What do git checkouts really mean?

Question

What are checkouts in git?

I know once you do checkout to a particular branch, the HEAD points to that branch. But what does it really mean? Does it mean I can then work on that branch? If yes, then, without checking out a branch, I am not able to work on it?

Also, what does remote checkout mean? How is it useful?

Answer 1

As you noted, HEAD is a label noting where you are in the commit tree. It moves with you when you move from one commit to another. git checkout <commit> is the basic mechanism for moving around in the commit tree, moving your focus (HEAD) to the specified commit.

The commit can be specified by any of a number of ways, commit hash, branch name, tag name, the relative syntax (HEAD^, HEAD~1, etc.) and so on. It is often useful to consider a checkout to be changing branches, and there are some options that work from that perspective, but they all reference commits.

To checkout a commit has some side affects other than moving HEAD around.

• The working directory is updated to the state of the checked out commit.
• if a branch name is specified, checkout makes that branch active. The active branch will move along with any new commits that are added.
  • with the -b option a new branch will be created based on the current commit and then made active.
  • with the --track option the checked out branch can be made aware of a remote branch
  • with the --orphan option a new branch is created (like with -b) but will not be based on any existing commit.

There are a few more options, which you can read about in the git checkout man-page, all of which revolve around moving from one commit to another -- just varying in what effect that move has in addition to moving HEAD.