GNU bug report logs - #26576
-v when used with -C

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Message #12 received at 26576-done <at> debbugs.gnu.org (full text, mbox):

From: Eric Blake <eblake <at> redhat.com>
To: 積丹尼 Dan Jacobson <jidanni <at> jidanni.org>,
 26576-done <at> debbugs.gnu.org
Subject: Re: bug#26576: -v when used with -C
Date: Thu, 20 Apr 2017 09:56:56 -0500

[Message part 1 (text/plain, inline)]

tag 26576 notabug
thanks

On 04/20/2017 09:39 AM, 積丹尼 Dan Jacobson wrote:
> You know if this only gets five lines,
> grep -C 2    ZZZ 00001.vcf|wc - 00001.vcf
>       5       5     197 -
>    1686    1731   83630 00001.vcf
> then this
> grep -C 2 -v ZZZ 00001.vcf|wc - 00001.vcf
>    1686    1731   83630 -
>    1686    1731   83630 00001.vcf
> should get all EXCEPT five lines.

Not necessarily true.  Let's simplify your example to something that
doesn't require knowing the contents of 00001.vcf:

$ seq 10 | grep -C 2    5
3
4
5
6
7

That says show all lines that match the regex '5', as well as (up to) 2
context lines on either side.  So we get a total output of five lines,
even though only one of those five lines actually matched.

Now the converse:

$ seq 10 | grep -C 2 -v 5
1
2
3
4
5
6
7
8
9
10

That says to show all lines that do not match the regex '5', as well as
(up to) 2 context lines on either side.  So we get a total output of ten
lines, but that is comprised of 4 matching lines, 1 context line, and 5
more matching lines (grep was smart enough to consolidate the two tail
lines after 4 and the two head lines before 6 into a single output line,
rather than displaying two independent chunks).

For further proof that -C and -v are correctly working together, try
something that excludes enough context lines to actually get two hunks:

$ seq 10 | grep -C 2 -v '[3-8]'
1
2
3
4
--
7
8
9
10

Now you're matching 2 lines, then 2 lines tail context, then a hunk
separator, then 2 lines head context, then 2 more matching lines.

Therefore, I'm tagging this as not a bug.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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