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detached HEAD explained

Git has something called a ‘detached HEAD’ that bears some explanation. You are told ‘do not commit on a detached HEAD’, and it is not always clear what or why this is.

This document is here for historical reasons, and because it provides a nice analogy. However, git concepts simplified should be a much nicer read for most people. It has a lot of nice pictures, and it also touches upon many other issues, not just on detached HEADs.

1 the analogy

Forget git for a bit and think about linked lists from your data structures course. Specifically, consider a singly linked list, where each node is created as a “child” of some existing node, and contains a link to that “parent” node. The words “child” and “parent” are chosen to be similar to git’s nomenclature for the relationship between a commit and its predecessor commit(s).

As with any list, you have a pointer to the node at the top of the list (we’ll call this pointer list_top). This would be the most recently created node, and so there’s a chain of “parent” links that can take you all the way to the first node created.

And you know how to add a new node to this linked list. In pseudo-code:

// insert a new node at 'list_top'
temp = malloc(...);         // ask for some memory for the struct
temp.body = ...;            // fill in the node info
temp.parent = list_top;     // fill in the backlink
list_top = temp;            // move list head to new node

Let’s also assume that all list operations are done with some temporary variable (that is, they never directly use list_top etc.).

Now suppose you move to the grandparent of the current list_top, to do something (like examine that node, print it out, whatever):

temp = list_top;
temp = temp.parent;         // parent of list_top
temp = temp.parent;         // grandparent

So far, no harm done; your list_top variable still points to the top of the list, and you can always do temp = list_top to go back to where you were for any further operations.

Now suppose you add a new node to the list on top of the current node:

// insert a new node at 'temp'
temp2 = malloc(...);        // ask for some memory for the struct
temp2.body = ...;           // fill in the node info
temp2.parent = temp;        // fill in the backlink
temp = temp2;               // move list head to new node

This is the same code as before, except temp has taken the place of list_top this time.

But ‘temp’ is a temporary variable, and unless you somehow save its value into some ‘global’ (or ‘static’) variable, you risk losing that commit you just made, oops, I mean the new node you added to the linked list :-)

2 reality check

A previous version of this article took the analogy too far into git territory, and I did not realise how many facts about git I had passed over/simplified until an extended review on irc by doener and jsquared.

So we’ll git real, if you’ll pardon the weak pun, and get on with understanding a ‘detached HEAD’.

In git, commits always go on top of HEAD. In terms of our linked list analogy, they ‘insert a new commit at HEAD’.

HEAD is, normally, a symbolic reference to [the tip of] a branch. For instance, if you do cat .git/HEAD on a brand new repository, you’ll get back ref: refs/heads/master. When you add a commit, git actually updates ‘master’, because that’s where HEAD points. You can see this by doing cat .git/refs/heads/master before and after making a commit. HEAD does not change (it’s only a symbolic reference) but ‘master’ does.

When you do a git checkout branchname, HEAD will now become a symbolic reference to ‘branchname’. This means cat .git/HEAD will return ref: refs/heads/branchname now. New commits will now go on ‘branchname’ instead of master, and correspondingly, the contents of .git/refs/heads/branchname will change.

However, when you checkout anything that is not a proper, local, branch name, then HEAD is no longer a symbolic reference to anything. Instead, it actually contains the SHA-1 hash (the commit id) of the commit you are switching to.

This is called a detached HEAD. Example commands that will cause your HEAD to become detached (ouch!) are[1]:

git checkout master^        # parent of master
git checkout HEAD~2         # grandparent of current HEAD
git checkout origin/master  # a non-local branch
git checkout tagname        # since you cant commit to a tag!

These will all make .git/HEAD contain the actual (40-hex-digit) hash of the corresponding commit instead of some string like ref: refs/heads/branch.

2.1 re-attaching the HEAD

If you made the classic error of checking out a remote branch (like git checkout origin/master) and making a few commits on it before realising something was wrong, you can recover quite easily:

git checkout -b newbranch
# or, in 2 steps: git branch newbranch; git checkout newbranch

To go back to the linked list analogy way up there, you just created a new ‘global’ variable to save off the value of ‘temp’ before some other operation overwrote it.

2.2 Auld Lang Syne

What if you didn’t realise this, and – after making those commits – blithely switched to another branch:

git checkout someoldbranch

HEAD is now a symbolic reference to ‘someoldbranch’, and its previous value (the SHA-1 representing the top commit you made on the detached HEAD) has now been overwritten.

So is it gone for good now, never to be seen again? Fortunately, no! The commit object itself is still safe out there somewhere, but you don’t know where! You have to find it…

Think back to these 2 lines in the analogy above, where the ‘current node’ was updated:

list_top = temp;            // in the first example
temp = temp2;               // in the second example

What if you had some sort of wrapper around that line, such that, every time this happened, the new value being assigned is also saved away somewhere, including information on what command called it and when. And let’s say this information is kept for 30 days. So, even if you lost all your pointers, you could check this saved list and the caller/time information to jog your memory of which one it was, and actually use that pointer value to recover the node you ‘lost’.

That’s the ‘reflog’ in git:

git reflog show HEAD@{now} -10

dcd215b... HEAD@{5 minutes ago}: commit (amend): 0-terminology: the malloc analogy added, plus
5ce8bfe... HEAD@{11 minutes ago}: commit: 0-terminology: the malloc analogy added, plus
3d93420... HEAD@{11 minutes ago}: rebase -i (pick): updating HEAD
7fdae94... HEAD@{11 minutes ago}: checkout: moving from master to 7fdae94815d6c676742c9984132b7b9e71a57f98
3d93420... HEAD@{13 minutes ago}: rebase -i (squash): updating HEAD
c55900c... HEAD@{13 minutes ago}: rebase -i (pick): updating HEAD
7fdae94... HEAD@{13 minutes ago}: checkout: moving from master to 7fdae94815d6c676742c9984132b7b9e71a57f98
e9955c8... HEAD@{14 minutes ago}: commit: s
97ab644... HEAD@{20 minutes ago}: commit: autogen
c55900c... HEAD@{23 minutes ago}: commit (amend): 0-terminology: the malloc analogy added, plus

Now you look at this, decide which one you want, and grab it:

git branch thank_God_its_safe 7fdae94
# like 'thank_God_its_safe = 0x7fdae94815d6c676742c9984132b7b9e71a57f98'
git checkout thank_God_its_safe

2.3 reality check – conclusion

There are a few other points that don’t fit the simple ‘linked list’ analogy, or present some additional information that may be useful:

2.4 footnotes

[1] Curiously, git checkout master^0 also gets you a detached HEAD. The command actually says “switch to the commit that master currently points to”. Now you’d think, since ‘master’ and ‘HEAD’ are currently pointing at the same commit, it shouldn’t be a detached HEAD, but the point is not that they happen to be pointing to the same commit. The point is that HEAD is no longer a symbolic ref (containing something like ref: refs/heads/master) and instead contains an explicit SHA-1 in it.