ROS 2 Nodes 

A ROS 2 Node is a piece of software/executable that uses ROS 2 to communicate with other ROS 2 nodes.
ROS 2 Nodes are the building blocks of any ROS 2 application.
For example, in a wall-following robot, one ROS 2 node could retrieve distance sensor values while another node controls the robot’s motors. These nodes communicate to move the robot.
Although it’s possible to write the entire application in a single node, having multiple nodes ensures modularity and fault tolerance— if one node crashes, it won’t crash the entire application.
A ROS 2 package can contain multiple ROS 2 nodes.
ROS 2 Nodes can be written in Python or C++.

Creating a ROS 2 Node 

Steps to Create a Node in Python 

Navigate to your ROS 2 package (for example, pkg_ros_basics):

cd ~/ros2_ws/src/pkg_ros_basics

OR

ros2 pkg create pkg_ros_basics
cd ~/ros2_ws/src/pkg_ros_basics

Create a folder called scripts for your Python scripts and navigate into the folder.

mkdir scripts
cd scripts

Create a Python script called node_hello_ros.py.

touch node_hello_ros.py

Open the script in a text editor and start editing.

gedit node_hello_ros.py

The first line of all Python ROS 2 scripts should be the shebang.

#!/usr/bin/env python3

Now write a ROS 2 node that prints “Hello World!” to the console.

#!/usr/bin/env python3
import rclpy
from rclpy.node import Node

class HelloWorldNode(Node):
    def __init__(self):
        super().__init__('hello_world_node')
        self.get_logger().info('Hello World!')

def main(args=None):
    rclpy.init(args=args)
    node = HelloWorldNode()
    rclpy.spin(node)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

Make your node executable.

chmod +x node_hello_ros.py

To add your newly created script, in CMakeLists.txt file
- Open CmakeLists.txt in your package
- add the following piece of code as follows

install(PROGRAMS
    scripts/node_hello_ros.py
    DESTINATION lib/${PROJECT_NAME}
)

Now to run your node:
- Open a terminal and run:

ros2 run pkg_ros_basics node_hello_ros.py

Command 

Command: ros2 run 

ros2 run allows you to run an executable in an arbitrary package from anywhere without having to give its full path or cd there first.

Usage:

ros2 run <package> <executable>

<package> is the name of the package you created using the ros2 pkg create command.

<executable> is the Python or C++ file you want to run.

To create an executable Python file 

After creating a package, create a folder in the package called scripts to store all Python files in that folder.

cd ~/ros2_ws/src/<package>
mkdir scripts

Create Python scripts by running this command in the scripts directory:

cd scripts
touch filename.py

Edit your Python file, and make it executable before running:

chmod +x filename.py

Command: ros2 node 

ros2 node provides command-line tools for displaying and debugging ROS 2 nodes.
Use the following command to list all active ROS 2 nodes:

ros2 node list

This will print the names of all active nodes.

To get information about a specific node, including its publications and subscriptions:

ros2 node info <node_name>

To kill (terminate) a node:

ros2 node kill <node_name>

ROS 2 Nodes

Creating a ROS 2 Node

Steps to Create a Node in Python

Command

Command: ros2 run

To create an executable Python file

Command: ros2 node