Understanding Crontab in Linux with 20 Useful Examples for Scheduling Tasks

Are you struggling to maintain and execute cron jobs on your Linux server with optimal frequency? As a dedicated system administrator or a savvy engineer, managing cron jobs can often be a recurring task in your day-to-day responsibilities. Understanding and remembering the syntax for configuring a new cron job can be a challenge, but it’s crucial for efficient server management.

Cron jobs are a fundamental component of server maintenance, providing a reliable method to automate repetitive tasks. Whether you’re working with CentOS, RHEL, or another Linux distribution, the crontab command is your go-to tool for editing and managing these jobs. Although the process of creating a cron job might seem straightforward, complexities can arise when juggling multiple users and diverse server environments. Fortunately, the crontab syntax is largely uniform across different Linux systems, enabling you to master task scheduling quickly.

This in-depth tutorial is tailored for system administrators and IT professionals looking to enhance their server management skills. You’ll discover 20 examples of using crontab for scheduling jobs, ranging from basic to advanced scenarios. Additionally, learn how to use the crontab for one-time future tasks, though for such requirements, the Linux ‘at’ command is often recommended.

Understanding Linux Crontab Syntax and Configuration

A crontab file’s format is deceptively simple: each line comprises six fields, delineated by spaces. These fields specify the minute (0-59), hour (0-23), day of the month (1-31), and so forth, determining when the job will execute. Wildcards, like asterisks, offer additional flexibility in scheduling.

 
[Minute] [Hour] [Day_of_the_Month] [Month_of_the_Year] [Day_of_the_Week] [Command to Execute]

Here’s a quick reference for crontab scheduling:

• Minute: Range 0-59
• Hour: Range 0-23
• Day of the Month: Range 1-31
• Month of the Year: Range 1-12 or abbreviations (e.g., Jan, Feb)
• Day of the Week: Range 0-7 (0 and 7 for Sunday)

Moreover, we’ll delve into how to set multiple values or ranges in your cron jobs, enhancing your server’s operational efficiency.

• Asterisks (*): For universal matching
• Multiple Values: Use commas for distinct values (e.g., 2,4,8)
• Range Definition: Specify ranges with a hyphen (e.g., 1-10)
• Multiple Ranges: Combine ranges with commas (e.g., Jan-Mar, Jul-Sep)

Stay tuned as we explore the nuances of cron job scheduling and management, ensuring your Linux servers operate smoothly and efficiently.

How to Add/Update Crontab

Crontab files are typically stored in the /etc/cron.d/ directory on Linux systems. The crontab command can be used to edit the crontab file.

crontab -e 

By default, it will edit the crontab entries of the currently logged-in user. To edit other user crontab use the command below:

crontab -u username -e 

Change the EDITOR environment variable to change your default editor.

How to List Crontab

To view the crontab entries of current users use the following command.

crontab -l 

Use -u followed by the username to view the crontab entries of the specified user.

crontab -u username -l 

20 Useful Examples for Scheduling Crontab

Here is the list of examples for scheduling cron jobs in a Linux system using crontab.

1. Schedule a cron to execute at 2 am daily.

   This will be useful for scheduling database backups on a daily basis.

   0 2 * * * bash /script/backup.sh
   

   • Asterisk (*) is used for matching all the records.
2. Schedule a cron to execute twice a day.

   The below example command will execute at 5 AM and 5 PM daily. You can specify multiple time stamps by comma-separated.

   0 5,17 * * * /scripts/script.sh
   

3. Schedule a cron to execute every minute.

   Generally, we don’t require any script to execute every minute but in some cases, you may need to configure it.

   * * * * *  /scripts/script.sh
   

4. Schedule a cron to execute every Sunday at 5 PM.

   This type of cron is useful for doing weekly tasks, like log rotation, etc.

   0 17 * * sun  /scripts/script.sh
   

5. Schedule a cron to execute every 10 minutes.

   If you want to run your script at 10 minutes intervals, you can configure it like the below. These types of crons are useful for monitoring.

   */10 * * * * /scripts/monitor.sh
   

   */10: means to run every 10 minutes. Same as if you want to execute on every 5 minutes use */5.

6. Schedule a cron to execute on selected months.

   Sometimes we are required to schedule a task to be executed for selected months only. Below example script will run in January, May, and August months.

   * * * jan,may,aug *  /script/script.sh
   

7. Schedule a cron to execute on selected days.

   If you required scheduling a task to be executed for selected days only. The below example will run on each Sunday and Friday at 5 PM.

   0 17 * * sun,fri  /script/script.sh
   

8. Schedule a cron to execute on the first Sunday of every month.

   To schedule a script to execute a script on the first Sunday only is not possible by time parameter, But we can use the condition in command fields to do it.

   0 2 * * sun  [ $(date +%d) -le 07 ] && /script/script.sh
   

9. Schedule a cron to execute every four hours.

   If you want to run a script on 4 hours intervals. It can be configured like below.

   0 */4 * * * /scripts/script.sh
   

10. Schedule a cron to execute twice every Sunday and Monday.

    To schedule a task to execute twice on Sunday and Monday only. Use the following settings to do it.

    0 4,17 * * sun,mon /scripts/script.sh
    

11. Schedule a cron to execute every 30 Seconds.

    To schedule a task to execute every 30 seconds is not possible by time parameters, But it can be done by scheduling the same cron twice as below.

    * * * * * /scripts/script.sh
    * * * * *  sleep 30; /scripts/script.sh
    

12. Schedule multiple tasks in a single cron.

    To configure multiple tasks with a single cron Can be done by separating tasks by the semicolon ( ; ).

    * * * * * /scripts/script.sh; /scripts/scrit2.sh
    

13. Schedule tasks to execute yearly ( @yearly ).

    @yearly timestamp is similar to “0 0 1 1 *“. It will execute a task on the first minute of every year, It may useful to send new year greetings 🙂

    @yearly /scripts/script.sh
    

14. Schedule tasks to execute monthly ( @monthly ).

    @monthly timestamp is similar to “0 0 1 * *“. It will execute a task in the first minute of the month. It may useful to do monthly tasks like paying the bills and invoicing to customers.

    @monthly /scripts/script.sh
    

15. Schedule tasks to execute Weekly ( @weekly ).

    A @weekly timestamp is similar to “0 0 * * sun“. It will execute a task in the first minute of the week. It may useful to do weekly tasks like the cleanup of the system etc.

    @weekly /bin/script.sh
    

16. Schedule tasks to execute daily ( @daily ).

    @daily timestamp is similar to “0 0 * * *“. It will execute a task in the first minute of every day, It may useful to do daily tasks.

    @daily /scripts/script.sh
    

17. Schedule tasks to execute hourly ( @hourly ).

    @hourly timestamp is similar to “0 * * * *“. It will execute a task in the first minute of every hour, It may useful to do hourly tasks.

    @hourly /scripts/script.sh
    

18. Schedule tasks to execute on system reboot ( @reboot ).

    @reboot is useful for those tasks which you want to run on your system startup. It will be the same as system startup scripts. It is useful for starting tasks in the background automatically.

    @reboot /scripts/script.sh
    

19. Redirect Cron Results to a specified email account.

    By default, cron sends details to the current user where cron is scheduled. If you want to redirect it to your other account, can be done by setup the MAIL variable like below

    crontab -l 
    
    MAIL=bob
    0 2 * * * /script/backup.sh
    

20. Take a backup of all cron to a plain text file.

    I recommend keeping a backup of all jobs entry in a file. This will help you to recover cron in case of accidental deletion.

    Check current scheduled cron:

    crontab -l 
    
    MAIL=rahul
    0 2 * * * /script/backup.sh
    

    Backup cron to text file:

    # crontab -l > cron-backup.txt
    # cat cron-backup.txt
    MAIL=rahul
    0 2 * * * /script/backup.sh
    

    Removing current scheduled cron:

    # crontab -r
    # crontab -l
    no crontab for root
    

    Restore crons from text file:

    # crontab cron-backup.txt
    # crontab -l
    MAIL=rahul
    0 2 * * * /script/backup.sh
    

Hope these examples helps you to understand the Crontab architecture and job scheduling. You can also visit another article containing more Crontab practical examples.

Conclusion

In conclusion, this tutorial offers an invaluable guide for both beginners and seasoned Linux users. Through 20 detailed examples, this article has illuminated the versatility and efficiency of crontab in managing routine tasks, thereby enhancing productivity and ensuring consistent execution of important jobs.

The journey through various cron job setups, from basic to advanced configurations, demonstrates how crontab can be tailored to fit a wide array of requirements. Whether it’s automating system maintenance, managing backups, or scheduling regular data processing tasks, crontab stands out as an essential tool in the Linux arsenal. The practical nature of the examples provided ensures that readers can directly apply these concepts in their daily operations.

Remember, mastering crontab is not just about automating tasks; it’s about optimizing your time and resources, which is crucial in the fast-paced world of technology.