A variety of Windows-like GUIs that vary in size, appearance, features, capabilities to support a wide range of embedded requirements. Yet despite this great diversity, all Linux implementations share a common core that includes the kernel, drivers, several popular GUIs, and utilities. Among these are a growing number of application-oriented Linux configurations and distributions that are tuned to specific applications. You may also opt to create your own flavor of embedded Linux, starting from a standard distribution and leaving out modules you don't need. Even so, you should consider jump-starting your efforts by beginning with someone else's working configuration, since the source code of their version will be available for that purpose.
OS runs as a task under a real-time kernel.
Of course, you could also dive in and modify Linux to convert it into a real-time operating system, because its source is openly available. In short, your customized Linux won't benefit from the continual Linux evolution. Although CPUs run at ever-increasing speeds, the world around them goes on at a constant speed. Therefore, real-time performance is becoming ever easier to achieve. The greater concern has become time to market and sophistication of functionality.