Linux Terminal: CTRL+D is like pressing ENTER
Honestly I had no idea what ctrl+d even did, I just knew it was a convenient way for me to close all the REPL programs I use. The fact that it is similar to pressing enter really surprised me, so I wanted to share this knowledge with you :)
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Ctl-D is the End-of-File character. Programs interpret it as "that's it, the input you were reading has finished", and react accordingly.
bash
$ cat You sound very nice :) You sound very nice :) Bye<ctl-d>Bye Oh wait, and cool too Oh wait, and cool too <ctl-d> $
The Ctl-D didn't end the file when i typed "Bye" :( it only worked when I pressed Ctl-D on its own line. So how does cat know that it should ignore the EOF character if there is some text that comes before it?
What Ctl-D does is flush the input to the program, and the program sees how big that input is. If the length of the input is 0 that is interpreted as EOF. So Ctl-D is like Enter because they both flush the input, but Ctl-D is unlike Enter because it does not append a newline before flushing, and as a consequence you can send empty input (aka an EOF "character") with Ctl-D.
When running cat this way, you are in "cooked mode". A ctrl-d does nothing on a non-empty line.
The shell usually runs in non-cokked, or raw, mode as well as nonblocking mode. Where it sees (nearly) every key you press as you press them. Which is why it " sees" the ctrl-d even when you are not on an empty line.
You can learn more here:
Interesting, I have not heard of these terms before. Thanks for sharing!
I think this adds the bit of nuance that was bugging me: using something like ncurses or vim, presumably when you press a key like ctrl-z or ctrl-d it actually sends the character to the app. It would feel a bit silly if the terminal intercepted the ctrl-d, flushed some buffer, and the program had to reverse engineer whether you pressed ctrl-d or enter or something.
For raw mode, I assume the app asks the tty to please forward some characters to the app. Otherwise, in the default cooked mode, the tty intercepts those control characters to call certain functions. I suppose some REPLs may choose to emulate a cooked mode on top of raw mode, and so they have to handle the x04 in the same way a tty would to keep it functioning like the user expects. I believe readline does something like this, which is why you had to use bash --noediting for ctrl-d to run the command. Good food for thought :)
I also have to say, naming it "cooked mode" is extremely funny as gen z. I love that
A ctrl-d does nothing on a non-empty line.
ctrl-d actually is flushing the buffer regardless of if the line is empty or not.
See my other comment for how you can observe it.
This!
It's merely a buffer flush, in case it's empty, the program handling the input can choose how to interpret, cat decides to do it as an EOF.
Reason why it also works as exit.