Crontab timed execution command under Linux

Crontab timed execution command under Linux

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table of Contents

1. Crontab overview and installation
2. Crontab rules
3. Common examples

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1. Crontab overview and installation

1. Crontab instruction is a command to execute timing tasks under Linux.

2. Check if crontab is installed on the server

rpm -qa | grep crontab

3. If it is not installed, execute the installation command

1. vixie-cron is the main program of cron;
2. crontabs is a program used to install, uninstall, or list tables used to drive the cron daemon.

yum -y install vixie-cron
yum -y install crontabs

4. The installation takes a long time to start and configure the service

service crond start     //启动服务
service crond stop      //关闭服务
service crond restart   //重启服务
service crond reload    //重新载入配置
service crond status    //查看crontab服务状态

5. Set up auto start

chkconfig --level 345 crond on

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2. Crontab rules

1. crontab file format

	*	 	*	 	*	 	*	 	* 			command
    分	    时      日      月       周（几）       命令

2. Special character interpretation

1. * : Represents the meaning of "every", for example, if the month field is *, it means that the command is executed every month
2. , : Indicates the meaning of separating time periods, for example, "1, 3, 5, 7, 9"
3. - : Indicates a time range, for example, "2-6" means "2, 3, 4, 5, 6".
4. /: Indicates the interval frequency of the time, for example, "0-23/2" means it will be executed every two hours. At the same time /it can be *used together, e.g. */ 10, if used in the minute field representing every 10 minutes.

3. Practice

1. Create a new shell file in the directory: test.sh, write

   #!/bin/bash
   
   echo "hello world!"
   

2. Give test.sh executable permissions

   chmod 755 test.sh
   

3. Execute and contab -ewrite timing tasks, and execute the test.sh script every minute.

   */1 * * * * /a8root/home/lijinwang/test/test.sh >> /a8root/home/lijinwang/test/test.log
   

4. result.
   

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3. Common examples

1. Execute the test.sh script at 3:30 a.m. and 12:20 noon every month

   30 3,12 * * * /root/test.sh >> /root/test.log
   

2. Execute test.sh script every 30 minutes every 6 hours every month

   30 */6 * * * /root/test.sh >> /root/test.log
   

3. Execute the test.sh script every 30 minutes every 2 hours from 8 am to 18 pm every month

   30 8-18/2 * * * /root/test.sh >> /root/test.log
   

4. Execute the test.sh script at 21:30 every night every month

   30 21 * * * /root/test.sh >> /root/test.log
   

5. Execute the test.sh script at 4:45 am on the 1st, 10th, and 22nd of each month

   45 4 1,10,22 * * /root/test.sh >> /root/test.log
   

6. Execute the test.sh script at 1:10 am on Monday and Sunday in August

   10 1 * 8 6,0 /root/test.sh >> /root/test.log
   

7. Execute the test.sh script every hour and hour on the hour every month

   00 */1 * * * /root/test.sh >> /root/test.log