Up until now, we have worked with standard type messages (e.g. strings). Let’s see how we can create our own custom messages in ROS.

Open a new terminal window, and type the following command to go to the noetic_basics_part_1 package:

roscd noetic_basics_part_1

Create a msg folder inside that package.

mkdir msg

Move inside that folder.

cd msg

Create a new .msg file

gedit noetic_basics_part_1_msg.msg

Add the following lines.

int32 A
int32 B
int32 C

Click Save and close out the window.

In the code above, we have written three message descriptions. Specifically, we have declared three signed 32-bit integers. The syntax for a message description is:

fieldtype fieldname

Now, go back to the package.

roscd noetic_basics_part_1

Open the package.xml file.

gedit package.xml

Uncomment these two lines by deleting the arrows on each side:

<build_depend>message_generation</build_depend>
<exec_depend>message_runtime</exec_depend>

Click Save, and then close the editor.

Now open CMakeList.txt.

gedit CMakeList.txt

Add message_generation inside the find_package(catkin REQUIRED COMPONENTS…) scope.

Also uncomment the add_message_files (…) lines and add the name of the message you just created.

Now uncomment the generate_messages(…) lines.

Since we made all these changes, we now need to build the package.

cd ~/catkin_ws/

catkin_make

Check to see if everything built properly by typing the following command:

rosmsg show noetic_basics_part_1/noetic_basics_part_1_msg

Here is what you should see:

You can also see the custom message you just created by typing this command:

rosmsg list

The message types are listed in alphabetic order by package name.

You can also just see a list of message types for just the noetic_basics_part_1 package.

rosmsg package noetic_basics_part_1

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.

How to Build a C++ Node

Now that we’ve created our two nodes, we need to build them. Building ROS nodes means getting them in a format that enables ROS to use them.

The first thing we need to do is to edit the CMakeLists.txt file inside the noetic_basics_part_1 package. 

The CMakeLists.txt file is the file inside the noetic_basics_part_1 package that was automatically generated when we created the package. CMakeLists.txt says how to build the code we wrote (i.e. the nodes we just created).

Let’s edit the CMakeLists.txt file for the noetic_basics_part_1 package. Open a new terminal window, and type this command:

roscd noetic_basics_part_1 

gedit CMakeLists.txt

You should see a file that looks like this:

Now add these lines to the bottom of the CMakeLists.txt file:

add_executable(simple_publisher_node src/simple_publisher_node.cpp)
target_link_libraries(simple_publisher_node ${catkin_LIBRARIES})

add_executable(simple_subscriber_node src/simple_subscriber_node.cpp)
target_link_libraries(simple_subscriber_node ${catkin_LIBRARIES})

In the code above, we have done the following:

• Create two executables, simple_publisher_node and simple_subscriber_node, which will by default go into the ~/catkin_ws/devel/lib/noetic_basics_part_1 folder.
• Make sure the nodes get linked to the catkin libraries (i.e. ROS-related libraries).

Now save and close the CMakeLists.txt file.

Open a new terminal window, and type the following commands to build all the nodes in the noetic_basics_part_1 package:

cd ~/catkin_ws

catkin_make 

That command above will build all nodes inside all the packages in your catkin workspace.

Now, open a new terminal window and go to the catkin_ws/devel/lib/noetic_basics_part_1/ folder.

cd ~/catkin_ws/devel/lib/noetic_basics_part_1/

Type dir to see the files listed. You will see the publisher and subscriber executables. Feel free to close the terminal window now.

How to Build a Python Node

Navigate to the scripts folder inside your package. 

roscd noetic_basics_part_1/scripts

Make the files executable. Type these commands, pressing Enter after each.

chmod +x simple_python_publisher.py

chmod +x simple_python_subscriber.py

Open a new terminal window, and type the following commands to build all the nodes in the noetic_basics_part_1 package:

cd ~/catkin_ws

catkin_make