Git is a distributed version-control system for tracking changes in source code during software development.
Git is open source software originally created in 2005 by Linus Torvalds, the famous creator of the Linux operating system kernel.
According to the latest survey, more than 70 percent of developers use Git, making it the most-used VCS in the world. Git is commonly used for both open source and commercial software development, with significant benefits for individuals, teams and businesses.
Git lets developers see the entire timeline of their changes, decisions, and progression of any project in one place. From the moment they access the history of a project, the developer has all the context they need to understand it and start contributing.
Developers work in every time zone. With a DVCS like Git, collaboration can happen any time while maintaining source code integrity. Using branches, developers can safely propose changes to production code.
Businesses using Git can break down communication barriers between teams and keep them focused on doing their best work. Plus, Git makes it possible to align experts across a business to collaborate on major projects.
Git is an example of a distributed version control system (DVCS) commonly used for open source and commercial software development. 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. DVCSs like Git don’t need a constant connection to a central repository, developers can work anywhere and collaborate asynchronously from any time zone.
On the contrary, version control systems like CVS or Subversion (also known as SVN) have only one single place for the full version history of the software as is common in once-popular version control systems.
Even though Git’s user interface is fairly similar to these other VCSs, Git stores and thinks about information in a very different way, and understanding these differences will help you avoid becoming confused while using it.
The major difference between Git and any other VCS is the way Git thinks about its data. Conceptually, most other systems store information as a list of file-based changes. These other systems (CVS, Subversion, Perforce, Bazaar, and so on) think of the information they store as a set of files and the changes made to each file over time (this is commonly described as delta-based version control).
Storing data as changes to a base version of each file.
Git doesn’t think of or store its data this way. Instead, Git thinks of its data more like a series of snapshots of a miniature filesystem. With Git, every time you commit, or save the state of your project, Git basically takes a picture of what all your files look like at that moment and stores a reference to that snapshot. To be efficient, if files have not changed, Git doesn’t store the file again, just a link to the previous identical file it has already stored. Git thinks about its data more like a stream of snapshots.
Storing data as snapshots of the project over time.
This is an important distinction between Git and nearly all other VCSs. It makes Git reconsider almost every aspect of version control that most other systems copied from the previous generation. This makes Git more like a mini filesystem with some incredibly powerful tools built on top of it, rather than simply a VCS.
Most operations in Git need only local files and resources to operate — generally no information is needed from another computer on your network. If you’re used to a CVCS where most operations have that network latency overhead, this aspect of Git will make you think that the gods of speed have blessed Git with unworldly powers. Because you have the entire history of the project right there on your local disk, most operations seem almost instantaneous.
For example, to browse the history of the project, Git doesn’t need to go out to the server to get the history and display it for you — it simply reads it directly from your local database. This means you see the project history almost instantly. If you want to see the changes introduced between the current version of a file and the file a month ago, Git can look up the file a month ago and do a local difference calculation, instead of having to either ask a remote server to do it or pull an older version of the file from the remote server to do it locally.
This also means that there is very little you can’t do if you’re offline or off VPN. If you get on an airplane or a train and want to do a little work, you can commit happily (to your local copy, remember?) until you get to a network connection to upload. If you go home and can’t get your VPN client working properly, you can still work. In many other systems, doing so is either impossible or painful. In Perforce, for example, you can’t do much when you aren’t connected to the server; in Subversion and CVS, you can edit files, but you can’t commit changes to your database (because your database is offline). This may not seem like a huge deal, but you may be surprised what a big difference it can make.
Everything in Git is checksummed before it is stored and is then referred to by that checksum. This means it’s impossible to change the contents of any file or directory without Git knowing about it. This functionality is built into Git at the lowest levels and is integral to its philosophy. You can’t lose information in transit or get file corruption without Git being able to detect it.
The mechanism that Git uses for this checksumming is called a SHA-1 hash. This is a 40-character string composed of hexadecimal characters (0–9 and a–f) and calculated based on the contents of a file or directory structure in Git. A SHA-1 hash looks something like this:
You will see these hash values all over the place in Git because it uses them so much. In fact, Git stores everything in its database not by file name but by the hash value of its contents.
When you do actions in Git, nearly all of them only add data to the Git database. It is hard to get the system to do anything that is not undoable or to make it erase data in any way. As with any VCS, you can lose or mess up changes you haven’t committed yet, but after you commit a snapshot into Git, it is very difficult to lose, especially if you regularly push your database to another repository.
This makes using Git a joy because we know we can experiment without the danger of severely screwing things up.
Pay attention now — here is the main thing to remember about Git if you want the rest of your learning process to go smoothly. Git has three main states that your files can reside in: modified, staged, and committed:
Modified means that you have changed the file but have not committed it to your database yet.
Staged means that you have marked a modified file in its current version to go into your next commit snapshot.
Committed means that the data is safely stored in your local database.
This leads us to the three main sections of a Git project: the working tree, the staging area, and the Git directory.
Working tree, staging area, and Git directory.
The working tree is a single checkout of one version of the project. These files are pulled out of the compressed database in the Git directory and placed on disk for you to use or modify.
The staging area is a file, generally contained in your Git directory, that stores information about what will go into your next commit. Its technical name in Git parlance is the “index”, but the phrase “staging area” works just as well.
The Git directory is where Git stores the metadata and object database for your project. This is the most important part of Git, and it is what is copied when you clone a repository from another computer.
The basic Git workflow goes something like this:
You modify files in your working tree.
You selectively stage just those changes you want to be part of your next commit, which adds only those changes to the staging area.
You do a commit, which takes the files as they are in the staging area and stores that snapshot permanently to your Git directory.
If a particular version of a file is in the Git directory, it’s considered committed. If it has been modified and was added to the staging area, it is staged. And if it was changed since it was checked out but has not been staged, it is modified.
This link has details on how to install Git in multiple operating systems:
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