# Specifying N revisions after the initial commit 7 messages from 2015-09-22 to 2015-09-22. Participants: Josh Boyer, Konstantin Khomoutov, Junio C Hamano, Theodore Ts'o. Thread: https://gitlist.dev/t/40403 ## Josh Boyer, 2015-09-22 18:32 Subject: Specifying N revisions after the initial commit Message-ID: URL: https://gitlist.dev/e/CA%2B5PVA40x8bxW63X%2Bb77FDO8btRBaVOg%3Dfq%2BZyX%3DbdJ%2BuEm9gA%40mail.gmail.com ``` 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, 2015-09-22 18:40 Subject: Re: Specifying N revisions after the initial commit Message-ID: <20150922214037.b6b48059f051fcd4060a5a85@domain007.com> URL: https://gitlist.dev/e/20150922214037.b6b48059f051fcd4060a5a85%40domain007.com In-Reply-To: ``` On Tue, 22 Sep 2015 14:32:19 -0400 Josh Boyer 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, 2015-09-22 19:10 Subject: Re: Specifying N revisions after the initial commit Message-ID: URL: https://gitlist.dev/e/CA%2B5PVA6YWJizRjseeJ4EpRya0Mpyrv3DoNsB3%3DGi%3DdOiAoDfHg%40mail.gmail.com In-Reply-To: <20150922214037.b6b48059f051fcd4060a5a85@domain007.com> ``` On Tue, Sep 22, 2015 at 2:40 PM, Konstantin Khomoutov wrote: > On Tue, 22 Sep 2015 14:32:19 -0400 > Josh Boyer 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, 2015-09-22 19:55 Subject: Re: Specifying N revisions after the initial commit Message-ID: URL: https://gitlist.dev/e/xmqqpp1a2pql.fsf%40gitster.mtv.corp.google.com In-Reply-To: ``` Josh Boyer 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). ``` ## Josh Boyer, 2015-09-22 20:11 Subject: Re: Specifying N revisions after the initial commit Message-ID: URL: https://gitlist.dev/e/CA%2B5PVA6zUXGs2R-MOoj5sBxwUqM7HP%3Dtx-0WZJzPf71MsjH9MA%40mail.gmail.com In-Reply-To: ``` On Tue, Sep 22, 2015 at 3:55 PM, Junio C Hamano wrote: > Josh Boyer 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, 2015-09-22 20:40 Subject: Re: Specifying N revisions after the initial commit Message-ID: <20150922204000.GC3318@thunk.org> URL: https://gitlist.dev/e/20150922204000.GC3318%40thunk.org In-Reply-To: ``` 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. :-) - Ted ``` ## Junio C Hamano, 2015-09-22 20:42 Subject: Re: Specifying N revisions after the initial commit Message-ID: URL: https://gitlist.dev/e/xmqq1tdq2nkd.fsf%40gitster.mtv.corp.google.com In-Reply-To: <20150922204000.GC3318@thunk.org> ``` Theodore Ts'o writes: > 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 ;-) ```