Scheduling tasks with Mac’s launchd (moving on from cron)

With the move to OSX 10.8.2 I have finally given up on cron as a task scheduler. This post is part a record to help me remember, and partly a tutorial for others.

Not that i think there’s anything wrong with cron … how simple is this:

*/2 * * * * /Users/ra/bash/daemonTest.sh

This means run this script every two minutes — you can get a quick intro to cron here .

Anyhow, the clever folks at apple want us to change to launchd.

They have been pushing on this for a while. As a result, every time i have a problem, someone on stackoverflow reminds me that cron’s been deprecated … It’s been going on so long i’m beginning to feel both both old and silly.

Thus, i decided to make a cup of tea and take on the docs.

The first thing i did was to make a script that would just make a log to tell me that the daemon was running. It’s the `daemonTest.sh` script cron called above.

#! /bin/bash

# cd to the logFiles dir
cd /Users/ra/logFiles/

# log date and time to file
echo $(date +"%Y-%b-%d %H:%M:%S") >> daemonLog.txt

Note that i have a ~/logFiles/ folder. If you don’t, this script will fail (make one with mkdir logFiles from $HOME)

Now we have the script, we need to make it executable.

So cd into the containting directory and do the following:

chmod a+x daemonTest.sh

Next, we need to create an xml file which contains our daemon’s instructions.

There are some examples in /Library/LaunchDaemons/ — check them out!

If i were you i’d open one of these in your text editor, and get ready with the cut and paste. Or you can cut and paste mine (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>com.mcj.testDaemon</string>
    <key>ProgramArguments</key>
    <array>
      <string>/Users/mcj/bash/scrapeLaunch.sh</string>
    </array>
    <key>StartInterval</key>
    <integer>120</integer>
  </dict>
</plist>

I saved this as: ~/launchDaemons/com.ra.testDaemon.plist

If you are going to make a few of these, i’d make an ~/launchDaemons/ directory and save it there — that’s what i’ve done.

Finally, we need to install the daemon. You may need super user (sudo) for this step.

cd ~/LaunchDaemons
sudo cp com.ra.testDaemon.plist /Library/LaunchDaemons
launchctl load -w /Library/LaunchDaemons/com.ra.testDaemon.plist

This should now be installed. To check, type the following in bash:

launchctl list

You should be able to see your daemon in the list that’s returned.

If all is going well, you will eventually have a series of timestamps in your logfile.

Let’s check it out (after waiting a bit).

cd ~/logFiles
cat daemonLog.txt

Okay, so assuming you have time stamps two minutes apart, this works. Success!

Now, it’s a pretty stupid script, so let’s get rid of it to stop it from clogging things up:

launchctl unload -w /Library/LaunchDaemons/com.ra.testDaemon.plist
sudo rm /Library/LaunchDaemons/com.ra.testDaemon.plist

If you want to be sure it’s been stopped, wait a while and cat daemonLog.txt again. The most recent time stamp should be around the time you removed the task.