Undoing Changes

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Overview

Questions

• How do I undo changes?

Objectives

• How do I roll back a single change?
• How do I get back to a specific state?

Challenge

Exercise: Creating a branch.

Create a new branch called hotfix. Create a new file and make 3-4 commits in that file or create 3-4 new files. Check the log to see the SHA of the last commit.

Give me a hint

You can use the touch command to create new files quickly.

Give me a hint

Use git add and git commit -m "your message" to save your changes.

Show me the solution

BASH

git checkout -b hotfix
touch a.txt
git add . && git commit -m "1st git commit: 1 file"
touch b.txt
git add . && git commit -m "2nd git commit: 2 file"
touch c.txt
git add . && git commit -m "3rd git commit: 3 file"
git status
git log --oneline

Git Revert

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Reverting undoes a commit by creating a new commit. This is a safe way to undo changes, as it has no chance of re-writing the commit history. For example, the following command will figure out the changes contained in the 2nd to last commit, create a new commit undoing those changes, and tack the new commit onto the existing project.

BASH

git revert HEAD~1
ls

git revert

Note that revert only backs out the atomic changes of the ONE specific commit (by default, you can also give it a range of commits but we are not going to do that here, see the help).

git revert does not rewrite history which is why it is the preferred way of dealing with issues when the changes have already been pushed to a remote repository.

Git Reset

---

Resetting is a way to move the tip of a branch to a different commit. This can be used to remove commits from the current branch. For example, the following command moves the hotfix branch backwards by two commits.

BASH

git checkout hotfix
git reset HEAD~1

git reset

The two commits that were on the end of hotfix are now dangling, or orphaned commits. This means they will be deleted the next time git performs a garbage collection. In other words, you’re saying that you want to throw away these commits.

git reset also reverts the commits but leaves the uncommitted changes in the repo.

BASH

git status
git restore b.txt

git reset is a simple way to undo changes that haven’t been shared with anyone else. It’s your go-to command when you’ve started working on a feature and find yourself thinking, “Oh crap, what am I doing? I should just start over.”

In addition to moving the current branch, you can also get git reset to alter the staged snapshot and/or the working directory by passing it one of the following flags:

–soft – The staged snapshot and working directory are not altered in any way.

–mixed – The staged snapshot is updated to match the specified commit, but the working directory is not affected. This is the default option.

–hard – The staged snapshot and the working directory are both updated to match the specified commit.

It’s easier to think of these modes as defining the scope of a git reset operation.

To just undo any uncommitted changes:

BASH

git status
git add c.txt
git status
git reset HEAD
git status

You can add and commit the changes or restore the file.

git reset can also work on a single file:

BASH

git reset HEAD~2 foo.txt

Git Checkout: A Gentle Way

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We already saw that git checkout is used to move to a different branch but is can also be used to update the state of the repository to a specific point in the projects history.

BASH

git checkout hotfix
git checkout HEAD~2

git checkout

This puts you in a detached HEAD state. AGHRRR!

Most of the time, HEAD points to a branch name. When you add a new commit, your branch reference is updated to point to it, but HEAD remains the same. When you change branches, HEAD is updated to point to the branch you’ve switched to. All of that means that, in these scenarios, HEAD is synonymous with “the last commit in the current branch.” This is the normal state, in which HEAD is attached to a branch.

The detached HEAD state is when HEAD is pointing directly to a commit instead of a branch. This is really useful because it allows you to go to a previous point in the project’s history. You can also make changes here and see how they affect the project.

BASH

echo "Welcome to the alternate timeline, Morty!" > new-file.txt
git add .
git commit -m "Create new file"
echo "Another line" >> new-file.txt
git commit -a -m "Add a new line to the file"
git log --oneline

If you haven’t made any changes or you have made changes but you want to discard them you can recover by switching back to your branch:

BASH

git checkout hotfix

Alternatively, you want to keep the changes:

BASH

git branch alt-history
git checkout alt-history

https://www.atlassian.com/git/tutorials/resetting-checking-out-and-reverting Also OMG: http://blog.kfish.org/2010/04/git-lola.html

Exercise: Undoing Changes

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Challenge

Exercise: Undoing Changes

1. Create a new branch called hotfix. Create a new file and make 3-4 commits in that file. Check the log to see the SHA of the last commit.
2. Revert the last commit that we just inserted. Check the history.
3. Completely throw away the last two commits [DANGER ZONE!!!]. Check the status and the log.
4. Undo another commit but leave it in the staging area. Check the status and log.
5. Wrap it up: add and commit the changes.

Show me the solution

Step 1:

BASH

git checkout -b hotfix
touch my_file.txt
echo "First line" > my_file.txt
git add my_file.txt
git commit -m "First commit"
echo "Second line" >> my_file.txt
git add my_file.txt
git commit -m "Second commit"
echo "Third line" >> my_file.txt
git add my_file.txt
git commit -m "Third commit"
git status
git log --oneline

Step 2:

BASH

git revert -m 1 <SHA>
git log

Step 3:

BASH

git reset HEAD~2 --hard
git status
git log

Step 4:

BASH

git reset HEAD~1
git status
git log

Step 5:

BASH

git add .
git commit -m "Message"

Key Points

• git reset rolls back the commits and leaves the changes in the files
• git reset --hard roll back and delete all changes
• git reset does alter the history of the project.
• You should use git reset to undo local changes that have not been pushed to a remote repository.
• git revert undoes a commit by creating a new commit.
• git revert should be used to undo changes on a public branch or changes that have already been pushed remotely.
• git revert only backs out a single commit or a range of commits.