GITFORMAT-INDEX(5)

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version 2.56.0 · 22 versions
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🌸 GITFORMAT-INDEX 💅 — Git Index Format, but make it make sense 💅

Okay bestie, so this is the gitformat-index page — it’s literally the manual for Git’s index file format, and we’re about to break it down so you can understand the whole vibe. No cap, it’s a lot of bytes, but we got this. ✨

✨ SYNOPSIS ✨

$GIT_DIR/index

That’s it. That’s the whole synopsis. The index file lives at $GIT_DIR/index — periodt. 💅

🎀 DESCRIPTION 🎀

It’s the Git index format. Simple as that. But like, not simple at all, so let’s get into it. 🫶

🔥 THE GIT INDEX FILE HAS THE FOLLOWING FORMAT 🔥

First things first, bestie:

  • All binary numbers are in network byte order. (Big-endian, if you’re fancy.)
  • In a repository using the traditional SHA-1, checksums and object IDs (object names) mentioned below are all computed using SHA-1. Similarly, in SHA-256 repositories, these values are computed using SHA-256.
  • Version 2 is described here unless stated otherwise. (We’ll shout out versions 3 and 4 when they matter, don’t worry.)

Here’s the structure, step by step:

🧚 1. A 12-byte header, consisting of:

  • 4-byte signature — the signature is { 'D', 'I', 'R', 'C' } (which stands for "dircache" — iconic, honestly).
  • 4-byte version number — the current supported versions are 2, 3 and 4.
  • 32-bit number of index entries — how many entries are in the index, as a 32-bit number. Slay.

🦋 2. A number of sorted index entries

These are sorted, and we’ll explain exactly how below. Keep reading, bestie. 😭

🌈 3. Extensions

Extensions are identified by signature. Optional extensions can be ignored if Git does not understand them. Here’s the layout:

  • 4-byte extension signature — if the first byte is 'A'..'Z', the extension is optional and can be ignored.
  • 32-bit size of the extension — how big the extension data is.
  • Extension data — the actual payload.

👑 4. Hash checksum over the content of the index file before this checksum

That’s a checksum over everything before it. It’s giving “trust but verify,” and honestly? We love to see it. 🔥

⭐ INDEX ENTRY ⭐

Okay so here’s the tea on index entries:

Index entries are sorted in ascending order on the name field, interpreted as a string of unsigned bytes (i.e. memcmp() order, no localization, no special casing of directory separator '/'). Entries with the same name are sorted by their stage field.

An index entry typically represents a file. However, if sparse-checkout is enabled in cone mode (core.sparseCheckoutCone is enabled) and the extensions.sparseIndex extension is enabled, then the index may contain entries for directories outside of the sparse-checkout definition. These entries have mode 040000, include the SKIP_WORKTREE bit, and the path ends in a directory separator.

Here’s the full field-by-field breakdown — every single byte, no skips:

  • 32-bit ctime seconds — the last time a file’s metadata changed. This is stat(2) data. 🕒
  • 32-bit ctime nanosecond fractions — also stat(2) data. Tiny time details, we love.
  • 32-bit mtime seconds — the last time a file’s data changed. stat(2) data again.
  • 32-bit mtime nanosecond fractions — yep, stat(2) data. ✨
  • 32-bit dev — stat(2) data.
  • 32-bit ino — stat(2) data.
  • 32-bit mode, split into (high to low bits):
    • 16-bit unused, must be zero.
    • 4-bit object type — valid values in binary are 1000 (regular file), 1010 (symbolic link) and 1110 (gitlink). 💀
    • 3-bit unused, must be zero.
    • 9-bit unix permission — only 0755 and 0644 are valid for regular files. Symbolic links and gitlinks have value 0 in this field.
  • 32-bit uid — stat(2) data.
  • 32-bit gid — stat(2) data.
  • 32-bit file size — this is the on-disk size from stat(2), truncated to 32-bit.
  • Object name for the represented object — the OID, basically.
  • A 16-bit flags field, split into (high to low bits):
    • 1-bit assume-valid flag
    • 1-bit extended flag (must be zero in version 2)
    • 2-bit stage (during merge)
    • 12-bit name length — if the length is less than 0xFFF; otherwise 0xFFF is stored in this field.
  • (Version 3 or later) A 16-bit field, only applicable if the "extended flag" above is 1, split into (high to low bits):
    • 1-bit reserved for future
    • 1-bit skip-worktree flag (used by sparse checkout)
    • 1-bit intent-to-add flag (used by git add -N)
    • 13-bit unused, must be zero.
  • Entry path name (variable length) — relative to top level directory (without leading slash). '/' is used as path separator. The special path components ".", ".." and ".git" (without quotes) are disallowed. Trailing slash is also disallowed. 🚫
The exact encoding is undefined, but the '.' and '/' characters are encoded in 7-bit ASCII and the encoding cannot contain a NUL byte (iow, this is a UNIX pathname). Bestie, that’s important. Don’t forget it. 🙈
  • (Version 4) In version 4, the entry path name is prefix-compressed relative to the path name for the previous entry (the very first entry is encoded as if the path name for the previous entry is an empty string). At the beginning of an entry, an integer N in the variable width encoding (the same encoding as the offset is encoded for OFS_DELTA pack entries; see linkgit:gitformat-pack[5]) is stored, followed by a NUL-terminated string S. Removing N bytes from the end of the path name for the previous entry, and replacing it with the string S yields the path name for this entry. It’s giving compression, honestly. 🎀
  • 1-8 nul bytes as necessary to pad the entry to a multiple of eight bytes while keeping the name NUL-terminated.
  • (Version 4) In version 4, the padding after the pathname does not exist. Periodt.
Interpretation of index entries in split index mode is completely different. See below for details. Don’t skip it, bestie. 💀

🌸 EXTENSIONS 🌸

🧚 Cache tree

Since the index does not record entries for directories, the cache entries cannot describe tree objects that already exist in the object database for regions of the index that are unchanged from an existing commit. The cache tree extension stores a recursive tree structure that describes the trees that already exist and completely match sections of the cache entries. This speeds up tree object generation from the index for a new commit by only computing the trees that are “new” to that commit. It also assists when comparing the index to another tree, such as HEAD^{tree}, since sections of the index can be skipped when a tree comparison demonstrates equality.

The recursive tree structure uses nodes that store a number of cache entries, a list of subnodes, and an object ID (OID). The OID references the existing tree for that node, if it is known to exist. The subnodes correspond to subdirectories that themselves have cache tree nodes. The number of cache entries corresponds to the number of cache entries in the index that describe paths within that tree’s directory.

The extension tracks the full directory structure in the cache tree extension, but this is generally smaller than the full cache entry list. Iconic efficiency. 👑

When a path is updated in index, Git invalidates all nodes of the recursive cache tree corresponding to the parent directories of that path. We store these tree nodes as being “invalid” by using "-1" as the number of cache entries. Invalid nodes still store a span of index entries, allowing Git to focus its efforts when reconstructing a full cache tree.

The signature for this extension is { 'T', 'R', 'E', 'E' }. Like the tree emoji but make it bytes. 🌳

A series of entries fill the entire extension; each of which consists of:

  • NUL-terminated path component — relative to its parent directory.
  • ASCII decimal number of entries in the index that is covered by the tree this entry represents (entry_count).
  • A space — ASCII 32. Literally just a space. 💅
  • ASCII decimal number that represents the number of subtrees this tree has.
  • A newline — ASCII 10.
  • Object name for the object that would result from writing this span of index as a tree.
An entry can be in an invalidated state and is represented by having a negative number in the entry_count field. In this case, there is no object name and the next entry starts immediately after the newline. When writing an invalid entry, -1 should always be used as entry_count. Don’t forget, bestie. 😭

The entries are written out in the top-down, depth-first order. The first entry represents the root level of the repository, followed by the first subtree — let’s call this A — of the root level (with its name relative to the root level), followed by the first subtree of A (with its name relative to A), and so on. The specified number of subtrees indicates when the current level of the recursive stack is complete. It’s giving recursion, and honestly? We’re obsessed. ✨

🫶 Resolve undo

A conflict is represented in the index as a set of higher stage entries. When a conflict is resolved (e.g. with "git add path"), these higher stage entries will be removed and a stage-0 entry with proper resolution is added.

When these higher stage entries are removed, they are saved in the resolve undo extension, so that conflicts can be recreated (e.g. with "git checkout -m"), in case users want to redo a conflict resolution from scratch. It’s literally the undo button for conflicts. Slay. 🔥

The signature for this extension is { 'R', 'E', 'U', 'C' }.

A series of entries fill the entire extension; each of which consists of:

  • NUL-terminated pathname — the entry describes (relative to the root of the repository, i.e. full pathname).
  • Three NUL-terminated ASCII octal numbers — entry mode of entries in stage 1 to 3 (a missing stage is represented by "0" in this field).
  • At most three object names of the entry in stages from 1 to 3 (nothing is written for a missing stage).

Conflict resolution core, no cap. 💅

🦋 Split index

In split index mode, the majority of index entries could be stored in a separate file. This extension records the changes to be made on top of that to produce the final index. It’s giving “divide and conquer,” honestly. 🎀

The signature for this extension is { 'l', 'i', 'n', 'k' }.

The extension consists of:

  • Hash of the shared index file — the shared index file path is $GIT_DIR/sharedindex.<hash>. If all bits are zero, the index does not require a shared index file. 💀
  • An ewah-encoded delete bitmap — each bit represents an entry in the shared index. If a bit is set, its corresponding entry in the shared index will be removed from the final index. Note: because a delete operation changes index entry positions, but we do need original positions in replace phase, it’s best to just mark entries for removal, then do a mass deletion after replacement. Strategic, we love. 🔥
  • An ewah-encoded replace bitmap — each bit represents an entry in the shared index. If a bit is set, its corresponding entry in the shared index will be replaced with an entry in this index file. All replaced entries are stored in sorted order in this index. The first "1" bit in the replace bitmap corresponds to the first index entry, the second "1" bit to the second entry and so on. Replaced entries may have empty path names to save space.
The remaining index entries after replaced ones will be added to the final index. These added entries are also sorted by entry name then stage. Okay bestie, that’s a lot of info, but you’re doing amazing. 💜

This is part 1 of 2, so don’t go anywhere — more follows. You’re literally doing so good. 🌈

🌸 UNTRACKED CACHE 🌸

okay so the untracked cache is literally the biggest slay — it saves the whole untracked file list plus the data you need to actually verify that cache is still valid. its signature is { 'U', 'N', 'T', 'R' }.

the extension starts with:

  • a sequence of NUL-terminated strings, preceded by the size of the sequence in variable width encoding. each string describes the environment where the cache can be used 💅
  • stat data of $GIT_COMMON_DIR/info/exclude. see the "Index entry" section from the ctime field until "file size"
  • stat data of core.excludesFile
  • 32-bit dir_flags (see struct dir_struct)
  • hash of $GIT_COMMON_DIR/info/exclude. a null hash means the file does not exist
  • hash of core.excludesFile. a null hash means the file does not exist
  • NUL-terminated string of per-dir exclude file name. this usually is .gitignore
  • the number of following directory blocks, in variable width encoding. if this number is zero, the extension ends here with a following NUL 🎀
  • a number of directory blocks in depth-first-search order, each one consists of:
    • the number of untracked entries, variable width encoding
    • the number of sub-directory blocks, variable width encoding
    • the directory name terminated by NUL
    • a number of untracked file/dir names terminated by NUL

the remaining data of each directory block is grouped by type:

  • an ewah bitmap, the n-th bit marks whether the n-th directory has valid untracked cache entries
  • an ewah bitmap, the n-th bit records the "check-only" bit of read_directory_recursive() for the n-th directory
  • an ewah bitmap, the n-th bit indicates whether hash and stat data is valid for the n-th directory and exists in the next data
  • an array of stat data. the n-th data corresponds with the n-th "one" bit in the previous ewah bitmap
  • an array of hashes. the n-th hash corresponds with the n-th "one" bit in the previous ewah bitmap
  • one NUL

🦋 FILE SYSTEM MONITOR CACHE 🦋

the file system monitor cache tracks files for which the core.fsmonitor hook has told us about changes. bestie, it's giving watchful guardian energy. the signature for this extension is { 'F', 'S', 'M', 'N' }.

the extension starts with:

  • 32-bit version number: the current supported versions are 1 and 2
  • (Version 1) 64-bit time: the extension data reflects all changes through the given time, which is stored as the nanoseconds elapsed since midnight, January 1, 1970 🕛
  • (Version 2) a null terminated string: an opaque token defined by the file system monitor application. the extension data reflects all changes relative to that token
  • 32-bit bitmap size: the size of the CE_FSMONITOR_VALID bitmap
  • an ewah bitmap, the n-th bit indicates whether the n-th index entry is not CE_FSMONITOR_VALID

✨ END OF INDEX ENTRY ✨

the End of Index Entry (EOIE) is used to locate the end of the variable length index entries and the beginning of the extensions. code can take advantage of this to quickly locate the index extensions without having to parse through all of the index entries. main character energy honestly — it just gets straight to the point.

because it must be able to be loaded before the variable length cache entries and other index extensions, this extension must be written last. periodt. the signature for this extension is { 'E', 'O', 'I', 'E' }.

the extension consists of:

  • 32-bit offset to the end of the index entries
  • hash over the extension types and their sizes (but not their contents). e.g. if we have a "TREE" extension that is N-bytes long, a "REUC" extension that is M-bytes long, followed by "EOIE", then the hash would be:
Hash("TREE" + <binary-representation-of-N> +
        "REUC" + <binary-representation-of-M>)

💜 INDEX ENTRY OFFSET TABLE 💜

the Index Entry Offset Table (IEOT) is used to help address the CPU cost of loading the index by enabling multi-threading the process of converting cache entries from the on-disk format to the in-memory format. it's literally a glow-up for load times 🔥. the signature for this extension is { 'I', 'E', 'O', 'T' }.

the extension consists of:

  • 32-bit version (currently 1)
  • a number of index offset entries, each consisting of:
    • 32-bit offset from the beginning of the file to the first cache entry in this block of entries
    • 32-bit count of cache entries in this block

🎀 SPARSE DIRECTORY ENTRIES 🎀

when using sparse-checkout in cone mode, some entire directories within the index can be summarized by pointing to a tree object instead of the entire expanded list of paths within that tree. an index containing such entries is a "sparse index". index format versions 4 and less were not implemented with such entries in mind. thus, for these versions, an index containing sparse directory entries will include this extension with signature { 's', 'd', 'i', 'r' }.

⚠️ like the split-index extension, tools should avoid interacting with a sparse index unless they understand this extension. no cap — if your tool doesn't get it, it should stay out of it. 🙈

🌈 GIT 🌈

part of the git(1) suite 💖

and that's a wrap, bestie! this little masterpiece came to you from Git 2.56.0, dated 2026-09-27, and it's the GITFORMAT-INDEX(5) page — now go forth and parse some indexes, you absolute icon 👑💅✨

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