Manual: losetup

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 , based on the original version from Theodore Ts’o .

REPORTING BUGS
       For bug reports, use the issue tracker .

AVAILABILITY
       The losetup command is part of the util-linux package which can be downloaded from Linux Kernel Archive
       .

util-linux 2.41.3					 2025-12-15						  LOSETUP(8)