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lsof

Lists open files and the processes that have them open.

lsof ("list open files") inspects the kernel's per-process file-descriptor tables and reports every open file, socket, pipe, and device. Because everything in Linux is a file, lsof is the universal tool for answering "what is holding this resource?" — from "what is listening on port 443?" (lsof -i :443) to "what is using my unmounted filesystem?" (lsof /mnt).

It ships on every Linux distribution and is essential debugging kit for system administrators.

Debian/Ubuntu: sudo apt install lsof
Fedora:        sudo dnf install lsof
Arch:          sudo pacman -S lsof