fuser — Identify processes using files or ports

Linux / Service and process management / fuser.html

Shows which processes hold a given file, directory or socket.

Basic Information

What is this command?

fuser reports the PIDs holding a file or directory open.

It is commonly used to find what blocks an unmount or which process owns a port.

What problems does it solve?

• Find the cause when umount reports busy.
• Identify the process holding a port.
• Clean up processes locking a file.
• Check whether a mount is in use.

Key features

Processes using a file

Pass a path to print the PIDs that have it open.

• When to use: Diagnosing a failed unmount
• What you get: PID list with access modes
• Limitations: Some entries are hidden without privileges

Port ownership

With -n and a tcp/udp spec it finds processes using a port.

• When to use: Resolving address-in-use errors
• What you get: The PID holding the port
• Limitations: Results depend on the network namespace

Combined signaling

The -k option sends a signal to those processes.

• When to use: When releasing a lock and cleaning up at once
• What you get: The target processes terminated
• Limitations: Risk of killing unintended processes

Installation and usage

Check whether it is installed

fuser -V

apt install psmisc # Debian/Ubuntu dnf install psmisc # Fedora/RHEL

Real-world examples

Who uses a file

Find processes using a mount point.

# Find processes using this mount point
fuser -vm /mnt/data

Port owner

Find what holds 8080/tcp.

# Identify the process on port 8080
fuser -n tcp 8080

Inspect then kill

Review targets, then signal them.

# Review, then terminate the holders
fuser -vm /mnt/data
fuser -km /mnt/data