Using Arduino to open file on Mac

I am brand new to Arduino, and a decade rusty on C++. I have the basics of on-board programming on the Arduino. What I'd like to do is use the Arduino boad as a multi-purpose trigger to launch various tasks on my computer via USB. For example, when the ping board detects a distance of less than 3 feet, open a video or audio file on the computer. I can send a command from the UNO via serial, but how can I get the computer to receive and process it? Any kick in the right direction is appreciated.

Run script on Mac which queries distance from arduino in endless loop.
If distance > 3 then open