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The course shows how to use Git, the popular open-source version control software, to manage changes to source code and text files. Using a step-by-step approach, author Kevin Skoglund presents the commands that enable efficient code management and reveals the fundamental concepts behind version control systems and the Git architecture. Discover how to track changes to files in a repository, review previous edits, and compare versions of a file; create branches to test new ideas without altering the main project; and merge those changes into the project if they work out. The course begins by demonstrating version control in a single-user, standalone context, before exploring how remote repositories allow users to collaborate on projects effectively.
There are five important version control systems that predate Git that I want us to look at. There have been many others over the years, but these are some of the most popular and the most influential. The first of these is SCCS, it wasn't the first, but it was the first to become popular. It was released in 1972 and was developed by AT&T and bundled free with Unix. Now Unix was also free, and as a consequence it spread quickly to places like universities, so therefore universities also taught their students how to do version control using SCCS. So if you were writing Unix programs that's what you learned to use.
And then when these students left the universities to go work in jobs, the Version Control System that they were familiar with, and they took with them, was SCCS, so you can see why it became very popular. We talked about very primitive version control earlier, that you might have something like a budget, and you might save version 1 of the budget and version 2 and version 3 just giving a different file name each time. When you do that you are actually saving the full document three different times. That's not the most efficient way to do it. What SCCS does is it keeps the original document, but then instead of saving the whole document a second time, it just saves a snapshot of what the changes were.
So if you want version 5 of the document, you just take version 1 and apply four sets of changes to it, to get to version 5, that's a much more efficient way to store the changes over time. So SCCS stayed dominant until the early '80s when RCS was developed, Revision Control System, and it just made lots of improvements over SCCS. For one thing it was cross-platform, whereas SCCS was Unix only. With the rise of the Personal Computer it was important to have a Version Control System that could work on PCs. It was also more intuitive, had a cleaner syntax with fewer commands, but more features.
And most importantly it was faster and a lot of its speed increase came from the fact that it used a smarter storage format whereas SCCS stored the original file and then kept track of all the changes that went after it, RCS flipped that around so that it kept the most recent file in its whole form, and if you wanted previous versions, then you applied the change snapshots to go backwards in time. If you think about it, that's a lot faster because most of the time what we want to work with is the current document. With SCCS if we wanted the current document, and there were 20 sets of changes, we had to pull up the original and then apply 20 sets of changes to get there, with RCS we can just bring up the original file, and it is already stored in its full state.
Now one of the problems with both SCCS and RCS was they only allowed you to work with an individual file one at a time. So you could track changes in a single file, but not in sets of files, or in a whole project. CVS allowed you to do that, that's for Concurrent Versions System. Now the real innovation now to CVS is not just the fact that you can work with multiple files, it's the concurrent part, the fact that we can have a place where we store our code, called a Code Repository, you can put that on a remote server, and more than one user can work on the same file at the same time, they can work concurrently.
With previous versions only one person could work with a file at a single time. So CVS adds a lot of features for users to be able to share their work and be able to update their file with changes that other people have made and placed in the remote repository. This idea of working with remote repositories was further improved upon with Apache Subversion or SVN for short. SVN was faster than CVS, and it allowed saving of non-text files, like images, whereas CVS couldn't do that. Most importantly, the big innovation for SVN was that it was tracking not just changes to files, or to groups of files, but actually watching what happened in a directory as a whole, watching the files in the directory and actually taking a snapshot of the directory not just the files.
Now it may seem like a small difference, but it's important. In CVS you would talk about having revision 7 of some file. In SVN, you talk about some file as it appears in revision 7. SVN could track the history of directories. For examples, CVS had a hard time if you renamed a file. SVM though would track that change with no problem. If you add a file directory, remove a file, rename it, it's watching the directory as a whole to see what happens and taking a snapshot of that, whereas CVS was just looking at a collection of individually named files.
CVS would also update files one at a time as it went to either apply or read back changes. SVN would instead do a transactional commit and apply all of the changes that happen to the directory or none of them at all. The snapshot was bigger than just the individual files, it was the entire directory, the entire set of changes that were happening to that directory at one time. It's a subtle but important difference. Now SVN stayed the most popular Version Control System for a very long time until Git came out, but there is one other Version Control System that I want us to look at before that, and that is BitKeeper SCM.
It was a closed source proprietary source code management tool. That means that a company owned it and sold it, the same way that Adobe sells Photoshop or Microsoft sells Word. One of the important features the BitKeeper had, and it wasn't the first to have it, but is Distributed Version Control. But before we get to that, let's talk a little bit more about this idea of being closed source, whereas all the other ones that we have been looking at for a while had been open source. The community version of BitKeeper was free, it had a few less features and some usage restrictions, but there was a version that they gave away for free, and that version was used for source code management of the Linux kernel from 2002 to 2005.
Now it was controversial to use a proprietary SCM for the Linux kernel because the Linux kernel is an open source project, no one owns it, whereas the SCM is owned and controlled by a company. So a lot of people objected saying, well what if they change the rules in the future, suddenly we're going to be screwed because we are stuck using this company's software? Well, guess what, in April 2005, the community version stopped being free and all those predictions came true. Now BitKeeper was never as popular CVS or SVN, but it is super important with the creation of Git because of its connection to Linux, because in April 2005, when the community version stopped being free, that's the same point at which Git was born, Git was created by Linus Torvalds, and you may recognize that name as the person who created Linux and still drives the development of the Linux kernel.
When BitKeeper stop being free they needed an alternative for managing their source code. Linus looked around, and it didn't like the other VCSs that were out there, like CVS and SVM, he did like some of the concepts of BitKeeper, but he thought he could do even better, so he wrote a new VCS from scratch. Now Git is Distributed Version Control, like BitKeeper, and we'll talk more about Distributed Version Control in the next movie. It's also open source and free, which is great for us, because it means that people like you and me can download it for free and use it for free with no license fees or anything like that.
It also means that because it's open source the community can see the source code and contribute to it as well. So they can submit bug fixes, add new features, all of those things we get to be the beneficiaries of because it's an open source project. It's also compatible with most platforms, Unix-like systems and Windows, and it's faster than most other Source Code Management tools. 100 times faster in some cases for some operations, and has better safeguards built into it to guard against data corruption, and we will talk about that a bit later. Now these improvements worked.
Git became a hit, as people discovered the power of Distributed Version Control as they got used to all of Git's nice features, Git has experienced an explosion in popularity. Now there is no official statistics on this but to give you an example, GitHub launched in 2008 to host Git source code repositories. In 2009, there were over 50,000 repositories with a 100,000 users. In 2011 just two years later, there were 2 million repositories with over a million users. So you can see this rapid growth in just two years.
So Git has definitely taken off. In the next movie, let's talk about Distributed Version Control and see why that's such an important feature.
Find answers to the most frequently asked questions about Git Essential Training.
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1. To add "-mmacosx-version-min=10.6" as described here:https://stackoverflow.com/questions/14268887/what-is-the-illegal-instruction-4-error-and-why-does-mmacosx-version-min-10https://stackoverflow.com/questions/10177038/illegal-instruction-4-shows-up-in-os-x-lion
2. Or to use the version of git that comes with Xcode, or to use homebrew to install git instead.http://superuser.com/questions/697144/installed-git-not-sure-how-to-get-it-working
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