The special symbol "HEAD" can always be used to refer to the current

branch. In fact, git uses a file named "HEAD" in the .git directory to

remember which branch is current:

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$ cat .git/HEAD

ref: refs/heads/master

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Examining an old version without creating a new branch

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The git-checkout command normally expects a branch head, but will also

accept an arbitrary commit; for example, you can check out the commit

referenced by a tag:

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$ git checkout v2.6.17

Note: moving to "v2.6.17" which isn't a local branch

If you want to create a new branch from this checkout, you may do so

(now or later) by using -b with the checkout command again. Example:

git checkout -b <new_branch_name>

HEAD is now at 427abfa... Linux v2.6.17

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The HEAD then refers to the SHA1 of the commit instead of to a branch,

and git branch shows that you are no longer on a branch:

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$ cat .git/HEAD

427abfa28afedffadfca9dd8b067eb6d36bac53f

git branch

* (no branch)

master

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In this case we say that the HEAD is "detached".

This can be an easy way to check out a particular version without having

to make up a name for a new branch. However, keep in mind that when you

switch away from the (for example, by checking out something else), you

can lose track of what the HEAD used to point to.