Appendix 3: The Loop Device

The loop device is a device driver that allows an image file to be mounted as

though it were a normal block device. The question that immediately jumps to

mind is,  So, how do I use this beast . As an example, let's look at mounting,

and examining the contents of, the rescue floppy image file.

First the CD ROM, or other device containing this image file needs to be

mounted like:

mount  r  t iso9660 /dev/scd0 /cdrom

Note:

The  r option declares the device to be read only. Also

/dev/scd0 assumes the drive is the first SCSI device. If the

machine uses a SoundBlaster Pro/Creative Labs combo for

the cdrom this device would be /dev/sbpcd0 instead. The

other possibilities depend on the particular interface being

used.

If this is an  Official CD  that was mounted, the file of interest is found as:

/cdrom/debian/hamm/main/disks i386/current/resc1440.bin

So the mount command using the loop device will look like:

mount  t msdos  o loop /../../resc1440.bin /mnt

After this an ls of /mnt will show all the files that will appear on the floppy disk

when this image file is transferred to it. This is a  live  file system. That is, it

can be modified just like any other read/write file system mounted as a block

device. The changes made to the file system while mounted become part of

the image file and will be reflected on the floppy constructed from that image.

Of course, in our example the file image resides on a read only medium so

changes aren't possible, but when the image file resides on a writable medium,

like the hard disk, then those kinds of changes become possible.

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