USB Python Script Issue Raspberry Pi OS

Question

I'm encountering an issue where I'm trying to trigger a Python script in the background upon plugging in up to four USB devices, all with the same USB vendor and product IDs.

To accomplish this, I've created a udev rule in /etc/udev/rules.d/myrule.rules as follows:

ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="1234", ATTR{idProduct}=="5678", ENV{SYSTEMD_WANTS}="usb-trigger.service"

1234 and 5689 are just examples.

Additionally, I've set up a systemd service in /etc/systemd/system/usb-trigger.service with the following configuration:

[Unit]
Description=USB Trigger Service

[Service]
Type=oneshot
ExecStart=/path/to/my/python/script/uart.py

[Install]
WantedBy=multi-user.target

However, the script is sometimes executed and sometimes not. Furthermore, if I plug in more than the first device or the second device, the background script fails to start. Occasionally, none of the devices work as expected.

The primary goal of my Python script is to generate a CSV file upon plugging in a USB device, save the data, and close the file when the device is unplugged. I need this script to handle up to four devices.

Here's the Python script I'm using:

python
import serial
import serial.tools.list_ports
import time
import csv
import datetime
import sys

def get_current_time():
    current_time = time.time()
    milliseconds = int((current_time - int(current_time)) * 1000)
    formatted_time = time.strftime("%H:%M:%S", time.localtime(current_time))
    formatted_time += f".{milliseconds:03d}"  # Adding milliseconds
    return formatted_time

def write_to_csv(writer, data):
    try:
        writer.writerow(data)
    except Exception as e:
        print(f"Error writing to CSV: {e}")

def find_com_port():
    try:
        com_ports = serial.tools.list_ports.comports()
        for port in com_ports:
            if port.device.startswith('/dev/ttyUSB') or port.device.startswith('/dev/ttyACM'):
                return port.device
    except Exception as e:
        print(f"Error finding COM port: {e}")

def main():
    try:
        while True:
            com_port = find_com_port()
            if com_port is not None:
                print(f"COM port found: {com_port}")
                break  # Exit the loop once a COM port is found
            else:
                print("No COM port found. Retrying...")
                time.sleep(1)      

        if com_port is None:
            print("No COM port found.")
            return

        filename = datetime.datetime.now().strftime("%Y-%m-%d_%H-%M-%S") + ".csv"
        bottle_id = None
        bottle_id_collected = False  # Flag to track if bottle ID has been collected

        with serial.Serial(port=com_port, baudrate=460800, timeout=2) as ser, open(filename, 'w', newline='') as file:
            print(f"Connected to {com_port} successfully.")
            writer = csv.writer(file)

            while True:
                received_bytes = ser.readline().decode('ascii', errors='replace').strip()
                if received_bytes:
                    if not bottle_id_collected and 'Bottle ID' in received_bytes:
                        bottle_id = received_bytes.split('Bottle ID')[1].strip()
                        writer.writerow(['Bottle ID', bottle_id])
                        writer.writerow(['Input 1', 'Input 2', 'Input 3', 'time stamp'])
                        bottle_id_collected = True
                    else:
                        parts = received_bytes.split(', ')
                        try:
                            numbers = [float(part) for part in parts]
                            data_row = numbers + [get_current_time()]
                            writer.writerow(data_row)
                            print(f"Writing to CSV: {data_row}") 
                            file.flush()  # Force flushing
                        except ValueError as e:
                            pass
                time.sleep(0.01)

    except serial.SerialException as e:
        print(f"Error opening serial port {com_port}: {e}")

    except KeyboardInterrupt:
        print("Program terminated by user.")

    except Exception as e:
        print(f"An error occurred: {e}")

    finally:
        try:
            if 'ser' in locals() and ser.is_open:
                ser.close()
        except Exception as e:
            print(f"Error closing serial port: {e}")

        try:
            if 'file' in locals() and not file.closed:
                file.close()


        except Exception as e:
            print(f"Error closing CSV file: {e}")

        sys.exit()

if __name__ == "__main__":
    main()

Any insights into why the script behaves inconsistently and how I can ensure it runs reliably upon plugging in multiple USB devices would be greatly appreciated. Thank you!

I configured a udev rule and systemd service to trigger a Python script upon plugging in multiple USB devices with identical IDs. I expected the script to reliably generate a CSV file for each device, handling up to four devices simultaneously. However, the script sometimes failed to start, especially with multiple devices plugged in, leading to inconsistent behavior.

Answer 1

I don't know whether your script will work but as posted it wouldn't even run.

The systemd service needs to have the full path to python.

The python script itself is missing a shebang (and needs to have run permission) unless it is called by python.

NOTE there are dozens of similar questions.