The crontab is used for running specific tasks on a regular interval. Linux crontab is similar to windows task schedules. Crontab is very useful for routine tasks like scheduling system scanning, daily backups etc. Crontab executes jobs automatically in the backend on a specified time and interval. In this tutorial, you will learn to uses of crontab with 20 useful examples for scheduling jobs. You can also use crontab for the tasks to run once in future only, but for any tasks to run once we recommends to use Linux at command.
If you do not have crontab installed on your system refer article Install Crontab in CentOS/RHEL.
Linux Crontab Syntax
Linux crontab has six fields. 1-5 fields defines the date and time of execution. The 6’th fields are used for command or script to be executed.The Linux crontab syntax are as following:
[Minute] [hour] [Day_of_the_Month] [Month_of_the_Year] [Day_of_the_Week] [command]
- Astrics (*) – Matches anything
- Define range – You can define range using the hypen like: 1-10 or 20-30 or sun-fri or feb-apr
- Define multiple range – You can define multiple ranges with command seprated like: jan-mar,jul-sep
How to Add/Edit Crontab
To add or update job in crontab, use below command. It will open crontab file in the editor where a job can be added/updated.
By default, it will edit crontab entries of current logged in user. To edit other user crontab use command as below
crontab -u username -e
Change EDITOR environment variable to change your default editor.
How to List Crontab
To view crontab entries of current user use the following command.
Use -u followed by username to view crontab entries of the specified user.
crontab -u username -l
20 Useful Crontab Examples
Here is the list of examples for scheduling cron job in a Linux system using crontab.
1. Schedule a cron to execute at 2am daily.
This will be useful for scheduling database backup on a daily basis.
0 2 * * * /bin/sh backup.sh
- Asterisk (*) is used for matching all the records.
2. Schedule a cron to execute twice a day.
Below example command will execute at 5 AM and 5 PM daily. You can specify multiple time stamp by comma separated.
0 5,17 * * * /scripts/script.sh
3. Schedule a cron to execute on every minutes.
Generally, we don’t require any script to execute on every minute but in some case, you may need to configure it.
* * * * * /scripts/script.sh
4. Schedule a cron to execute on 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 on every 10 minutes.
If you want to run your script on 10 minutes interval, can configure like below. These type of crons are useful for monitoring.
*/10 * * * * /scripts/monitor.sh
*/10: means to run on 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 required scheduling 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. 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 first sunday of every month.
To schedule a script to execute a script on 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 on every four hours.
If you want to run a script on 4 hours interval. It can be configured like below.
0 */4 * * * /scripts/script.sh
10. Schedule a cron to execute twice on 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 on every 30 Seconds.
To schedule a task to execute on every 30 seconds is not possible by time parameters, But it can be done by schedule same cron twice like below.
* * * * * /scripts/script.sh * * * * * sleep 30; /scripts/script.sh
12. Schedule a multiple tasks in single cron.
To configure multiple tasks with single cron, Can be done by separating tasks by the semicolon ( ; ).
* * * * * /scripts/script.sh; /scripts/scrit2.sh
13. Schedule tasks to execute on yearly ( @yearly ).
@yearly timestamp is similar to “0 0 1 1 *”. It will execute task on the first minute of every year, It may useful to send new year greetings 🙂
14. Schedule tasks to execute on 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.
15. Schedule tasks to execute on Weekly ( @weekly ).
@weekly timestamp is similar to “0 0 1 * mon”. It will execute a task in the first minute of the week. It may useful to do weekly tasks like the cleanup of system etc.
16. Schedule tasks to execute on 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.
17. Schedule tasks to execute on 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.
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.
19. Redirect Cron Results to 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 MAIL variable like below
# crontab -l MAIL=bob 0 2 * * * /script/backup.sh
20. Taking backup of all crons to plain text file.
I recommend keeping a backup of all jobs entry in a file. This will help you to recover crons 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
Thanks for reading this article, I hope it will help you to understand Crontab in Linux. For scheduling one time tasks you can also use Linux at command.