LOSETUP(8)

💖 LOSETUP(8) 💖

okay bestie, this is the losetup page from the System Administration corner of the manual — the one all about loop devices. sit down, grab a strawberry drink 🍓, and let's get you fluent.

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✨ NAME ✨

losetup — set up and control loop devices

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✨ SYNOPSIS ✨

Set up a loop device:

losetup [options] -f|loopdev file

Get info:

losetup [-l] [-a|loopdev]

losetup -j file [-o offset]

Recalibrate the size of a loop device:

losetup -c loopdev

Detach loop devices:

losetup -d loopdev ...

losetup -D

Prevent loop devices from getting listed:

losetup --remove loopdev ...

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🦋 DESCRIPTION 🦋

so losetup is the girl who does everything with loop devices: she associates loop devices with regular files or block devices, she detaches loop devices, and she queries the status of a loop device. the whole job description, no cap. 💅

if you give her only the loopdev argument, she shows you the status of that corresponding loop device. if you give her no option at all, she shows you all loop devices. main character energy, honestly.

ngl, heads up: the old output format (aka losetup -a) with those comma-delimited strings is deprecated — the --list output format is the moment now. the old one is giving 2011.

okay so this next bit is important and a little scary 😭. it is possible to create multiple independent loop devices for the same backing file. this setup may be dangerous, can cause data loss, corruption, and overwrites. use --nooverlap with --find during setup to avoid this problem. protect yourself, bestie. 🫶

also: the loop device setup is not an atomic operation when used with --find, and losetup does not protect this operation by any lock. the number of attempts is internally restricted to a maximum of 16. it is recommended to use for example flock(1) to avoid a collision in heavily parallel use cases. (iykyk — parallel chaos is real.)

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🌸 OPTIONS 🌸

first, the size tea ☕ — the size and offset arguments may be followed by the multiplicative suffixes KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB, EiB, ZiB and YiB (the "iB" is optional, e.g., "K" has the same meaning as "KiB") or the suffixes KB (=1000), MB (=1000*1000), and so on for GB, TB, PB, EB, ZB and YB.

• -a, --all — Show the status of all loop devices. Note that not all information is accessible for non-root users. See also --list. The old output format (as printed without --list) is deprecated. 💀
• -d, --detach loopdev... — Detach the file or device associated with the specified loop device(s). Note that since Linux v3.7 kernel uses "lazy device destruction". The detach operation does not return EBUSY error anymore if device is actively used by system, but it is marked by autoclear flag and destroyed later. Even if the device is not used, the loop device can be destroyed later. If you need to wait for a complete removal of the loop device, call udevadm settle after losetup. ⏳
• -D, --detach-all — Detach all associated loop devices. clean sweep, so satisfying.
• -f, --find [file] — Find the first unused loop device. If a file argument is present, use the found device as loop device. Otherwise, just print its name.
• --show — Display the name of the assigned loop device if the -f option and a file argument are present.
• -L, --nooverlap — Check for conflicts between loop devices to avoid the situation when the same backing file is shared between more loop devices. If the file is already used by another device then re-use the device rather than a new one. The option makes sense only with --find. 🫶
• -j, --associated file [-o offset] — Show the status of all loop devices associated with the given file.
• -o, --offset offset — The data start is moved offset bytes into the specified file or device. The offset may be followed by the multiplicative suffixes; see above. ⬆️
• --loop-ref string — Set the reference string. The backwardly compatible default is to use the backing filename as a reference in the loop setup ioctl (aka lo_file_name). This option overrides this default behavior and sets the reference to the string. The reference may be used by udevd in /dev/loop/by-ref. The Linux kernel does not use the reference at all, but it could be used by some old utils that cannot read the backing file from sysfs. The reference is readable only for the root user (see --output +REF) and it is restricted to 64 bytes.
• --sizelimit size — The data end is set to no more than size bytes after the data start. The size may be followed by the multiplicative suffixes; see above.
• -b, --sector-size size — Set the logical sector size of the loop device in bytes (since Linux 4.14). The option may be used when creating a new loop device as well as a stand-alone command to modify sector size of the already existing loop device.
• -c, --set-capacity loopdev — Force the loop driver to reread the size of the file associated with the specified loop device. recalibrate, queen. 📏
• -P, --partscan — Force the kernel to scan the partition table on a newly created loop device. Note that the partition table parsing depends on sector sizes. The default is sector size is 512 bytes, otherwise you need to use the option --sector-size together with --partscan.
• -r, --read-only — Set up a read-only loop device. ✋ no writing allowed.
• --direct-io[=on|off] — Enable or disable direct I/O for the backing file. The default is off. Specifying either --direct-io or --direct-io=on will enable it. But, --direct-io=off can be provided to explicitly turn it off.
• -l, --list — If a loop device or the -a option is specified, print the default columns for either the specified loop device or all loop devices; the default is to print info about all devices. See also --output, --noheadings, --raw, and --json.
• -O, --output column[,column]... — Specify the columns that are to be printed for the --list output. Use --help to get a list of all supported columns. 👑
• --output-all — Output all available columns.
• -n, --noheadings — Don't print headings for --list output format.
• --raw — Use the raw --list output format.
• -J, --json — Use JSON format for --list output.
• --remove loopdev... — Remove the specified loop device(s). The loop device must be unused (not associated with any backing file). This option removes the device node from the system. In most cases, --detach is sufficient for normal use. See the LOOP DEVICE LIFECYCLE section below. 🌈
• -h, --help — Display help text and exit.
• -V, --version — Display version and exit.

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🧚 LOOP DEVICE LIFECYCLE 🧚

Loop devices go through several states during their lifecycle — it's giving glow-up journey, honestly:

Creation

When losetup associates a loop device with a backing file, it automatically creates the device node (e.g., /dev/loop0) if it doesn't already exist. This happens on-demand when you use options like -f or specify a loop device explicitly. The device is created using the LOOP_CTL_ADD ioctl.

Detachment

The -d (--detach) option disassociates the backing file from the loop device using the LOOP_CLR_FD ioctl. After detachment, the loop device node remains in /dev and can be reused for another backing file. Use lsblk -a to see all loop device nodes, including unused ones.

Removal

The --remove option completely removes the loop device node from the system using the LOOP_CTL_REMOVE ioctl. The device must be detached first (not associated with any backing file). After removal, the device node disappears from /dev.

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🔐 ENCRYPTION 🔐

Cryptoloop is no longer supported in favor of dm-crypt. For more details see cryptsetup(8). rip to cryptoloop, but dm-crypt ate. 💅

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⭐ EXIT STATUS ⭐

losetup returns 0 on success, nonzero on failure. When losetup displays the status of a loop device, it returns 1 if the device is not configured and 2 if an error occurred which prevented determining the status of the device.

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📝 NOTES 📝

Since version 2.37 losetup uses LOOP_CONFIGURE ioctl to set up a new loop device by one ioctl call. The old versions use LOOP_SET_FD and LOOP_SET_STATUS64 ioctls to do the same. glow-up achieved. ✨

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💜 ENVIRONMENT 💜

• LOOPDEV_DEBUG=all — enables debug output.
• LIBSMARTCOLS_DEBUG=all — enables libsmartcols debug output.
• LIBSMARTCOLS_DEBUG_PADDING=on — use visible padding characters.
• LIBSMARTCOLS_JSON=compact|lines — Controls JSON output format when using --json. Supported values are compact for JSON output with minimal whitespace, and lines for JSON Lines format (one JSON object per line). If unset or set to any other value, pretty-printed JSON is used.

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🎀 FILES 🎀

• /dev/loop[0..N] — loop block devices
• /dev/loop-control — loop control device

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🌈 EXAMPLE 🌈

The following commands can be used as an example of using the loop device. look at her go 😭

# dd if=/dev/zero of=~/file.img bs=1024k count=10
# losetup --find --show ~/file.img
/dev/loop0
# mkfs -t ext2 /dev/loop0
# mount /dev/loop0 /mnt
...
# umount /dev/loop0
# losetup --detach /dev/loop0

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👑 AUTHORS 👑

Karel Zak <kzak@redhat.com>, based on the original version from Theodore Ts'o <tytso@athena.mit.edu>.

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🐛 REPORTING BUGS 🐛

For bug reports, use the issue tracker <https://github.com/util-linux/util-linux/issues>.

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🌟 AVAILABILITY 🌟

The losetup command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.

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and that's the whole page, bestie 🫶 go forth and loop responsibly. this was util-linux 2.42.4, dated 2026-09-21 — periodt. 💅✨🦋