At last the shell is running satisfyingly. Finishing it, I had two last problems. The first one is because Ubuntu finishes its lines with a lone line feed whereas the ChibiOS shell expects a carriage return before it. When it does not get it, it does not reply.
The second problem was that the shell not only returns feedback on the command, it also echoes the command itself. Which is then printed a second time to the user. I did not find another way around this that simply comparing the response to the answer and only print the different part. This may lead to some concurrency issues since the thread receiving the responses is different than the one sending them but even if we sometimes cannot wait to send a new command, at worst we will get the first command printed twice instead of once which is not very important.
There is also an older problem that I could not solve. When the client try connecting to the ESP32, it often fails on the call of the “connect” function from the standard socket API with “Network unreachable” error. I still do not know the reason.