Bad command or file name

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"Bad command or file name" was an infamous error message in early versions of Microsoft's MS-DOS operating system, which is based on a command line interface (in contrast with, for example, Windows, which is GUI-based).

The dreaded "Bad command or file name" message represented basically the same as the "Syntax error" message from BASIC interpreters of the 1980s, and both resulted from mistyping a command to be executed or file name to be loaded or saved. Novices, in general, and quite a few experienced users as well, had trouble understanding the message, so later operating systems changed this message; for instance, the Windows NT family uses "<typed-in string> is not recognized as an internal or external command, operable program, or batch file".