//po4a: entry man manual
= losetup(8)
:doctype: manpage
:man manual: System Administration
:man source: util-linux {release-version}
:page-layout: base
:command: losetup
:smartcols-json:

== NAME

losetup - set up and control loop devices

== 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_ ...

== DESCRIPTION

*losetup* is used to associate loop devices with regular files or block devices, to detach loop devices, and to query the status of a loop device. If only the _loopdev_ argument is given, the status of the corresponding loop device is shown. If no option is given, all loop devices are shown.

Note that the old output format (i.e., *losetup -a*) with comma-delimited strings is deprecated in favour of the *--list* output format.

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.

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.

== OPTIONS

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.

*-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 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.

*-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.

*--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.

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== LOOP DEVICE LIFECYCLE

Loop devices go through several states during their lifecycle:

*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_.

== ENCRYPTION

*Cryptoloop is no longer supported in favor of dm-crypt.* For more details see *cryptsetup*(8).

== 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.

== 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.

== ENVIRONMENT

*LOOPDEV_DEBUG*=all::
enables debug output.

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== FILES

_/dev/loop[0..N]_::
loop block devices

_/dev/loop-control_::
loop control device

== EXAMPLE

The following commands can be used as an example of using the loop device.

 # 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

== AUTHORS

mailto:kzak@redhat.com[Karel Zak], based on the original version from mailto:tytso@athena.mit.edu[Theodore Ts'o].

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