Git of every day

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Notions of git, important commands and use cases.

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Git of every day

  1. 1. of every day Notions and more Alan Descoins Tryolabs, 2014
  2. 2. Goals ● Better understand what we do every day ● Ask less questions (understand the man) ● Improve some practices ● Make fewer mistakes ● Do something super specific ;)
  3. 3. ● 2005, Linux kernel ● Efficiency, simple design ● Non-linear development (thousands of parallel branches) ● Completely distributed Origins and philosophy
  4. 4. Distributed ● Nearly all operations are local ● All the machines have ALL the information ● Contrast with SVN, CVS, etc.
  5. 5. Some theory ● git init initializes a repository ● Creates only one .git directory in the root (doesn’t pollute) ● Mini filesystem ● Snapshots, not differences!
  6. 6. Git and the content ● Tracking of content, NOT files ● Vocabulary: tracked / untracked ● Git objects ○ Blob (content) ○ Tree (structure) ○ Commit (points to a tree, + data) ○ Tag (points to a commit, + data) SHA-1
  7. 7. ● One file per “content” ● SHA-1 of the content (zlib) + a header ● Initially, loose object ● Then packfile (efficiency, heuristics…) ● Example: find .git/objects -type f Blob
  8. 8. Tree ● Like a directory structure ● Pointers to blobs or other trees ● Represents a complete snapshot of the state ● Example: git ls-tree <sha-1>
  9. 9. Commit ● Information about the trees ● Who, when, why saved the tree ● Points to the top level tree of the project ● Tree hierarchy ● Also SHA-1, different hash than the tree it points to
  10. 10. Commit (2)
  11. 11. Tags ● Pointer to a commit ● Additional information, signature ● So we don’t have to remember the SHA-1...
  12. 12. The Holy Trinity A content may be in three states: ● Committed ● Modified ● Staged
  13. 13. The Holy Trinity (2) Three parts of a project: ● Working directory ● Staging area (or index) ● .git (repository)
  14. 14. Basic commands ● git add - adds to the index (staging) ● git commit - saves the index together with author, message, date, etc. ● git status - shows the file with differences between the working dir, staging, and the untracked ● git diff - what changed but is not staged ● git diff --cached - what is going in the next commit ● git log
  15. 15. Avoiding the staging ● Sometimes, it may be easier to avoid the staging area ● git commit -a | git commit <archivo> ● Automatically add to the staging, then commit
  16. 16. Branches ● Simply a pointer to a commit ● As fast as it is to write 41 characters to a file (SHA-1 + n) ● By default master ● Special pointer HEAD (current branch) ● Example: cat .git/HEAD
  17. 17. Branches (2) ● git checkout to switch branches
  18. 18. References ● The HEAD pointer is a special case ● Tags too: “an unmovable branch” (lightweight tags) Example: find .git/refs
  19. 19. Remotes ● No central server ● Can add several remotes, external copies of the repo (URL) ● The famous origin :)
  20. 20. Remote branches ● Save the state of branches in remote repositories ● They are local, but cannot be moved, only when you do network operations (eg. git fetch). ● The form is <remote>/<branch> Example: cat .git/refs/remotes/origin/master
  21. 21. Fetch, push ● git fetch - sync and update remote branches (moves pointers) ● Eg. git fetch <remote> ● git pull - equivalent to git fetch and the git merge, merges the local branch with the changes in the remote ● git push - update the remote branch with the local data ● Eg. git push <remote> <branch>
  22. 22. Tracking branches ● Local branches can “track” remote ones ● Two possibilities: ○ git checkout --track <remote>/<branch> ○ git push --set-upstream <remote> <branch> ● Now, when pushing you don’t have to type the name of the remote and the branch :)
  23. 23. Merges ● Fast-forward: git is smart and only needs to move a pointer ● Integrate changes of one branch into another
  24. 24. Merges (2) ● Three way merge: commit not direct ancestor of what I am merging in (non linear) ● Git determines the best common ancestor ● A merge commit is created, special because it has more than one parent ● Conflicts...
  25. 25. Three way merge
  26. 26. Rebasing ● Another way to introduce changes of one branch into another ● Rewind & replay, rewrites the commits ● From a common ancestor ● History is always linear, even though it was work originally done in parallel
  27. 27. Rebasing (2) ● git rebase <new-base> moves the commits from the current branch to the new base ● git rebase -i <new-base> lets us select actions for every commit ● git pull --rebase
  28. 28. ● Never do rebase of a branch that is already pushed, or you pulled from somebody else ○ Duplicate commits if someone does merge ○ Arguable: if the remote branch is yours and used as backup ○ Use git push --force, if you are certain nobody else works in the branch ● If you rebase to get changes by other developers locally, do it as often as possible. Golden rules
  29. 29. Rebase vs Merge Why rebase? ● Clean, linear history ● Open-source collaboration, many demand it ● Easier for code review Why merge? ● Easier to solve conflicts (only once) ● Less risk
  30. 30. git reset ● Often used ● Used wrong, can cause loss of data (workdir unsafe) ● Three most used options: ○ --soft ○ --mixed ○ --hard
  31. 31. git reset... ● move the branch HEAD points to so it points to target commit (stop if --soft) ● then make the staging look like that (stop if --mixed, default option) ● then make the working dir look like that (if --hard, it is workdir usafe!)
  32. 32. Examples of git reset ● With HEAD in dev branch, do git reset master ○ now dev y master point to the same commit ● With HEAD in branch master, do git reset --hard origin/master ○ now my local master branch is identical to the remote (discard local changes) ● git reset --soft HEAD~ ○ Undo the last commit without losing changes
  33. 33. Unstaging files ● When doing git reset <file> we don’t specify a branch or commit SHA ● Equivalent to git reset --mixed HEAD <file> ● Moves a particular file out of staging, opposite of git add :)
  34. 34. git checkout ● git checkout <branch> is superficially similar to git reset --hard <branch> ● Two differences: ○ Workdir safe! :D Checks to avoid data loss. ○ reset moves the branch that HEAD points to, checkout moves the HEAD pointer itself.
  35. 35. git checkout of a file ● git checkout <branch> <file> ○ As with reset, now it doesn’t make sense for it to move HEAD. Replaces only <file> in the workdir (workdir usafe!). ○ Would be the same as git reset --hard <branch> <file> ○ But that command does not exist :)
  36. 36. Common situations (1) ● “I need to pull but there are changes I am working on, and I don’t want to commit yet” ○ git stash ○ It’s a stack ○ Apply with git stash pop.
  37. 37. Common situations (2) ● “There is a bug in a branch, I need to know what commit introduced it” ○ git bisect ○ Binary search, driven by git. Very easy! ○ git bisect start git bisect bad <commit> git bisect good <commit> ● Test on every step (git makes a checkout)
  38. 38. Common situations (3) ● “I made one (or several) commits in the wrong branch, haven’t yet pushed though!” ○ If the correct branch does not yet exist ■ git checkout -b <correct branch> ■ git reset over the old branch ○ If the branch already existed ■ git cherry-pick (or git rebase --onto) ■ git reset over the old branch
  39. 39. Common situations (4) ● “I have many changes in a file and only want to commit some of them” ○ git add -p or git commit -p ○ git will ask for every part, whether to add it or not
  40. 40. Common situations (5) ● “I need to change some commits” ● “I need to delete some commits” ● “I need to change the order of some commits” ○ git rebase -i (--interactive) ○ Easy to use ○ Rewrites history, asking us what to do for each commit
  41. 41. Common situations (6) ● “I am doing a merge with many conflicts and I want to stop it and get back to where I was before” ○ git merge --abort
  42. 42. (Images from git-scm and others ;) ) Didn’t learn git

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