Create and manage CRON jobs

Hello Friends,

There are lots of things in technical world which are done with more complexity while simple thing attracts me. I always think about simplest way to implement anything until it is really need to be complex with specific and justifiable reasons.

Well, here is my new article with simple thing but not very easily available as per my requirement. So sharing with you guys if could be of any help for someone.

Introduction
“The software utility Cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.”

So, in simple words Cron is a utility/software/application in unix based OS, just like task scheduler in windows based operating system. Purpose of cron job is to provide facility of automated task processing on specific time intervals. Cron jobs are widely used to setup and configure to run shell script on periodic interval which in turn triggers other applications or process to accomplish specific automated process flow.

So that is about cron jobs. Now point is how to access and manage these jobs and answer is Crontab. Crontab is just like a file which contains configuration of all cron jobs scheduled. 

The crontab is a list of commands that you want to run on a regular schedule, and also the name of the command used to manage that list. crontab stands for "cron table," because it uses the job scheduler. 

Let’s come to the practical. As we know unix operating system is command based OS. Most of the work done in Unix is done through commands. In the same way there are some commands which are useful to view and manage cron jobs.

To connect to server to access and manage cron jobs we need an application which helps to execute commands. PUTTY console is one of the option that i am using.

PuTTY is an SSH and telnet client, developed originally by Simon Tatham for the Windows platform. PuTTY is open source software that is available with source code and is developed and supported by a group of volunteers.

You can download PuTTY here

Below are the required details needed  to connect with Putty console.

Server IP - 10.xx.xx.xx
Username - admin
Password - admin

Once connected to server through putty console  we can access Crontab to view and add/edit Cron jobs through commands.  

Below are the  commands which are used to view and manage cron jobs in crontab

Crontab -l :-  This command is used to view list of all cron jobs created and scheduled.

Crontab -e :- This command is used to edit crontab to add or edit  existing cron job configuration.

Crontab -r :- This command is used to remove crontab and to delete configuration of all scheduled cron jobs.

There are few more commands available but above three are sufficient for now.

Now next step is to know about how to schedule a cron job to run on a specific interval.

When you run “crontab -e”command the crontab would be opened in edit mode but you need to press “Insert” key to make any modification in it.

Below are the key points to know while scheduling cron job.

1. Cron jobs are schedules in following fixed pattern
[MOD] [HOD] [DOM] [MON] [DOW] [COMMAND]
MOD - Minute of day - possible values : 0 - 59
HOD - Hour of day - possible values : 0 - 23
DOM - Day of month - possible values : 1 - 31
MON - month - possible values : 1 - 12
DOW - Day of week - possible values : 0 - 6 (from Sun to Sat)
COMMAND - Path of shell  script (.sh file) which need to be triggered on specified schedule   
For ex.

0 16 * * 0 /home/task.sh

Above configuration of cron job is scheduled to run shell script (task.sh) on 0th minute of 16th hour of every day of every month when day of week is Sunday.

Please note that - 
0 - indicates first instance only
* - indicates every instance

2. Press “Esc” key to exit from edit mode of crontab 
3. Then type “:wq” and hit enter key  to save changes in cron tab.

This will display that new cron tab is installing. You can verify you changes later through Crontab -l command.

So that’s all about crontab to create and manage cron jobs in short  but you can further explore topic for in-depth details.