Unix Commands are Case Sensitive
When interacting with the Unix operating system, one of the first things you need to know is that, unlike other computer systems you may be accustomed to, everything in Unix is case-sensitive. Be careful when you're typing in commands - whether a character is upper or lower case does make a difference. For instance, if you want to list your files with the `ls' command, if you enter LS you will be told "command not found."
Worse than the system not finding the command is the possibility that you will cause a function to be performed that you didn't want to happen.
For example, in the `vi' editor, `j' means to move down one line and `J' means to join two lines together. If you should leave "caps lock" on by accident and then start pressing the `j' key to move the cursor down through the text, you will find all the lines jumping up to join the previous lines.
This is simply a word to the wise to keep the case of characters in mind while working in Unix.