1LOSETUP(8) System Administration LOSETUP(8)
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6 losetup - set up and control loop devices
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9 Get info:
10 losetup [loopdev]
12 losetup -l [-a]
14 losetup -j file [-o offset]
16 Detach a loop device:
18 losetup -d loopdev...
20 Detach all associated loop devices:
22 losetup -D
24 Set up a loop device:
26 losetup [-o offset] [--sizelimit size] [--sector-size size]
28 [-Pr] [--show] -f|loopdev file
29 Resize a loop device:
31 losetup -c loopdev
33
35 losetup is used to associate loop devices with regular files or block
36 devices, to detach loop devices, and to query the status of a loop
37 device. If only the loopdev argument is given, the status of the cor‐
38 responding loop device is shown. If no option is given, all loop
39 devices are shown.
40 Note that the old output format (i.e., losetup -a) with comma-delimited
42 strings is deprecated in favour of the --list output format.
43 It's possible to create more independent loop devices for the same
45 backing file. This setup may be dangerous, can cause data loss, cor‐
46 ruption and overwrites. Use --nooverlap with --find during setup to
47 avoid this problem.
48 The loop device setup is not an atomic operation when used with --find,
50 and losetup does not protect this operation by any lock. The number of
51 attempts is internally restricted to a maximum of 16. It is recom‐
52 mended to use for example flock(1) to avoid a collision in heavily par‐
53 allel use cases.
54
57 The size and offset arguments may be followed by the multiplicative
58 suffixes KiB (=1024), MiB (=1024*1024), and so on for GiB, TiB, PiB,
59 EiB, ZiB and YiB (the "iB" is optional, e.g., "K" has the same meaning
60 as "KiB") or the suffixes KB (=1000), MB (=1000*1000), and so on for
61 GB, TB, PB, EB, ZB and YB.
62 -a, --all
65 Show the status of all loop devices. Note that not all informa‐
66 tion is accessible for non-root users. See also --list. The
67 old output format (as printed without --list) is deprecated.
68 -d, --detach loopdev...
70 Detach the file or device associated with the specified loop
71 device(s). Note that since Linux v3.7 kernel uses "lazy device
72 destruction". The detach operation does not return EBUSY error
73 anymore if device is actively used by system, but it is marked
74 by autoclear flag and destroyed later.
75 -D, --detach-all
77 Detach all associated loop devices.
78 -f, --find [file]
80 Find the first unused loop device. If a file argument is
81 present, use the found device as loop device. Otherwise, just
82 print its name.
83 --show Display the name of the assigned loop device if the -f option
85 and a file argument are present.
86 -L, --nooverlap
88 Check for conflicts between loop devices to avoid situation when
89 the same backing file is shared between more loop devices. If
90 the file is already used by another device then re-use the
91 device rather than a new one. The option makes sense only with
92 --find.
93 -j, --associated file [-o offset]
95 Show the status of all loop devices associated with the given
96 file.
97 -o, --offset offset
99 The data start is moved offset bytes into the specified file or
100 device. The offset may be followed by the multiplicative suf‐
101 fixes; see above.
102 --sizelimit size
104 The data end is set to no more than size bytes after the data
105 start. The size may be followed by the multiplicative suffixes;
106 see above.
107 -b, --sector-size size
109 Set the logical sector size of the loop device in bytes (since
110 Linux 4.14). The option may be used when create a new loop
111 device as well as stand-alone command to modify sector size of
112 the already existing loop device.
113 -c, --set-capacity loopdev
115 Force the loop driver to reread the size of the file associated
116 with the specified loop device.
117 -P, --partscan
119 Force the kernel to scan the partition table on a newly created
120 loop device. Note that the partition table parsing depends on
121 sector sizes. The default is sector size is 512 bytes, other‐
122 wise you need to use the option --sector-size together with
123 --partscan.
124 -r, --read-only
126 Set up a read-only loop device.
127 --direct-io[=on|off]
129 Enable or disable direct I/O for the backing file. The optional
130 argument can be either on or off. If the argument is omitted,
131 it defaults to off.
132 -v, --verbose
134 Verbose mode.
135 -l, --list
137 If a loop device or the -a option is specified, print the
138 default columns for either the specified loop device or all loop
139 devices; the default is to print info about all devices. See
140 also --output, --noheadings, --raw, and --json.
141 -O, --output column[,column]...
143 Specify the columns that are to be printed for the --list out‐
144 put. Use --help to get a list of all supported columns.
145 --output-all
147 Output all available columns.
148 -n, --noheadings
150 Don't print headings for --list output format.
151 --raw Use the raw --list output format.
153 -J, --json
155 Use JSON format for --list output.
156 -V, --version
158 Display version information and exit.
159 -h, --help
161 Display help text and exit.
162
165 Cryptoloop is no longer supported in favor of dm-crypt. For more
166 details see cryptsetup(8).
167
170 losetup returns 0 on success, nonzero on failure. When losetup dis‐
171 plays the status of a loop device, it returns 1 if the device is not
172 configured and 2 if an error occurred which prevented determining the
173 status of the device.
174
177 LOOPDEV_DEBUG=all
178 enables debug output.
179
182 /dev/loop[0..N]
183 loop block devices
184 /dev/loop-control
186 loop control device
187
189 The following commands can be used as an example of using the loop
190 device.
191 # dd if=/dev/zero of=~/file.img bs=1024k count=10
193 # losetup --find --show ~/file.img
194 /dev/loop0
195 # mkfs -t ext2 /dev/loop0
196 # mount /dev/loop0 /mnt
197 ...
198 # umount /dev/loop0
199 # losetup --detach /dev/loop0
200
202 Karel Zak <kzak@redhat.com>, based on the original version from
203 Theodore Ts'o <tytso@athena.mit.edu>
204
206 The losetup command is part of the util-linux package and is available
207 from https://www.kernel.org/pub/linux/utils/util-linux/.
208util-linux November 2015 LOSETUP(8)