1UDEV(7)                              udev                              UDEV(7)
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3
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NAME

6       udev - Dynamic device management
7

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.
27

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
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
43       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
49       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
53       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
57   Operators
58       "=="
59           Compare for equality. (The specified key has the specified value.)
60
61       "!="
62           Compare for inequality. (The specified key doesn't have the
63           specified value, or the specified key is not present at all.)
64
65       "="
66           Assign a value to a key. Keys that represent a list are reset and
67           only this single value is assigned.
68
69       "+="
70           Add the value to a key that holds a list of entries.
71
72       "-="
73           Remove the value from a key that holds a list of entries.
74
75       ":="
76           Assign a value to a key finally; disallow any later changes.
77
78   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
85       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
90       Please note that NUL is not allowed in either string variant.
91
92   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
99       ACTION
100           Match the name of the event action.
101
102       DEVPATH
103           Match the devpath of the event device.
104
105       KERNEL
106           Match the name of the event device.
107
108       KERNELS
109           Search the devpath upwards for a matching device name.
110
111       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
115       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.
119
120       SUBSYSTEM
121           Match the subsystem of the event device.
122
123       SUBSYSTEMS
124           Search the devpath upwards for a matching device subsystem name.
125
126       DRIVER
127           Match the driver name of the event device. Only set this key for
128           devices which are bound to a driver at the time the event is
129           generated.
130
131       DRIVERS
132           Search the devpath upwards for a matching device driver name.
133
134       ATTR{filename}
135           Match sysfs attribute value of the event device.
136
137           Trailing whitespace in the attribute values is ignored unless the
138           specified match value itself contains trailing whitespace.
139
140       ATTRS{filename}
141           Search the devpath upwards for a device with matching sysfs
142           attribute values. If multiple ATTRS matches are specified, all of
143           them must match on the same device.
144
145           Trailing whitespace in the attribute values is ignored unless the
146           specified match value itself contains trailing whitespace.
147
148       SYSCTL{kernel parameter}
149           Match a kernel parameter value.
150
151       ENV{key}
152           Match against a device property value.
153
154       CONST{key}
155           Match against a system-wide constant. Supported keys are:
156
157           "arch"
158               System's architecture. See ConditionArchitecture= in
159               systemd.unit(5) for possible values.
160
161           "virt"
162               System's virtualization environment. See systemd-detect-virt(1)
163               for possible values.
164
165           Unknown keys will never match.
166
167       TAG
168           Match against a device tag.
169
170       TAGS
171           Search the devpath upwards for a device with matching tag.
172
173       TEST{octal mode mask}
174           Test the existence of a file. An octal mode mask can be specified
175           if needed.
176
177       PROGRAM
178           Execute a program to determine whether there is a match; the key is
179           true if the program returns successfully. The device properties are
180           made available to the executed program in the environment. The
181           program's standard output is available in the RESULT key.
182
183           This can only be used for very short-running foreground tasks. For
184           details, see RUN.
185
186           Note that multiple PROGRAM keys may be specified in one rule, and
187           "=", ":=", and "+=" have the same effect as "==".
188
189       RESULT
190           Match the returned string of the last PROGRAM call. This key can be
191           used in the same or in any later rule after a PROGRAM call.
192
193       Most of the fields support shell glob pattern matching and alternate
194       patterns. The following special characters are supported:
195
196       "*"
197           Matches zero or more characters.
198
199       "?"
200           Matches any single character.
201
202       "[]"
203           Matches any single character specified within the brackets. For
204           example, the pattern string "tty[SR]" would match either "ttyS" or
205           "ttyR". Ranges are also supported via the "-" character. For
206           example, to match on the range of all digits, the pattern "[0-9]"
207           could be used. If the first character following the "[" is a "!",
208           any characters not enclosed are matched.
209
210       "|"
211           Separates alternative patterns. For example, the pattern string
212           "abc|x*" would match either "abc" or "x*".
213
214       The following keys can get values assigned:
215
216       NAME
217           The name to use for a network interface. See systemd.link(5) for a
218           higher-level mechanism for setting the interface name. The name of
219           a device node cannot be changed by udev, only additional symlinks
220           can be created.
221
222       SYMLINK
223           The name of a symlink targeting the node. Every matching rule adds
224           this value to the list of symlinks to be created.
225
226           The set of characters to name a symlink is limited. Allowed
227           characters are "0-9A-Za-z#+-.:=@_/", valid UTF-8 character
228           sequences, and "\x00" hex encoding. All other characters are
229           replaced by a "_" character.
230
231           Multiple symlinks may be specified by separating the names by the
232           space character. In case multiple devices claim the same name, the
233           link always points to the device with the highest link_priority. If
234           the current device goes away, the links are re-evaluated and the
235           device with the next highest link_priority becomes the owner of the
236           link. If no link_priority is specified, the order of the devices
237           (and which one of them owns the link) is undefined.
238
239           Symlink names must never conflict with the kernel's default device
240           node names, as that would result in unpredictable behavior.
241
242       OWNER, GROUP, MODE
243           The permissions for the device node. Every specified value
244           overrides the compiled-in default value.
245
246       SECLABEL{module}
247           Applies the specified Linux Security Module label to the device
248           node.
249
250       ATTR{key}
251           The value that should be written to a sysfs attribute of the event
252           device.
253
254       SYSCTL{kernel parameter}
255           The value that should be written to kernel parameter.
256
257       ENV{key}
258           Set a device property value. Property names with a leading "."  are
259           neither stored in the database nor exported to events or external
260           tools (run by, for example, the PROGRAM match key).
261
262       TAG
263           Attach a tag to a device. This is used to filter events for users
264           of libudev's monitor functionality, or to enumerate a group of
265           tagged devices. The implementation can only work efficiently if
266           only a few tags are attached to a device. It is only meant to be
267           used in contexts with specific device filter requirements, and not
268           as a general-purpose flag. Excessive use might result in
269           inefficient event handling.
270
271       RUN{type}
272           Specify a program to be executed after processing of all the rules
273           for the event. With "+=", this invocation is added to the list, and
274           with "=" or ":=", it replaces any previous contents of the list.
275           Please note that both "program" and "builtin" types described below
276           share a common list, so clearing the list with ":=" and "=" affects
277           both types.
278
279           type may be:
280
281           "program"
282               Execute an external program specified as the assigned value. If
283               no absolute path is given, the program is expected to live in
284               /usr/lib/udev; otherwise, the absolute path must be specified.
285
286               This is the default if no type is specified.
287
288           "builtin"
289               As program, but use one of the built-in programs rather than an
290               external one.
291
292           The program name and following arguments are separated by spaces.
293           Single quotes can be used to specify arguments with spaces.
294
295           This can only be used for very short-running foreground tasks.
296           Running an event process for a long period of time may block all
297           further events for this or a dependent device.
298
299           Note that running programs that access the network or mount/unmount
300           filesystems is not allowed inside of udev rules, due to the default
301           sandbox that is enforced on systemd-udevd.service.
302
303           Starting daemons or other long-running processes is not allowed;
304           the forked processes, detached or not, will be unconditionally
305           killed after the event handling has finished. In order to activate
306           long-running processes from udev rules, provide a service unit and
307           pull it in from a udev device using the SYSTEMD_WANTS device
308           property. See systemd.device(5) for details.
309
310       LABEL
311           A named label to which a GOTO may jump.
312
313       GOTO
314           Jumps to the next LABEL with a matching name.
315
316       IMPORT{type}
317           Import a set of variables as device properties, depending on type:
318
319           "program"
320               Execute an external program specified as the assigned value
321               and, if it returns successfully, import its output, which must
322               be in environment key format. Path specification,
323               command/argument separation, and quoting work like in RUN.
324
325           "builtin"
326               Similar to "program", but use one of the built-in programs
327               rather than an external one.
328
329           "file"
330               Import a text file specified as the assigned value, the content
331               of which must be in environment key format.
332
333           "db"
334               Import a single property specified as the assigned value from
335               the current device database. This works only if the database is
336               already populated by an earlier event.
337
338           "cmdline"
339               Import a single property from the kernel command line. For
340               simple flags the value of the property is set to "1".
341
342           "parent"
343               Import the stored keys from the parent device by reading the
344               database entry of the parent device. The value assigned to
345               IMPORT{parent} is used as a filter of key names to import (with
346               the same shell glob pattern matching used for comparisons).
347
348           This can only be used for very short-running foreground tasks. For
349           details see RUN.
350
351           Note that multiple IMPORT{} keys may be specified in one rule, and
352           "=", ":=", and "+=" have the same effect as "==". The key is true
353           if the import is successful, unless "!=" is used as the operator
354           which causes the key to be true if the import failed.
355
356       OPTIONS
357           Rule and device options:
358
359           link_priority=value
360               Specify the priority of the created symlinks. Devices with
361               higher priorities overwrite existing symlinks of other devices.
362               The default is 0.
363
364           string_escape=none|replace
365               Usually, control and other possibly unsafe characters are
366               replaced in strings used for device naming. The mode of
367               replacement can be specified with this option.
368
369           static_node=
370               Apply the permissions specified in this rule to the static
371               device node with the specified name. Also, for every tag
372               specified in this rule, create a symlink in the directory
373               /run/udev/static_node-tags/tag pointing at the static device
374               node with the specified name. Static device node creation is
375               performed by systemd-tmpfiles before systemd-udevd is started.
376               The static nodes might not have a corresponding kernel device;
377               they are used to trigger automatic kernel module loading when
378               they are accessed.
379
380           watch
381               Watch the device node with inotify; when the node is closed
382               after being opened for writing, a change uevent is synthesized.
383
384           nowatch
385               Disable the watching of a device node with inotify.
386
387           db_persist
388               Set the flag (sticky bit) on the udev database entry of the
389               event device. Device properties are then kept in the database
390               even when udevadm info --cleanup-db is called. This option can
391               be useful in certain cases (e.g. Device Mapper devices) for
392               persisting device state on the transition from initramfs.
393
394           log_level=level
395               Takes a log level name like "debug" or "info", or a special
396               value "reset". When a log level name is specified, the maximum
397               log level is changed to that level. When "reset" is set, then
398               the previously specified log level is revoked. Defaults to the
399               log level of the main process of systemd-udevd.
400
401               This may be useful when debugging events for certain devices.
402               Note that the log level is applied when the line including this
403               rule is processed. So, for debugging, it is recommended that
404               this is specified at earlier place, e.g., the first line of
405               00-debug.rules.
406
407               Example for debugging uevent processing for network interfaces.
408
409                   # /etc/udev/rules.d/00-debug-net.rules
410                   SUBSYSTEM=="net", OPTIONS="log_level=debug"
411
412       The NAME, SYMLINK, PROGRAM, OWNER, GROUP, MODE, SECLABEL, and RUN
413       fields support simple string substitutions. The RUN substitutions are
414       performed after all rules have been processed, right before the program
415       is executed, allowing for the use of device properties set by earlier
416       matching rules. For all other fields, substitutions are performed while
417       the individual rule is being processed. The available substitutions
418       are:
419
420       $kernel, %k
421           The kernel name for this device.
422
423       $number, %n
424           The kernel number for this device. For example, "sda3" has kernel
425           number 3.
426
427       $devpath, %p
428           The devpath of the device.
429
430       $id, %b
431           The name of the device matched while searching the devpath upwards
432           for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
433
434       $driver
435           The driver name of the device matched while searching the devpath
436           upwards for SUBSYSTEMS, KERNELS, DRIVERS, and ATTRS.
437
438       $attr{file}, %s{file}
439           The value of a sysfs attribute found at the device where all keys
440           of the rule have matched. If the matching device does not have such
441           an attribute, and a previous KERNELS, SUBSYSTEMS, DRIVERS, or ATTRS
442           test selected a parent device, then the attribute from that parent
443           device is used.
444
445           If the attribute is a symlink, the last element of the symlink
446           target is returned as the value.
447
448       $env{key}, %E{key}
449           A device property value.
450
451       $major, %M
452           The kernel major number for the device.
453
454       $minor, %m
455           The kernel minor number for the device.
456
457       $result, %c
458           The string returned by the external program requested with PROGRAM.
459           A single part of the string, separated by a space character, may be
460           selected by specifying the part number as an attribute: "%c{N}". If
461           the number is followed by the "+" character, this part plus all
462           remaining parts of the result string are substituted: "%c{N+}".
463
464       $parent, %P
465           The node name of the parent device.
466
467       $name
468           The current name of the device. If not changed by a rule, it is the
469           name of the kernel device.
470
471       $links
472           A space-separated list of the current symlinks. The value is only
473           set during a remove event or if an earlier rule assigned a value.
474
475       $root, %r
476           The udev_root value.
477
478       $sys, %S
479           The sysfs mount point.
480
481       $devnode, %N
482           The name of the device node.
483
484       %%
485           The "%" character itself.
486
487       $$
488           The "$" character itself.
489

SEE ALSO

491       systemd-udevd.service(8), udevadm(8), systemd.link(5)
492

NOTES

494        1. C-style escaped
495           https://en.wikipedia.org/wiki/Escape_sequences_in_C#Table_of_escape_sequences
496
497
498
499systemd 248                                                            UDEV(7)
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