Finding only disk drives using pyudev

Question

I'm looking to enumerate all hard disks on a computer using udev and specifically pyudev to enumerate everything:

import pyudev

context = pyudev.Context()

for device in context.list_devices(subsystem='block', DEVTYPE='disk'):
    print "{}, ({})".format(device.device_node, device.device_type)

This prints out the following:

/dev/sdb (disk)
/dev/sdc (disk)
/dev/sda (disk)
/dev/sr0 (disk)
/dev/loop0 (disk)
/dev/loop1 (disk)
/dev/loop2 (disk)
/dev/loop3 (disk)
/dev/loop4 (disk)
/dev/loop5 (disk)
/dev/loop6 (disk)
/dev/loop7 (disk)
/dev/ram0 (disk)
/dev/ram1 (disk)
/dev/ram10 (disk)
/dev/ram11 (disk)
/dev/ram12 (disk)
/dev/ram13 (disk)
/dev/ram14 (disk)
/dev/ram15 (disk)
/dev/ram2 (disk)
/dev/ram3 (disk)
/dev/ram4 (disk)
/dev/ram5 (disk)
/dev/ram6 (disk)
/dev/ram7 (disk)
/dev/ram8 (disk)
/dev/ram9 (disk)

Since I'm mainly concerned with actual drives and not optical disk drives, loopback devices, or ram devices, how can I filter my results down to only get real physical media?

Answer 1

Since the device management under udev is very flexible, it may depend on the udev rules installed in /lib/udev/rules.d or /etc/udev/rules.d that determine what keys you can filter on.

You can use udevadm to get a list of devices and their associated udev keys.

sudo udevadm info --export-db

Similarly, your script could be modified to output this information as follows:

#!/usr/bin/env python
import pyudev

context = pyudev.Context()

for device in context.list_devices(subsystem='block', DEVTYPE='disk'):
    for key, value in device.iteritems():
        print '{key}={value}'.format(key=key, value=value)
    print

This will allow you to determine what keys are available for filtering. On my Debian system, some of the entries look like this:

# Loop device
UDEV_LOG=3
DEVPATH=/devices/virtual/block/loop0
MAJOR=7
MINOR=0
DEVNAME=/dev/loop0
DEVTYPE=disk
SUBSYSTEM=block
UDISKS_PRESENTATION_NOPOLICY=1
DEVLINKS=/dev/block/7:0

#cdrom
UDEV_LOG=3
DEVPATH=/devices/pci0000:00/0000:00:01.1/host1/target1:0:0/1:0:0:0/block/sr0
MAJOR=11
MINOR=0
DEVNAME=/dev/sr0
DEVTYPE=disk
SUBSYSTEM=block
ID_CDROM=1
ID_CDROM_DVD=1
ID_CDROM_MRW=1
ID_CDROM_MRW_W=1
ID_SCSI=1
ID_VENDOR=VBOX
ID_VENDOR_ENC=VBOX\x20\x20\x20\x20
ID_MODEL=CD-ROM
ID_MODEL_ENC=CD-ROM\x20\x20\x20\x20\x20\x20\x20\x20\x20\x20
ID_REVISION=1.0
ID_TYPE=cd
ID_BUS=scsi
ID_PATH=pci-0000:00:01.1-scsi-1:0:0:0
GENERATED=1
UDISKS_PRESENTATION_NOPOLICY=0
DEVLINKS=/dev/block/11:0 /dev/scd0 /dev/disk/by-path/pci-0000:00:01.1-scsi-1:0:0:0 /dev/cdrom /dev/dvd
TAGS=:udev-acl:

# hard disk
UDEV_LOG=3
DEVPATH=/devices/pci0000:00/0000:00:0d.0/host2/target2:0:0/2:0:0:0/block/sda
MAJOR=8
MINOR=0
DEVNAME=/dev/sda
DEVTYPE=disk
SUBSYSTEM=block
ID_ATA=1
ID_TYPE=disk
ID_BUS=ata

It would be possible to filter on these key/value pairs to return just those disks which are real. The UDISKS_PRESENTATION_NOPOLICY entry for example, can be used to filter out loop devices. You can peruse the udev rules to determine how it tries to distinguish between real/virtual disks.

Something like this will filter out cdroms and loop devices:

devices = []
for device in context.list_devices(subsystem='block', DEVTYPE='disk'):
    # Filter out cd drives, loop devices.
    if device.get('ID_TYPE', '') == 'cd':
        continue
    if device.get('UDISKS_PRESENTATION_NOPOLICY', '0') == '1':
        continue
    devices.append(device)

print "HARD DISKS:"

Answer 2

Since udev only runs on Linux kernel (at least as of now), you could filter out by MAJOR number 8 which represents all SCSI/SATA disk driver based devices.

for device in context.list_devices(MAJOR='8'):
    if (device.device_type == 'disk'):
        print "{}, ({})".format(device.device_node, device.device_type)

On my system, your code outputs the following:

/dev/sda, (disk)
/dev/sdf, (disk)
/dev/sdb, (disk)
/dev/sdc, (disk)
/dev/sdd, (disk)
/dev/sde, (disk)
/dev/sr0, (disk)
/dev/loop0, (disk)
/dev/loop1, (disk)
/dev/loop2, (disk)
/dev/loop3, (disk)
/dev/loop4, (disk)
/dev/loop5, (disk)
/dev/loop6, (disk)
/dev/loop7, (disk)

After filtering by major number 8, I see the following output:

/dev/sda, (disk)
/dev/sdf, (disk)
/dev/sdb, (disk)
/dev/sdc, (disk)
/dev/sdd, (disk)
/dev/sde, (disk)

Note that you would also get USB hard drives and USB sticks in the list, since they also tend to use the same SCSI disk driver.

I'm not really sure if IDE hard drives are mapped as sdX or hdX with the latest 2.6 or 3.x kernels. Do not have an IDE hard drive to verify and long since I have had one. :D

UPDATE: The same device number page lists /dev/hdX to be the one used by IDE hard drives (and IDE cdroms too may be ?). If you want to filter these as well, I believe you could do something like this:

for device in context.list_devices(DEVTYPE='disk'):
    major = device['MAJOR']
    if major == '8' or major == '3':
        print "{}, ({})".format(device.device_node, device.device_type)