>%P</option></term>
 691           <listitem>
 692             <para>The node name of the parent device.</para>
 693           </listitem>
 694         </varlistentry>
 695
 696         <varlistentry>
 697           <term><option>$name</option></term>
 698           <listitem>
 699             <para>The current name of the device. If not changed by a rule, it is the
 700             name of the kernel device.</para>
 701           </listitem>
 702         </varlistentry>
 703
 704         <varlistentry>
 705           <term><option>$links</option></term>
 706           <listitem>
 707             <para>A space-separated list of the current symlinks. The value is
 708             only set during a remove event or if an earlier rule assigned a value.</para>
 709           </listitem>
 710         </varlistentry>
 711
 712         <varlistentry>
 713           <term><option>$root</option>, <option>%r</option></term>
 714           <listitem>
 715             <para>The udev_root value.</para>
 716           </listitem>
 717         </varlistentry>
 718
 719         <varlistentry>
 720           <term><option>$sys</option>, <option>%S</option></term>
 721           <listitem>
 722             <para>The sysfs mount point.</para>
 723           </listitem>
 724         </varlistentry>
 725
 726         <varlistentry>
 727           <term><option>$devnode</option>, <option>%N</option></term>
 728           <listitem>
 729             <para>The name of the device node.</para>
 730           </listitem>
 731         </varlistentry>
 732
 733         <varlistentry>
 734           <term><option>%%</option></term>
 735           <listitem>
 736           <para>The <literal>%</literal> character itself.</para>
 737           </listitem>
 738         </varlistentry>
 739
 740         <varlistentry>
 741           <term><option>$$</option></term>
 742           <listitem>
 743           <para>The <literal>$</literal> character itself.</para>
 744           </listitem>
 745         </varlistentry>
 746       </variablelist>
 747   </refsect1>
 748
 749   <refsect1>
 750     <title>See Also</title>
 751     <para>
 752       <citerefentry>
 753         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 754       </citerefentry>,
 755       <citerefentry>
 756         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 757       </citerefentry>,
 758       <citerefentry>
 759         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 760       </citerefentry>
 761     </para>
 762   </refsect1>
 763 </refentry>