git-external

You are tired of git-submodules? For some us cases, we all were tempted to scream at git submodules, since they do not work as svn:externals work.

This is an alternative to the alternative written by Daniel Cestary. I really like Daniel's concept, but I cannot force my users to install a Ruby gem. Therefore this git-external is cramped into a single file, which can be commited into your project repository as a ./init script.

Furthermore, git-external also supports Git, SVN and Git SVN remotes as externals. And provides a mechanism to self update.

Why?

git-external is a tool for including external repositories inside of a super repository or main repository. Is very similar to git submodule since it helps keeping some dependencies or sub-projects in different repositories and reuse them in many projects.

git external intends to mimic the behavior of svn:externals, providing an updatable reference to an external repository which the main repository doesn't need to know about.

Why not git submodule?

With git submodule, git supports to import other repositories as modules, , all of them are a git repository on their own.

The problem is each module has a commit id associated with it and every time the module is updated (by issuing a git pull inside the module) this causes a change on the supermodule therefor forcing you to make a new commit only for updating what is sometimes a dependency.

git submodule is a great tool, but it's not for everyone, neither is git-external.

How?

git-externalstores a file in the root of the repository called .gitexternals with a format very similar to the .gitmodules from git submodule (if not almost identical). This file keeps the information of the externals (meaning path and url). If you are interested in the format, look at the .gitexternals in this repository.

Each external is really a clone of the repository specified to git external add so you can do everything you would do on a normal git repository inside an external.

The path where the external's clone resides is added to the .gitignore file in order to keep it out of your way while you get your work done.

With the only attribute you prevent git-external to clone the repository by default. For these externals, the explicit clone command has to be used.

[external "repo"]
	path = local-path
	url = [email protected]:remote/repo.git
	branch = master
	only = update
	vcs = git

Branches

Within the branch = configuration, you cannot only give branch names, but also tag names. We use git checkout to guess what you want to checkout.

On update, branches are not switched, even if the external definition has changed. If you want to be sure to switch to the configured branch, use ./git-external clone.

Overrides

You can overide the settings for externals by putting an external section into your global git config. For example, if you want all possible forms of Github urls that are used in externals you can match up the external defnitions with this override in our ~/.gitconfig:

[external "bib-override"]
	match-url = "*github.com*luhsra/bib*"
	url = "[email protected]:luhsra/bib"

The match-* attributes are regular shell globs and matche against the corresponding attribute. All other keys override settings in the original external definition. A prominent example of such an override is to use always git-svn instead of svn:

[external "override-svn"]
     match-vcs = svn
     vcs = git-svn

Another problem that comes up with git-externals is often that you have multiple copies of the same repository, over and over again. However, git-external provides the possibility to symlink an already existing repository instead of cloning a new instance. This can also be employed by overrides:

[external "bib-override"]
   match-url = "*github.com*luhsra/bib*"
   symlink = ~/proj/SRA/bib

Instead of cloning always a new instance of the bib repository there exists only one instance of it that is always symlinked.

Getting Started

1. Install git-external into your git repo as ./init

     wget https://raw.githubusercontent.com/stettberger/git-external/master/bin/git-external -O init
     chmod u+x init
   

2. Add an external repository

    ./init add https://github.com/stettberger/ispositive.git is-positive-git
   

3. Initial clone or whenever you feel like an update is needed

    ./init