Re: Question about get-selections

[Daniel Burrows <dburrows@debian.org>]

On Thu, Apr 23, 2009 at 09:13:38AM +0000, "J.M.Roth" <lists+debianuser@roth.lu> was heard to say:
> > >Oh and I guess to make things really right one would have to use
> > >--get-selections '*'
> > >during the backup, or an additional
> > >dpkg --clear-selections
> > >before the restore. Just a thought.
> > 
> > --clear-selections before the restore makes sense.  But, any selection other 
> > than "install" isn't really necessary to reproduce.
> 
> If you consider a base system that you try to restore to some previously
> existing system that you took the backup from, that may be true. But if you want
> to 'convert' a generic system to some other system (that you took the backup
> from), it might be necessary to e.g. purge exim4 and install postfix, in case
> the package management wouldn't figure that out on its own.

  When I was working on this problem as a part-time sysadmin, I ended
up writing a big "script" for aptitude.  You can do this by (ab)using
the ability to enter commands at the aptitude prompt.  However, that
doesn't give you the ability to "snapshot" a state easily,
automatically and efficiently.  Probably not the greatest idea.

  Another option you could take would be to generate a pkgstates file.
You seem to be familiar with awk, so it shouldn't be hard.  All you
need is a bunch of stanzas like this, separated by blank lines:

Package: cyrus-admin-2.3
State: 3

  The "state" is a flag indicating the package's current state.  It's
written as an integer because I designed this code when I was young and
foolish. :-)  The relevant values are:

  1 -> Install
  2 -> Hold
  3 -> Remove
  4 -> Purge

  This is the file that "aptitude install" with no arguments uses to
decide what to install.  Generating it directly should give you a much
simpler and faster script.

  Similarly, you should be able to generate /var/lib/apt/extended_states
directly.  Each stanza there looks like:

Package: kpat
Auto-Installed: 1

  where of course "1" is "yes it is auto-installed".  You can also have
"0", but that means the same thing as not having a stanza at all for
that package.

  Please note, I haven't tried any of this myself!  And of course you'll
be at the mercy of any future changes in file format (not that I know
of any that are planned, and they'd have to be backwards-compatible
anyway...)

  Daniel