The command processor, or shell, is the user's interface to the operating system. It is responsible for parsing and carrying out user commands, including the loading and execution of other programs from a disk or other mass-storage device.
The default shell that is provided with MS-DOS is found in a file called COMMAND.COM. Although COMMAND.COM prompts and responses constitute the ordinary user's complete perception of MS-DOS, it is important to realize that COMMAND.COM is not the operating system, but simply a special class of program running under the control of MS-DOS.
COMMAND.COM can be replaced with a shell of the programmer's own design by simply adding a SHELL directive to the system-configuration file (CONFIG.SYS) on the system startup disk. The product COMMAND-PLUS from ESP Systems is an example of such an alternative shell.