I recently finished a project that involved generating lists of top ten tweets. It was scheduled to run twice a day, six days a week, and once on the day that the weekly top 40 list ran. This was easy to do as a cron job, as shown below:

00  8	* * *	/path/to/topten.py 
00 20	* * 1-6	/path/to/topten.py

Most of the examples of using launchd .plists only include jobs that occur once a day or on-demand, and the documentation doesn’t make it clear how complex task schedules are set up.

As usual, Stack Overflow provided a wealth of information, and I arrived at the following:

<key>StartCalendarInterval</key>
<array>
    <dict>
        <key>Weekday</key>
            <integer>123456</integer>
        <key>Hour</key>
            <integer>8</integer>
        <key>Minute</key>
            <integer>00</integer>
    </dict>
    <dict>
        <key>Weekday</key>
            <integer>123456</integer>
        <key>Hour</key>
            <integer>20</integer>
        <key>Minute</key>
            <integer>00</integer>
    </dict>
    <dict>
        <key>Weekday</key>
            <integer>0</integer>
        <key>Hour</key>
            <integer>8</integer>
        <key>Minute</key>
            <integer>00</integer>
    </dict>
</array>

As you can see, it is nowhere near as elegant as cron, but it works. It reminds me of a project where we opted for CSV files over XML because the tags were bigger than the data. These days, a JSON version of this would be nice.

The complete .plist file for scheduling this job is shown below:

<?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>local.toptentweets</string>
	<key>ProgramArguments</key>
	<array>
		<string>/path/to/topten.py</string>
	</array>
	<key>StartCalendarInterval</key>
	<array>
		<dict>
			<key>Weekday</key>
				<integer>123456</integer>
			<key>Hour</key>
				<integer>8</integer>
			<key>Minute</key>
				<integer>00</integer>
		</dict>
		<dict>
			<key>Weekday</key>
				<integer>123456</integer>
			<key>Hour</key>
				<integer>20</integer>
			<key>Minute</key>
				<integer>00</integer>
		</dict>
		<dict>
			<key>Weekday</key>
				<integer>0</integer>
			<key>Hour</key>
				<integer>8</integer>
			<key>Minute</key>
				<integer>00</integer>
		</dict>
	</array>
	<key>StandardOutPath</key>
		<string>/path/to/topten.log</string>
	<key>StandardErrorPath</key>
		<string>/path/to/topten.err</string>
</dict>
</plist>

Hopefully, this will save somebody time trying to implement advanced scheduling with launchd.