This is a Heroku buildpack for adding rar and unrar to your application.
More likely, you'll want to use it as part of a larger project, which needs to use rar and unrar. The easiest way to do this is with a multipack, where this is just one of the buildpacks you'll be working with.
$ cat .buildpacks
git://github.com/heroku/heroku-buildpack-ruby.git
git://github.com/akshaychinnan/heroku-buildpack-rarlab.git
$ heroku config:add BUILDPACK_URL=git://github.com/akshaychinnan/heroku-buildpack-multi.git
By this you can use rar and unrar command.
heroku config:add BUILDPACK_URL=git://github.com/akshaychinnan/heroku-buildpack-rarlab.git"