It's a bit long for a one-liner, but this should work.

for f in /media/johann/5461-000B/DCIM/100MEDIA/*.AVI; do num=${f%.AVI}; num=${num##*IMAG}; ffmpeg -i "$f" -ss 00:00:00 -t 00:00:20 "~/Public/240321/240321_$num.avi"; rm "$f"; done

This num=${f%.AVI}; num=${num##*IMAG}; extracts the number from the video filename. To make sure it's not deleting anything it shouldn't, you also might want to run it with rm -i "$f"

Oh, stewardess! I speak Klingon.

Five is right out

You really need to watch Red Dwarf

My entire life has vim key bindings. My window manager and browser all have vim key bindings. I work in vim. I write my shopping lists in vim.

I really can't overstate how ingrained vim is in my day to day life. Bram had a big impact.

It's called Title Case. There are different rules depending on which style manual you're using. Some people just capitalise everything. Some people don't use it at all.