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List Open Files: lsof and fuser
The function of these commands is very similar. The goal is to determine what processes have certain files open. lsof is freeware and thus freely compilable under all Unixes. It is available by default under Linux only.

The main difference between lsof and fuser is that lsof takes both files/filesystems and PIDs as arguments whereas fuser only accepts files/filesystems.

lsof:
lsof stands for list open files. It lists information about files that are currently open by processes.
In the absence of any options, lsof lists all open files belonging to all active processes

To list all open files for login name ``abe'', or user ID 1234 you would use the -u option. To list files being used by process 456, process 123, or process 789, you would use the -p flag. Putting these criteria together, we would have a command that looks like this:

lsof -p 456,123,789 -u 1234,abe

The command to list all open files on device /dev/hd4, would look like this:

lsof /dev/hd4

To find the process that has /u/abe/foo open, use:

lsof /u/abe/foo

fuser:
fuser is more widely available (under AIX, HP-UX, IRIX, Linux, Solaris, Tru64 UNIX, and others) and can only be run as root. It displays the process ID numbers 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 (these codes differ from vendor to vendor):
a if the process is using the file as its trace file in /proc (IRIX)
c if the process is using the file as its current directory
e if the process is using the file as the executable being run
f if the process is using the file as an open file (Linux)
m if the process is using the file as a mmaped or shared lib (Linux)
o if the process is using the file as an open file (IRIX, Solaris, Tru64 UNIX))
p if the process is using the file as the parent of its current directory (Tru64 UNIX)
r if the process is using the file as root directory
s if the process is using the file as a shared lib (AIX)
t if the process is using the file as its text file (Solaris)
y if the process is using the file as its controlling terminal (IRIX)