# /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 )

• */30 - Запускать каждые 30 минут
• crontab -l - вывести список заданий
• crontab -u - пользователь (root, somebody)