Linux is being used in many more consumer devices. Investigators should at least have a basic idea of what Linux is, how it is different than Windows and basic usage (because hackers certainly will). This free course from edX by the Linux Foundation will give a good overview of how the Linux operating system functions and its usage.
I am taking it right now. So far it gives a quick and interesting overview and differences between popular distributions. It looks like it goes into some basic usage from a desktop as well as command-line interface. It probably won't make you an expert, but it looks like a great place to start.
Once you have gone through this course, if you want to know more about investigations using Linux, I strongly recommend the book and resources at Linux LEO.