this post was submitted on 20 Mar 2025
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Linux Terminal: CTRL+D is like pressing ENTER (hackarcana.com)
submitted 22 hours ago by tuna@discuss.tchncs.de to c/linux@lemmy.ml
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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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[–] double_quack@lemm.ee 17 points 19 hours ago* (last edited 19 hours ago) (1 children)

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.

[–] tuna@discuss.tchncs.de 3 points 17 hours ago (2 children) 
$ 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.

[–] CasualTee@beehaw.org 4 points 13 hours ago

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:

[–] skepller@lemmy.world 0 points 13 hours ago* (last edited 11 hours ago)

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.