https://swcarpentry.github.io/shell-novice/
http://www.computersecuritystudent.com/HOME/index.html

http://freecomputerbooks.com/
http://www.freetechbooks.com/
http://www.freebookcentre.net/
http://www.onlineprogrammingbooks.com/
http://www.e-booksdirectory.com