$gazebo fails to work after installing gazebo_ros_pkgs

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Hi. I'm in a dilemma. After installing gazeborospkgs, the 'rosrun/roslaunch gazebo_ros' commands work well and bring a Gazebo GUI.However when I run alone the command $gazebo in a terminal, nothing happens. Just nothing! Neither Gazebo GUI which is expected, nor warning/error information in the terminbal. I think something might go wrong with the environment variables or else, but I can't figure out. It's really awkward, for gazebo is there but not avaiable to me. I really need some help. Thanks.