GNU bug report logs - #55652
please elaborate on diff -u -I RE

Previous Next

To add a comment to this bug, you must first unarchive it, by sending
a message to control AT debbugs.gnu.org, with unarchive 55652 in the body.
You can then email your comments to 55652 AT debbugs.gnu.org in the normal way.

Toggle the display of automated, internal messages from the tracker.

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

From: Harald Dunkel <harri <at> afaics.de>
To: bug-diffutils <at> gnu.org
Subject: please elaborate on diff -u -I RE
Date: Thu, 26 May 2022 11:15:39 +0200

Hi folks,

I have hoped that

	diff -I '^[[:space:]]*\#' file1 file2

would ignore comment lines, but it doesn't work as I expected.
Example:

	#!/bin/bash

	diff -version | head -1

	echo -e '\t# comment 1' >/tmp/x1
	echo -e '\t# comment 2' >/tmp/x2
	diff -u -I '^[[:space:]]*\#' /tmp/x1 /tmp/x2 && echo 'no diffs'

	echo -e '' >>/tmp/x1
	echo -e '' >>/tmp/x2
	diff -u -I '^[[:space:]]*\#' /tmp/x1 /tmp/x2 && echo 'still no diffs'

	echo -e 'hello, world' >>/tmp/x2
	diff -u -I '^[[:space:]]*\#' /tmp/x1 /tmp/x2 && echo 'no diffs'
	echo "diff --normal"
	diff -I '^[[:space:]]*\#' /tmp/x1 /tmp/x2 && echo 'no diffs'

Output:

	diff (GNU diffutils) 3.8
	no diffs
	still no diffs
	--- /tmp/x1     2022-05-26 10:54:36.187361365 +0200
	+++ /tmp/x2     2022-05-26 10:54:36.191361276 +0200
	@@ -1,2 +1,3 @@
	-       # comment 1
	+       # comment 2
	
	+hello, world
	diff --normal
	2a3
	> hello, world

This seems weird. Only 1 line has been appended to x2 before running
diff the third time. It does not match the pattern. How comes diff -u
reports 2 different lines, with an unchanged line in between? Is this
as expected?


Regards

Harri

Message #10 received at 55652-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Harald Dunkel <harri <at> afaics.de>
Cc: 55652-done <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#55652: please elaborate on diff -u -I RE
Date: Thu, 26 May 2022 18:53:20 -0700

On 5/26/22 02:15, Harald Dunkel wrote:
> How comes diff -u
> reports 2 different lines, with an unchanged line in between?

What else can diff do? You asked for context, and the context lines differ.


> Is this
> as expected? 

Yes, the manual says:

   However, '-I' only ignores the insertion or deletion of lines that
contain the regular expression if every changed line in the hunk--every
insertion and every deletion--matches the regular expression.  In other
words, for each nonignorable change, 'diff' prints the complete set of
changes in its vicinity, including the ignorable ones.


One might add an option to make diff behave differently in this case, 
although it's not clear what the "right" behavior would be. In the 
meantime perhaps you can get by with "diff -U0 -Iwhatever".

Message #13 received at 55652-done <at> debbugs.gnu.org (full text, mbox):

From: Harald Dunkel <harri <at> afaics.de>
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: 55652-done <at> debbugs.gnu.org
Subject: Re: [bug-diffutils] bug#55652: please elaborate on diff -u -I RE
Date: Fri, 27 May 2022 09:53:55 +0200

Hi Paul,

On 2022-05-27 03:53:20, Paul Eggert wrote:
> On 5/26/22 02:15, Harald Dunkel wrote:
>> How comes diff -u
>> reports 2 different lines, with an unchanged line in between?
> 
> What else can diff do? You asked for context, and the context lines differ.
> 

It could show the old ignored lines as unchanged, of course. This would allow
to apply the diff using patch. If it would show the new ignored lines instead,
then you could only reverse apply the diff, which is much less likely to be
useful. In the current implementation it is not useful at all.

> 
>> Is this
>> as expected?
> 
> Yes, the manual says:
> 
>      However, '-I' only ignores the insertion or deletion of lines that
> contain the regular expression if every changed line in the hunk--every
> insertion and every deletion--matches the regular expression.  In other
> words, for each nonignorable change, 'diff' prints the complete set of
> changes in its vicinity, including the ignorable ones.
> 

Where did you find this? The man page for diffutils 3.8 doesn't tell.


Regards

Harri

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.