An interactive shell
is one started without non-option arguments, unless `-s' is
specified, without specifiying the `-c' option, and
whose input and output are both
connected to terminals (as determined by isatty(3)),
or one started with the `-i' option.
An interactive shell generally reads from and writes to a user's terminal.
The `-s' invocation option may be used to set the positional parameters when an interactive shell is started.