Fuser (Unix)

The Unix command fuser is used to show which processes are using a specified computer file, file system, or Unix socket.

For example, to check process IDs and users accessing a USB drive:

The command displays the process identifiers (PIDs) of processes using the specified files or file systems. In the default display mode, each PID is followed by a letter denoting the type of access:

Only the PIDs are written to standard output. Additional information is written to standard error. This makes it easier to process the output with computer programs.

The command can also be used to check what processes are using a network port:

The command returns a non-zero code if none of the files are accessed or in case of a fatal error. If at least one access has succeeded, fuser returns zero. The output of "fuser" may be useful in diagnosing "resource busy" messages arising when attempting to unmount filesystems.

POSIX defines the following options:

psmisc adds the following options, among others: