List Topics and Their Types

Start Turtlesim and teleop in sourced terminals:

ros2 run turtlesim turtlesim_node
ros2 run turtlesim turtle_teleop_key

In a third sourced terminal:

ros2 topic list

You should see topics including /turtle1/cmd_vel, /turtle1/pose, /rosout, and /parameter_events.

Ask for types too

The official beginner tutorial uses:

ros2 topic list -t

Now each topic is paired with a type, for example:

/turtle1/cmd_vel [geometry_msgs/msg/Twist]

Why the type matters

Publishers and subscribers need to agree on the data structure moving through the topic.

A topic name alone does not tell you whether the data represents velocity, an image, a string, or something else.

Build a topic inventory

Record three topics:

The guess column is intentionally a guess. The next commands will let you replace guesses with evidence.

Checkpoint

Which command answers:

"What typed topic paths are active right now?"

Use the exact command, not a prose description.