The crontab is the default scheduling Application/program in most of the Linux/Nix boxes.
Crontab is a scheduling tool for programs/scripts/applications to run at defined intervals without administrator interventions.
Crontab = (Cron)oligical+(tab)le is nothing but time-table for executing some scripts at a given point of time

So where we require crontab?

Where there is no need for admin intervention such as taking daily backups, sending mails to particular user, monitoring network etc.

Coming to scheduling task, crontab uses clock daemon (/usr/sbin/cron or /usr/sbin/crond) that executes commands at specific times. The corn table for each user is located in /var/spool/cron/ with their names
suppose if you want to see cron table for root then the file you have to look for cron job for root is /var/spool/cron/root.

Crontab file content

The content of the file can be divided in to 6 fields(first 5 for specifying time and last one for executing scripts/app etc)
The first five fields are separated by either a space or a tab and represent the following unit’s, respectively:

* minutes (0-59)
* hour (0-23)
* day of the month (1-31)
* month of the year (1-12)
* day of the week (0-6, 0=Sunday)

Configuration of crontab for user

Example 1: Creating crontab for a user

#crontab -e -u username

-e is for specifying editing and -u for specifying user name when you are scheduling crontabs for other users
if users want to schedule their own he can just give (crontab -e)

I will take one example here to explain crontab,suppose I want to execute one script located in /var/test/ and save the output to /temp/script.out should be executed every 10th of each month at 9:43pm

#crontab -e -u surya

so here it will open a temporary in /tmp folder

43 21 10 * * /var/test/ > /temp/script.out

here * in month field and week field indicates any month and any week execute this script

after entering the values save and exit the file, A file will be created in as /var/spool/cron/surya with the above content

Example2: seeing crontab for a particular user

#crontab -l -u username


#crontab -l -u surya

If you are a user and want to see your crontab you can just give (crontab -l)

Example3: Removing crontab for particular user

This can be achieved in two ways
1)removing all the crontab entries for a particular user

#crontab -r -l username


#crontab -r -l surya

2)removing one task for particular user,for doing this we have to edit the crontab

#crontab -e -l username

Please see here some of the crontab issues

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Mr Surendra Anne is from Vijayawada, Andhra Pradesh, India. He is a Linux/Open source supporter who believes in Hard work, A down to earth person, Likes to share knowledge with others, Loves dogs, Likes photography. He works as Devops Engineer with Taggle systems, an IOT automatic water metering company, Sydney . You can contact him at surendra (@) linuxnix dot com.