When I try it like you typed it with a Capital 'C' in Cat it says 'Cat: not found'.
Can someone on a MT3G try this?
It just returns -18 if its returning 0 that's because there is no numerical entry at the end so theoretically it is working. Are you using stock keyboard or a keyboard emulator such as better keyboard?
Never used an mt3g before so somewhat ignorant of how it works. Oh and that should be a lower case C, my bad.