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I would like to boot a testing Ubuntu release from a directory (which contains a debootstrap based install).

As far as I understand I just need someway to tell the boot process (initrd scripts?) that it should chroot() into the specified dir immediately after mounting the root partition, and then resume the regular upstart/init start.

Could someone provide some instructions on how to achieve this?

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Check out my working example of it here… – kenn Feb 20 '14 at 15:37
up vote 3 down vote accepted

You can boot a sub directory by replacing your sbin init with a script which uses pivot_root, the process is sketchy and various people have reported failure as much as sucesses. So try it and see, but keep a live cd handy in case your system becomes unbootable.

Details here:

What we could do with, is an automatic system for being able to boot multiple roots from the same drive. It should be programatically possible to do.

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Rather than replace /sbin/init just add an /init script to root which does the pivot root. – Matt H Apr 22 '14 at 21:43

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