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On a Linux system, is it possible to use only user-space tools to create and manipulate partition images (which will probably only have ext2 filesystems on them) and then take those partition images to create a disk image?

I can use genext2fs to generate an image of an ext2 filesystem in userspace, but I can't find any tool that can manipulate disk images (e.g. creating partition tables or create a disk image from existing partition images etc.) without the need for root access. Currently, I'm using kpartx to mount the partitions inside a disk image, but because this requires root, it doesn't really make it suitable for scripting, and if there's an error in the build script which isn't trapped, the mapper devices remain in use.

Basically, I have three directories, and I want each directory to become a partition on a disk image, but I want to script this and I really do not want to use any tools that require root access (it seems like there is nothing in this process that absolutely needs to have root access).

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Partition creation

To make a partition table, you can use your common tools directly on the file instead of the device. Most standard tools should support this usage, like fdisk and parted. parted is great to be integrated in a script as it can take the commands as argument.

parted -s testing.img mklabel gpt
parted -s -a none testing.img mkpart ESP fat32 0 4M
parted -s -a none testing.img mkpart linux ext4 4M 10M

File system creation

I don't have a clean solution to mount or create a file system in a virtual partition. The right thing to do would probably be to have a fuse (Filesystem in Userspace) driver for partition tables. But I haven't found one at the moment.

However, mke2fs can take a -E offset=1234 option to build an ext2, ext3 or ext4 file system at an arbitrary offset within the file. You can give it the offset of your partition. However, be sure to give it the size of the file system as well since the default behavior seems buggy and write a file system with the size of the virtual disk by default, extending the file if needed.

mke2fs -E offset=4000256 testing.img 6316k

Unfortunately, not all the mkfs have such an option. mkfs.vfat doesn't. So another solution can be to create a separate file of the size of the partition, use mkfs on it and then use dd to copy the content into the full virtual disk.

dd if=testing.img of=testing.fat32.img bs=512 skip=34 count=7779
mkfs -t vfat testing.fat32.img
dd if=testing.fat32.img of=testing.img bs=512 seek=34 count=7779 conv=notrunc

Not the cleanest or fastest solution, but it works, it is quite generic and it doesn't require root privileges. The conversion option sparse can also be used to speed things up a bit by not writing longs sequences of zeros.

Mounting file systems

You can mount a file system in a partiton with the help of fuse provided it has a support for your file system. There is a standard support for ext file systems under the name fuse-ext2 and one for FAT under the name fusefat. They unfortunately don't take an offset option at the moment. So I guess you're bound to either using mount or the same dd trick as earlier.

The standard method with partx or kpartx, (or the -o offset option of mount) require root privileges. The dd solution is slow, complex and error-prompt.

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