Understanding ROS Nodes

1) First, please install this simulator:

  sudo apt-get install ros-hydro-ros-tutorials

Terminology:

Nodes: A node is an executable that uses ROS to communicate with other nodes.
Messages: ROS data type used when subscribing or publishing to a topic.
Topics: Nodes can publish messages to a topic as well as subscribe to a topic to receive messages.
Master: Name service for ROS (i.e. helps nodes find each other)
rosout: ROS equivalent of stdout/stderr
roscore: Master + rosout + parameter server (parameter server will be introduced later)

2) To use ROS, start by running the core:

  roscore

It will display a summary. If it fails to initialize, try changing the ownership of the root:

  sudo chown -R <your_username> ~/.ros

Then attempt to bring up roscore again.

3) In a new terminal (we will call this terminal 2) see what nodes are running:

  rosnode list

Likely, you will only see one called /rosout.

4) In terminal 2 try seeing more information about each node:

  rosnode info /rosout

This gives you information about the currently running nodes.

5) In a new terminal (terminal 3) we will initialize a new node.

To run a node, use rosrun [package_name] [node_name]:

  rosrun turtlesim turtlesim_node

A new window with a turtle will show up.

6) In terminal 2 (where we used rosnode list) run rosnode again:

  rosnode list

The nodes /rosout and /turtlesim now appear.

7) Close the turtle sim. In terminal 3 (where we had started the turtle sim) we will restart it again, but give the node a name:

  rosrun turtlesim turtlesim_node __name:=my_turtle

8) In terminal 2, look at the nodes again:

  rosnode list

You will see /rosout and /my_turtle. If not, try cleaning up the rosnode list:

  rosnode cleanup

Then try running the rosnode list command again.

9) In terminal 2, let us ping our simulation:

  rosnode ping my_turtle

This tests the simulation to see if it is in fact running. You will see time of replies printed to the command.

Navigation menu