Mount Command – Difference Between mount and mount -o loop

loop-devicemount

I have a iso file named ubuntu.iso.

I can mount it with the command: mount ubuntu.iso /mnt. After mounting it, I can see it from the outout of the command df -h: /dev/loop0 825M 825M 0 100% /mnt.

However, if I execute the command mount -o loop ubuntu.iso /mnt, I'll get the same result.

As I know, loop device allows us to visit the iso file as a device, I think this is why we add the option -o loop. But I can visit my iso file even if I only execute mount ubuntu.iso /mnt.

So I can't see the difference between mount and mount -o loop.

Best Answer

Both versions use loop devices, and produce the same result; the short version relies on “cleverness” added to mount in recent years. mount -o loop tells mount explicitly to use a loop device; it leaves the loop device itself up to mount, which will look for an available device, set it up, and use that. (You can specify the device too with e.g. mount -o loop=/dev/loop1.)

The cleverness is that, when given a file to mount, mount will automatically use a loop device to mount it when necessary — i.e., the file system isn’t specified, or libblkid determines that the file system is only supported on block devices (and therefore a loop device is needed to translate the file into a block device).

The loop device section of the mount man page has more details.

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