Summary: Linux has facilities that make it relatively easy to configure and build a custom kernel. The process has improved to the point that it has become a good confidence building exercise for developers new to Linux.Why would you want to build your own kernel? Perhaps to reduce the bloat of a stock distribution kernel, to support some peculiar feature of your machine, and maybe add DOS and Windows file system support. If you're an embedded developer, you'll probably need to configure a kernel to fit the resource constraints of your target environment.
