There was a problem after adding an environment variable to .bashrc

[CiaranYoung]

After following the instructions in the readme, you cannot use the 'gazebo' command alone to open the 'gazebo' command, you must use the 'roscore+rosrun gazebo_ros gazebo' or 'roslaunch gazebo_ros empty.world' command to open it. But commenting out the two lines added to the .bashrc can be opened with 'gazebo', and using 'gazebo --verbose' shows that the empty.world cannot be found