git-lfs/docs/spec.md

# Git LFS Specification

This is a general guide for Git LFS clients.  Typically it should be
implemented by a command line `git-lfs` tool, but the details may be useful
for other tools.

## The Pointer

The core Git LFS idea is that instead of writing large blobs to a Git repository,
only a pointer file is written.

* Pointer files are text files which MUST contain only UTF-8 characters.
* Each line MUST be of the format `{key} {value}\n` (trailing unix newline).
* Only a single space character between `{key}` and `{value}`.
* Keys MUST only use the characters `[a-z] [0-9] . -`.
* The first key is _always_ `version`.
* Lines of key/value pairs MUST be sorted alphabetically in ascending order
(with the exception of `version`, which is always first).
* Values MUST NOT contain return or newline characters.
* Pointer files MUST be stored in Git with their executable bit matching that
of the replaced file.
* Pointer files are unique: that is, there is exactly one valid encoding for a
  pointer file.

An empty file is the pointer for an empty file. That is, empty files are
passed through LFS without any change.

The required keys are:

* `version` is a URL that identifies the pointer file spec.  Parsers MUST use
simple string comparison on the version, without any URL parsing or
normalization.  It is case sensitive, and %-encoding is discouraged.
* `oid` tracks the unique object id for the file, prefixed by its hashing
method: `{hash-method}:{hash}`.  Currently, only `sha256` is supported.  The
hash is lower case hexadecimal.
* `size` is in bytes.

Example of a v1 text pointer:

```
version https://git-lfs.github.com/spec/v1
oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
size 12345
(ending \n)
```

Blobs created with the pre-release version of the tool generated files with
a different version URL.  Git LFS can read these files, but writes them using
the version URL above.

```
version https://hawser.github.com/spec/v1
oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
size 12345
(ending \n)
```

For testing compliance of any tool generating its own pointer files, the
reference is this official Git LFS tool:

**NOTE:** exact pointer command behavior TBD!

* Tools that parse and regenerate pointer files MUST preserve keys that they
don't know or care about.
* Run the `pointer` command to generate a pointer file for the given local
file:

    ```
    $ git lfs pointer --file=path/to/file
    Git LFS pointer for path/to/file:

    version https://git-lfs.github.com/spec/v1
    oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
    size 12345
    ```

* Run `pointer` to compare the blob OID of a pointer file built by Git LFS with
a pointer built by another tool.

  * Write the other implementation's pointer to "other/pointer/file":

    ```
    $ git lfs pointer --file=path/to/file --pointer=other/pointer/file
    Git LFS pointer for path/to/file:

    version https://git-lfs.github.com/spec/v1
    oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
    size 12345

    Blob OID: 60c8d8ab2adcf57a391163a7eeb0cdb8bf348e44

    Pointer from other/pointer/file
    version https://git-lfs.github.com/spec/v1
    oid sha256 4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
    size 12345

    Blob OID: 08e593eeaa1b6032e971684825b4b60517e0638d

    Pointers do not match
    ```

  * It can also read STDIN to get the other implementation's pointer:

    ```
    $ cat other/pointer/file | git lfs pointer --file=path/to/file --stdin
    Git LFS pointer for path/to/file:

    version https://git-lfs.github.com/spec/v1
    oid sha256:4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
    size 12345

    Blob OID: 60c8d8ab2adcf57a391163a7eeb0cdb8bf348e44

    Pointer from STDIN
    version https://git-lfs.github.com/spec/v1
    oid sha256 4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393
    size 12345

    Blob OID: 08e593eeaa1b6032e971684825b4b60517e0638d

    Pointers do not match
    ```

## Intercepting Git

Git LFS uses the `clean` and `smudge` filters to decide which files use it.  The
global filters can be set up with `git lfs install`:

```
$ git lfs install
```

These filters ensure that large files aren't written into the repository proper,
instead being stored locally at `.git/lfs/objects/{OID-PATH}` (where `{OID-PATH}`
is a sharded filepath of the form `OID[0:2]/OID[2:4]/OID`), synchronized with
the Git LFS server as necessary.  Here is a sample path to a file:

    .git/lfs/objects/4d/7a/4d7a214614ab2935c943f9e0ff69d22eadbb8f32b1258daaa5e2ca24d17e2393

The `clean` filter runs as files are added to repositories.  Git sends the
content of the file being added as STDIN, and expects the content to write
to Git as STDOUT.

* Stream binary content from STDIN to a temp file, while calculating its SHA-256
signature.
* Atomically move the temp file to `.git/lfs/objects/{OID-PATH}` if it does not
exist, and the sha-256 signature of the contents matches the given OID.
* Delete the temp file.
* Write the pointer file to STDOUT.

Note that the `clean` filter does not push the file to the server.  Use the
`git push` command to do that (lfs files are pushed before commits in a pre-push hook).

The `smudge` filter runs as files are being checked out from the Git repository
to the working directory.  Git sends the content of the Git blob as STDIN, and
expects the content to write to the working directory as STDOUT.

* Read 100 bytes.
* If the content is ASCII and matches the pointer file format:
  * Look for the file in `.git/lfs/objects/{OID-PATH}`.
  * If it's not there, download it from the server.
  * Write its contents to STDOUT
* Otherwise, simply pass the STDIN out through STDOUT.

The `.gitattributes` file controls when the filters run.  Here's a sample file that
runs all mp3 and zip files through Git LFS:

```
$ cat .gitattributes
*.mp3 filter=lfs -text
*.zip filter=lfs -text
```

Use the `git lfs track` command to view and add to `.gitattributes`.