Beginner
Learn Shell (learnshell.org)
The Command Line Crash Course (cli.learncodethehardway.org)
The Fundamentals of Bash Scripting (code.tutsplus.com)
Learning the Shell (linuxcommand.org)
Don't Fear the Command Line (readwrite.com)
Badassify Your Terminal and Shell (jilles.me)
Beginner's Guide to the Bash Terminal (www.youtube.com)
Difference Between Shell, Terminal, Command Line (askubuntu.com)
Command Line (devcenter.heroku.com)
Intermediate
Explain Shell (explainshell.com)
Advanced
Advanced Topics (www.tldp.org)
