Command to find users or processes using a specified file

fuser is a UNIX command used to show which processes,users are using a specified file, file system, or socket.

For example:

# fuser -m -u /mnt/usb1
/mnt/usb1: 1347c(root) 1348c(guido) 1349c(guido)

fuser displays the PIDs of processes using the specified files or file systems.
In the default display mode, each file name is followed by a letter denoting the type of access:

c
current directory.
e
executable being run.
f
open file.
F
open file for writing.
r
root directory.
m
mapped file or shared library