Working with Image Files

When working with virtual machine disk images, it is not uncommon for them to contain partitions, and not just a single filesystem which starts at block 0.

There are two basic ways to mount a filesystem contained within the former type of images:

Manually

Use fdisk -l to get the image's partition information
Calculate the byte offset of the partition of interest (multiplying the starting sector reported by fdisk by the sector size, usually 512)
Use mount -o loop,offset=${STARTING_OFFSET} -t ${FS_TYPE} to mount

# fdisk -l 2022-04-04-raspios-bullseye-armhf-lite.img
Disk 2022-04-04-raspios-bullseye-armhf-lite.img: 1,9 GiB, 2017460224 bytes, 3940352 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: 0x7d5a2870

Device                                      Boot  Start     End Sectors  Size Id Type
2022-04-04-raspios-bullseye-armhf-lite.img1        8192  532479  524288  256M  c W95 FAT32 (LBA)
2022-04-04-raspios-bullseye-armhf-lite.img2      532480 3940351 3407872  1,6G 83 Linux
# mount -o loop,offset=$(( 532480 * 512)) 2022-04-04-raspios-bullseye-armhf-lite.img /mnt
# losetup
/dev/loop0         0 272629760         1  0 /tmp/2022-04-04-raspios-bullseye-armhf-lite.img   0     512

Automatically (with udiskctl)

If your system has udiskctl installed, all of the contained filesystems can be mounted automatically using the udiskctl command:

$ udisksctl loop-setup --file 2022-04-04-raspios-bullseye-armhf-lite.img 
Mapped file 2022-04-04-raspios-bullseye-armhf-lite.img as /dev/loop8.
$ mount
/dev/loop0p1 on /media/username/boot type vfat (rw,nosuid,nodev,relatime,uid=1000,gid=1000,uhelper=udisks2, ...)
/dev/loop0p2 on /media/username/rootfs type ext4 (rw,nosuid,nodev,relatime,uhelper=udisks2)