Ubuntu18.04 gazebo和rviz的一些问题environment variable ‘TURTLEBOT3_MODEL‘ is not set.

1. Ubuntu18.04 运行gazebo9 ros_pkg not found
   输入命令：

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 ros-melodic-gazebo-ros-pkgs ros-melodic-gazebo-ros-control

2. [gazebo-2] process has died

killall gzserver
killall gzclient

3. /launch/turtlebot3_gmapping.launch:
   Invalid tag: environment variable ‘TURTLEBOT3_MODEL’ is not set.
   Arg xml is
   The traceback for the exception was written to the log file

输入命令即可：

export TURTLEBOT3_MODEL=burger

4. ERROR: cannot launch node of type [gmapping/slam_gmapping]: gmapping
   ROS path [0]=/opt/ros/melodi