threads / discuss / 40403

Specifying N revisions after the initial commit

Subject: Specifying N revisions after the initial commit

## tl;dr

7 messages between Sep 22, 2015 and Sep 22, 2015.

replies: 6people: 4as markdown or json

Josh Boyer· Sep 22, 2015, 18:32 UTC · lore

Hi All,

Please CC me as I'm not subscribed.

I was hoping someone could help me with the revision shorthand to get the commit sha of a commit N commits after the initial commit. Thus far I've figured out that to get the initial commit in a repository, you can use:

git rev-list --max-parents=0 HEAD

but I can't figure out how to get "give me the commit sha1 of the commit immediately after the initial commit", or for some number N. I could always do something like:

git rev-list HEAD | tail -2

to get both, but I was curious if there was a refspec shorthand for this that could be used. It seems that git's rev parsing is all built on going backwards in order (and probably rightfully so).

Thanks in advance.

josh

Konstantin Khomoutov· Sep 22, 2015, 18:40 UTC · re: Josh Boyer · lore

Re: Specifying N revisions after the initial commit

On Tue, 22 Sep 2015 14:32:19 -0400 Josh Boyer <jwboyer@gmail.com> wrote:

> Please CC me as I'm not subscribed.
> 
> I was hoping someone could help me with the revision shorthand to get
> the commit sha of a commit N commits after the initial commit.

What happens if right after the initial commit, there have been five branches created -- with no common commits except for the initial one?

That's the core limitation of the data model Git uses (and arguably any other DVCS system): all commits form a directed acyclic graph. The "directed" in that construct means that child commits contain a link to their parent commit (or commits) but not vice-versa.

Hence, given any particular commit, you're able to trace all of its ancestry, but the reverse is not possible.

Josh Boyer· Sep 22, 2015, 19:10 UTC · re: Konstantin Khomoutov · lore

Re: Specifying N revisions after the initial commit

On Tue, Sep 22, 2015 at 2:40 PM, Konstantin Khomoutov <kostix+git@007spb.ru> wrote:

Show 15 quoted lines

> On Tue, 22 Sep 2015 14:32:19 -0400
> Josh Boyer <jwboyer@gmail.com> wrote:
>
>> Please CC me as I'm not subscribed.
>>
>> I was hoping someone could help me with the revision shorthand to get
>> the commit sha of a commit N commits after the initial commit.
>
> What happens if right after the initial commit, there have been five
> branches created -- with no common commits except for the initial one?
>
> That's the core limitation of the data model Git uses (and arguably any
> other DVCS system): all commits form a directed acyclic graph.
> The "directed" in that construct means that child commits contain a
> link to their parent commit (or commits) but not vice-versa.

Hm. It has been so long since I've looked at the underlying model and git has proven to be so flexible on such a variety of things that I guess I forgot it was constructed through a DAG. The --reverse parameter to git-log and git-rev-parse had left me hopeful.

> Hence, given any particular commit, you're able to trace all of its
> ancestry, but the reverse is not possible.

That makes sense. I suppose I will have to resort to parsing output of git-rev-list or something. Thanks for the reminder.

josh

Junio C Hamano· Sep 22, 2015, 19:55 UTC · re: Josh Boyer · lore

Re: Specifying N revisions after the initial commit

Josh Boyer <jwboyer@gmail.com> writes:

Show 7 quoted lines

> On Tue, Sep 22, 2015 at 2:40 PM, Konstantin Khomoutov
> ...
>> Hence, given any particular commit, you're able to trace all of its
>> ancestry, but the reverse is not possible.
>
> That makes sense.  I suppose I will have to resort to parsing output
> of git-rev-list or something.  Thanks for the reminder.

I think Konstantin explained why it fundamentally does not make sense to ask "which one is the Nth one after the root". I am not sure how running rev-list and count its output would help, unless you are now solving a different problem (perhaps "find all the ones that are Nth after some root", which does have an answer).

Josh Boyer· Sep 22, 2015, 20:11 UTC · re: Junio C Hamano · lore

Re: Specifying N revisions after the initial commit

On Tue, Sep 22, 2015 at 3:55 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 15 quoted lines

> Josh Boyer <jwboyer@gmail.com> writes:
>
>> On Tue, Sep 22, 2015 at 2:40 PM, Konstantin Khomoutov
>> ...
>>> Hence, given any particular commit, you're able to trace all of its
>>> ancestry, but the reverse is not possible.
>>
>> That makes sense.  I suppose I will have to resort to parsing output
>> of git-rev-list or something.  Thanks for the reminder.
>
> I think Konstantin explained why it fundamentally does not make
> sense to ask "which one is the Nth one after the root".  I am not
> sure how running rev-list and count its output would help, unless
> you are now solving a different problem (perhaps "find all the ones
> that are Nth after some root", which does have an answer).

Oh, context would help, yes. In the case of the tree I'm parsing, I know for a fact that the commit history is entirely linear and will (should) always remain so. E.g.

A - B - C - D - E - F ... {N}

So yes, finding e.g. the second commit after the root is complicated for something resembling anything like a typical git repo, but this isn't like that. In other words, I can cheat. Or at least I'm pretty sure I can cheat :).

josh

Theodore Ts'o· Sep 22, 2015, 20:40 UTC · re: Josh Boyer · lore

Re: Specifying N revisions after the initial commit

On Tue, Sep 22, 2015 at 04:11:23PM -0400, Josh Boyer wrote:

Show 10 quoted lines

> Oh, context would help, yes.  In the case of the tree I'm parsing, I
> know for a fact that the commit history is entirely linear and will
> (should) always remain so.  E.g.
> 
> A - B - C - D - E - F ... {N}
> 
> So yes, finding e.g. the second commit after the root is complicated
> for something resembling anything like a typical git repo, but this
> isn't like that.  In other words, I can cheat.  Or at least I'm pretty
> sure I can cheat :).

I'd suggest making your script makes sure "git rev-list --merges A..N" doesn't output any commits, so you know for sure that the commit history is linear. That way you'll be certain that you can cheat. :-)

	   	    	     	    - Ted

Junio C Hamano· Sep 22, 2015, 20:42 UTC · re: Theodore Ts'o · lore

Re: Specifying N revisions after the initial commit

Theodore Ts'o <tytso@mit.edu> writes:

Show 15 quoted lines

> On Tue, Sep 22, 2015 at 04:11:23PM -0400, Josh Boyer wrote:
>> Oh, context would help, yes.  In the case of the tree I'm parsing, I
>> know for a fact that the commit history is entirely linear and will
>> (should) always remain so.  E.g.
>> 
>> A - B - C - D - E - F ... {N}
>> 
>> So yes, finding e.g. the second commit after the root is complicated
>> for something resembling anything like a typical git repo, but this
>> isn't like that.  In other words, I can cheat.  Or at least I'm pretty
>> sure I can cheat :).
>
> I'd suggest making your script makes sure "git rev-list --merges A..N"
> doesn't output any commits, so you know for sure that the commit
> history is linear.  That way you'll be certain that you can cheat.  :-)

There are histories with multiple roots without any merges, in which case you cannot ;-)