🚀 go-pugleaf

>The manuals say that the system when booting fires up /usr/etc/init.

The manuals lie.  "/sbin/init" is what's fired up.  (Well, actually, the
kernel searches several directories, but "/sbin" is the first one it
searches....)

The manual probably said "/etc/init" prior to SunOS 4.0, and got updated
to reflect the fact that programs in "/etc" moved to "/usr/etc".

>In my younger days a system came up with the root filesystem mounted but
>without /usr mounted.

Well, when the system *first* comes up, only root is mounted, but it
mounts "/usr" soon after that.

> What is the copy of init in /sbin for?

See above.

> Do I have to have /usr mounted at boot time?

See above.  It has to get mounted fairly early in the boot sequence,
because there's not a boatload of code on the root file system, by
default; the executables and shared libraries are largely on the "/usr"
file system.