Why does the script stop working after execution?

Question

linux

There is a TP-Link 3020 router with OpenWrt installed on it. A GSM modem is connected to the USB port of the router, which transmits the phone number via USB when an incoming call and opens the gate in parallel.
In /etc/rc.local script launch is registered The script
./test.sh &
itself

#!/bin/sh
cat /dev/ttyUSB0 | while read DATA; do
  if echo $DATA | grep -q +7; then
        DATEFORMAT=$(date '+%m/%d/%y %H:%M:%S')
        echo $DATA $DATEFORMAT >> /www/1.txt
  fi
done

The phone number and date are written to the 1.txt file, and after that the script stops working, after rebooting the device, it again writes the number 1 time and that's it.
I watched the running processes, after the call the process is unloaded. Can you tell me how to catch the error or what could be the problem?

Answer 1

Why does he have to keep working? The script describes one-time actions and there are no cycles.