在安装完ROS2之后，打开终端选择ROS1或ROS2之后会出现提示：

ROS_DISTRO was set to 'dashing' before .
Please make sure that the environment does not mix paths from different distributions.

或

ROS_DISTRO was set to 'melodic' before .
Please make sure that the environment does not mix paths from different distributions.

好像目前没有发现忽略这个问题出现的bug
解决方法：

修改环境变量，避免两个版本冲突

sudo vim /opt/ros/melodic/share/ros_environment/catkin_env_hook/1.ros_distro.sh

# generated from ros_environment/env-hooks/1.ros_distro.sh.in
#if [ -n "$ROS_DISTRO" -a "$ROS_DISTRO" != "melodic" ]; then
#  echo "ROS_DISTRO was set to '$ROS_DISTRO' before. Please make sure that the environment does not mix paths from different distributions."
#fi
#export ROS_DISTRO=melodic

ROS2:

sudo vim /opt/ros/dashing/share/ros_environment/environment/1.ros_distro.sh

# generated from ros_environment/env-hooks/1.ros_distro.sh.in
#if [ -n "$ROS_DISTRO" -a "$ROS_DISTRO" != "dashing" ]; then
#  echo "ROS_DISTRO was set to '$ROS_DISTRO' before. Please make sure that the environment does not mix paths from different distributions."
#fi
#export ROS_DISTRO=dashing

将里面的内容全部注释即可