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Tuesday, 13 March 2018

How to setup cron jobs in Ubuntu

 The Cron software utility is a time-based job scheduler in Unix-like operating systems. Cron allows Linux and Unix users to run commands or scripts at a given time and date. Once can schedule scripts to be executed periodically. It is usually used for system admin jobs such as backups or cleaning/tmp/ directories and more.

The following steps to be followed to set up a cron job in Ubuntu:

  • Connect to server and update the system:
    Before begin with setting up crontab connect the server and update the system software to the latest version available. We can do this by using below command:
    #apt-get update && #apt-get upgrade
  • Check if cron package is installed:
    To check if cron is installed, run the following command –
    #dpkg -l cron 
  • If cron is not installed, install the cron package on Ubuntu:
    One can Install the cron package with package Manager using the following command-

    #apt-get install cron
  • Verify if cron service is running:
    To check whether the cron service is running on the system, we can use the following command-
    #systemctl status cron
  • Configure cron job on ubuntu:
    In order to set up cron jobs, one needs to modify the /etc/crontab file which can be done by only root user. You can edit the crontab file with following text editor.

    Example:

    #nano /etc/crontab

Before we take example of cron tab execution let’s understand the common syntax of cron tab:

Syntax:

* * * * * /path/to/command arg1 arg2
OR
* * * * * /root/backup.sh

In the syntax first * stand for representing minutes [0-59]. Second * stands for representing hour[0-23]. Third * stand for representing day [0-31]. Fourth star stands for representing month[0-12]. Fifth * stand for representing day of the week[0-7].

 
After all step for installation of cron tab and understanding common syntax, let’s execute a cron tab with suitable example.

Example #1: If we want to schedule a backup on first day of each month at 9 PM, the following command performs this operation.

#crontab -e //install your cron job by running this command.
// Append the following entry.

0 9 1 * * /path/to/script/backup-script.sh

 
Example #2: Set up and run php script as cron job to run script every day at 10 AM.

#crontab -e //add cron job

// Append the following entry.
0 10 * * * /path/to/myphpscript.php

Following options are available in crontab:
crontab -l : List the all your cron jobs.
crontab -r : Delete the current cron jobs.

For more information about cron, one can check the manual pages using:

man cron
man crontab 

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