GNU bug report logs - #31537
Combining -v and -A/-B/-C options

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: "Garreau\, Alexandre" <galex-713 <at> galex-713.eu>
To: bug-grep <at> gnu.org
Subject: Combining -v and -A/-B/-C options
Date: Sun, 20 May 2018 03:15:43 +0200

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

It seems using a context option (-A (after-), -B (before-) or -C
(context)) when -v (invert-match) is in use just do nothing as well as
disabling the later.  I don’t see the utility of this behavior, as well
as I didn’t see it documented anywhere in the description of any of
these options.

I believe context options used in conjonction with invert-match option
should specify which lines *not to show* along with matched then not
shown lines.

For instance, while this command:

[without-2.sh (text/x-sh, inline)]

seq 1 100 | grep -v '2$'

[Message part 3 (text/plain, inline)]

does show the list of numbers going from 1 to 100, without those ending
with “2” in base 10, this one:

[without-2-and-3.sh (text/x-sh, inline)]

seq 1 100 | grep -v '2$' -A 1

[Message part 5 (text/plain, inline)]

should also hide the lines ending with “3” because they systematically
follow those ending with “2” (currently it just show all the numbers
from 1 to 100).

I didn’t look at if grep does act differently according the order
options are given, but if so maybe this behavior could be triggered only
if the context option were specified *after* the invert-match one?

Thank you for any attention!

Message #8 received at 31537 <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: "Garreau, Alexandre" <galex-713 <at> galex-713.eu>, 31537 <at> debbugs.gnu.org
Subject: Re: bug#31537: Combining -v and -A/-B/-C options
Date: Sun, 20 May 2018 18:03:04 -0700

Garreau, Alexandre wrote:
> I believe context options used in conjonction with invert-match option
> should specify which lines*not to show*  along with matched then not
> shown lines.

I can see it the other way. For example, "show all lines that lack signed 
integers, plus their context" can be done via a command like "grep -C1 -v -E 
'[-+][0-9]+([^.e]|$)'", and I don't see any easy way to do the same search under 
the changes you're proposing.

In essence you're proposing a new feature that I'd call "negative context"; 
i.e., if a line is not selected, then suppress the output of nearby lines that 
are selected. grep already has a lot of features and it's not clear this one is 
important enough to add.

> maybe this behavior could be triggered only
> if the context option were specified *after* the invert-match one

I'd prefer option order to not matter, at least when options do not simply 
override each other (as is the case here).

Message #11 received at 31537 <at> debbugs.gnu.org (full text, mbox):

From: "Garreau\, Alexandre" <galex-713 <at> galex-713.eu>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 31537 <at> debbugs.gnu.org
Subject: Re: bug#31537: Combining -v and -A/-B/-C options
Date: Mon, 21 May 2018 08:42:42 +0200

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

Le 20/05/2018 à 18h03, Paul Eggert a écrit :
> Garreau, Alexandre wrote:
>> I believe context options used in conjonction with invert-match option
>> should specify which lines*not to show*  along with matched then not
>> shown lines.
>
> I can see it the other way. For example, "show all lines that lack
> signed integers, plus their context" can be done via a command like
> "grep -C1 -v -E 
> '[-+][0-9]+([^.e]|$)'", and I don't see any easy way to do the same
> search under the changes you're proposing.

Oh in fact, I didn’t notice it actually had this effect… in fact when
only a few lines don’t match, especially when they’re not contiguous,
using -v with context options nearly equals not using grep at all… turns
out I narrowed enough my mind to miss this.

> In essence you're proposing a new feature that I'd call "negative
> context"; i.e., if a line is not selected, then suppress the output of
> nearby lines that are selected. grep already has a lot of features and
> it's not clear this one is important enough to add.

My memory is bad, but grepping at looking at my history for when I made
up this example says me I needed this feature for something like this:

[apt.sh (text/x-sh, inline)]

apt search programming ant | grep -v '^lib' -A 2 | less

[Message part 3 (text/plain, inline)]

Here I guess we’re in a case where sed would be better suited… like a
bunch of today’s features of grep (invert-match, personally I believe it
would naturally be handy with it): they both come from ed
after all. But this seems to be, as coincidently said today on #emacs
(and later outside I heard [1]), a case of:

[opposite.log (text/plain, inline)]

[CEST]
[03:16:37] <JordiGH> Don't you love it when you have commands like
[03:16:54] <JordiGH> do_the_thing --nope-this-option-makes-me-do-the-opposite-of-the-thing
[03:17:00] <JordiGH> Currently thinking of
[03:17:07] <JordiGH> ssh-keygen -R
[03:17:13] <chu> It's the Unix way
[03:17:14] <JordiGH> That -R makes it delete a key instead of generating one.
[03:17:55] <twb> ssh-keygen is a random grab-bag utility that does lots of things, and yes, it's dumb
[03:18:13] <JordiGH> It's the git way.
[03:19:16] <JordiGH> sort --random-sort
[…]
[03:19:24] <JordiGH> Isn't that, like, the opposite of sorting?
[PS : JordiGH agreed to be quoted in pm, and before that]
[06:37:04] <galex-713> twb: can I quote you too?
[06:38:11] <galex-713> chu: what about you then, too?
[06:38:41] <twb> galex-713: sure, you can quote that
[08:20:01] <chu> galex-713: it's fine

[Message part 5 (text/plain, inline)]

>> maybe this behavior could be triggered only
>> if the context option were specified *after* the invert-match one
>
> I'd prefer option order to not matter, at least when options do not
> simply override each other (as is the case here).

Agree, would add more complexity of use. And keywords are made to get
rid of order considerations anyway.

[1] Unix options that make commands do the opposite of their name: <https://mathstodon.xyz/@JordiGH/100064804426272107>

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