The index is a binary file (generally kept in .git/index) containing a sorted list of path names, each with permissions and the SHA1 of a blob object; git ls-files can show you the contents of the index:
git ls-files --stage
100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 .gitignore
100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 .mailmap
100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0 COPYING
100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0 Documentation/.gitignore
100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0 Documentation/Makefile
...
100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0 xdiff/xtypes.h
100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0 xdiff/xutils.c
100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0 xdiff/xutils.hNote that in older documentation you may see the index called the “current directory cache” or just the “cache”. It has three important properties:
- The index contains all the information necessary to generate a single (uniquely determined) tree object.
- The index enables fast comparisons between the tree object it defines and the working tree.
- It can efficiently represent information about merge conflicts between different tree objects, allowing each pathname to be associated with sufficient information about the trees involved that you can create a three-way merge between them.
The index is thus a sort of temporary staging area, which is filled with a tree which you are in the process of working on.
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Good explanation of how the Git index acts as a staging area between working directory and commits. One thing worth adding is that the index is not just a “temporary list” — it actually stores full file snapshots (blobs + metadata), which is why commands like
git diff --stagedcan show exact commit-ready content even before committing. In real workflows, this becomes important during partial commits and conflict resolution, where the index can hold multiple staged states for the same file.