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Git basics

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Surabhi Gupta
Surabhi Gupta

Git is a distributed version control system that provides faster performance compared to client-server systems like Perforce. It works by storing content and metadata in objects that are identified by cryptographic hashes, including blobs for file contents and trees and commits for snapshots. Developers can work locally and commit changes to their local repository, and integrate changes with remote repositories using branches and merging.

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Introduction to Surabhi Gupta
Fast, open-source, distributed source control system.
Client-Server vs Distributed models VCS SERVER Version 1 Version 2 Version 3 Version 1 Version 1 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3
Advantages of Git over P4 Perforce (Client-Server) Git (Distributed) Version management system Source control system Slow due to network latency and increased dependency on server calls Fast! Work locally, offline Intermediate work cannot be easily saved to P4 Various checkpoints for saving intermediate work Difficult to experiment Facilitates experimentation A merger is typically responsible for merging between branches The developer is responsible for merging their branch into master
Server for Git ❖ Github, Stash, CloudForge, etc are code management and collaboration tools for Git repos! ❖ They provide fine grained control over permissions, audit of commit history.! ❖ The distributed model of Git facilitates open source projects since individuals can easily fork off repos and merge the changes back in.
Scope of the talk ❖ Various roles require different levels of expertise in Git:! ❖ Manager ! ❖ Software Engineer/QA Engineer ! ❖ Merger/Release Engineer — consumer of git scripts! ❖ Develop scripts that extend git functionality — deep dive into git internals.! ❖ We will cover concepts and commands that will come in handy in your day-to-day work as a developer.! ❖ This talk is a road map of the Git world. Hopefully, it will whet your appetite for exploring the trails.
Roadmap ❖ Content hashing! ❖ Blobs to Branches! ❖ Staging and committing ! ❖ Remotes and pull requests! ❖ Merge conflicts! ❖ Git resources
Content Hashing ❖ Contents are referenced using their hashes: ! sha1(“blob ” + fileSize + “0” + fileContent)! echo “foobar” > foo.txt
 git hash-object foo.txt = sha1 (“blob 70foobarn”)! 323fae03f4606ea9991df8befbb2fca795e648fa! ❖ Fun fact: Renames are not stored in the repo. They’re computed by commands such as git diff, git merge, etc.
Blobs to trees ❖ A tree is an object that stores ! a) blob! b) subtree! ❖ Each of these contain metadata about their mode, type and name! ❖ A tree object can contain objects of type “blob” or “tree”.! ❖ Example modes: 100755 means it’s an executable file, 120000 specifies a symbolic link
Git Internals: Tree blob blob tree
Commit from trees ❖ A commit is a pointer to a tree! ❖ It is pointed to by one or more parent commits! ❖ It also contains metadata about its:! 1) Author ! 2) Committer
Git Internals: Commit parent! commit tree’ tree blob’ blob commit
Commits to trees parent! commit commit tree tree blob blob tree’ blob’ blob
Reuse of objects tree tree blob blob tree’ blob’ blob parent! commit commit Reusing blob/tree ! from elsewhere or … under-the-hood! object! sharing
Reuse of objects within a tree “B”“A” “C” “A” tree Blobs can be shared within! a single tree.
Multiple parents P1 P2 C
Multiple parents T1 B1 T2 B2 T3 B3 P1 P2 C Commits with multiple parents! have a one-to-one relationship with trees, ! similar to commits with single parents
Branch - pointer to a commit Master git branch
HEAD - pointer to the current commit HEAD git checkout C Master HEAD Master C C
All your codebase are belong to me ❖ git clone! ❖ git log Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Server/Remote You Peer
Our first commit ❖ echo “May the 4th” >> “force.txt”! ❖ git status! ❖ git add force.txt! ❖ git diff —cached! ❖ git commit -m “May the force be with you”
C3 C2 C1 C4 C3 master C2 C1 You Remote remotes/master master
C4 C3 C2 C1 You git push Remote C4 C3 C2 C1 origin/master master master
What if I made a mistake?
Undo unstaged changes force.txt git checkout — force.txt echo “new” >> force.txt Committed Staging! Area Unstaged! changes
Unstage changes force.txt force.txt git reset HEAD force.txt git add force.txt Committed Staging! Area Unstaged! changes
Uncommit changes force.txt force.txt git reset —soft HEAD^ git commit -m “Second commit” Committed Staging! Area Unstaged! changes
Typical workflow Typically, if your team has more than one person, you wouldn’t commit to master directly. Recommended workflow:! 1) Check out a private branch! 2) Commit to the branch, and regularly push to remote.! 3) When the work is complete, get a code review (likely via a pull request) and merge the branch into master
Step 1: Create a new branch git branch bugFix HEAD master bugFix HEAD master
Checkout said branch git checkout bugFix bugFix HEAD master bugFix HEAD master Current branch
Step 2: Feature development HEAD master B CbugFix master B CbugFix D Local Remote A A
Step 3: Merge into master A master B CbugFix D Remote A masterbugFix B E C New merge commit E Remote after! merge D gitk - show git graph
Can we do better? A master B CbugFix D We would like to modify the commit history to make it appear as if bugFix was based on commit D all along!
Rebase to the rescue ❖ Rebase allows you to replay a series of commits on top of a new base commit. ! ❖ Helps keep the commit history clean
Rebase in action A master B CbugFix D bugFix A D C* B* git rebase master bugFix B C master
Merge bugFix with master A D E masterbugFix A master C* bugFix B* D C* B* Merging the rebased branch bugFix ! into master. This merge is typically! triggered in the code management tool! (Github, Stash, etc) after a pull request! is approved.
Merge conflicts ❖ Situation: Conflicting modifications to a file that has changed since we checked it out! ❖ Two options: merge, rebase! ❖ On a private branch, it is recommended that you rebase. ! ❖ On a shared branch, merge is the way to go.
Changing the commit history ❖ “git commit —amend” rewrites the your last commit with the current changes instead of creating a new commit! ❖ Interactive rebase: git rebase -i! ❖ Swiss army knife of modifying history! ❖ Allows you to amend, squash, split, or skip commits as they're applied
Many roads, one destination ❖ There are often multiple ways to accomplish a task in Git, for example: git branch <branchName>
 git checkout <branchName>
 git checkout -b <branchName> git checkout -b <branchName> <remoteName>/<remoteBranch> git branch --track <branchName> <remoteName>/<remoteBranch> git fetch! git merge git pull
Give It a Try
Git Resources ❖ Learn by playing: http://pcottle.github.io/learnGitBranching/! ❖ Atlassian tutorial: https://www.atlassian.com/git/tutorials/ setting-up-a-repository/! ❖ Free CodeSchool course on Git: https://www.codeschool.com/ courses/git-real! ❖ StackOverflow is a great resource: http://stackoverflow.com/ questions/2706797/finding-what-branch-a-commit-came-from! ❖ Pro Git by Scott Chacon and Ben Straub: http://git-scm.com/ book/en/v2
Closing thoughts ❖ Git is a powerful source control tool designed to maximize the efficiency of the developer. Take full advantage of it!! ❖ We’ve only explored the tip of the iceberg. May the power of Git be with you.

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Git basics

  • 3. Client-Server vs Distributed models VCS SERVER Version 1 Version 2 Version 3 Version 1 Version 1 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3
  • 4. Advantages of Git over P4 Perforce (Client-Server) Git (Distributed) Version management system Source control system Slow due to network latency and increased dependency on server calls Fast! Work locally, offline Intermediate work cannot be easily saved to P4 Various checkpoints for saving intermediate work Difficult to experiment Facilitates experimentation A merger is typically responsible for merging between branches The developer is responsible for merging their branch into master
  • 5. Server for Git ❖ Github, Stash, CloudForge, etc are code management and collaboration tools for Git repos! ❖ They provide fine grained control over permissions, audit of commit history.! ❖ The distributed model of Git facilitates open source projects since individuals can easily fork off repos and merge the changes back in.
  • 6. Scope of the talk ❖ Various roles require different levels of expertise in Git:! ❖ Manager ! ❖ Software Engineer/QA Engineer ! ❖ Merger/Release Engineer — consumer of git scripts! ❖ Develop scripts that extend git functionality — deep dive into git internals.! ❖ We will cover concepts and commands that will come in handy in your day-to-day work as a developer.! ❖ This talk is a road map of the Git world. Hopefully, it will whet your appetite for exploring the trails.
  • 7. Roadmap ❖ Content hashing! ❖ Blobs to Branches! ❖ Staging and committing ! ❖ Remotes and pull requests! ❖ Merge conflicts! ❖ Git resources
  • 8. Content Hashing ❖ Contents are referenced using their hashes: ! sha1(“blob ” + fileSize + “0” + fileContent)! echo “foobar” > foo.txt
 git hash-object foo.txt = sha1 (“blob 70foobarn”)! 323fae03f4606ea9991df8befbb2fca795e648fa! ❖ Fun fact: Renames are not stored in the repo. They’re computed by commands such as git diff, git merge, etc.
  • 9. Blobs to trees ❖ A tree is an object that stores ! a) blob! b) subtree! ❖ Each of these contain metadata about their mode, type and name! ❖ A tree object can contain objects of type “blob” or “tree”.! ❖ Example modes: 100755 means it’s an executable file, 120000 specifies a symbolic link
  • 11. Commit from trees ❖ A commit is a pointer to a tree! ❖ It is pointed to by one or more parent commits! ❖ It also contains metadata about its:! 1) Author ! 2) Committer
  • 13. Commits to trees parent! commit commit tree tree blob blob tree’ blob’ blob
  • 14. Reuse of objects tree tree blob blob tree’ blob’ blob parent! commit commit Reusing blob/tree ! from elsewhere or … under-the-hood! object! sharing
  • 15. Reuse of objects within a tree “B”“A” “C” “A” tree Blobs can be shared within! a single tree.
  • 17. Multiple parents T1 B1 T2 B2 T3 B3 P1 P2 C Commits with multiple parents! have a one-to-one relationship with trees, ! similar to commits with single parents
  • 18. Branch - pointer to a commit Master git branch
  • 19. HEAD - pointer to the current commit HEAD git checkout C Master HEAD Master C C
  • 20. All your codebase are belong to me ❖ git clone! ❖ git log Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Version 1 Version 2 Version 3 Server/Remote You Peer
  • 21. Our first commit ❖ echo “May the 4th” >> “force.txt”! ❖ git status! ❖ git add force.txt! ❖ git diff —cached! ❖ git commit -m “May the force be with you”
  • 24. What if I made a mistake?
  • 25. Undo unstaged changes force.txt git checkout — force.txt echo “new” >> force.txt Committed Staging! Area Unstaged! changes
  • 26. Unstage changes force.txt force.txt git reset HEAD force.txt git add force.txt Committed Staging! Area Unstaged! changes
  • 27. Uncommit changes force.txt force.txt git reset —soft HEAD^ git commit -m “Second commit” Committed Staging! Area Unstaged! changes
  • 28. Typical workflow Typically, if your team has more than one person, you wouldn’t commit to master directly. Recommended workflow:! 1) Check out a private branch! 2) Commit to the branch, and regularly push to remote.! 3) When the work is complete, get a code review (likely via a pull request) and merge the branch into master
  • 29. Step 1: Create a new branch git branch bugFix HEAD master bugFix HEAD master
  • 30. Checkout said branch git checkout bugFix bugFix HEAD master bugFix HEAD master Current branch
  • 31. Step 2: Feature development HEAD master B CbugFix master B CbugFix D Local Remote A A
  • 32. Step 3: Merge into master A master B CbugFix D Remote A masterbugFix B E C New merge commit E Remote after! merge D gitk - show git graph
  • 33. Can we do better? A master B CbugFix D We would like to modify the commit history to make it appear as if bugFix was based on commit D all along!
  • 34. Rebase to the rescue ❖ Rebase allows you to replay a series of commits on top of a new base commit. ! ❖ Helps keep the commit history clean
  • 36. Merge bugFix with master A D E masterbugFix A master C* bugFix B* D C* B* Merging the rebased branch bugFix ! into master. This merge is typically! triggered in the code management tool! (Github, Stash, etc) after a pull request! is approved.
  • 37. Merge conflicts ❖ Situation: Conflicting modifications to a file that has changed since we checked it out! ❖ Two options: merge, rebase! ❖ On a private branch, it is recommended that you rebase. ! ❖ On a shared branch, merge is the way to go.
  • 38. Changing the commit history ❖ “git commit —amend” rewrites the your last commit with the current changes instead of creating a new commit! ❖ Interactive rebase: git rebase -i! ❖ Swiss army knife of modifying history! ❖ Allows you to amend, squash, split, or skip commits as they're applied
  • 39. Many roads, one destination ❖ There are often multiple ways to accomplish a task in Git, for example: git branch <branchName>
 git checkout <branchName>
 git checkout -b <branchName> git checkout -b <branchName> <remoteName>/<remoteBranch> git branch --track <branchName> <remoteName>/<remoteBranch> git fetch! git merge git pull
  • 40. Give It a Try
  • 41. Git Resources ❖ Learn by playing: http://pcottle.github.io/learnGitBranching/! ❖ Atlassian tutorial: https://www.atlassian.com/git/tutorials/ setting-up-a-repository/! ❖ Free CodeSchool course on Git: https://www.codeschool.com/ courses/git-real! ❖ StackOverflow is a great resource: http://stackoverflow.com/ questions/2706797/finding-what-branch-a-commit-came-from! ❖ Pro Git by Scott Chacon and Ben Straub: http://git-scm.com/ book/en/v2
  • 42. Closing thoughts ❖ Git is a powerful source control tool designed to maximize the efficiency of the developer. Take full advantage of it!! ❖ We’ve only explored the tip of the iceberg. May the power of Git be with you.