LinuxQuestions.org - Crontab Question

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Crontab Question

When I execute crontab -l

I get get this listing for one of the jobs:

5 0-20/4 * * *

I understand the first set is minutes and the second is hours, but what is the "/4?" Does that mean the job runs 5 minutes after the hour every 4 hours between midnight and 8pm? :)

Nope. It means "run every 4 hours in the range from 0 to 20. An excerpt from man 5 crontab explaining it all:

Code:

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 number’s 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".

Your response looks like what I explained in my first post (except the 5 minutes part). And, 0-20? Doesn't that mean midnight - 8pm?

Oh yes. Sorry. I read your post too quickly! Indeed the job will run at 00:05, 04:05, 08:05, 12:05, 16:05, 20:05. Hours in crontab are in the 24-hours format.