More info:
The recipes extensively use
the Calibre recipe API. The "recipe" file itself is just the tip of the iceberg. It functions to give a pointer to the source for the ebook to be created and it tweaks some parameters, but it doesn't have any of the code that does the real work. As user_none noted, you don't need the GUI parts of calibre, but you will need to be able to install the code that supports the API.