File: losetup.8

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.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
[\fIloopdev\fP]
.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
Set up a loop device:
.sp
.in +5
.B losetup
.RB [ \-o
.IR offset ]
.RB [ \-\-sizelimit
.IR size ]
.RB [ \-\-sector\-size
.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 with \fB\-\-find\fR during setup 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 \-d , " \-\-detach " \fIloopdev\fR...
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.
.TP
.BR \-D , " \-\-detach\-all"
Detach all associated loop devices.
.TP
.BR \-f , " \-\-find " "\fR[\fIfile\fR]"
Find the first unused loop device.  If a \fIfile\fR argument is present, use
the found device as loop device.  Otherwise, just print its name.
.IP "\fB\-\-show\fP"
Display the name of the assigned loop device if the \fB\-f\fP option and a
\fIfile\fP argument are present.
.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\fR " \fR[\fB\-o \fIoffset\fR]"
Show the status of all loop devices associated with the given \fIfile\fR.
.TP
.BR \-o , " \-\-offset " \fIoffset
The data start is moved \fIoffset\fP bytes into the specified file or device.  The \fIoffset\fP
may be followed by the multiplicative suffixes; see above.
.IP "\fB\-\-sizelimit \fIsize\fP"
The data end is set to no more than \fIsize\fP bytes after the data start.  The \fIsize\fP
may be followed by the multiplicative suffixes; see above.
.TP
.BR \-b , " \-\-sector-size " \fIsize
Set the logical sector size of the loop device in bytes (since Linux 4.14). The
option may be used when create a new loop device as well as stand-alone command
to modify sector size of the already existing loop device.
.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 \-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 \fB\-\-sector\-size\fR together with \fB\-\-partscan\fR.
.TP
.BR \-r , " \-\-read\-only"
Set up a read-only loop device.
.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 \-v , " \-\-verbose"
Verbose mode.
.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\-\-raw\fP, and \fB\-\-json\fP.
.TP
.BR \-O , " \-\-output " \fIcolumn\fR[,\fIcolumn\fR]...
Specify the columns that are to be printed for the \fB\-\-list\fP output.
Use \fB\-\-help\fR to get a list of all supported columns.
.TP
.B \-\-output\-all
Output all available columns.
.TP
.BR \-n , " \-\-noheadings"
Don't print headings for \fB\-\-list\fP output format.
.IP "\fB\-\-raw\fP"
Use the raw \fB\-\-list\fP output format.
.TP
.BR \-J , " \-\-json"
Use JSON format for \fB\-\-list\fP output.
.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=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
.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/.