What is a Version Control System? What is Git?

1. MILANO 1863
POLITECNICO
Version Control System
REASONS AND GIT
Carlo Bernaschina – carlo.bernaschina@polimi.it

2. Our day to day work can be described as follow:
• Create
• Save
• Edit
• Save again
• …
Background
DEVELOPMENT CYCLE

3. Generally we do not work alone.
This process is shared between peoples (developers).
Background
COLLABORATION
Donald:
• Create
• Save
• Edit
• Save again
• …
Emily:
• Create
• Save
• Edit
• Save again
• …

4. Collisions
We both changed the same file, which one should we
use?
Journaling
Who changed What? When? Why?
The Problem
COLLISIONS & JOURNALING

5. A Version Control System is not “just” a backup.
It is a tool that enables:
• Parallel Development
• Journaling
• Collaboration
The Solution
VERSION CONTROL SYSTEM

6. • CVS
• Centralized (Client/Server)
• Subversion “CVS Made Right”
• Atomic operations
• Git
• Decentralized
• Mercurial
• Linear History
Alternatives
FREE VERSION CONTROL SYSTEMS

7. Git is a Version Control System.
It was developed by Linus Torvalds in 2005 for the
development of the Linux Kernel.
It was originally meant to replace BitKeeper.
Git
A FREE AND OPEN SOURCE ALTERNATIVE
not actually directed to BitKeeper

8. • Take Concurrent Versions System (CVS) as an
example of what not to do; if in doubt, make the
exact opposite decision
• Support a distributed, BitKeeper-like workflow
• Include very strong safeguards against corruption,
either accidental or malicious
Linus Torvalds
The Original Idea
SOME GUIDING RULES

9. To create a repository you just need to type:
git init
it creates a repository inside the current folder.
Or:
git init <directory>
it creates a repository inside a specific directory.
git init
HOW DO I CREATE A REPOSITORY?

10. Let’s Git know who we are. Using:
git config user.name “<your name>"
git config user.email “<your email>"
These information will be attached to each commit
that we are going to do (journaling).
git config
WHO AM I?

11. Once we have produced some content let’s Git know
what do we want to save (stage for commit).
git add <file>
will stage the specified file for commit.
git add -A
will stage All the files for commit.
git add
WHAT AM I SAVING?

12. If you want to have a list of the currently
changed/staged files you can list them.
git status
Files can be in different states:
• at revision (no change since last commit)
• changed (added, edited or deleted)
• staged (changed and listed for commit)
git status
WHAT DID I DO?

13. Once we have staged all the files it is time so actually
commit the changes.
git commit
will open a command line editor and let you specify a
commit message.
git commit -m "<a meaningful commit message>"
the same but easier.
git commit
WHY DID I DO THAT?

14. Git stores your history of commits.
You move inside the history and start new parallel
development flows called branches.
History
WHAT HAPPENED?

15. You can always restore the last stored version of a file.
git checkout <file name>
All the local edits will be overwritten.
git checkout
HOW DO I COME BACK?

16. If you want to preserve the current status of the
repository you can create a new branch (a separate
development flow).
git checkout -b <new branch name>
It will clone the current status of the repository in a new
branch.
git checkout
HOW DO I START DEVELOPING A NEW FEATURE?

17. If you want to move between the available branches
you can checkout the one you want to work on.
git checkout <branch name>
It will set the selected branch as active.
git checkout
HOW DO I MOVE BETWEEN BRANCHES?

18. If you want to list all the available branches.
git branch
git branch
HOW DO I LIST MY BRANCHES?

19. If you want to integrate the changes done in a branch
into another.
Move to the branch you want to merge to and merge
them.
git checkout <target branch>
git merge <source branch>
git merge
HOW DO I INTEGRATE TWO BRANCHES?

20. If both the branches have a commit that changes the same
content it will generate a conflict.
the number of planets are
<<<<<<< HEAD
nine
=======
Eight
>>>>>>> branch-a
Manually fix them and then commit.
Conflicts
HOW DOES IT MERGE CONFLICTING CONTENT?

21. If you want to remove branches, just delete them.
git branch -d <branch name>
git branch -d
HOW DO I CLEAN UP?

22. We just worked with local repositories right now.
How can we collaborate.
Git is decentralized, the copies of the repositories on different
machines describe a graph.
A way to have a “central” copy of the repository that is
accessible to everyone without the need of particular setups is
using sites like:
• github.com
• gitlab.com
Distributed
HOW DO WE COLLABORATE?

23. If you want to copy a remote repository just clone it.
git clone <address of the repository>
It will copy the remote repository on the local machine.
git clone
HOW DO I COPY A REMOTE REPOSITORY?

24. If you want keep your clone updated you can pull the
changes from the remote one.
git pull
It will merge the remote changes inside the local
repository.
git pull <remote> <branch>
git pull
HOW DO I UPDATE MY COPY?

25. If you need/want to maintain sequentiality in the
commits adding your local changes after the others
you can ask the merge utility to apply them after the
others.
git pull --rebase
You will be asked to confirm some of your commits.
git pull --rebase <remote> <branch>
git pull --rebase
HOW DO I KEEP JOURNALING SEQUENTIAL?

26. If you want to send your local commits to a remote
repository you can push them.
git push
It will copy the local changes on the remote repository.
It requires the last commit in the remote repository to
be known in the local one.
git push <remote> <branch>
git push
HOW DO I UPLOAD MY CHANGES?

27. Do you want to learn more?
https://try.github.io
Let’s learn
ONLINE LEARNING TOOLS

28. • https://git-scm.com
• http://www.catb.org/esr/writings/version-
control/version-control.html
• https://www.youtube.com/watch?v=4XpnKHJAok8
• http://marc.info/?l=git&m=118143549107708
Reference