Cygwin ( SIG-win) is a POSIX-compatible programming and runtime environment that runs natively on Microsoft Windows. Under Cygwin, source code designed for Unix-like operating systems may be compiled with minimal modification and executed.The Cygwin installation directory has a directory layout that is similar to the root file system of Unix-like systems, with familiar directories, such as /bin, /home, /etc, /usr, /var.