CSE 391 Lecture 9 - courses.cs.washington.edu

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CSE 391Lecture 9

VersioncontrolwithGit

slidescreatedbyRuthAnderson,MartyStepp,BrettWortzman andZorah Fungimagesfromhttp://git-scm.com/book/en/http://www.cs.washington.edu/391/

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Problems Working Alone• Everdoneoneofthefollowing?

§ Hadcodethatworked,madeabunchofchangesandsavedit,whichbrokethecode,andnowyoujustwanttheworkingversionback…

§ Accidentallydeletedacriticalfile,hundredsoflinesofcodegone…§ Somehowmessedupthestructure/contentsofyourcodebase,andwanttojust“undo”thecrazyactionyoujustdid

§ Harddrivecrash!!!!Everything’s gone,thedaybeforedeadline.

• Possibleoptions:§ Saveas(MyClass-v1.java)

• Ugh.Justugh.Andnowasinglelinechangeresults induplicatingtheentirefile…

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Problems Working in teams• Whosecomputerstores the"official" copyoftheproject?

§ Canwestoretheprojectfilesinaneutral"official"location?

• Willwebeabletoread/writeeachother'schanges?§ Dowehavetherightfilepermissions?§ Letsjustemailchangedfilesbackandforth!Yay!

• Whathappensifwebothtrytoeditthesamefile?§ Billjustoverwrote afileIworkedonfor6hours!

• Whathappensifwemakeamistakeandcorruptanimportantfile?§ Isthereawaytokeepbackupsofourprojectfiles?

• HowdoIknowwhatcodeeachteammateisworkingon?

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Solution: Version Control• versioncontrolsystem:Softwarethattracksandmanageschangestoasetoffilesandresources.

• Youuseversioncontrolallthetime§ Builtintowordprocessors/spreadsheets/presentation software

• Themagical“undo”buttontakesyoubackto“theversionbeforemylastaction”

§ Wikis• Wikisareallaboutversioncontrol,managingupdates,andallowingrollbackstopreviousversions

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Software Version Control• Manyversioncontrolsystemsaredesignedandusedespeciallyforsoftwareengineeringprojects§ examples:CVS,Subversion(SVN),Git,Monotone,BitKeeper,Perforce

• helpsteamstoworktogetheroncodeprojects§ asharedcopyofallcodefilesthatalluserscanaccess§ keepscurrentversionsofallfiles,andbackupsofpastversions§ canseewhatfilesothershavemodifiedandviewthechanges§ managesconflictswhenmultipleusersmodifythesamefile

§ notparticulartosourcecode;canbeusedforpapers,photos,etc.• butoftenworksbestwithplaintext/codefiles

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Repositories• Repository(aka“repo”):alocationstoringacopyofallfiles.

§ youdon'teditfilesdirectlyintherepo;§ youeditalocalworkingcopyor“workingtree”§ thenyoucommit youreditedfilesintotherepo

• Theremaybeonlyonerepositorythatallusersshare(CVS,Subversion)

• Oreachusercouldalsohavetheirowncopyoftherepository(Git,Mercurial)

• Filesinyourworkingdirectorymustbeaddedtotherepoinordertobetracked.

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What to put in a Repo?• Everythingneeded tocreateyourproject:

§ Sourcecode(Examples: .java,.c,.h,.cpp )§ Buildfiles(Makefile, build.xml)§ Otherresourcesneeded tobuildyourproject:icons,textetc.

• ThingsgenerallyNOTputinarepo(thesecanbeeasilyre-createdandjusttakeupspace):§ Objectfiles(.o)§ Executables(.exe)§ Machine-specificconfigurationfiles

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Repository Location• Cancreatetherepositoryanywhere

§ Canbeonthesamecomputerthatyou’regoingtoworkon,whichmightbeokforapersonalprojectwhereyoujustwantrollbackprotection

• But,usuallyyouwanttherepositorytoberobust:§ Onacomputerthat’supandrunning24/7

• Everyonealwayshasaccesstotheproject§ Onacomputerthathasaredundantfilesystem(ie RAID)

• Nomoreworriesaboutthatharddiskcrashwipingawayyourproject!

• Options:§ attu,CSEGitLab,GitHub(doNOTuseGitHubforhomework!!!)

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Aside: So what is GitHub?• GitHub.com isasiteforonlinestorageof Git repositories.• Manyopensourceprojectsuseit,suchasthe Linuxkernel.• Youcangetfreespaceforopensourceprojectsoryoucanpayforprivateprojects.

• DoNOTuseGitHubtostoreyourhomework!!Question:DoIhavetouseGitHubtouseGit?Answer:No!• youcanuseGit completely locallyforyourownpurposes,or• youcanusetheCSEGitLab server,or• youcouldsharearepowithusersonthesamefilesystem(e.g.attu)aslongeveryone hastheneeded filepermissions.

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Git

HTTP://XKCD.COM/1597/

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Git Resources• Atthecommandline:(where<verb> = config,add,commit,etc.)

$ git help <verb> $ git <verb> --help $ man git-<verb>

• Freeon-linebook: https://git-scm.com/book/en/v2• Git tutorial:http://schacon.github.com/git/gittutorial.html• ReferencepageforGit:http://gitref.org/index.html• Git website:http://git-scm.com/

• Git forComputerScientists:http://eagain.net/articles/git-for-computer-scientists/

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History of Git• CameoutofLinuxdevelopment community• LinusTorvalds,2005• Initialgoals:

§ Speed§ Supportfornon-lineardevelopment (thousandsofparallelbranches)§ Fullydistributed§ AbletohandlelargeprojectslikeLinuxefficiently

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Git uses a distributed model

Centralized Model Distributed Model

(CVS, Subversion, Perforce) (Git, Mercurial)Result: Many operations are local

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Ways to use Git

Using Git on your own computer, one user

Using Git on multiple computers,multiple users or one user onmultiple computers

Possible servers:• CSE GitLab• attu• GitHub (NOT for homework!)

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Git Operations

Working changes

Changes you’re

prepping to commit

Local copy of the repo with your commitedchanges

Remoteshared

repo

also called “stage”

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Git Operations

git add

git commit

git push

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Basic WorkflowBasicGit workflow:

1. Modify filesinyourworkingdirectory.2. Stage files,addingsnapshotsofthemtoyourstagingarea.3. Doacommit,whichtakesthefilesastheyareinthestagingarea

andstoresthatsnapshotpermanentlytoyourlocalGit directory(yourlocalcopyoftherepo).

• Notes:§ Ifaparticularversionofafileisinyourlocalgit directory,it’sconsideredcommitted.§ Ifit’smodifiedbuthasbeenaddedtothestagingarea,itisstaged.§ Ifitwaschanged since itwascheckedoutbuthasnot beenstaged,itismodified.

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CSE GitLabEveryone inthiscoursehasbeengivenanaccountonCSEGitLabTouseCSEGitLab:1. LogontoCSEGitLab:https://gitlab.cs.washington.edu/

§ IMPORTANT:IfyouhaveaCSENetID usethat,otherwiseuseyourUWNetID

2. Addanssh key:https://gitlab.cs.washington.edu/help/ssh/README.md

§ FollowtheinstructionsinREADME.mdwhichsaytotype:ssh-keygen -t rsa -C "yourUWNetID@uw.edu" -b 4096

• Justhitreturntoacceptthedefaultfilelocationandyoudonotneedapassword so hitreturnbothtimes when prompted forapassword

§ Thentype: cat ~/.ssh/id_rsa.pub

§ Copy-pastethekeytothe'SSHKeys'sectionunder‘ProfileSettings'inyouruserprofileonCSEGitLab.Orgodirectlyhere:https://gitlab.cs.washington.edu/profile/keys

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Get ready to use Git!1. SetthenameandemailforGit tousewhenyoucommit:

$ git config --global user.name “Bugs Bunny”$ git config --global user.email bugs@gmail.com

§ Youcancallgit config –-list toverifytheseareset.§ ThesewillbesetgloballyforallGit projectsyouworkwith.§ Youcansetvariablesonaproject-onlybasisbynot usingthe--global flag.

• Thelatestversionofgit willalsopromptyouthatpush.default isnotset,youcanmakethiswarninggoawaywith:$ git config --global push.default simple

• Youcanalsosettheeditorusedforwritingcommitmessages:$git config --global core.editor emacs (itisvimbydefault)

vim tips: “a” add, “esc” when done adding, “wq:” to save and quitvim editor: http://www.gentoo.org/doc/en/vi-guide.xmlvim ref card: http://tnerual.eriogerg.free.fr/vimqrc.pdf

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Create a local copy of a repo2. Twocommonscenarios:(onlydooneofthese)

a) Tocloneanalreadyexistingrepotoyourcurrentdirectory:$ git clone <url> [local dir name]Thiswillcreateadirectorynamed localdir name,containingaworkingcopyofthefilesfromtherepo,anda.git directorywhichyoucanignore(usedtoholdthestagingareaandyourlocalrepo)

Example:git clone git@gitlab.cs.washington.edu:rea/superTest.git

b) TocreateaGit repoinyourcurrentdirectory:$ git initThiswillcreatea.git directoryinyourcurrentdirectorywhichyoucanignore(usedtoholdthestagingareaandyourlocalrepo).Thenyoucancommitfilesinyourcurrentdirectoryintothelocalrepo:$ git add file1.java$ git commit –m “initial project version”

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Git commandscommand description

git clone url [dir] copyagitrepositorysoyoucanaddtoitgit add files addsfilecontentstothestagingareagit commit recordsasnapshotofthestagingareagit status viewthestatusofyourfilesintheworking

directoryandstagingareagit diff showsdiffofwhatisstagedandwhatis

modifiedbutunstagedgit help [command] gethelpinfoaboutaparticularcommandgit pull fetchfromaremoterepoandtrytomerge

intothecurrentbranchgit push pushyournewbranchesanddatatoa

remoterepositoryothers: init, reset, branch, checkout, merge, log, tag

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Adding & Committing files1. Thefirsttimeweaskafiletobetracked,and everytimebefore

wecommitafilewemustaddittothestagingarea:$ git add README.txt hello.java

Thistakesasnapshotofthesefilesatthispointintimeandaddsittothestagingarea.Note:Tounstage achangeonafilebeforeyouhavecommittedit:$ git reset HEAD filename

2. Tomovestagedchangesintothelocalrepowecommit:$ git commit –m “Fixing bug #22”Note:Youcanedityourmostrecentcommitmessage (ifyouhavenotpushedyourcommityet)using:git commit –-amend

Note:Thesecommandsarejustactingonyourlocalversionofrepo.

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Use Good Commit Messages

HTTP://XKCD.COM/1296/

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Status and Diff• Toviewthestatus ofyourfilesintheworkingdirectoryandstagingarea:

$ git status or$ git status –s

(-s showsashortonelineversion)

• Toseedifference between yourworkingdirectoryandthestagingarea(Thisshowswhatismodifiedbutunstaged):

$ git diff

• Toseedifferencebetweenthestagingareaandyourlocalcopyoftherepo(Thisshowsstagedchanges):(--staged is synonymous)

$ git diff --cached

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After editing a file…$emacs rea.txt$git status#Onbranchmaster#Changesnotstagedforcommit:#(use"git add<file>..."toupdatewhatwillbecommitted)#(use"git checkout-- <file>..."todiscardchangesinworkingdirectory)##modified:rea.txt#nochangesaddedtocommit(use"git add"and/or"git commit-a")$git status-sMrea.txt ß Note:Misinsecondcolumn=“workingtree”$git diff ß Showsmodificationsthathavenot beenstaged.diff--git a/rea.txtb/rea.txtindex66b293d..90b65fd100644--- a/rea.txt+++b/rea.txt@@-1,2+1,4@@Here isrea's file.++Onenewlineadded.$git diff--cached ß Showsnothing,nomodificationshavebeenstagedyet.$

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After adding file to staging area…$git addrea.txt$git status#Onbranchmaster#Changestobecommitted:#(use"git resetHEAD<file>..."tounstage)##modified:rea.txt#$git status-sMrea.txt ß Note:Misinfirstcolumn=“stagingarea”$git diff ß Note:Showsnothing,nomodificationsthathavenot beenstaged.$git diff--cached ß Note:Showsstagedmodifications.diff--git a/rea.txtb/rea.txtindex66b293d..90b65fd100644--- a/rea.txt+++b/rea.txt@@-1,2+1,4@@Here isrea's file.++Onenewlineadded.

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Viewing logsToseealogofallchangesinyourlocalrepo:• $ git log or• $ git log --oneline (toshowashorterversion)

1677b2dEditedfirstlineofreadme258efa7Addedlinetoreadme0e52da7Initialcommit

• git log -5 (toshowonlythe5mostrecentupdates,etc.)

Note:changeswillbelistedbycommitID #,(SHA-1hash)Note:changesmadetotheremoterepobeforethelasttimeyoucloned/pulledfromitwillalsobeincludedhere

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Pulling and PushingGoodpractice:1. AddandCommit yourchangestoyourlocalrepo2. Pull fromremoterepo togetmostrecentchanges(fixconflictsif

necessary, thenaddandcommitthosechangestoyourlocalrepo)3. Push yourchangestotheremoterepo

Tofetchthemostrecentupdatesfromtheremoterepointoyourlocalrepo,andputthemintoyourworkingdirectory:$ git pull origin masterTopushyourchangesfromyourlocalrepototheremoterepo:$ git push origin masterNotes: origin =analiasfortheURLyouclonedfrom

master =theremotebranchyouarepullingfrom/pushingto,(thelocalbranchyouarepullingto/pushingfromisyourcurrentbranch)

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Avoiding Common ProblemsFromCSE331:http://courses.cs.washington.edu/courses/cse331/18wi/tools/versioncontrol.html#git-pitfalls

• Donotedittherepository(the .git directory)manually.Itwasn'tdesignedformodificationsbyhumans.

• Trynottomakemanydrasticchangesatonce.Instead,makemultiplecommits,eachofwhichhasasinglelogicalpurpose.Thiswillminimizemergeconflicts.Thisisgoodcodingpracticeingeneral.

• Always git pull beforeeditingafile.It'seasytoforgetthis.Ifyouforget,youmayendupeditinganoutdated version,whichcancausenastymergeconflicts.

• Don'tforget git push afteryouhavemadeandcommittedchanges.Theyarenotcopiedtotheremoterepositoryuntilyoudoapush.

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BranchingTocreateabranchcalledexperimental:• $ git branch experimental

Tolistallbranches:(*showswhichoneyouarecurrentlyon)• $ git branch

Toswitchtotheexperimentalbranch:• $ git checkout experimental

Lateron,changesbetween thetwobranchesdiffer,tomergechangesfromexperimental intothemaster:• $ git checkout master• $ git merge experimental

Note:git log --graph canbeusefulforshowingbranches.Note:Thesebranchesareinyourlocalrepo!

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SVN vs. Git• SVN:

§ centralrepositoryapproach– themainrepository istheonly“true”source,onlythemainrepositoryhasthecompletefilehistory

§ Userscheckoutlocalcopiesofthecurrentversion• Git:

§ Distributedrepositoryapproach– every checkoutoftherepository isafullfledgedrepository,completewithhistory

§ Greaterredundancyandspeed§ Branchingandmergingrepositories ismoreheavilyusedasaresult

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Wrap-up• You*will*useversioncontrolsoftwarewhenworkingonprojects,bothhereandinindustry§ Ratherfoolishnotto§ Advice:justsetuparepository,evenforsmallprojects,itwillsaveyoutimeandhassle

• HW9(Git)hasmoredetailsandwalksyouthroughcreatingaGitrepoandaddingtoasharedrepo.