UDEV in Linux

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UDEV may not officially stand for "user devices", but this definition can be helpful in understanding what UDEV does. Let's say you connect a Samsung device to your Linux machine with a USB cable. Let's use the LSUSB command to gather some data about the device.

[user1@server1 ]# lsusb
Bus 002 Device 007: ID 04e8:6860 Samsung Electronics Co., Ltd Galaxy (MTP)


Let's also use the mount command to see where the device is mounted.

[user1@server1 ]# mount
/dev/sdb1 on /media type fuseblk (rw, nosuid, nodev, allow_other, blksize=4096)


The kernel will emit a uevent. The uevent is picked up by UDEV. UDEV will then follow rules at one of these directories:

  • /etc/udev/rules.d
  • /run/udev/rules.d
  • /lib/udev/rules.d

As an example, when the Samsung device is connected to the Linux machine, the Linux machine will read the contents of the /lib/udev/rules.d/69-libmtp.rules file to know what to do with the USB flash drive. We can use the CAT command to read the contents of the /lib/udev/rules.d/69-libmtp.rules file, and then pipe the STDOUT through GREP, using device is 6860 from the LSUSB command.

[user1@server1 ]# cat /lib/udev/rules.d/69-libmtp.rules | grep 6860
ATTR{idVendor}=="04e8", ATTR{idProduct}=="6860", SYMLINK+="libmtp-%k", MODE="660", GROUP="audio", ENV{ID_MTP_DEVICE}="1", ENV{ID_MEDIA_PLAYER}="1"


Notice the Mode is 660 and the group is "audio." If we view the mounted device, we can see that the mode is 660 and hte group is audio.

[user1@server1 ]# ls -l /media
drw-rw---- 2 root audio 4096 Jan 1 00:01 Samsung

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