Emacs Package Synchronization

package_sync.el

(defun installed-packages ()
  "Return the list of installed packages."
  (mapcar 'car package-alist))

(defun save-package-list ()
  "Save the list of installed packages to a file."
  (interactive)
  (with-temp-file "~/Dropbox/.emacs-packages-installed.el"
    (insert (format "(defvar my-packages '%s)" 'installed-packages))))
  
(add-hook 'kill-emacs-hook 'save-package-list)

(load-file "~/Dropbox/.emacs-packages-installed.el")

(dolist (p my-packages)
  (when (not (package-installed-p p))
    (package-install p)))