How to split out pieces of a file while preserving git line history: The easy way with misleading commits
Last time, we split pieces of a file into separate files while preserving line history. We had to do some
git commit-tree magic to get the results we wanted. But there’s a way to do this with an octopus merge. You just have to make sure to keep the octopus happy.
Again, let’s use the same scratch repo as we did for the last few days. You can follow the same copy/paste script, or you can take your existing scratch repo and
git reset --hard ready to get it back into its “ready to start experimenting” state.
To do things the easy way, we create a branch for each file we want to split out.
git checkout -b 2f git mv foods fruits git commit --author="Greg <greg>" -m "create fruits from foods"
As before, we start by renaming
fruits. This ensures that when git traces the history of the
fruits file, it will follow the history back into the
Next, we edit the
fruits file so that it contains the lines we want to split out from
foods (so far so good), but we also regenerate the
foods file with only its final contents. We intend to delete the vegetables from the
foods file, so we’ll delete both the fruits and the vegetables. and the rest go back into the
>foods echo cheese >>foods echo eggs >>foods echo milk git add foods >fruits echo apple >>fruits echo grape >>fruits echo orange git commit --author="Greg <greg>" -am "split fruits from foods" git checkout -
This is completely misleading and looks like we’ve lost our minds. We are ostensibly splitting the fruits out from the foods, but we also threw away the veggies. Somebody looking at this commit in isolation will say, “Hey, what happened to all the veggies?”
But let’s keep going. Repeat the exercise by splitting out the veggies and throwing away the fruit.
git checkout -b 2v git mv foods veggies git commit --author="Greg <greg>" -m "create veggies from foods" git checkout 2f foods >veggies echo celery >>veggies echo lettuce >>veggies echo peas git commit --author="Greg <greg>" -am "split veggies from foods" git checkout -
To save ourselves some typing, we used a
git checkout 2f foods to say “Just give me the copy of
foods from the
Finally, on the main branch, we also edit the
foods file into its final form.
git checkout 2f foods git commit --author="Greg <greg>" -am "split fruits and veggies from foods"
This commit is also absurdly misleading because most of the contents of the
foods file simply vanished!
git merge 2f 2v Trying simple merge with 2f Trying simple merge with 2v Merge made by the 'octopus' strategy. fruits | 3 +++ veggies | 3 +++ 2 files changed, 6 insertions(+) create mode 100644 fruits create mode 100644 veggies
The result is now that all three files are at their desired final forms, with the desired final line attributions.
git blame fruits ^e7a114d foods (Alice 2019-09-16 07:00:00 -0700 1) apple 86348be4 foods (Bob 2019-09-16 07:00:01 -0700 2) grape 34eb5bd1 foods (Carol 2019-09-16 07:00:02 -0700 3) orange git blame veggies ^e7a114d foods (Alice 2019-09-16 07:00:00 -0700 1) celery 86348be4 foods (Bob 2019-09-16 07:00:01 -0700 2) lettuce 34eb5bd1 foods (Carol 2019-09-16 07:00:02 -0700 3) peas git blame foods ^e7a114d (Alice 2019-09-16 07:00:00 -0700 1) cheese 86348be4 (Bob 2019-09-16 07:00:01 -0700 2) eggs 34eb5bd1 (Carol 2019-09-16 07:00:02 -0700 3) milk
However, the way we got there is very strange, and includes quite a few extremely misleading commits. I don’t really recommend it. I recommend doing it the hard way with
git commit-tree. Yes, it’s harder, but it’s also much less misleading to people who come to the repo later.
Bonus chatter: The misleading commit on the main branch is necessary because of another bug in octopus merges: It silently ignores the
--no-ff flag. You can see this if you skip the extra commit on the main branch and try an octopus merge right away:
git merge --no-ff 2f 2v Fast-forwarding to: 2f Trying simple merge with 2v Merge made by the 'octopus' strategy. foods | 6 ------ fruits | 3 +++ veggies | 3 +++ 3 files changed, 6 insertions(+), 6 deletions(-) create mode 100644 fruits create mode 100644 veggies
Even though we said
--no-ff, the octopus merge fast-forwarded anyway. The result is that the
foods file failed to preserve line history.
git blame foods 61bca29b (Greg 2019-09-18 07:00:00 -0700 1) cheese 61bca29b (Greg 2019-09-18 07:00:00 -0700 2) eggs 61bca29b (Greg 2019-09-18 07:00:00 -0700 3) milk
All the lines got blamed on Greg, when they really should be blamed on Alice, Bob, and Carol.