1<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 2 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"> 4<head> 5<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> 6<style type="text/css"> 7<!-- 8 .green { color: #003300; } 9 .blue { color: #0000CC; } 10//--> 11</style> 12<title>Writing udev rules</title> 13</head> 14 15<body> 16 17<h1>Writing udev rules</h1> 18by Daniel Drake (dsd)<br /> 19Version 0.72<br /><br /> 20 21The most recent version of this document can always be found at: <br /> 22 23<a href="http://www.reactivated.net/writing_udev_rules.html">http://www.reactivated.net/writing_udev_rules.html</a> 24 25<h2>Contents</h2> 26<ul> 27<li>Introduction 28 <ul> 29 <li><a href="#about">About this document</a></li> 30 <li><a href="#history">History</a></li> 31 </ul> 32</li> 33<li>The concepts 34 <ul> 35 <li><a href="#terminology">Terminology: devfs, sysfs, nodes, etc.</a></li> 36 <li><a href="#why">Why?</a></li> 37 <li><a href="#builtin">Built-in persistent naming schemes</a></li> 38 </ul> 39</li> 40<li>Rule writing 41 <ul> 42 <li><a href="#syntax">Rule files and semantics</a></li> 43 <li><a href="#syntax">Rule syntax</a></li> 44 <li><a href="#basic">Basic rules</a></li> 45 <li><a href="#sysfsmatch">Matching sysfs attributes</a></li> 46 <li><a href="#strsubst">String substitutions</a></li> 47 <li><a href="#strmatch">String matching</a></li> 48 </ul> 49</li> 50<li>Finding suitable information from sysfs 51 <ul> 52 <li><a href="#sysfstree">The sysfs tree</a></li> 53 <li><a href="#udevinfo">udevinfo</a></li> 54 <li><a href="#sysfsalt">Alternative methods</a></li> 55 </ul> 56</li> 57<li>Advanced topics 58 <ul> 59 <li><a href="#ownership">Controlling permissions and ownership</a></li> 60 <li><a href="#external-naming">Using external programs to name devices</a></li> 61 <li><a href="#external-run">Running external programs on certain events</a></li> 62 <li><a href="#env">Environment interaction</a></li> 63 <li><a href="options">Additional options</a></li> 64 </ul> 65</li> 66<li>Examples 67 <ul> 68 <li><a href="#example-printer">USB Printer</a></li> 69 <li><a href="#example-camera">USB Camera</a></li> 70 <li><a href="#example-usbhdd">USB Hard Disk</a></li> 71 <li><a href="#example-usbcardreader">USB Hard Disk</a></li> 72 <li><a href="#example-pilot">USB Palm Pilot</a></li> 73 <li><a href="#example-cdrom">CD/DVD drives</a></li> 74 <li><a href="#example-netif">Network interfaces</a></li> 75 </ul> 76</li> 77<li>Testing and debugging 78 <ul> 79 <li><a href="#testing">Putting your rules into action</a></li> 80 <li><a href="#udevtest">udevtest</a></li> 81 </ul> 82</li> 83<li><a href="#author">Author and contact</a></li> 84</ul> 85 86<h2>Introduction</h2> 87 88<a name="about"></a> 89<h3>About this document</h3> 90 91<p> 92udev is targeted at Linux kernels 2.6 and beyond to provide a userspace solution for a dynamic /dev directory, with persistent device naming. The previous /dev implementation, <i>devfs</i>, is now deprecated, and udev is seen as the successor. udev vs devfs is a sensitive area of conversation - you should read <a href="http://kernel.org/pub/linux/utils/kernel/hotplug/udev_vs_devfs">this document</a> before making comparisons. 93</p> 94 95<p> 96Over the years, the things that you might use udev rules for has changed, as well as the flexibility of rules themselves. On a modern system, udev provides persistent naming for some device types out-of-the-box, eliminating the need for custom rules for those devices. However, some users will still require the extra level of customisation. 97</p> 98 99<p> 100This document assumes that you have udev installed and running OK with default configurations. This is usually handled by your Linux distribution. 101</p> 102 103<p> 104This document does not cover every single detail of rule writing, but does aim to introduce all of the main concepts. The finer details can be found in the udev man page. 105</p> 106 107<p> 108This document uses various examples (many of which are entirely fictional) to illustrate ideas and concepts. Not all syntax is explicitly described in the accompanying text, be sure to look at the example rules to get a complete understanding. 109</p> 110 111<a name="history"></a> 112<h3>History</h3> 113 114<ul> 115<ul>October 2nd 2006 v0.72: Fixed a typo in one of the example rules.</li> 116<li>June 10th 2006 v0.71: Misc changes based on recent feedback - thanks!</li> 117<li>June 3rd 2006 v0.7: Complete rework, to be more suited for the modern-day udev.</li> 118<li>May 9th 2005 v0.6: Misc updates, including information about udevinfo, groups and permissions, logging, and udevtest.</li> 119<li>June 20th 2004 v0.55: Added info on multiple symlinks, and some minor changes/updates.</li> 120<li>April 26th 2004 v0.54: Added some Debian info. Minor corrections. Re-reverted information about what to call your rule file. Added info about naming network interfaces.</li> 121<li>April 15th 2004 v0.53: Minor corrections. Added info about NAME{all_partitions}. Added info about other udevinfo tricks.</li> 122<li>April 14th 2004 v0.52: Reverted to suggesting using "udev.rules" until the udev defaults allow for other files. Minor work.</li> 123<li>April 6th 2004 v0.51: I now write suggest users to use their own "local.rules" file rather than prepending "udev.rules".</li> 124<li>April 3rd 2004 v0.5: Minor cleanups and preparations for possible inclusion in the udev distribution.</li> 125<li>March 20th 2004 v0.4: General improvements, clarifications, and cleanups. Added more information about writing rules for usb-storage.</li> 126<li>February 23rd 2004 v0.3: Rewrote some parts to emphasise how sysfs naming works, and how it can be matched. Updated rule-writing parts to represent udev 018s new SYSFS{filename} naming scheme. Improved sectioning, and clarified many points. Added info about KDE.</li> 127<li>February 18th 2004 v0.2: Fixed a small omission in an example. Updated section on identifying mass-storage devices. Updated section on nvidia.</li> 128<li>February 15th 2004 v0.1: Initial publication.</li> 129</ul> 130 131<h2>The concepts</h2> 132 133<a name="terminology"></a> 134<h3>Terminology: devfs, sysfs, nodes, etc.</h3> 135 136<p> 137A basic introduction only, might not be totally accurate. 138</p> 139 140<p> 141On typical Linux-based systems, the <i>/dev</i> directory is used to store file-like device <b>nodes</b> which refer to certain devices in the system. Each node points to a part of the system (a device), which might or might not exist. Userspace applications can use these device nodes to interface with the systems hardware, for example, the X server will "listen to" /dev/input/mice so that it can relate the user's mouse movements to moving the visual mouse pointer. 142</p> 143 144<p> 145The original <i>/dev</i> directories were just populated with every device that might possibly appear in the system. /dev directories were typically very large because of this. <b>devfs</b> came along to provide a more manageable approach (noticeably, it only populated /dev with hardware that is plugged into the system), as well as some other functionality, but the system proved to have problems which could not be easily fixed. 146</p> 147 148<p> 149<b>udev</b> is the "new" way of managing <i>/dev</i> directories, designed to clear up some issues with previous <i>/dev</i> implementations, and provide a robust path forward. In order to create and name <i>/dev</i> device nodes corresponding to devices that are present in the system, udev relies on matching information provided by <i>sysfs</i> with <i>rules</i> provided by the user. This documentation aims to detail the process of rule-writing, one of the only udev-related tasks that must (optionally) be performed by the user. 150</p> 151 152<p> 153<b>sysfs</b> is a new filesystem to the 2.6 kernels. It is managed by the kernel, and exports basic information about the devices currently plugged into your system. udev can use this information to create device nodes corresponding to your hardware. sysfs is mounted at <i>/sys</i> and is browseable. You may wish to investigate some of the files stored there before getting to grips with udev. Throughout this document, I will use the terms <i>/sys</i> and <i>sysfs</i> interchangeably. 154</p> 155 156<a name="why"></a> 157<h3>Why?</h3> 158 159<p> 160udev rules are flexible and very powerful. Here are some of the things you can use rules to achieve: 161</p> 162 163<ul> 164<li>Rename a device node from the default name to something else</li> 165<li>Provide an alternative/persistent name for a device node by creating a symbolic link to the default device node</li> 166<li>Name a device node based on the output of a program</li> 167<li>Change permissions and ownership of a device node</li> 168<li>Launch a script when a device node is created or deleted (typically when a device is attached or unplugged)</li> 169<li>Rename network interfaces</li> 170</ul> 171 172<p> 173Writing rules is <u>not</u> a workaround for the problem where no device nodes for your particular device exist. Even if there are no matching rules, udev will create the device node with the default name supplied by the kernel. 174</p> 175 176<p> 177Having persistently named device nodes has several advantages. Assume you own two USB storage devices: a digital camera and a USB flash disk. These devices are typically assigned device nodes <em>/dev/sda</em> and <em>/dev/sdb</em> but the exact assignment depends on the order which they were originally connected. This may cause problems to some users, who would benefit greatly if each device could be named persistently every time, e.g. <em>/dev/camera</em> and <em>/dev/flashdisk</em>. 178</p> 179 180<a name="builtin"></a> 181<h3>Built-in persistent naming schemes</h3> 182 183<p> 184udev provides persistent naming for some device types out of the box. This is a very useful feature, and in many circumstances means that your journey ends here: you do not have to write any rules. 185</p> 186 187<p> 188udev provides out-of-the-box persistent naming for storage devices in the <em>/dev/disk</em> directory. To view the persistent names which have been created for your storage hardware, you can use the following command: 189</p> 190 191<blockquote><pre># ls -lR /dev/disk</pre></blockquote> 192 193<p> 194This works for all storage types. As an example, udev has created <em>/dev/disk/by-id/scsi-SATA_ST3120827AS_4MS1NDXZ-part3</em> which is a persistent-named symbolic link to my root partition. udev creates <em>/dev/disk/by-id/usb-Prolific_Technology_Inc._USB_Mass_Storage_Device-part1</em> when I plug my USB flash disk in, which is also a persistent name. 195</p> 196 197<!-- FIXME: document CDROM persistent naming when things settle down --> 198 199<h2>Rule writing</h2> 200 201<a name="files"></a> 202<h3>Rule files and semantics</h3> 203 204<p> 205When deciding how to name a device and which additional actions to perform, udev reads a series of rules files. These files are kept in the <em>/etc/udev/rules.d</em> directory, and they all must have the <em>.rules</em> suffix. 206</p> 207 208<p> 209Default udev rules are stored in <em>/etc/udev/rules.d/50-udev.rules</em>. You may find it interesting to look over this file - it includes a few examples, and then some default rules proving a devfs-style /dev layout. However, you should not write rules into this file directly. 210</p> 211 212<p> 213Files in /etc/udev/rules.d/ are parsed in <b>lexical</b> order, and in some circumstances, the order in which rules are parsed is important. In general, you want your own rules to be parsed before the defaults, so I suggest you create a file at <em>/etc/udev/rules.d/10-local.rules</em> and write all your rules into this file. 214</p> 215 216<p> 217In a rules file, lines starting with "#" are treated as comments. Every other non-blank line is a rule. Rules cannot span multiple lines. 218</p> 219 220<p> 221One device can be matched by more than one rule. This has it's practical advantages, for example, we can write two rules which match the same device, where each one provides its own alternate name for the device. Both alternate names will be created, even if the rules are in separate files. It is important to understand that udev will <em>not</em> stop processing when it finds a matching rule, it will continue searching and attempt to apply every rule that it knows about. 222</p> 223 224<a name="syntax"></a> 225<h3>Rule syntax</h3> 226 227<p> 228Each rule is constructed from a series of key-value pairs, which are separated by commas. <b>match</b> keys are conditions used to identify the device which the rule is acting upon. When <u>all</u> match keys in a rule correspond to the device being handled, then the rule is applied and the actions of the <b>assignment</b> keys are invoked. Every rule should consist of at least one match key and at least one assignment key. 229</p> 230 231<p> 232Here is an example rule to illustrate the above: 233</p> 234 235<blockquote><pre>KERNEL=="hdb", NAME="my_spare_disk"</pre></blockquote> 236 237<p> 238The above rule includes one match key (<em>KERNEL</em>) and one assignment key (<em>NAME</em>). The semantics of these keys and their properties will be detailed later. It is important to note that the match key is related to its value through the equality operator (==), whereas the assignment key is related to its value through the assignment operator (=). 239</p> 240 241<a name="basic"></a> 242<h3>Basic Rules</h3> 243 244<p> 245udev provides several different match keys which can be used to write rules which match devices very precisely. Some of the most common keys are introduced below, others will be introduced later in this document. For a complete list, see the udev man page. 246</p> 247 248<ul> 249<li><b>KERNEL</b> - match against the kernel name for the device</li> 250<li><b>SUBSYSTEM</b> - match against the subsystem of the device</li> 251<li><b>DRIVER</b> - match against the driver name for the device</li> 252</ul> 253 254<p> 255After you have used a series of match keys to precisely match a device, udev gives you fine control over what happens next, through a range of assignment keys. For a complete list of possible assignment keys, see the udev man page. The most basic assignment keys are introduced below. Others will be introduced later in this document. 256</p> 257 258<ul> 259<li><b>NAME</b> - the name that shall be used for the device node</li> 260<li><b>SYMLINK</b> - a <b>list</b> of symbolic links which act as alternative names for the device node</li> 261</ul> 262 263<p> 264As hinted above, udev only creates one true device node for one device. If you wish to provide alternate names for this device node, you use the symbolic link functionality. With the <em>SYMLINK</em> assignment, you are actually maintaining a <em>list</em> of symbolic links, all of which will be pointed at the real device node. To manipulate these links, we introduce a new operator for appending to lists: <b>+=</b>. You can append multiple symlinks to the list from any one rule by separating each one with a space. 265</p> 266 267<blockquote><pre>KERNEL=="hdb", NAME="my_spare_disk"</pre></blockquote> 268 269<p> 270The above rule says: <em>match a device which was named by the kernel as hdb, and instead of calling it hdb, name the device node as my_spare_disk</em>. The device node appears at <em>/dev/my_spare_disk</em>. 271</p> 272 273<blockquote><pre>KERNEL=="hdb", DRIVER=="ide-disk", SYMLINK+="sparedisk"</pre></blockquote> 274 275<p> 276The above rule says: <em>match a device which was named by the kernel as hdb AND where the driver is ide-disk. Name the device node with the default name and create a symbolic link to it named sparedisk</em>. Note that we did not specify a device node name, so udev uses the default. In order to preserve the standard <em>/dev</em> layout, your own rules will typically leave the NAME alone but create some SYMLINKs and/or perform other assignments. 277</p> 278 279<blockquote><pre>KERNEL=="hdc", SYMLINK+="cdrom cdrom0"</pre></blockquote> 280 281<p> 282The above rule is probably more typical of the types of rules you might be writing. It creates two symbolic links at <em>/dev/cdrom</em> and <em>/dev/cdrom0</em>, both of which point at <em>/dev/hdc</em>. Again, no NAME assignment was specified, so the default kernel name (hdc) is used. 283</p> 284 285<a name="sysfsmatch"></a> 286<h3>Matching sysfs attributes</h3> 287 288<p> 289The match keys introduced so far only provide limited matching capabilities. Realistically we require much finer control: we want to identify devices based on advanced properties such as vendor codes, exact product numbers, serial numbers, storage capacities, number of partitions, etc. 290</p> 291 292<p> 293Many drivers export information like this into sysfs, and udev allows us to incorporate sysfs-matching into our rules, using the <em>SYSFS</em> key with a slightly different syntax. 294</p> 295 296<p> 297Here are some examples of rules which match data from sysfs. Further detail will be provided later in this document which will aid you in writing rules based on sysfs attributes. 298</p> 299 300<blockquote><pre> 301KERNEL=="sda", SYSFS{model}=="ST3120827AS", SYMLINK+="my_hard_disk" 302SUBSYSTEM=="block", SYSFS{size}=="234441648", SYMLINK+="my_disk" 303BUS=="usb", SYSFS{manufacturer}=="OLYMPUS", SYSFS{product}=="X250,D560Z,C350Z", SYMLINK+="camera" 304</pre></blockquote> 305 306<a name="strsubst"></a> 307<h3>String substitutions</h3> 308 309<p> 310When writing rules which will potentially handle multiple similar devices, udev's <em>printf-like string substitution operators</em> are very useful. You can simply include these operators in any assignments your rule makes, and udev will evaluate them when they are executed. 311</p> 312 313<p> 314The most common operators are <b>%k</b> and <b>%n</b>. %k evaluates to the kernel name for the device, e.g. "sda3" for a device that would (by default) appear at <em>/dev/sda3</em>. <b>%n</b> evaluates to the kernel number for the device (the partition number for storage devices), e.g. "3" for <em>/dev/sda3</em>. 315</p> 316 317<p> 318udev also provides several other substitution operators for more advanced functionality. Consult the udev man page after reading the rest of this document. There is also an alternative syntax for these operators - <b>$kernel</b> and <b>$number</b> for the examples above. For this reason, if you wish to match a literal % in a rule then you must write <b>%%</b>, and if you wish to match a literal $ then you must write <b>$$</b>. 319</p> 320 321<p> 322To illustrate the concept of string substitution, some example rules are shown below. 323</p> 324 325<blockquote><pre> 326KERNEL=="mice", NAME="input/%k" 327KERNEL=="loop0", NAME="loop/%n", SYMLINK+="%k" 328</pre></blockquote> 329 330<p> 331The first rule ensures that the mice device node appears exclusively in the <em>/dev/input</em> directory (by default it would be at <em>/dev/mice</em>). The second rule ensures that the device node named loop0 is created at <em>/dev/loop/0</em> but also creates a symbolic link at <em>/dev/loop0</em> as usual. 332</p> 333 334<p> 335The use of the above rules is questionable, as they all could be rewritten without using any substitution operators. The true power of these substitutions will become apparent in the next section. 336</p> 337 338<a name="strmatch"></a> 339<h3>String matching</h3> 340 341<p> 342As well as matching strings exactly, udev allows you to use shell-style pattern matching. There are 3 patterns supported: 343</p> 344 345<ul> 346<li><b>*</b> - match any character, zero or more times</li> 347<li><b>?</b> - match any character exactly once</li> 348<li><b>[]</b> - match any single character specified in the brackets, ranges are also permitted</li> 349</ul> 350 351<p> 352Here are some examples which incorporate the above patterns. Note the use of the string substitution operators. 353</p> 354 355<blockquote><pre> 356KERNEL=="fd[0-9]*", NAME="floppy/%n", SYMLINK+="%k" 357KERNEL=="hiddev*", NAME="usb/%k" 358</pre></blockquote> 359 360<p> 361The first rule matches all floppy disk drives, and ensures that the device nodes are placed in the <em>/dev/floppy</em> directory, as well as creating a symbolic link from the default name. The second rule ensures that hiddev devices are only present in the <em>/dev/usb</em> directory. 362</p> 363 364<h2>Finding information from sysfs</h2> 365 366<a name="sysfstree"></a> 367<h3>The sysfs tree</h3> 368 369<p> 370The concept of using interesting information from sysfs was briefly touched upon above. In order to write rules based on this information, you first need to know the names of the attributes and their current values. 371</p> 372 373<p> 374sysfs is actually a very simple structure. It is logically divided into directories. Each directory contains a number of files (<em>attributes</em>) which typically contain just one value. Some symbolic links are present, which link various parts of the tree together. 375</p> 376 377<p> 378Some directories are referred to as <em>top-level device paths</em>. These directories act as the top-level glue which chain other parts of sysfs to the device in question. Top-level device paths can be classified as sysfs directories which contain a <em>dev</em> file, the following command will list these for you: 379</p> 380 381<blockquote><pre># find /sys -name dev</pre></blockquote> 382 383<p> 384For example, on my system, the <em>/sys/block/sda</em> directory is the device path for my hard disk. It is chained to the controller through the <em>/sys/block/sda/device/</em> symbolic link, which is in turn chained to the device driver through the <em>/sys/block/sda/device/driver/</em> symbolic link. 385</p> 386 387<p> 388When you write rules based on sysfs information, you are simply matching attribute contents of some files in one part of the chain. For example, I can read the size of my hard disk as follows: 389</p> 390 391<blockquote><pre> 392# cat /sys/block/sda/size 393234441648 394</pre></blockquote> 395 396<p> 397In a udev rule, I could use SYSFS{size}=="234441648" to identify this disk. As udev iterates through the entire device chain, I could alternatively opt to match attributes in another part of the chain (e.g. attributes in <em>/sys/class/block/sda/device/</em>), however there are some caveats when dealing with different parts of the chain which are described later. 398</p> 399 400<p> 401Although this serves as a useful introduction as to the structure of sysfs and exactly how udev matches values, manually trawling through sysfs is both time consuming and unnecessary. 402</p> 403 404<a name="udevinfo"></a> 405<h3>udevinfo</h3> 406 407<p> 408Enter <em>udevinfo</em>, which is probably the most straightforward tool you can use to construct rules. All you need to know is the sysfs device path of the device in question. A trimmed example is shown below: 409</p> 410 411<blockquote><pre> 412# udevinfo -a -p /sys/block/sda 413 414<span class="green"> looking at device '/block/sda': 415 KERNEL=="sda" 416 SUBSYSTEM=="block" 417 SYSFS{stat}==" 128535 2246 2788977 766188 73998 317300 3132216 5735004 0 516516 6503316" 418 SYSFS{size}=="234441648" 419 SYSFS{removable}=="0" 420 SYSFS{range}=="16" 421 SYSFS{dev}=="8:0"</span> 422 423<span class="blue"> looking at device '/devices/pci0000:00/0000:00:07.0/host0/target0:0:0/0:0:0:0': 424 ID=="0:0:0:0" 425 BUS=="scsi" 426 DRIVER=="sd" 427 SYSFS{ioerr_cnt}=="0x0" 428 SYSFS{iodone_cnt}=="0x31737" 429 SYSFS{iorequest_cnt}=="0x31737" 430 SYSFS{iocounterbits}=="32" 431 SYSFS{timeout}=="30" 432 SYSFS{state}=="running" 433 SYSFS{rev}=="3.42" 434 SYSFS{model}=="ST3120827AS " 435 SYSFS{vendor}=="ATA " 436 SYSFS{scsi_level}=="6" 437 SYSFS{type}=="0" 438 SYSFS{queue_type}=="none" 439 SYSFS{queue_depth}=="1" 440 SYSFS{device_blocked}=="0"</span> 441</pre></blockquote> 442 443<p> 444As you can see, udevinfo simply produces a list of attributes you can use as-is as match keys in your udev rules. From the above example, I could produce (e.g.) either of the following two rules for this device: 445</p> 446 447<blockquote><pre> 448SUBSYSTEM=="block", <span class="green">SYSFS{size}=="234441648",</span> NAME="my_hard_disk" 449BUS=="scsi", <span class="blue">SYSFS{model}=="ST3120827AS",</span> NAME="my_hard_disk"</pre></blockquote> 450 451<p> 452You may have noted the use of colour in the above examples. This is to demonstrate that you generally can <em>not</em> mix-and-match attributes from different sections of the udevinfo output - your rule will not work. For example, the following rule is <em>invalid</em>: 453</p> 454 455<blockquote><pre> 456SUBSYSTEM=="block", <span class="green">SYSFS{size}=="234441648",</span> <span class="blue">SYSFS{model}=="ST3120827AS",</span> NAME="my_hard_disk"</pre></blockquote> 457 458<p> 459You are usually provided with a large number of attributes, and you must pick a number of them (from the same section) to construct your rule. In general, you want to choose attributes which identify your device in a persistent and human-recognisable way. In the examples above, I chose the size of my disk and its model number. I did not use meaningless numbers such as SYSFS{iodone_cnt}=="0x31737". 460</p> 461 462<p> 463Another point to note is that it is common for text attributes to appear in the udevinfo output to be padded with spaces (e.g. see ST3120827AS above). In your rules, you can either specify the extra spaces, or you can cut them off as I have done. 464</p> 465 466<p> 467The only complication with using udevinfo is that you are required to know the top-level device path (/sys/block/sda in the example above). This is not always obvious. However, as you are generally writing rules for device nodes which already exist, you can use udevinfo to look up the device path for you: 468</p> 469 470<blockquote><pre># udevinfo -a -p $(udevinfo -q path -n /dev/sda)</pre></blockquote> 471 472<a name="sysfsalt"></a> 473<h3>Alternative methods</h3> 474 475<p> 476Although udevinfo is almost certainly the most straightforward way of listing the exact attributes you can build rules from, some users are happier with other tools. Utilities such as <a href="http://www.kroah.com/linux/usb/">usbview</a> display a similar set of information, most of which can be used in rules. 477</p> 478 479<h2>Advanced topics</h2> 480 481<a name="ownership"></a> 482<h3>Controlling permissions and ownership</h3> 483 484<p> 485udev allows you to use additional assignments in rules to control ownership and permission attributes on each device. 486</p> 487 488<p> 489The <em>GROUP</em> assignment allows you to define which Unix group should own the device node. Here is an example rule which defines that the <em>video</em> group will own the framebuffer devices: 490</p> 491 492<blockquote><pre>KERNEL=="fb[0-9]*", NAME="fb/%n", SYMLINK+="%k", GROUP="video"</pre></blockquote> 493 494<p> 495The <em>OWNER</em> key, perhaps less useful, allows you to define which Unix user should have ownership permissions on the device node. Assuming the slightly odd situation where you would want <em>john</em> to own your floppy devices, you could use: 496</p> 497 498<blockquote><pre>KERNEL=="fd[0-9]*", OWNER="john"</pre></blockquote> 499 500<p> 501udev defaults to creating nodes with Unix permissions of 0660 (read/write to owner and group). If you need to, you can override these defaults on certain devices using rules including the <em>MODE</em> assignment. As an example, the following rule defines that the inotify node shall be readable and writable to everyone: 502</p> 503 504<blockquote><pre>KERNEL=="inotify", NAME="misc/%k", SYMLINK+="%k", MODE="0666"</pre></blockquote> 505 506<a name="external-naming"></a> 507<h3>Using external programs to name devices</h3> 508 509<p> 510Under some circumstances, you may require more flexibility than standard udev rules can provide. In this case, you can ask udev to run a program and use the standard output from that program to provide device naming. 511</p> 512 513<p> 514To use this functionality, you simply specify the absolute path of the program to run (and any parameters) in the <em>PROGRAM</em> assignment, and you then use some variant of the <em>%c</em> substitution in the NAME/SYMLINK assignments. 515</p> 516 517<p> 518The following examples refer to a fictional program found at <em>/bin/device_namer</em>. device_namer takes one command line argument which is the kernel name for the device. Based upon this kernel name, device_namer does its magic and produces some output to the usual <em>stdout</em> pipe, split into several parts. Each part is just a single word, and parts are separated by a single space. 519</p> 520 521<p> 522In our first example, we assume that device_namer outputs a number of parts, each one to form a symbolic link (alternative name) for the device in question. 523</p> 524 525<blockquote><pre>KERNEL=="hda", PROGRAM="/bin/device_namer %k", SYMLINK+="%c"</pre></blockquote> 526 527<p> 528The next example assumes that device_namer outputs two parts, the first being the device name, and the second being the name for an additional symbolic link. We now introduce the <em>%c{N}</em> substitution, which refers to part N of the output: 529</p> 530 531<blockquote><pre>KERNEL=="hda", PROGRAM="/bin/device_namer %k", NAME="%c{1}", SYMLINK+="%c{2}"</pre></blockquote> 532 533<p> 534The next example assumes that device_namer outputs one part for the device name, followed by any number of parts which will form additional symbolic links. We now introduce the <em>%c{N+}</em> substitution, which evaluates to part N, N+1, N+2, ... until the end of the output. 535</p> 536 537<blockquote><pre>KERNEL=="hda", PROGRAM="/bin/device_namer %k", NAME="%c{1}", SYMLINK+="%c{2+}"</pre></blockquote> 538 539<p> 540Output parts can be used in any assignment key, not only NAME and SYMLINK. The example below uses a fictional program to determine the Unix group which should own the device: 541</p> 542 543<blockquote><pre>KERNEL=="hda", PROGRAM="/bin/who_owns_device %k", GROUP="%c"</pre></blockquote> 544 545<a name="external-run"></a> 546<h3>Running external programs upon certain events</h3> 547 548<p> 549Yet another reason for writing udev rules is to run a particular program when a device is connected or disconnected. For example, you might want to execute a script to automatically download all of your photos from your digital camera when it is connected. 550</p> 551 552<p> 553Do not confuse this with the <em>PROGRAM</em> functionality described above. <em>PROGRAM</em> is used for running programs which produce device names (and they shouldn't do anything other than that). When those programs are being executed, the device node has not yet been created, so acting upon the device in any way is not possible. 554</p> 555 556<p> 557The functionality introduced here allows you to run a program after the device node is put in place. This program can act on the device, however it must not run for any extended period of time, because udev is effectively paused while these programs are running. One workaround for this limitation is to make sure your program immediately detaches itself. 558</p> 559 560<p> 561Here is an example rule which demonstrates the use of the <em>RUN</em> list assignment: 562</p> 563 564<blockquote><pre>KERNEL=="sdb", RUN+="/usr/bin/my_program"</pre></blockquote> 565 566<p> 567When <em>/usr/bin/my_program</em> is executed, various parts of the udev environment are available as environment variables, including key values such as <em>SUBSYSTEM</em>. You can also use the <em>ACTION</em> environment variable to detect whether the device is being connected or disconnected - ACTION will be either "add" or "remove" respectively. 568</p> 569 570<a name="env"></a> 571<h3>Environment interaction</h3> 572 573<p> 574udev provides an <em>ENV</em> key for environment variables which can be used for both matching and assignment. 575</p> 576 577<p> 578In the assignment case, you can set environment variables which you can then match against later. You can also set environment variables which can be used by any external programs invoked using the techniques mentioned above. A fictional example rule which sets an environment variable is shown below. 579</p> 580 581<blockquote><pre>KERNEL=="fd0", SYMLINK+="floppy", ENV{some_var}="value"</pre></blockquote> 582 583<p> 584In the matching case, you can ensure that rules only run depending on the value of an environment variable. Note that the environment that udev sees will not be the same user environment as you get on the console. A fictional rule involving an environment match is shown below. 585</p> 586 587<blockquote><pre>KERNEL=="fd0", ENV{an_env_var}=="yes", SYMLINK+="floppy"</pre></blockquote> 588 589<p> 590The above rule only creates the <em>/dev/floppy</em> link if $an_env_var is set to "yes" in udev's environment. 591</p> 592 593<a name="options"></a> 594<h3>Additional options</h3> 595 596<p> 597Another assignment which can prove useful is the <em>OPTIONS</em> list. A few options are available: 598</p> 599 600<ul> 601<li><b>all_partitions</b> - create all possible partitions for a block device, rather than only those that were initially detected</li> 602<li><b>ignore_device</b> - ignore the event completely</li> 603<li><b>last_rule</b> - ensure that no later rules have any effect</li> 604</ul> 605 606<p> 607For example, the rule below sets the group ownership on my hard disk node, and ensures that no later rule can have any effect: 608</p> 609 610<blockquote><pre>KERNEL=="sda", GROUP="disk", OPTIONS+="last_rule"</pre></blockquote> 611 612 613<h2>Examples</h2> 614 615<a name="example-printer"></a> 616<h3>USB Printer</h3> 617 618<p> 619I power on my printer, and it is assigned device node <em>/dev/lp0</em>. Not satisfied with such a bland name, I decide to use udevinfo to aid me in writing a rule which will provide an alternative name: 620</p> 621 622<blockquote><pre> 623# udevinfo -a -p $(udevinfo -q path -n /dev/lp0) 624looking at the device chain at '/sys/devices/pci0000:00/0000:00:02.1/usb3/3-3': 625BUS=="usb" 626SYSFS{manufacturer}=="EPSON" 627SYSFS{product}=="USB Printer" 628SYSFS{serial}=="L72010011070626380" 629</pre></blockquote> 630 631<p> 632My rule becomes: 633</p> 634 635<blockquote><pre>BUS=="usb", SYSFS{serial}=="L72010011070626380", SYMLINK+="epson_680"</pre></blockquote> 636 637<a name="example-camera"></a> 638<h3>USB Camera</h3> 639 640<p> 641Like most, my camera identifies itself as an external hard disk connected over the USB bus, using the SCSI transport. To access my photos, I mount the drive and copy the image files onto my hard disk. 642</p> 643 644<p> 645Not all cameras work in this way: some of them use a non-storage protocol such as cameras supported by <a href="http://www.gphoto.org/">gphoto2</a>. In the gphoto case, you do not want to be writing rules for your device, as is it controlled purely through userspace (rather than a specific kernel driver). 646</p> 647 648<p> 649A common complication with USB camera devices is that they usually identify themselves as a disk with a single partition, in this case <em>/dev/sdb</em> with <em>/dev/sdb1</em>. The sdb node is useless to me, but sdb1 is interesting - this is the one I want to mount. There is a problem here that because sysfs is chained, the useful attributes which udevinfo produces for /dev/sdb1 are identical to the ones for /dev/sdb. This results in your rule potentially matching <u>both</u> the raw disk and the partition, which is not what you want, your rule should be <b>specific</b>. 650</p> 651 652<p> 653To get around this, you simply need to think about what differs between sdb and sdb1. It is surprisingly simple: the name itself differs, so we can use a simple pattern match on the NAME field. 654</p> 655 656<blockquote><pre> 657# udevinfo -a -p $(udevinfo -q path -n /dev/sdb1) 658 looking at device '/devices/pci0000:00/0000:00:02.0/usb2/2-1/2-1:1.0/host6/target6:0:0/6:0:0:0': 659 ID=="6:0:0:0" 660 BUS=="scsi" 661 DRIVER=="sd" 662 SYSFS{rev}=="1.00" 663 SYSFS{model}=="X250,D560Z,C350Z" 664 SYSFS{vendor}=="OLYMPUS " 665 SYSFS{scsi_level}=="3" 666 SYSFS{type}=="0" 667</pre></blockquote> 668 669<p> 670My rule: 671</p> 672 673<blockquote><pre>KERNEL=="sd?1", BUS=="scsi", SYSFS{model}=="X250,D560Z,C350Z", SYMLINK+="camera"</pre></blockquote> 674 675<a name="example-usbhdd"></a> 676<h3>USB Hard Disk</h3> 677 678<p> 679A USB hard disk is comparable to the USB camera I described above, however typical usage patterns are different. In the camera example, I explained that I am not interested in the sdb node - it's only real use is for partitioning (e.g. with fdisk), but why would I want to partition my camera!? 680</p> 681 682<p> 683Of course, if you have a 100GB USB hard disk, it is perfectly understandable that you might want to partition it, in which case we can take advantage of udev's string substitutions: 684</p> 685 686<blockquote><pre>BUS=="usb", KERNEL=="sd*", SYSFS{product}=="USB 2.0 Storage Device", NAME="%k", SYMLINK+="usbhd%n"</pre></blockquote> 687 688<p> 689This rule creates symlinks such as: 690</p> 691 692<ul> 693<li><em>/dev/usbhd</em> - The fdiskable node</li> 694<li><em>/dev/usbhd1</em> - The first partition (mountable)</li> 695<li><em>/dev/usbhd2</em> - The second partition (mountable)</li> 696</ul> 697 698<a name="example-usbcardreader"></a> 699<h3>USB Card Reader</h3> 700 701<p> 702USB card readers (CompactFlash, SmartMedia, etc) are yet another range of USB storage devices which have different usage requirements. 703</p> 704 705<p> 706These devices typically do not inform the host computer upon media change. So, if you plug in the device with no media, and then insert a card, the computer does not realise, and you do not have your mountable sdb1 partition node for the media. 707</p> 708 709<p> 710One possible solution is to take advantage of the <em>all_partitions</em> option, which will create 16 partition nodes for every block device that the rule matches: 711</p> 712 713<blockquote><pre>BUS=="usb", SYSFS{product}=="USB 2.0 CompactFlash Reader", SYMLINK+="cfrdr%n", OPTIONS+="all_partitions"</pre></blockquote> 714 715You will now have nodes named: cfrdr, cfrdr1, cfrdr2, cfrdr3, ..., cfrdr15. 716 717<a name="example-pilot"></a> 718<h3>USB Palm Pilot</h3> 719 720<p> 721These devices work as USB-serial devices, so by default, you only get the <em>ttyUSB1</em> device node. The palm utilities rely on <em>/dev/pilot</em>, so many users will want to use a rule to provide this. 722</p> 723 724<p> 725<a href="http://www.clasohm.com/blog/one-entry?entry%5fid=12096">Carsten Clasohm's blog post</a> appears to be the definitive source for this. Carsten's rule is shown below: 726</p> 727 728<blockquote><pre>BUS=="usb", SYSFS{product}=="Palm Handheld", KERNEL=="ttyUSB*", SYMLINK+="pilot"</pre></blockquote> 729 730<p> 731Note that the product string seems to vary from product to product, so make sure that you check (using udevinfo) which one applies to you. 732</p> 733 734<a name="example-cdrom"></a> 735<h3>CD/DVD drives</h3> 736 737<p> 738I have two optical drives in this computer: a DVD reader (hdc), and a DVD rewriter (hdd). I do not expect these device nodes to change, unless I physically rewire my system. However, many users like to have device nodes such as <em>/dev/dvd</em> for convenience. 739</p> 740 741<p> 742As we know the KERNEL names for these devices, rule writing is simple. Here are some examples for my system: 743</p> 744 745<blockquote><pre> 746BUS=="ide", KERNEL=="hdc", SYMLINK+="dvd", GROUP="cdrom" 747BUS=="ide", KERNEL=="hdd", SYMLINK+="dvdrw", GROUP="cdrom" 748</pre></blockquote> 749 750<a name="example-netif"></a> 751<h3>Network interfaces</h3> 752 753<p> 754Even though they are referenced by names, network interfaces typically do not have device nodes associated with them. Despite that, the rule writing process is almost identical. 755</p> 756 757<p> 758It makes sense to simply match the MAC address of your interface in the rule, as this is unique. However, make sure that you use the <em>exact</em> MAC address as shown as udevinfo, because if you do not match the case exactly, your rule will not work. 759</p> 760 761<blockquote><pre> 762# udevinfo -a -p /sys/class/net/eth0 763 looking at class device '/sys/class/net/eth0': 764 SYSFS{address}=="00:52:8b:d5:04:48" 765</pre></blockquote> 766 767<p> 768Here is my rule: 769</p> 770 771<blockquote><pre>KERNEL=="eth*", SYSFS{address}=="00:52:8b:d5:04:48", NAME="lan"</pre></blockquote> 772 773<p> 774You will need to reload the net driver for this rule to take effect. You can either unload and reload the module, or simply reboot the system. You will also need to reconfigure your system to use "lan" rather than "eth0". I had some troubles getting this going (the interface wasn't being renamed) until I had completely dropped all references to eth0. 775After that, you should be able to use "lan" instead of "eth0" in any calls to ifconfig or similar utilities. 776</p> 777 778<h2>Testing and debugging</h2> 779 780<a name="testing"></a> 781<h3>Putting your rules into action</h3> 782 783<p> 784Assuming you are on a recent kernel with <em>inotify</em> support, udev will automatically monitor your rules directory and automatically pick up any modifications you make to the rule files. 785</p> 786 787<p> 788Despite this, udev will not automatically reprocess all devices and attempt to apply the new rule(s). For example, if you write a rule to add an extra symbolic link for your camera while your camera is plugged in, you cannot expect the extra symbolic link to show up right away. 789</p> 790 791<p> 792To make the symbolic link show up, you can either disconnect and reconnect your camera, or alternatively in the case of non-removable devices, you can run <b>udevtrigger</b>. 793</p> 794 795<p> 796If your kernel does not have inotify support, new rules will not be detected automatically. In this situation, you must run <b>udevcontrol reload_rules</b> after making any rule file modifications for those modifications to take effect. 797</p> 798 799<a name="udevtest"></a> 800<h3>udevtest</h3> 801 802<p> 803If you know the top-level device path in sysfs, you can use <b>udevtest</b> to show the actions which udev would take. This may help you debug your rules. For example, assuming you want to debug a rule which acts on <em>/sys/class/sound/dsp</em>: 804</p> 805 806<blockquote><pre> 807# udevtest /class/sound/dsp 808main: looking at device '/class/sound/dsp' from subsystem 'sound' 809udev_rules_get_name: add symlink 'dsp' 810udev_rules_get_name: rule applied, 'dsp' becomes 'sound/dsp' 811udev_device_event: device '/class/sound/dsp' already known, remove possible symlinks 812udev_node_add: creating device node '/dev/sound/dsp', major = '14', minor = '3', mode = '0660', uid = '0', gid = '18' 813udev_node_add: creating symlink '/dev/dsp' to 'sound/dsp' 814</pre></blockquote> 815 816<p> 817Note the <em>/sys</em> prefix was removed from the udevtest command line argument, this is because udevtest operates on device paths. Also note that udevtest is purely a testing/debugging tool, it does not create any device nodes, despite what the output suggests! 818</p> 819 820<a name="author"></a> 821<h2>Author and contact</h2> 822 823<p> 824This document is written by Daniel Drake <<a href="mailto:dan@reactivated.net">dan@reactivated.net</a>>. Feedback is appreciated. 825</p> 826 827<p> 828For support, you should mail the linux-hotplug mailing list: <a href="mailto:linux-hotplug-devel@lists.sourceforge.net">linux-hotplug-devel@lists.sourceforge.net</a>. 829</p> 830 831<p> 832Copyright (C) 2003-2006 Daniel Drake.<br /> 833This document is licensed under the <a href="http://www.gnu.org/licenses/gpl.html">GNU General Public License, Version 2</a>. 834</p> 835 836</body> 837</html> 838 839