Typing ifeffit at the system command prompt will start the basic IFEFFIT command-line program. You should get a set of messages and a command prompt that looks like this:
Ifeffit 1.2.5 Copyright (c) 2004 Matt Newville, Univ of Chicago
command-shell version 1.1 with GNU Readline
Ifeffit>
At this point, you're ready to start typing IFEFFIT commands at the
prompt. If you don't get such a prompt, consult the installation
instructions that come with the IFEFFIT distribution.