How to pass parameters from %prep to %build?

How to pass parameters from %prep to %build?

I have the following problem: In the %prep, I run a command that returns a
value. I want to be able to use this value in the %build.
I was not able to do it with macros, because %global gets its value on
init, and %declare is executed when called.
I also wasn't able to use bash parameters because they are not shared
between %prep and %build.
I have 2 workarounds: Do all in %prep, or save the vale to a file. I'm not
comfortable with any of these workarounds.
This seems to be a simple task. Am I missing something?
Thanks, Eran.