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 Unknown keys will never match.
167 TAG
169 Match against one of device tags. It can be used once a TAG key has
170 been set in one of the preceding rules. There may be multiple tags;
171 only one needs to match. If the operator is "!=", the token returns
172 true only if there is no tag matched.
173 TAGS
175 Search the devpath upwards for a device with matching tag. If the
176 operator is "!=", the token returns true only if there is no tag
177 matched.
178 TEST{octal mode mask}
180 Test the existence of a file. An octal mode mask can be specified
181 if needed.
182 PROGRAM
184 Execute a program to determine whether there is a match; the key is
185 true if the program returns successfully. The device properties are
186 made available to the executed program in the environment. The
187 program's standard output is available in the RESULT key.
188 This can only be used for very short-running foreground tasks. For
190 details, see RUN.
191 Note that multiple PROGRAM keys may be specified in one rule, and
193 "=", ":=", and "+=" have the same effect as "==".
194 RESULT
196 Match the returned string of the last PROGRAM call. This key can be
197 used in the same or in any later rule after a PROGRAM call.
198 Most of the fields support shell glob pattern matching and alternate
200 patterns. The following special characters are supported:
201 "*"
203 Matches zero or more characters.
204 "?"
206 Matches any single character.
207 "[]"
209 Matches any single character specified within the brackets. For
210 example, the pattern string "tty[SR]" would match either "ttyS" or
211 "ttyR". Ranges are also supported via the "-" character. For
212 example, to match on the range of all digits, the pattern "[0-9]"
213 could be used. If the first character following the "[" is a "!",
214 any characters not enclosed are matched.
215 "|"
217 Separates alternative patterns. For example, the pattern string
218 "abc|x*" would match either "abc" or "x*".
219 The following keys can get values assigned:
221 NAME
223 The name to use for a network interface. See systemd.link(5) for a
224 higher-level mechanism for setting the interface name. The name of
225 a device node cannot be changed by udev, only additional symlinks
226 can be created.
227 SYMLINK
229 The name of a symlink targeting the node. Every matching rule adds
230 this value to the list of symlinks to be created.
231 The set of characters to name a symlink is limited. Allowed
233 characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
234 sequences, and "\x00" hex encoding. All other characters are
235 replaced by a "_" character.
236 Multiple symlinks may be specified by separating the names by the
238 space character. In case multiple devices claim the same name, the
239 link always points to the device with the highest link_priority. If
240 the current device goes away, the links are re-evaluated and the
241 device with the next highest link_priority becomes the owner of the
242 link. If no link_priority is specified, the order of the devices
243 (and which one of them owns the link) is undefined.
244 Symlink names must never conflict with the kernel's default device
246 node names, as that would result in unpredictable behavior.
247 OWNER, GROUP, MODE
249 The permissions for the device node. Every specified value
250 overrides the compiled-in default value.
251 SECLABEL{module}
253 Applies the specified Linux Security Module label to the device
254 node.
255 ATTR{key}
257 The value that should be written to a sysfs attribute of the event
258 device.
259 SYSCTL{kernel parameter}
261 The value that should be written to kernel parameter.
262 ENV{key}
264 Set a device property value. Property names with a leading "." are
265 neither stored in the database nor exported to events or external
266 tools (run by, for example, the PROGRAM match key).
267 TAG
269 Attach a tag to a device. This is used to filter events for users
270 of libudev's monitor functionality, or to enumerate a group of
271 tagged devices. The implementation can only work efficiently if
272 only a few tags are attached to a device. It is only meant to be
273 used in contexts with specific device filter requirements, and not
274 as a general-purpose flag. Excessive use might result in
275 inefficient event handling.
276 RUN{type}
278 Specify a program to be executed after processing of all the rules
279 for the event. With "+=", this invocation is added to the list, and
280 with "=" or ":=", it replaces any previous contents of the list.
281 Please note that both "program" and "builtin" types described below
282 share a common list, so clearing the list with ":=" and "=" affects
283 both types.
284 type may be:
286 "program"
288 Execute an external program specified as the assigned value. If
289 no absolute path is given, the program is expected to live in
290 /usr/lib/udev; otherwise, the absolute path must be specified.
291 This is the default if no type is specified.
293 "builtin"
295 As program, but use one of the built-in programs rather than an
296 external one.
297 The program name and following arguments are separated by spaces.
299 Single quotes can be used to specify arguments with spaces.
300 This can only be used for very short-running foreground tasks.
302 Running an event process for a long period of time may block all
303 further events for this or a dependent device.
304 Note that running programs that access the network or mount/unmount
306 filesystems is not allowed inside of udev rules, due to the default
307 sandbox that is enforced on systemd-udevd.service.
308 Starting daemons or other long-running processes is not allowed;
310 the forked processes, detached or not, will be unconditionally
311 killed after the event handling has finished. In order to activate
312 long-running processes from udev rules, provide a service unit and
313 pull it in from a udev device using the SYSTEMD_WANTS device
314 property. See systemd.device(5) for details.
315 LABEL
317 A named label to which a GOTO may jump.
318 GOTO
320 Jumps to the next LABEL with a matching name.
321 IMPORT{type}
323 Import a set of variables as device properties, depending on type:
324 "program"
326 Execute an external program specified as the assigned value
327 and, if it returns successfully, import its output, which must
328 be in environment key format. Path specification,
329 command/argument separation, and quoting work like in RUN.
330 "builtin"
332 Similar to "program", but use one of the built-in programs
333 rather than an external one.
334 "file"
336 Import a text file specified as the assigned value, the content
337 of which must be in environment key format.
338 "db"
340 Import a single property specified as the assigned value from
341 the current device database. This works only if the database is
342 already populated by an earlier event.
343 "cmdline"
345 Import a single property from the kernel command line. For
346 simple flags the value of the property is set to "1".
347 "parent"
349 Import the stored keys from the parent device by reading the
350 database entry of the parent device. The value assigned to
351 IMPORT{parent} is used as a filter of key names to import (with
352 the same shell glob pattern matching used for comparisons).
353 This can only be used for very short-running foreground tasks. For
355 details see RUN.
356 Note that multiple IMPORT{} keys may be specified in one rule, and
358 "=", ":=", and "+=" have the same effect as "==". The key is true
359 if the import is successful, unless "!=" is used as the operator
360 which causes the key to be true if the import failed.
361 OPTIONS
363 Rule and device options:
364 link_priority=value
366 Specify the priority of the created symlinks. Devices with
367 higher priorities overwrite existing symlinks of other devices.
368 The default is 0.
369 string_escape=none|replace
371 When "replace", possibly unsafe characters in strings assigned
372 to NAME, SYMLINK, and ENV{key} are replaced. When "none", no
373 replacement is performed. When unset, the replacement is
374 performed for NAME, SYMLINK, but not for ENV{key}. Defaults to
375 unset.
376 static_node=
378 Apply the permissions specified in this rule to the static
379 device node with the specified name. Also, for every tag
380 specified in this rule, create a symlink in the directory
381 /run/udev/static_node-tags/tag pointing at the static device
382 node with the specified name. Static device node creation is
383 performed by systemd-tmpfiles before systemd-udevd is started.
384 The static nodes might not have a corresponding kernel device;
385 they are used to trigger automatic kernel module loading when
386 they are accessed.
387 watch
389 Watch the device node with inotify; when the node is closed
390 after being opened for writing, a change uevent is synthesized.
391 nowatch
393 Disable the watching of a device node with inotify.
394 db_persist
396 Set the flag (sticky bit) on the udev database entry of the
397 event device. Device properties are then kept in the database
398 even when udevadm info --cleanup-db is called. This option can
399 be useful in certain cases (e.g. Device Mapper devices) for
400 persisting device state on the transition from initrd.
401 log_level=level
403 Takes a log level name like "debug" or "info", or a special
404 value "reset". When a log level name is specified, the maximum
405 log level is changed to that level. When "reset" is set, then
406 the previously specified log level is revoked. Defaults to the
407 log level of the main process of systemd-udevd.
408 This may be useful when debugging events for certain devices.
410 Note that the log level is applied when the line including this
411 rule is processed. So, for debugging, it is recommended that
412 this is specified at earlier place, e.g., the first line of
413 00-debug.rules.
414 Example for debugging uevent processing for network interfaces:
416 # /etc/udev/rules.d/00-debug-net.rules
418 SUBSYSTEM=="net", OPTIONS="log_level=debug"
419 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
421 fields support simple string substitutions. The RUN substitutions are
422 performed after all rules have been processed, right before the program
423 is executed, allowing for the use of device properties set by earlier
424 matching rules. For all other fields, substitutions are performed while
425 the individual rule is being processed. The available substitutions
426 are:
427 $kernel, %k
429 The kernel name for this device.
430 $number, %n
432 The kernel number for this device. For example, "sda3" has kernel
433 number 3.
434 $devpath, %p
436 The devpath of the device.
437 $id, %b
439 The name of the device matched while searching the devpath upwards
440 for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
441 $driver
443 The driver name of the device matched while searching the devpath
444 upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
445 $attr{file}, %s{file}
447 The value of a sysfs attribute found at the device where all keys
448 of the rule have matched. If the matching device does not have such
449 an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
450 test selected a parent device, then the attribute from that parent
451 device is used.
452 If the attribute is a symlink, the last element of the symlink
454 target is returned as the value.
455 $env{key}, %E{key}
457 A device property value.
458 $major, %M
460 The kernel major number for the device.
461 $minor, %m
463 The kernel minor number for the device.
464 $result, %c
466 The string returned by the external program requested with PROGRAM.
467 A single part of the string, separated by a space character, may be
468 selected by specifying the part number as an attribute: "%c{N}". If
469 the number is followed by the "+" character, this part plus all
470 remaining parts of the result string are substituted: "%c{N+}".
471 $parent, %P
473 The node name of the parent device.
474 $name
476 The current name of the device. If not changed by a rule, it is the
477 name of the kernel device.
478 $links
480 A space-separated list of the current symlinks. The value is only
481 set during a remove event or if an earlier rule assigned a value.
482 $root, %r
484 The udev_root value.
485 $sys, %S
487 The sysfs mount point.
488 $devnode, %N
490 The name of the device node.
491 %%
493 The "%" character itself.
494 $$
496 The "$" character itself.
497
499 systemd-udevd.service(8), udevadm(8), systemd.link(5)
500
502 1. C-style escaped
503 https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences
504systemd 253 UDEV(7)