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DISSECTING GIT'S GUTS

@emilyxxie

  • For most people, Git is complete sorcery
  • Git happens 😆
  • Why dissect Git? Gain comfort, better mental model. E.g. what is at the heart of a branch? What is a detached HEAD state?
  • Git Porcelain: the high-level commands we use on a daily basis
  • Plumbing: the internal, low-level commands

Agenda

  • Git directory

  • Git objects

  • Git references

  • Git packfiles

  • Git directory

    • .git
    • git init puts the scaffolding in place needed to work with git on a project
    • A git clone just copies the .git folder
    • ls .git
      • objects
      • refs
      • …

  • Objects

    • Where the content is!
    • Git’s database
      • Content-addressable filesystem
      • Git gives you a key to retrieve content from the database
      • git hash-object -w hello_world.txt plumbing command, gives use the SHA-1! 💥
      • For a given set of content, you reliably generate the same hash
        • A fingerprint
      • git cat-file -p allows you to read from the database, -p makes the output pretty, -t by the way, returns the object’s type (e.g. blob, tree)
      • Git’s database stores compressed objects, compressed with zlib

  • Exercise

  • Cheat sheet

  • git hash-object -w bio.md

  • git cat-file -p 9d6b6928c57e27ddd64bcad64b1a53623c9867b2

  • Git doesn’t store diffs, for each version of a file, it installs an entire copy of the file (initially) but… [follow up later on this]

  • Trees

    • A complete snapshot of your directory
    • To create a tree… Need an index first (git update-index --add, a low-level git add), index lives in .git/index
    • git ls-files —stage shows the index
    • git write-tree
    • Note, trees can reference other trees (sub-directories)

  • Exercise: Make a tree!

git write-tree
0baa49142494d6bb979bcd36572662bdd62f2160
git cat-file -p 0baa49142494d6bb979bcd36572662bdd62f2160
100644 blob 9d6b6928c57e27ddd64bcad64b1a53623c9867b2	bio.md
040000 tree d35e12dfe16923b05e99fbf49c31e91c18eb1db5	subdir
git cat-file -p d35e12dfe16923b05e99fbf49c31e91c18eb1db5
100644 blob 8ab01e46b848c561cf37000562915c4ae4d116ba	barbaz.md
100644 blob 5ddc4a0635b7e92187567282868679136da1ec1f	foobar.md

  • Commit objects

    • git commit-tree (echo “first commit” | git commit-tree $SHA -p $PARENT_SHA)
    • We’ve done a git commit! Up until now, we have not committed anything, nuttin’!

  • Exercise: Commit an object!

echo "First commit" | git commit-tree 0baa49142494d6bb979bcd36572662bdd62f2160
cat >> foobar.md
git update-index --add foobar.md
git write-tree
echo "Second commit" | git commit-tree 4fa2797f288b553d0ed8cddb59b3719f817cdabe -p f6e6ec6a7806b894ccce06c57a6568798b11ab60
git cat-file -p ea99086abafac64f6598a3053d80c64a24fc3f04
tree 4fa2797f288b553d0ed8cddb59b3719f817cdabe
parent f6e6ec6a7806b894ccce06c57a6568798b11ab60
author Steve Winton <[email protected]> 1467069112 -0700
committer Steve Winton <[email protected]> 1467069112 -0700

Second commit

  • 🎉 We’ve now performed all the low-level plumbing commands for git add, git commit

  • Git is a both a DAG and a Merkle tree (Merkle DAG)

    • Merkle also used with Bitcoin

  • Git references

  • We have blobs, trees, commits as our low-level objects in our Git objects database

  • How do we keep track of what commit we want to work with?

    • Branches, but what is a branch?
    • How does Git know what goes with a branch?
    • Branches are just pointers, that reside in .git/refs/heads

  • git update-ref can be used to create a branch

    • git update-ref refs/heads/master $SHA
    • Just a file, containing the SHA the branch points to 😮

  • What about .git/HEAD?

    • Just a pointer to current branch
    • Detached head is when HEAD points to a commit, not to a branch
    • Any commits made on detached HEAD will not belong to a branch, and will eventually get pruned by Git
    • To recover: Checkout a branch

  • Exercise: References

 git update-ref refs/heads/master ea99086abafac64f6598a3053d80c64a24fc3f04
 cat .git/refs/heads/master
 git checkout -b my-feature-is-a-feature
 ls -a .git/refs/heads/
 cat .git/refs/heads/my-feature-is-a-feature
 cat bio.md
 cat >> bio.md
 git st
 git add bio.md
 git commit
 cat .git/refs/heads/my-feature-is-a-feature
  • Packfiles
    • git gc garbage collects
    • git count-objects, returns total no. of lose objects, should significantly shrink after git gc
    • git verify-pack

Resources

Slides?

  • This was recorded at Git Merge, recording available… 🔜?

Metadata


Mon Jun 27 15:05:30 PDT 2016