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] [-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.
-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.
--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 optional
argument can be either on or off. If the optional argument is
omitted, it defaults to on.
-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 at
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 2.38.1 2022-07-20 LOSETUP(8)
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