I have some articles on my web site typicalprogrammer.com that describe my experience and offer some opinions about freelancing.

I’ve worked with Unix since it came out, and cloud infrastructure since it became available. Most customers have fairly simple setups but you have to know about security, performance/networking, Linux configuration and maintenance, and how the various cloud providers work. System admin makes a good freelancing business because small/medium-size companies don’t usually have enough work to keep someone full-time, so either their programmers do it (badly and reluctantly) or they outsource.