system-type

system-type is a variable defined in `C source code'.
Its value is

gnu/linux

Documentation:
The value is a symbol indicating the type of operating system you are using.
Special values:
`gnu' compiled for a GNU Hurd system.
`gnu/linux' compiled for a GNU/Linux system.
`gnu/kfreebsd' compiled for a GNU system with a FreeBSD kernel.
`darwin' compiled for Darwin (GNU-Darwin, Mac OS X, ...).
`ms-dos' compiled as an MS-DOS application.
`windows-nt' compiled as a native W32 application.
`cygwin' compiled using the Cygwin library.
Anything else (in Emacs 24.1, the possibilities are: aix, berkeley-unix,
hpux, irix, usg-unix-v) indicates some sort of Unix system.