losetup - set up and control loop devices
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
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.
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.
Cryptoloop is no longer supported in favor of dm-crypt.
For more details see cryptsetup(8).
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.
- /dev/loop[0..N]
- loop block devices
- /dev/loop-control
- loop control device
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
Karel Zak <kzak@redhat.com>, based on the original version
from Theodore Ts'o <tytso@athena.mit.edu>
The losetup command is part of the util-linux package and is
available from https://www.kernel.org/pub/linux/utils/util-linux/.