Understanding Linux Cronjobs?

4 min readDec 16, 2023

Cronjob is a scheduled task that is defined in crontab. They are able to trigger automatically at a specified time. They are beneficial for housekeeping tasks in Linux such as restarting services, cleaning logs or unwanted data, having database backups, automatic certificate updates, and many more.

Who Plays Behind the Cronjobs?(crond.service)

The crond.service aka cron daemon plays behind the scene and keep monitoring the /etc/crontab file, and /etc/cron.*/ directories and /var/spool/cron/

crond.service by default coming as enabled but if it is disabled you can enable it by,

sudo systemctl enable crond.service

sudo systemctl start crond.service

Understanding Cronjob Structure

A cronjob can be written as a rule with 6 parts. Each part is separated by a blank space. The first five slots are allocated to set the time for the cronjob.

For example, let’s say you need to execute a script called “test.sh” at 10.30 am everyday,

30 10 * * * cd /home/randula/scripts/ && ./test.sh

Wild Cards and Special Characters

There are several wild cards and special characters used in cronjobs. The following table is a simplified summary of symbols used in cronjobs in Linux.

Crontab Example

Let’s have a look into sample cronjobs that we need to use in most cases when you managing a production server.

Daily Cronjobs Use Cases

shutdown the instance at 12.00 a.m
00 00 * * * sudo /sbin/shutdown -h now

Shutdown the instance if it on between 12 am- 7a.m
00 0–7 * * * sudo /sbin/shutdown -h now

Execute a script daily and save output
00 5 * * * cd /home/randula/scripts/ && ./test.sh >> ~/tmp/test.output

Clean logs older than 5 days
10 3 * * * find /home/randula/log -name “*.log*” -type f -mtime +5 -exec rm -f{} \;

Execute a script hourly
0 * * * * cd /home/randula/scripts/ && ./test.sh

Weekly Cronjobs Use cases

Execute a script on every Sunday
00 11 * * 0 cd /home/randula/scripts/ && ./test.sh

Execute a script on every weekday
00 11 * * 1–5 cd /home/randula/scripts/ && ./test.sh

Exercise

Try to write time patterns for the following scenarios

Every 30 minutes
2.Every hour
3.Every 2 hours
4.Every day at 2:45 PM
5.Every Thursday at 2:45 PM
6.13th of Every month
7.2:40 AM the third Saturday of every month
8.2:40 AM every Monday, Tuesday, Wednesday, Thursday and Friday
9.At 2:10 p.m. and at 2:44 p.m. every Wednesday in March.

Answers

1.Every 30 minutes
*/30 * * * *

2.Every hour
0 */1 * * *

3.Every 2 hours
0 */2 * * *

4.Every day at 2:45 PM
45 14 * * *

5.Every Thursday at 2:45 PM
45 14 * * 4
45 14 * * THU

6. 13 th of Every month
* * 13 * *

7. 2:40 AM the third Saturday of every month
Not possible

8. 2:40 AM every Monday, Tuesday, Wednesday, Thursday and Friday
40 2 * * 1–6

9. At 2:10 p.m. and at 2:44 p.m. every Wednesday in March
10,44 14 * 3 3

I found this tool, Crontab guru very useful to test and create cronjobs very easily.

How to Create or Edit a New Cronjob?

To start writing a new cronjob you can simply write

crontab -eYou can press ‘i’ to start editing the cronjob.

Once you have done adding the cronjob, you can save and exit the cronjob as same as you do in your editor.

ESC :qw Press Enter

If the crontab service is not activated, please start the service using,

sudo systemctl enable crond.service
sudo systemctl start crond.service

Red Hat Error : You are not allowed to use this program error RHEL

On Linux there are two files that control the access to crontab:

/etc/cron.allow
/etc/cron.deny

The above error occurs when your user is not allowed to for cronjobs. Hence you will need to add the login account to the /etc/cron.allow file.

Solution

sudo -i -u root
echo YOUR_USER > /etc/cron.allow

example, if your user is randula,

sudo -i -u root echo randula > /etc/cron.allow

this will fix the error.

Listing a cronjob

cronjob -l

AWS Cronjobs

Amazon AWS also provides similar functionality for cronjobs, call “schedule”. They canbe set up for a single run or run in sequences as same as cronjobs.

MigrationScheduleExpression: 
   Description: cron job or rate
   Default: 'cron(0/3 13-1 ? * * *)' 
   Type: 'String'

The scheduler consists of 6 parts to describe the duration of the event, whereas we had 5 parts at Linux cronjobs.

There are several other differences between Linux and AWS cronjobs.

Not like Linux cronjobs, you can’t put * for both “day of the month” and “day of the week” fields. It is prohibited here. You may use “?” for one of them instead.

The time is always calculated in UTC hence you need to convert the time correctly when creating the schedule

The weekdays are starting from 0, where 0 is Monday. Linux cronjobs consider 0 as Sunday.