2009-04-03
The (or a) problem with Unix manpages
Unix has three sorts of manpages: excellent ones that clearly answer your questions, good ones that answer your questions if you read them carefully, and bad ones that aren't worth reading because they don't answer your questions (at least in a way that you can understand).
The first sort are rare and obvious when encountered, which means that when you read a manpage you are generally trying to guess whether you are dealing with the second sort or the third sort. The problem is that there are significantly more of the third sort than there are of the second sort, so people (myself included) are trained into aggressively skimming manpages instead of reading them carefully, because usually reading carefully doesn't actually help and just wastes your time.
And this is a problem because then you run into a manpage that actually does answer your questions, except you didn't bother to read it carefully so you didn't notice (if you are lucky, you notice later). This is usually at least a bit embarrassing.
(This actually generalizes to other documentation, but I think that Unix manpages are a large source of this sort of thing, partly because they aggregate together in one spot a lot of documentation from a lot of different people.)
How to use Vixie cron to schedule at regular odd times
The traditional way of specifying that cron should run a command every ten minutes is to write out a list of every minute that the command is to run on:
0,10,20,30,40,50 * * * * /some/command
Vixie cron introduced a shorthand notation for this: '*/10'. We use
it quite a lot, since it is very convenient; it is both shorter and
more explicit about what is really going on, which means that we are
less likely to make mistakes.
(Quick, spot the error in '10,20,30,40,50'. Or is there even an error? Without additional information on what was actually intended, you can't know for sure.)
But suppose you want to run a command every ten minutes but offset
by two minutes (so that it runs at :02, :12, and so on) because,
for example, you already have another command that runs every ten
minutes and you don't want the two to clash. It turns out that Vixie
cron also has a short form for this, one that I find somewhat less
obvious: '2-59/10'.
(Per the documentation, this means 'every ten minutes starting at :02 and going to :59'; the '2-59' is the range and the '/10' is the skip interval. Cron starts at the start of the range and then skips forward the skip interval every time until it runs out of the range.)
You have to start the range at the offset that you want, in this case two minutes, but the end time doesn't have to be aligned with the skip interval. I end my ranges at 59 minutes by convention, because it's always good enough regardless of what the skip interval is.
(This is one of those entries that I write so that maybe I'll remember the logic the next time around, and if not I can look it up here. Although the manpage actually is quite clear if read carefully, now that I look at it closely.)