Do you run a cron job on your server every so many minutes? Are you a system administrator or engineer who has to manage cron jobs on a daily basis? Do you keep forgetting the syntax for setting up a new cron job? If so, read on.
Cron is an essential part of any system, as it automates various scheduled tasks. The crontab command is used to edit and manage cron jobs. Creating a cron job is not difficult, but it can become tricky when dealing with multiple users and environments. Crontab syntax is mostly the same across all Linux distributions and servers, so once you get the hang of it you’ll be scheduling tasks in no time!
In this tutorial, you will learn to use crontab with 20 useful examples for scheduling jobs. You can also use crontab for the tasks to run once in the future only, but for any tasks to run once we recommend using Linux at command.
If you do not have crontab installed on your system refer to article Install Crontab in CentOS/RHEL.
Linux Crontab Syntax
The format of a crontab file is very simple: each line contains six fields, separated by spaces. The first field specifies the minute (0-59) when the job will be run, the second field specifies the hour (0-23), the third field specifies the day of the month (1-31), and so on. Wildcards can also be used in crontab files. For example, an asterisk in the fourth field indicates that the job should be run every week.
<Minute> <Hour> <Day_of_the_Month> <Month_of_the_Year> <Day_of_the_Week> <command>
- Minute – A minute value can be between 0-59
- Hour – A hour value can be between 0-23
- Day_of_the_month – This value can between 1-31. For the months having fewer days will ignore remaining part
- Month_of_the_year – This can be between 1-12. You can also define this value with the first three alphabets of the month like jan, feb, mar, apr etc.
- Day_of_the_Week – This can be the value between 0-7. Where 0 and 7 for Sunday, 1 for Monday, 2 for Tuesday, and so on. You can also use the first three alphabets of days like sun, mon, tue, wed, etc.
Now, the below statements will describe how to define multiple values or ranges. Read below and understand.
- Astrics (*) – Matches anything
- Multiple values – Use the command (,) to define multiple values like 2,4,8 or sun,fri or jan,oct,dec etc.
- Define range – You can define range using the hyphen like: 1-10 or 20-30 or sun-fri or feb-apr
- Define multiple ranges – You can define multiple ranges with commands separated like: jan-mar,jul-sep
How to Add/Edit 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.
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.
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.
- Schedule a cron to execute at 2 am daily.
This will be useful for scheduling database backups on a daily basis.
0 2 * * * /bin/sh backup.sh
- Asterisk (*) is used for matching all the records.
- 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
- 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
- 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
- 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.
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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 🙂
- 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.
- 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.
- 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.
- 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.
- 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.
- 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 -lMAIL=bob 0 2 * * * /script/backup.sh
- 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 -lMAIL=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.