can't find gazebo /world dir

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hello everyone , i follow the tutorial below , but when i type "$ env | grep GAZEBO_RESOURCE_PATH " ,there are nothing happen . i can't see anything in my terminate.

World files are found within the /worlds directory of your Gazebo resource path. The location of this path depends on how you installed Gazebo and the type of system your are on. To find the location of your Gazebo resources, use the following command:

env | grep GAZEBO_RESOURCE_PATH

An typical path might be something like /usr/local/share/gazebo-1.9. Add /worlds to the end of the path and you should have the directory containing the world files Gazebo uses, including the mud.world file.

if anyone know why , help me please~