OverviewTeaching: 10 min
Exercises: 10 minQuestions
- How can I undo things?
- Learn to undo changes safely
- See when undone changes are permanently deleted and when they can be retrieved
One of the main points of version control is that you can go back in time to recover. Unlike this xkcd comic implies: https://xkcd.com/1597/
This lesson covers four types of undoing:
git commit --amend).
git reset --hard).
Your instructor will choose two to do two in this lesson.
When have you made a change, and had to go through much more trouble to figure out what you did, or fix it?
As long as you commit something once (or at least
git add it), you
can almost always go back to it, no matter what you do, within ~2
weeks. But you may need to ask stackoverflow or your local
guru… until that guru becomes you.
You do some work, and want to undo everything and go back to your last commit. You can always do that with:
git checkout -f master: go back to master, no matter what.
Or, you can undo things selectively, as you have already learned:
git checkout -por
git checkout $file(working dir)
git reset -por
git reset $file(staging area)
Clear your workspace
- If you have unstaged changes from earlier sections, remove them and get a clean working directory.
git statusshould report nothing.
Note: we’ve seen some time when
git checkout -f master doesn’t
work. Can you figure out when? If it doesn’t work, try
f960dd3was 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 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
- Inspect the history with
git log --oneline.
- Now try
git showon both the reverted and the newly created commit.
Modifying history? Isn’t a “commit” permanent?
The following two commands are dangerous if you use them without being careful. Use them only before you have pushed commits, until you have more practice!
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.
Exercise: Modify a previous commit
- Make an incomplete change to the recipe or a typo in your change,
git committhe incomplete/unsatisfactory change.
- Inspect the unsatisfactory but committed change with
git show. Remember the commit hash.
- Now complete/fix the change but instead of creating a new commit, add to the previous commit with
git commit --amend. What changed?
You can reset branch history to move your branch head back to some point in the past.
git reset --hard $commitwill force a branch head to any other point. All other changes are lost.
git grapha lot before and after.
Exercise: Destroy our experimentation in this episode
After we have experimented with reverts and amending, let us destroy all of that and get our repositories to a similar state.
- First, we will look at our history (
git graph) and find the last commit
$commitbefore our tests.
- Then, we will
git reset --hard $committo that.
git graphagain to see what happened.
$ 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 2bb9bb4 add half an onion 2d79e7e adding ingredients and instructions
Test your understanding
- What happens if you accidentally remove a tracked file with
git rm, is it gone forever?
- Is it OK to modify commits that nobody has seen yet?
- What situations would justify to modify the Git history and possibly remove commits?
- What is the difference between these commands?
$ git diff $ git diff --staged # or git diff --cached $ git diff HEAD $ git diff HEAD^
- It is not gone forever since
git rmcreates a new commit. You can simply revert it!
- If you haven’t shared your commits with anyone it can be alright to modify them.
- 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.
- 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.