Using fixed tty device path on Ubuntu

Why fix the tty device path?

Usually, you don’t need this but if you are working on a project that uses an embedded device with some peripherals, it’s nice to have a fixed device path since /dev/ttyUSB0 or /dev/ttyUSB1 can be any device.

Wouldn’t it be better if we have /dev/ttyMyPort0 always be the device 0 and /dev/ttyMyPort1 be the device 1?

How do I do this

Finding the unique attribute

We need to tell Linux that a specific device must go to /dev/ttyMyPort0. And the specific device can be distinguished by the following command.

udevadm info -a -n /dev/ttyUSB0

user@ubuntu:~$ udevadm info -a -n /dev/ttyUSB0

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/3610000.xhci/usb1/1-2/1-2.1/1-2.1:1.0/ttyUSB0/tty/ttyUSB0':
    KERNEL=="ttyUSB0"
    SUBSYSTEM=="tty"
    DRIVER==""

  looking at parent device '/devices/3610000.xhci/usb1/1-2/1-2.1/1-2.1:1.0/ttyUSB0':
    KERNELS=="ttyUSB0"
    SUBSYSTEMS=="usb-serial"
    DRIVERS=="ftdi_sio"
    ATTRS{latency_timer}=="16"
    ATTRS{port_number}=="0"

  looking at parent device '/devices/3610000.xhci/usb1/1-2/1-2.1/1-2.1:1.0':
    KERNELS=="1-2.1:1.0"
    SUBSYSTEMS=="usb"
    DRIVERS=="ftdi_sio"
    ATTRS{authorized}=="1"
    ATTRS{bAlternateSetting}==" 0"
    ATTRS{bInterfaceClass}=="ff"
    ATTRS{bInterfaceNumber}=="00"
    ATTRS{bInterfaceProtocol}=="ff"
    ATTRS{bInterfaceSubClass}=="ff"
    ATTRS{bNumEndpoints}=="02"
    ATTRS{interface}=="Quad RS232-HS"
    ATTRS{supports_autosuspend}=="1"

  ...
  ...
  ...

This will tell us device information such as which port is connected to the device. Try this command with different device paths. You will see some parameters are different from the others. In my case, I usually use KERNELS==1-2.1:1.0 which tells me that this device is connected to 1-2.1:1.0 physical port.

Tell Linux to use the fixed path that I want

Once we have a unique attribute of the device, it’s time to tell Linux that I want to use this device to be this path. Open /etc/udev/rules.d/99-usb-serial.rules with root permission and add something like the below.

KERNEL=="ttyUSB*",KERNELS=="1-2.1:1.0",SYMLINK+="ttyTelem"

KERNEL and KERNELS are the unique attributes of /dev/ttyUSB0 device. And SYMLINK tells Linux to make a symbolic link of /dev/ttyUSB0 to /dev/ttyTelem. You can add more attributes prior to SYMLINK to make a better separation.

Reboot your system or use below the commands to apply these changes immediately.

sudo udevadm control --reload-rules
sudo udevadm trigger

See if /dev/ttyTelem exists and if it is linked to /dev/ttyUSB0. If it doesn’t show up, the unique attributes might not be unique.