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https://github.com/capr/multigit

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https://github.com/capr/multigit

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README 

        
# multigit



#### layered git repositories

Writen by Cosmin Apreutesei. **Public Domain**.


Tested on **Linux**, **Windows** and **OSX**



Multigit allows checking out multiple git repositories overlaid 

onto the same directory.



It is useful for projects which are made of different components

that are developed separately, but which need to track files 

from different parts of the directory structure of the project.



This cannot be done using git submodules or git subtrees, which

only allow subprojects to be checked out into their own private 

subdirectories. Multigit allows repositories to track any file 

from any directory of the project structure, similar to a union 

filesystem, where each repository is a layer.



Some examples where this combination of change management

and module management could be useful:



  * manage customizations made to a web app in a separate repository.

  * putting your home directory under source control, one repo per app.

  * package manager for a project, dev platform or language.



## How do I install it?



Multigit is a simple shell script with no dependencies. You can either

put it somewhere in your PATH, or you can clone it everywhere

you want to create a multigit project in, and call it as `./mgit`

(or `mgit` on Windows).



To enable tab completion in bash, put `mgit-autocomplete.sh` somewhere

and source it in your `.bashrc` along with your git completion script

(don't call them, dot them). On Ubuntu, the git completion script is at

`/usr/share/bash-completion/completions/git` (you need to source

that manually).



## How do I use it?



Let's see a bare bones example:



	$ mkdir project

	$ cd project



	$ mgit init foo                # create layered repo foo

	$ mgit init bar                # create layered repo bar

	$ mgit ls                      # list layered repos

	foo

	bar



	$ echo > foo.txt               # create file foo.txt

	$ echo > bar.txt               # create file bar.txt

	$ mgit foo add -f foo.txt      # add foo.txt to project foo

	$ mgit bar add -f bar.txt      # add bar.txt to project bar

	$ mgit foo commit -m "init"    # commit on foo

	$ mgit bar commit -m "init"    # commit on bar



	$ ls

	foo.txt bar.txt                # foo.txt and bar.txt are in the same dir

	$ mgit foo ls-files

	foo.txt                        # but project foo only tracks foo.txt

	$ mgit bar ls-files

	bar.txt                        # and project bar only tracks bar.txt



Note the `-f` (force) when adding files to git. When creating a repo with

`mgit init foo`, the `.gitignore` file for foo is set to

`.mgit/foo.exclude` which defaults to `*`, which means that

all files are ignored by default, hence the need to add them with `-f`.

This is to prevent accidentally adding files of other projects with

`git add -A` and ending up with multiple projects tracking the same file.

To recover the convenience of `git -A` and the correct reporting of

untracked files, change the exclude files and add patterns that are

appropriate to each repo. Given that all repos now share the same

namespace, you need to be explicit about which parts of that namespace

are "reserved" for which repos.



You can use git direcly on your layered repos with a subshell:



	$ mgit foo

	Entering subshell: git commands will affect the repo 'foo'.

	Type `exit' to exit subshell.

	A  foo.txt



	[foo] $ git ls-files

	foo.txt

	[foo] $ exit

	$



Or you can type git commands directly without a subshell:



	$ mgit foo ls-files

	foo.txt



Or you can ditch multigit entirely and go bare git:



	$ export GIT_DIR=.mgit/foo/.git

	$ git ls-files

	foo.txt



## How do I clone repositories?



	$ mkdir project

	$ cd project



	$ mgit clone https://github.com/bob/foo

	$ mgit clone https://github.com/bob/bar



This will clone both repos into the current directory

(there's no "target directory" arg).



## But do I have to type the full URL every time?



No you don't. Since most of the submodules in a big project will probably

reside at a common base URL, you can make an alias for that base URL and

use that instead when cloning the submodules:



	$ mgit baseurl bob https://github.com/bob/  # writes `https://github.com/bob/` in .mgit/bob.baseurl

	$ mgit clone bob/foo bob/bar                # writes `bob` in .mgit/foo.origin and .mgit/bar.origin



Now that bob is _registered_ as a remote, and both foo's and bar's origins

are registered too (they are set to `bob`), next time it will be enough

to type `mgit clone foo bar`. Which brings us to the next question...



## How do I manage module collections?



Every time you clone a repo with `mgit clone`, a file `.mgit/foo.origin`

is created which contains the origin url of that repo, which allows you

to clone it by name alone. These files are not tracked by default,

they're just sitting there, but there's nothing stopping you from

creating a "meta" repo and start tracking them:



	$ mgit init meta

	$ mgit meta add -f .mgit/bob.baseurl   # add bob's baseurl to meta

	$ mgit meta add -f .mgit/foo.origin    # add foo's origin to meta

	$ mgit meta add -f .mgit/bar.origin    # add bar's origin to meta

	$ mgit meta commit -m "bob's place; foo and bar modules"



> The meta repo is like any other layered repo, it just so happens that it

tracks files from `.mgit` (and it doesn't have to be called meta either,

and it doesn't have to be the _only_ repo containing meta-information).



Maintaining a `meta` repo for a project allows the users of that project

to clone the meta repo and then clone all other repos

with `mgit clone-all` (and list them with `mgit ls-all`).

This makes for a simple way to manage and share module collections

that later can be cloned back wholesale.



Note that repos created with `mgit init foo` don't create an origin file.

You can add that yourself with `mgit origin foo https://github.com/bob/foo`

or simply `mgit origin foo bob` if you have previously added bob's baseurl.



## But this will always clone master. How do I lock versions?



	$ mgit release 1.0 update     # adds .mgit/1.0.release



This creates (or updates) a list with currently checked out versions

of all repos, effectively recording a snapshot of the entire project.

This snapshot can later be restored with:



	$ mgit clone-release 1.0



> Note: Existing repos will have to be removed first. This command

will refuse to overwrite them. Also, this will not create branches.

Your repos will be in "detached HEAD state". You can take it from there

with git, eg. with `mgit --all checkout -b release-1.0`.



Needless to say, you can add the .release file to your meta repo too,

just like with the .baseurl and .origin files before, so that other people

will be able to clone the project at that specific release point.



Another quick way to get a snapshot of the project without using .release

files is with:



	$ mgit --all ver

	foo=701d080 bar=0fefd96



And later clone the repos with:



	$ mgit clone foo=701d080 bar=0fefd96



Tag releases can be made with:



	$ mgit release 1.0 update tag



Tag releases only record the current tag as opposed to the exact

full version of the packages. This allows you to make stable releases

even when some packages are "unstable" (i.e. they contain commits above the

latest tag). It also enables the controversial practice of updating a tag's

target commit without having to make a new release.



## What else can it do?



command                               | description

------------------------------------- | --------------------------------------

`mgit st`                             | list modified files across all repos

`mgit --all -v status -s`             | like `mgit st` but shows the repos too

`mgit ls-unpushed`                    | list repos ahead of origin

`mgit ls-tracked`                     | list files and the repos who track them

`mgit ls-untracked`                   | list files untracked by any repo

`mgit ls-double-tracked`              | list files tracked by multiple repos

`mgit remove [--dry] REPO ...`        | remove repos from disk



There's more stuff, type `mgit` to see.



## How does this work anyway?



Simply by telling git to clone all the repositories into a common

work tree. The git trees are kept in `.mgit//.git` and the

work tree is always '.'.



This is such basic and useful functionality that it should

be built into `git clone` and `git init` really. As dead simple

as multigit is, it's still yet another script that you have to deploy.



## Simple? But it's a 600 lines script!



Don't worry about it, it's mostly fluff. The gist of it is only 6 lines:



mgit init foo:



	mkdir -p .mgit/foo

	export GIT_DIR=.mgit/foo/.git

	git init                                                  # create .mgit/foo/.git

	git config --local core.worktree ../../..                 # relative to GIT_DIR

	git config --local core.excludesfile .mgit/foo.exclude    # instead of .gitignore

	[ -f .mgit/foo.exclude ] || echo '*' > .mgit/foo.exclude  # "ignore all"



mgit foo ls-files:



	export GIT_DIR=.mgit/foo/.git    # set git to work on foo

	git ls-files                     # list files of foo



## What's its relation to the current directory?



Just like git, mgit scans the current directory and its parents for

a `.mgit` dir, and if found, it sets the current directory to that,

and everything else happens from there, except starting a subshell

and executing git commands which run in the current directory.



## Where is multigit used?



Multigit is the package manager for [luapower](https://luapower.com).

The [meta-package](https://github.com/luapower/luapower-repos) contains

the list of packages in the distribution and a few handy multigit plugins

specific to luapower.



## What multigit plugins?



Plugins allows extending multigit with project-specific scripts

that are exposed as multigit commands (like build scripts, etc.).

Plugin scripts can be included say, in the meta package of your project.

The way they work is very simple:



  * `mgit   ...` will try to run

    `.mgit/git-.sh ...` with `$GIT_DIR` set properly,

	 `$MULTIGIT_REPO` set to ``, `$PWD0` set to the

	 invocation path and current directory set to one directory where

	 the `.mgit` directory was found.



  * `mgit  ...` will try to run `.mgit/.sh`.



  * `mgit help` will try to `cat .mgit/*.help`, which is where you should

    place the help section of the added commands.



Look at [luapower-repos](https://github.com/luapower/luapower-repos)

for a real-world example of this.



## Environment



Multigit will use the following environment variables when performing

various tasks:



  * `$MULTIGIT_FETCH_OPTS` will be expanded and passed to `git fetch`

  * `$MULTIGIT_INIT_OPTS` will be expanded and passed to `git init`



## Limitations



  * No way to specify a different branch when cloning (must switch it after).

  * No way to specify a different branch when making releases.

  * No way to register multiple remotes for the same repo and specify

  the remote when cloning.



## Related efforts



There is a very similar project called [vcsh](https://github.com/RichiH/vcsh).

It even has a [video presentation](http://mirror.as35701.net/video.fosdem.org//2012/lightningtalks/vcsh.webm),

check it out.