Question

#!/bin/sh
INTERVAL=1
COUNT=0
while [ $COUNT -le 9 ]
do
    (( COUNT++ ))
    sleep $INTERVAL
    echo "count is $COUNT"
done

On execution.

$ sh ascript 
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found
count is 0
ascript: 9: COUNT++: not found

Answer 1

You probably want #!/bin/bash rather than #!/bin/sh at the top if you want to use bash-specific operations.

Your script does work fine here on my Mac, where sh is really just bash. If your sh is a real one you might not be so lucky.