作用: 计划任务主要是做一些周期性的任务，目前最主要的用途是定期备份数据
Schedule one-time tasks with at. 一次性调度执行 at Schedule recurring jobs with cron.
循环调度执行 cron Schedule recurring system jobs.
所有计划任务执行中的输出都会以邮件的方式发送给指定用户， 除非重定向一次性调度执行 at
语法格式： at <TIMESPEC> now +5min teatime tomorrow (teatime is 16:00) noon +4 days 5pm august 3 2018
例1:
[root@localhost ]# yum -y install at
[root@localhost ]# systemctl status atd
● atd.service - Job spooling tools
Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)
Active: inactive (dead)
[root@localhost nginx]# at now +1min
at> useradd ehome001
at> <EOT>
job 1 at Tue Jul 30 11:33:00 2019
Can't open /var/run/atd.pid to signal atd. No atd running?
[root@localhost ]# systemctl start atd //首先要启动服务
[root@fanhua ~]# at now +1min //一分钟后开始执行
at> useradd uuuu
at> <EOT> //Ctrl+D
job 1 at Sat Mar 21 22:34:00 2015
[root@fanhua ~]# id uuuu
例2:
[root@yang ~]# vim at.jobs
useradd u99
useradd u00
touch /a.txt
[root@yang ~]# at now +1min < at.jobs
例3:
1. 准备sudo用户
[root@youngfit ~]# useradd jack -G wheel
[root@youngfit ~]# visudo
Allows people in group wheel to run all commands
%wheel ALL=(ALL) ALL
Same thing without a password
%wheel ALL=(ALL) NOPASSWD: ALL //此步骤不加，很有可能出现问题
errors： sudo: no tty present and no askpass program specified
[root@youngfit ~]# su - jack
[jack@youngfit ~]$ sudo useradd yy
2. 以sudo 用户jack创建at任务
[jack@youngfit ~]$ vim jack.at
sudo useradd u200
sudo useradd u300
sudo touch /home/jack.txt
[jack@youngfit ~]$ at now +1min < jack.at
