losetup8

Usage

Overview

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 is possible to create multiple 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

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. Even if the device is not used, the loop device can be destroyed later. If you need to wait for a complete removal of the loop device, call udevadm settle after losetup.

-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]

-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 the reference string. The backwardly compatible default is to use the backing filename as a reference in the loop setup ioctl (aka lo_file_name). This option overrides this default behavior and sets the reference to the string. The reference may be used by udevd in /dev/loop/by-ref. The 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.

--direct-io[=on|off]

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.

-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.

--remove loopdev...

Remove the specified loop device(s). The loop device must be unused (not associated with any backing file). This option removes the device node from the system. In most cases, --detach is sufficient for normal use. See the LOOP DEVICE LIFECYCLE section below.

-h, --help

Display help text and exit.

-V, --version

Display version and exit.

Loop device lifecycle

Loop devices go through several states during their lifecycle:

Creation

Detachment

Removal

Encryption

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

Exit status

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

Since version 2.37 losetup uses LOOP_CONFIGURE ioctl to set up 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

LOOPDEV_DEBUG=all

LIBSMARTCOLS_DEBUG=all

LIBSMARTCOLS_DEBUG_PADDING=on

LIBSMARTCOLS_JSON=compact|lines

Files

/dev/loop[0..N]

/dev/loop-control

Examples

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