Write your first Cron Job with me.

For a long time, I was having an urge to write a cron job to automate some tasks. Here is a summary so you can do it too.

1. Little Background
2. Setup
3. Write your cron job

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1. Little Background

1. What is Cron?

In simple words, it is a scheduler

2. What does a scheduler do?

runs a specified task at a specified time

3. Where can you use it?

to automate any repeated task like syncing data or taking backups etc.

4. How to write Syntax for cron.

for writing a cron file you just need
(specify the time)+ (specify the task)

Cron syntax sample

In General to specify the time we have 5 parameters (* * * * *)

1. minute
2. hour
3. day (month)
4. month
5. day (Week)

Example
45 23 * * 6 /home/oracle/scripts/export_dump.sh

In addition to this we could use some arbitrary scheduler expression such as

1. @yearly
2. @monthly
3. @weekly
4. @daily

Example
@weekly /bin/script.sh
This will run the script once in a week.

Cron expression syntax is very important for it to work use websites like https://crontab.guru/ and ensure that it works properly.

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2. Setup

Although cron is installed in most Linux distributions you can check if it is present or not.
Just Type crontab and check.

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3. Write your cron Job

1. To see if any cron job already exists

crontab -l

2. Let's write a new cron job

crontab -e
and since cron uses vi by default you may use the below command to edit it using your favorite editor
EDITOR=/usr/bin/vim crontab -e

3. Running a sample cron job

Open up your crontab and write
0 0 * * * date >> /tmp/cron_output

This will run every day at 00:00 in 24-hour format or 12:00 am in 12-hour format and will output the current date to /tmp/cron_output.

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Tips

1. Tests your cron job before deploying.
2. Remember that cron jobs run on a separate thread(daemon thread) and sometimes may have different behavior than intended.

Stay tuned for more.
Next coming up: Sending Automated WhatsApp message using cron