Crontab command usage

Task scheduled crond Resident order
crond yes linux A command used to execute a program on a regular basis . When the operating system is installed , By default, the task scheduling command is started .crond Order that every minute be checked regularly for any work to be performed , If there is work to be performed, it will be automatically performed . and linux Task scheduling is mainly divided into the following two categories ：
1、 The work performed by the system ： What the system does periodically , Such as backup system data 、 Clean cache
2、 Work performed by individuals ： Work that a user does on a regular basis , For example, every 10 Minutes to check the mail server for new messages , These tasks can be set by each user  

Crontab yes UNIX Timing task trigger under the system , The permissions of its users are recorded in the following two documents ： 

/etc/cron.deny 

Users listed in this file are not allowed to use Crontab command  

/etc/cron.allow 

Users listed in this file are allowed to use Crontab command  

/var/spool/cron/ 

It's for all users crontab file  

/var/spool/cron/crontabs
/var/spool/cron/crontabs
Crontab The format of the command is ：crontab –l|-r|-e|-i [username], The meaning of its parameters is shown in Table 1 ： 


-l  Display the user's Crontab The content of the document  crontabl –l 

-i  Delete the user's Crontab Prompt before file  crontabl -ri 

-r  from Crontab Delete the user's Crontab file  crontabl -r 

-e  Edit the user's Crontab file  crontabl -e 

User created Crontab File in /var/spool/cron in , The file name is the same as the user name .
Its format is divided into six paragraphs , The first five sections are time setting sections , The sixth paragraph is the command segment to be executed ,
The format is as follows ：* * * * * 
The meaning of the time period is shown in Table 2 ： 

The first paragraph   For minutes  0—59 

The second paragraph   For hours  0—23 

The third paragraph   Representative date  1—31 

The fourth paragraph   Represents the month  1—12 

The fifth paragraph   The day of the week ,0 On behalf of Sunday  0—6