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 am okay assuming that the variable value does not contain single or double quotes, but cannot assume that command extensions are enabled.

I've tried the following, which works great in a .bat file, but fails when typed directly on the command line:

IF "%UNDEFINED%" == "" (echo yes) 

When that exact line is in a .bat file and executed, I see yes as the output. When I type it on the command line, the output is empty. I am testing this on Windows XP SP3, though my coworker sees the same results on Windows 7. This is the method suggested by http://support.microsoft.com/kb/121170 and http://www.robvanderwoude.com/battech_defined.php. I do not want to use IF DEFINED UNDEFINED (echo yes) because that won't work if command extensions are disabled.

The top-voted answer in the following post has led me to believe that this issue is related to how percent-expansion is handled differently in the "CmdLineParser" vs. the "BatchLineParser," but still has not led me to a solution: How does the Windows Command Interpreter (CMD.EXE) parse scripts?

Answer 1

Errr... just:

if defined your-var-name (      echo yarp ) else (     echo narp ) 

I should add, I do not believe this needs command extensions...