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From:   Junio C Hamano <gitster@pobox.com>
To:     ZheNing Hu <adlternative@gmail.com>
Cc:     Git List <git@vger.kernel.org>
Subject: Re: [QUESTION] how to diff one blob with nothing
References: <CAOLTT8TVGna+C9nYy9nj3h5bT7AdAT0SKtUM3YVz6h=KZhGHHg@mail.gmail.com>
        <xmqqo7k7c1yw.fsf@gitster.g>
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Date:   Wed, 26 Jul 2023 11:23:52 -0700
In-Reply-To: <CAOLTT8R84Zrtpd=j6o2V=Y_uD54XAS5EA7NWHsdfL+XCkD5cqA@mail.gmail.com>
        (ZheNing Hu's message of "Thu, 27 Jul 2023 02:00:17 +0800")
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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 =E2=80=9Dwrite=E2=80=9C 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.