Version Control with Git

Version Control
with Git

Luigi De Russis
Dipartimento di Automatica e Informatica
Politecnico di Torino
luigi.derussis@polito.it

How do you share and save data?
 “I’m working solo… and I only have one
computer”
What I need:
- backup;
- different saved versions;
- early and frequently saving.

What I can use:
- external hard drives;
- dedicated folder;
- Dropbox folder;
- …

2

 “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?

3

 “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;
- …

4

 “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?

5

 “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;
- …

6

 “I’m working in team”
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?

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?

7

Version Control Systems
Record changes to a file or a set of files over time so that you can
recall specific versions later

Three generations:
1. Local (RCS, SCCS)
2. Centralized (CVS, Subversion, Team
Foundation Server)
NOW
3. Distributed (Git, Mercurial)

8

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

9

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?

10

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

11

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

12

Centralized Version Control

 one central repository
 client-server relationship

Linguaggi e Ambienti Multimediali A 13

Distributed Version Control

 clients and server have the full copy of the repository
◦ local repositories clone a remote repository
 possible to have more than one server

Linguaggi e Ambienti Multimediali A 14

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

15

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

16

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

17

Projects using Git

Perl

18

Getting started with Git
 Standard installations
◦ http://git-scm.com/download
 On Windows, you can also use Git
Extensions
◦ http://code.google.com/p/gitextensions/
 For this course, Git is
◦ included in RailsInstaller
◦ integrated in Aptana Studio 3

19

Git by Example
SOMEWHERE IN THE USA

MARCO, ITALY DAVE, ENGLAND

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.
20

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:
git init --bare myproject.git

to initialize an empty git repository for the project.
21

Git by Example


When the central repository is ready, Dave
clones it on his computer:
git clone http://centralserver.com/myproject.git

22

Git by Example
git clone http://centralserver.com/myproject.git

 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 projectOne

23

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

24

The Staging area

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”
25

The Staging area: an example

STAGING AREA

 Imagine to modify an existing file in the
working copy
 The file is marked as “modified”
26


ADD

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
27


STAGING AREA

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

28

Git by Example


After adding main.rb to the Staging area, Dave
can commit the file to the local repository:

git commit -a -m “initial implementation”

29

Git by Example
git commit -a -m “initial implementation”

 store the current snapshot in the local repository
 -a
◦ also perform an add for modified files
◦ useless at this point
 -m “put a message here”
◦ perform the commit with a log message
◦ useful to know what you have committed
If Dave wants permanently to exclude from version
control some files in the project folder, he can add
them in the .gitignore file
30

Git by Example


Now Dave can push the data to the remote
repository:
git push origin master

where origin is the remote name and master is
the branch name
31

Git by Example
git push origin master

 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
◦ you have to push the branch explicitly the first
time
 Alternative command:
◦ git push --all
 After the first time, you can simply use:
◦ git push

32

Git by Example


After cloning the remote repository, Marco
can pull the data:
git pull

Now Marco has the code!
33

Pull and Fetch in Git
Fetch
remote repository
FETCH

instance to a local one
◦ previously, we called it “pull”

Pull
◦ perform fetch
◦ update the working copy

34

Git by Example


Marco wants to check the log to see the details:
git log

The result will be something like:
commit bcb39bee268a92a6d2930cc8a27ec3402ebecf0d
Author: Dave <dave@email.co.uk>
Date: Wed Mar 28 10:06:13 2012

initial implementation
35

Git by Example


Marco wants to check the log to see the details:
git log

The result will be something like:
commit bcb39bee268a92a6d2930cc8a27ec3402ebecf0d
Author: Dave <dave@email.co.uk>
SHA-1 hash for data integrity
Date: Wed Mar 28 10:06:13 2012

initial implementation
36

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 functionalites”

37

Git by Example


Meanwhile, Dave has been found some bugs
in the code. After fixing them, he commits:
git commit -a -m “bug fixing”

38

Git by Example


Dave tries to push his changes
git push

but something goes wrong:
To http://centralserver.com/myproject
! [rejected] master -> master (non-fast-forward)
error: failed to push some refs to ‘http://centralserver.com/myproject’

39

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

40

Git by Example
 Merge with conflicts
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.

 Git includes both Marco’s code and
Dave’s code with conflict markers to
delimit things
<<<<<<< HEAD
# Marco’s code here
=======
# Dave’s code here
>>>>>>> b77378f6eb0af44468be36a085c3fe06a80e0322
41

Git by Example


After (manually) resolving these conflicts,
Dave can push the changes:
git push

42

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]
 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]
 Change the last commit
◦ git commit --amend
43

Tags and Branches in a nutshell

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

44

 Tags
◦ 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
45

 Branches
◦ 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

46

Hosted Git
 To have (at least) one remote repository
 Alternative: to set up your Git server
 Most popular:
◦ Github https://github.com/
◦ Bitbucket https://bitbucket.org/
◦ Sourceforge http://sourceforge.net/
◦ (Microsoft) Codeplex
http://www.codeplex.com/
◦ Google Code http://code.google.com/

47

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

48

Bitbucket
 Similar to Github
 Less famous 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

49

References
 Git Reference
◦ http://gitref.org/
 Git - the simple guide
◦ http://rogerdudler.github.com/git-guide/
 Git User’s Manual
◦ http://schacon.github.com/git/user-manual.html
 Pro Git (online book)
◦ http://progit.org/book/
 Git Magic (online book)
◦ http://www-cs-students.stanford.edu/~blynn/gitmagic
 Git Screencast
◦ http://gitcasts.com/
 Version Control by Example (online book)
◦ http://www.ericsink.com/vcbe/
50

References
 Learn Git one commit at time
◦ http://gitready.com/
 The Git Parable
◦ http://tom.preston-werner.com/2009/05/19/the-git-
parable.html
 Interactive Git Cheatsheet
◦ http://ndpsoftware.com/git-cheatsheet.html
 A successful Git branching model
◦ http://nvie.com/posts/a-successful-git-branching-model/
 Some Git (graphical) clients
◦ http://guides.beanstalkapp.com/version-
control/clients.html#git-clients
 EGit, Eclipse plugin for Git
◦ http://www.eclipse.org/egit/
51

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/licenses/by-nc-sa/3.0/
52

Version Control with Git

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