Map a key to act like a combination of modifiers

Question

I've got a keyboard, where pressing M-C-S is uncomfortable (Meta and control are under the thumbs and Shift is on the far end of the keyboard). However, PageUp, PageDown, Home and End are in a very comfortable place. I never use any of those keys though, so I was thinking, if it was possible to remap, say, Home to act like M-C-S, that would make my life a lot easier. I would be even happier, if the solution wasn't Emacs-only, but on the system level (it is okay if it is Linux-only).

Answer 1

A pure Emacs solution that doesn't do exactly what you're asking, but close, would be to make e.g. Home be a key that adds the Ctrl, Shift and Alt modifiers to the next key that you press. That is, rather than press Home+key, you would press Homethen key.

Emacs has a built-in feature to add a modifier to the next key, bound to the unwieldy C-x @ letter by default, e.g. if you type C-x @ m a it's interpreted as M-a. This is implemented via function-key-map. Bindings in function-key-map are overridden by key bindings in most maps, including the global keymap; if you want your binding to take precedence, define it in input-decode-map instead. I've written a bit about it here.

You can adapt this mechanism to bind home to event-apply-control-meta-shift-modifiers.

(defun event-symbol-add-modifiers (symbol prefixes)
  (let* ((name (symbol-name symbol))
         (parts (split-string name "-"))
         (old-modifiers nil))
    (while parts
      (if (= (length (car parts)) 1)
          (setq old-modifiers (cons (aref (car parts) 0) old-modifiers)))
      (setq parts (cdr parts)))
    (while prefixes
      (if (not (memq (car prefixes) old-modifiers))
          (setq name (concat (list (car prefixes) ?-) name)))
        (setq prefixes (cdr prefixes)))
    (intern name)))
(defun event-apply-modifiers (event mask prefixes)
  (cond
   ((numberp event)
    (logior mask event))
   ((symbolp event)
    (event-symbol-add-modifiers event prefixes))
   ((consp event)
    (cons (event-symbol-add-modifiers (car event) prefixes) (cdr event)))
   (t event)))
(defun event-apply-control-meta-shift-modifiers (ignore-prompt)
  "\\<function-key-map>Add the Ctrl, Meta and Shift modifier to the following event.
For example, type \\[event-apply-control-meta-shift-modifiers] & to enter Ctrl-Meta-Shift-&."
  (vector (event-apply-modifiers (read-event) #xe000000 '(?C ?M ?S))))
(define-key input-decode-map [home] 'event-apply-control-meta-shift-modifiers)

(event-apply-modifiers is a highly simplified version of Emacs's event-apply-modifier and may be inferior in some circumstances. Minimally tested.)