zulip/docs/git/the-git-difference.md

How Git is different

Whether you're new to Git or have experience with another version control system (VCS), it's a good idea to learn a bit about how Git works. We recommend this excellent presentation Understanding Git from Nelson Elhage and Anders Kaseorg and the Git Basics chapter from Pro Git by Scott Chacon and Ben Straub.

Here are the top things to know:

• Git works on snapshots. Unlike other version control systems (e.g., Subversion, Perforce, Bazaar), which track files and changes to those files made over time, Git tracks snapshots of your project. Each time you commit or otherwise make a change to your repository, Git takes a snapshot of your project and stores a reference to that snapshot. If a file hasn't changed, Git creates a link to the identical file rather than storing it again.

• Most Git operations are local. Git is a distributed version control system, so once you've cloned a repository, you have a complete copy of that repository's entire history. Staging, committing, branching, and browsing history are all things you can do locally without network access and without immediately affecting any remote repositories. To make or receive changes from remote repositories, you need to git fetch, git pull, or git push.

• Nearly all Git actions add information to the Git database, rather than removing it. As such, it's hard to make Git perform actions that you can't undo. However, Git can't undo what it doesn't know about, so it's a good practice to frequently commit your changes and frequently push your commits to your remote repository.

• Git is designed for lightweight branching and merging. Branches are simply references to snapshots. It's okay and expected to make a lot of branches, even throwaway and experimental ones.

• Git stores all data as objects, of which there are four types: blob (file), tree (directory), commit (revision), and tag. Each of these objects is named by a unique hash, the SHA-1 hash of its contents. Most of the time you'll refer to objects by their truncated hash or more human-readable reference like HEAD (the current branch). Blobs and trees represent files and directories. Tags are named references to other objects. A commit object includes: tree id, zero or more parents as commit ids, an author (name, email, date), a committer (name, email, date), and a log message. A Git repository is a collection of mutable pointers to these objects called refs.

• Cloning a repository creates a working copy. Every working copy has a .git subdirectory, which contains its own Git repository. The .git subdirectory also tracks the index, a staging area for changes that will become part of the next commit. All files outside of .git is the working tree.

• Files tracked with Git have possible three states: committed, modified, and staged. Committed files are those safely stored in your local .git repository/database. Staged files have changes and have been marked for inclusion in the next commit; they are part of the index. Modified files have changes but have not yet been marked for inclusion in the next commit; they have not been added to the index.

• Git commit workflow is as follows. Edit files in your working tree. Add to the index (that is stage) with git add. Commit to the HEAD of the current branch with git commit.