[conspire] How to update packages when Deb is behind?

[Rick Moen]

Quoting Paul Zander (paulz at ieee.org):

> I have now edited sources.list.  Removing all of the lines commented out leaves the following.   All of these lines were previously present, but have been edited.
> 
>   deb http://deb.debian.org/debian/ stable main contrib non-free
>   deb http://security.debian.org/ stable/updates main contrib non-free
>   deb http://deb.debian.org/debian/ stable-updates main contrib non-free
>   deb http://www.deb-multimedia.org stable main non-free
> 
> BTW, the top of the file has this line generated during the original install.
> # deb cdrom:[Debian GNU/Linux testing _Jessie_ - Official Snapshot amd64 lxde-CD Binary-1 20140728-06:13]/ testing main
> If I read the release info correctly July of 2014, Jessie was "testing"

Ah, good point.  I hadn't picked up the implications of that.  Bearing
that in mind, it clarifies just slightly how this situation came about
originally:  It seems that, some time in 2015, you used CD media for 
a snapshot of Debian 8 'jessie' frozen to an ISO in back when it was the
branch on the Testing track.  Which explains why your system was
configured as if it were a rather out-of-date Debian Testing machine in
many but not all ways.

And also, if I'd paid attention to that line the first time, I'd have
known you installed an LXDE image, and not wasted your time asking you
about DE choice.  Ah, well.

By the way, source.list lines starting with 'deb cdrom:' get
automatically created whenever you install from an Official Debian
optical disc, but can thereafter be such a nuisance that it's extremely
common, in fact nearly universal, to comment those lines out.  That is,
the 'cdrom:' apt mechanism is a keyword saying 'Prompt the user to
re-insert the optical disc if it isn't already available for mounting.'
Basically, if you say 'apt-get install foo' and apt-get thinks the
highest-versioned installable package is on your installation optical
disc, you'll be prompted to pop it in again.  Don't know about you, but
at the moment installation ends, I really don't want the distro to
request the installation media again, but rather consult Internet
repos.

> If I use apt-get with the above info, I should hopefully be updating
> to "stretch".  Or, if I change "stable" to "testing" I would get the
> current instantiation of buster.

Correct -- with the nitpicking proviso that this refers to apt-get
operations that substantively change things, a point I'll clarify below.

> I still want check the versions for a bunch of apps, just to see if I
> might be headed into a mine field.  
> 
> Anything else before I jump off the cliff?

Well, first slow down and consider the two steps of the standard apt-get
dance.  'apt-get update' replaces your current catalogues of installable
software available at the apt sources mentioned in sources.list.  It
makes no other changes to your system, to the best of my understanding.
Therefore, you can do that _and not proceed further_ harmlessly.  So, do
that now.

Worst case, you could revert sources.list to its former contents and do
'apt-get update' again, which would take you back to pretty much the
situation you started with.

But, my point is, once you have refreshed your system's package catalogues,
you can then use Debian package to tools to query what is _now_
available, and what would _now_ happen if you were to command
installation of something.

Among other things, apt-get includes this option:
   -s, --simulate, --just-print, --dry-run, --recon, --no-act

But, frankly, I never use that, myself.  I just do things like
'apt-cache search', 'apt-cache depends', 'apt-cache showpkg', and
'apt-cache show' to explore what's available.  _Or_, just go at the
command line to /var/lib/apt/lists and look at the *Packages[.gz] files
using zless.  (zless is 'less' with a front-end sniffer that checks to
see if the file is gzipped, and, if so, gunzips it into RAM before
handing the contents to 'less' -- so it's never particularly bad 
idea to say 'zless' instead of 'less', especially for file globs that
might match to some gzipped files among others.)  Each such package
displays information about packages available from one of the various
apt sources you've referenced in sources.list, in alphabetical order by
package name.  Including version information.