6.3.1 What is an Interactive Shell?

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.