udev/writing rules

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Edit Log

 * 2005-10-05 Copied from http://www.reactivated.net/writing_udev_rules.html, with minor editing for clarity. Probably needs considerable format editing.
 * 2014-01-10 Updating to reflect changes in diagnostic utilities.
 * }

About this document
udev 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, devfs, is now deprecated, and udev is seen as the successor. udev vs devfs is a sensitive area of conversation - you should read this document before making comparisons.

Over 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.

This document assumes that you have udev installed and running OK with default configurations. This is usually handled by your Linux distribution.

This 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.

This 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.

Terminology: devfs, sysfs, nodes, etc.
A basic introduction only, might not be totally accurate.

On typical Linux-based systems, the /dev directory is used to store file-like device nodes 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.

The original /dev directories were just populated with every device that might possibly appear in the system. /dev directories were typically very large because of this. devfs 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.

udev is the "new" way of managing /dev directories, designed to clear up some issues with previous /dev implementations, and provide a robust path forward. In order to create and name /dev device nodes corresponding to devices that are present in the system, udev relies on matching information provided by sysfs with rules 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.

sysfs 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 /sys 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 /sys and sysfs interchangeably.

Why?
As stated above, writing rules for udev is an optional process. By default, you can plug a device in, and the a relevant node (e.g. /dev/sda for a mass-storage device) will be there, just like in previous /dev implementations.

However, udev allows you to customise the naming of device nodes. There are two reasons why you might want to do this: convenience, and persistent naming.

Take the example of using udev, so that when your printer is plugged in, it gets named as /dev/printer and also as the usual /dev/lp0. It's not only convenience (e.g. reading and interpreting "printer" as opposed to "lp0"), its a solution for non-persistent naming. Say that I have two printers - a HP laser printer and an Epson inkjet. When they are both plugged in and on, I have /dev/lp0 and /dev/lp1.

How do I know which node refers to which printer? There is no easy way. The first printer that got connected was assigned name "lp0", and the second "lp1". Plugging in my printers in a different order would swap the names here, and that would mess up my scripts that always expect my HP laser printer to be lp1.

However, if my HP laser printer got named lp_hp (as well as lpX) and my other printer got named lp_epson (as well as lpY), then my scripts could just refer to those names. udev magic can control this and ensure that these persistent names always point to the device that I intended.

For external mass-storage devices (e.g. usb hard disks), persistent naming is very helpful in that it allows you to hardcode accurate device paths into your /etc/fstab.

It is important to understand that writing rules is simply a means of customizing udev behaviour. Writing rules is not a workaround for the problem where no device nodes for your particular device exist. If no matching rules exist, udev will create the node anyway, using the name that was supplied by the kernel.

The basics of writing rules
When populating /dev, udev decides which nodes to include, and how to name them, by reading a series of rules files.

Default udev rules are stored in /etc/udev/rules.d/50-udev.rules. 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, to reduce hassle while updating your udev installation in the future.

Files in /etc/udev/rules.d/ are parsed in lexical order. udev will stop processing rules as soon as it finds a matching rule in a file for the new item of hardware that has been detected. It is important that your own rules get processed before the udev defaults, otherwise your own naming schemes will not take effect! I suggest that you keep your own rules in a file at /etc/udev/rules.d/10-local.rules (this doesn't exist by default - create it). As 10 comes before 50, you know that your rules will be looked at first. It is important that the filenames of your rule files end with the .rules suffix, otherwise they will not be used.

As your own rules will effectively mask out the udev defaults which create the base /dev layout, it is recommended that you also specify devfs-style names/symlinks for the rules you write, so that you get the sensible defaults plus your own names.

In rule files, lines starting with a "#" are treated as comments. Every uncommented line in the file corresponds to a rule.

Rules are composed of keys. Keys are seperated by commas. Some keys are used for reading and matching information, others are used for assigning information and performing actions.


 * 1) At least one identification key should be provided, which will match the rule to any number of devices in the system. These are listed in the later section:.
 * 2) At least one assignment key should be provided, to control how the resultant device node is created. These include NAME, SYMLINK, OWNER, GROUP and MODE, all of which are described in this document.

Common rules will use basic identification keys to determine the device to name, and then have a NAME assignement key to define the device node name. udev will only create one node for one device, so if you want it to be accessible through multiple nodes, then you have to specify the other nodes in the SYMLINK assignment key.

I'll take a slightly modified udev example rule to illustrate this:

BUS="usb", SYSFS{serial}="HXOLL0012202323480", NAME="lp_epson", SYMLINK="printers/epson_stylus"

The identification keys here are BUS and SYSFS{serial}. The assignment keys here are NAME and SYMLINK. udev will match this rule against a device that is connected through the USB bus and with a serial number of HXOLL0012202323480. Note that all (as opposed to any) specified keys must be matched for udev to use the rule to name a device.

udev will name this node lp_epson, and it will be located at /dev/lp_epson.

udev will also create a symlink to /dev/lp_epson, located at /dev/printers/epson_stylus (the printers directory will be automatically created). You can now print to your Epson printer by sending data to /dev/printers/epson_stylus or /dev/lp_epson.

Any rules that you have added or modified will not take effect until you notify udev of this. Make sure you remember to run the following every time you modify any rule files:


 * 1) udevstart

Additional automated customisation for NAME and SYMLINK parameters
In the NAME and SYMLINK parameters of your rules, you are able to use basic operators to assist the naming of devices. Hackers will know this sort of thing as printf-like string substitution. There are a number of operators which can compose some or all of your NAME/SYMLINK parameters. These operators refer to kernel-data relating to the device. Take this example:

BUS="usb", SYSFS{vendor}="FUJIFILM", SYSFS{model}="M100", NAME="camera%n"

The %n operator will be replaced with the "kernel number" for the camera device, to produce a NAME such as camera0, camera1, etc.

Another common operator is %k. This represents what the kernel would name the device, e.g. "hda1". You may often see rules which have NAME="%k" to produce the default names for the hardware. In these rules, customisation is usually done through the SYMLINK parameter.

A full list of operators, with explanations, can be found in the udev man page.

Using shell-style pattern matching in keys
You can use shell style pattern matching to provide even more flexibility when writing keys. Taking a default udev rule:

KERNEL="ts*", NAME="input/%k"

The * operator is used here, which matches literally anything - zero, one, or more characters of any kind. The rule literally says:

The ? operator is similar, and matches any single character (but not zero characters).

You can also use square brackets [ ] to match any single character. Direct quote from udev man page:

You can also specify ranges that can be matched, e.g. [0-9] would match any single digit. Using an example rule from a default udev installation:

KERNEL="fd[0-9]*", NAME="floppy/%n"

This rule says:

You can use these wildcards/pattern matches in any type of key, including both basic keys and sysfs-based identification (see below for explanations of these key types).

I have purposely left out some information on this topic (particularly the flexibility of using [ ] operators) that is out of the scope of basic rule-writing documentation. More information on this topic can be found in the udev man page.

Key-writing basics
udev provides a few basic key matching methods, and also provides flexible ways of matching information in SYSFS. A typical rule will match both normal keys (e.g. BUS and KERNEL), as well as SYSFS keys to differentiate between different hardware plugged in throught the same port.

You may be wondering, "How do I find the serial number of my printer? What is the model of my camera?". Rule writing isn't as hard as it sounds. The trickiest bit is finding your device in /sys, and deciding which info to use.

Identifying devices through basic keys
See the udev man page for more info on these keys.

The valid keys are:


 * BUS - match the bus type of the device.
 * KERNEL - match the kernel device name.
 * DRIVER - match the name of the kernel driver.
 * SUBSYSTEM - match the kernel subsystem name.
 * ID - match the device number on the bus (e.g. PCI bus ID).
 * PLACE - match the physical position where the device is plugged into (useful for USB).

The ID and PLACE keys do have their uses, but they are not commonly used in rules. This document focuses on using BUS and KERNEL keys, as well as SYSFS{...} keys (detailed in the next section). I will show how to use these keys by example.

For extra flexibility, udev also provides keys to call external scripts and examine their result, and to examine environment variables. This is out of scope of this document. Look at the udev man page for more details.

Identifying devices through SYSFS files
Background information: SYSFS stores many small files under a tree of directories which provide information about your hardware. One file typically contains just one "data item" - e.g. device name, manufacturer, or product ID.

Note that SYSFS{...} keys can be combined with the basic keys described in the previous section.

You can use keys in the format SYSFS{filename} to match specific info from SYSFS, where filename corresponds to a file in your SYSFS tree. For example, when my camera is connected, there is a file located at /sys/block/sda/device/model which contains "USB 2.0M DSC". To match this, I could use the following key: SYSFS{model} = "USB 2.0M DSC"

Note that any file in sysfs can be matched in this manner, but if you match more than one file (through multiple keys), then you must only match files that exist in the same directory. Typically, there will be several directories giving information about one device. You cannot mix and match (as shown by example below).

Luckily, the process of rule writing does not entail hunting through millions of files in SYSFS, the udevinfo utility does the hard work. This program is included in the udev distribution.

The first thing you need to do is find a directory somewhere in /sys that corresponds to your hardware, and includes a file named "dev", as udevinfo can only work on directories of this type. These directories are all found under either /sys/block or /sys/class - there is no point looking anywhere else! However, udevinfo will follow links through this directory and read info found from other sections of sysfs.

Once you have found a directory of this type, you can use the following command to assist you in the creation of writing keys for udev rules:


 * 1) udevadm info -a -p /sys/path/to/hardware/info

You may find that finding the correct place in /sys to run udevinfo on is not obvious. Chances are the device you just plugged in has already careted a device node (e.g. /dev/sda), in which case, udevinfo can be helpful! Taking the example of my /dev/sda node, running the following command will point you to the appropriate area of sysfs:


 * 1) udevadm info -q path -n /dev/sda

This returns: /block/sda

The output of the command (shown above) is telling me that the sysfs path to start at is /sys/block/sda. I would now run "udevinfo -a -p /sys/block/sda". These two commands can be chained together, like so:


 * 1) udevinfo -a -p $(udevinfo -q path -n /dev/sda)

Sidenote: You may notice that we previously provided full paths (/sys/some/path) to udevinfo beforehand, but now we are providing sysfs-relative paths (/some/path) by chaining these commands. This does not matter - both types of path are accepted.

Moving on to rule-writing, some snipped output of the results of "udevinfo -a -p /sys/block/sda" command is shown below, with colour added.

 follow the class device's "device" looking at the device chain at '/sys/devices/pci0000:00/0000:00:02.1/usb3/3-3/3-3:1.0/host0/0:0:0:0': BUS="scsi" ID="0:0:0:0" SYSFS{detach_state}="0" SYSFS{type}="0" SYSFS{max_sectors}="240" SYSFS{device_blocked}="0" SYSFS{queue_depth}="1" SYSFS{scsi_level}="3" SYSFS{vendor}="       " SYSFS{model}="USB 2.0M DSC   " SYSFS{rev}="1.00" SYSFS{online}="1"  looking at the device chain at '/sys/devices/pci0000:00/0000:00:02.1/usb3/3-3': BUS="usb" ID="3-3" SYSFS{detach_state}="0" SYSFS{bNumInterfaces}=" 1" SYSFS{bConfigurationValue}="1" SYSFS{bmAttributes}="c0" SYSFS{bMaxPower}=" 0mA" SYSFS{idVendor}="052b" SYSFS{idProduct}="1514" SYSFS{bcdDevice}="0100" SYSFS{bDeviceClass}="00" SYSFS{bDeviceSubClass}="00" SYSFS{bDeviceProtocol}="00" SYSFS{bNumConfigurations}="1" SYSFS{speed}="12" SYSFS{manufacturer}="Tekom Technologies, Inc" SYSFS{product}="USB 2.0M DSC"

The udevinfo tool provides a lot of information which you can simply copy-paste as udev rules. The reason that I have colour coded the above output is to point out that you generally cannot mix and match information from different parts of the udevinfo output. In the above output, I could not combine information from the different coloured sections - this is because each section of output refers to a different directory in SYSFS. For example, the following rule would not work:

BUS="scsi", SYSFS{manufacturer}="Tekom Technologies, Inc", NAME="%k"

This rule would not work because I am combining information found in the section beginning with BUS="scsi" (green) with information only found in the blue section. The rule would work if I used BUS="usb", sticking only to information found in the blue section above.

You will notice that a lot of information is not relevant for writing basic rules (there is so much of it!), you should generally be looking for information that you recognise and know will not change (e.g. model name).

Note that if you write your own rule to identify a device, the default devfs-style rules will not take effect! It is usually sensible to use NAME="%k" and specify your own extra names in the SYMLINK parameter so that you do not lose the default sensible names.

I will show three examples of this rule writing based on udevinfo output process below. I will then attempt to list some device-dependant tips and tricks for locating the correct info.

A reader wrote to me and informed me that he found KDE's control centre useful for writing rules. Apparently, information about USB devices (and others) can be found in the "Info Centre" section of the KDE Control Centre. This interface shows information such as serial number, vendor ID, etc. If you prefer a GUI-like approach, you might want to investigate this.

The current releases of gnome-volume-manager are unable to treat symlink-nodes as real devices. Conversely as described above, you may wish to specify your own naming in the NAME parameter and specify %k in the SYMLINK parameter.

The behaviour of your own rules masking the defaults can be overcome if you write multiple-SYMLINK style rules.

Using multiple SYMLINK style rules
Another recent feature is the ability to write rules that do not specify a NAME, but instead they simply specify SYMLINK keys. This allows you to avoid the issue where your own rules effectively mask the udev defaults.

Take the rule:

KERNEL="hdc", SYMLINK="dvd"

When udev finds this rule, it will take a mental note of it. Upon finding another rule matching the same device which also includes a NAME parameter, udev will create the node as specified by the NAME parameter, plus symbolic links as specified by the SYMLINK parameters of both rules. To put it into practical terms, when udev is naming nodes for my hdc device, it will use the default rules for block devices as usual, with the addition of my personal symlink "dvd".

Similarly to normal rules, rules of this type will only take effect if udev is able to find them before it finds a rule specifying a NAME parameter.

Controlling ownership and permissions As well as controlling the naming of the device nodes which are created, udev rules also allow you to control ownership and permission attributes on that device node.

The GROUP key allows you to define which unix group should own the device node. Here's an example from the udev defaults, which defines that the video group will own framebuffer (fb) devices:

KERNEL="fb[0-9]*", NAME="fb/%n", SYMLINK="%k", GROUP="video"

The OWNER key, perhaps less useful, allows you to define which unix user should own the device node. Assuming the slightly odd situation where you would want "john" to own your floppy devices, you could use:

KERNEL="fd[0-9]*", OWNER="john"

You'll notice in the above rule that we didn't specify any NAME or SYMLINK keys. This is similar to the multiple symlink style where udev will take a mental note that we want john to own floppy nodes, and will apply that ownership once it finds a rule which defines a NAME for the floppy device nodes.

Building on the style mentioned above, you can do even more flashy things. The udev defaults use the following rule to define that all the sound device nodes shall be owned by the "audio" group:

SUBSYSTEM="sound", GROUP="audio"

This prevents the need to excessively provide a GROUP="audio" key on every following rule which names sound devices.

udev defaults to creating nodes with unix permissions of 0660 (read/write to owner and group), which is configured by the default_mode setting inside /etc/udev/udev.conf. There may be some situations where you do not want to use the default permissions on your device node. Fortunately, you can easily override the permissions in your rules using the MODE assignment key. As an example, the following rule defines that the inotify node shall be readable and writable to everyone:

KERNEL="inotify", NAME="misc/%k", SYMLINK="%k", MODE="0666"

Example: Writing a rule for my USB printer After plugging in my printer, I started looking around some /sys directories for a relevant place to start. I didn't get anywhere, but I noticed that my printer had been given device node /dev/lp0. udevinfo was able to provide me with a useful path:

/class/usb/lp0
 * 1) udevinfo -q path -n /dev/lp0

Running "udevinfo -a -p /sys/class/usb/lp0" provided me with a heap of info, as usual. I picked out the relevant bits for unique device identification:

looking at the device chain at '/sys/devices/pci0000:00/0000:00:02.1/usb3/3-3': BUS="usb" SYSFS{manufacturer}="EPSON" SYSFS{product}="USB Printer" SYSFS{serial}="L72010011070626380"

My udev rule becomes:

BUS="usb", SYSFS{serial}="L72010011070626380", NAME="%k", SYMLINK="epson_680"

And my printer nodes exist at /dev/lp0 (or /dev/lp1 if another printer was plugged in beforehand) and /dev/epson_680 always points at the device node for that particular printer.

Example: Writing a rule for my USB-Storage digital camera Quick Intro: My camera identifies itself as an external SCSI hard disk (it uses the usb-storage driver which is also used by devices such as USB hard disks and flash-card readers). I can then mount the partition on that disk and copy images over. Not all cameras work like this - many require external software (e.g. gphoto2) to be able to access photos.

This one is a bit tricky. Several nodes are created by default when my camera is connected : /dev/sda and /dev/sda1, and possibly even /dev/sg1. This is an example where specifity is important - if your rule is not specific enough, it could match any of the above 3 nodes.

sda1 is the node that I would like as my /dev/camera, as that is what gets mounted. udevinfo did not point out any useful differences between sda, sda1, and sg1. I decided that a reliable way to differentiate between these 3 nodes would be to look at the KERNEL name.

A key such as KERNEL="sd?1" would match KERNEL names such as "sda1", "sdb1", "sdc1", and equally importantly, it will not match KERNEL names such as sda, sdb, or sg1. The purpose of this key is to ignore the /dev/sda and /dev/sg1 nodes. The device is a digital camera - I would not dream of fdisking it or anything like that, so these 2 nodes are pretty useless to me. The key attempts to capture the /dev/sda1 node, which is mountable and therefore useful!

As this node (sda1) is treated as a block device, looking in /sys/block would be a good place to start.

In my /sys/block, I have a directory named sda. In my /sys/block/sda, I have a directory named sda1. Both of these directories have dev files in, so they are OK to run udevinfo on. Running the following dumps a lot of information about my camera and the USB port it is connected through.


 * 1) udevinfo -a -p /sys/block/sda/sda1

In the udevinfo output, I also noticed this bit of useful and understandable information:

SYSFS{product}="USB 2.0M DSC"

So that gives me my rule. For completeness, I also include a BUS key (this was also found in the udevinfo output).

BUS="usb", SYSFS{product}="USB 2.0M DSC", KERNEL="sd?1", NAME="%k", SYMLINK="camera"

Now, when my camera is plugged in, it will be named /dev/sda1 (or, if sda1 isnt available, it might be called /dev/sdb1) and will always be correctly linked to from /dev/camera. The /dev/sda (or sdb) node still appears as normal, but the important thing is that my custom persistent "camera" symlink points to the mountable partition.

Additional notes on writing rules for USB storage Carl Streeter, the owner of a large USB hard disk, wrote to me and explained that unlike in my digital camera example, the /dev/sda node is useful to him. He pointed out that he does occasionally need to use tools such as fdisk and hdparm on that node.

Carl's rule is:

BUS="usb", KERNEL="sd*", SYSFS{product}="USB 2.0 Storage Device", NAME="%k", SYMLINK="usbhd%n"

This rule creates symlinks such as:


 * /dev/usbhd - The fdiskable node
 * /dev/usbhd1 - The first partition (mountable)
 * /dev/usbhd2 - The second partition (mountable)

We agreed that depending on the situation and device in question, there are reasons for both wanting and not wanting the non-mountable /dev/sda node. Use whichever setup suits you best.

Another difficult situation is having a multiple-slot USB-storage card reader. These types of device generally do not inform the host when new cards are plugged in or out, so plugging a card into an unused slot while the reader is plugged in will not create the extra device node needed for mounting! This problem also applies to other USB disks - e.g. if you create a new partition, the new partition node will not appear until you re-plug the device.

udev provides a solution here - it is able to create nodes for all partitions of a block device. For every rule that you specify, the block device will have all 16 partition nodes created. To achieve this, you can simply modify the NAME key, as shown below:

BUS="usb", SYSFS{product}="USB 2.0 Storage Device", NAME{all_partitions}="usbhd"

You will now have nodes named: usbhd, usbhd1, usbhd2, usbhd3, ..., usbhd15.

Writing convenience rules for my CD drives
I have two CD drives in my PC - a DVD reader, and a CD rewriter. My DVD is hdc and my CDRW is hdd. I would not expect this to change, unless I manually changed the cabling of my system.

Still, some people (myself included) like to have nodes such as /dev/dvd and /dev/cdrw for convenience. Since we know the "hdX" values for these drives, writing rules is simple. The examples below should be self explanatory.

BUS="ide", KERNEL="hdc", NAME="%k", SYMLINK="dvd cdroms/cdrom%n" BUS="ide", KERNEL="hdd", NAME="%k", SYMLINK="cdrw cdroms/cdrom%n"

You may have noticed that the default 50-udev.rules file contains a rule which runs a script to produces names for block devices. Do not be confused by this - as usual, because your own rules are located in a file which is processed before the default rules, the defaults will not be used when naming the hardware you have written rules for.

Writing a rule for your USB Visor Palm Pilot
These devices work as USB-serial devices, so by default, you only get the ttyUSB1 node. The user-space palm utilities rely on /dev/pilot, so you need to use a rule to create this. The following rule will do the job:

BUS="usb", SYSFS{product}="Palm Handheld", KERNEL="ttyUSB*", SYMLINK="pilot"

This was adapted from Carsten Clasohm's blog entry, which includes a useful discussion of the situation. You may also wish to add ownership and permission keys to the rule to suit your setup.

Writing a rule to name my network interface
An interesting new feature in recent udev versions is the ability to rename your network interfaces, like the nameif utility does. Network interfaces do not show up in /dev, but they are generally referenced by names (e.g. with ifconfig). Despite the differences, the rule writing process is almost identical.

As usual, udevinfo comes to our aid in rule-writing. In my example, I wish to rename my "eth0" network device (the following output is snipped):

looking at class device '/sys/class/net/eth0': SYSFS{address}="00:52:8b:d5:04:48"
 * 1) udevinfo -a -p /sys/class/net/eth0/

Every network adapter has its own unique MAC-address, so I chose to use this when writing my rule. This will not change, unless you change your network card. There is one caveat here: make sure you use the MAC address you obtain from udevinfo (as above), because it is case sensitive. Be careful when using utilities such as ifconfig as they will capitalize the letters.

An example rule is shown below:

KERNEL="eth*", SYSFS{address}="00:52:8b:d5:04:48", NAME="lan"

You 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. After that, you should be able to use "lan" instead of "eth0" in any calls to ifconfig or similar utilities.

Tips for finding the appropriate places in SYSFS
I'm looking for some more device specific tips here. Please contact me with any you can provide.


 * If the device you are looking to write rules for has created a device node under /dev, then you are in luck! Run the following command to get an appropriate /sys path: udevinfo -q path -n /dev/yournode
 * Always use udevinfo to assist the rule-writing process. Always use udevinfo to look under /sys/block or /sys/class (it will not start reading a chain from anywhere else).
 * If you get totally stuck, use the following command to find all "dev" files under /sys (udevinfo can work on directories containing this file): find /sys -iname dev
 * If your device is a flash-card reader, usb flash-drive, or digital camera that acts as usb-storage, that is created as /dev/sdX, then start looking in /sys/block/sdX.
 * If applicable, make sure you identify the difference between sdX and sdX1 in the above situation. This can be done with the key KERNEL="sd?1" to match sdX1, or KERNEL="sd?" to match sdX.
 * For USB printers that are created as /dev/lpX, then you should start looking in /sys/class/usb/lpX.
 * The usb scanner driver has recently been removed from the kernel and re-implemented in userspace (as part of the SANE package). You do not (and can not) write rules for this hardware as it does not rely on specific kernel drivers.
 * Remember that unfortunately, the kernel does not export information for all devices into sysfs, meaning that you simply can't write rules for some devices yet. On 20/02/04, the udev author stated that there are 162 drivers left to convert to sysfs.

Debugging your rules
If you have written rules and remembered to run udevstart but they do not appear to be taking effect, there are a couple of ways you can debug them.

The file /etc/udev/udev.conf contains a udev_log option. Setting this option to yes will cause udev to log some useful information about which rules are being applied to which nodes into the system logger. The logs will be included in /var/log/messages for most users.

Additionally, if you know the path in sysfs for the node you want to create, you can use udevtest to see a rundown on what udev would do with the node. For example:

version 056 looking at '/class/sound/dsp/' opened class_dev->name='dsp' configured rule in '/etc/udev/rules.d/50-udev.rules[132]' applied, added symlink '%k' configured rule in '/etc/udev/rules.d/50-udev.rules[132]' applied, 'dsp' becomes 'sound/%k' creating device node '/dev/sound/dsp', major = '14', minor = '3', mode = '0660', uid = '0', gid = '18'
 * 1) udevadm test /sys/class/sound/dsp/

The "test" option in udevadm is only a debugging/testing tool - it does not actually create the device node, even though it says it doing so!