What is Cron? All You Need To Know About Cron Jobs


In Unix-like Operating Systems, there is a software utility available to schedule different Jobs and it is known as Cron. In other words, it is simply a scheduler which can be used to run any script supported by the operating system periodically. It can be used to run different kind of tasks automatically on a time basis. For example, to execute a script, every 5 minutes, every day, every Tuesday @ 12:15 pm, etc. Cron can schedule any job upto the accuracy a minute. Cron Jobs can also be used to schedule one-time tasks.

One of the best examples of practical use of Cron Job Scheduling is to regularly backup your server files automatically. Let’s say there’s a script that you’ve to execute manually every time you wish to backup your website files. But if you’ll add that script in Cron Job Schedule, your backup will be done automatically regularly as per the set schedule. It can be used to send birthday messages to everybody sharp @ 12 on their respective birthday dates. It can be used to update the software regularly. Likewise, there are numerous ways in which Cron Jobs are used these days.

CronTab and Cron Commands

There’s a term CronTab here, which simply means Cron Table. It is basically a configuration file. It specifies the shell commands to be executed periodically as per the schedule defined in CronTab. As a web server may have multiple users, therefore, there may be multiple CronTab files as well. But usually, there is always a System Specific CronTab file as well. Only System Administrators have the right to edit System Specific CronTab files.

In every line of CronTab file, there’s a single Cron Job Instruction. An example CronTab instruction is given below.

0 0 * * 0 /home/gurmeet/my-project/index.php

This Cron Instruction tells the system to execute the PHP script at the path /home/gurmeet/my-project/index.php, periodically, every week at the midnight (12:00 am) of Sunday Morning. You might be confused as a beginner how the notation 0 0 * * 0 represents midnight of every Sunday Morning. Don’t worry we’ll understand it deeply later on in this article. But for now, let’s just think of it as the beauty of Cron Job Scheduling.

Cron Job Command Instruction

Each number of the command is separated by a space and followed by the complete path of the script to be executed. Furthermore, each number is basically a time unit. Try to understand this carefully

  • The first number represents a minute and can take value 0 to 59.
  • The second number represents an hour and can take value 0 to 23.
  • The third number represents the day of the month and can take value 1 to 31.
  • The four number represents the month of the year and can take value 1 to 12.
  • The fifth and last number represents the day of the week and can take value 0 to 6. The value 0 represents Sunday. Some Operating Systems allows the value 7 that represents Sunday.
  • * represents any time.

Now if you’ve understood the above timing specification logic used by the CronTab, you can create any periodic time period.

Let’s first decode the time of this instruction. 0 0 * * 0 /home/gurmeet/my-project/index.php

  • As the first and second number represents the minute and hour respectively and in this case, both are 0, therefore it the time of the day is 12:00 AM (Midnight) or simply 00:00.
  • The third and fourth number represents the day and the month respectively and in this case both values as *. This means that it can be any day or any month.
  • And as the fifth number represents day of the week and in this case, it is 0, so the day of the week should be Sunday.

Hence, the CronTab Command is justified. Let’s increase the complexity of the command a bit.

To run a command every 5 minutes, the command will be as defined below.

*/5 * * * *

You can also say, this command is for every fifth minute. The first number is */5 and it represents every 5 minutes. Likewise, the command for every second hour will be as defined below.

* */2 * * *

There are also some short commands to make your work easier. You can schedule some common regular intervals easily with them.

Predefined Cron Scheduling Commands

The commands are clearly specified in the Table image given above. The command 0 0 * * 0 /home/gurmeet/my-project/index.php we understood previously can shortly be written as @weekly /home/gurmeet/my-project/index.php

Cron Expressions can be made even more complex. For example, if you want to send an email to your subscribers every day only in July, October, and December. There is no possibility of telling Cron this time period using only the above-specified ways. Therefore Cron also supports some other symbols for Cron Expression. These symbols are * , - ? L W

Not every number place of the command allows all of these symbols. For better understanding, focus on the following table image.

Cron Expression Symbols

The symbols ? L W are non-standard and are only used in some systems and therefore we won’t talk about them.

0 * * 3-6 *

The above command tells the cron to run At 0 minute in every hour, every day but only from month 3 to 6 i.e. March through June.

0,15,30,45 * * * *

The above command schedule the cron job to run every 0, 15, 30 and 45 minute of the hour, every day, every month and every year and day of the week.

The symbol % doesn’t have anything to do with the timing and scheduling but wherever the command encounters first % symbol, all the data after it is used as standard input the script to be executed by the CronTab.

To make sure your command is correct to execute your scripts on the right time, periodically, you can make use of third party websites and tools to ensure the correctness.

It also shows you the commands for some of the most commonly used Cron Jobs Schedules and also tells the execution time of your custom typed Cron Command in Simple English.

CronTab Commands Formation

How to Use CronTab to Schedule Tasks?

There are two approaches in which you can schedule Tasks on your Linux based server or computer with CronTab Scheduling.

  • Command Line Approach
  • GUI Approach

Command Line Cron Jobs Scheduling

Here’s a link to a detailed tutorial of scheduling Cron Job Tasks with CronTab in Linux Operating Systems.

GUI Cron Jobs Scheduling

A lot of web servers and web hosting providers offer control panels like cPanel or Webuzo and along with it, they also some times offer A CronJob Scheduling Utility. These utilities provide GUI (Graphical User Interface) that lets you schedule various Cron Commands without opening Terminal or any other Command-Line Interface.

I’ve given below the screenshot of Cron Job Schedular available in Webuzo Control Panel of My Web Server.

GUI Cron Job Scheduler

Likewise, following is the screenshot of the CronJob Schedular Available in cPanel.

CPanel Control Job Scheduling

Suggested Readings:

Hope you found this article as useful. If so, do share it with others who might find it useful as well. If you’ve any questions related to Cron, feel free to ask us via the comments. Don’t forget to subscribe to WTMatter for latest updates.

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