Cron Job In Linux | Scheduling job with crontab in Linux

Cron job are used to schedule commands to be executed periodically. You can setup commands or scripts, which will repeatedly run at a set time. Cron is one of the most useful tool in Linux or UNIX like operating systems. The cron service (daemon) runs in the background and constantly checks the
/etc/crontab file, and /etc/cron.*/ (cron.d/,cron.daily/,cron.hourly/,cron.monthly/,cron.weekly/) directories. It also checks the /var/spool/cron/ directory. Each user can have their own crontab file.

Other than crontab there are 2 more services to schedule the job the "at" command and "batch" command. The "at" command is used to schedule a one-time task at a specific time. The batch command is used to schedule a one-time task to be executed when the systems load average drops below 0.8.

Install or create or edit my own cron jobs:

To edit your crontab file, type the following command at the UNIX / Linux shell prompt:

$ crontab -e

Syntax of crontab (field description)

The syntax is:

    1 2 3 4 5 /path/to/command arg1 arg2

OR

    1 2 3 4 5 /root/backup-script.sh

Where,

1: Minute (0-59)
2: Hours (0-23)
3: Day (0-31)
4: Month (0-12 [12 == December])
5: Day of the week(0-7 [7 or 0 == sunday])

/path/to/command - Script or command name to schedule

Easy to remember format:

Examples of Run backup cron job script:
If you wish to run a backup-script.sh daily at 2.00 AM then first install your cronjob by running the following command as following .
$ crontab -e
append the following entry at the end

0 2 * * * /root/backup-script.sh

save and close the crontab file . The backup-script.sh will run every day at 2.00 AM.

More examples:
To run /path/to/script.sh every 5 min , Enter following command:

*/5 * * * * /path/to/script.sh

To run /path/to/script.sh five minutes after midnight, every day, Enter following:

5 0 * * * /path/to/script.sh

To run /path/to/script.sh at 3:15 PM on the first of every month, Enter following:

15 14 1 * * /path/to/script.sh

To run /path/script.sh at 10 PM on weekdays(Mon,Tue,Wed,Thu,Fri), Enter following:

0 22 * * 1-5 /scripts/script.sh

To run /path/myscripts/script.pl at 23 minutes after midnight, 2am, 4am ..., everyday, Enter:

23 0-23/2 * * * /path/myscripts/script.pl

How do I use operators?

An operator allows you to specifying multiple values in a field. There are four operators:

The asterisk (*) :

This operator specifies all possible values for a field. For example, an asterisk in the hour time field would be equivalent to every hour or an asterisk in the month field would be equivalent to every month.

The comma (,) :

This operator specifies a list of values, for example: "1,5,10,15,20, 25".

The dash (-) :

This operator specifies a range of values, for example: "5-15" days , which is equivalent to typing "5,6,7,8,9,....,13,14,15" using the comma operator.

The separator (/) :

This operator specifies a step value, for example: "0-23/" can be used in the hours field to specify command execution every other hour. Steps are also permitted after an asterisk, so if you want to say every two hours, just use */2.