Extract system.img from Nexus 7 factory image

apt-get install lib32z1-dev
git clone https://github.com/anestisb/android-simg2img.git
cd android-simg2img/
cp simg2img /usr/bin/
simg2img system.img system.img.raw
mkdir ~/tmfs
mount -t ext4 -o loop system.img.raw ~/tmfs/

Alternately, you can install android-tools-fsutils package for simg2img and make_ext4fs.

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