gistfile1.el

(defun source (filename)
  "Update environment variables from a shell source file."
  (interactive "fSource file: ")

  (message "Sourcing environment from `%s'..." filename)
  (with-temp-buffer

    (shell-command (format "diff -u <(true; export) <(source %s; export)" filename) '(4))

    (let ((envvar-re "declare -x \\([^=]+\\)=\\(.*\\)$"))
      ;; Remove environment variables
      (while (search-forward-regexp (concat "^-" envvar-re) nil t)
        (let ((var (match-string 1)))
          (message "%s" (prin1-to-string `(setenv ,var nil)))
          (setenv var nil)))

      ;; Update environment variables
      (goto-char (point-min))
      (while (search-forward-regexp (concat "^+" envvar-re) nil t)
        (let ((var (match-string 1))
              (value (read (match-string 2))))
          (message "%s" (prin1-to-string `(setenv ,var ,value)))
          (setenv var value)))))
  (message "Sourcing environment from `%s'... done." filename))