programming-basics-materials/2-git-continued.md

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Project management 2. Git continued

Branches

• A Git repository can have multiple branches
• By default, a Git repository has only one branch
  • named usually main or master
• Using multiple branches makes it possible to work on new features step by step in their own feature branches
  • Meanwhile, the main branch is kept clean and in a working state
  • Only when the feature is completed, the branch is merged into the main branch!

Active branch

• Even though your local repository can have multiple branches, only one of them is active at a given time
• git status tells you firsthand which branch you are on:

  On branch main
  

git branch & git checkout

• You can create a new branch with git branch <branchname>
  • The new branch is not empty: it contains a copy of the code of the branch you executed this command in
• Note: git branch <branchname> does not make the branch active!
• To make the branch active, a.k.a "move" to the branch:
  • git checkout <branchname>.
  • For example, to move back to the main branch, use git checkout main

Local vs. remote branches

• git branch <branchname> only creates a local branch
• When you try to push changes from a new local branch for the first time, Git nags you that a matching remote branch doesn't yet exist
  • Git tells you how to create the remote branch:
    • git push --set-upstream origin <branchname>
    • Afterwards, git push pushes the changes to the matching remote branch

Extra branch commands

• Handy command: git checkout -b <branchname>
  • It's a shorthand for git branch <branchname> + git checkout <branchname>
• Get a list of local branches with git branch
  • ...and all branches (incl. the remote ones) with git branch -a
• Delete a local branch with git branch -d <branchname>
  • Remote branch can be deleted in the GitLab/etc website
    • ...or with git push origin -d <branchname>
  • Note: If deleted remote branches still show up in git branch -a, you can use the command git remote prune origin to remove them from the list.

git merge

• When a feature is done (and all the broken things fixed), you want to apply, i.e., merge your changes from the feature branch to the main branch

1. First, checkout the main branch with git checkout main
2. Then, do a git pull so you have the newest version of the main branch
   • (Someone else might have done changes to it while you were working on your feature!)
3. Then, merge your local feature branch to your local main branch with git merge <featurebranch>
   • It applies changes from <featurebranch> to the current active branch
   • This is where conflicts might happen (more about them later)
4. Then, push your changes to remote with git push

Merging with a pull/merge request

• The aforementioned procedure isn't what we usually do, though!
• main branch should be protected so we can't merge our new code there directly
  • Instead, we do the inverse.

1. First, checkout the main branch with git checkout main
2. Then, do a git pull so you have the newest version of the main branch
3. Then, checkout back to the feature branch.
4. Merge your local main branch to your local featurebranch with git merge main
5. Then, create a merge request on GitLab (it's pull request on GitHub—they're the same thing, just named from different points of view.)

Simplifying merging further

• Pro tip: If you're working on your feature branch, you can merge changes from the main branch without changing branches altogether.

1. Update your local main branch with git fetch origin main:main
2. Merge your local main branch to your local featurebranch with git merge main
3. Then, create a merge request/pull request

• Much easier this way!
  • But what is the merge request, anyway?

Merge request/pull request

• Merge/pull request is a formal process for merging your remote feature branch straight to the remote main branch
• This adds a layer of protection to the main branch: no direct merging to the main branch!
• Additionally, a code review can happen at this stage:
  • If a code maintainer thinks your merge request needs some modifications, you can make changes to the feature branch accordingly
  • A merge request is branch-specific, so when you push your changes to your feature branch, the merge request is automatically updated as well.
  • If this procedure is followed strictly, no bad code should get into the main branch!

GitHub pull request UI

• UI of a new pull request (Pull requests > New pull request) can be confusing...
  • base is the branch you're merging, compare is the branch you're merging into.

Exercise 1. Pushing onwards

Continue the exercise from Git Basics or create a new repository for these exercises.

1. Create a new branch (with a name new-feature, for instance) in your local repository.
2. Checkout the branch, make some changes to GitTest.md there, and push the changes to GitLab.
3. Then, merge the changes from your new-feature branch to the main branch by using a) git merge from command line b) a merge request in GitLab

GitLens

• To make the Git workflow easier, install the GitLens extension to VS code
  • It helps in managing conflicts, comparing branches or commits
• Install it from the Extensions panel (access with CTRL+SHIFT+X)
• Adds many new views to the source control tab

• For example:
  • Commits
  • Repositories
  • File History
  • Line History
  • Branches
  • Remotes

Conflicts

• Sometimes two people have made changes in the same lines of code!
  • This leads to a conflict.
• Let's assume we're trying to merge changes from featurebranch to main.
• If a conflict happens, the merge does not conclude automatically.
• Instead, we need to fix all the conflicts by hand and then conclude the merge with some commands.
• Conflicting lines of code are framed by some <<<< garbage ==== symbols >>>> we don't yet understand
• Before we can conclude the merge, we need to get rid of the garbage.

Garbage explained

<<<<<<< HEAD:Player.cs
  if (Input.anyKey) {
    return true;
  }
=======
  if (Input.anyKey)
      return true;
>>>>>>> featureBranch:Player.cs

• Current change is between <<<<<<< and =======
  • old code that was already in main
• Incoming change is between >>>>>>> and =======
  • new code coming from featurebranch
• Use your text editor to choose which (or some combination of both) you want to preserve

VS Code tools

• VS Code gives us tools to make conflict resolution a relatively quick process
• Choose your preference:
  • Accept Current Change (old code is preserved, new code is removed)
  • Accept Incoming Change (new code is preserved, old code is removed)
  • Accept Both Changes (both are preserved)

After resolving the conflict

• After resolving all conflicting files, use git add <filename> to add them to the commit
• Then use git commit to apply changes (without a message! no -m)
  • Close the automatically opened COMMITMSG file.
  • This finishes the merge.
• Then just git push to apply changes in the remote repository
• Note: If VS code is not configured properly as the Git's text editor, and you encounter an error, run git config core.editor code --wait

Example Git workflow with branches

Exercise 2. Fixing conflicts

Continue in the Exercise 1 repository.

1. Create a new branch in your local repository, but do not checkout it just yet.
2. On the main branch, make some changes to GitTest.md, and add & commit.
3. Then, checkout the new feature branch.
4. Make some other changes to GitTest.md to the same line as before, add & commit.
5. Then, merge the changes from the main branch to your feature branch by using git merge
6. Fix the ensuing conflicts, add & commit & push.

Undoing

• Git doesn't have a general "undo" command
• If you make a mistake, it is very case-specific what you need to do to fix it
  • See undo options here
  • Also, ohshitgit.com

git log & git checkout <hashcode>

• Use git log to see the commit history

  • Or git log --oneline for a more concise version
  • Press Q to quit the log view.
  • The newest changes are seen on top
  • On the left side of the commit message you see the hashcode of the commit
  • Use git checkout <hashcode> to "time travel" into the commit

• Note: If you have GitLens, check the Commits view in the Source control tab to see the commit history.

Reverting one file to a previous state

• Sometimes you want to revert just one file to its previous state
• You can use git checkout <commit-hash> -- <filename> to time travel just that file to how it was in that commit
• For this, you of course need to figure out the commit hash you want to return to
• Find that out in one of the following methods:
  • git log --oneline
  • VS Code: Source control > File history
  • Check commits from GitHub/GitLab

Exercise 3. Branching team effort

Work as a group for this assignment. Continue Exercise 2 from Git basics.

1. Every group member: create your own individual branch from the main branch
2. Then, make some changes to the GitTest.md file.
   • Do not tell other group members what you're going to change! :D
3. Add new files as well, at least one file per group member.
4. Then, merge the changes back to the main branch. Fix ensuing conflicts, if any appear.

Reading

• Pro Git book
• Oh shit Git
• Undo possibilities

Very extra: git rebase

• git merge creates a new commit for the merge process
• Sometimes that's undesirable, so an alternative is to use git rebase
• Unlike merge, rebase applies changes from the rebased branch one commit at a time
• Whenever there's a conflict:

1. After fixing the conflict, add the conflicting file with git add <filename>
2. Then continue the rebase process with git rebase --continue
3. If you want to disregard a conflicting commit, use git rebase --skip
4. If you get cold feet, you can cancel the rebase with git rebase --abort

Very extra: Interactive git rebase -i

• git rebase also has a hidden interactive mode used with git rebase -i
• This is a true swiss army knife of a Git tool for rewriting Git history
• It can:
  • Squash commits into one
  • Cherry-pick only certain commits
  • Reword commit messages
  • And more!
• See Hackernoon: Beginner’s Guide to Interactive Rebasing

Very very extra: Git submodules

• To add external code to your project from someone else's repository, Git has a neat system called submodules
• To add a submodule to your project, use git submodule add <submodule-url> <folder>
• To remove a submodule, use git rm <path-to-submodule>
• If you clone a project with submodules, you need to run git submodule update --init --recursive once.
• Note: If you don't want submodules to appear in the Source control tab of VS Code, go to settings and disable the Git: Detect Submodules setting.