(global-set-key) vs (define-key)

[Daniel Corbe]

I'm trying to extend eshell so I can use it to manage routers and
switches.  This necessitates changing a few important key-bindings that
equipment manufacturers tend to use.

The problem with the code below is ? works; however, both Ctrl-Z and TAB
stay bound to their original key bindings.

If I use global-set-key instead, TAB and Ctrl-Z do what they're supposed
to do.

I have a suspicion that (define-key) is not allowed to override global
bindings.  Is that true?  If so, what should I be using instead?

(define-derived-mode router-mode eshell-mode "Router"
  "Some key bindings and tools to make dealing with routers/switches easier."

  (define-key eshell-mode-map "?" 'router-mode-help-key)
  (define-key eshell-mode-map (kbd "C-z") 'router-mode-end-key)
  (define-key eshell-mode-map (kbd "TAB") 'self-insert-command))

(defun router-mode-help-key ()
  "Defines what happens when a user hits the ? key."
  (interactive)
  (insert "?")
  (eshell-send-input nil nil t))

(defun router-mode-end-key ()
  "This sends Ctrl-Z along to the shell."
  (interactive)
  (eshell-kill-input)
  (insert "end")
  (eshell-send-input))