Version Control with Git

Git
AN INTRODUCTION
Introduction to Git as a version control system:
concepts, main features and practical aspects.

How do you share and save data?
• “I’m working solo… and I only have one
computer”
3/13/2015 Version Control with Git 2
What I need:
- backup;
- different saved versions;
- early and frequently
saving.
What I can use:
- external hard drives;
- dedicated folder;
- Dropbox folder;
- …

• “I’m working solo… and I only have one
computer”
What if…
- … I forget to save a specific version
and then I need it?
- … I delete/loose a previous version?

• “I’m working solo… and I have more than one
computer”
What I need:
- backup;
- early and frequently saving;
- conventions on file names.
What I can use:
- USB memory sticks;
- Dropbox folder;
- shared folder;
- …

• “I’m working solo… and I have more than one
computer”
What if…
- … I forget to save a specific version
and then I need it?
- … I delete/loose a previous version?
- … I have different projects in the
“shared” workspace?
- … I forget to copy one version
between computers?

• “I’m working in team”
What I need:
- backup;
- early and frequently saving;
- shared conventions on file
names.
What I can use:
- USB memory sticks;
- Dropbox folder;
- e-mails;
- …

• “I’m working in team”
Other issues:
- who has the latest version?
- who has the right to edit?
- how to ensure that everyone
sees up-to-date versions of
everything?
- how to handle conflicts?
What if…
- … a team member forgets to save a
specific version and then someone
else needs it?
- … someone deletes/looses a version?
- … someone forgets to share a specific
version of the projects?

Version Control Systems
Three generations:
1. Local (RCS, SCCS)
2. Centralized (CVS, Subversion, Team Foundation
Server)
3. Distributed (Git, Mercurial)
Record changes to a file or a set of files over time so that
you can recall specific versions later
NOW

Basic Concepts
Repository
– place where you store all your
work
– contains every version of your work that has ever
existed
• files
• directories layout
• history
– can be shared by the whole team

Basic Concepts
Working copy
– a snapshot of the repository used
for… working
– the place where changes happens
– private, not shared by the team
– it also contains some metadata so that it can keep
track of the state of things
• has a file been modified?
• is this file new?
• has a file been deleted?

Basic Concepts
Commit
– the operation that
modifies the repository
– atomically performed by modern version control
tools
• the integrity of the repository is assured
– it is typical to provide a log message (or comment)
when you commit
• to explain the changes you have made
• the message becomes part of the history of the repository

Basic Concepts
Update
– update the working copy
with respect to the
repository
• apply changes from the repository
• merge such changes with the ones you have made to your
working copy, if necessary

Centralized Version Control
• one central repository
• client-server relationship

Distributed Version Control
• clients and server have the full copy of the repository
– local repositories clone a remote repository
• it is possible to have more than one server

More Basic Concepts
Push
– copy changesets from a
local repository instance
to a remote one
• synchronization between two repository instances

More Basic Concepts
Pull
remote repository
instance to a local one
• synchronization between two repository instances

Introducing… Git
• Distributed Version Control System
• Born
– on 2005 for the Linux kernel project
– to be used via command line
• Website: http://git-scm.com
• Highlights:
– free and open source
– strong support for non-linear development
– fully distributed
– efficient handling of large projects
– cryptographic authentication of history

Who uses Git?

Getting started with Git
• Standard installations
– http://git-scm.com/downloads
• Available for all the platform
• Git Graphical Applications
– http://git-scm.com/downloads/guis
• For this course, Git is
– included in RailsInstaller
– integrated in RubyMine

Installing Git
• Windows
– download and install Git from http://git-
scm.com/downloads
• Linux
– check if it is already installed
• open a terminal and type “git”
– otherwise, install it from your package manager or via
http://git-scm.com/downloads
• Mac
– check if it is already installed
• open a terminal and type “git”
– otherwise, install it from http://git-scm.com/downloads

Git by Example
Marco and Dave work for the same company, but
in two different countries.
They have to realize a new software project, so
they decided to make it in Ruby and to use Git for
version control.
SOMEWHERE IN THE USA
MARCO, ITALY DAVE, ENGLAND

Git by Example
Marco starts the project by creating a new Git
repository on the central server.
He goes into the project directory and types:
to initialize an empty Git repository for the project.
git init --bare myproject.git

Git by Example
When the central repository is ready, Dave create
a folder named myproject on his computer and
initializes a repository in it:
git init

Git by Example
• initializes an empty Git repository inside an
existing folder
• creates a .git directory inside it
• without parameters, typically
– on the server, it is used with the --bare parameter
git init

Git by Example
Dave writes some code in the myproject folder.
Before committing, Dave needs to really put the
created file under version control, by adding the
file to the Staging area:
git add main.rb

The Staging Area
• A sort of loading dock
• It can contain things that are neither in the working
copy nor in the repository instance
• Also called “index”
STAGING AREA

The Staging Area: an example
• Imagine to modify an existing file in the working
copy
• The file is marked as “modified”
STAGING AREA

• Before committing, the modified file needs to be
“staged”
– i.e., add a snapshot of it in the staging area
• Modified data has been marked in its current version to
go into the next commit snapshot
STAGING AREA
ADD

• Then, changes can be “committed”
– i.e., take the file from the staging area and store
permanently the snapshot in the local repository
STAGING AREA

Git by Example
If Dave wants permanently to exclude from
version control some files in the project folder, he
can add them in the .gitignore file
– such files and folders will not be staged

Git by Example
After adding main.rb to the Staging area, Dave
can commit the file to the local repository:
git commit -m “initial commit”

Git by Example
• store the current snapshot in the local
repository
• -m “put a message here”
– perform the commit with a log message
– useful to know what you have committed
• - a
– a useful parameter
– it performs an add for modified files, too
• useless at this point
git commit -m “initial commit”

Git by Example
Dave is ready to load the data to the remote
repository. He has to specify what is the remote
server to use:
git remote add origin http://centralserver.com/myproject.git

Git by Example
• add a new remote repository
– multiple remotes can exists
• for each remote, a name and an address is
specified
– origin is the “standard” name for indicating the
principal remote
– the address can be in the format http(s):// or git://
• remotes can also be removed, renamed, etc.
git remote add origin http://centralserver.com/myproject.git

Git by Example
Now Dave can push the data to the remote
repository:
where origin is the remote name and master is the
default branch name
git push -u origin master

Git by Example
• Git pushes only to matching branches
– for every branch that exists on the local side, the remote
side is updated if a branch of the same name already
exists there
• this behavior will change with Git 2.0
– you have to push the branch explicitly the first time
• -u
– set other information useful for other commands (e.g.,
pull)
• After the first time, you can simply use:
– git push
git push -u origin master

Git by Example
Marco starts working on the project and clones the
remote repository on his computer:
Now Marco has the code!
git clone http://centralserver.com/myproject.git

Git by Example
• creates a directory named myproject
• initializes a .git directory inside it
• pulls down all the data for that repository
• checks out a working copy of the latest version
If you want to clone the repository into a
directory with another name, you can specify that
as:
git clone http://centralserver.com/myproject.git
git clone http://centralserver.com/myproject.git first_project

Git by Example
Marco wants to see the details of what Dave did:
The result will be something like:
git log
commit bcb39bee268a92a6d2930cc8a27ec3402ebecf0d
Author: Dave <dave@email.co.uk>
Date: Wed Mar 12 10:06:13 2015
initial commit

Git by Example
Marco wants to see the details of what Dave did:
The result will be something like:
git log
commit bcb39bee268a92a6d2930cc8a27ec3402ebecf0d
Author: Dave <dave@email.co.uk>
Date: Wed Mar 12 10:06:13 2015
initial commit
bcb39bee268a92a6d2930cc8a27ec3402ebecf0d
SHA-1 hash for data integrity

Git by Example
• At this point, Marco edits the source code and
saves
• To see the pending changes, he can use:
– git status
• To see the difference between his version and
the previous one, he can use:
– git diff (--cached, to include staged files)
• Marco decides to commit and to push his work
git commit -a -m “added new functionalities”
git push

Git by Example
Meanwhile, Dave found some bugs in the code.
He looks for update on the central server and get
it (if any):
git pull

Pull and Fetch in Git
Fetch
remote repository
instance to a local one
– previously, we called it “pull”
Pull
– perform fetch
– update the working copy

Git by Example
However, no new data is available since Marco
has not yet pushed his changes.
So, Dave fixes the bugs, and commits:
git commit -a -m “bug fixing”

Git by Example
After some time, Dave tries to push his changes
but something goes wrong
git push
To http://centralserver.com/myproject
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to ‘http://centralserver.com/myproject’

Git by Example
• What happens?
– Git is not allowing Dave to push his changes because
Marco has already pushed something to the master
branch
• Solution:
– Dave has to do a pull, to bring in changes before
pushing his modifications
• Two possible scenarios:
– merging of files goes smoothly;
– merging of files generates conflicts.

Git by Example
• Merge with conflicts
• Git includes both Marco’s code and Dave’s code
with conflict markers to delimit things
From http://centralserver.com/myproject
b19f36c..b77378f master -> origin/master
Auto-merging main.rb
CONFLICT (content): Merge conflict in main.rb
Automatic merge failed; fix conflicts and then commit the result.
<<<<<<< HEAD
# Marco’s code here
=======
# Dave’s code here
>>>>>>> b77378f6eb0af44468be36a085c3fe06a80e0322

Git by Example
After (manually) resolving these conflicts, Dave is
able to push the changes:
git push

Other Useful Commands
• Operations on files
– Remove: git rm [filename]
– Move/rename: git mv [file-from] [file-to]
– Unstage some staged files: git reset HEAD [filename-
list]
– Unmodify a modified file: git checkout -- [filename]
• Change the last commit
– git commit --amend

Other Useful Commands
• Operations on remotes
– List: git remote (-v, to show the URLs)
– Add: git remote add [shortname] [url]
– Inspect: git remote show [remote-name]
– Rename: git remote rename [old-name] [new-name]
– Remove: git remote rm [remote-name]

Tags and Branches in a Nutshell
• Local and remote
• Do not push automatically
[Image from http://nvie.com/posts/a-successful-git-branching-model/]

Tags… in brief
• useful to mark release points
• two types:
– lightweight
– annotated (more complete)
• commands:
– git tag, shows the available existing tags
– git tag [tag-name], creates a lightweight tag
– git tag -a [tag-name] -m [message]‚ creates an
annotated tag
– tag show [tag-name], shows the tag data

Branches… in brief
• used to develop features isolated from each other
• the master branch is the “default” branch when you
create a repository
– you should use other branches for development and
merge them back to the master branch upon completion
• really lightweight in Git
• commands:
– git branch [branch-name], create a new branch
– git branch, lists all existing branches
– git checkout [branch-name], switches to the selected
branch
– git branch -d [branch-name], removes the selected branch

Hosted Git
• To have (at least) one remote repository
– alternative: set up your own Git server!
• Most popular:
– GitHub, https://github.com/
– Bitbucket, https://bitbucket.org/
– Sourceforge, http://sourceforge.net/
– CodePlex (by Microsoft), https://www.codeplex.com/

GitHub
• Slightly different than other code-hosting sites
– instead of being primarily based on the project, it is
user-centric
– social coding
• A commercial company
– charges for accounts that maintain private repository
– free account to host as many open source project as
you want
– free Micro plan for students
• 5 private repositories, unlimited public repositories
• https://education.github.com

Bitbucket
• Similar to GitHub
• Less used than GitHub, right now
• Mercurial support
• A commercial company
– free private and public repositories for small team
(up to 5 private collaborators)
– charges for project involving bigger team
– free for academia (also for students)
• unlimited public and private repositories
• unlimited users for single projects

GitHub Pages
• Website for your (GitHub) repository
– https://pages.github.com/
• We will use it for hosting your project
documentation
– everything realized in Modulo B
– your website will be reachable from
http://lam-2015.github.io/your-project-name
• FAQ
– https://help.github.com/categories/github-pages-
basics/

Homework(s)
• Create a personal GitHub account
– you can also ask for a “student discount” at
https://education.github.com
– we will require your username to set up your team
repositories for final projects
• Try Git!
– http://try.github.io/
– 15 minutes tutorial

References
• Git Reference
– http://gitref.org/
• Git - the simple guide
– http://rogerdudler.github.io/git-guide/
• Git Documentation
– http://git-scm.com/docs
• Pro Git (online book)
– http://git-scm.com/book
• Version Control by Example (online book)
– http://www.ericsink.com/vcbe/

References
• Try Git!
– http://try.github.io/
• Various Git resources
– https://help.github.com/articles/what-are-other-good-
resources-for-learning-git-and-github
• A successful Git branching model
– http://nvie.com/posts/a-successful-git-branching-model/
• Some Git (graphical) clients
– http://git-scm.com/downloads/guis
• EGit, Eclipse plugin for Git
– http://www.eclipse.org/egit/

Questions?
02NPYPD LINGUAGGI E AMBIENTI MULTIMEDIALI
Luigi De Russis
luigi.derussis@polito.it

License
• This work is licensed under the Creative Commons “Attribution-
NonCommercial-ShareAlike Unported (CC BY-NC-SA 3,0)” License.
• You are free:
– to Share - to copy, distribute and transmit the work
– to Remix - to adapt the work
• Under the following conditions:
– Attribution - You must attribute the work in the manner specified by the
author or licensor (but not in any way that suggests that they endorse you
or your use of the work).
– Noncommercial - You may not use this work for commercial purposes.
– Share Alike - If you alter, transform, or build upon this work, you may
distribute the resulting work only under the same or similar license to this
one.
• To view a copy of this license, visit
http://creativecommons.org/license/by-nc-sa/3.0/

Version Control with Git

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