Linux in the military

So it's no secret that some parts of the army in the USA and my country (UK) sometimes use legacy software like DOS for niche roles as they're robust including older versions of Windows.

But.. where does Linux fit in this? It's a kernel OS that's used in top of the line supercomputers, workstations, medical equipment and weather stations.

I imagine some aspects of this would be military secrets but how do they use it? I know that Linux was used for certain space projects with NASA but I'm talking about army applications.

TLDR : Does the penguin OS power shooty shooty machines and tanks