TOPIC: How does "use an external tool" work exactly?

I made a shell script to transform a mono sample to stereo and apply a convolution reverb on it. I call the script from anywhere and give the wav file as argument. This wav gets processed and replaced. I'm trying to use this script in Polyphone calling it with use an external tool and using as commandbut I only get a dialog "it wasn't possible to start the command". Any suggestions?

You could try to write this line (not tested):
You need to make sure that the script has execution rights and that it can take the argument into account

But you will have a problem if you separate the original mono sample into a stereo sample in your script. Polyphone will reload the exported mono sample itself after the script has been run: all extra files that would have been created will be ignored.