How to install ROS on mac OS X El Capitan?

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Dual booting will make your Ubuntu & ROS install work faster but you can risk breaking your current installation. You can always try first on a virtual machine (Virtual box). If the Virtual Machine approach becomes too slow, then you can always go for dual boot. I'd recommend have someone with experience setting up Dual boot on Mac to help you if possible.

Ubuntu 18.04 with ROS Melodic (as they are both LTS, Long Term Support). You can read more about ROS distributions here. Most packages have been ported from ROS Kinetic (which uses Ubuntu 16.04) to Melodic already, and if not, you can build them by source easily.