How can I map Ctrl-Alt-<letter> mappings in vim?

Question

My keybindings are consistent and work across different operating systems, terminal emulators, browsers, tmux, etc. There are no clashes, but this sometimes means remapping things in some instances.

Therefore my vim-tmux-navigator mappings are probably a little unusual:

nnoremap <silent> <C-M-h> :TmuxNavigateLeft<CR>
nnoremap <silent> <C-M-j> :TmuxNavigateDown<CR>
nnoremap <silent> <C-M-k> :TmuxNavigateUp<CR>
nnoremap <silent> <C-M-l> :TmuxNavigateRight<CR>

They work in nvim but I have never got them to work in vim, but since I got this answer a while ago, I now understand why.

To get the correct mappings that work in vim in that question for <M-S-j>, you can have to enter the code by typing C-v and then <M-S-j>, resulting in something that looks like ^[J (typing those characters does not result in the same code and they get highlighted in different colours).

This is not possible when the Ctrl key is being used in the mapping. Is there a way of getting this code somehow?

I have:

• nvim on Linux

• vim on Linux and Windows

• gvim on Linux and Windows

...available for input. It is only vim on Linux that this problem applies to, as there is no tmux used elsewhere other than with nvim on Linux, but the problem does not occur there as nvim accepts the syntax above.

Answer 1

The problem

For historical reasons, the Alt key can be configured to work in two different ways. When pressing a character with Alt held down, your terminal can either:

1. Set the 8th bit of that character, (traditional Meta behaviour)
2. First send the ESC character, and then the pressed character (Alt key meta emulation).

You have your terminal configured to use the second method, but Vim is expecting the first.

The answer you received to your other question was therefore slightly misleading. It is not true that you cannot ever use <A- mappings, it's just that your current configuration doesn't allow it.

There are various ways you can deal with this:

Reconfigure your terminal

You could change your terminal settings to treat Alt as a Meta key. Vim's documentation has some suggestions for how to do this, universally or just for Vim, at :help :map-alt-keys.

But if you don't want to touch your terminal configuration, there are ways to handle this inside of Vim:

Use 'modifyOtherKeys'

If your terminal supports it, you can switch on its modifyOtherKeys mode. This should be handled automatically by your terminfo, but if that's not set up properly there are some instructions at :help modifyOtherKeys for how to override the settings in your Vim config.

Otherwise, you can:

Tell Vim what to expect

If you don't want to reconfigure your terminal, you can explicitly inform Vim what keycodes to expect for your key presses:

" First tell Vim what keycodes to expect
" We use :execute so we don't have to enter literal character codes with <C-V>
execute "set <C-M-j>=\<Esc>\<C-j>"

" Then map them
nnoremap <C-M-j> :TmuxNavigateDown<CR>

There are a couple of big N.B.s here, though:

1. You have to set up all the keycodes you want to use, either with a Vimscript loop or one-at-a-time,
2. Specifically for <C-M- mappings, although the mapping fires correctly, the original behaviour of j is also triggered: the cursor moves down. I'm not sure why this occurs.

Bearing the above in mind, there is a fourth option:

Just map the keycodes directly

This is the solution you already tried. As you've discovered, entering the raw keycodes via Ctrl-V doesn't work for <C-M- keystrokes. But we know what keycodes you're going to receive, so there's an alternative that would be preferable anyway as it doesn't require adding literal control characters into your vimrc:

nnoremap <Esc><C-j> :TmuxNavigateDown<CR>