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 can
37 be used to override a system-supplied rules file with a local file if
38 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. Rule
40 files must have the 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}
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.
111 SYSCTL{kernel parameter}
113 Match a kernel parameter value.
114 KERNELS
116 Search the devpath upwards for a matching device name.
117 SUBSYSTEMS
119 Search the devpath upwards for a matching device subsystem name.
120 DRIVERS
122 Search the devpath upwards for a matching device driver name.
123 ATTRS{filename}
125 Search the devpath upwards for a device with matching sysfs
126 attribute values. If multiple ATTRS matches are specified, all of
127 them must match on the same device. Trailing whitespace in the
128 attribute values is ignored unless the specified match value itself
129 contains trailing whitespace.
130 TAGS
132 Search the devpath upwards for a device with matching tag.
133 ENV{key}
135 Match against a device property value.
136 CONST{key}
138 Match against a system-wide constant. Supported keys are:
139 "arch"
141 System's architecture. See ConditionArchitecture= in
142 systemd.unit(5) for possible values.
143 "virt"
145 System's virtualization environment. See systemd-detect-virt(1)
146 for possible values.
147 Unknown keys will never match.
149 TAG
151 Match against a device tag.
152 TEST{octal mode mask}
154 Test the existence of a file. An octal mode mask can be specified
155 if needed.
156 PROGRAM
158 Execute a program to determine whether there is a match; the key is
159 true if the program returns successfully. The device properties are
160 made available to the executed program in the environment. The
161 program's standard output is available in the RESULT key.
162 This can only be used for very short-running foreground tasks. For
164 details, see RUN.
165 Note that multiple PROGRAM keys may be specified in one rule, and
167 "=", ":=", and "+=" have the same effect as "==".
168 RESULT
170 Match the returned string of the last PROGRAM call. This key can be
171 used in the same or in any later rule after a PROGRAM call.
172 Most of the fields support shell glob pattern matching and alternate
174 patterns. The following special characters are supported:
175 "*"
177 Matches zero or more characters.
178 "?"
180 Matches any single character.
181 "[]"
183 Matches any single character specified within the brackets. For
184 example, the pattern string "tty[SR]" would match either "ttyS" or
185 "ttyR". Ranges are also supported via the "-" character. For
186 example, to match on the range of all digits, the pattern "[0-9]"
187 could be used. If the first character following the "[" is a "!",
188 any characters not enclosed are matched.
189 "|"
191 Separates alternative patterns. For example, the pattern string
192 "abc|x*" would match either "abc" or "x*".
193 The following keys can get values assigned:
195 NAME
197 The name to use for a network interface. See systemd.link(5) for a
198 higher-level mechanism for setting the interface name. The name of
199 a device node cannot be changed by udev, only additional symlinks
200 can be created.
201 SYMLINK
203 The name of a symlink targeting the node. Every matching rule adds
204 this value to the list of symlinks to be created.
205 The set of characters to name a symlink is limited. Allowed
207 characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
208 sequences, and "\x00" hex encoding. All other characters are
209 replaced by a "_" character.
210 Multiple symlinks may be specified by separating the names by the
212 space character. In case multiple devices claim the same name, the
213 link always points to the device with the highest link_priority. If
214 the current device goes away, the links are re-evaluated and the
215 device with the next highest link_priority becomes the owner of the
216 link. If no link_priority is specified, the order of the devices
217 (and which one of them owns the link) is undefined.
218 Symlink names must never conflict with the kernel's default device
220 node names, as that would result in unpredictable behavior.
221 OWNER, GROUP, MODE
223 The permissions for the device node. Every specified value
224 overrides the compiled-in default value.
225 SECLABEL{module}
227 Applies the specified Linux Security Module label to the device
228 node.
229 ATTR{key}
231 The value that should be written to a sysfs attribute of the event
232 device.
233 SYSCTL{kernel parameter}
235 The value that should be written to kernel parameter.
236 ENV{key}
238 Set a device property value. Property names with a leading "." are
239 neither stored in the database nor exported to events or external
240 tools (run by, for example, the PROGRAM match key).
241 TAG
243 Attach a tag to a device. This is used to filter events for users
244 of libudev's monitor functionality, or to enumerate a group of
245 tagged devices. The implementation can only work efficiently if
246 only a few tags are attached to a device. It is only meant to be
247 used in contexts with specific device filter requirements, and not
248 as a general-purpose flag. Excessive use might result in
249 inefficient event handling.
250 RUN{type}
252 Specify a program to be executed after processing of all the rules
253 for the event. With "+=", this invocation is added to the list, and
254 with "=" or ":=", it replaces any previous contents of the list.
255 Please note that both "program" and "builtin" types described below
256 use a single list, so clearing the list with ":=" and "=" affects
257 both types.
258 type may be:
260 "program"
262 Execute an external program specified as the assigned value. If
263 no absolute path is given, the program is expected to live in
264 /usr/lib/udev; otherwise, the absolute path must be specified.
265 This is the default if no type is specified.
267 "builtin"
269 As program, but use one of the built-in programs rather than an
270 external one.
271 The program name and following arguments are separated by spaces.
273 Single quotes can be used to specify arguments with spaces.
274 This can only be used for very short-running foreground tasks.
276 Running an event process for a long period of time may block all
277 further events for this or a dependent device.
278 Note that running programs that access the network or mount/unmount
280 filesystems is not allowed inside of udev rules, due to the default
281 sandbox that is enforced on systemd-udevd.service.
282 Starting daemons or other long-running processes is not allowed;
284 the forked processes, detached or not, will be unconditionally
285 killed after the event handling has finished. In order to activate
286 long-running processes from udev rules, provide a service unit and
287 pull it in from a udev device using the SYSTEMD_WANTS device
288 property. See systemd.device(5) for details.
289 LABEL
291 A named label to which a GOTO may jump.
292 GOTO
294 Jumps to the next LABEL with a matching name.
295 IMPORT{type}
297 Import a set of variables as device properties, depending on type:
298 "program"
300 Execute an external program specified as the assigned value
301 and, if it returns successfully, import its output, which must
302 be in environment key format. Path specification,
303 command/argument separation, and quoting work like in RUN.
304 "builtin"
306 Similar to "program", but use one of the built-in programs
307 rather than an external one.
308 "file"
310 Import a text file specified as the assigned value, the content
311 of which must be in environment key format.
312 "db"
314 Import a single property specified as the assigned value from
315 the current device database. This works only if the database is
316 already populated by an earlier event.
317 "cmdline"
319 Import a single property from the kernel command line. For
320 simple flags the value of the property is set to "1".
321 "parent"
323 Import the stored keys from the parent device by reading the
324 database entry of the parent device. The value assigned to
325 IMPORT{parent} is used as a filter of key names to import (with
326 the same shell glob pattern matching used for comparisons).
327 This can only be used for very short-running foreground tasks. For
329 details see RUN.
330 Note that multiple IMPORT{} keys may be specified in one rule, and
332 "=", ":=", and "+=" have the same effect as "==". The key is true
333 if the import is successful, unless "!=" is used as the operator
334 which causes the key to be true if the import failed.
335 OPTIONS
337 Rule and device options:
338 link_priority=value
340 Specify the priority of the created symlinks. Devices with
341 higher priorities overwrite existing symlinks of other devices.
342 The default is 0.
343 string_escape=none|replace
345 Usually, control and other possibly unsafe characters are
346 replaced in strings used for device naming. The mode of
347 replacement can be specified with this option.
348 static_node=
350 Apply the permissions specified in this rule to the static
351 device node with the specified name. Also, for every tag
352 specified in this rule, create a symlink in the directory
353 /run/udev/static_node-tags/tag pointing at the static device
354 node with the specified name. Static device node creation is
355 performed by systemd-tmpfiles before systemd-udevd is started.
356 The static nodes might not have a corresponding kernel device;
357 they are used to trigger automatic kernel module loading when
358 they are accessed.
359 watch
361 Watch the device node with inotify; when the node is closed
362 after being opened for writing, a change uevent is synthesized.
363 nowatch
365 Disable the watching of a device node with inotify.
366 db_persist
368 Set the flag (sticky bit) on the udev database entry of the
369 event device. Device properties are then kept in the database
370 even when udevadm info --cleanup-db is called. This option can
371 be useful in certain cases (e.g. Device Mapper devices) for
372 persisting device state on the transition from initramfs.
373 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
375 fields support simple string substitutions. The RUN substitutions are
376 performed after all rules have been processed, right before the program
377 is executed, allowing for the use of device properties set by earlier
378 matching rules. For all other fields, substitutions are performed while
379 the individual rule is being processed. The available substitutions
380 are:
381 $kernel, %k
383 The kernel name for this device.
384 $number, %n
386 The kernel number for this device. For example, "sda3" has kernel
387 number 3.
388 $devpath, %p
390 The devpath of the device.
391 $id, %b
393 The name of the device matched while searching the devpath upwards
394 for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
395 $driver
397 The driver name of the device matched while searching the devpath
398 upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
399 $attr{file}, %s{file}
401 The value of a sysfs attribute found at the device where all keys
402 of the rule have matched. If the matching device does not have such
403 an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
404 test selected a parent device, then the attribute from that parent
405 device is used.
406 If the attribute is a symlink, the last element of the symlink
408 target is returned as the value.
409 $env{key}, %E{key}
411 A device property value.
412 $major, %M
414 The kernel major number for the device.
415 $minor, %m
417 The kernel minor number for the device.
418 $result, %c
420 The string returned by the external program requested with PROGRAM.
421 A single part of the string, separated by a space character, may be
422 selected by specifying the part number as an attribute: "%c{N}". If
423 the number is followed by the "+" character, this part plus all
424 remaining parts of the result string are substituted: "%c{N+}".
425 $parent, %P
427 The node name of the parent device.
428 $name
430 The current name of the device. If not changed by a rule, it is the
431 name of the kernel device.
432 $links
434 A space-separated list of the current symlinks. The value is only
435 set during a remove event or if an earlier rule assigned a value.
436 $root, %r
438 The udev_root value.
439 $sys, %S
441 The sysfs mount point.
442 $devnode, %N
444 The name of the device node.
445 %%
447 The "%" character itself.
448 $$
450 The "$" character itself.
451
453 systemd-udevd.service(8), udevadm(8), systemd.link(5)
454systemd 246 UDEV(7)