Finding out which process is using a port on Linux

The simplest way to find out which process is using a port on Linux is to use the lsof command:

lsof -i :80

(-i tells lsof to list only files matched by the specified Internet address)

Example output (with nginx running on port 80):

COMMAND  PID     USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
nginx   1074     root    6u  IPv4  12011      0t0  TCP *:http (LISTEN)
nginx   1074     root    7u  IPv6  12012      0t0  TCP *:http (LISTEN)
nginx   1075 www-data    6u  IPv4  12011      0t0  TCP *:http (LISTEN)
nginx   1075 www-data    7u  IPv6  12012      0t0  TCP *:http (LISTEN)
nginx   1076 www-data    6u  IPv4  12011      0t0  TCP *:http (LISTEN)
nginx   1076 www-data    7u  IPv6  12012      0t0  TCP *:http (LISTEN)
nginx   1077 www-data    6u  IPv4  12011      0t0  TCP *:http (LISTEN)
nginx   1077 www-data    7u  IPv6  12012      0t0  TCP *:http (LISTEN)
nginx   1078 www-data    6u  IPv4  12011      0t0  TCP *:http (LISTEN)
nginx   1078 www-data    7u  IPv6  12012      0t0  TCP *:http (LISTEN)

If no matches are found, you'll get empty output with return code 1 (useful for scripting):

lsof -i :81 || echo "Nothing found."

Alternatively, you could filter the output from lsof -P, but this would require way more resources.

If for some reason you don't have losf already installed, you can install it with:

# deb-based (Debian, Ubuntu, Mint, Raspbian, Kali, etc.)
apt install lsof

# rpm-based (Fedora, CentOS, etc.)
dnf install lsof

# Arch
pacman -S lsof

# Alpine
apk add lsof

Alternatives

Alternatives you could use instead of lsof are fuser and netstat (I'll add more examples on this later).