GitNoon Lesson 4

Get Ready for Lesson 4

Login to your account on github.com
Make sure your blog has no uncommitted changes and that you are up to date with origin/main by checking the output of git status
If you missed the previous lesson, ask for help with the quick-setup on the next slide
- technoon.org/gitnoon/lesson_4/slides.html

Quick Setup

Configuring Git:

git config --global user.name "Your Name"
git config --global user.email "your.email@example.com"
git config --global core.editor nano
git config --global init.defaultBranch main
# For Linux or macOS:
git config --global core.autocrlf input
# For Windows:
git config --global core.autocrlf true
# If you're behind a corporate proxy on Windows
git config --global http.sslBackend schannel

Setting up a blog repository:

mkdir blog
cd blog
git init
echo "# Entry 1" > entry_1.md
git add entry_1.md
git commit -m "Add first blog entry"
git status

Creating a new public blog repository on GitHub, and pushing your local repo to it:

git remote add origin ssh://git@ssh.github.com:443/<username>/blog.git
git push -u origin main

Add the git graph alias for visualising branches:

git config --global alias.graph "log --all --graph --oneline --decorate"

Thanks

To the host for the great venue!
To our sponsors

Administrivia

Fire escapes
Toilets
Cleaning up after ourselves
Wi-Fi

Lunch Talk: Rewriting History

Git has features for rewriting commit history
These can be dangerous if used incorrectly
But, they can be very useful for:
- Keeping a tidy, readable commit history
- Fixing “Git messes”

Merging vs Rebasing

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch feature
  checkout feature
  commit id: "X"
  commit id: "Y"
  checkout main
  commit id: "C"
  commit id: "D"

Problem: feature conflicts with main, how do I update feature so that it will merge cleanly back into main?

Merging vs Rebasing

Option 1: git merge main

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch feature
  checkout feature
  commit id: "X"
  commit id: "Y"
  checkout main
  commit id: "C"
  commit id: "D"
  checkout feature
  merge main

Clutters history with merge commits
Especially if you update feature multiple times

Option 2: git rebase main

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature (old)"
  checkout "feature (old)"
  commit id: "X"
  commit id: "Y"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "feature (rebased)"
  checkout "feature (rebased)"
  commit id: "X*"
  commit id: "Y*"

X and Y are “replayed” on top of main
Keeps a linear history

Implications of Rebasing

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature (old)"
  checkout "feature (old)"
  commit id: "X"
  commit id: "Y"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "feature (rebased)"
  checkout "feature (rebased)"
  commit id: "X*"
  commit id: "Y*"

Every commit’s identifying SHA is determined by its content, message, author, date, and parent commit
Changing the parent commit changes the SHA, so Git will see the rebased commits as different commits!
If you already pushed feature, Git will stop you from overwriting the “old commits” unless you push with git push --force

When to Rebase?

Avoid rebasing if others are using the branch
- Otherwise they must rebase or reset to get the new history
- Personal feature branches are usually safe
Some teams avoid rewriting history to treat commits like an unchanging audit log
- This can come at the cost of a clean, readable history
Rebasing replays commits one-by-one, so conflicts can require multiple resolutions
- If in doubt, git rebase --abort and either:
  - Squash commits with an interactive rebase
  - Merge instead of rebasing

Fixing “Git Messes”

A bad rebase or merge might leave users feeling confused and like they’ve “messed up” their repo:

To collaborate on your colleague’s origin/feature, you fetch it and make commits in your local feature branch:

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "origin/feature" order: 1
  checkout "origin/feature"
  commit id: "X"
  commit id: "Y"
  branch "feature" order: 2
  checkout "feature"
  commit id: "Z"
  checkout main
  commit id: "C"
  commit id: "D"

Your colleague then rebases and force pushes to origin/feature, meaning your push will now fail:

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature" order: 2
  checkout "feature"
  commit id: "X"
  commit id: "Y"
  commit id: "Z"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "origin/feature" order: 1
  checkout "origin/feature"
  commit id: "X*"
  commit id: "Y*"

Mess: You fetch origin/feature, but instead of rebasing, you merge git merge origin/feature:

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature" order: 2
  checkout "feature"
  commit id: "X"
  commit id: "Y"
  commit id: "Z"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "origin/feature" order: 1
  checkout "origin/feature"
  commit id: "X*"
  commit id: "Y*"
  checkout "feature"
  merge "origin/feature"

Resolution Step 1: git reset Z to get back to the state prior to the merge:

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature" order: 2
  checkout "feature"
  commit id: "X"
  commit id: "Y"
  commit id: "Z"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "origin/feature" order: 1
  checkout "origin/feature"
  commit id: "X*"
  commit id: "Y*"

Resolution Step 2: Now git rebase origin/feature:

%%{init: { 'gitGraph': {'showCommitLabel': true} } }%%
gitGraph
  commit id: "A"
  commit id: "B"
  branch "feature (old)" order: 2
  checkout "feature (old)"
  commit id: "X"
  commit id: "Y"
  commit id: "Z"
  checkout main
  commit id: "C"
  commit id: "D"
  branch "origin/feature" order: 1
  checkout "origin/feature"
  commit id: "X*"
  commit id: "Y*"
  branch "feature" order: 3
  checkout "feature"
  commit id: "Z*"

General Process for Fixing Messes

Understand the current state of the repo
- git graph (aka git log --graph --all --oneline)
- git status
- Draw a Git graph on a piece of paper
Determine the desired state
- Draw a graph of what you’d like the repo to look like
- (if it’s not your repo, confirm your plan with the owner)
Draft a series of commands to get from the current state to the desired state
Make backups: copy the repo, make a new branch, etc.
Execute the commands, checking the repo state as you go

Your Git Tool Belt

Here’s a quick cheat sheet of useful (but potentially dangerous) commands that you can learn more about to help you sort out Git messes:

git cherry-pick: Replay a single commit on top of the current branch
git commit --amend: Instead of making a new commit, modify the last one
git rebase --interactive: Rewrite a branch by deleting, editing, or re-ordering commits
git reset: Force a branch to point to a different commit
- The --soft, --mixed, and --hard options determine what happens to your working directory and staging area
git restore: Update files in the working directory with content from a specific commit
git filter-repo: Power tool for rewriting history
- E.g. removing a file from every commit in the history

Is rewritten history truly gone?

Old commits might still exist in your local repo
- Once a commit is two weeks old and not part of any branch or tag, it may be removed by garbage collection (git gc)
- git reflog lists commits that HEAD has pointed to recently
  - Useful for recovering accidentally rewritten commits
If you’ve pushed a commit, it might still exist on the remote
- If you need to delete passwords, keys, or other sensitive data, contact the service provider
- GitHub has some unintuitive behaviours around forks and what happens when a private repo is made public that you should be aware of

Further Learning

Git for Ages 4 and Up
- Great visual explanation of Git branching, merging, rebasing, etc.
- Uses git checkout, while we’ve used the modern git switch and git restore
Learn Git Branching
- Interactive tutorial on branch-related git commands
Free e-book: Pro Git
- To learn more about Git commands for your “tool belt”
Try a graphical client, a text editor extension, a graphical git log like gitk, or a graphical diff tool like meld
Checkout more TechNoon courses: technoon.org

Tutorial Objectives

Make a pull request
Rebase a pull request
Complete a pull request

Homework

Make a pull request to a neighbour’s repo
1. Visit the repo’s GitHub page, and select Fork
2. Clone your fork and make a branch with some changes
3. Push your local branch up to your fork
4. Make a pull request from your fork to your neighbour’s repo
5. Good practice for contributing to open-source projects!
Continue your daily blog entries
- Instead of working directly on main, make each entry in a new branch, and merge them into main with pull requests