We’ve all been there: you want to umount a certain device and you’re being told the resource is still being used. Maybe you forgot to close the Nautilus window or maybe you are still accessing the device from the command line. To find out for sure, use fuser.
If, for example, you can’t umount your iPod, use
fuser -v -m /media/ipod
The output will be a list of processes and users currently accessing the device. Find the ID of the corresponding process and kill it with
kill -9 PID
Fuser can also tell us what processes currently access certain network ports:
fuser -v -n tcp 21
This will tell us what applications are using the FTP port.