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Fixing Mac OS X's "periodic" command

I've been discussing the key tasks that a Mac OS X user has to run to ensure that their system stays healthy and in tip-top shape, and one set that we've all agreed upon are the daily, weekly and monthly cron jobs. You can figure out when they're run with a simple grep command.

Dave's Answer: Here's the command:
$ grep -E '(daily|weekly|monthly)' crontab
# Run daily/weekly/monthly jobs.
15 3 * * * root periodic daily
30 4 * * 6 root periodic weekly
30 5 1 * * root periodic monthly
These entries are read, left-to-right, as minute, hour, day, month, day-of-month, so you can see that my daily cron job is run at 3:15am every night, my weekly job at 4:30am on the 6th day of each week (Sunday, because Unix starts counting at day = 0), and the monthly job is run at 5:30am on the first day of each month...

If you don't have your computer running at 3:15am every night - and you probably don't because either you shut it off or you have it "sleeping" - then an easy way to run the daily job is to duplicate the command:

$ periodic daily
But that's not a good solution! Why? Because the periodic script needs to run as the root user, and it's not smart enough to check and ensure that's the case. Instead, you get lots of weird errors about permissions being incorrect. To fix it, you need to actually run the command:
$ sudo periodic daily
But why not instead fix periodic to ensure that you're running as root? Well, one reason is that periodic can ostensibly be run as other users, but if you check out the man page, you'll see it's described: "The periodic program is intended to be called by cron(8) to execute shell scripts located in the specified directory."

How do you fix a system script to run the way you want? There are two approaches. One is that you can actually edit the periodic script itself, which involves you finding out where it lives:

$ which periodic
/usr/sbin/periodic
$ ls -l /usr/sbin/periodic
-r-xr-xr-x 1 root wheel 2936 12 Sep 20:18 /usr/sbin/periodic
$ file /usr/sbin/periodic
/usr/sbin/periodic: a /bin/sh - script text executable
You can see it's a shell script (as I suspected) and that it lives in the system directory /usr/sbin (I'm on Panther, btw, for all of this). The first few lines of the script is where we could add a simple test to ensure that it's being run as root, as the highlighted lines below show:
$ head -24 /usr/sbin/periodic
#!/bin/sh -
#
# $FreeBSD: src/usr.sbin/periodic/periodic.sh,v 1.9.2.7 2000/11/26 06:06:18 kris Exp $
#
# Run nightly periodic scripts
#
# usage: periodic { daily | weekly | monthly } - run standard periodic scripts
# periodic /absolute/path/to/directory - run periodic scripts in dir
#

usage () {
  echo "usage: $0 " 1>&2
  echo "or $0 { daily | weekly | monthly }" 1>&2
  exit 1
}

if [ $(id -u) -ne 0 ] ; then
  echo "You need to be root to run this command: use sudo first" 1>&2
  exit 1
fi

if [ $# -lt 1 ] ; then
  usage
fi
If you're not comfortable hacking into system shell scripts - which I can totally understand - then an alternative is to create a short wrapper script that you ensure is earlier in your path. It might look like this:
#!/bin/sh
# wrapper script for 'periodic' that ensures it's run as root

if [ $(id -u) -ne 0 ] ; then
  echo "You need to be root to run this command: use sudo first" 1>&2
  exit 1
fi
exec /usr/sbin/periodic "$@"
Short and sweet. One more step needed: check your path to ensure that it's in a directory that shows up prior to /usr/sbin or, even better, create an alias that ensures it's the one executed when you type in periodic at the command line:
alias periodic="$HOME/bin/periodic"
Drop this into your .bashrc or .cshrc (depending on your login shell), log out, log in again, and next time you're on the ball and remember to run your periodic commands, but forget to use sudo, you'll get the friendly error message:
$ periodic weekly
You need to be root to run this command: use sudo first
There's a lot more we can do with this idea, too, some of which is discussed - at length, with plenty of fun examples - in my new book Wicked Cool Shell Scripts.

I hope this article was helpful reading!



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Comments

I appreciate your thoughtful tip. Another very useful piece of information would be a date for the article's authoring. Apple changes it's OS quite frequently and following tips for an out-of-date system can wreak havoc.

In my particular case, I'm trying to ensure that my monthly periodic script runs on the 2nd of each month but my users' crontabs (including roots) do not have periodic included, my guess being that launchtl now has taken over such functions.

msq

Posted by: mistersquid at February 27, 2006 1:38 PM

hi this is ramana, i want hellp, script
to download files from remote server to local unix sever using shell script. pls help me.
thank u

Posted by: ramana at February 21, 2007 7:50 AM


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