调度执行
作用：周期性任务，主要用途是定期备份数据
Schedule ont-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
--未用root登录，导致执行失败
lyq@DESKTOP-40030BI:~$ vim jack.at
lyq@DESKTOP-40030BI:~$ cat jack.at
useradd u200
useradd u300
touch /home/'date +%F'_jack.txt
lyq@DESKTOP-40030BI:~$ at now +1min < jack.at
warning: commands will be executed using /bin/sh
job 2 at Sat Jul 13 14:42:00 2019
Can't open /var/run/atd.pid to signal atd. No atd running?
lyq@DESKTOP-40030BI:~$ id u200
id: ‘u200’: no such user
lyq@DESKTOP-40030BI:~$ date
Sat Jul 13 14:42:48 DST 2019
lyq@DESKTOP-40030BI:~$ id u777
id: ‘u777’: no such user
lyq@DESKTOP-40030BI:~$ id u300
id: ‘u300’: no such user
注意：sudo执行需要考虑tty问题
lyq@DESKTOP-40030BI:~$ visudo
visudo: /etc/sudoers: Permission denied
lyq@DESKTOP-40030BI:~$ sudo visudo
visudo: /etc/sudoers.tmp unchanged
crond
systemctl status crond.service
pa aux |grep crond
crond进程每分钟会处理一次计划任务
crontab -e
语法格式
Minutes Hours Day-of-Month Month Day-of-Week Command
* * * * * command
Minutes -- 0-59
Hours -- 0-23
Day-of-Month -- 1-31
Month -- 1-12 or jan feb mar apr
Day-of-Week -- 0-6 or Sunday=0 or 7
用户级
--存储路径
(base) root@dell:~ # ls /var/spool/cron
atjobs atspool crontabs
0 2 * * * /mysql_back.sh --每天2点整
0 2 1 * * /mysql_back.sh --每月1号2点整
0 2 14 2 * /mysql_back.sh --每月2月14号2点整
0 2 * * 7 /mysql_back.sh --每周日2点整
0 2 * 6 5 /mysql_back.sh --每年6月的周五2点整
0 2 14 * 7 /mysql_back.sh --每月14号2点整 或 每周日2点整 ，两个时间都执行
0 2 14 2 7 /mysql_back.sh --每月2月14号2点整 或 每周日2点整 ，两个时间都执行
0 2 2 * 5 /mysql_back.sh --每周五2点
*/5 * * * * /mysql_back.sh --每隔5分钟
0 2 1,4,6 * * /mysql_back.sh --每月1/4/6号的2点运行
0 2 5-9 * * /mysql_back.sh --每月5-9号的2点运行
0 * * * * /mysql_back.sh --
* * 2 * * /mysql_back.sh --2号那天，每分钟都执行，逻辑有错
* 2 * * * /mysql_back.sh --每天2点整的每一个分钟，逻辑有错
系统级
主要用途：
临时文件的清理 /tmp/var/tmp
系统信息的采集 sar
日志的轮转（切割）logrotate
--定义位置1
(base) root@dell:~ # cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m h dom mon dow user command
17 * * * * root cd / && run-parts --report /etc/cron.hourly
25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#
--定义位置2： /etc/cron.d/*
(base) root@dell:~ # ls /etc/cron.d
mdadm popularity-contest
(base) root@dell:~ # cat /etc/cron.d/mdadm
#
# cron.d/mdadm -- schedules periodic redundancy checks of MD devices
#
# Copyright © martin f. krafft <madduck@madduck.net>
# distributed under the terms of the Artistic Licence 2.0
#
# By default, run at 00:57 on every Sunday, but do nothing unless the day of
# the month is less than or equal to 7. Thus, only run on the first Sunday of
# each month. crontab(5) sucks, unfortunately, in this regard; therefore this
# hack (see #380425).
57 0 * * 0 root if [ -x /usr/share/mdadm/checkarray ] && [ $(date +\%d) -le 7 ]; then /usr/share/mdadm/checkarray --cron --all --idle --quiet; fi
crond仅仅会执行每小时定义的脚本 /etc/cron.hourly
crond 实际是启动anacrond进程
每小时的01分钟，crond进程会唤醒anacrond进程，防止某些任务被错过 /etc/anacrontab
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