C-M-... bindings in emacs -nw

Question

I have an issue when I run emacs with -nw: all my C-M- bindings stop working, as C (control) doesn't register, and only the M- binding is executed.

E.g:

  (global-set-key (kbd "C-M-.") 'vundo)

-- works in Emacs GUI, but with emacs -nw I get M-. executed. I have key chords start with C-M-<return>, in -nw mode I always get the message: M-RET is undefined.

The terminal emulator might be behind this? - I tried xterm, urxvt, alacritty and kitty, same everywhere.

My emacs config? I tried with emacs -Q -nw -> same issue.

What am I missing? Where is my Control going in terminal mode?

Answer 1

There are two issues at play here, both of which have to do with the terminal environment (tty):

1. Your terminal emulator (usually*) gets first dibs over any key combos that you enter, which can be a problem for any chords that happen to match a shortcut. For example, in terminator, C-M-w renames a window even in emacs tty mode. (In my version of doom emacs gui, this appends from the kill ring.) In this case, you won't have much choice except to find which shortcut could be interferring. I do not believe that is the main issue for the specific problem you listed though because 2) is more likely in play.

2. Terminal emulators sends characters not keystrokes. See this excellent discussion in the Unix SE for an overview. Also see further here that talks about making a terminal emulator to handle all key combos. But in summary: While it is possible to press a chord in emacs gui to enact some functionality and a different key for a different set of functionality, when we step into emacs tty, emacs could end up seeing the same characters on both keypresses. This is especially noticeable for chords involving any of the control keys**. In other words, the information that you pressed C is lost.

There are not any easy fixes to number 2 because there aren't standard escape code messages for all terminals and key combos as far as I can tell. You could approach the problem in a few ways:

• Use a config that minimizes use of the control key (possibly with a leader key setup, like spacemacs/doomemacs with the space bar).

• Check if your terminal emulator already solves this problem. As @db48x mentions below, xterm has the option modifyOtherKeys to send escape codes for ordinary chords. You can also use a solution involving xkb to send in meta chords to terminal, like a comment in this answer details.

• In the preferences of your terminal emulator, define a set of escape sequences that are tied to your key combo of choice. Then in your emacs config, put that escape sequence as the input for your command or as a input remap. You may end up with a command that looks similar to the ones in the linked post above (which is using xterm):

In ~/.Xresources to define behaviours with C, S, and M:

 Term.VT100.translations:       #override \n\
     Ctrl ~Shift ~Meta <key>Return: string("\033[73;5~") \n\
     Ctrl Shift ~Meta <key>percent: string("\033[37;6~") 

Then in emacs, translate into mapping (the above uses number representation to send in ESC [ n1 ; n2 ~ where n2 is the number we get if we treat shift as 2, meta as 3, control as 5, control + meta as 7, or add 1 to the other number wherever we added shift to the combo.

(define-key local-function-key-map "\033[37;6~" [(control ?L)])
(define-key local-function-key-map "\033[37;6~" [(control ?L)])

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*Not always though. Observe that you can enter C-d in emacs tty mode and not close your session. If you exit emacs and do the same on the terminal, you close the session. This also holds for other common chords like C-c and C-x.

**But you may find control keys and chords which are not affected. Note how C-M-q works in emacs tty for instance if you assign a chord to it.