Cron Jobs

A Cron Job is a Linux program that allows users to schedule the execution of a piece of software, often in the form of a shell script or a compiled executable.

Cron is typically used when you have a task that needs to be run on a fixed schedule, and/or to automate repetitive tasks like downloading files or sending emails.

At its most basic level, a cron job is an entry written into a table called the cron table, otherwise known as the crontab for short. This entry contains a schedule and a command to be executed.

The cron daemon (crond) looks for entries in the crontab to determine what jobs it should run, and when it should run them according to the specified schedule.

How Do Cron Jobs Work?

Most standard installations of cron consists of two commands:

• cron or crond, which is the daemon that runs the scheduling utility

• crontab, which is the command that allows you to edit the cron entries for your jobs

When you talk about a daemon from a Linux perspective, it’s a program that is running in the background and is noninteractive.

This means that the program does not accept any user input and doesn’t display output to the user.

The word daemon is historically used in a Unix/Linux context and is not a universal term across different operating systems.

The daemon will be running under the root user. You can run the following command to see if cron is running:

$ ps aux | grep cron
root     	617  0.0  0.0   9420  2800 ?    	Ss   17:00   0:00 /usr/sbin/cron -f

Cron Job Schedule Syntax

A basic crontab entry looks something like this, with the cron job schedule first, followed by the command to run:

*    *    *    *    *   /home/user/bin/somecommand.sh
|    |    |    |    |            |
|    |    |    |    |    Command or Script to execute
|    |    |    |    |
|    |    |    | Day of week(0-6 | Sun-Sat)
|    |    |    |
|    |    |  Month(1-12)
|    |    |
|    |  Day of Month(1-31)
|    |
|   Hour(0-23)
|
Min(0-59)

An asterisk (*) matches all values, so if you take a look at the example above, it specifies that /home/user/bin/somecommand.sh should be run at minutes 0-59 and hours 0-23 on days of the month 1-31 for months 1-12 on days of week 0-6 — or in other words "every minute".

Crontab Operators

Cron syntax also utilizes operators for performance. Operators are large inland that operates effectively on the Cron attribute values.

1. Asterisk operator (*)

The asterisk operator denotes any significance or already. Suppose you see an asterisk (*) in the Hour domain, it implies the job will be done every hour. It represents all values.

Utilization of this operator is to gather operating for the entire month or week.

1. Comma Operator (,)

You can stipulate a range of items for regurgitation using the comma operator. It also defines distinct unique values.

1. Hyphen Operator (-)

You can stipulate a set of outcomes using the hyphen operator. If you enter 2-5 in the Weekday domain, the assignment will execute every weekday (From Tuesday to Friday). It also represents a set of parameters.

1. Forward slash Operator (/)

The slash operator allows you to specify values that will be repeated over a specific interval between them. This operator can also be used to separate a number into various stages.

Managing Crontab Entries

Once cron is running, it checks for crontab entries in the following files every minute:

• /etc/crontab

• /var/spool/cron/crontabs/$USER (where $USER is the currently logged-in user).

• From the terminal, enter edit mode for your user’s crontab using the following command:

$ crontab -e

This command is used to remove the existing crontab directory.

$ crontab -r

Crontab -l command help you to show the crontab information.

$ crontab -l

It is used to modify the crontab files of other users. This alternative necessitates the use of system administrative access.

$ crontab -l

This directive is used to eliminate your existing crontab directory, prompting you before doing so.

$ crontab -i