What is Git & GitHub? – Revert

🇯🇵 日本語版：03_Revert.md

This section introduces git revert and explains how to undo files with Git.

Outline

• Preparing for Time Travel
  • Local Git
  • GitHub
  • Create Sample Files and Branches
• Revert: Rewind the Clock
  • View the History
  • GitHub - timeline branch
  • GitHub - timeline’s commits
• Visit the Past
• Go Back One Commit
• Go Back by Commit Hash
• Reset vs Revert
• Next Section

Preparing for Time Travel

Local Git

In the previous section, you created, merged, and deleted the develop branch.

Let’s get down to just the main branch. If you still have the develop branch, please delete it.

1. Move to the learning_git folder.

   cd ~/learning_git
   

2. Check the current Git status.

   git status
   

   On branch develop
   Your branch is up to date with 'origin/develop'.
   nothing to commit, working tree clean
   

3. Check the branches in the repository.

   git branch
   

   - develop
     main
   

4. Since you are on the develop branch, switch to main.

   git checkout main
   

   Switched to branch 'main'
   Your branch is up to date with 'origin/main'.
   

5. Delete the develop branch.

   git branch --delete develop
   

   warning: deleting branch 'develop' that has been merged to
            'refs/remotes/origin/develop', but not yet merged to HEAD.
   Deleted branch develop (was 4f98baf).
   

6. Let’s also delete the develop_file.md file.

   rm develop_file.md
   

GitHub

1. Go to the learning_git repository on GitHub.
   • github.com/UserName/learning_git
2. Check whether there is only one branch.
   • github.com/UserName/learning_git/branches
3. If there is a develop branch, delete the branch.
   • 

---

Create Sample Files and Branches

Create files for time travel.

1. Switch to the main branch.

   git checkout main
   

2. Create a timeline branch.

   git checkout -b timeline
   

   Switched to a new branch 'timeline'
   

3. Create the following files and commit them separately.

   • yr_1, yr_2, yr_3

   touch yr_1
   git add yr_1
   git commit -m "Year 1"
   

   touch yr_2
   git add yr_2
   git commit -m "Year 2"
   

   touch yr_3
   git add yr_3
   git commit -m "Year 3"
   

Revert: Rewind the Clock

View the History

You should have the following four files in the repository.

ls

README.md   yr_1   yr_2   yr_3

git log --oneline

• Lists the commits on the branch.

• Use the commit hash it shows for the checkout and revert commands.

  git log --oneline
  

  7a5bbf4 (HEAD -> timeline) Year 3
  5215f6d Year 2
  f10f791 Year 1
  03098e7 (origin/main, main) README file created
  

git push origin timeline

• The timeline branch is now pushed to GitHub.
• Visit the repository in your browser to confirm.

git push origin timeline

GitHub - timeline branch

GitHub - timeline’s commits

Visit the Past

git log --oneline

• Lists the commits on the branch.

• Use the commit hash it shows for the checkout and revert commands.

  git log --oneline
  

  7a5bbf4 (HEAD -> timeline, origin/timeline) Year 3
  5215f6d Year 2
  f10f791 Year 1
  03098e7 (origin/main, main) README file created
  

git checkout [commit hash]

• Turns your working directory into the exact same state as that [commit].
• Use it to check whether this is the commit you want to undo.

• Any changes you make in this state are not saved.

  git checkout f10f791
  

  Note: switching to 'f10f791'.
  
  You are in 'detached HEAD' state. You can look around, make experimental
  changes and commit them, and you can discard any commits you make in this
  state without impacting any branches by switching back to a branch.
  
  If you want to create a new branch to retain commits you create, you may
  do so (now or later) by using -c with the switch command. Example:
  
    git switch -c <new-branch-name>
  
  Or undo this operation with:
  
    git switch -
  
  Turn off this advice by setting config variable advice.detachedHead to false
  
  HEAD is now at f10f791 Year 1
  

  ls
  

  README.md  yr_1
  

Go Back One Commit

git revert HEAD

• Goes back one commit.

  git checkout timeline
  

  Previous HEAD position was f10f791 Year 1
  Switched to branch 'timeline'
  

  ls
  

  README.md  yr_1  yr_2  yr_3
  

  git revert HEAD
  

  Removing yr_3
  [timeline 450d385] Revert "Year 3"
  1 file changed, 0 insertions(+), 0 deletions(-)
  delete mode 100644 yr_3
  

  ls
  

  README.md  yr_1  yr_2
  

Go Back by Commit Hash

git log --oneline

2fb96f6 (HEAD -> timeline) Revert "Year 3"
7a5bbf4 (origin/timeline) Year 3
5215f6d Year 2
f10f791 Year 1
03098e7 (origin/main, main) README file created

Notice the 7a5bbf4 (origin/timeline) Year 3 line. It means the Revert "Year 3" commit has not been applied on GitHub (origin) yet.

git push

git push origin timeline

Enumerating objects: 3, done.
Counting objects: 100% (3/3), done.
Delta compression using up to 4 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (2/2), 240 bytes | 240.00 KiB/s, done.
Total 2 (delta 1), reused 0 (delta 0), pack-reused 0
remote: Resolving deltas: 100% (1/1), completed with 1 local object.
remote: This repository moved. Please use the new location:
remote:   https://github.com/ahandsel/learning_git_3.git
To https://github.com/ahandsel/learning_git_3.git
   f0c82a0..53a0f3e  timeline -> timeline

git log --oneline

727642d ( HEAD -> timeline \, origin/timeline ) Revert “Year 3”
f7cf1cb Year 3
f7fb07c Year 2
e4df7f2 Year 1
03098e7 ( origin/main \, main ) README file created

git revert f7fb07 Year 2 commit’s hash

git revert 5215f6d

Removing yr_2
[timeline 6c3367a] Revert "Year 2"
 1 file changed, 0 insertions(+), 0 deletions(-)
 delete mode 100644 yr_2

git log --oneline

f3fc335 ( HEAD -> timeline ) Revert “Year 2”

727642d ( origin/timeline ) Revert “Year 3”

f7cf1cb Year 3

f7fb07c Year 2

e4df7f2 Year 1

03098e7 ( origin/main \, main ) README file created

Revert

ls

README.md yr_1

git push origin timeline

git revert [commit hash]

git revert [commit hash]

• A forward-moving undo command.
• It reverses the changes made by the given [commit] and adds the result as a new commit.

Reset vs Revert

git reset [commit]	git revert [commit]
A backward-moving undo that deletes commits.	A forward-moving undo.
Goes back to a past [commit] and removes every commit made after it.	Creates a new commit at a past [commit].
This cleans everything up.	No commits are deleted.
However, the history of the deleted commits is lost.	Use it for public or shared repositories.

Next Section

Git Cheat Sheet - 04_CheatSheet_EN.md

List of Lecture Guides

README_EN.md ⚙️