Crontab Introduction: crontab

Crontab Introduction: crontab

1. Start and Stop Cron

Since Cron is a built-in service in Linux, you can use the following method to start and close the service:

/Sbin/service crond start // start the service
/Sbin/service crond stop // close the service
/Sbin/service crond restart // restart the service
/Sbin/service crond reload // reload the configuration

2. Cron configuration file

2.1 global configuration file

Crontab contains cron. hourly, cron. daily, cron. weekly, cron. monthly, cron. d and crontab and cron. deny files under the/etc directory.

Cron. daily is a job executed once a day, cron. weekly is a job executed once a week. cron. monthly is a job executed once a month, cron. hourly is a job executed every hour. cron. d is a task automatically and periodically required by the system, but it is not executed by hour, by day, by week, by month, so it is placed under this directory.

The/etc/crontab file is generally as follows:

01 *** root run-parts/etc/cron. hourly
02 4 *** root run-parts/etc/cron. daily
22 4 ** 0 root run-parts/etc/cron. weekly
42 4 1 ** root run-parts/etc/cron. monthly

We can add the required cron job to this file.

The/etc/cron. deny file is used to control which users are not allowed to use Crontab.

2.2 user configuration file

Each user has his own cron configuration file, which can be edited through crontab-e. Generally, after the cron configuration file is saved and exited, the system automatically stores the file in the/var/spool/cron/directory. The file is named after the user name.

The linux cron service reads all the content under/var/spool/cron,/etc/crontab,/etc/cron. d every minute.

3. Cron Command Format

Crontab [-u user] File
Crontab [-u user] {-l |-r |-e}

-U: Specifies a user.

-E: Execute the text editor to set the user (current user or specified user) time table. The specified text editor is vi.
-R: delete a user time table.
-L: list the user time table.

4. Cron File Format

* *** Command
Hour, day, month, and week commands

The 1st column indicates minute 1 ~ 59, represented by * or */1 per minute
The first column indicates the hour 1 ~ 23 (0 indicates 0 points)
The 3rd column indicates the date 1 ~ 31
The 4th column indicates the month 1 ~ 12
The Identification Number of column 5th is from day of the week to day ~ 6 (0 indicates Sunday)
6th columns of commands to run

5. Cron file usage instructions

5.1 General situation

When f1 is *, the program is executed every minute, when f2 is *, the program is executed every hour, and so on.

* ***/Bin/usershell: Execute/bin/usershell every minute
When f1 is a-B, it indicates that execution is performed from the minute a to the minute B. When f2 is a-B, it indicates that execution is performed from the hour a to the hour B, and so on.

0-12 *****/bin/usershell is executed every minute from 0 to 12 minutes every day/bin/usershell
When f1 is */n, it indicates that execution is performed every n minutes. If f2 is */n, it indicates that execution is performed every n hours, and so on.

**/2 ***/bin/usershell is executed every two hours every day./bin/usershell
When f1 is a, B, c ,... a, B, c ,... execute in minutes. f2 is a, B, c ,... a, B, c... execute in hours, and so on.

* 1, 3, 5, 7 ***/bin/usershell execute/bin/usershell once every 3, 3, 4, 7 every day

5.2. Conflict Logic

The date can be set to monthly or weekly. If two segments conflict, the command of the sixth segment will be executed when any one of the segments matches, for example

"30 4*5", will run on the first and 15th of every month plus every Friday at AM.

5.3. symbol "%"

"%" In the Cron file has the function of "ending command line", "line feed", and "redirection". If the special function of "%" is not required, escape using escape characters.

5.4. @ reboot

This does not need to be understood, in order to achieve the purpose of running after the boot, and only run once, in addition to this, it cannot be achieved through the first five segment settings.

@ Annually.

As for @ yearly, @ monthly, and so on, you can use the preceding five sections to set them.

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