How Git Log History Simplification Works
Authors: Ross Brodbeck
Git history simplification can be a confusing beast. 99% of the time you won't even know it exists, but every once in a while it will jump out of the dark corners of Git and bite you. In this article we'll explore what history simplification is and how it can cause confusion when looking at file history.
Let's start with a common scenario:
- You push a change to a file and then merge the change into master.
- Some of your colleagues also merge their branches to master.
- You come back some time later and notice your changes are missing.
- Looking for the culprit, you go look at the file history and notice... your changes aren't even listed!?
What is happening here is that Git commit history is a tree, and sometimes the chronological history is not the same as the actual file tree history. This is particularly true in cases where a merge commit reverts a file back to its original state. In that case, the default history view won't actually show you all changes, because technically the file didn't change.
In the above scenario, Git realizes it can simplify the history and the "changes" you are most likely looking for are removed from the log.
Unless you have run into it before, the result is often a lot of hair pulling and wondering Where the heck did my changes go?
History Simplification: On by Default
By default, running the log command on a file:
git log file.txt will automatically simplify history, possibly hiding some commits from its output. You can read more details over at the git log man page.
What makes this a tad more confusing is that history simplification does not occur if you just run
git log (because you are looking at all changes there is nothing to simplify).
In order to turn off history simplification, we need to use the command line switch
An Example of History Simplification
This is all nice and theoretical, but let's create our own example of history simplification so we can see how it works. First, let's look at a diagram of the history we are going to create:
As you can see, we are going to:
- Create a file.
- Add a line to that file in a branch (animals).
- Add a different line to that file in another branch (fruit).
- Merge branch animals back into master.
- Merge branch fruit back into master, and choose the entire copy of the file from the fruit branch.
- Check the history of the file.
As you will see, Git is going to simplify the history for us. The key here is step 5 -- we ignored all changes from the animal branch. Git will notice that our file essentially did not change between step 1 and step 5, and so it will only show us two history entries.
First we create the file and add it to our repo:
> cd sample > git init > echo "some content" > test.txt > git add test.txt > git commit -m "Initial commit"
Now we decide to append the text "donkeys" to the file in an animal branch:
> git checkout -b animals > echo "donkeys" >> test.txt > git commit -am "We have added an animal"
While we are experimenting, we decide maybe we want to go with fruit in our file instead, so we create a different branch and append the text "bananas" at the end of the file instead:
> git checkout master -b fruit > echo "bananas" >> test.txt > git commit -am "We have added a fruit"
Feeling satisfied with our changes, we decide to merge our animal branch back into master:
> git checkout master > git merge animals
Now let's look at the log for our
> git log test.txt
commit 6b33d99b996c430a60c9552b79245d1aa8320339 Author: hross <email@example.com> Date: Mon Feb 15 10:45:33 2016 -0500 We have added an animal commit 206613ccd9a54b055b184c7b6c16f2ece8067e51 Author: hross <firstname.lastname@example.org> Date: Mon Feb 15 10:44:18 2016 -0500 Initial commit
So far so good, right? Nothing looks out of the ordinary in our log output. Now let's say we changed our minds and decided to merge our fruit branch:
> git merge fruit
Auto-merging test.txt CONFLICT (content): Merge conflict in test.txt Automatic merge failed; fix conflicts and then commit the result.
Uh-oh, a merge conflict. After some consideration, we decide to use the entire
test.txt file from our fruit branch. Typically you would use some kind of text editor or merge tool, but we'll just recreate the entire file, since it's only two lines:
> echo "some content" > test.txt > echo "bananas" >> test.txt > git commit -am "Fixed merge conflict"
Now let's take a look at the history for our
> git log test.txt
commit fdd4dfd816c4efebc5bdb240f49e934e299db581 Author: hross <email@example.com> Date: Mon Feb 15 10:51:06 2016 -0500 We have added a fruit commit 206613ccd9a54b055b184c7b6c16f2ece8067e51 Author: hross <firstname.lastname@example.org> Date: Mon Feb 15 10:44:18 2016 -0500 Initial commit
Sure enough, we don't see any changes from our first experiment in the log, nor do we see our merge! Are they still there? Did Git eliminate the changes entirely?
> git log --full-history test.txt
As you can see, although it simplified the log without the
full-history flag, Git has kept all of our changes:
commit 5d0bb77a24e265dc154654fb3b5be331b53bf977 Merge: 6b33d99 fdd4dfd Author: hross <email@example.com> Date: Mon Feb 15 10:59:34 2016 -0500
Fixed merge conflict commit fdd4dfd816c4efebc5bdb240f49e934e299db581 Author: hross <firstname.lastname@example.org> Date: Mon Feb 15 10:51:06 2016 -0500 We have added a fruit commit 6b33d99b996c430a60c9552b79245d1aa8320339 Author: hross <email@example.com> Date: Mon Feb 15 10:45:33 2016 -0500 We have added an animal commit 206613ccd9a54b055b184c7b6c16f2ece8067e51 Author: hross <firstname.lastname@example.org> Date: Mon Feb 15 10:44:18 2016 -0500 Initial commit
Git History Simplification in a Nutshell
The thing about history simplification is that most of the time you will never notice it. But when a merge conflict goes wrong and you want to know what happened -- you may find yourself looking at the git log history and wondering where your changes went.
Now, instead of panicking, you know that:
- History simplification for files is turned on by default
--full-historyflag will give you a more comprehensive file history
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