Notes about an ROS Topic. This will describe a basic publisher and subscriber topic. Where to find the list of topics and the list of possible messages for a topic. How to install a new message for a topic.

rostopic list
rosmsg list
rostopic type <topic>
rostopic echo <topic>
rostopic info <topic>
rostopic hz <topic>

Publisher

Publish messages into a topic

Publish in Code

import rospy
from geometry_msgs.msg import Twist

rospy.init_node('robot_mover')      # Create Publisher
pub = rospy.Publisher('/cmd_move', Twist, queue_size=1)     # Set the data type for the publisher
rate = rospy.Rate(2)                # 2hz
move = Twist()                      # Create a Twist object
move.linear.x = 0.5                 # Move forward on x 0.5
move.angular.z = 0.5                # Move at an anglular rate of 0.5

while not rospy.is_shutdown():
    pub.publish(move)               # Publish the move
    rate.sleep()                    # Maintain the refresh rate

Publish on Commandline

Manually publish in command line

rostopic pub /cmd_move geometry_msgs/Twist "linear: x: 0.5 y: 0.0 z: 0.0 angular: x: 0.0 y: 0.0 z: 0.5"

rostopic pub <topic_name> <message_type> <value>

You can use the ‘double tab’ to autofil the value.

Subscriber

Read messages from a topic

import rospy
from std_msgs.msg import Int32

def callback(msg)
    print(msg.data)

rospy.init_node('topic_subscriber')
sub = rospy.Subscriber('/counter', Int32, callback)
rospy.spin()

Determine the message type of the topic

rostopic info /counter

This will return back the data type, publishers and subscribers for the given topic.

Create a Custom Data Type

Create a directory in the package

roscd my_new_package
mkdir msg

Create the Data type

Create a file within the new directory for the new data type Call it Vector.msg. Add the following in to the file

float32 magnitude
float32 direction

Edit CMakeLists.txt

Edit CMakeLists.txt in the package to find the new data type. Look for find_package(catkin REQUIRED COMPONENTS …)

find_package(catkin REQUIRED COMPONENTS
    rospy
    std_msgs
    message_generation
)

Again within CMakeLists.txt search for catkin_package(…) and uncomment the following line and append it if it contains other things passed CAKTIN_DEPENDS.

catkin_package(
        CATKIN_DEPENDS message_runtime
)

Again within CMakeLists.txt search for add_message_files(…) and uncomment it and add the new messages.

add_message_files(
    FILES
    Vector.msg
)

Again within CMakeLists.txt search for generate_messages(…) and uncomment it.

generate_messages(
    DEPENDENCIES
    std_msgs
)

Save the CMakeLists.txt file.

Edit package.xml

Edit package.xml in the package to find our new data type. Find build_depend…

<build_depend>message_generation</build_depend>
<run_depend>message_runtime</run_depend>

Compile Code

Go to your catkin workspace and run catkin_make

cd catkin_ws
catkin_make

Verify Data Type Exist

rosmsg show Vector

Stop Topic

rosnode kill /cmd_move

All the details for this can be found in this video.

ROS Topic