ROS2 Basics Exercise

Motivation

Our goal for this exercise is to go through the basic ROS2 commands to understand how they work. We will not be writing any code during this exercise, instead we will just run through a few commands in the shell.

Workspaces and packages

  1. Start by creating a new ROS2 workspace. As with catkin in ROS1, a workspace consists of a folder with a src/ subdirectory.

    mkdir -p ~/ros2_ws/src

  2. Clone the ROS2 demos examples repositories which we will use to run some examples. When building existing packages from source, it is important to verify the code version as the most recent version may have changes incompatible with the current release. Here we checkout the repositories to the eloquent release branch when cloned.

    cd ~/ros2_ws/src
git clone -b eloquent git@github.com:ros2/demos.git
git clone -b eloquent git@github.com:ros2/examples.git

  3. Install any required dependencies for the repositories using rosdep. This step is the same as in ROS1 since rosdep is installed as a system tool.

    cd ~/ros2_ws
rosdep install --ignore-src --from-paths src/

Building packages

ROS2 uses colcon as the build tool for ROS packages. Colcon is installed separately from ROS distributions as a pure Python package. It is always available in the user’s PATH without sourcing a workspace setup file.

  1. Run colcon -h to see a short help summary and a list of verbs that can be used. Run colcon build -h to see the help description for the build verb.

  2. Run colcon list and colcon info to see infomation about what packages are currently in the workspace. Colcon can be directed to ignore packages if a directory contains an empty file named COLCON_IGNORE.

  3. Build the workspace. This will require a setup script to be sourced so Ament can find the required dependent ROS packages. Check your .bashrc file and either change or remove any line that sources a ROS1 setup file at the end of the file. Remember if you change your .bashrc to start a terminal for it to take effect.

    source /opt/ros/eloquent/setup.bash
colcon build

    • Important: Unlike catkin tools, colcon looks for packages and will create its output folders wherever you run it. Be sure you are in the workspace root before running it.

    • Colcon does not currently have a clean verb. To rebuild a workspace from scratch, you must remove the generated outputs manually: rm -r build/ install/ log/

  4. Source the newly-built workspace. Inside the new install/ folder will be both setup.bash and local_setup.bash files. The setup.bash will configure a terminal environment to see the workspace packages as well as the environment the workspace was built in. local_setup.bash will add the workspace packages to the current environment.

    • If you keep your .bashrc to source /opt/ros/eloquent/setup.bash then you should only need to run source install/local_setup.bash

    • Otherwise, run source install/setup.bash

The ros2 command

ROS2 replaces the set of ros* command line tools with subcommands of a single ros2 command. Briefly we’ll run through some of ones that may be useful for a developer.

ros2 pkg

Shows information about ROS packages visible in the current environment

  • Use ros2 pkg list to see a list of all visible packages

  • Use ros2 pkg executables to see a list of executables made available by a package. E.g., ros2 pkg executables tf2_ros

  • Use ros2 pkg prefix to get the installation location of package

ros2 run

Runs executables provided by a package (ros2 run <package_name> <executable_file>)

  • Open two terminals sourced to the ros2_ws workspace.

  • In the first one, run ros2 run demo_nodes_cpp listener. Tab completion should be available so you don’t have to type the full package and executable names.

  • In the second, run ros2 run demo_nodes_cpp talker. You should now start to see a string message start to be printed repeatedly on both terminals. Leave both nodes running.

Note: You did not need to start a ROS master process before running nodes. The two nodes discovered each other on the network in a decentralized way as they started up.

ros2 node

Shows information about currently running ROS nodes.

  • Open a third terminal sourced to the ros_ws workspace.

  • Run ros2 node list to see a list of running nodes

  • Run ros2 node info /talker to see the topics and services the talker node is using

ros2 topic

Shows information about topics currently used by one or more nodes.

  • Run ros2 topic list to see currently published topics

  • Run ros2 topic info /chatter to see information about the topic the talker node is publishing

  • Run ros2 topic echo /chatter to locally subscribe to the /chatter topic and print it on the terminal

  • Run ros2 topic -h for a list of more available subcommands

ros2 service/srv

Shows information about ROS services.

  • Either stop the talker and listener nodes with Ctrl-C or open two more terminals.

  • In the first terminal, run ros2 run demo_nodes_cpp add_two_ints_server

  • In the second terminal, run ros2 service list -t to show currently available services and what type they are.

  • Run ros2 srv show example_interfaces/srv/AddTwoInts to see the service definition of the /add_two_ints service. Notice the names and types of the fields used in the service request.

  • Run ros2 service call /add_two_ints example_interfaces/srv/AddTwoInts '{a: 1, b: 2}' to manually call the service. The fields of the service request must be set using a YAML dictionary that specifies the field names and values.

ros2 launch

Start an installed launch file (ros2 launch <package_name> <launch_file>)

  • Close any currently running nodes

  • Run ros2 launch dummy_robot_bringup dummy_robot_bringup.launch.py. Again, tab completion should be able to help here. Notice the .py suffix on the launch file, it’s just a python script.

This starts five nodes that can inspected with the above command line tools. You might notice a /joint_states topic publishing changing values and a /scan topic publishing simulated 2D laser scanner data. Let’s visualize them.

RViz

RViz has been renamed to rviz2 and substantially rewritten for ROS2 but appears more-or-less the same from the user’s point-of-view.

  • In a free terminal, run rviz2 while the dummy robot launch file is still running.

  • Change the ‘Fixed Frame’ to ‘world’

  • Add a RobotModel display type. Expand the display options and set ‘Description Source’ to ‘Topic’ and ‘Description Topic’ to ‘/robot_description’. You should now see a robot moving on your screen.

  • Add a LaserScan display and have it display the ‘/scan’ topic. You should start to see 2D laser scan data appear as if the scanner were attached to the end of the robot. Getting intermittent transform errors is normal.