7.7 Git Tools - Reset Demystified

An easier way to think about reset and checkout is through the mental frame of Git being a content manager of three different trees. By “tree” here we really mean “collection of files”, not specifically the data structure. (There are a few cases where the index doesn’t exactly act like a tree, but for our purposes it is easier to think about it this way for now.)

Git as a system manages and manipulates three trees in its normal operation:

Git’s main purpose is to record snapshots of your project in successively better states, by manipulating these three trees.

Let’s visualize this process: say you go into a new directory with a single file in it. We’ll call this v1 of the file, and we’ll indicate it in blue. Now we run git init, which will create a Git repository with a HEAD reference which points to an unborn branch (master doesn’t exist yet).

At this point, only the Working Directory tree has any content.

Now we want to commit this file, so we use git add to take content in the Working Directory and copy it to the Index.

Then we run git commit, which takes the contents of the Index and saves it as a permanent snapshot, creates a commit object which points to that snapshot, and updates master to point to that commit.

If we run git status, we’ll see no changes, because all three trees are the same.

Now we want to make a change to that file and commit it. We’ll go through the same process; first we change the file in our working directory. Let’s call this v2 of the file, and indicate it in red.

If we run git status right now, we’ll see the file in red as “Changes not staged for commit,” because that entry differs between the Index and the Working Directory. Next we run git add on it to stage it into our Index.

At this point if we run git status we will see the file in green under “Changes to be committed” because the Index and HEAD differ – that is, our proposed next commit is now different from our last commit. Finally, we run git commit to finalize the commit.

Now git status will give us no output, because all three trees are the same again.

Switching branches or cloning goes through a similar process. When you checkout a branch, it changes HEAD to point to the new branch ref, populates your Index with the snapshot of that commit, then copies the contents of the Index into your Working Directory.

The reset command makes more sense when viewed in this context.

For the purposes of these examples, let’s say that we’ve modified file.txt again and committed it a third time. So now our history looks like this:

Let’s now walk through exactly what reset does when you call it. It directly manipulates these three trees in a simple and predictable way. It does up to three basic operations.

That covers the behavior of reset in its basic form, but you can also provide it with a path to act upon. If you specify a path, reset will skip step 1, and limit the remainder of its actions to a specific file or set of files. This actually sort of makes sense – HEAD is just a pointer, and you can’t point to part of one commit and part of another. But the Index and Working directory can be partially updated, so reset proceeds with steps 2 and 3.

So, assume we run git reset file.txt. This form (since you did not specify a commit SHA-1 or branch, and you didn’t specify --soft or --hard) is shorthand for git reset --mixed HEAD file.txt, which will:

So it essentially just copies file.txt from HEAD to the Index.

This has the practical effect of unstaging the file. If we look at the diagram for that command and think about what git add does, they are exact opposites.

This is why the output of the git status command suggests that you run this to unstage a file. (See Odstranění souboru z oblasti připravených změn for more on this.)

We could just as easily not let Git assume we meant “pull the data from HEAD” by specifying a specific commit to pull that file version from. We would just run something like git reset eb43bf file.txt.

This effectively does the same thing as if we had reverted the content of the file to v1 in the Working Directory, ran git add on it, then reverted it back to v3 again (without actually going through all those steps). If we run git commit now, it will record a change that reverts that file back to v1, even though we never actually had it in our Working Directory again.

It’s also interesting to note that like git add, the reset command will accept a --patch option to unstage content on a hunk-by-hunk basis. So you can selectively unstage or revert content.

Let’s look at how to do something interesting with this newfound power – squashing commits.

Say you have a series of commits with messages like “oops.”, “WIP” and “forgot this file”. You can use reset to quickly and easily squash them into a single commit that makes you look really smart. (Squashing Commits shows another way to do this, but in this example it’s simpler to use reset.)

Let’s say you have a project where the first commit has one file, the second commit added a new file and changed the first, and the third commit changed the first file again. The second commit was a work in progress and you want to squash it down.

You can run git reset --soft HEAD~2 to move the HEAD branch back to an older commit (the first commit you want to keep):

And then simply run git commit again:

Now you can see that your reachable history, the history you would push, now looks like you had one commit with file-a.txt v1, then a second that both modified file-a.txt to v3 and added file-b.txt. The commit with the v2 version of the file is no longer in the history.

Finally, you may wonder what the difference between checkout and reset is. Like reset, checkout manipulates the three trees, and it is a bit different depending on whether you give the command a file path or not.

Hopefully now you understand and feel more comfortable with the reset command, but are probably still a little confused about how exactly it differs from checkout and could not possibly remember all the rules of the different invocations.

Here’s a cheat-sheet for which commands affect which trees. The “HEAD” column reads “REF” if that command moves the reference (branch) that HEAD points to, and “HEAD” if it moves HEAD itself. Pay especial attention to the WD Safe? column – if it says NO, take a second to think before running that command.