Linux command line reference

Whether you are a newbie to Linux or you simply do not use it often enough for the commands to become old hat yet. This is a great resource to help you through the learning curve or if your simply have a terrible memory like me then you will find both these references very helpful.
The previous reference for practical Linux commands was surprisingly popular with over 3.5 million hits in nearly 5 years.
So they decided to start compiling another list of somewhat more involved/esoteric commands. Which can be found here.
Examples marked with • are valid/safe to paste without modification into a terminal, so
you may want to keep a terminal window open while reading this so you can cut & paste.