Am willing to run a script every 45 minute (not the :45th minute of every hour)
e.g. 10:00, 10:45, 11:30, 12:15, and so on.
*/45 * * * *
Am not sure this is the correct expression.
I suspect (edit: I'm pretty sure by now) that it doesn't do what you want: fields are separate, and */45
for minutes is nothing more than 0,45
. I would use the following three entries if */45
doesn't do the job:
0,45 0-23/3 * * *
30 1-23/3 * * *
15 2-23/3 * * *
If you take a look at entry.c
file in vixie cron sources, you'll notice that each field of each entry is parsed by get_list
and represented as bitmaps of allowed values for that field. That almost precludes any "smart" interpretation, as the distinction of */45
and 0,45
is lost at this stage... but there is a MIN_STAR
flag, set at the presence of *
in minutes (including */45
). So we take a look at cron.c
, a single place where MIN_STAR
is examined, to learn it's unrelated to our problem. Now we know for sure that */45
means "every 45th minute of every hour": 0:00, 0:45, 1:00, 1:45 and so on.
There were two answers here confidently stating the opposite, quoting an unfortunate passage in the manual:
Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2"
We are lucky to have a 24 hour day, containing even number of hours, making "every two hours from 0:00, each day" and "every two hours generally" indistinguishable. Too bad that the manual didn't go far enough to document non-trivial cases, making the impression that * */22
means every 22 hours. It does not. Star with a step is just a shorthand for a list of values in the field where it's used; it doesn't interact with other fields.
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