You're reading the documentation for a version of ROS 2 that has reached its EOL (end-of-life), and is no longer officially supported. If you want up-to-date information, please have a look at Jazzy.

Introspection with command line tools

ROS 2 includes a suite of command-line tools for introspecting a ROS 2 system.


The main entry point for the tools is the command ros2, which itself has various sub-commands for introspecting and working with nodes, topics, services, and more.

To see all available sub-commands run:

ros2 --help

Examples of sub-commands that are available include:

  • daemon: Introspect/configure the ROS 2 daemon

  • launch: Run a launch file

  • lifecycle: Introspect/manage nodes with managed lifecycles

  • msg: Introspect msg types

  • node: Introspect ROS nodes

  • param: Introspect/configure parameters on a node

  • pkg: Introspect ROS packages

  • run: Run ROS nodes

  • security: Configure security settings

  • service: Introspect/call ROS services

  • srv: Introspect srv types

  • topic: Introspect/publish ROS topics


To produce the typical talker-listener example using command-line tools, the topic sub-command can be used to publish and echo messages on a topic.

Publish messages in one terminal with:

$ ros2 topic pub /chatter std_msgs/String "data: Hello world"
publisher: beginning loop
publishing std_msgs.msg.String(data='Hello world')

publishing std_msgs.msg.String(data='Hello world')

Echo messages received in another terminal with:

$ ros2 topic echo /chatter
data: Hello world

data: Hello world

Behind the scenes

ROS 2 uses a distributed discovery process for nodes to connect to each other. As this process purposefully does not use a centralized discovery mechanism (like the ROS Master in ROS 1), it can take time for ROS nodes to discover all other participants in the ROS graph. Because of this, there is a long-running daemon in the background that stores information about the ROS graph to provide faster responses to queries, e.g. the list of node names.

The daemon is automatically started when the relevant command-line tools are used for the first time. You can run ros2 daemon --help for more options for interacting with the daemon.


The source code for the ros2 command is available at https://github.com/ros2/ros2cli

The ros2 tool has been implemented as a framework that can be extended via plugins. For example, the sros2 package provides a security sub-command that is automatically detected by the ros2 tool if the sros2 package is installed.