The content is useful!! There seems to be some repetition in the explanation..

tg1011

To those of you who are hesitating whether to learn Linux shell scripting or not, I will tell you one thing. 
Do you want to handle Linux in a cool way like a hacker? Can you imagine a hacker who doesn't know shell scripting? 
Then start right now!

guileschool

Linux Shell Script Master

입문

초급

중급이상

리눅스

Linux

셸 스크립트

Shell Script

Linux Basics

Shell Basic Commands

Shell script

Regular expressions and search tools

Comparison and loop statements

arrangement

Input/Output and Environment Variables

Useful utilities

Pipes and work control