1. Version Control Systems
Introduction to GIT
Version Control Systems

2. Version Control
Version control (a.k.a. revision control) is a system that records changes to a file or set of files over time
so that you can retrieve specific versions later
and manage a collection of files for use by a group project
Version Control can be:
Local
Centralized
Distributed

3. Local
A Local VCS uses a simple database to keep all the changes to files under revision control.

4. Centralized Version Control
Centralized Version Control Systems have a single server that contains all the versioned files, and a number of clients that check out files from that central place.

5. Distributed Version Control
In a distributed VCS, clients don’t just check out the latest snapshot of the files: they fully mirror the repository.

6. Distributed Version Control
GIT is a distributed version control system
A master authoritative server is only used if desired by a project
Every checked out copy is a repository
Version control can be used even when detached from server
Backup of a repository is trivial
Other distributed systems include
Mercurial
BitKeeper
Darcs
Bazaar

7. Git Advantages Resilience Speed Space Simplicity
No one repository has more data than any other
Speed
Very fast operations compared to most VCS
Space
Compression can be done across repository not just per file
Minimizes local size as well as push/pull data transfers
Simplicity
Object model is very simple
Large user base with robust tools
Open source, free

8. Some GIT Disadvantages
Definite learning curve, especially for those used to centralized systems
must pay attention to follow recommended procedures when several people are working on a project
Documentation mostly through man pages and web wikis
Windows support is not as well developed as for Linux/Unix

9. Git Workflow

10. Git Architecture Workspace (working directory) Index Object Database
A regular directory that contains a git repository (.git subdirectory)
Index
Stores information about current working directory and changes made to it
Object Database
Blobs (files)
Stored in .git/objects
Each indexed by unique hash
All files are stored as blobs
Commits
One object is stored for every commit
Contains hash of parent, name of author, time of commit, and hash of the current tree
Tags

11. Some Commands Getting a Repository Commits Getting information
git init
git clone
Commits
git add
git commit
Getting information
git help
git status
git diff
git log
git show

12. Our First Git Repository
mkdir gitRepo1
cd gitRepo1
git init
Creates an initial repository in the .git subdirectory
echo "Hello World" >hello.txt
git status
shows which files have been modified in working directory and/or index since the last commit

13. Our First Git Repository
(in directory gitRepo1)
git add .
adds all changed files in the current working directory to the index of the current repository
makes the index reflect the working tree
used to stage changes for a commit (decide which files should be committed prior to a commit)
git commit -m 'Check in #1'
Stores the current contents of the index in a new commit along with a log message describing the changes
A commit operation is what actually updates the repository

14. Working With Git echo "I love Git" >>hello.txt git diff
Shows what changes we have made
compares working tree with index by default
git status
Shows list of modified files
git add hello.txt
No changes shown now, as working tree and index are the same
git diff HEAD
Compares working tree with latest commit (HEAD)
git commit -m 'Check in #2'

15. Viewing History git log git show <OBJECT> git reflog
Shows a log of commits, with most recent first
Note the hash code, which identifies each commit
git show <OBJECT>
OBJECT can be a full or shortened hash code
git reflog
to show all updates that have occurred to the current branch

16. Undoing What is Done git checkout <branch>
Used to checkout a specific version/branch of the tree
git revert <commit>
Makes a new commit that reverses the effect of an earlier commit
Example: git revert HEAD
Does not delete the commit object, just applies a patch
Reverts can themselves be reverted!
git reset
Moves the working tree back to a certain specified version
Use the --force if you really want to ignore working changes
Ordinary git commands do not delete commit objects
It is very hard to shoot yourself in the foot!

17. Git and Tagging Tags are just human readable shortcuts for hashes
A tag can be created to refer to any commit
git tag –a <tag-name> [<commit>]
Creates a tag to refer to the given commit (HEAD by default)
git tag –l
List all tags
git show-ref --tags
Show all tags with their commit hashes

18. Branching
Multiple branches allow you to work on a new part of a system without disrupting the stable version of the system
Git branching is lightweight
Most of the code does not need to be copied
Tools for helping merge branches and changes easily
You are ALWAYS working on a branch
By default, you are working on a branch named “master”
Branches can be local or remote
Key commands
git branch
Lists all local branches available in the current repository
The current branch is highlighted with *

19. Using Branches git checkout -b newbranch git branch
A branch named newbranch is created and checked out
The new branch will start with the same content as the current branch
Make sure you have committed changes in the current branch before switching to a new branch
git branch
Check which branch is currently checked out
We can now make changes in one branch and then propagate changes using
git merge
git cherry-pick

20. Branching Example
Simple branching: start a new branch (mywork) to develop an experimental feature or work on fixing a bug
o--o--o <-- master \
a--b--c <-- mywork

21. Branching Example More work done on master branch
o--o--O--o--o--o <-- master \
a--b--c <-- mywork

22. Merging Example
Could merge changes from master into the current branch (mywork)
git merge master
o--o--O--o--o--o <-- master \
a--b--c--m<-- mywork

23. Merging Example o--o--O--o--o--m <-- master \ a--b--c <-- mywork
When an experimental branch has been adequately tested, we can merge it into the master branch
git checkout master
git merge mywork
o--o--O--o--o--m <-- master \
a--b--c <-- mywork

24. Cleaning Up git fsck git gc
Checks object database to make sure all is sane
Can show information about dangling objects
git gc
Cleans up repository and compress files
When used with --prune, cleans out dangling blobs
Can speed up larger repositories, but not often necessary for small projects

25. Using a Remote Repository
Replicate a remote repository
git clone
Get changes with
git fetch
git pull (fetches and merges)
Propagate changes with
git push
Protocols
Local filesystem
SSH
Rsync
HTTP
Git protocol

26. Cloning our Repository
git clone first-git-repo
Now have a full git repository to work with
Changes are pushed back with git push
Pushing changes WILL NOT change working copy on the repository being worked on
Branches can be based off of remote branches
git branch --track new-branch remote/branch
Remote configuration information stored in .git/config
Can have multiple remote backends!

27. Git for Software Versioning
Create convention to define default server
Developers clone from central server
Lots of tools for transmitting patches between developers
Being used for
Linux (obviously)
Ruby On Rails
Check out for a variety of hosted projects

28. Git for Backups Example: A directory needs regular backups
Could use Linux rsync, but unwieldy in size
Create Git repository for appropriate directory
regular local commits
regular push to backup location
Can store/retrieve simple revision history

29. Git for Configuration Management
Example: Apache configurations
Multiple environments (dev/test/production)
Each environment is a separate branch
Tags are defined for each release version
Minor differences between environments
IP Address
Log levels
Want to effectively move changes across environments

30. Further Resources http://git-scm.com/ Git project home page
user-manual.html

31. Key Git Files/Directories
~/.gitconfig
.git
In top level directory of repository
Contains all objects, commits, configuration, for project
.git/config has project specific configurations
.gitignore
Stored in directory for ignoring

32. Some Git Coolness NetBeans provides interface to use projects with Git
gitk
Linux GUI to review changes
bash/zsh completion
git instaweb
Used for starting HTTP process for browing project

33. Git and Other VCS Integrations with
Subversion
CVS
Darcs
Many others
Example of integration with Subversion
Use git-svn to fetch and commit push
Note initial fetch may take a long time as each commit is downloaded individually!
Use git commands for everything
Branches integrated with tags and branches

34. Git and Patch files git diff HEAD^^ git diff HEAD~10..HEAD~2
Show what has changed in last two commits
git diff HEAD~10..HEAD~2
Show what changed between 10 commits ago and two commits ago
git format-patch HEAD^^..HEAD
Will create individual patch files per commit
git apply to apply patches
git am to apply patches from an mbox
Can also compare
Between specific objects
To branches/tags