How to make apt-get upgrade to work non-interactively?

[Ivan Teliatnikov <ivan@geosci.usyd.edu.au>]

Dear list users,

I have re-configured debconf to do non-interactive and low critical
levels. Nevertheless occasionally upgrade will not proceed without
user's interaction. I have notices that under such circumstances  system
always suggests a meaningful default behaviour. Sometimes it is yes,
sometimes it is no. For example:

Configuration file `/etc/cups/client.conf'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ?  Your options are:
    Y or I  : install the package maintainer's version
    N or O  : keep your currently-installed version
      D     : show the differences between the versions
      Z     : background this process to examine the situation
 The default action is to keep your current version.
*** client.conf (Y/I/N/O/D/Z) [default=N] ?

I want apt-get to stick to the default, how do tell it to do so?


There are two option available for apt-get 

--trivial-only - assumes NO as an answer to any prompts
--assume-yes  - assumes Yes as an answer to any prompts

Both options is not exactly what I want? Is there such option as
use-default?

If such option does not exist, how can be automatic upgrade of large
number of machines can be achieved using just apt-get.

I know that one way is to use systemimager, but it is not always
possible if the machine pool is heterogeneous?
-- 
Ivan Teliatnikov <ivan@geosci.usyd.edu.au>
University of Sydney