bug#18808: implement 'tail -r' as synonym for 'tac'

[Bernhard Voelker]

On 10/24/2014 12:52 AM, Eric Blake wrote:
> On 10/23/2014 03:59 PM, Pádraig Brady wrote:
> [...].  So among existing POSIX-certified systems, 'tail -r' is
> the more popular extension at the moment.
> 
>> Saying that, I'm 50:50 for implementing `tail -r` for compat reasons.
> 
> Yes, this alone is reason enough to implement it in GNU, regardless of
> the direction POSIX takes.

I'm also worried about such an extension.  'tail -r' might be a useful
thing - when -r is the only option being used, but looking at all the
other options GNU tail already got over the years, adding -r doesn't
fit well.  You proposed to make -n available to tac and then jump into
it from 'tail -r'.  Well, what about -c? --follow={name|descriptor}?
--retry, --pid=PID?  To make -r fit well into tail, we'd have to add
every single feature of it to tac.  This sounds not like a good idea.
tail does a good job, and so does tac.  I'd almost prefer to fork+pipe
internally instead, similar to what e.g. 'sort --compress-program=PROG'
does.

Have a nice day,
Berny