The easiest way to prevent a broken build is to run your tests before you commit. This can be done with a simple git hook. Preventing your developers (and yourself) from breaking the build is as simple as putting this in your .git/hooks/pre-commit and making it executable: chmod +x .git/hooks/pre-commit.

#!/bin/sh
rake spec 2> /dev/null

This will stop the commit if the specs don’t pass.

This isn’t a replacement for a more robust CI system but it does make it a lot harder to do something stupid. Redirecting STDERR to /dev/null is optional but recommended since the STDERR output of failing specs isn’t useful. It you use Test::Unit instead of RSpec (for shame), use rake test instead. Likewise, anything that returns proper error codes (0 for success, > 0 for failure) can be used.

This is mainly useful if your specs take under a minute to run, otherwise it becomes tedious. If you have long-running specs, I suggest using a special task that runs an abridged set of core specs instead.