I regularly have 3D models that consist of a Boolean union of balls and cylinders (molecule models, star maps, mathematical designs), that I then want to convert to a STL model for printing at Shapeways. But I don't know how to easily do that! I already have the model as a series of floating point numbers (four for each ball, and seven for each cylinder), so I don't want to have to go into a graphical interface to click/drag points. Instead I want to convert my lists of floating point numbers to STL from the command line.
I thought this would be a trivial thing, but I cannot find what is the best tool for this. All I need, I think, is a pointer to what tools are most appropriate (although more detailed help will be appreciated :-)) I thought that "gmsh" might be good for this, but it is not immediately clear to me how to use it for that.
I would like the software for this to be "free", in the sense that it does not cost money. Also, I am not a Windows user, so I would like to be able to run this under Linux or macOS.