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Git Basic Concepts

Now that you have a basic understanding of version control systems in general, we can start experimenting with Git. Using Git as a VCS is a lot like working with a normal software project. You’re still writing code in files and storing those files in folders, only now you have access to a plethora of Git commands to manipulate those files.

For example, if you want to revert to a previous version of your project, all you have to do is run a simple Git command. This command will dive into Git’s internal database, figure out what your project looked like at the desired state, and update all the existing files in your project folder (also known as the working directory). From an external standpoint, it will look like your project magically went back in time.

This module explores the fundamental Git workflow: creating a repository, staging and committing snapshots, configuring options, and viewing the state of a repository. It also introduces the HTML website that serves as the running example for this entire tutorial. A very basic knowledge of HTML and CSS will give you a deeper understanding of the purpose underlying various Git commands but is not strictly required.

Create the Example Site

Before we can execute any Git commands, we need to create the example project. Create a new folder called my-git-repo to store the project, then add a file called index.html to it. Open index.html in your favorite text editor and add the following HTML.

<!DOCTYPE html> <html lang="en"> <head>   <title>A Colorful Website</title>   <meta charset="utf-8" /> </head> <body>   <h1 style="color: #07F">A Colorful Website</h1>   <p>This is a website about color!</p>          <h2 style="color: #C00">News</h2>   <ul>     <li>Nothing going on (yet)</li>   </ul> </body> </html>

Save the file when you’re done. This serves as the foundation of our example project. Feel free to open the index.html in a web browser to see what kind of website it translates to. It’s not exactly pretty, but it serves our purposes.

Initialize the Git Repository

Now, we’re ready to create our first Git repository. Open a command prompt (or Git Bash for Windows users) and navigate to the project directory by executing:

cd /path/to/my-git-repo

where /path/to/my-git-repo is a path to the folder created in the previous step. For example, if you created my-git-repo on your desktop, you would execute:

cd ~/Desktop/my-git-repo

Next, run the following command to turn the directory into a Git repository.

git init

This initializes the repository, which enables the rest of Git’s powerful features. Notice that there is now a .git directory in my-git-repo that stores all the tracking data for our repository (you may need to enable hidden files to view this folder). The .git folder is the only difference between a Git repository and an ordinary folder, so deleting it will turn your project back into an unversioned collection of files.

View the Repository Status

Before we try to start creating revisions, it would be helpful to view the status of our new repository. Execute the following in your command prompt.

git status

This should output something like:

# On branch master # # Initial commit # # Untracked files: #   (use "git add <file>..." to include in what will be committed) # #       index.html nothing added to commit but untracked files present (use "git add" to track)

Ignoring the On branch master portion for the time being, this status message tells us that we’re on our initial commit and that we have nothing to commit but “untracked files.”

An untracked file is one that is not under version control. Git doesn’t automatically track files because there are often project files that we don’t want to keep under revision control. These include binaries created by a C program, compiled Python modules (.pyc files), and any other content that would unnecessarily bloat the repository. To keep a project small and efficient, you should only track source files and omit anything that can be generated from those files. This latter content is part of the build process—not revision control.

Stage a Snapshot

So, we need to explicitly tell Git to add index.html to the repository. The aptly named git add command tells Git to start tracking index.html:

git add index.html git status

In place of the “Untracked files” list, you should see the following status.

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

We’ve just added index.html to the snapshot for the next commit. A snapshot represents the state of your project at a given point in time. In this case, we created a snapshot with one file: index.html. If we ever told Git to revert to this snapshot, it would replace the entire project folder with this one file, containing the exact same HTML as it does right now.

Git’s term for creating a snapshot is called staging because we can add or remove multiple files before actually committing it to the project history. Staging gives us the opportunity to group related changes into distinct snapshots—a practice that makes it possible to track the meaningful progression of a software project (instead of just arbitrary lines of code).

Commit the Snapshot

We have staged a snapshot, but we still need to commit it to the project history. The next command will open a text editor and prompt you to enter a message for the commit.

git commit

Type Create index page for the message, leave the remaining text, save the file, and exit the editor. You should see the message 1 files changed among a mess of rather ambiguous output. This changed file is our index.html.

As we just demonstrated, saving a version of your project is a two step process:

  1. Telling Git what files to include in the next commit.
  2. Recording the staged snapshot with a descriptive message.

Staging files with the git add command doesn’t actually affect the repository in any significant way—it just lets us get our files in order for the next commit. Only after executing git commit will our snapshot be recorded in the repository. Committed snapshots can be seen as “safe” versions of the project. Git will never change them, which means you can do almost anything you want to your project without losing those “safe” revisions. This is the principal goal of any version control system.

Screenshot_1

View the Repository History

Note that git status now tells us that there is nothing to commit, which means our current state matches what is stored in the repository. The git status command will only show us uncommitted changes—to view our project history, we need a new command:

git log

When you execute this command, Git will output information about our one and only commit, which should look something like this:

commit b650e4bd831aba05fa62d6f6d064e7ca02b5ee1b Author: unknown <user@computer.(none)> Date:   Wed Jan 11 00:45:10 2012 -0600      Create index page

Let’s break this down. First, the commit is identified with a very large, very random-looking string (b650e4b…). This is an SHA-1 checksum of the commit’s contents, which ensures that the commit will never be corrupted without Git knowing about it. All of your SHA-1 checksums will be different than those displayed in this tutorial due to the different dates and authors in your commits. In the next module, we’ll see how a checksum also serves as a unique ID for a commit.

Next, Git displays the author of the commit. But since we haven’t told Git our name yet, it just displays unknown with a generated username. Git also outputs the date, time, and timezone (-0600) of when the commit took place. Finally, we see the commit message that was entered in the previous step.

Configure Git

Before committing any more snapshots, we should probably tell Git who we are. We can do this with the git config command:

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

Be sure to replace Your Name and your.email@example.com with your actual name and email. The –global flag tells Git to use this configuration as a default for all of your repositories. Omitting it lets you specify different user information for individual repositories, which will come in handy later on.

Create New HTML Files

Let’s continue developing our website a bit. Start by creating a file called orange.html with the following content.

<!DOCTYPE html> <html lang="en"> <head>   <title>The Orange Page</title>   <meta charset="utf-8" /> </head> <body>   <h1 style="color: #F90">The Orange Page</h1>   <p>Orange is so great it has a   <span style="color: #F90">fruit</span> named after it.</p> </body> </html>

Then, add a blue.html page:

<!DOCTYPE html> <html lang="en"> <head>   <title>The Blue Page</title>   <meta charset="utf-8" /> </head> <body>   <h1 style="color: #00F">The Blue Page</h1>   <p>Blue is the color of the sky.</p> </body> </html>

Stage the New Files

Next, we can stage the files the same way we created the first snapshot.

git add orange.html blue.html git status

Notice that we can pass more than one file to git add. After adding the files, your status output should look like the following:

# On branch master # Changes to be committed: #   (use "git reset HEAD <file>..." to unstage) # #       new file:   blue.html #       new file:   orange.html

Try running git log. It only outputs the first commit, which tells us that blue.html and orange.html have not yet been added to the repository’s history. Remember, we can see staged changes with git status, but not with git log. The latter is used only for committed changes.

Screenshot_2

Commit the New Files

Let’s commit our staged snapshot:

git commit

Use Create blue and orange pages as the commit message, then save and close the file. Now, git log should output two commits, the second of which reflects our name/email configuration. This project history can be visualized as:

Screenshot_3

Each circle represents a commit, the red circle designates the commit we’re currently viewing, and the arrow points to the preceding commit. This last part may seem counterintuitive, but it reflects the underlying relationship between commits (that is, a new commit refers to its parent commit). You’ll see this type of diagram many, many times throughout this tutorial.

Modify the HTML Pages

The git add command we’ve been using to stage new files can also be used to stage modified files. Add the following to the bottom of index.html, right before the closing </body> tag:

<h2>Navigation</h2> <ul>   <li style="color: #F90">     <a href="orange.html">The Orange Page</a>   </li>   <li style="color: #00F">     <a href="blue.html">The Blue Page</a>   </li> </ul>

Next, add a home page link to the bottom of orange.html and blue.html (again, before the </body> line):

<p><a href="index.html">Return to home page</a></p>

You can now navigate between pages when viewing them in a web browser.

Stage and Commit the Snapshot

Once again, we’ll stage the modifications, then commit the snapshot.

git status git add index.html orange.html blue.html git status git commit -m "Add navigation links"

The -m option lets you specify a commit message on the command line instead of opening a text editor. This is just a convenient shortcut (it has the exact same effect as our previous commits).

Our history can now be represented as the following. Note that the red circle, which represents the current commit, automatically moves forward every time we commit a new snapshot.

Screenshot_4

Explore the Repository’s History

The git log command comes with a lot of formatting options, a few of which will be introduced throughout this tutorial. For now, we’ll just use the convenient –oneline flag:

git log --oneline

Condensing output to a single line is a great way to get a high-level overview of a repository. Another useful configuration is to pass a filename to git log:

git log --oneline blue.html

This displays only the blue.html history. Notice that the initial Create index page commit is missing, since blue.html didn’t exist in that snapshot.

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

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