Git Commit Reference and Checkout

HEAD (refers to commit 'f') | v e---f / a---b---c---d branch 'master' (refers to commit 'd') ^ | tag 'v2.0' (refers to commit 'b') ------------ In fact, we can perform all the normal Git operations. But, let's look at what happens when we then checkout `master`: ------------ $ git checkout master HEAD (refers to branch 'master') e---f | / v a---b---c---d branch 'master' (refers to commit 'd') ^ | tag 'v2.0' (refers to commit 'b') ------------ It is important to realize that at this point nothing refers to commit `f`. Eventually commit `f` (and by extension commit `e`) will be deleted by the routine Git garbage collection process, unless we create a reference before that happens. If we have not yet moved away from commit `f`, any of these will create a reference to it: ------------ $ git checkout -b foo # or "git switch -c foo" <1> $ git branch foo <2> $ git tag foo <3> ------------ <1> creates a new branch `foo`, which refers to commit `f`, and then updates `HEAD` to refer to branch `foo`. In other words, we'll no longer be in detached `HEAD` state after this command. <2> similarly creates a new branch `foo`, which refers to commit `f`, but leaves `HEAD` detached. <3> creates a new tag `foo`, which refers to commit `f`, leaving `HEAD` detached. If we have moved away from commit `f`, then we must first recover its object name (typically by using git reflog), and then we can create a reference to it. For example, to see the last two commits to which `HEAD` referred, we can use either of these commands: ------------ $ git reflog -2 HEAD # or $ git log -g -2 HEAD ------------ ARGUMENT DISAMBIGUATION ----------------------- When there is only one argument given and it is not `--` (e.g. `git checkout abc`), and when the argument is both a valid _<tree-ish>_ (e.g. a branch `abc` exists) and a valid _<pathspec>_ (e.g. a file or a directory whose name is "abc" exists), Git would usually ask you to disambiguate. Because checking out a branch is so common an operation, however, `git checkout abc` takes "abc" as a _<tree-ish>_ in such a situation. Use `git checkout -- <pathspec>` if you want to checkout these paths out of the index. EXAMPLES -------- === 1. Paths The following sequence checks out the `master` branch, reverts the

When switching from a detached HEAD state to a branch, commits without a reference are subject to garbage collection, but can be preserved by creating a new branch, tag, or reference, and Git provides options to recover and manage such commits, as well as to disambiguate between tree-ish and pathspec arguments during checkout operations.