Linux

I have just finished Bandit wargame from overthewire. Twenty-six levels packed with mini quizzes aimed at beginner Linux/security enthusiasts. First thirteen levels can be solved by using simple Linux commands with the goal of finding password hidden or encrypted in some file. Then it becomes interesting because we start to play with bits of networking knowledge like ssh, netcat, telnet etc. And last five are about cron jobs and writing bash scripts. I totally recommend this game.