Variable: diff-command
diff-command is a customizable variable defined in diff.el.gz.
Value
"diff"
Documentation
The command to use to run diff.
Source Code
;; Defined in /usr/src/emacs/lisp/vc/diff.el.gz
;;;###autoload
(defcustom diff-command "diff"
"The command to use to run diff."
:type 'string)