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October 17, 2014

What does the slash in crontab(5) actually do?

Filed under: UNIX & Linux — Tags: , , , — martin @ 2:16 pm

That’s a bit of a stupid question. Of course you know what the slash in crontab(5) does, everyone knows what it does.

I sure know what it does, because I’ve been a UNIX and Linux guy for almost 20 years.

Unfortunately, I actually didn’t until recently.

The manpage for crontab(5) says the following:

20141017150008

It’s clear to absolutely every reader that */5 * * * * in crontab means, run every 5 minutes. And this is the same for every proper divisor of 60, which there actually are a lot of: 2, 3, 4, 5, 6, 10, 12, 15, 20, 30

However, */13 * * * * does not mean that the job will be run every 13 minutes. It means that within the range *, which implicitly means 0-59, the job will run every 13th minute: 0, 13, 26, 39, 52. Between the :52 and the :00 run will be only 8 minutes.

Up to here, things look like a simple modulo operation: if minute mod interval equals zero, run the job.

Now, let’s look at 9-59/10 * * * *. The range starts at 9, but unfortunately, our naive modulo calculation based on wall clock time fails. Just as described in the manpage, the job will run every 10th minute within the range. For the first time at :09, after which it will run at :19 and subsequently at :29, :39, :49 and :59 and then :09 again.

Let’s look at a job that is supposed to run every second day at 06:00 in the morning: 0 6 */2 * *. The implied range in */2 is 1-31, so the job will run on all odd days, which means that it will run on the 31st, directly followed by the 1st of the following month. The transitions from April, June, September and November to the following months will work as expected, while after all other months (February only in leap years), the run on the last day of the month will be directly followed by one on the next day.

The same applies for scheduled execution on every second weekday at 06:00: 0 6 * * */2. This will lead to execution on Sunday, Tuesday, Thursday, Saturday and then immediately Sunday again.

So, this is what the slash does: It runs the job every n steps within the range, which may be one of the default ranges 0-59, 0-23, 1-31, 1-11 or 0-7, but does not carry the remaining steps of the interval into the next pass of the range. The “every n steps” rule works well with minutes and hours, because they have many divisors, but will not work as expected in most cases that involve day-of-month or day-of-week schedules.

But we all knew this already, didn’t we?

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3 Comments »

  1. I learnt this the day I wanted to create crontabs (that triggered something database related) on different web servers that spoke to a single backend database server but wanted to avoid loading the backend server all at once. So, i did:
    0-55/5 * * * … on the first web server
    1-56/5 * * * … on the second web server
    2-58/5 * * * …on the third web server …etc

    Comment by clonetwin — October 20, 2014 @ 2:00 pm

    • There’s no need to specify the end of the interval unless you explicitely need it.
      0/5 * * *
      1/5 * * *
      2/5 * * *
      or 9/10 * * *
      is sufficient.

      Comment by Stefan Seidel (@seidler2547) — October 21, 2014 @ 9:10 am

      • umm, yep, your’re right. Didn’t think of that. In any case, the range simply was more readable for me (and I hope to whoever had to look at that crontab later).

        Comment by clonetwin — October 21, 2014 @ 10:04 am


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