[App_rpt-users] Running a shell script from a DTMF command?

Hi gang. Basic question here: what is the magic incantation to run an arbitrary shell script from a DTMF command to an asterisk app_rpt system? E.g., I want to be able to dial *99999 or something and have my asterisk system run /usr/local/bin/whatever. I figure there has to be a way to do this, but I haven't been able to spot it. Thanks! Phil, N6TCT