How to find out whether remote port is open using network redirections

Find out whether remote port is open using Bourne Again SHell network redirections.

Examine whether port 22/TCP is open on 127.0.0.1.

$ PROTO="tcp";DST_ADDR="127.0.0.1";DST_PORT=22;(</dev/${PROTO}/${DST_ADDR}/${DST_PORT} && STATUS="OPEN" || STATUS="CLOSED OR UNAVAILABLE"; echo Port ${DST_PORT}/${PROTO^^} on ${DST_ADDR} is ${STATUS} ) 2>/dev/null

Port 22/TCP on 127.0.0.1 is OPEN

It is open, so read from the TCP socket using Bourne Again SHell.

$ timeout 1 cat </dev/tcp/127.0.0.1/22

SSH-2.0-OpenSSH_7.9p1 Debian-10+deb10u2

Find out whether port 53/UDP is open on 10.0.0.1.

$ PROTO="udp";DST_ADDR="10.1.0.1";DST_PORT=53;(</dev/${PROTO}/${DST_ADDR}/${DST_PORT} && STATUS="OPEN" || STATUS="CLOSED OR UNAVAILABLE"; echo Port ${DST_PORT}/${PROTO^^} on ${DST_ADDR} is ${STATUS} ) 2>/dev/null

Port 53/UDP on 10.1.0.1 is CLOSED OR UNAVAILABLE