I am confused how this works. I would assume `SECONDS` would just be a shell variable and it was first assigned `0` and then it should stay same, why did it keep counting the seconds?

    > SECONDS
    bash: SECONDS: command not found
    > SECONDS=0; sleep 5; echo $SECONDS;
    5
    > echo "Your command completed after $SECONDS seconds";
    Your command completed after 41 seconds
    > echo "Your command completed after $SECONDS seconds";
    Your command completed after 51 seconds
    > echo "Your command completed after $SECONDS seconds";
    Your command completed after 53 seconds