Cron – Why did the cron job run this month

cron

Today is November 1st 2016 or in (unambiguous) numerals, 2016-11-01.

I have a user cron job set up like this:

# m h  dom mon dow   command
33  3   1  */2  *    /home/user/...

It is supposed to run every other month on the first of the month at 3:33am, no matter what day of week that is, but for some reason it was run today, even though 11 is not divisible by 2.

Can someone explain me this? Is my assumption of divisibility by 2 wrong?

EDIT: I forgot to mention, I am running cron version "3.0pl1-127+deb8u1" on a Debian 8.6 "Jessie" machine.

Best Answer

The / is not an arithmetic expression, instead it describes "step values" over the allowed range of values. So, since months always start with 1 instead of 0, /2 would mean "take every other value", resulting in (1, 3, 5, 7, 9, 11).

This is also decribed in the manual page, although this is not terrible clear and easy to understand:

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".

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