ROS Tutorials

Understanding ROS Topics

http://wiki.ros.org/ROS/Tutorials/UnderstandingTopics

1. Setup

Launch ROS:

roscore

In a new terminal, launch the turtlesim node:

rosrun turtlesim turtlesim_node

In another terminal, launch the keyboard teleoperation:

rosrun turtlesim turtle_teleop_key

2. ROS Topics

First, install rqt_graph:

sudo apt-get install ros-melodic-rqt
sudo apt-get install ros-melodic-rqt-common-plugins

Next, we can launch rqt_graph:

rosrun rqt_graph rqt_graph

You can view some commands with:

rostopic -h

We can use the echo command to view the data published

rostopic echo /turtle1/cmd_vel

Now, the data that we publish to turtlesim_node through the turtle_teleop_key will appear in this terminal.

To view all topics currently subscribed to and published, use:

rostopic list -h

For a more detailed list, you can use the verbose (-v) option:

rostopic list -v

3. ROS Messages

Communication between ROS nodes takes place in ROS messages from publisher to subscriber. In this case, turtle_teleop_key is our publisher and turtlesim_node is our subscriber.

rostopic type /turtle1/cmd_vel

To view the details of the message, we can use rosmsg:

rosmsg show geometry_msgs/Twist

4. rostopic Continued

We can use messages to directly communicate with ROS nodes:

rostopic pub -l /turtle1/cmd_vel geometry_msgs/Twist --'[2.0, 0.0, 0.0]' '[0.0, 0.0, 1.8]'