is there a reason pre-commit.sample uses "git diff-index"?

is there a reason pre-commit.sample uses "git diff-index"?

[Robert P. J. Day]

  was going over some hooks and writing some tutorials for some of the
commit-related, client-side hooks, and was wondering (perhaps
stupidly) why the pre-commit.sample hook uses, as its last line:

  exec git diff-index --check --cached $against --
           ^^^^^^^^^^

as in, could this not be done equivalently with just git diff, not git
diff-index? i just did a quick test and it seems to do the same thing,
but i've never taken a close look at git diff-index before so i may
just be clueless about some important distinction.

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                  http://crashcourse.ca/dokuwiki

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Duy Nguyen]

On Thu, May 31, 2018 at 6:38 PM, Robert P. J. Day <rpjday@crashcourse.ca> wrote:
>
>   was going over some hooks and writing some tutorials for some of the
> commit-related, client-side hooks, and was wondering (perhaps
> stupidly) why the pre-commit.sample hook uses, as its last line:
>
>   exec git diff-index --check --cached $against --
>            ^^^^^^^^^^
>
> as in, could this not be done equivalently with just git diff, not git
> diff-index? i just did a quick test and it seems to do the same thing,
> but i've never taken a close look at git diff-index before so i may
> just be clueless about some important distinction.

git diff-index is "plumbing", designed for writing scripts. "git diff"
on the other hand is for users and its behavior may change even if it
breaks backward compatibility.
-- 
Duy

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Robert P. J. Day]

On Thu, 31 May 2018, Duy Nguyen wrote:

> On Thu, May 31, 2018 at 6:38 PM, Robert P. J. Day <rpjday@crashcourse.ca> wrote:
> >
> >   was going over some hooks and writing some tutorials for some of
> > the commit-related, client-side hooks, and was wondering (perhaps
> > stupidly) why the pre-commit.sample hook uses, as its last line:
> >
> >   exec git diff-index --check --cached $against --
> >            ^^^^^^^^^^
> >
> > as in, could this not be done equivalently with just git diff, not
> > git diff-index? i just did a quick test and it seems to do the
> > same thing, but i've never taken a close look at git diff-index
> > before so i may just be clueless about some important distinction.
>
> git diff-index is "plumbing", designed for writing scripts. "git
> diff" on the other hand is for users and its behavior may change
> even if it breaks backward compatibility.

  ah, this was a philosophical underpinning i was unaware of. i see
occasional explanations of git porcelain versus plumbing, but i don't
recall anyone simply stating that the plumbing is meant to have a
long-term stability that is not guaranteed for the porcelain.

  in any event, this does mean that, stability issues aside, "git
diff" would apparently have worked just fine for that hook.

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                  http://crashcourse.ca/dokuwiki

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Duy Nguyen]

On Thu, May 31, 2018 at 7:27 PM, Robert P. J. Day <rpjday@crashcourse.ca> wrote:
> On Thu, 31 May 2018, Duy Nguyen wrote:
>
>> On Thu, May 31, 2018 at 6:38 PM, Robert P. J. Day <rpjday@crashcourse.ca> wrote:
>> >
>> >   was going over some hooks and writing some tutorials for some of
>> > the commit-related, client-side hooks, and was wondering (perhaps
>> > stupidly) why the pre-commit.sample hook uses, as its last line:
>> >
>> >   exec git diff-index --check --cached $against --
>> >            ^^^^^^^^^^
>> >
>> > as in, could this not be done equivalently with just git diff, not
>> > git diff-index? i just did a quick test and it seems to do the
>> > same thing, but i've never taken a close look at git diff-index
>> > before so i may just be clueless about some important distinction.
>>
>> git diff-index is "plumbing", designed for writing scripts. "git
>> diff" on the other hand is for users and its behavior may change
>> even if it breaks backward compatibility.
>
>   ah, this was a philosophical underpinning i was unaware of. i see
> occasional explanations of git porcelain versus plumbing, but i don't
> recall anyone simply stating that the plumbing is meant to have a
> long-term stability that is not guaranteed for the porcelain.
>
>   in any event, this does mean that, stability issues aside, "git
> diff" would apparently have worked just fine for that hook.

I think there are also stuff like config variables which can change
porcelain command behavior but usually not plumbing. Command exit code
may be another area where porcelain and plumbing differ. But in this
particular case, I think "git diff" works fine (but still should not
be used unless you're just writing a quick throwaway script).
-- 
Duy

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Johannes Sixt]

Am 31.05.2018 um 19:27 schrieb Robert P. J. Day:
> On Thu, 31 May 2018, Duy Nguyen wrote:
>> git diff-index is "plumbing", designed for writing scripts. "git
>> diff" on the other hand is for users and its behavior may change
>> even if it breaks backward compatibility.
> 
>    ah, this was a philosophical underpinning i was unaware of. i see
> occasional explanations of git porcelain versus plumbing, but i don't
> recall anyone simply stating that the plumbing is meant to have a
> long-term stability that is not guaranteed for the porcelain.

So, there you have it. ;) Plumbing commands offer long-term stability. 
That is not just philosophical, but practically relevant.

>    in any event, this does mean that, stability issues aside, "git
> diff" would apparently have worked just fine for that hook.

It may have worked just fine. You should still not use it.

Didn't you say that you are teaching git and hooks? Then you should 
teach the right thing, and the right thing is to use plumbing for scripts.

-- Hannes

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Robert P. J. Day]

On Fri, 1 Jun 2018, Johannes Sixt wrote:

> Am 31.05.2018 um 19:27 schrieb Robert P. J. Day:
> > On Thu, 31 May 2018, Duy Nguyen wrote:
> >> git diff-index is "plumbing", designed for writing scripts. "git
> >> diff" on the other hand is for users and its behavior may change
> >> even if it breaks backward compatibility.
> >
> >    ah, this was a philosophical underpinning i was unaware of. i
> > see occasional explanations of git porcelain versus plumbing, but
> > i don't recall anyone simply stating that the plumbing is meant to
> > have a long-term stability that is not guaranteed for the
> > porcelain.
>
> So, there you have it. ;) Plumbing commands offer long-term
> stability. That is not just philosophical, but practically relevant.
>
> >    in any event, this does mean that, stability issues aside, "git
> > diff" would apparently have worked just fine for that hook.
>
> It may have worked just fine. You should still not use it.
>
> Didn't you say that you are teaching git and hooks? Then you should
> teach the right thing, and the right thing is to use plumbing for
> scripts.

  sure, i agree, but i don't recall *ever* running across the claim
that the "plumbing" commands had a long-term stability and backward
compatibility that the porcelain commands did not. is that mentioned
anywhere?

rday

-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                  http://crashcourse.ca/dokuwiki

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
========================================================================

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Stefan Beller]

On Thu, May 31, 2018 at 4:09 PM, Robert P. J. Day <rpjday@crashcourse.ca> wrote:
> On Fri, 1 Jun 2018, Johannes Sixt wrote:
>
>> Am 31.05.2018 um 19:27 schrieb Robert P. J. Day:
>> > On Thu, 31 May 2018, Duy Nguyen wrote:
>> >> git diff-index is "plumbing", designed for writing scripts. "git
>> >> diff" on the other hand is for users and its behavior may change
>> >> even if it breaks backward compatibility.
>> >
>> >    ah, this was a philosophical underpinning i was unaware of. i
>> > see occasional explanations of git porcelain versus plumbing, but
>> > i don't recall anyone simply stating that the plumbing is meant to
>> > have a long-term stability that is not guaranteed for the
>> > porcelain.
>>
>> So, there you have it. ;) Plumbing commands offer long-term
>> stability. That is not just philosophical, but practically relevant.
>>
>> >    in any event, this does mean that, stability issues aside, "git
>> > diff" would apparently have worked just fine for that hook.
>>
>> It may have worked just fine. You should still not use it.
>>
>> Didn't you say that you are teaching git and hooks? Then you should
>> teach the right thing, and the right thing is to use plumbing for
>> scripts.
>
>   sure, i agree, but i don't recall *ever* running across the claim
> that the "plumbing" commands had a long-term stability and backward
> compatibility that the porcelain commands did not. is that mentioned
> anywhere?

`man git`

LOW-LEVEL COMMANDS (PLUMBING)
       Although Git includes its own porcelain layer, its low-level commands
       are sufficient to support development of alternative porcelains.
       Developers of such porcelains might start by reading about git-update-
       index(1) and git-read-tree(1).

       The interface (input, output, set of options and the semantics) to
       these low-level commands are meant to be a lot more stable than
       Porcelain level commands, because these commands are primarily for
       scripted use. The interface to Porcelain commands on the other hand are
       subject to change in order to improve the end user experience.

One example that Junio seemed to worry about was 940a911f8ec (log:
if --decorate is not given, default to --decorate=auto, 2017-03-23), as git log
seems to be used pseudo-plumbing-ly as there is no good and equally powerful
plumbing command, so the likelihood of git-log calls in scripts out
there is high.

So maybe the community should strive to be more aggressive about
changing the porcelain interface for the better.

One thing that we discussed internally for example is changing the
output of the porcelain output of fetch, pull and push to give less
cryptic output, but rather a one line progress bar (and only show errors
when they occur). I think that would be an improvement, as I don't think
many people care about the exact numbers of objects transferred in
a push/fetch, but rather want to have an estimate of the time left for
example. Also a one line progress bar might save some precious screen
real estate.

Stefan

Re: is there a reason pre-commit.sample uses "git diff-index"?

[Junio C Hamano]

Stefan Beller <sbeller@google.com> writes:

> plumbing command, so the likelihood of git-log calls in scripts out
> there is high.
>
> So maybe the community should strive to be more aggressive about
> changing the porcelain interface for the better.

To me, these two paragraphs are being incoherent.

If plumbing these days lag behind "log" Porcelain and tempt script
writers more towards "log", we should aggressively reject attempts
to change the "log" Porcelain behaviour to keep it stable, until a
suitable plumbing that scripters can rely on catches up.

Or course, aggressively improving plumbing would be a good solution
to that problem as well ;-)