I can’t send escape sequences from keyboard, but can do it from another tty

escape-charactersterminaltty

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:

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

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

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

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.

Edit

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

Best Answer

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.

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Shell – Escape sequences in output of script called from ncurses application

The reason why an event-script fails to send a "growler" message is that mcabber closes the standard input, output and error streams when it runs an event command. You can see this in hooks.c:

  if ((pid=fork()) == -1) {
    scr_LogPrint(LPRINT_LOGNORM, "Fork error, cannot launch external command.");
    g_free(datafname);
    return;   
  }    
  if (pid == 0) { // child
    // Close standard file descriptors
    close(STDIN_FILENO);
    close(STDOUT_FILENO);
    close(STDERR_FILENO);
    if (execl(extcmd, extcmd, arg_type, arg_info, bjid, arg_data,
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      // scr_LogPrint(LPRINT_LOGNORM, "Cannot execute external command.");
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  g_free(datafname);

That makes the event script run without interfering with the streams used by mcabber.

There is no special ncurses mode intercepting the message (after all, tmux is already running as a terminfo application). You can probably work around the problem by redirecting your echo (preferably printf) to /dev/tty, e.g.,

#!/bin/sh
printf '\033Ptmux;\033\033]9;foobar\007\033\\' >/dev/tty

How to enable Control key combinations for GNU screen on putty

In its default configuration, PuTTY sends cursor keys in normal mode, e.g., ^[OA while screen sets the cursor keys to application mode, e.g., ^[[A.

That is done by screen's sending the escape sequence to turn on the keypad (in termcap ks, terminfo smkx). That happens to turn on the application mode for both the numeric keypad and the cursor keys, because both are present in the ks/smkx capability. screen uses this capability if it is present, when initializing the terminal. screen also follows up by sending the string in the termcap CS capability if it is set, hinting that the original design was to use terminal descriptions where only the keypad was initialized by ks/smkx.

You notice this difference because your key bindings expect the strings sent by the cursor keys in normal mode. If you modified the terminal description for screen to omit the part which changes the cursor keys, you could continue to use those bindings.

For instance, you could do this:

infocmp screen >foo
vi foo

and change the assignment

smkx=\E[?1h\E=,

smkx=\E=,

then

tic foo

to compile the modified description. If you do this as your (non-root) user, tic will put the compiled description in your ~/.terminfo directory. There are drawbacks to this approach, and using tput to obtain key-binding information is preferable. But it is simple.

Best Answer

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Shell – Escape sequences in output of script called from ncurses application

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