Joewiz, Don't believe him. Bash is still now!
Jump over to here:
https://www.mobileread.com/forums/sho...124219&page=17
At the bottom of the page is a chunk of script which I happened to post earlier today (well, it's still Wed. here).
Pure bash setup with a call to ebook-convert and all of the command line switches available.
For *multi* use, I suppose you could open another terminal, and start another thread there. It *should* work...but that could depend on how your OS provides library (.so) access to calling processes. I have an idea that the memory space for running processes is kept separate. But MMMV. (I'm going to try it just to see).
Otherwise, you will need to split your source files: can't have two 'for file in *;' processes looking at the same folder!