Update 8 Feb 2015

I came back to some projects after many months where I had used this method, and I was running into errors complaining about COMMIT_SHA is not an ancestor of commit when running the git subtree split command. I did some more research and found what I think is a superior method in the form of this shell script. The new method also allows the build directory to not be checked in to the git repository which I think keeps things cleaner.

So I packaged the shell script as an npm module so it can be installed to devDependencies and used it the same way as the previous method.

npm run-script

npm allows you to add arbitray commands to your package.json file and run them with npm run [command].

Here is a set of scripts that can be used to easily deploy a subdirectory of any project to the gh-pages branch on GitHub. This is useful for when your master branch is used to build a static site, but you need the static files to end up in the gh-pages branch.

There is one assumption:

a file build.js will output static to a directory _built/.

If that is true, then running npm run deploy will build your files, commit them and deploy them to the gh-pages branch. Simple!

package.json (new git-directory-deploy method)

First make sure the module is installed by running npm install git-directory-deploy --save-dev

{
  "scripts": {
    "build": "node build",
    "deploy": "git-directory-deploy --directory _build --branch gh-pages"
  }
}

package.json (old git subtree method)

{
  "scripts": {
    "build": "node build",
    "commit-built": "git add _built -A && git commit -m \"_built files at `date`\" -n > /dev/null 2>&1; exit 0",
    "ghpages": "git subtree split --prefix _built/ -b gh-pages && git push -f origin gh-pages:gh-pages && git branch -D gh-pages",
    "deploy": "npm run build && npm run commit-built && npm run ghpages"
  }
}