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How to test if remote TCP port is opened from Shell script?

nc will do the job. A more compact way to use it: 
nc -z <host> <port>
That way nc will only check if the port is open, exiting with 0 on success, 1 on failure.
For a quick interactive check (with a 5 seconds timeout):
nc -z -w5 <host> <port>; echo $?
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