Check if Postgresql is listening

Question

Given an IP Address and port number, is it possible to check if the machine with that IP address has Postgresql listening on the specified port? If so, how?

I just want

Answer 1

You can use, for example, nmap tool:

=$ sudo nmap -v -p 5930 127.0.0.1

Starting Nmap 6.00 ( http://nmap.org ) at 2013-06-25 19:28 CEST
Initiating SYN Stealth Scan at 19:28
Scanning localhost (127.0.0.1) [1 port]
Discovered open port 5930/tcp on 127.0.0.1
Completed SYN Stealth Scan at 19:28, 0.03s elapsed (1 total ports)
Nmap scan report for localhost (127.0.0.1)
Host is up (0.000045s latency).
PORT     STATE SERVICE
5930/tcp open  unknown

Read data files from: /usr/bin/../share/nmap
Nmap done: 1 IP address (1 host up) scanned in 0.08 seconds
           Raw packets sent: 1 (44B) | Rcvd: 2 (88B)

Alternatively you can just "SELECT 1" with psql, and check output:

=$ psql -h 127.0.0.1 -p 5930 -c "select 1"
 ?column? 
----------
        1
(1 row)

=$ psql -h 127.0.0.1 -p 5940 -c "select 1"
psql: could not connect to server: Connection refused
        Is the server running on host "127.0.0.1" and accepting
        TCP/IP connections on port 5940?