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