Why I can't send escape sequences from keyboard, but can do it from another tty?

Question

I am trying to understand how terminal works by doing different tricks, like writing from one tty to another, changing settings tty1 from tty2, etc. Also, I am trying to change a color by sending escape sequence from keyboard, directly. That is, not by echo -e '\e[0;31m' command, but by direct keyboard input. It doesn’t work.

I do this steps:

1. Open tty1 and tty2
2. In the tty2 put bash into the sleep mode, by sleep 10m. Type word 'one'.

3. Go to tty1, do echo -n ^[[0;31m > /dev/tty2. The first character ^[ is typed by this way Ctrl + v Esc
4. Return to tty2, type word 'two'. Yes - the color has been changed to red by command from another tty.

5. Repeat steps 3,4, but with green color and word 'three'

6. And finally, I am trying to send the escape sequence not by another tty, but from keyboard directly - by typing ^[[0;37m in tty2. I do everything the same way - Esc (Ctrl + v doesn't needed, because readline is sleeping), then [0;37m, but get this:

Question: Why does it work this way? All characters the same, terminal state the same, but in one case terminal get escape sequence, and in another case don't.

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Edit

The question was answered here: Echoed escape sequences doesn't interpreted in Linux tty.

Answer 1

When you run echo, you're sending output to the terminal. The terminal interprets escape sequences such as the one to change colors in the output that is sent to it. These escape sequences are meant to be sent by applications, this is why they are recognized in the output that comes from the application running in the terminal. Usually the application running in the terminal is the one started by the terminal emulator (your shell) and the ones started by this in turn, but if you run echo … >/dev/tty2 then echo is effectively “running in the terminal” (in the sense that its output is going to the terminal, which is what matters here).

When you press Esc [ etc., you're sending input to the terminal. The terminal does not interpret escape sequences such as the one to change colors in the input that it receives. The terminal does interpret escape sequences in input, but for a completely different purpose: they're a way to encode function key presses.

How do keyboard input and text output work? has some relevant background.

Answer 2

This works even in xterm in xorg:

stty -echoctl
sleep <a bit>

Then, on the "dead" terminal, you can directly press Escape and then

[32m

and the next keys you press is already green! If you kill sleep with ctrl-C your prompt stays clean...

The Question is a very special case -- a way of sneaking an escape sequence from keyboard to the screen, while readline/bash is in a coma.

WITH normal echoctl/ctlecho, after sleep the shell does echo keypresses (input), but also echoes the escape key directly as ^[ on the screen (output). Special meaning as 'control sequence introducer (CSI) is lost.

So isn't the (accepted) answer wrong?

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Here I press ctrl-C with the cursor on the "l" of "line", as if I decide to abort and not continue editing the line:

$  weird -cmd ^Cne
$  stty -echoctl
$  weird -cmd line
$  # nice but now it looks like it got exec'd

This seems to be the main effect of this stty echoctl option.

I just realized, an Escape press is just short for control-[ aka ^[. So it is logical for tty to just ECHO the ^[ char immediately and not wait for a sequence. After a sleep with normal echoctl on, the Home key generates ^[[H. Without echoctl i.e. after stty -echoctl, the Home key...puts the cursor top left of my xterm screen!

(When bash is awake, the Home key works: beginning of line.)

these two bind -p lines are builtin/terminfo xterm:

"\eOH": beginning-of-line
"\e[H": beginning-of-line