Environment setup
It's convenient if the ROS environment variables are automatically added to your bash session every time a new shell is launched:
echo "source /opt/ros/cturtle/setup.bash" >> ~/.bashrc . ~/.bashrc
If you have more than one ROS distribution installed, ~/.bashrc must only source the setup.bash for the version you are currently using.
If you just want to change the environment of your current shell, you can type:
source /opt/ros/cturtle/setup.bash