I was playing around with TemperatureMonitor on my parents' iMac, today, and decided to add support for it to mrtg-rmt.  Adding support was easy (included in the 1.8.3 release), but I didn't know exactly how to get the daemon started at boot.  Legacy support in OS X for the usual rc.local hacks probably exist, but I figured I would do it right.  So, I stumbled upon launchd, which is a subsystem in OS X that provides granular control to the starting and stopping of daemons and optionally can act as a full-blown xinetd/inetd replacement.

I started out by reading parts of the launchd.plist(5) and launchctl(1) manpages, and threw together a simple XML propertly list file for mrtg-rmt:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
 "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
     <key>Label</key>
     <string>com.prolixium.mrtg-rmt</string>
     <key>ProgramArguments</key>
     <array>
          <string>/usr/local/sbin/mrtg-rmt</string>
     </array>
     <key>RunAtLoad</key>
     <true/>
</dict>
</plist>

Apparently there are a couple locations for the plist files.  The system (ie, don't touch) ones are in /System/Library/LaunchDaemons, and the custom ones should go in /Library/LaunchDaemons.  I placed com.prolixium.mrtg-rmt.plist in there, and ran the following:

% sudo launchctl load -w /Library/LaunchDaemons/com.prolixium.mrtg-rmt.plist

Neat, it started up all by itself:

% ps ax | grep mrtg-rmt
  866   ??  Ss     0:00.04 /usr/bin/perl -w /usr/local/sbin/mrtg-rmt
  870 s001  S+     0:00.00 grep --color -i mrtg

The plist XML schema has several other nifty options, too, but I didn't really explore them.  The whole thing sure beats putting scripts into /etc/init.d!  launchd reminds me of Sun's SMF stuff, too - but somehow more elegant and easier to use…

Read more: Getting Started with launchd

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