HEAD in Git has a special significance. It refers to a reference to the last commit in the currently checked-out branch. In simpler terms, it’s a way for Git to know what the current working snapshot is. Whenever you perform Git operations such as committing new changes or branching, Git uses the
HEAD to know on what basis these operations should be performed.
Let’s delve a bit deeper to understand this in detail:
- Snapshot and commit references: Whenever you make a commit in Git, it takes a snapshot of your project files at that specific moment and saves it. Each commit has a unique ID, also known as a SHA-1 hash, which acts as a reference to that particular snapshot.
- Role of HEAD: Now, when you’re working on a project, Git needs to know what the current snapshot of your project is. In other words, it needs to know what the latest changes are that you’re working with. This is where the
HEADcomes in. The
HEADpoints to the latest commit of the current branch, letting Git know what the most recent snapshot is.
- Moving the HEAD: When you switch between different branches using
git checkout, the
HEADchanges its reference to the last commit of the checked-out branch. If you make a new commit,
HEADwill move along with the branch tip, pointing to the new commit.
- Detached HEAD: Normally,
HEADpoints to a branch name, which in turn points to a commit. But if you check out a specific commit that isn’t the latest commit in a branch, then
HEADwill point directly to that commit instead of a branch. This state is known as a “detached HEAD” state.
- HEAD^ and HEAD~: Git also provides handy references to access commits relative to HEAD. For example,
HEAD^1) refers to the commit before the current one,
HEAD^2) refers to the commit before that, and so on. Similarly,
HEAD~1is equivalent to
HEAD~2is equivalent to
HEAD^^, and so forth.
HEAD~can also be used to refer to commits multiple steps behind the current one in a more concise way. For instance,
HEAD~3refers to “3 commits before the current commit”.
Remember, having a clear understanding of the
HEAD and its movement between commits is essential in Git, especially when trying to navigate through different versions or states of a project.