Is there any rule that Linux Softwares shall be open-source?

I'm curious to know if the Softwares or tools made for Linux have to be open-source?

I was working on a tool to view and edit CAN dbc files (link in my profile) and people asked me to make it for free and I made it open source. Now, I have another idea which I'm yet to start and it's just for Linux and I'm thinking to put a price on it for advanced features. Is it okay if I do that? Would you be interested to try it out?