Note: for novices reading along at home, the notation ^X means hold down the ctrl key and type x (without shift).

ctrl-a though ctrl-z will send ASCII characters 1 through 26, which are called control characters (because they’re for controling things, and also because you can type them by holding down the control key).

^D is the EOF character.

Nope, Chuck Testa: there is no EOF character. ^*

“D” being the fourth letter of the alphabet, sends ASCII character 4, which (as you can see in man ascii) is called EOT or “end of transmission”.

$ stty -a | grep eof
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
$ man stty |grep -A1 eof |head -n2
       eof CHAR
              CHAR will send an end of file (terminate the input)

What this means is that the character specified after eof (by default ^D, aka EOT) is configured to be intercepted (by the tty driver) and, instead of that character being sent to the process reading standard input, the kernel will “send an end of file (terminate the input)”.

But “send an end of file” does not mean sending any character to the reading process: as the blog post explains, it actually (counterintuitively) means flushing the buffer - meaning, causing the read syscall to return with whatever is in the buffer currently.

It is confusing that this functionality is called eof, and the stty man page description of it is even more so, given that it (really!) does actually flush the contents of the buffer to read - even if the line buffer is not empty, in which case it is not actually indicating end-of-file!

You can confirm this is happening by running cat and typing a few characters and then hitting ^D. (cat will echo those characters, even though you have not hit enter yet.)

Or, you can pipe cat into pv and see that ^D also causes pv to receive the buffer contents prior to hitting enter.

I guess unix calls this eof because this function is most often used to flush an empty buffer, which is how you “send an end of file” to the reader.

The empty-read-means-EOF semantics are documented, among other places, in the man page for the read() syscall (man read):

On success, the number of bytes read is returned (zero indicates end of file)

If you want ^D to send an actual EOT character through to the reading process, you can escape it using the confusingly-named lnext function, which by default triggered by the ^V control character (aka SYN, “synchronous idle”, ASCII character 22 - note V is the 22nd letter of the alphabet):

$ man stty|grep lnext -A1
       * lnext CHAR
              CHAR will enter the next character quoted
$ stty -a|grep lnext
werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;

Try it: you can type echo " and then ^V and ^D and then "|xxd (and then enter) and you will see that this is sending ascii character 4.

You can also send it with echo -e '\x04'. Note that the EOT character does not terminate bash:

$ echo -e '\x04\necho see?'|xxd
00000000: 040a 6563 686f 2073 6565 3f0a            ..echo see?.
$ echo -e '\x04\necho see?'|bash
bash: line 1: $'\004': command not found
see?

As you can see, it instead interprets it as a command.

$ echo -e '#!/bin/bash\necho lmao' > ~/.local/bin/$(echo -en '\x04')
$ chmod +x ~/.local/bin/$(echo -en '\x04')
$ echo -e '\x04\necho see?'|bash
lmao
see?