, the shell, being a separate program, is not a part of the OS at all. In the UNIX world a number of shells are available, among them the Korn shell, the C-shell, the Bourne shell and the Bourne Again shell (yes, really). In DOS, the standard shell is COMMAND.COM, again nothing more than a program. As different versions of command.com came with different versions of DOS, each added new commands and new things that could be done by the user. For example, DOS 4's COMMAND.COM added theP switch to DEL to verify each deletion, and DOS 5's COMMAND.COM provided the ability to sort the output of the DIR command.
             An acronym for disk operating system, the term DOS can refer to any operating system, but is most often used as shorthand for MS-DOS. Originally developed by Microsoft for IBM, MS-DOS was the standard operating system for IBM-compatible computers. The initial version of DOS was somewhat uncomplicated and resembled another operating system called CP/M. Subsequent versions
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