Shulou(Shulou.com)06/02 Report--

First, the startup and shutdown of crontab service

Cron is a timed execution tool under linux that can run jobs without human intervention. Because Cron is a built-in service for Linux, but it does not start automatically, you can start and shut down the service in the following ways:

/ sbin/service crond start / / start the service

/ sbin/service crond stop / / shut down the service

/ sbin/service crond restart / / restart the service

/ sbin/service crond reload / / reload the configuration

You can also start this service automatically when the system starts:

At the end of the script / etc/rc.d/rc.local, add:

/ sbin/service crond start

Now that the Cron service is in progress, we can use it.

Second, crontab command parameters

The cron service provides a crontab command to set the cron service. Here are some parameters and instructions for this command:

Crontab-u / / sets a user's cron service. Generally, root users need this parameter when executing this command.

Crontab-l / / lists the details of a user's cron service

Crontab-r / / Delete a user's cron service

Crontab-e / / Edit a user's cron service

Third, crontab related documents

Files in the 1./var/spool/cron directory

Every time you use crontab-e to edit under a user, cron automatically generates a file with the same name as the user under / var/spool/cron. The user's cron information is recorded in this file. This file cannot be edited directly, but can only be edited with crontab-e. Read this file every time you start cron to check if you want to execute the commands in it. Therefore, there is no need to restart the cron service after this file is modified.

2./etc/crontab profile

The cron service not only reads all the files in / var/spool/cron once per minute, but also reads / etc/crontab once, so we can configure this file to do something with the cron service. Configuration with crontab is for a user, while editing / etc/crontab is a system-specific task. The file format of this file is:

SHELL=/bin/bash

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

MAILTO=root / / if there is an error, or if there is data output, the data will be sent to this account by email

The path where the HOME=/ user runs. Here is the root directory.

# run-parts

01 * root run-parts / etc/cron.hourly / / execute scripts in / etc/cron.hourly every hour

02 4 * root run-parts / etc/cron.daily / / execute scripts in / etc/cron.daily every day

22 4 * * 0 root run-parts / etc/cron.weekly / / execute scripts in / etc/cron.weekly weekly

42 4 1 * * root run-parts / etc/cron.monthly / / execute the scripts in / etc/cron.monthly every month

Note the parameter "run-parts". If you remove this parameter, you can write the name of a script to run instead of the folder name.

Files in the 3./etc/cron.d directory

This directory is used to hold any crontab files or scripts to be executed.

Fourth, crontab permission

Crontab permission question to / var/adm/cron/ next look, whether the files cron.allow and cron.deny exist

The usage is as follows:

1. If neither file exists, only root users can use the crontab command.

2. If cron.allow exists but cron.deny does not exist, only users listed in the cron.allow file can use the crontab command, and if root users are not there, root users cannot use crontab.

3. If cron.allow does not exist and cron.deny exists, only the users listed in the cron.deny file cannot use the crontab command, and other users can use it.

4. If both files exist, users listed in the cron.allow file and not listed in the cron.deny can use crontab, if both files have the same user

Depending on whether the user is in the cron.allow file, if the user is in cron.allow, you can use the crontab command.

Fifth, crontab time format

How to enter the command and time you need to execute in the crontab file. Each line in the file contains six fields, the first five of which are the specified time when the command is executed, and the last is the command to be executed.

Each field is separated by spaces or tabs. The format is as follows:

Minute hour day-of-month month-of-year day-of-week commands

Legal value 00-59 00-23 01-31 01-120-6 (0 is sunday) commands (representing the script to be executed)

In addition to numbers, there are several special symbols that are "*", "/" and "-", * represent all the numbers in the range of values, "/" means each, "/ 5" means every five units, "-" represents from a number to a number, "," separate several discrete numbers.

Basic format:

* command

Time-sharing, day, month and week order

Column 1 represents minutes 1: 59 per minute is represented by * or * / 1.

The second column represents the hour 1: 23 (0 means 0 o'clock)

The third column represents the date 1: 31

The fourth column represents the month from January to December.

The fifth column identification number is 0006 (0 means Sunday)

Column 6 commands to run

Some examples of crontab files:

# restart apache at 21:30 every night.

30 21 * / usr/local/etc/rc.d/lighttpd restart

# on the 1st, 10th and 22nd of each month

45-4-1-10-10-22 * / usr/local/etc/rc.d/lighttpd restart

# 06:10 every morning

10 6 * date

# every two hours

0 * / 2 * date

# every two hours between 11:00 and 8 a.m., 8 a.m.

023-7bat 2pm 8 * date

# on the 4th of every month and 11:00 from Monday to Wednesday

0 11 4 * mon-wed date

# 4: 00 a. M. in January

0 4 1 jan * date

Sixth, crontab is accurate to execute in seconds.

In many cases, we plan tasks to be executed in seconds, and according to the following methods, tasks can be easily executed in seconds. The following method combines # crontab-e * / bin/date > > / tmp/date.txt * sleep 10; / bin/date > > / tmp/date.txt * sleep 20; / bin/date > > / tmp/date.txt * sleep 30; / bin/date > > / tmp/date.txt * sleep 40 / bin/date > > / tmp/date.txt * sleep 50; / bin/date > > / tmp/date.txt

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