Question

I’m wanting to test wich distant port is open to know if I have to connect with telnet VNC Teamviewer or whatever.

I’ll have about 10 ports to test, and I’m doing a script for it.
At this point I’ve come with this code:

function testPort(){
        res=`nc -v $1 $2 < /dev/null`
        echo $res
        if [[ "$res" == *refused* ]]
        then
                echo "refused"
                return 0
        else
                echo "accepted"
                return 1
        fi

}

if test -z "$1"
then
        echo "What's the adress?"
        read IP
else
        IP="$1"
fi

testPort $IP 80

The result of echo $res is something like:

nc: connect to 192.168.0.110 port 80 (tcp) failed: Connection refused
RFB 003.889 Connection to 192.168.0.110 5900 port [tcp/vnc-server] succeeded!

But in any case I got the “accepted” displayed. I can’t figure out why. Can someone explain me where’s my mistake?

Answer 1

It’s because netcat writes its message to standard error, not standard output. So, the variable res is empty, and doesn’t match *refused*.

The reason you see the netcat message on the console is not because of the echo $res line, but because you aren’t capturing standard error, so it’s going to the console.

If you change the first line of testPort to:

    res=`nc -v $1 $2 < /dev/null 2>&1`

It should work.