An Ultimate Guide to Git and Github

Highlights of the article:  

1. Introduction to Git
2. Git Repository Structure
3. Github
4. Accessing Github central repository via HTTPS or ssh
5. Working with git – Important Git commands

Introduction to Git

For installation purposes on ubuntu, you can refer to this article: How to Install, Configure and Use GIT on Ubuntu? 

Git is a distributed version control system. So, What is a Version Control System? 

A version Control system is a system that maintains different versions of your project when we work in a team or as an individual. (system managing changes to files) As the project progresses, new features get added to it. So, a version control system maintains all the different versions of your project for you and you can roll back to any version you want without causing any trouble to you for maintaining different versions by giving names to it like MyProject, MyProjectWithFeature1, etc. 

Distributed Version control system means every collaborator(any developer working on a team project)has a local repository of the project in his/her local machine unlike central where team members should have an internet connection to every time update their work to the main central repository. 

So, by distributed we mean: the project is distributed. A repository is an area that keeps all your project files, images, etc. In terms of Github: different versions of projects correspond to commits. 
For more details on introduction to Github, you can refer: Introduction to Github 

Git Repository Structure

It consists of 4 parts:  

1. Working directory: This is your local directory where you make the project (write code) and make changes to it.
2. Staging Area (or index): this is an area where you first need to put your project before committing. This is used for code review by other team members.
3. Local Repository: this is your local repository where you commit changes to the project before pushing them to the central repository on Github. This is what is provided by the distributed version control system. This corresponds to the .git folder in our directory.
4. Central Repository: This is the main project on the central server, a copy of which is with every team member as a local repository.

All the repository structure is internal to Git and is transparent to the developer. 

Some commands which relate to repository structure:  

// transfers your project from working directory
// to staging area.
git add .

// transfers your project from staging area to 
// Local Repository.
git commit -m "your message here"


// transfers project from local to central repository.
// (requires internet)
git push

Github

Github basically is a for-profit company owned by Microsoft, which hosts Git repositories online. It helps users share their git repository online, with other users, or access it remotely. You can also host a public repository for free on Github. 

User share their repository online for various reasons including but not limited to project deployment, project sharing, open source contribution, helping out the community and many such.

Accessing Github central repository via HTTPS or SSH

Here, transfer project means transfer changes as git is very lightweight and works on changes in a project. It internally does the transfer by using Lossless Compression Techniques and transferring compressed files. Https is the default way to access Github central repository.  

• By git remote add origin http_url: remote means the remote central repository. Origin corresponds to your central repository which you need to define (hereby giving HTTPS URL) in order to push changes to Github.
• Via SSH: connect to Linux or other servers remotely.

If you access Github by ssh you don’t need to type your username and password every time you push changes to GitHub. 

Terminal commands: 

ssh-keygen -t rsa -b 4096 -C "your_email@example.com"
This does the ssh key generation using RSA cryptographic algorithm.

eval "$(ssh-agent -s)" -> enable information about local login session.

ssh-add ~/.ssh/id_rsa -> add to ssh key.
cat ~/.ssh/id_rsa (use .pub file if not able to connect)
add this ssh key to github.


Now, go to github settings -> new ssh key -> create key

ssh -T git@github.com -> activate ssh key (test connection)

Refresh your github Page.

Working with git – Important Git commands 

Git user configuration (First Step)  

git --version (to check git version)
git config --global user.name "your name here"
git config --global user.email "your email here"

These are the information attached to commits.  

Initialize directory 

git init 

initializes your directory to work with git and makes a local repository. .git folder is made (OR) 

git clone http_url 

This is done if we have an existing git repository and we want to copy its content to a new place.

Connecting to the remote repository 

git remote add origin http_url/ssh_url 

connect to the central repo to push/pull. pull means adopting the changes on the remote repository to your local repository. push merges the changes from your local repository to the remote repository.  

git pull origin master

One should always first pull contents from the central repo before pushing so that you are updated with other team members’ work. It helps prevent merge conflicts. Here, master means the master branch (in Git).  

Stash Area in git

git stash

Whichever files are present in the staging area, it will move that files to stash before committing it. 

git stash pop

Whenever we want files for commit from stash we should use this command.

git stash clear

By doing this, all files from stash area is been deleted.

Steps to add a file to a remote Repository: 

First, your file is in your working directory, Move it to the staging area by typing:  

git add -A (for all files and folders)
#To add all files only in the current directory
git add .

git status: here, untracked files mean files that you haven’t added to the staging area. Changes are not staged for commit means you have staged the file earlier than you have made changes in that files in your working directory and the changes need to be staged once more. Changes ready to be committed: these are files that have been committed and are ready to be pushed to the central repository.  

git status

git commit -a -m "message for commit"
-a: commit all files and for files that have been 
     staged earlier need not to be git add once more
-a option does that automatically.

git push origin master -> pushes your files to 
                         github master branch
git push origin anyOtherBranch -> pushes any 
                      other branch to github.
git log ; to see all your commits

git checkout commitObject(first 8 bits) file.txt-> 
revert back to this previous commit for file file.txt

Previous commits might be seen through the git log command. 

HEAD -> pointer to our latest commit.  

Ignoring files while committing

In many cases, the project creates a lot of logs and other irrelevant files which are to be ignored. So to ignore those files, we have to put their names in“.gitignore” file.  

touch .gitignore
echo "filename.ext" >>.gitignore
#to ignore all files with .log extension
echo "*.log" > .gitignore

Now the filenames written in the .gitignore file would be ignored while pushing a new commit. To get the changes between commits, commit, and working tree. 

git diff

The ‘git diff’ command compares the staging area with the working directory and tells us the changes made. It compares the earlier information as well as the current modified information. 

Branching in Git 

create branch ->
git branch myBranch
or
git checkout -b myBranch -> make and switch to the 
                                  branch myBranch

Do the work in your branch. Then, 

git checkout master ; to switch back to master branch

Now,  merge contents with your myBranch By: 

git merge myBranch (writing in master branch)

This merger makes a new commit. 

Another way 

git rebase myBranch

This merges the branch with the master in a serial fashion. Now,  

git push origin master

To remove  or delete a file 

To remove.  a file from the  Git repository we use 

git rm “file name”

To remove only from the staging area 

git rm –cached “ file name”

Undoing change

To change all the  files to as same as the previous commit then use

git checkout -f

Contributing to Open Source

Open Source might be considered as a way where user across the globe may share their opinions, customizations or work together to solve an issue or to complete the desired project together. Many companies host there repositories online on Github to allow access to developers to make changes to their product. Some companies(not necessarily all) rewards their contributors in different ways.

You can contribute to any open source project on Github by forking it, making desired changes to the forked repository, and then opening a pull request. The project owner will review your project and will ask to improve it or will merge it.