This is a quick introduction to the inner workings of Git, largely based on what I learned from reading Git Internals (https://peepcode.com/products/git-internals-pdf) and experimentation. It covers Git objects (blob, tree, commit, tag), references, and branches with incrementally building diagrams to help convey the concepts.

1. Git
Understanding
January 6, 2010
Jeff Kunkle

2. What’s Git?
Git is a free & open source, distributed version control
system designed to handle everything from small to very
large projects with speed and efficiency.
Every Git clone is a full-fledged repository with complete
history and full revision tracking capabilities, not dependent
on network access or a central server. Branching and
merging are fast and easy to do.

3. Working Directory
./
Rakefile
init.rb
lib
grant.rb

4. Working Directory
./
.git
Rakefile
init.rb
lib
grant.rb
>git init

5. Working Directory
./
.git What’s in here?
Rakefile
init.rb
lib
grant.rb
>git init

6. Objects

7. Git Objects
header object_type [content size]0
content of the file
can be text
content can be binary
can be whatever
Zlib::Deflate

8. Git Objects
header object_type [content size]0
content of the file
can be text
content can be binary
can be whatever
Zlib::Deflate
1 Blob 2 Tree 3 Commit 4 Tag

9. 1 Blobs

10. Working Directory Git Directory
./
Rakefile
init.rb
lib
grant.rb

11. Working Directory Git Directory
./
Rakefile
init.rb
lib
grant.rb
>git add .
>git commit -m “initial commit”

12. Working Directory Git Directory
./
Rakefile blob : 21a307
init.rb blob : 644eda
lib
grant.rb blob : a22a24
>git add .
>git commit -m “initial commit”

13. Working Directory Git Directory
./
Rakefile blob : 21a307
init.rb blob : 644eda
SHA-1 hashes
lib
of file content
grant.rb blob : a22a24
>git add .
>git commit -m “initial commit”

14. Blob Content

15. Blob Content
%w{ models }.each do |dir|
path = File.join(File.dirname(__FILE__), 'lib', 'app', dir)
$LOAD_PATH << path
ActiveSupport::Dependencies.load_paths << path
ActiveSupport::Dependencies.load_once_paths.delete(path)
end
>git cat-file -p 644eda

16. 2 Trees

17. Working Directory Git Directory
./
Rakefile blob : 21a307
init.rb blob : 644eda
lib
grant.rb blob : a22a24

18. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24

19. Tree Content

20. Tree Content
100644 blob 21a30738954b6bb164731d822efafa6c89c7bce7! Rakefile
100644 blob 644eda506db859e011ccbca5a06421ee76782ac7! init.rb
040000 tree 523fa41bd27fa29a00afb0bef6a10fde75aef501! lib
>git cat-file -p f2eb1e

21. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24

22. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24

23. 3 Commits

24. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24

25. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa

26. Commit Content

27. Commit Content
tree f2eb1e549e7e0d9cd5b7a580e63e9ce79d5a03ae
author Jeff Kunkle <jkunkle@nearinfinity.com> 1261341813 -0500
committer Jeff Kunkle <jkunkle@nearinfinity.com> 1261341813 -0500
initial commit
first commit
>git cat-file -p 3848fa

28. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa

29. Working Directory Git Directory
./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa

30. Commit Content
tree ac451e549e7e0d9cd5b7a580e63e9ce79d5b45a1
parent 3848fa7e91490a99b77590ff1385c4b3eebb3de3
author Jeff Kunkle <jkunkle@nearinfinity.com> 1261351813 -0500
committer Jeff Kunkle <jkunkle@nearinfinity.com> 1261351813 -0500
second commit
next commit

31. 4 Tags

32. ./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa

33. ./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa
tag : e9eff3
>git tag -a v1.0

34. Tag Content

35. Tag Content
object 3848fa7e91490a99b77590ff1385c4b3eebb3de3
type commit
tag v1.0
tagger Jeff Kunkle <jkunkle@ni.com> Tue Dec 29 21:02:04 2009 -0500
version 1.0
>git cat-file -p v1.0

36. ./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa
tag : e9eff3
>git tag -a v1.0

37. ./ tree : f2eb1e
Rakefile blob : 21a307
init.rb blob : 644eda
lib tree : 523fa4
grant.rb blob : a22a24
commit : 3848fa
tag : e9eff3
>git tag -a v1.0

38. References

39. Basic Data Model
tag
commit
tree
blob

40. Basic Data Model
tag
commit
Immutable
Git Objects
tree
blob

41. Basic Data Model
tag
branch commit
Immutable
Git Objects
tree
blob

42. Basic Data Model
tag
branch commit
Immutable
Git Objects
tree
blob

43. Basic Data Model
HEAD tag
branch commit
Immutable
Git Objects
tree
blob

44. Basic Data Model
HEAD tag
branch commit
Immutable
Git Objects
Mutable tree
References
blob

45. Working Directory
./
Rakefile
init.rb
lib
grant.rb

46. HEAD
branch tag
commit
tree blob
tree blob
blob

47. HEAD HEAD
branch tag branch
commit commit
tree blob tree
tree blob tree
blob blob
>git commit -a lib/grant.rb

48. HEAD HEAD HEAD
branch tag branch branch
commit commit commit
tree blob tree tree blob
tree blob tree blob
blob blob
>git commit -a Rakefile init.rb

49. Branches

50. branch branch
commit
tree C1
blob

51. master
C C

52. master
C C
idea
>git checkout -b idea

53. master
C C
C
idea
>git commit

54. master
C C
C
idea
>git checkout master

55. master
C C C
C
idea
>git commit

56. bug
master
C C C
C
idea
>git checkout -b bug

57. bug
master
C
C C C
C
idea
>git commit

58. bug
master
C
C C C
C
idea
>git commit
checkout master

59. bug
master
C
C C C
C
idea
>git commitbug
merge

60. bug
master
C
C C C C
C
idea
>git commitbug
merge

61. bug
master
C
C C C C
C
idea
>git commitbug bug
branch
merge -d

62. bug
master
C
C C C C
C
idea
>git commitbugidea
checkout
branch
merge -d bug

63. bug
master
C
C C C C
C C
idea
>git commitbugidea
checkout
branch
merge -d bug

64. bug
master
C
C C C C
C C C
idea
>git commitbugidea
checkout
branch
merge -d bug

65. bug
master
C
C C C C
C C C
idea
>git commitbugidea
checkout bug
branch
merge -dmaster

66. bug
master
C
C C C C
C C C
idea
>git commitbugidea
checkout
branch -dmaster
merge ideabug

67. bug
master
C
C C C C C
C C C
idea
>git commitbugidea
checkout
branch -dmaster
merge ideabug

68. bug
master
C
C C C C C
C C C
idea
>git commitbugidea
checkout idea
branch -dmaster
merge ideabug

69. Recommendations

70. Starting new project?

71. Starting new project?
Use Git

72. Using SVN or CVS?

73. Using SVN or CVS?
Switch to Git

74. Using Team Studio?

75. Using Team Studio?
Maybe switch to Git

76. Using MKS?

77. Using MKS?
Switch to Git
or CVS, or Subversion, or anything else

78. References
Good Better Best
*most diagrams in this presentation are based on Git internals