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Automating OS X System Maintenance with Anacron

The default install of OS X schedules the cron jobs specified in
/etc/{daily,weekly,monthly} to run at 3am, 4am, and 5am respectively.
This schedule is good in the sense that the tasks will run at a time
when the computer is unlikely to be in use. However, if the computer
is a laptop or office computer that is shut off at the end of the day,
these tasks (including rotating the logfiles, cleaning out /tmp, etc.)
will _never_ be run.

One solution to this problem is to put these system cron jobs under
anacron's control. Doing this has both an upside and a downside:

Upside:   The tasks _will_ be run once a day/week/month.
Downside: The precise time of day when this will happen is not
          guaranteed.

If you want to do this, you must make three changes to the crontabs:

1) Edit /sw/etc/anacrontab and uncomment the lines for
   cron.daily, cron.weekly, and cron.monthly:

##period   delay    job-identifier   command
   1        5       cron.daily       sh /etc/daily	2>&1 | tee /var/log/daily.out	| mail -s "`hostname` daily output"   root
   7       10       cron.weekly      sh /etc/weekly	2>&1 | tee /var/log/weekly.out	| mail -s "`hostname` weekly output"  root
  30       15       cron.monthly     sh /etc/monthly	2>&1 | tee /var/log/monthly.out | mail -s "`hostname` monthly output" root

2) Comment out the corresponding cron.daily, cron.weekly, cron.monthly
   lines in /etc/crontab:

# do daily/weekly/monthly maintenance
#15     3       *       *       *       root    sh /etc/daily   2>&1 | tee /var/log/daily.out   | mail -s "`hostname` daily output"   root
#30     4       *       *       6       root    sh /etc/weekly  2>&1 | tee /var/log/weekly.out  | mail -s "`hostname` weekly output"  root
#30     5       1       *       *       root    sh /etc/monthly 2>&1 | tee /var/log/monthly.out | mail -s "`hostname` monthly output" root


3) Add a line to /etc/crontab to run anacron at whatever schedule you
   would prefer. (Keep in mind that running anacron doesn't
   necessarily do anything; it merely checks to see if anything listed
   in /sw/etc/anacrontab is due to be run. If not, it just exits. 

   As an example, if you wanted it to run once a day at 8.30 in the
   morning you would add the line:

#min    hour    mday    month   wday    user    command
30	8	*	*	*	root	@PREFIX/sbin/anacron -s

   For more on the various permutations of this, consult the crontab.5
   manpage. 

   One other thing to note is that anacron is run by default at
   boot-time, so if the computer isn't on at the time you set in the
   line above, it'll still be guaranteed to run the next time you turn
   on the computer.