Understanding ROS Nodes

This tutorial introduces ROS graph concepts and discusses the use of roscore, rosnode, and rosrun commandline tools.

Quick Overview of Graph Concepts

  • 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)


A node really isn't much more than an executable file within a ROS package. ROS nodes use a ROS client library to communicate with other nodes. Nodes can publish or subscribe to a Topic. Nodes can also provide or use a Service.

Client Libraries

ROS client libraries allow nodes written in different programming languages to communicate:

  • rospy = python client library
  • roscpp = c++ client library


roscore is the first thing you should run when using ROS.

Please run:

... logging to /root/.ros/log/e8b674a4-bff3-11d3-af73-4ed34f3752c7/roslaunch-erlerobot-2990.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

started roslaunch server http://erlerobot:59324/
ros_comm version 1.9.44


 * /rosdistro
 * /rosversion


auto-starting new master
process[master]: started with pid [3003]

setting /run_id to e8b674a4-bff3-11d3-af73-4ed34f3752c7
process[rosout-1]: started with pid [3016]
started core service [/rosout]


rosnode displays information about the ROS nodes that are currently running. The rosnode list command lists these active nodes:

[email protected]:~# rosnode list

This showed us that there is only one node running: rosout. This is always running as it collects and logs nodes' debugging output.

The rosnode info command returns information about a specific node.

[email protected]:~#  rosnode info /rosout
Node [/rosout]
 * /rosout_agg [rosgraph_msgs/Log]

 * /rosout [unknown type]

 * /rosout/set_logger_level
 * /rosout/get_loggers

contacting node http://erlerobot:49274/ ...
Pid: 3016

Now, let's see some more nodes. For this, we're going to use rosrun to bring up another node.


rosrun allows you to use the package name to directly run a node within a package (without having to know the package path).


$ rosrun [package_name] [node_name]

In a new terminal:

[email protected]:~# rosrun autopilot_bridge mavlink.py --device /dev/ttyO4 --baudrate 115200
Starting mavlink <-> ROS interface over the following link:
  device:        /dev/ttyO4
  baudrate:        115200

Waiting for AP heartbeat

In another terminal:

[email protected]:~# rosnode list

One powerful feature of ROS is that you can reassign Names from the command-line.

Go back to the rosrun autopilot_bridge terminal and use ctrl-C (there seems to be a problem with this node so you might need to open a new terminal). Now let's re-run it, but this time use a Remapping Argument to change the node's name:

[email protected]:~# rosrun autopilot_bridge mavlink.py --device /dev/ttyO4 --baudrate 115200 __name:=my_autopilot
Starting mavlink <-> ROS interface over the following link:
  device:        /dev/ttyO4
  baudrate:        115200

Waiting for AP heartbeat

Now, if we go back and use rosnode list:

[email protected]:~# rosnode list

Let's use another rosnode command, ping, to test that it's up:

[email protected]:~# rosnode ping my_autopilot
rosnode: node is [/my_autopilot]
pinging /my_autopilot with a timeout of 3.0s
xmlrpc reply from http://erlerobot:39540/    time=15.213966ms
xmlrpc reply from http://erlerobot:39540/    time=13.329983ms
xmlrpc reply from http://erlerobot:39540/    time=12.695074ms
xmlrpc reply from http://erlerobot:39540/    time=13.059974ms
xmlrpc reply from http://erlerobot:39540/    time=12.760997ms
xmlrpc reply from http://erlerobot:39540/    time=12.681007ms
^Cping average: 13.290167ms


What was covered:

  • roscore = ros+core : master (provides name service for ROS) + rosout (stdout/stderr) + parameter server (parameter server will be introduced later)
  • rosnode = ros+node : ROS tool to get information about a node.
  • rosrun = ros+run : runs a node from a given package. Now that you understand how ROS nodes work, let's look at how ROS topics work. Also, feel free to press Ctrl-C to stop turtlesim_node.

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