Overview

Unlike the Git commands found on the previous posts in this series, in this article, I’ll focus on low-level Git commands.

Raison d’être

Usually, one won’t use low-level Git commands in a Git repository, but I included them here because these commands help one understand the Git object model.

The list

$ find .git/objects       # Return all files/folders under `.git/objects'
$ git cat-file -p {hash}  # Print the content to which {hash} corresponds
$ git hash-object {file}  # Return the hash of {file}
$ git hash-object --stdin # Return the hash of the standard input
$ git write-tree          # Return the hash of the tree of tracked contents
$ git ls-files -s         # List all tracked files and their hash
$ git ls-tree {hash}      # List the contents of a tree
$ git rev-parse {hash}    # Return the full SHA-1 of {hash}
$ git rev-parse HEAD      # Return the full SHA-1 of HEAD
$ git diff                # Show the difference between the working directory and the index
$ git diff --cached       # Show the difference between HEAD and the index
$ git diff HEAD           # Show the difference between HEAD and the working directory

It isn’t necessary to supply the full SHA-1 hash for {hash}—several hex digits at the beginning will be enough.

Facts learnt

1. An SHA-1 hash has 160 bits. Since 1 byte is equal 8 bit, it has 20 bytes. When it’s displayed, it has 40 hex-digits.
2. The blob for a file is independent of the file name, which was actually stored in a tree object. For example, the object ID of a file which contains only a single line “hello world” is 3b18e512dba79e4c8300dd08aeb37f8e728b8dad, so if the file is stored inside the .git directory as a blob, the blob will be named as .git/objects/3b/18e512dba79e4c8300dd08aeb37f8e728b8dad.
3. If a sub-directory is created in the dir directory, then a tree entry will be created in the tree object corresponding to dir.¹

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