The extension is optional and can be up to three characters long. You can use the extension to further identify a file or to describe what type of file it is, such as a text file or program file. When you use an extension, separate it from the filename with a period. For example, an MS-DOS filename might look like this:

DATA.TXT

Some application programs automatically add extensions to the files you create. These application programs use the extension to determine whether it is a compatible data file. Avoid using the same extensions as your application programs. Also, do not use uppercase and lowercase letters to distinguish between files. MS-DOS does not recognize the difference and displays all filenames in uppercase.

Certain extensions are reserved for program files and you must not use them for your data files. The reserved extensions are

.COM, .EXE, and .BAT. Files with these extensions are also sometimes called executable files.

The .BAT extension denotes a particular kind of executable file called a batch file. Batch files can be used to automate sequences of MS-DOS instructions. Even if you are not a programmer, you may want to create some batch files to assist you in your work. A particularly useful kind of batch file, called an autoexecute batch file (or “AUTOEXEC” file) is discussed later in this chapter.

Copying Files

You can use the COPY command to copy individual files or groups of files. COPY is an internal command; you can use it any time you see the MS-DOS command prompt.