[root@localhost ~]# more /etc/crontab

SHELL=/bin/bash

PATH=/sbin:/bin:/usr/sbin:/usr/bin

MAILTO=root

# For details see man  crontabs

# Example of job definition:

# .---------------- minute ( - )

# |  .------------- hour ( - )

# |  |  .---------- day of month ( - )

# |  |  |  .------- month ( - ) OR jan,feb,mar,apr ...

# |  |  |  |  .---- day of week ( - ) (Sunday= or ) OR sun,mon,tue,wed,thu,fri,sat

# |  |  |  |  |

# *  *  *  *  * user-name  command to be executed

From root@localhost.localdomain  Sat Feb  ::

Return-Path: <root@localhost.localdomain>

X-Original-To: root

Delivered-To: root@localhost.localdomain

Received: by localhost.localdomain (Postfix, from userid )

    id B340060ADA00; Sat,  Feb  :: + (CST)

From: "(Cron Daemon)" <root@localhost.localdomain>

To: root@localhost.localdomain

Subject: Cron <root@localhost> root ntpdate -u  192.168.10.10

Content-Type: text/plain; charset=UTF-

Auto-Submitted: auto-generated

Precedence: bulk

X-Cron-Env: <XDG_SESSION_ID=>

X-Cron-Env: <XDG_RUNTIME_DIR=/run/user/>

X-Cron-Env: <LANG=en_US.UTF->

X-Cron-Env: <SHELL=/bin/sh>

X-Cron-Env: <HOME=/root>

X-Cron-Env: <PATH=/usr/bin:/bin>

X-Cron-Env: <LOGNAME=root>

X-Cron-Env: <USER=root>

Message-Id: <.B340060ADA00@localhost.localdomain>

Date: Sat,  Feb  :: + (CST)

/bin/sh: root: command not found

[root@localhost ~]# whereis crontab

crontab: /usr/bin/crontab /etc/crontab /usr/share/man/man1/crontab..gz /usr/share/man/man5/crontab..gz

[root@localhost ~]# whereis clock

clock: /usr/sbin/clock /usr/share/man/man8/clock..gz

[root@localhost ~]# crontab -e

  * * *  /usr/sbin/ntpdate -u  192.168.10.10; /usr/sbin/clock -w