Congratulations! If you are at this step, you have an installed version of ROS on your system! To start using it, the system needs to know the location of the executable or binary files, as well as the other commands. To do this, normally you need to execute the next script; if you also install another ROS distro, you can work with both just by calling the script of the one you need each time, since this script simply sets your environment. Here, we use the one for ROS Kinetic, but just replace kinetic with indigo or jade, for example, if you want to try other distros:
$ source /opt/ros/kinetic/setup.bash
If you type roscore in the shell, you will see something starting up. This is the best test for finding out if you have ROS, and if it is installed correctly.
Note that if you open another terminal you also have to source the setup.bash file to set the environment variables to detect the ROS packages installed on your system. Otherwise, roscore or other ROS commands will not work. This is because the script must be sourced again to configure the environment variables, which include the path where ROS is installed, as well as other packages and additional paths for compiling new code properly.
It is very easy to solve this; you just need to add the script at the end of your .bashrc script file so that when you start a new shell, the script will execute and you will have the environment configured.
The .bashrc file is within the user home (/home/USERNAME/.bashrc). It has the configuration of the shell or terminal, and each time the user opens the terminal, this file is loaded. That way, you can add commands or configuration to make the user's life easy. For this reason, we will add the script at the end of the .bashrc file to avoid keying it in each time we open a terminal. We do this with the following command:
$ echo "source /opt/ros/kinetic/setup.bash" >> ~/.bashrc
To see the results, you have to execute the file using the following command, or close the current terminal and open another:
$ source ~/.bashrc
Some users need more than a single ROS distribution installed in their system, so you'll have several distros living in the same system and may need to switch between them. Your ~/.bashrc file must only source the setup.bash file of the version you are currently using, since the last call will override the environment set by the others.
For example, you might have the following lines in your .bashrc file:
…
source /opt/ros/indigo/setup.bash
source /opt/ros/jade/setup.bash
source /opt/ros/kinetic/setup.bash
…
The ROS Kinetic version will be executed in this case. Make sure that the version you are running is the last one in the file. It's also recommended to source a single setup.bash.
If you want to check the version used in a terminal, you can do so easily running the echo $ROS_DISTRO command.