--- title: Version Control (Git) description: How to use version control _properly_, and take advantage of it to save you from disaster, collaborate with others, and quickly find and isolate problematic changes. No more `rm -rf; git clone`. No more merge conflicts (well, fewer of them… url: https://subramanya.ai/2022/12/21/version-control/ tags: - Git - Version Control date: '2022-12-21' author: Subramanya N --- # Version Control (Git) Version control systems (VCSs) are tools used to track changes to source code (or other collections of files and folders). As the name implies, these tools help maintain a history of changes; furthermore, they facilitate collaboration. VCSs track changes to a folder and its contents in a series of snapshots, where each snapshot encapsulates the entire state of files/folders within a top-level directory. VCSs also maintain metadata like who created each snapshot, messages associated with each snapshot, and so on. Why is version control useful? Even when you're working by yourself, it can let you look at old snapshots of a project, keep a log of why certain changes were made, work on parallel branches of development, and much more. When working with others, it's an invaluable tool for seeing what other people have changed, as well as resolving conflicts in concurrent development. Modern VCSs also let you easily (and often automatically) answer questions like: - Who wrote this module? - When was this particular line of this particular file edited? By whom? Why was it edited? - Over the last 1000 revisions, when/why did a particular unit test stop working? While other VCSs exist, **Git** is the de facto standard for version control. This [XKCD comic](https://xkcd.com/1597/) captures Git's reputation: ![xkcd 1597](https://imgs.xkcd.com/comics/git.png) Because Git's interface is a leaky abstraction, learning Git top-down (starting with its interface / command-line interface) can lead to a lot of confusion. It's possible to memorize a handful of commands and think of them as magic incantations, and follow the approach in the comic above whenever anything goes wrong. While Git admittedly has an ugly interface, its underlying design and ideas are beautiful. While an ugly interface has to be _memorized_, a beautiful design can be _understood_. For this reason, we give a bottom-up explanation of Git, starting with its data model and later covering the command-line interface. Once the data model is understood, the commands can be better understood in terms of how they manipulate the underlying data model. # Git's data model There are many ad-hoc approaches you could take to version control. Git has a well-thought-out model that enables all the nice features of version control, like maintaining history, supporting branches, and enabling collaboration. ## Snapshots Git models the history of a collection of files and folders within some top-level directory as a series of snapshots. In Git terminology, a file is called a "blob", and it's just a bunch of bytes. A directory is called a "tree", and it maps names to blobs or trees (so directories can contain other directories). A snapshot is the top-level tree that is being tracked. For example, we might have a tree as follows: ``` (tree) | +- foo (tree) | | | + bar.txt (blob, contents = "hello world") | +- baz.txt (blob, contents = "git is wonderful") ``` The top-level tree contains two elements, a tree "foo" (that itself contains one element, a blob "bar.txt"), and a blob "baz.txt". ## Modeling history: relating snapshots How should a version control system relate snapshots? One simple model would be to have a linear history. A history would be a list of snapshots in time-order. For many reasons, Git doesn't use a simple model like this. In Git, a history is a directed acyclic graph (DAG) of snapshots. That may sound like a fancy math word, but don't be intimidated. All this means is that each snapshot in Git refers to a set of "parents", the snapshots that preceded it. It's a set of parents rather than a single parent (as would be the case in a linear history) because a snapshot might descend from multiple parents, for example, due to combining (merging) two parallel branches of development. Git calls these snapshots "commit"s. Visualizing a commit history might look something like this: ``` o <-- o <-- o <-- o ^ \ --- o <-- o ``` In the ASCII art above, the `o`s correspond to individual commits (snapshots). The arrows point to the parent of each commit (it's a "comes before" relation, not "comes after"). After the third commit, the history branches into two separate branches. This might correspond to, for example, two separate features being developed in parallel, independently from each other. In the future, these branches may be merged to create a new snapshot that incorporates both of the features, producing a new history that looks like this, with the newly created merge commit shown in bold: 

o <-- o <-- o <-- o <---- o
            ^            /
             \          v
              --- o <-- o
Commits in Git are immutable. This doesn't mean that mistakes can't be corrected, however; it's just that "edits" to the commit history are actually creating entirely new commits, and references (see below) are updated to point to the new ones. ## Data model, as pseudocode It may be instructive to see Git's data model written down in pseudocode: ``` // a file is a bunch of bytes type blob = array // a directory contains named files and directories type tree = map // a commit has parents, metadata, and the top-level tree type commit = struct { parents: array author: string message: string snapshot: tree } ``` It's a clean, simple model of history. ## Objects and content-addressing An "object" is a blob, tree, or commit: ``` type object = blob | tree | commit ``` In Git data store, all objects are content-addressed by their [SHA-1 hash](https://en.wikipedia.org/wiki/SHA-1). ``` objects = map def store(object): id = sha1(object) objects[id] = object def load(id): return objects[id] ``` Blobs, trees, and commits are unified in this way: they are all objects. When they reference other objects, they don't actually _contain_ them in their on-disk representation, but have a reference to them by their hash. For example, the tree for the example directory structure [above](#snapshots) (visualized using `git cat-file -p 698281bc680d1995c5f4caaf3359721a5a58d48d`), looks like this: ``` 100644 blob 4448adbf7ecd394f42ae135bbeed9676e894af85 baz.txt 040000 tree c68d233a33c5c06e0340e4c224f0afca87c8ce87 foo ``` The tree itself contains pointers to its contents, `baz.txt` (a blob) and `foo` (a tree). If we look at the contents addressed by the hash corresponding to baz.txt with `git cat-file -p 4448adbf7ecd394f42ae135bbeed9676e894af85`, we get the following: ``` git is wonderful ``` ## References Now, all snapshots can be identified by their SHA-1 hashes. That's inconvenient, because humans aren't good at remembering strings of 40 hexadecimal characters. Git's solution to this problem is human-readable names for SHA-1 hashes, called "references". References are pointers to commits. Unlike objects, which are immutable, references are mutable (can be updated to point to a new commit). For example, the `master` reference usually points to the latest commit in the main branch of development. ``` references = map def update_reference(name, id): references[name] = id def read_reference(name): return references[name] def load_reference(name_or_id): if name_or_id in references: return load(references[name_or_id]) else: return load(name_or_id) ``` With this, Git can use human-readable names like "master" to refer to a particular snapshot in the history, instead of a long hexadecimal string. One detail is that we often want a notion of "where we currently are" in the history, so that when we take a new snapshot, we know what it is relative to (how we set the `parents` field of the commit). In Git, that "where we currently are" is a special reference called "HEAD". ## Repositories Finally, we can define what (roughly) is a Git _repository_: it is the data `objects` and `references`. On disk, all Git stores are objects and references: that's all there is to Git's data model. All `git` commands map to some manipulation of the commit DAG by adding objects and adding/updating references. Whenever you're typing in any command, think about what manipulation the command is making to the underlying graph data structure. Conversely, if you're trying to make a particular kind of change to the commit DAG, e.g. "discard uncommitted changes and make the 'master' ref point to commit `5d83f9e`", there's probably a command to do it (e.g. in this case, `git checkout master; git reset --hard 5d83f9e`). # Staging area This is another concept that's orthogonal to the data model, but it's a part of the interface to create commits. One way you might imagine implementing snapshotting as described above is to have a "create snapshot" command that creates a new snapshot based on the _current state_ of the working directory. Some version control tools work like this, but not Git. We want clean snapshots, and it might not always be ideal to make a snapshot from the current state. For example, imagine a scenario where you've implemented two separate features, and you want to create two separate commits, where the first introduces the first feature, and the next introduces the second feature. Or imagine a scenario where you have debugging print statements added all over your code, along with a bugfix; you want to commit the bugfix while discarding all the print statements. Git accommodates such scenarios by allowing you to specify which modifications should be included in the next snapshot through a mechanism called the "staging area". # Git command-line interface To avoid duplicating information, we're not going to explain the commands below in detail. See the highly recommended [Pro Git](https://git-scm.com/book/en/v2) for more information. ## Basics The `git init` command initializes a new Git repository, with repository metadata being stored in the `.git` directory: ```console $ mkdir myproject $ cd myproject $ git init Initialized empty Git repository in .git $ git status On branch master No commits yet nothing to commit (create/copy files and use "git add" to track) ``` How do we interpret this output? "No commits yet" basically means our version history is empty. Let's fix that. ```console $ echo "hello, git" > hello.txt $ git add hello.txt $ git status On branch master No commits yet Changes to be committed: (use "git rm --cached ..." to unstage) new file: hello.txt $ git commit -m 'Initial commit' [master (root-commit) 4515d17] Initial commit 1 file changed, 1 insertion(+) create mode 100644 hello.txt ``` With this, we've `git add`ed a file to the staging area, and then `git commit`ed that change, adding a simple commit message "Initial commit". If we didn't specify a `-m` option, Git would open our text editor to allow us type a commit message. Now that we have a non-empty version history, we can visualize the history. Visualizing the history as a DAG can be especially helpful in understanding the current status of the repo and connecting it with your understanding of the Git data model. The `git log` command visualizes history. By default, it shows a flattened version, which hides the graph structure. If you use a command like `git log --all --graph --decorate`, it will show you the full version history of the repository, visualized in graph form. ```console $ git log --all --graph --decorate * commit 4515d17a167bdef0a91ee7d50d75b12c9c2652aa (HEAD -> master) Author: Subramanya N Date: Tue Dec 21 22:18:36 2020 -0500 Initial commit ``` This doesn't look all that graph-like, because it only contains a single node. Let's make some more changes, author a new commit, and visualize the history once more. ```console $ echo "another line" >> hello.txt $ git status On branch master Changes not staged for commit: (use "git add ..." to update what will be committed) (use "git checkout -- ..." to discard changes in working directory) modified: hello.txt no changes added to commit (use "git add" and/or "git commit -a") $ git add hello.txt $ git status On branch master Changes to be committed: (use "git reset HEAD ..." to unstage) modified: hello.txt $ git commit -m 'Add a line' [master 35f60a8] Add a line 1 file changed, 1 insertion(+) ``` Now, if we visualize the history again, we'll see some of the graph structure: ``` * commit 35f60a825be0106036dd2fbc7657598eb7b04c67 (HEAD -> master) | Author: Subramanya N | Date: Tue Dec 21 22:26:20 2020 -0500 | Add a line * commit 4515d17a167bdef0a91ee7d50d75b12c9c2652aa Author: Subramanya N Date: Tue Dec 21 22:18:36 2020 -0500 Initial commit ``` Also, note that it shows the current HEAD, along with the current branch (master). We can look at old versions using the `git checkout` command. ```console $ git checkout 4515d17 # previous commit hash; yours will be different Note: checking out '4515d17'. You are in 'detached HEAD' state. You can look around, make experimental changes and commit them, and you can discard any commits you make in this state without impacting any branches by performing another checkout. If you want to create a new branch to retain commits you create, you may do so (now or later) by using -b with the checkout command again. Example: git checkout -b HEAD is now at 4515d17 Initial commit $ cat hello.txt hello, git $ git checkout master Previous HEAD position was 4515d17 Initial commit Switched to branch 'master' $ cat hello.txt hello, git another line ``` Git can show you how files have evolved (differences, or diffs) using the `git diff` command: ```console $ git diff 4515d17 hello.txt diff --git c/hello.txt w/hello.txt index 94bab17..f0013b2 100644 --- c/hello.txt +++ w/hello.txt @@ -1 +1,2 @@ hello, git +another line ``` - `git help `: get help for a git command - `git init`: creates a new git repo, with data stored in the `.git` directory - `git status`: tells you what's going on - `git add `: adds files to staging area - `git commit`: creates a new commit - Write [good commit messages](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html)! - Even more reasons to write [good commit messages](https://chris.beams.io/posts/git-commit/)! - `git log`: shows a flattened log of history - `git log --all --graph --decorate`: visualizes history as a DAG - `git diff `: show changes you made relative to the staging area - `git diff `: shows differences in a file between snapshots - `git checkout `: updates HEAD and current branch ## Branching and merging Branching allows you to "fork" version history. It can be helpful for working on independent features or bug fixes in parallel. The `git branch` command can be used to create new branches; `git checkout -b ` creates and branch and checks it out. Merging is the opposite of branching: it allows you to combine forked version histories, e.g. merging a feature branch back into master. The `git merge` command is used for merging. - `git branch`: shows branches - `git branch `: creates a branch - `git checkout -b `: creates a branch and switches to it - same as `git branch ; git checkout ` - `git merge `: merges into current branch - `git mergetool`: use a fancy tool to help resolve merge conflicts - `git rebase`: rebase set of patches onto a new base ## Remotes - `git remote`: list remotes - `git remote add `: add a remote - `git push :`: send objects to remote, and update remote reference - `git branch --set-upstream-to=/`: set up correspondence between local and remote branch - `git fetch`: retrieve objects/references from a remote - `git pull`: same as `git fetch; git merge` - `git clone`: download repository from remote ## Undo - `git commit --amend`: edit a commit's contents/message - `git reset HEAD `: unstage a file - `git checkout -- `: discard changes # Advanced Git - `git config`: Git is [highly customizable](https://git-scm.com/docs/git-config) - `git clone --depth=1`: shallow clone, without entire version history - `git add -p`: interactive staging - `git rebase -i`: interactive rebasing - `git blame`: show who last edited which line - `git stash`: temporarily remove modifications to working directory - `git bisect`: binary search history (e.g. for regressions) - `.gitignore`: [specify](https://git-scm.com/docs/gitignore) intentionally untracked files to ignore # Miscellaneous - **GUIs**: there are many [GUI clients](https://git-scm.com/downloads/guis) out there for Git. We personally don't use them and use the command-line interface instead. - **Shell integration**: it's super handy to have a Git status as part of your shell prompt ([zsh](https://github.com/olivierverdier/zsh-git-prompt), [bash](https://github.com/magicmonty/bash-git-prompt)). Often included in frameworks like [Oh My Zsh](https://github.com/ohmyzsh/ohmyzsh). - **Editor integration**: similarly to the above, handy integrations with many features. [fugitive.vim](https://github.com/tpope/vim-fugitive) is the standard one for Vim. - **Workflows**: we taught you the data model, plus some basic commands; we didn't tell you what practices to follow when working on big projects (and there are [many](https://nvie.com/posts/a-successful-git-branching-model/) [different](https://www.endoflineblog.com/gitflow-considered-harmful) [approaches](https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow)). - **GitHub**: Git is not GitHub. GitHub has a specific way of contributing code to other projects, called [pull requests](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/about-pull-requests). - **Other Git providers**: GitHub is not special: there are many Git repository hosts, like [GitLab](https://about.gitlab.com/) and [BitBucket](https://bitbucket.org/). # Resources - [Pro Git](https://git-scm.com/book/en/v2) is **highly recommended reading**. Going through Chapters 1--5 should teach you most of what you need to use Git proficiently, now that you understand the data model. The later chapters have some interesting, advanced material. - [Oh Shit, Git!?!](https://ohshitgit.com/) is a short guide on how to recover from some common Git mistakes. - [Git for Computer Scientists](https://eagain.net/articles/git-for-computer-scientists/) is a short explanation of Git's data model, with less pseudocode and more fancy diagrams than these lecture notes. - [Git from the Bottom Up](https://jwiegley.github.io/git-from-the-bottom-up/) is a detailed explanation of Git's implementation details beyond just the data model, for the curious. - [How to explain git in simple words](https://smusamashah.github.io/blog/2017/10/14/explain-git-in-simple-words) - [Learn Git Branching](https://learngitbranching.js.org/) is a browser-based game that teaches you Git.