Let's say you have a script named foo.sh that will append "Hello World" to a txt file, like this.

#!/bin/bash
echo "Hello World - The current date and time is `date +%Y-%m-%s %H:%M:%S`" >> log.txt

The watch command can be used to run this script every x seconds. In this example, the foo.sh script will be run every 5 seconds.

watch -n 5 bash foo.sh

Something like this should be displayed.

Every 5.0s: foo.sh