Technically there used to be (still is?) such a thing as realtime Linux but not _for_ Linux (which you probably meant the userland?).

Regardless. There are many and far better alternatives to Linux for real time applications.

RT Preempt / Preempt-RT patch still exists. It slowly is being integrated into mainline as per everyone's wishes but it is still a thing.

The original RTLinux it also still usable and it provides it's own hard RT scheduler.

I've worked with RTLinux in automotive. It is only used in R&D and testing. It's much worse than INTime (windows rtos) or L4, which are usually used. I've even patched g++ to work on RTLinux eliminating all the dynamic, unreliable stuff.

People are using Linux when they need HW and driver support, e.g. gigabit Ethernet, firewire and such. RTOS vendors charging shitload of money on those drivers. I trust the Linux drivers more than the RTLinux scheduler or libc. But well, recently networking went to hell, so even there they start fucking up.

Pretty much anything that actually needs hard realtime is very likely running on a dedicated MCU/FPGA/ASIC. Linux is vastly easier to use for everything else, though. Navigation or communication doesn't need hard realtime, for example.

NI's cRIO devices use a real-time variant of Linux.

http://www.ni.com/white-paper/14627/en/