9.1 Git Internals - Plumbing and Porcelain
Plumbing and Porcelain
This book covers how to use Git with 30 or so verbs such as
remote, and so on. But because Git was initially a toolkit for a VCS rather than a full user-friendly VCS, it has a bunch of verbs that do low-level work and were designed to be chained together UNIX style or called from scripts. These commands are generally referred to as "plumbing" commands, and the more user-friendly commands are called "porcelain" commands.
The book’s first eight chapters deal almost exclusively with porcelain commands. But in this chapter, you’ll be dealing mostly with the lower-level plumbing commands, because they give you access to the inner workings of Git and help demonstrate how and why Git does what it does. These commands aren’t meant to be used manually on the command line, but rather to be used as building blocks for new tools and custom scripts.
When you run
git init in a new or existing directory, Git creates the
.git directory, which is where almost everything that Git stores and manipulates is located. If you want to back up or clone your repository, copying this single directory elsewhere gives you nearly everything you need. This entire chapter basically deals with the stuff in this directory. Here’s what it looks like:
$ ls HEAD branches/ config description hooks/ index info/ objects/ refs/
You may see some other files in there, but this is a fresh
git init repository — it’s what you see by default. The
branches directory isn’t used by newer Git versions, and the
description file is only used by the GitWeb program, so don’t worry about those. The
config file contains your project-specific configuration options, and the
info directory keeps a global exclude file for ignored patterns that you don’t want to track in a .gitignore file. The
hooks directory contains your client- or server-side hook scripts, which are discussed in detail in Chapter 7.
This leaves four important entries: the
index files and the
refs directories. These are the core parts of Git. The
objects directory stores all the content for your database, the
refs directory stores pointers into commit objects in that data (branches), the
HEAD file points to the branch you currently have checked out, and the
index file is where Git stores your staging area information. You’ll now look at each of these sections in detail to see how Git operates.