Crontab details

Here is the format of a line in crontab.

#minute, hour, day of month, month, day of week, command
*        *     *             *      *            command

Entries in a crontab Command Line

Field	Value
minute	0–59
hour	Based on a 24-hour clock; for example, 23 = 11 P.M.
day of month	1–31
month	1–12, or jan, feb, mar, etc.
day of week	0–7; where 0 and 7 are both Sunday; or sun, mon, tue, etc.
command	The command you want to run

If you see an asterisk in any column, cron runs that command for all possible values of that column. For example, an * in the minute field means that the command is run every minute during the specified hour(s). Consider another example, as shown here:

1  5  3  4  *  ls

This line runs the ls command every April 3 at 5:01 A.M. The asterisk in the day of week column simply means that it does not matter what day of the week it is; crontab still runs the ls command at the specified time.

The crontab file is flexible. For example, a 7–10 entry in the hour field would run the specified command at 7:00 A.M., 8:00 A.M., 9:00 A.M., and 10:00 A.M. A list of entries in the minute field such as: 0,5,10,15,20,25,30,35,40,45,50,55 would run the specified command every five minutes. The cron daemon also recognizes abbreviations for months and the day of the week.

The actual command is the sixth field. You can set up new lines with a percent (%) symbol. This is useful for formatting standard input. The example of a cron file follows formats input for an e-mail message:

# crontab -l
# Sample crontab file
#
# Force /bin/sh to be my shell for all of my scripts.
SHELL=/bin/sh
# Run 15 minutes past Midnight every Saturday
15 0 * * sat   $HOME/scripts/scary.script
# Do routine cleanup on the first of every Month at 4:30 AM
30 4 1 * *     /usr/scripts/removecores >> /tmp/core.tmp 2>>&1
# Mail a message at 10:45 AM every Friday
45 10 * * fri  mail -s "Project Update employees%Can I have a status
update on your project?%%Your Boss.%
# Every other hour check for alert messages
0 */2 * * * /usr/scripts/check.alerts

For more examples, review some of the scripts in the /etc/cron.daily directory.