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