Git workshop 33degree 2011 krakow

GitLuca MilanesioAndroid programmingSang ShinThe Productive ProgrammerNeal FordThe Power of RetrospectionLinda RisingIntroduction to ScalaHubert PlociniczakMain sponsor

AgendaSCM and Git ConceptsGit quick startBranching and mergingResolving conflictsReverting changesWorking with tagsGit remotesGit on the serverGit peer-to-peerGit democracy

Who’s that guy ?Luca MilanesioLMIT Limited – Director / co-founder of GitEnterprise.comJenkins (formerly Hudson) contributor since 2007founder of hudson-mobi.comOver 18 years of experience in Software and Services Development and Application LifecycleWorked for major UK, EU and US CustomersBanking, Retailers, Industry, Finance, Telecoms, Utilities, GovernmentGit enthusiast since 2009 and innovator in Vodafone Group Services Ltd

About SCM … remember ?SCM = Source Code ManagementMultiple revisions of files

Manage branches of development

Integrate the work of multiple teams together

… and much more Picture courtesyofglobalnerdy.com - Allrightskindlyreserved

Brief history of OpenSourceSCMsLocal SCMs (versions kept on local filesystem)SCCS (1972) … I was not yet born, don’t remember RCS (1982) the most widely used on UnixServer-based SCMs (central repository server)CVS (1990) first widely used SCM serverSubversion (2000) first widely Internet SCM… and then let’s to go distributed …DCVS (2002) who has ever used it ?Mercurial and Git (2005)

Centralised vs. distributed ?How many of you are using central repositories ?CVS-boys … raise your hands !SVN-guys … it’s your turn !How many are for distributed repositories ?Mercurial-scientists ... raise your hands !Git-explorers … brave men !

Who’s right ?Central SCMUnique “source of truth”Central back-upSeamless alignment for all developersSecurity and access controlDistributed SCMCommunity code-baseNo single-point-of-failurePeer-to-peer alignmentContinuous branching and merging

Why GIT ?It’s all about BitKeeper fault: they broke up with LinusTorvalds … and I’m not kidding Story:Linux Kernel SCM: BitKeeperApr 2005 – Linus writes PERL scripts for Linux Kernel SCMJul 2005 … Git 0.99 is out !Git principles:Allow the BitKeeper SCM workflowUse CVS as the “not-to-do” exampleCheck and prevent corruptionMake it FFF … FAST FAST FAST !

Let’s experimentGit in action !

Git installation (Ver. >=1.6)Linux (Gitfavourite of course !)Ubuntu: sudo apt-get install git-coreother Linux ? … best from source codehttp://git-scm.com/Mac OSXhttp://code.google.com/p/git-osx-installer/Windowshttp://code.google.com/p/msysgit/(sucks … but it’s your fault not using Unix)CygwinHIGHLY RECOMMENDED (mandatory IMHO)

Step 1 – Git repositorycreate project

$ git initStep 2 – Git identityDefine your full name

Everything stored in .git/configStep 3 – Add some files and commitCreate some files

Add to GIT (default = recursive)

CommitStep 4 – Inspect GIT logGet GIT history of commits

Display changesToo simple ? … let’s add some statesThree states of Git files1. Unstaged2. Staged3. Cleangit addgit commitPicture courtesyofProGit.org

State 1. unstagedCreate a new fileChange an existing fileCheck status

State 2. stagedAdd the two files to staging areaCheck status

State 3. work directory cleanCommit the staging areaCheck status

Display Git lifecycleAdd another file and make some changes$ gitgui

Stage with gitguiSelect files and chose “Stage to commit”

Commit with gitguiEnter commit message and click “commit”

Git stores the whole fileGit is different from SVN: no diffs, just whole filesGit stores changed files between snapshots(BitKeeperdocet)Picture courtesyofProGit.org

Git object types: blobsGit stores all files (and their versions) as objects (blobs)Each object has a SHA-1 160-bit identifierSHA-1 provides:Unique Global IDIntegrity checkExample:File content: “You're GIT too”SHA-1: bbecf72783dfba9e0243e13dbb5fb04ed39ed4e4 (Hex)Track content (not files)Automatically detect renames … cool !

SHA-1 ? WTF …Why LinusTorvalds has chosen SHA-1 hashing ?Need for track content globallySHA-1 collision probability is 1/251What happens if two files have same SHA-1 ?BOOM !What is the probability of it ?World’s population (7 BN people)sharing files of 10 times Linux Kernel Possible ? More likely to be hit by a 15 KM asteroid NOW !!!

Git object types: commits and treesGit tree identifies a snapshot (set of files)Git commit identifiesAuthor / CommitterCommit messageTimestampTreePicture courtesyofProGit.org

Git history: graph of commitsEvery commit points to its predecessorSeries of commits make Git repository historyPicture courtesyofProGit.org

Where are Git objects ?Git objects are in .git/objectsSHA-1 identify directory / file

Curious about Git objects ?Git objects are compressedUse git show to display content

Getting lost ?How to remember SHA-1 hashing codes ?How Git stores the “pointers” to Commit graph ?Git references are the solution !Head of the Git historyIntermediate tagsBranch pointsRelative points

Git referencesReferences: “labels” for Git SHA-1 commit IDsStored as files under .git/refsReference types:TagsHeads (branches)HEAD is a special ref:always points tohead of currentbranch.

How Git commits graph looks like ?ReferenceCommitTreePicture courtesyofProGit.org

What is a branch for Git ?Git named branch = reference to a commit ID (head of branch)Git supports “network” of commits, with named and unnamed branches… don’t know why Git reminds me some “underground” branches 

Real-life Git branchesThink I’m exaggerating ? Look at this example (it’s real, swear !)

Let’s practice on Git branches !

Wear “life jacket” firstGet Git bash extensions source git/contrib/completion/git-completion.bashRedefine promptexport PS1='\W$(__git_ps1 " (%s)") \$ ’... and your current branch is visible on your prompt: you will not get lost 

Creating branchesCreate branch  create a new ref to current commit

Switching branchUse git checkout to switch branchCurrent displayed branch changed to experimentalNote that HEAD points to experimental

Commit on branchAdd a new commit to experimental branchSee the new branch graph (gitk)

MergeWhen experiments are completed … merge back !Checkout masterMerge experimental

Git graph after mergeLet’s have a look on the result with gitkMerge-type applied: Fast-forward (move refs in history)Branch has been “flattened”Experimental just another ref to master

Git recursive-mergeLet’s create some divergenceChanges on both master and experimentalFast-forward merge = move branch ref to another commit ID

Git diverging branchesUse gitk --all to display all branchesNOTE: no args displays just current branchexperimental is really diverging from master

Git recursive mergeLet’s merge again with masterThis is a real merge folks !NOTE: Merge is a Git commit: you can associate a comment, or revert it later ! Don’t be scared by Git-managed merge 

Merge alternatives: rebasePicture courtesyofProGit.org

Git rebase in actionLet’s diverge again between master and experimentalMagic ! … rebase flattens the branching history

Git graph after rebaseExperimental is no more a diverging branchNOTE: Marconi’s test is on “unnamed branch” and experimental branch history has changed !

Merge alternatives: squashoriginC1C2C3C4C5C6C1C2C3C4C3C4C3’+C4’myworkoriginmywork

Git squash in actionGet branch changes but do not join themgit merge –squash experimentalcommit

Git branch graph after squashBranches are still divergingGet all the branch changes in a single commit(squash changes together)

Merge alternatives: cherry-pickoriginC1C2C3C4C5C6C1C2C3C4C5C6C6’myworkoriginmywork

Merge alternatives: cherry-pickGet individual commit IDsApply individual changes to another branch

Git branch graph after cherry-pickIndividual commit has been copied from tesla-testBoth branches are kept

Too easy ?… let’s create some conflict !

Merge conflictSame file changed, same range, two different branches

Resolving conflictGet list of conflicts with git statusDisplay and edit conflicted file

Completing mergeAdd edited file to stageCommit and finalise the merged commit… wasn’t that scary isn’t it ? 

Rebase / cherry-pick conflictSame file changed, same range, two different branches

Don’t panic … unnamed branchCheck in which branch you areCheck files in conflict

Resolve conflict and continue rebaseResolve conflictsContinue rebase

Got lost with commands, merge and conflicts ?

Git is powerful and dangerousGit has full control on historyAmend existing commitsRemove commitsRevert changesBe careful: you could destroy your history !Be even more careful: history revert is unrecoverableSCARY !!!!!

Change existing commitsGit commit support the “amend” option to overwrite committed dataWhat can be amendedFile changes Author / commentDateThe original commit will disappear: amend is NOT revertible (but just amended again)

Amend last commitLet’s display the last commit on master

Amend last commitLet’s change commit content

Git reset: back to the pastGit reset allows to:Put committed data back to the working dir  soft resetRemove completely committed data  hard resetGit reset is NOT revertibleYou want to reset ?Do you really need it ?Do you really want it ?What do you want to achieve ?… and then think again …

Soft resetRemove the last commit and put changes back to workdirHint:master~1 = reference to “one commit before master head”Commit is lost, but you still have the changes in workdir

Hard resetRemove commit and all the changes associatedCommit is lostFOREVER: there is no way to restore the data

Git revertGit revert allows to:Revert the changes and to workdirRevert the changes and create a “reverted commit”Git revert is revertibleRevert = negative commit (eliminates effect of reverted commit)

Display last commitLet’s display the last commit changes on master

Create reverted commitLet’s revert last commit

Now: how to revert the revert ?Reverted commit is a commitUse reset to eliminate the revert operation

Importance of Git tagsWhy using tags ? … yeah, you know it Why is MORE IMPORTANT in Git than in SVN ?Git commit IDs is SHA-1 hashing (WTF $!#@$!%@^!)Tags = reference to a commit (zero payload)Type of Git tagsLightweight tags (simple Git ref to a commit ID)Annotated tags (author, description, signature)

Lightweight tagsLet’s create a lightweight tag… that’s easier to remember than b7dbbe69f0be…. !

Fully annotate tagsFully annotated tags contains meta-data:TimestampAuthor name and e-mailDescriptionCommit IDGPG Digital SignatureCreate your private GPG Key-pair firstGPG Public Key identify your userGPG Private Key is used to sign content (tags, commits)Exchange GPG Public Key with your peers

Create annotated tagLet’s create a fully annotated and signed tagTags are just Git references

Git nature: peer-to-peer distributedGit designed to be distributedGlobal unique IDs for files and commits (SHA-1)Completely disconnected operationsRich set of merging capabilitiesCompression and integrity checkNatural way of using it is peer-to-peer… the Linus way, yeah

Remote Git repositoriesPoints to other’s people repositoryRemote Git servers / locationRemote Git branchesExample: clone GIT source code repository via “remote”git clone git://git.kernel.org/pub/scm/git/git.git

Inspecting remote pointersList of remote Git repositoriesName “origin” refers to remote Git repositoryList of remote Git branches

Push changes to remote Git repositoryAdd pointer to remote Git repositoryPush all local branches to remote Git repository

Getting remote Git repository updatesFetching remote changes with git fetchMerge (or rebase) changes

Getting changes: shortcutsUse git pull for merging with remote changesgit pull = git fetch + git merge… beaware of the branches merge mess !Use git fetch + git rebase with a macro: git updateupdate = !sh -c 'git stash clear && git stash && git fetch origin && git rebase origin/master && git stash pop’git update = git fetch + git rebase… flat and clean history … seems like SVN isn’t it ? 

Git repositories (local/remote) recap

Public Git Server: githubMany choices … but github is the best !Create your SSH Key-pairssh-keygen -trsa -b 2048Create your free acount on: https://github.com/signup/freeCreate your repository on:https://github.com/repositories/newAdd your remotegit remote add origin git@github.com:lucamilanesio/33degree.git

Private Git Server: GitEnterpriseMany choices … but this is the best for FREE Create your SSH Key-pair (optional: you can use HTTP/basic auth, firewall frendly)ssh-keygen -trsa -b 2048Create your free acount on: https://gitent-scm.com/gitent/users/SignUp.gitCreate your repository on:https://gitent-scm.com/gitent/repository/RepositoryCreation.gitAdd your remotegit remote add origin ssh://lmilanesio@gitent-scm.com/git/gitentdevelopment/33degree

3rd choice: make your own !Installed on your network, running on your hardwareNOTE: make daily backups … GIT is dangerous !Gitosis (http://eagain.net/gitweb/?p=gitosis.git)Users / Groups / KeysRepository managementEverything managed with Git Management Repository Gerrit (http://code.google.com/p/gerrit/)Full Web-based InterfaceUsers / Groups integrated with LDAP, OpenID, …Full repository and security managementFull code-review lifecycle management

Everybody fetch/pull from each otherAnnaJohnCathyLinusLukePeterNobody pushes: everybody fetch or pullEvery Git repository has the same importance

How does it work ?Run your own Git server with git daemonOthers can clone and fetch from your repositoryYou see them fetching

Git collaboration: dictator and lieutenants

Developers fetch/pull, dictator pushesgit pushDevelopers pull, Lieutenants integrateDictator get integration branches together: he is the only one that PUSH to GitPicture courtesyofProGit.org

GitHub variantgit pushDevelopers pull from “blessed” and have their own public GitIntegration manager is the DictatorRevolution allowed: developer nominates himself “new dictator”Picture courtesyofProGit.org

Unique central repositoryEverybody can push / pull from shared Git RepositoryCentral repository dies  elections of new repositoryPicture courtesyofProGit.org

Back to centralisation ? WTF ?Does it seems like SVN ? … much more guys Git Democracy vs. SVNHorizontal collaboration between developers (P2P)Continuous branching / merging“promotions” of changes through voting (Gerrit model)Control over integration / releaseDoes it seems like Git is mature for the Enterprise ?

Git workshop 33degree 2011 krakow

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