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Tree

A Tree object in Git represents a directory. Its contents include other tree objects and blob objects, allowing Git to keep track of the names and hierarchical structure of files and folders in a project.

Structure of a Tree Object​

A Tree object is structured as follows:

  1. Object Type: Always tree.
  2. Size: The size of the file content in bytes.
  3. Null Byte Separator: A null byte (\x00) separating the header from the content.
  4. Content: The main body containing the file structure.
  5. Null Byte Terminator: A null byte indicating the end of the content.

Body​

The body of the tree file is formatted as follows:

<fileType> <fileName>\x00<sha1Bytes>\x00

Notes on Size Calculation​

  • The header title (blob) and the final null byte do not count towards the total size.
  • The first null byte (\x00) is included in the size calculation.

Key Details​

  • SHA-1 Representation: The SHA-1 is stored as raw bytes rather than as a hexadecimal string.
  • Continuous Data: There is no explicit separation between each TreeLine; the next fileType directly follows the SHA-1 of the previous entry.
  • File Types: There are four file types commonly used:
    • 100644: Regular file (blob).
    • 100755: Executable file (blob).
    • 040000: Directory (tree) (although having this 0 at the beginning, must of the time this is dropped of the mode).

Example of a Tree File​

A tree object might appear as:

tree 122\x00100644file1.txt\x00<20-byte SHA-1>100755script.sh\x00<20-byte SHA-1>040000subdir\x00<20-byte SHA-1>\x00

Readable Breakdown​

For clarity, the example above can be visualized as:

tree 122\x00
100644 file1.txt\x00<20-byte SHA-1>
100755 script.sh\x00<20-byte SHA-1>
040000 subdir\x00<20-byte SHA-1>

However, it is crucial to understand that:

  1. No Line Separators: The data is stored as a continuous stream without line breaks.
  2. Separations: The only actual separator (\x20) is the space between the title (tree) and the size of the content.

Decoding the Tree Object​

To interpret a Tree object:

  1. Read the Header: Identify the object type (tree) and content size.
  2. Parse the Body:
    • Extract each file entry based on the file type, file name, and SHA-1.
    • Use the null bytes as markers to differentiate between components.

This structure ensures Git's ability to efficiently manage project directories and maintain their integrity through the SHA-1 hashes for version tracking.