Name

losetup — set up and control loop devices

Synopsis

Get info:

losetup loopdev

losetup −a

losetup −j file [ −o offset ]

Delete loop:

losetup −d loopdev...

Delete all used loop devices:

losetup −D

Print name of first unused loop device:

losetup −f

Setup loop device:

losetup [ −e | −E encryption ] [ −o offset ] [ −−sizelimit size ] [ −p pfd ] [−rP] −f [−−show] | loopdev file

Resize 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 corresponding loop device is shown.

OPTIONS

The size and offset arguments may be followed by binary (2^N) suffixes KiB, MiB, GiB, TiB, PiB and EiB (the "iB" is optional, e.g. "K" has the same meaning as "KiB") or decimal (10^N) suffixes KB, MB, GB, PB and EB.

−a, −−all

show status of all loop devices. Note that not all information are accessible for non-root users.

−c, −−set−capacity loopdev

force loop driver to reread 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)

−e, −E, −−encryption encryption_type

enable data encryption with specified name or number

−f, −−find

find the first unused loop device. If a file argument is present, use this device. Otherwise, print its name

−h, −−help

print help

−j, −−associated file

show status of all loop devices associated with 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

−p, −−pass−fd num

read the passphrase from file descriptor with number num instead of from the terminal

−P, −−partscan

force kernel to scan partition table on newly created loop device

−r, −−read−only

setup read-only loop device

−−show

print device name if the −f option and a file argument are present.

−v, −−verbose

verbose mode

ENCRYPTION

Cryptoloop is deprecated in favor of dm-crypt. For more details see cryptsetup (8). It is possible that all bug reports regarding to -E/-e options will be ignored.

It is possible to specify transfer functions (for encryption/decryption or other purposes) using one of the −E and −e options. There are two mechanisms to specify the desired encryption: by number and by name. If an encryption is specified by number then one has to make sure that the Linux kernel knows about the encryption with that number, probably by patching the kernel. Standard numbers that are always present are 0 (no encryption) and 1 (XOR encryption). When the cryptoloop module is loaded (or compiled in), it uses number 18. This cryptoloop module will take the name of an arbitrary encryption type and find the module that knows how to perform that encryption.

RETURN VALUE

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 from determining the status of the device.

FILES

/dev/loop[0..N]

loop block devices

/dev/loop-cotrol

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=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

AUTHORS

Karel Zak <kzak@redhat.com>, based on 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 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.