Who Pays the Bills?

I'll be honest, I wasn't in class the day they watched Revolution OS.  But I do have some thoughts to share on the open-source movement.  I love the open-source mentality that pervades the programming community.  It's one of the things that has impressed me the most during my short time as a programmer.  Fellow programers put up code and solutions for free on places like GitHub and Stack Overflow.  Being a programmer makes you a part of a vast network of compatriots each willing to help one another.  But can such a system stand on its own?  Is a purely open-source community sustainable?  You can't eat or take shelter in shared code.  I personally see open-source projects as something I would do in my spare time while making a living writing code for another entity.  I'm aware, without knowing specifics, that there are ways to monetize open-source projects but doesn't that go against the point of open-source?  I suppose open-source and commercial software must exist together.