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/
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