How to write Publisher Subscriber in ROS Noetic | Step by Step instructions

ROS1 Noetic -Publisher and Subscriber Example

This post tells about the implementation of a publisher and a subscriber in ROS Noetic.  

In this example, we have considered the following:

Example 1: 

One publisher (pub.py) and one Subscriber (Sub.py)

Topic: first_lecture

Message: String Message 

Example 2:

Two publishers (pub.py and pub1.py) and

Two subscribers (sub.py and sub1.py)

Topic: first_lecture 

See the following video for step by step instructions for running this example:

The following picture shows the nodes (The picture is taken by running the rqt_graph command)

Publisher Subscriber in ROS Noetic

How to run in ROS Noetic

Prerequsities 

1. Ubuntu Basic commands

2. PATH Setting in Ubuntu

3. Basic knowledge of Linux 

4. Programming fundamentals

To Install ROS1 Noetic in Ubuntu 20.04, you can refer the following video in my channel.

ROS is enabled with a catkin make system which perform the easier build operation and cross compilation enablement.

Create a workspace to store all our files.

/home/pradeepkumar/pradeep_ws 

To create a workspace:

Open a terminal 

$ mkdir -p pradeep_ws/src

$ cd pradeep_ws/

$ catkin_make 

Then set the path 

Copy the following line in the /home/pradeepkumar/.bashrc file 

source /home/pradeepkumar/pradeep_ws/devel/setup.bash

]alternative way to set the environment, but for every terminal to be opened, we need to do the following step.

$ source $HOME/pradeep_ws/devel/setup.bash

To create a Catkin package, here is the command

$ cd pradeep_ws/src

$ catkin_create_pkg ros_tutorial1 rospy std_msgs

$ cd pradeep_ws/src/ros_tutorial1

$ mkdir script

create the pub.py, pub1.py, sub.py and sub1.py (You can download the source code from https://github.com/tspradeepkumar/ROSNoteic 

$ chmod 777 *.py

ROS Noetic runs always with a master that means it should run roscore to run all the ros applications.

You need to open 5 Terminals and give the commands in each of the terminals

• $ roscore
• $ rosrun ros_tutorial1 pub.py
• $ rosrun ros_tutorial1 pub1.py
• $ rosrun ros_tutorial1 sub.py
• $ rosrun ros_tutorial1 sub1.py

Open a terminal and run the command roscore

$ roscore

Open another terminal for publisher  and run the following command 

$ rosrun ros_tutorial1 pub.py

You will get the output as ROS Noetic World

If ros_tutorial1 is not found, then set the following command and try again 

$ source $HOME/pradeep_ws/devel/setup.bash

Open one more ternminal for the subscriber and run the following command 

$ rosrun ros_tutorial1 sub.py

You will get the output as I learn about ROS Noetic World. 

Run the following command to know about the graph 

$ rqt_graph 

You can see the following graph (You need to untick the Debug checkbox, to see the complete nodes running in ROS Noetic) 

ROS Noetic Pub Sub