How to check programs and listening ports in Linux

Checking method: 1. Use the lsof command to check, the syntax is "sudo lsof -i -P -n | grep LISTEN" or "doas lsof -i -P -n | grep LISTEN"; 2. Use the netstat command To check the listening port and program, the syntax is "netstat -tulpn | grep LISTEN"; 3. Use the nmap command to check, the syntax is "sudo nmap -sT -O localhost".

#The operating environment of this tutorial: linux7.3 system, Dell G3 computer.

How to check the programs and listening ports in Linux

1. Open a terminal, such as a shell command window.

2. Run any of the following commands:

sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo nmap -sTU -O IP地址

Method 1: lsof command

The syntax is as follows:

$ sudo lsof -i -P -n
$ sudo lsof -i -P -n | grep LISTEN
$ doas lsof -i -P -n | grep LISTEN  ### OpenBSD

The output is as follows :

Look carefully at the last line of the above output:

sshd    85379     root    3u  IPv4 0xffff80000039e000      0t0  TCP 10.86.128.138:22 (LISTEN)

• sshd is a program The name

• 10.86.128.138 is the IP address of the sshd program binding (LISTEN)

• 22 is the TCP port being used (LISTEN)

• ##85379 is the process ID (PID) of the sshd task

Method 2: netstat command

You can use netstat as shown below to check the listening ports and programs.

Netstat syntax in Linux

$ netstat -tulpn | grep LISTEN

netstat syntax in FreeBSD/MacOS X

$ netstat -anp tcp | grep LISTEN
$ netstat -anp udp | grep LISTEN

netstat syntax in OpenBSD

$ netstat -na -f inet | grep LISTEN
$ netstat -nat | grep LISTEN

This command can also be used in windows ：

netstat -bano | more
netstat -bano | grep LISTENING
netstat -bano | findstr /R /C:"[LISTING]"

Method 3: nmap command

The syntax is as follows:

$ sudo nmap -sT -O localhost
$ sudo nmap -sU -O 192.168.2.13 ### 列出打开的 UDP 端口
$ sudo nmap -sT -O 192.168.2.13 ### 列出打开的 TCP 端口

The sample output is as follows:

You can merge TCP/UDP scanning with one command:

$ sudo nmap -sTU -O 192.168.2.13

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