Re: virtual package question

[Miles Fidelman <mfidelman@meetinghouse.net>]

Don Armstrong wrote:
> On Thu, 16 Oct 2014, Miles Fidelman wrote:
> > As I dig more into the whole question of installing alternate init systems,
> > I realize that I'm a bit confused about the behavior of apt- when it comes
> > to virtual packages; and I can't find any documentation that clarifies
> > things.
> You're looking for Policy §7.5, specifically:
>
>       To specify which of a set of real packages should be the default to
>       satisfy a particular dependency on a virtual package, list the real
>       package as an alternative before the virtual one.
>
> > I know I ran across this at some point when installing Postfix - but I
> > can't for the life of me remember how this all worked out, an can't
> > find documentation anywhere.
> postfix Provides: mail-transport-agent.
>
> Things that need an MTA depend on exim4 | mail-transport-agent; this is
> how the default is defined.

Thanks Don, which brings us to the other part of my question....

- how does one explicitly install a lower-priority alternative, and

- what, if anything, prevents the default alternative from being 
reinstalled during a future install (e.g, as part of a dependency 
fulfillment or an updgrade)?

Miles

--
In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra