.TH LOSETUP 8 "November 2015" "util-linux" "System Administration"
.SH NAME
losetup \- set up and control loop devices
.SH SYNOPSIS
.ad l
Get info:
.sp
.in +5
.B losetup
.I loopdev
.sp
.B losetup -l
.RB [ \-a ]
.sp
.B losetup -j
.I file
.RB [ \-o
.IR offset ]
.sp
.in -5
Detach a loop device:
.sp
.in +5
.B "losetup \-d"
.IR loopdev ...
.sp
.in -5
Detach all associated loop devices:
.sp
.in +5
.B "losetup \-D"
.sp
.in -5
Print the name of the first unused loop device:
.sp
.in +5
.B "losetup \-f"
.sp
.in -5
Set up a loop device:
.sp
.in +5
.B losetup
.RB [ \-o
.IR offset ]
.RB [ \-\-sizelimit
.IR size ]
.in +8
.RB [ \-Pr ]
.RB [ \-\-show ] " \-f" | \fIloopdev\fP
.I file
.sp
.in -13
Resize a loop device:
.sp
.in +5
.B "losetup \-c"
.I loopdev
.in -5
.ad b
.SH DESCRIPTION
.B 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
\fIloopdev\fP argument is given, the status of the corresponding loop
device is shown. If no option is given, all loop devices are shown.
.sp
Note that the old output format (i.e. \fBlosetup -a\fR) with comma-delimited
strings is deprecated in favour of the \fB--list\fR output format.
.sp
It's possible to create more independent loop devices for the same backing
file.
.B This setup may be dangerous, can cause data loss, corruption and overwrites.
Use \fB\-\-nooverlap\fR to avoid this problem.
.SH OPTIONS
The \fIsize\fR and \fIoffset\fR
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.
.TP
.BR \-a , " \-\-all"
Show the status of all loop devices. Note that not all information is accessible
for non-root users. See also \fB\-\-list\fR. The old output format (as printed
without \fB--list)\fR is deprecated.
.TP
.BR \-c , " \-\-set\-capacity " \fIloopdev
Force the loop driver to reread the size of the file associated with the
specified loop device.
.TP
.BR \-d , " \-\-detach " \fIloopdev\fR...
Detach the file or device associated with the specified loop device(s).
.TP
.BR \-D , " \-\-detach\-all"
Detach all associated loop devices.
.TP
.BR \-\-direct\-io [ =on | off ]
Enable or disable direct I/O for the backing file. The optional argument
can be either \fBon\fR or \fBoff\fR. If the argument is omitted, it defaults
to \fBon\fR.
.TP
.BR \-f , " \-\-find"
Find the first unused loop device. If a
.I file
argument is present, use the found device as loop device.
Otherwise, just print its name.
.TP
.BR \-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 \fB\-\-find\fP.
.TP
.BR \-j , " \-\-associated " \fIfile
Show the status of all loop devices associated with the given
.IR file .
.TP
.BR \-J , " \-\-json"
Use JSON format for \fB\-\-list\fP output.
.TP
.BR \-l , " \-\-list"
If a loop device or the \fB-a\fR 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 \fB\-\-output\fP, \fB\-\-noheadings\fP
\fB\-\-json\fP and \fB\-\-raw\fP.
.TP
.BR \-n , " \-\-noheadings"
Don't print headings for \fB\-\-list\fP output format.
.TP
.BR \-o , " \-\-offset " \fIoffset
The data start is moved \fIoffset\fP bytes into the specified file or device.
.TP
.BR \-O , " \-\-output " \fIcolumns
Specify the columns that are to be printed for the \fB\-\-list\fP output.
.TP
.BR \-P , " \-\-partscan"
Force the kernel to scan the partition table on a newly created loop device.
.IP "\fB\-\-raw\fP"
Use the raw \fB\-\-list\fP output format.
.TP
.BR \-r , " \-\-read\-only"
Set up a read-only loop device.
.IP "\fB\-\-sizelimit \fIsize\fP"
The data end is set to no more than \fIsize\fP bytes after the data start.
.IP "\fB\-\-show\fP"
Display the name of the assigned loop device if the
.B \-f
option and a
.I file
argument are present.
.TP
.BR \-v , " \-\-verbose"
Verbose mode.
.TP
.BR \-V , " \-\-version"
Display version information and exit.
.TP
.BR \-h , " \-\-help"
Display help text and exit.
.SH ENCRYPTION
.B Cryptoloop is no longer supported in favor of dm-crypt.
.B For more details see cryptsetup(8).
.SH RETURN VALUE
.B losetup
returns 0 on success, nonzero on failure. When
.B 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.
.SH FILES
.TP
.I /dev/loop[0..N]
loop block devices
.TP
.I /dev/loop-control
loop control device
.SH EXAMPLE
The following commands can be used as an example of using the loop device.
.nf
.IP
# 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
.fi
.SH ENVIRONMENT
.IP LOOPDEV_DEBUG=all
enables debug output.
.SH AUTHORS
Karel Zak <kzak@redhat.com>, based on the original version from
Theodore Ts'o <tytso@athena.mit.edu>
.SH AVAILABILITY
The losetup command is part of the util-linux package and is available from
https://www.kernel.org/pub/linux/utils/util-linux/.
