Training

For who, why ?

A version control system (VCS: version control system) helps to manage the history of the changes made by people, teams, or only one person who collaborate on a project.

When the project evolves, people can at any time, find an old version of the code.

Git answers these questions quickly

What changes have been made?
Who made the changes?
When were the changes made?
Why were the changes made?

Git is a decentralized source management software (DVCS) created in 2005 by Linus Torvald (creator of the Linux kernel). The sources are stored both on the computer of each contributor of the project, but also on a centralized server.

Figure 1. logo Git

Today, most of the open source or commercial projects developed use Git (More than 90% of developers, according stackoverflow insights).

Git allows access to all files, branches, versions (releases) or tags of a project to authorized people.

Basically you don’t need a centralized solution with Git, but online solutions have emerged like Github or Gitlab.

Figure 2. logo Github

Figure 3. logo Gitlab

These services extend Git and offer more features like pull requests, code reviews… Github was the precursor but today Gitlab is equivalent, and you can use the one you prefer.

As I had to make a choice for our courses, I choose Github. From now on, I will only talk about Github. Back up your sources regularly. At the end of our courses, you will have to provide me the URGithub URL of your repository which will contain the sources of your project.

What’s a repository ?

A Git repository contains all the files and directories associated with your project. It also contains all the history of changes made on each one. You can retrieve any changes made by any developer at any time.

Publishing changes is called a commit. Each commit is linked to another (this is a chained list).

Commits can be organized into several parallel branches.

Projects are stand-alone units, and anyone with a copy of the repository can access the entire code and its history. You can execute different operations like logging, cloning, branch creation, commit, merging…

Figure 4. Centralized repository, local repository

Install Git

If you use Windows, you can download Git here https://git-scm.com/downloads

On a Linux (Fedora)

sudo yum install git

On Linux (Debian, Ubuntu) use

sudo apt-get install git

To check your install, display the git version in terminal

~$ git --version

git version 2.34.1

First commands

Configure Git

It’s important to provide some information to Git, especially when you push your changes to a centralized repository. Use the git config operation to set your name and your email.

~$ git config --global user.name "Guillaume EHRET Dev-Mind"
~$ git config --global user.email "guillaume@dev-mind.fr"

By default, Git will create a branch called master when you create a new repository with the command git init. The terms master and slave are often used in computer industry’s. After many protests, these harmful and antiquated terms were no longer considered appropriate. Github used the term main branch. To use the same configuration, you can execute

~$  git config --global init.defaultBranch main

Clone a Git project

Go in your working directory, and in a terminal type

~$ git clone https://github.com/Dev-Mind/git-demo.git

The remote project git-demo is downloaded on your computer. You get all the history, all the files, all the branches …

For example you can run these commands

~$ cd git-demo
~$ git log

Initialize a Git repository

For a new project, it’s better to start from 0. Delete the directory to recreate it

~$ cd ..
~$ rm -rf git-demo
~$ mkdir git-demo
~$ cd git-demo

To link this directory to Git, launch

~$ git init

Initialized empty Git repository in /home/devmind/Workspace/java/git-demo/.git/

A .git directory has been added and it contains the different files, logs, traces

---
~$ ls -la
total 28
drwxr-xr-x  3 devmind devmind  4096 sept. 26 22:46 .
drwxr-xr-x 10 devmind devmind  4096 sept. 26 22:46 ..
drwxr-xr-x  8 devmind devmind  4096 sept. 26 22:46 .git
----

Note	Git will display a warning if you name the default branch master (default configuration). Read the chapter about Git configuration to set another default branch name.

Linking files to Git

Let’s start by creating files in this git-demo directory

~$ echo "Hello world" > hello.txt
~$ echo "<html><body><h1>Hello World</h1></body></html>" > hello.html
~$ echo "Hello world" > hello.md
~$ ls
hello.html  hello.txt  hello.md

Use git status to know the state

~$ git status

On branch main

No commits yet

Untracked files:
  (use "git add <file>..." to include in what will be committed)
	hello.html
	hello.md
	hello.txt

nothing added to commit but untracked files present (use "git add" to track)

Use the git add command to indicate that a new or multiple files will be handled by git.

~$ git add hello.txt
~$ git add .

Adds hello.txt file
Adds all the files

At this moment, our files are not saved in Git. They are in a staging area. We have only a snapshot of the changes

~$ git status

On branch main

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)
	new file:   hello.html
	new file:   hello.md
	new file:   hello.txt

Saving modifications

The following command saves the snapshot in the project history and completes the change tracking process. In short, a commit works like taking a photo. We freeze in time what we have done.

~$ git commit -m "First commit"

[main a7d5b84] First commit
 3 files changed, 3 insertions(+)
 create mode 100644 hello.html
 create mode 100644 hello.md
 create mode 100644 hello.txt

-m to add a message. It’s very important to be able to understand why a commit was made
Git displays the name of the branch ⇒ main and the hash of the commit` a7d5b84` (this is the number of the photo)

Our Git history is a living, ever-changing, searchable record that tells the story of how and why our code is the way it is. It’s important to explain the purpose of a commit.And try to commit often your work. It is a security if you want to restore your work. For example

Know the current state

We will delete the hello.txt file, update hello.md and add a hello.adoc file

~$ rm hello.md
~$ echo "My first Git example" > hello.md
~$ echo "Hello World" > hello.adoc

Launch the following command to know where Git is

~$ git status

Git should indicate that you have a modified file hello.md and an untracked file hello.adoc

~$ git add .
~$ git commit -m "Second commit"

[main 7b7d8e6] Second commit
 3 files changed, 2 insertions(+), 2 deletions(-)
 create mode 100644 hello.adoc
 delete mode 100644 hello.txt

~$ git status

On branch main
nothing to commit, working tree clean

Now all the changes are saved

See history

Use the log command

~$ git log

commit 7b7d8e69a06af284c9da7aa4a8c28835d23318fe (HEAD -> main)
Author: Guillaume EHRET Dev-Mind <guillaume@dev-mind.fr>
Date:   Wed Sep 26 23:22:46 2018 +0200

    Second commit

commit a7d5b843ebc65ac6e94c37872d6a936e1c03a6b5
Author: Guillaume EHRET Dev-Mind <guillaume@dev-mind.fr>
Date:   Wed Sep 26 23:08:00 2018 +0200

    First commit

We find our two commits, with the names we have set, at what time …

See current changes

We will update a file and run the diff command

~$ echo "Fichier Asciidoc" > hello.adoc
~$ git diff

You should see, what has been added and removed in the file.

You can use IDE (like IntelliJ) and its visual tools for tracking changes. For example

Go back in the history

You can use a reset to go back to the state of the last commit

For example

~$ echo "Hello World" > hello.adoc
~$ git status
On branch main
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
	modified:   hello.adoc

no changes added to commit (use "git add" and/or "git commit -a")

~$ git reset --hard
~$ git status
On branch main
nothing to commit, working tree clean

Working with branches

Generally on a project we have a main branch (generally called main). We can open different branches in parallel to fix bugs, add new features.

These branches are merged on the main branch when they are completes. For example when the code will be reviewed by co-coworkers and the app tested by users.

Figure 5. Branch Principle

See the current branch

Run the following command to see the current branch

~$ git branch
feat/hairs
* main

For the moment we have 2 local branches feat/hairs and main. Character * shows the current branch.

Create a branch

A branch is just a name without special characters or spaces. To create a branch we use git branch [name]

~$ git branch test
~$ git branch
* main
test

By default the created branch is not active (it does not have *)

A prefix is often used by convention, when you want to name a branch. for example

fix/1233-hair-color: a branch to fix a hair color problem. The ticket number of the bug tracker is often indicated
feat/add-glasses: a branch to add a feature
chore/upgrade-jquery: a branch to perform a technical task

Change the current branch

~$ git checkout test
~$ git branch
main
main
* test

The test branch is now the default

You can make a change and save it

~$ echo "Fichier Asciidoc updated" > hello.adoc
~$ git add .
~$ git commit -m "Third commit"

You can launch the following commands

~$ git log
~$ git checkout main
~$ git log

As we returned on the main branch we don’t see the last commit (only present in the test branch)

Compare 2 branches

We will reuse the git diff command but we specify the 2 branches separated by …

~$ git diff test...main

This command should show nothing because test is based on` main` and it is just ahead, it contains all main commits

~$ git diff main...test

This time, as test branch is the reference, Git detects that there are differences

~$ git log

commit 4529128a723e0a16cf405b218f37f2da58c5a9fd (HEAD -> test)  (1)
Author: Guillaume EHRET Dev-Mind <guillaume@dev-mind.fr>
Date:   Thu Sep 27 00:00:00 2018 +0200

    Third commit

commit 9fd87d1ffc654a74105f3f279032e7f88d3d265b (main)       (2)
Author: Guillaume EHRET Dev-Mind <guillaume@dev-mind.fr>
Date:   Wed Sep 26 23:51:10 2018 +0200

    Second  commit
...

Git indicates the HEAD of the test branch
Git displays the name of the parent branch =⇒ main and where it is in the history

You can go back to main and create another branch` test2`

~$ git checkout main
~$ git branch test2
~$ git checkout test2
~$ echo "Fichier toto" > toto.adoc
~$ git add .
~$ git commit -m "Fourth commit"

Merge 2 branches

If you want to post your changes from the test branch to` main`, you’re going to do a merge

~$ git checkout main
~$ git merge test

Delete a branch

You can delete a branch if this branch is not the active one

~$ git checkout test
~$ git branch -d test
error: Cannot delete branch 'test' checked out at '/home/devmind/Workspace/java/git-demo'

~$ git checkout main
~$ git branch -d test
Deleted branch test (was 9fd87d1).

Github

Link your local repository to a remote repository

You can install a centralized Git repository by using Github Enterprise, Gitlab Enterprise, Gogs.But you can also use an online service

Create an account under Github
Once you are logged in on Github, you can create your first repository

Figure 6. New project under Github

Github provides you commands to connect this remote repository to your local repository.

Figure 7. commands to connect this remote repository

To link your local project type the following commands

~$ git checkout main
~$ git remote add origin https://github.com/Dev-Mind/git-demo.git  (1)
~$ git push -u origin main                                       (2)

Git adds remote origin to your local project
The push command allows to send what you have on the current branch (local main branch) on the server

If you have a problem to use your login and password in the command line, you can follow this documentation to generate an access token. When you have a token you can use it as password

Github allows teams to synchronize throughout the life of a project. You can push any branch.

~$ git checkout feat/new_hair
~$ git push -u origin feat/new_hair

These commands

activate the feat/new_hair branch and
push the changes on Github.

If you want to retrieve local changes made by your colleagues do

~$ git pull

The git fetch command retrieves the history of all branches locally.

Git Project Workflow

When we are working in team, the workflow will be

Create a branch
We usually create a branch from main (or another branch). Some companies have, for example, a branch dev for the current developments, a branch prod that corresponding to what is in production. Branches created on these canonical branches must have a short life duration.

Add commits
Every small changes should be made as often as possible. Each commit, within a branch creates restoration points in the project history.

Open a pull request
When you pushed your changes on Github, you can open a pull request. This allows to discuss about the current development (for example with the people who will test)

Code Review
In a mature development team, your code is always read by another developers. Your code is often also tested by other people. Anyone can make returns on the Pull request. You can make changes on the branch and pushed other commits to fix the review remarks.

Merge +When everything is OK you can click the Merge button on the pull request.GitHub automatically performs the equivalent of a local 'git merge' operation.

Deploy
Teams can install your updates or continue development

Conclusion

There’s still a lot of things to say about Git and Github. Some concepts can be complex like rebase or conflict resolution. IDEs will simplify some tasks.You can find more resources on the web http://try.github.io/

This course is there to give you the basics in order to be able to realize the TP using Git. You are now able to save your sources, to share them, to find them easily from one computer to another.

So enjoy with Git