at跟crontab一样，都是执行定时计划任务的命令。但不同的是，crontab执行的循环的任务，而at执行的是一次性任务，任务执行完以后便失效。

设置任务：

at now + 1 week -f a.sh      #a.sh必须已存在

at 01:35 < my-at-jobs.txt    

at < my.txt 01:35            #也可以把时间写在后面

at now +2 minutes <<< "mkdir 123"

at now +10 minutes <<ENDMARKER

rm -rf 123

echo successful done > /var/log/messages

ENDMARKER

查看所有任务：

at -l

atq

查看单个任务：

at -c ID

删除任务：

atrm 23

at -r 1

关于时间，可参考的语法：

noon            12:00 PM October 18 2018

midnight    12:00 AM October 19 2018

teatime            4:00 PM October 18 2018

tomorrow    10:00 AM October 19 2018

noon tomorrow    12:00 PM October 19 2018

next week    10:00 AM October 25 2018

next monday    10:00 AM October 24 2018

fri            10:00 AM October 21 2018

NOV            10:00 AM November 18 2018

9:00 AM            9:00 AM October 19 2018

2:30 PM            2:30 PM October 18 2018

1430            2:30 PM October 18 2018

2:30 PM tomorrow    2:30 PM October 19 2018

2:30 PM next month    2:30 PM November 18 2018

2:30 PM Fri            2:30 PM October 21 2018

2:30 PM 10/21            2:30 PM October 21 2018

2:30 PM Oct 21            2:30 PM October 21 2018

2:30 PM 10/21/2018    2:30 PM October 21 2018

2:30 PM 21.10.14    2:30 PM October 21 2018

now + 30 minutes    10:30 AM October 18 2018

now + 1 hour    11:00 AM October 18 2018

now + 2 days    10:00 AM October 20 2018

4 PM + 2 days    4:00 PM October 20 2018

now + 3 weeks    10:00 AM November 8 2018

now + 4 months    10:00 AM February 18 2015

now + 5 years    10:00 AM October 18 2019