You can use vi to edit any text file. vi copies the file to be edited into a buffer (an area temporarily set aside in memory), displays the buffer (though you can see only one screenful at a time), and lets you add, delete, and change text. When you save your edits, vi copies the edited buffer back into a permanent file, replacing the old file of the same name. Remember that you are always working on a copy of your file in the buffer, and that your edits will not affect your original file until you save the buffer. Saving your edits is also called "writing the buffer," or more commonly, "writing your file."
vi is the UNIX command that invokes the vi editor for an existing file or for a brand new file. The syntax for the vi command is:
$ vi [filename ]
The brackets shown on the above command line indicate that the filename is optional. The brackets should not be typed. The $ is the UNIX prompt. If the filename is omitted, vi will open an unnamed buffer. You can assign the name when you write the buffer into a file. For right now, though, let's stick to naming the file on the command line.
A filename must be unique inside its directory. On older System V UNIX systems, it cannot exceed 14 characters in length (most common ...