The Undiscoverable/Command line
The Linux command line in any of the available shell programs is complex and entirely unintuitive. The standard Linux commands have numerous options, which they handle in inconsistent ways. Their names are short and cryptic. Almost none of this usage can be guessed.
The usual shell today, and the one provided in Terminal, is Bash, the Bourne-Again Shell. It is documented in Introduction to the Gnu/Linux Command Line, a free manual for beginners written and published by FLOSS Manuals, and in many other manuals, most aimed at more technical users.