Introduction to Git


Table of Contents

Git is an Open Source Distributed Version Control System, that came out of Linux development community. Git has a remote repository which is stored in a server and a local repository which is stored in the computer of each developer. This means that the code is not just stored in a central server, but the full copy of the code is present in all the developers’ computers. Git is a Distributed Version Control System since the code is present in every developer’s computer.

Initial goals:

  • Strong support for non-linear development
  • Distributed development
  • Compatibility with existent systems and protocols
  • Efficient handling of large projects
  • Cryptographic authentication of history
  • Toolkit-based design
  • Pluggable merge strategies
  • Garbage accumulates until collected
  • Periodic explicit object packing

Centralized Model

A central repository stores everything in one place. In a centralized model, you work on the code while connected to the server itself. This maintains a single source of truth. The example of centralized model are CVS, Subversion and Perforce.

Distributed Model

In a distributed model, every developer has their own repo. The example of distributed model are Git and Mercurial.

Git has two data structures:

  • a mutable index (also called stage or cache) that caches information about the working directory and the next revision to be committed
  • an immutable, append-only object database

Groups of Git commands are:

  • Setup and branch management
    init, checkout, branch
  • Modify
    add, delete, rename, commit
  • Get information
    status, diff, log
  • Create reference points
    tag, branch

Introduce yourself to Git

By entering these lines (with appropriate changes) you can configure the Git with your names:

git config --global user.name  "John Smith"
git config --global user.email  jsmith@example.com

You only need to do this once.

If you want to use a different name/email address for a particular project, you can change it for just that project cd to the project directory, use the above commands, but leave out the --global

Create a repository

For ceating a Git repository, go to the project directory and follow these steps:

  1. Type in git init
    • This creates the repository (a directory named .git)
    • You seldom (if ever) need to look inside this directory
  2. Type in git add .
    • The period at the end is part of this command!
    • Period means “this directory”
    • This adds all your current files to the repository
  3. Type in git commit –m "Initial commit"
    • You can use a different commit message, if you like

Clone a repository

For cloning a remote Git repository, you can use the clone command, as follows:

  • git clone URL
    these make an exact copy of the repository at the given URL into a directory with the same name under the current directory
  • git clone URL mypath
    these make an exact copy of the repository at the given URL into the local path mypath

All repositories are equal, But you can treat some particular repository (such as one on Github) as the “master” directory Typically, each team member works in his/her own repository, and “merges” with other repositories as appropriate

Repository

Your top-level working directory contains everything about your project

  • The working directory probably contains many subdirectories—source code, binaries, documentation, data files, etc.
  • One of these subdirectories, named .git, is your repository

At any time, you can take a “snapshot” of everything (or selected things) in your project directory, and put it in your repository

  • This “snapshot” is called a commit object
  • The commit object contains
    1. a set of files
    2. references to the “parents” of the commit object
    3. a unique “SHA1” name
  • Commit objects do not require huge amounts of memory

You can work as much as you like in your working directory, but the repository isn’t updated until you commit something

Making commits

If you create new files and/or folders, they are not tracked by Git unless you ask it to do so

git add newFile1 newFolder1 newFolder2 newFile2

Committing makes a "snapshot" of everything being tracked into your repository

  • git commit –m "a message telling what you have done is required"
  • git commit
    • This version opens an editor for you the enter the message
    • To finish, save and quit the editor

Commits and graphs

A commit is when you tell git that a change (or addition) you have made is ready to be included in the project

When you commit your change to git, it creates a commit object:

  • A commit object represents the complete state of the project, including all the files in the project
  • The very first commit object has no “parents”
  • Usually, you take some commit object, make some changes, and create a new commit object; the original commit object is the parent of the new commit object
    • Hence, most commit objects have a single parent
  • You can also merge two commit objects to form a new one
    • The new commit object has two parents

Hence, commit objects form a directed graph

Git is all about using and manipulating this graph

Working with Local Repository

A head is a reference to a commit object. The "current head" is called HEAD (all caps).

Usually, you will take HEAD (the current commit object), make some changes to it, and commit the changes, creating a new current commit object. This results in a linear graph: A → B → C → …→ HEAD

You can also take any previous commit object, make changes to it, and commit those changes. This creates a branch in the graph of commit objects.

Also, you can merge any previous commit objects. This joins branches in the commit graph.

Commit messages

In git, "Commits are cheap". Do them as often you can. When you commit, you must provide a one-line message stating what you have done

  • Terrible message: “Fixed a bunch of things”
  • Better message: “Corrected the calculation of median scores”

Commit messages can be very helpful, to yourself as well as to your team members. You can’t say much in one line, so commit often.

Choose an editor

When you commit, git will require you to type in a commit message. For longer commit messages, you will use an editor. The default editor is probably vim. To change the default editor:

git config --global core.editor /path/to/editor

You may also want to turn on colors:

git config --global color.ui auto

Working with others

All repositories are equal (distributed model), but usually there are one central repository in the cloud. Here’s what you normally do for working in a team:

  • Download the current HEAD from the central repository
  • Make your changes
  • Commit your changes to your local repository
  • Check to make sure someone else on your team hasn’t updated the central repository since you got it
  • Upload your changes to the central repository

and If the central repository has changed since you got it:

  • It is your responsibility to merge your two versions
    This is a strong incentive to commit and upload often!
  • Git can often do this for you, if there aren’t incompatible changes

Typical workflow

the typical workflow looks as follows:

  • git pull remote_repository
    Get changes from a remote repository and merge them into your own repository
  • git status
    See what is the current status
  • Work on your files (remember to add any new ones)
  • git commit –m “changes description”
  • git push