can `ros2 topic list` poll periodically?

edit

It doesn't exist in ROS2. If you're on Linux, you should be able to use some utilities to do what you want:

watch ros2 topic list will show the current topics (by default every 2 seconds) but won't accumulate like you want.

Try this two-liner shell (bash or bash-like) script to gather all the topics into a file called topics.txt:

> topics.txt

while true; do ros2 topic list | sort -u -o topics.txt topics.txt -; sleep 1; done

You can try using the option --spin-time, which will cause ros2 topic list to wait for the specified number of seconds before returning. Note, it will only wait if a daemon is not already running. E.g. when running ros2 topic list --spin-time 3 consecutively, only the first call will wait 3 seconds and the others will return immediately since the daemon was started by the first call.

You can force the discovery time to happen by first stopping the daemon:

ros2 daemon stop
ros2 topic list --spin-time 3

I'm not sure this is quite the feature you are looking for though. ros2 topic list does not provide a way to periodically poll for topics.