Experience with Linux is an important thing – a track record of tinkering and involvement in the open source world. Working in drivers, embedded Linux, etc. At this point companies are desperate for Linux talent. The most important thing to show is you've gotten hands-on with bits of the kernel, whichever ones are interesting to you personally. Time spent as a site reliability engineer or working in a DevOps environment is particularly attractive to employers these days, as are well rounded sys admin skills. Even if you just run Linux as your primary operating system and know how to tinker with your machine, you’re ahead of many candidates.