How find out which process is using a file in Linux?

You can use the fuser command, like:

fuser file_name

You will receive a list of processes using the file.

You can use different flags with it, in order to receive a more detailed output.

You can find more info in the fuser's Wikipedia article, or in the man pages.

@jim's answer is correct -- fuser is what you want.

Additionally (or alternately), you can use lsof to get more information including the username, in case you need permission (without having to run an additional command) to kill the process. (THough of course, if killing the process is what you want, fuser can do that with its -k option. You can have fuser use other signals with the -s option -- check the man page for details.)

For example, with a tail -F /etc/passwd running in one window:

ghoti@pc:~$ lsof | grep passwd
tail      12470    ghoti    3r      REG  251,0     2037 51515911 /etc/passwd

Note that you can also use lsof to find out what processes are using particular sockets. An excellent tool to have in your arsenal.