1UDEV(7) udev UDEV(7)
2
6 udev - Dynamic device management
7
9 udev supplies the system software with device events, manages
10 permissions of device nodes and may create additional symlinks in the
11 /dev directory, or renames network interfaces. The kernel usually just
12 assigns unpredictable device names based on the order of discovery.
13 Meaningful symlinks or network device names provide a way to reliably
14 identify devices based on their properties or current configuration.
15 The udev daemon, systemd-udevd.service(8), receives device uevents
17 directly from the kernel whenever a device is added or removed from the
18 system, or it changes its state. When udev receives a device event, it
19 matches its configured set of rules against various device attributes
20 to identify the device. Rules that match may provide additional device
21 information to be stored in the udev database or to be used to create
22 meaningful symlink names.
23 All device information udev processes is stored in the udev database
25 and sent out to possible event subscribers. Access to all stored data
26 and the event sources is provided by the library libudev.
27
29 The udev rules are read from the files located in the system rules
30 directory /usr/lib/udev/rules.d, the volatile runtime directory
31 /run/udev/rules.d and the local administration directory
32 /etc/udev/rules.d. All rules files are collectively sorted and
33 processed in lexical order, regardless of the directories in which they
34 live. However, files with identical filenames replace each other. Files
35 in /etc have the highest priority, files in /run take precedence over
36 files with the same name in /usr/lib. This can be used to override a
37 system-supplied rules file with a local file if needed; a symlink in
38 /etc with the same name as a rules file in /usr/lib, pointing to
39 /dev/null, disables the rules file entirely. Rule files must have the
40 extension .rules; other extensions are ignored.
41 Every line in the rules file contains at least one key-value pair.
43 Except for empty lines or lines beginning with "#", which are ignored.
44 There are two kinds of keys: match and assignment. If all match keys
45 match against their values, the rule gets applied and the assignment
46 keys get the specified values assigned.
47 A matching rule may rename a network interface, add symlinks pointing
49 to the device node, or run a specified program as part of the event
50 handling.
51 A rule consists of a comma-separated list of one or more key-value
53 pairs. Each key has a distinct operation, depending on the used
54 operator. Valid operators are:
55 "=="
57 Compare for equality.
58 "!="
60 Compare for inequality.
61 "="
63 Assign a value to a key. Keys that represent a list are reset and
64 only this single value is assigned.
65 "+="
67 Add the value to a key that holds a list of entries.
68 "-="
70 Remove the value from a key that holds a list of entries.
71 ":="
73 Assign a value to a key finally; disallow any later changes.
74 The following key names can be used to match against device properties.
76 Some of the keys also match against properties of the parent devices in
77 sysfs, not only the device that has generated the event. If multiple
78 keys that match a parent device are specified in a single rule, all
79 these keys must match at one and the same parent device.
80 ACTION
82 Match the name of the event action.
83 DEVPATH
85 Match the devpath of the event device.
86 KERNEL
88 Match the name of the event device.
89 NAME
91 Match the name of a network interface. It can be used once the NAME
92 key has been set in one of the preceding rules.
93 SYMLINK
95 Match the name of a symlink targeting the node. It can be used once
96 a SYMLINK key has been set in one of the preceding rules. There may
97 be multiple symlinks; only one needs to match.
98 SUBSYSTEM
100 Match the subsystem of the event device.
101 DRIVER
103 Match the driver name of the event device. Only set this key for
104 devices which are bound to a driver at the time the event is
105 generated.
106 ATTR{filename}, SYSCTL{kernel parameter}
108 Match sysfs attribute values of the event device. Trailing
109 whitespace in the attribute values is ignored unless the specified
110 match value itself contains trailing whitespace. Match a kernel
111 parameter value.
112 KERNELS
114 Search the devpath upwards for a matching device name.
115 SUBSYSTEMS
117 Search the devpath upwards for a matching device subsystem name.
118 DRIVERS
120 Search the devpath upwards for a matching device driver name.
121 ATTRS{filename}
123 Search the devpath upwards for a device with matching sysfs
124 attribute values. If multiple ATTRS matches are specified, all of
125 them must match on the same device. Trailing whitespace in the
126 attribute values is ignored unless the specified match value itself
127 contains trailing whitespace.
128 TAGS
130 Search the devpath upwards for a device with matching tag.
131 ENV{key}
133 Match against a device property value.
134 TAG
136 Match against a device tag.
137 TEST{octal mode mask}
139 Test the existence of a file. An octal mode mask can be specified
140 if needed.
141 PROGRAM
143 Execute a program to determine whether there is a match; the key is
144 true if the program returns successfully. The device properties are
145 made available to the executed program in the environment. The
146 program's standard output is available in the RESULT key.
147 This can only be used for very short-running foreground tasks. For
149 details, see RUN.
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 shell glob pattern matching and alternate
156 patterns. The following special characters are supported:
157 "*"
159 Matches zero or more 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 via the "-" character. For
168 example, to match on the range of all digits, the pattern "[0-9]"
169 could be used. If the first character following the "[" is a "!",
170 any characters not enclosed are matched.
171 "|"
173 Separates alternative patterns. For example, the pattern string
174 "abc|x*" would match either "abc" or "x*".
175 The following keys can get values assigned:
177 NAME
179 The name to use for a network interface. See systemd.link(5) for a
180 higher-level mechanism for setting the interface name. The name of
181 a device node cannot be changed by udev, only additional symlinks
182 can be created.
183 SYMLINK
185 The name of a symlink targeting the node. Every matching rule adds
186 this value to the list of symlinks to be created.
187 The set of characters to name a symlink is limited. Allowed
189 characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
190 sequences, and "\x00" hex encoding. All other characters are
191 replaced by a "_" character.
192 Multiple symlinks may be specified by separating the names by the
194 space character. In case multiple devices claim the same name, the
195 link always points to the device with the highest link_priority. If
196 the current device goes away, the links are re-evaluated and the
197 device with the next highest link_priority becomes the owner of the
198 link. If no link_priority is specified, the order of the devices
199 (and which one of them owns the link) is undefined.
200 Symlink names must never conflict with the kernel's default device
202 node names, as that would result in unpredictable behavior.
203 OWNER, GROUP, MODE
205 The permissions for the device node. Every specified value
206 overrides the compiled-in default value.
207 SECLABEL{module}
209 Applies the specified Linux Security Module label to the device
210 node.
211 ATTR{key}
213 The value that should be written to a sysfs attribute of the event
214 device.
215 SYSCTL{kernel parameter}
217 The value that should be written to kernel parameter.
218 ENV{key}
220 Set a device property value. Property names with a leading "." are
221 neither stored in the database nor exported to events or external
222 tools (run by, for example, the PROGRAM match key).
223 TAG
225 Attach a tag to a device. This is used to filter events for users
226 of libudev's monitor functionality, or to enumerate a group of
227 tagged devices. The implementation can only work efficiently if
228 only a few tags are attached to a device. It is only meant to be
229 used in contexts with specific device filter requirements, and not
230 as a general-purpose flag. Excessive use might result in
231 inefficient event handling.
232 RUN{type}
234 Add a program to the list of programs to be executed after
235 processing all the rules for a specific event, depending on "type":
236 "program"
238 Execute an external program specified as the assigned value. If
239 no absolute path is given, the program is expected to live in
240 /usr/lib/udev; otherwise, the absolute path must be specified.
241 This is the default if no type is specified.
243 "builtin"
245 As program, but use one of the built-in programs rather than an
246 external one.
247 The program name and following arguments are separated by spaces.
249 Single quotes can be used to specify arguments with spaces.
250 This can only be used for very short-running foreground tasks.
252 Running an event process for a long period of time may block all
253 further events for this or a dependent device.
254 Starting daemons or other long-running processes is not appropriate
256 for udev; the forked processes, detached or not, will be
257 unconditionally killed after the event handling has finished.
258 Note that running programs that access the network or mount/unmount
260 filesystems is not allowed inside of udev rules, due to the default
261 sandbox that is enforced on systemd-udevd.service.
262 LABEL
264 A named label to which a GOTO may jump.
265 GOTO
267 Jumps to the next LABEL with a matching name.
268 IMPORT{type}
270 Import a set of variables as device properties, depending on
271 "type":
272 "program"
274 Execute an external program specified as the assigned value
275 and, if it returns successfully, import its output, which must
276 be in environment key format. Path specification,
277 command/argument separation, and quoting work like in RUN.
278 "builtin"
280 Similar to "program", but use one of the built-in programs
281 rather than an external one.
282 "file"
284 Import a text file specified as the assigned value, the content
285 of which must be in environment key format.
286 "db"
288 Import a single property specified as the assigned value from
289 the current device database. This works only if the database is
290 already populated by an earlier event.
291 "cmdline"
293 Import a single property from the kernel command line. For
294 simple flags the value of the property is set to "1".
295 "parent"
297 Import the stored keys from the parent device by reading the
298 database entry of the parent device. The value assigned to
299 IMPORT{parent} is used as a filter of key names to import (with
300 the same shell glob pattern matching used for comparisons).
301 This can only be used for very short-running foreground tasks. For
303 details see RUN.
304 WAIT_FOR
306 Wait for a file to become available or until a timeout of 10
307 seconds expires. The path is relative to the sysfs device; if no
308 path is specified, this waits for an attribute to appear.
309 OPTIONS
311 Rule and device options:
312 link_priority=value
314 Specify the priority of the created symlinks. Devices with
315 higher priorities overwrite existing symlinks of other devices.
316 The default is 0.
317 string_escape=none|replace
319 Usually, control and other possibly unsafe characters are
320 replaced in strings used for device naming. The mode of
321 replacement can be specified with this option.
322 static_node=
324 Apply the permissions specified in this rule to the static
325 device node with the specified name. Also, for every tag
326 specified in this rule, create a symlink in the directory
327 /run/udev/static_node-tags/tag pointing at the static device
328 node with the specified name. Static device node creation is
329 performed by systemd-tmpfiles before systemd-udevd is started.
330 The static nodes might not have a corresponding kernel device;
331 they are used to trigger automatic kernel module loading when
332 they are accessed.
333 watch
335 Watch the device node with inotify; when the node is closed
336 after being opened for writing, a change uevent is synthesized.
337 nowatch
339 Disable the watching of a device node with inotify.
340 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
342 fields support simple string substitutions. The RUN substitutions are
343 performed after all rules have been processed, right before the program
344 is executed, allowing for the use of device properties set by earlier
345 matching rules. For all other fields, substitutions are performed while
346 the individual rule is being processed. The available substitutions
347 are:
348 $kernel, %k
350 The kernel name for this device.
351 $number, %n
353 The kernel number for this device. For example, "sda3" has kernel
354 number "3".
355 $devpath, %p
357 The devpath of the device.
358 $id, %b
360 The name of the device matched while searching the devpath upwards
361 for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
362 $driver
364 The driver name of the device matched while searching the devpath
365 upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
366 $attr{file}, %s{file}
368 The value of a sysfs attribute found at the device where all keys
369 of the rule have matched. If the matching device does not have such
370 an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
371 test selected a parent device, then the attribute from that parent
372 device is used.
373 If the attribute is a symlink, the last element of the symlink
375 target is returned as the value.
376 $env{key}, %E{key}
378 A device property value.
379 $major, %M
381 The kernel major number for the device.
382 $minor, %m
384 The kernel minor number for the device.
385 $result, %c
387 The string returned by the external program requested with PROGRAM.
388 A single part of the string, separated by a space character, may be
389 selected by specifying the part number as an attribute: "%c{N}". If
390 the number is followed by the "+" character, this part plus all
391 remaining parts of the result string are substituted: "%c{N+}".
392 $parent, %P
394 The node name of the parent device.
395 $name
397 The current name of the device. If not changed by a rule, it is the
398 name of the kernel device.
399 $links
401 A space-separated list of the current symlinks. The value is only
402 set during a remove event or if an earlier rule assigned a value.
403 $root, %r
405 The udev_root value.
406 $sys, %S
408 The sysfs mount point.
409 $devnode, %N
411 The name of the device node.
412 %%
414 The "%" character itself.
415 $$
417 The "$" character itself.
418
420 systemd-udevd.service(8), udevadm(8), systemd.link(5)
421systemd 239 UDEV(7)