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3 ways to time travel in Git to undo destructive mistakes

Zak Laughton on February 27, 2020

Raise your hand if you can relate: Emma Bostian 🐞 @emmabostian ...

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Slavius • Feb 28 '20 • Edited

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:

git pull
git merge
git rebase
git commit

To prevent running automatic GC on your git repos run:
git config --global gc.auto 0

Similarly 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 never git config --global gc.reflogExpire never

Zak Laughton • Feb 28 '20 • Edited

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.

git checkout my-new-branch
git branch my-new-branch-backup
# now reset, rebase, or otherwise destroy my-new-branch
# checkout my-new-branch-backup later to restore

Slavius • Mar 2 '20

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... :(

Aleksandr Hovhannisyan • Feb 28 '20

Good to know!

Aleksandr Hovhannisyan • Feb 28 '20

Came here thinking this was gonna cover generic things I already knew, but nope! I actually learned something :) That last one blew my mind:

git reset --hard HEAD@{5.minutes.ago}

I'll have to keep this in mind!

michelemauro • Feb 28 '20

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 🤷‍♂️