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11 <productname>udev</productname>
15 <refentrytitle>udev</refentrytitle>
16 <manvolnum>7</manvolnum>
20 <refname>udev</refname>
21 <refpurpose>dynamic device management</refpurpose>
24 <refsect1><title>DESCRIPTION</title>
25 <para>udev provides a dynamic device directory containing only the files for
26 actually present devices. It creates or removes device node files in the
27 <filename>/dev</filename> directory, or it renames network interfaces.</para>
29 <para>Usually udev runs as <citerefentry><refentrytitle>udevd</refentrytitle>
30 <manvolnum>8</manvolnum></citerefentry> and receives uevents directly from the
31 kernel if a device is added or removed from the system.</para>
33 <para>If udev receives a device event, it matches its configured rules
34 against the available device attributes provided in sysfs to identify the device.
35 Rules that match may provide additional device information or specify a device
36 node name and multiple symlink names and instruct udev to run additional programs
37 as part of the device event handling.</para>
40 <refsect1><title>CONFIGURATION</title>
41 <para>udev configuration files are placed in <filename>/etc/udev/</filename>
42 and <filename>/lib/udev/</filename>. All empty lines, or lines beginning with
43 '#' will be ignored.</para>
45 <refsect2><title>Configuration file</title>
46 <para>udev expects its main configuration file at <filename>/etc/udev/udev.conf</filename>.
47 It consists of a set of variables allowing the user to override default udev values.
48 The following variables can be set:</para>
51 <term><option>udev_root</option></term>
53 <para>Specifies where to place the device nodes in the filesystem.
54 The default value is <filename>/dev</filename>.</para>
59 <term><option>udev_log</option></term>
61 <para>The logging priority. Valid values are the numerical syslog priorities
62 or their textual representations: <option>err</option>, <option>info</option>
63 and <option>debug</option>.</para>
69 <refsect2><title>Rules files</title>
70 <para>The udev rules are read from the files located in the
71 default rules directory <filename>/lib/udev/rules.d/</filename>,
72 the custom rules directory <filename>/etc/udev/rules.d/</filename>
73 and the temporary rules directory <filename>/dev/.udev/rules.d/</filename>.
74 All rule files are sorted and processed in lexical order, regardless
75 in which of these directories they live.</para>
77 <para>Rule files are required to have a unique name, duplicate file names
78 are ignored. Files in <filename>/etc/udev/rules.d/</filename> have precedence
79 over files with the same name in <filename>/lib/udev/rules.d/</filename>. This
80 can be used to ignore a default rules file if needed.</para>
82 <para>Every line in the rules file contains at least one key value pair.
83 There are two kind of keys, match and assignment keys.
84 If all match keys are matching against its value, the rule gets applied and the
85 assign keys get the specified value assigned.</para>
87 <para>A matching rule may specify the name of the device node, add a symlink
88 pointing to the node, or run a specified program as part of the event handling.
89 If no matching rule is found, the default device node name is used.</para>
91 <para>A rule consists of a list of one or more key value pairs separated by
92 a comma. Each key has a distinct operation, depending on the used operator. Valid
96 <term><option>==</option></term>
98 <para>Compare for equality.</para>
103 <term><option>!=</option></term>
105 <para>Compare for inequality.</para>
110 <term><option>=</option></term>
112 <para>Assign a value to a key. Keys that represent a list, are reset
113 and only this single value is assigned.</para>
118 <term><option>+=</option></term>
120 <para>Add the value to a key that holds a list of entries.</para>
125 <term><option>:=</option></term>
127 <para>Assign a value to a key finally; disallow any later changes,
128 which may be used to prevent changes by any later rules.</para>
133 <para>The following key names can be used to match against device properties.
134 Some of the keys also match against properties of the parent devices in sysfs,
135 not only the device that has generated the event. If multiple keys that match
136 a parent device are specified in a single rule, all these keys must match at
137 one and the same parent device.</para>
140 <term><option>ACTION</option></term>
142 <para>Match the name of the event action.</para>
147 <term><option>DEVPATH</option></term>
149 <para>Match the devpath of the event device.</para>
154 <term><option>KERNEL</option></term>
156 <para>Match the name of the event device.</para>
161 <term><option>NAME</option></term>
163 <para>Match the name of the node or network interface. It can
164 be used once the NAME key has been set in one of the preceding
170 <term><option>SYMLINK</option></term>
172 <para>Match the name of a symlink targeting the node. It can
173 be used once a SYMLINK key has been set in one of the preceding
174 rules. There may be multiple symlinks; only one needs to match.
180 <term><option>SUBSYSTEM</option></term>
182 <para>Match the subsystem of the event device.</para>
186 <term><option>DRIVER</option></term>
188 <para>Match the driver name of the event device. Only set for devices
189 which are bound to a driver at the time the event is generated.</para>
193 <term><option>ATTR{<replaceable>filename</replaceable>}</option></term>
195 <para>Match sysfs attribute values of the event device. Trailing
196 whitespace in the attribute values is ignored, if the specified match
197 value does not contain trailing whitespace itself.
203 <term><option>KERNELS</option></term>
205 <para>Search the devpath upwards for a matching device name.</para>
210 <term><option>SUBSYSTEMS</option></term>
212 <para>Search the devpath upwards for a matching device subsystem name.</para>
217 <term><option>DRIVERS</option></term>
219 <para>Search the devpath upwards for a matching device driver name.</para>
224 <term><option>ATTRS{<replaceable>filename</replaceable>}</option></term>
226 <para>Search the devpath upwards for a device with matching sysfs attribute values.
227 If multiple <option>ATTRS</option> matches are specified, all of them
228 must match on the same device. Trailing whitespace in the attribute values is ignored,
229 if the specified match value does not contain trailing whitespace itself.</para>
234 <term><option>ENV{<replaceable>key</replaceable>}</option></term>
236 <para>Match against a device property value.</para>
241 <term><option>TEST{<replaceable>octal mode mask</replaceable>}</option></term>
243 <para>Test the existence of a file. An octal mode mask can be specified
249 <term><option>PROGRAM</option></term>
251 <para>Execute a program. The key is true, if the program returns
252 successfully. The device properties are made available to the
253 executed program in the environment. The program's output printed to
254 stdout, is available in the RESULT key.</para>
259 <term><option>RESULT</option></term>
261 <para>Match the returned string of the last PROGRAM call. This key can
262 be used in the same or in any later rule after a PROGRAM call.</para>
267 <para>Most of the fields support a shell style pattern matching. The following
268 pattern characters are supported:</para>
271 <term><option>*</option></term>
273 <para>Matches zero, or any number of characters.</para>
277 <term><option>?</option></term>
279 <para>Matches any single character.</para>
283 <term><option>[]</option></term>
285 <para>Matches any single character specified within the brackets. For
286 example, the pattern string 'tty[SR]' would match either 'ttyS' or 'ttyR'.
287 Ranges are also supported within this match with the '-' character.
288 For example, to match on the range of all digits, the pattern [0-9] would
289 be used. If the first character following the '[' is a '!', any characters
290 not enclosed are matched.</para>
295 <para>The following keys can get values assigned:</para>
298 <term><option>NAME</option></term>
300 <para>The name, a network interface should be renamed to, or the name
301 a device node should be named. Usually the kernel provides the defined
302 node name, or even creates and removes the node before udev receives
303 any event. Changing the node name from the kernel's default may result
304 in unexpected behavior and is not supported. Udev is only expected to
305 handle device node permissions and to create additional symlinks, which
306 do not conflict with the kernel device node names.</para>
311 <term><option>SYMLINK</option></term>
313 <para>The name of a symlink targeting the node. Every matching rule will add
314 this value to the list of symlinks to be created along with the device node.
315 Multiple symlinks may be specified by separating the names by the space
316 character. In case multiple devices claim the same name, the link will
317 always point to the device with the highest link_priority. If the current device
318 goes away, the links will be re-evaluated and the device with the next highest
319 link_priority will own the link. If no link_priority is specified, the order
320 of the devices, and which of them will own the link, is undefined. Claiming
321 the same name for a node and links may result in unexpected behavior and is
328 <term><option>OWNER, GROUP, MODE</option></term>
330 <para>The permissions for the device node. Every specified value overwrites
331 the compiled-in default value.</para>
336 <term><option>ATTR{<replaceable>key</replaceable>}</option></term>
338 <para>The value that should be written to a sysfs attribute of the
344 <term><option>ENV{<replaceable>key</replaceable>}</option></term>
346 <para>Set a device property value. Property names with a leading '.'
347 are not stored in the database or exported to external tool or events.</para>
352 <term><option>TAG</option></term>
354 <para>Attach a tag to a device. This is used to filter events for users
355 of libudev's monitor functionality, or to enumerate a group of tagged
356 devices. The implementation can only work efficiently if only a few
357 tags are attached to a device. It is only meant to be used in
358 contexts with specific device filter requirements, and not as a
359 general-purpose flag. Excessive use might result in inefficient event
365 <term><option>RUN</option></term>
367 <para>Add a program to the list of programs to be executed for a specific
368 device. This can only be used for very short running tasks. Running an
369 event process for a long period of time may block all further events for
370 this or a dependent device. Long running tasks need to be immediately
371 detached from the event process itself. If the option
372 <option>RUN{<replaceable>fail_event_on_error</replaceable>}</option> is
373 specified, and the executed program returns non-zero, the event will be
374 marked as failed for a possible later handling.</para>
375 <para>If no path is given, the program must be in
376 <filename>/lib/udev</filename>, otherwise the full path must be
378 <para>If the specified string starts with
379 <option>socket:<replaceable>path</replaceable></option>, all current event
380 values will be passed to the specified socket, as a message in the same
381 format the kernel sends an uevent. If the first character of the specified path
382 is an @ character, an abstract namespace socket is used, instead of an existing
384 <para>Program name and arguments are separated with spaces. To
385 include spaces in an argument, use single quotes. Please note
386 that this does not run through a shell.</para>
391 <term><option>LABEL</option></term>
393 <para>Named label where a GOTO can jump to.</para>
398 <term><option>GOTO</option></term>
400 <para>Jumps to the next LABEL with a matching name</para>
405 <term><option>IMPORT{<replaceable>type</replaceable>}</option></term>
407 <para>Import a set of variables as device properties,
408 depending on <replaceable>type</replaceable>:</para>
411 <term><option>program</option></term>
413 <para>Execute an external program specified as the assigned value and
414 import its output, which must be in environment key
415 format. Path specification, command/argument separation,
416 and quoting work like in <option>RUN</option>.</para>
420 <term><option>file</option></term>
422 <para>Import a text file specified as the assigned value, which must be in
423 environment key format.</para>
427 <term><option>db</option></term>
429 <para>Import a single property specified as the assigned value from the
430 current device database. This works only if the database is already populated
431 by an earlier event.</para>
435 <term><option>parent</option></term>
437 <para>Import the stored keys from the parent device by reading
438 the database entry of the parent device. The value assigned to
439 <option>IMPORT{parent}</option> is used as a filter of key names
440 to import (with the same shell-style pattern matching used for
445 <para>If no option is given, udev will choose between <option>program</option>
446 and <option>file</option> based on the executable bit of the file
452 <term><option>WAIT_FOR</option></term>
454 <para>Wait for a file to become available or until a 10
455 seconds timeout expires.</para>
460 <term><option>OPTIONS</option></term>
462 <para>Rule and device options:</para>
465 <term><option>link_priority=<replaceable>value</replaceable></option></term>
467 <para>Specify the priority of the created symlinks. Devices with higher
468 priorities overwrite existing symlinks of other devices. The default is 0.</para>
472 <term><option>event_timeout=</option></term>
474 <para>Number of seconds an event will wait for operations to finish, before it
475 will terminate itself.</para>
479 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
481 <para>Usually control and other possibly unsafe characters are replaced
482 in strings used for device naming. The mode of replacement can be specified
483 with this option.</para>
487 <term><option>watch</option></term>
489 <para>Watch the device node with inotify, when closed after being opened for
490 writing, a change uevent will be synthesised.</para>
498 <para>The <option>NAME</option>, <option>SYMLINK</option>, <option>PROGRAM</option>,
499 <option>OWNER</option>, <option>GROUP</option>, <option>MODE</option> and <option>RUN</option>
500 fields support simple printf-like string substitutions. The <option>RUN</option>
501 format chars gets applied after all rules have been processed, right before the program
502 is executed. It allows the use of device properties set by earlier matching
503 rules. For all other fields, substitutions are applied while the individual rule is
504 being processed. The available substitutions are:</para>
507 <term><option>$kernel</option>, <option>%k</option></term>
509 <para>The kernel name for this device.</para>
514 <term><option>$number</option>, <option>%n</option></term>
516 <para>The kernel number for this device. For example, 'sda3' has
517 kernel number of '3'</para>
522 <term><option>$devpath</option>, <option>%p</option></term>
524 <para>The devpath of the device.</para>
529 <term><option>$id</option>, <option>%b</option></term>
531 <para>The name of the device matched while searching the devpath upwards for
532 <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
538 <term><option>$driver</option></term>
540 <para>The driver name of the device matched while searching the devpath upwards for
541 <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
547 <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
549 <para>The value of a sysfs attribute found at the device, where
550 all keys of the rule have matched. If the matching device does not have
551 such an attribute, follow the chain of parent devices and use the value
552 of the first attribute that matches.
553 If the attribute is a symlink, the last element of the symlink target is
554 returned as the value.</para>
559 <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
561 <para>A device property value.</para>
566 <term><option>$major</option>, <option>%M</option></term>
568 <para>The kernel major number for the device.</para>
573 <term><option>$minor</option>, <option>%m</option></term>
575 <para>The kernel minor number for the device.</para>
580 <term><option>$result</option>, <option>%c</option></term>
582 <para>The string returned by the external program requested with PROGRAM.
583 A single part of the string, separated by a space character may be selected
584 by specifying the part number as an attribute: <option>%c{N}</option>.
585 If the number is followed by the '+' char this part plus all remaining parts
586 of the result string are substituted: <option>%c{N+}</option></para>
591 <term><option>$parent</option>, <option>%P</option></term>
593 <para>The node name of the parent device.</para>
598 <term><option>$name</option></term>
600 <para>The current name of the device node. If not changed by a rule, it is the
601 name of the kernel device.</para>
606 <term><option>$links</option></term>
608 <para>The current list of symlinks, separated by a space character. The value is
609 only set if an earlier rule assigned a value, or during a remove events.</para>
614 <term><option>$root</option>, <option>%r</option></term>
616 <para>The udev_root value.</para>
621 <term><option>$sys</option>, <option>%S</option></term>
623 <para>The sysfs mount point.</para>
628 <term><option>$tempnode</option>, <option>%N</option></term>
630 <para>The name of a created temporary device node to provide access to
631 the device from a external program before the real node is created.</para>
636 <term><option>%%</option></term>
638 <para>The '%' character itself.</para>
643 <term><option>$$</option></term>
645 <para>The '$' character itself.</para>
652 <refsect1><title>AUTHOR</title>
653 <para>Written by Greg Kroah-Hartman <email>greg@kroah.com</email> and
654 Kay Sievers <email>kay.sievers@vrfy.org</email>. With much help from
655 Dan Stekloff and many others.</para>
659 <title>SEE ALSO</title>
661 <refentrytitle>udevd</refentrytitle><manvolnum>8</manvolnum>
664 <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
665 </citerefentry></para>