I recently had a need to dynamically create an img file containing a FAT32 partition so its contents would be bootable on a PC. I had a heck of a time trying to put all the pieces together from different articles online. Alas, I came up with a working solution, so here it is. Enjoy!

This solution is for Linux. It does not work on Windows, but will work on Windows Subsystem for Linux (WSL).

This is a multi-step process outlined below:

1. Create the disk.img image file
   • Create empty disk.img file
   • Create partition table
   • Create partition
   • Format partition as FAT32 file system
2. Create the partition.img image file
   • Create empty partition.img file
   • Format the partition image file as FAT32 file system
3. Copy files into partition.img
4. Copy partition.img into the partition in disk.img

Create the disk.img image file

First, we want to create a blank img file.

The dd command is used to create a file of a specified size. The bs parameter is the block size, and count is the number of blocks. So if we want a 1GB file, we would do bs=1M count=1024. The if parameter is the input file, which in this case is /dev/zero. /dev/zero is a special file that provides as many null characters as you want. So we can use this to create a blank file. The of parameter is the output file, which in this case is disk.img.

$ sudo dd if=/dev/zero of=disk.img bs=1M count=1024
user@example:~/img_test$ sudo dd if=/dev/zero of=disk.img bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB, 1.0 GiB) copied, 0.707222 s, 1.5 GB/s

Now we need to create a GPT partition table in the img file.

We are using the parted tool to manage the disk partitions. The --script parameter tells parted to not prompt for user input and that we are running in a script for automation. The disk.img parameter is the path to the img file we created. The mktable command creates a new partition table. The gpt parameter specifies the type of partition table to create.

sudo parted --script disk.img mktable gpt

Create a FAT32 partition that spans the entire disk.

--align optimal ensures the partition is aligned to the optimal sector size for the disk. mkpart creates a partition. primary is the partition type. fat32 is the file system type. 2048s is the starting sector. 100% is the ending sector.

sudo parted --script disk.img --align optimal mkpart primary fat32 2048s 100%

We can name the partition if we want. This is optional.

name sets the name of the partition. 1 is the partition number. BOOT is the name.

sudo parted --script disk.img name 1 BOOT

We can print the partition table to verify everything is correct.

$ sudo parted --script disk.img print
Model:  (file)
Disk /home/user/disk.img: 1074MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  1073MB  1072MB               BOOT  msftdata

Create the partition.img image file

We will use dd to create another img file to represent the FAT32 partition. Make it the same size as disk.img minus a couple megabytes. This is to make up for the fact that the partition stars 2048 sectors in, and not at the beginning of the disk. If you don’t, you will get out of space errors when copying this partition into disk.img

$ sudo dd if=/dev/zero of=partition.img bs=1M count=1022
1022+0 records in
1022+0 records out
1071644672 bytes (1.1 GB, 1022 MiB) copied, 1.00757 s, 1.1 GB/s

Now we need to create a FAT32 file system on the partition image file.

We will use the mkfs.vfat command to do this. -F 32 specifies the FAT32 file system. -v is verbose output.

$ sudo mkfs.vfat partition.img -F 32 -v
mkfs.fat 4.2 (2021-01-31)
partition.img has 64 heads and 63 sectors per track,
hidden sectors 0x0000;
logical sector size is 512,
using 0xf8 media descriptor, with 2093049 sectors;
drive number 0x80;
filesystem has 2 32-bit FATs and 8 sectors per cluster.
FAT size is 2040 sectors, and provides 261117 clusters.
There are 32 reserved sectors.
Volume ID is 986efae3, no volume label.

Let us print the partition info to verify everything is correct.

$ sudo parted --script partition.img print
Model:  (file)
Disk /home/user/img_test/partition.img: 1072MB
Sector size (logical/physical): 512B/512B
Partition Table: loop
Disk Flags:

Number  Start  End     Size    File system  Flags
 1      0.00B  1072MB  1072MB  fat32

Copy files into partition.img

We can use the mcopy tool to copy files into the partition.img file.

To test this, I created the directory files, with 3 text files, and a directory inner_dir, that contains another text file.

```bash
$ sudo mcopy -i partition.img /home/user/files/* -spQmv ::/
Copying inner_dir
Copying text4.txt
Copying text1.txt
Copying text2.txt
Copying text3.txt

s indicates a recursive copy and will copy all directories and their contents. p preserves the file attributes. Q is valid when copying multiple files and will quit as soon as one copy fails. m preserves the file modification time. v is verbose output.

There is a b switch to use batch copy mode for large recursive copies. For me, it kept erroring with Internal error, size too big.

Copy partition.img into the partition in disk.img

Lastly, we need to copy the contents of the partition.img file into the partition in disk.img. We can do this with the dd command after mounting the partitions of the image file.

We will use kpartx to attach the partition in disk.img to a loop device. kpartx will create a loop device for each partition in the image file. We will use the -a switch to add the partition to the loop device. -v is verbose output.

$ sudo kpartx -av disk.img
add map loop5p1 (253:0): 0 2093056 linear 7:5 2048

We can verify the attachment by listing out block devices.

$ lsblk
NAME      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
loop0       7:0    0  59.2M  1 loop /snap/core20/1977
loop1       7:1    0  59.3M  1 loop /snap/core20/2019
loop2       7:2    0 109.6M  1 loop /snap/lxd/24326
loop3       7:3    0  35.5M  1 loop /snap/snapd/20102
loop4       7:4    0  35.5M  1 loop /snap/snapd/20298
loop5       7:5    0     1G  0 loop
└─loop5p1 253:0    0  1022M  0 part
sda         8:0    0     1T  0 disk
└─sda1      8:1    0  1024G  0 part /mnt/example
sdb         8:16   0    30G  0 disk
├─sdb1      8:17   0  29.9G  0 part /
└─sdb15     8:31   0    99M  0 part /boot/efi

We can see that loop5 represents the loop device for disk.img and loop5p1 is the partition we created. Now lets copy the contents of partition.img into the loop5p1 partition. Notice the mapper location. This is where kpartx attached the partition.

$ sudo dd if=partition.img of=/dev/mapper/loop5p1 bs=1M
1022+0 records in
1022+0 records out
1071644672 bytes (1.1 GB, 1022 MiB) copied, 2.15913 s, 496 MB/s

Now lets unmount the partition.

$ sudo kpartx -dv disk.img
del devmap : loop5p1
loop deleted : /dev/loop5

We can delete partition.img.

Lets print the partition info for disk.img once more to verify we’re done.

$ sudo parted --script disk.img print
Model:  (file)
Disk /home/sync-user/img_test/disk.img: 1074MB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system  Name  Flags
 1      1049kB  1073MB  1072MB  fat32        BOOT  msftdata

We have a gpt type partition table and 1 partition labeled BOOT formatted with the FAT32 file system! disk.img is ready to go!

References

• Layouting a disk image and copying files into it