# how to rebase backwards 5 messages from 2009-11-03 to 2009-11-06. Participants: bill lam, Junio C Hamano, David Kågedal. Thread: https://gitlist.dev/t/21460 ## bill lam, 2009-11-03 05:45 Subject: how to rebase backwards Message-ID: <20091103054510.GB7117@debian.b2j> URL: https://gitlist.dev/e/20091103054510.GB7117%40debian.b2j ``` I want to rebase a new branch onto an earlier commit original: E is a newly created branch from D and not yet modified E / -- A -- B -- C -- D new: what I wanted B'-- C' -- E for deployment / -- A -- B -- C -- D trunk so that I can continue to change all commits from B to C without affecting E I try (not sure if correct) doing it by git rebase --onto A B E but then there is a lot of conflict/both modified files. What will be the correct way to do it? -- regards, ==================================================== GPG key 1024D/4434BAB3 2008-08-24 gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3 ``` ## Junio C Hamano, 2009-11-03 07:43 Subject: Re: how to rebase backwards Message-ID: <7vocnkt5o4.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vocnkt5o4.fsf%40alter.siamese.dyndns.org In-Reply-To: <20091103054510.GB7117@debian.b2j> ``` bill lam writes: > I want to rebase a new branch onto an earlier commit > > original: > E is a newly created branch from D and not yet modified > > E > / > -- A -- B -- C -- D I sense a grave misconception here. If you have "not yet modified" anything at all since D, then E does *not* even exist. Remember, a branch is just a pointer pointing at a commit. You can have more than one such pointers pointing at the same commit. So, if you have been working on 'master' branch, building up to D, your history would look like this: ---A---B---C---D ^ master And you would "create a branch", say "hack", like this: $ git checkout -b hack What happens to your history when you do so is this: v hack ---A---B---C---D ^ master The same commit "D", pointed by two branches, 'master' and 'hack'. If you start editing files at this point, before making a commit, nothing in the history changes. But once you make a commit E while on branch 'hack' (since you ran "git checkout -b hack" above), the history changes into this: E /^ hack ---A---B---C---D ^ master > new: what I wanted > > B'-- C' -- E for deployment > / > -- A -- B -- C -- D trunk > > so that I can continue to change all commits from B to C without > affecting E > > I try (not sure if correct) doing it by > git rebase --onto A B E This asks to replay C, D and E in this order on top of A, to create C'--D'--E' / -- A which is different from what you described earlier. Assuming that you did actually commit to create E, after branching 'hack' off of 'master', i.e. your history looks like this: E /^ hack ---A---B---C---D ^ master the way to create what you want would actually be $ git checkout -b deploy master^ ;# branch deploy at "C" $ git cherry-pick hack ;# replay E on top of it which would make a history of this shape. v deploy E' E / /^ hack ---A---B---C---D ^ master If you need to later tweak B and C (only) for deployment branch, you could further rewrite B and/or C with $ git rebase -i A to rewrite everything since A leading to the tip of your current branch (i.e. E'), to result in v deploy B'--C'--E" / / E' E / / /^ hack ---A---B---C---D ^ master Now, you may not want to keep both 'hack' and 'deploy'. Perhaps you forked 'deploy' and modified for E but it was done on top of D that you did not want to include in 'deploy' by mistake. IOW, you might have done this: E /^ deploy ---A---B---C---D ^ master Another way to make 'deploy' point at the result of replaying E on top of C when your history looks like this is (assuming you are on 'deploy'): $ git reset --hard HEAD^^ ;# reset to "C" $ git cherry-pick @{1} ;# cherry-pick "E" That would give you a history of this shape: v deploy E' E / / ---A---B---C---D ^ master Note that the original commit "E" becomes dangling and it will eventually be garbage-collected. ``` ## bill lam, 2009-11-03 09:37 Subject: Re: how to rebase backwards Message-ID: <20091103093716.GD7117@debian.b2j> URL: https://gitlist.dev/e/20091103093716.GD7117%40debian.b2j In-Reply-To: <7vocnkt5o4.fsf@alter.siamese.dyndns.org> ``` Thank you for detail explanation. From what you described, I begin with master commit D, $ git checkout -b deploy $ git commit --allow-empty -m deploy E /^ deploy ---A---B---C---D ^ master $ git rebase -i A v deploy B'--C'--D'--E' / ---A---B---C---D ^ master since E is an empty commit, I suppose content of D' E' and D are identical at this point. Is that correct? If several months later, I forget which is the common ancestor for master and deploy, how do I generate the above graph or identify commit A as the common ancestor for these two branches? -- regards, ==================================================== GPG key 1024D/4434BAB3 2008-08-24 gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3 ``` ## David Kågedal, 2009-11-05 19:12 Subject: Re: how to rebase backwards Message-ID: <877hu4zsyq.fsf@lysator.liu.se> URL: https://gitlist.dev/e/877hu4zsyq.fsf%40lysator.liu.se In-Reply-To: <20091103093716.GD7117@debian.b2j> ``` bill lam writes: > Thank you for detail explanation. From what you described, I begin > with master commit D, > $ git checkout -b deploy > $ git commit --allow-empty -m deploy But why on earth would you want to use --allow-empty? There is no reason for that. Just let deploy be where it is when you created it (until you actually add something to it). > E > /^ deploy > ---A---B---C---D > ^ master > > $ git rebase -i A > > v deploy > B'--C'--D'--E' > / > ---A---B---C---D > ^ master > > since E is an empty commit, I suppose content of D' E' and D are > identical at this point. Is that correct? Assuming you didn't intentionally change it during rebase, yes. > If several months later, I forget which is the common ancestor for > master and deploy, how do I generate the above graph or identify > commit A as the common ancestor for these two branches? That is exactly what git does all the time. You can use many command. For instance git log --graph master...deply whill show you both branches, starting from their common ancestor. "git merge-base deploy master" will tell you that the common ancestor is. Etc. -- David Kågedal ``` ## bill lam, 2009-11-06 01:43 Subject: Re: how to rebase backwards Message-ID: <20091106014316.GC3928@debian.b2j> URL: https://gitlist.dev/e/20091106014316.GC3928%40debian.b2j In-Reply-To: <877hu4zsyq.fsf@lysator.liu.se> ``` On Thu, 05 Nov 2009, David Kågedal wrote: > command. For instance git log --graph master...deply whill show you both > branches, starting from their common ancestor. "git merge-base deploy > master" will tell you that the common ancestor is. Etc. David, thank you for this info. -- regards, ==================================================== GPG key 1024D/4434BAB3 2008-08-24 gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3 ```