A crontab task consists of four date/time fields and a command field. Every minute cron checks all crontabs for a match between the current date/time and their tasks. If there’s a match, the command is executed. The system crontab has an additional field “User” that tells cron with which user id the command should be executed.
The fields are: Min minute of execution, 0-59 Hour hour of execution, 0-23 Mday day of month of execution, 1-31 Month month of execution, 1-12 (or names) Wday day of week of execution, 1-7 (1 = monday, 7 = sunday, or names) Command Anything that can be launched from the command line Possible values: * matches all values, e.g. a * in month means: "every month" x-y matches the range x to y, e.g. 2-4 in Mday means "on the 2nd, 3rd, and 4th of the month" x/n in range x with frequency n, e.g. */2 in Hour means "every other hour" Months can be names: jan, Feb, mAr (case insensitive). (use 3 chars) Weekdays can be names, e.g. sun, Mon, tUe. (use 3 chars, no case)
Special Strings
These can be used to replace the 5 time values.
string meaning ------ ------- @reboot Run once, at startup. @yearly Run once a year, "0 0 1 1 *". @annually (same as @yearly) @monthly Run once a month, "0 0 1 * *". @weekly Run once a week, "0 0 * * 0". @daily Run once a day, "0 0 * * *". @midnight (same as @daily) @hourly Run once an hour, "0 * * * *".
Emailing Cron Output:
MAILTO="recipient@domain.com" MAILTO="" <- will send no email
Examples
# Run drupal cron every 15 minutes.. */15 * * * * /usr/local/bin/wget -q -O /dev/null http://www.mostlygeek.com/cron.php # Update ports/sources and ports db. (2 commands, one cron line) 34 3 * * * cvsup -g -L 0 /root/supfile ; portsdb -Uu
