First ROS2 Program

Let's create our first ROS2 program

Watch the video

Source the ROS setup.bash script

To use all the common ROS2 commands we need to source the setup script:

• Source the scripts - From the remote terminal, type:

source /opt/ros/humble/setup.bash

ROS2 Workspaces

In ROS we use workspaces to manage our robotics projects. They are essentially a folder, with a set of files that ROS expects to see. The great thing is that ROS makes it really easy to set these up with a single colcon build command.

• Create a new workspace - From the ROS2 container terminal, type:

cd /ros2
mkdir first_app
cd first_app
mkdir src
colcon build
ls

You will see a few new folders created - build, install and log

What is colcon?

colcon - collective construction

Colcon is a command line tool to improve the workflow of building, testing and using multiple software packages. It automates the process, handles the ordering and sets up the environment to use the packages.

Install folder

colcon created a new install folder; this contains a number of setup scripts that we will use to setup our terminal shell environment with the correct settings for this workspace.

You can look inside the setup files by using the cat command, eg cat setup.bash will show you (catalogue) the contents of the script.

Source the setup.bash script

Let’s go ahead and setup our terminal shell environment with our freshly created workspace bash setup scripts.

• Source the scripts - From the remote terminal, type:

source install/setup.bash

Fix permissions

You may need to fix the permissions within the container so that VSCode can write to these files and folders.

• change mode - From the ros2 terminal, type:

chmod 777 -R ../first_app

Create talker.py in src

Let’s create our first ROS2 python program.

• Select the src folder and Click New file - From the VSCode folder window, select the src folder and then click the New Files button
• Name the file - name the file talker.py

Paste this code

Let’s cheat a bit - paste the code below, we’ll explain it in the next section

• Paste this code - into the new file

#!/usr/bin/env python3
# My first Node!
import rclpy
from rclpy.node import Node

def main(args=None):
    rclpy.init(args=args)
    node = Node("py_test")
    node.get_logger().info("Hello ROS2")
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == "__main__":
    main()

Run the program

Let’s run it! Don’t worry we’ll explain exactly whats going on.

• Run the code - From the ROS2 container terminal, type:

colcon build
python3 src/talker.py

Congratulations! You’ve just written a ROS2 program.

What the code means

• import rclpy loads the ROS 2 Python library (ros client library for python).
• rclpy.init() starts ROS communications for this program.
• Node("py_test") creates a node called py_test. A node is a single program in a ROS system; a robot usually has lots of them, each doing one job (reading a sensor, driving the motors, planning a route).
• node.get_logger().info(...) prints a message through ROS’s logging system, with a timestamp and the node’s name.
• rclpy.spin(node) keeps the node running and handling events until you press CTRL + C.
• rclpy.shutdown() tidies up when the node stops.

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