Primitive error handling

>
> Hello,
> I have another "problem". In windows, my microchip is connected on port 7 (COM7) but I dunno which port number it is in
> Linux. It is connected in location /dev/ttyUSB0 but I dunno what is its number. I have tried this code to discover it:
>
> |n|
> n := SerialPort new.
> 1 to: 20 do: [:port | n openPort: port. (n isNil) ifFalse: [Transcript show: port]. n close]
>
> but it told me that no port after opening is nil (strange, I thought that on Windows when I tried to open non-existing port,
> it returned nil). So I have tried another code, reading string from this port by n readString and watching when it does
> primitiveFailed and when doesn't. But I want only this kind of code (but I dunno how is it named): (n readString
> isPrimitiveFail) ifFalse: [Transcript show: ...]. How can I do it? Thanks for response.