Urban Hafner

Ruby, Ruby on Rails, JavaScript programmer. Dabbling with Rust and agile methodologies on the side.

Using Launchd to Manage Long Running Processes on Mac OS X

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I recently had the need to have a long running, user defined process on my Mac. At first I thought about using Monit or Inspeqtor, but then Jérémy Lecour pointed out to me that I could just use the built in launchd.

Lauchd can automatically start processes on startup and it can monitor them and restart them should they abort. Adding one yourself is rather easy. You create a file in ~/Library/LaunchAgents in a certain format. Here’s one of mine:

gnugo13x13.plist
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<?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>gnugo13x13</string>
    <key>ProgramArguments</key>
    <array>
      <string>/Users/uh/bin/cgosGnuGo13x13</string>
    </array>
    <key>KeepAlive</key>
    <true/>
    <key>RunAtLoad</key>
    <true/>
    <key>StandardOutPath</key>
    <string>/Users/uh/Library/Logs/CGOS/gnugo13x13.stdout</string>
    <key>StandardErrorPath</key>
    <string>/Users/uh/Library/Logs/CGOS/gnugo13x13.stderr</string>
  </dict>
</plist>

Then you notify launchd of your new file by running lauchnctl load ~/Library/LaunchAgents/gnugo13x13.plist and you should see a new line in your system.log (accessible through Console.app). If all goes well then that’s all you will see there, but if starting the log didn’t work you will see that mentioned in the system.log, too.

Now let’s go through the interesting parts of that file. As you may have already guessed we essentially setup key value pairs here. An XML element key defines the key name and the next element defines the value.

Label is the name of your job. It needs to be unique and it is used in the system.log whenever there is something happening (stop, start, crash, …) with your job.

ProgramArguments is an array of strings that make up your system call. The first one is the path to the executable you want to run, and the others are command line arguments. If you don’t have any command line arguments you can just use Program. So, I probably should have used Program in my example file, but that’s the actual file from my system and it works, so why change it, right? ;)

KeepAlive is optional and means that launchd will restart your job should it terminate. RunAtLoad is necessary to automatically start your job when you turn on your computer.

The last two, StandardOutPath and StandardErrorPath should be self explanatory. They are paths to files that will be used to log the stdout and stderr of your job. There’s just one thing you need to keep in mind. The folder where these files reside needs to exist before you start the job. It will be created by launchd for you, but it will be owned by root and therefore the job won’t be able to write in there and the job will fail.

Detailed information on everything that you can do with launchd can be found at launchd.info.

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