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.git/index

.git/index is a binary file that contains a list of all the files being tracked (thus, it represents a virtual working tree. (Virtual: the index does not necessarily have to match the files in the working tree).
The index is also called staging area.
New files are added with git add, existing files removed with git rm.
A commit commits the files that are in the staging area.
The current index file for the repository.
An index file is usually not found in a bare repository.

Showing the content of the index

The content of the .git/index file can be shown with: 
git ls-files

Format of the index file

The format of the index file is documented in git's own repository under Documentation/technical/index-format.txt.

See also

The index file can be manipulated with git update-index.
git add, git-apply, git checkout-index
index
.git directory structure

Links

gin is a Python script that parses index files displays its content in readable form.

Index