MSYS, (an acronym for "Minimal SYStem"), is a command line environment based on bash, (the GNU Bourne Again Shell). This is complemented by a small collection of GNU utilities such as make, gawk, grep and sed, carefully selected to facilitate building of applications and programs which depend on traditionally UNIX tools. It is intended primarily to supplement MinGW, providing a convenient build environment for MinGW applications. However, with the addition of further user supplied tools, its capabilities may be extended to satisfy the needs of any user preferring a Bourne Shell command line interpreter, (CLI), as a panacea to the deficiencies of the cmd.exe shell.

An example would be building a library that uses the autotools build system. Users will typically run "./configure" then "make" to build it. The configure shell script requires a shell script interpreter which is not present on Windows systems, but provided by MSYS.

What MSYS is NOT

A common misunderstanding is that MSYS is "UNIX on Windows". MSYS by itself does not provide a compiler, nor a C library, nor any API features typical of UNIX or POSIX systems. Therefore, it does not offer any magical capability to port UNIX programs to MS-Windows without extensive modification, nor does it provide any UNIX specific functionality like case-sensitive file names. Users seeking such functionality are advised to consider Cygwin or Microsoft's Interix instead.