TIL: Mounting paritions from an img file

2025-04-13

My wife got me a little retro gaming device for my birthday today. I turned it on, played around a bit, then reset it. It wouldn’t boot. I did some digging and discovered that the SD card needed to be replaced. I mounted the card on my Linux box and used dd to back it up.

Here’s what I learned/remembered/will learn again:

dd is used to make direct copies of a device. You can use it to make a backup .img file of a device
lsblk lists block devices that may or may not be mounted
lsblk -o NAME,FSTYPE is handy cause it’ll show partitions and their file systems
fdisk -lu foo.img will show the partitions within the .img and their offsets

You can use a loopback mount to mount an .img so you can browse the files in it as if it were plugged in like a drive.

If you need to mount just one of the paritions, you need to run fdisk -lu like this:

❯ fdisk -lu ~/foo.img
Disk /home/jon/foo.img: 7.4 GiB, 7948206080 bytes, 15523840 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
Disklabel type: dos
Disk identifier: 0xc9f931c9

Device                      Boot    Start      End  Sectors  Size Id Type
/home/jon/foo.img1         32768   262143   229376  112M  b W95 FAT32
/home/jon/foo.img2        262144 13983743 13721600  6.5G 83 Linux
/home/jon/foo.img3      13983744 15523839  1540096  752M  7 HPFS/NTFS/exFAT

Suppose I wanted to mount just the Linux parition. I’d do that by using the start as an offset, and 512 as the sector size from the above fdisk call, with the mount command:

sudo mount -o loop,offset=$((512 * 13983743)) /home/jon/foo.img /mnt

Then it would be available at /mnt.

Side note: at first I went down the path of using losetup to create a loopback device until a Stack Overflow comment reminded me that the -o loop flag on mount can do that for you.