git internals

dbyrne23andMe

agenda

1.data ... in two commits

a. blobs

b. trees

c. commits

2.algorithms ... in six commitsa. commit

b. branch

c. merge

d. reset

e. rebase

objects

commits

trees

blobs

objects, blobs

$ git init objects_example

$ cd objects_example

$ echo "puts 'hello world'" > foo.rb

$ git add foo.rb

$ strings .git/index | grep hello

$ strings .git/index | grep foo.rb

foo.rb

objects, blobs$ find .git/objects -type file

.git/objects/aa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

$ git cat-file -t aaaa # what type of object is this?

blob

$ git cat-file -p aaaa # print the object

puts 'hello world'

objects, blobs$ echo "puts 'hello world'" > bar.rb

$ git add bar.rb

$ strings .git/index | grep bar.rb

bar.rb

$ find .git/objects -type file # no new blob

.git/objects/aa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

objects

$ git commit -m 'commit msg #1'

$ find .git/objects -type file

.git/objects/aa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

.git/objects/bb/bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

.git/objects/cc/cccccccccccccccccccccccccccccccccccccc

objects, trees$ git cat-file -t bbbb

tree

$ git cat-file -p bbbb

100644 blob aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa bar.rb

100644 blob aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa foo.rb

objects, commits$ git cat-file -t cccc

commit

$ git cat-file -p cccc

tree bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

author dbyrne <dennis.byrne@gmail.com> 1474171822 -0700

committer dbyrne <dennis.byrne@gmail.com> 1474171822 -0700

commit msg #1

c

a

b

foo.rb bar.rb

objects

$ mkdir dir

$ echo "puts 'hello world'" > dir/baz.rb

$ git add dir/baz.rb

$ git commit -m 'commit msg #2'

objects$ find .git/objects -type file

.git/objects/aa/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

.git/objects/bb/bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb

.git/objects/cc/cccccccccccccccccccccccccccccccccccccc

.git/objects/dd/dddddddddddddddddddddddddddddddddddddd <- new

.git/objects/ee/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee <- new

.git/objects/ff/ffffffffffffffffffffffffffffffffffffff <- new

objects$ git cat-file -p HEAD^{commit} # same as ffff

tree eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

parent cccccccccccccccccccccccccccccccccccccccc

author dbyrne <dennis.byrne@gmail.com> 1474172113 -0700

committer dbyrne <dennis.byrne@gmail.com> 1474172113 -0700

commit msg #2

objects

$ git cat-file -p HEAD^{tree} # same as eeee

100644 blob aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa bar.rb

040000 tree dddddddddddddddddddddddddddddddddddddddd dir

100644 blob aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa foo.rb

objects$ git cat-file -t dddd

tree

$ git cat-file -p dddd

100644 blob aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa baz.rb

c

a

b /

f

e /

d /dir

foo.rb

bar.rb

baz.rb

foo.rb bar.rb

objectsthere are three types of objects: blobs, trees & commits

blobs represent files

blobs are created with the add, not the commit command

blobs are snapshots, not deltas or patches

the add command records snapshots every time

a file can be both “staged” and “modified”

trees represent directories

trees point to blobs and/or trees

commits point to trees and most likely each other

a SHA can be abbreviated with a minimum of four chars

refs$ ls .git/HEAD

.git/HEAD <- important

$ tree .git/refs/

├── heads <- important

│ └── master

├── remotes

├── stash

└── tags

git commit$ git init refs_example

$ cd refs_example

$ echo "print 'commit #1'" > spam.py

$ git add spam.py

$ git commit -m 'commit msg #1'

[master (root-commit) 1111111] commit msg #1

git commit$ ls .git/objects/11

11111111111111111111111111111111111111

$ cat .git/refs/heads/master

1111111111111111111111111111111111111111

$ cat .git/HEAD

ref: refs/heads/master

git commit

1111

master

HEAD

top to bottom

5HEAD

branch

commit

tree

blob

git commit$ echo "print 'commit #2'" >> spam.py

$ git commit -am 'commit msg #2'

[master 2222222] commit msg #2

$ cat .git/HEAD

ref: refs/heads/master

git commit

1111

master

HEAD2222

git commit$ git show-ref master --abbrev

2222222 refs/heads/master

$ git cat-file -p 2222 | grep parent

parent 1111111111111111111111111111111111111111

git branch$ git branch feature_branch

$ ls .git/refs/heads/

feature_branch

master

$ git show-ref --abbrev

2222222 refs/heads/feature_branch

2222222 refs/heads/master

git branch

1111

master

HEAD2222

feature_branch

git checkout$ cat .git/HEAD

ref: refs/heads/master <- before

$ git checkout feature_branch

$ cat .git/HEAD

ref: refs/heads/feature_branch <- after

git checkout

1111

master

HEAD2222

feature_branch

working on a branch$ echo "print 'commit #3'" > eggs.py

$ git add eggs.py

$ git commit -m 'commit msg #3'

[feature_branch 3333333] commit msg #3

$ git show-ref --abbrev

3333333 refs/heads/feature_branch

2222222 refs/heads/master

working on a branch

1111

master

HEAD2222

feature_branch

3333

divergence$ git checkout master

$ echo "print 'commit #4'" > spam.py

$ git commit -am 'commit msg #4'

[master 4444444] commit msg #4

divergence

1111

master

HEAD2222

feature_branch

3333

4444

git merge$ git merge --no-edit feature_branch

Merge made by the 'recursive' strategy. <- important

eggs.py | 1 +

1 file changed, 1 insertion($)

create mode 100644 eggs.py

git merge

1111

master

HEAD2222

feature_branch

3333

4444

5555

git lol$ git log --graph --oneline --decorate

* 5555555 (HEAD -> master) Merge branch 'feature_branch'

|\

| * 3333333 (feature_branch) commit msg #3

* | 4444444 commit msg #4

|/

* 2222222 commit msg #2

* 1111111 commit msg #1

git merge$ git show-ref master --abbrev

5555555 refs/heads/master

$ git cat-file -p 5555 | grep parent

parent 4444444444444444444444444444444444444444 <- master

parent 3333333333333333333333333333333333333333 <- fe. br.

merge vs. rebase

record of what

actually happened

vs. story of how your

project was made

git reset (to a commit)

change index? change working dir? working dir safe?

git reset --soft SHA Yes No

git reset --mixed SHA No Yes

git reset --hard SHA Yes Yes

git reset$ git show-ref --abbrev

3333333 refs/heads/feature_branch

5555555 refs/heads/master

$ git reset --hard HEAD^

HEAD is now at 4444444 commit msg #4

$ git show-ref --abbrev

3333333 refs/heads/feature_branch

4444444 refs/heads/master

git reset

1111

master

HEAD2222

3333

4444

5555

feature_branch

feature_branch

git reset, simplified

1111

master

HEAD2222

3333

4444

git rebase$ git show-ref feature_branch --abbrev

3333333 refs/heads/feature_branch

$ git checkout feature_branch

$ git rebase master

First, rewinding head to replay your work on top of it...

Applying: commit msg #3

$ git show-ref feature_branch --abbrev

6666666 refs/heads/feature_branch

git rebase

1111

master

HEAD2222

feature_branch

3333

4444

6666

git rebase, simplified

1111

master

HEAD2222

feature_branch

4444 6666

git merge - fast forward$ git checkout master

$ git merge feature_branch

Updating 4444444..6666666

Fast-forward <- important

eggs.py | 1 $

1 file changed, 1 insertion($)

create mode 100644 eggs.py

git merge - fast forward

1111

master

HEAD2222

feature_branch

4444 6666

git merge - fast forward$ git show-ref --abbrev

6666666 refs/heads/feature_branch

6666666 refs/heads/master

$ ls

eggs.py

spam.py

git reflog$ git reflog

6666666 HEAD@{0}: merge feature_branch: Fast-forward

...6666666 HEAD@{3}: rebase: commit msg #3...4444444 HEAD@{6}: reset: moving to HEAD^

5555555 HEAD@{7}: merge feature_branch: Merge made by the 'recursive' strategy....

git fsck$ echo "lost?" >> spam.py && git add spam.py

$ echo "no" >> spam.py && git add spam.py

$ git fsck --unreachable

unreachable blob 7777777777777777777777777777777777777777

$ git cat-file -p 7777

print 'commit #4'

lost?

git gc$ git fsck --unreachable

unreachable blob 7777777777777777777777777777777777777777

$ git gc --prune=all

$ git fsck --unreachable

$ git cat-file -p 7777

fatal: Not a valid object name 7777

detached head state$ git checkout 5555555

<(HEAD detached at 5555555)> $ cat .git/HEAD

5555555555555555555555555555555555555555

<(HEAD detached at 5555555)> $ # look around, even do work

$ git checkout -b nostalgia

$ git branch

feature_branch master* nostalgia

trends