Heroku provides a read-only file system on their Bamboo stack, and a "scratch-pad" file system on their Cedar stack where changes are thrown away after the web request finishes.

I guess the user account has restricted access in what commands are available.

exec('sudo cat /etc/shadow') => RestrictedCodeException System Calls disabled

Also, while File.read('/etc/passwd') works, at least File.read('/etc/shadow') doesn't.