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11 <date>August 2005</date>
12 <productname>udev</productname>
16 <refentrytitle>udev</refentrytitle>
17 <manvolnum>7</manvolnum>
18 <refmiscinfo class="version"></refmiscinfo>
22 <refname>udev</refname>
23 <refpurpose>dynamic device management</refpurpose>
26 <refsect1><title>DESCRIPTION</title>
27 <para>udev provides a dynamic device directory containing only the files for
28 actually present devices. It creates or removes device node files in the
29 <filename>/dev</filename> directory, or it renames network interfaces.</para>
31 <para>Usually udev runs as <citerefentry><refentrytitle>udevd</refentrytitle>
32 <manvolnum>8</manvolnum></citerefentry> and receives uevents directly from the
33 kernel if a device is added or removed from the system.</para>
35 <para>If udev receives a device event, it matches its configured rules
36 against the available device attributes provided in sysfs to identify the device.
37 Rules that match may provide additional device information or specify a device
38 node name and multiple symlink names and instruct udev to run additional programs
39 as part of the device event handling.</para>
42 <refsect1><title>CONFIGURATION</title>
43 <para>udev configuration files are placed in <filename>/etc/udev/</filename>
44 and <filename>/lib/udev/</filename>. All empty lines, or lines beginning with
45 '#' will be ignored.</para>
47 <refsect2><title>Configuration file</title>
48 <para>udev expects its main configuration file at <filename>/etc/udev/udev.conf</filename>.
49 It consists of a set of variables allowing the user to override default udev values.
50 The following variables can be set:</para>
53 <term><option>udev_root</option></term>
55 <para>Specifies where to place the device nodes in the filesystem.
56 The default value is <filename>/dev</filename>.</para>
61 <term><option>udev_log</option></term>
63 <para>The logging priority. Valid values are the numerical syslog priorities
64 or their textual representations: <option>err</option>, <option>info</option>
65 and <option>debug</option>.</para>
71 <refsect2><title>Rules files</title>
72 <para>The udev rules are read from the files located in the
73 default rules directory <filename>/lib/udev/rules.d/</filename>,
74 the custom rules directory <filename>/etc/udev/rules.d/</filename>
75 and the temporary rules directory <filename>/dev/.udev/rules.d/</filename>.
76 All rule files are sorted and processed in lexical order, regardless
77 in which of these directories they live.</para>
79 <para>Rule files are required to have a unique name, duplicate file names
80 are ignored. Files in <filename>/etc/udev/rules.d/</filename> have precedence
81 over files with the same name in <filename>/lib/udev/rules.d/</filename>. This
82 can be used to ignore a default rules file if needed.</para>
84 <para>Every line in the rules file contains at least one key value pair.
85 There are two kind of keys, match and assignment keys.
86 If all match keys are matching against its value, the rule gets applied and the
87 assign keys get the specified value assigned.</para>
89 <para>A matching rule may specify the name of the device node, add a symlink
90 pointing to the node, or run a specified program as part of the event handling.
91 If no matching rule is found, the default device node name is used.</para>
93 <para>A rule may consist of a list of one or more key value pairs separated by
94 a comma. Each key has a distinct operation, depending on the used operator. Valid
98 <term><option>==</option></term>
100 <para>Compare for equality.</para>
105 <term><option>!=</option></term>
107 <para>Compare for non-equality.</para>
112 <term><option>=</option></term>
114 <para>Assign a value to a key. Keys that represent a list, are reset
115 and only this single value is assigned.</para>
120 <term><option>+=</option></term>
122 <para>Add the value to a key that holds a list of entries.</para>
127 <term><option>:=</option></term>
129 <para>Assign a value to a key finally; disallow any later changes,
130 which may be used to prevent changes by any later rules.</para>
135 <para>The following key names can be used to match against device properties.
136 Some of the keys also match against properties of the parent devices in sysfs,
137 not only the device that has generated the event. If multiple keys that match
138 a parent device are specified in a single rule, all these keys must match at
139 one and the same parent device.</para>
142 <term><option>ACTION</option></term>
144 <para>Match the name of the event action.</para>
149 <term><option>DEVPATH</option></term>
151 <para>Match the devpath of the event device.</para>
156 <term><option>KERNEL</option></term>
158 <para>Match the name of the event device.</para>
163 <term><option>NAME</option></term>
165 <para>Match the name of the node or network interface. It can
166 be used once the NAME key has been set in one of the preceding
172 <term><option>SUBSYSTEM</option></term>
174 <para>Match the subsystem of the event device.</para>
178 <term><option>DRIVER</option></term>
180 <para>Match the driver name of the event device. Only set for devices
181 which are bound to a driver at the time the event is generated.</para>
185 <term><option>ATTR{<replaceable>filename</replaceable>}</option></term>
187 <para>Match sysfs attribute values of the event device. Trailing
188 whitespace in the attribute values is ignored, if the specified match
189 value does not contain trailing whitespace itself.
195 <term><option>KERNELS</option></term>
197 <para>Search the devpath upwards for a matching device name.</para>
202 <term><option>SUBSYSTEMS</option></term>
204 <para>Search the devpath upwards for a matching device subsystem name.</para>
209 <term><option>DRIVERS</option></term>
211 <para>Search the devpath upwards for a matching device driver name.</para>
216 <term><option>ATTRS{<replaceable>filename</replaceable>}</option></term>
218 <para>Search the devpath upwards for a device with matching sysfs attribute values.
219 If multiple <option>ATTRS</option> matches are specified, all of them
220 must match on the same device. Trailing whitespace in the attribute values is ignored,
221 if the specified match value does not contain trailing whitespace itself.</para>
226 <term><option>ENV{<replaceable>key</replaceable>}</option></term>
228 <para>Match against a device property value.</para>
233 <term><option>TEST{<replaceable>octal mode mask</replaceable>}</option></term>
235 <para>Test the existence of a file. An octal mode mask can be specified
241 <term><option>PROGRAM</option></term>
243 <para>Execute a program. The key is true, if the program returns
244 successfully. The device properties are made available to the
245 executed program in the environment. The program's output printed to
246 stdout, is available in the RESULT key.</para>
251 <term><option>RESULT</option></term>
253 <para>Match the returned string of the last PROGRAM call. This key can
254 be used in the same or in any later rule after a PROGRAM call.</para>
259 <para>Most of the fields support a shell style pattern matching. The following
260 pattern characters are supported:</para>
263 <term><option>*</option></term>
265 <para>Matches zero, or any number of characters.</para>
269 <term><option>?</option></term>
271 <para>Matches any single character.</para>
275 <term><option>[]</option></term>
277 <para>Matches any single character specified within the brackets. For
278 example, the pattern string 'tty[SR]' would match either 'ttyS' or 'ttyR'.
279 Ranges are also supported within this match with the '-' character.
280 For example, to match on the range of all digits, the pattern [0-9] would
281 be used. If the first character following the '[' is a '!', any characters
282 not enclosed are matched.</para>
287 <para>The following keys can get values assigned:</para>
290 <term><option>NAME</option></term>
292 <para>The name of the node to be created, or the name the network interface
293 should be renamed to. Only one rule can set the node name, all later rules with
294 a NAME key will be ignored.</para>
299 <term><option>SYMLINK</option></term>
301 <para>The name of a symlink targeting the node. Every matching rule can add
302 this value to the list of symlinks to be created along with the device node.
303 Multiple symlinks may be specified by separating the names by the space
309 <term><option>OWNER, GROUP, MODE</option></term>
311 <para>The permissions for the device node. Every specified value overwrites
312 the compiled-in default value.</para>
317 <term><option>ATTR{<replaceable>key</replaceable>}</option></term>
319 <para>The value that should be written to a sysfs attribute of the
325 <term><option>ENV{<replaceable>key</replaceable>}</option></term>
327 <para>Set a device property value.</para>
332 <term><option>RUN</option></term>
334 <para>Add a program to the list of programs to be executed for a specific
335 device. This can only be used for very short running tasks. Running an
336 event process for a long period of time may block all further events for
337 this or a dependent device. Long running tasks need to be immediately
338 detached from the event process itself.</para>
339 <para>If the specifiefd string starts with
340 <option>socket:<replaceable>path</replaceable></option>, all current event
341 values will be passed to the specified socket, as a message in the same
342 format the kernel sends an uevent. If the first character of the specified path
343 is an @ character, an abstract namespace socket is used, instead of an existing
349 <term><option>LABEL</option></term>
351 <para>Named label where a GOTO can jump to.</para>
356 <term><option>GOTO</option></term>
358 <para>Jumps to the next LABEL with a matching name</para>
363 <term><option>IMPORT{<replaceable>type</replaceable>}</option></term>
365 <para>Import a set of variables as device properties,
366 depending on <replaceable>type</replaceable>:</para>
369 <term><option>program</option></term>
371 <para>Execute an external program specified as the assigned value and
372 import its output, which must be in environment key format.</para>
376 <term><option>file</option></term>
378 <para>Import a text file specified as the assigned value, which must be in
379 environment key format.</para>
383 <term><option>parent</option></term>
385 <para>Import the stored keys from the parent device by reading
386 the database entry of the parent device. The value assigned to
387 <option>IMPORT{parent}</option> is used as a filter of key names
388 to import (with the same shell-style pattern matching used for
393 <para>If no option is given, udev will choose between <option>program</option>
394 and <option>file</option> based on the executable bit of the file
400 <term><option>WAIT_FOR</option></term>
402 <para>Wait for a file to become available.</para>
407 <term><option>OPTIONS</option></term>
409 <para>Rule and device options:</para>
412 <term><option>last_rule</option></term>
414 <para>Stops further rules application. No later rules will have
419 <term><option>ignore_device</option></term>
421 <para>Ignore this event completely.</para>
425 <term><option>ignore_remove</option></term>
427 <para>Do not remove the device node when the device goes away. This may be
428 useful as a workaround for broken device drivers.</para>
432 <term><option>link_priority=<replaceable>value</replaceable></option></term>
434 <para>Specify the priority of the created symlinks. Devices with higher
435 priorities overwrite existing symlinks of other devices. The default is 0.</para>
439 <term><option>all_partitions</option></term>
441 <para>Create the device nodes for all available partitions of a block device.
442 This may be useful for removable media devices where media changes are not
447 <term><option>event_timeout=</option></term>
449 <para>Number of seconds an event will wait for operations to finish, before it
450 will terminate itself.</para>
454 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
456 <para>Usually control and other possibly unsafe characters are replaced
457 in strings used for device naming. The mode of replacement can be specified
458 with this option.</para>
466 <para>The <option>NAME</option>, <option>SYMLINK</option>, <option>PROGRAM</option>,
467 <option>OWNER</option>, <option>GROUP</option>, <option>MODE</option> and <option>RUN</option>
468 fields support simple printf-like string substitutions. The <option>RUN</option>
469 format chars gets applied after all rules have been processed, right before the program
470 is executed. It allows the use of device properties set by earlier matching
471 rules. For all other fields, substitutions are applied while the individual rule is
472 being processed. The available substitutions are:</para>
475 <term><option>$kernel</option>, <option>%k</option></term>
477 <para>The kernel name for this device.</para>
482 <term><option>$number</option>, <option>%n</option></term>
484 <para>The kernel number for this device. For example, 'sda3' has
485 kernel number of '3'</para>
490 <term><option>$devpath</option>, <option>%p</option></term>
492 <para>The devpath of the device.</para>
497 <term><option>$id</option>, <option>%b</option></term>
499 <para>The name of the device matched while searching the devpath upwards for
500 <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
506 <term><option>$driver</option></term>
508 <para>The driver name of the device matched while searching the devpath upwards for
509 <option>SUBSYSTEMS</option>, <option>KERNELS</option>, <option>DRIVERS</option> and <option>ATTRS</option>.
515 <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
517 <para>The value of a sysfs attribute found at the device, where
518 all keys of the rule have matched. If the matching device does not have
519 such an attribute, follow the chain of parent devices and use the value
520 of the first attribute that matches.
521 If the attribute is a symlink, the last element of the symlink target is
522 returned as the value.</para>
527 <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
529 <para>A device property value.</para>
534 <term><option>$major</option>, <option>%M</option></term>
536 <para>The kernel major number for the device.</para>
541 <term><option>$minor</option>, <option>%m</option></term>
543 <para>The kernel minor number for the device.</para>
548 <term><option>$result</option>, <option>%c</option></term>
550 <para>The string returned by the external program requested with PROGRAM.
551 A single part of the string, separated by a space character may be selected
552 by specifying the part number as an attribute: <option>%c{N}</option>.
553 If the number is followed by the '+' char this part plus all remaining parts
554 of the result string are substituted: <option>%c{N+}</option></para>
559 <term><option>$parent</option>, <option>%P</option></term>
561 <para>The node name of the parent device.</para>
566 <term><option>$name</option></term>
568 <para>The current name of the device node. If not changed by a rule, it is the
569 name of the kernel device.</para>
574 <term><option>$links</option></term>
576 <para>The current list of symlinks, separated by a space character. The value is
577 only set if an earlier rule assigned a value, or during a remove events.</para>
582 <term><option>$root</option>, <option>%r</option></term>
584 <para>The udev_root value.</para>
589 <term><option>$sys</option>, <option>%S</option></term>
591 <para>The sysfs mount point.</para>
596 <term><option>$tempnode</option>, <option>%N</option></term>
598 <para>The name of a created temporary device node to provide access to
599 the device from a external program before the real node is created.</para>
604 <term><option>%%</option></term>
606 <para>The '%' character itself.</para>
611 <term><option>$$</option></term>
613 <para>The '$' character itself.</para>
617 <para>The count of characters to be substituted may be limited by specifying
618 the format length value. For example, '%3s{file}' will only
619 insert the first three characters of the sysfs attribute</para>
623 <refsect1><title>AUTHOR</title>
624 <para>Written by Greg Kroah-Hartman <email>greg@kroah.com</email> and
625 Kay Sievers <email>kay.sievers@vrfy.org</email>. With much help from
626 Dan Stekloff and many others.</para>
630 <title>SEE ALSO</title>
632 <refentrytitle>udevd</refentrytitle><manvolnum>8</manvolnum>
635 <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
636 </citerefentry></para>