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As a version control system, Git empowers developers to efficiently manage branches by rebasing—moving or combining commits to a new base commit. However, rebasing can also have pitfalls, like conflicts or rebasing the wrong branch. In these cases, completely canceling the rebase and reverting to an older state may be necessary.

This comprehensive guide will cover best practices on when and how to fully undo a Git rebase, including:

• Common rebase scenarios that require cancellation
• Step-by-step instructions to reset using reflog
• How resetting compares to other revert methods
• Pros, cons, and alternatives to canceling
• Expert advice on rebase conflict resolution

Follow along for a detailed look at safely aborting rebases to rescue your commit history.

When Canceling a Rebase is Necessary

Before diving into the how-to, let‘s explore in which rebase scenarios cancelling and resetting becomes essential. The most common cases are:

Rebase Conflicts

As rebase tries applying commits from one branch onto another, it can result in merge conflicts between incompatible changes:

Auto-merging file.txt
CONFLICT (content): Merge conflict in file.txt
Failed to merge in the changes.
Patch failed at 0001 fix typo

If conflicts arise mid-rebase and seem too complex to resolve, aborting the rebase clears the conflicts while returning all files to their pre-rebase state.

Rebasing the Wrong Branch

It‘s also easy to accidentally rebase the incorrect feature branch, especially when juggling multiple branches. Resetting then brings back the original branch as if the errant rebase never happened.

History Rewrites Gone Awry

Developers may rebase to tidy up commit history before sharing a feature branch. If the resultscrambles commit messages or authors unacceptably, cancelling returns the history to its initial state.

Data Loss Concerns

In rare cases, rebasing with the --force option can cause data loss if existing commits are overwritten. Aborting the rebase before pushes mitigates this risk.

In all the above scenarios, aborting the rebase via git reset restores the branch while avoiding a corrupted Git history.

Step 1: Verify the Before & After Reflog

Our starting point to any rebase cancellation is the reflog—a chronological audit log tracking every change the HEAD reference pointer has moved to.

Check Reflog Before Rebase

Right before running an ill-fated rebase, the reflog captures the commit SHAs we want to return to:

$ git reflog
3e92170 HEAD@{0}: commit: Add README basics
9c80639 HEAD@{1}: pull: Fast-forward
ea67812 HEAD@{2}: clone: from https://github.com/user/repo

Here 3e92170 is the latest commit on our branch before things go wrong.

Confirm Reflog After Rebase

If we run git reflog again post-rebase, we clearly see new commits interleaved, like HEAD@{4}:

$ git reflog 
25db112 HEAD@{0}: rebase finished
78196aa HEAD@{1}: rebase: checkout 09234fc5
09234fc HEAD@{2}: rebase dev: checkout 78196aa
78196aa HEAD@{3}: commit: Update feature X
3e92170 HEAD@{4}: commit: Add README basics  
9c80639 HEAD@{5}: pull: Fast-forward

Now the key is spotting our original commit SHA (3e92170) among the rebase noise.

Reset Back Using Reflog

With the before/after SHAs identified via reflog, we can pick the one to reset back to, aborting the rebase entirely.

Step 2: Reset Commits with git reset

The git reset command erases commits, moving the HEAD and branch pointers back to a prior state.

$ git reset 3e92170 --hard
HEAD is now at 3e92170 Add README basics

The --hard flag also updates our working directory and staging area to match. This wholly reverts everything to the pre-rebase commit.

Compare With Other Reset Modes

Why use --hard over other reset modes?

• --soft – Updates HEAD but leaves both the index and working directory untouched.
• --mixed – Default option. Updates HEAD and index but leaves working directory unchanged.
• --hard – Updates HEAD, index, and working directory to match target commit.

For fully cancelling mid-rebase, --hard does the heavy lifting to restore all file states. Other modes risk leaving intermediate rebase commits or uncommitted changes cluttering things.

Verify Git Log Post-Reset

As a final check, we can view the Git log to ensure the botched rebase commits no longer appear:

  
$ git log --oneline
3e92170 Add README basics  
9c80639 Pull upstream changes 
ea67812 Initial commit

The log shows a linear history ending at the reflog SHA we reset to. Our rebase got erased!

Reset vs. Other Git Revert Methods

Before concluding, it‘s worth contrasting git reset against other commands that revert changes:

git revert

The git revert command generates a new commit that inverts an existing one:

$ git revert 78196aa 
$ git log --oneline 
856e112 Revert "Update feature X"
78196aa Update feature X
3e92170 Add README basics

Reverting adds commits on top of history rather than erasing, avoiding data loss. However, it doesn‘t help mid-rebase since the new commits just compound merge conflicts.

git checkout

For incomplete rebases, git checkout swaps out changes in the working directory:

  
$ git checkout HEAD^ file.txt
$ git add 
$ git rebase --continue

This lets you resolve conflicts file-by-file. But it still progresses the rebase forward. Resetting abandons entirely.

So for rebase cancellations, git reset proves uniquely suited to the task.

Pros of Canceling vs. Completing Rebases

Given alternatives like commit-by-commit conflict resolution, is aborting rebases via reset always optimal? Let‘s weigh the pros and cons.

Pros of Cancelling

Rebase cancellation brings several advantages:

• Avoids rebase commit conflicts overwhelming devs
• Prevents merge issues if rebased onto the wrong branch
• Restores original commit history instantly
• Complete undo reduces scope for Git tree corruption

For long-running rebases crossing hundreds of commits, cancelling may be the sanest route before trying again.

Pros of Completing

That said, sticking out a rebase has merits too:

• Resolve conflicts incrementally via git add/rebase
• Retain rebased changes by finishing
• More granular control over history rewriting

If conflicts seem isolated or you want to retain parts of the rebase, powering through may better suit your needs.

In summary, resetting provides an eject button for failed rebases, but working through conflicts enables more selectivity. Consider which approach best aligns with your goals.

Expert Guide To Resolving Rebase Conflicts

Since finishing rebases has advantages for selective conflict resolution, let‘s equip you with an expert-level guide to addressing merge issues:

Step 1: Investigate First Conflict

Upon hitting the first conflict, run git status to reveal which file needs resolution:

$ git status
rebase in progress; onto dd878a2
You are currently rebasing branch ‘feature‘ on ‘dd878a2‘.
  (fix conflicts and then run "git rebase --continue")
  (use "git rebase --skip" to skip this patch)
  (use "git rebase --abort" to check out the original branch)
Unmerged paths:
(use "git reset HEAD ..." to unstage)
(use "git add ..." to mark resolution)
both modified:   src/file.txt
Here we see src/file.txt as the problem file. 
Step 2: Review File Differences
Inspect the conflicting changes within the file itself:
<<<<<<>>>>>> dd878a2

The HEAD and dd878a2 commit versions appear split by conflict markers. This lets us manually integrate changes.
Step 3: Resolve Conflicts In-File
Decide how to merge the disparate changes from each branch commit. 
For example, we might wrap Dev 2‘s changes as a conditional around Dev 1‘s:
 
Dev 1 changes

Dev 2 changes  


The file now contains both sets of changes merged.
Step 4: Add/Continue Rebase
Stage the conflict resolution via git add, then continue the rebase:
 
$ git add file.txt
$ git rebase --continue  

Git applies all non-conflicting commits up through the next conflict.
Repeat Conflict Resolution
Follow steps 1-4 iteratively for each conflicted file until the rebase finishes.
While more labor intensive than aborting, this process allows salvaging work from aborted rebases. Use judiciously when appropriate. 
Common Rebasing Pitfalls & FAQs
Let‘s conclude by addressing frequent pitfalls and questions around rebase resets:
Will resetting undo commits forever?
No—the reflog storing your "lost" commits stays for 90 days by default before garbage collection. So you have a few months to recover commits using git reset.
How can I avoid needing resets?
Carefully check what branch you‘re rebasing onto before executing commands. Also rebase often on feature branches to expose conflicts incrementally vs. right before merging. 
Is there risk of losing data?
Resetting mixed with force-pushing rewritten commits can potentially make remote commit data unrecoverable. But used carefully in local branches, reset is quite safe thanks to reflog‘s local persistence.
And that‘s everything you need to know about completely cancelling botched Git rebases safely and like an expert!