Using Launchd on Mac!

It is often said that crontab is the simplest tool to set up a scheduling job on Unix-based systems including Mac. However, from my own experience, I learned a lard lesson that how could that supposedly simple job is not indeed.

Obviously, it should be. The process should be as straightforward as the word simple says.

The culprit was the way how crontab works.

Specifically, It doesn't respect the PATH you set on bash.sh nor the location where your desired script is located. More importantly, using Git was pain in my neck.

Despite my efforts to make it work, I couldn't let the script to make simple pull and push operation.

So, here comes launchd on Mac.

The main idea is the same - make a .plist extension file detailing scheduling jobs information into ~/Library/LaunchAgents folder. Then, use launchctl load command to load the job into the scheduler.

First off, plist file structure are as 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.launchd.cron</string>
    <key>ProgramArguments</key>
        <array>
            <string>bash</string> <!-- This line isn't required if *.sh is already executable -->
            <string>/Users/minpark/Documents/Github/nCOV-summary/cron_nCOV.sh</string>
        </array>
    <key>StartCalendarInterval</key>
    <dict>
      <key>Hour</key>
      <integer>15</integer>
      <key>Minute</key>
      <integer>00</integer>
    </dict>
  </dict>
</plist>

As the StartCalendarInterval section implies, the job is to be run daily at 3PM. For the launchd process, you set string value of Label section to be com.launchd.xxx, which is the filename of the file itself excluding the .plist extension.

Now, to register the job to launchd, as I mentioned earlier, you run the following commands on Terminal in the ~/Library/LaunchAgents folder.

launchctl load com.launchd.cron.plist

When you want to unregister, you just unload rather than load.

launchctl unload com.launchd.cron.plist

To check whether your desired job is registered indeed in launchd, you run the following.

launchctl list | grep com.launchd.cron

Here's what is shown in my terminal.

-	0	com.launchd.cron

First dash character means there's no currently running job, and the number 0 indicates the job ended successfully without any errors. When error occurs, a positive number will show up instead of 0.

For scheduling simple shell scripts, launchd doesn't have much advantage. Rather than writing one line in crontab -e command, you have to write down multi-line xml-type file. However, when it comes to Git, this can make your job a lot easier than crontab and that's where launchd shines.