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Any checkout of a commit that is not the name of one of your branches will get you a detached HEAD. A SHA1 which represents the tip of a branch still gives a detached HEAD.
If you want to keep changes made with a detached HEAD, just create a new branch and switch to it. You can create it right after arriving at a detached HEAD or after creating one or more commits. The result is the same. The only restriction is that you should do it before returning to your normal branch.
Oct 1, 2020. A detached HEAD occurs when you check out a commit that is not a branch. The term detached HEAD tells you that you are not viewing the HEAD of any repository. The HEAD is the most recent version of a branch. This is sometimes called the “tip of a branch”.
In order to checkout a Git tag, use the “git checkout” command and specify the tagname as well as the branch to be checked out. Note that you will have to make sure that you have the latest tag list from your remote repository.
Okay, first a few terms slightly oversimplified.
In git, a tag (like many other things) is what's called a treeish. It's a way of referring to a point in in the history of the project. Treeishes can be a tag, a commit, a date specifier, an ordinal specifier or many other things.
Now a branch is just like a tag but is movable. When you are "on" a branch and make a commit, the branch is moved to the new commit you made indicating it's current position.
Your HEAD is pointer to a branch which is considered "current". Usually when you clone a repository, HEAD will point to master which in turn will point to a commit. When you then do something like git checkout experimental, you switch the HEAD to point to the experimental branch which might point to a different commit.
Now the explanation.
When you do a git checkout v2.0, you are switching to a commit that is not pointed to by a branch. The HEAD is now "detached" and not pointing to a branch. If you decide to make a commit now (as you may), there's no branch pointer to update to track this commit. Switching back to another commit will make you lose this new commit you've made. That's what the message is telling you.
Usually, what you can do is to say git checkout -b v2.0-fixes v2.0. This will create a new branch pointer at the commit pointed to by the treeish v2.0 (a tag in this case) and then shift your HEAD to point to that. Now, if you make commits, it will be possible to track them (using the v2.0-fixes branch) and you can work like you usually would. There's nothing "wrong" with what you've done especially if you just want to take a look at the v2.0 code. If however, you want to make any alterations there which you want to track, you'll need a branch.
You should spend some time understanding the whole DAG model of git. It's surprisingly simple and makes all the commands quite clear.
Yes, it is normal. This is because you checkout a single commit, that doesnt have a head. Especially it is (sooner or later) not a head of any branch.
But there is usually no problem with that state. You may create a new branch from the tag, if this makes you feel safer :)
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