Linux中的定時自動執行功能（at,crontab）

概念

在Linux系統中，提供了兩種提前對工作進行安排的方式

• at 只執行一次

• crontab 周期性重復執行

通過對這兩個工具的應用可以讓我們在操作系統層面實現定時自動運行功能

實驗環境

操作系統：CentOS Linux release 7.3.1611 (Core)

IP： 192.168.230.134

at介紹

at命令會，並且會執行一次

此功能會使用到atd工具，需要保證其為啟動狀態

[root@localhost ~]# systemctl status atd
● atd.service - Job spooling tools
   Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2017-09-07 18:18:18 CST; 4h 2min ago
 Main PID: 921 (atd)
   CGroup: /system.slice/atd.service
 └─921 /usr/sbin/atd -f Sep 07 18:18:18 localhost.localdomain systemd[1]: Started Job spooling tools. Sep 07 18:18:18 localhost.localdomain systemd[1]: Starting Job spooling tools...

at使用

常用命令如下

[root@localhost ~]# at -help
Usage: at [-V] [-q x] [-f file] [-mMlbv] timespec ...
 at [-V] [-q x] [-f file] [-mMlbv] -t time  at -c job ...  atq [-V] [-q x]  at [ -rd ] job ...  atrm [-V] job ...  batch

時間格式：

格式	示例	意義
HH:MM	01:01	下一次1點1分執行
HH:MM YYYY-MM-DD	01:00 2017-09-08	指定具體年月時間執行
HH:MM[am/pm]+number[minutes/hours/days/weeks]	now+3 days	某時間點再加些時間后

at實例

創建測試用sh，並修改權限

[root@localhost ~]# vim test.sh echo '豆姐威武' [root@localhost ~]# chmod 755 test.sh [root@localhost ~]# ./test.sh 豆姐威武

查看當前時間

[root@localhost ~]# date
Thu Sep  7 23:19:51 CST 2017

使用at命令，設定23:21執行test.sh命令，注意以Ctrl+d結束

[root@localhost ~]# at 23:21
at> /root/test.sh
at> <EOT> job 5 at Thu Sep 7 23:21:00 2017

坐等一分鍾后，收到系統給的信息，查看時間以及到了設定時間

You have new mail in /var/spool/mail/root
[root@localhost ~]# date
Thu Sep  7 23:21:04 CST 2017

查看系統收到的信息

[root@localhost ~]# cat /var/spool/mail/root From [e-mail] Thu Sep 7 23:21:00 2017 Return-Path: <[e-mail]> X-Original-To: root Delivered-To: [e-mail] Received: by localhost.localdomain (Postfix, from userid 0) id A139B243D2; Thu, 7 Sep 2017 23:21:00 +0800 (CST) Subject: Output from your job 5 To: [e-mail] Message-Id: <[e-mail]> Date: Thu, 7 Sep 2017 23:21:00 +0800 (CST) From: [e-mail] (root) 豆姐威武

抄一個比較實用的腳本： 
定時關機

[root@localhost ~]# at 18:00 2017-09-10 at> /bin/sync at> /bin/sync at> /sbin/shutdown -h now at> <EOT> job 6 at Sun Sep 10 18:00:00 2017

將在2017/09/07 18：00自動關機

at命令的取消

查看當前at命令 
使用atq命令

[root@localhost ~]# atq 6 Sun Sep 10 18:00:00 2017 a root

取消命令 
使用atrm命令

[root@localhost ~]# atrm 6

crontab介紹

crontab命令會按照配置周期性的執行

他的記錄會保存在/var/log/cron中

此功能需要用到crond服務，需要保證其狀態正常

[root@localhost ~]# systemctl status crond ● crond.service - Command Scheduler Loaded: loaded (/usr/lib/systemd/system/crond.service; enabled; vendor preset: enabled) Active: active (running) since Thu 2017-09-07 18:18:18 CST; 5h 51min ago Main PID: 920 (crond) CGroup: /system.slice/crond.service └─920 /usr/sbin/crond -n Sep 07 18:18:18 localhost.localdomain systemd[1]: Started Command Scheduler. Sep 07 18:18:18 localhost.localdomain systemd[1]: Starting Command Scheduler... Sep 07 18:18:18 localhost.localdomain crond[920]: (CRON) INFO (RANDOM_DELAY will be ...) Sep 07 18:18:19 localhost.localdomain crond[920]: (CRON) INFO (running with inotify ...) Hint: Some lines were ellipsized, use -l to show in full. 

crontab使用

常用命令如下：

[root@localhost ~]# crontab -help
crontab: invalid option -- 'h'
crontab: usage error: unrecognized option
Usage:
 crontab [options] file
 crontab [options]
 crontab -n [hostname]

Options:
 -u <user> define user -e edit user's crontab -l list user's crontab -r delete user's crontab -i prompt before deleting -n <host> set host in cluster to run users' crontabs -c get host in cluster to run users' crontabs -s selinux context -x <mask> enable debugging

時間格式：

*    *    *    *    *
- - - - - | | | | | | | | | | | | | | +----- day of week (0 - 7) (Sunday=0 or 7) | | | +---------- month (1 - 12) | | +--------------- day of month (1 - 31) | +-------------------- hour (0 - 23) +------------------------- min (0 - 59)

crontab實例

還是使用之前創建的test.sh做實驗

使用crontab命令新建計划

先查看當前時間

[root@localhost ~]# date Thu Sep 7 23:52:12 CST 2017

創建計划，設定每天23：55執行 /root/test.sh腳本

[root@localhost ~]# crontab -e 55 23 * * * /root/test.sh no crontab for root - using an empty one crontab: installing new crontab

查看當前計划：

[root@localhost ~]# crontab -l 55 23 * * * /root/test.sh

坐等兩分鍾后：

[root@localhost ~]# date Thu Sep 7 23:55:07 CST 2017 You have new mail in /var/spool/mail/root

查看記錄：

[root@localhost ~]# cat /var/spool/mail/root From [e-mail] Thu Sep 7 23:55:01 2017 Return-Path: <[e-mail]> X-Original-To: root Delivered-To: [e-mail] Received: by localhost.localdomain (Postfix, from userid 0) id 45D1028EB7; Thu, 7 Sep 2017 23:55:01 +0800 (CST) From: "(Cron Daemon)" <[e-mail]> To: [e-mail] Subject: Cron <root@localhost> /root/test.sh Content-Type: text/plain; charset=UTF-8 Auto-Submitted: auto-generated Precedence: bulk X-Cron-Env: <XDG_SESSION_ID=52> X-Cron-Env: <XDG_RUNTIME_DIR=/run/user/0> X-Cron-Env: <LANG=en_US.UTF-8> X-Cron-Env: <SHELL=/bin/sh> X-Cron-Env: <HOME=/root> X-Cron-Env: <PATH=/usr/bin:/bin> X-Cron-Env: <LOGNAME=root> X-Cron-Env: <USER=root> Message-Id: <[e-mail]> Date: Thu, 7 Sep 2017 23:55:01 +0800 (CST) 豆姐威武

crontab計划的刪除

如果需要刪除一兩個計划，直接使用crontab -e進入編輯頁面刪除 
如果要清空計划,-r即可

[root@localhost ~]# crontab -l 55 23 * * * /root/test.sh [root@localhost ~]# crontab -r [root@localhost ~]# crontab -l no crontab for root

crontab補充

也可以直接編輯/etc/crontab文件來實現計划腳本

[root@localhost ~]# vim /etc/crontab SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root # For details see man 4 crontabs # Example of job definition: # .---------------- minute (0 - 59) # | .------------- hour (0 - 23) # | | .---------- day of month (1 - 31) # | | | .------- month (1 - 12) OR jan,feb,mar,apr ... # | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat # | | | | | # * * * * * user-name command to be executed

 

但是使用這種方式有時候不會立刻生效，需要重啟crond服務