Re: [QUESTION] how to diff one blob with nothing

From: Taylor Blau <me@ttaylorr.com>
Date: Jul 27, 2023, 17:46 UTC
Message-ID: <ZMKtcaN7xYaTtkcI@nand.local>
In-Reply-To: <xmqqila6cz0n.fsf@gitster.g>

On Wed, Jul 26, 2023 at 11:23:52AM -0700, Junio C Hamano wrote:

Show 25 quoted lines

> ZheNing Hu <adlternative@gmail.com> writes:
>
> >> If you haven't created one, of course it would fail.  It should help
> >> to do
> >>
> >>     $ git hash-object -w --stdin </dev/null
> >>
> >> before running
> >>
> >>     $ git diff 00750edc e69de29bb
> >>
> >
> > This is a viable solution, but it's a bit ugly since a read-only "diff"
> > requires ”write“ an empty blob.
>
> If you do not even have an empty blob, you have no business
> comparing some other blobs you have with it, do you?
>
> If you do not have a file with a single line "hello, world\n" (that
> hashes to 4b5fa63702dd96796042e92787f464e28f09f17d if written in a
> blob), then you cannot do "git diff 4b5fa637" with anything and
> expect it to work.  It is the same thing.
>
> Besides, if you _know_ you want to compare a blob X to emptyness,
> you are better of doing "git cat-file blob X" in the first place.

Yeah, exactly. In 346245a1bb6 (hard-code the empty tree object, 2008-02-13), the rationale was partly that having the empty tree object is useful for showing some diffs, such as for the initial commit.

But I can't think of a similar argument for the empty blob. Like Junio said, if you're purposefully diff-ing against the empty blob, wouldn't you simply want the entire contents anyway? If that's the case, cat-file seems like a much more appropriate tool.

Thanks, Taylor