Introduction to ROS¶

In this exercise, we will setup ROS to be used from the terminal, and start roscore.

In order to start programming in ROS, you should know how to install ROS on a new machine as well and check that the installation worked properly. This module will walk you through a few simple checks of your installed ROS system.

ROS Installation¶

As ROS is already preinstalled to your system, this section can be fast forwarded. Nevertheless, You are still encouredged to have a look on the installation instructions needed to get ROS going on a freshly installed Ubuntu Linux.

Testing the environment¶

We believe we have a good installation of ROS but let’s test it to make sure.

A good way to start is to ensure that environment variables like ROS_ROOT and ROS_PACKAGE_PATH are set:
```
printenv | grep ROS
```
If the variables are empty or do not contain paths to ROS workspaces you would like to work with, it is most likely that you might need to ‘source’ some setup.*sh files.
Let’s do that by entering:
```
source /opt/ros/noetic/setup.bash
```
Now we have sourced the default workspace for ROS Noetic.
Run the printenv | grep ROS command again and verify that the variables have been set.

Tip

If you are ever having problems finding or using your ROS packages make sure that you have your environment properly setup.

Note

In a “bare” ROS install, you will need to run this command on every new shell you open to have access to the ROS commands. One of the setup steps in a typical ROS install is to add that command to the end of your ~/.bashrc file, which is run automatically in every new terminal window.

To check that your .bashrc file has already been configured to source the ROS-noetic setup.bash script:
```
tail ~/.bashrc
```
For ease of this workshop, use the following oneliner to add automatic sourcing to the .bashrc If it is not listed yet:
```
echo "source /opt/ros/noetic/setup.bash" >> ~/.bashrc
```

Note

As one can observe, we sourced the ROS environment for a specific ROS distribution (Noetic). Using this sourcing process allows you to install several ROS distributions (e.g. Kinetic and Noetic) on the same computer and switch between them by sourcing the distribution-specific setup.bash file.

Navigating the ROS Filesystem¶

Once you have learned the basic commands for navigating Linux filesystem, it is a matter of adding a prefix ros and you are able to easily navigate between ROS packages in your sourced sourced workspaces.

In this section we will explore the following ROS commands:

rospack = ros + pack(age)
roscd = ros + cd
rosls = ros + ls

This naming pattern holds for many other ROS tools.

rospack Command¶

rospack allows you to get information about packages. In the following, we are only going to cover the find option, which returns the path to package. The command is used as follows:

rospack find [package_name]

In the terminal configured for ROS, type:
```
rospack find roscpp
```
rospack should return /opt/ros/noetic/share/roscpp for the package location.

roscd Command¶

Type: roscd roscpp, then type pwd.
You will notice that the working directory was changed to where the roscpp package is located.

rosls Command¶

With rosls, you can directly get the file listing of a given package.

Try it out with:
```
rosls laser_geometry
```
Files and folders from the laser_geometry package should be displayed.

Note

Keep in mind that all ROS tools will only find ROS packages that are within the directories listed in your ROS_PACKAGE_PATH. To see what is in your ROS_PACKAGE_PATH, type: echo $ROS_PACKAGE_PATH. A colon separated list of paths appears.

Starting roscore¶

roscore is a collection of programs that are pre-requisites of a ROS-based system. You must have a roscore running in order for ROS nodes to communicate. It is launched using:
```
roscore
```

There are several components that are started with roscore:

a ROS Master

a ROS Parameter Server

a rosout logging node

You will see ending with started core service [/rosout]. If you see roscore: command not found then you have not sourced your environment, please refer to the previous section to resource the environment.

Verify that the roscore is running. Open a new terminal window and enter:
```
rosnode list
```

You should see the logging node named /rosout

Press Ctrl+C in the first terminal window to stop roscore.
Try to obtain the node list again in the second terminal. Was the command sucessful this time?

Tip

Ctrl-C is the typical method used to stop most ROS commands.

Installing and Running your first ROS node¶

Many of the coolest and most useful capabilities of ROS already exist somewhere in its community. Often, stable resources exist as easily downloadable debian packages. Alternatively, you can download the “cutting edge” versions (usually hosted on Github), but then you would need to set up a catkin workspace and build the packages yourself. This is a topic of day 2. Today, we use apt to install a precompiled ROS packages from the ROS repository.

Install the package roscpp_tutorials, which can be found under metapackage ros_tutorials:
```
sudo apt install ros-noetic-ros-tutorials
```
Note

Pay attention to the distribution noetic in the command.
Use the commands you learned in section Navigating the ROS Filesystem to explore the location and content of the roscpp_tutorials package.
Let’s run a talker node from this package. For that, we use the following rosrun command:
```
rosrun roscpp_tutorials talker
```
Now, open another terminal and from the same package run the node listener.
If you receive hello world messages with the listener node then congratulations! You are successfully running two nodes in the ROS system with one sending text messages to another.
Now take another terminal window and try to run again:
```
rosnode list
```

To see which nodes are now published.

More information about the rosnode <http://wiki.ros.org/rosnode>