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()
         


        

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:"