GIT Tutorial: Part 3
YouTube Video Series:
Git tracks its history using a tree structure behind the scenes. Each entry in the tree is uniquely identified with a SHA hash. When playing with Git you may have seen something like:
What you are looking at is a stack of commits where the SHA hash uniquely identifies each commit. It can be thought of as a unique identifier like a primary key in a database.
Try the following to see some commits in your repository.
git log --oneline
The oldest is at the bottom and the newest at the top. What is not obvious is that the child item has a pointer to its parent(s). In the above scenario each child has one parent and it looks just like a list. Note: In the diagram below we are using alphabet letters instead of SHA hashes.
A commit can have one or more parents. When the commit has two or more parents it is essentially a merge where we have taken content from A and B and made C.
When we download a Git repository we can think of it being almost two separate items. First there are all the files in your project as then there is as an index that tracks the Sha references and which files they each relate to. This index is called the reflog. Not only does it contain these Sha references, it also can contain Tags (which are human-readable labels applied to a single Sha reference) and pointers that identify the Sha reference of each branch in the repository.
I highly recommend this book on Git as it goes really deep into how Git works and is well written.