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8 <productname>systemd</productname>
11 <contrib>Developer</contrib>
12 <firstname>Greg</firstname>
13 <surname>Kroah-Hartmann</surname>
14 <email>greg@kroah.com</email>
17 <contrib>Developer</contrib>
18 <firstname>Kay</firstname>
19 <surname>Sievers</surname>
20 <email>kay@vrfy.org</email>
26 <refentrytitle>udev</refentrytitle>
27 <manvolnum>7</manvolnum>
31 <refname>udev</refname>
32 <refpurpose>Dynamic device management</refpurpose>
35 <refsect1><title>Description</title>
36 <para>udev supplies the system software with device events, manages permissions
37 of device nodes and may create additional symlinks in the <filename>/dev</filename>
38 directory, or renames network interfaces. The kernel usually just assigns unpredictable
39 device names based on the order of discovery. Meaningful symlinks or network device
40 names provide a way to reliably identify devices based on their properties or
41 current configuration.</para>
43 <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
44 <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
45 the kernel whenever a device is added or removed from the system, or it changes its
46 state. When udev receives a device event, it matches its configured set of rules
47 against various device attributes to identify the device. Rules that match may
48 provide additional device information to be stored in the udev database or
49 to be used to create meaningful symlink names.</para>
51 <para>All device information udev processes is stored in the udev database and
52 sent out to possible event subscribers. Access to all stored data and the event
53 sources is provided by the library libudev.</para>
56 <refsect1><title>Rules Files</title>
57 <para>The udev rules are read from the files located in the
58 system rules directory <filename>/usr/lib/udev/rules.d</filename>,
59 the volatile runtime directory <filename>/run/udev/rules.d</filename>
60 and the local administration directory <filename>/etc/udev/rules.d</filename>.
61 All rules files are collectively sorted and processed in lexical order,
62 regardless of the directories in which they live. However, files with
63 identical filenames replace each other. Files in <filename>/etc</filename>
64 have the highest priority, files in <filename>/run</filename> take precedence
65 over files with the same name in <filename>/usr/lib</filename>. This can be
66 used to override a system-supplied rules file with a local file if needed;
67 a symlink in <filename>/etc</filename> with the same name as a rules file in
68 <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
69 disables the rules file entirely. Rule files must have the extension
70 <filename>.rules</filename>; other extensions are ignored.</para>
72 <para>Every line in the rules file contains at least one key-value pair.
73 Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
74 There are two kinds of keys: match and assignment.
75 If all match keys match against their values, the rule gets applied and the
76 assignment keys get the specified values assigned.</para>
78 <para>A matching rule may rename a network interface, add symlinks
79 pointing to the device node, or run a specified program as part of
80 the event handling.</para>
82 <para>A rule consists of a comma-separated list of one or more key-value pairs.
83 Each key has a distinct operation, depending on the used operator. Valid
87 <term><literal>==</literal></term>
89 <para>Compare for equality.</para>
94 <term><literal>!=</literal></term>
96 <para>Compare for inequality.</para>
101 <term><literal>=</literal></term>
103 <para>Assign a value to a key. Keys that represent a list are reset
104 and only this single value is assigned.</para>
109 <term><literal>+=</literal></term>
111 <para>Add the value to a key that holds a list of entries.</para>
116 <term><literal>-=</literal></term>
118 <para>Remove the value from a key that holds a list of entries.</para>
123 <term><literal>:=</literal></term>
125 <para>Assign a value to a key finally; disallow any later changes.</para>
130 <para>The following key names can be used to match against device properties.
131 Some of the keys also match against properties of the parent devices in sysfs,
132 not only the device that has generated the event. If multiple keys that match
133 a parent device are specified in a single rule, all these keys must match at
134 one and the same parent device.</para>
135 <variablelist class='udev-directives'>
137 <term><varname>ACTION</varname></term>
139 <para>Match the name of the event action.</para>
144 <term><varname>DEVPATH</varname></term>
146 <para>Match the devpath of the event device.</para>
151 <term><varname>KERNEL</varname></term>
153 <para>Match the name of the event device.</para>
158 <term><varname>NAME</varname></term>
160 <para>Match the name of a network interface. It can be used once the
161 NAME key has been set in one of the preceding rules.</para>
166 <term><varname>SYMLINK</varname></term>
168 <para>Match the name of a symlink targeting the node. It can
169 be used once a SYMLINK key has been set in one of the preceding
170 rules. There may be multiple symlinks; only one needs to match.
176 <term><varname>SUBSYSTEM</varname></term>
178 <para>Match the subsystem of the event device.</para>
182 <term><varname>DRIVER</varname></term>
184 <para>Match the driver name of the event device. Only set this key for devices
185 which are bound to a driver at the time the event is generated.</para>
189 <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
191 <para>Match sysfs attribute values of the event device. Trailing
192 whitespace in the attribute values is ignored unless the specified match
193 value itself contains trailing whitespace.
199 <term><varname>KERNELS</varname></term>
201 <para>Search the devpath upwards for a matching device name.</para>
206 <term><varname>SUBSYSTEMS</varname></term>
208 <para>Search the devpath upwards for a matching device subsystem name.</para>
213 <term><varname>DRIVERS</varname></term>
215 <para>Search the devpath upwards for a matching device driver name.</para>
220 <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
222 <para>Search the devpath upwards for a device with matching sysfs attribute values.
223 If multiple <varname>ATTRS</varname> matches are specified, all of them
224 must match on the same device. Trailing whitespace in the attribute values is ignored
225 unless the specified match value itself contains trailing whitespace.</para>
230 <term><varname>TAGS</varname></term>
232 <para>Search the devpath upwards for a device with matching tag.</para>
237 <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
239 <para>Match against a device property value.</para>
244 <term><varname>TAG</varname></term>
246 <para>Match against a device tag.</para>
251 <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
253 <para>Test the existence of a file. An octal mode mask can be specified
259 <term><varname>PROGRAM</varname></term>
261 <para>Execute a program to determine whether there
262 is a match; the key is true if the program returns
263 successfully. The device properties are made available to the
264 executed program in the environment. The program's standard output
265 is available in the <varname>RESULT</varname> key.</para>
266 <para>This can only be used for very short-running foreground tasks. For details,
267 see <varname>RUN</varname>.</para>
272 <term><varname>RESULT</varname></term>
274 <para>Match the returned string of the last <varname>PROGRAM</varname> call.
275 This key can be used in the same or in any later rule after a
276 <varname>PROGRAM</varname> call.</para>
281 <para>Most of the fields support shell glob pattern matching and
282 alternate patterns. The following special characters are supported:</para>
285 <term><literal>*</literal></term>
287 <para>Matches zero or more characters.</para>
291 <term><literal>?</literal></term>
293 <para>Matches any single character.</para>
297 <term><literal>[]</literal></term>
299 <para>Matches any single character specified within the brackets. For
300 example, the pattern string <literal>tty[SR]</literal>
301 would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
302 Ranges are also supported via the <literal>-</literal> character.
303 For example, to match on the range of all digits, the pattern
304 <literal>[0-9]</literal> could be used. If the first character
305 following the <literal>[</literal> is a <literal>!</literal>,
306 any characters not enclosed are matched.</para>
310 <term><literal>|</literal></term>
312 <para>Separates alternative patterns. For example, the pattern string
313 <literal>abc|x*</literal> would match either <literal>abc</literal>
314 or <literal>x*</literal>.</para>
319 <para>The following keys can get values assigned:</para>
320 <variablelist class='udev-directives'>
322 <term><varname>NAME</varname></term>
324 <para>The name to use for a network interface. See
325 <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
326 for a higher-level mechanism for setting the interface name.
327 The name of a device node cannot be changed by udev, only additional
328 symlinks can be created.</para>
333 <term><varname>SYMLINK</varname></term>
335 <para>The name of a symlink targeting the node. Every matching rule adds
336 this value to the list of symlinks to be created.</para>
337 <para>The set of characters to name a symlink is limited. Allowed
338 characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
339 sequences, and <literal>\x00</literal> hex encoding. All other
340 characters are replaced by a <literal>_</literal> character.</para>
341 <para>Multiple symlinks may be specified by separating the names by the
342 space character. In case multiple devices claim the same name, the link
343 always points to the device with the highest link_priority. If the current
344 device goes away, the links are re-evaluated and the device with the
345 next highest link_priority becomes the owner of the link. If no
346 link_priority is specified, the order of the devices (and which one of
347 them owns the link) is undefined.</para>
348 <para>Symlink names must never conflict with the kernel's default device
349 node names, as that would result in unpredictable behavior.
355 <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
357 <para>The permissions for the device node. Every specified value overrides
358 the compiled-in default value.</para>
363 <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
365 <para>Applies the specified Linux Security Module label to the device node.</para>
370 <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
372 <para>The value that should be written to a sysfs attribute of the
378 <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
380 <para>Set a device property value. Property names with a leading <literal>.</literal>
381 are neither stored in the database nor exported to events or
382 external tools (run by, for example, the <varname>PROGRAM</varname>
388 <term><varname>TAG</varname></term>
390 <para>Attach a tag to a device. This is used to filter events for users
391 of libudev's monitor functionality, or to enumerate a group of tagged
392 devices. The implementation can only work efficiently if only a few
393 tags are attached to a device. It is only meant to be used in
394 contexts with specific device filter requirements, and not as a
395 general-purpose flag. Excessive use might result in inefficient event
401 <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
403 <para>Add a program to the list of programs to be executed after
404 processing all the rules for a specific event, depending on
405 <literal>type</literal>:</para>
408 <term><literal>program</literal></term>
410 <para>Execute an external program specified as the assigned
411 value. If no absolute path is given, the program is expected
412 to live in <filename>/usr/lib/udev</filename>; otherwise, the
413 absolute path must be specified.</para>
414 <para>This is the default if no <replaceable>type</replaceable>
419 <term><literal>builtin</literal></term>
421 <para>As <varname>program</varname>, but use one of the
422 built-in programs rather than an external one.</para>
426 <para>The program name and following arguments are separated by spaces.
427 Single quotes can be used to specify arguments with spaces.</para>
428 <para>This can only be used for very short-running foreground tasks. Running an
429 event process for a long period of time may block all further events for
430 this or a dependent device.</para>
431 <para>Starting daemons or other long-running processes is not appropriate
432 for udev; the forked processes, detached or not, will be unconditionally
433 killed after the event handling has finished.</para>
438 <term><varname>LABEL</varname></term>
440 <para>A named label to which a <varname>GOTO</varname> may jump.</para>
445 <term><varname>GOTO</varname></term>
447 <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
452 <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
454 <para>Import a set of variables as device properties,
455 depending on <literal>type</literal>:</para>
458 <term><literal>program</literal></term>
460 <para>Execute an external program specified as the assigned value and
461 import its output, which must be in environment key
462 format. Path specification, command/argument separation,
463 and quoting work like in <varname>RUN</varname>.</para>
467 <term><literal>builtin</literal></term>
469 <para>Similar to <literal>program</literal>, but use one of the
470 built-in programs rather than an external one.</para>
474 <term><literal>file</literal></term>
476 <para>Import a text file specified as the assigned value, the content
477 of which must be in environment key format.</para>
481 <term><literal>db</literal></term>
483 <para>Import a single property specified as the assigned value from the
484 current device database. This works only if the database is already populated
485 by an earlier event.</para>
489 <term><literal>cmdline</literal></term>
491 <para>Import a single property from the kernel command line. For simple flags
492 the value of the property is set to <literal>1</literal>.</para>
496 <term><literal>parent</literal></term>
498 <para>Import the stored keys from the parent device by reading
499 the database entry of the parent device. The value assigned to
500 <option>IMPORT{parent}</option> is used as a filter of key names
501 to import (with the same shell glob pattern matching used for
506 <para>This can only be used for very short-running foreground tasks. For details
507 see <option>RUN</option>.</para>
512 <term><varname>WAIT_FOR</varname></term>
514 <para>Wait for a file to become available or until a timeout of
515 10 seconds expires. The path is relative to the sysfs device;
516 if no path is specified, this waits for an attribute to appear.</para>
521 <term><varname>OPTIONS</varname></term>
523 <para>Rule and device options:</para>
524 <variablelist class='udev-directives'>
526 <term><option>link_priority=<replaceable>value</replaceable></option></term>
528 <para>Specify the priority of the created symlinks. Devices with higher
529 priorities overwrite existing symlinks of other devices. The default is 0.</para>
533 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
535 <para>Usually control and other possibly unsafe characters are replaced
536 in strings used for device naming. The mode of replacement can be specified
537 with this option.</para>
541 <term><option>static_node=</option></term>
543 <para>Apply the permissions specified in this rule to the
544 static device node with the specified name. Also, for every
545 tag specified in this rule, create a symlink
547 <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
548 pointing at the static device node with the specified name.
549 Static device node creation is performed by systemd-tmpfiles
550 before systemd-udevd is started. The static nodes might not
551 have a corresponding kernel device; they are used to trigger
552 automatic kernel module loading when they are accessed.</para>
556 <term><option>watch</option></term>
558 <para>Watch the device node with inotify; when the node is
559 closed after being opened for writing, a change uevent is
564 <term><option>nowatch</option></term>
566 <para>Disable the watching of a device node with inotify.</para>
574 <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
575 <varname>PROGRAM</varname>, <varname>OWNER</varname>,
576 <varname>GROUP</varname>, <varname>MODE</varname>, and
577 <varname>RUN</varname> fields support simple string substitutions.
578 The <varname>RUN</varname> substitutions are performed after all rules
579 have been processed, right before the program is executed, allowing for
580 the use of device properties set by earlier matching rules. For all other
581 fields, substitutions are performed while the individual rule is being
582 processed. The available substitutions are:</para>
583 <variablelist class='udev-directives'>
585 <term><option>$kernel</option>, <option>%k</option></term>
587 <para>The kernel name for this device.</para>
592 <term><option>$number</option>, <option>%n</option></term>
594 <para>The kernel number for this device. For example,
595 <literal>sda3</literal> has kernel number <literal>3</literal>.
601 <term><option>$devpath</option>, <option>%p</option></term>
603 <para>The devpath of the device.</para>
608 <term><option>$id</option>, <option>%b</option></term>
610 <para>The name of the device matched while searching the devpath
611 upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
612 <option>DRIVERS</option>, and <option>ATTRS</option>.
618 <term><option>$driver</option></term>
620 <para>The driver name of the device matched while searching the
621 devpath upwards for <option>SUBSYSTEMS</option>,
622 <option>KERNELS</option>, <option>DRIVERS</option>, and
623 <option>ATTRS</option>.
629 <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
631 <para>The value of a sysfs attribute found at the device where
632 all keys of the rule have matched. If the matching device does not
633 have such an attribute, and a previous <option>KERNELS</option>,
634 <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
635 <option>ATTRS</option> test selected a parent device, then the
636 attribute from that parent device is used.
638 <para>If the attribute is a symlink, the last element of the
639 symlink target is returned as the value.
645 <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
647 <para>A device property value.</para>
652 <term><option>$major</option>, <option>%M</option></term>
654 <para>The kernel major number for the device.</para>
659 <term><option>$minor</option>, <option>%m</option></term>
661 <para>The kernel minor number for the device.</para>
666 <term><option>$result</option>, <option>%c</option></term>
668 <para>The string returned by the external program requested with
669 <varname>PROGRAM</varname>.
670 A single part of the string, separated by a space character, may be selected
671 by specifying the part number as an attribute: <literal>%c{N}</literal>.
672 If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
673 of the result string are substituted: <literal>%c{N+}</literal>.</para>
678 <term><option>$parent</option>, <option>%P</option></term>
680 <para>The node name of the parent device.</para>
685 <term><option>$name</option></term>
687 <para>The current name of the device. If not changed by a rule, it is the
688 name of the kernel device.</para>
693 <term><option>$links</option></term>
695 <para>A space-separated list of the current symlinks. The value is
696 only set during a remove event or if an earlier rule assigned a value.</para>
701 <term><option>$root</option>, <option>%r</option></term>
703 <para>The udev_root value.</para>
708 <term><option>$sys</option>, <option>%S</option></term>
710 <para>The sysfs mount point.</para>
715 <term><option>$devnode</option>, <option>%N</option></term>
717 <para>The name of the device node.</para>
722 <term><option>%%</option></term>
724 <para>The <literal>%</literal> character itself.</para>
729 <term><option>$$</option></term>
731 <para>The <literal>$</literal> character itself.</para>
738 <title>See Also</title>
741 <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
744 <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
747 <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>