Before use, a loop device must be connected to an existing file in the filesystem. The association provides the user with an API that allows the file to be used in place of a block special file (cf. device file system). Thus, if the file contains an entire file system, the file may then be mounted as if it were a disk device.
Files of this kind are often used for CD ISO images and floppy disc images. Mounting a file containing a filesystem via such a loop mount makes the files within that filesystem accessible. They appear in the mount point directory.
A loop device may allow some kind of data elaboration during this redirection. For example, the device may be the unencrypted version of an encrypted file. In such a case, the file associated with a loop device may be another pseudo-device. This is mostly useful when this device contains an encrypted file system. If supported, the loop device is in this case the decrypted version of the original encrypted file and can therefore be mounted as if it were a normal filesystem.
Uses of loop mounting 
After mounting a file that holds a filesystem, the files within the filesystem can be accessed through the usual filesystem interface of the operating system, without any need for special functionality, such as reading and writing to ISO images, in applications.
- managing and editing filesystem images meant for later normal use (especially CD or DVD images or installation systems);
- installing an operating system without repartitioning a drive;
- permanent segregation of data (for example, simulating removable media on a faster and more convenient hard disk or encapsulating encrypted filesystems).
Some confusion exists about the naming of the loop device under various operating systems. Various Unix-like operating systems provide the loop device functionality under different names.
In Linux, device names are encoded in the symbol table entries of their corresponding device drivers. The device is called "loop" device and device nodes are usually named /dev/loop0, /dev/loop1, etc. They can be created by the makedev script for the static device directory, dynamically by the facilities of the device filesystem (udev), or directly by the mknod command. The management user interface for the loop device is losetup and is part of the util-linux package.
Sometimes, the loop device is erroneously referred to as 'loopback' device, but this term is reserved for a networking device in the Linux kernel (cf. loopback). The concept of the 'loop' device is distinct from that of 'loopback', although similar in name.
In BSD-derived systems, such as NetBSD and OpenBSD, the loop device is called "virtual node device" or "vnd", and generally located at /dev/vnd0, /dev/rvnd0 or /dev/svnd0, etc., in the file system. The vnconfig program is used for configuration.
FreeBSD followed the same conventions as other BSD systems until release version 5, in which the loop device was incorporated into the memory disk driver ("md"). Configuration is now performed using the mdconfig program.
In Solaris/OpenSolaris, the loop device is called "loopback file interface" or lofi, and located at /dev/lofi/1, etc. SunOS has the lofiadm configuration program. The "lofi" driver supports read-only compression and read-write encryption. There is also a 3rd party "fbk" (File emulates Blockdevice) driver available for SunOS/Solaris since summer 1988.
Mac OS X implements a native image mounting mechanism as part of its random access disk device abstraction. The devices appear in /dev as regular disk devices; reads from and writes to those devices are sent to a user-mode helper process, which reads the data from the file or writes it to the file. In the user interface it is automatically activated by opening the disk image. OS X can handle disk (.dmg or .iso), CD-ROM or DVD images in various formats.
Loop mounting is not natively available on Microsoft Windows operating systems (until version Windows 7, where this functionality is natively implemented, and available through the diskpart utility). However, the facility is often added using third-party applications such as Daemon Tools and Alcohol 120%. Freely-available tools from VMware (Disk Mount Utility) and LTR Data (ImDisk) can also be used to achieve similar functionality.
In A2 BlueBottle loop mounting is available via the creation/installation of a so called virtual disk by one of the commands VirtualDisks.Create or VirtualDisks.Install.
Mounting a file containing a disk image on a directory requires two steps:
- association of the file with a loop device node,
- mounting of the loop device at a mount point directory
These two operations can be performed either using two separate commands, or through special flags to the mount command. The first operation may be performed by programs such as losetup in Linux, or lofiadm in SunOS. As an example, if
example.img is a regular file containing a filesystem and
/home/you/dir is a Linux user's directory, the superuser (root) may mount the file on the directory by executing the following two commands:
losetup /dev/loop0 example.img mount /dev/loop0 /home/you/dir
The second command mounts the device on the directory
/home/you/dir. The overall effect of executing these two commands is that the content of the file is used as a file system rooted at the mount point.
The mount utility is usually capable of handling the entire procedure:
mount -o loop example.img /home/you/dir
The device can then be unmounted with the following command:
umount /home/you/dir # or, after finding the associated loop number by e.g. mount | grep "/home/you/dir" # or losetup -a | grep example.img umount /dev/loop<N>
See also 
- Device file system
- Network block device used when the file to associate with the device is on a remote computer
- cloop - a special compressed version of a loop device i.e. for live CDs
- With similar name, but different concept, a loopback interface, or loopback device is network facility in UNIX-like operating systems
- Disk image
- mdconfig—configure and enable memory disks, FreeBSD manual page
- See for example: DiskPart: vdisk create select attach, install and boot Windows 7 from virtual disk drive and other relative articles on this thematics.
- losetup manual page
- lofiadm, Sun System Administration Command manual