Prerequisite

• A machine running Rocky Linux.
• Know how to use your favorite editor to modify the configuration file in the command line environment (this article will use vi).
• You have understood the basic knowledge of bash, python or other scripting/programming tools, and expect to run the script automatically.
• You have connected to your machine via ssh (it can be root user or ordinary user with UID greater than 1000)
• We think you are a very cool person.

cron introduction

GNU/Linux provides the cron system, which is a time-based cron jobs program for automated processes. It is simple, but quite powerful. Want a script or program to run at 5 pm every day? cron can do it. There are different branches (or variants) of cron, which have the same functions. In this document, cronie is used, and the version is 1.5.2. You can click here to find the latest version and update log.

cronie's description

• cronie -package name, Rocky Linux includes cronie by default;
• crontab -command to maintain crontab (task schedule) for each user;
• crond.service -cronie's daemon, you can manage the daemon by systemctl start | restart | stop | status ;
• /etc/crontab -Assign cron jobs to different users, usually we are more accustomed to using crontab -e . For example, if you are currently logged in as the root user, type crontab -e and you will see specific cron jobs in the file /var/spool/cron/root after saving.
• /var/log/cron * -Cronie's log, by default, does log rotation and ends with a date suffix. * Here means wildcard
• anacron -part of cronie . For more information about anacron, see anacron-automation commands.

crontab command

crontab is a command obtained after installation of the cronie package. Compared with anacron, it is more suitable for servers that work 7 * 24 hours a day. Common options of crontab are:

-e # edit crontab scheduled tasks
the -l # View crontab task
-r # delete all the current user's crontab tasks

Use of cronie

To allow different users to execute different commands (or scripts) at different times, they can be written into this file. However, usually we are more accustomed to using crontab -e.

shell > cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# 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

Parameter	Meaning	Value Range
The 1st*	The first minute of the hour	0-59
The 2nd*	Hour of the day	0-23
The 3rd*	Day of the month	1-31
The 4th*	The month of the year	1-12
The 5th*	Day of the week	0-7 (0 and 7 both indicate Sunday)

In this example, assuming you are performing this operation as the root user, type the following: crontab -e, this will bring up the timed tasks of the root user, if you use vi as the default system editor, press i key to enter the insert mode, enter the following content, # means this is a line of comment. Press Esc to exit insert mode, enter: wq (displayed at the bottom) to save and exit vi, which means to run the script once every night at 22:00. Obviously, this is a very simple example, and the situation can become very complicated when you need to elaborate.

# Nightly 10:00 backup system
00 22 *  *  * /usr/local/sbin/backup

Attention

The script needs to have execute permission (chmod +x) before cronie can run it.

Complex options

So far, the content discussed are very simple options, but how to complete more complex timed tasks?

# Suppose you want to run every 10 minutes backup script (may be impractical, however, it is only an example!) Throughout the day. To this end, the following will be written:
* /10 *  *  *  * /usr/local/sbin/backup
#What if you only want to run a backup every 10 minutes on Monday, Wednesday, and Friday? :
* /10 *  *  * 1,3,5 /usr/local/sbin/backup
# In addition to Saturdays and Sundays, once every 10 minutes, every day, how to back up?
* /10 *  *  * 1-5 /usr/local/sbin/backup

Special Symbols	Meaning
*	represents any time. For example, the first * means any minute, and the second * means any hour
,	stands for discontinuous time, such as "0 8,12,16 * * * ", which means that the command will be executed once every day at 8:00, 12:00, and 16:00
-	represents a continuous time range, such as "0 5 * * 1-6 ", which means that a command will be executed at five o'clock in the morning every day from Monday to Saturday
*/n	Represents how often the interval is executed, such as "/10 * * *" means that it is executed every 10 minutes

Attention

The smallest time unit that cronie can recognize is 1 minute; when using, for example, 30 4 1,15 * 5 command , it will cause the command to run on the 1st and 15th of each month and 4:30 in the morning of every Friday ; The output information of some scripts or commands will prevent the execution of timed tasks, and output redirection is required, such as this- */10 * * * * /usr/local/sbin/backup &> /dev/null

Q & A

1. /etc/crontab and crontab -e , is there any difference between the two methods? crontab -e does not need to specify a user (the currently logged-in user is used by default), while /etc/crontab needs to specify a user.
2. What should I do if the specified command or script is not executed correctly? Check the /var/log/cron* file, use journalctl -u crond.service to check the information about the daemon process, whether the script has x permission, etc., for troubleshooting.
3. In addition to cronie, what cron variants are there? dcron , the latest version is 4.5 (2011-50-01). fcron , the latest version is 3.3.0 (dev, 2016-08-14). bcron , the latest version is 0.11 (2015-08-12). cronsun , the latest version 0.3.5 (2018-11-20).

Summary

For Rocky Linux desktop users or system administrators, cronie is a very powerful tool. It allows you to automate tasks and scripts so you don't have to remember to run them manually. Although the basic knowledge is simple, the actual task may be complex. For more information about crontab, please visit crontab man page. You can also simply search for "crontab" on the Internet, which will provide you with a large number of search results and help you fine-tune the crontab expression.

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Last update: January 4, 2022

Author: tianci li

Contributors: Steven Spencer