umount — Unmount a filesystem

Linux / File and disk management / umount.html

Detaches a mounted filesystem so a device can be removed without losing data.

Basic Information

What is this command?

umount removes a mount point from the kernel's mount table. It flushes pending writes to the device, so it must be run before unplugging a USB stick or external disk.

It accepts either the device name (/dev/sdb1) or the mount point (/mnt/usb).

What problems does it solve?

• Safely detaching an external disk.
• Unmounting before running a filesystem check.
• Disconnecting an NFS or network share that is no longer needed.
• Remounting with different options.

Key features

Device or mount point

You can pass either the device path or the directory path.

• When to use: Scripts that only know the device name
• What you get: No need to look up the mount point
• Limitations: If a device is mounted twice, only one mount is removed

Detecting busy mounts

A busy error means the mount is in use; lsof or fuser finds the process.

• When to use: When you see a device is busy error
• What you get: Shows which process holds the mount
• Limitations: Forcing the unmount risks data loss

Lazy unmount

The lazy option detaches immediately and cleans up remaining references later.

• When to use: Disconnecting an unresponsive network mount
• What you get: The shell does not hang
• Limitations: Resources may linger until cleanup completes

Installation and usage

Check whether it is installed

which umount
umount --version

It is part of util-linux, so no separate installation is needed.

Real-world examples

Unmount by mount point

The most common form.

# Unmount /mnt/usb
sudo umount /mnt/usb

Find the process holding it

When unmount is refused, check which process is accessing it.

# Find the cause of busy
sudo lsof /mnt/usb
sudo fuser -vm /mnt/usb

Lazy unmount

Used for unresponsive remote mounts.

# Detach now, clean up later
sudo umount -l /mnt/nfs