定时任务：

[root@foundation21 ~]# systemctl status crond.service  首先查看定时服务是否开启

● crond.service - Command Scheduler

   Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled)

   Active: active (running) since Sun 2018-04-22 08:43:14 CST; 1h 15min ago

 Main PID: 1217 (crond)

   CGroup: /system.slice/crond.service

           └─1217 /usr/sbin/crond -n

Apr 22 08:43:14 foundation21.ilt.example.com systemd[1]: Started Command Sc...

Apr 22 08:43:14 foundation21.ilt.example.com systemd[1]: Starting Command S...

Apr 22 08:43:14 foundation21.ilt.example.com crond[1217]: (CRON) INFO (RAND...

Apr 22 08:43:14 foundation21.ilt.example.com crond[1217]: (CRON) INFO (runn...

Apr 22 09:55:01 foundation21.ilt.example.com crond[1217]: (root) RELOAD (/v...

Apr 22 09:56:01 foundation21.ilt.example.com crond[1217]: (root) RELOAD (/v...

Apr 22 09:59:01 foundation21.ilt.example.com crond[1217]: (root) RELOAD (/v...

Hint: Some lines were ellipsized, use -l to show in full.

[root@foundation21 ~]# crontab -u root -e    建立定时任务

no crontab for root - using an empty one

crontab: installing new crontab

[root@foundation21 ~]# crontab -u root -l     查看定时任务

54 21 * * * /bin/rm -fr /mnt/file*

[root@foundation21 ~]# cat /var/spool/cron/root   查看自己编写的内容，到自己的定时之后就自动执行命令。

格式说明：

54     21     *      *    *       /bin/rm -fr /mnt/file*

分钟  小时    天    月  周       意思是每天的九点五十四清空一次/mnt/目录下的file文件

[root@foundation21 ~]# crontab -u root -e    重新建立定时任务

crontab: installing new crontab

[root@foundation21 ~]# crontab -u root -l    查看定时任务

56 09 * * * /bin/rm -fr /mnt/file*

[root@foundation21 ~]# crontab -u root -r    删除定时任务

[root@foundation21 ~]# crontab -u root -l    查看定时任务已经被删除

no crontab for root

[root@foundation21 ~]# cat /var/spool/cron/root   查看文件内容，文件被删掉

cat: /var/spool/cron/root: No such file or directory

crontab定时命令的黑白名单：

[root@foundation21 ~]# crontab -e     超级用户可以执行定时任务

no crontab for root - using an empty one

crontab: installing new crontab

[root@foundation21 ~]# su - westos    切换到普通用户

Last login: Sun Apr 22 10:04:43 CST 2018 on pts/3

[westos@foundation21 ~]$ crontab -e   普通用户可以执行定时任务

no crontab for westos - using an empty one

crontab: installing new crontab

[westos@foundation21 ~]$ logout

[root@foundation21 ~]# su - linux      切换到普通用户

Last login: Sun Apr 22 10:04:51 CST 2018 on pts/3

[linux@foundation21 ~]$ crontab -e     普通用户可以执行定时任务

no crontab for linux - using an empty one

crontab: installing new crontab

[linux@foundation21 ~]$ logout

[root@foundation21 ~]# vim /etc/cron.deny

[root@foundation21 ~]# cat /etc/cron.deny   将westos用户加入黑名单

westos

[root@foundation21 ~]# su - westos

Last login: Sun Apr 22 10:14:14 CST 2018 on pts/0

[westos@foundation21 ~]$ crontab -e         不可以执行定时任务

You (westos) are not allowed to use this program (crontab)

See crontab(1) for more information

[westos@foundation21 ~]$ logout

[root@foundation21 ~]# su - linux

Last login: Sun Apr 22 10:14:31 CST 2018 on pts/0

[linux@foundation21 ~]$ crontab -e          不在黑名单用户可以执行定时任务

crontab: installing new crontab

[linux@foundation21 ~]$ logout

[root@foundation21 ~]# vim /etc/cron.deny   删除黑名单

[root@foundation21 ~]# vim /etc/cron.allow

[root@foundation21 ~]# cat /etc/cron.allow  将westos用户加入白名单

westos

[root@foundation21 ~]# su - linux

Last login: Sun Apr 22 10:16:00 CST 2018 on pts/0

[linux@foundation21 ~]$ crontab -e          不在白名单用户不可以执行定时任务

You (linux) are not allowed to use this program (crontab)

See crontab(1) for more information

[linux@foundation21 ~]$ logout

[root@foundation21 ~]# su - westos

Last login: Sun Apr 22 10:15:54 CST 2018 on pts/0

[westos@foundation21 ~]$ crontab -e         白名单用户可以执行定时任务

crontab: installing new crontab

[westos@foundation21 ~]$ logout

[root@foundation21 ~]# cat /etc/cron.allow

westos

[root@foundation21 ~]# vim /etc/cron.allow   删除白名单用户

定时任务的另外一种表示：

[root@foundation21 ~]# cd /etc/cron.d       切换路径

[root@foundation21 cron.d]# vim westos      

[root@foundation21 cron.d]# cat westos      建立五个文件

* * * * * root touch /mnt/xfl{1..5}

[root@foundation21 cron.d]# vim westos

[root@foundation21 cron.d]# cat westos

* * * * * root rm -fr /mnt/xfl{1..5}      删除五个文件

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