fuser

Fuser is a command-line utility found on many Unix-like systems that identifies which processes are using a given file, file system, or network endpoint. It reports the process IDs (PIDs) of processes that have the resource open and can display additional information such as the owning user and the command name when used with verbose options. Fuser determines usage by inspecting a system’s process table and open file descriptors.

Common uses include determining which processes must be stopped before unmounting a filesystem, deleting or modifying a file, or freeing a network port. It can also be employed for debugging or system administration to understand resource contention.

Typical usage examples are simple: fuser /path/to/file or fuser /mount/point. Adding the verbose option (for example, -v) yields more detail about each process. The -k option can be used to terminate all processes using the resource, sending signals to those processes as configured. For network resources, fuser can interpret a port or protocol with options such as -n tcp or -n udp, e.g., fuser -n tcp 80 to identify processes using port 80.

Fuser is commonly provided by the psmisc package on Linux distributions and is available on other Unix-like systems through equivalent packages or utilities. Access to information about processes owned by other users typically requires root privileges; without them, fuser shows only processes owned by the calling user. Related tools include lsof and ps, which offer alternative ways to view resource usage by processes.