1UDEV(7)                              udev                              UDEV(7)
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NAME

6       udev - Dynamic device management
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DESCRIPTION

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
16       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
24       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.
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RULES FILES

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
42       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
48       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
52       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
56       "=="
57           Compare for equality.
58
59       "!="
60           Compare for inequality.
61
62       "="
63           Assign a value to a key. Keys that represent a list are reset and
64           only this single value is assigned.
65
66       "+="
67           Add the value to a key that holds a list of entries.
68
69       "-="
70           Remove the value from a key that holds a list of entries.
71
72       ":="
73           Assign a value to a key finally; disallow any later changes.
74
75       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
81       ACTION
82           Match the name of the event action.
83
84       DEVPATH
85           Match the devpath of the event device.
86
87       KERNEL
88           Match the name of the event device.
89
90       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
94       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
99       SUBSYSTEM
100           Match the subsystem of the event device.
101
102       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
107       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
112       SYSCTL{kernel parameter}
113           Match a kernel parameter value.
114
115       KERNELS
116           Search the devpath upwards for a matching device name.
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118       SUBSYSTEMS
119           Search the devpath upwards for a matching device subsystem name.
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121       DRIVERS
122           Search the devpath upwards for a matching device driver name.
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124       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
131       TAGS
132           Search the devpath upwards for a device with matching tag.
133
134       ENV{key}
135           Match against a device property value.
136
137       TAG
138           Match against a device tag.
139
140       TEST{octal mode mask}
141           Test the existence of a file. An octal mode mask can be specified
142           if needed.
143
144       PROGRAM
145           Execute a program to determine whether there is a match; the key is
146           true if the program returns successfully. The device properties are
147           made available to the executed program in the environment. The
148           program's standard output is available in the RESULT key.
149
150           This can only be used for very short-running foreground tasks. For
151           details, see RUN.
152
153       RESULT
154           Match the returned string of the last PROGRAM call. This key can be
155           used in the same or in any later rule after a PROGRAM call.
156
157       Most of the fields support shell glob pattern matching and alternate
158       patterns. The following special characters are supported:
159
160       "*"
161           Matches zero or more characters.
162
163       "?"
164           Matches any single character.
165
166       "[]"
167           Matches any single character specified within the brackets. For
168           example, the pattern string "tty[SR]" would match either "ttyS" or
169           "ttyR". Ranges are also supported via the "-" character. For
170           example, to match on the range of all digits, the pattern "[0-9]"
171           could be used. If the first character following the "[" is a "!",
172           any characters not enclosed are matched.
173
174       "|"
175           Separates alternative patterns. For example, the pattern string
176           "abc|x*" would match either "abc" or "x*".
177
178       The following keys can get values assigned:
179
180       NAME
181           The name to use for a network interface. See systemd.link(5) for a
182           higher-level mechanism for setting the interface name. The name of
183           a device node cannot be changed by udev, only additional symlinks
184           can be created.
185
186       SYMLINK
187           The name of a symlink targeting the node. Every matching rule adds
188           this value to the list of symlinks to be created.
189
190           The set of characters to name a symlink is limited. Allowed
191           characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
192           sequences, and "\x00" hex encoding. All other characters are
193           replaced by a "_" character.
194
195           Multiple symlinks may be specified by separating the names by the
196           space character. In case multiple devices claim the same name, the
197           link always points to the device with the highest link_priority. If
198           the current device goes away, the links are re-evaluated and the
199           device with the next highest link_priority becomes the owner of the
200           link. If no link_priority is specified, the order of the devices
201           (and which one of them owns the link) is undefined.
202
203           Symlink names must never conflict with the kernel's default device
204           node names, as that would result in unpredictable behavior.
205
206       OWNER, GROUP, MODE
207           The permissions for the device node. Every specified value
208           overrides the compiled-in default value.
209
210       SECLABEL{module}
211           Applies the specified Linux Security Module label to the device
212           node.
213
214       ATTR{key}
215           The value that should be written to a sysfs attribute of the event
216           device.
217
218       SYSCTL{kernel parameter}
219           The value that should be written to kernel parameter.
220
221       ENV{key}
222           Set a device property value. Property names with a leading "."  are
223           neither stored in the database nor exported to events or external
224           tools (run by, for example, the PROGRAM match key).
225
226       TAG
227           Attach a tag to a device. This is used to filter events for users
228           of libudev's monitor functionality, or to enumerate a group of
229           tagged devices. The implementation can only work efficiently if
230           only a few tags are attached to a device. It is only meant to be
231           used in contexts with specific device filter requirements, and not
232           as a general-purpose flag. Excessive use might result in
233           inefficient event handling.
234
235       RUN{type}
236           Add a program to the list of programs to be executed after
237           processing all the rules for a specific event, depending on "type":
238
239           "program"
240               Execute an external program specified as the assigned value. If
241               no absolute path is given, the program is expected to live in
242               /usr/lib/udev; otherwise, the absolute path must be specified.
243
244               This is the default if no type is specified.
245
246           "builtin"
247               As program, but use one of the built-in programs rather than an
248               external one.
249
250           The program name and following arguments are separated by spaces.
251           Single quotes can be used to specify arguments with spaces.
252
253           This can only be used for very short-running foreground tasks.
254           Running an event process for a long period of time may block all
255           further events for this or a dependent device.
256
257           Starting daemons or other long-running processes is not appropriate
258           for udev; the forked processes, detached or not, will be
259           unconditionally killed after the event handling has finished.
260
261           Note that running programs that access the network or mount/unmount
262           filesystems is not allowed inside of udev rules, due to the default
263           sandbox that is enforced on systemd-udevd.service.
264
265           Please also note that ":=" and "=" are clearing both, program and
266           builtin commands.
267
268       LABEL
269           A named label to which a GOTO may jump.
270
271       GOTO
272           Jumps to the next LABEL with a matching name.
273
274       IMPORT{type}
275           Import a set of variables as device properties, depending on
276           "type":
277
278           "program"
279               Execute an external program specified as the assigned value
280               and, if it returns successfully, import its output, which must
281               be in environment key format. Path specification,
282               command/argument separation, and quoting work like in RUN.
283
284           "builtin"
285               Similar to "program", but use one of the built-in programs
286               rather than an external one.
287
288           "file"
289               Import a text file specified as the assigned value, the content
290               of which must be in environment key format.
291
292           "db"
293               Import a single property specified as the assigned value from
294               the current device database. This works only if the database is
295               already populated by an earlier event.
296
297           "cmdline"
298               Import a single property from the kernel command line. For
299               simple flags the value of the property is set to "1".
300
301           "parent"
302               Import the stored keys from the parent device by reading the
303               database entry of the parent device. The value assigned to
304               IMPORT{parent} is used as a filter of key names to import (with
305               the same shell glob pattern matching used for comparisons).
306
307           This can only be used for very short-running foreground tasks. For
308           details see RUN.
309
310       OPTIONS
311           Rule and device options:
312
313           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
318           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
323           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
334           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
338           nowatch
339               Disable the watching of a device node with inotify.
340
341           db_persist
342               Set the flag (sticky bit) on the udev database entry of the
343               event device. Device properties are then kept in the database
344               even when udevadm info --cleanup-db is called. This option can
345               be useful in certain cases (e.g. Device Mapper devices) for
346               persisting device state on the transition from initramfs.
347
348       The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
349       fields support simple string substitutions. The RUN substitutions are
350       performed after all rules have been processed, right before the program
351       is executed, allowing for the use of device properties set by earlier
352       matching rules. For all other fields, substitutions are performed while
353       the individual rule is being processed. The available substitutions
354       are:
355
356       $kernel, %k
357           The kernel name for this device.
358
359       $number, %n
360           The kernel number for this device. For example, "sda3" has kernel
361           number "3".
362
363       $devpath, %p
364           The devpath of the device.
365
366       $id, %b
367           The name of the device matched while searching the devpath upwards
368           for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
369
370       $driver
371           The driver name of the device matched while searching the devpath
372           upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
373
374       $attr{file}, %s{file}
375           The value of a sysfs attribute found at the device where all keys
376           of the rule have matched. If the matching device does not have such
377           an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
378           test selected a parent device, then the attribute from that parent
379           device is used.
380
381           If the attribute is a symlink, the last element of the symlink
382           target is returned as the value.
383
384       $env{key}, %E{key}
385           A device property value.
386
387       $major, %M
388           The kernel major number for the device.
389
390       $minor, %m
391           The kernel minor number for the device.
392
393       $result, %c
394           The string returned by the external program requested with PROGRAM.
395           A single part of the string, separated by a space character, may be
396           selected by specifying the part number as an attribute: "%c{N}". If
397           the number is followed by the "+" character, this part plus all
398           remaining parts of the result string are substituted: "%c{N+}".
399
400       $parent, %P
401           The node name of the parent device.
402
403       $name
404           The current name of the device. If not changed by a rule, it is the
405           name of the kernel device.
406
407       $links
408           A space-separated list of the current symlinks. The value is only
409           set during a remove event or if an earlier rule assigned a value.
410
411       $root, %r
412           The udev_root value.
413
414       $sys, %S
415           The sysfs mount point.
416
417       $devnode, %N
418           The name of the device node.
419
420       %%
421           The "%" character itself.
422
423       $$
424           The "$" character itself.
425

SEE ALSO

427       systemd-udevd.service(8), udevadm(8), systemd.link(5)
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431systemd 243                                                            UDEV(7)
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