Introduction to Git (Greg Lonnon)

Git Intro

agenda

source code management (SCM) tools

gitdesign

usage

Who is Greg?

SCM geek...Sccs,rcs,cvs,svn,clearcase,perforce, and now git

Master Architect @ HPFutureSmart product line enterprise imaging products

YouTube FutureSmart Video

I still use the tag...I don't care about IE...

SCM???

Attributes of a SCM toolTrack (folders and files)Individually or in sets (change sets)

Branch Independent changes

Common starting points

Release, feature development, etc...

Merge Combine 2..N branches into 1

HistoryWhat, when, why questions

SCM Evolution

tar/zip

RCS

SVN

CVS

clearcaseperforce

git/hg/bazaar

SCM: the new guys

git, hg, bazaar

peer to peer architecture

atomic change set basedfile changes are grouped together as a transaction

takes advantage of local machine featuresCPU and Disk

What is git?

One of the new guys

A tree based content management storage systemwith SCM'ish commands

Developed for Linux kernelLinus Torvald wrote the first version

Ported to Windows, OSX, any posix based OS

Many GUI clients

git Storage Design

git storage design is the key concept to understand

It's simple, based on...Cryptography theory

Database theory

Graph theory

Set theory

git storage (repository)

Object based Storage(Key,Data) tuple key = SHA1(data)

keys are 20 byte arrays.

SHA1 algorithm crypto algorithm

Datablobs : files, sym-links

trees : directories

commit : points to a tree, and other commits

tag : points to a commit

Git: blobs

#include int main(){printf(hello world); return 0;}

Blobs: files (text and binary), symlinks

git: tree

permission

object

hash

name

100644

blob

82354a1...

README

100755

blob

02342c5...

a.out

040000

tree

a67ef32...

02342c...

src

Trees: aka directories

git: commit

date

author

parents

comment

tree

Mon Dec 20 16:10:41 2010 -0700

joe developer

234ab23...

Fixed the border defect

89023a2...

git: tag

date

tagger

Mon Dec 20 16:10:41 2010 -0700

joe developer

234ab23...

commit

comment

Release 1.0

git: repo

C3C1C2

T1

B2B1

T1T2

B2

B1

T1B1

git initecho hello > B1git add B1git commit

echo world > B2git add B2git commit

C3C1C2

T2

B2B1

T3T2

B2

B1

T1B1

git initecho hello > B1git add B1git commit

mkdir T2git mv B1 B2 T2cp T2/B1 B3git add B3git commit

Time

git: repo

C3C1C2

T1

T1T1B1

C3C1C2

T2B2

T3

T1B1

git : DAG

Wikipedia...In mathematics and computer science, a directed acyclic graph (commonly abbreviated to DAG), is a directed graph with no directed cycles. That is, it is formed by a collection of vertices and directed edges, each edge connecting one vertex to another, such that there is no way to start at some vertex v and follow a sequence of edges that eventually loops back to v again.

git : DAG

C1C1C1C2C1C3C1C6C1C8

C4C5C7

masterHEAD

work_br

V1.0

git referencesbranches (master, work_br)

tags : immutable (V1.0)

git: DAG

C1C1masterHEAD

git initgit commit

git: DAG

C1C1masterHEAD

git branch work_br

work_br

git: DAG

C1C1C1C2

masterHEAD

git commit

work_br

git: DAG

C1C1C1C2C1C3

masterHEAD

work_br

git commit

git: DAG

C1C1C1C2C1C3

C4

masterHEAD

work_br

git checkout work_brgit commit

git: DAG

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

git commit

git: DAG

C1C1C1C2C1C3C1C6

C4C5

masterHEAD

work_br

git checkout mastergit merge work_br

git: DAG

C1C1C1C2C1C3C1C6

C4C5C7

masterHEAD

work_br

git checkout work_brgit commit

git: DAG

C1C1C1C2C1C3C1C6C1C8

C4C5C7

masterHEAD

work_br

git checkout mastergit commit

git repository

Each git repo contains complete historyAll git objects are stored in the repo

Peer to Peer Design

.git directoryobjects

referencesbranches, tags : files that contain commit hash

HEAD : current checkout hash

git clone

C1C1C2C1C3

C4C5

masterwork_br

C1C1C2C1C3

C4C5

origin/masterHEAD

origin/work_br

http:/github.com// origin

git clone http://github.com// local

git clone copies the remote objects/references to anew repositoryThe remote repo is nicknamed origin, a remote tracking branch is created to track origin/master

local

master

git fetch

C1C2C3C4C5masterwork_brC6

C1C2C3C4C5origin/masterHEAD

work_br


work_brC6

C7

git fetch origin

origin

local

local

Fetch moves C6 commit objects to the local repo, and updates the tracking branch origin/master

master

masterorigin/work_br

origin/work_br

git push


git push origin

origin

Push local to origin.The local work_br is a proper superset ofthe origin work_br branch, and the origin isupdated.The local master push fails, it is NOT a not a proper superset of the origin master branch


C7

origin


work_brC6

C7

local

masterorigin/work_br

git pull

git pullgit fetch + git merge

git: merge algorithms

git mergeMerges 1..N branches into current branch (HEAD)

git merge --squashgit merge with no parent tracking (merge tracking)

git cherry-pickCalculates the file differences of the commit and applies them to current branch (HEAD)

git rebaseMoves a related set of commits to a new base reference.

git merge

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

git merge work_br

C1C1C1C2C1C3C1C6

C4C5

masterHEAD

work_br

Merges 1 or more branch into the current branch.

git merge --squash

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

git merge --squash work_br

C1C1C1C2C1C3C1C6

C4C5

masterHEAD

work_br

Merges a branch, but does not track the merge.Useful in large repositories with a lot of activity.

git cherry-pick

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

git cherry-pick C5

C1C1C1C2C1C3

C4C5

work_br

C5'

master

HEAD

Calculates and applies the file changes of a commit. Useful for moving changes between a release branch and main development

git rebase

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

git rebase master

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

C4'C5'

Moves a family of commits to a new baseUseful for developers to refresh their branch.

git rebase: a closer look

C1C1C1C2C1C3

C4C5

masterHEAD

work_br

C4'C5'

d1d2d1d2Step 1: Apply d1 to C3

Step 2: Apply d2 to C4'

git commands
porcelain

Repositorygit init, git clone, git fetch, git pull, git push, git gc, git config

Working treegit checkout, git reset, git clean, git add, git commit

Branchgit branch, git tag, git merge, git rebase, git cherry-pick, git revert, git mergetool

Historygit log, git diff, git difftool, git reflog

git commands
plumbing

Git low level commandsEasy to parse output

Stable API

Use them for scripting git

100+ commands...

gitk

git gui

git hints

CommitWhen something compiles, passes a test, before refactoring, before going to lunch/home/sleep, or... when you have nothing else to do...

Use commits to separate out features, debug, temporary work, etc...

git != svn, don't treat it like svn...

Use git reflog to find lost commits

Create a sandbox (git init) and play...

git : Web Resources

Use google...http://git-scm.com/

Free Pro Git bookhttp://progit.org/book/

Free Webcastshttp://gitcasts.com/

http://oreillynet.com/pub/e/1394

Free Open Source Hosting (cheap commerical hosting)https://github.com/

Gitolite : git server supports complex security models

https://github.com/sitaramc/gitolite

Windows executableGit-extensions or msysgit

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Introduction to Git (Greg Lonnon)

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Transcript of Introduction to Git (Greg Lonnon)