latent-pixel / beginner_tutorials

ROS2 beginner tutorials

ROS2 Beginner Tutorials

This is a basic tutorial on writing and executing a Publisher-Subscriber pair using ROS2. Firstly, install all the dependencies required

Setting up and sourcing ROS

You will have to install ROS2 first, the instructions for which can be found on http://docs.ros.org/en/foxy/Installation.html. We will be using ROS2 Foxy for the purposes of this tutorial.

First, source your ROS2 environment:

source /opt/ros/foxy/setup.bash

Alternatively, you can add this to your system's .bashrc file to avoid sourcing ROS2 underlay each time a new terminal is opened:

echo "source /opt/ros/foxy/setup.bash" >> ~/.bashrc

Setting up the project

Now, create a workspace to work with new packages. This is done so that you don't have to build all the packages each time you try building a new package.

mkdir -p <your_dir>/ros2_ws/src
cd ros2_ws/src/

Download the code from the repository:

git clone [email protected]:Bhargav-Soothram/beginner_tutorials.git

Now rename the package using (this is for build purposes)

mv beginner_tutorials cpp_pubsub

Building the package

It is time to build the package! But before that, check if you have all the dependencies using resdep before going ahead:

rosdep install -i --from-path src --rosdistro foxy -y

Build the package:

cd ..
colcon build --packages-select cpp_pubsub

Running the package

Open a new terminal, navigate to ros2_ws, and source the setup files:

Publisher and Subscriber

. install/setup.bash

Start the talker node:

ros2 run beginner_tutorials talker

Open another terminal, source the setup files from inside ros2_ws again, and then start the listener node:

ros2 run beginner_tutorials listener

That is it, you have a publisher talking to a subscriber!

Service-Client

Now, let us see how to request for a change in the string that is being published. This can be accomplished using a service-client (request-response) paradigm. To request for a change in the string,

ros2 service call /modify_string cpp_pubsub/srv/ModifyString "{new_string: <your_string>}"

Using Launch files

We can use launch files to 'spin' multiple nodes at once with arguments passed to each of them. The arguments provided are the parameters to the nodes being executed. We have two parameters that can take user inputs,

• my_message #the new message to be published

• my_message_freq #time between successive messages in ms

• rosbag_record #whether or not to record a rosbag file for the current execution (exclusive to _launch_bag.py and not in _launch.py)

To do this, execute the following command:

ros2 launch cpp_pubsub _launch.py my_message:=<your_message> my_message_freq:=<your_message_frequency>

You can choose to give one or both the arguments at launch.

Testing the package (using colcon)

Open a new terminal, navigate to ros2_ws, and source the setup files as we did before.

Run the publisher as described in the "Publisher and Subscriber" section.

Now, use the following command to test the code:

colcon test --event-handlers console_direct+ --packages-select cpp_pubsub

Generating (recording) rosbag file

Open a new terminal, navigate to ros2_ws, and source the setup files as we did before.

Here, we have 2 launch files:

• _launch.py: Runs a publisher and subsriber with the given message
• _launch_bag.py: Inherits the functionality of _launch.py and adds the option to record rosbags

Sample Outputs

Attached are two sample outputs for different log levels: