how to mount a disk image and its partition in linux

The image was created by dd. if I had copied a partition, it would have been easy, but sometimes I want to preserve the state of the entire disk, and literally I have to jump through loops to get to my files.

$ dd if=/dev/sdb of=sdb.img

to mount the image, you need losetup

$ losetup /dev/loop1 sdb.img

remember /dev/loop1 is the entire disk, not a partition, so you cannot mount it directly to your file system. now find out where its partition is

$ parted /dev/loop1 unit B print

Model: Unknown (unknown)
Disk /dev/loop1: 4005527551B
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End          Size         Type     File system  Flags
1      32256B  4004904959B  4004872704B  primary  ext3          

Information: Don't forget to update /etc/fstab, if necessary.         

This tells us that the first partition starts at offset 32256

$ losetup -o 32256 /dev/loop2 /dev/loop1

mount the partition that we just setup as /dev/loop2

$ mount /dev/looop2 /mnt/tmp

You should see your file system, and you can modify it. Please remember to umount and sync so that the image can be copied back to a hard disk, which in my case is really a usb stick. This technique is quite useful for data recovery, virtualization, and so on.