sudo -s runs the shell specified in your
$SHELL environment variable as the superuser/root. You can specify another user using
$SHELL environment variable contains the path to the user's default login shell. The actual setting for the default shell program is usually in
etc/passwd. Depending on what you've done in your current session, the $SHELL variable may not contain the shell program you're currently using. If you login automatically with zsh for instance, but then invoke bash, $SHELL won't change from
Show the current user and shell program:
echo $(whoami) is logged in and shell is $0
whoami prints out the username the user is working under.
$0 contains the name/path of the currently running program (shell program in this case).