Linux Cron Utility

An experienced Linux sysadmin knows the importance of running the routine maintenance jobs in the background automatically.

Linux Cron utility is an effective way to schedule a routine background job at a specific time and/or day on an on-going basis.

Linux Crontab Format

MIN HOUR DOM MON DOW CMD

MIN = Minute field = 0 to 59
HOUR= Hour field = 0 to 23
DOM = Day of Month = 1-31
MON = Month field = 1-12
DOW = Day of Week = 0-6
CMD = Command = Any command to be executed.

crontab -e to edit crontab file
crontab -l to display crontab file 
crontab -r to remove crontab file
crontab -v to display the last time you edited your crontab file.
crontab -u used in conjunction with other options, this option allows you to modify or view the crontab file of user, when available, only administrator can use this option.

Example:-
vi task.sh
cal >> /home/oracle/test1.txt
:wq

make sure script has all the permissions

chmod 777 task.sh

we are going to run the task every minute

now go to the crontab

$crontab -e

* * * * * /home/oracle/task.sh

:wq

1. Scheduling a Job For a Specific Time

30 08 10 06 * /home/ramesh/full-backup

8-30 am
10th day
6th month (june)
* every day of the week

2. Schedule a Job For More Than One Instance (e.g. Twice a Day)

The following script take an incremental backup twice a day every day.

00 11,16 * * * /home/oracle/incremental-backup

00 = 0th minute
11,16 = 11 am, 4pm
* = every day
* = every month
* = every day of the week

3. Schedule a Job for Specific Range of Time (e.g. Only on Weekdays)

Cron Job everyday during working hours

00 09-18 * * * /home/ramesh/bin/check-db-status

00 = 0th minute
09-18 = 9am, 10am, 11am, 12am, 1pm, 2pm, 3pm, 4pm, 5pm, 6pm
* = every day
* = every month
* = every day of the week


00 09-18 * * 1-5 /home/ramesh/bin/check-db-status

00 = 0th minute
09-18 = 9am, 10am, 11am, 12am, 1pm, 2pm, 3pm, 4pm, 5pm, 6pm
* = every day
* = every month
1-5 = mon, tue, wed, thu and fri (working days)

4. How to View Crontab Entries?

$crontab -l

To view Root user Crontab entries


# crontab -l

To view other linux user's crontab entries

{username} -l

[root@dba ~]# crontab -u oracle -l
* * * * * /home/oracle/task.sh

5. How to Edit Crontab Entries?


$crontab -e

Edit Root Crontab entries

root@dba-db# crontab -e

To edit crontab entries of other Linux users, login to root and use -u 


root@dba-db# crontab -u oracle -e

6. Schedule a Job for Every Minute Using Crontab.

* * * * * CMD


when we specify */5 in minute field means every 5 minutes

7. Schedule a Background Cron Job For Every 10 Minutes.

*/10 * * * * /home/oracle/check-disk-space

Instead of specifying values in the 5 fields, we can specify it using a single 

keyword as mentioned below.

There are special cases in which instead of the above 5 fields you can use @ 

followed by a keyword — such as reboot, midnight, yearly, hourly.

keyword equivalent

@yearly 0 0 1 1 *

@daily 0 0 * * *

@hourly 0 * * * *

@reboot run at startup

8. Schedule a Job For First Minute of Every Year using @yearly

@yearly /home/oracle/red-hat/bin/annual-maintenance

9. Schedule a Cron Job Beginning of Every Month using @monthly

@monthly /home/oracle/suse/bin/tape-backup

10. Schedule a Background Job Every Day using @daily

@daily /home/oracle/arch-linux/bin/cleanup-logs "day started"

11. How to Execute a Linux Command After Every Reboot using @reboot?

@reboot CMD