How to delete the old git history?

Question

I have git repository with many, many (2000+) commits, for example:

                 l-- m -- n   
                /
a -- b -- c -- d -- e -- f -- g -- h -- i -- j -- k
                     \
                      x -- y -- z

and I want to truncate old log history - delete all commits from log history starting from (for example) commit "f" but as the beginning of repository.

How to do it?

Answer 1

In order not to lose some history; better first take a copy of your repository :). Here we go: (<f> is the sha of the commit f that you want to be the new root commit)

git checkout --orphan temp <f>      # checkout to the status of the git repo at commit f; creating a branch named "temp"
git commit -m "new root commit"     # create a new commit that is to be the new root commit
git rebase --onto temp <f> master   # now rebase the part of history from <f> to master onthe temp branch
git branch -D temp                  # we don't need the temp branch anymore

If you have a remote where you want to have the same truncated history; you can use git push -f. Warning this is a dangerous command; don't use this lightly! If you want to be sure that your last version of the code is still the same; you can run git diff origin/master. That should show no changes (since only the history changed; not the content of your files).

git push -f  

The following 2 commands are optional - they keep your git repo in good shape.

git prune --progress                 # delete all the objects w/o references
git gc --aggressive                  # aggressively collect garbage; may take a lot of time on large repos