HOWTO: Command-line test for whether or not a TCP port is open

Usually I just use `telnet <host_or_ip> <port>' to see if a port is open, but this doesn't lend itself to automation. One way to accomplish this automatically is

 nc -z <host_or_ip> <port> 1>/dev/null 2>&1; result=$?;
 if [ $result -eq 0 ]; then
     echo 'the port is open for tcp connections'
 else
     echo 'the port was closed'
 fi

If the port is open, the exit status will be `0', otherwise it will be `1'.

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