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git-lfs/docs/spec.md
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2015-04-20 11:11:03 -06:00

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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 SHOULD NOT have the executable bit set when checked-in in Git.

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. Currently, only sha256 is supported.
  • 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)

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!

  • Run git lfs pointer to generate a pointer file for the given local file.
  • Run git lfs pointer to compare the blob OID of the generated pointer files.
  • Tools that parse and regenerate pointer files MUST preserve keys that they don't know or care about.

Note: Earlier versions only contained the OID, with a # comment above it. Here's some ruby code to parse older pointer files.

# data is a string of the content
# last full line contains the oid
return nil unless data.size < 100
lines = data.
  strip.      # strip ending whitespace
  split("\n") # split by line breaks

# We look for a comment line, and the phrase `git-media` somewhere
lines[0] =~ /# (.*git-media|external)/ && lines.last

That code returns the OID, which should be on the last line. The OID is generated from the SHA-256 signature of the file's contents.

The Server

Git LFS needs a URL endpoint to talk to a remote server. A Git repository can have different Git LFS endpoints for different remotes. Here is the list of rules that Git LFS uses to determine a repository's Git LFS server:

  1. The lfs.url string.
  2. The remote.{name}.lfsurl string.
  3. Append /info/lfs to the remote URL. Only works with HTTPS URLs.

Here's a sample Git config file with the optional remote and Git LFS configuration options:

[core]
  repositoryformatversion = 0
[lfs]
  url = "https://github.com/github/git-lfs.git/info/lfs"
[remote "origin"]
  url = https://github.com/github/git-lfs
  fetch = +refs/heads/*:refs/remotes/origin/*
  lfsurl = "https://github.com/github/git-lfs.git/info/lfs"

Git LFS uses git credential to fetch credentials for HTTPS requests. Setup a credential cache helper to save passwords for future users.

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 init:

$ git lfs init

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.

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.
  • Check for the file at .git/lfs/objects/{OID-PATH}.
  • If it does not exist:
    • Queue the OID to be uploaded.
    • Move the temp file to .git/lfs/objects/{OID-PATH}.
  • 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 lfs sync command to do that.

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.
    • Read 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 runs all mp3 and zip files through Git LFS:

$ cat .gitattributes
*.mp3 filter=lfs -crlf
*.zip filter=lfs -crlf

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