main-site/content/post/renames-in-git-explained.md

title	date	draft	toc	tags	author
Renames in Git explained	2020-11-28T12:07:00Z	false	true	[tech git rename]	Gaël Depreeuw

Introduction

One of the questions I'm often asked when teaching or explaining Git is how Git handles file and/or directory renames. The short answer to this is: It doesn't.

The slightly longer answer is: It does, but probably not in the way you envision it?.

Let's first take a look at how Git works internally. If you don't quite understand everything which follows, I can recommend reading chapter 10 of the Git Pro Book 2nd. Edition.

Git stores content, not files

When you commit to a Git repository it basically does the following:

Create a blob object for every file in the index (a.k.a. the staging area). A blob object is created by taking the content of the file, prepending a header and compressing the result. A SHA-1 hash is then calculated for this object which will be used to identify the object. The object is stored in the aptly named object store (found in .git/objects). The first 2 characters of the hash (in hex format) are used as a directory within this store, while the remaining characters are the filename of the blob object.

Let's look at an example. Create a git repo somewhere and create a file.

git init foo
cd foo
echo "foo" >> foo.txt

If you look into your .git/objects directory, it will be empty, aside from two empty subdirectories. Let's create a blob out of this file now.

$ git hash-object -w foo.txt
257cc5642cb1a054f08cc83f2d943e56fd3ebe99

You will now find an object in the store at:

$ find .git/objects -type f
.git/objects/25/7cc5642cb1a054f08cc83f2d943e56fd3ebe99

Here's an interesting exercise: what happens if you rename the file and create the blob with the renamed file?

$ mv foo.txt bar.txt
$ git hash-object -w bar.txt
257cc5642cb1a054f08cc83f2d943e56fd3ebe99

That's right, nothing changed, this makes sense as we're only adding the content to the object store! So how does Git remember the file names?

Filenames are part of tree objects

Aside from blob objects, Git also creates tree objects. You can sort of compare it to the directories in your worktree, i.e. for each directory in your worktree, you will have a tree object. A tree object's content looks like:

<mode> <type> <hash>    <name>

You can create this one for yourself by doing:

$ git update-index --add --cacheinfo 100644 \
  257cc5642cb1a054f08cc83f2d943e56fd3ebe99 foo.txt
$ git write-tree
fcf0be4d7e45f0ef9592682ad68e42270b0366b4
$ git cat-file -p fcf0be4d7e45f0ef9592682ad68e42270b0366b4
100644 blob 257cc5642cb1a054f08cc83f2d943e56fd3ebe99    foo.txt

There are 3 different types (that I know of) which can be referred to in a tree object: blob, tree, commit. Blobs represent file content, tree represent other tree (i.e. subdirectories) and commits represent submodules (i.e the commit at which they are included). A commit is a type of object which is also present outside the tree objects. They contain the top tree object (representing the top level of your repository), a link to one or more parent commits and some meta data (author, commit msg, date, ...). Finally and for completion's sake, there is also an object for annotated tags, which contain the commit it is pointing too as well as some meta data.

Renaming

Armed with the knowledge about trees and blobs, it should be fairly easy to understand what happens if you rename a file. To make not make it easier to understand, consider a simple example: we just rename a file at the top level.

Note: more complex examples are just more time consuming to explain, but not to understand. The same principles apply.

In case of such a rename, when you commit this rename, your repository will be impacted as follows:

• The blob representing the file remains unchanged.
• The top level tree object changes as it now has a different file name.
• The commit object will point to the new tree. (It's parent will point to the old tree.)

Nowhere is there any special mention of a rename occuring. Remember, we're just storing content! As such Git is not aware of any name changes. This is why the short answer was: Git doesn't handle file renames. The repository itself has no notion of this action. It's just has content and a structure for that content.

However, that does not mean you lose your history when you rename a file.

How to see history of a renamed file

Git might not store information on renames in it repository but it does come packed with an algorithm that detects file renames. For every add/delete pair added to the index, it determines how alike the paired files are. If they are at least 50% alike, it considered the pair to have been a rename. If there are multiple possibilities it takes the highest percentage one. If multipe files have the same percentage, it picks one depending on the implementation.

Note: I believe, but am not sure, it basicaly takes the first alphabeticaly match in the last case.

By default git log -- <file> does not track accross renames. If you want to do see the history across renames, you will need to add the --follow option.

You can also define the treshold percentage to be different from 50%. This is done via the -M<n> or --find-renames=<n> option. See the git documentation for the correct syntax.

You can also turn off rename detection by doing --no-renames

Rename best practice

Because of the treshold and the cheapness of commits, it is recommended that when you rename a file/directory. You commit those renames first, before you continue working on the renamed file. This basically makes it so you can use a treshold of 100% all the time.

Why did Git do it this way

This is pure speculation, but here's my thoughts on it:

Filenames are actually part of the underlying file systems, so for a version controls system to support multiple file system they have to handle filenames in their own way. This includes renames. If you think about what this would require for Git, it would not be very straightforward: Git could have chosen to provide a command to store rename data, let's say: git rename fileA fileB, but what should this command do? We can image it could create new 'rename object, which would hold the blob hash and the name of the previous file. Now, every time you would walk through history, when you encounter this object type, you would need to remember this redirections. There's probably a lot of little nuances which are not immediately apparent though and it does not deal with one of the major drawbacks of this new command: What happens if the user forgets it and just does mv fileA fileB?

Well, we'd actually want to have some mechanism to detect this as a rename as once this is commited it becomes more difficult to undo this change (especially if we already pushed the commit!). So it sure would be nice if Git could somehow figure out that it was a rename. Which is exactly what they did. But now that we have this functionality, what actually is the point of the new command we wanted to implement? This is probably highly subjective, but to me it seems completely irrelevant now. Instead of having a command which can be forgotten and for which we need contigency, just use the contigency as the solution! It makes the behaviour a lot more consistent!

Can I fix my commit if I did change a lot of content after renaming

First, to prevent this: always check using git status whether are not the rename is being detected. Now, how to solve it?

It depends. If your commit is local only and it is the last commit, then you can fix this easily. There are many ways to to it, but one option is:

git mv <newname> <oldname> # Undo the file rename
git commit --amend # Commit the changes to the file
git mv <oldname> <newname> # Rename the file
git commit # Commit the rename

If you want to rename first and the changes second you can also do this, but it is a bit more complex:

git reset --soft HEAD~ # Go back one commit, but keep the changes
git restore --staged <oldname> <newname> # unstage the deletion and addition
git restore <oldname> # undelete the old file
mv <newname> <newname.tmp> # make a temp backup of the new file
git mv <oldname> <newname> # Rename the old file
git commit # commit the rename
cp <newname.tmp> <newname> # apply the new changes
git commit -a # Commit the changes

If the commit is already a couple of commits ago, you can do the same with an interactive rebase and doing either of the above at the correct time.

If you already pushed your commits you will have to check with the team if you can rewrite the history and push it. If this is not possible, you might need to find the right treshold to have Git mark it as a rename.

Summary

So in summary: no, Git does not store renames in its repository. Instead, for every add/delete pair in a commit, Git will do an similarity analysis and when they are X% alike (default 50%), it will assume a rename occured.

Some commands influenced by this are: git log, git diff and git merge. Options related to renames are:

-M=<n>, --find-renames=<n> # where n is the treshold percentage.
--no-renames # don't do any rename detection

It is best practise to handle renames in their own commits. Try to avoid renaming and modifying a file within the same commit.