man/udev.xml

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   6   SPDX-License-Identifier: LGPL-2.1-or-later
   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>
  27     <title>Description</title>
  28     <para>udev supplies the system software with device events, manages permissions
  29     of device nodes and may create additional symlinks in the <filename>/dev/</filename>
  30     directory, or renames network interfaces. The kernel usually just assigns unpredictable
  31     device names based on the order of discovery. Meaningful symlinks or network device
  32     names provide a way to reliably identify devices based on their properties or
  33     current configuration.</para>
  34
  35     <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
  36     <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
  37     the kernel whenever a device is added or removed from the system, or it changes its
  38     state. When udev receives a device event, it matches its configured set of rules
  39     against various device attributes to identify the device. Rules that match may
  40     provide additional device information to be stored in the udev database or
  41     to be used to create meaningful symlink names.</para>
  42
  43     <para>All device information udev processes is stored in the udev database and
  44     sent out to possible event subscribers. Access to all stored data and the event
  45     sources is provided by the library libudev.</para>
  46   </refsect1>
  47
  48   <refsect1>
  49       <title>Rules Files</title>
  50       <para>The udev rules are read from the files located in the system rules directories
  51       <filename>/usr/lib/udev/rules.d</filename> and <filename>/usr/local/lib/udev/rules.d</filename>, the
  52       volatile runtime directory <filename>/run/udev/rules.d</filename> and the local administration
  53       directory <filename>/etc/udev/rules.d</filename>.  All rules files are collectively sorted and
  54       processed in lexical order, regardless of the directories in which they live. However, files with
  55       identical filenames replace each other. Files in <filename>/etc/</filename> have the highest priority,
  56       files in <filename>/run/</filename> take precedence over files with the same name under
  57       <filename>/usr/</filename>. This can be used to override a system-supplied rules file with a local
  58       file if needed; a symlink in <filename>/etc/</filename> with the same name as a rules file in
  59       <filename>/usr/lib/</filename>, pointing to <filename>/dev/null</filename>, disables the rules file
  60       entirely. Rule files must have the extension <filename>.rules</filename>; other extensions are
  61       ignored.</para>
  62
  63       <para>Every line in the rules file contains at least one key-value pair.
  64       Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
  65       There are two kinds of keys: match and assignment.
  66       If all match keys match against their values, the rule gets applied and the
  67       assignment keys get the specified values assigned.</para>
  68
  69       <para>A matching rule may rename a network interface, add symlinks
  70       pointing to the device node, or run a specified program as part of
  71       the event handling.</para>
  72
  73       <para>A rule consists of a comma-separated list of one or more key-operator-value expressions.
  74       Each expression has a distinct effect, depending on the key and operator used.</para>
  75
  76       <refsect2>
  77         <title>Operators</title>
  78         <variablelist>
  79           <varlistentry>
  80             <term><literal>==</literal></term>
  81             <listitem>
  82               <para>Compare for equality. (The specified key has the specified value.)</para>
  83             </listitem>
  84           </varlistentry>
  85
  86           <varlistentry>
  87             <term><literal>!=</literal></term>
  88             <listitem>
  89               <para>Compare for inequality. (The specified key doesn't have the specified value, or the
  90               specified key is not present at all.)
  91               </para>
  92             </listitem>
  93           </varlistentry>
  94
  95           <varlistentry>
  96             <term><literal>=</literal></term>
  97             <listitem>
  98               <para>Assign a value to a key. Keys that represent a list are reset
  99               and only this single value is assigned.</para>
 100             </listitem>
 101           </varlistentry>
 102
 103           <varlistentry>
 104             <term><literal>+=</literal></term>
 105             <listitem>
 106               <para>Add the value to a key that holds a list of entries.</para>
 107             </listitem>
 108           </varlistentry>
 109
 110           <varlistentry>
 111             <term><literal>-=</literal></term>
 112             <listitem>
 113               <para>Remove the value from a key that holds a list of entries.</para>
 114             </listitem>
 115           </varlistentry>
 116
 117           <varlistentry>
 118             <term><literal>:=</literal></term>
 119             <listitem>
 120               <para>Assign a value to a key finally; disallow any later changes.</para>
 121             </listitem>
 122           </varlistentry>
 123         </variablelist>
 124       </refsect2>
 125
 126       <refsect2>
 127         <title>Values</title>
 128           <para>Values are written as double quoted strings, such as ("string").
 129           To include a quotation mark (") in the value, precede it by a backslash (\").
 130           Any other occurrences of a backslash followed by a character are not unescaped.
 131           That is, "\t\n" is treated as four characters:
 132           backslash, lowercase t, backslash, lowercase n.</para>
 133
 134           <para>The string can be prefixed with a lowercase e (e"string\n") to mark the string as
 135           <ulink url="https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences">C-style escaped</ulink>.
 136           For example, e"string\n" is parsed as 7 characters: 6 lowercase letters and a newline.
 137           This can be useful for writing special characters when a kernel driver requires them.</para>
 138
 139           <para>Please note that <constant>NUL</constant> is not allowed in either string variant.</para>
 140       </refsect2>
 141
 142       <refsect2>
 143         <title>Keys</title>
 144         <para>The following key names can be used to match against device properties.
 145         Some of the keys also match against properties of the parent devices in sysfs,
 146         not only the device that has generated the event. If multiple keys that match
 147         a parent device are specified in a single rule, all these keys must match at
 148         one and the same parent device.</para>
 149         <variablelist class='udev-directives'>
 150           <varlistentry>
 151             <term><varname>ACTION</varname></term>
 152             <listitem>
 153               <para>Match the name of the event action.</para>
 154             </listitem>
 155           </varlistentry>
 156
 157           <varlistentry>
 158             <term><varname>DEVPATH</varname></term>
 159             <listitem>
 160               <para>Match the devpath of the event device.</para>
 161             </listitem>
 162           </varlistentry>
 163
 164           <varlistentry>
 165             <term><varname>KERNEL</varname></term>
 166             <listitem>
 167               <para>Match the name of the event device.</para>
 168             </listitem>
 169           </varlistentry>
 170
 171           <varlistentry>
 172             <term><varname>KERNELS</varname></term>
 173             <listitem>
 174               <para>Search the devpath upwards for a matching device name.</para>
 175             </listitem>
 176           </varlistentry>
 177
 178           <varlistentry>
 179             <term><varname>NAME</varname></term>
 180             <listitem>
 181               <para>Match the name of a network interface. It can be used once the
 182               NAME key has been set in one of the preceding rules.</para>
 183             </listitem>
 184           </varlistentry>
 185
 186           <varlistentry>
 187             <term><varname>SYMLINK</varname></term>
 188             <listitem>
 189               <para>Match the name of a symlink targeting the node. It can
 190               be used once a SYMLINK key has been set in one of the preceding
 191               rules. There may be multiple symlinks; only one needs to match.
 192               </para>
 193             </listitem>
 194           </varlistentry>
 195
 196           <varlistentry>
 197             <term><varname>SUBSYSTEM</varname></term>
 198             <listitem>
 199               <para>Match the subsystem of the event device.</para>
 200             </listitem>
 201           </varlistentry>
 202
 203           <varlistentry>
 204             <term><varname>SUBSYSTEMS</varname></term>
 205             <listitem>
 206               <para>Search the devpath upwards for a matching device subsystem name.</para>
 207             </listitem>
 208           </varlistentry>
 209
 210           <varlistentry>
 211             <term><varname>DRIVER</varname></term>
 212             <listitem>
 213               <para>Match the driver name of the event device. Only set this key for devices
 214               which are bound to a driver at the time the event is generated.</para>
 215             </listitem>
 216           </varlistentry>
 217
 218           <varlistentry>
 219             <term><varname>DRIVERS</varname></term>
 220             <listitem>
 221               <para>Search the devpath upwards for a matching device driver name.</para>
 222             </listitem>
 223           </varlistentry>
 224
 225           <varlistentry>
 226             <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 227             <listitem>
 228               <para>Match sysfs attribute value of the event device.</para>
 229
 230               <para>Trailing whitespace in the attribute values is ignored unless the specified match value
 231               itself contains trailing whitespace.</para>
 232             </listitem>
 233           </varlistentry>
 234
 235           <varlistentry>
 236             <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 237             <listitem>
 238               <para>Search the devpath upwards for a device with matching sysfs attribute values. If
 239               multiple <varname>ATTRS</varname> matches are specified, all of them must match on the same
 240               device.</para>
 241
 242               <para>Trailing whitespace in the attribute values is ignored unless the specified match value
 243               itself contains trailing whitespace.</para>
 244             </listitem>
 245           </varlistentry>
 246
 247           <varlistentry>
 248             <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 249             <listitem>
 250               <para>Match a kernel parameter value.</para>
 251             </listitem>
 252           </varlistentry>
 253
 254           <varlistentry>
 255             <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 256             <listitem>
 257               <para>Match against a device property value.</para>
 258             </listitem>
 259           </varlistentry>
 260
 261           <varlistentry>
 262             <term><varname>CONST{<replaceable>key</replaceable>}</varname></term>
 263             <listitem>
 264               <para>Match against a system-wide constant. Supported keys are:</para>
 265               <variablelist>
 266                 <varlistentry>
 267                   <term><literal>arch</literal></term>
 268                   <listitem>
 269                     <para>System's architecture. See <option>ConditionArchitecture=</option> in
 270                     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 271                     for possible values.</para>
 272                   </listitem>
 273                 </varlistentry>
 274                 <varlistentry>
 275                   <term><literal>virt</literal></term>
 276                   <listitem>
 277                     <para>System's virtualization environment. See
 278                     <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 279                     for possible values.</para>
 280                   </listitem>
 281                 </varlistentry>
 282               </variablelist>
 283               <para>Unknown keys will never match.</para>
 284             </listitem>
 285           </varlistentry>
 286
 287           <varlistentry>
 288             <term><varname>TAG</varname></term>
 289             <listitem>
 290               <para>Match against a device tag.</para>
 291             </listitem>
 292           </varlistentry>
 293
 294           <varlistentry>
 295             <term><varname>TAGS</varname></term>
 296             <listitem>
 297               <para>Search the devpath upwards for a device with matching tag.</para>
 298             </listitem>
 299           </varlistentry>
 300
 301           <varlistentry>
 302             <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 303             <listitem>
 304               <para>Test the existence of a file. An octal mode mask can be specified
 305               if needed.</para>
 306             </listitem>
 307           </varlistentry>
 308
 309           <varlistentry>
 310             <term><varname>PROGRAM</varname></term>
 311             <listitem>
 312               <para>Execute a program to determine whether there is a match; the key is true if the program
 313               returns successfully. The device properties are made available to the executed program in the
 314               environment. The program's standard output is available in the <varname>RESULT</varname>
 315               key.</para>
 316
 317               <para>This can only be used for very short-running foreground tasks. For details, see
 318               <varname>RUN</varname>.</para>
 319
 320               <para>Note that multiple <varname>PROGRAM</varname> keys may be specified in one rule, and
 321               <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 322               <literal>==</literal>.</para>
 323             </listitem>
 324           </varlistentry>
 325
 326           <varlistentry>
 327             <term><varname>RESULT</varname></term>
 328             <listitem>
 329               <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 330               This key can be used in the same or in any later rule after a
 331               <varname>PROGRAM</varname> call.</para>
 332             </listitem>
 333           </varlistentry>
 334         </variablelist>
 335
 336         <para>Most of the fields support shell glob pattern matching and
 337         alternate patterns. The following special characters are supported:</para>
 338         <variablelist>
 339           <varlistentry>
 340             <term><literal>*</literal></term>
 341             <listitem>
 342               <para>Matches zero or more characters.</para>
 343             </listitem>
 344           </varlistentry>
 345           <varlistentry>
 346             <term><literal>?</literal></term>
 347             <listitem>
 348               <para>Matches any single character.</para>
 349             </listitem>
 350           </varlistentry>
 351           <varlistentry>
 352             <term><literal>[]</literal></term>
 353             <listitem>
 354               <para>Matches any single character specified within the brackets. For
 355               example, the pattern string <literal>tty[SR]</literal>
 356               would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 357               Ranges are also supported via the <literal>-</literal> character.
 358               For example, to match on the range of all digits, the pattern
 359               <literal>[0-9]</literal> could be used. If the first character
 360               following the <literal>[</literal> is a <literal>!</literal>,
 361               any characters not enclosed are matched.</para>
 362             </listitem>
 363           </varlistentry>
 364           <varlistentry>
 365             <term><literal>|</literal></term>
 366             <listitem>
 367               <para>Separates alternative patterns. For example, the pattern string
 368               <literal>abc|x*</literal> would match either <literal>abc</literal>
 369               or <literal>x*</literal>.</para>
 370             </listitem>
 371           </varlistentry>
 372         </variablelist>
 373
 374         <para>The following keys can get values assigned:</para>
 375         <variablelist class='udev-directives'>
 376           <varlistentry>
 377             <term><varname>NAME</varname></term>
 378             <listitem>
 379               <para>The name to use for a network interface. See
 380               <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 381               for a higher-level mechanism for setting the interface name.
 382               The name of a device node cannot be changed by udev, only additional
 383               symlinks can be created.</para>
 384             </listitem>
 385           </varlistentry>
 386
 387           <varlistentry>
 388             <term><varname>SYMLINK</varname></term>
 389             <listitem>
 390               <para>The name of a symlink targeting the node. Every matching rule adds
 391               this value to the list of symlinks to be created.</para>
 392               <para>The set of characters to name a symlink is limited. Allowed
 393               characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 394               sequences, and <literal>\x00</literal> hex encoding. All other
 395               characters are replaced by a <literal>_</literal> character.</para>
 396               <para>Multiple symlinks may be specified by separating the names by the
 397               space character. In case multiple devices claim the same name, the link
 398               always points to the device with the highest link_priority. If the current
 399               device goes away, the links are re-evaluated and the device with the
 400               next highest link_priority becomes the owner of the link. If no
 401               link_priority is specified, the order of the devices (and which one of
 402               them owns the link) is undefined.</para>
 403               <para>Symlink names must never conflict with the kernel's default device
 404               node names, as that would result in unpredictable behavior.
 405               </para>
 406             </listitem>
 407           </varlistentry>
 408
 409           <varlistentry>
 410             <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 411             <listitem>
 412               <para>The permissions for the device node. Every specified value overrides
 413               the compiled-in default value.</para>
 414             </listitem>
 415           </varlistentry>
 416
 417           <varlistentry>
 418             <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 419             <listitem>
 420               <para>Applies the specified Linux Security Module label to the device node.</para>
 421             </listitem>
 422           </varlistentry>
 423
 424           <varlistentry>
 425             <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 426             <listitem>
 427               <para>The value that should be written to a sysfs attribute of the
 428               event device.</para>
 429             </listitem>
 430           </varlistentry>
 431
 432           <varlistentry>
 433             <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 434             <listitem>
 435               <para>The value that should be written to kernel parameter.</para>
 436             </listitem>
 437           </varlistentry>
 438
 439           <varlistentry>
 440             <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 441             <listitem>
 442               <para>Set a device property value. Property names with a leading <literal>.</literal>
 443               are neither stored in the database nor exported to events or
 444               external tools (run by, for example, the <varname>PROGRAM</varname>
 445               match key).</para>
 446             </listitem>
 447           </varlistentry>
 448
 449           <varlistentry>
 450             <term><varname>TAG</varname></term>
 451             <listitem>
 452               <para>Attach a tag to a device. This is used to filter events for users
 453               of libudev's monitor functionality, or to enumerate a group of tagged
 454               devices. The implementation can only work efficiently if only a few
 455               tags are attached to a device. It is only meant to be used in
 456               contexts with specific device filter requirements, and not as a
 457               general-purpose flag. Excessive use might result in inefficient event
 458               handling.</para>
 459             </listitem>
 460           </varlistentry>
 461
 462           <varlistentry>
 463             <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 464             <listitem>
 465               <para>Specify a program to be executed after processing of all the rules for the event. With
 466               <literal>+=</literal>, this invocation is added to the list, and with <literal>=</literal> or
 467               <literal>:=</literal>, it replaces any previous contents of the list. Please note that both
 468               <literal>program</literal> and <literal>builtin</literal> types described below share a common
 469               list, so clearing the list with <literal>:=</literal> and <literal>=</literal> affects both
 470               types.</para>
 471
 472               <para><replaceable>type</replaceable> may be:</para>
 473               <variablelist>
 474                 <varlistentry>
 475                   <term><literal>program</literal></term>
 476                   <listitem>
 477                     <para>Execute an external program specified as the assigned
 478                     value. If no absolute path is given, the program is expected
 479                     to live in <filename>/usr/lib/udev</filename>; otherwise, the
 480                     absolute path must be specified.</para>
 481                     <para>This is the default if no <replaceable>type</replaceable>
 482                     is specified.</para>
 483                   </listitem>
 484                 </varlistentry>
 485                 <varlistentry>
 486                   <term><literal>builtin</literal></term>
 487                   <listitem>
 488                     <para>As <varname>program</varname>, but use one of the
 489                     built-in programs rather than an external one.</para>
 490                   </listitem>
 491                 </varlistentry>
 492               </variablelist>
 493
 494               <para>The program name and following arguments are separated by spaces. Single quotes can be
 495               used to specify arguments with spaces.</para>
 496
 497               <para>This can only be used for very short-running foreground tasks. Running an event process for
 498               a long period of time may block all further events for this or a dependent device.</para>
 499
 500               <para>Note that running programs that access the network or mount/unmount filesystems is not
 501               allowed inside of udev rules, due to the default sandbox that is enforced on
 502               <filename>systemd-udevd.service</filename>.</para>
 503
 504               <para>Starting daemons or other long-running processes is not allowed; the forked processes,
 505               detached or not, will be unconditionally killed after the event handling has finished. In order
 506               to activate long-running processes from udev rules, provide a service unit and pull it in from a
 507               udev device using the <varname>SYSTEMD_WANTS</varname> device property. See
 508               <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 509               for details.</para>
 510             </listitem>
 511           </varlistentry>
 512
 513           <varlistentry>
 514             <term><varname>LABEL</varname></term>
 515             <listitem>
 516               <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 517             </listitem>
 518           </varlistentry>
 519
 520           <varlistentry>
 521             <term><varname>GOTO</varname></term>
 522             <listitem>
 523               <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 524             </listitem>
 525           </varlistentry>
 526
 527           <varlistentry>
 528             <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 529             <listitem>
 530               <para>Import a set of variables as device properties, depending on
 531               <replaceable>type</replaceable>:</para>
 532
 533               <variablelist>
 534                 <varlistentry>
 535                   <term><literal>program</literal></term>
 536                   <listitem>
 537                     <para>Execute an external program specified as the assigned
 538                     value and, if it returns successfully,
 539                     import its output, which must be in environment key
 540                     format. Path specification, command/argument separation,
 541                     and quoting work like in <varname>RUN</varname>.</para>
 542                   </listitem>
 543                 </varlistentry>
 544                 <varlistentry>
 545                   <term><literal>builtin</literal></term>
 546                   <listitem>
 547                     <para>Similar to <literal>program</literal>, but use one of the
 548                     built-in programs rather than an external one.</para>
 549                   </listitem>
 550                 </varlistentry>
 551               <varlistentry>
 552                   <term><literal>file</literal></term>
 553                   <listitem>
 554                     <para>Import a text file specified as the assigned value, the content
 555                     of which must be in environment key format.</para>
 556                   </listitem>
 557                 </varlistentry>
 558                 <varlistentry>
 559                   <term><literal>db</literal></term>
 560                   <listitem>
 561                     <para>Import a single property specified as the assigned value from the
 562                     current device database. This works only if the database is already populated
 563                     by an earlier event.</para>
 564                   </listitem>
 565                 </varlistentry>
 566                 <varlistentry>
 567                   <term><literal>cmdline</literal></term>
 568                   <listitem>
 569                     <para>Import a single property from the kernel command line. For simple flags
 570                     the value of the property is set to <literal>1</literal>.</para>
 571                   </listitem>
 572                 </varlistentry>
 573                 <varlistentry>
 574                   <term><literal>parent</literal></term>
 575                   <listitem>
 576                     <para>Import the stored keys from the parent device by reading
 577                     the database entry of the parent device. The value assigned to
 578                     <option>IMPORT{parent}</option> is used as a filter of key names
 579                     to import (with the same shell glob pattern matching used for
 580                     comparisons).</para>
 581                   </listitem>
 582                 </varlistentry>
 583               </variablelist>
 584
 585               <para>This can only be used for very short-running foreground tasks. For details see
 586               <option>RUN</option>.</para>
 587
 588               <para>Note that multiple <varname>IMPORT{}</varname> keys may be specified in one rule, and
 589               <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 590               <literal>==</literal>. The key is true if the import is successful, unless <literal>!=</literal>
 591               is used as the operator which causes the key to be true if the import failed.</para>
 592             </listitem>
 593           </varlistentry>
 594
 595           <varlistentry>
 596             <term><varname>OPTIONS</varname></term>
 597             <listitem>
 598               <para>Rule and device options:</para>
 599               <variablelist class='udev-directives'>
 600                 <varlistentry>
 601                   <term><option>link_priority=<replaceable>value</replaceable></option></term>
 602                   <listitem>
 603                     <para>Specify the priority of the created symlinks. Devices with higher
 604                     priorities overwrite existing symlinks of other devices. The default is 0.</para>
 605                   </listitem>
 606                 </varlistentry>
 607                 <varlistentry>
 608                   <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 609                   <listitem>
 610                     <para>When <literal>replace</literal>, possibly unsafe characters in strings
 611                     assigned to <varname>NAME</varname>, <varname>SYMLINK</varname>, and
 612                     <varname>ENV{<replaceable>key</replaceable>}</varname> are replaced. When
 613                     <literal>none</literal>, no replacement is performed. When unset, the replacement
 614                     is performed for <varname>NAME</varname>, <varname>SYMLINK</varname>, but not for
 615                     <varname>ENV{<replaceable>key</replaceable>}</varname>. Defaults to unset.</para>
 616                   </listitem>
 617                 </varlistentry>
 618                 <varlistentry>
 619                   <term><option>static_node=</option></term>
 620                   <listitem>
 621                     <para>Apply the permissions specified in this rule to the
 622                     static device node with the specified name. Also, for every
 623                     tag specified in this rule, create a symlink
 624                     in the directory
 625                     <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 626                     pointing at the static device node with the specified name.
 627                     Static device node creation is performed by systemd-tmpfiles
 628                     before systemd-udevd is started. The static nodes might not
 629                     have a corresponding kernel device; they are used to trigger
 630                     automatic kernel module loading when they are accessed.</para>
 631                   </listitem>
 632                 </varlistentry>
 633                 <varlistentry>
 634                   <term><option>watch</option></term>
 635                   <listitem>
 636                     <para>Watch the device node with inotify; when the node is
 637                     closed after being opened for writing, a change uevent is
 638                     synthesized.</para>
 639                   </listitem>
 640                 </varlistentry>
 641                 <varlistentry>
 642                   <term><option>nowatch</option></term>
 643                   <listitem>
 644                     <para>Disable the watching of a device node with inotify.</para>
 645                   </listitem>
 646                 </varlistentry>
 647                 <varlistentry>
 648                   <term><option>db_persist</option></term>
 649                   <listitem>
 650                     <para>Set the flag (sticky bit) on the udev database entry
 651                     of the event device. Device properties are then kept in the
 652                     database even when
 653                     <command>udevadm info --cleanup-db</command> is called.
 654                     This option can be useful in certain cases
 655                     (e.g. Device Mapper devices) for persisting device state
 656                     on the transition from initramfs.</para>
 657                   </listitem>
 658                 </varlistentry>
 659                 <varlistentry>
 660                   <term><option>log_level=<replaceable>level</replaceable></option></term>
 661                   <listitem>
 662                     <para>Takes a log level name like <literal>debug</literal> or
 663                     <literal>info</literal>, or a special value <literal>reset</literal>. When a log
 664                     level name is specified, the maximum log level is changed to that level. When
 665                     <literal>reset</literal> is set, then the previously specified log level is
 666                     revoked. Defaults to the log level of the main process of
 667                     <command>systemd-udevd</command>.</para>
 668                     <para>This may be useful when debugging events for certain devices. Note that the
 669                     log level is applied when the line including this rule is processed. So, for
 670                     debugging, it is recommended that this is specified at earlier place, e.g., the
 671                     first line of <filename>00-debug.rules</filename>.</para>
 672                     <para>Example for debugging uevent processing for network interfaces.
 673                     <programlisting># /etc/udev/rules.d/00-debug-net.rules
 674 SUBSYSTEM=="net", OPTIONS="log_level=debug"</programlisting></para>
 675                   </listitem>
 676                 </varlistentry>
 677               </variablelist>
 678             </listitem>
 679           </varlistentry>
 680         </variablelist>
 681
 682         <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 683         <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 684         <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 685         and <varname>RUN</varname> fields support simple string substitutions.
 686         The <varname>RUN</varname> substitutions are performed after all rules
 687         have been processed, right before the program is executed, allowing for
 688         the use of device properties set by earlier matching rules. For all other
 689         fields, substitutions are performed while the individual rule is being
 690         processed. The available substitutions are:</para>
 691         <variablelist class='udev-directives'>
 692           <varlistentry>
 693             <term><option>$kernel</option>, <option>%k</option></term>
 694             <listitem>
 695               <para>The kernel name for this device.</para>
 696             </listitem>
 697           </varlistentry>
 698
 699           <varlistentry>
 700             <term><option>$number</option>, <option>%n</option></term>
 701             <listitem>
 702               <para>The kernel number for this device. For example, <literal>sda3</literal> has kernel number
 703               3.</para>
 704             </listitem>
 705           </varlistentry>
 706
 707           <varlistentry>
 708             <term><option>$devpath</option>, <option>%p</option></term>
 709             <listitem>
 710               <para>The devpath of the device.</para>
 711             </listitem>
 712           </varlistentry>
 713
 714           <varlistentry>
 715             <term><option>$id</option>, <option>%b</option></term>
 716             <listitem>
 717               <para>The name of the device matched while searching the devpath
 718                 upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 719                 <option>DRIVERS</option>, and <option>ATTRS</option>.
 720               </para>
 721             </listitem>
 722           </varlistentry>
 723
 724           <varlistentry>
 725             <term><option>$driver</option></term>
 726             <listitem>
 727               <para>The driver name of the device matched while searching the
 728                 devpath upwards for <option>SUBSYSTEMS</option>,
 729                 <option>KERNELS</option>, <option>DRIVERS</option>, and
 730                 <option>ATTRS</option>.
 731               </para>
 732             </listitem>
 733           </varlistentry>
 734
 735           <varlistentry>
 736             <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 737             <listitem>
 738               <para>The value of a sysfs attribute found at the device where
 739                 all keys of the rule have matched. If the matching device does not
 740                 have such an attribute, and a previous <option>KERNELS</option>,
 741                 <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 742                 <option>ATTRS</option> test selected a parent device, then the
 743                 attribute from that parent device is used.
 744               </para>
 745               <para>If the attribute is a symlink, the last element of the
 746                 symlink target is returned as the value.
 747               </para>
 748             </listitem>
 749           </varlistentry>
 750
 751           <varlistentry>
 752             <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 753             <listitem>
 754               <para>A device property value.</para>
 755             </listitem>
 756           </varlistentry>
 757
 758           <varlistentry>
 759             <term><option>$major</option>, <option>%M</option></term>
 760             <listitem>
 761               <para>The kernel major number for the device.</para>
 762             </listitem>
 763           </varlistentry>
 764
 765           <varlistentry>
 766             <term><option>$minor</option>, <option>%m</option></term>
 767             <listitem>
 768               <para>The kernel minor number for the device.</para>
 769             </listitem>
 770           </varlistentry>
 771
 772           <varlistentry>
 773             <term><option>$result</option>, <option>%c</option></term>
 774             <listitem>
 775               <para>The string returned by the external program requested with
 776               <varname>PROGRAM</varname>.
 777               A single part of the string, separated by a space character, may be selected
 778               by specifying the part number as an attribute: <literal>%c{N}</literal>.
 779               If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 780               of the result string are substituted: <literal>%c{N+}</literal>.</para>
 781             </listitem>
 782           </varlistentry>
 783
 784           <varlistentry>
 785             <term><option>$parent</option>, <option>%P</option></term>
 786             <listitem>
 787               <para>The node name of the parent device.</para>
 788             </listitem>
 789           </varlistentry>
 790
 791           <varlistentry>
 792             <term><option>$name</option></term>
 793             <listitem>
 794               <para>The current name of the device. If not changed by a rule, it is the
 795               name of the kernel device.</para>
 796             </listitem>
 797           </varlistentry>
 798
 799           <varlistentry>
 800             <term><option>$links</option></term>
 801             <listitem>
 802               <para>A space-separated list of the current symlinks. The value is
 803               only set during a remove event or if an earlier rule assigned a value.</para>
 804             </listitem>
 805           </varlistentry>
 806
 807           <varlistentry>
 808             <term><option>$root</option>, <option>%r</option></term>
 809             <listitem>
 810               <para>The udev_root value.</para>
 811             </listitem>
 812           </varlistentry>
 813
 814           <varlistentry>
 815             <term><option>$sys</option>, <option>%S</option></term>
 816             <listitem>
 817               <para>The sysfs mount point.</para>
 818             </listitem>
 819           </varlistentry>
 820
 821           <varlistentry>
 822             <term><option>$devnode</option>, <option>%N</option></term>
 823             <listitem>
 824               <para>The name of the device node.</para>
 825             </listitem>
 826           </varlistentry>
 827
 828           <varlistentry>
 829             <term><option>%%</option></term>
 830             <listitem>
 831             <para>The <literal>%</literal> character itself.</para>
 832             </listitem>
 833           </varlistentry>
 834
 835           <varlistentry>
 836             <term><option>$$</option></term>
 837             <listitem>
 838             <para>The <literal>$</literal> character itself.</para>
 839             </listitem>
 840           </varlistentry>
 841         </variablelist>
 842       </refsect2>
 843   </refsect1>
 844
 845   <refsect1>
 846     <title>See Also</title>
 847     <para>
 848       <citerefentry>
 849         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 850       </citerefentry>,
 851       <citerefentry>
 852         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 853       </citerefentry>,
 854       <citerefentry>
 855         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 856       </citerefentry>
 857     </para>
 858   </refsect1>
 859 </refentry>