gistfile1.el

(defun my-get-agent ()
  "Establish my registered ssh agent in this emacs shell *and* emacs session."
  (interactive)
  (let ((process (get-buffer-process (current-buffer)))
        beg end start-point)
    (if (not process)
        (error "Non-shell buffer")
      (goto-char (process-mark process))
      (insert "eval $(ga); echo SSH_AUTH_SOCK::$SSH_AUTH_SOCK::")
      (comint-send-input)
      (setq start-point (point))
      (let ((tries 0)
            got)
        (while (not got)
          (sleep-for .25)
          (goto-char (point-max))
          (setq tries (1+ tries))
          (if (> tries 40)
              (error "Failed to find agent socket id after 10 seconds"))
          (when (re-search-backward "SSH_AUTH_SOCK::\\(.*\\)::" start-point t)
            (setq got (buffer-substring (match-beginning 1) (match-end 1)))
            (setenv "SSH_AUTH_SOCK" got)
            (goto-char (process-mark process))
            (message "SSH_AUTH_SOCK: %s" (getenv "SSH_AUTH_SOCK"))))))))