OverTheWire
overthewire.org
Free security wargames that teach hacking and Linux concepts through levelled challenges you solve over SSH, starting with the beginner-friendly Bandit game.
Why it is useful
A hands-on way to learn the command line and security basics legally, on machines built for the purpose. Each level hands you the password to the next, so progress is built in. Bandit alone is a solid introduction to Linux.
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