chsh changes the user login shell. This determines the name of the user's initial login command. A normal user may only change the login shell for their own account, the super user may change the login shell for any account.
The only restrictions placed on the login shell is that the command name must be listed in /etc/shells, unless the invoker is the super-user, and then any value may be added. An account with a restricted login shell may not change their 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 every changing their login shell back to its original value.
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 field, or leave the line blank to use the current value. The current value is displayed between a pair of [? marks.