A crash course on Git and Gitlab

What?

Version control

version control (also known as revision control, source control, or source code management) is a class of systems responsible for managing changes to computer programs, documents, large web sites, or other collections of information.

source: https://en.wikipedia.org/wiki/Version_control

Why? (¹⁄₂)

Other people are using Git (network effect)

Notes: Open source repositories registered on Ohloh Source

Why? (²⁄₂)

Advantages

• Keep an organized history of a project. No more:

• Easy to collaborate: working simultaneously with the same file(s)

How?

Git

Software that handles source code versioning

https://git-scm.com/

Gitlab

A cloud service for remote hosting of git repositories + makes your life easier

https://about.gitlab.com/

Prerequisites

• Have Git installed

see: https://git-scm.com/book/en/v2/Getting-Started-Installing-Git

• Have a Gitlab account

see: https://docs.gitlab.com/ee/gitlab-basics/start-using-git.html

• Basic knowledge of the terminal (cd, ls, …)

see: https://ubuntu.com/tutorials/command-line-for-beginners#3-opening-a-terminal

Git 101

the big picture

• Each “commit” is a snapshot of the state of the project

You first local repository (¹⁄₂)

Create a local repository

mkdir my_repo
cd my_repo
git init #Initialized empty Git repository  

Create a file your first commit

echo "Some line" > file1.txt
git status #Look at the changes within your repository

Create your first commit

git add file1.txt
git commit -m "my first commit"
git status

You first local repository (²⁄₂)

See the changes

git log

See the changes on a graph

git log --oneline --graph

Case study 1: Undo some changes (¹⁄₂)

Introduce a bug in our “code”

echo "A bug" >> file1.txt
git status

Look at the differences with the previous version

git diff file1.txt

Commit the bug

git add file1.txt
git commit -m "commit with a bug"
git log --oneline

Case study 1: Undo some changes (²⁄₂)

Undo the bug

git revert <commit> #Find <commit> with git log --oneline

You should see “Revert “commit with a bug””:

git log --oneline

See:

• https://www.atlassian.com/git/tutorials/undoing-changes/git-revert
• https://www.atlassian.com/git/tutorials/undoing-changes/git-reset

The concept of branches

• You may want to experiment with a new feature
• You don’t want to mess with the current state of the project -> Create a new branch

Feature Branch Workflow

Philosophy

• The latest state of the project (“production-ready”) is on the branch “master” (also called “main”)
• new features -> create a new “feature branch”
• When new feature is ready -> merge the feature into the branch “master”

See: https://www.atlassian.com/git/tutorials/comparing-workflows/feature-branch-workflow

Case study 2: Create and merge a feature branch (¹⁄₂)

Create a new branch from the master/main branch

git branch #Look at the existing branches
git checkout master #Go to branch master (or main)
git checkout -b new-feature # Create a new branch called "new-feature"

Introduce a new feature

echo "A new feature" >> file1.txt
git status #Look at the changes within your repo

Create a commit

git add -A #add changes from all tracked and untracked files
git commit -m "my new feature"

Case study 2: Create and merge a feature branch (²⁄₂)

Merge the feature branch into the master branch.

git checkout master #Go to branch master (or main)
git merge new-feature

See the changes

git log --oneline --graph

(Optional) Delete the feature branch

git branch -d new-feature #delete branch "new-feature"
git branch #You should see only one branch

Gitlab 101

Why?

• So far, everything on your local machine
• Collaboration made easier with Gitlab
• “Truth” repo hosted by Gitlab

Source: https://nvie.com/posts/a-successful-git-branching-model/

Setup

General Setup

git config --global user.name "Your Username"
git config --global user.email "your.email@email.com"

(Optional) Specific Setup

See installationgit.pdf

Case study 3: Sync repository with Gitlab

Create a new repository on Gitlab:

• Go to https://gitlab.com/users/sign_in and login
• Create a new repository “tuto”

Clone the repository:

git clone git@gitlab.com:<your_username>/tuto.git
cd tuto

Create a README file and push

touch README.md
git add README.md
git commit -m "add README"
git push -u origin master

Important: the .gitignore file

• You don’t want to track every single file in your repository (.i.e. large output files)

• At the root of your repository, create a file .gitignore. Add the following:

# ignore all logs
*.log
# ignore file text2.txt
text2.txt

See: https://www.atlassian.com/git/tutorials/saving-changes/gitignore

Case study 4: Feature workflow on Gitlab (¹⁄₄)

Same as previously. But this time, need to “synchronize” with the central repository (on Gitlab)

• Go to the master branch and delete local changes (careful)

git checkout master #Go to the branch master on your local machine
git fetch origin #Update info on the remote branch master (from Gitlab)
git reset --hard origin/master #Remove your local modifications and use the latest version from Gitlab

• Create a new branch

git checkout -b new-feature

Case study 4: Feature workflow on Gitlab (²⁄₄)

• Add a feature

echo "Cool new feature" > file1.txt

• Create a new commit and synchronize with Gitlab

git add -A #Add all files
git commit -m "Message describing the commit" #Create a commit
git push -u origin new-feature

Case study 4: Feature workflow on Gitlab (³⁄₄)

Merge Request

This step is done directly on Gitlab:

• On the top bar, select Menu -> Projects and find your project
• On the left menu, select Merge requests
• In the top right, select New merge request
• Select a source (new-feature) and target branch (master) and then Compare branches and continue.
• Fill out the fields and select Create merge request

See: https://docs.gitlab.com/ee/topics/git/feature_branch_development.html

Case study 4: Feature workflow on Gitlab (⁴⁄₄)

Merge Request

• When everyone is satisfied the merge request: Click on Merge

• On your local machine

  git checkout master
  git pull
  

• See the merge in the log

  git log
  

Case study 5: Resolving a merge conflict (¹⁄₄)

Setting

• You are working on a new feature, you are ready to create a pull request, but the master branch has changed since

git checkout -b feature2
echo "Feature 1 different" > file1.txt
echo "Another new feature" > file2.txt
git add -A #Add all files
git commit -m "Feature 2"
git push -u origin feature2

git checkout master
echo "Change feature 1 as well" > file1.txt
git add -A #Add all files
git commit -m "master branch changing"
git push

Case study 5: Resolving a merge conflict (²⁄₄)

• Create a merge commit on Gitlab
• You should see “There are merge conflicts”

Option 1: Resolve conflict using Gitlab

• Click on “Resolve Conflicts”

Case study 5: Resolving a merge conflict (³⁄₄)

Option 2: Resolve conflict using the command line

• Click on “Merge locally” on follow the instructions:

• Step 1. Fetch and check out the branch for this merge request

  git fetch origin
  git checkout 'feature2'
  

• Step 2. Merge the branch and fix any conflicts that come up

  git fetch origin
  git checkout 'master'
  git merge --no-ff 'feature2' #merge feature2 into master
  

Case study 5: Resolving a merge conflict (⁴⁄₄)

You should see:

Auto-merging file1.txt
CONFLICT (content): Merge conflict in file1.txt
Automatic merge failed; fix conflicts and then commit the result.

• Step 3. Edit manually the files with merge conflicts:

<<<<<<< HEAD
Change feature 1 as well
=======
Feature 1 different
>>>>>>> feature2

Step 4. Push the result of the merge to GitLab

git add -A
git commit -m "resolving merge conflicts"
git push origin 'master'

Readings

• https://git-scm.com/book/en/v2
• https://www.atlassian.com/git/tutorials/what-is-version-control
• https://docs.gitlab.com/ee/topics/gitlab_flow.html