Sometimes you want to write a script checking if the server is up. If the server ip is 172.17.0.3 and the process listens to the port 9200 you can run:
echo -e '\035\nquit' | telnet 172.17.0.3 9200 | grep Connected && echo "it's up" || echo "it's down"
It will print it's up in case of success and it's down in case of failure.
Another way using unix file descriptors:
echo > /dev/tcp/172.17.0.3/9200 && echo "it's up" || echo "it's down"