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 ;;

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
        echo This shell is interactive