losetup

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
       device.   If only the loopdev argument is given, the status of the cor-
       responding 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
              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.

       --sizelimit size
              The data end is set to no more than size bytes  after  the  data
              start.

       -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.

       -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
              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 out-
              put.  Use --help to get a list of all supported 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
       details 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
       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

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)
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