After seeing a discussion in steemit.chat I wanted to go back through and add some sources to my old posts. Since this one is too old to edit, I'm leaving them here in a comment.
The man pages for Bash and all of the other commands I've used above appear to cover 100% the stuff that I put in. If you're not on a Linux or *nix system where you can view man pages, here are the links to some online:
Bash This does a decent job of explaining all of the built-in commands that I used (read, echo, while loops, for loops, parameter expansion). Some of the examples are pretty light, though. So I suggest googling some of the terms you might read in there to see examples on Stackoverflow.
convert The convert man page does an okay job, but you'll probably want to check out the ImageMagick site for examples.
readlink For discovering the path to a thing, I used it to get the absolute path to the script that was being run.
find While technically this explains the command, most mortals will want to look for examples on Stackoverflow.