UNIX is the most popular operating system for multi-user computers. It is also a command line interface and just to make it REALLY confusing, it is also case sensitive. To be case sensitive means the operating system recognizes a difference between upper case characters and lower case characters. For example, typing in ls (the UNIX command for listing your files) will give you a listing of your files, but if you type LS, you will get the message "command not found". Commands in UNIX are usually in lower case and most UNIX users will use lower case for file names to make this aspect of UNIX a little easier to handle. In fact, people who work both in a Windows environment and a UNIX environment find it easier to always use lower case, no matter where they are working. There are graphical shells to UNIX which make it act like a GUI. In this class, we will think of UNIX as mainly a command line operating system.