Efficiently test if a port is open on Linux?

Question

From a bash script how can I quickly find out whether a port 445 is open/listening on a server.

I have tried a couple of options, but I want something q

Answer 1

You can use netcat for this.

nc ip port < /dev/null

connects to the server and directly closes the connection again. If netcat is not able to connect, it returns a non-zero exit code. The exit code is stored in the variable $?. As an example,

nc ip port < /dev/null; echo $?

will return 0 if and only if netcat could successfully connect to the port.

Answer 2

I wanted to check if a port is open on one of our linux test servers. I was able to do that by trying to connect with telnet from my dev machine to the test server. On you dev machine try to run:

$ telnet test2.host.com 8080
Trying 05.066.137.184...
Connected to test2.host.com

In this example I want to check if port 8080 is open on host test2.host.com