READ THE ENVIRONMENT VARIABLE DURING THE COMPILE TIME IN NSIS - nsis

Question

HI Everyone As i was working with the NSIS code, i need to get the environment variable value during the compile time, I Found this code written in somewhere But am not getting understand what does 'Foo' means. & How can we execute the below code in compile time, in my compiler Am not getting the environment variable value with the below following code, Can anybody give a brief example about where we can execute this & How it works?
Edit : As you requested i was inserting the link of Code referred from this forum "https://stackoverflow.com/questions/22149007/nsis-how-to-check-at-compile-time-if-an-environment-variable-exists"
; NSIS 2+
!define DOLLAR $
!if "$%foo%" == "${DOLLAR}%foo%"
!echo "%foo% not set"
!endif
; NSIS 3+
!if "$%foo%" == "${U+24}%foo%"
!echo "%foo% not set"
!endif

Answer 1

This code tries to check if a environment variable has been set (on the computer where the compiler is executing). If it has not been set the compiler just returns the raw string (like a batch file does). The code you found escapes the $ to check if the raw string is returned.
Foo is just a placeholder name, meaning the %Foo% variable. If you want to check %windir% for example, use !if "$%windir%" == "${U+24}%windir%".
If you just want the value, just use $%windir%:
ReadEnvStr $0 WINDIR
MessageBox MB_OK "%windir% was $%windir% when this installer was compiled. On this machine it is $0"