How to extract .backup files created by android stock recovery?
As a first step, you will need to merge the various .backupN files into one by using the following command:

cat userdata_yyyymmdd_HHMMSS.backup* &gt; userdata_yyyymmdd_HHMMSS.backup

It looks like the header size of 512 bytes is the same across many devices, so try to extract using the following command:

dd if=userdata_xxxxxxxx_yyyyyy.backup bs=512 skip=1 | gunzip -c | tar xv

If this doesn't work, you can try the other method as described in this post:


Download the simg2img tool from here.
Convert the .backup file into an image using command simg2img userdata_xxxxxxxx_yyyyyy.backup userdata_xxxxxxxx_yyyyyy.backup.raw
Mount the image using command mount -t ext4 -o loop,rw userdata_xxxxxxxx_yyyyyy.backup.raw "data".


You should now be able to copy the contents of the backup image.

Alternatively, you can download the simg2img binary along with the script that automates the above steps using the link from the above mentioned post.