losetup Command in Linux: Syntax, Options & Loop Device Examples

Deepak Prasad
Tested on Ubuntu 25.04 (Plucky Puffin)
Package losetup 2.40.2 (util-linux 2.40.2-14ubuntu1)
losetup 2.40.2 (util-linux 2.40.2-14ubuntu1)
Applies to Ubuntu, Debian, RHEL, AlmaLinux, Fedora
Privilege sudo or root
Man page losetup(8)
Scope losetup attaches regular files or block devices to /dev/loop* kernel nodes so you can partition, format, and mount disk images without dedicated hardware. It also lists, resizes, and detaches loop devices from the shell.
Related guides Linux commands

losetup — quick reference

Set up loop devices

Map a file to a /dev/loopN block device so partition tools and mkfs can treat it like a disk.

When to use Command
Find the first free loop device name losetup -f
Attach a file to the first free loop device sudo losetup -f /path/to/image.img
Attach and print the chosen device (script friendly) sudo losetup -f --show /path/to/image.img
Attach to a specific loop device sudo losetup /dev/loop7 /path/to/image.img
Attach read-only sudo losetup -r -f --show /path/to/image.img
Limit visible size below the file length sudo losetup -f --show --sizelimit 5242880 /path/to/image.img
Start at a byte offset inside the backing file sudo losetup -o 1048576 -f --show /path/to/image.img
Scan partition table and create /dev/loopNp1 nodes sudo losetup -Pf --show /path/to/image.img

List and inspect

When to use Command
List all active loop devices (classic format) losetup -a
List in columnar table format (default when listing) losetup -l
List one device losetup -l /dev/loop7
JSON listing for scripts losetup -J
Show which loop device backs a file losetup -j /path/to/image.img
Pick specific columns losetup -O NAME,BACK-FILE,OFFSET -l

Detach and maintenance

When to use Command
Detach one loop device (umount first if mounted) sudo losetup -d /dev/loop7
Detach every loop device sudo losetup -D
Grow a loop device after enlarging the backing file sudo losetup -c /dev/loop7
Verbose attach/detach messages sudo losetup -v -f --show /path/to/image.img

Help and version

When to use Command
Show options losetup --help
Show util-linux version losetup -V

losetup — command syntax

Synopsis from losetup --help on Ubuntu 25.04 (util-linux 2.40.2):

text
losetup [options] [<loopdev>]
losetup [options] -f | <loopdev> <file>

Set up and control loop devices.

losetup configures kernel loop drivers; it does not mount filesystems. After attach, use mount on /dev/loopN or /dev/loopNp1. Setup and detach usually need sudo.

losetup — command examples

Essential Create an image and attach to the first free loop

Loop devices let you practice disk operations on a file under /tmp instead of a real disk.

bash
dd if=/dev/zero of=/tmp/loopfile bs=10M count=1 status=none
sudo losetup -f --show /tmp/loopfile

Sample output:

output
/dev/loop27

Verify the association:

bash
losetup -j /tmp/loopfile

Sample output:

output
/dev/loop27: [0040]:1264 (/tmp/loopfile)

Detach when finished:

bash
sudo losetup -d /dev/loop27
rm -f /tmp/loopfile
Essential List active loop devices

Use -a for the legacy one-line format or -l for columns.

bash
losetup -a
losetup -l

Sample -l row (device names vary):

text
/dev/loop27         0      0         0  0 /tmp/loopfile                                     0     512

Columns include NAME, OFFSET, SIZELIMIT, RO, BACK-FILE, and logical sector size.

Essential Find the first unused loop device

-f alone prints a name; combine with --show during attach to capture it in scripts.

bash
losetup -f

Sample output:

output
/dev/loop27
Common Read-only loop for ISOs or forensic images

-r sets the RO flag so the kernel rejects writes through the loop device.

bash
dd if=/dev/zero of=/tmp/ro.img bs=5M count=1 status=none
LOOP=$(sudo losetup -r -f --show /tmp/ro.img)
losetup -l $LOOP
sudo losetup -d $LOOP && rm -f /tmp/ro.img

Sample output:

output
NAME        SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE             DIO LOG-SEC
/dev/loop27         0      0         0  1 /tmp/ro.img           0     512

RO is 1 (read-only).

Common Size limit and byte offset

Expose only part of a large backing file — useful for nested images or skipping a header region.

bash
dd if=/dev/zero of=/tmp/big.img bs=10M count=1 status=none
LOOP=$(sudo losetup -f --show --sizelimit 5242880 /tmp/big.img)
losetup -l $LOOP
sudo losetup -d $LOOP
LOOP=$(sudo losetup -o 1048576 -f --show /tmp/big.img)
losetup -l $LOOP
sudo losetup -d $LOOP && rm -f /tmp/big.img

Sample SIZELIMIT / OFFSET values:

text
/dev/loop27   5242880      0 ...
/dev/loop27         0 1048576 ...
Common JSON output for automation

-J prints structured data for Ansible, CI parsers, or custom scripts.

bash
losetup -J | head -12

Sample fragment:

text
"back-file": "/tmp/loopfile",
         "dio": false,
         "log-sec": 512
Common Partition scan with -P

After partitioning an image with parted, -P creates /dev/loopNp1 partition nodes.

bash
dd if=/dev/zero of=/tmp/part.img bs=1M count=64 status=none
LOOP=$(sudo losetup -f --show /tmp/part.img)
sudo parted -s $LOOP mklabel gpt mkpart primary 1MiB 32MiB
sudo losetup -d $LOOP
LOOP=$(sudo losetup -Pf --show /tmp/part.img)
ls -la ${LOOP}*
sudo losetup -d $LOOP && rm -f /tmp/part.img

Sample output:

output
brw-rw---- 1 root disk  7, 27 ... /dev/loop27
brw------- 1 root root 259,  1 ... /dev/loop27p1
Advanced Detach failures — device busy

Detaching fails while a filesystem is still mounted on the loop or a partition child.

bash
sudo losetup -d /dev/loop7

Sample error:

text
losetup: /dev/loop7: detach failed: No such device or address

Run mount | grep loop7, umount the mount point, then retry losetup -d. For snap-held loops on Ubuntu desktops, identify the owner with losetup -l before forcing detach.

losetup — when to use / when not

Use losetup when Use something else when
  • You need a block device node for a disk image, ISO, or encrypted container file
  • You are testing parted or mke2fs safely on /tmp images
  • You must expose only part of a file (--offset, --sizelimit)
  • You want partition minors (loop0p1) without KVM or USB storage
  • You only need to mount a known filesystem image in one step → mount -o loop (kernel attaches a transient loop)
  • You are serving ISOs to VMs → hypervisor disk backends or qemu-img
  • You need network filesystem access → NFS, SSHFS, not loop
  • The image is LUKS-encrypted → cryptsetup open first, then losetup or direct map

losetup vs mount -o loop

losetup mount -o loop
Control Explicit device choice, offset, size, read-only Kernel picks a loop device during mount
Partition support -P exposes /dev/loopNp* Often needs losetup -P first for partition mounts
Best for Scripts, lab work, multi-step partition+format Quick single-filesystem mount

losetup — interview corner

What is a loop device?

A loop device is a kernel block driver that maps /dev/loopN to the blocks of a regular file (or another device). User tools such as mkfs, parted, and mount then talk to the file as if it were a disk.

A strong answer is:

"A loop device presents a file as a block device — I use losetup to attach images so I can partition or format them like real disks."

What does losetup -f --show do?

-f selects the first unused loop device. --show prints that name after a successful attach — handy in scripts: LOOP=$(sudo losetup -f --show image.img).

A strong answer is:

"-f picks a free loop; --show prints the device name after setup so scripts can capture it in a variable."

Why use -P / --partscan?

Without partition scanning, only /dev/loopN exists. -P asks the kernel to read the partition table inside the image and create /dev/loopNp1, p2, and so on — required before mounting an individual partition from a full-disk image.

A strong answer is:

"--partscan exposes partition devices like loop0p1 from a partitioned image — needed when the image contains a table, not just a bare filesystem."

Why does losetup -d fail with 'busy'?

Something still holds the device open — usually a mount, a dm-crypt mapping, or a process reading /dev/loopNp*. Unmount filesystems, close crypt mappings, then detach.

A strong answer is:

"The loop is still in use — I umount filesystems and close mappers before losetup -d."

When attach read-only with -r?

Use read-only loops for ISOs, golden images, or forensic copies where writes must not touch the backing file.

A strong answer is:

"-r sets the loop read-only — I use it for ISOs and audit images so nothing can modify the file underneath."

Troubleshooting

Symptom Likely cause What to try
failed to set up loop device: Device or resource busy File already attached losetup -j FILE then detach the listed device
detach failed Filesystem still mounted umount /dev/loopNp* then losetup -d
No loopNp1 after attach Partition scan not run Detach and reattach with -Pf
Wrong size seen by mkfs --sizelimit or --offset set losetup -l and adjust flags

References

Rohan Timalsina

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