Git cherry-pick syntax and merge branches

Question

So I have done countless cherry picks before and it seems that I must fail at life with this right now, I am trying to cherry pick from one branch to another which should be easy, how ever I get an error about it being a merge but not -m was given?

$ git cherry-pick a8c5ad438f6173dc34f6ec45bddcef2ab23285e0 error: Commit a8c5ad438f6173dc34f6ec45bddcef2ab23285e0 is a merge but no -m option was given. fatal: cherry-pick failed 

That looks wrong.......it should be:

$ git cherry-pick a8c5ad438f6173dc34f6ec45bddcef2ab23285e0 

Since when do I have to supply a -m function?

Answer 1

You have to supply -m if the commit is a merge commit, i.e. a commit with more than one parent.

Normally, what git cherry-pick REV does can be described as:

1. Take the changes between rev and its parent.

2. Apply these changes to the current HEAD and commit the result with rev's commit message.

A merge commit joins two lines of development. For example, one line implements widget, and the other line removes clutter. The merge gives you the code with the widget, sans the clutter.

Now consider step #1 of the cherry-pick process: git can't guess whether you want to remove the clutter or to implement the widget. Nor can you do both, because the information on how to do both is not contained inside a single merge commit, only the content of the resultant merged tree is.

The -m option allows you to tell git how to proceed. For example, if clutter removal happened on master and the merge commit was created using git merge WIDGET, then git cherry-pick -m 1 merged-commit will cherry-pick the new widget because diff between the merged tree and parent 1 (the last of clutter-removing commits) will have been exactly the widget addition. On the other hand, git cherry-pick -m 2 merge-commit will delete the clutter, because the difference between parent 2 (the last of the widget-adding commits) and merge-commit is exactly the clutter-removal missing from the widget branch.

Answer 2

The git is requesting you to specify parent number (-m), because your merge commit has two parents and git do not know which side of the merge should be considered the mainline. So using this option you can specify the parent number (starting from 1) of the mainline and cherry-pick in order to replay the change relative to the specified parent.

To find out your commit parents, try either:

git show --pretty=raw <merge_commit> 

or:

git cat-file -p <merge_commit> 

or even for better GUI visibility, try:

gitk <merge_commit> 

As result, you should get something like:

commit fc70b1e9f940a6b511cbf86fe20293b181fb7821 tree 8d2ed6b21f074725db4f90e6aca1ebda6bc5d050  parent 54d59bedb9228fbbb9d645b977173009647a08a9 = <parent1_commit> parent 80f1016b327cd8482a3855ade89a41ffab64a792 = <parent2_commit> 

Then check your each parent details by:

git show <parent1_or_2_commit> 

^{Add --stat to see list of modified files.}

Or use the following command to compare the changes (based on the above parent):

git diff <parent1_or_2_commit>..<commit> 

^{Add --stat to see list of modified files.}

or use the combined diff to compare the two parents by:

git diff --cc <parent1_commit> git diff --cc <parent2_commit> 

Then specify the parent number starting from 1 for your cherry-pick, e.g.

git cherry-pick -m 1 <merge_commit> 

Then run git status to see what's going on. If you don't want to commit the changes yet, add -n option to see what happens. Then when you're not happy, reset to HEAD (git reset HEAD --hard). If you'll get git conflicts, you'll probably have to solve them manually or specify merge strategy (-X), see: How to resolve merge conflicts in Git?