ASCII control characters in my terminal

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2024-10-31 14:30:08

Hello! I’ve been thinking about the terminal a lot and yesterday I got curious about all these “control codes”, like Ctrl-A, Ctrl-C, Ctrl-W, etc. What’s the deal with all of them?

Here’s a table of all 33 ASCII control characters, and what they do on my machine (on Mac OS), more or less. There are about a million caveats, but I’ll talk about what it means and all the problems with this diagram that I know about.

The first surprising thing about this diagram to me is that there are 33 control codes, split into (very roughly speaking) these categories:

There’s no real structure to which codes are in which categories, they’re all just kind of randomly scattered because this evolved organically.

(If you’re curious about readline, I wrote more about readline in entering text in the terminal is complicated, and there are a lot of cheat sheets out there)

Something else that I find a little surprising is that are only 33 control codes – A to Z, plus 7 more (@, [, \, ], ^, _, ?). This means that if you want to have for example Ctrl-1 as a keyboard shortcut in a terminal application, that’s not really meaningful – on my machine at least Ctrl-1 is exactly the same thing as just pressing 1, Ctrl-3 is the same as Ctrl-[, etc.

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