How do I force my iOS device to create a new backup on iCloud?

Because rm -i expects user's input from stdin, too. Try this:

echo "$MOUNTPOINTLIST" | while read onelineforrm; do rm -i "${onelineforrm}testfileforwrite" < /dev/tty; done

You can check this How to configure a Grunt? to configure grunt.

Follow above link it help to configure grunt.