Task scheduling of Linux task scheduling system

Linux at for run-time tasks

Crontab commands to control scheduled tasks

Crond System Services

Crond is a daemon that is used to periodically perform certain tasks or wait to handle certain events under Linux.

Similar to Scheduled tasks under Windows, this service tool is installed by default when the operating system is installed.

And the Crond process starts automatically, and the Crond process periodically checks to see if there are any tasks to perform.

If there is a task to perform, the task is automatically performed.

System task scheduling: The work to be performed by the system periodically, such as writing cache data to hard disk, log cleanup, etc. In the/etc directory there is a crontab file, this is the System Task Scheduler configuration file.

User Task scheduling: Users to perform regular work, such as user data backup, scheduled email reminders and so on. Users can use the Crontab tool to customize their own scheduled tasks. All user-defined crontab files are saved in the/var/spool/cron directory. Its file name is the same as the user name.

User Rights file

/etc/cron.deny

Description

The users listed in this file are not allowed to use the crontab command

File:

/etc/cron.allow

Description

Users listed in this file are allowed to use the crontab command

crontab What the file means:

In the crontab file created by the user, each line represents a task, each field of each row represents a setting, its format is divided into six fields, the first five is the time setting segment, and the sixth paragraph is the command segment to execute, in the following format:

Minute hour day Month Week command

Asterisk (*): represents all possible values, such as the month field if it is an asterisk, the command action is executed monthly after the constraints of other fields are met.

Comma (,): You can specify a list range with a comma-separated value, for example, "1,2,5,7,8,9"

Middle Bar (-): An integer range can be represented by a middle bar between integers, such as "2-6" for "2,3,4,5,6"

Forward slash (/): You can specify the interval frequency of the time with a forward slash, such as "0-23/2", which is performed every two hours. A forward slash can be used with asterisks, such as */10, if used in the minute field, indicating that every 10 minutes is performed

Second, Crond service

Install Crontab:

Yum Install Crontabs

Service Operation Instructions:

/sbin/service Crond Start//Startup service

/sbin/service Crond stop//Shut down service

/sbin/service crond Restart//Restart service

/sbin/service Crond Reload//Reload Configuration

View Crontab Service Status:

Service Crond Status

To start the Crontab service manually:

Service Crond Start

To see if the Crontab service is set to boot, execute the command:

Ntsysv

Add to boot auto start:

Chkconfig–level Crond on

Command parameters:

-u User: Used to set a user's crontab service, for example, "-u ixdba" means to set IXDBA user's crontab service, this parameter is usually run by the root user.

File:file is the name of the command file, which indicates that file is the Crontab task list and loaded into crontab. If this file is not specified on the command line, the crontab command will accept the commands typed on the standard input (keyboard) and load them into crontab.

-E: Edits the contents of a user's crontab file. If you do not specify a user, the crontab file for the current user is edited.

-L: Displays the contents of a user's crontab file, or displays the contents of the current user's crontab file if no user is specified.

-r: Deletes a user's crontab file from the/var/spool/cron directory and, if no user is specified, deletes the current user's crontab file by default.

-I: Give a confirmation prompt when deleting a user's crontab file.

Example 1: command is executed every 1 minutes

Command:

* * * * * command

Example 2:3rd and 15 minutes per hour of execution

Command:

3,15 * * * command

Example 3: Execution at 3rd and 15 minutes from 8 o'clock in the morning to 11.

Command:

3,15 8-11 * * command

Example 4:3rd and 15 minutes of every two-day 8 o'clock in the morning to 11-point execution

Command:

3,15 8-11 */2 * command

Example 5:3rd and 15 minutes of each Monday from 8 o'clock in the morning to 11.

Command:

3,15 8-11 * * 1 command

Example 6:21:30 restart of SMB per night

Command:

* * * * */ETC/INIT.D/SMB restart

Example 7:4:45 restart SMB per month for 1, 10, 22nd

Command:

4 1,10,22 * */ETC/INIT.D/SMB restart

Example 8:1:10 restart SMB per Saturday, Sunday

Command:

1 * * 6,0/ETC/INIT.D/SMB restart

Example 9: Restart SMB every 30 minutes from 18:00 to 23:00 daily

Command:

0,30 18-23 * * */ETC/INIT.D/SMB restart

Example 10: Every Saturday night at 11:00am restart SMB

Command:

0 * * 6/ETC/INIT.D/SMB restart

Example 11: Restart SMB every hour

Command:

* */1 * * * */ETC/INIT.D/SMB restart

Example 12: Restart SMB every hour from 11 o'clock to 7 in the morning

Command:

* 23-7/1 * * * */ETC/INIT.D/SMB restart

Example 13:4th per month with 11 points per Monday to Wednesday restart SMB

Command:

0 4 * MON-WED/ETC/INIT.D/SMB restart

Example 14:4-point restart of SMB on January 1

Command:

0 4 1 Jan */ETC/INIT.D/SMB restart

Example 15: Execution of scripts within/etc/cron.hourly directory per hour

Iv. Precautions for use

1. pay attention to the environment variable problem

Sometimes we create a crontab, but this task cannot be executed automatically, but it is not a problem to perform this task manually, which is usually caused by not configuring environment variables in the crontab file.

When defining multiple dispatch tasks in a crontab file, one of the issues that needs special attention is the setting of environment variables, because when we perform a task manually, it is done in the current shell environment, the program can certainly find the environment variable, and the system will not load any environment variables when it automatically executes the task schedule. Therefore, you need to specify all the environment variables that are required for the task to run in the crontab file, so that the system does not have a problem when it executes the Task Scheduler.

Don't assume that Cron knows the special circumstances you need, and it doesn't really know. So you have to make sure that you provide all the necessary path and environment variables in the shelll script, except for some auto-set global variables. So note the following 3 points:

1) Write the global path when the file path is involved in the script;

2) When script execution is used in Java or other environment variables, the environment variables are introduced through the source command, such as:

Cat start_cbp.sh

#!/bin/sh

Source/etc/profile

Export run_conf=/home/d139/conf/platform/cbp/cbp_jboss.conf

/usr/local/jboss-4.0.5/bin/run.sh-c MeV &

3) When the script is executed manually, but crontab is not executed. At this point, we must boldly suspect that environmental variables are the bane, and can try to directly introduce environmental variables in crontab to solve the problem. Such as:

0 * * * *. /etc/profile;/bin/sh/var/www/java/audit_no_count/bin/restart_audit.sh

2. take care to clean up the mail log for system users

Each task is scheduled to execute, the system will send the task output information in the form of e-mail to the current system users, so the cumulative, log information will be very large, may affect the normal operation of the system, so it is important to redirect each task.

For example, you can set the following form in the crontab file, ignoring the log output:

0 */3 * * */usr/local/apache2/apachectl restart >/dev/null 2>&1

"/dev/null 2>&1" means that the standard output is redirected to/dev/null and then the standard error is redirected to standard output, and standard errors are redirected to/dev/null because the standard output has been redirected to/dev/null. This will solve the problem of log output.

3. System-level task scheduling and user-level task scheduling

System-level task scheduling is mainly to complete some maintenance operations of the system, user-level task scheduling is mainly to complete the user-defined tasks, you can put the user-level task scheduling to the system-level task scheduling to complete (not recommended), but in turn, the root user's task scheduling operation can be through the "crontab– Uroot–e "To set, you can also write the dispatch task directly to the/etc/crontab file, it should be noted that if you want to define a scheduled restart of the system task, you must put the task into the/etc/crontab file, Even the task of creating a timed restart of the system under the root user is not valid.

4. Other Precautions

The newly created cron job will not execute immediately, at least 2 minutes. If you restart Cron, it will be executed immediately.

When the crontab suddenly fails, you can try/etc/init.d/crond restart solve the problem. Or check the log to see if a job has execution/error tail-f/var/log/cron.

Don't run crontab-r. It removes the user's crontab file from the crontab directory (/var/spool/cron). All crontab of the user have been deleted.

In Crontab, the% has a special meaning, indicating the meaning of the line break. If you want to use the words must be escaped \%, such as the frequently used date ' +%y%m%d ' in the crontab will not be executed, should be replaced by the date ' +\%y\%m\%d '.

Task scheduling of Linux task scheduling system