GNU (abbreviation for "Gnu's Not Unix"), a UNIX-like operating system promoted by the GNU Project, aims to establish a free software environment that is fully compatible with UNIX.

The plan to develop the GNU system was first launched by Richard Stallman in 1983. It is the earliest goal of the Free Software Foundation.

The most recent GNU system version, GNU 0.401, was released on April 1, 2011, using GNU Hurd as the operating system kernel.

Other kernels, most notably the Linux kernel, are also used in GNU systems.

Compilers are classified according to needs, for example, gcc and g++ are both gnu compilers.

gcc is a compiler for c language; g++ is a compiler for c++.