Re: tail +n does not work under Linux?

[Eric Blake]

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According to Christophe LYON on 5/12/2009 2:19 AM:
> On 11.05.2009 20:28, Bob Proulx wrote:
>> It is only because 'n' and 'N' are different cases of the same letter
>> that people's minds play tricks on them.
> 
> Indeed.
> Maybe a slight rewording of the man page could highlight this?
> (eg add another reference to the info page for this trick)

What do you think about the attached patch to the --help output (and thus
man page)?

> After reading those 2 paragraphs several times, there are still a few
> things not completely clear:
> - tail -COUNT is described as obsolete, but supported when it does not
> conflict with the new standard. But has _POSIX2_VERSION any influence on
> this support?

In the latest coreutils, there is no influence.  The POSIX folks made it
clear that while 'count -12' is no longer mandated (because it conflicts
with the argument guidelines, which state that 'app -12' is equivalent to
'app -1 -2' which is equivalent to 'app -2 -1' which is equivalent to 'app
- -21', but obviously 'count -12' and 'count -21' aren't equivalent), it is
also not forbidden.  Yes, there was a window of coreutils releases where
this syntax was outright rejected, depending on _POSIX2_VERSION, but that
is no longer the case.

> - tail +COUNT is supported on "older systems". Older than what? How do I
> know whether I am using a "older system" or not?

"older systems" being those that failed to define _POSIX2_VERSION, or
defined it to the older value, or when you are using the environment
variable to select the older behavior.

> 
> Is there a way to know which default POSIX version is used by coreutils?
> I have check the build logs, and saw nothing.

Look for something like this:

configure:12516: checking for desired default level of POSIX conformance
configure:12527: result: none-specified

In my case, I neither had the environment variable set at configure time,
nor did my system headers set _POSIX2_VERSION to either recognized value
(in other words, my system intentionally admits that it is not yet
compliant to either version).  I guess there are three accepted values
these days, now that POSIX 2008 is finalized; maybe our docs need to be
updated to mention the value required by the latest POSIX version.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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>From 81556e44f12489532bd5a0c445dab12db4f3b115 Mon Sep 17 00:00:00 2001
From: Eric Blake <address@hidden>
Date: Wed, 20 May 2009 06:32:10 -0600
Subject: [PATCH] tail: make --help less ambiguous

* src/tail.c (usage): Use -n CNT, not -n N, to avoid confusion.
Reported by Chrisophe Lyon.
---
 src/tail.c |   17 +++++++++--------
 1 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/src/tail.c b/src/tail.c
index fe34600..c58d830 100644
--- a/src/tail.c
+++ b/src/tail.c
@@ -1,5 +1,6 @@
 /* tail -- output the last part of file(s)
-   Copyright (C) 1989, 90, 91, 1995-2006, 2008 Free Software Foundation, Inc.
+   Copyright (C) 1989, 90, 91, 1995-2006, 2008-2009 Free Software
+   Foundation, Inc.

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -226,8 +227,8 @@ With no FILE, or when FILE is -, read standard input.\n\
 Mandatory arguments to long options are mandatory for short options too.\n\
 "), stdout);
      fputs (_("\
-  -c, --bytes=N            output the last N bytes; alternatively, use +N to\n\
-                           output bytes starting with the Nth of each file\n\
+  -c, --bytes=CNT          output the last CNT bytes; or, use +CNT to\n\
+                           skip the first CNT-1 bytes of each file\n\
 "), stdout);
      fputs (_("\
   -f, --follow[={name|descriptor}]\n\
@@ -237,8 +238,8 @@ Mandatory arguments to long options are mandatory for short 
options too.\n\
   -F                       same as --follow=name --retry\n\
 "), stdout);
      printf (_("\
-  -n, --lines=N            output the last N lines, instead of the last %d;\n\
-                           or use +N to output lines starting with the Nth\n\
+  -n, --lines=CNT          output the last CNT lines, instead of the last 
%d;\n\
+                           or use +CNT to skip the first CNT-1 lines\n\
       --max-unchanged-stats=N\n\
                            with --follow=name, reopen a FILE which has not\n\
                            changed size after N (default %d) iterations\n\
@@ -264,9 +265,9 @@ Mandatory arguments to long options are mandatory for short 
options too.\n\
      fputs (VERSION_OPTION_DESCRIPTION, stdout);
      fputs (_("\
 \n\
-If the first character of N (the number of bytes or lines) is a `+',\n\
-print beginning with the Nth item from the start of each file, otherwise,\n\
-print the last N items in the file.  N may have a multiplier suffix:\n\
+If the first character of CNT (the number of bytes or lines) is a `+',\n\
+print beginning with the CNT item from the start of each file, otherwise,\n\
+print the last CNT items in the file.  CNT may have a multiplier suffix:\n\
 b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024,\n\
 GB 1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.\n\
 \n\
-- 
1.6.3.rc3.2.g4b51