crontabの書き方とcrontabでのサーバ時刻合わせ設定 (AWS EC2 - Amazon Linux -- NTP) ref: http://qiita.com/azusanakano/items/4448bf805f006acda907

0hourly

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
01 * * * * root run-parts /etc/cron.hourly

file0.txt

# 時刻合わせ
0 * * * * /usr/sbin/ntpdate -bs ntp1.noah.idc.jp > /dev/null 2>&1

file1.txt

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/

# For details see man 4 crontabs

# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name command to be executed

file2.txt

# 分　時　日　月　曜日　実行ユーザ　実行コマンド

# 毎時15分に、「logger」コマンドで「/var/log/messages」にをログを書き出す設定
15 * * * * root logger crontab-test

file3.txt

# 時刻合わせ
0 * * * * /usr/sbin/ntpdate -bs ntp1.noah.idc.jp > /dev/null 2>&1