What's the Ubuntu's default shell?
For regular users it's Bash. For root it's Dash (sh).
In some distributions you do not have the bash shell. But you can enable that using the procedure below:
1) Check if 'bash' is available
2) If not, (you encounter an command not found error) install that:
$ pkg_add -r -v bash
3) Change the default shell to bash of any user:
$ chsh -s /bin/bash username
4) To check for the bash installation location, you can type:
$ which bash
You can refer my blog, for more information.