OSで場合わけする

;; open
(defun open ()
  "Let's open file!!"
  (interactive)
  (cond ((eq system-type 'darwin)
         (shell-command (concat "open " (buffer-file-name))))
        ((eq system-type 'cygwin)
         (message "It's Cygwin"))
        )
  )
(global-set-key "\C-co" 'open)

emacs - How to determine operating system in elisp? - Stack Overflow

Emacs Lisp基礎文法最速マスター - (rubikitch loves (Emacs Ruby CUI Books))

open コマンドで開くコマンドから学ぶ、とても簡単な Elisp。 - 日々、とんは語る。