Custom cron jobs are stored in /etc/crontab
Cron jobs cannot run more frequently than every minute
Editing Cron Jobs
To edit the crontab for the current user type…
To edit the crontab for the root user type…
sudo crontab -e
minute hour dom month dow user cmd
minute Minute of the hour the command will run on, and is between ‘0’ and ‘59’
hour Hour the command will run on, and is specified in the 24 hour clock, values must be between 0 and 23 (0 is midnight)
dom Day of Month, that you want the command run on, e.g. to run a command on the 19th of each month, the dom would be 19.
month The month a specified command will run on, it may be specified numerically (0-12), or as the name of the month (e.g. May)
dow The Day of Week that you want a command to be run on, it can also be numeric (0-7) or as the name of the day (e.g. sun).
user The user who runs the command.
cmd The command that you want run. This field may contain multiple words or spaces.
Run something every minute
*/1 * * * * root echo "This command is run every minute"
Run something every 5 minutes
*/5 * * * * root echo "This command is run every 5 minutes"