I really like Linux in general and decide to follow its career. Currently, I am trying to learn POSIX myself, hopefully it will be beneficial for my future as a Linux developer in general and game programmer in particular, since POSIX is a portable operating system programming interface. I expect I can use it in game development for interacting and manipulating the Linux system. However, if even it helps me little in the game development, I still like to learn this.

In my opinion, POSIX is counterpart to Win32 API. Can someone help to tell me what the common careers for Linux programmers in general and POSIX programmers in particular?
Is there any job related to Linux programming rather than being a system administration and understand telecommunication (something like CISCO certificates...)?
If then, what's the requirement?
How does Linux programmers make money for living and being rich if possible, because it seems that most people (probably including me) think that Linux = FREE OPEN SOURCE = No Money?

Some of the questions I'm still wondering.

There is plenty of commercial software for Linux so just because you write software for Linux doesn't mean you have to give it away. (Looking at the sales numbers for indie Linux games it seems quite obvious that Linux users want more games and that they are prepared to pay to get them, the market isn't big enough to make you rich but its big enough to be profitable if you play your cards right)

The possible careers related to Linux development are pretty much the same as for Windows development, most programmers are consultants these days and write custom software for their customers, the platform is mostly irrelevant. (You write the software for whatever platform the customer is using unless you are delivering a complete IT solution).

Getting rich is extremely difficult for a programmer regardless of what platform you develop for and you usually need to do more than just programming to become rich. (Its not the programmers at a company such as Blizzard that is becoming filthy rich, its the company owners (In some cases these are the same people though and when starting a company you need to either have money to put in or work really hard))

You can certainly get a job as a regular software developer working on Linux. I imagine not so much in the world of producing packaged software, but many web-based applications run on Linux since it's so popular for web-servers.
In fact any software in the server environment has a fair chance of using Linux (or Unix), so for instance a MMO server could sensibly be run on Linux even if the client is Windows-only.

How does Linux programmers make money for living and being rich if possible, because it seems that most people (probably including me) think that Linux = FREE OPEN SOURCE = No Money?

I am a professional software engineer who develops exclusively for Linux -- not including some hobby stuff, which tends to be cross-platform. I write free software for a living. That means the software is licensed under some variant of the GNU GPL (GPLv3, LGPLv2, some LGPLv2), made as a work for hire and the copyright is retained by my employer. I am paid a decent wage for my labours, and I get to work remotely from a home office in a log cabin in the back woods wilderness. It is the perfect job.

My employer earns revenue in a number of ways. We provide paid service contracts for organizations which use our Linux distribution, which by the way is also the major source of revenue for Microsoft. We provide custom-built Linux images for OEMs (for example Dell and Lenovo) so they can ship their hardware with Linux preinstalled, and they pay a license fee per image shipped (just like they do for Microsoft). We provide online services integrated into the desktop environment. In fact, our revenue model is pretty much the same as Microsoft's. the only real difference is we work with others and share our code. Microsoft doesn't, but neither of us make any money from source code. What's free is the person who can read or modify our code.

The best way to get on board as a professional Linux developer is to get involved in and start contributing to free software projects. Not only do you learn a lot by seeing how good code gets written, but you get your name out there. In the real world when it comes to getting a good job, it's 10% what you know and 90% who you know (or who knows you).