Command-line while loops

while true (return key)
do (return key)
sleep 1; echo $SECONDS (return key)
done (return key)

while true(return key)
do
sleep 1; clear; uptime; echo $SECONDS (return key)
done (return key)

while [ $SECONDS -le 86400 ] (return key)
do (return key)
sleep 1; clear;uptime;echo $SECONDS (return key)
done (return key)