conditional Variable assignment and shell call

    Hi,

    What is the best way to init variable conditionally, I just noticed
    that following code will always call shell. I would like to call
    the formail only if the variable is empty. How do I do that?

#   If you already have saved the subject, please preset this variable
#   to prevent extra shell call!
#
JA_SRV_SUBJECT={JA_SRV_SUBJECT:-`formail -xSubject:`}

:0
* condition follows....