Setting up a new Ubuntu 12.04 LTS server I kept getting the error below when trying to run the rvm command.

  $ rvm
  RVM can not be run with `set -o posix`

This was solved by changing the login shell for my user. It prompts for your password and shell which I changed from /bin/sh to /bin/bash.

  $ chsh
  Password: ******
  Changing the login shell for some_user
  Enter the new value, or press ENTER for the default
    Login Shell [/bin/sh]: /bin/bash