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Commit

A Commit object in Git represents a snapshot of the repository's state. It is stored as a binary blob in the Git object database and links to its corresponding Tree and parent commits (if any).

Structure of a Commit Object​

The header contains:

  1. Object Type: Always commit.
  2. Size: The size of the file content in bytes.
  3. Null Byte Separator: A null byte (\x00) separating the header from the content.

Body​

The body of a Commit object is structured as follows:

  1. Tree Line: Links the commit to the Tree object representing the directory structure: tree <tree_hash>

  2. Parent Line(s) (if any): Links to the parent commit(s). For merge commits, there will be one line per parent: parent <parent_hash>

  3. Author Line: Specifies the name, email, timestamp, and timezone of the commit's author: author <name> <email> <timestamp> <timezone_offset>

  4. Committer Line: Similar to the author line but refers to the person who committed the changes: committer <name> <email> <timestamp> <timezone_offset>

  5. Blank Line: A single blank line (\x0A) separates the metadata from the commit message.

  6. Commit Message: Contains the description of the changes made in this commit.

  7. Null Byte Terminator: A null byte (\x00) marking the end of the commit object.

Important Notes​

  • Line Separation: Each metadata line ends with a newline character (\x0A), except for the commit message.
  • Spaces in Metadata: There are spaces between elements in the metadata (e.g., between the author’s name and email).
  • Blank Line Placement: A blank line is required before the commit message.

Example Commit Object​

An example commit object might look like this:

commit 210\x00tree b4d2d0140eae40e6f7f6503aa4eb7a7330379bf9\x0A
parent 8c4d92b5dcb71e928f662b3b5de8219a8cb61ff0\x0A
author Linus Torvalds <linus.torvalds@linux.com> 1587032664 +0530\x0A
committer Linus Torvalds <linus.torvalds@linux.com> 1587032664 +0530\x0A
\x0A
Your code is trash\x00

Readable Breakdown​

For better understanding, the commit can be visualized as:

commit 210\x00

tree b4d2d0140eae40e6f7f6503aa4eb7a7330379bf9
parent 8c4d92b5dcb71e928f662b3b5de8219a8cb61ff0
author Linus Torvalds <linus.torvalds@linux.com> 1587032664 +0530
committer Linus Torvalds <linus.torvalds@linux.com> 1587032664 +0530

Your code is trash

Details to Note​

  1. Tree Hash: Links to the exact state of the directory.
  2. Parent Commit Hash: Ensures the continuity of the commit chain.
  3. Timestamps and Timezone: Provide temporal context for the commit.
  4. Commit Message: Summarizes the purpose of the commit.

Decoding a Commit Object​

To decode a commit object:

  1. Read the Header:

    • Extract the object type (commit).
    • Determine the size of the content.

  2. Parse the Body:

    • Extract the tree hash, parent hash(es), author, and committer information.
    • Identify the commit message after the blank line.

  3. Validate the Structure:

    • Ensure proper line breaks and separators are present.

By maintaining this structure, Git ensures each commit accurately represents the repository state, enabling precise version control and traceability.