Download your raspberry image of choice. In my case I needed the redsleeve distro.
$ fdisk -l raspi-v2-redsleeve-cli-0.3.img
Disk raspi-v2-redsleeve-cli-0.3.img: 1939 MB, 1939865600 bytes
255 heads, 63 sectors/track, 235 cylinders, total 3788800 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00014d34
Device Boot Start End Blocks Id System
raspi-v2-redsleeve-cli-0.3.img1 8192 122879 57344 c W95 FAT32 (LBA)
raspi-v2-redsleeve-cli-0.3.img2 122880 3788799 1832960 83 Linux
We want to be able to mount/modify the second partition on /mnt. Run:
$ sudo kpartx -a raspi-v2-redsleeve-cli-0.3.img
which should add a loop device in /dev/mapper:
$ ls /dev/mapper
control loop0p1@ loop0p2@ root@ swap1@
The second partition is now mapped to /dev/mapper/loop0p2. Mount:
$ sudo mount -o rw /dev/mapper/loop0p2 /mnt
$ cd /mnt
$ ls
bin/ boot/ dev/ etc/ home/ lib/ lost+found/ media/ mnt/ opt/ proc/ root/ sbin/ selinux/ srv/ sys/ tmp/ usr/ var/
We can now modify the disk image as we please. Remember to umount and
kpartx -d when done.
$ sudo umount /mnt
$ sudo kpartx -d raspi-v2-redsleeve-cli-0.3.img
loop deleted : /dev/loop0