Check if an environment variable is defined without command extensions and without using a batch file?

Question

I need to use a cmd.exe command line (cmd.exe is being called from the gyp build tool) to determine whether an environment variable is defined or not. How can I do this? I

Answer 1

OK, this took a bit, but I think I've figured it out. Try this:

SET UNDEFINED 2>Nul | Findstr/I "."
IF ERRORLEVEL 1  ECHO Not Defined.

This works for all cases AFAIK, and does not rely on any command extension features.