Deep inside Git

Git init

Init repository

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[ryao@macpro-gn07 deep-inside-git]> git init
Initialized empty Git repository in /Users/ryao/Workspaces/deep-inside-git/.git/

.git

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[ryao@macpro-gn07 deep-inside-git]> tree .git/
.git/
├── HEAD
├── config
├── description
├── hooks
│   ├── applypatch-msg.sample
│   ├── commit-msg.sample
│   ├── post-update.sample
│   ├── pre-applypatch.sample
│   ├── pre-commit.sample
│   ├── pre-push.sample
│   ├── pre-rebase.sample
│   ├── pre-receive.sample
│   ├── prepare-commit-msg.sample
│   └── update.sample
├── info
│   └── exclude
├── objects
│   ├── info
│   └── pack
└── refs
    ├── heads
    └── tags

8 directories, 14 files

This is the structure of .git directory. So basically:

  • HEAD: pointing to the current HEAD
  • hooks: web hooks that make git host programmable
  • logs: this is where log read from
  • objects: this is where commit information and committed files are stored
  • refs: this is where reflog read from

HEAD content

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[ryao@macpro-gn07 deep-inside-git]> cat .git/HEAD
ref: refs/heads/master
[ryao@macpro-gn07 deep-inside-git]> ll .git/refs/heads/master
ls: .git/refs/heads/master: No such file or directory

As we don’t have any commit yet, the file that referred by HEAD is not exist.

hash-object

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[ryao@macpro-gn07 deep-inside-git]> echo 'Rugal' | git hash-object --stdin
2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
[ryao@macpro-gn07 deep-inside-git]> echo 'Rugal' > README.md
[ryao@macpro-gn07 deep-inside-git]> cat README.md | git hash-object --stdin
2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a

In order to start our tutorial, let me introduce hash-object tool.
This tool is what git uses for computing hash for objects.
By testing the code above, we can see that the hash of a file is simply the hash of its content.

Git add

Updated objects folder

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[ryao@macpro-gn07 deep-inside-git]> git add -A && git status
On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

        new file:   README.md

[ryao@macpro-gn07 deep-inside-git]> tree .git/objects
.git/objects/
├── 2f
│   └── b811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
├── info
└── pack

By simply add the new file into staging area, we have a new object being added into .git/objects folder.
We notice the folder name along with the file name is 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a, which is exactly the same with the hash we calculated above.
From this section we realize, that once a file is staged, its information, type will be hashed and store into .git/objects under the folder/file of the hash.

Object content

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#original content
[ryao@macpro-gn07 deep-inside-git]> cat .git/objects/2f/b811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
xKOR0*MOV
#size
[ryao@macpro-gn07 deep-inside-git]> git cat-file -s 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
6
#type
[ryao@macpro-gn07 deep-inside-git]> git cat-file -t 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
blob
#content
[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
Rugal

Here another interesting tool comes, cat-file. We can use this tool to inspect objects information.
We will use this tool to show information of an object later.

Git commit

Commit

In this section we will commit our first file. Let’s monitor what will change. 

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[ryao@macpro-gn07 deep-inside-git]> git commit -m"Initial commit"
[master (root-commit) f73a6ae] Initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 README.md

[ryao@macpro-gn07 deep-inside-git]> cat .git/refs/heads/master
f73a6ae93b095b899fbdb3b2485f5829b9f460cf

After the first commit, we got a file master under .git/refs/heads.
The content of file is the hashcode of the commit.

Updated objects folder

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[ryao@macpro-gn07 deep-inside-git]> tree .git/objects
.git/objects
├── objects
├── 2f
│   └── b811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
├── 36
│   └── 2032c56bddec6ad5b639e16eeb594f92886516
├── f7
│   └── 3a6ae93b095b899fbdb3b2485f5829b9f460cf
├── info
└── pack

Git commit objects

Now let’s inspect the commit objects.

Commit information

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -t f73a6ae93b095b899fbdb3b2485f5829b9f460cf
commit
[ryao@macpro-gn07 deep-inside-git]> git cat-file -p f73a6ae93b095b899fbdb3b2485f5829b9f460cf
tree 362032c56bddec6ad5b639e16eeb594f92886516
author Rugal Bernstein <ryao@peakcontact.com> 1498230956 -0400
committer Rugal Bernstein <ryao@peakcontact.com> 1498230956 -0400

Initial commit

This object is of commit type, which means it contains information of a commit, including:

  • Hash of tree
  • author information
  • commiter information
  • commit message

Commit tree

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -t 362032c56bddec6ad5b639e16eeb594f92886516
tree
[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 362032c56bddec6ad5b639e16eeb594f92886516
100644 blob 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a    README.md

This object is of tree type. It contains the files involved in one commit.

  • File permission
  • File type
  • File hash
  • File name

Notice that file name and content are separate, which means file content can be reused. I can see this in later section.

Commit object content

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -t 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
blob
[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
Rugal

This object is of blob type. It contains the original file content.

Second Commit

Now let’s make another commit that includes difference content and different name from README.md. 

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[ryao@macpro-gn07 deep-inside-git]> echo 'Bernstein' > INSTALL.md && git add -A
[ryao@macpro-gn07 deep-inside-git]> cat INSTALL.md  | git hash-object --stdin
56757e169d62beeb6371e7f5d3bd6bd507edd2f6
[ryao@macpro-gn07 deep-inside-git]> tree .git/objects/
.git/objects/
├── 2f
│   └── b811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
├── 36
│   └── 2032c56bddec6ad5b639e16eeb594f92886516
├── 56
│   └── 757e169d62beeb6371e7f5d3bd6bd507edd2f6
├── f7
│   └── 3a6ae93b095b899fbdb3b2485f5829b9f460cf
├── info
└── pack

6 directories, 4 files

[ryao@macpro-gn07 deep-inside-git]> git commit -m"Add INSTALL.md file"
[master 800d7b9] Add INSTALL.md file
 1 file changed, 1 insertion(+)
 create mode 100644 INSTALL.md
[ryao@macpro-gn07 deep-inside-git]> git lg
* 800d7b9 - (HEAD -> master) Add INSTALL.md file (68 seconds ago) <Rugal Bernstein>
* f73a6ae - Initial commit (13 minutes ago) <Rugal Bernstein>

Now we have 3 more git objects. 

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[ryao@macpro-gn07 deep-inside-git]> tree .git/objects/
.git/objects/
├── 2f
│   └── b811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
├── 36
│   └── 2032c56bddec6ad5b639e16eeb594f92886516
├── 56
│   └── 757e169d62beeb6371e7f5d3bd6bd507edd2f6
├── 80
│   └── 0d7b9974fd1d4ad26791dfcf4bb0478c51c4da
├── c6
│   └── 919ff8ab7ff578ebb6995121d501aa645d0797
├── f7
│   └── 3a6ae93b095b899fbdb3b2485f5829b9f460cf
├── info
└── pack

8 directories, 6 files

And the master is pointing to the latest commit. 

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[ryao@macpro-gn07 deep-inside-git]> cat .git/refs/heads/master
800d7b9974fd1d4ad26791dfcf4bb0478c51c4da

Git commit objects

Commit information

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 800d7b9974fd1d4ad26791dfcf4bb0478c51c4da
tree c6919ff8ab7ff578ebb6995121d501aa645d0797
parent f73a6ae93b095b899fbdb3b2485f5829b9f460cf
author Rugal Bernstein <ryao@peakcontact.com> 1498231664 -0400
committer Rugal Bernstein <ryao@peakcontact.com> 1498231664 -0400

Add INSTALL.md file

This file not only has the same type of data as before, but also has a parent field indicates which commit it follows.

Commit tree

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -p c6919ff8ab7ff578ebb6995121d501aa645d0797
100644 blob 56757e169d62beeb6371e7f5d3bd6bd507edd2f6    INSTALL.md
100644 blob 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a    README.md

We have one more file in tree.

Commit object content

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 56757e169d62beeb6371e7f5d3bd6bd507edd2f6
Bernstein
[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a
Rugal

Override

Finally let’s do another commit, where we are going to override INSTALL.md file with the same content as README.md. 

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[ryao@macpro-gn07 deep-inside-git]> echo 'Rugal' > INSTALL.md && git commit -am"Override content of INSTALL.md"
[master 95ab7c4] Override content of INSTALL.md
 1 file changed, 1 insertion(+), 1 deletion(-)

[ryao@macpro-gn07 deep-inside-git]> git lg
* 95ab7c4 - (HEAD -> master) Override content of INSTALL.md (7 seconds ago) <Rugal Bernstein>
* 800d7b9 - Add INSTALL.md file (15 minutes ago) <Rugal Bernstein>
* f73a6ae - Initial commit (27 minutes ago) <Rugal Bernstein>

Git object content

Commit information

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 95ab7c4
tree 2d5cfaf5513d9dc6876124ff683241bb5c61e0ae
parent 800d7b9974fd1d4ad26791dfcf4bb0478c51c4da
author Rugal Bernstein <ryao@peakcontact.com> 1498232582 -0400
committer Rugal Bernstein <ryao@peakcontact.com> 1498232582 -0400

Override content of INSTALL.md

Not many difference than previous commit.

Commit tree

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[ryao@macpro-gn07 deep-inside-git]> git cat-file -p 2d5cfaf5513d9dc6876124ff683241bb5c61e0ae
100644 blob 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a    INSTALL.md
100644 blob 2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a    README.md

Something weird in this file, we can see both files have the same hash content.
This means git can reuse object to reduce repository size.

Git GC

Some more interesting tool.

We can use gc to compress objects into one pack/index. 

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[ryao@macpro-gn07 deep-inside-git]> git gc
Counting objects: 8, done.
Delta compression using up to 4 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (8/8), done.
Total 8 (delta 0), reused 8 (delta 0)
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[ryao@macpro-gn07 deep-inside-git]> tree .git/objects/
.git/objects/
├── info
│   └── packs
└── pack
    ├── pack-1ba47dfdc2c98b428c17082e4ee16e8c111c42ac.idx
    └── pack-1ba47dfdc2c98b428c17082e4ee16e8c111c42ac.pack

2 directories, 3 files

Git verify-pack

After compression, we can still see object content by using verify-pack. 

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[ryao@macpro-gn07 deep-inside-git]> git verify-pack -v .git/objects/pack/pack-1ba47dfdc2c98b428c17082e4ee16e8c111c42ac.idx
95ab7c4946063d036a84f677081271a8106407ac commit 255 178 12
800d7b9974fd1d4ad26791dfcf4bb0478c51c4da commit 244 172 190
f73a6ae93b095b899fbdb3b2485f5829b9f460cf commit 191 131 362
2fb811a4ca96b3d0ac9b4fb8aa3d96e6a809509a blob   6 15 493
2d5cfaf5513d9dc6876124ff683241bb5c61e0ae tree   75 61 508
c6919ff8ab7ff578ebb6995121d501aa645d0797 tree   75 82 569
56757e169d62beeb6371e7f5d3bd6bd507edd2f6 blob   10 19 651
362032c56bddec6ad5b639e16eeb594f92886516 tree   37 48 670
non delta: 8 objects
.git/objects/pack/pack-1ba47dfdc2c98b428c17082e4ee16e8c111c42ac.pack: ok
development
#git
Deep inside Git
https://rug.al/2017/2017-06-22-deep-inside-git/
Author
Rugal Bernstein
Posted on
June 22, 2017
Licensed under