ROS: First Node

let’s start making the “hello world” example. In this blog we are going to make a Node. very Simple Node. It wont be doing any interactive thing but will provide the good understanding about Node. First we will make Node using C++ language.

Procedure of making the Node (c++)

in Package_name/src folder add new document** by right click and rename it **node_name.cpp

write code

mention the file into Cmakelists.txt

build package/workspace “catkin_make”

node is ready to use

1.) blank document

in beginner_tutorials/src folder add new document** by right click and rename it **hello_world.cpp

2.) write code

#include "ros/ros.h"
/**
 * This tutorial demonstrates simplest node running on the ROS system.
 */
int main(int argc, char **argv)
{
  ros::init(argc, argv, "talker");
  ros::NodeHandle n;
  ros::Rate loop_rate(10);
  while (ros::ok())
  {
    ROS_INFO("hello world");

    loop_rate.sleep();
  }
  return 0;
}

This Node prints the info”Hello world” On the terminal window. Now let’s understand the first code.

#include "ros/ros.h" //must required header file for every ROS node

main loop starts

  ros::init(argc, argv, "first_node");          //node name is "first_node"
  ros::NodeHandle n;

[ These two lines you will be writing in each code of node. Just keep in mind that they initializes the node with the provided name and assigns node handle for the internal requirements of ROS environment.]

 ros::Rate loop_rate(10);                    //node update frequncy 10 Hz

[code will be executed with the prescribed frequency]

 while (ros::ok())                          // continuous loop
  {
    ROS_INFO("hello world");            //info displayed

    loop_rate.sleep();                      //wait
  }

[loop continuously]

The five different verbosity levels are, in order:

DEBUG
INFO
WARN
ERROR
FATAL

as per the need to display the state, verbosity level can be chosen.

//Note: [ after saving the file right click on document Go to the permissions tab, then tick the box `Execute: [ ] Allow executing file as program.]

3.) Cmakelists.txt editing

Now we have the file “hello_world.cpp” open file Cmakelists.txt and add these two lines

add_executable(hello_world src/hello_world.cpp)

target_link_libraries(hello_world ${catkin_LIBRARIES})

4.) Building the Package

In new terminal(Ctrl + Alt +T),run the following command

$ cd ~/catkin_ws

$ catkin_make

5.) Running the Node

$ roscore

in new terminal

$ rosrun beginner_tutorials hello_world

You will see terminal same as below. Screenshot from 2017-04-11 20:07:38 you can press Ctrl + C to stop the Node yeah! we just created and run the node. Now let’s learn something that will be very helpful in future to understand and the debug the current ROS system.

$ rostopic list

[It will list all the topics currently available on ROS system]

$ rosnode list