From http://en.wikipedia.org/wiki/Cron
cron is the time-based job scheduler in Unix-like computer operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration, though its general-purpose nature means that it can be used for other purposes, such as connecting to the Internet and downloading email.
Cron is driven by a crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a system wide crontab file (usually in /etc or a subdirectory of /etc) which only system administrators can edit.
Each line of a crontab file represents a job and is composed of a CRON expression, followed by a shell command to execute.
Example:
The following line causes the user program test.pl – ostensibly a Perl script – to be run every two hours, namely at midnight, 2am, 4am, 6am, 8am, and so on:
0 */2 * * * /home/username/test.pl
Predefined scheduling definitions
There are several special predefined values which can be used to substitute the CRON expression.
| Entry | Description | Equivalent To |
|---|---|---|
@yearly (or @annually) |
Run once a year, midnight, Jan. 1st | 0 0 1 1 * |
@monthly |
Run once a month, midnight, first of month | 0 0 1 * * |
@weekly |
Run once a week, midnight on Sunday | 0 0 * * 0 |
@daily |
Run once a day, midnight | 0 0 * * * |
@hourly |
Run once an hour, beginning of hour | 0 * * * * |
@reboot |
Run at startup |
* * * * * command to be executed ┬ ┬ ┬ ┬ ┬ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └───── day of week (0 - 6) (0 is Sunday, or use names) │ │ │ └────────── month (1 - 12) │ │ └─────────────── day of month (1 - 31) │ └──────────────────── hour (0 - 23) └───────────────────────── min (0 - 59)
View crontab: crontab -l
Edit crontab: crontab -e