What is a “loop device” when mounting?

Question

I am doing a mounting of an iso file and looking at this tutorial. They use the command:

$ mount -o loop disk1.iso /mnt/disk

I'm trying to understand the use of -o loop. So I have two questions:

When I look at the long man page for mount, it takes time to find that -o option. If I do man mount | grep "-o" I get and error, and when I look in the file I do not find info that "loop" is a command text for option -o. Where is that documented?

Also, what is the "loop device" concept for mounting? Even a good link to an explanation would be appreciated.

Answer 1

loop device is a pseudo ("fake") device (actually just a file) that acts as a block-based device. You want to mount a file (disk1.iso) that will act as entire filesystem, so you use loop.

The -o comes from the -options.

And the last thing, if you want to search for "-o" you need to escape the '-'. Try:

man mount | grep "\-o"

Answer 2

Traditionally, UNIX systems have had various types of nodes in their filesystems:

• directory
• file
• symlink
• block device
• character device
• FIFO
• UNIX domain socket

While there are now exceptions, generally block devices containing filesystems are mounted on directories.

Since you want to mount a file, you must first create a loop block device that is backed by the file. This can be done using losetup, but mount -o loop is a shortcut that handles that behind the scenes.