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

From: Eric Blake
Subject: bug#18808: implement 'tail -r' as synonym for 'tac'
Date: Thu, 23 Oct 2014 13:03:54 -0600
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On 10/23/2014 12:35 PM, Pádraig Brady wrote:
> Now the only reason I see would be for compat with solaris and BSD.
> I don't think they should have included that functionality though
> as there isn't much complementary with the other tail functions
> (being incompat with -f for example) and so is better split out as a separate 
> util.

Yeah, today's Austin Group call included a debate about whether
standardizing 'tac' would be smarter than 'tail -r', but the consensus
among those on the call was that since:
1. more implementations already have 'tail -r' than 'tac'
2. standardizing new requirements to an existing utility is easier than
adding a completely new utility (if only slightly, due to less paperwork)
3. GNU code tends to be the most adaptable

therefore, 'tail -r' won the vote, and the result of the meeting was an
action to me to see how adaptable GNU really is, and whether we can
indeed quickly implement 'tail -r'.

> The only small advantage of integration is a tiny perf benefit
> when specifying a number of lines, but really `tail -r -n$num $file`
> is of minimal improvement over `tac $file | head -n$num`
> (though would be better than `tail -n$num | tac` as it avoids buffering).

The fact that we DON'T have 'tac -n' is what intrigues me most; that's
the one thing that 'tail -r -n' can do in a single process that GNU
can't.  Of course, it is trivial to implement it by doing what tac has
always done and then stop output after -n is exceeded, and easy on
seekable files as well.  But on non-seekable files, it may lend to an
interesting optimization where knowing that the output will be limited
to n lines makes it easier to set up a rolling window of the most recent
n lines seen rather than having to dump out to a temporary file when the
file proves to be larger than the current hueristics of when tac stays
in memory.

> As a side note I see that solaris doesn't even support non seekable input mode
> (though BSD does):
>   $ printf "%s\n" 1 2 3 | tail -r -2
>   tail: cannot open input

Careful, which tail are you testing?  From your output, I'm guessing you
are trying to open the file named './-2' rather than limiting -r to a
line count.  We made several observations during the Austin Group that
/usr/bin/tail and /usr/xpg4/bin/tail behave differently.

$ /usr/bin/tail -rc
usage: tail [+/-[n][lbc][f]] [file]
       tail [+/-[n][l][r|f]] [file]
$ printf "%s\n" 1 2 3 | /usr/bin/tail -2r
$ printf "%s\n" 1 2 3 | /usr/xpg4/bin/tail -r -n2
$ printf "%s\n" 1 2 3 | /usr/xpg4/bin/tail -r -2

Curiously, Solaris usage also has a bug, as this command line SHOULD be
supported, per the output:

$ printf "%s\n" 1 2 3 | /usr/xpg4/bin/tail -r -c2
usage: tail [-f|-r] [-c number | -n number] [file]
       tail [+/-[number][lbc][f]] [file]
       tail [+/-[number][l][r|f]] [file]

But since it is not supported, the POSIX proposal does not require it,
and therefore, we don't have to worry about it either :)

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

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