Using Launchd for Cronjobs

Recently I needed to schedule some re-occurring tasks on one of our office-Mac Minis. While I had used launchd before for the obvious service tasks (think SysV init.d scripts) I hadn’t tried it’s cron-like features. Seems like it might safe me some time in the future as using launchd here turned out to be quite straight forward! Not that cron isn’t, but I wanted to try something different here 😉

The process is basically identical to what you do for programs that you want to keep running indefinitely with launchd. The only real difference is that you also set either the StartCalendarInterval or StartInterval property and disable things like RunAtLoad, KeepAlive, and WatchPaths.

A simple yet very annoying example job would say “Hello World” every morning at 09:51:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
  <key>Label</key>
  <string>me.h10n.helloworld</string>
  <key>ProgramArguments</key>
  <array>
    <string>/usr/bin/say</string>
    <string>Hello World</string>
  </array>
  <key>StartCalendarInterval</key>
  <dict>
    <key>Hour</key>
    <integer>9</integer>
    <key>Minute</key>
    <integer>51</integer>
  </dict>
</dict>
</plist>

(Put that into ~/Library/LaunchAgents/me.h10n.helloworld.plist and execute launchctl load ~/Library/LaunchAgents/me.h10n.helloworld.plist to experience it in all its beauty…)

StartCalendarInterval offers everything you’re used to from cron. It supports fields for Hour, Minute, Month, Day, and Weekday of a date and the values are integers with the same semantics as the integer variants in cron. If no value is set, it’s the equivalent to the wild-card character.

Sadly, it looks like defining ranges involves putting all matching intervals into a big array in order to keep the annoyance limited to weekends:

<key>StartCalendarInterval</key>
<array>
  <dict>
    <key>Hour</key>
    <integer>9</integer>
    <key>Minute</key>
    <integer>51</integer>
    <key>Weekday</key>
    <integer>6</integer>
  </dict>
  <dict>
    <key>Hour</key>
    <integer>9</integer>
    <key>Minute</key>
    <integer>51</integer>
    <key>Weekday</key>
    <integer>7</integer>
  </dict>
</array>

StartInterval, on the other hand, allows you to define intervals in seconds. So if I wanted to hear “hello world” every 10 seconds, this would do:

<key>StartInterval</key>
<integer>10</integer>

Pretty much the most annoying thing ever and an instant launchctl unload 😉

As I wrote earlier, everything else is the same compared to usual services. The distinction between the LaunchAgents and LaunchDaemons folders still applies and you can still put shared files into /Library/{LaunchAgents,LaunchDaemons} or their /System counterparts.

If you want to learn more of the details here, take a look at most excellent launchd.info or man launchd.plist 😃