Bad command or file name
COMMAND.COM, the primary user interface of MS-DOS, produces this error message when the first word of a command could not be interpreted. For MS-DOS, this word must be the name of an internal command, executable file or batch file, so the error message provided an accurate description of the problem but easily confused novices. Though the source of the error was often a mistyped command, the wording gave the impression that files named in later words were damaged or had illegal filenames. Later, the wording of the error message was changed for clarity. Windows NT displays the following error message instead (where "foo" is replaced by the word causing error):
"foo" is not recognized as an internal or external command, operable program or batch file
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