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Playing with Crons

By Er.Susheel
Before reading this page , I am assuming you are familiar with Linux . Just to recollect 
things crons are nothing but a scheduler 

Uses :

It is used to run a tasks at some predefined time OR we can say if want some task to get run at some time say 3 PM ,without requiring any manual work from us, then its comes very handy

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Crons User :
crons can be set using two types of users one is ROOT user and another is any normal user present on linux machine.

Server/Daemon :
       And crons in linux are managed by a server/daemon know as crond. If you are new to Linux
and want to learn about crons. let me tell you how to do that and to use crons for our daily tasks.

  
To set the crons using ROOT user
# crontab -l        (this will list crons of ROOT user if there is any present,like this below)


[root@master ~]# crontab -l
0 * * * * /etc/webmin/bandwidth/rotate.pl

Once you type above command you will find yourself in a editor ,mostly on vi editor (Editors are another topic which are not going to cover here)

Here IMP thing to understand the syntax of crons , which is as follows

** There are 6 coulmn in crons entries
** 1st coulmn --- >>  represent the Minute
** 2nd coulmn --- >> represent the hour
**3rd coulmn --- >> represent the Day of Month
**4th column --->> represent the Month
**5th column --->> represent Day of week
** 6th column --->> represent Command


Setting Up the crons now.

for this you need to run below command
#crontab -e             (this will invoke ROOT user cron list)

then we can set the cron as per our need , say for example , I want certain command to run every minute of every day and of every month of every day , then I will do this entry in crontab

* * * * * /bin/bash /root/test1.sh > test2.txt

Now, this command will direct the output of /root/test1.sh to the text file.

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