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>CONST{<replaceable>key</replaceable>}</varname></term>
 241           <listitem>
 242             <para>Match against a system-wide constant. Supported keys are:</para>
 243             <variablelist>
 244               <varlistentry>
 245                 <term><literal>arch</literal></term>
 246                 <listitem>
 247                   <para>System's architecture. See <option>ConditionArchitecture=</option> in
 248                   <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 249                   for possible values.</para>
 250                 </listitem>
 251               </varlistentry>
 252               <varlistentry>
 253                 <term><literal>virt</literal></term>
 254                 <listitem>
 255                   <para>System's virtualization environment. See
 256                   <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 257                   for possible values.</para>
 258                 </listitem>
 259               </varlistentry>
 260             </variablelist>
 261             <para>Unknown keys will never match.</para>
 262           </listitem>
 263         </varlistentry>
 264
 265         <varlistentry>
 266           <term><varname>TAG</varname></term>
 267           <listitem>
 268             <para>Match against a device tag.</para>
 269           </listitem>
 270         </varlistentry>
 271
 272         <varlistentry>
 273           <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 274           <listitem>
 275             <para>Test the existence of a file. An octal mode mask can be specified
 276             if needed.</para>
 277           </listitem>
 278         </varlistentry>
 279
 280         <varlistentry>
 281           <term><varname>PROGRAM</varname></term>
 282           <listitem>
 283             <para>Execute a program to determine whether there is a match; the key is true if the program
 284             returns successfully. The device properties are made available to the executed program in the
 285             environment. The program's standard output is available in the <varname>RESULT</varname>
 286             key.</para>
 287
 288             <para>This can only be used for very short-running foreground tasks. For details, see
 289             <varname>RUN</varname>.</para>
 290
 291             <para>Note that multiple <varname>PROGRAM</varname> keys may be specified in one rule, and
 292             <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 293             <literal>==</literal>.</para>
 294           </listitem>
 295         </varlistentry>
 296
 297         <varlistentry>
 298           <term><varname>RESULT</varname></term>
 299           <listitem>
 300             <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 301             This key can be used in the same or in any later rule after a
 302             <varname>PROGRAM</varname> call.</para>
 303           </listitem>
 304         </varlistentry>
 305       </variablelist>
 306
 307       <para>Most of the fields support shell glob pattern matching and
 308       alternate patterns. The following special characters are supported:</para>
 309       <variablelist>
 310         <varlistentry>
 311           <term><literal>*</literal></term>
 312           <listitem>
 313             <para>Matches zero or more characters.</para>
 314           </listitem>
 315         </varlistentry>
 316         <varlistentry>
 317           <term><literal>?</literal></term>
 318           <listitem>
 319             <para>Matches any single character.</para>
 320           </listitem>
 321         </varlistentry>
 322         <varlistentry>
 323           <term><literal>[]</literal></term>
 324           <listitem>
 325             <para>Matches any single character specified within the brackets. For
 326             example, the pattern string <literal>tty[SR]</literal>
 327             would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 328             Ranges are also supported via the <literal>-</literal> character.
 329             For example, to match on the range of all digits, the pattern
 330             <literal>[0-9]</literal> could be used. If the first character
 331             following the <literal>[</literal> is a <literal>!</literal>,
 332             any characters not enclosed are matched.</para>
 333           </listitem>
 334         </varlistentry>
 335         <varlistentry>
 336           <term><literal>|</literal></term>
 337           <listitem>
 338             <para>Separates alternative patterns. For example, the pattern string
 339             <literal>abc|x*</literal> would match either <literal>abc</literal>
 340             or <literal>x*</literal>.</para>
 341           </listitem>
 342         </varlistentry>
 343       </variablelist>
 344
 345       <para>The following keys can get values assigned:</para>
 346       <variablelist class='udev-directives'>
 347         <varlistentry>
 348           <term><varname>NAME</varname></term>
 349           <listitem>
 350             <para>The name to use for a network interface. See
 351             <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 352             for a higher-level mechanism for setting the interface name.
 353             The name of a device node cannot be changed by udev, only additional
 354             symlinks can be created.</para>
 355           </listitem>
 356         </varlistentry>
 357
 358         <varlistentry>
 359           <term><varname>SYMLINK</varname></term>
 360           <listitem>
 361             <para>The name of a symlink targeting the node. Every matching rule adds
 362             this value to the list of symlinks to be created.</para>
 363             <para>The set of characters to name a symlink is limited. Allowed
 364             characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 365             sequences, and <literal>\x00</literal> hex encoding. All other
 366             characters are replaced by a <literal>_</literal> character.</para>
 367             <para>Multiple symlinks may be specified by separating the names by the
 368             space character. In case multiple devices claim the same name, the link
 369             always points to the device with the highest link_priority. If the current
 370             device goes away, the links are re-evaluated and the device with the
 371             next highest link_priority becomes the owner of the link. If no
 372             link_priority is specified, the order of the devices (and which one of
 373             them owns the link) is undefined.</para>
 374             <para>Symlink names must never conflict with the kernel's default device
 375             node names, as that would result in unpredictable behavior.
 376             </para>
 377           </listitem>
 378         </varlistentry>
 379
 380         <varlistentry>
 381           <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 382           <listitem>
 383             <para>The permissions for the device node. Every specified value overrides
 384             the compiled-in default value.</para>
 385           </listitem>
 386         </varlistentry>
 387
 388         <varlistentry>
 389           <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 390           <listitem>
 391             <para>Applies the specified Linux Security Module label to the device node.</para>
 392           </listitem>
 393         </varlistentry>
 394
 395         <varlistentry>
 396           <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 397           <listitem>
 398             <para>The value that should be written to a sysfs attribute of the
 399             event device.</para>
 400           </listitem>
 401         </varlistentry>
 402
 403         <varlistentry>
 404           <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 405           <listitem>
 406             <para>The value that should be written to kernel parameter.</para>
 407           </listitem>
 408         </varlistentry>
 409
 410         <varlistentry>
 411           <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 412           <listitem>
 413             <para>Set a device property value. Property names with a leading <literal>.</literal>
 414             are neither stored in the database nor exported to events or
 415             external tools (run by, for example, the <varname>PROGRAM</varname>
 416             match key).</para>
 417           </listitem>
 418         </varlistentry>
 419
 420         <varlistentry>
 421           <term><varname>TAG</varname></term>
 422           <listitem>
 423             <para>Attach a tag to a device. This is used to filter events for users
 424             of libudev's monitor functionality, or to enumerate a group of tagged
 425             devices. The implementation can only work efficiently if only a few
 426             tags are attached to a device. It is only meant to be used in
 427             contexts with specific device filter requirements, and not as a
 428             general-purpose flag. Excessive use might result in inefficient event
 429             handling.</para>
 430           </listitem>
 431         </varlistentry>
 432
 433         <varlistentry>
 434           <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 435           <listitem>
 436             <para>Specify a program to be executed after processing of all the rules for the event. With
 437             <literal>+=</literal>, this invocation is added to the list, and with <literal>=</literal> or
 438             <literal>:=</literal>, it replaces any previous contents of the list. Please note that both
 439             <literal>program</literal> and <literal>builtin</literal> types described below use a single
 440             list, so clearing the list with <literal>:=</literal> and <literal>=</literal> affects both
 441             types.</para>
 442
 443             <para><replaceable>type</replaceable> may be:</para>
 444             <variablelist>
 445               <varlistentry>
 446                 <term><literal>program</literal></term>
 447                 <listitem>
 448                   <para>Execute an external program specified as the assigned
 449                   value. If no absolute path is given, the program is expected
 450                   to live in <filename>/usr/lib/udev</filename>; otherwise, the
 451                   absolute path must be specified.</para>
 452                   <para>This is the default if no <replaceable>type</replaceable>
 453                   is specified.</para>
 454                 </listitem>
 455               </varlistentry>
 456               <varlistentry>
 457                 <term><literal>builtin</literal></term>
 458                 <listitem>
 459                   <para>As <varname>program</varname>, but use one of the
 460                   built-in programs rather than an external one.</para>
 461                 </listitem>
 462               </varlistentry>
 463             </variablelist>
 464
 465             <para>The program name and following arguments are separated by spaces. Single quotes can be
 466             used to specify arguments with spaces.</para>
 467
 468             <para>This can only be used for very short-running foreground tasks. Running an event process for
 469             a long period of time may block all further events for this or a dependent device.</para>
 470
 471             <para>Note that running programs that access the network or mount/unmount filesystems is not
 472             allowed inside of udev rules, due to the default sandbox that is enforced on
 473             <filename>systemd-udevd.service</filename>.</para>
 474
 475             <para>Starting daemons or other long-running processes is not allowed; the forked processes,
 476             detached or not, will be unconditionally killed after the event handling has finished. In order
 477             to activate long-running processes from udev rules, provide a service unit and pull it in from a
 478             udev device using the <varname>SYSTEMD_WANTS</varname> device property. See
 479             <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 480             for details.</para>
 481           </listitem>
 482         </varlistentry>
 483
 484         <varlistentry>
 485           <term><varname>LABEL</varname></term>
 486           <listitem>
 487             <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 488           </listitem>
 489         </varlistentry>
 490
 491         <varlistentry>
 492           <term><varname>GOTO</varname></term>
 493           <listitem>
 494             <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 495           </listitem>
 496         </varlistentry>
 497
 498         <varlistentry>
 499           <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 500           <listitem>
 501             <para>Import a set of variables as device properties, depending on
 502             <replaceable>type</replaceable>:</para>
 503
 504             <variablelist>
 505               <varlistentry>
 506                 <term><literal>program</literal></term>
 507                 <listitem>
 508                   <para>Execute an external program specified as the assigned
 509                   value and, if it returns successfully,
 510                   import its output, which must be in environment key
 511                   format. Path specification, command/argument separation,
 512                   and quoting work like in <varname>RUN</varname>.</para>
 513                 </listitem>
 514               </varlistentry>
 515               <varlistentry>
 516                 <term><literal>builtin</literal></term>
 517                 <listitem>
 518                   <para>Similar to <literal>program</literal>, but use one of the
 519                   built-in programs rather than an external one.</para>
 520                 </listitem>
 521               </varlistentry>
 522              <varlistentry>
 523                 <term><literal>file</literal></term>
 524                 <listitem>
 525                   <para>Import a text file specified as the assigned value, the content
 526                   of which must be in environment key format.</para>
 527                 </listitem>
 528               </varlistentry>
 529               <varlistentry>
 530                 <term><literal>db</literal></term>
 531                 <listitem>
 532                   <para>Import a single property specified as the assigned value from the
 533                   current device database. This works only if the database is already populated
 534                   by an earlier event.</para>
 535                 </listitem>
 536               </varlistentry>
 537               <varlistentry>
 538                 <term><literal>cmdline</literal></term>
 539                 <listitem>
 540                   <para>Import a single property from the kernel command line. For simple flags
 541                   the value of the property is set to <literal>1</literal>.</para>
 542                 </listitem>
 543               </varlistentry>
 544               <varlistentry>
 545                 <term><literal>parent</literal></term>
 546                 <listitem>
 547                   <para>Import the stored keys from the parent device by reading
 548                   the database entry of the parent device. The value assigned to
 549                   <option>IMPORT{parent}</option> is used as a filter of key names
 550                   to import (with the same shell glob pattern matching used for
 551                   comparisons).</para>
 552                 </listitem>
 553               </varlistentry>
 554             </variablelist>
 555
 556             <para>This can only be used for very short-running foreground tasks. For details see
 557             <option>RUN</option>.</para>
 558
 559             <para>Note that multiple <varname>IMPORT{}</varname> keys may be specified in one rule, and
 560             <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 561             <literal>==</literal>. The key is true if the import is successful, unless <literal>!=</literal>
 562             is used as the operator which causes the key to be true if the import failed.</para>
 563           </listitem>
 564         </varlistentry>
 565
 566         <varlistentry>
 567           <term><varname>OPTIONS</varname></term>
 568           <listitem>
 569             <para>Rule and device options:</para>
 570             <variablelist class='udev-directives'>
 571               <varlistentry>
 572                 <term><option>link_priority=<replaceable>value</replaceable></option></term>
 573                 <listitem>
 574                   <para>Specify the priority of the created symlinks. Devices with higher
 575                   priorities overwrite existing symlinks of other devices. The default is 0.</para>
 576                 </listitem>
 577               </varlistentry>
 578               <varlistentry>
 579                 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 580                 <listitem>
 581                   <para>Usually, control and other possibly unsafe characters are replaced
 582                   in strings used for device naming. The mode of replacement can be specified
 583                   with this option.</para>
 584                 </listitem>
 585               </varlistentry>
 586               <varlistentry>
 587                 <term><option>static_node=</option></term>
 588                 <listitem>
 589                   <para>Apply the permissions specified in this rule to the
 590                   static device node with the specified name. Also, for every
 591                   tag specified in this rule, create a symlink
 592                   in the directory
 593                   <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 594                   pointing at the static device node with the specified name.
 595                   Static device node creation is performed by systemd-tmpfiles
 596                   before systemd-udevd is started. The static nodes might not
 597                   have a corresponding kernel device; they are used to trigger
 598                   automatic kernel module loading when they are accessed.</para>
 599                 </listitem>
 600               </varlistentry>
 601               <varlistentry>
 602                 <term><option>watch</option></term>
 603                 <listitem>
 604                   <para>Watch the device node with inotify; when the node is
 605                   closed after being opened for writing, a change uevent is
 606                   synthesized.</para>
 607                 </listitem>
 608               </varlistentry>
 609               <varlistentry>
 610                 <term><option>nowatch</option></term>
 611                 <listitem>
 612                   <para>Disable the watching of a device node with inotify.</para>
 613                 </listitem>
 614               </varlistentry>
 615               <varlistentry>
 616                 <term><option>db_persist</option></term>
 617                 <listitem>
 618                   <para>Set the flag (sticky bit) on the udev database entry
 619                   of the event device. Device properties are then kept in the
 620                   database even when
 621                   <command>udevadm info --cleanup-db</command> is called.
 622                   This option can be useful in certain cases
 623                   (e.g. Device Mapper devices) for persisting device state
 624                   on the transition from initramfs.</para>
 625                 </listitem>
 626               </varlistentry>
 627             </variablelist>
 628           </listitem>
 629         </varlistentry>
 630       </variablelist>
 631
 632       <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 633       <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 634       <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 635       and <varname>RUN</varname> fields support simple string substitutions.
 636       The <varname>RUN</varname> substitutions are performed after all rules
 637       have been processed, right before the program is executed, allowing for
 638       the use of device properties set by earlier matching rules. For all other
 639       fields, substitutions are performed while the individual rule is being
 640       processed. The available substitutions are:</para>
 641       <variablelist class='udev-directives'>
 642         <varlistentry>
 643           <term><option>$kernel</option>, <option>%k</option></term>
 644           <listitem>
 645             <para>The kernel name for this device.</para>
 646           </listitem>
 647         </varlistentry>
 648
 649         <varlistentry>
 650           <term><option>$number</option>, <option>%n</option></term>
 651           <listitem>
 652             <para>The kernel number for this device. For example, <literal>sda3</literal> has kernel number
 653             3.</para>
 654           </listitem>
 655         </varlistentry>
 656
 657         <varlistentry>
 658           <term><option>$devpath</option>, <option>%p</option></term>
 659           <listitem>
 660             <para>The devpath of the device.</para>
 661           </listitem>
 662         </varlistentry>
 663
 664         <varlistentry>
 665           <term><option>$id</option>, <option>%b</option></term>
 666           <listitem>
 667             <para>The name of the device matched while searching the devpath
 668               upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 669               <option>DRIVERS</option>, and <option>ATTRS</option>.
 670             </para>
 671           </listitem>
 672         </varlistentry>
 673
 674         <varlistentry>
 675           <term><option>$driver</option></term>
 676           <listitem>
 677             <para>The driver name of the device matched while searching the
 678               devpath upwards for <option>SUBSYSTEMS</option>,
 679               <option>KERNELS</option>, <option>DRIVERS</option>, and
 680               <option>ATTRS</option>.
 681             </para>
 682           </listitem>
 683         </varlistentry>
 684
 685         <varlistentry>
 686           <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 687           <listitem>
 688             <para>The value of a sysfs attribute found at the device where
 689               all keys of the rule have matched. If the matching device does not
 690               have such an attribute, and a previous <option>KERNELS</option>,
 691               <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 692               <option>ATTRS</option> test selected a parent device, then the
 693               attribute from that parent device is used.
 694             </para>
 695             <para>If the attribute is a symlink, the last element of the
 696               symlink target is returned as the value.
 697             </para>
 698           </listitem>
 699         </varlistentry>
 700
 701         <varlistentry>
 702           <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 703           <listitem>
 704             <para>A device property value.</para>
 705           </listitem>
 706         </varlistentry>
 707
 708         <varlistentry>
 709           <term><option>$major</option>, <option>%M</option></term>
 710           <listitem>
 711             <para>The kernel major number for the device.</para>
 712           </listitem>
 713         </varlistentry>
 714
 715         <varlistentry>
 716           <term><option>$minor</option>, <option>%m</option></term>
 717           <listitem>
 718             <para>The kernel minor number for the device.</para>
 719           </listitem>
 720         </varlistentry>
 721
 722         <varlistentry>
 723           <term><option>$result</option>, <option>%c</option></term>
 724           <listitem>
 725             <para>The string returned by the external program requested with
 726             <varname>PROGRAM</varname>.
 727             A single part of the string, separated by a space character, may be selected
 728             by specifying the part number as an attribute: <literal>%c{N}</literal>.
 729             If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 730             of the result string are substituted: <literal>%c{N+}</literal>.</para>
 731           </listitem>
 732         </varlistentry>
 733
 734         <varlistentry>
 735           <term><option>$parent</option>, <option>%P</option></term>
 736           <listitem>
 737             <para>The node name of the parent device.</para>
 738           </listitem>
 739         </varlistentry>
 740
 741         <varlistentry>
 742           <term><option>$name</option></term>
 743           <listitem>
 744             <para>The current name of the device. If not changed by a rule, it is the
 745             name of the kernel device.</para>
 746           </listitem>
 747         </varlistentry>
 748
 749         <varlistentry>
 750           <term><option>$links</option></term>
 751           <listitem>
 752             <para>A space-separated list of the current symlinks. The value is
 753             only set during a remove event or if an earlier rule assigned a value.</para>
 754           </listitem>
 755         </varlistentry>
 756
 757         <varlistentry>
 758           <term><option>$root</option>, <option>%r</option></term>
 759           <listitem>
 760             <para>The udev_root value.</para>
 761           </listitem>
 762         </varlistentry>
 763
 764         <varlistentry>
 765           <term><option>$sys</option>, <option>%S</option></term>
 766           <listitem>
 767             <para>The sysfs mount point.</para>
 768           </listitem>
 769         </varlistentry>
 770
 771         <varlistentry>
 772           <term><option>$devnode</option>, <option>%N</option></term>
 773           <listitem>
 774             <para>The name of the device node.</para>
 775           </listitem>
 776         </varlistentry>
 777
 778         <varlistentry>
 779           <term><option>%%</option></term>
 780           <listitem>
 781           <para>The <literal>%</literal> character itself.</para>
 782           </listitem>
 783         </varlistentry>
 784
 785         <varlistentry>
 786           <term><option>$$</option></term>
 787           <listitem>
 788           <para>The <literal>$</literal> character itself.</para>
 789           </listitem>
 790         </varlistentry>
 791       </variablelist>
 792   </refsect1>
 793
 794   <refsect1>
 795     <title>See Also</title>
 796     <para>
 797       <citerefentry>
 798         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 799       </citerefentry>,
 800       <citerefentry>
 801         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 802       </citerefentry>,
 803       <citerefentry>
 804         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 805       </citerefentry>
 806     </para>
 807   </refsect1>
 808 </refentry>