Interix is the name of an optional, full-featured POSIX and Unix environment subsystem for Microsoft's Windows NT-based operating systems. It is a component of the Services for Unix (SFU) release 3.0 and 3.5 (this last one is distributed freely). The most recent releases of Interix, 5.2 and 6.0, are components of the Windows Server 2003 R2, Windows Vista Enterprise and Ultimate editions and Windows Server 2008 under the name SUA  (Subsystem for Unix-based Applications). Version 6.1 is included in Windows 7 (Enterprise and Ultimate) and Windows Server 2008 R2 (all editions).
Like the Microsoft POSIX subsystem in Windows NT, Interix is an implementation of an environment subsystem running atop the Windows kernel, but with increased functionality. Interix provides numerous open source utilities.
The complete installation of Interix includes (at version 3.5):
The development environment includes support for C, C++ and Fortran. Threading is supported using the Pthreads model. Additional languages can be obtained (Python, Ruby, Tcl, etc.). Unix-based software packaging and build tools are available for installing or creating pre-build software packages.
Starting with release 5.2 (Server 2003/R2) the following capabilities were added:
With release 6.0 (Vista and Server 2008) the following new features were added:
Interix release 6.1 will be available for Windows Server 2008 R2 and Windows 7 for the Enterprise and Ultimate editions.
Traditional support is provided by Microsoft for a fee or service contract, though released hotfixes can usually be obtained free of charge from Microsoft. Alternative, free support can be found with the forums at the SUA Community site and with the FAQ.
Additional pre-built applications and development libraries can be obtain for free from the SUA Community site and the Debian-Interix site. These include commonly used applications not part of the Microsoft installation such as OpenSSH, Python and bash.
The SUA Community site includes for developers of Interix and the site has been contracted by Microsoft for several Interix and SUA updates of utilities and documentation.
This product began to be developed in 1996 under the product name OpenNT by Softway Systems, Inc. before the name was changed to Interix in 1998. The last release by Softway Systems was release 2.2. Softway Systems had virtually completed the next release code-named Firebrand in 1999 which became the basis of Interix 3.0.
Interix was acquired by Microsoft in 1999. Microsoft continued to distribute Interix 2.2 as a standalone product until 2002.
Interix release 3.0 was released as component of Services for Unix (SFU) 3.0 in 2002. Interix release 3.5 was released as component of SFU 3.5 in January, 2004.
Interix became integrated as component of the regular Windows OS distribution as a component of Windows Server 2003 R2 in December, 2005 at release 5.2 and is a component of the Windows Vista release as release 6.0 (RTM November, 2006). Windows Server 2008 has release 6.0. Windows 7 betas presently include SUA 6.1.