7.6 Git Tools - Rewriting History

Changing your most recent commit is probably the most common rewriting of history that you’ll do. You’ll often want to do two basic things to your last commit: simply change the commit message, or change the actual content of the commit by adding, removing and modifying files.

If you simply want to modify your last commit message, that’s easy:

The command above loads the previous commit message into an editor session, where you can make changes to the message, save those changes and exit. When you save and close the editor, the editor writes a new commit containing that updated commit message and makes it your new last commit.

If, on the other hand, you want to change the actual content of your last commit, the process works basically the same way — first make the changes you think you forgot, stage those changes, and the subsequent git commit --amend replaces that last commit with your new, improved commit.

You need to be careful with this technique because amending changes the SHA-1 of the commit. It’s like a very small rebase — don’t amend your last commit if you’ve already pushed it.

To modify a commit that is farther back in your history, you must move to more complex tools. Git doesn’t have a modify-history tool, but you can use the rebase tool to rebase a series of commits onto the HEAD they were originally based on instead of moving them to another one. With the interactive rebase tool, you can then stop after each commit you want to modify and change the message, add files, or do whatever you wish. You can run rebase interactively by adding the -i option to git rebase. You must indicate how far back you want to rewrite commits by telling the command which commit to rebase onto.

For example, if you want to change the last three commit messages, or any of the commit messages in that group, you supply as an argument to git rebase -i the parent of the last commit you want to edit, which is HEAD~2^ or HEAD~3. It may be easier to remember the ~3 because you’re trying to edit the last three commits, but keep in mind that you’re actually designating four commits ago, the parent of the last commit you want to edit:

Remember again that this is a rebasing command — every commit in the range HEAD~3..HEAD with a changed message and all of its descendants will be rewritten. Don’t include any commit you’ve already pushed to a central server — doing so will confuse other developers by providing an alternate version of the same change.

Running this command gives you a list of commits in your text editor that looks something like this:

It’s important to note that these commits are listed in the opposite order than you normally see them using the log command. If you run a log, you see something like this:

Notice the reverse order. The interactive rebase gives you a script that it’s going to run. It will start at the commit you specify on the command line (HEAD~3) and replay the changes introduced in each of these commits from top to bottom. It lists the oldest at the top, rather than the newest, because that’s the first one it will replay.

You need to edit the script so that it stops at the commit you want to edit. To do so, change the word `pick' to the word `edit' for each of the commits you want the script to stop after. For example, to modify only the third commit message, you change the file to look like this:

When you save and exit the editor, Git rewinds you back to the last commit in that list and drops you on the command line with the following message:

These instructions tell you exactly what to do. Type:

Change the commit message, and exit the editor. Then, run:

This command will apply the other two commits automatically, and then you’re done. If you change pick to edit on more lines, you can repeat these steps for each commit you change to edit. Each time, Git will stop, let you amend the commit, and continue when you’re finished.

You can also use interactive rebases to reorder or remove commits entirely. If you want to remove the “Add cat-file” commit and change the order in which the other two commits are introduced, you can change the rebase script from this:

to this:

When you save and exit the editor, Git rewinds your branch to the parent of these commits, applies 310154e and then f7f3f6d, and then stops. You effectively change the order of those commits and remove the “Add cat-file” commit completely.

It’s also possible to take a series of commits and squash them down into a single commit with the interactive rebasing tool. The script puts helpful instructions in the rebase message:

If, instead of “pick” or “edit”, you specify “squash”, Git applies both that change and the change directly before it and makes you merge the commit messages together. So, if you want to make a single commit from these three commits, you make the script look like this:

When you save and exit the editor, Git applies all three changes and then puts you back into the editor to merge the three commit messages:

When you save that, you have a single commit that introduces the changes of all three previous commits.

Splitting a commit undoes a commit and then partially stages and commits as many times as commits you want to end up with. For example, suppose you want to split the middle commit of your three commits. Instead of “Update README formatting and add blame”, you want to split it into two commits: “Update README formatting” for the first, and “Add blame” for the second. You can do that in the rebase -i script by changing the instruction on the commit you want to split to “edit”:

Then, when the script drops you to the command line, you reset that commit, take the changes that have been reset, and create multiple commits out of them. When you save and exit the editor, Git rewinds to the parent of the first commit in your list, applies the first commit (f7f3f6d), applies the second (310154e), and drops you to the console. There, you can do a mixed reset of that commit with git reset HEAD^, which effectively undoes that commit and leaves the modified files unstaged. Now you can stage and commit files until you have several commits, and run git rebase --continue when you’re done:

Git applies the last commit (a5f4a0d) in the script, and your history looks like this:

This changes the SHA-1s of the three most recent commits in your list, so make sure no changed commit shows up in that list that you’ve already pushed to a shared repository. Notice that the last commit (f7f3f6d) in the list is unchanged. Despite this commit being shown in the script, because it was marked as “pick” and was applied prior to any rebase changes, Git leaves the commit unmodified.

If you want to get rid of a commit, you can delete it using the rebase -i script. In the list of commits, put the word “drop” before the commit you want to delete (or just delete that line from the rebase script):

Because of the way Git builds commit objects, deleting or altering a commit will cause the rewriting of all the commits that follow it. The further back in your repo’s history you go, the more commits will need to be recreated. This can cause lots of merge conflicts if you have many commits later in the sequence that depend on the one you just deleted.

If you get partway through a rebase like this and decide it’s not a good idea, you can always stop. Type git rebase --abort, and your repo will be returned to the state it was in before you started the rebase.

If you finish a rebase and decide it’s not what you want, you can use git reflog to recover an earlier version of your branch. See Data Recovery for more information on the reflog command.

There is another history-rewriting option that you can use if you need to rewrite a larger number of commits in some scriptable way — for instance, changing your email address globally or removing a file from every commit. The command is filter-branch, and it can rewrite huge swaths of your history, so you probably shouldn’t use it unless your project isn’t yet public and other people haven’t based work off the commits you’re about to rewrite. However, it can be very useful. You’ll learn a few of the common uses so you can get an idea of some of the things it’s capable of.