ROS Activities

ROS Example 1: Publisher/Subscriber

The following example is tested on ROS Kinetic on Ubuntu 16.04 (Xenial).

Clone the repo:

In your terminal (ROS on Ubuntu OS), clone the repo from the link using this command:

git clone https://github.com/MeqdadDev/hello-ros1

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Preparing workspace

Copy the files in ros_demo/src/pub_sub_example into your src package directory.

As a result, src directory in your package should have: publisher.py, subscriber.py

Now, open your workspace in the code editor (VS Code). Let's assume you're working on meqdad_ws workspace, and your package name is ros_demo.

Open the terminal and change the directory to meqdad_ws as in the command below:

cd meqdad_ws/

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Compile packages

After adding publisher.py, subscriber.py into your src directory, run the compilation command for packages:

catkin_make

And finally, don't forget to run the ROS master node:

roscore

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Publisher

In this example, we assume the following settings:

• Publisher node name: publisher
• Topic name: session
• Message type: string

import rospy
from std_msgs.msg import String

def publisher():
    pub = rospy.Publisher('session', String, queue_size=10)
    rospy.init_node('publisher', anonymous=True)
    rate = rospy.Rate(1) # 1hz
    ctr = 0
    while not rospy.is_shutdown():
        hello_str = "Meqdad speaks %s" % ctr
        rospy.loginfo(hello_str)
        pub.publish(hello_str)
        rate.sleep()
        ctr+=1

if __name__ == '__main__':
    try:
        publisher()
    except rospy.ROSInterruptException:
        pass

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Subscriber

In this example, we assume the following settings:

• Subscriber node name: subscriber
• Topic name: session
• Message type: string

import rospy
from std_msgs.msg import String

def purpose_callback(message):
    rospy.loginfo(rospy.get_caller_id() + "\nStudent listens' %s", "\nReceived message: " + message.data)
    
def subscriber():
    rospy.init_node('subscriber', anonymous=True)

    rospy.Subscriber("session", String, purpose_callback)

    rospy.spin()

if __name__ == '__main__':
    subscriber()

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Explore Nodes and Topics

rosnode list

rostopic list

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RQT Graphical Represntation

You can draw a graphical representation for the running topics and connected nodes using rqt GUI plugin.

rosrun rqt_graph rqt_graph