In this article we will go over few options to check if a port is open in Linux machine

Using lsop

Command : sudo lsof -i -P -n | grep LISTEN
Output :

systemd-r    671    systemd-resolve   13u  IPv4    38126      0t0  TCP 127.0.0.53:53 (LISTEN)
mongod      1207            mongodb   11u  IPv4    52227      0t0  TCP *:27017 (LISTEN)
container   1318               root    8u  IPv4    46101      0t0  TCP 127.0.0.1:32933 (LISTEN)
dockerd     1599               root    3u  IPv6    49803      0t0  TCP *:2375 (LISTEN)
postgres    1624           postgres    5u  IPv4    47429      0t0  TCP *:5432 (LISTEN)
postgres    1624           postgres    6u  IPv6    47430      0t0  TCP *:5432 (LISTEN)
java        2202            jenkins  163u  IPv6    56411      0t0  TCP *:8080 (LISTEN)

Using netstat

Command : netstat -nlt
Output :

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:5432            0.0.0.0:*               LISTEN
tcp        0      0 127.0.0.1:32933         0.0.0.0:*               LISTEN
tcp        0      0 0.0.0.0:27017           0.0.0.0:*               LISTEN
tcp6       0      0 :::8083                 :::*                    LISTEN

Using nmap

Command : sudo nmap -sT -O localhost
Output :

Starting Nmap 7.60 ( https://nmap.org ) at 2020-06-21 21:38 EDT
Nmap scan report for localhost (127.0.0.1)
Host is up (0.000062s latency).
Not shown: 992 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
631/tcp  open  ipp
5432/tcp open  postgresql
8080/tcp open  http-proxy
8081/tcp open  blackice-icecap
8082/tcp open  blackice-alerts
8083/tcp open  us-srv
8088/tcp open  radan-http