A CRON script is a list of one or more commands to a computer operating system or application server that are to be executed at a specified time. Each command is executed when its triggering time arrives. In UNIX-based operating systems, the crontab command is a user interface command that creates or changes a file (called a crontab file). This file contains a list of UNIX shell commands, each with a specified time of execution. A CRON script is similar.