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 Print the name of the first unused loop device: losetup -f Set up a loop device: losetup [-o offset] [--sizelimit 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 to avoid this problem.
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. -c, --set-capacity loopdev Force the loop driver to reread the size of the file associated with the specified loop device. -d, --detach loopdev... Detach the file or device associated with the specified loop device(s). -D, --detach-all Detach all associated loop devices. --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. -f, --find Find the first unused loop device. If a file argument is present, use the found device as loop device. Otherwise, just print its name. -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 Show the status of all loop devices associated with the given file. -J, --json Use JSON format for --list output. -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 --json and --raw. -n, --noheadings Don't print headings for --list output format. -o, --offset offset The data start is moved offset bytes into the specified file or device. -O, --output columns Specify the columns that are to be printed for the --list output. -P, --partscan Force the kernel to scan the partition table on a newly created loop device. --raw Use the raw --list output format. -r, --read-only Set up a read-only loop device. --sizelimit size The data end is set to no more than size bytes after the data start. --show Display the name of the assigned loop device if the -f option and a file argument are present. -v, --verbose Verbose mode. -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=1MiB 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
LOOPDEV_DEBUG=all enables debug output.
Karel Zak <firstname.lastname@example.org>, based on the original version from Theodore Ts'o <email@example.com>
The losetup command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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