How to determine operating system in elisp?

Question

How do I programmatically determine which OS Emacs is running under in ELisp?

I would like to run different code in .emacs depending on the OS.

Answer 1

In a .emacs, there is not only the system-type, but also the window-system variable. This is useful when you want to choose between some x only option, or a terminal, or macos setting.