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