How to Run a C++ Node

Ok, now that we have built our nodes, let’s run them.

Open up a new terminal window and launch the ROS Master.

roscore

In a new terminal tab, type the following command to run the publisher node:

rosrun noetic_basics_part_1 simple_publisher_node

Here is the output:

Now, let’s run the subscriber node. Type the following command in a new terminal tab.

rosrun noetic_basics_part_1 simple_subscriber_node

You’ll notice that the syntax for running a node in ROS is:

rosrun package-name executable-name

In a new terminal tab, check to see which nodes are running.

rosnode list

The /rosout node is a special node that starts up automatically when you launch the ROS Master. It is responsible for managing ROS-specific output messages to the terminal window.

If you want to find out information about the publisher node, you can type:

rosnode info /simple_publisher_node

That command aboves gives you a full account of what the node is all about. To find out more information about the subscriber node, type:

rosnode info /simple_subscriber_node

There are a bunch of commands for ROS nodes.

To list all active topics, type:

rostopic list

You can see the /message topic is currently active.

Now try these commands, and see what you get:

rostopic info message

rostopic type /message

rostopic bw /message

You can find out what all these commands mean at the ROS website here.

To visualize the publish-subscribe relationships between the nodes, type the following command:

rqt graph

An incoming arrow indicates that a node subscribed to the topic. An outgoing arrow indicates that a node publishes to the topic.

You can close all terminal windows when you’re ready by pressing CTRL + C on your keyboard on each window.

How to Run a Python Node

In a new terminal window, launch ROS.

roscore

Open a new terminal window, and execute simple_python_publisher.py.

rosrun noetic_basics_part_1 simple_python_publisher.py

Here is the output:

Open another terminal window and type:

rosrun noetic_basics_part_1 simple_python_subscriber.py

To stop the program, type CTRL+C in all terminal windows.