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I am trying to schedule a job which starts 6 mins past the hr every hr and then runs every 15 mins. So it kicks off at 10:06 then 10:21, 10:36,10:51. I know one way is : 06,21,36,51 * * * * /bin/foo

However why doesn't something like this work: 06/15 * * * *

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Because no one found it useful enough to write the code and make sure it is unambiguous. Because that's the problem with this: You can try to add all kind of fancy syntaxes for adding times, but they must be easy to understand and precise. – Sven Aug 11 '10 at 22:06
up vote 3 down vote accepted

You actually want it separeted by comma with no space between it, so it takes place in all the minutes you want:

6,21,36,51 * * * * /bin/foo

or you could use it every 15 minutes which would look like this:

6-59/15 * * * * /bin/foo

For more information you can man crontab 5 or check this url

The 6/15 doesnt work because it doesnt know how to divide it, if you had it like 0-59/15 it could work but then it would not count the initial 6 that you wish for.

The initial number should be a range that will be divided by the number after the / to run every X minutes (where X is the number that comes after the /) within the range you gave it.

another example would be 0-40/5 which would run every 5 minutes until the 40 minutes.

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Would 6-59/15 have the right effect, do you think? – Slartibartfast Aug 12 '10 at 5:19
Yes i belive it would i won't say for certain as i dont make much use of irregular minutes like you want to but in theory yes it should work. – Prix Aug 12 '10 at 6:04

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