losetup

Updated

September 4, 2026

Overview

losetup associates regular files with loop devices (/dev/loopN) so the kernel can treat an image file like a block device—mount ISOs, inspect disk images, or access partitions inside a raw .img.

Syntax

losetup [options] [loopdev] [file]
losetup [options] -f file

Common Options

Option Description
-a, --all List active loop devices
-f, --find Find first unused loop device
-f --show Find, attach, print device path
-d, --detach Detach
-D, --detach-all Detach all (careful)
-j file Show loops for file
-r, --read-only Read-only
-P, --partscan Scan partition table → /dev/loopNp1…
-b SIZE Logical block size
-o OFFSET Data offset
--sizelimit Limit size
-L, --nooverlap Refuse overlapping

Key Use Cases

  1. Mount disk/ISO images
  2. Access partitions inside a raw disk image
  3. Filesystem experiments without real disks
  4. Detach leftover loops after failed scripts

Examples with Explanations

Attach and list

sudo losetup -fP --show disk.img
sudo losetup -a
lsblk

Mount ISO

sudo losetup -f --show -r image.iso
# or directly:
sudo mount -o loop,ro image.iso /mnt/iso

Modern mount -o loop calls losetup for you.

Partitions inside image

sudo losetup -fP --show disk.img
lsblk /dev/loop0
sudo mount /dev/loop0p1 /mnt/p1

Detach

sudo umount /mnt/p1
sudo losetup -d /dev/loop0
sudo losetup -j disk.img

One-liner recipes

# Attach, print, and lsblk
dev=$(sudo losetup -fP --show disk.img); echo "$dev"; lsblk "$dev"

# Clean orphan loops (review first!)
sudo losetup -a
# sudo losetup -D   # nuclear option

Notes & Pitfalls

  • Always umount before detach.
  • Sparse image files and sparse loop mappings can confuse free-space calculations.
  • Permission: root required for setup.
  • Snap/container environments may restrict loop devices.
  • For qcow2 use qemu-nbd or guestmount, not raw losetup alone.

2026-relevant notes

  • Cloud “custom image” workflows still use loop/nbd for injection of SSH keys and cloud-init.
  • Prefer udisksctl loop-setup on desktops for user-friendly attach.
  • losetup -P + lsblk is the standard recovery path for raw VM disks copied as files.

Additional Resources

  • man losetup
  • util-linux documentation