How can I change real time update rate and/or max step size of ros-gazebo physic from command line once ros-gazebo has started?

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Hi Rahul !

I never have tried using the command line to set the physics simulation properties of the world, always I had used the definition inside the ".world" to set the properties.

I know that using Gazebo with ROS (gazebo_ros package) you can find a service to set some properties (time_step, max_update_rate, gravity, etc). Then you can use "rosservice call /gazebo/set_physics_properties ..." and to check if finally, you can modify the simulation parameters that you wish.

I hope my answer can help you, let me know if you have a good result.