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3802a3d3 1<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
63749b1a 2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4
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5<refentry id="udev">
6 <refentryinfo>
7b86ada7 7 <title>udev</title>
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8 <productname>systemd</productname>
9 <authorgroup>
10 <author>
11 <contrib>Developer</contrib>
12 <firstname>Greg</firstname>
13 <surname>Kroah-Hartmann</surname>
14 <email>greg@kroah.com</email>
15 </author>
16 <author>
17 <contrib>Developer</contrib>
18 <firstname>Kay</firstname>
19 <surname>Sievers</surname>
20 <email>kay@vrfy.org</email>
21 </author>
22 </authorgroup>
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23 </refentryinfo>
24
25 <refmeta>
26 <refentrytitle>udev</refentrytitle>
27 <manvolnum>7</manvolnum>
28 </refmeta>
29
30 <refnamediv>
31 <refname>udev</refname>
eb75d0ed 32 <refpurpose>Dynamic device management</refpurpose>
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33 </refnamediv>
34
35 <refsect1><title>Description</title>
36 <para>udev supplies the system software with device events, manages permissions
37 of device nodes and may create additional symlinks in the <filename>/dev</filename>
38 directory, or renames network interfaces. The kernel usually just assigns unpredictable
39 device names based on the order of discovery. Meaningful symlinks or network device
40 names provide a way to reliably identify devices based on their properties or
41 current configuration.</para>
42
b79f817b 43 <para>The udev daemon, <citerefentry><refentrytitle>systemd-udevd.service</refentrytitle>
0e4fa2ab 44 <manvolnum>8</manvolnum></citerefentry>, receives device uevents directly from
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45 the kernel whenever a device is added or removed from the system, or it changes its
46 state. When udev receives a device event, it matches its configured set of rules
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47 against various device attributes to identify the device. Rules that match may
48 provide additional device information to be stored in the udev database or
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49 to be used to create meaningful symlink names.</para>
50
0e846301 51 <para>All device information udev processes is stored in the udev database and
63749b1a 52 sent out to possible event subscribers. Access to all stored data and the event
17fd0f60 53 sources is provided by the library libudev.</para>
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54 </refsect1>
55
eb75d0ed 56 <refsect1><title>Rules Files</title>
63749b1a 57 <para>The udev rules are read from the files located in the
12a362be 58 system rules directory <filename>/usr/lib/udev/rules.d</filename>,
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59 the volatile runtime directory <filename>/run/udev/rules.d</filename>
60 and the local administration directory <filename>/etc/udev/rules.d</filename>.
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61 All rules files are collectively sorted and processed in lexical order,
62 regardless of the directories in which they live. However, files with
e9dd9f95 63 identical filenames replace each other. Files in <filename>/etc</filename>
91418155 64 have the highest priority, files in <filename>/run</filename> take precedence
e0e009c0 65 over files with the same name in <filename>/usr/lib</filename>. This can be
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66 used to override a system-supplied rules file with a local file if needed;
67 a symlink in <filename>/etc</filename> with the same name as a rules file in
e0e009c0 68 <filename>/usr/lib</filename>, pointing to <filename>/dev/null</filename>,
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69 disables the rules file entirely. Rule files must have the extension
70 <filename>.rules</filename>; other extensions are ignored.</para>
63749b1a 71
feedf8f3 72 <para>Every line in the rules file contains at least one key-value pair.
909f413d 73 Except for empty lines or lines beginning with <literal>#</literal>, which are ignored.
bb31a4ac 74 There are two kinds of keys: match and assignment.
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75 If all match keys match against their values, the rule gets applied and the
76 assignment keys get the specified values assigned.</para>
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77
78 <para>A matching rule may rename a network interface, add symlinks
79 pointing to the device node, or run a specified program as part of
80 the event handling.</para>
81
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82 <para>A rule consists of a comma-separated list of one or more key-value pairs.
83 Each key has a distinct operation, depending on the used operator. Valid
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84 operators are:</para>
85 <variablelist>
86 <varlistentry>
6b76fa66 87 <term><literal>==</literal></term>
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88 <listitem>
89 <para>Compare for equality.</para>
90 </listitem>
91 </varlistentry>
92
93 <varlistentry>
6b76fa66 94 <term><literal>!=</literal></term>
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95 <listitem>
96 <para>Compare for inequality.</para>
97 </listitem>
98 </varlistentry>
99
100 <varlistentry>
6b76fa66 101 <term><literal>=</literal></term>
63749b1a 102 <listitem>
0e846301 103 <para>Assign a value to a key. Keys that represent a list are reset
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104 and only this single value is assigned.</para>
105 </listitem>
106 </varlistentry>
107
108 <varlistentry>
6b76fa66 109 <term><literal>+=</literal></term>
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110 <listitem>
111 <para>Add the value to a key that holds a list of entries.</para>
112 </listitem>
113 </varlistentry>
114
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115 <varlistentry>
116 <term><literal>-=</literal></term>
117 <listitem>
118 <para>Remove the value from a key that holds a list of entries.</para>
119 </listitem>
120 </varlistentry>
121
63749b1a 122 <varlistentry>
6b76fa66 123 <term><literal>:=</literal></term>
63749b1a 124 <listitem>
2bf11540 125 <para>Assign a value to a key finally; disallow any later changes.</para>
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126 </listitem>
127 </varlistentry>
128 </variablelist>
129
130 <para>The following key names can be used to match against device properties.
131 Some of the keys also match against properties of the parent devices in sysfs,
132 not only the device that has generated the event. If multiple keys that match
133 a parent device are specified in a single rule, all these keys must match at
134 one and the same parent device.</para>
ccc9a4f9 135 <variablelist class='udev-directives'>
63749b1a 136 <varlistentry>
909f413d 137 <term><varname>ACTION</varname></term>
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138 <listitem>
139 <para>Match the name of the event action.</para>
140 </listitem>
141 </varlistentry>
142
143 <varlistentry>
909f413d 144 <term><varname>DEVPATH</varname></term>
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145 <listitem>
146 <para>Match the devpath of the event device.</para>
147 </listitem>
148 </varlistentry>
149
150 <varlistentry>
909f413d 151 <term><varname>KERNEL</varname></term>
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152 <listitem>
153 <para>Match the name of the event device.</para>
154 </listitem>
155 </varlistentry>
156
157 <varlistentry>
909f413d 158 <term><varname>NAME</varname></term>
63749b1a 159 <listitem>
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160 <para>Match the name of a network interface. It can be used once the
161 NAME key has been set in one of the preceding rules.</para>
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162 </listitem>
163 </varlistentry>
164
165 <varlistentry>
909f413d 166 <term><varname>SYMLINK</varname></term>
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167 <listitem>
168 <para>Match the name of a symlink targeting the node. It can
169 be used once a SYMLINK key has been set in one of the preceding
170 rules. There may be multiple symlinks; only one needs to match.
171 </para>
172 </listitem>
173 </varlistentry>
174
175 <varlistentry>
909f413d 176 <term><varname>SUBSYSTEM</varname></term>
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177 <listitem>
178 <para>Match the subsystem of the event device.</para>
179 </listitem>
180 </varlistentry>
181 <varlistentry>
909f413d 182 <term><varname>DRIVER</varname></term>
63749b1a 183 <listitem>
2e7f69eb 184 <para>Match the driver name of the event device. Only set this key for devices
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185 which are bound to a driver at the time the event is generated.</para>
186 </listitem>
187 </varlistentry>
188 <varlistentry>
909f413d 189 <term><varname>ATTR{<replaceable>filename</replaceable>}</varname></term>
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190 <listitem>
191 <para>Match sysfs attribute values of the event device. Trailing
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192 whitespace in the attribute values is ignored unless the specified match
193 value itself contains trailing whitespace.
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194 </para>
195 </listitem>
196 </varlistentry>
197
198 <varlistentry>
909f413d 199 <term><varname>KERNELS</varname></term>
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200 <listitem>
201 <para>Search the devpath upwards for a matching device name.</para>
202 </listitem>
203 </varlistentry>
204
205 <varlistentry>
909f413d 206 <term><varname>SUBSYSTEMS</varname></term>
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207 <listitem>
208 <para>Search the devpath upwards for a matching device subsystem name.</para>
209 </listitem>
210 </varlistentry>
211
212 <varlistentry>
909f413d 213 <term><varname>DRIVERS</varname></term>
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214 <listitem>
215 <para>Search the devpath upwards for a matching device driver name.</para>
216 </listitem>
217 </varlistentry>
218
219 <varlistentry>
909f413d 220 <term><varname>ATTRS{<replaceable>filename</replaceable>}</varname></term>
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221 <listitem>
222 <para>Search the devpath upwards for a device with matching sysfs attribute values.
909f413d 223 If multiple <varname>ATTRS</varname> matches are specified, all of them
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224 must match on the same device. Trailing whitespace in the attribute values is ignored
225 unless the specified match value itself contains trailing whitespace.</para>
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226 </listitem>
227 </varlistentry>
228
7df0ed83 229 <varlistentry>
909f413d 230 <term><varname>TAGS</varname></term>
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231 <listitem>
232 <para>Search the devpath upwards for a device with matching tag.</para>
233 </listitem>
234 </varlistentry>
235
63749b1a 236 <varlistentry>
909f413d 237 <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
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238 <listitem>
239 <para>Match against a device property value.</para>
240 </listitem>
241 </varlistentry>
242
243 <varlistentry>
909f413d 244 <term><varname>TAG</varname></term>
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245 <listitem>
246 <para>Match against a device tag.</para>
247 </listitem>
248 </varlistentry>
249
250 <varlistentry>
909f413d 251 <term><varname>TEST{<replaceable>octal mode mask</replaceable>}</varname></term>
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252 <listitem>
253 <para>Test the existence of a file. An octal mode mask can be specified
254 if needed.</para>
255 </listitem>
256 </varlistentry>
257
258 <varlistentry>
909f413d 259 <term><varname>PROGRAM</varname></term>
63749b1a 260 <listitem>
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261 <para>Execute a program to determine whether there
262 is a match; the key is true if the program returns
63749b1a 263 successfully. The device properties are made available to the
f131770b 264 executed program in the environment. The program's standard output
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265 is available in the <varname>RESULT</varname> key.</para>
266 <para>This can only be used for very short-running foreground tasks. For details,
909f413d 267 see <varname>RUN</varname>.</para>
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268 </listitem>
269 </varlistentry>
270
271 <varlistentry>
909f413d 272 <term><varname>RESULT</varname></term>
63749b1a 273 <listitem>
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274 <para>Match the returned string of the last <varname>PROGRAM</varname> call.
275 This key can be used in the same or in any later rule after a
276 <varname>PROGRAM</varname> call.</para>
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277 </listitem>
278 </varlistentry>
279 </variablelist>
280
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281 <para>Most of the fields support shell glob pattern matching and
282 alternate patterns. The following special characters are supported:</para>
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283 <variablelist>
284 <varlistentry>
6b76fa66 285 <term><literal>*</literal></term>
63749b1a 286 <listitem>
954e9c52 287 <para>Matches zero or more characters.</para>
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288 </listitem>
289 </varlistentry>
290 <varlistentry>
6b76fa66 291 <term><literal>?</literal></term>
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292 <listitem>
293 <para>Matches any single character.</para>
294 </listitem>
295 </varlistentry>
296 <varlistentry>
6b76fa66 297 <term><literal>[]</literal></term>
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298 <listitem>
299 <para>Matches any single character specified within the brackets. For
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300 example, the pattern string <literal>tty[SR]</literal>
301 would match either <literal>ttyS</literal> or <literal>ttyR</literal>.
302 Ranges are also supported via the <literal>-</literal> character.
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303 For example, to match on the range of all digits, the pattern
304 <literal>[0-9]</literal> could be used. If the first character
305 following the <literal>[</literal> is a <literal>!</literal>,
306 any characters not enclosed are matched.</para>
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307 </listitem>
308 </varlistentry>
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309 <varlistentry>
310 <term><literal>|</literal></term>
311 <listitem>
312 <para>Separates alternative patterns. For example, the pattern string
313 <literal>abc|x*</literal> would match either <literal>abc</literal>
314 or <literal>x*</literal>.</para>
315 </listitem>
316 </varlistentry>
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317 </variablelist>
318
319 <para>The following keys can get values assigned:</para>
ccc9a4f9 320 <variablelist class='udev-directives'>
63749b1a 321 <varlistentry>
909f413d 322 <term><varname>NAME</varname></term>
63749b1a 323 <listitem>
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324 <para>The name to use for a network interface. See
325 <citerefentry><refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum></citerefentry>
326 for a higher-level mechanism for setting the interface name.
327 The name of a device node cannot be changed by udev, only additional
328 symlinks can be created.</para>
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329 </listitem>
330 </varlistentry>
331
332 <varlistentry>
909f413d 333 <term><varname>SYMLINK</varname></term>
63749b1a 334 <listitem>
933b5623 335 <para>The name of a symlink targeting the node. Every matching rule adds
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336 this value to the list of symlinks to be created.</para>
337 <para>The set of characters to name a symlink is limited. Allowed
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338 characters are <literal>0-9A-Za-z#+-.:=@_/</literal>, valid UTF-8 character
339 sequences, and <literal>\x00</literal> hex encoding. All other
340 characters are replaced by a <literal>_</literal> character.</para>
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341 <para>Multiple symlinks may be specified by separating the names by the
342 space character. In case multiple devices claim the same name, the link
343 always points to the device with the highest link_priority. If the current
344 device goes away, the links are re-evaluated and the device with the
345 next highest link_priority becomes the owner of the link. If no
346 link_priority is specified, the order of the devices (and which one of
347 them owns the link) is undefined.</para>
348 <para>Symlink names must never conflict with the kernel's default device
349 node names, as that would result in unpredictable behavior.
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350 </para>
351 </listitem>
352 </varlistentry>
353
354 <varlistentry>
909f413d 355 <term><varname>OWNER</varname>, <varname>GROUP</varname>, <varname>MODE</varname></term>
63749b1a 356 <listitem>
91418155 357 <para>The permissions for the device node. Every specified value overrides
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358 the compiled-in default value.</para>
359 </listitem>
360 </varlistentry>
361
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362 <varlistentry>
363 <term><varname>SECLABEL{<replaceable>module</replaceable>}</varname></term>
364 <listitem>
365 <para>Applies the specified Linux Security Module label to the device node.</para>
366 </listitem>
367 </varlistentry>
368
63749b1a 369 <varlistentry>
909f413d 370 <term><varname>ATTR{<replaceable>key</replaceable>}</varname></term>
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371 <listitem>
372 <para>The value that should be written to a sysfs attribute of the
373 event device.</para>
374 </listitem>
375 </varlistentry>
376
377 <varlistentry>
909f413d 378 <term><varname>ENV{<replaceable>key</replaceable>}</varname></term>
63749b1a 379 <listitem>
909f413d 380 <para>Set a device property value. Property names with a leading <literal>.</literal>
ed9c5fb7 381 are neither stored in the database nor exported to events or
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382 external tools (run by, for example, the <varname>PROGRAM</varname>
383 match key).</para>
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384 </listitem>
385 </varlistentry>
386
387 <varlistentry>
909f413d 388 <term><varname>TAG</varname></term>
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389 <listitem>
390 <para>Attach a tag to a device. This is used to filter events for users
391 of libudev's monitor functionality, or to enumerate a group of tagged
392 devices. The implementation can only work efficiently if only a few
393 tags are attached to a device. It is only meant to be used in
394 contexts with specific device filter requirements, and not as a
395 general-purpose flag. Excessive use might result in inefficient event
396 handling.</para>
397 </listitem>
398 </varlistentry>
399
400 <varlistentry>
909f413d 401 <term><varname>RUN{<replaceable>type</replaceable>}</varname></term>
63749b1a 402 <listitem>
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403 <para>Add a program to the list of programs to be executed after
404 processing all the rules for a specific event, depending on
405 <literal>type</literal>:</para>
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406 <variablelist>
407 <varlistentry>
408 <term><literal>program</literal></term>
409 <listitem>
410 <para>Execute an external program specified as the assigned
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411 value. If no absolute path is given, the program is expected
412 to live in <filename>/usr/lib/udev</filename>; otherwise, the
413 absolute path must be specified.</para>
414 <para>This is the default if no <replaceable>type</replaceable>
415 is specified.</para>
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416 </listitem>
417 </varlistentry>
418 <varlistentry>
419 <term><literal>builtin</literal></term>
420 <listitem>
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421 <para>As <varname>program</varname>, but use one of the
422 built-in programs rather than an external one.</para>
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423 </listitem>
424 </varlistentry>
425 </variablelist>
426 <para>The program name and following arguments are separated by spaces.
427 Single quotes can be used to specify arguments with spaces.</para>
b45f770f 428 <para>This can only be used for very short-running foreground tasks. Running an
2b76eb68 429 event process for a long period of time may block all further events for
b45f770f 430 this or a dependent device.</para>
bcddd5bf 431 <para>Starting daemons or other long-running processes is not appropriate
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432 for udev; the forked processes, detached or not, will be unconditionally
433 killed after the event handling has finished.</para>
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434 </listitem>
435 </varlistentry>
436
437 <varlistentry>
909f413d 438 <term><varname>LABEL</varname></term>
63749b1a 439 <listitem>
bcddd5bf 440 <para>A named label to which a <varname>GOTO</varname> may jump.</para>
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441 </listitem>
442 </varlistentry>
443
444 <varlistentry>
909f413d 445 <term><varname>GOTO</varname></term>
63749b1a 446 <listitem>
bcddd5bf 447 <para>Jumps to the next <varname>LABEL</varname> with a matching name.</para>
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448 </listitem>
449 </varlistentry>
450
451 <varlistentry>
909f413d 452 <term><varname>IMPORT{<replaceable>type</replaceable>}</varname></term>
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453 <listitem>
454 <para>Import a set of variables as device properties,
6b76fa66 455 depending on <literal>type</literal>:</para>
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456 <variablelist>
457 <varlistentry>
6b76fa66 458 <term><literal>program</literal></term>
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459 <listitem>
460 <para>Execute an external program specified as the assigned value and
461 import its output, which must be in environment key
462 format. Path specification, command/argument separation,
909f413d 463 and quoting work like in <varname>RUN</varname>.</para>
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464 </listitem>
465 </varlistentry>
466 <varlistentry>
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467 <term><literal>builtin</literal></term>
468 <listitem>
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469 <para>Similar to <literal>program</literal>, but use one of the
470 built-in programs rather than an external one.</para>
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471 </listitem>
472 </varlistentry>
473 <varlistentry>
6b76fa66 474 <term><literal>file</literal></term>
63749b1a 475 <listitem>
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476 <para>Import a text file specified as the assigned value, the content
477 of which must be in environment key format.</para>
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478 </listitem>
479 </varlistentry>
480 <varlistentry>
6b76fa66 481 <term><literal>db</literal></term>
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482 <listitem>
483 <para>Import a single property specified as the assigned value from the
484 current device database. This works only if the database is already populated
485 by an earlier event.</para>
486 </listitem>
487 </varlistentry>
c4f6dcc4 488 <varlistentry>
6b76fa66 489 <term><literal>cmdline</literal></term>
c4f6dcc4 490 <listitem>
ad9c70a8 491 <para>Import a single property from the kernel command line. For simple flags
909f413d 492 the value of the property is set to <literal>1</literal>.</para>
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493 </listitem>
494 </varlistentry>
63749b1a 495 <varlistentry>
6b76fa66 496 <term><literal>parent</literal></term>
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497 <listitem>
498 <para>Import the stored keys from the parent device by reading
499 the database entry of the parent device. The value assigned to
500 <option>IMPORT{parent}</option> is used as a filter of key names
eb75d0ed 501 to import (with the same shell glob pattern matching used for
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502 comparisons).</para>
503 </listitem>
504 </varlistentry>
505 </variablelist>
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506 <para>This can only be used for very short-running foreground tasks. For details
507 see <option>RUN</option>.</para>
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508 </listitem>
509 </varlistentry>
510
511 <varlistentry>
909f413d 512 <term><varname>WAIT_FOR</varname></term>
63749b1a 513 <listitem>
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514 <para>Wait for a file to become available or until a timeout of
515 10 seconds expires. The path is relative to the sysfs device;
516 if no path is specified, this waits for an attribute to appear.</para>
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517 </listitem>
518 </varlistentry>
519
520 <varlistentry>
909f413d 521 <term><varname>OPTIONS</varname></term>
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522 <listitem>
523 <para>Rule and device options:</para>
ccc9a4f9 524 <variablelist class='udev-directives'>
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525 <varlistentry>
526 <term><option>link_priority=<replaceable>value</replaceable></option></term>
527 <listitem>
528 <para>Specify the priority of the created symlinks. Devices with higher
529 priorities overwrite existing symlinks of other devices. The default is 0.</para>
530 </listitem>
531 </varlistentry>
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532 <varlistentry>
533 <term><option>string_escape=<replaceable>none|replace</replaceable></option></term>
534 <listitem>
535 <para>Usually control and other possibly unsafe characters are replaced
536 in strings used for device naming. The mode of replacement can be specified
537 with this option.</para>
538 </listitem>
539 </varlistentry>
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540 <varlistentry>
541 <term><option>static_node=</option></term>
542 <listitem>
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543 <para>Apply the permissions specified in this rule to the
544 static device node with the specified name. Also, for every
545 tag specified in this rule, create a symlink
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546 in the directory
547 <filename>/run/udev/static_node-tags/<replaceable>tag</replaceable></filename>
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548 pointing at the static device node with the specified name.
549 Static device node creation is performed by systemd-tmpfiles
550 before systemd-udevd is started. The static nodes might not
551 have a corresponding kernel device; they are used to trigger
552 automatic kernel module loading when they are accessed.</para>
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553 </listitem>
554 </varlistentry>
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555 <varlistentry>
556 <term><option>watch</option></term>
557 <listitem>
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558 <para>Watch the device node with inotify; when the node is
559 closed after being opened for writing, a change uevent is
560 synthesized.</para>
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561 </listitem>
562 </varlistentry>
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563 <varlistentry>
564 <term><option>nowatch</option></term>
565 <listitem>
566 <para>Disable the watching of a device node with inotify.</para>
567 </listitem>
568 </varlistentry>
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569 </variablelist>
570 </listitem>
571 </varlistentry>
572 </variablelist>
573
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574 <para>The <varname>NAME</varname>, <varname>SYMLINK</varname>,
575 <varname>PROGRAM</varname>, <varname>OWNER</varname>,
576 <varname>GROUP</varname>, <varname>MODE</varname>, and
577 <varname>RUN</varname> fields support simple string substitutions.
578 The <varname>RUN</varname> substitutions are performed after all rules
579 have been processed, right before the program is executed, allowing for
580 the use of device properties set by earlier matching rules. For all other
581 fields, substitutions are performed while the individual rule is being
582 processed. The available substitutions are:</para>
ccc9a4f9 583 <variablelist class='udev-directives'>
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584 <varlistentry>
585 <term><option>$kernel</option>, <option>%k</option></term>
586 <listitem>
587 <para>The kernel name for this device.</para>
588 </listitem>
589 </varlistentry>
590
591 <varlistentry>
592 <term><option>$number</option>, <option>%n</option></term>
593 <listitem>
909f413d 594 <para>The kernel number for this device. For example,
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595 <literal>sda3</literal> has kernel number <literal>3</literal>.
596 </para>
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597 </listitem>
598 </varlistentry>
599
600 <varlistentry>
601 <term><option>$devpath</option>, <option>%p</option></term>
602 <listitem>
603 <para>The devpath of the device.</para>
604 </listitem>
605 </varlistentry>
606
607 <varlistentry>
608 <term><option>$id</option>, <option>%b</option></term>
609 <listitem>
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610 <para>The name of the device matched while searching the devpath
611 upwards for <option>SUBSYSTEMS</option>, <option>KERNELS</option>,
612 <option>DRIVERS</option>, and <option>ATTRS</option>.
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613 </para>
614 </listitem>
615 </varlistentry>
616
617 <varlistentry>
618 <term><option>$driver</option></term>
619 <listitem>
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620 <para>The driver name of the device matched while searching the
621 devpath upwards for <option>SUBSYSTEMS</option>,
622 <option>KERNELS</option>, <option>DRIVERS</option>, and
623 <option>ATTRS</option>.
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624 </para>
625 </listitem>
626 </varlistentry>
627
628 <varlistentry>
629 <term><option>$attr{<replaceable>file</replaceable>}</option>, <option>%s{<replaceable>file</replaceable>}</option></term>
630 <listitem>
0e846301 631 <para>The value of a sysfs attribute found at the device where
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632 all keys of the rule have matched. If the matching device does not
633 have such an attribute, and a previous <option>KERNELS</option>,
634 <option>SUBSYSTEMS</option>, <option>DRIVERS</option>, or
635 <option>ATTRS</option> test selected a parent device, then the
636 attribute from that parent device is used.
637 </para>
638 <para>If the attribute is a symlink, the last element of the
639 symlink target is returned as the value.
640 </para>
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641 </listitem>
642 </varlistentry>
643
644 <varlistentry>
645 <term><option>$env{<replaceable>key</replaceable>}</option>, <option>%E{<replaceable>key</replaceable>}</option></term>
646 <listitem>
647 <para>A device property value.</para>
648 </listitem>
649 </varlistentry>
650
651 <varlistentry>
652 <term><option>$major</option>, <option>%M</option></term>
653 <listitem>
654 <para>The kernel major number for the device.</para>
655 </listitem>
656 </varlistentry>
657
658 <varlistentry>
659 <term><option>$minor</option>, <option>%m</option></term>
660 <listitem>
661 <para>The kernel minor number for the device.</para>
662 </listitem>
663 </varlistentry>
664
665 <varlistentry>
666 <term><option>$result</option>, <option>%c</option></term>
667 <listitem>
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668 <para>The string returned by the external program requested with
669 <varname>PROGRAM</varname>.
21188522 670 A single part of the string, separated by a space character, may be selected
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671 by specifying the part number as an attribute: <literal>%c{N}</literal>.
672 If the number is followed by the <literal>+</literal> character, this part plus all remaining parts
673 of the result string are substituted: <literal>%c{N+}</literal>.</para>
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674 </listitem>
675 </varlistentry>
676
677 <varlistentry>
678 <term><option>$parent</option>, <option>%P</option></term>
679 <listitem>
680 <para>The node name of the parent device.</para>
681 </listitem>
682 </varlistentry>
683
684 <varlistentry>
685 <term><option>$name</option></term>
686 <listitem>
220893b3 687 <para>The current name of the device. If not changed by a rule, it is the
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688 name of the kernel device.</para>
689 </listitem>
690 </varlistentry>
691
692 <varlistentry>
693 <term><option>$links</option></term>
694 <listitem>
e8b5e4e5 695 <para>A space-separated list of the current symlinks. The value is
2d74bc75 696 only set during a remove event or if an earlier rule assigned a value.</para>
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697 </listitem>
698 </varlistentry>
699
700 <varlistentry>
701 <term><option>$root</option>, <option>%r</option></term>
702 <listitem>
703 <para>The udev_root value.</para>
704 </listitem>
705 </varlistentry>
706
707 <varlistentry>
708 <term><option>$sys</option>, <option>%S</option></term>
709 <listitem>
710 <para>The sysfs mount point.</para>
711 </listitem>
712 </varlistentry>
713
714 <varlistentry>
220893b3 715 <term><option>$devnode</option>, <option>%N</option></term>
63749b1a 716 <listitem>
220893b3 717 <para>The name of the device node.</para>
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718 </listitem>
719 </varlistentry>
720
721 <varlistentry>
909f413d 722 <term><option>%%</option></term>
63749b1a 723 <listitem>
909f413d 724 <para>The <literal>%</literal> character itself.</para>
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725 </listitem>
726 </varlistentry>
727
728 <varlistentry>
909f413d 729 <term><option>$$</option></term>
63749b1a 730 <listitem>
909f413d 731 <para>The <literal>$</literal> character itself.</para>
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732 </listitem>
733 </varlistentry>
734 </variablelist>
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735 </refsect1>
736
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737 <refsect1>
738 <title>See Also</title>
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739 <para>
740 <citerefentry>
b79f817b 741 <refentrytitle>systemd-udevd.service</refentrytitle><manvolnum>8</manvolnum>
ad29a9f1 742 </citerefentry>,
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743 <citerefentry>
744 <refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum>
2b09983f 745 </citerefentry>,
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746 <citerefentry>
747 <refentrytitle>systemd.link</refentrytitle><manvolnum>5</manvolnum>
748 </citerefentry>
bcddd5bf 749 </para>
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750 </refsect1>
751</refentry>