How do I trivially linearize my git history?

Question

Question

Given that I have a commit like the following:

A - B - C - D - G - H      \         /       - E - F - 

How do I get a perfectly linear history following only the first parents?

Eg, I want to get:

A - B - C - D - G'- H' 

It is expected that the sha1's of G and H will change, as noted above. (For obvious reasons). If the sha1's for A, B, C, and D change as well, then I want to know why.

Background

The intention is to avoid potential merge commits that would arise when doing a naive git rebase -i.

The real point is to hopefully gain additional insight in order to determine which commits are in a particular branch, eg, if we have the following graph:

I - J - K - L - M  \     /   /   /    - N - O - P - 

Where N was merged into K, but O was merged with --strategy=ours into L, and P was merged into M.

I want to be able to linearlize my history so that such problematic commits can be identified, and inspected. I want to have a tree where can identify that O was not put into the upstream branch, even if I potentially identify N and P as being potentially missing from the upstream branch as well, by using git cherry, however any suggestions here would be appreciated.

Answer 1

As you said, the following command works:

git filter-branch --parent-filter 'cut -f 2,3 -d " "' 

Why?

The problem you pose is solved by transforming each merge commit with a simple commit: this will simply remove the feature branches that were merged, since they will become orphan.

Each commit has one or more parent commits. Merge commit are the one which get more than one. Git stores this in each commit object of the history.

The git filter-branch command, with the --parent-filter option, allows to rewrite every commit's parent, passed to the filter as -p SHA1, repeated if there are more than one parent. Our filter cuts the parent and forces every commit to have a single parent.

For bonus, here's how to do it manually on a precise commit, by re-creating a new commit:

• get the commit tree and the first parent

  tree=`git show -s --format=%T SHA1` parent=`git show -s --format=%P SHA1 | cut -d " " -f1` 

• make a new commit with the same tree, same message, and keep only the first parent as ancestor

  git show -s --format=%B SHA1 | git commit-tree $tree -p $parent