Problem with Roscore : Ros master URI is not defined

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Hello Guys,

First of all my OS is Ubuntu 16.04 with Ros Kinetic and Gazebo 7.0. I installed that with full desktop install.

i followed the tutorial on :

http://gazebosim.org/tutorials?cat=gu...

everything works fine until i got to the ros part. I worked with ros before with turtlebot package and there everything works fine too...

when i type in :

cd ~/velodyne_plugin/build
source /opt/ros/kinetic/setup.bash
gazebo ../velodyne.world

i got :

[FATAL] [1509043732.797014978]: ROS_MASTER_URI is not defined in the environment. Either type the following or (preferrably) add this to your ~/.bashrc file in order set up your local machine as a ROS master:

export ROS_MASTER_URI=http://localhost:11311

then, type 'roscore' in another shell to actually launch the master program.

when i echo ros_master_uri i got the local host address... i have no idea what i am doing wrong