man/udev.xml

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   7   Copyright © 2014 Jason St. John
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   9
  10 <refentry id="udev" xmlns:xi="http://www.w3.org/2001/XInclude">
  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
 115             <xi:include href="version-info.xml" xpointer="v217"/>
 116             </listitem>
 117           </varlistentry>
 118
 119           <varlistentry>
 120             <term><literal>:=</literal></term>
 121             <listitem>
 122               <para>Assign a value to a key finally; disallow any later changes.</para>
 123
 124             <xi:include href="version-info.xml" xpointer="v247"/>
 125             </listitem>
 126           </varlistentry>
 127         </variablelist>
 128       </refsect2>
 129
 130       <refsect2>
 131         <title>Values</title>
 132           <para>Values are written as double quoted strings, such as ("string").
 133           To include a quotation mark (") in the value, precede it by a backslash (\").
 134           Any other occurrences of a backslash followed by a character are not unescaped.
 135           That is, "\t\n" is treated as four characters:
 136           backslash, lowercase t, backslash, lowercase n.</para>
 137
 138           <para>The string can be prefixed with a lowercase e (e"string\n") to mark the string as
 139           <ulink url="https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences">C-style escaped</ulink>.
 140           For example, e"string\n" is parsed as 7 characters: 6 lowercase letters and a newline.
 141           This can be useful for writing special characters when a kernel driver requires them.</para>
 142
 143           <para>Please note that <constant>NUL</constant> is not allowed in either string variant.</para>
 144       </refsect2>
 145
 146       <refsect2>
 147         <title>Keys</title>
 148         <para>The following key names can be used to match against device properties.
 149         Some of the keys also match against properties of the parent devices in sysfs,
 150         not only the device that has generated the event. If multiple keys that match
 151         a parent device are specified in a single rule, all these keys must match at
 152         one and the same parent device.</para>
 153         <variablelist class='udev-directives'>
 154           <varlistentry>
 155             <term><varname>ACTION</varname></term>
 156             <listitem>
 157               <para>Match the name of the event action.</para>
 158             </listitem>
 159           </varlistentry>
 160
 161           <varlistentry>
 162             <term><varname>DEVPATH</varname></term>
 163             <listitem>
 164               <para>Match the devpath of the event device.</para>
 165             </listitem>
 166           </varlistentry>
 167
 168           <varlistentry>
 169             <term><varname>KERNEL</varname></term>
 170             <listitem>
 171               <para>Match the name of the event device.</para>
 172             </listitem>
 173           </varlistentry>
 174
 175           <varlistentry>
 176             <term><varname>KERNELS</varname></term>
 177             <listitem>
 178               <para>Search the devpath upwards for a matching device name.</para>
 179             </listitem>
 180           </varlistentry>
 181
 182           <varlistentry>
 183             <term><varname>NAME</varname></term>
 184             <listitem>
 185               <para>Match the name of a network interface. It can be used once the
 186               NAME key has been set in one of the preceding rules.</para>
 187             </listitem>
 188           </varlistentry>
 189
 190           <varlistentry>
 191             <term><varname>SYMLINK</varname></term>
 192             <listitem>
 193               <para>Match the name of a symlink targeting the node. It can be used once a SYMLINK key has
 194               been set in one of the preceding rules. There may be multiple symlinks; only one needs to
 195               match. If the operator is <literal>!=</literal>, the token returns true only if there is no
 196               symlink matched.</para>
 197             </listitem>
 198           </varlistentry>
 199
 200           <varlistentry>
 201             <term><varname>SUBSYSTEM</varname></term>
 202             <listitem>
 203               <para>Match the subsystem of the event device.</para>
 204             </listitem>
 205           </varlistentry>
 206
 207           <varlistentry>
 208             <term><varname>SUBSYSTEMS</varname></term>
 209             <listitem>
 210               <para>Search the devpath upwards for a matching device subsystem name.</para>
 211             </listitem>
 212           </varlistentry>
 213
 214           <varlistentry>
 215             <term><varname>DRIVER</varname></term>
 216             <listitem>
 217               <para>Match the driver name of the event device. Only set this key for devices
 218               which are bound to a driver at the time the event is generated.</para>
 219             </listitem>
 220           </varlistentry>
 221
 222           <varlistentry>
 223             <term><varname>DRIVERS</varname></term>
 224             <listitem>
 225               <para>Search the devpath upwards for a matching device driver name.</para>
 226             </listitem>
 227           </varlistentry>
 228
 229           <varlistentry>
 230             <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
 231             <listitem>
 232               <para>Match sysfs attribute value of the event device.</para>
 233
 234               <para>Trailing whitespace in the attribute values is ignored unless the specified match value
 235               itself contains trailing whitespace.</para>
 236             </listitem>
 237           </varlistentry>
 238
 239           <varlistentry>
 240             <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
 241             <listitem>
 242               <para>Search the devpath upwards for a device with matching sysfs attribute values. If
 243               multiple <varname>ATTRS</varname> matches are specified, all of them must match on the same
 244               device.</para>
 245
 246               <para>Trailing whitespace in the attribute values is ignored unless the specified match value
 247               itself contains trailing whitespace.</para>
 248             </listitem>
 249           </varlistentry>
 250
 251           <varlistentry>
 252             <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 253             <listitem>
 254               <para>Match a kernel parameter value.</para>
 255
 256             <xi:include href="version-info.xml" xpointer="v240"/>
 257             </listitem>
 258           </varlistentry>
 259
 260           <varlistentry>
 261             <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 262             <listitem>
 263               <para>Match against a device property value.</para>
 264             </listitem>
 265           </varlistentry>
 266
 267           <varlistentry>
 268             <term><varname>CONST{<replaceable>key</replaceable>}</varname></term>
 269             <listitem>
 270               <para>Match against a system-wide constant. Supported keys are:</para>
 271               <variablelist>
 272                 <varlistentry>
 273                   <term><literal>arch</literal></term>
 274                   <listitem>
 275                     <para>System's architecture. See <option>ConditionArchitecture=</option> in
 276                     <citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 277                     for possible values.</para>
 278
 279                   <xi:include href="version-info.xml" xpointer="v244"/>
 280                   </listitem>
 281                 </varlistentry>
 282                 <varlistentry>
 283                   <term><literal>virt</literal></term>
 284                   <listitem>
 285                     <para>System's virtualization environment. See
 286                     <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 287                     for possible values.</para>
 288
 289                     <xi:include href="version-info.xml" xpointer="v244"/>
 290                   </listitem>
 291                 </varlistentry>
 292                 <varlistentry>
 293                   <term><literal>cvm</literal></term>
 294                   <listitem>
 295                     <para>System's confidential virtualization technology. See
 296                     <citerefentry><refentrytitle>systemd-detect-virt</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 297                     for possible values.</para>
 298
 299                     <xi:include href="version-info.xml" xpointer="v254"/>
 300                   </listitem>
 301                 </varlistentry>
 302               </variablelist>
 303               <para>Unknown keys will never match.</para>
 304
 305               <xi:include href="version-info.xml" xpointer="v244"/>
 306             </listitem>
 307           </varlistentry>
 308
 309           <varlistentry>
 310             <term><varname>TAG</varname></term>
 311             <listitem>
 312               <para>Match against one of device tags. It can be used once a TAG key has been set in one of
 313               the preceding rules. There may be multiple tags; only one needs to match. If the operator is
 314               <literal>!=</literal>, the token returns true only if there is no tag matched.</para>
 315             </listitem>
 316           </varlistentry>
 317
 318           <varlistentry>
 319             <term><varname>TAGS</varname></term>
 320             <listitem>
 321               <para>Search the devpath upwards for a device with matching tag. If the operator is
 322               <literal>!=</literal>, the token returns true only if there is no tag matched.</para>
 323             </listitem>
 324           </varlistentry>
 325
 326           <varlistentry>
 327             <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
 328             <listitem>
 329               <para>Test the existence of a file. An octal mode mask can be specified
 330               if needed.</para>
 331             </listitem>
 332           </varlistentry>
 333
 334           <varlistentry>
 335             <term><varname>PROGRAM</varname></term>
 336             <listitem>
 337               <para>Execute a program to determine whether there is a match; the key is true if the program
 338               returns successfully. The device properties are made available to the executed program in the
 339               environment. The program's standard output is available in the <varname>RESULT</varname>
 340               key.</para>
 341
 342               <para>This can only be used for very short-running foreground tasks. For details, see
 343               <varname>RUN</varname>.</para>
 344
 345               <para>Note that multiple <varname>PROGRAM</varname> keys may be specified in one rule, and
 346               <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 347               <literal>==</literal>.</para>
 348             </listitem>
 349           </varlistentry>
 350
 351           <varlistentry>
 352             <term><varname>RESULT</varname></term>
 353             <listitem>
 354               <para>Match the returned string of the last <varname>PROGRAM</varname> call.
 355               This key can be used in the same or in any later rule after a
 356               <varname>PROGRAM</varname> call.</para>
 357             </listitem>
 358           </varlistentry>
 359         </variablelist>
 360
 361         <para>Most of the fields support shell glob pattern matching and
 362         alternate patterns. The following special characters are supported:</para>
 363         <variablelist>
 364           <varlistentry>
 365             <term><literal>*</literal></term>
 366             <listitem>
 367               <para>Matches zero or more characters.</para>
 368             </listitem>
 369           </varlistentry>
 370           <varlistentry>
 371             <term><literal>?</literal></term>
 372             <listitem>
 373               <para>Matches any single character.</para>
 374             </listitem>
 375           </varlistentry>
 376           <varlistentry>
 377             <term><literal>[]</literal></term>
 378             <listitem>
 379               <para>Matches any single character specified within the brackets. For
 380               example, the pattern string <literal>tty[SR]</literal>
 381               would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
 382               Ranges are also supported via the <literal>-</literal> character.
 383               For example, to match on the range of all digits, the pattern
 384               <literal>[0-9]</literal> could be used. If the first character
 385               following the <literal>[</literal> is a <literal>!</literal>,
 386               any characters not enclosed are matched.</para>
 387             </listitem>
 388           </varlistentry>
 389           <varlistentry>
 390             <term><literal>|</literal></term>
 391             <listitem>
 392               <para>Separates alternative patterns. For example, the pattern string
 393               <literal>abc|x*</literal> would match either <literal>abc</literal>
 394               or <literal>x*</literal>.</para>
 395
 396               <xi:include href="version-info.xml" xpointer="v217"/>
 397             </listitem>
 398           </varlistentry>
 399         </variablelist>
 400
 401         <para>The following keys can get values assigned:</para>
 402         <variablelist class='udev-directives'>
 403           <varlistentry>
 404             <term><varname>NAME</varname></term>
 405             <listitem>
 406               <para>The name to use for a network interface. See
 407               <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 408               for a higher-level mechanism for setting the interface name.
 409               The name of a device node cannot be changed by udev, only additional
 410               symlinks can be created.</para>
 411             </listitem>
 412           </varlistentry>
 413
 414           <varlistentry>
 415             <term><varname>SYMLINK</varname></term>
 416             <listitem>
 417               <para>The name of a symlink targeting the node. Every matching rule adds
 418               this value to the list of symlinks to be created.</para>
 419               <para>The set of characters to name a symlink is limited. Allowed
 420               characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
 421               sequences, and <literal>\x00</literal> hex encoding. All other
 422               characters are replaced by a <literal>_</literal> character.</para>
 423               <para>Multiple symlinks may be specified by separating the names by the
 424               space character. In case multiple devices claim the same name, the link
 425               always points to the device with the highest link_priority. If the current
 426               device goes away, the links are re-evaluated and the device with the
 427               next highest link_priority becomes the owner of the link. If no
 428               link_priority is specified, the order of the devices (and which one of
 429               them owns the link) is undefined.</para>
 430               <para>Symlink names must never conflict with the kernel's default device
 431               node names, as that would result in unpredictable behavior.
 432               </para>
 433             </listitem>
 434           </varlistentry>
 435
 436           <varlistentry>
 437             <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
 438             <listitem>
 439               <para>The permissions for the device node. Every specified value overrides
 440               the compiled-in default value.</para>
 441             </listitem>
 442           </varlistentry>
 443
 444           <varlistentry>
 445             <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
 446             <listitem>
 447               <para>Applies the specified Linux Security Module label to the device node.</para>
 448
 449             <xi:include href="version-info.xml" xpointer="v209"/>
 450             </listitem>
 451           </varlistentry>
 452
 453           <varlistentry>
 454             <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
 455             <listitem>
 456               <para>The value that should be written to a sysfs attribute of the
 457               event device.</para>
 458             </listitem>
 459           </varlistentry>
 460
 461           <varlistentry>
 462             <term><varname>SYSCTL{<replaceable>kernel parameter</replaceable>}</varname></term>
 463             <listitem>
 464               <para>The value that should be written to kernel parameter.</para>
 465
 466             <xi:include href="version-info.xml" xpointer="v220"/>
 467             </listitem>
 468           </varlistentry>
 469
 470           <varlistentry>
 471             <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
 472             <listitem>
 473               <para>Set a device property value. Property names with a leading <literal>.</literal>
 474               are neither stored in the database nor exported to events or
 475               external tools (run by, for example, the <varname>PROGRAM</varname>
 476               match key).</para>
 477             </listitem>
 478           </varlistentry>
 479
 480           <varlistentry>
 481             <term><varname>TAG</varname></term>
 482             <listitem>
 483               <para>Attach a tag to a device. This is used to filter events for users
 484               of libudev's monitor functionality, or to enumerate a group of tagged
 485               devices. The implementation can only work efficiently if only a few
 486               tags are attached to a device. It is only meant to be used in
 487               contexts with specific device filter requirements, and not as a
 488               general-purpose flag. Excessive use might result in inefficient event
 489               handling.</para>
 490             </listitem>
 491           </varlistentry>
 492
 493           <varlistentry>
 494             <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
 495             <listitem>
 496               <para>Specify a program to be executed after processing of all the rules for the event. With
 497               <literal>+=</literal>, this invocation is added to the list, and with <literal>=</literal> or
 498               <literal>:=</literal>, it replaces any previous contents of the list. Please note that both
 499               <literal>program</literal> and <literal>builtin</literal> types described below share a common
 500               list, so clearing the list with <literal>:=</literal> and <literal>=</literal> affects both
 501               types.</para>
 502
 503               <para><replaceable>type</replaceable> may be:</para>
 504               <variablelist>
 505                 <varlistentry>
 506                   <term><literal>program</literal></term>
 507                   <listitem>
 508                     <para>Execute an external program specified as the assigned
 509                     value. If no absolute path is given, the program is expected
 510                     to live in <filename>/usr/lib/udev</filename>; otherwise, the
 511                     absolute path must be specified.</para>
 512                     <para>This is the default if no <replaceable>type</replaceable>
 513                     is specified.</para>
 514                   </listitem>
 515                 </varlistentry>
 516                 <varlistentry>
 517                   <term><literal>builtin</literal></term>
 518                   <listitem>
 519                     <para>As <varname>program</varname>, but use one of the
 520                     built-in programs rather than an external one.</para>
 521
 522                   <xi:include href="version-info.xml" xpointer="v199"/>
 523                   </listitem>
 524                 </varlistentry>
 525               </variablelist>
 526
 527               <para>The program name and following arguments are separated by spaces. Single quotes can be
 528               used to specify arguments with spaces.</para>
 529
 530               <para>This can only be used for very short-running foreground tasks. Running an event process for
 531               a long period of time may block all further events for this or a dependent device.</para>
 532
 533               <para>Note that running programs that access the network or mount/unmount filesystems is not
 534               allowed inside of udev rules, due to the default sandbox that is enforced on
 535               <filename>systemd-udevd.service</filename>.</para>
 536
 537               <para>Starting daemons or other long-running processes is not allowed; the forked processes,
 538               detached or not, will be unconditionally killed after the event handling has finished. In order
 539               to activate long-running processes from udev rules, provide a service unit and pull it in from a
 540               udev device using the <varname>SYSTEMD_WANTS</varname> device property. See
 541               <citerefentry><refentrytitle>systemd.device</refentrytitle><manvolnum>5</manvolnum></citerefentry>
 542               for details.</para>
 543             </listitem>
 544           </varlistentry>
 545
 546           <varlistentry>
 547             <term><varname>LABEL</varname></term>
 548             <listitem>
 549               <para>A named label to which a <varname>GOTO</varname> may jump.</para>
 550             </listitem>
 551           </varlistentry>
 552
 553           <varlistentry>
 554             <term><varname>GOTO</varname></term>
 555             <listitem>
 556               <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
 557             </listitem>
 558           </varlistentry>
 559
 560           <varlistentry>
 561             <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
 562             <listitem>
 563               <para>Import a set of variables as device properties, depending on
 564               <replaceable>type</replaceable>:</para>
 565
 566               <variablelist>
 567                 <varlistentry>
 568                   <term><literal>program</literal></term>
 569                   <listitem>
 570                     <para>Execute an external program specified as the assigned
 571                     value and, if it returns successfully,
 572                     import its output, which must be in environment key
 573                     format. Path specification, command/argument separation,
 574                     and quoting work like in <varname>RUN</varname>.</para>
 575
 576                     <xi:include href="version-info.xml" xpointer="v199"/>
 577                   </listitem>
 578                 </varlistentry>
 579                 <varlistentry>
 580                   <term><literal>builtin</literal></term>
 581                   <listitem>
 582                     <para>Similar to <literal>program</literal>, but use one of the
 583                     built-in programs rather than an external one.</para>
 584
 585                   <xi:include href="version-info.xml" xpointer="v199"/>
 586                   </listitem>
 587                 </varlistentry>
 588               <varlistentry>
 589                   <term><literal>file</literal></term>
 590                   <listitem>
 591                     <para>Import a text file specified as the assigned value, the content
 592                     of which must be in environment key format.</para>
 593                   </listitem>
 594                 </varlistentry>
 595                 <varlistentry>
 596                   <term><literal>db</literal></term>
 597                   <listitem>
 598                     <para>Import a single property specified as the assigned value from the
 599                     current device database. This works only if the database is already populated
 600                     by an earlier event.</para>
 601                   </listitem>
 602                 </varlistentry>
 603                 <varlistentry>
 604                   <term><literal>cmdline</literal></term>
 605                   <listitem>
 606                     <para>Import a single property from the kernel command line. For simple flags
 607                     the value of the property is set to <literal>1</literal>.</para>
 608                   </listitem>
 609                 </varlistentry>
 610                 <varlistentry>
 611                   <term><literal>parent</literal></term>
 612                   <listitem>
 613                     <para>Import the stored keys from the parent device by reading
 614                     the database entry of the parent device. The value assigned to
 615                     <option>IMPORT{parent}</option> is used as a filter of key names
 616                     to import (with the same shell glob pattern matching used for
 617                     comparisons).</para>
 618                   </listitem>
 619                 </varlistentry>
 620               </variablelist>
 621
 622               <para>This can only be used for very short-running foreground tasks. For details see
 623               <option>RUN</option>.</para>
 624
 625               <para>Note that multiple <varname>IMPORT{}</varname> keys may be specified in one rule, and
 626               <literal>=</literal>, <literal>:=</literal>, and <literal>+=</literal> have the same effect as
 627               <literal>==</literal>. The key is true if the import is successful, unless <literal>!=</literal>
 628               is used as the operator which causes the key to be true if the import failed.</para>
 629             </listitem>
 630           </varlistentry>
 631
 632           <varlistentry>
 633             <term><varname>OPTIONS</varname></term>
 634             <listitem>
 635               <para>Rule and device options:</para>
 636               <variablelist class='udev-directives'>
 637                 <varlistentry>
 638                   <term><option>link_priority=<replaceable>value</replaceable></option></term>
 639                   <listitem>
 640                     <para>Specify the priority of the created symlinks. Devices with higher
 641                     priorities overwrite existing symlinks of other devices. The default is 0.</para>
 642                   </listitem>
 643                 </varlistentry>
 644                 <varlistentry>
 645                   <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
 646                   <listitem>
 647                     <para>When <literal>replace</literal>, possibly unsafe characters in strings
 648                     assigned to <varname>NAME</varname>, <varname>SYMLINK</varname>, and
 649                     <varname>ENV{<replaceable>key</replaceable>}</varname> are replaced. When
 650                     <literal>none</literal>, no replacement is performed. When unset, the replacement
 651                     is performed for <varname>NAME</varname>, <varname>SYMLINK</varname>, but not for
 652                     <varname>ENV{<replaceable>key</replaceable>}</varname>. Defaults to unset.</para>
 653                   </listitem>
 654                 </varlistentry>
 655                 <varlistentry>
 656                   <term><option>static_node=</option></term>
 657                   <listitem>
 658                     <para>Apply the permissions specified in this rule to the
 659                     static device node with the specified name. Also, for every
 660                     tag specified in this rule, create a symlink
 661                     in the directory
 662                     <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
 663                     pointing at the static device node with the specified name.
 664                     Static device node creation is performed by systemd-tmpfiles
 665                     before systemd-udevd is started. The static nodes might not
 666                     have a corresponding kernel device; they are used to trigger
 667                     automatic kernel module loading when they are accessed.</para>
 668                   </listitem>
 669                 </varlistentry>
 670                 <varlistentry>
 671                   <term><option>watch</option></term>
 672                   <listitem>
 673                     <para>Watch the device node with inotify; when the node is
 674                     closed after being opened for writing, a change uevent is
 675                     synthesized.</para>
 676                   </listitem>
 677                 </varlistentry>
 678                 <varlistentry>
 679                   <term><option>nowatch</option></term>
 680                   <listitem>
 681                     <para>Disable the watching of a device node with inotify.</para>
 682                   </listitem>
 683                 </varlistentry>
 684                 <varlistentry>
 685                   <term><option>db_persist</option></term>
 686                   <listitem>
 687                     <para>Set the flag (sticky bit) on the udev database entry of the event device. Device
 688                     properties are then kept in the database even when <command>udevadm info
 689                     --cleanup-db</command> is called.  This option can be useful in certain cases
 690                     (e.g. Device Mapper devices) for persisting device state on the transition from
 691                     initrd.</para>
 692
 693                     <xi:include href="version-info.xml" xpointer="v241"/>
 694                   </listitem>
 695                 </varlistentry>
 696                 <varlistentry>
 697                   <term><option>log_level=<replaceable>level</replaceable></option></term>
 698                   <listitem>
 699                     <para>Takes a log level name like <literal>debug</literal> or
 700                     <literal>info</literal>, or a special value <literal>reset</literal>. When a log
 701                     level name is specified, the maximum log level is changed to that level. When
 702                     <literal>reset</literal> is set, then the previously specified log level is
 703                     revoked. Defaults to the log level of the main process of
 704                     <command>systemd-udevd</command>.</para>
 705                     <para>This may be useful when debugging events for certain devices. Note that the
 706                     log level is applied when the line including this rule is processed. So, for
 707                     debugging, it is recommended that this is specified at earlier place, e.g., the
 708                     first line of <filename>00-debug.rules</filename>.</para>
 709                     <para>Example for debugging uevent processing for network interfaces:
 710                     <programlisting># /etc/udev/rules.d/00-debug-net.rules
 711 SUBSYSTEM=="net", OPTIONS="log_level=debug"</programlisting></para>
 712
 713                     <xi:include href="version-info.xml" xpointer="v248"/>
 714                   </listitem>
 715                 </varlistentry>
 716               </variablelist>
 717             </listitem>
 718           </varlistentry>
 719         </variablelist>
 720
 721         <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
 722         <varname>PROGRAM</varname>, <varname>OWNER</varname>,
 723         <varname>GROUP</varname>, <varname>MODE</varname>, <varname>SECLABEL</varname>,
 724         and <varname>RUN</varname> fields support simple string substitutions.
 725         The <varname>RUN</varname> substitutions are performed after all rules
 726         have been processed, right before the program is executed, allowing for
 727         the use of device properties set by earlier matching rules. For all other
 728         fields, substitutions are performed while the individual rule is being
 729         processed. The available substitutions are:</para>
 730         <variablelist class='udev-directives'>
 731           <varlistentry>
 732             <term><option>$kernel</option>, <option>%k</option></term>
 733             <listitem>
 734               <para>The kernel name for this device.</para>
 735             </listitem>
 736           </varlistentry>
 737
 738           <varlistentry>
 739             <term><option>$number</option>, <option>%n</option></term>
 740             <listitem>
 741               <para>The kernel number for this device. For example, <literal>sda3</literal> has kernel number
 742               3.</para>
 743             </listitem>
 744           </varlistentry>
 745
 746           <varlistentry>
 747             <term><option>$devpath</option>, <option>%p</option></term>
 748             <listitem>
 749               <para>The devpath of the device.</para>
 750             </listitem>
 751           </varlistentry>
 752
 753           <varlistentry>
 754             <term><option>$id</option>, <option>%b</option></term>
 755             <listitem>
 756               <para>The name of the device matched while searching the devpath
 757                 upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
 758                 <option>DRIVERS</option>, and <option>ATTRS</option>.
 759               </para>
 760             </listitem>
 761           </varlistentry>
 762
 763           <varlistentry>
 764             <term><option>$driver</option></term>
 765             <listitem>
 766               <para>The driver name of the device matched while searching the
 767                 devpath upwards for <option>SUBSYSTEMS</option>,
 768                 <option>KERNELS</option>, <option>DRIVERS</option>, and
 769                 <option>ATTRS</option>.
 770               </para>
 771             </listitem>
 772           </varlistentry>
 773
 774           <varlistentry>
 775             <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
 776             <listitem>
 777               <para>The value of a sysfs attribute found at the device where
 778                 all keys of the rule have matched. If the matching device does not
 779                 have such an attribute, and a previous <option>KERNELS</option>,
 780                 <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
 781                 <option>ATTRS</option> test selected a parent device, then the
 782                 attribute from that parent device is used.
 783               </para>
 784               <para>If the attribute is a symlink, the last element of the
 785                 symlink target is returned as the value.
 786               </para>
 787             </listitem>
 788           </varlistentry>
 789
 790           <varlistentry>
 791             <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
 792             <listitem>
 793               <para>A device property value.</para>
 794             </listitem>
 795           </varlistentry>
 796
 797           <varlistentry>
 798             <term><option>$major</option>, <option>%M</option></term>
 799             <listitem>
 800               <para>The kernel major number for the device.</para>
 801             </listitem>
 802           </varlistentry>
 803
 804           <varlistentry>
 805             <term><option>$minor</option>, <option>%m</option></term>
 806             <listitem>
 807               <para>The kernel minor number for the device.</para>
 808             </listitem>
 809           </varlistentry>
 810
 811           <varlistentry>
 812             <term><option>$result</option>, <option>%c</option></term>
 813             <listitem>
 814               <para>The string returned by the external program requested with
 815               <varname>PROGRAM</varname>.
 816               A single part of the string, separated by a space character, may be selected
 817               by specifying the part number as an attribute: <literal>%c{N}</literal>.
 818               If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
 819               of the result string are substituted: <literal>%c{N+}</literal>.</para>
 820             </listitem>
 821           </varlistentry>
 822
 823           <varlistentry>
 824             <term><option>$parent</option>, <option>%P</option></term>
 825             <listitem>
 826               <para>The node name of the parent device.</para>
 827             </listitem>
 828           </varlistentry>
 829
 830           <varlistentry>
 831             <term><option>$name</option></term>
 832             <listitem>
 833               <para>The current name of the device. If not changed by a rule, it is the
 834               name of the kernel device.</para>
 835             </listitem>
 836           </varlistentry>
 837
 838           <varlistentry>
 839             <term><option>$links</option></term>
 840             <listitem>
 841               <para>A space-separated list of the current symlinks. The value is
 842               only set during a remove event or if an earlier rule assigned a value.</para>
 843             </listitem>
 844           </varlistentry>
 845
 846           <varlistentry>
 847             <term><option>$root</option>, <option>%r</option></term>
 848             <listitem>
 849               <para>The udev_root value.</para>
 850             </listitem>
 851           </varlistentry>
 852
 853           <varlistentry>
 854             <term><option>$sys</option>, <option>%S</option></term>
 855             <listitem>
 856               <para>The sysfs mount point.</para>
 857             </listitem>
 858           </varlistentry>
 859
 860           <varlistentry>
 861             <term><option>$devnode</option>, <option>%N</option></term>
 862             <listitem>
 863               <para>The name of the device node.</para>
 864             </listitem>
 865           </varlistentry>
 866
 867           <varlistentry>
 868             <term><option>%%</option></term>
 869             <listitem>
 870             <para>The <literal>%</literal> character itself.</para>
 871             </listitem>
 872           </varlistentry>
 873
 874           <varlistentry>
 875             <term><option>$$</option></term>
 876             <listitem>
 877             <para>The <literal>$</literal> character itself.</para>
 878             </listitem>
 879           </varlistentry>
 880         </variablelist>
 881       </refsect2>
 882   </refsect1>
 883
 884   <refsect1>
 885     <title>See Also</title>
 886     <para>
 887       <citerefentry>
 888         <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
 889       </citerefentry>,
 890       <citerefentry>
 891         <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
 892       </citerefentry>,
 893       <citerefentry>
 894         <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
 895       </citerefentry>
 896     </para>
 897   </refsect1>
 898 </refentry>