No more crontab?

There are certain scripts that need to be run regularly. These are found in the /etc/periodic directory and used to be run by cron. If you examine /etc/crontab on Tiger, you'll find this:

 # The periodic and atrun jobs have moved to launchd jobs
 # See /System/Library/LaunchDaemons
 #
 # minute        hour    mday    month   wday    who     command
 
 

In /System/Library/LaunchDaemons you'll find (among other things)

 com.apple.periodic-daily.plist 
 com.apple.periodic-monthly.plist 
 com.apple.periodic-weekly.plist 
 
 

(On more modern versions, you won't find any /etc/crontab at all)

These appear to tell launchd when to run the various periodic tasks. Each has entries similar to this:

         <key>StartCalendarInterval</key>
         <dict>
                 <key>Hour</key>
                 <integer>6</integer>
                 <key>Minute</key>
                 <integer>15</integer>
         </dict>
 
 

So, this task is going to run at 6:15, right?

Maybe.

It will if your computer is never shut down and never goes to sleep. But for most of us, whose systems do sleep now and then, the actual start time will vary. Launchd apparently starts counting time at the last reboot, and counts nothing while sleeping. So, to make it simple, it seems that if you rebooted at midnight, and then had the system sleep for one hour, the task would run at 7:15, and if your waking and sleeping cycles are just right, it will never run.

Even without your accidental delays, Apple introduced another bug into launchd so that causes the daily script to run once per reboot only..

This bug is unfixed as I write this, but of course may be fixed when you read it. If it is still unfixed, you need to run these scripts manually, use cron (set to a time when you will be up and not asleep) or something like Anacron

See Ding, dong, the Cron is dead also.

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