Udev- How do I find out which subsystem a device belongs to?

Question

For example, I have device /dev/sdb. How do I find out it's udev subsystem?

Answer 1

$ udevadm info -q all -a /dev/sdb

NOTE: the above queries the UDEV database for device information, info, we're querying for all information, -q all, and we're walking all the attributes under /sys pertaining to this device label.

excerpt

   --attribute-walk|-a
       Print all sysfs properties of the specified device that can be used 
       in udev rules to match the specified device. It prints all devices
       along the chain, up to the root of sysfs that can be used in udev 
       rules.

Example

Here's my /dev/sda device.

$ udevadm info -q all -a /dev/sda | grep parent
walks up the chain of parent devices. It prints for every device
and the attributes from one single parent device.
  looking at parent device '/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0':
  looking at parent device '/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0':
  looking at parent device '/devices/pci0000:00/0000:00:1f.2/ata1/host0':
  looking at parent device '/devices/pci0000:00/0000:00:1f.2/ata1':
  looking at parent device '/devices/pci0000:00/0000:00:1f.2':
  looking at parent device '/devices/pci0000:00':

Differences between these 2 commands

@sepero's answer is showing information just pertaining to the UDEV device at the leaf node.

Sepero's answer

$ udevadm info -q all -n /dev/sda > udevadm_info_1.txt

• single udevadm output

slm's answer

My answer shows the attributes as it walks the entire /sys chain hierarchy of devices.

$ udevadm info -q all -a /dev/sda > udevadm_info_2.txt

• nested udevadm output

You can see the difference between these two.

$ diff -y udevadm_info_1.txt udevadm_info_2.txt  \
    > udevadm_info_diff.txt

• side by side diff of the above 2 files

NOTE: I'm running on Fedora 19, using version 204 of udevadm:

$ udevadm --version
204

Answer 2

udevadm info provides this kind of information.

If you specifically want the subsystem:

udevadm info -n /dev/sdb -q property | sed -n 's/SUBSYSTEM=//p'

When you're writing udev rules, the most useful command is

udevadm info -a -n /dev/sdb

This prints out rules that you can use to match the device in udev rules. The first block is about the device itself, and the subsequent blocks are about its ancestors in the device tree. The only caveat is that you cannot mix keys that correspond to different ancestors. For example, given this excerpt

KERNEL=="sdb"
SUBSYSTEM=="block"
…
KERNELS=="5:0:0:0"
SUBSYSTEMS=="scsi"
DRIVERS=="sd"
ATTRS{model}=="Yoyodyne Diskinator"
…
KERNELS=="0000:00:1f.2"
SUBSYSTEMS=="pci"
DRIVERS=="ahci"

then you can match this device with SUBSYSTEM=="block", SUBSYSTEMS=="scsi", ATTRS{model}="Yoyodyne Diskinator" or with SUBSYSTEM=="block", SUBSYSTEMS=="pci", DRIVERS="ahci" but not with SUBSYSTEM=="block", SUBSYSTEMS=="scsi", ATTRS{model}="Yoyodyne Diskinator", DRIVERS="ahci".

Another way to obtain information for use in rules is with

udevadm info -q property -n /dev/sdb --export

This prints device property values of the form KEY=VALUE which you can use as ENV{KEY}=="VALUE" in udev rules.

If your device doesn't have a /dev entry, you can refer to it with a path under /sys instead, e.g.

udevadm info -a -p /sys/block/sdb

(not a useful example in this case — this is useful for devices whose category isn't common and don't have an entry under /dev yet).

When you change udev rules, they are read automatically; however, they are only applied to devices that are subsequently plugged into the system. To apply the new rules to an already-connected device, use udevadm trigger (with options to restrict the application to certain devices).

Answer 3

$ udevadm info -q all -n /dev/sdb