How to duplicate a file while preserving git line history
Today, we’re going to duplicate a file while preserving git line history.
This could be useful if you want two copies of a component, say, one where you are doing a bunch of disruptive work, and another that remains largely unchanged. The project continues to use the old, stable version, but there’s a feature flag to switch to the new, exciting one. Eventually, you’ll make the new, exciting one the default version.
When you do this, you want the line history of the new version to be the same as the line history of the old version, because the new version is basically a fork of the old version.
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.
Let’s set up a scratch repo to demonstrate. I’ve omitted the command prompts so you can copy-paste this into your shell of choice and play along at home. (The timestamps and commit hashes will naturally be different.)
git init >foods echo apple git add foods git commit --author="Alice <alice>" -m created >>foods echo orange git commit --author="Bob <bob>" -am orange git blame foods ^62ef37c (Alice 2019-09-19 07:00:00 -0700 1) apple 335acb1b (Bob 2019-09-19 07:00:01 -0700 2) orange
We employ our standard trick: Create a branch where the desired new file appears to have been created via a rename of the original file. And then restore the original file.
git checkout -b dup git mv foods foods-new git commit --author="Greg <greg>" -m "duplicate foods to foods-new" git checkout HEAD~ foods git commit --author="Greg <greg>" -m "restore foods" git checkout -
On this branch, we renamed
foods-new. When git traces the history of the
foods-new file, it’ll see that the file was created via rename from
foods, so git will use
food‘s history to build the line history.
And then we bring back the original
foods file. We use the
git checkout HEAD~ foods command to restore the file from a specific commit, namely the commit before we renamed it away.
git merge --no-ff dup Merge made by the 'recursive' strategy. foods-new | 2 ++ 1 file changed, 2 insertions(+) create mode 100644 foods-new
dup branch deleted the
foods file, and then restored it. That means there was no net change to the file in the
dup branch, and even
git log won’t notice it by default. If you do a log of the
foods file, the merge doesn’t even show up.
git log --oneline foods ← the merge doesn't appear 335acb1 orange 62ef37c created
The line histories of the two files are identical, because the
foods-new was created at the same time an identical
foods file disappeared, which made git consider the operation to be a rename for the purpose of history tracking.
git blame foods ^62ef37c (Alice 2019-09-19 07:00:00 -0700 1) apple 335acb1b (Bob 2019-09-19 07:00:01 -0700 2) orange git blame foods-new ^62ef37c foods (Alice 2019-09-19 07:00:00 -0700 1) apple 335acb1b foods (Bob 2019-09-19 07:00:01 -0700 2) orange
I suggest to add “git” tag to this series. These posts contains useful tricks that I would very likely want to revisit later.
> When git traces the history of the foods-new file, it’ll see that the file was created via rename from foods, so git will use food‘s history to build the line history.
If only it would. Just this week, I wanted to merge master into a branch which had been forked 443 commits ago. The merge failed because of a modify/delete conflict, although this was really a rename. To save time I simply merged in both the parent of and the commit that performed the rename, at which point the file was now renamed locally, so I could merge in master.
You probably ran into the changed file limit where git uses a simpler algorithm tutto figure out renames. You can increase the limit to give git more time to figure it out.
Don’t remember the command-line argument for that anymore
Actually the file hadn’t been changed, but just to be sure, I tried it with find-renames=100 and that worked…