23. Git inside: Direct work with git objects

Goals

  • Explore the structure of the database objects
  • Using SHA1 hashes for searching the content in repository

Let us examine git objects with some tools.

01 Searching for the last commit

Run:

git hist --max-count=1

This command should find the last commit in the repository. SHA1 hash is probably different on our systems; however you should see something like this.

Result:

$ git hist --max-count=1
* 8029c07 2011-03-09 | Added index.html. (HEAD, master) [Marina Pushkova]

02 Display of the last commit

With SHA1 hash of a commit, as above...

Run:

git cat-file -t <hash>
git cat-file -p <hash>

I see this ...

Result:

$ git cat-file -t 8029c07
commit
$ git cat-file -p 8029c07
tree 096b74c56bfc6b40e754fc0725b8c70b2038b91e
parent 567948ac55daa723807c0c16e34c76797efbcbed
author Marina Pushkova <marina (at) githowto.com> 1299684476 -0500
committer Marina Pushkova <marina (at) githowto.com> 1299684476 -0500

Added index.html.

Note: If you specify the alias as «type» and «dump», as described in the corresponding lesson, you can enter commands git type and git dump instead of a long command (which I never memorize).

This displays the commit object, which is in the head of master branch.

03 Tree search

We can display the tree referenced in the commit. This should be a file description (top level) in our project (for a specific commit). Use the SHA1 hash of the tree string from the list above.

Run:

git cat-file -p <treehash>

Here is my tree ...

Result:

$ git cat-file -p 096b74c
100644 blob 28e0e9d6ea7e25f35ec64a43f569b550e8386f90	index.html
040000 tree e46f374f5b36c6f02fb3e9e922b79044f754d795	lib

I can see the index.html file and lib folder.

04 Display lib directory

Run:

git cat-file -p <libhash>

Result:

$ git cat-file -p e46f374
100644 blob c45f26b6fdc7db6ba779fc4c385d9d24fc12cf72	hello.html

There is a hello.html file.

05 Display hello.html file

Run:

git cat-file -p <hellohash>

Result:

$ git cat-file -p c45f26b
<!-- Author: Marina Pushkova (marina@githowto.com) -->
<html>
  <head>
  </head>
  <body>
    <h1>Hello, World!</h1>
  </body>
</html>

And there it is. Tree objects, commit objects and blob objects are displayed directly from the git repository. That's all there is - trees, blobs and commits.

06 Explore by yourself

The git repository can be explored manually. Try to manually find the original hello.html file from the first commit with help of SHA1 hash references in the last commit.

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