How to detect that emacs is in terminal-mode?

In my .emacs file, I have commands that only makes sense in graphical mode (like (set-frame-size (selected-frame) 166 100)). How do I run these only in graphical mode and not in terminal mode (i.e. emacs -nw).

Thanks!

Solution

The window-system variable tells Lisp programs what window system Emacs is running under. The possible values are

x: Emacs is displaying the frame using X.
w32: Emacs is displaying the frame using native MS-Windows GUI.
ns: Emacs is displaying the frame using the Nextstep interface (used on GNUstep and Mac OS X).
pc: Emacs is displaying the frame using MS-DOS direct screen writes.
nil: Emacs is displaying the frame on a character-based terminal.

From the doc.

Edit: it seems that window-system is deprecated in favor of display-graphic-p (source: C-h f window-system RET on emacs 23.3.1).

(display-graphic-p &optional DISPLAY)

Return non-nil if DISPLAY is a graphic display.
Graphical displays are those which are capable of displaying several
frames and several different fonts at once.  This is true for displays
that use a window system such as X, and false for text-only terminals.
DISPLAY can be a display name, a frame, or nil (meaning the selected
frame's display).

So what you want to do is :

(if (display-graphic-p)
    (progn
    ;; if graphic
      (your)
      (code))
    ;; else (optional)
    (your)
    (code))

And if you don't have an else clause, you can just:

;; more readable :)
(when (display-graphic-p)
    (your)
    (code))