Git Subtree Splitter

splitsh-lite replaces the subtree split Git built-in command to make splitting a monolithic repository to read-only standalone repositories easy and fast.

Why do I need this tool?

When starting a project, do you store all the code in one repository? Or are you creating many standalone repositories?

Both strategies work well and both have drawbacks as well. splitsh helps use both strategies by providing tools that automatically synchronize a monolithic repository to standalone repositories in real-time.

splitsh-lite is a sub-project that provides a faster implementation of the git subtree split command, which helps create standalone repositories for one or more sub-directories of a main repository.

If you want to learn more about monorepo vs manyrepos, watch this 4-minute lightning talk I gave at dotScale (or read the slides)... or watch the longer version from DrupalCon. "The Monorepo - Storing your source code has never been so much fun" is also a great resource.

Note If you currently have multiple repositories that you want to merge into a monorepo, use the tomono tool.

Installation

Docker (recommended)

The recommended way to use the splitter is via the official Docker image:

Manual Installation (not recommended)

To build the binary , you first need to install libgit2, preferably using your package manager of choice:

Via brew:
```
brew install libgit2@1.5
```
Via apt:
```
apt install libgit2-dev
```

Note that the last version of libgit2 supported (by git2go) is 1.5.

If you get libgit2 version 1.5, you're all set and jump to the build step below. If not, you first need to change the git2go version used in the code. Using the table on the libgit2 repository, figure out which version of the git2go you need based on the liggit2 library you installed. Let's say you need version v31:

sed -i -e 's/v34/v31/g' go.mod splitter/*.go
go mod tidy

On MacOS, export the following flags:

export LDFLAGS="-L/opt/homebrew/opt/libgit2@1.5/lib"
export CPPFLAGS="-I/opt/homebrew/opt/libgit2@1.5/include"
export PKG_CONFIG_PATH="/opt/homebrew/opt/libgit2@1.5/lib/pkgconfig"

Then, build the splitsh-lite binary:

go build -o splitsh-lite github.com/splitsh/lite

If everything goes fine, a splitsh-lite binary should be available in the current directory.

If you want to integrate splitsh with Git, install it like this (and use it via git splitsh):

cp splitsh-lite "$(git --exec-path)"/git-splitsh

Usage

Let's say you want to split the lib/ directory of a repository stored in the current directory from the current branch (bare or clone), run:

# Docker
docker run --rm -v $PWD:/data splitsh-lite --prefix=lib/

# Binary
splitsh-lite --prefix=lib/

The command outputs the sha1 of the split:

SHA1=`splitsh-lite --prefix=lib/`

The sha1 can be used to create a branch or to push the commits to a new repository.

Automatically create a branch for the split by passing a branch name via the --target option:

splitsh-lite --prefix=lib/ --target=heads/branch-name

If new commits are made to the repository, update the split by running the same command again. Updates are much faster as splitsh-lite keeps a cache of already split commits. Caching is possible as splitsh-lite guarantees that two splits of the same code always results in the same history and the same sha1s for each commit.

By default, splitsh-lite splits the currently checked out branch but you can split a different branch by passing it explicitly via the --origin flag (mandatory when splitting a bare repository):

splitsh-lite --prefix=lib/ --origin=origin/master

You don't even need to run the command from the Git repository directory:

# Docker
docker run --rm -v /path/to/repo:/data splitsh-lite --prefix=lib/ --origin=origin/1.0

# Binary
splitsh-lite --prefix=lib/ --origin=origin/1.0 --path=/path/to/repo

Available options:

--prefix is the prefix of the directory to split; you can put the split contents in a sub-directory of the target repository by using the --prefix=from:to syntax; split several directories by passing multiple --prefix flags;
--path is the path of the repository to split (current directory by default, or use the -v option of Docker when using the Docker image);
--origin is the Git reference for the origin (can be any Git reference like HEAD, heads/xxx, tags/xxx, origin/xxx, or any refs/xxx);
--target creates a reference for the tip of the split (can be any Git reference like heads/xxx, tags/xxx, origin/xxx, or any refs/xxx);
--progress displays a progress bar;
--scratch flushes the cache (useful when a branch is force pushed or in case of a cache corruption).

Migrating from `git subtree split`

Migrating from git subtree split to splith-lite is easy as both tools generate the same sha1s.

However, note that older versions of git subtree split used broken algorithms, and so generated different sha1s than the latest version. You can simulate those version via the --git flag. Use <1.8.2 or <2.8.0 depending on which version of git subtree split you want to simulate.

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