losetup(8) — Linux manual page


LOSETUP(8)                System Administration               LOSETUP(8)

NAME         top

       losetup - set up and control loop devices

SYNOPSIS         top

       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

DESCRIPTION         top

       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         top

       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

       -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.

           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

       -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.

           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 available columns.

       -n, --noheadings
           Don’t print headings for --list output format.

           Use the raw --list output format.

       -J, --json
           Use JSON format for --list output.

ENCRYPTION         top

       Cryptoloop is no longer supported in favor of dm-crypt. For more
       details see cryptsetup(8).

EXIT STATUS         top

       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         top

       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         top

           enables debug output.

FILES         top

           loop block devices

           loop control device

EXAMPLE         top

       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
           # mkfs -t ext2 /dev/loop0
           # mount /dev/loop0 /mnt
           # umount /dev/loop0
           # losetup --detach /dev/loop0

AUTHORS         top

       Karel Zak <kzak@redhat.com>, based on the original version from
       Theodore Ts’o <tytso@athena.mit.edu>.

REPORTING BUGS         top

       For bug reports, use the issue tracker at

AVAILABILITY         top

       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
       util-linux@vger.kernel.org. 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 man-pages@man7.org

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)