Linux.com | CLI Magic: Who Sed that?: “So what is Sed? I could lie and say it’s an editor for Field and Stream, but that would only be partially true. It’s the GNU Stream Editor. That is, the data to be edited is passed by the editor instead of the other way around. One time. Top to bottom. Start to finish. Get it? Sed acts on the file once, as it passes by.”

A little cutesy at times, but in general a nice intro to sed. It should get you started in any case.
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