How do I pass command line arguments to Dock items?

You have basically two options:

Option 1: Use Automator to create an application that in effect launches Chrome with command line arguments.

Start Automator and select to create an Application. Double-click Run Shell Script in the Library/Utilities folder and replace the text content — cat — with the following:

open -a "Google Chrome.app" --args -pinned-tab-count=4
# keep the .app suffix or will break with Parallels

Save anywhere you like.

To replace this application's icon, Get Info on your real Google Chrome, click on the icon on the top left, press Cmd-C, Get Info on your Chrome Automator app, click the icon, and press Cmd-V.

Since it's a different application, the Dock will display two Chrome applications when it's running: Chrome, and your Chrome launcher.

Option 2: Edit your application bundle to launch a script instead. This script will start the actual application, adding the command line argument.

Right-click Google Chrome.app and select Show Package Contents. Go to Contents/ and open Info.plist in Property List Editor/Xcode (Apple's developer tools), or a third party plist editor.

Look for the entry CFBundleExecutable or Executable File. Remember its value (e.g. firefox-bin for Firefox). Replace it with parameterized-app.sh.

Open Terminal and enter the following:

touch /Applications/Firefox.app/Contents/MacOS/parameterized-app.sh
open /Applications/Firefox.app/Contents/MacOS/parameterized-app.sh

An editor for the .sh file will open. Set the contents of the file to:

#!/usr/bin/env bash
exec /Applications/Firefox.app/Contents/MacOS/firefox-bin -ProfileManager

(using the actual executable's name you removed from Info.plist, adding the desired command-line arguments)

Save and close. In Terminal, enter the following:

chmod +x /Applications/Firefox.app/Contents/MacOS/parameterized-app.sh

Now, close Terminal and move your application (which must not be running right now) to a different folder and back again. This will update Launch Services, otherwise your changes will be ignored and irritate you immensely.

Now, when you open your application, it will actually execute the .sh file, which will in turn launch the actual executable file, sending the command line args along.

It will look and behave like you expect it to, but you will need to repeat this whenever you update your application, as this will generally replace the application bundle and all the changes you made.

You could write a script that launches Chrome, but that wouldn't be the application icon in the dock, and it would cause a separate Chrome icon to appear. So you'll have to create an application package.

First, make a copy of your Chrome app. Then, there are two approaches that might work; I'm not sure which will play better with Mac OS X and/or Chrome's self-update.

1. Make a 'launcher' that just calls Chrome with the arguments.

   • Open the app package (right-click → Show Package Contents in the Finder), and throw out everything but Info.plist, PkgInfo, and MacOS.

   • In the MacOS folder, delete the “Google Chrome” executable and replace it with a shell script that calls the real Chrome:

     #!/bin/sh
     exec '/Applications/Google Chrome.app/Contents/MacOS/Google Chrome' -pinned-tab-count=4
     

     Don't forget to chmod +x the script.
      

2. Modify the Chrome package you actually run.

   • In the MacOS folder, rename “Google Chrome” and write a shell script which invokes the real thing with the extra arguments:

     #!/bin/sh
     exec '/Applications/My Modified Google Chrome.app/Contents/MacOS/Google Chrome-real' -pinned-tab-count=4
     

   • I see the Chrome package has something about code signing; you might need to delete the CodeResources and/or _CodeSignature to get it to run with these changes.

This is all based on my understanding of Mac app packages; I haven't actually tested these changes on Chrome in particular. It is even posible that the Chrome executable on Mac doesn't support this option, as Mac applications are generally not expected to be run with controllable command-line options (unlike, for example, Windows, where applications are normally launched via shortcuts which can have embedded options).