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Important ROS Commands (02/09) – ROS Topic

Important ROS Commands Robot Operating System(ROS1/ROS2)

ROS Commands – ROS Topic

After describing “rosnode” command in previous blog, now we will discuss another important ROS command which is about “topic”. Since topic is a very important concept. To understand these commands you should be familiar with the “topic” concept. If you haven’t already familiar with it, please read ROS Terminologies and ROS Message Communication blogs.

“rostopic” command

The “rostopic” command is used to get the information about the topics and to print and publish the message with the specific topic name. Before exercising the commands, we will see what arguments are used with “rostopic” command.

Run the following command.

rostopic -h

In the figure below we can see number of commands. Each argument along with “rostopic” basically acts as a new command.

To exercise these commands, close all previously running nodes/terminals, and open three new terminals. Run following three commands on all three terminals separately.

Run in Terminal 1


Run in Terminal 2

rosrun turtlesim turtlesim_node

Run in Terminal 3

rosrun turtlesim turtle_teleop_key

Let’s exercise the commands.

“rostopic list” command

The “rostopic list” command is used to show the list of active topics.

Open a new terminal (say Terminal 4) and run the command

rostopic list

you will see the list of following topics






If we add the ‘-v’ option to the “rostopic list” command, it separates the published topics and the subscribed topics. In addition, it shows the message type for each topic.

Run in Terminal

rostopic list -v

The results in the terminal for above both commands are shown in figure below

“rostopic echo” command

The “rostopic echo” command print the content of a message on screen for a specific topic.

The structure of the command is as follows

rostopic echo [Topic_Name]

We will see later what type of a message is communicated through the topic. Here we will just execute this command and see the results.

The topic name is “/turtle1/pose”. We have already seen this topic in active topic list.

Run the command

rostopic echo /turtle1/pose

In figure below it shows the data for ‘x’, ‘y’, ‘theta, linear_velocity’, and ‘angular_velocity’ that constitutes the ‘/turtle1/pose’ topic in real-time.

“rostopic info” command

The “rostopic info” command is used to get the information about a particular topic. The structure of this command is as follows

rostopic info [Topic_Name]

Run the command to see the information about the “/turtle1/pose” topic.

rostopic info /turtle1/pose

Also, run the command to see the information about “turtle1/cmd_vel” topic.

rostopic info /turtle1/cmd_vel

Result of executing both commands is shown in figure below

“rostopic pub” command

Most of the time, we define publisher node in program, which publish the data periodically on a specific topic, and the subscriber node receives it. However, we also have the command “rostopic pub” to directly publish the message on command prompt with the specific topic name

The structure of the command is as follows

rostopic pub [Topic_Name] [Message_Type] [Parameter]

Now we publish a message with the topic name “/turtle1/cmd_vel” with a message type of “geometry_msgs/Twist”.

rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist -- '[3.0, 0.0, 0.0]' '[0.0, 0.0, 0.0]'

We run this command again with different values

rostopic pub -1 /turtle1/cmd_vel geometry_msgs/Twist -- '[1.0, 0.0, 0.0]' '[0.0, 0.0, 2.0]'

Here “-1” means the data will be published once.

linear parameter means that the object (in this case turtle) will move in x, y, or z direction with give speed, and angular parameter means, the object will rotate about x, y, or z –axis with specified speed.

(For example in first example the movement will be in x direction with the speed of 3.0 m per second, and in second example the movement will be with the speed of 1.0 m per second in the direction of x, and a rotation of 2 rad per second in around z-axis)

The figure below shows the execution of the commands and another image shows the movement of turtle after execution of the commands.

Since the turtle has subscribed the topic “turtle1/cmd_vel” of message type “geometry_msgs/Twist”, the turtle moved accordingly after receiving the published data values as shown below

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