On a cultural note, it's fun to be aware that GNU doesn't mean the software must be costless. It's perfectly valid to sell an OSS for a fee, but you are require

On a cultural note, it's fun to be aware that GNU doesn't mean the software must be costless. It's perfectly valid to sell an OSS for a fee, but you are required to provide the source code and your customer is free to sell/share it again. It's even written on the FSF's FAQ. I guess if you make something sufficiently "trade secret", your customer won't share it with the rest of the world.