Article directory
Preface
The main contents outlined in this article’s mind map:
1. Introduction to crontab
1.1 What is crontab
Crontab is a tool for timing tasks on Unix and Linux operating systems . It allows users to create and manage scheduled tasks to automatically run commands or scripts at specific intervals or points in time. Crontab is cron tablethe abbreviation of , cronwhich refers to a background process in Unix systems that is used to perform scheduled tasks.
A Crontab file contains a series of scheduled task entries, each of which defines when a task should be executed and the command or script to be run. These tasks can be scheduled in different time units such as minutes, hours, days, months, and weeks.
1.2 crontab command workflow
The crontab command is usually used to set up regularly executed tasks in Linux systems and store these tasks in crontab files. In order to use the crontab command, the system requires dependent crond 服务support. Normally, when the operating system is installed, the crond service is installed by default and starts automatically. The crond process will regularly check the task list in the crontab file every minute to determine whether there are tasks that need to be performed. If so, it will automatically perform these tasks.
The following topology diagram is easy to understand:
It can be summarized as follows: the crontab command can only be used after the crond service is installed. The crontab command edits scheduled tasks according to the specified format and saves them in the crontab file. The crond service will periodically check the scheduled task list in the crontab file every minute to execute the task.
1.3 Linux scheduled task classification
Task scheduling under Linux systems is divided into two categories: system task scheduling and user task scheduling .
- System task scheduling: work that the system performs periodically, 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.
- User task scheduling: User-defined tasks to be performed regularly. Users can use the crontab tool to customize their own scheduled tasks. All user-defined crontab files are saved in the directory
/var/spool/cron. The file name is consistent with the user name. For example, if I set up a scheduled task with the test user, then the corresponding crontab file is/var/spool/cron/test.
2. Detailed explanation of crontab usage
2.1 crond service installation
Under normal circumstances, the crond service will be installed by default and run automatically when installing the system.
Check whether the service is installed under centos or Red Hat system:
systemctl status crond
If Unit crond.service could not be found. is displayed, the crond service needs to be installed.
# crond 安装：
yum -y install crontabs
# 启动 crond 服务:
systemctl start crond
# 关闭 crond 服务:
systemctl stop crond
# crond设置开机自启动:
systemctl enable crond
# 重新载入配置
systemctl reload crond
# 查看 crontab 服务是否已经加入了开机启动
chkconfig crond --list
# 加入开机自动启动
chkconfig crond on
2.2 crontab file content analysis
As mentioned before, we have learned about using the crontab command to edit and save scheduled tasks into a crontab file. Now, let's take a closer look at the format of a scheduled task.
You can execute the following command to view the sample contents of the crontab file:
The first four lines are used to configure the environment variables for the crond task to run.
The SHELL variable in the first line specifies which shell the system should use, here it is bash
The PATH variable in the second line specifies the path for the system to execute the command.
The MAILTO variable in the third line specifies that crond's task execution information will be sent to the root user via email.
If the value of the MAILTO variable is empty, it means that task execution information will not be sent to the user.
The last line is the Crontab entry. The basic format is as follows:
* * * * * user-name command to be executed
The execution time of the task is determined by setting these five in the crontab file *. user-name is the user who executes the task, and command to be executed is the command or script task to be executed. Let’s take a closer look at what these five represent *. meaning.
Among them, the asterisk represents a wildcard, which means it can match any value. Each asterisk represents a different unit of time:
- The first asterisk represents the minute (0-59)
- The second asterisk indicates the hour (0-23)
- The third asterisk represents the day of the month (1-31)
- The fourth asterisk represents the month (1-12 or use abbreviation, such as 1 for January, 2 for February)
- The fifth asterisk represents the day of the week (0-7 or abbreviated, 0 and 7 both represent Sunday, 1 represents Monday, and so on)
In fact, in addition to *this character, there are other special characters used to meet different timing needs.
| special symbols | meaning | for example |
|---|---|---|
| *(Asterisk) | means any time | For example: * 8 * * * means to execute the command every minute at 8 o'clock every day |
| ,(comma) | Use commas to separate the values of the field. | For example: 10 8,10,12 * * *it means that the command will be executed every day at 8:10, 10:10, and 12:10 |
| -(middle bar) | Represents the range of values between two integers | For example: 10 8 * * 1-3 means to execute the command once every Monday to Wednesday at 8:10 |
| / (forward slash) | Indicates how often to execute | For example: * /10 * * * *means to execute the command every 10 minutes |
Summarize:
- It is best not to use week and day at the same time
- Timed tasks need to be annotated
- Can be directed to a log file or an empty file
- The scheduled task must be an absolute path, and the directory must exist to produce results.
- The crontab service must be started and running
2.3 crontab command usage
2.3.1 View scheduled task list
Use the following command to view the scheduled task list
[test@hecs]# crontab -l # 查看定时任务列表
2.3.2 Edit/Create scheduled tasks
In fact, in use, setting up Crontab scheduled tasks is very easy. Just switch to the user who executed the command and run crontab -ethe command. This will open a blank file where you can simply enter the task you want to schedule.
[test@hecs]# crontab -e # 编辑定时任务
#进入 crontab 编辑界面。会打开Vim编辑你的任务
*/10 * * * * /home/test/test.sh
/var/spool/cron/After editing and saving, a file named test will be generated in the directory. This file is the crontab file of the test user.
In the crontab [option] file command, file refers to the name of the command file, which means that file is used as the task list file of crontab and loaded into crontab. If the file name is not specified on the command line, this command will accept standard input ( Keyboard) and type them into crontab, for example: crontab -u test /root/jobs.txt means to load crontab with the task list in the /root/jobs.txt file.
2.3.3 Delete scheduled tasks
# 直接删除不提示
[test@hecs]# crontab -r
或
# 删除前给出提示确认
[test@hecs]# crontab -i
After executing the above command, /var/spool/cronthe user's crontab file will be deleted from the directory. If no user is specified, the current user's crontab file will be deleted by default.
Note : After executing the delete command, all scheduled tasks under this user will be deleted. If you just want to delete a certain scheduled task,crontab -ejust use the command to edit the crontab file and delete a certain task.
2.3.4 Other crontab related operations
(1) Back up crontab file
Execute the following command to back up the current crontab file
crontab -l > $HOME/mycron
(2) Recover lost crontab files
If you accidentally delete the crontab file, assuming you $HOMEstill have a backup in your own directory, you can copy it to /var/spool/cron/<username>, where is the user name. If the copy cannot be completed due to permission issues, you can use: crontab where is $HOMEthe file name of your copy in the directory.
Some crontab variations are a bit weird, so be careful when using the crontab command. If any options are omitted, crontab may open an empty file, or appear to be an empty file. Exit directly at this time, do not press Ctrl-D, otherwise you will lose the crontab file.
3. Precautions for crontab
3.1 Pay attention to environment variables
When defining multiple scheduling tasks in a crontab file, one issue that requires special attention is the setting of environment variables.
When the file path is involved in the script, write the global path;
when the script execution requires java or other environment variables, introduce the environment variables 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 &
When manually executing the script is OK, but the crontab is not executed. You can try to directly introduce environment variables in crontab to solve the problem.
0 * * * * . /etc/profile;/bin/sh /var/www/java/audit_no_count/bin/restart_audit.sh
3.2 System-level task scheduling and user-level task scheduling
The root user's task scheduling operation can be crontab -uroot -eset through , or the /etc/crontabscheduled task can be written directly to the file. It should be noted that if you want to define a task to restart the system regularly, you must put the task into /etc/crontabthe file, even if it is created under the root user A scheduled system restart task is also invalid.
3.3 Other notes and summary
- Environment variable problems, for example, crontab cannot recognize Java environment variables.
When crontab executes the shell, it can only recognize a few environment variables. Ordinary environment variables cannot be recognized, so when writing a shell, it is best to use export to re-declare the variables. Make sure the script executes. - It is best to use scripts to execute commands
- Add /bin/sh to script permissions, and standard path /server/scripts
- Time variables are escaped with backslashes, preferably in scripts
- Add comments to scheduled tasks
>/dev/null 2>&1 ==>&>/dev/nullDon’t print log files randomly- The scheduled task must be an absolute path, and the directory must exist to produce results.
- Avoid unnecessary programs and command output
- Add comments before scheduled tasks
- Package to the upper level of the file directory
- It is best not to use week and day at the same time
- Can be directed to a log file or an empty file
- The crontab service must be started and running
4. Debugging ineffective/invalid crontab
4.1 Solution to failed crontab debugging
When crontab suddenly fails, you can try /etc/init.d/crond restartto solve the problem. Or check the log to see if a job is executed/error reported tail -f /var/log/cron.
Don't run it randomly crontab -r, as this will delete the user's crontab file from the crontab directory (/var/spool/cron). After deleting all the crontabs of the user, they are gone.
It has a special meaning in crontab %, which means line break. If you want to use it, you must escape it \%. If it is often used, date +%Y%m%dit will not be executed in crontab and should be replaced withdate +\%Y\%m\%d
4.2 Reasons and solutions for scheduled tasks not being executed
The format of the scheduled task settings is correct, and manual execution is no problem, but the scheduled task is not executed. This situation is generally caused by the following reasons.
- The crond service is not started.
You need to first check whether the crond service is started. You can use the systemctl status crond command to check. If it is not started, just start it. - The script has no permissions.
If you want to execute the script without permissions, just add permissions through the chmod command. - File path problem:
When the file path is involved in the script to be executed by the scheduled task, the global path must be written, and the relative path cannot be written. - Environment variable issues:
When the script to be executed by a scheduled task uses java or other environment variables, the environment variables need to be introduced through the source command.
[test@vm1]# cat test.sh
#!/bin/bash
source /etc/profile
java -jar /home/test/test.jar
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About the Author
The number one master in the Linux field, with over 500,000 online fans; worked for a Fortune 500 foreign company; winner of the National Graduate Scholarship; 6 years of public account writing experience, more than 500 original articles, some of which have been read more than 1 million times; Entered the short video field in 2021 and produced more than 300 short video original copywritings. Some single videos have been played more than 3.5 million times.
