lsof stands for “list of open files.”
Using this command, thus, gives a list of all the open files.
Often crazy, ridiculously-named things are being used by programs and show up
in top, so using lsof can show where those files
originate.
$ lsof
lsof 5982 root mem REG 8,18 2981280 50856367 /usr/lib/locale/locale-archive
lsof 5982 root mem REG 8,18 138696 4981417 /lib/x86_64-linux-gnu/libpthread-2.23.so
lsof 5982 root mem REG 8,18 14608 4981420 /lib/x86_64-linux-gnu/libdl-2.23.so
You can also specify usernames with lsof -u username.