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 de-
vice. If only the loopdev argument is given, the status of the corre-
sponding 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, cor-
ruption and overwrites. Use --nooverlap with --find during setup to
avoid this problem.
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 informa-
tion 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 de-
vice(s). Note that since Linux v3.7 kernel uses "lazy device de-
struction". 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 de-
vice 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 suf-
fixes; 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 de-
vice 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, other-
wise 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 on.
-v, --verbose
Verbose mode.
-l, --list
If a loop device or the -a option is specified, print the de-
fault 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 out-
put. 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 de-
tails see cryptsetup(8).
RETURN VALUE
losetup returns 0 on success, nonzero on failure. When losetup dis-
plays 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.
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 de-
vice.
# 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
ENVIRONMENT
LOOPDEV_DEBUG=all
enables debug output.
AUTHORS
Karel Zak <kzak@redhat.com>, based on the original version from
Theodore Ts'o <tytso@athena.mit.edu>
AVAILABILITY
The losetup command is part of the util-linux package and is available
from https://www.kernel.org/pub/linux/utils/util-linux/.
util-linux November 2015 LOSETUP(8)