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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 flock1 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 create a new loop device as well as 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 argument is omitted, it defaults to 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.
-V, --version
Display version information and exit.
-h, --help
Display help text and exit.
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 AT redhat.com>, based on the original version from Theodore Ts'o
<tytso AT athena.edu>.
REPORTING BUGS
For bug reports, use the issue tracker at https://github.com/karelzak/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 2.37.2 2021-06-02 LOSETUP(8)
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