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Topic: How to Automate Tasks with cron Jobs in Linux (on the basis of time,date,month,year, etc)

Q. What is CRON JOB?
A. The cron command-line utility is a job scheduler on Unix-like operating systems. Users who set up and maintain software environments use cron to schedule jobs, also known as cron jobs, to run periodically at fixed times, dates, or intervals.

# Cron Jobs are stored in a location 
ls /etc/cron  #Press tab two times to list all the files

# Restart cron jobs
sudo systemctl status cron.service

# Cron job syntax
crontab -e: edits crontab entries to add, delete, or edit cron jobs.
crontab -l: list all the cron jobs for the current user.
crontab -u username -l: list another user's crons.
crontab -u username -e: edit another user's crons

# List crons
$ cron job example

#  What do **** mean
* *  * * * represents minute(s) hour(s) day(s) month(s) weekday(s), respectively.

*   *   *   *   *  sh /path/to/script/script.sh
|   |   |   |   |              |
|   |   |   |   |      Command or Script to Execute        
|   |   |   |   |
|   |   |   |   |
|   |   |   |   |
|   |   |   | Day of the Week(0-6)
|   |   |   |
|   |   | Month of the Year(1-12)
|   |   |
|   | Day of the Month(1-31)  
|   |
| Hour(0-23)  
|
Min(0-59)

# To list or check the running task use the command.
$ crontab -l

# To establish the task
$ crontab -e #choose nano and hit enter
 * * * * * (5 start denoted by abcde for explaining)
# a b c d e command
# a=minutes (00,..60)
# b=hour (01,02..12)
# c=date (01,02..10,11..30,31)
# d=month (jan,feb,mar,apr,may,jun,jul,aug,sep,oct,nov,dec)
# e=Day (sun,mon,tue,wed,thu,fri,sat)

# schadule the task
11 02 * * * touch /user/Desktop/file.txt

# It will create a file every day at 2 a.m. for 11 minutes. If you fill rest of the three dots you can set a specific date month and day as well.

# To stop the task you can use the command
$ crontab -r #close all schedules without any notification
$ crontab -i -r #close schedules with notifications

# Create user-specific task
$ crontab -u username -e #now again fill the box  as shown below
11 12 * * * mkdir cool #it will create the folder every day for 11 minutes and 12 pm

#repeat task after every 5 hours
0 */5 * * * touch /usr/bin/script

#Repeat the task after every 30 minutes
*/30 * * * * command

#Repeat multiple tasks after 30 minutes
*/30 * * * * command; command

Using special strings to save time on writing cron jobs
1)  @hourly timestamp corresponds to  0 * * * *
It will execute a task in the first minute of every hour.
@hourly /path/to/script

2) @daily timestamp is equivalent to  0 0 * * *
It executes a task in the first minute of every day (midnight). It comes in handy when executing daily jobs.
@daily /path/to/script

3) @weekly   timestamp is the equivalent to  0 0 1 * mon
It executes a cron job in the first minute of every week, whereby a  week starts on Monday.
@weekly /path/to/script

3) @monthly is similar to the entry 0 0 1 * *
It carries out a task in the first minute of the first day of the month.
@monthly /path/to/script

4) @yearly corresponds to 0 0 1 1 *
It executes a task in the first minute of every year and is useful in sending New Year greetings 🙂
@monthly /path/to/script