Bandit CTF is a great entry point to gain a better understanding of the UNIX command line and capture the flags. 

Capture the Flags are used by aspiring ethical hackers around the world to gain hands-on experience in detecting and exploiting vulnerabilities.

We will use go through Bandit to build upon our exisiting command line knowledge and learn more advanced tools.

As I don't want to spoil the answers, as asked by overthewire.org, I will simply details the commands used and the theory/concept behind it.