UDev for persistent symlink - ashBabu/Utilities GitHub Wiki

Persistent udev Symlink for Cameras.

Example: Intel RealSense D435i on Ubuntu 24.04

  • Run v4l2-ctl --list-devices
Intel(R) RealSense(TM) Depth Ca (usb-0000:00:0d.0-1.2):
        /dev/video0
        /dev/video1
        /dev/video2
        /dev/video3
        /dev/video4
        /dev/video5
        /dev/media0
        /dev/media1
Integrated_Webcam_HD: Integrate (usb-0000:00:14.0-6):
        /dev/video6
        /dev/video7
        /dev/video8
        /dev/video9
        /dev/media2
        /dev/media3

  • Of these /dev/video*, we dont know which one is the color stream. To identify that, run the following on all of the /dev/video* and look for the 'YUYV' in Pixel Format

  • v4l2-ctl --device=/dev/video4 --all | grep -i 'Format\|Name\|Pixel Format'

	Driver name      : uvcvideo
		Extended Pix Format
		Extended Pix Format
	Driver name      : uvcvideo
	Name             : Intel(R) RealSense(TM) Depth Ca
	  Link 0x02000010: from remote pad 0x100000a of entity 'Processing 7' (Video Pixel Formatter): Data, Enabled, Immutable
Format Video Capture:
	Pixel Format      : 'YUYV' (YUYV 4:2:2)

From the above, we know that /dev/video4 is the color stream from realsense camera

  • Get the unique device path (ID_PATH) for the video device:
  • udevadm info --name=/dev/video4 | grep ID_PATH or udevadm info -q property -n /dev/video4
Example output:
E: ID_PATH=pci-0000:00:0d.0-usb-0:1.2:1.4

  • Sometimes ID_PATH may not be enough to uniquely identify the device. Then other fields needs to be added in the udev rule

  • Create a udev rule for the RGB stream symlink:

  • sudo nano /etc/udev/rules.d/99-realsense-rgb.rules

  • Add the following line (replace with your ID_PATH):

  • SUBSYSTEM=="video4linux", ENV{ID_PATH}=="platform-0000:00:0d.0-usb-0:1.2:1.4", SYMLINK+="realsense-rgb”

  • Reload udev rules and trigger:

  • sudo udevadm control --reload-rules

  • sudo udevadm trigger

  • Verify the symlink:

  • ls -l /dev/realsense-rgb Should point to the correct /dev/video4 device

  • Use in OpenCV: cap = cv2.VideoCapture('/dev/realsense-rgb')

Tips:

  • If symlink does not appear, unplug and replug the USB or reboot.
  • Repeat for other streams (depth, IR) using their respective /dev/video* and ID_PATH.
  • Always verify device formats with v4l2-ctl.

OR

Add udev rules on host to avoid permission issues for USB devices

  • Plug in the USB device
  • Run lsusb which would show Bus 001 Device 005: ID 10c4:ea60 Silicon Labs CP210x UART Bridge
  • In the above 10c4 --> idVendor and ea60 --> idProduct
  • Or use udevadm info --name=/dev/ttyUSB0 --attribute-walk. Example output would look like
 looking at device '/devices/.../ttyUSB0':
    KERNEL=="ttyUSB0"
    ATTRS{idVendor}=="10c4"
    ATTRS{idProduct}=="ea60"
  • sudo nano /etc/udev/rules.d/99-usb-lidar.rules and add the following SUBSYSTEM=="tty", ATTRS{idVendor}=="10c4", ATTRS{idProduct}=="ea60", MODE="0666", SYMLINK+="lidar"
  • sudo udevadm control --reload-rules && sudo udevadm trigger
  • ls -l /dev/lidar and it should show lrwxrwxrwx 1 root root 7 Month xx hr:min /dev/lidar -> ttyUSB0
  • Run the docker with -v /dev/lidar:/dev/lidar
  • The above method is more robust but for testing, you could run the docker with --device=/dev/ttyUSB0:/dev/ttyUSB0