Basics

Use crontab -l to list existing items in crontab.
Use crontab -e to edit the crontab.
- If you have never added anything before a new file will be created.
- Pick an editor of your choice to edit the crontab.

Syntax

Syntax of crontab entry: * * * * * <username> command(s)
Commands can be followed by arguments.
Multiple commands can be added with &&.
The five stars stand for
- Minute (0-59)
- Hour (0-23)
- Day of the month (1-31)
- Month (1-12)
- Day of the week (0-7), where Sunday is 0 or 7.
Operators
- * any value, or each instance. for example, each minute, each hour, so on.
- , to specify a list of values. 1,3,5 in the day field to show monday, wednesday, and friday.
- - to specify a range of values. 8-20 in the hour field to set 8AM to 8PM.
- / to divide the frequency of occurence. */4 in the hour field, to set every 4 hours. 1-30/10 in the days field o set every ten days.
Examples
- 0 15 * * 1-5 every weekday at 3pm.
- 0 15 * * Mon every monday at 3pm.
- */5 * * * * every five minutes.
- 00 08-16 * * * everyday, at the start of every working hour from 8am to 4pm.
- 0 7 1-7 * 1 every first monday of the month at 7am.
- 15 9 1,15 * * at the 1st and 15th of every month at 9:15am.
Macros
- @yearly == 0 0 1 1 *
- @monthly == 0 0 1 * *
- @weekly == 0 0 * * 0
- @daily == 0 0 * * *
- @hourly == 0 * * * *
- @reboot