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 <
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：
noon12:00 PM October 18
midnight 12:00 AM October 19
teatime4:00 PM October 18
tomorrow 10:00 AM October 19
noon tomorrow 12:00 PM October 19
next week 10:00 AM October 25
next monday 10:00 AM October 24
fri10:00 AM October 21
NOV10:00 AM November 18
9:00 AM9:00 AM October 19
2:30 PM2:30 PM October 18
14302:30 PM October 18
2:30 PM tomorrow 2:30 PM October 19
2:30 PM next month 2:30 PM November 18
2:30 PM Fri2:30 PM October 21
2:30 PM 10/212:30 PM October 21
2:30 PM Oct 212:30 PM October 21
2:30 PM 10/21/ 2:30 PM October 21
2:30 PM 21.10.14 2:30 PM October 21
now + 30 minutes 10:30 AM October 18
now + 1 hour 11:00 AM October 18
now + 2 days 10:00 AM October 20
4 PM + 2 days 4:00 PM October 20
now + 3 weeks 10:00 AM November 8
now + 4 months 10:00 AM February 18
now + 5 years 10:00 AM October 18
