Netcat

https://www.digitalocean.com/community/tutorials/how-to-use-netcat-to-establish-and-test-tcp-and-udp-connections

nc -zvw 2 host.example.com 443
nc -w 1 -z -v host.example.com 443
[[ $(nc -w1 -z -v www.example.com 443 2>&1) == *"succeeded"* ]] && echo ok

server

nc -kl 8080

client

nc 10.x.x.x 8080