Git in a NutshellSort of...

Mathias Meyerwww.paperplanes.de

Ruby User Group Berlin

What the git?

• Git is not the next Subversion

• Git is a content tracker

• Git is almost a versioned file system

• Git is distributed

What the git?

• Git is a huge collection of commands

• Git is f***ing fast

Ancient History

• Developed to replace BitKeeper for the Linux kernel development

• Started out as a couple of scripts built by Linus Torvalds

• Used by Rails, Merb, Ubuntu, X.org, Wine, Rubinius and Linux (whoa!)

• Started out focused on development with a central maintainer

Basics

• Every working copy is a full-fledged repository

• Git only tracks content

• Content is identified by a hash

• Though that content has a name

• The same content is only stored once

Basics

• Everything is stored in one .git directory in the top level of your working copy

• Git uses an index to find changes in the working directory

• The index is a snapshot of a specific tree in the repository

Git Objects

• Git knows four types of objects

• Have SHA1 IDs

• Can be addressed with the six first bytes of the hash

• Are compressed with gzip

Commits

• Refers to a tree and usually a parent commit (usually the commit before the current one)

• Can have two or more parents, then they represent a merge

• Or no parent at all

• Stored with a log message

Tree

• A simple structure containing names of blobs and other trees

• Represents a snapshot of the source tree

Blob

• Contains data, e.g. file content

• Blobs don’t have names

• Their names are derived from the tree

• Will be found through associations from trees and commits

Tag

• Glued to a specific object

• Marks a specific point in the Git timeline with a name

HEAD

branch

commit

tree

tree

tree

blob

blob

blob

Git Branches

• master - default development branch

• origin - default upstream branch

• HEAD - current branch

Git Branches

• HEAD~2, master^1, wtf?

• ~N references the Nth generation grandparent of a commit

• ^N references the Nth parent of a commit, only useful for merges of two or more commits

• So....

Git Branches

• HEAD~2 is the second generation grand-parent of the last commit on the current branch

• master^2 is the second parent of the last commit on master

• master~2^2 is the second parent of the second generation grand-parent of the last commit on the current branch

Git Branches

Rrright.

Git Branches

• Branches are cheap and easy

• A branch is an ID pointing to a tree and a parent commit

• Merging is cheap, fast and almost painless

• Ergo: Branching in Git rocks

How Do I Get Git?

• MacPorts (port install git-core +svn)

• Git Installer for Leopard

• On every Linux distribution (apt-get|rpm install git-core)

• On Windows? I think so

How Do I Get Started?

• mkdir project.git

• cd project.git

• git init

• Start hacking

How Do I Get Started?

• Or go to GitHub

• Best GUI for Git evah

Everyday Git

• Some of the commonly used Git commands

• Called the Porcelain

Set up camp

• git config --global user.name “Mathias Meyer”

• git config --global user.email “meyer@paperplanes.de”

git clone

• Creates a local working copy of a remote project

• git clone git://github.com/mattmatt/macistrano.git

git checkout

• Checks out code from a branch

• Will overwrite your local changes

• git checkout -b will create a branch and check it out

git branch

• Shows, creates and deletes branches

• git branch master* show_me_the_money

• git branch new_branch

• git branch -d new_branch

git fetch

• Fetches all objects from a remote repository which are not in the local repository

git add

• Schedule changes in one or more files for the next commit

git rm

• Removes files

git mv

• Moves files and directories around

git commit

• Isn’t it obvious?

• It will check in your changes

• But only the ones you added

• Use git commit -a to check in all the changes without adding them

• git commit <file> will commit the file without the need to add it

git status

• Shows staged and unstaged changes

• staged = added

• unstaged = new, conflicts, changed

git push

• Pushes your changes to a remote repository

• git push origin pushes the current branch

• git push origin master pushes the master branch

git pull

• Different way to do a merge

• git pull = git fetch + git merge

git log

• Shows the commit history

• Can be verbose if you want it to

git merge

• Merges the changes from a branch into your current branch

• git merge rails_2_1

• And that’s that

• In case of conflicts, these need to be resolved and then committed

git tag

• Duh!

• Associates a tag with the

git stash• It’s like a clipboard in your repository

• Need to work on something else but don’t want to commit your local changes yet?

• git stash save will save all your changes

• git stash apply will reapply them after you’re done

• git stash list shows all the stashes

• git stash show shows the changes of a stash

git show

• Shows the details of a specified, or the last commit

git archive

• Creates an export of your repository

• Default is tar

• Convenient, no?

• git archive release_2_1 > release_2_1.tar

git ....

• There’s lots more

• Which you probably won’t need most of the time

• You use 20 or so for everyday work

• Some of them only on special occasions

• git-<tab> and start digging

Git-Svn

• The gateway drug for Subversion users

• Warning: You don’t want to use the svn command ever again

• Ever!

Git-Svn

• Basically a couple of Perl scripts

• Using the Subversion bindings

• Translates Subversion commits to Git commits and vice versa

Show me how!

• mkdir webistrano

• git svn init -s http://labs.peritor.com/svn/webistrano

• git svn fetch

• Get a coffee...

Everyday Git-Svn

• git checkout -b make_me_rich

• ...endless hours of coding...

• ...and writing tests...

• git add lib/monetize.rb spec/monetize_spec.rb

• git commit

Everyday Git-Svn - Merging

• git commit -a

• git checkout master

• git svn rebase

• git merge make_me_rich

• ...run tests...

• git svn dcommit

• git branch -d make_me_rich

Everyday Git-Svn - Merging

• The potentiel downside of using git merge:

• git svn dcommit will check in all the commits from the branch in one single svn commit

Everyday Git-Svn - Merging

• The alternative

• dcommit from the branch

• rebase the changes on master

Everyday Git-Svn - Merging

• git commit -a

• git svn rebase # on branch make_me_rich

• git svn dcommit # on branch make_me_rich

• git checkout master

• git svn rebase

• git branch -d make_me_rich

Some Git Awesomeness

• Find out what commits will be ci’d to svn

• git svn dcommit -n (rather sparse)

• git cherry -v trunk (full glory)

• Commit partial changes of a file

• git add --patch

But I don’t like the CLI

• Use git gui for a “nicer” interface

• Use Gitk to visualize commits and branches

• For both: brace yourself to be blinded

• Much nicer, less powerful: GitNub

• TextMate bundle!

In the end...

• Git will blow your brains out

• In several ways

• It takes a while to get used to it

• But it’s totally worth it

• Even if you’re still using Subversion as a central repository

The Bad Stuff

• Poor documentation, be prepared to spend some time on Google

• Complex beast, lots of small commands

• No API

Resources• Git Home (http://www.git.or.cz)

• Git User’s Manual (http://www.kernel.org/pub/software/scm/git/docs/user-manual.html)

• Git PeepCode (http://peepcode.com/products/git)

• Must-read: Git Internals (http://peepcode.com/products/git-internals-pdf)

• GitCasts (http://www.gitcasts.com/)