How to run OxiPNG with launchd and WatchPaths?

Question

I want to run oxipng *.png in /Users/user/Desktop/ immediately after a PNG screenshot (Cmd-Shift-3) is created in my Desktop folder.

I have the following plist file in ~/Library/LaunchAgents/ :

<?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>oxipng</string>
    <key>ProgramArguments</key>
    <array>
        <string>oxipng</string>
        <string>*.png</string>
    </array>
    <key>WatchPaths</key>
    <array>
        <string>/Users/user/Desktop/</string>
    </array>
</dict>
</plist>

Then, I run launchctl load ~/Library/LaunchAgents/oxipng.plist. However, the PNG are never modified after creation.

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How do I make oxipng run after a screenshot is saved to the Desktop folder?

Answer 1

Wildcards are expanded by shells only, so your plist just passes *.png to oxipng, a file which clearly doesn‘t exist. Create a shell script (which then runs cd ~/Desktop; oxipng *.png) and use launchd to trigger that.