When provisioning a device, it is useful to modify the disk image (
before flashing it to SD cards in order to edit necessary configurations.
In order to do this we can discover the partition offsets within the disk image
and mount the individual partition file systems based on these offsets.
In the example below we will use a BeagleBone Black disk image (
but the procedure is the same for all disk images.
We start off by looking at information about the image with
fdisk -l -u mender-beaglebone.sdimg
The output should look similar to the following:
Disk mender-beaglebone.sdimg: 384 MiB, 402653184 bytes, 786432 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: 0x7dea8cfc Device Boot Start End Sectors Size Id Type mender-beaglebone.sdimg1 * 49152 81919 32768 16M c W95 FAT32 (LBA) mender-beaglebone.sdimg2 81920 294911 212992 104M 83 Linux mender-beaglebone.sdimg3 294912 507903 212992 104M 83 Linux mender-beaglebone.sdimg4 524287 786431 262145 128M f W95 Ext'd (LBA) mender-beaglebone.sdimg5 524288 786431 262144 128M 83 Linux
In this example there are four partitions (plus an extended partition). Please see
Partition layout for a description of the
partitions Mender uses. The two Linux partitions in the middle, at device
.sdimg3, are the two rootfs partitions.
First, we need to know the sector size. This is shown by the second line of the output, in bold below:
Units = sectors of 1 * 512 = 512 bytes
The second piece of information we need is the start sector of the partition we want to mount.
This is the second column in the output from
fdisk. The start sector is shown in bold below for
our two rootfs partitions:
mender-beaglebone.sdimg2 81920 294911 212992 104M 83 Linux mender-beaglebone.sdimg3 294912 507903 212992 104M 83 Linux
In order to mount a partition we simply multiply the sector size and the start sector
and pass that to
mount. You can use
bash to do this calculation for you.
So in order to mount both the rootfs partitions in our example, we can run the following commands:
sudo mkdir /mnt/rootfs1 && sudo mkdir /mnt/rootfs2
sudo mount -o loop,offset=$((512*81920)) mender-beaglebone.sdimg /mnt/rootfs1
sudo mount -o loop,offset=$((512*294912)) mender-beaglebone.sdimg /mnt/rootfs2
Now you can modify the rootfs file systems in the paths
When finished, please remember to unmount so that the changes are written back to the
disk image, like the following:
sudo umount /mnt/rootfs1 && sudo umount /mnt/rootfs2
Optionally, you can also remove the created directories:
sudo rmdir /mnt/rootfs1 && sudo rmdir /mnt/rootfs2