https://www.atlassian.com/git/tutorials/refs-and-the-reflog/the-reflog

git show
    refs/heads/some-feature
        full-name ref
    some-feature
        short-name ref

after GC refs may be packed into the .git/packed_refs:

00f54250cf4e549fdfcafe2cf9a2c90bc3800285 refs/heads/feature
0e25143693cfe9d5c2e83944bbaf6d3c4505eb17 refs/heads/master
bb883e4c91c870b5fed88fd36696e752fb6cf8e6 refs/tags/v0.9


Special Refs

In addition to the refs directory, there are a few special refs that reside in the top-level .git directory. They are listed below:

HEAD
    The currently checked-out commit/branch.
FETCH_HEAD
    The most recently fetched branch from a remote repo.
ORIG_HEAD
    A backup reference to HEAD before drastic changes to it.
MERGE_HEAD
    The commit(s) that you’re merging into the current branch with git merge.
CHERRY_PICK_HEAD
    The commit that you’re cherry-picking.

These refs are all created and updated by Git when necessary.
For example, The git pull command first runs git fetch, which updates the FETCH_HEAD reference.
Then, it runs git merge FETCH_HEAD to finish pulling the fetched branches into the repository.
Of course, you can use all of these like any other ref, as I’m sure you’ve done with HEAD.

These files contain different content depending on their type and the state of your repository.
The HEAD ref can contain either a symbolic ref, which is simply a reference to another ref instead of a commit hash, or a commit hash.
For example, take a look at the contents of HEAD when you’re on the master branch:

git checkout master
cat .git/HEAD

This will output

ref: refs/heads/master

, which means that HEAD points to the refs/heads/master ref.
This is how Git knows that the master branch is currently checked out.
If you were to switch to another branch, the contents of HEAD would be updated to reflect the new branch.
But, if you were to check out a commit instead of a branch, HEAD would contain a commit hash instead of a symbolic ref.
This is how Git knows that it’s in a detached HEAD state.

For the most part, HEAD is the only reference that you’ll be using directly.
The others are generally only useful when writing lower-level scripts that need to hook into Git’s internal workings.


Symbolic Refs
git symbolic-ref ...


Every branch, tag, and other decoration is just a reference to the ID of a specific commit
(calculated with SHA-1, the secure hash algorithm).

git rev-parse <human-hash> or <branch-name>
	[pick out and massage parameters]
	to get a sys-id (long) from a human-readable (short) form

git rev-parse 'master^{tree}'
	.. commit tree

git describe --tags master
git describe --tags HEAD
	describe (find nearest tag) non-annotated tag