How to do Filter in Advanced printer settings

Joymaker

In the Printer Settings dialogue, Advanced tab, is a box entitled "Filter path and parameter". This is a really useful feature, to be able to post process the gcode after it is produced and before printing. I have used it to good effect on the Windows version.

Now I'm on the Mac version, 0.56. And I can't seem to get it to work. After failing with the more complex filter, I decided to put the ultimate trivial filter command into that box:

/bin/cp #in #out

which should actually change nothing, but simply deliver the gcode as it was. However, here's what happens: slic3r goes through all its regular steps. I see "starting postprocessor". And then nothing ever happens again. The gcode never appears in its window, and is never ready to print.

Does this feature WORK? Please can someone tell me an acceptable form of command to put in that box, that would work?