Using git on a daily basis, many times we fail to understand the ins and outs of how git internals really work – what is stored between commits, how is a diff encoded, what really happens when we use git init or how version control works (drum roll… 🥁), we will create a repo from scratch and see the wonders of the plumbing beneath porcelain.
This is our second post diving into the internals of git. If you missed our previous post, you can find it here.
In our previous post we covered the basic
git objects — blobs, trees, and commits. We explained that a blob holds the contents of a file. A tree is a directory-listing, containing blobs and/or sub-trees. A commit is a snapshot of our working directory, with some meta-data such as the time or the commit message. Additionally, we discussed branches and how they are implemented, as they are nothing but a named reference to a commit. (These really show why Git is valuable for version control).
So far we set the ground covering fundamentals, and now we’re ready to really Git going.
A Repo from Scratch
In order to deeply understand how
git internals work, we will create a repository, but this time — build it from scratch.
We won’t use
git add or
git commit which will enable us to get a better hands-on understanding in the process.
Setting Up .
Note — most posts with shell commands show UNIX commands. As in our previous post, I will provide commands for both Windows and UNIX, with screenshots from Windows, for the sake of variance. When the commands are exactly the same, I will provide them only once.
Let’s create a new working directory (also named “working tree”), and run git status within it:
git seems unhappy as we don’t have a
.gitfolder. The natural thing to do would be to simply create that directory:
Apparently, creating a
.git directory is just not enough. We need to add some content to that directory.
A git repository has mainly two components:
- A collection of objects — blobs, trees, and commits.
- A system of naming those objects — called references.
A repository may also contain other things, such as git hooks, but at the very least — it must include objects and references.
Let’s create a directory for the objects at
.git\objects and a directory for the references (in short: refs) at
.git\refs (on UNIX -based systems —
One type of reference is branches. Internally,
git calls branches by the name of heads. So we will create a directory for them —
This still doesn’t change
git know where to start when looking for a commit in the repository?
As explained in our previous post, it looks for
HEAD, which points to the current active branch (or commit, in some cases). So, we need to create
HEAD, which is just a file residing at
.git\HEAD.We can apply the following:
On Windows: >
echo ref: refs/heads/master > .git\HEAD
$ echo "ref: refs/heads/master" > .git/HEAD
⭐ So we now know how
HEAD is implemented — it’s simply a file, and its file contents describe what it points to.
Following the command above,
git status seems to change its mind:
git believes we are on a branch called
master, even though we haven’t created this branch. As mentioned in the previous post,
master is just a name. We could also make git believe we are on a branch called
banana if we wanted to:
We will switch back to
master for the rest of this post, just to adhere to the normal convention.
Now that we have our
.git directory ready, can we work our way to make a commit (again, without using
git add or
Plumbing vs porcelain commands
At this point, it would be helpful to make a distinction between two types of
git commands: plumbing and porcelain. The application of the terms oddly comes from toilets (yeah, these — 🚽), traditionally made of porcelain, and the infrastructure of plumbing (pipes and drains). We can say that the porcelain layer provides a user-friendly interface to the plumbing. Most people only deal with the porcelain. Yet, when things go (terribly) wrong, and as someone would want to understand why, they would have to roll-up their sleeves to check the plumbing. (Note: these terms are not mine, they are used very widely in
git uses this terminology in analogy, to separate the low-level commands that users don’t usually need to use directly (“plumbing” commands) from the more user-friendly high level commands (“porcelain” commands).
So far, we have dealt with porcelain commands —
git add or
git commit. Next, we transition to plumbing commands.
Starting with creating an object and writing it into the objects’ database of
git, residing within
.git\objects, we can find out the SHA-1 hash value of a blob object by using our first plumbing command,
git hash-object, in the following way:
> echo git is awesome | git hash-object --stdin
$ echo "git is awesome" | git hash-object --stdin
--stdin we are instructing
git hash-object to take its input from the standard input. This will provide us with the relevant SHA-1 hash value. In order to actually write that blob into
git’s object database, we can simply add the
-w switch for
git hash-object. Then, we can check the contents of
.git folder, and see that it changed.
We can now see the hash of our blob is —
54f6...36. We can also see that a directory has been created under
.git\objects, a directory named
54, and within it — a file by the name of
git actually takes the first two characters of the of the SHA-1 hash and uses them as the name of a working directory, and the remaining SHA-1 characters are used as the filename, for the file that actually contains the blob object.
Why is that so? Consider a fairly big repository, one that has 300,000 objects (blobs, trees, and commits) in its database. To look up a hash inside that list of 300,000 hashes can take a while. Thus,
git simply divides that problem by 256. To look up the hash above,
git would first look for the directory named
54 inside the director
.git\objects, which may have up to 256 directories (
FF). Then, it will search that working directory, narrowing down the search.
Back to our process of generating a commit. We have now created an object. What is the type of that object? We can use another plumbing command,
git cat-file -t (
-t stands for “type”), to check that out:
Not surprisingly, this object is a blob. We can also use
git cat-file -p(
-p stands for “pretty-print”) to see its contents:
This process of creating a blob usually happens when we add something to the staging area — that is, when we use
git add. Remember that
git creates a blob of the entire file that is staged. Even if a single character is modified or added (as we added
! in our example in the previous post), the file has a new blob with a new hash.
Will there be any change to
Apparently, no. Adding a blob object to
git’s internal database doesn’t change the status, as
git doesn’t know of any tracked or untracked files at this stage. We need to track this file — add it to the staging area. To do that, we can use the plumbing command
git update-index, like so:
git update-index --add --cacheinfo 100644 <blob-hash> <filename>.
Running the command above will result in a change to
Can you spot the change? A new file by the name of
index was created. This is it — the famous index (or staging area), is basically a file that resides within
So now that our blob has been added to the index, we expect
git status to look differently:
That’s interesting! Two things happened here.
First, we can see that
new_file.txt appears in green, in the
Changes to be committed area. That is so because the index now has
new_file.txt, waiting to be committed.
Second, we can see that
new_file.txt appears in red — because
git believes the file
my_file.txt has been deleted, and the fact that the file has been deleted is not staged. This happens as we added the blob with the contents
git is awesome to the objects’ database, and told the index that the new file
my_file.txt has the contents of that blob, but we never actually created that file. This can be easily solved by taking the contents of the blob, and writing them to our file system, to a file called
As a result, it will no longer appear in red by
So now it’s time to create a commit object from our staging area. As explained in our previous post, a commit object has a reference to a tree, so we need to create a tree. We can do it with the command
git write-tree,which records the contents of the index in a tree object. Of course, we can use
git cat-file -t to see that it’s indeed a tree:
And we can use
git cat-file -p to see its contents:
Great, so we created a tree, and now we need to create a commit object and adds a commit message that references this tree. To do that, we can use
git commit-tree <tree-hash> -m <commit message>:
You should now feel comfortable with the commands used to check the created object’s type, and print its contents:
Note that this commit doesn’t have a parent, because it’s the first commit. When we add another commit we will have to declare its parent — we will do so later.
The last hash that we got —
80e...8f, is a commit’s hash. We are actually very used to using these hashes — we look at them all the time. Note that this commit owns a tree object, with its own hash, which we rarely specify explicitly.
Will something change in
Why is that? Well, to know that our file has been committed,
git needs to know about the latest commit. How does
git do that? It goes to the
HEAD points to
master, but what is
master branch? We haven’t really created it yet. As we explained earlier in this post, a branch is simply a named reference to a commit. And in this case, we would like
master to refer to the commit with the hash
80e8ed4fb0bfc3e7ba88ec417ecf2f6e6324998f. We can achieve this by simply creating a file at
\refs\heads\master, with the contents of this hash, like so:
⭐ In sum, a branch is — just a file inside .
git\refs\heads, containing a hash of the commit it refers to.
git status and
git log seem to appreciate our efforts:
We have successfully created a commit without using porcelain commands! How cool is that? 🎉
Working with branches — under the hood
Just as we’ve created a repository and a commit without using
git add or
git commit, now we will create and switch between branches without using porcelain commands (
git branch or
git checkout). It’s perfectly understandable if you are excited, I am too 🙂
So far we only have one branch, named
master. To create another one with the name of
test (as the equivalent of
git branch test), we would need to simply create a file named test within .
git\refs\heads, and the contents of that file would be the same commit’s hash as the one
master points to.
If we use
git log, we can see that this is indeed the case — both
test point to this commit:
Let’s also switch to our newly created branch (the equivalent of
git checkout test). For that, we should change
HEAD to point to our new branch:
As we can see, both
git status and
git log confirm that
HEAD now points to
test, which is, therefore, the active branch.
We can now use the commands we have already used to create another file and add it to the index:
Using the commands above, we have created a file named
test.txt, with the content of
Testing, created a corresponding blob, and added it to the index. We also created a tree object representing the index.
It’s now time to create a commit referencing this tree. This time, we should also specify the parent of this commit — which would be the previous commit. We specify the parent using the
-p switch of
We have just created a commit, with a tree object as well as a parent, as we can see:
git log show us the new commit?
As we can see,
git log doesn’t show anything new. Why is that?🤔 Remember that
git log traces the branches to find relevant commits to show. It shows us now
test and the commit it points to, and it also shows
master which points to the same commit. That’s right — we need to change
test to point to our new commit. We can do that by simply changing the contents of
git log goes to
HEAD, which tells it to go to the branch
test, which points to commit
465...5e, which links back to its parent commit
Feel free to admire the beauty, we git you 😊
After setting the grounds in the first post with elementary terminology and
git internal know-how, in this post, we fearlessly deep-dived into
git; We stopped using porcelain commands and switched to plumbing commands. By using
echo and low-level commands such as
git hash-object, we were able to create a blob, add it to the index, create a tree of the index, and create a commit object pointing to that tree. We were also able to create and switch between branches. Kudos to those of you who tried this on their own!👏
Disclaimer: This post and the previous one on git internals, are part of our new Swimminars blog series (read more here to find out what Swimminars are). We plan to provide similar posts in the future, so please comment and let us know your thoughts or questions on your journey into
A lot has been written and said about
git. Specifically, I found these references to be useful: