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This article mainly introduces linux how to achieve crontab timing tasks, has a certain reference value, interested friends can refer to, I hope you can learn a lot after reading this article, the following let Xiaobian take you to understand.

The crontab command is used to submit and manage tasks that users need to execute periodically, similar to the scheduled tasks under windows. When the installation of the operating system is completed, this service tool is installed by default, and the crond process is automatically started. The crond process periodically checks whether there is a task to be executed every minute, and automatically executes the task if there is a task to be executed.

Crontab (option) (parameter) option

-e: edit the timer settings for this user

-l: lists the timer settings for this user

-r: delete the timer setting for this user

-u: specifies the name of the user to set the timer.

Task scheduling under Linux is divided into two categories: system task scheduling and user task scheduling.

System task scheduling: the work that the system periodically performs, such as writing cached data to the hard disk, log cleaning, etc. There is a crontab file in the / etc directory, which is the configuration file for system task scheduling. The / etc/crontab file includes the following lines:

-- View / etc/crontab file information

[root@12c01 ~] # cat / etc/crontab

SHELL=/bin/bash

PATH=/sbin:/bin:/usr/sbin:/usr/bin

MAILTO=root

HOME=/

# 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

Description:

The first four lines are the environment variables used to configure the crond task to run. The first line SHELL variable specifies which shell the system will use, here is bash, the second line PATH variable specifies the path to which the system executes the command, and the third line MAILTO variable specifies that the task execution information of crond will be sent to the root user by email. If the value of the MAILTO variable is empty, the task execution information will not be sent to the user. The HOME variable on the fourth line specifies the home directory to be used when executing a command or script.

User task scheduling: tasks that users perform on a regular basis, such as user data backup, scheduled email reminders, etc. Users can use the crontab tool to customize their scheduled tasks. All user-defined crontab files are saved in the / var/spool/cron directory. The file name is the same as the user name, and the user rights file is as follows:

/ etc/cron.deny the users listed in this file are not allowed to use the crontab command

/ etc/cron.allow users listed in this file are allowed to use the crontab command

/ var/spool/cron/ the directory where all users' crontab files are stored, named after the user name

Meaning of crontab file: in the crontab file created by the user, each line represents a task, and each field on each line represents a setting. Its format is divided into six fields. The first five paragraphs are the time setting section, and the sixth paragraph is the command segment to be executed. The format is as follows:

Minute hour day month week command order: time-sharing, day, month and week

Where: minute: represents minutes, which can be any integer from 0 to 59.

Hour: represents an hour, which can be any integer from 0 to 23.

Day: represents the date and can be any integer from 1 to 31.

Month: represents the month and can be any integer from 1 to 12.

Week: represents the day of the week, which can be any integer from 0 to 7, where 0 or 7 represents Sunday.

Command: the command to be executed can be a system command or a script file written by yourself.

The following special characters can also be used in each of the above fields:

Asterisk (*): represents all possible values. For example, if the month field is an asterisk, it means that the command operation is performed every month after the constraints of other fields are met.

Comma (,): you can specify a list range with values separated by commas, for example, "1, 2, 5, 7, 8, 9"

Middle bar (-): you can use the middle bar between integers to represent a range of integers, for example, "2-6" means "2pens 3pr 4je 5pl 6"

Forward slash (/): the interval frequency of the time can be specified with a forward slash, for example, "0-23gam2" indicates that it is executed every two hours. At the same time, a forward slash can be used with an asterisk, such as * / 10, which, if used in the minute field, means that it is executed every ten minutes.

Example:

1. Execution of the 3rd and 15th minutes of each hour: 3 command 15 *

2. Execution at the 3rd and 15th minutes from 8am to 11:00: 3Jing 15 8-11 * command

3. Execution of the 3rd and 15th minutes from 8 a.m. to 11:00 every two days: 3 command 15 8-11 * / 2 * *

4. Execution of the 3rd and 15th minutes from 8am to 11:00 every Monday: 3 command 15 8-11 * * 1

5. Restart smb at 21:30 every night: 30 21 * / etc/init.d/smb restart

6. Restart smb at 4: 45 on the 1st, 10th and 22nd of each month: 45 41, 10, 10, 22 * * / etc/init.d/smb restart

7. Restart smb at 1:10 every Saturday and Sunday: 10 1 * * 6 smb 0 / etc/init.d/smb restart

8. Restart smb every 30 minutes between 18: 00 and 23: 00 every day: 0Jing 30 18-23 * / etc/init.d/smb restart

9. Pm restarts smb at 11:00 every Saturday night: 023 * 6 / etc/init.d/smb restart

10. Restart smb every hour: * * / 1 * / etc/init.d/smb restart

Restart smb every other hour between 11:00 and 7 a.m.: * 23-7swap 1 * / etc/init.d/smb restart

12. Restart smb on the 4th of each month and 11:00 from Monday to Wednesday: 0 11 4 * mon-wed / etc/init.d/smb restart

13. Restart smb at 4: 00 on January 1: 0 41 jan * / etc/init.d/smb restart

14. Execute the scripts in the / etc/cron.hourly directory every hour: 01 * root run-parts / etc/cron.hourly

Add:

A. Check the status of crontab service:

Service crond status

B. Start the crontab service status:

Service crond start

C. Disable crontab service status:

Service crond stop

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