Have you ever been working in the terminal and found yourself repeating the same command many times? Delegate that work to the computer.
watch comes with Linux and can be installed on OSX via homebrew. It executes a program periodically, defaulting to every two seconds.
We used it today while writing a database backup script. Instead of checking our dump directory every time a cron job executed, we ran watch ls, and watched the script succeed or fail with live updates.
man watch for more information.