jetson nano overlay read only filesystem (example /var folder)

overlay_ro_var.sh

# To prevent SD card corruption due powercuts etc. without /boot/inird
# this code can be used to protect any directory on your filesystem
# directory you know has lots of I/O 

# Overlay mounts a union filesystem (or directory tree) where upper directory is protected
# This is an example for /var folder making it read only 
# rw is done only on RAM (/dev/shm) hence logs etc. are lost on reboot

# create folders needed by overlay filesystem on /dev/shm (RAM)
mkdir -p /dev/shm/var_upper /dev/shm/var_workdir /dev/shm/var_overlay
mkdir -p /var_
# since /var will be 'hidden' by overlay mouting 'over' it
# we need it 'visible' somewhere to be usable as lowerdir 
# mount --bind does that to /var_
mount --bind /var /var_
sudo mount -t overlay overlay -o lowerdir=/var_,upperdir=/dev/shm/var_upper,workdir=/dev/shm/var_workdir /var

# This code is safe for reboot since unbind will be done automatically 
# and everything will got back to its default  
# can be replicated to any other folder that needs read-only protection