First userspace executalbe run at boot up

[Valdis.Kletnieks at vt.edu]

On Tue, 27 Aug 2013 11:00:00 +0200, Matthias Brugger said:

> All of them try to execute init by invoking run_init_process.
> So my question is, why does the rdinit= parameter exist, I suppose for
> reasons of compatibility with older kernel versions. But why it is set
> to "/init"?

Consider the case where you have an initrd that contains an "init" program
that launches your system - but you also have a "busybox" program for rescue
purposes.  If things get hosed, you can 'rdinit=/busybox' to get into
rescue mode.

> Searching in the web, I found information saying that the first file,
> the Linux kernel executes after boot is "/sbin/init". As far as I
> understand that's not correct.

It's semi-correct, in that /sbin/init is traditionally what gets launched
after the initrd/initramfs has done its magic.  Also, research the case
where there is neither an initrd or initramfs provided.
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