How to create the image file

[UPDATE: 2018.12.11.]
I’m sorry for the late correction, the approval request of comments landed in the SPAM. Stefan Naumann and Wojciech Franczyk pointed out correctly:

“Hi. In the point 3 you are creating FAT filesystem on the disk image, but you should have it created only on the partition. This is corrupting the image. You can check it trying fdisk -l test.img after performing the point 3 – you will get no partitions.

To fix it we first need to map the partition to /dev:
sudo losetup –offset 1048576 -f test.img
offset value is the start sector of the partition [2048] multiplied by sector size [512] to get bytes.

And create FAT filesystem on the partition, not disk:
sudo mkfs.vfat /dev/loop0

I’m leaving the solution here because I had exactly this problem as I needed valid whole disk image (to boot it), not only the partition 🙂
Nice tutorial thought, thanks for that, It helped me. Cheers.”

1. Create a file filled with zeros:

   $ dd if=/dev/zero of=test.img count=50 bs=1M

   This command makes a 50 MB image file. Change the “count” argument for different size.

2. Create the partition (and partition table):

   $ fdisk test.img
   
   Command (m for help): o
   Building a new DOS disklabel with disk identifier 0x46ac6035.
   
   Command (m for help): n
   Partition type:
     p primary (0 primary, 0 extended, 4 free)
     e extended
   Select (default p): <Enter>
   Using default response p
   Partition number (1-4, default 1): <Enter>
   First sector (2048-99999, default 2048):
   Using default value 2048
   Last sector, +sectors or +size{K,M,G} (2048-99999, default 99999): <Enter>
   Using default value 99999
   Partition 1 of type Linux and of size 47.8 MiB is set
   
   Command (m for help): t
   Selected partition 1
   Hex code (type L to list all codes): c
   
   Changed type of partition 'Linux' to 'W95 FAT32 (LBA)'
   
   Command (m for help): w
   The partition table has been altered!
   
   Syncing disks

3. Create the FAT file system in the image

   $ mkfs.vfat test.img 
   mkfs.fat 3.0.22 (2013-07-19)

 How to mount the image and copy files

1. Create a directory for mounting

   $ sudo mkdir /mnt/test

2. Mount the image

   $ sudo mount test.img /mnt/test

   Now you can copy/delete files in /mnt/test directory which will be written into the image file.

3. After file operations unmount the image

   $ sudo umount /mnt/test

4. Delete the directory

   $ sudo rmdir /mnt/test