gazebo9降级到gazebo7

$ sudo apt-get remove gazebo9
$ sudo apt-get install libignition-math2 libsdformat4
$ sudo apt-get install ros-kinetic-gazebo7*

$ sudo apt-get install libgazebo7

sudo sh -c 'echo "deb http://packages.osrfoundation.org/gazebo/ubuntu-stable `lsb_release -cs` main" > /etc/apt/sources.list.d/gazebo-stable.list'
wget http://packages.osrfoundation.org/gazebo.key -O - | sudo apt-key add -
 sudo apt-get update
 sudo apt-get install gazebo7 gazebo7-common 
  sudo apt-get install gazebo7 gazebo7-common --fix-missing

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原文链接：https://blog.****.net/CAIYUNFREEDOM/article/details/105198681
$ sudo apt-get install gazebo7 gazebo7-common 

rosdep install --from-paths src --ignore-src -y