shell-command-on-region-example.el

;; Create and switch to a test buffer.
(with-current-buffer (get-buffer-create "*Foo*")
  ;; Ensure it's empty, for testing purposes.
  (setf (buffer-string) "")
  ;; Insert some test text
  (insert "hello, world!")
  ;; Now use `shell-command-on-region' to manipulate it.
  (shell-command-on-region (point-min) (point-max) "tr a-z A-Z" (current-buffer) t))