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What is CronTab? What does it do? 



What if you want to execute a file daily at particular time?Running it manually daily is 
absolutely not a good idea..! 
Then What do you do?For this kind of manual works,To Automate it,
there is a daemon in linux called "CRONTAB".

Crontab:            

Crontab called as CRON TABle Where 
cron is a daemon to run periodically on a given time and Table consists of scheduled tasks,
jobs 
i.e, Crons.

Crontab Options: 
 
crontab -l : lists all the crontabs scheduled.                                                    
crontab -e : Edits existing cron or else create new cron.                                    
crontab -r : Removes scheduled cron jobs.                                                  
crontab -i -r: Prompts before deleting user's crontab i.e,asks your permission. 
crontab -u username -l : lists all the crons of a particular user. 
 
Setting up a cron:
Press Ctrl+Alt+t to open the terminal and paste following command.
 
crontab -e  
 
Syntax : 
 
 *         *        *                *           *  /home/SendEmail.php 
 
 * means: 
 
 (minute)   (hour)    (day of month)   (month)   (day of week)  /Path_of_the/file/to_be_executed
 
Allowed Values: 
 
(0-59)     (0-23)    (1-31)           (1-12)     (0-6) 
  
->Above cron runs run for every minute
 
What is the difference between day/month and day/week? 
If Day/Month : 5 and Day/Week :5 then cron gets executed on 5th date of specified month 
and 5th(Friday) of every week in the specified month. 
 Day/Month is Once a Month. 
 Day/Week is 4 times i.e, no.of weeks in a month having friday.
 
Cron Special keywords:

@yearly (or) @manually - Runs once a yearly at 12.00 AM on January 1st.
@monthly               - Runs once every month 1st at 12.00 AM. 
@weekly                - Runs once a week at 12.00 AM on Sunday.
@daily                 - Runs once daily at midnight 12.00AM 
@hourly                - Runs once an hour at the start.
@reboot                - Runs on starting up the system. 


 Where does crontab present in linux  OS? 


 Linux      :    /var/spool/cron/crontabs/   
Try examples here..                https://crontab.guru/  


 


Examples: 

                                                              




MinuteHour      Day/Month Month      Day/Week  Executes at                                            


*****At every minute.


4511-18**1-6At minute 45 past every hour from 11 through 18 on every day-of-week from Monday through Saturday.


5,4512,3*1-51-5At minute 5 and 45 past hour 12 and 3 on every day-of-week from Monday through Friday in every month from January through May.   











































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