Table of Contents
Let’s assume you created a disk image with
dd on a linux computer like
$ sudo dd if=/dev/sda of=disk.img bs=4M status=progress conv=fsync
There are several partitions in that image and we want to access the linux
filesystem on it. For reference, I’ll bring in some old backup I made from a
Raspberry Pi. That backup is taken from a 8GB sdcard, which is 2.6GB compressed
When uncompressed, look at the partition table with fdisk:
$ fdisk -l disk.img Festplatte disk.img: 7,4 GiB, 7948206080 Bytes, 15523840 Sektoren Einheiten: Sektoren von 1 * 512 = 512 Bytes Sektorgröße (logisch/physikalisch): 512 Bytes / 512 Bytes E/A-Größe (minimal/optimal): 512 Bytes / 512 Bytes Festplattenbezeichnungstyp: dos Festplattenbezeichner: 0x1b7f4bbb Gerät Boot Anfang Ende Sektoren Größe Kn Typ disk.img1 8192 532479 524288 256M c W95 FAT32 (LBA) disk.img2 532480 15523839 14991360 7,1G 83 Linux
We will refer to this output later again.
Using losetup #
The output of fdisk is not that important to us, unless we have an unknown disk image that we need to inspect first. I already know the partitions. The first is the FAT32 partition used for UEFI and the second is the root file system.
Creating and mounting the loop device #
$ sudo losetup --partscan --find --show disk.img /dev/loop1
The second line is the output of the program. I used losetup already today, so
this is not loop0 but loop1. You may get
Mount the new virtual loop device to the directory that you like. This is
~/tmp in my case.
$ sudo mount /dev/loop1p2 tmp
Removing the loop device #
$ sudo umount tmp $ sudo losetup -d /dev/loop1
From the output above, we see that
532480 is the starting unit of the linux
filesystem in this image file. Further above you see the Units (Einheiten):
1 Unit is 1 sector of 512 Bytes.
I use a german speaking computer, so you might look for Start or Offset or Beginning—you know what to look for…
We calculate the needed offset like:
532480 * 512 = 272629760
And the resulting command is
$ sudo mount -o loop,offset=272629760 disk.img tmp/
A remount is simple as
$ sudo umount tmp
When do you need this stuff #
I often create quick and dirty (big) card images from my Raspberry Pies. They are saved and easy to copy over to another storage (because they are a single file).
If you have less space,
dd is probably not the best method to create a disk
partimage for example creates images from partitions, but it only
saves the used data from that partition. Those images are smaller.