An asterisk (*) specifies that the command should be executed at every instance during the given time period. A * in the Hour field in the cron job will cause the command to be executed every hour. To execute the command at multiple instances of a time period, specify the time intervals separated by a comma in this time field. For example, to run the command at the 5^th and 10^th minute, enter 5,10 in the Minute field. A slash (divide by) symbol will cause the command to run as per a division of the time. For example 0-30/6 in the Minutes field will run a command every 5 minutes during the first half of each hour. The string */12 in the Hours field will run a command every other hour.

Cron jobs are executed as the user who created crontab. If you need to execute commands that require higher privileges, such as shutting down the computer, run the crontab command as root.

The commands specified in a cron job are written with the full path to the command. This is because cron does not source your .bashrc, so the environment in which a cron job is executed is different from the bash shell we execute on a terminal. Hence, the PATH environment variable may not be set. If your command requires certain environment variables, you must explicitly set them.