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

[Eric Blake]

On 10/23/2014 03:59 PM, Pádraig Brady wrote:
>> 1. more implementations already have 'tail -r' than 'tac'
> 
> More implementations but definitely not more installations.
> POSIX should prefer the most popular interface in my biased opinion.

Hey, I like being biased towards free software, as well :)  Popularity
does have some impact when considering what to standardize, and the
Austin Group has been very good at considering GNU behavior to at least
ensure that new requirements in the standard do not conflict with GNU.

But to be pedantic, probably a majority of POSIX-certified installations
have 'tail -r' (maybe as many as 100%? 'tail -r' dates back to at least
1980, before POSIX was even a dream, and is on more flavors of Unix than
just Solaris) while very few POSIX-certified installations have 'tac'
(no one has yet certified a GNU/Linux platform, to my knowledge; and
while some POSIX systems like Solaris include various GNU tools in a
default installation, GNU tac doesn't tend to be one of them).

Sadly, even though many pieces of GNU software try hard to comply with
POSIX, no one has yet ironed out all the remaining warts to use GNU as a
POSIX-certified system (and it's not a trivial task; some of the warts
are embedded fairly deeply in the kernel, such as behavior of
unlink("symlink-to-dir/") being different than the current POSIX
requirements).  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.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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