After having to manually reconfigure all of my dotfiles for the 3rd or 4th time, I decided it was finally time to put them into version control. While I was poking around the internet to see how other people did this, I came across this post on Hacker News that described how to use Git’s “bare repository” feature to track my dotfiles without turning my home folder into a Git repo.
This sounded great, but I had never heard of a bare repository before, and I wanted to understand more before I committed all of my dotfiles to this thing. After sifting through piles of blog posts and StackOverflow answers, I remained unenlightened. I eventually figured it out by digging deeper into Git’s storage model, and decided to write up the pieces that were missing from the other explanations I found. I hope this post will be helpful to anybody else who wants to know what the heck a bare repository is and when you would want to use one.
What is a bare Git repository?
I assume that you’ve worked with a Git repo before. You’ve probably run
git init to create a new repository on your local machine or run
git clone to download a repo from GitHub/GitLab/BitBucket. You’ve probably even worked on different branches, made pull requests, and gone back to an old commit to figure out what you were thinking when you added that code.
That repo you’ve been using is a non-bare repository. It contains two pieces of state:
- A snapshot of all of the files in the repository (in Git jargon, this is the “working tree”)
- A history of all changes made to all the files that have ever been in the repository (there doesn’t seem to be a concise piece of Git jargon that encompasses all of this)
The snapshot is what you probably think of as your project: your code files, build files, helper scripts, and anything else you version with Git. I call this a snapshot because it’s what your files happen to look like at one point in time. If you check out a different commit, the files change: you’re looking at a snapshot from a different point in time.
The history is the state that allows you to check out that other commit and get a complete snapshot of what the files in your repository looked like when that commit was added. It consists of a bunch of data structures that are internal to Git that you’ve probably never interacted with directly.
In a non-bare repository, the history is stored in the
.git folder at the top level of your repository (I encourage you to poke around in it by running
tree .git1 - go ahead, I’ll wait). The most important piece to understand here is that the history doesn’t just store metadata (e.g. “User U added this many lines to File F at Time T as part of Commit C”), it also stores data (e.g. “User U added these exact lines to File F”).
The key idea of a bare repository is that you don’t actually need to have the snapshot. Git keeps the snapshot around because it’s convenient for humans and other non-Git processes that want to interact with your code2, but the snapshot is just duplicating state that’s already in the history. This is why you can delete all of the files in your snapshot and then restore them with
git reset --hard: the data is still in the history even if you delete the snapshot.
A bare repository is a Git repository that does not have a snapshot. It just stores the history. It also happens to store the history in a slightly different way3, but that’s not nearly as important.
A bare repository will still store your files (remember, the history has enough data to reconstruct the state of your files at any commit). You can even create a non-bare repository from a bare repository: if you
git clone a bare repository, Git will automatically create a snapshot for you in the new repository (if you want a bare repository, use
git clone --bare).
So why would we use a bare Git repository?
Almost every explanation I found of bare repositories mentioned that they’re used for centralized storage of a repository that you want to share between multiple users4. Basically, if you wanted to write your own GitHub/GitLab/BitBucket, your centralized service would store each repo as a bare repository. But why? How does not having a snapshot connect to sharing?
The answer is that there’s no need to have a snapshot if the only service that’s interacting with your repo is Git. Basically, the snapshot is a convenience for humans and non-Git tools, but Git only interacts with the history. Your centralized Git hosting service will only interact with the repos through Git commands, so why bother materializing snapshots all the time? The snapshots only take up extra space for no gain5.
Using a bare Git repository to track dotfiles
Bringing us back full-circle, we can now look at why you’d want a bare Git repo for your dotfiles.
We already covered the fact that Git repos don’t need to store a snapshot of the files tracked by the repo. More generally, Git repos don’t need to store that snapshot in the same folder as the history. You can store your history in one folder and your snapshot in a different folder.
This is why it’s nice to use a bare repo to track dotfiles (or any set of files that already exist in some location that you don’t want to turn into a Git repo): you can leave the dotfiles where they are (so that they’ll get picked up by the different tools that use them) while still getting the benefits of version control. Your history will live in a separate folder, while your dotfiles (your snapshot) can live wherever they need to be. You don’t need to manually link dotfiles from your Git repo to their correct locations, and you don’t need to turn your home directory into a Git repo6.
Here’s how you can set yourself up with a bare Git repo for your dotfiles7. At a high level, we’re creating a bare repo in a new directory to store the history of our dotfiles. Then we tell Git that the snapshot for that repo actually lives in your home directory, not the directory that contains the bare repo’s history (this lets us keep your dotfiles in your home directory rather than the directory that has the history). We won’t commit the entire home directory (which would take up a lot of space and probably put some important secrets on the internet), just the pieces that you want to add to version control.
Create a new bare Git repo to store the history for your dotfiles.
git init --bare $HOME/dotfiles
Tell Git that it should use your home directory as the snapshot for this bare Git repo.
alias dotgit='git --git-dir=$HOME/dotfiles/ --work-tree=$HOME'
--git-dirtells Git where the history lives and
--work-treetells Git where the snapshot lives (the snapshot is called the “working tree” in Git jargon). We create an aliased command here so that you don’t have to specify these options manually every time you want to add a dotfile to your Git repo. If you want to use this command in the future, you should add this line to your
.zshrcor whatever dotfile you use for aliases.
Tell Git that this repo should not display all untracked files (otherwise
git statuswill include every file in your home directory, which will make it unusable).
dotgit config status.showUntrackedFiles no
Set up a remote repo to sync your dotfiles to (I use GitHub).
dotgit remote add origin firstname.lastname@example.org:GregOwen/dotfiles.git
Whenever you want to add a new dotfile to your Git repo, use your aliased Git command with your special options set.
dotgit add ~/.gitconfig dotgit commit -m "Git dotfiles" dotgit push origin master
Copying your dotfiles to another machine
To download your dotfiles onto a new machine, do the following:
Clone your repo onto the new machine as a non-bare repository. You need a non-bare repository on the new machine since you’re trying to move the actual dotfiles (that is, the snapshot of your repo) onto the new machine, not just the history.
git clone \ --separate-git-dir=$HOME/dotfiles \ email@example.com:GregOwen/dotfiles.git \ dotfiles-tmp
--separate-git-dirtells Git that the history should live in
$HOME/dotfileseven though the snapshot will live in
dotfiles-tmp(which is just an arbitrary temporary directory that we’ll delete later once we’ve moved the dotfiles into their proper locations).
Copy the snapshot from your temporary directory to the correct locations on your new machine.
rsync --recursive --verbose --exclude '.git' dotfiles-tmp/ $HOME/
This command copies every file in
dotfiles-tmpand its subdirectories into the corresponding locations in your home directory (so
dotfiles-tmp/.gitconfigwill be copied to
dotfiles-tmp/.emacs.d/init.elwill be copied to
Remove the temporary directory. Now that we’ve copied over the snapshot to the correct locations in your actual home directory, we can delete the old snapshot.
rm -rf dotfiles-tmp
At this point, your new machine has the dotfiles in the correct locations in your home directory and is tracking their history in
~/dotfiles, which is exactly the same state that your original machine was in after Step 1 of the initial setup. To allow your new machine to track changes to the dotfiles, just follow the steps you followed on your original machine, starting with Step 2.
If you need to remember these commands in the future and would like a handy cheat sheet, see GitHub user Siilwyn’s writeup here.
You can also get a high-level (if somewhat jargony) explanation of what all the subfolders of
.gitdo by running
git help repository-layout
For example, you could theoretically edit your files by changing Git’s object store directly. Your build process could theoretically read Git’s object stores to get the current state of all code files and use that to build an executable. That would be super painful, though, and it would require any process that operates on a Git repository to be able to understand Git’s storage model. As a corollary, it would also make Git’s storage model an external-facing interface, which would make it much harder for the developers of Git to change the storage model without breaking everything. You can see why Git provides a snapshot by default. ↩
In a non-bare repository, all of the history is stored in subdirectories of the
.gitdirectory (e.g. for a project called
MyProject, this data would be in
MyProject/.git/refs, etc.). In a bare repository, the history is stored in multiple top-level directories at the project root (e.g.
MyProject/refs, etc.). The layout is otherwise the same. ↩
Also, it doesn’t really make sense for a shared central repository to store a snapshot. If I’m working on one branch of the repo and you’re working on a different branch, which branch should the central repository store a snapshot of? This may seem surprising, since GitHub shows you a snapshot when you go to github.com/my-repo, but GitHub generates that snapshot on the fly when you access that page, rather than storing it permanently with the repo (this means that GitHub only needs to generate a snapshot when you ask for it, rather than keeping one updated every time anybody pushes any changes). ↩
There’s no reason that this would be impossible, but it would be annoying because it would mean that every subdirectory of your home directory would be part of the home git repo, so accidental git commands would be valid rather than triggering the helpful “you’re not in a Git repo” error. ↩
I tested all these commands on MacOS and Ubuntu, but they should work for any Unix-like operating system. ↩