An interactive shell
is one started without non-option arguments, unless -s is
specified, without specifying the -c option, and
whose input and error 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.