[SOLVED] Determine serial port based on USB description (bash)

Wim Sturkenboom · 06-18-2024, 06:16 AM

Hi all,

Not sure if this is the correct section to ask; moderators feel free to move it.

I have a scenario where I need to identify the serial port (/dev/ttyACMx) of a USB device based on USB description. There are a number of these devices connected to a computer; they have the same VID and PID but differ in USB description.

Is there a single bash command that I can use?

Without that, the process that I visualise is to use lsusb to find the connected devices by VID/PID to see if they are connected, use a filter on the output of dmesg (probably read /var/log/dmesg using tac) and find the usb description and associated device (/dev/ttyACMx). The sequence can be reversed, not sure what's better but I will figure that out in time.

michaelk · 06-19-2024, 03:42 PM

You should be able to write a udev rule to assign persistent device names based on some unique description for each serial port.

Code:

SUBSYSTEM=="tty",ENV{descriptor}=="descriptor value",SYMLINK+="unique_device_name"

Reference that particular USB serial port as /dev/unique_device_name

Based on the output of udevadm for each device.

udevadm info /dev/ttyACMx

I have not played with udev rules in a long time...

Wim Sturkenboom · 06-20-2024, 05:52 AM

Thanks for the reply and for the pointer to udevdm. I do not think that I need to modify rules; I also find it a little risky as I don't have experience with them (if that is what you suggested)

I however found a way. What I know about the devices is that they are always /dev/ttyACMx. So for starters I came up with the below which basically gives me all required info

Code:

for device in /dev/ttyACM*; do udevadm info --name $device; done

The output for ttyACM1

Code:

P: /devices/pci0000:00/0000:00:1d.7/usb1/1-5/1-5.4/1-5.4.2/1-5.4.2:1.0/tty/ttyACM1
N: ttyACM1
S: serial/by-id/usb-SparkFun_SparkFun_Pro_Micro22-if00
S: serial/by-path/pci-0000:00:1d.7-usb-0:5.4.2:1.0
E: DEVLINKS=/dev/serial/by-id/usb-SparkFun_SparkFun_Pro_Micro22-if00 /dev/serial/by-path/pci-0000:00:1d.7-usb-0:5.4.2:1.0
E: DEVNAME=/dev/ttyACM1
E: DEVPATH=/devices/pci0000:00/0000:00:1d.7/usb1/1-5/1-5.4/1-5.4.2/1-5.4.2:1.0/tty/ttyACM1
E: ID_BUS=usb
E: ID_MODEL=SparkFun_Pro_Micro22
E: ID_MODEL_ENC=SparkFun\x20Pro\x20Micro22
E: ID_MODEL_FROM_DATABASE=NM10/ICH7 Family USB2 EHCI Controller
E: ID_MODEL_ID=9204
E: ID_PATH=pci-0000:00:1d.7-usb-0:5.4.2:1.0
E: ID_PATH_TAG=pci-0000_00_1d_7-usb-0_5_4_2_1_0
E: ID_PCI_CLASS_FROM_DATABASE=Serial bus controller
E: ID_PCI_INTERFACE_FROM_DATABASE=EHCI
E: ID_PCI_SUBCLASS_FROM_DATABASE=USB controller
E: ID_REVISION=0100
E: ID_SERIAL=SparkFun_SparkFun_Pro_Micro22
E: ID_TYPE=generic
E: ID_USB_CLASS_FROM_DATABASE=Miscellaneous Device
E: ID_USB_DRIVER=cdc_acm
E: ID_USB_INTERFACES=:020200:0a0000:
E: ID_USB_INTERFACE_NUM=00
E: ID_USB_PROTOCOL_FROM_DATABASE=Interface Association
E: ID_VENDOR=SparkFun
E: ID_VENDOR_ENC=SparkFun
E: ID_VENDOR_FROM_DATABASE=Intel Corporation
E: ID_VENDOR_ID=1b4f
E: MAJOR=166
E: MINOR=1
E: SUBSYSTEM=tty
E: USEC_INITIALIZED=19858834082

I now need to puzzle how to extract the correct info; I did not find a direct way in the udevadm man page. It's basically the ID_MODEL and/or ID_MODEL_ENC. grep and awk come to mind although it's ages ago that I used *awk* ( the book is already dusted of

)

Note
I can modify the MODEL_ID with the code that I program in the USB device. So each USB device has its own model.

Wim Sturkenboom · 06-22-2024, 10:28 AM

The devices no identify themselves slightly different

/dev/ttyACM0

Code:

E: ID_MODEL=SparkFun_Pro_Micro__1_
E: ID_MODEL_ENC=SparkFun\x20Pro\x20Micro\x20\x281\x29

and /dev/ttyACM2

Code:

E: ID_MODEL=SparkFun_Pro_Micro__2_
E: ID_MODEL_ENC=SparkFun\x20Pro\x20Micro\x20\x282\x29

After a serious battle with hex values I came up with the below solution

Code:

# check for arguments
if [ $# != 1 ]
then
  echo "No argument"
  exit -1
fi

emptyText=""

# count number of matching devices
count=0
# final port (device) to use
port=""

# loop through all ttyACM devices
for device in /dev/ttyACM*;
do
  # get info for devicesw 
  udevDevice=$(udevadm info --name $device)
  #echo "udevDevice = "
  #echo "$udevDevice"
  # grep what we're looking for
  udevGrep=$(echo -e "$udevDevice" |grep "$1");
  #echo "udevGrep = "
  #echo "$udevGrep"

  # if we have a match
  if [ "$udevGrep" != "$emptyText" ]
  then
    # assign to port
    port=$device
    #echo "$device"
    #echo "$result"
    # update count of matching devices
    ((count++))
  fi
done

# check results
if [[ $count -eq 0 ]]
then
  echo "No matching device found"
  exit -2
else
  if [[ $count -gt 1 ]]
  then
    echo "Multiple devices found"
    echo "$count"
    exit -3
  else
    echo -n "Device '"
    echo -n "$1"
    echo -n "' on port '"
    echo -n "$port"
    echo "'"
  fi
fi

exit 0

For now it will do; I might have overlooked some possible scenarios and the use of $1 as regexp might need a bit of fine tuning.

Typical output

Code:

$ ./findtty.sh "Micro (2)"
Device 'Micro (2)' on port '/dev/ttyACM2'

$ ./findtty.sh "Micro"
Multiple devices found
2

Again thanks for the pointer to udevadm.