apt and do-release-upgrade question - what's sources.list.distUpgrade for?

[Liam Proven]

On Mon, 20 Jul 2020 at 11:38, Chris Green <cl at isbd.net> wrote:

AFAICS:

>     -rw-rw-r-- 1 root root 3315 Jul 20 10:22 sources.list

The above is your active config file

>     -rw-rw-r-- 1 root root 3304 Jul 20 10:22 sources.list.distUpgrade

This one (& others with the same extension) is a backup made when you
did an upgrade of the distribution -- today, I guess...?

>     -rw-rw-r-- 1 root root 3304 Nov 24  2019 sources.list.save

This is a backup made last November, perhaps when you ran some script
that added a repo to sources.list and so it kept an unmodified copy.

Adding an extension is standard DOS/Windows practice. When I was a DOS
greenhorn in 1988, I was taught that, before I edited a config file, I
should copy it to $name.BAK so I had something to fall back upon if my
change didn't work and I could not remember how to revert it manually,
or if some accident over-wrote the file.

So it became almost muscle memory, like the bikers' "lifesaver"
over-the-shoulder glance, to never ever do:

EDLIN CONFIG.SYS

but instead always do

COPY CONFIG.SYS *.BAK
EDLIN CONFIG.SYS

(This is an example of the sort of handy wildcard usage that is legal
in the DOS/NT shells, but illegal in the *nix shell, which is why
after >30 years, I _still_ prefer the Windows command line.)

This habit was inherited from VAX-VMS which did it automatically:

EDT CONFIG.SYS

... would automatically create CONFIG.SYS;1 and edit in a copy.

DOS can't do this -- no double extensions, no file versioning -- so
instead it changed the extension. The programmers who wrote MS-DOS
took a lot of "inspiration" (*coughs in a diplomatic manner*) from
DR's CP/M. CP/M itself took "inspiration" from DEC OS/8 and other DEC
minicomputer OSes -- that's where the 8-dot-3 filenames and many of
the actual file extensions (.COM, .EXE, .SYS etc.) came from.

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