I've read some questions about the difference of TTY, console, terminal, etc here and on Askubuntu. Someone mentioned restricted shell, like rbash or bash -r. I first fought it is good to give someone very restricted access to a computer, but it is very easy to break out. Just start bash. So what's the usage of rbash, when you can break out so easily or are other shells more secure (sh -r)? 
What's the usage of rbash?
bash;shell;restricted shell
Restricted shells like rbash need careful setup; eg setting a PATH variable that points to a directory (e.g. /usr/local/rbin) that only has the commands you want to let people run.  Make sure the user can't modify their home directory so they can't change startup scripts.  At that point the user is limited.If you allow the standard /bin and /usr/bin directories to be on the user PATH then, as you've found out, people can easily escape the restrictions.