6.3.2 Is this Shell Interactive?

To determine within a startup script whether or not Bash is running interactively, test the value of the ‘-’ special parameter. It contains i when the shell is interactive. For example:

     case "$-" in
     *i*)	echo This shell is interactive ;;
     *)	echo This shell is not interactive ;;
     esac

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:

     if [ -z "$PS1" ]; then
             echo This shell is not interactive
     else
             echo This shell is interactive
     fi