Have you ever want to switch a pin on/off from serial monitor? and did you ever need to push message to (let's say...) Bluetooth Module just by writing via serial monitor?
Then you are came to the right place!
I made a sketch that has ability to switch every single I/O pin just by writing a little command to Serial Monitor, no manual program required.
It's also allow you to forward message to second serial port (in Arduino, it's Serial1) for supported board.
Beside of that, this sketch also useful if you want to keep heavy-lifting logic on PC program then send command to Arduino via COM port to do things with hardware.
Anyway, it's available on GitHub and it is called "Arduino CLI", check it out :)