cat < /dev/null > /dev/tcp/<hostname or ip>/<port>; echo $?

check open network port with cat

Trying to check for an open port and missing netcat or nmap? This is the lowest common denominator way to verify a port is accessible from one server to another. This will give you a pretty quick return of 0 if it works. If it fails, it will just hang and takes awhile to timeout. I usually ctrl+c the command. "echo ?$" will give you an exit code other then 0 after you exit.
Sample Output
cat < /dev/null > /dev/tcp/www.msn.com/89; echo $?
cat < /dev/null > /dev/tcp/8.8.8.8/53; echo $?

0
By: pborowicz
2018-02-14 15:51:51

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