[CentOS-mirror] Cron help

David Richardson david.richardson at utah.edu
Wed Nov 9 14:01:09 EST 2011

On Wed, 9 Nov 2011, Joao Paulo Saldanha wrote:

> Please,
> I need help setting up a cron to run rsync every 3 hours. Can someone 
> help me create this?
> Wait.
> Thank.


To run a program every 3 hours, use an entry like this:
0 */3 * * * /run/this/program

Cron will run the program when all the fields match.

More explanation (from "man 5 crontab"):

The time and date fields are:

field          allowed values
-----          --------------
minute         0-59
hour           0-23
day of month   1-31
month          1-12 (or names, see below)
day of week    0-7 (0 or 7 is Sun, or use names)

A field may be an asterisk (*), which always stands for "first-last".

Ranges of numbers are allowed.  Ranges are two numbers separated with a 
hyphen.  The specified range is inclusive.  For example, 8-11 for an 
"hours" entry specifies execution at hours 8, 9, 10 and 11.

Lists are allowed.  A list is a set of numbers (or ranges) separated by 
commas.  Examples: "1,2,5,9", "0-4,8-12".

Step values can be used in conjunction with ranges.  Following a range 
with "<number>" specifies skips of the numberâs value through the range. 
For example, "0-23/2"  can be used in the hours field to specify command 
execution every other hour (the alternative in the V7 standard is 
"0,2,4,6,8,10,12,14,16,18,20,22").  Steps are also permitted after an 
asterisk, so if you want to say "every two hours", just use "*/2".

Hope this helps,

David Richardson <david.richardson at utah.edu>
Center for High Performance Computing at the University of Utah
"Perry the Platypus? I swear, out of all the aquatic mammals I
hang with, you are truly the most uncooperative." -- Phineas and Ferb

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