Periodic planning work

cron process

cron： It's a process . It can make linux Execute a command periodically .
crontab It's an order , You can set linux Execute a command periodically  .
-u： Set the periodic work of a user --root jurisdiction ;
-e： edit crontab Work content of 
-l： Look up crontab Work content of 
-r： Remove all crontab Work content of , To remove only one item , please 
 use -e To edit 

• Explanation of sentence format

1. Sentence order minute hour day month week command

Sequential column	Means
minute	Represents minutes , It can be from 0 To 59 Any integer between .
hour	For hours , It can be from 0 To 23 Any integer between .
day	Means day , It can be from 1 To 31 Any integer between .
month	Represents the month , It can be from 1 To 12 Any integer between .
week	Day of the week ,0 To 7 The whole number between ,0 or 7 On behalf of Sunday
command	Commands to execute , It can be a system command , You can also write your own feet

2. Special symbols

Special symbols	Means
* ( asterisk }	Accept at any time , Represents any time
, ( comma )	Stands for separated fields
- ( minus sign }	For a period of time
/ n	that n On behalf of the digital , That is, every n Unit interval execution means

• crontab give an example

Achieve the goal	example
Every time 1 Once per minute	/1 * * * * command
At the end of every hour 15,30,45 Minute execution	15,30,45 * * * * command
7 Point to 10 Point of the first 10、 The first 50 Minute execution	10,50 7-10 * * * command
Every other day 7-11 Point of the first 20 And the 40 Minute execution	20,40 7-11 */ 1 * * command
On Monday 7 Point to 11 Point of the first 25 And the 50 Minute execution	25,50 7-11 * * 1 command
Daily 3:30 perform	30 3 * * * command
monthly 1、10、 20 Japanese 3: 30 perform	30 31,10,20 * * command
Every Saturday 、 Sunday's 3:30 perform	30 3 * * 6,7 command
Every day 9 Point to 16 Every 15 Minute execution	/15 9-16 ** * command

crontab To configure

1. crontab The configuration file

first line SHELL Variable specifies which system to use shell, Here is bash.

The second line PATH Variable specifies the path of the system to execute the command .

The third line MAILTO The variable specifies crond The task execution information of will be sent to root user , If MAILTO The value of the variable is null , The task execution information is not sent to the user .

2. crontab Configuration principle

When the user uses crontab This command is used to create a work schedule after , The work will be recorded in /var/spool/cron/ It went to the , And it is judged by the account number

Such as : lewis use User use crontab after , His work will be recorded in /ar/spool/cron/lewis in . in addition ,cron Every operation will be recorded in /var/log/cron In this log file .

• crond service

crond The service detects every minute , therefore cron Will read every minute /etc/crontab And /var/spool/cron The data content inside , therefore , As long as you finish editing /etc/crontab This file , And after storing it , that cron The configuration of will automatically run .

If you have finished modifying crontab Or add crontab There is no immediate implementation , So you can use systemctl restart crond.service restart