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How to use Launchd?

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By Sangjun Sim
1 min read
How to use Launchd?

Long story short,launchd is the framework to automate background jobs on the Mac OS system. It is analogous to systemd in Linux. This requires a plist file that encodes what Mac should do. The plist file follows the syntax of xml language. The example and its interpretation are at the end of this note. Assuming that you wrote a nice plist file for automation, you should:

  1. Save com.user.code.plist in ~/Library/LaunchAgents/.
  2. Load the plist file to launchctl that manages launchd.
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launchctl load ~/Library/LaunchAgents/com.user.code.plist

That’s it. However, if one edits the plist file, one should unload and “reload” the file:

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# com.user.code.plist must be edited
launchctl unload ~/Library/LaunchAgents/com.user.code.plist

# Reload the plist file
launchctl load ~/Library/LaunchAgents/com.user.code.plist

If one wants to see the loading status:

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launchctl list | grep code

Trying out how to write a plist file

For example, the plist file below executes DropboxUpdater every hour (See $3600$ in StartInterval key as integer).

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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>AbandonProcessGroup</key>
<true/>
<key>AssociatedBundleIdentifiers</key>
<array>
<string>com.dropbox.DropboxUpdater</string>
</array>
<key>Label</key>
<string>com.dropbox.DropboxUpdater.wake</string>
<key>LimitLoadToSessionType</key>
<string>Aqua</string>
<key>ProgramArguments</key>
<array>
<string>/Users/user/Library/Application Support/Dropbox/DropboxUpdater/Current/DropboxUpdater.app/Contents/MacOS/DropboxUpdater</string>
<string>--wake-all</string>
<string>--enable-logging</string>
<string>--vmodule=*/components/update_client/*=2,*/chrome/updater/*=2</string>
</array>
<key>StartInterval</key>
<integer>3600</integer>
</dict>
</plist>

There are several keys one should know in order to understand what this file does. First one is <key>LimitLoadToSessionType</key>. This is set as Aqua, which means that it is only activated when logged in. The second one is ProgramArguments. This is the execution point. If you write the path of a file (such as ...Library/Application Support/.../DropboxUpdater.app/Contents/MacOS/DropboxUpdater), then it is run with two arguments wrapped in <string>. Here, one can notice that Dropbox seems to use Chromium when leaving logs.

Programming, Automation
automation launchd
This post is licensed under CC BY 4.0 by the author.