Undoing Changes

Estimated time: 30 minutes

Overview

Questions

• How do I undo changes to a repository?

Objectives

• To understand how to undo changes before making a commit
• To understand how to undo changes after making a commit

Undoing Changes

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• One of the purposes of version control is to let you undo changes
• There are different changes we might want to undo:

• Changes to the working tree
• Change files staged for the next commit
• Changes made in previous commits
• History ( !! ) (not covered here)

Undoing Changes to Working Tree

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• The simplest!
• Github Desktop: right-click -> Discard Changes
• CLI: git restore <file>...

• For example: git restore README.md
• If you forget, run git status for a reminder

• DANGER : git isn't tracking these files, so there's no way to undo this undo!

Undoing Staged Changes

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Graphical interface

Command line

Stage/unstage

You can even stage/unstage individual lines

• CLI: git restore --staged <file>...

• For example: git restore --staged README.md
• If you forget, run git status for a reminder

• This is exactly the opposite of git add <file>...

Commit Hashes

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• Each commit in the history has a unique identifier associated with it, the commit hash
• This is a long string that looks something like this: 0efd1cb9e37318404b76de7c99e26fbef16ef3a3
• You only ever need the first 7 characters of the ID!
• It is used in any Git operation that needs to refer to a specific commit

BASH

klcm500@matscrn my_first_repository % git log
commit 0efd1cb9e37318404b76de7c99e26fbef16ef3a3 (HEAD -\> main, origin/main, origin/HEAD)
Merge: c267c36 77caf66
Author: Killian Murphy \<killian.murphy@york.ac.uk\>
Date:   Mon Jan 23 16:43:31 2023 +0000]

Reverting A Commit

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Graphical interface

Command line

• GitHub desktop can’t revert many commits at once
• We can easily revert commits one-by-one!
• Switch to the “History” tab and right-click the latest commit

Click this

• We now have a new commit that reverts the previously made changes
• We can play around with this and see if it works
• If we are happy with the reversion, we can push to reflect the reversion on GitHub
• With GitHub Desktop, we need to do this for each commit we want to revert, in reverse order
• With the command line, reverting multiple commits at once is possible - not covering that today

• First need to identify the ID of the commit we want to revert - the last commit we made. Take a look at git log output: commit f7cfe07b8d7691241ebca90c19b187da142350c1 (HEAD -\> main). We only need the first 7 characters of this ID string!
• Then we run: git revert f7cfe07. Since we are making a new commit which reverts the previous commit, we are prompted for a label and a message. This is a good opportunity to document why we are reverting a commit
• If we are happy with the reversion, we can push the changes as with any other commit