Some useful and simple examples of crontab entries.

This is a typical crontab:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

#backup local mysql databases
59 23   * * *   root    /sbin/2v_bck_mysql 

What does it means?

59 23   * * *   root    /sbin/2v_bck_mysq
| |     | | |   |       |
| |     | | |   |       +----- Command
| |     | | |   +----- User executing command
| |     | | |
| |     | | +----- Day of week(options: 0-6, 0 = Sun ...) 
| |     | +------- Month (options: 1-12) 
| |     +--------- Day of month (options: 1-31) 
| |
| +----------- Hour (options: 0-23)
+------------- Min (optiions: 0-59) 

Other examples:

0 1 * * 1,2,3 /opt/bin/script1.sh
0 1 * * 1-3 /opt/bin/script1.sh

Both options will run the script at  01:00 on Mon, Tue and Wed.

 0-59/5 *   * * *   root updatedb 

This script executes updatedb every 5 minute everyday of the year