Block device whose date is backed by a file.
Type, major, minor: block, 7, X for /dev/loopX
loop is a kernel module. Ubuntu 14.04 embeds it in the kernel.
Can be used to mount regular files that contain filesystems:
dd if=/dev/zero of=a.ex2 bs=1024 count=64
echo y | mke2fs -t ext2 a.ex2
mkdir -p d
sudo mount -o loop a.ex2 d
# Do stuff. sudo needed: permissions for d change when mounted.
echo a | sudo tee d/a
sudo umount d
mkdir -p e
sudo mount -o loop a.ex2 e
ls e
sudo umount e
A less magical way to do sudo mount a.ex2 d -o loop is:
losetup /dev/loop2 a.ex2
sudo mount /dev/loop2 d
Or to avoid hardcoding it:
loop="$(losetup -f --show a.ex2)"
sudo mount "$loop" d
A common use case is to mount a file that is a backup dump from a partition, e.g.:
dd if=/dev/sda5 of=a.img bs=1024
sudo mount a.img d/
The image to be mounted must contain a single filesystem (partition). If you've dumped the entire hard disk, e.g. with:
dd if=/dev/sda of=a.img bs=1024
it is not going to work. What to do in that case:
- http://stackoverflow.com/questions/1419489/loopback-mounting-individual-partitions-from-within-a-file-that-contains-a-parti
- http://unix.stackexchange.com/questions/9099/reading-a-filesystem-from-a-whole-disk-image
- http://superuser.com/questions/117136/how-can-i-mount-a-partition-from-dd-created-image-of-a-block-device-e-g-hdd-u
- http://askubuntu.com/questions/69363/mount-single-partition-from-image-of-entire-disk-device
Associate loop devices with files.
TODO: what system call is used to implement it?
Loop devices know what file they point to. Make a given loop device point to a given file:
losetup /dev/loop2 a.img
--offset: each loop device has an associated offset. If given, the device representing the file starts at a given offset within the file.
losetup --offset 32256 /dev/loop2 harddrive.img
Application: mount a given partition of a filesystem. But this can now be done better with -P.
If !=0, this automatically creates one loopXpY device per partition when you losetup: