What is cron in Linux ?

Cron is a utility in Linux used to schedule commands or scripts to run at a specific time and frequency automatically. Cron is short for chronograph which is a timekeeping device.

How do I check if cron is running on my Linux system ?

You can check if cron is running on your Linux system by running the command systemctl status cron. This will show you the status of the cron service and whether it is running or not.

How do I list all the cron jobs for a specific user in Linux?

To list all the cron jobs for a specific user in Linux, you can run the command crontab -l -u username . This will show you the crontab file for the specified user, which lists all the scheduled tasks for that user.

How do I remove a cron job in Linux?

To remove a cron job in Linux, you can edit the crontab file using the crontab -e command and remove the line that corresponds to the job you want to remove. Alternatively, you can run the command crontab -r to remove all the cron jobs for the current user.

Top 10 Cron Examples - Run Every Hour , Every 24 Hour , Every Weekend and Every Nth Minute.

Author - Sanjay

Introduction

Cron is a command that runs jobs or tasks on a schedule. It's essentially a background task management system for Linux services and configurations. Of course, the Cron daemon was developed for Windows OS too but the above-mentioned thing doesn't mean you cannot implement cron in Linux; it just has limitations.

In this article we will be discussing some of the reltime Cron examples used for shell scripting in Linux systems, how these examples can be used for automating processes and scheduling jobs with cron tools.

Introduction
Pre-Requisistes
Cron Job Syntax
Cron Example Variations
Run a Cron Job In Every 30 Min
Run a Cron Job Every 12 Hour or 2 Hour .
Run a Cron Job Every Hour Except 5 Am
Run a Cron Job every 24 hours at midnight
Run a cron job every weekend Saturday and Sunday .
Points to remember about Cron Jobs
Conclusion

Pre-Requisistes

A Linux server or a Desktop with root privilages.

Cron Job Syntax

An (*) explains an expression should execute for every time unit for example star in a our field denote for every hour .
A (?) explains to execute an expression to execute as per random value and ignore the field values .
A(-) denotes the range so let say you want to execute an expression between first and second month of the day. So we write as 1-2 . It defines the range for any value wheather month , year and hour.

Have a look at the image below for better understanding of the Cron Job Syntax.

Cron Example Variations

Automated Backups Using Cron Job everyday Midnight

Backing up critical data is essential. Schedule an automated backup every day at midnight . Here is the cron example

0 0 * * * /path/to/your_backupscript.sh

Cleanup Temporary files using Cron

Cron example to free up disk space by running a cleanup script every Friday at 5:30 PM:

17 - Represents time in 24 Hr format 5- Represents Day of the Week

30 17 * * 5 /path/to/your_cleanupscript.sh

Run a Cron Job in Every Half an Hour

We are going to use step operator to run a cron every half an hour as shown

*/30 * * * *

Run a Cron Job In Every 30 Min

We are going to use step operator to run a cron every 30 min as shown

*/30 * * * *

Run a Cron Job Every 12 Hour or 2 Hour .

We are going to see w cron job to run every nth hour . In this case every other hour .

0 */2 * * *

In Case we need to run every Odd Hours , we can write logic as below

0 1-23/2 * * *

Run a Cron Job Every Hour Except 5 Am

In this case we are going to use , separated . In comma separated we have specified range first then we have specified the time rangle except the time we dont want.

0 0-4, 6-23 * * *

Run a Cron Job every 24 hours at midnight

Running cron job every 24 Hrs at midnight is below

0 0 * * *

Run a cron job every weekend Saturday and Sunday .

We are going to write a cron job to run in weekends only At 00:00 only on Saturday and Sunday.

0 0 * * 6,0

Points to remember about Cron Jobs

Always try to run Cron Job in Servers In an unified time zone such as UTC .
Cron job have minimal possible time of greater than 60 Sec . So cron tabs are not suitable for time frames in seconds , it doesnt provide you that much granularity . If you want that level of granularity then cron tab are not proper solution .
Crontabs doesnt have any retry mechanisms , so if you are willling to have some more control on error conditions we need to handle in our scripts.
Be cautious not to overload your system with too many concurrent cron jobs. Prioritize critical tasks and stagger less crucial ones.

Conclusion

We have seen an exhaustive list of examples of cron jobs. In Order to mae the job easier we have create a Cron Expresion Generator which is helpful in generating complex cron expressions . Let us know about your thoughts.

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