1UDEV(7) udev UDEV(7)
2
6 udev - dynamic device management
7
9 udev provides a dynamic device directory containing only the files for
10 actually present devices. It creates or removes device node files in
11 the /dev directory, or it renames network interfaces.
12 Usually udev runs as udevd(8) and receives uevents directly from the
14 kernel if a device is added or removed from the system.
15 If udev receives a device event, it matches its configured rules
17 against the available device attributes provided in sysfs to identify
18 the device. Rules that match may provide additional device information
19 or specify a device node name and multiple symlink names and instruct
20 udev to run additional programs as part of the device event handling.
21
23 udev configuration files are placed in /etc/udev/ and /lib/udev/. All
24 empty lines, or lines beginning with ´#´ will be ignored.
25 Configuration file
27 udev expects its main configuration file at /etc/udev/udev.conf. It
28 consists of a set of variables allowing the user to override default
29 udev values. The following variables can be set:
30 udev_root
32 Specifies where to place the device nodes in the filesystem. The
33 default value is /dev.
34 udev_log
36 The logging priority. Valid values are the numerical syslog
37 priorities or their textual representations: err, info and debug.
38 Rules files
40 The udev rules are read from the files located in the default rules
41 directory /lib/udev/rules.d/, the custom rules directory
42 /etc/udev/rules.d/ and the temporary rules directory
43 /dev/.udev/rules.d/. All rule files are sorted and processed in lexical
44 order, regardless in which of these directories they live.
45 Rule files are required to have a unique name, duplicate file names are
47 ignored. Files in /etc/udev/rules.d/ have precedence over files with
48 the same name in /lib/udev/rules.d/. This can be used to ignore a
49 default rules file if needed.
50 Every line in the rules file contains at least one key value pair.
52 There are two kind of keys, match and assignment keys. If all match
53 keys are matching against its value, the rule gets applied and the
54 assign keys get the specified value assigned.
55 A matching rule may specify the name of the device node, add a symlink
57 pointing to the node, or run a specified program as part of the event
58 handling. If no matching rule is found, the default device node name is
59 used.
60 A rule consists of a list of one or more key value pairs separated by a
62 comma. Each key has a distinct operation, depending on the used
63 operator. Valid operators are:
64 ==
66 Compare for equality.
67 !=
69 Compare for inequality.
70 =
72 Assign a value to a key. Keys that represent a list, are reset and
73 only this single value is assigned.
74 +=
76 Add the value to a key that holds a list of entries.
77 :=
79 Assign a value to a key finally; disallow any later changes, which
80 may be used to prevent changes by any later rules.
81 The following key names can be used to match against device properties.
83 Some of the keys also match against properties of the parent devices in
84 sysfs, not only the device that has generated the event. If multiple
85 keys that match a parent device are specified in a single rule, all
86 these keys must match at one and the same parent device.
87 ACTION
89 Match the name of the event action.
90 DEVPATH
92 Match the devpath of the event device.
93 KERNEL
95 Match the name of the event device.
96 NAME
98 Match the name of the node or network interface. It can be used
99 once the NAME key has been set in one of the preceding rules.
100 SYMLINK
102 Match the name of a symlink targeting the node. It can be used once
103 a SYMLINK key has been set in one of the preceding rules. There may
104 be multiple symlinks; only one needs to match.
105 SUBSYSTEM
107 Match the subsystem of the event device.
108 DRIVER
110 Match the driver name of the event device. Only set for devices
111 which are bound to a driver at the time the event is generated.
112 SECLABEL{module}
114 Applies the specified Linux Security Module label to the device
115 node. Note that at this time only selinux is supported.
116 ATTR{filename}
118 Match sysfs attribute values of the event device. Trailing
119 whitespace in the attribute values is ignored, if the specified
120 match value does not contain trailing whitespace itself.
121 KERNELS
123 Search the devpath upwards for a matching device name.
124 SUBSYSTEMS
126 Search the devpath upwards for a matching device subsystem name.
127 DRIVERS
129 Search the devpath upwards for a matching device driver name.
130 ATTRS{filename}
132 Search the devpath upwards for a device with matching sysfs
133 attribute values. If multiple ATTRS matches are specified, all of
134 them must match on the same device. Trailing whitespace in the
135 attribute values is ignored, if the specified match value does not
136 contain trailing whitespace itself.
137 ENV{key}
139 Match against a device property value.
140 TEST{octal mode mask}
142 Test the existence of a file. An octal mode mask can be specified
143 if needed.
144 PROGRAM
146 Execute a program. The key is true, if the program returns
147 successfully. The device properties are made available to the
148 executed program in the environment. The program´s output printed
149 to stdout, is available in the RESULT key.
150 RESULT
152 Match the returned string of the last PROGRAM call. This key can be
153 used in the same or in any later rule after a PROGRAM call.
154 Most of the fields support a shell style pattern matching. The
156 following pattern characters are supported:
157 *
159 Matches zero, or any number of characters.
160 ?
162 Matches any single character.
163 []
165 Matches any single character specified within the brackets. For
166 example, the pattern string ´tty[SR]´ would match either ´ttyS´ or
167 ´ttyR´. Ranges are also supported within this match with the ´-´
168 character. For example, to match on the range of all digits, the
169 pattern [0-9] would be used. If the first character following the
170 ´[´ is a ´!´, any characters not enclosed are matched.
171 The following keys can get values assigned:
173 NAME
175 The name of the node to be created, or the name the network
176 interface should be renamed to.
177 SYMLINK
179 The name of a symlink targeting the node. Every matching rule adds
180 this value to the list of symlinks to be created.
181 The set of characters to name a symlink is limited. Allowed
183 characters are [0-9A-Za-z#+-.:=@_/], valid utf8 character
184 sequences, and "\x00" hex encoding. All other characters are
185 replaced by a _ character.
186 Multiple symlinks may be specified by separating the names by the
188 space character. In case multiple devices claim the same name, the
189 link always points to the device with the highest link_priority. If
190 the current device goes away, the links are re-evaluated and the
191 device with the next highest link_priority becomes the owner of the
192 link. If no link_priority is specified, the order of the devices
193 (and which one of them owns the link) is undefined.
194 Symlink names must never conflict with the kernels default device
196 node names, as that would result in unpredictable behavior.
197 OWNER, GROUP, MODE
199 The permissions for the device node. Every specified value
200 overwrites the compiled-in default value.
201 ATTR{key}
203 The value that should be written to a sysfs attribute of the event
204 device.
205 ENV{key}
207 Set a device property value. Property names with a leading ´.´ are
208 not stored in the database or exported to external tool or events.
209 RUN
211 Add a program to the list of programs to be executed for a specific
212 device. This can only be used for very short running tasks. Running
213 an event process for a long period of time may block all further
214 events for this or a dependent device. Long running tasks need to
215 be immediately detached from the event process itself. If the
216 option RUN{fail_event_on_error} is specified, and the executed
217 program returns non-zero, the event will be marked as failed for a
218 possible later handling.
219 If the specified string starts with socket:path, all current event
221 values will be passed to the specified socket, as a message in the
222 same format the kernel sends an uevent. If the first character of
223 the specified path is an @ character, an abstract namespace socket
224 is used, instead of an existing socket file.
225 LABEL
227 Named label where a GOTO can jump to.
228 GOTO
230 Jumps to the next LABEL with a matching name
231 IMPORT{type}
233 Import a set of variables as device properties, depending on type:
234 program
236 Execute an external program specified as the assigned value and
237 import its output, which must be in environment key format.
238 file
240 Import a text file specified as the assigned value, which must
241 be in environment key format.
242 db
244 Import a single property specified as the assigned value from
245 the current device database. This works only if the database is
246 already populated by an earlier event.
247 cmdline
249 Import a single property from the kernel commandline. For
250 simple flags the value of the property will be set to ´1´.
251 parent
253 Import the stored keys from the parent device by reading the
254 database entry of the parent device. The value assigned to
255 IMPORT{parent} is used as a filter of key names to import (with
256 the same shell-style pattern matching used for comparisons).
257 If no option is given, udev will choose between program and file
259 based on the executable bit of the file permissions.
260 WAIT_FOR
262 Wait for a file to become available.
263 OPTIONS
265 Rule and device options:
266 ignore_device
268 Ignore this event completely.
269 ignore_remove
271 Do not remove the device node when the device goes away. This
272 may be useful as a workaround for broken device drivers.
273 link_priority=value
275 Specify the priority of the created symlinks. Devices with
276 higher priorities overwrite existing symlinks of other devices.
277 The default is 0.
278 all_partitions
280 Create the device nodes for all available partitions of a block
281 device. This may be useful for removable media devices where
282 media changes are not detected.
283 event_timeout=
285 Number of seconds an event will wait for operations to finish,
286 before it will terminate itself.
287 string_escape=none|replace
289 Usually control and other possibly unsafe characters are
290 replaced in strings used for device naming. The mode of
291 replacement can be specified with this option.
292 watch
294 Watch the device node with inotify, when closed after being
295 opened for writing, a change uevent will be synthesised.
296 nowatch
298 Disable the watching of a device node with inotify.
299 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE and RUN fields support
301 simple printf-like string substitutions. The RUN format chars gets
302 applied after all rules have been processed, right before the program
303 is executed. It allows the use of device properties set by earlier
304 matching rules. For all other fields, substitutions are applied while
305 the individual rule is being processed. The available substitutions
306 are:
307 $kernel, %k
309 The kernel name for this device.
310 $number, %n
312 The kernel number for this device. For example, ´sda3´ has kernel
313 number of ´3´
314 $devpath, %p
316 The devpath of the device.
317 $id, %b
319 The name of the device matched while searching the devpath upwards
320 for SUBSYSTEMS, KERNELS, DRIVERS and ATTRS.
321 $driver
323 The driver name of the device matched while searching the devpath
324 upwards for SUBSYSTEMS, KERNELS, DRIVERS and ATTRS.
325 $attr{file}, %s{file}
327 The value of a sysfs attribute found at the device, where all keys
328 of the rule have matched. If the matching device does not have such
329 an attribute, follow the chain of parent devices and use the value
330 of the first attribute that matches. If the attribute is a symlink,
331 the last element of the symlink target is returned as the value.
332 $env{key}, %E{key}
334 A device property value.
335 $major, %M
337 The kernel major number for the device.
338 $minor, %m
340 The kernel minor number for the device.
341 $result, %c
343 The string returned by the external program requested with PROGRAM.
344 A single part of the string, separated by a space character may be
345 selected by specifying the part number as an attribute: %c{N}. If
346 the number is followed by the ´+´ char this part plus all remaining
347 parts of the result string are substituted: %c{N+}
348 $parent, %P
350 The node name of the parent device.
351 $name
353 The current name of the device node. If not changed by a rule, it
354 is the name of the kernel device.
355 $links
357 The current list of symlinks, separated by a space character. The
358 value is only set if an earlier rule assigned a value, or during a
359 remove events.
360 $root, %r
362 The udev_root value.
363 $sys, %S
365 The sysfs mount point.
366 $tempnode, %N
368 The name of a created temporary device node to provide access to
369 the device from a external program before the real node is created.
370 %%
372 The ´%´ character itself.
373 $$
375 The ´$´ character itself.
376
378 Written by Greg Kroah-Hartman greg@kroah.com and Kay Sievers
379 kay.sievers@vrfy.org. With much help from Dan Stekloff and many others.
380
382 udevd(8), udevadm(8)
383udev 09/06/2016 UDEV(7)