running some external program only if it exists

You can easily run external programs from emacs; one way to do it is by using the shell-command-function. However, as discussed here, if you are using emacs in different environments, you might want to check first if the program is available, and give a warning if it's not there.

One simple helper function you can use in your .emacs for this, could be something like:

(defun djcb-shell-command-maybe (exe &optional paramstr)
  "run executable EXE with PARAMSTR, or warn if EXE's not available; eg. "
  " (djcb-shell-command-maybe \"ls\" \"-l -a\")"
  (if (executable-find exe)
    (shell-command (concat exe " " paramstr))
    (message (concat "'" exe "' not found found; please install"))))

Of course, this function is not very useful by itself, but we'll use it in later entries, and of you can use it in your own scripts.