[michaelmdresser]

Over the Wire's "Bandit" game was my tool of choice for teaching the introduction to shell for my university's Cybersecurity Club a few semesters in a row. It didn't feel like a chore because of the gamification and the go-your-own-pace approach -- the hints are also a nice touch. I highly recommend it to build some shell muscles and maybe learn something you didn't already know.

For something more advanced with less help available, I recommend Hack The Box [1] which frequently rotates challenge "boxes" with new combinations of configuration and vulnerability. I haven't touched it in a few years, so take that with a grain of salt. It used to be the case that you had to "hack" the website (very easy) in order to sign up for an account, but it appears that that may no longer be the case :(

[1] https://www.hackthebox.com/