### Overview

Teaching: 10 min
Exercises: 10 min
Questions
• How can I undo things?
Objectives
• Learn to undo changes safely
• See when undone changes are permanently deleted and when they can be retrieved

## Undoing things

• Commits that are part of any branch will not get lost.
• Files which were added and later removed can always be recovered.
• In Git we can modify, reorder, squash, and remove commits and also these actions can be undone.
• Some commands can permanently delete uncommitted changes. In doubt always commit first.
• Some commands modify history. This is OK for local commits but may not be OK for commits shared with others.

• If you have unstaged changes from earlier sections, remove them with git checkout <filename>.
• We will see in more detail below how git checkout works.

### Reverting commits

• Imagine we made a few commits.
• We realize that the latest commit f960dd3 was a mistake and we wish to undo it:
$git log --oneline f960dd3 (HEAD -> master) not sure this is a good idea dd4472c we should not forget to enjoy 2bb9bb4 add half an onion 2d79e7e adding ingredients and instructions  A safe way to undo the commit is to revert the commit with git revert: $ git revert f960dd3


This creates a new commit that does the opposite of the reverted commit. The old commit remains in the history:

$git log --oneline d62ad3e (HEAD -> master) Revert "not sure this is a good idea" f960dd3 not sure this is a good idea dd4472c we should not forget to enjoy 2bb9bb4 add half an onion 2d79e7e adding ingredients and instructions  You can revert any commit, no matter how old it is. It doesn’t affect other commits you have done since then - but if they touch the same code, you may get a conflict (which we’ll learn about later). ## Exercise: Revert a commit • Create a commit. • Revert the commit with git revert. • Inspect the history with git log --oneline. • Now try git show on both the reverted and the newly created commit. ### Adding to the previous commit Sometimes we commit but realize we forgot something. We can amend to the last commit: $ git commit --amend


This can also be used to modify the last commit message.

Note that this will change the commit hash. This command modifies the history. This means that we never use this command on commits that we have shared with others.

### Undo unstaged/uncommitted changes

git checkout <file> is a command that permanently deletes changes that were unstaged/uncommitted!

## Exercise: Modify without staging

1. Make a silly change to the recipe, do not stage it or commit it.
2. Inspect the change with git status and git diff.
3. Now undo the change with git checkout <file>.
4. Verify that the change is gone with git status and git diff.

## Exercise: Modify after staging

1. Make a reasonable change to the recipe, stage it.
2. Make a silly change after you have staged the reasonable change.
3. Inspect the situation with git status, git diff, git diff --staged, and git diff HEAD.
4. Now undo the silly change with git checkout <file>.
5. Inspect the new situation with git status, git diff, git diff --staged, and git diff HEAD.

1. What happens if you accidentally remove a tracked file with git rm, is it gone forever?
2. Is it OK to modify commits that nobody has seen yet?
3. What situations would justify to modify the Git history and possibly remove commits?
4. What is the difference between these commands?
$git diff$ git diff --staged (or git diff --cached)
$git diff HEAD$ git diff HEAD^


## Solution

1. It is not gone forever since git rm creates a new commit. You can simply revert it!
2. If you haven’t shared your commits with anyone it can be alright to modify them.
3. If you have shared your commits with others (e.g. pushed them to GitHub), only extraordinary conditions would justify modifying history. For example to remove sensitive or secret information.
4. The different commands show changes between different file states:
$git diff - Show what has changed but hasn't been staged yet via git add.$ git diff --staged - Show what has been staged but not yet committed.
$git diff HEAD - Show what has changed since the last commit.$ git diff HEAD^ - Show what has changed since the commit before the latest commit.


### Clean history

We will be using the guacamole recipe example throughout the lesson and it will be easy to follow if the learners, instructor and the teaching material contains identical repositories with same number of commits. For this purpose we recommend to reset the guacamole repository to an earlier state using the git reset command. This command is one of the destructive commands in Git, so use with caution.

$git log --oneline d62ad3e (HEAD -> master) Revert "not sure this is a good idea" f960dd3 not sure this is a good idea dd4472c we should not forget to enjoy 2bb9bb4 add half an onion 2d79e7e adding ingredients and instructions$ git reset --hard dd4472c

HEAD is now at dd4472c we should not forget to enjoy

\$ git log --oneline

dd4472c (HEAD -> master) we should not forget to enjoy