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 directories /usr/lib/udev/rules.d and /usr/local/lib/udev/rules.d, the
31 volatile runtime directory /run/udev/rules.d and the local
32 administration directory /etc/udev/rules.d. All rules files are
33 collectively sorted and processed in lexical order, regardless of the
34 directories in which they live. However, files with identical filenames
35 replace each other. Files in /etc/ have the highest priority, files in
36 /run/ take precedence over files with the same name under /usr/. This
37 can be used to override a system-supplied rules file with a local file
38 if needed; a symlink in /etc/ with the same name as a rules file in
39 /usr/lib/, pointing to /dev/null, disables the rules file entirely.
40 Rule files must have the extension .rules; other extensions are
41 ignored.
42 Every line in the rules file contains at least one key-value pair.
44 Except for empty lines or lines beginning with "#", which are ignored.
45 There are two kinds of keys: match and assignment. If all match keys
46 match against their values, the rule gets applied and the assignment
47 keys get the specified values assigned.
48 A matching rule may rename a network interface, add symlinks pointing
50 to the device node, or run a specified program as part of the event
51 handling.
52 A rule consists of a comma-separated list of one or more
54 key-operator-value expressions. Each expression has a distinct effect,
55 depending on the key and operator used.
56 Operators
58 "=="
59 Compare for equality. (The specified key has the specified value.)
60 "!="
62 Compare for inequality. (The specified key doesn't have the
63 specified value, or the specified key is not present at all.)
64 "="
66 Assign a value to a key. Keys that represent a list are reset and
67 only this single value is assigned.
68 "+="
70 Add the value to a key that holds a list of entries.
71 "-="
73 Remove the value from a key that holds a list of entries.
74 ":="
76 Assign a value to a key finally; disallow any later changes.
77 Values
79 Values are written as double quoted strings, such as ("string"). To
80 include a quotation mark (") in the value, precede it by a backslash
81 (\"). Any other occurrences of a backslash followed by a character are
82 not unescaped. That is, "\t\n" is treated as four characters:
83 backslash, lowercase t, backslash, lowercase n.
84 The string can be prefixed with a lowercase e (e"string\n") to mark the
86 string as C-style escaped[1]. For example, e"string\n" is parsed as 7
87 characters: 6 lowercase letters and a newline. This can be useful for
88 writing special characters when a kernel driver requires them.
89 Please note that NUL is not allowed in either string variant.
91 Keys
93 The following key names can be used to match against device properties.
94 Some of the keys also match against properties of the parent devices in
95 sysfs, not only the device that has generated the event. If multiple
96 keys that match a parent device are specified in a single rule, all
97 these keys must match at one and the same parent device.
98 ACTION
100 Match the name of the event action.
101 DEVPATH
103 Match the devpath of the event device.
104 KERNEL
106 Match the name of the event device.
107 KERNELS
109 Search the devpath upwards for a matching device name.
110 NAME
112 Match the name of a network interface. It can be used once the NAME
113 key has been set in one of the preceding rules.
114 SYMLINK
116 Match the name of a symlink targeting the node. It can be used once
117 a SYMLINK key has been set in one of the preceding rules. There may
118 be multiple symlinks; only one needs to match. If the operator is
119 "!=", the token returns true only if there is no symlink matched.
120 SUBSYSTEM
122 Match the subsystem of the event device.
123 SUBSYSTEMS
125 Search the devpath upwards for a matching device subsystem name.
126 DRIVER
128 Match the driver name of the event device. Only set this key for
129 devices which are bound to a driver at the time the event is
130 generated.
131 DRIVERS
133 Search the devpath upwards for a matching device driver name.
134 ATTR{filename}
136 Match sysfs attribute value of the event device.
137 Trailing whitespace in the attribute values is ignored unless the
139 specified match value itself contains trailing whitespace.
140 ATTRS{filename}
142 Search the devpath upwards for a device with matching sysfs
143 attribute values. If multiple ATTRS matches are specified, all of
144 them must match on the same device.
145 Trailing whitespace in the attribute values is ignored unless the
147 specified match value itself contains trailing whitespace.
148 SYSCTL{kernel parameter}
150 Match a kernel parameter value.
151 ENV{key}
153 Match against a device property value.
154 CONST{key}
156 Match against a system-wide constant. Supported keys are:
157 "arch"
159 System's architecture. See ConditionArchitecture= in
160 systemd.unit(5) for possible values.
161 "virt"
163 System's virtualization environment. See systemd-detect-virt(1)
164 for possible values.
165 "cvm"
167 System's confidential virtualization technology. See systemd-
168 detect-virt(1) for possible values.
169 Unknown keys will never match.
171 TAG
173 Match against one of device tags. It can be used once a TAG key has
174 been set in one of the preceding rules. There may be multiple tags;
175 only one needs to match. If the operator is "!=", the token returns
176 true only if there is no tag matched.
177 TAGS
179 Search the devpath upwards for a device with matching tag. If the
180 operator is "!=", the token returns true only if there is no tag
181 matched.
182 TEST{octal mode mask}
184 Test the existence of a file. An octal mode mask can be specified
185 if needed.
186 PROGRAM
188 Execute a program to determine whether there is a match; the key is
189 true if the program returns successfully. The device properties are
190 made available to the executed program in the environment. The
191 program's standard output is available in the RESULT key.
192 This can only be used for very short-running foreground tasks. For
194 details, see RUN.
195 Note that multiple PROGRAM keys may be specified in one rule, and
197 "=", ":=", and "+=" have the same effect as "==".
198 RESULT
200 Match the returned string of the last PROGRAM call. This key can be
201 used in the same or in any later rule after a PROGRAM call.
202 Most of the fields support shell glob pattern matching and alternate
204 patterns. The following special characters are supported:
205 "*"
207 Matches zero or more characters.
208 "?"
210 Matches any single character.
211 "[]"
213 Matches any single character specified within the brackets. For
214 example, the pattern string "tty[SR]" would match either "ttyS" or
215 "ttyR". Ranges are also supported via the "-" character. For
216 example, to match on the range of all digits, the pattern "[0-9]"
217 could be used. If the first character following the "[" is a "!",
218 any characters not enclosed are matched.
219 "|"
221 Separates alternative patterns. For example, the pattern string
222 "abc|x*" would match either "abc" or "x*".
223 The following keys can get values assigned:
225 NAME
227 The name to use for a network interface. See systemd.link(5) for a
228 higher-level mechanism for setting the interface name. The name of
229 a device node cannot be changed by udev, only additional symlinks
230 can be created.
231 SYMLINK
233 The name of a symlink targeting the node. Every matching rule adds
234 this value to the list of symlinks to be created.
235 The set of characters to name a symlink is limited. Allowed
237 characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
238 sequences, and "\x00" hex encoding. All other characters are
239 replaced by a "_" character.
240 Multiple symlinks may be specified by separating the names by the
242 space character. In case multiple devices claim the same name, the
243 link always points to the device with the highest link_priority. If
244 the current device goes away, the links are re-evaluated and the
245 device with the next highest link_priority becomes the owner of the
246 link. If no link_priority is specified, the order of the devices
247 (and which one of them owns the link) is undefined.
248 Symlink names must never conflict with the kernel's default device
250 node names, as that would result in unpredictable behavior.
251 OWNER, GROUP, MODE
253 The permissions for the device node. Every specified value
254 overrides the compiled-in default value.
255 SECLABEL{module}
257 Applies the specified Linux Security Module label to the device
258 node.
259 ATTR{key}
261 The value that should be written to a sysfs attribute of the event
262 device.
263 SYSCTL{kernel parameter}
265 The value that should be written to kernel parameter.
266 ENV{key}
268 Set a device property value. Property names with a leading "." are
269 neither stored in the database nor exported to events or external
270 tools (run by, for example, the PROGRAM match key).
271 TAG
273 Attach a tag to a device. This is used to filter events for users
274 of libudev's monitor functionality, or to enumerate a group of
275 tagged devices. The implementation can only work efficiently if
276 only a few tags are attached to a device. It is only meant to be
277 used in contexts with specific device filter requirements, and not
278 as a general-purpose flag. Excessive use might result in
279 inefficient event handling.
280 RUN{type}
282 Specify a program to be executed after processing of all the rules
283 for the event. With "+=", this invocation is added to the list, and
284 with "=" or ":=", it replaces any previous contents of the list.
285 Please note that both "program" and "builtin" types described below
286 share a common list, so clearing the list with ":=" and "=" affects
287 both types.
288 type may be:
290 "program"
292 Execute an external program specified as the assigned value. If
293 no absolute path is given, the program is expected to live in
294 /usr/lib/udev; otherwise, the absolute path must be specified.
295 This is the default if no type is specified.
297 "builtin"
299 As program, but use one of the built-in programs rather than an
300 external one.
301 The program name and following arguments are separated by spaces.
303 Single quotes can be used to specify arguments with spaces.
304 This can only be used for very short-running foreground tasks.
306 Running an event process for a long period of time may block all
307 further events for this or a dependent device.
308 Note that running programs that access the network or mount/unmount
310 filesystems is not allowed inside of udev rules, due to the default
311 sandbox that is enforced on systemd-udevd.service.
312 Starting daemons or other long-running processes is not allowed;
314 the forked processes, detached or not, will be unconditionally
315 killed after the event handling has finished. In order to activate
316 long-running processes from udev rules, provide a service unit and
317 pull it in from a udev device using the SYSTEMD_WANTS device
318 property. See systemd.device(5) for details.
319 LABEL
321 A named label to which a GOTO may jump.
322 GOTO
324 Jumps to the next LABEL with a matching name.
325 IMPORT{type}
327 Import a set of variables as device properties, depending on type:
328 "program"
330 Execute an external program specified as the assigned value
331 and, if it returns successfully, import its output, which must
332 be in environment key format. Path specification,
333 command/argument separation, and quoting work like in RUN.
334 "builtin"
336 Similar to "program", but use one of the built-in programs
337 rather than an external one.
338 "file"
340 Import a text file specified as the assigned value, the content
341 of which must be in environment key format.
342 "db"
344 Import a single property specified as the assigned value from
345 the current device database. This works only if the database is
346 already populated by an earlier event.
347 "cmdline"
349 Import a single property from the kernel command line. For
350 simple flags the value of the property is set to "1".
351 "parent"
353 Import the stored keys from the parent device by reading the
354 database entry of the parent device. The value assigned to
355 IMPORT{parent} is used as a filter of key names to import (with
356 the same shell glob pattern matching used for comparisons).
357 This can only be used for very short-running foreground tasks. For
359 details see RUN.
360 Note that multiple IMPORT{} keys may be specified in one rule, and
362 "=", ":=", and "+=" have the same effect as "==". The key is true
363 if the import is successful, unless "!=" is used as the operator
364 which causes the key to be true if the import failed.
365 OPTIONS
367 Rule and device options:
368 link_priority=value
370 Specify the priority of the created symlinks. Devices with
371 higher priorities overwrite existing symlinks of other devices.
372 The default is 0.
373 string_escape=none|replace
375 When "replace", possibly unsafe characters in strings assigned
376 to NAME, SYMLINK, and ENV{key} are replaced. When "none", no
377 replacement is performed. When unset, the replacement is
378 performed for NAME, SYMLINK, but not for ENV{key}. Defaults to
379 unset.
380 static_node=
382 Apply the permissions specified in this rule to the static
383 device node with the specified name. Also, for every tag
384 specified in this rule, create a symlink in the directory
385 /run/udev/static_node-tags/tag pointing at the static device
386 node with the specified name. Static device node creation is
387 performed by systemd-tmpfiles before systemd-udevd is started.
388 The static nodes might not have a corresponding kernel device;
389 they are used to trigger automatic kernel module loading when
390 they are accessed.
391 watch
393 Watch the device node with inotify; when the node is closed
394 after being opened for writing, a change uevent is synthesized.
395 nowatch
397 Disable the watching of a device node with inotify.
398 db_persist
400 Set the flag (sticky bit) on the udev database entry of the
401 event device. Device properties are then kept in the database
402 even when udevadm info --cleanup-db is called. This option can
403 be useful in certain cases (e.g. Device Mapper devices) for
404 persisting device state on the transition from initrd.
405 log_level=level
407 Takes a log level name like "debug" or "info", or a special
408 value "reset". When a log level name is specified, the maximum
409 log level is changed to that level. When "reset" is set, then
410 the previously specified log level is revoked. Defaults to the
411 log level of the main process of systemd-udevd.
412 This may be useful when debugging events for certain devices.
414 Note that the log level is applied when the line including this
415 rule is processed. So, for debugging, it is recommended that
416 this is specified at earlier place, e.g., the first line of
417 00-debug.rules.
418 Example for debugging uevent processing for network interfaces:
420 # /etc/udev/rules.d/00-debug-net.rules
422 SUBSYSTEM=="net", OPTIONS="log_level=debug"
423 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
425 fields support simple string substitutions. The RUN substitutions are
426 performed after all rules have been processed, right before the program
427 is executed, allowing for the use of device properties set by earlier
428 matching rules. For all other fields, substitutions are performed while
429 the individual rule is being processed. The available substitutions
430 are:
431 $kernel, %k
433 The kernel name for this device.
434 $number, %n
436 The kernel number for this device. For example, "sda3" has kernel
437 number 3.
438 $devpath, %p
440 The devpath of the device.
441 $id, %b
443 The name of the device matched while searching the devpath upwards
444 for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
445 $driver
447 The driver name of the device matched while searching the devpath
448 upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
449 $attr{file}, %s{file}
451 The value of a sysfs attribute found at the device where all keys
452 of the rule have matched. If the matching device does not have such
453 an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
454 test selected a parent device, then the attribute from that parent
455 device is used.
456 If the attribute is a symlink, the last element of the symlink
458 target is returned as the value.
459 $env{key}, %E{key}
461 A device property value.
462 $major, %M
464 The kernel major number for the device.
465 $minor, %m
467 The kernel minor number for the device.
468 $result, %c
470 The string returned by the external program requested with PROGRAM.
471 A single part of the string, separated by a space character, may be
472 selected by specifying the part number as an attribute: "%c{N}". If
473 the number is followed by the "+" character, this part plus all
474 remaining parts of the result string are substituted: "%c{N+}".
475 $parent, %P
477 The node name of the parent device.
478 $name
480 The current name of the device. If not changed by a rule, it is the
481 name of the kernel device.
482 $links
484 A space-separated list of the current symlinks. The value is only
485 set during a remove event or if an earlier rule assigned a value.
486 $root, %r
488 The udev_root value.
489 $sys, %S
491 The sysfs mount point.
492 $devnode, %N
494 The name of the device node.
495 %%
497 The "%" character itself.
498 $$
500 The "$" character itself.
501
503 systemd-udevd.service(8), udevadm(8), systemd.link(5)
504
506 1. C-style escaped
507 https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences
508systemd 254 UDEV(7)