Going through my old linux and macOS cheatsheets, I found this,
youtube-dl; it lets you rip youtube vids and download them.
sudo apt-get update && sudo apt-get install youtube-dl youtube-dl <youtube vid URL>
It's also available on macOS. If you use brew, you can get it with this
brew update brew install youtube-dl youtube-dl <youtube vid URL>
I think you can also do this in Windows with Cygwin, but I haven't tried that.
UPDATE:added in the comments. Windows users can also get this via a Python package. You can get the EXE from rg3/youtube-dl