How to map CMD+Alt alone to the command?

Question

I am trying to map Cmd+Alt to start searching in vim (/). The only question is: how would I do this? I know to map Alt in vimscript is by typing <A> but I do not know how to do Cmd.

I also want to press the keys at the same time (not one after the other)

Answer 1

This is not possible. In the terminal, no key code is generated for modifiers alone.

Gvim also has no support for this, partly because there is no valid way to represent the key using the map commands.

Answer 2

Looking at the keycodes, towards the end of the table is:

<M-...>         alt-key or meta-key             meta alt <M-
<A-...>         same as <M-...>                 <A-
<D-...>         command-key (Macintosh only)    <D-

Combinations would look like <D-M-x>.

Though I don't think a combination of modifiers alone can be mapped (Ctrl-Shift,Cmd-Alt, etc.).

Answer 3

Map Cmd+Opt if pressed alone to / with Karabiner Elements

1. Install Karabiner Elements

2. Install following custom complex modification:

{
  "title": "Change Cmd_L and Option_L to Slash (/)",
  "rules": [
    {
      "description": "Change Command_L + Option_L (Alt) to slash (/) if pressed at the same time and without any other key.",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "left_option",
            "modifiers": {
              "mandatory": [
                "left_command"
              ],
              "optional": [
                "any"
              ]
            }
          },
          "to_if_alone": [
            {
              "key_code": "slash"
            }
          ],
          "to": [
            {
              "key_code": "left_option",
                "modifiers": [
                    "left_command"
                 ]
            }
          ]
        },
        {
          "type": "basic",
          "from": {
            "key_code": "left_command",
            "modifiers": {
              "mandatory": [
                "left_option"
              ],
              "optional": [
                "any"
              ]
            }
          },
          "to_if_alone": [
            {
              "key_code": "slash"
            }
          ],
          "to": [
            {
              "key_code": "left_command",
              "modifiers": [
                    "left_option"
                 ]
            }
          ]
        }
      ]
    }
  ]
}

i) Copy json file to the correct location:

$ cp cmd-opt-alone-to-slash.json ~/.config/karabiner/assets/complex_modifications

ii) Import rules from Karabiner-Elements Preferences. Karabiner-Elements Preferences > Complex Modifications > Rules > Add rule