ci.sh

#!/bin/bash -x

# This should be your "script/ci" checked into version control, and then wired as your sole build step in Jenkins.
#
# Simplifying Assumptions:
#
#  * You build all branches
#  * You want to deploy all branches.  
#  * You wired up an SSH key to your CI server appropriately so it can talk to your deployment target(s) via Cap

set -e
script/test # or bundle exec rake, or whatever

echo "Beginning deploy from $GIT_BRANCH...."
# Do some git clean up and checkout and pull a branch
# Jenkins by default will be in a detached HEAD state, which Capistrano doesn't like
git remote prune origin
git fetch origin
git branch -t -f local/$GIT_BRANCH $GIT_BRANCH
git checkout local/$GIT_BRANCH
git pull
bundle exec cap deploy

deploy.rb

# ...meanwhile, in your Capistrano config somewhere:

# configure your deployments to always go from the current branch
set(:current_branch) { `git branch`.match(/\* (\S+)\s/m)[1] || raise("Couldn't determine current branch") }
set :branch,  defer { current_branch }