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: 

Minute	Hour	Day/Month	Month	Day/Week	Executes at
*	*	*	*	*	At every minute.
45	11-18	*	*	1-6	At minute 45 past every hour from 11 through 18 on every day-of-week from Monday through Saturday.
5,45	12,3	*	1-5	1-5	At 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.