gistfile1.el

 2 down vote
  

I don't have uptime on my system, so I can't test this for you. But it should give you an idea. Seems to work on my system with ps substituted for uptime.

Perhaps someone else will offer a simpler or cleaner solution. You might also look at call-process or start-process instead of shell-command-to-string --- start-process is async. You might also want to consider using an idle timer --- the code here can slow Emacs down considerably, since it calls uptime each time the mode line is updated.

(setq-default 
 mode-line-format
 (list " " 'mode-line-modified
       "--" 'mode-line-buffer-identification
       "--" 'mode-line-modes
       'mode-line-position
       "--" '(:eval (shell-command-to-string "uptime"))
       "-%-"))

Here is another approach, which doesn't seem to slow things down noticeably:

(defun bar ()
  (with-current-buffer (get-buffer-create "foo")
    (erase-buffer)
    (start-process "ps-proc" "foo" "uptime")))

(setq foo (run-with-idle-timer 30 'REPEAT 'bar))

(setq-default 
 mode-line-format
 (list " " 'mode-line-modified
       "--" 'mode-line-buffer-identification
       "--" 'mode-line-modes
       'mode-line-position
       "--" '(:eval (with-current-buffer (get-buffer-create "foo")
                      (buffer-substring (point-min) (point-max))))
       "-%-"))