ROS 2 Topic Command

  • ros2 topic is a command-line tool for displaying debug information about ROS 2 Topics, including publishers, subscribers, publishing rates, and ROS Messages.

  • Use

ros2 topic --help

to get the help for the ros2 topic command.

ros2 topic list

  • ros2 topic list returns a list of all topics currently being subscribed to and published.

  • In one terminal, ensure your ROS 2 environment is sourced and then run:

ros2 topic list

The above will give you a list of the default ROS 2 topics.

  • For this tutorial, we will also use turtlesim. Please run in a new terminal:

ros2 run turtlesim turtlesim_node

You will see a small blue box with a turtle inside it.

  • Now, check again the ros2 topic list command in another terminal and observe the topics being subscribed to or published.

ros2 topic type, info, and ros2 interface

type

Communication on topics happens by sending ROS 2 messages between nodes. To communicate, the publisher and subscriber must use the same message type. You can determine the message type being used on a topic with the ros2 topic type command.

ros2 topic type [topic_name]

From the turtlesim node, we know that there are several topics being published/subscribed, such as:

  • /turtle1/cmd_vel

  • /turtle1/pose

  • /turtle1/color_sensor

Let’s check the message type associated with the topic /turtle1/cmd_vel. Enter the following command:

ros2 topic type /turtle1/cmd_vel

ros2 interface show

  • The message type associated with /turtle1/cmd_vel is geometry_msgs/msg/Twist. To see the message details, you can use the ros2 interface show command:

ros2 interface show geometry_msgs/msg/Twist

  • A message consists of fields and constants. Fields represent the data type, and constants hold predefined values.

  • You will see two fields under geometry_msgs/msg/Twist:

    • geometry_msgs/Vector3 linear : Describes the linear velocities for all three axes.

    • geometry_msgs/Vector3 angular : Describes the angular velocities for all three axes.

ros2 topic info

  • This command provides more detail about a topic, including both the message type and the nodes publishing and subscribing to it.

ros2 topic info /turtle1/cmd_vel

ros2 topic pub

  • ros2 topic pub is used to publish data onto a topic currently advertised.

  • Usage:

ros2 topic pub [topic] [msg_type] [args]

  • Let’s move the turtle in the simulator by publishing a command to /turtle1/cmd_vel:

ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.5}}"

  • The above command will rotate the turtle clockwise around its z-axis.

  • However, the turtle will only rotate briefly because the command is sent only once. To continuously send the command, use the –rate argument.

ros2 topic pub --rate 10 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.5}}"

  • The turtle will now rotate continuously at a rate of 10Hz. You can always get more information about the pub command by typing:

ros2 topic pub --help

ros2 topic echo

  • ros2 topic echo shows the data being published on a topic.

  • Usage:

ros2 topic echo [topic]

  • Let’s check the pose information of the turtle in turtlesim, which is published on the /turtle1/pose topic. Run:

ros2 topic echo /turtle1/pose

  • You will see the x, y, z, and theta values (pose) of the turtle being published in real time.