Cygwin

Cygwin is a large collection of GNU and Open Source tools that provides a Unix-like environment on Windows. It includes a dynamic-link library, cygwin1.dll, which implements a POSIX compatibility layer by translating Unix system calls to their Windows equivalents, and a package suite of common utilities and development tools.

The project comprises the POSIX emulation layer (the DLL) and a package manager plus a repository of software. Users install the desired packages into a Cygwin root directory (commonly C:\cygwin or C:\cygwin64) and access the environment from a Windows console or an X server for graphical applications.

Cygwin is not a full Linux environment or a virtual machine. It provides portability for existing Unix software by translating API calls at run time, while running on the Windows kernel and using Windows processes. It is designed to ease porting and development, rather than to emulate a complete Linux system.

Typical uses include running shell scripts, building software with a Unix-like toolchain, and using familiar commands such as bash, grep, and ssh on Windows. Some daemons and graphical programs can run under Cygwin, often requiring additional configuration or an X server for display.

History and status: Cygwin was originally developed by Cygnus Solutions to ease Unix-to-Windows porting and later came under Red Hat after an acquisition. Development today is maintained by a combination of open-source contributors and corporate teams. It remains a widely used option for POSIX-like development on Windows, though newer approaches like Windows Subsystem for Linux offer alternative paths.