#Bash 的 losetup 命令

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
       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
       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 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 optional
           argument can be either on or off. If the optional 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 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.

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 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
       LOOPDEV_DEBUG=all
           enables debug output.

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

AUTHORS
       Karel Zak <[email protected]>, based on the original version from
       Theodore Ts’o <[email protected]>.

REPORTING BUGS
       For bug reports, use the issue tracker at
       https://github.com/util-linux/util-linux/issues.

AVAILABILITY
       The losetup command is part of the util-linux package which can be
       downloaded from Linux Kernel Archive
       <https://www.kernel.org/pub/linux/utils/util-linux/>.

util-linux 2.39.3                 2023-12-01                        LOSETUP(8)