Introduction to git

Introduction To GIT
Rob Di Marco
Philly Linux Users Group
July 14, 2008

A Brief History of Git
• Linus uses BitKeeper to manage Linux code
• Ran into BitKeeper licensing issue
 Liked functionality
 Looked at CVS as how not to do things
• April 5, 2005 - Linus sends out email showing
first version
• June 15, 2005 - Git used for Linux version control

Git is Not an SCM
Never mind merging. It's not an SCM, it's a
distribution and archival mechanism. I bet
you could make a reasonable SCM on top
of it, though. Another way of looking at it is
to say that it's really a content-addressable
filesystem, used to track directory trees.
Linus Torvalds, 7 Apr 2005
http://lkml.org/lkml/2005/4/8/9

Centralized Version Control
• Traditional version control system
 Server with database
 Clients have a working version
• Examples
 CVS
 Subversion
 Visual Source Safe
• Challenges
 Multi-developer conflicts
 Client/server communication

Distributed Version Control
• Authoritative server by
convention only
• Every working
checkout is a
repository
• Get version control
even when detached
• Backups are trivial
• Other distributed
systems include
 Mercurial
 BitKeeper
 Darcs
 Bazaar

Git Advantages
• Resilience
 No one repository has more data than any other
• Speed
 Very fast operations compared to other VCS (I’m looking at you
CVS and Subversion)
• 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 userbase with robust tools

Some GIT Disadvantages
• Definite learning curve, especially for those used
to centralized systems
 Can sometimes seem overwhelming to learn
• Documentation mostly through man pages
• Windows support can be an issue
 Can use through Cygwin
 Also have the msysgit project

Git Architecture
• Index
 Stores information about current working directory and changes
made to it
• Object Database
 Blobs (files)
• Stored in .git/objects
• Indexed by unique hash
• All files are stored as blobs
 Trees (directories)
 Commits
• One object for every commit
• Contains hash of parent, name of author, time of commit, and hash
of the current tree
 Tags

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

Our First Git Repository
• mkdir first-git-repo && cd first-
git-repo
• git init
 Creates the basic artifacts in the .git directory
• echo “Hello World” > hello.txt
• git add .
 Adds content to the index
 Index reflects the working version
 Must be run prior to a commit
• git commit -a -m ‘Check in number
one’

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

Working With Git
• echo “I love Git” >> hello.txt
• git diff
 Shows changes we have made
• git status
 Shows list of modified files
• git add hello.txt
• git diff
 No changes shown as diff compares to the index
• git diff HEAD
 Now can see the changes in working version
• git status
• git commit -m ‘Second commit’

Viewing What Has Changed
• git log
 Note the hash code for each commit.
• git show <OBJECT>
 Can use full or shortened hash
• git reflog to see all changes that have
occurred

Git and Patch files
• git diff HEAD^^
 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

Undoing What is Done
• git checkout
 Used to checkout a specific version/branch of the tree
• git reset
 Moves the tree back to a certain specified version
 Use the --force to ignore working changes
• git revert
 Reverts a commit
 Does not delete the commit object, just applies a
patch
 Reverts can themselves be reverted!
• Git never deletes a commit object
 It is very hard to shoot yourself in the foot!

Git and Tagging
• Tags are just human readable shortcuts for
hashes
• Branches can be made from any commit
• git tag <tag-name>

Branching
• Git branching is lightweight
 No massive copying a la CVS/Subversion
 Tools for helping merge branches and changes easily
• You are ALWAYS on a branch
• Branches can be local or remote
• Key commands
 git branch
 git merge
 git cherry-pick
• Allows you to choose specific commits to apply
• You can edit the commits while cherry picking

Using Branches
• git checkout -b branch
• git checkout -b devel/branch
• git branch
 Lists all local branches available
• We can now make changes in one branch
and propagate change using
 git merge
 git cherry-pick

Rebasing Example
• Simple branching
o--o--o <-- origin

a--b--c <-- mywork

Rebasing Example
• Work done on origin branch
o--o--O--o--o--o <-- origin

a--b--c <-- mywork

Rebasing Example
• Could merge changes into branch
• git merge origin

a--b--c--m<-- mywork

Rebasing Example
• Rebasing moves branch point
• git rebase origin

a`--b`--c`

Cleaning Up
• git fsck
 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 really speed up larger repositories

Using Remote
• Use git clone to replicate
repository
• Get changes with
 git fetch (fetches and
merges)
 git pull
• Propagate changes with
 git push
• Protocols
 Local filesystem
 SSH
 Rsync
 HTTP
 Git protocol

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!

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 http://github.com for a variety of hosted
projects

Git for Backups
• Example: Directory needs regular backups
 Could use rsync but unwieldy in size
• Create Git repository for appropriate
directory
 Regular local commits
 Regular push to backup location
 Get simple revision heistory

Git for Configuration Management
• Example: Apache configurations
 Multiple environments (dev/test/production)
 Minor differences between environments
• IP Address
• Log levels
 Want to effectively move changes across
environments

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

Some Git Coolness
• bash/zsh completion
• Gitk
 GUI to review changes
• git instaweb
 Used for starting HTTP process for browing
project

Further Resources
• http://git.or.cz/
 http://git.or.cz/course/cvs.html (For CVS
users)
 http://git.or.cz/course/svn.html (For SVN users)
• http://www.kernel.org/pub/software/scm/git/docs/user
• http://jonas.iki.fi/git_guides/HTML/git_guide/

Introduction to git

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