Detailed explanation of crontab scheduled execution command under Linux

In LINUX, periodic tasks are usually handled by the cron daemon process [ps -ef | grep cron]. Cron reads one or more configuration files that contain command lines and the times they are called.

The cron configuration file is called "crontab", which is short for "cron table".

1. Cron service

Cron is a scheduled execution tool under Linux that can run jobs without human intervention.
service crond start //Start the service
service crond stop //Shut down the service
service crond restart //Restart service
service crond reload //Reload configuration
service crond status //Check service status

2. Cron looks for configuration files in 3 places:

1. /var/spool/cron/ This directory stores crontab tasks for each user, including root. Each task is named after the creator. For example, the crontab task created by tom corresponds to the file /var/spool/cron/tom. Generally, a user has at most one crontab file.

3. /etc/crontab This file is responsible for arranging crontabs for system maintenance and other tasks set by the system administrator.

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 

•MAILTO=root: This means, when an error occurs in a routine command in the /etc/crontab file, to whom will the error message or the message displayed on the screen be sent? Since root cannot receive emails on the client side, I usually change this e-mail to my own account so that I can always know the status of the system!
•01 * * * * root run-parts /etc/cron.hourly: In the commands after the #run-parts line, we can find that the five numbers are followed by root. This line means "the execution level is root identity." Of course, you can also change this line to other identities! And run-parts means that the following /etc/cron.hourly is "all executable files in a directory (/etc/cron.hourly)", that is, at 01 minute of every hour, the system will execute all executable files in the /etc/cron.hourly directory as root! The following three lines have similar meanings! You can go to /etc/ and have a look. These four directories are already preset in the system! You can write the commands you need to execute every day directly to /etc/cron.daily, and you don’t need to use the crontab -e program!

4. /etc/cron.d/ This directory is used to store any crontab files or scripts to be executed.

5. Permissions (?)

Crontab permission problem Go to /var/adm/cron/ and check if the files cron.allow and cron.deny exist
Usage is as follows:

1. If both files do not exist, only the root user can use the crontab command.
2. If cron.allow exists but cron.deny does not, only users listed in the cron.allow file can use the crontab command. If the root user is not in it, the root user 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, while other users can use it.
4. If both files exist, users listed in the cron.allow file but not in cron.deny can use crontab. If the same user exists in both files, the user in the cron.allow file will prevail. If the user exists in cron.allow, the crontab command can be used.

In AIX, ordinary users have crontab permissions by default. If you want to restrict users from using crontab, you need to edit /var/adm/cron/cron.deny
In HP-UNIX, ordinary users do not have crontab permissions by default. To grant ordinary users crontab permissions, you can edit

6. Create a cron script

Step 1: Write a cron script file and name it crontest.cron.
15,30,45,59 * * * * echo "xgmtest....." >> xgmtest.txt means that the print command will be executed once every 15 minutes. Step 2: Add a scheduled task. Execute the command "crontab crontest.cron". Finish the third step: "crontab -l" to check whether the scheduled task is successful or check whether the corresponding cron script is generated in /var/spool/cron

Note: This operation directly replaces the crontab of the user, rather than adding a new one

7. Crontab usage

The crontab command is used to install, remove, or list the tables used to drive the cron daemon. The user puts the command sequence to be executed in the crontab file to get it executed.
Each user can have his or her own crontab file. The crontab file in /var/spool/cron cannot be created or modified directly. The crontab file is created by the crontab command

How to enter the commands and time to be executed in the crontab file. Each line in this file includes six fields, the first five fields specify the time when the command is to be executed, and the last field 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 values: 00-59 00-23 01-31 01-12 0-6 (0 is Sunday)
In addition to numbers, there are several special symbols, namely "*", "/", "-", "", "*" represents all numbers within the value range, "/" means every, "/5" means every 5 units, "-" represents from a certain number to a certain number, and "," separates several discrete numbers.

-l Display the current crontab on standard output.
-r Delete the current crontab file.
-e Edit the current crontab file using the editor specified by the VISUAL or EDITOR environment variable. When you finish editing and exit, the edited file will be automatically installed.

8. Examples:

Every morning at 6:00

0 6 * * * echo "Good morning." >> /tmp/test.txt //Note that with just echo, you cannot see any output on the screen because cron emails any output to root's mailbox.

Every two hours

0 */2 * * * echo "Have a break now." >> /tmp/test.txt

Every two hours between 11pm and 8am and at 8am

0 23-7/2，8 * * * echo "Have a good dream" >> /tmp/test.txt

On the 4th of every month and every Monday to Wednesday at 11:00 a.m.

0 11 4 * 1-3 command line

January 1, 4 a.m.

0 4 1 1 * command line SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root //If an error occurs or data is output, the data will be sent to this account as an email HOME=/

Execute the script in /etc/cron.hourly every hour

01 * * * * root run-parts /etc/cron.hourly

Execute the script in /etc/cron.daily every day

02 4 * * * root run-parts /etc/cron.daily

Execute the script in /etc/cron.weekly every week

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

Execute the script in /etc/cron.monthly every month

42 4 1 * * root run-parts /etc/cron.monthly

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

Execute the command at 5 min, 15 min, 25 min, 35 min, 45 min, and 55 min at 4, 5, and 6 p.m. every day.

5, 15, 25, 35, 45, 55, 16, 17, 18 * * * command

The system will enter maintenance mode and restart at 3:00 pm every Monday, Wednesday and Friday.

00 15 * * 1，3，5 shutdown -r +5

At 10 and 40 minutes past every hour, execute the command innd/bbslin in the user directory:

10,40 * * * * innd/bbslink

Execute the bin/account command in the user directory at 1 minute every hour:

1 * * * * bin/account

At 3:20 every morning, execute the following two commands in the user directory (each command is separated by ;):

20 3 * * * (/bin/rm -f expire.ls logins.bad;bin/expire$#@62;expire.1st)

In January and April of every year, at 3:12 and 3:55 on the 4th to 9th, execute the command /bin/rm -f expire.1st and add the result to the mm.txt file (the mm.txt file is located in the user's own directory).

12,55 3 4-9 1,4 * /bin/rm -f expire.1st$#@62;$#@62;mm.txt

Summarize

The above is a detailed explanation of the crontab command for scheduled execution of tasks under Linux introduced by the editor. I hope it will be helpful to everyone. If you have any questions, please leave me a message and the editor will reply to you in time. I would also like to thank everyone for their support of the 123WORDPRESS.COM website!