I'm running into a situation where a cron job I thought was running every 55 minutes is actually running at 55 minutes after the hour and at the top of the hour. Actually, it's not a cron job, but it's a PHP scheduling application that uses cron syntax.
When I ask this application to schedule a job every 55 minutes, it creates a crontab line like the following.
*/55 * * * *
This crontab line ends up not running a job every 55 minutes. Instead a job runs at 55 minutes after the hours, and at the top of the hour. I do not desire this. I've run this though a cron tester, and it verifies the undesired behavior is correct cron behavior.
This leads me to looking up what the / actually means. When I looked at the cron manual I learned the slash indicated "steps", but the manual itself is a little fuzzy on that that means
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".
The manual's description ("specifies skips of the number's value through the range") is a little vague, and the "every two hours" example is a little misleading (which is probably what led to the bug in the application)
So, two questions:
How does the unix cron program use the "step" information (the number after a slash) to decide if it should skip running a job? (modular division? If so, on what? With what conditions deciding a "true" run, and which decisions not? Or is it something else?)
Is it possible to configure a unix cron job to run every "N" minutes?
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".
The "range" being referred to here is the range given before the /, which is a subrange of the range of times for the particular field. The first field specifies minutes within an hour, so */... specifies a range from 0 to 59. A first field of */55 specifies all minutes (within the range 0-55) that are multiples of 55 -- i.e., 0 and 55 minutes after each hour.
Similarly, 0-23/2 or */2 in the second (hours) field specifies all hours (within the range 0-23) that are multiples of 2.
If you specify a range starting other than at 0, the number (say N) after the / specifies every Nth minute/hour/etc starting at the lower bound of the range. For example, 3-23/7 in the second field means every 7th hour starting at 03:00 (03:00, 10:00, 17:00).
This works best when the interval you want happens to divide evenly into the next higher unit of time. For example, you can easily specify an event to occur every 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, or 30 minutes, or every 1, 2, 3, 4, 6, or 12 hours. (Thank the Babylonians for choosing time units with so many nice divisors.)
Unfortunately, cron has no concept of "every 55 minutes" within a time range longer than an hour.
If you want to run a job every 55 minutes (say, at 00:00, 00:55, 01:50, 02:45, etc.), you'll have to do it indirectly. One approach is to schedule a script to run every 5 minutes; the script then checks the current time, and does its work only once every 11 times it's called. Such a script would need to allow for some drift; a cron job scheduled for 00:00 might actually run a second or two later, or more if the system is heavily loaded or something goes wrong.
Or you can use multiple lines in your crontab file to run the same job at 00:00, 00:55, 01:50, etc. -- except that a day is not a multiple of 55 minutes. If you don't mind having a longer or shorter interval once a day, week, or month, you can write a program to generate a large crontab with as many entries as you need, all running the same command at a specified time.