3.0: Customisation – Emacs Lisp and init.el

Q3.0.1: What version of Emacs am I running?

How can init.el determine which of the family of Emacsen I am using?

To determine if you are currently running GNU Emacs 18, GNU Emacs 19, XEmacs 19, XEmacs 20, or Epoch, and use appropriate code, check out the example given in etc/sample.init.el. There are other nifty things in there as well!

For all new code, you can use the variables running-xemacs and running-sxemacs or something like

(when (featurep 'sxemacs)
  ...)
(when (featurep 'xemacs)
  ...)

as well.

Please note, that you should double check these values, if you intend to use xemacs-exclusive features. Both, the variable running-xemacs and the form (featurep 'xemacs) evaluate to t within SXEmacs. This is due to compatibility reasons.