Raise your hand if you can relate:
*Honest job interview*
Interviewer: βWhatβs your biggest weakness?β
Me: βSometimes I fuck up my rebase so badly I delete the repo and clone a new copy π₯πββοΈβ21:10 PM - 25 Feb 2020
πππππ
We've all been there! It's easy to do a well-meaning git rebase or git reset only to find everything broken afterwards. Then, since your changes rewrote history, the only way to go back is to delete everything and re-clone the repository to start over again. I used to do this all the time!
But what if I told you there was a better way? What if you could time travel back to a point before you did those destructive changes without deleting anything? It turns out, git has built-in tools for this such as reflog, ORIG_HEAD, and gitrevisions, and they're surprisingly easy to use!
In this article, I'll show you how to use these nifty shortcuts to undo some of those history-changing moves and get back to the proper timeline!
Setup
If you only want to read the examples, you can skip to the next section. But if you want to follow along with the examples on your own terminal, run each line below as a command in your terminal to set up the demo repository:
mkdir reflog-demo && cd reflog-demo && git init
echo "file 1 content" > file1.txt && git add . && git commit -m "add file1"
echo "file 2 content" > file2.txt && git add . && git commit -m "add file2"
git checkout -b my-new-branch
echo "new branch file content" > newBranchFile.txt && git add . && git commit -m "add newBranchFile"
git checkout master
echo "file 3 content" > file3.txt && git add . && git commit -m "add file3"
git checkout my-new-branch
Examples
Here's an overview of the starting git history (using the command git log --oneline --graph --all):
There is a master branch with 3 files, each added in a separate commit. There is also a separate my-new-branch branch. This branch also has the first two files, but then splits off from master and has its own third file.
And here are all the files in my-new-branch:
This is the desired state. When we mess it up in the examples below, our goal is to bring the repository back to this state.
1. Reset using a reflog reference
Let's say we're on my-new-branch and we rebase onto master:
But Agh! We realize this was a mistake! Maybe the code stopped working after the rebase. Maybe there were some weird merge conflicts during the rebase and everything got messy. Either way, we want things to be the way they were before, but here is what our new git log shows:
...and all our my-new-branch files are combined with the master files:
Apparently, our original commit (6cde040) is gone. How do we go back? We could delete everything and re-clone, but we probably don't have a remote copy of our most recent changes to clone. We could manually create a new branch off the old commit and re-add/delete the files that were there, but what a hassle!
Instead, try running git reflog to see some helpful points in history:
Git reflog shows everywhere our current working HEAD was at, including commits that were removed with destructive commands such as git reset and git rebase. Simply pick the commit before our destructive changes, do a hard reset to the gitrevision number (the one with the curly braces), and everything is reset to the way it was before the rebase!
git reset --hard HEAD@{3}
After running this command, our git log shows us we're back to our original state with the original commits and the original files:
2. Reset to ORIG_HEAD
"But Zak," you might say, "that reflog output is kind of confusing, and it's hard to tell exactly which reflog line to go back to."
Agreed! Luckily, there's an EVEN EASIER way!
git reset --hard ORIG_HEAD
ORIG_HEAD automatically points to the state before the most recent destructive change, so we can easily undo our most recent rebase or reset with a single command! Running git reset --hard ORIG_HEAD here does the exact same thing as running git reset --hard HEAD@{3} in the example above without needing to hunt down a specific revision!
3. Reset to a relative time
πππWARNING: THIS IS REALLY COOL
Resetting to ORIG_HEAD is great, but what if we've really gone down a rabbit hole of destructive changes. Let's take the rebase from the last example, and add a few more steps to it:
git rebase master
git reset --hard HEAD~2
git commit --amend -m 'shablagoo!'
Why did we do this? I don't know Β―\_(γ)_/Β―. Someone on Stack Overflow probably said it was a good idea, so we tried it on a whim and, well, things got weird:
The commits are all out of order, and we are missing files. How can we undo this??? We made several destructive changes, so ORIG_HEAD isn't going to work. We can check git reflog, but there's so many changes that happened that it might be hard to find the right one. All we want is for our branch to look like it did 5 minutes ago before the changes.
Try this:
git reset --hard HEAD@{5.minutes.ago}
...and afterwards, you'll see the branch in the exact state it was 5 minutes ago:
Yes, you read that right: This command literally tells Git in plain English to return to some time in the past, and Git does it!
This is using a powerful concept in git called gitrevisions. Some other examples include: HEAD@{1.day.2.hours.ago}, HEAD@{yesterday}, HEAD@{2020-01-01 18:30:00}, HEAD@{12:43}. In other words: TIME TRAVEL IS POSSIBLE IN GIT!!!
Caveats
The techniques above are powerful, but there are a few limitations to keep in mind:
-
Only works for your local terminal β your
reflogand gitrevisions are stored locally, but aren't shared when you push to a remote repository - Only works for committed files β if you've deleted uncommitted files, these aren't stored in Git anywhere. Good reason to commit early and often
Recap
So, to summarize, here are some ways to fix your repo after you reset or rebase your code into an unusable state:
- Use
git reflogto pick a point in history, then undo your recent changesgit reset --hard HEAD@{<number>} - As a shortcut, use
git reset --hard ORIG_HEADto undo the most recent destructive operation - Reset to
<refname>@<relative time>to time travel back to a happier state (e.g.git reset --hard HEAD@{10.minutes.ago})
I hope this helps someone out there! Let me know in the comments if you have any questions or any other go-to methods for undoing git mistakes!
Top comments (11)
Please note that you would only be able to achieve all this if your git repo was not garbage cleaned (yes, git has its own Garbage Collector).
There is an optional git config variable called gc.reflogExpireUnreachable that defaults to 30 days that clears all unreachable reflogs (not pointing to/from any live reflogs - a.k.a. lost commits).
Git garbage collection is run automatically on several frequently used commands:
To prevent running automatic GC on your git repos run:
git config --global gc.auto 0Similarly you could set to never expire reflog entries which would make your git repo to grow much larger than usually:
git config --global gc.pruneExpire nevergit config --global gc.reflogExpire never
Thanks for pointing this out! These tools are best used for resets to a recent state (within a few days at the most).
From my understanding, the default expiration is 90 days, but the impact is the same: if you have really old changes that you might want to use later, these should probably be reliably saved in a commit somewhere.
If you know ahead of time you want a reliable long-lasting backup of your branch state, you can create a backup branch before making changes.
The thing is some tools enable/update/run git GC automatically (and sometimes even without you knowing). That might kill your whole reflog history and any attempts to restore... :(
Good to know!
Came here thinking this was gonna cover generic things I already knew, but nope! I actually learned something :) That last one blew my mind:
I'll have to keep this in mind!
Or you could just use a sane SCM system, like Mercurial that, by design, does not destroy history so easily: it is impossible in hg to remotely damage a shared repository π€·ββοΈ
Git Bisect is a really powerful tool too when you don't really know when something started to fail.
Exactly the same here! Awesome article. Thanks for teaching me something new π
Great writeup. Learned new things that i never thought ππ love the time traveling π
Great article! Well written and humorous, with great examples.
... Could you do one on how to rebase and squash commits?
Glad you liked it! Rebasing and squashing is definitely on my list of things I'd like write about. Thanks for the request, and stay tuned...