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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENCRYPTION | EXIT STATUS | NOTES | ENVIRONMENT | FILES | EXAMPLE | AUTHORS | REPORTING BUGS | AVAILABILITY |
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LOSETUP(8) System Administration LOSETUP(8)
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]
[--loop-ref name] [-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.
--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.
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.
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.
LOOPDEV_DEBUG=all
enables debug output.
/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 <[email protected]>, based on the original version from
Theodore Ts’o <[email protected]>.
For bug reports, use the issue tracker at
https://github.com/util-linux/util-linux/issues.
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/>. This page
is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have
a bug report for this manual page, send it to
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-10.) If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page,
or you have corrections or improvements to the information in
this COLOPHON (which is not part of the original manual page),
send a mail to [email protected]
util-linux 2.41.devel-537-e... 2024-06-13 LOSETUP(8)
Pages that refer to this page: loop(4), cryptsetup(8), e2image(8), mount(8), umount(8)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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