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Me: Dr James Hetherington -- UCL Research Software Development Team --

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1 Me: Dr James Hetherington -- UCL Research Software Development Team -- -- programming

2 Version Control and Issue Tracking Managing code inventory –“When did I introduce this bug?” –Undoing mistakes Working with other programmers –How can I merge my work with Jim’s? What’s the most important bug to fix next?

3 What is version control? (Solo version) Do some programming > my_vcs commit Program some more –Realise mistake > my_vcs rollback –Mistake is undone Syntax here is example only!

4 What is version control? (team version) … wait … Join the team > my_vcs checkout do some programming > my_vcs commit Do some programming … more programming… > my_vcs commit … more programming … > my_vcs commit Error again… Create some code > my_vcs commit …wait… >my_vcs update Do some programming … program some more > my_vcs commit Oh Noes! Error message! > my_vcs update > my_vcs merge > my_vcs commit More programming… Sue Jim

5 Centralised VCS concepts There is one, linear history of changes on the server or repository Each revision has a unique identifier You have a working copy You update the working copy to match the state of the repository You commit your changes to the repository If you someone else has changed it you have to resolve conflicts between your changes and the repository, and then commit

6 Centralised VCS Server With All Committed Versions Client At v4 Client At v4+ Client At v3

7 Centralised VCS solo workflow svn checkout vim svn commit touch mynewfile.yml svn add mynewfile.yml vim mynewfile.yml svn commit Commands for this in Subversion: Time

8 Centralised VCS Team workflow: no conflicts Jim’s commands: svn checkout vim svn commit Sue’s commands: svn co svn update cat # Sue can see changes vim svn commit

9 Centralised VCS with conflicts Jim’s commands: svn update vim svn commit Sue’s commands: svn up vim svn commit svn: Out of date: ’’ svn up vim svn ci

10 Resolving conflicts On update, you get a prompt like: > svn update Conflict discovered in ’'. Select: (p) postpone, (df) diff-full, (e) edit, (mc) mine-conflict, (tc) theirs-conflict, (s) show all options: If you choose (e) or (p) the conflicted file will look something like: > cat previous content <<<<<<<.mine Sue’s content ======= Jim’s content >>>>>>>.r4 Previous content You edit to fix this, then save.

11 Revisiting history You can update to a particular revision –svn up -r 3 You can see the differences between your working area and a revision –svn diff (to current repository most recent version) –svn diff –r 3 You can see which files you’ve changed or added –svn status You can get rid of changes to a file –svn revert

12 Distributed and Centralized Version Control Centralized: –Some server contains the remote version –Your computer has your copy –To switch back to an old copy you need the internet –E.g. cvs, subversion (svn) Distributed: –Every user has a version of the full history –Users can synchronize their history with each other –Having a central “master” copy is a policy option Most groups do this –E.g. git, mercurial (hg), bazaar (bzr)

13 Distributed VCS In principle: Master copy (with v0,1,2,4,5) Sue’s copy (with v0,1,2,3) Phil’s copy (with v0,1,2,4,5,6) Jim’s copy (with v0,1,2,4,5) In practice:

14 Pragmatic distributed VCS Subversion svn checkout svn commit svn up svn status svn diff Git git clone git commit -a git push git pull git status git diff

15 Why go distributed? Easy to start a repository (no server needed) Easy to start a server Can work without an internet connection Better merges Easy branching More widespread support Why not go distributed? More complex commands Easier to get confused!

16 Distributed VCS concepts (1) Each revision has a parent that it is based on These revisions form a graph The most recent in each copy is the head or tip Each revision has a globally unique hash-code –E.g. in Sue’s copy revision 43 is ab3578d6 –Jim thinks that is revision 38 When you bring in information from a remote the histories might conflict –Histories from different copies are merged together

17 Distributed VCS concepts (2) You have a working copy You pick a subset of the files in your working copy to add to the next commit: these go into the staging area or index When you commit, you commit: –from the staging area –to the local repository You push to remote repositories to share or publish your changes You pull or fetch to bring in from a remote


19 Distributed VCS solo workflow Create a file git init git add. git commit create a new file and edit git add git commit Only changes to file2 get into commit Edit both files git commit -a Commands for this in Git:

20 Distributed VCS solo with publishing git clone Edit a few files git add --update git commit git push Edit a few files git commit -a git push Commands for this in Git:

21 Distributed VCS Team workflow: no conflicts Jim’s commands: git init git add mycode git commit git remote add --track master origin git push git fetch git merge git commit –a git push Sue’s commands: git clone git commit –a git push git pull

22 Distributed team workflow with conflicts Jim’s commands: git commit -a git push Error: ! [rejected] git pull git commit –a git push Sue’s commands: git commit –a git push git commit –a git push git commit -a git commit –a git pull

23 Really distributed: more than one remote git remote add sue ssh:// add a second remote git remote list available remotes git push sue git push origin push to a specific remote

24 Working with branches

25 Working with branches in git > git branch * master > git branch experiment > git branch * master experiment > git checkout experiment > git branch master * experiment

26 Sharing branches in git git push origin experiment publish the branch to remote git push -u origin experiment publish the branch to remote(first time) git branch -r discover branches on remote(s) git checkout origin/experiment get a new branch from a remote

27 Merging and deleting branches git checkout master switch back to master branch git merge experiment take all the changes from experiment into master exactly like merging someone else’s work git branch -d experiment the experiment is done, get rid of local branch git push --delete experiment git rid of the branch on the remote

28 Working with branches You should have a development branch and a stable branch You should create temporary branches for experimental changes If you release code to others, you should make a release branch –Then you can make fixes to bugs they find –And control which of your work goes in the release


30 Tagging You should tag working versions You should produce real science only with specific tagged versions, and note which one

31 Tagging git tag –a v1.3 add a tag, labelling last commit git tag –a v1.3 ab48dc tag an old commit git push --tags publish the tags to origin

32 Branching and tagging in subversion You can do branches and tags in subversion too –But it’s harder –svn doesn’t have real branches or tags, instead you make copies of code inside the repo –and you can merge between the copies –It works, but it’s cleaner in git –see subversion references if you need this

33 More comparisons Subversion svn up –r54 svn revert svn revert –depth=infinity. Git git checkout –r ab39d git checkout git reset –hard But git can do more, e.g.: git reset HEAD^ will undo the last commit from your local repository (providing you haven’t pushed) Both git and svn have many options – have a look on the web!

34 Other version control systems vcs –Really old! –Works by “locking” files instead of resolving conflicts cvs –Very like svn hg –“mercurial” –Very like git

35 Hosting a server In git, any repository can be a remote for pulls –Just use git pull ssh://theircomputer/theirrepo –There are problems with pushing to someone else’s working repo: don’t! –You can, however, create a git repo with git init --bare –This bare repo has no working directory, –use it as a remote for push and pull via ssh:// In subversion, the procedure is more complicated –You have to configure a server ‘daemon’

36 Hosting a server in the cloud There are many services which allow you to create git, mercurial, and subversion repositories online –Typically free for open source –Typically a fee for private repositories I recommend GitHub –Create an account at –Students can get five free private repositories at –Can interact with GitHub repositories as either svn or git Bitbucket is also good

37 Working with GitHub

38 Set up ssh keys

39 Create repository

40 Social coding



43 Browse changes


45 Comment on and discuss code

46 Issue tracking Your code has bugs (defects) Your code has things you want to do (enhancements) The best way to keep track of all this is with an issue tracker

47 Issue tracking

48 Anatomy of an issue Type –Defect, enhancement, task Severity –Critical, blocker, major, minor, trivial Owner Status –Open, fixed, duplicate, blocked, under review, won’t fix, invalid, new… Estimated time and time spent? Tags

49 Timeline of an issue

50 Some questions Public or private issues Organising issues into milestones Estimate effort? Who can close an issue? Review processes Integration with version control

51 Some issue trackers Trac Redmine Jira GitHub’s issue tracker Bitbucket’s issue tracker

52 Issues on GitHub

53 The Pull Request


55 Conclusions Tools can make your development easier, safer, more reliable, more correct, and more collaborative They can be complicated and take time to learn Learn by practicing –Use the tools –Pick an open source project on github or bitbucket and start contributing

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