wait-for-server-start.sh

#!/bin/bash

echo "Waiting for servers to start..."
while true; do
  curl -f http://localhost:8000/health > /dev/null 2> /dev/null
  if [ $? = 0 ]; then
    echo "Frontend started"

    curl -f http://localhost:3000/health > /dev/null 2> /dev/null
    if [ $? = 0 ]; then
      echo "Backend started"
      break
    fi
  fi

  sleep 10
  echo -n .
done

What should be echo'd / returned if a service has failed. E.G. it's has started, but doesn't function correctly (based on 'bad' response, like a null when some endpoint should return data)?