What Is CronJob?
Cron is a job scheduler that runs automatically in most Unix-like operating systems. You can schedule Cron to run when you want it to or schedule jobs to run at specified times.
You can use Cron to schedule jobs to run on specific dates or times. This allows you to automate tasks such as running commands or shell scripts at specified times.
It allows you to automate routine tasks such as performing system maintenance or managing users, downloading files from the internet, or sending emails regularly. It is a daemon, i.e., an application that runs in the background and is always running on a server.
Cron executes what is called CronJobs. Cron is named after the Greek god that represents the cycle of time, called Chronos.
What Is the Application Of CronJobs?
You can use CronJobs to automate repetitive tasks, such as cleaning up databases or creating statistics on users who visit a website.
CronJobs can run single commands, or they can run sequences of commands repeatedly.
Some applications enable you to automatically update RSS feeds, publish new content to a website on a specified date, create multiple invoices for specific customers, or automate newsletter distribution. You can schedule the backup of a database using CronJobs.
What Is the Structure and Syntax of a CronTab File?
Cron uses special files called crontabs, which contain a list of cronjobs to be executed. CronTab stands for Cron Table.
Every line in a CronTab file is a CronJob. It contains commands that are executed in the background. They look like rows of columns separated by a space.
An individual line in a CronTab file specifies when and how often a script or command should be run.
A CronTab file ignores lines beginning with a blank line or a single character, such as a #. Comments that begin with # are considered to be user comments.
It is possible to declare an environment variable or define a CronJob on activated lines in a CronTab file.
Lines with active comments cannot be commented on. It is not possible to add comments to active lines.
It is easy to access the CronTab file by typing crontab-e in a command-line interpreter.
The commands in a CronTab file are checked periodically to ensure they are valid and will run successfully during the specified time. The CronTab file has the basic structure of having a row of columns that look like a table.
* * * * * command to execute
Five columns in the table contain time values defining when a command should be executed. The following are listed from left to right:
- Minutes, specified as 0 – 59.
- Hours are specified as 0 – 23.
- Day, specified as being from 1 – 31.
- Monthly, specified as 1 – 12.
- Every day is listed as 0 – 7; 0 or 7 is Sunday.
There are three types of wildcards for each of these values: “*” for any time (i.e., anytime on any day, hour, or minute), “*/n” for execution every n minutes, and “n,x,y” for execution at n, x, or y minutes.
If you specify the script that is running, it will run whenever you specify it, or if you specify the script that will be executed when you specify it, it will be executed when you specify it.
When the current time, the month, and minute are the same as the time at the beginning of the month or weekday, then CronJob is executed.
Some Cron implementations support special strings. These strings are used instead of the first five fields in the CronTab file.
These strings replace the first five fields of a table containing the current time. A script is run at a specific frequency by each string.
- @yearly / @annually: Runs once a year on January 1st at midnight (0 01 1 *).
- @monthly: Executed on the first of every month at midnight (0 0 1 * *)
- @weekly: Executed once a week at midnight on Sundays (0 0 * * 0).
- @daily: Executes once a day at midnight (0 0 * * *)
- @hourly: Execution occurs every hour (0 * * * * *)
- @reboot: Runs once when the system starts
Examples of CronJob
CronJobs are routine tasks run daily at 2 a.m., such as backing up a database. CronTab files will contain the following information:
0 2 * * */bin/sh backup.sh
There may be times when task execution must be planned only for specific months. An example script that runs in January, May, and August looks like this:
* * * 1,5,8 */bin/sh script.sh
This CronTab file might look like this if you need tasks to run only on selected days at specific times, for example, every Sunday and Friday at 5 pm:
0 17 * * 0,5/bin/sh script.sh
What Is the Importance of CronJob for Online Marketing?
You can automate various tasks in your online marketing using CronJobs, an automated job-generating software based on your website’s content management system (CMS). This can improve your efficiency.
It is possible to automate many time-consuming tasks using CronJobs, such as creating statistics for a website and automatically sending them to a recipient at certain intervals.
You can use CronJobs with CMSs to calculate metrics or send newsletters as part of a marketing campaign using CronJobs.
What is * in CronJob?
Asterisks * are used as wildcards in Cron. * Specifies when a task should be executed, whether in a minute, hour, day, weekday, or month.
What Is the Usage of Cron?
The Cron program allows you to schedule jobs and automate recurring tasks, such as the distribution of newsletters.
What is CronTab?
Individual CronJobs are stored in CronTabs.
How To Schedule a CronJob for backup every day at 12 PM?
Here is an example of a scheduled backup that occurs every morning at 12 PM.:
0 12 * * */bin/sh backup.sh