Where's *the* ROS executable?

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Short Answer: No.

Longer Answer:

ROS isn't just an executable, it's a platform. ROS is made up of many smaller components that do everything from interfacing with hardware, path planning, localization, filtering, and decision making.

In order to run a robot, you have to pick and choose from which of these parts (divided into stacks and nodes) to see what applies to the project that you are working on. Each of these nodes is programmed in some mixture of C/C++/Python, with a few other languages mixed in. Which a node may have some executables associated with it, they are generally designed to operate as part of a complete system.

You typically put all of these parts together in launch file, which then systematically launches all of the nodes that you need for your application. These are launched in conjunction with a ROS core, that keeps everything communicating and synchronized.

On another point, the common "face" of ROS is RVIZ, which is part of the visualization stack. This is what provides the pretty GUI that you see in many YouTube videos and images. RVIZ is displaying the underlying data that the nodes are passing back and forth to each other.