Elisp function for copying Emacs-selection to the X primary selection (clipboard)

gistfile1.el

(defun yank-to-x-clipboard ()
  (interactive)
  (if (region-active-p)
      (progn
        (shell-command-on-region 
         (region-beginning) (region-end) 
         "xsel -pi")
        (message "Yanked region to clipboard!")
        (deactivate-mark))
    (message "You havn't marked any text.")))