What version of Linux do I have? Find out at command line

Originally published at: https://www.cyberciti.biz/faq/what-version-of-linux-do-i-have-find-out-at-command-line/

I am a new Linux system user and developer. What version of Linux do I have? What’s my OS and How do I find out from the command line? How do I check Linux version from the GUI or the CLI?