The GNU ed command is a line editor. It’s considered the standard Unix text editor because it was the very first text editor for Unix, and so it was (and generally still is) available on any POSIX system. In some ways, it’s easy to tell that it was the first because, in many ways, it’s extremely rudimentary. Unlike most other text editors, it doesn’t open in a window or screen of its own, and in fact, by default, it doesn’t even prompt the user for input. On the other hand, its near lack of any interface can also be a strength.
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