How to check if port is in use on Linux or Unix

sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here

Option #1: lsof command
$ sudo lsof -i -P -n
$ sudo lsof -i -P -n | grep LISTEN
$ doas lsof -i -P -n | grep LISTEN ### [OpenBSD] ###

Linux netstat syntax
netstat -tulpn | grep LISTEN
watch netstat -tulpn
sudo ss -tulw
sudo ss -tulwn


netstat -tulpn |grep tcp |grep -v 127.0.0.1 |grep -v 192.168.101.36
-v is invert matching (every thing except 127.0.0.1 and 192.168.101.36)

Where ss command options are as follows:

• -t : Show only TCP sockets on Linux
• -u : Display only UDP sockets on Linux
• -l : Show listening sockets. For example, TCP port 22 is opened by SSHD server.
• -p : List process name that opened sockets
• -n : Don’t resolve service names i.e. don’t use DNS

FreeBSD/MacOS X netstat syntax

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

OpenBSD netstat syntax

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

Option #3: nmap command

The syntax is:
$ sudo nmap -sT -O localhost
$ sudo nmap -sU -O 192.168.2.13 ##[ list open UDP ports ]##
$ sudo nmap -sT -O 192.168.2.13 ##[ list open TCP ports ]##

You can combine TCP/UDP scan in a single command:
$ sudo nmap -sTU -O 192.168.2.13

A note about Windows users

You can check port usage from Windows operating system using following command:
netstat -bano | more
netstat -bano | grep LISTENING
netstat -bano | findstr /R /C:"[LISTEING]"

Ref:
unix-linux-check-if-port-is-in-use-command