Need to have only shadows in image file

[Shurlin]

Does anyone know how to render shadows only or save shadows in image file different from main render image?

[Dusan_Pavlicek]

Turn the background color to white and use a VRayMtlWrapper material with a "Matte surface" option checked. Apply this material to all objects in the scene.
Don't leave the Base material of VRayMtlWrapper empty, set it e.g. to the standard VRay material.
That's it.