$ Linux Command

Files

losetup

Set up and control loop devices for mounting files in images.

loopdevicemountimageiso

Additional Notes

losetup associates a loop device (e.g., /dev/loop0) with a regular file, allowing the file to be treated as a block device. This is how you mount disk images, ISO files, and filesystem images without writing them to physical media.

The kernel's loop device driver transparently translates block-level read/write operations on the loop device into file read/write operations on the backing file. Each loop device (/dev/loopN) can be associated with one file at a time.

Syntax

losetup [options] [loop-device] [backing-file]

Parameters

  • loop-device: The loop device to use (e.g., /dev/loop0). If omitted, the kernel automatically finds a free device.
  • backing-file: The file to associate with the loop device (e.g., disk.img, file.iso).

Common Options

  • -f, --find: Find the first unused loop device.
  • -a, --all: Show all loop devices and their backing files.
  • -j file, --associated file: Show the loop device associated with a specific backing file.
  • -d, --detach: Detach the loop device from its backing file.
  • -D, --detach-all: Detach all currently associated loop devices.
  • -o offset, --offset offset: Start at an offset within the file (in bytes).
  • --sizelimit size: Limit the data size on the loop device.
  • -P, --partscan: Force the kernel to scan the partition table on the loop device.
  • -r, --read-only: Set up the loop device as read-only.
  • -e encryption, --encryption encryption: Enable encryption (deprecated, use cryptsetup instead).
  • -L, --nooverlap: Avoid overlapping loop device detach/setup operations.

Examples

losetup -f

Show the first available loop device.

losetup -f disk.img

Associate a free loop device with disk.img.

losetup /dev/loop0 disk.img

Explicitly associate /dev/loop0 with disk.img.

losetup -a

List all currently associated loop devices.

losetup -d /dev/loop0

Detach a specific loop device.

losetup -D

Detach all loop devices.

losetup -f -P disk.img

Associate a free loop device with partition scanning enabled.

losetup -r -f iso-file.iso

Set up a loop device in read-only mode for an ISO file.

mount /dev/loop0 /mnt

Mount the filesystem through the loop device.

Practical Notes

  • Modern Linux supports mounting files directly with mount -o loop file.iso /mnt, which internally calls losetup.
  • Use -P when the file contains a partition table (like a full disk image) so each partition appears as /dev/loop0p1, /dev/loop0p2, etc.
  • Loop devices are limited in number; check /dev/loop-control or cat /proc/sys/dev/loop/max_loop for the maximum.
  • Always detach loop devices with losetup -d when done to free kernel resources.