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 2014
midnight 12:00 AM October 19 2014
teatime 4:00 PM October 18 2014
tomorrow 10:00 AM October 19 2014
noon tomorrow 12:00 PM October 19 2014
next week 10:00 AM October 25 2014
next monday 10:00 AM October 24 2014
fri 10:00 AM October 21 2014
NOV 10:00 AM November 18 2014
9:00 AM 9:00 AM October 19 2014
2:30 PM 2:30 PM October 18 2014
1430 2:30 PM October 18 2014
2:30 PM tomorrow 2:30 PM October 19 2014
2:30 PM next month 2:30 PM November 18 2014
2:30 PM Fri 2:30 PM October 21 2014
2:30 PM 10/21 2:30 PM October 21 2014
2:30 PM Oct 21 2:30 PM October 21 2014
2:30 PM 10/21/2014 2:30 PM October 21 2014
2:30 PM 21.10.14 2:30 PM October 21 2014
now + 30 minutes 10:30 AM October 18 2014
now + 1 hour 11:00 AM October 18 2014
now + 2 days 10:00 AM October 20 2014
4 PM + 2 days 4:00 PM October 20 2014
now + 3 weeks 10:00 AM November 8 2014
now + 4 months 10:00 AM February 18 2015
now + 5 years 10:00 AM October 18 2019