MPlayerX supports Shell script command line and AppleScript as its integration to the OS.
For Shell script
Besides the names in the table below, the strings declared in this file could also be used as name.
|url||the url of the media
|file||the path of the media file
A real world example
MPlayerX just started to support AppleScript in 1.0.17, its implementation still stays in primary status.
Usually you could use AppleScript Editor to look up all the functions provided by MPlayerX, for geeks, the source code is here.