CMake : how to use bash command in CMakeLists.txt

I'm wondering how I can use bash command in CMakeLists.txt. What I want is to get the number of processor retrieved using :

export variable=`getconf _NPROCESSORS_ONLN`

and set NB_PROCESSOR to variable using something like :

SET (NB_PROCESSOR variable)

So my question is how can I use the getconf command in CMakeLists.txt and how can I use the result (stored in variable) in CMake SET command?