How to schedule crontab in Unix Operating Systems

How to schedule crontab in Unix Operating Systems.

 
If you need to run some script at a specific time in a Unix box and wish to do so without manually executing it you could leverage on a program called cron. Through cron you can schedule a script to be executed at any specific time you wish. However the lowest value of time that can be scheduled is minute.

Crontab - Crontab is where you configure what interval and which script to execute.

To see crontab entry.
[root@myserver]# crontab -l

To edit crontab entry.
[root@myserver]# crontab -e


How to schedule cron to execute a script.
You have to edit the crontab to the below format in order for cron to execute your script at your desired time.
*  *  *  *  *  /root/script.sh
| | |  | |
| | |  | |
| | |  | |_____  day of week (0 - 6) (0 to 6 are Sun to Sat)
| | |  |_______ month (1 - 12)
| | |_________ day of month (1 - 31)
| |___________ hour (0 - 23)
|_____________ min (0 - 59)


Examples.
To execute script /root/script.sh every Saturday 1am.
0 1 * * 5 /root/script.sh

To execute script /root/script.sh on every weekdays at 2am.
0 2 * * 1-5 /root/script.sh

To execute script /root/script.sh every 15min daily.
0,15,30,45 * * * * /root/script.sh

To execute script /root/script.sh every 15min daily and output the output of /root/script.sh to /root/script_output.txt.
0,15,30,45 * * * * /root/script.sh >> /root/script_output.txt 2>&1
*Note: 2>&1 means standard error, STDERR (2) store to standard output, STDOUT (1). & infront of 1 (STDOUT) means
           the standard output is stored to the file stated which in this case is /root/script_output.txt.

To execute script /root/script.sh every 15min daily and tell it not to show any output.
0,15,30,45 * * * * /root/script.sh >> /dev/null 2>&1
*Note: /dev/null is commonly referred to black hole where everything directed to it is discarded.


Default location of crontab for all users in most Unix operating system.
The default location where crontab is stored is at /var/spool/cron*
For Linux it is at /var/spool/cron/
For Solaris, AIX and HP-UX it is at /var/spool/cron/crontabs/
0 (0)
Article Rating (No Votes)
Rate this article
Attachments
There are no attachments for this article.
Comments
There are no comments for this article. Be the first to post a comment.
Full Name
Email Address
Security Code Security Code
Related Articles RSS Feed
RHEL: Create a local RPM repository
Viewed 1037 times since Sun, May 27, 2018
List of 10 Must Know Oracle Database Parameters for Database Administrator
Viewed 6043 times since Thu, Jun 21, 2018
Manage Linux Password Expiration and Aging Using chage
Viewed 1164 times since Tue, Sep 11, 2018
What UUIDs can do for you
Viewed 472 times since Tue, Jul 17, 2018
Set Up SSH Tunneling on a Linux / Unix / BSD Server To Bypass NAT
Viewed 86 times since Fri, May 15, 2020
stunnel bacula
Viewed 439 times since Fri, Sep 28, 2018
Top 10 darmowych i publicznych serwerów DNS
Viewed 819 times since Tue, May 22, 2018
OEL 7 – How to disable IPv6 on Oracle Linux 7
Viewed 3393 times since Fri, Aug 3, 2018
RHEL: What is "SysRq key" and how to use it
Viewed 1256 times since Sat, Jun 2, 2018
LVM: Extend an existing Logical Volume / Filesystem
Viewed 762 times since Sat, Jun 2, 2018