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

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Thu Apr 29 2021 06:18:24 GMT+0000 (Coordinated Universal Time)

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sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo ss -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here
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https://www.cyberciti.biz/faq/unix-linux-check-if-port-is-in-use-command/