What is HEAD in Git?

Question

You see the Git documentation saying things like

The branch must be fully merged in HEAD.

But what is Git HEAD exac

Answer 1

I'd just like to detail a few things in Greg Hewgil's accepted answer. According to the Git Pocket Guide

Branch:

the branch itself is defined as all points reachable in the commit graph from the named commit (the “tip” of the branch).

HEAD: A special type of Ref

The special ref HEAD determines what branch you are on...

Refs

Git defines two kinds of references, or named pointers, which it calls “refs”:

• A simple ref, which points directly to an object ID (usually a commit or tag)
• A symbolic ref (or symref), which points to another ref (either simple or symbolic)

As Greg mentioned, HEAD can be in a "detached state". So HEAD can be either a simple ref (for a detached HEAD) or a symref.

if HEAD is a symbolic ref for an existing branch, then you are “on” that branch. If, on the other hand, HEAD is a simple ref directly naming a commit by its SHA-1 ID, then you are not “on” any branch, but rather in “detached HEAD” mode, which happens when you check out some earlier commit to examine.