Node

Nodes are processes:

  • used for calculations,
  • that run inside robotic application,
  • grouped into packages,
  • that communicate with each other (topics, servers, actions, parameter servers).

Why to use nodes:

  • reduce code complexity,
  • the code is more error-resistant,
  • use of different programming languages.

New node

Nodes are scripts that are located inside scripts folder. 

roscd rpi_test
mkdir scripts
cd scripts
Create new script my_first_node.py: 
touch my_first_node.py

Change the access permissions of the file to executable: 

chmod +x my_first_node.py

Open script with Visual Studio Code: 

code my_first_node.py

The minimal working example for node:

my_fist_node.py
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#!/usr/bin/env python

import rospy

if __name__ == '__main__':
  rospy.init_node('my_first_python_node')
  rospy.loginfo('This node has been started.')
  rospy.sleep(1)
  print('Exit now')

Only one node with specific name can be run at a time. If you want to run more instances of the same node, change: 

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  rospy.init_node('my_first_python_node', anonymous=True)

To run, write 

python my_first_node.py

New node - class

my_fist_node_class.py
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#!/usr/bin/env python

import rospy

class hello_world():

  def __init__(self):
    # init variables
    self.delay = 5
    self.ctrl_c = False
    rospy.on_shutdown(self.shutdownhook)

  def shutdownhook(self):
    # works better than the rospy.is_shutdown()
    # this code is run at ctrl + c
    rospy.loginfo('This node has been terminated.')
    self.ctrl_c = True

  def test_node(self): 
    rospy.loginfo('This node has been started.')
    rospy.sleep(self.delay)
    print('Exit now')

if __name__ == '__main__':
  # initialise node
  rospy.init_node('my_first_python_node')
  # initialise class
  first_node = hello_world()
  try:
    first_node.test_node()
  except rospy.ROSInterruptException:
    pass

DEBUG

  • rosrun <pkg name> <node name> - run specific node
  • rosnode list - list of all active nodes
  • rosnode info <node name> - information about hte node
  • rosnode kill <node name> - shut down node
  • rosnode ping <node name> - ping node (check, if it is working)