The options which apply to the chsh command are:
-s, --shell SHELL
If the -s option is not selected, chsh operates in an interactive fashion, prompting the user with the current login shell. Enter the new value to change the shell, or leave the line blank to use the current one. The current shell is displayed between a pair of [ ] marks.
The only restriction placed on the login shell is that the command name must be listed in /etc/shells, unless the invoker is the superuser, and then any value may be added. An account with a restricted login shell may not change her login shell. For this reason, placing /bin/rsh in /etc/shells is discouraged since accidentally changing to a restricted shell would prevent the user from ever changing her login shell back to its original value.
chfn(1), login.defs(5), passwd(5).