linux.recipes - Your Linux Command Reference
Unix Toolbox
A collection of Unix/Linux/BSD commands and tasks which are useful for IT work or for advanced users, a compact and practical reference.
BashSheet - Greg's Wiki
bobbyiliev/introduction-to-bash-scripting: Free Introduction to Bash Scripting eBook
Free Introduction to Bash Scripting eBook.
The Bash Guide
A complete guide for newcomers and advanced users to correct usage and deepen understanding of the bash shell language.
Linux Commands - A practical reference
A linux command line cheat sheet