internals

Git Objects

Git objects

$ ls .git/

branches

config

description

HEAD

hooks

index

info

logs

objects

refs

The blob

$ git init

Initialized empty Git repository in /tmp/test/.git/

$ echo "test_content" | git hash-object -w -t blob --stdin

915e94ff1ac3818f1e458534b0228a12a99cd6c5

$ ls .git/objects/91/5e94ff1ac3818f1e458534b0228a12a99cd6c5

.git/objects/91/5e94ff1ac3818f1e458534b0228a12a99cd6c5

$ cat .git/objects/91/5e94ff1ac3818f1e458534b02* | zlib_inflate -d

blob 13\0test_content

The tree

tree [content size]\010644 blob a906cbREADME10755 blob 6f4e32run04000 tree 1f7a4e src

$ git commit file

[master dbaf944] This is a commit message.

$ cat .git/objects/db/af944a4a9eb72af64042b1e3a128936000dfc2 |

zlib_inflate -d

commit 318

tree 47ec7a250164a21cb14eb64618c3a903db0b7420

parent 402b26df0644f09fc62842c0a4a44a0a3345c530

author Manu <m.cupcic@criteo.com> 1380977766 +0200

committer Manu <m.cupcic@criteo.com> 1380977766 +0200

This is a commit message.

It explains what the hell this commit is about

The commit

The commit

• Is identified by• a snapshot of the repo state (the tree).• parent commit(s)• a commit message

• Is immutable• Has a deterministic hash (SHA1)• Commits form a linked list: the history

Git References

$ cat .git/refs/heads/master

dbaf944a4a9eb72af64042b1e3a128936000dfc2

$ cat .git/HEAD

ref: refs/heads/master

$ echo "dbaf944" > .git/refs/heads/newbranch

$ git checkout newbranch

Switched to branch 'newbranch'

Git References

$ git tag 1.0

dbaf944a4a9eb72af64042b1e3a128936000dfc2

$ cat .git/refs/tags/1.0

dbaf944a4a9eb72af64042b1e3a128936000dfc2

Take home message

• Git stores a snapshot of the whole repo at each commit.

• The SHA1 of a commit depends only on its content,

message, committer and parent(s).

• A git branch/tag is a 40 digits hex number stored in a

file.

git pull –rebase

git stash

git rebase -i

git pack

git config

git refspecs

git refloggit fsck

git log (advanced stuff)

git add -p

git reset

Things we can play with