IBM recommends that you name environment variables in all uppercase when you intend to use them with a system running Cygwin.
When you use the Cygwin interface to create a UNIX®-like shell environment on Windows®, be aware that Cygwin converts all of your environment variables to uppercase names. This is a normal convention for Cygwin, but it may cause confusion if you define mixed case or lowercase environment variable names within the system, then find that when accessed from Cygwin, the names are all uppercase.