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 RESULT
167 Match the returned string of the last PROGRAM call. This key can be
168 used in the same or in any later rule after a PROGRAM call.
169 Most of the fields support shell glob pattern matching and alternate
171 patterns. The following special characters are supported:
172 "*"
174 Matches zero or more characters.
175 "?"
177 Matches any single character.
178 "[]"
180 Matches any single character specified within the brackets. For
181 example, the pattern string "tty[SR]" would match either "ttyS" or
182 "ttyR". Ranges are also supported via the "-" character. For
183 example, to match on the range of all digits, the pattern "[0-9]"
184 could be used. If the first character following the "[" is a "!",
185 any characters not enclosed are matched.
186 "|"
188 Separates alternative patterns. For example, the pattern string
189 "abc|x*" would match either "abc" or "x*".
190 The following keys can get values assigned:
192 NAME
194 The name to use for a network interface. See systemd.link(5) for a
195 higher-level mechanism for setting the interface name. The name of
196 a device node cannot be changed by udev, only additional symlinks
197 can be created.
198 SYMLINK
200 The name of a symlink targeting the node. Every matching rule adds
201 this value to the list of symlinks to be created.
202 The set of characters to name a symlink is limited. Allowed
204 characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
205 sequences, and "\x00" hex encoding. All other characters are
206 replaced by a "_" character.
207 Multiple symlinks may be specified by separating the names by the
209 space character. In case multiple devices claim the same name, the
210 link always points to the device with the highest link_priority. If
211 the current device goes away, the links are re-evaluated and the
212 device with the next highest link_priority becomes the owner of the
213 link. If no link_priority is specified, the order of the devices
214 (and which one of them owns the link) is undefined.
215 Symlink names must never conflict with the kernel's default device
217 node names, as that would result in unpredictable behavior.
218 OWNER, GROUP, MODE
220 The permissions for the device node. Every specified value
221 overrides the compiled-in default value.
222 SECLABEL{module}
224 Applies the specified Linux Security Module label to the device
225 node.
226 ATTR{key}
228 The value that should be written to a sysfs attribute of the event
229 device.
230 SYSCTL{kernel parameter}
232 The value that should be written to kernel parameter.
233 ENV{key}
235 Set a device property value. Property names with a leading "." are
236 neither stored in the database nor exported to events or external
237 tools (run by, for example, the PROGRAM match key).
238 TAG
240 Attach a tag to a device. This is used to filter events for users
241 of libudev's monitor functionality, or to enumerate a group of
242 tagged devices. The implementation can only work efficiently if
243 only a few tags are attached to a device. It is only meant to be
244 used in contexts with specific device filter requirements, and not
245 as a general-purpose flag. Excessive use might result in
246 inefficient event handling.
247 RUN{type}
249 Add a program to the list of programs to be executed after
250 processing all the rules for a specific event, depending on "type":
251 "program"
253 Execute an external program specified as the assigned value. If
254 no absolute path is given, the program is expected to live in
255 /usr/lib/udev; otherwise, the absolute path must be specified.
256 This is the default if no type is specified.
258 "builtin"
260 As program, but use one of the built-in programs rather than an
261 external one.
262 The program name and following arguments are separated by spaces.
264 Single quotes can be used to specify arguments with spaces.
265 This can only be used for very short-running foreground tasks.
267 Running an event process for a long period of time may block all
268 further events for this or a dependent device.
269 Starting daemons or other long-running processes is not appropriate
271 for udev; the forked processes, detached or not, will be
272 unconditionally killed after the event handling has finished.
273 Note that running programs that access the network or mount/unmount
275 filesystems is not allowed inside of udev rules, due to the default
276 sandbox that is enforced on systemd-udevd.service.
277 Please also note that ":=" and "=" are clearing both, program and
279 builtin commands.
280 In order to activate long-running processes from udev rules,
282 provide a service unit, and pull it in from a udev device using the
283 SYSTEMD_WANTS device property. See systemd.device(5) for details.
284 LABEL
286 A named label to which a GOTO may jump.
287 GOTO
289 Jumps to the next LABEL with a matching name.
290 IMPORT{type}
292 Import a set of variables as device properties, depending on
293 "type":
294 "program"
296 Execute an external program specified as the assigned value
297 and, if it returns successfully, import its output, which must
298 be in environment key format. Path specification,
299 command/argument separation, and quoting work like in RUN.
300 "builtin"
302 Similar to "program", but use one of the built-in programs
303 rather than an external one.
304 "file"
306 Import a text file specified as the assigned value, the content
307 of which must be in environment key format.
308 "db"
310 Import a single property specified as the assigned value from
311 the current device database. This works only if the database is
312 already populated by an earlier event.
313 "cmdline"
315 Import a single property from the kernel command line. For
316 simple flags the value of the property is set to "1".
317 "parent"
319 Import the stored keys from the parent device by reading the
320 database entry of the parent device. The value assigned to
321 IMPORT{parent} is used as a filter of key names to import (with
322 the same shell glob pattern matching used for comparisons).
323 This can only be used for very short-running foreground tasks. For
325 details see RUN.
326 OPTIONS
328 Rule and device options:
329 link_priority=value
331 Specify the priority of the created symlinks. Devices with
332 higher priorities overwrite existing symlinks of other devices.
333 The default is 0.
334 string_escape=none|replace
336 Usually, control and other possibly unsafe characters are
337 replaced in strings used for device naming. The mode of
338 replacement can be specified with this option.
339 static_node=
341 Apply the permissions specified in this rule to the static
342 device node with the specified name. Also, for every tag
343 specified in this rule, create a symlink in the directory
344 /run/udev/static_node-tags/tag pointing at the static device
345 node with the specified name. Static device node creation is
346 performed by systemd-tmpfiles before systemd-udevd is started.
347 The static nodes might not have a corresponding kernel device;
348 they are used to trigger automatic kernel module loading when
349 they are accessed.
350 watch
352 Watch the device node with inotify; when the node is closed
353 after being opened for writing, a change uevent is synthesized.
354 nowatch
356 Disable the watching of a device node with inotify.
357 db_persist
359 Set the flag (sticky bit) on the udev database entry of the
360 event device. Device properties are then kept in the database
361 even when udevadm info --cleanup-db is called. This option can
362 be useful in certain cases (e.g. Device Mapper devices) for
363 persisting device state on the transition from initramfs.
364 The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
366 fields support simple string substitutions. The RUN substitutions are
367 performed after all rules have been processed, right before the program
368 is executed, allowing for the use of device properties set by earlier
369 matching rules. For all other fields, substitutions are performed while
370 the individual rule is being processed. The available substitutions
371 are:
372 $kernel, %k
374 The kernel name for this device.
375 $number, %n
377 The kernel number for this device. For example, "sda3" has kernel
378 number "3".
379 $devpath, %p
381 The devpath of the device.
382 $id, %b
384 The name of the device matched while searching the devpath upwards
385 for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
386 $driver
388 The driver name of the device matched while searching the devpath
389 upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
390 $attr{file}, %s{file}
392 The value of a sysfs attribute found at the device where all keys
393 of the rule have matched. If the matching device does not have such
394 an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
395 test selected a parent device, then the attribute from that parent
396 device is used.
397 If the attribute is a symlink, the last element of the symlink
399 target is returned as the value.
400 $env{key}, %E{key}
402 A device property value.
403 $major, %M
405 The kernel major number for the device.
406 $minor, %m
408 The kernel minor number for the device.
409 $result, %c
411 The string returned by the external program requested with PROGRAM.
412 A single part of the string, separated by a space character, may be
413 selected by specifying the part number as an attribute: "%c{N}". If
414 the number is followed by the "+" character, this part plus all
415 remaining parts of the result string are substituted: "%c{N+}".
416 $parent, %P
418 The node name of the parent device.
419 $name
421 The current name of the device. If not changed by a rule, it is the
422 name of the kernel device.
423 $links
425 A space-separated list of the current symlinks. The value is only
426 set during a remove event or if an earlier rule assigned a value.
427 $root, %r
429 The udev_root value.
430 $sys, %S
432 The sysfs mount point.
433 $devnode, %N
435 The name of the device node.
436 %%
438 The "%" character itself.
439 $$
441 The "$" character itself.
442
444 systemd-udevd.service(8), udevadm(8), systemd.link(5)
445systemd 245 UDEV(7)