How to check if Emacs is in GUI mode (and execute `tool-bar-mode` only then)?

I have this line in my .emacs file:

(tool-bar-mode 0)

because I hate the toolbars in my GUI emacs (/Applications/Emacs.app/Contents/MacOS/Emacs).

But when I start up my other, text-based emacs in the terminal (/opt/local/bin/emacs) it complains about that command:

Symbol's function definition is void: tool-bar-mode

How can I add an if condition so that it executes the tool-bar-mode command only when I'm in the GUI emacs?

Thanks!

Okay, found it myself. Just add

;; turn off toolbar
(if window-system
    (tool-bar-mode 0))

another way to do it would be:

 (if (functionp 'tool-bar-mode) (tool-bar-mode 0))

like this, the function is called only if it exists

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