Just spent too much time figuring out how to show a Growl message at a particular time in Mac OS X.  Seems like “cron” has been abandoned in place of launchd, so I had to write my first .plist file, which was simpler than expected.

Here are the steps:

1. First install Growl and growlnotify if you need to.  (download here).  Note for pre-Lion, growlnotify is in Extras in the Growl download.

2. make a script that shows your reminder, such as my schoolClock.sh:

/usr/local/bin/growlnotify -s -m "Time to pick up Isabel!" --appIcon "/Applications/iCal.app"

3. open up “Property List Editor” (which is included in XCode dev tools) and make a plist file in ~/Library/LaunchAgents

I called mine “com.immuexa.schoolClock.plist” and it looks like this:

(click image to see it, note the location and name of the script)

4. run this command in Terminal:

launchctl load -w ~/Library/LaunchAgents/com.immuexa.schoolClock.plist

Do this and you’ll get a message saying “Time to pick up Isabel at 3pm each day”.

For those that don’t have Property List Editor handy, you can use any text editor to create:

<?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.immuexa.schoolClock</string>
    <key>ProgramArguments</key>
    <array>
        <string>/Users/immuexa/cmd/schoolClock.sh</string>
    </array>
    <key>StartCalendarInterval</key>
    <dict>
        <key>Hour</key>
        <integer>15</integer>
        <key>Minute</key>
        <integer>0</integer>
    </dict>
</dict>
</plist>