From: Lukas Tenbrink <lukastenbrink@gmail.com>
Date: Wed, 03 May 2023 21:26:17 GMT
Subject: Re: git diff -U0 header off-by-one error when deleting no lines
Message-ID: <7F1955DC-0296-470F-96F7-7BB96860D4BD@gmail.com>
In-Reply-To: <xmqqednxp0ys.fsf@gitster.g>

> This might be unintuitive, but it is specified by POSIX.  
> 
> Open
> 
>  https://pubs.opengroup.org/onlinepubs/9699919799/
> 
> and look for "@@".
> 
>    "@@-%s+%s@@", <file1 range>, <file2 range>
> 
>    Each <range> field shall be of the form:
> 
>    "%1d", <beginning line number>
> 
>    or:
> 
>    "%1d,1", <beginning line number>
> 
>    if the range contains exactly one line, and:
> 
>    "%1d,%1d", <beginning line number>, <number of lines>
> 
>    otherwise.
> 
>    If a range is empty, its beginning line number shall be the
>    number of the line just before the range, or 0 if the empty
>    range starts the file.
> 
> So a patch that adds lines to an empty file would look like
> 
>    $ diff -U0 /dev/null test1
>    --- /dev/null   2023-04-29 22:24:54.395999895 -0700
>    +++ test1       2023-05-03 14:02:27.718960038 -0700
>    @@ -0,0 +1,3 @@
>    +A
>    +B
>    +C
> 
> just as specified.  Note "-0,0" that says "there was no line before
> this hunk before this change".


That is indeed fairly unintuitive, but I appreciate the detailed response. I shall adapt my scripts.
Thanks and best wishes,

Lukas