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Understanding ROS 2 parameters

Goal: Learn how to get, set, save and reload parameters in ROS 2.

Tutorial level: Beginner

Time: 5 minutes

Contents

Background
Prerequisites
Tasks
Summary
Next steps

Background 

A parameter is a configuration value of a node. You can think of parameters as node settings. A node can store parameters as integers, floats, booleans, strings and lists. In ROS 2, each node maintains its own parameters. All parameters are dynamically reconfigurable, and built off of ROS 2 services.

Prerequisites 

This tutorial uses the turtlesim package.

As always, don’t forget to source ROS 2 in every new terminal you open.

Tasks 

1 Setup 

Start up the two turtlesim nodes, /turtlesim and /teleop_turtle.

Open a new terminal and run:

ros2 run turtlesim turtlesim_node

Open another terminal and run:

ros2 run turtlesim turtle_teleop_key

2 ros2 param list 

To see the parameters belonging to your nodes, open a new terminal and enter the command:

ros2 param list

You will see the node namespaces, /teleop_turtle and /turtlesim, followed by each node’s parameters:

/teleop_turtle:
  scale_angular
  scale_linear
  use_sim_time
/turtlesim:
  background_b
  background_g
  background_r
  use_sim_time

Every node has the parameter use_sim_time; it’s not unique to turtlesim.

Based on their names, it looks like /turtlesim’s parameters determine the background color of the turtlesim window using RGB color values.

To determine a parameter’s type, you can use ros2 param get.

3 ros2 param get 

To display the type and current value of a parameter, use the command:

ros2 param get <node_name> <parameter_name>

Let’s find out the current value of /turtlesim’s parameter background_g:

ros2 param get /turtlesim background_g

Which will return the value:

Integer value is: 86

Now you know background_g holds an integer value.

If you run the same command on background_r and background_b, you will get the values 69 and 255, respectively.

4 ros2 param set 

To change a parameter’s value at runtime, use the command:

ros2 param set <node_name> <parameter_name> <value>

Let’s change /turtlesim’s background color:

ros2 param set /turtlesim background_r 150

Your terminal should return the message:

Set parameter successful

And the background of your turtlesim window should change colors:

Setting parameters with the set command will only change them in your current session, not permanently. However, you can save your settings and reload them the next time you start a node.

5 ros2 param dump 

You can “dump” all of a node’s current parameter values into a file to save them for later by using the command:

ros2 param dump <node_name>

To save your current configuration of /turtlesim’s parameters, enter the command:

ros2 param dump /turtlesim

Your terminal will return the message:

Saving to:  ./turtlesim.yaml

You will find a new file in the directory your workspace is running in. If you open this file, you’ll see the following contents:

turtlesim:
  ros__parameters:
    background_b: 255
    background_g: 86
    background_r: 150
    use_sim_time: false

Dumping parameters comes in handy if you want to reload the node with the same parameters in the future.

6 ros2 param load 

You can load parameters from a file to a currently running node using the command:

ros2 param load <node_name> <parameter_file>

To load the ./turtlesim.yaml file generated with ros2 param dump into /turtlesim node’s parameters, enter the command:

ros2 param load /turtlesim ./turtlesim.yaml

Your terminal will return the message:

Set parameter background_b successful
Set parameter background_g successful
Set parameter background_r successful
Set parameter use_sim_time successful

7 Load parameter file on node startup 

To start the same node using your saved parameter values, use:

ros2 run <package_name> <executable_name> --ros-args --params-file <file_name>

This is the same command you always use to start turtlesim, with the added flags --ros-args and --params-file, followed by the file you want to load.

Stop your running turtlesim node so you can try reloading it with your saved parameters, using:

ros2 run turtlesim turtlesim_node --ros-args --params-file ./turtlesim.yaml

The turtlesim window should appear as usual, but with the purple background you set earlier.

Summary 

Nodes have parameters to define their default configuration values. You can get and set parameter values from the command line. You can also save the parameter settings to a file to reload them in a future session.

Next steps 

Jumping back to ROS 2 communication methods, in the next tutorial you’ll learn about actions.