losetup(8) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ENCRYPTION | EXIT STATUS | NOTES | ENVIRONMENT | FILES | EXAMPLE | AUTHORS | AVAILABILITY | COLOPHON

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]
                    [-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 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         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

       LOOPDEV_DEBUG=all
              enables debug output.

FILES         top

       /dev/loop[0..N]
              loop block devices

       /dev/loop-control
              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
              /dev/loop0
              # 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>

AVAILABILITY         top

       The losetup command is part of the util-linux package and is
       available from
       https://www.kernel.org/pub/linux/utils/util-linux/.

COLOPHON         top

       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
       2021-03-21.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-19.)  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                    November 2015                   LOSETUP(8)

Pages that refer to this page: loop(4)mount(8)umount(8)