It is essential to choose a good key for every functioning udev rule. Here are some examples of standard keys:

The keys ID and PLACE can be useful, but usually the keys BUS, KERNEL, and SYSFS{...} are used. The udev configuration also provides keys that call external scripts and evaluate their results. Further details about this can be found in man udev.

The file system sysfs stores small files with hardware information in a directory tree. Each file generally is only given one item of information, such as the device name, the vendor, or the serial number. Each of these files can be used as the value of a key. To use several SYSFS keys in one rule, however, you can only use files in the same directory.

udevinfo can be a useful tool here. You just have to find one subdirectory of /sys that refers to the relevant device and is given a file dev. These directories are all located under /sys/block or /sys/class.

If a device node already exists for the device, udevinfo can again reduce the amount of work you need to do. The command udevinfo -q path -n /dev/sda outputs /block/sda. This means that the directory you are looking for is /sys/block/sda. Now call udevinfo with the command udevinfo -a -p /sys/block/sda. The two commands can also be combined, as in udevinfo -a -p `udevinfo -q path -n /dev/sda`. The following is an extract from the output:

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"

From the output information, look for suitable keys that will not change. Remember that you cannot normally use keys from different directories.