[Ignition Edifice] Newbie misunderstanding gazebo topics ?

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Hi everyone,

[Ignition edifice, installed by binary on Ubuntu 20.04]

I'm trying to work through the tutorials in an attempt to understand the codebase. I may be misunderstanding how gazebo works - some pointers would be appreciated.

I've got a basic diff-drive robot example generally working. A Key Publisher and Triggered Publisher seem to work together - the model / simulation responds.

However, (see below), if I call

ign topic --list

multiple times, I seem to get different results.

If I then call something like this directly from the command line

ign topic -t "/cmd_vel" -m ignition.msgs.Twist -p "linear: {x:0.5}, angular: {z: 0.05}"

sometimes it works, but, more often, it does not. (The key publisher / triggered publisher combo in the simulation itself does seem to work all the time).

I was expecting the list of topics to remain the same (at least with nothing else happening). I'd expect the calls to this topic from the command line to have an effect all the time, or at least come back with some kind of failure message / timeout. This seems a bit nondeterministic.

Maybe as a newbie I'm just expecting the wrong thing. Pointers would be appreciated.

Otherwise, any steers on how to 'debug' what's going on, or not get into this state in the first place, would be helpful.

Thank you