1.atd和crond两个任务管理程序的区别

atd是一次性的执行了一次之后就不再执行，crond是周期性质的可以循环重复的执行定时任务

⒉.指定在2023/04/15 09: 00将时间写入testmail.txt文件中

[root@server fox]# at 9:00 2023-04-15
warning: commands will be executed using /bin/sh
at Sat Apr 15 09:00:00 2023
at> date >> testmail.txt
at> <EOT>
job 3 at Sat Apr 15 09:00:00 2023

3.指定每天凌晨4∶00将该时间点之前的日志信息备份到个目录下(/var/log/messages )，备份后日志文件名显示格式logfileYY-MM-DD HH-MM

[root@server ~]# vim /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root# 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 
* 4 * * * root/bin/cp  /root/testmail.txt /var/log/messages/logfile `date +\%Y-\%m-\%d-\%H-\%M-\%S`