[TUHS] why does tar have the tape device hard coded into it and why is it mt1 instead of mt0

[random832@fastmail.com (Random832)]

On 2015-12-11, John Cowan <cowan at mercury.ccil.org> wrote:
> Will Senn scripsit:
>
>> Thanks, that'll teach me to read the man pages more carefully even
>> when the command is familiar in its modern form. 
>
> Note that if -f is not given, modern tars still default to the tape
> drive, /dev/st0 on Linux or /dev/sa0 on *BSD.

That depends on the tar. A lot these days default to stdin (or
stdout as appropriate to the operation). Many don't even support
the "drive number" option. OSX knows nothing of the options, and
Ubuntu's GNU tar says:

tar: Options '-[0-7][lmh]' not supported by *this* tar

GNU tar has the capability, but it's apparently not a very
well-used feature, since the ./configure script it ships with
has a bug in a sed command meant to convert e.g. "/dev/st0" to
"/dev/st" [prefix for 0-7 to be appended to].

Star switched in 1982, according to the author. And in any case
even when POSIX did define tar, it only said the default was
"system-dependent".

The only one I could find that keeps this alive is bsdtar
(libarchive) which apparently will even use something called
\\.\tape0 if compiled for Windows. But it doesn't actually
support the 0-7 options.

Well, that and OpenSolaris tar. Which apparently gets devices
_and_ blocking factors corresponding to arguments 0-7 from
/etc/default/tar:

> archive0=/dev/rmt/0		20	0
> archive1=/dev/rmt/0n		20	0
> archive2=/dev/rmt/1		20	0
> archive3=/dev/rmt/1n		20	0
> archive4=/dev/rmt/0		126	0
> archive5=/dev/rmt/0n		126	0
> archive6=/dev/rmt/1		126	0
> archive7=/dev/rmt/1n		126	0

(The third argument is a size parameter; you're supposed to use
it if you define one of the devices to be a floppy drive.)