1. Introduction to Version Control with Git CSC/ECE 517, Fall 2014 A joint project of the CSC/ECE 517 staff, including Titus Barik, Gaurav Tungatkar, Govind Menon, and Krunal Jhaveri

2. Local version control: RCS Keep many copies of files Error prone RCS stores deltas Version 3 Version 2 Version 1 File check out

3. Centralized Version Control If you need to work with other programmers … Version 3 Version 2 Version 1 File check out File check out Computer B Computer A

4. File Server vs. Version-Control Server At first glance, the client-server architecture of a version-control system looks much like a typical file server. So why do we need version control?why do we need version control

5. File-Sharing Issues Image: Version Control with Subversion The problem is that users are stepping on each other’s feet!

6. Approach 1: Lock, Modify, Unlock Image: Version Control with Subversion 1.Locking may cause administrative problems. 2.Locking may cause unnecessary serialization. 3.Locking may create a false sense of security.

7. Approach 2: Copy-Modify-Merge Image: Version Control with Subversion Sounds chaotic, but in practice, runs extremely smoothly. Question: When is locking necessary?

8. Answer these questionsAnswer Give one advantage of using a version-control server for source-code management over using a fileserver. Explain how locking can cause administrative problems. Explain how locking can create a false sense of security. With copy-modify -merge, when is locking necessary? Exercise 1

9. Branches and Tags Trunk: Location where main development occurs. Branches: Location used to isolate changes to another development line (e.g., experimental features). Tags: Snapshot of the content (e.g., RTM, service packs, EOL). Image: http://en.wikipedia.org/wiki/Subversion_(software)

10. Traditional Repository Format Image: Version Control with Subversion A Subversion repository layout—typical of older version-control systems. The folder names are just a convention, and have no special meaning to the repository.

11. Creating a Branch—by Copying Image: Version Control with Subversion In Subversion, the underlying mechanism of a branch is implemented by performing a simple directory copy.

12. Exercise 2 Answer these questions about branches.Answer these questions oSuppose, in fixing a bug, you modify three lines of code in two source files. Should you create a new branch? Why or why not? oWhich would probably be more common, branches or tags? oWhat are some of the risks of copying files in a repository? How do version-control systems minimize this risk?

13. Distributed Version Control Clients don’t check out individual files; they mirror the repository. What’s the advantage? Version 3 Version 2 Version 1 Version 3 Version 2 Version 1 Version 3 Version 2 Version 1 File Computer AComputer B

14. Git Came out of the Linux project, in 2005. Simple design Strong support for non-linear development (thousands of parallel branches) Fully distributed Able to handle large projects like the Linux kernel efficiently (speed and data size)

15. Integrity & Checksums Everything checksummed with an SHA-1 hash –40-character string –composed of hex characters –calculated based on the contents of a file or directory structure in Git Example –24b9da6552252987aa493b52f8696cd6d3b00373 –But, you don’t have to type the whole SHA … Git knows everything by hash, not filename

16. Snapshots, not Diffs See http://git-scm.com/book/ch1-3.html Every time you commit, Git takes a snapshot of your files. Files that have not changed are not copied.   Almost all ops are local browse history commit

17. 3 States of a File in Git Modified Staged Committed working directory staging area git directory (repository) check out the project stage files commit

18. File Status Lifecycle unmodifiedmodifiedstaged edit the file stage the file untracked add the file remove the file

19. Checking Status To check the status of your files: $ git status # On branch master nothing to commit (working directory clean) Creating new files $ vim README $ git status # On branch master # Untracked files: # (use "git add..." to include in what will be committed) # # README nothing added to commit but untracked files present (use "git add" to track)

20. Checking status, cont. Begin to track the file: $ git add README The file is now tracked: $ git status # On branch master # Changes to be committed: # (use "git reset HEAD..." to unstage) # # new file: README # For more info: http://git-scm.com/book/ch2-2.html

21. Remotes On a project, you may be working with several remote directories. “Origin” is the server you cloned your repository from $ git clone git://github.com/schacon/ticgit.git Initialized empty Git repository in /private/tmp/ticgit/.git/ remote: Counting objects: 595, done. remote: Compressing objects: 100% (269/269), done. remote: Total 595 (delta 255), reused 589 (delta 253) Receiving objects: 100% (595/595), 73.31 KiB, done. Resolving deltas: 100% (255/255), done. $ cd ticgit $ git remote origin http://git-scm.com/book/ch2-5.html

22. Pulling, pushing to remote $ git fetch [remote-name] E.g., git fetch origin git push origin master

23. Common Workflow using Git Centralized workflow … http://git-scm.com/book/ch5-1.html Integration-manager workflow … Common use cases: http://git-scm.com/book/ch5-2.html

24. Pull requests After you’ve finished a project, you need to notify the maintainer. This is done via a pull request. You say which repository to pull from, and give a summary of your changes. http://git-scm.com/book/ch5-2.html

25. Guidelines for Commits What happens if you download a repo in a zip file, do your project, then save it with a single commit? (Think of someone else trying to merge your changes with another programmer’s changes.)

26. Your code Repository code … a = a + b … a = c … Is the difference because— you changed a = c to a = a + b, or because someone else changed a = a + b to a = c while you were working on your project?

27. Guidelines for Commits Which is worse, Downloading the repo as a zip file, and being scrupulously careful to make multiple commits with reasonable commit comments, or Downloading the repo with its commit history, but committing your whole project in one commit? Why? Of course, you shouldn’t do either!

28. Guidelines for Commits In your work, save the commit history. Each commit should be on one topic. A commit comment should be 1 line, certainly no more than one sentence.

29. Exercise 3 Visit https://github.ncsu.edu/grmenon/versionControl https://github.ncsu.edu/grmenon/versionControl Clone the repository using the HTTPS clone url $ git clone [https_url] // clone an existing repo $ git branch // List branches

30. Exercise 3, cont. $ git branch $ git branch [unityId] // create a new branch from the current HEAD $ git branch $ git checkout [unityId] // switch to that branch

31. Exercise 3, cont. Add a new file (you can take a look at the test file and create a new test) $ git status $ git add [filename] $ git status $ git commit -m “Commit message” $ git push origin [unityId] // push on to the branch on the remote

32. Exercise 3, cont. [Pause here, until next week.] $ git fetch origin // Update other branches $ git merge origin/master //Merge any new changes on master into current branch

33. Exercise 3, cont. $ git checkout master $ git merge [unityId] // merge changes from your branch back into master

34. Exercise 3, cont. Commit 1 Commit 2Commit 1 Your new branch Master branch Commit 1Commit 2Commit 1Commit 1.5 Local Remote (Origin) Commit 1.5

35. Exercise 3, cont. Commit 1 Commit 2 Your new branch Local Remote (Origin) Commit 1.5 Commit 1 Commit 2Commit 1.5 Commit 1 Commit 2Commit 1.5

36. Exercise 3, cont. Commit 1 Commit 2 Master branch Local Remote (Origin) Commit 1.5 Commit 1 Commit 2Commit 1.5 Commit 1 Commit 2Commit 1.5