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copied!<p>The Git book contains an article on <a href="http://shafiulazam.com/gitbook/7_the_git_index.html" rel="noreferrer">what an index includes</a>:</p> <blockquote> <p>The index is a binary file (generally kept in <code>.git/index</code>) containing a sorted list of path names, each with permissions and the SHA1 of a blob object; <code>git ls-files</code> can show you the contents of the index:</p> </blockquote> <pre><code>$ git ls-files --stage 100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 .gitignore 100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 .mailmap </code></pre> <p>The <a href="http://www.kernel.org/pub/software/scm/git/docs/technical/racy-git.txt" rel="noreferrer">Racy git problem</a> gives some more details on that structure:</p> <blockquote> <p>The index is one of the most important data structures in git.<br> It represents a virtual working tree state by recording list of paths and their object names and serves as a staging area to write out the next tree object to be committed.<br> The state is "virtual" in the sense that it does not necessarily have to, and often does not, match the files in the working tree.</p> </blockquote> <hr> <p>To see more, cf. "<a href="https://github.com/git/git/blob/master/Documentation/technical/index-format.txt" rel="noreferrer"><strong>git/git/Documentation/technical/index-format.txt</strong></a>":</p> <h2>The Git index file has the following format</h2> <blockquote> <p>All binary numbers are in network byte order.<br> <strong>Version 2</strong> is described here unless stated otherwise.</p> <ul> <li>A 12-byte header consisting of: <ul> <li>4-byte <strong>signature</strong>:<br> The signature is { '<code>D</code>', '<code>I</code>', '<code>R</code>', '<code>C</code>' } (stands for "<strong><code>dircache</code></strong>")</li> <li>4-byte <strong>version number</strong>:<br> The current supported versions are 2, 3 and 4.</li> <li>32-bit number of index entries.</li> </ul></li> <li>A number of sorted <strong><a href="https://github.com/git/git/blob/867b1c1bf68363bcfd17667d6d4b9031fa6a1300/Documentation/technical/index-format.txt#L38" rel="noreferrer">index entries</a></strong>.</li> <li><strong>Extensions</strong>:<br> Extensions are identified by signature.<br> Optional extensions can be ignored if Git does not understand them.<br> Git currently supports cached tree and resolve undo extensions. <ul> <li>4-byte extension signature. If the first byte is '<code>A</code>'..'<code>Z</code>' the extension is optional and can be ignored.</li> <li>32-bit size of the extension</li> <li>Extension data</li> </ul></li> <li>160-bit SHA-1 over the content of the index file before this checksum.</li> </ul> </blockquote> <hr> <p><a href="https://stackoverflow.com/users/1483503/mljrg">mljrg</a> <a href="https://stackoverflow.com/users/1483503/mljrg">comments</a>:</p> <blockquote> <p>If the index is the place where the next commit is prepared, why doesn't "<code>git ls-files -s</code>" return nothing after commit?</p> </blockquote> <p>Because <strong>the index represents what is being tracked</strong>, and right after a commit, what is being tracked is identical to the last commit (<code>git diff --cached</code> returns nothing). </p> <p>So <code>git ls-files -s</code> lists all files tracked (object name, mode bits and stage number in the output).</p> <p>That list (of element tracked) is initialized with the content of a commit.<br> When you switch branch, the index content is reset to the commit referenced by the branch you just switched to.</p>

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