Utilities
find
Holy crap find, I always have to look at your man page. Especially on macOS.
du
The command du -cks * | sort -rn | head is very helpful.
scron
Have to try this one: scron
identify
identify -format "%[w]x%[h]\n"
gives you the width and height of an image.
Shell scripting
Here’s a good tutorial: Shell Scripting Tutorial