check availability of specific port in docker-entrypoint by using netcat (example for elasticsearch)

entrypoint.sh

#!/usr/bin/env bash

echo "Stalling for Elasticsearch..."
for i in {30..0}; do
    nc -q 1 elasticsearch 9200 2>/dev/null && break
    echo "Elasticsearch init process in progress..."
    sleep 1
done

if [ "$i" = 0 ]; then
    echo >&2 "Seems like Elasticsearch is not up and running! aborting..."
    exit 1
fi