LOSETUP(8)		     System Administration		    LOSETUP(8)

NAME
       losetup - set up and control loop devices

SYNOPSIS
       Get info:

       losetup [loopdev]

       losetup -l [-a]

       losetup -j file [-o offset]

       Detach a loop device:

       losetup -d loopdev ...

       Detach all associated loop devices:

       losetup -D

       Set up a loop device:

       losetup [-o offset] [--sizelimit size] [--sector-size size] [--loop-ref
       name] [-Pr] [--show] -f|loopdev file

       Resize a loop device:

       losetup -c 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’s possible to create more 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 reference string. The backwardly compatible default is to use
	   the backing filename as a reference in loop setup ioctl (aka
	   lo_file_name). This option can overwrite this default behavior and
	   set the reference to the string. The reference may be used by udevd
	   in /dev/loop/by-ref. 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.

       -v, --verbose
	   Verbose mode.

       -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.

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 setup 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.

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

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

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/>.

util-linux 2.41.2		  2025-09-22			    LOSETUP(8)