The command line is a textual prompt for a command.

It's also used to describe the textual interface to the system, as in, "I dropped to the command line and ran the application."

And actually most people just say CommandLine, when they mean typing something into a UnixShell or MicrosoftWindows command or cmd window.

There's an interesting sense of ''height'' when referring to the command line, as illustrated in the quote above.  There's a sense that the command line is "lower" in the system, closer to the base of the system.
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CategoryCommandLine