Git

Git
A distributed version control system
Oct 8, 2013

Version control systems
 Version control (or revision control, or source control) is all
about managing multiple versions of documents, programs, web
sites, etc.
 Almost all “real” projects use some kind of version control
 Essential for team projects, but also very useful for individual projects
 Some well-known version control systems are CVS, Subversion,
Mercurial, and Git
 CVS and Subversion use a “central” repository; users “check out” files,
work on them, and “check them in”
 Mercurial and Git treat all repositories as equal
 Distributed systems like Mercurial and Git are newer and are
gradually replacing centralized systems like CVS and Subversion
2

Why version control?
 For working by yourself:
 Gives you a “time machine” for going back to earlier versions
 Gives you great support for different versions (standalone,
web app, etc.) of the same basic project
 For working with others:
 Greatly simplifies concurrent work, merging changes
 For getting an internship or job:
 Any company with a clue uses some kind of version control
 Companies without a clue are bad places to work
3

Why Git?
 Git has many advantages over earlier systems such as
CVS and Subversion
 More efficient, better workflow, etc.
 See the literature for an extensive list of reasons
 Of course, there are always those who disagree
 Best competitor: Mercurial
 I like Mercurial better
 Same concepts, slightly simpler to use
 In my (very limited) experience, the Eclipse plugin is easier to
install and use
 Much less popular than Git
4

Download and install Git
 There are online materials that are better than any that I could
provide
 Here’s the standard one:
http://git-scm.com/downloads
 Here’s one from StackExchange:
http://stackoverflow.com/questions/315911/git-for-beginners-the-
definitive-practical-guide#323764
 Note: Git is primarily a command-line tool
 I prefer GUIs over command-line tools, but…
 The GIT GUIs are more trouble than they are worth (YMMV)
5

Introduce yourself to Git
 Enter these lines (with appropriate changes):
 git config --global user.name "John Smith"
 git config --global user.email jsmith@seas.upenn.edu
 You only need to do this once
 If you want to use a different name/email address for a
particular project, you can change it for just that project
 cd to the project directory
 Use the above commands, but leave out the --global
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Create and fill a repository
1. cd to the project directory you want to use
2. Type in git init
 This creates the repository (a directory named .git)
 You seldom (if ever) need to look inside this directory
1. Type in git add .
 The period at the end is part of this command!
 Period means “this directory”
 This adds all your current files to the repository
1. Type in git commit –m "Initial commit"
 You can use a different commit message, if you like
7

Clone a repository from elsewhere
 git clone URL
 git clone URL mypath
 These make an exact copy of the repository at the given URL
 git clone git://github.com/rest_of_path/file.git
 Github is the most popular (free) public repository
 All repositories are equal
 But you can treat some particular repository (such as one on Github) as
the “master” directory
 Typically, each team member works in his/her own repository,
and “merges” with other repositories as appropriate
8

The repository
 Your top-level working directory contains everything about
your project
 The working directory probably contains many subdirectories—source
code, binaries, documentation, data files, etc.
 One of these subdirectories, named .git, is your repository
 At any time, you can take a “snapshot” of everything (or selected
things) in your project directory, and put it in your repository
 This “snapshot” is called a commit object
 The commit object contains (1) a set of files, (2) references to the
“parents” of the commit object, and (3) a unique “SHA1” name
 Commit objects do not require huge amounts of memory
 You can work as much as you like in your working directory, but
the repository isn’t updated until you commit something
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init and the .git repository
 When you said git init in your project directory, or
when you cloned an existing project, you created a
repository
 The repository is a subdirectory named .git containing
various files
 The dot indicates a “hidden” directory
 You do not work directly with the contents of that directory;
various git commands do that for you
 You do need a basic understanding of what is in the repository
10

Making commits
 You do your work in your project directory, as usual
 If you create new files and/or folders, they are not tracked by Git unless you
ask it to do so
 git add newFile1 newFolder1 newFolder2 newFile2
 Committing makes a “snapshot” of everything being tracked into your
repository
 A message telling what you have done is required
 git commit –m “Uncrevulated the conundrum bar”
 git commit

This version opens an editor for you the enter the message

To finish, save and quit the editor
 Format of the commit message
 One line containing the complete summary
 If more than one line, the second line must be blank
11

Commits and graphs
 A commit is when you tell git that a change (or addition) you
have made is ready to be included in the project
 When you commit your change to git, it creates a commit object
 A commit object represents the complete state of the project, including all
the files in the project
 The very first commit object has no “parents”
 Usually, you take some commit object, make some changes, and create a
new commit object; the original commit object is the parent of the new
commit object

Hence, most commit objects have a single parent
 You can also merge two commit objects to form a new one

The new commit object has two parents
 Hence, commit objects form a directed graph
 Git is all about using and manipulating this graph
12

Working with your own repository
 A head is a reference to a commit object
 The “current head” is called HEAD (all caps)
 Usually, you will take HEAD (the current commit object), make
some changes to it, and commit the changes, creating a new
current commit object
 This results in a linear graph: A  B  C  … HEAD
 You can also take any previous commit object, make changes to
it, and commit those changes
 This creates a branch in the graph of commit objects
 You can merge any previous commit objects
 This joins branches in the commit graph
13

Commit messages
 In git, “Commits are cheap.” Do them often.
 When you commit, you must provide a one-line
message stating what you have done
 Terrible message: “Fixed a bunch of things”
 Better message: “Corrected the calculation of median scores”
 Commit messages can be very helpful, to yourself as
well as to your team members
 You can’t say much in one line, so commit often
14

Choose an editor
 When you “commit,” git will require you to type in a
commit message
 For longer commit messages, you will use an editor
 The default editor is probably vim
 To change the default editor:
 git config --global core.editor /path/to/editor
 You may also want to turn on colors:
 git config --global color.ui auto
15

Working with others
 All repositories are equal, but it is convenient to have one central
repository in the cloud
 Here’s what you normally do:
 Download the current HEAD from the central repository
 Make your changes
 Commit your changes to your local repository
 Check to make sure someone else on your team hasn’t updated the central
repository since you got it
 Upload your changes to the central repository
 If the central repository has changed since you got it:
 It is your responsibility to merge your two versions

This is a strong incentive to commit and upload often!
 Git can often do this for you, if there aren’t incompatible changes
16

Typical workflow
 git pull remote_repository
 Get changes from a remote repository and merge them into
your own repository
 git status
 See what Git thinks is going on
 Use this frequently!
 Work on your files (remember to add any new ones)
 git commit –m “What I did”
 git push
17

Multiple versions
18
Initial commit
Second commit
Third commit
Bob gets a copy
Fourth commit
Merge
Bob’s commit

Keeping it simple
 If you:
 Make sure you are current with the central repository
 Make some improvements to your code
 Update the central repository before anyone else does
 Then you don’t have to worry about resolving conflicts or
working with multiple branches
 All the complexity in git comes from dealing with these
 Therefore:
 Make sure you are up-to-date before starting to work
 Commit and update the central repository frequently
 If you need help: https://help.github.com/
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The End
20
When I say I hate CVS with a passion, I have to also
say that if there are any SVN [Subversion] users in
the audience, you might want to leave. Because my
hatred of CVS has meant that I see Subversion as
being the most pointless project ever started. The
slogan of Subversion for a while was "CVS done
right", or something like that, and if you start with
that kind of slogan, there's nowhere you can go.
There is no way to do CVS right.
--Linus Torvalds, as quoted in Wikipedia

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