[derefr]

I'm assuming you mean a shell into the iPad's own Unix environment. You could build something for iOS that works like Crouton does on ChromeOS just fine. (Just, nobody has done it yet, for some reason. The iPad Pro is plenty powerful enough to run VMs!)

[matthewbauer]

I don't think it's as easy as crouton. There's no chroot or user namespace equivalent for XNU.

[saagarjha]

iSH does this by emulating Linux rather than virtualizing it.