Setting a variable from a shellcommand

[Chip Seraphine]

I may be obtuse, but short of writing a module how can I set a cfengine 
variable from the output of a shellcommand?  I could have sworn I've 
seen it done before, but I can't find it.

I want to do set a variable (call it $(my_ypserver)) to the output of 
the /usr/bin/ypwhich command.  A module seems like overkill for this....