How to check whether a port is occupied in LINUX

I have never been able to figure out whether the port is occupied. After asking many people, I finally figured it out. Now let me summarize:

1.netstat -anp |grep port number

As follows, I take 3306 as an example, netstat -anp |grep 3306 (note here, I am operating as a normal user, so I added sudo. If you are operating as a root user, you can view it without adding sudo), as shown in Figure 1:

In Figure 1, the monitoring status is LISTEN, which means it is occupied. The last column shows that it is occupied by the service mysqld. Check the specific port number. As long as there is a line like the one in the figure, it means it is occupied.

2.netstat -nultp (no need to add port number here)

This command is used to view all currently used ports, as shown in Figure 2:

As you can see from the picture, my port 82 is not occupied.

3.netstat -anp |grep 82 to check the usage of port 82, as shown in Figure 3:

It can be seen that there is no LISTEN line, so it means that it is not occupied. Note that the LISTENING in the figure does not mean that the port is occupied. Do not confuse it with LISTEN. When checking the specific port, you must see the line of tcp, port number, and LISTEN to indicate that the port is occupied.

View the port number occupied by the process in Linux

More information about reserved ports can be found in the /etc/services file on Linux.

You can use the following six methods to view port information.

• ss: can be used to dump socket statistics.
• netstat: can display a list of open sockets.
• lsof: can list open files.
• nmap: is a network detection tool and port scanner.
• systemctl: is the control manager and service manager of the systemd system.

Next we will find out the port number used by the sshd daemon.

Method 1: Using the ss command

[root@elk3 ~]# ss -tlnp | grep sshd

Method 2: Use the netstat command

[root@elk3 ~]# netstat -tnlp | grep ssh

Method 3: Using lsof command

You can also check using the port number.

[root@elk3 ~]# netstat -lntp | grep 860

Method 4: Using nmap command

[root@elk3 ~]# nmap -sV -p 22 localhost

In most cases, the above output will not show the actual port number of the process. At this time, it is recommended to use the following journalctl command to check the detailed information in the log file.

[root@elk3 ~]# journalctl | grep -i ssh

Dec 01 21:43:36 elk3 systemd[1]: Starting OpenSSH server daemon...

Dec 01 21:43:36 elk3 sshd[860]: Server listening on 0.0.0.0 port 22.

Dec 01 21:43:36 elk3 sshd[860]: Server listening on :: port 22.

Dec 01 21:43:36 elk3 systemd[1]: Started OpenSSH server daemon.

Dec 01 21:44:19 elk3 sshd[1106]: Accepted password for root from 192.168.122.1 port 49316 ssh2

Dec 01 21:44:19 elk3 sshd[1106]: pam_unix(sshd:session): session opened for user root by (uid=0)

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